Development of a Sensitive SERS Method for Label-Free Detection of Hexavalent Chromium in Tea Using Carbimazole Redox Reaction

Tea plants absorb chromium-contaminated soil and water and accumulate in tea leaves. Hexavalent chromium (Cr⁶⁺) is a very toxic heavy metal; excessive intake of tea containing Cr⁶⁺ can cause serious harm to human health. A reliable and sensitive surface-enhanced Raman spectroscopy (SERS) method was developed using Au@Ag nanoparticles as an enhanced substrate for the determination of Cr⁶⁺ in tea. The Au@AgNPs coated with carbimazole showed a highly selective reaction to Cr⁶⁺ in tea samples through a redox reaction between Cr⁶⁺ and carbimazole. The Cr⁶⁺ in the contaminated tea sample reacted with methimazole-the hydrolysate of carbimazole-to form disulfide, which led to the decrease in the Raman intensity of the peak at 595 cm^-1. The logarithm of the concentration of Cr⁶⁺ has a linear relationship with the Raman intensity at the characteristic peak and showed a limit of detection of 0.945 mg/kg for the tea sample. The carbimazole functionalized Au@AgNPs showed high selectivity in analyzing Cr⁶⁺ in tea samples, even in the presence of other metal ions. The SERS detection technique established in this study also showed comparable results with the standard ICP-MS method, indicating the applicability of the established technique in practical applications.

Biofilm-Templated Heteroatom-Doped Carbon-Palladium Nanocomposite Catalyst for Hexavalent Chromium Reduction

In this study, we report an interdisciplinary and novel strategy toward biofilm engineering for the development of a biofilm-templated heteroatom-doped catalytic system through bioreduction and biofilm matrix-facilitated immobilization of the in situ-formed catalytic nanoparticles followed by controlled pyrolysis. We showed that (i) even under room temperature and bulk aerobic conditions, Shewanella oneidensis MR-1 biofilms reduced Pd(II) to form Pd(0) nanocrystals (∼10 to 20 nm) that were immobilized in the biofilm matrix and in cellular membranes, (ii) the MR-1 biofilms with the immobilized Pd(0) nanocrystals exhibited nanocatalytic activity, (iii) exposure to Pd(II) greatly increased the rate of cell detachment from the biofilm and posed a risk of biofilm dispersal, (iv) controlled pyrolysis (carbonization) of the biofilm led to the formation of a stable heteroatom-doped carbon-palladium (C-Pd) nanocomposite catalyst, and (v) the biofilm-templated C-Pd nanocomposite catalyst exhibited a high Cr(VI) reduction activity and maintained a high reduction rate over multiple catalytic cycles. Considering that bacteria are capable of synthesizing a wide range of metal and metalloid nanoparticles, the biofilm-templated approach for the fabrication of the catalytic C-Pd nanocomposite we have demonstrated here should prove to be widely applicable for the production of different nanocomposites that are of importance to various environmental applications.

[Adsorption and Photocatalytic Removal of Chromium on High-index TiO2 Facet]

Chromium (Cr) contamination caused by industrial manufacturing poses a severe challenge in the environment. Titanium dioxide (TiO₂) has potential application in Cr removal due to its adsorption and photocatalytic performance. High-index TiO₂ with exposed {201} facet was synthesized using the solvothermal method and characterized by SEM, TEM, XRD, and XPS. The adsorption of Cr(Ⅲ/Ⅵ) and photocatalytic reduction of Cr(Ⅵ) on TiO₂{201} was examined for the removal from water. The synthesized TiO₂{201} was constructed by a dandelion-like hierarchical structure. The adsorption isotherms of Cr(Ⅲ) and Cr(Ⅵ) on TiO₂{201} conformed to the Langmuir model, with maximum adsorption capacities of 22.7 mg·g^-1 and 13.2 mg·g^-1, respectively. The best fitted results from the Freundlich model show that the adsorption of Cr(Ⅲ) and Cr(Ⅵ) on TiO₂{201} were favorable with the parameter of 1/n less than 0.5. The results of photocatalytic reduction show that TiO₂{201} can reduce Cr(Ⅵ) to Cr(Ⅲ) under UV irradiation, and Cr(Ⅲ) was further precipitated on the surface of TiO₂ in the form of Cr(OH)₃ and Cr₂O₃, which was evidenced by XPS characterization. To explore the mechanism of photocatalytic reduction of Cr(Ⅵ), the effect of scavengers for photogenerated holes (EDTA-2Na) and electrons (KBrO₃) on Cr(Ⅵ) reduction was studied, and the results suggested that photogenerated electrons were the main reductant.