Expecting to Realize Virus Testing at Home
Printable Antigen-Antibody Tests Use Optical Sensor Developed by Kyushu University

The research group of Assistant Professor Hiroaki Yoshioka, Professor Yuji Oki, and graduate student Abdul Nacil of the Faculty of Information Science and Electrical Engineering, Kyushu University, has succeeded in developing an optical sensor that does not require heat treatments and which can be used for antigen-antibody testing that can be printed with the same technology as commercially available inkjet printers, which they accomplished by combining a special polymer (Nissan Chemical Corporation), in which biotin used for the antigen-antibody reaction can be surface printed at room temperature, with original, disk-shaped micro-laser element printing technology. It is expected that testing that can show results on the spot and that simple, at-home testing will now be realized. Published in Optical Materials Express.

One of the optical sensors using the antigen-antibody reaction is a method of using disc-shaped micro-laser elements consisting of a micro-optical resonator with a size of approx. 0.1 nanometers that traps light well. When the target substance for detection adheres to this disc-shaped micro-laser element, the antigen or antibody contained in the element binds to the target substance for detection and, by causing an antigen-antibody reaction, the optical spectrum of the laser generated from the element changes and the target substance can be detected. In theory, ultra-sensitive sensors capable of detecting even a single virus can be detected.

In particular, the optical sensor, which uses an antigen-antibody reaction based on the binding of avidin and biotin molecules, is outstanding in terms of sensitivity to and selectivity for the target substance to be detected, so biosensing research that combines this with micro-optical resonators is actively being conducted. In order to use this avidin-biotin binding reaction in a micro-optical resonator, biotinylation is required to covalently bond biotin to the surface of the micro-optical resonator, and this can be achieved by using surface printing on the micro-optical resonator to bind a functional group compatible with biotin's covalent bonding. However, the fabrication of micro-optical resonators for biosensing, which typically employ materials such as polymers and silica (glass), usually request heat or acid treatments, and, after being fabricated, surface reforming is performed to expose the functional groups that can bind to biotin, so it cannot be realized without using processes that perform complicated chemical processing or without using multi-process equipment. In other words, the base micro-optical resonators needed to be prepared at a semiconductor factory, etc., and the materials that can be used were limited due to the processing burden.

The research group first used an original inkjet printing method for a newly developed low-viscosity special polymer (provided by Nissan Chemical Corporation), which is characterized by an easily biotinylated of hydrophilic carboxyl functional groups and a chain of CF2 hydrophobic fluorinated functional groups, to mold and print micro-laser elements (micro-disk lasers) based on disk-shaped micro-optical resonators for biosensing. Next, the shape of the manufactured laser elements and the basic characteristics of the laser were evaluated, and the basic performance of the biotinylated micro-laser elements was confirmed. In evaluating the sensing, the absorption of streptavidin, which is the detection target, was compared by observing the shift in the laser's oscillation spectrum via the use of two micro-laser elements, one that was biotinylated and one that was not. As a result, it was observed, when not biotinylated, that avidin was randomly layered on the surface of the micro-laser elements and the spectrum continued to shift, but, when biotinylated, the spectral shift corresponded to that of a single avidin layer formed by binding to the surface-printed biotin. In other words, this clearly confirmed the avidin-biotin reaction in which the avidin is supplemented only with biotin, and successfully demonstrated label-free sensing.

With the establishment of the biotinylation method, it can be said that in the future a large number of avidin-modified antibodies can be attached to micro-disk surfaces to provide a base for a variety of sensing, and, additionally, the technology of applying biotinification treatments to polymers that can be used for inkjet printing is expected to have ripple effects on other optical micro-resonators and biosensing coatings.

In a general virus test, it often takes time to send a sample to an institute with dedicated equipment, such as PCR equipment, and obtain a result. However, when this micro-laser element technology for biosensing, done via inkjet printing methods without the use of heat treatments and which can be used completely at room temperature and pressure, matures and becomes a portable device, it opens up the possibility of immediately and on the spot conducting simple virus tests via an antigen-antibody reaction, and, ultimately, it will be possible to continuously conduct virus tests at home. Furthermore, because the sensors can be printed either at home or at a destination, just as with a handy printer, mass and repeated testing can be easily carried out, even in economically disadvantaged countries and regions. In the future, it will be important to do significant work to improve the technology's performance, such as through quantitative measurements, identification evaluations, and sensitivity evaluations/optimizations, and to develop portable devices for practical use.

In the future, the research group will proceed with the development of organic topological optical resonators in ongoing projects, and, by combining the foundations of this biosensing with organic topological optical resonators, will lead to the establishment of more performant sensing technology.

According to Yoshioka, "This inkjet printing method, which can print laser elements, does not require a special environment like a laboratory and can be used at room temperature and pressure. As such, a variety of substances can be blended in from the ink stage and the possible range of applications is very wide, so I am looking forward to further research and development of this technology in the future."