Ligand‐Engineered Hydrophilic Perovskite Enabling Surface Potential‐Driven Anions Exchange for Multicolor Biosensing

The difficulty in designing zwitterionic ligands impedes the water-dispersed CsPbX₃ perovskite nanocrystals (NCs) and their application as fast anion exchange (FAE) probes in biosensing. This study proposes a design paradigm for zwitterionic ligands predicated upon revealing the mechanism of the S_N2 reaction between unsaturated alkylamines (Cn') and haloalkanoic acids (HAAs). Among them, the C=C bond can enhance the nucleophilicity of Cn' and promote the electrostatic adsorption of HAAs onto Cn', i.e., the geometric preorganization process, thereby initiating the S_N2 reaction. Moreover, an appropriate "bridge" length enables HAAs to balance the geometric preorganization process and the Sigma hole intensity of the C-Br bond. Zwitterionic ligands derived from oleylamine (C18') and 5-bromovaleric acid (5-BVA) endow CsPbBr₃ NCs with water dispersibility, an almost 100 % photoluminescence quantum yield, and enhanced surface potential, facilitating the capture of halide ions and driving the FAE reaction. Using AgI nanoparticles (NPs) as latent anion exchangers, a third FAE strategy is presented for multicolor biosensing. Such a robust biosensing strategy can generate wavelength shift and chromatic difference for biological target molecules, exemplified by H₂S, and is ultimately applicable to multicolor assay in biological, environmental and food samples, demonstrating the immense potential of perovskite-based FAE probes in biosensing.