Simple extraction of organosolv lignin from rice husks and its application in N,S-doped carbon quantum dots as fluorescent sensor for Fe3+ ions

The conversion of lignin into high-value-added materials not only helps alleviate the energy crisis but also provides an efficient waste management solution. In this study, we present a simple extraction method of organosolv lignin from rice husks and demonstrate its application as a precursor for synthesising nitrogen and sulphur co-doped carbon quantum dots (N,S-LCQDs). The prepared N,S-LCQDs exhibited strong blue fluorescence with excellent photostability and were successfully employed as a selective fluorescent sensor for Fe3+ ions in aqueous solutions. Spectroscopic analysis revealed that the N,S-LCQDs displayed excitation-dependent emission behaviour with a quantum yield of approximately 9.7%. The fluorescence intensity of N,S-LCQDs decreased progressively with increasing Fe3+ concentration, exhibiting a good linear relationship over the range of 0–100 μM and a detection limit of 0.91 μM. The sensing mechanism was thoroughly investigated using Stern-Volmer analysis at different temperatures and fluorescence lifetime measurements. This work not only provides a sustainable approach for valorising rice husk biomass but also develops a cost-effective, sensitive, and selective fluorescent probe for Fe3+ detection in environmental and biological samples.