Aptamer-based thin film gold electrode modified with gold nanoparticles and carboxylated multi-walled carbon nanotubes for detecting oxytetracycline in chicken samples

Enhanced quenching efficiency of UiO-66-NH2 over UiO-66 to engineer high-performance fluorescence aptasensor for oxytetracycline monitoring

Oxytetracycline (OTC), a broad class of antibacterial medication, has been widely utilized in poultry husbandry and fish farming. However, the presence of OTC residues in the food has become a matter of great concern. These residues have the potential to induce the emergence and spread of bacterial resistance, which can have far-reaching consequences for public health. In this work, we focused on the comparison of the quenching efficiency of UiO-66 and its amino group-modified counterpart UiO-66-NH2, towards the fluorescence of nitrogen-doped graphene quantum dots (NGQDs) labeled aptamer (NGQDs-apt). It was found that UiO-66-NH2 demonstrated a more potent fluorescence quenching ability towards NGQDs-apt, as high as 98.8 %, compared to 69.7 % for UiO-66 without amino functionalization. This enhanced performance is attributed to the improved interaction between the amino groups on UiO-66-NH2 and the NGQDs-apt, facilitating more effective electron transfer. Consequently, UiO-66-NH2 was selected as the optimal quencher for the development of a highly sensitive aptasensor. This aptasensor demonstrates a quantitative detection range for OTC spanning from 0.001 to 200 μM with an exceptionally low detection limit of 0.25 nM, showing simplicity, accuracy, stability and great potential for antibiotic residue detection in food samples. This work addresses the urgent need to develop reliable methods for detecting OTC residues in food due to the potential of OTC to trigger bacterial resistance, which is crucial for food safety and public health.

Recent Advances in Electrochemical Aptasensors for Detection of Clinical and Veterinary Drugs

Nowadays, aptamer-based biosensors and electrochemical measurements represent one of the efficient tools for the detection of drugs in both medical and veterinary. Precise trace values analysis of chemicals, especially drugs, plays a crucial role in food and therapeutic safety evaluations that are often time-consuming and costly. Ultimately, accurate determination of therapeutic medications like antibiotics in food, environmental resources, and biological matrices is very important for protecting public health and drug monitoring (TDM) for effective treatment. This review highlights recent advancements in electrochemical aptasensors as an innovative approach offering high sensitivity, specificity, and rapid detection of clinical and veterinary drugs at lower costs. We provide a comprehensive overview of the advancements and discuss the challenges and prospects of electrochemical aptasensing in drug residue detection across various samples.

A novel electrochemical sensor based on ꞵ-cyclodextrin/bismuth oxybromide/multi-walled carbon nanotubes modified electrode with in situ addition of tetrabutylammonium bromide for the simultaneous detection and degradation of tebuconazole

A novel electrochemical sensor-based glassy carbon electrode (GCE) was fabricated and applied to simultaneous detection and degradation of tebuconazole (TBZ) for the first time. The GCE was consecutively modified by multi-walled carbon nanotubes (MWCNTs), bismuth oxybromide (BiOBr), ꞵ-cyclodextrin (ꞵ-CD), and in situ addition of tetrabutylammonium bromide (TBABr). The detection was based on the decreasing of Bi signal at its anodic potential (Epa) of 0.05 V. Under the optimum conditions, the modified electrode exhibited a linear response to TBZ in the concentration range 1-100 μg L-1 with a detection limit of 0.9 μg L-1. TBZ was firstly adsorbed on the surface of the modified electrode through host-guest molecule interactions of the ꞵ-CD. The adsorption was further enhanced by the large surface area of BiOBr and MWCNTs. The adsorbed TBZ on the electrode surface hindered the electron transfer of Bi, thus decreasing the oxidation of Bi. In addition, the in situ addition of tetrabutylammonium bromide (TBABr) enriched TBZ via electrostatic interactions, increasing its detection sensitivity. The fabricated electrochemical sensor was applied to determine TBZ in water and soil samples from rice fields with recoveries of 80.5-100.5% and 87.6-112%, respectively. Furthermore, the degradation of TBZ on the modified electrode was studied under a solar light simulator. The degradation percentage (100%) of TBZ (50 µg L-1) was achieved in 5 min owing to the excellent photocatalytic properties of BiOBr.

Monitoring of trace oxytetracycline using a porphyrin-MOF layer-based electrochemical aptasensor

A two-dimensional porphyrin-MOF nanolayer was developed to construct an electrochemical aptasensor for monitoring oxytetracycline from 0.01 pg mL-1 to 0.1 ng mL-1. This aptasensor exhibited high sensitivity, outstanding selectivity, good stability, fine reproducibility, and quantitative detection ability in real samples.

Bi2MoO6/MWCNTs-COOH/GCE electrochemical probe for ultrasensitively and selectively detecting antibiotic norfloxacin

Antibiotics have emerged as a class of contaminants of concern globally, raising widespread worry and unease, primarily because of their inappropriately use and endless migration in food chains. Electrochemical ultrasensitive and selective determination of norfloxacin (NOR) using bismuth-based bimetallic salt bismuth molybdate (Bi2MoO6) and carboxylated multi-walled carbon nanotube (MWCNTs-COOH) hybrid electrode as sensing platforms is presented hereby. The electrocatalytic ability of the electrodes was verified to be enhanced by the synergy effect of both Bi2MoO6 and MWCNTs-COOH using cyclic voltammetry and electrochemical impedance spectroscopy. Linear scan voltammetry was applied to detect NOR. The oxidation peak current was linearly correlated with NOR concentration of 0.03-10 μM and the low limit detection of the optimized method was 6.7 nM (S/N = 3). In addition, it was verified that Bi2MoO6/MWCNTs-COOH/GCE probe had good stability, reproducibility, repeatability and selectivity. The spiked recovery method for NOR in milk and lake water samples showed recoveries of 94.8 - 96.9% and 90.2 - 98.3%, respectively, which can be used to determine NOR in real samples with high sensitivity. The preparation of Bi2MoO6/MWCNTs-COOH/GCE provides a new prototype for probing NOR detection at nanomolar concentration and safeguarding antibiotic contamination in environmental and food chains.

Colorimetric aptasensor utilizing MOF-235 with exceptional peroxidase-like activity for the detection of oxytetracycline residues in raw milk

In this work, a simple, convenient and cost-effective colorimetric aptasensor was successfully constructed for the detection of antibiotic residues in raw milk based on the property that aptamer (Apt) synergistically enhances the catalase-like activity of MOF-235. Under optimised conditions, the proposed colorimetric aptasensor exhibited a wide detection range (15-1500 nM) with a low detection limit (6.92 nM). Furthermore, the proposed aptasensor demonstrated high selectivity, good resistance to interference and storage stability. The proposed aptasensor was validated by spiking recovery in camel milk, cow milk and goat milk with satisfactory recoveries, which demonstrated the great potential of the aptasensor for further application in real food samples, and also suggested that MOF-235 can be used as a potential universal platform to build a sensitive detection platform for other targets.

Design and fabrication of a competitive lateral flow assay using gold nanoparticle as capture probe for the rapid and on-site detection of penicillin antibiotic in food samples

Lateral flow assays (LFAs) are among the utmost cost-efficient, paper-based point-of-care (POC) diagnostic devices. Herein, we have reported the fabrication of a competitive LFA for on-site detection of penicillin. Various parameters such as Ab concentration for conjugation, Pen-BSA conjugate concentration, pore size of membrane, and blocking buffer were standardised for the fabrication of LFA. Different concentrations of penicillin (1 pM-1 mM) were added to the sample pad to observe the color intensity. The visual detection limit (LOD) achieved from the LFA was 10 nM for Penicillin that correlated with the LOD calculated from the 'ColorGrab' colorimeter application. Additionally, LFA showed insignificant cross reactivity with other β-lactam antibiotics and were also validated with spiked food samples such as milk, meat and egg. Hence, the fabricated LFA can be successfully utilised for the POC detection of penicillin in food samples on large scale.

Enhancing the sensitivity and stability of electrochemical aptamer-based sensors by AuNPs@MXene nanocomposite for continuous monitoring of biomarkers

Electrochemical aptamer-based (E-AB) sensors offer exciting potential for real-time tracking of various biomarkers, such as proteins and small molecules, due to their exceptional selectivity and adaptability. However, most E-AB sensors rely on planar gold structures, which inherently limit their sensitivity and operational stability for continuous monitoring of biomarkers. Although gold nanostructures have recently enhanced E-AB sensor performance, no studies have explored the combination of gold nanostructure with other types of nanomaterials for continuous molecular monitoring. To fill this gap, we employed gold nanoparticles and MXene Ti3C2 (AuNPs@MXene), a versatile nanocomposite, in designing an E-AB sensor targeted at vascular endothelial growth factor (VEGF), a crucial human signaling protein. Remarkably, the AuNPs@MXene nanocomposite achieved over thirty-fold and half-fold increases in active surface area compared to bare and AuNPs-modified gold electrodes, respectively, significantly elevating the analytical capabilities of E-AB sensors during continuous operation. After a systematic optimization and characterization process, the newly developed E-AB sensor, powered by AuNPs@MXene nanocomposite, demonstrated both enhanced stability and heightened sensitivity. Overall, our findings open new avenues for the incorporation of nanocomposites in E-AB sensor design, enabling the creation of more sensitive and durable real-time monitoring systems.

Comparative Analysis of pH and Target-Induced Conformational Changes of an Oxytetracycline Aptamer in Solution Phase and Surface-Immobilized Form

To date, numerous aptamer-based biosensing platforms have been developed for sensitive and selective monitoring of target analytes, relying on analyte-induced conformational changes in the aptamer for the quantification of the analyte and the conversion of the binding event into a measurable signal. Despite the impact of these conformational rearrangements on sensor performance, the influence of the environment on the structural conformations of aptamers has rarely been investigated, so the link between parameters directly influencing aptamer folding and the ability of the aptamer to bind to the target analyte remains elusive. Herein, the effect a number of variables have on an aptamer's 3D structure was examined, including the pH of the buffering medium, as well as the anchoring of the aptamer on a solid support, with the use of two label-free techniques. Circular dichroism spectroscopy was utilized to study the conformation of an aptamer in solution along with any changes induced to it by the environment (analyte binding, pH, composition and ionic strength of the buffer solution), while quartz crystal microbalance with dissipation monitoring was employed to investigate the surface-bound aptamer's behavior and performance. Analysis was performed on an aptamer against oxytetracycline, serving as a model system, representative of aptamers selected against small molecule analytes. The obtained results highlight the influence of the environment on the folding and thus analyte-binding capacity of an aptamer and emphasize the need to deploy appropriate surface functionalization protocols in sensor development as a means to minimize the steric obstructions and undesirable interactions of an aptamer with a surface onto which it is tethered.

An electrochemical impedimetric platform formed by a CNT@UiO-66 nanocomposite for quantitative analysis of oxytetracycline

Functional materials are considered one of the most critical factors in constructing high-performance electrochemical aptasensors in the sensing field. In this work, the microporous Zr-MOF UiO-66 (UiO = Universitetet i Oslo) is selected for assembly with carbon nanotubes (CNTs) to prepare a CNT@UiO-66 composite. The as-synthesized CNT@UiO-66 composite has a high surface area, excellent stability, good electrical conductivity, and abundant Zr(IV) sites, conferring it great potential for application in fabricating high-performance electrochemical aptasensors. It is gratifying that this electrochemical impedimetric aptasensor can detect trace oxytetracycline (OTC) from 0.01 to 0.7 pg mL-1 with a low limit of detection (LOD) of 1.48 fg mL-1. Meanwhile, this fabricated sensor based on CNT@UiO-66 has fine stability, excellent selectivity, and available reproducibility. In particular, the CNT@UiO-66-based aptasensor can quantitatively detect the OTC concentration in real samples.

Label-free "signal-off" PEC aptasensor for determination of kanamycin based on 3D nanoflower-like FeIn2S4/CdS Z-scheme heterostructures

Highly photoactive 3D nanoflower-like FeIn2S4/CdS heterostructures were synthesized by hydrothermal treatment and low-temperature cation exchange. The FeIn2S4/CdS displayed 14.5 times signal amplification in contrast to FeIn2S4 alone. It was applied as a photoactive substrate to construct a label-free photoelectrochemical (PEC) aptasensor for ultrasensitive determination of kanamycin (KAN). Under the optimal conditions, the constructed PEC aptasensor displayed a wide linear range (5.0 × 10-4 ~ 5.0 × 101 ng mL-1) and a low detection limit (S/N = 3) of 40.01 fg mL-1. This study provides some constructive insights for preparation of advanced photoactive materials and exhibits great potential for quantitative determination of antibiotics in foods and environmental samples.

Current Trends in Nanomaterials-Based Electrochemiluminescence Aptasensors for the Determination of Antibiotic Residues in Foodstuffs: A Comprehensive Review

Veterinary pharmaceuticals have been recently recognized as newly emerging environmental contaminants. Indeed, because of their uncontrolled or overused disposal, we are now facing undesirable amounts of these constituents in foodstuff and its related human health concerns. In this context, developing a well-organized environmental and foodstuff screening toward antibiotic levels is of paramount importance to ensure the safety of food products as well as human health. In this case, with the development and progress of electric/photo detecting, nanomaterials, and nucleic acid aptamer technology, their incorporation-driven evolving electrochemiluminescence aptasensing strategy has presented the hopeful potentials in identifying the residual amounts of different antibiotics toward sensitivity, economy, and practicality. In this context, we reviewed the up-to-date development of ECL aptasensors with aptamers as recognition elements and nanomaterials as the active elements for quantitative sensing the residual antibiotics in foodstuff and agriculture-related matrices, dissected the unavoidable challenges, and debated the upcoming prospects.

Size-controlling preparation of covalent organic framework nanospheres for electrochemical impedimetric aptasensing of oxytetracycline

Covalent organic framework (COF) nanospheres with controlled size of ∼500 and ∼1100 nm were successfully prepared by adjusting the HOAc amount in the synthetic system. The as-synthesized COFs have large conjugate aromatic skeleton, excellent stability, abundant pore, and uniform morphology. These advantages of COFs are benefit for immobilizing aptamers to fabricate the targeted electrochemical aptasensor. The commonly used oxytetracycline (OTC) is an analytic model to explore the sensing performance of the COF-based aptasensor, indicating that the smaller COF (∼500 nm) is more conducive to acquiring the sensitive sensor than that of the larger COF (∼1100 nm). Moreover, the limitation of detection of the COF (∼500 nm)-based aptasensor is calculated to be 7.4 fg mL^-1 using the response impedance signal. Additionally, the aptamer-based biosensor has fine reproducibility, good stability, excellent specificity, and available usability even in real samples.

Modified electrochemical aptasensor for ultrasensitive detection of tetracycline: In silico and in vitro studies

An ultrasensitive electrochemical aptasensor based on a glassy carbon electrode, modified by carbon nanofibers and carboxylated multi-walled carbon nanotubes was fabricated to detect tetracycline in food samples. The affinity of antibiotics, including kanamycin, tetracycline, ampicillin, and sulfadimethoxine toward desired sequences of aptamers and the stability of antibiotic-aptamer complexes were studied using molecular docking and molecular dynamic simulations. Moreover, the highest affinity and most stable complex were observed for tetracycline in complex with kanamycin-specific aptamer (KAP). Finally, KAP was used to develop an aptasensor. The central composite design (CCD) was used to optimize effective parameters. The biosensor achieved a wide dynamic linear range (1.0 × 10^-17-1.0 × 10^-5 M) and a low limit of detection (2.28 × 10^-18 M) under optimized conditions using differential pulse voltammetry. Using the developed aptasensor, tetracycline residues in milk samples were determined.

AgPdNFs and AuNOs@GO nanocomposites for T-2 toxin detection by catalytic hairpin assembly

T-2 toxin is the most potent and toxic mycotoxin, produced by various Fusarium species that can potentially affect human health, and widely exists in field crops and stored grain. In this work, an electrochemical aptasensor with nonenzymatic signal amplification strategy for the detection of T-2 toxin is presented, using noble metal nanocomposites and catalytic hairpin assembly as signal amplification strategy. Silver palladium nanoflowers and gold octahedron nanoparticles@graphene oxide nanocomposites are used for synergistic amplification of electrical signals. Simultaneously, the catalytic hairpin assembly strategy based on artificial molecular technology was introduced to further amplify the signal. Under optimal conditions, T-2 toxin was measured within a linear concentration range 1 × 10^-2 ~ 1 × 10⁴ pg·mL^-1 with an extremely low detection limit of 6.71 fg·mL^-1. The aptasensor exhibited high sensitivity, good selectivity, satisfactory stability, and excellent reproducibility. Moreover, this method had high accuracy in detecting T-2 toxin in beer sample. The encouraging results show the potential application in foodstuff analysis. A dual signal amplification electrochemical biosensor for the detection of T-2 toxins was constructed, through the signal amplification of noble metal nanomaterials and CHA strategy.

Bibliometrics Analysis of Research Progress of Electrochemical Detection of Tetracycline Antibiotics

Tetracycline is a broad-spectrum class of antibiotics. The use of excessive doses of tetracycline antibiotics can result in their residues in food, posing varying degrees of risk to human health. Therefore, the establishment of a rapid and sensitive field detection method for tetracycline residues is of great practical importance to improve the safety of food-derived animal foods. Electrochemical analysis techniques are widely used in the field of pollutant detection because of the simple detection principle, easy operation of the instrument, and low cost of analysis. In this review, we summarize the electrochemical detection of tetracycline antibiotics by bibliometrics. Unlike the previously published reviews, this article reviews and analyzes the development of this topic. The contributions of different countries and different institutions were analyzed. Keyword analysis was used to explain the development of different research directions. The results of the analysis revealed that developments and innovations in materials science can enhance the performance of electrochemical detection of tetracycline antibiotics. Among them, gold nanoparticles and carbon nanotubes are the most used nanomaterials. Aptamer sensing strategies are the most favored methodologies in electrochemical detection of tetracycline antibiotics.

Construction of Electrochemical Sensors for Antibiotic Detection Based on Carbon Nanocomposites

Excessive antibiotic residues in food can cause detrimental effects on human health. The establishment of rapid, sensitive, selective, and reliable methods for the detection of antibiotics is highly in demand. With the inherent advantages of high sensitivity, rapid analysis time, and facile miniaturization, the electrochemical sensors have great potential in the detection of antibiotics. The electrochemical platforms comprising carbon nanomaterials (CNMs) have been proposed to detect antibiotic residues. Notably, with the introduction of functional CNMs, the performance of electrochemical sensors can be bolstered. This review first presents the significance of functional CNMs in the detection of antibiotics. Subsequently, we provide an overview of the applications for detection by enhancing the electrochemical behaviour of the antibiotic, as well as a brief overview of the application of recognition elements to detect antibiotics. Finally, the trend and the current challenges of electrochemical sensors based on CNMs in the detection of antibiotics is outlined.

Electrochemical Aptasensors for Antibiotics Detection: Recent Achievements and Applications for Monitoring Food Safety

Antibiotics are often used in human and veterinary medicine for the treatment of bacterial diseases. However, extensive use of antibiotics in agriculture can result in the contamination of common food staples such as milk. Consumption of contaminated products can cause serious illness and a rise in antibiotic resistance. Conventional methods of antibiotics detection such are microbiological assays chromatographic and mass spectroscopy methods are sensitive; however, they require qualified personnel, expensive instruments, and sample pretreatment. Biosensor technology can overcome these drawbacks. This review is focused on the recent achievements in the electrochemical biosensors based on nucleic acid aptamers for antibiotic detection. A brief explanation of conventional methods of antibiotic detection is also provided. The methods of the aptamer selection are explained, together with the approach used for the improvement of aptamer affinity by post-SELEX modification and computer modeling. The substantial focus of this review is on the explanation of the principles of the electrochemical detection of antibiotics by aptasensors and on recent achievements in the development of electrochemical aptasensors. The current trends and problems in practical applications of aptasensors are also discussed.

Fabrication of AuNPs/MWCNTS/Chitosan Nanocomposite for the Electrochemical Aptasensing of Cadmium in Water

Cadmium (Cd²⁺) is one of the most toxic heavy metals causing serious health problems; thus, designing accurate analytical methods for monitoring such pollutants is highly urgent. Herein, we report a label-free electrochemical aptasensor for cadmium detection in water. For this, a nanocomposite combining the advantages of gold nanoparticles (AuNPs), carbon nanotubes (CNTs) and chitosan (Cs) was constructed and used as immobilization support for the cadmium aptamer. First, the surface of a glassy carbon electrode (GCE) was modified with CNTs-CS. Then, AuNPs were deposited on CNTs-CS/GCE using chrono-amperometry. Finally, the immobilization of the amino-modified Cd-aptamer was achieved via glutaraldehyde cross-linking. The different synthesis steps of the AuNPs/CNTs/CS nano assembly were characterized by cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS) was employed for cadmium determination. The proposed biosensor exhibited excellent performances for cadmium detection at a low applied potential (−0.5 V) with a high sensitivity (1.2 KΩ·M⁻¹), a detection limit of 0.02 pM and a wide linear range (10⁻¹³–10⁻⁴ M). Moreover, the aptasensor showed a good selectivity against the interfering ions: Pb²⁺; Hg²⁺ and Zn²⁺. Our electrochemical biosensor provides a simple and sensitive approach for Cd²⁺ detection in aqueous solutions, with promising applications in the monitoring of trace amounts of heavy metals in real samples.

A New Covalent Organic Framework of Dicyandiamide-Benzaldehyde Nanocatalytic Amplification SERS/RRS Aptamer Assay for Ultratrace Oxytetracycline with the Nanogold Indicator Reaction of Polyethylene Glycol 600

In this paper, dicyandiamide (Dd) and p-benzaldehyde (Bd) were heated at 180 °C for 3 h to prepare a new type of stable covalent organic framework (COF) DdBd nanosol with high catalysis. It was characterized by molecular spectroscopy and electron microscopy. The study found that DdBd had a strong catalytic effect on the new indicator reaction of polyethylene glycol 600 (PEG600)-chloroauric acid to form gold nanoparticles (AuNPs). AuNPs have strong resonance Rayleigh scattering (RRS) activity, and in the presence of Victoria Blue B (VBB) molecular probes, they also have a strong surface-enhanced Raman scattering (SERS) effect. Combined with a highly selective oxytetracycline (OTC) aptamer (Apt) reaction, new dual-mode scattering SERS/RRS methods were developed to quantitatively analyze ultratrace OTC. The linear range of RRS is 3.00 × 10-3 -6.00 × 10-2 nmol/L, the detection limit is 1.1 × 10-3 nmol/L, the linear range of SERS is 3.00 × 10-3-7.00 × 10-2 nmol/L, and the detection limit is 9.0 × 10-4 nmol/L. Using the SERS method to analyze OTC in soil samples, the relative standard deviation is 1.35-4.78%, and the recovery rate is 94.3-104.9%.

Molecularly imprinted fluoroprobes doped with Ag nanoparticles for highly selective detection of oxytetracycline in real samples

A molecularly imprinted polymer (MIP), which is synthesized by a nanomolding process around a template, has emerged as a promising analytical tool for environmental quality monitoring and food safety test. In this work, a fluoroprobe with Ag-doped MIP nanolayer (16 nm thickness) is successfully prepared for the highly selective detection of oxytetracycline (OTC) in real samples (i.e. Yangtze River water, swine urine). In the MIP nanolayer, two functional monomers (i.e. 4-(2-acrylamidoethylcarbamoyl)-3-fluorophenylboronic acid, methacrylic acid) synergistically constitute the specific recognition sites. Meanwhile, the doped Ag enhances the detection sensitivity (with a detection limit of 5.38 nM) and accelerates the detection rate (within 2.5 min) even in real samples. Therefore, the present study paves the way for the preparation of MIP-based fluoroprobes, showing great prospects in environmental quality and food safety tests.

Electrochemical Biosensors in Food Safety: Challenges and Perspectives

Safety and quality are key issues for the food industry. Consequently, there is growing demand to preserve the food chain and products against substances toxic, harmful to human health, such as contaminants, allergens, toxins, or pathogens. For this reason, it is mandatory to develop highly sensitive, reliable, rapid, and cost-effective sensing systems/devices, such as electrochemical sensors/biosensors. Generally, conventional techniques are limited by long analyses, expensive and complex procedures, and skilled personnel. Therefore, developing performant electrochemical biosensors can significantly support the screening of food chains and products. Here, we report some of the recent developments in this area and analyze the contributions produced by electrochemical biosensors in food screening and their challenges.

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

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

A fluorescent sensing strategy for ultrasensitive detection of oxytetracycline in milk based on aptamer-magnetic bead conjugate, complementary strand of aptamer and PicoGreen

Misuse of antibiotics in animal husbandry and presence of their residues in animal foods is a serious crisis worldwide and thus, monitoring the level of them in food samples is vital for human health. Herein, a fluorescent aptasensor was developed for highly sensitive quantification of oxytetracycline (OTC) in food samples. This method is based on OTC aptamer conjugated to magnetic beads, functioned as recognition element, complementary strand of OTC aptamer, and PicoGreen (PG) as a sensitive double-stranded DNA (dsDNA) fluorescent dye. Formation of OTC aptamer-magnetic bead conjugate provides the opportunity of sample condensation and separation technology. Additionally, the presence of complementary strand leads to significant fluorescence signal alteration of aptasensor in the presence or absence of target and a noteworthy improvement of the aptasensor sensitivity. In the absence of target, complementary strand could bind to aptamer and form dsDNA on the surface of magnetic bead. As a consequence, adding PG to the sample leads to observation of high fluorescence signal from sample. In contrast, once OTC is added to the sample, it binds to OTC aptamer-magnetic bead complex and prevents hybridization of OTC aptamer and its complementary strand. Hence, after addition of PG to the sample, a weak fluorescence intensity is measured. Under optimized conditions, the linear ranges for OTC detection were 0.2-2 nM and 2-800 nM. The detection limit was calculated to be as low as 0.15 nM for the fabricated aptasensor. Besides the great sensitivity, proposed method demonstrated superior specificity towards OTC once it was used against several antibiotics. More significantly, the recovery rates of OTC in milk ranged from 96.46% to 101.5%, implying the great feasibility of designed sensor as well as its potential to be employed for analysis of OTC in real samples.

Recent Advances in Aptamer Sensors

Recently, aptamers have attracted attention in the biosensing field as signal recognition elements because of their high binding affinity toward specific targets such as proteins, cells, small molecules, and even metal ions, antibodies for which are difficult to obtain. Aptamers are single oligonucleotides generated by in vitro selection mechanisms via the systematic evolution of ligand exponential enrichment (SELEX) process. In addition to their high binding affinity, aptamers can be easily functionalized and engineered, providing several signaling modes such as colorimetric, fluorometric, and electrochemical, in what are known as aptasensors. In this review, recent advances in aptasensors as powerful biosensor probes that could be used in different fields, including environmental monitoring, clinical diagnosis, and drug monitoring, are described. Advances in aptamer-based colorimetric, fluorometric, and electrochemical aptasensing with their advantages and disadvantages are summarized and critically discussed. Additionally, future prospects are pointed out to facilitate the development of aptasensor technology for different targets.

Highly luminescent water-dispersed silicon quantum dots for fluorometric determination of oxytetracycline in milk samples

A fluorescent probe based on silicon quantum dots (SiQDs) was developed for the selective and sensitive detection of oxytetracycline (OTC) via the inner filter effect (IFE). The water-soluble fluorescent SiQD was synthesized based on the reaction of 3-Aminopropyltriethoxysilane (APTES) and sodium citrate as precursors by the one-pot hydrothermal process. The strong fluorescence emission of quantum dots (QDs) was obtained at 440 nm when excited at 350 nm and OTC had a broad absorption band between 200 and 400 nm. The excitation spectrum of SiQDs was completely overlapped with the absorption spectrum of OTC. The light at an excitation wavelength of QDs absorbed by OTC caused a decrease in fluorescence intensity with an increase in the concentration of OTC. Under optimal conditions, the linear concentration range was 0.92–9.2 µg mL1 with a detection limit (LOD; S/N = 3) of 0.19 µg mL ^-1 . The proposed method was applied to the determination of OTC in milk samples and satisfactory recoveries (98.8–100.5%) with low RSD % values (0.93–2.31%) were achieved. This simple, selective, sensitive, rapid, and cheap method can be used as a promising tool for OTC analysis in food safety.

A wrinkled structure of gold film greatly improves the signaling of electrochemical aptamer-based biosensors

Electrochemical aptamer-based (E-AB) sensors provide a great opportunity towards the goal of point-of-care and wearable sensing technologies due to their good sensitivity and selectivity. Nevertheless, the output signals from this sensor class remain low when sensors are interrogated via square-wave voltammetry. This low signaling limits the sensor's precision for its capability to detect small changes in target concentrations. To circumvent this, we proposed here the use of a readily shrink-induced, wrinkled Au-coating polyolefin film to immobilize a greater number of DNA probes and thus improve the signaling. Specifically, wrinkled gold film exhibits a 5.5-fold increase of surface area in comparison to the unwrinkled ones. Using these substrates we fabricated a set of E-AB sensors of three biological molecules, including kanamycin, doxorubicin and ATP. We achieved up to 10-fold increase in its current and also good accuracies within ±20% error in the target concentration range across 2 orders of magnitude.

A wrinkled gold substrate greatly improves the signaling of electrochemical aptamer-based biosensors, achieving up to 10-fold increase in signals.

Advances in Electrochemical Aptasensors Based on Carbon Nanomaterials

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

Low-Dimension Nanomaterial-Based Sensing Matrices for Antibiotics Detection: A Mini Review

Antibiotics, a kind of secondary metabolite with antipathogen effects as well as other properties, are produced by microorganisms (including bacterium, fungi, and actinomyces) or higher animals and plants during their lives. Furthermore, as a chemical, an antibiotic can disturb the developmental functions of other living cells. Moreover, it is impossible to avoid its pervasion into all kinds of environmental media via all kinds of methods, and it thus correspondingly becomes a trigger for environmental risks. As described above, antibiotics are presently deemed as a new type of pollution, with their content in media (for example, water, or food) as the focus. Due to their special qualities, nanomaterials, the most promising sensing material, can be adopted to produce sensors with extraordinary detection performance and good stability that can be applied to detection in complicated materials. For low-dimensional (LD) nanomaterials, the quantum size effect, and dielectric confinement effect are particularly strong. Therefore, they are most commonly applied in the detection of antibiotics. This article focuses on the influence of LD nanomaterials on antibiotics detection, summarizes the application of LD nanomaterials in antibiotics detection and the theorem of sensors in all kinds of antibiotics detection, illustrates the approaches to optimizing the sensitivity of sensors, such as mixture and modification, and also discusses the trend of the application of LD nanomaterials in antibiotics detection.

Ternary Z-scheme heterojunction of Bi SPR-promoted BiVO4/g-C3N4 with effectively boosted photoelectrochemical activity for constructing oxytetracycline aptasensor

Developing photoactive materials with wide spectral response is critical to improve the sensitivity of PEC biosensors. Herein, a sensitive photoelectrochemical (PEC) aptasensor was fabricated based on Bi surface plasmon resonance (SPR)-promoted BiVO₄/g-C₃N₄ (Bi/BiVO₄/g-C₃N₄) as photoactive material for the detection of oxytetracycline (OTC). Ternary Z-scheme Bi/BiVO₄/g-C₃N₄ heterojunction exhibited widest spectral response and best PEC activity compared to g-C₃N₄, BiVO₄, Bi/BiVO₄, and BiVO₄/g-C₃N₄. The wide spectral response and high PEC activity could be attributed to three reasons: Firstly, the SPR effect of Bi could greatly increase light harvesting; Secondly, Bi served as an electron conduction bridge between BiVO₄ and g-C₃N₄ to form Z-scheme structure, significantly accelerating the separation of photogenerated carriers; Thirdly, the synergism of Z-scheme heterojunction and the SPR effect of Bi efficiently boosted the photoelectric response. Based on the above sensitization strategies, the proposed PEC aptasensor for OTC determination showed a wide linear range of 0.01-1000 nM and a low detection limit (S/N = 3) of 3.3 × 10^-3 nM. Moreover, the high stability, satisfactory repeatability and favorable practicability of the fabricated PEC aptasensor revealed the potential applications for accurate monitoring of antibiotics in environmental media.

Impedimetric aptasensor based on highly porous gold for sensitive detection of acetamiprid in fruits and vegetables

A novel electrochemical aptasensor modified with highly porous gold and aptamer was prepared for the determination of acetamiprid in fruits and vegetables. Highly porous gold was synthesized by electroreduction at -4 V in an electrolyte containing 2.5 mol/L NH₄Cl and 10 mmol/L HAuCl₄. Acetamiprid-binding aptamer was immobilized on highly porous gold by self-assembly. Acetamiprid could be captured by aptamer on the sensing interface, resulting in an increment of electron transfer resistance. Thanks to the large specific surface area of highly porous gold and the high affinity of aptamer, the aptasensor exhibited a highly sensitive impedance response for acetamiprid. Under optimal condition, the aptasensor displayed a linear response for acetamiprid in the concentration range of 0.5-300 nmol/L, and the detection limit was 0.34 nmol/L. Furthermore, the aptasensor showed high selectivity, good reproducibility and stability. Finally, the aptasensor was applied for the determination of acetamiprid in fruits and vegetables with satisfactory results.

Electrochemical aptasensors based on the gold nanostructures

Electrochemical aptasensors as novel diagnostic tools have attracted sufficient research interest in biomedical sciences. In this review, recent leading trends about gold (Au) nanostructures based electrochemical aptasensors have been collected, reviewed, and compared. Here, the considered electrochemical aptasensors were categorized based on the analytes and diagnostic techniques. Pharmaceutical analytes and biomolecules were reviewed in a separate section consisting of a variety of antibiotics, analgesics, and other biomolecules. Various aptasensors have also measured toxins, ions, and hazardous chemicals, and the findings of them have also been reviewed. Many aptasensors have been designed to detect different disease biomarkers that will play an essential role in the future of early diagnosis of diseases. Pathogen microorganisms have been considered as the analyte in several designed electrochemical aptasensors in recent researches, and their results have been reviewed and discussed as another section. Important aspects considered in the review of the mentioned aptasensors were the type of analyte, features of the aptamer as the biorecognition element, type of Au nanostructures, diagnostic technique, diagnostic mechanism, detection range and the limit of detection (LOD). In the last section, an in-depth analysis has been provided based on the crucial features of all included aptasensors.