Metallic Fingerprints of Carbon: Label-Free Tracking and Imaging of Graphene in Plants

Detection and quantification of carbon nanomaterials are extremely challenging, especially under the background interference of carbon. Here, we propose a new label-free method to quantify, track, and in situ image graphene and graphene oxide (GO) in plants based on their inherent metallic impurities as fingerprints. We show the ubiquity and high stability of inherent metallic fingerprints of graphene and GO obtained from different exposure routes under the natural environments, which enables the materials to be easily quantified and in situ imaged by high-sensitivity (laser ablation) inductively coupled plasma mass spectrometry. The method was applied to investigate the uptake and spatial distribution of graphene and GO in soybean plants. The plants were cultivated in graphene or GO solutions for 7 days, and the indicative elements (Ni or Mn) in different parts of plants were monitored and imaged. We found that graphene and GO showed different distribution patterns in plants (the highest uptake percentages in root up to 14.4% for graphene and 47.8% for GO), and high concentration of material exposure might cause excessive accumulation of materials in roots which blocked their further transport to the other parts of plants. The present method is more straightforward, accessible, and economical than normally used isotopic or metal-labeling methods. It also avoids the uncertainties or alterations of properties caused by the labeling process and thus has great promise in analysis and risk assessment of carbon nanomaterials.