Magnetic nanosensor compositions and bioanalytical assays therefor

A method for detection of an analyte in a sample, the method comprising:

• (a) providing a fluid sample and one or more binding moieties, the binding moieties responsive to a target analyte or analog thereof and placing the sample and binding moieties under conditions and for a sufficient period of time to allow analyte in the sample to bind to binding moiety;
• (b) providing nanosensors comprising magnetic nanoparticles linked to analyte or an analog thereof;
• (c) placing the pre-incubated sample and binding moiety and the nanosensors in a container under conditions and for a sufficient period of time to allow analyte linked to nanosensors to bind to and compete off the binding moiety from the analyte in the sample;
• (d) placing the container in proximity to an NMR detector;
• (e) measuring one or more relaxivity parameters of the sample in the container;
• and
• (f) determining one or more attributes relative to the sample;
• wherein the nanoparticle, binding moiety and analyte or analog thereof linked to the nanoparticle are size optimized to confer optimal relaxation measurements.