Development of an integrated microspectrometer using arrayed waveguide grating (AWG)

With non-invasive properties and high sensitivities, portable optical biosensors are extremely desirable for point-of-care (POC) applications. Lab-on-a-chip technology such as microfluidics has been treated as an ideal approach to integrate complex sample processing and analysis units with optical detection elements. Spectroscopic sensing (such as fluorescence, Raman and absorption spectroscopy) remains the most highly developed, widely applied, optical technique. However, conventional spectroscopic sensing systems still rely on bulky and expensive dispersive components such as spectrophotometers in a well established laboratory. The work in this thesis is to develop an integrated dispersive component in combination with a microfluidic chip, providing a portable and inexpensive platform for on-chip spectroscopic sensing. In this study, an arrayed waveguide grating (AWG) design developed for telecommunication is re-engineered and utilized to realise a compact, dispersive optical component operating in the visible spectral region. The AWG devices operating in the visible region (λ_c=680 nm) are designed and fabricated with flame hydrolysis deposited (FHD) silica waveguide material. The micro-spectrometer in this proof of concept study has a small (1 cm x 1 cm) footprint and 8 output channels centred on different wavelengths. A series of fabrication issues and challenges are investigated and discussed for the specific AWG device. Subsequently, a sample cuvette is formed by using lithographic technique and dry etching process. Following this, a PDMS chip with microfluidic channels is bonded with the AWG device, leading to an integrated AWG-microfluidic platform. To the best of the author’s knowledge, this is the first work to integrate a visible AWG device and a microfluidic chip towards spectroscopic sensing. The monolithic integrated AWG microspectrometer–microfluidic platform is demonstrated for fluorescence spectroscopic analysis. Signals from the output channels detected on a camera chip can be used to re-create the complete fluorescence spectrum of an analyte. By making fluorescence measurements of (i) mixed quantum dot solutions, (ii) an organic fluorophore (Cy5) and (iii) the propidium iodide (PI)-DNA assay, the results obtained illustrate the unique advantages of the AWG platform for simultaneous, quantitative multiplex detection and its capability to detect small spectroscopic shifts. Although the current system is designed for fluorescence spectroscopic analysis, in principle, it can be implemented for other types of analysis, such as Raman spectroscopy. Fabricated using established semiconductor industry methods, this miniturised platform holds great potential to create a handheld, low cost biosensor with versatile detection capability. Also, the AWG device design is modified with focusing properties that enable localised spectroscopic measurements. Micro-beads based, multiplexed fluorescence detection is performed with the AWG + CCD system and the results have demonstrated capabilities of using the adapted AWG device for localised, multiplexed fluorescence detections, opening up potential applications in the field of cell sorting and single cell analysis. Furthermore, the AWG-microfluidic device is investigated for absorption spectroscopy measurement. As a test system, the pH dependence of the absorption spectra of bromophenol blue is measured to illustrate how an AWG device could be used as a colorimetric pH sensor. Overall, it is believed that the AWG technology holds great potential to realise a compact, integrated spectroscopic biosensor for point-of-care applications.