Iontophoretic transport through porous membranes using scanning electrochemical microscopy: application to in vitro studies of ion fluxes through skin

Mechanistic Study of the Conductance and Enhanced Single-Molecule Detection in a Polymer-Electrolyte Nanopore

Solid-state nanopores have been widely employed in the detection of biomolecules, but low signal-to-noise ratios still represent a major obstacle in the discrimination of nucleic acid and protein sequences substantially smaller than the nanopore diameter. The addition of 50% poly(ethylene) glycol (PEG) to the external solution is a simple way to enhance the detection of such biomolecules. Here, we demonstrate with finite-element modeling and experiments that the addition of PEG to the external solution introduces a strong imbalance in the transport properties of cations and anions, drastically affecting the current response of the nanopore. We further show that the strong asymmetric current response is due to a polarity-dependent ion distribution and transport at the nanopipette tip region, leading to either ion depletion or enrichment for few tens of nanometers across its aperture. We provide evidence that a combination of the decreased/increased diffusion coefficients of cations/anions in the bath outside the nanopore and the interaction between a translocating molecule and the nanopore-bath interface is responsible for the increase in the translocation signals. We expect this new mechanism to contribute to further developments in nanopore sensing by suggesting that tuning the diffusion coefficients of ions could enhance the sensitivity of the system.

Electrical aspects of skin as a pathway to engineering skin devices

Skin is one of the indispensable organs for life. The epidermis at the outermost surface provides a permeability barrier to infectious agents, chemicals, and excessive loss of water, while the dermis and subcutaneous tissue mechanically support the structure of the skin and appendages, including hairs and secretory glands. The integrity of the integumentary system is a key for general health, and many techniques have been developed to measure and control this protective function. In contrast, the effective skin barrier is the major obstacle for transdermal delivery and detection. Changes in the electrical properties of skin, such as impedance and ionic activity, is a practical indicator that reflects the structures and functions of the skin. For example, the impedance that reflects the hydration of the skin is measured for quantitative assessment in skincare, and the current generated across a wound is used for the evaluation and control of wound healing. Furthermore, the electrically charged structure of the skin enables transdermal drug delivery and chemical extraction. This paper provides an overview of the electrical aspects of the skin and summarizes current advances in the development of devices based on these features.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Surface-tracked scanning ion conductance microscopy: A novel imaging technique for measuring topography-correlated transmembrane ion transport through porous substrates

Ion transport through porous substrates is ubiquitous in biological and synthetic materials, and fundamental for chemical separation, drug delivery and bio-sensing. Contemporary imaging techniques for simultaneously characterizing topography and ion transport through porous substrates are limited in range and resolution. In this paper, we demonstrate 'surface-tracked scanning ion conductance microscopy' as a technique to image topography of a porous substrate and simultaneously measure voltage-driven transmembrane ion transport. This technique uses the principles of 'shear-force tracking' to image the surface of a polycarbonate track-etch membrane, and chronoamperometry to reconstruct topography-correlated transmembrane ion transport through the membrane at different transmembrane potentials. Spatial transmembrane transport through individual pores is modeled using Goldman-Hodgkin-Katz (GHK) theory to examine the effects of shear-force modulation on magnitude of transmembrane currents recorded with a nanopipette. The modeled transmembrane current through the porous membrane is compared with experimental behavior, and discrepancies between predicted values and measured data are outlined. The proposed surface-tracked imaging mode allows for rapid assessment (approximately 7 s/μm²) of interfacial processes at the nanoscale and addresses a bottleneck for stable, large-area characterization of porous substrates using scanning ion conductance microscopy.

Characterization of Membrane Patch-Ion Channel Probes for Scanning Ion Conductance Microscopy

Integration of dual-barrel membrane patch-ion channel probes (MP-ICPs) to scanning ion conductance microscopy (SICM) holds promise of providing a revolutionized approach of spatially resolved chemical sensing. A series of experiments are performed to further the understanding of the system and to answer some fundamental questions, in preparation for future developments of this approach. First, MP-ICPs are constructed that contain different types of ion channels including transient receptor potential vanilloid 1 and large conductance Ca2⁺ -activated K⁺ channels to establish the generalizability of the methods. Next, the capability of the MP-ICP platforms in single ion channel activity measurements is proved. In addition, the interplay between the SICM barrel and the ICP barrel is studied. For ion channels gated by uncharged ligands, channel activity at the ICP barrel is unaffected by the SICM barrel potential; whereas for ion channels that are gated by charged ligands, enhanced channel activity can be obtained by biasing the SICM barrel at potentials with opposite polarity to the charge of the ligand molecules. Finally, a proof-of-principle experiment is performed and site-specific molecular/ionic flux sensing is demonstrated at single-ion-channel level, which show that the MP-ICP platform can be used to quantify local molecular/ionic concentrations.

Bioinspired Protein Channel-Based Scanning Ion Conductance Microscopy (Bio-SICM) for Simultaneous Conductance and Specific Molecular Imaging

The utility of stochastic single-molecule detection using protein nanopores has found widespread application in bioanalytical sensing as a result of the inherent signal amplification of the resistive pulse method. Integration of protein nanopores with high-resolution scanning ion conductance microscopy (SICM) extends the utility of SICM by enabling selective chemical imaging of specific target molecules, while simultaneously providing topographical information about the net ion flux through a pore under a concentration gradient. In this study, we describe the development of a bioinspired scanning ion conductance microscopy (bio-SICM) approach that couples the imaging ability of SICM with the sensitivity and chemical selectivity of protein channels to perform simultaneous pore imaging and specific molecule mapping. To establish the framework of the bio-SICM platform, we utilize the well-studied protein channel α-hemolysin (αHL) to map the presence of β-cyclodextrin (βCD) at a substrate pore opening. We demonstrate concurrent pore and specific molecule imaging by raster scanning an αHL-based probe over a glass membrane containing a single 25-μm-diameter glass pore while recording the lateral positions of the probe and channel activity via ionic current. We use the average channel current to create a conductance image and the raw current-time traces to determine spatial localization of βCD. With further optimization, we believe that the bio-SICM platform will provide a powerful analytical methodology that is generalizable, and thus offers significant utility in a myriad of bioanalytical applications.

Transport of redox probes through single pores measured by scanning electrochemical-scanning ion conductance microscopy (SECM-SICM)

We report scanning electrochemical microscopy-scanning ion conductance microscopy (SECM-SICM) experiments that describe transport of redox active molecules which emanate from single pores of a track-etch membrane. Experiments are performed with electrodes which consist of a thin gold layer deposited on one side of a nanopipet. Subsequent insulation of the electrode with parylene results in a hybrid electrode for SECM-SICM measurements. Electrode fabrication is straightforward and highly parallel. For image collection, ionic current measured at the nanopipet both controls the position of the electrode with respect to the membrane surface and reports the local conductance in the vicinity of the nanopipet, while faradaic current measured at the Au electrode reports the presence of redox-active molecules. Application of a transmembrane potential difference affords additional control over migration of charged species across the membrane.

Effects of pipette modulation and imaging distances on ion currents measured with scanning ion conductance microscopy (SICM)

Local conductance variations can be estimated by measuring ion current magnitudes with scanning ion conductance microscopy (SICM). Factors which influence image quality and quantitation of ion currents measured with SICM have been evaluated. Specifically, effects of probe-sample separation and pipette modulation have been systematically studied for the case of imaging conductance variations at pores in a polymer membrane under transmembrane concentration gradients. The influence of probe-sample separation on ion current images was evaluated using distance-modulated (ac) feedback. Approach curves obtained using non-modulated (dc) feedback were also recorded to determine the relative influence of pipette-generated convection by comparison of ion currents measured with both ac and dc feedback modes. To better interpret results obtained, comparison to a model based on a disk-shaped geometry for nanopores in the membrane, as well as relevant position-dependent parameters of the experiment is described. These results advance our current understanding of conductance measurements with SICM.

Diffusive flux and magnetic manipulation of nanoparticles through porous membranes

Measurement of transport of nanometer scale particles through porous media is important to begin to understand the potential environmental impacts of nanomaterials. Using a diffusion cell with two compartments separated by either a porous alumina or polycarbonate membrane as a model system, diffusive flux through mesoporous materials is examined. Experiments are performed as a function of particle size, pore diameter, and solvent, and the particle fluxes are monitored by the change in absorbance of the solution in the receiving cell. Using the measured extinction coefficient and change in absorbance of the solution as a function of time, the fluxes of 3, 8, and 14 nm diameter CoFe(2)O(4) particles are determined as they are translocated across pores with diameters 30, 50, 100, and 200 nm in hexane and aqueous solutions. In general, flux decreases with increasing particle size and increases with pore diameter. We find that fluxes are faster in aqueous solutions than in hexane, which is attributed to the hydrophilic nature of the porous membranes and differences in wettability. The impact of an applied magnetic flux gradient, which induces magnetization and motion, on permeation is also examined. For larger membrane pore diameters, applied magnetic fluxes increase the rate of transport of 14 nm CoFe(2)O(4) particles more than that of 3 or 8 nm diameter particles, reflecting their differences in susceptibility. However, larger particles are excluded from membranes with small diameter pores, consistent with magnetic interparticle attractions that reversibly induce magnetic aggregation.

Diffusion and reaction in microbead agglomerates

Scanning electrochemical microscopy has been used to analyze the flux of p-aminonophenol (PAP) produced by agglomerates of polymeric microbeads modified with galactosidase as a model system for the bead-based heterogeneous immunoassays. With the use of mixtures of enzyme-modified and bare beads in defined ratio, agglomerates with different saturation levels of the enzyme modification were produced. The PAP flux depends on the intrinsic kinetics of the galactosidase, the local availability of the substrate p-aminophenyl-beta-D-galactopyranoside (PAPG), and the external mass transport conditions in the surrounding of the agglomerate and the internal mass transport within the bead agglomerate. The internal mass transport is influenced by the diffusional shielding of the modified beads by unmodified beads. SECM in combination with optical microscopy was used to determine experimentally the external flux. These data are in quantitative agreement with boundary element simulation considering the SECM microelectrode as an interacting probe and treating the Michaelis-Menten kinetics of the enzyme as nonlinear boundary conditions with two independent concentration variables [PAP] and [PAPG]. The PAPG concentration at the surface of the bead agglomerate was taken as a boundary condition for the analysis of the internal mass transport condition as a function of the enzyme saturation in the bead agglomerate. The results of this analysis are represented as PAP flux per contributing modified bead and the flux from freely suspended galactosidase-modified beads. These numbers are compared to the same number from the SECM experiments. It is shown that depending on the enzyme saturation level a different situation can arise where either beads located at the outer surface of the agglomerate dominate the contribution to the measured external flux or where the contribution of buried beads cannot be neglected for explaining the measured external flux.

Analytical expressions for quantitative scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM), is a recent analytical technique in electrochemistry, which was developed in the 1990s and uses microelectrodes to probe various surfaces. Even with the well-known disc microelectrodes, the system geometry is not as simple as in regular electrochemistry. As a consequence even the simplest experiments, the so-called positive and negative feedback approach curves, cannot be described with exact analytical expressions. This review gathers all the analytical expressions available in the SECM literature in steady-state feedback experiments. Some of them are claimed as general expressions, other are presented as approximate. Their validity is discussed in the light of the current understanding and computer facilities.

Scanning electrochemical microscopy in neuroscience

This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular.

Electroosmotic flow and its contribution to iontophoretic delivery

Iontophoresis is the movement of charged molecules in solution under applied current using pulled multibarrel glass capillaries drawn to a sharp tip. The technique is generally nonquantitative, and to address this, we have characterized the ejection of charged and neutral species using carbon-fiber electrodes attached to iontophoretic barrels. Our results show that observed ejections are due to the sum of iontophoretic and electroosmotic forces. With the use of the neutral, electroactive molecule 2-(4-nitrophenoxy) ethanol (NPE), which is only transported by electroosmotic flow (EOF), a positive correlation between the amount ejected and the diameter of each barrel's tip was found. In addition, using various charged and neutral electroactive compounds we found that, when each compound is paired with the EOF marker, the percentage of the ejection due to EOF remains constant. This percentage varies for each pair of compounds, and the differences in mobility are positively correlated to differences in electrophoretic mobility. Overall, the results show that capillary electrophoresis (CE) can be used to predict the percentage of ejection that will be due to EOF. With this information, quantitative iontophoresis is possible for electrochemically inactive drugs by using NPE as a marker for EOF.

One-step immobilization of glucose oxidase in a silica matrix on a Pt electrode by an electrochemically induced sol-gel process

We demonstrate here that the electrochemical generation of hydroxyl ions and hydrogen bubbles can be used to induce the synthesis of enzyme- or protein-encapsulated 3D porous silica structure on the surface of noble metal electrodes. In the present work, the one-step synthesis of a glucose oxidase (GOD)-encapsulated silica matrix on a platinum electrode is presented. In this process, glucose oxidase was mixed with ethanol and TEOS to form a doped precursory sol solution. The electrochemically generated hydrogen bubbles at negative potentials assisted the formation of the porous structure of a GOD-encapsulated silica gel, and then the one-step immobilization of enzyme into the silica matrix was achieved. Scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM) characterizations showed that the GOD-encapsulated silica matrix adhered to the electrode surface effectively and had an interconnected porous structure. Because the pores started at the electrode surface, their sizes increased gradually along the distance away from the electrode and reached maximum at the solution side, and effective mass transport to the electrode surface could be achieved. The entrapped enzyme in the silica matrix retained its activity. The present glucose biosensor had a short response time of 2 s and showed a linear response to glucose from 0 to 10 mM with a correlation coefficient of 0.9932. The detection limit was estimated to be 0.01 mM at a signal-to-noise ratio of 3. The apparent Michaelis-Menten constant (K m app) and the maximum current density were determined to be 20.3 mM and 112.4 microA cm-2, respectively. The present method offers a facile way to fabricate biosensors and bioelectronic devices in situ.

Scanning electrochemical microscopy for direct imaging of reaction rates

Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.

Remote in vivo imaging of human skin corneocytes by means of an optical fiber bundle

Human corneocytes forming the outermost layer of the epidermis (stratum corneum) were imaged in vivo by epifluorescence through a coherent optical fiber bundle. A very simple and rapid method to remotely visualize the cells forming this protective layer of the skin is presented. After the topical application of fluorescein, the distal face of an optical fiber bundle is gently applied perpendicularly onto the labeled skin (contact mode). Remote fluorescence images of the corneocytes are acquired in 50 ms through the bundle comprising 30 000 individually cladded 3.5 microm diameter optical fibers. The very short focal distance which is an intrinsic characteristic of such bundles, allows visualizing only the most superficial monolayer of cells in contact with the external environment. An image displays about 400-500 cells directly on the human body. The size and the arrangement of the corneocytes can thus be acquired and analyzed in a very simple and easy way. The method is flexible and can be used for any location on the human body. Using a gradient-index lens objective (magnification 2.8x) fused to the distal face of the bundle allows the shape of the corneocytes to be better resolved. In addition, the working distance is 300 microm and hence this second approach works in a noncontact imaging mode. Both approaches are complementary and allow providing instantaneously either a global view of the cells with a possible statistical determination of their area or morphological information, which are essential for dermatology and cosmetic sciences. Finally, to improve the quality and the contrast of the recorded images, we tested silica nanoparticles containing fluorescein. In brief, this diagnostic method is nontoxic, painless, easy to use, noninvasive, and nondestructive.

Regulation and characterization of the polarity of cells embedded in a reconstructed basement matrix using a three-dimensional micro-culture system

Three cell lines, that is, the human breast cancer cell line (MCF-7) and the human mammary epithelial cell line (S-1) and its malignant form (T4-2) were embedded in a reconstituted basement membrane (Matrigel) that had 20-nL pyramid-shaped silicon microstructures. The proliferative behavior of the MCF-7 cells was dependent on the surrounding conditions (2-D, collagen gel, or Matrigel), whereas the respiratory activity of a single cell (F(c)) was almost identical under different culture conditions. The F(c) value changed with cellular polarity. The F(c) value for the S-1 cells was observed to decrease slightly, whereas that of the T4-2 cells increased 2 days after cultivation in the microstructures within the Matrigel. However, when the T4-2 cells were cultured in the presence of tyrphostin AG 1478 (T4-2 tyr) to inhibit epidermal growth factor (EGF) signaling, the F(c) value decreased slightly and remained almost constant for an additional 1 week; this was similar to the behavior of the S-1 cells. Further, fluorescence images showed that the T4-2 tyr cells formed polar structures that were similar to those formed by the S-1 cells whereas the T4-2 cells did not form such structures. These results indicate that cellular polarity can be assessed by measuring cellular respiratory activity.

Scanning electrochemical microscopy in the 21st century

The fundamentals of and recent advances in scanning electrochemical microscopy (SECM) are described. The focus is on applications of this method to studies of systems and processes of active current interest ranging from nanoelectrochemistry to electron transfer reactions and electrocatalysis to biological imaging.

Scanning electrochemical microscopy: principles and applications to biophysical systems

This review highlights numerous and wide ranging biophysical and biochemical applications of scanning electrochemical microscopy (SECM). SECM instrumentation and theoretical modelling, necessary for experimental interpretation, are outlined, followed by a detailed discussion of the diverse applications of this technique. These include the measurement of flow through membranes, the determination of kinetic parameters of reactions, the investigation of the permeability of small molecules in tissues and monitoring biological processes, such as the production of oxygen or nitric oxide by cells. The significant impact of micro-electrochemical techniques on our understanding of basic physicochemical processes at biologically relevant interfaces is also considered. Studies reviewed include transport across and within bilayers and monolayers. Recent advances in SECM include the combination of SECM with other techniques, such as atomic force microscopy and optical microscopy. These developments are highlighted, along with prospects for the future.

Modeling Steady-State Experiments with a Scanning Electrochemical Microscope Involving Several Independent Diffusing Species Using the Boundary Element Method

The BEM algorithm developed earlier for steady-state experiments in the scanning electrochemical microscopy (SECM) feedback mode has been expanded to allow for the treatment of more than one independently diffusing species. This allows the treatment of substrate-generation/tip-collection SECM experiments. The simulations revealed the interrelation of sample layout, local kinetics, imaging conditions, and the quality of the obtained SECM images. Resolution in the SECM SG/TC images has been evaluated, and it depends on several factors. For most practical situations, the resolution is limited by the diffusion profiles of the sample. When a dissolved compound is converted at the sample (e.g., oxygen reduction or enzymatic reaction at the sample), the working distance should be significantly larger than in SECM feedback experiments (ca. 3 r(T) for RG = 5) in order to avoid diffusional shielding of the active regions on the sample by the UME body. The resolution ability also depends on the kinetics of the active regions. The best resolution can be expected if all the active regions cause the same flux. In one simulated example, which might mimic a possible scenario of a low-density protein array, considerable compromises in the resolving power, were noted when the flux from two neighboring spots differs by more than a factor of 2.

Biological applications of scanning electrochemical microscopy: chemical imaging of single living cells and beyond

Recent applications of scanning electrochemical microscopy (SECM) to studies of single biological cells are reviewed. This scanning probe microscopic technique allows the imaging of an individual cell on the basis of not only its surface topography but also such cellular activities as photosynthesis, respiration, electron transfer, single vesicular exocytosis and membrane transport. The operational principles of SECM are also introduced in the context of these biological applications. Recent progress in techniques for high-resolution SECM imaging are also reviewed. Future directions, such as single-channel detection by SECM, high-resolution imaging with nanometer-sized probes, and combined SECM techniques for multidimensional imaging are also discussed.

Guanylyl cyclase-dependent chemotaxis of endothelial cells in response to nitric oxide gradients

Nitric oxide (NO) is an important regulator of angiogenesis and neovascularization. The nature of endothelial cell motility responses to NO was examined using a Boyden chamber method. NO generated via decomposition of either DEA/NO or DETA/NO produced increases in human umbilical vein endothelial cell (HUVEC) chemotaxis, which were completely abrogated by ODQ, a soluble guanylyl cyclase inhibitor. Measurements of NO either directly by chemiluminescence or its chemistry with diaminofluorescein revealed that chemotaxis was driven by subtle NO gradients between the lower and the upper wells in this system. In addition to diffusion and volatilization from the upper chambers, the data showed that HUVEC consumption of NO contributed to these sustained gradients. Comparison of DEA/NO- and DETA/NO-mediated responses suggested that the persistence of spatial NO gradients is as significant as the absolute magnitude of NO exposure per unit time. The findings suggest that subnanomolar NO gradients are sufficient to mobilize endothelial cell migration into hypoxic tissue during neovascularization events, such as in wound healing and cancer.

Alternating Current Impedance Imaging of Membrane Pores Using Scanning Electrochemical Microscopy

Alternating current impedance imaging of a 6-microm thick membrane containing conical-shaped pores (60-nm and 2.5-microm diameter openings) using scanning electrochemical microscopy (SECM) is described. Impedance images of the pore openings were obtained by rastering a glass-sealed conically shaped Pt tip (approximately 1-microm radius) above the membrane surface, while measuring the total impedance between the tip and a large area Pt electrode located on the opposite side of the membrane. Individual pore openings in the high pore density membrane (approximately 8 x 10(4) pores/cm2) are observed in the SECM impedance image. The image contrast is due to the decrease in tip and membrane resistance, in the vicinity of the pore opening. An equivalent circuit for the SECM cell and membrane is proposed and evaluated against the measured SECM imaging impedance. Criteria for employing SECM in impedance mode to image membranes are discussed.

Nanowire probes for high resolution combined scanning electrochemical microscopy - atomic force microscopy

We describe a method for the production of nanoelectrodes at the apex of atomic force microscopy (AFM) probes. The nanoelectrodes are formed from single-walled carbon nanotube AFM tips which act as the template for the formation of nanowire tips through sputter coating with metal. Subsequent deposition of a conformal insulating coating, and cutting of the probe end, yields a disk-shaped nanoelectrode at the AFM tip apex whose diameter is defined by the amount of metal deposited. We demonstrate that these probes are capable of high-resolution combined electrochemical and topographical imaging. The flexibility of this approach will allow the fabrication of nanoelectrodes of controllable size and composition, enabling the study of electrochemical activity at the nanoscale.