[a] Bipolar electrochemistry is employed to demonstrate the formation of goldn anoparticle size gradients on planar surfaces. By controlling the electric field in aH AuCl 4 -containing electrolyte, gold was reduced onto 10 nm diameter particles immobilized on pre-modifiedt hiolatedb ipolar electrode (BPE) templates, resulting in larger particles towards the more cathodic direction. As the gold deposition wast he dominating cathodic reaction, the increased size of the nanoparticles also reflected the current distribution on the bipolar electrode. The size gradients werea lso combined with as econd gradient-forming technique to establish nanoparticle surfaces with orthogonal size and density gradients, resulting in aw ide range of combinations of small/large and few/many particles on as ingle bipolar electrode. Such surfaces are valuable in, for example, cellmateriali nteraction and combinatorial studies, where al arge number of conditions are probeds imultaneously.An umber of new and interesting applications of bipolar electrochemistry have recently appeared, involving small objects and surfacem odifications, [1][2][3][4][5][6][7] as well as sensors and detectors. [8][9][10][11][12] In such experiments,t he object or surface (the bipolar electrode, BPE) is placed in an electrolyte and exposed to an electric field established via two feedere lectrodes. Provided the BPE is conducting and the potential difference across it is sufficiently high, anodic and cathodic reactions can be induced with part of the cell current passingt hrough the BPE. With respect to larger objectsl ike mm-or cm-sizeds urfaces, formation of compositional gradients of materials, [13] such as semiconductors, [14] polymers, [15] metals, [16] and self-assembled monolayers and proteins [6,7] have been generated. In addition, bipolar gradients related to energy conversion have been presented, [17,18] and very recently,w eh ave also shown the possibility to swiftlys creen the corrosion properties of stainless steels functioning as BPEs. [19,20] An equally versatile method to modify surface chemical and morphological properties is the irreversible deposition of nanoparticles. Nanoparticle-coateds urfaces have been used to explore biological phenomenas uch as cell adhesion [21] and improvedb iosensing, [22] as well as energy-harvesting solar devices.[23] We have previously investigated how variation of particle deposition conditions tune the self-organization of gold nanoparticles, especially the distance between them, on dithiol-modified gold surfaces [24] in order to create regular [25] and anisotropic (gradient) [26] nanoparticle arrays for cell-screening purposes.H erein we extend this work by demonstrating the use of bipolar electrochemistry to gradually increase the size of surface immobilized gold nanoparticles, and also to form two orthogonal gradients in size and density on as ingle BPE. We also discuss how the gold deposition can be linked to the BPE current density distribution during the experiment.To first demonstrate the bipolar depo...
