pH-Switchable phenylalanine-templated copper nanoclusters: CO₂ probing and efficient peroxidase mimicking activity

Inter-cluster conversion through the strategic tuning of external stimuli and thereby modulation of the optical properties of metal nanoclusters (MNCs) is an emerging domain for exploration. Herein, we report the preparation of blue-emitting CuNCs using phenylalanine (Phe) as a template under acidic conditions (pH ∼ 4). The as-prepared CuNCs exhibit a sequential tuning of the photophysical properties upon varying the pH of the solution from pH ∼4 to pH ∼12. Blue-emitting CuNCs (B-CuNCs, λ_em = 410 nm) are systematically converted to cyan-emitting CuNCs (C-CuNCs, λ_em = 490 nm) with a large red-shifted emission maximum by 80 nm as a function of pH. Our present investigation delineates an unprecedented switchability of the photoluminescence (PL) properties of the CuNCs with the variations of the pH from pH ∼4 to pH ∼12. Both the Phe-templated CuNCs (B-CuNCs and C-CuNCs) were broadly characterized by various spectroscopic and morphological techniques. The X-ray photoelectron spectroscopy (XPS) studies reveal the presence of different oxidation states in the metallic core of B-CuNCs and C-CuNCs. These results in turn substantiate the pH-induced intercluster conversion of CuNCs through the substantial change in their core composition as well as valence states. Owing to the pH sensitivity, the CuNCs act as an efficient and highly sensitive probe for CO₂, and quantitative estimation of the dissolved CO₂ in the form of bicarbonate ions has been achieved through the enhancement of the PL intensity, wherein a very low value of the limit of detection (LOD) of ∼60 μM was obtained. Furthermore, we demonstrated that the CuNCs act as an efficient bio-catalyst with peroxidase mimicking enzymatic activity which has been investigated using OPD as a substrate under physiological conditions (pH ∼7.4 and temperature ∼37 °C). The mechanistic investigations confirmed that the oxidation of OPD mainly proceeds through the generation of hydroxyl radicals (˙OH). We hope the present investigations shed light on a multidimensional aspect of MNCs and uncover an upsurging recent interest in MNCs to act as an artificial enzyme.