First microlasers capable of detecting individual molecules and ions could one day aid diagnosis

Image Credit: Courtesy of University of Exeter

Scientific Frontline: Extended "At a Glance" Summary: Single-Molecule Microlaser Biosensors

The Core Concept: Researchers have developed microscopic glass bead lasers—measuring between 0.1mm and 0.01mm—capable of acting as highly sensitive optical biosensors. These microlasers can detect materials at an unprecedented scale, identifying individual molecules and single atomic ions.

Key Distinction/Mechanism: The microlasers operate using whispering gallery modes (WGM), where trapped light continuously circles the inner boundary of the glass sphere. When combined with gold nanorods that create nanometer-scale "hot spots," the binding of a single molecule or ion slightly alters the beatnote frequency of the clockwise and counterclockwise laser waves, which researchers measure using self-heterodyne beatnote detection.

Origin/History: The breakthrough was led by Professor Frank Vollmer and Dr. Samir Vartabi Kashanian at the University of Exeter’s Living Systems Institute, funded by the Engineering and Physical Sciences Research Council (EPSRC).

Major Frameworks/Components: 

• Whispering Gallery Modes (WGM): A phenomenon where optical waves travel in a circular path around a concave surface, creating a highly sensitive resonant cavity.
• Plasmonic Enhancement: The use of gold nanorods on the laser's surface to compress and concentrate light into nanometer-scale hot spots, amplifying the signal of single-molecule interactions.
• Self-Heterodyne Beatnote Detection: A technique used to detect minute frequency shifts caused by molecular binding rather than measuring barely perceptible shifts in the light directly.

Branch of Science: Optical Physics, Biophysics, Biochemistry, and Medical Diagnostics.

Future Application: This technology paves the way for advanced "lab-on-a-chip" devices capable of instant medical testing, rapid viral detection, and identifying small structural changes in proteins related to enzyme activity or cellular signaling.

Why It Matters: Achieving single-ion and single-molecule sensitivity represents a massive leap in optical biosensing capabilities. This precision allows for the ultra-early diagnosis of conditions such as cancer and dementia, fundamentally advancing our understanding of the mechanisms that underpin disease development at the molecular level.

Scientists have created the first microlasers capable of detecting individual molecules and even single atomic ions, a breakthrough that could significantly advance early disease diagnosis and molecular-scale medical testing. 

Microlasers are tiny glass beads measuring around just 0.1 mm (the width of a human hair) to 0.01mm – (the length of a single bacterium). With a central cavity that acts like a tiny mirror, they emit and bounce light in a circular motion around the bead. This circular path of trapped light is known as whispering gallery modes (WGM) laser technology. Light continuously circulates around the sphere’s inner boundary, enabling the device to detect extremely small disturbances on its surface. Previous research has shown that such microlasers can even be inserted into living cells, acting as optical barcodes to track cellular movement inside organisms. Now, researchers at the University of Exeter’s Living Systems Institute have published their work in Nature Photonics. Funded by the Engineering and Physical Sciences Research Council, the paper opens new possibilities for microlaser biosensing technology, including “lab-on-a-chip” technology capable of instant medical testing and diagnosis. 

Physicist Professor Frank Vollmer, at the University of Exeter’s Living Systems Institute, led the work. He said: “For the first time, we’ve created microlasers capable of detecting materials smaller than ever before – on the scale of individual atoms and molecules. This is an exciting innovation because it moves us closer to a new generation of lab-on-a-chip devices, which could diagnose conditions such as cancers or dementia early, and be used for swift-testing viruses. It could also enable us to detect small structural changes in proteins, such as those associated with enzyme activity and protein signaling, which no technology is capable of currently. Such an advance would mean a huge leap in our understanding of the mechanisms underpinning processes such as disease development.” 

The research team used a number of different techniques to enhance the lasers to become sensitive to single-molecule and even single atomic ions. The microlaser itself is highly precise, able to register tiny changes in the light circulating inside it. The researchers then added gold nanorods to the surface, which concentrate light into tiny nanometer-scale ‘hot spots’, compressing it down to the size of molecules and amplifying the effect of a single molecule or ion binding there. 

Finally, they used a technique called self-heterodyne beatnote detection. When a molecule or ion binds at one of these nanometer-scale hot spots, it slightly changes the beatnote frequency produced by the clockwise and counterclockwise laser waves inside the sphere. Rather than measuring a barely perceptible shift in light directly, the system detects this tiny change in frequency. 

By tracking several laser beatnotes at once, the researchers can confirm the activity of single-molecule events across multiple signals. This improves the reliability of the system and strengthens its ability to detect and verify molecular interactions with high confidence. 

The work was led by the University of Exeter’s Living Systems Institute. Study co-author Dr Samir Vartabi Kashanian said: “The Living Systems Institute brings physicists, biologists and chemists together under one roof. That stimulating interdisciplinary environment allows us to cross boundaries between physics, chemistry and biology, which is essential for translating advances in optical physics into practical biosensing applications.” 

Published in journal: Nature Photonics

Title: Single-atomic-ion detection with plasmon-enhanced whispering-gallery-mode microlasers

Authors: Samir Vartabi Kashanian, and Frank Vollmer

Source/Credit: University of Exeter | Louise Vennells

Reference Number: phy032526_01

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