Plasmonic gadolinium oxide nanomatryoshkas: bifunctional magnetic resonance imaging enhancers for photothermal cancer therapy

Abstract Nanoparticle-assisted laser-induced photothermal therapy (PTT) is a promising method for cancer treatment; yet, visualization of nanoparticle uptake and photothermal response remain a critical challenge. Here, we report a magnetic resonance imaging-active nanomatryoshka (Gd2O3-NM), a multilayered (Au core/Gd2O3 shell/Au shell) sub-100 nm nanoparticle capable of combining T1 MRI contrast with PTT. This bifunctional nanoparticle demonstrates an r1 of 1.28 × 108 mM–1 s–1, an MRI contrast enhancement per nanoparticle sufficient for T1 imaging in addition to tumor ablation. Gd2O3-NM also shows excellent stability in an acidic environment, retaining 99% of the internal Gd(3). This report details the synthesis and characterization of a promising system for combined theranostic nanoparticle tracking and PTT.

Potassium carbonate anhydrous (K2CO3) was purchased from Fisher, mPEG-Thiol (MW = 10000) from Laysan Bio, Inc. and NaOH from Fisher was used without purification. 50 nm gold colloids citrate NanoXact were purchased from NanoComposix. Aqua regia was used to clean all glassware and stir bars, followed by thorough rinsing with distilled water, ethanol, and Milli-Q water in the final step. Milli-Q water (18.2 MΩ.cm at 25 °C, Millipore) was used to prepare all solutions and reagents without further purification.
Au/Gd2O3/Au Nanomatryoshkas (NMs) synthesis. Au/Gd2O3/Au NMs were prepared by growing a shell of Gd2O3 around Au cores followed by growth of an outer Au shell. Briefly, citratecapped Au spheres (50 nm) were dispersed in 0.02 M sodium oleate (NaOA) and heated at 80 ºC for 1 hr. The resultant Au-NaOA was centrifuged at 3000 G for 20 min and redispersed in water.
For a standard reaction, 30 mL of Au-NaOA was added to 300 mL of Milli-Q water and vortexed. Then, 7.8 mL of hexamethylene tetramine (HMT) (0.1 M) and 12 mL of Gd(III)-nitrate (0.01 M) were added, vortexed for 1 min followed by sonication for 15 min. The reaction mixture was then heated at 80 ºC for 1 hr in a sealed container and left overnight. The Au/Gd2O3 solution was then centrifuged at 3000 K G for 20 min and resuspended in 15 mL of EtOH. 150 µL of 10 % APTES (v/v in EtOH) was added and gently stirred for 12 hr at room temperature then dialyzed in EtOH for 12 hr. The Au/Gd2O3-APTES solution was concentrated in water and added to 40 mL Duff colloid that was prepared in advance (2 weeks) 1 and 750 µL of NaCl (1 M). The solution was sonicated for 35 min and left undisturbed for 24 hr. A continuous Au outer shell was grown and then redispersed in mPEG-thiol (10 k), following the previously reported protocol. 2 The concentration of Gd(III) in the NMs was determined using inductively coupled plasma mass spectroscopy (ICP-MS). The measurements were performed in a PerkinElmer NexION 300. The Courant stability factor (~0.99) for the simulated model was realized by setting the simulation time step to dt = 0.02 fs. 5 The extinction spectra were extracted using total-field scattered-field approach over the wavelength range of interest with a regular plane-wave with a pulse length of 75 fs.
Photothermal heating of Gd2O3-NM. A solution of Gd2O3-NM in H2O was prepared (1 x 10 9 NP/mL) in an insulated glass beaker equipped with a stir bar and thermocouple. A second thermocouple was used to measure the ambient temperature. An 808 nm diode laser was set to 3 W/cm 2 and positioned above the stirring solution. The solution was irradiated for 2 min and the temperature of the solution and ambient temperature was recorded. As a control, the beaker was then filled with water and irradiated under the same conditions.

Instrumentation
Transmission electron microscopy (TEM) was performed using a JEOL 1230 operating at 80 kV. High-resolution STEM with high-angle annular dark field (STEM-HAADF) and and energydispersive X-ray element mapping were used for elemental analysis. Cary 5000 UV/Vis/NIR Varian spectrophotometer was used to measure the extinction spectra of the nanoparticles. DLS