Fabrication And Applications Of The Surface-Functionalized Cellulose Materials

With the rapid development of the global economy and industry, nonrenewable resources such as coal, petroleum and natural gas has been excessively consumed, giving rise to the serious environmental pollution, therefore, more and more attention was paid to the renewable resources. Among them, cellulose substance is the richest raw material in the word, which possesses good biocompatibility, biodegradability, flexibility, nontoxicity and porosity, hence the research regarding the functionalization and application of cellulose materials becomes one of the hottest spots in material research community. Meanwhile, various functional cellulose materials like membrane materials, luminescent materials, smart responsive materials, hydrophobic materials, antibacterial materials, gel materials and medical materials have been largely explored, being the focus in the scope of cellulose science.Herein, employing natural cellulose substance (ordinary quantitative filter paper) as substrate and metal alkoxide (titanium(IV)n-butoxide or zirconium(IV)n-butoxide) as precursor, an ultrathin metaloxide (TiO2or ZrO2) gel film was first deposited onto cellulose nanofibers of filter paper by means of a surface sol-gel process. The deposited metaloxide gel layer activated the relatively inert surface of cellulose fibers and favord the following immobilization of different functional molecules or guest substances onto its surface, which endowed the cellulose materials with desired functionalities and properties, resulting in lots of advanced functional cellulose materials. The details are described as follows.(1) Oligonecleotide-functionalized cellulose material:Employing zirconium(IV) n-butoxide as precursor, uniform ultrathin zirconia gel film was first deposited on each cellulose nanofiber in bulk filter paper by a facile surface sol-gel process. Relying on the large surface area and strong affinity of zirconia for phosphate group, terminally phosphated probe DNA was abundantly immobilized on the cellulose fibers so as to convert the composite into a biofunctional material for sensitive, selective and repetitive recognition for the corresponding complementary target DNA at a nanomolar level. By contrast, in spite of the viability of immobilization of probe DNA and recognition of target DNA on quartz plate, the amount of captured probe DNA or recognized target DNA on such flat substrate, was much less than that captured or recognized on filter paper, resulting in relatively insensitive recognition event. Moreover, control experiments on bare filter paper (without zirconia nanocoating) suggested that the zirconia gel film was essential for probe DNA immobilization and the subsequent target DNA recognition.(2) Cellulose based colorimetric cysteine (Cys) chemosensor:Employing zirconium(Ⅳ) n-butoxide as precursor, uniform ultrathin zirconia gel film was first deposited on each cellulose nanofiber in bulk filter paper by a facile surface sol-gel process. Immobilization of ruthenium dye-Hg2+complex (N719-Hg2+complex) monolayer on such ultrathin zirconia gel film coated cellulose nanofibers of filter paper was conducted to fabricate a solid-phase sensing device with high sensitivity, selectivity and reversibility for colorimetric detection of Cys in aqueous media. This assay relied on the specific mercury displacement by cysteine from the immobilized N719-Hg2+complex. The dissociation of N719and Hg2+caused by such mercury displacement engendered obvious orange-to-purple color change of the zirconia gel layer and N719-Hg2+complex modified filter paper as well as the corresponding band shift from480nm to520nm in UV-vis spectra, hence realizing the colorimetric detection of cysteine. The detection limit of this assay is20μM by the naked eye and the selectivity for Cys against interference of the other19natural amino acids and their mixture is extremely high. Additionally, the deposition of the zirconia gel film onto cellulose fibers was crucial, and without the metal oxide layer, immobilization of the N719-Hg2+complex and subsequent Cys sensing would not be accomplished.(3) Antibacterial cellulose material:organic/inorganic hybrid materials comprised of titania/chitosan composite were uniformly coated on cellulose nanofibers of filter paper by alternately depositing titania gel layer and chitosan layer for a given number of times. The resultant surface-modified cellulose nanofibers possessed a cable-like core-shell structure, showing cellulose nanofiber core adhered with flat and homogeneous titania/chitosan composite shell, whose average thickness is14nm for10-cycle deposition of titania/chitosan bilayer. By adsorbing silver ions with titania/chitosan composite modified paper followed by reducing the immobilized silver ions with UV irradiation, another kind of organic/inorganic hybrid nanocoating composed of titania/chitosan/silver composite was uniformly deposited on cellulose nanofibers of filter paper. Such resultant surface-decorated cellulose nanofibers also possessed a core-shell structure, exhibiting cellulose nanofiber core coated with14-nanometer-thick titania/chitosan/Ag composite shell for10-cycle titania/chitosan bilayer deposition as well as polydispersed nanosized silver particles (4-20nm in diameter) well distributed on the shell. Moreover, antibacterial tests suggested that the titania gel film coated cellulose papers and the titania/chitosan composite modified cellulose papers exhibited moderate antibacterial activity against E. coli and S. aureus with the antibacterial effects not more than60%, while the titania/chitosan/silver composite decorated cellulose papers nearly disinfected all the inoculated bacteria due to the high loading of Ag NPs, showing splendid antibacterial activity.(4) Catalytic cellulose material:Employing titanium(IV) n-butoxide as precursor, uniform ultrathin titania gel film was first deposited on each cellulose nanofiber in bulk filter paper by a facile surface sol-gel process. Then, cationic polyelectrolyte layer PDDA as well as the two-component layer containing anionic polyelectrolyte PSS and KAuCl4were alternatively self-assembled onto the titania gel film pre-coated cellulose nanofibers of filter paper through electrostatic interaction. Followed by treatment with aqueous solution of NaBH4, the immobilized KAuCl4molecules were in situ reduced to Au NPs with the diameters about5nm, thus yielding Au NPs modified cellulose material, which inherited the original morphological characteristics of virgin cellulose substance with the hierarchically fibrous structures and exhibited typical SPR band around520nm in UV-vis spectra. Such Au NPs modified cellulose material also showed satisfactory catalytic activity toward the reduction reaction of4-nitrophenol (4-NP) with NaBH4in aqueous environment. By filtering10mL aqueous solution containing4-NP (0.1mM) mixed with NaBH4(10mM) through the Au NPs modified cellulose material (cellulose/(TiO2)5/Au@(PDDA/PSS)10) for one time, the conversion ratio of4-NP reached up to74.0%, which could be further promoted to98.7%after triple filtrations. In sharp contrast, pure filter paper, the titania gel film coated paper and the titania/PDDA/PSS multilayer modified paper displayed neither catalytic effect toward such reduction reaction nor apparent physical adsorption of4-NP.