Surface Studies of Aminoferrocene Derivatives on Gold: Electrochemical Sensors for Chemical Warfare Agents

The cystamine conjugate [(BocNH)Fc(CO)CSA]2 was prepared by coupling cystamine with the N-protected ferrocene amino acid derivative BocHN-Fc-COOH and was
fully characterized by spectroscopic methods and by
single-crystal X-ray diffraction. The cystamine conjugate
forms films on gold substrates, which upon deprotection
of the amino group, react with chemical warfare agent
(CWA) mimics, upon which the redox properties of the
Fc group are affected significantly. Cyclic voltammetry
shows 50(5) mV anodic shifts of the Fc redox potentials
after exposure to EtSCH2CH2Cl, a simulant for sulfur
mustard HD (MA), and (NC)(EtO)2P(O), a simulant for
nerve agent Tabun (NA). Exposure to MA and NA causes
an increase in 2.3 and 4.5 ng mass, respectively, in QCM
which indicates ca. 70% efficiency in Boc-deprotection.
Ellipsometry measured a film thickness increase from
6(±1) Å for the deprotected film to 10(±4) Å for the film
modified with MA and to 7(±2) Å for the film modified
with NA. AFM measurements show changes in the thickness and morphology of the film after reaction with MA
and NA. The surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and clearly show the attachment
of the cystamine conjugate on the surface and its reaction
with CWA mimics.