Figure 4. Plain-stick representation of a CTAB molecule with an hydro-dynamic radius a^CTAB ~ 2.3 nm (a).

A schematic of the electro-oxidation reaction of adrenaline in a solution: in water, adrenaline is separated into adrenaline-quinone and adreno-chrome at the surface of the gate electrode of platinum, from this an ion of hydrogen H⁺ is released that immediately forms the super-molecular complex H₃O⁺ (b). The modulation current I_M, that is, the maximum drain-source measured on a time interval, is reported for CTAB (c) and adrenaline (d) as a function of position on the device and gate voltage V_gs. For both molecules, the intensity of current is larger at the border of the device compared to the center. The difference of current between adrenaline and CTAB (e) reveals that adrenaline is transported to the periphery of the pattern with an increased efficiency compared to CTAB: the discrepancy between adrenaline and CTAB may be ascribed to the sole difference in mass between those species. The scheme in (f) depicts the main drivers of transportation for a solute trace in a drop: the experimental conditions generate within the drop Marangoni convective flows due to the curvature of the drop, which combine with diffusion and the external applied electric field and these are the forces exerted on the particle in solution. The balance/unbalance between these forces is dependent on particle size and would ultimately determine the field of motion of the particle within the drop. The simulated radial distribution of an analytical trace after 3 s from deposition, where the size of the particles constituting the trace has been varied from a = 0.02 nm (g) to a = 2 nm (h). Smaller particles distribute preferentially laterally in the drop differently from larger particles.