Lagdhir, J.; Bhalerao, S.; Parmar, B.; Bhatia, D.

Conventional fluorescent imaging probes, including organic dyes and semiconductor quantum dots, suffer from inherent limitations such as photobleaching, cytotoxicity, poor aqueous dispersibility, and complex synthetic routes, necessitating the development of next-generation nanoscale fluorophores suitable for biological imaging. Carbon dots (CDs) have emerged as a compelling alternative owing to their nanoscale dimensions, tunable photoluminescence, excellent biocompatibility, and amenability to green synthesis from biomass-derived precursors. Herein, we report a comparative synthesis and systematic physicochemical evaluation of nitrogen doped and undoped carbon dots derived from chamomile (Matricaria chamomilla L) extract, prepared via solvothermal and microwave-assisted routes. Among the four synthesized variants CM ST U, CM ST N, CM MW U, and CM MW N the solvothermally synthesized nitrogen doped carbon dots (CM STN) exhibited markedly superior optical performance, characterized by a high fluorescence quantum yield of 57.2%, which is among the highest reported for biomass-derived nitrogen-doped carbon dots. Comprehensive characterization using UV visible spectroscopy, photoluminescence (PL) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential analysis, and atomic force microscopy (AFM) confirmed the nanoscale dimensions (~8.3 nm), surface-rich functional groups, successful nitrogen incorporation (10.86 %), and moderate colloidal stability. Photoluminescence stability studies across seven solvent systems including biologically relevant media phosphate-buffered saline (PBS), Dulbecco's modified Eagle's medium (DMEM), and serum-free medium (SFM) demonstrated sustained fluorescence emission over 72 hours. In vitro cytotoxicity assessment using the MTT assay on RPE1 retinal pigment epithelial cells confirmed high cell viability (>70%) across a broad concentration range over multiple exposure durations. Collectively, these results establish CM ST-N as a highly fluorescent, biocompatible, and colloidally stable nanoprobe with strong potential for fluorescence-based bioimaging applications.