ABSTRACTGold nanorods (AuNRs) show surface plasmon absorption bands in the near-infrared region. This characteristic property has stimulated utilization of gold nanorods as novel nanoprobes for noninvasive bioimaging, such as photoacoustic tomography. Herein, we discuss the synthesis of a series of gold nanorods coated with pH-responsive polymers to investigate the effect of the surface structure and zeta potential of nanoparticles on cellular uptake via a surface charge-mediated endocytic pathway. The surface of the gold nanorods was modified with polyethylene glycol (PEG@AuNR) and tertiary amine derivatives, specifically, diethylaminoethyl ester (1@AuNRs), its amide analog (2@AuNRs), and dimethylaminoethyl ester (3@AuNRs). It was found that the pH-sensitivity of1@AuNRs was relatively high and the surface was positively charged at lower pH. In contrast, the tertiary amino group of1@AuNRs was deprotonated to form an electrostatically neutral surface at higher pH. The pH-responsive gold nanorods were incubated with A549 cells (human lung cancer cells) to quantify the amount of cellular uptake using inductively coupled plasma mass spectrometry. The results indicate that1@AuNRs can be taken up efficiently in the cells, and thereafter, slowly flow out of the cells. Interestingly, only small amounts of the amide analog (2@AuNRs) were taken into the cells, suggesting minor structural changes may affect the interaction between the cell surface and AuNRs. This study highlights a potential application of pH-sensitive nanorods as a probe for bioimaging the acidic environment of tumors.
