State Photocurrent Research Articles

Effect of the Heterovalent Sc3+ and Nb5+ Doping on Photoelectrochemical Behavior of Anatase TiO2

In this study, we explored the effect of either Nb or Sc doping at a concentration range of 0.0–1.0 at.% on the physical–chemical and photoelectrochemical behavior of TiO2 anatase electrodes. This behavior was characterized by work function, flat band potential, donor density, spectral dependence of photocurrent and stationary photocurrent measurements. All experimental results are interpreted in terms of the formation of the shallow delocalized polaron states in the case of Nb doping and deep acceptor states induced by Sc doping on TiO2 anatase.

A Nano-Biohybrid-Based Bio-Solar Cell to Regulate the Electrical Signal Transmission to Living Cells for Biomedical Application.

Bio-solar cells are studied as sustainable and biocompatible energy sources with significant potential for biomedical applications. However, they are composed of light-harvesting biomolecules with narrow absorption wavelengths and weak transient photocurrent generation. In this study, a nano-biohybrid-based bio-solar cell composed of bacteriorhodopsin, chlorophyllin, and Ni/TiO2 nanoparticles is developed to overcome the current limitations and verify the possibility of biomedical applications. Bacteriorhodopsin and chlorophyllin are introduced as light-harvesting biomolecules to broaden the absorption wavelength. As a photocatalyst, Ni/TiO2 nanoparticles are introduced to generate a photocurrent and amplify the photocurrent generated by the biomolecules. The developed bio-solar cell absorbs a broad range of visible wavelengths and generates an amplified stationary photocurrent density (152.6nAcm-2 ) with a long lifetime (up to 1month). Besides, the electrophysiological signals of muscle cells at neuromuscular junctions are precisely regulated by motor neurons excited by the photocurrent of the bio-solar cell, indicating that the bio-solar cell can control living cells by signal transmission through other types of living cells. The proposed nano-biohybrid-based bio-solar cell can be used as a sustainable and biocompatible energy source for the development of wearable and implantable biodevices and bioelectronic medicines for humans.

Stimulation of medial amygdala GABA neurons with kinetically different channelrhodopsins yields opposite behavioral outcomes.

The medial amygdala (MeA) receives pheromone information about conspecifics and has crucial functions in social behaviors. A previous study showed that activation of GABA neurons in the postero-dorsal MeA (MeApd) with channelrhodopsin-2H134R (ChR2) stimulates inter-male aggression. When performing these experiments using the faster channelrhodopsinH134R,E123T (ChETA), we find the opposite behavioral outcome. A systematic comparison between the two channelrhodopsin variants reveals that optogenetic activation of MeApd GABA neurons with ChETA suppresses aggression, whereas activation under ChR2 increases aggression. Although the mechanism for this paradoxical difference is not understood, we observe that activation of MeApd GABA neurons with ChR2 causes larger plateau depolarizations, smaller action potentials, and larger local inhibition than with ChETA. Thus, the channelrhodopsin variant used for invivo optogenetic experiments can radically influence the behavioral outcome. Future work should continue to study the role of specific sub-populations of MeApd GABA neurons in aggression control.

Rhodopsin Channel Activity Can Be Evaluated by Measuring the Photocurrent Voltage Dependence in Planar Bilayer Lipid Membranes.

The studies of the functional properties of retinal-containing proteins often include experiments in model membrane systems, e.g., measurements of electric current through planar bilayer lipid membranes (BLMs) with proteoliposomes adsorbed on one of the membrane surfaces. However, the possibilities of this method have not been fully explored yet. We demonstrated that the voltage dependence of stationary photocurrents for two light-sensitive proteins, bacteriorhodopsin (bR) and channelrhodopsin 2 (ChR2), in the presence of protonophore had very different characteristics. In the case of the bR (proton pump), the photocurrent through the BLM did not change direction when the polarity of the applied voltage was switched. In the case of the photosensitive channel protein ChR2, the photocurrent increased with the increase in voltage and the current polarity changed with the change in the voltage polarity. The protonophore 4,5,6,7-tetrachloro-2-trifluoromethyl benzimidazole (TTFB) was more efficient in the maximizing stationary photocurrents. In the presence of carbonyl cyanide-m-chlorophenylhydrazone (CCCP), the amplitude of the measured photocurrents for bR significantly decreased, while in the case of ChR2, the photocurrents virtually disappeared. The difference between the effects of TTFB and CCCP was apparently due to the fact that, in contrast to TTFB, CCCP transfers protons across the liposome membranes with a higher rate than through the decane-containing BLM used as a surface for the proteoliposome adsorption.

Stationary photocurrent generation from bacteriorhodopsin-loaded lipo-polymersomes in polyelectrolyte multilayer assembly on polyethersulfone membrane.

Vesicles constructed of either synthetic polymers alone (polymersomes) or a combination of polymers and lipids (lipo-polymersomes) demonstrate excellent long-term stability and ability to integrate membrane proteins. Applications using lipo-polymersomes with integrated membrane proteins require suitable supports to maintain protein functionality. Using lipo-polymersomes loaded with the light-driven proton pump bacteriorhodopsin (BR), we demonstrate here how the photocurrent is influenced by a chosen support. In our study, we deposited BR-loaded lipo-polymersomes in a cross-linked polyelectrolyte multilayer assembly either directly physisorbed on gold electrode microchips or cross-linked on an intermediary polyethersulfone (PES) membrane covalently grafted using a hydrogel cushion. In both cases, electrochemical impedance spectroscopic characterization demonstrated successful polyelectrolyte assembly with BR-loaded lipo-polymersomes. Light-induced proton pumping by BR-loaded lipo-polymersomes in the different support constructs was characterized by amperometric recording of the generated photocurrent. Application of the hydrogel/PES membrane support together with the polyelectrolyte assembly decreased the transient current response upon light activation of BR, while enhancing the generated stationary current to over 700nA/cm2. On the other hand, the current response from BR-loaded lipo-polymersomes in a polyelectrolyte assembly without the hydrogel/PES membrane support was primarily a transient peak combined with a low-nanoampere-level stationary photocurrent. Hence, the obtained results demonstrated that by using a hydrogel/PES support it was feasible to monitor continuously light-induced proton flux in biomimetic applications of lipo-polymersomes. Graphical abstract.

MerMAIDs: a family of metagenomically discovered marine anion-conducting and intensely desensitizing channelrhodopsins

Channelrhodopsins (ChRs) are algal light-gated ion channels widely used as optogenetic tools for manipulating neuronal activity. ChRs desensitize under continuous bright-light illumination, resulting in a significant decline of photocurrents. Here we describe a metagenomically identified family of phylogenetically distinct anion-conducting ChRs (designated MerMAIDs). MerMAIDs almost completely desensitize during continuous illumination due to accumulation of a late non-conducting photointermediate that disrupts the ion permeation pathway. MerMAID desensitization can be fully explained by a single photocycle in which a long-lived desensitized state follows the short-lived conducting state. A conserved cysteine is the critical factor in desensitization, as its mutation results in recovery of large stationary photocurrents. The rapid desensitization of MerMAIDs enables their use as optogenetic silencers for transient suppression of individual action potentials without affecting subsequent spiking during continuous illumination. Our results could facilitate the development of optogenetic tools from metagenomic databases and enhance general understanding of ChR function.

Electrical properties, substrate specificity and optogenetic potential of the engineered light-driven sodium pump eKR2

A new microbial rhodopsin class that actively transports sodium out of the cell upon illumination was described in 2013. However, poor membrane targeting of the first-identified sodium pump KR2 in mammalian cells has hindered the direct electrical investigation of its transport mechanism and optogenetic application to date. Accordingly, we designed enhanced KR2 (eKR2), which exhibits improved membrane targeting and higher photocurrents in mammalian cells to facilitate molecular characterization and future optogenetic applications. Our selectivity measurements revealed that stationary photocurrents are primarily carried by sodium, whereas protons only play a minor role, if any. Combining laser-induced photocurrent and absorption measurements, we found that spectral changes were not necessarily related to changes in transport activity. Finally, we showed that eKR2 can be expressed in cultured hippocampal mouse neurons and induce reversible inhibition of action potential firing with millisecond precision upon illumination with moderate green-light. Hence, the light-driven sodium pump eKR2 is a reliable inhibitory optogenetic tool applicable to situations in which the proton and chloride gradients should not be altered.

Complex Photochemistry within the Green-Absorbing Channelrhodopsin ReaChR

Channelrhodopsins (ChRs) are light-activated ion channels widely employed for photostimulation of excitable cells. This study focuses on ReaChR, a chimeric ChR variant with optimal properties for optogenetic applications. We combined electrophysiological recordings with infrared and UV-visible spectroscopic measurements to investigate photocurrents and photochemical properties of ReaChR. Our data imply that ReaChR is green-light activated (λmax = 532 nm) with a non-rhodopsin-like action spectrum peaking at 610 nm for stationary photocurrents. This unusual spectral feature is associated with photoconversion of a previously unknown light-sensitive, blue-shifted photocycle intermediate L (λmax = 495 nm), which is accumulated under continuous illumination. To explain the complex photochemical reactions, we propose a symmetrical two-cycle-model based on the two C15=N isomers of the retinal cofactor with either syn- or anti-configuration, each comprising six consecutive states D, K, L, M, N, and O. Ion conduction involves two states per cycle, the late M- (M2) with a deprotonated retinal Schiff base and the consecutive green-absorbing N-state that both equilibrate via reversible reprotonation. In our model, a fraction of the deprotonated M-intermediate of the anti-cycle may be photoconverted—as the L-state—back to its inherent dark state, or to its M-state pendant (M′) of the syn-cycle. The latter reaction pathway requires a C13=C14, C15=N double-isomerization of the retinal chromophore, whereas the intracircular photoconversion of M back to D involves only one C13=C14 double-bond isomerization.

Controllable stationary photocurrents generated from a bacteriorhodopsin/upconversion nanoparticle-based bionanosystem under NIR illumination.

In the past few decades, tremendous effort has been dedicated to develop bacteriorhodopsin (bR)-based photo-electronic devices for generating a stationary photocurrent and further for use as a component of artificial retinas in constant illumination sensing. However, an IR-triggered stationary photocurrent with controllable amplitudes has never been realized to date. Herein, NaYF4:Yb,Er and NaYF4:Yb,Tm upconversion nanoparticles (UCNPs) with green and blue emissions, respectively, were synthesized and further incorporated with bR to build a bionanosystem. Under 980 nm NIR irradiation the UCNPs function as internal green and blue light sources to initiate the photocycle and speed up the transition of bR from M410 to the ground state, consequently accelerating the bR photocycle for the generation of a stationary photocurrent. Moreover, the photocurrent profile could be tailored by changing the blue/green emission intensity ratio. The mechanism is analysed to explore the scientific insights. The system consisting of controllable blue and green light sources may not only hold great promise to construct new types of bR-based optical devices, but also offers a useful setup to investigate the fundamental science underlying the bR photoresponse.

Kinetics of Water Oxidation at TiO2 Nanotube Arrays at Different pH Domains Investigated by Electrochemical and Light-Modulated Impedance Spectroscopy

The electronic properties of a TiO2-nanotube (NT) array used as a photoelectrode for water oxidation at neutral and basic pH were characterized by combining complementary measurement techniques: transient photocurrents, stationary photocurrent–voltage curves, photo-electrochemical impedance spectroscopy (PEIS), and intensity-modulated photocurrent spectroscopy (IMPS). Transient measurements point out the slow chemical modification of the TiO2 surface when going from dark to light, essentially around neutral pH. After this transient period, a new stationary state of the TiO2 surface is established, allowing small amplitude perturbation techniques (PEIS and IMPS) to be applied to obtain information on transfer and recombination kinetics and on surface states contribution. The relevant information was obtained via theoretical models for the PEIS and IMPS responses, involving physical parameters with values extracted by nonlinear least-squares fitting. The main conclusions taken from our experiments include t...

Influence of surface potential barriers on the dependence of photocurrent on intensity of exciting light in cadmium sulfide films with a nanostructured surface

The influence of the photoexcitation intensity on the value of the stationary photocurrent in pure and Li-doped pyrolitic CdS films with a nanostructured surface has been studied. The brightness dependence of the photocurrent of the structures under study is approximated by a power function whose power index is a decay function of the excitation intensity. The unusual peculiarity of the photocurrent dependence on illumination intensity is explained by a modulation of the surface potential barriers for the drift and recombination of nonequilibrium charge carriers.

Photoelectrochemical Studies and Capacitance Measurements during the Nitride Passivation of InP in Liquid Ammonia (-55°C)

This work presents a fundamental photo-electrochemical process at the interface SC/electrolyte. The photoelectrochemical passivation of n-InP is reported in liquid ammonia (-55 °C) during an anodic treatment[1]. The extraction of InP sample from the solvent is successfully performed at ambient temperature. Thanks to XPS (X-ray Photoelectron Spectroscopy), a reproducible polyphosphazene like film is reported on InP (-[(H2N)-P=N]n-)[2]. A soft anodic electrochemical treatment under illumination is applied at the interface to provide a gradual controlled coverage ratio of the surface by the film. This quantitative study requires a chronopotentiometry process with a constant anodic current of 1µA.cm-2 under illumination. An anodic charge around 0.3 mC.cm-2 is enough to ensure a complete coverage of the surface by the film. According to the anodic charge a gradual striking flat band potential shift is observed (Fig. 1). A flat C-2 = f(V) is extended over 1 volt leading to a high interfacial capacitance (» 2µF.cm-2). From the analysis of capacitance-potential curves, the interface appears to be under accumulation conditions after the film formation. A significant drop of the resulting photo potential is then observed (Fig. 2). However, under a strong positive overvoltage (» 1V), a stationary anodic photocurrent is recovered. [1] A-M. Gonçalves, N. Mézailles, C. Mathieu, P. Le Floch, A. Etcheberry, « «Fully Protective yet functionalizable monolayer onto InP” Chemistry of Materials.22(2010) 3114-3120. [2] A-M. Gonçalves, C.Njel, C. Mathieu, D.Aureau, A. Etcheberry. « Phosphazene like film formation on InP in liquid ammonia (223K) . Thin Solid Films 538 (2013) 21–24. Figure 1

Photoelectrochemical Evidence of Cu2O/TiO2 Nanotubes Hetero-Junctions formation and their Physicochemical Characterization

Cu2O/TiO2 nanotubes heterojunctions were fabricated by electrochemical deposition of cuprous oxide on TiO2 nanotubes arrays grown by anodizing. X-ray diffraction and Raman Spectroscopy analysis allows for identification of Cu2O, whose morphological features were studied by Scanning Electron Microscopy as a function of the charge circulated during the electrodeposition step. Photoelectrochemical measurements in aqueous solutions evidenced a red shift of the light absorption threshold of TiO2 nanotubes due to the presence of cuprous oxide even for very low circulated charges, while electrochemical impedance measurements proved a significant reduction of the electrode impedance due the presence of Cu2O. Photoelectrochemical measurements in non aqueous solutions allow to reduce reverse bias across the junction and to measure stationary cathodic photocurrent, thus confirming the formation of p-type Cu2O inside the nanotubes.

Photogalvanic current in electron gas over a liquid helium surface

We study the stationary surface photocurrent in 2D electron gas near the helium surface. Electron gas is assumed to be attracted to the helium surface due to the image attracting force and an external stationary electric field. The alternating electric field has both vertical and in-plane components. The photogalvanic effect originates from the periodic transitions of electrons between quantum subbands in the vertical direction caused by a normal component of the alternating electric field accompanied by synchronous in-plane acceleration/deceleration due to the electric field in-plane component. The effect needs vertical asymmetry of the system. The problem is considered taking into account a friction caused by the electron-ripplon interaction. The photocurrent resonantly depends on the field frequency. The resonance occurs at field frequencies close to the distance between well subbands. The resonance is symmetric or antisymmetric depending on the kind (linear or circular) of polarization.

Photogalvanic current in a parabolic well

We study the in-plane stationary photocurrent in a parabolic potential well. The well has vertical asymmetry due to inhomogeneous distribution of scatterers. The electric field of light has both vertical and in-plane components. The photogalvanic effect originates from the periodic oscillation of electrons in a vertical direction caused by the normal component of the alternating electric field with simultaneous in-plane acceleration/deceleration by the in-plane component of electric field. The problem is considered in classical approximation assuming inhomogeneously-distributed friction. Photocurrent has a resonance character. Resonance occurs at light frequencies close to a characteristic well frequency. The effect of in-plane magnetic field is also studied.

Correlated electron dynamics in quantum-dot sensitized solar cell: Kadanoff-Baym versus Markovian approach

Electron dynamics in quantum-dot sensitized solar cells is studied in the nonequilibrium Green's function approach. To solve Kadanoff-Baym (KB) equations we propose a method based on two-time spectral expansion of nonequilibrium Green's functions. The method is very efficient for stationary photocurrent calculations. The calculated photocurrent is compared with the computations based on the Markovian master equations. It is shown that the Markovian approximation overestimates the photocurrent versus the numerical solutions of the exact KB equations. The discrepancy can reach as much as one order of magnitude and, therefore, the Markovian approximation can overestimate the solar cell efficiency by one order of magnitude.

Bacteriorhodopsin-based photo-electrochemical cell

A simple solution-based electrochemical cell has been constructed and successfully employed in the detection of the photoelectric response upon photoexcitation of bacteriorhodopsin (bR) without external bias. Commercially-available indium tin oxide (ITO) glasses served as the optical windows and electrodes. Small amounts of bR suspensions (∼100 μL) were utilized as the photovoltaic medium to generate the proton gradient between two half-cells separated by a molecular porous membrane. Continuous broadband visible light ( λ > 380 nm) and a short-pulse 532-nm laser were employed for the photoexcitation of bR. Upon the modulated cw broadband irradiation, an instantaneous rise and decay of the current was observed. Our observations of the pH-dependent photocurrent are consistent with previous reports in a bR thin film configuration, which also showed a polarity inversion at pH 5–6. This is due to the change of the priority of the proton release and proton uptake in the photocycle of bR. Studies on the ionic strength effect were also carried out at different KCl concentrations, which resulted in the acceleration of the rise and decay of the photoelectric response. This was accompanied by a decrease in the stationary photocurrent at higher KCl concentrations in the broadband excitation experiments. The solution-based electrochemical cell uses aqueous medium, which is required for the completion of the bR proton pumping function. Due to the generation of the stationary current, it is advantageous to convert solar energy into electricity without the need of film-based photovoltaic devices with external bias.

Plasmonic Field Enhancement of the Bacteriorhodopsin Photocurrent during Its Proton Pump Photocycle

The proton pump photocycle of bacteriorhodopsin (bR) produces photocurrent on a microsecond time scale which is assigned to the deprotonation step forming the M(412) intermediate. The return of the M(412) intermediate to the bR ground state (bR(570)) has two pathways: (1) thermally via multiple intermediates (which takes 15 ms) or (2) by a more rapid and direct process by absorbing blue light (which takes hundreds of nanoseconds). By using nanoparticles (Ag, Ag-Au, and Au NPs) having different surface plasmon resonance extinction spectra, it is found that Ag NPs whose spectrum overlaps best with the M(412) absorption regions enhance the stationary photocurrent 15 times. This large enhancement is proposed to be due to the accelerated photoexcitation rate of the M(412) (in the presence of the plasmon field of the light in this region) as well as short-circuiting of the photocycle, increasing its duty cycles.

A photoelectrochemical and spectroscopic study of phenol and catechol oxidation on titanium dioxide nanoporous electrodes

Abstract The photoelectrochemical behavior of TiO2 nanoporous thin film electrodes has been studied in the presence of phenol and catechol. Voltammograms, both in the dark and under illumination, as well as photocurrent transients were combined with Attenuated Total Reflection Infrared measurements. Particular attention has been paid to the way that photocurrent is sensitive to the organics concentration. A non-monotonic, unusual tendency has been observed in all cases. For low organics concentration (lower than 0.1 mM), there is a deactivation of the electrode toward the water photooxidation process. Parallel spectroscopic measurements prove that, in this concentration range, the amount of adsorbed phenol (or catechol) is very small (undetectable), which supports the existence of a low coverage of surface active sites for the water photooxidation process. In the high concentration range, an increase of the organics concentration leads to an unexpected decrease in the stationary photocurrent. This behavior, attributed to both the build-up of an intermediate acting as a recombination center and a fouling of the oxide surface, has rarely been observed in TiO2 heterogeneous photocatalysis.

Stationary drag photocurrent caused by strong effective running wave in quantum wires: Quantization of current

The stationary current induced by a strong running potential wave in one-dimensional system is studied. Such a wave can result from illumination of a straight quantum wire with special grating or spiral quantum wire by circular-polarized light. The wave drags electrons in the direction correlating with the direction of the system symmetry and polarization of light. In a pure system the wave induces minibands in the accompanied system of reference. We study the effect in the presence of impurity scattering. The current is an interplay between the wave drag and impurity braking. It was found that the drag current is quantized when the Fermi level gets into energy gaps.