Integrated Plastic Microfluidic Devices with ESI-MS for Drug Screening and Residue Analysis

Numerical Modeling of Physical Cell Trapping in Microfluidic Chips

Microfluidic methods have proven to be effective in separation and isolation of cells for a wide range of biomedical applications. Among these methods, physical trapping is a label-free isolation approach that relies on cell size as the selective phenotype to retain target cells on-chip for follow-up analysis and imaging. In silico models have been used to optimize the design of such hydrodynamic traps and to investigate cancer cell transmigration through narrow constrictions. While most studies focus on computational fluid dynamics (CFD) analysis of flow over cells and/or pillar traps, a quantitative analysis of mechanical interaction between cells and trapping units is missing. The existing literature centers on longitudinally extended geometries (e.g., micro-vessels) to understand the biological phenomenon rather than designing an effective cell trap. In this work, we aim to make an experimentally informed prediction of the critical pressure for a cell to pass through a trapping unit as a function of cell morphology and trapping unit geometry. Our findings show that a hyperelastic material model accurately captures the stress-related softening behavior observed in cancer cells passing through micro-constrictions. These findings are used to develop a model capable of predicting and extrapolating critical pressure values. The validity of the model is assessed with experimental data. Regression analysis is used to derive a mathematical framework for critical pressure. Coupled with CFD analysis, one can use this formulation to design efficient microfluidic devices for cell trapping and potentially perform downstream analysis of trapped cells.

Three-Dimensional In Vitro Cell Culture Models for Efficient Drug Discovery: Progress So Far and Future Prospects

Despite tremendous advancements in technologies and resources, drug discovery still remains a tedious and expensive process. Though most cells are cultured using 2D monolayer cultures, due to lack of specificity, biochemical incompatibility, and cell-to-cell/matrix communications, they often lag behind in the race of modern drug discovery. There exists compelling evidence that 3D cell culture models are quite promising and advantageous in mimicking in vivo conditions. It is anticipated that these 3D cell culture methods will bridge the translation of data from 2D cell culture to animal models. Although 3D technologies have been adopted widely these days, they still have certain challenges associated with them, such as the maintenance of a micro-tissue environment similar to in vivo models and a lack of reproducibility. However, newer 3D cell culture models are able to bypass these issues to a maximum extent. This review summarizes the basic principles of 3D cell culture approaches and emphasizes different 3D techniques such as hydrogels, spheroids, microfluidic devices, organoids, and 3D bioprinting methods. Besides the progress made so far in 3D cell culture systems, the article emphasizes the various challenges associated with these models and their potential role in drug repositioning, including perspectives from the COVID-19 pandemic.

A Review: Sample Preparation and Chromatographic Technologies for Detection of Aflatoxins in Foods

As a class of mycotoxins with regulatory and public health significance, aflatoxins (e.g., aflatoxin B1, B2, G1 and G2) have attracted unparalleled attention from government, academia and industry due to their chronic and acute toxicity. Aflatoxins are secondary metabolites of various Aspergillus species, which are ubiquitous in the environment and can grow on a variety of crops whereby accumulation is impacted by climate influences. Consumption of foods and feeds contaminated by aflatoxins are hazardous to human and animal health, hence the detection and quantification of aflatoxins in foods and feeds is a priority from the viewpoint of food safety. Since the first purification and identification of aflatoxins from feeds in the 1960s, there have been continuous efforts to develop sensitive and rapid methods for the determination of aflatoxins. This review aims to provide a comprehensive overview on advances in aflatoxins analysis and highlights the importance of sample pretreatments, homogenization and various cleanup strategies used in the determination of aflatoxins. The use of liquid-liquid extraction (LLE), supercritical fluid extraction (SFE), solid phase extraction (SPE) and immunoaffinity column clean-up (IAC) and dilute and shoot for enhancing extraction efficiency and clean-up are discussed. Furthermore, the analytical techniques such as gas chromatography (GC), liquid chromatography (LC), mass spectrometry (MS), capillary electrophoresis (CE) and thin-layer chromatography (TLC) are compared in terms of identification, quantitation and throughput. Lastly, with the emergence of new techniques, the review culminates with prospects of promising technologies for aflatoxin analysis in the foreseeable future.

Detection of Aflatoxins in Different Matrices and Food-Chain Positions

Aflatoxins, produced mainly by filamentous fungi Aspergillus flavus and Aspergillus parasiticus, are one of the most carcinogenic compounds that have adverse health effects on both humans and animals consuming contaminated food and feed, respectively. Aflatoxin B1 (AFB1) and aflatoxin B2 (AFB2) as well as aflatoxin G1(AFG1) and aflatoxin G2 (AFG2) occur in the contaminated foods and feed. In the case of dairy ruminants, after the consumption of feed contaminated with aflatoxins, aflatoxin metabolites [aflatoxin M1 (AFM1) and aflatoxin M2 (AFM2)] may appear in milk. Because of the health risk and the official maximum limits of aflatoxins, there is a need for application of fast and accurate testing methods. At present, there are several analytical methods applied in practice for determination of aflatoxins. The aim of this review is to provide a guide that summarizes worldwide aflatoxin regulations and analytical methods for determination of aflatoxins in different food and feed matrices, that helps in the decision to choose the most appropriate method that meets the practical requirements of fast and sensitive control of their contamination. Analytical options are outlined from the simplest and fastest methods with the smallest instrument requirements, through separation methods, to the latest hyphenated techniques.

Microfluidic systems for hydrodynamic trapping of cells and clusters

Microfluidic devices have been widely applied to trapping and isolation of cells and clusters for controllable intercellular environments and high-throughput analysis, triggering numerous advances in disease diagnosis and single-cell analysis. Passive hydrodynamic cell trapping is one of the simple and effective methods that has been gaining attention in recent years. Our aim here is to review the existing passive microfluidic trapping approaches, including microposts, microfiltration, microwells, and trapping chambers, with emphasis on design principles and performance. We summarize the remarkable advances that hydrodynamic trapping methods offer, as well as the existing challenges and prospects for development. Finally, we hope that an improved understanding of hydrodynamic trapping approaches can lead to sophisticated and useful platforms to advance medical and biological research.

A paper-based colorimetric microfluidic sensor fabricated by a novel spray painting prototyping process for iron analysis

A novel, simple, and low-cost spray painting technique has been developed for the fabrication of microfluidic paper-based devices. The devices that we developed utilize aerosol spray paint to build hydrophobic barriers and employ a hole puncher to obtain paper-based patterned layers and paper dots without using any specialized instruments (e.g., without a laser cutter). The entire manufacturing process is extremely simple, inexpensive, and rapid, which means that it can be applied broadly. Furthermore, the application of the device to iron detection was demonstrated. A linear relationship between the colour value and the iron concentration was observed from 0 to 0.02 g/L. The developed microfluidic paper-based device for iron detection exhibited a low detection limit (0.00090 g/L), good selectivity, and acceptable recovery.

Detection of Pathogenic Microorganisms by Microfluidics Based Analytical Methods

Microfluidics based biochemical analysis shows distinctive advantages for fast detection of pathogenic microorganisms. This Feature summarizes the progress in the past decade on microfluidic methods for purification and detection of pathogenic bacteria and viruses as well as their applications in food safety control, environmental monitoring, and clinical diagnosis.

Recent Advances in Mycotoxin Determination for Food Monitoring via Microchip

Mycotoxins are one of the main factors impacting food safety. Mycotoxin contamination has threatened the health of humans and animals. Conventional methods for the detection of mycotoxins are gas chromatography (GC) or liquid chromatography (LC) coupled with mass spectrometry (MS), or enzyme-linked immunosorbent assay (ELISA). However, all these methods are time-consuming, require large-scale instruments and skilled technicians, and consume large amounts of hazardous regents and solvents. Interestingly, a microchip requires less sample consumption and short analysis time, and can realize the integration, miniaturization, and high-throughput detection of the samples. Hence, the application of a microchip for the detection of mycotoxins can make up for the deficiency of the conventional detection methods. This review focuses on the application of a microchip to detect mycotoxins in foods. The toxicities of mycotoxins and the materials of the microchip are firstly summarized in turn. Then the application of a microchip that integrates various kinds of detection methods (optical, electrochemical, photo-electrochemical, and label-free detection) to detect mycotoxins is reviewed in detail. Finally, challenges and future research directions in the development of a microchip to detect mycotoxins are previewed.

Iontronics

Iontronics is an emerging technology based on sophisticated control of ions as signal carriers that bridges solid-state electronics and biological system. It is found in nature, e.g., information transduction and processing of brain in which neurons are dynamically polarized or depolarized by ion transport across cell membranes. It suggests the operating principle of aqueous circuits made of predesigned structures and functional materials that characteristically interact with ions of various charge, mobility, and affinity. Working in aqueous environments, iontronic devices offer profound implications for biocompatible or biodegradable logic circuits for sensing, ecofriendly monitoring, and brain-machine interfacing. Furthermore, iontronics based on multi-ionic carriers sheds light on futuristic biomimic information processing. In this review, we overview the historical achievements and the current state of iontronics with regard to theory, fabrication, integration, and applications, concluding with comments on where the technology may advance.

Capacitive deionization on-chip as a method for microfluidic sample preparation

Desalination as a sample preparation step is essential for noise reduction and reproducibility of mass spectrometry measurements. A specific example is the analysis of proteins for medical research and clinical applications. Salts and buffers that are present in samples need to be removed before analysis to improve the signal-to-noise ratio. Capacitive deionization is an electrostatic desalination (CDI) technique which uses two porous electrodes facing each other to remove ions from a solution. Upon the application of a potential of 0.5 V ions migrate to the electrodes and are stored in the electrical double layer. In this article we demonstrate CDI on a chip, and desalinate a solution by the removal of 23% of Na(+) and Cl(-) ions, while the concentration of a larger molecule (FITC-dextran) remains unchanged. For the first time impedance spectroscopy is introduced to monitor the salt concentration in situ in real-time in between the two desalination electrodes.

Real-time cellular exometabolome analysis with a microfluidic-mass spectrometry platform

To address the challenges of tracking the multitude of signaling molecules and metabolites that is the basis of biological complexity, we describe a strategy to expand the analytical techniques for dynamic systems biology. Using microfluidics, online desalting, and mass spectrometry technologies, we constructed and validated a platform well suited for sampling the cellular microenvironment with high temporal resolution. Our platform achieves success in: automated cellular stimulation and microenvironment control; reduced non-specific adsorption to polydimethylsiloxane due to surface passivation; real-time online sample collection; near real-time sample preparation for salt removal; and real-time online mass spectrometry. When compared against the benchmark of "in-culture" experiments combined with ultraperformance liquid chromatography-electrospray ionization-ion mobility-mass spectrometry (UPLC-ESI-IM-MS), our platform alleviates the volume challenge issues caused by dilution of autocrine and paracrine signaling and dramatically reduces sample preparation and data collection time, while reducing undesirable external influence from various manual methods of manipulating cells and media (e.g., cell centrifugation). To validate this system biologically, we focused on cellular responses of Jurkat T cells to microenvironmental stimuli. Application of these stimuli, in conjunction with the cell's metabolic processes, results in changes in consumption of nutrients and secretion of biomolecules (collectively, the exometabolome), which enable communication with other cells or tissues and elimination of waste. Naïve and experienced T-cell metabolism of cocaine is used as an exemplary system to confirm the platform's capability, highlight its potential for metabolite discovery applications, and explore immunological memory of T-cell drug exposure. Our platform proved capable of detecting metabolomic variations between naïve and experienced Jurkat T cells and highlights the dynamics of the exometabolome over time. Upregulation of the cocaine metabolite, benzoylecgonine, was noted in experienced T cells, indicating potential cellular memory of cocaine exposure. These metabolomics distinctions were absent from the analogous, traditional "in-culture" UPLC-ESI-IM-MS experiment, further demonstrating this platform's capabilities.

Chip-based nanoelectrospray mass spectrometry for protein characterization

In the last several years, significant progress has been made in the development of microfluidic-based analytical technologies for proteomic and drug discovery applications. Chip-based nanoelectrospray coupled to a mass spectrometer detector is one of the recently developed analytical microscale technologies. This technology offers unique advantages for automated nanoelectrospray including reduced sample consumption, improved detection sensitivity and enhanced data quality for proteomic studies. This review presents an overview and introduction of recent developments in chip devices coupled to electrospray mass spectrometers including the development of the automated nanoelectrospray ionization chip device for protein characterization. Applications using automated chip-based nanoelectrospray ionization technology in proteomic and bioanalytical studies are also extensively reviewed in the fields of high-throughput protein identification, protein post-translational modification studies, top-down proteomics, biomarker screening by pattern recognition, noncovalent protein-ligand binding for drug discovery and lipid analysis. Additionally, future trends in chip-based nanoelectrospray technology are discussed.

Electrical field-induced extraction and separation techniques: promising trends in analytical chemistry--a review

Sample preparation is an important issue in analytical chemistry, and is often a bottleneck in chemical analysis. So, the major incentive for the recent research has been to attain faster, simpler, less expensive, and more environmentally friendly sample preparation methods. The use of auxiliary energies, such as heat, ultrasound, and microwave, is one of the strategies that have been employed in sample preparation to reach the above purposes. Application of electrical driving force is the current state-of-the-art, which presents new possibilities for simplifying and shortening the sample preparation process as well as enhancing its selectivity. The electrical driving force has scarcely been utilized in comparison with other auxiliary energies. In this review, the different roles of electrical driving force (as a powerful auxiliary energy) in various extraction techniques, including liquid-, solid-, and membrane-based methods, have been taken into consideration. Also, the references have been made available, relevant to the developments in separation techniques and Lab-on-a-Chip (LOC) systems. All aspects of electrical driving force in extraction and separation methods are too specific to be treated in this contribution. However, the main aim of this review is to provide a brief knowledge about the different fields of analytical chemistry, with an emphasis on the latest efforts put into the electrically assisted membrane-based sample preparation systems. The advantages and disadvantages of these approaches as well as the new achievements in these areas have been discussed, which might be helpful for further progress in the future.

When Medicine Meets Engineering-Paradigm Shifts in Diagnostics and Therapeutics

During the last two decades, the manufacturing techniques of microfluidics-based devices have been phenomenally advanced, offering unlimited potential for bio-medical technologies. However, the direct applications of these technologies toward diagnostics and therapeutics are still far from maturity. The present challenges lay at the interfaces between the engineering systems and the biocomplex systems. A precisely designed engineering system with narrow dynamic range is hard to seamlessly integrate with the adaptive biological system in order to achieve the design goals. These differences remain as the roadblock between two fundamentally non-compatible systems. This paper will not extensively review the existing microfluidic sensors and actuators; rather, we will discuss the sources of the gaps for integration. We will also introduce system interface technologies for bridging the differences to lead toward paradigm shifts in diagnostics and therapeutics.

Microfluidic Paper-Based Analytical Devices (μPADs) and Micro Total Analysis Systems (μTAS): Development, Applications and Future Trends

Microfluidic paper-based analytical devices and micro total analysis systems are relatively new group of analytical tools, capable of analyzing complex biochemical samples containing macromolecules, proteins, nucleic acids, toxins, cells or pathogens. Within one analytical run, fluidic manipulations like transportation, sorting, mixing or separation are available. Recently, microfluidic devices are a subject of extensive research, mostly for fast and non-expensive biochemical analysis but also for screening of medical samples and forensic diagnostics. They are used for neurotransmitter detection, cancer diagnosis and treatment, cell and tissue culture growth and amplification, drug discovery and determination, detection and identification of microorganisms. This review summarizes development history, basic fabrication methods, applications and also future development trends for production of such devices.

Detecting proteins in microfluidic channels decorated with liquid crystal sensing dots

In this paper, we report the integration of liquid crystal (LC) dots on microfluidic channels as microscopic protein sensors. Flexibility of patterning LC dots on a surface to fit small microfluidic channels is achieved by using inkjet printing technology. These LC dots (1 pL) remain stable when they are subjected to flowing buffer solution at a high flow velocity (v ≥ 0.198 cm/s). When the buffer solution contains protein, such as bovine serum albumin (BSA), it causes a change in the orientational ordering of the LC dots as indicated by a distinct dark-to-bright transition in the optical appearance of the LC dots. Moreover, we are able estimate the concentration of BSA by simply counting the number of bright LC dot sections. This microscopic protein sensor has potential applications in the real-time detection and quantification of proteins in aqueous solutions. This detection method is advantageous because protein labeling and complex instrumentation are not required.

Current development of microfluidic immunosensing approaches for mycotoxin detection via capillary electromigration and lateral flow technology

Mycotoxin contamination in the food chain has caused serious health issues in humans and animals. Thus, a rapid on-site and lab-independent detection method for mycotoxins, such as aflatoxins (AFTs), is desirable. Microfluidic chip based immunosensor technology is one of the most promising methods for fast mycotoxin assays. In this review, we cover the major microfluidic immunosensors used for mycotoxin analysis, via flow-through (capillary electromigration) and lateral flow technology. Sample preparation from different matrices of agricultural products and foodstuffs is summarized. The choice of materials, fabrication strategies, and detection methods for microfluidic immunosensors are further discussed in detail. The sensors application in mycotoxin determination is also outlined. Finally, future challenges and opportunities are discussed.

Cytotoxicity of quantum dots assay on a microfluidic 3D-culture device based on modeling diffusion process between blood vessels and tissues

In this work, a novel quantum dot (QD) cytotoxicity assay platform on a microfluidic three-dimensional (3D) culture device via imitating the diffusion process between blood vessels and tissues was developed. The device is composed of a main channel and two sets of cell culture chambers. The cell culture chambers were located at different distances from the main channel and were divided into "close chambers" and "far chambers". HepG2 cells were cultured in an agarose matrix under 3D conditions and kept at high viability for at least three days. Fluorescein sodium and fluorescein isothiocyanate conjugated to bovine serum albumin (FITC-BSA) were used as models to demonstrate the diffusion process between main channel and cell culture chambers. QD cytotoxicity was evaluated by determining cell apoptosis, intracellular reactive oxygen species (ROS) and glutathione (GSH) with specific fluorescence probes. Cell autophagy inhibitor 3-methyladenine (3-MA) could reduce cell apoptosis at low concentrations of QDs, which proves that cell autophagy plays a key role in QD cytotoxicity. The effect of a series of 3-MA solutions on cell apoptosis at QD concentration of 40 μg mL(-1) was investigated, which showed that the percentage of cell apoptosis decreased ∼15% from 0 to 12 mM 3-MA. The device shows potential as a high-throughput, low-cost and time-saving platform and constructs a more vivid biomimetic microenvironment for the QD cytotoxicity study.

Size selective DNA transport through a nanoporous membrane in a PDMS microfluidic device

A microfluidic counter current dialysis device for size based purification of DNA is described. The device consists of two polydimethylsiloxane (PDMS) channels separated by a track etched polycarbonate membrane with a 50 nm pore size. Recovery of fluorescein across the membrane was compared with 10 and 80 nucleotide (nt) ssDNA to characterize the device. Recovery of all three analytes improved with decreasing flow rate. Size selectivity was observed. Greater than 2-fold selectivity between 10 nt and 80 nt ssDNA was observed at linear velocities less than 3mm s(-1). Increasing the ionic strength of the buffer increased transport across the membrane. Recovery of 80 nt ssDNA increased over 4-fold by adding 30 mM NaCl to the buffer. The effect was size dependent as 10 nt showed a smaller increase while the recovery of fluorescein was largely unaffected by increasing the ionic strength of the buffer.

Towards monitoring real-time cellular response using an integrated microfluidics-matrix assisted laser desorption ionisation/nanoelectrospray ionisation-ion mobility-mass spectrometry platform

The combination of microfluidic cell trapping devices with ion mobility-mass spectrometry offers the potential for elucidating in real time the dynamic responses of small populations of cells to paracrine signals, changes in metabolite levels and delivery of drugs and toxins. Preliminary experiments examining peptides in methanol and recording the interactions of yeast and Jurkat cells with their superfusate have identified instrumental set-up and control parameters and online desalting procedures. Numerous initial experiments demonstrate and validate this new instrumental platform. Future outlooks and potential applications are addressed, specifically how this instrumentation may be used for fully automated systems biology studies of the significantly interdependent, dynamic internal workings of cellular metabolic and signalling pathways.

Rapid prototyping of microstructures by soft lithography for biotechnology

This chapter describes the methods and specific procedures used to fabricate microstructures by soft lithography. These techniques are useful for the prototyping of devices useful for applications in biotechnology. Fabrication by soft lithography does not require specialized or expensive equipment; the materials and facilities necessary are found commonly in biological and chemical laboratories in both academia and industry. The combination of the fact that the materials are low-cost and that the time from design to prototype device can be short (< 24 h) makes it possible to use and to screen rapidly devices that also can be disposable. Here we describe the procedures for fabricating microstructures with lateral dimensions as small as 1 mum. These types of microstructures are useful for microfluidic devices, cell-based assays, and bioengineered surfaces.

Fully integrated three-dimensional electrodes for electrochemical detection in microchips: fabrication, characterization, and applications

A scalable and rather inexpensive solution to producing microanalytical systems with "on-chip" three-dimensional (3D) microelectrodes is presented in this study, along with applicability to practical electrochemical (EC) detection scenarios such as preconcentration and interferant removal. This technique to create high-aspect-ratio (as much as 4:1) gold microstructures in constrained areas involved the modification of stud bump geometry with microfabricated silicon molds via an optimized combination of temperature, pressure, and time. The microelectrodes that resulted consisted of an array of square pillars approximately 18 microm tall and 20 microm wide on each side, placed at the end of a microfabricated electrophoresis channel. This technique increased the active surface area of the microelectrodes by as much as a factor of 50, while mass transfer and, consequently, preconcentration collection efficiencies were increased to approximately 100%, compared to approximately 30% efficiency for planar nonmodified microelectrodes (samples that were used included the neurotransmitters dopamine and catechol). The 3D microelectrodes were used both in a stand-alone configuration, for direct EC detection of model catecholamine analytes, and, more interestingly, in dual electrode configurations for EC sample processing prior to detection downstream at a second planar electrode. In particular, the 3D electrodes were shown to be capable of performing coulometry or complete (100%) redox conversion of analyte species over a wide range of concentrations, from 4.3 microM to 4.4 mM, in either plug-flow or continuous-flow formats.

Emerging optofluidic technologies for point-of-care genetic analysis systems: a review

This review describes recently emerging optical and microfluidic technologies suitable for point-of-care genetic analysis systems. Such systems must rapidly detect hundreds of mutations from biological samples with low DNA concentration. We review optical technologies delivering multiplex sensitivity and compatible with lab-on-chip integration for both tagged and non-tagged optical detection, identifying significant source and detector technology emerging from telecommunications technology. We highlight the potential for improved hybridization efficiency through careful microfluidic design and outline some novel enhancement approaches using target molecule confinement. Optimization of fluidic parameters such as flow rate, channel height and time facilitates enhanced hybridization efficiency and consequently detection performance as compared with conventional assay formats (e.g. microwell plates). We highlight lab-on-chip implementations with integrated microfluidic control for "sample-to-answer" systems where molecular biology protocols to realize detection of target DNA sequences from whole blood are required. We also review relevant technology approaches to optofluidic integration, and highlight the issue of biomolecule compatibility. Key areas in the development of an integrated optofluidic system for DNA hybridization are optical/fluidic integration and the impact on biomolecules immobilized within the system. A wide range of technology platforms have been advanced for detection, quantification and other forms of characterization of a range of biomolecules (e.g. RNA, DNA, protein and whole cell). Owing to the very different requirements for sample preparation, manipulation and detection of the different types of biomolecules, this review is focused primarily on DNA-DNA interactions in the context of point-of-care analysis systems.

Recent progress of online sample preconcentration techniques in microchip electrophoresis

Microchip electrophoresis (MCE) has been advanced remarkably by the applications of several separation modes and the integration with several chemical operations on a single planer substrate. MCE shows superior analytical performance, e.g., high-speed analysis, high resolution, low consumption of reagents, and so on, whereas low-concentration sensitivity is still one of the major problems. To overcome this drawback, various online sample preconcentration techniques have been developed in MCE over the past 15 years, which have successfully enhanced the detection sensitivity in MCE. This review highlights recent developments in online sample preconcentration in MCE categorized on the basis of "dynamic" and "static" methods. The dynamic techniques including field amplified stacking, ITP, sweeping, and focusing have been easily applied to MCE, which provide effective enrichments of various analytes. The static techniques such as SPE and filtration have also been combined with MCE. In the static techniques, extremely high preconcentration efficiency can be obtained, compared to the dynamic methods. This review provides comprehensive tables listing the applications and sensitivity enhancement factors of these preconcentration techniques employed in MCE.

Analytical characterization of microfabricated SU-8 emitters for electrospray ionization mass spectrometry

We present a detailed optimization and characterization of the analytical performance of SU-8-based emitters for electrospray ionization mass spectrometry (ESI/MS). The improved SU-8 fabrication process presented here enhances patterning accuracy and reduces the time and cost of fabrication. All emitters are freestanding and enable sample delivery by both pressure-driven and spontaneous flows. The optimized emitter design incorporates a sharp, double-cantilevered tip implemented to the outlet of an SU-8 microchannel and provides highly sensitive ESI/MS detection. Moreover, the optimized design allows the use of relatively large microchannel dimensions (up to 200 x 50 microm(2), w x h) without sacrificing the detection sensitivity. This is advantageous with a view of preventing emitter clogging and enabling reproducible analysis. The measured limits of detection for the optimized emitter design were 1 nM for verapamil and 4 nM for Glu-fibrinopeptide B with good quantitative linearities between 1 nM and 10 microM (R(2) = 0.9998) for verapamil and between 4 nM and 3 microM (R(2) = 0.9992) for Glu-fibrinopeptide B. The measured tip-to-tip repeatability for signal intensity was 14% relative standard deviation (RSD) (n = 3; 5 microM verapamil) and run-to-run repeatability 4-11% RSD (n = 4; 5 microM verapamil) for all individual emitters tested. In addition, long-term stability of < 2% RSD was maintained for timescales of 30 min even under free flow conditions. SU-8 polymer was also shown to be chemically stable against most of the tested electrospray solvents.

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.

Integrated membrane filters for minimizing hydrodynamic flow and filtering in microfluidic devices

Microfluidic devices have gained significant scientific interest due to the potential to develop portable, inexpensive analytical tools capable of quick analyses with low sample consumption. These qualities make microfluidic devices attractive for point-of-use measurements where traditional techniques have limited functionality. Many samples of interest in biological and environmental analysis, however, contain insoluble particles that can block microchannels, and manual filtration prior to analysis is not desirable for point-of-use applications. Similarly, some situations involve limited control of the sample volume, potentially causing unwanted hydrodynamic flow due to differential fluid heads. Here, we present the successful inclusion of track-etched polycarbonate membrane filters into the reservoirs of poly(dimethylsiloxane) capillary electrophoresis microchips. The membranes were shown to filter insoluble particles with selectivity based on the membrane pore diameter. Electrophoretic separations with membrane-containing microchips were performed on cations, anions, and amino acids and monitored using conductivity and fluorescence detection. The dependence of peak areas on head pressure in gated injection was shown to be reduced by up to 92%. Results indicate that separation performance is not hindered by the addition of membranes. Incorporating membranes into the reservoirs of microfluidic devices will allow for improved analysis of complex solutions and samples with poorly controlled volume.

Forensic DNA Analysis on Microfluidic Devices: A Review

The advent of microfluidic technology for genetic analysis has begun to impact forensic science. Recent advances in microfluidic separation of short-tandem-repeat (STR) fragments has provided unprecedented potential for improving speed and efficiency of DNA typing. In addition, the analytical processes associated with sample preparation--which include cell sorting, DNA extraction, DNA quantitation, and DNA amplification--can all be integrated with the STR separation in a seamless manner. The current state of these microfluidic methods as well as their advantages and potential shortcomings are detailed. Recent advances in microfluidic device technology, as they pertain to forensic DNA typing, are discussed with a focus on the forensic community.

Integration of nanoporous membranes for sample filtration/preconcentration in microchip electrophoresis

Microfluidic devices integrating membrane-based sample preparation with electrophoretic separation are demonstrated. These multilayer devices consist of 10 nm pore diameter membranes sandwiched between two layers of PDMS substrates with embedded microchannels. Because of the membrane isolation, material exchange between two fluidic layers can be precisely controlled by applied voltages. More importantly, since only small molecules can pass through the nanopores, the integrated membrane can serve as a filter or a concentrator prior to microchip electrophoresis under different design and operation modes. As a filter, they can be used for separation and selective injection of small analytes from sample matrix. This has been effectively applied in rapid determination of reduced glutathione in human plasma and red blood cells without any off-chip deproteinization procedure. Alternatively, in the concentrator mode, they can be used for online purification and preconcentration of macromolecules, which was illustrated by removing primers and preconcentrating the product DNA from a PCR product mixture.

Microfluidic technologies for MALDI-MS in proteomics

The field of microfluidics continues to offer great promise as an enabling technology for advanced analytical tools. For biomolecular analysis, there is often a critical need to couple on-chip microfluidic sample manipulation with back-end MS. Though interfacing microfluidics to MS has been most often reported through the use of direct ESI-MS, there are compelling reasons for coupling microfluidics to MALDI-MS as an alternative to ESI-MS for both online and offline analysis. The intent of this review is to provide a summary of recent developments in the integration of microfluidic systems with MALDI-MS, with an emphasis on applications in proteomics. Key points are summarized, followed by a review of relevant technologies and a discussion of outlook for the field.

Membranes and microfluidics: a review

The integration of mass transport control by means of membrane functionality into microfluidic devices has shown substantial growth over the last 10 years. Many different examples of mass transport control have been reported, demonstrating the versatile use of membranes. This review provides an overview of the developments in this area of research. Furthermore, it aims to bridge the fields of microfabrication and membrane science from a membrane point-of-view. First the basic terminology of membrane science will be discussed. Then the integration of membrane characteristics on-chip will be categorized based on the used fabrication method. Subsequently, applications in various fields will be reviewed. Considerations for the use of membranes will be discussed and a checklist with selection criteria will be provided that can serve as a starting point for those researchers interested in applying membrane-technology on-chip. Finally, opportunities for microfluidics based on proven membrane technology will be outlined. A special focus in this review is made on the membrane properties of polydimethylsiloxane (PDMS), since this material is frequently used nowadays in master replication.

The uptake of O3 by myristic acid-oleic acid mixed particles: evidence for solid surface layers

The oleic acid ozonolysis in mixed oleic and myristic acid particles was studied in a flow tube reactor using single particle mass spectrometry. The change in reactivity was investigated as a function of the myristic acid concentration in these 2 micron particles. For pure oleic acid aerosol, the reactive ozone uptake coefficient, gamma, was found to be 3.4 (+/-0.3) x 10(-4) after taking secondary reactions into account. At the myristic acid crystallization point, where only 2.5% of the particle is in the solid phase, the uptake coefficient was reduced to 9.7 (+/-1.0) x 10(-5). This dramatic drop in the uptake coefficient is explained by the presence of a crystalline monolayer of myristic acid, through which ozone diffusion is reduced by several orders of magnitude, relative to liquid oleic acid. Scanning electron microscope images of the mixed particles confirm that the particle surface is crystalline when the myristic acid mole fraction exceeds 0.125. The findings of these experiments illustrate that particle morphology is important to understanding the reactivity of species in a mixed particle. The decay of myristic acid during the course of ozonolysis is explained in terms of a reaction with stabilized Criegee intermediates, which attack the acidic groups of the oleic and myristic acids with equal rate constants.

Microfabricated devices: A new sample introduction approach to mass spectrometry

Instrument miniaturization is one way of addressing the issues of sensitivity, speed, throughput, and cost of analysis in DNA diagnostics, proteomics, and related biotechnology areas. Microfluidics is of special interest for handling very small sample amounts, with minimal concerns related to sample loss and cross-contamination, problems typical for standard fluidic manipulations. Furthermore, the small footprint of these microfabricated structures leads to instrument designs suitable for high-density, parallel sample processing, and high-throughput analyses. In addition to miniaturized systems designed with optical or electrochemical detection, microfluidic devices interfaced to mass spectrometry have also been demonstrated. Instruments for automated sample infusion analysis are now commercially available, and microdevices utilizing chromatographic or capillary electrophoresis separation techniques are under development. This review aims at documenting the technologies and applications of microfluidic mass spectrometry for the analysis of proteomic samples.

Selective protein removal and desalting using microchip CE

This paper describes the on-line sample pretreatment and analysis of proteins and peptides with a poly(methylmethacrylate) (PMMA) microfluidic device (IonChip). This chip consists of two hyphenated electrophoresis channels with integrated conductivity detectors. The first channel can be used for sample preconcentration and sample clean-up, while in the second channel the selected compounds are separated. Isotachophoresis (ITP) combined with zone electrophoresis (CZE) was used to preconcentrate a myoglobin sample by a factor of about 65 before injection into the second dimension and to desalt a mixture of six proteins with 100 mM NaCl. However, ITP-CZE could not be used for the removal of two proteins from a protein/peptide sample since the protein zone in the ITP step was too small to remove certain compounds. Therefore, we used CZE-CZE for the removal of proteins from a protein/peptide mixture, thereby injecting only the peptides into the second CZE separation channel.

A multilayer poly(dimethylsiloxane) electrospray ionization emitter for sample injection and online mass spectrometric detection

An ESI emitter made of poly(dimethylsiloxane) interfaces on-chip sample preparation with MS detection. The unique multilayer design allows both the analyte and the spray solutions to reside on the device simultaneously in discrete microfluidic environments that are spatially separated by a polycarbonate track-etched, nanocapillary array membrane (NCAM). In direct spray mode, voltage is applied to the microchannel containing a spray solution delivered via a syringe pump. For injection, the spray potential is lowered and a voltage is applied that forward biases the membrane and permits the analyte to enter the spray channel. Once the injection is complete, the bias potential is switched off, and the spray voltage is increased to generate the ESI of the injected analyte plug. Consecutive injections of a 10 microM bovine insulin solution are reproducible and produce sample plugs with limited band broadening and high quality mass spectra. Peptide signals are observed following transport through the NCAM, even when the peptide is dissolved in solutions containing up to 20% seawater. The multilayer emitter shows great potential for performing multidimensional chemical manipulations on-chip, followed by direct ESI with negligible dead volume for online MS analysis.

Electrospray interfacing of polymer microfluidics to MALDI-MS

The off-line coupling of polymer microfluidics to MALDI-MS is presented using electrospray deposition. Using polycarbonate microfluidic chips with integrated hydrophobic membrane electrospray tips, peptides and proteins are deposited onto a stainless steel target followed by MALDI-MS analysis. Microchip electrospray deposition is found to yield excellent spatial control and homogeneity of deposited peptide spots, and significantly improved MALDI-MS spectral reproducibility compared to traditional target preparation methods. A detection limit of 3.5 fmol is demonstrated for angiotensin. Furthermore, multiple electrospray tips on a single chip provide the ability to simultaneously elute parallel sample streams onto a MALDI target for high-throughput multiplexed analysis. Using a three-element electrospray tip array with 150 microm spacing, the simultaneous deposition of bradykinin, fibrinopeptide, and angiotensin is achieved with no cross talk between deposited samples. In addition, in-line proteolytic digestion of intact proteins is successfully achieved during the electrospray process by binding trypsin within the electrospray membrane, eliminating the need for on-probe digestion prior to MALDI-MS. The technology offers promise for a range of microfluidic platforms designed for high-throughput multiplexed proteomic analyses in which simultaneous on-chip separations require an effective interface to MS.

An integrated microfluidic device in polyester for electrophoretic analysis of amino acids

A precolumn reaction chamber was integrated into a polyester microfluidic device with a miniaturized detection system. The reaction chamber was designed to be a zigzag channel, 70 microm in width, 8 mm in length, followed by a wider straight channel, 150 microm in width, 2 mm in length. The detection system is composed of an embedded light-emitting diode (LED), an integrated optical fiber, and a photomultiplier tube (PMT). A success in amino acid analysis using the integrated microchemical analysis device proved that the precolumn reaction chamber was compatible with the integrated detection system. Three kinds of amino acids, arginine, glycine, and phenylalanine, mixed and reacted with 7-fluoro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-F) in the precolumn reaction chamber to produce fluorescent products, were separated by micellar eletrokinetic chromatography (MEKC) and detected by LED-excited fluorescence. The detection limits for arginine, glycine, and phenylalanine were 1, 1, and 0.5 mM, respectively, which can be improved by further optimizations of the reaction system and detection system.