Designing tunable optical metamaterials is one of the great challenges in photonics. Strategies for reversible tuning of nanoengineered devices are currently being sought through electromagnetic or piezo effects. For example, bottom-up self-assembly of nanoparticles at solid | liquid or liquid | liquid interfaces can be used to tune optical responses by varying their structure either chemically or through applied voltage. Here, we report on a fully reversible tunable-color mirror based on a TiN-coated Ag substrate immersed in an aqueous solution of negatively charged Au-nanoparticles (NPs). Switching electrode potential can be used to fully control the assembly/disassembly of NPs at the electrode | electrolyte interface within a 0.6 V wide electrochemical window. The plasmon coupling between the electrode and the adsorbed NP array at high positive potentials produces a dip in the optical reflectance spectrum, creating the “absorber” state. Desorption of NPs at low potentials eliminates the dip, returning the system to the reflective “mirror” state. The intensity and wavelength of the dip can be finely tuned through electrode-potential and electrolyte concentration. The excellent match between the experimental data and the theory of optical response for such system allows us to extract valuable information on equilibrium and kinetic properties of NP-assembly/disassembly. Together with modeling of the latter, this study promotes optimization of such meta-surfaces for building electrotunable reflector devices.

Self-assembling arrays of metallic nanoparticles at liquid|liquid or liquid|solid interfaces could deliver new platforms for tuneable optical systems. Such systems can switch between very-high and very-low reflectance states upon assembly and disassembly of nanoparticles at the interface, respectively. This encourages creation of electro-variably reversible mirror/window nanoplasmonic devices. However, the response time of these systems is usually limited by the rate-of-diffusion of the nanoparticles in the liquid, towards the interface and back. A large time-constant implies slow switching of the system, challenging the practical viability of such a system. Here we introduce a smart alternative to overcome this issue. We propose obtaining fast switching via electrically-induced rotation of a two-dimensional array of metal nanocuboids tethered to an ITO substrate. By applying potential to the ITO electrode the orientation of nanocuboids can be altered, which results in conversion of a highly-reflective nanoparticle layer into a transparent layer (or vice versa) within sub-second timescales. A theoretical method is developed based on the quasi-static effective-medium approach to analyse the optical response of such arrays, which is verified against full-wave simulations. Further theoretical analysis and estimates based on the potential energy of the nanoparticles in the two orientations corroborate the idea that voltage-controlled switching between the two states of a nanoparticle assembly is a viable option

A novel system for blood type testing is developed based on the first molecularly imprinted nanoparticles specific to oligosaccharides.

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