S. Sarkar, K. Mathwig, S. Kang, A. F. Nieuwenhuis and S. G. Lemay
Redox Cycling Without Reference Electrode
Analyst 139 (2014) 6052.
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The reference electrode is a key component in electrochemical measurements, yet it remains a challenge to implement a reliable reference electrode in miniaturized electrochemical sensors. Here we explore experimentally and theoretically an alternative approach based on redox cycling which eliminates the reference electrode altogether. We show that shifts in the solution potential caused by the lack of reference can be understood quantitatively, and determine the requirements for accurate measurements in miniaturized systems in the absence of a reference electrode.

L. Rassaei, K. Mathwig, S. Kang, H. A. Heering and S. G. Lemay
Integrated Biodetection in a Nanofluidic Device
ACS Nano 8 (2014) 8278.
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The sensing of enzymatic processes in volumes at or below the scale of single cells is challenging but highly desirable in the study of biochemical processes. Here we demonstrate a nanofluidic device which combines an enzymatic recognition element and electrochemical signal transduction within a six femtoliter volume. Our approach is based on localized immobilization of the enzyme tyrosinase in a microfabricated nanogap electrochemical transducer. The enzymatic reaction product quinone is localized in the confined space of a nanochannel in which efficient redox cycling also takes place. Thus the sensor allows the sensitive detection of minute amounts of product molecules generated by the enzyme in real time. This method is ideally suited for the study of ultra-small volume systems such as the contents of individual biological cells or organelles.

K. Mathwig, T. J. Aartsma, G. W. Canters and S. G. Lemay
Nanoscale Methods for Single-Molecule Electrochemistry
Annual Review of Analytical Chemistry 7 (2014) 383.
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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 partly 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.

E. Kätelhön, K. J. Krause, K. Mathwig, S. G. Lemay and B. Wolfrum
Noise Phenomena Caused by Reversible Adsorption in Nanoscale Electrochemical Devices
ACS Nano 8 (2014) 4924.
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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.

D. Mampallil, K. Mathwig, S. Kang and S. G. Lemay
Reversible Adsorption of Outer-Sphere Redox Molecules at Pt Electrodes
The Journal of Physical Chemistry Letters 5 (2014) 636.
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Adsorption often dominates the response of nanofluidic systems due to their high surface-to-volume ratios. Here we harness this sensitivity to investigate the reversible adsorption of outer-sphere redox species at electrodes, a phenomenon that is easily overlooked in bulk measurements. We find that, even though adsorption does not necessarily play a role in the electron-transfer process, such adsorption is nevertheless ubiquitous for the widely used outer-sphere species. We investigate the physical factors driving adsorption and find that this counter-intuitive behavior is mediated by the anionic species in the supporting electrolyte, closely following the well-known Hofmeister series. Our results provide foundations both for theoretical studies of the underlying mechanisms and for contriving strategies to control adsorption in micro/nanoscale electrochemical transducers where surface effects are dominant.

S. Kang, A. F. Nieuwenhuis, K. Mathwig, D. Mampallil and S. G. Lemay
Electrochemical Single-Molecule Detection in Aqueous Solution using Self-Aligned Nanogap Transducers
ACS Nano 7 (2013) 10931.
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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.

K. Mathwig, S. Schlautmann, S. G. Lemay and J. Hohlbein
A Novel Parallel Nanomixer for High-Throughput Single-Molecule Fluorescence Detection
Proceedings of the 17th International Conference on Miniaturized Systems for Chemistry and Life Science, Freiburg, Germany, Oct. 27 – 31 (2013) 1385.
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This paper introduces a novel fluidic device based on syringe-driven flow of fluorescent species through a parallel array of nanochannels, in which the geometrical confinement enables long observation times of non-immobilized molecules. Extremely low flow rates are achieved by operating the array of nanochannels in parallel with a larger microchannel. The addition of a second microfluidic inlet allows for mixing different species in a well-defined volume, enabling the study of irreversible reactions such as DNA synthesis in real-time using single-molecule fluorescence resonance energy transfer. Devices are fabricated in glass with the purpose of high-throughput single-molecule fluorescence detection.

K. Mathwig and S. G. Lemay
Mass transport in electrochemical nanogap sensors
Electrochimica Acta 112 (2013) 943.
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Nanofluidic thin-layer cells based on redox cycling allow for extremely sensitive electrochemical detection. Here we establish a physical mass-transfer model for analyte molecules in these transducers which takes into account advective and diffusive transport of both oxidized and reduced species as well as reversible dynamic adsorption at the sensor surfaces. We use finite-element modeling to determine the transient response of nanogap sensors; numerically we predict that the response time can be reduced substantially by pressure-driven advection while the faradaic limiting current remains unaffected by this flow for all experimentally accessible flow rates.