Impedimetric Immobilized DNA-Based Sensor for Simultaneous Detection of Pb2+, Ag+, and Hg2+

Fluorescent aptasensors for sensitive detection of lead ions based on structure-switching DNA beacon probe and exonuclease I-mediated signal amplification

Herein, two simple fluorescent signal-on sensing strategies for detecting lead ions (Pb2+) were established based on structure-switching aptamer probes and exonuclease-assisted signal amplification strategies. Two hairpin-structure fluorescent probes with blunt-ended stem arms were designed by extending the base sequence of Pb2+ aptamer (PS2.M) and labelling the probes with FAM (in probe 1) and 2-aminopurine (2-AP) (in probe 2), respectively. In method 1, graphene oxide (GO) was added to adsorb probe 1 and quench the fluorescence emission of FAM to achieve low fluorescent background. In method 2, fluorescent 2-AP molecule inserted into the double-stranded DNA of probe 2 was quenched as a result of base stacking interactions, leading to a simplified, quencher-free approach. The addition of Pb2+ can induce the probes to transform into G-quadruplex structures, exposing single DNA strands at the 3' end (the extended sequences). This exposure enables the activation of exonuclease I (Exo I) on the probes, leading to the cleavage effect and subsequent release of free bases and fluorophores, thereby resulting in amplified fluorescence signals. The two proposed methods exhibit good specificity and sensitivity, with detection limits of 0.327 nM and 0.049 nM Pb2+ for method 1 and method 2, respectively, and have been successfully applied to detect Pb2+ in river water and fish samples. Both detection methods employ the structure-switching aptamer probes and can be completed in two or three steps without the need for complex analytical instruments. Therefore, they have a broad prospect in the sensitive and simple detection of lead ion contamination in food and environmental samples.

Development of a fluorescent DNA sensor for dual detection of heavy metal ions utilising DAPI in distinct buffers

The DNA-based biosensor utilises a thymine/guanine(T/G)-rich ODN-4 scaffold with 4',6-diamidino-2-phenylindole(DAPI) as a fluorescent emissary to monitor mercury/lead(Hg(II)/Pb(II)) ions simultaneously. Key to its bifocal detection capability is the twin unbound cytosine(C) bases strategically bridging the G-quadruplex and T-rich sequences, enabling their synergistic interplay. It facilitates the recognition of Hg(II)/Pb(II) ions, characterised by high specificity, and effectively mitigates interference from silver(Ag(I)). The G-quadruplex, guided by the C bases, induces a conformational transition in T-Hg(II)-T complexes, resulting in intense fluorescence. Pb(II) causes a spatial shift in the G-quadruplex, relaxing the T-Hg(II)-T base pairs and attenuating the fluorescence signal. The ODN-4 exhibits a robust, linear correlation with Hg(II) concentration (4.09 nmol/L to 1000 nmol/L) and Pb(II) concentration (3.22 nmol/L to 5 μmol/L). Recovery rates in milk, tap water, and rice water specimens with both ions validate method accuracy (Hg(II): 95.19% to 104.68%, Pb(II): 98.20% to 103.46%). It holds promising prospects for practical food analysis.

Cytosine-rich mismatched DNA aptamer combined with superparamagnetic photonic crystal sensing material for the specific visual detection of silver ions

DNA aptamer superparamagnetic photonic crystals (DSPCs), enriched with a highly selective cytosine-rich mismatched single-stranded DNA aptamer (CRDA), were successfully employed in a novel visual detection strategy for the detection of silver ions (Ag+). The technologies of superparamagnetic colloidal nanospheres (SCNs), DNA aptamer, and photonic crystals were combined to fabricate DPSCs. The aptamer was immobilized via electrostatic adsorption with amino groups that were chemically introduced on the surface of the SCNs, forming D-NH-SCNs. The detection is achieved by forming an Ag+ complex (C-Ag+-C) between Ag+ and D-NH-SCN. The DSPCs assembled under a magnetic field by D-NH-SCNs effectively detected Ag+ in the range of 1 μg/L to 5 mg/L, corresponding to the critical concentration range for heavy metals in drinking water. During the detection, the DSPC exhibited a wavelength blueshift from 652.8 nm to 626.4 nm (26.4 nm), as well as changes in reflection intensity. Notably, when detecting Ag+, a change in DSPC color from orange to yellow was observed. In summary, the developed visual detection material facilitates direct Ag + sensing. In the future, different DNA aptamers will be modified further to detect various targets in the fields of medicine, environmental monitoring, and food safety.

Studies on the Aptasensor Miniaturization for Electrochemical Detection of Lead Ions

Lead poses severe effects on living organisms, and since Pb2+ ions tend to accumulate in different organs, it is crucial to monitor Pb2+ concentration in samples such as water and soil. One of the approaches is the utilization of biosensors combined with aptamer-based layers for the electrochemical detection of lead ions. Herein, we present the studies of applying miniaturized screen-printed transducers as solid surfaces to fabricate aptamer layers. As the research is the direct continuation of our previous studies regarding the use of gold disk electrodes, the working parameters of elaborated aptasensors were defined, including the range of linear response (10-100 nM), selectivity as well as stability, regeneration, and feasibility of application for the analysis of real samples. This was achieved using voltammetric techniques including cyclic and square-wave voltammetry in the presence of methylene blue redox indicator.

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.

Development of a new near-infrared, spectrophotometric, and colorimetric probe based on phthalocyanine containing mercaptoquinoline unit for discriminative and highly sensitive detection of Ag+, Cu2+, and Hg2+ ions

A new near-infrared, spectrophotometric, and colorimetric probe based on a phthalocyanine-containing mercaptoquinoline unit (MQZnPc) has been constructed and utilized for discriminative and highly selective/sensitive detection of Ag⁺, Cu²⁺, and Hg²⁺ ions by using proper masking agents like EDTA, KI, and NaCl. The probe only responds to Ag⁺, Cu²⁺, and Hg²⁺ among the tested ions without any interference. The probe performs quite well (the limit of detection: 160 ppb, 148 ppb, and 276 ppb of Ag⁺, Cu²⁺, and Hg²⁺ions for UV-Vis, and 15 ppb, 37 ppb, and 467 ppb of Ag⁺, Cu²⁺, and Hg²⁺ ions for fluorescence, respectively), and has a fast response time (150 sec, 90 sec, and 90 sec of Ag⁺, Cu²⁺, and Hg²⁺ions for UV-Vis, and 300 sec, 240 sec, and 90 sec Ag⁺, Cu²⁺, and Hg²⁺ions for fluorescence, respectively). The probe also displays a colorimetric feature for UV-Vis and smartphone applications. Based on a single probe, Ag⁺, Cu²⁺, and Hg²⁺ ions which are the main toxic water contaminants could be recognized very quickly and colorimetrically with high recovery values in tap water samples. This study stands out with its unique properties compared to the related studies in the literature.

Finding the Lost Dissociation Constant of Electrochemical Aptamer-Based Biosensors

Electrochemical aptamer-based (E-AB) biosensors afford real-time measurements of the concentrations of molecules directly in complex matrices and in the body, offering alternative strategies to develop innovative personalized medicine tools. While different electroanalytical techniques have been used to interrogate E-AB sensors (i.e., cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry) to resolve the change in electron transfer of the aptamer's covalently attached redox reporter, square-wave voltammetry remains a widely used technique due to its ability to maximize the redox reporter's faradic contribution to the measured current. Several E-AB sensors interrogated with this technique, however, show lower aptamer affinity (i.e., μM-mM) even in the face of employing aptamers that have high affinities (i.e., nM-μM) when characterized using solution techniques such as isothermal titration calorimetry (ITC) or fluorescence spectroscopy. Given past reports showing that E-AB sensor's response is dependent on square-wave interrogation parameters (i.e., frequency and amplitude), we hypothesized that the difference in dissociation constants measured with solution techniques stemmed from the electrochemical interrogation technique itself. In response, we decided to compare six dissociation constants of aptamers when characterized in solution with ITC and when interrogated on electrodes with electrochemical impedance spectroscopy, a technique able to, in contrast to square-wave voltammetry, deconvolute and quantify E-AB sensors' contributions to the measured current. In doing so, we found that we were able to measure dissociation constants that were either separated by 2-3-fold or within experimental errors. These results are in contrast with square-wave voltammetry-measured dissociation constants that are at the most separated by 2-3 orders of magnitude from ones measured by ITC. We thus envision that the versatility and time scales covered by electrochemical impedance spectroscopy offer the highest sensitivity to measure target binding in electrochemical biosensors relying on changes in electron-transfer rates.

Heavy Metal Ions Detection Using Nanomaterials-Based Aptasensors

Heavy metals ions as metallic pollutants are a growing global issue due to their adverse effects on the aquatic ecosystem, and human health. Unfortunately, conventional detection methods such as atomic absorption spectrometry exhibit a relatively low limit of detection and hold numerous disadvantages, and therefore, the development of an efficient method for in-situ and real-time detection of heavy metal residues is of great importance. The aptamer-based sensors offer distinct advantages over antibodies and emerged as a robust sensing platform against various heavy metals due to their high sensitivity, ease of production, simple operations, excellent specificity, better stability, low immunogenicity, and cost-effectiveness. The nucleic acid aptamers in conjugation with nanomaterials can bind to the metal ions with good specificity/selectivity and can be used for on-site monitoring of metal ion residues. This review aimed to provide background information about nanomaterials-based aptasensor, recent advancements in aptamer conjunction on nanomaterials surface, the role of nanomaterials in improving signal transduction, recent progress of nanomaterials-based aptasening procedures (from 2010 to 2022), and future perspectives toward the practical applications of nanomaterials-based aptasensors against hazardous metal ions for food safety and environmental monitoring.

Recent Advancements in Electrochemical Biosensors for Monitoring the Water Quality

The release of chemicals and microorganisms from various sources, such as industry, agriculture, animal farming, wastewater treatment plants, and flooding, into water systems have caused water pollution in several parts of our world, endangering aquatic ecosystems and individual health. World Health Organization (WHO) has introduced strict standards for the maximum concentration limits for nutrients and chemicals in drinking water, surface water, and groundwater. It is crucial to have rapid, sensitive, and reliable analytical detection systems to monitor the pollution level regularly and meet the standard limit. Electrochemical biosensors are advantageous analytical devices or tools that convert a bio-signal by biorecognition elements into a significant electrical response. Thanks to the micro/nano fabrication techniques, electrochemical biosensors for sensitive, continuous, and real-time detection have attracted increasing attention among researchers and users worldwide. These devices take advantage of easy operation, portability, and rapid response. They can also be miniaturized, have a long-life span and a quick response time, and possess high sensitivity and selectivity and can be considered as portable biosensing assays. They are of special importance due to their great advantages such as affordability, simplicity, portability, and ability to detect at on-site. This review paper is concerned with the basic concepts of electrochemical biosensors and their applications in various water quality monitoring, such as inorganic chemicals, nutrients, microorganisms' pollution, and organic pollutants, especially for developing real-time/online detection systems. The basic concepts of electrochemical biosensors, different surface modification techniques, bio-recognition elements (BRE), detection methods, and specific real-time water quality monitoring applications are reviewed thoroughly in this article.

Nucleic acid-based electrochemical biosensors for rapid clinical diagnosis: advances, challenges, and opportunities

Clinical diagnostic tests should be quick, reliable, simple to perform, and affordable for diagnosis and treatment of diseases. In this regard, owing to their novel properties, biosensors have attracted the attention of scientists as well as end-users. They are efficient, stable, and relatively cheap. Biosensors have broad applications in medical diagnosis, including point-of-care (POC) monitoring, forensics, and biomedical research. The electrochemical nucleic acid (NA) biosensor, the latest invention in this field, combines the sensitivity of electroanalytical methods with the inherent bioselectivity of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The NA biosensor exploits the affinity of single-stranded DNA/RNA for its complementary strand and is used to detect complementary sequences of NA based on hybridization. After the NA component in the sensor detects the analyte, a catalytic reaction or binding event that generates an electrical signal in the transducer ensues. Since 2000, much progress has been made in this field, but there are still numerous challenges. This critical review describes the advances, challenges, and prospects of NA-based electrochemical biosensors for clinical diagnosis. It includes the basic principles, classification, sensing enhancement strategies, and applications of biosensors as well as their advantages, limitations, and future prospects, and thus it should be useful to academics as well as industry in the improvement and application of EC NA biosensors.

Recent Applications of Pillar[n]arene-Based Host-Guest Recognition in Chemosensing and Imaging

Pillar[n]arene is a novel kind of synthetic supramolecular macrocyclic host characterized by its particular pillar-shaped structure consisting of an electron-rich cavity and two finely adjustable rims. Benefiting from its rigid structure, facile synthesis, ease of functionalization, and outstanding host-guest chemistry, pillar[n]arene shows great potential for diverse applications. Significantly, the host-guest recognition of pillar[n]arene provides a novel approach for chemosensing and imaging. Herein, this Review critically and comprehensively reviews the applications of pillar[n]arene-based host-guest recognition in chemosensing and imaging. The sensing and imaging mechanisms as well as the unique roles and advantages of pillar[n]arene-based host-guest recognition are summarized. In addition, preparations of hybrid materials based on pillar[n]arene and inorganic materials are also introduced comprehensively in the light of chemosensing and imaging. Finally, current challenges and perspectives on pillar[n]arene-based host-guest recognition in chemosensing and imaging are outlined.

Detecting Pb2+by a 'turn-on' fluorescence sensor based on DNA functionalized magnetic nanocomposites

Sensitive and selective detection of the lead ion (Pb²⁺) plays an important role in terms of both human health and environmental protection, as the heavy metal is fairly ubiquitous and highly toxic. The highly stable fluorescence biosensor is composed of Fe₃O₄@TiO₂core-shell nanocomposites, functionalized with a carboxyl fluorescein labeled DNA. The morphology, physical and chemical properties of the sensing nanomaterials were studied by transmission electron microscopy, FT-IR spectroscopy (FT-IR), x-ray powder diffraction and vibrating sample magnetometer. UV-visible and fluorescence spectroscopy were used to characterize the fluorescein functionalized magnetic nanoparticles. The performance of Pb²⁺detection displayed an excellent linearity (R² = 0.995) in the range of 10^-10to 5 × 10^-9ppm with a detection limit of 10^-10ppm, based on the optimization of the fabrication process and aptamers' specification. The fluorescence biosensor has an accurate response, excellent recoveries and high adsorbent capacities. It was successfully applied for the determination of Pb²⁺in contaminated water and serum samples; the detection of limit in both media were 10^-10ppm. These features ensure the potential use of aptamer functionalized magnetic nanocomposites as a new class of non-toxic biocompatible sensors for biological and environmental applications.

Recyclable Target Metal-Enhanced Fluorometric Naked Eye Aptasensor for the Detection of Pb2+ and Ag+ Ions Based on the Structural Change of CaSnO3@PDANS-Constrained GC-Rich ssDNA

Reliable, label-free, and ultraselective detection of Pb2+ and Ag+ ions is of paramount importance for toxicology assessment, human health, and environmental protection. Herein, we present a novel recyclable fluorometric aptasensor based on the Pb2+ and Ag+-induced structural change of the GC-rich ssDNA (guanine cytosine-rich single-strand DNA) and the differences in the fluorescence emission of acridine orange (AO) from random coil to highly stable G-quadruplex for the detection of Pb2+ and Ag+ ions. More interestingly, the construction and principle of the aptasensor explore that the GC-rich ssDNA and AO can be strongly adsorbed on the CaSnO3@PDANS surface through the π-π stacking, hydrogen-bonding, and metal coordination interactions, which exhibit high fluorescence quenching and robust holding of the GC-rich ssDNA. However, in the presence of Pb2+, the specific G-rich ssDNA segment could form a stable G-quadruplex via G4-Pb2+ coordination and capture of AO from the CaSnO3@PDANS surface resulting in fluorescence recovery (70% enhancement). The subsequent addition of Ag+ ion induces coupled cytosine base pairs in another segment of ssDNA to get folded into a duplex structure together with the G-quadruplex, which highly stabilizes the G-quadruplex resulting in the maximum recovery of AO emission (99% enhancement). When the Cys@Fe3O4Nps are added to the above solution, the sensing probe was restored by complexation between the Cys in the Cys@Fe3O4Nps and target metal ions, resulting in the fabrication of a highly sensitive recyclable Pb2+ and Ag+ assay with detection limits of 0.4 and 0.1 nM, respectively. Remarkably, the Cys@Fe3O4Nps can also be reused after washing with EDTA. The utility of the proposed approach has great potential for detecting the Pb2+ and Ag+ ions in environmental samples with interfering contaminants.

Metal-dependent electrochemical discrimination of DNA quadruplex sequences

Films of four different DNA quadruplex-forming (G4) sequences (c-KIT, c-MYC, HTelo, and BCL2) on gold surfaces were investigated by electrochemical impedance spectroscopy (EIS) to evaluate whether they evoke unique electrochemical responses that can be used for their identification. This could render EIS an alternative means for the determination of G4 sequences of unknown structure. Towards, this end, cation-dependent topology changes in the presence of either K⁺, K⁺ in combination with Li⁺, or Pb²⁺ in the presence of Li⁺ were first evaluated by circular dichroism (CD) spectroscopy, and electrochemical studies were performed subsequently. As a result, G4-sequence specific charge transfer resistance (R_CT) patterns were in fact observed for each G4 sequence, allowing their discrimination by EIS.

Simultaneous detection of mercury (II), lead (II) and silver (I) based on fluorescently labelled aptamer probes and graphene oxide

We have developed a fluorescence quantitative analysis method for the simultaneous detection of Hg²⁺, Pb²⁺ and Ag⁺ based on fluorescently labelled nucleic acid aptamer probes and graphene oxide (GO). By this method, three nucleic acid aptamer probes (P_Hg, P_Pb, P_Ag) were designed. The carboxyl fluorescein (FAM), tetramethyl-6-carboxyrhodamine (TAMRA) and cyanine-5 (Cy-5) were respectively selected as fluorophore of aptamer probes, and GO was chosen as quencher. In general, these probes were on free single-stranded state and adsorbed on the surface of GO via π-π interactions, which brought fluorophores of probes and GO into close proximity. Due to the fluorescence resonance energy transfer occurred between fluorophores and GO, the fluorescence was quenched and fluorescence signals were all weak. Under the optimal condition, fluorescence intensities of three fluorophores exhibited a good linear dependence on corresponding ions concentration. The detection limit for Hg²⁺, Pb²⁺ and Ag⁺ were 0.2, 0.5 and 2 nmol/L (3σ, n = 11). Average recoveries of this method were 97.56-104.92%, which indicated the method had a high accuracy and low detection limit. In addition, this proposed method has good selectivity, and there was no crosstalk effect among these probes.

Nanoscale Ion Regulation in Wood-Based Structures and Their Device Applications

Ion transport and regulation are fundamental processes for various devices and applications related to energy storage and conversion, environmental remediation, sensing, ionotronics, and biotechnology. Wood-based materials, fabricated by top-down or bottom-up approaches, possess a unique hierarchically porous fibrous structure that offers an appealing material platform for multiscale ion regulation. The ion transport behavior in these materials can be regulated through structural and compositional engineering from the macroscale down to the nanoscale, imparting wood-based materials with multiple functions for a range of emerging applications. A fundamental understanding of ion transport behavior in wood-based structures enhances the capability to design high-performance ion-regulating devices and promotes the utilization of sustainable wood materials. Combining this unique ion regulation capability with the renewable and cost-effective raw materials available, wood and its derivatives are the natural choice of materials toward sustainability.

Highly selective and rapid detection of silver ions by using a "turn on" non-fluorescent cysteine stabilized gold nanocluster probe

Cysteine is widely used as a stabilizer for the preparation of fluorescent gold nanoclusters (Au NCs) with different fluorescence properties. Herein, by using cysteine as a stabilizer and controlling the synthesis conditions, a new non-fluorescent cysteine stabilized gold nanocluster (Cys-Au NCs) probe was prepared and a new strategy for "turning on" the fluorescence of the Cys-Au NCs was studied for rapid and selective detection of silver ions. In this strategy, the addition of silver ions to non-fluorescent Cys-Au NCs solution could quickly induce a visible fluorescence "turn on" phenomenon in 30 s. Further studies indicated that this fluorescence "turn on" phenomenon is specific for silver ions and the "turn on" fluorescence intensity has a linear relationship with the amount of silver ions in the range from 3.0 to 30.0 μM. Therefore, the non-fluorescent Cys-Au NCs were applied to the detection of silver ions in environmental water samples and a limit of detection (LOD) of 0.26 μM was obtained. This research sheds light on new applications of Au NCs and proposes a simple, rapid, sensitive, and visual method for the detection of metal ions.

Fluorescence and electrochemical assay for bimodal detection of lead ions based on Metal-Organic framework nanosheets

The accurate measurement of heavy metal ions is essential for human health and environmental protection. Here, we report the design of a simple and convenient bimodal strategy for signal-on, label-free lead ion detection in environmental samples based on two-dimensional metal-organic framework (2D-MOF) nanosheets. 2D-MOFs have different affinities toward guanine-rich DNA (ssGDNA) and the G-quadruplex, allowing these structures to be distinguished. The nanosheets were also used as quenchers for fluorescent lead ion detection. Using lead ions to induce G-quadruplex formation from ssGDNA, a simple fluorescence resonance energy transfer (FRET) strategy was developed for lead ion detection; the detection limit was 3.3 nM. Based on changes in the GDNA configuration, the FRET system was converted into an electrochemical sensor for lead ion assays using an electrode modified with the 2D-MOF nanosheets. Electrochemical impedance spectroscopy showed a high sensitivity and a low limit of detection (i.e., 8.7 pM) of the electrode. The adaptability of the bimodal mechanism was verified through the successful detection of lead ions in tap water and fertilizer samples, and the method accuracy was demonstrated through inductively coupled plasma analysis. The developed bimodal device is cost-effective, highly sensitive, and allows for convenient operation, thereby rendering it a promising and reliable system for the detection of lead ions in environmental samples.

Review of recent progress on DNA-based biosensors for Pb2+ detection

Lead (Pb) is a highly toxic heavy metal of great environmental and health concerns, and interestingly Pb²⁺ has played important roles in nucleic acids chemistry. Since 2000, using DNA for selective detection of Pb²⁺ has become a rapidly growing topic in the analytical community. Pb²⁺ can serve as the most active cofactor for RNA-cleaving DNAzymes including the GR5, 17E and 8-17 DNAzymes. Recently, Pb²⁺ was found to promote a porphyrin metalation DNAzyme named T30695. In addition, Pb²⁺ can tightly bind to various G-quadruplex sequences inducing their unique folding and binding to other molecules such as dyes and hemin. The peroxidase-like activity of G-quadruplex/hemin complexes was also used for Pb²⁺ sensing. In this article, these Pb²⁺ recognition mechanisms are reviewed from fundamental chemistry to the design of fluorescent, colorimetric, and electrochemical biosensors. In addition, various signal amplification mechanisms such as rolling circle amplification, hairpin hybridization chain reaction and nuclease-assisted methods are coupled to these sensing methods to drive up sensitivity. We mainly cover recent examples published since 2015. In the end, some practical aspects of these sensors and future research opportunities are discussed.

Surface chemistry tuning the selectivity of carbon nanodots towards Hg2+ recognition

Fluorescence quenching of carbon nanodots by metal ions has been extensively applied for the determination of oligonucleotides, proteins, small molecules and metal ions. However, the problem of poor selectivity originating from the coordination of surface oxygen-containing groups to many kinds of metal ions has limited the prosperity of carbon nanodots in detection field. Herein, the specific recognition of carbon nanodots to Hg²⁺ is controlled by rational regulation of the surface structure of carbon nanodots. Passivation of the surface carboxyl and hydroxyl groups plays a decisive role in inhibiting the binding of metal ions with carbon nanodots. Upon the attachment of Hg²⁺ specific recognition unit, carbon nanodots exhibited a high selectivity to Hg²⁺. This work facilitates to rationally design the surface structure of carbon nanodots to obtain the desirable selective recognition ability.

Discrimination of copper and silver ions based on the label-free quantum dots

A simple and fast method for copper ions (Cu²⁺) and silver ions (Ag⁺) detection was established with cadmium telluride quantum dots (CdTe QDs) as fluorescent probes. In the presence of Cu²⁺ or Ag⁺, the fluorescence intensity of TGA-CdTe QD can be significantly quenched, which fitted a linear relationship between the fluorescence quenching degree (F₀-F)/F₀ and the concentration of metal ions. In this work, the lowest detected concentration for Cu²⁺ and Ag⁺ was 35.0 nM and 25.3 nM, respectively. In addition, the differentiation of Cu²⁺ and Ag⁺ at different concentrations was realized with the principal component analysis (PCA). Furthermore, Cu²⁺ was successfully detected in body fluids. This method provides a good potential for copper ions and silver ions detection with simplicity, rapidity, and excellent selectivity.

Ratiometric-enhanced G-Quadruplex Probes for Amplified and Mix-to-Read Detection of Mercury Pollution in Aquatic Products

Mercury, as a global toxic pollutant, is easy to be accumulated in aquatic products and poses a great threat to human health. In this work, we proposed a mix-to-read, label-free, and robust assay for detecting mercury pollution in aquatic products by engineering a ratiometric-enhanced G-quadruplex probe. The transformation from the G-quadruplex to a hairpin-like structure allows us to confer a ratiometric and leveraged response to Hg²⁺, amplifying the signal-to-background ratio for Hg²⁺ detection. Hg²⁺ response was further improved by screening parallel- and antiparallel-, single-, and multiple-stranded G-quadruplex structures. Compared to the common aptamer probes, the ratiometric-enhanced G-quadruplex probe increased the sensitivity for Hg²⁺ detection by 4.7 times. This proposed sensing system allowed a simple and one-tube homogenous detection of Hg²⁺ at room temperature using a single unlabeled DNA sequence. Its application for Hg²⁺ detection in fish and shrimp conferred satisfactory recovery rates ranging from 98.5 to 105.9%. The label-free and mix-to-read assay is promising for the onsite detection of mercury pollution and facilitating food safety of aquatic products.

Dual Colorimetric Sensor for Hg2+/Pb2+ and an Efficient Catalyst Based on Silver Nanoparticles Mediating by the Root Extract of Bistorta amplexicaulis

Environmental pollution derivated from toxic metals and organic toxins is becoming a serious issue worldwide because of their harmful effects on the ecosystem and human health. Here we are reporting an extremely selective and cost-effective colorimetric sensor for simultaneous recognition of Hg2+ and Pb2+ by using green synthesized silver nanoparticles (AgNPs) mediated from the environmental friendly roots extract of Bistorta amplexicaulis. Biogenic synthesized AgNPs were well-characterized by various spectroscopic techniques e.g., UV-vis, FT-IR, XRD, AFM, and Zetasizer. The photophysical potential of synthesized AgNPs toward common metal cations was explored via absorption spectroscopy and colorimetric assay. The hypsochromic shift in the SPR band of AgNPs can easily be detected through naked eyes vision from dark brown to light yellow in the case of Hg2+. A substantial reduction in the absorbance of AgNPs was recorded upon mixing with Pb2+. AgNPs based colorimetric sensor is highly sensitive toward Hg2+ and Pb2+ with a limit of detection (LOD) of 8.0 × 10-7 M and 2.0 × 10-7 M for Hg2+ and Pb2+, respectively. Furthermore, AgNPs showed promising catalytic activity for the degradation of methyl orange dye. These results demonstrate that Bistorta amplexicaulis stabilized silver nanoparticles have potential applications as a colorimetric sensor and an effective catalyst for the degradation of methyl orange.

Detection of biological mercaptan by DNA functionalized room temperature phosphorescent quantum dot nanocomposites

In this study, green low-toxicity Mn-doped Zns (Mn-Zns) room-temperature phosphorescent (RTP) quantum dots (QDs) (PQDs) nanocomposites (DNA-PQDs) were prepared under the optimal conditions by using single-stranded DNA (PS-C-ssDNA) rich of cytosine C and Thioguanine G (PS) as the template. DNA-PQDs interact with Ag⁺ to form AgN bonds and further produce C-Ag⁺-C conjugates. As a result, DNA-PQDs cluster together and induce the phosphorescent exciton energy transfer (PEET), resulting in quenching of room-temperature phosphorescent of DNA-PQDs. Nevertheless, Ag⁺ tends to form AgS bonds with biological mercaptan when it is added in, so that Ag⁺ falls from C-Ag⁺-C. DNA-PQDs changed from aggregation to looseness and RTP is recovered accordingly. On this basis, RTP detection of biological mercaptan is realized. Since this sensor system has RTP properties based on DNA-PQDs, it is very applicable to detection of mercaptan compounds in biological fluids.

Surface Immobilized Nucleic Acid-Transcription Factor Quantum Dots for Biosensing

Immobilization of biosensors on surfaces is a key step toward development of devices for real-world applications. Here the preparation, characterization, and evaluation of a surface-bound transcription factor-nucleic acid complex for analyte detection as an alternative to conventional systems employing aptamers or antibodies are described. The sensor consists of a gold surface modified with thiolated Cy5 fluorophore-labeled DNA and an allosteric transcription factor (TetR) linked to a quantum dot (QD). Upon addition of anhydrotetracycline (aTc)-the analyte-the TetR-QDs release from the surface-bound DNA, resulting in loss of the Förster resonance energy transfer signal. The sensor responds in a dose-dependent manner over the relevant range of 0-200 µm aTc with a limit of detection of 80 nm. The fabrication of the sensor and the subsequent real-time quantitative measurements establish a framework for the design of future surface-bound, affinity-based biosensors using allosteric transcription factors for molecular recognition.

A review on nanostructure-based mercury (II) detection and monitoring focusing on aptamer and oligonucleotide biosensors

Heavy metal ion pollution is a severe problem in environmental protection and especially in human health due to their bioaccumulation in organisms. Mercury (II) (Hg²⁺), even at low concentrations, can lead to DNA damage and give permanent harm to the central nervous system by easily passing through biological membranes. Therefore, sensitive detection and monitoring of Hg²⁺ is of particular interest with significant specificity. In this review, aptamer-based strategies in combination with nanostructures as well as several other strategies to solve addressed problems in sensor development for Hg²⁺ are discussed in detail. In particular, the analytical performance of different aptamer and oligonucleotide-based strategies using different signal improvement approaches based on nanoparticles were compared within each strategy and in between. Although quite a number of the suggested methodologies analyzed in this review fulfills the standard requirements, further development is still needed on real sample analysis and analytical performance parameters.

Recent Advances and Applications of Microfluidic Capillary Electrophoresis: A Comprehensive Review (2017-Mid 2019)

Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).

A low-cost paper-based aptasensor for simultaneous trace-level monitoring of mercury (II) and silver (I) ions

Mercury (Hg²⁺) and silver (Ag⁺) ions possess the harmful effects on public health and environment that makes it essential to develop the sensing techniques with great sensitivity for the ions. Metal ions commonly coexist in the different biological and environmental systems. Hence, it is an urgent demand to design a simple method for the simultaneous detection of metal ions, peculiarly in the case of coexisting Hg²⁺ and Ag⁺. This study introduces a low-cost paper-based aptasensor to monitor Hg²⁺ and Ag⁺, simultaneously. The strategy of the sensing array is according to the conformational changes of Hg²⁺- and Ag⁺-specific aptamers and their release from the GO surface after the injection of the target sample on the sensing platform. Through monitoring the fluorescence recovery changes against the concentrations of the ions, Hg²⁺ and Ag⁺ can be determined as low as 1.33 and 1.01 pM. The paper-based aptasensor can simultaneously detect the ions within about 10 min. The aptasensor is applied prosperously to monitor Hg²⁺ and Ag⁺ in human serum, water, and milk. The designed aptasensor with the main advantages of simplicity and feasibility holds the supreme potential to develop a cost-effective sensing method for environmental monitoring, food control, and human diagnostics.

Recent Developments of Flexible and Stretchable Electrochemical Biosensors

The skyrocketing popularity of health monitoring has spurred increasing interest in wearable electrochemical biosensors. Compared with the traditionally rigid and bulky electrochemical biosensors, flexible and stretchable devices render a unique capability to conform to the complex, hierarchically textured surfaces of the human body. With a recognition element (e.g., enzymes, antibodies, nucleic acids, ions) to selectively react with the target analyte, wearable electrochemical biosensors can convert the types and concentrations of chemical changes in the body into electrical signals for easy readout. Initial exploration of wearable electrochemical biosensors integrates electrodes on textile and flexible thin-film substrate materials. A stretchable property is needed for the thin-film device to form an intimate contact with the textured skin surface and to deform with various natural skin motions. Thus, stretchable materials and structures have been exploited to ensure the effective function of a wearable electrochemical biosensor. In this mini-review, we summarize the recent development of flexible and stretchable electrochemical biosensors, including their principles, representative application scenarios (e.g., saliva, tear, sweat, and interstitial fluid), and materials and structures. While great strides have been made in the wearable electrochemical biosensors, challenges still exist, which represents a small fraction of opportunities for the future development of this burgeoning field.

Electrochemical Anion Sensing: Supramolecular Approaches

Anions play a vital role in a broad range of environmental, technological, and physiological processes, making their detection/quantification valuable. Electroanalytical sensors offer much to the selective, sensitive, cheap, portable, and real-time analysis of anion presence where suitable combinations of selective (noncovalent) recognition and transduction can be integrated. Spurred on by significant developments in anion supramolecular chemistry, electrochemical anion sensing has received considerable attention in the past two decades. In this review, we provide a detailed overview of all electroanalytical techniques that have been used for this purpose, including voltammetric, impedimetric, capacititive, and potentiometric methods. We will confine our discussion to sensors that are based on synthetic anion receptors with a specific focus on reversible, noncovalent interactions, in particular, hydrogen- and halogen-bonding. Apart from their sensory properties, we will also discuss how electrochemical techniques can be used to study anion recognition processes (e.g., binding constant determination) and will furthermore provide a detailed outlook over future efforts and promising new avenues in this field.

A glass nanopore ionic sensor for surface charge analysis

Surface charge-based nanopore characterization techniques unfold unique properties and provide a powerful platform for a variety of sensing applications. In this paper, we have proposed a nanoconfined inner wall surface charge characterization method with glass nanopores. The glass nanopores were functionalized with DNA aptamers that were designed for mercury (Hg²⁺) ion immobilization by forming thymine–Hg²⁺–thymine structures. The surface charge of the nanopores was modulated by surface chemistry and Hg²⁺ ion concentrations and analysed by combining zeta potential measurements on glass slides and the ionic current rectification ratio of the nanopores. Also, 1 pM Hg²⁺ ions could be detected by the nanopores.

Surface charge-based nanopore characterization techniques unfold unique properties and provide a powerful platform for a variety of sensing applications.

Photoelectrochemical aptasensor for lead(II) by exploiting the CdS nanoparticle-assisted photoactivity of TiO2 nanoparticles and by using the quercetin-copper(II) complex as the DNA intercalator

A photoelectrochemical (PEC) aptasensor for Pb(II) detection is described. A nanocomposite consisting of CdS (2.5 μm) and TiO₂ nanoparticles (10 nm) was used as a photoactive material, and gold nanochains (Au NCs) as the support for immobilization of the Pb(II)-binding aptamer. The quercetin-copper(II) complex was further employed as the intercalator for the improvement of the photoactivity by embedding it into dsDNA. In the presence of Pb(II), a Pb(II)-stabilized G-quadruplex was formed between Pb(II) and DNA S1. This is accompanied by unwinding of the dsDNA and the release of the quercetin-copper(II) complex from the surface of the sensor. This results in a decrease of the photocurrent that drops linearly from 5.0 × 10^-12 to 1.0 × 10^-8 mol·L^-1 Pb(II) concentration range with a detection limit of 1.6 × 10^-12 mol·L^-1. The method was applied to the determination of Pb(II) in various samples and gave satisfactory results. Graphical abstractA photoelectrochemical aptasensor was fabricated for the detection of Pb(II) based on CdS-TiO₂ nanocomposite modified indium tin oxide (ITO) electrode. Gold nanochains (AuNCs) were used as anchor to immobilize the aptamers S1 and S2 that form a double helix structure by DNA hybridization. After embedding of quercetin-copper(II) complex as intercalator and electron donor, the concentrations of Pb(II) were determined by the changes of photocurrents.

Fluorescence Anisotropy Reduction of An Allosteric G-Rich Oligonucleotide for Specific Silver Ion and Cysteine Detection Based on the G-Ag+-G Base Pair

Silver is a common heavy metal, and the detection of silver ion (Ag⁺) is of great importance because of its wide application and hazardous effect on the environment and human health. However, it is a great challenge to produce a large fluorescence anisotropy (FA) change for small molecules (e.g, Ag⁺). Herein, we describe a novel fluorescence anisotropy reduction approach for the sensitive and specific detection of Ag⁺. The feasibility of this method is demonstrated through screening a number of guanine-rich oligonucleotide probes. By selectively labeling the oligonucleotides with a single fluorophore tetramethylrhodamine (TMR), the reduction in FA response is associated with the conformation change from the unfolded to a hairpin-like folded structure by inducing formation of the intermolecular G-Ag⁺-G base pair, which diminishes the interaction between guanine and TMR by photoinduced electron transfer (PET). The change in FA allows the selective detection of Ag⁺ at a concentration as low as 0.5 nM with a dynamic range from 2.0 to 100 nM. The interference from the other 14 metal ions with a 100-fold even to a 1000-fold excess amount is negligible. This simple and cost-effective probe was further explored to determine cysteine (Cys) based on competing with a guanine-rich oligonucelotide for Ag⁺-binding.

Optical aptasensing of mercury(II) by using salt-induced and exonuclease I-induced gold nanoparticle aggregation under dark-field microscope observation

An optical method for determination of Hg(II) is described that exploits the aggregation of gold nanoparticles (AuNPs) under dark-field microscope (DFM) observation. This assay is based on the use of a Hg(II)-specific aptamer, AuNPs modified with complementary DNA strands, and exonuclease I (Exo I). In the absence of Hg(II), the added dsDNA prevents salt-induced aggregation of the green-colored AuNPs. If Hg(II) is added, the aptamer will capture it to form T-Hg(II)-T pairs, and the complementary strand is digested by Exo I. On addition of a solution of NaCl, the AuNPs will aggregate. This is accompanied by a color change from green to orange/red) in the dark-field image. By calculating the intensity of the orange/red dots in the dark-field image, concentration of Hg(II) can be accurately determined. The limit of detection is as low as 36 fM, and response is a linear in the 83 fM to 8.3 μM Hg(II) concentration range. Graphical abstract Schematic representation of a colorimetric assay for Hg(II) based on the use of a mercury(II)-specific aptamer, gold nanoparticles modified with complementary DNA strands, and exonuclease I.

The functionalized ruthenium(II) polypyridine complexes for the highly selective sensing of mercury ions

A series of new ruthenium(II) polypyridine complexes appending with thioether groups were designed, synthesized and characterized. The sensing ability of the complexes toward mercury ions were studied by electronic absorption and emission spectra, and the reaction of the complexes with mercury ions were also confirmed by ESI mass spectroscopy and ¹HNMR spectroscopy. The thioether groups would react with mercury ion fast to form aldehyde group leading to the significant change in the spectra. The color of the complex changed from yellow to orange after addition of mercury ions, and the color of the emission changed from red orange to dark red with a large red shift (~80 nm). Importantly, these kinds of ruthenium(II) complexes show a unique recognition of mercury ions over other metal ions. The complexes with more thioether groups also showed a better sensitivity toward mercury ions, this is good strategy for the further design of the new phosphorescent probes for sensing of mercury ions.

Synthesis and characterization of novel bithiazolidine derivatives-capped CdTe/CdS quantum dots used as a novel Hg2+ fluorescence sensor

In the present study, (E)-2,2'-(4,4'-dioxo-2,2'-dithioxo-2H,2'H-[5,5'-bithiazolylidene]-3,3'(4H,4'H)-diyl) bis(3-mercaptopropanoic acid) (DTM) as a new derivative of thiazolidine was synthesis and characterized for the detrtmination of Hg²⁺ ions. Then, the CdTe@CdS QDs and DTM capped CdTe@CdS QDs were produced. The DTM-CdTe@CdS/QDs used as an effective fluorescence sensing material due to the selective interaction of DTM with Hg (II). The results indicated that the DTM-CdTe@CdS/QDs shows strong fluorescence emissions in the absence of mercury ions and efficiently quenched in presence of Hg², with the formation of a strong and stable complex between Hg²⁺ and DTM. Experimental results showed that under optimal conditions, Hg²⁺ could be detected with a detection limit of 0.08 nM in a linear range from 0.3 nM to 21 nM. The constructed aptasensor illustrated the high selectivity for mercury ions even in the presence of the other interfering metal ions when their concentration ratio was more than 300 times. The satisfactory results illustrated that the designed fluorescence sensor as a sensitive, reliable and easy to use approach could be applied for the facile and rapid determination of Hg²⁺ in tap water.