Diamond microchips for fast chromatography of proteins

Abstract

As a material for bioanalysis polycrystalline diamond surfaces have attractive characteristics, such as the highest thermal conductivity, optical transparency, chemical inertness, and by choice, electrical insulation or semiconduction. In addition, diamond has the smallest lattice constant of all solid materials, its permeability is excellent, and it only allows slow diffusion of the small ions Li + and B +. This paper is one step in making diamond microfluidic systems, e.g. chromatography. To study the wetting properties of diamond surfaces, the normally hydrophobic diamond was dry- and wet-oxidised to become hydrophilic. It was possible to tailor the contact angle to a chosen value between 5 and 65°. After a week of ageing, the contact angle of the surface of a hydrophilic diamond surface’s has changed from 5 to just below 25° where it stabilises. Using a silicon-based microengineering technology, we are able to make relatively long and narrow diamond capillaries. To investigate the limitations of sacrificial silicon etching, when enclosed by diamond, we opened up capillaries with cross-section areas of 2000 μm 2. It was then possible to etch the silicon deeper than 30 mm. Using the same technique, microchips with 40 mm straight columns (cross-section areas of 4000 and 7000 μm 2, respectively) for chromatography were fabricated. The channels of the diamond chips were filled with continuous polymer beds and used for anion-exchange chromatography of proteins. A sample of myoglobin (horse) conalbumin, ovalbumin (chicken), and trypsin inhibitor (soybean) was separated in about 30 s and a sample of hemoglobin A 0 and β-lactoglobulin A was separated in less than 20 s. The preparation of the continuous columns is quite straightforward, since an aqueous solution of selected monomers is simply pressed into the chip channel. Following polymerisation, a mechanically rigid polymeric stationary phase is formed. The continuous bed was found to be uniformly polymerised, also close to the inner wall, and the resolution of the chromatograms were comparable with separations on continuous beds synthesised in both fused-silica capillaries and quartz microchips.