Ti-co-Ce oxide decorated chitosan fiber oxidase mimic identified for portable monitoring of S^2-, CrO₄^2- and Fe²⁺ assisted with a smartphone

Environmental safety and protection is one of the most concerned topics nowadays. To conveniently monitor toxic S^2-/CrO₄^2- and to regulate bioactive Fe²⁺, Ti-co-Ce oxide decorated chitosan fiber (Ti-co-Ce ox@CC) was developed using microwave-assisted hydrothermal method. The integration of chitosan fiber and nano Ti-co-Ce oxide endowed Ti-co-Ce ox@CC with superior oxidase-like activity and improved water-dispersibility. It could quickly accelerate the oxidization of 3,3',5,5'-tetramethyl benzidine (TMB) with V_max/K_m of 6.67 × 10^-8 M·s^-1/0.0178 mM, much better than these (4.31 × 10^-8 M·s^-1/0.0471 mM) for Ti-co-Ce oxide only. Trace S^2-, CrO₄^2- or Fe²⁺ could selectively alter the oxidase-like activity with clear hyperchromic or hypochromic effect. Under the optimized conditions, there expressed excellent linear relationships between A₆₅₂ and c_S^2-, c_CrO4^2- or c_Fe²⁺ with ideal limits of detection (LOD) of 7.8 × 10^-9 M, 5.0 × 10^-8 M and 6.5 × 10^-8 M respectively. What's more, the red-green-blue (RGB) values analyzed by a smartphone also expressed nice linear relationships with satisfactory LODs of 3.8 × 10^-8 M, 7.7 × 10^-8 M and 9.2 × 10^-8 M for S^2-, CrO₄^2- and Fe²⁺. The proposed Ti-co-Ce ox@CC sensing platform will provide a promising potential for on-site intelligent identification of S^2-, CrO₄^2- and Fe²⁺ in practice.