Optical sensors based on inhomogeneous waveguiding in the walls of capillaries (‘capillary waveguide optrodes’)

Abstract

Glass capillaries with a chemically sensitive coating on the inner surface are used for optical chemical sensing in gaseous and liquid samples. The light from an LED or a laser is coupled into and out of the capillary under a well-defined angle. The coated glass capillary acts as an inhomogeneous waveguide, in which light is guided in both the glass and the coating. The portion of the light that is absorbed in the chemically sensitive coating is a function of the concentration of a chemical analyte. Typical relative light-intensity signal changes with this type of optical interrogation are as high as 98%. Such capillary waveguide optrodes possess several attractive features: (a) they can serve as sample cavities or flow cells for gases and liquids; (b) they are suitable for direct sampling; (c) their relative signal change can be optimized by adjusting the optical pathlength; (d) the optical pathlength is much longer compared to other optrode designs based on absorption or reflection, therefore the coatings can be made thinner, and less loaded with chemically active components; (e) the sensors are hardly cross-sensitive to color and turbidity of the sample; (f) cheap light sources and detectors can be used in connection with these novel capillary waveguide optrodes.