Light-addressable electrochemistry at semiconductor electrodes: redox imaging, mask-free lithography and spatially resolved chemical and biological sensing

Spatial confinement of electrochemical reactions at solid/liquid interfaces is a mature area of research, and a central theme from cell biology to analytical chemistry. Monitoring or manipulating the kinetics of a charge transfer reaction in 2D is generally achieved using scanning electrochemical microscopy or multielectrode arrays; techniques that rely on moving physical probes, or on a network of electrical connections. This tutorial is introducing concepts and instruments to confine faradaic electrochemical reactions in 2D, without resorting to the mechanical movement of a probe, and with the simple design of one semiconducting electrode, one electrical lead and a single-channel potentiostat. We provide a theoretical background of semiconductor electrochemistry, and describe the use of localised visible light stimuli on photoconductor/liquid and semiconductor/liquid interfaces to address electrical conductivity ‒ hence chemical reactivity ‒ only at one specific site defined by the experimentalist. This enables to shift the tenet of one electrode/one wire, toward one wire/many electrodes. We discuss the applications of this emerging platform in the context of surface chemistry patterning, redox imaging, chemical and biological sensing, generating chemical gradients, electrocatalysis, nanotechnology and cell biology.


Introduction
Theoretical and historical background of semiconductor photoelectrochemistry.
(2) Spatial confinement of electrochemical reactions: advantages of using light over traditional methods (scanning probes techniques and multielectrode arrays).
(3) Applications of the light-addressable electrochemistry concept in: -Mask-free micropatterning of metals, metal oxides, polymers and molecules on semiconductor and photoconductor surfaces; -Chemical analysis by amperometric detection of adsorbed or diffusive analytes; -Electrochemical microscopy; -Generation of chemical gradients; -Cell biology.
(4) Practical aspects of a light-addressable electrochemistry experiment: choice of photoelectrode material, experimental parameters and experimental designs, such as light pointer vs micromirrors or microprojectors, backside vs frontside illumination (at a dry interface vs across an electrolyte).

Glossary
Semiconductor photoelectrochemistry: the study of chemical processes that are coupled to the flow of electricity at a semiconductor/liquid interface under electromagnetic (generally visible) radiation.
Light-addressable electrochemistry: localised (2D) changes to the thermodynamics and/or kinetics of an electrochemical reaction. The geometrical characteristics of the 2D pattern are specified on an unstructured semiconductor electrode by a light stimulus.
Photoeffects at semiconductors: the generation and recombination of free charge carriers in a semiconductor exposed to supra band gap radiation, and their movement under the influence of an electric field or a concentration gradient.
Photoelectrochemical microscopy: techniques to image the magnitude and dynamics of localised charge transfer heterogeneous reactions.
Photoelectrode: electrode capable of assisting electrochemical reactions when illuminated.
Photoanode/photocathode: photoelectrode giving an anodic/cathodic photocurrent, whose open circuit potential tends to shift cathodic or anodic (respectively) of the dark value upon illumination.   Figure   48 2a-c and explained briefly below (for more detailed information 49 the reader is referred to the bibliography). 6 In a semiconductor   bias for an n-type semiconductor/liquid electrolyte interface. As an obvious consequence of the initial disparity between the concentration of electron and holes, electrode illumination will result in negligible changes in the concentration of majority charge carriers, but will induce a significant increase in the concentration of minority carriers. Photoeffects will therefore be more pronounced in depleted electrodes, that is, when light -generated minority carriers will migrate toward the electrode surface and dominate charge transfer and charge transport. The photoexcited electrons move into the bulk of the semiconductor, while a flux of holes migrate to the interface. A hole-mediated oxidation of a soluble electron donor is shown in (c). In an n-type electrode the bands are bent upward and are assumed to be pinned at the silicon-monolayer interface. (d) Illustration of the principle by which a faradaic process is light-assisted at a semiconductor electrode in the presence a SCL. Here the depleted SCL is used to spatially confine an electro-oxidation reaction by removing a surface kinetic barrier only at the illumination site.
(e) Bias regime required to perform light-addressable electrochemistry (photooxidations) on an n-type semiconductor. Photoeffects would dominate charge transfer in a p-type material that is biased cathodic of its flat band potential (Efb).
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54
The treatment of the electron transfer kinetics at 55 semiconductor electrodes has been pioneered by Gerischer and 56 Lewis. 17 The dark current-potential relationship is given by the 57 second order rate law:    ,  as generated, preventing them to diffuse away from the 3 generation point (Fig. 4b). With backside illumination the 4 carriers must cross a relatively long path until they reach the 5 electroactive species (Fig. 4a).

6
However, despite its poorer 2D resolution, backside 7 illumination is sometimes preferable, such as for the analysis   (Fig. 7a-b). For 54 example, Cu 2+ ions are reduced to Cu + under potentiostatic 55 control to form the Cu2O nanocrystals after hydrolysis (Fig. 7a). 56 We found the spatial resolution to be a function not only of the 57 illuminated area, but also of the amount of charge being

66
We used the projector and adjustments of these three variable 67 to build complex patterns over large areas (Fig 7d), and this has 68 created a platform perfectly suited to investigate the electrical 69 properties of single polyhedral nanocrystals, 11     this possibility by generating pH micro-gradients using a digital 30 micromirror device and an a-Si photoelectrode (Fig. 11a). 27

31
Protons and hydroxide anions were locally generated by the 32 light-addressed chlorine hydrolysis and reduction of water, 33 respectively (Fig. 11a). The formed gradients were imaged by 34 fluorescence microscopy in the presence of a fluorescent pH 35 indicator (Fig. 11b). The digital micromirror device allows the 36 projection of light patterns that can be converted into pH 37 gradient patterns, with a resolution of few micrometers. The 38 dynamic interplay of polarization and light allows for the 39 superposition of pH gradients ( Fig. 11c and d).