Quantifying the levels and oxidative potential of submicron carbon black in plant leaves

Abstract

Large amounts of the submicron carbon black are emitted in urban areas. Here, we developed a quantitative method using dual-wavelength ultraviolet–visible spectroscopy and Raman spectroscopy (respectively) for carbon black with amorphous structures and with ordering in a graphene sheet. Further, the dithiothreitol (DTT) assay and the ascorbic acid (AA) assay, were used to assess the oxidative potential (OP) of water extracts of leaf samples. Humic acids were adopted as a reference for the OP comparisons with carbon black in both assays. Using dual-wavelength ultraviolet–visible spectroscopy to determine carbon black with amorphous structures, our results achieved coefficients of determination greater than 0.99 in the linear range between 0.5 and 200 μg mL−1. Determination of carbon black with ordering in a graphene sheet using dual-wavelength Raman spectroscopy showed a linear in the range from 2 to 200 μg mL−1. The contents of carbon black with amorphous structures in tree leaves of passive bio-monitoring samplers were found to be in the range of 0.27–0.32 mg m−2, while the carbonaceous particles ordered with a graphene sheet were undetected. The OP of water extracts of leaves varied from 29 to 38 nmol min−1 mg−1 determined by DTT assay, and 16–30 nmol min−1 mg−1 by AA assay, respectively. The water-soluble ions enhanced the OP of submicron carbon black due to the presence of sp2-hybridized carbon atoms. These findings can help frame measures to mitigate health risks from nano-carbon black emitted from varied sources.