Abstract: Nanometer-scale sensors useful in the detection of biological and chemical entities are formed from a protein rod body portion 41, a magnetic particle 42 affixed to the protein rod body portion 41; and an analyte binding moiety 43 disposed on the protein rod body portion 41 at a location remote from the magnetic particle 42, The analyte binding moiety specifically binds to the analyte 44 to form a sensor-analyte complex. The protein rod body portion may be formed from a tail fiber protein from a T even bacteriophage or a derivative thereof. Interaction of the analyte with the sensor will change the overall shape and size, and thus the ability of the sensor to move within a liquid sample in response to an applied magnetic field. Interaction can therefore be observed as a change in magnetic susceptibility or in relaxation time compared to that of a sensor in the absence of the analyte.