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.