Metal ion detection using functional nucleic acids and nanomaterials

DNA-templated fluorescent metal nanoclusters and their illuminating applications

After the discovery of DNA during the mid-20th century, a multitude of novel methodologies have surfaced which exploit DNA for its various properties. One such recently developed application of DNA is as a template in metal nanocluster formation. In the early years of the new millennium, a group of researchers found that DNA can be adopted as a template for the binding of metal nanoparticles that ultimately form nanoclusters. Three metal nanoclusters have been studied so far, including silver, gold, and copper, which have a plethora of biological applications. This review focuses on the synthesis, mechanisms, and novel applications of DNA-templated metal nanoclusters, including the therapies that have employed them for their wide range of fluorescent properties, and the future perspectives related to their development by exploiting machine learning algorithms and molecular dynamics simulation studies.

Enzyme-free dual amplification biosensor based on functional nucleic acid and CDs/CoOOH for detection of leukemia fusion gene

Acute promyelocytic leukemia (APL) is an acute myeloid leukemia (AML) with a specific fusion gene target, PML/RARα fusion gene (PML/RARα), which is formed by the translocation of chromosomes 15 and 17. Detection of PML/RARα is the most reliable parameter for the diagnosis, treatment adjustment, efficacy evaluation, prognosis analysis and relapse prediction of APL. In this study, a novel biosensor was constructed for rapid enzyme-free detection of PML/RARα using DNAzyme and carbon dots/cobalt oxhydroxide nanosheet complexs (CDs/CoOOH). In the detection system, the separated DNAzymes could specifically recognize and bind together by the PML/RARα to form a complete DNAzyme for shearing hairpin probe (HP), then generated trigger, which was the first signal amplification. Then, trigger could hybridize with the capture probe (CP) anchored to streptavidin (SA) modified microplate as well as fluorescence quenching signal probe (SP@CDs/CoOOH). Finally, ascorbic acid (AA) was added to decompose CoOOH and the fluorescence of CDs was released, which was the second signal amplification. Through the dual signal amplification of DNAzyme and CDs/CoOOH, PML/RARα could be detected quickly and sensitively, which overcame the limitation of protein enzyme in traditional fluorescence methods, showing potential clinical application value in the diagnosis and treatment of leukemia.

Design of a Synthetic Strategy to Achieve Enhanced Fluorescent Carbon Dots with Sulfur and Nitrogen Codoping and Its Multifunctional Applications

Here, a rational strategy to achieve multifunctional N, S codoped carbon dots (N, S-CDs) is reported, aiming to improve the photoluminescence quantum yields (PLQYs) of the CDs. The synthesized N, S-CDs have excellent stability and emission properties independent of excitation wavelength. Through the introduction of S element doping, the fluorescence emission of CDs is red-shifted from 430 to 545 nm, and the corresponding PLQYs can be greatly enhanced from 11.2% to 65.1%. It is found that the doping of S elements causes an increase in the size of CDs and an elevated graphite N content, which may be the key factors to cause the redshift of fluorescence emission. Furthermore, the introduction of S element also serves to suppress the nonradiative transitions, which may be responsible for the elevated PLQYs. Besides, the synthesized N, S-CDs have certain solvent effect and can be applied to detect water content in organic solvents, and have strong sensitivity to alkaline environment. More importantly, the N, S-CDs can be used to achieve an "on-off-on" dual detection mode between Zr4+ and NO2 - . In addition, N, S-CDs combinedwith polyvinylpyrrolidone (PVP) can also be utilized as fluorescent inks for anti-counterfeiting applications.

A ratiometric fluorescence platform for lead ion detection via RNA cleavage-inhibited self-assembly of three-arm branched junction

Heavy metal pollution can pose a threat to food safety and human health, and accurate quantification of heavy metal ions is a vital requirement. Emerging DNA nanostructures-based biosensors offer attractive tools toward ultra-sensitive or rapid analysis of heavy metal ions. However, the problems including complex design, severe reaction conditions and undesirable reliability are inevitable obstacle in advancing their extension and application. Herein, a ratiometric fluorescent platform was established for monitoring lead ion (Pb²⁺) in food based on dual Förster resonance energy transfer (FRET) and RNA cleavage-inhibited self-assembly of three-arm branched junction (TBJ). GR-5 DNAzyme was employed for Pb²⁺ recognition, and enzyme-free amplification technique catalytic hairpin assembly (CHA) served to form FRET probes-carried TBJ. The substrate strand (S) of DNAzyme triggered the generation of CHA-TBJ, and Pb²⁺-responsive cleavage of S hindered the assembly of CHA-TBJ, causing opposite changes in the FRET states of FAM/BHQ1 and ROX/BHQ2 pairs. The fluorescence responses were recorded through synchronous fluorescence spectrometry to indicate Pb²⁺ concentration, allowing sensitive and reliable identification of Pb²⁺ in the linear range of 0.05-5 ng mL^-1 with the detection limit of 0.03 ng mL^-1. The Pb²⁺ detection can be achieved under conventional reaction conditions, simple mixing procedures and one-step measurement operation. The approach can afford excellent specificity for Pb²⁺ against competing metal ions, and can be applied to analyze Pb²⁺ in tea samples with satisfactory results. This facile fluorescence platform shows a capable method for Pb²⁺ detection, and provides new avenue in the development of ratiometric approaches and DNAzyme strategies for monitoring heavy metal pollution, facilitating the transformation of DNAzyme-based biosensors for food safety control.

Engineering DNAzyme strategies for fluorescent detection of lead ions based on RNA cleavage-propelled signal amplification

Based on the high recognition ability and flexible programmability of GR5 DNAzyme, two fluorescent biosensors were engineered for amplified detection of Pb²⁺ via incorporating Ti₃C₂T_X MXenes and embedding 2-aminopurine (2-AP), respectively. The quencher-required approach relied on the DNA affinity and fluorescence quenching ability of Ti₃C₂T_X MXenes. Benefiting from the low background signal modulated by Ti₃C₂T_X MXenes, the sensitive determination of Pb²⁺ was achieved in the linear range of 0.2-10 ng mL^-1 with the limit of detection (LOD) of 0.05 ng mL^-1. The quencher-free approach combined the fluorescent trait of 2-AP embedded in DNA structure, and the RNA cleavage-propelled digestion process of Exonuclease I (Exo I) for signal amplification, indicating the sensitive detection of Pb²⁺ with the LOD as low as 0.02 ng mL^-1 in the linear range of 0.1-10 ng mL^-1. Both DNAzyme assays exhibited simple procedures, favorable specificity, rapid analysis, and satisfactory application in standard reference materials (lead in drinking water) and spiked water samples. The two fluorescent biosensors established in this work would not only provide theoretic fundament for DNA adsorption of Ti₃C₂T_X MXenes and the design of 2-AP-embedded DNAzyme assays, but also hold a great potential for on-site monitoring of lead pollution in water samples.

A visible-light-responsive DiSCn(3)-type fluorescent probe for the rapid, sensitive, and specific detection of tin(II) ions in aqueous solution

Visible-light-excited fluorescent probes with excellent water solubility are of significant research importance in the biomedical field. In this work, a series of visible-light-responsive carbocyanine fluorescent compounds [DiSCn(3)] are synthesized from 2-methylbenzothiazole and are employed as fluorescent probes for detecting tin(II) ions. Upon addition of tin(II) ions to an aqueous solution of compounds DiSCn(3)-3 and 4, both exhibit considerable changes in their absorption and fluorescent intensity, respectively. These spectral changes may correspond to the binding between tin(II) ions and the sulfur atom in the benzothiazole unit to form a metal complex. This indicates that DiSCn(3)-3 and 4 can be used as promising fluorescent chemical sensors for detecting tin(II) ions in aqueous solution via visible-light excitation.

DNAzyme-based biosensors for mercury (Ⅱ) detection: Rational construction, advances and perspectives

Mercury contamination is one of the most severe issues in society due to its threats to public health and the ecological system. However, traditional methods for mercury ion detection are still limited by their time-consuming procedures, requirement of expensive instruments, and low selectivity. In recent decades, tremendous progress has been made in the development of functional nucleic acid-based, especially DNAzyme sensors for mercury (Ⅱ) (Hg²⁺) determination, including RNA-cleaving DNAzymes and G-quadruplex-based DNAzymes in particular. Researchers have heavily studied the construction of Hg²⁺ sensors, mainly originating from in vitro selection-derived DNAzymes, by incorporating T-Hg²⁺-T recognition moieties in existing DNAzyme scaffolds, and interfacing Hg²⁺-sensitive sequences with nanomaterials. In the last case, the employment of materials (as quenchers, signal transducers and DNA immobilizers) enriches the application scenarios of current Hg²⁺-DNAzymes, due to a combination of their functions. We summarize a broad range of sensing approaches, including optical, electrochemical, and other sensing methods, and compare their features. This review elaborates on the rational design strategies for engineering DNAzymes to selectively sense Hg²⁺, critically discusses their properties in different application scenarios, and summarizes recent advances in this field. Additionally, current progress, challenges and future perspectives are also discussed. This minireview provides deeper insights into the chemistry of these functional nucleic acids when working with Hg²⁺, explains the design ideas of DNAzyme-sensors in each platform, and reveals potential opportunities in developing more advanced DNAzyme sensors for the highly selective and sensitive recognition of Hg²⁺. ENVIRONMENTAL IMPLICATION: Mercury is one of the most toxic metallic contaminants due to its high toxicity, non-biodegradability, and serious human health risks when accumulated in the body. In the recent decade, intensive studies have focused on exploring mercury sensors by combining DNAzymes with various sensing methods, paving a promising avenue to gain ultra-high sensitivity and selectivity. However, so far, no review has introduced the recent advances on DNAzyme-based sensors for mercury detection in a critical way. In this review, we comprehensively summarized the studies on DNAzyme-based sensors for mercury detection using various sensing techniques including optical, electrochemical and other sensing methods.

Adsorption of DNA Oligonucleotides by Self-Assembled Metalloporphyrin Nanomaterials

Porphyrin assemblies have controllable morphology, high biocompatibility, and good optical properties and were widely used in biomedical diagnosis and treatment. With the development of DNA biotechnology, combining DNA with porphyrin assemblies can broaden the biological applications of porphyrins. Porphyrin assemblies can serve as nanocarriers for DNA, although the fundamental interactions between them are not well understood. In this work, zinc meso-tetra(4-pyridyl)porphyrin (ZnTPyP) assemblies were prepared in the presence of various surfactants and at different pH values, yielding a variety of aggregation forms. Among them, the hexagonal stacking form exposes more pyridine substituents, and the hydrogen bonding force between the substituents and the DNA bases allows the DNA to be quickly adsorbed on the surface of the assemblies. The effects of DNA sequence and length were systematically tested. In particular, the adsorption of duplex DNA was less efficient compared to the adsorption of single-stranded DNA. This fundamental study is useful for the further combination of DNA and porphyrin assemblies to prepare new functional hybrid nanomaterials.

DNAzyme-Functionalized Nanomaterials: Recent Preparation, Current Applications, and Future Challenges

DNAzyme-nanomaterial bioconjugates are a popular hybrid and have received major attention for diverse biomedical applications, such as bioimaging, biosensor development, cancer therapy, and drug delivery. Therefore, significant efforts are made to develop different strategies for the preparation of inorganic and organic nanoparticles (NPs) with specific morphologies and properties. DNAzymes functionalized with metal-organic frameworks (MOFs), gold nanoparticles (AuNPs), graphene oxide (GO), and molybdenum disulfide (MoS₂ ) are introduced and summarized in detail in this review. Moreover, the focus is on representative examples of applications of DNAzyme-nanomaterials over recent years, especially in bioimaging, biosensing, phototherapy, and stimulation response delivery in living systems, with their several advantages and drawbacks. Finally, the perspective regarding the future directions of research addressing these challenges is also discussed and highlighted.

Development of gold nanoparticles-aptamer nanocomposite for multiplexed analysis of antibiotics and design of molecular logic gates

The widespread use of antibiotics caused severe problems of antibiotic residues in foodstuffs and water, posing a serious threat to public health and thus urging the development of sensitive, selective, and rapid detection methods for antibiotics. In this study, a fluorescence resonance energy transfer (FRET)-based system is developed for the multiplexed analysis of chloramphenicol (CAP) and streptomycin (Strep) with detection limits of 2.51 and 8.69μg l^-1, respectively. The FRET-based system consists of Cy3-tagged anti-CAP aptamer-conjugated gold nanoparticles (AuNPs) (referred to as AuNPs-AptCAP) and Cy5-tagged anti-Strep aptamer-conjugated AuNPs (referred to as AuNPs-AptStrep). In addition, AuNPs-AptCAP and AuNPs-AptStrep have been demonstrated to serve as signal transducers for implementing a series of logic operations such as YES, NOT, INH, OR, (2-4)-Decoder and even more complicated multi-level logic gates (OR-INH). Based on the outputs of logic operations, it could be figured out whether targeted analytes were present or not, thus enabling multiplex sensing and evaluation of pollution status. This proof of concept study might provide a new route for the enhanced sensing performance to distinguish different pollution status as well as the design of molecular mimics of logic elements to demonstrate better applicability.

Self-paired dumbbell DNA -assisted simple preparation of stable circular DNAzyme and its application in Pb2+ sensor

During its development in recent decades, DNAzyme has become a promising candidate for application in biosensor field. However, it still suffers from the problem of thermodynamic and biological instability such as nuclease digestion, which limits its applications in complex samples. Here we have presented a simple and common strategy to resolve this problem by engineering the linear DNAzyme into a circular shape DNAzyme based on the integration of substrate and enzyme parts into one single-stranded sequence. This circular DNAzyme system is indeed endowed with excellent stability due to the stable intramolecular double-stranded formation and extraordinary resistance to nuclease digestion due to the closed structure. We demonstrated that this circular DNAzyme system gained excellent stability and could active under conditions across a broader range of temperature, salt concentrations, and pH. Depending on this circular DNAzyme, combing with Terminal deoxynucleotidyl transferase (TdT)-generated G-quadruplexes, a label free colorimetric sensing platform for Pb²⁺ quantitation was developed, and a detection limit of 0.085 nM was achieved. Then the enzyme digestion cycle amplification was introduced to further improve the sensitivity of the sensing system, an ultralow detection limit of 0.0015 nM for this fluorescence method was achieved. Based on the two sensing platforms, ultrasensitive analysis of Pb²⁺ in environmental water and food samples was successfully realized. It is anticipated that this stable circular DNAzyme design will be helpful for trace detection in complex samples.

Logic Gates Based on DNA Aptamers

DNA bio-computing is an emerging trend in modern science that is based on interactions among biomolecules. Special types of DNAs are aptamers that are capable of selectively forming complexes with target compounds. This review is devoted to a discussion of logic gates based on aptamers for the purposes of medicine and analytical chemistry. The review considers different approaches to the creation of logic gates and identifies the general algorithms of their creation, as well as describes the methods of obtaining an output signal which can be divided into optical and electrochemical. Aptameric logic gates based on DNA origami and DNA nanorobots are also shown. The information presented in this article can be useful when creating new logic gates using existing aptamers and aptamers that will be selected in the future.

"Plug and Play" logic gate construction based on chemically triggered fluorescence switching of gold nanoparticles conjugated with Cy3-tagged aptamer

Gold nanoparticles (AuNPs) conjugated with Cy3-tagged aptamer which can specifically recognize chloramphenicol (CAP) (referred to as AuNPs-AptCAP) are described. CAP can trigger the configuration change of CAP binding aptamer, and thus switching the fluorescence of AuNPs-AptCAP through changing the efficiency of the fluorescence resonance energy transfer (FRET) system with Cy3 as donors and AuNPs as recipients. AuNPs-AptCAP exhibits a linear range of CAP concentrations from 26.0 to 277 μg L^-1 with a limit of detection of 8.1 μg L^-1 when Cy3 was excited at 530 nm and emission was measured at 570 nm. More importantly, AuNPs-AptCAP can be utilized as signal transducers for the build-up of a series of logic gates including YES, PASS 0, INH, NOT, PASS 1, and NAND. Utilizing the principle of a metal ion-mediated fluorescence switch together with a strong metal ion chelator, the fluorescence of AuNPs-AptCAP could be modulated by adding metal ions and EDTA sequentially. Therefore, a "Plug and Play" logic system based on AuNPs-AptCAP has been realized by simply adding other components to create new logic functions. This work highlights the advantages of simple synthesis and facile fluorescence switching properties, which will provide useful knowledge for the establishment of molecular logic systems. Graphical abstract.

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.

DNA-Templated Fluorescent Nanoclusters for Metal Ions Detection

DNA-templated fluorescent nanoclusters (NCs) have attracted increasing research interest on account of their prominent features, such as DNA sequence-dependent fluorescence, easy functionalization, wide availability, water solubility, and excellent biocompatibility. Coupling DNA templates with complementary DNA, aptamers, G-quadruplex, and so on has generated a large number of sensors. Additionally, the preparation and applications of DNA-templated fluorescent NCs in these sensing have been widely studied. This review firstly focuses on the properties of DNA-templated fluorescent NCs, and the synthesis of DNA-templated fluorescent NCs with different metals is then discussed. In the third part, we mainly introduce the applications of DNA-templated fluorescent NCs for sensing metal ions. At last, we further discuss the future perspectives of DNA-templated fluorescent NCs in the synthesis and sensing metal ions in the environmental and biological fields.

Colorimetric Technique for Antimony Detection Based on the Use of Gold Nanoparticles Conjugated with Poly-A Oligonucleotide

A simple and rapid positive–negative colorimetric approach to determine the presence of antimony ions based on the use of gold nanoparticles conjugated with oligonucleotide (poly-A sequence) is developed. Colorimetric measurements reveal that the aggregates of modified gold nanoparticles were afforded after adding antimony ions, thus changing the solution color from pink to blue. The results of aptamer’s interaction on the gold nanoparticle surface with the target analyte can be detected either by photometry or by the naked eye. The realized assay provides rapid (2 min), sensitive (detection limit 10 ng/mL), specific, and precise (variation coefficient less than 3.8%) detection of antimony (III) in drinking water.

Disposable Sensors in Diagnostics, Food, and Environmental Monitoring

Disposable sensors are low-cost and easy-to-use sensing devices intended for short-term or rapid single-point measurements. The growing demand for fast, accessible, and reliable information in a vastly connected world makes disposable sensors increasingly important. The areas of application for such devices are numerous, ranging from pharmaceutical, agricultural, environmental, forensic, and food sciences to wearables and clinical diagnostics, especially in resource-limited settings. The capabilities of disposable sensors can extend beyond measuring traditional physical quantities (for example, temperature or pressure); they can provide critical chemical and biological information (chemo- and biosensors) that can be digitized and made available to users and centralized/decentralized facilities for data storage, remotely. These features could pave the way for new classes of low-cost systems for health, food, and environmental monitoring that can democratize sensing across the globe. Here, a brief insight into the materials and basics of sensors (methods of transduction, molecular recognition, and amplification) is provided followed by a comprehensive and critical overview of the disposable sensors currently used for medical diagnostics, food, and environmental analysis. Finally, views on how the field of disposable sensing devices will continue its evolution are discussed, including the future trends, challenges, and opportunities.

Nicking enzyme-assisted signal-amplifiable Hg2+ detection using upconversion nanoparticles

A highly specific and sensitive isothermal method for mercury detection using DNA-conjugated upconversion nanoparticles is reported. A single-stranded DNA containing thymine bases, used as the Hg²⁺-capturing element through the formation of thymine-Hg²⁺-thymine complex, is covalently attached to the NaYF₄: Yb^3+, Tm³⁺ nanoparticles. Luminescence resonance energy transfer takes place between the NaYF₄: Yb^3+, Tm³⁺ nanoparticles as donor and DNA-intercalating SYBR Green I as the acceptor upon excitation of 980 nm. The sensitivity and selectivity toward Hg²⁺ are enhanced using the nicking enzyme, Nt. Alwl, which leads to signal amplification. By monitoring the ratio of acceptor emission to a reference peak, the presence of Hg²⁺ ions are quantitatively determined with a lower detection limit of 0.14 nM, which is much lower than the US Environmental Protection Agency (EPA) limit of Hg²⁺ in drinking water.

Progress in rapid optical assays for heavy metal ions based on the use of nanoparticles and receptor molecules

This review (with 230 refs.) covers recent progress in rapid optical assays for heavy metals (primarily lead and mercury as the most relevant) based on the use of nanoparticles and receptor molecules. An introduction surveys the importance, regulatory demands (such as maximum permissible concentrations) and potential and limitations of various existing methods. This is followed by a general discussion on the use of nanoparticles in optical assays of heavy metals (including properties, basic mechanisms of signal generation). The next sections cover methods for the functionalization of nanoparticles with (a) sulfur-containing compounds (used for modification of nanoparticles or added to the reaction medium), (b) nitrogen-containing compounds (such as amino acids, polypeptides, and heterocyclic molecules), and (c) oxygen-containing species (such as hydroxy and carbonyl compounds). This is continued by a specific description of specific assays based on the use of aptamers as receptors, on the use of deoxyribozymes as synthetic reaction catalysts, of G-quadruplex aptamers, of aptamers in logic gate-type of assays of linear (unstructured) aptamers ("hairpins"), and on the use of aptamers in lateral flow assays. A next section covers assays based on the employment of antibodies as receptors (used in the immunoassay development). The properties of various nanoparticles and their applicability in optical assays are also discussed in some detail. Final sections discuss the selectivity of assays, potential interferences by other cations, methods for their elimination, and also matrix effects and approaches for sample pretreatment. A concluding section discusses current challenges and future trends. Analysis based on enzyme inhibition assay is not treated here but enzyme-like action of some receptor molecules such as DNAzymes is discussed. Graphical abstract Schematic presentation of main principles of application of various nanoparticles with receptor molecules (S-, N-, O-containing, heterocyclic compounds, proteins, antibody, aptamers) for heavy metals ions detection. The included methods cover optical assays with description of mechanisms of interactions and signal generation.

An Ultrasensitive Colorimetric Strategy for Detection of Cadmium Based on the Peroxidase-like Activity of G-Quadruplex-Cd(II) Specific Aptamer

We rationally designed an ultrasensitive and label-free sensing platform for determination of cadmium (Cd). The sensing platform contains G-quadruplex-Cd(II) specific aptamer (GCDSA) constructed by incorporating G-rich sequence at the end of 5' and the critical domain of the Cd-4 aptamer. GCDSA designed act as both a special recognition sequence for Cd²⁺ and a signal DNAzyme. In absence of Cd²⁺, GCDSA may mainly exist in a random coil sequence. Upon addition of Cd²⁺, GCDSA could probably be induced to fold into a G-quadruplex structure. The generation of plentiful active G-quadruplex interacts with hemin to form a peroxidase-like DNAzyme, leading to increased absorbance signal of the sensing system. ΔA was directly proportional to the two segments of concentrations for Cd²⁺, with the detection of limit of 0.15 nM. The proposed method avoids the labeled oligonucleotides and allows directly quantitative analysis of the samples by cheap instruments, with an excellent dynamic range.

Metallic-semiconducting junctions create sensing hot-spots in carbon nanotube FET aptasensors near percolation

Easily fabricated random network carbon nanotube field-effect transistors (CNT-FETs) have benefitted from improved separation techniques to deliver CNTs with current formulations providing at least 99% semiconducting tube content. Amongst the most promising applications of this device platform are electronic biosensors, where the network conduction is affected through tethered probes such as aptamers which act as molecular scale electrostatic gates. However, the prevailing assumption that these biosensor devices would be optimized if metallic tubes were entirely eliminated has not been examined. Here, we show that metallic-semiconducting junctions in aptasensors are sensing hotspots and that their impact on sensing is heightened by the CNT network's proximity to percolation. First, we use a biased conducting AFM tip to gate a CNT-FET at the nanoscale and demonstrate that the strongest device response occurs when gating at metallic-semiconducting junctions. Second, we resolve the target sensitivity of an aptasensor as a function of tube density and show heightened sensitivity at densities close to the percolation threshold. We find the strongest sensing response where the 1% of metallic tubes generate a high density of metallic-semiconducting junctions but cannot form a percolated metallic path across the network. These findings highlight the critical role of metallic tubes in CNT-FET biosensor devices and demonstrate that network composition is an important variable to boost the performance of electronic biosensors.

Simultaneous detection and determination of mercury (II) and lead (II) ions through the achievement of novel functional nucleic acid-based biosensors

The serious threats of mercury (Hg²⁺) and lead (Pb²⁺) ions for the public health makes it important to achieve the detection methods of the ions with high affinity and specificity. Metal ions usually coexist in some environment and foodstuff or clinical samples. Therefore, it is very necessary to develop a fast and simple method for simultaneous monitoring the amount of metal ions, especially when Hg²⁺ and Pb²⁺ coexist. DNAzyme-based biosensors and aptasensors have been highly regarded for this purpose as two main groups of the functional nucleic acid (FNA)-based biosensors. In this review, we summarize the recent achievements of functional nucleic acid-based biosensors for the simultaneous detection of Hg²⁺ and Pb²⁺ ions in two main optical and electrochemical groups. The tremendous interest in utilizing the various nanomaterials is also highlighted in the fabrication of the FNA-based biosensors. Finally, some results are presented based on the advantages and disadvantages of the studied FNA-based biosensors to compare their validation.

A highly selective fluorescence sensing platform for nanomolar Hg(II) detection based on cytosine derived quantum dot

Inspired by low toxicity and good biocompatibility of biomass derived quantum dot (QD), we herein developed a cytosine derived quantum dot, namely cyt-dot, via a one-step hydrothermal synthesis. The as-prepared cyt-dot emits blue fluorescence (FL) containing abundant oxygen (20.6at.%) and nitrogen (24.1at.%) contents. The cyt-dot based sensing platform shows exclusive selectivity for Hg(II) while being insensitive towards Fe(III) and Ag(I), which are important interference that usually cannot be ruled out. The detection limit for Hg(II) is of 11nM, which is very close to the guideline value of 10nM allowed by the U.S. Environmental Protection Agency in drinking water. In real water sample analyses, the present sensing platform can fulfil satisfied recoveries ranging from 100% to 108%. Besides, the acidity of solution has almost no effect on the sensing performance of the cyt-dot in a pH range of 5-8, suggesting its potential applications in sensing and bio-imaging.

Preparation of enzyme-functionalized carbon nanotubes and their application in glucose and Fe2+ detection through “turn on” and “turn off” approaches

Herein, we report the preparation of enzyme-conjugated carbon nanotubes for the detection of Fe²⁺ and glucose with enhanced signal intensity.