Abstract
In this study, a simple and amplified colorimetric assay is developed for the detection of the enzymatic activity of glucose oxidase (GOx) based on in situ formation of a photoswitchable oxidase mimetic of PO43−-capped CdS quantum dots (QDs). GOx catalyzes the oxidation of 1-thio-β-d-glucose to give 1-thio-β-d-gluconic acid which spontaneously hydrolyzes to β-d-gluconic acid and H2S; the generated H2S instantly reacts with Cd2+ in the presence of Na3PO4 to give PO43−-stabilized CdS QDs in situ. Under visible-light (λ ≥ 400 nm) stimulation, the PO43−-capped CdS QDs are a new style of oxidase mimic derived by producing some active species, such as h+, •OH, O2•− and a little H2O2, which can oxidize the typical substrate (3,3,5,5-tetramethylbenzydine (TMB)) with a color change. Based on the GOx-triggered growth of the oxidase mimetics of PO43−-capped CdS QDs in situ, we developed a simple and amplified colorimetric assay to probe the enzymatic activity of GOx. The proposed method allowed the detection of the enzymatic activity of GOx over the range from 25 μg/L to 50 mg/L with a low detection limit of 6.6 μg/L. We believe the PO43−-capped CdS QDs generated in situ with photo-stimulated enzyme-mimicking activity may find wide potential applications in biosensors.
Highlights
Natural enzymes play an important role in biochemistry due to their high substrate specificity and high catalytic efficiency in catalyzing various meaningful reactions
We report an advanced method to detect the enzymatic activity of glucose oxidase (GOx) based on the GOx generation of S2 ́ anions followed by interaction with Cd2+ /Na3 PO4 to give PO4 3 ́ -capped CdS quantum dots (QDs) in situ
The size of the CdS QDs catalyzed by GOx is about 2–3 nm (Figure 1A) as confirmed high resolution transmission electron microscopy (TEM)
Summary
Natural enzymes play an important role in biochemistry due to their high substrate specificity and high catalytic efficiency in catalyzing various meaningful reactions. Searching for artificial enzyme mimics with good stability and high catalytic capability is of great interest and urgently needed. Gao et al [2] reported that Fe3 O4 nanoparticles (NPs) possessed an intrinsic peroxidase-like activity in 2007, which opened the door for developing various nanoscale materials such as enzyme mimetics in the biochemical field. Many manufactured nanomaterials have been found to possess peroxidase-like activity including noble metals [3,4], metal oxides [5], carbon materials [6,7,8,9] and so on. Owning to the prominent advantages of low cost, high stability, ease of storage, and tunability in catalytic activity, these nanomaterial-based mimicking enzymes are promising candidates for natural enzymes in biological and biomedical applications [10,11,12]. Almost all of these nanomaterial-based peroxidase mimetics are Molecules 2016, 21, 902; doi:10.3390/molecules21070902 www.mdpi.com/journal/molecules
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