Highly Stable Surface Functionalization of Microgas Chromatography Columns Using Layer-by-Layer Self-Assembly of Silica Nanoparticles

Abstract

A controllable and high-yield surface functionalization of silicon microchannels using layer-by-layer (LbL) self-assembly of SiO 2 nanoparticles (SNPs) is presented. The application of SNPs (45 nm average diameter) coating as a stationary phase for chromatographic separation is also demonstrated with surface functionalization using chloroalkylsilanes. This method facilitates a simple, low-cost, and parallel processing scheme that also provides homogeneous and stable nanoparticle-based stationary phases with ease of control over the coating thickness. The SNP-functionalized microfabricated columns with either single capillary channels (1 m long, 150 μm wide, 240 μm deep) or very narrow multicapillary channels (25 cm long, 30 μm wide, 240 μm deep, 16 parallel channels) successfully separated a multicomponent gas mixture with a wide range of boiling points with high reproducibility. B ecause of recent advancements and the emergence of microelectromechanical systems (MEMS), energy-efficient integrated microgas chromatography (μGC) systems have attracted considerable attention. This system, upon complete realization, could expand the range of applications for real-time and rapid on-site analysis at a lower cost.1−10 As a key component in μGC systems, conventional meters-long separation columns are necessarily replaced with microfabricated silicon columns on the order of a few square centimeters. The separation ability and efficiency of the columns are directly related to the quality of the stationary phase. Using conventional coating techniques on microfabricated columns (e.g., dynamic or static coatings of functionalized polysiloxane polymers), especially for highaspect-ratio (HAR) rectangular-shaped capillary channels, present major challenges toward obtaining stable, reproducible, and uniform coatings. 11There have been progressive research efforts toward the development of coating techniques that yield chemically inert, thermally stable, selective, and robust stationary phases. Specifically, nanotechnology-based phases have opened up countless prospects for applications in conventional as well as microfabricated separation columns with nanoscale controllability, simplicity, and flexibility.12,13 Recently, carbon nanotubes 14,15 and thiol-encapsulated gold nanoparticles 16 have been utilized for gas chromatography (GC) separations. Our group has reported on the capability of thiol monolayers on electrodeposited gold surfaces as a stationary phase for microfabricated columns by combining nanofabrication and microfabrication techniques. 17,18The excellent adsorption properties of silica for organics has long been demonstrated as a stationary phase medium for conventional columns (silica gel, Celite) 19 and, more recently, by employing the MEMS-compatible sputtering technique for microfabricated columns.20 SiO 2 nanoparticles (SNPs), because of their small and uniform size, high surface area, chemical inertness, and thermal stability, are excellent candidates for stationary phases. SNPs have also been dispe...

Width-Modulated Microfluidic Columns for Gas Separations

“…In the present work, this process is terminated after 10 bilayers of PAH and SNPs. Similar to our earlier work for very narrow channel-width (20 µm) multicapillary columns [25], the original concentration of SNPs suspension is reduced to a third in order to achieve a uniform film thickness. At the end, the devices are thoroughly purged by passing DI water and dry nitrogen at low pressure for 15 minutes.…”

“…Next, dry nitrogen and deionized (DI) water are passed through the devices to remove any dirt gathered during dicing. The complete details of layer-by-layer (LbL) coating of SNPs can be found in an earlier report [25]. Briefly, the pH values of polyallylamine hydrochloride (PAH) solution (Sigma-Aldrich) and SNPs suspension (Nissan Chemical) are first adjusted to 7.0 (±0.1) and 9.0 (±0.1), respectively, by adding HCl and NaOH solutions respectively.…”

“…Additionally, innovative research work on deposition of nontraditional adsorbent based stationary phase coatings for rectangular microchannels has also been performed. This includes the integration of carbon nanotubes [23], self-assembled thiol monolayers [20], [24], silica nanoparticles [25] and MEMS compatible sputtered silica [26] thin-films inside the anisotropically etched channels.…”

Zebra GC: A fully integrated micro gas chromatography system

“…These components have been previously developed by our research group [18,[26][27][28][29][30] and the following sections briefly describe the fabrication process, schematically shown in Figure 3 …”

Optical sensor for pH monitoring using a layer-by-layer deposition technique emphasizing enhanced stability and re-usability

“…Prakash et al [21] discussed applying nanoparticles to achieve an adequate sensitivity and stability with the modification of the sensors (or biosensors) with nanomaterials such as gold and/or silver nanoparticles [6,22,23], carbon nanomaterials [24,25] and silica nanoparticles [26] and these have shown considerable promise. Putzbach et al reported that the immobilization of enzymes improves stability of the biosensor discussed [27].…”