Nanoscale channels and electrodes for electrochemical measurements exhibit extreme surface-to-volume ratios and a correspondingly high sensitivity to even weak degrees of surface interactions. Here, we exploit the potential-dependent reversible adsorption of outer-sphere redox species to modulate in space and time their concentration in a nanochannel under advective flow conditions. Induced concentration variations propagate downstream at a species-dependent velocity. This allows one to amperometrically distinguish between attomole amounts of species based on their time-of-flight. On-demand concentration pulse generation, separation, and detection are all integrated in a miniaturized platform.
A cationic diode is fabricated by hot-pressing a commercial cation-conducting ionomer membrane (Fumasep FKS-30) onto a polyethylene terephthalate (PET) substrate with microhole of 5, 10, 20, or 40 μm diameter. Both, symmetric (ionomer on both sides) and asymmetric (ionomer only on the working electrode side) cases are investigated in a 4-electrode measurement cell. A 5-electrode measurement cell in generator-collector mode is employed to directly detect competing proton transport through the ionomer. Only the asymmetric device allows ion current rectification to be observed. With decreasing microhole diameter the rectification effect increases. With increasing electrolyte concentration (for aqueous HCl, NaCl, LiCl, NH4Cl, MgCl2, CaCl2) the rectification effect diminishes. Competition between cation transport and proton transport is observed in all cases. A qualitative impedance model is developed to diagnose the quality and performance of these cationic diodes.
P. de Haan, M. A. Ianovska, K. Mathwig, H. Bouwmeester and E. Verpoorte, 21st International Conference on Miniaturized Systems for Chemistry and Life Sciences, Savannah, Georgia, USA, Oct. 22-16 (2017) 1593.
Gut-on-a-chip models have gained attention as replacements for other cell-based assays or animal studies in drug development or toxicological studies. These models aim to provide a more accurate representation of the in vivo situation in form and function; however, no digestive processes have been included in these systems so far. This work describes a miniaturized digestive system based on artificial digestive juices that digest liquid samples in a series of three microreactors. After optimization of the pH value of juices and mixtures, samples leading to fluorescent products were digested to demonstrate enzyme functionality and to determine kinetic parameters.
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.