Electronic desalting for controlling the ionic environment in droplet-based biosensing platforms

Abstract

The ability to control the ionic environment in saline waters and aqueous electrolytes is useful for desalination as well as electronic biosensing. We demonstrate a method of electronic desalting at micro-scale through on-chip micro electrodes. We show that, while desalting is limited in bulk solutions with unlimited availability of salts, significant desalting of ≥1 mM solutions can be achieved in sub-nanoliter volume droplets with diameters of ∼250 m. Within these droplets, by using platinum-black microelectrodes and electrochemical surface treatments, we can enhance the electrode surface area to achieve>99% and 41% salt removal in 1 mM and 10 mM salt concentrations, respectively. Through self-consistent simulations and experimental measurements, we demonstrate that conventional double-layer theory over-predicts the desalting capacity and, hence, cannot be used to model systems that are mass limited or undergoing significant salt removal from the bulk. Our results will provide a better understanding of capacitive desalination, as well as a method for salt manipulation in high-throughput droplet-based microfluidic sensing platforms.

Graphene and Two-Dimensional Materials for Biomolecule Sensing

“…Given the 2D attribute of graphene and the electrostatic influence of biomolecule-related electrical charges and currents from the third dimension, there would naturally be limitations to biomolecule sensing. The d Debye has been indicated as a limitation (66,122), e.g., in aqueous media, given that ∼ d Debye (nm) ∼ ; indeed, with typical extracellular or intracellular fluid concentrations at the order of 100 mM, it would seem that only analytes <1 nm in extent could be monitored. However, such a limitation is not borne out experimentally, where charge modulation at a distance more than 30 times the d Debye was detected (99).…”

Droplet-based Biosensing for Lab-on-a-Chip, Open Microfluidics Platforms

“…Theoretical work in Reference [ 47 ] shows that droplets can be appreciably desalted for physiological concentrations using high-surface area electrodes. Droplet desalting for such a system, has been experimentally demonstrated in Reference [ 46 ] for concentrations .…”

“…As we will see in Section 3 , droplets offer a fundamentally different approach to desalting: Due to finite number of ions in a sub-nL droplet, it is possible to temporarily desalt the droplet electrically near the sensor region (graphically shown in Figure 1 (c2)) to maximize the sensitivity. Swaminathan et al demonstrated a method for localized electronic desalting on a field effect transistor (FET) biosensor by using on-chip polarizable electrodes to locally deplete salt ions near the sensor region [ 46 ]. Theoretical analysis by Dak et al shows that such approach could lead to a 250X improvement of the detection limit [ 47 ].…”

On paper characterisation of droplet and evaporation study using impedance spectroscopy

“…During the process of evaporation, a change in impedance is produced as a result of the presence of ionic species in the solution. 40 Several investigations on the process of liquid evaporation have been carried out on substrates consisting of silicon, glass, and ITO glass. [41][42][43] However, the application of impedance analysis to examine the process of liquid evaporation on a hydrophilic solid substrate has not yet been investigated.…”