Semiconducting single-walled carbon nanotubes (s-SWNTs) fluoresce in the near-infrared, a spectral window where tissue is transparent, making them ideal for biosensing applications. Prior studies demonstrated that s-SWNTs can be used as fluorophores and can also be made sensitive to single molecules by polymer encapsulation. Metallic SWNTs (m-SWNTS), on the other hand, do not fluoresce, which has caused them to be overlooked in biological applications. However, m-SWNTs surprisingly still host strong optical transitions, which are ideal for use in photoacoustic microscopy. This technique uses a light pulse that is absorbed in a medium and converts it into sound waves that are then recorded using an ultrasound transducer. The positioning of the laser in the tissue affects the intensity of the signal detected by the ultrasound and is highest at the wavelength where peak absorption occurs. In this study, we demonstrate for the first time that m-SWNTs can indeed give a photoacoustic response. When combining this data with measurements of the optical absorbance spectra of bare m-SWNTs and polymer encapsulated m-SWNTs before and after the addition of dopamine, we mapped the absorbance spectrum of m-SWNTs. This is just the beginning step to measuring the brain’s chemical interactions using the conversion of light into sound. Future work includes measuring the optical absorbance of equal diameter m-SWNTs and measuring the dopamine-induced photoacoustic response of polymer encapsulated m-SWNTs.
