Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu

Organic Field-Effect Transistors for Interfacial Chemistry: Monitoring Reactions on SAMs at the Solid-Liquid Interface

Chemical modification of self-assembled monolayers (SAMs) at the solid-liquid interface can effectively impart SAMs with desired functions on demand. However, appropriate methods to monitor organic reactions at the solid-liquid interface have not yet been established. Therefore, this perspective introduces an extended-gate type organic field-effect transistor (EG-OFET)-based detector to monitor chemical reactions at the interface between SAMs on the extended-gate electrode of the OFET and an aqueous solution containing reactants. The EG-OFET is operated by applying gate voltages, enabling the monitoring of organic reactions on the extended-gate electrode through changes in transistor characteristics. Leveraging its amplification ability, the EG-OFET enables the sensitive detection of slight differences in product properties accompanied by variations in the charge and/or dipole moment of the SAM caused by chemical reactions at the interface. This perspective summarizes strategies, including those combined with chemometrics and microfluidic technologies, for monitoring irreversible and reversible chemical reactions at the solid-liquid interface.

Mitigating dithiothreitol interference to gold/thiol interface in electrochemical detection of cathepsin B activity toward multiplex protease analysis

Proteases are overexpressed at various stages of conditions such as cancers and thus can serve as biomarkers for disease diagnosis. Electrochemical techniques to detect the activity of extracellular proteases have gained attraction due to their multiplexing capability. Here we employ an electrochemical approach based on a 3 × 3 gold (Au) microelectrode array (MEA) functionalized with (2-aminoethyl)ferrocene (AEF) tagged specific peptide substrates to monitor cathepsin B (CB) protease activity. Cleavage of these peptide substrates by proteases leads to an exponential decay in the alternating current voltammetry (ACV) signal. The protease activity is represented by the inverse of the decay time constant (1/τ), which is equal to (kcat/KM)[CB] based on the heterogeneous Michaelis-Menton model. However, the thiol/Au chemisorption linking AEF-peptide to gold electrodes is susceptible to interference by the protease activation reagent dithiothreitol (DTT), causing the peptides to desorb from the Au surface during continuous ACV measurement. This induces a false signal decay, masking the protease activity and reducing the sensor sensitivity. To address this, DTT is removed after activating CB using centrifugal filtration while EDTA is incorporated to maintain the enzyme activity. This allows accurate CB proteolysis kinetics and clarifies the roles of EDTA and DTT in activation. The intrinsic substrate-dependent cleavage by CB to three different peptide substrates has been demonstrated with the MEA chip, showcasing the potential for rapid activity profiling of multiple proteases. The study highlights the importance of understanding the interference of active bioreagents to the thiol/Au interface in broad redox-tagged electrochemical biosensors.

Direct Electrooxidation of Ethylene to Ethylene Glycol over 90% Faradaic Efficiency Enabled by Cl- Modification of the Pd Surface

Direct electrochemical ethylene-to-ethylene glycol (C2H4-to-EG) conversion can potentially reduce the consumption of fossil fuels and the emission of carbon dioxide (CO2) compared with the traditional thermo-catalytic approach. Palladium (Pd) prepared by electrodeposition is represented as a promising electrocatalyst; however, it exhibits low Ethylene glycol (EG) current density (<4 mA cm-2), Faradaic efficiency (<60%), and productivity (<10 μmol h-1), hindering practical applications. Herein, we report a nanodendrite palladium catalyst supported on a large-area gas diffusion electrode. This catalyst gives high EG current density (12 mA cm-2) and productivity (227 μmol h-1) but low Faradaic efficiency (65%). With further Cl- ions modification, Faradaic efficiency increased to a record-high value of 92%, and EG current density (18 mA cm-2) and productivity (∼340 μmol h-1) were also promoted. Experimental data suggest that the strong electron-withdrawing feature of Cl- reduces the oxidation ability of in situ generated Pd-OH species, inhibiting EG overoxidation to glycol aldehyde. Meanwhile, Cl- alters EG adsorption configuration─from parallel and dual-site coordination to vertical and single-site coordination─over the Pd surface, thus preventing C-C bond cleavage of EG to CO2. In addition, Cl- adsorption facilitates the generation of Pd-OH active species to improve catalytic activity. This work demonstrates the great potential of surface ion modification for improving activity and selectivity in direct electrochemical C2H4-to-EG conversion, which may have implications for diverse value-added chemicals electrosynthesis.

Hydrolytic, Thermal, and Electrochemical Stability of Thiol- and Terminal Alkyne-Based Monolayers on Gold: A Comparative Study

The terminal alkyne-Au interaction is emerging as a promising adsorbing bonding motif for organic monolayers, allowing it to be used for installing antifouling layers and/or recognition elements on gold surfaces for biosensing applications. In contrast to the well-known thiol-on-gold monolayers, the long-term hydrolytic, thermal, and electrochemical stability of the alkyne-Au bond remains relatively unexplored. Insight into these is, however, essential to deliver on the promise of the alkyne-Au bond for (bio)sensing applications, and to see under which conditions they might replace thiolate-gold bonds, if the latter are insufficiently stable due to, e.g., biological thiol exchange. Therefore, these stabilities were investigated for monolayers on Au substrates formed from 1-octadecanethiol and 1-octadecyne. Additionally, monodentate and tridentate alkyne-based adsorbates were designed to investigate the effect of multivalency on the stability. The hydrolytic stability over time in four aqueous media and the thermal stability in air were evaluated using static water contact angle measurements and X-ray photoelectron spectroscopy. Electrochemical oxidative desorption potentials were also assessed by cyclic voltammetry. All three tests indicate that the monovalent terminal alkyne monolayers on gold are slightly less stable than their thiolate analogs, which we could attribute to a lower packing density but still sufficiently stable to be applied in biosensing in the gut, while multivalency can further improve this. Our work provides insight into the stability of terminal alkynes under different conditions, better enabling the use of terminal alkyne-Au interactions in biosensors.

Modified Working Electrodes for Organic Electrosynthesis

Organic electrosynthesis has gained much attention over the last few decades as a promising alternative to traditional synthesis methods. Electrochemical approaches offer numerous advantages over traditional organic synthesis procedures. One of the most interesting aspects of electroorganic synthesis is the ability to tune many parameters to affect the outcome of the reaction of interest. One such parameter is the composition of the working electrode. By changing the electrode material, one can influence the selectivity, product distribution, and rate of organic reactions. In this Review, we describe several electrode materials and modifications with applications in organic electrosynthetic transformations. Included in this discussion are modifications of electrodes with nanoparticles, composite materials, polymers, organic frameworks, and surface-bound mediators. We first discuss the important physicochemical and electrochemical properties of each material. Then, we briefly summarize several relevant examples of each class of electrodes, with the goal of providing readers with a catalog of electrode materials for a wide variety of organic syntheses.

Beyond the Gold-Thiol Paradigm: Exploring Alternative Interfaces for Electrochemical Nucleic Acid-Based Sensing

Nucleic acid-based electrochemical sensors (NBEs) use oligonucleotides as affinity reagents for the detection of a variety of targets, ranging from small-molecule therapeutics to whole viruses. Because of their versatility in molecular sensing, NBEs are being developed broadly for diagnostic and biomedical research applications. Benchmark NBEs are fabricated via self-assembly of thiol-based monolayers on gold. Although robust for rapid prototyping, thiol monolayers suffer from limitations in terms of stability under voltage modulation and in the face of competitive ligands such as thiolated molecules naturally occurring in biofluids. Additionally, gold cannot be deployed as an NBE substrate for all biomedical applications, such as in cases where molecular measurements coupled to real-time, under-the-sensor tissue imaging is needed. Seeking to overcome these limitations, the field of NBEs is pursuing alternative ligands and electrode surfaces. In this perspective, I discuss new interface fabrication strategies that have successfully achieved NBE sensing, or that have the potential to allow NBE sensing on conductive surfaces other than gold. I hope this perspective will provide the reader with a fresh view of how future NBE interfaces could be constructed and will serve as inspiration for the pursuit of collaborative developments in the field of NBEs.

Spectroelectrochemical determination of thiolate self-assembled monolayer adsorptive stability in aqueous and non-aqueous electrolytes

Self-assembled monolayers (SAM) are ubiquitous in studies of modified electrodes for sensing, electrocatalysis, and environmental and energy applications. However, determining their adsorptive stability is crucial to ensure robust experiments. In this work, the stable potential window (SPW) in which a SAM-covered electrode can function without inducing SAM desorption was determined for aromatic SAMs on gold electrodes in aqueous and non-aqueous solvents. The SPWs were determined by employing cyclic voltammetry, attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and surface plasmon resonance (SPR). The electrochemical and spectroscopic findings concluded that all the aromatic SAMs used displayed similar trends and SPWs. In aqueous systems, the SPW lies between the reductive desorption and oxidative desorption, with pH being the decisive factor affecting the range of the SPW, with the widest SPW observed at pH 1. In the non-aqueous electrolytes, the desorption of SAMs was observed to be slow and progressive. The polarity of the solvent was the main factor in determining the SPW. The lower the polarity of the solvent, the larger the SPW, with 1-butanol displaying the widest SPW. This work showcases the power of spectroelectrochemical analysis and provides ample future directions for the use of non-polar solvents to increase SAM stability in electrochemical applications.

Layered Sn-Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement

Operando electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (EC ATR-SEIRAS) is a valuable method for a fundamental understanding of electrochemical interfaces under real operating conditions. The applicability of this method depends on the ability to tune the optical and catalytic properties of an electrode film, and it thus requires unique optimization for any given material. Motivated by the growing interest in Sn-based electrocatalysts for selective reduction of CO2 to formate species, we investigate several Sn thin-film synthesis routes for the resulting SEIRA signal response. We compare the SEIRA performance of thermally evaporated metallic Sn to a series of Sn-based films on top of a SEIRA-active Au substrate (metallic Sn, oxide-derived metallic Sn, and metal oxide SnOx). Using alkanethiol self-assembled monolayers as a probe, we find that electrodepositing metallic catalyst films on top of SEIRA-active Au substrates yield higher signal relative to thermal evaporation as well as higher signal than the independent SEIRA-active Au underlayer. These observations come despite the fact that thermally evaporated Sn has a significantly higher surface roughness (and thus higher adsorbate population), suggesting specific SEIRA-magnifying effects for the stacked films. Finally, we applied these films to observe the electrochemical conversion of CO2. Differences are observed in spectral features based on the composition of the electrode being either metallic or oxide-derived metallic Sn, implying differences in their respective reaction pathways.

Enhanced CO2 Reactive Capture and Conversion Using Aminothiolate Ligand-Metal Interface

Metallic catalyst modification by organic ligands is an emerging catalyst design in enhancing the activity and selectivity of electrocatalytic carbon dioxide (CO2) reactive capture and reduction to value-added fuels. However, a lack of fundamental science on how these ligand-metal interfaces interact with CO2 and key intermediates under working conditions has resulted in a trial-and-error approach for experimental designs. With the aid of density functional theory calculations, we provided a comprehensive mechanism study of CO2 reduction to multicarbon products over aminothiolate-coated copper (Cu) catalysts. Our results indicate that the CO2 reduction performance was closely related to the alkyl chain length, ligand coverage, ligand configuration, and Cu facet. The aminothiolate ligand-Cu interface significantly promoted initial CO2 activation and lowered the activation barrier of carbon-carbon coupling through the organic (nitrogen (N)) and inorganic (Cu) interfacial active sites. Experimentally, the selectivity and partial current density of the multicarbon products over aminothiolate-coated Cu increased by 1.5-fold and 2-fold, respectively, as compared to the pristine Cu at -1.16 VRHE, consistent with our theoretical findings. This work highlights the promising strategy of designing the ligand-metal interface for CO2 reactive capture and conversion to multicarbon products.

Combining Renewable Electricity and Renewable Carbon: Understanding Reaction Mechanisms of Biomass-Derived Furanic Compounds for Design of Catalytic Nanomaterials

ConspectusDespite the growing deployment of renewable energy conversion technologies, a number of large industrial sectors remain challenging to decarbonize. Aviation, heavy transport, and the production of steel, cement, and chemicals are heavily dependent on carbon-containing fuels and feedstocks. A hopeful avenue toward carbon neutrality is the implementation of renewable carbon for the synthesis of critical fuels, chemicals, and materials. Biomass provides an opportune source of renewable carbon, naturally capturing atmospheric CO2 and forming multicarbon linkages and useful chemical functional groups. The constituent molecules nonetheless require various chemical transformations, often best facilitated by catalytic nanomaterials, in order to access usable final products.Catalyzed transformations of renewable biomass compounds may intersect with renewable energy production by offering a means to utilize excess intermittent electricity and store it within chemical bonds. Electrochemical catalytic processes can often offer advantages in energy efficiency, product selectivity, and modular scalability compared to thermal-driven reactions. Electrocatalytic reactions with renewable carbon feedstocks can further enable related processes such as water splitting, where value-adding organic oxidation reactions may replace the evolution of oxygen. Organic electroreduction reactions may also allow desirable hydrogenations of bonds without intermediate formation of H2 and need for additional reactors.This Account highlights recent work aimed at gaining a fundamental understanding of transformations involving biomass-derived molecules in electrocatalytic nanomaterials. Particular emphasis is placed on the oxidation of biomass derived furanic compounds such as furfural and 5-hydroxymethylfurfural (HMF), which can yield value-added chemicals, including furoic acid (FA), maleic acid (MA), and 2,5-furandicarboxylic acid (FDCA) for renewable materials and other commodities. We highlight advanced implementations of online electrochemical mass spectrometry (OLEMS) and vibrational spectroscopies such as attenuated total reflectance surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS), combined with microkinetic models (MKMs) and quantum chemical calculations, to shed light on the elementary mechanistic pathways involved in electrochemical biomass conversion and how these paths are influenced by catalytic nanomaterials. Perspectives are given on the potential opportunities for materials development toward more efficient and selective carbon-mitigating reaction pathways.