Article pubs.acs.org/Langmuir
Chiral Nematic Stained Glass: Controlling the Optical Properties of Nanocrystalline Cellulose-Templated Materials Joel A. Kelly,† Kevin E. Shopsowitz,† Jun Myun Ahn,† Wadood Y. Hamad,‡ and Mark J. MacLachlan*,† †
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 CelluForce Inc., 3800 Wesbrook Mall, Vancouver, British Columbia, Canada V6S 2L9
‡
S Supporting Information *
ABSTRACT: Chiral nematic mesoporous materials decorated with metal nanoparticles have been prepared using the templated self-assembly of nanocrystalline cellulose (NCC). By adding small quantities of ionic compounds to aqueous dispersions of NCC and tetramethoxysilane (TMOS), the helical pitch of the chiral nematic structure could be manipulated in a manner complementary to the ratio of NCC/TMOS previously demonstrated by our group. We have studied the transformation of these ion-loaded composites into high surface area mesoporous silica and carbon films decorated with metal nanoparticles through calcination and carbonization, respectively. This general and straightforward approach to prepare chiral nematic metal nanoparticle assemblies may be useful in a variety of applications, particularly for their chiral optical properties.
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INTRODUCTION The self-assembly of rodlike mesogens into chiral nematic phases has been the subject of intense research since the discovery of brilliant iridescence from liquid crystalline cholesterol derivatives.1 Selective reflection of light due to the periodic helical structure in these phases has been exploited in a wide variety of applications, such as thermometers, polarizing mirrors, low-threshold lasers, and display technologies.2 The self-assembly process for rod-shaped polyelectrolytes into chiral nematic phases is frequently described using the theory of Stroobants, Lekkerkerker, and Odijk (SLO) to account for the interplay between entropic considerations (mainly related to the mesogen’s aspect ratio) and electrostatic interactions.3 Interestingly, it has been shown that ionic strength can be used to modulate the self-assembly process for some mesogens through two factors: (1) adding excess ions decreases the effective mesogen diameter, thereby reducing the favorable excluded volume entropy associated with forming an anisotropic phase and increasing the critical concentration required for the chiral nematic phase to form, and (2) increased ionic strength causes a decrease in the helical pitch of the chiral nematic phase, implying an increase in chiral interactions due to a decrease in double-layer thickness.4 Nanocrystalline cellulose (NCC), derived from strong acid hydrolysis of cellulosic biomass, exhibits lyotropic chiral nematic behavior.5−8 Its rodlike morphology (with typical average dimensions of 5−10 nm by 100−250 nm depending on the cellulose source), negatively charged surface (due to sulfate ester groups when sulfuric acid is used to prepare NCC), and inherent chirality associated with its D-glucose building blocks © 2012 American Chemical Society
can give rise to left-handed chiral nematic phases at high concentrations and in dried films. These films with chiral nematic organization are capable of reflecting left-handed circularly polarized light across the near-IR and visible spectrum. There is widespread interest in NCC for developing new materials, such as superhydrophobic membranes, aerogels, or catalytic supports.8−12 Gray and co-workers have demonstrated that adding small quantities of monovalent ionic compounds to NCC suspensions can be used to decrease the chiral nematic helical pitch, and thus blue-shift the reflected photonic color, consistent with SLO theory.4,13−15 Recently, our group has shown that chiral nematic NCC can be used as a template to prepare a variety of new sol−gel derived materials with chiral nematic structure and optical properties.16−20 Removal of the NCC yields mesoporous freestanding films that retain the chiral nematic structure and optical properties of the NCC template. The loading of these sol−gel precursors inside the NCC chiral nematic composite is a convenient way to tailor the photonic properties. For instance, increasing the proportion of silica (up to a remarkable 60 wt % SiO2) tends to increase the helical pitch in the chiral nematic structure and thus produce a red-shift in the reflected wavelength. However, varying the proportion of inorganic material in the composite also affects the resulting materials’ surface area, thickness, and mechanical properties. Received: October 22, 2012 Revised: November 26, 2012 Published: November 27, 2012 17256
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Figure 1. (a) Specular reflectance from HAuCl4-loaded NCC/SiO2 composite films. (b) Maximum reflected wavelength as a function of ion concentration. (c) Polarizing optical micrographs (10× magnification) of HAuCl4-loaded NCC/TMOS dispersions showing a characteristic fingerprint texture for 0−5 mM loading. (d) Photograph of HAuCl4-loaded NCC/SiO2 composite films. (e) Gelation of NCC/TMOS dispersions at a high ionic strength.
with chiral nematic order, we demonstrate that the induced circular dichroism (CD) signal arising from chiral plasmonic nanoparticle-decorated films is sensitive to the local changes in the chemical environment, which could find use in CD-based sensing.
In many applications that might exploit these high surface area materials, the porous structure acts as a support upon which an active species is installed (e.g., catalyst, fluorophore, or sensing group).21 A convenient, one-pot method to accomplish this for other mesoporous silica systems is to load a suitable precursor directly to a lyotropic mixture of surfactant and sol−gel precursors.22−25 Formation of the mesoporous silica support and decoration with size-controlled nanoparticles is carried out by evaporation-induced self-assembly and subsequent calcination, reduction, and/or other chemical transformation. Here, we investigate the influence of ionic strength on the self-assembly of NCC/silica sol−gel films and the transformation of these ion-loaded composites into chiral nematic mesoporous materials. Tailoring the color reflected by the chiral nematic material by adjusting the ionic strength is complementary to the control achieved by varying the silica loading. Depending on the ionic compound used to influence self-assembly, thermal processing of the mesoporous silica or carbon can be exploited to prepare metal nanoparticles decorated within the pores of these materials (analogous to the preparation of gold nanoparticles in stained glass by medieval artisans26). As an application utilizing this general and convenient method to prepare metal nanoparticle assemblies
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EXPERIMENTAL SECTION
Materials and Characterization. Reagents, including metal salts (chloroauric acid trihydrate, silver nitrate, potassium tetrachloroplatinate, chloroplatinic acid hexahydrate, potassium ferricyanide, iron sulfate heptahydrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, and zinc nitrate hexahydrate), were purchased from Aldrich [with the exception of tetramethylorthosilicate (TMOS), Acros, 99% and 49% hydrofluoric acid (HF), J.T. Baker, 99.99%] as reagent purity or higher and used as received. NCC was obtained from fully bleached, commercial kraft softwood pulp and prepared as previously described.17 The final concentration of NCC in the aqueous suspension was 3 wt % (pH 2.4). Ultraviolet−visible/near-infrared spectroscopy was carried out on a Cary 5000 UV−vis/NIR spectrophotometer. Reflectance spectroscopy was conducted with an Ocean Optics spectrometer equipped with a UV−vis reflectance probe. Polarized optical microscopy was performed on an Olympus BX41 microscope with perpendicular polarizers. Circular dichroism spectra were collected using a JASCO J710 spectropolarimeter. Small pieces of films were mounted between microscope slides perpendicular to the beam path. Nitrogen 17257
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Figure 2. (a) UV−vis spectra of a 1 mM HAuCl4-loaded film before and after calcination at 540 °C showing the characteristic plasmonic absorption and blue-shift in the chiral nematic photonic band. (b) Photograph of pink Au- and yellow Ag-decorated films. (c) N2 isotherm and BJH pore-size distribution (inset) for mesoporous silica prepared with and without 10 mM HAuCl4 loading. (d) SEM of 1 mM HAuCl4-loaded silica decorated with Au nanoparticles after calcination. (e) TEM of 1 mM HAuCl4-loaded silica indicates the Au nanoparticles are distributed throughout the structure. (f) HRTEM of a well-resolved Au nanoparticle shows a lattice spacing of 2.3 Å, assignable to the (111) plane of fcc Au.
of the resulting composite film to blue-shift from ∼975 to 450 nm (Figure 1, panels a and b). The induced blue-shift is observed for a variety of ionic compounds, including mono-, di-, and trivalent species. The magnitude of the blue-shift in reflectance as a function of ion concentration scales with the change in ionic strength; 1 mM of HAuCl4 gives a reflected wavelength of ca. 700 nm, whereas the same concentration of H2PtCl6 gives a reflected wavelength of ca. 600 nm. At low ionic strength, polarized optical microscopy (POM) of the NCC/TMOS dispersion shows a characteristic chiral nematic fingerprint texture, which is lost as the ionic strength increases (e.g., 1 mM vs 7.5 mM HAuCl4, Figure 1c). At high ionic strength [e.g., 10 mM Ni(NO3)2], the NCC/TMOS mixtures tend to gel (Figure 1e) and are transparent upon drying. Scanning electron microscopy (SEM) of films prepared from gelled dispersions [e.g., with 2.5 mM Fe(NO3)3, Figure S1 of the Supporting Information] confirms the loss of chiral ordering and shows a disordered layered structure. It is noteworthy that the reflectance peak of the chiral nematic composite can still be red-shifted by increasing the proportion of silica in the material, as we previously found with the pure silica materials. At 5 mM HAuCl4 loading, varying the TMOS loading by 25% (from 28 up to 40 wt % SiO2 assuming full condensation of TMOS) causes the reflected wavelength to red-shift from ca. 425 to 520 nm (Figure 1b, right side). Thus, varying the ionic strength and the proportion of silica allows complementary control of the reflected wavelength. Previously, Gray and co-workers have shown that increasing the ionic strength of pure NCC dispersions using monovalent ionic compounds causes an increase in the critical concentration required for the formation of the chiral nematic phase.4 As well, a similar blue-shift in the reflected color in the resulting dried film was observed due to a decrease in the chiral nematic helical pitch.14 They observed the critical concentration increases with the atomic number of the monovalent cation added, consistent with a decrease in the effective NCC hydrodynamic radius due to decreasing hydration of surfacebound ions. We observed a similar general trend in our NCC/
adsorption data were obtained at 77 K using a Micromeritics ASAP 2000 analyzer; degassed sample weights of ca. 100 mg were used for all measurements. The pore-size distributions were derived from the adsorption branches using the Barrett−Joyner−Halenda (BJH) method. SEM experiments were conducted on a Hitachi S4700 electron microscope on samples sputter-coated with 5 nm of gold. TEM micrographs were obtained with a FEI Technai at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were obtained from finely ground samples using a Bruker D8 Advance diffractometer equipped with a Cu Kα X-ray source. Preparation of Metal-Loaded Chiral Nematic Films. Chiral nematic NCC/silica composites were prepared in a similar method as previously described17 but with the addition of an ionic metal complex. In a typical procedure, 20 mL of a 3 wt % NCC aqueous dispersion was sonicated for 15 min, mixed with TMOS (0.8 mL, 5.4 mmol), and stirred for 1 h. An aliquot of 0.25 M aqueous metal salt solution [e.g., HAuCl4, K2PtCl4, K3Fe(CN)6] was dispensed using a micropipette, and the reaction mixture was stirred for a few minutes until a homogeneous dispersion was obtained. Portions of this solution were transferred to polystyrene Petri dishes and left to evaporate to dryness under ambient conditions (ca. 24 h). HAuCl4-loaded films were dried in the dark, as HAuCl4 is known to be sensitive to ambient light. Removal of the cellulose by calcination was carried out under flowing air by heating the composite films to 540 °C at 120 °C h−1 and holding for 6 h; a typical yield of 35% silica was obtained. Au nanoparticles could be isolated by etching 0.1 g Au-loaded calcined films in 10 mL of an ethanolic 2% HF/1% dodecanethiol solution for 1 h and purified by three cycles of centrifugation and washing with ethanol. Mesoporous carbon films were synthesized by pyrolyzing the composite films under flowing nitrogen, heating to 900 °C at 120 °C h−1, and holding for 6 h, with a typical yield of 45%. The silica from carbonized films was removed by etching with 2 M sodium hydroxide at 90 °C for 4 h and rinsing the films with copious amounts of DI water, with a typical yield of 31%.
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RESULTS AND DISCUSSION
Figure 1 shows the effect of varying ionic strength on the chiral nematic self-assembly of NCC/silica composites. For example, the addition of 1−5 mM of HAuCl4 to the NCC/TMOS dispersion before evaporation causes the reflected wavelength 17258
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Figure 3. Normalized CD (left) and UV−vis (right) spectra of metal nanoparticle-decorated chiral nematic silica films: (a) Ag nanoparticles prepared by varying loadings of AgNO3 (soaked in ethanol). (b) Ag-decorated films soaked in various solvents (1 mM AgNO3 loading). (c) Audecorated films soaked in various solvents (1 mM HAuCl4 loading). (d) Ligand-capped Au-decorated films soaked in ethanol (1 mM HAuCl4 loading).
nanoparticles. Assuming complete decomposition of HAuCl4 into Au nanoparticles, 1 to 5 mM loadings in the composites yields 1.2 to 5.7 wt % Au in the mesoporous silica films. A 1 mM HAuCl4-loaded sample shows a blue-shift in the photonic reflection from ca. 750 nm to ca. 400 nm alongside the formation of a strong plasmonic resonance at ca. 520 nm, which is characteristic of nanoparticulate gold (Figure 2a). The blueshift of the reflection is attributed to both a decrease in the average refractive index of the material when cellulose is removed from the silica and contraction of the helical pitch that occurs with further condensation of the silica. Likewise, calcination of a 1 mM H2PtCl6-loaded sample gives a black film decorated with ca. 7 nm Pt nanoparticles [estimated by Xray diffraction (XRD), Figure S2 of the Supporting Information]. Other silica-supported metal nanoparticles can be prepared by immersing the calcined films in a concentrated solution of NaBH4. For example, reducing a 1 mM AgNO3-
SiO2 composites; for example, the reflected wavelength of K2PtCl4-loaded films is blue-shifted in comparison to the H2PtCl6-loaded films. Furthermore, the induced blue-shift appears more sensitive to the atomic number of the cation than that of the anion [e.g., a larger blue-shift is noted for the addition of Fe(NO3)3 over K3Fe(CN)6], likely due to the influence of negatively charged sulfate NCC surface groups. Thus, for the purpose of preparing chiral nematic materials decorated with metal nanoparticles, the highest loading of suitable ionic nanoparticle precursors while retaining the chiral nematic structure is accomplished using the acid salts of chlorometallate anions (i.e., HAuCl4, H2PtCl6, etc.). Calcination of HAuCl4-loaded composites decomposes the NCC, leaving mesoporous silica decorated with Au nanoparticles (Figure 2). UV−vis spectroscopy verified that the chiral nematic organization of the template is retained in the mesoporous silica and that the gold was converted into metal 17259
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loaded calcined sample yields a yellow film with a plasmonic resonance centered at ca. 420 nm that is characteristic of Ag nanoparticles (Figure 2b, vide infra). The effect of increasing ionic strength on the surface area of these mesoporous materials was investigated by nitrogen adsorption−desorption (Figure 2c and Table S1 of the Supporting Information). The HAuCl4-loaded films retain a type IV isotherm with significant hysteresis, indicative of mesoporosity. Loading the composite with 10 mM HAuCl4 resulted in a decrease in the BET surface area from 769 m2/g to 654 m2/g and a slight broadening of the BJH pore size distribution. Typical pore volumes were 0.52−0.66 cm3/g. The change in mesoporosity could be due to a combination of Au nanoparticles blocking the pores, a decrease in the spacing between neighboring NCC rods due to the increase in ionic strength, or simply the increased density of the gold/silica relative to pure silica. SEM images of 1 mM HAuCl4-loaded calcined films show that the twisting, layered structure of the chiral nematic silica is preserved and is decorated with Au nanoparticles throughout its entirety (Figure 2d). Although mostly large Au particles (ca. 70−100 nm) are observed at the surface of the film, transmission electron microscopy (TEM) of Au nanoparticles isolated after the removal of the silica by HF etching suggests a log-normal distribution with a mean diameter of 28 ± 21 nm (n = 175, Figure S3 of the Supporting Information), comparable with other reported polydispersities for Au nanoparticles prepared in mesoporous silica via a direct method.25 TEM analyses of HAuCl4-loaded calcined films show a mixture of faceted and pseudospherical Au particles embedded in the mesoporous silica (Figure 2e). High-resolution TEM (HRTEM) of a well-resolved 27 nm Au nanoparticle shows a lattice spacing of 2.3 Å, consistent with the (111) plane of crystalline Au (Figure 2f). XRD of Au-loaded films (Figure S4 of the Supporting Information) confirms the presence of fcc Au and shows an increase in the intensity of Au reflections with higher HAuCl4 loading relative to the broad amorphous silica feature at ca. 20° 2θ. The breadth of the Au peaks did not narrow significantly with increasing HAuCl4 loading, with Scherrer analysis of the (111) reflection giving an average grain size of 35 nm (within the range suggested by TEM). By pyrolyzing these ion-loaded NCC/SiO2 composites under nitrogen and subsequent removal of silica, these materials can also be conveniently transformed into mesoporous carbon decorated with metal nanoparticles (Figure S5 of the Supporting Information). For example, pyrolysis of a 1 mM HAuCl4-loaded composite at 900 °C yields mesoporous carbon with a high surface area (BET surface area of 1612 m2/g, total pore volume of 1.39 cm3/g) decorated with ca. 38 nm Au nanoparticles (estimated by XRD). Similarly, pyrolysis of a 1 mM H2PtCl6 composite at 900 °C under an atmosphere of 10% H2/90% N2 (used to reduce Pt ions) gave a BET surface area of 1382 m2/g, a total pore volume of 1.24 cm3/g, and a Pt nanoparticle diameter of ca. 6 nm. As a potential application utilizing this general approach to load metal nanoparticles inside chiral nematic materials, we investigated the induced circular dichroism (CD) signals arising from Ag and Au nanoparticles inside chiral nematic silica (Figure 3). Previously, we have shown that Ag nanoparticles prepared by a two-step reduction process inside chiral nematic silica display strong bisignated CD signals due to plasmon− plasmon Coulombic interactions between neighboring nanoparticles inside of the chiral network.20 In comparison to other
reports of induced CD signals from chiral ligand-stabilized nanoparticles,27−29 this phenomenon arises purely through the chiral organization of the nanoparticles.30−34 The induced CD signal changed significantly upon soaking in water, suggesting these materials might hold advantages over conventional plasmon absorption-based sensors. For the materials described in this paper, the ability to control the chiral nematic pitch and nanoparticle loading in one step allows systematic study of the sensitivity of the induced CD signals to precursor loading, solvent dielectric constant, and the presence of capping ligands (Figure 3). For example, increasing the loading of AgNO3 from 0.5 to 1.5 mM results in a broadening in the plasmonic absorption of the resulting Ag nanoparticle-decorated films, likely due to an increase in the nanoparticle size and polydispersity and a decrease in interparticle spacing.35 In comparison, the CD spectra of these samples soaked in ethanol show a strong feature centered at ca. 510 nm, displaying a negative Cotton effect that changes in intensity with increasing AgNO3-loading (Figure 3a). At higher loading, another feature with a negative Cotton effect appears at ca. 340 nm. Likewise, although soaking the pores of a 1 mM AgNO3-loaded film with solvents of increasing dielectric constants produces a slight red-shift in the absorption spectrum, a significant change is noted between films soaked in water versus ethanol and DMSO (Figure 3b). It is important to note that the Ag nanoparticle plasmon resonance in these samples overlaps with the chiral nematic reflection in these materials (which gives strong positive CD signals and is also sensitive to changes in refractive index, see Figure S6 of the Supporting Information for CD spectra of dry films), which likely influences the change in CD spectra in addition to chiral interactions between neighboring nanoparticles. In comparison, CD spectra of Au nanoparticles prepared from 1 mM HAuCl4-loaded composites show a positive Cotton effect in the plasmon band centered at ca. 550 nm, separated from a positive feature at ca. 450 nm due to the chiral nematic reflection. Fan and Govorov have calculated that the sign of the Cotton effect in chiral plasmonic nanoparticle assemblies can flip between positive and negative depending on the geometry of the nanoparticle complex.30 When the Au decorated films are immersed in solvents, the absorption spectra of the films show a slight red-shift with increasing refractive index. Upon soaking in DMSO, the chiral nematic CD band disappears completely (due to the close match in refractive index between silica and DMSO17), while the lobes of the plasmon band shift and change in relative intensity. As an aside, we did not observe any appreciable CD signal from the Au nanoparticles etched from the silica using HF, confirming that the CD signals arise purely through organization inside the chiral nematic structure. A common sensing motif using plasmonic nanoparticles is to use a binding event at the nanoparticle surface to induce a change in the plasmon absorption band (e.g., through aggregation). This colorimetric response has proven very sensitive for a range of analytes, such as DNA, proteins, and heavy metals.36,37 To explore the effect of surface-bound ligands on the induced CD signals from these chiral assemblies of metal nanoparticles, we soaked Au-loaded films in dodecanethiol and cetyltrimethylammonium bromide (CTAB), two species with high affinities for the nanoparticle surface. Absorption spectra showed that binding of CTAB induced a blue-shift in the plasmon band, whereas dodecanethiol induced a red-shift. Conversely, CD spectra of the CTAB-treated sample showed a red-shift of ca. 25 nm in the 17260
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(5) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose nanocrystals: Chemistry, self-assembly, and applications. Chem. Rev. 2010, 110, 3479−3500. (6) Peng, B. L.; Dhar, N.; Liu, H. L.; Tam, K. C. Chemistry and applications of nanocrystalline cellulose and its derivatives: A nanotechnology perspective. Can. J. Chem. Eng. 2011, 89, 1191−1206. (7) Hamad, W. Y., In Sustainable Production of Fuels, Chemicals and Fibers from Forest Biomass. Zhu, J. Y., Zhang, X., Pan, X. J., Eds.; Oxford University Press: Oxford, 2011; pp 301−321. (8) Klemm, D.; Kramer, F.; Moritz, S.; Lindström, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: A new family of nature-based materials. Angew. Chem., Int. Ed. 2011, 50, 5438−5466. (9) Korhonen, J. T.; Kettunen, M.; Ras, R. H. A.; Ikkala, O. Hydrophobic nanocellulose aerogels as floating, sustainable, reusable, and recyclable oil absorbents. ACS Appl. Mater. Interfaces 2011, 3, 1813−1816. (10) Sehaqui, H.; Zhou, Q.; Ikkala, O.; Berglund, L. A. Strong and tough cellulose nanopaper with high specific surface area and porosity. Biomacromolecules 2011, 12, 3638−3644. (11) Mendez, J.; Annamalai, P. K.; Eichhorn, S. J.; Rusli, R.; Rowan, S. J.; Foster, E. J.; Weder, C. Bioinspired mechanically adaptive polymer nanocomposites with water-activated shape-memory effect. Macromolecules 2011, 44, 6827−6835. (12) Cirtiu, C. M.; Dunlop-Brière, A. F.; Moores, A. Cellulose nanocrystallites as an efficient support for nanoparticles of palladium: Application for catalytic hydrogenation and Heck coupling under mild conditions. Green Chem. 2011, 13, 288−291. (13) Dong, X. M.; Kimura, T.; Revol, J. F.; Gray, D. G. Effects of ionic strength on the isotropic-chiral nematic phase transition of suspensions of cellulose crystallites. Langmuir 1996, 12, 2076−2082. (14) Revol, J.-F.; Godbout, D. L.; Gray, D. G. Solidified Liquid Crystals of Cellulose with Optically Variable Properties. U.S. Patent 5,629,055, 1997. (15) Pan, J.; Hamad, W.; Straus, S. K. Parameters affecting the chiral nematic phase of nanocrystalline cellulose films. Macromolecules 2010, 43, 3851−3858. (16) Shopsowitz, K. E.; Hamad, W. Y.; MacLachlan, M. J. Flexible and iridescent chiral nematic mesoporous organosilica films. J. Am. Chem. Soc. 2012, 134, 867−870. (17) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; MacLachlan, M. J. Free-standing mesoporous silica films with tunable chiral nematic structures. Nature 2010, 468, 422−425. (18) Shopsowitz, K. E.; Stahl, A.; Hamad, W. Y.; MacLachlan, M. J. Hard templating of nanocrystalline titanium dioxide with chiral nematic ordering. Angew. Chem., Int. Ed. 2012, 51, 6886−6890. (19) Shopsowitz, K. E.; Hamad, W. Y.; MacLachlan, M. J. Chiral nematic mesoporous carbon derived from nanocrystalline cellulose. Angew. Chem., Int. Ed. 2011, 50, 10991−10995. (20) Qi, H.; Shopsowitz, K. E.; Hamad, W. Y.; MacLachlan, M. J. Chiral nematic assemblies of silver nanoparticles in mesoporous silica thin films. J. Am. Chem. Soc. 2011, 133, 3728−3731. (21) Ariga, K.; Vinu, A.; Yamauchi, Y.; Ji, Q.; Hill, J. P. Nanoarchitectonics for mesoporous materials. Bull. Chem. Soc. Jpn. 2012, 85, 1−32. (22) Dag, Ö .; Samarskaya, O.; Coombs, N.; Ozin, G. A. The synthesis of mesostructured silica films and monoliths functionalised by noble metal nanoparticles. J. Mater. Chem. 2003, 13, 328−334. (23) Tura, C.; Coombs, N.; Dag, Ö . One-pot synthesis of CdS nanoparticles in the channels of mesostructured silica films and monoliths. Chem. Mater. 2005, 17, 573−579. (24) King, N. C.; Blackley, R. A.; Zhou, W.; Bruce, D. W. The preparation by true liquid crystal templating of mesoporous silicates containing nanoparticulate metals. Chem. Commun. 2006, 3411−3413. (25) Gutiérrez, L.-F.; Hamoudi, S.; Belkacemi, K. Synthesis of gold catalysts supported on mesoporous silica materials: Recent developments. Catalysts 2011, 1, 97−154. (26) Hunt, L. B. The true story of Purple of Cassius. Gold Bull. 1976, 9, 134−139.
negative lobe of the plasmon band originally centered at 524 nm, whereas dodecanethiol showed a red-shift of ca. 15 nm in the positive lobe originally centered at 593 nm. In addition to the shifts in wavelengths observed in the CD spectra of these materials, we also observe a qualitative change in intensity that is challenging to quantify, owing to the fragility of the films. We are actively pursuing a greater understanding of how the induced CD signals from these materials change with variation in the local chemical environment and the utility of this phenomenon in sensing applications. In conclusion, the ionic strength of NCC/TMOS dispersions can be used to control the helical pitch of the resulting selfassembled chiral nematic NCC/silica composites. Through calcination or carbonization, mesoporous silica or carbon films that have chiral nematic organization and are decorated with metal nanoparticles can be prepared in a straightforward manner. The Ag and Au nanoparticles included in the silica show induced CD signals associated with their plasmon resonances, and preliminary data shows that these signals are sensitive to changes in their local environments and may serve as the basis for sensing applications. We anticipate these nanoparticle-decorated chiral nematic materials could find use in a variety of applications such as nonlinear optics, sensing, or catalysis.
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ASSOCIATED CONTENT
S Supporting Information *
SEM of achiral NCC/SiO2 composite loaded with 2.5 mM Fe(NO3)3, XRD of Pt-decorated SiO2 films, TEM and size histogram of Au nanoparticles after HF etching, XRD of calcined Au-loaded SiO2 films, N2 gas adsorption characterization of calcined Au-loaded SiO2 films, characterization of carbonized samples, CD spectra of dry Ag nanoparticle-loaded SiO2 films. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.A.K. and K.E.S. thank NSERC for postdoctoral and graduate fellowships, respectively. We thank CelluForce Inc. and NSERC for supporting this project.
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REFERENCES
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