Nanoscale memory provided by thermoreversible stochastically structured polymer aggregates on mica

AVISHAY PELAH, SILVIO LUDUENA, ELIZABETH JARES-ERIJMAN, IGAL SZLEIFER , LIA PIETRASANTA Y THOMAS JOVIN

Mar del Plata, Argentina

Simposio; Fourth Latin American Symposium on Scanning Probe Microscopy; 2007

LASPMStimuli-responsive polymers, also known as “smart” polymers, are of great utility in fields such as biomedicine and nanotechnology. These materials exhibit substantial changes in their properties in response to relatively minor alterations in the mic

Stimuli-responsive polymers, also known as “smart” polymers, are of great utility in fields such as biomedicine and nanotechnology. These materials exhibit substantial changes in their properties in response to relatively minor alterations in the microenvironment. The different stimuli inducing such transitions include shifts in temperature, ionic strength, and pH. Poly(Nisopropylacrylamide), PNIPAM, is a water-soluble polymer with a lower critical solution temperature (LCST) of 32 °C.1 Heating an aqueous solution above the LCST leads to dehydration of the polymer chains by expelling water, resulting in a thermoreversible coil-to-globule phase transition and chain aggregation. Due to their unique thermoresponsive properties, polymers based on N-isopropylacrylamide have been at the focus of numerous studies 2-10 and applications such as bioanalytical assays,3,6,11 manipulation of aggregate size,4 and the control of enzymatic activity,7 colloidally stable particles when the temperature is raised above the LCST.12 These particles are stabilized by electrostatic repulsion between sulfate groups originating from the persulfate initiator.12 In the present study, carried out by temperaturecontrolled tapping mode atomic force microscopy (AFM) in water, we demonstrate a new and intriguing “memory” property of PNIPAM-based polymer aggregates formed on a mica surface, namely the memorization of their positions and shapes over numerous cooling and heating cycles despite the loss of apparent structure at the lower temperature. These structures evolve in the course of heating via the incorporation of polymer from the overlying solution and by the coalescence of neighboring aggregates. The phenomenon has been characterized and rationalized by quantitative image analysis and has numerous practical implications. (1) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163-249.(2) Bergbreiter, D. E.; Caraway, J. W. J. Am. chem. Soc. 1996, 118, 6092-6093.(3) Hoffman, A. S. Clin. Chem. 2000, 46, 1478-1486.(4) Kulkarni, S.; Schilli, C.; Mu¨ller, A. H. E.; Hoffman, A. S.; Stayton, P. S. Bioconjugate Chem. 2004, 15, 747-753.(5) Li, C.; Gunari, N.; Fischer, K.; Janshoff, A.; Schmidt, M.  Angew. Chem., Int. Ed.2004, 43, 1101-1104.(6) Malmstadt, N.; Hyre, D. E.; Ding, Z.; Hoffman, A. S.; Stayton, P. S. Bioconjugate Chem. 2003, 14, 575-580.(7) Shimoboji, T.; Larenas, E.; Fowler, T.; Hoffman, A. S.; Stayton, P. S. Bioconjugate Chem. 2003, 14, 517-525.(8) Stayton, P. S.; Shimoboji, T.; Long, C.; Chilkoti, A.; Chen, G.; Harris, J. M.; Hoffman, A. S. Nature 1995, 378, 472-474.(9) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Okano, T.; Sakurai, T. Bioconjugate Chem. 1993, 4, 42-46.(10) Zhu, M.; Wang, L.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656-2657.(11) Ding, Z.; Chen, G.; Hoffman, A. S. Bioconjugate Chem. 1996, 7, 121-125.(12) Chan, K.; Pelton, R.; Zhang, J. Langmuir 1999, 15, 4018-4020.