A sandwich dipstick assay for ATP detection based on split aptamer fragments

Noncompetitive Determination of Small Analytes by Sandwich-Type Lateral Flow Assay Based on an Aptamer Kissing Complex

Here, we present the proof-of-concept of a lateral flow assay (LFA) that is capable of detecting small-molecule targets in a noncompetitive manner by deploying a sandwich-type format based on the aptamer kissing complex (AKC) strategy. A fluorescently labeled hairpin aptamer served as the signaling agent, while a specific RNA hairpin grafted onto the strip served as the capture element. The hairpin aptamer switched from an unfolded to a folded form in the presence of the target, resulting in kissing interactions between the loops of the reporter and the capture agents. This design triggered a target-dependent fluorescent signal at the test line. The AKC-based LFA was developed for the detection of adenosine, achieving a detection limit in the micromolar range. The assay revealed the presence of the same analyte in urine. The method also proved effective with another small molecule (theophylline). We believe that the AKC-based LFA approach could overcome many of the shortcomings associated with conventional signal-off methods and competitive processes.

Dual-signal fluorescence aptasensing system for adenosine triphosphate assisting by MoS2 nanosheets

Adenosine triphosphate (ATP) has an irreplaceable role in the maintenance of many physiological processes and biological functions, and can be employed as an indicator of many diseases. In this work, we constructed a simple and sensitive dual-signal fluorescence aptasensing system for ATP detection with berberine as the signal reporter, ATP-aptamer as the recognition unit and MoS2 nanosheets as the signal amplification. In the absence of ATP, berberine can bind to the single-stranded DNA (ssDNA) of ATP-aptamer and selectively assemble on the surface of MoS2 nanosheets, leading to the fluorescence quenching of bererbine based on the fluorescence resonance energy transfer, denoted by "OFF". Accordingly, the fluorescence anisotropy signal is enhanced due to restriction on rotate of the fluorescent probe and denoted as "ON". Conversely, in the presence of ATP, it specifically interacts with ATP-aptamer and switches the free-curled single-stranded of ATP-aptamer to the G-quadruplex structure of ATP-aptamer/ATP/berberine, causing the detachment from the surface of the MoS2 nanosheet. Accordingly, the fluorescence signal was reversed from "OFF" to "ON", and the fluorescence anisotropy signal was turned "ON" to "OFF". The developed aptasensing system achieved a desirable sensitivity of 40.0 nM with fluorescent mode, and of 20.8 nM with fluorescent anisotropic mode. The sensing system has demonstrated high quality detection performance in human serum sample, and obtained the satisfactory recovery results for fluorescent of 93.0-108.5%, fluorescent anisotropic of 96.4-106.7%.

A novel fluorescent biosensor based on affinity-enhanced aptamer-peptide conjugate for sensitive detection of lead(II) in aquatic products

Lead contamination is a major concern in food safety and, as such, many lead detection methods have been developed, especially aptamer-based biosensors. However, the sensitivity and environmental tolerance of these sensors require improvement. A combination of different types of recognition elements is an effective way to improve the detection sensitivity and environmental tolerance of biosensors. Here, we provide a novel recognition element, an aptamer-peptide conjugate (APC), to achieve enhanced affinity of Pb²⁺. The APC was synthesized from Pb²⁺ aptamers and peptides through clicking chemistry. The binding performance and environmental tolerance of APC with Pb²⁺ was studied through isothermal titration calorimetry (ITC); the binding constant (K_a) was 1.76*10⁶ M^-1, indicating that the APC's affinity was increased by 62.96% and 802.56% compared with the aptamers and peptides, respectively. Besides, APC demonstrated better anti-interference (K⁺) than aptamer and peptide. Through the molecular dynamics (MD) simulation, we found that more binding sites and stronger binding energy between APC with Pb²⁺are the reasons for higher affinity between APC with Pb²⁺. Finally, a carboxyfluorescein (FAM)-labeled APC fluorescent probe was synthesized and a fluorescent detection method for Pb²⁺ was established. The limit of detection of the FAM-APC probe was calculated to be 12.45 nM. This detection method was also applied to the swimming crab and showed great potential in real food matrix detection.

Improving the selective naked-eye detection of COVID-19 mediated by simultaneously using three different target oligonucleotides coated on plasmonic AuNPs/hexagonal Ag@AuNPs

The localized surface plasmon resonance (LSPR) phenomenon provides a versatile property in biosensor technology. This uncommon feature was utilized to produce a homogeneous optical biosensor to detect COVID-19 by the naked-eye readout. In this work, we synthesized two types of plasmonic nanoparticles: (i) AuNPs and (ii) hexagonal core-shell nanoparticles-Au shell on AgNPs (Au@AgNPs). We report herein the development of two colorimetric biosensors employing the efficient targeting and the binding ability for three regions of the COVID-19 genome, that is, S-gene, N-gene and E-gene, at the same time. Two AuNPs and Ag@AuNPs individually coated with three different targets oligonucleotide sequence (TOs) (AuNPs-TOs-mix and Ag@AuNPs-TOs-mix) for simultaneous detection of S-gene, N-gene and E-gene of the COVID-19 virus, using the LSPR and naked-eye methods in the laboratory and biological samples. The target COVID-19 genome RNA detected using the AuNPs-TOs-mix and Ag@AuNPs-TOs-mix can achieve the same sensitivity. The detection ranges by the AuNPs-TOs-mix and Ag@AuNPs-TOs-mix are both sufficiently improved in equal amounts in comparison to any of the AuNPs-TOs and Ag@AuNPs-TOs. The sensitivity of the current COVID-19 biosensors were 94% and 96% based on the number of positive samples detected for AuNPs-TOs-mix and Ag@AuNPs-TOs-mix, respectively. Moreover, all the real-time PCR confirmed negative samples obtained the same results by the biosensor; accordingly, the specificity of this approach got to 100%. The current study reports a selective, reliable, reproducible and visual 'naked-eye' detection of COVID-19, devoid of the requirement of any sophisticated instrumental techniques.Communicated by Ramaswamy H. Sarma.

Near-Infrared Fluorescent Nanoprobes for Adenosine Triphosphate-Guided Imaging in Cancer and Fatty Liver Mice

Adenosine triphosphate (ATP), as an indispensable biomolecule, is the main energy source of cells and is used as a marker for diseases such as cancer and fatty liver. It is of great significance to design a near-infrared fluorescent nanoprobe with excellent performance and apply it to various disease models. Here, a near-infrared fluorescent nanoprobe (ZIF-90@SiR) based on a zeolitic imidazole framework is proposed. The fluorescent nanoprobes are synthesized by encapsulating the dye (SiR) into the framework of ZIF-90. Upon the addition of ATP, the structure of the ZIF-90@SiR nanoprobe is disrupted and SiR is released to generate near-infrared fluorescence at 670 nm. In the process of ATP detection, ZIF-90@SiR shows high sensitivity and good selectivity. Moreover, the ZIF-90@SiR nanoprobe has good biocompatibility due to its low toxicity to cells. It is used for fluorescence imaging of ATP in living cells and thus distinguishing normal cells and cancer cells, as well as distinguishing fatty liver cells. Due to excellent near-infrared fluorescence properties, the ZIF-90@SiR nanoprobe can not only distinguish normal mice and tumor mice but also differentiate normal mice and fatty liver mice for the first time.

In Situ Monitoring of Intracellular ATP Variation Based on a Thermoregulated Polymer Nanocomposite

It is necessary to develop reliable chemiluminescence strategies for determination of intracellular adenosine triphosphate (ATP), which is vital in life science and clinical diagnosis. However, the current chemiluminescence methods based on firefly luciferase suffered from low delivery efficiency, unsatisfied targeting performance, and autohydrolysis in living biosystem. To circumvent these drawbacks, a thermoresponsive polymer nanocomposite modified with firefly luciferase and ATP aptamer (PFLNC@aptamer) was fabricated, which targeted ATP and determined the intracellular ATP levels via measuring the chemiluminescence signals at different temperatures. The PFLNC@aptamer exhibited capability for the enzymolysis efficiency regulation, increased 21.0% with temperature change from 37.0 to 25.0 °C. The ATP detection limit was 3.3 nM with a linear relationship from 10.0 nM to 0.1 mM. Moreover, the thermoresponsive nanocomposite could also effectively avoid the interference during delivering firefly luciferase into the living cells and effectively discriminate ATP via the immobilized ATP aptamer, which further confirmed its reliability for practical applications. It paves a specific avenue for effective intracellular ATP monitoring in fundamental and applied research.

Capture-SELEX: Selection Strategy, Aptamer Identification, and Biosensing Application

Small-molecule contaminants, such as antibiotics, pesticides, and plasticizers, have emerged as one of the substances most detrimental to human health and the environment. Therefore, it is crucial to develop low-cost, user-friendly, and portable biosensors capable of rapidly detecting these contaminants. Antibodies have traditionally been used as biorecognition elements. However, aptamers have recently been applied as biorecognition elements in aptamer-based biosensors, also known as aptasensors. The systematic evolution of ligands by exponential enrichment (SELEX) is an in vitro technique used to generate aptamers that bind their targets with high affinity and specificity. Over the past decade, a modified SELEX method known as Capture-SELEX has been widely used to generate DNA or RNA aptamers that bind small molecules. In this review, we summarize the recent strategies used for Capture-SELEX, describe the methods commonly used for detecting and characterizing small-molecule-aptamer interactions, and discuss the development of aptamer-based biosensors for various applications. We also discuss the challenges of the Capture-SELEX platform and biosensor development and the possibilities for their future application.

Recent developments of aptamer-based lateral flow assays for point-of-care (POC) diagnostics

In the field of lateral flow assay (LFA), the application of aptamer as a bioreceptor has been implemented to overcome the limitations of antibodies, such as tedious in vivo processes, short shelf-life, and functionalization issues. To address these limitations aptamer-based LFA (ALFA) is preferred to antibody-based LFA that produces higher sensitivity and specificity. In principle, aptamers have a strong affinity towards their targets like small, large, and non-immunogenic molecules because of their high affinity, sensitivity, low dissociation constant, cost-effectiveness, and flexible nature. Thus, ALFA can be considered an efficient biosensor model for its superior portability, rapid detection with quick turnaround time, and usability by a non-technical person at any location with simple visual output. This review concisely overviews ALFA, its principles, formats, aptamer selection process, and biomedical applications. In addition, the critical components to design, develop, test, and amplify signals to create ALFA are discussed in brief. In addition, the aspects of conceptualization of ALFA product transforming from bench-side laboratory design and fabrication to commercial market are addressed in detail.

Split aptamer acquisition mechanisms and current application in antibiotics detection: a short review

Antibiotic contamination is becoming a prominent global issue. Therefore, sensitive, specific and simple technology is desirable the demand for antibiotics detection. Biosensors based on split aptamer has gradually attracted extensive attention for antibiotic detection due to its higher sensitivity, lower cost, false positive/negative avoidance and flexibility in sensor design. Although many of the reported split aptamers are antibiotics aptamers, the acquisition and mechanism of splitting is still unknow. In this review, six reported split aptamers in antibiotics are outlined, including Enrofloxacin, Kanamycin, Tetracycline, Tobramycin, Neomycin, Streptomycin, which have contributed to promote interest, awareness and thoughts into this emerging research field. The study introduced the pros and cons of split aptamers, summarized the assembly principle of split aptamer and discussed the intermolecular binding of antibiotic-aptamer complexes. In addition, the recent application of split aptamers in antibiotic detection are introduced. Split aptamers have a promising future in the design and development of biosensors for antibiotic detection in food and other field. The development of the antibiotic split aptamer meets many challenges including mechanism discovery, stability improvement and new biosensor development. It is believed that split aptamer could be a powerful molecular probe and plays an important role in aptamer biosensor.

Core-satellite assembly of gold nanoshells on solid gold nanoparticles for a color coding plasmonic nanosensor

We present core-satellite assemblies comprising a solid gold nanoparticle as the core and hollow decahedral gold nanoshells as satellites for tuning the optical properties of the plasmonic structure for sensing. The core-satellite assemblies were fabricated on a substrate via the layer-by-layer assembly of nanoparticles linked by DNA. We used finite-difference time-domain simulations to help guide the geometrical design, and characterized the optical properties and morphology of the solid-shell nanoparticle assemblies using darkfield microscopy, single-nanostructure spectroscopy, and scanning electron microscopy. Plasmon coupling yielded resonant peaks at longer wavelengths in the red to near-infrared range for solid-shell assemblies compared with solid-solid nanoparticle assemblies. We examined sensing with the solid-shell assemblies using adenosine triphosphate (ATP) as a model target and ATP-aptamer as the linker. Binding of ATP induced disassembly and led to a decrease in the scattering intensity and a color change from red to green. The new morphology of the core-satellite assembly enabled plasmonic color-coding of multiplexed sensors. We demonstrate this potential by fabricating two types of assemblies using DNA linkers that target different molecules - ATP and a model nucleic acid. Our work expands the capability of chip-based plasmonic nanoparticle assemblies for the analysis of multiple, different types of biomolecules in small sample sizes including the microenvironment and single cells.

A Cooperatively Activatable DNA Nanoprobe for Cancer Cell-Selective Imaging of ATP

DNA-based nanoprobes have attracted extensive interest in the field of bioanalysis. Notably, engineered DNA nanoprobes that can respond to multiple pathological parameters are desirable to detect targets precisely. Here we design a split aptamer/DNAzyme (aptazyme)-based DNA probe for fluorescence detection of ATP and further develop a cooperatively activatable DNA nanoprobe for tumor-specific imaging of ATP in vivo. The DNA nanoprobes comprising split aptazyme-coated MnO₂ nanovectors have high stability and are synergistically activated by multiple biomarkers, GSH and ATP. Upon stimuli by overexpressed GSH in tumor cells, this DNA nanoprobe can release the aptazyme and self-supply cofactor Mn²⁺ of the DNAzyme. Sequentially, intracellular ATP induces the proper folding of the split ATP aptamer and Mn²⁺-dependent DNAzyme, which activates the specific cleavage of substrate and generates the optical readout signal. This nanoprobe exhibits remarkable resistance to enzymatic degradation, satisfactory biosafety, identifies ATP specifically within cancer cells, and selectively lights up solid tumors. Our research provides a reliable method for ATP imaging in cancer cells and opens a new avenue for biochemical research and highly accurate disease diagnosis.

Oligonucleotide aptamers for pathogen detection and infectious disease control

During an epidemic or pandemic, the primary task is to rapidly develop precise diagnostic approaches and effective therapeutics. Oligonucleotide aptamer-based pathogen detection assays and control therapeutics are promising, as aptamers that specifically recognize and block pathogens can be quickly developed and produced through simple chemical synthesis. This work reviews common aptamer-based diagnostic techniques for communicable diseases and summarizes currently available aptamers that target various pathogens, including the SARS-CoV-2 virus. Moreover, this review discusses how oligonucleotide aptamers might be leveraged to control pathogen propagation and improve host immune system responses. This review offers a comprehensive data source to the further develop aptamer-based diagnostics and therapeutics specific for infectious diseases.

Advances and Challenges in Small-Molecule DNA Aptamer Isolation, Characterization, and Sensor Development

Aptamers are short oligonucleotides isolated in vitro from randomized libraries that can bind to specific molecules with high affinity, and offer a number of advantages relative to antibodies as biorecognition elements in biosensors. However, it remains difficult and labor-intensive to develop aptamer-based sensors for small-molecule detection. Here, we review the challenges and advances in the isolation and characterization of small-molecule-binding DNA aptamers and their use in sensors. First, we discuss in vitro methodologies for the isolation of aptamers, and provide guidance on selecting the appropriate strategy for generating aptamers with optimal binding properties for a given application. We next examine techniques for characterizing aptamer-target binding and structure. Afterwards, we discuss various small-molecule sensing platforms based on original or engineered aptamers, and their detection applications. Finally, we conclude with a general workflow to develop aptamer-based small-molecule sensors for real-world applications.

Development of nucleic acid aptamer-based lateral flow assays: A robust platform for cost-effective point-of-care diagnosis

Lateral flow assay (LFA) has made a paradigm shift in the in vitro diagnosis field due to its rapid turnaround time, ease of operation and exceptional affordability. Currently used LFAs predominantly use antibodies. However, the high inter-batch variations, error margin and storage requirements of the conventional antibody-based LFAs significantly impede its applications. The recent progress in aptamer technology provides an opportunity to combine the potential of aptamer and LFA towards building a promising platform for highly efficient point-of-care device development. Over the past decades, different forms of aptamer-based LFAs have been introduced for broad applications ranging from disease diagnosis, agricultural industry to environmental sciences, especially for the detection of antibody-inaccessible small molecules such as toxins and heavy metals. But commercial aptamer-based LFAs are still not used widely compared with antibodies. In this work, by analysing the key issues of aptamer-based LFA design, including immobilization strategies, signalling methods, and target capturing approaches, we provide a comprehensive overview about aptamer-based LFA design strategies to facilitate researchers to develop optimised aptamer-based LFAs.

Point-of-Need Diagnostics for Foodborne Pathogen Screening

Foodborne illness is a major public health issue that results in millions of global infections annually. The burden of such illness sits mostly with developing countries, as access to advanced laboratory equipment and skilled lab technicians, as well as consistent power sources, is limited and expensive. Current gold standards in foodborne pathogen screening involve labor-intensive sample enrichment steps, pathogen isolation and purification, and costly readout machinery. Overall, time to detection can take multiple days, excluding the time it takes to ship samples to off-site laboratories. Efforts have been made to simplify the workflow of such tests by integrating multiple steps of foodborne pathogen screening procedures into a singular device, as well as implementing more point-of-need readout methods. In this review, we explore recent advancements in developing point-of-need devices for foodborne pathogen screening. We discuss the detection of surface markers, nucleic acids, and metabolic products using both paper-based and microfluidic devices, focusing primarily on developments that have been made between 2015 and mid-2020.

Engineering a Reversible Fluorescent Probe for Real-Time Live-Cell Imaging and Quantification of Mitochondrial ATP

Real-time imaging and quantification of adenosine triphosphate (ATP) fluctuation in cells are significant for understanding the relationship between energy metabolism and cell functions. However, few synthetic fluorescent probes have been reported to tackle this challenge due to lack of accurate fluorescence readout and suitable response concentration. Herein we designed and synthesized a ratiometric fluorescent probe (Rh6G-ACFPN) for quantitatively detecting the fluctuation of mitochondrial ATP in living cells. Rh6G-ACFPN selectively and reversibly responds to ATP with an ideal dissociation constant (K_d) of 4.65 mM (3-10 mM: the range of mitochondrial ATP concentrations). Live-cell imaging allows us to directly monitor the dynamic changes of mitochondrial ATP in high temporal resolution. Moreover, for the first time, mitochondrial ATP in normal and cancer cells lines was successfully quantified and discriminated. These results demonstrate the versatility of Rh6G-ACFPN as a useful imaging tool to elucidate the function of mitochondrial ATP in living cells.

Lateral flow assay using aptamer-based sensing for on-site detection of dopamine in urine

A lateral flow assay using DNA aptamer-based sensing for the detection of dopamine in urine is reported. Dopamine duplex aptamers (hybridized sensor with capture probe) are conjugated to 40-nm Au nanoparticles (AuNPs) with 20T linkers. The detection method is based on the dissociation of the duplex aptamer in the presence of dopamine, with the sensor part undergoing conformational changes and being released from the capture part. Hybridization between the complementary DNA in the test line and the conjugated AuNP-capture DNA produces a red band, whose intensity is related to the dopamine concentration. The minimum detectable concentration obtained by ImageJ analysis was <10 ng/mL (65.2 nM), while the visual limit of detection is estimated to be ~50 ng/mL (normal range of dopamine in urine of 52-480 ng/mL or 0.3-3.13 μM). No cross reactivity to other stress biomarkers in urine was confirmed. These results indicate that this robust and user-friendly point-of-care biosensor has significant potential for providing a cost-effective alternative for dopamine detection in urine.

Advances in functional nucleic acid based paper sensors

This article provides an extensive review of paper-based sensors that utilize functional nucleic acids, particularly DNA aptamers and DNAzymes, as recognition elements.

An aptamer-based colorimetric lateral flow assay for the detection of human epidermal growth factor receptor 2 (HER2)

An aptamer-based colorimetric lateral flow assay was developed for the detection of human epidermal growth factor receptor 2 (HER2). In this study, two approaches were examined using HER2 binding aptamers and gold nanoparticles. The first method used was a solution-based adsorption-desorption colorimetric approach wherein aptamers were adsorbed onto the gold nanoparticle surface. Upon the addition of HER2, HER2 binds specifically with its aptamer, releasing the gold nanoparticles. Addition of NaCl then induces the formation of gold nanoparticle aggregates. This leads to a color change from red to blue and a detection limit of 10 nM was achieved. The second method used an adsorption-desorption colorimetric lateral flow assay approach wherein biotin-modified aptamers were adsorbed onto the gold nanoparticle surface in the absence of HER2. In the presence of HER2, HER2 specifically binds with its aptamer leading to release of the gold nanoparticles. These solutions were applied to the lateral flow assay format and a detection limit of 20 nM was achieved. Both colorimetric and lateral flow assays are inexpensive, simple, rapid to perform and produce results visible to the naked-eye.

Novel strategy to improve the sensing performances of split ATP aptamer based fluorescent indicator displacement assay through enhanced molecular recognition

Split aptamer strategy was often used to improve the sensitivity of aptasensor. However, traditional split aptamer strategy can not be directly used to improve the label-free aptamer based Thioflavin T (ThT) displacement assay for ATP because the split ATP aptamer display much lower enhancement effects on the fluorescence of ThT than intact aptamer. In order to address this issue, this is the first report using G-rich DNA sequence to enhance the affinity of the two split ATP aptamer halves to ThT and offer lower limit of detection (LOD), wider linear range and higher selectivity through the enhanced molecular recognition. Compared to the intact aptamer/ThT complex, the ensemble of two G-rich split ATP aptamer fragments/ThT are higher fluorescent. Consequently, G-rich sequences would improve the fluorescent signal and thus the sensing performance of the proposed assay. In the optimized conditions, the LOD of the proposed fluorescent ATP aptasensor is 2 nM, which is lower than the reported ThT/ATP aptamer based methods. Additionally, our aptasensor has a wider dynamic linear range (0.1 μM - 120 μM) and higher selectivity. The proposed aptasensor has been successfully applied to detect ATP in 15% human serum. More importantly, the current study not only provides a novel method for ATP assay but also presents a way to construct a label-free split aptamer based fluorescent sensor for other species where aptamer can be generated.

A colorimetric ATP assay based on the use of a magnesium(II)-dependent DNAzyme

A colorimetric assay for ATP is described that uses a strategy that combines the concept of split Mg(II)-dependent DNAzyme, split aptamer, and hybridization-induced aggregation of gold nanoparticles (AuNPs). Both ATP aptamer and Mg(II)-dependent DNAzyme are split into two fragments which are allocated to two well-designed DNA probes. The probes also possess mutually complementary stem sequences and spacer sequences. In the presence of ATP, the separated DNAzyme sequences in the two probes assemble via the synchronous recognition of ATP with two fragments of the aptamer. Then, the activated DNAzyme catalyzes multiple cycles of the cleavage of its substrate DNA sequence. The latter acts as a linker and induces the aggregation of two types of ssDNA-modified AuNP through the hybridization between the complementary sequences. Thus, the color of the AuNP solution remains red. However, in the absence of ATP, the detached aptamer cannot induce the assembly of DNAzyme to cleave the linker DNA. This results in the aggregation of AuNP and a concomitant color transition from red to purple. This ATP assay, performed at a wavelength of 530 nm, has a linear detection range that extends from 10 pM to 100 nM, with a detection limit of 5.3 pM. It was applied to the detection of ATP in human serum. Conceivably, the strategy has a wide scope in that it may be applied to the colorimetric detection of various other analytes through the split aptamer configuration. Graphical abstract Schematic presentation of colorimetric assay for adenosine triphosphate (ATP) based on the use of a split Mg(II)-dependent DNAzyme, a split aptamer, and by exploiting the hybridization-induced aggregation of gold nanoparticles that leads to a color change from red to purple.

An aptamer and functionalized nanoparticle-based strip biosensor for on-site detection of kanamycin in food samples

A lateral flow strip biosensor for fast, sensitive, low-cost and on-site detection of kanamycin was developed by using kanamycin-specific aptamer-modified gold nanoparticles (AuNPs-apt) as a probe and oligonucleotide DNA1-modified silver nanoparticles (AgNPs-DNA1) as a signal amplification element. Through the complementary sequences of DNA1 and the aptamer, the AgNP-DNA1-apt-AuNPs complex can be formed and further captured on the test zone of the strip, where a capture probe DNA2 complementary to the 3'-terminal of DNA1 was immobilized. In the presence of kanamycin, it can competitively bind to the aptamer, and then inhibit the formation of the complex and the accumulation of AuNPs on the test zone. AuNPs-apt can finally be captured on the control zone via the specific binding between biotin and streptavidin. The assay avoids multiple incubation and washing steps and can be completed within 10 min. By observing the color change of the test zone, a qualitative detection for kanamycin can be achieved by the naked eye, with the visual limit of 35 nM. Meanwhile, a linear detection range of 1-30 nM with a low detection limit of 0.0778 nM for quantitative analysis can be achieved by using a scanning reader. The lateral flow strip biosensor exhibited high specificity and stability. Moreover, it was applied to detect kanamycin in various food samples, indicating its great potential in field testing.

How to Improve Sensitivity of Sandwich Lateral Flow Immunoassay for Corpuscular Antigens on the Example of Potato Virus Y?

A simple approach was proposed to decrease the detection limit of sandwich lateral flow immunoassay (LFIA) by changing the conditions for binding between a polyvalent antigen and a conjugate of gold nanoparticles (GNPs) with antibodies. In this study, the potato virus Y (PVY) was used as the polyvalent antigen, which affects economically important plants in the Solanaceae family. The obtained polyclonal antibodies that are specific to PVY were characterized using a sandwich enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). For LFIA, the antibodies were conjugated with GNPs with a diameter of 17.4 ± 1.0 nm. We conducted LFIAs using GNP conjugates in a dried state on the test strip and after pre-incubation with a sample. Pre-incubating the GNP conjugates and sample for 30 s was found to decrease the detection limit by 60-fold from 330 ng∙mL^-1 to 5.4 ng∙mL^-1 in comparison with conventional LFIA. The developed method was successfully tested for its ability to detect PVY in infected and uninfected potato leaves. The quantitative results of the proposed LFIA with pre-incubation were confirmed by ELISA, and resulted in a correlation coefficient of 0.891. The proposed approach is rapid, simple, and preserves the main advantages of LFIA as a non-laboratory diagnostic method.

A Personal Glucose Meter for Label-Free and Washing-Free Biomolecular Detection

We developed a label-free and washing-free method for biomolecular detection using a personal glucose meter (PGM). ATP was selected as a model target, and cascade enzymatic reactions promoted by hexokinase and pyruvate kinase were adopted to link the amount of ATP to glucose that is detectable by a hand-held PGM. In principle, the presence of target ATP enables hexokinase to catalyze the conversion of glucose to glucose 6-phosphate by providing a phosphate group to glucose, and thus the amount of glucose is decreased in proportion to the amount of ATP. In addition, adenosine 5'-diphosphate (ADP), which is generated after hexokinase-catalyzed enzymatic reaction, is recovered to ATP by a pyruvate kinase enzyme. The regenerated ATP is again supplemented to catalyze multiple rounds of cascade enzymatic reactions, leading to signal amplification. As a result, the change of glucose amount that is inversely proportional to ATP amount is simply measured by a hand-held PGM. By employing this strategy, we successfully determined ATP down to 49 nM with high selectivity even in real samples such as tap water, human serum, and bovine urine. Importantly, the developed system does not require expensive modification and washing steps but is conveniently operated with a commercially available PGM, which would pave the way for the development of a simple and cost-effective sensing platform.

A Novel Fluorescent Biosensor for Adenosine Triphosphate Detection Based on a Metal⁻Organic Framework Coating Polydopamine Layer

In this work, a novel and sensitive fluorescent biosensor based on polydopamine coated Zr-based metal–organic framework (PDA/UiO-66) is presented for adenosine triphosphate (ATP) detection. This PDA/UiO-66 nanoparticle which holds a great potential to be excellent fluorescence quencher can protect the 6-carboxyfluorescein (FAM)-labeled probe from cleaved by DNase I dispersed in solution and the flurescence of labeled FAM is quenched. When ATP molecules exist, aptamers on the PDA/UiO-66 nanoparticles can hybridize with ATP molecule to form complex structure that will be desorbed from the PDA/UiO-66 and digested by DNase I. After that, the released ATP molecule can react with another aptamer on the PDA/UiO-66 complexes, then restarts a new cycle. Herein, the excellent strong fluorescence quenching ability and uploading more amount of aptamer probes of PDA/UiO-66 composites make them efficient biosensors, leading to a high sensitivity with detection limit of 35 nM. Compared with ATP detection directly by UiO-66-based method, the LOD is about 5.7 times higher with PDA/UiO-66 nanoparticle. Moreover, the enhanced biocompatibility and bioactivity with PDA layer of the composites render a proposed strategy for clinical diagnosis field of detecting small biological molecules in vivo in the future.

Triple-helix molecular switch-based aptasensors and DNA sensors

Utilization of traditional analytical techniques is limited because they are generally time-consuming and require high consumption of reagents, complicated sample preparation and expensive equipment. Therefore, it is of great interest to achieve sensitive, rapid and simple detection methods. It is believed that nucleic acids assays, especially aptamers, are very important in modern life sciences for target detection and biological analysis. Aptamers and DNA-based sensors have been widely used for the design of various sensors owing to their unique features. In recent years, triple-helix molecular switch (THMS)-based aptasensors and DNA sensors have been broadly utilized for the detection and analysis of different targets. The THMS relies on the formation of DNA triplex via Watson-Crick and Hoogsteen base pairings under optimal conditions. This review focuses on recent progresses in the development and applications of electrochemical, colorimetric, fluorescence and SERS aptasensors and DNA sensors, which are based on THMS. Also, the advantages and drawbacks of these methods are discussed.

A Lateral Flow Strip Based Aptasensor for Detection of Ochratoxin A in Corn Samples

Ochratoxin A (OTA) is a mycotoxin identified as a contaminant in grains and wine throughout the world, and convenient, rapid and sensitive detection methods for OTA have been a long-felt need for food safety monitoring. Herein, we presented a new competitive format based lateral flow strip fluorescent aptasensor for one-step determination of OTA in corn samples. Briefly, biotin-cDNA was immobilized on the surface of a nitrocellulose filter on the test line. Without OTA, Cy5-labeled aptamer combined with complementary strands formed a stable double helix. In the presence of OTA, however, the Cy5-aptamer/OTA complexes were generated, and therefore less free aptamer was captured in the test zone, leading to an obvious decrease in fluorescent signals on the test line. The test strip showed an excellent linear relationship in the range from 1 ng·mL⁻¹ to 1000 ng·mL⁻¹ with the LOD of 0.40 ng·mL⁻¹, IC₁₅ value of 3.46 ng·mL⁻¹ and recoveries from 96.4% to 104.67% in spiked corn samples. Thus, the strip sensor developed in this study is an acceptable alternative for rapid detection of the OTA level in grain samples.

A fluorescence aptasensor based on two-dimensional sheet metal-organic frameworks for monitoring adenosine triphosphate

In the present study, a facile fluorescence aptasensor based on two-dimensional sheet metal-organic frameworks of N,N-bis(2-hydroxyethyl)dithiooxamidato copper(II) (H₂dtoaCu) was developed for the sensitive detection of adenosine triphosphate (ATP). The sensing mechanism was based on the noncovalent interaction between FAM-labeled (fluorescein amidite) ATP aptamers and H₂dtoaCu. In the absence of ATP, the FAM-labeled aptamer readily adsorbs onto H₂dtoaCu, mainly via π-π stacking and hydrogen bond interactions between the nucleotide bases and the H₂dtoaCu surface, leading to the reduction of fluorescence intensity of the FAM by photoinduced electron transfer (PET). In the presence of ATP, the FAM-labeled aptamer specifically forms ATP-binding aptamer complexes which exhibit only weak adsorption on the H₂dtoaCu surface. Thus, the fluorescence of the FAM-labeled ATP aptamer remained largely unchanged. The fluorescence aptasensor exhibited a good linear relationship between the fluorescence intensity and the logarithm concentration of ATP over a range of 25-400 nM, with a detection limit of 8.19 nM (3S/N). ATP analogs such as guanosine triphosphate, uridine triphosphate, and cytidine triphosphate have negligible effect on the aptasensor performance due to the high selectivity of the ATP aptamer to its target, showing promising potential in real sample analysis.

Fluorescence Resonance Energy Transfer-Based DNA Nanoprism with a Split Aptamer for Adenosine Triphosphate Sensing in Living Cells

We have developed a DNA nanoprobe for adenosine triphosphate (ATP) sensing in living cells, based on the split aptamer and the DNA triangular prism (TP). In which nucleic acid aptamer was split into two fragments, the stem of the split aptamer was respectively labeled donor and acceptor fluorophores that underwent a fluorescence resonance energy transfer if two ATP molecules were bound as target molecule to the recognition module. Hence, ATP as a target induced the self-assembly of split aptamer fragments and thereby brought the dual fluorophores into close proximity for high fluorescence resonance energy transfer (FRET) efficiency. In the in vitro assay, an almost 5-fold increase in F_A/F_D signal was observed, the fluorescence emission ratio was found to be linear with the concentration of ATP in the range of 0.03-2 mM, and the nanoprobe was highly selective toward ATP. For the strong protecting capability to nucleic acids from enzymatic cleavage and the excellent biocompatibility of the TP, the DNA TP nanoprobe exhibited high cellular permeability, fast response, and successfully realized "FRET-off" to "FRET-on" sensing of ATP in living cells. Moreover, the intracellular imaging experiments indicated that the DNA TP nanoprobe could effectively detect ATP and distinguish among changes of ATP levels in living cells. More importantly, using of the split aptamer and the FRET-off to FRET-on sensing mechanism could efficiently avoid false-positive signals. This design provided a strategy to develop biosensors based on the DNA nanostructures for intracellular molecules analysis.