Determining the state of radionuclides in polymersomes using perturbed angular correlation spectroscopy

Polymersomes, polymeric nano-carriers composed of amphiphilic block copolymers, are promising candidates for transporting (radio)pharmaceuticals to tumour cells. While methods have been developed for trapping both diagnostic and therapeutic amounts of radionuclides in these polymersomes, there is not much known about the loading process and the state of the radionuclide in the polymersomes. At this point perturbed angular correlation (PAC) spectroscopy becomes essential, as it is a useful method for gaining information about the chemical surroundings and physical structures of different gamma-emitting radioisotopes. PAC is a gamma ray spectroscopy technique which can be used for the investigation of hyperfine interactions. A hyperfine interaction is the interaction between a nucleus and its environment. Hyperfine interactions arise in the intermediate state of a decaying radionuclide. This decay is then perturbed with a time-dependent factor and can be measured from the angular correlation function. The angular correlation function is expressed as the number of coincidences per unit time as function of angle. The number of coincidences is measured using the PAC spectroscopy. The polymersomes are composed of poly(butadiene-b-ethylene oxide) block copolymers, and the labelling has been achieved by transportation of the radionuclide, complexed to a lipophilic ligand, through the hydrophobic bilayer into the aqueous cavity containing 0.5 M of KH2PO4. A sufficient amount of the radionuclides was encapsulated in the polymersomes (>90 % loading efficiency). Subsequent to the loading, the sample was immersed in liquid nitrogen, after it was counted on the PAC setup for 6 hours at 90 and 180 angles. To confirm a working PAC setup, measurements on directional correlation and a simple perturbed system are performed. Excellent agreement with literature has been observed for cobalt-60 directional correlation measurements (G22 of 0.1016 vs 0.1020 in [18]). The hafnium metal measurements, which only undergo a static interaction, are compared with literature [39]. The results give an indication of a reliable PAC setup. Final measurements are performed with indium-111. The results obtained for the loaded radionuclides in polymersomes indicate that differences could be distinguished. The results indicate that the radionuclides will move from the bilayer towards the core of the polymersome. There was no clear bonding observed between the chelator and the radionuclides. The used side of the polymersomes (100 nm, 800 nm) are expected not to influence the measurement.Medical Physics, RIH, RIDBioMechanical EngineeringMechanical, Maritime and Materials Engineerin