In this work, silica nanoparticles covered with an initiator silane were grafted with novel biocompatible and functionalizable zwitterionic poly [[3-(acryloylamino)propyl] (2-carboxyethyl)dimethylammonium] (polyCBAA) via atomic transfer radical polymerization. The stability of these particles in protein solutions and their functionalization were investigated. Dynamic light scattering and transmission electron microscopy were employed to characterize these particles. Results indicate that the polyCBAA-modified nanoparticles; are stable for at least 72 h in both negative and positive protein solutions. The size distribution remains the same before and after protein incubation. Moreover, our results show that polyCBAA-modified silica nanoparficics can be easily functionalized. This makes these particles ideal candidates for future applications in targeted drug delivery and diagnostics in complex media.