Detecting Arbitrary DNA Mutations Using Graphene Oxide and Ethidium Bromide

Recent Progress in Single-Nucleotide Polymorphism Biosensors

Single-nucleotide polymorphisms (SNPs), the most common form of genetic variation in the human genome, are the main cause of individual differences. Furthermore, such attractive genetic markers are emerging as important hallmarks in clinical diagnosis and treatment. A variety of destructive abnormalities, such as malignancy, cardiovascular disease, inherited metabolic disease, and autoimmune disease, are associated with single-nucleotide variants. Therefore, identification of SNPs is necessary for better understanding of the gene function and health of an individual. SNP detection with simple preparation and operational procedures, high affinity and specificity, and cost-effectiveness have been the key challenge for years. Although biosensing methods offer high specificity and sensitivity, as well, they suffer drawbacks, such as complicated designs, complicated optimization procedures, and the use of complicated chemistry designs and expensive reagents, as well as toxic chemical compounds, for signal detection and amplifications. This review aims to provide an overview on improvements for SNP biosensing based on fluorescent and electrochemical methods. Very recently, novel designs in each category have been presented in detail. Furthermore, detection limitations, advantages and disadvantages, and challenges have also been presented for each type.

Dicationic styryl dyes for colorimetric and fluorescent detection of nucleic acids

Nucleic acid staining dyes are important tools for the analysis and visualizing of DNA/RNA in vitro and in the cells. Nevertheless, the range of commercially accessible dyes is still rather limited, and they are often very costly. As a result, finding nontoxic, easily accessible dyes, with desirable optical characteristics remains important. Styryl dyes have recently gained popularity as potential biological staining agents with many appealing properties, including a straightforward synthesis procedure, excellent photostability, tunable fluorescence, and high fluorescence quantum yield in the presence of nucleic acid targets with low background fluorescence signals. In addition to fluorescence, styryl dyes are strongly colored and exhibit solvatochromic properties which make them useful as colorimetric stains for low-cost and rapid testing of nucleic acids. In this work, novel dicationic styryl dyes bearing quaternary ammonium groups are designed to improve binding strength and optical response with target nucleic acids which contain a negatively charged phosphate backbone. Optical properties of the newly synthesized styryl dyes have been studied in the presence and absence of nucleic acid targets with the aim to find new dyes that can sensitively and specifically change fluorescence and/or color in the presence of nucleic acid targets. The binding interaction and optical response of the dicationic styryl dyes with nucleic acid were superior to the corresponding monocationic styryl dyes. Applications of the developed dyes for colorimetric detection of DNA in vitro and imaging of cellular nucleic acids are also demonstrated.

Highly Sensitive Detection for Mercury Ions Using Graphene Oxide (GO) Sensors

The mercury ion (Hg²⁺) is one of the heavy metal ions, and its presence in trace amounts can cause physiological damage to an organism. Traditional methods of Hg²⁺ detection have been useful but have also had numerous limitations and challenges, and as a result, it is important to design new and sophisticated methods that can aid in the detection of Hg²⁺. In this paper, two fluorescent dyes, carboxyfluorescein (FAM) and SYBR Green I, were used to label and intercalate DNA probes immobilized on the surface of graphene oxide (GO) for sensors to detect Hg²⁺. FAM and SYBR Green I dye share close excitation and emission wavelength spectra, which can promote and amplify the detection of signals, and also increase the limit of detection (LOD). The results showed that the limit of detection in this method was 0.53 nM. Moreover, when the sensors with double amino groups on the surface of GO were carried out to detect Hg²⁺, a limit of detection was improved to 0.43 nM. The sensors were then applied in the real sample. The results show that this method has a promising potential in Hg²⁺ detection.

2'-O-Methyl molecular beacon: a promising molecular tool that permits elimination of sticky-end pairing and improvement of detection sensitivity

An innovative 2'-O-methyl molecular beacon (MB) has been designed and prepared with improved thermal stability and unique nuclease resistance. The employment of 2'-O-methyl MBs helps efficiently suppress the background signal, while DNase I is responsible for the signal amplification and elimination of sticky-end pairing. The coupled use of 2'-O-methyl MBs and DNase I makes it possible to develop an enzyme-aided strategy for amplified detection of DNA targets in a sensitive and specific fashion. The analysis requires only mix-and-measure steps that can be accomplished within half an hour. The detection sensitivity is theoretically determined as 27.4 pM, which is nearly 200-fold better than that of the classic MB-based assay. This proposed sensing system also shows desired selectivity. All these features are of great importance for the design and application of MBs in biological, chemical, and biomedical fields.

Improving the sensitivity and selectivity of a DNA probe using graphene oxide-protected and T7 exonuclease-assisted signal amplification

The accurate analysis of single-nucleotide polymorphisms is of great significance for clinical detection and diagnosis. Based on the hybridization hindrance caused by graphene oxide (GO) and hairpin probe, we report a T7 Exo-assisted cyclic amplification technique to distinguish single-base mismatch for highly sensitive and selective detection of mutant-type DNA. When the mutant-type target is completely complementary to the probe, the T7 Exo hydrolyzes the probe and releases the fluorescent molecule from the GO surface, resulting in a fluorescence signal. Conversely, when the wild-type mismatch target is present, the weak hybridization prevents the release of FAM-labeled probe from the GO surface. Therefore, the FAM-labeled probe cannot be degraded efficiently by T7 Exo, and the fluorescence is still quenched by GO. The detection limit of the proposed method can be as low as 34 fM due to the cyclic signal amplification. The experimental results showed that the established method could be used to detect single-nucleotide polymorphisms accurately and sensitively at low cost.

A simple aptasensor for Aβ40 oligomers based on tunable mismatched base pairs of dsDNA and graphene oxide

β-amyloid 1-40 oligomers (Aβ₄₀O) is considered to be one of the important biomarkers for the diagnosis and treatment of Alzheimer's disease (AD). To explore a method with excellent performance is favorable for measuring the low concentration of Aβ₄₀O in AD patients. Here, we developed a simple and fast method with a double stranded DNA (dsDNA)/graphene oxide (GO) based sensor, which was a fluorescent probe for a highly sensitive detection of Aβ₄₀O down to 0.1 nM with a linear detectable range from 0.1 nM to 40 nM. The proposed sensor effectively reduced non-specific adsorption and improved the specificity of detection because of the covalent conjugation of a binding DNA (bDNA) containing Aβ₄₀O-targeting aptamer (Apt_Aβ) onto GO surface, as well as the optimization of the number of mismatch base pairs of dsDNA. Moreover, AD patients and healthy persons were distinguished by this present method. All advantages of this method are exactly what the clinical detection of AD biomarkers need. This novel aptasensor might pave a way towards the early diagnosis of AD.

Graphene oxide and enzyme-assisted dual-cycling amplification method for sensitive fluorometric determination of DNA

A fluorometric method is described for the determination of DNA. It involves the use of graphene oxide (GO), exonuclease III (Exo III), and two specially designed fluorophore-labeled hairpin probes (HP1 and HP2). Different from other GO-based DNA assays, the method takes advantage of the distinct binding abilities of GO with hairpin DNA probes and single nucleotides. GO serves as a strong quencher for fluorescent labels to ensure a very low background signal. Two reaction cycles mediated by Exo III are employed to enhance the signals. The combination of GO-induced quenching and Exo III-mediated dual regeneration of analytes leads to a detection limit as low as 1 pM for the model analyte human hemochromatosis protein (HFE) gene. The method is also applicable for the determination of HFE gene spiked into fetal bovine serum. Graphical abstract Schematic representation of a GO-based, Exo III-assisted method for dual-signal amplified detection of DNA, for which human haemochromatosis protein (HFE) gene is designed as the model target. The assay involves graphene oxide (GO), exonuclease (Exo III), and two specially designed, fluorophore-labelled hairpin probes (HP1 and HP2).

3' end of eae gene-based fluorescence DNA nanosensor for detection of E. coli O157:H7

Escherichia coli O157:H7 bacterium as a zoonotic pathogen is one of the most important causative agents of foodborne illnesses worldwide. Due to the serious concerns in public health and enormous economic losses in agriculture and food industry, it is very necessary to develop novel technology-based methods for sensitive and rapid detection of this bacterium in contaminated resources. In this study, a sensitive and selective fluorescence DNA nanosensing platform based on graphene oxide (GO) and the 3' end of eae gene as specific sequence was developed for the detection of E. coli O157:H7. In this platform, fluorescence resonance energy transfer (FRET) process between GO- and FAM-labeled eae gene probe was used for the diagnosis of E. coli O157:H7. Following the immobilization of the eae gene probe on GO, fluorescence emission of FAM was quenched. In hybridization reaction, by adding the complementary DNA, fluorescence emission of FAM was significantly increased and recovered to 93%. The performance of sensor for detection of E. coli O157:H7 genomic DNA was determined 10 pg genomic DNA per 1 ml Tris-HCl hybridization buffer which was significantly more sensitive than PCR method. In conclusion, the results indicated that GO eae gene-based nanosensor has potential to be developed as a rapid and sensitive diagnostic device besides PCR methods for the detection of E. coli O157:H7 bacteria.

A dual-cycling fluorescence scheme for ultrasensitive DNA detection through signal amplification and target regeneration

In this work, we propose an ultrasensitive fluorescence strategy for DNA detection. This method utilizes a molecular beacon (MB), a hairpin probe (HP), and an enzyme to trigger dual-cycling reactions (cycles I and II). In cycle I, the target is repeatedly used to amplify the fluorescence emission through hybridizations with the MB and cleavage reactions achieved by the enzyme. In cycle II, hybridization reactions between the HP and a segment of the MB continuously regenerate the target to trigger more cycle I reactions, leading to an enhanced fluorescent signal. The detection limit of the method is determined to be as low as 50 fM within 45 min, which is 2 to 3 orders of magnitude lower than that of the conventional fluorescence strategies. The method also shows a high selectivity over mismatched and random DNA sequences. The signal amplification mechanism of the strategy offers insights into constructing efficient and ultrasensitive biosensors for various applications.

Thioflavin T specifically brightening “Guanine Island” in duplex-DNA: a novel fluorescent probe for single-nucleotide mutation

Here, we found that Thioflavin T (ThT) could specifically bind with a G-GGG unit (named as “Guanine Island”) in double stranded DNA (ds-DNA).

Ultraspecific Multiplexed Detection of Low-Abundance Single-Nucleotide Variants by Combining a Masking Tactic with Fluorescent Nanoparticle Counting

To be able to detect simultaneously multiple single-nucleotide variants (SNVs) with both ultrahigh specificity and low-abundance sensitivity is of pivotal importance for molecular diagnostics and biological research. In this contribution, we for the first time developed a multiplex SNV detection method that combines the masking tactic with fluorescent nanoparticle (FNP) counting based on the sandwich design. The method presents a rivaling performance due to its advantageous features: the masking reagent was designed to hybridize with an extremely large amount of the wild-type sequence to render the assay with high specificity; FNP counting provides a sensitive multiplexed SNV detection; the sandwich design facilitates an easy separation to make the detection free of interferences from the matrix. For single SNV target discrimination, including the 6 most frequently occurring DNA KRAS gene mutations and 2 possible RNA KRAS gene mutations as well as 11 artificial mutations, the discrimination factor ranged from 204 to 1177 with the median being 545. Among the tested 19 SNVs, abundances as low as 0.05% were successfully identified in 14 cases, and an abundance as low as 0.1% was identified for the remaining 5 cases. For multiplexed detection of SNVs in the KRAS gene, abundances as low as 0.05-0.1% were achieved for multiple SNVs occurring at the same and different codons. As low as 0.05% low-abundance detection sensitivity was also achieved for PCR amplicons of human genomic DNA extracted from cell samples. This proposed method presents the potential for ultrahigh specific multiplexed detection of SNVs with low-abundance detection capability, which may be applied to practical applications.

Transition Metal Dichalcogenide Nanosheets for Visual Monitoring PCR Rivaling a Real-Time PCR Instrument

Monitoring the progress of polymerase chain reactions (PCRs) is of critical importance in bioanalytical chemistry and molecular biology. Although real-time PCR thermocyclers are ideal for this purpose, their high cost has limited their applications in resource-poor areas. Direct visual detection would be a more attractive alternative. To monitor the PCR amplification, DNA-staining dyes, such as SYBR Green I (SG), are often used. Although these dyes give higher fluorescence when binding to double-stranded DNA products, they also yield strong background fluorescence in the presence of a high concentration of single-stranded (ss) DNA primers. In this work, we screened various nanomaterials and found that graphene oxide (GO), reduced GO, molybdenum disulfide (MoS₂), and tungsten disulfide (WS₂) can quench the fluorescence of nonamplified negative samples while still retaining strong fluorescence of positive ones. The signal ratio of positive-over-negative samples was enhanced by around 50-fold in the presence of these materials. In particular, MoS₂ and WS₂ nearly fully retained the fluorescence of the positive samples. The mechanism for MoS₂ and WS₂ to enhance PCR signaling is attributed to the adsorption of both the ssDNA PCR primers and SG with an appropriate strength. MoS₂ can also suppress nonspecific amplification caused by excess polymerase. Finally, this method was used to detect extracted transgenic soya GTS 40-3-2 DNA after PCR amplification. Compared with the samples without nanomaterials, the addition of MoS₂ could better distinguish the concentration difference of the template DNA, and the sensitivity of visual detection rivaled that from a real-time PCR instrument.

A Target-Lighted dsDNA-Indicator for High-Performance Monitoring of Mercury Pollution and Its Antagonists Screening

As well-known, the excessive discharge of heavy-metal mercury not only destroys the ecological environment, bust also leads to severe damage of human health after ingestion via drinking and bioaccumulation of food chains, and mercury ion (Hg²⁺) is designated as one of most prevalent toxic metal ions in drinking water. Thus, the high-performance monitoring of mercury pollution is necessary. Functional nucleic acids have been widely used as recognition probes in biochemical sensing. In this work, a carbazole derivative, ethyl-4-[3,6-bis(1-methyl-4-vinylpyridium iodine)-9H-carbazol -9-yl)] butanoate (EBCB), has been synthesized and found as a target-lighted DNA fluorescent indicator. As a proof-of-concept, Hg²⁺ detection was carried out based on EBCB and Hg²⁺-mediated conformation transformation of a designed DNA probe. By comparison with conventional nucleic acid indicators, EBCB held excellent advantages, such as minimal background interference and maximal sensitivity. Outstanding detection capabilities were displayed, especially including simple operation (add-and-read manner), ultrarapidity (30 s), and low detection limit (0.82 nM). Furthermore, based on these advantages, the potential for high-performance screening of mercury antagonists was also demonstrated by the fluorescence change of EBCB. Therefore, we believe that this work is meaningful in pollution monitoring, environment restoration and emergency treatment, and may pave a way to apply EBCB as an ideal signal transducer for development of high-performance sensing strategies.

Direct Detection of Nucleic Acid with Minimizing Background and Improving Sensitivity Based on a Conformation-Discriminating Indicator

As is well-known, the nucleic acid indicator-based strategy is one of the major approaches to monitor the nucleic acid hybridization-mediated recognition events in biochemical analysis, displaying obvious advantages including simplicity, low cost, convenience, and generality. However, conventional indicators either hold strong self-fluorescence or can be lighted by both ssDNA and dsDNA, lacking absolute selectivity for a certain conformation, always with high background interference and low sensitivity in sensing; and additional processing (e.g., nanomaterial-mediated background suppression, and enzyme-catalyzed signal amplification) is generally required to improve the detection performance. In this work, a carbazole derivative, EBCB, has been synthesized and screened as a dsDNA-specific fluorescent indicator. Compared with conventional indicators under the same conditions, EBCB displayed a much higher selective coefficient for dsDNA, with little self-fluorescence and negligible effect from ssDNA. Based on its superior capability in DNA conformation-discrimination, high sensitivity with minimizing background interference was demonstrated for direct detection of nucleic acid, and monitoring nucleic acid-based circuitry with good reversibity, resulting in low detection limit and high capability for discriminating base-mismatching. Thus, we expect that this highly specific DNA conformation-discriminating indicator will hold good potential for application in biochemical sensing and molecular logic switching.

Metal ion detection using functional nucleic acids and nanomaterials

Metal ion detection is critical in a variety of areas. The past decade has witnessed great progress in the development of metal ion sensors using functional nucleic acids (FNAs) and nanomaterials. The former has good recognition selectivity toward metal ions and the latter possesses unique properties for enhancing the performance of metal ion sensors. This review offers a summary of FNA- and nanomaterial-based metal ion detection methods. FNAs mainly include DNAzymes, G-quadruplexes, and mismatched base pairs and nanomaterials cover gold nanoparticles (GNPs), quantum dots (QDs), carbon nanotubes (CNTs), and graphene oxide (GO). The roles of FNAs and nanomaterials are introduced first. Then, various methods based on the combination of different FNAs and nanomaterials are discussed. Finally, the challenges and future directions of metal ion sensors are presented.

Carbon nanostructures in biology and medicine

Carbon nanostructures have unique physical, chemical, and electrical properties, which have attracted great interest from scientists. Carbon dots, carbon nanotubes, graphene and other carbon nanomaterials are being successfully implemented in electrochemical sensing, biomedical and biological imaging.

An enzyme-free surface plasmon resonance biosensing strategy for detection of DNA and small molecule based on nonlinear hybridization chain reaction

A label-free and enzyme-free surface plasmon resonance (SPR) biosensing strategy has been developed for highly sensitive and specific detection of target DNA by employing the nonlinear hybridization chain reaction (HCR) amplification. Nonlinear HCR is a hairpin-free system in which double-stranded DNA monomers could dendritically assemble into highly branched nanostructure upon introducing a trigger sequence. The target DNA partly hybridizes with capture probe on the gold sensing chip and the unpaired fragment of target DNA works as a trigger to initiate the nonlinear HCR, forming a chain-branching growth of DNA dendrimer by self-assembly. Real-time amplified SPR response is observed upon the introduction of nonlinear HCR system. The method is capable of detecting target DNA at the concentration down to 0.85 pM in 60min with a dynamic range from 1 pM to 1000 pM, and could discriminate target DNA from mismatched sequences. This biosensing strategy exhibits good reproducibility and precision, and has been successfully applied for detection of target DNA in complex sample matrices. In addition, the nonlinear HCR based SPR biosensing methodology is extended to the detection of adenosine triphosphate (ATP) by aptamer recognition. Thus, the versatile method might become a potential alternative tool for biomolecule detection in medical research and early clinical diagnosis.

Graphene oxide: strategies for synthesis, reduction and frontier applications

In this review article, we describe a general introduction to GO, its synthesis, reduction and some selected frontier applications. Its low cost and potential for mass production make GO a promising building block for functional hybrid materials.

Assembly of substituted phenanthridines via a cascade palladium-catalyzed coupling reaction, deprotection and intramolecular cyclization

A practical method for one-pot synthesis of substituted phenanthridines is described. Via this method, a series of substituted phenanthridines are obtained in good yields with remarkable functional groups compatibility.