A portable sensor for visual monitoring of Fe.sup.2+ and H.sub.2O.sub.2, two-dimensional Co.sub.3O.sub.4 modified by nano-IrO.sub.2 (IrO.sub.2@2D Co.sub.3O.sub.4) was prepared in this work, for the first time, with the help of microwave radiation at 140 °C, which was further stabilized onto common test strips. The present IrO.sub.2@2D Co.sub.3O.sub.4 possessed superior dual-function enzyme-like activity with low toxicity and excellent biocompatibility. Especially, trace Fe.sup.2+ and H.sub.2O.sub.2 could exclusively alter their enzyme-like catalytic activity with discriminating hyperchromic or hypochromic effect, i.e., from blue to colorless or to dark blue for both IrO.sub.2@2D Co.sub.3O.sub.4 dispersion and its functionalized test strips. The linear regression equations were A.sub.652 = 0.5940 - 0.00041 c.sub.Fe.sup.2+ (10.sup.-8 M, R.sup.2 = 0.9927) for Fe.sup.2+ and âA.sub.652 = 0.0023 c.sub.H2O2 + 0.00025 (10.sup.-7 M, R.sup.2 = 0.9982) for H.sub.2O.sub.2, respectively. When applied to visual monitoring of aqueous Fe.sup.2+ and intercellular H.sub.2O.sub.2, the recoveries were 101.2 ~ 102.5% and 95.8 ~ 103.7% with detection limits of 1.25 x 10.sup.-8 mol/L and 1.02 x 10.sup.-7 mol/L, respectively, far below the permitted values in drinking water set by the World Health Organization. The mechanisms for the enhancing enzyme-mimetic activity of IrO.sub.2@2D Co.sub.3O.sub.4 and its selective responses to Fe.sup.2+ and H.sub.2O.sub.2 were investigated in detail. Graphical