Potential modulated absorbance spectroscopy: an investigation of the potential distribution at a CdS nanoparticle modified electrode

Opportunities and challenges for electrochemistry in studying the electronic structure of nanocrystals

We review the state-of-the-art of determining the electronic structure of nanocrystals in thin films by electrochemistry and emphasize the benefits of correlating electrochemical with spectroscopic methods to this end.

Quantum-dot-based photoelectrochemical sensors for chemical and biological detection

Quantum-dot-based photoelectrochemical sensors are powerful alternatives for the detection of chemicals and biochemical molecules compared to other sensor types, which is the primary reason as to why they have become a hot topic in nanotechnology-related analytical methods. These sensors basically consist of QDs immobilized by a linking molecule (linker) to an electrode, so that upon their illumination, a photocurrent is generated which depends on the type and concentration of the respective analyte in the immediate environment of the electrode. The present review provides an overview of recent developments in the fabrication methods and sensing concepts concerning direct and indirect interactions of the analyte with quantum dot modified electrodes. Furthermore, it describes in detail the broad range of different sensing applications of such quantum-dot-based photoelectrochemical sensors for inorganic and organic (small and macro-) molecules that have arisen in recent years. Finally, a number of aspects concerning current challenges on the way to achieving real-life applications of QD-based photochemical sensing are addressed.

Potential-Modulated, Attenuated Total Reflectance Spectroscopy of Poly(3,4-ethylenedioxythiophene) and Poly(3,4-ethylenedioxythiophene Methanol) Copolymer Films on Indium−Tin Oxide

We report the first application of a potential-modulated spectroelectrochemical ATR (PM-ATR) instrument utilizing multiple internal reflections at an optically transparent electrode to study the charge-transfer kinetics and electrochromic response of adsorbed films. A sinusoidally modulated potential waveform was applied to an indium-tin oxide (ITO) electrode while simultaneously monitoring the optical reflectivity of thin (2-6 equivalent monolayers) copolymer films of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-ethylenedioxythiophene methanol) (PEDTM), previously characterized in our laboratory. At high modulation frequencies the measured response of the polymer film is selective toward the fastest electrochromic processes in the film, presumably those occurring within the first adsorbed monolayer. Quantitative determination of the electrochromic switching rate, derived from the frequency response of the attenuated reflectivity, shows a linear decrease in the rate, from 11 x 10(3) s(-1) to 3 x 10(3) s(-1), with increasing proportions of PEDTM in the copolymer, suggesting that interactions between the methanol substituent on EDTM and the ITO surface slow the switching process by limiting the rate of conformational change in the polymer film.

Nanoparticle-Mediated Electron Transfer Across Ultrathin Self-Assembled Films

The electrochemical behavior of arrays of Au nanoparticles assembled on Au electrodes modified by 11-mercaptoundecanoic acid (MUA) and poly-L-lysine (PLYS) was investigated as a function of the particle number density. The self-assembled MUA and PLYS layers formed compact ultrathin films with a low density of defects as examined by scanning tunneling microscopy. The electrostatic adsorption of Au particles of 19 +/- 3 nm on the PLYS layer resulted in randomly distributed arrays in which the particle number density is controlled by the adsorption time. In the absence of the nanoparticles, the dynamics of electron transfer involving the hexacynoferrate redox couple is strongly hindered by the self-assembled film. This effect is primarily associated with a decrease in the electron tunneling probability as the redox couple cannot permeate through the MUA monolayer at the electrode surface. Adsorption of the Au nanoparticles dramatically affects the electron-transfer dynamics even at low particle number density. Cyclic voltammetry and impedance spectroscopy were interpreted in terms of classical models developed for partially blocked surfaces. The analysis shows that the electron transfer across a single particle exhibits the same phenomenological rate constant of electron transfer as for a clean Au surface. The apparent unhindered electron exchange between the nanoparticles and the electrode surface is discussed in terms of established models for electron tunneling across metal-insulator-metal junctions.

Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals

The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 +/- 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 +/- 0.1) x 10(10) particles cm(-2), with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.