Hamza Shakeel

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

A controllable and high-yield surface functionalization of silicon microchannels using layer-by-layer (LbL) self-assembly of SiO2 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.

Self-Patterned Gold-Electroplated Multicapillary Gas Separation Columns With MPG Stationary Phases

Gas chromatography (GC) is a powerful analytical method to accurately and reliably identify the constituents of a complex mixture. There are numerous efforts to miniaturize GC system, in general, and separation columns, in particular, for rapid, dependable, and portable on-site analysis. This paper reports the development of two diverse self-patterned gold electrodeposition fabrication techniques for high-aspect-ratio microfluidic channels including multicapillary GC columns. First approach involves geometry-dependent tuned electroplating conditions to self-pattern gold along the vertical sidewalls without any deposition on horizontal top and bottom surfaces, while the second method provides highly conformal gold deposition inside the 3-D microchannels. Both reported approaches do not require a postdeposition patterning step while affording at the same time excellent bonding and stationary phase coating yields. The ability of thiol to self-assemble on gold surface is also utilized to form monolayer-protected gold (MPG) surfaces and is used as a stationary phase for micro GC. To evaluate the chromatographic performance of both schemes, 250-μm -deep 30-μm-wide 25-cm-long microfabricated multicapillary columns (μMCCs) with 16 channels are functionalized by self-assembly of octadecanethiol (C18H37 SH) to form the MPG surface. With about 7300-plate/m theoretical plates, these columns demonstrate the highest reported separation efficiency on 16-channel μMCCs and are capable of separating complex gas mixtures containing compounds with wide range of boiling points.

Width-Modulated Microfluidic Columns for Gas Separations

Microgas chromatography separation columns typically employ channels with fixed cross sections. In this paper, we demonstrate a new class of unidirectional microfabricated width-modulated columns (μWMC) that afford improved chromatographic efficiency, resolution, and retention times compared with corresponding constant width (120 μm) bidirectional columns. Two new μWMCs architectures are introduced: a linearly variable column (LVC) and a stepgradient column (SGC). The width of a 1-m long, 250-μm-deep LVC is gradually reduced from 120 to 20 μm at 1 μm/cm. While that of a 1-m-long SGC is modulated in five steps (120, 95, 70, 45, and 20 μm) each with a length of 20 cm. The effect of inlet selection (120 or 20 μm) on chromatographic performance is also evaluated. Moreover, with our improved fabrication process, multiple serially connected devices are simultaneously coated for the first time with highly stable silica nanoparticles utilizing layer-by-layer technique enabling constant film thickness.

Semipacked separation columns with monolayer protected gold stationary phases for microgas chromatography

Semi-packed columns (SPC), as a new class of gas chromatography columns, offer several advantages like increased surface area and sample capacity compared to open columns and the elimination of the differential flows observed in multicapillary columns. We present herein for the first time SPCs with monolayer protected gold stationary phases by utilizing the ability of octadecanethiol (C18H37SH) to self-assemble on deposited gold layers. We employed two selective gold deposition approaches followed by thiol functionalization; 1) our previously reported high-yield double-doped self-patterned process and 2) e-beam evaporation of gold followed by the lift-off process. By utilizing both schemes, we produced 1m-long, 250μm-deep, 190μm-wide semipacked columns with circular micropillars of 20 μm-diameters and 20 μm-post spacing. Both approaches afford the separation of complex gas mixtures with wide range of boiling points. However, the shadowing effect inherent in e-beam evaporation specifically for high-aspect-ratio structures results in incomplete gold coverage and limits its application in separation columns with microposts.

High-performance multicapillary gas separation columns with MPG stationary phases

This paper reports the development of microfabricated multicapillary columns (μMCC) with self-assembled stationary phases for high-performance gas separation. The fabrication process merging a double-doped MEMS fabrication process and nanotechnology provides highly reproducible and conformal gold deposition inside 250µm-deep, 30µm-wide, 25cm-long µMCC with 16 channels. This technique also affords excellent bonding and coating yields (∼95%). To evaluate chromatographic performance, μMCCs were functionalized by self-assembly of octadecanethiol (C18H37SH) to form monolayer-protected-gold (MPG) surface. These columns demonstrated 7300 plates/m, the highest reported on μMCCs, and separated a complex gas mixture containing compounds with wide range of boiling points.

Analysis of thickness and quality factor of a double paddle oscillator at room temperature

In this paper, we evaluate the quality (Q) factor and the resonance frequency of a double paddle oscillator (DPO) with different thickness using analytical, computational and experimental methods. The study is carried out for the 2nd anti-symmetric resonance mode that provides extremely high experimental Q factors on the order of 105. The results show that both the Q factor and the resonance frequency of a DPO increase with the thickness at room temperature.

Width-modulated microgas chromatography separation columns with silica nanoparticles stationary phase

Microgas chromatography separation columns typically employ channels with fixed cross sections. In this article, we demonstrate unidirectional microfabricated width-modulated columns (μWMC) that afford improved chromatographic efficiency, resolution and retention times compared to corresponding constant width bidirectional columns. The width of 1m-long, 250 μm-deep μWMC is gradually reduced from 120 μm to 20 μm at 1 μm/cm. The effect of inlet selection (120 μm or 20 μm) on chromatographic performance is also evaluated. Moreover, with our improved fabrication process, multiple serially connected devices are simultaneously coated for the first time with highly-stable silica nanoparticles utilizing layer-by-layer technique thus enabling constant film thicknesses.

Measurements of enthalpy of sublimation of Ne, N2, O2, Ar, CO2, Kr, Xe, and H2O using a double paddle oscillator

We report precise experimental values of the enthalpy of sublimation (ΔHs ) of quenched condensed films of neon (Ne), nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2), krypton (Kr), xenon (Xe), and water (H2O) vapor using a single consistent measurement platform. The experiments are performed well below the triple point temperature of each gas and fall in the temperature range where existing experimental data is very limited. A 6 cm2 and 400 µm thick double paddle oscillator (DPO) with high quality factor (Q ≈ 4 × 105 at 298K) and high frequency stability (33 parts per billion) is utilized for the measurements. The enthalpies of sublimation are derived by measuring the rate of mass loss during temperature programmed desorption. The mass change is detected due to change in the resonance frequency of the self-tracking oscillator. Our measurements typically remain within 10% of the available literature, theory, and National Institute of Standards and Technology (NIST) Web Thermo Tables (WTT) values, but are performed using an internally consistent method across different gases.

An integrated chromatographychip for rapid gas separation and detection

This paper reports the first implementation of a highly sensitive micro discharge photoionization detector (μDPID) in a silicon-glass architecture and its monolithic integration with a high performance micro separation column. The new detector requires a two-mask fabrication process, is universal, non-destructive, low power (<;2.5mW), and insensitive to flow and temperature variations. It has yielded a minimum detection limit of ~10pg which is on par with the widely used destructive flame ionization detector. The integrated chip comprises a newly developed semi-packed column with atomic layer deposited (ALD) stationary phases and is capable of multi-analyte gas mixture separation and their trace level identification while allowing the use of temperature programming for enhanced separation performance and speed.