(Invited) Development of Biosensors Based on Graphene-Coated Porous Silica Spheres
Authors: Itoh Tetsuji 
Published: December 2024

During the onset of the COVID-19 pandemic, the public was reminded of how quickly routine diagnostic tests could overwhelm the health-care system when no other options are available. As mass adoption of the at-home rapid tests were developed and accepted thereby relieving the first responders to care for more critically ill patients, it sparked a resurgence of interest in affordable point-of-care (POC) diagnostics. Currently, besides a handful of common POC devices (e.g., glucose sensor, pregnancy tests and COVID test) there are not many commercially successful examples due to complex design, high production costs, and material stability for field deployment.
In this study, we demonstrate that graphene coated porous silica spheres (G/PSS) can be a simple and affordable material for target sensing. Porous carbons particles are low-cost materials widely used as electrode materials in applications such as electric double layer capacitors, batteries, biofuel cells, and biosensors. In the above applications, the non-uniformity of the particles does not interfere with their performance. However, for target sensing, the reduction of noise is generally achieved by working with uniform material. In our case, we developed a method to synthesize uniformly coated graphene on uniformly sized porous silica. The triple pore structure and increased surface area enable, the diffusion of enzymes, mediators, and substrates for rapid enzyme electrode reactions - the mesopores provide a reaction field, the micropores trapping mediators, and the macropores providing substrate, mediator, and enzyme diffusion. Of particular interests to the audience are three key areas: 1) the development of porous silica spheres with controllable particle size and pore size, 2) the graphene chemistry to deposit graphene on to the particles as developed and 3) fine-tuned by our group for this application, and the fabrication technical to prepare electrodes using screen-printing. We succeeded in developing a sensor chip with G/PSS arranged on the surface with high efficiency by selecting a mediator that can smoothly transfer electrons to and from the carbon, which enables various sensing applications. Furthermore, to expand the portfolio of new and improved components that increase the field usability of POC diagnostic devices, we demonstrate that Prussian blue (PB) dispersed in G/PSS functions as an effective and affordable reference electrode as a promising alternative to industry standards. The solubility product of PB is approximately only 1/1031 of that of AgCl, rendering PB a water-insoluble electrode that is free from metal leaching in small devices. The support material, G/PSS, provides conductive mesopores that accommodate insulating PB clusters, ensuring effective contact between the PB clusters and electrolytes and stable potential. Furthermore, the monodisperse spherical shape of G/PSS is beneficial for the production of a reference electrode for POC devices through a low-cost, highly reproducible screen-printing process. This study successfully demonstrated the feasibility and stability of the prototype small biosensor, opening the door for future improvements in the device’s ability to perform similar sensing in complex media such as biological samples.