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

Abstract

Congo red is toxic to humans and the environment and persists in the environment for long periods. Therefore, developing a rapid detection method for Congo red is crucial. In this study, iron oxide quantum dots (IOQDs) were synthesized and employed for dual-mode detection (colorimetric and smartphone-based) of Congo red in real samples. Upon mixing with Congo red, the IOQDs induce a color change in the solution due to the strong intermolecular interactions between Congo red and the IOQDs, making them practical as colorimetric sensors. To further increase the on-site detection capabilities of IOQDs, images of the sensor platform were captured using a smartphone, and the color data were analyzed with a dedicated APP. As a colorimetric sensor, the ultraviolet–visible (UV–vis) absorbance response exhibited good linearity for Congo red concentrations between 2 and 50 μM, with a detection limit of 0.89 μM. The smartphone-based sensor also provided highly quantitative results, showing a linear relationship between Congo red concentrations and the blue-to-red (B/R) channel ratio, with a detection limit of 0.58 μM. Moreover, this dual-mode method demonstrated better selectivity for Congo red than other colorimetric sensors, even in the presence of other red dyes. The proposed method is convenient, fast, low-cost, and suitable for real-sample applications.