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Researchers have developed a 3D micro-printed sensor for highly sensitive on-chip biosensing. The sensor, which is based on a polymer whispering-gallery-mode microlaser, opens new opportunities for developing high-performance, cost-effective lab-on-a-chip devices for early disease diagnosis.

"In the future, these whispering-gallery-mode microlaser sensors could be integrated into a microfluidic chip to enable a new generation of lab-on-chip devices for ultrasensitive quantitative detection of multiple biomarkers," said research team leader A. Ping Zhang from Hong Kong Polytechnic University in Hong Kong, China.

"This could be used for early diagnosis of diseases such as cancers and Alzheimer's disease or for fighting major health crises, such as the COVID-19 pandemic."

In the journal Optics Letters, researchers their new microlaser sensor design, which overcomes many of the challenges that have made it difficult to integrate this type of sensor into lab-on-a-chip systems that could be used for point-of-care medical tests.

The researchers also show that the sensor's unique Limacon-shaped disk microcavity enables the detection of extremely small concentrations of human immunoglobulin G (IgG), a common antibody found in blood and other body fluids.

"This innovative microlaser sensor was possible because of our in-house 3D micro-printing technology," said Zhang. "It enables rapid printing of the specially designed 3D whispering-gallery-mode microcavity and high-precision trimming of the suspended microdisk."

Getting microlaser sensors onto a chip

Optical whispering-gallery-mode microlaser sensors work by trapping light in tiny microcavities. When the target molecules bind to the cavity, they cause slight changes in the laser's frequency, allowing highly sensitive biodetection.

One challenge in using these sensors in real-life applications is that coupling light into them typically requires a tapered optical fiber with a diameter smaller than 2 microns. Such tiny fibers are difficult to align and susceptible to various environmental disturbances. This has posed a barrier to integrating such microlaser sensors into lab-on-a-chip devices for real-time, high-sensitivity detection of biomolecules.

Using the light that emits from the microlaser sensor itself is a promising alternative to delivering it via tapered optical fibers, but the circular microcavities of conventional whispering-gallery-mode microlasers make it difficult to efficiently collect the light. This limits how well the sensor's signal can be read.

Printing precision biosensors

To solve this issue, the researchers designed a whispering-gallery-mode microlaser sensor with a Limacon-shaped suspended microdisk. This design gives the sensor a low lasing threshold and produces directional light emission, improving efficiency and making on-chip integration more practical.

Using their in-house 3D micro-printing technology, which has the advantages of high resolution and high flexibility, the researchers were able to rapidly print arrays of whispering-gallery-mode microlaser biosensors.

Experiments showed that the biosensors exhibited a very low lasing threshold of 3.87 μJ/mm² and a narrow lasing linewidth of about 30 pm. The sensors were able to detect IgG with a limit of just attograms per milliliter, showing their potential for ultralow detection of biomarkers used in early disease diagnosis.

Next, the researchers plan to integrate the microlaser sensors into a microfluidic chip to develop optofluidic biochips that could be used for rapid and quantitative detection of multiple disease biomarkers simultaneously.