Supplementary Methods
Synthesis of pyropheophorbide-lipidIn a standard reaction, 100 nmol of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Avanti Polar Lipids), 50 nmol pyropheophorbide (prepared from Spirulina Pacifica algae as described previously;Zheng et al., Bioconj. Chem., 2002, 13-392), 50 nmol of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (Sigma), 25 nmol of 4-(dimethylamino) pyridine (Sigma) and 50 µL of N,Ndiisopropylethylamine (Sigma) were combined in 10 mL of anhydrous dichloromethane. The reaction mixture was stirred at room temperature under argon in the dark for 48 hours. The solvent was evaporated and the residue was subjected to thin layer chromatography purification (20 x 20 cm pre-coated silica gel plate with fluorescent indicator, 1.5 mm in thickness). Chloroform-methanol-glacial acetic acid-water 65:25:8:2 (volume ratio) was used as the solvent. The major band with R f =0.4 was isolated from the plate and eluted giving a final yield of 45%. Recently, we found that improved purification could be achieved by using diol modified silica (Sorbtech) and eluting the product with 8% methanol in DCM after washing out impurities with 2% and 5% methanol in DCM. The pyropheophrobide-lipid was then dried under
Converting nanoparticles or monomeric compounds into larger supramolecular structures by endogenous or external stimuli is increasingly popular because these materials are useful for imaging and treating diseases. However, conversion of microstructures to nanostructures is less common. Here, we show the conversion of microbubbles to nanoparticles using low-frequency ultrasound. The microbubble consists of a bacteriochlorophyll-lipid shell around a perfluoropropane gas. The encapsulated gas provides ultrasound imaging contrast and the porphyrins in the shell confer photoacoustic and fluorescent properties. On exposure to ultrasound, the microbubbles burst and form smaller nanoparticles that possess the same optical properties as the original microbubble. We show that this conversion is possible in tumour-bearing mice and could be validated using photoacoustic imaging. With this conversion, our microbubble can potentially be used to bypass the enhanced permeability and retention effect when delivering drugs to tumours.
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