Highly Sensitive Fluorescent\nand Colorimetric pH Sensor\nBased on Polyethylenimine-Capped Silver Nanoclusters

Abstract

Silver nanoclusters capped by hyperbranched polyethylenimine\n(PEI)\nhave been developed as a highly sensitive fluorescent and colorimetric\npH sensor. The probe responds rapidly to pH fluctuations and has such\nabsorption characteristics that the color changes from the colorless\nor a nearly colorless state to a colored state with increasing acidity,\nso PEI-capped Ag nanoclusters could be used as a color indicator for\ncolorimetric pH detection. Quantitatively, the fluorescence intensity\nof PEI-capped Ag nanoclusters exhibits a linear fashion over the pH\nrange of 5.02–7.96 and increases by around 10-fold approximately\nwith greater fluorescence at higher pH values. The repulsion development\nand conformational change of PEI with decreasing pH induce the aggregation\nof Ag nanoclusters, leading to an obvious color change and fluorescence\nquenching of Ag nanoclusters at low pH values. As expected, the pH\nprobe is also sensitive to the different buffer solutions, except\nfor those containing some anions that could react with Ag nanoclusters.\nBesides, the ionic strength of the buffers has a little influence\non the pH-responsive behavior. Our pH sensor with nanoscaled physical\ndimensions would be a promising candidate in the applications in biological,\nmedical, and pharmaceutical fields.