Electrochemical quantification of Ag 2 S quantum dots: evaluation of different surface coating ligands for bacteria determination.

In this work, novel silver sulphide quantum dots (Ag ₂ S QD) are electrochemically quantified for the first time. The method is based on the electrochemical reduction of Ag ⁺ to Ag ⁰ at -0.3 V on screen-printed carbon electrodes (SPCEs), followed by anodic stripping voltammetric oxidation that gives a peak of currents at +0.06 V which represents the analytical signal. The optimized methodology allows the quantification of water-stabilized Ag ₂ S QD in the range of approximately 2 × 10 ⁹ -2 × 10 ¹² QD·mL ^-1 with a good reproducibility (RSD: 5%). Moreover, as proof-of-concept of relevant biosensing application, Ag ₂ S QD are evaluated as tags for Escherichia coli (E. coli) bacteria determination. Bacteria tagged with QD are separated by centrifugation from the sample solution and placed on the SPCE surface for quantitative analysis. The effect of two different Ag ₂ S QD surface coating/stabilizing agents on both the voltammetric response and the bacteria sensing is also evaluated. 3-mercaptopropionic acid (3-MPA) is studied as model of short length coating ligand with no affinity for the bacteria, while boronic acid (BA) is evaluated as longer length ligand with chemical affinity for the polysaccharides present in the peptidoglycan layer on the bacteria cells surface. The biosensing system allows to detect bacteria in the range 10 ^-1 -10 ³ bacteria·mL ^-1 with a limit of detection as low as 1 bacteria·mL ^-1 . This methodology is a promising proof-of-concept alternative to traditional laboratory-based tests, with good sensitivity and short time and low cost of analysis. Graphical abstractNovel silver sulphide quantum dots (Ag ₂ S QD) are electrochemically quantified for the first time. Moreover, Ag ₂ S QD are evaluated as tags for Escherichia coli bacteria determination. The effect of two different QD surface coating ligands is also evaluated.