Channel-specific coatings on microfabricated chips

Low-input and multiplexed microfluidic assay reveals epigenomic variation across cerebellum and prefrontal cortex

Extensive effort is under way to survey the epigenomic landscape of primary ex vivo tissues to establish normal reference data and to discern variation associated with disease. The low abundance of some tissue types and the isolation procedure required to generate a homogenous cell population often yield a small quantity of cells for examination. This difficulty is further compounded by the need to profile a myriad of epigenetic marks. Thus, technologies that permit both ultralow input and high throughput are desired. We demonstrate a simple microfluidic technology, SurfaceChIP-seq, for profiling genome-wide histone modifications using as few as 30 to 100 cells per assay and with up to eight assays running in parallel. We applied the technology to profile epigenomes using nuclei isolated from prefrontal cortex and cerebellum of mouse brain. Our cell type-specific data revealed that neuronal and glial fractions exhibited profound epigenomic differences across the two functionally distinct brain regions.

Protein immobilization techniques for microfluidic assays

Microfluidic systems have shown unequivocal performance improvements over conventional bench-top assays across a range of performance metrics. For example, specific advances have been made in reagent consumption, throughput, integration of multiple assay steps, assay automation, and multiplexing capability. For heterogeneous systems, controlled immobilization of reactants is essential for reliable, sensitive detection of analytes. In most cases, protein immobilization densities are maximized, while native activity and conformation are maintained. Immobilization methods and chemistries vary significantly depending on immobilization surface, protein properties, and specific assay goals. In this review, we present trade-offs considerations for common immobilization surface materials. We overview immobilization methods and chemistries, and discuss studies exemplar of key approaches-here with a specific emphasis on immunoassays and enzymatic reactors. Recent "smart immobilization" methods including the use of light, electrochemical, thermal, and chemical stimuli to attach and detach proteins on demand with precise spatial control are highlighted. Spatially encoded protein immobilization using DNA hybridization for multiplexed assays and reversible protein immobilization surfaces for repeatable assay are introduced as immobilization methods. We also describe multifunctional surface coatings that can perform tasks that were, until recently, relegated to multiple functional coatings. We consider the microfluidics literature from 1997 to present and close with a perspective on future approaches to protein immobilization.

Chemical cytometry on microfluidic chips

Chemical cytometry, referring to the analysis of the chemical contents in individual cells, has been in intensive study since Kennedy's first work that was published in Science. The early researches relied on fine-tip capillaries to capture the cells and do the analyses, which were lab- and time-intensive and required high skills of operation. The emergence of microfluidics has greatly spurred this research field and a great number of research papers have been published in the last decades. Highly integrated microfluidic chips have been developed to capture multiple single cells, lyse them, perform chemical reactions in enclosed microchambers, separate contents by CE and detect chemical species in individual cells. This review focuses on the development of relevant components and their integration for on-chip chemical cytometry.

Novel dense CO2 technique for beta-galactosidase immobilization in polystyrene microchannels

In this study we design new fabrication techniques and demonstrate the potential of using dense CO2 for facilitating crucial steps in the fabrication of polymeric lab-on-a-chip microdevices by embedding biomolecules at temperatures well below the polymer's glass transition temperature (T(g)). These new techniques are environmentally friendly and done without the use of a clean room. Carbon dioxide at 40 degrees C and between 4.48 and 6.89 MPa was used to immobilize the biologically active molecule, beta-galactosidase (beta-gal), on the surface of polystyrene microchannels. To our knowledge, this is the first time dense CO2 has been used to directly immobilize an enzyme in a microchannel. beta-gal activity was maintained and shown via a fluorescent reaction product, after enzyme immobilization and microchannel capping by the designed fabrication steps at 40 degrees C and pressures up to 6.89 MPa.

Controlled proteolysis of normal and pathological prion protein in a microfluidic chip

A microreactor for proteinase K (PK)-mediated protein digestion was developed as a step towards the elaboration of a fully integrated microdevice for the detection of pathological prion protein (PrP). PK-grafted magnetic beads were immobilized inside a polydimethylsiloxane (PDMS) microchannel using a longitudinal magnetic field parallel to the flow direction and a magnetic field gradient, thereby forming a matrix for enzymatic digestion. This self-organization provided uniform pore sizes, a low flow resistance and a strong reaction efficiency due to a very thin diffusion layer. The microreactor's performance was first evaluated using a model substrate, succinyl-ala-ala-ala-paranitroanilide (SAAAP). Reaction kinetics were typically accelerated a hundred-fold as compared to conventional batch reactions. Reproducibility was around 98% for on-chip experiments. This microsystem was then applied to the digestion of prion protein from brain tissues. Controlled proteolysis could be obtained by varying the on-chip flow rate, while a complete proteolysis of normal protein was achieved in only three minutes. Extracts from normal and pathological brain homogenates were finally compared and strong discrimination between normal and pathological samples was demonstrated.

Immobilized capillary enzyme reactor based on layer-by-layer assembling acetylcholinesterase for inhibitor screening by CE

A method for creating an immobilized capillary acetylcholinesterase (AChE) reactor based on a layer-by-layer (LBL) assembly for inhibitor screening is described. The unique capillary AChE reactor was easily prepared by the instrument in three steps: first, a 0.5 cm long plug of a solution of the cationic polyelectrolyte polydiallyldimethylammonium (PDDA) was injected into the capillary to produce a positively charged coating on the surface of the capillary; subsequently, the enzyme solution with the same plug length was injected into the capillary and incubated for 10 min to immobilize the enzyme on the capillary wall via electrostatic interaction; third, PDDA solution with the same plug length was injected again into the capillary to cover the immobilized enzyme by forming PDDA-AChE-PDDA sandwich-like structure. The enzyme reactor can be easily renewed after removing the immobilized enzyme by flushing the column with 1 M NaCl solution. Activity of the immobilized enzyme can be assayed simply by carrying out an electrophoretic separation, i.e., the substrate solution was injected and incubated for a short time, followed by applying a voltage to separate the product from the unreacted substrate. The measured peak area of the product then represented the enzyme activity. For enzyme inhibitor screening, the mixture solution of the substrate and the inhibitor was injected and assayed the reduction of the enzyme activity. The immobilized enzyme could withstand 100 consecutive assays by only losing 10% activity. The reproducibility in terms of time-to-time, day-to-day, and batch-to-batch was measured with RSD% less than 4.7%. Furthermore, the screening system was validated by a known inhibitor. Finally, screening a small compound library containing four known AChE inhibitors and 42 natural extracts was demonstrated, and species with inhibition activity can be straightforwardly identified with the system.

Glucose microfluidic biosensors based on immobilizing glucose oxidase in poly(dimethylsiloxane) electrophoretic microchips

Here we reported a novel microfluidic biosensor with an on-column immobilized enzyme microreactor. The fabrication approach of this biosensor is simple and the enzyme microreactors with controlled sizes can be placed at any desired position on the microchip. Taking glucose oxidase (GOx) as an example, electroosmotic flow (EOF) as a driving force and amperometry as a detection method, the performance of biosensors were modulated by changing the length of enzyme reactor from 0.5 cm to 3 cm, and the linear ranges were changed from 0-8.0 mM to 0-30.0 mM with the detection limits from 42 microM to 6.5 microM. The enzyme reactor remained its 65% activity after 23 days storage. It also showed good anti-interference ability and was used to quantify glucose in human serum samples.

Development of monolithic enzymatic reactors in glass microchips for the quantitative determination of enzyme substrates using the example of glucose determination via immobilized glucose oxidase

A one-step procedure for the immobilization of glucose oxidase in fused-silica capillaries and in glass microchips was developed based on enzyme entrapment in a polyacrylamide-based monolithic column. The inner capillary surface was silanized with gamma-methacryloxypropyltrimethoxysilane (gamma-MAPS) to allow covalent binding of the gel to the surface. The composition of the polymer was optimized to prevent the formation of bubbles, allow liquid transportation by electroosmotic flow and to maintain the enzymatic activity. These requirements resulted in the addition of polyethylene glycol and poly(acrylic acid) to the acrylamide mixture. The gel containing the enzyme was formed in situ in the capillaries, respectively, in one channel of the microchip. In the microchip, it was limited to the sample injection channel by accordingly controlled silanization of the inner capillary surface. Glucose was detected via the amperometric determination of hydrogen peroxide. A linear correlation between signals and glucose concentration was observed from 0.05 to 1.1 mM glucose with a correlation coefficient of 0.999. The enzymatic monolithic microreactor showed no loss of activity during 8 h of continuous use and during storage in the running buffer at 4 degrees C for about 2 months. Interferents, such as ascorbic acid, were separated from the analyte electrophoretically, so that glucose could be quantified in diluted juices.

Immobilized microfluidic enzymatic reactors

The use of enzymes for cleavage, synthesis or chemical modification represents one of the most common processes used in biochemical and molecular biology laboratories. The continuing progress in medical research, genomics, proteomics, and related emerging biotechnology fields leads to exponential growth of the applications of enzymes and the development of modified or new enzymes with specific activities. Concurrently, new technologies are being developed to improve reaction rates and specificity or perform the reaction in a specific environment. Besides large-scale industrial applications, where typically a large processing capacity is required, there are other, much lower-scale applications, benefiting form the new developments in enzymology. One such technology is microfluidics with the potential to revolutionize analytical instrumentation for the analyses of very small sample amounts, single cells or even subcellular assemblies. This article aims at reviewing the current status of the development of the immobilized microfluidic enzymatic reactors (IMERs) technology.

Monolayer-functionalized microfluidics devices for optical sensing of acidity

This paper describes the integration of opto-chemosensors in microfluidics networks. Our technique exploits the internal surface of the network as a platform to build a sensing system by coating the surface with a self-assembled monolayer and subsequently binding a fluorescent sensing molecule to the monolayer. Fluorescent molecules were used that can switch between a fluorescent and a non-fluorescent state, depending on the acidity of the surrounding solution. Two systems were investigated. The first employs surface confinement of a Rhodamine B dye in a glass micro channel that serves as a molecular switch in organic solutions. Upon rinsing the micro channels with acidic or basic solutions it was possible to switch between the fluorescent and non-fluorescent forms reversibly. Moreover, this system could be used to monitor the mixing of two solutions of different acidity along the micro channel. To widen the scope of optical sensing in micro channels an Oregon Green dye derivative was immobilized, which functions as a sensing molecule for pH differences in aqueous solutions. In this case, a hybrid system was used consisting of a glass slide and PDMS channels. The fluorescence intensity was found to be directly correlated to the pH of the solution in contact, indicating the possibility of using such a system as a pH sensor. These systems allow real-time measurements and can be easily implemented in micro- and nanofluidics systems thus enabling analysis of extremely small sample volumes in a fast and reproducible manner.

Chip electrochromatography

Electrochromatography (EC) in microfluidic chips is emerging as an attractive alternative to capillary electrophoresis (CE) for on-chip separations. This review summarizes recent developments in the rapidly growing area of chip electrochromatography with a focus on "column" technologies. Relevant achievements are summarized according to the types of stationary phase used for the separations including open channels, microfabricated structures, and channels packed with beads or containing a porous monolith. The advantages and disadvantages of each, as well as practical aspects of their application, are discussed. The analytical performance of these devices is demonstrated with separations involving various families of compounds mostly in the reversed-phase chromatographic mode.

Bacterial P450-catalyzed polyketide hydroxylation on a microfluidic platform

The incorporation of a multicomponent, cofactor-dependant P450 into a microfluidic biochip is demonstrated. The PikC hydroxylase Streptomyces venezuelae was incorporated into a PDMS-based microfluidic channel. The enzyme was immobilized to Ni-NTA agarose beads via in situ attachment following the addition of the beads to the microchannel. The enzyme loading was approximately 6 microg per mg of beads resulting in a microchannel loading of 10.7 mg/mL. This high enzyme loading enabled the rapid hydroxylation of the macrolide YC-17 to methymycin and neomethymycin in about equal amounts with a conversion of >90% at a flow rate of 70 nL/min. This high reactivity allowed rapid hydroxylation reactions to be performed with short residence times, which is critical for complex enzymes with limited inherent stability.

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.

Total Protein Determinations by Particle Beam/Hollow Cathode Optical Emission Spectroscopy

A novel method for quantitative total protein determinations is presented. Total protein content is determined by particle beam/hollow cathode optical emission spectroscopy (PB/HC-OES) through monitoring of carbon atomic emission. The PB/HC-OES offers such advantages as ease of operation, exclusion of labor-intensive sample pretreatment processes, rapid analysis, high sensitivity, and low detection limit. The method could also be adapted to be integrated to current microfluidics devices. Parametric optimization for sample introduction, nebulization, desolvation, and hollow cathode source conditions is performed for the analysis of aqueous bovine serum albumin solutions. Response curves of C (I) 193.0-nm emission were obtained under the optimized conditions with both 10% HCl and 100 micromL KCl added to the sample matrix as potential carriers. The detection limit for triplicate injections of bovine serum albumin standards was found to be on the single-nanogram level with 200-microL injections. The addition of KCl significantly improved the sensitivity, supporting the proposed "carrier effect" of chloride salts in the particle transport process. Results obtained here suggest a range of applications for the use of the PB/HC-OES source for total protein determinations; emphasis here is future use in assessing protein quantification in microfluidic systems.

Multienzyme catalysis in microfluidic biochips

The attachment of enzymes to glass microfluidic channels has been achieved using a highly reactive poly(maleic anhydride-alt-alpha-olefin) (PMA)-based coating that is supplied to the microchannel in a toluene solution. The PMA reacts with 3-aminopropyltriethoxysilane groups linked to the glass surface to form a matrix that enables additional maleic anhydride groups to react with free amino groups on enzymes to give a mixed covalent-noncovalent immobilization support. Using a simple T-channel microfluidic design, with reaction channel dimensions of 200 microm wide (at the center), 15 microm deep, and 30 mm long giving a reaction volume of 90 nL, soybean peroxidase (SBP) was attached at an amount up to 0.6 microg/channel. SBP-catalyzed oxidation of p-cresol was performed in aqueous buffer (with 20% [v/v], dimethylformamide) containing H(2)O(2), with microfluidic transport enabled by electroosmotic flow (EOF). Michaelis-Menten kinetics were obtained with K(m) and V(max) values of 0.98 mM and 0.21 micromol H(2)O(2) converted/mg SBP per minute, respectively. These values are nearly identical to nonimmobilized SBP kinetics in aqueous-DMF solutions in 20-microL volumes in 384-well plates and 5-mL reaction volumes in 20-mL scintillation vials. These results indicate that SBP displays intrinsically native activity even in the immobilized form at the microscale, and further attests to the mild immobilization conditions afforded by PMA. Bienzymic and trienzymic reactions were also performed in the microfluidic biochip. Specifically, a combined Candida antarctica lipase B-SBP bienzymic system was used to convert tolyl acetate into poly(p-cresol), and an invertase-glucose oxidase SBP trienzymic system was used to take sucrose and generate H(2)O(2) for SBP-catalyzed synthesis of poly(p-cresol).

Determination of endogenous extracellular signal-regulated protein kinase by microchip capillary electrophoresis

The application of microchip capillary electrophoresis (CE) to the assay of extracellular signal-regulated protein kinase (ERK) is presented. In this assay, ERK catalyzes the transfer of gamma-phosphate from adenosine 5(')-triphosphate to the threonine residue of a fluorescently labeled nonapeptide (APRTPGGRR), and the phosphorylated and nonphosphorylated peptides were detected by fluorescence. The phosphorylated and nonphosphorylated peptides and the internal standard were separated within 20s, and the increase in magnitude of the phosphorylated peptide peak was monitored to assess ERK activity. ERK reactions were prepared off-chip and analyzed on a single-lane glass microchip fabricated by standard methods. It was demonstrated that microchip CE could be used to measure endogenous amounts of ERK by spiking known concentrations of recombinant ERK2 into the lysates of serum-starved human umbilical vein endothelial cells (HUVEC) and recovering between 90 and 100% for all samples. Endogenous ERK activity was determined by microchip where HUVEC were stimulated with 500pM vascular endothelial growth factor (VEGF) at different times before cell lysis. The results showed a transient VEGF-mediated ERK activation that peaked at 10min, which was consistent with previous reports using conventional techniques. The microchip assay provided a rapid, accurate, and precise alternative to conventional methods of determining endogenous ERK activity.

Microfluidic peroxidase biochip for polyphenol synthesis

An enzyme-containing microfluidic biochip has been developed for the oxidative polymerization of phenols. The biochip consists of a simple T-junction with two feed reservoirs 20 mm apart and a microreaction channel 30 mm long. The channel is 15 microm deep and 200 microm wide at the center, giving a reaction volume of 90 nL. The biochip was fabricated using conventional photolithographic methods on a glass substrate etched using a HF-based solution. Fluid transport was enabled using electroosmotic flow. Soybean peroxidase was used as the phenol oxidizing catalyst, and in the presence of p-cresol and H(2)O(2), essentially complete conversion of the H(2)O(2) (the limiting substrate) occurred in the microchannel at a flow rate of ca. 290 nL/min. Thus, peroxidase was found to be intrinsically active even upon dramatic scale-down as achieved in microfluidic reactors. These results were extended to a series of phenols, thereby demonstrating that the microfluidic peroxidase reactor may have application in high-throughput screening of phenolic polymerization reactions for use in phenolic resin synthesis. Finally, rapid growth of poly(p-cresol) on the walls of the microreaction channel could be performed in the presence of higher H(2)O(2) concentrations. This finding suggests that solution-phase peroxidase catalysis can be used in the controlled deposition of polymers on the walls of microreactors.

Protein proteolysis and the multi-dimensional electrochromatographic separation of histidine-containing peptide fragments on a chip

This paper reports a system for three-dimensional electrochromatography in a chip format. The steps involved included trypsin digestion, copper(II)-immobilized metal affinity chromatography [Cu(II)-IMAC] selection of histidine-containing peptides, and reversed-phase capillary electrochromatography of the selected peptides. Trypsin digestion and affinity chromatography were achieved in particle-based columns with a microfabricated frit whereas reversed-phase separations were executed on a column of collocated monolithic support structures. Column frits were designed to maintain constant cross sectional area and path length in all channels and to retain particles down to a size of 3 microm. Cu(II)-IMAC selection of histidine-containing peptides from standard peptide mixtures and protein digests followed by reversed-phase chromatography of the selected peptides was demonstrated in the electrochromatography mode. The possibility to run a comprehensive proteomic analysis by combining trypsin digestion, affinity selection, and a reversed-phase separation on chips was shown using fluorescein isothiocyanate-labeled bovine serum albumin as an example.

High-throughput peptide mass mapping using a microdevice containing trypsin immobilized on a porous polymer monolith coupled to MALDI TOF and ESI TOF mass spectrometers

An enzymatic microreactor with a volume of 470 nL has been prepared by immobilizing trypsin on a 10 cm long reactive porous polymer monolith located in a 100 microm i.d. fused silica capillary. This reactor affords suitable degrees of digestion of proteins even after very short residence times of less than 1 min. The performance is demonstrated with the digestion of eight proteins ranging in molecular mass from 2848 to 77 754. The digests were analyzed using mass spectrometry in two modes: off-line MALDI and in-line nanoelectrospray ionization. The large numbers of identified peptides enable a high degree of sequence coverage and positive identification of the proteins. The extent of sequence coverage decreases as the molecular mass of the digested protein increases.

Mass spectrometric detection in chromatography. Trends and perspectives

The newest results in the use of miscellaneous mass spectrometric detection methods in various chromatographic techniques are compiled and critically evaluated. Examples of application in clinical chemistry, health care, and in the analysis of pharmaceuticals, environmental pollutants, foods and food products are given. The benefits and drawbacks of MS detection are discussed, and future trends are briefly discussed.

Microfluidic chips for clinical and forensic analysis

This review gives an overview of developments in the field of microchip analysis for clinical diagnostic and forensic applications. The approach chosen to review the literature is different from that in most microchip reviews to date, in that the information is presented in terms of analytes tested rather than microchip method. Analyte categories for which examples are presented include (i) drugs (quality control, seizures) and explosives residues, (ii) drugs and endogenous small molecules and ions in biofluids, (iii) proteins and peptides, and (iv) analysis of nucleic acids and oligonucleotides. Few cases of microchip analysis of physiological samples or other "real-world" matrices were found. However, many of the examples presented have potential application for these samples, especially with ongoing parallel developments involving integration of sample pretreatment onto chips and the use of fluid propulsion mechanisms other than electrokinetic pumping.