Sodium alginate-modified laser-induced graphene microalgal biosensor for sensitive monitoring of PS-II-inhibiting herbicides.

The widespread use of herbicides and their persistent occurrence in aquatic environments necessitate the development of rapid, sensitive, and biologically relevant monitoring strategies. In this study, a sodium alginate-modified laser-induced graphene (LIG)-based microalgal biosensor is developed for the photoelectrochemical monitoring of Photosystem-II (PS-II)-targeting herbicides. The platform integrates intact Nannochloropsis sp. Cells immobilized within a biocompatible sodium alginate matrix on a conductive LIG electrode, enabling direct monitoring of herbicide-induced inhibition of photosynthetic activity. The modified LIG architecture enhances electron-transfer efficiency and signal stability, while sodium alginate improves cell immobilization and operational robustness. The proposed biosensor demonstrates low nanomolar detection limits of 60.86 nM for monuron and 178.32 nM for propazine. In comparison, diuron exhibited comparatively higher detection limits (225 nM for Nannochloropsis sp. and 900 nM for Chlorella sp.), reflecting analyte-dependent sensitivity. These herbicides inhibit PS-II, disrupting photosynthetic electron transport in microalgae and generating measurable photoelectrochemical responses. Owing to its PS-II-based sensing mechanism, the platform is particularly suited for the rapid toxicity screening of PS-II-inhibiting herbicides rather than the selective identification of individual compounds in mixed samples. The biosensor showed excellent reproducibility, cross-platform consistency, and reliable performance in environmentally relevant matrices. Overall, the developed sodium alginate-LIG microalgal platform provides a simple, cost-effective, and sustainable approach for on-site assessment of herbicide toxicity and environmental water quality monitoring.