Colorimetric and Smartphone-Based Dual-Mode Rapid\nDetection of Congo Red Using Iron Oxide Quantum Dots

Congo red is toxic\nto humans and the environment and persists in\nthe environment for long periods. Therefore, developing a rapid detection\nmethod for Congo red is crucial. In this study, iron oxide quantum\ndots (IOQDs) were synthesized and employed for dual-mode detection\n(colorimetric and smartphone-based) of Congo red in real samples.\nUpon mixing with Congo red, the IOQDs induce a color change in the\nsolution due to the strong intermolecular interactions between Congo\nred and the IOQDs, making them practical as colorimetric sensors.\nTo further increase the on-site detection capabilities of IOQDs, images\nof the sensor platform were captured using a smartphone, and the color\ndata were analyzed with a dedicated APP. As a colorimetric sensor,\nthe ultraviolet–visible (UV–vis) absorbance response\nexhibited good linearity for Congo red concentrations between 2 and\n50 μM, with a detection limit of 0.89 μM. The smartphone-based\nsensor also provided highly quantitative results, showing a linear\nrelationship between Congo red concentrations and the blue-to-red\n(B/R) channel ratio, with a detection limit of 0.58 μM. Moreover,\nthis dual-mode method demonstrated better selectivity for Congo red\nthan other colorimetric sensors, even in the presence of other red\ndyes. The proposed method is convenient, fast, low-cost, and suitable\nfor real-sample applications.