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Methionine (Met) is one of sulfur-containing essential amino acids, it plays an indispensable role in physiological processes. Its quantitative and sensitive determination is imperative. However, currently available analytical approaches for determination of Met are scarce and also exhibit many disadvantages, such as poor selectivity for Met towards other amino acids and biothiols. In this work, a label-free colorimetric biosensor based on bio-dots assembly-triggered aggregation of gold nanoparticles (AuNPs) for detecting Met is firstly developed. In the presence of Met, Met effectively prevents the bio-dots assembly-induced aggregation of AuNPs since that AuNPs are protected inside the Met layer. This colorimetric biosensor provides a simple, fast, highly sensitive and highly selective method for Met detection. It presents lower detection limit of 3.2 nM and exhibits highly selectivity toward bio-thiols surprisingly. Furthermore, this proposed biosensor has been successfully applied for the determination of Met in human serum. [ABSTRACT FROM AUTHOR]

In this paper, we present a colorimetric method for the assay of DNA methyltransferase (MTase) activity based on strand displacement amplification (SDA). In our study, a well-designed hairpin DNA I (HPI) containing the sequence of 5′-CCGG-3′ is specifically recognized by CpG methyltransferase (M.SssI) and HpaII endonuclease. The methylated HPI is able to coexist with all the DNA and enzymes in the solution while the unmethylated HPI can be cleaved into single-stranded DNA (ssDNA) fragments. The amplification can be triggered by the HpaII digestion products hybridization with another hairpin structure DNA II (HPII) to form a duplex, which would be replaced by probe DNA, leading to the aggregation of gold nanoparticles (AuNPs). Simultaneously, ssDNA fragments released from the duplex, and triggered the cycle anew. Varying concentrations of M.SssI in the solution therefore would lead to differences of absorption and color changed from red to pale. A linear response was obtained when the M.SssI concentration ranging from 0.2 to 50 U mL −1 with a detection limit of 0.08 U mL −1 . In addition, the developed assay in this study can also be applied to screen the inhibitors of M.SssI. [ABSTRACT FROM AUTHOR]

A novel and sensitive ratiometric fluorescent assay for monitoring kinase activity based on carboxypeptidase Y (CPY) digestion by using Au nanoclusters (AuNCs) and CdSe/ZnS QDs@SiO 2 as the dual-emission nanoparticles is developed. This approach can not only sensitively monitor the kinase activity with affording built-in correction which avoids environmental interferences, but also realize visual detection. Under optimal conditions, a linear relationship between the fluorescence intensity ratio (I 415 /I 630 ) of CdSe/ZnS QDs@SiO 2 @peptide-AuNCs and the protein kinase A (PKA) concentration in a range of 0.01 to 40 U mL −1 with a detection limit of 0.004 U mL −1 (3 σ ) is obtained. The feasibility of this CdSe/ZnS QDs@SiO 2 @peptide-AuNCs-based ratiometric sensor has been further demonstrated by assessment of kinase inhibition by ellagic acid in cell lysates. The IC 50 value (inhibitor concentration producing 50% inhibition) for ellagic acid is estimated to be 0.08 μM. The detection of other enzymes can also be realized with precise design of the peptide substrate sequences. The present assay is universal and visual for kinase assay and promises potential application in kinase-related biochemical fundamental research and inhibitor screening. [ABSTRACT FROM AUTHOR]