Highly selective and sensitive fluorescence sensing of nanomolar Zn ions in aqueous medium using Calix[4]arene passivated Carbon Quantum Dots based on fluorescence enhancement: Real-time monitoring and intracellular investigation.

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 Calix[4]arene derivative by hydrothermal route to detect biologically pertinent Zn 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. Zn ions get apprehended in the cavity formed due to passivation by Calix[4]arene moeity on the exterior of CDs having profuse -NH groups. Addition of Zn 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 Zn 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 Zn ions. The proposed sensor was also applied for fluorescence imaging of intracellular Zn ions.