Ultrasensitive electrochemical sensor for Hg2+ by using hybridization chain reaction coupled with Ag@Au core–shell nanoparticles

A novel electrochemical aptasensor based on core-shell nanomaterial labeling for simultaneous detection of acetamiprid and malathion

At present, due to the coexistence of multiple pesticides in vegetables and the enhanced toxicity, a simultaneous detection method for multiple pesticides is urgently needed. In this work, two types of core-shell nanomaterials, Ag-Au core-shell nanoparticles (Ag@Au NPs) and Cu2O-Au core-shell nanoparticles (Cu2O@Au NPs), were synthesized and labeled with acetamiprid aptamer and malathion aptamer to prepare two novel electroactive signal probes, respectively. The two probes were hybridized on the surface of the electrode by the principle of base complementary pairing between the aptamers and the thiolated DNA oligonucleotide sequences, and a dual-signal electrochemical aptasensor for the simultaneous detection of acetamiprid and malathion was established by modified glassy carbon electrode (GCE). The limits of detection (LOD) were calculated to be 43.7 pg mL-1 for acetamiprid and 63.4 pg mL-1 for malathion. The aptasensor determined acetamiprid and malathion in spinach and rape with the recovery rates of 88.9%-112.5% and 98.0%-114.1%, respectively.

Biosensors based on core-shell nanoparticles for detecting mycotoxins in food: A review

Mycotoxins are toxic metabolites produced by fungi in the process of infecting agricultural products, posing serious threat to the health of human and animals. Thus, sensitive and reliable analytical techniques for mycotoxin detection are needed. Biosensors equipped with antibodies or aptamers as recognition elements and core-shell nanoparticles (NPs) for the pre-treatment and detection of mycotoxins have been extensively studied. By comparison with monocomponent NPs, core-shell nanostructures exhibit unique optical, electric, magnetic, plasmonic, and catalytic properties due to the combination of functionalities and synergistic effects, resulting in significant improvement of sensing capacities in various platforms, such as surface-enhanced Raman spectroscopy, fluorescence, lateral flow immunoassay and electrochemical sensors. This review focused on the development of core-shell NPs based biosensors for the sensitive and accurate detection of mycotoxins in food samples. Recent developments were categorised and summarised, along with detailed discussion of advantages and shortcomings. The future potential of utilising core-shell NPs in food safety testing was also highlighted.

DNA tetrahedral scaffold-corbelled 3D DNAzyme walker for electrochemiluminescent aflatoxin B1 detection

The reaction efficiency of surface-based DNA walker can directly affect the properties of a biosensor. Herein, three-dimensional (3D) DNAzyme walker were first fixed on the top of DNA tetrahedral scaffold to improve the immobilization efficiency. Ferrocene (Fc) that labeled at substrate strand ends effectively quenched the electrochemiluminescence (ECL) signal of Ru(bpy)₂(cpaphen)²⁺, yielding the sensor in a "signal-off" state. Upon the addition of aflatoxin B₁ (AFB₁), 3D DNAzyme walker was activated and fueled by Na⁺, accordingly releasing Fc and recovering the ECL signal of Ru(bpy)₂(cpaphen)²⁺. Due to the high movement efficiency of such 3D DNAzyme walker, ultrasensitive detection of AFB₁ was achieved in the range of 1.0 fg mL^-1-10 ng mL^-1, with a detection limit of 0.58 fg mL^-1. Moreover, satisfactory results were obtained while detecting AFB₁ in corn and peanut samples, suggesting it has a potential application in food safety analysis.

A catalytic hairpin assembly-based Förster resonance energy transfer sensor for ratiometric detection of ochratoxin A in food samples

Ochratoxin A (OTA) poses severe risks to the environment and human health, making the development of an accurate and sensitive analytical method for OTA detection essential. In this study, a catalytic hairpin assembly (CHA)-based Förster resonance energy transfer (FRET) aptasensor was developed to detect OTA using carbon quantum dots (CDs) and 6-carboxy-fluorescein (FAM) as dual signal readout. In the presence of OTA, the aptamer specifically interacted with OTA to release the helper DNA (HP), which could open the hairpin structure of FAM-labeled hairpin DNA 1 (H₁-FAM) modified on the surface of gold nanoparticles (AuNPs). CHA between H₁-FAM and hairpin H₂ labeled with CDs (H₂-CDs) can release HP for the next cycle, resulting in the occurrence of FRET with CDs as the energy donor and FAM as the energy acceptor. According to the ratio of F_CDs/F_FAM, the proposed aptasensor showed a wide linear range from 5.0 pg/mL to 3.0 ng/mL and a low detection limit of 1.5 pg/mL for OTA detection. Moreover, satisfactory results were obtained for OTA detection in rice, suggesting the potential application of this sensor in food safety analysis.

In Situ Assembly of Nanomaterials and Molecules for the Signal Enhancement of Electrochemical Biosensors

The physiochemical properties of nanomaterials have a close relationship with their status in solution. As a result of its better simplicity than that of pre-assembled aggregates, the in situ assembly of nanomaterials has been integrated into the design of electrochemical biosensors for the signal output and amplification. In this review, we highlight the significant progress in the in situ assembly of nanomaterials as the nanolabels for enhancing the performances of electrochemical biosensors. The works are discussed based on the difference in the interactions for the assembly of nanomaterials, including DNA hybridization, metal ion-ligand coordination, metal-thiol and boronate ester interactions, aptamer-target binding, electrostatic attraction, and streptavidin (SA)-biotin conjugate. We further expand the range of the assembly units from nanomaterials to small organic molecules and biomolecules, which endow the signal-amplified strategies with more potential applications.

An electrochemical aptasensor for the milk allergen β-lactoglobulin detection based on a target-induced nicking site reconstruction strategy

Food allergy is an immune system reaction to a particular food, milk being the most common one. β-Lactoglobulin (β-Lg) is the main ingredient of milk protein and the main cause of infant milk allergy. On such an occasion, the determination of β-Lg is very important and the electrochemical sensors are a good alternative for this purpose since they are sensitive, selective and inexpensive. In this work, an electrochemical aptasensor was fabricated for the quantitative detection of β-Lg in hypoallergenic formula (HF) milk. A tri-functional hairpin (HP) was designed, which was composed of an aptamer sequence, a nicking site and a DNA sequence (T1). In the absence of β-Lg, the aptamer part hybridized with T1 to form a stable stem-loop structure. However, in the presence of β-Lg, the capture of the aptamer sequence towards β-Lg caused the reconstruction of HP and thus the nicking sites were exposed. Then, the nicking enzyme was activated and T1 could be released, which bound with the end of the hairpin 1-methylene blue (HP1-MB)/HP2-MB conjugation on the Au nanoparticle (AuNP) modified electrode surface. Thus, the insulating property of the electrode was enhanced and the current response of MB decreased, which built the quantitative basis for β-Lg detection. In this way, the proposed aptasensor exhibited a wide linear range of 0.01-100 ng mL^-1 and a low detection limit of 5.7 pg mL^-1. This aptasensor also displayed high selectivity, reproducibility and stability, and became a promising platform for β-Lg detection in real food samples.

Highly Sensitive Colorimetric Naked-Eye Detection of HgII Using a Sacrificial Metal-Organic Framework

This study has developed a specific, easy, and novel approach to designing a sacrificial metal-organic framework (MOF) that can detect and measure the amount of Hg²⁺ in aqueous and nonaqueous solutions using the naked eye. The functionalized [Zn(oba)(RL3)_0.5]_n·1.5DMF (TMU-59) provides the ability of simple visual assessment or colorimetric readout without sophisticated analytical equipment. Because of the special interaction with Hg²⁺, degradation of the structure of this unique MOF causes the solution to change color from colorless to a pink that is easily recognizable to the naked eye. The presence of a methyl group plays a major role in naked-eye detection by a qualitative sensor. Furthermore, this qualitative sensor data for the production of a simple, instant, and portable red, green, and blue (RGB)-based quantitative sensor were used to determine the concentration of Hg²⁺ in different specimens. As a turn-off fluorescence sensor, this unique structure is also capable of detecting Hg²⁺ at very low concentrations (the limit of detection is 0.16 ppb). To the best of our knowledge, TMU-59 is the first MOF-based naked-eye sensor that can successfully and specifically display the presence of Hg²⁺ through a major color change.

An electrochemical Hg2+ sensor based on signal amplification strategy of target recycling

In this work, an unmodified homogeneous electrochemical sensor based on electrochemical bonding and catalytic hairpin assembly (CHA) was first constructed for the high sensitivity detection of Hg²⁺. Herein, tetraferrocene, a synthesized compound, was used as a signal marker that modified both ends of the hairpin probe to amplify the electrochemical signal. The interaction of T-Hg²⁺-T could induce the catalytic self-assembly of hairpins by means of auxiliary DNA. The rigid DNA triangle that was formed easily reaches the electrode and induced Au-S self-assembly assisted by potential, allowing tetraferrocene to reach the electrode surface and generate a sensitive electrochemical signal. CHA and tetraferrocene signal markers accomplished dual signal amplification, and the limit of detection was 0.12 pM. Differential pulse voltammetry experiments in the presence of tetraferrocene redox indicator show that the linear response range of electrochemical biosensors to mercury ions is 0.2-2000 pM, This technology offers good selectivity and high recognition efficiency for the detection of mercury ions and has broad application prospects in actual sample detection.

One-step and ultrasensitive ATP detection by using positively charged nano-gold@graphene oxide as a versatile nanocomposite

A versatile nanocomposite was simply prepared based upon the electrostatic adsorption of positively charged gold nanoparticles with negatively charged graphene oxide (nano-gold@GO), and utilized as a novel fluorescence quenching platform for ultrasensitive detection of adenosine triphosphate (ATP). In the designed system, DNA-stabilized Ag nanoclusters (DNA/AgNCs) were used as fluorescent probes, DNA duplex was formed in the presence of ATP, and they can electrostatically adsorb onto the surface of nano-gold@GO to quench the fluorescence signal. Upon the addition of exonuclease III (Exo III), the DNA duplex would be hydrolyzed into DNA fragments and resulted in the recovery of the fluorescence signals due to the diffusion of AgNCs away from nano-gold@GO. Based on these, sensitive detection of ATP was realized with a detection range of 5.0 pM-20 nM. Notably, a good recovery in the range of 94-104% was obtained when detecting ATP in human serum samples, indicating a promising application value in early disease diagnosis. Graphical abstract A functional positively charged nano-gold@graphene oxide was fabricated and utilized as an enhanced fluorescence quenching platform for the detection of ATP, coupled with exonuclease III-assisted signal amplification.

NanoPOP: Solution-Processable Fluorescent Porous Organic Polymer for Highly Sensitive, Selective, and Fast Naked Eye Detection of Mercury

Fluorescence-based detection is one of the most efficient and cost-effective methods for detecting hazardous, aqueous Hg²⁺. We designed a fluorescent porous organic polymer (TPA-POP-TSC), with a "fluorophore" backbone and a thiosemicarbazide "receptor" for Hg²⁺-targeted sensing. Nanometer-sized TPA-POP-TSC spheres (nanoPOP) were synthesized under mini-emulsion conditions and showed excellent solution processability and dispersity in aqueous solution. The nanoPOP sensor exhibits exceptional sensitivity (K_sv = 1.01 × 10⁶ M^-1) and outstanding selectivity for Hg²⁺ over other ions with rapid response and full recyclability. Furthermore, the nanoPOP material can be easily coated onto a paper substrate to afford naked eye-based Hg²⁺-detecting test strips that are convenient, inexpensive, fast, highly sensitive, and reusable. Our design takes advantage of the efficient and selective capture of Hg²⁺ by thiosemicarbazides (binding energy = -29.84 kJ mol^-1), which facilitates electron transfer from fluorophore to bound receptor, quenching the sensor's fluorescence.

Hierarchical nanomaterials via biomolecular self-assembly and bioinspiration for energy and environmental applications

Bioinspired synthesis offers potential green strategies to build highly complex nanomaterials by utilizing the unique nanostructures, functions, and properties of biomolecules, in which the biomolecular recognition and self-assembly processes play important roles in tailoring the structures and functions of bioinspired materials. Further understanding of biomolecular self-assembly for inspiring the formation and assembly of nanoparticles would promote the design and fabrication of functional nanomaterials for various applications. In this review, we focus on recent advances in bioinspired synthesis and applications of hierarchical nanomaterials based on biomolecular self-assembly. We first discuss biomolecular self-assembly towards biological nanomaterials, in which the mechanisms and ways of biomolecular self-assembly as well as various self-assembled biomolecular nanostructures are demonstrated. Secondly, the bioinspired synthesis strategies including molecule-molecule interaction, molecule-material recognition, molecule-mediated nucleation and growth, and molecule-mediated reduction/oxidation are introduced and discussed. Meanwhile, typical examples and discussions on how biomolecular self-assembly inspires the formation of hierarchical hybrid nanomaterials are presented. Finally, the applications of bioinspired nanomaterials in biofuel cells, light-harvesting systems, batteries, supercapacitors, catalysis, water/air purification, and environmental monitoring are presented and discussed. We believe that this review will be very helpful for readers to understand the self-assembly of biomolecules and the biomimetic/bioinspired strategies for synthesizing hierarchical nanomaterials on the one hand, and on the other hand to design novel materials for extended applications in nanotechnology, materials science, analytical science, and biomedical engineering.

Dual-type responsive electrochemical biosensor for the detection of α2,6-sialylated glycans based on AuNRs-SA coupled with c-SWCNHs/S-PtNC nanocomposites signal amplification

In this study, a dual-type responsive electrochemical biosensor was developed for the quantitative detection of α2,6-sialylated glycans (α2,6-sial-Gs), a potential biomarker of tumors. The gold nanorods (AuNRs), which exhibited great specific surface area, as well as good biocompatibility, was synthesized by the way of seed growth method. Furthermore, a biotin-streptavidin (biotin-SA) system was introduced to improve the immunoreaction efficiency. Accordingly, a label-free biosensor was fabricated based on AuNRs-SA for the quick detection of α2,6-sial-Gs by recording the signal of differential pulse voltammetry (DPV). Furthermore, to expand the ultrasensitive detection of α2,6-sial-Gs, a carboxylated single-walled carbon nanohorns/sulfur-doped platinum nanocluster (c-SWCNHs/S-PtNC) was synthesized for the first time as a novel signal label, which showed an excellent catalytic performance. The usage of c-SWCNHs/S-PtNC could significantly amplify the electrochemical signal recorded by the amperometric i-t curve. Herein, a sandwich type biosensor was constructed by combining the AuNRs-SA on the electrode and c-SWCNHs/S-PtNC (signal amplifier). The label-free biosensor possessed a linear range from 5 ng mL^-1 to 5 μg mL^-1 with a detection limit of 0.50 ng mL^-1, and the sandwich-type biosensor possessed a wide linear range from 1 fg mL^-1 to 100 ng mL^-1 with a detection limit of 0.69 fg mL^-1. Furthermore, the biosensor exhibited excellent recovery and stability, indicating its potential for use in actual samples.

Application of hairpin DNA-based biosensors with various signal amplification strategies in clinical diagnosis

Biosensors have been commonly used in biomedical diagnostic tools in recent years, because of a wide range of application, such as point-of-care monitoring of treatment and disease progression, drug discovery, commonly use food control, environmental monitoring and biomedical research. Additionally, development of DNA biosensors has been increased enormously over the past few years as confirmed by the large number of scientific publications in this field. A wide range of techniques can be used for the development of DNA biosensors, such as DNA nano-machines and various signal amplification strategies. This article selectively reviews the recent advances in DNA base biosensors with various signal amplification strategies for detection of cancer DNA and microRNA, infectious microorganisms, and toxic metal ions.

Electrochemical mercury biosensors based on advanced nanomaterials

This review presents an overview of the synthesis strategies and electrochemical performance of recently developed nanomaterials for the Hg²⁺ assay.

Stabilization of noble metal nanostructures for catalysis and sensing

Noble metal nanocrystals have been widely used as active components in catalysis and chemical/bio-sensing. The sizes, structures, and shapes of noble metal nanocrystals are crucial to their electronic, optical, and catalytic properties. However, metal nanocrystals tend to lose their structural and morphological properties when they are subjected to thermal and chemical treatment. Therefore, stabilization of noble metal nanostructures remains a challenge. In this feature article, we present our recent efforts on the stabilization of noble metal nanocrystals, i.e., using inorganic and non-metal solids as supports and physical barriers, protecting the nanocrystal surface by a metal coating, and forming alloys with other metals. At the end of this review, we provide our perspectives on the future development of effective methods for nanocrystal stabilization.

Sensitive immunoassay of von Willebrand factor based on fluorescence resonance energy transfer between graphene quantum dots and Ag@Au nanoparticles

Graphene quantum dots (GQDs) and core-shell Ag@Au nanoparticles (Ag@Au NPs) were synthetized and they were characterized by transmission electron microscope and X-ray photoelectron spectra, respectively. Von Willebrand factor antibody (vWF Ab) was bound on Ag@Au NPs to construct Ag@Au-Ab nanocomposites (Ag@Au-Ab NCs). The fluorescence of GQDs could be effectively quenched by the prepared nanocomposites owing to fluorescence resonance energy transfer (FRET). The immunoreaction between vWF and Ag@Au-Ab NCs resulted in the declined FRET efficiency and a degree of fluorescence recovery of GQDs. The fluorescence intensity change was found to be proportional to the logarithm of the vWF concentration in the range of 0.1 pg mL^-1-10 ng mL^-1 with a detection limit of 30 fg mL^-1. The proposed fluorescence sensor was employed to investigate the relationship between the release of vWF and the oxidation-injury degree of vascular endothelial cells. The experimental results indicate that the vWF content in the growth medium was enhanced and the cell injury was intensified when the contact time of the cells with H₂O₂ was increased.

Selective reduction-based, highly sensitive and homogeneous detection of iodide and melamine using chemical vapour generation-atomic fluorescence spectrometry

A selective reduction-based method was proposed for the sensitive detection of iodide and melamine using chemical vapour generation (CVG) coupled with atomic fluorescence spectrometry (AFS).

Shell-encoded Au nanoparticles with tunable electroactivity for specific dual disease biomarkers detection

The exploration of electroactive labelling with tailorable and strong differential pulse voltammetry (DPV) responses is of great importance in accurate and sensitive screening of a panel of biomarkers related to cancer. Herein, shell-encoded gold nanoparticles (Au NPs) are fabricated and give rise to shell species-dominated DPV peak potentials. Two independent DPV peaks appear at -0.08V for Au@Cu₂O core-shell NPs and 0.26V for Au@Ag core-shell NPs. Shell-encoded Au NPs drastically exhibit shell thickness-tunable amplified peak currents. The non-interfering and amplified DPV responses enable shell-encoded Au NPs to be an alternative electrochemical signal amplifier for dual screening of carcinoembryonic antigen (CEA) and alpha-fetoprotein (AFP). The limits of detection (LODs) are calculated to be 1.8pg/mL for CEA and 0.3pg/mL for AFP. In comparison to the parallel single-analyte assays, shell-encoded Au NPs engineered electrochemical aptasensors offer multiplexing capability and show significant prospects in biomedical research and early diagnosis of diseases.

Ultrasensitive and selective electrochemical biosensor for detection of mercury (II) ions by nicking endonuclease-assisted target recycling and hybridization chain reaction signal amplification

In this paper, a novel and signal-on electrochemical biosensor based on Hg²⁺- triggered nicking endonuclease-assisted target recycling and hybridization chain reaction (HCR) amplification tactics was developed for sensitive and selective detection of Hg²⁺. The hairpin-shaped capture probe A (PA) contained a specific sequence which was recognized by nicking endonuclease (NEase). In the presence of Hg²⁺, probe B (PB) hybridized with PA to form stand-up duplex DNA strands via the Hg²⁺ mediated thymine-Hg²⁺-thymine (T-Hg²⁺-T) structure, which automatically triggered NEase to selectively digest duplex region from the recognition sites, spontaneously dissociating PB and Hg²⁺ and leaving the remnant initiators. The released PB and Hg²⁺ could be reused to initiate the next cycle and more initiators were generated. The long nicked double helices were formed through HCR event, which was triggered by the initiators and two hairpin-shaped signal probes labeled with methylene blue, resulting in a significant signal increase. Under optimum conditions, the resultant biosensor showed the high sensitivity and selectivity for the detection of Hg²⁺ in a linear range from 10 pM to 50nM (R²=0.9990), and a detection limit as low as 1.6 pM (S/N=3). Moreover, the proposed biosensor was successfully applied in the detection of Hg²⁺ in environment water samples with satisfactory results.

Highly effective target converting strategy for ultrasensitive electrochemical assay of Hg2+

An electrochemical sensing system based on a highly effective Hg²⁺ converting strategy and RCA has been developed for the ultrasensitive detection of Hg²⁺.

A facile and green strategy for preparing newly-designed 3D graphene/gold film and its application in highly efficient electrochemical mercury assay

In this work, we report a facile and green strategy for in situ and one step preparation of a novel 3D graphene/gold (G/Au) film. Triggering with unique driving force from hydrothermal growth, a 3D interlaced graphene framework with hierarchically porous structures was directly attached on a gold substrate pretreated with a self-assembled monolayer. Simultaneously, highly dispersive Au nanoparticles with tunable morphologies were anchored on the framework utilizing generated graphene as an endogenous reductant. Newly-designed 3D G/Au film possessed excellent properties of significantly large specific area, good electrical conductivity, high structure stability and substrate binding strength, etc. As a paradigm, an electrochemical Hg²⁺ biosensor was constructed on 3D G/Au film, in which an exonuclease III-assisted target recycling was introduced. Impressively, an ultralow detection limit of 50 aM (S/N=3), a wide linear range from 0.1 fM to 0.1μM, a high selectivity and a good reliability for Hg²⁺ assay in real water and serum samples were realized using prepared biosensor. It is highly envisioned that this work opens the door towards simply fabricating varying types of 3D graphene based hybrid films, and such G/Au film will have widespread applications in electroanalysis and electrocatalysis.