| dc.description.abstract | 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. | en |
