Date Published: December 01, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Bijay Kumar Poudel, Jong Oh Kim, Jeong Hoon Byeon.
Gold (Au) agglomerates (AGs) are reassembled using Triton X‐100 (T) and doxorubicin (D) dissolved in ethanol under 185 nm photoirradiation to form TAuD nanovesicles (NVs) under ambient gas flow conditions. The positively charged Au particles are then electrostatically conjugated with the anionic chains of TD components via a flowing drop (FD) reaction. Photoirradiation of the droplets in a tubular reactor continues the photophysicochemical reactions, resulting in the reassembly of Au AGs and TD into TAuD NVs. The fabricated NVs are electrostatically collected onto a polished aluminum rod in a single‐pass configuration. The dispersion of NVs is employed for bioassays to confirm uptake by cells and accumulation in tumors. The chemo‐photothermal activity is determined both in vitro and in vivo. Different combinations of components are also used to fabricate NVs using the FD reaction, and these NVs are suitable for gene delivery as well. This newly designed gaseous single‐pass process results in the reassembly of Au AGs for incorporation with TD without the need of batch wet chemical reactions, modifications, separations, or purifications. Thus, this process offers an efficient platform for preparing biofunctional Au nanostructures that requires neither complex physicochemical steps nor special storage techniques.
Plasmonic gold (Au)‐based nanostructures (NSs) are frequently employed for therapeutic and diagnostic applications because they are chemically inert, can be easily prepared, are highly biocompatible, and have tunable optical properties (i.e., localized surface plasmon resonance (LSPR) effect).1 In particular, the strong light absorption of NSs in the near‐infrared (NIR) region generates nonradiative energy dissipation through Landau damping, producing sufficient heat for photothermal hyperthermia therapies. NSs have been extensively employed in such therapies using a variety of treatment modes and different bioactive molecules, such as drugs and targeting agents.2 Changing the shape of Au NSs has been reported to manipulate the LSPR wavelength from visible to NIR. Compared with the very limited red‐shift in LSPR absorption resulting from increasing Au size (the simplest method), this change results in stronger optical behavior in photothermal therapy and optical diagnosis.3 Au nanorods (NRs) are frequently employed to achieve red‐shifting or broadening, and the absorption spectra can be tuned by controlling the aspect ratios of Au.4 More recently, Au NRs have been reported to limit cell viability because of physical features, such as sharp edges, tips, and residues (e.g., cetyltrimethylammonium bromide), and have low in vivo retention time caused by high aspect ratios resulting from their anisotropic characteristics.5, 6 To address these issues, Au nanovesicles (NVs) have been deliberately fabricated to induce strong plasmonic coupling between primary particles of Au agglomerates (AGs), thereby increasing their LSPR absorption for effective NIR‐induced photothermal therapy.7, 8 Furthermore, the optical properties of other Au NSs, such as nanostars, nanocages, nanoshells, and nanopopcorns, also have been manipulated or tuned to allow for their effective biomedical application.9
The formation of hybrid droplets from the FD reaction was verified by measuring the size distribution of Au AGs, TD droplets, and their merged forms (hybrid droplets) using a scanning mobility particle sizer (SMPS, 3936, TSI, USA) before solvent extraction in the denuder (Figure S1, Supporting Information). The AGs were formed by Brownian motion (thermal collision behavior) of primary Au particles (formed by the condensation of spark‐ablated Au vapors) in the presence of compressed nitrogen gas flow. The TD droplets were generated by collison atomization of an ethanolic TD solution by injecting another compressed nitrogen gas flow. The hybrid droplets were prepared by direct injection of the Au AG‐laden gas flow into the collison atomization device as the working fluid, where the AGs were incorporated into the TD solution to be atomized as the hybrid droplets. The hybrid droplets were directed to a tubular reactor under 185 nm UV irradiation. The size distributions of the AG, TD, and hybrid droplets, measured using SMPS, are shown in Figure2. The results are presented as the geometric mean diameter (GMD), geometric standard deviation (GSD), and total number concentration (TNC). The hybrid droplet size also showed a unimodal configuration (located between the distributions of AG and TD) similar to individual AG and TD cases, although there were significant differences between the AG and TD cases. This result implied that nearly all AGs were included in the TD droplets during the atomization process; thus, this product was suitable for use in the subsequent FD reaction. Interestingly, the GSD of hybrid droplets was significantly lower (narrower) than that of the TD droplet, possibly caused by the reassembly of AGs with TD components (intervening and subsequent conjugating) under photoirradiation, including shattering of the AGs during atomization.23 This result suggested that the photophysicochemical reassembly of Au AGs is a suitable route for fabricating functionalized Au NSs.
D‐loaded Au NVs were fabricated via the FD reaction in an ambient single‐pass configuration without the use of any hydrothermal chemical processes. The application of photon energy higher than that of the Au work function to hybrid droplets containing Au AGs and bioactive molecules (T and D) caused the reassembly of Au AGs and electrostatic conjugation between positively charged Au surfaces and negatively charged functional groups of bioactive molecules, resulting in the formation of biofunctional NVs. The NVs demonstrated biocompatibility, broadband photoconversion, and pH‐ and NIR‐triggerable D release. Thus, the NVs were suitable for testing chemo‐photothermal therapies under NIR irradiation both in vitro (with increased intracellular uptake) and in vivo (with preferential tumor accumulation). Apoptosis‐dominant antitumor effects without significant necrosis were demonstrated by western blotting, and minimal systemic toxicity was confirmed by the weight‐change profiles of tumor‐bearing mice during treatment. Furthermore, NVs (lipid molecules instead of T) also were successfully fabricated using the FD reaction, and these NVs demonstrated NIR‐triggered drug release and efficient gene delivery. Thus, this work offers a generalizable on‐demand platform technology for fabricating numerous biofunctional NSs from combinations of photoionizable metals with bioactive molecules in a gaseous, single‐pass configuration.
Fabrication of TAuD NVs: As shown in Figure S1 in the Supporting Information, Au AGs were produced via a laboratory‐made spark ablation reactor (volume, 42.8 cm3) comprising two Au rods (AU‐172561, Nilaco, Japan) and were continuously carried by nitrogen gas (99.999% purity; 3 L min−1) to a collision atomizer to be injected into a solution containing TD (0.10 mg of D and 26.75 mg of T in 1 mL ethanol) for the FD reaction. Hybrid droplets (e.g., Au AGs/TD) from the atomizer were exposed for 7.8 s to 185 nm wavelength photoirradiation (E = 6.2 eV, I = 0.14 J m−2 s−1; 3SC‐9‐A0, UVP, UK) to eject electrons from primary Au particles (work function, 5.1 eV) in the AGs. The surfaces of positively charged Au were electrostatically conjugated with negatively charged groups in TD, resulting in the reassembly of Au AGs forming TAuD NVs (Figure 1). The solvent was extracted from the hybrid droplets as they passed through a denuder containing pelletized activated carbons and silica gel. The NVs were charged with gaseous positive ions in a field charging configuration (pin (+4 kV)‐to‐ring (ground)), and subsequently collected on a polished aluminum rod under an electric field (−2.7 kV cm−1) via electrostatic attraction. The collecting rod was then immersed in PBS under ultrasonication (40 k Hz) for 5 min to release the NVs from the rod, forming an NV dispersion that was used in bioassays.
The authors declare no conflict of interest.