Zeta potential of microfluidic substrates: 1. Theory, experimental techniques, and effects on separations

This paper summarizes theory, experimental techniques, and the reported data pertaining to the zeta potential of silica and silicon with attention to use as microfluidic substrate materials, particularly for microchip chemical separations. Dependence on cation concentration, buffer and cation type, pH, cation valency, and temperature are discussed. The Debye-Hückel limit, which is often correctly treated as a good approximation for describing the ion concentration in the double layer, can lead to serious errors if it is extended to predict the dependence of zeta potential on the counterion concentration. For indifferent univalent electrolytes (e.g., sodium and potassium), two simple scalings for the dependence of zeta potential on counterion concentration can be derived in high- and low-zeta limits of the nonlinear Poisson-Boltzman equation solution in the double layer. It is shown that for most situations relevant to microchip separations, the high-zeta limit is most applicable, leading to the conclusion that the zeta potential on silica substrates is approximately proportional to the logarithm of the molar counterion concentration. The zeta vs. pH dependence measurements from several experiments are compared by normalizing the zeta based on concentration.     

Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers: 2. Slip and interfacial water structure

We discuss the structure of water at hydrophobic interfaces from the standpoint of its impact on electrokinetic phenomena in microfluidic devices fabricated from hydrophobic polymers such as Teflon or Zeonor. Water structuring at hydrophobic interfaces has been described as a source of interfacial charge (see Part 1, this issue), and dewetting phenomena, whether via depletion layers or nanobubbles, contribute to slip and enhanced apparent electrokinetic potentials. Issues concerning the impact of hydrodynamic slip and the role of diffuse interfacial structures are discussed. These issues are coupled with each other and with interfacial charge concerns, providing challenges for measurements of individual parameters.     

Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers: 1. The origins of charge

This paper combines new experimental data for electrokinetic characterization of hydrophobic polymers with a detailed discussion of the putative origins of charge at water-hydrophobe interfaces. Complexities in determining the origin of charge are discussed in the context of design and modeling challenges for electrokinetic actuation in hydrophobic microfluidic devices with aqueous working fluids. Measurements of interfacial charge are complicated by slip and interfacial water structuring phenomena (see Part 2, this issue). Despite these complexities, it is shown that (i) several hydrophobic materials, such as Teflon and Zeonor, have predictable electrokinetic properties and (ii) electrokinetic data for hydrophobic microfluidic systems is most consistent with the postulate that hydroxyl ion adsorption is the origin of charge.     

Zeta potential orientation dependence of sapphire substrates

The zeta potential of planar sapphire substrates for three different crystallographic orientations was measured by a streaming potential technique in the presence of KCl and (CH3)4NCl electrolytes. The streaming potential was measured for large single crystalline C-plane (0001), A-plane (1120), and R-plane (1102) wafers over a full pH range at three or more ionic strengths ranging from 1 to 100 mM. The roughness of the epi-polished wafers was verified using atomic force microscopy to be on the order of atomic scale, and X-ray photoelectron spectroscopy (XPS) was used to ensure that the samples were free of silica and other contaminants. The results reveal a shift in the isoelectric point (iep) of the three samples by as much as two pH units, with the R-plane surface exhibiting the most acidic behavior and the C-plane samples having the highest iep. The iep at all ionic strengths was tightly centered around a single pH for each wafer. These values of iep are substantially different from the range of pH 8-10 consistently reported in the literature for alpha-Al2O3 particles. Particle zeta potential measurements were performed on a model powder using phase analysis light scattering, and the iep was confirmed to occur at pH 8. Modified Auger parameters (MAP) were calculated from XPS spectra of a monolayer of iridium metal deposited on the sapphire by electron beam deposition. A shift in MAP consistent with the observed differences in iep of the surfaces confirms the effect of surface structure on the transfer of charge between the Ir and sapphire, hence accounting for the changes in acidity as a function of crystallographic orientation.     

Zeta potential determination by streaming current modelization and measurement in electrophoretic microfluidic systems

Electrophoresis in capillary and microfluidic systems, used in analytical chemistry to separate charged species, are quite sensitive to surface phenomena in terms of separation performances. In order to improve theses performances, new surface functionalization techniques are required. There is a need for methods to provide fast and accurate quantification about surface charges at liquid/solid interfaces. We present a fast, simple, and low-cost technique for the measurement of the zeta-potential, via the modelization and the measurement of streaming currents. Due to the small channel cross section in microfluidic devices, the streaming current modelization is easier than the streaming potential measurement. The modelization combines microfluidic simulations based on the Navier-Stokes equation and charge repartition simulations based on the Poisson-Boltzmann equation. This method has been validated with square and circular cross section shape fused-silica capillaries and can be easily transposed to any lab-on-chip microsystems.     

Zeta potential of particle bilayers on mica: a streaming potential study

The streaming potential of mica covered by bilayers of latex particles was measured using the parallel-plate channel cell. The size of the first latex (A500) bearing amidine charged groups was 503 nm and the second latex (L800) bearing sulfonate groups was 810 nm (at pH 5.5 and an ionic strength of 10(-2)M). The A500 latex exhibited an isoelectric point at pH 10.5, whereas the L800 latex was strongly negative at all pH. Mica sheets were precovered first by the A500 latex particles under diffusion transport conditions. The coverage of this supporting layer was regulated between 0.02 and 0.5 by changing the bulk concentration of latex and the deposition time. Then, the second layer of the L800 latex of regulated coverage up to 0.55 was deposited under the diffusion transport. The coverage of particles and their distributions in both layers were determined by a direct enumeration of particles by optical microscopy under wet conditions and by AFM. It was shown that the structure of the L800 particle layers and the maximum coverage were in accordance with theoretical simulations performed according to the random sequential adsorption (RSA) model. After forming bilayers of desired composition and structure, streaming potential measurements were carried out. The influence of the mica substrate, the supporting layer coverage, and its zeta potential on the apparent zeta potential of bilayers was systematically studied. It was established that for a bilayer coverage exceeding 0.20, the net zeta potential became independent of the substrate and the supporting layer zeta potentials. Then, the asymptotic values of the zeta potential of the bilayer approach 1/√2=0.71 of the bulk zeta potential of the particles forming the external (second) layer. This behavior was interpreted theoretically in terms of the electrokinetic model derived previously for monolayers. It was also concluded that results obtained in this work can be exploited for interpretation of polyelectrolyte film formation in the layer by layer (LbL) processes and protein adsorption pertinent to the antigen/antibody interactions.     

Zeta potential measurement of calcium carbonate

The problem of scaling, which one finds in industrial heat exchangers, particularly in atmospheric coolers in nuclear power stations, depends on calcium carbonate deposits from fresh water. To better understand this phenomenon, we have examined the eventual implication of superficial electric charge of precipitated crystal nuclei. After a bibliographical review showing a fundamental divergence from already published results, this paper describes an experimental plant to measure the zeta potential in controlled conditions of thermodynamic equilibrium, oversaturation, or undersaturation of a CaCO(3)-H(2)O-CO(2) system taking into account simultaneously the three phases: gas, liquid, and solid. The zeta potential is measured by a crystalline-plug method with calcite or aragonite crystals. The potential cancels at thermodynamic equilibrium and is always negative for other conditions, in particular for oversaturation where the possibility of scaling exists. The analysis of these results suggests that the potential determining ions of the system are Ca(2+) and HCO(-)(3).     

Zeta potential measurement for air bubbles in protein solutions

Protein adsorption at gas-liquid interfaces is important in a number of processes including foam formation in bioreactors, foam fractionation for protein recovery, and production of protein based food and drinks. The physical properties of the gas-liquid interface will influence foam stability; important properties will include both surface rheological and electrokinetic properties. While surface rheological properties of gas-protein solution interfaces have been reported, there are no published values for electrokinetic properties at such interfaces. In this paper, zeta potential values of gas bubbles in solutions of three proteins, measured using a microelectrophoresis technique, are reported. The three proteins chosen were BSA, beta-casein, and lysozyme; these proteins have all been used previously in protein foaming studies. The effect of protein concentration and ionic strength is considered. For BSA and beta-casein, zeta potential was found to increase with increasing protein concentration and ionic strength. For air bubbles in lysozyme solutions, measured zeta potential was zero. zeta potential values for air bubbles in some binary protein mixtures are also presented.     

Zeta potential of shape-controlled TiO2 nanoparticles with surfactants

Zeta potential of shape- and size-controlled TiO₂ nanoparticles obtained with the introduction of surfactants during synthesis was measured at different pH values. A unique finding is that TiO₂ nanoparticles shaped by sodium dodecyl sulfate (SDS) have double isoelectric points (IEPs), while other shape-controlled TiO₂ nanoparticles have only one IEP. The double IEPs might be due to the complex chemistry of TiO₂ nanoparticles with the presence of impurities (Na, S, C, etc.). At neutral pH, shape- and size-controlled TiO₂ nanoparticles have more negative zeta potential values and lower IEPs than TiO₂ nanoparticles obtained without the addition of surfactants during synthesis and the commercial anatase TiO₂ nanoparticles Degussa P-25. The lower IEPs could be attributed to the presence of carbonates on particle surfaces. The IEP value of TiO₂ nanoparticles increases with an increase in calcination temperature. The results suggest that the zeta potential of TiO₂ nanoparticles can be manipulated with the addition of surfactants during the synthesis process.     

Zeta potential of dodecyltrimethylammonium hydroxide micelles in water

 The electrophoretical mobility of dodecyltrimethyl-ammonium hydroxide micelles has been measured at two different concentrations giving values similar to that determined in other surfac-tants. There is a good agreement between micelle ionization degrees computed from zeta potential measurements and those from ion-selective electrodes experiments. This demonstrates that electrophoresis experiments may be replaced by the simpler ion-selective electrode measurements to determine micelle surface potential. It has also been concluded that ion-selective electrodes detect only the non-micellised ions, that only free ions contribute to the intermicellar solution ionic strength, and micelles do not affect the result, and that the dependence of the electrophoretic mobility on the soap concentration is due to the reduction of the micelle net charge when the ionic strength of the intermicellar solution arises. Received: 2 December 1996 Accepted: 24 February 1997     

Surface modification on microfluidic devices with 2-methacryloyloxyethyl phosphorylcholine polymers for reducing unfavorable protein adsorption

Surface modification of polymer materials for preparing microfluidic devices including poly(dimethyl siloxane) (PDMS) was investigated with phospholipids polymers such as poly(2-methacryloyloxylethyl phosphorylcholine(MPC)-co-n-butyl methacrylate) (PMB) and poly(MPC-co-2-ethylhexyl methacrylate-co-2-(N,N-dimethylamino)ethyl methacrylate) (PMED). The hydrophilicity of every surface on the polymer materials modified with these MPC polymers increased and the value of zeta-potential became close to zero. The protein adsorption on the polymer materials with and without the surface modification was evaluated using a protein mixture of human plasma fibrinogen and serum albumin. Amount of proteins adsorbed on these polymeric materials showed significant reduction by the surface modification with the MPC polymers compared to the uncoated surfaces ranging from 56 to 90%. Furthermore, we successfully prepared PDMS-based microchannel which was modified by simple coating with the PMB and PMED. The modified microchannel also revealed a significant reduction of adsorption of serum albumin. We conclude that the MPC polymers are useful for reducing unfavorable protein adsorption on microfluidic devices.     

Rapid prototyping polymers for microfluidic devices and high pressure injections

Multiple methods of fabrication exist for microfluidic devices, with different advantages depending on the end goal of industrial mass production or rapid prototyping for the research laboratory. Polydimethylsiloxane (PDMS) has been the mainstay for rapid prototyping in the academic microfluidics community, because of its low cost, robustness and straightforward fabrication, which are particularly advantageous in the exploratory stages of research. However, despite its many advantages and its broad use in academic laboratories, its low elastic modulus becomes a significant issue for high pressure operation as it leads to a large alteration of channel geometry. Among other consequences, such deformation makes it difficult to accurately predict the flow rates in complex microfluidic networks, change flow speed quickly for applications in stop-flow lithography, or to have predictable inertial focusing positions for cytometry applications where an accurate alignment of the optical system is critical. Recently, other polymers have been identified as complementary to PDMS, with similar fabrication procedures being characteristic of rapid prototyping but with higher rigidity and better resistance to solvents; Thermoset Polyester (TPE), Polyurethane Methacrylate (PUMA) and Norland Adhesive 81 (NOA81). In this review, we assess these different polymer alternatives to PDMS for rapid prototyping, especially in view of high pressure injections with the specific example of inertial flow conditions. These materials are compared to PDMS, for which magnitudes of deformation and dynamic characteristics are also characterized. We provide a complete and systematic analysis of these materials with side-by-side experiments conducted in our lab that also evaluate other properties, such as biocompatibility, solvent compatibility, and ease of fabrication. We emphasize that these polymer alternatives, TPE, PUMA and NOA, have some considerable strengths for rapid prototyping when bond strength, predictable operation at high pressure, or transitioning to commercialization are considered important for the application.     

Zeta potential of ion-conductive membranes by streaming current measurements

Surface charge properties have a significant influence on membrane retention and fouling performance. As a key parameter describing the surface charge of membranes used in aqueous applications, zeta potential measurements on membranes of various types have attracted great attention. During the zeta potential characterization of a series of ion-conductive sulfonated poly(sulfone) membranes, it was found that the measured streaming current varied with the thickness of the sample, which is not predicted by the classical Smoluchowski equation. Moreover, for higher conductivity membranes with an increased concentration of sulfonate groups, the zeta potential tended toward zero. It was determined that the influence of membrane bulk conductance on the measured streaming current must be taken into account in order to correctly interpret the streaming current data for ion-conductive polymers and understand the relationship between membrane chemical composition and zeta potential. Extrapolating the measured streaming current to a membrane thickness of zero has proven to be a feasible method of eliminating the error associated with measuring the zeta potential on ion conductive polymer membranes. A linear resistance model is proposed to account for the observed streaming currents where the electrolyte channel is in parallel with the ion-conductive membranes.     

Transient ζ‐potential measurements in hydrophobic,TOPAS microfluidic substrates

We utilize time‐resolved electrokinetic measurements in order to study the electrokinetic properties of silica and TOPAS microfluidic channels as a function of the time history of the fluid–solid interface. In pressure‐driven flow through TOPAS microchannels, the ζ‐potential as inferred from streaming potential measurements decays exponentially by a factor of 1.5 with a characteristic decay time of 3 h after the initial formation of the fluid–solid interface. A similar exponential decay is observed immediately after water is exchanged for ethanol as the solvent in the system. In electroosmotically driven flow through TOPAS microchannels, the ζ‐potential as inferred through current monitoring experiments was constant in time. No electrokinetic transients were observed in silica microchannels under these flow conditions.     

Disposable microfluidic substrates: transitioning from the research laboratory into the clinic

As more microfluidic applications emerge for clinical diagnostics, the choice of substrate and production method must be considered for eventual regulatory approval. In this review, we survey recent developments in disposable microfluidic substrates and their fabrication methods. We note regulatory approval for disposable microfluidic substrates will be more forthcoming if the substrates are developed with the United States Pharmacopeia's biocompatibility compliance guidelines in mind. We also review the recent trend in microfluidic devices constructed from a hybrid of substrates that takes advantage of each material's attributes.     

Zeta potential study of the oxidation of copper sulfide minerals

The zeta potential of the copper sulfide minerals, chalcocite, covellite, chalcopyrite, bornite, enargite and tennantite was measured as a function of pH and oxidising conditions. The changes in zeta potential observed in this study are consistent with the presence of a copper hydroxide layer covering a metal-deficient sulfur-rich surface and with the extent of this copper hydroxide coverage increasing with oxidation conditions. The existence of these surface species and their percentage were also confirmed by X-ray photoelectron spectroscopy. Analysis of the zeta potential data revealed that during the acid titration of the minerals, dissolution of the surface copper hydroxide layer occurs at pH values less than 8 while during the base titration, precipitation of copper hydroxide on the mineral surface is observed at pH values higher than 6. Hysteresis between the zeta potential acid and base titration curves was only observed in oxidising conditions and is attributed to the dissolution of the minerals at acidic pH values. The following ranking for the oxidation of these minerals is obtained: chalcocite>tennantite>enargite>bornite>covellite>chalcopyrite.     

Fluorescent microporous organic polymers: potential testbed for optical applications

Fluorescent microporous organic polymers (MOPs) are an emerging class of porous organic materials with considerable technological relevance due to great promise in a range of applications, in addition to typical usage in gas storage and catalysis. In this respect, innovative optical materials have been developed based on MOPs. In this Concept article, a brief appraisal of the fabrication methodologies of fluorescent MOPs in the form of bulk solid as well as in nanoparticles, their optical attributes, and promising application potentials are presented.     

Zeta potential of microbubbles in aqueous solutions: electrical properties of the gas-water interface

Microbubbles are very fine bubbles and appropriate for the investigation of the gas-water interface electrical charge, because of their long stagnation, due to slow buoyancy, in the electrophoresis cell observation area. This study investigated the zeta potential of microbubbles in aqueous solutions and revealed that the bubbles were negatively charged under a wide range of pH conditions. The potential was positive under strong acidic conditions, and the inorganic electrolytes decrease the potential by increasing the amount of counterions within the slipping plane. OH(-) and H(+) are crucial factors for the charging mechanism of the gas-water interface, while other anions and cations have secondary effects on the zeta potential, because counterions are attracted by the interface charge. The addition of a small amount of propanol and butanol provided significant information for considering the mechanism of the gas-water interface charge. Even though these alcohols did not have any electrical charge, they had a strong effect on the gas-water interface charge and dispersed the zeta potential of the microbubbles in the aqueous solution. These alcohols tended to adsorb to the interface and affect the hydrogen-bonding network at the interface, so that it was concluded that the gas-water interface electrical charge must be related to the difference of the construction of the hydrogen-bonding network between the bulk water and the gas-water interface.     

Electrokinetic studies of cellulosic fibres I. Zeta potential of fibres dyed with reactive dyes

Summary The influence of reaction between a reactive dye molecule and cotton cellulose on the zeta potential of the cellulosic fibres has been studied by streaming potential method. As a result of such a reaction the negative value of zeta potential was decreased and the extent of this decrease was proportional to the amount of dye entered into the reaction. With respect to the influence of various structural features of a reactive dye molecule on the zeta potential of dyed fibres, it was observed that the zeta potential was independent of the type of reactive system but was very much dependent on the number of –SO₃H groups in the dye molecule and the chemical constitution of the chromophoric system in the dye structure.A bifunctional reactive dye (Remazol Black B —C. I. Reactive Black 5) has been shown to form crosslinks between cellulose chains and bring about considerable lowering of negative zeta potential of the dyed fibres as compared to the equivalent amount of same dye but in the modified form to act as a monofunctional reactive dye. This bifunctional dye was also observed to induce H-bonding in the vicinity of the sites of cross-links. Trichloropyrimidine type of dyes have been found to be neither completely monofunctional nor fully bifunctional but seem to lie in between the monofunctional and bifunctional types of dyes.

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Zusammenfassung Der Einfluß der Reaktion zwischen einem reaktiven Farbstoffmolekül und Baumwollzellulose auf das-Potential der Zellulosefasern wurde mit der Strömungspotential-Methode untersucht. Als Ergebnis einer solchen Reaktion sank der negative-Potentialwert ab, und die Abnahme war proportional dem Betrag an Farbstoff, der in Reaktion trat. Im Hinblick auf den Einfluß verschiedener struktureller Eigenschaften eines reaktiven Farbstoffmoleküls auf das-Potential der gefärbten Faser wurde festgestellt, daß letzteres unabhängig vom Typ des Reaktions-systems war, abet sehr abhängig von der Zahl der –SO₃H-Gruppen im Farbstoffmolekül und der Konstitution des chromophoren Systems.Ein zweifunktionaler reaktiver Farbstoff (Remazol Black B — C. I. Reactive Black 5) bildet Vernetzungen zwischen Zelluloseketten und erniedrigt beträchtlich das negative-Potential der gefärbten Fasern, verglichen mit einem Farbstoff in modifizierter Form, so daß er als monofunktioneller Farbstoff wirkt. Dieset zweifunktionale Farbstoff induziert auch Wasserstoffbrücken in der Nachbarschaft der Vernetzungsstellen. Farbstoffe von Trichlorpyrimidin-Typ sind weder streng monofunktionell noch voll bifunktionell. Sie liegen in der Mitte.

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With 11 figures     

Lamination-based rapid prototyping of microfluidic devices using flexible thermoplastic substrates

Transposing highly sensitive DNA separation methods (such as DNA sequencing with high read length or the detection of point mutations) to microchip format without loss of resolution requires fabrication of relatively long (approx. 10 cm) microchannels along with sharp injection bands. Conventional soft lithography methods, such as mold casting or hot-embossing in a press, are not convenient for fabricating long channels. We have developed a lamination-based replication technique for rapid fabrication of sealed microfluidic devices with a 10 cm long, linear separation channel. These devices are fabricated in thin cyclo-olefin copolymer (COC) plastic substrates, thus making the device flexible and capable of assuming a range of 3-D configurations. Due to the good optical properties of COC, this new family of devices combines multiple advantages of planar microfluidics and fused-silica capillaries.