Generation and characterization of quinolone-specific DNA aptamers suitable for water monitoring

Label-free liquid crystal-based optical detection of norfloxacin using an aptamer recognition probe in soil and lake water

Norfloxacin (NOX), a broad spectrum fluoroquinolone (FQ) antibiotic, is commonly detected in environmental residues, potentially contributing to biological drug resistance. In this paper, an aptamer recognition probe has been used to develop a label-free liquid crystal-based biosensor for simple and robust optical detection of NOX in aqueous solutions. Stimuli-receptive liquid crystals (LCs) have been employed to report aptamer-target binding events at the LC-aqueous interface. The homeotropic alignment of LCs at the aqueous-LC interface is due to the self-assembly of the cationic surfactant cetyltrimethylammonium bromide (CTAB). In the presence of the negatively charged NOX aptamer, the ordering changes to planar/tilted. On addition of NOX, the aptamer-NOX binding causes redistribution of CTAB at the LC-aqueous interface and the homeotropic orientation is restored. This results in a bright-to-dark optical transition under a polarized optical microscope (POM). This optical transition serves as a visual indicator to mark the presence of NOX. The devised aptasensor demonstrates high specificity with a minimum detection limit of 5 nM (1.596 ppb). Moreover, the application of the developed aptasensor for the detection of NOX in freshwater and soil samples underscores its practical utility in environmental monitoring. This proposed LC-based method offers several advantages over conventional detection techniques for a rapid, feasible and convenient way to detect norfloxacin.

Photocurrent switchable dual-target bioassay: Signal distinction and interface reconfiguration via pH-responsive triplex DNA programming

Most multiplexed photoelectrochemical (PEC) sensors require additional instrumentation and cumbersome electrode modification and surface partitioning, which limits their portability and instrument miniaturization. Herein, a pH-responsive programmable triple DNA nanomachine was developed for constructing a reconfigurable multiplex PEC sensing platform. By programming the base sequence, T-A·T-riched triple DNA was designed to construct integrated nano-controlled release machine (INCRM) for simultaneous recognition of multiple targets. The INCRM enables to recognize two targets in one step, and sequentially separate the signal labels by pH adjustment. The detached signal label catalyzes glucose to produce gluconic acid, causing the C-riched DNA fold into a triple structure on the electrode surface. As a result, one target can be detected relying on the enhanced photocurrent due to accelerated electron transfer between the CdS QD labeled at the end of C-riched DNA and the electrode. The triplex DNA dissociation in pH 7.4 buffer reconfigures the electrode interface, which can be continued to detect another target. The feasibility of the multiplexed sensor is verified by the detection of extensively coexisting antibiotics enrofloxacin (ENR) and ciprofloxacin (CIP). Under the optimal conditions, wide linear range (10 fg/mL ∼ 1 μg/mL) and low detection limit (3.27 fg/mL and 9.60 fg/mL) were obtained. The pH-regulated programmable triplex DNA nanomachine-based sensing platform overcomes the technical difficulties of conventional multiplexed PEC assay, which may open the way for miniaturization of multiplexed PEC sensors.

Screening of Cross-Reactive Aptamers for the Detection of 24 Quinolones by Using a Liebig's Law-Guided Parallel-Series Strategy

Quinolones, a widely used class of antibiotics, present significant environmental and health concerns if they excessively remain in the environment and in food. Aptamers specific to quinolones can be applied as bioreceptors for the detection of quinolone residues in the environment and food. The quinolone family contains dozens of different individuals that share the same core structure coupled with various substituents at six different positions. The diversity and complexity of the substitution sites make it a challenge to choose a set of representative molecules that encompass all the desired sites and preserve the core molecular framework for the screening of quinolone-specific aptamers via systematic evolution of ligands by exponential enrichment (SELEX). To address this challenge, we introduce a novel parallel-series strategy guided by Liebig's law for isolating quinolone-specific cross-reactive aptamers by using the library-immobilized SELEX method. Through this approach, we successfully identified 5 aptamers (Apt.AQ01-Apt.AQ05) with high binding affinity and excellent specificity to 24 different quinolone individuals. Among them, Apt.AQ03 showcased optimal performance with affinities ranging from 0.14 to 1.07 μM across the comprehensive set of 24 quinolones, exhibiting excellent specificity against nontarget interferents. The binding performance of Apt.AQ03 was further characterized with microscale thermophoresis, circular dichroism spectra, and an exonuclease digestion assay. By using Apt.AQ03 as a bioreceptor, a fluorescence resonance energy transfer (FRET) aptasensor was developed for the detection of 24 quinolones in milk, achieving a remarkable detection limit of 14.5-21.8 ng/mL. This work not only establishes a robust and effective strategy for selecting cross-reactive aptamers applicable to other small-molecule families but also provides high-quality aptamers for developing various high-throughput and reliable methods for the detection of multiple quinolone residues in food.

A review on magnetic beads-based SELEX technologies: Applications from small to large target molecules

This review summarizes the stepwise strategy and key points for magnetic beads (MBs)-based aptamer selection which is suitable for isolating aptamers against small and large molecules via systematic evolution of ligands by exponential enrichment (SELEX). Particularities, if any, are discussed according to the target size. Examples targeting small molecules (<1000 Da) such as xenobiotics, toxins, pesticides, herbicides, illegal additives, hormones, and large targets such as proteins (biomarkers, pathogens) are discussed and presented in tabular formats. Of special interest are the latest advances in more efficient alternatives, which are based on novel instrumentation, materials or microelectronics, such as fluorescence MBs-SELEX or microfluidic chip system-assisted MBs-SELEX. Limitations and perspectives of MBs-SELEX are also reviewed. Taken together, this review aims to provide practical insights into MBs-SELEX technologies and their ability to screen multiple potential aptamers against targets from small to large molecules.

Dual-mode colorimetric and photothermal aptasensor for detection of kanamycin using flocculent platinum nanoparticles

Chitosan (CS)-stabilized platinum nanoparticles (CS/PtNPs) were employed to develop a novel aptamer-based dual-mode colorimetric and photothermal biosensor for selective detection of kanamycin (KAN). As a peroxidase-like catalyst, the CS/PtNPs showed outstanding catalytic activity for the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2). As a stabilizing agent, CS excelled at fixing the KAN binding aptamer on the surface of the CS/PtNPs, amplifying their catalytic activity and enhancing colloidal dispersion and stability. The oxidized TMB (TMBox) functioned as a signal for the colorimetric, photothermal aptasensor because of its observable absorbance of light in the visible and near-infrared (NIR) regions. When light from a NIR laser was absorbed by the TMBox in the reaction solution, heat was generated in inverse proportion to the KAN concentration. The developed colorimetric and photothermal modes of the aptasensor showed a linear detection range of 0.1-50 and 0.5-50 μM, with a limit of detection (LOD) of 0.04 and 0.41 μM, respectively. Moreover, the aptasensor successfully determined KAN concentrations in spiked milk samples, verifying the reliability and reproducibility in practical applications. The dual-mode aptasensor based on CS/PtNPs for KAN detection, utilizing both color change and heat generation signals through a single probe (TMBox), demonstrates rapid response, simplicity in operation, cost-effectiveness, and high sensitivity. In addition, unlike typical immunoassays, this aptamer-based peroxidase-like nanozyme activation and inhibition strategy required no washing process, which was very effective in terms of reducing the time required for an assay and sustaining a high sensitivity.

Sensing Levofloxacin with an RNA Aptamer as a Bioreceptor

To combat the growing threat of antibiotic resistance, environmental testing for antibiotic contamination is gaining an increasing role. This study aims to develop an easy-to-use assay for the detection of the fluoroquinolone antibiotic levofloxacin. Levofloxacin is used in human and veterinary medicine and has been detected in wastewater and river water. An RNA aptamer against levofloxacin was selected using RNA Capture-SELEX. The 73 nt long aptamer folds into three stems with a central three-way junction. It binds levofloxacin with a Kd of 6 µM and discriminates the closely related compound ciprofloxacin. Furthermore, the selection process was analyzed using a next-generation sequencing approach to better understand the sequence evolution throughout the selection. The aptamer was used as a bioreceptor for the development of a lateral flow assay. The biosensor exploited the innate characteristic of RNA Capture-SELEX to select aptamers that displace a complementary DNA oligonucleotide upon ligand binding. The lateral flow assay achieved a limit of visual detection of 100 µM. While the sensitivity of this assay constrains its immediate use in environmental testing, the present study can serve as a template for the selection of RNA aptamer-based biosensors.

Graphene oxide mediated CdSe quantum dots fluorescent aptasensor for high sensitivity detection of fluoroquinolones

In view of the urgent need for fluoroquinolones contamination detection in the fields of food safety, a novel aptasensor based on the fluorescence quenching property of graphene oxide (GO) and the fluorescence characteristic of cadmium selenide quantum dots (CdSe QDs) was developed for fluoroquinolones highly sensitive detection in this work. The CdSe QDs with carboxyl-rich surface were synthesized successfully and fluoresced at 525 nm under the optimal excitation light of 366 nm. Based on the hydrophobic and π-π stacking between GO and aptamer, aptamer labeled by CdSe QDs fluorescence (CdSe QDs-apt) were adsorbed by GO and the fluorescence of CdSe QDs was quenched. After the aptamer combined specifically with fluoroquinolones, greater specific force lead to the desorption of CdSe QDs-apt from GO and fluorescence recovery. Represented by Ciprofloxacin (CIP), a member of fluoroquinolones, the fluorescence emission increased with the increasing of CIP concentrations from 8 nM to 500 nM, and the detection limit was 0.42 nM. The spiked recoveries in real samples of honey and milk were 91.5-96.9 % and 90.3-95.2 %, respectively, indicating that the aptasensor was reliable. Moreover, the fluorescence responses of multiple members of fluoroquinolones were found to be consistent, denoting that the fluorescence aptasensor can be used to detect the total amount of multiple members of fluoroquinolones. These results showed that the aptasensor can be used as a promising platform for fluoroquinolones detection.

Electrospun nanofiber-based indicatorpaper sensing platform for fluorescence and visualization detection of norfloxacin

Norfloxacin (NOR) residues in water pose a serious threat to human health via the food chain, necessitating the development of a rapid on-site antibiotic detection technique. In this work, we utilize electrostatic spinning technology that combines polyacrylonitrile (PAN) fibers and adenosine triphosphate (ATP)-rare earth metal Tb3+ complexes (ATP/Tb) to construct a new ternary film-based sensor for sensitive, quick, and convenient field testing of NOR in water. The operating mechanism is that the ternary system produces gradually enhanced bright green fluorescence at increasing concentrations of NOR. The unique fluorescence property of the ternary systems is attributed to the use of ATP, rather than the commonly used adenosine monophosphate (AMP), to coordinate with Tb3+, which avoided the possible fluorescence quenching from competitive water binding. Benefiting from the PAN nanofiber's superior stability, acid, and alkali resistance, and flexibility as support, the ternary system exhibited a good linear response to NOR in a wide dynamic range of 0.04-30 μM at the detection limit of 16 nM. Additionally, the combination of a smartphone color recognition app allows for quick on-scene NOR detection. This film sensing strategy is instructive for the development of smart and portable sensing platforms for real-time detection of analytes such as antibiotics, pesticide residues, and hazardous materials in water bodies.

A novel electrochemical aptasensor based on eco-friendly synthesized titanium dioxide nanosheets and polyethyleneimine grafted reduced graphene oxide for ultrasensitive and selective detection of ciprofloxacin

Developing a rapid, sensitive, and efficient analytical method for the trace-level determination of highly concerning antibiotic ciprofloxacin (CIP) is desirable to guarantee the safety of human health and ecosystems. In this work, a novel electrochemical aptasensor based on polyethyleneimine grafted reduced graphene oxide and titanium dioxide (rGO/PEI/TiO2) nanocomposite was constructed for ultrasensitive and selective detection of CIP. Through the in-situ electrochemical oxidation of Ti3C2Tx nanosheets, TiO2 nanosheets with good electrochemical response were prepared in a more convenient and eco-friendly method. The prepared TiO2 nanosheets promote charge transferring on electrode interface, and [Fe(CN)6]3-/4- as electrochemical active substance can be electrostatically attracted by rGO/PEI. Thus, electrochemical detection signal of the aptasensor variates a lot after specific binding with CIP, achieving working dynamic range of 0.003-10.0 μmol L-1, low detection limit down to 0.7 nmol L-1 (S/N = 3) and selectivity towards other antibiotics. Additionally, the aptasensor exhibited good agreement with HPLC method at 95% confidence level, and achieved good recoveries (96.8-106.3%) in real water samples, demonstrating its suitable applicability of trace detection of CIP in aquatic environment.

Aptamer-based colorimetric and lateral flow assay approaches for the detection of toxic metal ions, thallium(i) and lead(ii)

Thallium(i) and lead(ii) ions are heavy metals and extremely toxic. These metals are environmental pollutants, posing a severe risk to the environment and human health. In this study, two approaches were examined using aptamer and nanomaterial-based conjugates for thallium and lead detection. The first approach utilized an in-solution adsorption-desorption approach to develop colorimetric aptasensors for the detection of thallium(i) and lead(ii) using gold or silver nanoparticles. The second approach was the development of lateral flow assays, and their performance was tested with thallium (limit of detection is 7.4 μM) and lead ion (limit of detection is 6.6 nM) spiked into real samples. The approaches assessed are rapid, inexpensive, and time efficient with the potential to become the basis for future biosensor devices.

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.

Fluorescent aptasensor for the ultrasensitive detection of antibiotic residue in food samples based on dumbbell DNA-mediated signal amplification

Sensitive and reliable detection of antibiotics is of great significance for environmental and food safety due to its high risk in trace concentrations. Herein, we developed a fluorescence sensing system for chloramphenicol (CAP) detection based on dumbbell DNA-mediated signal amplification. Two hairpin dimers (2H1 and 2H2) were employed as the building blocks to construct the sensing scaffolds. The CAP-aptamer binding in another hairpin H0 can liberate the trigger DNA, which then activates the cyclic assembly reaction between 2H1 and 2H2. The separation of FAM and BHQ in the formed product of cascaded DNA ladder yields a high fluorescence signal for CAP monitoring. Compared with the monomer hairpin assembly between H1 and H2, the dimer hairpin assembly between 2H1 and 2H2 exhibits enhanced signal amplification efficiency and reduced reaction time. The developed CAP sensor showed a wide linear range from 10 fM to 10 nM with a detection limit of 2 fM. Importantly, this sensing platform has been successfully applied to the determination of CAP in fish, milk, and water samples with satisfactory recovery and accuracy. With the advantages of high sensitivity, mix-and-read pattern, and robustness, our proposed CAP sensor can be used as a simple and routine tool for the detection of trace amounts of antibiotic residues.

Nucleic acid aptamers as aptasensors for plant biology

Our knowledge of cell- and tissue-specific quantification of phytohormones is heavily reliant on laborious mass spectrometry techniques. Genetically encoded biosensors have allowed spatial and some temporal quantification of phytohormones intracellularly, but there is still limited information on their intercellular distributions. Here, we review nucleic acid aptamers as an emerging biosensing platform for the detection and quantification of analytes with high affinity and specificity. Options for DNA aptamer technology are explained through selection, sequencing analysis and techniques for evaluating affinity and specificity, and we focus on previously developed DNA aptamers against various plant analytes. We suggest how these tools might be applied in planta for quantification of molecules of interest both intracellularly and intercellularly.

Review on the Selection of Aptamers and Application in Paper-Based Sensors

An aptamer is a synthetic oligonucleotide, referring to a single-stranded deoxyribonucleic acid or ribonucleic acid ligand produced by synthesis from outside the body using systematic evolution of ligands by exponential enrichment (SELEX) technology. Owing to their special screening process and adjustable tertiary structures, aptamers can bind to multiple targets (small molecules, proteins, and even whole cells) with high specificity and affinity. Moreover, due to their simple preparation and stable modification, they have been widely used to construct biosensors for target detection. The paper-based sensor is a product with a low price, short detection time, simple operation, and other superior characteristics, and is widely used as a rapid detection method. This review mainly focuses on the screening methods of aptamers, paper-based devices, and applicable sensing strategies. Furthermore, the design of the aptamer-based lateral flow assay (LFA), which underlies the most promising devices for commercialization, is emphasized. In addition, the development prospects and potential applications of paper-based biosensors using aptamers as recognition molecules are also discussed.

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

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

Aptasensing of ciprofloxacin residue using graphene oxide modified with gold nanoparticles and branched polyethyleneimine

Precise monitoring of antibiotic residues in aqueous solution is of vital significance for safeguarding the environment and food resources. Herein, a convenient platform was fabricated for the electrochemical assay of ciprofloxacin (CFX) in real milk samples using aminated aptamer and graphene oxide nanogold-functionalized branched polyethyleneimine (GO-PEI-AuNPs) nanocomposite. For the first time, a gold electrode was modified with GO-PEI-AuNPs. The modified surface endowed excellent electrochemical substrates with large surface areas, excellent electron transfer rates, and suitable capabilities to firmly attach high amounts of aptamer. After further modification of substrate with CFX specific aptamer a recognition probe enabling selective and sensitive determination of CFX was realized. All of the aptasensor fabrication steps were surveyed via cyclic voltammetry techniques. The construction and morphology of the GO-PEI-AuNPs composite were evaluated by UV-Vis spectroscopy, transmission electron microscopy, and field emission scanning electron microscopy. Under optimal conditions, the suggested scaffold can offer an acceptable linear range of 0.001 to 100 μM and a low limit of quantification of 0.001 μM for selective and sensitive monitoring of CFX in real samples. The effectiveness of the apta-assay was confirmed by detection of CFX in pasteurized and local milk samples for which suitable analytical results were achieved. It is expected that the developed substrate can be facilely extended to other aptamer-based multiplex screening platforms in actual food and environmental samples.

A convenient platform was fabricated for the electrochemical assay of ciprofloxacin using aminated aptamer immobilized in GO-PEI-AuNPs nanocomposite.

Fluorescence Assay for Detecting Four Organophosphorus Pesticides Using Fluorescently Labeled Aptamer

In this work, we reported a rapid and sensitive fluorescence assay in homogenous solution for detecting organophosphorus pesticides by using tetramethylrhodamine (TAMRA)-labeled aptamer and its complementary DNA (cDNA) with extended guanine (G) bases. The hybridization of cDNA and aptamer drew TAMRA close to repeated G bases, then the fluorescence of TAMRA was quenched by G bases due to the photoinduced electron transfer (PET). Upon introducing the pesticide target, the aptamer bound to pesticide instead of cDNA because of the competition between pesticide and cDNA. Thus, the TAMRA departed from G bases, resulting in fluorescence recovery of TAMRA. Under optimal conditions, the limits of detection for phorate, profenofos, isocarbophos, and omethoate were 0.333, 0.167, 0.267, and 0.333 µg/L, respectively. The method was also used in the analysis of profenofos in vegetables. Our fluorescence design was simple, rapid, and highly sensitive, which provided a means for monitoring the safety of agricultural products.

Recent Advances in Aptasensors For Rapid and Sensitive Detection of Staphylococcus Aureus

The infection of Staphylococcus aureus (S.aureus) and the spread of drug-resistant bacteria pose a serious threat to global public health. Therefore, timely, rapid and accurate detection of S. aureus is of great significance for food safety, environmental monitoring, clinical diagnosis and treatment, and prevention of drug-resistant bacteria dissemination. Traditional S. aureus detection methods such as culture identification, ELISA, PCR, MALDI-TOF-MS and sequencing, etc., have good sensitivity and specificity, but they are complex to operate, requiring professionals and expensive and complex machines. Therefore, it is still challenging to develop a fast, simple, low-cost, specific and sensitive S. aureus detection method. Recent studies have demonstrated that fast, specific, low-cost, low sample volume, automated, and portable aptasensors have been widely used for S. aureus detection and have been proposed as the most attractive alternatives to their traditional detection methods. In this review, recent advances of aptasensors based on different transducer (optical and electrochemical) for S. aureus detection have been discussed in details. Furthermore, the applications of aptasensors in point-of-care testing (POCT) have also been discussed. More and more aptasensors are combined with nanomaterials as efficient transducers and amplifiers, which appears to be the development trend in aptasensors. Finally, some significant challenges for the development and application of aptasensors are outlined.

Selection, Characterization, and Optimization of DNA Aptamers against Challenging Marine Biotoxin Gymnodimine-A for Biosensing Application

Gymnodimines (GYMs), belonging to cyclic imines (CIs), are characterized as fast-acting toxins, and may pose potential risks to human health and the aquaculture industry through the contamination of sea food. The existing detection methods of GYMs have certain defects in practice, such as ethical problems or the requirement of complicated equipment. As novel molecular recognition elements, aptamers have been applied in many areas, including the detection of marine biotoxins. However, GYMs are liposoluble molecules with low molecular weight and limited numbers of chemical groups, which are considered as “challenging” targets for aptamers selection. In this study, Capture-SELEX was used as the main strategy in screening aptamers targeting gymnodimine-A (GYM-A), and an aptamer named G48nop, with the highest KD value of 95.30 nM, was successfully obtained by screening and optimization. G48nop showed high specificity towards GYM-A. Based on this, a novel aptasensor based on biolayer interferometry (BLI) technology was established in detecting GYM-A. This aptasensor showed a detection range from 55 to 1400 nM (linear range from 55 to 875 nM) and a limit of detection (LOD) of 6.21 nM. Spiking experiments in real samples indicated the recovery rate of this aptasensor, ranging from 96.65% to 109.67%. This is the first study to report an aptamer with high affinity and specificity for the challenging marine biotoxin GYM-A, and the new established aptasensor may be used as a reliable and efficient tool for the detection and monitoring of GYMs in the future.

Novel ssDNA aptamer-based fluorescence sensor for perfluorooctanoic acid detection in water

Per- and polyfluoroalkyl substances (PFAS) are widely detected environmental contaminants, and there is a great need for development of sensor technologies for rapid and continuous monitoring of PFAS. In this study, we have developed fluorescence based aptasensor that can possibly monitor perfluorooctanoic acid (PFOA) in water with limit of detection (LOD) of 0.17 μM. This is first to report the successful isolation of PFAS binding ssDNA aptamers. The obtained aptamer selectively binds PFOA with dissociation constant (K_D) of 5.5 μM. Specific aptamer binding sites to PFOA were identified and the length of the fluorinated carbons was a key binding factor rather than the functional group. The aptamer binding to structurally similar PFAS compounds (i.e., perfluorocarboxylic acids and perfluorosulfonic acids with 4-8 carbon chains) was also investigated; the aptamer K_D values were 6.5 and 3.3 μM for perfluoroheptanoic acid and perfluorohexanesulfonic acid, respectively, while other analogs did not bind to the aptamer. The presence of major inorganic ions and dissolved organic matter had negligible influences on the aptamer performance (<14% at a 10 mM concentration), and the aptamer performance was also robust in real wastewater effluent conditions, with a K_D of 7.4 μM for PFOA. Fluorescence-based aptasensor developed in this study is adequate in monitoring PFOA levels in water contaminated with the accident spills and heavy usage of fire-fighting foams near the industrial sites and military bases. More importantly, the study opens up new capability of aptasensors to efficiently monitor the trace amount of various PFAS compounds and other fluorinated alternatives in natural and engineered water environments.

An enhanced fluorescent probe through the strategy of using MgWO4 nanosheets to enhance terbium ion luminescence for highly sensitive and point-of-care visual quantitative testing of ciprofloxacin integrated with a low-cost smartphone-based platform

In this work, MgWO₄ nanosheets have been successfully synthesized by a simple hydrothermal method, and the morphology and composition of the MgWO₄ nanosheets are characterized by SEM, TEM, XPS, and UV-vis. The results show that the as-prepared MgWO₄ nanosheets have a triclinic symmetry phase and an obvious two-dimensional layered structure. Studies have shown that the MgWO₄ semiconductor nanosheets can adjust the energy level, which significantly enhances the visible light absorption and the separation and transfer of photogenerated electrons, which facilitates the generation of photogenerated electrons. We use this feature to boost a terbium ion (Tb³⁺)-ciprofloxacin (CIP) system to enhance the luminescence, so as to achieve highly sensitive detection of CIP. The mechanism of Tb³⁺-MgWO₄ for CIP detection is the effective energy transfer from WO₄^2- in MgWO₄ nanosheets to Tb³⁺-CIP, thereby enhancing the characteristic emission of Tb³⁺ and enhancing the sensitivity of CIP detection. The linear response of the Tb³⁺-MgWO₄ enhanced fluorescent probe is in the range from 10 nM to 20 μM, and the detection limit (LOD) is 2 nM. In addition, we tested the recovery in spiked river water and mouse serum. Experiments have shown that the recovery of spiked samples is 97-102.2%, while the relative standard deviation (RSD) is less than 5.53%. The possible interfering substances in environmental samples will not interfere with the detection of CIP with the Tb³⁺-MgWO₄ enhanced fluorescent probe. At the same time, the development of a smartphone-based portable device provides the possibility of CIP on-site detection. Our work provides a simple, fast, highly sensitive and stable method for detecting CIP in living organisms and the environment. Importantly, the strategy of MgWO₄ nanosheet boosted terbium ion luminescence expands the application range of semiconductor nanomaterials.

A sensitive aptasensor based on rolling circle amplification and G-rich ssDNA/terbium (III) luminescence enhancement for ofloxacin detection in food

As the light-harvesting "antenna", G-rich oligonucleotides (such as the G-quadruplex) can interact with lanthanide (III) to bring a luminescent enhancement response. In this study, phenomenon of luminescent enhancement of G-triplex/terbium (III) (G3/Tb³⁺) and interaction between G3 and Tb³⁺ were first reported and characterized. Based on G3/Tb³⁺ luminescence, a label-free aptasensor for the detection of ofloxacin (OFL) residues in the food was developed. The OFL triggered the action of rolling circle amplification (RCA) allowed for the amplification product of G3-forming sequences in the single-stranded DNA, which promoted the conformational transition of the G3/Tb³⁺ complexes once the addition of Tb³⁺. Under the optimal conditions, the logarithmic correlation between the G3/Tb³⁺ luminescence intensity and the concentration of OFL was found to be linear in the range of 5-1000 pmol L^-1 (R² = 0.9949). The limit of detection was 0.18 pmol L^-1 (3σ/slope). Additionally, the good recoveries of 90.19-108.89 % and the relative standard deviations values of 0.59-5.87 % were obtained in the application of the aptasensor detecting OFL in the practical samples. These results confirmed that the present aptasensor has a good analytical performance and bright prospect for detecting ofloxacin residues in food.

Hybrid Antibody-Aptamer Assay for Detection of Tetrodotoxin in Pufferfish

The marine toxin tetrodotoxin (TTX) poses a great risk to public health safety due to its severe paralytic effects after ingestion. Seafood poisoning caused by the consumption of contaminated marine species like pufferfish due to its expansion to nonendemic areas has increased the need for fast and reliable detection of the toxin to effectively implement prevention strategies. Liquid chromatography-mass spectrometry is considered the most accurate method, although competitive immunoassays have also been reported. In this work, we sought to develop an aptamer-based assay for the rapid, sensitive, and cost-effective detection of TTX in pufferfish. Using capture-SELEX combined with next-generation sequencing, aptamers were identified, and their binding properties were evaluated. Finally, a highly sensitive and user-friendly hybrid antibody–aptamer sandwich assay was developed with superior performance compared to several assays reported in the literature and commercial immunoassay kits. The assay was successfully applied to the quantification of TTX in pufferfish extracts, and the results obtained correlated very well with a competitive magnetic bead-based immunoassay performed in parallel for comparison. This is one of the very few works reported in the literature of such hybrid assays for small-molecule analytes whose compatibility with field samples is also demonstrated.

[Ru(bpy)3]2+@Ce-UiO-66/Mn:Bi2S3 Heterojunction and Its Exceptional Photoelectrochemical Aptasensing Properties for Ofloxacin Detection

A photoelectrochemical (PEC) aptasensor on basis of [Ru(bpy)₃]²⁺@Ce-UiO-66/Mn:Bi₂S₃ composites was constructed for detecting ofloxacin (OFL). First, Ce-UiO-66, prepared by a solvothermal method, had Zr⁴⁺-Zr³⁺ and Ce⁴⁺-Ce³⁺ intervalence cycles to increase the charge separation efficiency. Subsequently, Ce-UiO-66 was further modified by [Ru(bpy)₃]²⁺ and Mn:Bi₂S₃ cosensitization to improve the photoelectric activity. [Ru(bpy)₃]²⁺ not only broadened the range of light absorbed but also reacted with an electron donor to maintain the photoelectric conversion process. Among the [Ru(bpy)₃]²⁺@Ce-UiO-66/Mn:Bi₂S₃ heterojunction, Mn:Bi₂S₃ was a photosensitizer, which maximized the efficiency of the electron-hole separation and significantly improved photocurrent. Then, an aptamer was used as a biorecognition unit for OFL-specific detection. Under the best conditions, the PEC aptasensor realized the sensitive detection of OFL, with a detection range of 0.01-100 nmol/L and a detection limit of 6 pmol/L. In addition, the constructed PEC OFL sensor showed good reproducibility, stability, and specificity.

Aptamer-based diagnostic and therapeutic approaches in animals: Current potential and challenges

Fast and precise diagnosis of infectious and non-infectious animal diseases and their targeted treatments are of utmost importance for their clinical management. The existing biochemical, serological and molecular methods of disease diagnosis need improvement in their specificity, sensitivity and cost and, are generally not amenable for being used as points-of-care (POC) device. Further, with dramatic changes in environment and farm management practices, one should also arm ourselves and prepare for emerging and re-emerging animal diseases such as cancer, prion diseases, COVID-19, influenza etc. Aptamer – oligonucleotide or short peptides that can specifically bind to target molecules – have increasingly become popular in developing biosensors for sensitive detection of analytes, pathogens (bacteria, virus, fungus, prions), drug residues, toxins and, cancerous cells. They have also been proven successful in the cellular delivery of drugs and targeted therapy of infectious diseases and physiological disorders. However, the in vivo application of aptamer-mediated biosensing and therapy in animals has been limited. This paper reviews the existing reports on the application of aptamer-based biosensors and targeted therapy in animals. It also dissects the various modifications to aptamers that were found to be successful in in vivo application of the aptamers in diagnostics and therapeutics. Finally, it also highlights major challenges and future directions in the application of aptamers in the field of veterinary medicine.

Aptamer-based lateral flow assay on-site biosensors

The lateral flow assay (LFA) is a widely used paper-based on-site biosensor that can detect target analytes and obtain test results in several minutes. Generally, antibodies are utilized as the biorecognition molecules in the LFA. However, antibodies selected using an in vivo process not only may risk killing the animal hosts and causing errors between different batches but also their range is restricted by the refrigerated conditions used to store them. To avoid these limitations, aptamers screened by an in vitro process have been studied as biorecognition molecules in LFAs. Based on the sandwich or competitive format, the aptamer-based LFA can accomplish on-site detection of target analytes. Since aptamers have a distinctive ability to undergo conformational changes, the adsorption-desorption format has also been exploited to detect target analytes in aptamer-based LFAs. This paper reviews developments in aptamer-based LFAs in the last three years for the detection of target analytes. Three formats of aptamer-based LFAs, i.e., sandwich, competitive, and adsorption-desorption, are described in detail. Based on these formats, signal amplification strategies and multiplexed detection are discussed in order to provide an overview of aptamer-based LFAs for on-site detection of target analytes. In addition, the potential commercialization and future perspectives of aptamer-based LFAs for rapid detection of SARS-CoV-2 are given to support the COVID-19 pandemic.

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

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

Application of Nanotechnology in Analysis and Removal of Heavy Metals in Food and Water Resources

Toxic heavy metal contamination in food and water from environmental pollution is a significant public health issue. Heavy metals do not biodegrade easily yet can be enriched hundreds of times by biological magnification, where toxic substances move up the food chain and eventually enter the human body. Nanotechnology as an emerging field has provided significant improvement in heavy metal analysis and removal from complex matrices. Various techniques have been adapted based on nanomaterials for heavy metal analysis, such as electrochemical, colorimetric, fluorescent, and biosensing technology. Multiple categories of nanomaterials have been utilized for heavy metal removal, such as metal oxide nanoparticles, magnetic nanoparticles, graphene and derivatives, and carbon nanotubes. Nanotechnology-based heavy metal analysis and removal from food and water resources has the advantages of wide linear range, low detection and quantification limits, high sensitivity, and good selectivity. There is a need for easy and safe field application of nanomaterial-based approaches.

Screening and application of a broad-spectrum aptamer for acyclic guanosine analogues

Acyclic guanosine analogues, a class of widely used antiviral drugs, can cause chronic toxicity and virus resistance. Therefore, it is essential to establish rapid and accurate methods to detect acyclic guanosine analogues. In this study, five acyclic guanosine analogues (acyclovir, famciclovir, ganciclovir, penciclovir, and valaciclovir) were used as positive targets to obtain broad-spectrum aptamers through Capture-SELEX technology. Real-time quantitative PCR (Q-PCR) was used to monitor the aptamer SELEX process. After the sixteen rounds of selection against mixed targets, sequences were obtained by high-throughput sequencing (HTS). Furthermore, a broad-spectrum aptamer, named CIV6, was found as the higher performance aptamer that was suitable for five acyclic guanosine analogues by graphene oxide (GO) polarization and fluorescence assay. Finally, the aptamer CIV6 was used to construct GO fluorescence assay to detect five acyclic guanosine analogues. The limits of detection (LOD) of acyclovir, famciclovir, ganciclovir, penciclovir, and valaciclovir were 0.48 ng·mL^-1, 0.53 ng·mL^-1, 0.50 ng·mL^-1, 0.56 ng·mL^-1, and 0.38 ng·mL^-1, respectively.

Sensitive aptasensing of ciprofloxacin residues in raw milk samples using reduced graphene oxide and nanogold-functionalized poly(amidoamine) dendrimer: An innovative apta-platform towards electroanalysis of antibiotics

The constant need of humans and animals for food resources was led to overuse of antibiotics as vital medicines. In this regard, we are now facing major concern about the risks on the food safety and environment owing to their uncontrolled disposal. Hence, the progress of simple and sensitive approaches for fast monitoring of antibiotic levels is highly desirable. Here, we aimed to describe a new sensitive and easy-to use strategy based on electrochemical single off apta-assay toward ciprofloxacin (CFX). A novel interface using 3D Au-PAMAM/rGO have been designed via full electrochemically technique on the surface of glassy carbon electrodes (GCE) and evaluated with cyclic voltammetry method. Firstly, rGO with large amount of active functional groups as substrate was fabricated on the GCE electrode. Thereby, the 3D Au-PAMAM nanocomposite was synthesized and covalently electrodeposited onto the rGO-GCE modified surface. The structure and morphology of 3D Au-PAMAM were studied using UV-Vis spectroscopy, Field emission scanning electron microscopes (FE-SEM) and Transmission electron microscopy. Also, FE-SEM and Energy-dispersive X-ray spectroscopy (EDS) have been carry out to illustrate surface morphology of electrodes. The obtained results from square wave voltammetry, different pulse voltammetry, and chronoamperometry techniques implied that the suggested scaffold could be used as facile bio-device toward antibiotic detection with low limit of quantification (LLOQ) of 1 nM and a linear range of 1 μM-1 nM. Interestingly, the suggested aptasensor is successfully used to measure CFX residues in local and pasteurized milk samples. It can be deduced that Apt/rGO/3D Au-PAMAM/GCE as a novel biocompatible interface could offer suitable, cost-effective, reliable, rapid, and user-friendly sensing device for direct determination of CFX in real samples.

Colorimetric Detection of Kanamycin Residue in Foods Based on the Aptamer-Enhanced Peroxidase-Mimicking Activity of Layered WS2 Nanosheets

Although the colorimetric methods can easily meet the demands of point-of-care and ease-of-use for antibiotic detection, they still face many challenges in the accuracy and stability of assay. Herein, a facile and stable colorimetric aptasensor is first developed for kanamycin residue detection based on the aptamer-enhanced peroxidase-mimicking activity of layered WS₂ nanosheets. The investigation confirmed that aptamer sequences can improve the affinity of nanosheets to the chromogenic substrate 3,3'',5,5''-tetramethylbenzidine, resulting in a significant increase of the peroxidase-mimicking activity. Under the optimal conditions, the limit of detection of the proposed colorimetric aptasensor for kanamycin was determined to be as low as 0.6 μM, and such an aptasensor displays excellent selectivity against other competitive antibiotics. Moreover, further studies have verified the applicability of the established colorimetric aptasensor in several actual samples, indicating that the aptasensor may have bright application prospects for kanamycin detection in livestock husbandry and agriculture samples.

Capture-SELEX for aptamer selection: A short review

The emerging aptamer, developed through the systematic evolution of ligands by exponential enrichment (SELEX) process, has revolutionized and facilitated the discoveries in basic research. Among all SELEX technology, Capture-SELEX is a variant of the in vitro selection process, which is suitable for isolating aptamers against small molecules. Capture-SELEX library was developed to enable the immobilization of the oligonucleotides instead of the target molecules during the aptamer selection process. The review provides an update on the recent-advances in this new screening method with particular emphasis on key points of capture protocol and its applications. The limitations and the prospects of the Capture-SELEX are also discussed. We hope that present review will inspire more researchers to understand the selection problems from the perspective of Capture-SELEX. Moreover, it will open new pave to improve the efficiency and success of screening to meet the growing demand for aptasensor discovery in small molecules.

Biosensors for wastewater-based epidemiology for monitoring public health

Public health is attracting increasing attention due to the current global pandemic, and wastewater-based epidemiology (WBE) has emerged as a powerful tool for monitoring of public health by analysis of a variety of biomarkers (e.g., chemicals and pathogens) in wastewater. Rapid development of WBE requires rapid and on-site analytical tools for monitoring of sewage biomarkers to provide immediate decision and intervention. Biosensors have been demonstrated to be highly sensitive and selective tools for the analysis of sewage biomarkers due to their fast response, ease-to-use, low cost and the potential for field-testing. This paper presents biosensors as effective tools for wastewater analysis of potential biomarkers and monitoring of public health via WBE. In particular, we discuss the use of sewage sensors for rapid detection of a range of targets, including rapid monitoring of community-wide illicit drug consumption and pathogens for early warning of infectious diseases outbreaks. Finally, we provide a perspective on the future use of the biosensor technology for WBE to enable rapid on-site monitoring of sewage, which will provide nearly real-time data for public health assessment and effective intervention.

Colorimetric detection of ciprofloxacin in aqueous solution based on an unmodified aptamer and the aggregation of gold nanoparticles

A colorimetric method is described for detection of the antibiotic ciprofloxacin (CIP) in aqueous solutions based on an unmodified CIP-aptamer and gold nanoparticles (AuNPs), which are regarded as a probe and an indicator, respectively. In the absence of CIP, aptamers can hybridize with poly dimethyl diallyl ammonium chloride (PDDA) to form a 'duplex' structure through electrostatic interactions, and AuNPs remain dispersed in solution, leading to a red solution and an obvious absorption peak at 520 nm. Contrarily, the CIP-aptamer can specifically bind to CIP after the introduction of CIP into the solution. Then PDDA is thus free to aggregate AuNPs. The solution turns blue from red accordingly, presenting a strong absorption at 650 nm. Hence, the concentration of CIP can be quantified through the changes of the absorption. This sensitive and selective colorimetric method for CIP detection has a good linear response (R = 0.9935) in the 20 to 300 nM CIP concentration range, with a limit of detection (LOD) of 0.215 nM. Importantly, the proposed aptasensor demonstrates great application potential in CIP determination in aqueous samples.

Investigation of a Truncated Aptamer for Ofloxacin Detection Using a Rapid FRET-Based Apta-Assay

In this work, we describe the use of a new truncated aptamer for the determination of ofloxacin (OFL), being a principal quinolone commonly used in both human and animal healthcare. Since the affinity of a 72-mer ssDNA sequence has been previously described without further investigations, this paper demonstrates the first computational prediction of the binding motif between this aptamer and OFL through in silico molecular docking studies. Besides, we suggest the application of the characterized recognition mechanism in a simple FRET (Förster Resonance Energy Transfer) pattern for the rapid aptasensing of the quinolone of interest. Accordingly, our approach harnesses the fluorescence quenching of the fluorescein-tagged aptamer (FAM-APT) induced by its partial hybridization to a tetramethyl rhodamine-labelled complementary ssDNA (TAMRA-cDNA). In such a structure, dye labels brought into close proximity act as a FRET pair. Upon ofloxacin addition, an affinity competition occurs to form a more stable FAM-APT/OFL complex, thus unquenching the FAM-APT signal. Interestingly, the recovered fluorescence intensity was found to correlate well with the antibiotic's concentrations in the range of 0.2-200 μM in HEPES buffer, with a linear response that ranged between 0.2 and 20 μM. The rapid apta-assay achieved limits of detection and quantification of 0.12 and 0.40 μM, respectively. The truncated aptamer has also shown an improved specificity toward OFL than other quinolones, compared to the original full-length aptamer described in previous works. Finally, the practical application of the developed apta-assay was successfully confirmed to detect OFL quinolone in spiked milk samples, with satisfactory recoveries ranging between 97.4% and 111.4%.

A time-resolved luminescence aptasensor of ofloxacin based on rolling circle amplification and magnetic separation

A novel sensitive, competitive, and time-resolved luminescence sensor for the detection of ofloxacin (OFL) was developed in this study. The sensor used OFL-specific aptamer as a recognition molecule and rolling circle amplification (RCA) as a signal amplification tool. In this way, the time-resolved luminescence can not only avoid background noise from sample, but also provide robust luminescence for detection. Besides, the separation and enrichment of target veterinary drug can be conducted assisted by magnetic treatment. Under optimal conditions, the logarithmic correlation between the concentration of OFL and the luminescence intensity was found to be linear in the range of 5 × 10^-11-5 × 10^-8 mol L^-1 (R² = 0.9988), with a detection limit (LOD) of 32.1 pmol L^-1. Furthermore, this method was applied to the determination of OFL in chicken and pork samples, exhibiting good recovery (72.5-100%) and repeatability (RSD < 10.0%). These results confirm that this novel established method has good application potential for the detection of OFL in food samples.

Moxifloxacin Hydrochloride Electrochemical Detection at Gold Nanoparticles Modified Screen-Printed Electrode

It appeared that either the carbon paste or the screen-printed carbon electrodes that were modified with gold nanoparticles (AuNPs) gave rise to the largest current responses after a rapid screening of various nanomaterials as modifiers of carbon composite electrodes in view of designing an electrochemical sensor for Moxifloxacin Hydrochloride (Moxi). The screen-printed electrode (SPE) support was preferred over the carbon paste one for its ability to be used as disposable single-use sensor enabling the circumvention of the problems of surface fouling encountered in the determination of Moxi. The response of AuNPs modified SPE to Moxi was investigated by cyclic voltammetry (CV) (including the effect of the potential scan rate and the pH of the medium), chronoamperometry, and differential pulse voltammetry (DPV) after morphological and physico-chemical characterization. DPV was finally applied to Moxi detection in phosphate buffer at pH 7, giving rise to an accessible concentration window ranging between 8 µM and 0.48 mM, and the detection and quantification limits were established to be 11.6 µM and 38.6 µM, correspondingly. In order to estimate the applicability of Moxi identification scheme in actual trials, it was practiced in a human baby urine sample with excellent recoveries between 99.8 % and 101.6 % and RSDs of 1.1-3.4%, without noticeable interference.

Fluorescence assay for three organophosphorus pesticides in agricultural products based on Magnetic-Assisted fluorescence labeling aptamer probe

There has been increasing recent concern about the agricultural use of organophosphorus pesticides. A rapid and sensitive fluorescence assay for the detection of three organophosphorus pesticides has therefore been developed using 6-carboxy-fluorescein labeling aptamer as the probe and functionalized magnetic nanoparticles as the separation carrier. The aptamer hybridized with complementary DNA conjugated on the surface of the magnetic nanoparticles to form a magnetic aptamer-complementary DNA complex. Upon introducing the target organophosphorus pesticide, the aptamer departed from the complementary DNA, resulting in the fluorescence signal. Under optimized conditions, the limits of detection (LODs, S/N = 3) for trichlorfon, glyphosate, and malathion were 72.20 ng L-1, 88.80 ng L-1, and 195.37 ng L-1, respectively. The method was applied for the detection of trichlorfon, glyphosate, and malathion in spiked lettuce and carrot samples. The recoveries were in the range of 79.4%-118.7%, which were in good agreement with those obtained by gas chromatography, and the relative standard deviations were also acceptable. The method therefore has high sensitivity, so provides a means for the detection of multiple organophosphorus pesticides.

Predicting Future Prospects of Aptamers in Field-Effect Transistor Biosensors

Aptamers, in sensing technology, are famous for their role as receptors in versatile applications due to their high specificity and selectivity to a wide range of targets including proteins, small molecules, oligonucleotides, metal ions, viruses, and cells. The outburst of field-effect transistors provides a label-free detection and ultra-sensitive technique with significantly improved results in terms of detection of substances. However, their combination in this field is challenged by several factors. Recent advances in the discovery of aptamers and studies of Field-Effect Transistor (FET) aptasensors overcome these limitations and potentially expand the dominance of aptamers in the biosensor market.

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

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

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

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

Fluorescent aptasensor for ofloxacin detection based on the aggregation of gold nanoparticles and its effect on quenching the fluorescence of Rhodamine B

This paper proposes the idea of building a fluorescent biosensor for ofloxacin (OFL) determination in aqueous and milk samples by label-free OFL-specific aptamer, gold nanoparticles (AuNPs) and Rhodamine B (RB). In the absence of OFL, AuNPs are coated with OFL aptamer and maintain dispersed in the high concentration of NaCl. The dispersed AuNPs could reduce the strong fluorescence intensity of RB efficiently. By contrast, in the presence of OFL, OFL is combined with aptamer to form stable compounds, causing the aptamers separated from the surface of AuNPs, thus AuNPs would be exposed in the solution. And the aggregated AuNPs will not quench the fluorescence intensity of RB. Through the distinction of the fluorescence intensity, the concentration of OFL could be detected in aqueous and milk samples quantitatively. The convenient and specific fluorescent assay for OFL is established with a linear range (R = 0.9907) from 20 to 300 nM and a detection limit of 1.66 nM in aqueous solution, and a linear range (R = 0.9963) from 20 to 300 nM and a detection limit of 4.61 nM (1.66 μg/L) in milk samples. With the good sensitivity and selectivity, this biosensor has good application potential to detect OFL in food and environmental samples.