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.
In nanofluidic electrochemical sensors based on redox cycling, zeptomole quantities of analyte molecules can be detected as redox-active molecules travel diffusively between two electrodes separated by a nanoscale gap. These sensors are employed to study the properties of multiferrocenylic compounds in nonpolar media, 2,3,4-triferrocenylthiophene and 2,5-diferrocenylthiophene, which display well-resolved electrochemically reversible one-electron transfer processes. Using stochastic analysis, we are able to determine – as a function of the oxidation states of a specific redox couple – the effective diffusion coefficient as well as the faradaic current generated per molecule; all in a straightforward experiment requiring only a mesoscopic amount of molecules in a femtoliter compartment. It was found that diffusive transport is reduced for higher oxidation states and that analytes yield very high currents per molecule of 15 fA.
Y. Rong, Q. Song, K. Mathwig, E. Madrid, D. He, R. G. Niemann, P. J. Cameron, S. E.C. Dale, S. Bending, M. Carta, R. Malpass-Evans, N. B. McKeown, F. Marken, Electrochemistry Communications 69 (2016) 41.
“Ionic diode” (or current rectification) effects are potentially important for a range of applications including water purification. In this preliminary report, we observe novel ionic diode behaviour of thin (300 nm) membranes based on a polymer of intrinsic microporosity (PIM-EA-TB) supported on a poly-ethylene-terephthalate (PET) film with a 20 μm diameter microhole, and immersed in aqueous electrolyte media. Current rectification effects are observed for half-cells with the same electrolyte solution on both sides of the membrane for cases where cation and anion mobility differ (HCl, other acids, NaOH, etc.) but not for cases where cation and anion mobility are more alike (LiCl, NaCl, KCl, etc.). A pH-dependent reversal of the ionic diode effect is observed and discussed in terms of tentatively assigned mechanisms based on both (i) ion mobility within the PIM-EA-TB nano-membrane and (ii) a possible “mechanical valve effect” linked to membrane potential and electrokinetic movement of the membrane as well as hydrostatic pressure effects.
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.
The diffusive mass transport of individual redox molecules was probed experimentally in microfabricated nanogap electrodes. The residence times for molecules inside a well-defined detection volume were extracted and the resulting distribution was compared with quantitative analytical predictions from random-walk theory for the time of first passage. The results suggest that a small number of strongly adsorbing sites strongly influence mass transport at trace analyte levels.
G. A. Kalkman, Y. Zhang, E. Monachino, K. Mathwig, M. E. Kamminga, P. Pourhossein, P. E. Oomen, S. A. Stratmann, Z. Zhao, A. M. van Oijen, E. M. J. Verpoorte and R. C. Chiechi Bisecting Microfluidic Channels with Metallic Nanowires Fabricated by Nanoskiving ACS Nano 10 (2016) 2852.
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 (< 5V). Further, passive functionality is demonstrated by visualizing individual, fluorescently labelled DNA molecules attached to the wires. We measure rates of flow and show that, compared to surface-bound DNA strands, elongation saturates at lower rates of flow and background fluorescence from non-specific binding is reduced.
H. R. Zafarani, K. Mathwig, E. J. R. Sudhölter and L. Rassaei Electrochemical redox cycling in a new nanogap sensor: Design and simulation Journal of Electroanalytical Chemistry 760 (2016) 42.
We propose a new geometry for nanogap electrochemical sensing devices. These devices consist of two closely spaced side-by-side electrodes which work under redox cycling conditions. Using finite element simulations, we investigate the effects of different geometric parameters on the redox cycling signal amplification to gain insight into the electrochemical sensing performance of the device design. This will allow optimizing the sensor performance of devices to be fabricated in the future.
S. Sarkar, A. F. Nieuwenhuis, S. Kang, K. Mathwig and S. G. Lemay
Integrated Microfluidics of Electrochemical Nanogap Sensors
Proceedings of the 19th International Conference on Miniaturized Systems for Chemistry and Life Science, Gyeongju, Korea, Oct. 25 – 29 (2015) 1522.
Owing to the broad applicability of electrochemical sensors in the biomedical field, there has been considerable interest in incorporating electrochemical sensors into lab-on-a-chip platforms. Such sensors can be miniaturized easily and are relatively stable and robust. In this work, an effective method of incorporating fluid flow with nanoscale electrochemical sensors is presented.
K. Mathwig, Q. Chi, S. G. Lemay and L. Rassaei Handling and Sensing of Single Enzyme Molecules: From Fluorescence Detection Towards Nanoscale Electrical Measurements ChemPhysChem 17 (2015) 452.
Classical methods to study single enzyme molecules have provided valuable information about distribution of conformational heterogeneities, reaction mechanisms, and transients in enzymatic reactions when individual molecules instead of an averaging ensemble are studied. Here, we highlight major advances in all-electrical single enzyme studies with a focus on recent micro- and nanofluidic tools which offer new ways of handling and studying small numbers of molecules or even single enzyme molecules. We particularly emphasize nanofluidic devices which enable the integration of electrochemical transduction and detection.