On-Fiber Printed Sensor for Label-Free Detection of Bacteria

Research, fiber optics, and 3D printing in new techniques to fight old nemesis

Published: Wednesday, March 23, 2022 - 12:01

According to the World Health Organization (WHO), antibiotic resistance is now one of the greatest threats to global health, food safety, and development. This immunity of bacteria is a natural evolutionary process, and that is further accelerated by the misuse of antibiotic treatments. For more precise treatment of infections, in situ characterization of the infected region would be ideal because it would allow specific treatment against the pathogens.

With this in mind, researchers at the Imperial College London developed a novel fiberoptics Raman spectroscopy sensor for the label-free identification of bacteria using Nanoscribe’s microfabrication technology.

Gold-coated fiber optical SERS probe: The green color of the probe is a diffraction phenomenon of the microstructured surface. Image: J. A. Kim, Imperial College London

The invention of antibiotics is certainly one of the great milestones of modern medicine and has relieved many people’s fears of infectious disease such as tuberculosis or pneumonia. However, experts in medicine and biology are again focusing on the fight against bacteria infections. Due to the high use or even misuse of antibiotics, bacteria become increasingly resistant to their treatment, and even minor infections can once again become life-threatening. To counteract this, antibiotics should only be administered when necessary and in accordance with the respective disease pattern.

Schematic illustration of the fiber-based SERS probe. A laser light is coupled into the optical fiber and excites signal hotspots on the nanostructured surface of the fiber probe. In interaction with the analyte, the enhanced Raman signal (SERS signal) is generated and collected by the optical fiber. Image: J. A. Kim, Imperial College London

In situ bacteria characterization by fiberoptics SERS probe

Scientists at Imperial College London are addressing this global challenge by developing a fiber-optic sensor for the label-free detection and characterization of bacteria. This miniaturized sensor, based on surface-enhanced Raman spectroscopy (SERS), is the first of its kind and can potentially be integrated into medical endoscopes for in situ analysis of inflamed tissue.

Study to optimize the design of a single voxel array. The voxel spacing is varied, and the effect on the SERS signal is analyzed. Impressively small spacing distances of 400 nm could be resolved in this study. However, the maximum SERS signal was found at a spacing of 700 nm. Image: J. A. Kim, Imperial College London

Raman spectroscopy is a powerful analytical technique for organic and biological samples, allowing the characterization of samples such as bacteria based on their individual spectral fingerprint. The inherently weak nature of Raman scattering can be enhanced by metallized micro- and nanostructured surfaces, creating signal hotspots that interact with the sample.

In their study, the scientists 3D-printed these micro- and nanostructures on facets of the optical fibers using two-photon polymerization (2PP), then coated them with a thin layer of gold. For SERS measurements, a laser light is coupled into the optical fiber and excites signal hotspots on the nanostructured surface of the fiber probe. In interaction with the analyte, the SERS signal is generated and collected by the optical fiber.

Microstructures printed directly onto an optical fiber for an optical sensor that uses the principle of surface-enhanced Raman spectroscopy to detect bacteria. Image: J. A. Kim, Imperial College London

Rapid prototyping with short design iteration cycles

In a first-design study, the scientists analyzed the SERS effect of various micro- and nanopatterns printed on a planar glass substrate using Nanoscribe’s 2PP technology. A hexagonally arranged single-voxel array proved to be the most efficient sample. With rapid design iterations, the researchers further optimized the spacing between the individual voxels of the array and fabricated samples with impressively small spacing distances of only 400 nanometers, which is challenging but in fact still feasible with 2PP-based microfabrication.

A second design that proved to be efficient for SERS measurements was a microspike array that guides and concentrates the signal hotspots at the apexes. Specifically printed on fibers, this design showed increased mechanical stability compared to the single-voxel array and was further investigated for the detection of E. coli bacteria.

For the final fiberoptic SERS probe, the researchers printed the optimized microstructures directly onto the facets of the fiber and demonstrated its analytical capabilities by detecting unlabeled E. coli bacteria.

Nanoscribe’s solution for tilt, compensated and aligned on-fiber printing

For the on-fiber printed SERS probe, the researchers had to overcome several fabrication challenges. First, they designed a custom fiber holder that allows printing on the facet of a fiber. Then, the printed object must be perfectly aligned to the core of the optical fiber to excite the microfabricated Raman hotspots. A remaining challenge, especially for filigree structures such as the single-voxel array, is the compensation of a potentially tilted substrate surface. The tilted substrate surface of the optical fiber resulted in a poor yield of the SERS active microstructures.

To drive innovations in photonics and applications of medical instrumentation and optical sensing, such as the intriguing fiberoptic SERS probe, Nanoscribe recently introduced its latest 3D printer, Quantum X align. With its proprietary on-fiber print set and tilt correction in all spatial directions, the new 3D printer may already provide the answer to the challenges of the on-fiber printed SERS probes and pave the way for further improvements and new innovations.

Read the scientific publication here: “Fiber-Optic SERS Probes Fabricated Using Two-Photon Polymerization For Rapid Detection of Bacteria.”

First published Feb. 24, 2022, on Nanoscribe News.

About The Author

Nanoscribe

A member of the Bico Group, Nanoscribe is a spinoff of the Karlsruhe Institute of Technology (KIT) pioneering 3D microfabrication. Developing products and services based on its two-photon polymerization technology, Nanoscribe empowers cutting-edge science to drive industrial innovations in a wide variety of sectors such as micro-optics, micromechanics, biomedical engineering, and photonics technologies.