Centrifugal-driven, reduced-dimension, planar chromatography.

A fundamental problem with efficiency in capillary action driven planar chromatography results from diminishing flow rates as development proceeds, giving rise to molecular diffusion related band dispersion for most sample types. Overpressure and electrokinetic means to speed flow have been used successfully in TLC. We explore the use of centrifugal force (CF) to drive flow for reduced-dimension planar platforms (ultra-TLC, low micrometer features, and nano-TLC, nanoscale features). The silicon wafer platforms have two forms of continuous 2D arrays created by either photolithography or metal dewetting followed by deep reactive ion etching and coated with porous SiO2 . The flow pattern is unusual with co-planar flows above and within the arrays. The effects of parameters such as spin rate, solvent type, and surface character on flow rates is established and can be substantially greater than capillary action flow. Using fluorescent dyes, we investigate retardation factors and chromatographic plate height; the latter falls in the low to sub-micrometer range. To the best of our knowledge, we demonstrate the first analytical separations performed in pillar arrays using CF to augment solvent flow.

Evaluation of Porous Silicon Oxide on Silicon Microcantilevers for Sensitive Detection of Gaseous HF.

Sensitive detection of harmful chemicals in industrial applications is pertinent to safety. In this work, we demonstrate the use of a sensitive silicon microcantilever (MC) system with a porous silicon oxide layer deposited on the active side of the MCs that have been mechanically manipulated to increase sensitivity. Included is the evaluation of porous silicon oxide present on different geometries of MCs and exposed to varying concentrations of hydrogen fluoride in humid air. Profilometry and the signal generated by the stress-induced porous silicon oxide (PSO) coating and bending of the MC were used as methods of evaluation.

Chemical Sensor Based Upon Stress-Induced Changes in the Permeability of a Magnetoelastic Wire.

We introduce a chemical sensing technology, named ChIMES (Chemical Identification through Magneto-Elastic Sensing), that can detect a broad range of targets and that has the capability of untethered communication through a metallic or nonmetallic barrier. These features enable many applications in which penetrations into the sampled environment are unwanted or infeasible because of health, safety, or environmental concerns, such as following the decomposition of a dangerous material in a sealed container. The sensing element is passive and consists of a target response material hard-coupled to a magnetoelastic wire. When the response material encounters a target, it expands, imposing mechanical stress on the wire and altering its magnetic permeability. Using a remote excitation-detection coil set, the changes in permeability are observed by switching the magnetic domains in the wire and measuring the modifications in the Faraday voltage as the stress is varied. Sensors with different response materials can be arrayed and interrogated individually. We describe the sensor and its associated instrumentation, compare the performance of several types of wire, and evaluate analytical metrics of single and arrayed ChIMES sensors against a suite of volatile organic compounds.

Ultra-thin layer chromatography with integrated silver colloid-based SERS detection.

Simplified lab-on-a-chip techniques are desirable for quick and efficient detection of analytes of interest in the field. The following work involves the use of deterministic pillar arrays on the micro-scale as a platform to separate compounds, and the use of Ag colloid within the arrays as a source of increased signal via surface enhanced Raman spectroscopy (SERS). One problem traditionally seen with SERS surfaces containing Ag colloid is oxidation; however, our platforms are superhydrophobic, reducing the amount of oxidation taking place on the surface of the Ag colloid. This work includes the successful separation and SERS detection of a fluorescent dye compounds (resorufin and sulforhodamine 640), fluorescent anti-tumor drugs (Adriamycin and Daunomycin), and purine and pyrimidine bases (adenine, cytosine, guanine, hypoxanthine, and thymine).

Retention in Porous Layer Pillar Array Planar Separation Platforms.

This work presents the retention capabilities and surface area enhancement of highly ordered, high-aspect-ratio, open-platform, two-dimensional (2D) pillar arrays when coated with a thin layer of porous silicon oxide (PSO). Photolithographically prepared pillar arrays were coated with 50-250 nm of PSO via plasma-enhanced chemical vapor deposition and then functionalized with either octadecyltrichlorosilane or n-butyldimethylchlorosilane. Theoretical calculations indicate that a 50 nm layer of PSO increases the surface area of a pillar nearly 120-fold. Retention capabilities were tested by observing capillary-action-driven development under various conditions, as well as by running one-dimensional separations on varying thicknesses of PSO. Increasing the thickness of PSO on an array clearly resulted in greater retention of the analyte(s) in question in both experiments. In culmination, a two-dimensional separation of fluorescently derivatized amines was performed to further demonstrate the capabilities of these fabricated platforms.

Manipulating the inter pillar gap in pillar array ultra-thin layer planar chromatography platforms.

An advantage of separation platforms based on deterministic micro- and nano-fabrications, relative to traditional systems based on packed beds of particles, is the exquisite control of all morphological parameters. For example, with planar platforms based on lithographically-prepared pillar arrays, the size, shape, height, geometric arrangement, and inter pillar gaps can be independently adjusted. Since the inter pillar gap is expected to be important in determining resistance to mass transfer in the mobile phase as well as the flow rate, which influences the mass transfer effect and axial diffusion, we herein study the effect of reducing inter pillar gaps on capillary action-based flow and band dispersion. Atomic layer deposition is used to narrow the gap between the pillars for photo-lithographically defined pillar arrays. The plate height of gap-adjusted arrays is modeled based on predicted and observed flow rates. A reduction in the flow rate with smaller gaps hinders the efficiency in the modeled case and is correlated with actual separations. A conclusion is drawn that simultaneously reducing both the gap and the pillar diameter is the best approach in terms of improving the chromatographic efficiency.

Nanopillar Based Enhanced-Fluorescence Detection of Surface-Immobilized Beryllium.

The unique properties associated with beryllium metal ensures the continued use in many industries despite the documented health and environmental risks. While engineered safeguards and personal protective equipment can reduce risks associated with working with the metal, it has been mandated by the Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA) that the workplace air and surfaces must be monitored for toxic levels. While many methods have been developed to monitor levels down to the low µg/m(3), the complexity and expense of these methods have driven the investigation into alternate methodologies. Herein, we use a combination of the previously developed fluorescence Be(II) ion detection reagent, 10-hydroxybenzo[h]quinoline (HBQ), with an optical field enhanced silicon nanopillar array, creating a new surface immobilized (si-HBQ) platform. The si-HBQ platform allows the positive control of the reagent for demonstrated reusability and a pillar diameter based tunable enhancement. Furthermore, native silicon nanopillars are overcoated with thin layers of porous silicon oxide to develop an analytical platform capable of a 0.0006 µg/L limit of detection (LOD) using sub-µL sample volumes. Additionally, we demonstrate a method to multiplex the introduction of the sample to the platform, with minimal 5.2% relative standard deviation (RSD) at 0.1 µg/L, to accommodate the potentially large number of samples needed to maintain industrial compliance. The minimal sample and reagent volumes and lack of complex and highly specific instrumentation, as well as positive control and reusability of traditionally consumable reagents, create a platform that is accessible and economically advantageous.

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.

Superhydrophobic analyte concentration utilizing colloid-pillar array SERS substrates.

The ability to detect a few molecules present in a large sample is of great interest for the detection of trace components in both medicinal and environmental samples. Surface enhanced Raman spectroscopy (SERS) is a technique that can be utilized to detect molecules at very low absolute numbers. However, detection at trace concentration levels in real samples requires properly designed delivery and detection systems. The following work involves superhydrophobic surfaces that have as a framework deterministic or stochastic silicon pillar arrays formed by lithographic or metal dewetting protocols, respectively. In order to generate the necessary plasmonic substrate for SERS detection, simple and flow stable Ag colloid was added to the functionalized pillar array system via soaking. Native pillars and pillars with hydrophobic modification are used. The pillars provide a means to concentrate analyte via superhydrophobic droplet evaporation effects. A ≥ 100-fold concentration of analyte was estimated, with a limit of detection of 2.9 × 10(-12) M for mitoxantrone dihydrochloride. Additionally, analytes were delivered to the surface via a multiplex approach in order to demonstrate an ability to control droplet size and placement for scaled-up uses in real world applications. Finally, a concentration process involving transport and sequestration based on surface treatment selective wicking is demonstrated.

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.

Surface enhanced Raman scattering imaging of developed thin-layer chromatography plates.

A method for hyphenating surface enhanced Raman scattering (SERS) and thin-layer chromatography (TLC) is presented that employs silver-polymer nanocomposites as an interface. Through the process of conformal blotting, analytes are transferred from TLC plates to nanocomposite films before being imaged via SERS. A procedure leading to maximum blotting efficiency was established by investigating various parameters such as time, pressure, and type and amount of blotting solvent. Additionally, limits of detection were established for test analytes malachite green isothiocyanate, 4-aminothiophenol, and Rhodamine 6G (Rh6G) ranging from 10(-7) to 10(-6) M. Band broadening due to blotting was minimal (∼10%) as examined by comparing the spatial extent of TLC-spotted Rh6G via fluorescence and then the SERS-based spot size on the nanocomposite after the blotting process. Finally, a separation of the test analytes was carried out on a TLC plate followed by blotting and the acquisition of distance × wavenumber × intensity three-dimensional TLC-SERS plots.

Silicon nanopillars as a platform for enhanced fluorescence analysis.

The importance of fluorescent detection in many fields is well established. While advancements in instrumentation and the development of brighter fluorophore have increased sensitivity and lowered the detection limits of the method, additional gains can be made by manipulating the local electromagnetic field. Herein we take advantage of silicon nanopillars that exhibit optical resonances and field enhancement on their surfaces and demonstrate their potential in improving performance of biomolecular fluorescent assays. We use electron beam lithography and wafer scale processes to create silicon nanoscale pillars with dimensions that can be tuned to maximize fluorescence enhancement in a particular spectral region. Performance of the nanopillar based fluorescent assay was quantified using two model bioaffinity systems (biotin-streptavidin and immunoglobulin G-antibody) as well as covalent binding of fluorescently tagged bovine serum albumin (BSA). The effects of pillar geometry and number of pillars in arrays were evaluated. Color specific and pillar diameter dependent enhancement of fluorescent signals is clearly demonstrated using green and red labels (FITC, DyLight 488, Alexa 568, and Alexa 596). The ratios of the on pillar to off pillar signals normalized by the nominal increase in surface area due to nanopillars were found to be 43, 75, and 292 for the IgG-antibody assay, streptavidin-biotin system, and covalently attached BSA, respectively. Applicability of the presented approaches to the detection of small numbers of molecules was evaluated using highly diluted labeled proteins and also control experiments without biospecific analytes. Our analysis indicates that detection of fewer than 10 tagged proteins is possible.

Lithography-free approach to highly efficient, scalable SERS substrates based on disordered clusters of disc-on-pillar structures.

We present a lithography-free technological strategy that enables fabrication of large area substrates for surface-enhanced Raman spectroscopy (SERS) with excellent performance in the red to NIR spectral range. Our approach takes advantage of metal dewetting as a facile means to create stochastic arrays of circular patterns suitable for subsequent fabrication of plasmonic disc-on-pillar (DOP) structures using a combination of anisotropic reactive ion etching (RIE) and thin film deposition. Consistent with our previous studies of individual DOP structures, pillar height which, in turn, is defined by the RIE processing time, has a dramatic effect on the SERS performance of stochastic arrays of DOP structures. Our computational analysis of model DOP systems confirms the strong effect of the pillar height and also explains the broadband sensitivity of the implemented SERS substrates. Our Raman mapping data combined with SEM structural analysis of the substrates exposed to benzenethiol solutions indicates that clustering of shorter DOP structures and bundling of taller ones is a likely mechanism contributing to higher SERS activity. Nonetheless, bundled DOP structures appeared to be consistently less SERS-active than vertically aligned clusters of DOPs with optimized parameters. The latter are characterized by average SERS enhancement factors above 10(7).

Surface enhanced Raman spectroscopy for microfluidic pillar arrayed separation chips.

Numerous studies have addressed the challenges of implementing miniaturized microfluidic platforms for chemical and biological separation applications. However, the integration of real time detection schemes capable of providing valuable sample information under continuous, ultra low volume flow regimes has not fully been addressed. In this report we present a chip based chromatography system comprising of a pillar array separation column followed by a reagent channel for passive mixing of a silver colloidal solution into the eluent stream to enable surface enhanced Raman spectroscopy (SERS) detection. Our design is the first integrated chip based microfluidic device to combine pressure driven separation capability with real time SERS detection. With this approach we demonstrate the ability to collect distinctive SERS spectra with or without complete resolution of chromatographic bands. Computational fluidic dynamic (CFD) simulations are used to model the diffusive mixing behaviour and velocity profiles of the two confluent streams in the microfluidic channels. We evaluate the SERS spectral band intensity and chromatographic efficiency of model analytes with respect to kinetic factors as well as signal acquisition rates. Additionally, we discuss the use of a pluronic modified silver colloidal solution as a means of eliminating contamination generally caused by nanoparticle adhesion to channel surfaces.

Nanotransfer printing using plasma etched silicon stamps and mediated by in situ deposited fluoropolymer.

This communication describes a simple method that uses a thin film of octafluorocyclobutane (OFCB) polymer for efficient nanoscale transfer printing (nTP). Plasma polymerization of OFCB produces a Teflon-like fluoropolymer which strongly adheres and conformally covers a 3-D inorganic stamp. The inherently low surface energy of in situ deposited OFCB polymer on nanoscale silicon features is demonstrated as a unique nanocomposite stamp to fabricate various test structures with improved nTP feature resolution down to sub-100 nm.

Enclosed pillar arrays integrated on a fluidic platform for on-chip separations and analysis.

Due to the difficulty of reliably producing sealed 3-D structures, few researchers have tackled the challenges of creating pillar beds suitable for miniaturized liquid phase separation systems. Herein, we describe an original processing sequence for the fabrication of enclosed pillar arrays integrated on a fluidic chip which, we believe, will further stimulate interest in this field. Our approach yields a mechanically robust enclosed pillar system that withstands mechanical impacts commonly incurred during processing, sealing and operation, resulting in a design particularly suitable for the research environment. A combination of a wafer-level fabrication sequence with chip-level elastomer bonding allows for chip reusability, an attractive and cost efficient advancement for research applications. The characteristic features in the implemented highly ordered pillar arrays are scalable to submicron dimensions. The proposed fluidic structures are suitable for handling picolitre sample volumes and offer prospects for substantial improvements in separation efficiency and permeability over traditional packed and monolithic columns. Our experimental observations indicate plate heights as low as 0.76 microm for a 10 mm long pillar bed. Theoretical calculations confirm that ordered pillar arrays with submicron pore sizes combine superior analysis speed, picolitre sample volumes, high permeability and reasonably large plate numbers on a small footprint. In addition, we describe a fluidic interface that provides streamlined coupling of the fabricated structures with off-chip fluidic components.

Aluminum oxide nanostructured microcantilever arrays for nanomechanical-based sensing.

Novel nanostructured microcantilever (MC) surfaces were developed by modifying the active side of the MCs with aluminum oxide nanoparticles (AONP) for purposes of enhancing sensitivity in nanomechanical-based sensing. Uniform layers of AONP were spin coated and chemically immobilized on the surfaces of MCs with tetramethoxysilane (TMOS) as a cross-linker. Optimization studies on MC modification were performed for better surface uniformity and higher surface area, based on scanning electron microscope (SEM) images. The AONP-modified MC array (MCA) were subsequently functionalized by being immersed in parallel configured capillaries filled with different reagents for immobilizing chemical or biological receptors onto the MC surfaces. A MCA prepared for chemical sensing was exposed to the samples made of headspace vapor of different volatile organic compounds (VOCs). The characteristic response signatures for each gas phase VOC analyte showed substantial diversity. Immersion time in the capillary and the chemical nature of the reagents used for functionalization were both optimized to achieve the highest sensitivity and long-term reproducibility in nanomechanical responses to the test analytes. A second MCA functionalized with two different immunological receptors was prepared and exposed to three biological analytes in the liquid phase, with a highly selective response obtained for each analyte. Fluorescence microscope images and FT-IR spectra were used in this work to validate the controlled, variable chemical nature of the MC surfaces.

High-aspect-ratio, silicon oxide-enclosed pillar structures in microfluidic liquid chromatography.

The present paper discusses the ability to separate chemical species using high-aspect-ratio, silicon oxide-enclosed pillar arrays. These miniaturized chromatographic systems require smaller sample volumes, experience less flow resistance, and generate superior separation efficiency over traditional packed bed liquid chromatographic columns, improvements controlled by the increased order and decreased pore size of the systems. In our distinctive fabrication sequence, plasma-enhanced chemical vapor deposition (PECVD) of silicon oxide is used to alter the surface and structural properties of the pillars for facile surface modification while improving the pillar mechanical stability and increasing surface area. The separation behavior of model compounds within our pillar systems indicated an unexpected hydrophobic-like separation mechanism. The effects of organic modifier, ionic concentration, and pressure-driven flow rate were studied. A decrease in the organic content of the mobile phase increased peak resolution while detrimentally effecting peak shape. A resolution of 4.7 (RSD = 3.7%) was obtained for nearly perfect Gaussian shaped peaks, exhibiting plate heights as low as 1.1 and 1.8 µm for fluorescein and sulforhodamine B, respectively. Contact angle measurements and DART mass spectrometry analysis indicate that our employed elastomeric soft bonding technique modifies pillar properties, creating a fortuitous stationary phase. This discovery provides evidence supporting the ability to easily functionalize PECVD oxide surfaces by gas-phase reactions.

Landfill siloxane gas sensing using differentiating, responsive phase coated microcantilever arrays.

Landfill biogases are being utilized more frequently as a new source of fuel energy. Volatile siloxane compounds usually contained in landfill biogases will form siloxane residues when the gases are burned, which significantly increases abrasion of combustion engines. Research on detection of siloxanes in landfill gas has been active during recent years with the principal analytical technique being gas chromatography/mass spectrometry (GC/MS). In our present work, we introduce a less expensive, compact methodology that employs microcantilever (MC) arrays for sensitive nanomechanical-based gas-phase sensing of the siloxanes. The cantilevers on the MC array were differentially coated on the active, nanostructured side with different responsive phases, and composite responses (magnitude of siloxane-induced MC bending) for four siloxanes were collected that exhibited selective signatures to aid in recognizing each siloxane. Limits of detection (LODs) derived from linear calibration plots were down to the sub-parts-per-million range, a sensitivity that is comparable with that of GC/MS reported by other researchers. Studies were performed in rather inert helium environment and a realistic matrix, and the overall response profiles and LODs were similar for both matrixes. A 5 week long-term reproducibility study illustrates the stability of the MC array. Moreover, the portable character of the MC array setup makes our method a very promising way to facilitate in-field detection of siloxanes in landfill gas in the future.

Characterization and detection of uranyl ion sorption on silver surfaces using surface enhanced Raman spectroscopy.

The study of the chemical behavior of uranyl species and its rapid detection is of primary environmental and nonproliferation concern. Herein, we report on a surface enhanced Raman spectroscopic study of uranyl ion (UO(2)(2+)) sorption onto the thermally vapor deposited silver particle surface. The ability of vibrational spectroscopy to characterize surface phenomenon and the remarkable sensitivity of surface enhanced Raman spectroscopy (SERS) have been introduced as an appropriate combination for the surface characterization and detection of UO(2)(2+) onto the silver surface. The appearance of symmetric stretching frequency of UO(2)(2+) around 700 cm(-1) and the disappearance of the 854 cm(-1) band are attributed to the development of a chemical bond between silver surface and uranyl species. The effects of temperature, solute-surface interaction time, and pH have been studied using silver modified polypropylene filter (PPF) substrates. Results show that under appropriate conditions, the concentration of uranyl ion as low as 20 ng/mL can be easily detected using the discussed SERS approach without any surface modification of silver nanoparticles. Moreover, an alternative SERS approach of uranyl detection is demonstrated using nanolithographically fabricated SERS substrates.