Selection and identification of streptomycin-specific single-stranded DNA aptamers and the application in the detection of streptomycin in honey

A highly sensitive luminescent aptasensor utilizing MOF-74-co encapsulation of luminol in a 'turn-on' mode for streptomycin detection

This study focused on detecting streptomycin (STR) residues using a luminescent aptasensor encapsulated with aptamer. Utilizing MOF-74-Co with peroxidase-like activity, luminol was enclosed in its pores. The specific STR aptamer acted as a gatekeeper, ensuring excellent performance. Upon exposure to STR, the aptamers detached, releasing luminol and amplifying the luminescent signal through MOF-74-Co catalytic activity. A linear relationship between fluorescence intensity and STR concentration (50 nM ∼ 5 × 106 nM) was established, with a limit of detection of 0.065 nM. The sensor exhibited high selectivity for STR even in the presence of other aminoglycoside antibiotics. Applied to tea, egg, and honey samples, the sensor showed recovery rates of 91.38-100.2%, meeting safety standards. This MOF-based aptasensor shows promise for detecting harmful residues.

Aptamer Clicked Poly(ferrocenylsilanes) at Au Nanoparticles as Platforms with Multiple Function[†]

Aptamers are widely used in biosensing due to their specific sensitivity toward many targets. Thus, gold nanoparticle (AuNP) aptasensors are subject to intense research due to the complementary properties of aptamers as sensing elements and AuNPs as transducers. We present herein a novel method for the functional coupling of thrombin-specific aptamers to AuNPs via an anionic, redox-active poly(ferrocenylsilane) (PFS) polyelectroyte. The polymer acts as a co-reductant and stabilizer for the AuNPs, provides grafting sites for the aptamer, and can be used as a redox sensing element, making the aptamer-PFS-AuNP composite (aptamer-AuNP) a promising model system for future multifunctional sensors. The aptamer-AuNPs exhibit excellent colloidal stability in high ionic strength environments owing to the combined electrosteric stabilizing effects of the aptamer and the PFS. The synthesis of each assembly element is described, and the colloidal stability and redox responsiveness are studied. As an example to illustrate applications, we present results for thrombin sensitivity and specificity using the specific aptamer.

Development of Novel Peptide-Modified Silver Nanoparticle-Based Rapid Biosensors for Detecting Aminoglycoside Antibiotics

The detection and monitoring of aminoglycoside antibiotics (AGAs) have become of utmost importance due to their widespread use in human and animal therapy, as well as the associated risks of exposure, toxicity, and the emergence of antimicrobial resistance. In this study, we successfully synthesized casein hydrolysate peptides-functionalized silver nanoparticles (CHPs@AgNPs) and employed them as a novel colorimetric analytical platform to demonstrate remarkable specificity and sensitivity toward AGAs. The colorimetric and spectral response of the CHPs@AgNPs was observed at 405 and 520 nm, showing a linear correlation with the concentration of streptomycin, a representative AGA. The color changes from yellow to orange provided a visual indication of the analyte concentration, enabling quantitative determination for real-world samples. The AgNP assay exhibited excellent sensitivity with dynamic ranges of approximately 200-650 and 100-700 nM for streptomycin-spiked tap water and dairy whey with limits of detection found to be ∼98 and 56 nM, respectively. The mechanism behind the selective aggregation of CHPs@AgNPs in the presence of AGAs involves the amine groups of the target analytes acting as molecular bridges for electrostatic coupling with hydroxyl or carboxyl functionalities of adjacent NPs, driving the formation of stable NP aggregates. The developed assay offers several advantages, making it suitable for various practical applications. It is characterized by its simplicity, rapidity, specificity, sensitivity, and cost-effectiveness. These unique features make the method a promising tool for monitoring water quality, ensuring food safety, and dealing with emergent issues of antibiotic resistance.

Aptamer Sensors for the Detection of Antibiotic Residues- A Mini-Review

Food security is a global issue, since it is closely related to human health. Antibiotics play a significant role in animal husbandry owing to their desirable broad-spectrum antibacterial activity. However, irrational use of antibiotics has caused serious environmental pollution and food safety problems; thus, the on-site detection of antibiotics is in high demand in environmental analysis and food safety assessment. Aptamer-based sensors are simple to use, accurate, inexpensive, selective, and are suitable for detecting antibiotics for environmental and food safety analysis. This review summarizes the recent advances in aptamer-based electrochemical, fluorescent, and colorimetric sensors for antibiotics detection. The review focuses on the detection principles of different aptamer sensors and recent achievements in developing electrochemical, fluorescent, and colorimetric aptamer sensors. The advantages and disadvantages of different sensors, current challenges, and future trends of aptamer-based sensors are also discussed.

Selection and colorimetric application of ssDNA aptamers against metamitron based on magnetic bead-SELEX

Metamitron (MTM) is a typical and widely used triazine herbicide in agricultural production. Its moderate toxicity and high residue in the environment have deleterious impacts on human health. The establishment of a rapid and efficient MTM detection method is of great significance. In this study, a magnetic-bead SELEX (systematic evolution of ligands by exponential enrichment) system was developed to select the MTM aptamers with high affinity and specificity. Through 10 rounds of screening, six candidate aptamers with the highest abundance were obtained by high-throughput sequencing. The homology, secondary structure, and affinity analyses were performed. The aptamer named MTM-6 was selected as the optimal aptamer with the dissociation constant (K_d) value of 16 nM. Then, a colorimetric detection method for MTM based on aptamer MTM-6 and the aggregation of gold nanoparticles (AuNPs) induced by NaCl was established with a linear range from 20 to 1000 nM (R = 0.9966) and a limit of detection (LOD) of 4.58 nM. The average recovery rate of MTM in the application of actual aqueous samples ranged from 95.40 to 107.83% with a relative standard deviation (RSD) from 1.11 to 3.48%. With considerable sensitivity and specificity, this colorimetric aptasensor is convenient and efficient, and shows bright application potential in MTM detection in aqueous samples.

Selection and identification of a DNA aptamer for fluorescent detection of netilmicin

Netilmicin (NET) is an antibiotic widely used in healthcare and agriculture, but it can accumulate in the environment to threat human health. Netilmicin (NET) is an antibiotic used for veterinary purposes, for human therapy and for agricultural purposes. Therefore, there is a need to develop high-sensitive measuring methods to detect NET. Aptamer-based detecting methods are highly sensitive, inexpensive, and portable. In this study, we developed an aptamer-based fluorescence method to detect and quantify NET. NET was first conjugated to magnetic beads by amidation reaction and then NET-coated beads were used as the stationary phase to isolate aptamers by systematic evolution of ligands by exponential enrichment (SELEX) screening method. After ten rounds of SELEX screening, 32 aptamers with NET-binding affinity were obtained and the candidate aptamer APT-21 was finally chosen by comprehensively comparing their secondary structure characters and NET-binding affinity. APT-21 bound to NET with high affinity (Kd = 194.1 nmol/L) and high specificity that it displayed low cross-binding activities on 7 different structural analogs. We also developed a fluorometric assay using SYBR Green I (SG-I) and the APT-21. Key experimental parameters were optimized, including buffer system, SG-I and APT-21 reaction time, SG-I concentration, and aptamer concentration, to improve the detecting sensitivity. Our results suggest that the low limit of detection (LOD) of this method reached a low level of 1.95 nM and it also exhibited a good linear range up to 200 nM. Moreover, we successfully applied our method to detect the NET spiked in tap water and river water with good recoveries in the range from 97% to 111%. In conclusion, our current study isolated a NET-specific aptamer and developed an aptamer-based quantification method, which is promising to apply to detect NET in environmental samples.

Advanced screening and tailoring strategies of pesticide aptamer for constructing biosensor

The rapid development of aptamers has helped address the challenges presented by the wide existed pesticides contaminations. Screening of aptamers with excellent performance is a prerequisite for successfully constructing biosensors, while further tailoring of aptamers with enhanced activity greatly improved the assay performance. Firstly, this paper reviewed the advanced screening strategies for pesticides aptamers, including immobilization screening that preserves the native structures of targets, non-immobilized screening based on nanomaterials, capillary electrophoresis-systematic evolution of ligands by exponential enrichment (CE-SELEX), virtual screening in silico, high-throughput selection, and rational secondary library generation methods, which contributed significantly to improve the success rate of screening, reduce the screening time, and ensure aptamer binding affinity. Secondly, the precise tailoring strategies for pesticides aptamers were modularly elaborated, containing deletion, splitting, elongation, and fusion, which provided various advantages like cost-efficiency, enhanced binding affinity, and new derived functional motifs. Thirdly, the developed aptamer-based biosensors (aptasensors) for pesticide detection were systematically reviewed according to the different signal output modes. Finally, the challenges and future perspectives of pesticide detection are discussed comprehensively.

Three-dimensional modeling of streptomycin binding single-stranded DNA for aptamer-based biosensors, a molecular dynamics simulation approach

Streptomycin (STR) an aminoglycoside antibiotic which is used against bacteria in human and animal infection, have serious side effects on different parts of human body. Therefore, there is a crucial need to detect trace amount of it in serum and food products. Aptamers are oligonucleotides or peptides, which bind their targets with high affinity and specificity. These properties make aptamers as suitable candidates for biosensing applications. A 79-mer ss-DNA aptamer was applied for the detection of small amount of STR in various aptasensors. But there is no structural information on the STR-binding aptamer and molecular details underlying the aptamer-STR binding remain unexplored. In this study we provided a 3D-structural model for 79-mer ss-DNA aptamer from the sequence. Using docking program and molecular dynamics (MD) simulation we predicted the binding pocket of ss-DNA aptamer. Our results show STR streptose ring is buried within the groove of DNA model and capped by non Watson-Crick bases. STR interacts with aptamer through forming stable hydrogen bonds. Our computational findings are in fair agreement with experimental results. With the atomic structural details, we gained new insight into the Apt-STR binding interaction that can help to further optimize aptamer efficiency in biosensing applications.Communicated by Ramaswamy H. Sarma.

Systematic Evolution of Ligands by Exponential Enrichment Technologies and Aptamer-Based Applications: Recent Progress and Challenges in Precision Medicine of Infectious Diseases

Infectious diseases are considered as a pressing challenge to global public health. Accurate and rapid diagnostics tools for early recognition of the pathogen, as well as individualized precision therapy are essential for controlling the spread of infectious diseases. Aptamers, which were screened by systematic evolution of ligands by exponential enrichment (SELEX), can bind to targets with high affinity and specificity so that have exciting potential in both diagnosis and treatment of infectious diseases. In this review, we provide a comprehensive overview of the latest development of SELEX technology and focus on the applications of aptamer-based technologies in infectious diseases, such as targeted drug-delivery, treatments and biosensors for diagnosing. The challenges and the future development in this field of clinical application will also be discussed.

Recent advances in aptamer-based sensors for aminoglycoside antibiotics detection and their applications

Aminoglycoside antibiotics (AAs) have been extensively applied in medical field and animal husbandry owing to desirable broad-spectrum antibacterial activity. Excessive AAs residues in the environment can be accumulated in human body through food chain and cause detrimental effect on human health. The establishment of highly specific, simple and sensitive detection methods for monitoring AAs residues is highly in demand. Aptasensor using aptamer as the biological recognition element is the efficient and promising sensing method for detection of AAs. In this review, we have made a summary of specific aptamers sequences against AAs. Subsequently, we provide a systematical and comprehensive overview of modern techniques in aptasensors for detection of AAs according to optical aptasensors as well as electrochemical aptasensors and further summarize their advantages and disadvantages to compare their applications. In addition, we present an overview of practical applications of aptasensors in sample detection of AAs. Moreover, the current challenges and future trends in this field are also included to reveal a promising perspective for developing novel aptasensors for AAs.

Advances in Gold Nanoparticles-Based Colorimetric Aptasensors for the Detection of Antibiotics: An Overview of the Past Decade

Misuse of antibiotics has recently been considered a global issue because of its harmful effects on human health. Since conventional methods have numerous limitations, it is necessary to develop fast, simple, sensitive, and reproducible methods for the detection of antibiotics. Among numerous recently developed methods, aptasensors are fascinating because of their good specificity, sensitivity and selectivity. These kinds of biosensors combining aptamer with colorimetric applications of gold nanoparticles to recognize small molecules are becoming more popular owing to their advantageous features, for example, low cost, ease of use, on-site analysis ability using naked eye and no prerequisite for modern equipment. In this review, we have highlighted the recent advances and working principle of gold nanoparticles based colorimetric aptasensors as promising methods for antibiotics detection in different food and environmental samples (2011–2020). Furthermore, possible advantages and disadvantages have also been summarized for these methods. Finally, the recent challenges, outlook, and promising future perspectives for developing novel aptasensors are also considered.

A split aptamer (SPA)-based sandwich-type biosensor for facile and rapid detection of streptomycin

As antibiotic pollution is gaining prominence as a global issue, the demand for detection of streptomycin (STR), which is a widely used antibiotic with potential human health and ecological risks, has attracted increasing attention. Aptamer-based biosensors have been developed for the detection of STR in buffers and samples, however, the non-target signals due to the conformational variation of free aptamers possibly affect their sensitivity and stability. In this study, by introducing the STR-specific split aptamer (SPA), a sensitive evanescent wave fluorescent (EWF) biosensor is developed for the sandwich-type based detection of STR. The standard calibration curve obtained for STR has a detection limit of 33 nM with a linear range of 60-526 nM. This biosensor exhibited good selectivity, reliable reusability for at least 100 times measurements, and high recovery rates for spiked water samples; moreover, all detection steps are easy-to-operate and can be completed in 5 min. Therefore, it exhibits great promise for actual on-site environmental monitoring. Additionally, without introducing any other oligonucleotides or auxiliary materials, this SPA-based biosensing method shows potential as a simple, sensitive, and low-cost manner for the detection of other small molecular targets.

The isolation of a DNA aptamer to develop a fluorescent aptasensor for the thiamethoxam pesticide

Aptamers, which are called chemical antibodies for their high affinity and specificity to targets, have great potential as analytical tools to detect pesticides. In this work, a DNA aptamer for thiamethoxam was isolated by an improved SELEX (systematic evolution of ligands by exponential enrichment) strategy, in which the ssDNA library was fixed on streptavidin-agarose beads through a short biotin labeled complementary strand. After 13 rounds of selection, the random ssDNA pool was successfully enriched. Three sequences were chosen as aptamer candidates through sequencing and analysis and were transformed into fluorescent probes to evaluate their interactions with thiamethoxam. A fluorescent turn-on aptasensor for thiamethoxam based on the best aptamer (FAM-Thi13) and a short quenching strand were further designed and showed a quantitative linear range from 10 to 1000 nM with a detection limit of 1.23 nM for thiamethoxam. Molecular docking and molecular dynamics were used to investigate the binding site of the main probe of the aptasensor (FAM-Thi13) and thiamethoxam. Satisfactory results were also obtained in quantifying thiamethoxam in environmental water samples by the developed fluorescent aptasensor.

Application of Multiplexed Aptasensors in Food Contaminants Detection

The existence of contaminants in food poses a serious threat to human health. In recent years, aptamer sensors (aptasensors) have been developed rapidly for the detection of food contaminants because of their high specificity, design flexibility, and high efficiency. However, the development of high-throughput, highly sensitive, on-site, and cost-effective methods for simultaneous detection of food contaminants is still restricted due to multiple signal overlap or mutual interference and cross-reaction between different analytes with similar molecular structures. To overcome these problems, this Review summarizes some effective strategies from the articles published in recent years about multiplexed aptasensors for the simultaneous detection of food contaminants. This work focuses on the application of multiplexed aptasensors to simultaneously detect antibiotics, pathogens, and mycotoxins in food. These aptasensors mainly contain fluorescent aptasensors, electrochemical aptasensors, surface-enhanced Raman scattering-based aptasensors, microfluidic chip aptasensors, and paper-based multiplexed aptasensors. In addition, this Review also covers the application of nucleic acid cycle amplification and nanomaterial amplification strategies to improve the detection sensitivity. Finally, the limitations and challenges in the design of multiplexed aptasensor are also taken into account.

Advances in the Application of Aptamer Biosensors to the Detection of Aminoglycoside Antibiotics

Antibiotic abuse is becoming increasingly serious and the potential for harm to human health and the environment has aroused widespread social concern. Aminoglycoside antibiotics (AGs) are broad-spectrum antibiotics that have been widely used in clinical and animal medicine. Consequently, their residues are commonly found in animal-derived food items and the environment. A simple, rapid, and sensitive detection method for on-site screening and detection of AGs is urgently required. In recent years, with the development of molecular detection technology, nucleic acid aptamers have been successfully used as recognition molecules for the identification and detection of AGs in food and the environment. These aptamers have high affinities, selectivities, and specificities, are inexpensive, and can be produced with small batch-to-batch differences. This paper reviews the applications of aptamers for AG detection in colorimetric, fluorescent, chemiluminescent, surface plasmon resonance, and electrochemical sensors for the analysis in food and environmental samples. This study provides useful references for future research.

Kanamycin Adsorption on Gold Nanoparticles Dominates Its Label-Free Colorimetric Sensing with Its Aptamer

A short kanamycin-binding aptamer has been widely used for detecting kanamycin. One of the popular signaling methods is based on the color change of gold nanoparticles (AuNPs) to develop label-free colorimetric biosensors. The general perception was that aptamer binding to its target would inhibit aptamer adsorption by the AuNPs. This inhibited adsorption results in the aggregation of the AuNPs and a color change upon addition of salt. However, the potential adsorption of kanamycin was ignored. Herein, we carefully studied the adsorption of kanamycin on AuNPs and performed a comprehensive analysis using two mutated aptamers and a randomly sequenced DNA which were not supposed to bind kanamycin. In addition, a total of six antibiotics were studied over a wide concentration range. As low as 90 nM kanamycin can induce the aggregation of 3 nM citrate-capped AuNPs, indicating very strong adsorption of kanamycin. The color change was independent of DNA sequence, and all the tested sequences showed a similar color response, regardless of aptamer. Among the different antibiotics, kanamycin and streptomycin induced a color change but not the other four. Our results support an alternative mechanism that kanamycin and streptomycin adsorption by the AuNPs was the main reason for the color change instead of aptamer binding.

Screening and application of a truncated aptamer for high-sensitive fluorescent detection of metronidazole

Aptamer-based biosensors have been widely constructed and applied to detect diverse targets. Metronidazole is a widely used broad-spectrum antibacterial drug, whose residue has multiple risks to human health. Herein, metronidazole-specific aptamers were selected from a random ssDNA library with the full length of 79 nucleotides (nt) based on DNA library-immobilized magnetic beads SELEX technology. After ten rounds of selection, four aptamers with highly similar secondary structures were selected, among which AP32, with the lowest dissociation constants, was chosen as the optimal aptamer for further optimization. Then a semi-rational post-SELEX truncation was carried out based on the secondary structure analysis and molecular docking, as well as affinity assessment. Redundant nucleotides in AP32 were stepwise removed without the decrease of affinity. Following such strategy, a truncated aptamer AP32-4 with the length of only 15 nt was eventually screened. The dissociation constant of 77.22 ± 11.27 nM is almost equivalent to the original AP32. Furthermore, an aptamer-based fluorescent biosensor for metronidazole was constructed based on AP32-4. With the help of exonuclease-assisted target-recycling amplification, the biosensor exhibits a linear detection range of 25-800 nM, and the detection limit of 10.50 nM. The biosensor was applied to detect metronidazole in honey samples. The results show that not only an efficient strategy for screening robust and practicable aptamers, but also an ultrahigh sensitive detection platform for metronidazole were established.

Silver nanoparticle probe for colorimetric detection of aminoglycoside antibiotics: picomolar-level sensitivity toward streptomycin in water, serum, and milk samples

BACKGROUND

The low cost of aminoglycoside (AMG) antibiotics facilitates their excessive use in animal husbandry and the agriculture sector. This scenario has led to the occurrence of residues in the food chain. After several years of AMG use in antibacterial therapy, resistance to streptomycin has begun to appear. Most of the detection methods developed for AMG antibiotics lacks specificity. A broad target specific nanoprobe would be ideal for detecting the entire class of AMGs. A rapid and sensitive method for the detection of AMGs is urgently needed.

RESULTS

Gallic acid-coated silver nanoparticles (AgNPs) were demonstrated as a nanoprobe for the colorimetric detection of AMGs (yellow to orange / red). A linear dynamic range of 50-650 pmol L^-1 was achieved readily by ratiometric spectrophotometry (A₅₆₀ /A₄₀₀ ) with a limit of detection (LOD) as low as 36 pmol L^-1 . The amine-groups of the AMGs function as molecular linkers, so that electrostatic coupling interactions between neighboring particles drive the formation of AgNP aggregates. The assay can also be applied for the determination of streptomycin residues in serum and milk samples.

CONCLUSION

This study revealed the potential of an AgNP probe for the rapid and cost-effective detection of low-molecular-weight target analytes, such as the AMGs. A ligand-induced aggregation of AgNPs coated with gallic acid was reported to be a rapid and sensitive assay for AMGs. Analysis of streptomycin was demonstrated with excellent picomolar-level sensitivity. Thus, the validated method can find practical applications in the ultrasensitive detection of AMGs in complex and diagnostic settings. © 2019 Society of Chemical Industry.

Amplified oxygen reduction signal at a Pt-Sn-modified TiO2 nanocomposite on an electrochemical aptasensor

In this work, a metallic composite with strong electrocatalytic property was designed by uniformly decorating Pt and Sn nanoparticles on the surface of TiO₂ nanorods (Pt-Sn@TiO₂). A detection scheme was then developed based on a dual signal amplification strategy involving the Pt-Sn@TiO₂ composite and exonuclease assisted target recycling. The Pt-Sn@TiO₂ composite exhibited an enhanced oxygen reduction current owing to the synergistic effect between Pt and Sn, as well as high exposure of Pt (111) crystal face. Initially, a Pt-Sn@TiO₂ modified glassy carbon electrode produced an amplified electrochemical signal for the reduction of dissolved oxygen in the analyte solution. Next, a DNA with a complementary sequence to a streptomycin aptamer (cDNA) was immobilised on the Pt-Sn@TiO₂ modified electrode, followed by the streptomycin aptamer that hybridised with cDNA. The corresponding oxygen reduction current was diminished by 51% attributable to the hindrance from the biomolecules. After a mixture of streptomycin and RecJ_f exonuclease was introduced, both the streptomycin-aptamer complex and the cDNA were cleaved from the electrode, making the Pt-Sn and Pt (111) surface available for oxygen reduction. RecJ_f would also release streptomycin from the streptomycin-aptamer complex, allowing it to complex again with aptamers on the electrode. This has then promoted a cyclic amplification of the oxygen reduction current by 85%, which is quantitatively related to streptomycin. Under optimal conditions, the aptasensor exhibited a linear range of 0.05-1500 nM and a limit of detection of 0.02±0.0045 nM streptomycin. The sensor was then used in the real-life sample detection of streptomycin to demonstrate its potential applications to bioanalysis.

Self-Assembled Microgels for Sensitive and Low-Fouling Detection of Streptomycin in Complex Media

In terms of detection of antibiotics within complex media, the nonspecific adsorption is an enormous challenge and antifouling sensing interfaces capable of reducing the nonspecific adsorption from complex biological samples are highly desirable. In this work, a novel antifouling electrochemical immunosensor was explored based on the self-assembly of two kinds of poly( N-isopropylacrylamide) microgels on the surface of graphene oxide for sensitive detection of streptomycin (STR). The microgels modified with glycidyl methacrylate (GMA) and zwitterionic liquid 1-propyl-3-vinylimidazole sulfonate (PVIS) were prepared. The microgels with GMA were used by combining specific recognition of anti-STR. The rapid specific binding of antigen and anti-STR resulted in a decrease of current density to generate electrochemical responsive signals. Zwitterionic liquid-modified microgels were used for antifouling, which can form stronger hydration and show excellent antifouling ability. As a result, we achieved efficient and sensitive detection of STR in the complex sample with evidently resisted nonspecific adsorption effect, the wide linear range toward STR was from 0.05 to 100 ng mL^-1, with a detection limit down to 1.7 pg mL^-1. The immunosensor based on the surface functionalization of microgels showed promising applications for the detection of antibiotics in complex media.

An electrochemical aptasensor for streptomycin based on covalent attachment of the aptamer onto a mesoporous silica thin film-coated gold electrode

An electrochemical method is described for the determination of streptomycin (STR). It is making use of a gold electrode coated with a thin mesoporous silica film (MSF). In addition, silver nanoparticles were coated on the MSF to increase the surface area, to bind a large amount of aptamer (Apt), and to improve the electrical conductivity. In the presence of STR, it will bind to the Apt and hinder the diffusion of the redox probe hexacyanoferrate through the nanochannels of the mesoporous film. The aptasensor, best operated at a working potential of 0.22 V (vs. Ag/AgCl) has a linear response in the 1 fg.mL^-1 to 6.2 ng.mL^-1 STR concentration range. The detection limit is 0.33 fg.mL^-1. The assay was successfully validated by analyzing spiked samples of milk and blood serum. Graphical abstract Voltammetric assay of streptomycin (STR) by using a Fe(CN)₆^3-/4- probe. The aptamer was immobilized on a gold electrode modified with a mesoporous silica thin film (MSF) that was functionalized with (3-aminopropyl) triethoxysilane (APTES) and silver nanoparticles (AgNP). Incubation with STR leads to a decrease of the current.

Recent developments of photoelectrochemical biosensors for food analysis

Recent developments of photoelectrochemical biosensors for food analysis are summarized and the future prospects in this field are discussed.

Application of Gold-Nanoparticle Colorimetric Sensing to Rapid Food Safety Screening

Due to their unique optical properties, narrow size distributions, and good biological affinity, gold nanoparticles have been widely applied in sensing analysis, catalytic, environmental monitoring, and disease therapy. The color of a gold nanoparticle solution and its maximum characteristic absorption wavelength will change with the particle size and inter-particle spacing. These properties are often used in the detection of hazardous chemicals, such as pesticide residues, heavy metals, banned additives, and biotoxins, in food. Because the gold nanoparticles-colorimetric sensing strategy is simple, quick, and sensitive, this method has extensive applications in real-time on-site monitoring and rapid testing of food quality and safety. Herein, we review the preparation methods, functional modification, photochemical properties, and applications of gold nanoparticle sensors in rapid testing. In addition, we elaborate on the colorimetric sensing mechanisms. Finally, we discuss the advantages and disadvantages of colorimetric sensors based on gold nanoparticles, and directions for future development.

Electrochemical detection of tobramycin based on enzymes-assisted dual signal amplification by using a novel truncated aptamer with high affinity

An aptamer with the length of only 15 nucleotides specific for tobramycin was obtained through rationally designed truncation from a previously reported long sequence. The structural and binding properties of the aptamer were characterized. The dissociation constant (K_d) was determined to be 42.12 nM, indicating high affinity of the aptamer for tobramycin. Then an electrochemical sensor based on this aptamer was developed, which employed an enzymes-assisted dual signal amplification cycle through target recycling and strand-displacement DNA polymerization. A hairpin probe containing the aptamer sequence was designed and used to start the production cycle of a short ssDNA fragment in the presence of tobramycin, with the help of phi29 DNA polymerase and nicking endonuclease Nt.AlwI. The ssDNA fragment was captured by a signal transduction probe modified on gold electrode to form a triple-helix structure. With the help of [Ru(NH₃)₆]³⁺, a significant electrochemical signal was observed in differential pulse voltammetry (DPV). Under the optimal conditions, the current in DPV is linearly related with the concentration of tobramycin in the range of 10-200 nM, and the detection limit is 5.13 nM. The electrochemical sensor showed high specificity for tobramycin when it was challenged by other antibiotics. In addition, the constructed sensor was used to detect tobramycin in milk and water samples, and showed satisfactory performance. Therefore, the screened aptamer as well as the developed sensor has great application prospects in the fields of food safety control, medical test and environment monitoring.

Critical Review: DNA Aptasensors, Are They Ready for Monitoring Organic Pollutants in Natural and Treated Water Sources?

There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

Aptamer-Based Biosensors for Antibiotic Detection: A Review

Antibiotic resistance and, accordingly, their pollution because of uncontrolled usage has emerged as a serious problem in recent years. Hence, there is an increased demand to develop robust, easy, and sensitive methods for rapid evaluation of antibiotics and their residues. Among different analytical methods, the aptamer-based biosensors (aptasensors) have attracted considerable attention because of good selectivity, specificity, and sensitivity. This review gives an overview about recently-developed aptasensors for antibiotic detection. The use of various aptamer assays to determine different groups of antibiotics, like β-lactams, aminoglycosides, anthracyclines, chloramphenicol, (fluoro)quinolones, lincosamide, tetracyclines, and sulfonamides are presented in this paper.

Multiplexed aptasensor based on metal ions labels for simultaneous detection of multiple antibiotic residues in milk

A dual-target electrochemical aptasensor was developed for the simultaneous detection of multiple antibiotics based on metal ions as signal tracers and nanocomposites as signal amplification strategy. Metal ions such as Cd²⁺ and Pb²⁺ could generate distinct differential pulse voltammetry (DPV) peaks. When targets were present, kanamycin (KAN) and streptomycin (STR) as models, the KAN aptamer (KAP) and STR aptamer (STP) were released from their complementary strands, with more change of Cd²⁺ and Pb²⁺ corresponding to peak currents. At the same time, complementary strand of KAP (cKAP) and STP (cSTP) were linked with the poly (A) structure (cSTP-PolyA-cKAP) to increase their conformational freedom. Graphitized multi-walled carbon nanotubes (MWCNT_Gr) and carbon nanofibers-gold nanoparticles (CNFs-AuNPs) as a biosensor platform enhanced the surface area to capture a large amount of cSTP-PolyA-cKAP, thus amplifying the detection response. Under the optimal conditions, the aptasensor could detect KAN and STR as low as 74.50 pM and 36.45 pM respectively with the range from 0.1 to 100 nM and exhibited excellent selectively. Moreover, this aptasensor showed promising applications for the detection of other analytes by changing corresponding aptamers.

Optical and Electrochemical Aptasensors for Sensitive Detection of Streptomycin in Blood Serum and Milk

Detection and quantitation of antibiotic residues in blood serum and foodstuffs are in great demand. We have developed aptasensors for detection of streptomycin using electrochemical and optical methods. In the first method, an electrochemical aptasensor was developed for sensitive and selective detection of streptomycin, based on combination of exonuclease I (Exo I), complementary strand of aptamer (CS), arch shaped structure of aptamer (Apt)-CS conjugate, and gold electrode. The designed electrochemical aptasensor exhibited high selectivity toward streptomycin with a limit of detection (LOD) as low as 11.4 nM. Moreover, the developed electrochemical aptasensor was successfully used to detect streptomycin in milk and serum with LODs of 14.1 and 15.3 nM, respectively. In the second method, fluorescence quenching and colorimetric aptasensors were designed for detection of streptomycin based on aqueous gold nanoparticles (AuNPs) and double-stranded DNA (dsDNA). In the absence of streptomycin, aptamer/FAM-labeled complementary strand dsDNA is stable, resulting in the aggregation of AuNPs by salt bridge and an obvious color change from red to blue and strong emission of fluorescence. The colorimetric and fluorescence quenching aptasensors showed excellent selectivity toward streptomycin with limit of detections as low as 73.1 and 47.6 nM, respectively. The presented aptasensors were successfully used to detect streptomycin in milk and serum. For serum, LODs were determined to be 58.2 and 102.4 nM for fluorescence quenching and colorimetric aptasensors, respectively. For milk, LODs were calculated to be 56.2 and 108.7 nM for fluorescence quenching and colorimetric aptasensors, respectively.

Amplified detection of streptomycin using aptamer-conjugated palladium nanoparticles decorated on chitosan-carbon nanotube

A streptomycin-specific aptamer was used as a receptor molecule for ultrasensitive quantitation of streptomycin. The glassy carbon (GC) electrode was modified with palladium nanoparticles decorated on chitosan-carbon nanotube (PdNPs/CNT/Chi) and aminated aptamer against streptomycin. Modification of the sensing interface was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDS), wavelength-dispersive X-ray spectroscopy (WDX), cyclic voltammetry (CVs), and electrochemical impedance spectroscopy (EIS). The methodologies applied for designing the proposed biosensor are based on target-induced conformational changes of streptomycin-specific aptamer, leading to detectable signal change. Sensing experiments were performed in the streptomycin concentration range from 0.1 to 1500 nM in order to evaluate the sensor response as a function of streptomycin concentration. Based on the results, the charge transfer resistance (R_ct) values increased proportionally to enhanced streptomycin content. The limit of detection was found to be as low as 18 pM. The superior selectivity and affinity of aptamer/PdNPs/CNT/Chi modified electrode for streptomycin recognition made it favorable for versatile applications such as streptomycin analysis in real samples.

Aptamer based fluorometric sulfamethazine assay based on the use of graphene oxide quantum dots

The authors have developed a homogeneous "off-on" fluorometric method for the determination of the antibiotic sulfamethazine (SMZ). Aptamer against SMZ was labeled with graphene oxide quantum dots upon which the Graphene oxide quenched the blue fluorescence of the GOQDs. On addition of SMZ, the aptamers will bind SMZ and this will cause the release of GOQDs. As a result, fluorescence will be regenerated. Fluorescence, best measured at excitation/emission wavelengths of 365/455 nm, increases linearly in the 8 pg·mL^-1 to 60 ng·mL^-1 SMZ concentration range, with a 5 pg·mL^-1 detection limit. The method is reliable and was successfully applied to the determination of SMZ in spiked milk samples, with recoveries ranging from 89 to 96% depending on analyte concentration. Graphical abstract Graphene oxide quantum dots (GOQDs) were covalently bound to the aptamer (apt) against sulfamethazine (SMZ) and adsorbed on the surface of graphene oxide (GO). This results in quenching of the fluorescence of GOQDs. On addition of SMZ, fluorescence is restored due to the release of GOQD@apt from GO.

Gold nanoparticle based photometric determination of tobramycin by using new specific DNA aptamers

A magnetic bead-based SELEX was applied to identify 37 single-stranded DNA aptamers specific for tobramycin after ten rounds of selection. The aptamers were classified into nine families according to sequence analysis. Among them, several aptamers with typical sequences were selected and their dissociation constants (K_ds) were determined by a fluorescent method. An aptamer termed "Ap 32", with a K_d value of 56.8 ± 4.6 nM, possesses the highest affinity and satisfactory specificity. Theoretical modeling showed that nucleotides 14-18 and 26-29 play a most significant role in the interaction between aptamer and tobramycin. Subsequently, the sequence of Ap 32 was optimized through rationally designed truncation. The truncated aptamer Ap 32-2 consists of 34 nucleotides and has a K_d that is similar to the original one. It was chosen as the optimal aptamer for use in the assay and was immobilized on gold nanoparticles. On addition of tobramycin, the color turns from red to purple. The findings were used to design a photometric assay (best performed at 520 nm) that has a linear response in the 100 nM to 1.4 μM concentration range, with a 37.9 nM detection limit. The method was successfully applied to the determination of tobramycin in (spiked) honey samples. Graphical abstract A 34-nucleotide aptamer specific for tobramycin was obtained through magnetic beads-based systematic evolution of ligands by exponential enrichment (SELEX) and structural analysis-based rational post-SELEX truncation, and then applied to the determination of tobramycin using a gold nanoparticle-based photometric assay.

A novel electrochemical aptasensor for highly sensitive and quantitative detection of the streptomycin antibiotic

In the present study, we report a facile approach to employ gold nanoparticle (AuNPs) and thiol graphene quantum dots (GQD-SH) as the nanomaterial for ultrasensitive detection of streptomycin (STR). Based on this strategy, a GQD-SH was immobilized onto the surface of a glassy carbon electrode (GCE). AuNPs have been immobilized on SH groups of GQDs through bonding formation of AuS and Apt have been loaded on the electrode surface through the interaction between thiol group of aptamer. By incubating STR as a target onto the surface of the prepared Apt/AuNPs/GQD-SH/GCE as a proposed nanoaptasensor, the Apt/STR complex was formed and the changes of the electrochemical signal were evaluated with the EIS technique. The proposed nanoaptasensor showed wide linear range from 0.1 to 700pgml^-1. Finally, the proposed nanoaptasensor was successfully applied for the determination of STR in real samples and satisfactory results were obtained.

Screening Criteria for Qualified Antibiotic Targets in Unmodified Gold Nanoparticles-Based Aptasensing

In designing unmodified gold nanoparticles-based aptasensing (uGA) assays for antibiotics, we find that some antibiotics can adsorb directly on gold nanoparticles (GNP) regardless of the presence of aptamers, which have been long overlooked in the past. Some adsorptions, however, would strongly disturb the charge distribution on the GNP surface, break up the static colloidal profile, and thus generate false positive colorimetric signals. To identify antibiotics qualified for uGA assays, we established two rational screening criteria for antibiotic targets relying on their oil-water partition coefficients (log P values) and net physiological charges: log P > 0 and charge ≤0. A good agreement of the GNP color change was obtained between the two criteria-based predictions and the actual tests using six representative antibiotics. The proposed criteria help to shed light on GNP-target interactions, which is significant for developing novel GNP-based colorimetric assays with high reliability.

Point-of-care testing for streptomycin based on aptamer recognizing and digital image colorimetry by smartphone

The rapid detection of antibiotic residual in everyday life is very important for food safety. In order to realize the on-site and visual detection of antibiotic, a POCT method was established by using digital image colorimetry based on smartphone. Streptomycin was taken as the analyte model of antibiotics, streptomycin aptamer preferentially recognized analyte, and the excess aptamer hybridized with the complementary DNA to form the dsDNA. SYBR Green I combined with the dsDNA and then emitted obvious green fluorescence, thus the fluorescence intensity decreased with the increasing of streptomycin concentration. Then a smartphone-based device was constructed as the fluorescence readout. The smartphone camera acquired the images of the fluorescence derived from the samples, and the Touch Color APP installed in smartphone read out the RGB values of the images. There was a linear relationship between the G values and the streptomycin concentrations in the range of 0.1-100µM. The detection limit was 94nM, which was lower than the maximum residue limit defined by World Health Organization. The POCT method was applied for determining streptomycin in chicken and milk samples with recoveries in 94.1-110%. This method had the advantages of good selectivity, simple operation and on-site visualization.

Electrochemical aptasensors for contaminants detection in food and environment: Recent advances

The growing number of contaminants requires the development of new analytical tools to meet the increasing demand for legislative actions on food safety and environmental pollution control. In this context, electrochemical aptamer-based sensors appear promising among all biosensors because they permit multiplexed analysis and provide fast response, sensitivity, specificity and low cost. The aim of this review is to give the readers an overview of recent important achievements in the development of electrochemical aptamer-based biosensors for contaminant detection over the last two years. Special emphasis is placed on aptasensors based on screen-printed electrodes which show a substantial improvement of analytical performances.