Chia Jung Lu

Multiple-stage microfabricated preconcentrator-focuser for micro gas chromatography system

The design, fabrication, and characterization of a multiple-stage Si microfabricated preconcentrator-focuser (/spl mu/PCF) for a micro gas chromatography (/spl mu/GC) system that can provide real-time quantification and identification of complex organic vapor mixtures are presented. The /spl mu/PCF consists of a Si microheater loaded with Carbopack B, Carbopack X, and Carboxen 1000 carbon adsorbent granules, and a Si micromachined cover plate. Deep reactive ion etching is utilized to produce mechanically robust fluidic interconnection adapters hermetically sealed to fused silica capillary tubing for connection to the other components in the /spl mu/GC. This three-stage device is designed to capture compounds spanning up to 4 orders of magnitude in volatility. The dead volume, thermal mass, heating efficiency, and pressure drop of the three-stage /spl mu/PCF are improved significantly over its single-stage /spl mu/PCF predecessor. We demonstrate the successful capture, desorption, and high-resolution chromatographic separation of a mixture of 30 common organic vapors using our three-stage /spl mu/PCF in a conventional GC system. The peak width at half height is <2.05 s for all compounds after elution from the GC column.

Microfabricated preconcentrator-focuser for a microscale gas chromatograph

The design, fabrication, and testing of a preconcentrator-focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph. The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 /spl mu/m (h)/spl times/50 /spl mu/m (w)/spl times/3000 /spl mu/m (l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, /spl sim/200 /spl mu/m in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly.

Organic vapour sensing using localized surface plasmon resonance spectrum of metallic nanoparticles self assemble monolayer

The response of localized surface plasmon resonance (LSPR) spectra of gold and silver nanoparticles, and gold nanoshells to organic vapors was investigated. The surface area of nanomaterials was sufficiently high for quantitative adsorption of volatile organic compounds (VOCs). Surface adsorption and condensation of VOCs caused the environmental refractive index to increase from n=1.00 in pure air to as high as n=1.29 in near saturated toluene vapor. The extinction and wavelength shift of the LSPR spectra were very sensitive to changes in the surface refractive index of the nanoparticles. Responses of the LSPR band were measured with a real-time UV-vis spectrometer equipped with a CCD array detector. The response of silver nanoparticles to organic vapors was most sensitive in changes in extinction, while gold nanoshells exhibited red-shifts in wavelength ( approximately 250nm/RIU) when exposed to organic vapors. The LSPR spectral shifts primarily were determined by the volatility and refractive indices of the organic species. The T(90) response time of the VOC-LSPR spectrum was less than 3s and the response was completely reversible and reproducible.

A vapor sensor array using multiple localized surface plasmon resonance bands in a single UV–vis spectrum

This research reports a sensor array consisting of three types of surface-modified nanoparticles that exhibit localized surface plasmon resonance (LSPR) extinctions at different wavelength regions of a UV-vis spectrum. By simultaneously measuring the ensemble of the LSPR bands of the three nano-materials, response patterns were obtained at different regions of a UV-vis spectrum. Three types of nano-metals used in this study were Ag nanoparticles, Au nanoparticles and Au nano-shells. The center wavelengths of their LSPR bands were 427, 534 and 772nm respectively, after they were each modified with decanethiol, naphthalene thiol and 2-mercaptobenzothiazole to create chemical selectivity. The average absolute difference in the absorbance of each LSPR band was used as the signal for each sensor. Reversible, rapid (approximately 8s) and wide linear range responses were observed with this sensor array. Nine volatile organic compounds (VOCs) with various functional groups were tested with this sensor array and differentiable groups of response patterns were obtained. The detection limits were as low as 16ppm for anisole.

A micro GC detector array based on chemiresistors employing various surface functionalized monolayer-protected gold nanoparticles

Aspects of the design, fabrication, and characterization of a chemiresistor type of microdetector for use in conjunction with gas chromatograph are described. The detector was manufactured on silicon chips using microelectromechanical systems (MEMS) technology. Detection was based on measuring changes in resistance across a film comprised of monolayer-protected gold nanoclusters (MPCs). When chromatographic separated molecules entered the detector cell, the MPC film absorbed vapor and undergoes swelling, then the resistance changes accordingly. Thiolates were used as ligand shells to encapsulate the nano-gold core and to manipulate the selectivity of the detector array. The dimensions of the μ-detector array were 14(L)×3.9(W)×1.2(H)mm. Mixtures of eight volatile organic compounds with different functional groups and volatility were tested to characterize the selectivity of the μ-detector array. The detector responses were rapid, reversible, and linear for all of the tested compounds. The detection limits ranged from 2 to 111ng, and were related to both the compound volatility and the selectivity of the surface ligands on the gold nanoparticles. Design and operation parameters such as flow rate, detector temperature, and width of the micro-fluidic channel were investigated. Reduction of the detector temperature resulted in improved sensitivity due to increased absorption. When a wider flow channel was used, the signal-to-noise ratio was improved due to the larger sensing area. The extremely low power consumption and small size makes this μ-detector array potentially useful for the development of integrated μ-GC.

A novel micropreconcentrator employing a laminar flow patterned heater for micro gas chromatography

A simple micromachined process based on one photomask is developed for a novel micropreconcentrator (µPCT) used in a micro gas chromatograph (µGC). Unique thick silver heating microstructures with a high surface area for microheater of µPCT are fabricated by combining the microfluidic laminar flow technique and the Tollens’ reaction within a microchannel. Silver deposition using this laminar flow patterning technique provides a higher deposition rate and easier microfabrication compared to conventional micromachined technologies for thick metal microstructures (>200 µm). An amorphous and porous carbon film that functions as an adsorbent is grown on microheaters inside the microchannel. The µPCT can be heated to >300 °C rapidly by applying a constant electrical power of ∼1 W with a heating rate of 10 °C s−1. Four volatile organic compounds, acetone, benzene, toluene and xylene, are collected through the proposed novel µPCTs and separated successfully using a 17 m long gas chromatography column. The peak widths at half height (PWHHs) of the four compounds are relatively narrow ( 38 000 for benzene and toluene.

A preconcentrator chip employing μ-SPME array coated with in-situ-synthesized carbon adsorbent film for VOCs analysis

We report the design, fabrication, and evaluation of a μ-preconcentrator chip that utilizes an array of solid-phase microextraction (SPME) needles coated with in-situ-grown carbon adsorbent film. The structure of the SPME needle (diameter=100 μm, height=250 μm) array inside the sampling chamber was fabricated using a deep reactive-ion etching (DRIE) process to enhance the attachable surface area for adsorbent film. Heaters and temperature sensors were fabricated onto the back of a μ-preconcentrator chip using lithography patterning and a metal lift-off process. The devices were sealed by anodic bonding and diced prior to the application of the adsorbent film. An adsorbent precursor, cellulose was dissolved in water and dynamically coated onto the SPME needle array. The coated cellulose film was converted into a porous carbon film via pyrolysis at 600 °C in a N(2) atmosphere. The surface area of the carbon adsorbent film was 308 m(2)/g, which is higher than that of a commercial adsorbent Carbopack X. A preconcentration factor as high as 13,637-fold was demonstrated using toluene. Eleven volatile organic compounds (VOCs) of different volatilities and functional groups were sampled and analyzed by GC-FID, and the desorption peak widths at half height were all less than 2.6 s after elution from a 15m capillary GC column. There was no sign of performance degradation after continuous operation for 50 cycles in air.

A multidimensional micro gas chromatograph employing a parallel separation multi-column chip and stop-flow μGC × μGCs configuration

A dual-chip, multidimensional micro gas chromatographic module was designed, built and evaluated. Column chips were fabricated on a silicon wafer with an etched rectangular channel 100 μm (width) × 250 μm (depth) using a deep reactive ion etching (DRIE) process. The column chip for the first GC dimension was 3 m long and was coated with polydimethylsiloxane (DB-1) as the stationary phase. The columns on the second dimensional chip were etched with the same width and depth as the first chip, but the flow channel was split into three parallel columns, 1 m long, on the same sized silicon chip (i.e., 3 cm × 3 cm). These three parallel columns on the second chip were coated with polyethylene oxide (DB-Wax), trifluoropropylpolymethylsilicone (OV-210) and cyanopropylmethylphenylmethylpolysilicone (OV-225), accordingly, in order to provide diversified chromatographic retention. These two chips were connected via a stop-flow configuration to simultaneously generate multiple two-dimensional gas chromatograms for every analysis. This stop-flow μGC × μGCs design allowed the first column to function as a pre-separator and as a sequencing injector for the second parallel-separation chip. Fifteen volatile organic compounds with boiling points that ranged from 80-131 °C with various functional groups were tested using this μGC × μGCs module. Three discrete 2-D chromatograms were generated simultaneously, which demonstrated the advantages of simultaneously combining GC × GC with parallel separation GCs in microchip chromatography. The total traveling length in the column was only 4 m for each eluted peak and fully resolved separation was achieved through the cross reference among triplet 2-D chromatograms.

A vapor response mechanism study of surface-modified single-walled carbon nanotubes coated chemiresistors and quartz crystal microbalance sensor arrays

This paper compares the selectivity and discusses the response mechanisms of various surface-modified, single-walled carbon nanotube (SWCNT)-coated sensor arrays for the detection of volatile organic compounds (VOCs). Two types of sensor platforms, chemiresistor and quartz crystal microbalance (QCM), were used to probe the resistance changes and absorption masses during vapor sensing. Four sensing materials were used in this comparison study: pristine, acidified, esterified, and surfactant (sodium dodecyl sulfate, SDS)-coated SWCNTs. SWCNT-coated QCMs reached the response equilibrium faster than the chemiresistors did, which revealed a delay diffusion behavior at the inter-tube junction. In addition, the calibration lines for QCMs were all linear, but the chemiresistors showed curvature calibration lines which indicated less effectiveness of swelling at high concentrations. While the sorption of vapor molecules caused an increase in the resistance for most SWCNTs due to the swelling, the acidified SWCNTs showed no responses to nonpolar vapors and a negative response to hydrogen bond acceptors. This discovery provided insight into the inter-tube interlocks and conductivity modulation of acidified SWCNTs via a hydrogen bond. The results in this study provide a stepping-stone for further understanding of the mechanisms behind the vapor selectivity of surface-modified SWCNT sensor arrays.

Novel Gas Chromatographic Detector Utilizing the Localized Surface Plasmon Resonance of a Gold Nanoparticle Monolayer inside a Glass Capillary

This paper presents the design, assembly, and evaluation of a novel gas chromatographic detector intended to measure the absorbance of the localized surface plasmon resonance (LSPR) of a gold nanoparticle monolayer in response to eluted samples from a capillary column. Gold nanoparticles were chemically immobilized on the inner wall of a glass capillary (i.d. 0.8 mm, length = 5-15 cm). The eluted samples flowed through the glass capillary and were adsorbed onto a gold nanoparticle surface, which resulted in changes in the LSPR absorbance. The LSPR probing light source used a green light-emitting diode (LED; λ(center) = 520 nm), and the light traveled through the glass wall of the capillary with multiple total reflections. The changes in the light intensity were measured by a photodiode at the rear of the glass capillary. The sensitivity of this detector can be improved by using a longer spiral glass capillary. The detector is more sensitive when operated at a lower temperature and at a slower carrier velocity. The calibration lines of 8 preliminary test compounds were all linear (R(2) > 0.99). The detection limits (3σ) ranged from 22 ng (n-butanol) to 174 ng (2-pentanone) depending on the volatility of the chemicals and the affinity to the citrate lignads attached to the gold nanoparticle surface. This detector consumed a very low amount of energy and could be operated with an air carrier gas, which makes this detector a promising option for portable GC or μGC.