Tunable electrochemical switch of the optical properties of metallic nanoparticles

Host-guest complexation: a convenient route for the electroreduction of diazonium salts in aqueous media and the formation of composite materials

Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.

Gold nanoparticles with externally controlled, reversible shifts of local surface plasmon resonance bands

We have achieved reversible tunability of local surface plasmon resonance in conjugated polymer functionalized gold nanoparticles. This property was facilitated by the preparation of 3,4-ethylenedioxythiophene (EDOT) containing polynorbornene brushes on gold nanoparticles via surface-initiated ring-opening metathesis polymerization. Reversible tuning of the surface plasmon band was achieved by electrochemically switching the EDOT polymer between its reduced and oxidized states.

Active plasmonic devices with anisotropic optical response: a step toward active polarizer

Control of the optical properties of metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). It is shown that the quenching of localized surface plasmon (LSP) sustained by oblate particles depends of the frequency of the LSP resonance. This effect is attributed to the variation of the CP dielectric function with wavelength. As a consequence, prolate arrays show total quenching of the LSP resonance along the major axis of the particles whereas modulation and moderate damping are observed along the minor axis. Combining electroactive conducting polymer and prolate NP makes it possible to design active plasmonic devices with anisotropic optical response upon CP switching. In the present case, such devices can be used as active filters or polarizers.

Active molecular plasmonics: controlling plasmon resonances with molecular switches

A gold nanodisk array, coated with bistable, redox-controllable [2]rotaxane molecules, when exposed to chemical oxidants and reductants, undergoes switching of its plasmonic properties reversibly. By contrast, (i) bare gold nanodisks and (ii) disks coated with a redox-active, but mechanically inert, control compound do not display surface-plasmon-based switching. Along with calculations based on time-dependent density functional theory, these experimental observations suggest that the nanoscale movements within surface-bound "molecular machines" can be used as the active components in plasmonic devices.

Optoelectronic properties of nanostructured ensembles controlled by biomolecular logic systems

A nanostructured system composed of enzyme-functionalized silica microparticles, ca. 74 microm, and gold-coated magnetic nanoparticles, 18 +/- 3 nm, modified with pH-sensitive organic shells was used to process biochemical signals and transduce the output signal into the changes of the optoelectronic properties of the assembly. The enzymes (glucose oxidase, invertase, esterase) covalently bound to the silica microparticles performed Boolean logic operations AND/OR processing biochemical information received in the form of chemical input signals resulting in changes of the solution pH value. Dissociation state of the organic shells on the gold-coated magnetic nanoparticles was controlled by pH changes generated in situ by the enzyme logic systems. The charge variation on the organic shells upon the reversible protonation/dissociation process resulted in the changes of the gold layer localized surface plasmon resonance energy (LSPR), thus producing optical changes in the system. The proton transfer process allowed the functional coupling of the information processing enzyme systems with the signal transducing gold-coated magnetic nanoparticles providing their cooperative performance. Magnetic properties of the gold-coated magnetic nanoparticles allowed separation of the signal-transducing nanoparticles from the enzyme-modified signal processing silica microparticles. The reversible system operation was achieved by the Reset function, returning the pH value and optical properties of the system to the initial state. This process was biocatalyzed by another immobilized enzyme (urease) activated with a biochemical signal. The studied approach opens the way to novel optical biosensors logically processing multiple biochemical signals and "smart" multisignal responsive materials with logically switchable optical properties.