Smartphone-assisted multiple-mode assay of ascorbic acid using cobalt oxyhydroxide nanoflakes and carbon quantum dots

A multi-mode detection platform for ascorbic acid (AA) based on cobalt oxyhydroxide nanoflakes (CoOOH) was designed in this study. CoOOH has enzyme-mimicking properties and can oxidize 3,3′,5,5′-tetramethyl benzidine (TMB) into a blue colored oxidation product (TMBox). CoOOH nanoflakes are selectively reduced to Co 2+ by AA, which results in the loss of oxidation of CoOOH and a gradual color evolution from blue to colorless and a UV–vis absorption decrease of TMBox. The AA can be detected in the range of 0.2–20 μM with a limit of detection (LOD) of 0.067 μM (UV–vis model). In addition, carbon quantum dots (CQDs) with blue fluorescence were prepared using a one-step hydrothermal method and employed as the signal output. When nanocomposites of CoOOH nanoflakes and CQDs were formed by hybridization, the fluorescence of CQDs was quenched by CoOOH nanoflakes via fluorescence resonance energy transfer (FRET) and readily recovered by AA, representing an “on–off-on” response and a variation in color from black to light blue under a 365-nm UV lamp. The calibration curve was linear in the range of 0.1–30 μM with a limit of detection (LOD) of 0.031 μM (Fluorescence model). Two smartphone-based colorimetric devices were designed for analyzing the chromogenic solutions. The red, green, and blue values of the colored solutions were obtained directly using an Android application (Color Grab). The LOD of two smartphone colorimetry models are 0.16 μM and 0.19 μM, and linear range are 0.5–16 μM, 0.75–20 μM respectively. The multi-mode detection platform was successfully applied to the detection of AA in fruits with acceptable recoveries from 90.5% to 107%, a relative standard deviation (RSD) ranging from 1.6% to 5.0%. Compared with the fluorescence and UV–vis modes, the smartphone-integrated colorimetric platform was found to be simple, portable, fast, and inexpensive.