Upconversion fluorescent nanoprobe based on the 4-NP reversible structure for a wide range of pH determination

The accurate detection of pH value has received increasing attention in various fields. However, most reported probes show pH values in the acidic or alkaline range, and work within a narrow pH range. Herein, core@shell@SiO2@4-NP, as a pH probe, responds to different fluorescence signals of acid and base under 980 nm excitation. The emission peak area ratio at 360 nm and 345 nm (denoted as R(I-360nm/I-345nm)) shows a linear relationship with pH (R = -0.11916 x pH + 1.4856). Therefore, a wide pH range in the solution can be quantitatively detected via R(I-360nm/I-345nm). In addition, core@shell@SiO2@4-NP has good upconversion fluorescence reversibility between pH = 2.0 and pH = 10.0. These advantages make the proposed upconversion fluorescent nanoprobe based on the 4-NP reversible structure a promising solution for on-line pH detection under different conditions.