This thesis focuses on the symbiotic effect between CdSe/ZnS core-shell QDs or CdSe core QDs and their organic ligands, and the advantages of this effect to improve the functionality of the QD or/and of the ligand, or create a new functionality of the device. Thus,
1. the nanoparticle surface capping with organic ligands can allow the QD i) to remain stable in organic solvents or water, by providing steric or ionic repulsion between the nanoparticles, ii) to preserve/enhance their emissive properties (passivation of the surface defects, isolation from quenchers), and/or iii) to add functionality to the nanoparticle.
2. the spherical shape of the nanoparticle makes the location of an elevated number of ligands on the QD surface possible. Therefore, it is possible to have a high local concentration of a functional group at the QD periphery, in an otherwise diluted solution.
3. the combined action of the QD and the ligand can permit the encapsulation/interdigitation of other molecules, bringing them closer to the QD surface. In addition, the functional ligand can reversibly modify the nanoparticles emissive properties and this can be used, for example, for sensing applications.
There is great interest on the passivation of CdSe core and CdSe/ZnS core-shell QDs with organic ligands possessing a mercapto group at one end, because of the high affinity of this anchoring group to the QD surface, mainly when it is as thiolate. Nevertheless, exchange of the ligands of the QDs (usually prepared with amino ligands) by thiol (core) or thiolates (core-shell) causes a remarkable reduction of the luminescent properties of both type of QDs. In the case of the core QDs, the thiol traps the hole of the excited QD, and therefore prevents the electron-hole recombination. The low emission of the core-shell QD has been attributed to the alteration of the QD shell during the ligand exchange.
We have developed a new strategy to obtain highly fluorescent organic- soluble and water-soluble CdSe/ZnS QDs passivated with thiolates, based on the
ligand plays a key role, by dissociating the thiol in the vicinity to the QD surface. The replacement of the long chain amines by thiols was performed under mild conditions, and it allows preserve or even improve the QD luminescent properties. This methodology was tested in home-made and commercially available nanoparticles. Moreover, the QDs were more photostable than those capped with amine.
These results are of interest for constructing new devices of application in sensing or molecular recognition, based on energy or electron transfer processes. Thus, that strategy was used in the preparation of highly fluorescent water-soluble CdSe/ZnS QDs capped with a bifunctional ligand, with a mercapto group (the anchoring group) at one end and a benzophenone moiety at the other. The QD was used to develop a supramolecular system, based on ketoprofen-functionalized CdSe/ZnS QDs and pyrene-modified cyclodextrin (CD). The ketoprofen units play a key role, ensuring the molecular recognition between the nanoparticle and the cyclodextrin and, at the same time, places the pyrene unit near to the nanoparticle surface, resulting in QD fluorescence quenching. This symbiotic effect was successfully used for molecular sensing of different analytes. The sensing of this easy-to-prepare and effective hybrid system is based in the recovery of the QD fluorescence intensity in response to different analytes. In addition of this, the methodology proposed allows the individual recovery of all the components used in the mentioned system, which is highly desirable both from the commercial and environmental points of view.
As stated before, the QD can be used as nano-carrier of a functional group, leading to increase of the local concentration of the functional group in an otherwise diluted solution. In this thesis, CdSe QDs were used as carriers of pyrene moieties, and, as a consequence, making the formation of the pyrene excimer possible by using a very low concentration of the pyrene ligand. The CdSe QD luminescence was drastically reduced by using a pyrene ligand with mercapto group as the anchoring group. The system was used to study the influence of the QD on the photostability of the pyrene in chlorinated solvents and the results were
compared to those of the free pyrene. These studies evidenced that the combination of chloroform and light produces a high photodegradation of all the pyrene systems, while they were highly stable in dichloromethane.
Finally, we studied highly fluorescent QDs-organogels prepared from a pseudopeptidic macrocycle and both types of QDs, i.e. CdSe and CdSe/ZnS capped with different ligands. These hybrid organogels were easily prepared, by heating the organogelator in toluene, followed by the addition of the QDs and the cooling of the system to room temperature. The presence of the QD in the organogel system decreases the critical concentration of the gelator needed to form stable and thermoreversible organogels, without affecting significantly the internal fibrillar network of the organogel. In addition, the effect of the organogel on the luminescent properties of the QDs depends on the presence of the ZnS shell. The photophysical properties of CdSe/ZnS core-shell QDs are preserved in the organogel media, while in the case of CdSe core QDs, an enormous increase of the fluorescence intensity was observed. We demonstrate that the macrocycle interacts with the CdSe QD ligand even before gelation, by using common methodologies, such as IR and NMR spectroscopies. This is the first time that the interaction between an organogel and a QD has been reported.