Terahertz for Tracking the Cadmium Heavy Metal Accumulation During Pak Choi (Brassica chinensis) Food Production Processes

Detecting heavy metal–phytochelatin complexes in plants would simplify identification of cultivars with both high tolerance and enrichment abilities for heavy metals. In this study, we introduced a novel approach for the characterization and detection of Cd–PC2 complexes using terahertz spectroscopy, density functional theory, circular dichroism, and chemometrics, where the phytochelatin2 (PC2) was innovatively used as a natural probe to amplify terahertz signals of the trace Cd. Results showed that PC2 chelates Cd2+ to form Cd(PC2)2 complex; the Cd connected to thiols of the PC2 and changed β-turn and random coil of the PC2 peptide chain to β-sheet, which presented as terahertz vibrations of the PC2 around 1.03 and 1.71 THz being suppressed. This study demonstrated that the terahertz spectroscopy can be utilized to visualize and quantify chelated Cd in pak choi (Brassica chinensis) leaves, with a detection limit down to 1.151 ppm, and suggests that terahertz information of the complexes can be used as a simpler alternative to classical indices for phytoextraction performance evaluation and cultivar identification.