Microfluidic technologies for MALDI-MS in proteomics

Microfluidic chips: recent advances, critical strategies in design, applications and future perspectives

Microfluidic chip technology is an emerging tool in the field of biomedical application. Microfluidic chip includes a set of groves or microchannels that are engraved on different materials (glass, silicon, or polymers such as polydimethylsiloxane or PDMS, polymethylmethacrylate or PMMA). The microchannels forming the microfluidic chip are interconnected with each other for desired results. This organization of microchannels trapped into the microfluidic chip is associated with the outside by inputs and outputs penetrating through the chip, as an interface between the macro- and miniature world. With the help of a pump and a chip, microfluidic chip helps to determine the behavioral change of the microfluids. Inside the chip, there are microfluidic channels that permit the processing of the fluid, for example, blending and physicochemical responses. Microfluidic chip has numerous points of interest including lesser time and reagent utilization and alongside this, it can execute numerous activities simultaneously. The miniatured size of the chip fastens the reaction as the surface area increases. It is utilized in different biomedical applications such as food safety sensing, peptide analysis, tissue engineering, medical diagnosis, DNA purification, PCR activity, pregnancy, and glucose estimation. In the present study, the design of various microfluidic chips has been discussed along with their biomedical applications.

[Recent advances in microchip liquid chromatography]

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3D-Printed electrochemical sensor-integrated transwell systems

This work presents a 3D-printed, modular, electrochemical sensor-integrated transwell system for monitoring cellular and molecular events in situ without sample extraction or microfluidics-assisted downstream omics. Simple additive manufacturing techniques such as 3D printing, shadow masking, and molding are used to fabricate this modular system, which is autoclavable, biocompatible, and designed to operate following standard operating protocols (SOPs) of cellular biology. Integral to the platform is a flexible porous membrane, which is used as a cell culture substrate similarly to a commercial transwell insert. Multimodal electrochemical sensors fabricated on the membrane allow direct access to cells and their products. A pair of gold electrodes on the top side of the membrane measures impedance over the course of cell attachment and growth, characterized by an exponential decrease (~160% at 10 Hz) due to an increase in the double layer capacitance from secreted extracellular matrix (ECM) proteins. Cyclic voltammetry (CV) sensor electrodes, fabricated on the bottom side of the membrane, enable sensing of molecular release at the site of cell culture without the need for downstream fluidics. Real-time detection of ferrocene dimethanol injection across the membrane showed a three order-of-magnitude higher signal at the membrane than in the bulk media after reaching equilibrium. This modular sensor-integrated transwell system allows unprecedented direct, real-time, and noninvasive access to physical and biochemical information, which cannot be obtained in a conventional transwell system.

Liquid Beam Desorption Mass Spectrometry for the Investigation of Continuous Flow Reactions in Microfluidic Chips

In this work, we present the combination of microfluidic chips and mass spectrometry employing laser-induced liquid beam ionization/desorption. The developed system was evaluated with respect to stable beam generation and laser parameters as well as solvent compatibility. The device was exemplarily applied to study a vinylogous Mannich reaction performed in continuous flow on chip. Fast processes can be observed with this technique which in the future could be beneficial for studying intermediates or contribute to the elucidation of reaction mechanisms.

Microfluidics for genome-wide studies involving next generation sequencing

Next-generation sequencing (NGS) has revolutionized how molecular biology studies are conducted. Its decreasing cost and increasing throughput permit profiling of genomic, transcriptomic, and epigenomic features for a wide range of applications. Microfluidics has been proven to be highly complementary to NGS technology with its unique capabilities for handling small volumes of samples and providing platforms for automation, integration, and multiplexing. In this article, we review recent progress on applying microfluidics to facilitate genome-wide studies. We emphasize on several technical aspects of NGS and how they benefit from coupling with microfluidic technology. We also summarize recent efforts on developing microfluidic technology for genomic, transcriptomic, and epigenomic studies, with emphasis on single cell analysis. We envision rapid growth in these directions, driven by the needs for testing scarce primary cell samples from patients in the context of precision medicine.

Advances in coupling microfluidic chips to mass spectrometry

Microfluidic technology has shown advantages of low sample consumption, reduced analysis time, high throughput, and potential for integration and automation. Coupling microfluidic chips to mass spectrometry (Chip-MS) can greatly improve the overall analytical performance of MS-based approaches and expand their potential applications. In this article, we review the advances of Chip-MS in the past decade, covering innovations in microchip fabrication, microchips coupled to electrospray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-MS. Development of integrated microfluidic systems for automated MS analysis will be further documented, as well as recent applications of Chip-MS in proteomics, metabolomics, cell analysis, and clinical diagnosis.

Recent advances in microfluidics combined with mass spectrometry: technologies and applications

Instrument miniaturization is one of the critical issues to improve sensitivity, speed, throughput, and to reduce the cost of analysis. Microfluidics possesses the ability to handle small sample amounts, with minimal concerns related to sample loss and cross-contamination, problems typical for standard fluidic manipulations. Moreover, the native properties of microfluidics provide the potential for high-density, parallel sample processing, and high-throughput analysis. Recently, the coupling of microfluidic devices to mass spectrometry, especially electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), has attracted an increasing interest and produced tremendous achievements. The interfaces between microfluidics and mass spectrometry are one of the primary focused problems. In this review, we summarize the latest achievements since 2008 in the field of the technologies and applications in the combining of microfluidics with ESI-MS and MALDI-MS. The integration of several analytical functions on a microfluidic device such as sample pretreatment and separations before sample introduction into the mass spectrometer is also discussed.

The future of microfluidic assays in drug development

BACKGROUND

Microfluidic technology emerges as a convenient route to applying automated and reliable assays in a high-throughput manner with low cost.

OBJECTIVE

This review aims to answer questions related to the capabilities and potential applications of microfluidic assays that can benefit the drug development process and extends an outlook on its future trends.

METHODS

This article reviews recent publications in the field of microfluidics, with an emphasis on novel applications for drug development.

RESULTS/CONCLUSION

Microfluidics affords unique capabilities in sample preparation and separation, combinatorial synthesis and array formation, and incorporating nanotechnology for more functionalities. The pharmaceutical industry, facing challenges from limited productivity and accelerated competition, can thus greatly benefit from applying new microfluidic assays in various drug development stages, from target screening and lead optimization to absorption distribution metabolism elimination and toxicity studies in preclinical evaluations, diagnostics in clinical trials and drug formulation and manufacturing process optimization.

Microfluidic device fabrication by thermoplastic hot-embossing

Due to their low cost compatibility with replication-based fabrication methods, thermoplastics represent an exceptionally attractive family of materials for the fabrication of lab-on-a-chip platforms. A diverse range of thermoplastic materials suitable for microfluidic fabrication is available, offering a wide selection of mechanical and chemical properties that can be leveraged and further tailored for specific applications. While high-throughput embossing methods such as reel-to-reel processing of thermoplastics is an attractive method for industrial microfluidic chip production, the use of single chip hot embossing is a cost-effective technique for realizing high-quality microfluidic devices during the prototyping stage. Here we describe methods for the replication of microscale features in two thermoplastics, polymethylmethacrylate (PMMA) and polycarbonate (PC), using hot embossing from a silicon template fabricated by deep reactive-ion etching.

Microfluidic devices for high-throughput proteome analyses

Over the last decades, microfabricated bioanalytical platforms have gained enormous interest due to their potential to revolutionize biological analytics. Their popularity is based on several key properties, such as high flexibility of design, low sample consumption, rapid analysis time, and minimization of manual handling steps, which are of interest for proteomics analyses. An ideal totally integrated chip-based microfluidic device could allow rapid automated workflows starting from cell cultivation and ending with MS-based proteome analysis. By reducing or eliminating sample handling and transfer steps and increasing the throughput of analyses these workflows would dramatically improve the reliability, reproducibility, and throughput of proteomic investigations. While these complete devices do not exist for routine use yet, many improvements have been made in the translation of proteomic sample handling and separation steps into microfluidic formats. In this review, we will focus on recent developments and strategies to enable and integrate proteomic workflows into microfluidic devices.

Sample preconcentration in open microchannels combined with MALDI-MS

In this work, a method for preconcentrating samples in 1 cm long, 50-150 μm wide open microchannels is presented. Platinum electrodes were positioned at the channel ends, voltage was applied, and charged analyte was preconcentrated at the oppositely charged side during continuous supply of sample. The preconcentration was initially studied in a closed system, where an influence on the analyte position from a pH gradient, generated by water electrolysis, was observed. In the open channel, the analyte distribution after preconcentration was evaluated using MALDI-MS with the channel as MALDI target. MALDI matrix was applied with an airbrush or by electrospray matrix deposition and by using the latter technique higher degrees of crystallization in the channels were obtained. After preconcentrating a 1 nM cytochrome c solution for 5 min, corresponding to a supplied amount of 1.25 fmol, a signal on the cathodic channel end could be detected. When a solution of cytochrome c trypsin digest was supplied, the peptides were preconcentrated at different positions along the channel depending on their charge.

Surface acoustic wave nebulization facilitating lipid mass spectrometric analysis

Surface acoustic wave nebulization (SAWN) is a novel method to transfer nonvolatile analytes directly from the aqueous phase to the gas phase for mass spectrometric analysis. The lower ion energetics of SAWN and its planar nature make it appealing for analytically challenging lipid samples. This challenge is a result of their amphipathic nature, labile nature, and tendency to form aggregates, which readily precipitate clogging capillaries used for electrospray ionization (ESI). Here, we report the use of SAWN to characterize the complex glycolipid, lipid A, which serves as the membrane anchor component of lipopolysaccharide (LPS) and has a pronounced tendency to clog nano-ESI capillaries. We also show that unlike ESI SAWN is capable of ionizing labile phospholipids without fragmentation. Lastly, we compare the ease of use of SAWN to the more conventional infusion-based ESI methods and demonstrate the ability to generate higher order tandem mass spectral data of lipid A for automated structure assignment using our previously reported hierarchical tandem mass spectrometry (HiTMS) algorithm. The ease of generating SAWN-MS(n) data combined with HiTMS interpretation offers the potential for high throughput lipid A structure analysis.

Lab-on-a-plate: extending the functionality of MALDI-MS and LDI-MS targets

We review the literature that describes how (matrix-assisted) laser desorption/ionization (MA)LDI target plates can be used not only as sample supports, but beyond that: as functional parts of analytical protocols that incorporate detection by MALDI-MS or matrix-free LDI-MS. Numerous steps of analytical procedures can be performed directly on the (MA)LDI target plates prior to the ionization of analytes in the ion source of a mass spectrometer. These include homogenization, preconcentration, amplification, purification, extraction, digestion, derivatization, synthesis, separation, detection with complementary techniques, data storage, or other steps. Therefore, we consider it helpful to define the "lab-on-a-plate" as a format for carrying out extensive sample treatment as well as bioassays directly on (MA)LDI target plates. This review introduces the lab-on-plate approach and illustrates it with the aid of relevant examples from the scientific and patent literature.

A solid-phase bioreactor with continuous sample deposition for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

We report the development of a solid-phase proteolytic digestion and continuous deposition microfluidic chip platform for low volume fraction collection and off-line matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Tryptic peptides were formed in an on-chip bioreactor and continuously deposited onto a MALDI target plate using a motor-driven xyz stage. The bioreactor consisted of a 4 cm × 200 µm × 50 µm microfluidic channel with covalently immobilized trypsin on an array of 50 µm diameter micropost structures with a 50 µm edge-to-edge inter-post spacing. A 50 µm i.d. capillary tube was directly attached to the end of the bioreactor for continuous sample deposition. The MALDI target plate was modified by spin-coating a nitrocellulose solution containing a MALDI matrix on the surface prior to effluent deposition. Protein molecular weight standards were used for evaluating the performance of the digestion and continuous deposition system. Serpentine sample traces 200 µm wide were obtained with a 30 fmol/mm quantity deposition rate and a 3.3 nL/mm volumetric deposition rate.

Microfluidic devices for bioapplications

Harnessing the ability to precisely and reproducibly actuate fluids and manipulate bioparticles such as DNA, cells, and molecules at the microscale, microfluidics is a powerful tool that is currently revolutionizing chemical and biological analysis by replicating laboratory bench-top technology on a miniature chip-scale device, thus allowing assays to be carried out at a fraction of the time and cost while affording portability and field-use capability. Emerging from a decade of research and development in microfluidic technology are a wide range of promising laboratory and consumer biotechnological applications from microscale genetic and proteomic analysis kits, cell culture and manipulation platforms, biosensors, and pathogen detection systems to point-of-care diagnostic devices, high-throughput combinatorial drug screening platforms, schemes for targeted drug delivery and advanced therapeutics, and novel biomaterials synthesis for tissue engineering. The developments associated with these technological advances along with their respective applications to date are reviewed from a broad perspective and possible future directions that could arise from the current state of the art are discussed.

Incubated protein reduction and digestion on an electrowetting-on-dielectric digital microfluidic chip for MALDI-MS

Localized heating of droplets on an electrowetting-on-dielectric (EWOD) chip has been implemented and shown to accelerate trypsin digestion reaction rates, sample drying, and matrix crystallization for matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Achieving this involved extending the functionality of previous EWOD droplet-based techniques by developing a multifunctional electrode with closed-loop temperature control, while minimizing overall system complexity and addressing challenges associated with rapid evaporation. For the EWOD chip design, we discuss the performance of multifunctional surface electrodes for actuation, localized Joule heating, and thermistic temperature sensing. Furthermore, a hydrophilic pattern is formed in the multifunctional electrode to control the location of an evaporating droplet on the electrode. To demonstrate the capabilities and limitations of this technique, we performed three experiments and measured the results using MALDI-MS: (i) insulin disulfide reductions in dithiothreitol (DTT) over a range of heater temperatures (22-70 °C) to show how reaction rates can be affected by thermal control, (ii) insulin disulfide reductions at 130 °C in dimethyl sulfoxide (DMSO) to demonstrate a reaction in a high boiling point solvent, and (iii) tryptic digestions of cytochrome c at 22 and 40 °C to show that heated droplets can yield reasonably higher peptide sequence coverage than unheated droplets. Although they do not decouple the effects of changing temperatures and concentrations, these experiments verified that thermal cycling by EWOD electrodes accelerates reaction rates in liquid droplets in air.

Mixed-mode electrokinetic and chromatographic peptide separations in a microvalve-integrated polymer chip

A cycloolefin polymer chip supporting the concatenation of isoelectric focusing (IEF) and reversed-phase liquid chromatography (RPLC) is demonstrated for high throughput two dimensional peptide separations. A unique benefit of the mixed-mode platform is the ability of IEF to act as a highly concentrating electrokinetic separation mode for effective isolation of sample components prior to RPLC. The thermoplastic chip contains integrated high pressure microvalves, enabling uniform sample transfer from the IEF channel to multiple parallel RPLC channels, gradient elution from each RPLC column, and hydrodynamic isolation between the separation dimensions. The reusable system is shown to provide efficient 2-D separations together with facile interfacing with MALDI-MS, suggesting a new path towards effective peptide analysis from complex samples.

Proteomics of lung cell biology and pulmonary disease

Proteomics has the goal of defining the complete protein complement of biological systems, which can then be analyzed in a comparative fashion to generate informative data regarding protein expression and function. Proteomic analyses can also facilitate the discovery of biomarkers that can be used to diagnose and monitor disease severity, activity and therapeutic response, as well as to identify new targets for drug development. A major challenge for proteomics, however, has been detecting low-abundance proteins in complex biological fluids. This review summarizes how proteomic analyses have advanced lung cell biology and facilitated the identification of new mechanisms of disease pathogenesis in respiratory disorders, such as asthma, cystic fibrosis, lung cancer, acute lung injury and sarcoidosis. The impact of nanotechnology and microfluidics, as well as studies of post-translational modifications and protein-protein interactions (the interactome), are considered. Furthermore, the application of systems-biology approaches to organize and analyze data regarding the lung proteome, interactome, genome, transcriptome, metabolome, glycome and small RNAome (regulatory RNAs), should facilitate future conceptual advances regarding lung cell biology, disease pathogenesis, biomarker discovery and drug development.

HIV diagnostics: challenges and opportunities

Accurate HIV diagnostic testing continues to pose challenges, but there are also opportunities for assay performance improvements in key areas for specific intended-use settings. The genetic diversity of HIV can result in false and discordant results in assays that fail to detect new variant strains. The use of antiretroviral therapies has resulted in drug-resistant variants that require monitoring by sequencing and genotyping methods. Nucleic acid testing is the most sensitive and reliable platform for detection, but it is costly and limited to centralized testing facilities, making implementation difficult in resource-limited settings where HIV has hit the hardest. Rapid antibody tests suitable for point-of-care use are becoming more accessible in resource-limited settings, but these tests may not detect HIV during the acute infection stage. Emerging antigen/antibody combination assays are viable alternatives to nucleic acid testing for diagnosis of recent infections. Although patient monitoring (e.g., via CD4+ T-cell count and viral load determination) and infant diagnoses still rely on clinical laboratory-based testing, point-of-care options are being developed. There are other technical challenges to HIV diagnostic testing and emerging biodetection technologies that may be able to address them, but they are not yet proven.

Microchip technology in mass spectrometry

Microfabrication of analytical devices is currently of growing interest and many microfabricated instruments have also entered the field of mass spectrometry (MS). Various (atmospheric pressure) ion sources as well as mass analyzers have been developed exploiting microfabrication techniques. The most common approach thus far has been the miniaturization of the electrospray ion source and its integration with various separation and sampling units. Other ionization techniques, mainly atmospheric pressure chemical ionization and photoionization, have also been subject to miniaturization, though they have not attracted as much attention. Likewise, all common types of mass analyzers have been realized by microfabrication and, in most cases, successfully applied to MS analysis in conjunction with on-chip ionization. This review summarizes the latest achievements in the field of microfabricated ion sources and mass analyzers. Representative applications are reviewed focusing on the development of fully microfabricated systems where ion sources or analyzers are integrated with microfluidic separation devices or microfabricated pums and detectors, respectively. Also the main microfabrication methods, with their possibilities and constraints, are briefly discussed together with the most commonly used materials.

Microfluidic Device for Coupling Capillary Electrophoresis and Matrix-Assisted Laser Desorption Ionization-Mass Spectrometry

We have designed and fabricated a polydimethylsiloxane (PDMS) microfluidic device for coupling capillary electrophoresis (CE) and matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS). The coupling is advantageous in biological research because CE has the power of separating analytes in a sample based on mobility difference and MALDI-MS provides accurate and sensitive mass analysis of the analytes. The goal is realized by fractionating the separated analytes inside the microfluidic device and pushing the analyte fractions into open reservoirs. Each analyte fraction is then mixed with a matrix solution and deposited on a MALDI target for MALDI-MS. Therefore, a two-step analysis of analytes in the form of CE-MALDI-MS is achieved by using the microfluidic device.

Surface plasmon resonance immuno assays - A perspective

Human growth hormone (GH) represents an extremely challenging task from an anti-doping viewpoint. GH is an endogenously produced substance, present at very low levels in circulation (for the most abundant 22kDa isoform approximately 50pM in plasma and 100fM in urine) either as monomer or homo- and heterodimers, comprises a family of distinct isoforms, and obeys a pulsatile secretion routine that is affected by many different internal and external factors. Upon administration of the recombinant, single-isoform pharmaceutical, the feedback mechanism reduces the endogenous heterogeneity resulting in altered ratios between the different GH isoforms. Thus, measuring the isoform ratios through immuno assays appears the approach of choice. Conventional assays do not provide information on isoform-specific association and dissociation events of the individual primary antibody-isoform or isoform-secondary antibody interactions. This particular information can be obtained using the technology of surface plasmon resonance (SPR) which enables monitoring of biomolecular interactions in a dynamic and label-free setting. In this paper the different aspects of SPR are described, how the technology may be beneficial for understanding today's anti-GH immunoassays, and whether the approach could be employed for measuring GH in the near future.

Miniaturizing sample spots for matrix-assisted laser desorption/ionization mass spectrometry

The trend of miniaturization in bioanalytical chemistry is shifting from technical development to practical application. In matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), progress in miniaturizing sample spots has been driven by the needs to increase sensitivity and speed, to interface with other analytical microtechnologies, and to develop miniaturized instrumentation.We review recent developments in miniaturizing sample spots for MALDI-MS. We cover both target modification and microdispensing technologies, and we emphasize the benefits with respect to sensitivity, throughput and automation.We hope that this review will encourage further method development and application of miniaturized sample spots for MALDI-MS, so as to expand applications in analytical chemistry, protein science and molecular biology.

Microfluidics with MALDI analysis for proteomics--a review

Various microfluidic devices have been developed for proteomic analyses and many of these have been designed specifically for mass spectrometry detection. In this review, we present an overview of chip fabrication, microfluidic components, and the interfacing of these devices to matrix-assisted laser desorption ionization (MALDI) mass spectrometry. These devices can be directly coupled to the mass spectrometer for on-line analysis in real-time, or samples can be analyzed on-chip or deposited onto targets for off-line readout. Several approaches for combining microfluidic devices with analytical functions such as sample cleanup, digestion, and separations with MALDI mass spectrometry are discussed.

Environmental proteomics: a paradigm shift in characterizing microbial activities at the molecular level

The increase in sequencing capacity led to a new wave of metagenomic projects, enabling and setting the prerequisite for the application of environmental proteomics technologies. This review describes the current status of environmental proteomics. It describes sample preparation as well as the two major technologies applied within this field: two-dimensional electrophoresis-based environmental proteomics and liquid chromatography-mass spectrometry-based environmental proteomics. It also highlights current publications and describes major scientific findings. The review closes with a discussion of critical improvements in the area of integrating experimental mass spectrometry technologies with bioinformatics as well as improved sample handling.

Microfluidic chips for mass spectrometry-based proteomics

Microfluidic devices coupled to mass spectrometers have emerged as excellent tools for solving the complex analytical challenges associated with the field of proteomics. Current proteome identification procedures are accomplished through a series of steps that require many hours of labor-intensive work. Microfluidics can play an important role in proteomic sample preparation steps prior to mass spectral identification such as sample cleanup, digestion, and separations due to its ability to handle small sample quantities with the potential for high-throughput parallel analysis. To utilize microfluidic devices for proteomic analysis, an efficient interface between the microchip and the mass spectrometer is required. This tutorial provides an overview of the technologies and applications of microfluidic chips coupled to mass spectrometry for proteome analysis. Various approaches for combining microfluidic devices with electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) are summarized and applications of chip-based separations and digestion technologies to proteomic analysis are presented.

Development of an automated digestion and droplet deposition microfluidic chip for MALDI-TOF MS

An automated proteolytic digestion bioreactor and droplet deposition system was constructed with a plastic microfluidic device for off-line interfacing to matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The microfluidic chips were fabricated in poly(methyl methacrylate) (PMMA), using a micromilling machine and incorporated a bioreactor, which was 100 microm wide, 100 microm deep, and possessed a 4 cm effective channel length (400 nL volume). The chip was operated by pressure-driven flow and mounted on a robotic fraction collector system. The PMMA bioreactor contained surface immobilized trypsin, which was covalently attached to the UV-modified PMMA surface using coupling reagents N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) and hydroxysulfosuccinimide (sulfo-NHS). The digested peptides were mixed with a MALDI matrix on-chip and deposited as discrete spots on MALDI targets. The bioreactor provided efficient digestion of a test protein, cytochrome c, at a flow rate of 1 microL/min, producing a reaction time of approximately 24 s to give adequate sequence coverage for protein identification. Other proteins were also evaluated using this solid-phase bioreactor. The efficiency of digestion was evaluated by monitoring the sequence coverage, which was 64%, 35%, 58%, and 47% for cytochrome c, bovine serum albumin (BSA), myoglobin, and phosphorylase b, respectively.

Trypsin entrapped in poly(diallyldimethylammonium chloride) silica sol-gel microreactor coupled to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

An enzyme-immobilized capillary microreactor for rapid protein digestion and proteomics analysis is reported. The inner surface of the fused-silica capillary was coated with poly(diallyldimethylammonium chloride) (PDDA)-entrapped silica sol-gel matrix, followed by assembly of trypsin onto the PDDA-modified surface via electrostatic adsorption. The immobilization parameters such as PDDA content in the sol-gel matrix, trypsin concentration and pH were investigated in detail. Protein samples including beta-casein, myoglobin and cytochrome c could be effectively digested and electrophoretically separated simultaneously in such a modified capillary. Just 2.26 ng (corresponding to 0.10-0.14 picomole) of sample was sufficient for on-line capillary electrophoresis peptide mapping. The efficiency of the digestion was further demonstrated by digestion of a human liver cytoplasm sample and 253 proteins were identified in one unique run.

Microfluidics for drug discovery and development: from target selection to product lifecycle management

Microfluidic technologies' ability to miniaturize assays and increase experimental throughput have generated significant interest in the drug discovery and development domain. These characteristics make microfluidic systems a potentially valuable tool for many drug discovery and development applications. Here, we review the recent advances of microfluidic devices for drug discovery and development and highlight their applications in different stages of the process, including target selection, lead identification, preclinical tests, clinical trials, chemical synthesis, formulations studies and product management.

Nanotechnology in proteomics

In genomics, the ability to amplify rare transcripts has enabled rapid advances in the understanding of gene expression patterns in human disease. The inability to increase the copy number and to detect the signal of rare proteins as unique species in biological samples has hindered the ability of proteomics to dissect human disease with the same complexity as genomic analyses. Advances in nanotechnology have begun to allow researchers to identify low-abundance proteins in samples through techniques that rely upon both nanoparticles and nanoscale devices. Coupled with rapid advances made in protein identification and isolation over the past decade, currently available technology enables more effective multiplexing and improved signal-to-noise, which enhances detection of low-abundance proteins in cellular and tissue lysates significantly. Techniques, including nanowires, nanocantilevers, bio-barcoding and surface-enhanced Raman spectroscopy, permit the detection of proteins into the low attomolar range, where many biologically important cellular processes occur. In this review, we summarize several such techniques, highlight their implementation in current protein research and comment on their potential role in future proteomic investigations and biomedical applications.

A decade of microfluidic analysis coupled with electrospray mass spectrometry: an overview

This review presents a thorough overview covering the period 1997-2006 of microfluidic chips coupled to mass spectrometry through an electrospray interface. The different types of fabrication processes and materials used to fabricate these chips throughout this period are discussed. Three 'eras' of interfaces are clearly distinguished. The earliest approach involves spraying from the edge of a chip, while later devices either incorporate a standard fused-silica emitter inserted into the device or fully integrated emitters formed during chip fabrication. A summary of microfluidic-electrospray devices for performing separations and sample pretreatment steps before sample introduction into the mass spectrometer is also presented.

Capillary electrophoresis–mass spectrometry for the analysis of intact proteins

Developments in the fields of protein chemistry, proteomics and biotechnology have increased the demand for suitable analytical techniques for the analysis of intact proteins. In 1989, capillary electrophoresis (CE) was combined with mass spectrometry (MS) for the first time and its potential usefulness for the analysis of intact (i.e. non-digested) proteins was shown. This article provides an overview of the applications of CE-MS within the field of intact protein analysis. The principles of the applied CE modes and ionization techniques used for CE-MS of intact proteins are shortly described. It is shown that separations are predominantly carried out by capillary zone electrophoresis and capillary isoelectric focusing, whereas electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most popular ionization techniques used for interfacing. The combination of CE with inductively coupled plasma (ICP) MS for the analysis of metalloproteins is also discussed. The various CE-MS combinations are systematically outlined and tables provide extensive overviews of the applications of each technique for intact protein analysis. Selected examples are given to illustrate the usefulness of the CE-MS techniques. Examples include protein isoform assignment, single cell analysis, metalloprotein characterization, proteomics and biomarker screening. Finally, chip-based electrophoresis combined with MS is shortly treated and some of its applications are described. It is concluded that CE-MS represents a powerful tool for the analysis of intact proteins yielding unique separations and information.