• A novel fluorescence turn-on sensor for Cr3+ detection is developed using FRET between rhodamine B and gold nanoparticles, achieving a low detection limit of 0.95 nM.
• The sensor operates via Cr3+-induced aggregation of AuNPs, leading to fluorescence recovery and a visible color change to black, enabling rapid and simple detection.
• The method offers high sensitivity, selectivity, and ease of preparation, making it suitable for environmental and biological monitoring of Cr3+.
• The proposed sensor addresses limitations of traditional techniques by providing a low-cost, real-time, and non-time-consuming alternative for Cr3+ analysis.