| Gold clusters protected with self-assembled monolayers of sodium, 10-mercaptodecanesulfonate, HS(CH2)10SO 3Na, and 11-mercaptoundecanylphosphonic acid, HS(CH2) 11PO3H2, were synthesized to provide negatively charged nanoparticles. The negatively charged end groups of the sulfonate- and phosphonate-monolayers protected clusters, SO3-MPCs and PO 3-MPCs, allowed the particles to be water soluble in addition to be stable in air, and can be dried and redissolved without change. Modifications of experimental conditions, namely the thiol:gold ratio, gold concentration and the type of surfactant, yielded MPCs of 1.8--5.7 nm core diameters with a broad range of size dispersity, and solubilities. Transmission electron microscopy, proton nuclear magnetic resonance (1H NMR), UV-Visible and infrared spectroscopies, thermogravimetric and zeta-potential analyses were consistent with nanoparticle formation. The chain conformation, dynamic behavior and ordering of SO3-MPCs and PO3-MPCs were further investigated by solid-state NMR. Despite the high surface curvature of the nanoparticles and the bulky end groups, medium length chains of SO 3-MPCs (C10) and PO3-MPCs (C11) showed a high degree of organization and restricted mobility when compare to long chains of alkanethiolate MPCs (C18). The more extensive hydrogen bonding interactions between the phosphonic acid groups led to a reduced chain mobility and a higher thermal stability. The PO3-MPCs exhibit a high order-disorder transition, partial decomposition and a very marked hysteresis in the chain recrystallization similar to MPCs stabilized by HS(CH2)nCO2H. Solid-state 31P-NMR allowed studies of the hydrogen-bonded self-assembled structure in the PO3-MPCs as well as in the bulk phosphonic acid. The 1.8 nm-diameter SO3-MPCs produced large aggregates in presence of poly(diallyldimethyl-ammonium chloride) (PDADMAC). However, the encapsulation of individual particles was favored when the same polyelectrolyte was adsorbed on the 3.1 nm-diameter PO3-MPCs, which to date is the smallest reported coated particle size. |
