Reversible enhancement of fluorescence in acidic pH driven by tryptophan stabilized copper nanoclusters and its application in bioimaging

Switchable catalysis and CO2 sensing by reduction resistant, luminescent copper-thiolate complexes

Metal-thiolate complexes have been the focus of research for several years because of their unique photophysical properties and their use as a precursor for synthesizing various well-defined metal nanoclusters. A rational understanding of their structure-property relationship is necessary to realize their full potential in practical applications. Herein, we demonstrate the synthesis of a unique copper-thiolate complex with reversibly switchable catalytic and photoluminescence (PL) properties. The as-synthesized complex at basic pH (Complex B) showed cyan PL with a strong peak at ∼488 nm (cyan) and a small shoulder peak at ∼528 nm (green). When the pH of the complex was changed to acidic (Complex A), the PL was switched to light green. Such pH-responsive PL properties were demonstrated to be useful for pH and CO₂ sensing. The switchable properties originate from their two distinct structural states at two different pHs. We found that Complex A was resistant to high concentrations of a strong reducing agent, and had an intermediate oxidation state of copper (Cu⁺) with good thermodynamic stability. Furthermore, the switchable catalytic property was investigated with a 4-nitrophenol reduction and 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction. The reduction kinetics followed pseudo-first-order, where the catalytic activity was enhanced by more than 10³ times when Complex B was switched to Complex A. A similar trend was also observed for TMB oxidation. Our design strategy demonstrates that redox switchable metal-thiolate complexes could be a powerful candidate for a plethora of applications.

A highly selective fluorescent sensor for chlortetracycline based on histidine-templated copper nanoclusters

In this study, histidine-protected copper nanoclusters (Cu NCs@His) were established by using a one-pot method, which histidine and ascorbic acid were applied as the template and reducing agent, respectively. The as-developed Cu NCs@His endued green emission wavelength at 494 nm with the excitation of 378 nm. The Cu NCs@His exhibited green fluorescence under UV light (365 nm). Using Cu NCs@His as a pattern nanosensor, the fluorescent "turn off" mechanism was fabricated for the determination of chlortetracycline in the light of the linear decrease of fluorescence intensities around 494 nm. The chlortetracycline conducted as a quencher, leading to reveal an excellent linear relationship between ln(F₀/F) of Cu NCs@His and chlortetracycline concentrations with the range of 0.5-200 μM, and the detection limit was 0.876 μM. The fluorescence quenching of Cu NCs@His revealed excellent selectivity for chlortetracycline over other potential interfering substances in the human body. This strategy was exhibited to be a convenient sensing platform for the detection of chlortetracycline in real medical samples, which could unfold a brand new and direct system for the sensing of chlortetracycline in real samples.