WO2025199012A1

WO2025199012A1 - Seeded synthesis of low dispersity nanospheres and their assembly into photonic crystals

Info

Publication number: WO2025199012A1
Application number: PCT/US2025/020195
Authority: WO
Inventors: Robert Macfarlane; Theodore HUECKEL; Hayim S. SIMS; Steven NGO; Kyungtae Kim
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 2024-03-18
Filing date: 2025-03-17
Publication date: 2025-09-25
Anticipated expiration: 2026-09-18

Abstract

A method of synthetizing a photonic crystal is disclosed. The method uses nanoparticles to seed the creation of siloxane-based nanoparticles, having a diameter as small as 20 nm. The seed nanoparticles may be gold nanoparticles and the silane may be 3-(trimethoxysilyl)propyl methacrylate (TPM). These siloxane-based nanoparticles are separated from a basic solution and are used to create the photonic crystal. This may be achieved using drop casting, centrifugation or another method. The siloxane-based nanoparticles are then assembled into a photonic crystal. The photonic crystal may reflect light in the visible spectrum in an angle independent manner.

Description

SEEDED SYNTHESIS OF LOW DISPERSITY NANOSPHERES AND THEIR ASSEMBLY INTO PHOTONIC CRYSTALS

This application claims priority of U.S. Provisional Patent Application Serial No. 63/566, 421, filed March 18, 2024, the disclosure of which is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

This invention was made with government support under CHE2304909 and DMR2204222 awarded by the National Science Foundation. The government has certain rights in the invention.

Field

This disclosure describes methods of synthesizing nanocrystals and photonic crystals.

Background

Photonic crystals have the capability to manipulate light in ways that conventional materials cannot. By exerting control over light propagation, photonic crystals as components of optical devices or materials can achieve superior performance compared to traditional electronic devices, particularly in terms of speed, while also mitigating issues related to heat susceptibility. However, the fabrication of such architectures presents a significant challenge due to the fact that these photonic crystals must have precisely controlled internal structures at length scales commensurate with the wavelengths of light being manipulated ( features typically sized between -20-300 nm) . While sel f-assembly approaches in which a set of uni form colloidal particles are induced to organize into photonic crystal structures are possible , proper fabrication of these photonic crystals via sel f-assembly requires colloidal particle building blocks with uni formity in particle si ze and shape . Synthesizing these uni formly shaped particles that can self-assemble into predefined structures remains challenging . Common colloidal materials used for the sel f-assembly of photonic crystals , such as silica and polystyrene , encounter limitations related to particle sphericity and dispersity at this scale , which hinder their applicability in precise self-assembly processes . The challenges associated with uni form synthesis of these colloidal systems arise due to the random nature of particle nucleation and growth processes .

Two step syntheses that convert initial high-dispersity batches of colloidal particles into batches of particles with lower dispersity are more demanding, and can produce inconsistent results in terms of particle si ze and shape distribution . Additionally, colloidally assembled particles of , for example , polystyrene and silica often produce mechanically weak solids upon removal of solvent used during the assembly process .

Therefore , it would be beneficial if there were a method of synthesi zing colloidal particles with low dispersity, such that these particles can be used for the fabrication of photonic crystals via self-assembly . Further, it would be advantageous i f these particles could be synthesized such that they are easily assembled into photonic crystals , and also modi fied pre- or postassembly to impart additional functionality or stability to the crystals , thereby enabling their use in a wider range of potential applications .

Summary

A method of synthetizing a photonic crystal is disclosed . The method uses nanoparticles to seed the creation of siloxane-based nanoparticles , having a diameter as small as 20 nm . The seed nanoparticles may be gold nanoparticles and the silane used to produce the siloxane may be 3- ( trimethoxysilyl ) propyl methacrylate ( TPM) . These siloxane-based nanoparticles are separated from a basic solution and are used to create the photonic crystal . This may be achieved using drop casting, centri fugation or another method . The siloxane-based nanoparticles are then assembled into a photonic crystal . The photonic crystal may reflect light in the visible spectrum in an angle independent manner .

According to one embodiment , a method of making a photonic crystal is disclosed . The method comprises : a . providing seed nanoparticles ; b . mixing the seed nanoparticles with a basic solution and a silane to produce siloxane-coated seed nanoparticles ; c . solidifying the siloxane-coated seed nanoparticles to form monodisperse nanoparticles comprising polymeri zed siloxane-coated seed nanoparticles ; d . optionally repeating b and c with dif ferent concentrations of the seed nanoparticles and the silane ; e . separating the monodisperse nanoparticles from the basic solution; f . washing the separated monodisperse nanoparticles ; and g . producing a photonic crystal from the separated monodisperse nanoparticles . In some embodiments , the method comprises hydrolyzing the silane in an aqueous solution to produce a hydrolyzed silane (siloxane) prior to mixing with the seed nanoparticles. In some embodiments, the seed nanoparticles are citrate capped gold nanoparticles (AuNPs) . In some embodiments, the seed nanoparticles are metallic, inorganic, or organic nanoparticles, including silver nanoparticles (AgNPs) , silicon-based nanoparticles (SiNPs) , platinum nanoparticles (PtNPs) , palladium nanoparticles (PdNPs) , silica nanoparticles (SiO₂) , and polystyrene nanoparticles (PS) . In some embodiments, the seed nanoparticles are coated using citrate, PVP (polyvinylpyrrolidone) , CTAB ( cetyltrimethylammonium bromide) , or PEG (polyethylene glycol. In some embodiments, the seed nanoparticles have a diameter between 5nm and 200 nm. In some embodiments, the silane comprises 3- ( trimethoxysilyl ) propyl methacrylate (TPM) . In some embodiments, the silane comprises triethoxysilyl or trimethoxysilyl derivatives. In some embodiments, solidifying the siloxane-coated seed nanoparticles is performed by adding a radical initiator to the siloxane-coated seed nanoparticles. In certain embodiments, the radical initiator comprises azobisisobutyronitrile (AIBN) , benzoyl peroxide, potassium persulphate, V-50, and other water soluble or water insoluble radical initiators. In some embodiments, solidifying the siloxane-coated seed nanoparticles is achieved using silane crosslinking via base or acid catalysis to induce self-hardening. In some embodiments, the siloxane-coated seed nanoparticles are heated or exposed to UV irradiation during the solidifying. In some embodiments, the separating is performed using centrifugation. In some embodiments, the separating is performed using gravity separation, filtration, or depletion-based methods. In some embodiments, a method of separating is based on a desired monodisperse nanoparticles size and dispersion stability. In some embodiments, separated monodisperse nanoparticles are washed with water, ethanol, methanol or isopropanol. In some embodiments, the photonic crystal is produced by drop casting the separated monodisperse nanoparticles on a substrate and drying the separated monodisperse nanoparticles. In certain embodiments, the photonic crystal has variable wavelength coloration. In some embodiments, the photonic crystal is produced by centrifugation. In some embodiments, the seed nanoparticles are gold nanoparticles (AuNP) , the silane is 3- ( trimethoxysilyl ) propyl methacrylate (TPM) and the basic solution comprises NH3, wherein 100 pL of 5% v/v NH₃, 20 ml 1 nM AuNP, and hydrolyzed TPM are mixed to produce TPM-coated AuNP. In some embodiments, a ratio of a concentration of seed nanoparticles and silane is varied. In some embodiments, the monodisperse nanoparticles have a selected size. In some embodiments, the photonic crystal is embedded in a hydrogel. In some embodiments, the siloxane is functionalized by grafting polymer to a surface. In certain embodiments, the polymer comprises acrylates, methacrylates, siloxanes, polyethylene glycol (PEG) , polystyrene (PS) , or other relevant functionalized polymers, including fluorescent groups (e.g., rhodamine, fluorescein) , crosslinkable groups (e.g., acrylates, vinyl, epoxy) , or reactive groups for further modification (e.g., amine, thiol, carboxyl) .

According to another embodiment, a method of making a photonic crystal is disclosed. The method comprises: a. providing seed nanoparticles; b. mixing the seed nanoparticles with a basic solution and a silane to produce siloxane-coated seed nanoparticles; c. optionally repeating b and c with different concentrations of the seed nanoparticles and the silane; d. separating the siloxane-coated seed nanoparticles from the basic solution; e. washing the separated siloxane-coated seed nanoparticles; and f. producing a photonic crystal from the separated siloxane-coated seed nanoparticles. In some embodiments, the method comprises hydrolyzing the silane in an aqueous solution to produce a hydrolyzed silane (siloxane) prior to mixing with the seed nanoparticles.

According to another embodiment, a photonic crystal is disclosed, wherein the photonic crystal is made using any of the methods described above. In some embodiments, the photonic crystal reflects light of a wavelength between 300 to 3000 nm in an angle independent manner. In some embodiments, the wavelength is determined based on particle size. In some embodiments, the photonic crystal reflects a narrow bandwidth of light in the visible spectrum, such that the photonic crystal appears as a single, well-defined color across the entire photonic crystal. In some embodiments, the photonic crystal reflects yellow, red, green, or green light.

Brief Description of the Drawings

For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which:

FIG. 1A shows the low dispersity colloidal building blocks through seeded growth of emulsions;

FIGs. 1B-1C show two examples of droplet size;

FIG. ID shows the tunability of droplet diameter by varying certain parameters;

FIGs. 2A-2B show structural coloration according to two embodiments ; FIGs . 3A-3B show SEM and optical analysis of centri fugally assembled photonic crystals , respectively;

FIGs . 4A-4B show the embedding of the crystals in flexible gels , and their response to stress ; and

FIG . 5 shows a flowchart illustrating the procedure to create the photonic crystal .

Detailed Description

This disclosure describes a novel method for synthesi zing colloidal nanoparticles with an asymmetric core-shell structure using a seeded growth approach . This method provides an ability to produce colloidal particles that are smaller and more monodisperse than methods documented in existing literature for synthesi zing colloids of this composition . One distinctive feature of the synthesi zed material is its exhibition of highly uni form and angle-independent structural color when the particles are sedimented via centri fugation . This unique optical property is distinct from similar materials that exhibit structural coloration that is viewing angle-dependent .

This method allows the creation of a new set of low-dispersity colloidal building blocks in the appropriate si ze regime for photonics technologies . Additionally, assemblies of these building blocks represent a class of structurally colored materials with potential applications in pigment and display technologies , sensors , and security features that require consistent color appearance from all viewing angles . FIG. 5 shows the method of synthesizing these nanocrystals according to one embodiment. FIG. 1A shows some of the major steps in this method. First, seed nanoparticles are provided (see Box 500 in FIG. 5) . These seed nanoparticles may be gold nanospheres. In other embodiments, the seed nanoparticles may be other metallic, inorganic, or organic nanoparticles, including silver nanoparticles (AgNPs) , silicon-based nanoparticles (SiNPs) , platinum nanoparticles (PtNPs) , palladium nanoparticles (PdNPs) , silica nanoparticles (SiO₂) , and polystyrene nanoparticles (PS) . These seed nanoparticles may be coated using citrate or other surfactants such as PVP (polyvinylpyrrolidone) , CTAB ( cetyltrimethylammonium bromide) , or PEG (polyethylene glycol) and other surfactants and small molecules which provide stability to the nanoparticles or modulate wetting onto the seed particles. The seed nanoparticles may have a diameter between 5 nm and 200 nm, in some embodiments.

Next, a silane may be hydrolyzed (see Box 510 of FIG. 5) . The top panel of FIG. 1A shows the chemical representation of the silane monomer 3- ( trimethoxysilyl ) propyl methacrylate (TPM) . While TPM is described in this disclosure, in other embodiments, other silanes, such as triethoxysilyl or trimethoxysilyl derivatives may be used. The TPM may be hydrolyzed with water to form hydrolyzed TPM (H-TPM) or a hydrolyzed silane. In other embodiments, the silane is not hydrolyzed.

TPM or H-TPM (or other silane) may then be mixed with the seed nanoparticles in a basic solution, containing bases such as NHj, NaOH, KOH, and other bases (see Box 520) to produce siloxane- coated particles. This is also shown in the middle panel of FIG. 1A. In some embodiments, the concentration of the base is adjusted to control coating thickness and stability . The liquid siloxane particles are driven to a highly spherical shape due to surface tension . As shown in the bottom panel , in some embodiments , liquid droplets are solidified by hardening the liquid through a secondary polymeri zation using a radical initiator ( see Box 530 in FIG . 5 ) . In one embodiment, the radical initiator may be azobisisobutyronitrile (AIBN) , benzoyl peroxide , potassium persulphate , V-50 , and other water soluble or water insoluble radical initiators . In some embodiments , the liquid droplets are heated or exposed to UV irradiation during this step . In another embodiment, the hardening is achieved using silane crosslinking via base or acid catalysis to induce sel f-hardening . This results in siloxane-coated monodisperse nanoparticles .

As can be seen in FIG . IB, droplets with diameters as small as 20 nm may be formed with one TPM droplet per seed nanoparticle . Note, as shown in FIG . 1C, larger diameter droplets may also be created .

FIG . ID shows a graph that illustrates the relationship between the concentration of seed nanoparticles , the concentration of silane and the diameter of the droplets . This graph was created using gold as the seed nanoparticles and TPM as the silane . Note that, in general , a higher concentration of seed nanoparticles results in smaller diameter droplets , while a higher concentration of the silane results in larger diameter droplets . By varying these two concentrations , droplets with diameters between 20 nm and 400 nm were produced . However, larger diameter droplets may be formed by increasing the concentration of the silane . This graph illustrates that a single growth stage generates droplets with exceptionally low dispersity and tunable diameters based on experimental conditions .

After the siloxane-coated monodisperse nanoparticles are formed, they are separated from the reaction mixture ( see Box 540 in FIG . 5 ) . This may be done using centri fuge, gravity separation, filtration or depletion-based method . The separation method may be selected based on desired particle si ze and dispersion stability .

Then, as shown in Box 550 of FIG . 5 , the separated siloxane- coated monodisperse nanoparticles are washed with water or other solvents that maintain colloidal stability, such as ethanol , methanol , or isopropanol . Alternatively, the solvent may be replaced with a monomer such as (Hydroxyethyl ) methacrylate (HEMA) or other species for embedding, such as crosslinkable polymers , to facilitate further integration into composite materials .

Once the siloxane-coated monodisperse nanoparticles are separated, they may be assembled to produce a photonic crystal , as shown in Box 560 of FIG . 5 . In one embodiment , this assembly is performed by drop casting the siloxane-coated monodisperse nanoparticles on a substrate such as glass , silicon, or polymer films and drying the particles to produce a photonic crystal with variable wavelength coloration . In other embodiments , alternative assembly methods may be utilized . These alternative assembly methods include depletion, centri fugation, or concentrating the particles to drive self-assembly at high densities . Note that i f centri fugation is used, photonic crystals with angle independent coloration are produced . In some embodiments, as shown in Box 570, the crystallized siloxane-coated monodisperse nanoparticles are then embedded within hydrogels or other media such as PDMS (polydimethylsiloxane) , agarose, or PVA (polyvinyl alcohol) to tune optical and mechanical properties. In one embodiment, a tertiary polymerization is carried out to crosslink monomer or polymer in the continuous phase. In some embodiments, this produces photonic crystals embedded in hard plastic or elastic hydrogel.

As shown in Box 580 of FIG. 5, the liquid TPM state may optionally be functionalized by grafting polymer to the surface, specifically pluronics though the adsorption of a hydrophilic core into the emulsion.

Note that the functionalization of the TPM liquid state by polymer grafting may be achieved using polymers of different lengths and functional end groups such as acrylates, methacrylates, siloxanes, polyethylene glycol (PEG) , polystyrene (PS) , or other relevant functionalized polymers, including fluorescent groups (e.g., rhodamine, fluorescein) , crosslinkable groups (e.g., acrylates, vinyl, epoxy) , or reactive groups for further modification (e.g., amine, thiol, carboxyl) .

In summary, disclosed herein is a chemical process that synthesizes nanoparticles with a core-shell structure that are uniquely uniform in size, wherein the diameter is in the range of ~20-3000 nm and may be intentionally controlled, as shown in FIG. ID. The particles are synthesized via a controlled seeded growth method, where a silane, such as 3- ( trimethoxysilyl ) propyl methacrylate (TPM) , forms spherical emulsions around seed particles . The following are some of the structural features of the nanoparticles produced by the disclosed method .

Because the method comprises a seeded growth process , each nanoparticle contains both a single seed nanoparticle (which may be between 5nm and 200 nm; typically ~20 nm) and a spherical siloxane component ( such as TPM) . The seed nanoparticle is located at the surface of the TPM sphere .

Due to the TPM' s liquid state , the resulting nanoparticles are highly spherical , and the use of nanosi zed seed particles as seeds produce uniquely small droplets . For comparison, conventional synthetic methods to make TPM particles typically only produce highly spherical and low si ze dispersity colloids at si ze regimes above ~300nm, or irregularly shaped and poorly si ze- controlled colloids at smaller sizes .

The low dispersity and regularity of the nanoparticle ' s shapes allows for facile formation of colloidal crystals . For example , when these nanoparticles are centri fuged, they settle into an FCC crystal structure . Importantly, when the nanoparticles have a diameter in the range of -20-300 nm, these nanocrystals have inherent structural coloration that would not be observed i f the particles were larger or less regularly shaped .

This structural coloration exhibits highly uni form and distinct coloration, meaning the photonic crystals appear as a single , well-defined color across the entire sample . This is in contrast to prior colloidal crystals that typically showed mixed coloration based on the orientation of dif ferent crystalline grains within the sample. In some embodiments, the photonic crystals reflect light predominantly of a wavelength between 300 to 3000 nm in an angle independent manner, based on nanoparticle spacing in the crystal.

As noted above, these nanocrystals may then be assembled into larger crystals having an FCC crystal structure. Specifically, through sedimentation via centrifugation, a polycrystalline material forms with grain sizes of approximately 20 microns. As shown in FIG. 2A, the crystals display angle independent structural coloration and can be tuned across the visible spectrum by utilizing TPM droplet particles with diameters ranging from 180 nm (Blue) to 200 nm (green) to 210 nm (yellow) to 230 nm (red) . The use of larger TPM droplet particles can expand this wavelength range to larger wavelengths.

After centrifugation, these larger crystals are slowly dried to retain their microscopic order. As seen in FIG. 3A, SEM analysis reveals roughly 20 micron domain sizes responsible for photonic effects in angle independent photonic crystals. Further, FIG. 3B shows that the dried crystals retain coloration (which in this figure was green) , though hues shift due to refractive index differences between the air and water media.

As noted above, the photonic crystals may be formed in other ways. For example, larger crystals formed through slowly evaporating drop-casted particles display angle dependent coloration, as shown in FIG. 2B. This coloration may vary across the visible spectrum based on viewing angle. FIG. 2B shows the particles change from pink (leftmost box) to brown (second box) to yellow-green (third box) and to bluish-purple (rightmost box) , depending on viewing angle.

As shown in FIG. 4A, polymer functionalized TPM can be grafted into a polymer matrix. FIGs. 4B-4C show optical images of photonic crystals embedded within a hydrogel. FIG. 4C shows a green coloration. FIG. 4D shows the reaction of the photonic crystals to stress. A slice of hydrogel is subjected to pressure by compression between glass slides. The largely independent red color (leftmost box) shifts to green (second box) , then to blue (third box) as the lattice is compressed. Release of the pressure returns the gel to its original color (rightmost box) .

Note that the solidification step (Box 530) is described as optional. Note that if this step is omitted, the separation (Box 540) , the washing (Box 550) , the assembly (Box 560) and optional embedding (Box 560) may still proceed as described above using the liquid siloxane-coated seed nanoparticles. However, since the nanoparticles are liquid, the separating may not include a drying process .

The following describes one embodiment of the material and method used for creating the nanocrystals. Note that different seed nanoparticles may be used, and this is one example.

Materials :

3- (Trimethoxysilyl ) propyl methacrylate (TPM, 98%) ; ammonium hydroxide (25% NH₃ in water) ; and azobis ( isobutyronitrile ) (AIBN, >98%) Citrate capped gold nanoparticles (~20 nm) (AuNP) were synthesized following the Turkevich method. All solutions were prepared in a medium of deionized water using Millipore Filtration. (Hydroxyethyl ) methacrylate (HEMA, 99%) is filtered through an alumina column prior to use.

Methods :

Preparation of Seed-Nucleated Samples :

Emulsification and Polymerization of TPM followed a modified protocol described by Neilbloom et. al. (Neibloom, D., Bevan, M. A., & Frechette, J. (2021) . Droplet Formation and Growth Mechanisms in Reaction-Induced Spontaneous Emulsification of 3- ( Trimethoxysilyl ) Propyl Methacrylate. Langmuir: the ACS Journal of Surfaces and Colloids, 37 (39) , 11625-11636. which is incorporated herein by reference.

First, 1 mL TPM was added to 100 mL of water in a 250 mL Erlenmeyer flask and left to stir for 1 hour at 250 rpm to hydrolyze. Second, 20 ml of 1 nM AuNP was added to 25 ml beaker, stirring at 300 rpm. Third, 100 pL of 5% v/v NH₃ was added to the beaker, followed immediately by 1 ml of hydrolyzed TPM solution. Fourth, beakers were left to stir for 1.5 hours to grow TPM onto AuNP. Fifth, 0.02 grams AIBN radical initiator was added to the beaker, and the solution was transferred to a 30 ml vial and left overnight at 75°C to polymerize. Finally, for comparison purposes, steps 2-4 were repeated with alternate concentrations of AuNP and H-TPM. Particle Washing and Substrate Composition :

First, solutions were separated into two vials of equal weight . Second, centrifugation took place for 20 minutes at 3000 ref . Third, supernatant was pipetted into liquid waste, and vials were re- filled to 12 . 5mL level with fresh deioni zed water . Steps 2-3 were repeated 3 times for all samples to ensure complete wash of particles . Finally, the washed samples were drop cashed onto the substrate and allowed to dry for 1 hour prior to inspection .

Imaging :

H-TPM crystals were imaged by an optical imaging microscope .

Dried samples were imaged with a scanning electron microscope ( SEM) . Results were analyzed using ImageJ software to determine color intensity and size .

This method has many advantages . This disclosed method solves the problem of prior works ' inability to synthesize TPM particles in the size regime relevant for photonics applications . While other colloidal particles can be synthesized in this si ze range , the composition of TPM makes it useful and versatile for many technologies or research studies that cannot be conducted with other comparable particle syntheses ( e . g . , polymer or silica colloids ) . The assemblies of these structures exhibit monolithic, angle-dependent color variation, which is distinct from prior colloidal crystals that produce structural color that changes depending on the angle between the light source and the viewer . This angle-insensitive coloration is necessary for any photonic crystal application where light sources or viewing angles are not fixed ( e . g . , displays and sensors ) The method disclosed herein also provides an ability to finely tune particle size in the 20-400nm size range , the achievement of nanoscopic and monodisperse emulsions , and the production of angle-independent structural color . Benefits include improved guality and performance in color-based applications , reduced variability in manufacturing, and potential for new application development due to the unique optical properties .

Further, unlike other colloidal nanoparticle fabrication methods using TPM, this process permits access to the highly desirable 20-400nm si ze regime for low dispersity spherical particles used in the production of photonic crystals ; it also can be used to produce TPM colloids over a much wider size range (up to 3000 nm in diameter ) . Additionally, this method makes anisotropic structures that are not solely comprised of TPM, but also contain a gold nanoparticle seed . Finally, the assembly of these colloids produces high quality crystals that have angle independent structural color, a necessity for the use of colloidal photonic crystals .

The present disclosure is not to be limited in scope by the specific embodiments described herein . Indeed, other various embodiments of and modi fications to the present disclosure , in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings . Thus , such other embodiments and modifications are intended to fall within the scope of the present disclosure . Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose , those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes . Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein .

Claims

What is claimed is:

1. A method of making a photonic crystal, the method comprising : a. providing seed nanoparticles; b. mixing the seed nanoparticles with a basic solution and a silane to produce siloxane-coated seed nanoparticles ; c. solidifying the siloxane-coated seed nanoparticles to form monodisperse nanoparticles comprising polymerized siloxane-coated seed nanoparticles; d. optionally repeating b and c with different concentrations of the seed nanoparticles and the silane; e. separating the monodisperse nanoparticles from the basic solution; f. washing the separated monodisperse nanoparticles; and g. producing a photonic crystal from the separated monodisperse nanoparticles.

2. The method of claim 1, further comprising hydrolyzing the silane in an aqueous solution to produce a hydrolyzed silane (siloxane) prior to mixing with the seed nanoparticles .

3. The method of claim 1, wherein the seed nanoparticles are citrate capped gold nanoparticles (AuNPs) .

4. The method of claim 1, wherein the seed nanoparticles are metallic, inorganic, or organic nanoparticles, including silver nanoparticles (AgNPs) , silicon-based nanoparticles (SiNPs) , platinum nanoparticles (PtNPs) , palladium nanoparticles (PdNPs) , silica nanoparticles (SiO₂) , and polystyrene nanoparticles (PS) .

5. The method of claim 1, wherein the seed nanoparticles are coated using citrate, PVP (polyvinylpyrrolidone) , CTAB

( cetyltrimethylammonium bromide) , or PEG (polyethylene glycol .

6. The method of claim 1, wherein the seed nanoparticles have a diameter between 5nm and 200 nm.

7. The method of claim 1, wherein the silane comprises 3- ( trimethoxysilyl ) propyl methacrylate (TPM) .

8. The method of claim 1, wherein the silane comprises triethoxysilyl or trimethoxysilyl derivatives.

9. The method of claim 1, wherein solidifying the siloxane- coated seed nanoparticles is performed by adding a radical initiator to the siloxane-coated seed nanoparticles.

10. The method of claim 9, wherein the radical initiator comprises azobisisobutyronitrile (AIBN) .

11. The method of claim 9, wherein the radical initiator comprises benzoyl peroxide, potassium persulphate, V-50, and other water soluble or water insoluble radical initiators .

12. The method of claim 1, wherein solidifying the siloxane-coated seed nanoparticles is achieved using silane crosslinking via base or acid catalysis to induce selfhardening .

13. The method of claim 1, wherein the siloxane-coated seed nanoparticles are heated or exposed to UV irradiation during the solidifying.

14. The method of claim 1, wherein the separating is performed using centrifugation.

15. The method of claim 1, wherein the separating is performed using gravity separation, filtration, or depletion-based methods.

16. The method of claim 1, wherein a method of separating is based on a desired monodisperse nanoparticles size and dispersion stability.

17. The method of claim 1, wherein the separated monodisperse nanoparticles are washed with water.

18. The method of claim 1, wherein the separated monodisperse nanoparticles are washed with ethanol, methanol or isopropanol.

19. The method of claim 1, wherein the photonic crystal is produced by drop casting the separated monodisperse nanoparticles on a substrate and drying the separated monodisperse nanoparticles.

20. The method claim 19, wherein the photonic crystal has variable wavelength coloration.

21. The method of claim 1, wherein the photonic crystal is produced by centrifugation.

22. The method of claim 1, wherein the seed nanoparticles are gold nanoparticles (AuNP) , the silane is 3-

( trimethoxysilyl ) propyl methacrylate (TPM) and the basic solution comprises NH3, wherein 100 pL of 5% v/v NH₃, 20 ml 1 nM AuNP, and hydrolyzed TPM are mixed to produce TPM- coated AuNP.

23. The method of claim 1, wherein a ratio of a concentration of seed nanoparticles and silane is varied.

24. The method of claim 1, wherein the monodisperse nanoparticles have a selected size.

25. The method of claim 1, wherein the photonic crystal is embedded in a hydrogel.

26. The method of claim 1, wherein the siloxane is functionalized by grafting polymer to a surface.

27. The method of claim 26, wherein the polymer comprises acrylates, methacrylates, siloxanes, polyethylene glycol (PEG) , polystyrene (PS) , or other relevant functionalized polymers, including fluorescent groups (e.g., rhodamine, fluorescein) , crosslinkable groups (e.g., acrylates, vinyl, epoxy) , or reactive groups for further modification (e.g., amine, thiol, carboxyl) .

28. The photonic crystal produced by the method of any one of claims 1-27.

29. The photonic crystal of claim 28, wherein the photonic crystal reflects light of a wavelength between 300 to 3000 nm in an angle independent manner.

30. The photonic crystal of claim 29, wherein the wavelength is determined based on particle size.

31. The photonic crystal of claim 28, wherein the photonic crystal reflects a narrow bandwidth of light in the visible spectrum, such that the photonic crystal appears as a single, well-defined color across the entire photonic crystal .

32. The photonic crystal of claim 31, wherein the photonic crystal reflects yellow, red, green, or green light.

33. A method of making a photonic crystal, the method comprising : a. providing seed nanoparticles; b. mixing the seed nanoparticles with a basic solution and a silane to produce siloxane-coated seed nanoparticles ; c. optionally repeating b and c with different concentrations of the seed nanoparticles and the silane; d. separating the siloxane-coated seed nanoparticles from the basic solution; e. washing the separated siloxane-coated seed nanoparticles; and f. producing a photonic crystal from the separated siloxane-coated seed nanoparticles.

34. The method of claim 33, further comprising hydrolyzing the silane in an aqueous solution to produce a hydrolyzed silane (siloxane) prior to mixing with the seed nanoparticles .

35. The method of claim 33, wherein the seed nanoparticles are citrate capped gold nanoparticles (AuNPs) .

36. The method of claim 33, wherein the seed nanoparticles are metallic, inorganic, or organic nanoparticles, including silver nanoparticles (AgNPs) , silicon-based nanoparticles (SiNPs) , platinum nanoparticles (PtNPs) , palladium nanoparticles (PdNPs) , silica nanoparticles (SiO₂) , and polystyrene nanoparticles (PS) .

37. The method of claim 33, wherein the seed nanoparticles are coated using citrate, PVP (polyvinylpyrrolidone) , CTAB

( cetyltrimethylammonium bromide) , or PEG (polyethylene glycol .

38. The method of claim 33, wherein the seed nanoparticles have a diameter between 5nm and 200 nm.

39. The method of claim 33, wherein the silane comprises 3- ( trimethoxysilyl ) propyl methacrylate (TPM) .

40. The method of claim 33, wherein the silane comprises triethoxysilyl or trimethoxysilyl derivatives.

41. The method of claim 33, wherein the separating is performed using centrifugation.

42. The method of claim 33, wherein the separating is performed using gravity separation, filtration, or depletion-based methods.

43. The method of claim 33, wherein a method of separating is based on a desired monodisperse nanoparticles size and dispersion stability.

44. The method of claim 33, wherein the separated siloxane-coated seed nanoparticles are washed with water.

45. The method of claim 33, wherein the separated siloxane-coated seed nanoparticles are washed with ethanol, methanol or isopropanol.

46. The method of claim 33, wherein the photonic crystal is produced by centrifugation.

47. The method of claim 33, wherein a ratio of a concentration of seed nanoparticles and silane is varied.

48. The method of claim 33, wherein the siloxane-coated seed nanoparticles have a selected size.

49. The method of claim 33, wherein the photonic crystal is embedded in a hydrogel.

50. The method of claim 33, wherein the siloxane is functionalized by grafting polymer to a surface.

51. The method of claim 50, wherein the polymer comprises acrylates, methacrylates, siloxanes, polyethylene glycol (PEG) , polystyrene (PS) , or other relevant functionalized polymers, including fluorescent groups (e.g., rhodamine, fluorescein) , crosslinkable groups (e.g. , acrylates, vinyl, epoxy) , or reactive groups for further modification (e.g. , amine, thiol, carboxyl) .

52. The photonic crystal produced by the method of any one of claims 33-51.

53. The photonic crystal of claim 52, wherein the photonic crystal reflects light of a wavelength between 300 to 3000 nm in an angle independent manner.

54. The photonic crystal of claim 53, wherein the wavelength is determined based on particle size.

55. The photonic crystal of claim 52, wherein the photonic crystal reflects a narrow bandwidth of light in the visible spectrum, such that the photonic crystal appears as a single, well-defined color across the entire photonic crystal .

56. The photonic crystal of claim 55, wherein the photonic crystal reflects yellow, red, green, or green light.