A BSA-squaraine hybrid system for selectively detecting Ag+ in absolute PBS and sequential construction of logic functions

A novel near infrared (NIR) fluorescent hybrid system ( PD-SQ ⊂ BSA ) assembled from squaraine dye PD-SQ and bovine serum albumin (BSA) was developed for selective detection of Ag + over other metal ions in absolute PBS. The detection limit of PD-SQ ⊂ BSA for Ag + is calculated to be about 8.99   ×   10 −8   M, which is lower than the guidelines for drinking water set by the WHO. PD-SQ alone shows very weak fluorescence due to aggregation-caused quenching (ACQ) property. BSA can cause PD-SQ to partially disaggregate through electrostatic and hydrophobic interactions, but various metal ions including Ag + cannot do. Interestingly, the presence of both BSA and Ag + can trigger PD-SQ to almost completely disaggregate through synergic binding of multiple S-Ag linkings derived from the biothiol moieties in BSA and PD-SQ with Ag + , resulting in obvious turn-on fluorescence change. Absorption and field emission scanning electron microscope (FESEM) spectra have been used to prove the conversion of PD-SQ from aggregates to monomers. In addition, based on the fluorescence spectra changes of PD-SQ ⊂ BSA system upon addition of Hg 2+ and ethylenediaminetetraacetic acid (EDTA), logic circuits including AND and INHIBIT logic gates with reset function were successfully constructed. This type of fluorescent system shows great potential for disease diagnosis, environmental protection, and information protection at the molecular scale.