Light can be used to modulate the behaviour of cells like neurons for applications in clinical therapeutics and for fundamental single-cell studies. However, most of the methods available today to do this are invasive, require cells to be genetically modified and are not precise enough. A new technique that makes use of free-standing photoactivated coaxial silicon nanowires containing atomic gold on their surfaces could help overcome these problems.
Photobiology is the study of the effect of light on organisms that have evolved over millions of years to enhance the amount of light they absorb thanks to specialized machinery. Vision in animals and energy harvesting for photosynthesis in plants are two well-known examples.
Researchers have recently extended this concept to artificially make cells and living organisms more sensitive to light. New work by a team of researchers at the University of Chicago in the US makes use of silicon nanowires measuring just a few hundred nanometres across and consisting of a p-type (boron-doped) core and an n-type (phosphorus-doped) shell lined with surface atomic gold.
When illuminated with light, photoexcited charge carriers (electrons and holes) are produced in the wires. These carriers then separate at the core-shell junction with the photogenerated holes migrating to the core and the electrons being trapped at the surface of a nanowire by the surface gold.
Cathode current depolarizes neuronal membrane
“These electrons then participate in cathodic electrochemical reactions in a surrounding electrolyte solution, generating a cathodic current,” explains team leader Bozhi Tian. “When we then interface the coaxial nanowires with a target neuronal membrane, this current depolarizes the membrane, mimicking the effect of a nerve impulse and causing the neuron to fire an action potential.”
Surprisingly, a single nanowire can prompt this neuronal firing, he adds.
The researchers say they successfully tested out their technique on primary rat dorsal root ganglion neurons.
Towards clinical therapeutics
“This tool could be used for both fundamental single bioelectric studies and clinical therapeutics,” Tian tells nanotechweb.org. “Silicon strongly absorbs light in the near-infrared, a wavelength of light that deeply penetrates biological tissue, which means that the nanowires could be used to stimulate peripheral nerves (lying as far as 1 cm below the skin) if injected into tissue. This could ultimately allow for non-invasive treatment of diseases characterized by severe neuropathic pain, such as diabetic peripheral neuropathy, for example.”
The researchers, reporting their work in Nature Nanotechnology doi:10.1038/s41565-017-0041-7, say they will now try to use these nanowires to modulate voltage in both photo-excitable and non-excitable cellular systems for fundamental bioelectric studies. “We also plan to target specific cell types after modifying their surfaces and develop non-invasive treatments in mouse models of neuropathic diseases,” says Ramya Parameswaran, lead author of this study.