Synthesis and Characterization of Dye-Labeled Poly(methacrylic acid) Grafted Silica Nanoparticles Lei Wang and Brian C. Benicewicz* Department of Chemistry and Biochemistry and USC NanoCenter, University of South Carolina, Columbia, South Carolina 29208, United States * S Supporting Information ABSTRACT: The synthesis of dye-labeled poly(methacrylic acid) (PMAA) grafted silica nanoparticles was studied. Surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization of tert-butylmethacrylate (tBuMA) was conducted on dye-labeled CPDB coated silica nanoparticles followed by sequential removal of the thiocarbonylthio end groups and the tert-butyl moieties. Additionally, as a more straightforward strategy, direct polymerization of methacrylic acid on silica nanoparticles with a diameter size as small as 15 nm was conducted via the RAFT polymerization technique. A variety of PMAA brushes with different lengths and densities were prepared on nanoparticle surfaces via surface-initiated RAFT polymerization with excellent control and surface grafting densities as high as 0.65 chains/ nm 2 . The grafted PMAA was methylated by trimethylsilyldiazomethane to conduct organic phase GPC characterization. The dye- labeled PMAA grafted nanoparticles provide a platform to bind biomolecules and to track the movement of the nanoparticles in biological systems. R eversible addition-fragmentation chain transfer (RAFT) polymerization has been recognized as an important reversible addition radical polymerization (RDRP) technique to prepare polymers with controllable molecular weight and low polydispersities since its invention by Moad and co-workers in 1998. 1 RAFT polymerization has many advantages, such as being adaptable to almost all free radical polymerizable monomers, without participation of inorganic catalysts and mild operation conditions. Polymer-grafted nanoparticles are of great interest because of their applications in chemosensors, coatings, and organic light- emitting devices (OLEDs). 2 The RAFT polymerization technique has emerged as a powerful tool to modify nanoparticle surfaces with functional polymers containing predetermined molecular weights due to the straightforward attachment chemistry and controllable surface graft density. Poly(methacrylic acid) (PMAA) and other polymers made from acid-containing monomers represent an important class of stimuli-responsive polymers and have been widely used in membrane transport, 3 biomedical applications, 4 coatings, 5 and sensors. 6 There are few reports about the synthesis of PMAA or other multi-acid-containing polymers on nanoparticle surfaces. For example, Brittain et al. 7 synthesized poly(tert-butylacrylate) brushes on silica surface by atom transfer radical polymerization (ATRP), followed by pyrolysis at 200 °C, resulting in PAA- grafted silica substrates. Genzer et al. 8 prepared poly(tert- butylacrylate) grafted silicon wafer by ATRP, followed by acid hydrolysis of the polymer to form the immobilized PAA chains. Zhao et al. 9 sequentially prepared poly(tert-butylacrylate) brushes by ATRP and polystyrene brushes by nitroxide- mediated radical polymerization (NMRP) on the surface of silica nanoparticles. Subsequent deprotection of the tert-butyl moieties with trimethylsilane iodide (TMSI) led to environ- mentally responsive nanoparticle materials. To avoid the toxicity issue of residual copper from ATRP catalysts in bioapplications, Benicewicz et al. 10 prepared PMAA-grafted silica nanoparticles by surface-initiated RAFT polymerization of tert-butyl methacrylate, followed by deprotection of the tert- butyl groups by TMSI. Very few groups have conducted direct surface-initiated RAFT polymerization of methacrylic acid or other acid containing monomers on nanoparticle surfaces. One particular challenge is maintaining good dispersibility of the polymer grafted nanoparticles using small size substrate nanoparticles. Generally, smaller size nanoparticles agglomerate more readily than larger particles. Thus, the size and nature of the substrate nanoparticles are important issues affecting the final dispersi- bility of polymer grafted nanoparticles. Charpentier et al. 11 used a RAFT agent with a carboxylic acid group to modify TiO 2 nanoparticles and conducted the surface-initiated polymer- ization of acrylic acid. Yusa et al. 12 synthesized poly(6- (acrylamide)hexanoic acid chains on 11 μm (diameter) size silica particles. The polymer-grafted particles flocculated at low pHs (pH = 3) and dispersed in water at high pHs (pH = 10). However, the large (11 μm diameter) particles are much easier Received: December 18, 2012 Accepted: January 29, 2013 Published: February 7, 2013 Letter pubs.acs.org/macroletters © 2013 American Chemical Society 173 dx.doi.org/10.1021/mz3006507 | ACS Macro Lett. 2013, 2, 173-176