Silicon (100) electrodes resistant to oxidation in aqueous solutions: an unexpected benefit of surface acetylene moieties

Terminal Deuterium Atoms Protect Silicon from Oxidation

In recent years, the hybrid silicon-molecular electronics technology has been gaining significant attention for applications in sensors, photovoltaics, power generation, and molecular electronics devices. However, Si-H surfaces, which are the platforms on which these devices are formed, are prone to oxidation, compromising the mechanical and electronic stability of the devices. Here, we show that when hydrogen is replaced by deuterium, the Si-D surface becomes significantly more resistant to oxidation when either positive or negative voltages are applied to the Si surface. Si-D surfaces are more resistant to oxidation, and their current-voltage characteristics are more stable than those measured on Si-H surfaces. At positive voltages, the Si-D stability appears to be related to the flat band potential of Si-D being more positive compared to Si-H surfaces, making Si-D surfaces less attractive to oxidizing OH- ions. The limited oxidation of Si-D surfaces at negative potentials is interpreted by the frequencies of the Si-D bending modes being coupled to that of the bulk Si surface phonon modes, which would make the duration of the Si-D excited vibrational state significantly less than that of Si-H. The strong surface isotope effect has implications in the design of silicon-based sensing, molecular electronics, and power-generation devices and the interpretation of charge transfer across them.

Evaluation of Mono and Bimetallic Ferrocene-Based 1,2,3-Triazolyl Compounds as Burning Rate Catalysts for Solid Rocket Motor

We reported mono and bimetallic ferrocene-based 1,2,3-triazolyl compounds as potential burning rate catalysts in their neutral and ionic forms. All complexes reported here were characterized using 1H and 13C NMR, elemental analysis, and Mössbauer spectroscopy, which was performed for neutral and oxide compounds. The complexes present quasireversible redox potentials with higher oxidative ability than ferrocene and catocene under the same conditions. The complexes were tested as catalysts on the thermal decomposition of ammonium perchlorate (AP) and examined by a differential scanning calorimetry technique to gain further knowledge about their catalytic behavior. Compound 1 causes a decrease of the high-temperature decomposition (HTD) of AP positively, decreasing the decomposition temperature of AP to 345 °C and consequently increasing the energy release to 1939 J·g-1. We took the residues from the pans after testing from the DSC to elucidate the underlying reaction pathways. We obtained the size of the nanostructures formed after thermal decomposition of AP determined by the TEM technique. The diameter and size distribution of iron oxide nanoparticles formed depend on the alkyl sidechain of the triazolium ring, which induces the formation of nanoparticles with a double diameter and size distribution compared to their neutral analogues, suggesting that the possible intermediate for the mechanism degradation of AP by ferrocene derivatives is nanoscale Fe2O3 or similar oxides.

A click chemistry amplified nanopore assay for ultrasensitive quantification of HIV-1 p24 antigen in clinical samples

Despite major advances in HIV testing, ultrasensitive detection of early infection remains challenging, especially for the viral capsid protein p24, which is an early virological biomarker of HIV-1 infection. Here, To improve p24 detection in patients missed by immunological tests that dominate the diagnostics market, we show a click chemistry amplified nanopore (CAN) assay for ultrasensitive quantitative detection. This strategy achieves a 20.8 fM (0.5 pg/ml) limit of detection for HIV-1 p24 antigen in human serum, demonstrating 20~100-fold higher analytical sensitivity than nanocluster-based immunoassays and clinically used enzyme-linked immunosorbent assay, respectively. Clinical validation of the CAN assay in a pilot cohort shows p24 quantification at ultra-low concentration range and correlation with CD4 count and viral load. We believe that this strategy can improve the utility of p24 antigen in detecting early infection and monitoring HIV progression and treatment efficacy, and also can be readily modified to detect other infectious diseases.

On-Surface Azide-Alkyne Cycloaddition Reaction: Does It Click with Ruthenium Catalysts?

Owing to its simplicity, selectivity, high yield, and the absence of byproducts, the "click" azide-alkyne reaction is widely used in many areas. The reaction is usually catalyzed by copper(I), which selectively produces the 1,4-disubstituted 1,2,3-triazole regioisomer. Ruthenium-based catalysts were later developed to selectively produce the opposite regioselectivity─the 1,5-disubstituted 1,2,3-triazole isomer. Ruthenium-based catalysis, however, remains only tested for click reactions in solution, and the suitability of ruthenium catalysts for surface-based click reactions remains unknown. Also unknown are the electrical properties of the 1,4- and 1,5-regioisomers, and to measure them, both isomers need to be assembled on the electrode surface. Here, we test whether ruthenium catalysts can be used to catalyze surface azide-alkyne reactions to produce 1,5-disubstituted 1,2,3-triazole, and compare their electrochemical properties, in terms of surface coverages and electron transfer kinetics, to those of the compound formed by copper catalysis, 1,4-disubstituted 1,2,3-triazole isomer. Results show that ruthenium(II) complexes catalyze the click reaction on surfaces yielding the 1,5-disubstituted isomer, but the rate of the reaction is remarkably slower than that of the copper-catalyzed reaction, and this is related to the size of the catalyst involved as an intermediate in the reaction. The electron transfer rate constant (ket) for the ruthenium-catalyzed reaction is 30% of that measured for the copper-catalyzed 1,4-isomer. The lower conductivity of the 1,5-isomer is confirmed by performing nonequilibrium Green's function computations on relevant model systems. These findings demonstrate the feasibility of ruthenium-based catalysis of surface click reactions and point toward an electrical method for detecting the isomers of click reactions.

Metal-Insulator-Semiconductor Anodes for Ultrastable and Site-Selective Upconversion Photoinduced Electrochemiluminescence

Photoinduced electrochemiluminescence (PECL) allows the electrochemically assisted conversion of low-energy photons into high-energy photons at an electrode surface. This concept is expected to have important implications, however, it is dramatically limited by the stability of the surface, impeding future developments. Here, a series of metal-insulator-semiconductor (MIS) junctions, using photoactive n-type Si (n-Si) as a light absorber covered by a few-nanometer-thick protective SiO_x /metal (SiO_x /M, with M=Ru, Pt, and Ir) overlayers are investigated for upconversion PECL of the model co-reactant system involving the simultaneous oxidation of tris(bipyridine)ruthenium(II) and tri-n-propylamine. We show that n-Si/SiO_x /Pt and n-Si/SiO_x /Ir exhibit high photovoltages and record stabilities in operation (35 h for n-Si/SiO_x /Ir) for the generation of intense PECL with an anti-Stokes shift of 218 nm. We also demonstrate that these surfaces can be employed for spatially localized PECL. These unprecedented performances are extremely promising for future applications of PECL.

A Transparent Semiconducting Surface for Capturing and Releasing Single Cells from a Complex Cell Mixture

Selective isolation of individual target cells from a heterogeneous population is technically challenging; however, the ability to retrieve single cells can have high significance in various aspects of biological research. Here, we present a new photoelectrochemical surface based on a transparent electrode that is compatible with high-resolution fluorescence microscopy for isolating individual rare cells from complex biological samples. This is underpinned by two important factors: (i) careful design of the electrode by patterning discrete Au disks of micron dimension on amorphous silicon-indium tin oxide films and (ii) orthogonal surface chemistry, which modifies the patterned electrode with self-assembly layers of different functionalities, to selectively capture target cells on the Au disks and resist cell binding to the amorphous silicon surface. The co-stimulation of the surface using light from a microscope and an electric potential triggers the reductive desorption of the alkanethiol monolayer from the Au disks to release the single cells of interest from the illuminated regions only. Using circulating tumor cells as a model, we demonstrate the capture of cancer cells on an antibody-coated surface and selective release of single cancer cells with low expression of epithelial cell adhesion molecules.

Modulating the Conduction Band Energies of Si Electrode Interfaces Functionalized with Monolayers of a Bay-Substituted Perylene Bisimide

The confinement of π-conjugated chromophores on silicon (Si) electrode surfaces is a powerful approach to engineer electroresponsive monolayers relevant to microelectronics, electrocatalysis, and information storage and processing. While common strategies to functionalize Si interfaces exploit molecularly dissolved building blocks, only a handful number of studies have leveraged the structure-function relationships of π-aggregates to tune the electronic structures of hybrid monolayers at Si interfaces. Herein, we show that the semiconducting properties of n-type monolayers constructed on Si electrodes are intimately correlated to the initial aggregation state of π-conjugated chromophore precursors derived from bay-substituted perylene bisimide (PBI) units. Specifically, our study unravels that for n-type monolayers engineered using PBI π-aggregates, the cathodic reduction potentials required to inject negative charge carriers into the conduction bands can be stabilized by 295 mV through reversible switching of the maximum anodic potential (MAP) that is applied during the oxidative cycles (+0.5 or +1.5 V vs Ag/AgCl). This redox-assisted stabilization effect is not observed with n-type monolayers derived from molecularly dissolved PBI cores and monolayers featuring a low surface density of the redox-active probes. These findings unequivocally point to the crucial role played by PBI π-aggregates in modulating the conduction band energies of n-type monolayers where a high MAP of +1.5 V enables the formation of electronic trap states that facilitate electron injection when sweeping back to cathodic potentials. Because the structure-function relationships of PBI π-aggregates are shown to modulate the semiconducting properties of hybrid n-type monolayers constructed at Si interfaces, our results hold promising opportunities to develop redox-switchable monolayers for engineering nonvolatile electronic memory devices.

Effect of Electric Fields on Silicon-Based Monolayers

Electric fields can induce bond breaking and bond forming, catalyze chemical reactions on surfaces, and change the structure of self-assembled monolayers on electrode surfaces. Here, we study the effect of electric fields supplied either by an electrochemical potential or by conducting atomic force microscopy (C-AFM) on Si-based monolayers. We report that typical monolayers on silicon undergo partial desorption followed by the oxidation of the underneath silicon at +1.5 V vs Ag/AgCl. The monolayer loses 28% of its surface coverage and 55% of its electron transfer rate constant (k_et) when +1.5 V electrochemical potential is applied on the Si surface for 10 min. Similarly, a bias voltage of +5 V applied by C-AFM induces complete desorption of the monolayer at specific sites accompanied by an average oxide growth of 2.6 nm when the duration of the bias applied is 8 min. Current-voltage plots progressively change from rectifying, typical of metal-semiconductor junctions, to insulating as the oxide grows. These results define the stability of Si-based organic monolayers toward electric fields and have implication in the design of silicon-based monolayers, molecular electronics devices, and on the interpretation of charge-transfer kinetics across them.

Non-Ideal Cyclic Voltammetry of Redox Monolayers on Silicon Electrodes: Peak Splitting is Caused by Heterogeneous Photocurrents and Not by Molecular Disorder

Over the last three decades, research on redox-active monolayers has consolidated their importance as advanced functional material. For widespread monolayer systems, such as alkanethiols on gold, non-ideal multiple peaks in cyclic voltammetry are generally taken as indication of heterogeneous intermolecular interactions─namely, disorder in the monolayer. Our findings show that, contrary to metals, peak multiplicity of silicon photoelectrodes is not diagnostic of heterogeneous intermolecular microenvironments but is more likely caused by photocurrent being heterogeneous across the monolayer. This work is an important step toward understanding the cause of electrochemical non-idealities in semiconductor electrodes so that these can be prevented and the redox behavior of molecular monolayers, as photocatalytic systems, can be optimized.

Hydroelementation of diynes

This review highlights the hydroelementation reactions of conjugated and separated diynes, which depending on the process conditions, catalytic system, as well as the type of reagents, leads to the formation of various products: enynes, dienes, allenes, polymers, or cyclic compounds. The presence of two triple bonds in the diyne structure makes these compounds important reagents but selective product formation is often difficult owing to problems associated with maintaining appropriate reaction regio- and stereoselectivity. Herein we review this topic to gain knowledge on the reactivity of diynes and to systematise the range of information relating to their use in hydroelementation reactions. The review is divided according to the addition of the E-H (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) bond to the triple bond(s) in the diyne, as well as to the type of the reagent used, and the product formed. Not only are the hydroelementation reactions comprehensively discussed, but the synthetic potential of the obtained products is also presented. The majority of published research is included within this review, illustrating the potential as well as limitations of these processes, with the intent to showcase the power of these transformations and the obtained products in synthesis and materials chemistry.

Light in Electrochemistry

Electrochemistry represents an important analytical technique used to acquire and assess chemical information in detail, which can aid fundamental investigations in various fields, such as biological studies. For example, electrochemistry can be used as simple and cost-effective means for bio-marker tracing in applications, such as health monitoring and food security screening. In combination with light, powerful spatially-resolved applications in both the investigation and manipulation of biochemical reactions begin to unfold. In this article, we focus primarily on light-addressable electrochemistry based on semiconductor materials and light-readable electrochemistry enabled by electrochemiluminescence (ECL). In addition, the emergence of multiplexed and imaging applications will also be introduced.

Functionalized Silicon Electrodes Toward Electrostatic Catalysis

Oriented external electric fields are now emerging as "smart effectors" of chemical changes. The key challenges in experimentally studying electrostatic catalysis are (i) controlling the orientation of fields along the reaction axis and (ii) finely adjusting the magnitudes of electrostatic stimuli. Surface models provide a versatile platform for addressing the direction of electric fields with respect to reactants and balancing the trade-off between the solubility of charged species and the intensity of electric fields. In this mini-review, we present the recent advances that have been investigated of the electrostatic effect on the chemical reaction on the monolayer-functionalized silicon surfaces. We mainly focus on elucidating the mediator/catalysis role of static electric fields induced from either solid/liquid electric double layers at electrode/electrolyte interfaces or space charges in the semiconductors, indicating the electrostatic aspects is of great significance in the semiconductor electrochemistry, redox electroactivity, and chemical bonding. Herein, the functionalization of silicon surfaces allows scientists to explore electrostatic catalysis from nanoscale to mesoscale; most importantly, it provides glimpses of the wide-ranging potentials of oriented electric fields for switching on/off the macroscale synthetic organic electrochemistry and living radical polymerization.

Substrate Materials for Biomolecular Immobilization within Electrochemical Biosensors

Electrochemical biosensors have potential applications for agriculture, food safety, environmental monitoring, sports medicine, biomedicine, and other fields. One of the primary challenges in this field is the immobilization of biomolecular probes atop a solid substrate material with adequate stability, storage lifetime, and reproducibility. This review summarizes the current state of the art for covalent bonding of biomolecules onto solid substrate materials. Early research focused on the use of Au electrodes, with immobilization of biomolecules through ω-functionalized Au-thiol self-assembled monolayers (SAMs), but stability is usually inadequate due to the weak Au-S bond strength. Other noble substrates such as C, Pt, and Si have also been studied. While their nobility has the advantage of ensuring biocompatibility, it also has the disadvantage of making them relatively unreactive towards covalent bond formation. With the exception of Sn-doped In2O3 (indium tin oxide, ITO), most metal oxides are not electrically conductive enough for use within electrochemical biosensors. Recent research has focused on transition metal dichalcogenides (TMDs) such as MoS2 and on electrically conductive polymers such as polyaniline, polypyrrole, and polythiophene. In addition, the deposition of functionalized thin films from aryldiazonium cations has attracted significant attention as a substrate-independent method for biofunctionalization.

Solvent Effects in Halogen and Hydrogen Bonding Mediated Electrochemical Anion Sensing in Aqueous Solution and at Interfaces

Sensing anionic species in competitive aqueous media is a well-recognised challenge to long-term applications across a multitude of fields. Herein, we report a comprehensive investigation of the electrochemical anion sensing performance of novel halogen bonding (XB) and hydrogen bonding (HB) bis-ferrocene-(iodo)triazole receptors in solution and at self-assembled monolayers (SAMs), in a range of increasingly competitive aqueous organic solvent media (ACN/H2 O). In solution, the XB sensor notably outperforms the HB sensor, with substantial anion recognition induced cathodic voltammetric responses of the ferrocene/ferrocenium redox couple persisting even in highly competitive aqueous solvent media of 20 % water content. The response to halides, in particular, shows a markedly lower sensitivity to increasing water content associated with a unique halide selectivity at unprecedented levels of solvent polarity. The HB sensor, in contrast, generally displayed a preference towards oxoanions. A significant surface-enhancement effect was observed for both XB/HB receptive films in all solvent systems, whereby the HB sensor generally displayed larger responses towards oxoanions than its halogen bonding analogue.

Surface Functionalization of a γ-Graphyne-like Carbon Material via Click Chemistry

Surface functionalization of carbon materials is of interest in many research fields, such as electrocatalysis, interfacial engineering, and supercapacitors. As an emerging carbon material, γ-graphyne has attracted broad attention. Herein, we report that the surface functionalization of a γ-graphyne-like carbon material (γ-G1) is achieved by immobilizing functional groups via the click chemistry. Texture analysis of aberration-corrected microscopy, X-ray photoelectron spectroscopy, and electrochemistry confirm the successful surface modification of γ-G1 through a strong covalent linkage 1,2,3-triazole. The direct linkage of functional groups on γ-G1 via the click chemistry represents a general method for preparing other functional materials by using γ-graphyne-like materials as a skeleton.

Leveraging the Assembly of a Rylene Dye to Tune the Semiconducting Properties of Functionalized n-Type, Hybrid Si Interfaces

The functionalization of silicon electrodes with π-conjugated chromophores opens new avenues to engineer hybrid semiconducting interfaces relevant to information storage and processing. Notably, molecularly dissolved π-conjugated units, such as ferrocene derivatives, are traditionally exploited as building blocks to construct well-defined interfaces that establish electrochemically addressable platforms with which to investigate electron transfer properties and charge storage capabilities. In contrast, planar π-conjugated building blocks such as naphthalene diimide (NDI) cores enable the formation of solvated aggregates equipped with emergent electronic structures not manifested by the parent, molecularly dissolved building blocks. To interrogate the extent to which the aggregated states of π-conjugated chromophores can be leveraged to regulate the n-type semiconducting properties of functionalized electrodes, we have devised an amphiphilic rylene core (NDI) that demonstrates a non-negligible degree of aggregation in an aqueous medium. Characterization of the electronic structures of the NDI-derived aggregates using a combination of electrochemistry, reductive titration experiments, and spectroelectrochemistry unveils the existence of π-anion stacks, the formation of which is contingent on the initial concentration of NDI building blocks. We show that grafting n-doped NDI aggregates on silicon electrode precursors equipped with a high density of anchoring groups by means of "click" reaction enables the formation of the hybrid Si-NDI electrode (Si-NDI-15@1) that facilitates electron injection by more than 400 mV when compared to Si interfaces constructed from molecularly dissolved NDI units. Furthermore, the engineering of a Si precursor surface characterized by a low density of anchoring groups provides additional proof to highlight that the potentiometric properties recorded for Si-NDI-15@1 originate from NDI units, evidencing a non-negligible degree of aggregation. The present work delivers tools to manipulate the potentiometric properties of functionalized electrodes by leveraging on the electronic structures of aggregated, π-conjugated precursors.

Monitoring the heterogeneity in single cell responses to drugs using electrochemical impedance and electrochemical noise

Impedance spectroscopy is a widely used technique for monitoring cell-surface interactions and morphological changes, typically based on averaged signals from thousands of cells. However, acquiring impedance data at the single cell level, can potentially reveal cell-to-cell heterogeneity for example in response to chemotherapeutic agents such as doxorubicin. Here, we present a generic platform where light is used to define and localize the electroactive area, thus enabling the impedance measurements for selected single cells. We firstly tested the platform to assess phenotypic changes in breast cancer cells, at the single cell level, using the change in the cell impedance. We next show that changes in electrochemical noise reflects instantaneous responses of the cells to drugs, prior to any phenotypical changes. We used doxorubicin and monensin as model drugs and found that both drug influx and efflux events affect the impedance noise signals. Finally, we show how the electrochemical noise signal can be combined with fluorescence microscopy, to show that the noise provides information on cell susceptibility and resistance to drugs at the single cell level. Together the combination of electrochemical impedance and electrochemical noise with fluorescence microscopy provides a unique approach to understanding the heterogeneity in the response of single cells to stimuli where there is not phenotypic change.

Functionalized Silicon Electrodes in Electrochemistry

Avoiding the growth of SiO_x has been an enduring task for the use of silicon as an electrode material in dynamic electrochemistry. This is because electrochemical assays become unstable when the SiO_x levels change during measurements. Moreover, the silicon electrode can be completely passivated for electron transfer if a thick layer of insulating SiO_x grows on the surface. As such, the field of silicon electrochemistry was mainly developed by electron-transfer studies in nonaqueous electrolytes and by applications employing SiO_x-passivated silicon-electrodes where no DC currents are required to cross the electrode/electrolyte interface. A solution to this challenge began by functionalizing Si-H electrodes with monolayers based on Si-O-Si linkages. These monolayers have proven very efficient to avoid SiO_x formation but are not stable for a long-term operation in aqueous electrolytes due to hydrolysis. It was only with the development of self-assembled monolayers based on Si-C linkages that a reliable protection against SiO_x formation was achieved, particularly with monolayers based on α,ω-dialkynes. This review discusses in detail how this surface chemistry achieves such protection, the electron-transfer behavior of these monolayer-modified silicon surfaces, and the new opportunities for electrochemical applications in aqueous solution.

Quantitative Analysis of Cyclic Voltammetry of Redox Monolayers Adsorbed on Semiconductors: Isolating Electrode Kinetics, Lateral Interactions, and Diode Currents

The design of devices whose functions span from sensing their environments to converting light into electricity or guiding chemical reactivity at surfaces often hinges around a correct and complete understanding of the factors at play when charges are transferred across an electrified solid-liquid interface. For semiconductor electrodes in particular, published values for charge-transfer kinetic constants are scattered. Furthermore, received wisdom suggests slower charge-transfer kinetics for semiconductors than for metal electrodes. We have used cyclic voltammetry of ferrocene-modified silicon photoanodes and photocathodes as the experimental model system and described a systematic analysis to separate charge-transfer kinetics from diode effects and interactions between adsorbed species. Our results suggest that literature values of charge-transfer kinetic constants at semiconductor electrodes are likely to be underestimates of their actual values. This is revealed by experiments and analytical models showing that the description of the potential distribution across the semiconductor-monolayer-electrolyte interface has been largely oversimplified.

Amorphous silicon on indium tin oxide: a transparent electrode for simultaneous light activated electrochemistry and optical microscopy

Herein is reported a new type of transparent electrode, prepared by depositing a thin layer of amorphous silicon film on indium tin oxide, which enables photoswitchable electrochemistry and optical imaging to be performed simultaneously. This offers the opportunity to visualise a spatially controlled electrochemical event on an unstructured electrode surface.

DNA-Hybridization Detection on Si(100) Surfaces Using Light-Activated Electrochemistry: A Comparative Study between Bovine Serum Albumin and Hexaethylene Glycol as Antifouling Layers

Light can be used to spatially resolve electrochemical measurements on a semiconductor electrode. This phenomenon has been explored to detect DNA hybridization with light-addressable potentiometric sensors and, more recently, with light-addressable amperometric sensors based on organic-monolayer-protected Si(100). Here, a contribution to the field is presented by comparing sensing performances when bovine serum albumin (BSA) and hexaethylene glycol (OEG₆) are employed as antifouling layers that resist nonspecific adsorption to the DNA-modified interface on Si(100) devices. What is observed is that both sensors based on BSA or OEG₆ initially allow electrochemical distinction among complementary, noncomplementary, and mismatched DNA targets. However, only surfaces based on OEG₆ can sustain electroactivity over time. Our results suggest that this relates to accelerated SiO _x formation occasioned by BSA proteins adsorbing on monolayer-protected Si(100) surfaces. Therefore, DNA biosensors were analytically explored on low-doped Si(100) electrodes modified on the molecular level with OEG₆ as an antifouling layer. First, light-activated electrochemical responses were recorded over a range of complementary DNA target concentrations. A linear semilog relation was obtained from 1.0 × 10^-11 to 1.0 × 10^-6 mol L^-1 with a correlation coefficient of 0.942. Then, measurements with three independent surfaces indicated a relative standard deviation of 4.5%. Finally, selectivity tests were successfully performed in complex samples consisting of a cocktail mixture of four different DNA sequences. Together, these results indicate that reliable and stable light-activated amperometric DNA sensors can be achieved on Si(100) by employing OEG₆ as an antifouling layer.

Light-Addressable Ion Sensing for Real-Time Monitoring of Extracellular Potassium

We report here on a light addressable potassium (K⁺ ) sensor where light illumination of a semiconducting silicon electrode substrate results in a localized activation of the faradaic electrochemistry at the illuminated spot. This allows one, by electrochemical control, to oxidize surface bound ferrocene moieties that in turn trigger K⁺ transfer from the overlaid K⁺ -selective film to the solution phase. The resulting voltammetric response is shown to be K⁺ -selective, where peak position is a direct function of K⁺ activity at the surface of electrode. This concept was used to measure extracellular K⁺ concentration changes by stimulating living breast cancer cells. The associated decrease of intracellular K⁺ level was confirmed with a fluorescent K⁺ indicator. In contrast to light addressable potentiometry, the approach introduced here relies on dynamic electrochemistry and may be performed in tandem with other electrochemical analysis when studying biological events on the electrode.

Switchable Interfaces: Redox Monolayers on Si(100) by Electrochemical Trapping of Alcohol Nucleophiles

Organic electrosynthesis is going through its renaissance but its scope in surface science as a tool to introduce specific molecular signatures at an electrode/electrolyte interface is under explored. Here, we have investigated an electrochemical approach to generate in situ surface-tethered and highly-reactive carbocations. We have covalently attached an alkoxyamine derivative on an Si(100) electrode and used an anodic bias stimulus to trigger its fragmentation into a diffusive nitroxide (TEMPO) and a surface-confined carbocation. As a proof-of-principle we have used this reactive intermediate to trap a nucleophile dissolved in the electrolyte. The nucleophile was ferrocenemethanol and its presence and surface concentration after its reaction with the carbocation were assessed by cyclic voltammetry. The work expands the repertoire of available electrosynthetic methods and could in principle lay the foundation for a new form of electrochemical lithography.

A photoelectrochemical platform for the capture and release of rare single cells

For many normal and aberrant cell behaviours, it is important to understand the origin of cellular heterogeneity. Although powerful methods for studying cell heterogeneity have emerged, they are more suitable for common rather than rare cells. Exploring the heterogeneity of rare single cells is challenging because these rare cells must be first pre-concentrated and undergo analysis prior to classification and expansion. Here, a versatile capture & release platform consisting of an antibody-modified and electrochemically cleavable semiconducting silicon surface for release of individual cells of interest is presented. The captured cells can be interrogated microscopically and tested for drug responsiveness prior to release and recovery. The capture & release strategy was applied to identify rare tumour cells from whole blood, monitor the uptake of, and response to, doxorubicin and subsequently select cells for single-cell gene expression based on their response to the doxorubicin.

Many cell capture systems exist but the characterisation and controlled release of single cells is a challenge. Here, the authors report on the development of a duel trigger release system using a combination of photo and electro triggers to allow for light based analysis without unwanted release.

Reproducible flaws unveil electrostatic aspects of semiconductor electrochemistry

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor’s space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfer at semiconductors as well as aiding the study of electrostatics on chemical reactivity.

Most electrical devices must pass charges across semiconductor interfaces, yet redox-active molecular behavior obscures comprehension of these processes. Here, the authors develop a model to describe redox processes on semiconductor surfaces and gauge these interactions electrochemically.

Single-molecule electrical contacts on silicon electrodes under ambient conditions

The ultimate goal in molecular electronics is to use individual molecules as the active electronic component of a real-world sturdy device. For this concept to become reality, it will require the field of single-molecule electronics to shift towards the semiconducting platform of the current microelectronics industry. Here, we report silicon-based single-molecule contacts that are mechanically and electrically stable under ambient conditions. The single-molecule contacts are prepared on silicon electrodes using the scanning tunnelling microscopy break-junction approach using a top metallic probe. The molecular wires show remarkable current–voltage reproducibility, as compared to an open silicon/nano-gap/metal junction, with current rectification ratios exceeding 4,000 when a low-doped silicon is used. The extension of the single-molecule junction approach to a silicon substrate contributes to the next level of miniaturization of electronic components and it is anticipated it will pave the way to a new class of robust single-molecule circuits.

The next level of miniaturization of electronic circuits calls for a connection between current single-molecule and traditional semiconductor processing technologies. Here, the authors show a method to prepare metal/molecule/silicon diodes that present high current rectification ratios exceeding 4,000.

Copper Contamination of Self-Assembled Organic Monolayer Modified Silicon Surfaces Following a "Click" Reaction Characterized with LAPS and SPIM

A copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) reaction combined with microcontact printing was used successfully to pattern alkyne-terminated self-assembled organic monolayer-modified silicon surfaces. Despite the absence of a copper peak in X-ray photoelectron spectra, copper contamination was found and visualized using light-addressable potentiometric sensors (LAPS) and scanning photo-induced impedance microscopy (SPIM) after the "click"-modified silicon surfaces were rinsed with hydrochloric acid (HCl) solution, which was frequently used to remove copper residues in the past. Even cleaning with an ethylenediaminetetraacetic acid (EDTA) solution did not remove the copper residue completely. Different strategies for avoiding copper contamination, including the use of bulky chelators for the copper(I) catalyst and rinsing with different reagents, were tested. Only cleaning of the silicon surfaces with an EDTA solution containing trifluoroacetic acid (TFA) after the click modification proved to be an effective method as confirmed by LAPS and SPIM results, which showed the expected potential shift due to the surface charge introduced by functional groups in the monolayer and allowed, for the first time, imaging the impedance of an organic monolayer.

Coupled Thermodynamic and Kinetic Changes in the Electrochemistry of Ferrocenyl Monolayers Induced by Light

We report on the light-induced systematic changes to the thermodynamics and kinetics of ferrocene units attached to a n-type silicon(100) photoelectrode. Both the reaction rate and the energetics of the charge transfer are simultaneously affected by changes in the intensity of the incident light. Cyclic voltammetry shows that increases in the intensity of illumination can drive the redox process toward less positive potentials, with a downhill shift in E_1/2 of ca. 160 mV by increasing the light intensity from 3 to 94 mW cm^-2. However, this thermodynamic shift is paralleled by an increase in the kinetics of the charge transfer. This latter observation-light-induced kinetic effects at monolayers on silicon electrodes-is made possible only by the stability of the surface chemistry construct. Furthermore, electrochemical impedance measurements showed that the electrodes exhibit faster electron-transfer kinetics under illumination than previously reported for ferrocene-terminated highly doped silicon (around 1 order of magnitude faster). An explanation for the kinetic effects is proposed on the basis of the consistent increase in photogenerated charge carriers inside silicon and the enlarged potential difference between the valence band of silicon and the surface-attached ferrocene.

Molecular Monolayers for Electrical Passivation and Functionalization of Silicon-Based Solar Energy Devices

Silicon-based solar fuel devices require passivation for optimal performance yet at the same time need functionalization with (photo)catalysts for efficient solar fuel production. Here, we use molecular monolayers to enable electrical passivation and simultaneous functionalization of silicon-based solar cells. Organic monolayers were coupled to silicon surfaces by hydrosilylation in order to avoid an insulating silicon oxide layer at the surface. Monolayers of 1-tetradecyne were shown to passivate silicon micropillar-based solar cells with radial junctions, by which the efficiency increased from 8.7% to 9.9% for n⁺/p junctions and from 7.8% to 8.8% for p⁺/n junctions. This electrical passivation of the surface, most likely by removal of dangling bonds, is reflected in a higher shunt resistance in the J-V measurements. Monolayers of 1,8-nonadiyne were still reactive for click chemistry with a model catalyst, thus enabling simultaneous passivation and future catalyst coupling.

TEMPO Monolayers on Si(100) Electrodes: Electrostatic Effects by the Electrolyte and Semiconductor Space-Charge on the Electroactivity of a Persistent Radical

This work demonstrates the effect of electrostatic interactions on the electroactivity of a persistent organic free radical. This was achieved by chemisorption of molecules of 4-azido-2,2,6,6-tetramethyl-1-piperdinyloxy (4-azido-TEMPO) onto monolayer-modified Si(100) electrodes using a two-step chemical procedure to preserve the open-shell state and hence the electroactivity of the nitroxide radical. Kinetic and thermodynamic parameters for the surface electrochemical reaction are investigated experimentally and analyzed with the aid of electrochemical digital simulations and quantum-chemical calculations of a theoretical model of the tethered TEMPO system. Interactions between the electrolyte anions and the TEMPO grafted on highly doped, i.e., metallic, electrodes can be tuned to predictably manipulate the oxidizing power of surface nitroxide/oxoammonium redox couple, hence showing the practical importance of the electrostatics on the electrolyte side of the radical monolayer. Conversely, for monolayers prepared on the poorly doped electrodes, the electrostatic interactions between the tethered TEMPO units and the semiconductor-side, i.e., space-charge, become dominant and result in drastic kinetic changes to the electroactivity of the radical monolayer as well as electrochemical nonidealities that can be explained as an increase in the self-interaction "a" parameter that leads to the Frumkin isotherm.

π-Conjugated bis(terpyridine)metal complex molecular wires

Bottom-up approaches have gained significant attention recently for the creation of nano-sized, ordered functional structures and materials. Stepwise coordination techniques, in which ligand molecules and metal sources are reacted alternatively, offer several advantages. Coordination bonds are stable, reversible, and self-assembling, and the resultant metal complex motifs may contain functionalities unique to their own characteristics. This review focuses on metal complex wire systems, specifically the bottom-up fabrication of linear and branched bis(terpyridine)metal complex wires on electrode surfaces. This system possesses distinct and characteristic electronic functionalities, intra-wire redox conduction and excellent long-range electron transport ability. This series of comprehensive studies exploited the customizability of bis(terpyridine)metal complex wires, including examining the influence of building blocks. In addition, simple yet effective electron transfer models were established for redox conduction and long-range electron transport. A fabrication technique for an ultra-long bis(terpyridine)metal complex wire is also described, along with its properties and functionalities.

Advances in click chemistry for silica-based material construction

Click chemistry is undoubtedly the most powerful 1,3-dipolar cycloaddition reaction in organic synthesis.

Connecting electrodes with light: one wire, many electrodes

The requirement of a wire to each electrode is central to the design of any electronic device but can also be a major restriction. For example it entails space restrictions and rigid device architecture in multi-electrode devices. The finite space that is taken up by the array of electrical terminals and conductive pads also severely limits the achievable density of electrodes in the device. Here it is shown that a travelling light pointer can be used to form transient electrical connections anywhere on a monolithic semiconductor electrode that is fitted with a single peripheral electrical terminal. This is achieved using hydrogen terminated silicon electrodes that are modified with well-defined organic monolayers. It is shown that electrochemical information can be either read from or written onto these surfaces. Using this concept it is possible to form devices that are equivalent to a conventional electrode array but that do not require a predetermined architecture, and where each element of the array is temporally "connected" using light stimulus; a step change in capability for electrochemistry.

"Click" Patterning of Self-Assembled Monolayers on Hydrogen-Terminated Silicon Surfaces and Their Characterization Using Light-Addressable Potentiometric Sensors

Two potential strategies for chemically patterning alkyne-terminated self-assembled monolayers (SAMs) on oxide-free silicon or silicon-on-sapphire (SOS) substrates were investigated and compared. The patterned surfaces were validated using a light-addressable potentiometric sensor (LAPS) for the first time. The first strategy involved an integration of photolithography with "click" chemistry. Detailed surface characterization (i.e. water contact angle, ellipsometry, AFM, and XPS) and LAPS measurements showed that photoresist processing not only decreases the coverage of organic monolayers but also introduces chemically bonded contaminants on the surfaces, thus significantly reducing the quality of the SAMs and the utility of "click" surface modification. The formation of chemical contaminants in photolithography was also observed on carboxylic acid- and alkyl-terminated monolayers using LAPS. In contrast, a second approach combined microcontact printing (μCP) with "click" chemistry; that is azide (azido-oligo(ethylene glycol) (OEG)-NH2) inks were printed on alkyne-terminated SAMs on silicon or SOS through PDMS stamps. The surface characterization results for the sample printed with a flat featureless PDMS stamp demonstrated a nondestructive and efficient method of μCP to perform "click" reactions on alkyne-terminated, oxide-free silicon surfaces for the first time. For the sample printed with a featured PDMS stamp, LAPS imaging showed a good agreement with the pattern of the PDMS stamp, indicating the successful chemical patterning on non-oxidized silicon and SOS substrates and the capability of LAPS to image the molecular patterns with high sensitivity.