Tapered optical fiber sensor (TOFS) devices are attractive as biosensors due to their higher sensitivity, accurate measurement capabilities, and real-time operation. The tapered region allows the evanescent electromagnetic (EM) field to extend outside the fiber to enable the detection of minute changes in the refractive index in close proximity to the tapered region. The sensing is achieved using appropriate functionalized tapered fiber surfaces. In this work, a second generation (2G) automated compact TOFS system developed in our lab is tested, and repeatable and stable signals are obtained proving that this device potentially can serve as a portable bio/chemical sensor in the future. Preliminary simulations, using a FFT based split-step beam propagation method, of optical propagation through a tapered fiber leading to the detected signal as a function of scanning wavelength and its phase shift with cladding refractive index are presented.
A practical tapered optical fiber (TOF) biosensing system was developed for label-free detection using antigen-antibody pairs with repeatable results and a very high degree of sensitivity. This was done by attaching molecular recognition agents to a tapered fiber surface for augmenting sensitivity and specificity of analyte. The entire system included three main parts: a tunable laser, a tapered fiber, and an optical detector. Light from an unpolarized tunable fiber laser was introduced into the tapered fiber from one end, and the transmitted intensity was detected by a photodetector. In the tapered fiber area, the evanescent electromagnetic field, which extends outside the fiber, was able to detect minute changes in the refractive index caused by antigen-antibody pairs. Recorded data was analyzed using an innovative Fourier analysis method to find phase changes, which are directly related to the biomolecular concentration coated on fiber, from which antibody-antigen concentrations are obtained. Two experiments were performed to confirm the concept using two very different agents. The first was the protein Interleukin-8 (IL-8). Repeatable results with a sensitivity of 10 pg/mL were achieved. The second was human coronavirus OC43 (HCoV-OC43), a surrogate viral particle for SARS-CoV-2, with a sensitivity of 50 viruses/mL. Critical sources of error were identified and addressed for the purpose of using the device for real clinical diagnosis in various real-life environments, where viruses can reside in water, phosphate-buffer solution, or saliva, the most popular three environments in real clinical diagnosis. Our device was designed according to the principle that only one specific kind of antibody and antigen can be combined together. The device demonstrated good accuracy to chosen analyte(s) tailored to specific applications and offered the potential to develop a point-of-care device used in clinics, as well as for detecting a variety of viruses and biocontaminants. The reproducibility of TOFs was confirmed through multiple fabrications and consistent results.
Head and neck squamous cell carcinoma (HNSCC) is a fatal disease with 650,000 newly diagnosed cases worldwide. Early detection of this cancer improves survival significantly. Salivary biomarker analysis demonstrates that screening of HNSCC is possible and potentially improves survival. Interleukin-8 (IL-8), a cytokine produced in abundance by the HNSCC stem cells, induces epithelial-mesenchymal transformation (EMT), enhances metastasis, and can be used as a salivary biomarker for HNSCC screening. A healthy individual has 300 – 500 picogram/ml (pg/ml) in saliva, while HNSCC patients have 1700 – 2500 pg/ml. We have developed a novel tapered optical fiber biosensing system for label-free detection of antigens by attaching a molecular recognition agent to a tapered fiber surface for augmenting sensitivity and specificity of the analyte. In a tapered fiber, the evanescent electromagnetic field, which extends outside the fiber, is able to detect minute changes of the refractive index caused by the environment. The evanescent field intensity exponentially decreases with distance away from the surface of the fiber on a length scale in the order of the wavelength of light. Light from a tunable fiber laser is introduced into the tapered fiber from one end and the transmitted intensity is detected by a photodetector. Received data is then analyzed using Fourier transformation to find phase changes related to the biomolecular coating of the fiber, which is directly related to the antigen concentration. Real-time measurement of antigen concentrations at the level of 10 pg/ml is shown to be possible by successful integration of hardware and software systems. This approach has the potential to develop a pointof-care device to be used in the clinics.
In our previous work, the induced force on a microbubble due to a focused laser beam has been studied by deriving a general optical force model. It has also been shown that the microbubble, without and with agglomerated metallic nanoparticles, can be steered using a low power laser beam. An equivalent optical force for a microbubble surrounded by nanoparticles has been also calculated and presented in our previous work. While only optical forces acting on the microbubbles and nanoparticles have so far been considered, in this work the effect of surface tension force is considered and calculated using the temperature gradient generated by the focused trapping laser beam and compared with optical forces. Experiments are performed to observe the behavior of the microbubbles in the vicinity of a focused laser beam for five different wavelengths.
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