Highly Ordered Silicon Pillar Arrays As Platforms for Planar Chromatography

Abstract

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.

Advances in and prospects of microchip liquid chromatography

“…Silicon wafers with 1-3 μm diameter, 15-20 μm tall cylindrical pillars in a 1 cm × 3 cm array (spacing of 2-6 μm between pillars), modified with a C18 reversed phase coating, were tested for efficiency as thin-layer chromatography (TLC) plates [64,65]. Compared to commercial TLC plates, chromatographic efficiency was ~3-5 times better for the pillar array [64]. To further improve performance, pillar dimension were reduced to 0.2-0.4 μm (diameter) by 1-2 μm (height) to which aided in stacking and focusing effects that reduced band broadening.…”

Centrifugal‐driven, reduced‐dimension, planar chromatography

“…Separation experiments were performed with samples composed of laser dyes consisting of Coumarin 540 (Lambda Physik, MA, USA, 1 × 10 −5 M), Coumarin 440 (Sigma–Aldrich, MO, USA, 1 × 10 −5 M), Sulforhodamine 640 (Exciton, OH, USA, 1 × 10 −6 M) in 60% methanol 40% water. Samples were applied to the pillar arrays using a 5 μL HPLC syringe and a CCD camera via a contact transfer spotting method . As with the mobile‐phase velocity comparisons, the mobile phase was delivered using a syringe pump and a capillary tube.…”

“…However, most efforts produced a creek‐like flow development. The four sample spots around the centroid of the array took advantage of the superhydrophobic behavior of the arrays and were performed as described previously . At times, the sample spots could be <300 μm in diameter.…”

“…Our previous work with fabricated silicon pillar arrays, another stationary phase for UTLC, has shown improved efficiency by replacing the relatively polydisperse and heterogeneous packing particles in traditional structures with periodic pillar arrays fabricated via photolithography (PL) . It was demonstrated that plate height (H) is improved due to the lack of eddy diffusion, less resistance to mass transfer in the mobile phase with smaller pillar diameters and interpillar gaps, as well as less molecular diffusion band broadening due to greater permeability.…”

“…More recently, we have reduced the scale of the planar chromatographic platforms to nanodimensions using electron beam lithography and metal dewetting (DW) fabrication methods . With both the UTLC pillar arrays and the nanoscale cases the flow is considerably faster than for traditional TLC, however efficiency is still limited during most of the development by molecular diffusion . Hence, we explore the use of centrifugal force (CF), a forced‐flow technique, to augment flow in reduced‐dimension planar chromatography.…”