Formation of Gold Nanoparticle Size and Density Gradients via Bipolar Electrochemistry

Graphical Abstract

Gradients in nanoparticle diameter: Bipolar electrodeposition is used to form controlled size gradients of surface-immobilized gold nanoparticles (five separate SEM images from one single electrode are presented in the figure). The increased size in the cathodic direction reflects the extent of reduction of AuCl₄⁻, a process imposed by an external electric field.

Abstract

Bipolar electrochemistry is employed to demonstrate the formation of gold nanoparticle size gradients on planar surfaces. By controlling the electric field in a HAuCl₄-containing electrolyte, gold was reduced onto 10 nm diameter particles immobilized on pre-modified thiolated bipolar electrode (BPE) templates, resulting in larger particles towards the more cathodic direction. As the gold deposition was the dominating cathodic reaction, the increased size of the nanoparticles also reflected the current distribution on the bipolar electrode. The size gradients were also combined with a second gradient-forming technique to establish nanoparticle surfaces with orthogonal size and density gradients, resulting in a wide range of combinations of small/large and few/many particles on a single bipolar electrode. Such surfaces are valuable in, for example, cell-material interaction and combinatorial studies, where a large number of conditions are probed simultaneously.