Apertured impedance microchips: Surface modification and evaluation using high performance liquid chromatography

The present study evaluates the performance and potential utility of apertured impedance microchips (AIMs). The electronics-based, dielectric constant detection devices feature a novel layered architecture with aluminum metal/silicon-oxide/silicon layers, where the top metal layer has apertures and the middle oxide layer has wells. This layered yet open geometry enables molecules to have access to detection of electric fields induced by a voltage applied between the top metal/bottom silicon electrode layers. Our design rationale surmised that the AIM deviceʹs large footprint area and thin nanoscale oxide layer should enable impedance detection of molecules with high sensitivity in a variety of solvents. The present study confirms this hypothesis and explores the effects that simple surface modifications have on the deviceʹs response. Specifically, the affinity between various analytes and the deviceʹs surface is evaluated when: (a) the aluminum top electrode is hydrated and rendered hydrophilic by immersing the microchip in boiling water, and (b) the silicon substrate is stripped of oxide and rendered hydrophobic (hydrogen-terminated) by hydrofluoric acid (HF) wet etching. Both types of devices were incorporated into a high performance liquid chromatography (HPLC) system already equipped with an ultra-violet–visible (UV–vis) detector. A range of analytes was injected using both normal and reverse phase modes; and the signals generated by each microchip device and UV–vis detector were recorded simultaneously and compared. The microchip devices’ responses were found to vary for analytes according to the surface modification used. The second surface modification method of exposing bare silicon surface showed greater sensitivity than the first method for all the analytes. Both types of devices were remarkably sensitive to organic salts tested, i.e., their sensitivity was significantly greater than that of the UV–vis detector. To demonstrate the novel dielectric constant capability of AIM, as a case study, the HF-treated device was used to detect enantiomers of a synthesized chiral molecule separated by a chiral column using an insulating eluent, namely hexanes/isopropanol.