A novel near-infrared fluorescent and colorimetric probe for selective detection of Ag⁺ and Hg²⁺

It is particularly important to develop effective and specific detection methods for harmful metal ions such as the silver ion (Ag⁺) and mercury ion (Hg²⁺). In this article, a new near-infrared (NIR) fluorescent probe (N-FP) based on intramolecular charge transfer (ICT) effect was designed and synthesised, which exhibited the characteristics of large Stokes shift (163 nm) and excellent stability. Addition of Ag⁺ or Hg²⁺ to the N-FP solution in ethanol/water (9:1, v/v) caused remarkable enhancement of fluorescence emission at 661 nm, bathochromic shift of ultraviolet-visible absorption wavelength, and colour change from orange to red or purple. While adding other metal ions including Li⁺, Na⁺, K⁺, Ag⁺, Cu²⁺, Fe²⁺, Zn²⁺, Co²⁺, Ni²⁺, Mn²⁺, Sr²⁺, Ca²⁺, Mg²⁺, Al³⁺, Cr³⁺ and Fe³⁺ did not bring about substantial spectral and colour change. The detection limit of the N-FP for Ag⁺ and Hg²⁺ was calculated to be 1.1 and 0.72 μM, respectively. The N-FP could be used to recognise Ag⁺ and Hg²⁺ in a wide pH range of 1–10. The sensing mechanism was proposed and demonstrated by proton nuclear magnetic resonance (¹H-NMR), high-performance liquid chromatography (HPLC) and mass spectrometry (MS) measurements.