Redox feedback mechanisms can be exploited in electroanalytical detection right to the limit of single molecules being observed. The process relies on anode and cathode being placed extremely close together to minimize diffusion time. In addition to the more complex and expensive nanofabrication tools, there are attempts of “benchtop” microgap and nanogap fabrication to exploit deposition and etch reactions in the assembly. An overview is given summarizing recent methodology development and emerging applications in electroanalysis. One important implication of a very close anode-to-cathode distance is migration of ions in a strong electric field when no electrolyte is used, leading to ion accumulation and a change in signal amplification. Phenomena of this type and geometry/functional implications are considered.
C. Fijen, M. Fontana, S. G. Lemay, K. Mathwig and J. Hohlbein, bioRxiv (2017) 20179.
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.
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.
We introduce a nanofluidic mixing device entirely fabricated in glass for the fluorescence detection of single molecules. The design consists of a nanochannel T-junction and allows the continuous monitoring of chemical or enzymatic reactions of analytes as they arrive from two independent inlets. The fluorescently labeled molecules are tracked before, during and after they enter the mixing region, and their reactions with each other are observed by means of optical readout such as Förster Resonance Energy Transfer (FRET). Our method can be used for analyzing the kinetics of DNA annealing in a high-parallelized fashion.
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.
Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a phenomenon in which an excited state—formed by an electron-transfer reaction between electrogenerated species in the vicinity of a working electrode—emits light. Although the first detailed studies of ECL were reported in the 1960s, ECL publications have revealed an exponential growth worldwide thanks to advances in nanotechnology, photoelectrochemistry, and spectroscopy.
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.
A thin film of Nafion®, of approximately 5 µm thickness, asymmetrically deposited onto a 6 µm thick film of poly(ethylene terephthalate) (PET) fabricated with a 5, 10, 20, or 40 µm 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
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 two femtoliter 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 modelling is employed to simulate the electrochemical processes in the nanogap sensor, and the results are in good agreement with those of experiments.
The use of nanoscale electrodes is beneficial in microfluidic sensing applications, as a high sensitivity and efficient mass transport can be achieved. Facile preparation of millimeter-long gold nanowires is possible using nanoskiving. Single nanowires were suspended over a glass microchannel, which was closed with a complementary PDMS microchannel. Using the nanowire as an electrode, cyclic voltammograms of ferrocene were recorded, with the electroactive solution flowing through the channel at different rates. Nanowires suspended in the center of the flow profile exhibit higher current responses than nanowires on the bottom of microchannels, due to efficient analyte transport towards the electrode surface.