Synthesis of N-doped carbon quantum dot and its application on sensing

The aim of this work is to synthesize and characterize nitrogen-doped carbon quantum dots (N-CQDs) and investigate their opto-electrical, photonic and thermal sensing properties. N-CQDs were prepared by mixing 2 g of citric acid with 0.02 g of urea and heating the mixture in a 1100 W microwave oven at 200–250 °C for 5 min. The resulting dark-brown powder was analyzed using FTIR, which confirmed the presence of functional groups at 641.24 cm⁻¹ (–P–O, –S–O, aromatic –CH), 1177.18 cm⁻¹ (–O–C linkages), 1721.05 cm⁻¹ (C = C), 2938.71 cm⁻¹ (CH₂/CH₃), and a distinct peak at 1585.43 cm⁻¹ corresponding to C = N stretching, indicating successful nitrogen doping. In the XRD patterns is display a peak at around 20°, indicating and ordered carbon atom in N-CQDs. Optical characterization through the visible spectrometer and PL studies at different temperatures along with laser exposure displayed a good tunability was observed. The concentration-dependent absorption and transmittance of N-CQDs were observed to have a maximum value at concentration 100%. Photoluminescence intensity of samples was apparently temperature dependent and the highest intensity was obtained at 60 °C. The treated samples have a higher transmittance than resistance and short-time-treated ones on laser irradiation condition. The FTO glass based electrode was fabricated for sensing applications, and the prototype device was proposed to investigate opto-electrical and thermal responses. The voltage-dependent luminescence gradually enhanced with the applied voltage from 1 V to 5 V, demonstrating good field-induced emission properties. Thermal response revealed a decrease of resistance from 144.9 Ω at 30 °C to 106.4 Ω at 60 °C, semiconductor-like behaviour was observed. The results show that N-CQDs have tunable photoluminescence, stable thermosensitivity and controllable charge transport properties, which are promising device for low-cost sensors, temperature monitoring and quantum biosensing application.