Electrochemiluminescent lead biosensor based on GR-5 lead-dependent DNAzyme for Ru(phen)3(2+) intercalation and lead recognition

Nanohybrid of antimonene@Ti3C2Tx-based electrochemical aptasensor for lead detection

Lead ions (Pb2+), as one of many common heavy metallic environmental pollutants, can cause serious side-effects and result in chronic poisoning to people's health, so it is highly significant to monitor Pb2+ efficiently and sensitively. Here, we proposed an antimonene@Ti3C2Tx nanohybrid-based electrochemical aptamer sensor (aptasensor) for high sensitive Pb2+ determination. The sensing platform of nanohybrid was synthesized by ultrasonication, possessing the advantages of both antimonene and Ti3C2Tx, which not only can vastly enlarge the sensing signal of the proposed aptasensor, but also greatly simplified its manufacturing flow, because antimonene can strongly interact with aptamer through noncovalently bound. The surface morphology and microarchitecture of the nanohybrid were perused by several methods such as scanning electron microscope (SEM), energy-dispersive X-ray mapping spectroscopy (EDS), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscope (AFM). Under optimal empirical conditions, the proposed aptasensor exhibited a wide linear correlation of the current signals with the logarithm of CPb2+ (Log CPb2+) over the span from 1 × 10-12 to 1 × 10-7 M and provided a trace discernment limit of 3.3 × 10-13 M. Moreover, the constructed aptasensor displayed superior repeatability, great consistency, eminent selectivity, and beneficial reproducibility, implying its extreme potential application for water quality control and the environmental monitoring of Pb2+.

In situ synthesis of label-free electrochemical aptasensor-based sandwich-like AuNPs/PPy/Ti3C2Tx for ultrasensitive detection of lead ions as hazardous pollutants in environmental fluids

The monitoring of hazardous pollutants in environmental fluids is one of main stretaegy in investigation of water and soil quality. Metal ions are one of main and dangerius materials in water sampels and one of the main causes of environmental problems. Therefore, many of environmental researchers focused on fabrication of highly sensitive sensor to ion hazardous pollutants environmental fluids. The encapsulation of 2D MXenes with other stable materials has proven to be an effective method for enhancing their stability and electrochemical properties. In this work, a sandwich-like nanocomposite structure, AuNPs/PPy/Ti₃C₂T_x, was designed and synthesized via a facile method of one-step layer-by-layer self-assembly. The morphology and structure of the prepared nanocomposites are characterized with various methods such as scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Ti₃C₂T_x as a substrate played a significant role in the synthesis and alignment of PPy and AuNPs growth. The nanocomposites have maximized the benefits of the inorganic AuNPs and organic PPy materials, enhancing their stability and electrochemical performance. Meanwhile, AuNPs have given the nanocomposite the ability to form covalent bonds with biomaterials via the Au-S bond. Thus, a novel electrochemical aptasensor was developed based on AuNPs/PPy/Ti₃C₂T_x for the sensitive and selective detection of Pb²⁺. It demonstrated a wide linear range from 5 × 10^-14 to 1 × 10^-8 M with a low LOD of 1 × 10^-14 M (S/N = 3). Additionally, the developed aptasensor exhibited excellent selectivity and stability and successfully used to sensing of Pb²⁺ in environmental fluids such as NongFu Spring and tap water.

Porphyrin-Based Covalent Organic Framework Thin Films as Cathodic Materials for "On-Off-On" Photoelectrochemical Sensing of Lead Ions

Currently, cathodic photoelectrochemical (PEC) sensors, which could effectively reduce background interference, are urgently required for ultrasensitive environmental monitoring. Herein, porphyrin-based covalent organic framework (TAPP-COF) thin films were fabricated via a bottom-up growth approach on the liquid/liquid interface and applied as a photocathode material to "on-off-on" PEC sensing of Pb²⁺. Benefitting from the unique charge channels of COFs and the good photoelectric properties of porphyrin, the as-prepared TAPP-COF thin films presented an improved photocathodic current, with a strongly enhanced "signal-on" response with low background. Then, CdSe@SiO₂ quantum dots (QDs), as a quenching agent, were introduced through a hybridization chain reaction (HCR) to obtain a "signal off" PEC response. Afterward, with the introduction of target Pb²⁺, CdSe@SiO₂ QDs were detached from TAPP-COF thin films, and the PEC response transformed into a signal-on state. Benefiting from the multiple-quenching and steric hindrance effect of CdSe@SiO₂ QDs and the photocathodic property of TAPP-COFs, accurate monitoring of Pb²⁺ in a wide detection range from 0.05 to 1000 nM with a lower detection limit of 0.012 nM was realized based on the proposed on-off-on PEC approach. Notably, the methodology provides an efficient platform for ultrasensitive determination of heavy metal ions, which would play a significant role in environmental monitoring and public safety fields.

Modification of screen-printed gold electrode with 1,4-dithiothreitol: application to sensitive voltammetric determination of Sudan II

Objectives

The aim of this study is to investigate the electrochemical behavior of Sudan II (SuII) using a screen-printed gold electrode (SPGE) modified with 1,4-dithiothreitol (DTT) and to determine the amount of Sudan II by voltammetry.

Materials and Methods

A DTT-modified screen-printed gold electrode (DTT/SPGE) was fabricated and its application for differential pulse voltammetric (DPV) determination of SuII was reported. Fourier transform infrared spectroscopy (FT-IR), cyclic voltammetry and electrochemical impedance spectroscopy were used for the characterization of the modified electrode. The effects of instrumental and chemical parameters were optimized for the determination of SuII. The fabricated electrode was used for the analysis of SuII in fortified and real samples. High-performance liquid chromatography was preferred as a reference method for the evaluation of the obtained voltammetric results.

Results

The electrochemical studies and FT-IR demonstrated that the SPGE was modified with DTT. The obtained peak current at DTT/SPGE was 6.67 times higher than that recorded with SPGE. At the optimized conditions of DPV in pH = 2.5 of H2SO4, the oxidation peak current of SuII was proportional to its concentration in range: 0.001–1.500 μmol l–1 with a detection limit of 0.0002 μmol l–1 (S/N = 3). For the analysis of SuII, 101.67%–104.33% of recovery percentage was obtained.

Conclusions

A new electrode was successfully improved for the determination of SuII. This highly selective and sensitive electrode supplied the fast determination of SuII in ketchup, chili sauce and salsa dip sauce. In addition, voltammetric and chromatographic results are found to be consistent.

A solvent sensitive coumarin derivative coupled with gold nanoparticles as selective fluorescent sensor for Pb2+ ions in real samples

A coumarin based fluorescent molecule, 3-amino-2-cynano-3-(7-diethylamino-2-oxo-2H-chromen-3-yl)-acrylic acid ethyl ester (1) has been synthesized and characterised. Photophysical studies of 1 exhibit polarity dependent shift of its emission maxima which have been explained on the basis the existence of polar excited state of the molecule. Combination of compound 1 and citrate capped AuNPs (AuNPs/1 conjugate) has been used as a sensing tool for heavy metals. AuNPs/1 conjugate has been found to detect Pb²⁺ selectively by naked-eye color change as well as fluorescence enhancement. On addition of molecule 1 to gold nanoparticles solution, the color of the solution becomes reddish followed by quenching in fluorescence intensity. With gradual addition of Pb²⁺, the solution of AuNPs/1 conjugate becomes violet accompanied by a fluorescence enhancement. Excited state lifetime measurement revealed that compound 1 exhibits very fast decay pattern in aqueous medium whereas in AuNPs medium the lifetime of 1 increases. Upon addition of Pb²⁺ ions to that AuNPs/1 solution the lifetime of 1 decreases again. Based on the experimental observations the mechanism of sensing of lead has been proposed thoroughly. Initially compound 1 gets absorbed on the surface of the spherical gold nanoparticles. When Pb²⁺ is added, probably gold nanoparticles aggregates to form bigger particles by releasing compound 1 from its surface to show fluorescence enhancement.

Ultrasensitive Detection of Pb2+ Based on a DNAzyme and Digital PCR

In this study, an ultrasensitive detection method for aqueous Pb²⁺ based on digital polymerase chain reaction (dPCR) technology and a Pb²⁺-dependent DNAzyme was developed. In the presence of Pb²⁺, the Gr-5 DNAzyme was activated and catalyzed the hydrolytic cleavage of the substrate strand, resulting in an increase in the amount of template DNA available for dPCR and a resultant change in the number of droplets showing a positive signal. Moreover, the detection system was found to be sensitive and stable in environmental sample detection. In summary, an ultrasensitive quantitative detection method for Pb²⁺ within environmental substrates was established.

Sensitivity limits of biosensors used for the detection of metals in drinking water

Even when present in very low concentrations, certain metal ions can have significant health impacts depending on their concentration when present in drinking water. In an effort to detect and identify trace amounts of such metals, environmental monitoring has created a demand for new and improved methods that have ever-increasing sensitivities and selectivity. This paper reviews the sensitivities of over 100 recently published biosensors using various analytical techniques such as fluorescence, voltammetry, inductively coupled plasma techniques, spectrophotometry and visual colorimetric detection that display selectivity for copper, cadmium, lead, mercury and/or aluminium in aqueous solutions.

QCM-nanomagnetic beads biosensor for lead ion detection

A QCM biosensor combined with NMBs has been proposed for Pb²⁺detection with a lower detection limit of 0.3 pM.

A review of electrogenerated chemiluminescent biosensors for assays in biological matrices

Electrogenerated chemiluminescence (ECL) is the production of light via electron transfer reactions between electrochemically produced reagents. ECL-based biosensors use specific biological interactions to recognize an analyte and produce a luminescent signal. Biosensors fabricated with novel biorecognition species have increased the number of analytes detected. Some of these analytes include peptides, cells, enzymes and nucleic acids. ECL biosensors are selective, simple, sensitive and have low detection limits. Traditional methods use ruthenium complexes or luminol to generate ECL. Nanomaterials can be incorporated into ECL biosensors to improve efficiency, but also represent a new class of ECL emitters. This article reviews the application of ruthenium complex, luminol and nanomaterial-based ECL biosensors to making measurements in biological matrices over the past 4 years.

Spinach RNA aptamer detects lead(II) with high selectivity

Spinach RNA aptamer contains a G-quadruplex motif that serves as a platform for binding and fluorescence activation of a GFP-like fluorophore. Here we show that Pb(2+) induces formation of Spinach's G-quadruplex and activates fluorescence with high selectivity and sensitivity. This device establishes the first example of an RNA-based sensor that provides a simple and inexpensive tool for Pb(2+) detection.

Rapid amperometric detection of trace metals by inhibition of an ultrathin polypyrrole-based glucose biosensor

A sensitive and reliable inhibitive amperometric glucose biosensor is described for rapid trace metal determination. The biosensor utilises a conductive ultrathin (55 nm thick) polypyrrole (PPy) film for entrapment of glucose oxidase (GOx) to permit rapid inhibition of GOx activity in the ultrathin film upon exposure to trace metals, resulting in reduced glucose amperometric response. The biosensor demonstrates a relatively fast response time of 20s and does not require incubation. Furthermore, a complete recovery of GOx activity in the ultrathin PPy-GOx biosensor is quickly achieved by washing in 2mM EDTA for only 10s. The minimum detectable concentrations achieved with the biosensor for Hg(2+), Cu(2+), Pb(2+) and Cd(2+) by inhibitive amperometric detection are 0.48, 1.5, 1.6 and 4.0 µM, respectively. Also, suitable linear concentration ranges were achieved from 0.48-3.3 µM for Hg(2+), 1.5-10 µM for Cu(2+), 1.6-7.7 µM for Pb(2+) and 4-26 µM for Cd(2+). The use of Dixon and Cornish-Bowden plots revealed that the suppressive effects observed with Hg(2+) and Cu(2+) were via non-competitive inhibition, while those of Pb(2+) and Cd(2+) were due to mixed and competitive inhibition. The stronger inhibition exhibited by the trace metals on GOx activity in the ultrathin PPy-GOx film was also confirmed by the low inhibition constant obtained from this analysis. The biosensor was successfully applied to the determination of trace metals in tap water 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.

Cleavage-based hybridization chain reaction for electrochemical detection of thrombin

In the present work, we constructed a new label-free "inter-sandwich" electrochemical aptasensor for thrombin (TB) detection by employing a cleavage-based hybridization chain reaction (HCR). The designed single-stranded DNA (defined as binding DNA), which contained the thrombin aptamer binding sequence, a DNAzyme cleavage site and a signal reporter sequence, was first immobilized on the electrode. In the absence of a target TB, the designed DNAzymes could combine with the thrombin aptamer binding sequence via complementary base pairing, and then Cu(2+) could cleave the binding DNA. In the presence of a target TB, TB could combine with the thrombin aptamer binding sequence to predominantly form an aptamer-protein complex, which blocked the DNAzyme cleavage site and prevented the binding DNA from being cleaved by Cu(2+)-dependent DNAzyme. As a result, the signal reporter sequence could leave the electrode surface to trigger HCR with the help of two auxiliary DNA single-strands, A1 and A2. Then, the electron mediator hexaammineruthenium (III) chloride ([Ru(NH3)6](3+)) was embedded into the double-stranded DNA (dsDNA) to produce a strong electrochemical signal for the quantitative measurement of TB. For further amplification of the electrochemical signal, graphene reduced by dopamine (PDA-rGO) was introduced as a platform in this work. With this strategy, the aptasensor displayed a wide linearity in the range of 0.0001 nM to 50 nM with a low detection limit of 0.05 pM. Moreover, the resulting aptasensor exhibited good specificity and acceptable reproducibility and stability. Because of these factors, the fabrication protocol proposed in this work may be extended to clinical application.

Electrochemiluminescence-based detection method of lead(II) ion via dual enhancement of intermolecular and intramolecular co-reaction

A novel analytical method to design a highly selective and sensitive detection technique for lead(II) ions (Pb(2+)) detection was developed based on an electrochemiluminescence (ECL) sensor, taking advantage of the high specificity of the aptamer for Pb(2+) and the use of both intermolecular and intramolecular co-reaction to achieve signal enhancement. For sensing interface construction, L-cysteine (Cys) and gold nanostructured layers were electrodeposited on the electrode surface successively, which afforded a large surface area to anchor massive thiol-terminated auxiliary probes (APs) via a thiol-Au interaction. Then, a DNA duplex was generated based on the hybridization of the APs with capture probes (CPs, Pb(2+) specific aptamers). In the presence of Pb(2+), Pb(2+)-induced aptamers were released from the DNA duplex via the formation of a Pb(2+)-stabilized G-quadruplex, accompanied by leaving the single CPs on the sensing interface. Herein, the ruthenium(ii) complexes with functional groups of -COOH (Ru-COOH) were covalently bonded on the polyamidoamine dendrimers with amine end groups (PAMAM), which were capped by the high-index-faceted Au nanoparticles (HIFAuNPs) to obtain the ECL signal labels of Ru-PAMAM-HIFAuNPs. Then, the detection probes (DPs) of amino-terminated Pb(2+) specific aptamers were tagged with the Ru-PAMAM-HIFAuNPs. It was demonstrated that the covalent bonding of PAMAM and Ru-COOH could generate a self-enhanced ECL luminophore by an intramolecular co-reaction and the use of a Cys layer modified electrode could enhance the ECL by the intermolecular co-reaction of Cys and Ru-COOH, which lead to a significant enhancement of the ECL response. Based on this analytical method, the ECL signal increased with Pb(2+) concentration which presented a linear relationship in the range 1.0 × 10(-13)-1.0 × 10(-7) M with the detection limit of 4.0 × 10(-14) M. The proposed approach was also successfully utilized for the determination of Pb(2+) in soil samples.

Recent advances in electrochemiluminescence

The great success of electrochemiluminescence (ECL) for in vitro diagnosis (IVD) and its promising potential in light-emitting devices greatly promote recent ECL studies. More than 45% of ECL articles were published after 2010, and the first international meeting on ECL was held in Italy in 2014. This critical review discusses recent vibrant developments in ECL, and highlights novel ECL phenomena, such as wireless ECL devices, bipolar electrode-based ECL, light-emitting electrochemical swimmers, upconversion ECL, ECL resonance energy transfer, thermoresponsive ECL, ECL using shape-controlled nanocrystals, and ECL as an ion-selective electrode photonic reporter, a paper-based microchip, and a self-powered microfluidic ECL platform. We also comment on the latest progress in bioassays, light-emitting devices and, the computational approach for the ECL mechanism study. Finally, perspectives and key challenges in the near future are addressed (198 references).

"Signal-on" photoelectrochemical sensing strategy based on target-dependent aptamer conformational conversion for selective detection of lead(II) ion

A "signal-on" photoelectrochemical sensing strategy for selective determination of Pb(2+) is designed on the basis of the combination of Pb(2+)-induced conformational conversion, the amplified effect of reduced graphene oxide (RGO) and resonance energy transfer between CdS quantum dots (QDs) and gold nanoparticles (AuNPs). The RGO/CdS/aptamer platform is constructed via a stepwise modification method, and characterized by electrochemical impedance spectroscopy. In the absence of Pb(2+), the AuNP-labeled DNA, as a signal quenching element, can be introduced by hybridization with aptamer on the surface of sensing platform, which quenches the photocurrent of QDs via an energy transfer process. Upon addition of Pb(2+), the aptamer is induced into a G-quadruplex structure, which can greatly hinder the hybridization between aptamer and AuNP-labeled DNA due to the competitive occupation of binding sites and steric effect, leading to the recovery of photocurrent. Under optimized conditions, this "signal-on" photoelectrochemical biosensor shows a linear relationship between photocurrent variation and the logarithm of Pb(2+) concentration in the range of 0.1-50 nM with a detection limit of 0.05 nM. Meanwhile, it also exhibits good selectivity for Pb(2+) over other interfering ions, and is successfully applied to the detection of Pb(2+) in environmental water samples. By substituting the aptamers with other sequences, this proposed strategy could be conveniently extended to detect different targets as versatile photoelectrochemical devices.

Functional nucleic-acid-based sensors for environmental monitoring

Efforts to replace conventional chromatographic methods for environmental monitoring with cheaper and easy to use biosensors for precise detection and estimation of hazardous environmental toxicants, water or air borne pathogens as well as various other chemicals and biologics are gaining momentum. Out of the various types of biosensors classified according to their bio-recognition principle, nucleic-acid-based sensors have shown high potential in terms of cost, sensitivity, and specificity. The discovery of catalytic activities of RNA (ribozymes) and DNA (DNAzymes) which could be triggered by divalent metallic ions paved the way for their extensive use in detection of heavy metal contaminants in environment. This was followed with the invention of small oligonucleotide sequences called aptamers which can fold into specific 3D conformation under suitable conditions after binding to target molecules. Due to their high affinity, specificity, reusability, stability, and non-immunogenicity to vast array of targets like small and macromolecules from organic, inorganic, and biological origin, they can often be exploited as sensors in industrial waste management, pollution control, and environmental toxicology. Further, rational combination of the catalytic activity of DNAzymes and RNAzymes along with the sequence-specific binding ability of aptamers have given rise to the most advanced form of functional nucleic-acid-based sensors called aptazymes. Functional nucleic-acid-based sensors (FNASs) can be conjugated with fluorescent molecules, metallic nanoparticles, or quantum dots to aid in rapid detection of a variety of target molecules by target-induced structure switch (TISS) mode. Although intensive research is being carried out for further improvements of FNAs as sensors, challenges remain in integrating such bio-recognition element with advanced transduction platform to enable its use as a networked analytical system for tailor made analysis of environmental monitoring.

Sensitive and reusable electrochemiluminescent aptasensor achieved with diblock oligonucleotides immobilized solely through preferential adenine-Au interaction

Strong Au-S chemistry to self-assemble thiolated oligonucleotides at gold electrode is an efficient strategy to construct electrochemiluminescent (ECL) aptasensor. However, it remains challenging to precisely control the orientation and conformation of surface-tethered oligonucleotides and to reuse ECL aptasensor because of the narrow electrochemical window of thiolated DNA film on Au surface (below ~0.80 V versus Ag/AgCl). Here, we demonstrate adenine/thymine diblock oligonucleotides (d(Am-Tn)) to substitute DNA-SH in DNA immobilization for constructing ECL aptasensor. As a proof-of-principle, thrombin was used to present the properties of the proposed sensor. The as-formed ECL aptasensor had a wide electrochemical window and good stability (decreased 5.38% after 200 cyclic potential cycles, 0-1.2 V versus Ag/AgCl). Moreover, the aptasensor exhibited an extremely low detection limit (0.017 pM) and offered good selectivity toward thrombin. This detection limit was at least one order of magnitude lower than those of previous methods for thrombin. Additionally, the ECL aptasensor was reusable (n = 3) and showed good reproducibility (relative standard derivation, 4.7% (n = 6)). We believe that the strategy demonstrated here provides a good platform for DNA immobilization in constructing ECL even electrochemical aptasensor for the detection of targets in clinical analysis conveniently.

An electrochemiluminescence aptasensor for thrombin using graphene oxide to immobilize the aptamer and the intercalated [Formula: see text] probe

The immobilization of aptamer and the introduction of signal molecule are two keys for the development of electrochemiluminescence (ECL) aptasensor. Herein, the immobilization strategy with graphene oxide (GO) and a functional oligonucleotide (FO) are used to develop a sensitive aptasensor with the detection of thrombin as a model. After GO is attached on glass carbon or gold electrodes through physical adsorption, the amino-tagged aptamer is immobilized on the electrode surface via an amide linkage between the amino group at the end of aptamer and the carboxyl groups on GO. The FO is designed to contain two parts: the complementary strand and an intermolecular duplex for the intercalation of Ru(phen)₃²⁺ as ECL probe. The hybridization between aptamer and its complementary part at FO achieves the introduction of Ru(phen)₃²⁺ probe onto the electrode surface for high ECL emission. The hybrid between aptamer and thrombin leads to the release of FO containing the intercalated Ru(phen)₃²⁺ probe. Correspondingly, the decreased ECL emission is used to quantify thrombin. The concentration-dependent response of thrombin is observed between 0.90 pM and 226 pM with a detection limit of 0.40 pM. While GO is used to immobilize the aptamer with various electrodes, such as glass carbon electrode and gold electrode in this work, GO can also preconcentrate TPrA on its surface to improve the sensitivity. The well-designed label-free ECL aptasensor strategy can be easily extended to other targets via the selection of their aptamers.