Microwave-Assisted Synthesis of Functionalized Carbon Nanospheres Using Banana Peels: pH-Dependent Synthesis, Characterization, and Selective Sensing Applications

This work presents a microwave-based green synthesis method for producing carbon nanospheres (CNSs) and investigates the impact of presynthesis pH on their size and assembly. The resulting CNSs are monodispersed, averaging 35 nm in size, and exhibit notable characteristics including high water solubility, photostability, and a narrow size distribution, achieved within a synthesis time of 15 min. The synthesized CNS features functional groups such as −OH, −COOH, –NH, −C–O–C, ═C–H, and −CH. This diversity empowers the CNS for various applications including sensing. The CNS exhibits a distinct UV peak at 282 nm and emits intense fluorescence at 430 nm upon excitation at 350 nm. These functionalized CNSs enable selective and specific sensing of Cu2+ ions and the amino acid tryptophan (Trp) in aqueous solutions. In the presence of Cu2+ ions, static-based quenching of CNS fluorescence was observed due to the chelation-enhanced quenching (CHEQ) effect. Notably, Cu2+ ions induce a substantial change in UV spectra alongside a red-shift in the peak position. The limits of detection and quantification for Cu2+ ions with CNS are determined as 0.73 and 2.45 μg/mL, respectively. Additionally, on interaction with tryptophan, the UV spectra of CNS display a marked increase in the peak at 282 nm, accompanied by a red-shift phenomenon. The limits of detection and quantification for l-tryptophan are 4.510 × 10–3 and 1.50 × 10–2 μg/mL, respectively, indicating its significant potential for biological applications. Furthermore, the practical applicability of CNSs is demonstrated by their successful implementation in analyzing real water samples and filter paper-based examination, showcasing their effectiveness for on-site sensing.