The application is directed to a sensor element for use in a magnetochemical sensor, which comprises a first gel-like material (10), especially a swellable hydrogel such as polyacrylamide, and a magnetic second material (20) in the form of particles, especially magnetite (Fe304) nanoparticles, embedded in the first material along with a receptor for an alalyte, eg glucose. Changes in magnetic field due to interaction with the analyte are detected with GMR element (50). Applications include biosensors, DNA testing devices, and high throughput screening.