Figure 6.

(a) Photograph and absorption spectra of colorimetric reactions under different conditions: (i) 3,3,5,5-tetramethylbenzidine (TMB) + H₂O₂; (ii) TMB + NiCo₂O₄; (iii) TMB + H₂O₂ + NiCo₂O₄; (iv) TMB + H₂O₂ + NiO; (v) TMB + H₂O₂ + Co₃O₄. TMB concentration- (b) and pH- (c) dependent peroxidase-like activity of hierarchical NiCo₂O₄ hollow sphere. Experimental conditions: 5 μL of 1 mg·mL⁻¹ enzyme mimetic dispersions were incubated with 3 mL of pre-prepared sodium citrate buffer solution (0.1 M, pH 4.5) with the presence of H₂O₂ (0.02 M) and TMB (0.08 mM).