Kanamycin detection based on the catalytic ability enhancement of gold nanoparticles

Dual-ligand lanthanide metal-organic framework based ratiometric fluorescent platform for visual monitoring of aminoglycoside residues in food samples

Herein, a dual-emission Eu metal-organic framework (Eu-MOF) is prepared and used as the ratiometric fluorescence probe for ultrasensitive detection of aminoglycoside antibiotics (AGs). Due to the strong hydrogen bond interactions between AGs and Eu-MOF, the blue emission is enhanced while the red emission has little fluctuation in Eu-MOF with the addition of AGs, thus a good linear relationship with the logarithm of AGs concentrations from 0.001 to 100 μg/mL can be established for quantitative analysis. Good sensitivity with the detection limit of 0.33 ng/mL for apramycin, 0.32 ng/mL for amikacin and 0.30 ng/mL for kanamycin is achieved. The proposed assay demonstrates good selectivity and applicability for determination of AGs in real milk and honey samples. The Eu-MOF materials are further fabricated as fluorescent test papers for facile visual detection. The as-established ratio fluorescence platform offers a portable and economical way for rapid monitoring AGs residues in complex food samples.

Synergistic effect of photoelectrochemical aptasensor based on staggered gap ZnO/BiFeO3 heterojunction coupled with cDNA-CdS sensitizer enabling ultrasensitive assay of kanamycin

Using staggered-gap ZnO/BiFeO3 heterojunction as photoactive materials and cDNA-CdS as the sensitizer for sensitive Kanamycin (KAN) detection, we have created a unique signal-off biosensing platform. The ZnO/BiFeO3 heterojunction provides active sites for aptamer loading and enhances photocurrent responsiveness. Rapid interfacial charge transfer and the separation efficiency of photo-generated carriers are enhanced by sensitization of the ternary heterojunction ZnO/BiFeO3/CdS. Signal-amplified quenching occurs when sensitizers are replaced with sterically hindered KAN. Because of the aptamer's greater affinity for KAN, the replacement of CdS causes a decrease in photocurrent response. Additionally, the weakly conductive aptamer-KAN complex causes steric hindrance, which exacerbates the photoelectrochemical signal-damping effect even more. The photoelectrochemical aptasensor exhibits excellent selectivity and stability, detecting KAN within the range of 0.00005825-0.233 μg/mL with a detection limit of 0.0466 ng/mL (S/N = 3). This work demonstrates the potential of perovskite oxides and their heterostructures for advanced photoelectrochemical biosensing applications.

A colorimetric aptasensor for detecting ochratoxin A based on label-free aptamer and gold nanozyme

In this study, the aptamer of ochratoxin A (OTA) increased the negative charge density on the surface of gold nanoparticles (AuNPs) and promoted the release of hydroxyl radicals and Au3+ to enhance the peroxidase-like activity of the AuNPs. The OTA bound only to the aptamer and did not adsorb non-specifically to the AuNPs. Based on these two conclusions, a label-free colorimetric aptasensor was successfully developed, enabling the precise detection of OTA within the concentration range of 10-600 nM, with a remarkably low detection limit of 6.20 nM. The colorimetric aptasensor was applied to detect OTA in oats, corn, soybeans, rice, and glutinous rice.

Sensitive lateral flow immunoassay strips based on Fe3+-chelated polydopamine nanospheres for the detection of kanamycin

As kanamycin (KAN) residue in animal products is harmful to consumers, a rapid and sensitive method for KAN detection needs to be established. KAN monoclonal antibody (KAN-mAb, 1D11) with the half maximal inhibitory concentration of 1.16 ng/mL was prepared in this study. A one-pot method was used to synthesize Fe³⁺-chelated polydopamine nanospheres (Fe@PDANs) with excellent characteristics of strong light absorption. The novel label of Fe@PDANs and KAN-mAb was used to develop a lateral flow immunoassay (LFIA) for the sensitive detection of KAN. The limit of detection of the Fe@PDANs-based LFIA (Fe@PDANs-LFIA) for KAN was 0.0191 ng/mL, which was 2.75 times lower than PDANs-based LFIA. Furthermore, the Fe@PDANs-LFIA was successfully applied to detect KAN in pork, milk, and honey samples, with recoveries ranging from 93.75% to 113.80% (coefficient of variation < 10%). Therefore, Fe@PDANs have potential for the detection of analytes in LFIA.

A simple tag-free fluorometric aptasensing assay for sensitive detection of kanamycin

A novel label-free and enzyme-free fluorescence aptasensing assay that uses Sybr Green I (SGI) as the signal indicator for the kanamycin determination was designed. An aptamer-complementary strand (Apt/CP) conjugate was formed, which provided the intercalation sites for SGI and, therefore, a considerable fluorescent signal. The introduction of the target led to the separation of Apt from CP due to the high affinity of Apt toward kanamycin. Hence, the suitable intercalation gaps reduced, which resulted in a decrease in the generated fluorescent signal. Under optimized conditions, a broad linear concentration range from 0.05 μM to 20 μM and a limit of detection of 11.76 nM were obtained, confirming the ability of the fabricated aptasensor for sensitive and specific kanamycin detection in real samples such as milk and human serum. The aptasensing method has the potential to be extensively employed in the food industry and veterinary science due to its simplicity, sensitivity, user-friendly, and capability of on-site detection of kanamycin.

Porous-nanozyme-based colorimetric sensor for rapid detection of kanamycin in foods under neutral condition

Most colorimetric methods based on nanozymes need to have excellent performance under acidic condition, so they are currently facing some challenges in the field of food hazard detection. Herein, a facile and rapid colorimetric sensor for kanamycin (KAN) detection in foods under neutral condition has been designed using the peroxidase-mimic activity of porous nanozyme like Co₃ O₄ nanodisk. Further investigations showed that the interaction mechanism between porous Co₃ O₄ nanodisk and substrates belongs to a ping-pong model, and the inhibition type of KAN on the peroxidase-mimic activity is noncompetitive inhibition. The constructed sensor has good sensitivity for KAN with the limit of 57 nM, and the color changes can be discerned visually when KAN exceeds 0.5 µM. Besides, the colorimetric sensor obtains excellent recovery results in chicken serum, milk, honey, and pork, which shows that the proposed sensing strategy can provide a rapid and convenient detection method for KAN in foods under neutral condition. PRACTICAL APPLICATION: The established sensing strategy can rapidly distinguish whether KAN residue exceeds the permissible level within 10 min, which meets the requirement for on-site monitoring of antibiotics in foods, and also open up a new idea for other hazards detection under neutral condition.

Poly(N,N-dimethylacrylamide)-stabilized gold nanoparticles as nanozymes with enhancement of catalytic activity for detection of lomefloxacin

Recently, polymer-protected gold nanoparticles (AuNPs) have attracted extensive attention due to their good catalytic activities. However, how to regulate their catalytic activities by changing the polymer chain morphologies or the interactions between the ligands and the analytes through external stimuli is still a great challenge. This study describes a simple synthesis of AuNPs capped by thermo-responsive poly(N,N-dimethylacrylamide) (PDMAM). In comparison with three kinds of PDMAMs@AuNPs, PDMAM-2@AuNPs exhibited better peroxidase-mimic ability via the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with hydrogen peroxide (H₂O₂) to generate oxidized TMB (oxTMB). Interestingly, its catalytic activity could be regulated by changing environmental temperature. Importantly, the addition of the antibiotic lomefloxacin endowed the PDMAM-2@AuNPs with enhancement in catalytic efficiency due to electrostatic interactions and the increased levels of reactive oxygen species. Based on this principle, a protocol for highly selective and sensitive monitoring of lomefloxacin has been constructed with the color change from pale blue to deep blue. The ultraviolet-visible absorbance of oxTMB at the wavelength of 650 nm showed a good linear relationship with antibiotic concentration in the range of 0.25-10.0 µM (R² = 0.990). The limit of detection was 0.1 µM. The practical application of the proposed protocol with the promoted peroxidase-mimic activity for the measurement of lomefloxacin in capsules was realized.

Monitoring of parathion methyl using a colorimetric gold nanoparticle-based acetylcholinesterase assay

A colorimetric gold nanoparticles (AuNPs)-based acetylcholinesterase (AChE) assay was designed for the first time to measure the concentration of parathion-methyl (PM) in lake water samples. In this assay, the analyte PM inhibited the hydrolysis of acetylthiocholine (ATCh) by AChE, preventing the formation of thiocholine (TCh) that would otherwise react with the AuNPs catalyst and deactivate the catalyst. Therefore, in the presence of PM, the AuNPs catalyzed the oxidation of the 3,3',5,5'-tetramethylbenzidine (TMB) colorimetric indicator to oxTMB, inducing a visual color change from colorless to blue. However, in the absence of PM, AChE hydrolyzed ATCh to TCh, which then reacted with the AuNPs, preventing the oxidation of TMB to oxTMB and rendering the solution colorless. Therefore, the change in the color of the analyte solution indicated the presence of PM, and the absorbance of the resulting solution was measured by UV-Vis spectroscopy to calculate the concentration of PM after generation of a calibration curve. This method was then employed using the smartphone app Color Picker, which converted the color information from the photos of the solution into digital red (R), green (G), and blue (B) values. The ratio of green (G) to blue (B) (G/B) was then plotted against the corresponding concentration to calculate the standard curve, whose regression equation was expressed by y = -0.012x + 1.02 (ng/mL), and the coefficient of determination (R²) was 0.97. In addition, this method was also used to determine the amount of PM in real lake water samples with recovery of 90.2-133.3%.

Synergistic function of Au NPs/GeO2 nanozymes with enhanced peroxidase-like activity and SERS effect to detect choline iodide

A novel Au NPs/GeO₂ nanozymes are developed as Surface-Enhanced Raman Scattering (SERS) substrates with the promising prospect for detection ChI. Herein, it is discovered that both Au NPs and GeO₂ nanozymes have peroxidase-like activity, catalyzing colorless 3,3',5,5'-tetramethylbenzidine (TMB) to produce blue TMB_ox. Interestingly, compared with single Au NPs or GeO₂ nanozymes, the Au NPs/GeO₂ nanozymes show stronger peroxidase-like activity, and significantly ameliorated SERS signal of TMB_ox. The mentioned two enhancements are ascribed to a positive synergistic function of Au NPs/GeO₂ nanozymes. Surprisingly, choline iodide (ChI) can inhibit the positive synergy in Au NPs/GeO₂ nanozymes, and slow down the reaction of TMB-H₂O₂-Au NPs/GeO₂ system. On this foundation, a new Au NPs/GeO₂ SERS technique with high sensitivity, label-free detection method of choline iodide (ChI) is established, suggesting that Au NPs/GeO₂ nanozymes have the potential application of water environment.

Modulating the catalytic activity of gold nanoparticles using amine-terminated ligands

Nanozymes have broad applications in theranostics and point-of-care tests. To enhance the catalytic activity of nanozymes, the conventional strategy is doping metals to form highly active nanoalloys. However, high-quality and stable nanoalloys are hard to synthesize. Ligand modification is a powerful strategy to achieve chemoselectivity or bioactivity by changing the surface chemistry. Here, we explore different ligands to enhance the catalytic activity of nanozymes, e.g., gold nanoparticles (AuNPs). We systematically studied the impacts on the enzymatic activity of AuNPs by ligand engineering of surface chemistry (charge, group, and surface distance). Our work established critical guidelines for surface modification of nanozymes. The amine group favors higher activity of AuNPs than other groups. The flexible amine-rich ligand enhances the catalytic activity of AuNPs in contrast to other ligands and unmodified AuNPs. Using a proof-of-concept model, we screened many candidate ligands to obtain polyamine-AuNPs, which have strongly enhanced peroxidase-like activity and 100 times enhanced sensitivity compared to unmodified AuNPs. The strategy of enhancing the catalytic activity of AuNPs using ligands will facilitate the catalysis-related applications of nanozymes in biology and diagnostics.

Surface ligand engineering can precisely modulate the catalytic activity of nanozymes from inactive to highly active.

l-Proline-methyl ester derivative-modulated synthesis of gold nanoclusters with promoted peroxidase-mimic activity for monitoring of ofloxacin

Ligands greatly affect the catalytic-properties of AuNCs-nanozymes in TMB oxidation. Adding ofloxacin enhanced the POD-mimic-activity of POMe@AuNCs upon greater ROS yield. A protocol was proposed for monitoring serum ofloxacin.

Multiply-amplified strategy for the ultrasensitive detection of kanamycin via aptamer-triggered three-dimensional G-quadruplex/Ni-Fe layered double oxide frame networks

In this work, a multiply-amplified peroxidase-like colorimetric strategy was proposed for the high-specific recognition and ultrasensitive detection of kanamycin (Kana). Based on two Kana-aptamer triggered sequential reactions, G-quadruplex (G4) and DNA (hairpins) modified Ni-Fe layered double oxides (LDOs) could be obtained simultaneously. Later, a three-dimensional G4/LDO frame networks, as a novel DNAzyme, with enhanced peroxidase-like catalytic activity was assembled through electrostatic interaction. This DNAzyme catalyzed 3,3',5,5'-tetramethylbenzidine oxidation for the colorimetric detection of Kana. The enhancement principle was discussed and the charge transfer process during the catalytic reaction was investigated. Under the optimal experiment conditions, the proposed method exhibited high sensitivity, where the linear range is from 10 fM to 10 nM (r² = 0.992), and the limit of detection is 3 fM (S/N = 3). The practicability of this assay was demonstrated by successfully application of residual Kana detection in genuine milk and urine samples.

Fluorescence method for kanamycin detection based on the conversion of G-triplex and G-quadruplex

A versatile fluorescence scaffold was constructed by connecting a G-triplex sequence (G31) with G-rich DNA (aptamer of kanamycin) and using thioflavin T (ThT) as the fluorescent molecule. With the assistance of an aptamer, the G-quadruplex DNA structure was fabricated using G31 as three strands and the aptamer as the fourth strand. Due to the parallel planar morphology of the final products, which was favorable for ThT binding and which restricted the rotation of the aromatic rings of ThT, the fluorescence signal intensity of ThT was significantly enhanced. Because of the specific interaction of aptamer and kanamycin, in addition to the greater ability for kanamycin to bind with G-triplex than ThT, the conformation of G-quadruplex DNA was changed; in addition, ThT was dissociated from the aptamer-G31, and therefore a 'turn-on' to 'turn-off' detection principle was applied for kanamycin detection, which yielded reasonable sensitivity and selectivity. The detection range was 50-2000 nM, with a limit of detection of 1.05 nM. Our proposed method was thus successfully applied for kanamycin determination in pork, chicken, and beef samples, and satisfactory results were obtained.

Fluorescent PEI@Pd nanoclusters: facile synthesis and application

Metal nanoclusters (NCs) have recently emerged as a novel class of luminescent nanomaterials and held significant potential in analytical chemistry. In this work, novel polyethyleneimine stabilized palladium nanoclusters (PEI-Pd NCs) were synthesized by chemical reduction at 60 °C for 6 h, and used as a fluorescent nanosensor for the detection of oxytetracycline (OTC). The spectral characteristics, surface structure and morphology of the Pd NCs were studied. The selectivity and stability of the nanosensor were also investigated. The experimental results showed that the Pd NCs had good biocompatibility, stability and photobleaching resistance in aqueous solution. The fluorescence quenching effect showed a good linear relationship with the degree of fluorescence quenching of Pd NCs and OTC in the range of 25-440 nM, with a correlation coefficient of 0.99. The limit of detection (LOD) of the proposed nanosensor for OTC was calculated to be 22 nM. The mechanism of determination was thought to be an inner filter effect (IFE) between OTC and Pd NCs. Based on this, we have established a new nanosensing analysis method for detecting OTC.

PCN-222@g-C3N4 cathodic materials for "signal-off" photoelectrochemical sensing of kanamycin sulfate

A novel cathodic photoelectrochemical (PEC) sensor was developed for the ultrasensitive detection of kanamycin sulfate (KAM) based on the g-C3N4 coupled zirconium-based porphyrinic metal-organic framework (PCN-222). Photocathodes made by double n-type semiconductors, which was attributed to the transfer of electrons and holes from g-C3N4 broad band to PCN-222 with narrow band gap. The photocurrent decreased when KAM was added, which was conducive to the construction of the PEC sensor. Then, the PCN-222@g-C3N4 was used as a photosensitive platform to construct a label-free strategy and ultrasensitive detection of KAM with wide linear range from 1 to 1000 nM and a low detection limit of 0.127 nM. Moreover, this sensing platform shows good selectivity, favourable reproducibility and brilliant stability. The reported sensors provided great potential for the detection of KAM in actual samples.

Usefulness of Hybrid Magnetoliposomes for Aminoglycoside Antibiotic Residues Determination in Food Using an Integrated Microfluidic System with Fluorometric Detection

A new microfluidic approach using hybrid magnetoliposomes (h-MLs) containing hydrophobic magnetic nanoparticles (Fe₃O₄@AuNPs-C12SH) and encapsulated N-acetylcysteine has been developed in this research to determine aminoglycoside antibiotic (AAG) residues in food using o-phthalaldehyde. Four AAGs, kanamycin, streptomycin, gentamicin, and neomycin, have been used as model analytes. The h-MLs have been used for reagent preconcentration and were retained using an external electromagnet device in the reaction/detection zone in a microfluidic system, inserted into the sample chamber of a conventional fluorimeter. The formation of a fluorescent isoindole derivate caused an increase in the luminescence signal, which was proportional to the analyte concentration. The dynamic range of the calibration graph was 0.1-1000 μmol L^-1, expressed as AAG concentration, with an 8.7 nmol L^-1 limit of detection for kanamycin and a sampling frequency of 8 h^-1. The method was applied to determine AAG residues in milk and meat samples with recovery values between 87.2 and 107.4%.

An electrochemical platform based on a hemin-rGO-cMWCNTs modified aptasensor for sensitive detection of kanamycin

Kanamycin (KANA) residue in meat is particularly harmful to public health and there is an urgent need to establish a fast, accurate and low-cost method to determinate KANA in food quality control. In this paper, hemin-reduced graphene oxide-carboxylated multiwalled carbon nanotubes (hemin-rGO-cMWCNTs) were designed and prepared, and the characteristics of hemin-rGO-cMWCNTs are presented. After that, an aptamer/hemin-rGO-cMWCNTs sensor for determination of KANA was developed. The electrochemical characteristics were studied by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). Under optimal conditions, the sensitive response of the aptasensor towards KANA presented a wide concentration range of 10-9 to 10-6 M and a low detection limit of 0.36 nM (S/N = 3). Meanwhile, the aptasensor showed prominent selectivity, high stability and acceptable reproducibility in the application of KANA detection. In addition, the aptasensor detection in real samples correlated well with that obtained by liquid chromatograph mass spectrometer (LCMS).

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.

A signal-on fluorescent aptasensor based on gold nanoparticles for kanamycin detection

This study describes the development, verification and practical application of an aptasensor for the fluorometric detection of kanamycin. Using the nucleic acid aptamer with FAM fluorescent group as the conjugate, using gold nanoparticles as the fluorescence dynamic quenching source, a fluorescence sensor was fabricated through the signal-on method for the micro-detection of kanamycin. The nucleic acid chimera is connected to the fluorophore, and the gold nanoparticles are used as the fluorescence dynamic quenching source under actual conditions. The detection limit of kanamycin is 0.1 pM, and the detection range is 0.1 pM to 0.1 μM. This biosensor works satisfactorily in complex samples with no impurities, which gives this method an obvious advantage over other analytical methods. In addition, the mechanism of action between gold nanoparticles/FAM-aptamer/kanamycin is discussed and studied in depth here. It provides a more thorough analysis and more application possibilities for fluorescence-aptamer biosensing.

Regulating Valence States of Gold Nanocluster as a New Strategy for the Ultrasensitive Electrochemiluminescence Detection of Kanamycin

Monitoring of kanamycin residue has attracted considerable attention owing to the potential harm caused by the abuse of kanamycin. However, the detection of kanamycin has been limited owing to its electrochemical and optical inertness. Herein, we report a facile and highly efficient electrochemiluminescence (ECL) strategy for the detection of kanamycin based on the valence state effect of gold nanocluster (AuNC) probes. It is proven that Au⁰ in chemically reduced AuNCs (CR-AuNCs) could be oxidized to Au^I via the redox reaction between kanamycin and CR-AuNCs in the presence of H₂O₂, resulting in ECL quenching due to the valence state change of CR-AuNCs. Because the ECL of the AuNC probes is sensitively affected by the valence state, excellent sensitivity for kanamycin was achieved without any signal amplification operation and aptamers. A preferable linear-dependent curve was acquired in the detection range from 1.0 × 10^-11 to 3.3 × 10^-5 M with an extremely low detection limit of 1.5 × 10^-12 M. The proposed kanamycin sensing platform is very simple and shows high selectivity and an extremely broad linear range detection of kanamycin. Furthermore, the proposed sensing platform can detect kanamycin in milk samples with excellent recoveries. Therefore, this sensing strategy provides an effective and facile way to detect kanamycin and can help promote the understanding of the constructed mechanism of the AuNC-based ECL system, thus greatly broadening its potential application in ECL fields.

Bioactive Coatings Based on Hydroxyapatite, Kanamycin, and Growth Factor for Biofilm Modulation

The occurrence of opportunistic local infections and improper integration of metallic implants results in severe health conditions. Protective and tunable coatings represent an attractive and challenging selection for improving the metallic devices' biofunctional performances to restore or replace bone tissue. Composite materials based on hydroxyapatite (HAp), Kanamycin (KAN), and fibroblast growth factor 2 (FGF2) are herein proposed as multifunctional coatings for hard tissue implants. The superior cytocompatibility of the obtained composite coatings was evidenced by performing proliferation and morphological assays on osteoblast cell cultures. The addition of FGF2 proved beneficial concerning the metabolic activity, adhesion, and spreading of cells. The KAN-embedded coatings exhibited significant inhibitory effects against bacterial biofilm development for at least two days, the results being superior in the case of Gram-positive pathogens. HAp-based coatings embedded with KAN and FGF2 protein are proposed as multifunctional materials with superior osseointegration potential and the ability to reduce device-associated infections.

Label-free exonuclease I-assisted signal amplification colorimetric sensor for highly sensitive detection of kanamycin

A label-free colorimetric method based on exonuclease I (Exo I)-assisted signal amplification with protamine as a medium was developed for analysis of kanamycin. In this study, a double-stranded DNA (dsDNA) probe was tailored by manipulating an aptamer and its complementary DNA (cDNA) ensuring detection of target with high selectivity and excellent sensitivity. Herein, protamine could not only combine with negatively charged gold nanoparticles but also interaction with polyanion DNA. Upon addition of target kanamycin, the target-aptamer complex was formed and the cDNA was released. Thus, both aptamer and cDNA could be digested by Exo I, and the captured kanamycin was liberated for triggering target recycling and signal amplification. Under optimized conditions, the proposed colorimetric method realized a low detection limit of 2.8 × 10^-14 M along with a wide linear range plus excellent selectivity. Our strategy exhibited enormous potentials for fabricate various kinds of biosensors based on target-induced aptamer configuration changes.

Application of Multiplexed Aptasensors in Food Contaminants Detection

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

DNAzyme-powered DNA walking machine for ultrasensitive fluorescence aptasensing of kanamycin

A DNAzyme-powered DNA walking machine was constructed to develop the fluorescence aptasensing for sensitive detection of kanamycin. The aptamer for kanamycin is partially hybridized with complementary DNA (cDNA) modified on magnetic beads (MBs). The specific interaction of target and aptamer triggered the cDNA to be free tentatively, which captured walker DNA. Then the autonomous motion of DNA walker on MBs surface was propelled via DNAzyme digestion of recognition sites. The signal probe was separated, and the amplified fluorescence signal was achieved by the accumulation of the signal probe. Kanamycin was used as a model analyte, and the developed assay achieves a detection limit of 0.00039 ng·mL^-1 (S/N = 3) within a linear detection range from 0.001 to 2000 ng·mL^-1. This aptasensing strategy can be extended for detection of other antibiotics by adapting corresponding target recognition aptamer sequence. Graphical abstract The fluorescence aptasensing for sensitive detection of kanamycin based on DNAzyme-powered DNA walking machine was constructed.

Designing signal-on sensors by regulating nanozyme activity

Nanozymes are nanomaterials with enzyme-like activities. Compared to natural enzymes, nanozymes are more stable and cost-effective, and they have unique properties due to their nanoscale size and surface chemistry. In this review, we summarize 'signal-on' nanozyme-based sensors for detecting metal ions, anions, small molecules and proteins. Since protein-based enzymes are already highly active, they were used to detect their inhibitors, resulting in 'signal-off' sensors. On the other hand, for nanozymes, target molecules were detected either as a promotor of nanozyme activity or for its ability to selectively remove nanozyme inhibitors. In both cases, 'signal-on' detection was achieved. We classify the commonly used nanozymes based on their composition such as metal oxide, gold nanoparticles and other nanomaterials, most of which belong to the oxidase, peroxidase and catalase mimics. The nanozymes can catalyze the oxidation of colorless or non-fluorescent substrates to produce a visual or fluorescent signal. Based on this, this article presents some typical 'turn-on' and 'turn-off-on' sensors, and we critically review their design principles. At the end, further perspectives for the nanozyme-based sensors are outlined.

An efficient LSPR method to quantitatively detect dimethoate: Development, characterization and evaluation

In recent years, there has been growing concern among consumers about pesticide contamination in fruits. Therefore, rapid, reliable, and consistent detection methods for OPPs, especially dimethoate, are crucially needed. The existing quantitative methods for detecting dimethoate are not suitable for rapid measuring system such as the dimethoate samples from two channels. Hence this paper examines the utilization of a dual-channel system for utilize the absorption variations of the Localized Surface Plasmon Resonance (LSPR) bands of gold nanoparticles (AuNPs) were investigate for detection of dimethoate. Under optimized conditions, the relationship between concentrations of dimethoate and absorbance ratios (A(520)/A(640)) was linearly found in the concentration range of 10–100 nM. Result from the experiment shows that both channels exhibit a linear correlation coefficient as high as 0.97 and a limit of detection (LOD) as low as 5.5 nM. This LSPR detection system was characterized by testing the dimethoate in apple samples and the recovery rates were found to be in the range of 85.90% to 107.37%. The proposed dual-channel LSPR system for detecting dimethoate creating a new approach for detecting organophosphate insecticide in agricultural fields. It could lay the foundation for designing a high-throughput analysis of the insecticides using a wavelength division multiplexing switch (WDMS).

Recent Advances in Optical Detection of Aminoglycosides

Aminoglycosides (AGs) are broad-spectrum antibiotics used in both human infection and animal medicine. The overuse of AGs causes undesirable residues in food, leading to serious health problems due to food chain accumulation. In recent years, various methods have been developed to determine AGs in food. Among these methods, fluorescent (FL), colorimetric and chemiluminescent (CL) optical methods possess advantages such as their simple instrumentation, low cost, simple operation, feasibility of realizing visualization, and smartphone imaging. This mini-review summarizes optical assays for the detection of AGs in food developed in recent years. The detection principles for different categories are discussed. Then, the amplification techniques for the ultrasensitive detection of AGs are introduced. We also discuss multiplex methods for the simultaneous detection of AGs. Finally, the challenges and future prospects are discussed in the Conclusions and Perspectives section.

Conjugation of antibodies and aptamers on nanozymes for developing biosensors

Nanozymes are engineered nanomaterials with enzyme-like activities. Over the past decade, impressive progresses on nanozymes in biosensing have been made due to their unique advantages of high stability, low cost, and easy modification compared to natural enzymes. For many biosensors, it is critical to conjugate nanozymes to affinity ligands such as antibodies and aptamers. Since different nanomaterials have different surface properties, conjugation methods need to be compatible with these properties. In addition, the effect of biomolecules on nanozyme activity needs to be considered. In this review, we first categorized nanozyme-based biosensors into four parts, respectively describing noncovalent and covalent modifications with antibodies and aptamers. Meanwhile, recent advances in antibody and aptamer labeled nanozyme biosensors are summarized, and the methods of their conjugation are further illustrated. Finally, conclusions and future perspectives for the development and application of nanozyme bioconjugates are discussed.

Gold nanoparticles mediated designing of versatile aptasensor for colorimetric/electrochemical dual-channel detection of aflatoxin B1

This work is aimed to develop of a new class of versatile aptasensor to specifically detect aflatoxin B1 (AFB1) using dual-channel detection method. To achieve this objective, gold nanoparticles (AuNPs) having peroxidase-like activity and capability of promoting silver deposition were used as the versatile label for both colorimetric and electrochemical techniques. First of all, aptamer (apt) modified Fe₃O₄@Au magnetic beads (MBs-apt) and cDNA modified AuNPs (cDNA-AuNPs) were prepared to use as capture probes and signal probes, respectively. Taking advantage of hybridization reaction between apt and cDNA, these two probes were coupled with each other to generate MBs-apt/cDNA-AuNPs bioconjugations. The high affinity between apt and AFB1 made cDNA-AuNPs detached from MBs-apt, and the released signal probes were separated and collected using an external magnetic field and used for both colorimetric and electrochemical detection channels. The dual-channel signals were directly proportional to logarithm of AFB1 concentration within the ranges of 5-200 ng mL^-1 and 0.05-100 ng mL^-1. The detection limit can reach as low as 35 pg mL^-1 and 0.43 pg mL^-1 for colorimetric and electrochemical channel, respectively. Moreover, the proposed aptasensor has been successfully applied to determine AFB1 in corn samples with satisfactory results. This dual-channel detection method can not only improve the detection precision and diversity significantly, but also can reduce the false-negative and-positive rates in food quality monitoring. We believe we have provided a general strategy with the convincing dual-readout mode which possess great promising in all of the aptamer related fields.

Colorimetric biosensing of nopaline synthase terminator using Fe3O4@Au and hemin-functionalized reduced graphene oxide

In this paper, we present a simple and label-free colorimetric biosensor for detection of the nopaline synthase (NOS) terminator in genetically modified (GM) plants. The "signal on" colorimetric biosensor was developed using a nanocomposite consisted of gold nanoparticles doped magnetic Fe₃O₄ nanoparticles (Fe₃O₄@Au NP), capture probe DNA (cDNA), and hemin-functionalized reduced graphene oxide nanosheets (H-GN). The nanocomposite was successfully prepared by means of Au-S bonds and the strong π interactions between cDNA and H-GN. The sensing approach is based on the excellent peroxidase-mimicking activity of H-GN and its different electrostatic interactions with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). In presence of the target NOS, the cDNA in the nanocomposite will hybridize with its complementary sequence, and form dsDNA structure. Due to the weak π interactions between dsDNA and H-GN, a portion of H-GN will be released from the surface of Fe₃O₄@Au NPs and transferred into solution. After magnetic separation was performed, the supernatant was incubated with 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. The released H-GN can catalyze the oxidation reaction of TMB and turn the colorless solution blue. This "signal-on" colorimetric biosensor shows a broad linear range of 0.5-100 nM for the target NOS, with a 0.19 nM detection limit. The application of the biosensor for determination of NOS segments in samples of GM and non-GM tomatoes shows that it can discriminate between GM and non-GM plants. The reliability of the method for samples of NOS-spiked GM tomato suggests satisfactory recoveries in the range of 93.6%-94.2%.

The Growing Interest in Development of Innovative Optical Aptasensors for the Detection of Antimicrobial Residues in Food Products

The presence of antimicrobial residues in food-producing animals can lead to harmful effects on the consumer (e.g., allergies, antimicrobial resistance, toxicological effects) and cause issues in food transformation (i.e., cheese, yogurts production). Therefore, to control antimicrobial residues in food products of animal origin, screening methods are of utmost importance. Microbiological and immunological methods (e.g., ELISA, dipsticks) are conventional screening methods. Biosensors are an innovative solution for the development of more performant screening methods. Among the different kinds of biosensing elements (e.g., antibodies, aptamers, molecularly imprinted polymers (MIP), enzymes), aptamers for targeting antimicrobial residues are in continuous development since 2000. Therefore, this review has highlighted recent advances in the development of aptasensors, which present multiple advantages over immunosensors. Most of the aptasensors described in the literature for the detection of antimicrobial residues in animal-derived food products are either optical or electrochemical sensors. In this review, I have focused on optical aptasensors and showed how nanotechnologies (nanomaterials, micro/nanofluidics, and signal amplification techniques) largely contribute to the improvement of their performance (sensitivity, specificity, miniaturization, portability). Finally, I have explored different techniques to develop multiplex screening methods. Multiplex screening methods are necessary for the wide spectrum detection of antimicrobials authorized for animal treatment (i.e., having maximum residue limits).

Engineering Multivalence Aptamer Probes for Amplified and Label-Free Detection of Antibiotics in Aquatic Products

Excessive use of antibiotics in aquatic products is a serious problem for food safety and human health, and on-site detection of antibiotics is highly demanded. Herein, we proposed multivalence aptamer probes, allowing sensitive, label-free, and homogeneous detection of antibiotics in different aquatic products. Compared to commonly used aptamers, multivalence aptamer probes can provide multiple binding sites and a higher affinity for target molecules, and the iterative binding on different binding sites contributes to an amplified recognition effect, sharply increasing the response and sensitivity of aptamer probes. The 2-valence aptamer probes conferred a limit of detection of 0.097 nM for kanamycin detection, where it is estimated that their sensitivity is enhanced 12 times compared to 1-valence aptamer probes. Meanwhile, multivalence aptamer probes allowed us to specifically identify kanamycin among other antibiotics. It could detect kanamycin residual in aquatic products including river eel and puffer fish, as well as tap water with high precision. A multivalence design strategy of aptamer probes would significantly improve the detection performance of aptamers, facilitating the translation of aptamer for food safety control.

Colorimetric method for glucose detection with enhanced signal intensity using ZnFe2O4-carbon nanotube-glucose oxidase composite material

In this paper, a composite material comprised of ZnFe2O4 nanomaterial, carbon nanotubes (CNT) and glucose oxidase (GOD) was synthesized and used for glucose detection. ZnFe2O4-CNT was formed by a one-step solvothermal approach using acid-treated CNT as precursor, then GOD was linked to it by coupling reaction between -NH2 and -COOH. After addition of glucose, which is oxidized by GOD, the intermediate product (H2O2) further oxidizes the 3,3',5,5'-tetramethylbenzidine (TMB) substrate and forms a blue product. This process was accelerated in the presence of peroxidase-mimic ZnFe2O4 nanomaterial and the detected signal intensity was correspondingly enhanced. The linear detection range of glucose was 0.8 to 250 μM, with a limit of detection of 0.58 μM. This may originate from (1) the limited diffusion of intermediate species, which resulted in enhanced local concentrations of reaction compounds; (2) enhanced electron transmission among CNT, GOD and ZnFe2O4; (3) the synergistic enhancement of catalytic activity of ZnFe2O4 compared with other metal oxides; (4) the high loading capacity of ZnFe2O4-CNT for GOD molecules, because of its high surface-to-volume ratio. Meanwhile, this method has reasonable selectivity, stability and reusability and can be used for real serum detection, which may be useful for the development of sensitive biosensors.

Detection of Malachite Green using a colorimetric aptasensor based on the inhibition of the peroxidase-like activity of gold nanoparticles by cetyltrimethylammonium ions

A specific and sensitive colorimetric aptasensor is described for the determination of Malachite Green (MG). It is exploiting the inhibition of the peroxidase-like activity of gold nanoparticles (AuNPs). The AuNPs act as enzyme mimics that catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to yield a dark blue solution. The catalytic activity is inhibited by hexadecyl trimethyl ammonium ion, specifically by cetyltrimethylammonium bromide (CTAB), which causes the aggregation of AuNPs. If a (negatively charged) RNA-aptamer against MG is added, it binds to the positively charged CTAB and prevents aggregation. This enhances the enzyme mimicking activity of the AuNPs and leads to the formation of a dark blue solution. However, in the presence of MG, the aptamer binds to MG, and leads to the aggregation of AuNPs again. The aggregated AuNPs possess a light blue color. A colorimetric method (best performed at 650 nm) was work out that can detect MG in a concentration range from 10 to 500 nmol L^-1. The detection limit based on 3σ/k criterion is 1.8 nmol L^-1. The assay is highly specific and accurate. Recoveries from spiked real samples (aquaculture water) ranged from 80% to 120%. Graphical abstract Based on the inhibition of cetyltrimethyal ammonium ion and the enhancement of RNA-aptamer, the differences of the peroxidase-like activities of AuNPs can be greatly enlarged with and without MG, by which a colorimetric aptasensor can be constructed for the detection of Malachite Green (MG).

A simple colorimetric probe based on anti-aggregation of AuNPs for rapid and sensitive detection of malathion in environmental samples

In this study, a simple colorimetric probe was developed for rapid and highly sensitive detection of malathion based on gold nanoparticles (AuNPs) anti-aggregation mechanism. A certain amount of NaOH can cause the aggregation of citrate-stabilized AuNPs due to the electrostatic interactions, and the color of AuNP solution changes from wine-red to gray. While in the presence of malathion, malathion is easily hydrolyzed in a strong alkali environment (pH > 9), followed by the production of a mass of negative charges, and thus the aggregated AuNPs turns to well-dispersed and the color of AuNP solution changes from gray to wine-red. This characteristic change can be visualized with the naked eye and quantitatively detected by an ultraviolet-visible (UV-Vis) spectrometer. Under optimized conditions, this probe exhibited a linear response to malathion in the concentration range of 0.05-0.8 μM with a limit of detection (LOD) down to 11.8 nM. The probe also showed good specificity for malathion detection in the presence of other interfering pesticide residues. Furthermore, the probe was successfully employed to detect malathion in environmental samples, with a recovery of 94-107% and a relative standard deviation (RSD) less than 8%. The results demonstrated that the proposed colorimetric probe based on anti-aggregation of AuNPs could be used for quantitative analysis of malathion and provided great potential for malathion determination in environmental samples.

A fluorometric aptamer method for kanamycin by applying a dual amplification strategy and using double Y-shaped DNA probes on a gold bar and on magnetite nanoparticles

A simple and highly sensitive fluorometric method is described for the determination of the antibiotic kanamycin (Kana) in food. Dual signal amplification is accomplished by making use of double Y-shaped aptamer DNA probes acting as a capture probes and signal amplification probes. The DNA probes were immobilized on a gold bar and on a magnetic bar, respectively. On addition of Kana, the Y-shaped aptamer probe captures Kana and then is disassembled to release two single-stranded DNAs. These trigger target recycling and HCR between the two bars simultaneously. As a result, many long duplex DNA chains are formed in the supernatant. After pulling out the bars and adding the fluorescent intercalating probe SYBR Green I, strong fluorescence (with excitation/emission peaks at 497/525 nm) is induced. The use of such double Y-shaped DNA probes obviously overcomes the unspecific signal amplification by HCR which increases selectivity and sensitivity. This is due to the fact that the hairpin of HCR is separated in being present in different arms of the Y-shaped probe. Under the optimal conditions, the assay has a limit of 0.45 pg·mL^-1 for Kana. It was applied to analyze spiked milk, fish and pork samples. Graphical abstract The scheme represents a sensitive fluorometric aptamer-based method to detect kanamycin (Kana). It is making use of a double stirring bar-assisted dual amplification strategy with zero background. Abbreviations: apt: aptamer, AuNPs: gold nanoparticles, HCR: hybridization chain reaction.

Voltammetric kanamycin aptasensor based on the use of thionine incorporated into Au@Pt core-shell nanoparticles

A signal-on aptasensor is described for the voltammetric determination of kanamycin (KANA). Au@Pt core-shell nanoparticles with large surface and good electrical conductivity were synthetized and act as both a conductive material and as the carrier for complementary strands (CS2) and thionine (TH). In the presence of KANA, the electrochemical response of TH changes due to hybridization between CS1 immobilized on the electrode and the Au@Pt-CS2/TH system. The peak current increases linearly with the logarithm of the KANA concentration in the range from 1 pM to 1 μM, and the limit of detection is 0.16 pM. The sensor was characterized in terms of selectivity, reproducibility and stability, and satisfactory results were obtained. It was also utilized for the determination of KANA in (spiked) chicken samples. The recoveries (95.8-103.2%) demonstrate the potential of the method for KANA detection in real samples. Graphical abstract A signal-on aptasensor for kanamycin (KANA) was developed by using Au@Pt core-shell nanoparticles as nanocarrier for probe aptamer and as a sensing probe.

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

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

Enzyme- and label-free electrochemical aptasensor for kanamycin detection based on double stir bar-assisted toehold-mediated strand displacement reaction for dual-signal amplification

It is critically important to detect antibiotic residues for monitoring food safety. In this study, an enzyme- and label-free electrochemical aptasensor for antibiotics, with kanamycin (Kana) as a typical analyte, was developed based on a double stir bar-assisted toehold-mediated strand displacement reaction (dSB-TMSDR) for dual-signal amplification. First, we modified two gold electrodes (E-1 and E-2) with different DNA probes (S1/S2 hybrid probe in E-1 and DNA fuel strand S3 in E-2). In the presence of Kana, an S1/S2 probe can be disassembled from E-1 to form an S2/Kana complex in supernatant. The S2/Kana could react with S3 on E-2 to form S2/S3 hybrid and release Kana through TMSDR. After then, the target recycling was triggered. Subsequently, the formed S2/S3 hybrid can also trigger a hybridization chain reaction (HCR). Consequently, the dual-signal amplification strategy was established, which resulted in many long dsDNA chains on E-2. The chains can associate with methylene blue (MB) as redox probes to produce a current response for the quantification of Kana. The assay exhibited high sensitivity and specificity with a detection limit at 16 fM Kana due to the dual-signal amplification. The double stir bars system can both increase phase separation and prevent leakage of DNA fuel to reduce background interference. Moreover, it allows flexible sequence design of the TMSDR probes. The assay was successfully employed to detect Kana residues in food and showed potential application value in food safety detection.