Optical biosensors facilitate advances in various disciplines of biological research by enabling the exploration of questions that are difficult to address with other methods. While many optical biosensors are based on genetically engineered proteins, synthetic optical sensors have also made important contributions. Among these synthetic biosensors, SWCNTs possess useful photophysical attributes that make them particularly well-suited for applications in biology. SWCNT-based biosensors have been developed for a wide range of analytes, including reactive oxygen species , small biomolecules and lipids , neuropeptides , proteins , disease biomarkers , and even bacteria and viruses .