Surface engineered quantum dots in photoelectrochemistry and supramolecular assembly

This thesis demonstrates the power of chemical surface engineering in the design and fabrication of functional hybrid materials made of Quantum Dots. The small size of the QDs, in the range of 1 to 10 nm, and related stability in solution, require a careful consideration of a proper surface chemistry for the ligand shell. By a judicious choice of the coating one can remarkably influence the physicochemical and photophysical properties of the semiconductor nanocrystals as well as design and engineer new generations of advanced nanoscale materials. This work describes in detail the synthetic approaches to chemical surface functionalization of QDs with electroactive ligands, including ferrocenyl thiols and poly(ferrocenylsilanes), and with B-cyclodextrin (B-CD) ligands suitable for supramolecular host-guest assembly. These functional ligands are shown to be important components in the engineering of new types of QD hybrid materials. The influence of the electroactive ligands on the optical properties of QDs was investigated by spectroscopic and electrochemical methods. These investigations gave an important insight into the quenching mechanisms of QDs by ferrocene and to the fundamental electron transfer processes in hybrid materials composed of QDs and electro-active ligands. Additionally, ferrocene groups located on the QD surface were shown to be able to take part in host-guest complexation reactions with B-cyclodextrin in solution. This ability was useful in the phase transfer of hydrophobic nanoparticles between solvents of markedly different polarities. The complexation ability of B-CD-functionalized QDs and adamantyl dendrimers was exploited for the preparation of supramolecular multilayer structures on surfaces. Surface bound QDs were shown to be able to transduce optically the binding events to the B-CD cavity, a proof-of-principle for a sensor design. This thesis demonstrates that both FRET and ET can be used as the transduction mechanisms. Thus, proper surface design and engineering of QDs gives unique opportunities to obtain the new class of hybrid materials using numerous functionalization approaches and surface chemistries.
Chapter 2 introduces the most important terms and definitions related to QDs, and describes the QD structure, synthesis and photophysical properties. Chemical engineering of the QD surface ligand shell via ligand exchange and methods for the immobilization of QDs on surfaces were discussed. The analytical tools for the characterization of QDs were introduced, and finally some applications of QDs in optoelectronics, sensing and life sciences were described.
Chapter 3 describes the investigation of the photoluminescence quenching of QDs by ferrocene. Ferrocenyl thiols with variable length of the alkyl thiol were coated on the surface of CdSe/ZnS QDs. The ferrocene was shown to be a good quencher of QD luminescence and the quenching efficiency depended on the distance between ferrocene and the QD. By decreasing the spacer length the quenching efficiency was shown to increase. A quenching mechanism involving a hole transfer between the photoexcited QD and the ferrocene was proposed. Electrochemical characterization of TOPO-coated and ferrocenyl-coated QDs in non-aqueous solution is presented in Chapter 4. Cathodic reduction and anodic oxidation processes involving the QD HOMO and LUMO levels as well as defect states were identified by cyclic voltammetry. The electrochemical bandgap was estimated from the anodic and cathodic redox peaks and found to match well the optical bandgap estimated from the absorption spectrum. Cyclic voltammetry showed that the redox potentials of the QDs are modified due to the presence of ferrocene on the surface of the QD. A new electrochemical peak associated with oxidation of ferrocene appeared on the cyclic voltammograms and the anodic and cathodic redox peaks of the QDs were shifted to more negative potentials due to the presence of the Fc ligand. Concurrent shift of the ferrocene redox peaks indicates that our system displayed features of a “molecular hybrid”, where both the QD and the ligand influence each other.
In Chapter 5 ferrocene-coated QDs were shown to undergo reversible phase transfer to and from the aqueous phase upon formation/release of inclusion complexes of B-cyclodextrin with the ferrocene units on the surface of the QDs. The release has been achieved by decomplexation via competitive reaction of B-cyclodextrin with naphthalene and adamantane derivatives. The emission of QDs in the water phase was quenched due to the presence of the cyclodextrin. The use of adamantane as the decomplexating agent resulted in clear and transparent solutions of QDs in the chloroform phase unlike for the case of naphthalene where aggregation of the QDs was observed.
Chapter 6 describes two routes to obtain QD patterns on surfaces using multivalent supramolecular host-guest interactions. To this end, B-CD-functionalized QDs were obtained by covalent coupling of amine-terminated B-CD to carboxy-functionalized QDs. The QD/B-CD materials were employed in the formation of a multilayer structure using dendrimeric supramolecular “glue”. The stability of the resulting patterns was demonstrated by further formation of host-guest complexes with the unused B-CD cavities on the QD surface. In particular, the luminescence of QDs was modulated by complexation reactions with ferrocene-functionalized dendrimers printed across the QD patterns. Due to the presence of ferrocene the luminescence of the QDs visibly decreased.
Chapter 7 describes how the supramolecular multilayer assemblies could be used as sensing platforms using FRET as the signal transduction mechanism. The vacant B-CD cavities located on the surface of the QDs formed host-guest complexes with adamantyl-functionalized dyes. Fluorescence microscopy, spectroscopy and FLIM experiments clearly demonstrated that a FRET process was occurring only in the areas where the analyte formed host-guest complexes with the B-CD functionalized QDs. Finally, the modification of QDs with poly(ferrocenylsilanes) (PFS) using a “grafting to” approach is described
In Chapter 8. two thiol end-functionalized PFS polymers of molar masses of 7750 g/mol and 12500 g/mol were attached to the nanocrystal’s surface via ligand exchange reaction. The resulting QD/polymer hybrid material displayed spectral shifts in the absorbance and luminescence, which were accompanied by a decrease in the luminescence. The obtained QD/polymer conjugates were investigated using diffusion ordered H NMR spectroscopy. This characterization technique gives evidence for the presence of the polymer chains on the QD surface.