Development of lignin-based photo-responsive moiety hybrid compounds with the photophysical & photochemical properties studies and the applications

Abstract

Lignin is the most abundant renewable aromatic carbon on Earth. It is also an excellent alternative source to provide the essential chemical precursors or polyphenolic carbon skeleton in the production of functional materials. Upon the advancements in lignocellulosic biomass refinery, various novel chemical building blocks and the lignin based functional materials have been derived from lignin biopolymer. Among these functional materials, only few of them have explored the integration of photo-responsive chemical moieties with lignin as the synthesis strategy. In this dissertation, two classical photo-responsive chemicals, i.e., porphyrin and photoacid were hybridized with alkali lignin and synthesized into novel functional nanomaterials (i.e., AL-CTPP and AL-Por-PP). The grafted chemical moieties introduced outstanding photo-physiochemical features to lignin, of which demonstrated its abilities to enhance or/and control the photo-physiochemical functions of the graft chemical moieties. When the porphyrin integrated with the lignin polymer in AL-CTPP, this combination dramatically reduced the challenging aggregate-caused-quenching effects in fluorescence of porphyrin moieties. AL-CTPP obtained >50 folds stronger fluorescent intensity than that of porphyrin monomer in aqueous environment (pH7). In AL-Por-PP, lignin's primitive structural advantages were further applied to harness the excited-state proton transfer (ESPT) dynamics of the grafted photoacid moieties. AL-Por-PP nanohybrids showed its abilities to retard the ESPT from pH 1 to pH 12. Moreover, by systematically fine adjusting the lignin content in AL-Por-PP, the desirable fluorescent intensities ratio of dual emissions from different forms of excited photoacid could be obtained. AL-Por-PP demonstrated an outstanding selectivity of fluorescent responses to an essential index of asthma and lung cancer, bisulfite ions, in the concentration range of 1 to 20 μM (R2 = 0.99). ESPT retardation of AL-Por-PP were selectively terminated by bisulfite ions and leading the strong fluorescence due to the ESPT process of photoacid. AL-Por-PP showed outstanding biocompatibility as over 94 % of cells were viable even 360 μg mL−1 of AL-Por-PP were applied for 24 hours incubation, hence demonstrated to be as a potential fluorescent live cell imaging agent for in vivo bisulfite concentrations detection. Finally, with comparable quantum yield of photo-trigger singlet oxygen generation, AL-Por-PP demonstrated robust structural advantages from the design of synthetic approach to achieve the multifunctionality as a smart functional material. This dissertation offers a sustainable and promising approach for extending the functionalities of lignin and highlights the prospects of valorizing lignin and plant extracts for biomass-based photo-responsive molecular device development and applications.