Integrated Fluorescence Sensing in a Digital Microfluidic System Using Thin Film Silicon Photodetectors

Abstract

Advances in the development of miniaturized, autonomous general-purpose sensing systems for applications such as medical diagnostics, biological and chemical analysis, and point-of-care testing have driven the emergence of lab-on a-chip (LOC) systems, which integrate sample preparation and sensing. To realize LOC systems, enabling technologies are needed to carry out sample preparation and manipulation at the chip-scale, and sensing technologies that can be integrated with the chip-scale fluidic sample preparation platform.The integration of sample preparation and sensing is key to LOC systems. Electrowetting-on-dielectric (EWD) fluidic technology enables digital droplet manipulation at the chip-scale with standard microfabrication manufacturing techniques, with the advantages of non-bulky systems, low cost and portability [1]. EWD system have been integrated with optical, mechanical and electrochemical sensing mechanisms to realize a miniaturized LOC [2]–[4].Fluorescence sensing is one of the most widely used types of analyte sensing for biochemical targets due to its high sensitivity and specificity [2]. Intensity-based fluorescence sensing provides fast, localized detection that can be correlated to the concentration of the test analyte, thus providing quantitative detection information. Most current microfluidic LOC systems that utilize fluorescent sensing use large external microscopes or bulky filters and lenses for extracting the quantitative information, thus compromising on the portability and cost-effectiveness. The realization of optical fluorescence sensing integrated directly into microfluidic-based LOC systems can enable the systems to become more self-contained, portable, and potentially low cost.Fluorescence sensing is an effective method of identifying target species, and the integration of fluorescence sensing into a programmable digital microfluidic platform is the goal of the thesis work described herein. Silicon photodetectors (PDs) are an excellent choice for optical sensing due to their high responsivity at visible wavelengths, which corresponds to the emission wavelengths of many fluorophores. Additionally, using Si PDs enables the use of standard microfabrication processes that can be scaled for mass manufacturing. Furthermore, using thin film Si photodetectors provides the ability to be bonded to a chosen substrate for further system integration in small spaces where conventional photodetectors will not fit, without significantly altering the spatial configuration of the region where the sensing occurs.This dissertation presents the design and fabrication of annular, thin film Si photodetectors heterogeneously bonded to a pyrex substrate for fluorescence sensing, with a longer-term goal of integration into an EWD to realize a LOC system. Herein we design, fabricate, and test the performance of PD fluorescence sensors with thin film Si PD testing, followed by PD integration into an EWD microfluidic system. This system is simple and low cost, because it does not utilize filters to block the optical pump beam selectively from the PD, but rather, uses a novel optical design to suppress the fluorescence pump signal with the bottom plate of the microfluidic system, so that the signal to noise ratio (SNR) of the fluorescence signal to the pump signal is high. This device has the potential to be applied as a miniature, non-invasive, multi-target sensor.[Figure 1: Cross-section schematic of the integrated thin film Si fluorescence sensor with a bottom plate designed for integration with an EWD system. PD thickness not drawn to scale.]The target EWD system for integration with the fluorescence sensor has a top plate and a bottom plate, with the thin film Si PD integrated onto the top plate. The PD sensor is a thin film annular silicon PD bonded to the pyrex top plate, as shown in Figure 1, which can also serve as a ground electrode for the microfluidic platform. The optical pump signal, delivered herein through an optical fiber, is coupled to the droplet under test (containing the fluorophore) through the pyrex, through a thin metallization on the pyrex to minimize stray light, and then through the aperture in the photodetector. Light delivery can also be carried out using an integrated optical (LED or laser) source with or without an optical waveguide. The droplet is sandwiched between the top plate and the bottom plate. Fluorophore concentrations ranging from 0.3 μM to 20 μM were tested with different bottom plate substrates and herein, we discuss high SNR detection for droplet sizes as low as 10 μL over a time period of microseconds. Next, the integration of the fluorescence sensor with a digital microfluidic system is discussed. Finally, the performance of the sensor is evaluated, followed by conclusions and thoughts for future work.