Abstract: A bio-sensor that includes (a) an optical fiber in which the surface of the fiber comprises a hydrogel polymer that includes a functional group comprising a first member of a binding pair; and (b) an excitation light source coupled to the fiber. When the fiber contacts a solution comprising a second member of the binding pair labeled with a light-emitting label, the first member binds to the second member, resulting in the emission of a detectable signal. Alternatively, the first member of the binding pair is provided with the light-emitting label.