Green synthesis of carbon dots derived from biomass of Polyalthia bullata root extract as colorimetric sensor for selective detection of Fe3+ ions

In this study, carbon dots (CDs) were successfully synthesized using a one-step hydrothermal approach at 200 °C for 15 h using a biomass from the root extract of Polyalthia bullata as a carbon source. The synthesis process did not involve any chemical reagents or surface passivation. The calculated quantum yield (QY) of the prepared CDs in comparison with a quinine sulfate was 7.55 %. Transmission electron microscopy (TEM) revealed that the CDs possessed a spherical shape and narrow size distribution with an average size of ∼3.34 ± 0.76 nm. The hydrodynamic diameter was 39.55 nm using the DLS technique with a PDI value of 0.206. UV–Vis absorption spectrum revealed two absorption peaks at 252 nm and 324 nm which attributed to π-π∗ transition of C=C bonds and n-π∗ transition of C=O bonds in CDs, respectively. The CDs exhibited remarkable fluorescence (FL) with excitation and emission wavelengths of 380 nm and 470 nm, respectively. The CDs showed excitation-dependent FL emission spectra upon excitation wavelengths from 300 to 500 nm with a blue shift. The average FL lifetime of the CDs was 5.634 ns. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) identified numerous functional groups on the surface of CDs, such as carbonyl, hydroxyl, and carboxyl groups. The CDs were employed as a nanoprobe for Fe3+ ions detection using a colorimetric approach by measuring absorbance at 200–700 nm. The sensitivity of the synthesized CDs towards Fe3+ ions was examined at concentrations from 2.5 to 200 µM. Within the Fe3+ concentration range of 0–57 µM, an excellent linear relationship with coefficient of determination (R2) of 0.9926 was observed, yielding a limit of detection of 0.186 µM. The detection method demonstrated satisfactory recovery rates when tested on real water samples such as lake, farm, and tap water.