Surface Ligand Influences the Cu Nanoclusters as a Dual Sensing Optical Probe for Localized pH Environment and Fluoride Ion

Functional metal nanomaterials, especially in the nanocluster (NC) size regime, with strong fluorescence, aqueous colloidal stability, and low toxicity, necessitate their application potential in biology and environmental science. Here, we successfully report a simple cost-effective method for red-/green-color-emitting protein/amino-acid-mediated Cu NCs in an aqueous medium. As-synthesized Cu NCs were characterized through UV-Vis absorption spectroscopy, fluorescence spectroscopy, time-resolved photoluminescence, dynamic light scattering, zeta potential, transmission electron microscopy and X-ray photoelectron spectroscopy. The optical properties of both Cu NCs responded linearly to the variation in pH in the neutral and alkaline ranges, and a robust pH reversible nature (between pH 7 and 11) was observed that could be extended to rapid, localized pH sensor development. However, a contrasting pH response nature between protein-Cu NCs and amino acid-Cu NCs was recorded. The alteration in protein secondary structure and strong binding nature of the surfactants were suggested to explain this behavior. Furthermore, we investigated their use as an efficient optical probe for fluoride ion detection. The limit of detection for protein-Cu NCs is 6.74 mu M, whereas the limit of detection for amino acid-Cu NCs is 4.67 mu M. Thus, it is anticipated that ultrasmall Cu NCs will exhibit promise in biological and environmental sensing applications.

Automated summary

We have developed a simple and cost-effective method for the synthesis of red-/green-color-emitting protein/amino acid-mediated copper nanoclusters (Cu NCs) in water. These Cu NCs exhibited strong fluorescence, aqueous colloidal stability, and low toxicity, making them suitable for applications in biology and environmental science. The optical properties of the Cu NCs responded linearly to changes in pH, and they showed a robust pH reversible nature between pH 7 and 11, which has potential for rapid and localized pH sensing. Additionally, the Cu NCs were found to be efficient optical probes for fluoride ion detection, with different limits of detection for protein-Cu NCs and amino acid-Cu NCs.