Iron core–gold shell (Fe@Au) nanoparticles are prominent for their magnetic and optical properties, which are especially beneficial for biomedical uses. Some experiments were carried out to produce Fe@Au particles with a one-step synthesis method, Ultrasonic Spray Pyrolysis (USP), which is able to produce the particles in a continuous process. The Fe@Au particles were produced with USP from a precursor solution with dissolved Iron (III) chloride and Gold (III) chloride, with Fe/Au concentration ratios ranging from 0.1 to 4. The resulting products are larger Fe oxide particles (mostly maghemite Fe2O3), with mean sizes of about 260–390 nm, decorated with Au nanoparticles (AuNPs) with mean sizes of around 24–67 nm. The Fe oxide core particles are mostly spherical in all of the experiments, while the AuNPs become increasingly irregular and more heavily agglomerated with lower Fe/Au concentration ratios in the precursor solution. The resulting particle morphology from these experiments is caused by surface chemistry and particle to solvent interactions during particle formation inside the USP system.

To monitor the progress and prevent the spread of the COVID-19 pandemic in real time and outside laboratories, it is essential to develop effective tests that can ensure rapid, selective, and reliable diagnosis of infected persons in different environments. Key in this regard is the lateral flow immunoassays (LFIAs) that can detect the presence of the SARS-CoV-2 virus quickly, with the aid of nanoparticles (NPs) and specific proteins. We report the use of gold (Au) NPs AuNPs synthesised from a gold(iii) chloride tetrahydrate precursor in a USP device and collected in a suspension composed of deionised water with polyvinylpyrrolidone as a stabiliser and cryoprotectant. In combination with freeze-drying of the AuNPs’ suspension to achieve water elimination, improved stability, and the target concentration, they exhibit the necessary properties for use as markers in LFIA rapid diagnostic tests. This was confirmed by complementary characterisation determined by using the techniques including inductively coupled plasma-optical emission spectrometry, dynamic light scattering method and zeta-potential, ultraviolet-visible spectroscopy, X-ray diffraction, scanning electron microscopy with energy dispersion spectrometer, and transmission electron microscopy, as well as with the preparation of a prototype LFIA test strip with AuNPs. Thus, such AuNPs, as well as the USP method, show promise for the development of new markers for use in LFIA.

The Ni/Y2O3 catalyst showed high catalytic activity. Based on this, the aim of this study was to create Ni/Y2O3 nanocomposites powder with two innovative technologies, Ultrasonic Spray Pyrolysis (USP) and lyophilisation. In the USP process, thermal decomposition of the generated aerosols in an N2/H2 reduction atmosphere caused a complete decomposition of the nickel (II) nitrate to elemental Ni, which became trapped on the formed Y2O3 nanoparticles. The Ni/Y2O3 nanocomposite particles were captured via gas washing in an aqueous solution of polyvinylpyrrolidone (PVP) in collection bottles. PVP was chosen for its ability to stabilise nano-suspensions and as an effective cryoprotectant. Consequently, there was no loss or agglomeration of Ni/Y2O3 nanocomposite material during the lyophilisation process. The Ni/Y2O3 nanocomposite powder was analysed using ICP-MS, SEM-EDX, and XPS, which showed the impact of different precursor concentrations on the final Ni/Y2O3 nanocomposite particle composition. In a final step, highly concentrated Ni/Y2O3 nanocomposite ink (Ni/Y2O3 > 0.140 g/mL) and test coatings from this ink were prepared by applying them on a white matte photo paper sheet. The reflection curve of the prepared Ni/Y2O3 nanocomposite coating showed a local maximum at 440 nm with a value of 39% reflection. Given that Ni is located on the surface of the Ni/Y2O3 nanocomposite in the elemental state and according to the identified properties, tests of the catalytic properties of this coating will be performed in the future.

A coupled process of ultrasonic spray pyrolysis and lyophilisation was used for the synthesis of dried gold nanoparticles. Two methods were applied for determining their melting temperature: uniaxial microcompression and differential scanning calorimetry (DSC) analysis. Uniaxial microcompression resulted in sintering of the dried gold nanoparticles at room temperature with an activation energy of 26–32.5 J/g, which made it impossible to evaluate their melting point. Using DSC, the melting point of the dried gold nanoparticles was measured to be around 1064.3°C, which is close to pure gold. The reason for the absence of a melting point depression in dried gold nanoparticles was their exothermic sintering between 712 and 908.1°C.

Gold nanoparticles (AuNPs) have now been used in skin care creams for several years, with marketed anti-aging, moisturizing, and regenerative properties. Information on the harmful effects of these nanoparticles is lacking, a concern for the use of AuNPs as cosmetic ingredients. Testing AuNPs without the medium of a cosmetic product is a typical method for obtaining this information, which is mainly dependent on their size, shape, surface charge, and dose. As these properties depend on the surrounding medium, nanoparticles should be characterized in a skin cream without extraction from the cream’s complex medium as it may alter their physicochemical properties. The current study compares the sizes, morphology, and surface changes of produced dried AuNPs with a polyvinylpyrrolidone (PVP) stabilizer and AuNPs embedded in a cosmetic cream using a variety of characterization techniques (TEM, SEM, DLS, zeta potential, BET, UV–vis). The results show no observable differences in their shapes and sizes (spherical and irregular, average size of 28 nm) while their surface charges changed in the cream, indicating no major modification of their primary sizes, morphology, and the corresponding functional properties. They were present as individually dispersed nanoparticles and as groups or clusters of physically separated primary nanoparticles in both dry form and cream medium, showing suitable stability. Examination of AuNPs in a cosmetic cream is challenging due to the required conditions of various characterization techniques but necessary for obtaining a clear understanding of the AuNPs’ properties in cosmetic products as the surrounding medium is a critical factor for determining their beneficial or harmful effects in cosmetic products.