Nano- and micropores functionalized with responsive or reactive materials offer defined high rates of mass transport which are beneficial for applications in sensing, nanofluidics and catalysis.^1,2 They can also act as model systems for material characterization and fabrication.^2,3 However, precisely controlling the functionalization reactions inside such tiny volumes is a challenge. Moreover, it is frequently desired to deposit the material at a specific location within the pore, for example, on the inside wall or just at the opening. When the functionalization requires electrodeposition, achievement of such site specificity is difficult as it necessitates a continuous electrical contact that is exposed only at the desired sites. Bipolar electrochemistry is a powerful tool capable of localized electrodeposition in the absence of an electrical contact.⁴ It offers a precise way to grow materials on 3D substrates.

In our work, two electrodes are placed on either side of a conductive nanoporous membrane and a potential bias is applied (1-4 V). Redox reactions, one cathodic and one anodic, occur at either end of the nanopore. One reaction is chosen such that it forms a porous material which is selectively electrodeposited at one pore orifice. These capped pores allow the study of the properties of this porous material, such as conductivity, reactivity, stability and surface charge.

Figure 1 shows porous manganese dioxide (MnO₂) that was selectively electrodeposited at one end of a pore by bipolar electrochemistry. Manganese oxides are interesting materials for sensing, catalysis, supercapacitor and battery applications.⁵ Their well-defined porous structures exhibit high specific surface areas and lattices that allow the intercalation of ions, such as lithium. We report on the mechanism of formation and growth rate of these MnO₂ structures on gold nanopores as studied by electrochemistry and complementary techniques. We also report electrochemical measurements of the synthesized material, including its permselectivity towards lithium, potassium and sodium ions. Finally, we address the performance of this material for applications in batteries and catalysis.

1. Xu, Y.; Jin, S.; Xu, H.; Nagai, A.; Jiang, D. Chem. Soc. Rev. 2013, 42, 8012-8031.
2. Wang, C. M.; Kong, D. L.; Chen, Q.; Xue, J. M. Front. Mater. Sci. 2013, 7(4), 335-349.
3. Baker, L. A.; Choi, Y.; Martin, C. R. Curr. Nanosci. 2006, 2(3), 243-255.
4. Fosdick, S. E.; Knust, K. N.; Scida, K.; Crooks, R. M. Angew. Chem. Int. Ed. 2013, 52, 10438-10456.
5. Brock, S. L.; Duan, N.; Tian, Z. R.; Giraldo, O.; Zhou, H.; Suib, S. L. Chem. Mater. 1998, 10, 2619-2628.