Magnetic movement of biological fluid droplets

Magnetic fields can be used to control the movement of aqueous drops on non-patterned, silicon nanowire superhydrophobic surfaces. Drops of aqueous and biological fluids are controlled by introducing magnetizable carbonyl iron microparticles into the liquid. Key elements of operations such as movement, coalescence, and splitting of water and biological fluid drops, as well as electrochemical measurement of an analyte are demonstrated. Superhydrophobic surfaces were prepared using vapor–liquid–solid (VLS) growth systems followed by coating with a perfluorinated hydrocarbon molecule. Drops were made from aqueous and biological fluid suspensions with magnetizable microparticle concentrations ranging from 0.1 to 10 wt%.     

Electrocatalytic detection of insulin at RuOx/carbon nanotube-modified carbon electrodes

A bilayer surface coating, prepared by electrodepositing ruthenium oxide (RuOx) onto a carbon nanotube (CNT) layer, offers dramatic improvements in the stability and sensitivity of voltammetric and amperometric measurements of insulin compared to the individual (CNT or RuOx) coated electrodes. The enhanced electrocatalytic activity towards insulin is indicated from lowering the potential of the oxidation process (starting around 0.35 versus Ag/AgCl) and the substantially higher sensitivity over the entire potential range. A wide linear dynamic range (10-800 nM) was achieved with a detection limit of 1 nM. The marked electrocatalytic activity of the RuOx/CNT coating towards insulin is coupled with a greatly enhanced stability. For example, the insulin amperometric response of the RuOx/CNT-coated electrodes is highly stable, with 97% of the initial activity remaining after 60 min stirring of 2 × 10⁻⁶ M solution (compared to significantly faster current diminutions at the RuOx- or CNT-coated surfaces). The results suggest great promise for miniaturized sensors and detectors for monitoring insulin.     

Discrete microfluidics with electrochemical detection

A droplet-based electrochemical digital magnetofluidics system has been developed. The system relies on the magnetic movement, in air, of different aqueous microdroplets containing magnetic microparticles--serving as the 'sample', 'blank', 'wash' and 'reagent' solutions--into and out of a three-electrode assembly. The movement of all droplets was controlled using the magnetic fields generated by three separate external magnets positioned below the superhydrophobic surface. Square-wave voltammetry was used for rapid measurements of dopamine in multiple successive microdrops with minimal cross talk. The ability of the droplet-based electrochemical microfluidic system to manipulate microliter solutions was also illustrated in bioassays of glucose, involving the merging of enzyme (GOx) and substrate droplets, followed by chronoamperometric measurements of the hydrogen peroxide product in the merged droplet. Variables of the new electrochemical digital magnetomicrofluidic technique were examined and optimized. The new droplet-based electrochemical microfluidic system offers a promising platform for automated clinical diagnostics and drug discovery.