Electrokinetic transport of CTAB induces multiphasic behavior during capillary adsorption and desorption.

Cationic surfactant coatings (e.g., CTAB) are commonly used in CE to control EOF and thereby improve separation efficiencies. However, our understanding of surfactant adsorption and desorption dynamics under EOF conditions is limited. Here, we apply automated zeta potential analysis to study the adsorption and desorption kinetics of CTAB in a capillary under different transport conditions: diameter, length, voltage alternation pattern and frequency, and applied pressure. In contrast to other studies, we observe slower kinetics at distinct capillary wall zeta potential ranges. Within these ranges, which we call "stagnant regimes," the EOF mobility significantly counteracts the electrophoretic (EP) mobility of CTA+ and hinders the net transport. By constructing a numerical model to compare with our experiments and recasting our experimental data in terms of the net CTA+ transport volume normalized by surface area, we reveal that the EP mobility of CTA+ and the capillary surface-area-to-volume ratio dictate the zeta potential range and the duration of the stagnant regime and thereby govern the overall reaction kinetics. Our results indicate that further transport-oriented studies can significantly aid in the understanding and design of electrokinetic systems utilizing CTAB and other charged surfactants.

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels.

In this study, we present nanofluidic diodes fabricated from straight glass nanochannels and functionalized using bio-inspired polydopamine (PDA) and poly-L-lysine (PLL) coatings. The resulting PDA coatings are shown to be asymmetric due to a combination of transport considerations which can be leveraged to provide a measure of control over the effective channel geometry. By subsequently introducing a layer of amine-bearing PLL chains covalently bound to the PDA, we enhance heterogeneities in the charge and ion distributions within the channel and enable significant current rectification between forward-bias and reverse-bias modes; our PDA-PLL-coated channels yielded a rectification ratio greater than 1000 in a 100 nm channel filled with 0.01× phosphate-buffered saline solution (PBS). We further demonstrated that at higher ionic strength conditions, reducing the solution pH increased the number of protonated amines within the PLL layer, amplifying the charge disparities along the channel and leading to greater rectification. As nanofluidic diodes with bipolar surface charge distributions tend to provide superior performance compared to those with a single wall charge polarity, we imposed a more bipolar charge distribution in our devices by partially coating our PDA-PLL-coated channels with negatively charged polyacrylic acid (PAA). These enhanced bipolar channels exhibited greater current rectification than the PDA-PLL-coated channels, reaching rectification ratios in excess of 100 even in more physiologically-relevant 1× PBS solutions. Our fabrication approach and the results herein provide a promising platform from which the scientific community can build upon in the relentless endeavor for improved sensitivity in biosensors and other analytical devices.

Electrooxidation of Phenol on Polyelectrolyte Modified Carbon Electrodes for Use in Insulin Pump Infusion Sets.

BACKGROUND: Many type 1 diabetes patients using continuous subcutaneous insulin infusion (CSII) suffer from the phenomenon of unexplained hypoglycemia or "site loss." Site loss is hypothesized to be caused by toxic excipients, for example, phenolic compounds within insulin formulations that are used as preservatives and stabilizers. Here, we develop a bioinspired polyelectrolyte-modified carbon electrode for effective electrooxidative removal of phenol from insulin and eventual incorporations into an infusion set of a CSII device. METHODS: We modified a carbon screen printed electrode (SPE) with poly-L-lysine (PLL) to avoid passivation due to polyphenol deposition while still removing phenolic compounds from insulin injections. We characterized these electrodes using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) and compared their data with data from bare SPEs. Furthermore, we performed electrochemical measurements to determine the extent of passivation, and high-performance liquid chromatography (HPLC) measurements to confirm both the removal of phenol and the integrity of insulin after phenol removal. RESULTS: Voltammetry measurements show that electrode passivation due to polyphenol deposition is reduced by a factor of 2X. HPLC measurements confirm a 10x greater removal of phenol by our modified electrodes relative to bare electrodes. CONCLUSION: Using bioinspired polyelectrolytes to modify a carbon electrode surface aids in the electrooxidation of phenolic compounds from insulin and is a step toward integration within an infusion set for mitigating site loss.

Predicting ion concentration polarization and analyte stacking/focusing at nanofluidic interfaces.

We report on the investigation of electropreconcentration phenomena in micro-/nanofluidic devices integrating 100 µm long nanochannels using 2D COMSOL simulations based on the coupled Poisson-Nernst-Planck and Navier-Stokes system of equations. Our numerical model is used to demonstrate the influence of key governing parameters such as electrolyte concentration, surface charge density, and applied axial electric field on ion concentration polarization (ICP) dynamics in our system. Under sufficiently extreme surface-charge-governed transport conditions, ICP propagation is shown to enable various transient and stationary stacking and counter-flow gradient focusing mechanisms of anionic analytes. We resolve these spatiotemporal dynamics of analytes and electrolyte ICP over disparate time and length scales, and confirm previous findings that the greatest enhancement is observed when a system is tuned for analyte focusing at the charge, excluding microchannel, nanochannel electrical double layer (EDL) interface. Moreover, we demonstrate that such tuning can readily be achieved by including additional nanochannels oriented parallel to the electric field between two microchannels, effectively increasing the overall perm-selectivity and leading to enhanced focusing at the EDL interfaces. This approach shows promise in providing added control over the extent of ICP in electrokinetic systems, particularly under circumstances in which relatively weak ICP effects are observed using only a single channel.

Real-Time Zeta Potential Analysis of Microchannel Surfaces during Aminosilane Deposition and Exposure Using Current Monitoring.

Surface coatings are extensively used in capillary electrophoresis to increase separation efficiency and resolution. The stability of these coatings across a wide pH range is desirable to achieve repeatable migration times; therefore, a comprehensive understanding of coating degradation timescales is needed. We present a novel platform for automated zeta potential analysis based upon current monitoring that delivers improved time resolution over the existing methods. Using our platform, we measure the zeta potential continuously during aminosilane coating reactions and infer changes in the surface composition. We found that the change in the zeta potential after coating depended on the monomer type and solvent, while its stability was influenced by the coating solvent and exposure pH. Our versatile platform provides an elegant approach for evaluating the molecular composition, reactivity, and stability of surfaces in real time.

Molecular Design of a New Diboronic Acid for the Electrohydrodynamic Monitoring of Glucose.

A new dicationic diboronic acid structure, DBA2+, was designed to exhibit good affinity (Kd ≈1 mm) and selectivity toward glucose. Binding of DBA2+ to glucose changes the pKa of DBA2+ from 9.4 to 6.3, enabling opportunities for detection of glucose at physiological pH. Proton release from DBA2+ is firmly related to glucose concentrations within the physiologically relevant range (0-30 mm), as verified by conductimetric monitoring. Negligible interference from other sugars (for example, maltose, fructose, sucrose, lactose, and galactose) was observed. These results demonstrate the potential of DBA2+ for selective, quantitative glucose sensing. The nonenzymatic strategy based on electrohydrodynamic effects may enable the development of stable, accurate, and continuous glucose monitoring platforms.

Fluorescence-Based Observation of Transient Electrochemical and Electrokinetic Effects at Nanoconfined Bipolar Electrodes.

Bipolar electrodes (BPEs) are conductors that, when exposed to an electric field, polarize and promote the accumulation of counterionic charge near their poles. The rich physics of electrokinetic behavior near BPEs has not yet been rigorously studied, with our current understanding of such bipolar effects being restricted to steady-state conditions (under constant applied fields). Here, we reveal the dynamic electrokinetic and electrochemical phenomena that occur near nanoconfined BPEs throughout all stages of a reaction. Specifically, we demonstrate, both experimentally and through numerical modeling, that the removal of an electric field produces solution-phase charge imbalances in the vicinity of the BPE poles. These imbalances induce intense and short-lived nonequilibrium electric fields that drive the rapid transport of ions toward specific BPE locations. To determine the origin of these electrokinetic effects, we monitored the movement and fluorescent behavior (enhancement or quenching) of charged fluorophores within well-defined nanofluidic architectures via real-time optical detection. By systematically varying the nature of the fluorophore, the concentration of the electrolyte, the strength of the applied field, and oxide growth on the BPE surface, we dissect the ion transport events that occur in the aftermath of field-induced polarization. The results contained in this work provide new insights into transient bipolar electrokinetics that improve our understanding of current analytical platforms and can drive the development of new micro- and nanoelectrochemical systems.

Confinement effects on DNA hybridization in electrokinetic micro- and nanofluidic systems.

Spatial confinement, within cells or micro- and nanofabricated devices, impacts the conformation and binding kinetics of biomolecules. Understanding the role of spatial confinement on molecular behavior is important for comprehending diverse biological phenomena, as well as for designing biosensors. Specifically, the behavior of molecular binding under an applied electric field is of importance in the development of electrokinetic biosensors. Here, we investigate whether confinement of DNA oligomers in capillary electrophoresis impacts the binding kinetics of the DNA. To infer the role of confinement on hybridization dynamics, we perform capillary electrophoresis measurements on DNA oligomers within micro- and nanochannels, then apply first-order reaction dynamics theory to extract kinetic parameters from electropherogram data. We find that the apparent dissociation constants at the nanoscale (i.e., within a 100 nm channel) are lower than at the microscale (i.e., within a 1 µm channel), indicating stronger binding with increased confinement. This confirms, for the first time, that confinement-based enhancement of DNA hybridization persists under application of an electric field.

DNA-Stabilized Silver Nanoclusters as Specific, Ratiometric Fluorescent Dopamine Sensors.

Neurotransmitters are small molecules that orchestrate complex patterns of brain activity. Unfortunately, there exist few sensors capable of directly detecting individual neurotransmitters. Those sensors that do exist are either unspecific or fail to capture the temporal or spatial dynamics of neurotransmitter release. DNA-stabilized silver nanoclusters (DNA-AgNCs) are a new class of biocompatible, fluorescent nanostructures that have recently been shown to offer promise as biosensors. In this work, we identify two different DNA sequences that form dopamine-sensitive nanoclusters. We demonstrate that each sequence supports two distinct DNA-AgNCs capable of providing specific, ratiometric fluorescent sensing of dopamine concentration in vitro. DNA-Ag nanoclusters therefore offer a novel, low-cost approach to quantification of dopamine, creating the potential for real-time monitoring in vivo.

An Experimental Approach to Systematically Probe Charge Inversion in Nanofluidic Channels.

Charge inversion of the surfaces of nanofluidic channels occurs in systems with high-surface charge and/or highly charged ions and is of particular interest because of applications in biological and energy conversion systems. However, the details of such charge inversion have not been clearly elucidated. Specifically, although we can experimentally and theoretically show charge inversion, understanding at what conditions charge inversion occurs, as well how much the charge-inverting ions change the surface, are not known. Here, we show a novel experimental approach for uniquely finding both the ζ-potential and adsorption time of charge inverting ions in aqueous nanofluidic systems.

Two-Dimensional Electric Double Layer Structure with Heterogeneous Surface Charge.

In this work we present a systematic study of the lateral (parallel to the wall) and normal (perpendicular to the wall) nanostructure of the electric double layer at a heterogeneous interface between two regions of different surface charges, often found in nanoscale electrochemical devices. Specifically, classical density functional theory (DFT) is used to probe a cation concentration range of 10 mM to 1 M, for valences of +1, + 2, and +3, and a diameter range of 0.15-0.9 nm over widely varying surface charges (between -0.15 and +0.15 C/m2). The DFT results predict significant lateral and normal nanostructure in the form of ion concentration oscillations. These results are directly compared with those from Poisson-Boltzmann theory, showing significant deviation between the two theories, not only in the concentration profiles, but also in the sign of the electrostatic potential.

Optimal MEMS device for mobility and zeta potential measurements using DC electrophoresis.

We have developed a novel microchannel geometry that allows us to perform simple DC electrophoresis to measure the electrophoretic mobility and zeta potential of analytes and particles. In standard capillary geometries, mobility measurements using DC fields are difficult to perform. Specifically, measurements in open capillaries require knowledge of the hard to measure and often dynamic wall surface potential. Although measurements in closed capillaries eliminate this requirement, the measurements must be performed at infinitesimally small regions of zero flow where the pressure driven-flow completely cancels the electroosmotic flow (Komagata Planes). Furthermore, applied DC fields lead to electrode polarization, further questioning the reliability and accuracy of the measurement. In contrast, our geometry expands and moves the Komagata planes to where velocity gradients are at a minimum, and thus knowledge of the precise location of a Komagata plane is not necessary. Additionally, our microfluidic device prevents electrode polarization because of fluid recirculation around the electrodes. We fabricated our device using standard MEMS fabrication techniques and performed electrophoretic mobility measurements on 500 nm fluorescently tagged polystyrene particles at various buffer concentrations. Results are comparable to two different commercial dynamic light scattering based particle sizing instruments. We conclude with guidelines to further develop this robust electrophoretic tool that allows for facile and efficient particle characterization.

A universal design for a DNA probe providing ratiometric fluorescence detection by generation of silver nanoclusters.

DNA-stabilized silver nanoclusters (AgNCs), the fluorescence emission of which can rival that of typical organic fluorophores, have made possible a new class of label-free molecular beacons for the detection of single-stranded DNA. Like fluorophore-quencher molecular beacons (FQ-MBs) AgNC-based molecular beacons (AgNC-MBs) are based on a single-stranded DNA that undergoes a conformational change upon binding a target sequence. The new conformation exposes a stretch of single-stranded DNA capable of hosting a fluorescent AgNC upon reduction in the presence of Ag(+) ions. The utility of AgNC-MBs has been limited, however, because changing the target binding sequence unpredictably alters cluster fluorescence. Here we show that the original AgNC-MB design depends on bases in the target-binding (loop) domain to stabilize its AgNC. We then rationally alter the design to overcome this limitation. By separating and lengthening the AgNC-stabilizing domain, we create an AgNC-hairpin probe with consistent performance for arbitrary target sequence. This new design supports ratiometric fluorescence measurements of DNA target concentration, thereby providing a more sensitive, responsive and stable signal compared to turn-on AgNC probes. Using the new design, we demonstrate AgNC-MBs with nanomolar sensitivity and singe-nucleotide specificity, expanding the breadth of applicability of these cost-effective probes for biomolecular detection.

(Almost) Stationary Isotachophoretic Concentration Boundary in a Nanofluidic Channel Using Charge Inversion.

The present work is an experimental study of a new means to induce a quasi-stationary boundary for concentration or separation in a nanochannel induced by charge inversion. Instead of using pressure-driven counter-flow to keep the front stationary, we exploit charge inversion by a highly charged electrolyte, Ru(bpy)3Cl2, that changes the sign of the zeta potential in part of the channel from negative to positive. Having a non-charge inverting electrolyte (MgCl2) in the other part of the channel and applying an electric field can create a standing front at the interface between them without added dispersion due to an externally applied pressure-driven counterflow. The resulting slow moving front position can be easily imaged optically since Ru(bpy)3Cl2 is fluorescent. A simple analytical model for the velocity field and front axial position that reproduces the experimental location of the front shows that the location can be tuned by changing the concentration of the electrolytes (and thus local zeta potential). Both of these give the charge inversion-mediated boundary significant advantages over current methods of concentration and separation and the method is, therefore, of particular importance to chemical and biochemical analysis systems such as chromatography and separations and for enhancing the stacking performance of field amplified sample injection and isotachophoresis. By choosing a non-charge inverting electrolyte other than MgCl2, either this electrolyte or the Ru(bpy)3Cl2 solution can be made to be the leading or trailing electrolyte.

Electrophoretic mobility of spherical particles in bounded domain.

In this study, we improve on our 3D steady-state model of electrophoretic motion of spherical particles in bounded fluidic channels (Liu et al., 2014) to include the effect of nonsymmetric electrolytes, and further validate this improved model with detailed comparisons to experimental data. Specifically, we use the experimentally-measured particle mobilities from the work of Semenov et al. (2013), Napoli et al. (2011), and Wynne et al. (2012) to determine the corresponding particle zeta potentials using our model, and compare these results with classical theory. Incorporating the effects of nonsymmetric electrolytes, EDL polarization, and confinement, we show that our improved model is applicable to a wide range of practical experimental conditions, for example, particles that have high zeta potentials in a bounded channel filled with nonsymmetric electrolyte solutions, where classical theory is not applicable. In addition, we find that when electrolyte concentration is comparable to the concentration of hydronium or hydroxide ions, the complicated composition of ions increases the particle mobility. Finally, increased electrophoretic mobility can be observed when buffer solutions (phosphate or borate) were used as electrolyte solutions in experiments as opposed to simple symmetric electrolytes.

Changes in Spectra and Conformation of Hairpin DNA-Stabilized Silver Nanoclusters Induced by Stem Sequence Perturbations.

It is well-known that even small perturbations of the DNA sequence can drastically and unpredictably disrupt or alter the fluorescence of DNA-stabilized silver nanoclusters (DNA-AgNCs). Understanding how the structure of DNA affects the nanocluster that it stabilizes is the key to rationalizing such effects. We approach this challenge by strategically modifying the stem sequence of a hairpin DNA that hosts a spectrally pure, red-emitting nanocluster. Most of our modifications (base composition, sequence orientation, and loop location) reduce AgNC fluorescence in purity and shift it in wavelength, but one modification (appending poly(thymidine) to the 3' end of the stem) is inert with respect to fluorescence. Microfluidic capillary electrophoresis reveals that all of the modifications induce conformational changes of the DNA and that the original, spectrally pure nanocluster exists in two structurally distinct conformations. Interestingly, appending five or more thymidines, despite having no effect on fluorescence, eliminates this structural degeneracy. To explain this result, we propose that the original spectrally pure cluster is stabilized by a pair of hairpins whose stems can arrange in either a cis or trans orientation. Finally, we quantify the extent to which thymidine appendages of different lengths can be used to fine-tune the electrophoretic mobility of DNA-AgNC.

A simple microfluidic aggregation analyzer for the specific, sensitive and multiplexed quantification of proteins in a serum environment.

Portable and low-cost platforms for protein biomarker detection are highly sought after for point of care applications. We demonstrate a simple microfluidic device for the rapid, electrically-based detection of proteins in serum. Our aggregation analyzer relies on detecting the protein-induced aggregation of sub-micron particles, using a one-step procedure followed by a fast, particle-by-particle measurement with a very high count rate. This enables the rapid and precise quantification of C-Reactive protein levels, within the clinically relevant range, using unprocessed human serum and a disposable microfluidic device; no optics are involved in the implementation. Due to the single particle detection format and the use of microfluidics, only a small volume of serum (~50 nL) is needed to complete the analysis. The method can be easily extended to multiplexed biomarker detection by combining an assay using differently sized particles, each targeting a separate protein. We illustrate this by using two sizes of latex beads and demonstrating the simultaneous detection of two different proteins in a serum environment with minimal cross-interference. This confirms that our aggregation analyzer platform provides a simple and straightforward method for multiplexed biomarker detection in a low cost, portable design.

Electrocavitation in nanofluidics: unique phenomenon and fundamental platform.

In this paper, we will highlight one phenomenon unique to nanofluidics: electrocavitation. Electrocavitation is defined as cavitation induced by electric fields. Cavitation in general occurs in a liquid when it is subjected to a pressure below its vapor pressure, where the liquid can break apart and form a cavity (bubble). This is frequently seen in macroscale systems, for example, rotating propeller blades on the turbines of ships or water columns in the xylem of trees. Electrocavitation in nanochannels was first reported when researchers applied electric fields within nanochannels containing electrolytes discontinuous in conductivity and found that bubbles formed within the channel. The reasons to highlight electrocavitation to both the lab-on-a-chip community and those interested in the fundamental understanding of cavitation in general are detailed below.

Quantitative characterization of the colloidal stability of metallic nanoparticles using UV-vis absorbance spectroscopy.

Plasmonic nanoparticles are used in a wide variety of applications over a broad array of fields including medicine, energy, and environmental chemistry. The continued successful development of this material class requires the accurate characterization of nanoparticle stability for a variety of solution-based conditions. Although many characterization methods exists, there is an absence of a unified, quantitative means for assessing the colloidal stability of plasmonic nanoparticles. We present the particle instability parameter (PIP) as a robust, quantitative, and generalizable characterization technique based on UV-vis absorbance spectroscopy to characterize colloidal instability. We validate PIP performance with both traditional and alternative characterization methods by measuring gold nanorod instability in response to different salt (NaCl) concentrations. We further measure gold nanorod stability as a function of solution pH, salt, and buffer (type and concentration), nanoparticle concentration, and concentration of free surfactant. Finally, these results are contextualized within the literature on gold nanorod stability to establish a standardized methodology for colloidal instability assessment.