This study synthesized carbon quantum dots (CQDs) with green photoluminescence through a hydrothermal method that utilized mulberry juice as the carbon source. The influence of fruit ripeness on the physical and chemical properties, focusing on the fluorescence spectra, has been explored. Fourier-transform infrared spectroscopy (FT-IR) and energy dispersive X-ray analysis (EDX) showed that there were oxygen-containing groups, and X-ray diffraction (XRD) showed that the carbon quantum dots (CQDs) were graphitic. The results revealed that the CQDs had an average size of around 7.4nm and 9.7nm for unripe and ripe mulberry juice, respectively. These CQDs emitted green light at 500nm and 510nm in unripe and ripe mulberry juice, respectively, when excited at a wavelength of 400nm. The prepared CQDs exhibited excitation-dependent photoluminescence (PL) emission behavior, demonstrating their dependence on the excitation light. The impact of fruit ripeness on optical properties was explored by examining fluorescent spectra from different fruits (including tomato and blackberry), demonstrating comparable behaviors observed in mulberry fruit. In addition, the prepared CQDs were utilized as a fluorescent sensor with high specificity to detect Cu²⁺ ions. The detection limit (DL) for this sensor was determined to be 0.2687µM, and the limit of qualification (LOQ) is 0.814µM. The linear range for detection lies between 0.1 and 1µM. The selectivity of the CQDs towards Cu²⁺ ions was confirmed by recording the PL response for Cu²⁺ ions compared to the weak response of other metal ions. According to these results, the CQDs can be applied in various cellular imaging and biology applications, bio-sensing, optoelectronics, and sensors.

Cesium lead bromide (CsPbBr₃) nanocrystals exhibit remarkable optoelectronic properties and exceptional stability. As a result, they have garnered significant interest for their potential applications in various fields, including solar cells, light-emitting devices, photodetectors, and lasers. Despite its resistance to moisture, oxygen, and heat compared to other perovskite materials, CsPbBr₃ still faces challenges maintaining its structural and optical stability over extended periods. This study proposes a robust solution to enhance and improve simultaneously the photoluminescence intensity and stability of CsPbBr₃ nanocrystals. The solution involves doping the perovskite precursor with green-synthesized carbon quantum dots (CQDs) and tri-n-octyl phosphine (TOP). The results indicate that the photoluminescence intensity of the perovskite nanocrystals (NCs) is sensitive to varying CQD ratios. A high photoluminescence intensity enhancement of 45 was achieved at the optimal CQDs ratio. The synthesized perovskite NCs/CQDs also demonstrated improved stability by adding TOP into the mixture. After storage in the air for 45days, the mixed perovskite NCs maintained their performance, which was almost unchanged. Solar cell devices based on the modified perovskite NCs showed a power conversion of 7.74. The devices also demonstrated a significant open-circuit voltage (VOC), with the most successful device achieving a VOC of 1.193V, an Isc of 10.5748mA cm⁻², and a fill factor (FF) of 61. This study introduces a cost-effective method for producing high-quality all-inorganic optoelectronic devices with enhanced performance and stability.