Use of Metal Nanostructure Arrays to Develop Flexible Biosensors for Rapid Point-of-Care Diagnosis Device
Authors: Li Szu-Ying , Yi Shang-Yi , Wan Dehui 
Published: February 2020

The accurate and rapid determination of hydrogen peroxide (H2O2) is of great importance in many applications such as clinical diagnosis, bioanalysis, and food safety. Moreover, H2O2 is a side product of specific enzymatic reactions. For instance, glucose could be oxidized to produce gluconic acid and H2O2 in the presence of glucose oxidase (GOx). Up until now, there have already been many methods to detect H2O2, including spectrophotometry, electroanalysis, and fluorometry. However, these techniques require expensive or sophisticated instruments. Therefore, low-cost, easy-to-use, sensitive H2O2 biosensors are necessary. In this decade, researchers have developed various approaches to detect the H2O2, such as enzyme-linked immunosorbent assay (ELISA). A common enzyme, horseradish peroxidase (HRP), could catalyze the oxidation of the substrate 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) by H2O2 to develop a green color in aqueous solution. But the enzyme has many environmental limitations, including temperature, pH value, and complicated purification process. As a result, enzyme-mimic materials have emerged. Among them, the noble metal nanoparticles (NPs) are eye-catching due to their unique properties such as strong surface plasmon resonance (SPR) absorption from the visible to near-infrared region, high stability, low cost, large surface-to-volume ratio, and high catalytic activity. Furthermore, NPs have been widely used in diverse fields ranging from ultrasensitive sensing, nanophotonics, solar cells to enzymatic reactions.
In this study, we have developed two systems to detect H2O2 without any complicated equipments. Both of them are simple, low cost, and disposable. Most importantly, we could distinguish the color change by naked eyes. By these advantages, the sensors could be applied as point-of-care platforms and clinical diagnoses. The intrinsic catalytic ability of palladium nanoparticles in many specific reactions have been confirmed. Herein, our first system utilizes an enzymatic approach by using our hollow palladium nanostructure (PdNS) to catalyze the oxidization of ABTS by H2O2. In this case, we have successfully immobilized a nanoparticle array onto a commercial polymer substrate, polyethylene terephthalate (PET), via a wet-chemical method. By mixing H2O2 with ABTS in an acetate buffer sollution (pH=4.6) and dropping the solution onto a PdNS-coated PET plate, we could observe the increasing darkness of green color with increasing H2O2 concentration by naked eyes. The high catalytic activity may attribute to the hollow structure with high surface area, and the intrinsic catalytic ability of Pd.
In another system, we immobilized hollow gold nanostructure (AuNS) onto PET substrates, and then dropped the H2O2 solution directly onto the AuNS-coated PET plate. Because of the strong oxidizing ability of H2O2, the silver atoms in AuNS would be oxidized to silver ions and the AuNS would be transformed to a more porous structure. Owing to the changes in structure/morphology, different colors were obtained in various H2O2 concentrations by naked eyes. In summary, the two efficient sensing platforms show great sensitivity and obvious color changes. Moreover, combining specific enzymes or antibody with our platforms creates the potential for clinical diagnosis such as glucose or prostatic specific antigen (PSA) sensing. In the future, the applications of the sensing systems could extend to other fields that relate to H2O2.