Effects of alkaline hydrolysis and dynamic coating on the electroosmotic flow in polymeric microfabricated channels

Development of microfluidic concentrator using ion concentration polarization mechanism to assist trapping magnetic nanoparticle-bound miRNA to detect with Raman tags

MicroRNAs (miRNAs) are small noncoding single-stranded ribonucleic acid molecules. This type of endogenous oligonucleotide could be secreted into the circulation and exist stably. The detection of specific miRNAs released by cancer cells potentially provides a noninvasive means to achieve early diagnosis and prognosis of cancers. However, the typical concentration of miRNAs in blood is below the ultratrace level. This study uses a simple thermoplastic microfluidic concentration device based on an ion concentration polarization mechanism to perform enrichment and cleanup and Raman sensing beads to determine miRNA quantitatively. One sample solution containing target miRNA molecules having been hybridized with two nucleotide probes, where one probe is on a Raman tag of a nanoaggregate embedded bead (NAEB) and the other probe is on a magnetic nanoparticle (MNP), is first filled into the device. When an external field is applied across a cation exchange membrane stationed in the middle conduit of the device, the MNP-miRNA-NAEB complexed particles are enriched near the membrane edge of the cathode side. The concentrated complexed particles are further trapped using an external magnet to perform washing steps to remove excess noncomplexed NAEBs. When cleanup steps are accomplished, the remaining complexed particles are loaded into one detection capillary to acquire Raman signals from the sensing beads. Compared with that using a conventional magnetic trapping device, the cleanup time is shortened from nearly an hour to less than 10 min. Sample loss during the washing steps becomes more controllable, resulting in adequate standard curve linearity (R > 0.99) ranging from 1 to 100 pM.

Electroosmosis through α-Hemolysin That Depends on Alkali Cation Type

We demonstrate experimentally the existence of an electroosmotic flow (EOF) through the wild-type nanopore of α-hemolysin in a large range of applied voltages and salt concentrations for two different salts, LiCl and KCl. EOF controls the entry frequency and residence time of small neutral molecules (β-cyclodextrins, βCD) in the nanopore. The strength of EOF depends on the applied voltage, on the salt concentration, and, interestingly, on the nature of the cations in solution. In particular, EOF is stronger in the presence of LiCl than KCl. We interpret our results with a simple theoretical model that takes into account the pore selectivity and the solvation of ions. A stronger EOF in the presence of LiCl is found to originate essentially in a stronger anionic selectivity of the pore. Our work provides a new and easy way to control EOF in protein nanopores, without resorting to chemical modifications of the pore.

Bacterial outer membrane protein analysis by electrophoresis and microchip technology

Outer membrane proteins are indispensable components of bacterial cells and participate in several relevant functions of the microorganisms. Changes in the outer membrane protein composition might alter antibiotic sensitivity and pathogenicity. Furthermore, the effects of various factors on outer membrane protein expression, such as antibiotic treatment, mutation, changes in the environment, lipopolysaccharide modification and biofilm formation, have been analyzed. Traditionally, the outer membrane protein profile determination was performed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Converting this technique to capillary electrophoresis format resulted in faster separation, lower sample consumption and automation. Coupling capillary electrophoresis with mass spectrometry enabled the fast identification of bacterial proteins, while immediate quantitative analysis permitted the determination of up- and downregulation of certain outer membrane proteins. Adapting capillary electrophoresis to microchip format ensured a further ten- to 100-fold decrease in separation time. Application of different separation techniques combined with various sensitive detector systems has ensured further opportunities in the field of high-throughput bacterial protein analysis. This review provides an overview using selected examples of outer membrane proteins and the development and application of the electrophoretic and microchip technologies for the analysis of these proteins.

Cationic amylopectin derivatives as additives for analysis of proteins in capillary electrophoresis

Positively charged amylopectin, which is a major constituent of cationic starch, was used to modify the inner surface of fused-silica capillaries by addition to the running solution, which was subsequently employed in CE. Capillaries filled with cationic amylopectin derivatives were shown to generate a stable reversed EOF in the investigated range of pH 4-8. Among the additives studied, quaternary ammonium amylopectin derivatives with high amino and low hydroxypropyl groups showed fast electroosmotic mobility and very effectively suppressed the adsorption of proteins. The run-to-run and batch-to-batch repeatability of the procedures were satisfactory with RSDs of 0.5% and 2.4%, respectively. A basic protein, alpha-chymotrypsinogen, migrated within 6 min and the theoretical plate number of it reached 560 000 plates/m.

Thermoplastic microchannel fabrication using carbon dioxide laser ablation

We report the procedures of machining microchannels on Vivak co-polyester thermoplastic substrates using a simple industrial CO(2) laser marker. To avoid overheating the substrates, we develop low-power marking techniques in nearly anaerobic environment. These procedures are able to machine microchannels at various aspect ratios. Either straight or serpent channel can be easily marked. Like the wire-embossed channel walls, the ablated channel surfaces become charged after alkaline hydrolysis treatment. Stable electroosmotic flow in the charged conduit is observed to be of the same order of magnitude as that in fused silica capillary. Typical dynamic coating protocols to alter the conduit surface properties are transferable to the ablated channels. The effects of buffer acidity on electroosmotic mobility in both bare and coated channels are similar to those in fused silica capillaries. Using video microscopy we also demonstrate that this device is useful in distinguishing the electrophoretic mobility of bare and latex particles from that of functionalized ones.

Analysis of amino acids and proteins using a poly(methyl methacrylate) microfluidic system

Plastic microchips are very promising analytical devices for the high-speed analysis of biological compounds. However, due to its hydrophobicity, their surface strongly interacts with nonpolar analytes or species containing hydrophobic domains, resulting in a significant uncontrolled adsorption on the channel walls. This paper describes the migration of fluorescence-labeled amino acids and proteins using the poly(methyl methacrylate) microchip. A cationic starch derivative significantly decreases the adsorption of analytes on the channel walls. The migration time of the analytes was related to their molecular weight and net charge or pI of the analytes. FITC-BSA migrated within 2 min, and the theoretical plate number of the peak reached 480,000 plates/m. Furthermore, proteins with a wide range of pI values and molecular weights migrated within 1 min using the microchip.

Effect of iron restriction on outer membrane protein composition ofPseudomonas strains studied by conventional and microchip electrophoresis

In this study the outer membrane protein (OMP) composition of six Pseudomonas aeruginosa strains had been analyzed by conventional CE and microchip electrophoresis. Bacterial OMPs are important virulence factors and play a significant role in the pathogenesis of infectious diseases. Changes in their composition might refer to the altered pathogenic properties and antibiotic sensitivity of a certain strain. Pathogenic bacteria invading the human host have to multiplicate under iron-restricted conditions that induce changes in the OMP composition. High-molecular-weight OMPs have to be expressed, which serve as receptors for the iron-siderophore complexes. OMP patterns of bacteria obtained by the two different methods in this study were similar, all major proteins could be detected by both techniques, and the molecular weights showed good correlations, although direct comparison of the peak areas is not straightforward due to the different detection methods (UV and LIF). Changes in OMP composition under iron restriction could be detected, and appearance of a 92 kDa protein in all six P. aeruginosa strains and a 94 kDa protein in the KT 2 strain could be demonstrated. Besides that up- and downregulation of certain proteins could be also detected. The increased separation speed, picoliters of sample consumption, baseline separation achieved more frequently by this method--especially in the high-molecular-weight region--showed the advantages of microchip electrophoresis in the analysis of clinical samples.

Microchip electrophoretic protein separation using electroosmotic flow induced by dynamic sodium dodecyl sulfate-coating of uncoated plastic chips

Separation of sodium dodecyl sulfate (SDS)-protein complexes is difficult on plastic microchips due to protein adsorption onto the wall. In this paper, we elucidated the reasons for the difficulties in separating SDS-protein complexes on plastic microchips, and we then demonstrated an effective method for separating proteins using polymethyl methacrylate (PMMA) microchips. Separation difficulties were found to be dependent on adsorption of SDS onto the hydrophobic surface of the channel, by which cathodic electroosmotic flow (EOF; reversed flow) was generated. Our developed method effectively utilized the reversed flow from this cathodic EOF as a driving force for sample proteins using permanently uncoated but dynamic SDS-coated PMMA microchips. High-speed (6 s) separation of proteins and peptides up to 116 kDa was successfully achieved using this system.

A polymeric master replication technology for mass fabrication of poly(dimethylsiloxane) microfluidic devices

A protocol of producing multiple polymeric masters from an original glass master mold has been developed, which enables the production of multiple poly(dimethylsiloxane) (PDMS)-based microfluidic devices in a low-cost and efficient manner. Standard wet-etching techniques were used to fabricate an original glass master with negative features, from which more than 50 polymethylmethacrylate (PMMA) positive replica masters were rapidly created using the thermal printing technique. The time to replicate each PMMA master was as short as 20 min. The PMMA replica masters have excellent structural features and could be used to cast PDMS devices for many times. An integration geometry designed for laser-induced fluorescence (LIF) detection, which contains normal deep microfluidic channels and a much deeper optical fiber channel, was successfully transferred into PDMS devices. The positive relief on seven PMMA replica masters is replicated with regard to the negative original glass master, with a depth average variation of 0.89% for 26-microm deep microfluidic channels and 1.16% for the 90 mum deep fiber channel. The imprinted positive relief in PMMA from master-to-master is reproducible with relative standard deviations (RSDs) of 1.06% for the maximum width and 0.46% for depth in terms of the separation channel. The PDMS devices fabricated from the PMMA replica masters were characterized and applied to the separation of a fluorescein isothiocyanate (FITC)-labeled epinephrine sample.

Wall coating for capillary electrophoresis on microchips

This review article with 116 references describes recent developments in the preparation of wall coatings for capillary electrophoresis (CE) on a microchip. It deals with both dynamic and permanent coatings and concentrates on the most frequently used microchip materials including glass, poly(methyl methacrylate), poly(dimethyl siloxane), polycarbonate, and poly(ethylene terephthalate glycol). Characterization of the channel surface by measuring electroosmotic mobility and water contact angle of the surface is included as well. The utility of the microchips with coated channels is demonstrated by examples of CE separations on these chips.

Dependence of buffer acidity and surfactant chain-length on electro-osmotic mobility in thermoplastic microchannels

In this paper, we report the dependence of buffer pH and coating surfactant chain-length on electro-osmotic (EO) mobility in co-polyester microchannels. Thermoplastics co-polyester hydrolyzes to anionic functionality to create electrical double layer on the micro-channel walls. These negatively charged sites are partially or completely screened when long-chain surfactants are added into the buffer. This ancillary technique to modify surface charge polarity to avoid analyte adsorption is known as dynamic coating. We develop a theory to predict the EO mobility tendency on buffer acidity considering the combination of pH-dependent surfactant aggregation and surface dissociation. Our findings of pH-dependent EO mobility in coated channels, using three types of quaternary ammonium surfactants, lauryltrimethyammonium bromide (LTAB), trimethyl (tetradecyl) ammonium bromide (TTAB), and cetyltrimethyammonium bromide (CTAB), agree with our theoretical prediction. We also explain the chain-length dependence of mobility with a collaborative adsorption mechanism of surfactant aggregates.

In-channel dual-electrode amperometric detection in electrophoretic chips with a palladium film decoupler

An electrophoretic microchip integrated with a Pd-film decoupler and a series-dual electrode was proven practical (200-800 V/cm) for routine amperometric detection. In fluidic systems, amperometric enhancement of parallel-opposed dual-electrode detection is due to redox cycling of analytes between the electrodes. We, however, found that the oxidation current of catecholamines was enhanced significantly (1.9-3.8 folds) by switching from the single electrode mode to dual-series mode. This novel finding was unexpected because the unidirectional flow characteristic of the microfluidic system should eliminate the possibility for analytes physically migrating back and forth between the upstream and downstream electrodes. We attribute the enhancement to turbulence generated by impinging of the flow onto the edge of the downstream electrode. The linear range, sensitivity, limit of detection (S/N = 3) and number of theoretical plates for DA and CA are, respectively, 0.5-50 microM, 47 pA/microM, 0.25 microM, 7000 m(-1) and 1.0-100 microM, 28 pA/microM, 0.49 microM, 15,000 m(-1).

Introduction to micro-analytical systems: bioanalytical and pharmaceutical applications

This review presents a brief overview of recent developments in miniaturization of analytical instruments utilizing microfabrication technology. The concept 'Micro-Total Analysis Systems micro-TAS)', also termed 'Lab-on-a-chip', and the latest progresses in the development of microfabricated separation devices and on-chip detection techniques are discussed. Applications of micro-analytical methods to bioanalytical and pharmaceutical studies are also described, including chemical reactions, assays, and analytical separations of biomolecules in micro-scale.

Comparison of standard capillary and chip separations of sodium dodecylsulfate-protein complexes

Conditions for converting a set of five standard proteins to electrochemically active sodium dodecylsulfate (SDS) complexes were worked out with the aim of using such complexes for conductivity detection with a a chip electrophoresis system. The results obtained were compared with standard capillary electrophoresis (37 cm (effective length 30 cm) x 75 microm I.D. capillary, 10 kV, negative polarity at the inlet). The chip separations were run at 500 V per chip (100 V/cm) as compared to the standard capillary arrangement, which was run at 266.6 V/cm. For the capillary set-up the protein complexes were prepared in aqueous solution (Milli-Q water) made 10 mM with respect to SDS. If the SDS concentration was increased to 50 mM, the separation in the capillary was incomplete. On the other hand with the chip system both approaches yielded acceptable results. The chip separations were slightly (but not distinctly) shorter and offered better separations than the standard set-up. The concentration of the surfactant used for the preparation the complexes results in alternations of the elution sequence, which is preserved if the chip separation is used instead of the capillary set-up. Apparently the full capacity of protein-SDS binding is not exploited for the preparation of the adducts.

Fabrication of microchannel structures in fluorinated ethylene propylene

A new technique for fabrication of channel structures with diameters down to 13 microm in fluorinated ethylene propylene (also known as poly(tetrafluoroethylene-co-hexafluoropropylene), FEP) is described. The technique is based on the unique property of a dual-layer fluoropolymer tubing consisting of an outer layer of poly(tetrafluoroethylene) (PTFE) and an inner layer of FEP. When heated (>350 degrees C), the outer PTFE layer shrinks while the inner FEP layer melts, resulting in filling of all empty space inside the tubing with FEP. The channel structures are formed using tungsten wires as templates that are pulled out after completion of the shrinking and melting process. While several analytical devices have been reproducibly prepared and shown to function, this report describes a single example. A microreactor coupled to an electrochemical flow cell detects the biuret complex of the natively electroinactive peptide des-Tyr-Leu-enkephalin.

Control of electroosmotic flow in laser-ablated and chemically modified hot imprinted poly(ethylene terephthalate glycol) microchannels

The fabrication of microchannels in poly(ethylene terephthalate glycol) (PETG) by laser ablation and the hot imprinting method is described. In addition, hot imprinted microchannels were hydrolyzed to yield additional charged organic functional groups on the imprinted surface. The charged groups are carboxylate moieties that were also used as a means for the further reaction of different chemical species on the surface of the PETG microchannels. The microchannels were characterized by fluorescence mapping and electroosmotic flow (EOF) measurements. Experimental results demonstrated that different fabrication and channel treatment protocols resulted in different EOF rates. Laser-ablated channels had similar EOF rates (5.3+/-0.3 x 10(-4) cm(2)/Vs and 5.6+/-0.4 x 10(-4) cm(2)/Vs) to hydrolyzed imprinted channels (5.1+/-0.4 x 10(-4) cm(2)/Vs), which in turn demonstrated a somewhat higher flow rate than imprinted PETG channels that were not hydrolyzed (3.5+/-0.3 x 10(-4) cm(2)/Vs). Laser-ablated channels that had been chemically modified to yield amines displayed an EOF rate of 3.38+/- 0.1 x 10(-4) cm(2)/Vs and hydrolyzed imprinted channels that had been chemically derivatized to yield amines showed an EOF rate of 2.67+/-0.6 cm(2)/Vs. These data demonstrate that surface-bound carboxylate species can be used as a template for further chemical reactions in addition to changing the EOF mobility within microchannels.

Polymer based micro-reactors

In this paper, we describe the fabrication technologies necessary for the production of polymer-based micro-fluidic devices. These technologies include hot embossing as a micro-structuring method as well as so-called back-end processes to complete the micro-devices. Applications such as capillary electrophoresis, micro-mixers and nanowell plates are presented.

Fabrication of polyester microchannels and their applications to capillary electrophoresis

Inexpensive and disposable polyester microchips were fabricated through photolithographic and wet-chemical etching procedure, followed by replication using an imprinting method at room temperature. Laboratory-scale laser-induced fluorescence equipment was employed as a detection system. The generation of electroosmotic flow (EOF) on the polyester channels was discussed in this paper. Surfactants in the running buffer had a significant effect on the EOF depending on their types. The epsilon potential of the electric double layer formed by adsorbing sodium lauryl sulfate molecules on the wall of polyester channels seemed to be constant within the buffer pH investigated. EOF could also be suppressed to zero by adding polyoxyethylene 23 lauryl ether into the running buffer. The separation of two laser dyes was obtained using polyester chips through both micellar electrokinetic chromatography and capillary zone electrophoresis. The polyester channels modified with 10-undecen-1-ol exhibited a dramatically high-separation efficiency compared with the conventional fused-silica capillary tubes.

Continuous concentration of bacteria in a microfluidic flow cell using electrokinetic techniques

A novel method for the concentration of bacterial solutions is presented that implements electrokinetic techniques, zone electrophoresis (ZE) and isoelectric focusing (IEF), in a microfluidic device. The method requires low power (< 3e-5 W) and can be performed continuously on a flowing stream. The device consists of two palladium electrodes held in a flow cell constructed from layers of polymeric film held together by a pressure-sensitive adhesive. Both ZE and IEF are performed with carrier-free solutions in devices in which the electrodes are in intimate contact with the sample fluid. IEF experiments were performed using natural pH gradients; no carrier ampholyte solution was required. Experiments performed in buffer alone resulted in significant electroosmotic flow. Pretreatment of the sample chamber with bleach followed by a concentrated solution of cationic detergent effectively suppressed electroosmotic flow.

Recent developments in electrokinetically driven analysis on microfabricated devices

This review is devoted to the rapid developments in the field of microfluidic separation devices in which the flow is electrokinetically driven, and where the separation element forms the heart of the system, in order to give an overview of the trends of the last three years. Examples of microchip layouts that were designed for various application areas are given. Optimization of mixing and injection strategies, designs for the handling of multiple samples, and capillary array systems show the enormous progress made since the first proof-of-concept papers about lab-on-a-chip devices. Examples of functional elements for on-chip preconcentration, filtering, DNA amplification and on-chip detection indicate that the real integration of various analytical tasks on a single microchip is coming into reach. The use of materials other than glass, such as poly(dimethylsiloxane) and polymethylmethacrylate, for chip fabrication and detection methods other than laser-induced fluorescence (LIF) detection, such as mass spectrometry and electrochemical detection, are described. Furthermore, it can be observed that the separation modes known from capillary electrophoresis (CE) in fused-silica capillaries can be easily transferred to the microchip platform. The review concludes with an overview of applications of microchip CE and with a brief outlook.