(172b) pH-Dependent Controlled Clustering of Polymer-Functionalized Gold and Magnetite Core-Shell and Janus Nanoparticles

In recent years, there has been increasing interest in the potential applications of functionalized nanoparticles, with a concomitant strong scientific focus on their fundamental properties. These functionalized nanoparticles, prominent among which are gold(Au) and magnetic (Fe3O4) nanoparticles, are applied in a wide range of fields, including drug delivery systems, diagnostics, gene analysis, proteomics, affinity purifications, quantum dots, and the like. There are undoubtedly still many gains to be made in the synthesis, functionalization, characterization and applications of these systems. Gold nanoparticles display electromagnetic resonance due to collective oscillations of their free electrons known as plasmons. For particles which are small compared to the wavelength of the incident light, plasmons are excited when light with an appropriate wavelength is shone on them. This results in a strong absorption at that wavelength. Because of plasmon behavior, different conditions of aggregation result in different colors; which helps us for a visual determination of the aggregation condition of particles. On the other hand, the magnetite nanoparticles can be controlled by an external magnetic field. It is very important to prepare nanoparticles with a well defined size distribution and with surface functionalization because of their ferrohydrodynamic behavior. In this report, we present polymer functionalized gold and magnetite nanoparticles. Two forms of functionalization have been employed which resulted in the core shell type and the Janus type nanoparticles.

Poly acrylic acid (PAA)-coated core-shell gold and magnetite nanoparticles were prepared by surface initiated atomic transfer radical polymerization (ATRP) (namely "grafting from method") as has been the recent trend in synthesizing core-shell type nanoparticles. ATRP is a well-known technique for creating a narrow polymer distribution which allows us to make core-shell nanoparticles with a precisely designed and high density polymer shell. The methods of attaching polymer on the surface of nanoparticles (namely "grafting onto method") tend to lead to a relatively low graft density because of the steric repulsions. In addition, PAA is a well-known and a very useful polymer because of its numerous carboxyl groups. We can control the aggregation and dispersion by changing pH because of the isoelectric point for PAA(pKa=4.5). Also these carboxylic groups on the surface help in the easy functionalization of the nanoparticles. We examined the PAA coated core shell structure of nanoparticles by TEM, TGA, FT-IR, GPC, DLS, UV/Visible spectroscopy (for gold particles) and Zeta potential measurement and their pH dependent behaviors by DLS, Zeta potential and UV/Visible spectroscopy (for gold particles) at pH values from 10 to 2.

Also, we have recently developed methods for the synthesis of a new set of functional nanoparticles with asymmetric surface properties "C Janus Particles. In previous studies, Janus particles have been prepared for particles on the order of 100s of nanometers, but never before at the 10 nm scale. Specifically, we have prepared nanoparticles that have a composite structure consisting of a nanocolloid and a soft material. The colloid size is approximately 5-10 nm and the functional polymer is on only one side of the colloid. This Janus particle can have a surfactant-like structure if the remaining colloid surface has different properties than the tail polymer. Consequently, Janus nanoparticles can self-assemble into micelle-like structures depending on medium conditions such as pH, temperature, concentration and additional salt. We characterized Janus particles by TEM, DLS, and Zeta Potential.

Preparation and characterization of the core-shell type of nanoparticles

Poly(acrylic acid) coated core-shell type of gold and magnetite nanoparticles were prepared by deprotection of poly(trimethylsilyl acrylate)(TMSA) which was polymerized on the gold nanoparticles by ATRP (by using the grafting from method). Gold nanoparticles were prepared in the presence of 11-mercaptoundecanol(MUD) and subsequently esterified with 2-bromoisobutyryl bromide (BIB) for ATRP initiation (Mandal et al.,NANO lett. 2, 3(2002)), and the following polymerization of TMSA occurred on the surface of gold particles. Magnetite nano particles were prepared following the organic synthesis route proposed by Sun et al.(JACS 126, 273(2004)). Ligand exchange reaction of galactic acid was done and subsequently esterified with BIB for ATRP initiation and the following polymerization of TMSA occurred on the surface of magnetite nanoparticles (Langmuir, 23, 2158 (2007)).

TEM images show the size of gold and magnetite to be about 10nm and 5nm respectively. We observed core-shell structure of both gold and magnetite nanoparticles with 2-3nm PAA shell. At pH greater than 4.5, the PAA coated nanoparticles were well-dispersed for at least 1 month. At acid condition smaller than pH 4.5, these particles aggregated due to a decrease in their negative zeta potential. The optical properties of the clustered PAA coated gold nanoparticles exhibit red shift, together with damping and broadening of the surface plasmon features at pH 2. We have investigated this clustering behavior by UV/visible spectroscopy and DLS as a function of time. These clusters slowly re-dispersed on changing the pH from 2 to 7.

Preparation and characterization of the Janus particle

Our Janus nanoparticles consist of either magnetite or gold crystals coated on one side with a pH-dependant polymer (PAA) that is charged at high pH but not at low pH, with the other side functionalized by a second polymer that is always charged, independent of the solution pH. Therefore, these Janus particles can be dispersed individually at high pH values, but can self assemble at low pH value. The first step in the preparation of Janus nanoparticels consists in preparing nanoparticles(magnetite and gold) with size approximately 5-10 nm, following which they are coated them poly acrylic acid (PAA), both of which have pKa values below neutral pH. In particular, these nano colloids have high negative surface charges of about -50mV at pH10 and are uncharged at a pH of 2. Next, large positively charged silica beads with diameters of approximately 700nm are prepared using a silane coupling agent with a trimethyl ammonium group. The positively charged silica beads are then dispersed in the nanoparticle suspension at high pH, and coated by a monolayer of nanoparticles through electrostatic adsorption. The surfaces of the nanoparticles facing the silica beads are protected from further functionalization while amino-end functionalized polystyrene sodium sulfonate (PSSNa Mw:90kD) are attached to the exposed surfaces of the nanoparticles through amidation chemistry. The recovery of the Janus particles thus synthesized is achieved by increasing the pH of the solution thereby inducing a charge reversal on silica bead surface. The pH dependent aggregation behaviors of both polymer-coated magnetic nanoparticles and Janus magnetic nanoparticles have been measured by dynamic light scattering (DLS). The results for the Janus nanoparticles show that self assembly does occur, and a stable dispersion of clusters of approximately 80 nm in diameter can be formed at pH 2. On the other hand, nanoparticles without the tail polymer become uncharged, aggregate and precipitate at pH values less than about 4, i.e., less than the pKa of the coating polymer. It is noted that the sizes of self-assembled Janus nanoparticle clusters change depending on their concentration, from a low value of 15 nm at low concentration (corresponding to individually dispersed nanoparticles) to a higher value of 95 nm at high concentration; this behavior is akin to that observed near the critical micelle concentration noted for conventional molecular and polymer micellar systems.