the present invention is a novel , ultra - small tip , internal referenced , amperometric microbiosensor that uses an immobilized biological interface to measure the concentration of an analyte in a specimen . it consists of a casing that narrows to an aperture having a diameter at the tip no greater than 4 μm ; enclosed within the casing a reference electrode and a working electrode both immersed in electrolyte ; within the aperture , an inner polymer film , an immobilized biological interface layer , and an outer specimen - compatible , non - virulent polymer film . another important feature of the present invention is that the microbiosensor can readily be encased in a durable protective sheath . the microbiosensor is especially useful in situ for specimens that cannot be mixed , such as in situ compounds in unmixed fluid , or semi - solid specimens . the microbiosensor provides 90 % response time less than 5 seconds and typically about 1 second , less than 5 % change in output current due to changes in the stirring rate , and the ability to measure in viscous , semi - solid or porous - solid specimens with a spatial resolution as small as 30 μm .