Selection, characterization, and biosensing application of high affinity congener-specific microcystin-targeting aptamers

Development of a Multiplexed Electrochemical Aptasensor for the Detection of Cyanotoxins

In this study, we report a multiplexed platform for the simultaneous determination of five marine toxins. The proposed biosensor is based on a disposable electrical printed (DEP) microarray composed of eight individually addressable carbon electrodes. The electrodeposition of gold nanoparticles on the carbon surface offers high conductivity and enlarges the electroactive area. The immobilization of thiolated aptamers on the AuNP-decorated carbon electrodes provides a stable, well-orientated and organized binary self-assembled monolayer for sensitive and accurate detection. A simple electrochemical multiplexed aptasensor based on AuNPs was designed to synchronously detect multiple cyanotoxins, namely, microcystin-LR (MC-LR), Cylindrospermopsin (CYL), anatoxin-α, saxitoxin and okadaic acid (OA). The choice of the five toxins was based on their widespread presence and toxicity to aquatic ecosystems and humans. Taking advantage of the conformational change of the aptamers upon target binding, cyanotoxin detection was achieved by monitoring the resulting electron transfer increase by square-wave voltammetry. Under the optimal conditions, the linear range of the proposed aptasensor was estimated to be from 0.018 nM to 200 nM for all the toxins, except for MC-LR where detection was possible within the range of 0.073 to 150 nM. Excellent sensitivity was achieved with the limits of detection of 0.0033, 0.0045, 0.0034, 0.0053 and 0.0048 nM for MC-LR, CYL, anatoxin-α, saxitoxin and OA, respectively. Selectivity studies were performed to show the absence of cross-reactivity between the five analytes. Finally, the application of the multiplexed aptasensor to tap water samples revealed very good agreement with the calibration curves obtained in buffer. This simple and accurate multiplexed platform could open the window for the simultaneous detection of multiple pollutants in different matrices.

Selection, characterization, and biosensing applications of DNA aptamers targeting cyanotoxin BMAA

Scientists have established a connection between environmental exposure to toxins like β-N-methylamino-l-alanine (BMAA) and a heightened risk of neurodegenerative disorders. BMAA is a byproduct from certain strains of cyanobacteria that are present in ecosystems worldwide and is renowned for its bioaccumulation and biomagnification in seafood. The sensitivity, selectivity, and reproducibility of the current analytical techniques are insufficient to support efforts regarding food safety and environment monitoring adequately. This work outlines the in vitro selection of BMAA-specific DNA aptamers via the systematic evolution of ligands through exponential enrichment (SELEX). Screening and characterization of the full-length aptamers was achieved using the SYBR Green (SG) fluorescence displacement assay. Aptamers BMAA_159 and BMAA_165 showed the highest binding affinities, with dissociation constants (Kd) of 2.2 ± 0.1 μM and 0.32 ± 0.02 μM, respectively. After truncation, the binding affinity was confirmed using a BMAA-conjugated fluorescence assay. The Kd values for BMAA_159_min and BMAA_165_min were 6 ± 1 μM and 0.63 ± 0.02 μM, respectively. Alterations in the amino proton region studied using solution nuclear magnetic resonance (NMR) provided further evidence of aptamer-target binding. Additionally, circular dichroism (CD) spectroscopy revealed that BMAA_165_min forms hybrid G-quadruplex (G4) structures. Finally, BMAA_165_min was used in the development of an electrochemical aptamer-based (EAB) sensor that accomplished sensitive and selective detection of BMAA with a limit of detection (LOD) of 1.13 ± 0.02 pM.

A Single Set of Well-Designed Aptamer Probes for Reliable On-site Qualitative and Ultra-Sensitive Quantitative Detection

Aptamer-based probes are pivotal components in various sensing strategies, owing to their exceptional specificity and versatile programmable structure. Nevertheless, numerous aptamer-based probes usually offer only a single function, limiting their capacity to meet the diverse requirements of multi-faceted sensing systems. Here, we introduced supersandwich DNA probes (SSW-DNA), designed and modified on the outer surface of nanochannels with hydrophobic inner walls, enabling dual functionality: qualitative detection for on-site analysis and quantitative detection for precise analysis. The fragmented DNAs resulting from the target recognition, are subsequently identified through lateral flow assays, enabling robust on-site qualitative detection of microcystin-LR with an impressively low limit of detection (LOD) at 0.01 μg/L. Meanwhile, the nanochannels enable highly sensitive quantification of microcystin-LR through the current analysis, achieving an exceptionally low LOD at 2.5×10-7  μg/L, with a broad dynamic range spanning from 1×10-6 to 1×102  μg/L. Furthermore, the process of target recognition introduces just a single potential error propagation, which reduces the overall risk of errors during the entire qualitative and quantitative detection process. This sensing strategy broadens the scope of applications for aptamer-based composite probes, holding promising implications across diverse fields, such as medical diagnosis, food safety, and environmental protection.

Application of aptamer-conjugated graphene oxide for specific enrichment of microcystin-LR in Achatina fulica prior to matrix-assisted laser desorption ionization mass spectrometry

Microcystin-LR (MC-LR), as a hepatotoxin, can cause liver swelling, hepatitis, and even liver cancer. In this study, MC-LR aptamer (Apt-3) modified graphene oxide (GO) was designed to enrich MC-LR in white jade snail (Achatina fulica) and pond water, followed by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) analysis. Results indicated that the Apt-3/PEG/GO nanocomposites were highly specific to MC-LR, and the detection limit of MALDI-MS was 0.50 ng/mL. Moreover, the MC-LR can be released from nanocomposites at 75°C, thus, the reuse of Apt-3/PEG/GO is realized. Real sample analysis indicated that the Apt-3/PEG/GO nanocomposites coupled with MALDI-MS were efficient in detecting trace amounts of MC-LR in real samples. With the merits of being low cost, reusable, and easy to besynthesized, this Apt-3/PEG/GO MALDI-MS is expected to be comprehensively applied by anchoring suitable aptamers for different targets.

Photoelectrochemical aptasensing with methylene blue filled Ni-MOFs nanocomposite by spatial confinement for microcystin-LR detection

Microcystin LR (MC-LR) is a hazardous cyanotoxin produced by cyanobacteria during freshwater eutrophication, which can cause liver cancer. Here, a photoelectrochemical (PEC) aptasensor based on methylene blue (MB)-loaded Ni-MOF composite (Ni-MOF/MB) with spatial confinement was constructed for the sensitive detection of MC-LR. Ni-MOF with two-dimensional sheet structure was prepared via a liquid-liquid interface synthesis method with environmental-friendly solvent and milder reaction conditions. Benefiting from the uniform pore size, Ni-MOF acted as reaction platform to anchor the photosensitive molecule MB. The electron donor, ascorbic acid (AA), was produced by alkaline phosphatase (ALP) loaded on DNA strand catalyzing ascorbic acid phosphate. The generated AA was absorbed by Ni-MOF/MB, thereby effectively improving the utilization of AA and avoiding the external environment interferences to enlarge the photocurrent of MB. For analysis, ALP-labeled aptamer can specifically recognize MC-LR by forming a complex to strip from aptasensor, thus leading to a  decreased photocurrent. The developed PEC aptasensor offered a linear range of 10 fM-100 pM with a detection limit of 6 fM. It was successfully employed for detecting MC-LR in farm water and fish meat, and the results were validated by ultrahigh-performance liquid chromatography-mass spectrometry. This method presents a new idea of MOF-limited domain for PEC aptasensing.

Rapid and Easy Detection of Microcystin-LR Using a Bioactivated Multi-Walled Carbon Nanotube-Based Field-Effect Transistor Sensor

In this study, we developed a multi-walled carbon nanotube (MWCNT)-based field-effect transistor (MWCNT-FET) sensor with high sensitivity and selectivity for microcystin-LR (MC-LR). Carboxylated MWCNTs were activated with an MC-LR-targeting aptamer (MCTA). Subsequently the bioactivated MWCNTs were immobilized between interdigitated drain (D) and source (S) electrodes through self-assembly. The top-gated MWCNT-FET sensor was configured by dropping the sample solution onto the D and S electrodes and immersing a Ag/AgCl electrode in the sample solution as a gate (G) electrode. We believe that the FET sensor's conduction path arises from the interplay between the MCTAs, with the applied gate potential modulating this path. Using standard instruments and a personal computer, the sensor's response was detected in real-time within a 10 min time frame. This label-free FET sensor demonstrated an impressive detection capability for MC-LR in the concentration range of 0.1-0.5 ng/mL, exhibiting a lower detection limit of 0.11 ng/mL. Additionally, the MWCNT-FET sensor displayed consistent reproducibility, a robust selectivity for MC-LR over its congeners, and minimal matrix interferences. Given these attributes, this easily mass-producible FET sensor is a promising tool for rapid, straightforward, and sensitive MC-LR detection in freshwater environments.

Laser induced graphene electrochemical aptasensor based on tetrahedral DNA for ultrasensitive on-site detection of microcystin-LR

The development of accurate and reliable sensor for on-site detection of microcystin-LR (MC-LR), one of hazardous environmental pollutants, is highly required. Herein, a laser induced graphene (LIG)-based electrochemical aptasensor for sensitive on-site detection of MC-LR was reported. LIG electrode, the substrate of aptasensor, was prepared via thermal transfer with ethylene-vinyl acetate copolymer, and LIG acted as quasi-reference electrode to replace conventional Ag/AgCl electrode for better operability and robustness. LIG electrode provided large surface area to assemble tetrahedral DNA to absorb methylene blue (MB) for the signal amplification. For detection, the specific recognition of MC-LR with aptamer led to the stripping of tetrahedral DNA complex and further the decreased redox current of MB (IMB). Consequently, the fabricated aptasensor offered high analytical performance for MC-LR detection with a linear range of 1 × 10-2-1 × 105 pM and a detection limit of 3 × 10-3 pM, which was successfully used for water sample analysis with comparable reliability and accuracy of standard method. Furthermore, a portable detection platform by coupling of LIG-based electrochemical aptasensor with electrochemical workstation was constructed for on-site detection of MC-LR. This work offers a novel method for the on-site monitoring of MC-LR, which promotes the investigation of LIG-based electrochemical biosensing in the field of environmental analysis.

Label-Free Chemiresistive Sensors Based on Self-Assembled Ti3C2Tx MXene Films for Monitoring of Microcystin-LR in Water Samples

Herein, we propose a label-free chemiresistive sensor for the highly sensitive and selective detection of microcystin (MC)-LR in water samples. The sensor uses a layer-by-layer (LBL) assembled conductive film consisting of Ti3C2Tx nanosheets as the sensing channel. It is further modified by using an aptamer for the specific recognition of MC-LR. The response signal is based on the change in resistance of the conductive channel upon binding of MC-LR with the aptamer. Our novel strategy is the first concept proposed for immobilizing the aptamer containing -SH on the channel surface through a Ti-S bond under weakly alkaline condition. The resulting sensor is highly sensitive and stable for the detection of MC-LR, with a detection limit of 0.18 ng L-1 and a wide linear range from 1 to 104 ng L-1. We used the sensor to continuously monitor MC-LR released by cultivated Microcystis aeruginosa, showing a strong relationship between MC-LR and cell density. Furthermore, the sensor was successfully used to measure MC-LR in freshwater lakes with moderate algal blooms, and the results agreed well with those obtained by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. The present study provides a reliable method for highly sensitive and selective detection of MC-LR in environmental waters.

Dual-amplification system based on CRISPR-Cas12a and horseradish peroxidase-tethered magnetic microspheres for colorimetric detection of microcystin-LR

A novel dual-amplification system based on CRISPR-Cas12a and horseradish peroxidase (HRP) was developed for colorimetric determination of MC-LR. This dual-amplification was accomplished by combining the nuclease activity of CRISPR-Cas12a with the redox activity of HRP. HRP linked to magnetic beads through an ssDNA (MB-ssDNA-HRP) was used to induce a color change of the 3,3',5,5'-tetramethylbenzidine (TMB)-H₂O₂ chromogenic substrate solution. Specific binding of MC-LR with its aptamer initiated the release of a complementary DNA (cDNA), which was designed to activate the trans-cleavage activity of CRISPR-Cas12a. Upon activation, Cas12a cut the ssDNA linker in MB-ssDNA-HRP, causing a reduction of HRP on the magnetic beads. Consequently, the UV-Vis absorbance of the HRP-catalyzed reaction was decreased. The dual-signal amplification facilitated by CRISPR-Cas12a and HRP enabled the colorimetric detection of MC-LR in the range 0.01 to 50 ng·mL^-1 with a limit of detection (LOD) of 4.53 pg·mL^-1. The practicability of the developed colorimetric method was demonstrated by detecting different levels of MC-LR in spiked real water samples. The recoveries ranged from 86.2 to 118.5% and the relative standard deviation (RSD) was 8.4 to 17.6%. This work provides new inspiration for the construction of effective signal amplification platforms and demonstrates a simple and user-friendly colorimetric method for determination of trace MC-LR.

Aptasensors Are Conjectured as Promising ALT and AST Diagnostic Tools for the Early Diagnosis of Acute Liver Injury

Abnormal levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in human serum are the most sensitive indicator of hepatocellular damage. Because liver-related health problems are directly linked to elevated levels of ALT and AST, it is important to develop accurate and rapid methods to detect these enzymes for the early diagnosis of liver disease and prevention of long-term liver damage. Several analytical methods have been developed for the detection of ALT and AST. However, these methods are based on complex mechanisms and require bulky instruments and laboratories, making them unsuitable for point-of-care application or in-house testing. Lateral flow assay (LFA)-based biosensors, on the other hand, provide rapid, accurate, and reliable results, are easy to operate, and are affordable for low-income populations. However, due to the storage, stability, batch-to-batch variations, and error margins, antibody-based LFAs are considered unaffordable for field applications. In this hypothesis, we propose the selection of aptamers with high affinity and specificity for the liver biomarkers ALT and AST to build an efficient LFA device for point-of-care applications. Though the aptamer-based LFA would be semiquantitative for ALT and AST, it would be an inexpensive option for the early detection and diagnosis of liver disease. Aptamer-based LFA is anticipated to minimize the economic burden. It can also be used for routine liver function tests regardless of the economic situation in each country. By developing a low-cost testing platform, millions of patients suffering from liver disease can be saved.

Construction of a rapid electrochemical biosensor consisting of a nanozyme/aptamer conjugate for waterborne microcystin detection

Microcystin-LR (MC-LR) is a hepatotoxin generated by the excessive proliferation of cyanobacteria, which is a threat to humans and wildlife. Therefore, rapid detection of MC-LR is an important challenge. This study describes a rapid electrochemical biosensor comprising nanozymes and aptamers. Alternating current electrothermal flow (ACEF) significantly reduced the MC-LR detection period to 10 min. We also used MnO₂/MC-LR aptamer conjugates to improve the sensitivity to MC-LR detection. Here, MnO₂ amplified the electrochemical signal and the aptamer showed high selectivity for MC-LR. Under the optimal conditions, the limit of detection (LOD) and selectivity in freshwater were detected using cyclic voltammetry and differential pulse voltammetry. As a result, an LOD of 3.36 pg mL^-1 was observed in the linear concentration range of 10 pg mL^-1 to 1 μg mL^-1. This study quickly and sensitively detected MC-LR in a situation where it causes serious damage worldwide. In addition, the ACEF technology introduction is the first example of MC-LR detection, suggesting a wide range of possibilities for MC-LR biosensors.

Recent Advances in Aptasensing Strategies for Monitoring Phycotoxins: Promising for Food Safety

Phycotoxins or marine toxins cause massive harm to humans, livestock, and pets. Current strategies based on ordinary methods are long time-wise and require expert operators, and are not reliable for on-site and real-time use. Therefore, it is urgent to exploit new detection methods for marine toxins with high sensitivity and specificity, low detection limits, convenience, and high efficiency. Conversely, biosensors can distinguish poisons with less response time and higher selectivity than the common strategies. Aptamer-based biosensors (aptasensors) are potent for environmental monitoring, especially for on-site and real-time determination of marine toxins and freshwater microorganisms, and with a degree of superiority over other biosensors, making them worth considering. This article reviews the designed aptasensors based on the different strategies for detecting the various phycotoxins.

Sensitive Identification of Microcystin-LR via a Reagent-Free and Reusable Electrochemical Biosensor Using a Methylene Blue-Labeled Aptamer

We report a methylene blue (MB)-modified electrochemical aptamer (E-AB) sensor for determining microcystin-LR (MC-LR). The signal transduction of the sensor was based on changes in conformation and position of MB induced by the binding between MC-LR and the modified aptamer probe. In the absence of MC-LR, an aptamer probe was considered partially folded. After combining aptamer and MC-LR, the configuration of the aptamer probe changed and facilitated the electron transfer between MB and the electrode surface. As a result, an increased current response was observed. We optimized the parameters and evaluated the electrochemical performance of the sensor using square wave voltammetry (SWV). MC-LR was measured from 1.0 to 750.0 ng/L with a detection limit of 0.53 ng/L. The reliability of the method was verified by the determination of MC-LR in environmental real samples, such as pond water and tap water. Moreover, we demonstrated that this reagent-less biosensor could be regenerated and reused after rinsing with deionized water with good accuracy and reproducibility. As a reusable and regenerable E-AB sensor, this rapid, reagent-free, and sensitive sensing platform will facilitate routine monitoring of MC-LR in actual samples.

Combating small molecule environmental contaminants: detection and sequestration using functional nucleic acids

Small molecule contaminants pose a significant threat to the environment and human health. While regulations are in place for allowed limits in many countries, detection and remediation of contaminants in more resource-limited settings and everyday environmental sources remains a challenge. Functional nucleic acids, including aptamers and DNA enzymes, have emerged as powerful options for addressing this challenge due to their ability to non-covalently interact with small molecule targets. The goal of this perspective is to outline recent efforts toward the selection of aptamers for small molecules and describe their subsequent implementation for environmental applications. Finally, we provide an outlook that addresses barriers that hinder these technologies from being widely adopted in field friendly settings and propose a path forward toward addressing these challenges.

Recent advances in nanomaterials-based optical and electrochemical aptasensors for detection of cyanotoxins

The existence of cyanotoxins poses serious threats to human health, it is highly desirable to develop specific and sensitive methods for rapid detection of cyanotoxins in food and water. Due to the distinct advantages of aptamer including high specificity, good stability and easy preparation, various aptamer-based sensors (aptasensors) have been proposed to promote the detection of cyanotoxins. In this review, we summarize recent advance in optical and electrochemical aptasensors for cyanotoxins sensing by integrating with versatile nanomaterials or innovative sensing strategies, such as colorimetric aptasensors, fluorescent aptasensors, surface enhancement Raman spectroscopy-based aptasensors, voltammetric aptasensors, electrochemical impedance spectroscopy-based aptasensors and photoelectrochemical aptasensors. We highlight the accomplishments and advancements of aptasensors with improved performance. Furthermore, the current challenges and future prospects in cyanotoxins detection are discussed from our perspectives, which we hope to provide more ideas for future researchers.

Label-Free Detection of Saxitoxin with Field-Effect Device-Based Biosensor

Saxitoxin (STX) is a highly toxic and widely distributed paralytic shellfish toxin (PSP), posing a serious hazard to the environment and human health. Thus, it is highly required to develop new STX detection approaches that are convenient, desirable, and affordable. This study presented a label-free electrolyte-insulator-semiconductor (EIS) sensor covered with a layer-by-layer developed positively charged Poly (amidoamine) (PAMAM) dendrimer. An aptamer (Apt), which is sensitive to STX was electrostatically immobilized onto the PAMAM dendrimer layer. This results in an Apt that is preferably flat inside a Debye length, resulting in less charge-screening effect and a higher sensor signal. Capacitance-voltage and constant-capacitance measurements were utilized to monitor each step of a sensor surface variation, namely, the immobilization of PAMAM dendrimers, Apt, and STX. Additionally, the surface morphology of PAMAM dendrimer layers was studied by using atomic force microscopy and scanning electron microscopy. Fluorescence microscopy was utilized to confirm that Apt was successfully immobilized on a PAMAM dendrimer-modified EIS sensor. The results presented an aptasensor with a detection range of 0.5–100 nM for STX detection and a limit of detection was 0.09 nM. Additionally, the aptasensor demonstrated high selectivity and 9-day stability. The extraction of mussel tissue indicated that an aptasensor may be applied to the detection of STX in real samples. An aptasensor enables marine toxin detection in a rapid and label-free manner.

An electrochemical PAH-modified aptasensor for the label-free and highly-sensitive detection of saxitoxin

Saxitoxin (STX), is one of the most dangerous and widespread paralytic shellfish toxins, causing a severe threat to the ecosystem and human health. So, it is important and highly essential to develop novel techniques for STX detection in a convenient, desirable, and low-cost manner. Herein, this study developed an electrolyte-insulator-semiconductor (EIS) sensor covered with a layer-by-layer prepared, positively-charged weak polyelectrolyte layer of poly (allylamine hydrochloride) (PAH) for the label-free detection of STX. The specific aptamer (Apt) sensitive to STX was electrostatically adsorbed onto the PAH layer. This leads to a preferentially flat orientation of the Apt within the Debye length, thus yielding a reduced charge-screening influence and a higher sensor signal. Each step of sensor surface modification, i.e. PAH adsorption, immobilization of Apt, and attachment of STX, was monitored by capacitance-voltage (C-V) and constant-capacitance (ConCap) measurements. Furthermore, atomic force microscopy (AFM) was employed to characterize the surface morphology and roughness of the PAH layer. Fluorescence microscopy was used to confirm the effective immobilization of Apt onto the PAH-modified EIS sensor. The results showed that the detection range of this aptasensor for STX detection was 0.5-100 nM and the detection limit was as low as 0.05 nM. Furthermore, this aptasensor showed good selectivity and 9 days' stability. The mussel tissue extraction test suggested that this aptasensor can be used to detect STX in real samples. This aptasensor provides a convenient approach for moderate, rapid, and label-free detection of marine biological toxins.

Aptamers functionalized hybrid nanomaterials for algal toxins detection and decontamination in aquatic system: Current progress, opportunities, and challenges

Purification and detection of algal toxins is the most effective technique to ensure that people have clean and safe drinking water. To achieve these objectives, various state-of-the-art technologies were designed and fabricated to decontaminate and detect algal toxins in aquatic environments. Amongst these technologies, aptamer-functionalized hybrid nanomaterials conjugates have received significant consideration as a result of their several benefits over other methods, such as good controllable selectivity, low immunogenicity, and biocompatibility. Because of their excellent properties, aptamer-functionalized hybrid nanomaterials conjugates are one of several remarkable agents. Several isolated aptamer sequences for algal toxins are addressed in this review, as well as aptasensor and decontamination aptamer functionalized metal nanoparticle-derived hybrid nanocomposites applications. In addition, we present diverse aptamer-functionalized hybrid nanomaterial conjugates designs and their applications for sensing and decontamination.

A novel CRISPR/Cas14a system integrated with 2D porphyrin metal-organic framework for microcystin-LR determination through a homogeneous competitive reaction

Selective and sensitive detection of microcystin-LR (MC-LR) is of vital importance because of its high toxicity and broad distribution. Herein, a novel and versatile fluorescence sensor (Cas14-pMOFs fluorescence sensor) was developed by combining the CRISPR/Cas14a system with a 2D porphyrin metal-organic framework nanosheets (2D-pMOFs) for MC-LR determination. The designed CRISPR/Cas14a system was activated by the unbound complementary DNA (cDNA), which was positively correlated with MC-LR concentration. Furthermore, the activated Cas14a protein was utilized to indiscriminately cleave the FAM-labeled single-stranded DNA (ssDNA-FAM), which was pre-absorbed on Cu-TCPP(Fe) nanosheets. Because of the desorption of the cleaved ssDNA-FAM, the pre-quenched fluorescence signal was recovered. Owing to the excellent performance in quantifying cDNA using this Cas14-pMOFs fluorescence sensor with a limit of detection (LOD) of 0.12 nM, this Cas14-pMOFs fluorescence sensor was able to detect MC-LR in a range from 50 pg/mL to 1 μg/mL with the LOD of 19 pg/mL. This work not only provided a new insight for the exploration of fluorescence sensors based on 2D-pMOFs coupled with CRISPR/Cas14a, but also, demonstrated its universality in both nucleic acid and non-nucleic acid targets determination.

Detection of Microcystin-LR in the Cells and Natural Lake Water Samples by A Unique Fluorescence-Based Method

Microcystin-LR (MC-LR) is widely distributed in natural lakes and could strongly inhibit protein phosphatase activity; it is also a potent liver tumor promoter. Over the last two decades, tremendous efforts have been devoted to enhance the detection of MC-LR in water samples. However, the traditional method is complex and costly, and achieving the fast, sensitive, and accurate determination of MC-LR in the cells and natural lakes by using fluorescence signal changes is fairly difficult. Our work explores novel fluorescent probes that are capable of concurrently analyzing and detecting MC-LR in the cells and water. In this study, we introduce, for the first time, 5-AF and 6-AF as small-molecule fluorescent probes suitable for MC-LR detection in the cells and water samples based on fluorescence signal changes. We titrated 5-AF and 6-AF with MC-LR in pure water, scanned the fluorescence of the sample, and then obtained the equation the fluorescence intensity versus MC-LR concentration curve. MC-LR in lake water samples was crudely purified, and then 5-AF was added to measure its fluorescence peak. The fluorescence intensity of 5-AF is significantly enhanced with increasing MC-LR concentration. This enhancement trend is stable and could be mathematically modeled. We also comprehensively analyzed the mechanism and recognition principle of the probe response to MC-LR in natural lake water. Moreover, we believe that 5-AF may be capable of detecting exogenous MC-LR in cells. The results of this study reveal that these unique fluorescent probes may be applied to construct near-infrared fluorescent probes that could detect MC-LR levels in vivo.

High-sensitivity small-molecule detection of microcystin-LR cyano-toxin using a terahertz-aptamer biosensor

We demonstrate the rapid and highly sensitive detection of a small molecule, microcystin-LR (MC-LR) toxin using an aptasensor based on a terahertz (THz) emission technique named the terahertz chemical microscope (TCM). The main component of the TCM is the sensing plate, which consists of a thin silicon layer deposited on a sapphire substrate, with a natural SiO₂ layer formed on the top of the Si layer. The DNA aptamer is linked to the oxidized top surface of the silicon layer by a one-step reaction (click chemistry) between the DBCO-labeled aptamer and an azido group that binds to the surface. Using density functional theory (DFT) calculations, the number of active sites on the surface has been estimated to be 3.8 × 10¹³ cm^-2. Aptamer immobilization and MC-LR binding have been optimized by adjusting the aptamer concentration and the binding buffer composition. When MC-LR binds with the DNA aptamer, it causes a change in the chemical potential at the surface of the sensing plate, which leads to a change in the amplitude of the THz signal. Compared with other bio-sensing methods such as surface plasmon resonance (SPR), TCM is a rapid assay that can be completed in 15 min (10 min incubation and 5 min data acquisition). Moreover, our results show that the aptamer-based TCM can detect MC-LR with an excellent detection limit of 50 ng L^-1, which is 20 times more sensitive compared with SPR measurements of MC-LR.

Aptamer-functionalized metal-organic framework-based electrospun nanofibrous composite coating fiber for specific recognition of ultratrace microcystin in water

A novel aptamer@AuNPs@UiO-66-NH₂ electrospun nanofibrous coating fiber for specific recognition of microcystin-LR (MC-LR) was proposed by using electrospinning, metal-organic frameworks (MOF) seed growth and AuNPs bridging aptamer strategies. Characterization of morphology, structure and stability of the obtained affinity nanofibrous coating fiber were investigated. High loading of MOFs and aptamers on the nanofibrous fiber were achieved and successfully applied for accurate identification of MC-LR by solid-phase microextraction (SPME) coupled with LC-MS. Highly specific recognition of MC-LR with little interference of analogs was achieved with extremely low LOD (0.004 ng/mL), good precision (CV% < 11.0%) and low relative error (RE% = -1.5% to -10.0%), which was rather better than that of the traditional SPME or SPE protocols. Satisfactory recoveries of MC-LR were obtained in the range of 92.0-96.8% (n = 3) in fortified tap water, raw pond water and river water samples. This work revealed an attractive alternative access to specific recognition and super-sensitive analysis of MC-LR in water.

Visible light-driven self-powered aptasensors for ultrasensitive Microcystin-LR detection based on the carrier density effect of N-doped graphene hydrogel/hematite Schottky junctions

In this work, a novel visible light-driven self-powered photoelectrochemical (PEC) platform was designed based on 3D N-doped graphene hydrogel/hematite nanocomposites (NGH/Fe₂O₃) via a facile one-pot hydrothermal route. The coupling NGH with Fe₂O₃ could generate a Schottky junction, which promoted the separation of charges. Moreover, Mott-Schottky measurements validated that the carrier concentration achieved by NGH/Fe₂O₃ was about 3.4 × 10³ times in comparison to that of pure Fe₂O₃, which was beneficial for efficient charge transfer. Owing to the carrier density effect and Schottky junction, the photocurrent of the as-fabricated NGH/Fe₂O₃ nanocomposites was 6.9-fold higher than that of pure Fe₂O₃. On the basis of such excellent Schottky junctions, an ultrasensitive visible light-induced self-powered PEC aptasensor was developed using a Microcystin-LR (MC-LR) aptamer. The as-fabricated PEC aptasensor displayed good analytical performance toward MC-LR detection in terms of wide linear range (1 pM-5 nM), low detection limit (0.23 pM, S/N = 3), excellent selectivity and high stability. This new strategy can provide a way for regulating nanostructures for more sensitive PEC sensors by increasing the carrier density.

Sequestration and Removal of Multiple Small-Molecule Contaminants Using an Optimized Aptamer-Based Ultrafiltration System

Small-molecule toxins pose a significant threat to human health and the environment, and their removal is made challenging by their low molecular weight. Aptamers show promise as affinity reagents for binding these toxins, and recently, aptamers have been utilized for both sensing and remediation applications. We found that functionalization of ultrafiltration membranes with aptamers provides a convenient scaffold for toxin sequestration, but our initial efforts in this area were limited by low functionalization efficiencies and the ability to only capture a single target molecule. Herein, we describe detailed optimization of our aptamer-functionalized ultrafiltration membrane system and subsequent use for simultaneous removal of multiple small-molecule toxins. We examine multiple critical components involved in fabricating and functionalizing the membranes, including PEG polymer molecular weight for membrane fabrication, grafting conditions for pMAA attachment, and coupling reagents for aptamer functionalization. This screening enabled us to identify a set of unique conditions in which we were able to achieve high flux, near quantitative yield for DNA attachment, and effective overall depletion of both toxins and bacterial cells. Furthermore, we demonstrate the attachment of multiple aptamers and subsequent parallel removal of atrazine, bisphenol A, and microcystin-LR in a complex lake water matrix. Our rigorous evaluation resulted in depletion of multiple small-molecule toxins, contaminants, and microorganisms, demonstrating the potential of aptamer-functionalized membranes as point-of-use decontamination systems.

Sensitive Electrochemical Detection of Microcystin-LR in Water Samples Via Target-Induced Displacement of Aptamer Associated [Ru(NH3)6]3

In this study, we demonstrate the successful development of an electrochemical aptamer-based sensor for point-of-use detection and quantification of the highly potent microcystin-LR (MC-LR) in water. The sensor uses hexaammineruthenium(III) chloride ([Ru(NH₃)₆]³⁺) as redox mediator, because of the ability of the positively charged (3+) molecule to associate with the phosphate backbone of the nucleic acids. We quantitatively measure the target-induced displacement of aptamer associated, or surface confined, [Ru(NH₃)₆]³⁺ in the presence of MC-LR. Upon the addition of MC-LR in the water, surface-confined [Ru(NH₃)₆]³⁺ dissociates, resulting in less faradaic current from the reduction of [Ru(NH₃)₆]³⁺ to [Ru(NH₃)₆]²⁺ Sensing surfaces of highly packed immobilized aptamers were capable of recording decreasing square wave voltammetry (SWV) signals after the addition of MC-LR in buffer. As a result, SWV recorded substantial signal suppression within 15 min of target incubation. The sensor showed a calculated limit of detection (LOD) of 9.2 pM in buffer. The effects of interferents were minimal, except when high concentrations of natural organic matter (NOM) were present. Also, the sensor performed well in drinking water samples. These results indicate a sensor with potential for fast and specific quantitative determination of MC-LR in drinking water samples. A common challenge when developing electrochemical, aptamer-based sensors is the need to optimize the nucleic acid aptamer in order to achieve sensitive signaling. This is particularly important when an aptamer experiences only a small or localized conformational change that provides only a limited electrochemical signal change. This study suggests a strategy to overcome that challenge through the use of a nucleic acid-associated redox label.

Cell density-dependent regulation of microcystin synthetase genes (mcy) expression and microcystin-LR production in Microcystis aeruginosa that mimics quorum sensing

As the secondary metabolites of cyanobacterial harmful algal blooms (Cyano-HABs), microcystins (MCs) were generated under various environmental and cellular conditions. The understanding of the causes of MCs generation is of great interest in the field of water treatment and environmental science. In this work, we studied how Microcystis aeruginosa (FACHB-905) cell densities affect the MCs synthetase genes (mcy) expression, microcystin-LR (MC-LR) and quorum sensing molecules (Acyl-homoserine lactones (AHLs)) production. An electrochemical sensor was developed here for sensitive and quantitative detection of MC-LR that cultured at different cell densities. The results showed that mcy expression and MC-LR concentration started to increase when the cell density reached ca. 22 × 10⁶ cells/mL, and was significantly increased with increasing cell densities. Moreover, the up-regulation of AHLs with increasing cell densities revealed that MC-LR is quorum sensing-mediated. Our results undoubtedly confirmed that MC-LR was produced in a cell density-dependent way that mimics quorum sensing, and the minimum cell density (ca. 22 × 10⁶ cells/mL) that was required to produce MC-LR was provided and offered a reference standard for the prevention and control of MCs pollution in the actual water environment.

Electrochemical impedance biosensor for detection of saxitoxin in aqueous solution

Saxitoxin is a cyanotoxin which is very harmful to human health; the concentration limit in drinking water is only 3 μg/L. Therefore, a simple, fast, sensitive, low-cost, and specific method for its detection, quantification, and monitoring in water bodies is needed to avoid adverse effects on animal and human health. In this work, we developed an electrochemical impedimetric biosensor using a specific aptamer as recognition element for saxitoxin detection. This method allies the superior sensing characteristics of aptamers with the nondestructive, label-free, and easy working principles of the electrochemical impedance technique. The device presented sensitivity for detecting saxitoxin concentrations above 0.3 μg/L, with high selectivity in negative control experiments, demonstrating a promising alternative for water toxin detection.

Visualizing of AuNPs protection aptamer from DNase I enzyme digestion based on Nanopipette and its use for Microcystin-LR detection

A simple and effective fluorescence platform has been established for visualizing nanomaterials' protective effect of DNA from cellular enzyme digestion based on nanopipette. In a proof-of-concept trial, gold nanoparticles (AuNPs) protect aptamer was designed, and it used for Microcystin-LR (MC-LR) sensitive detection. In the absence of MC-LR, FAM-labeled aptamers were combined on AuNPs, resulting in weak fluorescence emission. In the presence of MC-LR, aptamer bound with MC-LR. The formed complex leaves the surface of AuNPs. With the addition of the deoxyribonuclease I (DNase I) enzyme, the aptamer was selectively cleavaged, and MC-LR was released as an additional target molecule to achieve signal amplification and obtain strong fluorescence intensity. At the optimized conditions, a wide linear range (0.25 nM-20 nM) of fluorescence response for MC-LR was obtained. Further, by electrochemically manipulation MC-LR and DNase I inside confining nanopipette, which is filled with aptamer/AuNPs. The fluorescence intensity change with the aptamer and AuNPs interaction, these results directly visualize the process of DNA cleavage, and the interaction with AuNPs can effectively prevent the cleavage at the nanoscale confinement. This convenient nanoscale device provides new kinetic information about the dynamic chemical processes at a single-molecule level.

Aptamers and Aptamer-Coupled Biosensors to Detect Water-Borne Pathogens

Aptamers can serve as efficient bioreceptors for the development of biosensing detection platforms. Aptamers are short DNA or RNA oligonucleotides that fold into specific structures, which enable them to selectively bind to target analytes. The method used to identify aptamers is Systematic Evolution of Ligands through Exponential Enrichment (SELEX). Target properties can have an impact on aptamer efficiencies. Therefore, characteristics of water-borne microbial targets must be carefully considered during SELEX for optimal aptamer development. Several aptamers have been described for key water-borne pathogens. Here, we provide an exhaustive overview of these aptamers and discuss important microbial aspects to consider when developing such aptamers.

Multiplexed SERS Detection of Microcystins with Aptamer-Driven Core-Satellite Assemblies

We describe surface-enhanced Raman spectroscopy (SERS) aptasensors that can indirectly detect MC-LR and MC-RR, individually or simultaneously, in natural water and in algal culture. The sensor is constructed from nanoparticles composed of successive layers of Au core-SERS label-silver shell-gold shell (Au@label@Ag@Au NPs), functionalized on the outer Au surface by MC-LR and/or MC-RR aptamers. These NPs are immobilized on asymmetric Au nanoflowers (AuNFs) dispersed on planar silicon substrates through DNA hybridization of the aptamers and capture DNA sequences with which the AuNFs are functionalized, thereby forming core-satellite nanostructures on the substrates. This construction led to greater electromagnetic (EM) field enhancement of the Raman label-modified region, as supported by finite-difference time-domain (FDTD) simulations of the core-satellite assembly. In the presence of MC-LR and/or MC-RR, the aptamer-functionalized NPs dissociate from the AuNFs because of the stronger affinity of the aptamers with the MCs, which decreases the SERS signal, thus allowing indirect detection of the MCs. The improved SERS sensitivity significantly decreased the limit of detection (LOD) for separate MC-LR detection (0.8 pM) and for multiplex detection (1.5 pM for MC-LR and 1.3 pM for MC-RR), compared with other recently reported SERS-based methods for MC-LR detection. The aptasensors show excellent selectivity to MC-LR/MC-RR and excellent recoveries (96-105%). The use of these SERS aptasensors to monitor MC-LR production over 1 week in a culture medium of M. aeruginosa cells demonstrates the applicability of the sensors in a realistic environment.

DNA nanotechnology: A recent advancement in the monitoring of microcystin-LR

The Microcystin-Leucine-Arginine (MC-LR) is the most toxic and widely distributed microcystin, which originates from cyanobacteria produced by water eutrophication. The MC-LR has deleterious effects on the aquatic lives and agriculture, and this highly toxic chemical could severely endanger human health when the polluted food was intaken. Therefore, the monitoring of MC-LR is of vital importance in the fields including environment, food, and public health. Utilizing the complementary base pairing between DNA molecules, DNA nanotechnology can realize the programmable and predictable regulation of DNA molecules. In analytical applications, DNA nanotechnology can be used to detect targets via target-induced conformation change and the nano-assemblies of nucleic acids. Compared with the conventional analytical technologies, DNA nanotechnology has the advantages of sensitive, versatile, and high potential in real-time and on-site applications. According to the molecular basis for recognizing MC-LR, the strategies of applying DNA nanotechnology in the MC-LR monitoring are divided into two categories in this review: DNA as a recognition element and DNA-assisted signal processing. This paper introduces state-of-the-art analytical methods for the detection of MC-LR based on DNA nanotechnology and provides critical perspectives on the challenges and development in this field.

Impedimetric Microcystin-LR Aptasensor Prepared with Sulfonated Poly(2,5-dimethoxyaniline)–Silver Nanocomposite

This paper presents a novel impedimetric aptasensor for cyanobacterial microcystin-LR (L, l-leucine; R, l-arginine) (MC-LR) containing a 5′ thiolated 60-mer DNA aptamer (i.e., 5′-SH-(CH2)6GGCGCCAAACAGGACCACCATGACAATTACCCATACCACCTCATTATGCCCCATCT CCGC-3′). A nanocomposite electrode platform comprising biocompatible poly(2,5-dimethoxyaniline) (PDMA)-poly(vinylsulfonate) (PVS) and silver nanoparticle (Ag0) on a glassy carbon electrode (GCE), i.e., (GCE/PDMA–PVS–Ag0) was used in the biosensor development. Small-angle X-ray scattering (SAXS) spectroscopic analysis revealed that the PDMA–PVS–Ag0 nanocomposites were polydispersed and contained embedded Ag0. Electrochemical impedance spectroscopy (EIS) responses of the aptasensor gave a dynamic linear range (DLR) and limit of detection (LOD) values of 0.01–0.1 ng L−1 MC-LR and 0.003 ng L−1 MC-LR, respectively. The cross-reactivity studies, which was validated with enzyme-linked immunosorbent assay (ELISA), showed that the aptasensor possesses excellent selectivity for MC-LR.

Single-stranded DNA probe paired aptasensor with extra dye binding sites to enhance its fluorescence response in the presence of a target compound

The purpose of this study is to investigate the possibility of improving the performance of a DNA binding dye water quenching based aptasensor without changing or truncating the aptamer. To demonstrate the possibility of increasing the change in fluorescence of the aptasensor by pairing it with a suitable ssDNA probe, three ssDNA probes (probe 1, 2, and 3) were employed and the fluorescence from the bound dyes was measured. This showed that ssDNA probe 2 created the most additional binding sites. By varying the target compound concentration (0, 0.05, 0.5, 5, 50, and 500 mg L⁻¹ 4-n-nonylphenol), the corresponding change in the fluorescence signal of the unpaired and ssDNA probe paired aptasensors were measured and compared over a range of emission wavelengths. The response of all three ssDNA probe paired aptasensors showed good fit (R² = 0.88–0.92) to a logarithmic response. The sensitivity of the aptasensor paired with ssDNA probe 2 was improved by ∼60%, whereas that of the aptasensor paired with ssDNA probe 3 was only improved by a marginal ∼3%. This study is a demonstration of using an appropriate ssDNA probe to increase the number of binding sites and hence the performance of a DNA binding dye and water quenched aptasensor. It is a possibility that can be extended to similar aptasensors without having to change or truncate the aptamer.

Principle of an ssDNA paired aptasensor where extra dye binding sites are created to enhance its fluorescence response.

Immunoaffinity Extraction and Alternative Approaches for the Analysis of Toxins in Environmental, Food or Biological Matrices

The evolution of instrumentation in terms of separation and detection allowed a real improvement of the sensitivity and analysis time. However, the analysis of ultra-traces of toxins in complex samples requires often a step of purification and even preconcentration before their chromatographic analysis. Therefore, immunoaffinity sorbents based on specific antibodies thus providing a molecular recognition mechanism appear as powerful tools for the selective extraction of a target molecule and its structural analogs to obtain more reliable and sensitive quantitative analysis in environmental, food or biological matrices. This review focuses on immunosorbents that have proven their efficiency in selectively extracting various types of toxins of various sizes (from small mycotoxins to large proteins) and physicochemical properties. Immunosorbents are now commercially available, and their use has been validated for numerous applications. The wide variety of samples to be analyzed, as well as extraction conditions and their impact on extraction yields, is discussed. In addition, their potential for purification and thus suppression of matrix effects, responsible for quantification problems especially in mass spectrometry, is presented. Due to their similar properties, molecularly imprinted polymers and aptamer-based sorbents that appear to be an interesting alternative to antibodies are also briefly addressed by comparing their potential with that of immunosorbents.

Influence of microfabrication on digital PCR performance in bead-based microwell array assays

Digital PCR (dPCR) is a highly sensitive analytical technique used to quantify DNA targets. Detection sensitivity can be further enhanced by capturing target sequences onto beads for preconcentration and sample cleanup prior to analysis in microfluidic microwell arrays. However, robust digital analysis requires individual beads to be interrogated within individual wells. Fabricating microwells with dimensions ≤ 3 μm is challenging, and the high surface area-to-volume ratio of the wells leaves PCR susceptible to inhibition stemming from materials used during device processing. This report describes the development of a microfabrication procedure to create ultralow-volume wells (100 fL) for bead-based dPCR and characterize the effects of microprocessing materials on assay performance. Standard microfabrication protocols used for creating microelectronics resulted in devices with nanoscopic debris originating from photoresists used during processing. A model dPCR assay was developed to characterize the effects of this debris, which revealed variable PCR inhibition. Debris within microwells attenuated digital and analog assay signals to a greater extent than debris on the device surface. Spatial heterogeneity of debris across devices was quantified to characterize regional PCR inhibition and intra- and inter-device variability. Ultimately, a fabrication procedure was developed to create pristine microfluidic arrays using dual processes to remove positive resist and forgoing use of negative resist entirely, which enabled robust amplification with digital signals matching theoretical predictions. Results from this work catalog the unique performance artifacts from device microfabrication and provide a guide for future studies seeking to conduct robust, high-sensitivity bead-based dPCR assays. Graphical abstract.

Probing the influence of graphene oxide sheets size on the performance of label-free electrochemical biosensors

The integration of graphene materials into electrochemical biosensing platforms has gained significant interest in recent years. Bulk quantities of graphene can be synthesized by oxidation of graphite to graphite oxide and subsequent exfoliation to graphene oxide (GO). However, the size of the resultant GO sheets changes from the parent graphite yielding a polydispersed solution of sizes ranging from a few nanometers to tens of micrometers. Here, we investigate the direct effect of GO sheets sizes on biosensor performance. We separated different GO sheets sizes, and we characterized them via atomic force, scanning electron, Raman and X-ray photoelectron spectroscopies and solid state nuclear magnetic resonance (NMR). As proof of concept, the sensing performance of these GO samples was probed using a well-known ssDNA aptasensor against microcystin-LR toxin and an immunosensor against β-lactoglobulin. The resulting aptasensors and immunosensors are fabricated by using covalent attachment and physical adsorption. We found that the aptasensors fabricated using physical adsorption, the binding signal variation was dramatically increased with increasing the GO sheet size. In contrast, for the aptasensor fabricated using covalent immobilization, the binding signal variation decreased with increasing GO sheet size. However, for the β-lactoglobulin immunosensors, the optimum signals were observed at intermediate GO sheet size. GO sheet size could enhance or inhibit the sensitivity of the graphene-based electrochemical sensors. Our results demonstrate that controlling the size of GO sheets may have a profound impact in specific biosensing applications.

Anti-VCAM-1 and Anti-IL4Rα Aptamer-Conjugated Super Paramagnetic Iron Oxide Nanoparticles for Enhanced Breast Cancer Diagnosis and Therapy

The surface protein overexpressed on cancer cells can be used as biomarkers for early detection of specific diseases. Anti-VCAM-1 and anti-IL4Rα DNA aptamers specific to VCAM-1 and IL4Rα receptors that are overexpressed in 4T1 tumor-bearing mice could be used as potential biomarker for both diagnostic and therapeutic applications in cancer biology. Cell Viability and luciferase assay of 4T1-Luc2 cancer cells in the presence of anti-VCAM-1 ssDNA or anti-IL4Rα RNA aptamers was assessed by monitoring the changes in the absorbance and the fluorescence of Alamar blue dye. The aptamer-conjugated SPIO magnetic beads, used for the selective targeting to tumor sites, were monitored using noninvasive MRI and Bioluminescence imaging (BLI). Cell viability and luciferase assays showed that both anti-VCAM-1 and anti-IL4Rα aptamers favor the depletion of cancer cells and limit tumor progression. Microscopic analyses confirmed that the target specific aptamers significantly trigger tumor cell apoptosis and limit cancer cell growth in vitro. The intravenous injection of SPIO nanoparticle-conjugated aptamers were further confirmed using noninvasive MRI and Bioluminescence imaging. Anti-VCAM1 and anti-IL4Rα aptamers, specific to VCAM-1 and IL4Rα receptors overexpressed in 4T1-Luc2 tumor-bearing mice, were used as diagnostic and therapeutic tools.

Recent advances in nanocomposite-based electrochemical aptasensors for the detection of toxins

Toxins are one of the major threatening factors to human and animal health, as well as economic growth. There is therefore an urgent demand from various communities to develop novel analytical methods for the sensitive detection of toxins in complex matrixes. Among the as-developed toxin detection strategies, nanocomposite-based aptamer sensors (termed as aptasensors) show tremendous potential for combating toxin pollution; in particular electrochemical (EC) aptasensors have received significant attention because of their unique advantages, including simplicity, rapidness, high sensitivity, low cost and suitability for field-testing. This paper reviewed the recently published approaches for the development of nanocomposite-/nanomaterial-based EC aptasensors for the detection of toxins with high assaying performance, and their potential applications in environmental monitoring, clinical diagnostics, and food safety control by summarizing the detection of different types of toxins, including fungal mycotoxins, algal toxins and bacterial enterotoxins. The effects of nanocomposite properties on the detection performance of EC aptasensors have been fully addressed for supplying readers with a comprehensive understanding of their improvement. The current technical challenges and future prospects of this subject have also been discussed.

A Fluorescence and Surface-Enhanced Raman Spectroscopic Dual-Modal Aptasensor for Sensitive Detection of Cyanotoxins

The ability to detect trace analytes without necessitating solid surface attachment or complicated processing steps would facilitate the translation of sensors for monitoring environmental toxins in the field. To address a critical unmet need in fresh water ecology, we have developed a dual-modal aptamer-based biosensor (aptasensor), featuring fluorescence and surface-enhanced Raman spectroscopy (SERS), for sensitive and selective detection of hepatotoxin microcystin-LR (MC-LR). The rational sensor design is based on the high affinity of the cyanine (Cy3) dye-modified complementary DNA (Cy3-cDNA) strand toward the plasmonic gold nanostars (GNSs) in comparison to the Cy3-cDNA/aptamer duplex. The preferential binding of MC-LR toward the MC-LR-specific aptamer triggers the dissociation of Cy3-cDNA/aptamer duplexes, which switches the Cy3's fluorescence "off" and SERS "on" due to the proximity of Cy3 dye to the GNS surface. Both fluorescence and SERS intensities are observed to vary linearly with the MC-LR concentration over the range of investigation. We have achieved high sensitivity and excellent specificity with the aptasensor toward MC-LR, which can be attributed to the fluorescence quenching effect, significant SERS enhancement by the GNSs, and the high affinity of the aptamer toward the MC-LR analytes. We further demonstrate the applicability of the present aptasensor for detection of MC-LR in a diverse set of real water samples with high accuracy and excellent reproducibility. With further refinement, we believe that the aptamer-driven complementary assembly of the SERS and fluorescence sensing constructs can be applied for rapid, multiplexed, and robust measurements of environmental toxins in the field.

Marine Toxins Detection by Biosensors Based on Aptamers

Marine toxins cause great harm to human health through seafood, therefore, it is urgent to exploit new marine toxins detection methods with the merits of high sensitivity and specificity, low detection limit, convenience, and high efficiency. Aptasensors have emerged to replace classical detection methods for marine toxins detection. The rapid development of molecular biological approaches, sequencing technology, material science, electronics and chemical science boost the preparation and application of aptasensors. Taken together, the aptamer-based biosensors would be the best candidate for detection of the marine toxins with the merits of high sensitivity and specificity, convenience, time-saving, relatively low cost, extremely low detection limit, and high throughput, which have reduced the detection limit of marine toxins from nM to fM. This article reviews the detection of marine toxins by aptamer-based biosensors, as well as the selection approach for the systematic evolution of ligands by exponential enrichment (SELEX), the aptamer sequences. Moreover, the newest aptasensors and the future prospective are also discussed, which would provide thereotical basis for the future development of marine toxins detection by aptasensors.

In vitro selection of DNA aptamers and their integration in a competitive voltammetric biosensor for azlocillin determination in waste water

The uncontrolled usage of veterinary antibiotics has led to their widespread pollution in waterways and milk products. Potential impact of antibiotic residues on the environment and human health such as increased antibiotic resistance of microorganisms and triggering allergic reactions in humans have been reported. In this work, we developed a highly selective and sensitive voltammetric aptasensor for on-step, sensitive and low cost detection of azlocillin antibiotic, one of the broad spectrum β-lactam antibiotics. The successful selection of DNA aptamers against azlocillin was accomplished using systemic evolution of ligands by exponential enrichment (SELEX) method. Fluorescence-binding assays showed dissociation constant of 55 nM for one of the selected aptamers (Az9). This aptamer was used to construct a competitive voltammetric aptasensor for azlocillin. A limit of detection of 1.2 pg/mL as well as remarkable selectivity against potential interfering agents, including amoxicillin, were achieved. This signal-off competitive sensor takes 30-50 min to complete the quantification of the target antibiotic. The sensor was challenged by detecting the target directly in complex environments such as tap and waste water where good recovery percentages were achieved.

Fabrication of ultra-sensitive photoelectrochemical aptamer biosensor: Based on semiconductor/DNA interfacial multifunctional reconciliation via 2D-C3N4

In this work, we fabricate a novel bismuth vanadate/two dimensional-carbon nitride/deoxyribonucleic acid (BiVO₄/2D-C₃N₄/DNA) aptamer photoelectrochemical (PEC) sensor, and this sensor provides a record detection sensitivity area (5 × 10^-7 μg/L - 10 μg/L) for Microcystin-LR (MC-LR). Meanwhile, except for MC-LR detection, this sensor presents highly sensitivity for tumor marker, heavy metal ion, antibiotic also by changing the DNA aptamer. Photo charge dynamic and theory calculation results reveal that 2D-C₃N₄ is a key material for multifunctional interface reconciliation of this PEC aptamer sensor. Firstly, it can serve as photogenerated hole oriented-transfer medium from the BiVO₄ photoanode to the detective target; In addition, 2D-C₃N₄ with large area of π electron cloud can fix the DNA aptamer parallelly by π-π bonding with the nucleic acid in the DNA aptamer to shorten the hole transfer distance from the semiconductor to target. So that, a record MC-LR detection sensitivity has been achieved by the 2D-C₃N₄ modified BiVO₄/DNA aptamer sensor.