An amperometric aptasensor for ultrasensitive detection of sulfadimethoxine based on exonuclease-assisted target recycling and new signal tracer for amplification

A dual-signal mode electrochemical aptasensor based on tetrahedral DNA nanostructures for sensitive detection of citrinin in food using PtPdCo mesoporous nanozymes

Citrinin (CIT) is a mycotoxin with nephrotoxicity and hepatotoxicity, presenting a significant threat to human health that is often overlooked. Therefore, a dual-signal mode (DPV and SWV) aptasensor for citrinin (CIT) detection was constructed based on tetrahedral DNA nanostructures (TDN) in this study. Furthermore, PtPdCo mesoporous nanozymes exhibit catalase-like catalytic functions, generating significant electrochemical signals through a Fenton-like reaction. Meanwhile their excellent Methylene Blue (MB) loading capability ensures independent dual signal outputs. The RecJf exonuclease-assisted (RecJf Exo-assisted) process can expand the linear detection range, enabling further amplification of the signal. Under optimized conditions, the constructed aptaensor exhibited excellent detection performance with limits of detection (LODs) of 7.67 × 10-3 ng·mL-1 (DPV mode) and 1.57 × 10-3 ng·mL-1 (SWV mode). Due to its multiple signal amplification and highly accurate dual-signal mode detection capability, this aptasensor shows promising potential for the in situ detection.

Isolation of aptamers with excellent cross-reactivity and specificity to sulfonamides towards a ratiometric fluorescent aptasensor for the detection of nine sulfonamides in seafood

Sulfonamides (SAs) is a class of antibiotics that extensively used for treating infectious diseases in livestock industries and aquaculture. Thus, it is urgent need to obtain the bio-receptor, which has excellent cross-reactivity and specificity to SAs, for developing high-throughput methods for the determination of multiple SAs even all commonly-used SAs, to realize the quick screening/detection of total SAs in animal-derived foods. We herein isolated several SAs-specific cross-reactive aptamers by using a library-immobilized SELEX with multi-SAs parallel selection strategy. Two of the isolated aptamers (Sul-01 and Sul-04) can specifically recognize and bind seven SAs respectively with higher binding affinity and no interference of non-sulfonamide antibiotics, and thus can be applied as bio-receptors for developing high-throughput aptasensors for the quick screening/detection of multiple SAs. By using the mixture of Sul-01 and Sul-04 as bio-receptor, a ratiometric fluorescent aptasensor was created for the quick detection of nine SAs including sulfamethoxydiazine (SMD), sulfapyridine (SPD), sulfaquinoxaline (SQ), sulfathiazole (ST), sulfamonomethoxine (SMM), sulfamerazine (SMR), sulfaguanidine (SG), sulfamethazine (SMZ) and sulfadiazine (SD) with a detection limit (LOD) of 0.10-0.50 μM, or total of above nine SAs with a LOD of 0.20 μM. The fluorescent aptasensor was successfully applied to detect each or total of SMD, SPD, SQ, ST, SMM, SMR, SG, SMZ and SD in fish samples with a recovery of 83 %-92 % and a relative standard deviation (RSD, n = 5) < 5 %. This study not only provided several promising bio-receptors for the development of diverse high-throughput aptasensors to achieve the quick screening of multiple SAs residues, but also provided a simple, stable and sensitive method for the quick screening of SMD, SPD, SQ, ST, SMM, SMR, SG, SMZ and SD in seafood.

Electrochemical biosensor for rapid and sensitive monitoring of sulfadimethoxine based on nanoporous carbon and aptamer system

In this study, a straightforward electrochemical aptasensor was developed to detect sulfadimethoxine (SDM). It included a glassy carbon electrode decorated by boron nitride quantum dots (BNQDs) and aptamer-functionalized nanoporous carbon (APT/CZ). CZ was first synthesized by calcinating a zeolitic imidazolate framework (ZIF-8). Then, the electroactive dye methylene blue (MB) was entrapped inside its pores. By attaching aptamer to the CZ surface, APT/CZ acted as a bioguard, which prevented the MB release. Therefore, the electrochemical signal of the entrapped MB was high in the absence of SDM. Introducing SDM caused the conformation of aptamers to change, and a large number of MB was released, which was removed by washing. Therefore, the detection strategy was done based on the change in the electrochemical signal intensity of MB. The aptasensor was applied to detect SDM at a concentration range of 10-17 to 10-7 M with a detection limit of 3.6 × 10-18 M.

A novel sandwich-type electrochemical immunosensor for sensitive detection of zearalenone using NG/PDDA/HNTs and Ti-MOF-KB composites for signal amplification

In this work, a novel sandwich-type electrochemical immunosensor based on signal amplification was developed for the ultrasensitive detection of zearalenone (ZEA). The composite consisting of poly(diallyldimethylammonium chloride) functionalized nitrogen doped graphene, halloysite nanotubes and toluidine blue (Tb/NG/PDDA/HNTs), was synthesized for the first time. Then it was modified with gold nanoparticles (AuNPs) to bind the secondary antibody (Ab2, 10 μg mL-1) and form the tracer label. In addition, ketjen black (KB) was doped into titanium-based metal-organic framework (Ti-MOF), which provided large specific surface area and employed as the sensing platform to increase the immobilization of the primary antibody (Ab1, 10 μg mL-1). This immunosensor showed a wide linear range for ZEA from 10 fg mL-1 to 100 ng mL-1 with a limit of detection (LOD) as low as 0.57 fg mL-1, which was below the maximum tolerable levels (50∼100 µg kg-1) set by the United Nations Food and Agriculture Organization (FAO).

A novel and sensitive electrochemical aptasensor for sulfadimethoxine detection based on the triple helix/exonuclease I-assisted double-amplification strategy

In this paper, a novel and sensitive electrochemical aptasensor for sulfadimethoxine (SDM) detection has been designed based on the triple helix structure/exonuclease I (Exo I)-assisted double signal amplification strategy. The aptamer probe (Apt) hybridizes with the signal transduction probe (STP) on the electrode to form a rigid double-stranded DNA (dsDNA) structure, so that the STP remains upright and methylene blue (MB) on the STP is far away from the electrode surface, resulting in a delicate current signal. In the presence of SDM, the SDM and Apt combine into a complex, leading to the transfer of the Apt and the exposure of the STP. Meanwhile, the added Exo I can digest the Apt to realize the cyclic amplification of SDM. After the addition of the signal probe (SP), a triple helix structure between the SP and STP is formed under acidic conditions, and MB on the STP and SP collide with the electrode surface to generate a strong electrochemical signal. The proposed aptasensor combines the features of the triple helix structure and Exo I to achieve double signal amplification for the sensitive detection of SDM with a wide linear range of 0.05-1000 ng mL-1 and a low detection limit of 0.02 ng mL-1. Furthermore, it has been successfully used to detect SDM in milk and lake water samples.

Aggregation-induced enhancement of pyrene-based metal-organic framework as a new electrochemiluminescence emitter for ultrasensitive detection of sulfadimethoxine

In this study, a signal-on electrochemiluminescence (ECL) aptasensor for ultrasensitive detection of sulfadimethoxine (SDM) was constructed based on a competitive aptamer strategy. Specifically, cerium-metal-organic framework (Ce-MOF) with large specific surface area and excellent electrical conductivity was combined with gold nanoparticles (AuNPs) to form a substrate, followed by the immobilisation of double-stranded DNA (dsDNA) via AuN bonds. In the presence of SDM, the aptamer tended to form an aptamer-SDM complex, which caused dsDNA to dissociate. After release of the aptamer, the capture probe (CP) combined with the tracer label to enhance the ECL signal. As expected, the prepared sensor displayed an ideal linear response range from 10.0 fg mL-1 to 100 ng mL-1 with a limit of detection (LOD) of 1.28 fg mL-1 and successfully detected SDM in milk and quality control samples.

An upconversion biosensor based on inner filter effect for dual-role recognition of sulfadimethoxine in aquatic samples

Sulfadimethoxine (SDM) as an extensively employed veterinary drug causes potential threats to human health. Herein, a dual recognition mode novel upconversion fluorescence biosensor was designed based on inner filter effect (IFE) to sensitively and rapidly detect SDM in aquatic samples. Aldehyde-functionalized magnetic nanoparticles (MNPs) were applied to recognize and capture SDM, followed by specifically bond with biotin-labeled aptamers. The upconversion nanoparticles and the colored products resulting from the enzyme-catalyzed oxidation of 3,3,5,5-tetramethylbenzidine exhibited an IFE quenching process. Under the optimal condition, the results displayed the fluorescence intensity was correlated with the concentration of SDM within the range of 0.5-1000 ng⋅mL-1 achieving a low limit of detection of 0.13 ng⋅mL-1. The SDM detection system was further employed in the spiked aquatic samples with good recoveries (88.41-96.78 %). Consequently, the constructed fluorescence biosensor provided broad prospects for accuracy and rapid detection of SDM.

A signal "on-off-on"-type electrochemiluminescence aptamer sensor for detection of sulfadimethoxine based on Ru@Zn-oxalate MOF composites

An "on-off-on"-type electrochemiluminescence (ECL) aptamer sensor based on Ru@Zn-oxalate metal-organic framework (MOF) composites is constructed for sensitive detection of sulfadimethoxine (SDM). The prepared Ru@Zn-oxalate MOF composites with the three-dimensional structure provide good ECL performance for the "signal-on." The MOF structure with a large surface area enables the material to fix more Ru(bpy)₃²⁺. Moreover, the Zn-oxalate MOF with three-dimensional chromophore connectivity provides a medium which can accelerate excited-state energy transfer migration among Ru(bpy)₃²⁺ units, and greatly reduces the influence of solvent on chromophore, achieving a high-energy Ru emission efficiency. The aptamer chain modified with ferrocene at the end can hybridize with the capture chain DNA1 fixed on the surface of the modified electrode through base complementary pairing, which can significantly quench the ECL signal of Ru@Zn-oxalate MOF. SDM specifically binds to its aptamer to separate ferrocene from the electrode surface, resulting in a "signal-on" ECL signal. The use of the aptamer chain further improves the selectivity of the sensor. Thus, high-sensitivity detection of SDM specificity is realized through the specific affinity between SDM and its aptamer. This proposed ECL aptamer sensor has good analytical performance for SDM with low detection limit (27.3 fM) and wide detection range (100 fM-500 nM). The sensor also shows excellent stability, selectivity, and reproducibility, which proved its analytical performance. The relative standard deviation (RSD) of SDM detected by the sensor is between 2.39 and 5.32%, and the recovery is in the range 97.23 to 107.5%. The sensor shows satisfactory results in the analysis of actual seawater samples, which is expected to play a role in the exploration of marine environmental pollution.

Broad-Specificity Aptamer of Sulfonamides: Isolation and Its Application in Simultaneous Detection of Multiple Sulfonamides in Fish Sample

Sulfonamide antibiotics (SAs) are widely used in animal husbandry and aquaculture, and the excess residues of SAs in animal-derived foods will harm the health of consumers. In reality, various SAs were alternately used in animal husbandry and aquaculture, and thus, it is urgent need to develop simple and high-throughput methods for simultaneously detecting multiple SAs or groups of SAs in order to realize rapid screening of total SAs residues in animal-derived foods. We herein isolated a broad-specificity aptamer for SAs by using a multi-SAs systematic evolution of ligands by exponential enrichment (SELEX) strategy. The isolated broad-specificity aptamer has a higher binding affinity to five different SAs including sulfaquinoxaline (SQ), sulfamethoxypyridazine (SMPZ), sulfametoxydiazine (SMD), sulfachloropyridazine (SCP), and sulfapyridine (SPD) and, thus, can be used as a bioreceptor for developing various high-throughput methods for the simultaneous detection or rapid screening of above five SAs. Based on the isolated broad-specificity aptamer and Cy7 (diethylthiatricarbocyanine) displacement strategy, a colorimetric aptasensor was developed for the simultaneous detection of SQ, SMPZ, SMD, SCP, and SPD with a visual detection limit of 2.0-5.0 μM and a spectrometry detection limit of 0.2-0.5 μM. The colorimetric aptasensor was successfully used to detect SQ, SMPZ, SMD, SCP, and SPD in fish muscle with a recovery of 82%-92% and a RSD (n = 5) < 7%. The success of this study provided a promising bioreceptor for developing various high-throughput methods for on-site rapid screening of multiple SAs residues, as well as a simple method for the rapid and cost-effective screening of total SQ, SMPZ, SMD, SCP, and SPD in seafood.

Application of Nanomaterial Modified Aptamer-Based Electrochemical Sensor in Detection of Heavy Metal Ions

Heavy metal pollution resulting from significant heavy metal waste discharge is increasingly serious. Traditional methods for the detection of heavy metal ions have high requirements on external conditions, so developing a sensitive, simple, and reproducible detection method is becoming an urgent need. The aptamer, as a new kind of artificial probe, has received more attention in recent years for its high sensitivity, easy acquisition, wide target range, and wide use in the detection of various harmful substances. The detection platform that an aptamer-based electrochemical biosensor (E-apt sensor) provides is a new approach for the detection of heavy metal ions. Nanomaterials are particularly important in the construction of E-apt sensors, as they can be used as aptamer carriers or sensitizers to stimulate or inhibit electrochemical signals, thus significantly improving the detection sensitivity. This review summarizes the application of different types of nanomaterials in E-apt sensors. The construction methods and research progress of the E-apt sensor based on different working principles are systematically introduced. Moreover, the advantages and challenges of the E-apt sensor in heavy metal ion detection are summarized.

Incorporating Fullerenes in Nanoscale Metal-Organic Matrixes: An Ultrasensitive Platform for Impedimetric Aptasensing of Tobramycin

The rational design and preparation of available fullerene@metal-organic matrix hybrid materials are of profound significance in electrochemical biosensing applications due to their unique photoelectric properties. In this work, C₆₀@UiO-66-NH₂ nanocomposites serve as greatly promising materials to modify electrodes and fix aptamers, resulting in a remarkable electrochemical aptasensor for impedimetric sensing of tobramycin (TOB). Nanoscale composites have preferable electroactivity and small particle size with more exposed functional sites, such as Zr(IV) and -NH₂, to immobilize aptamers for enhanced detection performance. As we know, most of the electrochemical impedance aptasensors require a long time to complete the detection process, but this prepared biosensor shows the rapid quantitative identification of target TOB within 4 min. This work expands the synthesis of functional fullerene@metal-organic matrix hybrid materials in electrochemical biosensing applications.

Facile design of multifunction-integrated linear oligonucleotide probe with multiplex amplification effect for label-free and highly sensitive GMO biosensing

Well-defined structures and compositions of nucleic acids afford oligonucleotide probes with unique chemical properties and biological functions for various biosensing applications. Herein, a unique and special oligonucleotide probe, named multifunction-integrated linear oligonucleotide probe (MI-LOP), was facile designed and reported for label-free and turn-on fluorescent detection of the codon component of genetically modified organisms (GMOs). The MI-LOP contains four different functional regions including recognition of target, serving as polymerization template, and creating polymerization primer-linked G-quadruplex (PP-G-quadruplex). Without the aid of any other oligonucleotides, the introduction of target DNA can make each function of the MI-LOP executed one-by-one, during which the species of target DNA, target analogue, and PP-G-quadruplex can be cyclically utilized and in turn induce a multiplex signal amplification responsible for substantial collection of the G-quadruplex moieties under isothermal conditions. The stable G-quadruplexes can combine with N-methyl mesoporphyrin IX (NMM) and function as efficient fluorescence light-up probes, rapidly leading to a dramatic increase in the fluorescence intensity for the amplified detection of the target codon component. Our results strongly demonstrate that the developed MI-LOP with multiplex amplification effect confers the sensing strategy a high sensitivity and specificity for quantitative and qualitative detection of the target codon. And it has also been successfully applied for analyzing target codon in the complex extractions of soybean. The achievements highlight the significance of using oligonucleotide probes as promising analytical tools to promote the basic biochemical research and help in food and environmental analysis.

Nucleic acid-based electrochemical biosensor: Recent advances in probe immobilization and signal amplification strategies

With the increasing importance of accurate and early disease diagnosis and the development of personalized medicine, DNA-based electrochemical biosensor has attracted broad scientific and clinical interests in the past decades due to its unique hybridization specificity, fast response time, and potential for miniaturization. In order to achieve high detection sensitivity, the design of DNA electrochemical biosensors depends critically on the improvement of the accessibility of target molecules and the enhancement of signal readout. Here, we summarize the recent advances in DNA probe immobilization and signal amplification strategies with a special focus on DNA nanostructure-supported DNA probe immobilization method, which provides the opportunity to rationally control the distance between probes and keep them in upright confirmation, as well as the contribution of functional nanomaterials in enhancing the signal amplification. The next challenge of biosensors will be the fabrication of point-of-care devices for clinical testing. The advancement of multidisciplinary areas, including nanofabrication, material science, and biochemistry, has exhibited profound promise in achieving such portable sensing devices. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures.

Target-Mediated 5'-Exonuclease Digestion of DNA Aptamers with RecJ to Modulate Rolling Circle Amplification for Biosensing

We report a new method for biosensing based on the target-mediated resistance of DNA aptamers against 5'-exonuclease digestion, allowing them to act as primers for rolling circle amplification (RCA). A target-bound DNA strand containing an aptamer region on the 5'-end and a primer region on the 3'-end is protected from 5'-exonuclease digestion by RecJ exonuclease in a target-dependent manner. As the protected aptamer is at the 5'-end, the exposed primer on the 3'-end can participate in RCA in the presence of a circular template to generate a turn-on sensor. Without target, RecJ digests the primer and prevents RCA from occurring, allowing quantitative fluorescence detection of both thrombin, a protein, and ochratoxin A (OTA), a small molecule, at picomolar concentrations.

Recent trends in nanomaterial-based signal amplification in electrochemical aptasensors

Ultrasensitive biosensors have become a necessity in the world of scientific research, and several signal enhancement strategies have been employed to attain exceptionally low detection limits. Nanotechnology turns out to be a strong contender for signal amplification, as they can be employed as platform modifiers, catalysts, carriers or labels. Here, we have described the most recent advancements in the utilization of nanomaterials as signal amplification components in aptamer-based electrochemical biosensors. We have briefly reviewed the methods that utilized nanomaterials, namely gold and carbon, as well as nanocomposites such as: graphene, carbon nanotubes, quantum dots, and metal-organic frameworks.

Contribution of Nanomaterials to the Development of Electrochemical Aptasensors for the Detection of Antimicrobial Residues in Food Products

The detection of antimicrobial residues in food products of animal origin is of utmost importance. Indeed antimicrobial residues could be present in animal derived food products because of animal treatments for curative purposes or from illegal use. The usual screening methods to detect antimicrobial residues in food are microbiological, immunological or physico-chemical methods. The development of biosensors to propose sensitive, cheap and quick alternatives to classical methods is constantly increasing. Aptasensors are one of the major trends proposed in the literature, in parallel with the development of immunosensors based on antibodies. The characteristics of electrochemical sensors (i.e., low cost, miniaturization, and portable instrumentation) make them very good candidates to develop screening methods for antimicrobial residues in food products. This review will focus on the recent advances in the development of electrochemical aptasensors for the detection of antimicrobial residues in food products. The contribution of nanomaterials to improve the performance characteristics of electrochemical aptasensors (e.g., Sensitivity, easiness, stability) in the last ten years, as well as signal amplification techniques will be highlighted.

Amplified photocurrent signal for fabricating photoelectrochemical sulfadimethoxine aptasensor based on carbon nitride photosensitization with visible/near-infrared light responsive zinc phthalocyanine

Developing effective analytical method for sulfadimethoxine (SDM) detection is highly desirable and vitally crucial for protecting environment safety and human health. Herein, a highly selective and sensitive photoelectrochemical (PEC) aptasensor for accurate detection of SDM was proposed, which employed zinc phthalocyanine/graphitic carbon nitride (ZnPc/CN) nanocomposite as photosensitive material. The ZnPc/CN nanocomposite was constructed by modification of CN nanosheet with visible/near-infrared light responsive photosensitizer ZnPc. The introduction of ZnPc into CN exhibited amplified PEC response, which was 5.7 and 18.3 times than pure ZnPc and CN, attributed to the enhanced light harvesting ability and improved photoelectric conversion efficiency of such nanocomposite. By using ZnPc/CN and sulfadimethoxine (SDM) aptamer as PEC response material and specific probe, a PEC aptasensor was established for SDM detection. The aptamer was connected to the surface of chitosan/ZnPc/CN/ITO through the formation of phosphoramidate bonds between the amino group of the chitosan and phosphate group of the aptamer at 5' end. The fabricated aptasensor displayed good detection linearity of 0.1 ~ 300 nM and low detection limit of 0.03 nM (S/N = 3) under optimized conditions, and the potential applicability of the PEC aptasensor was confirmed by detecting SDM in milk powder samples.

An electrochemical immunosensor coupling a bamboo-like carbon nanostructure substrate with toluidine blue-functionalized Cu(ii)-MOFs as signal probes for a C-reactive protein assay

In this paper, a novel sandwich immunosensor based on a toluidine blue (Tb) loaded metal organic framework (Cu(ii)-HKUST-1/Tb) as the signal element and a nitrogen-doped 3D carbon nanostructure as the electrode substrate was constructed for the detection of C-reactive protein (CRP). Tb as an electrochemically active agent usually forms a polymer by aggregation in the solvent, causing a poor electrochemical response. Therefore, in order to overcome this obstacle, Cu(ii)-HKUST-1 with a porous nanostructure and large specific surface area as a carrier could adsorb a large number of Tb molecules on its surface to improve its electrochemical performance. In addition, the high electron transfer efficiency of the N-doped bamboo-like carbon nanotubes (CoFe/N-GCT) improves the sensitivity of the biosensor. Differential pulse voltammetry (DPV) was used to detect the current signal of Tb at -0.2 V. The current response increased with the increase in concentration of CRP, ranging from 0.5 to 200 ng mL-1. The detection limit is 166.7 pg mL-1 (S/N = 3). Moreover, the proposed biosensor can be applied in real serum sample detection. It has potential applications in the field of biomedicine assays.

A reusable colorimetric assay based on mixed valence state Ce-MOF@Pt nanoparticles for highly sensitive detection of visfatin

In this work, a colorimetric assay for visfatin detection is described. The mixed valence state Ce-MOF (MVCM) modified by platinum nanoparticles (Pt NPs) is used as a novel catalyst. MVCM exhibits excellent intrinsic peroxidase-like catalytic activity due to the spontaneous recycling of the Ce(III)/Ce(IV) system. Pt NPs serve not only as a carrier of the -NH₂-modified single strand DNA (S1) but also as a synergistic catalyst of MVCM. The capture probe (S2) attached to the streptavidin-modified magnetic beads (Mag-SA) could combine with the aptamer to form the Mag-SA/S2/aptamer complex. When in the presence of the target visfatin, aptamer specifically combines with the visfatin, which induces the release of Mag-SA/S2 from the Mag-SA/S2/aptamer complex. At this time, the MVCM@Pt/S1 complex connects with the released Mag-SA/S2, which quickly catalyzes the 3,3,3',3'-tetramethylbenzidine (TMB), leading to a color change. Under optimal conditions, the absorbance increases linearly when the concentration ranges from 1 ng mL^-1 to 100 ng mL^-1, and the detection limit is as low as 0.11 ng mL^-1. Furthermore, Mag-SA/S2 can be reused at least five times by using the uracil-DNA glycosylase (UDG) and an external magnetic field. The proposed method shows satisfying reproducibility, stability, specificity, and sensitivity, and it was successfully applied to detect visfatin in spiked human serum samples. Thus, it has great potential for clinical research, detection, and catalytic applications.

An efficient electrochemical assay for miR-3675-3p in human serum based on the nanohybrid of functionalized fullerene and metal-organic framework

Idiopathic pulmonary fibrosis (IPF) is an interstitial lung disease with unclear pathogenesis, for which diagnosis has been a great challenge. Recent researches have revealed that miR-3675-3p is a promising biomarker for IPF diagnosis. Herein, the present work describes a novel electrochemical microRNA biosensor for rapid and sensitive detection of miR-3675-3p based on multiple signal amplification strategies. First of all, fullerene (C₆₀) is doped with poly(amidoamine) (PAMAM)-functionalized metal-organic framework (MOF) to form a new nanohybrid of C₆₀@PAMAM-MOF, which exhibits more remarkable redox activity compared with the other two synthesized C₆₀-based nanohybrids when triggered by tetraoctylammonium bromide (TOAB). C₆₀@PAMAM-MOF also possesses a large specific surface area and abundant amino groups to anchor Au nanoparticles (AuNPs) for the immobilization of signal probe (SP) to form tracer label and enhance the electrochemical response signal. In addition, core@shell Au-Pt nanoparticles (Au@PtNPs) are absorbed on chitosan-acetylene black (CS-AB) to act as sensing platform, which can promote electron transfer and increase the loading of capture probe (CP). Under optimum conditions, the proposed biosensor displays a wide linear range for miR-3675-3p from 10 fM to 10 nM, with a limit of detection (LOD) as low as 2.99 fM. More significantly, this biosensor shows a lower LOD and wider linear range than that of qRT-PCR, and its trial application in human serum shows favorable results, which exhibits a promising prospect for IPF diagnosis.

Advances in Electrochemical Aptasensors Based on Carbon Nanomaterials

Carbon nanomaterials offer unique opportunities for the assembling of electrochemical aptasensors due to their high electroconductivity, redox activity, compatibility with biochemical receptors and broad possibilities of functionalization and combination with other auxiliary reagents. In this review, the progress in the development of electrochemical aptasensors based on carbon nanomaterials in 2016–2020 is considered with particular emphasis on the role of carbon materials in aptamer immobilization and signal generation. The synthesis and properties of carbon nanotubes, graphene materials, carbon nitride, carbon black particles and fullerene are described and their implementation in the electrochemical biosensors are summarized. Examples of electrochemical aptasensors are classified in accordance with the content of the surface layer and signal measurement mode. In conclusion, the drawbacks and future prospects of carbon nanomaterials’ application in electrochemical aptasensors are briefly discussed.

Graphene-family materials in electrochemical aptasensors

The study of graphene-based carbon nanocomposites has remarkably increased in recent years. Functionalized graphene-based nanostructures, including graphene oxide and reduced graphene oxide, have great potential as new innovative electrode materials in the fabrication of novel electrochemical sensors. Electrochemical sensors based on aptamers attracted great attention because of their high sensitivity and selectivity, and simple instrumentation, as well as low production cost. Aptamers as a potent alternative to antibodies are functional nucleic acids with a high tendency to specific analytes. Electrochemical aptasensors show specific recognition ability for a wide range of analytes. Although aptamers are selected in vitro in contrast to antibodies, they are interesting due to advantages like high stability, easy chemical modifications, and the potential to be employed in nanostructured device fabrication or electrochemical sensing devices. Recently, new nanomaterials have shown a significant impact on the production of electrochemical sensors with high efficiency and performance. This review aims to give an outline of electrochemical aptasensors based on the graphene family materials and discuss the detection mechanism in this type of aptasensors. The present review summarizes some of the recent achievements in graphene-based aptasensors and includes their recent electroanalytical applications. Graphical Abstract Graphical Abstract.

Computer-Aided aptamer design for sulfadimethoxine antibiotic: step by step mutation based on MD simulation approach

This study introduces a computational method to design a new aptamer with higher binding affinity to a special target in comparison with the experimentally available aptamers. The method is called step by step mutation based on MD simulation, which includes some steps. First, MD simulation is performed for the SELEX-introduced (native) aptamer in the presence of the target. Afterwards, conformational factor (P_i) is calculated for the simulated system, which obtains the affinity of the aptamer residues to the target. A nucleotide exchange is done for the residue with the least P_i parameter to the nucleotide with the highest P_i value that results in a mutant aptamer. MD simulation is performed for the target-mutant complex, and P_i values are calculated again. The nucleotide exchange is performed similarly, and the designing process is proceeded repeatedly that results in a mutant with the improved specificity to the target. The aptamer affinity to the target is also determined in each step through calculating the binding Gibbs energy (ΔG_Bind) as a reliable parameter. The introduced strategy is utilized efficiently to design a mutant aptamer with improved specificity toward sulfadimethoxine (SDM) antibiotic as a case study. The great difference in the ΔG_Bind values about 579.856 kJ mol^-1 highlights that the M5 mutant possesses the improved specificity toward SDM in comparison with the native aptamer. Besides, the selectivity of the M5 aptamer toward SDM is examined among some conventional interfering compounds by using MD simulation that confirms the applicability of the designed aptamer for further experimental studies.

Highly Selective Wearable Smartsensors for Vapor/Liquid Amphibious Methanol Monitoring

A wearable screen-printed electrochemical smartsensor with excellent selectivity for methanol quantification has been developed. This smartsensor consists of a printable sensing system modified with platinum (Pt) confined in a reduced graphene oxide (rGO) matrix, as well as a compact electronic interface for data collection. The real-time electrochemical signal from methanol could be remotely detected and transmitted to a smartphone by blue tooth. It performs good environmental adaptability of vapor/liquid amphibious behaviors. Owing to the uniform distribution of Pt loading on the rGO nanosheets, this sensor demonstrates high selectivity, sensitivity, stability, and recoverability both in vapor and liquid during temperature or humidity diversification, compared with other resistance-based sensors. It also achieves good bending and stretching performance, and it could be a possible candidate device for the quantification of methanol in different environments.

A dichromatic label-free aptasensor for sulfadimethoxine detection in fish and water based on AuNPs color and fluorescent dyeing of double-stranded DNA with SYBR Green I

A dichromatic label-free aptasensor was described for sulfadimethoxine (SDM) detection. Compared with the binding of SDM-aptamer to SDM, the higher affinity of aptamer to cDNA may result in the hybridization of dsDNA. In the presence of SDM, the aptamer specifically binds to SDM, leading to a blue color of AuNPs in deposit and fluorescence at 530 nm in supernatant after adding cDNA and SGI. With no target of SDM, AuNPs protected with the aptamer re-disperse in PBS with a red color, and no fluorescence occurs in supernatant. Based on the principle, SDM can be quantitatively detected through both fluorescent emission and AuNPs color changes with recoveries ranging from 99.2% to 102.0% for fish and from 99.5% to 100.5% for water samples. An analytical linear range of 2-300 ng mL^-1 was achieved with the detection limits of 3.41 ng mL^-1 for water and 4.41 ng g^-1 for fish samples (3σ, n = 9).

Electrochemical aptasensor for sulfadimethoxine detection based on the triggered cleavage activity of nuclease P1 by aptamer-target complex

Herein, a simple and selective electrochemical method was developed for sulfadimethoxine detection based on the triggered cleavage activity of nuclease P1 by the formation of aptamer and sulfadimethoxine conjugate. After probe DNA was immobilized on gold electrode surface, aptamer DNA labeled with biotin at its 5'-terminal was then captured on electrode surface through the hybridization reaction between probe DNA and aptamer DNA. The formed double-stranded DNA (dsDNA) can block the digestion activity of Nuclease P1 towards the single-stranded probe DNA. Then, the anti-dsDNA antibody was further modified on electrode surface based on the specific interaction between dsDNA and antibody. Due to the electrostatic repulsion effect and steric-hindrance effect, a weak electrochemical signal was obtained at this electrode. However, in the presence of sulfadimethoxine, it can interact with aptamer DNA, and then the formation of dsDNA can be blocked. As a result, the probe DNA at its single-strand state can be digested by Nuclease P1, which leads to the failure of the immobilization of anti-dsDNA antibody. At this state, a strong electrochemical signal was obtained. Based on the change of the electrochemical signal, sulfadimethoxine can be detected with linear range of 0.1-500 nmol/L. The detection limit was 0.038 nmol/L. The developed method possesses high detection selectivity and sensitivity. The applicability of this method was also proved by detecting sulfadimethoxine in veterinary drug and milk with satisfactory results.

Trimetallic signal amplification aptasensor for TSP-1 detection based on Ce-MOF@Au and AuPtRu nanocomposites

In this work, an aptamer was used as the target capturing agent and a trimetallic signal amplification strategy based on Ce-MOF@Au and AuPtRu NPs was demonstrated for the sensitive detection of TSP-1. Herein, the synthesized AuPtRu nanocomposite (AuPtRu NPs) not only acts as the catalyst for catalyzing hydrogen peroxide but also acts as a nanocarrier for capturing the -NH₂ termination single strand DNA (S1) to obtain the signal probe (SP, AuPtRu nanocomposite/S1). Then, SP was efficiently linked into TSP-1 aptamers with the addition of complementary linking strands to form M1 (SP/aptamer). The Ce-MOF@Au nanocomposites were obtained by in situ reduction and used as GCE electrode modification materials. The -NH₂-modified capture probe (CP) DNA was immobilized on the surface of Ce-MOF@Au nanocomposites for hybridizing SP. In the presence of the target TSP-1, the aptamer recognizes the target and binds strongly so that SP is released from the prepared M1 and then hybridized with CP. When the detection solution contains an electrochemical matrix of H₂O₂, AuPtRu NPs can oxidize H₂O₂ to obtain an enhanced signal. Furthermore, the proposed aptasensor has a very low LOD of 0.13 fg mL^-1 TSP-1 in the detection range of 1 fg mL^-1 to 10 ng mL^-1. Moreover, the proposed platform also has application implications for other potential targets.

An aptamer based voltammetric biosensor for endotoxins using a functionalized graphene and molybdenum disulfide composite as a new nanocarrier

Lipopolysaccharides (LPS), known as endotoxins, can cause a strong inflammatory response and lead to multiple organ failure in severe cases. This work reports a simple label-free voltammetric aptasensor for highly sensitive determination of LPS using a polyethyleneimine (PEI) functionalized reduced graphene oxide (rGO) and molybdenum disulfide (MoS₂) composite (PEI-rGO-MoS₂) as a new nanocarrier for electroactive toluidine blue (TB). The PEI-rGO-MoS₂ nanocomposite with high electrical conductivity and large specific surface area can greatly increase the loading of TB and facilitate electron transfer from TB to an electrode. Then gold nanoparticles (AuNPs) were utilized to immobilize a thiolated LPS binding aptamer (LBA), which not only exhibited excellent biocompatibility, but also significantly amplified the electrochemical signal of TB. The proposed aptasensor exhibited high sensitivity for LPS and showed a wide linear range from 5.0 × 10^-5 ng mL^-1 to 2.0 × 10² ng mL^-1 with a low limit of detection (LOD) of 3.01 × 10^-5 ng mL^-1, which overcame the shortcomings of traditional detection methods and achieved fast and accurate detection of LPS. Moreover, it exhibited excellent recovery and specificity upon spiking LPS in serum samples, indicating that this method has promising application in the field of trace analysis of LPS in clinical detection.