Seeing the Virus: Colorimetric Sensors Identify Airborne Threats by Color

September 17, 2020 by Jake Hertz

Scientists at the Gwangju Institute of Science and Technology (GIST) in Korea have developed a new technology that visualizes the invisible.

The COVID-19 pandemic has spurred the search for technologies that safeguard individuals from contagious pathogens. For maximum safety, the real question becomes a matter of prevention: how do we avoid the spread of pathogens in the first place?

This can be particularly tricky, given that most viruses and other airborne hazards are invisible to the naked eye. In response to this, scientists at GIST in Korea have been looking into ways to make airborne hazards and pathogens visible via colorimetry. 


Colorimetry: A Brief Overview 

Put simply, colorimetric sensors are a type of optical sensor that changes color when influenced by external stimuli. Any change in a physical or chemical environment, from humidity to the presence of pathogens, can be considered a stimulus. Therefore, colorimetric sensors can be used for a wide variety of applications. 


Gradient-based colorimetric sensing

Gradient-based colorimetric sensing. Image used courtesy of Chenwen Lin et. al

These devices can operate in vastly different ways. Most operate on the principle that characteristic complexes offer specific absorption patterns based on different wavelengths of the EM spectrum (i.e color). The presence of a certain material will reveal itself based on its perceived color—a function of its material properties.

Other methods, like those used for commercial gas sensors, include miniaturized metal-oxide-semiconductor and electrochemical sensors.

A simple example of a colorimetric sensor that most people are familiar with is that of a PH test strip for a pool or hot tub. The pads on the test strip will change color based on the presence and concentrations of certain chemicals, letting the user know whether or not the water is safe to enter. 


A PH test strip is a common example of a colorimetric sensor

A PH test strip is a common example of a colorimetric sensor. Image used courtesy of Simplex Health


Unfortunately, one of the major limitations with currently-available colorimetric sensors is that they require complex structures with intricate fabrication procedures, making them less suitable for cheap, mass-scale production. On top of this, they suffer from slow response times and unsaturated colors.


Colorimetric Sensors Detect Faint Airborne Substances 

To address this issue, researchers at GIST in Korea have developed a new method of producing colorimetric results.

This method works by employing a thin layer of viruses called M13 bacteriophages. This type of virus has the ability to change its physical structure, and thus its optical properties, in response to changes in the surrounding environment.

The researchers genetically engineered the M13 bacteriophages by combining them with a highly lossy ultra-thin resonance promoter layer (HLRP) as the substrate. They then maximized the resonance of the virus layer by modifying the substrate, such that the viruses became sensitive toward specific airborne substances.


Schematic of a colorimetric sensor display showing an insensitive and sensitive color difference

Schematic of a colorimetric sensor display showing an insensitive and sensitive color difference. Image used courtesy of Young Jin Yoo et al.


This method makes it possible to detect the presence of chemicals at very low concentrations—as low as tens of parts per billion. “Through optimization of the virus layer deposition, the virus layer was coated with ultra-thin dimension, which enhanced the detection rate," explains head researcher Professor Song. "The HLRP with resonance enhancement was applied to obtain a distinct color even with a nanometer-scale thickness change in the M13 bacteriophage virus layer. Consequently, the color change was maximized by optimized resonance conditions.” 


A New Staple in Public Health?

Researchers at GIST are optimistic, claiming this new design affords the ability to mass-produce colorimetric sensors in a way previously impossible. Many safety-critical applications, such as detecting harmful industrial chemicals or assessing air quality, stand to benefit from this advance.

Professor Song concludes, “In the future, advances in genetic engineering will enhance the sensitivity of the sensors and extend their applicability to the medical industry, where they could be used as diagnostic kits for detecting specific viruses and pathogens.”