A visual detection method for Salmonella Typhimurium based on aptamer recognition and nanogold labeling

Signal-off colorimetric and signal-on fluorometric dual-mode aptasensor for ultrasensitive detection of Salmonella Typhimurium using graphitic carbon nitride

Food safety is severely burdened by the prevalence of foodborne pathogens and the diseases they cause, necessitating the development of rapid, easy-to-use, highly sensitive, and reliable detection methods. Here, a signal-off colorimetric and signal-on fluorometric dual-mode detection method for Salmonella Typhimurium (S. typhimurium) was developed based on its unique interaction with aptamer DNA and graphitic carbon nitride (GCN). In the absence of a target Salmonella species, 6-carboxyfluorescein (FAM)-labeled aptamers are adsorbed on the surface of GCN primarily via a π-π interaction, resulting in reduced fluorescence of FAM through GCN-mediated quenching as well as improved peroxidase-like activity of GCN to generate intense blue color through facilitated electrostatic attraction between the negatively charged aptamer and positively charged 3,3',5,5'-tetramethylbenzidine (TMB) substrate. The introduction of S. typhimurium to the sample solution causes the detachment of the aptamer from GCN due to its higher affinity for S. typhimurium than GCN, thereby rapidly reducing the colorimetric signal and recovering the fluorescence. We successfully determined the number of S. typhimurium using this method in a remarkably short duration (10-30 min), highlighting its rapidity. The limit of detection values for S. typhimurium were as low as 8 and 3 CFU/mL when using colorimetric and fluorometric methods, respectively. Moreover, this method can be used to detect S. typhimurium spiked in real vegetable extract and milk with high reproducibility and reliability. This method serves as a convenient route to the rapid, sensitive, selective, and reliable detection of pathogens from complex food samples, with the potential to replace conventional yet laborious methods currently in use.

Selection and application of highly specific Salmonella typhimurium aptamers against matrix interference

In practical applications, the structure and performance of aptamers can be influenced by the presence of sample matrices, which interferes with the specific binding between the aptamer and its target. In this work, to obtain aptamer chains resistant to matrix interference, four typical food matrices were introduced as negative selection targets and selection environments in the process of selecting aptamers for Salmonella typhimurium using the systematic evolution of ligands by exponential enrichment (SELEX) technology. As a result, some highly specific candidate aptamers for Salmonella typhimurium (BB-34, BB-37, ROU-8, ROU-9, ROU-14, ROU-24, DAN-3, NAI-12, and NAI-21) were successfully obtained. Based on the characterization results of secondary structure, affinity, and specificity of these candidate aptamers, ROU-24 selected in the pork matrix and BB-34 selected in the binding buffer were chosen to develop label-free fluorescence aptasensors for the sensitive and rapid detection of the Salmonella typhimurium and verify the performance against matrix interference. The ROU-24-based aptasensor demonstrated a larger linear range and better specificity compared to the BB-34-based aptasensor. Meanwhile, the recovery rate of the ROU-24-based aptasensor in real sample detection (ranging from 94.2% to 110.7%) was significantly higher than that of the BB-34-based aptasensor. These results illustrated that the negative selection of food matrices induced in SELEX could enhance specific binding between the aptamer and its target and the performance against matrix interference. Overall, the label-free fluorescence aptasensors were developed and successfully validated in different foodstuffs, demonstrating a theoretical and practical basis for the study of aptamers against matrix interference.

Selection of DNA Aptamers Against Neisseria gonorrhoeae Causing Sexually Transmitted Infection (STI)

Neisseria gonorrhoeae (NG) is the second most common bacterial sexually transmitted infection (STI) worldwide. Gonorrhoea is a very serious infection because if untreated, it can lead to significant ramifications to reproductive, maternal, & newborn health and increase transmission of HIV. Infections are very often asymptomatic and symptoms when present manifest differently in men and women. The cornerstone of gonorrhoea control is to assure rapid diagnosis and prompt treatment of patients to prevent the onward spread of infection. The resource-rich settings are utilizing nucleic acid amplification tests (NAATs) for diagnosis, whereas resource-limited settings like ours where laboratory infrastructure is lacking, reliance is placed on syndromic approach. In view of the limitations of each, there is a compelling need for development of a point of care test (POCT). Aptamers offer such potential. These are short oligonucleotides that bind to its target with high affinity and specificity and therefore can be maneuvred for use in diagnostics. In this study, we performed live cell-SELEX (Systematic Evolution of Ligands by EXponential enrichment) to select 12 single-stranded DNA (ssDNA) aptamers that bind strongly to a cocktail of Neisseria gonorrhoeae strains, with Kd values ranging from 8.58 to 596 nM. Gold nanoparticle (GNP) assay revealed that one of the aptamers, E8 19 was highly specific for Neisseria gonorrhoeae (Kd = 24.5 nM). More importantly, it did not demonstrate any binding to Neisseria meningitidis and commensal Neisseria sp. The identified aptamer holds much promise for the development of a rapid test for diagnosis of gonorrhoea.

Bioaffinity Nanoprobes for Foodborne Pathogen Sensing

Bioaffinity nanoprobes are a type of biosensor that utilize the specific binding properties of biological molecules, such as antibodies, enzymes, and nucleic acids, for the detection of foodborne pathogens. These probes serve as nanosensors and can provide highly specific and sensitive detection of pathogens in food samples, making them an attractive option for food safety testing. The advantages of bioaffinity nanoprobes include their ability to detect low levels of pathogens, rapid analysis time, and cost-effectiveness. However, limitations include the need for specialized equipment and the potential for cross-reactivity with other biological molecules. Current research efforts focus on optimizing the performance of bioaffinity probes and expanding their application in the food industry. This article discusses relevant analytical methods, such as surface plasmon resonance (SPR) analysis, Fluorescence Resonance Energy Transfer (FRET) measurements, circular dichroism, and flow cytometry, that are used to evaluate the efficacy of bioaffinity nanoprobes. Additionally, it discusses advances in the development and application of biosensors in monitoring foodborne pathogens.

Biosensors, modern technology for the detection of cancer-associated bacteria

Cancer is undoubtedly one of the major human challenges worldwide. A number of pathogenic bacteria are deemed to be potentially associated with the disease. Accordingly, accurate and specific identification of cancer-associated bacteria can play an important role in cancer control and prevention. A variety of conventional methods such as culture, serology, and molecular-based methods as well as PCR and real-time PCR have been adopted to identify bacteria. However, supply costs, machinery fees, training expenses, consuming time, and the need for advanced equipment are the main problems with the old methods. As a result, advanced and modern techniques are being developed to overcome the disadvantages of conventional methods. Biosensor technology is one of the innovative methods that has been the focus of researchers due to its numerous advantages. The main purpose of this study is to provide an overview of the latest developed biosensors for recognizing the paramount cancer-associated bacteria.

Recent Advances in Electrochemical Biosensors for the Detection of Salmonellosis: Current Prospective and Challenges

Salmonellosis is a major cause of foodborne infections, caused by Salmonella, posing a major health risk. It possesses the ability to infiltrate the food supply chain at any point throughout the manufacturing, distribution, processing or quality control process. Salmonella infection has increased severely and requires effective and efficient methods for early monitoring and detection. Traditional methods, such as real-time polymerase chain reaction and culture plate, consume a lot of time and are labor-intensive. Therefore, new quick detection methods for on-field applications are urgently needed. Biosensors provide consumer-friendly approaches for quick on-field diagnoses. In the last few years, there has been a surge in research into the creation of reliable and advanced electrochemical sensors for the detection of Salmonella strains in food samples. Electrochemical sensors provide extensive accuracy and reproducible results. Herein, we present a comprehensive overview of electrochemical sensors for the detection of Salmonella by focusing on various mechanisms of electrochemical transducer. Further, we explain new-generation biosensors (microfluidics, CRISPR- and IOT-based) for point-of care applications. This review also highlights the limitations of developing biosensors in Salmonella detection and future possibilities.

Metal-Organic Frameworks-Based Sensors for Food Safety

Food contains a variety of poisonous and harmful substances that have an impact on human health. Therefore, food safety is a worldwide public concern. Food detection approaches must ensure the safety of food at every step of the food supply chain by monitoring and evaluating all hazards from every single step of food production. Therefore, early detection and determination of trace-level contaminants in food are one of the most crucial measures for ensuring food safety and safeguarding consumers' health. In recent years, various methods have been introduced for food safety analysis, including classical methods and biomolecules-based sensing methods. However, most of these methods are laboratory-dependent, time-consuming, costly, and require well-trained technicians. To overcome such problems, developing rapid, simple, accurate, low-cost, and portable food sensing techniques is essential. Metal-organic frameworks (MOFs), a type of porous materials that present high porosity, abundant functional groups, and tunable physical and chemical properties, demonstrates promise in large-number applications. In this regard, MOF-based sensing techniques provide a novel approach in rapid and efficient sensing of pathogenic bacteria, heavy metals, food illegal additives, toxins, persistent organic pollutants (POPs), veterinary drugs, and pesticide residues. This review focused on the rapid screening of MOF-based sensors for food safety analysis. Challenges and future perspectives of MOF-based sensors were discussed. MOF-based sensing techniques would be useful tools for food safety evaluation owing to their portability, affordability, reliability, sensibility, and stability. The present review focused on research published up to 7 years ago. We believe that this work will help readers understand the effects of food hazard exposure, the effects on humans, and the use of MOFs in the detection and sensing of food hazards.

Dual-mode aptasensor for simultaneous detection of multiple food-borne pathogenic bacteria based on colorimetry and microfluidic chip using stir bar sorptive extraction

A dual-mode aptasensor using colorimetry and microfluidic chip (MC) together with stir bar sorptive extraction (SBSE) has been developed for firstly qualifying samples contaminated with Vibrio parahaemolyticus (V.P) and Salmonella typhimurium (S.T), then precisely determine both of them in positive samples. For this purpose, the aptamer-streptavidin encoded probes (Apt-SAEs) corresponding to different bacteria were prepared in advance. Then, a stir bar modified with 4-mercaptophenylboronic acid (MPBA) was made to extract bacteria together with Apt-SAE probes. The binding event of aptamer and target triggered the formation of two sandwich structures containing Apt-SAE, V.P or S.T. The concentration of bacteria could be enriched by 1000 times within 15 min to avoid long-time enrichment process. Finally, the stir bar was immersed in the 3,3',5,5'-Tetramethylbenzidine (TMB)-H₂O₂ solution for color development. The color could be observed by naked eyes to judge whether the analytes were present. The colorless samples were judged to be negative. For the positive samples, the adsorbed encoded probes corresponding to different bacteria would be eluted from the stir bar and rapidly analyzed by the MC. Under the optimized conditions, 100 CFU/mL of V.P or S.T or both of them could be observed by colorimetry and 35 CFU/mL of them could be detected (S/N = 3) by the MC. The assay has significant application value for on-site screening and multiple detection of food-borne pathogenic bacteria.

An enzyme-mediated universal fluorescent biosensor template for pathogen detection based on a three-dimensional DNA walker and catalyzed hairpin assembly

An enzyme-mediated universal fluorescent biosensor template for rapid detection of pathogens was developed based on the strategy of a three-dimensional (3D) DNA walker and catalyzed hairpin assembly (CHA) reaction. In the bacterial recognition step, a strand displacement reaction between bacteria and the double-stranded complex caused the release of the walker strand. The walker strand triggered the DNA walker to produce an enzyme fragment, and the DNA walker used gold nanoparticles (AuNPs) as the track to provide an excellent DNA ligand anchoring area. In the CHA step, the enzyme fragment induced the CHA cycle to yield fluorescence signals, which greatly enhanced the conversion ratio of trigger DNA and the sensitivity of the fluorescent biosensor. The effect of the distance and density of the DNA ligand was studied by adjusting the length of poly-adenine (PolyA), and was further explored by its reaction kinetics. By comparing the maximum reaction rate (Vmax), Michaelis constant (Km) and turnover number (Kcat), the optimized PolyA probe was assessed and identified. In this work, the optimized PolyA-DNA probe exhibited an outstanding sensitivity in Salmonella typhimurium (S. ty) detection, which is 11.9 times and 4.6 times higher than those of the SH-DNA and the MCH treated SH-DNA. Meanwhile, a detection limit of 28.1 CFU mL-1 was achieved in Escherichia coli (E. coli) detection. Furthermore, the biosensor achieved good selectivity and high repeatability with recoveries of 91%-115% for real sample detection. Considering these advantages, this template has great potential as a routine tool for pathogen detection and has wide applications in the field of global public health and food safety.

Colorimetric immunoassay for rapid detection of Staphylococcus aureus based on etching-enhanced peroxidase-like catalytic activity of gold nanoparticles

A novel colorimetric immunoassay for the detection of Staphylococcus aureus (S. aureus) based on a combination of immunomagnetic separation and signal amplification via etching-enhanced peroxidase-like catalytic activity of gold nanoparticles (AuNPs) was developed. Nanoconjugates composed of gold and iron oxide nanoparticles were synthesized and further modified with antiS. aureus immunoglobulin Y (IgY), which was used for the selective enrichment and rapid separation of target bacteria in complex matrices. AuNPs functionalized with antiS. aureus aptamer were used as an artificial enzyme which has peroxidase-like catalysis activity. Catalytic activity of AuNPs is inhibited by modifying aptamer. However, catalysis of modified AuNPs remarkably enhanced by hydrogen peroxide etching. Based on collecting unbound modified AuNPs in the supernatant and 3,3',5,5'-tetramethylbenzidine-hydrogen peroxide reporting system, the yellow color of solution decreases linearly with increasing the concentration of S. aureus ranging from 10 to 10⁶ cfu/mL. The limit of detection is 10 cfu/mL, and total detection time is 65 min. The recoveries of the S. aureus spiked in food samples are 88.2-119.8%. Schematic illustration of colorimetric method for detection of S. aureus based on the IgY-Fe₃O₄/Au nanocomposites as capture probes and apt-AuNPs as artificial enzyme with etching-enhanced peroxidase-like catalytic activity.

Quantitative Analysis of Salmonella typhimurium Based on Elemental-Tags Laser-Induced Breakdown Spectroscopy

Current rapid bacterial detection methods are dedicated to the classification and identification of bacteria. However, there is still a lack of a method for specific quantitative analysis of certain bacteria. In this work, a method based on elemental-tags laser-induced breakdown spectroscopy (ETLIBS) was developed for the rapid and specific quantitative analysis of Salmonella typhimurium (S. ty). Elemental tags were first synthesized by assembling copper nanoparticles (CuNPs) with poly(thymine) (poly-T) template that linked with the aptamer sequence. Under the specific recognition of the aptamer, S. ty can be fully combined with the elemental tags within 30 min to achieve labeling. Afterward, the silicon nanowires (SiNWs) array modified with Au@Ag nanoparticles (SiNWs-Au@Ag) was employed to capture S. ty in 30 min. Attributed to the rapid analysis superiority of ETLIBS mapping, 100 spectra of SiNWs-Au@Ag/S. ty/CuNPs can be obtained in 5 min. It was found that the peak area of the Cu(I) atomic emission line at 324.75 nm fitted by the Voigt profile was linearly related to the bacterial concentration in the range of 10²-10⁶ CFU/mL(R² = 0.978). Furthermore, ETLIBS mapping achieved a low limit of detection (LOD) of 61 CFU/mL and showed good selectivity to S. ty compared with other bacteria. Besides, the method exhibited preeminent detection performance in spiked samples with the recoveries of 87-113%. With the advantages of rapidity, high efficiency, and specificity, the proposed method is expected to be a powerful tool for bacterial detection.

Colorimetric aptasensor for detecting Salmonella spp., Listeria monocytogenes, and Escherichia coli in meat samples

In this study, we successfully developed a simple and rapid method for simultaneous detection of Salmonella spp., Listeria monocytogenes, and Escherichia coli using gold nanoparticles and the aptamer aptasensor. We screened 25 specific DNA aptamer candidates against these pathogens using whole-cell Systematic Evolution of Ligands by EXponential enrichment. Among them, Ap6 was selected due to its low energy minimization values of -12.25 and -27.67 kcal/mol derived from MFold and RNAFold analysis, respectively. The assay presented in this study allowed the visual colorimetric detection of labeled colloidal gold nanoparticles as well as determination of UV absorbance at 625 and 525 nm under optimized conditions. The detection limit of this aptasensor was as less as 10⁵ CFU/ml. A random investigation of 50 meat samples, including ham and chicken sausages, collected from the local market revealed 96% accuracy, 96% specificity, and 100% sensitivity of the assay. The colorimetric aptasensor can accomplish one-step detection without pre-culture, DNA extraction, and amplification. Hence, it is an easy, rapid, specific, and qualitative assay that can be used as a point-of-care testing to directly detect multiplex foodborne pathogens.

Detection of Salmonella in Food Matrices, from Conventional Methods to Recent Aptamer-Sensing Technologies

Rapid detection of the foodborne pathogen Salmonella in food processing is of crucial importance to prevent food outbreaks and to ensure consumer safety. Detection and quantification of Salmonella species in food samples is routinely performed using conventional culture-based techniques, which are labor intensive, involve well-trained personnel, and are unsuitable for on-site and high-throughput analysis. To overcome these drawbacks, many research teams have developed alternative methods like biosensors, and more particularly aptasensors, were a nucleic acid is used as biorecognition element. The increasing interest in these devices is related to their high specificity, convenience, and relative rapid response. This review aims to present the advances made in these last years in the development of biosensors for the detection and the quantification of Salmonella, highlighting applications on meat from the chicken food chain.

Development of rapid dipstick assay for food pathogens, Salmonella, by optimized parameters

Salmonella is among the very important pathogens threating the human and animal health. Rapid and easy detection of these pathogens is crucial. In this context, antibody (Ab) based lateral flow assays (LFAs) which are simple immunochromatographic point of care test kits were developed by gold nanoparticles (GNPs) as labelling agent for Salmonella detection. For that purpose some critical parameters such as reagent concentrations on the capture zones, conjugate concentrations and ideal membrane type needed for LFAs for whole cell detection were tested for naked eye analysis. Therefore, prepared LFAs were applied to the live and heat inactivated cells when they were used alone or included in different bacterial mixtures. Among the test platforms, membrane 180 (M180) was found as an ideal membrane and 36 nm GNPs showed highly good labelling in the developed LFAs. Diluted conjugates and low concentrations of reagents affected the test signal negatively. Salmonella was detected in different bacterial mixtures, selectively in 4-5 min. The best recognized species by used Ab were S. enteritidis and S. infantis. 5 × 105 S. typhimurium cells were also determined as a limit of detection of this study with mentioned parameters.

Ω-Shaped Fiber-Optic Probe-Based Localized Surface Plasmon Resonance Biosensor for Real-Time Detection of Salmonella Typhimurium

A novel, Ω-shaped fiber-optic localized surface plasmon resonance (FOLSPR) biosensor was designed for sensitive real-time and label-free bacterial detection. The designed Ω-shaped fiber-optic probe exhibits an outstanding sensitivity, due to the effect of unique geometry on performance. The results show that refractive index (RI) sensitivity of the Ω-shaped fiber-optic probe is 14 times and 2.5 times higher than those of the straight-shaped and the U-shaped FOLSPR, respectively. In addition, the reason for the geometry and the bending radius effects on RI sensitivity was discussed by investigating the relationship between RI sensitivity and the bending area. The results show that RI sensitivity was enhanced with the increase of bending area, and the best RI sensitivity obtained by Ω-shaped FOLSPR was 64.582 (a.u.)/RIU. Combined with this newly designed Ω-shaped FOLSPR biosensor, a real-time, label-free, sensitive, and highly selective bacterial detection method was established. In this work, the aptamers immobilized on the surface of FOLSPR could specifically capture Salmonella Typhimurium, resulting in an intense change of the absorption peak. In line with this principle, the FOLSPR biosensor achieved high detection sensitivity for Salmonella Typhimurium down to 128 CFU/mL within a linear range from 5 × 10² to 1 × 10⁸ CFU/mL and showed good selectivity for Salmonella Typhimurium detection compared to other bacteria. Furthermore, the FOLSPR biosensor was successfully applied to the detection of Salmonella Typhimurium in a chicken sample with the recoveries of 85-123%. With these characteristics, the novel biosensor is a potential alternative tool in food analysis and environmental monitoring.

Advances in aptasensors for the detection of food contaminants

Food safety is a global health objective, and foodborne diseases represent a major crisis in health. Techniques that are simple and suitable for fast screening to detect and identify pathogenic factors in the food chain are vital to ensure food safety. At present, a variety of analytical methods have been reported for the detection of pathogenic agents. Whereas the sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic bio-recognition elements, such as aptamers. Owing to the specific folding capability of aptamers in the presence of an analyte, aptasensors have substantially and successfully been exploited for the detection of a wide range of small and large molecules (e.g., toxins, antibiotics, heavy metals, bacteria, viruses) at very low concentrations. Here, we review the use of aptasensors for the development of highly sensitive and affordable detection tools for food analysis.

Selection of aptamers against pathogenic bacteria and their diagnostics application

Aptamers are short nucleotide sequences which can specifically bind to a variety of targets with high affinity. They are identified and selected via systematic evolution of ligands by exponential enrichment (SELEX). Compared to antibodies, aptamers offer several advantages including easy labeling, high stability and lower cost. Those advantages make it possible to be a potential for use as a recognition probe to replace antibody in the diagnostic field. This article is intended to provide a comprehensive review, which is focused on systemizing recent advancements concerning SELEX procedures, with special emphasis on the key steps in SELEX procedures. The principles of various aptamer-based detections of pathogenic bacteria and their application are discussed in detail, including colorimetric detection, fluorescence detection, electrochemical detection, lateral flow strip test, mass sensitive detection and PCR-based aptasensor. By discussing recent research and future trends based on many excellent publications and reviews, we attempt to give the readers a comprehensive view in the field of aptamer selection against pathogenic bacteria and their diagnostics application. Authors hope that this review will promote lively and valuable discussions in order to generate new ideas and approaches towards the development of aptamer-based methods for application in pathogenic bacteria diagnosis.

A sensitive assay based on specific aptamer binding for the detection of Salmonella enterica serovar Typhimurium in milk samples by microchip capillary electrophoresis

The detection of Salmonella enterica serovar Typhimurium (S. Typhimurium) is very important for the prevention of food poisoning and other infectious diseases. Here we reported a simple and sensitive strategy to test S. Typhimurium by microchip capillary electrophoresis couple with laser-induced fluorescence (MCE-LIF) based on the specific reaction between the bacterium and corresponding aptamers. Based on the differences in charge to mass ratio between bacteria-aptamer complexes and free aptamers, a separation of the complexes and free aptamers could be obtained by MCE. The optimal parameters of the specific reaction including fluorescent dye concentration, Mg²⁺ concentration, incubation time, and pH of incubation solution were carefully investigated. Meanwhile, a non-specific DNA was exploited as a contrast for the detection of S. Typhimurium. Under the optimal conditions, a rapid separation of the bacteria-aptamer complex and free aptamers was achieved within 135 s with a limit of detection (S/N = 3) of 3.37 × 10² CFU mL^-1. This method was applied for the detection of S. Typhimurium in fresh milk samples and a recovery rate of 95.8% was obtained. The experimental results indicated that the specific aptamers are of enough biostability and the established method could be used to analyze S. Typhimurium in foods.

A new phosphothreonine lyase electrochemical immunosensor for detecting Salmonella based on horseradish peroxidase/GNPs-thionine/chitosan

In the current study, a novel double-layer gold nanoparticles- electrochemical immunosensor electrode (DGN-EIE) immobilized with Salmonella plasmid virulence C (SpvC) antibody was developed. To increase the fixed quantity of antibodies and electrochemical signal, an electrochemical biosensing signal amplification system was utilized with gold nanoparticles-thionine-chitosan absorbing horseradish peroxidase (HRP). In addition, the SpvC monoclonal antibodies (derived from Balb/c mice) were prepared and screened with a high affinity to SpvC. To evaluate the quality of DGN-EIE, the amperometric I-t curve method was applied to determine Salmonella in PBS. The results showed that the response current had a good linear correlation with the bacterial quantity ranged from 1.0 × 10¹-5.0 × 10⁴ cfu/mL. The lowest detection limit was found at 5 cfu/mL. Furthermore, the proposed immunosensor has been demonstrated with high sensitivity, good selectivity and reproducibility. Apparently, DGN-EIE may be a very useful tool for monitoring the bacteria.

Recent advances in nanoparticle based aptasensors for food contaminants

Food safety and hazard analysis is a prime concern of human life, thus quality assessment of food and water is the need of the day. Recent advances in nano-biotechnology play a significant role in providing possible solutions for developing highly sensitive and affordable detection tools for food analysis. Nanomaterials based aptasensors hold great potential to overcome the drawbacks of conventional analytical techniques. Aptamers comprise a novel class of highly specific bio-recognition elements which are produced by SELEX (systematic evolution of ligands by exponential enrichment) process. They bind to target molecules by folding into 3D structures that can discriminate different chiral compounds. The flexibility in making modifications in aptamers contribute to the design of biosensors, enabling the generation of bio-recognition elements for a wide variety of target molecules. Nanomaterials such as metal nanoparticles, metal nanoclusters, metal oxide nanoparticles, metal and carbon quantum dots, graphene, carbon nanotubes and nanocomposites enable higher sensitivity by signal amplification and introduce several novel transduction principles such as enhanced chemiluminescence, fluorescence, Raman signals, electrochemical signals, enhanced catalytic activity, and super-paramagnetic properties to the biosensor. Although there are a few reviews published recently which deal with the potential of aptamers in various fields, none are devoted exclusively to the potential of aptasensors based on nanomaterials for the analysis of food contaminants. Hence, the current review discusses several transduction systems and their principles used in aptamer based nanosensors which have been developed in the past five years, the challenges faced in their designing, along with their strengths and limitations.

Single-Stranded DNA Aptamers against Pathogens and Toxins: Identification and Biosensing Applications

Molecular recognition elements (MREs) can be short sequences of single-stranded DNA, RNA, small peptides, or antibody fragments. They can bind to user-defined targets with high affinity and specificity. There has been an increasing interest in the identification and application of nucleic acid molecular recognition elements, commonly known as aptamers, since they were first described in 1990 by the Gold and Szostak laboratories. A large number of target specific nucleic acids MREs and their applications are currently in the literature. This review first describes the general methodologies used in identifying single-stranded DNA (ssDNA) aptamers. It then summarizes advancements in the identification and biosensing application of ssDNA aptamers specific for bacteria, viruses, their associated molecules, and selected chemical toxins. Lastly, an overview of the basic principles of ssDNA aptamer-based biosensors is discussed.

Aptamer-conjugated silver nanoparticles for electrochemical dual-aptamer-based sandwich detection of staphylococcus aureus

Staphylococcus aureus (S. aureus) is one of the most important human pathogens and causes numerous illnesses. In this study, we report a sensitive and highly selective dual-aptamer-based sandwich immunosensor for the detection of S. aureus. In this bioassay system, a biotinylated primary anti-S.aureus aptamer was immobilized on streptavidin coated magnetic beads (MB), which serves as a capture probe. A secondary anti-S.aureus aptamer was conjugated to silver nanoparticles (Apt-AgNP) that sensitively reports the detection of the target. In the presence of target bacterium, an Apt/S.aureus/apt-AgNP sandwich complex is formed on the MB surface and the electrochemical signal of AgNPs followed through anodic stripping voltammetry. The proposed sandwich assay benefits from advantageous of a sandwich assay for increased specificity, MB as carriers of affinity ligands for solution-phase recognition and fast magnetic separation, AgNPs for signal amplification, and an electrochemical stripping voltammetry read-out as a simple and sensitive detection. The electrochemical immunosensor shows an extended dynamic range from 10 to 1×10(6) cfu/mL with a low detection limit of 1.0 cfu/mL (S/N=3). Furthermore, the possible interference of other analog bacteria was studied. To assess the general applicability of this sensor, we investigated the quantification of S. aureus in real water samples. The results were compared to the experimental results obtained from a plate counting method, which demonstrated an acceptable consistency.

Gold nanoparticle-based enzyme-linked antibody-aptamer sandwich assay for detection of Salmonella Typhimurium

Enzyme-linked immunosorbent assay (ELISA) provides a convenient means for the detection of Salmonella enterica serovar Typhimurium (STM), which is important for rapid diagnosis of foodborne pathogens. However, conventional ELISA is limited by antibody-antigen immunoreactions and suffers from poor sensitivity and tedious sample pretreatment. Therefore, development of novel ELISA remains challenging. Herein, we designed a comprehensive strategy for rapid, sensitive, and quantitative detection of STM with high specificity by gold nanoparticle-based enzyme-linked antibody-aptamer sandwich (nano-ELAAS) method. STM was captured and preconcentrated from samples with aptamer-modified magnetic particles, followed by binding with detector antibodies. Then nanoprobes carrying a large amount of reporter antibodies and horseradish peroxidase molecules were used for colorimetric signal amplification. Under the optimized reaction conditions, the nano-ELAAS assay had a quantitative detection range from 1 × 10(3) to 1 × 10(8) CFU mL(-1), a limit of detection of 1 × 10(3) CFU mL(-1), and a selectivity of >10-fold for STM in samples containing other bacteria at higher concentration with an assay time less than 3 h. In addition, the developed nanoprobes were improved in terms of detection range and/or sensitivity when compared with two commercial enzyme-labeled antibody signal reporters. Finally, the nano-ELAAS method was demonstrated to work well in milk samples, a common source of STM contamination.

Detection and characterization of cancer cells and pathogenic bacteria using aptamer-based nano-conjugates

Detection and characterization of cells using aptamers and aptamer-conjugated nanoprobes has evolved a great deal over the past few decades. This evolution has been driven by the easy selection of aptamers via in vitro cell-SELEX, permitting sensitive discrimination between target and normal cells, which includes pathogenic prokaryotic and cancerous eukaryotic cells. Additionally, when the aptamer-based strategies are used in conjunction with nanomaterials, there is the potential for cell targeting and therapeutic effects with improved specificity and sensitivity. Here we review recent advances in aptamer-based nano-conjugates and their applications for detecting cancer cells and pathogenic bacteria. The multidisciplinary research utilized in this field will play an increasingly significant role in clinical medicine and drug discovery.

Development of ssDNA aptamers for the sensitive detection of Salmonella typhimurium and Salmonella enteritidis

Salmonella enterica subsp. enterica ser. enteritidis and Salmonella enterica subsp. enterica ser. typhimurium are the most common and severe food-borne pathogens responsible for causing salmonellosis in humans and animals. The development of an early and ultra-sensitive detection system is the first critical step in controlling this disease. To accomplish this, we used the cell systematic evolution of ligands by exponential enrichment (Cell-SELEX) technique to identify single-stranded DNA (ssDNA) aptamers to be used as detection probes that can specifically bind to S. enteritidis and S. typhimurium. A total of 12 target-specific ssDNA aptamers were obtained through ten rounds of Cell-SELEX under stringent selection conditions, and negative selection further enhanced the selectivity among these aptamers. Aptamer specificity was investigated using the gram-negative bacteria E. coli and P. aeruginosa and was found to be much higher towards S. enteritidis and S. typhimurium. Importantly, three candidate aptamers demonstrated higher binding affinities and the dissociation constants (Kd) were found to be in the range of nanomolar to submicromolar levels. Furthermore, individual aptamers were conjugated onto polyvalent directed aptamer polymer, which led to 100-fold increase in binding affinity compared to the individual aptamers alone. Taken together, this study reports the identification of higher affinity and specificity ssDNA aptamers (30mer), which may be useful as capture and detection probes in biosensor-based detection systems for salmonellosis.

A sensitive gold nanoparticle-based colorimetric aptasensor for Staphylococcus aureus

In this study, a gold nanoparticle-based colorimetric aptasensor for Staphylococcus aureus (S. aureus) using tyramine signal amplification (TSA) technology has been developed. First, the biotinylated aptamer specific for S. aureus was immobilized on the surface of the wells of the microtiter plate via biotin-avidin binding. Then, the target bacteria (S. aureus), biotinylated-aptamer-streptavidin-HRP conjugates, biotinylated tyramine, hydrogen peroxide and avidin-catalase were successively introduced into the wells of the microtiter plate. After that, the existing catalase consumed the hydrogen peroxide. Finally, the freshly prepared gold (III) chloride trihydrate was added, the color of the reaction production would be changed and the absorbance at 550 nm could be measured with a plate reader. Under optimized conditions, there was a linear relationship between the absorbance at 550 nm and the concentration of S. aureus over the range from 10 to 10(6) cfu mL(-1) (with an R² of 0.9947). The limit of the developed method was determined to be 9 cfu mL(-1).