A dual-mode fluorometric/colorimetric sensor for sulfadimethoxine detection based on Prussian blue nanoparticles and carbon dots.

A dual-mode (colorimetric/fluorescence) nanoenzyme-linked immunosorbent assay (NLISA) was developed based on Au-Cu nanocubes generating Prussian blue nanoparticles (PBNPs). It is expected that this method can be used to detect the residues of sulfonamides in the field, and solve the problem of long analysis time and high cost of the traditional method. Sulfadimethoxine (SDM) was selected as the proof-of-concept target analyte. The Au-Cu nanocubes were linked to the aptamer by amide interaction, and the Au-Cu nanocubes, SDM and antibody were immobilized on a 96-well plate using the sandwich method. The assay generates PBNPs by oxidising the Cu shells on the Au-Cu nanocubes in the presence of hydrochloric acid, Fe3+ and K3[Fe (CN)6]. In this process, the copper shell undergoes oxidation to Cu2+ and subsequently Cu2 + further quenches the fluorescence of the carbon point. PBNPs exhibit peroxidase-like activity, oxidising 3,3',5,5'-tetramethylbenzidine (TMB) to OX-TMB in the presence of H2O2, which alters the colorimetric signal. The dual-mode signals are directly proportional to the sulfadimethoxine concentration within the range 10- 3~10- 7 mg/mL. The limit of detection (LOD) of the assay is 0.023 ng/mL and 0.071 ng/mL for the fluorescent signal and the colorimetric signal, respectively. Moreover, the assay was successfully applied to determine sulfadimethoxine in silver carp, shrimp, and lamb samples with satisfactory results.

Nanozyme based on ZIF-8 for the colorimetric detection of sulfonamides in cow milk.

A simple and time-saving colorimetric method was developed to quantify sulfonamides (SAAs) in milk via inhibition of the human carbonic anhydrase II (hCAII)-like activity of ZIF-8 that can hydrolyze p-nitrophenyl acetate (pNPA) to p-nitrophenol (pNP), following the color change from yellow to colorless. Effects of different reaction conditions, including pH, temperature, amount of ZIF-8, and incubation time, were investigated. The value of Michaelis-Menten constant (Km) is measured to be 0.15 mM, which exhibits high affinity to pNPA. The IC50 (0.17, 0.24, and 0.60 mM) and inhibition constant (Ki) (0.09, 0.13, and 0.33 mM) of sulfamethazine (SD), sulfadimethoxine (SDM), and sulfathiazole (ST) on ZIF-8 were measured, respectively. Moreover, the activity of ZIF-8 remains more than 90.0% of its initial activity after 30 days' storage. The colorimetric method for SD, SDM, and ST determination was established at the linear ranges of 6.3-750.0 µM (1.75-208.75 mg/kg), 6.3-750.0 µM (1.96-232.75 mg/kg), and 5.0-1250.0 µM (1.28-319.15 mg/kg) with limit of detection of 4.3, 3.2, and 3.9 µΜ (1.2, 0.99, and 0.96 mg/kg), respectively. In addition, the spiked recoveries of SAAs in milk sample are in the range of 81.6%-106.7% with RSD less than 6.5%. In short, the developed colorimetric method can achieve rapid analysis of SAAs in milk with simple operations.

Salt-resistant nanosensor for fast sulfadimethoxine tracing based on oxygen-doped g-C3N4 nanoplates.

A novel oxygen-doped g-C3N4 nanoplate (OCNP) structure that can serve as an efficient sulfadimethoxine (SDM) sensing platform has been developed. Taking advantage of its inherent oxygen-containing functional groups and 2D layered structure with π-conjugated system, OCNP exhibits effective radiative recombination of surface-confined electron-hole pairs and efficient π-π interaction with SDM. This causes rapid fluorescence response and thus ensures the fast and continuous monitoring of SDM. Based on the fluorescence experiments and band structure calculation, the mechanism of the SDM-induced quenching phenomenon was mainly elucidated as the photoinduced electron transfer process under a dynamic quenching mode. Under optimized conditions, the as-proposed nanosensor, which emitted strong fluorescence at 375 nm with an excitation wavelength at 255 nm, presents an excellent analytical performance toward SDM with a wide linear range from 3 to 60 µmol L-1 and a detection limit of 0.85 µmol L-1 (S/N = 3). In addition, this strategy exhibits satisfactory recovery varied from 94 to 103% with relative standard derivations (RSD) in the range 0.9 to 6.8% in real water samples. It also shows marked tolerability to a series of high concentrations of metals and inorganic salts. This strategy not only broadens the application of oxygen-doped g-C3N4 nanomaterial in antibiotic sensing field but also presents a promising potential for on-line contaminant tracing in complex environments.

Determination of sulfadimethoxine in milk with aptamer-functionalized Fe3 O4 /graphene oxide as magnetic solid-phase extraction adsorbent prior to HPLC.

An aptamer (Apt) functionalized magnetic material was prepared by covalently link Apt to Fe3 O4 /graphene oxide (Fe3 O4 /GO) composite by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide, and then characterized by FTIR spectroscopy, X-ray diffraction, and vibration sample magnetometry. The obtained composite of Fe3 O4 /GO/Apt was employed as magnetic solid-phase extraction adsorbent for the selective preconcentration of sulfadimethoxine prior to analysis by high-performance liquid chromatography. Under the optimal conditions (sample pH of 4.0, sorbent dosage of 20 mg, extraction time of 3 h, and methanol-5% acetic acid solution as eluent), a good linear relationship was obtained between the peak area and concentration of sulfadimethoxine in the range of 5.0 to 1500.0 µg/L with correlation coefficient of 0.9997. The limit of detection (S/N = 3) was 3.3 µg/L. The developed method was successfully applied to the analysis of sulfadimethoxine in milk with recoveries in the range of 75.9-92.3% and relative standard deviations less than 8.1%. The adsorption mechanism of Fe3 O4 /GO/Apt toward sulfadimethoxine was studied through the adsorption kinetics and adsorption isotherms, and the results show that the adsorption process fits well with the pseudo-second-order kinetic model and the adsorbate on Fe3 O4 /GO/Apt is multilayer and heterogeneous.

Universal Design of Structure-Switching Aptamers with Signal Reporting Functionality.

Structure-switching aptamers (SSAs) offer a promising recognition element for sensor development. However, the generation of SSAs via in vitro aptamer selection technologies or postselection engineering is challenging. Inspired by the two-domain structure of antibodies, we have devised a simple, universal strategy for engineering aptamers into SSAs with signal reporting functionality. These constructs consist of a "constant" domain, comprising a split DNAzyme G-quadruplex (G4) region for signal transduction, and a "variable" domain, comprising an aptamer sequence capable of specific target binding. In the absence of target, the G4-SSA construct folds into a parallel G4 structure with high peroxidase catalytic activity. Target binding disrupts the G4 structure, resulting in low enzymatic activity. We demonstrate that this change in DNAzyme activity enables sensitive and specific colorimetric detection of diverse targets including Hg2+, thrombin, sulfadimethoxine, cocaine, and 17ß-estradiol. G4-SSAs can also achieve label-free fluorescence detection of various targets using a specific G4-binding dye. We demonstrate that diverse aptamers can be readily engineered into G4-SSA constructs independent of target class, binding affinity, aptamer length, or structure. This design strategy could broadly extend the power, accessibility, and utility of numerous SSA-based biosensors.

Detection of sulfadimethoxine using optical images of liquid crystals.

A label-free method for sulfadimethoxine (SDM) detection using an aptamer-based liquid crystal biosensor is developed. The sensor probe is fabricated by immobilizing amine-functionalized aptamers onto the glass slide decorating mixed self-assembled layers of triethoxysilylbutyraldehyde (TEA) and N,N-dimethyl-n-octadecyl-3-aminopropyltrimethoxysilylchloride (DMOAP). Liquid crystals (LCs) are supported on the surface and serve as response elements, which assume the homeotropic alignment and cause a dark optical appearance under crossed polarizers. In the presence of SDM, the formation of SDM-aptamer compounds induces a notable change in the topographical structure of the surface, which disturbs the original homeotropic orientation of LCs and results in a bright optical appearance. A detection limit of 10 µg L-1 is obtained, which is far lower than the maximum residue limit (100 µg L-1 in China). This facile method shows good specificity for SDM detection and may have great potential for detecting other small molecules.

Voltammetric aptasensor for sulfadimethoxine using a nanohybrid composed of multifunctional fullerene, reduced graphene oxide and Pt@Au nanoparticles, and based on direct electron transfer to the active site of glucose oxidase.

This work describes a voltammetric and ultrasensitive aptasensor for sulfadimethoxine (SDM). It is based on signal amplification by making use of a multifunctional fullerene-doped reduced graphene oxide nanohybrid. The nanohybrid was coated with poly(diallyldimethylammonium chloride) to obtain a material (P-C60-rGO) with large specific surface area and a unique adsorption ability for loading it with glucose oxidase (GOx). The coating also facilitates the direct electron transfer between the active site of GOx and the glassy carbon electrode (GCE). The P-C60-rGO were then modified with Pt@Au nanoparticles, and the thiolated SDM-binding aptamer was immobilized on the nanoparticles. On exposure of the modified GCE to a solution containing SDM, it binds to the aptamer. The results were recorded through the signal responses generated from the redox center of GOx (FAD/FADH2) by cyclic voltammetry at a scan rate of 100 mV·s-1 from -0.25 to -0.65 V. Accordingly, The sensor has good specificity and stability, and response is linear in the 10 fg·mL-1 to 50 ng·mL-1 SDM concentration range with a detection limit of 8.7 fg·mL-1. Graphical abstract Schematic presentation of an electrochemical aptasensor for sulfadimethoxine (SDM) using multifunctional fullerene-doped graphene (C60-rGO) nanohybrids for amplification. The limit of detection for SDM is as low as 8.7 fg·mL-1.

Screening of antibiotic residues in fresh milk of Kathmandu Valley, Nepal.

The prevalence of two groups of antibiotics; namely penicillin and sulfonamides was studied in fresh milk available in Kathmandu Valley of Nepal. The milk samples (n = 140) were collected from three different sources; individual farmers, cottage dairies and organized dairies of Kathmandu valley. Qualitative and semi-quantitative analysis with rapid screening kits revealed that 23% samples were positive for antibiotic residues in the fresh milk for penicillin and sulfonamide groups (1-256 µg/kg). High performance liquid chromatography (HPLC) analyses detected 81% samples positive for amoxicillin (68-802 µg/kg), 41% for sulfadimethoxine (31-69 µg/kg), 27% for penicillin G (13-353 µg/kg), and 12% for ampicillin (0.5-92 µg/kg). Due to the precision and accuracy of liquid chromatography method, it detected more positive samples and consequently presented higher prevalence than the rapid screening kits. The antibiotic residues were found above the maximum residue limits that presented serious threat to consumer health and raised a serious concern regarding the implementation and monitoring of international regulations in developing countries.

A colorimetric aptasensor for sulfadimethoxine detection based on peroxidase-like activity of graphene/nickel@palladium hybrids.

A sensitive, rapid and label-free colorimetric aptasensor for sulfadimethoxine (SDM) detection was developed based on the tunable peroxidase-like activity of graphene/nickel@palladium nanoparticle (Gr/Ni@Pd) hybrids. The addition of the SDM aptamer could inhibit the peroxidase-like catalytic activity of the hybrids. However, the target SDM and aptamer could be triggered tightly and recover the catalytic activity of the Gr/Ni@Pd hybrids. Due to the peroxidase-like catalytic activity, Gr/Ni@Pd could catalyze the decomposition of H2O2 with releasing hydroxyl radicals which further oxidized reagent 3, 3', 5, 5'-Tetramethylbenzidine (TMB) to oxTMB accompanied with a colorless-to-blue color change. The original color change could be applied to obtain quantitative detection of SDM, due to the relationship between the concentration of the target and the color difference. As a result, this approach performed a linear response for SDM from 1 to 500 ng/mL with a limit detection of 0.7 ng/mL (S/N = 3) under the optimized conditions and realized the detection of SDM in spiked lake water samples. Therefore, this colorimetric aptasensor was an alternative assay for SDM detection in real water. Moreover, with its design principle, this work might be applied to detecting other small molecule by employing appropriate aptamer.

Tissue/fluid correlation study for the depletion of sulfadimethoxine in bovine kidney, liver, plasma, urine, and oral fluid.

Sulfonamides are among the oldest, but still effective, antimicrobial veterinary medicines. In steers and dairy cows, the sulfonamides are effective in the treatment of respiratory disease and general infections. Sulfadimethoxine (SDM) has been approved by US Food and Drug Administration (FDA) for use in steers and dairy cows with a tolerance of 100 ng/g (ppb) in edible tissues and 10 ppb in milk. The detection of SDM residue above tolerance in the animal slaughtered for food process will result in the whole carcass being discarded. This report describes a comprehensive depletion study of SDM (and its main metabolite) in plasma, urine, oral fluid, kidney, and liver. In this study, nine steers were injected intravenously with the approved dose of SDM; the loading dose was 55 mg/kg, followed by 27.5 mg/kg dose at 24 h and again at 48 h. Fluids (blood, urine, and saliva) and tissue (liver and kidney) samples were collected at intervals after the last dose of SMD. The combination of laparoscopic serial sampling technique with the liquid chromatography/mass spectrometry method provided the data to establish the tissue/fluid correlation in the depletion of SMD. A strong correlation and linearity of the log-scale concentration over time in the depletion stage has been confirmed for kidney, liver, and plasma.

Validation of a LC-MS/MS method for the quantitative analysis of four antibiotics in pig tissues and plasma to assess the risk of transfer of residues to edible matrices after exposure to cross-contaminated feed.

Cross-contamination between medicated and non-medicated feed can occur during production, processing, transport or storage of animal feed. This may lead to the presence of low concentrations of antibiotics in supposedly drug-free feed for food production animals, which potentially could also harm consumers due to residues. In addition, consumption of sub-therapeutic concentrations of antibiotics may increase the risk of emergence of resistant bacteria. In this study, LC-MS/MS methods were developed to quantify four antibiotics (sulfadimethoxine, oxytetracycline, trimethoprim and amoxicillin) in several pig matrices, i.e. plasma, muscle, liver, kidneys and faeces. All methods were validated using the accuracy profile, except for amoxicillin in faeces, for which extraction could not be optimised for low concentrations. These methods were then applied as part of an animal study during which several pigs received contaminated feed at a concentration corresponding to 2% of therapeutic dose, in order to evaluate the risk of the presence of residues in animal faeces and tissues. The results showed that sulfadimethoxine is well absorbed and accumulates in the muscle, kidneys and liver, where concentrations were higher than the maximum residue limits (MRLs) authorised in EU legislation. Conversely, oxytetracycline was mostly found in faeces as its oral absorption is very low. Trimethoprim concentrations were slightly higher than the tolerated MRL in the kidneys, but they were below this level in the other tissues. Finally, amoxicillin concentrations remained below the lower limit of quantification of the methods in all matrices.

Detection of fluoroquinolone and sulfonamide residues in poultry eggs in Kunming city, southwest China.

Antibiotic residues contained in poultry eggs pose threat to human health. However, the classes and concentrations of antibiotics in poultry egg in southwestern China is unknown due to insufficient monitoring and research. A total of 513 egg samples were collected from supermarkets and farm markets in Kunming city in 2020 and the levels of 7 antibiotics were analyzed using ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method. The linear correlation coefficients were above 0.990 for all antibiotics tested. The limits of detection and limits of quantification in poultry eggs were 0.002 to 0.010 µg/g and 0.007 to 0.033 µg/g, respectively. The average recoveries of the 7 analytes from poultry egg samples were 80.00 to 128.01%, with relative standard deviations of less than 13.97%. A total of 93 (18.13%) samples tested positive for antibiotics, with the highest concentration being 2.48 µg/g. The concentration range of ofloxacin, danofloxacin, difloxacin, sulfadimethoxine, sulfamonomethoxine, sulfamethoxypyridazine, and sulfamethoxazole in poultry eggs was 0.01 to 0.37 µg/g, 0.06 to 0.48 µg/g, 0.05 to 0.29 µg/g, 0.03 to 0.16 µg/g, 0.06 to 1.00 µg/g, 0.05 to 0.37, and 0.07 to 2.48 µg/g, respectively. Sulfamonomethoxine was detected from hen eggs with the highest concentration level at 1.00 µg/g. Sulfamethoxazole was detected with the highest concentration level from both duck and quail eggs, at 1.87 and 2.48 µg/g, respectively. The antibiotic with the highest residue level in pheasant eggs was danofloxacin, which was 0.37 µg/g. Sulfamethoxypyridazine was identified in 30 samples with the highest positive rate of 5.85%, sulfadimethoxine was identified in 3 samples with the lowest positive rate of 0.58%. We observed that 7 targeted antibiotic residues in quail eggs and 3 targeted antibiotic residues in pheasant eggs. We also found that there were antibiotic residues in free-range hen eggs and the concentration was not low. The antibiotic with the highest residue level in free-range eggs was sulfamonomethoxine, which was 1.00 µg/g. These findings suggest that continual antibiotic residue monitoring of poultry eggs is essential in China.

Two fluoroalcohols as components of basic buffers for liquid chromatography electrospray ionization mass spectrometric determination of antibiotic residues.

Two fluoroalcohols--1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HFTB)--were evaluated for the first time as volatile buffer acids in the basic mobile phase for reversed-phase chromatography with electrospray ionization-mass spectrometric (LC-ESI-MS) detection of five antibiotics. Chromatographic separation as well as positive and negative ion ESI-MS intensities using these novel buffer components were compared to traditional buffer systems. Overall, the highest signal intensities and best chromatographic separation for the five antibiotics (ciprofloxacin, norfloxacin, ofloxacin, sulfadimethoxine and sulfamethoxazole) were achieved using 5 mM HFIP as the buffer acid to methanol : water mobile phase (pH of the aqueous component adjusted to 9.0 with ammonium hydroxide). Comparable results were achieved using 5 mM HFTB (pH adjusted to 9.0 with ammonium hydroxide). The suitability of HFIP for analysis of antibiotic residues in lettuce is demonstrated.

Stress degradation study on sulfadimethoxine and development of a validated stability-indicating HPLC assay.

Sulfadimethoxine was subjected to different International Conference on Harmonisation prescribed stress conditions (hydrolysis, oxidation and photolysis). A stability-indicating high-performance liquid chromatography method was developed for analysis of the drug in the presence of its major degradation products. It involved a Knauer Eurospher C18 thermostated column at 25°C, and 9.57 mM phosphate buffer (pH adjusted to 2.0 with orthophosphoric acid)-acetonitrile (70:30 v/v) as mobile phase. The mobile phase flow rate and sample volume injected were 1.2 mL/min and 20 µL respectively. The selected wavelength for the determination was 248 nm. The method was validated for its linearity, precision, accuracy, specificity and selectivity, and then applied to the assay of sulfadimethoxine in pharmaceutical formulations. The results of the study show that sulfadimethoxine is highly sensitive to basic hydrolysis and oxidation. The mechanisms and schemas of hydrolytic, oxidative and photolytic degradation are also studied. The method developed, which separates all of the most degradation products, is simple, accurate, precise and specific. Thus, it can be applied to study the stability of veterinary preparations containing sulfadimethoxine.

Validation of a liquid chromatography method for the simultaneous determination of sulfadimethoxine and trimethoprim and application to a stability study.

A liquid chromatography method is described for the simultaneous determination of sulfadimetoxine and trimethoprim from a veterinary formulation at the proportion of 187 mg and 40 mg respectively in presence of some excipient. The solution was subjected to different International Conference On Harmonisation prescribed stress conditions (hydrolysis, oxidation and photolysis). A stability-indicating high-performance liquid chromatography method was developed for the analysis of active substances in presence of their major degradation products. It involved a Knauer Eurospher C18 thermostated column at 25 degrees C, and 9.57 mM phosphate buffer (pH adjusted to 2.0 with orthophosphoric acid)-acetonitrile (70:30 v/v) as mobile phase. The mobile phase flow rate and sample volume injected were 1.2 mL/min and 20 microL, respectively. The selected wavelength for the determination was 248 nm. The method was validated for linearity, precision, accuracy and specificity, and then applied to a stability study of sulfadimetoxine and trimethoprim in the veterinary solution packaged in high density polyethylene plastic bottles of 1 L and 100 mL thermosealed and no thermosealed and corked by a white cap, at both accelerated and long-term conditions required by the International Conference On Harmonisation. The method developed, which separates all of the most degradation products formed under variety of conditions, proved to be simple, accurate, precise and specific. The results of the stress degradation show that the solution is more sensitive to hydrolysis. The stability studies carried out on three batches of each presentation show that the finished product remains stable for six months.

A dichromatic label-free aptasensor for sulfadimethoxine detection in fish and water based on AuNPs color and fluorescent dyeing of double-stranded DNA with SYBR Green I.

A dichromatic label-free aptasensor was described for sulfadimethoxine (SDM) detection. Compared with the binding of SDM-aptamer to SDM, the higher affinity of aptamer to cDNA may result in the hybridization of dsDNA. In the presence of SDM, the aptamer specifically binds to SDM, leading to a blue color of AuNPs in deposit and fluorescence at 530 nm in supernatant after adding cDNA and SGI. With no target of SDM, AuNPs protected with the aptamer re-disperse in PBS with a red color, and no fluorescence occurs in supernatant. Based on the principle, SDM can be quantitatively detected through both fluorescent emission and AuNPs color changes with recoveries ranging from 99.2% to 102.0% for fish and from 99.5% to 100.5% for water samples. An analytical linear range of 2-300 ng mL-1 was achieved with the detection limits of 3.41 ng mL-1 for water and 4.41 ng g-1 for fish samples (3σ, n = 9).

Determination of sulfadimethoxine and 4N-acetylsulfadimethoxine in bovine plasma, urine, oral fluid, and kidney and liver biopsy samples obtained surgically from standing animals by LC/MS/MS.

A quantitative method was developed and validated to measure the concentration of sulfadimethoxine (SDM) and its major metabolite, (4)N-acetylsulfadimethoxine (AcSDM), in bovine tissues and body fluids. Liquid chromatography/tandem mass spectrometry (LC/MS/MS) gave quantitative results for these two analytes in extracts from bovine plasma, urine, oral fluid, kidney, and liver, using SDM-d(4) as internal standard (I.S.). The lower limit of quantitation (LLOQ) for both analytes in these matrices was validated at 2, 100, and 5 ng/mL in plasma, urine, and oral fluid respectively, and 10 ng/g in both kidney (cortex) and liver. The overall accuracy (average of 4 levels) is, for plasma, 104% (SDM) and 95% (AcSDM), with standard deviation of 9% (SDM) and 15% (AcSDM); for urine, 100% (SDM) and 106% (AcSDM), with standard deviation of 5% (SDM) and 6% (AcSDM); for oral fluid, 103% (SDM) and 103% (AcSDM), with standard deviation of 4% (SDM) and 4% (AcSDM); for kidney, 101% (SDM) and 111% (AcSDM), with standard deviation of 7% (SDM) and 6% (AcSDM); and for liver, 99% (SDM) and 115% (AcSDM), with standard deviation of 11% (SDM) and 9% (AcSDM). C18 SPE cartridges were used to clean-up these matrices, except for urine which was diluted directly with buffer before analysis by LC/MS/MS.

Label-Free Fluorescence-Based Aptasensor for the Detection of Sulfadimethoxine in Water and Fish.

Fluorescence-based aptasensors possess high sensitivity but are complicated and usually require multistep labeling and modification in method design, which severely limit the practical applications. Here, a label-free fluorescence-based aptasensor, consisting of aptamer, gold nanoparticles (AuNPs), and cadmium telluride (CdTe) quantum dots (QDs), was developed for the detection of sulfadimethoxine (SDM) in water and fish based on the specific recognition of SDM-aptamer and the inner filter effect of QDs and AuNPs. In the absence of a target, AuNPs dispersed in salt solution because of the aptamer protection, which could effectively quench the fluorescence emission of QDs, while in the presence of SDM, AuNPs aggregated due to the specific recognition of SDM-aptamer to SDM, which resulted in fluorescence recovery. A linear response of SDM concentrations in the range of 10-250 ng mL-1 ( R2 = 0.99) was obtained, and the detection limit was 1.54 ng mL-1 (3σ, n = 9), far below the maximum residue limit (100 ng mL-1) of SDM in edible animal tissues regulated by China and the European Commission. The fluorescence-based aptasensor was applied to the detection of SDM in aquaculture water and fish samples with high accuracy, excellent precision, and ideal selectivity. The results indicated that the developed aptasensor was simple in design, easy to operate, and could be used to detect rapidly and accurately SDM in water and fish samples.

Electrochemical aptasensor for sulfadimethoxine detection based on the triggered cleavage activity of nuclease P1 by aptamer-target complex.

Herein, a simple and selective electrochemical method was developed for sulfadimethoxine detection based on the triggered cleavage activity of nuclease P1 by the formation of aptamer and sulfadimethoxine conjugate. After probe DNA was immobilized on gold electrode surface, aptamer DNA labeled with biotin at its 5'-terminal was then captured on electrode surface through the hybridization reaction between probe DNA and aptamer DNA. The formed double-stranded DNA (dsDNA) can block the digestion activity of Nuclease P1 towards the single-stranded probe DNA. Then, the anti-dsDNA antibody was further modified on electrode surface based on the specific interaction between dsDNA and antibody. Due to the electrostatic repulsion effect and steric-hindrance effect, a weak electrochemical signal was obtained at this electrode. However, in the presence of sulfadimethoxine, it can interact with aptamer DNA, and then the formation of dsDNA can be blocked. As a result, the probe DNA at its single-strand state can be digested by Nuclease P1, which leads to the failure of the immobilization of anti-dsDNA antibody. At this state, a strong electrochemical signal was obtained. Based on the change of the electrochemical signal, sulfadimethoxine can be detected with linear range of 0.1-500 nmol/L. The detection limit was 0.038 nmol/L. The developed method possesses high detection selectivity and sensitivity. The applicability of this method was also proved by detecting sulfadimethoxine in veterinary drug and milk with satisfactory results.

An amperometric aptasensor for ultrasensitive detection of sulfadimethoxine based on exonuclease-assisted target recycling and new signal tracer for amplification.

The risks caused by veterinary drug residues in animal foodstuffs are of great concern to the public. Accordingly, this work reported an amperometric aptasensor for highly sensitive detection of sulfadimethoxine (SDM). Functionalised fullerene (C60)-doped graphene (C60-rGO) nanohybrid was designed and prepared to load electroactive toluidine blue (Tb) through the π-π stacking, forming a C60-rGO-Tb nanocomposite. Furthermore, the as-prepared nanocomposite was decorated with gold nanoparticles and used for the immobilization of signal probes to form a new signal tracer, which was coupled with exonuclease-catalyzed target recycling for amplification. To construct the aptasensor, a thiolated double-stranded DNA (dsDNA) of aptamer-capture probe complex was immobilised on a gold electrode surface through strong Au-S bond. In the presence of SDM, the aptamer preferred to form an aptamer-SDM complex, which led to the dissociation of dsDNA. Then aptamer could be selectively digested by RecJf exonuclease, resulting in liberated SDM molecules to participate in the next reaction cycling and achieve signal amplification. Then, capture probes released from the cyclic processes were hybridized with the signal tracer, which could further enhance electrochemical signal responses. On the basis of cascade signal amplification strategies, the proposed aptasensor exhibited a wide linear range from 10 fg/mL to 10 ng/mL for SDM with high sensitivity, good selectivity and satisfactory stability.