Preparation and Characterization of Low Dispersity Anionic Multiresponsive Core-Shell Polymer Nanoparticles J. P. Pinheiro,* ,†,‡ Leila Moura, § Remco Fokkink, ‡ and J. P. S. Farinha* ,§ † CMQE/IBB, Departamento de Química e Farmacia/Faculdade de Ciê ncias e Tecnologia, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal ‡ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Wageningen, The Netherlands § Centro de Química-Física Molecular and INInstitute of Nanoscience and Nanotechnology, Instituto Superior Te ́ cnico, 1049-001 Lisboa, Portugal * S Supporting Information ABSTRACT: We prepared anionic multistimuli responsive core- shell polymer nanoparticles with very low size dispersity. By using either acrylic acid (AA) or methacrylic acid (MA) as a comonomer in the poly(N-isopropyl acrylamide) (PNIPAM) shell, we are able to change the distribution of negative charges in the nanoparticle shell. The particle size, volume phase transition temperature, and aggregation state can be modulated using temperature, pH, or ionic strength, providing a very versatile platform for applications in sensors, medical diagnostics, environmental remediation, etc. The nanoparticles have a glassy poly(methyl methacrylate) (PMMA) core of ca. 40 nm radius and a cross-linked PNIPAM anionic shell with either AA or MA comonomers. The particles, p(N-AA) and p(MA-N), respectively, have the same total charge but different charge distributions. While the p(MA-N) particles have the negative charges preferentially distributed toward the inner shell, in the case of the p(N-AA) particles the charge extends more to the particle outer shell. The volume phase transition temperature (T VPT ) of the particles is affected by the charge distribution and can be fine-tuned by controlling the electrostatic repulsion on the particle shell (using pH and ionic strength). By suppressing the particle charge we can also induce temperature- driven particle aggregation. ■ INTRODUCTION Responsive nanoparticles (RNP) change their properties, such as dimension, structure, interactions, or aggregation state, in response to external stimuli (temperature, pH, pressure, ionic strength, etc.). 1-10 The simplicity and versatility of preparation of polymer RNPs of different sizes, shapes, and surface properties have led to many new materials with promising applications in a range of areas from sensors, to removal of toxic substances, medical diagnostics, intelligent catalysts, microreactors, multiresponsive coatings, etc. 11-13 For temperature-responsive polymers in water the balance between segment-segment interactions and segment-solvent interactions can be shifted by temperature changes, inducing a volume phase transition of the material. One such material is poly(N-isopropylacrylamide) (PNIPAM), which was first used to produce cross-linked thermosensitive microgel par- ticles. 7,8,14-17 The polymer-water interactions in the case of PNIPAM decrease upon increasing the temperature above the lower critical solution temperature (LCST), which is 32 °C for PNIPAM alone, and ca. 31-35 °C in nanoparticles. 18-22 At this temperature, the PNIPAM chains undergo a reversible volume phase transition from a solvated coil to a collapsed globule. Below the LCST the chain is hydrophilic and therefore highly swollen with water, adopting a coil conformation that results from the balance between the hydrophobic interactions between isopropyl groups and the hydrogen bonding between water and the polymer amide groups. 23 Above the LCST, the PNIPAM chains collapse to a globular conformation because they are partially dehydrated due to the attractive interactions between the hydrophobic isopropyl groups. 24 This triggers an entropically favorable hydrophobic aggregation of the polymer segments that causes the polymer to deswell. 23-28 When PNIPAM-based materials are functionalized with pH- responsive groups, such as weak polyelectrolytes with acid or basic functional groups (carboxylic, phosphoric, or amino functional groups), their volume phase transition can be tuned over an even wider range of environmental conditions to generate fast and targeted swelling responses to multiple external stimuli (i.e., temperature, pH, ionic strength). In polymer RNPs with carboxylic acid groups, the change in the pH of the media changes the ionization degree of the polyelectrolyte moiety so that at higher pH, the osmotic pressure due to the increased ionization will result in swelling of the particles, while at lower pH the particles have less charge and therefore are less expanded. 29 Received: November 18, 2011 Revised: February 23, 2012 Published: February 23, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 5802 dx.doi.org/10.1021/la2045477 | Langmuir 2012, 28, 5802-5809