MEMS fabricated nanopores and micropores functionalized with chromate-selective solvent polymeric membrane

Daniel C. Rieck

There is an impetus in the global regulatory industry to develop new monitoring technologies targeting pollutants such as chromate, since current practice requires using expensive technologies such as inductively coupled plasma-mass spectroscopy, and is therefore labor intensive and time-consuming. The Ion Selective Electrode (ISE) is a technology that, despite its numerous benefits, has not been adapted to regulatory monitoring of heavy metals because of its shortcomings in sensitivity and selectivity. One approach to solving this problem is to combine miniaturized ISEs with lab-on-a-chip preconcentration and preseparation technologies. In this thesis, I present a manuscript submitted for publication in Sensors and Actuators B: Chemical that treats fabrication methods for producing nano- and micro-sized chromate-selective ISEs compatible with such lab-on-a-chip separation technology; characterizes these sensors; and compares their function to coated wire electrodes. We used UV lithography and focused ion beam to make single micropores in SU-8 photoresist and nanopores in Si3N4 thin films, respectively, functionalizing them with solvent polymeric membrane. We used a membrane formulation consisting of 7.7:62.2:31.1 wt % Aliquat336:2-NPOE:PVC. Our coated wire electrode arrays exhibited a response slope of −61.7±2.4 mV decade−1, limit of detection (LOD) of 3.0×10−6, and potentiometric selectivity coefficients ranging from 1.3×10−2 for SCN− to 5.0×10−5 for SO3 2−. A nano-scale ISE 100 nm in diameter achieved a response slope of −65.2±4.1 mV decade−1 and a LOD of 1.8×10−5 M, versus −58.6±5.6 mV decade−1 and 2.1×10−5 for a micro-scale ISE 30 µm in diameter. Response times averaged 29 s for the nano-scale ISE and 40 s for the micro-scale ISE. Electrical resistance measurements demonstrated working ranges of GΩ for the micro-scale ISEs and up to TΩ for the nano-scale ISEs. Predicted pore diameters based on these measurements showed -3% and +18% agreements with actual diameters for a 100 nm nanopore and a 30 µm micropore, respectively. Atomic force microscopy imaging of the micro-scale ISE revealed a properly formed micropore and cast membrane, with exposed membrane diameter exceeding that of the pore opening by 1.7 times. AFM was found to be incapable of distinguishing nano-scale ISEs from Si3N4 thin film surface features and dust particles.

MEMS fabricated nanopores and micropores functionalized with chromate-selective solvent polymeric membrane

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