Fundamentals and Applications of Label-Free FLOWER-Based Sensing for Ultra-sensitive Detection of Single Particles and Molecules

Abstract

Detection techniques for single particles and molecules play a crucial role in advancing basic science, disease diagnostics, and nanomaterial investigations. Despite traditional fluorescence-based methods being powerful tools for single molecule detection, they face limitations such as a restricted range of molecular probes, as well as issues related to photoblinking and photobleaching. Whispering gallery mode (WGM) optical microcavities emerge as sensitive tools for label-free biomolecular sensing due to their ultrahigh quality (Q) factor and small mode volume. In the Frequency Locked Optical Whispering Evanescent Resonator (FLOWER) system, particles binding on the WGM optical resonator induces a resonance shift, recorded through the frequency locking method.This dissertation investigates the properties of the FLOWER system through numerical simulations. Key parameters, including the Q-factor and frequency modulation depth, are discussed concerning their impact on the Signal-to-Noise Ratio (SNR) of FLOWER. The FLOWER system is identified as being limited by shot noise from the receiver and intensity noise from the probe laser. Using median filter and step-fitting algorithms, FLOWER demonstrates the capability to detect resonance shifts as small as 0.05 attometers at one-millisecond intervals. Furthermore, the microtoroid functionalized with the T1R2/T1R3 heterodimer sweet taste receptor, is utilized in experimental assays to investigate the binding of sweet ligands. The research utilizes FLOWER for label-free measurement of the sweet ligand binding response to explores the influence of G-proteins on receptor activation, Insights gained contribute to a deeper understanding of G-protein-coupled receptor (GPCR) signaling pathways. Additionally, the microtoroid can be employed in photothermal microscopy for single-particle detection. The microtoroid-based photothermal microscopy can spatially detect 5 nm diameter quantum dots (QDs) with an outstanding SNR exceeding 10,000. Integration with an amplitude modulated pump laser and a Proportional-Integral-Derivative (PID) controller significantly reduces noise, enhancing signal stability. The photothermal microscopy demonstrates a remarkable capability to detect a minimum heat dissipation of 0.75 pW. below the detectable level from single dye molecules. The photothermal microscopy exhibits the potential to be applied in various fields, including biological sciences, nanotechnology, materials science, chemistry, and medicine.