MEMS-Based Gas Chromatography Columns with Nano-Structured Stationary Phases Bassam Alfeeli, Syed Ali, Vaibhav Jain+, Reza Montazami++, James Heflin*, and Masoud Agah# VT MEMS Laboratory, Electrical and Computer Engineering + Macromolecular Science and Engineering ++ Materials Science and Engineering *Department of Physics Virginia Polytechnic Institute and State University Blacksburg, VA, USA # correspondence:
[email protected] to separate volatiles of interest in a gas mixture based on their boiling points or their polarities.
Abstract—Gas chromatography columns produced using microelectromechanical systems (MEMS) technology makes it possible to achieve very narrow widths as it improves the separation efficiency. However, coating these columns to obtain a uniform stationary phase is challenging using traditional techniques. This paper presents an important step to address this issue by merging MEMS and nanotechnology and reports, for the first time, MEMS-based columns coated with functionalized gold nanoparticles (FGNPs) by a modified layer-by-layer (LbL) deposition technique. The FGNPs were made stable by coating the etched silicon surface with a thin layer of silicon nitride. These films are of particular interest for GC as they provide a homogeneous phase with nanometer resolution and a fast mass transfer rate. The 1m-long MEMS column with FGNP stationary phase was successful in separating a mixture of 6 alkanes in less than 6 minutes.
I.
There are two conventional methods for the production of wall-coated open-tubular fused silica columns, static and dynamic coating. In static coating, the entire column is filled with the stationary phase solution and then the solvent is evacuated leaving behind a thin polymer layer on the wall of column. In dynamic coating, a plug of stationary phase solution is pushed through the column by the flow of a nonreactive gas. After the plug is expelled, the excess solvent is evaporated by continued gas flow [4]. These coating techniques suitable for capillary columns are unable to produce homogenous films on MEMS rectangular columns especially as their width is decreased to achieve higher separation efficiencies. This paper presents an important step to address this issue by merging MEMS and nanotechnology and reports, for the first time, MEMS-based columns coated with functionalized gold nanoparticles (FGNPs) by a modified layer-by-layer (LbL) deposition technique.
INTRODUCTION
Gas chromatography (GC) is a powerful analytical technique used for the analysis of complex gas mixtures in breath diagnostics, environmental monitoring, and forensics. In GC analysis, the components of a gas mixture are separated, identified, and their concentrations quantified. Although conventional GC systems are known to deliver accurate analyses, their size, fragility, cost, and power consumption limits their direct use in field analysis [1].
II.
μGC COLUMN DEVELOPMENT
A. MEMS Fabrication The fabrication process starts with spinning and patterning an 8µm-thick layer of photoresist (PR9260). The pattern was then etched using deep reactive ion etching (DRIE) to form 1m-long, 150µm-wide, 240µm-deep separation columns. Using plasma enhanced chemical vapor deposition (PECVD), a 250nm layer of silicon nitride was deposited in the etched channels. The wafer was then coated with the nanostructured stationary phase as explained in the next section. The columns were sealed by bonding the silicon wafer to a Pyrex glass wafer at 1250V and 250°C. The wafer was diced into individual devices. Each device
Microfabricated GC columns [2, 3] consume less power, have fast analysis times, and are suitable for field-use applications making them a promising technology for lab-onchip systems. These columns are undergoing a steady evolution resulting in continuous improvements in their performance. Microfabricated columns typically comprise of high-aspect-ratio rectangular channels etched in silicon and laid out in circular or square-spiral configuration. The channels are capped with a Pyrex substrate using anodic bonding. The columns are then coated by a stationary phase This work was supported by the U.S. National Science Foundation under award ECCS 0601456
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IEEE SENSORS 2008 Conference
was connected on both sides with 200μm-OD, 100μm-ID deactivated fused silica tubing using a high temperature silica based bonding agent. Fig. 1 summarizes the major steps in the fabrications process and Fig. 2 shows scanning electron microscopy images of an etched column at different angles. B. Nanotechnology Coating The LbL assembly method is a well known technique which allows detailed structural and thickness control at the molecular level [5]. The recent application of nanoparticles in the field of separation science has received much attention for use as a stationary phase material for fast separations [6, 7]. The capability of FGNPs to form self-assembled monolayers (SAM) along with their processability and suspensibility in organic solvents makes them a good candidate to form stationary phases for tubular gas chromatography [8, 9]. However, this is the first time that the FGNPs have been used as a possible stationary phase for MEMS- based columns.
Figure 2. SEM images of an etched column at different angles
The channels were coated according to the process illustrated in Fig. 3. The wafer was washed with NaOH to charge the surface with OH-groups. This was followed by treatment with 2mM solution of aminopropyltrimethoxysilane (APTMS) in ethanol. GNPs 20nm in diameter were attached to the amine group of the APTMS by immersing the wafer in a GNP solution for 8-10 hours. The final step in coating involved attaching the alkane group to GNPs by introducing 2mM solution of octadecylthiol (C18H35SH,) in hexane and bonding the GNP to the thiol functional group of the alkane.
Figure 1. Process flow for the fabrication of the MEMS columns
Figure 3. Deposition method for the FGNPs monolayer
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III.
Fig. 5 shows chromatograms from 6-component test mixture listed in Table I along with their boiling points. The column was able to separate the components at both temperature ramps, 10˚C/min and 30˚C/min, in under 12 minutes and 6 minutes respectively. Integrating heaters and sensors with these columns and using fast temperature programming can considerably reduce the analysis times for complex gas mixtures. A blank MEMS column tested under identical conditions did not show significant analyte retention.
RESULTS AND DISCUSSION
A. Coating Stability The stability of the GNPs was tested on three different kinds of surfaces, bare silicon, PECVD oxide, and PECVD nitride. After depositing the GNPs, the columns were heated to 250˚C while maintaining a constant flow of nitrogen gas over them. It was observed that the nanoparticles tend to agglomerate on the silicon and oxide surfaces after the heat treatment as shown in Fig. 4. This detrimental phenomenon was not observed on the silicon nitride surface in which the GNPs were found to be the most stable. The difference in the packing density of the silanes on silicon-nitride, as compared to bare silicon and PECVD oxide surfaces, is likely to result in a more stable stationary phase.
IV.
CONCLUSIONS
MEMS based GC column technology could make sophisticated, easy-to-use, and portable chemical detection systems possible. One of the main challenges in this technology is coating conformability and stability. Merging MEMS and nanotechnology is one possible way to overcome such challenge. FGNPs have been shown to be a good prospect as a stationary phase for rectangular MEMS column. It has been found that FGNPs are most stable on PECVD silicon nitride surfaces compared to bare silicon and PECVD silicon oxide surfaces. This phenomenon could be attributed to the difference in the packing density of the silanes on siliconnitride.
Figure 5. Separation of alkanes achieved under two different temperature ramps
Figure 4. Images of FGNPs before and after heating on (a) Bare Silicon, (b) PECVD silicon oxide, and (c) PECVD silicon nitride, red circles indicate agglomeration of FGNPs
TABLE I.
B. Separation Performance All the testing was performed using a conventional HP5890 GC oven with an HP7673 autosampler. The MEMS columns were connected to the GC oven using the capillary tubing. The columns were first conditioned at 150˚C for one hour. After conditioning, the columns were injected with 1µL mixtures of straight chained alkanes at a split ratio of 200:1and at 35˚C. The column temperature was then ramped at 10˚C/min for one test and at 30˚C/min for another test to confirm the results.
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COMPONENT LIST USED FOR COLUMN TESTING
Label
Compound
C8 C9 C10 C12 C14 C16
n-Octane n-Nonane n-Decane n-Dodecane n-Tetradecane n-Hexadecane
Boiling point (°C) 125.52 151 174.1 216.2 253.5 287
Studies are underway to improve the performance of the coating by employing different processing techniques and to characterize the efficiency and capacity of this new class of μGC stationary phases.
[4]
ACKNOWLEDGMENT
[5]
The authors thank Prof. Larry T. Taylor and Dr. Mehdi Ashraf-Khorassani of the Department of Chemistry at Virginia Tech for their technical assistance. This work was performed in part at the MicrON Semiconductor Fabrication Facilities and the Nanoscale Characterization and Fabrication Laboratory, both at Virginia Tech.
[6]
[7] [8]
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