Online Monitoring Strategies for Colorimetric Detection of Cadmium Ions and pH Based on Gold Nanomaterials with a Low-Cost Color Sensor

Gold triangular nanoplates (AuTNPs) and gold nanorods (AuNRs) have been widely used in colorimetric sensing due to their shape-dependent localized surface plasmon resonance properties. Herein, we report new colorimetric detection strategies based on a low-cost RGB colorimetry system equipped with an industrial-grade color sensor and provide two demonstrations for the potential applications. The first is the online detection of cadmium ions (Cd2+) through the Cd2+-promoted etching of glutathione-stabilized AuTNPs that was initialized by H2O2 and iodide. For the first time, the etching time corresponding to the inflection point of the first derivative of the Red channel transmission curve was used for calibration. The method was finally applied in the detection of Cd2+ in fortified rice samples. In the second demonstration, the RGB values continuously acquired using the instrument were converted to HSV (hue, saturation, and value) space, which was successfully employed in pH detection via etching of AuNRs. Importantly, the overaged AuNRs with poor homogeneity could be discerned by the method at the beginning of analysis, which is helpful in achieving reliable measurements. Our results suggest that integration of the low-cost portable instrument and the new strategies has the promising potential for sensitive, accurate, precise, and good-linearity detection.