Calix[n]arene-based Monolayer Protected Clusters (MPCs): Synthesis, Studies and Applications

This thesis deals with the synthesis, characterization and study of metallic nanoparticles (M = Au, Pd, Ag, Cd-Se) functionalized on their surface with calix[n]arene derivatives. These hybrid organic-inorganic materials, more commonly called "monolayer-protected clusters" (MPCs), have recently attracted a growing scientific interest for their potential applications in catalysis, in the realization of optical devices, electrical or magnetic sensors or more in general in the field of nanotechnology. In each of these applications the control, during the preparation, of the size of the nanoparticles obtained is important. The idea of preparing MPCs stabilized on their surface with macrocyclic receptors came from the possibility of combining the recognition properties of calix[n]arene derivatives with the typical properties of metal colloids in order to obtain "smart" material capable of interesting functions, that are able to perform actions in response to external chemical or electrochemical stimuli. In the first phase of the thesis it has developed new synthetic methodologies of calix[n]arene derivatives functionalized at the lower rim with alkyl chain bearing, at their ends, thiols functional groups suitable for covalent binding on metal surfaces. In a second phase it has developed reproducible synthetic methodologies for the preparation of metal nanoparticles having a core size between 0.8 -7 nm functionalized with calix[n]arene derivatives. This study has allowed us to evaluate various experimental parameters that lead to obtain size controlled and monodisperse gold nanoparticles, such as reaction temperature, duration, stochiometry of thiols and gold etc.. These studies have highlighted in particular the "denticity" of the ligand (number of thiolic chains on macrocycle) on the size and stability of such systems. All prepared nonosystems are studied by microscopic (TEM), superficial analysis (XPS, XRD), spettrometric techniques (NMR, UV-vis) and chemical techniques (elemental analysis and TGA). These thiolic derivatives were also used for the surface coating of Cd-Se semiconductor nanoparticles for the preparation of Quantum Dots-in collaboration with the University of Miami (USA). Such synthesized organic-inorganic hybrid materials were subsequently used for processes of self-assembly in solution exploiting the receptor capabilities of calix[n]arene derivatives on the surface with piridinium salts. These studies, conducted at the University of Konstanz (Germany), had been conducted using optical techniques (UV-vis and Dynamic Light Scattering) and have shown the aggregation processes in solution and the structural properties of such systems.Questo lavoro di tesi riguarda la sintesi, la caratterizzazione e lo studio di nanoparticelle metalliche ( M = Au, Pd, Ag e Cd-Se) funzionalizzate sulla loro superficie con derivati macrociclici di tipo calix[n]arenico. Questi materiali ibridi organici-inorganici, più comunemente chiamati “monolayer-protected clusters” (MPCs), hanno recentemente attirato un crescente interesse scientifico per le loro potenziali applicazioni nella catalisi, nella realizzazione di congegni ottici, elettrici o magnetici, nella sensoristica o più in generale nel campo delle nanotecnologie. In ciascuna di queste applicazioni è di primaria importanza il controllo, durante la fase di preparazione, delle dimensioni delle nanoparticelle ottenute. L’idea di preparare MPCs stabilizzati superficialmente con recettori macrociclici nasce dalla possibilità di coniugare le capacità recettoriali dei derivati calix[n]arenici con le proprietà tipiche dei colloidi metallici in modo da realizzare materiali “intelligenti” capaci di interessanti funzioni, cioè in grado di effettuare azioni in risposta a stimoli esterni di tipo chimico o elettrochimico. Nella prima fase della tesi si sono sviluppate le metodologie sintetiche di derivati calix[n]arenici opportunamente sul bordo inferiore con catene alchiliche recanti alle loro estremità gruppi funzionali tiolici adatti all’ancoraggio covalente dei macrocicli sulle superfici metalliche. In una seconda fase si è passati allo sviluppo di valide e riproducibili metodologie di sintesi volte alla preparazione di nanoparticelle metalliche di dimensioni tra 0.8 -7 nm, funzionalizzate con i diversi derivati calix[n]arenici preparati. Questo studio ha permesso di valutare diversi paramentri sperimentali che portano all’ottenimento di nanoparticelle monodisperse di oro di dimensione controllata quali ad esempio la temperatura di reazione, durata, rapporto stechiometrico legante tiolico e aurato ecc. Questi studi hanno evidenziato in modo particolare la “identicità” del legante(numero di catene tioliche presenti sul macrociclo) sulle dimensioni e sulla stabilità di tali sistemi. Tutti i nanosistemi preparati sono stati caratterizzati mediante tecniche microscopiche (TEM), di analisi superficiale (XPS,XRD), spettrometriche (NMR, UV-vis) e chimiche (analisi elementare e TGA). Alcuni derivati tiolici sono stati inoltre impiegati per il coating superficiale di nanoparticelle semiconduttrici di Cd-Se per la preparazione di Quantum-Dots in collaborazione con l’University of Miami (USA). I materiali ibridi organici-inorganici sintetizzati sono stati successivamente utilizzati per processi di self-assembly in soluzione sfruttando le capacità recettoriali dei derivati calix[n]arenici posti sulla superfice nei confronti di sali di piridinio elettrochimicamente attivi. Questi studi, effettuati presso l’Univeristà di Costanza (Germania) sono stati condotti mediante tecniche ottiche (UV-vis e Dynamic Light Scattering) che hanno permesso di constatare gli effettivi processi di aggregazione in soluzione e di valutarne le proprietà strutturali

Guest Controlled Assembly of Gold Nanoparticles Coated with Calix[4]arene Hosts

Gold nanoparticles protected with thiolate Calix[4]arenes hosts were synthesized through an exchange reaction in toluene, starting from tetraoctyl ammonium bromide stabilized gold nanoparticles having a mean core size of 6 nm. In low polar solvents, these new Calix[4]arenecoated nanoparticles are able to self-assemble through supramolecular interactions with dialkyl dipyridinium-based guests (2−3). The guest-induced self-assembly process between nanoparticles has been studied using UV−vis spectroscopy, dynamic light scattering, and TEM measurements. The size and the solubility of the aggregates strongly depend on the length and rigidity of the bifunctional guest used as “supramolecular linker” between the nanoparticles. In particular, the long and flexible guest 2 gives rise to superaggregates of nanoparticles that remain soluble in common low polar solvents

Luminescence quenching in supramolecular assemblies of quantum dots and bipyridinium dications

We have investigated the ability of two bipyridinium dications, with either octyl or decyl groups on their nitrogen atoms, to quench the luminescence of CdSe–ZnS core–shell quantum dots coated by either tri-n-octylphosphine oxide or a tris(phenylureido)calix[6]arene. Our studies demonstrate that both bipyridinium dications adsorb on the surface of the quantum dots with association constants ranging from 104 to 107 M−1 and quench the luminescence of the inorganic nanoparticles with rate constants ranging from 108 to 109 s−1. The association constants of these supramolecular assemblies vary significantly with the counterions of the bipyridinium dications and the ligands on the nanoparticle surface. Their quenching rate constants vary with the length of the alkyl chains appended to the bipyridinium core and, once again, the ligands on the nanoparticle surface. Furthermore, our studies show that the addition of a calix[6]arene able to compete with the quantum dots for the bipyridinium quenchers restores the original luminescence intensity of the nanoparticles. Indeed, the supramolecular association of the calix[6]arene with the bipyridinium dication removes the quencher from the nanoparticle surface and, hence, is transduced into a luminescent enhancement

Guest Controlled Assembly of Gold Nanoparticles Coated with Calix[4]arene Hosts

Gold nanoparticles protected with thiolate Calix[4]arenes hosts were synthesized through an exchange reaction in toluene, starting from tetraoctyl ammonium bromide stabilized gold nanoparticles having a mean core size of 6 nm. In low polar solvents, these new Calix[4]arenecoated nanoparticles are able to self-assemble through supramolecular interactions with dialkyl dipyridinium-based guests (2−3). The guest-induced self-assembly process between nanoparticles has been studied using UV−vis spectroscopy, dynamic light scattering, and TEM measurements. The size and the solubility of the aggregates strongly depend on the length and rigidity of the bifunctional guest used as “supramolecular linker” between the nanoparticles. In particular, the long and flexible guest 2 gives rise to superaggregates of nanoparticles that remain soluble in common low polar solvents

Ascorbic acid content in apple pulp, peel, and monovarietal cloudy juices of 64 different cultivars

<p>The present work was designed to compare the vitamin C (ascorbic acid, AsA) content of pulp, peel, and juice of 64 apple cultivars. These cultivars were carefully identified as ‘true to type’ by molecular genetic tools, grown in the same site under identical conditions and processed by a standardized protocol. Twenty-one of them, accounting for more than 95% of the apple production of South Tyrol, were chosen to represent the current market, 16 were old or local cultivars formerly grown in the area, and 27 were new cultivars, including 15 with scab resistance and 12 with red flesh fruit. For the determination of the AsA content, a new High-Performance Liquid Chromatography with Diode-Array Detection method was developed and validated. While old cultivars stood out for their high AsA content in pulp and peel, the red-fleshed cultivars are the ones maintaining most of their AsA content during processing. Our data thus suggest a potential for old and red-fleshed cultivars for healthy juices or further processed food components.</p

Mead fermentation monitoring by proton transfer reaction mass spectrometry and medium infrared probe

Mead is a traditional alcoholic beverage similar to wine, but obtained by the fermentation of a diluted solution of honey. The rate of fermentation is generally monitored by the measurement of a set of physicochemical variables such as pH, titratable acidity, Brix degrees, sugars and ethanol concentration. This work aims at developing a new monitoring method for alcoholic fermentations that is based on two on-line approaches: a proton transfer reaction mass spectrometry (PTR-MS) and a fibre optic coupled attenuated total reflection (FTIR-ATR) spectroscopy. Microfermentations are performed on 100 mL musts in isothermal conditions at 20 °C. Musts consist on diluted honey solutions (24 Bx) with pollen (0.4 % w/v) and yeast (Saccharomyces cerevisiae subsp. bayanus). The effect of flavour enhancers [chilli (Capsicum annuum), clove (Eugenia caryophyllata) and a mixture of both] on the rate of fermentation was also evaluated. The results show that clove inhibits fermentation, whereas chilli increases the rate of fermentation. PTR-MS and FTIR-ATR are simple, fast and nondestructive techniques able to monitor the fermentation process without the need of sample preparation, extraction or pre-concentration steps