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_____________________________________________________________ Results and Discussion The duration of a pulse has to allow the system to respond to the perturbation invoked by applying a certain pulse profile. In order to achieve the highest SAM formation efficiency, the duration of a given potential pulse should be long enough to allow for an appropriate concentration gradient to form and a whole thiol molecule to be brought to the electrode surface. By this the formation of the Au-S bond is achieved regardless of the orientation of a thiol molecule. However, the pulse time should be also short enough to allow for a high number of pulsing cycles. Figure 3.35 demonstrates the influence of the pulse duration on the SAM formation kinetics. For the investigated system (MCU, ΔE = 400 mV) 10 ms pulse duration manifests as the optimal pulse time. Prolonging the pulse time leads to a less efficient ion stirring and with this a slower immobilization kinetics (Figure 3.35, yellow and green curves). On the other hand, decreasing the pulse time prolongs the time necessary to obtain a compact monolayer (Figure 3.35, blue curve) even though the number of stirring cycles during the overall immobilization time is increased. In order to demonstrate the influence of the length of the investigated thiol derivative on the optimization of the potential-pulse profile for potential-assisted immobilization, Figure 3.36 compares alkylthiols of three different lengths: MCH (6 carbon atoms), MCU (11 carbon atoms) and MCHD (mercaptohexadecanol, 16 carbon atoms). In all three cases, a more efficient ion stirring and faster SAM formation kinetics are achieved using a higher pulse potential difference, suggesting that the optimal potential difference does not depend on the molecule length as long as the chosen pulse duration is appropriate. In contrast, the optimal pulse duration depends on the molecule length, more precisely, on its diffusion coefficient. For the immobilization of short and intermediately long molecules (Figure 3.36, a and b) the 10 ms pulse duration is sufficiently long. Prolonging the pulse time leads to a lower number of pulse cycles per time and slower adsorption kinetics. The optimal potential-pulse assisted immobilization procedure for short and intermediate thiols employs the 0.5/-0.2 V pulse profile with 10 ms pulse duration. In case of longer thiols such as MCHD (Figure 3.36, c) 10 ms is too short to allow for the whole alkyl chain to be brought to the interface, which results in a slower adsorption kinetics. By prolonging the pulse duration to 10 s a much faster SAM formation rate is observed. Therefore, the optimal immobilization kinetics of longer thiols is achieved using the 0.5/-0.2 V pulse profile with 10 s pulse duration. At these conditions the capacitance reaches a plateau within only 5 min. 3.3 Importance of controlling the surface 74
_____________________________________________________________ Results and Discussion Figure 3.36. Optimization of the potential-pulse assisted SAM formation kinetics for three alkylthiol derivatives of different length: a) MCH, b) MCU, and c) MCHD. All measurements were performed as explained in Figure 3.34. Figure adapted from ref. 89 . According to the Helmholtz model of the double layer the interfacial capacitance of a SAMmodified electrode is inversely proportional to the thickness of the SAM, that is, the length of the alkyl chain of the thiol derivative 43 : C = εε 0 δ (3.8) 3.3 Importance of controlling the surface 75
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Jambrec Daliborka - 2016 DISSERTATI
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Acknowledgements For me, the PhD st
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5.1 Materials and consumables …..
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18. Canaria C. A.; So J.; Maloney J
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microarray using a biotinylated int
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