Digital Nucleic Acid Signal Amplification Platform for Highly Sensitive DNA Mutation Analysis

G-quadruplex embedded in semi-CHA reaction combined with invasive reaction for label-free detection of single nucleotide polymorphisms

G-quadruplex/thioflavin T (G4/THT) is one of the ideal label-free fluorescent light-emitting elements in the field of biosensors due to its good programmability and adaptability. However, the unsatisfactory luminous efficiency of single-molecule G4/THT limits its more practical applications. Here, we developed a G4 embedded semi-catalytic hairpin assembly (G4-SCHA) reaction by rationally modifying the traditional CHA reaction, and combined with the invasive reaction, supplemented by magnetic separation technology, for label-free sensitive detection of single nucleotide polymorphisms (SNPs). The invasive reaction enabled specific recognition of single-base mutations in DNA sequences as well as preliminary signal cycle amplification. Then, magnetic separation was used to shield the false positive signals. Finally, the G4-SCHA was created for secondary amplification and label-free output of the signal. This dual-signal amplified label-free biosensor has been shown to detect mutant targets as low as 78.54 fM. What's more, this biosensor could distinguish 0.01 % of the mutant targets from a mixed sample containing a large number of wild-type targets. In addition, the detection of real and complex biological samples also verified the practical application value of this biosensor in the field of molecular design breeding. Therefore, this study improves a label-free fluorescent light-emitting element, and then proposes a simple, efficient and universal label-free SNP biosensing strategy, which also provides an important reference for the development of other G4/THT based biosensors.

FRAME: flap endonuclease 1-engineered PAM module for precise and sensitive modulation of CRISPR/Cas12a trans-cleavage activity

CRISPR/Cas12a system, renowned for its precise recognition and efficient nucleic acid cleavage capabilities, has demonstrated remarkable performance in molecular diagnostics and biosensing. However, the reported Cas12a activity regulation methods often involved intricate CRISPR RNA (crRNA) structural adjustments or costly chemical modifications, which limited their applications. Here, we demonstrated a unique enzyme activity engineering strategy using flap endonuclease 1 (FEN1) to regulate the accessibility of the protospacer adjacent motif (PAM) module in the double-stranded DNA activator (FRAME). By identifying the three-base overlapping structure between the target inputs and substrate, FEN1 selectively cleaved and released the 5'-flap containing the 'TTTN' sequence, which triggered the secondary cleavage of FEN1 while forming a nicked PAM, ultimately achieving the sensitive switching of Cas12a's activity. The FRAME strategy exemplified the 'two birds with one stone' principle, as it not only precisely programmed Cas12a's activity but also simultaneously triggered isothermal cyclic amplification. Moreover, the FRAME strategy was applied to construct a sensing platform for detecting myeloperoxidase and miR-155, which demonstrated high sensitivity and specificity. Importantly, it proved its versatility in detecting multiple targets using a single crRNA without redesign. Collectively, the FRAME strategy opens up a novel avenue for modulating Cas12a's activity, promising immense potential in the realm of medical diagnostics.

Hairpin-Empowered Invasive Reaction Combined with Catalytic Hairpin Assembly Cascade Amplification for the Specific Detection of Single-Nucleotide Polymorphisms

Single-nucleotide polymorphism (SNP) is widely used in the study of disease-related genes and in the genetic study of animal and plant strains. Therefore, SNP detection is crucial for biomedical diagnosis and treatment as well as for molecular design breeding of animals and plants. In this regard, this article describes a novel technique for detecting SNP using flap endonuclease 1 (FEN 1) as a specific recognition element and catalytic hairpin assembly (CHA) cascade reaction as a signal amplification strategy. The mutant target (MT) was hybridized with a biotin-modified upstream probe and hairpin-type downstream probe (DP) to form a specific three-base overlapping structure. Then, FEN 1 was employed for three-base overlapping structure-specific recognition, namely, the precise SNP site identification and the 5' flap of DP dissociation. After dissociation, the hybridized probes were magnetically separated by a streptavidin-biotin complex. Especially, the ability to establish such a hairpin-type DP provided a powerful tool that could be used to hide the cut sequence (CS) and avoid false-positive signals. The cleaved CS initiated the CHA reaction and allowed superior fluorescence signal generation. Owing to the high specificity of FEN 1 for single base recognition, only the MT could be distinguished from the wild-type target and mismatched DNA. Owing to the dual signal amplification, as low as 0.36 fM MT and 1% mutation abundance from the mixtures could be detected, respectively. Furthermore, it could accurately identify SNPs from human cancer cells, as well as soybean leaf genome extracts. This strategy paves the way for the development of more precise and sensitive tools for diagnosing early onset diseases as well as molecular design breeding tools.

Double base mismatches mediated catalytic hairpin assembly for enzyme-free single-base mutation detection: integrating signal recognition and amplification in one

Single nucleotide polymorphism (SNP) biosensors are emerging rapidly for their promising applications in human disease prevention diagnosis, treatment, and prognosis. However, it remains a bottleneck in equipping simple and stable biosensors with the traits of high sensitivity, non-enzyme, and low cost. Double base mismatches mediated chain displacement reactions have attracted fascinating advantages of tailorable thermodynamics stability, non-enzyme, and excellent assembly compliance to involvement in SNP identification. As the base mismatch position and amount in DNA sequence can be artificially adjusted, it provides plenty of selectivity and specificity for exploring perfect biosensors. Herein, a biosensor with double base mismatches mediated catalytic hairpin assembly (CHA) is designed via one base mismatch in the toehold domain and the other base mismatch in the stem sequence of hairpin 1 (H1) by triggering CHA reaction to achieve selective amplification of the mutation target (MT) and fluorescence resonance energy transfer (FRET) effect that is composed of Cy3 and Cy5 terminally attached H1 and hairpin 2 (H2). Depending on the rationally designed base mismatch position and toehold length, the fabricated biosensors show superior SNP detection performance, exhibiting a good linearity with high sensitivity of 6.6 fM detection limit and a broad detection abundance of 1%. The proposed biosensor can be used to detect the KRAS mutation gene in real samples and obtain good recoveries between 106 and 116.99%. Remarkably, these extendible designs of base mismatches can be used for more types of SNP detection, providing flexible adjustment based on base mismatch position and toehold length variations, especially for their thermodynamic model for DNA-strand displacement reactions.

Triplex DNA-based aggregation-induced emission probe: A new platform for hybridization chain reaction-based fluorescence sensing assay

The hybridization chain reaction (HCR), as one of the nucleic acid amplification technologies, is combined with fluorescence signal output with excellent sensitivity, simplicity, and stability. However, current HCR-based fluorescence sensing methods still have some defects such as the blocking effect of the HCR combination with fluorophores and the aggregation-caused quenching (ACQ) phenomenon of traditional fluorophores. Herein, a triplex DNA-based aggregation-induced emission probe (AIE-P) was designed as the fluorescent signal transduction, which is able to provide a new platform for HCR-based sensing assay. The AIE-P was synthesized by attaching the AIE fluorophores to terminus of the oligonucleotide through amido bond, and captured the products of HCR to form triplex DNA. In this case, the AIE fluorophores were located in close proximity to generate fluorescence. This assay provided turn-on fluorescence efficiency with a high signal-to-noise ratio and excellent amplification capability to solve the shortcoming of HCR-based fluorescence sensing methods. It enabled sensitive detection of Vibrio parahaemolyticus in the range of 102-106 CFU mL-1, and with a low limit of detection down to 39 CFU mL-1. In addition, this assay expressed good specificity and practicability. The triplex DNA-based AIE probe forms a universal molecular tool for developing HCR-based fluorescence sensing methods.

Dual-color nanospheres based on aggregation-induced emission and catalytic hairpin assembly for simultaneous imaging of acrylamide and miR-21 in living cells

Acrylamide (AA) is a heat-processed potent food carcinogen that is widely used in industry, posing a significant risk to human health. Therefore, it is necessary to investigate the toxic effects and mechanism of AA. miR-21 is a representative biomarker during AA-induced carcinogenesis. Here, dual-color aggregation-induced emission nanoparticles (AIENPs) were developed for the detection and simultaneous imaging of AA and miR-21. AIENPs were synthesized by combining aggregation-induced emission (AIE) dyes and a poly (styrene-co-maleic anhydride) (PSMA) amphiphilic polymer modified with hairpin DNA. Upon AA intervention and aptamer recognition, cDNA was dissociated, leading to miR-21 overexpression and initiating the catalytic hairpin assembly cycle. Consequently, fluorescence quenching was observed due to FRET between AIENPs and labeled quenchers. The relative fluorescence intensities of dual-color AIENPs displayed good linear relationships with logarithmic AA and miR-21 concentrations. Moreover, there was a gradual decrease in dual-color AIENP fluorescence as the HepG2 cell concentration of AA (0-500 μM) and stimulation time (0-12 h) increased, making it possible to simultaneously image AA and AA-induced miR-21. The findings of this work are valuable for revealing the cytotoxic mechanism of AA.

Rationally Designed Dual Base Pair Mismatch Enables Toehold-Mediated Strand Displacement to Efficiently Recognize Single-Nucleotide Polymorphism without Enzymes

The efficiency of the enzyme-free toehold-mediated strand displacement (TMSD) technique is often insufficient to detect single-nucleotide polymorphism (SNP) that possesses only single base pair mismatch discrimination. Here, we report a novel dual base pair mismatch strategy enabling TMSD biosensing for SNP detection under enzyme-free conditions when coupled with catalytic hairpin assembly (CHA) and fluorescence resonance energy transfer (FRET). The strategy is based on a competitive strand displacement reaction mechanism, affected by the thermodynamic stability originating from rationally designed dual base pair mismatch, for the specific recognition of mutant-type DNA. In particular, enzyme-free nucleic acid circuits, such as CHA, emerge as a powerful method for signal amplification. Eventually, the signal transduction of this proposed biosensor was determined by FRET between streptavidin-coated 605 nm emission quantum dots (605QDs, donor) and Cy5/biotin hybridization (acceptor, from CHA) when incubated with each other. The proposed biosensor displayed high sensitivity to the mutant target (MT) with a detection concentration down to 4.3 fM and led to high discrimination factors for all types of mismatches in multiple sequence contexts. As such, the application of this proposed biosensor to investigate mechanisms of the competitive strand displacement reaction further illustrates the versatility of our dual base pair mismatch strategy, which can be utilized for the creation of a new class of biosensors.

Nanosensor Based on the Dual-Entropy-Driven Modulation Strategy for Intracellular Detection of MicroRNA

The entropy-driven strategy has been proposed as a milestone work in the development of nucleic acid amplification technology. With the characteristics of an enzyme-free, isothermal, and relatively simple design, it has been widely used in the field of biological analysis. However, it is still a challenge to apply entropy-driven amplification for intracellular target analysis. In this study, a dual-entropy-driven amplification system constructed on the surface of gold nanoparticles (AuNPs) is developed to achieve fluorescence determination and intracellular imaging of microRNA-21 (miRNA-21). The dual-entropy-driven amplification strategy internalizes the fuel chain to avoid the complexity of the extra addition in the traditional entropy-driven amplification strategy. The unique self-locked fuel chain system is established by attaching the three-stranded structure on two groups of AuNPs, where the Cy5 fluorescent label was first quenched by AuNPs. After the target miRNA-21 is identified, the fuel chain will be automatically unlocked, and the cycle reaction will be driven, leading to fluorescence recovery. The self-powered and waste-recycled fuel chain greatly improves the automation and intelligence of the reaction process. Under the optimal conditions, the linear response range of the nanosensor ranges from 5 pM to 25 nM. This nanoreaction system can be used to realize intracellular imaging of miRNA-21, and its good specificity enables it to distinguish tumor cells from healthy cells. The development of the dual-entropy-driven strategy provides an integrated and powerful way for intracellular miRNA analysis and shows great potential in the biomedical field.

An all-in-one strategy for bisulfite-free DNA methylation detection by temperature-programmed enzymatic reactions

The fragmentation and low concentration of cell-free DNA (cfDNA) pose higher challenges for the cfDNA methylation detection technologies. Conventional bisulfite conversion-based methods are inadequate for cfDNA methylation analysis due to cumbersome operation and exacerbating cfDNA degradation. Herein, we proposed temperature-programmed enzymatic reactions for cfDNA methylation analysis in a single tube. Endonuclease was used to mildly recognize DNA methylation to avoid the degradation of cfDNA. And two stages of amplification reactions significantly improved the detection sensitivity for GC-rich sequence. With vimentin as the target, the detection sensitivity was 10 copies of methylated DNA. Meanwhile, the proposed method can accurately quantify the methylation level of target sequence from 1000-fold of unmethylated DNA background. Further, the methylated vimentin gene in 20 clinical plasma samples was successfully detected. The results shown significant differences in methylation levels of the vimentin gene between healthy volunteers and colorectal cancer patients. These results lead us to believe that the proposed method has great application potential for DNA methylation analysis as a complement to bisulfite conversion-based methods.

A protein enzyme-free strategy for fluorescence detection of single nucleotide polymorphisms using asymmetric MNAzymes

To establish protein enzyme-free and simple approach for sensitive detection of single nucleotide polymorphisms (SNPs), the nucleic acid amplification reactions were developed to reduce the dependence on protein enzymes (polymerase, endonuclease, ligase). These methods, while enabling highly amplified analysis for the short sequences, cannot be generalized to long genomic sequences. Herein, we develop a protein enzyme-free and general SNPs assay based on asymmetric MNAzyme probes. The multi-arm probe (MNAzyme-9_M-13) with two asymmetric recognition arms, containing a short (9 nt) and a long (13 nt) arm, is designed to detect EGFR T790 M mutation (MT). Owing to the excellent selectivity of short recognition arm, MNAzyme-9_M-13 probe can efficiently avoid interferences from wild-type target (WT) and various single-base mutations. Through a one-pot mixing, MNAzyme-9_M-13 probe enables the sensitive detection of MT, without protein enzyme or multi-step operation. The calculated detection limit for MT is 0.59 nM and 0.83%. Moreover, this asymmetric MNAzyme strategy can be applied for SNPs detection in long genomic sequences as well as short microRNAs (miRNAs) only by changing the low-cost unlabeled recognition arms. Therefore, along with simple operation, low-cost, protein enzyme-free and strong versatility, our asymmetric MNAzyme strategy provides a novel solution for SNPs detection and genes analysis.

Chemical Cyclic Amplification: Hydroxylamine Boosts the Fenton Reaction for Versatile and Scalable Biosensing

Nucleic acid detection is undoubtedly one of the most important research fields to meet the medical needs of genetic disease diagnosis, cancer treatment, and infectious disease prevention. However, the practical detection methods based on biological amplification are complex and time-consuming and require highly trained operators. Herein, we report a simple, rapid, and sensitive method for the nucleic acid assay by fluorescence or naked eye using chemical cyclic amplification. The addition of hydroxylamine (HA) during the Fenton reaction can continuously generate hydroxyl radicals (^•OH) via Fe³⁺/Fe²⁺ cycle, termed as "hydroxylamine boosts the Fenton reaction (Fenton-HA system)". Meanwhile, the reducing substances, such as terephthalic acid or o-phenylenediamine, react with ^•OH to generate oxidized substances that can be recognized by the naked eye or detected by fluorescence so as to realize the detection of Fe³⁺. The concentration of Fe³⁺ has a good linear relationship with fluorescence intensity in the range of 0.1 to 100 nM, and the limit of detection is calculated to be 0.03 nM (S/N = 3). Subsequently, Fe was introduced into the nucleic acid hybridization system after the Fe source was transformed into Fe³⁺, and the nucleic acids were indirectly determined by this method. This Fenton-HA system was used for sensing HIV-DNA and miRNA-21 to verify the validity of this method in nucleic acid detection. The detection limits were as low as 2.5 pM for HIV-DNA and 3 pM for miRNA-21. We believe that our work has unlocked an efficient signal amplification strategy, which is expected to develop a new generation of highly sensitive chemical biosensors.

Advances in optical counting and imaging of micro/nano single-entity reactors for biomolecular analysis

Ultrasensitive detection of biomarkers is of paramount importance in various fields. Superior to the conventional ensemble measurement-based assays, single-entity assays, especially single-entity detection-based digital assays, not only can reach ultrahigh sensitivity, but also possess the potential to examine the heterogeneities among the individual target molecules within a population. In this review, we summarized the current biomolecular analysis methods that based on optical counting and imaging of the micro/nano-sized single entities that act as the individual reactors (e.g., micro-/nanoparticles, microemulsions, and microwells). We categorize the corresponding techniques as analog and digital single-entity assays and provide detailed information such as the design principles, the analytical performance, and their implementation in biomarker analysis in this work. We have also set critical comments on each technique from these aspects. At last, we reflect on the advantages and limitations of the optical single-entity counting and imaging methods for biomolecular assay and highlight future opportunities in this field.