Smart graphene paper allows for rapid testing

The distinct advantage of rapid pregnancy and COVID-19 tests lies in their simplicity, allowing individuals to easily conduct the tests themselves in virtually any location. These tests rely on microfluidic techniques, where aqueous solutions travel through a paper test strip aided by capillary forces. Throughout this process, antibodies capture specific substances like virus particles or pregnancy hormones, concentrating them at a designated spot. A staining system subsequently reveals the increasingly concentrated target substance as a visible stripe.

Despite the simplicity and dependability of this fundamental principle, interpreting the results through visual assessment can pose challenges. The uncertainty of discerning whether a line is truly present or merely a figment of our imagination has likely crossed our minds since the onset of the COVID-19 pandemic.

The innovation from the ETH team precisely addresses this issue. They have devised a method to incorporate conductive electrodes directly within the test strip paper. When the target substance is captured, it initiates an electronic signal, resulting in significantly faster, more sensitive, and more accurate measurements.

Chih-​​Jen Shih and Andrew deMello share a passion: “Our biggest incentive is to improve basic chemical and biological experiments in ways that create new scientific opportunities.”

The research groups have achieved this very objective with their rapid tests. By integrating cost-effective and straightforward paper-based microfluidics with the precision and sensitivity of electronic measurement techniques, a wide array of analytical applications can benefit. These applications encompass patients being empowered to monitor blood biomarkers, on-site sampling of soil, air, and water, and swift disease testing in remote regions of the world, all accomplished within minutes. This extensive range of potential applications encompasses virtually all chemical, biological, and medical analyses conducted in aqueous solutions.

Efforts made in the past to integrate detection electrodes into affordable paper-based chemistry faced a significant challenge due to a fundamental characteristic of conductive materials. In essence, electrical conductors have limited interaction with water, impeding the flow of samples and reaction mixtures within a paper strip. Overcoming this obstacle and creating a reliable technology that functions effectively even in underdeveloped regions demanded a combination of expertise.

To develop the new rapid test, the researchers from ETH Zurich combined their respective areas of expertise. Shih's group contributed their knowledge on generating conductivity within the paper itself, while deMello's group brought their expertise in microfluidic systems to the collaboration. By pooling their skills, they were able to successfully create the innovative rapid test.

The foundation of this invention lies in employing a laser to transform the sugar polymers found in cellulose, the main component of the paper, into graphene. Graphene, a unique form of carbon, exhibits conductivity and is regarded as a promising electronic material for the future.

Through the process of laser conversion, the cellulose molecules undergo decomposition into their elemental components, including carbon, oxygen, and hydrogen. This transformation can be likened to the caramelisation of sugar commonly observed in household settings. However, unlike the prolonged heating of sugar in a pan, which results in the formation of ordinary carbon lacking electrical conductivity, the scientists from ETH Zurich harnessed the power of the laser to restructure the carbon atoms within cellulose, ultimately yielding conductive graphene.

Merely producing graphene within cellulose paper would not suffice since, similar to most other conductive materials, graphene possesses hydrophobic properties, repelling water and hindering its flow through the material. However, the researchers have ingeniously fine-tuned the laser energy to enable controlled decomposition of cellulose into graphene. This process preserves the original porosity of the cellulose while ensuring that individual oxygen groups on the cellulose surface remain intact within the graphene regions.

The presence of oxygen groups within the graphene areas allows for interaction with water molecules, ensuring that the electrodes exhibit comparable wettability to the rest of the paper. Furthermore, these oxygen groups offer the opportunity for chemical bonding with reporter molecules. Consequently, when a virus particle interacts with a detection antibody on the electrode, an electronic signal is generated, enabling rapid and sensitive detection.

To achieve their desired results, the researchers made two key adjustments to the laser energy. Firstly, they treated the paper with flame retardants to prevent excessive charring or burning of the cellulose when subjected to the laser energy. Secondly, they reduced the laser power and employed multiple pulses, ensuring that the energy delivered to the paper per unit area was decreased. These modifications allowed for precise control over the laser process and prevented unwanted damage to the paper while still achieving the desired graphene formation.

Shih and deMello went beyond merely establishing the scientific validity of paper electrodes. Their aim was to create a practical and usable product. With this objective in mind, they applied the principle to practical applications and significantly streamlined the production process of the analytical paper strips. They have achieved impressive results, as they can now produce 176 sensors from a single A4 sheet of paper in just 90 minutes, with each unit costing a mere $0.02. This substantial advancement in efficiency and affordability brings their invention closer to widespread practical implementation.

For Shih, the environment at ETH Zurich played a decisive role in the invention: “We’re part of the Department of Chemistry and Applied Biosciences. As engineers, we’re directly inspired by the cutting-​edge research being done all around us.”

The specific approach for making their invention available to society and marketing it is yet to be determined by the two chemical engineers. Considering the extensive scope of potential applications, a licensing model could be a viable option. Fortunately, the scientists can leverage the conducive environment at ETH Zurich to explore various avenues and strategies in this regard.

deMello says: “The people at ETH transfer have a lot of experience in protecting intellectual property and negotiating licensing agreements.”

Irrespective of the specific path taken to transform the invention into tangible products, the underlying significance remains that a vast number of individuals worldwide are poised to reap the benefits of this groundbreaking innovation from ETH Zurich. This advancement, which involves integrating electrodes directly into the test strips, holds immense potential for a wide range of applications. From enhancing medical treatments to optimising agricultural practices and facilitating seamless infection monitoring, this advancement elevates the capabilities of paper fluidics to unprecedented heights.