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).

It is now believed that the near-resonance excitation of plasmonic nanoparticles is necessary to increase the Raman signal of nearby molecules. Consequently, for surface-enhanced Raman scattering (SERS) applications, researchers seek to synthesize rationally designed nanoparticles with plasmon resonances (PRs) close to the excitation wavelength. However, existing experiments show contradicting results for the dependence of the SERS enhancement on the PR wavelength. Here, we used the etching method to prepare a set of Au nanorod (AuNR) colloids with a decreasing aspect ratio. The shape morphology of the AuNRs and their concentration and width were kept constant, while the plasmon resonance was progressively decreased from 925 to 650 nm. The AuNRs were functionalized with 1,4-nitrobenzenethiol (NBT), and SERS spectra of the colloids were measured under 785 nm laser excitation. The nanorod concentration (∼7 × 1010 mL–1) was quantified by the atomic absorption spectroscopy and spectrophotometry combined with TEM statistical data and T-matrix simulations. The number of adsorbed NBT molecules per one nanorod (∼104) corresponded to the effective footprint of ∼0.55 nm2 and was close to the monolayer packing density with the topological polar surface area of NBT at 0.468 nm2. The plasmon peak position correlated weakly with the SERS response; specifically, the ratio between the SERS intensities for on- and off-resonance excitation was below 1.5. This observation contradicts the current understanding of the electromagnetic contribution to the SERS signal. In particular, our simulations agreed with the experimental data for plasmon resonance wavelengths of 785–925 nm, but for shorter wavelengths the simulations predicted an order-of-magnitude decrease in the averaged enhancement factor. In contrast to this finding, the shape morphology strongly affected the SERS response. Specifically, when the initial cigarlike AuNRs were further overgrown to yield dumbbell morphology, their SERS intensity increased 5-fold. Finally, we show that the SERS background spectra can be attributed to both the photoluminescence from AuNR ensemble and the elastic light scattering of a very weak laser background by the same AuNR ensemble.

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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.

This article analyzes data on the diversity of shapes and sizes of nanoparticles obtained by green synthesis from HAuCl 4 , AgNO 3 , Na 2 SeO 3 , and Na 2 SiO 3 by using xylotrophic and humus basidiomycetes and soil bacteria. The formation of nanoparticles of various shapes and sizes was controlled by changing the bioreduction conditions, including culture type and age, growth medium, culture liquid, mycelial extract, isolated proteins, and incubation time. Biogenic selenium nanoparticles were represented exclusively by spheres whose size varied from 20 to 550 nm, depending on the culture. Autoclaving of selenium nanoparticles fabricated with bacterial cultures yielded nanowires with a width of 20−150 nm and a length of more than 10 μm. With bacterial culture liquids, silicon nanospheres were synthesized, ranging in size from very small (5−15 nm) to relatively large (250 nm) particles. The use of the Agaricus culture liquid made it possible to obtain mesoporous particles with a size of 30−60 nm. The size and shape of the fabricated gold and silver nanoparticles were very diverse, depending on the bioreduction conditions, and were represented by regular and irregular spheres; large balls; hexagonal, tetragonal, and triangular prisms; tetrahedrons; and nanobelts. The particle size ranged from 1−10 to 200−500 nm.