Polyethylenimine-protected green-emission copper nanoclusters as highly effective fluorescent and colorimetric nanoprobe for selective cobalt ions and temperature sensing

Excessive uptake of Co²⁺ is harmful to one's physical health and should not be ignored. Herein, a polyethylenimine (PEI) protected hydrophilic copper nanoclusters (PEI-CuNCs) with strong green fluorescence emission around 510 nm was apace synthesized employing a one-pot method without hyperthermia. Interestingly, the as-prepared water-soluble PEI-CuNCs can be specifically quenched by Co²⁺ at pH 6.0, with a wide detection range (0-500 μM) and a sensitive detection limit of 14.9 nM, which was lower than the maximum level in the body. Besides, the colorimetric detection of Co²⁺ could be additionally realized based upon the typical yellow color of PEI-CuNCs changed to baby-pink color of the PEI-Co²⁺ complex. Furthermore, the PEI-CuNCs was employed in fabrication portable test strip for visual detection of Co²⁺ by capturing the change in fluorescence color, which can be ascribed to the coordination interaction between Co²⁺ and amine groups in PEI, and also the aggregated quenching of large PEI-CuNCs-Co²⁺ particles formation. Moreover, the PEI-CuNCs displayed excellent reversible thermo-responsive within a temperature range of 20-65 °C. It is worth mentioning that the PEI-CuNCs exhibited low bio-toxicity and excellent cell permeability when selectively detecting Co²⁺ in living cells by fluorescence microscopy imaging. Armed with these engaging properties, the sensing system paved a new avenue for the effective development of a convenient fluorescence colorimetry sensor for general assessment of potential risks and specific assessment of human security.