WO2010051325A1 - A hybrid nanostructure composed of a natural photosystem and semiconductor nanoparticles - Google Patents

Abstract

A hybrid nanostructure comprises at least one semiconductor nanoparticle bound to a photosynthetic unit of a photosynthetic organism, and the methods of making and using the hybrid nanostructure. The semiconductor nanoparticle and the binding between the nanoparticle and the photosynthetic unit are suitable for an efficient forster energy transfer of excitons from the nanoparticle to the photosynthetic unit. The hybrid nanostructure is suitable for light energy harvesting.

Description

TITLE:

A HYBRID NANOSTRUCTURE COMPOSED OF A NATURAL PHOTOSYSTEM AND SEMICONDUCTOR NANOPARTICLES

BACKGROUND OF THE INVENTION

[0001] 1. Field Of The Invention

[0002] This invention relates generally to the field of an artificial photosynthetic system for solar fuel production, or a hybrid photoactive nano structure suitable for an enhanced rate of photochemical or photoelectronic energy production.

[0003] 2. Description Of The Related Art

[0004] Green plants, purple bacteria, cyanobacteria and other photosynthetic bacteria capture and utilize sunlight by means of molecular electronic complexes or photosynthetic units, reaction centers (RCs) that are embedded in their membranes. Photosynthetic RC complexes in plants, algae and of a variety of bacterial species convert the electromagnetic energy of sunlight into chemical potential energy. Although RCs from different photosynthetic organisms vary in their structure and composition, they are always composed of complexes of pigments and proteins. In eukaryotes and prokaryotes, pigment-protein complexes use light energy to drive a series of electron transfer reactions that are coupled to the translocation of protons across a charge-impermeable membrane. The established proton electrochemical gradient is then used to drive the synthesis of adenosine triphosphate (ATP) and other energy-dependent processes. [0005] The simplest and best understood RC is that found in purple bacteria, which can be taken as a model of all the photosynthetic reaction centers. In the intact photosynthetic system of purple bacteria (Rb. sphaeroides, for example), light energy is first absorbed, in the form of an excited electronic state (exciton), by the antenna BChIs and carotenoids that reside in the light-harvesting (LH) pigment-protein complexes surrounding the RC in the membrane. Then, excitons (or electron-hole pairs) are transferred via foster resonance energy transfer (FRET) to specialist chlorophyll cofactors in RCs, where the excitons become dissociated into their constituent carriers. In the next step, photo-excited electrons and holes are used in chemical transformationslO. [0006] The straightforward way to construct an artificial photosynthetic device for practical solar fuels production is to mimic the structural and functional organization of the natural photosynthetic machinery. Such a photosynthetic device should include, as one of its basic features, a built-in antenna for efficient light harvesting and excitation energy transfer to RCs for charge separation.

[0007] Based on the principle of photosynthesis, a variety of artificial antenna systems have been synthesized. These systems utilized supramolecular chemistry in the form of light-harvesting dendrimers, which incorporated porphyrins, other organic fluorophores, other organometallic complexes, and other chromophores (Gust, D., Moore, T. A. & Moore, A. L.,Acc. Chem. Res. 34, 40-48 (2001); Balzani, V., Credi, A. & Venturi, M., ChemSusChem 1, 26-58 (2008)). Although efficient excitation energy transfer was obtained in such systems, the use of organic fluorophores in light harvesting systems suffer from their narrow light-collecting spectral windows, small Stokes shifts and low photostability.

BRIEF SUMMARY OF THE INVENTION

[0008] Broadly, the present invention provides for a wide spectral artificial photosynthetic system suitable light energy harvesting and excitation energy transfer. [0009] Preferably, the system is a hybrid of inorganic semiconductor compound and a natural photosynthetic unit of a photosynthetic organism. The photosynthetic unit can be a reaction center (RC), a chloroplast, or a thylakoid. The semiconductor nanoparticle is bound to the photosynthetic unit so that the semiconductor nanoparticle transfers electron-hole pairs (excitons) to the photosynthetic unit through a Forster Resonance Energy Transfer (FRET) coupling. The photosynthetic organism is a purple bacterium, a cyanobacterium, a green plant, or another similar photosynthetic organism. [0010] Preferably, the semiconductor nanoparticles are bound to one or more sites of the photosynthetic unit, wherein the sites comprise electron donor sites, electron acceptor sites, or both. More preferably, A Forster radii (Rp) of the hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit, wherein the FRET efficiency (E₀) = 1/(1+(R/Rp)⁶) and wherein R_min =(D_SN+ D_PU)/2; D_SN is the diameter of the semiconductor nanoparticle, and Dpu is the diameter of the photosynthetic unit. Or more specifically, the efficient FRET of excitons occurs when peak intensities of exciton emission of the semiconductor nanoparticle are reduced when the semiconductor nanoparticle is suitably bound to the photosynthetic unit.

[0011] Preferably, the semiconductor nanoparticle is bound to the photosynthetic unit through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof. The length of the biolinker should be suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center.

[0012] According to some embodiments of the present invention, the biolinker is a bifunctional connecting molecule. Preferably, the biolinker comprises a carbodiimide group, a carboxylic group, an amino group, or a mixture thereof. Specifically, the biolinker assists in forming one or more covalent bonding between the semiconductor nanoparticle and the photosynthetic unit.

[0013] In a preferred embodiment, the semiconductor nanoparticle is suitable for an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit. Preferably, the diameter of the semiconductor nanoparticle is in the range of about 2nm to about 20nm. More preferably, the semiconductor nanoparticles include, but are not limited to, quantum dots, nanowire, nanorods, or a mixture thereof. Unlimited examples of the quantum dot suitable for the present invention are a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof. [0014] According to some embodiments of the present invention, the photosynthetic unit can be a reaction center. The reaction center is capable of sequential electron transfer, and it includes one or more quinones.

[0015] A preferred method of making the hybrid nano structure of the present invention includes (a) preparing a first aqueous solution comprising at least one photosynthetic unit (PU) of a photosynthetic organism; (b) preparing a second aqueous solution comprising one or more semiconductor nanoparticles; (c) adding the first solution to the second solution; (d) agitating the mixture of step c until the hybrid nanostructure is formed. Within the hybrid, the semiconductor nanoparticles efficiently supply electron-hole pairs (excitons) to the RC through a Forster Resonance Energy Transfer (FRET) coupling.

[0016] Preferably, the first aqueous solution and the second aqueous solution are substantially compatible with each other to form the hybrid, wherein the integration of the semiconductor nanoparticle into the hybrid does not interfere with the process of sequential electron transfer.

[0017] More preferably, the Forster radii (R_F) of the resulting hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the quantum dot to the reaction center, wherein the FRET efficiency (Eo) = 1/(1+(R/R_F)⁶) and wherein R_min =(D_SN+ D_PU)/2; D_SN is the diameter of the semiconductor nanoparticle, and Dpu is the diameter of the photosynthetic unit.

[0018] In a further embodiment, the first aqueous solution further comprises a compound capable of being an electron donor (the electron donor compound). The electron donor compound keeps the quinones fully reduced and the reaction center photochemically inactive. Moreover, in the presence of the electron donor, the reaction center is stable against photo-oxidation.

[0019] Preferably, the electron donor is selected from the group consisting of an electron donating dye, an electron donating compound, or a mixture thereof. Unlimited examples of the electron donor are a condensed polycyclic compound, an amine compound, a quaternary amine compound, an aniline derivative, a nitrogen-containing heterocyclic compound, a sulfur-containing heterocyclic compound, an oxygen- containing heterocyclic compound, a nitrogen- and oxygen-atom containing heterocyclic compound, a nitrogen- and sulfur-atom containing heterocyclic compound, an oxygen- and sulfur-atom containing heterocyclic compound or a sulfur-, nitrogen- and oxygen- atom containing heterocyclic compound. More preferably, the electron donor is sodium- ascorbate.

[0020] Most preferably, the photosynthetic units and the semiconductor nanoparticles are in a molar ratio that ensures sufficient excitons is transferred from the semiconductor nanoparticles to the photosynthetic units. Specifically, the molar ratio is sufficiently small so that A(λ_eXC) is greater than 1, wherein A is an enhancement factor; λe_XC is an excitation wavelength, and A (λe_XC) = PLh_yb_πd/PLpu, wherein the PLhybπd is ^a photoluminescence peak intensity for the hybrid, and the PLpu is a photolumiescence peak intensity for the photosynthetic unit. [0021] According to some embodiments of the method, the photosynthetic unit comprises at least one reaction center (RC). The reaction center comprises one or more quinones, and wherein the reaction center is capable of sequential electron transfer. [0022] According to some embodiments of the method, the second aqueous solution further comprises one or more stabilizing agents. The stabilizing agent forms a coating around the semiconductor nanoparticle to stabilize and solubilize the semiconductor nanoparticle in the second aqueous solution. Preferably, the thickness of the coating suitable to ensure that the FRET of exciton from the semiconductor nanoparticle to the photosynthetic unit is efficient. The stabilizing agent can be, but is not limited to, a mercapto-compound, another similar stabilizing molecule, or a mixture thereof. Specifically, the stabilizing agent can be a thio-containing polyethylene glycol molecule; a cysteine moiety; a thioglycolic acid; or a combination thereof. [0023] In addition, the fluorescent wavelengths of the semiconductor nanoparticles are sufficiently overlapped with absorption wavelengths of the photosynthetic unit to ensure an efficient Forster Resonance Energy Transfer (FRET) of excitons from the semiconductor nanoparticles to the photosynthetic units. [0024] Preferably, the semiconductor nanoparticle includes a quantum dot, a nanowire, a nanorod, or a mixture thereof. The quantum dot can be, but is not limited to, a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof; and wherein the quantum dot comprises a quantum dot fluorescent core. The quantum dots are bound to the reaction center through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof. [0025] In a further embodiment of the method, the second aqueous solution further comprises a linking agent, wherein the linking agent becomes a biolinker when the hybrid is formed. Preferably, the length of the biolinker is suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center. The linking agent is a compound containing a carbodiimide functional group with a formula of RN=C=NR (the carbodiimide compound), wherein R comprises a carboxylic function group, an amino function group, or a mixture thereof. The carbodiimide compound assists in forming a covalent bond between the quantum dot and the reaction center. More preferably, the linking agent can also be the stabilizing agent. [0026] In addition, the binding of the quantum dot to the reaction center can be shown by a reduction in peak intensities of exciton emissions of the quantum dot when the hybrid is formed.

[0027] Embodiments of the invention address some or all of the concerns with the prior art. This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention may be obtained by reference to the following description of the preferred embodiments thereof in connection with the attached drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0028] Fig. 1 is a schematic illustration of the hybrid nanostructure (the hybride) of an embodiment of the present invention. The hybrid comprises a quantum dot nanoparticle (the semiconductor nanoparticle) and a purified (or isolated) reaction center of a purple bacteria Rb. sphaeroides. Photons are absorbed by both the reaction center and the quantum dot. An exciton from the quantum dot 11 is transferred to the reaction center 12a via forster resonance energy transfer (FRET), and the excitons inside the reaction center 12a relax to the lowest energy level located at the special pair 12b (P or P870). [0029] Fig. 2a is an energy level diagram of an embodiment of the present invention, illustrating energy relaxation inside a reaction center after a quantum dot supplies excitations to the reaction center via FRET. The excitation quickly relaxes to P870-Qy level. The relaxation times are taken from the article by Balzani et al. (Balzani et al., 2008, ChemSusChem 1, 26-28). [0030] Fig. 2b illustrates an energy level diagram of an embodiment of the present invention as shown in Example 1, illustrating molar extinction coefficients of the reaction centers and QD570-CdSe emitting at 570nm, and also illustrating the photoluminescent intensity of the quantum dot.

[0031] Fig. 3a is an illustrative diagram of normalized photoluminescent (PL) emission intensity of CdSe/ZnS QD530 and QD570 at the exciton wavelength as a function of the reaction center (RC) concentration shown in RC/QD ratios for the embodiment of Fig. 1. QD 530 refers to the quantum dot emits at 530nm, while QD 570 refers the quantum dots emits at 570nm. The data are taken from Example 1. [0032] Fig. 3b is a diagram for the embodiment of Fig. 1 as shown in Example 1, illustrating photoluminescent (PL) intensity of QD530-CdSe/ZnS, a reaction center (RC) and a hybrid of QD-RC as a function of time in ps for either the QD region (520-65nm) or for the RC region (>700nm). The diagram demonstrates the fluorescent decay kinetics of QD530-CdSe/ZnS, the reaction center (RC) and the hybrid of QD-RC for both the QD region and the RC region. All samples were excited with 266 nm picosecond laser pulses. Kinetics were obtained in two spectral regions: 520-650 nm (QD-region) and >700 nm (RC -region). For the RC -region, RCs data are shown as a black curve and the data for QD-RC complex are depicted as a red curve. For the QD region, QDs data are given by a black curve and QD-RC complexes data are shown with a red curve. Grey lines show exponential fits at long times which includes only the slow exponents from Table 1.

[0033] Figs. 4 a-b are diagrams illustrating the FRET and optical enhancement in the hybrid of CdTe QDs and reaction centers using data from Example 1. Fig. 4a illustrates quenching of excitonic PL of QDs as a function of the RC concentration. PL excitation wavelength was 450 nm. The concentration of QD530 was 0.5 μM and concentration of QD570 was 0.25 μM. The volume of RCs for each addition step RCs was 4.8 μL and 2.4 μL respectively. Dilution factors as well as inner filter and re- absorption effects were taken into account (see SI). Fig. 4b illustrates an increase of the PL peak at 910 nm associated with the PL emission for the BChI special pair (P870) of RC. Figs. 4c-4d show the summary of the data for an enhancement of PL emission from RC. The enhancement coefficients (colored squares) as a function of the excitation wavelength are given for the 910-nm PL band of P870 for complexes based on CdTe QD530 and QD570; Na-ascorbate (10 mM) was added to prevent RCs from photo- oxidation. Solid curves are calculated from Eq. 3 for the corresponding complexes. Panel d also includes the solar spectrum. Eq. 3 is shown as

Λi_o™ (Λχ_C ) = 1 + £ ^{■ IQ} = 1 + E - X - ^{£QD exc} . Moreover, Eq. 3 is substantially the

I RC (AJ ZRAAXC )

( _{k ε} same as Eq. S7, which is shown as A₉₁₀ (λ ) = 1 + _ ^FRET ^-

T 4- J? P

\ ¹QD ^τ "-FRET ^C«C

[0034] Fig. 5a is a diagram of the embodiment of Fig. 1 as shown in Example 1, illustrating optical densities of the reaction center in various concentrations of Na Ascorbate (mM) as a function of wavelength (nm). The concentration of the reaction center (RC) is about 1.7μM. This UV- vis spectrum of the RC illustrates the attribution of some of the bands. Na-ascorbate (or Na Ascorbate or NaAsc) is a reduction agent or an electron donor compound.

[0035] Fig. 5b is a diagram of the embodiment of Fig. 1 as shown in Example 1, illustrating photoluminescent intensities (counts) of the reaction center in various concentrations of Na-ascorbate (mM) as a function of wavelength (nm). The concentration of the reaction center (RC) is about 1.7μM. [0036] Fig. 5c is a diagram of the embodiment of Fig. 1 as shown in Example 1, illustrating peak amplitudes (counts) of the reaction center in various concentrations of Na Ascorbate as a function of various concentrations of Na Ascorbate (mM) at a wavelength of 860nm and at a wavelength of 920nm. The concentration of the reaction center (RC) is about 1.7μM. PL bands with the maxima at 860 nm and -910 nm, from the Panel b, are fitted with Gaussian curves and the amplitudes of these bands are plotted as a function of concentration of Na-ascorbate.

[0037] Figs. 6a-d are diagrams of absorption and fluorescent emission spectra of

CdSe/ZnS core/shell and CdTe QDs for the embodiment of Fig. 1 as shown in Example 1. More specifically, Figs. 6a-6b illustrate the absorption spectra of CdSe/ZnS and CdTe QDs respectively, in which excitation coefficients (M ¹Cm^"1) of the QDs are plotted as a function of the wavelength (nm). Figs. 6c-6d illustrate the fluorescent emission spectra of CdSe/ZnS and CdTe QDs respectively, in which PL intensities (normalized-Fig. 6c or not normalized-Fig. 6d) of the QDs are plotted as a function of the wavelength (nm). [0038] Fig. 7a is a diagram illustrative of a quenching of fluorescent intensities of

QD upon assembly with the reaction center in various molar ratios of RC:QD for the embodiment of Fig. 1 as shown in Example 1. The molar ratios of RC:QD are formulated with different amounts of the RCs and the QDs as shown in Table 4a. In the diagram, the PL intensities (a.u.) of hybrid nanostructures with various molar ratios of RC:QD are plotted as a function of wavelength (nm).

[0039] Fig. 7b is a diagram illustrative the effective of an electron donor, 1OmM

Na-ascorbate, on the PL of CdTe QDs for the embodiment of Fig. 1 in Example 1. In the diagram, the PL intensities (a.u.) of CdTe QDs in either water or in 1OmM Na-ascorbate are plotted as a function of wavelength (nm). [0040] Fig. 8a is a diagram illustrating absorbances at 870nm of the RC suspension without Na-ascorbate as a function of time (s). The arrow shows a laser pulse at 532 nm for 20ns at 1OmJ.

[0041] Fig. 8b is a diagram illustrating absorbances at 870nm of the RC suspension in presence of 1OmM Na-ascorbate as a function of time (s). The arrow shows a laser pulse at 532 nm for 20ns at 1OmJ.

[0042] Fig. 8c is a diagram illustrating absorbances at 870nm of the RC suspension after adding it to an equimolar QD530 suspension in the presence of 1OmM Na-ascorbate as a function of time (s). The arrow shows a laser pulse at 532 nm for 20ns at 1OmJ.

[0043] Fig. 9a is a diagram illustrating absorbances of the hybrid nanostructures of Example 2 as a function of wavelength (nm). The hybrid nanostructures are chemically-conjugated complexes of RCs with QD520 (black line) and of RCs with

QD570 (red line).

[0044] Fig. 9b is a diagram illustrating absorbances of the QD570/RCs hybrid nanostructure, QDs, and the RCs of Example 2 as a function of wavelength (nm). This is an illustrative example of deconvolution of UV-vis spectra of the QD570/RCs hybrid (conjugate) (black) into that of the QDs' (red) and RCs' (blue) components. The result of fitting procedure is shown by a green line.

[0045] Figs. 9c-9d are UV-vis spectra for noncovalent-bound (blue) and conjugated (black) hybrids (complexes) for QD520/RCs (Fig. 9c) and QD570/RCs (Fig.

9d) respectively. The conjugated hybrid or complex was formed through a covalent bonding. The two types of hybrids yielded very similar UV-vis spectra, proving that the relative concentrations of QDs and RCs were the same in the electrostatic and covalent hybrids.

[0046] Fig. 10 is a comparative diagram of fluorescence in terms of % PL intensities of free QDs, the non-covalent bound hybrid, and the covalent bound hybrid. More specifically, it is a Comparison of photoluminescence quenching for noncovalent bounded and covalent bound QDs/RCs hybrids for QD520 (black) and QD570 (red).

There was a 35- and 18-fold increase in quenching efficiency for covalent bound hybrids, respectively.

[0047] In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity.

However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or terms similar thereto are often used. They are not limited to direct connection, but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. DETAILED DESCRIPTION OF THE INVENTION

[0048] Broadly, the present invention provides for a wide spectral artificial photosynthetic system suitable light energy harvesting and excitation energy transfer.

[0049] A. Hybrid Nano structure Containing at Least One Semiconductor

Nanoparticle and a Photosvnthetic Unit

[0050] The present invention provides a synthetic photosynthetic system, which is a hybrid nanostructure of inorganic semiconductor nanoparticle (NP) and a natural photosynthetic unit (PU) of a photosynthetic organism. The NP is bound to the photosynthetic unit so that the NP transfers electron-hole pairs (excitons) to the PU through a Forster Resonance Energy Transfer (FRET) coupling. The photosynthetic organism can be a purple bacterium, a cyanobacterium, a green plant, or another similar photosynthetic organism. [0051] In a natural photosynthetic organism, such as green plants or cyanobacteria, light is initially absorbed by antenna protein-pigment complexes, and then transferred to specialized photosynthetic units, in which the captured light energy is converted into chemical energy. Such reaction centers are often found in thylokoids or chloroplast. The photosynthetic units can be thylakoids, chloroplast, or reaction centers (RCs). Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. It is the site of the light-dependent reactions of photosynthesis.

[0052] The photosynthetic reaction centers, also called the reaction centers, are a part of the thylakoid. A reaction center is a complex of three types of protein, which becomes the site where molecular excitations from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as chlorophyll and pheophytin molecules. These absorb light, promoting an electron to a higher energy level within a pigment through a process of sequential electron transfer. [0053] The natural photosynthetic system typically include both reaction centers

(or photosynthetic units) and antenna chlorophylls. These components (RC and antenna chlorophylls) are built into a membrane that holds the system components. The antenna chlorophylls serve to absorb photos and to deliver them to the reaction center via the forster resonance energy transfer (FRET). By attaching or bonding semiconductor nanoparticles to the antenna, the enhancement effect can also be archived. [0054] The rate of the optical generation of excitons inside a photo synthetic unit can be greatly increased through conjugation with semiconductor nanoparticles. The enhancement stems from much larger optical absorption cross section of a semiconductor. The resulting enhanced generation of excitons can be utilized for chemical transformation or for generating photocurrents.

[0055] More specifically, incident photons from the light can be absorbed by both the semiconductor nanoparticle and the photosynthetic unit. Excitons optically generated in the semiconductor nanoparticle can recombine inside the semiconductor nanoparticle or can be transferred to the photosynthetic unit. The transfer process occurs via the foster resonance energy transfer (FRET) system. The FRET comes from the coulomb interaction and does not require tunnel coupling between two objects. The excitation in the photosynthetic unit undergoes fast energy relaxation and ends up at the special pair, such as P700 for cyanobacteria. Once the excitation becomes trapped at the special pair, the electron and hole are very efficiently separated.

[0056] However, such a separation does not occur in a closed state of a reaction center isolated without any membrane as shown in Example 1. According to some embodiments of the present invention, the photosynthetic unit can be a reaction center without a membrane or a reaction center with a membrane. The reaction center is capable of sequential electron transfer, and it includes one or more quinones. [0057] When a reaction center is isolated without any membrane intact, the close state is necessary to avoid photo-oxidation of the reaction center. This close state of the reaction center occurs when an experimentally isolated reaction center (without membrane) is stabilized by an electron donor (a compound capable donating electrons, see section B for more details), which also keeps the quinones in the reaction center fully reduced. Such a closed state is not as efficient as the reaction center in a non-closed state (the reaction center with membrane) because in the closed state, a slight thermal relaxation follows the absorption of the photons. [0058] Preferably, the semiconductor nanoparticles are bound to one or more sites of the photosynthetic unit, wherein the sites comprise electron donor sites, electron acceptor sites, or both. More preferably, a Forster radii (Rp) of the hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit, wherein the FRET efficiency (_EO) = 1/(1+(R/R_F ⁾⁶) and wherein R_min =(D_SN+ D_PU)/2; D_SN is the diameter of the semiconductor nanoparticle, and Dpu is the diameter of the photosynthetic unit. The value of an _RF for any given hybrid nanostructure is substantially stable or the same. Accordingly, to reduce R_m1n, the diameter of the semiconductor nanoparticle (D_SN) should be increased. In other words, to ensure an efficient FRET of excitons, it is desirable to use semiconductor nanoparticles with smaller diameters.

[0059] On the other hand, it is desirable to use semiconductor nanoparticles with larger diameters to increase quantum yield of the hybrid. The hybrid might have a disadvantage of a reduced quantum yield, owning to recombination of excitons in a semiconductor nanoparticle. While not wishing to be bound by theory, it is presently believed that the excitons of a semiconductor nanoparticle have a chance to recombine because the time of FRET from a semiconductor nanoparticle to a photosynthetic unit is relatively long. The reasons for the relatively long time for the process NP -> PU are the following: (1) the absorption cross section of a photosynthetic unit is not very large, and (2) the NP-PU center-to-center distance is relatively long since it is dictated by the size of the photosynthetic unit. In order to increase the quantum yield of a hybrid structure, the effective distance between the semiconductor nanoparticle (NP) and the photosynthetic unit (PU) should be decreased. This can be accomplished by using semiconductor nanoparticles with smaller size, using shorter biolinkers, or not using any biolinkers. Further, to obtain an increased quantum yield, more semiconductor nanoparticles can be attached to the photosynthetic unit.

[0060] Accordingly, the diameter of the semiconductor nanoparticle is preferably in the range of about 2nm to about 20nm. More preferably, the semiconductor nanoparticles include, but are not limited to, quantum dots, nanowire, nanorods, or a mixture thereof. Unlimited examples of the quantum dot suitable for the present invention are a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof. [0061] In addition, the efficient FRET of excitons occurs when peak intensities of exciton emission of the semiconductor nanoparticle are reduced when the semiconductor nanoparticle is suitably bound to the photosynthetic unit. This phenomenon is also a quenching of the exciton emission of the semiconductor nanoparticle, which is used to control the binding of the semiconductor nanoparticle to the photosynthetic unit in Example 1 (note, in Example 1, the semiconductor nanoparticle is the quantum dot, and the photosynthetic unit is the reaction center without the membrane). [0062] Preferably, the semiconductor nanoparticle is bound to the photosynthetic unit through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof. The length of the biolinker should be suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center.

[0063] The binding can be separated into either a noncovalent binding or a covalent binding. The non-covalent binding uses an electrostatic force or interaction, a molecular force or interaction, a hydrophobic force or interaction, a hydrogen bond, a physical interaction, and a combination thereof. The physical interaction can be absorbance, entrapment, swelling, adherence, and other similar interactions. For example, a nanoparticle may be coated with organic molecules, and the photosynthetic unit can be adsorbed onto the organic molecule coating of the nanoparticle (Lee et al., J. Phys. Chem. B. 2000). It is importantly to know that although a coating of the organic molecule might be applied onto the nanoparticle, the organic molecule is not the biolinker in a sense that no covalent binding occurs to bind the nanoparticle to the photosynthetic unit. Other forces or interactions stated above assist in binding the nanoparticle to the photosynthetic unit. However, because these forces are less stable than that of a covalent bond, the non-covalent binding is less stable also.

[0064] The covalent binding can be either a direct binding or an indirect binding, both of which can be the result of using a biolinker. To create a biolinker, a linking agent is added to the semiconductor nanoparticle solution, and the linking agent becomes a biolinker when the hybrid is formed. [0065] The direct binding includes the direct covalent binding between the nanoparticle and the function groups of the polypeptides in the photosynthetic unit. Sometimes, to ensure covalent binding, the polypeptides are genetically modified to include these function group. At other times, the covalent binding can occur without any genetic modification to the polypeptide, but with the help of one or more biolinkers. The indirect binding includes binding one functional group of a bifunctional connecting molecule (one type of the biolinkers) to the photosynthetic unit, and then binding the other function group of the bifunctional connecting molecule to the nanoparticle. [0066] The biolinker can include the bifunctional mercapto-compounds, such as thio-containing polyethyle glycol (PEG), a compound containing a carbodiimide function group with a formal for RN=C=NR, in which R can be a carboxylic function group, an amino function group, or a mixture thereof, or both. Other suitable biolinkers can also be used provided that it can create one or more covalent bonds between the photosynthetic unit and the semiconductor nanoparticle either directly or indirectly. The length of the biolinker should be suitable to ensure an efficient FRET of excitons in the hybrid. Decreasing the [0067] According to some embodiments of the present invention, the biolinker is a bifunctional connecting molecule. Preferably, the biolinker comprises a carbodiimide group, a carboxylic group, an amino group, or a mixture thereof. Specifically, the biolinker assists in forming one or more covalent bonding between the semiconductor nanoparticle and the photosynthetic unit. As explained above, the length of the biolinker along the diameter of the semiconductor nanoparticle are both critical to the efficiency of a FRET of exciton for any given hybrid nanostructure. They are important to ensure that FRET radii (Rp) is larger than R_min.

[0068] In a preferred embodiment of the present invention, the semiconductor nanoparticle is a quantum dot while the photosynthetic unit is a reaction center without any membrane as shown in Example 1. [0069] Advances in high-temperature inorganic synthesis enabled production of monodispersed quantum dots such as highly photoluminescent (PL) CdSe/ZnS core/shell or CdTe nanocrystals. The light absorption by quantum dots appears as a quasi- continuous superposition of peaks with orders of magnitude higher extinction coefficients than for organic molecules. Quantum dots are ultrastable against photobleaching and the quantum confinement effect yields PL emission energy that varies as a function of the size of the quantum dot. The spectral width and position of the optical emission bands of the quantum dot can be tailored to meet requirements by controlling its size and surface functionality. In Example 1, photoluminescent quantum dots were used as artificial antennas that absorbed light very efficiently in a wide range of photon energies (within the solar spectrum) and were able to transfer the harvested energy to RC chlorophyll cofactors with only one or more non-covalent bondings.

[0070] In Example 1, the reaction center complex of Rb. sphaeroides was purified from natural light-harvesting complexes. Strongly-absorbing light and strongly emitting photoluminescent CdSe/ZnS or CdTe QDs were then assembled with the RCs through one or more non-covalent bindings (Fig. 1). In order to develop an efficient hybrid material operating in the FRET regime, the photoluminescent colors (diameters) of the QDs (donors of energy) were carefully selected to be optically coupled with the RC pigment chromophores (acceptors). The results of Example 1 showed that although the reaction centers had their own light-harvesting system with a significant absorption cross section, properly designed quantum dots could easily dominate over the intrinsic absorption of the RCs. [0071] Comparing to non-covalent bindings, the covalent binding between the semiconductor nanoparticles (the quantum dots) and the photosynthetic center (the reaction center) can be a more stable and controller binding. In addition, as shown by Fig. 10, the covalent binding between the quantum dot and the reaction center can create a 18 to 35 fold increase in quenching efficiency (the rate of quenching of photoluminescent intensities for the quantum dot after binding it to the RC to form the hybrid).

[0072] B. Method of Making a Hybrid Nanostructure Containing At Least

One Semiconductor Nanoparticle and a Photosynthetic Unit

[0073] A preferred method of making the hybrid nanostructure of the present invention includes (a) preparing a first aqueous solution comprising at least one photosynthetic unit (PU) of a photosynthetic organism; (b) preparing a second aqueous solution comprising one or more semiconductor nanoparticles; (c) adding the first solution to the second solution; and (d) agitating the mixture of step c until the hybrid nanostructure is formed. Within the hybrid, the semiconductor nanoparticles efficiently supply electron-hole pairs (excitons) to the RC through a Forster Resonance Energy Transfer (FRET) coupling.

[0074] Preferably, the Forster radii (R_F) of the resulting hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the quantum dot to the reaction center, wherein the FRET efficiency (Eo) = 1/(1+(R/R_F)⁶) and wherein R_min =(D_SN+ Dpu)/2; D_SN is the diameter of the semiconductor nanoparticle, and Dpu is the diameter of the photosynthetic unit.

[0075] According to some embodiments of the method, the photosynthetic unit comprises at least one reaction center (RC). The reaction center comprises one or more quinones, and wherein the reaction center is capable of sequential electron transfer. [0076] Figure 1 shows an embodiment of a QD/RC hybrid 10 of the present invention using one or more non-covalent bindings. The purified RC 12a from purple bacteria Rb. sphaeroides is assembled with the PL semiconductor QD 11. The selection of appropriate surface functionalisation of the QDs 11 and the procedure for their electrostatic assembling with RCs 12a for the preparation of hybrid material 10 are presented in Example 1.

[0077] The RC 12a is composed from cofactors (building blocks) arranged into two (active 13 and inactive 14) membrane-spanning branches. The active branch is responsible for electron-hole separation and sequential transmembrane electron transfer. Each branch (A 13 or B 14 in Fig. 1) consists of one molecule of bacteriochlorophyll (B_A 15 or B_B 16), one bacteriopheophytin (H_A 17 or H_B 18) and one quinone (Q_A 19a or Q_B 19b). Only one of the branches (branch A 13) is active in catalyzing electron transfer across the RC 12a. Both branches are connected to a key element of the RC which is a dimer of BChI molecules, the so-called "special pair" 12b (P or P870). [0078] In nature, a solar photon first creates an excitation in the RC 12a and then this excitation relaxes towards P 12b where the electron separates from the hole in 2-3 ps. In the next step, this electron moves along the electron-transfer chain towards Q_A 19a. Afterwards, this electron takes part in chemical transformations eventually leading to synthesis of high-energy molecules such as ATP which fills the majority of the energy needs of the bacterium.

[0079] However, the environmental conditions in Example 1, with isolated RCs in water, are different to those in nature where RCs are built into a membrane inside a bacterium. In water, an isolated RC is a delicate object and can be easily photo-oxidized even under the very weak illumination on the order of 0.02 mW/cm². To avoid photo- oxidation and to keep P870 in the reduced form in the dark and under light illumination, Na-ascorbate molecules were used as a source of electrons. Other suitable electron donor compounds can also be used (see below for details).

[0080] When added to RCs, Na-ascorbate supplies electrons to the photo-ionized

P870⁺ species thus leading to the RC reduction reaction P870⁺ -> P870. So, by adding Na- ascorbate, the so-called "closed state" of the RC, P870/Q_A ^~Q_B ^~ were created, in which the quinones are fully reduced. When the closed state of RC absorbs a photon, the electron and hole are not separated; instead, they recombine in the P870, creating the PL band at 910 nm (Fig. 2a). Importantly, the closed state (P870/Q_A Q_B ) is stable against photo- oxidation at experimental light fluxes (12.5 mW/cm²) and its energy structure is very similar to that of the natural state P870/Q_AQ_B in the operational photosynthetic systems. [0081] The resulting hybrid of QDs and RCs can use the QDs to optically enhance the exciton absorption of the RCs in the hybrid (Figs. 2a-2b). The energy structure of Fig. 2a reflects and explains the RC absorption section presented in Fig. 2b. In particular, Figs. 2a-b show that at the comparable molar concentrations, the QD 570 absorption at wavelength of <570nm is stronger than that for RCs, and therefore, optical enhancement of RCs is possible in the hybrid with non-covalent bindings. [0082] According to some embodiments of the method of the present invention, the first aqueous solution and the second aqueous solution are substantially compatible with each other to form the hybrid, wherein the integration of the semiconductor nanoparticle into the hybrid does not interfere with the process of sequential electron transfer. [0083] In a further embodiment, the first aqueous solution further comprises a compound capable of being an electron donor (the electron donor compound). The electron donor compound keeps the quinones fully reduced and the reaction center photochemically inactive. Moreover, in the presence of the electron donor, the reaction center is stable against photo-oxidation. [0084] Preferably, the electron donor is selected from the group consisting of an electron donating dye, an electron donating compound, or a mixture thereof. Unlimited examples of the electron donor are a condensed polycyclic compound, an amine compound, a quaternary amine compound, an aniline derivative, a nitrogen-containing heterocyclic compound, a sulfur-containing heterocyclic compound, an oxygen- containing heterocyclic compound, a nitrogen- and oxygen-atom containing heterocyclic compound, a nitrogen- and sulfur-atom containing heterocyclic compound, an oxygen- and sulfur-atom containing heterocyclic compound or a sulfur-, nitrogen- and oxygen- atom containing heterocyclic compound. More preferably, the electron donor is sodium- ascorbate. [0085] As discussed above, the hybrid might have a disadvantage of a reduced quantum yield, owning to recombination of excitons in a semiconductor nanoparticle. Increase in quantum yield can be accomplished by using semiconductor nanoparticles with smaller size, using shorter biolinkers, or not using any biolinkers. Further, to obtain an increased quantum yield, more semiconductor nanoparticles can be attached to the photosynthetic unit.

[0086] As such, the photosynthetic units and the semiconductor nanoparticles should be in a molar ratio that ensures sufficient excitons are transferred from the semiconductor nanoparticles to the photosynthetic units. Specifically, the molar ratio of the photosynthetic unit (PU) to the nanoparticles (SN) is sufficiently small so that A(λ_eXC) is greater than 1, wherein A is an enhancement factor; λ_eXC is an excitation wavelength, and A (λe_XC) = PLh_ybπd/PLpu, wherein the PLhybπd is a photoluminescence peak intensity for the hybrid, and the PLpu is a photolumiescence peak intensity for the photosynthetic unit. Smaller molar ratios of PU/SN suggest that more semiconductor nanoparticles should be bound to a photosynthetic unit.

[0087] In addition to adding more semiconductor nanoparticles to bind to a photosynthetic unit, the semiconductor nanoparticle suspension (the second aqueous solution) should stabilize the semiconductor nanoparticle to prevent any possible quenching of exciton emission of the nanoparticle before the hybrid formation. Accordingly, the second aqueous solution further comprises one or more stabilizing agents. The stabilizing agent forms a coating around the semiconductor nanoparticle to stabilize and solubilize the semiconductor nanoparticle in the second aqueous solution. Preferably, the thickness of the coating is suitable to ensure that the FRET of exciton from the semiconductor nanoparticle to the photosynthetic unit is efficient. The stabilizing agent can be, but is not limited to, a mercapto-compound, another similar stabilizing molecule, or a mixture thereof. Specifically, the stabilizing agent can be a thio- containing polyethylene glycol molecule; a cysteine moiety; a thioglycolic acid; or a combination thereof. [0088] In addition, the fluorescent wavelengths of the semiconductor nanoparticles are sufficiently overlapped with absorption wavelengths of the photosynthetic unit to ensure an efficient Forster Resonance Energy Transfer (FRET) of excitons from the semiconductor nanoparticles to the photosynthetic units. [0089] Preferably, the semiconductor nanoparticle includes a quantum dot, a nanowire, a nanorod, or a mixture thereof. The quantum dot can be, but is not limited to, a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof; and wherein the quantum dot comprises a quantum dot fluorescent core. The quantum dots are bound to the reaction center through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof. For more detailed description of the quantum dots, please see section A.

[0090] In a further embodiment of the method, the second aqueous solution further comprises a linking agent, wherein the linking agent becomes a biolinker when the hybrid is formed. Preferably, the length of the biolinker is suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center. The linking agent is a compound containing a carbodiimide functional group with a formula of RN=C=NR (the carbodiimide compound), wherein R comprises a carboxylic function group, an amino function group, or a mixture thereof. The carbodiimide compound assists in forming a covalent bond between the quantum dot and the reaction center. More preferably, the linking agent can also be the stabilizing agent. [0091] In addition, the binding of the quantum dot to the reaction center can be shown by a reduction in peak intensities of exciton emissions of the quantum dot when the hybrid is formed.

[0092] C. Examples

[0093] The present invention is further illustrated by the following examples which are illustrative of some embodiments of the invention and are not intended to limit the scope of the invention in any way:

[0094] Example 1

[0095] This example illustrates preparation of a hybrid nanostructure composed of one or more quantum dots and a reaction center without any biolinker.

[0096] Procedure:

[0097] I. Reaction Centres Preparation and Characterisation. The cells of wild-type nonsulfur purple bacterium Rhodobacter sphaeroides were grown in liquid Ormerod culture medium under anaerobic conditions in a luminostat at ~30°C for 4-6 days. Chromatophores (membranes containing photo synthetic apparatus) were isolated from sonicated cells by centrifugation and were incubated in the dark for 30 min at 4°C in 0.01 M sodium phosphate buffer (pH 7.0) containing 0.5% zwitterion detergent lauryl dimethylamine oxide (LDAO). After that, the chromatophore suspension was centrifuged (144,000g, 60 min, 4°C). The supernatant was brought to 22% saturation with a solution of ammonium sulphate. The precipitate containing RC was dissolved in 0.01 M sodium phosphate buffer (pH 7.2) containing 0.05% LDAO and subjected to chromatography on a hydroxyapatite column. These procedures were described in more detail elsewhere. (Zakharova, N. I. & Churbanova, I. Yu. Biochemistry (Moscow), 65, 149-159 (2000)). The resulting RC preparation was suspended in 0.01 M sodium phosphate buffer (pH 7.2) containing 0.05% LDAO and dialyzed against 0.01 M Tris-HCl buffer (pH 7.8) containing 0.1% anionic detergent sodium cholate. Following this procedure, RC preparations were more tolerant to long-term storage at a temperature of - 80⁰C. [0098] It should be mentioned that the detergent LDAO induced precipitation of QDs from aqueous solutions, but the amount of the LDAO in the resulting QD-RC solution was insignificantly small so as to precipitate the QDs from aqueous solutions. The LDAO was used only in the 25μM RC stock solution. Along with RCs, the LDAO were diluted in a range of about 25 times to 250 times when the RC stock solution was added to the QD stock solutions. After such a dilution, there was little to none of the LDAO in the final QD-RC mixture to cause any precipitation of QDs.

[0099] The quality of the quinone acceptors in the RC preparation was checked by the kinetics of dark P⁺-form reduction at room temperature under short-flash excitation. The kinetics curve usually reveals a single exponential dark decay with a rate constant of about 0.9 s^"1 (typical for recombination reaction between QB and P⁺), that proved the presence of both QA and QB acceptors in the structure of purified RCs.

[00100] The purity of RC pigment-protein complexes was characterized by the ratio of intensities of UV-absorption bands of the protein at 280 nm (A_2So) to the absorption of bacteriochlorophyll at 800 nm (Agoo) which was close to 1.3-1.4 (Fig. 5a), slightly higher than the value of 1.2 for absolutely pure RC (Feher, G., & Okamura, M. Y. In: Photosynthetic bacteria (Clayton, R. K. & Sistrom, W. R., Eds.), pp. 349-386, Plenum Press, New York (1978)). This fact means that a small quantity of pigment impurity is present in the RC preparation. This impurity mostly consists of the LH-2 pigment-protein complex absorbing at 600 nm, 800 nm and 850 nm and, additionally, provides a contribution in absorption in the carotinoids' region at 400-500 nm (Fig. 5a).

[00101] Although this LH-2 impurity is normally on the level of one-to-five LH-2 complexes per 1000 purified RCs, its fluorescence quantum yield is much higher than that of P870 (Pflock. T., Dezi. M., Venturoli. G., Cogdell. R., Kohler. J. & Oellerich. S. Photosynth. Res. 95, 291-298 (2008)). So, even such small amounts of resting LH-2 may provide a fluorescence band at 860 nm comparable or even greater in intensity than that for P870 emitting PL at 910 nm (Figs. 5b-5c). [00102] It should be mentioned that as a result of the RC purification procedure, the spectral contribution from LH-I (which absorbs at 870 nm and fluoresces at 900 nm) was completely eliminated. Indeed, no fluorescence at 900 nm was detected in the oxidized RC, and also the fluorescence band at 910 nm due to the P870 form was absent. [00103] It should also be mentioned that in the presence of the 10 mM Na-ascorbate, RC is known to be always perfectly operational (bio-photochemically active). At the much higher concentrations of Na-ascorbate (>50 mM) or upon addition of some quantities of much stronger reduction agents, such as Na-dithionite, the RC may be transformed into the "closed trap" form where RC is not photochemically active and P870 is always reduced. Fig. 5a shows that addition of Na-ascorbate induces a gradual increase of the 870 nm band of P870 state of the BChI special pair which achieves saturation at 10 mM concentration of Na- ascorbate.

[00104] The PL band at -910 nm in Fig. 5b corresponds to the P870 BChI dimer

"special pair" whereas the PL band at -860 nm corresponds to the resting non-purified amounts of LH-2 (normal resting amount for LH-2 impurity). Addition of Na-ascorbate induces reduction of P870 reflected in the gradual increase of the -910 nm PL band accompanied by a decrease of the 860 nm PL band; both processes achieve saturation at 10 mM concentration of Na-ascorbate (Figs. 5b-5c).

[00105] II. Synthesis, solubilization and characterization of QDs. The CdTe quantum dots have a core of cadmium and telluride and are capped with a stabiliser, thioglycolic acid (TGA) (Rogach, et al, J. Phys. Chem. C, 111, 14628-14637 (2007)). The CdSe/ZnS QDs have a core of cadmium and selenium with a zinc sulphide shell and treated with DL-cysteine (Sukhanova, A., et al. Lab. Invest. 82, 1259-1262 (2002)) or solubilized in aqueous solutions using the mixture of three-functional thiol-containing polyethyleneglycol (PEG) molecules terminated by the carboxylic or hydroxyl groups using the procedure similar to that published recently (Williams, et al. Small, doi: smll.200900744C (2009); Liu et al., J. Am. Chem. Soc. 130, 1274-1284 (2008)). The mixtures of the polymers used for QDs solubilization were adjusted to provide the best binding affinity of QDs with the RCs. Surface treatment of QDs with low molecular weight mercapto-compounds (e.g. with DL-cysteine), instead of generally used encapsulation of nanoparticles within the additional organic PEG shell, yield water-soluble CdSe/ZnS QDs of the smallest possible diameters (Choi, et al. Nat Biotechnol. 25, 1165-1170 (2007)). CdTe QDs are easier and cheaper to synthesise than CdSe/ZnS core/shell QDs, especially for use in biological systems, however they are generally less stable than CdSe/ZnS QDs.

[00106] Absorption and PL emission spectra of the synthesized and solubilized

CdSe/ZnS and CdTe QDs are shown in Fig. 6.

[00107] The diameters and extinction coefficients of CdSe/ZnS QDs were derived using the empirical fitting functions as described in the reference article by Yu et al. (Yu, et al., Chem. Mater. 15, 2854-2860 (2003)):

D = (1.6122 ^• 10^"9U⁴ - (2.6575 ^• + (1.6242 ^• 10^"3)/l² - (0.4277)1 + 41.57,

£ = 1600 - ΔE -

D³, the diameters and extinction coefficients of CdTe QDs were calculated as described in Yu, et al., Chem. Mater. 15, 2854-2860 (2003):

D = (9.8127 x 10^"7U³ - (1.7147 x 10^"3U² + (1.0064U - 194.84, £ = 3450 AE (D)²⁴, where D (in nm) is the diameter of the nanocrystals, ε (in cm^~lM^~l ) is the molar extinction coefficient, λ and AE are the wavelength and transition energy corresponding to the first absorption peak and the unit are in nm and eV.

[00108] Hydrodynamic diameters of QDs were determined with the Dynamic Light Scattering approach and ζ-potentials of their surfaces were determined using the Malvern Zetasizer Nano ZS instrument according to supplier protocol. The data of QDs characterization are presented in Tables 1 and 2.

[00109] Table 1 1 Physicochemical Properties of Quantum Dots Used in the

Study: CdSe/ZnS QDs Solubilized with Low-Molecular- Weight Aminoacid DL- Cystein and CdTe Quantum Dots Covered with Thioglycolic Acid

* Diameters of the fluorescing CdTe and CdSe cores are calculated according to Ref . S9

** Hydrodynamic diameters are measured using the Dynamic Light Scattering (DLS) technique as described in Supplementary Methods. The presented values are the results of the average of three independent experiments for each QD sample. Errors represent the standard deviation for these three independent experiments with each sample.

[00110] Table 2 I Physicochemical Properties and Relative Capacities to be

Quenched by Reaction Centers for CdSe/ZnS QD570 Encapsulated with Mixtures of PEG-Based Polymers of Different Charges.

* Percent of QDs' initial PL intensity for RC/QD=2 molar ratios.

** Addition of RCs to positively charged QDs provoked irreversible QDs aggregation. [00111] III. Preparation of QD-RC hybrid materials. QDs with controlled zeta potential of their surfaces were assembled with the RCs. The assembling procedure was controlled by analysis of quenching of the QDs fluorescence. QD-RC assemblies were prepared in the 10 mM of Na-ascorbate buffer (pH 6.8) by adding appropriate amounts of RCs to 0.5 μM QDs solution in water. Different quantities of RCs were added to QDs samples with different surface functionalities to obtain 500 μl of the reaction mixture as shown in Table 3. All types of QDs were initially kept at the same concentrations in order to have different molar ratios of RC-QD complexes, those indicated in the Figure legends, upon addition of different quantities of RCs. Molar ratios of RC to QDs were discretely varied among samples from 0:1 to 10:1. After addition of RCs, reaction mixtures were allowed to self-assemble under gentle agitation for 30 min at room temperature.

[00112] The QD-RC complexes were further purified through gel filtration on a

Superdex 200 resin (GE Healthcare) columns equilibrated with 10 mM of Na-ascorbate buffer (pH 6.8). All collected fractions were analyzed by UV-vis and only those containing QD-RC complexes were used for the further experiments.

[00113] The individual samples were then diluted with 10 mM Na-ascorbate solution to a total volume of 2.5 mL. The solutions were placed in a 10 mm optical path quartz fluorescence cells (Hellma, Madrid). Control spectra collected from RCs in the absence of QD donors were recorded and subtracted from the solution spectra to adjust for parasitic contribution of RC preparations upon direct excitation. To avoid inner filtering effects, QD, RC and QD-RC preparations with the lowest possible optical densities at the excitation line were used in the present example. A procedure used for accounting and correction of the data on the effect of internal filter is described in below.

[00114] Table 3a: Compositions of Various Molar Ratios of RC (reaction center) to QD (quantum dot) for CdTe QD570 for Example 1

*Plain water was used for the 0.00 ratio — Sample #1.

**The total amount of the mixture of RC & QD was maintained at 500μl for all samples.

[00115] Table 3b: Compositions of Various Molar Ratios of RC (reaction center) to QD (quantum dot) for CdTe QD530 for Example 1

*The total amount of the mixture of RC & QD was maintained at 500μl for all samples.

[00116] IV. Steady-State Fluorescence Measurements of QDs in the Presence of Reaction Centres. Titration of CdSe/ZnS or CdTe QDs of different fluorescence colours (diameters) with RCs demonstrates that QDs show very similar behaviour in their quenching by RCs (Figs. 7a-7b). So, QDs diameters and good overlapping of their fluorescence bands with the absorption bands of RC ensure FRET from QDs to RC.

[00117] It is important to note that all QDs-RC preparations and titrations described in the text and in the SI were done in 10 mM Na-ascorbate. So a correction for the reduction in QDs emission upon addition of Na-ascorbate was not necessary. Also, in NIR PL measurements, the control scans of QDs were also performed in Na-ascorbate. Furthermore, the contribution from QDs was calculated by the least-squares fitting procedure - so all changes in QDs emission were taken into account automatically during this procedure. Nevertheless, Fig.7b demonstrates the direct effect of 10 mM Na-ascorbate on the PL of QDs. This effect is <5% indicating that the properties of QDs are not significantly affected by the Na-ascorbate molecules.

[00118] V. Fluorescence Decay Kinetics Measurements. Fluorescence decay kinetics were measured using a laboratory-based picosecond pulse fluorometer. Excitations with 532, 355 or 266 nm harmonics of the main (λ=1064 nm) wavelength were provided by the pulse-periodic Nd: YAG laser with an electrooptic control of the synchronisation process which generated light pulses of a duration of 20 ps at with a frequency of 0 - 100 Hz. Laser radiation was amplified up to an energy of 0.3 mJ. After pulse excitation, fluorescence emission was measured with a streak-camera AGAT SF-3 connected with a multichannel detector C7041 (Hamamatsu) and computer. The time response of the recording system was ~2 ps. Each experimental curve is an average of 50 accumulations.

[00119] VI. Time-resolved Fluorescence Measurements: RCs in the Presence of

QDs and QDs in the presence of RCs. Time resolved pico-second kinetics of the fluorescence decay of RC and QDs were analyzed in a two-exponential approach, according to the following formula:

where A and τ₁₍₂₎ are the amplitudes and characteristic decay times, respectively, for each of the two exponents used in the approximation. All experimental data were fitted with a nonlinear least square procedure using the Marquadt-Levenberg algorithm.

[00120] Kinetics of fluorescence decays for RCs, QDs and their complexes were analyzed separately in the 560-650 nm spectral regions of fluorescence of the QDs (named "QD-region") and in the >700 nm spectral region of fluorescence of the RCs (named "RC- region") in order to understand how the lifetimes and intensities of fluorescence of QDs and RCs were affected by their interaction in the QD-RC complexes. To estimate the total fluorescence for each sample (F), the fitting function was integrated, where: F = A₁T₁ + A₂T₂ [00121] Experimental results obtained for the RC-region and QD-region of spectra and results of the data treatment for the experiments with the CdSe/ZnS QD530, QD570 and QD600 are summarized in Table 4.

[00122] Table 4 I Analysis of Kinetics of Fluorescence Decay for QDs and their

Complexes with RC.

RC-region of optical spectrum QD-region of optical spectrum

A1 fl% τi A2 f₂ ^o/ > τ₂ F A1 f i% τi A2 f₂% X2 F

Experiment 1

RC 3,3 5 43 1 ,6 95 1838 3012

QD530 5,6 21 216 1 ,1 79 4187 5808

QD₊RC 4,3 20 433 3,1 80 2361 9279 5,6 10 60 1 ,4 90 2236 3570

Experiment 2

RC 1 ,5 33 274 0,5 67 1648 1269

QD570 2,7 19 131 0,5 81 2941 1761

QD+RC 0,2 1 1 16 2,2 99 971 2120 0,5 7 147 0,7 93 1345 1052

Experiment 3

RC 12,9 22 157 4,8 78 1479 9183

QD600 174,4 8 292 61 ,5 92 >10000 >660000

QD+RC 31 ,9 23 1 10 6,1 77 1941 15258 381 ,5 21 230 66,1 79 4886 410889

[00123] A₁, J₁ and τ, are the fluorescence amplitudes, weighted residuals (normalized to 100%) and characteristic decay times, respectively; T₁ are given in ps. Fractional contributions to the total

(time-integrated) emission intensity for each component ( /_; ) were estimated as

A ^₁

/i = - - ^■ 100% ; /₂ = - A - τ₂ ^{■ ■} 100% .

A₁ - T₁ + A₂ - T₂ ^{' "} ' A₁ - T^ A₂ - T₂

F = A₁T₁ + A₂T₂ is an estimated time -integrated fluorescence intensity. Relative amplitudes ( f_t ) were calculated to compare fluorescence intensities between samples. QD530 and QD570 and their complexes were excited with 266-nm laser pulses and QD600 and its complexes where excited with the 532-nm laser pulses. The values of τ₂ in decay kinetics of QD600 were difficult to calculate precisely, but it was evaluated to be >10 ns, providing approximate values for F>6.6 x 10⁵.

[00124] One of the universal characteristics of QDs is that they exhibit a bi- exponential decay giving rise to two radiative lifetime components. The shorter lifetime is usually around several nanoseconds and can be attributed to the intrinsic recombination of initially populated core states, whereas the longer lifetime is usually around tens of nanoseconds and its origin is still in question. A recent study by Wang et al, 2003, Nano Letters 3, 1103 explained this longer lifetime component, suggesting an involvement of the surface states in the recombination process in colloidal QDs.

[00125] The data in Table 4 shows that the interaction of RCs with QDs leads to an increase of fluorescence intensity (F) in the region of the fluorescence of RCs (RC -region) accompanied by a decrease of fluorescence intensity in the region of the fluorescence of QDs (QD-region). Moreover, the characteristic life-time of the slow component of the fluorescence of QDs was strongly decreased when the complex of QDs with RCs was formed. [00126] Taking into account that the intensity of excitation and concentrations of QDs and RCs were the same in all experiments, one may conclude that the increase of fluorescence intensity for RCs accompanied by the quenching of fluorescence of QDs is determined by the resonance energy transfer from QDs (donors) to RCs (acceptors). This conclusion is confirmed by the decrease of life-time of the slow component of fluorescence decay (τ₂) from QDs. Although analysis of the fast components is difficult due to the low fluorescence intensity, the data do not show significant variations of these components. [00127] An overall increase of RCs fluorescence within the QDs/RCs complexes may be calculated from FR_C/FR_C+QD (measured in RC -region) and was found to be 1.66 for QD600 and QD570 and 3.08 for QD530. The decrease of fluorescence for all QDs used was found to be around 1.6.

[00128] One can conclude that two factors should be taken into account in order to explain the QD-RC energy transfer. The first one is the relative ratios and steric factors of the QD-RC interaction which certainly affect the QDs fluorescence quenching and the second one is the spectral factor of efficient overlapping of QDs fluorescence bands with the electronic transitions of chromophores of RCs. It is clear that energy migration is more efficient in QD530-RC complexes than that for QD570-RC and QD600-RC complexes.

[00129] VII. Time-Resolved Study of Photo-Bleaching of QD-RC Complexes with Na-Ascorbate Demonstrates that the Reduction of P870⁺ Occurs within 10-100 ms. Fig. 8 shows kinetics of laser induced absorption changes at 870 nm in suspensions of RC without or with Na-ascorbate and after assembling of QDs and RCs in equimolar concentrations. [00130] The data show that >90% of RCs contain both quinone acceptors. Kinetics of the RC reduction in the dark contains presumably only one component with the τ ~1 sec (Fig. 8a). This long relaxation time comes from a slow process of returning of the electron from quinones to the P molecule (P870⁺/Q_aQ_b ^~ -» P870/Q_aQ_b). Addition of the 10 mM of Na- ascorbate (Fig. 8b) significantly increases the rate of reaction of RC reduction in the dark (τ=80 msec). In this process, Na-ascorbate molecules donate electrons to the photo-oxidized P870⁺. It is essential to note that the presence of the QDs does not affect the ability of the Na- ascorbate to reduce the photooxidized P870 by donation of the electron to the BChI special pair (Fig. 8c). In Fig. 8c, QDs were added to RC after addition of the Na-ascorbate. Fig. 8c shows that the kinetic behaviors for RCs and QD-RCs are very similar.

[00131] VIII. Instrumentation for Steady-State Spectroscopic Measurements.

Absorption spectra were recorded on a Varian Cary50Conc UV-visible spectrophotometer. The protein concentrations were estimated from the sample absorbance at 800 nm using an excitation coefficient of 288 000 M^-1Cm^"1. Near- infra red (NIR) photoluminescence spectra were recorded on a FLS920 fluorescence spectrometer (Edinburgh Instruments) equipped with a Hamamatsu R5509 NIR photomultiplier tube.

[00132] IX. Comparison with other experiments on FRET between QDs and molecules. Bio-sensors based on FRET are bio-conjugates designed to detect the presence of certain molecules (analyte) in the solution (Medintz, et al. Nature Materials, 4, 435-446 (2005); Somers, et al., Chem. Soc. Rev. 36, 579-591 (2007); Snee, et al. J. Am. Chem. Soc, 128-129, 13320 (2007)). The FRET efficiency in these structures depends on the concentration of analyte molecules. Analyte molecules can induce formation or dissociation of QD-dye complexes with a fast FRET or a change in the efficiency of FRET in the assembled QD-dye structure. The motivation of the present example was different - enhancement of light harvesting in important photosynthetic molecules. In contrast to strongly-emitting dye molecules, photosynthetic complexes are not efficient light emitters. Instead they are able to separate charges and store optical energy. It is interesting to compare FRET in Example 1 and in the existing bio-sensor structures in the above references. Overall, the parameters characterizing FRET in or structures and the structures reported in Ref. Error! Bookmark not defined, are comparable. The QD-dye complexes in the article by Medintz, et al had the FRET radii R_F = 4.1 - 4.5 nm . In the present example, the calculated FRET radii are somewhat greater: R_F = 6.1 - 6.4 nm for CdSe/ZnS QDs and R_F = 5.3 -5.4 nm for CdTe QDs. The relatively large FRET radii in the structures originate from the relatively strong absorption by the RCs which comprise strongly-absorbing BChI dimer and two BChI monomers. Simultaneously, the sizes of QDs with efficient FRET in the present example (2.5 - 3.lnm ) are very similar to the QD dimensions in Ref.Error! Bookmark not defined., which were in the range of 2.7 - 3.0 nm . Important difference between the hybrid of the present example and the structure of the paper by Medintz, et al. is in sizes of the attached molecules. RCs in the present example are much larger than dye molecules in the article by Medintz, et al.

[00133] X. Equations for the FRET rates. The FRET rate is given by the standard equation (see Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3d Ed., Springer Science + Business Media, LLC (2006)):

k —L &)

^{FRET ~} T [ R ' β^{Λ J} (Sl)

_{R6 ^} 9000 - In(IO) - φ_QD - fc^z

\ F_QD(λ) - ε_RC (λ) ■ λ⁴dλ,

^F 1 12988.- Λ W-⁵ .- N N_A .- n n⁴ where τ_QD is the PL lifetime of the QD, R is the donor-acceptor distance, R_F being the

Fόrster radius, K² = 2 / 3 is the dipole orientation factor, «= 1.33 is the refractive index of the medium (water), N _A is Avogadro's number, F_QD (λ) is the normalized donor emission spectrum, ε_RC(λ) is the acceptor (RC) molar extinction coefficient in cm^~lM^~l , and φ_QD is the fluorescence quantum yield of the QD donor. The FRET efficiency, E₀ , is defined by Eq. 1 in the main text. Using the simplest rate model (see text below), Eq. S2 is obtained: pjexciton , .

E = 1 - ^QD-^RC = ^k™^ET , = (S2)

^PLQD ^kFRET ^{+ τ}QD 1 + [R / R_p )

This equation shows that the FRET efficiency strongly depends on the ratio R l R_F .

[00134] XI. Geometrical corrections to FRET. [00135] The quenching of PL in a donor- accepter system may also come from (1) the so-called inner-filter effect and (2) the re-absorption by the acceptor component. The inner- filter effect originates from absorption of incident light by the acceptor component when acceptor molecules are added to a solution. The re-absorption is an attenuation of the emitted photons by the acceptor components. These effects can create quenching that is not associated with FRET. Therefore, these effects must be removed from the spectra of PL quenching shown in Figs. 3a and 4a. For this, a corrected normalized intensity is introduced:

PJ

PT normalized corrected = k - _jy ^Q _j ^D+RC '

"^LQD where PL_QD+RC(QD) are the exciton PL intensity of QD-RC complexes and QDs, respectively. The correction coefficient is:

4 emiss

(1 - 10^"^ )(1 - 10^"^^" ) A£ QD₊+_sR_cC ^• A ^A *_QQ^e k = - D+RC

A£ - A%r (1 - 10-4S- )(1 - 10-«^"sc ) where the absorbencies of the components at excitation/emission wavelengths are defined in the usual way:

A exc _ / 1 N j . A emiss _ / 1 N J

"-QD ^c QD \^/L excitation ' ^VQD ^' ™%)D ^QD ^ emiss ' ^VQD ^l »

-AQD+RC ~

(^excitation ' ^{' C}QD ^""^" ^RC ^excitation ) ^{' C} RC J ^' ^'

^{Λ e}m^iss - Yp ( 2 \ _r A- _P ( 2 \ r 1 / ^QD+RC ^~ \_^C QD \^/lemιss ' ^LQD ^{τ C} RC \^Λemιss > ^L RC J ^l' where L = lcm, and / = 2mm are optical paths of light in the vertical and horizontal directions, respectively. L and / are the dimensions of the cuvette. In the present example, relatively small concentrations of acceptor molecules were used that created very small geometrical effects. For example, for the curves in Fig. 4a, the correction factor: k < 1.07 for QD570 and k < 1.13for QD530. Overall, the geometrical effect on quenching of QD530 for RC:QD=1: 1 is about 7%; for the same conditions, the correction for QD570 is about 3%. The correction factor is close to unity since the absorption and re-absorption by RCs in the conditions of the present example are quite small. For example, at λ_excιtatιon = 450nm , ^εRc(Kxcιtatιon) ' ^C _RC ' ^L ~ 0-^{05 and} 0-^{025 for c} Re = ⁰-^{5 and} ^ .25 μM , respectively.

[00136] XII. Excitation dynamics. Figure 2a of the main text presents an energy structure of the (QD₁ -RC₁ ) complex comprising one QD and one RC. The RC and QD are coupled via FRET. The room-temperature relaxation dynamics of excitations in the QD-RC complex can be described by a system of classical rate equations. In the complex QD₁ -RC₁ , the exciton population of a QD is given by the equation: dm

~ \ ^QD ~*~ ^FRET ) ^{' 171}QD ~*~ I QD ' (^3)

[00137] where m_βD is the number of photo-generated excitons in the QD and I_QD is the rate of generation of excitons inside the QD. For this rate, it can be wήtten: I _QD = σ_QD(λ_exc) - Flux / hύ)_exc , where Flux and θ)_exc are the excitation flux and frequency, respectively, and <J_QD{λ_exc) is the QD absorption cross section. The excitation kinetics for the RC is described by:

"^■n PSlO-Qy _ 7 , T , T ( Q Λ \

^"T - ~^KRC ^{' n}pm)-Qy ^{+ 1} RC ^{+ K}FRET ^{' 171}QD ' ^⁴J where n_p&10__Qy is the number of excitations in the state P870-Qy and

I_RC ^{= σ} _Rc{K_xc^ ' Flux l ϊι(θ_exc is the rate of optical generation of excitations in the RC. fc_sc is the rate of recombination for the state P870-Qy; this rate is a sum of two terms, k_RC = k_Rc,raιi + k_Rc,non-_ra^ where ^_c>(rad)lI0B__rad are the radiative and non-radiative rates, respectively. The last term in Eq. S4 describes the FRET process in the coupled QD-RC complex. It is interesting to note that Eq. S4 assumes that an excitation inside a RC relaxes very quickly to the lowest excited state of P870-Qy. This assumption is well justified since the intrinsic relaxation times of the RC are rather short, -100 fs (see Fig. 2a of the main text). [00138] By solving Eqs. S3 and S4 for the stationary conditions

(dn_QD I dt = dn_p&70__Qy I dt = 0 ), an equation for the population of the P870-Qy state is obtained: k_FRFT I Flux

^TQD ^{+ K}FRET J ^{n ω}exc [00139] This quantity allows us to calculate the emission intensity from the RC at

910nm: PL^°^nm = k_RCtFad - n_PK1Q__Qy . Then, the PL intensities for a RC and QD-RC complex at 910nm and find the enhancement factor is calculated by Eq. S5:

A ( 7 /(J n

Aio™ (Λ« (∞⁾

[00140] By using molar extinctions, instead of cross sections, the following equation is obtained from Eq. S5: V₁ ^LQD ^{~ K}FRET ^CRC J

where ε_{RC ι}{λ) = — —σ_{RC ι}(λ) and ε_QD(ω) should be in M^~lcm^~l and σ_{RC ι}{λ) m m² .

[00141] Along with Eq. 3, the equation S7 was also employed to evaluate the enhancement factor and obtained good agreement with the experimental data (see Fig. 4). The equation S7 is substantially the same as Eq. 3. However, the above calculations were done for the particular case of the QD₁ -RC₁ complex. In the experiment, it is possible to have statistical distribution of complexes QD_n - RC_1n with arbitrary n and m. For this general case, similar equations may be used. Assume now that the fraction of QDs bound to RCs is y = N_QD,bound I N o_{p ft*} » where N_{QD tot} is the total number of QDs in a solution and N_QDt>ound is the number of QDs bound to RCs. Then, the FRET efficiency of the solution becomes: pj exciton -,

E = \ - p^Lj^Qex^Dc-it^Ron^C = y ^ i ^ , -1 = y J . E₀ U ,

1 "-FRET ^{τ L}QD where E₀ is the FRET efficiency in the complex QD₁ -RC₁ (see eq. S2), and PL~_C and PU_QQ""¹ are the peak intensities of QD exciton emission for QD-RC complexes and individual

QDs, respectively. Simultaneously, an enhancement coefficient for the solution of RC-QD complexes takes the form:

Al_Omn(4J - ' OS)

where x = N_{QD tot} I N _{RC tot} is a molar ratio of the components; here N _{RC tot} is the total number of RCs in the solution. I_QD = ε_QD ^■ N_{QD tot} ^■ L and I_RC = ε_RC ^■ N_{RC tot} ^■ L are the absorbances of the QDs and RCs, respectively; L is the optical path. Equation S 8 can be used for the general case when y = N_{QD bound} I N_QDfot and x = N_QDfi>t I N_{RC m} are arbitrary.

[00142] Discussion:

[00143] Figure 3a shows that electrostatic assembling of CdSe/ZnS QDs of different diameters (colors) and RCs lead to efficient quenching of exciton emission of QDs which suggests an efficient FRET effect from QDs to RCs. Quenching for QD600 is weaker than that for QD530 and 570; it can be attributed to an increased thickness of the polymer shell of QD600 that results in reduced FRET. The Fόrster radii ( R_F ), calculated from the standard theory are 6.2, 6.1 and 6.4 nm for QD570, QD530, and QD600 dots, respectively (for details of calculation, see SI). It should be noted that the calculated FRET radius for QD600 is larger because of a stronger QD-RC spectral overlap. To calculate R_F the following experimental values of the quantum yield were used: 0.5, 0.68, and 0.45 for the three types of QDs. The efficiency of FRET is conveniently described with the parameter E:

_T)jexcιton E _{= χ} _ ^QD-RC _{( 1 )} p - γexciton ^{7 v /}

where PL™_RC and PL⁶ _Q ^*0" are the peak intensities of QD exciton emission for QD-RC complexes and individual QDs, respectively. Within the standard theory, the FRET efficiency for the complex involving one QD and one RC (complex "QD₁ -RC₁ ") can be expressed as: E₀ = 1/ Ϊ1 + (R / R_F)^{β ~}\ , where R is the QD-RC spatial separation (see SI). Thus, it is clear that the FRET efficiency ( E₀ ) decays very fast with distance. Therefore, FRET is efficient (i.e. E - I ) if R_F ≤ R . Neglecting the thickness of organic shell around QDs, it is estimated that the minimum separation .R_1111n = (D_{QD core} + D_RC ) / 2 , where D_{QD core} is the QD diameter and D_RC is the RC size; the X-ray diameter of RC is about 5nm. Then, the estimated minimum QD-RC distances R₁₁₁₁₁₁ = 3.75 - 4. %nm for the CdSe QDs. It can be seen that these numbers are smaller than the estimated FRET radii. This opens the possibility to realize efficient FRET in the present example. The real separation can be greater than ^₁₁₁₁₁₁ because of the polymer shell of the QDs. For example, the relatively weak FRET for QD600 can be attributed to the increased thickness of this polymer shell.

[00144] Important time-resolved data on PL decay kinetics of QDs alone and in the complexes with RCs are shown in Fig. 3b. PL decay kinetics were recorded in two spectral regions, where the first region corresponds to the PL emission from QD530 used (520-650 nm) and the second region covers the weak PL emission from the RC (>700 nm). This second spectral region covers the fluorescence spectrum of RC with a peak at 910 nm corresponding to emission from the BChI special pair (form P870). Fig. 3b, clearly shows the key signatures of FRET: the lifetimes of QDs emission in the "QD- region" become shorter when QDs are assembled with RCs whereas the lifetimes of RCs emission in the "RC-region" become longer upon their interaction with QDs. Simultaneously, the time-integrated emission (F) decreases for the QD-region and grows for the RC-region (see Table 5).

[00145] Table 5 I Summary of decay times and PL amplitudes from time- resolved studies of CdSe/ZnS quantum dots and reaction centers.

RC-region of optical spectrum QD-region of optical spectrum

Sample/Parameters

Ti F Ti T2 F

Experiment 1

RC 43 1838 3012

QD530 216 4187 5808

QD+RC 433 2361 9279 60 2236 3570

Experiment 2

RC 274 1648 1269

QD570 131 2941 1761 QD+RC 116 971 2120 147 1345 1052

Experiment 3

RC 157 1479 9183

QD600 292 > 10000 >660000 QD+RC 110 1941 15258 230 4886 410889

7₁₍₂₎ are decay times for fast (slow) components in ps and F is the time -integrated intensity of fluorescence. The values of T₂ in decay kinetics of QD600 were difficult to be calculated precisely, but it was evaluated to be >10 ns providing us with approximate value for F>660000.

[00146] It is worth mentioning that an efficient FRET from QDs to photosynthetic

RCs found in this work is not limited to the CdSe/ZnS nanocrystals. Another convenient material system is CdTe QDs. CdTe QDs are synthesized directly in aqueous solution and have thioglycolic acid functionality on the surface. Importantly, these QDs have smaller hydrodynamic diameters compared to the CdSe/ZnS nanoparticles (see SI). It is then not surprising that these nanocrystals may demonstrate comparable and even more efficient FRET upon assembly with RCs (Fig. 4a). Figs. 4a-4b show that the exciton emission of QDs decreases with the number of added RCs (Fig. 4a) whereas the characteristic emission of RC at 910 nm grows upon addition of QDs (Fig. 4b). The calculated FRET radii for the CdTe QD530 and QD570 are ~ 5.4 and 5.3 nm. These radii are significantly longer than the expected QD-RC distances estimated as (D_{QD core} + D_RC) 12 . Again, this ensures fast FRET which was indeed observed. An enhancement factor for the PL emission of BChI special pair in RCs at 910 nm may be defined as pj9Wnm

A ( 2 Λ = ^βD-RC

^lOnm VW / r>_j910mn Γ^LRC , (2) where PL⁹^^nm _RC(RC) PL_QD__RC and PL_RC are the 910 nm PL peak intensities for QD-RC complex and for individual RCs, respectively. The enhancement factor strongly depends on the excitation wavelength, λ_exc . The experimentally determined factors A(λ_exc) are found to be larger than unity forλ_exc < λ_{QD excιton} , where λ_{QD excιton} is the exciton wavelength of QDs. The enhancement factor ^_10n can also be written as A_910111n = n_QC__RC I n_RC , where n_RC and n_QC__RC are the numbers of electron-hole pairs generated at the special pair in RC and in the QD-RC hybrid material, respectively. Therefore, this factor is proportional to the number of electron-hole pairs arriving at the special pair of RC. Since A(λ_exc ) > 1 , the hybrid structures of the present invention demonstrate optical enhancement, i.e. the number of electron-hole pairs arriving to the special pair of RC is increased. The physical reason for the optical enhancement of RCs is that additional electron-hole pairs arriving to the RC from QDs via the FRET coupling. The complex QD570-RC shows stronger enhancement (up to 3 fold at λ_exc = 550nm ) than that for

QD530-RC (Fig. 4) what may be explained by the larger absorption of QD570. Also, at smaller RC/QD molar ratios, the enhancement effect increases because more QDs interact with a single RC and, therefore, more energy can be transferred from QDs to a single RC. [00147] To support the above explanations and as a consistency check, the enhancement factor using experimental extinctions and FRET efficiencies was calculated. From a simple rate model, Eq. 3 is obtained:

A_9iOnm(λ_exc) = \ + E - ^u = l + E - x - ^u (3)

I Re (AJ £RC (A_XC ) where /_βD(sc) are the absorbances of QDs and RCs, ε_QD(RC) are the extinctions, and x is the QD/RC molar ratio. Figures 4c and 4d show overall good correspondence of measured and calculated results. The calculation reproduces both trends well: enhancement of emission at λ_exc < λ_{QD excιton} and increase of A(λ_exc) for small RC/QD ratios. The enhancement occurs at the wavelengths at which the QDs strongly absorb [00148] Fig. 4 also shows that the enhancement due to the FRET from QD530 and

QD570 to RCs occurs at the excitation wavelengths present in the solar spectrum. Stronger overlap between the absorptions of QDs and solar spectrum can be tailored by choosing nanocrystals with suitable optical spectra. In addition, CdTe QDs used in the present example have relatively small radii and, therefore, moderate absorptions. Further optimization of the light-harvesting complexes can be done using QDs with larger sizes having much stronger absorptions⁶ or by forming hybrid complexes from RCs and nanowires or nanorods which typically have very strong absorptions.

[00149] It was mentioned above that the RC system of the present example was in a state of P870/Q_a ^~Q_b ^". To keep this state non-oxidized, an efficient supply of electrons from Na-ascorbate is needed. One can imagine that attached QDs can block the delivery of electrons to the special pair. However, important time-resolved PL kinetic studies show that this is not the case (see Fig. S4 in SI). Delivery of electrons to the special pair remains fast in the QD-RC complexes.

[00150] Potential applications of hybrid materials built from nanocrystals and biological molecules include such emerging fields as sensing and light harvesting. In sensor structures, nanocrystals conjugated with a bio-molecule have optical emission sensitive to the environmental parameters. In light-harvesting nanocrystal complexes coupled by FRET, the absorbed optical energy flows from absorbing elements to the acceptor region and, in this way, becomes concentrated. In contrast to the light-harvesting structures reported in previous publications, the hybrids of the present example incorporate biological photosynthetic reaction centers coupled with nanocrystals by FRET. Nanocrystals in our complexes transfer optical energy to photosynthetic elements which are able to split electron-hole pairs and initiate chemical transformations. [00151] This study represents the first example of the efficient transfer of excitation energy harvested by nanoparticles to a complex biological photosynthetic system. The results are important for applications because they open the potential for the use of nanocrystals as a light-harvesting built-in antenna for artificial photosynthesis. In green plants the reaction center of Photosystem II has a charge separation site very similar to that of the bacterial reaction center. So, from the fundamental point of view, the results of efficient nano-bio energy transfer from QDs to the bacterial RCs offer one interesting possibility for the utilization of nanocrystals to enhance the efficiency of the photosynthetic biological function. Moreover, the theoretical estimates predict much stronger enhancements (upon further optimizations of the hybrid complexes. The predicted enhancement of light-harvesting of solar radiation with properly optimized hybrids can be as large as five times. It is worth mentioning that the enhancement of biological functions of natural photosynthetic systems, if realized, will make a real impact on energy-related technologies.

[00152] Example 2

[00153] This example illustrates preparation of a hybrid nanostructure composed of one or more quantum dots covalently bound to the reaction center.

[00154] Procedure:

[00155] CdSe/ZnS QDs emitting PL at 520 nm or 570 nm and solubilized with

PEG-COOH were used. The quantum dots were then covalently bound to (also called conjugated with) the reaction centers (RCs) using one or more compounds containing one or more carbodiimide function group (the carbodiimide compound). In the presence of the carbodiimide compound, the COOH-groups of QDs reacted with the NH2-groups of RCs to form a covalent peptide bond.

[00156] The detail covalent binding (conjugation) procedure of Example 2 is described on page 195 of the book by Greg T. Hermanson (Greg T. Hermanson, Bioconjugate Techniques, 2nd edition, Academic Press (An imprint of Elsevier), 2008, p. 195, ISBN: 978-0-12-370501-3). Other bioconjugation procedures listed on pages 192- 195 of the book can also be used.

[00157] Results and Discussion: [00158] Fig. 9a is a diagram illustrating absorbances of the hybrid nanostructures of Example 2 as a function of wavelength (nm). The hybrid nanostructures are chemically-conjugated complexes of RCs with QD520 (black line) and of RCs with QD570 (red line). [00159] Fig. 9b is a diagram illustrating absorbances of the QD570/RCs hybrid nanostructure, QDs, and the RCs of Example 2 as a function of wavelength (nm). This is an illustrative example of deconvolution of UV-vis spectra of the QD570/RCs hybrid (conjugate) (black) into that of the QDs' (red) and RCs' (blue) components. The result of fitting procedure is shown by a green line. [00160] Figs. 9c-9d are UV-vis spectra for noncovalent-bound (blue) and conjugated (black) hybrids (complexes) for QD520/RCs (Fig. 9c) and QD570/RCs (Fig. 9d) respectively. The conjugated hybrid or complex was formed through a covalent bonding. The two types of hybrids yielded very similar UV-vis spectra, proving that the relative concentrations of QDs and RCs were the same in the electrostatic and covalent hybrids. [00161] Fig. 10 is a comparative diagram of fluorescence in terms of % PL intensities of free QDs, the non-covalent bound hybrid, and the covalent bound hybrid. More specifically, it is a Comparison of photoluminescence quenching for noncovalent bounded and covalent bound QDs/RCs hybrids for QD520 (black) and QD570 (red). There was a 35- and 18-fold increase in quenching efficiency for covalent bound hybrids, respectively.

[00162] Where QD520 are concerned, Table 6 shows that their quenching efficiency in a covalent conjugate with RCs was found to be 34.8 times greater than that for the QD-RC hybrid through non-covalent bindings. Fig. 10 shows that for QD580, their quenching efficiency in a covalent conjugate with RCs was found to be around 18 times greater than the QD-RC hybrid through non-covalent bindings.

[00163] Table 61 Composition of QDs/RCs hybrids by covalent bindings for

Example 2 and the corresponding photoluminescence quenching efficiencies

[00164] Embodiments of the invention address some or all of the concerns with the prior art. This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention may be obtained by reference to the following description of the preferred embodiments thereof in connection with the attached drawings.

[00165] This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.

Claims

1. A method of making a hybrid nano structure suitable for energy harvesting and transfer, comprising: a. preparing a first aqueous solution comprising at least one photosynthetic unit (PU) of a photosynthetic organism; b. [preparing a second aqueous solution comprising one or more semiconductor nanoparticles; c. adding the first solution to the second solution; d. agitating the mixture of step c until the hybrid nanostructure is formed, wherein the semiconductor nanoparticles efficiently supply electron-hole pairs (excitons) to the RC through a Forster Resonance Energy Transfer (FRET) coupling.

2. The method according to claim 1, wherein the photosynthetic organism is a purple bacterium, a cyanobacteria, a green plant, or another similar photosynthetic organism.

3. The method according to claim 1, wherein the semiconductor nanoparticle is suitable for an efficient Forster Resonance Energy Transfer (FRET) from the semiconductor nanoparticle to the photosynthetic unit.

4. The method according to claim 3, wherein the photosynthetic center is capable of performing a process of sequential electron transfer.

5. The method according to claim 4, wherein the first aqueous solution and the second aqueous solution are substantially compatible with each other to form the hybrid, wherein the integration of the semiconductor nanoparticle into the hybrid does not interfere with the process of sequential electron transfer.

6. The method according to claim 3, wherein a Forster radii (R_F) of the hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit, wherein the FRET efficiency (Eo) = 1/(1+(R/R_F)⁶) and wherein R_min =(D_SN+ D_PU)/2; D_SN is the diameter of the semiconductor nanoparticle, and D_PU is the diameter of the photo synthetic unit.

7. The method according to claim 6, wherein the photosynthetic units and the semiconductor nanoparticles are in a molar ratio that ensures sufficient excitons is transferred from the semiconductor nanoparticles to the photosynthetic units.

8. The method according to claim 7, wherein the molar ratio is sufficiently small so that A(λ_eχ_c) is greater than 1, wherein A is an enhancement factor; λ_exc is an excitation wavelength, and A (λe_XC) = PLh_ybn_</PLpu, wherein the PLhybnd is ^a photoluminescence peak intensity for the hybrid, and the PL_PU is a photolumiescence peak intensity for the photosynthetic unit.

9. The method according to claim 1, wherein the photosynthetic unit comprises at a chromatophore, a thylakoid, a chloroplast, or a reaction center (RC).

10. The method according to claim 9, wherein the reaction center comprises one or more quinones, and wherein the reaction center is capable of sequential electron transfer.

11. The method according to claim 10, wherein the first aqueous solution further comprises a compound capable of being an electron donor (the electron donor compound).

12. The method according to claim 11, wherein the electron donor compound keeps the quinones fully reduced and the reaction center photochemically inactive.

13. The method according to claim 12, wherein the electron donor is selected from the group consisting of an electron donating dye, an electron donating compound, or a mixture thereof.

14. The method according to claim 13, wherein the electron donating compound is selected from the group consisting of a condensed polycyclic compound, an amine compound, a quaternary amine compound, an aniline derivative, a nitrogen-containing heterocyclic compound, a sulfur-containing heterocyclic compound, an oxygen- containing heterocyclic compound, a nitrogen- and oxygen-atom containing heterocyclic compound, a nitrogen- and sulfur-atom containing heterocyclic compound, an oxygen- and sulfur-atom containing heterocyclic compound or a sulfur-, nitrogen- and oxygen- atom containing heterocyclic compound.

15. The method according to claim 12, wherein the electron donor is sodium- ascorbate.

16. The method according to claim 12, wherein the reaction center is stable against photo-oxidation.

17. The method according to claim 1, wherein the second aqueous solution further comprises one or more stabilizing agents.

18. The method according to claim 17, wherein the stabilizing agent forms a coating around the semiconductor nanoparticle to stabilize and solubilize the semiconductor nanoparticle in the second aqueous solution.

19. The method according to claim 18, wherein a thickness of the coating is suitable to ensure that the FRET of exciton from the semiconductor nanoparticle to the photosynthetic unit is efficient.

20. The method according to claim 17, wherein the stabilizing agent is a mercapto- compound, another similar stabilizing molecule, or a mixture thereof.

21. The method according to claim 20, wherein the mercapto compound is a thio- containing polyethylene glycol molecule; a cysteine moiety; a thioglycolic acid; or a combination thereof.

22. The method according to claim 20, wherein the semiconductor nanoparticle is suitable for an efficient Forster Resonance Energy Transfer (FRET) from the semiconductor nanoparticle to the photosynthetic unit.

23. The method according to claim 21, wherein fluorescent wavelengths of the semiconductor nanoparticles are sufficiently overlapped with absorption wavelengths of the photosynthetic unit to ensure an efficient Forster Resonance Energy Transfer (FRET) of excitons from the semiconductor nanoparticles to the photosynthetic units.

24. The method according to claim 23, wherein the photosynthetic unit comprises at least one reaction center (RC), and wherein the reaction center comprises one or more quinones, and wherein the reaction center is capable of sequential electron transfer.

25. The method according to claim 24, wherein the semiconductor nanoparticle comprises a quantum dot, a nanowire, a nanorod, or a mixture thereof.

26. The method according to claim 25, wherein the quantum dot comprises a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof; and wherein the quantum dot comprises a quantum dot fluorescent core.

27. The method according to claim 26, wherein the quantum dots is bound to the reaction center through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof.

28. The method according to claim 27, wherein the second aqueous solution further comprises a linking agent, wherein the linking agent becomes a biolinker when the hybrid is formed.

29. The method according to claim 28, wherein a length of the biolinker is suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center.

30. The method according to claim 28, wherein the linking agent is a compound containing a carbodiimide functional group with a formula of RN=C=NR (the carbodiimide compound), wherein R comprises a carboxylic function group, an amino function group, or a mixture thereof.

31. The method according to claim 30, wherein the linking agent is the stabilizing agent.

32. The method according to claim 30, wherein the carbodiimide compound assists in forming a covalent bond between the quantum dot and the reaction center.

33. The method according to claim 26, wherein the binding of the quantum dot to the reaction center is shown by a reduction in peak intensities of exciton emissions of the quantum dot when the hybrid is formed.

34. A hybrid nanostructure (the hybrid), comprising a. at least one semiconductor nanoparticle; and

b. a photosynthetic unit of a photosynthetic organism;

wherein the semiconductor nanoparticle is bound to the photosynthetic unit so that the semiconductor nanoparticle transfers electron-hole pairs (excitons) to the photosynthetic unit through a Forster Resonance Energy Transfer (FRET) coupling.

35. The hybrid according to claim 34, wherein the photosynthetic organism is a purple bacterium, a cyanobacteria, a green plant, or another similar photosynthetic organism.

36. The hybrid according to claim 34, wherein the semiconductor nanoparticles are bound to one or more sites of the photosynthetic unit, wherein the sites comprise electron donor sites, electron acceptor sites, or both.

37. The hybrid according to claim 36, wherein a Forster radii (R_F) of the hybrid nanostructure is larger than R_min to ensure an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit, wherein the FRET efficiency (Eo) = 1/(1+(R/R_F)⁶) and wherein R_min =(D_SN+ D_PU)/2; D_SN is the diameter of the semiconductor nanoparticle, and D_PU is the diameter of the photosynthetic unit.

38. The hybrid according to claim 37, wherein the efficient FRET of exciton occurs when peak intensities of exciton emission of the semiconductor nanoparticle are reduced when the semiconductor nanoparticle is suitably bound to the photosynthetic unit.

39. The hybrid according to claim 37, wherein the semiconductor nanoparticle is bound to the photosynthetic unit through an electrostatic force, a molecular force, a hydrophobic force, a biolinker, or a combination thereof.

40. The hybrid according to claim 39, wherein a length of the biolinker is suitable for an efficient Forster Resonance Energy Transfer (FRET) of excitons from the quantum dot to the reaction center.

41. The hybrid according to claim 40, wherein the biolinker is a bifunctional connecting molecule.

42. The hybrid according to claim 40, wherein the biolinker comprises a carbodiimide group, a carboxylic group, an amino group, or a mixture thereof.

43. The hybrid according to claim 40, wherein the biolinker assists in forming one or more covalent bonding between the semiconductor nanoparticle and the photosynthetic unit.

44. The hybrid according to claim 39, wherein the semiconductor nanoparticle is suitable for an efficient FRET of excitons from the semiconductor nanoparticle to the photosynthetic unit.

45. The hybrid according to claim 44, wherein a diameter of the semiconductor nanoparticle is in the range of about 2nm to about 20nm.

46. The hybrid according to claim 44, wherein the semiconductor nanoparticles comprise quantum dots, nanowire, nanorods, or a mixture thereof.

47. The hybrid according to claim 46, wherein the quantum dot is a CdSe/ZnS quantum dot, a CdTe quantum dot, or a mixture thereof.

48. The hybrid according to claim 34, wherein the photosynthetic unit comprise at least one reaction center.

49. The hybrid according to claim 48, wherein the reaction center, comprise one or more quinones, and wherein the reaction center is capable of sequential electron transfer.

50. The hybrid according to claim 49, wherein the reaction center is kept photochemically inactive by one or more electron donor compounds, wherein the electron donor compound also keeps the quinones of the reaction center fully reduced.

51. The hybrid according to claim 34, wherein the hybrid nanostructure is suitable for light energy harvesting and excitation energy transfer.