Slow Electron Transfer Research Articles

Electrical stimulation accelerated phenanthrene biodegradation coupling with nitrate reduction in groundwater

Phenanthrene (PHE) and nitrate co-contamination widely exists in the groundwater. Anaerobic bioremediation of the two suffered from the slow removal rate and limited electron transfer within microbes. Herein electrical stimulation (ES) was proved to stimulate the simultaneous removal of PHE and nitrate anaerobically. High removal of PHE and nitrate (90.94 % and 100 %, respectively) was obtained within 24 h in the ES-bioreactor, two times higher than the control anaerobic bioreactor (32.16 %). ES-bioreactor exhibited a good PHE removal performance within initial PHE ranging from 5 to 25 mg/L. In ES-bioreactor, extracellular polymeric substances increased PHE bioavailability and acted as protective capsules. Nicotinamide adenine dinucleotide (NADH) and cytochrome C (cyt-C) facilitated the extracellular and intracellular electron exchange during PHE biotransformation. ES significantly selected Acetobacterium, PHE-degraders (f_Bacteroidetes_vadin HA17, Limnobacter and Stenotrophomonas) and electrogens (Clostrium_sensu_stricto_13) to participate in PHE decomposition. Acetobacterium decomposed complex organics to acetate, proving accessible substrates for electrogens to maintain an efficient electron flow. Enzymes regulating bacterial chemotaxis, flagellar assembly and carbohydrate metabolism were up-regulated with ES, suggesting the indispensable role of such enzymatic processes in ES-bioreactor. Overall, this work demonstrates ES can promote anaerobic bioremediation of PHE and nitrate in groundwater and, thus, opens up a new bioelectrochemistry application scenario.

Aggregation of Charge Acceptors on Nanocrystal Surfaces Alters Rates of Photoinduced Electron Transfer.

Semiconductor nanocrystals (NCs) interfaced with molecular ligands that function as charge and energy acceptors are an emerging platform for the design of light-harvesting, photon-upconverting, and photocatalytic materials. However, NC systems explored for these applications often feature high concentrations of bound acceptor ligands, which can lead to ligand-ligand interactions that may alter each system's ability to undergo charge and energy transfer. Here, we demonstrate that aggregation of acceptor ligands impacts the rate of photoinduced NC-to-ligand charge transfer between lead(II) sulfide (PbS) NCs and perylenediimide (PDI) electron acceptors. As the concentration of PDI acceptors is increased, we find the average electron transfer rate from PbS to PDI ligands decreases by nearly an order of magnitude. The electron transfer rate slowdown with increasing PDI concentration correlates strongly with the appearance of PDI aggregates in steady-state absorption spectra. Electronic structure calculations and molecular dynamics (MD) simulations suggest PDI aggregation slows the rate of electron transfer by reducing orbital overlap between PbS charge donors and PDI charge acceptors. While we find aggregation slows electron transfer in this system, the computational models we employ predict ligand aggregation could also be used to speed electron transfer by producing delocalized states that exhibit improved NC-molecule electronic coupling and energy alignment with NC conduction band states. Our results demonstrate that ligand aggregation can alter rates of photoinduced electron transfer between NCs and organic acceptor ligands and should be considered when designing hybrid NC:molecule systems for charge separation.

Artificial visible light-induced H2O2 production using polymeric K/O-doped carbon nitride as a catalyst

In an attempt to improve the decrease in H2O2 yield, resulting from slow electron transfer, availability of limited active sites for reaction, and low O2 activation of pristine polymeric carbon nitride (PCN), we fabricated a K/O-modified PCN photocatalyst with a high specific surface area and high O2 activation ability. A synergy between K and O dopants in PCN aided in reducing the reaction energy barrier of the •OOH intermediate species, thereby allowing PCN to exhibit a high H2O2 yield under white LED light 4428.3 μmol g-1h−1), almost 28 times higher in comparison to that obtained for the pristine PCN. The solar-to-chemical conversion (SCC) efficiencies in the presence of modified PCN were respectively equal to 0.05 % and 0.52 % under white LED light in pure water and ethanol solution (ethanol, 10 vol%). Additionally, the modified PCN exhibited a quite wide optical response. The apparent quantum yields (AQYs) of our modified PCN in ethanol were equal to 23.52 % and 1.19 % at 385 and 520 nm, respectively. Polymeric K/O-doped carbon nitride can therefore be utilized as a potential catalyst for artificial visible light-induced H2O2 production.

Scanning Optical Spectroelectrochemistry: Applications in Protein Redox Potential Measurements

AbstractThe accurate measurement of redox potentials of small molecules is a relatively straightforward task using electrochemical methods such as cyclic voltammetry. However, proteins, in most cases, are not amenable to the same approach due to slow heterogeneous electron transfer and the possibility of denaturing at the electrode surface. This necessitates the use of small molecular weight redox mediators to facilitate electron transfer. This leads to spectroelectrochemical techniques where the applied electrochemical potential is coupled to a spectroscopic signal of the protein. Traditionally this is done at different applied (fixed) potentials akin to an electrochemical titration, but the time required for electrochemical equilibrium to be established, and its consistent application, are major sources of experimental error. Here we have utilised a continuously scanning potential synchronised with time‐resolved UV‐vis spectroscopy to provide an automated approach that can be used to measure protein redox potentials accurately in an expedient manner. The test cases are the heme proteins cytochrome c and myoglobin. The scope and limitations of the method are discussed.

Preparation of NiMoO4 nanoarrays electrodes with optimized morphology and internal crystal water for efficient supercapacitors and water splitting

NiMoO4 shows good electrochemical properties for its multiple oxidation states. However, in practical application, NiMoO4 suffers from poor ion diffusion ability and slow electron transfer rate. In this work, the NiMoO4 nanoarrays with three different morphologies of sheets, rods, and fibers on the nickel foam substrate are successfully prepared, according to the difference of nucleation rate of NiMoO4 in different polar solvents during hydrothermal synthesis. Among them, NiMoO4 nanorods (NiMoO4-NRs) own the highest electrochemical activity due to their higher active surface area utilization. Furthermore, benefiting from the adequate crystal water inside, NiMoO4·1/2H2O-NRs exhibit good ionic diffusion ability. When examined for electrochemical performance experiments, NiMoO4·1/2H2O-NRs can achieve a rapid and reversible Faraday reaction, displaying a high area specific capacity of 2450 mC cm−2 at 1 mA cm−2. The asymmetric supercapacitor (ASC) using NiMoO4·1/2H2O-NRs as a positive electrode is assembled, which achieves a high energy density of 75.1 Wh kg−1 at 849.9 W kg−1 and excellent capacitance retention of 86.4% after 10,000 cycles. Moreover, NiMoO4·1/2H2O-NRs also present outstanding water electrolysis activity. It can afford ultralow overpotential in 1 M KOH solution (166 mV at 10 mA cm−2 for HER and 360 mV at 50 mA cm−2 for OER) and the current density retention of 99.4% (HER) as well as 89.1% (OER) after 10 h continuous electrolysis. The NiMoO4·1/2H2O-NRs designed in our work with optimal surface topography and moderate crystal water have a broad application prospect in both supercapacitors and electrolysis of water.

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation.

Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe(II) is considered as the main electron donor for O2 activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HAred) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HAred suspension. Solid HAred transferred electrons rapidly to O2 via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HAred to O2 for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

Extracellular electron transfer-dependent Cr(VI)/sulfate reduction mediated by iron sulfide nanoparticles

The slow electron transfer rate is a bottleneck to the biological wastewater treatment. This study evaluated the concomitant biotransformation and nonenzymatic reduction of Cr(VI) mediated by sulfate reducing bacteria (SRB), especially for the reinforcing Cr(VI) reduction via accelerating the electron transfer by the in-situ biosynthesized iron sulfide nanoparticles (FeS NPs). The kinetic results showed that 10mg/L Cr(VI) was completely removed by pre-cultured FeS NPs within 7h with kCr(VI) of 2.6×10-4s-1, one magnitude higher than that without FeS NPs. Despite its competing electron to postpone sulfate reduction, the reduction of Cr(VI) was markedly improved via nonenzymatic reactions by the sulfide, the product of sulfate reduction. In the reinforcing system (bio-FeS NP@SRB), the bio-FeS NPs served as an electronic bypass conduit for CoQ could significantly amplify the electron flux, and switch the Cr(VI) reduction from intracellular space to extracellular environment, which had a great detoxification effect on the microorganisms, eventually markedly promoted electron transfer extracellularly and the reduction of Cr(VI). After the long-term acclimatization, Desulfovibrio became the dominant bacteria at the genus level and accounted for the relative abundance of 32%. This study provides an alternative to use biogenic FeS NPs for Cr(VI) remediation.

Recent Advances of Nanomaterials-Based Molecularly Imprinted Electrochemical Sensors.

Molecularly imprinted polymer (MIP) is illustrated as an analogue of a natural biological antibody-antigen system. MIP is an appropriate substrate for electrochemical sensors owing to its binding sites, which match the functional groups and spatial structure of the target analytes. However, the irregular shapes and slow electron transfer rate of MIP limit the sensitivity and conductivity of electrochemical sensors. Nanomaterials, famous for their prominent electron transfer capacity and specific surface area, are increasingly employed in modifications of MIP sensors. Staying ahead of traditional electrochemical sensors, nanomaterials-based MIP sensors represent excellent sensing and recognition capability. This review intends to illustrate their advances over the past five years. Current limitations and development prospects are also discussed.

Co3-xO4/NiO with abundant Ni3+ active sites for boosting oxygen evolution reaction

Electrocatalytic water splitting has received widespread attention. However, due to the slow reaction kinetics and complex electron transfer process, the oxygen evolution reaction (OER) occurring at the anode has become a bottleneck. Herein, the metal-defective Co3-xO4 was selected as the electron-acceptor carrier and Co3-xO4/NiO was synthesized by a simple two-step method. The results show that Co3-xO4/NiO has a low overpotential of 240 mV and 320 mV at 10 mA·cm−2 and 100 mA·cm−2, a low Tafel slope of 64 mV/dec, and the electrochemical surface area (ECSA) of Co3-xO4/NiO is as high as 1033.3 cm2. Moreover, the decrease in activity after the 60 h stability testing is negligible. Combining experiments and theoretical calculations, the high activity and stability of Co3-xO4/NiO is attributed to the tunable electronic structure, and more electrons are transferred from NiO to Co3-xO4 which leads to the in-situ generation of more Ni3+ as active sites. The Co3-xO4/NiO with abundant of Ni3+ can effectively regulate the oxygen-containing intermediates (OH*, O* and OOH*) with appropriate binding energy, reduce Gibbs free energy change of rate-limiting step, and accelerate OER reaction kinetics. This work provides an efficient strategy to generate and stabilize Ni3+ active sites and confirms the key catalytic role of Ni3+ in the OER process.

Revealing the Role of Electronic Doping for Developing Cocatalyst-Free Semiconducting Photocatalysts.

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.

Innocent buffers reveal the intrinsic pH- and coverage-dependent kinetics of the hydrogen evolution reaction on noble metals

<h2>Summary</h2> The hydrogen evolution reaction (HER) is a critical transformation for a renewable energy economy and an ideal testbed for refining mechanistic models of interfacial electrocatalysis. The HER is less efficient in alkaline than in acidic electrolytes; however, the mechanistic origin of this is widely debated. Activation-controlled kinetics at intermediate pH are non-existent due to pH swings or buffer convolution, impeding mechanistic analysis. We identify formate and borate as buffers that minimally perturb the HER kinetics on Au and Pt and report activation-controlled kinetic data between pH 1 and 12. We find that the reaction order in H₃O⁺ is fractional across the pH range as a result of intermediate metal–H coverage and propose that efficiency loss arises from slow proton-coupled electron transfer (PCET) of H₂O versus H₃O⁺. These studies highlight the role of surface coverage in electrocatalytic kinetics and motivate strategies to promote efficient PCET in alkaline media.

Ultrafast separation of multiexcitons within core/shell quantum dot hybrid systems.

We investigated the electron transfer processes in methylene blue-CdTe and methylene blue-CdTe/CdSe complexes by steady state and femtosecond transient absorption spectroscopy by selective excitation of the quantum dot (QD) moiety. The ultrafast electron transfer is accelerated by the shell growth due to the separation of the charge carriers and the resulting increase of electron density in the shell. Transmission electron microscope images show that the successive addition of shell material deforms the spherical QDs until they adopt a tetrapodal shape. The increased donor-acceptor distance in the tetrapodal CdTe/CdSe QDs leads to a slower electron transfer. Photon flux dependent transient absorption measurements indicate the separation of two electrons for the QDs with a thin shell and thus demonstrate that charge carrier multiplication can be directly utilized for increased charge transfer in this type of QD hybrid system.

Inhibited interlayer electron transfer in metal ion linked multilayers on mesoporous metal oxide films

The incorporation of dye-electron donor motifs on metal oxide surfaces is a common strategy to increase dye regeneration rate, slow recombination, and improve overall dye-sensitized device performance. Here we used zinc ion linked multilayer assembly of a dye (N3) and a cobalt polypyridyl electron donor (Co) on TiO2 as an alternative to traditional covalently linked dye-donor systems. The formation of the TiO2-N3-Zn-Co multilayer was monitored using UV–Vis and ATR-IR and we introduced inductively coupled plasma mass spectrometry as a means of quantifying the surface loading of spectroscopically obscured molecules like Co. Interestingly, and contrary to the original intent, the addition of Co electron donor impeded the dye-sensitized device in nearly every performance metric. Transient absorption measurements indicate that both excited state quenching of N3 by Co and Co to N3+ electron transfer are slower than the intrinsic excited state decay and recombination, respectively. We suggest that the geometric restriction imposed by the metal ion linked architecture sufficiently slows electron transfer such that Co is effectively inert in this assembly. While this bilayer did not improve device performance, these insights may guide new design strategies that will improve performance and/or will be of use in other applications where electron transfer is not desired but say communication of spin and/or magnetic moment is needed.

MoC quantum dots embedded in ultra-thin carbon film coupled with 3D porous g-C3N4 for enhanced visible-light-driven hydrogen evolution

The main purpose of photocatalytic hydrogen evolution is to fundamentally solve the problem of energy shortage and environmental pollution. In this experiment, based on the adjustment of the density of MoC quantum dots (MoC-QDs) on carbon film and the morphology of 3D porous g-C3N4, this exploring aimed at exploring the influence of various morphology combinations on the hydrogen evolution performance. Using bulk g-C3N4 (CN-B) as a reference, the tubular and honeycomb g-C3N4 were prepared by solvothermal and chemical vapor deposition methods. The experimental results show that the hydrogen production of MoCT-7 and MoCH-5 composite catalysts are 1.54 and 1.76 times that of MoCB-15, respectively, which proves that the ingenious morphology matching is an effective strategy to improve the catalytic performance. It is believed that CN-H and CN-T have unique slow photo effect and directional electron transfer characteristics, respectively. The well-dispersed MoC-QDs provide abundant active sites, and the thin carbon film is conducive to the adsorption of dye molecules and electrons transfer. This experiment is expected to provide some references for the in-depth study of g-C3N4 and the design of novel photocatalysts.

Reversible or Irreversible Catalysis of H+/H2 Conversion by FeFe Hydrogenases.

Studies of molecular catalysts traditionally aim at understanding how a certain mechanism allows the reaction to be fast. A distinct question, which has only recently received attention in the case of bidirectional molecular catalysts, is how much thermodynamic driving force is required to achieve fast catalysis in either direction of the reaction. "Reversible" catalysts are bidirectional catalysts that work either way in response to even a small departure from equilibrium and thus do not waste input free energy as heat; conversely, "irreversible" catalysts require a large driving force to proceed at an appreciable rate [Fourmond et al. Nat. Rev. Chem. 2021, 5, 348-360]. Numerous mechanistic rationales for these contrasting behaviors have been proposed. To understand the determinants of catalytic (ir)reversibility, we examined the steady-state, direct electron transfer voltammetry of a particular FeFe hydrogenase, from Thermoanaerobacter mathranii, which is very unusual in that it irreversibly catalyzes H2 oxidation and production: a large overpotential is required for the reaction to proceed in either direction [Land et al. Chem. Sci. 2020, 11, 12789-12801]. In contrast to previous hypotheses, we demonstrate that in this particular enzyme catalytic irreversibility can be explained without invoking slow interfacial electron transfer or variations in the mechanism: the observed kinetics is fully consistent with the same catalytic pathway being used in both directions of the reaction.

The hydrophobicity of mutations targeting D1:Val219 modifies formate and diuron binding in the quinone-Fe-acceptor complex of Photosystem II.

The D1:Val219 residue of Photosystem II in the cyanobacterium Synechocystis sp. PCC 6803 was mutated to alanine or isoleucine, creating the V219A and V219I mutants, respectively. Oxygen evolution was slowed in these mutants, while chlorophyll a fluorescence induction assays indicated slowed electron transfer. As previously observed [Erickson J.M., Rahire, M., Rochaix, J.-D. and Mets. L. (1985) Science, 228, 204-207], the V219I mutant was resistant to 3,4-dichloro-1,1-dimethyl urea (DCMU); however, the V219A strain displayed no DCMU resistance. Additionally, the V219A strain was less sensitive to the addition of formate than the control, while the V219I strain was more sensitive to formate. Both mutant strains were susceptible to photodamage and required protein synthesis for recovery. We hypothesize that the sensitivity to DCMU and the extent of bicarbonate-reversible formate-induced inhibition, as well as the capacity for recovery in cells following photodamage, are influenced by the hydrophobicity of the environment associated with the Val219 residue in D1.

Electrochemical, EPR and Quantum Chemical Study of Pyrene-Cyclobutene Conjugates

Various biodegradable organic semiconductors are currently searched and developed for e.g. thin film transistors (OTFTs), organic light-emitting diodes (OLEDs), photovoltaic cells, sensors, radiofrequency identification tags etc. This work is focused on precursors for organic field-effect transistors – OFETs which would be based on a tetrasubstituted double bond serving as the central organic "core" (without heavy metals) and bearing photo- or redox active centers interconnected by π-conjugated bridges. These variably delocalized systems exhibit intramolecular electronic communication (charge transfer, polarizability) depending on the oxidation or excitation state.In the first part of the project, the tetrasubstituted double bond as the core was substituted by cyclobutene and cyclohexene. These "central" molecules bear various aromatic and/or unsaturated pendants, where dominating redox centre(s) are pyrene derivatives.Pyrene represents here an important structural unit which could be involved in π-delocalized systems. It can be electrochemically (and reversibly) oxidized and reduced, therefore it represents important redox center which can be used as a redox probe for more detailed investigations of electron distribution. In addition to this, pyrene exhibit fluorescence properties.For this aim, an extended series of pyrene conjugates based on the cyclobutene and cyclohexene as model precursors was synthesized and their redox properties as well as the relationship between their structure and delocalization (electron distribution) were electrochemically investigated using rotation disk voltammetry, polarography and cyclic voltammetry and their in-situ combination with simultaneously recorded EPR spectra. The experiments were performed in non-aqueous media using AN and DMF as solvents and glassy carbon and mercury (DME, HMDE) as working electrodes in order to obtain thermodynamically relevant data not influenced by solvent or electrode material. The experiments were accompanied by comparison – correlation with quantum chemical calculations.For the fundamental redox characterization, the first oxidation (Eox1) and the first reduction (Ered1) potentials are the most important. In addition to this, their difference correlates with HOMO-LUMO gap and reflects also the spectroscopic - photophysical properties. Nevertheless, we tried to analyze and interpret even the other redox steps in order to outline the redox mechanism and to understand the fate of the molecule after the primary electron transfer, that means follow-up reactions and the geometry changes connected with the changing extent of electron delocalization. Detailed analysis of electrochemical data gives information about intramolecular electron communication and reveals relationship between structure and redox properties, which is important for their tuning and further application.Although pyrene and cyclobutene derivatives seem to be totally explored, their redox properties and ET-initiated reaction mechanisms are not clear and the respective literature is missing. It is generally accepted that pyrene being reduced forms reversibly a stable radical anion and at more negative potentials unstable dianion. However, even the primary radical anion undergoes follow-up reaction yielding the same product like the dianion exhibiting thus most probably disproportionation. As the final product, we expect the 4,5-dihydro derivative, but its proof was not still successful. Gradual reduction of two directly interconnected pyrenes (1,1-dimer) involves partial irreversibility due to the slow intramolecular electron transfer caused by mutual twisting and thus limited electron communication.Similarly, electrochemical reduction of cyclobutene ring was not performed, up to now. Theoretically (based on the analogy) the ring should be reductively open giving rise the cis-disubstituted double bond, our experiments, however, point to the saturation of the double bond preserving the cyclobutane ring. In any case, it was proved that the cyclobutene bridging unit represents extension of delocalized system showing partial reversibility of its first reduction step.Whereas electrochemical oxidation of pyrenes and other aromatic substituents results in anodic polymerization and blocking of the electrode surface, the detailed reduction mechanisms are still under investigation.AcknowledgementsThis work is supported by grant GAČR 18-12150 S. The authors are grateful to Tomáš Tobrman and Peter Polák (University of Chemistry and Technology Prague) for organic synthesis of the compounds. Figure 1

Nickel(II) Bis(diethyldithiocarbamate) As a Redox Mediator in Dye Sensitized Solar Cells

Since the first report of a dye sensitized solar cell (DSSC) in 1991, the 2e- redox cycle of triiodide/iodide served as the archetype for redox mediators. However, major shortcomings of I3 -/I- include corrosion with metal surfaces and its redox potential of ERedox = -0.34 V vs Fc+/0, which limits the maximum photovoltage to ~0.8 V. Increasing ERedox using alternative mediators has been a primary research focus. Although [CoIII/II(bpy)3]3+/2+ and its derivatives have been shown to possess more positive ERedox values, these mediators suffer from slow electron transfer kinetics. Here we present the combination of a multielectron redox cycle with a tunable transition metal complex which could provide the best solution for increasing ERedox while maintaining a 2e- redox cycle. This research focuses on using [NiIV(dtc)3]+/NiII(dtc)2 as a redox mediator in DSSCs where dtc- is a bidentate dithiocarbamate ligand, which undergoes 2e- oxidation from Ni(II) to Ni(IV) during dye regeneration. Steady state photoluminescence quenching of [Ru(bpy)3]2+ shows comparable behavior between Ni(dtc)2 and iodide with a Ksv = 1.72×10-9 M-1 for Ni(dtc)2 compared with 3.67×10-9 M-1 for I-. Photoelectrochemical measurements were made using FTO/TiO2 electrodes sensitized with the N3 dye in the presence of I- or Ni(dtc)2. Cyclic voltammetry experiments were carried out using a 0.001 M Ni(dtc)2 with external light source (white light LED) with different light intensities ranging from 0 to 125 mW/cm2. Similar currents were obtained in the case of Ni(dtc)2 compared to iodide for all measured light intensities. The measured open-circuit potential was also found to be larger in case of Ni(dtc)2, consistent with the ERedox = 0.01 V for the Ni(IV/II) couple. This value is 0.32 V larger than I3 -/I- and could produce a theoretical photovoltage value of 1.15 V which is 0.14 V more than the maximum value reported so far with a cobalt based mediator. These studies show that [NiIV(dtc)3]+/Ni(dtc)2 has the potential to serve as an alternative redox mediator to I3 -/I-.

Electrochemiluminescence at 3D Printed Titanium Electrodes.

The fabrication and electrochemical properties of a 3D printed titanium electrode array are described. The array comprises 25 round cylinders (0.015 cm radius, 0.3 cm high) that are evenly separated on a 0.48 × 0.48 cm square porous base (total geometric area of 1.32 cm2). The electrochemically active surface area consists of fused titanium particles and exhibits a large roughness factor ≈17. In acidic, oxygenated solution, the available potential window is from ~-0.3 to +1.2 V. The voltammetric response of ferrocyanide is quasi-reversible arising from slow heterogeneous electron transfer due to the presence of a native/oxidatively formed oxide. Unlike other metal electrodes, both [Ru(bpy)3]1+ and [Ru(bpy)3]3+ can be created in aqueous solutions which enables electrochemiluminescence to be generated by an annihilation mechanism. Depositing a thin gold layer significantly increases the standard heterogeneous electron transfer rate constant, ko, by a factor of ~80 to a value of 8.0 ± 0.4 × 10−3 cm s−1 and the voltammetry of ferrocyanide becomes reversible. The titanium and gold coated arrays generate electrochemiluminescence using tri-propyl amine as a co-reactant. However, the intensity of the gold-coated array is between 30 (high scan rate) and 100-fold (slow scan rates) higher at the gold coated arrays. Moreover, while the voltammetry of the luminophore is dominated by semi-infinite linear diffusion, the ECL response is significantly influenced by radial diffusion to the individual microcylinders of the array.

Electron Transfer in Films of Atomically Precise Gold Nanoclusters

In applications of monolayer-protected gold clusters, assessing the dynamic behavior of the capping monolayer is crucial, as this determines the cluster’s effective size and electron-transfer (ET) properties. Here, we describe a systematic study on the effect of the monolayer thickness on the ET between molecular Au25(SR)180 nanoclusters in films. The length of the ligands protecting the Au core was varied by using a series of linear-chain thiolate (SCnH2n+1) protected Au25 clusters, where n = 3, 4, 5, 6, 7, 8, and 10. The effect of branching was assessed by using the 2-methyl-1-butanethiolate-protected Au25 cluster. Conductivity measurements were carried out on dry films obtained by drop-casting Au25(SR)180 solutions onto interdigitated gold electrodes. Changing the alkyl chain from C3 to C10 induces a smooth decrease in the film conductivity by 3.5 orders of magnitude. By using electrochemically determined Stokes radii, the conductivity results were transformed into the corresponding ET rate constants (kETs) between neighboring clusters. kET exponentially depends on the distance, and the data show that the average Au center-to-center distance in the film is not larger than the sum of two Stokes radii. This indicates that linear alkyl chains hold a detectable degree of fluidity in films, although not as much as in solution, as shown by the trend of the corresponding heterogeneous standard rate-constant values. For both physical states, these conclusions are precisely confirmed by the much slower ET rate determined for the Au25 cluster protected by the hindered thiolate. ET in films was also studied as a function of temperature, and the results were analyzed in the framework of the ET theory. We found that the outer and inner reorganization energies increase with n. For these ETs, the electronic coupling resonance between the reactant and product states, HRP, ranges from 12 (n = 3) to 0.7 cm–1 (n = 10) and therefore ETs are nonadiabatic. The data also show that 3/4 of the effect of distance on kET is related to HRP, while the rest is associated with the intrinsic barrier and thus the distance dependence of the outer and inner reorganization energies.