Conclusion

The underlying mechanism of the exact interaction between analyte and polymer/SWCNT is not fully understood [54], [101], [254]. In this chapter we performed complementary exper-iments to investigate four possible sensing mechanisms: (1) a pure adsorption-based mech-anism, (2) a sensing mechanism with a major role of a redox-reaction oxidizing either the SWCNT (3) or the polymer, and (4) fluorescence change due to the conformational change of the polymer upon analyte binding.

Our data did not support a pure adsorption-based mechanism (1). We calculated the free area on the surface of SWCNTs for different polymer wrappings based on the solvatochromic shifts ∆E_ii. This method is based on the idea that the optical properties of SWCNTs are medium depended. Our results demonstrated no correlation of polymer coverage and the observed fluorescence changes.

Second mechanism involves redox-reaction with the SWCNT (2) or the polymer (3). We were able to eliminate the involvement of reactive oxygen species (ROS) in the observed fluorescence change by adding them to our experiments. There was no significant change between ’normal’ fluorescence responses and fluorescence responses for sensor + ROS mix-tures. Electron transfer to and from the SWCNT itself was also excluded. Populating the conduction band or depopulating the valence band is known to quench both absorption and fluorescence emission [237], [255]. Our results showed no changes in the absorption spectra before and after the addition of an analyte. Furthermore, the response pattern was the same for all chiralities, which also indicates that the oxidation potential of SWCNT is not relevant for the sensing mechanism. Moreover, since the brightening of polymer/SWCNTs

is reversible, it is not likely that these molecules introduce permanent covalent defects in the carbon lattice. Therefore, the fluorescence increase is not caused by the removal of dark excitons via covalent chemistry [104]. Meanwhile, a mechanism that would non-covalently perturb the sp² network is in agreement with the observed patterns. Hence, we can exclude a stoichiometric redox reaction, however a catalytic involvement of redox active species could still influence exciton dynamics.

Results for investigation of the direct electron transfer to or from the polymer (3) were mixed. For various DNA sequences this hypothesis would be sustainable. Guanine acts as an electron sink [235] and guanine-rich DNA wrapping showed the highest fluorescence change in our experiments. However, for this theory to be satisfactory demonstrated, it should explain other sensor responses by showing how the polymer is either accepting or donating the electrons. However, for PAA it is difficult to imagine which functional groups could be reduced. Moreover, since the fluorescence change is reversible, a possible backward reaction makes a purely redox mechanism improbable [54]. It is therefore possible that the redox mechanism is either not involved in the general sensing or only contributes to the overall florescence change in case of DNA-sequences and similar polymers.

The role of redox potential is ambiguous in our data. The highest responses were indeed ob-served for molecules with the lowest reduction potential (<−0.4 V). However, the inversion of the argument does not work, as not all molecules with that redox potential induce the same fluorescence response. Moreover, the specific fluorescence changes range from +250%

to−50% depending on the exact wrapping polymer, which again emphasizes the import role of the organic phase. Therefore, our results support the idea that the redox potential is in-volved in the sensing mechanism, but the redox potential alone cannot explain the response patterns. Interestingly, the responses to reducing molecules also seem to correlate with the charge of the polymer. For negatively charged polymers we observed a fluorescence increase, while for positively charged polymer the same analytes caused a fluorescence decrease.

Finally, the last possible mechanism is a conformational change of the polymer that affects exciton decay routes. It was shown that small changes in molecule distribution around the SWCNT could change the exciton decay routes, and consequently the SWCNT fluorescence [256]. Experimentally, it is difficult to distinguish if a perturbation of the polymer around SWCNT is caused by a redox reaction or by any other interaction between the polymer and the analytes. The finding that PAA (with no apparent groups that could be reduced) is also sensitive to redox molecules supports this theory. Furthermore, the sensor response for PAA showed the same pattern as for DNA-wrappings, however with a lower magnitude. It is possible that in the case of PAA the interactions between functional groups and analyte are responsible for the conformational change of the polymer, while for DNA the conformational change is caused by both polymer-analyte interaction and a catalytic redox reaction.

Figure 31: Proposed sensing mechanism based on conformational change of the poly-mer around SWCNT. Dopamine pulls phosphate groups from the DNA-backbone towards the SWCNT surface. Phosphate groups then take the place of quenching sites which are thus removed. This shift increases SWCNT fluorescence (MD simulations).

Adapted from [56].

Furthermore, MD simulations conducted by Lela Vukovic and Klaus Schulten support the hypothesis that interaction of the analyte with the wrapping polymer can change the local potential experienced by excitons and consequently change the fluorescence quantum yield of SWCNTs [56]. The proposed mechanism suggests that dopamine interacts via two hydroxy groups of the chatecholamine unit with the phosphate groups of the DNA backbone, as shown in Figure 31. The phosphate groups (PO⁻₄) are pulled closer to the surface of SWCNT and move the water molecules aside. The change in the local exciton potentials induced by dopamine binding lies in the range of several kcal/mol.

In summary, we demonstrated that the organic phase of SWCNT-based sensors plays a crucial role, especially in case of redox-active compounds. This work has both practical and theoretical implications. First, we created a library of several polymers and analytes to choose from for various applications. Second, we investigated several possible sensing mechanisms and our results support the explanation that observed response patterns are caused by conformational changes of the polymer around SWCNTs.