Nanomaterials and composites containing metal oxides are increasingly being used in human health, cosmetics and food packaging applications, which may give rise to the evolution of reactive species on nanosurfaces by autoxidation with air O₂. In recent years, novel analytical approaches have been developed for total antioxidant capacity (TAC) determination, with special emphasis on nanoparticle-based sensors due to their low cost, easy use, rapid response and high precision. A nano-manganese oxide (nano-MnO_x)-based spectrophotometric method was developed for the indirect capacity measurement of antioxidant compounds, based on the measurement of the decrease in the color intensity (at λ_max = 650 nm) of 3,3′,5,5′-tetramethylbenzidine cations (TMB⁺) in the presence of hydrophilic and lipophilic antioxidants. TMB⁺ cations were obtained via a redox reaction between MnO_x NPs and the TMB chromophore in open air, and antioxidants caused a decrease of TMB⁺ color intensity. MnO_x NPs could be easily and rapidly prepared at low cost. The linearity, repeatability and recovery parameters defining reliability and precision were tested to validate the sensing method. Depending on the type of tested antioxidant, the limit of detection (LOD) values were in the range of 1.23 × 10⁻⁹ to 1.71 × 10⁻⁷ mol L⁻¹. The developed nanoprobe was applied to ternary synthetic mixtures of antioxidants to yield expected and found TAC values (as trolox-equivalents) in agreement. The potential interferents such as benzoic acid, citric acid, glucose and mannitol did not adversely affect the TAC determination. To test real samples, the method of standard addition of pure antioxidant compounds was applied to real samples such as green tea and orange juice.