Nanotechnology and chip level systems for pressure driven liquid chromatography and emerging analytical separation techniques: A review

Liquid phase detection in the miniature scale. Microfluidic and capillary scale measurement and separation systems. A tutorial review

Microfluidic and capillary devices are increasingly being used in analytical applications while their overall size keeps decreasing. Detection sensitivity for these microdevices gains more importance as device sizes and consequently, sample volumes, decrease. This paper reviews optical, electrochemical, electrical, and mass spectrometric detection methods that are applicable to capillary scale and microfluidic devices, with brief introduction to the principles in each case. Much of this is considered in the context of separations. We do consider theoretical aspects of separations by open tubular liquid chromatography, arguably the most potentially fertile area of separations that has been left fallow largely because of lack of scale-appropriate detection methods. We also examine the theoretical basis of zone electrophoretic separations. Optical detection methods discussed include UV/Vis absorbance, fluorescence, chemiluminescence and refractometry. Amperometry is essentially the only electrochemical detection method used in microsystems. Suppressed conductance and especially contactless conductivity (admittance) detection are in wide use for the detection of ionic analytes. Microfluidic devices, integrated to various mass spectrometers, including ESI-MS, APCI-MS, and MALDI-MS are discussed. We consider the advantages and disadvantages of each detection method and compare the best reported limits of detection in as uniform a format as the available information allows. While this review pays more attention to recent developments, our primary focus has been on the novelty and ingenuity of the approach, regardless of when it was first proposed, as long as it can be potentially relevant to miniature platforms.

[Recent advances in microchip liquid chromatography]

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Fast analysis using pillar array columns: Quantification of branched-chain α-keto acids in human plasma samples

Branched-chain α-keto acids (BCKAs, namely, α-ketoisovaleric acid (KIV), α-ketoisocaproic acid (KIC), and α-keto-β-methylvaleric acid (KMV)) are related to many diseases such as myeloid leukemia, liver cancer, and diabetes mellitus. A rapid quantitative analytical method for BCKAs using pillar array columns was developed. α-Keto acids were labeled with 1,2-diamino-4,5-methylenedioxybenzene (DMB), followed by their separation on octadecylsilane-treated pillar array columns with MeOH/H₂O as the mobile phase. Five DMB-labelled α-keto acids including the internal standard were separated in 160 s. The lower limits of quantification for DMB-α-keto acids were 2-5 μM. The intra- and interday precisions were 2.9-6.6 % and 5.2-10.7 %, respectively. The developed method was applied to BCKA quantification in human plasma samples; KIV, KIC, and KMV concentrations were determined to be 13.8, 24.2, and 15.2 μM, respectively. The method realized rapid, sensitive, and precise analysis of BCKAs and can be applied for clinical diagnosis.

Heat Transport of Electrokinetic Flow in Slit Soft Nanochannels

Soft nanochannels are defined as nanochannels with a polyelectrolyte layer (PEL) on the rigid walls. In the present study, the thermal transport properties of the fluids through slit soft nanochannels are investigated under the combined influences of pressure-driven and streaming potential. Based on the analytical solutions of electric potential and velocity distributions, a dimensionless temperature of electrolyte solution in soft nanochannels is obtained by resolving the energy equation. Then, a finite difference method is used to compute the energy equation and test the validity of the analytical solution. Results show that the temperature increases with the decrease of dimensionless velocity and the heat transfer rate for rigid nanochannel are higher than that for the soft one. Moreover, we find the total entropy generation decreases with the increases of the ratio K_λ of the electrical double layer (EDL) thickness in PEL to the EDL thickness on the solid wall.

Chemical Separation on Silver Nanorods Surface Monitored by TOF-SIMS

The article introduces a possible chemical separation of a mixture of two compounds on the metal nanorods surface. A silver nanorods surface has been prepared by controlled electrochemical deposition in anodic alumina oxide (AAO) template. Rhodamine 6G and 4-aminothiophenol have been directly applied to the sampling point on a silver nanorods surface in an aliquot mixture. The position of the resolved compounds was analysed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) which measured the fragments and the molecular ions of the two compounds separated on the silver nanorods surface. Rhodamine 6G has been preconcentrated as 1.5 mm radial from the sampling point while 4-aminothiophenol formed a continuous self-assembled monolayer on the silver nanorods surface with a maximum molecular ion intensity at a distance of 0.5 mm from the sampling point. The separation of the single chemical components from the two-component mixture over the examined silver nanostructured films could clearly be shown. A fast separation on the mentioned nanotextured films was observed (within 50 s). This procedure can be easily integrated into the micro/nanofluidic systems or chips and different detection systems can be applied.

Advances in and prospects of microchip liquid chromatography

This review focuses on recent advances in the field of microfluidic liquid chromatography from January 2013 through April 2015. Articles are organized by the type of stationary phase support focusing on device fabrication, column preparation, and use for specific applications. Additionally, a comprehensive table comparing chromatographic figures of merit for the work described is included as Appendix A as a reference for readers.

Evaluation of 5 µm Superficially Porous Particles for Capillary and Microfluidic LC Columns

Large-size (4–5 µm) superficially porous particles yield lower plate heights (e.g., the minimal reduced plate height or h_min ≈ 1.5) than fully porous particles of a similar size when packed into large-bore columns. This property allows for better chromatographic performance without the higher pressures required for smaller particles. This study explores the use of such particles in microfluidic LC columns where materials and fitting pressure limits can constrain the size of particle used. The theoretically predicted performance improvements compared to fully porous particles were not demonstrated in capillary columns (with h_min ≈ 2 for both particle types), in agreement with previous studies that examined smaller superficially porous particles. Microfluidic columns were then compared to capillary columns. Capillary columns significantly outperformed microfluidic columns due to imperfections imposed by microfluidic channel asymmetry and world-to-chip connection at the optimal flow rate; however, superficially porous particles packed in microfluidic LC columns had flatter plate height versus flow rate curves indicating potential for better performance at high reduced velocities.

Modular microfluidics for point-of-care protein purifications

Biochemical separations are the heart of diagnostic assays and purification methods for biologics. On-chip miniaturization and modularization of separation procedures will enable the development of customized, portable devices for personalized health-care diagnostics and point-of-use production of treatments. In this report, we describe the design and fabrication of miniature ion exchange, size exclusion and affinity chromatography modules for on-chip clean-up of recombinantly-produced proteins. Our results demonstrate that these common separations techniques can be implemented in microfluidic modules with performance comparable to conventional approaches. We introduce embedded 3-D microfluidic interconnects for integrating micro-scale separation modules that can be arranged and reconfigured to suit a variety of fluidic operations or biochemical processes. We demonstrate the utility of the modular approach with a platform for the enrichment of enhanced green fluorescent protein (eGFP) from Escherichia coli lysate through integrated affinity and size-exclusion chromatography modules.

Nanoscale pillar arrays for separations

The work presented herein evaluates silicon nano-pillar arrays for use in planar chromatography. Electron beam lithography and metal thermal dewetting protocols were used to create nano-thin layer chromatography platforms. With these fabrication methods we are able to reduce the size of the characteristic features in a separation medium below that used in ultra-thin layer chromatography; i.e. pillar heights are 1-2 μm and pillar diameters are typically in the 200-400 nm range. In addition to the intrinsic nanoscale aspects of the systems, it is shown they can be further functionalized with nanoporous layers and traditional stationary phases for chromatography; hence exhibit broad-ranging lab-on-a-chip and point-of-care potential. Because of an inherent high permeability and very small effective mass transfer distance between pillars, chromatographic efficiency can be very high but is enhanced herein by stacking during development and focusing while drying, yielding plate heights in the nm range separated band volumes. Practical separations of fluorescent dyes, fluorescently derivatized amines, and anti-tumor drugs are illustrated.

Wicking nanopillar arrays with dual roughness for selective transport and fluorescence measurements

Silicon nanopillars are important building elements for innovative nanoscale systems with unique optical, wetting, and chemical separation functionalities. However, technologies for creating expansive pillars arrays on the submicron scale are often complex and with practical time, cost, and method limitations. Herein we demonstrate the rapid fabrication of nanopillar arrays using the thermal dewetting of Pt films with thicknesses in the range from 5 to 19 nm followed by anisotropic reactive ion etching (RIE) of the substrate materials. A second level of roughness on the sub-30 nm scale is added by overcoating the silicon nanopillars with a conformal layer of porous silicon oxide (PSO) using room temperature plasma enhanced chemical vapor deposition (PECVD). This technique produced environmentally conscious, economically feasible, expansive nanopillar arrays with a production pathway scalable to industrial demands. The arrays were systematically analyzed for size, density, and variability of the pillar dimensions. We show that these stochastic arrays exhibit rapid wicking of various fluids and, when functionalized with a physiosorbed layer of silicone oil, act as a superhydrophobic surface. We also demonstrate high brightness fluorescence and selective transport of model dye compounds on surfaces of the implemented nanopillar arrays with two-tier roughness. The demonstrated combination of functionalities creates a platform with attributes inherently important for advanced separations and chemical analysis.

The application of a new microfluidic device for the simultaneous identification and quantitation of midazolam metabolites obtained from a single micro-litre of chimeric mice blood

RATIONALE

Improvements in the design of low-flow highly sensitive chromatographic ion source interfaces allow the detection and characterisation of drugs and metabolites from smaller sample volumes. This in turn improves the ethical treatment of animals by reducing both the number of animals needed and the blood sampling volumes required.

METHODS

A new microfluidic device combining an ultra-high pressure liquid chromatography (UHPLC) analytical column with a nano-flow electrospray source is described. All microfluidic, gas and electrical connections are automatically engaged when the ceramic microfluidic device is inserted into the source enclosure. The system was used in conjunction with a hybrid quadrupole-time-of-flight mass spectrometer.

RESULTS

The improved sensitivity of the system is highlighted in its application in the quantification and qualification of midazolam and its metabolites detected in whole blood from chimeric and wild-type mice. Metabolite identification and full pharmacokinetic profiles were obtained from a single micro-litre of whole blood at each sampling time and significant pharmacokinetic differences were observed between the two types of mice.

CONCLUSIONS

Improvements in the enhanced ionisation efficiency from the microfluidic device in conjunction with nanoUHPLC/MS was sufficiently sensitive for the identification and quantification of midazolam metabolites from a single micro-litre of whole blood. Detection of metabolites not previously recorded from the chimeric mouse in vivo model was made.

Highly ordered silicon pillar arrays as platforms for planar chromatography

Unlike HPLC, there has been sparse advancement in the stationary phases used for planar chromatography. Nevertheless, modernization of planar chromatography platforms can further highlight the technique's ability to separate multiple samples simultaneously, utilize orthogonal separation formats, image (detect) separations without rigorous temporal demands, and its overall simplicity. This paper describes the fabrication and evaluation of ordered pillar arrays that are chemically modified for planar chromatography and inspected by fluorescence microscopy to detect solvent development and analyte bands (spots). Photolithography, in combination with anisotropic deep reactive ion etching, is used to produce uniform high aspect ratio silicon pillars. The pillar heights, diameters, and pitch variations are approximately 15-20 μm, 1-3 μm, and 2-6 μm, respectively, with the total pillar array size typically 1 cm × 3 cm. The arrays are imaged using scanning electron microscopy in order to measure the pillar diameter and pitch as well as analyze the pillar sidewalls after etching and stationary phase functionalization. These fluidic arrays will enable exploration of the impact on mass transport and chromatographic efficiency caused by altering the pillar array morphology. A C18 reverse stationary phase (RP), common RP solvents that are transported by traditional but uniquely rapid capillary flow, and Rhodamine 6G (R6G) as the preliminary analyte are used for this initial evaluation. The research presented in this article is aimed at understanding and overcoming the unique challenges in developing and utilizing ordered pillar arrays as a new platform for planar chromatography: focusing on fabrication of expansive arrays, studies of solvent transport, methods to create compatible sample spots, and an initial evaluation of band dispersion.

Micro flow reactor chips with integrated luminescent chemosensors for spatially resolved on-line chemical reaction monitoring

Real-time chemical reaction monitoring in microfluidic environments is demonstrated using luminescent chemical sensors integrated in PDMS/glass-based microscale reactors. A fabrication procedure is presented that allows for straightforward integration of thin polymer layers with optical sensing functionality in microchannels of glass-PDMS chips of only 150 μm width and of 10 to 35 μm height. Sensor layers consisting of polystyrene and an oxygen-sensitive platinum porphyrin probe with film thicknesses of about 0.5 to 4 μm were generated by combining spin coating and abrasion techniques. Optimal coating procedures were developed and evaluated. The chip-integrated sensor layers were calibrated and investigated with respect to stability, reproducibility and response times. These microchips allowed observation of dissolved oxygen concentration in the range of 0 to over 40 mg L(-1) with a detection limit of 368 μg L(-1). The sensor layers were then used for observation of a model reaction, the oxidation of sulphite to sulphate in a microfluidic chemical reactor and could observe sulphite concentrations of less than 200 μM. Real-time on-line monitoring of this chemical reaction was realized at a fluorescence microscope setup with 405 nm LED excitation and CCD camera detection.

Exploring new scaling regimes for streaming potential and electroviscous effects in a nanocapillary with overlapping electric double layers

In this paper, we unravel new scaling regimes for streaming potential and electroviscous effects in a nanocapillary with thick overlapping Electric Double Layers (EDLs). We observe that the streaming potential, for a given value of the capillary zeta (ζ) potential, varies with the EDL thickness and a dimensionless parameter R, quantifying the conduction current. Depending on the value of R, variation of the streaming potential with the EDL thickness demonstrates distinct scaling regimes: one can witness a Quadratic Regime where the streaming potential varies as the square of the EDL thickness, a Weak Regime where the streaming potential shows a weaker variation with the EDL thickness, and a Saturation Regime where the streaming potential ceases to vary with the EDL thickness. Effective viscosity, characterizing the electroviscous effect, obeys the variation of the streaming potential for smaller EDL thickness values; however, for larger EDL thickness the electroosmotic flow profile dictates the electroviscous effect, with insignificant contribution of the streaming potential.

Recent developments in microfluidic chip-based separation devices coupled to MS for bioanalysis

In recent years, the development of microfluidic chip separation devices coupled to MS has dramatically increased for high-throughput bioanalysis. In this review, advances in different types of microfluidic chip separation devices, such as electrophoresis- and LC-based microchips, as well as 2D design of microfluidic chip-based separation devices will be discussed. In addition, the utilization of chip-based separation devices coupled to MS for analyzing peptides/proteins, glycans, drug metabolites and biomarkers for various bioanalytical applications will be evaluated.

A microfluidic device for preparing next generation DNA sequencing libraries and for automating other laboratory protocols that require one or more column chromatography steps

Library preparation for next-generation DNA sequencing (NGS) remains a key bottleneck in the sequencing process which can be relieved through improved automation and miniaturization. We describe a microfluidic device for automating laboratory protocols that require one or more column chromatography steps and demonstrate its utility for preparing Next Generation sequencing libraries for the Illumina and Ion Torrent platforms. Sixteen different libraries can be generated simultaneously with significantly reduced reagent cost and hands-on time compared to manual library preparation. Using an appropriate column matrix and buffers, size selection can be performed on-chip following end-repair, dA tailing, and linker ligation, so that the libraries eluted from the chip are ready for sequencing. The core architecture of the device ensures uniform, reproducible column packing without user supervision and accommodates multiple routine protocol steps in any sequence, such as reagent mixing and incubation; column packing, loading, washing, elution, and regeneration; capture of eluted material for use as a substrate in a later step of the protocol; and removal of one column matrix so that two or more column matrices with different functional properties can be used in the same protocol. The microfluidic device is mounted on a plastic carrier so that reagents and products can be aliquoted and recovered using standard pipettors and liquid handling robots. The carrier-mounted device is operated using a benchtop controller that seals and operates the device with programmable temperature control, eliminating any requirement for the user to manually attach tubing or connectors. In addition to NGS library preparation, the device and controller are suitable for automating other time-consuming and error-prone laboratory protocols requiring column chromatography steps, such as chromatin immunoprecipitation.

Fast and quantitative analysis of branched-chain amino acids in biological samples using a pillar array column

In this study, a fast and quantitative determination method for branched-chain amino acids (BCAAs), namely leucine, isoleucine, and valine, was developed using a pillar array column. A pillar array column with low-dispersion turns was fabricated on a 20 × 20-mm(2) microchip using multistep ultraviolet photolithography and deep reactive ion etching. The BCAAs were fluorescently labeled with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), followed by reversed-phase separation on the pillar array column. The NBD derivatives of the three BCAAs and an internal standard (6-aminocaproic acid) were separated in 100 s. The calibration curves for the NBD-BCAAs had good linearity in the range of 0.4-20 μM, using an internal standard. The intra- and interday precisions were found to be in the ranges of 1.42-3.80 and 2.74-6.97%, respectively. The accuracies for the NBD-BCAA were from 90.2 to 99.1%. The method was used for the analysis of sports drink and human plasma samples. The concentrations of BCAAs determined by the developed method showed good agreements with those determined using a conventional high-performance liquid chromatography system. As BCAAs are important biomarkers of some diseases, these results showed that the developed method could be a potential diagnostic tool in clinical research.

Amperometric noise at thin film band electrodes

Background current noise is often a significant limitation when using constant-potential amperometry for biosensor application such as amperometric recordings of transmitter release from single cells through exocytosis. In this paper, we fabricated thin-film electrodes of gold and conductive polymers and measured the current noise in physiological buffer solution for a wide range of different electrode areas. The noise measurements could be modeled by an analytical expression, representing the electrochemical cell as a resistor and capacitor in series. The studies revealed three domains; for electrodes with low capacitance, the amplifier noise dominated, for electrodes with large capacitances, the noise from the resistance of the electrochemical cell was dominant, while in the intermediate region, the current noise scaled with electrode capacitance. The experimental results and the model presented here can be used for choosing an electrode material and dimensions and when designing chip-based devices for low-noise current measurements.

Microfluidic chip-based liquid chromatography coupled to mass spectrometry for determination of small molecules in bioanalytical applications

The development and integration of microfabricated liquid chromatography (LC) microchips have increased dramatically in the last decade due to the needs of enhanced sensitivity and rapid analysis as well as the rising concern on reducing environmental impacts of chemicals used in various types of chemical and biochemical analyses. Recent development of microfluidic chip-based LC mass spectrometry (chip-based LC-MS) has played an important role in proteomic research for high throughput analysis. To date, the use of chip-based LC-MS for determination of small molecules, such as biomarkers, active pharmaceutical ingredients (APIs), and drugs of abuse and their metabolites, in clinical and pharmaceutical applications has not been thoroughly investigated. This mini-review summarizes the utilization of commercial chip-based LC-MS systems for determination of small molecules in bioanalytical applications, including drug metabolites and disease/tumor-associated biomarkers in clinical samples as well as adsorption, distribution, metabolism, and excretion studies of APIs in drug discovery and development. The different types of commercial chip-based interfaces for LC-MS analysis are discussed first and followed by applications of chip-based LC-MS on biological samples as well as the comparison with other LC-MS techniques.

Chip-based separation devices coupled to mass spectrometry

The hyphenation of miniaturized separation techniques like chip electrophoresis or chip chromatography to mass spectrometry (MS) is a highly active research area in modern separation science. Such methods are particularly attractive for comprehensive analysis of complex biological samples. They can handle extremely low sample amounts, with low solvent consumption. Furthermore they provide unsurpassed analysis speed together with the prospect of integrating several functional elements on a single multifunctional platform. In this article we review the latest developments in this emerging field of technology and summarize recent trends to face current and future challenges in chip-based biochemical analysis.

Integration of pillar array columns into a gradient elution system for pressure-driven liquid chromatography

A gradient elution system for pressure-driven liquid chromatography (LC) on a chip was developed for carrying out faster and more efficient chemical analyses. Through computational fluid dynamics simulations and an experimental study, we found that the use of a cross-Tesla structure with a 3 mm mixing length was effective for mixing two liquids. A gradient elution system using a cross-Tesla mixer was fabricated on a 20 mm × 20 mm silicon chip with a separation channel of pillar array columns and a sample injection channel. A mixed solution of water and fluorescein in methanol was delivered to the separation channel 7 s after the gradient program had been started. Then, the fluorescence intensity increased gradually with the increasing ratio of fluorescein, which showed that the gradient elution worked well. Under the gradient elution condition, the retention times of two coumarin dyes decreased with the gradient time. When the gradient time was 30 s, the analysis could be completed in 30 s, which was only half the time required compared to that required for an isocratic elution. Fluorescent derivatives of aliphatic amines were successfully separated within 110 s. The results show that the proposed system is promising for the analyses of complex biological samples.

Silicon nanopillars for field-enhanced surface spectroscopy

Silicon nanowire and nanopillar structures have drawn increased attention in recent years due in part to their unique optical properties. Herein, electron beam lithography combined with reactive-ion etching is used to reproducibly create individual silicon nanopillars of various sizes, shapes, and heights. Finite difference time domain analysis predicts local field intensity enhancements in the vicinity of appropriately sized and coaxially illuminated silicon nanopillars of approximately 2 orders of magnitude. While this level of enhancement is modest when compared to plasmonic systems, the unique advantage of the silicon nanopillar resonators is that they enhance optical fields in substantially larger volumes. By analyzing experimentally measured strength of the silicon Raman phonon line (500 cm(-1)), it was determined that nanopillars produced local field enhancements that are consistent with these predictions. Additionally, we demonstrate that a thin layer of Zn phthalocyanine on the nanopillar surface with a total amount of <30 attomoles produced prominent Raman spectra, yielding enhancement factors (EFs) better than 2 orders of magnitude. Finally, silicon nanopillars of cylindrical and elliptical shapes were labeled with different fluorophors and evaluated for their surface-enhanced fluorescence (SEF) capability. The EFs derived from analysis of the acquired fluorescence microscopy images indicate that silicon nanopillar structures can provide enhancements comparable or even stronger than those typically achieved using plasmonic SEF structures without the limitations of the metal-based substrates, such as fluorescence quenching and an insufficiently large probe volume. It is anticipated that dense arrays of silicon nanopillars will enable SEF assays with extremely high sensitivity, while a broader impact of the reported phenomena is anticipated in photovoltaics, subwavelength light focusing, and fundamental nanophotonics.

A microfabricated micropillar liquid chromatographic chip monolithically integrated with an electrospray ionization tip

We present the first monolithically integrated silicon/glass liquid chromatography-electrospray ionization microchip for mass spectrometry. The microchip is fabricated by bonding a silicon wafer, which has deep reactive ion etched micropillar-filled channels, together with a glass lid. Both the silicon channel and the glass lid have a through-wafer etched sharp tip that produces a stable electrospray. The microchip is also compatible with laser induced fluorescence (LIF) detection, due to the glass lid. Separation of drugs in less than 5 minutes using either SiO(2) (normal phase) or C(18) coated (reversed-phase) pillars with good sensitivity was demonstrated with mass spectrometric detection as well as separation of fluorescent compounds with LIF detection.