Immunochemical methods are frequently based on the indirect detection of analytes by means of labels with specific properties. They can be used in the diagnostics of diseases, in food control or monitoring of toxic contaminants or pathogens. While routinely used techniques (e. g. luminescence or enzyme-based immunoassays) suffer from limitations, such as interferences of the luminescent nanoparticles and fluorescence bleaching or low stability of enzymatic labels, laser-induced breakdown spectroscopy (LIBS) as a readout technique offers fast analysis based on the elemental composition with a comparable sensitivity and no need of the specific label properties.

Magnetic microbeads (MBs) composed of an iron-based core and an outer layer of polymers or organic ligands, represent alternative solid phase for biomolecule immobilization. Being the only magnetic component within the system, they bring advantages, such as the possibility of analyte separation and preconcentration by applying external magnetic field. Due to the multi-elemental detection capability of LIBS, the characteristic elemental signature of a label (nanoparticle) and MB can be detected simultaneously, thus the presence of MBs and the immunoassay complex (analyte, antibodies, label) are confirmed in one analysis. In this work, we employed the MB-based immunoassay with photon-upconversion nanoparticles (UCNPs) as labels in the detection of prostate specific antigen (PSA). The possibility of the internal standardization of the Y II spectral line LIBS signal by means of the Fe I spectral line signal was explored in a proof-of-concept experiment where the simulation of a pipetting error was successfully supressed.