Vision loss due to the degeneration of photoreceptor cells in the retina occurs in millions of people every year worldwide through diseases such as retinitis pigmentosa and age-related macular degeneration. This research aims to aid in the creation of an artificial retinal implant to replace these lost receptor cells in order to regain vision. This project focuses on the technology inspired by dye sensitised solar cell groups that use natural dyes as chromophores. Dyes bound to the surface of TiO₂ nanoparticles absorb photons of light which results in the excitation of an electron. This electron is then transferred to the TiO₂ nanostructured electrode, leaving a hole at the electrolyte surface. In our work, devices with TiO₂ nanoparticle layers, decorated with chromophores derived from fruits and vegetables, with photoresponses in the blue, green, and red spectral regions are intended to work as photocapacitors upon excitation with light. The devices aim to mimic a natural process of colour-selective photoexcitation in the eye. Our devices are envisioned to create a photoinduced voltage spike that will be sufficient to stimulate retinal neurons - specifically bipolar cells. The retinal neuron network is assumed to still be functional be after photoreceptor degradation.

The natural dyes that have been investigated include pomegranate, various berries, beetroot, red cabbage, and spinach. These juices have provided the different chromophores: anthocyanins, betalain, and chlorophyll. Like the rod and cone cells responsible for light absorption in the human retina, each of these chromophores have different absorption spectra with different peaks. Chlorophyll was found to be too unstable however juices containing anthocyanin chromophores from blackberry and red cabbage juice solution gave absorption spectra peaking at 574 nm and 586 nm respectively. Those with betalains from beetroot juice solution gave a peak around 448 nm. From these absorption peaks, we consider that red cabbage chromophores will be able to emulate the function of long (red absorbing) human cone cells which peak at 564 nm, blackberry chromophores will emulation medium (green absorbing) human cone cells which peak at 534 nm, and beetroot chromophores, with more blue-shifted response, could emulate the short (blue absorbing) human cone cells response which peaks at 420 nm. Analysis of photocurrent spectra data demonstrates that devices stained with these juice solutions show peaks in the correct spectral ranges. Devices synthesised using beetroot, blackberry, and red cabbage juice solution show peak responsivity at 428 nm, 542 nm, and ≈600 nm respectively. Selective excitations using pulsed LEDs with emission bands overlapping with strongest absorption bands of the chromophores pulsed at 20 ms in a 100 ms period and power density of 40 µW mm−2 have yielded transient photocurrent and photovoltage measurements of devices stained using blackberry, red cabbage and beetroot with values of -351 nA, 364 nA, and -192 nA and -5.1 mV, -5.5 mV, and -3.1 mV respectively. Although these values did not exceed -20 mV (the amount of depolarisation bipolar cells in a functioning retina will experience when being activated by adjacent photoreceptor cells) longer pulses of 350 ms have shown photovoltage outputs to increase to -90 mV, -49 mV, and -60 mV for beetroot, blackberry, and red cabbage juice solutions respectively, highlighting the potential of the devices after further optimisation.

This study indicates the feasibility of the use of chromophores from natural dyes in devices may be able to produce photovoltages high enough to stimulate bipolar cells of the retina. Future steps in the investigation to ensure this have been discussed and include i) improving the stability of chromophores, ii) optimisation of the semi-conducting layer, iii) growing and imaging of nerve cells on the device surface.