Fabrication of quartz microchips with optical slit and development of a linear imaging UV detector for microchip electrophoresis systems

Recent progress of microfluidic chips in immunoassay

Microfluidic chip technology is a technology platform that integrates basic operation units such as processing, separation, reaction and detection into microchannel chip to realize low consumption, fast and efficient analysis of samples. It has the characteristics of small volume need of samples and reagents, fast analysis, low cost, automation, portability, high throughout, and good compatibility with other techniques. In this review, the concept, preparation materials and fabrication technology of microfluidic chip are described. The applications of microfluidic chip in immunoassay, including fluorescent, chemiluminescent, surface-enhanced Raman spectroscopy (SERS), and electrochemical immunoassay are reviewed. Look into the future, the development of microfluidic chips lies in point-of-care testing and high throughput equipment, and there are still some challenges in the design and the integration of microfluidic chips, as well as the analysis of actual sample by microfluidic chips.

Real-Time Intrinsic Fluorescence Visualization and Sizing of Proteins and Protein Complexes in Microfluidic Devices

Optical detection has become a convenient and scalable approach to read out information from microfluidic systems. For the study of many key biomolecules, however, including peptides and proteins, which have low fluorescence emission efficiencies at visible wavelengths, this approach typically requires labeling of the species of interest with extrinsic fluorophores to enhance the optical signal obtained - a process which can be time-consuming, requires purification steps, and has the propensity to perturb the behavior of the systems under study due to interactions between the labels and the analyte molecules. As such, the exploitation of the intrinsic fluorescence of protein molecules in the UV range of the electromagnetic spectrum is an attractive path to allow the study of unlabeled proteins. However, direct visualization using 280 nm excitation in microfluidic devices has to date commonly required the use of coherent sources with frequency multipliers and devices fabricated out of materials that are incompatible with soft lithography techniques. Here, we have developed a simple, robust, and cost-effective 280 nm LED platform that allows real-time visualization of intrinsic fluorescence from both unlabeled proteins and protein complexes in polydimethylsiloxane microfluidic channels fabricated through soft lithography. Using this platform, we demonstrate intrinsic fluorescence visualization of proteins at nanomolar concentrations on chip and combine visualization with micron-scale diffusional sizing to measure the hydrodynamic radii of individual proteins and protein complexes under their native conditions in solution in a label-free manner.

A new soft lithographic route for the facile fabrication of hydrophilic sandwich microchips

Manufacturing materials are an essential element for the fabrication of microfluidic chips. PDMS, the most widely used polymeric material, is associated with apparent disadvantages such as hydrophobic nature, while other materials also suffer from some limitations. In this paper, a new soft lithographic route was proposed for the facile manufacturing of hydrophilic sandwich microchips, using bisphenol A based epoxy acrylate (BABEA) as a new patterning material. The BABEA copolymers are hydrophilic, highly transparent in visible range while highly untransparent when the wavelength is less than 290 nm, and of high replication fidelity. By combining with appropriate monomers, including glycidyl methacrylate, methylmethacrylate, and acrylic acid, the copolymers contain active functional groups, which allows for easy postmodification for desirable functional units. A fabrication procedure was proposed for manufacturing hybrid quartz/BABEA copolymer/quartz microchips. In the procedure, no micromachining equipments, wet etching, or imprinting techniques were involved, making the fabrication approach applicable in ordinary chemistry laboratories. The performance of the prepared microchips was demonstrated in terms of CIEF with UV-whole channel imaging detection. The hydrophilic microchannel ensures stable focusing while the polymeric middle layer acts as a perfectly aligned optical slit for whole channel UV absorbance detection.

Label-free real-time imaging in microchip free-flow electrophoresis applying high speed deep UV fluorescence scanning

We report on label-free monitoring of microfluidic free-flow electrophoresis (μFFE) separations in real-time using a custom built high speed deep UV laser scanner. In combination with a novel layout realized in fused silica (FS) FFE chips the setup was successfully applied for continuous separations and detection of unlabeled analytes including native proteins by space-resolved intrinsic deep UV fluorescence scanning.

Localized flexible integration of high-efficiency surface enhanced Raman scattering (SERS) monitors into microfluidic channels

We report here a facile approach for flexible integration of high efficiency surface enhanced Raman scattering (SERS) monitors in a continuous microfluidic channel. In our work, femtosecond laser direct writing was adopted for highly localizable and controllable fabrication of the SERS monitor through a multi-photon absorption (MPA) induced photoreduction of silver salt solution. The silver substrate could be shaped into designed patterns, and could be precisely located at the desired position of the microchannel bed, giving the feasibility for real-time detection during reactions. SEM and TEM images show that the silver substrates were composed of crystallized silver nanoplates with an average thickness of 50 nm. AFM results reveal that the substrates were about 600 nm in height and the surface was very rough. As representative tests for SERS detection, p-aminothiophenol (p-ATP) and flavin adenine dinucleotide (FAD) were chosen as probing molecules for microfluidic analysis at visible light (514.5 nm) excitation, exhibiting an enhancement factor of ~10(8). In addition, the combination of the SERS substrate with the microfluidic channel allows detection of inactive analytes through in situ microfluidic reactions.

Refractive index sensor based on a 1D photonic crystal in a microfluidic channel

A refractive index sensor has been fabricated in silicon oxynitride by standard UV lithography and dry etching processes. The refractive index sensor consists of a 1D photonic crystal (PhC) embedded in a microfluidic channel addressed by fiber-terminated planar waveguides. Experimental demonstrations performed with several ethanol solutions ranging from a purity of 96.00% (n = 1.36356) to 95.04% (n = 1.36377) yielded a sensitivity (Δλ/Δn) of 836 nm/RIU and a limit of detection (LOD) of 6 × 10(-5) RIU, which is, however, still one order of magnitude higher than the theoretical lower limit of the limit of detection 1.3 × 10(-) (6) RIU.

Capillary and microchip electrophoresis in microdialysis: recent applications

The theme of this review is to highlight the importance of microscale electrophoretic-based separation systems in microdialysis (microD). The ability of CE and MCE to yield very rapid and highly efficient separations using just nanolitre volumes of microdialysate samples will also be discussed. Recent advances in this area will be highlighted, by illustration of some exciting new applications while the need for further innovation will be covered. The first section briefly introduces the concept of microD sampling coupled with electrophoresis-based separation and the inherent advantages of this approach. The following section highlights some specific applications of CE separations in the detection of important biomarkers such as low-molecular-weight neurotransmitters, amino acids, and other molecules that are frequently encountered in microD. Various detection modes in CE are outlined and some of the advantages and drawbacks thereof are discussed. The last section introduces the concepts of micro-total analysis systems and the coupling of MCE and microD. Some of the latest innovations will be illustrated. The concluding section reflects on the future of this important chemical alliance between microD and CE/MCE.

Fabrication of a hybrid PDMS/SU-8/quartz microfluidic chip for enhancing UV absorption whole-channel imaging detection sensitivity and application for isoelectric focusing of proteins

A poly(dimethylsiloxane)(PDMS)/SU-8/quartz hybrid chip was developed and applied in the isoelectric focusing (IEF) of proteins with ultraviolet (UV) absorbance-based whole-channel imaging detection (UV-WCID). Each hybrid chip was made of three layers: a PDMS flat top substrate, a bottom quartz substrate and a middle layer of SU-8 photoresist. The SU-8 serves two purposes: it contains the microchannel used for IEF separation, and acts as an optical slit that absorbs UV light below 300 nm improving detection sensitivity in WCID. The novel hybrid design demonstrates a two to three times improvement in sensitivity over a comparable PDMS/PDMS design. In addition, the hybrid chip exhibits increased heat dissipation due to the superior thermal conductivity of the bottom quartz substrate allowing for larger electric fields to be used in separations. The hybrid design with IEF-UV-WCID was successful in resolving a complicated sample, hemoglobin control, with high fidelity.

Determination of adrenal steroids by microfluidic chip using micellar electrokinetic chromatography

This paper describes a sensitive and convenient method to separate progesterone, 17alpha-hydroxy progesterone, cortexolone, hydrocortisone and cortisone, all of which are steroids and have similar structures, using microfluidic chip-based technology with UV detection at 252 nm. We successfully obtained high-speed separation of the five steroids within 70 s in optimized microfluidic controls and micellar electrokinetic chromatography (MEKC) separation conditions. Fairly good linearity with correlation coefficient of over 0.98 from 10 or 20 to 100 mg/l steroid chemicals was obtained. The limits of detection obtained at a signal to noise ratio of 3 were from 3.89 to 7.80 mg/l. The values of the relative standard deviation (RSD) were 0.98-1.34% for repetitive injection (n = 12) and the intraday and interday RSDs were below 6%. The highly stable response reflected the feasibility of this method.

Direct observation of dispersion and mixing processes in microfluidic systems

The diffusion phenomena, dispersion and mixing processes of the sample solute (Basic Blue 3 dye and KMnO4 aqueous solutions) were directly observed in laminar flow in glass microchannels. Quasi steady-state UV-visible absorption spectrometry was carried out using CCD camera images of the colored sample dispersion and mixing processes, and the absorbance change (DeltaAbs) was discussed based on the dimensionless parameter, tau which represents the flow time renormalized to the diffusion coefficient and the channel cross section. It was found that DeltaAbs showed almost the same tau dependence, even though the solutions and the microchannel sizes differed in laminar flow, if the microchannel fabrication method was the same. On the basis of this fundamental result, the total microchannel length required for the reaction of 2,3-diaminonaphthalene (DAN) and NO2- at a flow rate of 2 microL min(-1) was calculated, and the obtained value ( approximately 100 mm) showed very good agreement with our previous microchip research. It was concluded that both results were useful for designing the microchannel width, depth and length to control the chemical reaction time in recent microfluidic systems.

Photonic crystal resonator integrated in a microfluidic system

We report on a novel optofluidic system consisting of a silica-based 1D photonic crystal, integrated planar waveguides, and electrically insulated fluidic channels. An array of pillars in a microfluidic channel designed for electrochromatography is used as a resonator for on-column label-free refractive index detection. The resonator was fabricated in a silicon oxynitride platform, to support electro-osmotic flow, and operated at lambda=1.55 microm. Different aqueous solutions of ethanol with refractive indices ranging from n=1.3330 to 1.3616 were pumped into the column/resonator, and the transmission spectra were recorded. Linear shifts of the resonant wavelengths yielded a maximum sensitivity of Deltalambda/Deltan=480 nm/RIU (refractive index unit), and a minimum difference of Deltan=0.007 RIU was measured.

Two-photon excited fluorescence detection at 420 nm for label-free detection of small aromatics and proteins in microchip electrophoresis

Two photon excited (TPE) fluorescence detection was applied to native fluorescence detection of aromatics in microchip electrophoresis (MCE). This technique was evaluated as an alternative to common one photon excitation in the deep UV spectral range. TPE enables fluorescence detection of unlabeled aromatic compounds, even in non-deep UV-transparent microfluidic chips. In this study, we demonstrate the proof of concept of native TPE fluorescence detection of small aromatics in commercial microfluidic glass chips. Label-free TPE fluorescence detection of native proteins and small aromatics in MCE was achieved within the micromolar concentration range, utilising 420 nm excitation light.

A hybrid microdevice with a thin PDMS membrane on the detection window for UV absorbance detection

We exploited a PDMS-quartz hybrid microchip with a thin PDMS membrane on the concave detection window for UV absorbance detection. The thickness of the PDMS membrane is about 100 mum, with high UV transmittance. As compared to a PDMS-quartz hybrid chip with a common detection window, the proposed one exhibited over an one order of magnitude sensitivity enhancement, and an about two orders of magnitude S/N increase for gastrodin (p-hydroxymethylphenyl-beta-D-glucopyranoside). In addition, the limit of the detection wavelength has been extended from 240 to 210 nm, which is otherwise impossible for a traditional PDMS-quartz hybrid microchip. This kind of microchip has the potential for a large range of applications in an integrated microfluidic system with UV detection.

Transcriptome analysis device based on liquid phase detection by fluorescently labeled nucleic acid probes

Personalized medicine based on genetic information has been proposed as an attractive medical treatment. It is supported by the rapid development of the gene diagnosis utilizing on-chip analysis technology. This study reports characterizations of mRNA detection device by its hybridization with 2'-O-methyl oligoribonucleotide probe in a liquid phase, which eliminates time-consuming processes such as removing non-hybridized probes in conventional solid phase detection. In order to achieve a high sensitivity in the fluorescent detection of the target mRNA, we have optimized detection channel depth and width for a microfluidic device, and investigated polydimethyl siloxiane components and observation setup to minimize background noise. Adopting optimized channel dimensions and setup for the probe, the fluorescent intensity increased 3.3-fold at the lowest detectable concentration of 7.8 nM.

Deep UV laser-induced fluorescence detection of unlabeled drugs and proteins in microchip electrophoresis

Deep UV fluorescence detection at 266-nm excitation wavelength has been realized for sensitive detection in microchip electrophoresis. For this purpose, an epifluorescence setup was developed enabling the coupling of a deep UV laser into a commercial fluorescence microscope. Deep UV laser excitation utilizing a frequency quadrupled pulsed laser operating at 266 nm shows an impressive performance for native fluorescence detection of various compounds in fused-silica microfluidic devices. Aromatic low molecular weight compounds such as serotonin, propranolol, a diol, and tryptophan could be detected at low-micromolar concentrations. Deep UV fluorescence detection was also successfully employed for the detection of unlabeled basic proteins. For this purpose, fused-silica chips dynamically coated with hydroxypropylmethyl cellulose were employed to suppress analyte adsorption. Utilizing fused-silica chips permanently coated with poly(vinyl alcohol), it was also possible to separate and detect egg white chicken proteins. These data show that deep UV fluorescence detection significantly widens the application range of fluorescence detection in chip-based analysis techniques.

Unconventional detection methods for microfluidic devices

The direction of modern analytical techniques is to push for lower detection limits, improved selectivity and sensitivity, faster analysis time, higher throughput, and more inexpensive analysis systems with ever-decreasing sample volumes. These very ambitious goals are exacerbated by the need to reduce the overall size of the device and the instrumentation - the quest for functional micrototal analysis systems epitomizes this. Microfluidic devices fabricated in glass, and more recently, in a variety of polymers, brings us a step closer to being able to achieve these stringent goals and to realize the economical fabrication of sophisticated instrumentation. However, this places a significant burden on the detection systems associated with microchip-based analysis systems. There is a need for a universal detector that can efficiently detect sample analytes in real time and with minimal sample manipulation steps, such as lengthy labeling protocols. This review highlights the advances in uncommon or less frequently used detection methods associated with microfluidic devices. As a result, the three most common methods - LIF, electrochemical, and mass spectrometric techniques - are omitted in order to focus on the more esoteric detection methods reported in the literature over the last 2 years.

Development and optimization of a lab-on-a-chip device for the measurement of trace nitrogen dioxide gas in the atmosphere

We propose the use of lab-on-a-chip technology for measuring gaseous chemical pollutants, and describe the development of a microchip for the detection of nitrogen dioxide (NO2) in air. A microchip fabricated from quartz glass has been developed for handling the following three functions, gas absorption, chemical reaction and fluorescence detection. Channels constructed in the microchip were covered with porous glass plates, allowing nitrogen dioxide to penetrate into the triethanolamine (TEA) flowing within the microchannel beneath. The nitrogen dioxide was then mixed with TEA and reacted with a suitable fluorescence reagent in the chemical reaction chamber in the microchip. The reacted solution was then allowed to flow into the fluorescence detection area to be excited by an ultraviolet light-emitting diode (UV-LED), and the fluorescence was detected using a photomultiplier tube (PMT). The reaction time, reagent concentration, pH, flow rate and other measurement conditions were optimised for analysis of nitrogen dioxide in air. Preliminary studies with standardized test solutions revealed quantitative measurements of nitrite ion (NO2-), which corresponded to atmospheric nitrogen dioxide in the range of 10-80 ppbv.

Planar thin film device for capillary electrophoresis

Hollow tubular microfluidic channels were fabricated on quartz substrates using sacrificial layer, planar micromachining processes. The channels were created using a bottom-up fabrication technique, namely patterning a photoresist/aluminum sacrificial layer and depositing SiO(2) over the substrate. The photoresist/aluminum layer was removed by etching first with HCl/HNO(3), followed by etching in Nano-Strip, a more stable form of piranha (H(2)SO(4)/H(2)O(2)) stripper. Rapid separation of fluorescently labeled amino acids was performed on a device made with these channels. The fabrication process presented here provides unique control over channel composition and geometry. Future work should allow the fabrication of highly complex and precise devices with integrated analytical capabilities essential for the development of micro-total analysis systems.

Single-step quantitation of DNA in microchip electrophoresis with linear imaging UV detection and fluorescence detection through comigration with a digest

We demonstrate a convenient single-step quantitation technique for double-stranded DNA (dsDNA) fragments in polymerase chain reaction (PCR) products based on microchip capillary electrophoresis (micro-CE)/UV or fluorescence detection. PCR products of polymorphisms on the human Y-chromosome related to spermatogenic failure did not need purification. They were premixed and comigrated with a DNA digest whose concentration was known. Hydroxyethyl cellulose (HEC) dissolved in 5x Tris-borate-EDTA (5x TBE, pH 8.3) was used as a separation matrix in a linear polyacrylamide-coated quartz microchip, while mixed poly(ethyl oxides) (PEOs) of different molar-masses dissolved in 1 x TBE (pH 8.3) containing 1 ng/microl ethidium bromide was used as a separation matrix in an uncoated poly(methyl methacrylate) (PMMA) microchip. Elution profiles were monitored under either real-time linear imaging UV detection in the snapshot mode where the total separation time is fixed, or light-emitting diode (LED) confocal fluorescence detection in the finishline mode where solutes migrate over the same separation length. It is found that, in both modes, a linear relation exists between the peak areas (A) and the multiplication of the digest concentrations (C) and the fragment sizes (L) in a DNA restrictive digest. Using the comigration electropherogram of a single-step experiment, the concentrations of PCR products were directly determined using the A versus LC linear relationship. The sole condition to obey is that the chosen digest has different fragment sizes with the PCR products of interest. This condition is easy to obey, because micro-CE owns high separation ability, and many digests are commercially available. The recovery of the technique was between 98 and 105%. The R.S.D. for chip-to-chip concentration measurements was less than 6.0% (n = 6). Hence, the technique was accurate and reliable for DNA assays.

Rapid Enantioseparation of 1-Aminoindan by Microchip Electrophoresis with Linear-Imaging UV Detection

Chiral separations of 1-aminoindan (AI) by cyclodextrin electrokinetic chromatography (CDEKC) were investigated on microfluidic quartz chips. By using a microchip electrophoresis (MCE) instrument equipped with a linear-imaging UV detector, the separation process of the enantiomeric compounds was observed. When sulfated beta-cyclodextrin was employed as a chiral selector, the baseline separation of AI could be achieved within 1 min with a high repeatability. The relative standard deviation of the migration time was less than 6%. The fastest separation was achieved in 14 s, utilizing a separation length of only 6.1 mm. These results show that the MCE analysis employing a linear imaging UV detector has a significant potential for fast chiral analysis.

Separation of proteins by zone electrophoresis on-line coupled with isotachophoresis on a column-coupling chip with conductivity detection

This feasibility study deals with the separations of proteins by an on-line combination of zone electrophoresis (ZE) with isotachophoresis (ITP) on a poly(methylmethacrylate) column-coupling (CC) chip with integrated conductivity detection. ITP and ZE provided specific analytical functions while performing the cationic mode of the separation. ITP served, mainly, for concentrations of proteins and its concentrating power was beneficial in reaching a low dispersion transfer (injection) of the proteinous constituents, loaded on the CC chip in a 960 nL volume, into the ZE separation stage. This was complemented by an electrophoretically driven removal of the sample constituents migrating in front of the focused proteins from the separation system before the ZE separation. On the other hand, ZE served as a final separation (destacking) method and it was used under the separating conditions providing the resolutions and sensitive conductivity detections of the test proteins. In this way, ITP and ZE cooperatively contributed to low- or sub-microg/mL concentration detectabilities of proteins and their quantitations at 1-5 microg/mL concentrations. However, a full benefit in concentration detectabilities of proteins, expected from the use of the ITP-ZE combination, was not reached in this work. Small adsorption losses of proteins and detection disturbances in the ZE stage of separation, very likely due to trace constituents concentrated by ITP, appear to set limits in the detection of proteins in our experiments. The ITP-ZE separations were carried out in a hydrodynamically closed separation compartment of the chip with suppressed hydrodynamic and electroosmotic flows of the electrolyte solutions. Such transport conditions, minimizing fluctuations of the migration velocities of the separated constituents, undoubtedly contributed to highly reproducible migrations of the separated proteins (fluctuations of the migration time of a particular protein were typically 0.5% RSD in repeated ITP-ZE runs).

Microchip electrophoresis-based separation of DNA

Miniaturized instruments have developed very quickly in the last decade. This review is focused on the microchip electrophoresis-based separation of DNA. Fundamentals, including the chip format, substrates and fabrication technologies, fluid control, as well as various detection methods, are summarized. Array electrophoresis microchip and the on-chip integration of electrophoresis with other systems are introduced as well. In addition, the application of microchip electrophoresis in DNA sizing, genetic analysis and DNA sequencing are also presented in this paper.

Monolithic integration of optical waveguides for absorbance detection in microfabricated electrophoresis devices

The fabrication and performance of an electrophoretic separation chip with integrated optical waveguides for absorption detection is presented. The device was fabricated on a silicon substrate by standard microfabrication techniques with the use of two photolithographic mask steps. The waveguides on the device were connected to optical fibers, which enabled alignment free operation due to the absence of free-space optics. A 750 microm long U-shaped detection cell was used to facilitate longitudinal absorption detection. To minimize geometrically induced band broadening at the turn in the U-cell, tapering of the separation channel from a width of 120 down to 30 microm was employed. Electrical insulation was achieved by a 13 microm thermally grown silicon dioxide between the silicon substrate and the channels. The breakdown voltage during operation of the chip was measured to 10.6 kV. A separation of 3.2 microM rhodamine 110, 8 microM 2,7-dichlorofluorescein, 10 microM fluorescein and 18 microM 5-carboxyfluorescein was demonstrated on the device using the detection cell for absorption measurements at 488 nm.

Isotachophoresis separations of enantiomers on a planar chip with coupled separation channels

The use of a poly(methylmethacrylate) chip, provided with a pair of on-line coupled separation channels and on-column conductivity detectors, to isotachophoresis (ITP) separations of optical isomers was investigated. Single-column ITP, ITP in the tandem-coupled columns, and concentration-cascade ITP in the tandem-coupled columns were employed in this investigation using tryptophan enantiomers as model analytes. Although providing a high production rate (about 2 pmol of a pure tryptophan enantiomer separated per second), single-column ITP was found suitable only to the analysis of samples containing the enantiomers at close concentrations. A 94-mm separation path in ITP with the tandem-coupled separation channels made possible a complete resolution of a 1.5 nmol amount of the racemic mixture of the enantiomers. However, this led only to a moderate extension of the concentration range within which the enantiomers could be simultaneously quantified. The best results in this respect were achieved by using a concentration-cascade of the leading anions in the tandem-coupled separation channels. Here, a high production rate, favored in the first separation channel, was followed by the ITP migration of the enantiomers in the second channel under the electrolyte conditions enhancing their detectabilities. In dependence on the migration configuration of the enantiomers, this technique made possible their simultaneous determinations when their ratios in the loaded sample were 35:1 or less (D-tryptophan a major constituent) and 70:1 or less (L-tryptophan a major constituent).