A novel"turn-off"fluorescence assay based on acid-copper nanoclusters in deep eutectic solvent micelles for...

As mixtures, deep eutectic solvent (DES) is designability. By adjusting the long alkyl chain hydrogen bond acceptors (HBAs) or hydrogen bond donors (HBDs), the DES displays surfactant characteristics and can form micelles. Hence, a novel, simple, facile and green natural organic acids capped copper nanoclusters (Aci-CuNCs) was synthesized and the spectrum behavior of Aci-CuNCs in DES micelles was researched. It was found that the surfactant-like DES can form micellar co-aggregation with Aci-CuNCs, resulting in the fluorescence (FL) intensive of Aci-CuNCs increase. Corresponding performance of spectral properties of Aci-CuNCs in DES medium were systematically studied by fourier transform infrared spectrometer, 3D FL spectroscopy, FL emission/excitation spectra, ultraviolet absorption spectroscopy. In the mechanism exploration part, on the one hand, the existence of micellar co-aggregation was confirmed by the conductivity, the mass effect of DES, dynamic light scattering and transmission electron microscopy. On the other hand, the influence of different kinds of DESs (types of HBAs/HBDs, molar ratio) and some possible factors (ionic strength and temperature) were discussed in detail to investigate the main driving forces for the formation of micellar co-aggregates. The results of mechanism exploration prove that the long alkyl chain of DES is amphiphilic which can form micellar co-aggregation with Aci-CuNCs through hydrogen bonding. The DES micelle provides Aci-CuNCs with a relatively stable and closed micro-environment which can effectively prevent collisions with water molecules and weakening of fluorescence intensity. On the basic of the above research, a "turn-off" fluorimetric method based on Aci-CuNCs in DES medium was applied for the determination of Fe³⁺. Under the optimum conditions, the assay worked in the Fe³⁺ concentration ranges from 1 to -20 μM and had a detection limit of 0.0374 μM. Method validation study illustrates the proposed system can provide a good accuracy, repeatability and stability conditions. Furthermore, the real sample analysis result demonstrates that no obvious matrix effect is found. As a consequence, the FL assays (Aci-CuNCs-based DES) composed of natural organic acid capped CuNCs and green solvent DES provides a simple, gentle and environmentally friendly method for the detection of iron ions.