Application Note 103: Electrochemical SPR: Metal Deposition/Stripping and Reorganization of Electroactive Organic Thin Films

In addition to studies of redox-induced conformational changes of surface-bound proteins (cf. Application Note #102), electrochemical SPR can be used to quantify the amount of metal electrodeposited onto a surface and reorganization of organic thin films upon redox reactions. A BI-SPR 1000 was used to quantify the amount of Cu deposited and stripped during Cu²⁺ reduction/Cu oxidation in a CuSO₄ solution (FIG. 1). In performing this experiment, a polyetheretherketone cell with an internal volume of 0.5 mL was used. A Pt wire and an Ag/AgCl, which fit tightly into the holes in the cell, served as the auxiliary and reference electrodes, respectively [1].

Integrating charges under the reduction peak in FIG. 1a yielded the thickness of the deposited copper film to be approximately 13 nm. On the basis of the Fresnel equation and a four-layer (glass/Au/Cu/H2O) model for the Au sensor chip (FIG. 1b), the thicknesses of the copper film was found to be 15 nm, which agrees with the voltammetric data remarkably well.

In a separate experiment, electrochemical SPR was used to investigate the changes in orientations of ferrocene (Fc)-terminated alkanethiol self-assembled monolayers (SAMs)[2]. Such an electroactive organic thin film is a model system for probing interfacial electron transfer reactions [3], studies of molecular electronics, and construction of liquid crystal devices [4]. The potential at which Fc groups are oxidized is more positive than the point of zero charge. As a result, the ferrocenium cations are repelled by the positively charged electrodes. As shown by the schematics (FIG. 2), such repulsion changes the orientation or reorganization of the Fc-terminated alkanethiol SAM, decreasing the tilt angle of the alkyl chain. To compensate the charges at the SAM terminus, anions in the solution will form ion-pairs with the ferrocenium cations. This ion-pairing process results in a change in the refractive index of the SAM. The film reorganization and ion-pairing processes can be monitored by electrochemical SPR in real time (FIG 3). By quantifying the number of ions and the extent of solvation involved in the ion-pairing process, the infinitesimal changes shown in the two different orientations can be accurately determined.

The film thickness variation of the FcC11SH SAM in 0.1 M HClO4 was deduced to be 0.09 nm. This is in good agreement with the ellipsometric results [5]. The ability of determining such tiny thickness variations demonstrates the superb sensitivity of SPR for interrogating orientation changes associated with ultrathin organic films and the amenability of SPR for in-situ coupling with electrochemistry.

Author: Nguyen Ly | Biosensing Instrument | Published May 1, 2010