© 2014 American Chemical Society. The development of sensitive and easy-to-use biosensors that allow an adequate characterization of specific weak biological interactions like carbohydrate-lectin interactions still remains challenging today. Nanoparticles functionalized with carbohydrates are one of the most powerful systems for studying carbohydrate-lectin interactions, because they mimic the multivalent presentation of carbohydrates encountered in nature, for example when viruses and bacteria bind to cells. On the basis of the model system glucose-Concanavalin A (ConA), we explore the application of Transient Magnetic Birefringence (TMB) to study these weak interactions, using glucose-functionalized colloidal magnetite nanoparticles (NPs) as probes. We demonstrate that the binding dynamics can be monitored and derive a model to obtain the apparent cooperativity. For our studies, we use nanoparticles of 6 and 8 nm in diameter. The ConA-generated response shows apparent cooperativity, due to the cross-linking of nanoparticles by the ConA tetramer which has four binding sites. Cooperativity is higher for 6 nm NPs, possibly due to a better accessibility of all four ConA binding sites on smaller NPs, enhancing cross-linking. For this system, we find a detection limit of 3-23 nM. (Chemical Equation Presented). ; This work has been supported by the Spanish Ministry of Education and Science (NAN2004-09125-C07-02), the Spanish National Research Council (projects 200550F0172 and 2009UY0024), the Spanish Ministry of Industry (FIT-010000-2006-98), the Spanish Ministry of Science and Innovation (CTQ2008-03739/PPQ), and the European Research Council (Starting Grant NANOPUZZLE). M.K. gratefully acknowledges support through a PhD fellowship I3P financed by the Spanish National Research Council and the European Commission. M.M. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (CONSOLIDER-NANOBIOMED). ; Peer Reviewed