Guanine nanowire based amplification strategy: Enzyme-free biosensing of nucleic acids and proteins

Formation of Novel Octuplex DNA Molecules from Guanine Quadruplexes

Guanine quadruplex (G4)-DNA structures have sparked the interest of many scientists due to their important biological roles and their potential use in molecular nanoelectronics and nanotechnology. The high guanine content in G4-DNA endows it with mechanical stability, robustness, and improved charge transport properties-attractive attributes for a molecular nanowire. The self-driven formation of a novel G4-DNA-based nanostructure, coined guanine octuplex (G8)-DNA, is reported herein. Atomic force microscopy and scanning tunneling microscopy characterization of this molecule reveal its organized coiled-coil structure, which is found to be stable under different temperatures and surrounding conditions. G8-DNA exhibits enhanced stiffness, mechanical and thermodynamic stability when compared to its parent G4-DNA. These, along with its high guanine content, make G8-DNA a compelling new molecule, and a highly prospective candidate for molecular nanoelectronics.

Colorimetric determination of DNA using an aptamer and plasmonic nanoplatform

A facile plasmonic nanoplatform was developed for rapid and sensitive determination of nucleic acid. Hg²⁺-regulated molecular beacon (MB, hairpin) containing rich thymine (T) bases at both ends is used as the probe. A hairpin structure can be formed in the MB probe due to the strong binding of Hg²⁺ to T. However, in the presence of target DNA, the hairpin structure is opened owing to target DNA-specific hybridization with the aptamer. Simultaneously, the opened MB interacts with poly(diallyldimethylammonium chloride) (PDDA) and hinders PDDA-induced aggregation of AuNPs, accompanied by a color change from blue to red and a decrease in absorption ratio (A₆₂₀/A₅₂₀). Hence, a good linear relationship was observed between the decreased absorption ratio (A₆₂₀/A₅₂₀) and DNA concentration ranging from 0.02 to 2 nmol/L with a low detection limit of 4.42 pmol/L. Moreover, this nanoplatform has been successfully utilized to discriminate between perfect target and mismatch sequences. More importantly, the bioassay is simple, versatile, rapid, and cost-effective compared with other common methods, which holds great promise for clinical diagnosis and biomedical application. Graphical abstract.

GC ends control topology of DNA G-quadruplexes and their cation-dependent assembly

GCn and GCnCG, where n = (G2AG4AG2), fold into well-defined, dimeric G-quadruplexes with unprecedented folding topologies in the presence of Na+ ions as revealed by nuclear magnetic resonance spectroscopy. Both G-quadruplexes exhibit unique combination of structural elements among which are two G-quartets, A(GGGG)A hexad and GCGC-quartet. Detailed structural characterization uncovered the crucial role of 5'-GC ends in formation of GCn and GCnCG G-quadruplexes. Folding in the presence of 15NH4+ and K+ ions leads to 3'-3' stacking of terminal G-quartets of GCn G-quadruplexes, while 3'-GC overhangs in GCnCG prevent dimerization. Results of the present study expand repertoire of possible G-quadruplex structures. This knowledge will be useful in DNA sequence design for nanotechnological applications that may require specific folding topology and multimerization properties.

An ultrasensitive guanine wire-based resonance light scattering method using G-quadruplex self-assembly for determination of microRNA-122

An enzyme-free resonance light scattering (RLS) method is described for the determination of microRNA-122. A guanine nanowire (G-wire) is used that consists of a predesigned DNA1 and a G-quadruplex sequence DNA2. These hybridize with microRNA-122 and partially hybridize with DNA2. After formation of stable double strands with DNA1, DNA2 is released. On addition of K⁺ and Mg²⁺ ions, the G-quadruplex sequences undergo self-assembly to form long filamentous G-wires. This increases the intensity of RLS. A 6.1 pM detection limit was obtained, and the linear response covers the 50 pM to 300 nM microRNA concentration range. The method was successfully applied to the quantitation of microRNA-122 in hepatocellular carcinoma cell lysates. Conceivably, this assay can be extended to other RLS methods for biomarker detection by simply changing the sequence of DNA1. Graphical abstract The G-quadruplex sequences of DNA2 were locked with DNA1. The G-quadruplex fragments of DNA2 were released after the hybridization of microRNA-122 with DNA1. These liberated G-quadruplex sequences were self-assembled into long filamentous guanine nanowires (G-wires) which increased resonance light intensity in the presence of Mg²⁺.

A Thioflavin T-induced G-Quadruplex Fluorescent Biosensor for Target DNA Detection

The detection of disease-related DNA is of great significance for early and accurate diagnosis and therapy. In this work, we successfully achieved the sensitive detection of target DNA based on a thioflavin T (ThT)-induced G-quadruplex fluorescent biosensor. ThT, a water-soluble fluorescent dye, can induce G-rich sequences to form G-quadruplexes and obtain an obviously enhanced fluorescence. In this work, it was employed to construct a biosensor for the detection of HIV. When the target HIV existed, the hairpin DNA probes would be opened in succession and release the completely exposed G-rich sequence to combine with ThT. The simple and rapid biosensor performed satisfactory selectivity; it also exhibited sensitivity with a detection limit of 2.4 nM. With good performance in human serum, we believe that this optical biosensor has the potential to be applied to the practical detection of target DNA.

High-resolution AFM structure of DNA G-wires in aqueous solution

We investigate the self-assembly of short pieces of the Tetrahymena telomeric DNA sequence d[G₄T₂G₄] in physiologically relevant aqueous solution using atomic force microscopy (AFM). Wire-like structures (G-wires) of 3.0 nm height with well-defined surface periodic features were observed. Analysis of high-resolution AFM images allowed their classification based on the periodicity of these features. A major species is identified with periodic features of 4.3 nm displaying left-handed ridges or zigzag features on the molecular surface. A minor species shows primarily left-handed periodic features of 2.2 nm. In addition to 4.3 and 2.2 nm ridges, background features with periodicity of 0.9 nm are also observed. Using molecular modeling and simulation, we identify a molecular structure that can explain both the periodicity and handedness of the major G-wire species. Our results demonstrate the potential structural diversity of G-wire formation and provide valuable insight into the structure of higher-order intermolecular G-quadruplexes. Our results also demonstrate how AFM can be combined with simulation to gain insight into biomolecular structure.

DNA and RNA G-quadruplexes can stack to form higher-order structures called G-wires. Here the authors report high-resolution AFM images of higher-order DNA G-quadruplexes in aqueous solution that could impact the design of G-wire based nanodevices and the understanding of G-wires in biology.

Formation of G-wires, bimolecular and tetramolecular quadruplex: Cation-induced structural polymorphs of G-rich DNA sequence of human SYTX gene

An exceptional property of auto-folding into a range of intra- as well as intermolecular quadruplexes by guanine-rich oligomers (GROs) of promoters, telomeres and various other genomic locations is still one of the most attractive areas of research at present times. The main reason for this attention is due to their established in vivo existence and biological relevance. Herein, the structural status of a 20-nt long G-rich sequence with two G5 stretches (SG20) is investigated using various biophysical and biochemical techniques. Bioinformatics analysis suggested the presence of a 17-nt stretch of this SG20 sequence in the intronic region of human SYTX (Synaptotagmin 10) gene. The SYTX gene helps in sensing out the Ca²⁺ ion, causing its intake in the pre-synaptic neuron. A range of various topologies like bimolecular, tetramolecular and guanine-wires (nano-wires) was exhibited by the studied sequence, as a function of cations (Na⁺ /K⁺ ) concentration. UV-thermal denaturation, gel electrophoresis, and circular dichroism (CD) spectroscopy showed correlations and established a cation-dependent structural switch. The G-wire formation, in the presence of K⁺ , may further be explored for its possible relevance in nano-biotechnological applications.

A dual-cycling biosensor for target DNA detection based on the toehold-mediated strand displacement reaction and exonuclease III assisted amplification

Based on the toehold-mediated strand displacement reaction and exonuclease III assisted amplification, a sensitive and simple target DNA biosensor was established.

Label-Free Photoelectrochemical "Off-On" Platform Coupled with G-Wire-Enhanced Strategy for Highly Sensitive MicroRNA Sensing in Cancer Cells

MicroRNA (miRNAs) quantification, especially at low abundance, is vital for disease diagnosis, prognosis, and therapy. Herein we develop a distinctive label-free "off-on" configuration for photoelectrochemical (PEC) sensing platform fabrication, coupled with DNA four-way junction (4J) architecture as well as G-wire superstructure for signal amplification. In addition, ultrathin copper phosphate nanosheets (CuPi NSs) coating Au nanoparticles (Au-CuPi NSs) serve as a highly efficient photocathode substrate. To improve the sensitivity, and avoid the false positive signals, the quencher, gold nanoparticles (GNPs), is utilized to switch off the PEC signal because of the commendable surface plasmon resonance (SPR) absorption. Subsequently, ingenious DNA 4J architecture is applied to export proportional c-myc regions for target quantification. Assisted with the G-wire superstructure formation, the enhancer 5,10,15,20-tetra(4-sulfophenyl)-21H,23H-porphyrin (TSPP) is coupled on the substrate to switch on the PEC signal, thus realizing the miRNA assay with persuasive accuracy, high sensitivity, and low detection limit. In addition, we execute the miRNA detection in prostate carcinoma cell line 22Rv1, and acquire desirable quantitative capability. Remarkably, the prepared PEC sensing platform not only realizes the highly efficient miRNAs quantification, but also uncovers a marvelous horizon for sensing platform fabrication.

A label-free electrochemical sensor for detection of mercury(II) ions based on the direct growth of guanine nanowire

A simple, sensitive and label-free electrochemical sensor is developed for detection of Hg(2+) based on the strong and stable T-Hg(2+)-T mismatches. In the presence of Mg(2+), the parallel G-quadruplex structures could be specifically recognized and precipitated in parallel conformation. Therefore, the guanine nanowire was generated on the electrode surface, triggering the electrochemical H2O2-mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB). In this research, a new method of signal amplification for the quantitative detection of Hg(2+) was described based on the direct growth of guanine nanowire via guanine nanowire. Under optimum conditions, Hg(2+) was detected in the range of 100 pM-100 nM, and the detection limit is 33 pM. Compared to the traditional single G-quadruplex label unit, this electrochemical sensor showed high sensitivity and selectivity for detecting Hg(2+).