Directed assembly of nano and microscale particles is of great interest and has widespread applications in various fields including electronics, nanomaterials and tissue engineering. Bottom-up tissue engineering is motivated by the occurrence of repeating functional units in vivo. The bottom-up approach requires novel techniques to assemble engineered functional units as building blocks at a high speed with spatial control over three-dimensional (3D) micro-architecture. Here, we report a magnetic assembler that utilizes nanoparticles and microscale hydrogels as building blocks to create 3D complex multi-layer constructs via external magnetic fields using different concentrations of magnetic nanoparticles. This approach holds potential for 3D assembly processes that could be utilized in various tissue engineering and regenerative medicine applications.

Electrochemical double-layer capacitors built from nanoporous electrodes can have such a high ratio of electrode surface area to pore volume that charging the capacitor can deplete the salt from the liquid volume. This can result in increased resistance, resulting in a slow, nonlinear charging rate of which quantitative understanding is limited. In some cases, this effect is masked by an external solution resistance or by the transport of salt into the pore from an external reservoir. However, in forms relevant to a compact energy storage device, the phenomenon can have an important effect on charging time and linearity, and understanding it is important for such design. We have observed salt depletion effects by using dealloyed gold, which has well-defined 10 nm pores and a chemically well-understood surface, and by minimizing the amount of external salt within range of diffusion. Good correspondence is observed with a modified de Levie model that accounts for reduced local conductivity due to salt depletion. The model’s assumption that the Stern layer (ions closely bound to the pore wall) makes a low contribution to conductance in the pore is validated by experimental data.

The classical model for porous electrodes reported by De Levie several decades ago (and expanded upon since then) was developed mainly to describe pores with micrometer-scale diameters. Presumably it will break down as pore diameters approach atomic dimensions. Mesoporous gold formed by dealloying is a valuable test platform for this because its 10 nm pores are on the boundary of this expected breakdown and because the electrochemical and surface properties of gold are relatively well understood. The De Levie model works for these electrodes at high salt concentrations, but under dilute conditions, there is not enough salt locally to charge the interface, increasing real impedance on intermediate time scales. Specific adsorption on pore walls can cause a similar increase and also cause an effective mobility decrease, tunable through electrolyte choice and the use of alkanethiol monolayers. These effects are not expected in micrometer-scale pores and are important considerations when designing devices with nanoporous electrodes.

In this study, nanoporous gold supercapacitors were produced by electrochemical dealloying of gold-silver alloy. Scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed completion of the dealloying process and generation of a porous gold material with approximately 10 nm diameter pores. Cyclic voltammetry and chronoamperometry of the nanoporous gold electrodes indicated that these materials exhibited supercapacitor behavior. The storage capacity of the electrodes measured by chronoamperometry was approximately 3 mC at 200 mV. Electrochemical storage and voltage-controlled delivery of two model pharmacologic agents, benzylammonium and salicylic acid, was demonstrated. These results suggest that capacitance-based storage and delivery of pharmacologic agents may serve as an alternative to conventional drug delivery methods.