Optical fiber biochemical sensors based on micro-/nano-structured polymeric coatings

Abstract

Fiber-optic sensors have become one of the most enabling sensing technologies due to their distinctive advantages, such as small size, low cost, multiplexed detection and remote control capabilities. In recent years, new fiber-optic sensors for chemical and biological sensing have been drawn remarkable attentions due to their biocompatible properties as well as miniature size and high sensitivity. In this thesis, we present three kinds of new fiber-optic biochemical sensors through micro-/nano-engineering their polymeric coatings and demonstrate their applications in biological sensing. We firstly developed a high-performance fiber-optic pH sensor through fabricating a nanoporous polyelectrolyte complex (PEC) film on the surface of a thin-core fiber modal interferometer (TCFMI) device. A monolayer self-assembly technique has been applied to make a high-quality PEC nano-structured film on the surface of TCFMI devices. Experimental results reveal that the sensor with nanoporous PEC film has a close sensitivity with the sensor coated with non-porous PEC film, but the former shows much fast response due to the nanoporous structures of the sensing film. The fabricated sensors break the common "trade-off" rule in sensor fabrication and show both superior pH sensing performance and fast response. The second type of the developed fiber-optic biochemical sensor is a highly sensitive long-period grating (LPG) pH sensor. PAA ionic hydrogel has been periodically patterned on the surface of a tapered optical fiber with a diameter of 30 m by using our own-developed maskless exposure setup and formed a strain-modulated grating structure. Experimental results revealed that this new kind of LPG pH sensor has much higher sensitivity than conventional LPG sensors whose grating structures are written in the photosensitive fiber core. In order to exploit the fiber sensing technology in biological fields, we have developed a TCFMI biosensor based on polyelectrolyte multilayer nanocoatings for label-free DNA hybridization detection. With the layer-by-layer (LbL) self-assembly technique, multi-layered (PEI/PAA)₅(PEI/ssDNA)₁ sensing film has been coated on the surface of a TCFMI sensor for DNA detection. The sensor performance has been tested by using different kinds of ssDNA solutions with a concentration of 1 M. Experimental results showed that the fabricated fiber-optic DNA sensor can precisely identify the number of match bases of ssDNA chains. Finally, LPG sensors have been fabricated in small-diameter single mode fiber (SDSMF) and integrated into a microfluidic chip to develop an optofluidic biochip. With the point-by-point grating fabrication technique, LPG has been inscribed in a SDSMF by using high-intensity UV laser pulses. LbL self-assembly technique was then adopted to coat (PEI/PAA)₉(PEI/GOD)₁ multilayer sensing film on the LPG surface for glucose (GO) sensing. The influence of sensing film thickness on the GO sensors' performance was studied and compared. The fiber-optic GO sensor was finally integrated into the microfluidic channel to produce a biochip. Experimental results showed that the fiber sensor integrated biochip can detect GO concentration as low as 1 nM. Due to its point-of-care and real time analysis capabilities, such an ultrasensitive biochip has the potential for clinical applications.