Self-Assembled Monolayers of Cystamine and Cysteamine on Gold Studied by XPS and Voltammetry

Abstract

The formation of self-assembled chemisorbed layers of cystamine, cysteamine, and 4-aminothiophenol on gold has been studied by XPS and voltammetry. These compounds, often used in the preparation of biosensors and modified electrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethanolic solutions. The results of the XPS experiments reveal that a shoulder on the S 2p3/2 peak (situated at 162.1 eV) develops at 161.3 eV upon increasing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. This shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption times also give rise to increased amounts of oxidized carbon and sulfur in the films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments with different takeoff angles. The main N 1s peak for cystamine is shifted toward higher binding energies for increasing adsorption times while two prominent nitrogen peaks are generally seen for cysteamine. For cysteamine, increasing adsorption times result in an increase of the main nitrogen component at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations for these experimental findings are discussed. Cystamine, cysteamine, and 4-aminothiophenol films on gold are shown to be irreversibly oxidized in the gold oxide formation region. On the basis of evaluation of the oxidation charge, surface coverages of approximately 1 × 10-9 mol/cm2 were obtained for adsorption times between 5 min and 1 week.

Insights into the surface and redox properties of single-walled carbon nanotube—cobalt(II) tetra-aminophthalocyanine self-assembled on gold electrode

“…1(B). The cysteamine SAM exhibited two strong components for the N (1s) peak at 399.5 and 401 eV, which is in agreement with literature for cysteamine SAMs [56].…”

“…1 shows the comparative survey X-ray photoelectron spectra of the (i) bare; (ii) Au-AET; (iii) Au-AET-SWCNT; and (iv) Au-AET-SWCNT-CoTAPc. The S (2p) peak at 162.5 eV is assigned to the normal gold-sulphur bond [16], [55], [56], [57], [58] and [59]. The presence of the N atoms (in the BE region of 395-405 eV) in the SAMs are evident in Fig.…”

CHCA-modified Au nanoparticles for laser desorption ionization mass spectrometric analysis of peptides

“…This result can be explained by the high proton affinity of the amine group in cysteamine. With a pKa value of 10.8 [37,38], cysteamine molecules on AuNPs surface play the role of proton predators instead of proton donors [22]. Thus, because of the poor AuNPs-to-sample energy-transfer, Au ions themselves, not peptides, tend to be ionized when a laser pulse is applied.…”

Comparative Study of Tetrahydrothiophene and Thiophene Self Assembled Monolayers on Au(111): Structure and Molecular Orientation

“…20 Our previous XPS studies showed that the S atoms both in the THT and TP SAMs were chemically bound to the Au(111) surface, 13,14,20 since the observed peak positions were almost the same as those of the alkanethiol SAMs. 21,22 This means that the THT and TP molecules were bound to the Au(111) surface through the S-Au chemical bond, as shown in Figure 1. Although both TP and THT molecules have a five-membered heterocyclic ring, TP has a π-conjugated ring and THT has an aliphatic cyclic ring without a π-conjugated system.…”