Voltammetric determination of lead(II) by using exonuclease III and gold nanoparticles, and by exploiting the conformational change of the complementary strand of an aptamer

A Novel Aptamer-Imprinted Polymer-Based Electrochemical Biosensor for the Detection of Lead in Aquatic Products

Lead contamination in aquatic products is one of the main hazard factors. The aptasensor is a promising detection method for lead ion (Pb(II)) because of its selectivity, but it is easily affected by pH. The combination of ion-imprinted polymers(IIP) with aptamers may improve their stability in different pH conditions. This paper developed a novel electrochemical biosensor for Pb(II) detection by using aptamer-imprinted polymer as a recognition element. The glassy carbon electrode was modified with gold nanoparticles and aptamers. After the aptamer was induced by Pb(II) to form a G-quadruplex conformation, a chitosan-graphene oxide was electrodeposited and cross-linked with glutaraldehyde to form an imprint layer, improving the stability of the biosensor. Under the optimal experimental conditions, the current signal change (∆I) showed a linear correlation of the content of Pb(II) in the range of 0.1-2.0 μg/mL with a detection limit of 0.0796 μg/mL (S/N = 3). The biosensor also exhibited high selectivity for the determination of Pb(II) in the presence of other interfering metal ion. At the same time, the stability of the imprinted layer made the sensor applicable to the detection environment with a pH of 6.4-8.0. Moreover, the sensor was successfully applied to the detection of Pb(II) in mantis shrimp.

Recent Achievements in Electrochemical and Optical Nucleic Acids Based Detection of Metal Ions

Recently nucleic acids gained considerable attention as selective receptors of metal ions. This is because of the possibility of adjusting their sequences in new aptamers selection, as well as the convenience of elaborating new detection mechanisms. Such a flexibility allows for easy utilization of newly emerging nanomaterials for the development of detection devices. This, in turn, can significantly increase, e.g., analytical signal intensity, both optical and electrochemical, and the same can allow for obtaining exceptionally low detection limits and fast biosensor responses. All these properties, together with low power consumption, make nucleic acids biosensors perfect candidates as detection elements of fully automatic portable microfluidic devices. This review provides current progress in nucleic acids application in monitoring environmentally and clinically important metal ions in the electrochemical or optical manner. In addition, several examples of such biosensor applications in portable microfluidic devices are shown.

An overview of Structured Biosensors for Metal Ions Determination

The determination of metal ions is important for nutritional and toxicological assessment. Atomic spectrometric techniques are highly efficient for the determination of these species, but the high costs of acquisition and maintenance hinder the application of these techniques. Inexpensive alternatives for metallic element determination are based on dedicated biosensors. These devices mimic biological systems and convert biochemical processes into physical outputs and can be used for the sensitive and selective determination of chemical species such as cations. In this work, an overview of the proposed biosensors for metal ions determination was carried out considering the last 15 years of publications. Statistical data on the applications, response mechanisms, instrumentation designs, applications of nanomaterials, and multielement analysis are herein discussed.

An Electrochemical Aptasensor for Pb2+ Detection Based on Metal-Organic-Framework-Derived Hybrid Carbon

A new double-shelled carbon nanocages material was synthesized and developed an aptasensor for determining Pb²⁺ in aqueous solution. Herein, nanoporous carbon materials derived from core–shell zeolitic imidazolate frameworks (ZIFs) demonstrated excellent electrochemical activity, stability, and high specificity surface area, consequently resulting in the strong binding with aptamers. The aptamer strands would be induced to form G-quadruplex structure when Pb²⁺ was introduced. Under optimal conditions, the aptasensor exhibited a good linear relationship of Pb²⁺ concentration ranging from 0.1 to 10 μg L⁻¹ with the detection limits of 0.096 μg L⁻¹. The feasibility was proved by detecting Pb²⁺ in spiked water samples and polluted soil digestion solution. The proposed aptasensor showed excellent selectivity and reproducibility, indicating promising applications in environmental monitoring.

A label-free aptasensor for turn-on fluorescent detection of ochratoxin A based on aggregation-induced emission probe

A novel label-free fluorescence aptasensor used for the detection of ochratoxin A (OTA) is presented in this study. When aggregated on the surface of DNA aptamer, aggregation-induced emission (AIE) fluorescence probe presents turn-on fluorescence property. The method proposed in this article was based on an AIE probe, 4, 4-(1E,1E)-2, 2-(anthracene-9, 10-diyl) bis (ethene-2, 1-diyl) bis (N, N, N-trimethylbenzenaminium iodide) (DSAI). With OTA present, the aptamer will combine with OTA and the conformation of the aptamer will switch to an antiparallel G-quadruplex from the initial random coil, which obstructs its digestion by Exo I. After the solution is added with DSAI, DSAI will aggregate on the surface of the aptamer/OTA complex and produces a strong emission. In the range of 5 to 500 ng · ml^-1 OTA concentrations, the fluorescence increases with a linear logarithm relationship. The detection limit is 1.9 ng · ml^-1. This method was used to detect OTA in spiked real samples, with recoveries and RSDs in the range of 92.2% to 106.3%, and 2.7% to 5.2%, respectively.

Recent advances in the development of electrochemical aptasensors for detection of heavy metals in food

Heavy metal contamination in environment and food has attracted intensive attention from the public since it poses serious threats to ecological system and human health. Traditional detection methods for heavy metals such as atomic absorption spectrometry have a fairly low detection limit, but the methods have many limitations and disadvantages. Therefore, it is of significance to develop a rapid technology for real-time and online detection of heavy metals. The electrochemical aptasensor-based technology is promising in the detection of heavy metals with advantages of high sensitivity, specificity, and accuracy. Although its development is rapid, more researches should be carried out before this technology can be used for on-site detection. In this review, the origin, basic principles and development of electrochemical aptasensors are introduced. The applications of nanomaterials and electrochemical aptasensors for the detection of heavy metals (mainly mercury, lead, cadmium, and arsenic) are summarized. The research and application tendency of electrochemical aptasensors for detection of heavy metals are prospected.

An electrochemical aptasensor based on gold@polypyrrole composites for detection of lead ions

This work describes an electrochemical aptasensor for determination of lead ions (Pb²⁺). Composites prepared from gold nanoparticles and polypyrrole (Au@PPy) with good electrical conductivity were used to modify the surface of a screen printed carbon electrode for amplifying the current signal. Single strand DNA was immobilized on the electrode and binds lead(II) as confirmed by cyclic voltammetry at voltage of -0.2 V~0.6 V. Differential pulse voltammetry, measured at 0.10 V (vs.  Ag/AgCl), was used to monitor the interaction between aptamer and lead(II) using hexacyanoferrate as an electrochemical probe. In the presence of Pb²⁺, the aptamer forms a G-quadruplex, and the peak current is increased. By this method, Pb²⁺ can be detected in the range of 0.5-10 nM with a low detection limit of 0.36 nM. The aptasensor was successfully applied to the determination of Pb²⁺ in polluted soil and baby's nail. The method showed outstanding sensitivity and selectivity in detecting Pb²⁺, therefore is considered to have great potential in developing an environmental monitoring platform. Graphical abstract Schematic illustration of Pb²⁺ detection procedure and principle using an electrochemical aptasensor.

Triple-helix molecular switch-based aptasensors and DNA sensors

Utilization of traditional analytical techniques is limited because they are generally time-consuming and require high consumption of reagents, complicated sample preparation and expensive equipment. Therefore, it is of great interest to achieve sensitive, rapid and simple detection methods. It is believed that nucleic acids assays, especially aptamers, are very important in modern life sciences for target detection and biological analysis. Aptamers and DNA-based sensors have been widely used for the design of various sensors owing to their unique features. In recent years, triple-helix molecular switch (THMS)-based aptasensors and DNA sensors have been broadly utilized for the detection and analysis of different targets. The THMS relies on the formation of DNA triplex via Watson-Crick and Hoogsteen base pairings under optimal conditions. This review focuses on recent progresses in the development and applications of electrochemical, colorimetric, fluorescence and SERS aptasensors and DNA sensors, which are based on THMS. Also, the advantages and drawbacks of these methods are discussed.

Cu x O@DNA sphere-based electrochemical bioassay for sensitive detection of Pb2

The paper describes a one-step synthetic method to chemically reduce cupric sulfate by ascorbic acid in the presence of DNA strands to directly produce Cu _x O@DNA spheres. The DNA strands act as template to assist the preparation of Cu _x O, and also are capable of specifically binding  Pb(II) ions. The Cu _x O@DNA spheres possess high specific surface area and strong bioaffinity. They can be directly employed as platform for detecting Pb²⁺ sensitively. Electrochemical impedance spectroscopy data showed that the assay exhibits high sensitivity and a wide linear analytical range that extends from 0.1 to 100 nM, and the detection limit is 6.8 pM at a signal-to-noise ratio of 3. The assay is selective, acceptably reproducible, stable, and well feasible for the detection of Pb²⁺ in blood serum. Graphical abstract Schematic presentation of the preparation of DNA-templated Cu _x O spheres (Cu _x O@DNA) for use in electrochemical detection of Pb²⁺. The assay exhibits detection limit of 6.8 pM, high selectivity, acceptable reproducibility, stability, and good applicability for Pb²⁺ detection.

An aptamer-based colorimetric lead(II) assay based on the use of gold nanoparticles modified with dsDNA and exonuclease I

The authors describe a colorimetric method for the sensitive and selective detection of Pb(II). It is based on the use exonuclease I (Exo I), a Pb(II)-binding aptamer bound to gold nanoparticles (AuNPs), and a DNA strand that complementary to the aptamer. In the absence of Pb(II), the dsDNA on the AuNPs prevents aggregation of the AuNPs in the presence of NaCl. In the presence of Pb(II), however, the aptamer binds Pb(II) and complementary strand is released and digested by Exo I. As a result, the solution of AuNPs undergoes a color change from red to purple if salt is added to the sample. The assay is selective for Pb(II) and has a limit of detection as low as 2.4 nM. It was successfully applied to the determination of Pb(II) in spiked tap water. Graphical abstract Schematic presentation of the aptamer based method for Pb²⁺ detection via salt-induced aggregation of gold nanoparticles and colorimetric quantitation.

Fluorometric aptamer assay for ochratoxin A based on the use of single walled carbon nanohorns and exonuclease III-aided amplification

The authors describe an aptamer based assay for the food mycotoxin ochratoxin A (OTA). It is based on the use of exonuclease III (Exo III) which assists in signal amplification, and of single-walled carbon nanohorns (SWCNHs) which act as quenchers of fluorescence. The detection scheme employs a hairpin probe (HP) and a signal probe (SP) labeled with carboxyfluorescein (FAM) at its 5'-end. The fluorescence of intact SPs (best measured at excitation/emission wavelengths of 495/518 nm) is quenched by SWCNHs. The HP contains the OTA-specific aptamer sequence and is partially complementary to the SP. After addition of OTA, the aptamer binds OTA and thus exposes a single-stranded sequence that can hybridize with the SP. Exo III digests the SP to liberate the free fluorophore labels. The damaged SPs no longer are adsorbed by the SWCNHs so that fluorescence is no longer quenched. The method has a detection range that is linear from 10 nM to 1000 nM (with a correlation coefficient of 0.997). The limit of detection (LOD), calculated on the basis of a signal to noise ratio of 3, is 4.2 nM. The procedure was validated by the quantitation of OTA in spiked real samples and were found to be free of interference by the sample matrix. Recoveries ranged from 93.8 to 113.0% in beer and from 92.0 to115.9% in red wine. Graphical abstract After adding ochratoxin A (OTA), the aptamer region in hairpin probe (HP) combined with OTA and thus exposed a single-stranded sequence to hybridize with signal probe (SP). Exonuclease III (Exo III) digested SP to liberate the free fluorophore (FAM).