Nanocapillary Array Membranes Research Articles

Transport of multicomponent, multivalent electrolyte solutions across nanocapillaries

Electrokinetic transport of aqueous solutions containing multiple ionic species in surface charge governed nanofluidic flows has seen limited investigation with most experimental and modeling efforts emphasizing symmetric, monovalent electrolytes. In this work, numerical models coupling steady-state Poisson–Nernst–Planck and Stokes equations along with experimental investigations were developed to characterize electrokinetic transport of potassium phosphate buffer, containing K+, H2PO4 −, and HPO4 2− across positively charged nanocapillary array membranes with 10 nm diameter nanocapillaries, sandwiched between a source and permeate reservoir. While systematically increasing phosphate buffer concentration from 0.2 to 10 mM, 0.14 mM of methylene blue (MB) dye was added to the source reservoir to study the dominating transport mechanism under a potential bias (0–0.75 V). Experiments provided validation of numerical results that elucidate fundamental transport mechanisms as a function of ion type, buffer concentration, and externally applied potential. The nanocapillary exhibits permselectivity toward anions at lower buffer concentrations (0.2, 1 mM) and was more selective for HPO4 2− in comparison with H2PO4 −. Transport of K+, H2PO4 −, and HPO4 2− was dominated by electromigration, with negligible effects of diffusion and convection at all buffer concentrations. However, transport of MB+ was dominated by diffusion at 0.2 mM buffer concentration under all potential bias conditions. Significant effects of electromigration appeared at high potential biases (0.5–0.75 V) for 1 and 10 mM bulk buffer concentrations. Additionally, in the multicomponent ion system, at all concentrations, the vast majority of the current was carried by the phosphate buffer ions and not the MB ions.

Study of ionic transport through metalized nanoporous membranes functionalized with self-assembled monolayers

Ionic transport through synthetic nanoporous membranes has received great attention in applications related to biosensing, fuel cell, and desalination. Past work has demonstrated that charge selectivity can be achieved by applying a potential across a metallized conductive membrane. However, challenges arise for improving charge selectivity as a result of irreversibility of the system from the anion adsorption at the membrane surface. This study demonstrates how charge selectivity can be improved with the presence of a well grown self-assembled monolayer (SAM), which can aid in applications that use chemical separation processes based on the surface charge of the nanopore.In this work, the transport and selectivity properties of gold-coated conductive nano-capillary-array-membranes (NCAMs) are studied using charged species methyl viologen paraquat, (MV2+) and napathalenedisulfonate disodium salt (NDS2−). The selectivity coefficients for MV2+ and NDS2− increased with the functionalization of undecanethiol on the gold-coated NCAM surface. With the presence of a SAM, the selectivity coefficients increased by 44% for MV2+ and 200% for NDS2−. The influence of ionic transport from the diffuse layer potential at the walls and surface of the nanopore is also discussed.

Enhanced Mass Transport of Electroactive Species to Annular Nanoband Electrodes Embedded in Nanocapillary Array Membranes

Electroosmotic flow (EOF) is used to enhance the delivery of Fe(CN)(6)(4-)/Fe(CN)(6)(3-) to an annular nanoband electrode embedded in a nanocapillary array membrane, as a route to high efficiency electrochemical conversions. Multilayer Au/polymer/Au/polymer membranes are perforated with 10(2)-10(3) cylindrical nanochannels by focused ion beam (FIB) milling and subsequently sandwiched between two axially separated microchannels, producing a structure in which transport and electron transfer reactions are tightly coupled. The middle Au layer, which contacts the fluid only at the center of each nanochannel, serves as a working electrode to form an array of embedded annular nanoband electrodes (EANEs), at which sufficient overpotential drives highly efficient electrochemical processes. Simultaneously, the electric field established between the EANE and the QRE (>10(3) V cm(-1)) drives electro-osmotic flow (EOF) in the nanochannels, improving reagent delivery rate. EOF is found to enhance the steady-state current by >10× over a comparable structure without convective transport. Similarly, the conversion efficiency is improved by approximately 10-fold compared to a comparable microfluidic structure. Experimental data agree with finite element simulations, further illustrating the unique electrochemical and transport behavior of these nanoscale embedded electrode arrays. Optimizing the present structure may be useful for combinatorial processing of on-chip sample delivery with electrochemical conversion; a proof of concept experiment, involving the generation of dissolved hydrogen in situ via electrolysis, is described.

Electrokinetic control of fluid transport in gold-coated nanocapillary array membranes in hybrid nanofluidic–microfluidic devices

The introduction of metallic elements into microfluidic devices that support electrokinetic transport creates several fundamental issues relative to the high conductivity of the metal, which can act as a current shunt, causing profound effects on the transport process. Here we examine the use of Au-coated nanocapillary array membranes (Au NCAMs) as electrically addressable fluid control elements in multi-layer microfluidic architectures. Three alternative methods for fluid injection across Au NCAMs are presented: electrokinetic injection across NCAMs with Au coated on one side (asymmetric NCAM), electrokinetic injection across NCAMs with an embedded Au layer (symmetric NCAM), and field-free electroosmotic flow (EOF) pumping across either type of Au NCAM. Injection efficiency across asymmetric NCAMs depends on the orientation of the asymmetric membrane relative to the driving potential. Efficient injections are enabled when the Au coating is on the receiving side of the membrane, however, some distortion of the injected volume element is observed, especially with large injection potentials. These results for asymmetric membranes agree qualitatively with two-dimensional numerical simulations of injections across a single slit pore, which suggest that the direction-selective transport behavior is related to electrophoretic transport of the anionic fluorescein probe. Reproducible, high quality injections are also achieved in symmetric Au NCAMs having an embedded gold nanoband region within the nanopores. Nanoband Au NCAMs are excellent candidates for a range of applications, including high efficiency electrochemical sensing, electrochemically catalyzed conversion or pretreatment and label free sensing utilizing extraordinary optical transmission. EOF pumping could be an alternative to electrokinetic injections in some applications, however, this approach is only useful for relatively large pore sizes (>400 nm) and presents considerably worse sample spreading via Taylor dispersion.

Multidimensional Separation of Chiral Amino Acid Mixtures in a Multilayered Three-Dimensional Hybrid Microfluidic/Nanofluidic Device

Microscale total analysis systems (microTAS) allow high-throughput analyses by integrating multiple processes, parallelization, and automation. Here we combine unit operations of microTAS to create a device that can perform multidimensional separations using a three-dimensional hybrid microfluidic/nanofluidic device composed of alternating layers of patterned poly(methyl methacrylate) and nanocapillary array membranes constructed from nuclear track-etched polycarbonate. Two consecutive electrophoretic separations are performed, the first being an achiral separation followed by a chiral separation of a selected analyte band. Separation conditions are optimized for a racemic mixture of fluorescein-isothiocyanate-labeled amino acids, serine and aspartic acid, chosen because there are endogenous D-forms of these amino acids in animals. The chiral separation is implemented using micellar electrokinetic chromatography using beta-cyclodextrin as the chiral selector and sodium taurocholate as the micelle-forming agent. Analyte separation is monitored by dual-beam laser-induced fluorescence detection. After separation in the first electrophoretic channel, the preselected analyte is sampled by the second-stage separation using an automated collection sequence with a zero-crossing algorithm. The controlled fluidic environment inherent to the three-dimensional architecture enables a series of separations in varying fluidic environments and allows sample stacking via different background electrolyte pH conditions. The ability to interface sequential separations, selected analyte capture, and other fluidic manipulations in the third dimension significantly improves the functionality of multilayer microfluidic devices.

Single nanopore transport of synthetic and biological polyelectrolytes in three-dimensional hybrid microfluidicnanofluidic devices.

This paper presents a study of electrokinetic transport in single nanopores integrated into vertically stacked three-dimensional hybrid microfluidicnanofluidic structures. In these devices, single nanopores, created by focused ion beam (FIB) milling in thin polymer films, provide fluidic connection between two vertically separated, perpendicular microfluidic channels. Experiments address both systems in which the nanoporous membrane is composed of the same (homojunction) or different (heterojunction) polymer as the microfluidic channels. These devices are then used to study the electrokinetic transport properties of synthetic (i.e., polystyrene sulfonate and polyallylamine) and biological (i.e., DNA) polyelectrolytes across these nanopores using both electrical current measurements and confocal microscopy. Both optical and electrical measurements indicate that electro-osmotic transport is predominant over electrophoresis in single nanopores with d>180 nm, consistent with results obtained under similar conditions for nanocapillary array membranes.

Ionic Transport Regimes for Nanoscale Transport towards the Development of Low Energy Water Desalination Membranes

AbstractNew materials, methods, and membranes are being developed for applications in water purification. One of the model systems that can be used for fundamental studies in nanoscale transport phenomena for new membrane technologies are nanocapillary array membranes (NCAMs). Toward developing more efficient membranes for water desalination, parameters such as the concentration polarization region which are influenced by the unstirred layers, surface properties (e.g., surface charge and surface energy) of the nanocapillaries, and the electric double layer (EDL) which mediates transport across NCAMs must be better understood. In this paper, a series of parametric experiments that were conducted to better understand the relative importance of unstirred layers with respect to the transport across nanocapillaries are described. Bulk salt concentration and potential drop across the NCAMs, were varied in a systematic manner to determine the influence EDL thickness and electromigration on transport regimes for ionic permeation across NCAMs. Based on previously developed methodologies, the experiments reported here were conducted in a permeation cell with an NCAM separating two reservoirs containing potassium phosphate buffer with a concentration range from 200 μM-10 mM. Methylene blue (MB) is used as an organic marker and the transport is quantified by tracking MB concentration in each reservoir with UV/VIS spectroscopy.

Enzymatic activity of surface-immobilized horseradish peroxidase confined to micrometer- to nanometer-scale structures in nanocapillary array membranes

Horseradish peroxidase (HRP) was immobilized on the planar surfaces and inside the cylindrical nanopores of nanocapillary array membranes (NCAMs) to study how the enzyme-catalyzed oxidation of a fluorigenic substrate, Amplex Red (AR), to fluorescent resorufin by hydrogen peroxide is influenced by confinement. Because AR was also found to be converted to resorufin photolytically at high laser fluences, a modified laser-induced fluorescence protocol was developed to characterize the enzyme-catalyzed reaction in the absence of interference from the photolytic reaction. Surface-immobilized HRP was studied in two environments: bound to the surface of a microfluidic channel, and bound to the interior of cylindrical nanopores in NCAMs connecting crossed microfluidic channels. HRP was immobilized through reaction of solvent-accessible primary amines with the epoxy group of the methyl methacrylate-glycidyl methacrylate copolymer synthesized in either planar or annular geometries to construct the test structures for enzymatic activity. HRP immobilized on planar surfaces shows high activity (approximately 10 microM min(-1)) meaning that the copolymer membrane exhibits good potential for immobilizing the enzyme, especially since active structures are obtained in a one-step reaction. HRP was also immobilized inside nanopores via physisorption. Enzymatic reactions inside the nanopores were characterized and compared to finite element simulations of a modified Eley-Rideal mechanism to bracket the value of the overall rate constant for the confined enzyme. Reaction velocities were estimated to be approximately 10-fold higher in the nanopores than for the same enzyme bound to a planar microfluidic surface.

Perturbation of Microfluidic Transport Following Electrokinetic Injection through a Nanocapillary Array Membrane: Injection and Biphasic Recovery

Ionic transport in nanopores is dependent on the nature of the electrical communication between the pores and the surrounding environment. A particularly useful fluidic device structure uses nanopores in nanocapillary array membranes (NCAMs) as electrically switchable valves between vertically separated microfluidic channels. In the off-state, the gate isolates the fluidic environments in the microchannels, but when the appropriate forward-bias voltage is applied, it selectively allows ions and analytes to move between the microchannels. However, the populations of species in the microfluidic channels are perturbed from their steady-state values due to ion accumulation and depletion effects. Experiments conducted here characterize the electrical conduction along the length of a microfluidic channel, and laser-induced fluorescence probes the formation of a high- and low-concentration regions of fluorescent dye before and after application of forward- and reverse-bias voltage pulses in both small (a = 10 nm...

Fluidic communication between multiple vertically segregated microfluidic channels connected by nanocapillary array membranes

Hybrid microfluidic/nanofluidic devices offer unique capabilities for manipulating and analyzing minute volumes of expensive or hard-to-obtain samples. Here, multilayer poly-(methyl methacrylate) microchips, with multiple spatially isolated microfluidic channels interconnected by nanocapillary array membranes (NCAMs), are fabricated using an adhesive contact printing process. The NCAMs, positioned between the microfluidic channel layers, add functionality to the inter-microchannel fluid transfer unit operation. They do so because the transport of specific analytes through the NCAM can be controlled by adjusting the ionic strength, the polarity of the applied bias, the surface charge density, and the pore size. A simplified, floating injection technique for NCAM-coupled nanofluidic devices is described and compared with conventional biased injection. In the floating injection approach, a voltage is applied across the injection channel and the slight electric field extension at the cross-section is used to transfer analytes through the nanopores to the separation channel. Floating injection excels in plug reproducibility, separation resolution, and operation simplicity, although it decreases assay throughput relative to biased injection. Floating injection can avoid the uneven distribution of analytes in the microfluidic channel that sometimes results from biased injection because of the volume mismatch between NCAM nanopore transport capacity and the supply of fluid. Moreover, the pressure-driven flow caused by the mismatch of the EOFs in the microfluidic channels connected by an NCAM must be considered when using NCAMs with pore diameters below 50 nm.

Electrokinetically driven fluidic transport in integrated three-dimensional microfluidic devices incorporating gold-coated nanocapillary array membranes

Electrokinetically driven fluid transport was evaluated within three-dimensional hybrid nanofluidic-microfluidic devices incorporating Au-coated nanocapillary array membranes (NCAMs). Gold NCAMs, prepared by electroless gold deposition on polymeric track-etched membranes, were susceptible to gas bubble formation if the interfacial potential difference exceeded approximately 2 V along the length of the gold region. Gold membranes were etched to yield 250 microm wide coated regions that overlap the intersection of two orthogonal microfluidic channels in order to minimize gas evolution. The kinetics of electrolysis of water at the opposing ends of the gold region was modeled and found to be in satisfactory agreement with experimental measurements of the onset of gas bubble formation. Conditions to achieve electrokinetic injection across Au-coated NCAMs were identified, with significant reproducible injections being possible for NCAMs modified with this relatively thin gold stripe. Continuous gold films led to suppressed injections and to a variety of ion enrichment/depletion effects in the microfluidic source channel. The suppression of injections was understood through finite element modeling which revealed the presence of a significant electrophoretic velocity component in opposition to electroosmotic flow at the edge of the Au-dielectric regions.

Surface Immobilization of Catalytic Beacons Based on Ratiometric Fluorescent DNAzyme Sensors: A Systematic Study

DNAzyme-based catalytic beacons have the potential for sensing a large number of relevant analytes. Thus, a systematic investigation of factors affecting their performance when immobilized into gold-coated nanocapillary array membranes (NCAMs) was undertaken. Enzyme immobilization times were varied to determine that as little as 15 min was sufficient for ratiometric detection of Pb2+-specific activity, while immobilization density saturated after 1.5 h. Immobilization of the DNAzymes into NCAMs with 600 nm pore size resulted in higher immobilization efficiency and higher enzymatic activity than that with 200 nm pore size. A poly-T linker length between the tethering thiol and first oligonucleotide, used to extend the DNAzyme above the backfilling mercaptohexanol (MCH) monolayer, had no effect on DNAzyme activity. The backfilling method of immobilization, involving backfilling followed by hybridization, was found most effective for DNAzyme activity compared to immobilization of hybridized DNAzyme complex (a 67% loss of activity) or concurrent enzyme and MCH immobilization (75% loss of activity). The backfilling MCH monolayer provided approximately 3.5 times increase in activity compared to DNAzyme assembled without MCH, and was over 5 times more active than shorter and longer backfilling molecules tested. The immobilized DNAzyme retained its optimized performance at 50 mM NaCl. Finally, the generalized immobilization and ratiometric procedure was employed for a uranyl-specific DNAzyme with 25 +/- 15 times increase in ratio observed. These findings form a firm basis on which practical applications of catalytic beacons can be realized, including sensors for both Pb2+ and UO22+ ions.

Direct Immobilization of Fab‘ in Nanocapillaries for Manipulating Mass-Limited Samples

Interfacing nanoscale elements into a microfluidic device enables a new range of fluidic manipulations. Nanocapillary array membranes (NCAMs), consisting of thin (5 microm < d < 20 microm) membranes containing arrays of nanometer diameter (10 nm < a < 500 nm) pores, are a convenient method of interfacing vertically separated microchannels in microfluidic devices that allow the external control of analyte transport between microfluidic channels. To add functionality to these nanopores beyond simple fluid transport, here we incorporate an antibody-based molecular recognition element onto the pore surface that allows selective capture, purification, and release of specific analytes from a mixture. The pores are fabricated by electroless plating of gold into the nanopores of an NCAM (Au-NCAM). An antibody is then immobilized on the Au-NCAM via gold-thiol chemistry as a thiolated fragment of antigen-binding (Fab') prepared by direct digestion of the antibody followed by reduction of the disulfide linkage on the hinge region. The successful immobilization and biological activity of the resultant Fab' through this protocol is verified on planar gold by fluorescence microscopy, scanning electron microscopy, and atomic force microscopy. Selective capture and release of human insulin is verified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The relative mass spectral peak intensities for insulin versus nonantigenic peptides increase more than 20-fold after passing through the Fab'-Au-NCAM relative to the control Au-NCAM. The affinity-tagged Au-NCAM can be incorporated into microfluidic devices to allow the concentration, capture, and characterization of analytes in complex mixtures with high specificity.

Three-dimensional integrated microfluidic architectures enabled through electrically switchable nanocapillary array membranes.

The extension of microfluidic devices to three dimensions requires innovative methods to interface fluidic layers. Externally controllable interconnects employing nanocapillary array membranes (NCAMs) have been exploited to produce hybrid three-dimensional fluidic architectures capable of performing linked sequential chemical manipulations of great power and utility. Because the solution Debye length, kappa(-1), is of the order of the channel diameter, a, in the nanopores, fluidic transfer is controlled through applied bias, polarity and density of the immobile nanopore surface charge, solution ionic strength and the impedance of the nanopore relative to the microfluidic channels. Analyte transport between vertically separated microchannels can be saturated at two stable transfer levels, corresponding to reverse and forward bias. These NCAM-mediated integrated microfluidic architectures have been used to achieve highly reproducible and tunable injections down to attoliter volumes, sample stacking for preconcentration, preparative analyte band collection from an electrophoretic separation, and an actively-tunable size-dependent transport in hybrid structures with grafted polymers displaying thermally-regulated swelling behavior. The synthetic elaboration of the nanopore interior has also been used to great effect to realize molecular separations of high efficiency. All of these manipulations depend critically on the transport properties of individual nanocapillaries, and the study of transport in single nanopores has recently attracted significant attention. Both computation and experimental studies have utilized single nanopores as test beds to understand the fundamental chemical and physical properties of chemistry and fluid flow at nanometer length scales.

Temperature-Controlled Flow Switching in Nanocapillary Array Membranes Mediated by Poly(N-isopropylacrylamide) Polymer Brushes Grafted by Atom Transfer Radical Polymerization

We report actively controlled transport that is thermally switchable and size-selective in a nanocapillary array membrane (NCAM) prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane. A smooth Au layer on the membrane surface, which is key to obtaining a uniform polymer film, was prepared by thermal evaporation of approximately 50 nm Au on both exterior surfaces. After evaporation, the inner diameter of the pore is reduced slightly, but the NCAM retains a narrow pore size distribution. PNIPPAm brushes with 10-30 nm (dry film) thickness were grafted onto the Au surface through surface-initiated atom transfer radical polymerization (ATRP) using a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. Molecular transport through the PNIPAAm polymer brush-modified NCAMs was investigated by real-time fluorescence measurements using fluorescein isothiocyanate (FITC)-labeled dextrans ranging from 4.4 to 282 kDa in membranes with variable initial pore diameters (80, 100, and 200 nm) and different PNIPAAm thicknesses. Manipulating the temperature of the NCAM through the PNIPAAm lower critical solution temperature (LCST) causes large, size-dependent changes in the transport rates. Over specific ranges of probe size, transport is completely blocked below the LCST but strongly allowed above the LCST. The combination of the highly uniform PNIPAAm brush and the monodisperse pore size distribution is critical in producing highly reproducible switching behavior. Furthermore, the reversible nature of the switching raises the possibility of using them as actively controlled filtration devices.

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.

Incorporation of a DNAzyme into Au-coated nanocapillary array membranes with an internal standard for Pb(ii) sensing

A Pb(ii)-specific DNAzyme has been successfully incorporated into Au-coated polycarbonate track-etched (PCTE) nanocapillary array membranes (NCAMs) by thiol-gold immobilization. Incorporation of the DNAzyme into the membrane provides a substrate-bound sensor using a novel internal control methodology for fluorescence-based detection of Pb(ii). A non-cleavable substrate strand, identical to the cleavable DNAzyme substrate strand except the RNA-base is replaced by the corresponding DNA-base, is used for ratiometric comparison of intensities. The cleavable substrate strand is labeled with fluorescein, and the non-cleavable strand is labeled with a red fluorophore (Cy5 or Alexa 546) for detection after release from the membrane surface. This internal standard based ratiometric method allows for real-time monitoring of Pb(ii)-induced cleavage, as well as standardizing variations in substrate size, solution detection volume, and monolayer density. The result is a Pb(ii)-sensing structure that can be stored in a prepared state for 30 days, regenerated after reaction, and detect Pb(ii) concentrations as low as 17 nM (3.5 ppb).

Nanofluidics and the role of nanocapillary array membranes in mass-limited chemical analysis

Integrated microfluidic structures, comprised of three-dimensional assemblies of microfluidic channels, can effect sequentially-linked analytical operations with mass-limited samples. This three-dimensional operation is enabled by electrically-switchable nanocapillary array membranes with novel transport properties.

Modeling and Simulation of Ionic Currents in Three-Dimensional Microfluidic Devices with Nanofluidic Interconnects

Electrokinetic fluid flow in nanocapillary array (NCA) membranes between vertically separated microfluidic channels offers an attractive alternative to using mechanical action to achieve fluidic communication between different regions of lab-on-a-chip devices. By adjusting the channel diameter, a, and the inverse Debye length, к, and applying the appropriate external potential, the nanochannel arrays, can be made to behave like digital fluidic switches, and the movement of molecules from one side of the array to the other side can be controlled. However, inherent differences in ionic mobility lead to non-equilibrium ion populations on the downstream side, which, in turn, shows up through transient changes in the microchannel conductance. Here we describe coupled calculations and experiments in which the electrical properties of a microfluidic–nanofluidic hybrid architecture are simulated by a combination of a compact model for the bulk electrical properties and iterative self-consistent solutions of the coupled Poisson, Nernst–Planck, and Navier–Stokes equations to recover the detailed ion motion in the nanopores. The transient electrical conductivity in the microchannel, after application of a forward bias pulse to the NCA membrane, is recovered in quantitative detail. The surface charge density of the nanopores and the capacitance of the membrane, which are critical determinants of electrokinetic flow through NCA, fall out of the analysis in a natural way, providing a clear mechanism to determine these critically important parameters.

Profiling pH Gradients Across Nanocapillary Array Membranes Connecting Microfluidic Channels

Nanocapillary array membranes (NCAMs), comprised of thin (d approximately 5-10 microm) nuclear track-etched polycarbonate sheets containing approximately 10(8) cm(-2) nearly parallel nanometer-diameter capillaries, may act to gate fluid transport between microfluidic channels to effect, for example, sample collection. There is interest in H+-transport across these NCAMs because there is significant practical interest in being able to process analyte-containing samples under different pH conditions in adjacent layers of an integrated microfluidic circuit and because protons, with their inherently high mobility, present a challenge in separating microfluidic environments with different properties. To evaluate the capability of NCAMs to support pH gradients, the proton transport properties of NCAMs were studied using laser scanning confocal fluorescence microscopy (LSCFM). Spatiotemporal maps of [H+] in microfluidic channels adjacent to the NCAMs yield information regarding diffusive and electrokinetic transport of protons. The NCAMs studied here are characterized by a positive zeta potential, zeta > 0, so at small nanocapillary diameters, the overlap of electrical double layers associated with opposite walls of the nanocapillary establish an energy barrier for either diffusion or electrokinetic transport of cations through the nanometer-diameter capillaries due to the positive charge on the nanocapillary surface. Proton transfer through an NCAM into microchannels is reduced for pore diameters, d < or = 50 nm and ionic strengths I < or = 50 mM, while for large pore diameters or solution ionic strengths, the incomplete overlap of electric double layer allows more facile ionic transfer across the membranes. These results establish the operating conditions for the development of multilevel integrated nanofluidic/microfluidic architectures which can support multidimensional chemical analysis of mass-limited samples requiring sequential operations to be implemented at different pH values.