Gold nanoparticles (GNPs) and GNP-based multifunctional nanocomposites are the subject of intensive studies and biomedical applications. This minireview summarizes our recent efforts in analytical and theranostic applications of engineered GNPs and nanocomposites by using plasmonic properties of GNPs and various optical techniques. Specifically, we consider analytical biosensing; visualization and bioimaging of bacterial, mammalian, and plant cells; photodynamic treatment of pathogenic bacteria; and photothermal therapy of xenografted tumors. In addition to recently published reports, we discuss new data on dot immunoassay diagnostics of mycobacteria, multiplexed immunoelectron microscopy analysis of Azospirillum brasilense, materno-embryonic transfer of GNPs in pregnant rats, and combined photodynamic and photothermal treatment of rat xenografted tumors with gold nanorods covered by a mesoporous silica shell doped with hematoporphyrin.
Lateral flow immunoassay (LFIA) is a rapid and simple point-of-care method for the detection of various analytes. In the colorimetric sandwich format, the reaction outcome is a colored test zone line formed by Au nanoparticle (AuNP) conjugates captured by bound analyte molecules. Although nanoparticle design is crucial for the assay sensitivity, the correlation between the test zone brightness and the number and size of captured AuNPs has not been studied in detail. To fill this gap, we used an unprecedented set of 10 spherical and monodisperse AuNPs with diameters d ranging from 16 to 115 nm. The calculated optical properties are in excellent agreement with two-layered Mie theory for Au cores coated with 3 nm CTAC shells with a refractive index of 1.5. Different concentrations of AuNPs (ICP-MS and UV–vis measurements) were spotted onto a nitrocellulose LFIA membrane, and the color intensity of the spots was measured and analyzed with RGB and HSV color parameters. The minimal detected spot intensity was proportional to the surface nanoparticle density and the particle volume. The derived size dependence means that extinction rather than scattering is the main physical mechanism behind spot brightness. The limit of detection (LOD) in terms of the surface nanoparticle density scaled like the inverse third power of the particle size (more precisely, like ∼d –3.1) and was about 7 × 107 and 1.5 × 105 particles/mm2 for 16 and 115 nm AuNPs, respectively. We analyzed an ideal LFIA format, when one analyte molecule delivers just one AuNP to the test zone. In this case, the theoretical LODs were in the pg/mL range for a typical LFIA format with 0.1 mL of a 25 kDa analyte. In practice, these estimations could be increased by 2 orders of magnitude because of the larger ratio of analyte to captured AuNPs. This strongly reduced the assay sensitivity to the the ng/mL level. Although an ideal LFIA predicts a strong increase in the assay sensitivity with the AuNP size (scales like ∼d 3.1), this improvement could be compensated for in part by an increase in the number of ineffective analyte molecules bound to the AuNP surface (scales like ∼d 2).
At present, the discrete dipole approximation (DDA) is the most popular technique to simulate optical responses from various nanoparticles, including gold nanorods with a complex morphology. However, the DDA code demands considerable computer resources and computation time if one needs orientation and ensemble averaging. Here, we introduce a new T-matrix-solvable and flexible geometrical model for nanorods that can be implemented on a usual office PC to calculate the optical properties of a statistical nanorod ensemble with random orientations. The model is used for accurate calculations of the orientation-averaged extinction spectra for transmission electron microscopy (TEM) based ensembles of gold nanorods with account taken of statistical variations in their shape and size. The simulated spectra are in good agreement with experimental measurements provided that the surface electron scattering constant A s is about 0.3. Our ensemble-based estimation of the surface-scattering constant is close to A s evaluations from single-rod scattering (Novo et al. Phys. Chem. Chem. Phys. 2006, 8, 3540−3546) and single-sphere absorption (Berciaud et al. Nano Lett. 2005, 5, 515−518) experiments.
Au, Ag, Se, and Si nanoparticles were synthesized from aqueous solutions of HAuCl4, AgNO3, Na2SeO3, and Na2SiO3 with extra- and intracellular extracts from the xylotrophic basidiomycetes Pleurotus ostreatus, Lentinus edodes, Ganoderma lucidum, and Grifola frondosa. The shape, size, and aggregation properties of the nanoparticles depended both on the fungal species and on the extract type. The bioreduction of the metal-containing compounds and the formation rate of Au and Ag nanoparticles depended directly on the phenol oxidase activity of the fungal extracts used. The biofabrication of Se and Si nanoparticles did not depend on phenol oxidase activity. When we used mycelial extracts from different fungal morphological structures, we succeeded in obtaining nanoparticles of differing shapes and sizes. The cytotoxicity of the noble metal nanoparticles, which are widely used in biomedicine, was evaluated on the HeLa and Vero cell lines. The cytotoxicity of the Au nanoparticles was negligible in a broad concentration range (1–100 µg/mL), whereas the Ag nanoparticles were nontoxic only when used between 1 and 10 µg/mL.
The recently developed laser-induced cell transfection mediated by Au nanoparticles is a promising alternative to the well-established lipid-based transfection or to electroporation. Optoporation is based on the laser plasmonic heating of nanoparticles located near the cell membrane. However, the uncontrollable cell damage from intense laser pulses and from random attachment of nanoparticles may be crucial for transfection. We present a novel plasmonic optoporation technique that uses Au nanostar layers immobilized in culture microplate wells. HeLa cells were grown directly on Au nanostar layers, after which they were subjected to continuous-wave 808 nm laser irradiation. An Au monolayer density ~15 μg/cm and an absorbed energy of about 15 to 30 J were found to be optimal for optoporation. Propidium iodide molecules were used as model penetrating agent. The transfection efficiency evaluated using fluorescence microscopy for HeLa cells transfected with pGFP under optimized optoporation conditions (95% ± 5%) was similar to the efficiency of TurboFect. The technique's efficiency (295 ± 10 relative light units, RLU), demonstrated by transfecting HeLa cells with the pCMV-GLuc 2 control plasmid, was greater than that obtained by transfection of HeLa cells with the TurboFect agent (220 ± 10 RLU). The cell viability in plasmonic optoporation (92% ± 7%), too, was greater than that in transfection with TurboFect (75% ± 7%).
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