The Effect of Tapering the Optical Fiber on Evanescent Wave Measurements

Complex Fiber Micro-Knots

Fiber micro-knots are a promising and a cheap solution for advanced fiber-based sensors. We investigated complex fiber micro-knots in theory and experiment. We compared the measured spectral response and present an analytical study of simple micro-knots with double twists, twin micro-knots, figure-eight micro-knots, and tangled micro-knots. This research brings the simple fabrication process and robustness of fiber micro-knots into the world of complex resonators which may lead to novel optical devices based on fiber micro-knots.

Optical fiber-based fluorescent viscosity sensor

Molecular rotors are a unique group of viscosity-sensitive fluorescent probes. Several recent studies have shown their applicability as nonmechanical fluid viscosity sensors, particularly in biofluids containing proteins. To date, molecular rotors have had to be dissolved in the fluid for the measurement to be taken. We now show that molecular rotors may be covalently bound to a fiber-optic tip without loss of viscosity sensitivity. The optical fiber itself may be used as a light guide for emission light (external illumination of the tip) as well as for both emission and excitation light. Covalently bound molecular rotors exhibit a viscosity-dependent intensity increase similar to molecular rotors in solution. An optical fiber-based fluorescent viscosity sensor may be used in real-time measurement applications ranging from biomedical applications to the food industry.

Array biosensor for detection of biohazards

A fluorescence-based biosensor has been developed for simultaneous analysis of multiple samples for multiple biohazardous agents. A patterned array of antibodies immobilized on the surface of a planar waveguide is used to capture antigen present in samples; bound analyte is then quantified by means of fluorescent tracer antibodies. Upon excitation of the fluorophore by a small diode laser, a CCD camera detects the pattern of fluorescent antibody:antigen complexes on the waveguide surface. Image analysis software correlates the position of fluorescent signals with the identity of the analyte. This array biosensor has been used to detect toxins, toxoids, and killed or non-pathogenic (vaccine) strains of pathogenic bacteria. Limits of detection in the mid-ng/ml range (toxins and toxoids) and in the 10(3)-10(6) cfu/ml range (bacterial analytes) were achieved with a facile 14-min off-line assay. In addition, a fluidics and imaging system has been developed which allows automated detection of staphylococcal enterotoxin B (SEB) in the low ng/ml range.

Antibody immobilization using heterobifunctional crosslinkers

Covalent attachment of functional proteins to a solid support is important for biosensors. One method employs thiol-terminal silanes and heterobifunctional crosslinkers such as N-succinimidyl 4-maleimidobutyrate (GMBS) to immobilize proteins through amino groups onto glass, silica, silicon or platinum surfaces. In this report, several heterobifunctional crosslinkers are compared to GMBS for their ability to immobilize active antibodies onto glass cover slips at a high density. Antibodies were immobilized at densities of 74-220 ng/cm2 with high levels of specific antigen binding. Carbohydrate-reactive crosslinkers were also compared to GMBS using a fiber optic biosensor to detect fluorescently-labeled antigen. At the concentrations tested, the antibodies immobilized with carbohydrate-reactive crosslinkers bound more antigen than GMBS immobilized antibodies as indicated by the fluorescence signal.

An evanescent wave biosensor--Part I: Fluorescent signal acquisition from step-etched fiber optic probes

A fiber-optic biosensor capable of remote continuous monitoring has recently been designed. To permit sensing at locations separate from the optoelectronic instrumentation, long optical fibers are utilized. An evanescent wave immuno-probe is prepared by removing the cladding near the distal end of the fiber and covalently attaching antibodies to the core. Probes with a radius unaltered from that of the original core inefficiently returned the signal produced upon binding the fluorescent-labelled antigen. To elucidate the limiting factors in signal acquisition, a series of fibers with increasingly reduced probe core radius was examined. The results were consistent with the V-number mismatch, the difference in mode carrying capacity between the clad and unclad fiber, being a critical factor in limiting signal coupling from the fiber probe. However, it was also delineated that conditions which conserve excitation power, such that power in the evanescent wave is optimized, must also be met to obtain a maximal signal. The threshold sensitivity for the optimal step-etched fiber probe was improved by over 20-fold in an immunoassay, although, it was demonstrated that signal acquisition decreased along the probe length, suggesting that a sensor region of uniform radius is not ideal.

An evanescent wave biosensor--Part II: Fluorescent signal acquisition from tapered fiber optic probes

A biosensor was developed using antibodies, fluorescence and the evanescent wave to detect antigen binding at the surface of an optical fiber. Cladding was removed from the core along the distal end of a step-index optical fiber, and recognition antibodies were immobilized on the declad core to form the probe sensing region. Immersing the declad probe in aqueous solution creates a V-number mismatch between the immersed probe and the clad fiber. Probes created with reduced sensing region radius exhibited improved response by decreasing the V-number mismatch. Tapering the radius of this region has further improved probe response. Ray tracing analysis of the tapered probe demonstrated that the evanescent wave penetration depth increases along the length of the taper. Experiments correlating position of refraction along the taper with launch angle at the proximal end were realized in the ray tracing model. An evanescent wave immunoassay was performed with a series of the tapered fiber probes, each tapered from the fiber core radius (100 microns) to different end radii. An end radius of 29 microns was found to produce maximal signal from the tapered probe. Factors leading to the determination of the optimized probe are discussed.

Evanescent-absorption coefficient for diffuse source illumination: uniform- and tapered-fiber sensors

A comparative study of evanescent-wave fiber-optic absorption sensors based on uniform and tapered fibers has been carried out. The expressions for an effective evanescent-absorption coefficient have been derived for diffused or Lambertian source illumination. It has been shown that the sensitivity of sensors depends on the numerical aperture of the fiber, the taper ratio, and the refractive index of the absorbing fluid. The higher the sensitivity the smaller the range of functional refractive indices of the fluid. In the case of taper, which fiber (with a low or high numerical aperture) has maximum sensitivity depends on the refractive index of the fluid.

Regeneration of immobilized antibodies on fiber optic probes

The regeneration of antibodies covalently immobilized to an optical fibre surface was investigated by dissociation of the antibody-antigen complex with three different solvents: (a) an acidic solution (0.1 M glycine hydrochloride in 50% (v/v) ethylene glycol, pH 1.75), (b) a basic solution (0.05 M tetraethylamine in 50% (v/v) ethylene glycol, pH 11.0) and (c) 50% (v/v) ethanol in PBS. The fibres coated with polyclonal rabbit anti-goat antibody against a large protein retained 70% and 65% of the original signal after five consecutive regenerations with acidic and basic solvent systems, respectively. The fibres coated with monoclonal mouse anti-trinitrobenzene antibody specific for a small organic molecule, retained over 90% of the original signal when regenerated with basic and ethanol solutions. This study evaluated regeneration and reuse of antibody-coated fibre optic biosensors as a means of reducing routine laboratory analysis costs and time.

Detection of Clostridium botulinum toxin A using a fiber optic-based biosensor

A rapid, sensitive, analytical method for the detection of Clostridium botulinum toxin has been developed. The fiber optic-based biosensor utilizes the evanescent wave of a tapered optical fiber for signal discrimination. A 50 mW argon-ion laser, which generates laser light at 514 nm, is used in conjunction with an optical fiber probe that is tapered at the distal end. Antibodies specific for C. botulinum are covalently attached to the surface of the tapered fiber. The principle of the system is a sandwich immunoassay using rhodamine-labeled polyclonal anti-toxin A immunoglobin G (IgG) antibodies for generation of the specific fluorescent signal. Various anti-toxin antibodies were immobilized to the fibers. Affinity-purified polyclonal horse anti-toxin A antibodies performed better than the IgG fraction from the same horse serum or than the monoclonal anti-toxin A antibody BA11-3. Botulinum toxin could be detected within a minute, at concentrations as low as 5 ng/ml. The reaction was highly specific and no response was observed against tetanus toxin.