Fig. 1: Overview of the processes influencing current measurements.
a Macroscopic relationship between the glucose-oxidase-functionalized electrode (red), bare-hydrogel-coated platinum electrode (black), and spheroid (blue) all contained within a single hanging-drop compartment allowing single-spheroid-resolution measurements of glucose concentrations. b Enzymatic conversion of glucose yielding hydrogen peroxide (H2O2) by the glucose oxidase (GOx) enzyme modeled with an x-axis extending from the platinum electrode surface (x = 0) to the hydrogel-medium interface (x = H). We assume first-order reaction kinetics by the GOx enzyme with a given rate vg. The proportion of active enzyme in the functionalized hydrogel is defined by a “biosensor number” \(\tilde B\), where a high number implies a consuming surface, and a low number implies a consuming disk. c Electrode reaction allowing for amperometric readout of H2O2 concentrations in the hanging-drop compartment, where the current is proportional to the derivative of H2O2 concentration at the platinum electrode surface. This electrode reaction also occurs in the GOx-functionalized hydrogel (b). d Glucose consumption by the spheroid modeled with an r-axis defined from the spheroid center (r = 0) to the spheroid-medium interface (r = R). We assume first-order reaction kinetics by the cells in the spheroid with a given rate vS. The proportion of active cells in the spheroid is defined by a “spheroid number” \(\tilde S\), where a high number implies a consuming shell, and a low number implies a consuming sphere. e Over time, diffusion drives the transport of glucose toward the glucose-consuming GOx-functionalized electrode and the glucose-consuming spheroid. It also drives the transport of H2O2 from the H2O2-producing GOx-functionalized electrode toward the bare-hydrogel-coated electrode. f Evaporation of the hanging drop drives upconcentration of the analytes (glucose and H2O2), which, in turn, increases the current.
