Aubergine to Blackberry: Nature Derived Chromophores to Mimic Photosensitive Neurons in Human Eye

Visual information processing in vertebrates begins with light detection by the eye’s retina cone and rod cells. Tri-colour cones, typical for primates, respond to blue, green and red/yellow spectral bands, whereas rods cell are extremely sensitive to low light intensities, providing grey-scale vision. Mimicking behaviour of these cells with optoelectronic devices opens up opportunities for neural stimulation, artificial retina, superhuman vision devices, and self-powered optical sensors. Nature is also rich in conjugated, organic semiconductor molecules, with various bandgaps, thus offering colour-specific responses in the visible spectral range.<br/>In this work we explore fruit- and vegetable-derived conjugated chromophores from various berries, including raspberry, blackberry, elderberry, blueberry, as well as aubergine, beetroot, red cabbage and spinach, targeting photo-induced response in three spectral bands, red, green and blue. Self-assembly binding properties of these molecules allow to build chromophores-decorated TiO2 nano-porous structures inspired by dye-sensitised solar cell technologies. We demonstrate photo-capacitor devices employing the chromophores, interfaced with biological electrolyte (phosphate buffered saline), showing spiking photocurrent (photovoltage) response under low-intensity pulsed light excitation.<br/>Absorption spectra of chromophores were found to be in the visible part of the spectrum, with chlorophyll derived from spinach peaking at 440 nm, and with anthocyanins and betalains chromophores’ peaks at around 520 nm. The photocurrent spectral response of the samples was correlated with that of absorption; however, some spectral shifts were observed. Photo-current response signals were clearly present for all samples, with magnitude from few tens to hundreds of nA when exposed to pulsed-light flash cycles with a pulse length of few tens of milliseconds. This is comparable to typical biological response times. The results suggest that nature- derived conjugated chromophores present feasible environmentally-friendly semiconductors for opto-stimulated devices with colour-selectivity. They also produce photo -current and -voltage responses corresponding to hyperpolarisation and depolarisation of cell membranes associated with neurons’ spiking behaviour.