Colorimetric and smartphone-based dual modality sensing platform for detection of ascorbic acid

Ascorbic acid (AA) is a vital vitamin possessing antioxidant properties which is used to prevent and treat diseases like scurvy and cancer. Therefore, precise detection of ultra-low concentrations of AA is one of the crucial challenges and is also important for therapeutic applications. We have evaluated for the first time the nanozymatic activity of ZnO-Pd nanobolts for the detection of AA. The present study has developed a nanozyme-based colorimetric detection assay for AA using a chromogenic substrate 3,3′,5,5′-Tetramethylbenzidine (TMB). ZnO-Pd nanobolts exhibited strong peroxidase mimetic activity by oxidizing chromogenic substrate TMB. Colorless TMB was oxidised to blue color ox-TMB by the action of ZnO-Pd nanobolts in the presence of hydrogen peroxide (H₂O₂). The detection of AA was based on a turn-off approach where addition of AA to the reaction mixture induced a reduction reaction. This reduces the ox-TMB to TMB causing the blue color of the solution to fade. The assay was tested with multiple serum containing substances and showed high specificity toward AA. The sensitivity of the assay was studied over a linear range of 0.05–128 µM, and the results were analysed by measuring the intensity of the colored solution using UV-Vis spectroscopy. The LOD and LOQ of the assay were found to be 0.031 µM and 0.094 µM respectively. Additionally, smartphone analysis was carried out for attaining a relationship between the color intensity of the reaction solution and concentrations of the AA. The method was used for determination of AA in vitamin C tablets for assessing the practical application of the assay.