Turn-Off Detection of Reactive Oxidative Species and Turn-On Detection of Antioxidants Using Fluorescent Copper Nanoclusters

The present work has been undertaken with an aim to design and develop a material-based nanoscale fluorescence sensory system for the selective and sensitive detection of both highly reactive oxidative species (hROS) and antioxidants in a single chemical entity by exploiting two optically distinguishable useful signals. For this purpose, water-soluble, chitosan-capped fluorescent copper nanoclusters (CuNCs) have been synthesized and characterized by using conventional methods. The sensory action of the CuNCs for several analytes of interest has been investigated at both ensemble-averaged and single-particle levels by exploiting spectroscopic and microscopic techniques. The steady-state fluorescence studies have revealed that CuNCs can act as a very efficient turn-off sensor for hROS (•OH and ClO–) and also a turn-on sensor for antioxidants (ascorbic acid and glutathione). Interestingly, it has been demonstrated that the present sensory system is quite effective in detecting antioxidants in commercial fruit juice and human blood samples at low concentration levels. The feasibility of the hROS sensing ability of CuNCs at single-particle levels is also demonstrated with the help of fluorescence correlation spectroscopy (FCS). Analysis of the data obtained from X-ray photoelectron spectroscopy (XPS) has clearly indicated that the mechanism of turn-off sensing is due to the oxidation of Cu(0)NCs to Cu(II) by hROS. On the other hand, fluorescence studies have revealed that the turn-on sensing mechanism is due to the presence of antioxidants, which prevents the oxidation of CuNCs by hROS. More interestingly, investigation has also demonstrated that the optical output signals of the probe–analyte interaction during the hROS/antioxidant signaling can successfully be exploited to construct NAND and IMPLICATION logic gates. Essentially, the outcome of the present investigation demonstrates that CuNCs not only can be effectively used as a nanoscale sensor for both hROS and antioxidants but also have the potential to be used for electronics and medical diagnostics purposes.