Detection of silver ions by a novel NBD-based colorimetric chemosensor with the intramolecular charge transfer character

Development of a selective colorimetric method for Ag⁺ is important because excessive accumulation of Ag⁺ in the environment or human body causes significant damage. Therefore, a novel NBD-based colorimetric chemosensor NPB (7-nitro-N-(2-phenoxyphenyl)benzo[c][1,2,5]oxadiazol-4-amine) was designed for detecting silver ions. NPB could detect Ag⁺ with a distinctive bathochromic shift in absorption spectra and color change from pale brown to pink. The stoichiometric binding ratio between NPB and Ag⁺ was confirmed to be a 1:1 by Job plot analysis. The detection limit of Ag⁺ was found to be 3.60 × 10⁻⁶ M. Importantly, NPB showed an improved intramolecular charge transfer (ICT) in complexation with Ag⁺, which was demonstrated through DFT calculations. The ICT process change was in good agreement with the experimental results of the spectroscopic response. In addition, NPB could be applied to the detection of Ag⁺ through a change of color in test kits.