Conventional photolithography methods were used for the fabrication of the microfluidic devices. Gold electrodes were patterned from Au (100 nm) and Ti-coated (40 nm) glass substrates (Deposition Research Lab, Inc). The width of each Au electrode was 50 µm and the spacing between two was 10 µm. There were two pairs of sensing electrodes inside each microfluidic channel. The distance between these two pairs was 1000 µm. The fluidic channels were formed by casting poly(dimethylsiloxane) (PDMS, Sylgard 184) into moulds made using the photoresist SU-8. The detailed fabrication parameters are included in the Supporting Information (section 1 Microfluidic chip fabrication). The height and width of microfluidic channels were characterised by an optical profilometer (ContourX-100, Bruker). The photolithography technique provided high resolution and fidelity for both the PDMS channels and Au electrodes. Figure shows optical microscopy and 3D profilometer images of the fabricated electrodes and channels.