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Our contribution to Graphene Revolution in biomedical research

 

Being simply a support structure or an electrode/biosensor is not good enough for graphene, when it can truly rule the world.

With this thought in mind, we developed a new technology that allows graphene (together with light) to modulate and control the fate and activities functions of a live cell. 

To do so, we took advantage of the fact that graphene can efficiently convert light into electricity via a hot-carrier multiplication process on a femtosecond timescale.    

Graphene is a zero-band gap semiconductor (i.e., neither metal nor semiconductor), and its electrons behave as “massless” quasiparticles. The crucial difference between graphene and conventional semiconductor materials is that in graphene light produces “hot” ballistic electrons that transfer their energy through a very efficient carrier–carrier scattering process, leading to multiple hot-carrier generation over a wide range of light frequencies. Additionally, the mean free path of photogenerated “hot” ballistic electrons can be up to 1 micron, which provides enhanced flexibility in spatial positioning of G-biointerfaces near cells.

The result: Graphene-mediated optical stimulation (GraMOS) technology based on optoelectronic properties of graphene and biophysical properties of live cells.

Mechanism

When a cell is positioned near a light-illuminated graphene surface, photo-generated electrons in from graphene will be able to change the cell membrane potential, as they displace cations near the graphene/cell membrane interface due to the capacitive coupling effect between the cell membrane and the surface of graphene-related materials.

This process will first lead to membrane depolarization, then to activation of voltage-gated ion channels, and, finally, to action potential generation in excitable cells. 

As a result, a graphene-based optoelectronic biointerface (G-biointerface) can provide optical stimulation of cells via the external light-generated electric field that interacts with the transmembrane field gradient, leading to cell activation in a physiological manner.

Properties of GraMOS

  • can be activated by a wide range of light wavelengths.

  • can be activated by the light intensities at the same range as required for optogenetic activation;

  • does not require genetic modification of cells;

  • provides physiological stimulation via a capacitive mechanism rather than exogenous optogenetics-driven ion currents;

  • demonstrates exceptional biocompatibility which appears to be enabled by exceptional electrical conductivity, mechanical properties, and micro-scale topographic features of graphene-based biointerfaces

  • exhibits super-fast kinetics which is determined by the femtosecond lifetime of photogenerated electrons in graphene.

It means that there is no charge accumulation on GraMOS biointerfaces, and cells can be activated by switching the light on/off with the superb temporal resolution. Further, the diffusion of short-lived photogenerated electrons beyond the illuminated area is minimal, resulting in the spatial resolution defined by dimensions of a light signal.

  • no Faradaic effects

Faradaic charge transfer between photo-generated electrons and ions is accompanied by redox reactions, resulting in changes in the solution pH value. We found that pH values of extracellular solution were not affected during two-hour continuous light illumination (4.6 mW/mm2) of immersed G-biointerfaces, which strongly suggest that the Faradaic mechanismis not involved. Moreover, since we did not apply voltage bias, and the lifetimes of photogenerated electrons in graphene are in the picosecond range, photogenerated currents are expected to be negligible.

  • No thermal effects

Direct light-induced heat-mediated effects on cells are unlikely in our experimental conditions, because a) ~1,000-times higher light intensity is required for heat effects; b) light-induced heat usually produces gradually increasing changes in cell properties during continuous light exposure, while we observed a steady-state response. The presence of graphene in our experiments did not change this status quo, as we found no changes in the surface temperature of G-coated coverslips during light illumination. This finding was expected, because graphene is a highly efficient light-to-electricity converter. Due to due to zero-band gap and strong electron-electron interactions in graphene, photogenerated electrons are poorly coupled to the graphene surface and preferentially distribute their energy to multiple secondary electrons rather than produce lattice heating.