IgG and IgM differentiation in a particle-based agglutination assay by control over antigen surface density.

Point-of-care (POC) testing offers fast and on-site diagnostics and can be crucial against many infectious diseases and in screening. One remaining challenge in serological POC testing is the quantification of immunoglobulin G (IgG) and immunoglobulin M (IgM). Quantification of IgG/IgM can be important to evaluate immunity and to discriminate recent infections from past infections and primary infections from secondary infections. POC tests such as lateral flow immunoassays allow IgG and IgM differentiation; however, a remaining limitation is their incapacity to provide quantitative results. In this work, we show how samples containing IgG or IgM can be distinguished in a nanoparticle-based agglutination biosensing assay by tuning the density of antigens on the nanoparticles' surface. We employ direct STochastic Optical Reconstruction Microscopy to quantify the accessible SARS-CoV-2 trimeric spike proteins conjugated to magnetic nanoparticles at a single-particle level and gain insight into the protein distribution provided by the conjugation procedure. Furthermore, we measure the anti-SARS-CoV-2 IgG/IgM induced agglutination using an optomagnetic readout principle. We show that particles with high antigen density have a relatively higher sensitivity toward IgM compared to IgG, whereas low antigen density provides a relatively higher sensitivity to IgG. The finding paves the way for its implementation for other agglutination-based serology tests, allowing for more accurate disease diagnosis.

Probing Temperature Responsivity of Microgels and Its Interplay with a Solid Surface by Super-Resolution Microscopy and Numerical Simulations.

Super-resolution microscopy has become a powerful tool to investigate the internal structure of complex colloidal and polymeric systems, such as microgels, at the nanometer scale. An interesting feature of this method is the possibility of monitoring microgel response to temperature changes in situ. However, when performing advanced microscopy experiments, interactions between the particle and the environment can be important. Often microgels are deposited on a substrate, since they have to remain still for several minutes during the experiment. This study uses direct stochastic optical reconstruction microscopy (dSTORM) and advanced coarse-grained molecular dynamics simulations to investigate how individual microgels anchored on hydrophilic and hydrophobic surfaces undergo their volume phase transition with temperature. We find that, in the presence of a hydrophilic substrate, the structure of the microgel is unperturbed and the resulting density profiles quantitatively agree with simulations performed under bulk conditions. Instead, when a hydrophobic surface is used, the microgel spreads at the interface and an interesting competition between the two hydrophobic strengths,monomer-monomer vs monomer-surface,comes into play at high temperatures. The robust agreement between experiments and simulations makes the present study a fundamental step to establish this high-resolution monitoring technique as a platform for investigating more complex systems, these being either macromolecules with peculiar internal structure or nanocomplexes where molecules of interest can be encapsulated in the microgel network and controllably released with temperature.