Collecting fine particles with optical fiber — Osaka Metropolitan University develops 3D trapping technology

Specially Appointed Assistant Professor Kota Hayashi, Professor Takuya Iida, Professor Shiho Tokonami, and their colleagues from the Research Institute for Light-induced Acceleration System (RILACS), Osaka Metropolitan University have developed an optical condensation module in which a fiber-based metallic nano-thin film is coated onto the tip of an optical fiber. The group demonstrated that by generating bubbles at a desired location and driving high-speed three-dimensional convection, 10,000 bacteria and nano/micro fluorescent polystyrene particles can be efficiently concentrated with just one minute of laser irradiation. Because the module is low-cost and easy to arrange in arrays, it is expected to contribute not only to microorganism testing and biomolecule measurement, but also to high-throughput preprocessing of diverse biological samples such as nucleic acids and proteins. The findings were published in Communications Physics.

Research and development of various bioanalytical technologies has been advancing in recent years, but concentrating dilute samples remains a major challenge in high-sensitivity detection. The research group has been developing optical condensation methods that use heat-driven convection (light-induced convection) and bubbles generated by laser irradiation of metallic nanostructures to concentrate and detect various biological substances within a few minutes. Specifically, they developed honeycomb substrates and bubble-mimetic substrates with micro-scale periodic structures, as well as nanobowl substrates with nano-scale multiporous structures. From this they reported that bacteria could be concentrated with a high survival rate of over 80% and that antigen-antibody reactions could be accelerated without damaging proteins. However, the assembly efficiency of concentrated bacteria was only a few percent at best, and improving the concentration rate remained a priority. Moreover, when bubbles are generated on a substrate, the convection driven by the substrate slows down, which limits the range, direction, and speed at which dispersed particles can be transported—a bottleneck that constrained further improvements in assembly efficiency.

The group's newly developed fiber-based optical condensation module incorporates a metallic nano-thin film formed on the tip of an optical fiber. When a near-infrared laser is introduced into the fiber inserted at any position within a sample, the metallic nanostructure at the fiber tip acts as a heat source, generating bubbles and convection that induce optical condensation at the fiber tip. As a result, dispersed particles can be assembled at an assembly efficiency exceeding 10%.

More specifically, just one minute of laser irradiation from the fiber-based optical condensation module was sufficient to induce optical condensation and collect 10,000 particles out of approximately 90,000 microparticles (1 micron in diameter) in a 20-microliter liquid sample, with similar results also obtained for Escherichia coli. In both cases, successful assembly at the fiber tip was demonstrated at an assembly efficiency of up to around 10%, and calculations suggest that as few as 10 bacteria in the liquid could potentially be detected.

First, the fiber-based optical condensation module was inserted into a dispersion liquid containing microparticles, and bubbles were generated at any desired position in the liquid, away from the substrate, successfully concentrating the particles. The condensation factor achieved was approximately 30 times higher than that of the conventional method using a flat gold nano-thin film.

A theoretical analysis was conducted to investigate the physical factors behind this high condensation factor. The analysis revealed that the convection occurring during optical condensation flows at high speed (several millimeters per second) from all three-dimensional directions surrounding the bubble toward the fiber tip. This high-speed three-dimensional convection is thought to be responsible for the assembly efficiency exceeding 10%, a level unachievable with conventional two-dimensional optical condensation substrates. By changing the insertion position of the fiber-based optical condensation module, it also becomes possible to clarify how liquid-gas interfaces (such as droplet surfaces) and solid-liquid interfaces (such as substrate or light-condensing cell surfaces) affect optical condensation. In fact, during this study, placing the fiber-based optical condensation module on a substrate enabled convection to be driven horizontally, parallel to the substrate, which had not been achieved before, leading to the discovery of new phenomena such as particle assembly not only around the bubble but also around the fiber module itself, as well as particle movement in directions away from the heat source.

Since the system can also be easily arrayed, it holds promise for achieving ultra-high-throughput preprocessing in compact and quick multi-sample measurement systems in the future. In particular, because this fiber-based optical condensation module can concentrate microorganisms and biomolecules in liquids simply by being inserted into a sample, it may be possible to omit the large optical systems previously required for high-performance optical condensation or to make the observation optics more compact, enabling simple field detection of microorganisms in food and beverages, and harmful fine particles in the environment, within just a few minutes. The module could also be used for identifying bacterial species through selective bacterial concentration via antibody or aptamer modification of the fiber tip, as well as for collecting concentrated bacteria or biomolecules for detailed subsequent analysis using state-of-the-art technologies such as mass spectrometry systems or next-generation sequencers.

Beyond cells, bacteria, and viruses, the technique can also be applied to bionanoparticles such as extracellular vesicles (EVs), as well as various biomolecular biomarkers including disease markers such as proteins and genes. High-throughput simultaneous multi-sample measurement combining this technology with plate wells is also anticipated.

Journal Information
Publication: Communications Physics
Title: Highly efficient three-dimensional optical condensation of nano- and micro-particles using a gold-coated optical fibre module
DOI: 10.1038/s42005-025-02480-9