Tailoring Graphene Quantum Dots through the Pyrolysis of Citric Acid, Phenylalanine, and Tryptophan: Effect of Precursor Ratios on Synthesis Efficiency and Properties

Graphene Quantum Dots (GQDs) are gaining significant attention due to their unique optical, electronic, and biocompatible properties, making them ideal candidates for applications in bioimaging, sensing, and drug delivery. This study explores the synthesis of GQDs derived from citric acid (CA), phenylalanine (Phe), and tryptophan (Trp) using a pyrolysis method, where GQDs were synthesized using 2.0 g of CA with varying amounts of Phe (0.75 g, 0.50 g, 0.25 g) and Trp (0.25 g, 0.50 g, 0.75 g), corresponding to GQDs1, GQDs2, and GQDs3, respectively. The influence of precursor composition on the structural, optical, and physicochemical properties of GQDs was analyzed. Particle size measurements showed a hydrodynamic diameter range of 0.89 nm to 1.5 nm, with increasing Trp content leading to larger particles and a broader size distribution, reflected in polydispersity index (PDI) values of 0.221, 0.312, and 0.368 for GQDs1, GQDs2, and GQDs3, respectively. Zeta potential analysis revealed values of -21.4 mV, -12.2 mV, and -7.5 mV for GQDs1, GQDs2, and GQDs3, respectively, indicating reduced surface charge with higher Trp content, which may affect colloidal stability. Optical characterization showed π→π* (~230–270 nm) and n→π* (~300–350 nm) transitions in the UV-Vis spectra, with varying absorbance intensities across samples. Fluorescence spectroscopy confirmed strong emission properties, which were highly dependent on precursor ratios. Quantum yield (QY) values were 32.2%, 95.5%, and 75.6% for GQDs1, GQDs2, and GQDs3, respectively, highlighting the role of nitrogen doping in fluorescence enhancement. These findings demonstrate that controlled precursor composition can fine-tune GQD properties, offering potential for optoelectronic, bioimaging, and sensing applications. Further exploration of functionalization strategies could enhance their practical utility.