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Commercial resin microbeads are widely applied in ion exchange and extraction. Here, a single anion-selective and phosphate binding resin microbead (FerrIX™) is mounted into an epoxy membrane and investigated by 4-electrode membrane voltammetry and membrane impedance spectroscopy. Anion transport properties are observed to dominate associated with three distinct potential domains: (I) a low bias ohmic potential domain (dominant at high electrolyte concentration), (II) a concentration polarisation potential domain, and (III) an over-limiting potential domain. Voltammetric responses show transient diffusion-migration features at higher scan rates and quasi-steady state features at lower scan rates. Inherent microbead conductivity is shown to be linked to two resistive elements, electrolyte concentration dependent and independent, in series. The effects of phosphate binding are revealed as transient pattern in impedance spectroscopy data. Preliminary data suggest phosphate concentration-dependent peak features in the imaginary impedance versus frequency plot due to phosphate binding into the microbead. Graphical abstract

Ionic rectifier membranes or devices generate uni-directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nano- and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.

Nanofluidic electrochemical devices confine the volume of chemical reactions to femtoliters. When employed for light generation by electrochemiluminescence (ECL), nanofluidic confinement yields enhanced intensity and robust luminescence. Here, we investigate different ECL pathways, namely coreactant and annihilation ECL in a single nanochannel and compare light emission profiles. By high-resolution imaging of electrode areas, we show that different reaction schemes produce very different emission profiles in the unique confined geometry of a nanochannel. The confrontation of experimental results with finite element simulation gives further insight into the exact reaction ECL pathways. We find that emission strongly depends on depletion, geometric exclusion, and recycling of reactants in the nanofluidic device.

Membrane materials with semipermeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed polyacrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semipermeability, also the ionic current rectification effect switches for phPAN materials deposited asymmetrically onto a 5, 10, 20, or 40 μm diameter microhole in a 6 μm thick polyethylene-terephthalate (PET) film substrate. Therefore, ionic rectifier behavior can be tuned and used to monitor and characterize semipermeability. Effects of electrolyte type and concentration and pH (relative to the zeta potential at approximately 3.1) are investigated by voltammetry, chronoamperometry, and impedance spectroscopy. A computational model provides good qualitative agreement with the observed electrolyte concentration data. High rectification effects are observed for both cations (pH > 3.1) and anions (pH < 3.1) but only at relatively low ionic strengths.

The polymer of intrinsic microporosity PIM-EA-TB provides a molecularly rigid micropore structure containing tertiary amine sites and is shown here to interact with hydrogen bonding guest molecules such as caffeic acid. Voltammetric data with a PIM-EA-TB film on glassy carbon electrodes show that in both acidic solution (pH 2; PIM-EA-TB is protonated) and in neutral solution (pH 6; PIM-EA-TB is not protonated) caffeic acid is slowly accumulated into the microporous host. Binding constants are estimated and suggested to be linked to hydrogen bonding causing accumulation of caffeic acid. When employing PIM-EA-TB as an asymmetric membrane coated onto a 5 μm thick Teflon support film with 10 μm diameter microholes (using either a single microhole or a 10×10 array of microholes), binding of caffeic acid is shown to cause a modulation of the ionic current without affecting the pH-dependent ionic diode behaviour. Two complementary types of effects of caffeic acid guests are discussed based on blocking anion diffusion pathways and based on removal of positive charges. The caffeic acid transport mechanism/efficiency is investigated in view of selective molecular pumping.

We suspended a single nanoskived gold nanowire in a microfluidic channel. In this preliminary report, a 200 nm-diameter nanowire was used as an electrode to perform hydrodynamic voltammetry in the center of solution flow. Suspended nanowires exhibit superior current response due to highly efficient mass transport in the area of fastest flow.

Microhole fluidic ionic diodes based on asymmetric deposits of charged ionomer membranes (e.g., Nafion or polymers of intrinsic microporosity) on microhole supports yield high rectification ratios for ionic transport. They are fabricated without the need for complex micro- or nano-structuring, and show potential for future applications in desalination and biosensing. Here, we propose an explanation for the functional principle for this type of materials-based ionic diode. A predictive computational model for ionic diode switching is based on finite element analysis. It is employed to determine the influence of diode geometry as well as type and concentration of aqueous electrolyte on the rectification behavior.

Ionic diode phenomena occur at asymmetric ionomer | aqueous electrolyte microhole interfaces. Depending on the applied potential, either an “open” or a “closed” diode state is observed switching between a high ion flow rate and a low ion flow rate. Physically, the “open” state is associated mainly with conductivity towards the microhole within the ionomer layer and the “closed” state is dominated by restricted diffusion-migration access to the microhole interface opposite to the ionomer. In this report we explore a “heterojunction” based on an asymmetric polymer of intrinsic microporosity (PIM) | Nafion ionomer microhole interface. Improved diode characteristics and current rectification are observed in aqueous NaCl. The effects of creating the PIM | Nafion micro-interface are investigated and suggested to lead to novel sensor architectures.

In ion-annihilation electrochemiluminescence (ECL), luminophore ions are generated by oxidation as well as reduction at electrodes surfaces, and subsequently recombine into an electronically excited state, which emits light. The intensity of the emitted light is often limited by the kinetic rate of recombination of the luminophore ion species. Recombination or annihilation rates are high ranging up to approximately 10(10) M-1 s(-1) and can be difficult to determine using scanning electrochemical microscopy or high-frequency oscillations of an electrode potential. Here, we propose determining annihilation kinetics by measuring the relative change of the emitted light intensity as a function of luminophore concentration. Using finite element simulations of annihilation ECL in a geometry of two closely spaced electrodes biased at constant potentials, we show that, with increasing concentrations, luminescence intensity crosses over from a quadratic dependence on concentration to a linear regime-depending on the rate of annihilation. Our numerical results are applicable to scanning electrochemical microscopy as well as nanofluidic electrochemical devices to determine fast ion-annihilation kinetics.

Single-molecule detection schemes offer powerful means to overcome static and dynamic heterogeneity inherent to complex samples. However, probing biomolecular interactions and reactions with high throughput and time resolution remains challenging, often requiring surface-immobilized entities. Here, we introduce glass-made nanofluidic devices for the high-throughput detection of freely-diffusing single biomolecules by camera-based fluorescence microscopy. Nanochannels of 200 nm height and a width of several micrometers confine the movement of biomolecules. Using pressure-driven flow through an array of parallel nanochannels and by tracking the movement of fluorescently labelled DNA oligonucleotides, we observe conformational changes with high throughput. In a device geometry featuring a T-shaped junction of nanochannels, we drive steady-state non-equilibrium conditions by continuously mixing reactants and triggering chemical reactions. We use the device to probe the conformational equilibrium of a DNA hairpin as well as to continuously observe DNA synthesis in real time. Our platform offers a straightforward and robust method for studying reaction kinetics at the single-molecule level.

The generation of stable enhanced light emission by electrochemiluminescence in microfabricated nanofluidic electrochemical devices is demonstrated for the first time by exploiting nanogap amplification., Microfabricated nanofluidic electrochemical devices offer a highly controlled nanochannel geometry; they confine the volume of chemical reactions to the nanoscale and enable greatly amplified electrochemical detection. Here, the generation of stable light emission by electrochemiluminescence (ECL) in transparent nanofluidic devices is demonstrated for the first time by exploiting nanogap amplification. Through continuous oxidation and reduction of [Ru(bpy)3]2+ luminophores at electrodes positioned at opposite walls of a 100 nm nanochannel, we compare classic redox cycling and ECL annihilation. Enhanced ECL light emission of attomole luminophore quantities is evidenced under ambient conditions due to the spatial confinement in a 10 femtoliter volume, resulting in a short diffusion timescale and highly efficient ECL reaction pathways at the nanoscale.

Single-molecule detection schemes offer powerful means to overcome static and dynamic heterogeneity inherent to complex samples. Probing chemical and biological interactions and reactions with high throughput and time resolution, however, remains challenging and often requires surface-immobilized entities. Here, utilizing camera-based fluorescence microscopy, we present glass-made nanofluidic devices in which fluorescently labelled molecules flow through nanochannels that confine their diffusional movement. The first design features an array of parallel nanochannels for high-throughput analysis of molecular species under equilibrium conditions allowing us to record 200.000 individual localization events in just 10 minutes. Using these localizations for single particle tracking, we were able to obtain accurate flow profiles including flow speeds and diffusion coefficients inside the channels.A second design featuring a T-shaped nanochannel enables precise mixing of two different species as well as the continuous observation of chemical reactions. We utilized the design to visualize enzymatically driven DNA synthesis in real time and at the single-molecule level. Based on our results, we are convinced that the versatility and performance of the nanofluidic devices will enable numerous applications in the life sciences.

We report a strategy for the fabrication of a new type of electrochemical nanogap transducer. These nanogap devices are based on signal amplification by redox cycling. Using two steps of electron-beam lithography, vertical gold electrodes are fabricated side by side at a 70 nm distance encompassing a 20 attoliter open nanogap volume. We demonstrate a current amplification factor of 2.5 as well as the possibility to detect the signal of only 60 analyte molecules occupying the detection volume. Experimental voltammetry results are compared to calculations from finite element analysis.

Interference or crosstalk of coexisting redox species results in overlapping of electrochemical signals, and it is a major hurdle in sensor development. In nanogap sensors, redox cycling between two independently biased working electrodes results in an amplified electrochemical signal and an enhanced sensitivity. Here, we report new strategies for selective sensing of three different redox species in a nanogap sensor of a 2 fL volume. Our approach relies on modulating the electrode potentials to define specific potential windows between the two working electrodes; consequently, specific detection of each redox species is achieved. Finite element modeling is employed to simulate the electrochemical processes in the nanogap sensor, and the results are in good agreement with those of experiments.

Electrofluorochromic molecules share the unique property that their fluorescence changes as a function of their oxidation state. This makes them interesting from a fundamental perspective as molecular dyads are designed and synthesized to tune the interplay of electrochemical and luminescent properties of molecules. Electrofluorochromic systems also find applications in sensing because a fluorescent signal can be detected with high sensitivity. Moreover, in the recent years the interest in redox-switchable fluorescent polymers has strongly increased due to their applicability in display devices. Here, we review electrofluorochromic molecules and polymers; we emphasize their structures and functional principles and point to specific applications.

This paper describes the fabrication of millimeter-long gold nanowires that bisect the center of microfluidic channels. We fabricated the nanowires by nanoskiving and then suspended them over a trench in a glass structure. The channel was sealed by bonding it to a complementary poly(dimethylsiloxane) structure. The resulting structures place the nanowires in the region of highest flow, as opposed to the walls, where it approaches zero, and expose their entire surface area to fluid. We demonstrate active functionality, by constructing a hot-wire anemometer to measure flow through determining the change in resistance of the nanowire as a function of heat dissipation at low voltage (

This paper reports on a microfluidic differential impedance cytometer, which uses a bandwidth compensation technique together with a small detection volume and multi-frequency analysis to achieve an increased sensitivity. The bandwidth compensation technique allows for measurements within small bandwidths by accounting for the increased signal amplitude dependence on the particle speed. We demonstrate detection and clear baseline discrimination of polystyrene beads with diameters of 1 μm and 2 μm and the discrimination of 5 μm beads from yeast cells of similar size. We show that using multiple frequencies in parallel significantly improves the discrimination performance of the cytometer., Procedia Engineering, 25, Eurosensors XXV

Helical nanofibers and nanotubes of inorganic materials have application potential in various fields, such as catalysis, sensors, and functional and smart systems. Currently available helical nanomaterials include coiled carbon nanotubes, carbon nanocoils, silica nanosprings, silicon carbide nanosprings, and helical transition-metal (Ti, Ta, V) oxide nanotubes. The synthesismethods for helical nanomaterials rely mainly on catalytic chemical vapor deposition (CVD) and template approaches. The template method is usually applied to the synthesis of helical oxide nanotubes because it is relatively difficult to fabricate such tubular structures directly by CVD or physical vapor deposition (PVD). Carbon nanocoils are the most successfully synthesized by CVD with good reproducibility and high yields. Previously, helical metal and oxide nanoandmicrostructures have been synthesized by electrodeposition, electroless deposition, and sol–gel methods with carbon nanocoils or microcoils as templates. However, these coating methods do not provide convenient and simple control over the thickness and uniformity of the coatings because the templates have low chemical reactivity, small diameters, and particularly large surface curvature. Moreover, certain pretreatments and suitable surfactants are required to improve the wettability and chemical reactivity of the templates. Atomic layer deposition (ALD) is a powerful growth technique for high-quality films. It utilizes the sequential exposure of reactants to substrates to achieve layer-by-layer film growth, which allows atomic-scale thickness control. Compared to traditional PVD or CVD methods, it shows outstanding advantages including precise thickness control

A thin film of Nafion®, of approximately 5 microm thickness, asymmetrically deposited onto a 6 microm thick film of poly(ethylene terephthalate) (PET) fabricated with a 5, 10, 20, or 40 microm microhole, is shown to exhibit prominent ionic diode behaviour involving cation charge carrier ("cationic diode"). The phenomenon is characterized via voltammetric, chronoamperometric, and impedance methods. Phenomenologically, current rectification effects are comparable to those observed in nano-cone devices where space-charge layer effects dominate. However, for microhole diodes a resistive, a limiting, and an over-limiting potential domain can be identified and concentration polarization in solution is shown to dominate in the closed state.

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.

We theoretically investigate reversible adsorption in electrochemical devices on a molecular level. To this end, a computational framework is introduced, which is based on 3D random walks including probabilities for adsorption and desorption events at surfaces. We demonstrate that this approach can be used to investigate adsorption phenomena in electrochemical sensors by analyzing experimental noise spectra of a nanofluidic redox cycling device. The evaluation of simulated and experimental results reveals an upper limit for the average adsorption time of ferrocene dimethanol of ∼200 μs. We apply our model to predict current noise spectra of further electrochemical experiments based on interdigitated arrays and scanning electrochemical microscopy. Since the spectra strongly depend on the molecular adsorption characteristics of the detected analyte, we can suggest key indicators of adsorption phenomena in noise spectroscopy depending on the geometric aspect of the experimental setup.

Distributed feedback lasers with first-order vertical grating based on AlInGaAs-InAs-InP quantum-dash lasers were fabricated by electron beam lithography and Cl2-Ar reactive ion etching with an electron cyclotron resonance source. Low threshold currents and single-mode operation with sidemode suppression ratios of 48 dB and a direct modulation bandwidth of 5.5 GHz were demonstrated

Electrochemical detection of individual molecular tags in nanochannels may enable cost-effective, massively parallel analysis and diagnostics platforms. Here we demonstrate single-molecule detection of prototypical analytes in aqueous solution based on redox cycling in 40 nm nanogap transducers. These nanofluidic devices are fabricated using standard microfabrication techniques combined with a self-aligned approach that minimizes gap size and dead volume. We demonstrate the detection of three common redox mediators at physiological salt concentrations.

We introduce all-electrical cross-correlation spectroscopy of molecular number fluctuations in nanofluidic channels. Our approach is based on a pair of nanogap electrochemical transducers located downstream from each other in the channel. When liquid is driven through this device, mesoscopic fluctuations in the local density of molecules are transported along the channel. We perform a time-of-flight measurement of these fluctuations by cross-correlating current-time traces obtained at the two detectors. Thereby we are able to detect ultralow liquid flow rates below 10 pL/min⁡. This method constitutes the electrical equivalent of fluorescence cross-correlation spectroscopy.

Laterally coupled DFB lasers based on quantum dashes were fabricated by FIB lithography. CW-powers above 30 mW, high SMSRs, a modulation bandwidth of 7.6 GHz and singlemode emission from 1.5 to 1.9 μm were demonstrated. (2 pages)

Nanofluidic thin-layer cells count among the most sensitive electrochemical sensors built to date. Here we study both experimentally and theoretically the factors that limit the response time of these sensors. We find that the key limiting factor is reversible adsorption of the analyte molecules to the surfaces of the nanofluidic system, a direct consequence of its high surface-to-volume ratio. Our results suggest several means of improving the response time of the sensor, including optimizing the device geometry and tuning the electrode biasing scheme so as to minimize adsorption.

This paper presents an improved technique to fabricate porous membranes from macroporous silicon as a starting material. A crucial step in the fabrication process is the dissolution of silicon from the backside of the porous wafer by aqueous potassium hydroxide to open up the pores. We improved this step by biasing the silicon wafer electrically against the KOH. By monitoring the current–time characteristics a good control of the process is achieved and the yield is improved. Also, the etching can be stopped instantaneously and automatically by short-circuiting Si and KOH. Moreover, the bias-assisted etching allows for the controlled fabrication of silicon dioxide tube arrays when the silicon pore walls are oxidized and inverted pores are released.

Biodegradable extracellular matrices (ECMs) consisting of mechanically stable arrays of aligned poly(lactide) nanorods (nanorod diameter ≈ 200 nm; lattice constant ≈ 500 nm) with areas up to 9 cm2 were prepared by nondestructive extraction from recyclable self-ordered nanoporous alumina hard templates. Fibroblasts formed dense tissue layers on the heparin/gelatin activated nanorod arrays, showed excellent adhesion and exhibited a highly elongated morphology such as in natural tissue. The synthetic approach reported here combining advantages of top-down lithography (well-defined topography) and self-assembly (low costs, high throughput, feature size in the 100 nm range) may yield ECMs for biomedical applications. Remarkably, on microrod arrays with four times larger feature sizes the fibroblasts were significantly less elongated and their proliferation was strongly reduced.

The uniformity of the properties of array elements was evaluated by statistical analysis of microscopic images of array structures, assuming that the brightness of the array elements correlates quantitatively or qualitatively with a microscopically probed quantity. Derivatives and autocorrelation functions of cumulative frequency distributions of the object brightnesses were used to quantify variations in object properties throughout arrays. Thus, different specimens, the same specimen at different stages of its fabrication or use, and different imaging conditions can be compared systematically. As an example, we analyzed scanning electron micrographs of microrod arrays and calculated the percentage of broken microrods.

Laterally coupled, complex distributed feedback lasers based on AlInGaAs/InAs/InP quantum dash layers were fabricated by maskless focused ion beam lithography. Continuous-wave powers above 30 mW at room temperature, sidemode suppression ratios of 44 dB and a modulation bandwidth of 7.6 GHz were demonstrated.