Figure 2.

a: Miniaturized array of sensors for measuring DO concentration (UMEA), pH (ISFET), and cell density (conductivity sensor). b: UMEA electrodes, showing individual electrodes formed by photo patterning polyimide to give 114 µm × 4 µm diameter apertures spaced 50 µm apart. UMEA electrodes were previously fabricated by sputtering platinum (with a tantalum adhesion layer) on an oxidized silicon substrate to give two 200 µm × 1,000 µm electrodes. The device shown recorded currents of ∼5 to 30 nA over the nitrogen to air range of DO, which corresponds to ∼0.05 to 0.3 nmol/h oxygen consumption. At 50% DO in a 50 µL reactor with k_La of 100 h^-1, this corresponds to 0.03% of the oxygen transfer rate (OTR). In later work, they were dissatisfied with the signal strength of the UMEA, leading them to operate it as a single electrode (Krommenhoek et al., 2008). This increased maximum current to ∼700 nA. Any effect on mass transfer dependence was not published. At 50% air DO, consumption of oxygen corresponds to ∼1% OTR. Problems with foaming agent addition causing changes in signal magnitude were also reported. Response time was not reported. Figure is reproduced with permission from Krommenhoek et al. (2007) (Copyright 2007 American Chemical Society).