Odo’s peptide-modified carbon nanotube field-effect transistors selectively detect volatile organic compound biomarkers in exhaled breath at concentrations as low as 0.5 parts-per-billion, a 1,000-fold improvement over traditional “eNoses.” This lets us measure biomarkers to include hydrogen sulfide, indole, butyric acid, and aldehyde inflammatory markers, indicators for specific microbiome niches and biological processes in the body that were previously hidden.