Development of integrated optofluidic sensors

Microfluidics is the technology for fluid-based operations at micrometre scale for controlling or detection purpose with the advantages of less reagents consumption, easier automation and lower costs. When the optical components are combined with microfluidics, the interaction between fluid and light can be exploited in the optofluidic system formed for versatile applications in physics, chemistry or biology. The integration of the microphotonic components rather than coupling the external macrocomponents with microfluidics devices hold the promises to improve the functionality, compactness and portability of optofluidic systems. This thesis describes the realisation of integrated optofluidic sensors based on three different approaches. A Fabry-Perot optofluidic sensor with a large detection depth, a three-dimensional photonic crystal optofluidic sensor for highly localised detection, as well as an optical tweezer based sensor that can take measurements at arbitrary positions in microfluidic systems were demonstrated. Experimental measurements were compared with theoretical calculations and showed good agreement. The disposable Fabry-Perot optofluidic sensor was fabricated by the hot embossing method in a convenient and cost-efficient manner. A Fabry-Perot cavity was integrated within a microfluidic channel to form a robust optofluidic sensor with a large sensing depth. Fluid refractive index detection with a sensitivity of 2.13× 10−3 was realised by the shift of the Fabry-Perot resonance positions. Both the sensing component and the microfluidic platform were fabricated by hot embossing method in a single process without the need of post-assembling. The all-plastic platform, the single process fabrication method and robust performance of the optofluidic sensor holds great promises towards disposable point-of-care applications. The 3D photonic crystal optofluidic sensor was proposed specifically for applications that require highly localised detection. The sensor was fabricated by femtosecond laser direct writing in a polymer substrate. Woodpile photonic crystal structures were fabricated due to their ability to provide good photon confinement and the flexibility of bandgap tuning. The change in the fluid refractive index inside a microchannel was detected by the shift of the photonic bandgap position. In addition, a planar defect was introduced to the 3D photonic crystal to form a microcavity. The localised defect mode inside the microcavity was also utilised for refractive index sensing. A sensitivity of 6×10−3 was achieved by the 3D photonic crystal sensor which shows an improvement on the sensitivity of one-dimensional and two-dimensional counterparts (∼ 10−2). The optical tweezer sensor is a non-invasive optofluidic probe that can be used at arbitrary positions in microfluidic systems of various internal geometries. Compared to conventional rheometry that is limited to measurements in macrosystems, the optical tweezer sensor is non-invasive and highly localised and is suitable for more complex microfluidic environments. In addition, characterisation of large particles, up to tens of micrometre scale that other microrheological techniques fail to deal with, can be performed by this sensor. In our experiments, optical tweezers were incorporated with straight and curved microfluidic channels for velocity measurements and shear stress detection. Particles of various diameters were characterised by their velocity and the shear stress. To our knowledge it is the first shear stress sensor in microfluidics that is based on optical tweezers. The research in this thesis explores designing, fabrication and sensing techniques regarding the integration of optical components into microfluidic systems. With three optofluidic sensors demonstrated, it is anticipated that the work will spur more innovative optical sensing solutions for microfluidics and in turn open new horizons for the next generation optofluidic devices with compact configurations and more functionality.