PNNL

Abstract

Poly(propylene carbonate) (PPC) decomposes at high temperature to release CO2. This CO2-generation temperature of PPC can be reduced down to less than 80 °C with the aid of a photoacid generator (PAG). In the present work, we demonstrate that using an additional helper component, surface plasmonic gold nanorods (GNRs), the PPC degradation reaction can also be initiated by near infrared (NIR) irradiation. For this purpose, a PPC-containing nanoparticle formulation was developed in which PPC-based amphiphilic block copolymers (BCPs), poly(poly(ethylene glycol) methacrylate-b-propylene carbonate-b-poly(ethylene glycol) methacrylate) (PPEGMA-PPC-PPEGMA), were self-assembled with GNRs and PAG molecules via solvent exchange; under NIR irradiation GNRs produce heat that can cause PPC to decompose into CO2, and PAG (after UV pre-treatment) catalyzes this PPC degradation process. Two PPEGMA-PPC-PPEGMA materials were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization and used for this study: PPEGMA7.3K-PPC5.6K-PPEGMA7.3K (“G7C6G7”), and PPEGMA2.1K-PPC5.6K-PPEGMA2.1K (“G2C6G2”). Addition of surfactant(CTAB)-coated GNRs dispersed in water to a G2C6G2 solution in dimethylformamide (DMF) produced individually G2C6G2-encapsulated GNRs, whereas the same solvent exchange procedure resulted in the formation of polymer-coated GNR clusters when G7C6G7 was used as the encapsulating material. Both G7C6G7- and G2C6G2-encapsulated GNRs were stable against agglomeration for at least 30 days in 150 mM NaCl. GNR/G2C6G2 NPs exhibited a surface plasmon resonance peak at 511 nm (which was the same as that for CTAB-coated GNRs). The clustered morphology of G7C6G7-encasulated GNRs caused a blue shift of the absorbance maximum to 697 nm. As a consequence, GNR/G2C6G2 NPs showed a greater absorbance/heat generation rate under NIR irradiation than did GNR/G7C6G7 NPs. The incorporation of PAG into the PPC coating domain did not alter the morphological and plasmonic properties of the BCP-encapsulated GNR systems. PAG-loaded G2C6G2- and G7C6G7-coated GNRs generated significant amounts of CO2 under NIR irradiation; the rate of CO2 generation was about 4.2 times higher with the GNR/G2C6G2+PAG sample than the rate with the GNR/G7C6G7+PAG sample. Both GNR/G7C6G7+PAG and GNR/G2C6G2+PAG systems produced ultrasound (US) contrast enhancement effects under continuous exposure to NIR light for > 20 minutes; contrast enhancement was more spatially uniform for the GNR/G2C6G2+PAG sample. These results support the potential utility of PPC as a CO2-generating contrast agent in ultrasound imaging applications.