A micro-electro-mechanical system (MEMS) device is described having a membrane which can be induced to resonate and the frequency of its resonance can be monitored. Chemical moieties can be attached to the membrane, and these moieties can be selected such that they have an affinity for molecules of interest, especially biological molecules of interest. When molecules of interest bind to the moieties they increase the mass of the membrane and thereby change the frequency of the membrane's resonance. By monitoring the resonance one can obtain an indication of the presence of the molecules of interest and in some circumstances an indication of the approximate concentration of these molecules. In addition, several types of moieties having affinities for several different molecules of interest can be placed on the membrane in such a way that the sensor can detect the presence of several different types of molecules of interest and distinguish which ones may be present and which ones may be absent.