Functionalization of Cellulose Nanocrystals with Inorganic Nanoparticles
In recent years, cellulose nanocrystals (CNCs) possess attractive features including good mechanical strength, high surface area, low density, crystalline rod-like structure, biocompatibility, biodegradability, and sustainability. In addition, the presence of sulphate ester groups on the surface of CNCs induces colloidal stability through electrostatic repulsion, and the abundant reactive primary hydroxyl groups offer the possibility for further modifications. These inherent properties make CNCs a good supportive substrate for loading inorganic nanoparticles in aqueous media. However, aggregation of CNCs and inorganic nanoparticles are often observed for CNC-inorganic nanohybrids. The aggregation greatly decreases surface area, limits the accessibility of reagents to inorganic nanoparticles, thereby, restricting potential applications. Furthermore, the formation of aggregates can alter the optical, catalytic and chemical characteristics of inorganic nanoparticles. In this study, stable synthetic strategies including physical adsorption, polymer encapsulation, and in-situ deposition were utilized to address the issue of aggregation most commonly observed with these nanoparticles. Fe3O4 nanoparticles, Au nanoparticles and CdS quantum dots were loaded onto CNC surface to achieve advanced applications, taking advantage of their attractive magnetic, catalytic and optical properties respectively. Superparamagnetic Fe3O4 nanoparticles have been widely used in numerous applications. We demonstrated the synthesis of Fe3O4 nanoparticles through coprecipitation method with further stabilization provided by polyvinylpyrrolidone. The Fe3O4 nanoparticles effectively adsorbed onto the surface of CNCs to yield CNC@Fe3O4. A silica shell with controllable thickness was then coated onto CNC@Fe3O4 to protect from the oxidation of the Fe3O4 nanoparticles. Transmission and scanning electron microscopic images confirmed good deposition of Fe3O4 nanoparticles on CNCs rods with a uniform silica coating. Thermal gravimetric analysis confirmed that silica coating significantly enhanced the thermal stability of CNCs, where the onset decomposition temperature of CNC@Fe3O4@SiO2 hybrids increased by 60 °C compared to pristine CNCs. Further application of the CNC@Fe3O4@SiO2 was achieved by grafting β-cyclodextrin (β-CD) onto the silica shell. β-CDmodified CNC@Fe3O4@SiO2 nanorods were found to show good adsorption toward two model pharmaceutical residues: procaine hydrochloride and imipramine hydrochloride. The adsorption capacities of the two residual drugs were calculated to be 13.0 ± 0.09 mg/g and 14.8 ± 0.16 mg/g respectively. Generation 6 poly (amido amine) dendrimer-grafted cellulose nanocrystals (CNC-PAMAM) were obtained via the carbodiimide-mediated amidation method to introduce pH-responsive and fluorescent properties, and metal affinities groups for stabilizing inorganic nanoparticles. The pHresponsive system was investigated and confirmed by zeta potential analyses, UV-vis transmittance data, and interactions between CNC-PAMAM and surfactants were measured by isothermal titration calorimetry. At pH ≤ 4, well-dispersed aqueous dispersions were obtained due to the electrostatic repulsion from protonated amine groups on PAMAM. At pH ≥ 10, stable aqueous dispersions were attributed to the abundant presence of negatively charged carboxylate and sulphate ester groups on CNCs. In addition, strong blue fluorescent emission was observed and investigated using fluorescent spectrophotometry. Moreover, the fluorescent behaviors of CNC-PAMAM were largely influenced by the formation of aggregates. Well-dispersed Au nanoparticles (AuNPs) of 2-4 nm were loaded onto CNC-PAMAM by reducing HAuCl4 using NaBH4 with PAMAM playing the role of nanoreactors. The dendrimer-grafted CNC system was also demonstrated to be an effective reducing agent and stabilizer for the synthesis of AuNPs. The impact of temperature, pH values, and CNC-PAMAM concentration on size distribution of AuNPs was studied. A very small size distribution of 10-20 nm was achieved under pH 3.3 at 25 °C with a CNC-PAMAM concentration of 0.008 wt.%. The as-prepared nanocomposites displayed superior catalytic properties with turnover frequencies of up to 5400 h-1 towards the reduction of 4- nitrophenol to 4-aminophenol. The enhanced catalytic performance may be attributed to the improved dispersibility and accessibility of AuNPs within the PAMAM dendrimer domain. Thus, we succeeded in demonstrating the versatility of CNC-PAMAM both as an effective nanoreactor and a reducing agent for AuNPs. Quantum dots (QDs) are attractive in bioimaging application because of their inherent size-tunable emission, flexible excitation wavelength, and good photochemical stability. Particular attention was devoted to the aqueous phase synthesis of QDs that avoids the use of organic solvents and subsequent tedious phase transfer procedures. For the first time, carboxylated CNCs were used as templates to stabilize CdS QDs and CdS@ZnS core-shell QDs in aqueous phase. High colloidal stability was achieved with sufficient negative charge on the CNC surface, and the coordination of Cd2+ to carboxylate groups allowed in-situ nucleation and growth of QDs on CNC surface. The influences of CdS/CNC ratio, pH, and ZnS/CdS ratio on colloidal stability and photoluminescence property were also studied. The results showed that products with excellent colloidal stability and the highest photoluminescence intensity could be obtained at pH 8 with a CdS/CNC weight ratio of 0.19 and a ZnS/CdS molar ratio of 1.5. The as-prepared CNC/CdS@ZnS exhibited long-term colloidal and optical stability. Using biocompatible CNCs as stabilizers, the products have been demonstrated to exhibit low adverse cytotoxicity effects towards HeLa cells, and can serve as promising red-emitting fluorescent bioimaging probes. Stable colloids are promising building blocks for fabricating functional thin films using the layerby- layer (LBL) self-assembly method. We report for the first time the use of CdS QD-functionalized CNC colloids for fabricating nanothin films via LBL self-assembly. Both negatively- and positivelycharged CNC/QD nanohybrids with high colloidal stability and narrow particle size distribution were synthesized, so electrostatic interaction between the two building blocks can form during the coating process. The controllable LBL coating process was confirmed by SEM images and ellipsometry data. The rigid structure of CNCs leads to nanoporous structured films, and the coated poly(ethylene terephthalate) (PET) substrates display high transmittance (above 70%) over the entire range of visible light and strong hydrophilicity with contact angles around 40. Most significantly, both tunable structural colours from thin-film interference and adjustable photoluminescence colours from embedded QDs were achieved. These coated PET substrates showed good flexibility, and strong stability in both water and ethanol. The modified PET films displayed exciting applications in anticounterfeiting and security protocols with structural colours from thin-film interference and photoluminescence from QDs. In conclusion, Fe3O4, AuNPs, and CdS QDs were uniformly loaded onto the surface of CNCs through different aqueous synthetic protocols. The well-developed CNC@inorganic nanoparticle systems displayed promising applications as efficient adsorbents for drug removal, catalysts for chemical reduction of 4-nitrophenol, fluorescent emitting bioimaging probes for cancer cells, and structural nanobuilding blocks for thin films.