Preparation and characterization of radical-coated nanoparticles
Abstract
The topic of this dissertation is the surface modification and functionalization of inorganic nanoparticles with free radicals. These particles have a diameter up to ca. 15 nm and consist of an inorganic core with an organic surface coating comprising open shell organic TEMPO radicals. Chapter 1 describes synthetic methods for the preparation of gold nanoparticles (AuNPs) and introduces the range of analytical methods for nanoparticle characterization. A survey of previous work on radical-coated nanoparticles is presented and the objectives of the current studies are outlined, highlighting the range of different bonding strategies for attaching the TEMPO radical to the AuNP surface. Chapter 2 describes the synthesis and characterization of citrate-coated AuNPs of 18 nm diameter. Treatment of these citrate-stabilized AuNPs with a water-soluble TEMPO radical [WS-TEMPO]Br led to aggregation at high [WS-TEMPO]Br concentrations identified by a long wavelength absorption but is suppressed at low concentration (10-4 M). Treatment with [WS-TEMPO]Br unexpectedly led to bromide/citrate exchange on the AuNP surface which was determined by IR spectroscopy and EDS. EPR studies on the resultant 14 nm nanoparticles reveal low concentrations of WS-TEMPO on the surface and these particles appear stable up to 345 K. Chapter 3 describes a one-pot reaction to covalently bond TEMPO radicals to the AuNP surface via an Au-S bond generated from TEMPO-thioacetate precursors or via a TEMPO-disulfide derivative. IR spectroscopy was used to identify cleavage of the thioacetate bond to generate a thiolate-bound TEMPO radical. TEM images reveal spherical AuNPs of ca. 3 nm diameter with EPR studies revealing high coverage of TEMPO radicals on the nanoparticle surface. Variable temperature UV/vis and EPR spectroscopies show that the effect of heat on these radical-coated AuNPs appears sensitive to the length of the alkyl chain. Thus, heat can variously lead to either an increase or decrease in AuNP size on heating. Chapter 4 examines the use of non-covalent, hydrophobic, dispersion driven interactions to tether TEMPO radicals to the AuNP. N-Octyl thiolate-coated AuNPs were prepared and treated with n-C8H17-O-TEMPO to afford 18 nm diameter nanoparticles. EPR studies reveal only low coverage of TEMPO radical on the surface. Preliminary studies showed that these radical-coated AuNPs could be used in the thermal polymerization of styrene, suggesting that free-radical moderation of the polymerization process is operative. Chapter 5 examines the synthesis of a more complex core topology, comprising a superparamagnetic iron core and gold outer shell of 9 nm diameter. This was then coated using a covalent S-bound thiolate-TEMPO surface. Unlike the S-bonded AuNPs described in Chapter 3, surface coating with TEMPO radicals is low. This system was confirmed by IR, EDX, PXRD, UV-vis and EPR spectroscopy. Chapter 6 summarises the findings of the current studies and reviews potential areas for future exploitation.