https://phys.org/news/2018-07-biosensors-implantable.html

Some of the most promising optical biosensors currently being developed are made using single-walled carbon nanotubes. The near-infraredlight emission of the carbon nanotubes lies within the optical transparency window of biological materials. This means water, blood, and tissue such as skin do not absorb the emitted light, making these biosensors ideal for implantable sensing applications. These sensors can thus be placed underneath the skin and the optical signal can still be detected without the need to have electrical contacts piercing through the surface.

However, the omnipresence of salts in biofluids creates a pervasive challenge in designing the implantable devices. Fluctuations in salt concentrations that naturally occur in the body have been shown to affect the sensitivity and selectivity of optical sensors based on single-walled carbon nanotubes wrapped with single-stranded DNA.