They are using an electrode coated in an antibody that binds with cortisol to do the detection, and needed the counter electrode to be really close to keep sensitivity high and noise low.
At a few microns apart, interdigitated fingers were too course, so the team created a sandwich structure consisting of, starting at the bottom, metal-insulator-metal-insulator, then etched holes in the top surface through three layers to expose the bottom metal (see artists impression).
This gave them a series of wells with electrodes separated only by the thickness of an insulation layer – 40nm in this case.
The proof-of-concept has 25 wells in an array, with lower and upper electrodes lead off in opposite directions to allow connections to be made.
Read out requires a impedance frequency spectrum to be measured, so one electrode is driven by a frequency sweep from 1Hz to 1MHz, while the signal at the other electrode is amplified then converted down to 0Hz using a pair of mixers driven with sine and cosine versions of the input signal.
“Our new sensor produces an accurate and reliable response that allows a continuous readout of cortisol levels for real-time analysis,” said Rutgers research engineer Mehdi Javanmard. “It has great potential to be adapted to non-invasive cortisol measurement in other fluids such as saliva and urine.”
The technique avoids attaching ‘label’ molecules to the cortisol.
“The fact that molecular labels are not required eliminates the need for instruments like optical microscopes and plate readers, making the readout instrumentation something you can measure ultimately in a small pocket-sized box or even fit onto a wristband one day,” said Javanmard.
Rutgers University worked with the University of Minnesota, Minneapolis and the University of Pennsylvania, Philadelphia.
The work is covered in ‘Single-step label-free nanowell immunoassay accurately quantifies serum stress hormones within minutes‘ a paper published in Science Advances.