Pelaz, B. Del Pino, P. Maffre, P. Hartmann, R. Gallego, M. Rivera-Fernández, S. Fuente, De La Nienhaus, G. U. Parak, W. J. Surface Functionalization of Nanoparticles with Polyethylene Glycol: Effects on Protein Adsorption and Cellular Uptake ACS Nano 10.1021/acsnano.5b01326 2015 2015