Lateral flow immunoassays for detecting biomarkers in body fluids are simple, quick, inexpensive point-of-care tests widely used in disease surveillance, such as during the coronavirus disease 2019 (COVID-19) pandemic. Improvements in sensitivity would increase their utility in healthcare, food safety, and environmental control. Recently, biofunctional magnetic nanoclusters have been used to selectively label target proteins, which allows their detection and quantification with a magneto-inductive sensor. This type of detector is easily integrated with the lateral flow immunoassay format. Pneumolysin is a cholesterol-dependent cytolysin and one of the most important protein virulence factors of pneumonia produced by Streptococcus pneumoniae. It is recognized as an important biomarker for diagnosis in urine samples. Pneumonia is the infectious disease that causes the most deaths globally, especially among children under five years and adults over 65 years, most of them in low- and middle-income countries. There especially, a rapid diagnostic urine test for pneumococcal pneumonia with high sensitivity and specificity would be helpful in primary care. In this work, a lateral flow immunoassay with magnetic nanoclusters conjugated to anti-pneumolysin antibodies was combined with two strategies to increase the technique’s performance. First, magnetic concentration of the protein before the immunoassay was followed by quantification by means of a mobile telephone camera, and the inductive sensor resulted in detection limits as low as 0.57 ng (telephone camera) and 0.24 ng (inductive sensor) of pneumolysin per milliliter. Second, magnetic relocation of the particles within the test strip after the immunoassay was completed increased the detected signal by 20%. Such results obtained with portable devices are promising when compared to non-portable conventional pneumolysin detection techniques such as enzyme-linked immunosorbent assays. The combination and optimization of these approaches would have excellent application in point-of-care biodetection to reduce antibiotic misuse, hospitalizations, and deaths from community-acquired pneumonia.

The flow behavior of practically structureless ferrofluids was tuned over a large range by adding well-defined amounts of magnetite nanoparticle clusters. Magnetite nanoparticle clusters with hydrophobic coating were dispersed in high colloidal stability transformer oil based ferrofluid samples, initially free of clusters. The resulting ferrofluid composition consists both of single (approx. 7 nm mean size) and clustered (280 nm average size) magnetite nanoparticles, each of them in a prescribed amount. This new approach allows a precise evaluation of the role of clusters on the ferrofluid flow behavior. The magnetite nanoparticle clusters obtained by a solvothermal method have a saturation magnetization of 75 emu g ⁻¹ and a remnant magnetization of 0.2 emu g ⁻¹ due to the large size (∼25 nm) of the magnetite nanoparticles in their composition. Long bundles of thin elongated needle like aggregates of magnetic nanocluster particles are formed in a magnetic field and produce a significant magnetorheological response of the bidisperse magnetic suspensions observed already for moderate applied magnetic field values. The dynamic yield stress and magnetoviscous effect are analyzed and compared with previous results concerning suspensions of micrometer size iron particles in a ferrofluid carrier. The viscoelastic properties and the interactions between the components of ferrofluid based magnetite nanoparticle cluster suspensions were evaluated by creep tests in zero and non-zero applied magnetic field. The bidisperse magnetic nanocomposite suspensions investigated provide a promising new formulation of MR fluids with improved kinetic stability to be used in seismic dampers and flow control devices.