Visual detection of cyanide ions by membrane-based nanozyme assay

In this paper, we report a simple one-step synthesis of well-dispersed amorphous cobalt hydroxide/oxide-modified graphene oxide (CoO_xH-GO) possessing peroxidase-like catalytic activity, and its application for the detection of H₂O₂, glucose, and CN^– ions. CoO_xH is formed and deposited in situ on the GO surface through the reaction between GO (size ~ 240 nm) and Co²⁺ in basic solution at room temperature. We investigated the enzyme-mimicking activity of the CoO_xH-GO nanohybrid in detail via the H₂O₂-mediated oxidation of Amplex Red (AR) to form fluorescent resorufin. The peroxidase-like activity of CoO_xH-GO is utilized herein for the quantitation of H₂O₂ in a wide concentration range, from 100 nM to 100 μM. When coupled with glucose oxidase (GOD), the AR/CoO_xH-GO system can determine glucose level in blood samples. Interestingly, cyanide ions (CN^–) significantly inhibit the catalytic activity of the CoO_xH-GO nanohybrid, which allows for the construction of a probe for the detection of CN^– in water samples and laboratory wastes. We fabricated a membrane-based CoO_xH-GO probe for the visual detection of CN^– by preparing a thin film of CoO_xH-GO on a positively charged and porous nylon membrane (N⁺M). The CoO_xH-GO/N⁺M operates on the principle that CN^– inhibits the catalytic activity of CoO_xH-GO towards the H₂O₂-mediated oxidation of AR to form reddish resorufin on the membrane. The intensity of the red color of the membrane decreases with increasing CN^– concentration, which can be easily observed with the naked eye at the nanomolar level. This cost-effective sensing system allows for the rapid and simple determination of the concentrations of CN⁻ in complicated wastewater samples.