Fluorescence Of DNA-AgNCs Research Articles

How G-quadruplex topology and loop sequences affect optical properties of DNA-templated silver nanoclusters

In the study of the fabrication of DNA-templated silver nanoclusters (DNA-Ag NCs), how templates affect the fluorescence of the nanoclusters remains unclear, and it has been a challenge to understand the correlation between the properties of the DNA template and the Ag NCs. In this respect, based on the rational design of a series of structurally defined intramolecular G-quadruplexes, we prepared G-quadruplex-templated Ag NCs with a defined G-tetrad-to-silver ratio of 1:2. We evaluated the effect of G-quadruplex topology and loop sequences on the fluorescence of DNA-Ag NCs using circular dichroism, and extinction and emission spectroscopy. G-quadruplex templates with an anti-parallel topology were found to produce Ag NCs with stronger fluorescence compared with parallel and hybrid configurations. Loop bases adjacent to G-tetrads have a more significant impact on the fluorescence of Ag NCs compared with those in the middle of the loop, with adenine largely exhibiting an enhancement effect and thymine being detrimental. Generally, G-quadruplexes having an anti-parallel topology with adenine in the loop adjacent to the G-tetrad would be good templates for producing highly fluorescent Ag NCs. This is the first study to focus on the correlation between G-quadruplex topology/sequence and the optical properties of Ag NCs. We hope that the results of this study will facilitate a more in-depth understanding of correlation between G-quadruplex templates and Ag NCs, and help to understand and utilize their unique attributes.

HIV-related DNA detection through switching on hybridized quenched fluorescent DNA-Ag nanoclusters.

In this paper, DNA containing six cytosines as the formation site for silver nanoclusters (Ag NCs) was adopted as a template for preparing fluorescent DNA-Ag NCs. For the first time, it was found that the fluorescence of DNA-Ag NCs could be quenched after hybridization with their complementary sequence. On the basis of this new phenomenon, we designed a sequence C1 that was completely complementary to human immunodeficiency virus (HIV) DNA, and probe DNA which was partially complementary to C1 for the synthesis of DNA-Ag NCs. The fluorescence of DNA-Ag NCs was quenched after hybridization with C1 and the DNA-Ag NCs/C1 composite was formed, while C1 could be dissociated away from the DNA-Ag NCs by HIV DNA through a strand exchange reaction due to the stronger affinity between HIV DNA and C1, which could switch on the quenched Ag NCs, thus a new "off-on" fluorescence method for HIV DNA detection was developed. In the experiment, the Ag NCs formation site of DNA, the number of base pairs, and the pH and salt concentration of binding buffer were optimized. Under the optimum conditions, the limit of detection for HIV DNA was obtained to be 3.18 nM (3σ/N, n = 7) with the linear range of 15-150 nM for the 150 nM DNA-Ag NCs/C1 probe. Besides, the probe showed excellent specificity to HIV DNA, and even distinguished one nucleotide mismatched HIV DNA.

A peptide with a cysteine terminus: probe for label-free fluorescent detection of thrombin activity.

Thrombin has been implicated in atherosclerotic disease development. However, thrombin activity detection is currently limited because of the lack of convenient fluorescent probes. We developed a label-free fluorescent method to assay thrombin activity on the basis of a designed peptide probe with a thrombin-cleavable peptide sequence and a cysteine terminus. The peptide probe can be conjugated to DNA-templated silver nanoclusters (DNA-AgNCs) through Ag-S bonding; as a result, the fluorescence of DNA-AgNCs was enhanced. As the DNA-AgNCs-peptide conjugate was adsorbed to graphene oxide (GO), the enhanced fluorescence of DNA-AgNCs was quenched. Once the peptide probe was cleaved by thrombin, the resulting release of the DNA-AgNCs from the surface of GO restored the enhanced fluorescence. Thrombin can be determined with a linear range of 0.0-50.0 nM with a detection limit of 1 nM. The thrombin-sensitive probe with a cysteine terminus may be developed into probes to detect other proteases.

Oligonucleotide-stabilized fluorescent silver nanoclusters for sensitive detection of biothiols in biological fluids

In this work, we reported a simple and sensitive method to detect biothiols, such as cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), using fluorescent silver nanoclusters (Ag NCs) stabilized by single-stranded DNA (DNA-Ag NCs) as probes. The photoluminescence intensity of DNA-Ag NCs was found to be quenched effectively with the increase of biothiols concentration due to the formed nonfluorescent coordination complex between DNA-Ag NCs and biothiols, resulting in the shift-to-red of emission wavelength. But the fluorescence of DNA-Ag NCs was not changed in the presence of other amino acids at 10-fold higher concentration. Satisfactory detection limits and linear relationships of Cys, GSH and Hcy were obtained, respectively. The resulted plots exhibited good linear relationships in the range from 8.0 × 10 −9 to 1.0 × 10 −7 mol L −1 ( R 2 = 0.984) for Cys, 8.0 × 10 −9 to 1.0 × 10 −7 mol L −1 ( R 2 = 0.983) for GSH, and 2.0 × 10 −6 to 6.0 × 10 −7 mol L −1 ( R 2 = 0.999) for Hcy, respectively; the detection limits of Cys, GSH and Hcy were 4.0 nmol L −1, 4.0 nmol L −1, and 0.2 μmol L −1, respectively. The method was successfully used for the detection of biothiols in human plasma samples.