A pH-regulated single-material fluorescence sensor array with tunable response patterns for discrimination and...

A pH-regulated analytical strategy was developed for the discrimination and concentration-dependent analysis of metal ions using a single-material fluorescence sensor array based on nitrogen and sulfur co-doped carbon dots (NS-CDs). The NS-CDs were synthesized through an l-cysteine-assisted alkaline reaction, which increased the quantum yield from 3.80% to 9.59% and introduced abundant surface functional groups for metal ion interactions. Under different pH conditions, the same sensing material generated distinct fluorescence response patterns toward Hg²⁺, Cr⁶⁺, and Mn⁷⁺, enabling array-based recognition. Mechanistic investigations indicated that the differential responses originated from coordination, redox processes, and inner filter effect-related contributions, which were supported by XPS, FTIR, fluorescence lifetime analysis, and density functional theory calculations. By combining pH modulation with multivariate statistical analysis, including principal component analysis, linear discriminant analysis, and hierarchical cluster analysis, accurate discrimination of the three metal ions was achieved. Leave-one-out cross-validation provided high classification accuracy (>93%), while logistic regression-based machine-learning classification achieved a predictive accuracy of 96.0%. The proposed sensing strategy also enabled concentration-dependent analysis and showed reliable performance in real water samples, affording an overall identification accuracy of 92.6% for unknown samples. This work provides a simple and effective analytical methodology for constructing single-material sensor arrays with tunable response diversity for metal ion detection in practical water samples.