By Pranjal Malewar 23 Aug, 2024
Collected at: https://www.techexplorist.com/detecting-single-5-nm-quantum-dot-microtoroid-resonator-photothermal-microscopy/88337/
Conventional techniques to detect single molecules mainly depended on fluorescence techniques. They require labeling target molecules.
In contrast, photothermal microscopy is a promising label-free, noninvasive imaging technique. It measures small changes in how light bends around a sample because the sample absorbs light and causes temperature changes in its surroundings.
Because of their ultra-high quality (Q) factors, Whispering gallery mode (WGM) resonators are ultra-sensitive temperature sensors.
Using microtoroid WGM optical resonators, scientists from the Wyant College of Optical Sciences and the Department of Biomedical Engineering at the University of Arizona have recently demonstrated a label-free ultra-sensitive photothermal microscopy technique. This technique can detect single nanoparticles as small as 5 nm quantum dots with a signal-to-noise ratio (SNR) exceeding 10,000.
Because of this newly demonstrated method, photothermal sensitivity significantly improved- achieving a detection limit of 0.75 pW in heat dissipation.
Scientists have previously developed a system called the Frequency Locked Optical Whispering Evanescent Resonator (FLOWER). The system uses frequency locking to track the resonance shift of optical microcavities.
This photothermal microscopy system uses FLOWER to detect the photothermal signal. It measures how the resonance changes when a free-space pump laser is applied. The pump laser, modulated at 203.7 Hz, shines on the microtoroid through a scanning system with a galvo mirror.
The FLOWER detected the resonance shift oscillation at 203.7 Hz. A lock-in amplifier is used to measure its amplitude.
A 2D scan of the pump laser creates a photothermal image of the microtoroid. High photothermal spots in the image help to detect single nanoparticles.
Scientists noted, “Moreover, future advancements may involve spectroscopy measurements by varying the pump laser’s wavelength or exciting with different wavelengths to enable multicolor imaging. In future work, although not necessary for many applications, we can combine this system with specific capture agents such as aptamers or sorbent polymers to enhance specificity.”
Integrating FLOWER with photothermal microscopy paves the way toward real-time observation of dynamic changes and interactions of target molecules.
According to scientists, this FLOWER-based photothermal microscopy will act as a versatile platform for label-free imaging and single-molecule detection. Its demonstrated high sensitivity and discrimination capabilities pave the way for advancements in nanoscale imaging and characterization techniques.
Journal Reference:
- Hao, S., Suebka, S. & Su, J. Single 5-nm quantum dot detection via microtoroid optical resonator photothermal microscopy. Light Sci Appl 13, 195 (2024). DOI: 10.1038/s41377-024-01536-9
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