Flexible optoelectronic biointerfaces using quantum dots and pseudocapacitive materials for photoelectric stimulation of neurons

Bioelectronic medicine offers significant benefits against a wide variety of neurological disorders such as Parkinson's disease, depression, retinal degeneration, and hearing loss. Today neurostimulation constitutes a market size on the order of several billion dollars and it is one of the gold-standards practiced in neuroscience field for unraveling the working principles of brain. Fundamentally, it utilizes electrical stimulation to control and modulate the neural signals at single cell and network level. The conventional electrical stimulation devices use wires to induce potential differences for producing ionic currents at the abiotic-biotic interface. However, wired electrodes increase the surgical invasiveness and complexity, and adds to the surgery time of implants. Moreover, wires can be dislocated and fractured due to unintentional tissue movements, which can lead to unwanted side effects. Alternatively, optoelectronic biointerfaces convert light to photovoltages and provide localized and nongenetic neural stimulation without requiring wired systems. However, little attention has been paid to the applicability of non-silicon optoelectronics in neural stimulation. Colloidal quantum dots (QDs) exhibit promising optoelectronic properties for neural stimulation such as high absorption coefficient, which allows fabrication of flexible and light-weight devices; tunable absorption profile, which facilitates extension of frequency spectrum from visible to near-infrared (NIR) region; and excellent optical stability, which minimizes the photobleaching and photodegradation effects during long-term use. Despite the ubiquitous use of QDs in different optoelectronic applications (e.g., light-emitting devices, solar cells, and photodetectors), their use in neural stimulation has been limited. This thesis investigates the use of QDs for developing novel photovoltaic neurostimulators that can be built on flexible substrates with ultra-thin layers; have high photoresponsivity with improved charge injection; and offer a wide operation window that covers visible and near-infrared (NIR) spectrum. So far, most neural stimulation studies by optoelectronic biointerfaces focused on activation of neurons by inducing action potentials. However, neural silencing is also beneficial for examination of neural mechanisms. For that, we demonstrate that indium phosphide (InP)-based QDs can be integrated into two different photovoltaic device architectures to build neural stimulating and silencing photoelectrodes. We nanoengineer InP/ZnS core/shell nanostructure and device structure to maximize the Faradaic photocurrent to control neural activity. Instead of Faradaic charge injection, which raises concerns due to the possibility of irreversible chemical reactions at the electrode-electrolyte interface, capacitive charge injection is a safer mechanism because of the charging/discharging of the double layer capacitance, which does not involve direct charge transfer between the electrode and electrolyte. For that, we introduced a QD-polymer donor-acceptor nanoheterojunction to generate capacitive photocurrent and control the faradaic and capacitive components of the photoresponse that leads to capacitive-dominant charge injection by optimizing the nanoheterojunction parameters. One of the main drawbacks of capacitive photostimulation electrodes has been the limited photogenerated charge densities due to the fast photocurrent spikes by the double-layer. We show that this problem can be overcome by integrating supercapacitor ruthenium oxide (RuO2) into a photocapacitive device architecture. RuO2 integration leads to over-an-order-of-magnitude improved charge injection density owing to the high interfacial capacitance of RuO2 resulting from reversible redox reactions. This enables safe photostimulation of hippocampal neurons with light intensities below 1 mW mm-2 via visible light. Furthermore, shifting the photoresponse spectrum to NIR wavelengths is advantageous to enhance the light penetration depth into the tissues. For that we demonstrate NIR-sensitive QD-based neurostimulators in a flexible and ultrathin device structure that operates within the ocular safety limits. The findings reported in this thesis are important in terms of improving efficiency, safety, and applicability of QD-based optoelectronic neural interfaces in different neurostimulation scenarios. Demonstrated high photoresponsivity levels, safe charge injection mechanisms, and NIR-operation are especially valuable for building safe and efficient optoelectronic neuroprostheses. Thus, this thesis paves the way toward a QD-based photovoltaic retinal implant against blindness due to degeneration of photoreceptors.