Label-free aptamer-based colorimetric detection of mercury ions in aqueous media using unmodified gold nanoparticles as colorimetric probe

Aptamer as capture agent in enzyme-linked apta-sorbent assay (ELASA) for ultrasensitive detection of Aflatoxin B1

Aflatoxin B₁ (AFB₁), is one of the most toxic mycotoxins found to contaminate various food commodities like cereals, dried fruits, tree nuts, spices and crude vegetable oils. In spite of considerable progress in analytical techniques, there is still a need to develop rapid and highly sensitive detection platforms for AFB₁. In this study, AFB₁ specific aptamer was used as a capture molecule to develop an enzyme-linked apta-sorbent assay (ELASA) for ultrasensitive detection of AFB₁. Under optimized conditions, the assay had a linear detection range from 1 μg to 1 pg with a limit of detection (LOD) of 1 pg/mL in buffer. Conventional ELISA with AFB₁ hapten as the capture agent (LOD = 10 pg/mL) was also carried out to compare the results with the present method. Recovery studies in food samples like dried red chillies, groundnut and pepper using both the methods was found to be in the range of 88.49-106.4% at 10 ng/mL and 87.4% to 95.8% at 5 ng/mL for ELASA and 76.56-127.68% at 10 ng/mL and 82-101.2% at 5 ng/mL for ELISA. Higher detection (10 fold) and better recovery using ELASA suggest that the method could offer an early, ultrasensitive, high-throughput, qualitative and semi-quantitative detection of AFB₁ in contaminated food samples.

Pyrophosphate ion-triggered competitive displacement of ssDNA from a metal-organic framework and its application in fluorescent sensing of alkaline phosphatase

Nanomaterial/aptamer assembly has been extensively explored in the detection of various targets, but some limitations still exist in its practical applications, especially time consumption and low-efficient detachment of the aptamer from the nanomaterial surface. In this work, we demonstrated the ligand role of pyrophosphate ion (PPi) in the competitive displacement of ssDNA from the nanoscaffold surface. For this purpose, a fluorescein-labeled ssDNA (F-DNA) and a mixed valence state cerium (Ce³⁺/Ce⁴⁺)-based MOF (MVCM) were employed as the signal response unit and nanoscaffold, respectively. Benefiting from the existence of Ce⁴⁺, the MVCM exhibits an ultrahigh quenching efficiency (more than 90%) to F-DNA fluorescence, which is 3-fold higher than that of the MOF with Ce³⁺ only. However, it was found that PPi can effectively suppress the quenching effect of the MVCM by competitive coordination with the MVCM to displace F-DNA. Different from the conventional target-induced conformational change of aptamers, the PPi-triggered displacement assay is independent of the ssDNA sequence and can be rapidly completed in just 2 min. The displacement assay is also highly sensitive, even at a PPi concentration as low as 55 nM. In contrast to PPi, however, the phosphate ions and other anions cannot displace F-DNA from the MVCM surface to switch on the F-DNA fluorescence. Inspired by this fact, the PPi-triggered displacement assay was further applied in the detection of alkaline phosphatase (ALP). The detection limit toward ALP was obtained at 0.18 mU mL^-1. Moreover, the accurate determination of ALP concentration in serum samples indicates the applicability of this sensing system in detecting real samples.

Advances in aptasensors for the detection of food contaminants

Food safety is a global health objective, and foodborne diseases represent a major crisis in health. Techniques that are simple and suitable for fast screening to detect and identify pathogenic factors in the food chain are vital to ensure food safety. At present, a variety of analytical methods have been reported for the detection of pathogenic agents. Whereas the sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic bio-recognition elements, such as aptamers. Owing to the specific folding capability of aptamers in the presence of an analyte, aptasensors have substantially and successfully been exploited for the detection of a wide range of small and large molecules (e.g., toxins, antibiotics, heavy metals, bacteria, viruses) at very low concentrations. Here, we review the use of aptasensors for the development of highly sensitive and affordable detection tools for food analysis.

Functionalized Silver Nano-Sensor for Colorimetric Detection of Hg2+ Ions: Facile Synthesis and Docking Studies

In the present study, we describe the facile synthesis of silver nanoparticles (AgNPs) and their nanostructures functionalized with 2-aminopyrimidine-4,6-diol (APD-AgNPs) for Hg²⁺ ion detection. The promising colorimetric response of APD-AgNPs to detect Hg²⁺ ions was visible with naked eyes and spectroscopic changes were examined by using a UV-Visible spectrophotometer. The aggregation of APD-AgNPs upon addition of Hg²⁺ ions was due to the chelation effect of the functionalized nanostructures and results in a color change from pale brown to deep yellow color. The probing sensitivity was observed within five minutes with a detection limit of about 0.35 µM/L. The TEM images of APD-AgNPs showed polydispersed morphologies with hexagonal, heptagonal and spherical nanostructures with an average size between 10 to 40 nm. Furthermore, the sensing behavior of APD-AgNPs towards Hg²⁺ ions detection was investigated using docking and interaction studies.

Revelation of susceptibility differences due to Hg(II) accumulation in Streptococcus pyogenes against CX-AgNPs and Cefixime by atomic force microscopy

Solution based method for the formation of chemically modified silver nanoparticles (CX-AgNPs) using Cefixime as stabilizing and reducing agent was developed. The CX-AgNPs were characterized by AFM, UV-visible, FT-IR and MALDI-TOF MS. Bactericidal efficiency of CX-AgNPs and Cefixime against Streptococcus pyogenes was evaluated. Afterwards, susceptibility differences of Streptococcus pyogenes due to accumulation of Hg(II) against CX-AgNPs and Cefixime were estimated and validated through Atomic force microscopy. Selectivity and sensitivity of CX-AgNPs against Hg(II) was evaluated in a systematic manner. The CX-AgNPs was titrated against optically silent Hg(II) which induced enhancement in the SPR band of CX-AgNPs. The increase in intensity of SPR band of CX-AgNPs was determined to be proportionate to the concentration of Hg(II) in the range of 33.3-700µM obeying linear regression equation of y = 0.125x + 8.962 with the detection limit of 0.10µM and the coefficient of determination equals to 0.985 (n = 3). The association constant Ka of CX-AgNPs-Hg(II) was found to be 386.0095mol^-1dm³ by using the Benesi Hildebrand plot.

In situ growth of gold nanoparticles on Hg2+-binding M13 phages for mercury sensing

Mercury poses a serious threat to human health and the ecosystem. Its pollution is still prevalent in developing areas, which calls for the development of a simple on-site method for Hg²⁺ detection. Plasmonic nanosensors for mercury, especially those based on gold nanoparticles (AuNPs), have been increasingly developed due to the flourish of nanotechnology in the last decade. However, the limitation on either selectivity or stability hindered their practical applications. Herein, by taking advantage of the unique optical properties of AuNPs and the versatility of M13 phages, a novel Hg²⁺ sensing strategy is proposed. AuNPs grew in situ on the surface of Hg²⁺-binding M13 phages at room temperature and the resulting AuNP-phage networks were directly used for mercury sensing. Hg²⁺ was selectively captured by M13 phages indwelling in the networks and gathered around AuNPs, followed by the reduction into Hg(0) and deposition on the AuNP surfaces, wherein it resulted in a blue shift of the SPR band of AuNPs and an increase in the absorbance. An LOD of 8 × 10^-8 mol L^-1 was achieved based on the quantification of the absorption ratio of AuNPs at 525 and 650 nm. As the Hg²⁺ recognition was double guaranteed by the capture of Hg²⁺-binding phages as well as the unique affinity between mercury and gold, the sensing system showed a high selectivity and a superior interference tolerance capability, facilitating its practical applications in environmental water bodies without deterioration of the sensing performance.

A review: Aptamer-based analytical strategies using the nanomaterials for environmental and human monitoring of toxic heavy metals

Recent developments in biotechnology offer the new methods for the sensitive detection of heavy metals based on the affinity and specificity of aptamers, as nucleic acid ligands selected from random sequence pools in vitro. Heavy metals have received considerable importance as the most toxic metallic pollutants which may cause serious environmental damages. They are classified as trace elements because of their presence in trace concentrations in various environmental matrices. Thus, the precise and sensitive methods to detect heavy metals are important to ensure human and environment safety. Aptamers as the biological probes, show high binding affinity which can often be directly translated into high detection sensitivity. On the other hand, high selectivity and stability make them possible to detect a wide range of targets, especially metallic ions. This review provides current progress of aptamers for environmental and biological monitoring of heavy metals using the nanomaterials mainly in two groups: (i) aptamer based biosensors (aptasensors) and (ii) aptamer based biosorbents (aptasorbents). The introduction of nanomaterials can efficiently increase the immobilization quantity of aptamers. Furthermore, they play an important role in the orientation and assembly density controlling of aptamers for the optimized recognition ability.

A label-free and universal platform for antibiotics detection based on microchip electrophoresis using aptamer probes

A novel label-free, universal, and high throughput aptasensor was developed based on a microchip electrophoresis (MCE) platform for automatic detection of antibiotic residues in food. Firstly, chloramphenicol (CAP) was employed as a model to be captured by its aptamer probe (Apt). Then, the partial complementary oligonucleotide of CAP's aptamer (C-DNA) was introduced into the reaction system. Because the Apt-CAP complex can't further hybrid with free C-DNA, the amount of hybrid Apt-C-DNA double strand DNA (dsDNA) was less than that without adding the target. Finally, the above mixture was introduced into the microchip electrophoresis (MCE) platform for detection, both dsDNA and Apt-CAP can be separated and produce different fluorescence signals in the MCE. In a certain concentration range, the ratio of signal between dsDNA and Apt-CAP (I_dsDNA/I _Apt-CAP) was proportional to the concentration of targets. Under the optimum conditions, the ratio showed a satisfactory linearity range from 0.008 to 1ng/mL of CAP with a detection limit of 0.003ng/mL. Thus, a universal MCE-based assay was developed for quantifying CAP automatically. The method was also successfully applied in the different food samples for CAP detection, which showed a good recovery (Milk: 91.1-108%, Fish: 86.1-114%) and the results were consistent with that of ELISA. This method owned many merits as follows: firstly, MCE was a high throughput screening platform and the detection time is limited to 3min for each sample. Secondly, the aptamer probes can be directly used for detection without labeling any signal tag which can facilitate the preparation procedures of probes. Thirdly, the operation was easy just by the following steps: firstly, the mixture of aptamer probes were incubated followed adding C-DNA; then measurement was performed. Moreover, the assay with MCE platform can be used to detect other targets just by changing the corresponding aptamer probe; it can even realize simultaneous detection when the targets have aptamers with different number of base pairs. Above all, it's a high- throughput and prospective method which can be applied in high throughput screening of antibiotics in food safety.

Utilization of unmodified gold nanoparticles for label-free detection of mercury (II): Insight into rational design of mercury-specific oligonucleotides

Colorimetric detection of mercury (II) with the use of DNA oligonucleotides and unmodified gold nanoparticles (AuNPs) as indicators has been extensively studied. This study provides in-depth insights into the rational design of mercury-specific oligonucleotides (MSO) in the biosensing system. The leftover bases of MSO, as a result of the formation of T-Hg²⁺-T base pairs, can adsorb on the AuNPs and hinder their aggregation at concentrations of salt. This phenomenon was directly verified by the changes in particle sizes characterized by dynamic light scattering for the first time. Based on these findings, we proposed a rational design for the MSO with approximately 20-fold improvement in detection sensitivity. The detection limit of the proposed assay decreased to 15nM with a linear working range from 50nM to 300nM for Hg²⁺. The cross-reactivity against eight other metal ions was negligible compared with the response to Hg²⁺. Considering the diverse applications of AuNPs with oligonucleotides, this study can serve as a good reference and provides important implications in sensing and DNA-directed nanoparticle assembly.

Colorimetric detection of streptomycin in milk based on peroxidase-mimicking catalytic activity of gold nanoparticles

A novel and effective colorimetric aptasensor based on enhanced peroxidase-mimicking catalytic ability of AuNPs was proposed for streptomycin detection.

A Portable Smart-Phone Readout Device for the Detection of Mercury Contamination Based on an Aptamer-Assay Nanosensor

The detection of environmental mercury (Hg) contamination requires complex and expensive instruments and professional technicians. We present a simple, sensitive, and portable Hg²⁺ detection system based on a smartphone and colorimetric aptamer nanosensor. A smartphone equipped with a light meter app was used to detect, record, and process signals from a smartphone-based microwell reader (MR S-phone), which is composed of a simple light source and a miniaturized assay platform. The colorimetric readout of the aptamer nanosensor is based on a specific interaction between the selected aptamer and Hg²⁺, which leads to a color change in the reaction solution due to an aggregation of gold nanoparticles (AuNPs). The MR S-phone-based AuNPs-aptamer colorimetric sensor system could reliably detect Hg²⁺ in both tap water and Pearl River water samples and produced a linear colorimetric readout of Hg²⁺ concentration in the range of 1 ng/mL-32 ng/mL with a correlation of 0.991, and a limit of detection (LOD) of 0.28 ng/mL for Hg²⁺. The detection could be quickly completed in only 20 min. Our novel mercury detection assay is simple, rapid, and sensitive, and it provides new strategies for the on-site detection of mercury contamination in any environment.

Mercury detection based on label-free and isothermal enzyme-free amplified fluorescence platform

A novel and convenient biosensor for Mercury (II) detection was developed based on toehold-mediated strand displacement isothermal enzyme-free amplification (EFA) technology and label-free fluorescence platform using Sybr Green Ι (SG) and graphene oxide (GO). The method is highly sensitive and selective, and the logarithmically related Hg²⁺ linearity range is from 0.1 to 50nM with a detection limit 0.091nM. Moreover, our strategy is simple, enzyme-free, and inexpensive and can be applied to detect spiked Hg²⁺ in environmental water samples with good recovery and accuracy, which demonstrates that the biosensor has a good potential in the environment surveys in the future.

Simple and rapid chemiluminescence aptasensor for Hg2+ in contaminated samples: A new signal amplification mechanism

Detection of ultralow concentration of heavy metal ion Hg²⁺ is important for human health protection and environment monitoring because of the gradual accumulation in environmental and biological fields. Herein, we report a convenient chemiluminescence (CL) biosensing platform for ultrasensitive Hg²⁺ detection by signal amplification mechanism from positively charged gold nanoparticles ((+)AuNPs). It is based on (+)AuNPs charge effect and aptamer conformation change induced by target to stimulate the generation of CL in the presence of H₂O₂ and luminol without high salt medium. Notably particularly, the typical problem of the high salt medium from (-) AuNPs system, like influencing aptamers' bind with target and hindering CL reaction can be effectively addressed through the direct introduction of (+)AuNPs. Therefore, the proposed biosensing exhibits a high sensitivity toward target Hg²⁺ with a detection limit of 16 pM, which is far below the limit (10nM) defined by the U.S. Environmental Protection Agency in drinkable water, and is about 10-fold lower than the previously reported aptamer-based assays for Hg²⁺. This sensing platform provides a simple, rapid, and cost-effective approach for label-free sensitive detection of Hg²⁺. Moreover, it is universal for the detection of other targets. Undoubtedly, such a direct utilizing of (+)AuNPs' charge effect will provide a new signal amplification way for label-free aptamer-based CL analysis.

Acousto-microfluidics for screening of ssDNA aptamer

We demonstrate a new screening method for obtaining a prostate-specific antigen (PSA) binding aptamer based on an acoustofluidic separation (acoustophoreis) technique. Since acoustophoresis provides simultaneous washing and separation in a continuous flow mode, we efficiently obtained a PSA binding aptamer that shows high affinity without any additional washing step, which is necessary in other screening methods. In addition, next-generation sequencing (NGS) was applied to accelerate the identification of the screened ssDNA pool, improving the selecting process of the aptamer candidate based on the frequency ranking of the sequences. After the 8^th round of the acoustophoretic systematic evolution of ligands by exponential enrichment (SELEX) and following sequence analysis with NGS, 7 PSA binding ssDNA aptamer-candidates were obtained and characterized with surface plasmon resonance (SPR) for affinity and specificity. As a result of the new SELEX method with PSA as the model target protein, the best PSA binding aptamer showed specific binding to PSA with a dissociation constant (K_d) of 0.7 nM.

Aptamer-aptamer linkage based aptasensor for highly enhanced detection of small molecules

The multi-target colorimetric aptasensors can be easily fabricated by using two different aptamer sequences. However, there have been no research reports about improvement or enhancing of colorimetric signals based on the aggregation properties of AuNPs. Herein, we report a simple and efficient method to control and enhance the function of the multi-target aptasensor using an aptamer-aptamer linkage method. The aptasensor was developed for highly sensitive multiple-target detection of small molecules. The extension of aptamer DNA sequences using this method resulted in the enhanced analytical sensitivity of this aptasensor in sensing applications for two small molecule targets. Furthermore, the mechanism of the interaction between DNA aptamer and AuNPs was studied by measuring the zeta potential to explain the enhancement of the sensitivity of this multi-target aptasensor. The limit of detection of this multi-target aptasensor was found to be 1 nM and 37 nM for kanamycin (KAN) and chlortetracycline (CHLOR), respectively. It is 25-fold lower than in the previous report using an AuNP-based sensor for defining the limit of detection (LOD) of KAN and five times lower than the LOD for CHLOR. This aptasensor has great potential in the simultaneous detection of a wide range of KAN and CHLOR concentrations.

A simple and rapid chemiluminescence aptasensor for acetamiprid in contaminated samples: Sensitivity, selectivity and mechanism

Ultralow concentration and selective detection of pesticide residue is important to evaluate the environmental and biological pollution and the threat to human health which single component pesticide can bring. Herein, we report an amplified chemiluminescence (CL) sensing platform for ultrasensitive and selective acetamiprid (widely used pesticide) detection. It is based on aptamer's high binding affinity to target and the relevance between AuNPs' morphology and its catalytic effect to stimulate the generation of CL in the presence of H2O2 and luminol. Moreover, AuNPs morphological slight change induced by aptamers' conformation during targets binding could lead to the significant change of catalytic properties. Therefore, the proposed sensing platform for pesticide residue exhibited a high sensitivity toward acetamiprid with a detection limit of 62pM, which was about 100-fold lower than that of other aptamer-based sensor for acetamiprid detection. Because of the intrinsic specificity of aptamer's recognization, this sensing platform has high selectivity. So, this sensing platform provides a label-free and cost-effective approach for sensitive and selective detection of single component pesticide residue. More importantly, this CL method was successfully used to determine acetamiprid in real contaminated samples.

Colorimetric determination of melamine in milk using unmodified silver nanoparticles

Melamine is nitrogen rich chemical compound used as an adulterant in dairy products by unscrupulous people to increase the apparent protein content. This incident prompted the researchers to develop simple methods for easy detection of melamine in food samples. In the present paper, we report a simple and sensitive colorimetric method for detection of melamine in milk based on silver nanoparticles. This method relies upon the principle that melamine causes the aggregation of silver nanoparticles, resulting in abrupt color change from yellow to red under optimized conditions. The concentration of melamine in adulterated sample can be quantitated by monitoring the absorption spectra of silver nanoparticles using ultraviolet-visible (UV-Vis) spectrometer. The present colorimetric method which utilizes silver nanoparticles of 35 nm can reliably detect melamine down to a concentration of 0.04 mg l(-1).

Colorimetric detection based on localised surface plasmon resonance of gold nanoparticles: Merits, inherent shortcomings and future prospects

Localised surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been exploited for two decades in analytical science and has proven to be a powerful tool for the detection of various kinds of substances including small molecules, ions, macro biomolecules and microbes. Detection can be performed by visual colour change observations, photometry or resonance light scattering. A wide range of applications have been studied in the areas of environmental, pharmaceutical and biological analysis and clinical diagnosis. In this article, some fundamental aspects and important applications involving LSPR of AuNPs are reviewed. Several inherent shortcomings of these techniques and possible strategies to circumvent them are discussed.

A novel aptasensor based on single-molecule force spectroscopy for highly sensitive detection of mercury ions

We have developed a novel aptasensor based on single-molecule force spectroscopy (SMFS) capable of detecting mercury ions (Hg(2+)) with sub-nM sensitivity. The single-strand (ss) DNA aptamer used in this work is rich in thymine (T) and readily forms T-Hg(2+)-T complexes in the presence of Hg(2+). The aptamer was conjugated to an atomic force microscope (AFM) probe, and the adhesion force between the probe and a flat graphite surface was measured by single-molecule force spectroscopy (SMFS). The presence of Hg(2+) ions above a concentration threshold corresponding to the affinity constant of the ions for the aptamer (about 5 × 10(9) M(-1)) could be easily detected by a change of the measured adhesion force. With our chosen aptamer, we could reach an Hg(2+) detection limit of 100 pM, which is well below the maximum allowable level of Hg(2+) in drinking water. In addition, this aptasensor presents a very high selectivity for Hg(2+) over other metal cations, such as K(+), Ca(2+), Zn(2+), Fe(2+), and Cd(2+). Furthermore, the effects of the ionic strength and loading rate on the Hg(2+) detection were evaluated. Its simplicity, reproducibility, high selectivity and sensitivity make our SMFS-based aptasensor advantageous with respect to other current Hg(2+) sensing methods. It is expected that our strategy can be exploited for monitoring the pollution of water environments and the safety of potentially contaminated food.

Platinum(II)-Oligonucleotide Coordination Based Aptasensor for Simple and Selective Detection of Platinum Compounds

Wide use of platinum-based chemotherapeutic regimens for the treatment for carcinoma calls for a simple and selective detection of platinum compound in biological samples. On the basis of the platinum(II)-base pair coordination, a novel type of aptameric platform for platinum detection has been introduced. This chemiluminescence (CL) aptasensor consists of a designed streptavidin (SA) aptamer sequence in which several base pairs were replaced by G-G mismatches. Only in the presence of platinum, coordination occurs between the platinum and G-G base pairs as opposed to the hydrogen-bonded G-C base pairs, which leads to SA aptamer sequence activation, resulting in their binding to SA coated magnetic beads. These Pt-DNA coordination events were monitored by a simple and direct luminol-peroxide CL reaction through horseradish peroxidase (HRP) catalysis with a strong chemiluminescence emission. The validated ranges of quantification were 0.12-240 μM with a limit of detection of 60 nM and selectivity over other metal ions. This assay was also successfully used in urine sample determination. It will be a promising candidate for the detection of platinum in biomedical and environmental samples.

Functional nucleic acid-based sensors for heavy metal ion assays

Heavy metal contaminants such as lead ions (Pb(2+)), mercury ions (Hg(2+)) and silver ions (Ag(+)) can cause significant harm to humans and generate enduring bioaccumulation in ecological systems. Even though a variety of methods have been developed for Pb(2+), Hg(2+) and Ag(+) assays, most of them are usually laborious and time-consuming with poor sensitivity. Due to their unique advantages of excellent catalytic properties and high affinity for heavy metal ions, functional nucleic acids such as DNAzymes and aptamers show great promise in the development of novel sensors for heavy metal ion assays. In this review, we summarize the development of functional nucleic acid-based sensors for the detection of Pb(2+), Hg(2+) and Ag(+), and especially focus on two categories including the direct assay and the amplification-based assay. We highlight the emerging trends in the development of sensitive and selective sensors for heavy metal ion assays as well.

Recent advances in nanoparticle based aptasensors for food contaminants

Food safety and hazard analysis is a prime concern of human life, thus quality assessment of food and water is the need of the day. Recent advances in nano-biotechnology play a significant role in providing possible solutions for developing highly sensitive and affordable detection tools for food analysis. Nanomaterials based aptasensors hold great potential to overcome the drawbacks of conventional analytical techniques. Aptamers comprise a novel class of highly specific bio-recognition elements which are produced by SELEX (systematic evolution of ligands by exponential enrichment) process. They bind to target molecules by folding into 3D structures that can discriminate different chiral compounds. The flexibility in making modifications in aptamers contribute to the design of biosensors, enabling the generation of bio-recognition elements for a wide variety of target molecules. Nanomaterials such as metal nanoparticles, metal nanoclusters, metal oxide nanoparticles, metal and carbon quantum dots, graphene, carbon nanotubes and nanocomposites enable higher sensitivity by signal amplification and introduce several novel transduction principles such as enhanced chemiluminescence, fluorescence, Raman signals, electrochemical signals, enhanced catalytic activity, and super-paramagnetic properties to the biosensor. Although there are a few reviews published recently which deal with the potential of aptamers in various fields, none are devoted exclusively to the potential of aptasensors based on nanomaterials for the analysis of food contaminants. Hence, the current review discusses several transduction systems and their principles used in aptamer based nanosensors which have been developed in the past five years, the challenges faced in their designing, along with their strengths and limitations.

A structure-switchable aptasensor for aflatoxin B1 detection based on assembly of an aptamer/split DNAzyme

An ultrasensitive, colorimetric and homogeneous strategy for aflatoxin B1 (AFB1) detection, which uses a DNA aptamer and two split DNAzyme halves, has been developed. Split halves of a hemin-binding DNAzymes is combined with an AFB1 aptamer to generate a homogeneous colorimetric sensor that undergoes an AFB1 induced DNA structural change. In the absence of AFB1, the split probes have peroxidase mimicking DNAzyme activity associated with catalysis of a color change reaction. Specific recognition of AFB1 by the aptamer component leads to structural deformation of the aptamer-DNAzyme complex, which causes splitting of the DNAzyme halves and a reduction in peroxidase mimicking activity. Therefore, a decrease of colorimetric signal arising from the catalytic process takes place upon in the presence of AFB1 in a concentration dependent manner in the 0.1-1.0 × 10(4) ng/mL range and with a colorimetric detection limit of 0.1 ng/mL. The new assay system exhibits high selectivity for AFB1 over other mycotoxins and can be employed detect the presence of AFB1 in ground corn samples. Overall, the strategy should serve as the basis for the development of rapid, simple and low-cost methods for detection of mycotoxins.

Label-free Protein Detection Based on the Heat-Transfer Method--A Case Study with the Peanut Allergen Ara h 1 and Aptamer-Based Synthetic Receptors

Aptamers are an emerging class of molecules that, because of the development of the systematic evolution of ligands by exponential enrichment (SELEX) process, can recognize virtually every target ranging from ions, to proteins, and even whole cells. Although there are many techniques capable of detecting template molecules with aptamer-based systems with high specificity and selectivity, they lack the possibility of integrating them into a compact and portable biosensor setup. Therefore, we will present the heat-transfer method (HTM) as an interesting alternative because this offers detection in a fast and low-cost manner and has the possibility of performing experiments with a fully integrated device. This concept has been demonstrated for a variety of applications including DNA mutation analysis and screening of cancer cells. To the best our knowledge, this is the first report on HTM-based detection of proteins, in this case specifically with aptamer-type receptors. For proof-of-principle purposes, measurements will be performed with the peanut allergen Ara h 1 and results indicate detection limits in the lower nanomolar regime in buffer liquid. As a first proof-of-application, spiked Ara h 1 solutions will be studied in a food matrix of dissolved peanut butter. Reference experiments with the quartz-crystal microbalance will allow for an estimate of the areal density of aptamer molecules on the sensor-chip surface.

Selective colorimetric sensors based on the monitoring of an unmodified silver nanoparticles (AgNPs) reduction for a simple and rapid determination of mercury

In this work, selective colorimetric sensors for simple and rapid detection of Hg(II) ions based on the monitoring of an unmodified silver nanoparticles (AgNPs) reduction were developed. The average diameter of synthesized AgNPs was 8.3±1.4nm which was characterized by transmission electron microscopy (TEM). The abrupt change in absorbance of the unmodified AgNPs was observed which progressively decreased and slightly shifted to the blue wavelength as the concentration of Hg(II) increased, indicating the oxidation of Ag(0) to Ag(I) occurred. It appears that the AgNPs were oxidized by Hg(II), resulting in disintegration of the AgNPs into smaller particles as well as mediating the reduction of Hg(II) to Hg(0) adsorbed onto the surface of AgNPs. The adsorption of Hg(0) resulted in the lack of sufficient charges on AgNPs surfaces due to the decrease in the surface coverage of negatively charged citrate molecules, which then leaded to enlargement of AgNPs. The calibration curve of this technique was demonstrated from 0.5 to 7ppm (r(2)=0.995), the limit of detection (LOD) was 0.06ppm (SDblank/slope of calibration curve) with the precision (RSD, n=4) of 3.24-4.53. Interestingly, the results show a significant enhance in the Hg(II) analytical sensitivity when Cu(II) is doped onto the unmodified AgNPs, which improves the quantitative detection limit to 0.008ppm. In addition, greater selectivity toward Hg(II) compared with the other metal ions tested was observed. Furthermore, the percentage recoveries of spiked drinking water, tap water and SRM1641d (mercury in water) were in acceptable range with a good precision (RSD) which were in agreement with the values obtained from graphite furnace atomic absorption spectrometer (GFAAS). The technique proposed in this study provides a rapid, simple, sensitive and selective detection method for Hg(II) in water samples.

Sensitive colorimetric detection of cyromazine in cucumber samples by using label-free gold nanoparticles and polythymine

Cyromazine (CYR) can cause serious damage to the organs of animals or human beings, and it was found to bind to polythymine (polyT10) via multiple hydrogen bonding interactions. Based on this novel finding, a highly sensitive and simple colorimetric method was developed for CYR detection by using label-free gold nanoparticles (AuNPs) and polyT10. Under the optimized conditions, excellent linearity was acquired for CYR within the range of 1-500 ng mL(-1). In addition, the spectra and color changes of the AuNP solution were measured by spectrophotometry and observed by the naked eye, and the results showed that as low as 1 and 5 ng mL(-1) of CYR could be detected, depending upon the measurement methods. Afterwards, cucumber was selected to investigate the sample matrix effect and a sample pretreatment procedure was developed with simple homogenization and filtration. Even after 200 times dilution, the limit of detection (LOD) and limit of quantitation (LOQ) reached 252 ng g(-1) and 500 ng g(-1), respectively. The LOD and LOQ satisfied the Chinese requirement for the maximum residue limit (MRL), which is 0.5-1 μg g(-1) of CYR in most vegetables. The assay also showed a good average recovery of 83.7-104.8% with the RSD of less than 7% and good selectivity for cyromazine over other pesticides that may exist in vegetable samples. The method proposed in this study was simple, fast, and highly sensitive and accurate, and the test result with this method was visible to the naked eye. Therefore, it could be used for routine determination of CYR residues in cucumber samples.

Simplified aptamer-based colorimetric method using unmodified gold nanoparticles for the detection of carcinoma embryonic antigen

A new colorimetric aptasensor for the detection of CEA was developed. Aptamer-based colorimetric method with nanoparticles was used for the detection of CEA. The colorimetric aptasensor has potential for the detection of other proteins or nucleotides.

Aptamer and its potential applications for food safety

Accompanied by industrial globalization, rapid urbanization, and population increment, mass production and staple trading for food consumption are upsoaring continuously, foodborne disease resulted from various food safety issues is currently a crucial public health concern worldwide, which has not only created a great burden on both economy and society, but also greatly threatened the sustainability of mankind's livelihood and human reproduction. In order to better ensure food safety and thus effectively curb the occurrence of foodborne diseases, the development and evolving of inspection strategies are indispensable measures for quality assurance and conformity assessment. Nowadays, as complementary measures to and with advantageous merits over classic analytical methods, highly specific and selective aptamer-based assays have found their increasingly important roles in various domains of food analysis. This critical review summarizes the advantages of aptamer as compared with antibody, introduces important evolving variants of systematic evolution of ligands by exponential enrichment (SELEX), and presents an overview of potential aptamer applications for food safety.

A facile label-free colorimetric aptasensor for acetamiprid based on the peroxidase-like activity of hemin-functionalized reduced graphene oxide

A facile aptasensor has been developed for the colorimetric detection of acetamiprid by using the hemin-functionalized reduced graphene oxide (hemin-rGO) composites. The as-prepared hemin-rGO composites possessed both the ability of rGO to physically adsorb the aptamers and the peroxidase-like activity of hemin that could catalyse 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2, to produce a solution with blue color. The well-dispersed hemin-rGO composites coagulated completely at the proper salt concentration; however, the coagulation of hemin-rGO was vanished when abundant aptamers were adsorbed on its surface because the attached negatively charged DNA backbone increased individual hemin-rGO electrostatic repulsion. In the detection scheme, acetamiprid with different concentrations was firstly incubated with the same amount of aptamer. The more acetamiprid in the tested solution, the less free aptamers were absorbed on the hemin-rGO surface, making the composites coagulate to a higher degree in the presence of the optimum NaCl concentration. As a consequence, the content of hemin-rGO in the supernatant was decreased after centrifugation, which catalysed oxidation of TMB in the presence of H2O2 to produce light blue color with a low absorbance. The color variation relavant to the acetamiprid concentration can be judged by the naked eyes and easily monitored by the inexpensive UV-vis spectrometer. Such designed aptasensor displayed a linear response for acetamiprid in the range from 100nM to 10μM with a detection limit of 40nM (S/N=3). This colorimetric aptasensing platform offers great advantages including the simple operation process, low-cost portable instrument, and user-friendly applications.