Retina-inspired artificial synapses and near-infrared optoelectronic neurostimulation devices using quantum dots

Neurons are the building blocks of the nervous system and are responsible for generating, receiving, and transmitting electrical and chemical signals. By replicating the functionality of biological neurons, it is possible to achieve response capabilities that mimic human abilities. Such biomimetic signal generation and transmission at bio-electronic interfaces can restore, replace, or enhance biological functions. In addition, it can provide efficient event-driven processing and high-accuracy perception without separate memory and processing units. Neuromorphic bioelectronics, inspired by the functions of neurons, have the potential to enable biomimetic communication with cells. These systems necessitate operation in aqueous environments, generation of sufficient ionic currents for neurostimulation, and plasticity. In this thesis, a novel compact neuromorphic synapse is introduced, combining photodetection, memory, and neurostimulation functionalities within a single device. Utilizing cell-interfacing InP/ZnS quantum dots, this device achieves artificial photoreception through photo-faradaic charge-transfer mediated plasticity. The device induces excitatory post-synaptic currents that exhibit paired-pulse facilitation and post-tetanic potentiation in hippocampal neurons, simulating the natural biomimetic temporal summation of neuronal signals. Additionally, this thesis explores the potential of silver bismuth sulfide (AgBiS2) colloidal nanocrystals for photovoltaic retina implants. AgBiS2 nanocrystals, known for their chemical stability and absence of toxic heavy metals, have an exceptional absorption coefficient (>105 cm-1) in the near-infrared (NIR) spectrum, surpassing silicon and many of inorganic nanocrystals such as PbS, CdTe, and perovskite. An ultrathin AgBiS2 nanocrystal layer (24 nm) is integrated into a water-stable photovoltaic bioelectronic device, which produces high-level capacitive photocurrent (2.17 ± 0.2 mA.cm-2) and ionic charge (10.97 ± 0.7 µC.cm-2) in artificial cerebrospinal fluid. The long-term stability of the device and non-toxicity to neurons underscore its suitability for bioelectronic applications. Electrophysiological studies on primary hippocampal neurons reveal that these devices can elicit neuron firing at NIR intensity levels well within ocular safety limits. To sum up, this thesis demonstrated the first toxic heavy-metal-free nanocrystal-based optoelectronic biointerface operating within the NIR spectral window. Moreover, it shows that quantum dots hold high promise for unconventional and biomimetic control of cells for future retina implants.