Cobalt-Doped\nCarbon Quantum Dots with Peroxidase-Mimetic\nActivity for Ascorbic Acid Detection through Both Fluorometric and\nColorimetric Methods

Abstract

In this work, we fabricated cobalt-doped\ncarbon quantum dots (Co-CQDs)\nby a one-pot hydrothermal method with cobalt tetraphenylporphyrin\nand 1,2-ethanediamine as precursors. The morphology and structure\nof the Co-CQDs were characterized through transmission electron microscopy,\nX-ray diffraction spectra, Fourier transform infrared, and X-ray photoelectron\nspectra. The Co-CQDs emitted intense blue luminescence under ultraviolet\nirradiation and exhibited a typical excitation-dependent emission\nproperty. Moreover, they can act as a fluorescent probe for the detection\nof Fe³⁺ and ascorbic acid (AA) with high selectivity and\nsensitivity through an “on–off–on” mode.\nThe limit of detection (LOD) of Fe³⁺ was measured as 38\nμM (S/N = 3). The quenched emission of carbon quantum dots can\nbe recovered with the addition of ascorbic acid (AA) to the Co-CQDs/Fe³⁺ system. The change of fluorescence was linear with the concentration\nof AA (0.6–1.6 mM) with the LOD of 18 μM. Furthermore,\nthe Co-CQDs exhibited high oxidase-like catalytic behavior, which\ncould convert transparent 3,3′,5,5′-tetramethylbenzidine\n(TMB) into blue ox-TMB by dissolved oxygen. After adding ascorbic\nacid to the Co-CQDs/TMB system, the blue color of the solution faded\ndue to the reduction of blue ox-TMB to colorless TMB. Based on this\nphenomenon, the Co-CQDs were capable of detecting AA (10–400\nμM) with the LOD of 0.27 μM. The fluorometric and colorimetric\nassays based on the Co-CQDs for the AA detection were then successfully\napplied in fresh fruits. Furthermore, the high biocompatibility of\nthe Co-CQDs against HeLa cells was verified by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium\nbromide (MTT) method. Thus, the Co-CQDs could be used as a powerful\ntool for the detection of AA in real samples through a dual-mode method.