Photonic crystal fiber sensor for biochemical applications
Wong, Wei Chang
Date of Issue2014
School of Chemical and Biomedical Engineering
Recently, optical fiber sensor has aroused great interest in the region of sensing technologies. Offering various advantages such as being immune to electromagnetic interference, remote sensing capability, compactness, chemical resistance and flexibility, unsurprisingly, it has been considered as a potential alternative to an electrical counterpart. A key factor influencing performance of an optical fiber sensor is its inherent physical structure, influencing its optical characteristics and subsequently the sensing performance. The key focus of this thesis is to fabricate and design a novel optical fiber structure, offering improvements such as ease of use and sensitivity in sensing performance. The possibilities of integrating the structure in biochemical sensing applications such as humidity, pH and immuno-sensing applications would be explored and demonstrated. The thesis begins with a thorough analysis of the characteristic of photonic crystal fiber (PCF) modal interferometer based on collapsed silica region. This is a structure first proposed by Villatoro. J's group and exhibited promising performance in sensing applications. Its refractive index (RI) sensing performance and interference fringes were analyzed. Through a simple mathematical derivation of the interference, it was proposed that sensing performance improvement is possible with the incorporation of a cavity ringdown (CRD) loop developed by our group previously. A 5 em long PCF interferometer based CRD sensor gave a RI detection limit of 7.8x10·5 RIU for RI range of 1.3512 to 1.3600, a two-fold increase after the enhancement. Regrettably, a high curvature cross sensitivity limit of 4. 85 x 1 o·3 m -t was also demonstrated. This places a stringent criterion on the handling of the sensor, compromising its suitability in biochemical application. One possible solution is to reduce the physical length of the sensor to limit the curvature. However, this would affect the interference period and sensitivity. Instead, the interferometer was coated with thin layer of gold for surface plasmon excitation. A 1 em device demonstrated a high RI sensitivity of 2250 nmiRIU due to the presence of higher order cladding mode excited. The surface plasmon resonance (SPR) sensor was functionalized with mixed self-assembled monolayer of thiols and protein G, and immobilized with bovine immunoglobulin G (lgG). A detection limit of 0.267 mg// anti-IgG was exhibited by the sensor. Problems such as lifting off of gold film after repeated usage and the expensive sputtering process of gold prevented the full endorsement of the device in biochemical applications. Based on the knowledge of the PCF interferometer, the key objective of this thesis was realized through making the tip of a Michelson PCF interferometer round with electric arcing. With the cladding mode excited upon reflection from the round tip, a novel optical sensor is formed. This is the first time to the author's knowledge, the cladding mode can be excited based on a round tip to improve sensing performance. With a higher order cladding mode, one of the optical path experiences an increase in phase delay. This allows reduction in the length of the interferometer while ensuring interference fringes could still be recorded in the 1500-1600 nm spectral window. High RI sensitivity of 262.38 nm/RIU was achieved while the temperature sensitivity remained low at 0.76x10-5 RIUfC for a 1 mm PCF modal interferometer. One disadvantage of the interferometer is the formation of Fabry Perot interference due to the small RI difference in the cores' RI between singe mode fiber (SMF) and PCF, which requires a digital low pass filter for removal. The positive outcome of the novel structure prompted for development of a simple application, in the form of functionalizing with a layer of polymeric film. This allows an initial evaluation of the practicality of the structure in biochemical sensing application. A humidity sensitive material, polyvinyl alcohol (PVA) film was coated on the structure for relative humidity (RH) sensing. High RH sensitivity of 0.60 nmi%RH was obtained and the sensor exhibited fast response with rising time and falling time of 300 ms and 500 ms, respectively. Hence, a novel structure has been developed and exhibiting promising results for humidity sensing application. It is of upmost importance to acknowledge that whilst the novel structure has the potential as a biochemical sensor, it suffers from poor signal to noise ratio and noise from Fabry Perot interference. Improvement has to be implemented on the structure before other biochemical applications can be developed. Poor signal to noise ratio is a challenge for the sensor's measurements to be conducted in a liquid medium. From Frensel's equation, having a high RI difference between mediums will increase the intensity of the reflected light. A layer of tantalum pentoxide film was coated onto the sensor through a sol-gel process. High RI of this transparent film increased the reflectivity of the sensor, leading to better signal to noise ratio. From the experimental results of the tantalum oxide coated interferometer, a mathematical model was proposed, showing good correlation between the computed and experimental data. With this model and knowing the RI of the coated material, the thickness of the film coated on the sensor can be predicted. A detection limit of 8. 79x 1 o-5 RIU was experimentally determined which is a near two times increases in RI sensitivity before oxide coating. The problem of poor signal to noise ratio has thus been alleviated with the implementation of a tantalum pentoxide, increasing the newly developed structure's feasibility as a biochemical sensor. The tantalum oxide coated interferometer was developed into an immunosensor as it was functionalized with 5 bilayers of polyelectrolytes, Poly(allylamine hydrochloride), PAH and poly(sodium 4-styrenesulfonate), PSS, followed by binding of bovine lgG to the PSS terminated sensor. A minimum concentration of 1 mg// of anti-IgG was achieved. Similar to the SPR sensor, the highly correlated fitting of the pseudo- first order kinetic equation and Langmuir isotherm, confirmed the success of interferometer for immunosensing. To eliminate the Fabry Perot ~nterference disturbance, different structures based on cladding mode excitation were examined. Core-offset and waist enlarged bitaper interferometers were demonstrated. The former exhibited RI detecting limit of 8.35 X 1 o-s RIU but the Fabry Perot interference remains a problem. However, this interference was not present in the latter as the interferometer managed to achieve a RI sensitivity of 145.2 nm/RIU. Similarly, with the coating of tantalum pentoxide, the interferometer exhibited an improved RI sensitivity of 302.6 nrn!RIU. This motivated us to develop a pH sensor out of this interferometer. Functionalized with a layer of thermally cross-linked PVA/PAA hydrogel, the pH sensor exhibited a pH sensitivity of 1.58 nm/pH for pH range of pH 4.02 to pH 6.03 with time response of 1 0 s in rising time and 11 s in falling time. With the use of tantalum pentoxide coating to improve signal to noise ratio and the implementation of waist enlarged bitaper to excite the cladding mode, a final novel structure displaying improved sensing performance is thus realized.