Improving Lyophilization and Long-Term Stability of Gold Nanostars for Photothermal Applications

Gold nanostars (AuNS) are anisotropic metal nanoparticles that display morphology-dependent optical properties. These include strong extinction coefficients in the near-infrared region of the spectrum and near-field enhancements. AuNS have been widely used for biomedical applications, including sensing, imaging, and photothermal therapy. To promote the use of AuNS for commercial (pre)clinical purposes, the stability and shelf life need to be maximized. In pharmaceutics, lyophilization (also known as freeze-drying) is commonly employed to extend shelf life and facilitate logistics. However, lyophilization tends to induce stress on nanoparticles, causing irreversible aggregation. In this work, we studied the use of sucrose, glucose, sorbitol, poly(vinylpyrrolidone) (PVP), and poly(ethylene glycol) (PEG) as cryoprotectants during AuNS lyophilization, as well as their effect on particle shelf life, photothermal properties, and contrast agent performance. We explored three different AuNS that were obtained via one-pot synthesis with biocompatible 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid (EPPS), 4-(2-hydroxyethyl) piperazine-1-ethanesulfonic acid (HEPES), and 3-(N-morpholino) propanesulfonic acid (MOPS) as the seedless synthetic protocol did not require cytotoxic surfactants that may complicate biological applications. As a reference, we used citrate-coated spherical gold nanoparticles (AuNPs). While AuNS synthesized with EPPS and HEPES could be lyophilized and resuspended in water in the absence of a cryoprotectant, the use of cryoprotectants (particularly sucrose) facilitated their reconstitution and improved shelf life. AuNPs and AuNS synthesized with MOPS irreversibly aggregated during lyophilization and required PVP for resuspension and long-term storage. The use of selected cryoprotectants preserved the photothermal characteristics and photoacoustic imaging signal generation of the nanoparticles after freeze-drying. Taken together, these findings provide insights into improving the stability and shelf life of nanoparticles, potentially contributing to the development of AuNS as biomedical products.