NL2009442A - Method for depositing metal nanoparticles on a surface, surface fabricated with the method, and the application thereof. - Google Patents

Description

Method for depositing metal nanoparticles on a surface, surface fabricated with the method, and the application thereof.

The subject matter of the invention is a method for depositing metal nanoparticles on an electrode surface, the surface so coated, and the application thereof as a platform for measurements with techniques making use of the surface plasmon resonance, i.e., the surface enhanced Raman scattering (SERS), and the localized surface plasmon resonance (LSPR).

Reduction of metal ions that are present in an electrolyte solution, e.g., Au, Ag, Cu, Ni, Pt and Pd ions, by applying potential to the electrode, leads to deposition of nanoparticles of these metals on the electrode surface. The properties of said nanoparticles, i.e., their size, shape, density and distribution on the surface, depend on the sort of the substrate used, the metal salt concentration and the type of the electrolyte, as well as the electrochemical technique used, and consequently on the applied voltage, duration and number of individual stages of the experiment (e.g., the pulses when using pulse techniques, or cycles when using cyclic techniques), emulsion formation, the use of porous membranes, adsorption phenomena of colloidal particles, e.g., organic polymers or oxide materials [Welch and Compton 2006, Analytical and Bioanalytical Chemistry 384(3): 601-619; Dryfe 2006, Physical Chemistry Chemical Physics 8(16): 1869-1883; Cheng and Schiffrin 1996, Journal of the Chemical Society-Faraday Transactions 92(20): 3865-3871],

In the state of the art there are essentially a few approaches to electrodeposition of nanoparticles on conducting substrates. For example, in the approach disclosed in the patent applications US 2008081388 (A1), US 7449757, US 20090101996 (A1), and in the patent application WO/2009/137694 (PCT/US2009/043167), the metal ions containing salt is dissolved in an acidic electrolyte solution, and the nanoparticles are deposited by application of a potential on the substrate immersed in that solution. The properties of the electrodeposited particles, such as their size and amount, are strictly dependent on the magnitude of the applied potential, the number and width of pulses (when using pulse techniques), and the additional substance used to speed up the metal electrodeposition reaction. Another methodology relies on electrochemical growth of metal nanostructures in matrices (mostly made of anodised aluminium oxide, as described in the patent applications US 20060038990 and US 20110045230).

The surface fabricated with the method according to the present invention can be used, for instance, as so called platform for localized surface plasmon resonance (LSPR) studies. LSPR is an optical phenomenon resulting from interaction of light with conducting nanoparticles of sizes smaller than the wavelength of the incident light. Electric component of the incident lightwave can collectively excite the electrons in the conduction band. As a result, surface plasmons oscillate with certain resonance frequency that is strictly dependent on the size, dimensions, and composition of the nanoparticles, as well as on the distance between them and on the dielectric properties of the surrounding medium [E. Petryayeva, U.J. Krull 2011, Analytica Chimica Acta 706(1): 8-24, doi:10.1016/j.aca.2011.08.020], LSPR allows for detection on the single molecule level and working with very small analyte volumes. Due to generation of a strong and localised electromagnetic field that can be utilised also by other techniques (including, e.g., surface enhanced Raman scattering (SERS), metal-enhanced fluorescence (MEF) on metal films and nanoparticles) it is possible to design integrated biosensors that will allow for making use of several methods mentioned above in a study of a sample deposited on one and the same substrate.

The surface enhanced Raman spectroscopy is a spectroscopic technique to measure intensity of light in ultraviolet, visible and near infrared spectral regions that is inelastically scattered on molecules adsorbed on surfaces of certain metals (e.g. Ag, Au, or Cu) with nanometer roughness features (10-100 nm) [Kneipp, Kneipp, Itzkan, Dasari, Feld 1997, Physical Review Letters 78: 1667-1670; Nie, Emory 1997, Science 275: 1102-1106], It has been one of the most intensely developed spectroscopic techniques in the recent decade, as it allows to enhance the effective Raman scattering cross section of molecules adsorbed on a metal surface by several orders of magnitude (102-106, and for certain systems even 108-1015) [Kneipp, Kneipp, Itzkan, Dasari, Feld 1997, Physical Review Letters 78: 1667-1670; Nie, Emory 1997, Science 275: 1102-1106] as compared with the effective Raman scattering cross section of non-adsorbed molecules [Herne, Ahern, Garrell 1991, Journal of American Chemical Society 113(3): 846-854; Thornton, Force 1991, Applied Spectroscopy 45(9): 1522-1526],

The SERS signal enhancement depends on a number of factors, including the effective Raman scattering cross section, the frequency of excitation radiation, the chemical origin of a molecule, and primarily on the type of metal surface whereon the molecule is adsorbed, and on the degree of surface roughness. These roughness features are responsible for the electromagnetic mechanism of enhancement that is the dominant SERS enhancement mechanism [Kambhampati, Child, Foster, Campion 1998, Journal of

Chemical Physics 108: 5013-5026],

First studies on the use of electrochemically deposited nanoparticles as a platform for surface enhanced Raman scattering studies were performed for gold and silver nanoparticles [Plieth, Dietz, Anders, Sandmann, Meixner, Weber, Kneppe 2005, Surface Science 597(1-3): 119-126], Reported nanoparticles were deposited from the aqueous phase using a two-pulse technique on the ITO electrode and glassy carbon. Another example of the use of nanoparticles as SERS platform is electrodeposition of nanoparticles on earlier prepared substrates, e.g.: periodical silicon microhole arrays [Luo, Chen, Zhang, He, Zhang, Yuan, Zhang, Lee 2009, Journal of Physical Chemistry C 113(21): 9191-9196], or hexagonal pattern on a platinum electrode [Geun Hoi Gu, Jung Sang Suh 2010, Journal of Physical Chemistry C 114(16): 7258-7262], In the former case, gold and silver nanoparticles were deposited from the aqueous phase using a chronopotentiometric technique at constant current density, while in the latter silver nanoparticles were deposited from the organic phase (ethyl alcohol) with an alternating current voltammetry.

Despite that various substrates called also grounds or platforms can be used, there is still a problem of fabricating surfaces yielding a strong enhancement of the SERS spectrum and reproducibility thereof at each point of the surface (and also yielding a well isolated signal coming from surface plasmons). This are extremely important characteristics of universal active surfaces, especially considering the application of the technique in biomedical studies or in biosensor design [Liu, Lu, Kim, Doll, and Lee 2005, Advanced Materials 17(22): 2683-2688; Domke, Zhang, and Pettinger 2007, Journal of the American Chemical Society 129: 6708-6709; Gunawidjaja, Peleshanko, Ko, and Tsukruk 2008, Advanced Materials 20(8): 1544-1549], It is also important that the resulting surfaces are produced using reagents that are not environmentally harmful.

Since the method according to the present invention allows for coating of large area surfaces with nanoparticles, it became possible to fabricate platforms large enough to accommodate two or more types of studied molecules/objects (e.g., biomolecules, bacteriophages, etc.) at different locations on the same platform, and to measure simultaneously their SERS and/or LSPR spectra. While the literature is replete with reports and patent applications, no method has been known to date that would enable such a measurement, guaranteeing at the same time reproducibility of the spectra for a given surface morphology. A suitably developed surface, and appropriate size and distribution of the silver nanoparticles on the platform allows one to perform reproducibly studies and to obtain a very good coefficient of Raman signal enhancement, as well as a strong signal from surface plasmons.

In the state of the art there are known surfaces for SERS measurements based on nanoparticles, nanowires or nanoprisms reported, e.g., in the following patents and patent applications: US 4005229, W02009/035479, US 20080096005, US 20050147963, W02008/09/4089. There are also known surfaces for LSPR

measurements that make use of electrodeposited metallic dots, US 20060279738). Universal platforms are not known.

According to the invention, the method for depositing metal nanoparticles on a surface, wherein the said surface is a working electrode and is at least in part immersed in an aqueous solution of a metal salt, said solution being the electrolyte solution, and the said nanoparticles are deposited on the said surface by application of electric potential, is characterised in that • the said metal salt contains an addition of an organic salt, preferably with reducing properties, • a reference electrode, preferably in the form of an Ag|AgCI electrode, calomel electrode, or Ag wire, and a counter electrode, preferably platinum or gold electrode, are used, • electric potential is applied using a cyclic voltammetry technique, • the working electrode is being polarised in a potential range from E^ to E2, where the potential Ei is from +0.2 V to -0.2 V, preferably from +0.2 V to 0 V, most preferably 0 V, the potential E2 is from -0.3 V to -1.2, preferably from -0.6 V to -1 V, most preferably -0.8 V, the polarisation rate is from 1 to 100 mV/s, preferably 5, 10, 50, or 100 mV/s, and the number of cycles is from 1 to 50, preferably from 20 to 50.

Preferably, the said metal salt is a salt of a metal selected from the group comprising Cu, Ag, Au, Al, and Pt.

Preferably, the concentration of the said metal salt, soluble in the electrolyte solution, is from 0.1 mM to 100 mM, and more preferably about 1-10 mM, most preferably 1 mM.

| n a preferred example of embodiment of the invention, the said electrolyte solution contains an organic salt, or an organic acid.

Preferably, the said organic salt has an anion selected from the group comprising lactones, e.g., C6H706', and most preferably has the anion C3H4(OH)(COO')3· According to the invention, the concentration of the said organic salt in the said aqueous solution is not less than 0.05 mM, and preferably from 1 mM to 1 M.

In the method according to the invention the said surface comprises preferably indium tin oxide, ITO, fluorine tin oxide, FTO, glassy carbon, GC, or gold, Au.

The method according to the invention may additionally comprise the stage of washing the said surface in water or ethanol.

The invention comprises also a surface coated with nanoparticles, fabricated with the above method. According to the invention, the surface may be of any size, and in particular larger than 5 mm2.

Preferably, the average size of nanoparticles on the surface is from 5 nm to 300 nm.

Preferably, the surface coverage is from 1% to 60%, more preferably from 15% to 55%, and most preferably from 25% to 40%.

According to the invention, the surface fabricated with the above method is preferably used as a platform for surface enhanced Raman scattering (SERS), and for localized surface plasmon resonance (LSPR) measurements.

So far, patent applications have been filed for the inventions (Polish patent applications no. P-391456 and P-392364, unpublished to date) reporting fabrication of stable metal nanoparticles that are resistant to mechanical stresses. The method according to the present invention allows for reproducible fabrication of substrates with specific coverage of nanoparticles of given sizes. In addition, the substrate pre-treatment to assure better adhesion of nanoparticles is not necessary, and the obtained nanoparticles do not require stabilisation with organic films.

The present invention is the only stationary method allowing for uniform coating of large area surfaces effectively, reproducibly (both within single surface and between individual surfaces), while maintaining full control over the size of electrodeposited particles and the surface coverage. The electrodeposition of silver nanoparticles on a light-transparent ITO electrode allowed for establishing a versatile platform for surface plasmon resonance studies, both in the form of SERS and LSPR.

Application of an addition of an organic salt as well as of the potential ranges and polarisation rates as indicated above allows for fabricating surfaces that are particularly suitable to use as a platform for surface enhanced Raman scattering (SERS), and for localized surface plasmon resonance (LSPR) measurements. At the same time, the parameters of these platforms are exceptionally preferable, which is demonstrated in the enclosed embodiment examples.

The present invention is now explained more in detail in examples of embodiment, with reference to the accompanying figures, wherein:

Fig. 1 shows a scanning electron microscope image of an ITO surface with electrodeposited silver nanoparticles according to the invention,

Fig.2 shows randomly selected SERS spectra of 4-aminothiophenol recorded at four different points of the surface (b) and (a) the second derivative of SERS spectra,

Fig. 3 shows a table of coefficients of linear correlation between the second derivatives of the recorded spectra,

Fig.4 shows SERS spectra of 4-aminothiophenol recorded on a freshly fabricated substrate and three months after the substrate has been fabricated,

Fig. 5 shows (a) a normal Raman spectrum of choline (10'3 M) and (b) a SERS spectrum of choline adsorbed from a 10'6 M aqueous solution, on a surface according to the invention,

Fig. 6 shows UV-Vis spectra of a surface according to the invention surrounded by media with various dielectric properties, and

Fig. 7 shows UV-Vis spectra of a surface in air: (a) pure surface according to the invention, (b) surface with deposited TEOS gel, (c) surface with adsorbed biotinylated T7 bacteriophage, and (d) surface with adsorbed biotinylated T7 bacteriophage bound to avidin.

Below the method for depositing nanoparticles on a surface according to the invention is described in detail, using an example of deposition of silver nanoparticles.

Reagents and materials used

Reagents: organic salts, silver nitrate, and electrode materials: ITO (indium tin oxide), FTO (fluorine tin oxide), GC (glassy carbon), Au are commercially available.

In a preferred example of embodiment of the invention, silver nanoparticles were obtained using a cyclic voltammetry technique. It is a one-step method for producing silver nanoparticles directly and durably deposited of an electrode substrate. According to the invention, the method consists in preparing the reactor by immersing the electrode in an aqueous electrolyte solution. The solution contains a salt of a metal from the group IB (Cu, Ag, Au), or IIIA (Al), or VIII (Pt) at a concentration of 1 mM, and anions from the group of lactones, preferably C3H4(OH)(COO')3, at a concentration of 0.001-1 M. The electron transfer between the electrode and the silver ion leads to electrodeposition of silver nanoparticles on the electrode surface.

Nanoparticles are deposited with the use of any potentiostats, Ag|AgCI, calomel, Ag wire reference electrodes, and platinum, gold counter electrodes used in the state of the art. The deposition is performed in a typical electrochemical cell, where the reference electrode, the counter electrode, and the working electrode are immersed in the aqueous electrolyte solution.

According to the invention, silver nanoparticles were deposited using cyclic voltammetry in the potential range from +0.2 V to -1.2 V relative to the reference electrode - Ag wire, more preferably in the range from 0 V to -0.8 V. The polarisation rates used were from 5 mV/s to 100 mV/s, preferably 5, 10, 20, 50, 100 mV/s, most preferably 10 mV/s. Potential scan was repeated from 1 to 50 times (cycles), preferably from 20 to 50 times. Upon completion of the electrodeposition, the electrodes with silver nanoparticles were washed in water.

Using the method for electrodepositing silver nanoparticles according to the invention one obtains nanoparticles characterised in that the size thereof may be from 5 nm to 300 nm. The electrode coverage by the nanoparticles may be from 25 to 40%. Using the method according to the invention one can coat an electrode surface of any area.

To test the adhesion of silver nanoparticles to the substrate, the electrodes were immersed alternating^ in water and ethanol, and subsequently exposed to ultrasound for 30 minutes.

Method for recording SERS spectra

Raman spectra were recorded with a high resolution InVia (Ranishaw) confocal Raman microspectrometer. The wavelength of excitation light used in measurements was 785nm. The scattered light was analysed in the spectrometer with a diffraction grating, and the intensity for each energy was recorded by a sensitive CCD detector. The magnification of the lens focusing the laser beam on the sample was 50x. The spatial resolution was better than 1 pm, and the spectral resolution was about 1 cm'1. The power of the laser used for measurements ranged from 1 mW to 3 mW for SERS measurements, and 150 mW while recording normal Raman spectra.

Reproducibility for one platform - means reproducibility of the SERS spectra recorded on that platform at its different points (agreement as to the intensity and band positions in the SERS spectra recorded under identical measurement conditions). The parameter was determined based on the computed Pearson’s linear correlation coefficient.

Reproducibility for different platforms - means reproducibility of the SERS spectra recorded on different platforms (agreement as to the intensity and band positions in the SERS spectra recorded under identical measurement conditions). The parameter was determined based on the computed Pearson’s linear correlation coefficient.

Pearson’s linear correlation coefficient - the quantitative evaluation of correlation between spectrograms (data series) made use of the Pearson’s linear correlation coefficient applied to the second derivative of the function representing the spectrogram. For that purpose a Golay-Savitzky filter was used that was obtained by fitting the data locally with a polynomial. In doing so, the trinomial coefficients were obtained for each value that was the centre of the window, and consequently the filtered spectrogram intensity values were determined, together with the values of derivatives to the second order inclusive. Subsequently, the windows for extreme data values were defined by appropriately extending a given series with the first and the last values. In doing so, new series of filtered values, and of the derivative values with the same length as the input series were obtained. Then, a table of coefficients of linear correlation between the second derivatives of the spectrograms was compiled, and the mean value of the correlation coefficients was computed. The said mean value is between 0 and 1.

Preferred examples of embodiments of the invention

Example 1 A working ITO electrode was placed in an electrochemical cell, so as to maintain it in a contact with an aqueous electrolyte solution. The solution contained 1 mM silver nitrate and 1 mM trisodium citrate. Cyclic voltammetry was applied in the potential range from 0 V to -0.8 V. 50 cycles have been performed with a polarisation rate 10 mV/s. Upon completion of the electrodeposition, the working electrode was washed in water and ethanol. The electrodeposited silver nanoparticles according to example 1 are shown in an SEM image (Fig. 1). Analysis of the image allows for determining that the average particle size is 91 nm, and the surface coverage is 47%. Placing the electrode in water and exposing it to ultrasound have no effect on the electrode coverage by the nanoparticles.

Example 2

It was proceeded similarly as in example 1, while changing the content of trisodium citrate in the aqueous electrolyte solution to 10 mM.

Example 3

Preparation of substrates for SERS measurements.

It was proceeded identically as in example 1. The plate so fabricated was cleaned by washing it with water and ethanol. The surface so prepared was placed for 24 hours in a solution of the analyte of specific concentration. After taking out of the solution, the surface with adsorbed analyte was ready for SERS measurements.

Example 4 A SERS surface according to examples 1 and 3 was immersed in a 10'6 M solution of 4-aminothiophenol. Then, the platform was dried and 30 Raman spectra were recorded from different points of the surface. Fig. 3 shows randomly selected SERS spectra of 4-aminothiophenol recorded at four different points of the surface (b), and (a) the second derivative of the SERS spectra.

The spectra were recorded during 10 s, using a 785 nm excitation line with a power of 2.5 mW.

In a preferred embodiment, the spectra recorded at different points of the platform are identical. They include strong bands at frequencies: 1593, 1491, 1006, 1087, 465 and 388 cm'1, whereas the relative intensities in each recorded spectrum are virtually the same.

Subsequently, the enhancement factor for 4-aminothiophenol adsorbed on the SERS surface was estimated with the expression: EF= ( IsERs/lRaman)/ (NsERs/NRaman) where:

Isers is the measured integral band intensity in the SERS spectrum of the molecules adsorbed on the surface, lRaman is the measured integral band intensity in the Raman spectrum of the molecules in solution, NRaman denotes the number of molecules in the solution ’’illuminated” with laser light to obtain the Raman spectrum,

Nsers- denotes the numbers of adsorbed molecules ’’illuminated” with laser light to obtain the SERS spectrum, NRaman was determined using the expression: N Raman = NA X C X Df X ΤΤΓ2 where:

Na Avogadro number, 6,02x1 O23, c molar concentration of the solution,

Df focal depth; Df = 2A/NA2, where for the 785 nm line, NA, i.e., the lens aperture, is 0.55 yielding Df = 5 pm, πτ2 geometrical cross section of molecules

Nsers was estimated based on the surface coverage, assuming that the molecules are adsorbed on the surface in a form of a monomolecular film, and taking into account the size of the area illuminated by the laser.

Nsers = Nm x A

where:

Nm number of molecules in the stock solution used for adsorption; A area illuminated by the laser, where A= π x S, wherein S denotes the size of the laser spot that is 5 pm2 for a 785 nm line used in the measurements and the lens magnification 50x.

In a preferred example of embodiment of a SERS surface according to the invention, the estimated enhancement factor (EF) for p-aminothiophenol is 1.8 x 107. Example 5

In another preferred example of embodiment of the invention the reproducibility of the recorded spectra was tested for various surfaces fabricated with the same method. The spectra of 4-aminothiophenol adsorbed on 25 consecutive surfaces for SERS measurements, such as those fabricated in example 1, were recorded. Table 1 presents the linear correlation coefficients between the second derivatives of the recorded spectrograms. The first row and the first column of the table contain series identifiers.

In a preferred example of embodiment of a SERS surface according to the invention the correlation coefficients are very high. The mean value of the correlation coefficients is 0.96.

Example 6

In another preferred example of embodiment of the invention the stability of fabricated surfaces for SERS measurements was tested. 4-aminothiophenol molecules from a 10'6 M solution had been adsorbed on a surface for SERS measurements freshly fabricated according to example 1, and a SERS spectrum was recorded (Fig. 4a). A second surface fabricated according to example 1 was left for three months in a cabinet under oxygen conditions. After three months the surface was immersed in a 10'6 M 4- aminothiophenol solution and a SERS spectrum was recorded (Fig. 4b). Preferably, the surface for the SERS measurements displays high stability. The SERS spectra recorded after three months lost only 10% of their intensity.

Example 7

In another preferred example of embodiment of the invention the performance of fabricated SERS surfaces in biological studies was tested. Choline molecules from 10'6 M solutions were adsorbed on a surface for SERS measurements (Fig. 5b). The wavelength of the excitation light used in the measurements was 785 nm, the laser power on the sample was 3 mW, and the spectrum accumulation time was about 50 s.

Preferably, the surface for SERS measurements yields strong enhancement of the SERS spectrum of choline molecules adsorbed from an even 10'6 M aqueous solution thereof.

Claims (13)

A method for depositing metal nanoparticles on a surface, said surface being a working electrode and at least partially immersed in a liquid solution of a metal salt, said solution being the electrolyte solution and said nanoparticles deposited on said surface surface by applying an electric potential, characterized in that • said metal salt contains an addition of an organic salt, preferably with reducing properties, and • a reference electrode, preferably in the form of an Ag | AgCI electrode, a calomel electrode or an Ag wire and a counter electrode, preferably a platinum or gold electrode, are used, • where electrical potential is applied using a cyclic voltametric technique, • where the working electrode is polarized in a potential range from E1 to E2, where the potential Eί is from +0.2 V to -0.2 V, preferably from +0.2 V to 0 V, more in particular 0 V, the potential E2 being from -0.3 V to -1.2, preferably from -0, 6 V to -1 V, and most preferably -0.8 V, wherein the polarization speed is from 1 to 100 mV / s, preferably 5, 10, 50 or 100 mV / s, and the number of cycles is located from 1 to 50, preferably from 20 to 50. A method according to claim 1, characterized in that said metal salt is a salt of a metal selected from the group comprising Cu, Ag, Au, Al, and Pt. Method according to claim 1 or 2, characterized in that the concentration of said metal salt dissolved in the electrolyte solution is from 0.1 mM to 100 mM, preferably around 1-10 mM, more in particular 1 mM . Method according to one of the preceding claims, characterized in that said electrolyte solution contains an organic salt or an organic acid. A method according to claim 4, characterized in that said electrolyte solution contains an organic salt, said organic salt containing an anion selected from the group comprising lactones, preferably C 6 H 70 6 ', and more in particular the anion C 3 H 4 (OH ) (COO ') 3 · Method according to claim 4 or 5, characterized in that the concentration of said organic salt in said liquid solution is no more than 0.05 mM and preferably from 1 mM to 1 M. Method according to one of the preceding claims, characterized in that the surface to be covered preferably comprises indium tin oxide (ITO), fluorine tin oxide (FTO), glassy carbon (GC) or gold (Au). Method according to one of the preceding claims, characterized in that it additionally comprises the step of washing the surface to be covered in water or ethanol. A surface covered with nanoparticles obtained according to the method according to one of the preceding claims. The surface according to claim 9, characterized in that it is larger than 5 mm 2. A surface according to claim 9 or 10, characterized in that the average size of the nanoparticles on the surface is between 5 nm and 300 nm. A surface according to claim 9, 10 or 11, characterized in that the covered surface is between 1% and 60%, more in particular between 15% and 55% and more in particular between 25% and 40%. Use of the surface according to claim 9, 10, 11 or 12 as a platform for surface enhanced Raman scattering (SERS), or for localized surface plasmon resonance measurements (LSPR).