Biological application of water-based electrochemically synthesized CuO leaf-like arrays: SERS response modulated by the positional isomerism and interface type

Cupric oxide leaf-like nanostructures (CuONSs) (average dimensions: 80–180 nm in width and 400–750 nm in length) were synthesized via anodic electrochemical dissolution of copper in an ethanol solution containing LiCl electrolyte and water. Ultraviolet-visible (UV-Vis), Fourier-transform infrared (FT-IR), and Raman spectroscopies as well as scanning electron microscope (SEM), high-resolution transmission electron microscopy with energy dispersive X-ray (HD-TEM-EDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) were used to explore the metal surface plasmon, size, rheology, and structure of CuONSs. Then, pyridine α-aminophosphinic acid isomers (α-, β-, and γ-NHPy) were synthesized and assembled on the CuONS/air and CuONS/aqueous solution interfaces at the pH level of solution = 7. Differences in adsorption and thus in the spectral response resulting from positional isomerism were examined by surface-enhanced Raman scattering (SERS) with an excitation wavelength of 785 nm. The manner of interaction of the investigated isomers with CuONSs in an aqueous solution was discussed in detail and compared with that at the CuONS/air interface. For γ-NHPy, at the CuONS/water interface, the time-dependent changes in the spectral profile were observed and analyzed. For β-NHPy at the CuONS/air interface, tip-enhanced Raman scattering (TERS) measurements were performed. These measurements allowed observing single molecule behavior and avoiding interference from the molecule's surrounding environment.