Selective and Practical Graphene-Based Arsenite Sensor at 10 ppb

Arsenic detection in field water samples at concentrations of relevance with affordable and simple equipment is of global interest. We report a biomimetic electrode using electrochemically reduced graphene oxide (ERGO) for highly selective and sensitive reagent-free arsenite (As3+) detection in field water samples, down to the ten parts per billion level, enabling measurement of drinking water quality affordably for millions of arsenic-affected people. This electronically and structurally optimized ERGO electrode shows selective detection of As3+ in both phosphate buffered saline (PBS, pH ∼7) and field water samples, even though more than 100 times larger conductivity and total dissolved solids (TDS), respectively, are present in them. Raman and FTIR spectroscopies were used to understand the mechanism of selectivity and sensitivity. The sensing mechanism involved two processes, namely, selective binding of As3+ with the -COOH groups of ERGO followed by its electro-oxidation by an applied potential. Density functional theory (DFT) and force-field calculations were used to obtain crucial insights into the site selectivity and mechanism of oxidation of As3+. A two-electron transfer process from As3+ to ERGO followed by associative O ligand addition to As3+ by a ketone oxygen atom, and concomitant regeneration of -COOH group is presented. The ion selectivity depends both on structural and electronic factors. First, the compact pyramidal-shaped As3+ species may closely approach the edge -COOH functional group to a greater extent than the other ions enabling covalent binding of the As center with the ketone O atom. Furthermore, closer proximity of the lowest unoccupied molecular orbital (LUMO) acceptor level of the positively charged ERGO and the highest occupied molecular orbital (HOMO) donor level of the As3+ species suggests that a uniquely selective resonant charge-transfer effect occurs between the As3+ species and ERGO.