Adsorption kinetics of proteins in plastic microfluidic channels: Real-time monitoring of lysozyme adsorption by pulsed streaming potentials

Deciphering the Dynamic Assembling-Disassembling of Small Molecules on Solid/Liquid Interfaces within Microchannels by Pulsed Streaming Potential Measurement

Assembling small molecules at liquid/solid interfaces is relatively common and contributes to many unique properties of the interface. However, such an assembling process can be dynamic depending on the concentration of the molecule and the properties of the solid and liquid themselves, which poses serious challenges on the accurate evaluation of the assembling processes. Herein, we report a convenient way for in situ and real-time monitoring of assembling-disassembling of small-molecule surfactants on the surface of microchannels using pulsed streaming potential (SP) measurement based on the variation of surface charge. With this technique, five distinctive kinetic regimes, each responsible for a characteristic molecular behavior, can be differentiated during a typical assembling-disassembling cycle. Significant difference of the assembling-disassembling process was clearly reflected for surfactants with hydrophobic tails of only a two -CH2- difference (C16TAB/C18TAB and D10DAB/D12DAB). The relative SP (Er) value is positively correlated with the molecular weight at a concentration of 0.1 mM for the same kinds of surfactants. Moreover, the assembling kinetics of D10DAB exhibits an "overshoot effect" at high concentration, which means morphology adjustment. The consequences of such assembling/disassembling of these molecules for electrophoretic separation, protein immobilization, and photocatalysis in a microchannel were investigated through dynamic characterization, which proves its potential as a tool for dynamic solid/liquid interface characterization.

Facile Evaluation of Nanoparticle-Protein Interaction Based on Charge Neutralization with Pulsed Streaming Potential Measurement

Exploration of simple and universal methods to quantitatively measure nanoparticle (NP)-protein interaction is of great importance. In this work, pulsed streaming potential (SP) measurement has been used to evaluate the interaction between NPs and proteins within microchannels. Graphene oxide (GO) and SiO₂ NPs were selected to represent two kinds of NPs. Lysozyme and common blood proteins, including albumin V, γ-globulins, and fibrinogen, were used as model proteins. The linear relationship between the initial adsorption rate (S = dE_r/dt) and the concentration of proteins was observed. Combined with the Hill equation, the microscopic dissociation constant (K_D) and the Hill coefficient (n) between NPs and proteins were calculated based on the relationship between S and the concentration of each protein. The concentration of free proteins which have not interacted with the NPs in the NPs-protein mixture could also be measured. The influence of pH, conductivity, and ionic strengths of the incubation buffer on the interaction between GO and lysozyme was evaluated based on the constant K_D. The interaction intensity between NPs and proteins was defined as charge neutralization efficiency Q_C, which could be calculated from the value of S. It takes only 150 s to get the whole set of data under the optimized experiment parameters. The measurement solely depends on the surface charge, no intrinsic fluorescence is required for either the NPs or the proteins, and no labeling or immobilization process is involved as well.

A microfluidic device for the batch adsorption of a protein on adsorbent particles

A microfluidic platform or “microfluidic batch adsorption device” is presented, which performs two sets of 9 parallel protein incubations with/without adsorbent particles to achieve an adsorption isotherm of a protein in a single experiment.

Use of Pulsed Streaming Potential with a Prepared Cationic Polyelectrolyte Layer to Detect Deposition Kinetics of Graphene Oxide and Consequences of Particle Size Differences

The deposition kinetics of graphene oxide (GO) onto poly(ethylene imine) (PEI) layer was characterized in situ with pulsed streaming potential (SP) measurement, and it was found that the initial rate constant (k_i) was dependent on the size of GO with same surface charge density at a fixed concentration under controlled experimental conditions. Assuming the deposition was controlled by diffusion at the initial stage, k_i is proportional to R_h^-2/3, where R_h is the hydrodynamic radius. By flushing a GO solution through a capillary coated with PEI, the initial change rate of relative SP (dE_r/dt) was obtained in 20 s and k_i was measured with five different concentrations in about 2 min. Three GO samples of different sizes obtained from the same batch of raw material were characterized with pulsed SP to get k_i values, and their sizes were verified with atomic force microscopy and dynamic light scattering. The experimental results are consistent with the predicted effects of the size of NPs on their deposition kinetics.

Facile real-time evaluation of the stability of surface charge under regular shear stress by pulsed streaming potential measurement

The applicability of the pulsed streaming potential measurement for real-time evaluation of stability of assembled layers based on the relative zeta potential change rate S_R was demonstrated.

Integrated biochip for label-free and real-time detection of DNA amplification by contactless impedance measurements based on interdigitated electrodes

Here, we introduce an integrated biochip which offers accurate thermal control and sensitive electrochemical detection of DNA amplification in real-time. The biochip includes a 10-μl microchamber, a temperature sensor, a heater, and a contactless impedance biosensor. A pair of interdigitated electrodes is employed as the impedance biosensor and the products of the amplification are determined directly through tracing the impedance change, without using any labels, redox indicators, or probes. Real-time monitoring of strand-displacement amplification and rolling circle amplification was successfully performed on the biochip and a detection limit of 1 nM was achieved. Amplification starting at an initial concentration of 10 nM could be discriminated from that starting at 1 nM started concentration as well as from the negative control. Since an insulation layer covers the electrodes, the electrodes are spared from erosion, hydrolysis and bubble formation on the surface, thus, ensuring a long lifetime and a high reusability of the sensor. In comparison to bench-top apparatus, our chip shows good efficiency, sensitivity, accuracy, and versatility. Our system requires only simple equipments and simple skills, and can easily be miniaturized into a micro-scale system. The system will then be suitable for a handheld portable device, which can be applied in remote areas. It covers merits such as low cost, low-power consumption, rapid response, real-time monitoring, label-free detection, and high-throughput detection.

Isothermal amplification methods for the detection of nucleic acids in microfluidic devices

Diagnostic tools for biomolecular detection need to fulfill specific requirements in terms of sensitivity, selectivity and high-throughput in order to widen their applicability and to minimize the cost of the assay. The nucleic acid amplification is a key step in DNA detection assays. It contributes to improving the assay sensitivity by enabling the detection of a limited number of target molecules. The use of microfluidic devices to miniaturize amplification protocols reduces the required sample volume and the analysis times and offers new possibilities for the process automation and integration in one single device. The vast majority of miniaturized systems for nucleic acid analysis exploit the polymerase chain reaction (PCR) amplification method, which requires repeated cycles of three or two temperature-dependent steps during the amplification of the nucleic acid target sequence. In contrast, low temperature isothermal amplification methods have no need for thermal cycling thus requiring simplified microfluidic device features. Here, the use of miniaturized analysis systems using isothermal amplification reactions for the nucleic acid amplification will be discussed.

Miniaturized isothermal nucleic acid amplification, a review

Micro-Total Analysis Systems (µTAS) for use in on-site rapid detection of DNA or RNA are increasingly being developed. Here, amplification of the target sequence is key to increasing sensitivity, enabling single-cell and few-copy nucleic acid detection. The several advantages to miniaturizing amplification reactions and coupling them with sample preparation and detection on the same chip are well known and include fewer manual steps, preventing contamination, and significantly reducing the volume of expensive reagents. To-date, the majority of miniaturized systems for nucleic acid analysis have used the polymerase chain reaction (PCR) for amplification and those systems are covered in previous reviews. This review provides a thorough overview of miniaturized analysis systems using alternatives to PCR, specifically isothermal amplification reactions. With no need for thermal cycling, isothermal microsystems can be designed to be simple and low-energy consuming and therefore may outperform PCR in portable, battery-operated detection systems in the future. The main isothermal methods as miniaturized systems reviewed here include nucleic acid sequence-based amplification (NASBA), loop-mediated isothermal amplification (LAMP), helicase-dependent amplification (HDA), rolling circle amplification (RCA), and strand displacement amplification (SDA). Also, important design criteria for the miniaturized devices are discussed. Finally, the potential of miniaturization of some new isothermal methods such as the exponential amplification reaction (EXPAR), isothermal and chimeric primer-initiated amplification of nucleic acids (ICANs), signal-mediated amplification of RNA technology (SMART) and others is presented.

Quantifying surface-accessible quaternary charge for surface modified coatings via streaming potential measurements

Prior research established that P[AB]-copolyoxetane polyurethanes with soft blocks having A = trifluoroethoxy (CF(3)CH(2)-O-CH(2)-, 3FOx) and B = dodecylammonium-butoxy (C12) are highly effective as polymer surface modifiers (PSMs). These PSMs displayed high contact antimicrobial efficiency against spray challenge that was attributed to surface concentration of quaternary charge. Herein, using a novel cell design and polymer coating process, streaming potential (SP) measurements are reported for estimating accessible surface charge density. Fused-silica capillaries were embedded in flat polypropylene sheets, and the inner capillary walls were coated with neat HMDI-BD(30)-P[(3FOx)(C12)-87:13-5100] (PU-1) and 1 wt % PU-1 in HMDI-BD(50)-PTMO-1000 (base polyurethane 2). Effects of annealing (60 degrees C) and electrolyte flow cycles on near-surface quaternary charge concentration were determined. Neat PU-1 had a constant SP that was cycle-independent and actually increased on annealing. As-cast 1 wt % PU-1 showed initial SPs about half those for neat PU-1, with substantial attenuation over 16 measurement cycles. SPs for annealed 1 wt % PU-1 displayed lower initial values that attenuated rapidly over multiple cycles. Zeta potentials and surface charge densities were calculated from SPs and discussed relative to contact antimicrobial properties. Tapping mode atomic force microscopy (TM-AFM) imaging was employed for investigation of 1 wt % PU-1 surface morphology. Microscale phase separation occurs on annealing 1 wt % PU-1 for 24 h at 60 degrees C. Surprisingly, phase separation was also observed after short immersion of 1 wt % PU-1 coatings in water. The morphological changes are correlated with instability of near-surface charge found by SP measurements. A model is proposed for near-surface spinodal decomposition of metastable as-cast 1 wt % PU-1. The formation of a fluorous modifier rich phase apparently sequesters near-surface quaternary charge and accounts for temporal instability of antimicrobial properties. The results are important in providing a facile method for screening polycation-based, contact antimicrobial coatings for accessible charge density and in assessing durability.