Harnessing fluorescent nano-material to visualize microfluidics

• Main Author: Xiao, Lian
• Other Authors: Sun Handong
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials::Photonics and optoelectronics materials; DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/88166; http://hdl.handle.net/10220/47311
• Institution: Nanyang Technological University

Microfluidics have been considered as a remarkable platform to explore fluid dynamics at micro scale as well as the versatile applications. Fluid flow visualization is the prerequisite of investigating a microfluidic system regarding both fundamental fluid mechanics and practical applications, but currently utilized fluorescent materials cannot meet the requirement for microfluidic visualization, which extremely obstructs their widespread practical applications. This thesis aims to address the issues relevant to microfluidic visualization. The major results are summarized as below. For biologic related microfluidic studies, such as bio-system mimic, pharmaceutical synthesis, drug delivery etc., the fluorescent tracer must have the combined characters of photo stability, chemical inertness, bio-compatibility, and mostly, low cost. Carbon dots fulfill all the requirements of microfluidic bio-applications, thus have been considered to be applicable for the fluid flow visualization. However, the contentious emission mechanism of carbon dots hinders the applications of these materials. Therefore, we first investigate systematically the emission mechanism of carbon dots by employing polarization anisotropy spectroscopy, electric-field modulation spectroscopy, and time-resolved photoluminescence (PL) measurements. Our results offered strong evidences that carbon dot emission originates from the self-trapped excitons, where the mobilization of the hot carriers is substantially obstructed due to the presence of a strong local potential field and thus the relaxation and decay process of the hot carriers are largely quelled. Our exploration furnishes an insight into the emission mechanism of carbon dots, which enhances our awareness of these novel materials. The understanding of carbon dots emission allows us to apply these novel materials into microfluidic visualization. For the first time, we document fluorescent carbon dots as a game-changer, applicable in versatile fluidic environment for the visualization in microfluidics with unprecedented advantages, such as photostability, chemical inertness, relative high imaging intensity, biocompatibility, environmental friendliness and above all, low cost. We have achieved a high fluorescent imaging speed up to 2500 frames per second under a normal continuous wave (CW) laser by utilizing carbon dots in microfluidics. In addition to the interface visualization, carbon dots based micro-particles (also called seeding particles), which enable quantitative investigation of bio-fluidic dynamics at micro-scale with a substantially lower cost, which is inaccessible by traditionally adopted fluorescent dye based seeding particles. Our findings hold profound influences to microfluidic investigations and may even lead to revolutionary changes to the relevant industries. The high speed dynamics at micro scale also finds various applications such as high efficiency sorting, chemical reaction, fast and high efficient mixing etc. Consequently, high-speed capability is highly demanded for micro particle image velocimetry (µPIV). To elevate the speed, current solutions dominantly focus on the illumination sources---more powerful lasers. Yet, the achievable speed is still insufficient to capture fast and complex flow fields. Hence, we have conceptualized and proposed an alternative solution of achieving ultra-fast quantitative measurement at micro-scale by self-assembling CsPbBr3 quantum dots into fluorescent seeding particles at micron scale. Through the synthesized particles, we are able to perceive and measure the flow patterns of fast dynamics. Illuminated by a normal continuous wave laser (CW) with a power of only 67 mW, we have created a fluorescent imaging speed up to 10,000 frames per second, which to the best of our knowledge, is the fastest record with a normal CW laser. We show that the introduction of CsPbBr3 QDs and the novel self-assembling method are keys to the final ultra-fast quantitative micro flow measurement, which reveals an alternative guide-line for the development of µPIV technology and shows a novel direction for fluorescent tracer particles as well. The thesis is composed of 6 Chapters. The general fluid physics, experimental techniques utilized in this thesis are presented in Chapter 1 and Chapter 2 respectively. Chapter 3 explore the emission mechanism of carbon dots. The demonstration in Chapter 4 imply the capability of carbon dots based microfluidic visualization, especially for the bio-microfluidics. To address the high speed quantitatively velocity field measurement issue in microfluidics, we document the CsPbBr3 Perovskite microparticles, as displayed in Chapter 5. The summary of the thesis and the prospects of microfluidics are shown in last Chapter (Chapter 6).