Dual-emissive gold nanoclusters for label-free and separation-free ratiometric fluorescence sensing of 4-nitrophenol based on the inner filter effect

Abstract

Highly sensitive detection of 4-nitrophenol (4-NP), one of the most hazardous and toxic phenols, has been highly sought-after. In this study, we found that 4-NP could selectively quench the 410 nm emission of the dityrosine (diTyr) residues of BSA-protected gold nanoclusters (GNCs@BSA) rather than the conventional red emission of GNCs@BSA via the inner filter effect (IFE). This finding helped us to develop a label-free and separation-free ratiometric fluorescence method for sensing of 4-NP with red emissive GNCs as the endogenous reference for the first time. This ratiometric sensor exhibited rapid response to 4-NP within 1 min. The photoluminescence (PL) intensity ratio of the 410 nm emission of the diTyr residues to the 630 nm emission of the GNCs (F₄₁₀/F₆₃₀) showed a linear relationship with 4-NP concentration in the range of 50 to 5000 nM (R² = 0.991) with a limit of detection (LOD) (S/N = 3) of 13.8 nM (1.9 ng mL⁻¹), remarkably lower than the concentration limit released by the US Environmental Protection Agency (EPA) for drinking water (60 ng mL⁻¹). Also, the ratiometric sensor showed selectivity for 4-NP over other eight common phenol derivatives. Furthermore, the ratiometric sensor was applied to analyze the trace 4-NP spiked in the tap and river water samples and the satisfied results were acquired, showing promising potential of the ratiometric sensor for practical applications.