Insights into the effect of aggregation on lignin fluorescence and its application for microstructure analysis

As a natural fluorescent material, the fluorescent property and mechanism of lignin were elusive until now, which hindered the high value application of lignin fluorescence. Herein, we firstly probed the previous studies on lignin fluorescence and the results indicated that lignin microstructure was an important factor for its complex fluorescence property because of fluorophore interaction and aggregation behavior. Following the rules, lignin fluorescence was explored by analyzing its aggregation fluorescence behaviors and basic fluorescence properties based on the theory of traditional conjugated luminescence and aggregation-induced emission. It was demonstrated that intermicellar aggregation of loose lignin micelle made no substantial effect on lignin fluorescence, while intramicellar aggregation could induce the enhancement of lignin fluorescence before the micellar compactness exceeded a critical value. Combined with the physicochemical structures and fluorescence properties of lignin, aggregation-induced conjugation from phenylpropane units was believed as the main sources of the visible emission of lignin and different phenylpropane aggregates consequently formed the multi-fluorophore system in lignin micelle. Furthermore, lignin aggregation fluorescence behavior has great potential in its microstructure analysis and a case study of pH/ionic strength-induced solution behavior analysis was presented. This work provided a totally new prospective for lignin fluorescence.