Ratiometric photothermal detection of silver ions using diimmonium salts

Selective detection and determination of silver ions are tricky due to the poor reactivity and the interference of various metal ions and halides. Only a few cases have been reported concerning sulfur-based functionality. This work proposes silver-selective probes exhibiting peculiar colorimetric and photothermal responses without the need for sulfur-functional groups. Tetrakis(4-dialklyaminophenyl)1,4-phenylenediamine containing hexyl and methoxytriethyleneglycol substituents, PTS1 and PTS2, respectively, are selectively oxidized by silver ions to form dicationic diimmonium salts. The resulting quinone diimminium salts show strong absorption in the near-infrared (NIR) region with intense color changes. The striking color change is observed from pale brown to intense olive green with immediate responses. PTS1 shows a higher binding constant and nanomolar detection limit, whereas PTS2 shows a sub-micromolar detection limit. Job's plot studies reveal the stoichiometric ratio as 1:2 (PTS1: Ag⁺). Due to the strong NIR absorption of the oxidized PTS probes, the temperature rise is monitored, after adding silver ions, by a thermal camera and in-contact thermal sensors. The concentration of the actual samples can be determined based on a calibration plot with a temperature change as a function of silver ion concentration. The current results offer highly selective and ultrasensitive probes for silver ions under multiple detection mediums, including naked eye and photothermal detection.