Oxidase-like activity of polymer-coated cerium oxide nanoparticles

Abstract

Inorganic enzyme? Ceria nanoparticles exhibit unique oxidase-like activity at acidic pH values. These redox catalysts can be used in immunoassays (ELISA) when modified with targeting ligands (see picture; light blue and yellow structures are nanoparticles with attached ligands). This modification allows both for binding and for detection by the catalytic oxidation of sensitive colorimetric dyes (e.g. TMB).

Figure 1

Formation of color product due to subtrate oxidation is pH dependent. a) Photographs show production of color product upon addition of nanoceria to TMB, AzBTS and DOPA at pH 4.0. b) Oxidation of TMB is pH dependent with the optimum activity at pH 4.0. buffer pH 4.0

Figure 2

Nanoceria-promoted oxidation of TMB is size-dependent. At pH 4.0, smaller size ceria nanoparticles show higher oxidase acivity than larger size nanoparticles.

Figure 3

Comparison between traditional ELISA (a) and nanoceria-based ELISA (b). In traditional ELISA, a HRP-antibody is utilized as secondary antibody that upon hydrogen peroxide treatment facilitates the oxidation of TMB, resulting in color development. Meanwhile, in a nanoceria- based ELISA, the oxidase activity of nanoceria facilitates the direct oxidation of TMB without the need of HRP or hydrogen peroxide.

Figure 4

Nanoceria-mediated ELISA detection of folate receptor expressing cells. Folate-nanoceria associating with A-549 cells which overexpress the folate receptor effectively oxidizes TMB.

Figure 5

Nanoceria based immunoassay is sensitive to the number of folate positive cells (folate receptor expressing cells A-549).