Open-Source Benchtop Magnetophotometer for Characterizing the Magnetic Susceptibility of Suspended Nanoparticles

Magnetic nanoparticles (MNPs) form the foundation of many technologies, frequently serving to purify proteins or cells from a biological sample or to remove environmental contaminants. Their success relies on their magnetic response, which allows them to be easily controlled in a liquid or solution. Therefore, the magnetic susceptibility provides one metric for assessing the suitability of a MNP for a given application. Unfortunately, conventional methods for measuring the magnetic susceptibility relies on instrumentation that characterizes the MNPs as a dry powder. Because MNPs are typically used in suspension, the measured value may be different from their behavior in suspension, thus providing inaccurate readings. Here, we present the design and validation of a magnetophotometer (MAP), an instrument that characterizes the effective magnetic susceptibility of suspended MNPs via differential optical spectroscopy, providing a more relevant measure of MNPs' magnetic properties. As part of this work, we developed a mathematical model to calculate the effective magnetic susceptibility from the MAP data and validated the model using measurements with a range of magnetic nanoparticles, including particles with different material compositions, sizes, architectures, and surface coatings. Finally, we demonstrate that MAP testing is nondestructive by successfully characterizing bioconjugated particles without damaging the bioactivity of the surface bioconjugation, providing a path for in-line quality control assessment.