Shaurya Prakash

Nanofluidics: Systems and Applications

Nanofluidics presents growing and exciting opportunities for conducting fundamental studies for processes and systems that govern molecular-scale operations in science and engineering. In addition, nanofluidics provides a rapidly growing platform for developing new systems and technologies for an ever-growing list of applications. This review presents a summary of the transport phenomena in nanofluidics with a focus on several systems and applications important to problems of public health and welfare. Special emphasis is afforded to the role of the electric double layer and the molecular-scale interactions that occur within confined nanoscale systems.

Theory, fabrication and applications of microfluidic and nanofluidic biosensors.

Biosensors are a broad array of devices that detect the type and amount of a biological species or biomolecule. Several different types of biosensors have been developed that rely on changes to mechanical, chemical or electrical properties of the transduction or sensing element to induce a measurable signal. Often, a biosensor will integrate several functions or unit operations such as sample extraction, manipulation and detection on a single platform. This review begins with an overview of the current state-of-the-art biosensor field. Next, the review delves into a special class of biosensors that rely on microfluidics and nanofluidics by presenting the underlying theory, fabrication and several examples and applications of microfluidic and nanofluidic sensors.

OH concentration profiles over alumina, quartz, and platinum surfaces using laser-induced fluorescence spectroscopy in low-pressure hydrogen/oxygen flames

ABSTRACT Laser-induced fluorescence measurements of OH radical concentration are performed in the post-flame gaseous region of a stoichiometric H2/O2 mixture combusting at 20 Torr. The measurements are taken near surfaces of three different materials [α-alumina (Al2O3), quartz (SiO2), and platinum (Pt) on alumina] at different surface temperatures (from ∼700 to 1100 K). A comparative analysis of some of the factors affecting OH recombination rate are provided as a function of material and temperature. Of the samples tested, Al2O3 is observed to be the least active whereas Pt on Al2O3 is the most active, with SiO2 at an intermediate level with respect to OH recombination rate on surfaces indicating a quantitative difference between the two relatively inert materials. The result of these differences in recombination rates is that the overall OH number density in the gaseous region within millimeters of the surface was observed to differ by over one order of magnitude between Pt and Al2O3 under nearly identical conditions.

A Three-State Nanofluidic Field Effect Switch

We report a three-state nanofluidic field effect switch in an asymmetrically gated device with a forward (positive), off (zero), and a reverse (negative) current state for tunable control of ionic transport by systematically controlling the gate potential. The embedded gate electrode allows for modulation of the ionic current through the 16 nm deep channels as a function of electrolyte concentration and gate electrode location for a fixed streamwise potential.

Determining nanocapillary geometry from electrochemical impedance spectroscopy using a variable topology network circuit model.

Solid-state nanopores and nanocapillaries find increasing use in a variety of applications including DNA sequencing, synthetic nanopores, next-generation membranes for water purification, and other nanofluidic structures. This paper develops the use of electrochemical impedance spectroscopy to determine the geometry of nanocapillaries. A network equivalent circuit element is derived to include the effects of the capacitive double layer inside the nanocapillaries as well as the influence of varying nanocapillary radius. This variable topology function is similar to the finite Warburg impedance in certain limits. Analytical expressions for several different nanocapillary shapes are derived. The functions are evaluated to determine how the impedance signals will change with different nanocapillary aspect ratios and different degrees of constriction or inflation at the capillary center. Next, the complex impedance spectrum of a nanocapillary array membrane is measured at varying concentrations of electrolyte to separate the effects of nanocapillary double layer capacitance from those of nanocapillary geometry. The variable topology equivalent circuit element model of the nanocapillary is used in an equivalent circuit model that included contributions from the membrane and the measurement apparatus. The resulting values are consistent with the manufacturers specified tolerances of the nanocapillary geometry. It is demonstrated that electrochemical impedance spectroscopy can be used as a tool for in situ determination of the geometry of nanocapillaries.

Dynamic Response of AFM Cantilevers to Dissimilar Functionalized Silica Surfaces in Aqueous Electrolyte Solutions

The dynamic response of an oscillating microcantilever with a gold-coated tip interacting with dissimilar functionalized silica surfaces was studied in electrolyte solutions with pH ranging from 4 to 9. Silica surfaces were chemically modified, yielding dissimilar surfaces with -Br, -NH(2), and -CH(3) functional group terminations. The relative hydrophobicity of the surfaces was characterized by contact angle measurements. The surface charge of the functionalized surfaces was first probed with commonly used static AFM measurements and serves as a reference to the dynamic response data. The amplitude and phase of the cantilever oscillation were monitored and used to calculate the effective interaction stiffness and damping coefficient, which relate to the electrical double layer interactions and also to distance-dependent hydrodynamic damping at the solid/water interface. The data for the dynamic response of the AFM over silica surfaces as a function of chemical functionalization and electrolyte pH show that the effective stiffness has a distinctive dependence on the surface charge of functionalized silica surfaces. The hydrodynamic damping also correlates strongly with the relative hydrophobicity of the surface. The data reported here indicate that interfacial properties can be strongly affected by changing the chemical composition of surfaces.

Characterization of ionic transport at the nanoscale

This paper reports on the development of a multi-layer microscale impedance measurement system with integrated working, counter, and reference electrodes that can be used to probe transport at the nanoscale. System fabrication and testing are carried out to demonstrate the feasibility of such a system for characterizing transport through nanocapillary array membranes (NCAMs). Results indicate that transport through NCAMs is a complex phenomenon, and impedance does not scale linearly with either pore diameter or ionic concentration. Use of a microscale construct for probing ionic transport at the nanoscale appears to be a promising path forward with further development.

Field effect nanofluidics

Nanoscale fluid transport through conduits in the 1-100 nm range is termed as nanofluidics. Over the past decade or so, significant scientific and technological advances have occurred in the domain of nanofluidics with a transverse external electrical signal through a dielectric layer permitting control over ionic and fluid flows in these nanoscale conduits. Consequently, this special class of nanofluidic devices is commonly referred to as field effect devices, analogous to the solid-state field effect transistors that form the basis for modern electronics. In this mini-review, we focus on summarizing the recent developments in field effect nanofluidics as a discipline and evaluate both tutorially and critically the scientific and technological advances that have been reported, including a discussion on the future outlook and identifying broad open questions which suggest that there are many breakthroughs still to come in field-effect nanofluidics.

Fabrication of Centimeter Long, Ultra-Low Aspect Ratio Nanochannel Networks in Borosilicate Glass Substrates

Nanofluidic devices have a broad range of applications resulting from the dominance of surface-fluid interactions. Examples include molecular gating, sample preconcentration, and sample injection. Manipulation of small fluid samples is ideal for micro total analysis systems or lab on chip devices which perform multiple unit operations on a single chip. In this paper, fabrication procedures for two different ultra-low aspect ratio (ULAR) channel network designs are presented. The ULAR provides increased throughput compared to higher aspect ratio features with the same critical dimensions. Channel network designs allow for integration between microscale and nanoscale fluidic networks. A modified calcium assisted glass–glass bonding procedure was developed to fabricate chemically uniform, all glass nanochannels. A polydimethylsiloxane (PDMS)-glass adhesive bonding procedure was also developed as adhesive bonding allows for more robust fabrication with lower sensitivity to surface defects. The fabrication schemes presented allow for a broad array of available parameters for facile selection of device fabrication techniques depending on desired applications for lab on chip devices.

Cation Dependent Surface Charge Regulation in Gated Nanofluidic Devices

Surface charge governs nanoscale aqueous electrolyte transport, both in engineered analytical systems and in biological entities such as ion channels and ion pumps as a function of ion type and concentration. Embedded electrodes in a nanofluidic channel, isolated from the fluid in the channel by a dielectric layer, act as active, tunable gates to systematically modify local surface charge density at the interface between the nanochannel surface and the aqueous electrolyte solution, causing significant changes in measured nanochannel conductance. A systematic comparison of transport of monovalent electrolytes [potassium chloride (KCl), sodium chloride (NaCl)], 2:1 electrolytes [magnesium chloride (MgCl2), calcium chloride (CaCl2)], and electrolyte mixtures (KCl + CaCl2) through a gated nanofluidic device was performed. Ion-surface interactions between divalent Ca2+ and Mg2+ ions and the nanochannel walls reduced the native surface charge density by up to ∼4-5 times compared to the monovalent cations. In electrolyte mixtures, Ca2+ was the dominating cation with nanochannel conductance independent of KCl concentration. Systematic changes in local electrostatic surface state induced by the gate electrode are impacted by the divalent cation-surface interactions, limiting modulation of the local surface potential by the gate electrode and resulting in cation dependent nanoscale ion transport as seen through conductance measurements and numerical models.