  Emerging concepts for neuromorphic computing, bioelectronics, and
brain-computer interfacing inspire new research avenues aimed at understanding
the relationship between oxidation state and conductivity in unexplored
materials. Here, we present ruthenium Prussian blue analogue (RuPBA), a mixed
valence coordination compound with an open framework structure and ability to
conduct both ionic and electronic charge, for flexible artificial synapses that
reversibly switch conductance by more than four orders of magnitude based on
electrochemically tunable oxidation state. Retention of programmed states is
improved by nearly two orders of magnitude compared to the extensively studied
organic polymers, thus reducing the frequency, complexity and energy costs
associated with error correction schemes. We demonstrate dopamine detection
using RuPBA synapses and biocompatibility with neuronal cells, evoking
prospective application for brain-computer interfacing. By application of
electron transfer theory to in-situ spectroscopic probing of intervalence
charge transfer, we elucidate a switching mechanism whereby the degree of mixed
valency between N-coordinated Ru sites controls the carrier concentration and
mobility, as supported by DFT.
