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.

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 (ki) 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, ki is proportional to Rh-2/3, where Rh is the hydrodynamic radius. By flushing a GO solution through a capillary coated with PEI, the initial change rate of relative SP (dEr/dt) was obtained in 20 s and ki 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 ki 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.

Extraction-Free, Direct Determination of Caffeine in Microliter Volumes of Beverages by Thermal Desorption-Gas Chromatography Mass Spectrometry.

An extraction-free method requiring microliter (µL) volumes has been developed for the determination of caffeine in beverages. Using a pyrolysis-gas chromatography mass spectrometry system, the conditions required for the direct thermal desorption-gas chromatography mass spectrometry (TD-GC/MS) determination of caffeine were optimised. A 5 µL aliquot was introduced to the thermal desorption unit, dried, and thermally desorbed to the GC/MS. The response was linear over the range 10 to 500 µg/mL (R 2 = 0.996). The theoretical limit of detection (3 σ) was 0.456 µg/mL. No interferences were recorded from endogenous beverage components or from commonly occurring drugs, such as nicotine, ibuprofen, and paracetamol. Replicate caffeine determinations on fortified latte style white coffee and Pepsi Max® gave mean recoveries of 93.4% (%CV = 4.1%) and 95.0% (%CV = 0.98%), respectively. Good agreement was also obtained with the stated values of caffeine for an energy drink and for Coca-Cola®. These data suggest that the method holds promise for the determination of caffeine in such samples.

Double-helix micro-channels on microfluidic chips for enhanced continuous on-chip derivatization followed by electrophoretic separation.

Micro-channels that contain a special inner structure are critical for efficient mixing and chemical reactions. In this paper, we described the facile fabrication of an integrated microchip with double-helix type micro-channels to improve mixing efficiency and to facilitate multi-step derivatization reactions prior to electrophoretic separation. With a prepared microchip, reagents, samples and reaction products could be driven through micro-channels by siphon, and no other pumping device was necessary. To test its performance, reductive amination of aldehydes with 8-aminonaphthalene-1,3,6-trisulfonate acid disodium (ANTS) was attempted via microchip electrophoresis with laser induced fluorescence (LIF). The effect of the geometry of the reaction micro-channel on the reaction's efficiency was evaluated. Under the selected conditions, successful derivatization of five aldehydes was realized for highly reproducible analysis. The relative standard deviations of the peak areas for 30 consecutive injections were in the range of 0.28-1.61%. The method was applied for the determination of aldehydes in real samples with standard addition recoveries of 87.8-102.8%. Good tolerance of organic solvents was achieved, and the proposed method can potentially be employed for rapid screening of excessively added aldehyde food flavoring.

Fast and interference-free determination of glyphosate and glufosinate residues through electrophoresis in disposable microfluidic chips.

With an increasing concern on food safety, fast screening of residues of widespread herbicides becomes necessary. Herein we report a microchip electrophoresis system with laser induced fluorescence (LIF) detection for rapid and sensitive analysis of glyphosate (GLYP) and glufosinate (GLUF) residues. Disposable cyclic olefin copolymer microchips and a low-cost LIF detector were employed to minimize the cost of the analysis. Systematic optimization of experimental conditions was performed to achieve highly efficient analysis. Under the selected condition, GLYP and GLUF were efficiently resolved from sample matrices with a buffer containing 10 mmol/L borax and 2.0% (m/v) hydroxypropyl cellulose at pH 9.0. The number of theoretical plates of 1.0×10(6) m(-1) was attained for both analytes. Derivatization at lower concentrations (<10 µg/L) was also examined, successful detection of 0.34 µg/L GLYP and 0.18 µg/L GLUF was confirmed. The system was applied for the determination of both analytes in real samples without any preconcentration involved. Recoveries of GLYP and GLUF spiked in these samples were 84.0-101.0% and 90.0-103.0%, respectively.

Charge tunable zwitterionic polyampholyte layers formed in cyclic olefin copolymer microchannels through photochemical graft polymerization.

Zwitterionic layers immobilized on various surfaces exhibit ideal biocompatibility and antifouling capability, but direct immobilization of zwitterionic molecules provides limited choice of surface charges. In this paper, the formation of charge tunable zwitterionic polyampholyte layers onto the surface of microfluidic channels of cyclic olefin copolymer by photochemical graft polymerization of mixed acrylic monomers, [2-(acryloyloxy) ethyl] trimethyl ammonium chloride and 2-acrylamido-2-methyl-1-propanesulfonic, under UV illumination was reported. With this method, surface charge of the resulting modification layers could be tailored through the initial monomer ratio and reaction conditions. The incorporation of both monomers into the grafted layers was confirmed by X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR). The results indicate that the modified layers are hydrophilic with contact angles of 33.0-44.3°, and the isoelectric points of the modified layers can be tuned from <3 to >9 simply by adjusting the monomer ratios. Elimination of the nonspecific adsorption of proteins on the zwitterionic layers thus formed was proved by fluorescent microscopy and streaming potential measurement. The uniformity of the modified layers was verified through a comparison of electrophoresis inside the modified and native microchannels. A whole blood coagulation time measurement was performed to show its applicability.