Surface-Modified Metal Oxides for Ultrasensitive Electrochemical Detection of Organophosphates, Heavy Metals, and Nutrients

One method for reliable on-site detection of water pollutants is to use highly sensitive and specific nonenzymatic electrochemical sensors containing agglomeration-free detecting agents specific to the water pollutants. We describe a facile and highly effective method to fabricate electrocatalytic electrodes with ZrO2, MnO2, and MgO that are well dispersed within the ink used to print these electrodes. The inorganic metal oxides serve as detecting agents with specific affinity toward fenitrothion (FT, an organophosphate), lead (Pb2+, a heavy-metal ion), and nitrite (NO2–, a nutrient), respectively. Our bioinspired method utilizes a catechol molecule, 3,4-dihydroxybenzaldehyde (DHBA), and film-forming polymer chitosan (CHIT). Redox-active DHBA strongly adsorbs the metal oxide through bidentate bonding, and CHIT-linked DHBA assists in stable film formation. This metal oxide surface modification through the DHBA-CHIT chemistry enhances the electronic, film-forming, and nanoparticle dispersing properties of the electrode modifying ink. The ink is drop-cast on nitrogen–sulfur co-doped activated carbon-coated carbon nanotubes (NS-AC-MWCNT) on a modified glassy carbon electrode (GCE). The resulting sensor consists of metal oxide as a detecting agent and NS-AC-MWCNT as a supportive electrode. The technique enhances the total surface area of the X-DHBA-CHIT/NS-AC-MWCNT/GCE electrode (where X denotes ZrO2, MnO2, or MgO) and its electrocatalytic properties. The ZrO2-DHBA-CHIT/NS-AC-MWCNT/GCE sensor has an excellent detection limit of 1.69 nM for 0.01–40 μM FT, which exceeds that of enzyme-based sensing. It is reliable for lake water with a detection limit of 11.3 nM FT, attributed to its excellent antifouling ability, and has negligible interference from commonly found chemicals in drinking water. Similarly, the MnO2-DHBA-CHIT/NS-AC-MWCNT/GCE and MgO-DHBA-CHIT/NS-AC-MWCNT/GCE sensors show excellent Pb2+ and NO2– electrochemical detection compared to unmodified electrodes. This simple, effective, non-hazardous methodology can be extended to modify the surfaces of other metal or metal oxide detecting agents.