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Optically tunable chiral nematic mesoporous cellulose films† Maik Schlesinger,a Wadood Y. Hamadb and Mark J. MacLachlan*a Demand for sustainable functional materials has never been larger. The introduction of functionality into pure cellulose might be one step forward in this field as it is one of the most abundant natural biopolymers. In this paper, we demonstrate a straightforward and scalable way to produce iridescent, mesoporous cellulose membranes with tunable colors and porosity. Concomitant assembly of cellulose nanocrystals (CNCs) and condensation of silica precursors results in CNC–silica composites with chiral nematic structures and tunable optical properties. Removal of the stabilizing silica matrix by alkaline or acid treatment gives access to novel chiral nematic mesoporous cellulose (CNMC) films. Importantly, the
Received 30th March 2015, Accepted 7th May 2015
optical properties and the mesoporosity can be controlled by either varying the silica-to-CNC ratio,
DOI: 10.1039/c5sm00745c
introduce additional functionality, CNMC has been used to stabilize gold nanoparticles with three different concentrations by wet impregnation. These materials are stable in water and can potentially
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function in sensors, tissue engineering or functional membranes.
or by varying the substrate used during the evaporation-induced self-assembly process. In order to
Introduction Coloration is ubiquitous in nature and arises for diverse functions in animals and plants. In plants, for example, chlorophyll is a green-colored pigment optimized to harvest energy from the sun for photosynthesis, and flavonoids – yellow pigments – provide visual cues for animal pollinators.1–4 In animals, coloration is necessary for distraction, camouflage and signaling, and mainly arises from pigments, dyes or structural colors.4–8 Structural colors occur from the interference of visible light by interaction with microstructures. This coloration, known as iridescence, is responsible for the brilliant coloration of many butterfly wings, bird feathers, fish scales, and beetle shells.9,10 Materials chemists are now actively working to mimic structural coloration in synthetic opals and other designer materials. Chiral nematic (cholesteric) liquid crystals organize with a structure where the director rotates with a specific pitch. Light is selectively reflected from chiral nematic liquid crystals with a wavelength that depends on their pitch and their refractive index. While synthetic chiral nematic liquid crystals have been known since 1888,11 the discovery of this structure in nature has been relatively recent. In fact, many arthropods obtain their
a
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1, Canada. E-mail: [email protected] b FPInnovations, 2665 East Mall, Vancouver, British Columbia, V6T 1Z4, Canada † Electronic supplementary information (ESI) available: SEM images, IR, UV/vis, CD and EDX spectra, PXRD data, photographs, tables with analytical data. See DOI: 10.1039/c5sm00745c
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coloration from a chiral nematic organization of chitin fibrils, as discussed below. Cellulose nanocrystals (CNCs) are isolated from the acidcatalyzed hydrolysis of plant, or animal, biomass12 and have widths of B5–15 nm and a length of B100–300 nm (depending on starting raw material).13–15 It was known since the 1950s that solutions of CNCs could form liquid crystalline phases in water.13,16 Revol et al. first showed that this phase was a chiral nematic phase,13 and later demonstrated that the structure of the lyotropic liquid crystal could be retained after water evaporation, giving iridescent films of CNCs.17 In the films, CNCs are organized into layers in which the rods are aligned, but rotate through the stack with a characteristic pitch. This organization resembles the Bouligand structure of beetle shells and arthropod shells, and has been termed ‘‘chiral nematic structure’’.18,19 The pitch of the chiral nematic order of CNC films, which affects their color, is influenced by ultrasonication,20 preparation process,14 ionic strength,21 counter ions,22,23 drying temperature,24 and magnetic fields.20 Once formed, films of CNCs are non-porous and are subject to redissolving in water due to the surface charges present on the nanocrystals. It is known that partial defunctionalization of pre-assembled CNC films, e.g. by desulfation of sulfated CNCs,25 significantly increases their water stability but all attempts to obtain chiral nematic phases from desulfated CNCs failed so far.15 (It is worth noting that chiral nematic phases have been observed in solution for CNCs functionalized with PEG amides,26 surfactants,27 or obtained via persulfate oxidation.28) Another way to retain and
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stabilize the chiral nematic ordering of CNCs is to co-assemble them with other components during the evaporation-induced self-assembly (EISA).29–31 Thus, composite materials containing silicates,29,32–34 acrylamides,35,36 phenol-37 and aminoformaldehydes38,39 were successfully synthesized, and these can be further transformed into chiral nematic mesoporous silica,29,32,34 carbon40 or formaldehyde resins.37,39 Further studies have shown that the mesoporosity of these films allows the incorporation of liquid crystals and metal nanoparticles, resulting in changes of their physical properties like thermal behavior and color.41–43 This behavior makes them attractive for applications such as novel sensor and display materials.41,43 Developing new functional materials based entirely on cellulose is attractive as the materials are available from a renewable feedstock. Giese et al. recently showed that degradation of ureaformaldehyde (UF) resins in CNC–UF composite materials yields mesoporous photonic cellulose, a stable chiral nematic cellulosic material.44 This material is able to change color in different solvents due to a unique swelling behavior, but it has proven difficult to control the color of the final material. A very recently published study showed that this material can be used to stabilize gold nanoparticles accompanied by a change in color.45 However, the color of the starting material as well as of the gold composite material is mainly fixed, and it is difficult to control either the porosity or the pitch of the structure to tune the properties of the films. Here, we present the synthesis of novel chiral nematic mesoporous cellulose (CNMC) with tunable optical properties. By taking advantage of the developments to produce CNC–silica composites with controlled architectures, we demonstrate that removal of the silica from these composites gives iridescent, mesoporous, desulfated CNC films with chiral nematic order. We report detailed studies of the synthetic approach to these materials and the properties of the CNMC. The CNMC materials are of interest for coloration in consumer packaging, for security features, for membrane applications, and for chromatography. Additionally, we demonstrate through the incorporation of gold nanoparticles that the CNMC is a good host material, producing chiroptical materials that may be useful for sensing.
Fig. 1 Schematic preparation procedure for CNMC and gold functionalized CNMC.
Higher proportion of silica in the composite results in an increase of the helical pitch due to repulsive interactions between the negatively charged silica species and the CNCs as well as a greater silica wall thickness.29 Additionally, the physical properties of the mesoporous films obtained after removal of cellulose, such as specific surface area and pore volume, are affected by changing the proportion of CNCs and silica. Another method to change the reflection of chiral nematic films is by casting the film on different substrates.30 In this case a constant CNC/silica ratio results in identical physical properties of the silica after cellulose removal, the only difference being the helical pitch. Silica reacts with concentrated alkali hydroxides to form water-soluble silicates.47 But, cellulose is also known to undergo mercerization upon treatment with base.48–53 Thus, we investigated the treatment of CNC–silica composites with alkali hydroxide solutions to examine how this would affect the composites. We were specifically capable of removing the silica completely without affecting the CNC organization, Fig. 1. Time-dependent UV/vis measurements were performed to investigate changes of the reflection band of CNC/Si-1 (lmax = 831 nm) after treatment with a 4.2 wt% (1.8 M) LiOH(aq) solution at room temperature (Fig. S1, ESI†). The investigations showed a step-by-step blue-shifting of the UV/vis reflectance band as the treatment was extended, finally showing a reflection band at 498 nm after 48 h (Fig. 2). Circular dichroism (CD) spectroscopy was used to identify the origin of the UV/vis reflectance band. The experiments showed positive CD signals at similar wavelengths to the UV/vis reflectance bands, confirming the presence and stabilization of the typical CNC-based
Results and discussion Chiral nematic mesoporous cellulose The evaporation-induced self-assembly (EISA) is the most important step on the way to chiral nematic structures obtained from cellulose nanocrystals (CNCs). Above a critical concentration, CNCs organize into a chiral nematic phase, which is retained in the solid film upon drying. The peak wavelength (lmax) of light reflected from the chiral nematic structure depends on the angle of incident light (y), the average refractive index (navg) of the material and the helical pitch (P):17,46 lmax = navgPsin(y)
(1)
with CNC–silica composites, the pitch of the chiral nematic structure is influenced by the proportion of the components.
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Fig. 2 Normalized CD (bottom) and UV/vis spectra (top) of CNC/Si-1(2h) (dotted lines), CNC/Si-1(48h) (dashed lines) and CNC/Si-HF (solid lines).
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left-handed helical assembly (Fig. 2). The influence of linear dichroism can be neglected as the CD spectra remain virtually unchanged after rotating the samples perpendicular to the light beam. Thermogravimetric analyses (TGA) were performed to identify the reason for the step-by-step blue shift of the reflectance (Fig. S2, ESI†). For CNC/Si-1 a two-step mass loss of 54.5% is obtained whereas CNC/Si-1(30m) and CNC/Si-1(2h) lost 67.6% and 86.5% of its mass, indicating a removal of 28.8% and 70.3% silica, respectively. Finally, CNC/Si-1(48h) gave a 100% mass loss, demonstrating that all of the inorganic matrix was removed. The results were confirmed by infrared (IR) spectroscopy which shows the absence of the Si–OH bending (792 cm 1) and Si–O–Si stretching vibrations (994 cm 1) for CNC/Si-1(48h), Fig. S3 (ESI†).54 IR spectroscopy also showed the expected bands for cellulose, such as vibrations from pyranose ring structures (850–1190 cm 1), O–H and C–H bending (1270–1465 cm 1), C–H (2994 cm 1) and O–H stretching (3316 cm 1) vibrations. Energy dispersive X-ray spectroscopy (EDX) confirmed the complete removal of silica (Fig. S3, ESI†) and CHNS analysis showed the formation of a de-sulfated cellulose material. These experiments demonstrate that treatment of the CNC–silica composites leads to removal of the silica, leaving a purely cellulosic material that retains the chiral nematic structure of the composite. We call these materials chiral nematic mesoporous cellulose (CNMC). NaOH and KOH solutions can also be used for the synthesis of CNMC. However, under identical synthetic conditions (48 h treatment, 1.8 M aqueous solutions, room temperature) the UV/vis reflection bands of the final CNMC materials are redshifted (LiOH o NaOH o KOH), Fig. S4 (ESI†). It’s not clear why there is a dependence on the counterion. For comparison, the assembly of CNC films from sulfated CNCs is influenced by the nature of the cations that charge-balance the sulfate ester groups on the CNC surface; the effect was attributed to increased van der Waals’ radii of the counterions, which results in an increased helical pitch.22 Our data suggest that the pitch of the CNMC is actually changing while the silica is being removed. We also investigated the treatment of CNC/Si-1 with a 5% hydrofluoric acid solution (in 1 : 1 H2O : EtOH) to remove silica. After only 15 min, the UV/vis reflectance band had already blueshifted to 471 nm (CNC/Si-HF). CD spectroscopy verified that this reflection arises from a left-handed helical assembly (Fig. 2). TGA showed a 100% mass loss indicating the quantitative removal of silica which was additionally confirmed by EDX, IR spectroscopy (Fig. S2 and S3, ESI†) and CHNS analysis. Six different polymorphs of cellulose are known, namely I, II, IIII, IIIII, IVI and IVII. For cellulose I, two suballomorphs (Ia and Ib) are identified.55,56 Powder X-ray diffraction (PXRD) of the CNC starting material showed that they are cellulose Ib, a form that is typical for cellulose from higher plants (Fig. S5, ESI†). As well, the CNC–silica composites show cellulose Ib phase. PXRD studies of the CNMC materials (CNC/Si-1(48h) and CNC/Si-HF) reveal that they are also cellulose Ib (Fig. 3).14,15 Thus, no phase transition from cellulose I to cellulose II took place, a transformation that is known to occur in highly alkaline solutions.48,57 Ruland–Rietveld analysis of the CNMC samples showed that
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Fig. 3 PXRD patterns (black lines) of CNC/Si-1(48h) (top) and CNC/Si-1-HF (bottom). The diffraction patterns are resolved between 101 2y and 901 2y into crystalline peaks and amorphous background. The refinement was done using the Ruland–Rietveld analytical approach with the crystal structure of cellulose Ib (CCDC 810597; red lines). Differences between observed and refined pattern are represented in blue lines, and expected peak positions of cellulose Ib (CCDC 810597) are given in magenta. The degree of crystallinity was determined to be B84% (CNC/Si-1(48h)) and B87% (CNC/Si-1-HF).
CNC/Si-1(48h) and CNC/Si-HF have B84% and B87% crystallinity, respectively. These values are comparable to pure CNC films (B90%),14 showing that the crystallites are not adversely affected by the acid or base treatment. For comparison, mesoporous cellulose obtained from CNC–UF composites shows a lower degree of crystallinity (B70%).44 Polarized optical microscopy (POM) and scanning electron microscopy (SEM) studies of CNC/Si-1(48h) and CNC/Si-HF showed the retention of the chiral nematic assembly without any apparent distortions. The typical birefringent texture is still present when the films are viewed under crossed polarizers by POM (Fig. 4a and b). SEM images of the films (Fig. 4c and d) show a layered structure that is typically observed for films with chiral nematic order. Notably, a left-handed helical assembly is apparent in the counter-clockwise direction of the twisting when the films are viewed under high magnification. Thus, the order of the chiral nematic assembly is not affected by either acid or base treatment. Taken together, these data show that removal of silica from CNC–silica composite materials does not adversely affect the
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Fig. 4 POM images (a, b) under crossed polarizers of CNC/Si-1(48h) and SEM images of (c) CNC/Si-1(48h) and (d) CNC/Si-1-HF.
structural order of the starting CNC, but stabilizes it by creating desulfated surfaces. The CNCs remain highly crystalline in the films with cellulose Ib form which results in higher brittleness compared to mesoporous photonic cellulose.44 The films show substantially improved stability in water when compared with pure films – even heating in boiling water overnight did not result in dissolution of the films. Nitrogen adsorption measurements of CNC/Si-1(48h) and CNC/Si-HF dried under ambient conditions reveal non-ideal type IV isotherms with strong hysteresis effects (Fig. S6, ESI†). The Brunauer–Emmett–Teller (BET) surface areas were calculated to be 44 m2 g 1 (CNC/Si-1(48h)) and 54 m2 g 1 (CNC/Si-1-HF). These results indicate a mesoporous behavior but with low surface area, likely due to collapse of the structure when the silica was removed. Thus, to improve the porosity, the samples were soaked in ethanol and subsequently dried with supercritical carbon dioxide (sc-CO2) to give highly mesoporous materials. As shown in Fig. 5, type IV isotherms with BET surface areas of 278 m2 g 1 (CNC/Si-1(48h)) and 268 m2 g 1 (CNC/Si-1-HF) with average pore volumes of 0.69 cm3 g 1 (CNC/ Si-1(48h)) and 0.52 cm3 g 1 (CNC/Si-1-HF) were obtained. The calculated Barrett–Joyner–Halenda (BJH) pore-size distributions are around 8.4 nm (CNC/Si-1(48h)) and 6.2 nm (CNC/Si-1-HF), Fig. 5, inset. The nitrogen adsorption studies reveal a slight influence of the silica removal procedure on the physical properties of the final material, but in every case, CNMC shows a higher mesoporosity than mesoporous photonic cellulose produced from CNC/UF.44 It is notable that sc-CO2 drying results in strongly iridescent films including a slight red-shift of the UV/vis reflection band from the dry state (e.g., to 516 nm for CNC/Si-1(48h)), though the films show lower translucence (Fig. S7, ESI†), suggesting a lessordered microstructure. As mentioned, some physical properties can be adjusted by changing the CNC to silica ratio used in preparing the CNC– silica composites. Thus, another CNC–silica composite material with higher silica content was synthesized (CNC/Si-2). The influence of the higher silica content in CNC/Si-2 can clearly be seen by its UV/vis reflection band, which is strongly red-shifted
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Fig. 5 N2 adsorption isotherms and BJH pore-size distributions (inset) of CNC/Si-1(48h) (top) and CNC/Si-1-HF (bottom) after sc-CO2 drying.
to 1183 nm compared to CNC/Si-1 (Fig. S8, ESI†). Removal of the silica from CNC/Si-2 by LiOH treatment (4.2 wt%, 48 h) gave access to CNC/Si-2(48h). The UV/vis reflection band is shifted to 678 nm due to the removal of silica, but is still at longer wavelengths compared to CNC/Si-1(48h). These findings show the possibility to control the color of reflectance of CNMC by varying the silica content. In addition, SEM and CD measurements confirm the retention of the chiral nematic assembly in CNC/Si-2(48h) (Fig. S8, ESI†). Nitrogen adsorption measurements of sc-CO2 dried CNC/Si-2(48h) also reveal an influence of the higher silica content (Fig. S9, ESI†). The BET surface area (314 m2 g 1), the average pore volume (1.24 cm3 g 1) and the BJH pore-size distribution (15.7 nm) are all increased compared to CNC/Si-1(48h), which is a result of the removal of the silica with a greater wall thickness. Thus, by increasing the proportion of silica employed in the CNC–silica composite, the resulting CNMC films show larger surface areas, pore volumes, and pore sizes after silica removal. As expected, this is opposite to the trend observed for chiral nematic silica films obtained by removing the cellulose from CNC–silica composites, where the porosity decreases with higher silica content.29 In order to obtain CNMC materials with identical porosity but different optical properties, CNC–silica composite materials with identical silica to CNC ratios were synthesized. The reflected colors were controlled by the use of different substrates,30 such as
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nematic assembly of CNMC, incorporation of gold nanoparticles (Au NPs) represents an interesting pathway to obtain monodisperse Au NPs within a chiral nematic environment. Gold functionalized cellulose materials have attracted substantial interest as they might serve as sensors, catalysts, membranes, and chromatography materials.58–65
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Incorporation of gold nanoparticles
Fig. 6 Normalized CD (bottom) and UV/vis spectra (top) of CNMC-Al (dotted lines), CNMC-CA (dashed lines) and CNMC-PS (solid lines).
polystyrene (CNC/Si-PS), cellulose acetate (CNC/Si-CA) and aluminum (CNC/Si-Al). The UV/vis reflectance bands of these composite films vary between 1175 nm (CNC/Si-PS), 804 nm (CNC/Si-CA) and B220 nm (CNC/Si-Al), Fig. S10 (ESI†). After removal of the silica with LiOH, CNMC films with UV/vis reflection bands of 474 nm (CNMC-PS), 421 nm (CNMC-CA) and o200 nm (CNMC-Al) were obtained (Fig. 6 and Fig. S10, ESI†). (It was not possible to accurately measure the reflection peak for the CNMC-Al samples owing to UV absorption by the cellulose.) Retention of the lefthanded chiral nematic structure after silica removal was confirmed by CD and SEM measurements (Fig. 6 and Fig. S11, ESI†). Finally, the porosity of the sc-CO2 dried films was determined by nitrogen adsorption (Fig. S12, ESI†). The films show a type IV isotherm with almost identical BET surface areas, average pore volumes and BJH pore-size distributions (Table 1). Thus, it is possible to prepare chiral nematic mesoporous cellulose samples with identical porosity, but different helical pitches (and, hence, colors). The new films of CNMC have potential for incorporation into sensors or security features, as they can change color upon swelling in different solvents. Such investigations were carried out with CNC/Si-2(48h) as a representative example. As shown in Fig. S13 (ESI†) the UV/vis reflection band is red-shifted to 886 nm in ethanol and 1003 nm in water. This behavior is attributed to a change in the helical pitch of the chiral nematic structure since the refractive indices of ethanol and water are similar. The color changes are reversible, and the original color is recovered upon drying. Thus, CNMC might be a useful material for security features with a high opacity in dried state and a high translucence if wetted. So far, we showed the synthesis of novel CNMC materials with tunable optical and physical properties starting from CNC–silica composites. As a result of the mesoporosity and the chiral Table 1 BET surface areas, pore volumes and BJH pore-width distributions of CNC/Si-PS, CNC/Si-CA and CNC/Si-Al
Sample
BET-Ao [m2 g 1]
Pore volume [cm3 g 1]
BJH pore-width [nm]
CNC/Si-PS CNC/Si-CA CNC/Si-Al
290 287 289
0.94 0.93 0.92
11.0 11.0 10.7
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Au NPs were loaded into CNMC by using the wet impregnation method. CNC/Si-2(48h), dried from sc-CO2, was soaked in 5 mM (CNMC-Au-5), 10 mM (CNMC-Au-10) and 100 mM (CNMC-Au-100) aqueous solutions of HAuCl4 for 1 h to ensure maximum loading. The Au(III) ions were finally reduced to Au(0) by the addition of an aqueous NaBH4 solution. The incorporation of Au NPs does not result in a strong color change but the final material shows a weak metallic luster (Fig. S14, ESI†). Thermogravimetric analyses under air (heating rate 10 1C min 1) of the CNMC-Au samples showed a sharp mass loss beginning at around 300 1C (Fig. S15, ESI†). The decomposition was almost complete at around 450 1C, showing the acceleration of cellulose decomposition by the presence of Au NPs. Such a behavior is well known for metal NP–cellulose composite materials.45,66,67 The final mass losses were calculated to 99.2% (CNMC-Au-5), 99.0% (CNMC-Au-10) and 98.7% (CNMC-Au-100) representing a loading of the CNMC with 0.8 wt%, 1.0 wt% and 1.3 wt% Au NPs, respectively (assuming no oxidation of the Au NPs). Nitrogen adsorption measurements of sc-CO2 dried CNMCAu-5, CNMC-Au-10 and CNMC-Au-100 were performed to study the influence of the Au NP incorporation on the porosity of CNMC. The BET surface areas were determined to be around 280 m2 g 1 with average pore volumes around 1 cm3 g 1 whereas the BJH pore size distributions show a pore width of approx. 13.5 nm (Fig. S16 and Table S1, ESI†). Thus, these results reveal that the amount of loading does not significantly affect the porosity of the CNMC. However, compared to CNC/Si-2(48h) the BET surface areas and average pore widths are slightly decreased, consistent with incorporated particles. Scanning electron microscopy studies have shown a slight distortion of the chiral nematic assembly, which is attributed to the supercritical drying before the loading (Fig. S17, ESI†). Such a behavior is also known to occur for mesoporous photonic cellulose.44 Transmission electron micrographs were obtained for CNMC-Au-10 and reveal the stabilization of spherical, almost monodisperse Au NPs with a diameter of 25 1 nm (Fig. 7 and Fig. S18, ESI†). Thus, the mesopores of CNMC stabilize isolated Au NPs without the addition of any surfactants similar to the observations for Au NP decorated photonic cellulose.45 The obtained particle sizes for CNMC-Au-10 are comparable to Au NPs on chiral nematic silica composites (28 21 nm) but show a much lower polydispersity.42 Several trials to obtain suitable TEM images or even stable sections of CNMC-Au-5 and CNMC-Au-100 failed due to separation issues during sectioning and measuring of the cellulose–resin composites (see comment in Experimental section). However, TEM images of CNMC-Au-10 at low magnification reveal a low amount
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Fig. 7 TEM images of CNMC-Au-10.
of loading as already determined from TGA measurements. These findings are confirmed by PXRD as peaks for gold were only obtained for CNMC-Au-100 where the crystallite size was calculated to be 34.1 nm (Fig. S19, ESI†). For CNMC-Au-5 and CNMC-Au-10, a combination of too low loading and small crystallite sizes hinders the formation of a necessary amount of diffraction centers. However, the degree of crystallinity for the CNC crystallites was additionally calculated to be between B80% (CNMC-Au-10) and B86% (CNMC-Au-5) by using the Ruland–Rietveld analytical approach. These results indicate that the incorporation of Au NPs does not adversely affect the crystallinity of the CNMC. In addition to those analyses, the identity and distribution of the Au NPs were examined by EDX measurements (Fig. S20, ESI†). The EDX spectra clearly show the presence of gold in all of the materials and EDX mapping confirmed their homogeneous distribution. Finally, the gold-loaded CNMC samples were analyzed by optical spectroscopy to determine the interaction of the surface plasmon resonance (SPR) of the Au NPs with the still intact chiral nematic structure of CNMC. As shown in Fig. 8 small deviations are obtained for the UV/vis reflection bands depending on the amount of gold loading. CNMC-Au-5 shows a UV/vis reflection band at 491 nm assigned to the chiral nematic structure whereas those bands are shifted to longer wavelengths for CNMC-Au-10 (495 nm) and CNMC-Au-100 (514 nm). Additionally, CNMC-Au-10 and CNMC-Au-100 show a second UV/vis band at 609 nm and 598 nm, respectively. These additional UV/vis bands are attributed to the SPR of the incorporated
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Au NPs, and the intensity of the signal is stronger for CNMC-Au-100 than for CNMC-Au-10 as a result of a higher amount of loading. In addition, CD spectroscopy confirmed the left-handed helical assembly of all gold-loaded CNMC materials due to signals with positive ellipticity (Fig. 8). These broad CD signals centered at 484 nm (CNMC-Au-5), 495 nm (CNMC-Au-10) and 532 nm (CNMC-Au-100) confirm the origin of the UV/vis reflection bands and show a small influence of the amount of gold loading. To eliminate the influence of linear dichroism effects, samples where additionally rotated perpendicular to the light beam, resulting in similar CD spectra. For CNMC-Au-100, an additional positive CD signal at 571 nm is obtained which is assigned to the SPR of the Au NPs. Thus, the SPR of the Au NPs interacts with the chiral nematic CNMC to give plasmonic chiroptical activity. Similar observations were obtained for metal loaded chiral nematic silica and cellulose samples.41,42,45,68 However, to clarify that the chiroptical properties are a result of the organization of the Au NPs within the chiral nematic environment of CNMC, we first prepared a CNC–silica composite without chiral nematic ordering by acidification during the EISA process.41 Treatment with an aqueous 4.2 wt% LiOH solution over 48 h gave access to a non-chiral nematic cellulose material which was additionally loaded with Au NPs using an identical synthetic procedure than for CNMC-Au-100. The obtained material only shows one UV/vis band at 581 nm for the SPR of the Au NPs and lacks the broad UV/vis reflection band due to the missing chiral nematic ordering (Fig. S21, ESI†). These observations are confirmed by CD spectroscopy where no signal was detected (Fig. S21, ESI†). The results indicate that the observed chiroptical properties of CNMC-Au-100 are not simply due to interactions between the surface of the Au NPs and the molecular chirality of the cellulose surface. Two research groups have independently investigated the co-assembly of gold nanorods with cellulose nanocrystals.68–70 The interaction of surface plasmon resonances of gold nanorods with the chiral nematic assembly of CNC was shown to yield tunable chiroptical activities.68 The responses were dependent on the amount of gold loading and the size of the gold nanorods, resulting in positive CD signals between 500 nm and 700 nm, similar to the results presented in this study. However, the authors also pointed out that interference effects might influence the observed CD signals. In the relatively new field of nanoparticleloaded chiral nematic composite materials, only a few publications are available that describe possible mechanisms of the origin of chiroptical activities.71–75 A recent review article also points out that severe conditions of helicity and interparticle interactions have to be met for enhancement of chiroptical activities.76 Thus, further research in this field is needed to understand the obtained chiroptical effects in detail as those materials might give access to attractive optical effects and devices.77
Experimental Materials and instrumentation Fig. 8 Normalized CD (bottom) and UV/vis spectra (top) of CNMC-Au-5 (solid lines), CNMC-Au-10 (dashed lines) and CNMC-Au-100 (dotted lines).
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All compounds were used as received without any further purification. Aqueous suspensions of cellulose nanocrystals
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(CNCs) were supplied from CelluForce Inc. as an acidic form (CNC-H: 4.1 wt%, pH = 2.52). The CNC suspensions were obtained by using a previously described procedure.29 Thermogravimetric analysis was done with a Pyris 6 (PerkinElmer) under air with a heating rate of 10 1C min 1. CD spectra were obtained on a J-710 spectropolarimeter (JASCO) by mounting the films between two quartz slides perpendicular to the beam path. As mentioned in the manuscript, some samples were rotated in order to confirm that the results were not dominated by linear dichroism effects. UV-vis/ NIR spectra were obtained on a Cary 5000 UV-vis/NIR spectrophotometer by using the same sample preparation techniques as for CD spectroscopy. Nitrogen adsorption isotherms were acquired at 77 K using a Micromeritics ASAP 2020 analyzer. Pore-size distributions were calculated from the adsorption branches using the Barrett–Joyner–Halenda (BJH) method. Electron microscopy studies were conducted on a Hitachi S4700 (SEM) with sputter-coated samples (5.0 nm of gold/ palladium 60/40) and on a Hitachi H7600 electron microscope (TEM). For our TEM experiments we used HM20 resins as a support for the cellulose samples. However, several tries to section the prepared materials failed. The cellulose samples separated from the resin after sectioning due to incomplete penetration. Numerous attempts to use Spurr’s resin instead of the HM20 resin or even cryosectioning were also unsuccessful. We were only able to get a few stable sections of sample CNMC-Au-10 which were used as a representative sample in this study. The nanoparticle size distributions were calculated from TEM images using the LINCE (linear intercept) software (TU Darmstadt). The quoted error represents the standard deviation in the particle size distribution. Powder X-ray diffraction studies were measured on a D8 Advance (Bruker) using Cu-Ka irradiation and a NaI scintillation detector. The deconvolution of the peaks of the diffraction patterns was done by the use of DIFFRACplus TOPAS software (Bruker-AXS) on the basis of the Rietveld refinements.78 The degree of crystallinity Xc (the fraction in weight occupied by the crystallites) was calculated using Ruland’s theoretical approach.79 Critical point drying with supercritical CO2 was carried out on an autosamdry-815 critical point dryer (Tousimis Research Corporation). Synthesis of CNMC CNC–silica films were synthesized by a similar procedure to that previously described.29 Usually, polystyrene dishes with a diameter of 60 mm were used. After mixing the appropriate amount of CNC suspension and Si(OMe)4 for 1 h, 3.65 mL of the mixture was placed onto the dishes under ambient conditions for EISA. CNC–silica composite films were synthesized according to a literature procedure. The helical pitch of the CNC–silica composite was varied by using different ratios of Si(OMe)4 : CNC on polystyrene dishes (CNC/Si-1: 2.0 mL TMOS per 1 mg CNC; CNC/Si-2: 3.0 mL TMOS per 1 mg CNC). In addition, the helical pitch of the CNC–silica composite was varied by using different substrates like aluminum (CNC/Si-Al), cellulose acetate (CNC/Si-CA) and polystyrene (CNC/Si-PS) with an identical Si(OMe)4 : CNC ratio of 1.8 mL TMOS per 1 mg CNC.
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For those experiments, the borders of polystyrene dishes (60 mm in diameter) were mounted onto aluminum foil or cellulose acetate sheets to obtain films with an identical size. Then the same EISA procedure as above was used. The chiral nematic composite films obtained after complete evaporation of water were treated with a 4.2 wt% LiOH(aq) solution for 48 h or with 5% HF(aq) for 15 min to remove the silica and obtain pure cellulose films (10 mL solution per 100 mg composite). The cellulose films were extensively washed with a 1 : 1 ethanol/water mixture and finally dried from ethanol under ambient conditions or by supercritical carbon dioxide. Metal nanoparticle incorporation 10 mg of supercritical carbon dioxide-dried CNC/Si-2(48h) were soaked in 5 mL of 5 mM, 10 mM or 100 mM aqueous solutions of HAuCl4 for 1 h. Afterwards the films were washed several times with water and ethanol and dried under ambient conditions. After reduction with a freshly prepared 200 mM NaBH4(aq) solution (10 mL) for 1 h, the films were extensively washed with water and ethanol, and finally dried under ambient conditions. Synthesis of achiral CNC–silica composite The achiral CNC–silica composite was synthesized by adding a 1 M aqueous HCl solution to the CNC suspension until a pH of 2 was reached. After mixing of the acidic CNC suspension with Si(OMe)4 (3.0 mL Si(OMe)4 per 1 mg CNCs), the mixture was cast into polystyrene dishes (60 mm in diameter; 3.65 mL of mixture per dish), yielding achiral composite films after evaporation of the water. After removal of the silica by treatment with a 4.2 wt% LiOH(aq) solution for 48 h, the obtained film was washed (H2O, EtOH), dried with supercritical CO2 and finally loaded with Au NPs using the procedure given in the manuscript (100 mM aqueous HAuCl4 solution).
Conclusions In summary, we developed synthetic routes to obtain novel chiral nematic mesoporous cellulose (CNMC) materials from CNC–silica composites. The optical and physical properties of the final material can be readily adjusted. Thus, CNMC materials with different helical pitches but identical porosity can be obtained. As well, CNMC materials with very high surface areas (up to 314 m2 g 1) and large average pore widths (up to 15.7 nm) are accessible. Characteristically, the CNMC materials show a high degree of crystallinity that is comparable to pure CNC films indicating no adverse effect of the preparation process. In addition, the materials were used to stabilize gold nanoparticles whose size can be controlled by changing the concentration of the gold precursor solution. These composite materials show chiroptical properties due to the interaction of the surface plasmon resonance of the gold nanoparticles with the chiral nematic environment provided by the CNMC host. The new CNMC and Au@CNMC composite materials presented here have potential utility in novel biosensors, security features, membranes, and chromatography materials.
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Acknowledgements We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and CelluForce Inc. for their support. M.S. is grateful to the German Academic Exchange Service (DAAD) for a postdoctoral fellowship.
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Cite this: DOI: 10.1039/c5sm00745c
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Optically tunable chiral nematic mesoporous cellulose films† Maik Schlesinger,a Wadood Y. Hamadb and Mark J. MacLachlan*a Demand for sustainable functional materials has never been larger. The introduction of functionality into pure cellulose might be one step forward in this field as it is one of the most abundant natural biopolymers. In this paper, we demonstrate a straightforward and scalable way to produce iridescent, mesoporous cellulose membranes with tunable colors and porosity. Concomitant assembly of cellulose nanocrystals (CNCs) and condensation of silica precursors results in CNC–silica composites with chiral nematic structures and tunable optical properties. Removal of the stabilizing silica matrix by alkaline or acid treatment gives access to novel chiral nematic mesoporous cellulose (CNMC) films. Importantly, the
Received 30th March 2015, Accepted 7th May 2015
optical properties and the mesoporosity can be controlled by either varying the silica-to-CNC ratio,
DOI: 10.1039/c5sm00745c
introduce additional functionality, CNMC has been used to stabilize gold nanoparticles with three different concentrations by wet impregnation. These materials are stable in water and can potentially
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function in sensors, tissue engineering or functional membranes.
or by varying the substrate used during the evaporation-induced self-assembly process. In order to
Introduction Coloration is ubiquitous in nature and arises for diverse functions in animals and plants. In plants, for example, chlorophyll is a green-colored pigment optimized to harvest energy from the sun for photosynthesis, and flavonoids – yellow pigments – provide visual cues for animal pollinators.1–4 In animals, coloration is necessary for distraction, camouflage and signaling, and mainly arises from pigments, dyes or structural colors.4–8 Structural colors occur from the interference of visible light by interaction with microstructures. This coloration, known as iridescence, is responsible for the brilliant coloration of many butterfly wings, bird feathers, fish scales, and beetle shells.9,10 Materials chemists are now actively working to mimic structural coloration in synthetic opals and other designer materials. Chiral nematic (cholesteric) liquid crystals organize with a structure where the director rotates with a specific pitch. Light is selectively reflected from chiral nematic liquid crystals with a wavelength that depends on their pitch and their refractive index. While synthetic chiral nematic liquid crystals have been known since 1888,11 the discovery of this structure in nature has been relatively recent. In fact, many arthropods obtain their
a
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1, Canada. E-mail: [email protected] b FPInnovations, 2665 East Mall, Vancouver, British Columbia, V6T 1Z4, Canada † Electronic supplementary information (ESI) available: SEM images, IR, UV/vis, CD and EDX spectra, PXRD data, photographs, tables with analytical data. See DOI: 10.1039/c5sm00745c
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coloration from a chiral nematic organization of chitin fibrils, as discussed below. Cellulose nanocrystals (CNCs) are isolated from the acidcatalyzed hydrolysis of plant, or animal, biomass12 and have widths of B5–15 nm and a length of B100–300 nm (depending on starting raw material).13–15 It was known since the 1950s that solutions of CNCs could form liquid crystalline phases in water.13,16 Revol et al. first showed that this phase was a chiral nematic phase,13 and later demonstrated that the structure of the lyotropic liquid crystal could be retained after water evaporation, giving iridescent films of CNCs.17 In the films, CNCs are organized into layers in which the rods are aligned, but rotate through the stack with a characteristic pitch. This organization resembles the Bouligand structure of beetle shells and arthropod shells, and has been termed ‘‘chiral nematic structure’’.18,19 The pitch of the chiral nematic order of CNC films, which affects their color, is influenced by ultrasonication,20 preparation process,14 ionic strength,21 counter ions,22,23 drying temperature,24 and magnetic fields.20 Once formed, films of CNCs are non-porous and are subject to redissolving in water due to the surface charges present on the nanocrystals. It is known that partial defunctionalization of pre-assembled CNC films, e.g. by desulfation of sulfated CNCs,25 significantly increases their water stability but all attempts to obtain chiral nematic phases from desulfated CNCs failed so far.15 (It is worth noting that chiral nematic phases have been observed in solution for CNCs functionalized with PEG amides,26 surfactants,27 or obtained via persulfate oxidation.28) Another way to retain and
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stabilize the chiral nematic ordering of CNCs is to co-assemble them with other components during the evaporation-induced self-assembly (EISA).29–31 Thus, composite materials containing silicates,29,32–34 acrylamides,35,36 phenol-37 and aminoformaldehydes38,39 were successfully synthesized, and these can be further transformed into chiral nematic mesoporous silica,29,32,34 carbon40 or formaldehyde resins.37,39 Further studies have shown that the mesoporosity of these films allows the incorporation of liquid crystals and metal nanoparticles, resulting in changes of their physical properties like thermal behavior and color.41–43 This behavior makes them attractive for applications such as novel sensor and display materials.41,43 Developing new functional materials based entirely on cellulose is attractive as the materials are available from a renewable feedstock. Giese et al. recently showed that degradation of ureaformaldehyde (UF) resins in CNC–UF composite materials yields mesoporous photonic cellulose, a stable chiral nematic cellulosic material.44 This material is able to change color in different solvents due to a unique swelling behavior, but it has proven difficult to control the color of the final material. A very recently published study showed that this material can be used to stabilize gold nanoparticles accompanied by a change in color.45 However, the color of the starting material as well as of the gold composite material is mainly fixed, and it is difficult to control either the porosity or the pitch of the structure to tune the properties of the films. Here, we present the synthesis of novel chiral nematic mesoporous cellulose (CNMC) with tunable optical properties. By taking advantage of the developments to produce CNC–silica composites with controlled architectures, we demonstrate that removal of the silica from these composites gives iridescent, mesoporous, desulfated CNC films with chiral nematic order. We report detailed studies of the synthetic approach to these materials and the properties of the CNMC. The CNMC materials are of interest for coloration in consumer packaging, for security features, for membrane applications, and for chromatography. Additionally, we demonstrate through the incorporation of gold nanoparticles that the CNMC is a good host material, producing chiroptical materials that may be useful for sensing.
Fig. 1 Schematic preparation procedure for CNMC and gold functionalized CNMC.
Higher proportion of silica in the composite results in an increase of the helical pitch due to repulsive interactions between the negatively charged silica species and the CNCs as well as a greater silica wall thickness.29 Additionally, the physical properties of the mesoporous films obtained after removal of cellulose, such as specific surface area and pore volume, are affected by changing the proportion of CNCs and silica. Another method to change the reflection of chiral nematic films is by casting the film on different substrates.30 In this case a constant CNC/silica ratio results in identical physical properties of the silica after cellulose removal, the only difference being the helical pitch. Silica reacts with concentrated alkali hydroxides to form water-soluble silicates.47 But, cellulose is also known to undergo mercerization upon treatment with base.48–53 Thus, we investigated the treatment of CNC–silica composites with alkali hydroxide solutions to examine how this would affect the composites. We were specifically capable of removing the silica completely without affecting the CNC organization, Fig. 1. Time-dependent UV/vis measurements were performed to investigate changes of the reflection band of CNC/Si-1 (lmax = 831 nm) after treatment with a 4.2 wt% (1.8 M) LiOH(aq) solution at room temperature (Fig. S1, ESI†). The investigations showed a step-by-step blue-shifting of the UV/vis reflectance band as the treatment was extended, finally showing a reflection band at 498 nm after 48 h (Fig. 2). Circular dichroism (CD) spectroscopy was used to identify the origin of the UV/vis reflectance band. The experiments showed positive CD signals at similar wavelengths to the UV/vis reflectance bands, confirming the presence and stabilization of the typical CNC-based
Results and discussion Chiral nematic mesoporous cellulose The evaporation-induced self-assembly (EISA) is the most important step on the way to chiral nematic structures obtained from cellulose nanocrystals (CNCs). Above a critical concentration, CNCs organize into a chiral nematic phase, which is retained in the solid film upon drying. The peak wavelength (lmax) of light reflected from the chiral nematic structure depends on the angle of incident light (y), the average refractive index (navg) of the material and the helical pitch (P):17,46 lmax = navgPsin(y)
(1)
with CNC–silica composites, the pitch of the chiral nematic structure is influenced by the proportion of the components.
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Fig. 2 Normalized CD (bottom) and UV/vis spectra (top) of CNC/Si-1(2h) (dotted lines), CNC/Si-1(48h) (dashed lines) and CNC/Si-HF (solid lines).
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left-handed helical assembly (Fig. 2). The influence of linear dichroism can be neglected as the CD spectra remain virtually unchanged after rotating the samples perpendicular to the light beam. Thermogravimetric analyses (TGA) were performed to identify the reason for the step-by-step blue shift of the reflectance (Fig. S2, ESI†). For CNC/Si-1 a two-step mass loss of 54.5% is obtained whereas CNC/Si-1(30m) and CNC/Si-1(2h) lost 67.6% and 86.5% of its mass, indicating a removal of 28.8% and 70.3% silica, respectively. Finally, CNC/Si-1(48h) gave a 100% mass loss, demonstrating that all of the inorganic matrix was removed. The results were confirmed by infrared (IR) spectroscopy which shows the absence of the Si–OH bending (792 cm 1) and Si–O–Si stretching vibrations (994 cm 1) for CNC/Si-1(48h), Fig. S3 (ESI†).54 IR spectroscopy also showed the expected bands for cellulose, such as vibrations from pyranose ring structures (850–1190 cm 1), O–H and C–H bending (1270–1465 cm 1), C–H (2994 cm 1) and O–H stretching (3316 cm 1) vibrations. Energy dispersive X-ray spectroscopy (EDX) confirmed the complete removal of silica (Fig. S3, ESI†) and CHNS analysis showed the formation of a de-sulfated cellulose material. These experiments demonstrate that treatment of the CNC–silica composites leads to removal of the silica, leaving a purely cellulosic material that retains the chiral nematic structure of the composite. We call these materials chiral nematic mesoporous cellulose (CNMC). NaOH and KOH solutions can also be used for the synthesis of CNMC. However, under identical synthetic conditions (48 h treatment, 1.8 M aqueous solutions, room temperature) the UV/vis reflection bands of the final CNMC materials are redshifted (LiOH o NaOH o KOH), Fig. S4 (ESI†). It’s not clear why there is a dependence on the counterion. For comparison, the assembly of CNC films from sulfated CNCs is influenced by the nature of the cations that charge-balance the sulfate ester groups on the CNC surface; the effect was attributed to increased van der Waals’ radii of the counterions, which results in an increased helical pitch.22 Our data suggest that the pitch of the CNMC is actually changing while the silica is being removed. We also investigated the treatment of CNC/Si-1 with a 5% hydrofluoric acid solution (in 1 : 1 H2O : EtOH) to remove silica. After only 15 min, the UV/vis reflectance band had already blueshifted to 471 nm (CNC/Si-HF). CD spectroscopy verified that this reflection arises from a left-handed helical assembly (Fig. 2). TGA showed a 100% mass loss indicating the quantitative removal of silica which was additionally confirmed by EDX, IR spectroscopy (Fig. S2 and S3, ESI†) and CHNS analysis. Six different polymorphs of cellulose are known, namely I, II, IIII, IIIII, IVI and IVII. For cellulose I, two suballomorphs (Ia and Ib) are identified.55,56 Powder X-ray diffraction (PXRD) of the CNC starting material showed that they are cellulose Ib, a form that is typical for cellulose from higher plants (Fig. S5, ESI†). As well, the CNC–silica composites show cellulose Ib phase. PXRD studies of the CNMC materials (CNC/Si-1(48h) and CNC/Si-HF) reveal that they are also cellulose Ib (Fig. 3).14,15 Thus, no phase transition from cellulose I to cellulose II took place, a transformation that is known to occur in highly alkaline solutions.48,57 Ruland–Rietveld analysis of the CNMC samples showed that
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Fig. 3 PXRD patterns (black lines) of CNC/Si-1(48h) (top) and CNC/Si-1-HF (bottom). The diffraction patterns are resolved between 101 2y and 901 2y into crystalline peaks and amorphous background. The refinement was done using the Ruland–Rietveld analytical approach with the crystal structure of cellulose Ib (CCDC 810597; red lines). Differences between observed and refined pattern are represented in blue lines, and expected peak positions of cellulose Ib (CCDC 810597) are given in magenta. The degree of crystallinity was determined to be B84% (CNC/Si-1(48h)) and B87% (CNC/Si-1-HF).
CNC/Si-1(48h) and CNC/Si-HF have B84% and B87% crystallinity, respectively. These values are comparable to pure CNC films (B90%),14 showing that the crystallites are not adversely affected by the acid or base treatment. For comparison, mesoporous cellulose obtained from CNC–UF composites shows a lower degree of crystallinity (B70%).44 Polarized optical microscopy (POM) and scanning electron microscopy (SEM) studies of CNC/Si-1(48h) and CNC/Si-HF showed the retention of the chiral nematic assembly without any apparent distortions. The typical birefringent texture is still present when the films are viewed under crossed polarizers by POM (Fig. 4a and b). SEM images of the films (Fig. 4c and d) show a layered structure that is typically observed for films with chiral nematic order. Notably, a left-handed helical assembly is apparent in the counter-clockwise direction of the twisting when the films are viewed under high magnification. Thus, the order of the chiral nematic assembly is not affected by either acid or base treatment. Taken together, these data show that removal of silica from CNC–silica composite materials does not adversely affect the
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Fig. 4 POM images (a, b) under crossed polarizers of CNC/Si-1(48h) and SEM images of (c) CNC/Si-1(48h) and (d) CNC/Si-1-HF.
structural order of the starting CNC, but stabilizes it by creating desulfated surfaces. The CNCs remain highly crystalline in the films with cellulose Ib form which results in higher brittleness compared to mesoporous photonic cellulose.44 The films show substantially improved stability in water when compared with pure films – even heating in boiling water overnight did not result in dissolution of the films. Nitrogen adsorption measurements of CNC/Si-1(48h) and CNC/Si-HF dried under ambient conditions reveal non-ideal type IV isotherms with strong hysteresis effects (Fig. S6, ESI†). The Brunauer–Emmett–Teller (BET) surface areas were calculated to be 44 m2 g 1 (CNC/Si-1(48h)) and 54 m2 g 1 (CNC/Si-1-HF). These results indicate a mesoporous behavior but with low surface area, likely due to collapse of the structure when the silica was removed. Thus, to improve the porosity, the samples were soaked in ethanol and subsequently dried with supercritical carbon dioxide (sc-CO2) to give highly mesoporous materials. As shown in Fig. 5, type IV isotherms with BET surface areas of 278 m2 g 1 (CNC/Si-1(48h)) and 268 m2 g 1 (CNC/Si-1-HF) with average pore volumes of 0.69 cm3 g 1 (CNC/ Si-1(48h)) and 0.52 cm3 g 1 (CNC/Si-1-HF) were obtained. The calculated Barrett–Joyner–Halenda (BJH) pore-size distributions are around 8.4 nm (CNC/Si-1(48h)) and 6.2 nm (CNC/Si-1-HF), Fig. 5, inset. The nitrogen adsorption studies reveal a slight influence of the silica removal procedure on the physical properties of the final material, but in every case, CNMC shows a higher mesoporosity than mesoporous photonic cellulose produced from CNC/UF.44 It is notable that sc-CO2 drying results in strongly iridescent films including a slight red-shift of the UV/vis reflection band from the dry state (e.g., to 516 nm for CNC/Si-1(48h)), though the films show lower translucence (Fig. S7, ESI†), suggesting a lessordered microstructure. As mentioned, some physical properties can be adjusted by changing the CNC to silica ratio used in preparing the CNC– silica composites. Thus, another CNC–silica composite material with higher silica content was synthesized (CNC/Si-2). The influence of the higher silica content in CNC/Si-2 can clearly be seen by its UV/vis reflection band, which is strongly red-shifted
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Fig. 5 N2 adsorption isotherms and BJH pore-size distributions (inset) of CNC/Si-1(48h) (top) and CNC/Si-1-HF (bottom) after sc-CO2 drying.
to 1183 nm compared to CNC/Si-1 (Fig. S8, ESI†). Removal of the silica from CNC/Si-2 by LiOH treatment (4.2 wt%, 48 h) gave access to CNC/Si-2(48h). The UV/vis reflection band is shifted to 678 nm due to the removal of silica, but is still at longer wavelengths compared to CNC/Si-1(48h). These findings show the possibility to control the color of reflectance of CNMC by varying the silica content. In addition, SEM and CD measurements confirm the retention of the chiral nematic assembly in CNC/Si-2(48h) (Fig. S8, ESI†). Nitrogen adsorption measurements of sc-CO2 dried CNC/Si-2(48h) also reveal an influence of the higher silica content (Fig. S9, ESI†). The BET surface area (314 m2 g 1), the average pore volume (1.24 cm3 g 1) and the BJH pore-size distribution (15.7 nm) are all increased compared to CNC/Si-1(48h), which is a result of the removal of the silica with a greater wall thickness. Thus, by increasing the proportion of silica employed in the CNC–silica composite, the resulting CNMC films show larger surface areas, pore volumes, and pore sizes after silica removal. As expected, this is opposite to the trend observed for chiral nematic silica films obtained by removing the cellulose from CNC–silica composites, where the porosity decreases with higher silica content.29 In order to obtain CNMC materials with identical porosity but different optical properties, CNC–silica composite materials with identical silica to CNC ratios were synthesized. The reflected colors were controlled by the use of different substrates,30 such as
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nematic assembly of CNMC, incorporation of gold nanoparticles (Au NPs) represents an interesting pathway to obtain monodisperse Au NPs within a chiral nematic environment. Gold functionalized cellulose materials have attracted substantial interest as they might serve as sensors, catalysts, membranes, and chromatography materials.58–65
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Incorporation of gold nanoparticles
Fig. 6 Normalized CD (bottom) and UV/vis spectra (top) of CNMC-Al (dotted lines), CNMC-CA (dashed lines) and CNMC-PS (solid lines).
polystyrene (CNC/Si-PS), cellulose acetate (CNC/Si-CA) and aluminum (CNC/Si-Al). The UV/vis reflectance bands of these composite films vary between 1175 nm (CNC/Si-PS), 804 nm (CNC/Si-CA) and B220 nm (CNC/Si-Al), Fig. S10 (ESI†). After removal of the silica with LiOH, CNMC films with UV/vis reflection bands of 474 nm (CNMC-PS), 421 nm (CNMC-CA) and o200 nm (CNMC-Al) were obtained (Fig. 6 and Fig. S10, ESI†). (It was not possible to accurately measure the reflection peak for the CNMC-Al samples owing to UV absorption by the cellulose.) Retention of the lefthanded chiral nematic structure after silica removal was confirmed by CD and SEM measurements (Fig. 6 and Fig. S11, ESI†). Finally, the porosity of the sc-CO2 dried films was determined by nitrogen adsorption (Fig. S12, ESI†). The films show a type IV isotherm with almost identical BET surface areas, average pore volumes and BJH pore-size distributions (Table 1). Thus, it is possible to prepare chiral nematic mesoporous cellulose samples with identical porosity, but different helical pitches (and, hence, colors). The new films of CNMC have potential for incorporation into sensors or security features, as they can change color upon swelling in different solvents. Such investigations were carried out with CNC/Si-2(48h) as a representative example. As shown in Fig. S13 (ESI†) the UV/vis reflection band is red-shifted to 886 nm in ethanol and 1003 nm in water. This behavior is attributed to a change in the helical pitch of the chiral nematic structure since the refractive indices of ethanol and water are similar. The color changes are reversible, and the original color is recovered upon drying. Thus, CNMC might be a useful material for security features with a high opacity in dried state and a high translucence if wetted. So far, we showed the synthesis of novel CNMC materials with tunable optical and physical properties starting from CNC–silica composites. As a result of the mesoporosity and the chiral Table 1 BET surface areas, pore volumes and BJH pore-width distributions of CNC/Si-PS, CNC/Si-CA and CNC/Si-Al
Sample
BET-Ao [m2 g 1]
Pore volume [cm3 g 1]
BJH pore-width [nm]
CNC/Si-PS CNC/Si-CA CNC/Si-Al
290 287 289
0.94 0.93 0.92
11.0 11.0 10.7
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Au NPs were loaded into CNMC by using the wet impregnation method. CNC/Si-2(48h), dried from sc-CO2, was soaked in 5 mM (CNMC-Au-5), 10 mM (CNMC-Au-10) and 100 mM (CNMC-Au-100) aqueous solutions of HAuCl4 for 1 h to ensure maximum loading. The Au(III) ions were finally reduced to Au(0) by the addition of an aqueous NaBH4 solution. The incorporation of Au NPs does not result in a strong color change but the final material shows a weak metallic luster (Fig. S14, ESI†). Thermogravimetric analyses under air (heating rate 10 1C min 1) of the CNMC-Au samples showed a sharp mass loss beginning at around 300 1C (Fig. S15, ESI†). The decomposition was almost complete at around 450 1C, showing the acceleration of cellulose decomposition by the presence of Au NPs. Such a behavior is well known for metal NP–cellulose composite materials.45,66,67 The final mass losses were calculated to 99.2% (CNMC-Au-5), 99.0% (CNMC-Au-10) and 98.7% (CNMC-Au-100) representing a loading of the CNMC with 0.8 wt%, 1.0 wt% and 1.3 wt% Au NPs, respectively (assuming no oxidation of the Au NPs). Nitrogen adsorption measurements of sc-CO2 dried CNMCAu-5, CNMC-Au-10 and CNMC-Au-100 were performed to study the influence of the Au NP incorporation on the porosity of CNMC. The BET surface areas were determined to be around 280 m2 g 1 with average pore volumes around 1 cm3 g 1 whereas the BJH pore size distributions show a pore width of approx. 13.5 nm (Fig. S16 and Table S1, ESI†). Thus, these results reveal that the amount of loading does not significantly affect the porosity of the CNMC. However, compared to CNC/Si-2(48h) the BET surface areas and average pore widths are slightly decreased, consistent with incorporated particles. Scanning electron microscopy studies have shown a slight distortion of the chiral nematic assembly, which is attributed to the supercritical drying before the loading (Fig. S17, ESI†). Such a behavior is also known to occur for mesoporous photonic cellulose.44 Transmission electron micrographs were obtained for CNMC-Au-10 and reveal the stabilization of spherical, almost monodisperse Au NPs with a diameter of 25 1 nm (Fig. 7 and Fig. S18, ESI†). Thus, the mesopores of CNMC stabilize isolated Au NPs without the addition of any surfactants similar to the observations for Au NP decorated photonic cellulose.45 The obtained particle sizes for CNMC-Au-10 are comparable to Au NPs on chiral nematic silica composites (28 21 nm) but show a much lower polydispersity.42 Several trials to obtain suitable TEM images or even stable sections of CNMC-Au-5 and CNMC-Au-100 failed due to separation issues during sectioning and measuring of the cellulose–resin composites (see comment in Experimental section). However, TEM images of CNMC-Au-10 at low magnification reveal a low amount
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Fig. 7 TEM images of CNMC-Au-10.
of loading as already determined from TGA measurements. These findings are confirmed by PXRD as peaks for gold were only obtained for CNMC-Au-100 where the crystallite size was calculated to be 34.1 nm (Fig. S19, ESI†). For CNMC-Au-5 and CNMC-Au-10, a combination of too low loading and small crystallite sizes hinders the formation of a necessary amount of diffraction centers. However, the degree of crystallinity for the CNC crystallites was additionally calculated to be between B80% (CNMC-Au-10) and B86% (CNMC-Au-5) by using the Ruland–Rietveld analytical approach. These results indicate that the incorporation of Au NPs does not adversely affect the crystallinity of the CNMC. In addition to those analyses, the identity and distribution of the Au NPs were examined by EDX measurements (Fig. S20, ESI†). The EDX spectra clearly show the presence of gold in all of the materials and EDX mapping confirmed their homogeneous distribution. Finally, the gold-loaded CNMC samples were analyzed by optical spectroscopy to determine the interaction of the surface plasmon resonance (SPR) of the Au NPs with the still intact chiral nematic structure of CNMC. As shown in Fig. 8 small deviations are obtained for the UV/vis reflection bands depending on the amount of gold loading. CNMC-Au-5 shows a UV/vis reflection band at 491 nm assigned to the chiral nematic structure whereas those bands are shifted to longer wavelengths for CNMC-Au-10 (495 nm) and CNMC-Au-100 (514 nm). Additionally, CNMC-Au-10 and CNMC-Au-100 show a second UV/vis band at 609 nm and 598 nm, respectively. These additional UV/vis bands are attributed to the SPR of the incorporated
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Au NPs, and the intensity of the signal is stronger for CNMC-Au-100 than for CNMC-Au-10 as a result of a higher amount of loading. In addition, CD spectroscopy confirmed the left-handed helical assembly of all gold-loaded CNMC materials due to signals with positive ellipticity (Fig. 8). These broad CD signals centered at 484 nm (CNMC-Au-5), 495 nm (CNMC-Au-10) and 532 nm (CNMC-Au-100) confirm the origin of the UV/vis reflection bands and show a small influence of the amount of gold loading. To eliminate the influence of linear dichroism effects, samples where additionally rotated perpendicular to the light beam, resulting in similar CD spectra. For CNMC-Au-100, an additional positive CD signal at 571 nm is obtained which is assigned to the SPR of the Au NPs. Thus, the SPR of the Au NPs interacts with the chiral nematic CNMC to give plasmonic chiroptical activity. Similar observations were obtained for metal loaded chiral nematic silica and cellulose samples.41,42,45,68 However, to clarify that the chiroptical properties are a result of the organization of the Au NPs within the chiral nematic environment of CNMC, we first prepared a CNC–silica composite without chiral nematic ordering by acidification during the EISA process.41 Treatment with an aqueous 4.2 wt% LiOH solution over 48 h gave access to a non-chiral nematic cellulose material which was additionally loaded with Au NPs using an identical synthetic procedure than for CNMC-Au-100. The obtained material only shows one UV/vis band at 581 nm for the SPR of the Au NPs and lacks the broad UV/vis reflection band due to the missing chiral nematic ordering (Fig. S21, ESI†). These observations are confirmed by CD spectroscopy where no signal was detected (Fig. S21, ESI†). The results indicate that the observed chiroptical properties of CNMC-Au-100 are not simply due to interactions between the surface of the Au NPs and the molecular chirality of the cellulose surface. Two research groups have independently investigated the co-assembly of gold nanorods with cellulose nanocrystals.68–70 The interaction of surface plasmon resonances of gold nanorods with the chiral nematic assembly of CNC was shown to yield tunable chiroptical activities.68 The responses were dependent on the amount of gold loading and the size of the gold nanorods, resulting in positive CD signals between 500 nm and 700 nm, similar to the results presented in this study. However, the authors also pointed out that interference effects might influence the observed CD signals. In the relatively new field of nanoparticleloaded chiral nematic composite materials, only a few publications are available that describe possible mechanisms of the origin of chiroptical activities.71–75 A recent review article also points out that severe conditions of helicity and interparticle interactions have to be met for enhancement of chiroptical activities.76 Thus, further research in this field is needed to understand the obtained chiroptical effects in detail as those materials might give access to attractive optical effects and devices.77
Experimental Materials and instrumentation Fig. 8 Normalized CD (bottom) and UV/vis spectra (top) of CNMC-Au-5 (solid lines), CNMC-Au-10 (dashed lines) and CNMC-Au-100 (dotted lines).
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All compounds were used as received without any further purification. Aqueous suspensions of cellulose nanocrystals
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(CNCs) were supplied from CelluForce Inc. as an acidic form (CNC-H: 4.1 wt%, pH = 2.52). The CNC suspensions were obtained by using a previously described procedure.29 Thermogravimetric analysis was done with a Pyris 6 (PerkinElmer) under air with a heating rate of 10 1C min 1. CD spectra were obtained on a J-710 spectropolarimeter (JASCO) by mounting the films between two quartz slides perpendicular to the beam path. As mentioned in the manuscript, some samples were rotated in order to confirm that the results were not dominated by linear dichroism effects. UV-vis/ NIR spectra were obtained on a Cary 5000 UV-vis/NIR spectrophotometer by using the same sample preparation techniques as for CD spectroscopy. Nitrogen adsorption isotherms were acquired at 77 K using a Micromeritics ASAP 2020 analyzer. Pore-size distributions were calculated from the adsorption branches using the Barrett–Joyner–Halenda (BJH) method. Electron microscopy studies were conducted on a Hitachi S4700 (SEM) with sputter-coated samples (5.0 nm of gold/ palladium 60/40) and on a Hitachi H7600 electron microscope (TEM). For our TEM experiments we used HM20 resins as a support for the cellulose samples. However, several tries to section the prepared materials failed. The cellulose samples separated from the resin after sectioning due to incomplete penetration. Numerous attempts to use Spurr’s resin instead of the HM20 resin or even cryosectioning were also unsuccessful. We were only able to get a few stable sections of sample CNMC-Au-10 which were used as a representative sample in this study. The nanoparticle size distributions were calculated from TEM images using the LINCE (linear intercept) software (TU Darmstadt). The quoted error represents the standard deviation in the particle size distribution. Powder X-ray diffraction studies were measured on a D8 Advance (Bruker) using Cu-Ka irradiation and a NaI scintillation detector. The deconvolution of the peaks of the diffraction patterns was done by the use of DIFFRACplus TOPAS software (Bruker-AXS) on the basis of the Rietveld refinements.78 The degree of crystallinity Xc (the fraction in weight occupied by the crystallites) was calculated using Ruland’s theoretical approach.79 Critical point drying with supercritical CO2 was carried out on an autosamdry-815 critical point dryer (Tousimis Research Corporation). Synthesis of CNMC CNC–silica films were synthesized by a similar procedure to that previously described.29 Usually, polystyrene dishes with a diameter of 60 mm were used. After mixing the appropriate amount of CNC suspension and Si(OMe)4 for 1 h, 3.65 mL of the mixture was placed onto the dishes under ambient conditions for EISA. CNC–silica composite films were synthesized according to a literature procedure. The helical pitch of the CNC–silica composite was varied by using different ratios of Si(OMe)4 : CNC on polystyrene dishes (CNC/Si-1: 2.0 mL TMOS per 1 mg CNC; CNC/Si-2: 3.0 mL TMOS per 1 mg CNC). In addition, the helical pitch of the CNC–silica composite was varied by using different substrates like aluminum (CNC/Si-Al), cellulose acetate (CNC/Si-CA) and polystyrene (CNC/Si-PS) with an identical Si(OMe)4 : CNC ratio of 1.8 mL TMOS per 1 mg CNC.
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For those experiments, the borders of polystyrene dishes (60 mm in diameter) were mounted onto aluminum foil or cellulose acetate sheets to obtain films with an identical size. Then the same EISA procedure as above was used. The chiral nematic composite films obtained after complete evaporation of water were treated with a 4.2 wt% LiOH(aq) solution for 48 h or with 5% HF(aq) for 15 min to remove the silica and obtain pure cellulose films (10 mL solution per 100 mg composite). The cellulose films were extensively washed with a 1 : 1 ethanol/water mixture and finally dried from ethanol under ambient conditions or by supercritical carbon dioxide. Metal nanoparticle incorporation 10 mg of supercritical carbon dioxide-dried CNC/Si-2(48h) were soaked in 5 mL of 5 mM, 10 mM or 100 mM aqueous solutions of HAuCl4 for 1 h. Afterwards the films were washed several times with water and ethanol and dried under ambient conditions. After reduction with a freshly prepared 200 mM NaBH4(aq) solution (10 mL) for 1 h, the films were extensively washed with water and ethanol, and finally dried under ambient conditions. Synthesis of achiral CNC–silica composite The achiral CNC–silica composite was synthesized by adding a 1 M aqueous HCl solution to the CNC suspension until a pH of 2 was reached. After mixing of the acidic CNC suspension with Si(OMe)4 (3.0 mL Si(OMe)4 per 1 mg CNCs), the mixture was cast into polystyrene dishes (60 mm in diameter; 3.65 mL of mixture per dish), yielding achiral composite films after evaporation of the water. After removal of the silica by treatment with a 4.2 wt% LiOH(aq) solution for 48 h, the obtained film was washed (H2O, EtOH), dried with supercritical CO2 and finally loaded with Au NPs using the procedure given in the manuscript (100 mM aqueous HAuCl4 solution).
Conclusions In summary, we developed synthetic routes to obtain novel chiral nematic mesoporous cellulose (CNMC) materials from CNC–silica composites. The optical and physical properties of the final material can be readily adjusted. Thus, CNMC materials with different helical pitches but identical porosity can be obtained. As well, CNMC materials with very high surface areas (up to 314 m2 g 1) and large average pore widths (up to 15.7 nm) are accessible. Characteristically, the CNMC materials show a high degree of crystallinity that is comparable to pure CNC films indicating no adverse effect of the preparation process. In addition, the materials were used to stabilize gold nanoparticles whose size can be controlled by changing the concentration of the gold precursor solution. These composite materials show chiroptical properties due to the interaction of the surface plasmon resonance of the gold nanoparticles with the chiral nematic environment provided by the CNMC host. The new CNMC and Au@CNMC composite materials presented here have potential utility in novel biosensors, security features, membranes, and chromatography materials.
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Acknowledgements We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and CelluForce Inc. for their support. M.S. is grateful to the German Academic Exchange Service (DAAD) for a postdoctoral fellowship.
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Soft Matter
