Inner filter effect-modulated ratiometric fluorescence aptasensor based on competition strategy for zearalenone detection in cereal crops: Using mitoxantrone as quencher of CdTe QDs@SiO2

Efficient detection of aklomide/nitromide based on a sensor of MPA capped CdTe QDs using fluorescence quenching method

Based on the fluorescence sensor of 3-Mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs), two novel detection methods for aklomide and nitromide were developed. The MPA-CdTe QDs were synthesized and characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), ultraviolet visible (UV-vis) and fluorescence (FL). The quenchings were all static. The binding constants (Ka) at different temperatures were obtained. Electrostatic forces were the main forces for the two bindings. For the detection of aklomide and nitromide, under the optimal conditions, the effects of some metal ions, glucose, bovine serum albumin (BSA) and congeneric drug on the determination were explored. The standard equations were established and the limits of detection (LOD) were 0.0215 and 0.0388 μg mL-1 (3S0/S), repectively. The methods were applied to analyse the samples of chicken and duck, the recoveries were 99.41 % - 101.24 % with RSDs of 0.29 % - 1.19 % (n = 5).

Recent progress in optical and electrochemical aptasensor technologies for detection of aflatoxin B1

AFB1 (Aflatoxin B1) contamination is becoming a global concern issue due to its extraordinary occurrence, severe toxicity, as well as the great influence on the economic losses, food safety and environment. Therefore, it is desirable to develop novel analytical techniques for simple, rapid, accurate, and even point-of-care testing of AFB1. Fortunately, aptamer, considered as a new generation bioreceptor and even superior to classic antibody and enzyme, has been emerged remarkable application in food hazards detection. Correspondingly, aptasensors have been well-established toward AFB1 determination with outstanding performance. In this article, we first discuss and summarize the recent progress in optical and electrochemical aptasensors to monitor AFB1 over the past three years. In particular, the embedding of advanced nanomaterials for their improved analytical performance is highlighted. Furthermore, the critical analysis on various signal transduction strategies for aptasensors construction is discussed. Finally, we reveal the challenges and provide our opinion in future opportunities for aptasensor development.

A quantum dot aptamer fluorescent sensor based on magnetic graphene oxide for the detection of zearalenone

As an estrogenic mycotoxin found in a wide range of agricultural crops, the toxicity of zearalenone (ZEN) poses a serious risk to human health. Accordingly, to achieve rapid detection of zearalenone in complex samples, an aptamer fluorescence sensor based on magnetic graphene oxide was developed. Compared with traditional methods, this technique has the virtues of simple operation, low cost, and reliable performance. Magnetic graphene oxide (MGO) was synthesized as a fluorescent bursting agent, using a chemical precipitation approach by depositing Fe3O4 on the surface of graphene oxide. As a fluorescent probe, an aptamer coupling with CdTe quantum dots and zearalenone was used. Following the specific binding of zearalenone and the aptamer, the affinity interaction between the fluorescent probe and MGO was weakened, resulting in the recovery of fluorescence and making the qualitative and quantitative analysis of zearalenone available via fluorescence intensity determination. The results indicated that the method's linear range was 5-120 μg L-1 and its detection limit was 2.9 μg L-1. In addition, the recoveries varied from 76.4 to 118.8% for crop samples, with relative standard deviations (RSDs) between 0.8 and 9.5%, which suggests an effective method for the separation and detection of mycotoxins in actual environmental samples.

Recent advances of ratiometric sensors in food matrices: mycotoxins detection

The public health problem caused by mycotoxins contamination has received a great deal of attention worldwide. Mycotoxins produced by filamentous fungi widely distributed in foodstuffs can cause adverse impacts on humans and livestock, posing serious health threats. Particularly worth mentioning is that mycotoxins can accumulate in organisms and be enriched through the food chain. Improving early trace detection and control from the source is a more desirable approach than the contaminated food disposal process to ensure food safety. Conventional sensors are susceptible to interference from various components in intricate food matrices when detecting trace mycotoxins. The application of ratiometric sensors avoids signal fluctuations, and reduce background influences, which casts new light on developing sensors with superior performance. This work is the first to provide an overview of the recent progress of ratiometric sensors in the detection of mycotoxins in intricate food matrices, and highlight the output types of ratiometric signal with respect to accurate quantitative analysis. The prospects of this field are also included in this paper and are intended to have key ramifications on the development of sensing detection conducive to food safety.

In-depth interpretation of aptamer-based sensing on electrode: Dual-mode electrochemical-photoelectrochemical sensor for the ratiometric detection of patulin

Electrochemical aptasensors have been extensively used to quantify food contaminants (e.g., mycotoxin) by using high-affinity aptamer for target recognition. Yet, analytical performance of aptasensors using different aptamers can be varied for the same target. Here, four aptamers with different sequences (i.e., A22, A34, A42, and A45) of patulin (PAT) were selected to estimate sensing behaviors at electrodes with electrochemical (EC) and photoelectrochemical (PEC) assays. Synergistic effect of steric hindrance and electron transfer distance was found to significantly affect EC and PEC response for PAT at aptasensors fabricated with A22, A34, A42, or A45. Eventually, A22 emerged to be the optimal aptamer for aptasensing, despite the highest affinity of A42 to PAT. The A22-based EC-PEC dual-mode ratiometric aptasensor offered a linear range of 50 fg mL^-1 - 500 ng mL^-1 with a detection limit of 30 fg mL^-1 for PAT, and it was applied to apple product (i.e., juice, puree) analysis.

Advanced biosensors for mycotoxin detection incorporating miniaturized meters

Advanced biosensors, considered as emerging technologies, are capable of accurate, quantitative and real-time analysis for point-of-care testing (POCT) applications. Moreover, the integrating of miniaturized meters into these advanced biosensors makes them ideally appropriate for portable, sensitive and selective detection of biomolecules. Miniaturized meters including PGMs (personal glucose meters), thermometer, pressuremeter, pH meter, etc. are the most accurate devices and wide availability in the market, exhibiting a promising potential towards detection of small molecule mycotoxins. In this article, we introduce and analyze the recent advancements for sensing of mycotoxins measured by handheld meters since the first report in 2012. Furthermore, limitations and challenges for versatile meters application against mycotoxins in food matrix are highlighted. By overcoming the bottleneck problems, we believe the miniaturized meters-based biosensor platform will provide great possibilities for mycotoxins analysis and launch them to the market.

Dual-quenching effects of methylene blue on the luminophore and co-reactant: Application for electrochemiluminescent-electrochemical ratiometric zearalenone detection

Methylene blue (MB) is a common multifunctional indicator, which can be applied as a quencher for electrochemiluminescence (ECL) analysis as well as a classical redox probe. Although it is relatively prevalent for MB to study the mechanism with Ru-based luminophores in ECL systems, there are few studies on the effects between MB and co-reactants. In this work, we proposed the first investigation of MB on the luminophore and co-reactant of the self-enhanced ECL composites (nitrogen-doped graphene quantum dots on Ru(bpy)₃²⁺-doped silica nanoparticles, NGQDs-Ru@SiO₂), respectively. The relatively narrow ECL spectrum of luminophore (Ru@SiO₂) and the suitable ultraviolet-visible absorption spectrum of MB led to the ECL resonance energy transfer between them, meanwhile the appropriate energy levels among them facilitated the electron transfer, resulting in a decreased ECL signal (quench mode I). Additionally, the co-reactant (NGQDs) was prone to π-π conjugation with MB due to its abundant π-electrons, which reduced the concentration of NGQDs' intermediates and triggered a weakened ECL signal (quench mode II). Therefore, the dual-quenching effects are ingeniously integrated and designed in one ECL-electrochemical (ECL-EC) ratiometric aptasensor for zearalenone detection, for demonstrating its efficacy in enhancing the sensitivity, which is 4.8-fold higher than Ru@SiO₂ alone. This innovative ratiometric aptasensor achieved a relatively wide linear range from 1.0 × 10^-15 to 5.0 × 10^-8 g mL^-1, and obtained a low detection limit of 8.5 × 10^-16 g mL^-1. Our proposed dual-quenching interactions between MB and NGQDs-Ru@SiO₂ will open a new prospective for ECL-EC ratiometric aptasensor, which further broaden the application in sensitive and precise analysis of mycotoxins.

Ratiometric fluorescence probe integrated with smartphone for visually detecting lipopolysaccharide

A portable instrument-free detection method for lipopolysaccharide (LPS) analysis was developed based on dual-emission ratiometric fluorescence sensing system. Herein, red-emitting Au nanoclusters (Au NCs) were as reference probe, while blue-emitting fluorescent silica quantum dots (Si QDs) were as response probe. Additionally, the aptamer of LPS was covalently grafted to the surface of Si QDs in order to specific recognize the LPS. According to the changes of fluorescence intensityratio (FL ratio, I_{461 nm}/I_{643 nm}) with the concentrations of LPS, the linear equation was fitted with the range of 50-3000 ng/mL, and the limit of detection (LOD) was 29.3 ng/mL. As a practical application, this method was employed to analyze LPS in normal saline with the recovery rate of 97.7-103.8 %. The color picker platform in the smartphone was used to transform the detection picture to the process of Red, Green and Blue (RGB) for visual detection of LPS. The low-cost and easy-carry method reported here presents broad merits for the visually quantitative detection of LPS.

Photoelectrochemical and visual dual-mode sensor for efficient detection of Cry1Ab protein based on the proximity hybridization driven specific desorption of multifunctional probe

Currently, the development of sensitive and visual strategy for Cry1Ab detection, particularly using a switchable dual-mode detection system based on a single component, remains a great challenge. Here, a photoelectrochemical (PEC) and visual dual-mode sensor was designed for Cry1Ab detection based on a proximity hybridization driven multifunctional probe. In the presence of Cry1Ab, specific desorption of the antibody-DNA conjugate was achieved via sufficient proximity hybridization, leading to the selective release of the multifunctional signal probe, i.e., antibody-labeled single-stranded DNA-gold nanoparticles (Ab₁-S1-AuNPs). The released Ab₁-S1-AuNPs reduced the photocurrent signal and produced a colored response, thereby achieving PEC and visual dual-mode detection based on a single component. Owing to the different signal generation mechanisms, two independent signals were obtained simultaneously, which provided self-verification to improve reliability and accuracy. Taking advantage of the PEC sensitive detection and visual prediction, the dual-mode sensor achieved efficient detection of the Cry1Ab protein. The developed sensor was successfully used to determine Cry1Ab in corn, wheat, and soil samples with satisfactory results. This method offers a promising biosensing platform for the on-site detection of Cry1Ab protein.

Engineering the Signal Transduction between CdTe and CdSe Quantum Dots for in Situ Ratiometric Photoelectrochemical Immunoassay of Cry1Ab Protein

Controllable modulation of a response mode is extremely attracting to fabricate biosensor with programmable analytical performances. Here, we reported a proof-of-concept ratiometric photoelectrochemical (PEC) immunoassay of Cry1Ab protein based on the signal transduction regulation at the sensing interface. A sandwich-type PEC structure was designed so that gold nanorods sensitized quantum dots to fix primary antibody (Au NRs/QDs-Ab₁) and methylene blue sensitized QDs to combine a second antibody (MB/QDs-Ab₂), which served as photoelectric substrate and signal amplifier, respectively. Unlike common recognition element, such a sandwich-type PEC structure allowed for the in situ generation of two specific response signals. For analysis, Cry1Ab captured by Au NRs/QDs-Ab₁ led to a decreased photocurrent (I_Cry1Ab), while the subsequently anchored MB/QDs-Ab₂ produced another photocurrent (I_MB). Noteworthy, by taking advantage of the different energy band gaps of QDs, varying locations of CdTe and CdSe QDs could realize different signal transduction strategies (i.e., Mode 1 and Mode 2). Investigations on data analysis of I_Cry1Ab and I_MB via different routes demonstrated the superior analytical performances of ratiometry (Mode 1). Consequently, the ratiometric PEC immunosensor offered a linear range of 0.01-100 ng mL^-1 with a detection limit of 1.4 pg mL^-1. This work provides an efficient strategy for in situ collection of multiple photocurrents to design ratiometric PEC sensors.

A label-free visual aptasensor for zearalenone detection based on target-responsive aptamer-cross-linked hydrogel and color change of gold nanoparticles

ZEN-responsive hydrogel was prepared using ZEN aptamer and a cationic conjugated skeleton which was formed by the electrostatic interaction between acrylamide and poly (diallyldimethylammonium chloride) (PDDA). In the presence of ZEN, the binding of aptamer and ZEN caused the hydrogel to dissociate, releasing the gold nanoparticles (AuNPs) encapsulated in the hydrogel into supernatant, whose color changes serving as response instructions were observed by eyes. To realize low dose visual detection, the TMB-H₂O₂ was used for quantitative readout by AuNPs released from the hydrogel which can efficiently catalyze the reaction to generate a obvious change.There was a good linear relationship between the changed absorbance and concentration of ZEN within 2.5-100 ng/mL, a detection limit of 0.98 ng/mL, recovery rates of 98.8%-101.3% and 99.8%-101.5% for corns and beer, respectively. This developed sensing strategy would provide a promising application for other mycotoxins by replacing the corresponding aptamer sequences.

Electrochemical aptasensing strategy based on a multivariate polymertitanium-metal-organic framework for zearalenone analysis

An electrochemical aptasensing strategy was developed with a novel bioplatform based on a multivariate titanium metal-organic framework, i.e. MTV polyMOF(Ti), to detect zearalenone (ZEN). MTV polyMOF(Ti) was prepared by using mixed linkers of polyether polymer (pbdc-xa or L₈, pbdc = poly(1,4-benzenedicarboxylate) and 1,4-benzenedicarboxylic acid (H₂bdc or L₀) as well as tetrabutyl titanate as nodes (MTV polyMOF(Ti)-L_8,0). Compared with Ti-MOFs synthesized by using the single ligand of L₈ or L₀, MTV polyMOF(Ti)-L_8,0 shows more porous structure assembled with multilayered nanosheets. In light of the improved electrochemical activity and strong bioaffinity to the aptamer, the aptasensor based on MTV polyMOF(Ti)-L_8,0 shows excellent performance for detecting ZEN with the ultralow detection limit at fg mL^-1 level in the linear range of 10 fg mL^-1 to 10 ng mL^-1, along with good selectivity, reproducibility, stability, regenerability, and applicability.

Evanescent Wave Optical-Fiber Aptasensor for Rapid Detection of Zearalenone in Corn with Unprecedented Sensitivity

Zearalenone (ZEN) is a common mycotoxin pollutant found in agricultural products. Aptamers are attractive recognition biomolecules for the development of mycotoxin biosensors. Even though numerous aptasensors have been reported for the detection of ZEN in recent years, many of them suffer from problems including low sensitivity, low specificity, tedious experimental steps, high-cost, and difficulty of automation. We report here the first evanescent wave optical-fiber aptasensor for the detection of ZEN with unprecedented sensitivity, high specificity, low cost, and easy of automation. In our aptasensor, a 40-nt ZEN-specific aptamer (8Z₃₁) is covalently immobilized on the fiber. The 17-nt fluorophore Cy5.5-labeled complementary DNA strand and ZEN competitively bind with the aptamer immobilized on the fiber, enabling the signal-off fluorescent detection of ZEN. The coating of Tween 80 enhanced both the sensitivity and the reproducibility of the aptasensor. The sensor was able to detect ZEN spiked-in the corn flour extract with a semilog linear detection range of 10 pM-10 nM and a limit of detection (LOD, S/N = 3) of 18.4 ± 4.0 pM (equivalent to 29.3 ± 6.4 ng/kg). The LOD is more than 1000-fold lower than the maximum ZEN residue limits set by China (60 μg/kg) and EU (20 μg/kg). The sensor also has extremely high specificity and showed negligible cross-reactivity to other common mycotoxins. In addition, the sensor was able to be regenerated for 28 times, further decreasing its cost. Our sensor holds great potential for practical applications according to its multiple compelling features.

Ratiometric fluorescence sensing with logical operation: Theory, design and applications

The construction of ratiometric fluorescence sensing logic systems has gradually become a hot topic in fluorescence analysis, due to the multi-target analysis potential of logic operations and the high specificity and selectivity of ratiometric fluorescence sensing. In this paper, the basic principles of various logic functions implemented in ratiometric fluorescence detection are discussed in the context of sensing mechanisms, and the strategies for constructing logic systems in different ratiometric fluorescence sensing application areas are summarized. Although there are limitations such as cumbersome operations and complicated experiments, ratiometric fluorescence sensing logic circuits that combine the visualization of logic operations and the accuracy of ratiometric fluorescence are still worthy of in-depth study. This review may be useful for researchers interested in the construction of logic operations based on ratiometric fluorescence sensing applications.

An ultra-high-sensitivity electrochemiluminescence aptasensor for Pb2+ detection based on the synergistic signal-amplification strategy of quencher abscission and G-quadruplex generation

Signal amplification provides an effective way to improve detection performance. Herein, an ultrasensitive electrochemiluminescence (ECL) aptasensor for Pb²⁺ detection was developed based on a dual signal-amplification strategy of the abscission of a quencher and the generation of a G-quadruplex by one-step and simultaneous way. Nitrogen-doped carbon quantum dots linked with complementary DNA (cDNA-NCQDs) at the sensing interface was applied as the quencher of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl)ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine system to minimize the ECL signal due to the intermolecular hydrogen bond-induced energy-transfer process. Upon the addition of Pb²⁺, its specific binding with the aptamer triggered the abscission of cDNA-NCQDs, accompanied by the formation of G-quadruplex on the surface of the electrode, both of which amplified the intensity of the light emission. The ECL amplification efficiency induced by the above two mechanisms (78.6%) was valuably greater than that of their sum value (69.3%). This synergistic effect resulted in high detection sensitivity of the ECL aptasensor, which allowed to thereby obtain Pb²⁺ measurements in the range of 1 fM - 10 nM with an ultra-low detection limit of 0.19 fM. The Pb²⁺-mediated synergistic signal-amplification ECL strategy can provide a new approach for integrating various amplification strategies.

Signal enhancing strategies in aptasensors for the detection of small molecular contaminants by nanomaterials and nucleic acid amplification

Small molecular contaminants (such as mycotoxins, antibiotics, pesticide residues, etc.) in food and environment have given rise to many biological and ecological toxicities, which has attracted worldwide attention in recent years. Meanwhile, due to the advantages of aptamers such as high specificity and stability, easy synthesis and modification, as well as low cost and immunogenicity, various aptasensors for the detection of small molecular contaminants have been flourishing. An aptasensor as a whole is composed of an aptamer-based target recognizer and a signal transducer, which are fields of concentrated research. In the practical detection applications, in order to achieve the quantitative detection of small molecular contaminants at low abundance in real samples, a large number of signal enhancing strategies have been utilized in the development of aptasensors. Recent years is a vintage period for efficient signal enhancing strategies of aptasensors by the aid of nanomaterials and nucleic acid amplification that are applied in the elements for target recognition and signal conversion. Therefore, this paper meticulously reviews the signal enhancing strategies based on nanomaterials (including the (quasi-)zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanomaterials) and nucleic acid amplification (including enzyme-assisted nucleic acid amplification and enzyme-free nucleic acid amplification). Furthermore, the challenges and future trends of the abovementioned signal enhancing strategies for application are also discussed in order to inspire the practitioners in the research and development of aptasensors for small molecular contaminants.

A novel electrochemical aptasensor based on layer-by-layer assembly of DNA-Au@Ag conjugates for rapid detection of aflatoxin M1 in milk samples

Aflatoxin M₁ (AFM₁) is a common toxin in dairy products that causes acute and chronic human health disorders. Thus, the development of a rapid and accurate AFM₁ detection method is of vital importance for food safety monitoring. This work was to develop a novel electrochemical aptasensor for sensitive and specific determination of AFM_1. The dendritic-like nanostructure was formed on the gold electrode surface by layer-by-layer assembly of gold-silver core-shell nanoparticles modified with DNA conjugates. In the presence of AFM₁, the specific recognition between AFM₁ and Apt caused the disassociation of the DNA controlled dual Au@Ag conjugates from the surface of the electrode, causing less methylene blue to bind to the surface and weakening the electrochemical signal. The more AFM₁ there is, the weaker the electrochemical signal. Transmission electron microscope results showed that the successfully synthesized Au@Ag nanoparticles exhibited a core-shell structure with Au as core and Ag as shell, and their average diameter was about 30 nm. Under optimal conditions, the electrochemical aptasensor showed a wide detection ranging from 0.05 ng mL^-1 to 200 ng mL^-1, and a low detection limit of 0.02 ng mL^-1. Moreover, the proposed strategy has been successfully applied to the detection of AFM₁ in cow, goat, and sheep milk samples with satisfactory recoveries ranging from 91.10% to 104.05%. This work can provide a novel rapid detection method for AFM₁, and also provide a new sensing platform for the detection of other toxins.

Boron-doped and serine and histidine-functionalized graphene quantum dots with strong yellow fluorescence emissions for highly sensitive detection of carbofuran in cucumber and cabbage

The weak fluorescence of graphene quantum dots upon visible-light excitation limits their applications.