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

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.