Typical fluorescent biosensors use fluorescently labeled ssDNA for target recognition and nanomaterials for signal transduction. Herein, we propose a reverse sensing strategy that Mo 5 N 6 nanosheets are used for target recognition while fluorescein (FAM)-labeled ssDNA only serves for signal generation. We discover that Mo 5 N 6 nanosheets show high fluorescence quenching ability (>95%) and selective recognition for sodium hexametaphosphate (SHMP). After FAM-labeled ssDNA is adsorbed on Mo 5 N 6 nanosheets, the fluorescence is quenched due to the photoinduced electron transfer (PET) effect between FAM and Mo 5 N 6 nanosheets. SHMP can specifically displace the adsorbed FAM-labeled ssDNA from Mo 5 N 6 nanosheets, resulting in more than 80% fluorescence recovery on addition of 5 μmol L −1 SHMP. This biosensor can sensitively detect SHMP down to 150 nmol L −1 and selectively recognize SHMP over glucose, lactose, common amino acids, Zn 2+ , Mg 2+ , Ca 2+ and other phosphates (such as Na 2 HPO 4 , sodium pyrophosphate, sodium tripolyphosphate). This biosensor also shows great potential for the detection of SHMP in bacon sample. This work not only provides a facile sensitive and selective biosensor for SHMP but also exploits the application of transition metal nitrides in the field of sensing and biosensing.
2, 2′-azinobis-3-ethylbenzthiazoline-6-sulfonate (ABTS), 3, 3′, 5, 5′-tetramethylbenzidine (TMB), o-phenylenediamine (OPD) are typical chromogenic substrates to evaluate the activity of peroxidase-like nanozymes. However, these substrates are toxic, unstable, and the coloration reactions are slow. Herein, the classical starch-sodium iodide reagent was utilized as the chromogenic substrate to evaluate peroxidase-like activity of nanozymes. Starch-sodium iodine is less toxic, more stable and faster color reaction than ABTS, TMB and OPD. Under the catalysis of peroxidase-like nanozymes such as Mo 5 N 6 nanosheets , graphene oxide nanosheets, Fe 3 O 4 nanoparticles , CeO 2 nanoparticles, iodide ion (I – ) is oxidized by H 2 O 2 to produce blue color complex (starch-I 2 ). Taking Mo 5 N 6 nanosheets as the peroxidase-like nanozyme, the color reaction completed within 5 min at room temperature (25 ºC), and the chromogenic substrate remained stable within 30 days. The maximum reaction rate (V max ) using starch-sodium iodine as the chromogenic substrate was more than 100-fold higher than that of TMB. Based on this, a sensitive colorimetric sensor and facile portable agarose gel test kits were proposed and applied for the detection of H 2 O 2 residue in liquid milk samples with fast response.
Mo 5 N 6 nanosheets were synthesized by a nickel-induced growth method and were found to possess peroxidase-like activity in acidic condition and catalase-like activity in weak basic condition. In acidic condition, Mo 5 N 6 nanosheets can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H 2 O 2 to form a blue color product (TMB OX ). At the co-existence of 4-aminophenol (4-AP), 4-AP can react with H 2 O 2 and TMB OX , resulting in the decrease of TMB OX and the fading of blue color. Therefore, a facile, sensitive colorimetric method for the quantitative detection of 4-AP was developed. The linear range for 4-AP was 1.0 to 80.0 μmol⋅L ‒1 ( R 2 = 0.999), and the detection limit was 0.56 μmol⋅L ‒1 based on 3 σ / k . Resorcinol, aniline, humic acid, and common ions and anions in surface water did not interfere the determination of 4-AP. This colorimetric method was applied to measure the 4-AP in real water sample from Wulong River in Fujian Province of China. The relative standard deviation for the determination of 4-AP was ranged from 0.03 to 1.88%, and the recoveries from spiked samples were ranged between 99.2 and 107.6%. The determination results were consistent with those obtained by HPLC. Graphical abstract
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