Nanotextured superhydrophobic electrodes enable detection of attomolar-scale DNA concentration within a droplet by non-faradaic impedance spectroscopy

Abstract

Label-free, rapid detection of biomolecules in microliter volumes of highly diluted solutions (sub-femtomolar) is of essential importance for numerous applications in medical diagnostics, food safety, and chem-bio sensing for homeland security. At ultra-low concentrations, regardless of the sensitivity of the detection approach, the sensor response time is limited by physical diffusion of molecules towards the sensor surface. We have developed a fast, low cost, non-faradaic impedance sensing method for detection of synthetic DNA molecules in DI water at attomolar levels by beating the diffusion limit through evaporation of a micro-liter droplet of DNA on a nanotextured superhydrophobic electrode array. Continuous monitoring of the impedance of individual droplets as a function of evaporation time is exploited to dramatically improve the sensitivity and robustness of detection. Formation of the nanostructures on the electrode surface not only increases the surface hydrophobicity, but also allows robust pinning of the droplet contact area to the sensor surface. These two features are critical for performing highly stable impedance measurements as the droplet evaporates. Using this scheme, the detection limit of conventional non-faradaic methods is improved by five orders of magnitude. The proposed platform represents a step-forward towards realization of ultra-sensitive lab-on-chip biomolecule detectors for real time point-of-care application. Further works are however needed to ultimately realize the full potential of the proposed approach to appraise biological samples in complex buffer solutions rather than DI water.

Evaporation-induced stimulation of bacterial osmoregulation for electrical assessment of cell viability

“…In addition, as a reference, we analyzed another group of cells that lost their osmoregulation ability due to heat treatment and therefore, are "osmoregulatory dead" (defined as "dead" throughout this work). Overall, we find that the droplet-based impedance sensors (25)(26)(27)(28) can selectively differentiate live vs. dead E. coli cells with 10-15% accuracy (comparable with the colony counting method) in less than 20 min.…”

“…As explained in detail elsewhere (25,28), there is a universal behavior of the sensor signal with respect to the initial droplet volume and total evaporation time. The method only relies on the normalized differential conductance (correlated to the ionic modulation), and therefore, the experimental results are insensitive to evaporation rates.…”

“…As the droplet evaporates, the reduction in volume forces the analytes toward the sensor surface. Evaporation-induced beating of the diffusion limit results in much higher sensitivity and shorter response time of this droplet-based sensor compared with the classical impedance sensors (25). During evaporation, concentration of the droplet's solutes increases, and so does the osmotic pressure.…”

“…Fabrication procedure of superhydrophobic nickel microelectrodes is explained in detail in our previous papers (25,26,28). The biosensor has a two-electrode configuration, with each electrode being an array of electroplated nickel fins (microscale structures) with width of a = 10 μm, height of H = 10 μm, spacing of b = 20 μm, and inplane length of H z = 4 mm patterned and deposited on a glass substrate.…”

Fabricating Superhydrophobic Polymeric Materials for Biomedical Applications

“…This permanent or semi-permanent water repellency is a useful property that is employed to prepare self-cleaning surfaces 7 , microfluidic devices 8 , anti-fouling cell/protein surfaces 9,10 , drag-reducing surfaces 11 , and drug delivery devices [12][13][14][15] . Recently, stimuli-responsive superhydrophobic materials are described where the nonwetted to wetted state is triggered by chemical, physical, or environmental cues (e.g., light, pH, temperature, ultrasound, and applied electrical potential/current) 14,[16][17][18][19][20] , and these materials are finding use for additional applications [21][22][23][24][25] .…”