Enzyme-mimicking (nanozyme)-based biosensors are attractive owing to their unique catalytic efficiency, multifunctionality, and tunable activity, but examples of oxidase-like nanozymes are quite rare. Herein, we demonstrated that histidine-capped gold nanoclusters (His@AuNCs) possessed intrinsic oxidase-like activity, which could directly oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to blue colored ox-TMB without H 2 O 2. The assembly of reduced graphene oxide (RGO) with His@AuNCs could further improve its oxidase-like activity and the His@AuNCs/RGO nanocomposites had a lower Michaelis constant (K m) and higher catalytic constant (K cat) for TMB oxidation. Furthermore, compared to other nanomaterials, the as-prepared His@AuNCs/RGO also exhibited enhanced electrocatalytic activity toward TMB. Interestingly, nitrite inhibited the catalytic (chromogenic) and electrocatalytic processes of His@AuNCs/RGO in the oxidation of TMB. The oxidase-like and electrocatalytic activity of His@AuNCs/RGO was evaluated with nitrite and TMB as substrates, and the results indicated that TMB and nitrite might share the same catalytic active sites. On the basis of these findings, a colorimetric and electrochemical sensor was developed with the His@AuNCs/RGO composite as an oxidase mimic for determination of nitrite with linear ranges of 10-500 μM and 2.5-5700 μM, respectively. The developed method was successfully applied to the detection of nitrites in real samples. The present work suggests that the oxidase-like nanozyme is not only suitable for colorimetric assay but also for development of electrochemical sensors in bioanalysis.

An ethanol extract prepared from the leaves of Cyclocarya paliurus, also known as sweet tea, which is one of the most popular teas utilized in traditional Chinese medicine, exhibited significant cytotoxicity against human lung and breast cancer cells. Using a bioassay-guided fractionation, we purified a pentacyclic triterpenoid, hederagenin, which exhibited superior and selective cytotoxicity against human breast and lung cancer cells. Evaluation of the structure-activity relationship between hederagenin and seven other pentacyclic triterpenoids revealed that the C3 hydroxyl group, the C17 carboxyl group and the Δ (12,13) double bond could be important active groups for the bioactivity of pentacyclic triterpenoids, whereas introduction of a hydroxyl group at C2 or C23 might reduce their bioactivity. We also investigated the cytotoxic activity of hedeargenin and demonstrated that it induces apoptosis, increases the cell membrane permeability, reduces the mitochondria potential, and suppresses NF-κB activation.

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Schematic illustration of a Hg2+ colorimetric sensor based on the Hg2+ triggered peroxidase-like activity of GNP@CDs.

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