Label-free chemiresistive biosensor for mercury (II) based on single-walled carbon nanotubes and structure-switching DNA

Carbon Nanotube (CNT)-Based Biosensors

This review focuses on recent advances in the application of carbon nanotubes (CNTs) for the development of sensors and biosensors. The paper discusses various configurations of these devices, including their integration in analytical devices. Carbon nanotube-based sensors have been developed for a broad range of applications including electrochemical sensors for food safety, optical sensors for heavy metal detection, and field-effect devices for virus detection. However, as yet there are only a few examples of carbon nanotube-based sensors that have reached the marketplace. Challenges still hamper the real-world application of carbon nanotube-based sensors, primarily, the integration of carbon nanotube sensing elements into analytical devices and fabrication on an industrial scale.

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.

Solid-State Chemiresistors from Two-Dimensional MoS2 Nanosheets Functionalized with l-Cysteine for In-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water

Sensing of metal contaminants at ultralow concentrations in aqueous environments is vital in today's overpopulated world, with an extremely stringent limit (<5 ppb) for Cd2+ ions in drinking water. Here, we utilize sonochemically exfoliated molybdenum disulfide (MoS2) nanosheets functionalized with l-cysteine (Cys) as highly sensitive and selective two-dimensional (2D) materials for solid-state chemiresistors. We specifically targeted Cd2+ ions due to their high toxicity at low concentrations. MoS2-Cys nanosheets are fabricated using an ad hoc, low-complexity, one-pot synthesis method. Porous MoS2-Cys thin films with a high surface area are assembled from these nanosheets. Two-terminal chemiresistors incorporating MoS2-Cys films are demonstrated to be preferentially sensitive to Cd2+ ions at neutral pH, irrespective of other metal ions present in water flowing through the device. A 5 ppb concentration of the Cd2+ ions in the water stream increases the device resistivity by 20 times. Our devices operate at broad (1-500 ppb) range and fast (∼1 s) response times. Cd2+ is selectively detected because of preferential, size-driven adsorption at the interstitials between l-cysteine functional groups, combined with pH-controlled charge transfer that removes electronic gap states from MoS2. MoS2-Cys-based chemiresistors can be deployed in-line to detect metal ions without any need for additional offline measurements.

Metal Cation Detection in Drinking Water

Maintaining a clean water supply is of utmost importance for human civilization. Human activities are putting an increasing strain on Earth’s freshwater reserves and on the quality of available water on Earth. To ensure cleanliness and potability of water, sensors are required to monitor various water quality parameters in surface, ground, drinking, process, and waste water. One set of parameters with high importance is the presence of cations. Some cations can play a beneficial role in human biology, and others have detrimental effects. In this review, various lab-based and field-based methods of cation detection are discussed, and the uses of these methods for the monitoring of water are investigated for their selectivity and sensitivity. The cations chosen were barium, cadmium, chromium, copper, hardness (calcium, magnesium), lead, mercury, nickel, silver, uranium, and zinc. The methods investigated range from optical (absorbance/fluorescence) to electrical (potentiometry, voltammetry, chemiresistivity), mechanical (quartz crystal microbalance), and spectrometric (mass spectrometry). Emphasis is placed on recent developments in mobile sensing technologies, including for integration into microfluidics.

Changing the Blood Test: Accurate Determination of Mercury(II) in One Microliter of Blood Using Oriented ZnO Nanobelt Array Film Solution-Gated Transistor Chips

The measurement of ultralow concentrations of heavy metal ions (HMIs) in blood is challenging. A new strategy for the determination of mercury ions (Hg²⁺ ) based on an oriented ZnO nanobelt (ZnO-NB) film solution-gated field-effect transistor (FET) chip is adopted. The FET chips are fabricated with ZnO-NB film channels with different orientations utilizing the Langmuir-Blodgett (L-B) assembly technique. The combined simulation and I-V behavior results show that the nanodevice with ZnO-NBs parallel to the channel has exceptional performance. The sensing capability of the oriented ZnO-NB film FET chips corresponds to an ultralow minimum detectable level (MDL) of 100 × 10^-12 m in deionized water due to the change in the electrical double layer (EDL) arising from the synergism of the field-induced effect and the specific binding of Hg²⁺ to the thiol groups (-SH) on the film surface. Moreover, the prepared FET chips present excellent selectivity toward Hg²⁺ , excellent repeatability, and a rapid response time (less than 1 s) for various Hg²⁺ concentrations. The sensing performance corresponds to a low MDL of 10 × 10^-9 m in real samples of a drop of blood.

Helical Assembly of Flavin Mononucleotides on Carbon Nanotubes as Multimodal Near-IR Hg(II)-Selective Probes

The development of novel methods to detect mercury is of paramount importance owing to the impact of this metal on human health and the environment. We observed that flavin mononucleotide (FMN) and its helical assembly with a single-walled carbon nanotube (SWNT) selectively bind Hg²⁺ arising from HgCl₂ and MeHgCl. Absorption spectroscopic studies show that FMN preferentially forms a 2:1 rather than a 1:1 complex with Hg²⁺ at high FMN concentrations. On the basis of the analogy to the thymine-Hg-thymine complex, it is proposed that the 2:1 complex between FMN and Hg²⁺ comprises a Hg-bridged pair of FMN groups, regardless of the presence of SWNT. Upon addition of as little as a few hundred nanomoles of Hg²⁺, both FMN and FMN-SWNT exhibit absorption and photoluminescence (PL) changes. Moreover, FMN-SWNT displays simultaneous multiple sigmoidal changes in PL of SWNT tubes having different chiral vectors. Assessment of binding affinities using the Hill equation suggests that 2:1 and 1:1 complexes form between Hg²⁺ and FMN groups on the FMN-SWNT. Theoretical calculations indicate that optical changes of the FMN-SWNT originate from Hg-mediated conformational changes occurring on the helical array of FMN on the SWNT. High-resolution transmission electron microscopy revealed that the presence of Hg²⁺ in complexes with the FMN-SWNT enables visualization of helical periodic undulation of FMN groups along SWNT without the need for staining. Circular dichroism (CD) study revealed that FMN-SWNT whose CD signal mainly originates from FMN decreases dichroic bands upon the addition of Hg²⁺ owing to the formation of a centrosymmetric FMN-Hg-FMN triad on SWNT. The binding mode specificity and multimodal changes observed in response to Hg²⁺ ions suggest that systems based on FMN-SWNT can serve as in vivo NIR beacons for the detection of various mercury derivatives.

An oligonucleotide-functionalized carbon nanotube chemiresistor for sensitive detection of mercury in saliva

Divalent mercuric (Hg(2+)) ion and monomethyl mercury (CH3Hg(+)) are two forms of mercury that are known to be highly toxic to humans. In this work, we present a highly selective, sensitive and label-free chemiresistive biosensor for the detection of both, Hg(2+) and CH3Hg(+) ions using DNA-functionalized single-walled carbon nanotubes (SWNTs). The SWNTs were functionalized with the capture oligonucleotide, polyT, using a linker molecule. The polyT was hybridized with polyA to form a polyT:polyA duplex. Upon exposure to mercury ions, the polyT:polyA duplex dehybridizes and a T-Hg(2+)-T duplex is formed. This structure switch leads to the release of polyA from the SWNT surface and correspondingly a change in the resistance of the chemiresistive biosensor is observed, which is used to quantify the mercury ion concentration. The biosensor showed a wide dynamic range of 0.5 to 100 nM for the detection of CH3Hg(+) ions in buffer solution with a sensitivity of 28.34% per log (nM) of CH3Hg(+). Finally, real world application of the biosensor was demonstrated by the detection of Hg(2+) and CH3Hg(+) ions in simulated saliva samples spiked with a known concentration of mercury ions.

Metal ion detection using functional nucleic acids and nanomaterials

Metal ion detection is critical in a variety of areas. The past decade has witnessed great progress in the development of metal ion sensors using functional nucleic acids (FNAs) and nanomaterials. The former has good recognition selectivity toward metal ions and the latter possesses unique properties for enhancing the performance of metal ion sensors. This review offers a summary of FNA- and nanomaterial-based metal ion detection methods. FNAs mainly include DNAzymes, G-quadruplexes, and mismatched base pairs and nanomaterials cover gold nanoparticles (GNPs), quantum dots (QDs), carbon nanotubes (CNTs), and graphene oxide (GO). The roles of FNAs and nanomaterials are introduced first. Then, various methods based on the combination of different FNAs and nanomaterials are discussed. Finally, the challenges and future directions of metal ion sensors are presented.

Novel carbon nanotube (CNT)-based ultrasensitive sensors for trace mercury(II) detection in water: A review

Infamous for "Mad hatter syndrome" and "Minamata disease", mercury (Hg) is ranked high on the Agency for Toxic Substances and Disease Registry's priority list of hazardous substances for its potent neurologic, renal, and developmental toxicities. Most typical exposures are via contaminated water and food. Although regulations and advisories are exercised at various levels, Hg pollution from both natural and anthropogenic sources has remained a major public health and safety concern. Rapid detection of solvated aqueous Hg²⁺ ions at low levels is critical for immediate response and protection of those who are vulnerable (young children, pregnant and breast-feeding women) to acute and chronic exposures to Hg²⁺. Various types of sensors capable of detecting Hg in water have been developed. In particular, the novel use of engineered carbon nanotubes (CNTs) has garnered attention due to their specificity and sensitivity towards Hg²⁺ detection in solution. In this focused review, we describe the sensitivity, selectivity and mechanisms of Hg²⁺ ion sensing at trace levels by employing CNT-based various sensor designs, and appraise the open literature on the currently applied and "proof-of-concept" methods. Five different types of CNT-based sensor systems are described: potentiometric, DNA-based fluorescence, surface plasmon resonance (SPR), colorimetric, and stripping voltammetric assays. In addition, the recognized merits and shortcomings for each type of electrochemical sensors are discussed. The knowledge from this succinct review shall guide the development of the next generation CNT-based biochemical sensors for rapid Hg²⁺ detection in the environment, which is a significant first step towards human health risk analysis of this legacy toxicant.

An electrochemically reduced graphene oxide chemiresistive sensor for sensitive detection of Hg2+ ion in water samples

Divalent mercuric (Hg²⁺) ion is one of the most prevalent forms of mercury species in waters with high toxicity and bioaccumulation in the human body, for which sensitive and selective detection methods are highly necessary to carry out its recognition and quantification. Here an electrochemically reduced graphene oxide (RGO) based chemiresistive sensor was constructed and used for the detection of Hg²⁺ ion in various water samples. Monolayer GO sheets were assembled onto interdigitated electrodes, followed by reduction through linear sweep voltammetry and then modification with a single-stranded DNA aptamer. The electrochemically derived RGO based sensor showed selective response to as low as 0.5nMHg²⁺ ion in presence of other metal ions and matrices. A comparison between chemiresistive sensors prepared with electrochemically and chemically derived RGO showed that the former had better response performance for sensing Hg²⁺ ion. The proposed method provides a simple tool for rapid, selective and sensitive monitoring of Hg²⁺ ion in environmental samples.

Small-Molecule Binding Aptamers: Selection Strategies, Characterization, and Applications

Aptamers are single-stranded, synthetic oligonucleotides that fold into 3-dimensional shapes capable of binding non-covalently with high affinity and specificity to a target molecule. They are generated via an in vitro process known as the Systematic Evolution of Ligands by EXponential enrichment, from which candidates are screened and characterized, and then used in various applications. These applications range from therapeutic uses to biosensors for target detection. Aptamers for small molecule targets such as toxins, antibiotics, molecular markers, drugs, and heavy metals will be the focus of this review. Their accurate detection is needed for the protection and wellbeing of humans and animals. However, the small molecular weights of these targets, including the drastic size difference between the target and the oligonucleotides, make it challenging to select, characterize, and apply aptamers for their detection. Thus, recent (since 2012) notable advances in small molecule aptamers, which have overcome some of these challenges, are presented here, while defining challenges that still exist are discussed.

Carbon nanomaterial-based electrochemical biosensors for label-free sensing of environmental pollutants

Carbon allotropes such as graphene and carbon nanotubes, have been incorporated in electrochemical biosensors for highly sensitive and selective detection of various analytes. The superior physical and electrical properties like high carrier mobility, ambipolar electric field effect, high surface area, flexibility and their compatibility with microfabrication techniques makes these carbon nanomaterials easy to integrate in field-effect transistor (FET)/chemiresistor type configuration which is suitable for portable and point-of-use/field-deployable sensors. This review covers the synthesis of carbon nanostructures (graphene and CNTs) and their integration into devices using various fabrication methods. Finally, we discuss the recent reports showing different sensing platforms that incorporate biomolecules like enzymes, antibodies and aptamers as recognition elements for fabrication of simple, low cost, compact biosensors that can be used for on-site, rapid environmental monitoring of environmental pollutants like pathogens, heavy metals, pesticides and explosives.

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.

Label-Free Electrical Immunosensor for Highly Sensitive and Specific Detection of Microcystin-LR in Water Samples

Microcystin-LR (MCLR) is one of the most commonly detected and toxic cyclic heptapeptide cyanotoxins released by cyanobacterial blooms in surface waters, for which sensitive and specific detection methods are necessary to carry out its recognition and quantification. Here, we present a single-walled carbon nanotube (SWCNTs)-based label-free chemiresistive immunosensor for highly sensitive and specific detection of MCLR in different source waters. MCLR was initially immobilized on SWCNTs modified interdigitated electrode, followed by incubation with monoclonal anti-MCLR antibody. The competitive binding of MCLR in sample solutions induced departure of the antibody from the antibody-antigen complexes formed on SWCNTs, resulting in change in the conductivity between source and drain of the sensor. The displacement assay greatly improved the sensitivity of the sensor compared with direct immunoassay on the same device. The immunosensor exhibited a wide linear response to log value of MCLR concentration ranging from 1 to 1000 ng/L, with a detection limit of 0.6 ng/L. This method showed good reproducibility, stability and recovery. The proposed method provides a powerful tool for rapid and sensitive monitoring of MCLR in environmental samples.

Hybrids of Nucleic Acids and Carbon Nanotubes for Nanobiotechnology

Recent progress in the combination of nucleic acids and carbon nanotubes (CNTs) has been briefly reviewed here. Since discovering the hybridization phenomenon of DNA molecules and CNTs in 2003, a large amount of fundamental and applied research has been carried out. Among thousands of papers published since 2003, approximately 240 papers focused on biological applications were selected and categorized based on the types of nucleic acids used, but not the types of CNTs. This survey revealed that the hybridization phenomenon is strongly affected by various factors, such as DNA sequences, and for this reason, fundamental studies on the hybridization phenomenon are important. Additionally, many research groups have proposed numerous practical applications, such as nanobiosensors. The goal of this review is to provide perspective on biological applications using hybrids of nucleic acids and CNTs.

A novel biosensor for silver(i) ion detection based on nanoporous gold and duplex-like DNA scaffolds with anionic intercalator

A novel biosensor for silver(i) ion detection based on nanoporous gold and duplex-like DNA scaffolds with anionic intercalator.

mga genosensor for early detection of human rheumatic heart disease

The 5' amino-labeled DNA probe complementary to mga gene of Streptococcus pyogenes was immobilized on carboxylated multiwall carbon nanotubes electrode and hybridized with 0.1-100 ng/6 μl single-stranded genomic DNA (ssG-DNA) of S. pyogenes from throat swab of suspected rheumatic heart disease (RHD) patients. Electrochemical response was measured by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance (EI). The sensitivity of the sensor was 106.03 (μA/cm(2))/ng and limit of detection (LOD) was found 0.014 ng/6 μl with regression coefficient (R(2)) of 0.921 using DPV. The genosensor was characterized by FTIR and SEM, and electrode was found stable for 6 months on storage at 4 °C with 5-6 % loss in initial DPV current. mga genosensor is the first report on RHD sensor which can save life of several suspected patients by early diagnosis in 30 min.

Label-free chemiresistive biosensor for mercury (II) based on single-walled carbon nanotubes and structure-switching DNA

Herein, we present a sensitive, selective, and facile label-free DNA functionalized single-walled carbon nanotube (SWNT)-based chemiresistive biosensor for the detection of Hg(2+). SWNTs were functionalized with Hg(2+) binding 15-bases long polyT oligonucleotide through covalent attachment using a bilinker molecule. The polyT was further hybridized with polyA to form a polyT-polyA duplex. When exposed to Hg(2+) the polyT-polyA duplex was dehybridized combined with switching of polyT structure, leading to change in resistance/conductance of the SWNT chemiresistor device. The device provided a significant response within 100 to 1000 nM of Hg(2+) concentration with a 6.72 × 10(-3) nM(-1) sensitivity.