Photostability of gold nanorods upon endosomal confinement in cultured cells

The photoinstability of plasmonic particles remains one remarkable obstacle before their clinical penetration as powerful contrast agents, for instance, in photoacoustic imaging. In particular, gold nanorods easily revert to nanospheres and so lose their best optical features under exposure to few-nanoseconds-long laser pulses. While this issue is attracting much attention and stimulating ad hoc solutions, such as the addition of rigid shells, the biological environment may cause even more instability. For instance, a frequent outcome of the interaction between this type of particles and malignant or immune cells is their tight confinement into endocytic vesicles. In this study, we assess whether this configuration may make an adverse impact on the photostability of gold nanorods, due to the effect of heat confinement. We compare experimental measurements from a limited set of representative samples and verify their relevance by the use of numerical simulations. Under conditions that are typical for photoacoustic microscopy, we estimate the threshold fluence for the onset of photoinstability to remain around 7 mJ center dot cm(-2), independent of the distance among neighboring particles, within accessible limits. Then, we simulate the effect of pulse duration in our model of endocytic confinement. Only in a mu s regime of lesser potential for biomedical optics do we predict this configuration to destabilize the gold nanorods, still by as little as 15-20%. Our results span from the femtosecond up to the continuous wave regimes of irradiation and suggest that the biological interface does not pose a major threat on the photostability of plasmonic particles for most biomedical applications, including the photoacoustic imaging and photothermal ablation of cancer.