Single-Molecule Spectroelectrochemistry (SMS-EC)

Sweet light o' mine: Photothermal and photodynamic inactivation of tenacious pathogens using conjugated polymers

Each year a rising number of infections can not be successfully treated owing to the increasing pandemic of antibiotic resistant pathogens. The global shortage of innovative antibiotics fuels the emergence and spread of drug resistant microbes. Basic research, development, and applications of alternative therapies are urgently needed. Since the 90´s, light-mediated therapies have promised to be the next frontier combating multidrug-resistance microbes. These platforms have demonstrated to be a reliable, rapid, and efficient alternative to eliminate tenacious pathogens while avoiding the emergence of resistance mechanisms. Among the materials showing antimicrobial activity triggered by light, conjugated polymers (CPs) have risen as the most promising option to tackle this complex situation. These materials present outstanding characteristics such as high absorption coefficients, great photostability, easy processability, low cytotoxicity, among others, turning them into a powerful class of photosensitizer (PS)/photothermal agent (PTA) materials. Herein, we summarize and discuss the advances in the field of CPs with applications in photodynamic inactivation and photothermal therapy towards bacteria elimination. Additionally, a section of current challenges and needs in terms of well-defined benchmark experiments and conditions to evaluate the efficiency of phototherapies is presented.

Emerging Optical Microscopy Techniques for Electrochemistry

An optical microscope is probably the most intuitive, simple, and commonly used instrument to observe objects and discuss behaviors through images. Although the idea of imaging electrochemical processes operando by optical microscopy was initiated 40 years ago, it was not until significant progress was made in the last two decades in advanced optical microscopy or plasmonics that it could become a mainstream electroanalytical strategy. This review illustrates the potential of different optical microscopies to visualize and quantify local electrochemical processes with unprecedented temporal and spatial resolution (below the diffraction limit), up to the single object level with subnanoparticle or single-molecule sensitivity. Developed through optically and electrochemically active model systems, optical microscopy is now shifting to materials and configurations focused on real-world electrochemical applications.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

Fluorescence-Amplified Detection of Redox Turnovers in Supported Lipid Bilayers Illuminates Redox Processes of α-Tocopherol

Electron-transfer processes in lipid membranes are key to biological functions, yet challenging to study because of the intrinsic heterogeneity of the systems. Here, we report spectro-electrochemical measurements on indium tin oxide-supported lipid bilayers toward the selective induction and sensing of redox processes in membranes. Working at neutral pH with a fluorogenic α-tocopherol analogue, the dynamics of the two-electron oxidation of the chromanol to a chromanone and the rapid thermal decay of the latter to a chromoquinone are recorded as a rapid surge and drop in intensity, respectively. Continuous voltage cycling reveals rapid chromoquinone two-electron, two-proton reduction to dihydrochromoquinone at negative bias, followed by slow regeneration of the former at positive bias. The kinetic parameters of these different transitions are readily obtained as a function of applied potentials. The sensitivity and selectivity afforded by the reported method enables monitoring signals equivalent to femtoampere currents with a high signal-to-background ratio. The study provides a new method to monitor membrane redox processes with high sensitivity and minimal concentrations and unravels key dynamic aspects of α-tocopherol redox chemistry.

Electrochemical Zero-Mode Waveguide Potential-Dependent Fluorescence of Glutathione Reductase at Single-Molecule Occupancy

Understanding functional states of individual redox enzymes is important because electron-transfer reactions are fundamental to life, and single-enzyme molecules exhibit molecule-to-molecule heterogeneity in their properties, such as catalytic activity. Zero-mode waveguides (ZMW) constitute a powerful tool for single-molecule studies, enabling investigations of binding reactions up to the micromolar range due to the ability to trap electromagnetic radiation in zeptoliter-scale observation volumes. Here, we report the potential-dependent fluorescence dynamics of single glutathione reductase (GR) molecules using a bimodal electrochemical ZMW (E-ZMW), where a single-ring electrode embedded in each of the nanopores of an E-ZMW array simultaneously serves to control electrochemical potential and to confine optical radiation within the nanopores. Here, the redox state of GR is manipulated using an external potential control of the Au electrode in the presence of a redox mediator, methyl viologen (MV). Redox-state transitions in GR are monitored by correlating electrochemical and spectroscopic signals from freely diffusing MV/GR in 60 zL effective observation volumes at single GR molecule average pore occupancy, ⟨n⟩ ∼ 0.8. Fluorescence intensities decrease (increase) at reducing (oxidizing) potentials for MV due to the MV-mediated control of the GR redox state. The spectroelectrochemical response of GR to the enzyme substrate, i.e., glutathione disulfide (GSSG), shows that GSSG promotes GR oxidation via enzymatic reduction. The capabilities of E-ZMWs to probe spectroelectrochemical phenomena in zL-scale-confined environments show great promise for the study of single-enzyme reactions and can be extended to important technological applications, such as those in molecular diagnostics.

Multi-dimensional designer catalysts for negative emissions science (NES): bridging the gap between synthesis, simulations, and analysis

Negative emissions technologies will play a critical role in limiting global warming to sustainable levels. Electrocatalytic and/or photocatalytic CO2 reduction will likely play an important role in this field moving forward, but efficient, selective catalyst materials are needed to enable the widespread adoption of these processes. The rational design of such materials is highly challenging, however, due to the complexity of the reactions involved as well as the large number of structural variables which can influence behavior at heterogeneous interfaces. Currently, there is a significant disconnect between the complexity of materials systems that can be handled experimentally and those that can be modeled theoretically with appropriate rigor and bridging these gaps would greatly accelerate advancements in the field of Negative Emissions Science (NES). Here, we present a perspective on how these gaps between materials design/synthesis, characterization, and theory can be resolved, enabling the rational development of improved materials for CO2 conversion and other NES applications.

Nanoelectrochemistry in the study of single-cell signaling

Label-free biosensing has been the dream of scientists and biotechnologists as reported by Vollmer and Arnold (Nat Methods 5:591-596, 2008). The ability of examining living cells is crucial to cell biology as noted by Fang (Int J Electrochem 2011:460850, 2011). Chemical measurement with electrodes is label-free and has demonstrated capability of studying living cells. In recent years, nanoelectrodes of different functionality have been developed. These nanometer-sized electrodes, coupled with scanning electrochemical microscopy (SECM), have further enabled nanometer spatial resolution study in aqueous environments. Developments in the field of nanoelectrochemistry have allowed measurement of signaling species at single cells, contributing to better understanding of cell biology. Leading studies using nanoelectrochemistry of a variety of cellular signaling molecules, including redox-active neurotransmitter (e.g., dopamine), non-redox-active neurotransmitter (e.g., acetylcholine), reactive oxygen species (ROS), and reactive nitrogen species (RNS), are reviewed here.

Single-Molecule Fluorescence Microscopy for Probing the Electrochemical Interface

The electrochemical interface is an ultrathin interfacial region between the electrode and solution where electrochemical reactions occur. The study of the electrochemical interface continues to be one of the most exciting directions in modern electrochemistry research. Much of our existing knowledge about the electrochemical interface comes from ensemble measurements and ex situ imaging of the electrode surface. Due to its enormous complexity and highly dynamic nature, however, new imaging tools that can probe the interface in situ with ultrahigh spatial and temporal resolution and single-molecule sensitivity are apparently needed. Single-molecule fluorescence microscopy (SMFM) has emerged as a powerful tool that is uniquely suited for studying the electrochemical interface. In this mini-review, we first give a brief overview of various existing SMFM methods for studying electrochemical problems. We then discuss several exciting research topics involving the use of SMFM methods for studying surface-immobilized molecules, single freely diffusing molecules, single molecules as catalytic reaction indicators, and single-molecule labeling and imaging of interfacial nanobubbles. We anticipate that we will continue to see a rapid increase in publications on stochastic electrochemistry of single molecules and nanoparticles. The increased use of SMFM will likely bring new information to our study of the electrochemical interface.

Single-molecule biosensors: Recent advances and applications

Single-molecule biosensors serve the unmet need for real time detection of individual biological molecules in the molecular crowd with high specificity and accuracy, uncovering unique properties of individual molecules which are hidden when measured using ensemble averaging methods. Measuring a signal generated by an individual molecule or its interaction with biological partners is not only crucial for early diagnosis of various diseases such as cancer and to follow medical treatments but also offers a great potential for future point-of-care devices and personalized medicine. This review summarizes and discusses recent advances in nanosensors for both in vitro and in vivo detection of biological molecules offering single-molecule sensitivity. In the first part, we focus on label-free platforms, including electrochemical, plasmonic, SERS-based and spectroelectrochemical biosensors. We review fluorescent single-molecule biosensors in the second part, highlighting nanoparticle-amplified assays, digital platforms and the utilization of CRISPR technology. We finally discuss recent advances in the emerging nanosensor technology of important biological species as well as future perspectives of these sensors.

Observing Transient Bipolar Electrochemical Coupling on Single Nanoparticles Translocating through a Nanopore

We report the observation of transient bipolar electrochemical coupling on freely moving 40 nm silver nanoparticles. The use of an asymmetric nanoelectrochemical environment at the nanopore orifice, for example, an acid inside the pipette and halide ions in the bulk, enabled us to observe unusually large current blockages of single Ag nanoparticles. We attribute these current blockages to the formation of H₂ nanobubbles on the surface of Ag nanoparticles due to the coupled faradaic reactions, in which the reduction of protons and water is coupled to the oxidation of Ag and water under potentials higher than 1 V. The appearance of large current blockages was strongly dependent on the applied voltage and the choice of anions in the bulk solution. The correlation between large current blockages with the oxidation of Ag nanoparticles and their nanopore translocation was further supported by simultaneous fluorescence and electric recordings. This study demonstrates that transient bipolar electrochemistry can take place on small metal nanoparticles below 50 nm when they pass through nanopores where the electric field is highly localized. The use of a nanopore and the resistive-pulse sensing method to study transient bipolar electrochemistry of nanoparticles may be extended to future studies in ultrafast electrochemistry, nanocatalyst screening, and gas nucleation on nanoparticles.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Single-Molecule Electrochemistry on a Porous Silica-Coated Electrode

Here we report the direct observation and quantitative analysis of single redox events on a modified indium-tin oxide (ITO) electrode. The key in the observation of single redox events are the use of a fluorogenic redox species and the nanoconfinement and hindered redox diffusion inside 3-nm-diameter silica nanochannels. A simple electrochemical process was used to grow an ultrathin silica film (∼100 nm) consisting of highly ordered parallel nanochannels exposing the electrode surface from the bottom. The electrode-supported 3-nm-diameter nanochannels temporally trap fluorescent resorufin molecules resulting in hindered molecular diffusion in the vicinity of the electrode surface. Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules can be studied using total-internal reflection fluorescence (TIRF). The rate constants of adsorption and desorption processes of resorufin were characterized from single-molecule analysis to be (1.73 ± 0.08) × 10^-4 cm·s^-1 and 15.71 ± 0.76 s^-1, respectively. The redox events of resorufin to the non-fluorescent dihydroresorufin were investigated by analyzing the change in surface population of single resorufin molecules with applied potential. The scan-rate-dependent molecular counting results (single-molecule fluorescence voltammetry) indicated a surface-controlled electrochemical kinetics of the resorufin reduction on the modified ITO electrode. This study demonstrates the great potential of mesoporous silica as a useful modification scheme for studying single redox events on a variety of transparent substrates such as ITO electrodes and gold or carbon film coated glass electrodes. The ability to electrochemically grow and transfer mesoporous silica films onto other substrates makes them an attractive material for future studies in spatial heterogeneity of electrocatalytic surfaces.

Tip-Enhanced Raman Voltammetry: Coverage Dependence and Quantitative Modeling

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was employed for the first time to observe nanoscale spatial variations in the formal potential, E^0', of a surface-bound redox couple. TERS cyclic voltammograms (TERS CVs) of single Nile Blue (NB) molecules were acquired at different locations spaced 5-10 nm apart on an indium tin oxide (ITO) electrode. Analysis of TERS CVs at different coverages was used to verify the observation of single-molecule electrochemistry. The resulting TERS CVs were fit to the Laviron model for surface-bound electroactive species to quantitatively extract the formal potential E^0' at each spatial location. Histograms of single-molecule E^0' at each coverage indicate that the electrochemical behavior of the cationic oxidized species is less sensitive to local environment than the neutral reduced species. This information is not accessible using purely electrochemical methods or ensemble spectroelectrochemical measurements. We anticipate that quantitative modeling and measurement of site-specific electrochemistry with EC-AFM-TERS will have a profound impact on our understanding of the role of nanoscale electrode heterogeneity in applications such as electrocatalysis, biological electron transfer, and energy production and storage.

Semiempirical modeling of electrochemical charge transfer

Nanoelectrochemical experiments using detection based on tip enhanced Raman spectroscopy (TERS) show a broad distribution of single-molecule formal potentials E°′ for large π-conjugated molecules; theoretical studies are needed to understand the origins of this distribution. In this paper, we present a theoretical approach to determine E°′ for electrochemical reactions involving a single molecule interacting with an electrode represented as a metal nanocluster and apply this method to the Ag₂₀–pyridine system. The theory is based on the semiempirical INDO electronic structure approach, together with the COSMO solvation model and an approach for tuning the Fermi energy, in which the silver atomic orbital energies are varied until the ground singlet state of Ag₂₀–pyridine matches the lowest triplet energy, corresponding to electron transfer from the metal cluster to pyridine. Based on this theory, we find that the variation of E°′ with the structure of the Ag₂₀–pyridine system is only weakly correlated with changes in either the ground-state interaction energy or the charge-transfer excited-state energies at zero applied potential, which shows the importance of calculations that include an applied potential in determining the variation of formal potential with geometry. Factors which determine E°′ include wavefunction overlap for geometries when pyridine is close to the surface, and electrostatics when the molecule-cluster separation is large.

Simultaneous Spectroscopic and Topographic Imaging of Single-Molecule Interfacial Electron-Transfer Reactivity and Local Nanoscale Environment

The fundamental information related to the energy flow between molecules and substrate surfaces as a function of surface site geometry and molecular structure is critical for understanding interfacial electron-transfer (ET) dynamics. The inhomogeneous nanoscale molecule-surface and molecule-molecule interactions are presumably the origins of the complexity in interfacial ET dynamics; thus, identifying the environment of molecules at nanoscale is crucial. We have developed atomic force microscopy (AFM) correlated single-molecule fluorescence intensity/lifetime imaging microscopy (AFM-SMFLIM) capable of identifying and characterizing individual molecules distributed across the heterogeneous surface at the nanometer length scale. Using the novel AFM-SMFLIM imaging, we are able to obtain nanoscale morphology and interfacial ET dynamics at a single-molecule level. Moreover, the observed blinking behavior and lifetime of each molecule in combination with the topography of the environment at nanoscale provide the location of each molecule on the surface (TiO2 vs cover glass) at nanoscale and the coupling strength of each molecule with TiO2 nanoparticles.

Single occupancy spectroelectrochemistry of freely diffusing flavin mononucleotide in zero-dimensional nanophotonic structures

Zero-mode waveguides (ZMW) have the potential to be powerful confinement tools for studying electron transfer dynamics at single molecule occupancy conditions. Flavin mononucleotide contains an isoalloxazine chromophore, which is fluorescent in the oxidized state (FMN) while the reduced state (FMNH2) exhibits dramatically lower light emission, i.e. a dark-state. This allows fluorescence emission to report the redox state of single FMN molecules, an observation that has been used previously to study single electron transfer events in surface-immobilized flavins and flavoenzymes, e.g. sarcosine oxidase, by direct wide-field imaging of ZMW arrays. Single molecule electron transfer dynamics have now been extended to the study of freely diffusing molecules using fluorescence measurements of Au ZMWs under single occupancy conditions. The Au in the ZMW serves both as an optical cladding layer and as the working electrode for potential control, thereby accessing single molecule electron transfer dynamics at μM concentrations. Consistent with expectations, the probability of observing single reduced molecules increases as the potential is scanned negative, E(appl) < E(eq), and the probability of observing emitting oxidized molecules increases at E(appl) > E(eq). Different single molecules exhibit different electron transfer properties as reflected in the position of E(eq) and the distribution of E(eq) among a population of FMN molecules. Two types of actively-controlled electroluminescence experiments were used: chronofluorometry experiments, in which the potential is alternately stepped between oxidizing and reducing potentials, and cyclic potential sweep fluorescence experiments, analogous to cyclic voltammetry, these latter experiments exhibiting a dramatic scan rate dependence with the slowest scan rates showing distinct intermediate states that are stable over a range of potentials. These states are assigned to flavosemiquinone species that are stabilized in the special environment of the ZMW nanopore.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Charge-Transfer Dynamics of Fluorescent Dye-Sensitized Electrodes under Applied Biases

The development of dye-sensitized solar cells requires an in-depth understanding of the interfacial charge-transfer dynamics that take place between dye sensitizers and semiconductors. Here, we describe a prototype system to probe these dynamics by monitoring in real time the fluorescence of two organic sensitizers, a perylene and a squaraine, bound to a SnO2 semiconductor thin film as a function of potentiostatic control of the Fermi level. The two different sensitizer fluorophores characterized by vastly different redox potentials undergo similar fluorescence modulation with applied bias, an indication that the density of states of the semiconductor largely influences the charge-transfer dynamics while energetics play a minimal role. We further show that the rate of photodegradation of the perylene sensitizer with applied bias provides a suitable marker to study the rate of charge injection and charge recombination. Taken together, our results demonstrate a suitable platform to visualize and study charge-transfer dynamics on films and constitute a step toward achieving single-molecule resolution in our quest to decipher the static and dynamic heterogeneity of charge-transfer dynamics in dye-sensitized photoanodes.

Self-induced redox cycling coupled luminescence on nanopore recessed disk-multiscale bipolar electrodes

We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy)32/3+/tri-n-propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH3)63+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.

Single-molecule interfacial electron transfer dynamics of porphyrin on TiO2 nanoparticles: dissecting the interfacial electric field and electron accepting state density dependent dynamics

Single-molecule photon-stamping spectroscopy correlated with electrochemical techniques was used to dissect interfacial electron transfer dynamics by probing an m-ZnTCPP molecule anchored to a TiO₂ NP surface.

Voltage-controlled fluorescence switching of a single redox protein

Heterogeneous electron transfer (ET) of the redox protein, wild-type azurin (wt-Az) from Pseudomonas aeruginosa, was monitored at the single-molecule (SM) level by fluorescence resonance energy transfer (FRET), one electron at a time. Azurin molecules were labeled with an organic fluorophore (Cy5), and the FRET-coupling between Cy5 and the redox center (copper) was used to study ET to a semi-transparent, 10nm thin gold electrode in an optical configuration. By using a confocal microscope and a bipotentiostat for control of the electrode potential, the oxidation and reduction processes of individual Az-Cy5 molecules were monitored. In the oxidized state of the redox center of the azurin molecule, the fluorescence emission of the covalently attached Cy5 was largely quenched by FRET ('off'-state), whereas the emission was recovered upon reduction ('on'-state). The work presented here, shows directly controlled single redox switching events of an individual redox protein and its thermodynamic dispersion. We show that the distribution of midpoint potentials (E0) of individual azurin molecules peaks at 45.7±0.5 mV with a full width at half maximum of 15 mV vs saturated calomel electrode (SCE).

Spectroscopic investigation of electrochemically charged individual (6,5) single-walled carbon nanotubes

Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift.

Nanoscale methods for single-molecule electrochemistry

The development of experiments capable of probing individual molecules has led to major breakthroughs in fields ranging from molecular electronics to biophysics, allowing direct tests of knowledge derived from macroscopic measurements and enabling new assays that probe population heterogeneities and internal molecular dynamics. Although still somewhat in their infancy, such methods are also being developed for probing molecular systems in solution using electrochemical transduction mechanisms. Here we outline the present status of this emerging field, concentrating in particular on optical methods, metal-molecule-metal junctions, and electrochemical nanofluidic devices.

Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.

Fluorescence-enabled electrochemical microscopy with dihydroresorufin as a fluorogenic indicator

Recently, we introduced a new electrochemical imaging technique called fluorescence-enabled electrochemical microscopy (FFEM). The central idea of FEEM is that a closed bipolar electrode is utilized to electrically couple a redox reaction of interest to a complementary fluorogenic reaction converting an electrochemical signal into a fluorescent signal. This simple strategy enables one to use fluorescence microscopy to observe conventional electrochemical processes on very large electrochemical arrays. The initial demonstration of FEEM focused on the use of a specific fluorogenic indicator, resazurin, which is reduced to generate highly fluorescent resorufin. The use of resazurin has enabled the study of analyte oxidation reactions, such as the oxidation of dopamine and H₂O₂. In this report, we extend the capability of FEEM to the study of cathodic reactions using a new fluorogenic indicator, dihydroresorufin. Dihydroresorufin is a nonfluorescent molecule, which can be electrochemically oxidized to generate resorufin. The use of dihydroresorufin has enabled us to study a series of reducible analyte species including Fe(CN)₆^3– and Ru(NH₃)₆³⁺. Here we demonstrate the correlation between the simultaneously recorded fluorescence intensity of resorufin and electrochemical oxidation current during potential sweep experiments. FEEM is used to quantitatively detect the reduction of ferricyanide down to a concentration of approximately 100 μM on a 25 μm ultramicroelectrode. We also demonstrate that dihydroresorufin, as a fluorogenic indicator, gives an improved temporal response and significantly decreases diffusional broadening of the signal in FEEM as compared to resazurin.

Potential-dependent single molecule blinking dynamics for flavin adenine dinucleotide covalently immobilized in zero-mode waveguide array of working electrodes

Single molecules exhibit a set of behaviors that are characteristic and distinct from larger ensembles. Blinking is one such behavior that involves episodic transitions between luminescent and dark states. In addition to the common blinking mechanisms, flavin adenine dinucleotide (FAD), a cofactor in many common redox enzymes, exhibits blinking by cycling between a highly fluorescent oxidized state and a dark reduced state. In contrast to its behavior in flavoenzymes, where the transitions are coupled to chemical redox events, here we study single FAD molecules that are chemically immobilized to the Au region of a zero-mode waveguide (ZMW) array through a pyrroloquinoline quinone (PQQ) linker. In this structure, the Au functions both to confine the optical field in the ZMW and as the working electrode in a potentiostatically controlled 3-elecrode system, thus allowing potential-dependent blinking to be studied in single FAD molecules. The subset of ZMW nanopores housing a single molecule were identified statistically, and these were subjected to detailed study. Using equilibrium potential, E(eq), values determined from macroscopic planar Au electrodes, single molecule blinking behavior was characterized at potentials E < E(eq), E - E(eq), and E > E(eq). The probability of observing a reduced (oxidized) state is observed to increase (decrease) as the potential is scanned cathodic of E(eq). This is understood to reflect the potential-dependent probability of electron transfer for single FAD molecules. Furthermore, the observed transition rate reaches a maximum near E(eq) and decreases to either anodic or cathodic values, as expected, since the rate is dependent on having significant probabilities for both redox states, a condition that is obtained only near E(eq).

Interfacial charge transfer events of BODIPY molecules: single molecule spectroelectrochemistry and substrate effects

BODIPY dye single molecules on nanostructured substrates are studied with a single molecule spectroelectrochemistry technique to reveal the heterogeneous charge transfer mechanism.

Imaging fluorescent nanoparticles to probe photoinduced charging of a semiconductor-solution interface

Optically transparent semiconductors allow simultaneous control of interfacial electrical potential and spectroscopic observation of chemistry near the electrode surface. Care must be taken, however, to avoid unwanted photoexcitation-induced charging of the semiconductor electrode that could influence the results. In this work, we investigate the in situ surface charging by photoexcitation well below the band gap of an optically transparent semiconductor, indium-tin oxide (ITO) electrode. Using total-internal-reflection fluorescence microscopy, the population of ~100-nm negatively charged carboxylate-polystyrene fluorescent nanoparticles at an ITO-aqueous solution interface could be monitored in situ. At positive applied potentials (~0.7 V versus Ag/AgCl), nanoparticles accumulate reversibly in the electrical double-layer of the ITO surface, and the interfacial nanoparticle populations increase with 488-nm excitation intensity. The potential sensitivity of nanoparticle population exhibited no dependence on excitation intensity, varied from 0.1 to 10 W cm(-2), while the onset potential for particle accumulation shifted by as much as 0.3 V. This shift in surface potential appears to be due to photoexcitation-induced charging of the ITO, even though the excitation radiation photon energy, ~2.4 eV, is well below the primary band gap of ITO, >3.5 eV. A kinetic model was developed to determine the photon order of electron-hole generation relative to the electron-hole recombination. The photoexcitation process was found to be first-order in photon flux, suggesting one-photon excitation of an indirect band gap or defect sites, rather than two-photon excitation into the direct band gap. A control experiment was conducted with red-fluorescent carboxylate-polystyrene particles that were counted using 647-nm excitation, where the photon energy is below the indirect band gap or defect site energy and where the optical absorption of the film vanishes. Red illumination between 1 and 15 W cm(-2) produced no detectable shifts in the onset accumulation potential, which is consistent with the negligible optical absorption of the ITO film at this longer wavelength.

Single-molecule electrochemistry: present status and outlook

The development of methods for detecting and manipulating matter at the level of individual macromolecules represents one of the key scientific advancements of recent decades. These techniques allow us to get information that is largely unobtainable otherwise, such as the magnitudes of microscopic forces, mechanistic details of catalytic processes, macromolecular population heterogeneities, and time-resolved, step-by-step observation of complex kinetics. Methods based on optical, mechanical, and ionic-conductance signal transduction are particularly developed. However, there is scope for new approaches that can broaden the range of molecular systems that we can study at this ultimate level of sensitivity and for developing new analytical methods relying on single-molecule detection. Approaches based on purely electrical detection are particularly appealing in the latter context, since they can be easily combined with microelectronics or fluidic devices on a single microchip to create large parallel assays at relatively low cost. A form of electrical signal transduction that has so far remained relatively underdeveloped at the single-molecule level is the direct detection of the charge transferred in electrochemical processes. The reason for this is simple: only a few electrons are transferred per molecule in a typical faradaic reaction, a heterogeneous charge-transfer reaction that occurs at the electrode's surface. Detecting this tiny amount of charge is impossible using conventional electrochemical instrumentation. A workaround is to use redox cycling, in which the charge transferred is amplified by repeatedly reducing and oxidizing analyte molecules as they randomly diffuse between a pair of electrodes. For this process to be sufficiently efficient, the electrodes must be positioned within less than 100 nm of each other, and the analyte must remain between the electrodes long enough for the measurement to take place. Early efforts focused on tip-based nanoelectrodes, descended from scanning electrochemical microscopy, to create suitable geometries. However, it has been challenging to apply these technologies broadly. In this Account, we describe our alternative approach based on electrodes embedded in microfabricated nanochannels, so-called nanogap transducers. Microfabrication techniques grant a high level of reproducibility and control over the geometry of the devices, permitting systematic development and characterization. We have employed these devices to demonstrate single-molecule sensitivity. This method shows good agreement with theoretical analysis based on the Brownian motion of discrete molecules, but only once the finite time resolution of the experimental apparatus is taken into account. These results highlight both the random nature of single-molecule signals and the complications that it can introduce in data interpretation. We conclude this Account with a discussion on how scientists can overcome this limitation in the future to create a new experimental platform that can be generally useful for both fundamental studies and analytical applications.

Efficient Analysis of Single Molecule Spectroscopic Data via MATLAB

A MATLAB program is developed to help analyze single molecule spectroscopic data obtained with a standard inverted optical microscope. The described software has provisions to mitigate the effects of high background signals present in such data sets and greatly streamlines their analysis. Efficient single molecule image analysis and statistical blinking analysis are enabled with these programs to support single molecule imaging in an inverted configuration.

Tracking of single charge carriers in a conjugated polymer nanoparticle

The motion of individual charge carriers in organic nanostructures was tracked by fluorescence microscopy. A twinkling effect is observed in fluorescence microscopy of single conjugated polymer nanoparticles, that is, small displacements in the fluorescence spot of single nanoparticles of the conjugated polymer PFBT are observed over time. There is evidence that superquenching by the charge carrier induces a dark spot in the nanoparticle, which moves with the carrier, resulting in the observed displacements in the fluorescence. Zero-field mobilities of individual charge carriers consistent with highly trapped polarons were obtained from tracking experiments.