Azodicarbonamide (ADA) has been extensively used as a flour additive due to its oxidizing and bleaching properties. However, the World Health Organization (WHO) reported that exposure to ADA could lead to allergies, asthma, and respiratory irritations. ADA is also readily decomposed into biurea and semicarbazide. Semicarbazide is a water-soluble white solid with known genotoxicity and carcinogenicity. For these reasons, many countries have set the maximum allowable amount of ADA in flour, some even went ahead to ban the use of ADA completely.
The current analytical methods for detecting ADA can be classified into indirect and direct methods. Indirect methods measure the semicarbazide hydrochloride produced from ADA, which include enzyme-linked immunosorbent assays, liquid chromatography coupled with mass spectrometry (LC−MS), and fluorescent assay. Direct methods, such as high-performance LC (HPLC), infrared spectrometry, capillary electrophoresis, surface-enhanced Raman spectroscopy (SERS), and colorimetric assay, detect the presence of ADA directly. While all of these methods provide highly accurate detection, they all require sophisticated instruments and complex operation steps, limiting their uses in point-of-care testing.
With the widespread use of the smartphone, the integration of laser/light sensor-based devices in smartphones have been increasingly common. Photoacoustic (PA) detection is a promising method for sensing liquids, solids or gases as the signal depends on the optical absorption of the sample, without being affected by reflected and scattered light. Inspired by this technique, Lan et. al developed a novel smartphone-integrated PA platform. Prussian blue (PB) has been used as a strong PA sensing agent owing to its high molar extinction coefficient. In the presence of Ag+ ions, the PA signal would rapidly decrease due to the conversion from PB to Ag4[Fe (CN)6] nanoparticles. Glutathione (GSH) can restore the PA signal to baseline by the ion bindings of Ag+ ions with its thiol functional group. However, ADA can also oxidize glutathione disulfide (GSSG), resulting in the release of Ag+ from GSH. Such changes in the PA signal can be captured by the PA probe and analyzed by the platform.
Figure 1: Working Principle of the Portable Photoacoustic Platform Based on Smartphone Readouts.
The subsequent studies confirmed the feasibility of using such a detecting system. It was found that when Ag+ and GSH coexisted, the intensities of the corresponding absorption peak and PA signal could be almost completely recovered, which was attributed to the chelation between GSH and Ag+; when ADA was added in the presence of GSH and Ag+, the recovered absorption peak and PA signal were quenched again, demonstrating that ADA causes GSH to lose its chelating ability. This could also be directly observed for colour changes in the PB test strips. In the controlled testing settings, the measured concentration of ADA has only an 8% deviation compared with the known concentration, indicating the high analytical precision and reliability of the established point-of-care testing method.
Figure 2: (A) PA signals of the PB NPs/Ag+/GSH system with different concentrations of ADA. (B) Relationship between the PA signal and ADA concentration.
Overall, the team developed a simple and quantitative smartphone-based method for rapid ADA testing. This system provides a solid foundation for the effectiveness of these legal provisions and the safety of flour-related products for consumption. Such work certainly has the potential to be used for quantitative point-of-care testing by local communities in developing regions to improve healthcare, environmental safety, and food quality.
The finding of this research has been published Analytical Chemistry: Guo, L.; Zhao, D.-M.; Chen, S.; Yu, Y.-L.; Wang, J.-H. Smartphone-Integrated Photoacoustic Analytical Device for Point-of-Care Testing of Food Contaminant Azodicarbonamide. Anal. Chem. 2022, 94 (40). https://doi.org/10.1021/acs.analchem.2c03319
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