Iridium nanoclusters for highly efficient p-nitroaniline fluorescence sensor

A facile strategy was proposed for the preparation of two fluorescent iridium nanoclusters of IrNCs-1 and IrNCs-2 by heating IrCl₃ in N,N-dimethylacetamide (DMA) and N-Methylformanilide (MFA), respectively. The DMA and MFA are amide compounds and contain electron-rich oxygen and nitrogen atoms. They can be used as electron donors and a fraction of electron charge was transferred to Ir atom to form ligand-to-metal charge transfer (LMCT) complexes. The Ir atoms can be stabilized in LMCT complexes via accepting and localizing the electron density. These LMCT complexes with excellent fluorescent property were defined as the IrNCs. They exhibited obvious excitation wavelength-dependent emission behavior. The emission bands of two IrNCs showed red-shifting with the increasing of excitation wavelength based on the effect of “inhomogeneous broadening”. The IrNCs-1 possessed concentration-dependent emission behavior. With the decreasing of IrNCs-1 concentrations, the fluorescence spectra showed remarkable blue-shifting due to the destruction of intermolecular hydrogen bonds between IrNCs-1. In the presence of p-nitroaniline (p-NA), the fluorescence of IrNCs-1 was significantly quenched and the fluorescence bands red-shifted strongly. According to the Stern-Volmer quenching behavior under different reaction temperature and absorbance spectra of IrNCs-1 in the presence and absence of p-NA, the dynamic quenching mechanism was included. Based on these findings, the fluorescence sensor for p-NA determination was developed with the linear range of 0.1–40 μM and the limit of detection (LOD) of 50 nM. The proposed sensor was successfully applied in the p-NA determination of tap water and lake water samples, suggesting that the novel IrNCs will be a promising candidate as a luminescence sensor.