Reversible Adsorption of Outer-Sphere Redox Molecules at Pt Electrodes

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 counterintuitive 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

Integrated Biodetection in a Nanofluidic Device

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 that 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

Noise Phenomena Caused by Reversible Adsorption in Nanoscale Electrochemical 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

Stochastic Amperometric Fluctuations as a Probe for Dynamic Adsorption in Nanofluidic Electrochemical Systems

Adsorption of analyte molecules is ubiquitous in nanofluidic channels due to their large surface-to-volume ratios. It is also difficult to quantify due to the nanometric scale of these channels. We propose a simple method to probe dynamic adsorption at electrodes that are embedded in nanofluidic channels or which enclose nanoscopic volumes. The amperometric method relies on measuring the amplitude of the fluctuations of the redox cycling current that arise when the channel is diffusively coupled to a bulk reservoir. We demonstrate the versatility of this new method by quantifying adsorption for several redox couples, investigating the dependence of adsorption on the electrode potential and studying the effect of functionalizing the electrodes with self-assembled monolayers of organothiol molecules bearing polar end groups. These self-assembled monolayer coatings are shown to significantly reduce the adsorption of the molecules on to the electrodes. The detection method is not limited to electrodes in nanochannels and can be easily extended to redox cycling systems that enclose very small volumes, in particular scanning electrochemical microscopy with nanoelectrodes. It thus opens the way for imaging spatial heterogeneity with respect to adsorption, as well as rational design of interfaces for redox cycling based sensors

Stochasticity in Single-Molecule Nanoelectrochemistry: Origins, Consequences, and Solutions

Electrochemical detection of single molecules is being actively pursued as an enabler of new fundamental experiments and sensitive analytical capabilities. Most attempts to date have relied on redox cycling in a nanogap, which consists of two parallel electrodes separated by a nanoscale distance. While these initial experiments have demonstrated single-molecule detection at the proof-of-concept level, several fundamental obstacles need to be overcome to transform the technique into a realistic detection tool suitable for use in more complex settings (<i>e.g.</i>, studying enzyme dynamics at single catalytic event level, probing neuronal exocytosis, <i>etc.</i>). In particular, it has become clearer that stochasticitythe hallmark of most single-molecule measurementscan become the key limiting factor on the quality of the information that can be obtained from single-molecule electrochemical assays. Here we employ random-walk simulations to show that this stochasticity is a universal feature of all single-molecule experiments in the diffusively coupled regime and emerges due to the inherent properties of Brownian motion. We further investigate the intrinsic coupling between stochasticity and detection capability, paying particular attention to the role of the geometry of the detection device and the finite time resolution of measurement systems. We suggest concrete, realizable experimental modifications and approaches to mitigate these limitations. Overall, our theoretical analyses offer a roadmap for optimizing single-molecule electrochemical experiments, which is not only desirable but also indispensable for their wider employment as experimental tools for electrochemical research and as realistic sensing or detection systems

Noise Characteristics of Nanoscaled Redox-Cycling Sensors: Investigations Based on Random Walks

We investigate noise effects in nanoscaled electrochemical sensors using a three-dimensional simulation based on random walks. The presented approach allows the prediction of time-dependent signals and noise characteristics for redox cycling devices of arbitrary geometry. We demonstrate that the simulation results closely match experimental data as well as theoretical expectations with regard to measured currents and noise power spectra. We further analyze the impact of the sensor design on characteristics of the noise power spectrum. Specific transitions between independent noise sources in the frequency domain are indicative of the sensor-reservoir coupling and can be used to identify stationary design features or time-dependent blocking mechanisms. We disclose the source code of our simulation. Since our approach is highly flexible with regard to the implemented boundary conditions, it opens up the possibility for integrating a variety of surface-specific molecular reactions in arbitrary electrochemical systems. Thus, it may become a useful tool for the investigation of a wide range of noise effects in nanoelectrochemical sensors

Modulating Selectivity in Nanogap Sensors

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

Deformability Assessment of Waterborne Protozoa Using a Microfluidic-Enabled Force Microscopy Probe - Fig 1

<p>Oocyst measurement process <b>(a)</b> Schematic of FluidFM setup. <b>(b)</b> Typical force-distance curve showing zero (no contact), exponential (initial contact), and linear (hard contact) increase of force. <b>(c-e)</b> An oocyst (indicated by dashed blue oval) is selected, measured and released.</p

Cryptosporidium deformability - data

The files within the ZIP archive are in CSV format and can be readily accessed by Microsoft Excel, MATLAB, and many other programs. CSV files were recorded by the CyUI software from Cytosurge AG (Glattbrug, Switzerland). "spec.fwd.csv" and "spec.bwd.csv" files refer to the approach or retraction phases of the force spectroscopy measurements, respectively. Only the "spec.fwd.csv" (approach) files were used in the data analysis. Cantilever spring constants, deflection sensitivities, and sampling frequencies are specified in the header of the csv files. The initial cantilever-substrate separation was 10 micron for the C. muris and 5.0-5.3 micron for the C. parvum measurements

Height and spring constant data for untreated, heat-treated and freeze-thawed <i>C</i>. <i>parvum</i> oocysts obtained using FluidFM.

<p>Height <b>(a)</b> and spring constant <b>(b)</b> data for untreated (<i>n</i> = 184), heat-treated (<i>n</i> = 224) and freeze-thawed <i>C</i>. <i>parvum</i> oocysts (<i>n</i> = 131) are represented by boxplots which display medians and quartiles for the respective distributions; the upper and lower error bar caps are indicative of the maximum and minimum recorded values. *** Significantly different from untreated group (<i>p</i> < 0.001). ^## Significantly different from heat-treated group (<i>p</i> < 0.01). The distributions of height <b>(c)</b> and spring constant <b>(d)</b> are presented using histograms.</p