Specific and simultaneous detection of micro RNA 21 and let-7a by rolling circle amplification combined with lateral flow strip

Hybrid chain reaction nanoscaffold-based functional nucleic acid nanomaterial cascaded with rolling circle amplification for signal enhanced miRNA let-7a detection

A novel functional nucleic acid (FNA) nanomaterial based on hybrid chain reaction (HCR) nanoscaffolds is proposed to solve the problem of time superposition and repeated primer design in sensitive miRND detection using cascade amplification technique. Rolling circle amplification (RCA) was cascaded with the prepared FNA nanomaterials for miRNA let-7a (as a model target) sensitive detection by lateral flow assay (LFA). Under the optimal conditions, the proposed RCA-FNA-LFA assay demonstrated the specificity and accuracy for miRNA let-7a detection with a detection limit of 1.07 pM, which increased sensitivity by nearly 20 times compared with that of RCA -LFA assay. It is worth noting that the non-target-dependent self-assembly process of HCR nanoscaffolds does not take up the whole detection time, thus, less time is taken than that of the conventional cascaded method. Moreover, the proposed assay does not need to consider the system compatibility between two kinds of isothermal amplification techniques. As for detection of different miRNAs, only the homologous arm of the padlock probe of RCA needs to be changed, while the FNA nanomaterial does not need any change, which greatly simplifies the primer design of the cascaded amplification techniques. With further development, the proposed RCA-FNA-LFA assay might achieve more sensitive and faster results to better satisfy the requirements of clinical diagnosis combing with more sensitive labels or small strip reader.

A lateral flow assay for miRNA-21 based on CRISPR/Cas13a and MnO2 nanosheets-mediated recognition and signal amplification

The point-of-care testing (POCT) of miRNA has significant application in medical diagnosis, yet presents challenges due to their characteristics of high homology, low abundance, and short length, which hinders the achievement of quick detection with high specificity and sensitivity. In this study, a lateral flow assay based on the CRISPR/Cas13a system and MnO2 nanozyme was developed for highly sensitive detection of microRNA-21 (miR-21). The CRISPR/Cas13a cleavage system exhibits the ability to recognize the specific oligonucleotide sequence, where two-base mismatches significantly impact the cleavage activity of the Cas13a. Upon binding of the target to crRNA, the cleavage activity of Cas13a is activated, resulting in the unlocking of the sequence and initiating strand displacement, thereby enabling signal amplification to produce a new sequence P1. When applying the reaction solution to the lateral flow test strip, P1 mediates the capture of MnO2 nanosheets (MnO2 NSs) on the T zone, which catalyzes the oxidation of the pre-immobilized colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) on the T zone and generates the blue-green product (ox-TMB). The change in gray value is directly proportional to the concentration of miR-21, allowing for qualitative detection through visual inspection and quantitative measurement using ImageJ software. This method achieves the detection of miR-21 within a rapid 10-min timeframe, and the limit of detection (LOD) is 0.33 pM. With the advantages of high specificity, simplicity, and sensitivity, the lateral flow test strip and the design strategy hold great potential for the early diagnosis of related diseases.

Microfluidic chip immunoassay based on rolling circle amplification and G-quadruplex/Thioflavin T for multiplex detection of CTX I

A label-free immunoassay based on rolling circle amplification (RCA) and G-quadruplex/Thioflavin T (G4/ThT) is proposed to realize the sensitive detection of carboxy-terminal cross-linked fragment of type I collagen (CTX I) for bone loss. Under the optimal conditions, as low as 38.02 pg/mL of CTX I can be detected. To improve the detecting throughput and simplify the operation, a microfluidic chip was designed, fabricated, and used for CTX I detection based on the proposed assay. The detection can be completed with only a single on-chip magnetic separation step, which was easy to operate, less time-consuming, and has only low reagent consumption. The limit of detection was 131.83 pg/mL by observing with fluorescence microscope. With further improvement of detection equipment, the sensitivity of on-chip detection can be improved. It can be expected that the proposed RCA/G4/ThT immunoassay for sensitive and high-throughput automated detection of CTX I might be chosen as a potential analytical tool for clinical osteoporosis diagnosis and in-orbit bone loss detection.

Point-of-Care Testing for the Detection of MicroRNAs: Towards Liquid Biopsy on a Chip

Point-of-care (PoC) testing is revolutionizing the healthcare sector improving patient care in daily hospital practice and allowing reaching even remote geographical areas. In the frame of cancer management, the design and validation of PoC enabling the non-invasive, rapid detection of cancer markers is urgently required to implement liquid biopsy in clinical practice. Therefore, focusing on stable blood-based markers with high-specificity, such as microRNAs, is of crucial importance. In this work, we highlight the potential impact of circulating microRNAs detection on cancer management and the crucial role of PoC testing devices, especially for low-income countries. A detailed discussion about the challenges that should be faced to promote the technological transfer and clinical use of these tools has been added, to provide the readers with a complete overview of potentialities and current limitations.

Comparison of DNA-Gold Nanoparticle Conjugation Methods: Application in Lateral Flow Nucleic Acid Biosensors

Lateral flow nucleic acid biosensors (LFNABs) have attracted extensive attention due to their rapid turnaround time, low cost, and results that are visible to the naked eye. One of the key steps to develop LFNABs is to prepare DNA-gold nanoparticle (DNA-AuNP) conjugates, which affect the sensitivity of LFNABs significantly. To date, various conjugation methods-including the salt-aging method, microwave-assisted dry heating method, freeze-thaw method, low-pH method, and butanol dehydration method-have been reported to prepare DNA-AuNP conjugates. In this study, we conducted a comparative analysis of the analytical performances of LFNABs prepared with the above five conjugation methods, and we found that the butanol dehydration method gave the lowest detection limit. After systematic optimization, the LFNAB prepared with the butanol dehydration method had a detection limit of 5 pM for single-strand DNA, which is 100 times lower than that of the salt-aging method. The as-prepared LFNAB was applied to detect miRNA-21 in human serum, with satisfactory results. The butanol dehydration method thus offers a rapid conjugation approach to prepare DNA-AuNP conjugates for LFNABs, and it can also be extended to other types of DNA biosensors and biomedical applications.

A universal lateral flow assay for microRNA visual detection in urine samples

MicroRNAs (miRNAs) have been emerged as novel and significant biomarkers in liquid biopsy that can be found in different body fluids. Several techniques have been developed and applied for miRNAs analysis, including nucleic acid-based amplification methods, next generation sequencing, DNA microarrays and new genome-editing methods. These methods, however, are time-consuming and require expensive instruments and specially trained personnel. Biosensors, on the other hand, are alternative and valuable analytical/diagnostic tools due to their simplicity, cost-effectiveness, rapid analysis and ease of use. Several biosensors, especially nanotechnology-based ones, have been developed for miRNA analysis that are based either on target amplification or signal amplification and target re-cycling for sensitive detection. At this point of view, we have introduced a new and universal lateral flow assay in combination with reverse transcription - polymerase chain reaction (RT-PCR) and gold nanoparticles as reporters for the detection of miR-21 and miR-let-7a in human urine. It is the first time that such a biosensor has been applied to the detection of microRNAs in urine. As low as 10²-10³ copies of miR-21 and 10^2--10⁴ copies of miR-let-7a added in urine were detectable by the proposed lateral flow assay with great specificity and repeatability (%CVs <4.5%).

A dual-labeled fluorescence quenching lateral flow assay based on one-pot enzyme-free isothermal cascade amplification for the rapid and sensitive detection of pathogens

Rapid detection of nucleic acids is integral for clinical diagnostics, especially if a major public-health emergency occurs. However, such detection cannot be carried out efficiently in remote areas limited by medical resources. Herein, a dual-labeled fluorescence resonance energy transfer (FRET) lateral flow assay (LFA) based on one-pot enzyme-free cascade amplification was developed for rapid, convenient, and sensitive detection of open reading frame (ORF)1ab of severe acute respiratory syndrome-coronavirus-2. The catalyzed hairpin assembly (CHA) reaction of two well-designed hairpin probes was initiated by a target sequence and generated a hybridization chain reaction (HCR) initiator. Then, HCR probes modified with biotin were initiated to produce long DNA nanowires. After two-level amplification, the cascade-amplified product was detected by dual-labeled lateral flow strips. Gold nanoparticles (AuNPs)-streptavidin combined with the product and then ran along a nitrocellulose membrane under the action of capillary force. After binding with fluorescent microsphere-labeled-specific probes on the T line, a positive signal (red color) could be observed. Meanwhile, AuNPs could quench the fluorescence of the T line, and an inverse relationship between fluorescence intensity and the concentration of the CHA-HCR-amplified product was formed. The proposed strategy achieved a satisfactory limit of detection of 2.46 pM for colorimetric detection and 174 fM for fluorescent detection, respectively. Benefitting from the features of being one-pot, enzyme-free, low background, high sensitivity, and selectivity, this strategy shows great potential in bioanalysis and clinical diagnostics upon further development.

Electrical potential-assisted DNA-RNA hybridization for rapid microRNA extraction

Analysis of microRNAs (miRNAs) is important in cancer diagnostics and therapy. Conventional methods used to extract miRNA for analysis are generally time-consuming. A novel approach for rapid and sensitive extraction of miRNAs is urgently need for clinical applications. Herein, a novel strategy based on electrical potential-assisted DNA-RNA hybridization was designed for miRNA extraction. The entire extraction process was accomplished in approximately 3 min, which is much shorter than the commercial adsorption column method, at more than 60 min, or the TRIzol method, at more than 90 min. Additionally, the method offered the advantages of simplicity and specificity during the extraction process by electrical potential-assisted hybridization of single-stranded DNA and RNA. Taking let-7a as an example, satisfactory results were achieved for miRNA extraction in serum, demonstrating the applicability in miRNA nucleic acid amplification.

Isothermal nucleic acid amplification technology for rapid detection of virus

While the research field and industrial market of in vitro diagnosis (IVD) thrived during and post the COVID-19 pandemic, the development of isothermal nucleic acid amplification test (INAAT) based rapid diagnosis was engendered in a global wised large measure as a problem-solving exercise. This review systematically analyzed the recent advances of INAAT strategies with practical case for the real-world scenario virus detection applications. With the qualities that make INAAT systems useful for making diagnosis relevant decisions, the key performance indicators and the cost-effectiveness of enzyme-assisted methods and enzyme-free methods were compared. The modularity of nucleic acid amplification reactions that can lead to thresholding signal amplifications using INAAT reagents and their methodology design were examined, alongside the potential application with rapid test platform/device integration. Given that clinical practitioners are, by and large, unaware of many the isothermal nucleic acid test advances. This review could bridge the arcane research field of different INAAT systems and signal output modalities with end-users in clinic when choosing suitable test kits and/or methods for rapid virus detection.

High sensitivity and rapid detection of hepatitis B virus DNA using lateral flow biosensors based on Au@Pt nanorods in the absence of hydrogen peroxide

Au@Pt nanorods with enhanced oxidase-like activity were designed and used as signal probes to construct LFBs for the high sensitivity detection of hepatitis B virus DNA (HBV-DNA).

Recent advances in biological detection with rolling circle amplification: design strategy, biosensing mechanism, and practical applications

Rolling circle amplification (RCA) is a simple and isothermal DNA amplification technique that is used to generate thousands of repeating DNA sequences using circular templates under the catalysis of DNA polymerase.

Recent advances of fluorescent biosensors based on cyclic signal amplification technology in biomedical detection

The cyclic signal amplification technology has been widely applied for the ultrasensitive detection of many important biomolecules, such as nucleic acids, proteins, enzymes, adenosine triphosphate (ATP), metal ions, exosome, etc. Due to their low content in the complex biological samples, traditional detection methods are insufficient to satisfy the requirements for monitoring those biomolecules. Therefore, effective and sensitive biosensors based on cyclic signal amplification technology are of great significance for the quick and simple diagnosis and treatment of diseases. Fluorescent biosensor based on cyclic signal amplification technology has become a research hotspot due to its simple operation, low cost, short time, high sensitivity and high specificity. This paper introduces several cyclic amplification methods, such as rolling circle amplification (RCA), strand displacement reactions (SDR) and enzyme-assisted amplification (EAA), and summarizes the research progress of using this technology in the detection of different biomolecules in recent years, in order to provide help for the research of more efficient and sensitive detection methods.

A portable and quantitative detection of microRNA-21 based on cascade enzymatic reactions with dual signal outputs

MicroRNAs (miRNAs) are physiological status-related molecules which can be used as biomarkers for diseases, such as cancers. The point-of-care testing (POCT) of miRNAs has great application potential in early diagnosis and process monitoring of diseases. In this paper, a fast and dual signal outputs detection for microRNA-21 (miRNA-21) was established by using both personal glucose meter (PGM) and fluorescence spectrometer. In such an assay protocol, a dual-functional hairpin structure was rationally designed to recognize miRNA-21 and serve as the carrier of the reporter adenosine monophosphate (AMP). The hairpin structure can be specifically degraded by exonuclease T (Exo T) after hybridization with the target miRNA-21, releasing a large amount of AMP as the reporter. Then a smart signal conversion machinery composed of four enzymes and the corresponding substrates was employed to produce dual output signals through enzymatic cascade reactions. The machinery includes two parts: an adenosine triphosphate (ATP) generation system and a glucose consumption/NADPH production system. The produced AMP in the former step triggers the production of ATP, and subsequently the consumption of glucose and the production of NADPH. The changes of both glucose and NADPH are proportional to the concentration of miRNA-21, and can be determined by PGM and fluorescence spectrometer, respectively. Besides, the build-in substrate-recycling mechanism achieves signal amplification of the cascade enzymatic reactions. Under the optimal experimental conditions, the PGM signal is linearly correlated with the concentration of miRNA-21 in the range from 5 to 150 nM, with the limit of detection (LOD) of 3.65 nM. The LOD of fluorescence detection mode is even lowered to 0.03 nM. The miRNA-21-spiked serum samples, as well as the actual serum samples from cancer patients, have been successfully detected by this detection strategy. Thus the established assay provides a POCT solution for cancer diagnosis and prognosis.

Multiplex Assay of Viruses Integrating Recombinase Polymerase Amplification, Barcode-Anti-Barcode Pairs, Blocking Anti-Primers, and Lateral Flow Assay

A multiplex assay based on recombinase polymerase amplification (RPA) and lateral flow test (LFT) is a desirable tool for many areas. This multiplex assay could be efficiently realized using single-stranded (ss) DNAs located in separate zones on the test strip and bound complementary ssDNA tags of double-stranded (ds) DNA amplicons. Here, we investigate how to enrich multiplex assay capabilities using ssDNAs. Bifunctional oligonucleotide probes integrating (1) a forward primer for RPA, (2) a C9 spacer to stop polymerase, and (3) a ssDNA tag for binding at test strip are developed. The amplicons have a unique individual ssDNA tag at one end and a universal label of fluorescein introducing through a reverse primer at the other end. A conjugate of gold nanoparticles (GNP) with antibodies to fluorescein is used to detect all amplicons. The remainder of primers after RPA interacting with GNP conjugate was found to be a limiting factor for sensitive and specific multiplex assay. The addition of anti-RPA-primers before the use of test strips was proposed to simply and effectively eliminate remaining primers. This approach was successfully applied for the detection of three priority plant RNA viruses: potato virus Y (PVY), -S (PVS) and potato leafroll virus (PLRV). The total time of the assay is 30 min. The multiplex RPA-LFT detected at least 4 ng of PVY per g of plant leaves, 0.04 ng/g for PVS, and 0.04 ng/g for PLRV. The testing of healthy and infected potato samples showed concordance between the developed assay and reverse transcription-polymerase chain reaction. Thus, the capabilities of the proposed universal modules (ssDNA anchors, bifunctional probes, and blocking anti-primers) for multiplex detection of RNA analytes with high specificity and sensitivity were demonstrated.

Ultra-sensitive and high efficiency detection of multiple non-small cell lung cancer-related miRNAs on a single test line in catalytic hairpin assembly-based SERS-LFA strip

Accurate quantification of multiple miRNAs biomarkers in body fluid is still a challenge for early screening of cancer. Herein, by catalytic hairpin assembly as a signal amplification strategy, we designed a novel surface-enhanced Raman scattering (SERS)-lateral flow assay (LFA) strip for ultrasensitive detection of miR-21 and miR-196a-5p in non-small cell lung cancer (NSCLC) urine on a single test (T) line. 4-mercaptobenzoic acid or 5,5'-dithiobis-2-nitrobenzoic acid as Raman molecules was labeled and two hairpin DNA sequence was modified gold nanocages (GNCs) were designed as two SERS tags. Through target miRNA-triggered catalytic hairpin assembly (CHA), the double-stranded DNAs (H1-H2 complex) formed by SERS tags and the related hairpin-structured DNA sequence 2 (H2) were immobilized on a single T line of SERS-LFA strip. This generated abundant "hot spots" because of the formation of numerous H1-H2 complex thus facilitated the SERS measurement. Through this method, two kinds of miRNAs were analyzed, resulting in limits of detection of 2.08 pM and 3.31 pM for miR-21 in PBS buffer and human urine, 1.77 pM and 2.18 pM for miR-196a-5p in PBS buffer and human urine. Significantly, the SERS-LFA strip exhibited high specificity and good repeatability toward miRNAs. The whole detection time was only 30 min, which means that the high detection efficiency of the strip. The clinical feasibility of the proposed method was also evaluated by detecting the levels of miR-21 and miR-196a-5p in urine samples from NSCLC patients and healthy subjects. The developed SERS-LFA strip has wide application prospect in biomedical research, drug development and early clinical diagnosis.

Padlock probe-based rolling circle amplification lateral flow assay for point-of-need nucleic acid detection

Sensitive, reliable and cost-effective detection of pathogens has wide ranging applications in clinical diagnostics and therapeutics, water and food safety, environmental monitoring, biosafety and epidemiology. Nucleic acid amplification tests (NAATs) such as PCR and isothermal amplification methods provide excellent analytical performance and significantly faster turnaround times than conventional culture-based methods. However, the inherent cost and complexity of NAATs limit their application in resource-limited settings and the developing world. To help address this urgent need, we have developed a sensitive method for nucleic acid analysis based on padlock probe rolling circle amplification (PLRCA), nuclease protection (NP) and lateral flow detection (LFA), referred to as PLAN-LFA, that can be used in resource-limited settings. The assay involves solution-phase hybridization of a padlock probe to target, sequence-specific ligation of the probe to form a circular template that undergoes isothermal rolling circle amplification in the presence of a polymerase and a labeled probe DNA. The RCA product is a long, linear concatenated single-stranded DNA that contains binding sites for the labeled probe. The sample is then exposed to a nuclease which selectively cleaves single-stranded DNA, the double-stranded labeled probe is protected from nuclease digestion and detected in a lateral flow immunoassay format to provide a visual, colorimetric readout of results. We have developed specific assays targeting beta-lactamase resistance gene for monitoring of antimicrobial resistance and Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2, the novel coronavirus discovered in 2019) using the PLAN-LFA platform. The assay provides a limit of detection of 1.1 pM target DNA (or 1.3 × 106 copies per reaction). We also demonstrate the versatility and robustness of the method by performing analysis on DNA and RNA targets, and perform analysis in complex sample matrices like saliva, plant tissue extract and bacterial culture without any sample pretreatment steps.

Nanotechnology in emerging liquid biopsy applications

Liquid biopsy is considered as the most attractive alternative to traditional tissue biopsies. The major advantages of this approach lie in the non-invasive procedure, the rapidness of sample collection and the potential for early cancer diagnosis and real-time monitoring of the disease and the treatment response. Nanotechnology has dynamically emerged in a wide range of applications in the field of liquid biopsy. The benefits of using nanomaterials for biosensing include high sensitivity and detectability, simplicity in many cases, rapid analysis, the low cost of the analysis and the potential for portability and personalized medicine. The present paper reports on the nanomaterial-based methods and biosensors that have been developed for liquid biopsy applications. Most of the nanomaterials used exhibit great analytical performance; moreover, extremely low limits of detection have been achieved for all studied targets. This review will provide scientists with a comprehensive overview of all the nanomaterials and techniques that have been developed for liquid biopsy applications. A comparison of the developed methods in terms of detectability, dynamic range, time-length of the analysis and multiplicity, is also provided.

Multichannel Paper Chip-Based Gas Pressure Bioassay for Simultaneous Detection of Multiple MicroRNAs

Simultaneous detection of multi-biomarkers not only enhances the accuracy of disease diagnosis but also improves detection efficiency and reduces cost. It is vital to achieve portable, simple, low-cost, and simultaneous detection of biomarkers for point-of-care (POC) diagnostics in a low-resource setting. Herein, a multichannel paper chip-based gas pressure bioassay was developed for the simultaneous detection of multiple biomarkers by combining multichannel paper chips with a portable gas pressure meter. Four DNA tetrahedral probes (DTPs) were used as capture probes and were immobilized in different detection zones of the paper chips to improve hybridization efficiency and reduce nonspecific adsorption. The formation of a sandwich structure between target microRNAs (miRNAs), the capture probe, and platinum nanoparticles (PtNPs)-modified complementary DNA (PtNPs-cDNA) transformed biomolecular recognition into quantitative detection of gas pressure. Four lung cancer-related miRNAs were detected simultaneously by a portable gas pressure meter. There is a good linear relationship between gas pressure and the logarithm of miRNA concentration in the range of 10 pM to 100 nM. Compared with single-stranded DNA capture probe, the signal-to-noise (S/N) of DNA tetrahedral probes improved more than 3 times. Using ring-oven washing, the unbound reagents in all channels of the paper chip were simultaneously and continuously washed away, leading to a more cheap, simple, and fast separation than magnetic separation. Therefore, it offers a promising multichannel paper chip-based gas pressure bioassay for portable and simultaneous detection of multiple biomarkers.

Ultrasensitive Detection of Nasopharyngeal Carcinoma-Related MiRNA through Garland Rolling Circle Amplification Integrated Catalytic Hairpin Assembly

MiRNA is reported to be closely related to nasopharyngeal carcinoma and has the potential to be a biomarker for the early diagnosis of nasopharyngeal carcinoma. However, the detection of miRNAs remains to be improved, given their complexity and low sensitivity. Herein, we propose here a novel miRNA detection method through the integration of garland RCA and CHA. In detail, the method is composed of two important signal amplification processes. For the first signal amplification process, the target miRNA could initiate garland RCA and then generate a nicking site on the products with the assistance of Nb.BbvCI enzymes. Afterward, a CHA process is induced with a designed H probe through the two signal amplification processes; the method exhibited a much-improved sensitivity. At last, we believe that this method is a promising approach capable of being applied in screening, diagnosing, and prognosticating multiple diseases.

Rapid developments in lateral flow immunoassay for nucleic acid detection

Recently, lateral flow assay (LFA) for nucleic acid detection has drawn increasing attention in the point-of-care testing fields. Due to its rapidity, easy implementation, and low equipment requirement, it is well suited for use in rapid diagnosis, food authentication, and environmental monitoring under source-limited conditions. This review will discuss two main research directions of lateral flow nucleic acid tests. The first one is the incorporation of isothermal amplification methods with LFA, which ensures an ultra-high testing sensitivity under non-laboratory conditions. The two most commonly used methodologies will be discussed, namely Loop-mediated Isothermal Amplification (LAMP) and Recombinase Polymerase Amplification (RPA), and some novel methods with special properties will also be introduced. The second research direction is the development of novel labeling materials. It endeavors to increase the sensitivity and quantifiability of LFA testing, where signals can be read and analyzed by portable devices. These methods are compared in terms of limits of detection, detection times, and quantifiabilities. It is anticipated that future research on lateral flow nucleic acid tests will focus on the integration of the whole testing process into a microfluidic system and the combination with molecular diagnostic tools such as clustered regularly interspaced short palindromic repeats to facilitate a rapid and accurate test.

Entropy-driven amplification strategy-assisted lateral flow assay biosensor for ultrasensitive and convenient detection of nucleic acids

Accurate, sensitive and rapid nucleic acid tests are important to implement timely treatment measures and control the spread of disease. Herein, we developed a novel portable platform for highly sensitive and specific detection of nucleic acids by integrating an entropy-driven amplification strategy into lateral flow assay (LFA) biosensor. We find that introducing an entropy-driven amplification strategy yields bright intensities on the test line of LFA stirp, which results in improved sensitivity for targeted nucleic acid detection. The developed LFA biosensor showed good reproducibility, specificity and sensitivity for target DNA and H1N1-RNA detection with a low detection limit of 1.43 pM and 2.02 pM, respectively. Its practical potential was also verified by detecting the target nucleic acid in human serum. More importantly, the design of an entropy-driven amplification strategy in this portable platform retained the convenient, rapid and low-cost characterizations of LFA biosensor due to the compact amplification principle and the elimination of enzyme use. Thus, we believe that this assay biosensor will certainly report its own position in the timely detection of nucleic acid, especially when the medical environment and resources are fewer.

Recent advances in sensitivity enhancement for lateral flow assay

Conventional lateral flow assay (LFA) is typically performed by observing the color changes in the test lines by naked eyes, which achieves considerable commercial success and has a significant impact on the fields of food safety, environment monitoring, disease diagnosis, and other applications. However, this qualitative detection method is not very suitable for low levels of disease biomarkers' detection. Although many nanomaterials are used as new labels for LFA, additional readers limit their application to some extent. Fortunately, a lot of work has been done for improving the sensitivity of LFA. In this review, currently reported LFA sensitivity enhancement methods with an objective evaluation are summarized, such as sample pretreatment, the change of flow rate, and label evolution, and future development direction and challenges of LFAs are discussed.

Recent advances in rolling circle amplification-based biosensing strategies-A review

Rolling circle amplification (RCA) is an efficient enzymatic isothermal reaction that using circular probe as a template to generate long tandem single-stranded DNA or RNA products under the initiation of short DNA or RNA primers. As a simplified derivative of natural rolling circle replication which synthesizes copies of circular nucleic acids molecules such as plasmids, RCA amplifies the circular template rapidly without thermal cycling and finds various applications in molecular biology. Compared with other amplification strategies, RCA has many obvious advantages. Firstly, because of the strict complementarity required in ligation of a padlock probe, it endows the RCA reaction with high specificity and can even be utilized to distinguish single base mismatches. Secondly, through the introduction of multiple primers, exponential amplification can be achieved easily and leads to a good sensitivity. Thirdly, RCA products can be customized by manipulating circular templates to generate functional nucleic acids such as aptamer, DNAzymes and restriction enzyme sites. Moreover, the RCA has good biocompatibility and is especially suitable for in situ detection. Therefore, RCA has attracted considerable attention as an efficient and potential tool for highly sensitive detection of biomarkers. Herein, we comprehensively introduce the fundamental principles of RCA technology, summarize it from three aspects including initiation mode, amplification mode and signal output mode, and discuss the recent application of RCA-based biosensor in this review.

Point-of-care cancer diagnostic devices: From academic research to clinical translation

Early and timely diagnosis of cancer plays a decisive role in appropriate treatment and improves clinical outcomes, improving public health. Significant advances in biosensor technologies are leading to the development of point-of-care (POC) diagnostics, making the testing process faster, easier, cost-effective, and suitable for on-site measurements. Moreover, the incorporation of various nanomaterials into the sensing platforms has yielded POC testing (POCT) platforms with enhanced sensitivity, cost-effectiveness and simplified detection schemes. POC cancer diagnostic devices provide promising platforms for cancer biomarker detection as compared to conventional in vitro diagnostics, which are time-consuming and require sophisticated instrumentation, centralized laboratories, and experienced operators. Current innovative approaches in POC technologies, including biosensors, smartphone interfaces, and lab-on-a-chip (LOC) devices are expected to quickly transform the healthcare landscape. However, only a few cancer POC devices (e.g. lateral flow platforms) have been translated from research laboratories to clinical care, likely due to challenges include sampling procedures, low levels of sensitivity and specificity in clinical samples, system integration and signal readout requirements. In this review, we emphasize recent advances in POC diagnostic devices for cancer biomarker detection and discuss the critical challenges which must be surmounted to facilitate their translation into clinical settings.

A label-free mass spectrometry detection of microRNA by signal switching from high-molecular-weight polynucleotides to highly sensitive small molecules

MicroRNAs (miRNAs) are associated with various cellular processes and have been recognized as potential biomarkers for many human diseases. The sensitive and accurate determination of miRNA expression levels in biological specimens is highly significant for understanding their biological functions and clinical diagnosis. Mass spectrometry (MS) has shown its potential to study bioactive molecules, however, direct MS analysis of miRNAs is often hampered by limited sensitivity. For sensitive detection of miRNAs, indirect methods are generally employed through the use of DNA probes labeled with peptides or metal elements. In this work, we proposed a novel MS-based label-free strategy for miRNA quantification. A dual-amplification system was developed by using a padlock probe containing the poly(thymine) sequence in combination with rolling circle amplification (RCA). The specific recognition of target miRNA by the padlock probes produced long single-stranded DNAs containing poly (adenine) segments, which guaranteed the specificity of detection and realized primary amplification. Then the RCA products were extracted and treated with acid to release a large number of free adenines as reporter molecules for secondary signal amplification. Overall, the quantification of target miRNA was carried out by signal switching from high-molecular-weight RCA products to highly sensitive small molecule of adenine. The developed method achieved a linear detection range from 200 amol to 100 fmol for miRNA-21 with a limit of detection of 50 amol, and successfully applied to detect endogenous miRNA-21 levels from lung cancer cells. Overall, the present study provides a sensitive, specific MS-based method for miRNA detection and holds great potential for further application of MS technology to detect other biomarkers in biomedical research and early clinical diagnosis.

An enzyme-free electrochemical biosensor based on target-catalytic hairpin assembly and Pd@UiO-66 for the ultrasensitive detection of microRNA-21

MicroRNA-21 (miR-21) has been widely investigated as important biomarkers for cancer diagnosis and treatment. Herein, a highly sensitive nonenzymatic electrochemical biosensor based on Pd@metal-organic frameworks (Pd@UiO-66) and target-catalytic hairpin assembly (CHA) with target recycling approach has been proposed for the detection of miR-21. The proposed biosensor integrates the efficient CHA strategy and excellent electrocatalytic performance of Pd@UiO-66 nanocomposites. The concentration of miRNA-21 is related to the amount of the adsorbed electrocatalyst, leading to the different electrochemical signals for readout towards paracetamol (AP). This biosensor shows a low limit of detection of 0.713 fM with the dynamic range of 20 fM -600 pM under the optimal experimental conditions, providing a powerful platform for detecting miR-21. Furthermore, the designed biochemical self-assembly strategy of this electrochemical biosensor is promising candidate for potential applications in the analysis of other important genetic biomarkers for early diagnosis of cancers.

High-sensitive sensing of plant microRNA by integrating click chemistry with an unusual on-bead poly(T)-promoted transcription amplification

MicroRNAs (miRNAs) act as pivotal regulators in plants. Therefore, sensing strategies with high specificity and high sensitivity are desired for plant miRNA analysis in order to unveil the exact biofunctions of miRNAs. Toward this goal, a fluorescent assay is developed based on a two-step signal amplification strategy. In the first step, target miRNA-templated cycling click nucleic acid ligation is employed for target recognition and amplification, the product of which can bind to magnetic microbeads (MBs) and introduce the T7 promoter sequence to the surface. In the second step, the poly(T) containing transcription template partially hybridizes with the T7 promoter sequence on the ligated strand and then regulates the on-bead transcription in a cycling manner with the participation of T7 RNA polymerase. Surprisingly, different from other reported templates, the poly(T) template improves the transcription efficiency to an unexpectedly high level by releasing ultra-long RNA chains in the reaction system. Finally, the RNA intercalating dye, RiboGreen, is utilized to specifically light up the as-produced RNA chains for low-background signal readout. Benefiting from the elaborate design, the detection limit of plant miRNA is down to ∼0.1 amol. This strategy provides a highly specific and highly sensitive platform for plant miRNA detection, which promises potential in the practical applications of miRNA-related biofunctions.

A fluorescence assay for microRNA let-7a by a double-stranded DNA modified gold nanoparticle nanoprobe combined with graphene oxide

A fluorescence switching platform was developed to monitor target microRNA let-7a by coupling dsDNA–AuNPs with the GO nanosheet.

Simultaneous Nucleic Acids Detection and Elimination of Carryover Contamination With Nanoparticles-Based Biosensor- and Antarctic Thermal Sensitive Uracil-DNA-Glycosylase-Supplemented Polymerase Spiral Reaction

The current report devised a novel isothermal diagnostic assay, termed as nanoparticle-based biosensor (NB)- and antarctic thermal sensitive uracil-DNA-glycosylase (ATSU)-supplemented polymerase spiral reaction (PSR; NB-ATSU-PSR). The technique merges enzymatic digestion of carryover contaminants and isothermal nucleic acid amplification technique (PSR) for simultaneous detection of nucleic acid sequences and elimination of carryover contamination. In particular, nucleic acid amplification and elimination of carryover contamination are conducted in a single pot and, thus, the use of a closed-tube reaction can remove undesired results due to carryover contamination. For demonstration purpose, Klebsiella pneumoniae is employed as the model to demonstrate the usability of NB-ATSU-PSR assay. The assay's sensitivity, specificity, and practical feasibility were successfully evaluated using the pure cultures and sputum samples. The amplification products were detectable from as little as 100 fg of genomic DNAs and from ~550 colony-forming unit (CFU) in 1 ml of spiked sputum samples. All K. pneumoniae strains examined were positive for NB-ATSU-PSR detection, and all non-K. pneumoniae strains tested were negative for the NB-ATSU-PSR technique. The whole process, including rapid template preparation (20 min), PSR amplification (60 min), ATSU treatment (5 min), and result reporting (within 2 min), can be finished within 90 min. As a proof-of-concept methodology, NB-ATSU-PSR technique can be reconfigured to detect various target nucleic acid sequences by redesigning the PSR primer set.

Specific discrimination and universal signal amplification for RNA detection by coupling toehold exchange with RCA through nucleolytic conversion of a structure-switched hairpin probe

Herein, we combined toehold exchange with ligation-free rolling circle amplification (RCA) by programming nucleolytic conversion of hairpin probe into sensors, allowed for both high specific recognition and universal signal amplification for RNA detection. The rational engineered HP ensured highly specific recognition based on toehold exchange and allowed the pre-formed circular template for RCA to be shared for different RNAs detection. Generally, detecting different RNA requires different circular template for signal amplification. In this paper, the circular template for RCA was independent of the sequences of the target, so the signal amplification system was an universal one for different RNAs detection. Taking miRNA let-7d as a model target, this method showed a wide linear range from 1 fM to 1 nM with a detection limit of 0.46 fM and exhibited a remarkable selectivity even in distinguishing homologous miRNAs with 1-nt or 2-nt difference. To evaluate the potential of the method, it was applied to analysis the let-7d concentration in human serum, total RNA, and cell lysates. In conclusion, we believe this method exhibits potential for both specific discrimination and universal signal amplification for RNA analysis in complex matrices.

Intracellular fluorometric determination of microRNA-21 by using a switch-on nanoprobe composed of carbon nanotubes and gold nanoclusters

A sensitive and rapid fluorometric "switch on" assay is described for the detection of microRNA-21. It is based on the use of a fluorescence resonance energy transfer pair consisting of lysozyme-modified gold nanoclusters (Lys-Au NCs) and carbon nanotubes (CNTs). The Lys-Au NCs can be synthesized by a microwave-assisted technique within 2.5 min. They were modified with the ss-DNA probe (a 22-mer) for microRNA-21. Once the ss-DNA associates with the CNTs due to π stacking, the orange-red fluorescence (with excitation/emission peaks at 500/610 nm) is quenched. Nevertheless, the quenched fluorescence can be recovered after addition of microRNA-21 because of the stronger affnity between ss-DNA and microRNA-21. On the basis of the fluorescence recovery at 610 nm caused by microRNA-21, the latter can be quantified in the 0.01 to 100 nM concentration range, with a 36 pM detection limit. The method was applied to the determination of microRNA-21 in spiked serum with recoveries ranging from 98.6% to 110.0%. It also enables normal and cancer cells to be differentiated by direct imaging of intracellular microRNA-21. Graphical abstract A sensitive "switch on" FRET-based fluorometric assay for microRNA-21 is described. It is based on the use of  lysozyme-modified gold nanoclusters (Lys-Au NCs) and carbon nanotubes (CNTs) as energy donor and energy acceptor, respectively.