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.

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.

Surface Modification of Silicon Pillar Arrays To Enhance Fluorescence Detection of Uranium and DNA.

There is an ever-growing need for detection methods that are both sensitive and efficient, such that reagent and sample consumption is minimized. Nanopillar arrays offer an attractive option to fill this need by virtue of their small scale in conjunction with their field enhancement intensity gains. This work investigates the use of nanopillar substrates for the detection of the uranyl ion and DNA, two analytes unalike but for their low quantum efficiencies combined with the need for high-throughput analyses. Herein, the adaptability of these platforms was explored, as methods for the successful surface immobilization of both analytes were developed and compared, resulting in a limit of detection for the uranyl ion of less than 1 ppm with a 0.2 µL sample volume. Moreover, differentiation between single-stranded and double-stranded DNA was possible, including qualitative identification between double-stranded DNA and DNA of the same sequence, but with a 10-base-pair mismatch.

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.

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.

Resonance modes, cavity field enhancements, and long-range collective photonic effects in periodic bowtie nanostructures.

The discovery of single-molecule sensitivity via surface-enhanced Raman scattering on resonantly excited noble metal nanoparticles has brought an increasing interest in its applications to the molecule detection and identification. Periodic gold bowtie nanostructures have recently been shown to give a large enhancement factor sufficient for single molecule detection. In this work, we simulate the plasmon resonance for periodic gold bowtie nanostructures. The difference between the dipole and the quadrupole resonances is described by examining the magnitude and phase of electric field, the bound surface charge, and the polarization. The gap size dependence of the field enhancement can be interpreted by considering cavity field enhancement. Also, additional enhancement is obtained through the long-range collective photonic effect when the bowtie array periodicity matches the resonance wavelength.

An integrated portable Raman sensor with nanofabricated gold bowtie array substrates for energetics detection.

An integrated field-portable surface enhanced Raman scattering (SERS) sensing system has been developed and evaluated for quantitative analysis of energetics such as perchlorate (ClO(4)(-)) and trinitrotoluene (TNT) at environmentally relevant concentrations and conditions. The detection system consists of a portable Raman spectrometer equipped with an optical fiber probe that is coupled with novel elevated gold bowtie nanostructural arrays as a sensitive and reproducible SERS substrate. Using the standard addition technique, we show that ClO(4)(-) and TNT can be quantified at concentrations as low as 0.66 mg L(-1) (or ~6.6 µM) and 0.20 mg L(-1) (~0.9 µM), respectively, in groundwater samples collected from selected military sites. This research represents the first step toward the development of a field SERS sensor which may permit rapid, in situ screening and analysis for various applications including national security, chemical, biological and environmental detection.

Free-standing optical gold bowtie nanoantenna with variable gap size for enhanced Raman spectroscopy.

We describe plasmonic interactions in suspended gold bowtie nanoantenna leading to strong electromagnetic field (E) enhancements. Surface-enhanced Raman scattering (SERS) was used to demonstrate the performance of the nanoantenna. In addition to the well-known gap size dependence, up to 2 orders of magnitude additional enhancement is observed with elevated bowties. The overall behavior is described by a SERS enhancement factor exceeding 10(11) along with an anomalously weak power law dependence of E on the gap size in a range from 8 to 50 nm that is attributed to a plasmonic nanocavity effect occurring when the plasmonic interactions enter a strongly coupled regime.

Surface-enhanced Raman spectroscopy substrates created via electron beam lithography and nanotransfer printing.

The development of quantitative, highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates requires control over size, shape, and position of metal nanoparticles. Despite the fact that SERS has gained the reputation as an information-rich spectroscopy for detection of many classes of analytes, in some isolated instances down to the single molecule detection limit, its future development depends critically on techniques for nanofabrication. Herein, an unconventional nanofabrication approach is used to produce efficient SERS substrates. Metallic nanopatterns of silver disks are transferred from a stamp onto poly(dimethysiloxane) (PDMS) to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via electron beam lithography (EBL) of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated onto the stamp to overcome the adhesive nature of the EBL resist and to function as a releasing layer. Subsequently, Ag is physically vapor deposited onto the stamp at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by scanning electron microscopy. Transferred Ag nanodisk-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. There are observed optimal conditions involving both Ag and cyclodextrin thickness. The SERS response of metallic nanodisks of various shapes and sizes on the original stamp is compared to the corresponding nTP created substrates with similar trends observed. Limits of detection for crystal violet and Mitoxantrone are approximately 10(-8) and 10(-9) M, respectively. As an innovative feature of this approach, we demonstrate that physical manipulation of the PDMS post-nTP can be used to alter morphology, e.g., to change internanodisk spacing. Additionally, stamps are shown to be reusable after the nTP process, adding the potential to scale-up regular morphology substrates by a stamp-and-repeat methodology.

Multiplexed microfluidic surface-enhanced Raman spectroscopy.

Over the past few decades, surface-enhanced Raman spectroscopy (SERS) has garnered respect as an analytical technique with significant chemical and biological applications. SERS is important for the life sciences because it can provide trace level detection, a high level of structural information, and enhanced chemical detection. However, creating and successfully implementing a sensitive, reproducible, and robust SERS active substrate continues to be a challenging task. Herein, we report a novel method for SERS that is based upon using multiplexed microfluidics (MMFs) in a polydimethylsiloxane platform to perform parallel, high throughput, and sensitive detection/identification of single or various analytes under easily manipulated conditions. A facile passive pumping method is used to deliver Ag colloids and analytes into the channels where SERS measurements are done under nondestructive flowing conditions. With this approach, SERS signal reproducibility is found to be better than 7%. Utilizing a very high numerical aperture microscope objective with a confocal-based Raman spectrometer, high sensitivity is achieved. Moreover, the long working distance of this objective coupled with an appreciable channel depth obviates normal alignment issues expected with translational multiplexing. Rapid evaluation of the effects of anion activators and the type of colloid employed on SERS performance are used to demonstrate the efficiency and applicability of the MMF approach. SERS spectra of various pesticides were also obtained. Calibration curves of crystal violet (non-resonant enhanced) and Mitoxantrone (resonant enhanced) were generated, and the major SERS bands of these analytes were observable down to concentrations in the low nM and sub-pM ranges, respectively. While conventional random morphology colloids were used in most of these studies, unique cubic nanoparticles of silver were synthesized with different sizes and studied using visible wavelength optical extinction spectrometry, scanning electron microscopy, and the MMF-SERS approach.