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Indocyanine green (ICG) was incorporated into poly(ethylenimine)/(phenylthio) acetic acid (PEI/PTA) ion pair self-assembly (IPSAM) as a photosensitizer. PEI/PTA/ICG IPSAM was more stable against a thermal dissolution as the PTA and ICG content were higher. The zeta potential of the IPSAM without ICG (the amino group(A)/carboxyl group(C) molar ratio was 5.3:4.7) was a strong positive charge. Upon the addition of a small amount of ICG (A/C/ICG molar ratio was 5.3:4.7:0.01), it changed to a strong negative charge. The IPSAM on TEM micrographs was found as almost sphere-like nanoparticles and its diameter was around 50 nm. The doxorubicin-loaded IPSAM on CLSM micrographs was found as nano-sized circular red domains, indicating that the anti-cancer drug was successfully loaded in the IPSAM without disrupting the shape. When A to C molar ratio was 4:6, the ICG-incorporated IPSAM released its cargo appreciably under NIR irradiation even though the temperature of the IPSAM suspension could hardly reach the upper critical solution temperature (UCST) under NIR irradiation, suggesting that the release took place mainly by the radical oxygen species (ROS)-induced oxidation of PTA. When A to C molar ratio was 5.3:4.7, the temperature of the IPSAM suspension could reach the UCST under NIR irradiation and the release degree increased in proportion to the temperature increase, indicating the release took place mainly by the temperature change. According to the FT-IR spectroscopy, the NIR irradiation-triggered release could be attributed to the heat and ROS generated by ICG under NIR irradiation.

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