Synthesis And Applications Of Cellulose Nanocrystals And Its Nanocomposites

Cellelose nanocrystals (CNs) is a kind of new biomass nanomaterilas, which is suit for the application in polymer composites as nanofiller due to its high purity, high crystallite, high Young Modulus and mechanical strength. In addition, CNs also has properties belonging to biomaterials, such as low density, biodegradable, biocompatible and renewable. The chemical modification of CNs and the prepation of hybrid material with CNs and inorganic functional nanomaterials would give more special properties and make CNs show broad applications in many fields.The preparation and chemical modification of cellulose nanocrystals (CNs) were investigated in this paper. Furthermore, the CNs were used as templates to synthesize silver nanoparticles (AgNPs), Ag-Pd alloy and Fe₂O₃, respectively. The applications of CNs/AgNPs, CNs/Ag-Pd and CNs/Fe₂O₃ composites used as multifunctional fillers for polymers, labels for DNA electrochemical sensor and adsorbing materials for water treatment were investigated. The innovations of this paper are synthesis of CNs by TEMPO/NaClO/NaBr oxidation of alkali treated cellulose; synthesis of CNs/Ag-Pd and CNs/Fe₂O₃ composites materials; applications of the composites as labels of novle electrochemical DNA biosensor and water treatment materials.CNs with size about 100-200 nm in length and 10-20 nm in width was prepared from MCC as raw material by H2SO4 hydrolysis. XRD results show that CNs and MCC are the same crystal phase. Carboxylated cellulose nanocrystals (CCNs) was synthesized from CNs by surface chemical modifications using the TEMPO/NaClO/NaBr (TEMPO, 2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation system. The crystal phase of CCNs was the same as the MCC. Furthemore, CNs was prepared from NaOH treated MCC using TEMPO/NaClO/NaBr oxidation system without acid. Morphology of this kind of CNs was the same as the CNs synthesized by H2SO4 hydrolysis.Nanocomposites composed of CCNs and silver nanoparticles (AgNPs) were prepared by liquid phase chemical methods and used as bi-functional nanofillers to blend with waterborne polyurethane (WPU) and polyving akohol (PVA). The characterizations indicated that glass transition temperatures (Tg) and thermal stability of WPU-based composites increased with increasing CCNs content. Tensile strength of WPU-based films increased significantly with filling, but tensile strength decreased with further addition of CCNs. The elongation at break decreased obviously with increasing CCNs content. In contrast with WPU composites, glass transition temperatures (Tg) of PVA composites decreased and thermal stability increased with increasing CCNs content. Tensile strength of PVA-based films increased significantly with filling CCNs. More importantly, WPU/CCNs/AgNPs and PVA/CCNs/AgNPs composite films showed strong antibacterial activities against E. coli and S. aureus.Grafting of the probe DNA onto the CCNs was carried out via carboxyl groups covalently coupled with the -NH2 moiety of the probe sequence by amide linkage (-CONH-) in the presence of EDC and NHS to prepare CCNs/AgNPs-probe DNA probe. The electrochemistry response of target PAT DNA, noncomplementary sequence and blank measurement illustrated that DNA biosensor with CCNs/AgNPs as the probe DNA label had good selectivity for the PAT gene fragment detection. Signal of silver had a linear relationship with the logarithmic value of the PAT gene fragment concentration ranging from 1.0×10^-10 mol/L to 1.0×10^-7 mol/L and the detection limit was 2.3×10^-11 mol/L. These results indicated that CCNs could be used for immobilization of DNA biomolecular and the CCNs/AgNPs nanocomposites could be used as labels of target DNA for electrical detection of PAT gene fragment.Synthesis of Ag-Pd alloy nanopaticles was carried out with CCNs suspension as reaction system by co-reducing metallic cations using NaBH4. Alloy particles with a size less than monometallic AgNPs and PdNPs were readily prepared and dispersed well. The average size of alloy nanoparticles decreased as the increasing molar ratio of Ag/Pd. After hybridization between target DNA and probe DNA, two signals belong to the Ag and Pd could be detected when the CCNs/Ag-Pd nanocomposites as probe DNA labels. The CCNs/Ag-Pd nanocomposites can be used as labels in DNA electrochemical sensor.Iron oxide nanoparticles with different morphologies were synthesized by hydrothermal reaction in the presence of CNs as stabilizing agent. The composites composed of iron oxide and cellulose were characterised by TEM, XRD and FT-IR. The adsorptive capacities of MCC, CNs, Fe₂O₃ and CNs/Fe₂O₃ composite to Cd²⁺, Pb²⁺, Ni²⁺, Cr₂O₇^2-, AsO₄^3- and PO₄^3- were investigated. Comparing with the monocomponet materials, CNs/Fe₂O₃ composite show better adsorptive capacities to the anions and cations.