Carbon Quantum Dots (CDs) owing to its unique photophysical properties have attracted immense consideration of the researchers in last decade to explore its applications in the fields of bio-imaging and sensing. In the present work, we have developed a novel CDs functionalized with Calixarene derivative by hydrothermal route to detect biologically pertinent Zn2+ ions. Surface modification of CDs enhances fluorescence efficiency apart from introducing active surface sites broadening its sensor capabilities. The obtained surface-functionalized CDs exhibits a strong blue fluorescence under UV light (365 nm) with a quantum yield (Φ) of 56% which is appreciably good. Zn2+ ions get apprehended in the cavity formed due to passivation by Calixarene moeity on the exterior of CDs having profuse –NH2 groups. Addition of Zn2+ ions into the solution of CDs results in remarkable enhancement in the fluorescence intensity attributed to Photo-induced Electron Transfer (PET) “OFF” phenomenon. The CDs based nanosensor exhibited a wide linear response with a detection limit of 7.34 nM. Significantly, CDs were shown to display insignificant cytotoxicity, decent biocompatibility, and virtuous resistance to photo-bleaching, good stability over a wide range of pH, insignificant variation in the emission intensity in the presence of high concentration of salt and excellent bio-labeling ability when tested on E. coli cell lines. Additionally, the detection results of Zn2+ ions obtained in real samples were in complete agreement with that obtained by Atomic Absorption Spectroscopy (AAS), signifying the prospective application of the proposed probe as a selective and sensitive sensor for Zn2+ ions. The proposed sensor was also applied for fluorescence imaging of intracellular Zn2+ ions.
Kaur, H., Raj, P., Sharma, H., Verma, M., Singh, N., & Kaur, N. Highly selective and sensitive fluorescence sensing of nanomolar Zn2+ ions in aqueous medium using Calix  arene passivated Carbon Quantum Dots based on fluorescence enhancement: Real-time monitoring and intracellular investigation. Analytica Chimica Acta, 2018,1009, 1-11.