Porosity-Based Silicon Nanowires for Optoelectronic Modulation

Nanoscale photoelectrochemical modulation holds great potential for intracellular bioelectric interrogation, offering significant insights into cellular function. Silicon nanowires (SiNWs) are promising candidates for this purpose, as they can be engineered to exhibit photoelectrochemical and photothermal effects through methods such as doping-based homojunctions and porosity-based heterojunctions. Additionally, SiNWs can spontaneously internalize into various cell types, providing versatility in morphology, surface functionalization, and composition. As a result, SiNWs serve as leadless probes for intracellular electrical and mechanical measurements, enabling the induction of calcium fluxes and monitoring of cellular mechanical forces. However, to facilitate the widespread use of SiNWs as leadless intracellular bioelectronic probes, their optoelectronic performance must be enhanced to enable optical modulation at feasible power densities. Furthermore, the underlying mechanisms of nanoscale optoelectronic modulation are not well understood, limiting their broader applicability. To fully utilize SiNWs as tools for intracellular investigation, it is crucial to resolve this underlying mechanism. The complex relationship between photoelectrochemical devices and their surrounding medium must be studied in the context of their nanoscale size. In this regard, optoelectronic modulation may involve photoelectrochemical, photothermal, and/or ROS-mediated mechanisms; however, the exact mechanism remains unclear.<br/>Traditionally, doping-based heterojunction SiNWs are created using chemical vapor deposition (CVD), which generates deep traps in silicon and reduces electron-hole charge separation. In This study, we used top-down methods for producing perfectly single crystallin SiNWs with porosity-based heterojunctions, employing nanosphere lithography and stain etching, which offers advantages at the biointerface. The porous layer modulates the band structure in situ, enabling an efficient photoelectrochemical response and reduces stiffness, which minimizes mechanical mismatch with biological cells. Moreover, the single crystallinity of both the bulk and pores-silicon eliminates the deep traps within polysilicon. The resulting SiNWs have uniform diameters and consistent outcomes. To induce intracellular calcium fluxes, SiNWs were allowed to spontaneously internalized. and cells were then loaded with calcium dye, and a Spinning Disk Confocal was used to optically stimulate and record calcium fluxes simultaneously. When comparing the two fabrication methods, cells exhibited higher calcium velocity due to stimulation with the top-down method, showing less reduction in cell reaction over prolonged duration of up to 5 days. Regarding the intracellular calcium mechanism, our results show that stimulating n-i-p and p-i-n SiNWs (producing photoelectrochemical and photothermal effects) led to higher dF/F values and higher calcium propagation velocities compared to i-i-i SiNWs (thermal effects only). This suggests that the photoelectrochemical reaction was the dominant effect. Additionally, depleting intracellular calcium storage with 5µM Thapsigargin eliminated calcium fluxes in response to optical modulation of p-i-n SiNWs, confirming the internal source of calcium transients. Furthermore, both Cell-SiNW hybrids and SiNW-free cells showed elevated oxidative stress levels with and without stimulation, indicating baseline oxidative stress due to the presence of nanowires regardless of their type. SiNWs exhibit promising potential in optoelectronic biointerfaces, highlighting the need for SiNWs in bioelectric interrogation and studying intracellular cell activity.