Ph.D. THESIS ABSTRACT SYNTHESIS, STEREOCHEMISTRY AND ...

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Radu Gropeanu – <strong>Ph</strong>D Thesis Abstract Also, it is likely that both compounds bind to the surface via at least two of their sulphurs, as disulfides are known to dissociate upon adsorption on gold. 26 The absence of CH deformation bands between 1300 and 1400 cm -1 suggests that 32 is not oriented perpendicular to the surface but is rather parallel to the surface. In this orientation the C-H deformation modes of methylenes will have a low intensity. It is thus possible that compound 32 is anchored to the surface by both 1,2-dithiolane rings. However, this would require an important reorganization of the optimized structure shown in Figure 8. a b c Figura 8. Possible orientations of compounds 30 (a) and 32 (b) and (c) adsorbed on Au(111) suggested by the analysis of its IR spectra. B. Adsorption on gold electrodes and interaction with gold nanoparticles The interaction between some (poly)spiranes having 1,2-dithiolane units and gold nanoparticles (GNP) was studied for the beginning using UV-Vis spectroscopy. Thus, the UV- Vis spectra of the organic ligands in chloroform and of the GNP (water solution) were recorded, as well as the spectra of the samples obtained by mixing cyclic disulfides with GNP solution (Figure 9). The main feature of the samples spectra is the decay of the plasmodic resonance peak. For example, in short time (20 minutes) the absorbance in the 500-550 nm region of the spectra was five times reduced in the case of interaction between 32 and GNP (Figure 9d). Also, the electronic absorption peak of the organic ligand (330 mn) has disappeared (Figure 9c). Absorbanta 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 440 460 480 500 520 540 560 580 600 620 λ(nm) Absorbanta 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 100 200 300 400 500 600 700 800 900 1000 λ(nm) Absorbanta 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 200 300 400 500 600 λ(nm) Absorbanta 0.042 0.040 0.038 0.036 0.034 0.032 0.030 0.028 0.026 0.024 0.022 Au+STS-1min Au+STS-5min Au+STS-20 min 0.020 440 460 480 500 520 540 560 580 600 620 a b c d Figure 9. UV-Vis spectra of the a) water dispersion of GNPs, b) chloroform solution of 32, c) GNPs functionalized with 32, and d) GNPs functionalized with 32, recorded at three different times These facts suggest an aggregation process of the nanoparticles, which is sustained by the transmission electronic microscope (80 kV) measurements (Figures 10). a b c Figure 10. Transmission electronic microscope image of the a) GNPs suspension, b) GNPs functionalized with 30, and c) GNPs functionalized with 32 26 Fenter, P.; Eberhardt, A.; Eisenberg, P. Science 1994, 266, 1216 λ(nm) 14
Radu Gropeanu – <strong>Ph</strong>D Thesis Abstract A method which is suitable for the study of the adsorption of the cyclic disulfides on the gold surface is the cyclic voltammetry using a gold electrode as working electrode. After the cleaning operations, the cyclic voltammetry of the standard system [K4Fe(CN)6 in 0.1 M KCl, Figure 11a] was performed and compared with the cyclic voltammograms run with the gold electrodes functionalized with each of cyclic disulfide 25-27, 30, 32. The absence of a redox process has shown that the spirane monolayer adsorbed on the surface of the gold electrode acts as a dielectric isolator (Figure 11b). a b Figure 11. Cyclic voltammogram of a solution of 3 mM K4Fe(CN)6 in 0.1 M KCl using a) bare gold electrode (10 mV/sec) and b) urface functionalized gold electrode with 32 (50 mV/sec) 31 has two cyclic disulfide units per molecule and, therefore, was tested for the functionalization of the gold electrode surface with GNP. Two methods of the preparation were tried: Method A: the electrode treated with the organic ligand was immersed in the GNP solution, and Method B: the electrode was immersed into the solution obtained by mixing of GNP with organic ligand at different pH values (5.5, 7 and 11). I / mA I / mA 0.04 0.03 0.02 0.01 0.00 -0.01 -0.02 -0.03 0.06 0.04 0.02 0.00 -0.02 -0.04 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Potential /V vs. Ag/AgCl 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Potential / V vs. Ag/AgCl -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 a b Figure 12. Cyclic voltammograms of a) a solution of 3 mM K4Fe(CN)6 in 0.1 M KCl (buffered at pH 5.5) using surface functionalized gold electrode with 32 and GNP (10 minutes immersion, pH 7, by Method A (50 mV/sec)), and b) a solution of 3 mM K4Fe(CN)6 in 0.1 M KCl (buffered at pH 7) using surface functionalized gold electrode with 32 and GNP (pH 7) by Method B (50 mV/sec) These functionalized electrodes were subjected to cyclic voltammetry of the standard solution. The results showed us that only the Method B run at pH 7 was successful (Figure 12), the voltammograms presenting a reversible redox process. Comparing the voltammograms, we can remark that for the GNP-32-modified gold electrode the current maxima are more than two times higher that the corresponding peaks for the non-functionalized gold electrode, meaning that the modification of the surface is increasing the electron transfer between solution and electrode. I / mA 0.025 0.020 0.015 0.010 0.005 0.000 -0.005 -0.010 I / mA 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.15 0.10 0.05 0.00 -0.05 -0.10 Potential / V vs. Ag/AgCl Potential / V, vs. Ag/AgCl 15
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