A Metal‐Free, Dual‐Mode Colorimetric and Fluorometric Sensing of Rhodamine‐B Using Multifaceted Biogenic Carbon Quantum Dots: A Biocompatible Probe for Biological and Plant Confocal Imaging

In this study, we present the green synthesis and multifunctional potential of amino-functionalized carbon quantum dots (ACQDs) derived from Withania somnifera (ashwagandha) for the sensitive detection of Rhodamine B (RhB), a toxic dye commonly found in adulterated food and cosmetic products. The ACQDs exhibited quasi-spherical morphology with a uniform size distribution (2.82 ± 0.07 nm), abundant surface functional groups (-OH, -COOH, -NH₂), and strong photoluminescence. Structural and optical analyses confirmed their excellent stability across a range of environmental conditions, including pH, UV exposure, and saline media. A dual-mode sensing of RhB was achieved via visible colorimetric change and fluorescence quenching, with a low quantification limit (LOQ) of 1.9 μgL^-1. Mechanistic studies revealed that both static and dynamic quenching, involving electrostatic interactions, hydrogen bonding, and π-π stacking, contributed to the sensing process. The ACQDs also demonstrated notable antioxidant activity (IC50 = 0.168 mg mL^-1) and low cytotoxicity (< 90 μg mL^-1) in NIH-3 T3 cells, confirming their biocompatibility. Furthermore, their efficient internalization into plant tissues highlights their suitability for plant imaging. These results establish ACQDs as a promising nanomaterial for applications in environmental monitoring, food safety, biomedical diagnostics, and agricultural research.