DNAzymes a series of nucleotide sequences
DNAzymes, a series of nucleotide sequences with specific catalytic activity, have been reported frequently applied in detection assays for such as ions and amino acids, due to the advantage in stability, high cost-effectiveness and modification convenience, by comparison with protein enzymes. The DNA probe in this assay has three main components and two catalytic functions, which respectively are DNA Cu2+ induced self-cleaving domain and G-rich sequence. The core of this assay mechanism is the step-by-step activation of the catalytic domains, that relates the colorimetric signal and the quantity of the l-histidine. The colorimetric signal of this assay is generated from the H2O2-mediated oxidation of ABTS catalyzed by the G-quadruplex–hemin complex, with a maximal Nystatin signal at a wavelength of 419 nm [7,50]. The G-quadruplex–hemin complex has a distinct HRP-mimicking catalysis activity, resulting from the ability to enhance the basicity of the bound porphyrin in the hemin, which is a metalloporphyrin, a peroxidase. In the absence of Cu2+, the DNA probe presents strong intramolecular structure, much more stable than the G-quadruplex resulting from the folding of G-rich DNA sequence. The activation of the self-cleaving, which is induced by the metal ion, makes the formation of G-quadruplex possible. With the addition of the l-histidine which contains imidazole group easy to bind the Cu2+, the l-histidine-Cu2+ is formed [51,52], leading to the inhibition of the formation of G-quadruplex, and then the correlation between the quantity of l-histidine and the absorbance is established.
Results and Discussion
Conclusions In conclusion, this method of l-histidine detection is convenient and sensitive with a detection limit of 50 nM and a wide linear range from 50 nM to 50 μM. Furthermore, in this assay, utilizing the unique characteristic of the l-histidine (imidazole group), a rigorous selectivity is reached in this method. Practically, the method can apply to the detection of real samples. That the proposed assay is label-free, and the self-cleaving character of the DNA probe, both lead to the low cost of this method. Moreover, the colorimetric detection operation is very simple without any further modifications. Altogether, a label-free, low-cost, and sensitive detection method for l-histidine is addressed.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21205142), The Research Innovation Program for Graduates of Central South University (2016zzts580, 2017zzts347).
Introduction As a group of cyclic heptapeptide compounds, microcystins can be produced by cyanobacteria from the serious eutrophication of domestic and industrial wastewater due to the worldwide urbanization and industrialization . Microcystin-LR (MC-LR, L = leucine, R = arginine), as the most toxic and commonly detected microcystins among more than 80 congeners, has an international threshold of 1 μg/L by the World Health Organization (WTO), while the daily intake value for MC-LR can be only accepted lower than 0.04 μg kg−1 . Due to its tremendous damage to animal and human, it is vital to monitor MC-LR in water resources such as lake and river. Nowadays, different well-established methodologies have been established to monitor MC-LR such as liquid chromatography-tandem mass spectrometry , high-performance liquid chromatography , and protein phosphatase inhibition assay , but they also face many shortcomings including the sophisticated instrument, the time-consuming and heavy laboratory work, or the complicated pretreatment. Recently, the promising technique for MC-LR detection is the construction of different immunosensors based on the efficient and specific recognition between antibody and antigen, such as electrochemical immunosensor , photoelectrochemical immunosensor  and fluorescence immunosensor . However, as for the on-site test, it is still necessary to pursue a more convenient, feasible and visualized method to detect MC-LR.