An Efficient Method for the Surface Functionalization of Luminescent Quantum Dots with Lipoic Acid Based Ligands

We describe herein an operationally advantageous general methodology for efficiently activating lipoic acid based compounds, a family of popular surface ligands for semiconductor nanocrystals, through the use of a borohydride exchange resin, and the use of the activated species to replace the native surface ligands of quantum dots. The procedure enabled phase transfer of the nanocrystals between polar and aqueous media and, if unsubstituted lipoic acid was used, a facile adjustment of their solubility in a wide range of solvents with varying polarity (from hexane to water). We show that the protocol is applicable to different types of nanocrystals and a variety of lipoic acid based ligands, and that the resulting quantum dots maintain their optical properties, in particular, an intense luminescence, and long-term colloidal stability

An efficient method for the surface functionalization of luminescent quantum dots with lipoic acid-based ligands

We describe an operationally advantageous general methodology to efficiently activate lipoic acid-based compounds - a family of popular surface ligands for semiconductor nanocrystals - by the use of a borohydride exchange resin, and the use of the activated species to replace the native surface ligands of quantum dots. The procedure enables the phase transfer of the nanocrystals between polar and aqueous media and, if unsubstituted lipoic acid is used, a facile adjustment of their solubility in a wide range of solvents with varying polarity (from hexane to water). We show that the protocol is applicable to different types of nanocrystals and a variety of lipoic acid-based ligands, and that the resulting quantum dots maintain their optical properties - in particular, an intense luminescence - and long term colloidal stability

Multicomponent nanodevices based on molecular and nanocrystal moieties

Nanoscience is an emerging and fast-growing field of science with the aim of manipulating nanometric objects with dimension below 100 nm. Top down approach is currently used to build these type of architectures (e.g microchips). The miniaturization process cannot proceed indefinitely due to physical and technical limitations. Those limits are focusing the interest on the bottom-up approach and construction of nano-objects starting from “nano-bricks” like atoms, molecules or nanocrystals. Unlike atoms, molecules can be “fully programmable” and represent the best choice to build up nanostructures. In the past twenty years many examples of functional nano-devices able to perform simple actions have been reported. Nanocrystals which are often considered simply nanostructured materials, can be active part in the development of those nano-devices, in combination with functional molecules. The object of this dissertation is the photophysical and photochemical investigation of nano-objects bearing molecules and semiconductor nanocrystals (QDs) as components. The first part focuses on the characterization of a bistable rotaxane. This study, in collaboration with the group of Prof. J.F. Stoddart (Northwestern University, Evanston, Illinois, USA) who made the synthesis of the compounds, shows the ability of this artificial machine to operate as bistable molecular-level memory under kinetic control. The second part concerns the study of the surface properties of luminescent semiconductor nanocrystals (QDs) and in particular the effect of acid and base on the spectroscopical properties of those nanoparticles. In this section is also reported the work carried out in the laboratory of Prof H. Mattoussi (Florida State University, Tallahassee, Florida, USA), where I developed a novel method for the surface decoration of QDs with lipoic acid-based ligands involving the photoreduction of the di-thiolane moiety

The effect of protons on CdSe and CdSe-ZnS nanocrystals in organic solution

none4noCore and core−shell quantum dots are covered with a layer of organic ligands which prevents aggregation and eliminates surface defects, thus enhancing the photophysical properties and stability of the material. These ligands are usually Lewis bases and can therefore be affected by the presence of acid in the surrounding environment. We synthesized core CdSe and core−shell CdSe−ZnS quantum dots with various shell thicknesses and different organic ligands, and we investigated the effect of acid and base on their photophysical properties. In dilute CHCl3 solution, the organic ligands can be protonated upon addition of acid and detached from the surface of the nanoparticles. As a consequence, the nanoparticles aggregate and their luminescence is quenched. Aggregated particles can be partly disgregated and the luminescence restored by deprotonation of the free ligands with a base. Since the presence of organic ligands on the surface is an essential characteristic of quantum dots, these effects should be taken into consideration when designing quantum dot-based sensors.mixedT. Avellini; M. Amelia; A. Credi; S. SilviT. Avellini; M. Amelia; A. Credi; S. Silv

Hybrids of semiconductor quantum dot and molecular species for photoinduced functions

Semiconductor quantum dots are inorganic nanocrystals which, because of their unique size-dependent electronic properties, are of high potential interest for the development of light-responsive nanodevices. Their surface can be chemically modified, by either covalent or non-covalent approaches, in order to interface them with molecular units endowed with specific physico-chemical properties. Photoinduced electron- and energy-transfer processes between quantum dots and attached molecular species offer versatile strategies to implement functionalities such as photosensitized processes, and luminescence sensing and switching. In this review we will discuss the strategies underlying the rational construction of this kind of multicomponent species, and we will illustrate a few examples taken from our own research

Functional Supramolecular Systems Controlled by Light

Molecules can be used as building blocks for the assembly of multicomponent (supramolecular) structures exhibiting novel and complex functions that derive from the cooperation of simpler functions performed by each component. These systems, if suitably designed, behave as devices and machines of nanometric size. In analogy to their macroscopic counterparts, molecular devices and machines need energy to operate and signal to communicate with the operator. Light provides an answer to this dual requirement: photons are indeed both quanta of energy and elements of information (by means of the spectroscopic techniques). In this chapter we describe a few recent examples developed in our laboratory with the aim to show that, in the frame of research on supramolecular photochemistry, the design and construction of nanoscale devices and machines capable of performing useful light-induced functions can indeed be attempted. They are (a) molecular machines capable of undergoing light-controlled unidirectional linear movements, (b) dendrimers in which predetermined energy transfer processes occur, (c) quantum dots that behave as luminescent chemosensors, and (d) assemblies of molecules that upon light inputs and/or outputs can perform logic functions. These systems represent some small, although interesting, advancements in exploiting the peculiar properties of light and its interaction with matter to obtain useful functions

Effect of Protons on CdSe and CdSe–ZnS Nanocrystals in Organic Solution

Core and core–shell quantum dots are covered with a layer of organic ligands which prevents aggregation and eliminates surface defects, thus enhancing the photophysical properties and stability of the material. These ligands are usually Lewis bases and can therefore be affected by the presence of acid in the surrounding environment. We synthesized core CdSe and core–shell CdSe–ZnS quantum dots with various shell thicknesses and different organic ligands, and we investigated the effect of acid and base on their photophysical properties. In dilute CHCl₃ solution, the organic ligands can be protonated upon addition of acid and detached from the surface of the nanoparticles. As a consequence, the nanoparticles aggregate and their luminescence is quenched. Aggregated particles can be partly disgregated and the luminescence restored by deprotonation of the free ligands with a base. Since the presence of organic ligands on the surface is an essential characteristic of quantum dots, these effects should be taken into consideration when designing quantum dot-based sensors

Redox properties of CdSe and CdSe–ZnS quantum dots in solution

Semiconductor quantum dots (QDs) are inorganic nanoparticles which, because of their unique size-dependent electronic properties, are of high potential interest for the construction of functional nanodevices. Photoinduced electron transfer is a versatile mechanism used to implement light-induced functionalities in multicomponent (supra)molecular assemblies. Indeed, QDs can be employed as active components in new generations of these systems. The rational design of the latter, however, requires prior knowledge of the photo-physical properties and redox potentials of the nanocrystals. Here we discuss the results of recent systematic electrochemical investigations aimed at understanding the structural factors that regulate the redox properties of CdSe core and CdSe–ZnS core–shell QDs

Modulation of the solubility of luminescent semiconductor nanocrystals through facile surface functionalization

The solubility of luminescent quantum dots in solvents from hexane to water can be finely tuned by the choice of the countercations associated with carboxylate residues present on the nanocrystal surface. The resulting nanocrystals exhibit long term colloidal and chemical stability and maintain their photophysical properties