Colour Shifting Nanomedicines Built from Highly-Ordered Dye Assemblies

Charron, Danielle 1, 2 ;  Liu, Richard 1, 3 ;  Zheng, Gang 1, 2, 4

1. Princess Margaret Cancer Research Centre, University Health Network, Toronto ON; 2. Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto ON; 3. Faculty of Science, McGill University, Montreal QC; 4. Department of Medical Biophysics, University of Toronto, Toronto ON

Porphyrins are a class of dyes that include the natural pigments heme and chlorophyll. As a result of their endogenous origin and strong absorption within the visible and near infrared region, porphyrins have been extensively studied as agents for phototherapy and optical imaging. We have expanded these applications by building highly-ordered porphyrin J-aggregate assemblies within the membrane of lipid-based nanomedicines called liposomes. J-aggregation is a phenomenon that sometimes arises when dyes organize in a head-to-tail arrangement, and results in a strongly red-shifted and sharpened absorption band and similarly shifted fluorescence emission. In this way, porphyrin J-aggregation encodes intact and disrupted liposomes with unique optical signals, or colours. Porphyrin colour shifting could be used to ratiometrically and responsively monitor nanomedicines in vivo by photoacoustic or fluorescence imaging, or to deliver photodynamic and photothermal therapies in a structure-dependent and wavelength-selective manner. As a nanoscale phenomenon, J-aggregation is inherently sensitive to temperature and protein adsorption in vivo. Therefore, simultaneously achieving stable porphyrin J-aggregation and responsive colour shifting is a complex balancing act. In this project, we investigated the influence of the liposome host lipid on porphyrin J-aggregate formation and stability in biological conditions. Two porphyrin variants called pyropheophorbide (Pyro) and bacteriochlorophyll (BChl) were studied, both derived from photosynthetic bacteria. We found that rigid host lipids enhance porphyrin J-aggregation, but the extent of this influence varies depending on the porphyrin variant. In addition, Pyro was found to form J-aggregates in a concentration-dependent manner, while BChl exhibited a threshold dependence. We hypothesize that these differences stem from the number of aggregated porphyrins required for efficient J-aggregation. Circular dichroism measurements suggest a possible structural explanation as Pyro and BChl form J-aggregates exhibiting opposite chirality. Fundamental studies to elucidate porphyrin nano-organization and behaviour in lipid membranes will be essential to guiding further optimization of this technology.