Sensor 2 (materials). To expand the sensing range of the device to particles smaller than 2 m, an additional RPS sensor was needed, termed sensor 2. Within the literature, a range of materials have been tested for RPS applications, and some of the most common solid state materials are SiN and SiO2. In keeping with our philosophy to create an easy to fabricate and assemble device, we opted to first create pores in SiN or SiO2 using a dielectric breakdown procedure. Also referred to as controlled dielectric breakdown, this approach was first reported for nanopore fabrication by Kwok et al. and produces nanopores by applying a potential across a thin membrane (<50 nm). The advantages of dielectric breakdown are the low cost and ease of assembly of the fabrication setup. The technique is commonly used for analysing and sequencing DNA, and has also been used to produce arrays within a microfluidic channel. The process of making and testing the SiN and SiO2 pores was carried out in a series of stages. Before being placed into the fluidic chip, the membrane holder used to form the pores also allowed the pores to be tested as RPS sensors by placing Ag/AgCl electrodes either side of the membrane connected to a current amplifier to the setup. Thus, to confirm pore formation, the translocation of λ-DNA through the pore was measured (figure ). Upon detection of λ-DNA, the pore was enlarged under an applied electric field to allow further detection of nanomaterials (Figure ). Once the pore had been enlarged it could be used to measure the translocation events of nanoparticles, an example of which is shown in figure . The successfully fabricated pores were able to detect different sized particles, however they were limited < 5 runs each pore, and subsequent washing with buffer and ethanol could not restore translocation events. Not all the pores survived, as the enlarging process caused the pores to become brittle, crack or produce a baseline noise level that made them unusable (data not shown). The success rate of enlarging pores to sizes capable of detecting nanomaterials > 100nm was 14%. When a suitable pore was found, they were only capable of running a few samples before blocking and attempts to restore them proved unsuccessful, thus they were not able to be transferred into the fluidic chip. Whilst easy to setup, the controlled dielectric breakdown process proved to be inconsistent. An alternative fabrication method for SiN and SiO2 pores is to use a focused ion beam (FIB) to fabricate single nano-or micropores in thin membranes of a desired size from 200 nm to 5 m (in diameter) as determined by SEM (see figure ). FIB produced consistently shaped pores regardless of their size. In contrast to the dielectric breakdown method, pores were produced in all of the membranes milled via FIB and, as a result, far more pores could progress onto the next stages of testing. However, the pores were found to be difficult to wet, as they had not been created within an electrolyte solution (as was the case with the electric breakdown setup). Before they were placed into the fluidic chip the FIB-milled pores were first tested in a static liquid cell with nanoparticles to see if translocations could be observed. Figure , shows the pulse distribution for two FIB pores and the relationship between pulse frequency and concentration. The success rate of detecting particles with FIB-milled pores prior to their insertion within the fluidic device was circa 45% of the pores that were made, this number may reflect the diameter of the pore and RPS user ability. FIB pores of different diameters and thicknesses may result in different efficiencies.