You-Yong Yuan, Cheng-Qiong Mao, Xiao-Jiao Du, Jin-Zhi Du, Feng Wang, and Jun Wang.Surface Charge Switchable Nanoparticles Based on Zwitterionic Polymer for Enhanced Drug Delivery to Tumor.Adv. Mater. 2012, 24, 5476–5480. 

Two faced nanoparticles: A zwitterionic polymer-based nanoparticle with response to tumor acidity is developed for enhanced drug delivery to tumors. The nanoparticles are neutrally charged at physiological conditions and show prolonged circulation time; after leaking into tumor sites, in the acidic extracellular tumor environment (pH(e)), nanoparticles are activated and become positively charged and are therefore efficiently taken up by tumor cells, leading to enhanced therapeutic effects in cancer treatment.