Illuminating the Underlying Mechanism of Intracellular Optoelectronic Modulation Using Silicon Nanowires

New bioelectrical modulation techniques are constantly improving the spatial resolution of electronic extra- and intracellular biointerfaces using different micro and nanomaterials. Although substrate bound micro and nanoelectrodes are extremely useful, they are limited in the sense of delivering bioelectrical modulation to cells that are inside a 3D tissue construct, or in vivo. Thus, many leadless technologies are being developed. One of the most promising approaches for leadless electrical modulation with high spatial resolution is the use of optoelectronic nanomaterials that can transduce optical illumination to an electrical output. Free standing silicon nanowires (SiNWs) with coaxial p-i-n junction are ideal for minimally invasive intracellular biointerface as they are soft, flexible, and thus can spontaneously internalize into many different cell types. They have been used by us and others to deliver extra- and intracellular bioelectrical modulation to neurons, oligodendrocytes, cardiomyocytes, cardiac myofibroblasts and more. However, the underlying mechanism of how SiNWs transduce the optical energy to an electrical modulation was not yet fully determined and is still under debate. In this context, the optical effect may be photothermal via capacitive membrane currents or transient membrane poration. On the other hand, the optical modulation induces a photoelectrochemical response which may produce reactive oxygen species (ROS) that can trigger local calcium release from neighboring organelles (ER or mitochondria). Alternatively, the photoelectrochemical response can generate a local change in the cytosol electrical potential which may induce an ionic response via voltage gated ion channels. Clearly, to disseminate this technology within the relevant scientific community, it is essential to decipher the underlaying mechanisms that govern SiNWs based optical response.<br/>In this study we used different approaches to assess the potential contribution of the proposed mechanisms. First, we used p-i-n and n-i-p coaxial core-shell SiNWs which possess photoelectrochemical properties to investigate the effect of opposing anodic/cathodic biointerfaces. Moreover, we used similar core-shell i-i-i SiNWs to test the contribution of photothermal effect without any photoelectrochemical response. We used calcium imaging and a tailored made algorithms to quantify the calcium flux in response to the modulation. We found that both p-i-n and n-i-p diode junctions resulted in comparable optical responses which were significantly higher than the i-i-i photothermal response. This suggests that the photoelectrochemical effect is dominant over photothermal response.<br/>To investigate whether the calcium source is internal or external, we used Thapsigargin to deplete intracellular calcium stores. Alternatively, we used calcium free media to eliminate any extracellular calcium and compared the cellular responses to optical modulation. We found that the eliminating intracellular calcium abolished any optical response, while extracellular calcium elimination had no apparent effect. Thus, we conclude that intracellular calcium stores are the source for the intracellular modulation.<br/>To investigate the contribution of ROS, we used ROS sensitive dyes to monitor changes in ROS upon optical modulation. Although a small ROS increase was observed in cells baring internalized SiNWs, intracellular ROS did not change upon optical modulation. This suggests that local ionic response is the dominant mechanism for inducing intracellular calcium release from organelles.<br/>To further understand this phenomenon, we will use different specific channel blockers to identify the channels that play a role in calcium release upon optical modulation. We believe that the understanding of this mechanism will facilitate the dissemination of this technology into bioelectronic research and open new avenues for studying bioelectronic communication in 3D microenvironment and in vivo.