β-Cyclodextrin coated porous Pd@Au nanostructures with enhanced peroxidase-like activity for colorimetric and paper-based determination of glucose.

β-cyclodextrin-functionalized porous Pd@Au nanostructures (β-CD-Pd@Au) with intrinsic and enhanced peroxidase-like activity were successfully synthesized by a two-step method. The synthesized β-CD-Pd@Au can efficiently catalyze the oxidation of various substrates, such as 3,3',5,5'-tetramethylbenzidine (TMB), mixture of 4-amino antipyrine (4-AAP) and 3,5-dichloro-2-hydroxy acid sodium (DHBS) (4-AAP/DHBS), and mixture of 4-AAP and N-Ethyl-N-(3-sulfopropyl)-3-methyl-aniline sodium salt (TOPS) (4-AAP/TOPS), by H ₂ O ₂ to generate visual blue, purple, and pink color, respectively. The UV-vis absorbance peak of the three β-CD-Pd@Au catalyzed the chromogenic reaction system located at 650 nm, 510 nm, and 550 nm, respectively. The β-CD-Pd@Au-catalyzed TMB-H ₂ O ₂ chromogenic reaction exhibited higher absorbance intensity, catalytic efficiency, and color stability in comparison to 4-AAP/DHBS-H ₂ O ₂ and 4-AAP/TOPS-H ₂ O ₂ chromogenic reactions. The catalytic activity of β-CD-Pd@Au was enhanced about 4-fold compared to that of Pd@Au in terms of K _cat for H ₂ O ₂ . Using TMB as chromogenic substrate, a colorimetric assay was fabricated for the determination of H ₂ O ₂ with a detection limit of 2.78 μM (absorbance at 650 nm). The colorimetric determination of glucose with a detection limit of 9.28 μM was further achieved by coupling with glucose oxidase enzymatic reaction, indicating the versatility of the β-CD-Pd@Au-based detection strategy. A paper-based detection method coupled with smartphone for fast visual and instrument-free detection of glucose was further developed. Finally, the developed colorimetric assay and paper-based detection method were successfully applied to the determination of glucose in human serum sample. Graphical abstract.