Modulating the fluorescence properties of carbon dots by varying the stoichiometric ratios of citric acid and urea during the synthesis

Varying the ratios of citric acid and urea during the synthesis of carbon dots (CDs) provides a strategy to modulate their fluorescence properties. The impact of different precursor ratios on the resulting CDs' size, surface chemistry, and structure was investigated using X-ray diffraction (XRD) analysis and confirmed through transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) was employed to study the influence of precursor ratios on the types and densities of surface functional groups. The optical properties of the CDs were examined using a UV–visible absorption spectrometer, photoluminescence excitation (PLE) spectra, and photoluminescence spectroscopy (PL). Smaller CDs generally exhibit higher energy emissions, resulting in bluer light, while larger CDs tend to emit lower energy light, appearing redder. By adjusting the precursor ratios, it is possible to induce shifts in fluorescence emission, leading to CDs with different fluorescence colours. Furthermore, the antimicrobial activity of CDs against bacterial pathogens was assessed, and their potential for promoting plant growth was explored by coating seeds with concentrated and diluted CDs solutions.