Immobilization of iron hydroxide/oxide on reduced graphene oxide: Peroxidase-like activity and selective detection of sulfide ions

We prepared nanocomposites of amorphous iron hydroxide/oxide immobilized on reduced graphene oxide (FeO_xH-rGO) with peroxidase-like activity for the detection of sulfide (S^2-) ions. FeO_xH-rGO nanocomposites were prepared by reaction of GO (size ∼ 300 nm) partially reduced by ultraviolet irradiation with Fe²⁺ in Tris-borate solution (5.0 mM, pH 7.0). The amorphous FeO(OH) and Fe(OH)₂ were immobilized on rGO to form FeO_xH-rGO nanocomposites. The as-prepared FeO _xH-rGO nanocomposites exhibited peroxidase-like catalytic activity in the H₂O₂-mediated oxidation of Amplex Red (AR) to fluorescent resorufin. Our AR/FeO_xH-rGO probe allowed the detection of H₂O₂ down to 50 nM within 10 min under microwave irradiation (170 W). The catalytic activity of FeO_xH-rGO was significantly suppressed in the presence of S^2- because of the formation of FeS on the FeO_xH-rGO nanocomposites' surfaces. The H₂O₂/AR-FeO_xH-rGO probe provided a limit of detection (signal-to-noise ratio = 3) of 50 nM for S^2- with high selectivity (>100-fold) with respect to other anions. Taking advantage of their high stability and selectivity, we employed our H₂O ₂/AR-FeO_xH-rGO probe for the detection of S^2- in hot spring samples (75.1-619.5 μM) and the results showed good correlation (r = 0.98) with results from inductively coupled plasma mass spectrometry. This label-free, rapid, and simple sensing system shows great potential for the detection of S^2- ions in real samples.