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Methylation of DNA at carbon 5 of cytosines is the most common epigenetic modification of human genome. Due to its critical role in many normal cell processes such as growth and development, any aberrant methylation pattern in a particular locus may lead to abnormal functions and diseases such as cancer. Development of methods to detect methylation state of DNA which may eliminate labor-intensive chemical or enzymatic treatments has received considerable attention in recent years. Herein, we report a DNA methylation detection procedure based on fluorescence turn-on strategy. Target sequence was selected from Sept9 promoter region that has been reported as one of the most frequently methylated sites in colorectal cancer. Probe DNA was designed to be complementary to this sequence with an additional six cytosines in the middle to form an internal loop to host silver nanoclusters. The fluorescence intensity of the synthesized silver nanoclusters with the duplexes of probe-non-methylated target was significantly different from that of probe-methylated target. The fluorescence enhanced with increasing the methylated DNA concentration with a linear relation in the range of 1.0 × 10 -8  M to 5.0 × 10 -7  M with the detection limit of 8.2 × 10 -9  M, and quenched with non-methylated ones. The method was very specific in the presence of non-complementary sequences with maximum similarity of 40%. Circular dichroism spectra indicated that silver ions significantly affected the structure of methylated and non-methylated DNA into different extents which could further influence the nanocluster fluorescence. Finally, a method was introduced to meet the concerns in the applicability of the proposed method in real situation., Competing Interests: Declaration of competing interest The authors confirm that there are no financial or non-financial conflicts of interest associated with this manuscript that could have influenced its outcomes., (Copyright © 2020 Elsevier B.V. All rights reserved.)

A DNA-silver nanocluster with two distinct emissions is devised, in which this unique modality has been exploited to develop a novel nanosensor for transgenic DNA detection. TEM and fluorescence analysis revealed the formation of Ag nanoclusters with a size of around 2 nm, which exhibit dual-emissions at 550 nm (green) and 630 nm (red). Moreover, in the presence of the target sequence (CaMV 35S promoter) from the transgenic plant, the nanoclusters showed an enhancement in the green emission and a reduction in the red emission. This property provided a ratiometric-sensing platform which lacks unavoidable noises. The ratio of green to red fluorescence emission (G/R) of the nanoclusters exhibited a linear relation with the target concentration in the range 10 to 1000 nM. However, the control DNA did not affect this ratio, which clearly confirmed the selective response of the designed nanosensor. This sensing platform had a detection limit of 1.5 nM and identified the DNA of transgenic soybeans within a short time. The mechanistic evaluation of the nanoclusters further revealed the role of protonated cytosine bases in the dual emission behavior. Finally, unique features of the designed nanosensor may improve the current approaches for the development and manufacturing of GMO detection tools.

The extent of DNA structural perturbation by silver ions is different in methylated and non-methylated DNA. Here, we explored the interaction of eight convenient DNA interacting molecules with methylated and non-methylated short GC rich oligonucleotides in the presence and absence of silver ions. Acridine orange, DAPI, Doxorubicin, Ethidium bromide, Hoechst 33342, Methylene blue, PicoGreen, and Propidium iodide are tested for their ability to distinguish methylated and non-methylated DNA. Among them, Ethidium bromide, Methylene blue, and PicoGreen were able to discriminate between methylated and non-methylated DNA, while DAPI and Hoechst 33342 were only able to discriminate with the aid of silver ions. A detection method is proposed using Ethidium bromide in which the silver-treated sample of DNA exposed different fluorescence intensity from the untreated one on the base of its methylation state. This phenomenon was sequence-dependent and could provide a sensing platform with a detection limit of about 4fi0 nM.

Determining methylation state of a particular DNA sequence is an essential task in many epigenetic investigations. Here a facile method based on silver nanocluster (AgNCs) fluorescence enhancement is presented. Target sequences were selected from Sept9 promoter region that its hypermethylation is demonstrated as a reliable biomarker of colorectal cancer. Probe DNA was complementary to a 25 nucleotide of the target region and possessed twelve additional cytosines in the middle to grant the formation of AgNCs. After probe strands were hybridized with methylated and non-methylated targets separately, AgNCs were synthesized, and their fluorescence intensities were recorded. Fluorescence intensity enhanced when the target strands were methylated and quenched when they were non-methylated. The Linear range of fluorescence enhancement was from 1.0 × 10 - 7 M to 5.0 × 10 - 7 M with the detection limit of 7.6 × 10 - 8 M. Sensor specificity was checked with non-complementary strands with the maximum similarity of 40%. Further experiments explored various characteristics of methylated and non-methylated DNAs carrying AgNC and indicated that structure of methylated and non-methylated DNAs was affected differently by silver ions that could then influence AgNC fluorescence. This effect was strongly sequence-dependent, and either fluorescence enhancement or quenching was observed with two different sequences., (Copyright © 2018 Elsevier B.V. All rights reserved.)