DNA Modified Fe3O4@Au Magnetic Nanoparticles as Selective Probes for Simultaneous Detection of Heavy Metal Ions

Determination of Caffeine in Energy Drinks Using a Composite Modified Sensor Based on Magnetic Nanoparticles

A new voltammetric sensor (BDDE/Nafion@Fe3O4/BiF) was fabricated by applying a nanocomposite drop of Fe3O4 magnetic nanoparticles in Nafion onto the polished boron-doped diamond electrode (BDDE) surface. Then, after drying (5 min at room temperature), the electrode was electrochemically modified with bismuth film (BiF) during in situ analysis. The Nafion@Fe3O4/BiF modification of the BDDE contributes to the acquisition of the highest differential-pulse adsorptive stripping voltammetric (DPAdSV) signals of caffeine (CAF) due to the improvement of electron transfer and the increase in the number of active sites on which CAF can be adsorbed. The DPAdSV signals exhibited a linearly varied oxidation peak with the CAF concentration range between 0.5 and 10,000 nM, leading to the 0.043 and 0.14 nM detection and quantification limits, respectively. The practical applicability of the DPAdSV procedure using the BDDE/Nafion@Fe3O4/BiF was positively confirmed with commercially available energy drinks.

Rapid and broad-spectrum detection of salmonella in various environments by magnetic enrichment-surface enhanced Raman spectroscopy

Rapid, accurate, and broad-spectrum detection of pathogenic bacteria in food is very important and an emerging need for increasing food safety concerns. Here, we present an ultrasensitive Surface-enhanced Raman spectroscopy (SERS) based aptasensor, which consists of core-shell structure (Ag@Au) and magnetic composites (Fe3O4@Au@Ag). 4-mercaptobenzoic acid (4MBA) as Raman reporter was embedded in Ag@Au nanoparticles (Ag@4MBA@Au) based on an internal standard. The combination of Ag@4MBA@Au and Fe3O4@Au@Ag are used to form the aptasensor with strongest Raman signal. Upon exposure to salmonella, the interaction between the SERS nanotags and the magnetic substrate was disrupted, causing an inverse correlation between the Raman intensity of 4MBA and the salmonella concentration. The result shows a wide linear range from 10 to 108 cfu/mL with five subtypes salmonella (S. Typhimurium, S. Enteritidis, S. Kentucky, S. Indiana, S. NO) and a low limit of detection of 1.782, 1.637, 1.941, 1.632, 1.875 cfu/mL, respectively. When applied to the detection of salmonella in actual samples (lake water, milk, shrimp), no pretreatment is required, and the results show no significant difference compared to the standard.

Bicycled Hairpin Assembly-Mediated SERS/Colorimetric Dual-Mode Sensor for the Detection of Dexamethasone in Milk and Pork

The presence of dexamethasone (DEX) residues and the illegal addition of DEX to food have garnered widespread public concern over recent decades. Based on the bicycled hairpin assembly (BHA) mechanism and the catalytic oxidase effect of urchin-shaped Au-Ag alloy loaded with Pt nanoparticles (Au@Ag@Pt), an accurate and ultrasensitive surface-enhanced Raman scattering (SERS)/colorimetric dual-mode method for DEX detection is pioneered. When DEX is introduced into the sensor system, the BHA cycle begins. The Fe3O4 nanosphere loaded with Au nanoparticles (Fe3O4@Au) surface facilitates the anchoring of a large amount of Au@Ag@Pt, resulting in synergistic signal amplification and facile magnetic separation from the complex system. Finally, the collected product effectively catalyzes the conversion of 3,3',5,5'-tetramethylbenzidine (TMB) to the oxidized product, producing a strong SERS response, an ultraviolet-visible absorption signal, and color deepening. Our dual-mode platform achieves ultrasensitive detection of DEX, achieving a detection limit (LOD) of 6.61 × 10-17 M within the broad range of 10-16 to 10-9 M for SERS and a LOD of 8.81 × 10-13 and 6.98 × 10-13 M within the range of 10-12-10-5 M for the colorimetric method. Additionally, this platform shows high selectivity and satisfactory recovery rates for DEX in milk and pork samples, with recoveries of 92.3%-106.5% (RSDs <8.35%) for SERS and 95.2%-107.6% (RSDs <8.81%) for colorimetric analysis. This study demonstrates that by combining the BHA amplification process with multifunctional Au@Ag@Pt and Fe3O4@Au substrates, the DEX content in actual food samples can be accurately determined using a dual-mode approach that combines the high sensitivity of SERS with the simplicity of colorimetric analysis.

Peptides in the detection of metal ions

By means of their specific interactions with different metal ions, naturally occurring proteins control structures and functions of many biological processes and functions in organisms. In view of natural metallopeptides, scientists have proposed artificial peptides which coordinate with metal ions through their functional groups either for introducing a special reactivity or for constructing various sensors. However, the design of new peptide ligands requires a deep understanding of the structures, assembly properties, and dynamic behaviors of such peptides. This review briefly describes detection strategies of metal ions via coordination to the binding sites in peptides. The principles and functions of sensing systems are described as well. We also highlight some examples of a metal-induced peptide self-assembly with relevance to biotechnology applications.

Green Synthesis of Magnetic Fe2O3 Nanoparticle with Chenopodium glaucum L. as Recyclable Heterogeneous Catalyst for One-Pot Reactions and Heavy Metal Adsorption

The growth of the environment depends upon developing greener and ecological methods for managing pollutants and contamination from industrial wastewater, which causes significant effects on human health. The removal of these pollutants from wastewater using nanomaterials covers an ecological method that is free from expensive and secondary pollution. In this report, we developed magnetic iron nanoparticles from Chenopodium glaucum (CG), which showed excellent adsorption capacity at pH 5 for selective Hg2+ and Pb2+ metal ions among various heavy metal ions, with maximum adsorption capacities of 96.9 and 94.1%, respectively. These metals' adsorption process conforms to the Langmuir model, which suggests that monolayer adsorption transpires on CG-Fe2O3 nanoparticles. CG-Fe2O3 nanoparticles also act as an efficient and recyclable heterogeneous catalyst for one-pot synthesis of xanthene derivatives, yielding products with high yields (up to 97%) and excellent purity (crystalline form) within a short timeframe (6 min) using microwave irradiations (at 120 W).

Comparative Study of Catalytic Activity of Recyclable Au/Fe3O4 Microparticles for Reduction Of 2,4-Dinitrophenol and Anionic, Cationic Azo Dyes

We synthesized Au/Fe3O4 microparticles. Initially, citrate-capped Fe3O4 micro-sized particles were synthesized by the co-precipitation method with an excess amount of trisodium citrate. Gold ions were reduced on the surface of citrate-capped Fe3O4 and grew as gold sub-microparticles with an average diameter of 210 nm on the surface. The characteristic SPR peak of gold nanoparticles on the surface of Fe3O4 was detected at 584 nm, whereas the absorption in the near-infrared region was increased. SEM images has proved that the synthesized Au/Fe3O4 composite microparticles has an average diameter of 1.7 micrometers. The results of XRD patterns proved the existence of both crystal phases of Fe3O4 and Au particles. To investigate the catalytic activity, the reaction rate constant of reduction of 2,4-dinitrophenol (2,4-DNP) and degradation of Congo red (CR), and methylene blue (MB) with NaBH4 in the presence of Au/Fe3O4 catalyst was monitored by UV-Vis spectroscopy. The initial reaction rate constant calculated from the change in characteristic peak absorptions of 2,4-dinitrophenol was 3.97×10-3 s-1, while the reaction rate constants for the degradation of CR and MB were 9.72×10-3 s-1 and 14.25×10-3 s-1 respectively. After 5 cycles, Au/Fe3O4 microparticles preserved 99 % of the reaction rate constant, exhibiting considerable recycling efficiency in the reduction of nitro groups.

Modified Gold Screen-Printed Electrodes for the Determination of Heavy Metals

Screen-printed electrodes (SPEs) are reliable, portable, affordable, and versatile electrochemical platforms for the real-time analytical monitoring of emerging analytes in the environmental, clinical, and agricultural fields. The aim of this study was to evaluate the electrochemical behavior of gold screen-printed electrodes (SPGEs) modified with molecules containing amino (Tr-N) or α-aminophosphonate (Tr-P) groups for the selective and sensitive detection of the toxic metal ions Pb2+ and Hg2+ in aqueous samples. After optimizing the analytical parameters (conditioning potential and time, deposition potential and time, pH and concentration of the supporting electrolyte), anodic square wave stripping voltammetry (SWASV) was used to evaluate and compare the electrochemical performance of bare or modified electrodes for the detection of Hg2+ and Pb2+, either alone or in their mixtures in the concentration range between 1 nM and 10 nM. A significative improvement in the detection ability of Pb2+ ions was recorded for the amino-functionalized gold sensor SPGE-N, while the presence of a phosphonate moiety in SPGE-P led to greater sensitivity towards Hg2+ ions. The developed sensors allow the detection of Pb2+ and Hg2+ with a limit of detection (LOD) of 0.41 nM and 35 pM, respectively, below the legal limits for these heavy metal ions in drinking water or food, while the sensitivity was 5.84 µA nM-1cm-2 and 10 µA nM-1cm-2, respectively, for Pb2+ and Hg2+. The reported results are promising for the development of advanced devices for the in situ and cost-effective monitoring of heavy metals, even in trace amounts, in water resources.

Recent advances and trends in innovative biosensor-based devices for heavy metal ion detection in food

Low-cost, reliable, and efficient biosensors are crucial in detecting residual heavy metal ions (HMIs) in food products. At present, based on distance-induced localized surface plasmon resonance of noble metal nanoparticles, enzyme-mimetic reaction of nanozymes, and chelation reaction of metal chelators, the constructed optical sensors have attracted wide attention in HMIs detection. Besides, based on the enrichment and signal amplification strategy of nanomaterials on HMIs and the construction of electrochemical aptamer sensing platforms, the developed electrochemical biosensors have overcome the plague of low sensitivity, poor selectivity, and the inability of multiplexed detection in the optical strategy. Moreover, along with an in-depth discussion of these different types of biosensors, a detailed overview of the design and application of innovative devices based on these sensing principles was provided, including microfluidic systems, hydrogel-based platforms, and test strip technologies. Finally, the challenges that hinder commercial application have also been mentioned. Overall, this review aims to establish a theoretical foundation for developing accurate and reliable sensing technologies and devices for HMIs, thereby promoting the widespread application of biosensors in the detection of HMIs in food.

Design and optimization of a cost-effective paper-based voltammetric sensor for the determination of trinitrotoluene (TNT) utilizing cysteamine-linked Fe3O4 @Au nanocomposite

This paper presents the development and optimization of a cost-effective paper electrochemical sensor for the detection of TNT using Fe3O4-Au core-shell nanoparticles modified with cysteamine (Fe3O4@Au/CA). The sensor was constructed by modifying a graphite paste with the aforementioned nanoparticles, which facilitated the formation of a Meisenheimer complex between cysteamine and TNT as an electron donor and an electron acceptor, respectively. The central composite design was employed to optimize four key parameters pH, modifier percentage, contact time, and buffer type to enhance the performance of the sensor. The detection limit was found to be 0.5 nM of TNT, while the linear range of the electrode response spanned from 0.002 μM to 10 μM. The simplicity and low cost of the sensor make it highly attractive for practical applications, particularly in scenarios where rapid and on-site TNT detection is required.

Flexible electronics for heavy metal ion detection in water: a comprehensive review

Flexible electronics offer a versatile, rapid, cost-effective and portable solution to monitor water contamination, which poses serious threat to the environment and human health. This review paper presents a comprehensive exploration of the versatile platforms of flexible electronics in the context of heavy metal ion detection in water systems. The review overviews of the fundamental principles of heavy metal ion detection, surveys the state-of-the-art materials and fabrication techniques for flexible sensors, analyses key performance metrics and limitations, and discusses future opportunities and challenges. By highlighting recent advances in nanomaterials, polymers, wireless integration, and sustainability, this review aims to serve as an essential resource for researchers, engineers, and policy makers seeking to address the critical challenge of heavy metal contamination in water resources. The versatile promise of flexible electronics is thoroughly elucidated to inspire continued innovation in this emerging technology arena.

Distinguishable Magnetic Reporter Coordination with Buoyancy-Magnetism Separation for Immobilization-Free Dual-Target Electrochemical Immunosensing

Simultaneous sensitive and precise determination of multibiomarkers is of great significance for improving detection efficiency, reducing diagnosis and treatment expenses, and elevating survival rates. However, the development of simple and portable biosensors for simultaneous determination of multiplexed targets in biological fluids still faces challenges. Herein, a unique and versatile immobilization-free dual-target electrochemical biosensing platform, which combines distinguishable magnetic signal reporters with buoyancy-magnetism separation, was designed and constructed for simultaneous detection of carcinoembryonic (CEA) and α-fetoprotein (AFP) in intricate biological fluids. To construct such distinguishable magnetic signal reporters with signal transduction, amplification, and output, secondary antibodies of CEA and AFP were respectively functionalized on methylene blue (MB) and 6-(ferrocenyl)hexanethiol (FeC) modified Fe3O4@Au magnetic nanocomposites. Meanwhile, a multifunctional flotation probe with dual target recognition, capture, and isolation capability was prepared by conjugating primary antibodies (Ab1-CEA, Ab1-AFP) to hollow buoyant microspheres. The target antigens of CEA and AFP can trigger a flotation-mediated sandwich-type immunoreaction and capture a certain amount of the distinguishable magnetic signal reporter, which enables the conversion of the target CEA and AFP quantities to the signal of the potential-resolved MB and FeC. Thus, the MB and FeC currents of magnetically adsorbed distinguishable magnetic reporters can be used to determine the CEA and AFP targets simultaneously and precisely. Accordingly, the proposed strategy exhibited a delightful linear response for CEA and AFP in the range of 100 fg·mL-1-100 ng·mL-1 with detection limits of 33.34 and 17.02 fg·mL-1 (S/N = 3), respectively. Meanwhile, no significant nonspecific adsorption and cross-talk were observed. The biosensing platform has shown satisfactory performance in the determination of real clinical samples. More importantly, the proposed approach can be conveniently extended to universal detection just by simply substituting biorecognition events. Thus, this work opens up a new promising perspective for dual and even multiple targets and offers promising potential applications in clinical diagnosis.

A simultaneous strategy with multiple-signal amplification and self-calibration for ultrasensitive assay of miRNA-21 based on 3D MNPs-IL-rGO-AuNPs

MicroRNA-21 (miRNA-21) is a significant biomarker for the development and progression of diverse cancers but is present in relatively low concentrations. Detecting such low-abundance molecules accurately can be challenging, especially in early-stage cancers where the concentration may be even lower. Herein, a self-calibration biosensing platform based on 3D novel MNPs-IL-rGO-AuNPs nanocomposites was successfully established for the ultrasensitive detection of miRNA-21. Duplex-specific nuclease (DSN) was introduced to recognize perfectly matched duplexes and trigger target recycling, enhancing the specificity and sensitivity of the biosensor. DSN-assisted target recycling, in conjunction with magnetic separation enrichment and high-performance MNPs-IL-rGO-AuNPs, collectively formed a multiple-signal amplification strategy. The obtained biosensor could output dual signals in both electrochemical and fluorescent modes, enabling self-correcting detection to enhance the accuracy. The obtained dual-mode biosensor prepared exhibited a wide detection range from 5 fM to 100 nM with a remarkably low LOD of 1.601 fM. It accomplished the sensitive evaluation of miRNA-21 in total RNA extracted from various human cancer cell lines and normal cell lines. Additionally, the greatly satisfactory outcomes in the analysis of human serum samples suggested that the proposed biosensor was a powerful screening candidate in early clinical diagnosis of cancer.

Development of a SERS aptasensor for the determination of L-theanine using a noble metal nanoparticle-magnetic nanospheres composite

An ultrasensitive surface-enhanced Raman spectroscopy (SERS) aptamer sensor (aptasensor) using a noble metal nanoparticle-magnetic nanospheres composite was developed for L-theanine detection. It makes use of Fe3O4@Au MNPs and Au@Ag NPs embedded with the Raman reporter 4-mercaptobenzoic acid (4MBA). Au@4MBA@Ag NPs modified by aptamer and Fe3O4@Au MNPs modified by cDNA created the aptasensor with the strongest Raman signal of 4MBA through the specific binding of the aptamer. With the preferred binding of L-theanine aptamer to L-theanine, Au@4MBA@Ag NPs were released from Fe3O4@Au MNPs, causing a linear decrease in SERS intensity to achieve the SERS detection of the L-theanine. The SERS peak of 4MBA at 1078 cm-1 was used for quantitative determination. SERS intensity showed a good log-linear relationship within the range 10-10 to 10-6 M of L-theanine. The aptasensor has a high selectivity for L-theanine compared with other twelve tested analytes. Hence, this aptasensor is a promising analytical tool for L-theanine detection. The developed method was applied to the analysis of real samples, demonstrating excellent performance. The comparison with the standard liquid chromatography mass spectrometry method showed an error within 20%.

Electrochemiluminescence Sensor Based on CeO2 Nanocrystalline for Hg2+ Detection in Environmental Samples

The excessive concentration of heavy-metal mercury ions (Hg2+) in the environment seriously affects the ecological environment and even threatens human health. Therefore, it is necessary to develop rapid and low-cost determination methods to achieve trace detection of Hg2+. In this paper, an Electrochemiluminescence (ECL) sensing platform using a functionalized rare-earth material (cerium oxide, CeO2) as the luminescent unit and an aptamer as a capture unit was designed and constructed. Using the specific asymmetric matching between Hg2+ and thymine (T) base pairs in the deoxyribonucleic acid (DNA) single strand, the "T-Hg-T" structure was formed to change the ECL signal, leading to a direct and sensitive response to Hg2+. The results show a good linear relationship between the concentration and the response signal within the range of 10 pM-100 µM for Hg2+, with a detection limit as low as 0.35 pM. In addition, the ECL probe exhibits a stable ECL performance and excellent specificity for identifying target Hg2+. It was then successfully used for spiked recovery tests of actual samples in the environment. The analytical method solves the problem of poor Hg2+ recognition specificity, provides a new idea for the efficient and low-cost detection of heavy-metal pollutant Hg2+ in the environment, and broadens the prospects for the development and application of rare-earth materials.

Synthesis of pH-Sensitive Nitrogen-Doped Carbon Dots with Biological Imaging Function and Their Application in Cu2+ and Fe2+ Determination by Ratiometric Fluorescent Probes

pH-sensitive nitrogen-doped carbon dots (N-CDs) were synthesized using immature seeds of elm trees as a carbon source and ethylenediamine as a coreactant through a facile one-step hydrothermal method. The N-CDs were characterized using fluorescence spectroscopy, fluorescence lifetime, ultraviolet-visible absorption, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy, as well as transmission electron microscopy. The N-CDs displayed excellent fluorescence properties and responded to pH changes. The N-CDs exhibited low toxicity and good biocompatibility and had the potential to be used for the biological imaging of HeLa cells and mung bean sprouts. Utilizing the mechanism of fluorescence resonance energy transfer, ratiometric fluorescent probes were prepared by simple mixing of N-CDs and fluorexon in a Britton-Robinson buffer solution. The ratiometric fluorescent probe was used to detect Cu2+ and Fe2+. The linear equations were RCu = -0.0591[Q] + 3.505 (R2 = 0.992) and RFe = -0.0874[Q] + 3.61 (R2 = 0.999). The corresponding limits of detection were 0.5 and 0.31 μM, respectively. The good results had been obtained in the actual samples detection.

A cell-surface-anchored DNA probe coupled with hybridization chain reaction enzyme-free dual signal amplification for sensitive electrochemical detection of the cellular microenvironment

The cellular microenvironment plays key roles in regulating physiological processes. However, it is still a challenge to detect it with quantification. Here, a simple, biocompatible, and universal strategy based on cell surface-anchored specific DNAzymes and hybridization chain reaction enzyme-free signal amplification for cellular microenvironment electrochemical detection is presented. In this strategy, the cell could be captured on the surface of the electrode via aptamer-target recognition. On the other hand, the DNAzyme hybridized with the substrate strand as a metal ion probe was anchored on the surface of the cell. In the presence of metal ions, the substrate strand could be cleaved into two fragments by the DNAzyme and released from the cell surface. Then, the DNA modified gold nanoparticles (AuNPs) could be captured on the electrode. Subsequently, an alternative hybridization reaction of two hairpin probes was triggered by the carried initiators forming nicked double helices. For signal readout, hemin could be inserted into the double-helix DNA long chain via electrostatic interaction, which could electro-reduce hydrogen peroxide to generate an electrochemical signal. Based on the intrinsic advantages of DNAzymes, including rapid kinetics, high sensitivity, and high selectivity, and the signal amplification strategy, this method should be able to monitor and semi-quantify target metal ions in the cellular microenvironment. Furthermore, this method shows potential for various targets by employing different DNA probes in the cellular microenvironment, providing a platform for bioanalysis.

Emerging Frontiers in Nanotechnology for Precision Agriculture: Advancements, Hurdles and Prospects

This review article provides an extensive overview of the emerging frontiers of nanotechnology in precision agriculture, highlighting recent advancements, hurdles, and prospects. The benefits of nanotechnology in this field include the development of advanced nanomaterials for enhanced seed germination and micronutrient supply, along with the alleviation of biotic and abiotic stress. Further, nanotechnology-based fertilizers and pesticides can be delivered in lower dosages, which reduces environmental impacts and human health hazards. Another significant advantage lies in introducing cutting-edge nanodiagnostic systems and nanobiosensors that monitor soil quality parameters, plant diseases, and stress, all of which are critical for precision agriculture. Additionally, this technology has demonstrated potential in reducing agro-waste, synthesizing high-value products, and using methods and devices for tagging, monitoring, and tracking agroproducts. Alongside these developments, cloud computing and smartphone-based biosensors have emerged as crucial data collection and analysis tools. Finally, this review delves into the economic, legal, social, and risk implications of nanotechnology in agriculture, which must be thoroughly examined for the technology’s widespread adoption.

A Review of Analytical Techniques for the Determination and Separation of Silver Ions and Its Nanoparticles

Many articles have already been published dealing with silver ions and its nanoparticles, but mostly from the environmental and toxicological point of view. This article is a review focused on the various analytical techniques and detection platforms used in the separation and determination of mentioned above species, especially on the trace concentration level. Commonly used are optical methods because of their high sensitivity and easy automation. The separation methods are mainly used for the separation and preconcentration of silver particles. Their combination with other analytical techniques, mainly inductively coupled plasma mass spectrometry (ICP-MS) leads to very low detection limits of analysis. The electrochemical methods are also powerful and perspective mainly because of the fabrication of new sensors designed for silver determination. All methods may be combined with each other to achieve a synergistic improvement of analytical parameters with an impact on sensitivity, selectivity and reliability. The paper comprises a review of all three types of analytical methods on the determination of trace quantities of silver ions and its nanoparticles.

A Novel Minidumbbell DNA-Based Sensor for Silver Ion Detection

Silver ion (Ag⁺) is one of the most common heavy metal ions that cause environmental pollution and affect human health, and therefore, its detection is of great importance in the field of analytical chemistry. Here, we report an 8-nucleotide (nt) minidumbbell DNA-based sensor (M-DNA) for Ag⁺ detection. The minidumbbell contained a unique reverse wobble C·C mispair in the minor groove, which served as the binding site for Ag⁺. The M-DNA sensor could achieve a detection limit of 2.1 nM and sense Ag⁺ in real environmental samples with high accuracy. More importantly, the M-DNA sensor exhibited advantages of fast kinetics and easy operation owing to the usage of an ultrashort oligonucleotide. The minidumbbell represents a new and minimal non-B DNA structural motif for Ag⁺ sensing, allowing for the further development of on-site environmental Ag⁺ detection devices.

Durable Nanocellulose-Stabilized Emulsions of Dithizone/Chloroform in Water for Hg2+ Detection: A Novel Approach for a Classical Problem

The use of dithizone (DTZ) for colorimetric heavy-metal detection is approximately one century old. However, its pending stability issues and the need for simple indicators justify further research. Using cellulose nanofibers, we attained DTZ-containing emulsions with high stability. These emulsions had water (at least 95 wt %) and acetic acid (1-8 mL/L) conforming the continuous phase, while dispersed droplets of diameter <1 μm contained chloroform-solvated DTZ (3 wt %). The solvation cluster was computed by molecular dynamics simulations, suggesting that chloroform slightly reduces the dihedral angle between the two sides of the thiocarbazone chain. Nanocellulose concentrations over 0.2 wt % sufficed to obtain macroscopically homogeneous mixtures with no phase separation. Furthermore, the rate of degradation of DTZ in the nanocellulose-stabilized emulsion did not differ significantly from a DTZ/chloroform solution, outperforming DTZ/toluene and DTZ/acetonitrile. Not only is the emulsion readily and immediately responsive to mercury(II), but it also decreases interferences from other ions and from natural samples. Unexpectedly, neither lead(II) nor cadmium(II) triggered a visual response at trace concentrations. The limit of detection of these emulsions is 15 μM or 3 mg/L, exceeding WHO limits for mercury(II) in drinking water, but they could be effective at raising alarms.

A SERS-Fluorescence dual-signal aptasensor for sensitive and robust determination of AFB1 in nut samples based on Apt-Cy5 and MNP@Ag-PEI

Food aflatoxin B1 (AFB1) contamination greatly threatens human health and its sensitive determination is imperative. In this study, a surface-enhanced Raman scattering (SERS) and fluorescence dual-signal aptasensor was constructed for sensitive AFB1 detection in peanuts, walnuts, and almonds samples. Fluorescent dye cy5 was used as fluorophore and Raman reporter, while polyethyleneimine modified Ag coating magnetic nanoparticles (MNP@Ag-PEI) were utilized to absorb the cy5 modified aptamer (apt-cy5). Results indicated that linear ranges of 0.001-1000 ng/mL and 0.2-20,000 ng/mL with detection limits of 0.45 pg/mL and 0.135 ng/mL for the SERS and fluorescence methods were obtained, respectively, and AFB1 detection in the nut samples using the aptasensor achieved satisfactory recoveries of 95.2%-108.6% for SERS and 94.7%-109.7% for fluorescence. Compared with other mono signal detection, the established aptasensor facilely fused the merits of the two signals and improved the detection accuracy and flexibility.

An ultrasensitive electrochemical biosensor for bisphenol A based on aptamer-modified MrGO@AuNPs and ssDNA-functionalized AuNP@MBs synergistic amplification

Bisphenol A (BPA) is a harmful endocrine disruptor, sensitive and rapid quantification of BPA is highly desirable. In this work, a novel synergistic signal-amplifying electrochemical biosensor was developed for BPA detection by using a recognition probe (RP) constructed by BPA aptamer modified gold nanoparticles-loaded magnetic reduced graphene oxide (Aptamer-MrGO@AuNPs), and a signal probe (SP) constructed by BPA aptamer-complementary single-stranded DNA (ssDNA) functionalized methylene blue (MB)-loaded gold nanoparticle (ssDNA-AuNP@MBs). The RP and SP can self-assemble to form a stable RP-SP complex through complementary base pairing. The current intensity of the biosensor correlates with the number of RP-SP complexes. In the presence of BPA, the BPA aptamer can capture BPA with high selectivity and affinity, form an RP-BPA complex and dissociate the RP-SP complex to release SP, resulting in a decrease in the current signal intensity of the biosensor. A single AuNP could be loaded with multiple BPA aptamers and MBs, which improves the recognition efficiency and enhances the signal intensity. Due to the magnetic properties of MrGO@AuNPs, the magnetic separation and adsorption of RP or RP-SP complex is very convenient, enabling all reaction processes to be carried out in solution, which not only improves the mass transfer efficiency, but also simplifies the operation. Under optimal conditions, the developed biosensor had a detection limit as low as 0.141 pg/mL and had been successfully applied to the detection of real environmental water samples. Therefore, the synergistic signal amplification strategy of RP and SP has potential value in the detection of trace pollutants in the water environment.

DNA-functionalized gold nanoparticles: Modification, characterization, and biomedical applications

With the development of technologies based on gold nanoparticles (AuNPs), bare AuNPs cannot meet the increasing requirements of biomedical applications. Modifications with different functional ligands are usually needed. DNA is not only the main genetic material, but also a good biological material, which has excellent biocompatibility, facile design, and accurate identification. DNA is a perfect ligand candidate for AuNPs, which can make up for the shortcoming of bare AuNPs. DNA-modified AuNPs (DNA-AuNPs) have exciting features and bright prospects in many fields, which have been intensively investigated in the past decade. In this review, we summarize the various approaches for the immobilization of DNA strands on the surface of AuNPs. Representative studies for biomedical applications based on DNA-AuNPs are also discussed. Finally, we present the challenges and future directions.

Overview on the Design of Magnetically Assisted Electrochemical Biosensors

Electrochemical biosensors generally require the immobilization of recognition elements or capture probes on the electrode surface. This may limit their practical applications due to the complex operation procedure and low repeatability and stability. Magnetically assisted biosensors show remarkable advantages in separation and pre-concentration of targets from complex biological samples. More importantly, magnetically assisted sensing systems show high throughput since the magnetic materials can be produced and preserved on a large scale. In this work, we summarized the design of electrochemical biosensors involving magnetic materials as the platforms for recognition reaction and target conversion. The recognition reactions usually include antigen-antibody, DNA hybridization, and aptamer-target interactions. By conjugating an electroactive probe to biomolecules attached to magnetic materials, the complexes can be accumulated near to an electrode surface with the aid of external magnet field, producing an easily measurable redox current. The redox current can be further enhanced by enzymes, nanomaterials, DNA assemblies, and thermal-cycle or isothermal amplification. In magnetically assisted assays, the magnetic substrates are removed by a magnet after the target conversion, and the signal can be monitored through stimuli-response release of signal reporters, enzymatic production of electroactive species, or target-induced generation of messenger DNA.

A highly sensitive electrochemical biosensor for Hg2+ based on entropy-driven DNA walker-based amplification

Herein, a sensitive electrochemical biosensor based on an enzyme-free and entropy-driven DNA walker is presented for the determination of Hg²⁺. This biosensor uses Hg²⁺ as a key to induce a mismatch between thymine-rich oligonucleotides to start the DNA walker, and it utilizes the entropy change of the sensing system to continuously drive the hybridization of oligonucleotides as a driving force for its walking. As the DNA walker runs, the detection signal is amplified to improve the sensitivity of the biosensor. Square wave voltammetry (SWV) of this biosensor shows a linear response of the methylene blue (MB) oxidation signal with an increase of Hg²⁺ concentration in the range of 0 to 80 nM with a detection limit of 0.136 nM, which satisfactorily meets the sensitivity requirement of the U.S. Environmental Protection Agency (EPA). The biosensor also exhibits excellent selectivity over a spectrum of interfering ions and performs well in real water samples, suggesting that it is a promising candidate for Hg²⁺ detection.

A review on magnetic sensors for monitoring of hazardous pollutants in water resources

Water resources have long been of interest to humans and have become a serious issue in all aspects of human life. The disposal of hazardous pollutants in water resources is one of the biggest global concerns and poses many risks to human health and aquatic life. Therefore, the control of hazardous pollutants in water resources plays an important role, when it comes to evaluating water quality. Due to low toxicity, good electrical conductivity, facile functionalization, and easy preparation, magnetic materials have become a good alternative in recent years to control hazardous pollutants in water resources. In the present study, the idea of using magnetic sensors in controlling and monitoring of pharmaceuticals, pesticides, heavy metals, and organic pollutants have been reviewed. The water pollutants in drinking water, groundwater, surface water, and seawater have been discussed. The toxicology of water hazardous pollutants has also been reviewed. Then, the magnetic materials were discussed as sensors for controlling and monitoring pollutants. Finally, future remarks and perspectives on magnetic nanosensors for controlling hazardous pollutants in water resources and environmental applications were explained.

Application of Nanomaterial Modified Aptamer-Based Electrochemical Sensor in Detection of Heavy Metal Ions

Heavy metal pollution resulting from significant heavy metal waste discharge is increasingly serious. Traditional methods for the detection of heavy metal ions have high requirements on external conditions, so developing a sensitive, simple, and reproducible detection method is becoming an urgent need. The aptamer, as a new kind of artificial probe, has received more attention in recent years for its high sensitivity, easy acquisition, wide target range, and wide use in the detection of various harmful substances. The detection platform that an aptamer-based electrochemical biosensor (E-apt sensor) provides is a new approach for the detection of heavy metal ions. Nanomaterials are particularly important in the construction of E-apt sensors, as they can be used as aptamer carriers or sensitizers to stimulate or inhibit electrochemical signals, thus significantly improving the detection sensitivity. This review summarizes the application of different types of nanomaterials in E-apt sensors. The construction methods and research progress of the E-apt sensor based on different working principles are systematically introduced. Moreover, the advantages and challenges of the E-apt sensor in heavy metal ion detection are summarized.

Construction of branched DNA-based nanostructures for diagnosis, therapeutics and protein engineering

Branched DNA with multibranch-like anisotropic topology serves as a promising and powerful building block in constructing multifunctional-integrated nanomaterials in a programmable and controllable manner. Recently, a series of branched DNA-based functional nanomaterials were developed by elaborate molecular design. In this review, we focused on the construction of branched DNA-based nanostructures for biological and biomedical applications. First, the molecular design and synthesis method of branched DNA monomer were briefly described. Then, the construction strategies of branched DNA-based nanostructures were categorially discussed, including target-triggered polymerization, enzymatic extension and hybrid assembly. Finally, the biological and biomedical applications including diagnosis, therapeutics and protein engineering were summarized. We envision that the review will contribute to the further development of branched DNA-based nanomaterials with great application potential in the field of biomedicine, thus building a new bridge between material chemistry and biomedicine.

Catalytic hairpin activated gold-magnetic/gold-core-silver-shell rapid self-assembly for ultrasensitive Staphylococcus aureus sensing via PDMS-based SERS platform

Staphylococcus aureus (S. aureus) has been identified as a marker of food contamination, closely associated with human health. This work designs a sensitive and rapid bio-detection strategy for S. aureus based on hybridization chain reaction-assisted surface enhanced Raman scattering (HCR-assisted-SERS) signal amplification. In this approach, the interaction between the aptamer (Apt) and its partial complementary DNA strands (cDNA) fabricated on the surface of gold-assisted magnetic nanoparticles (Au-MNPs) and the subsequent detachment of the cDNA results in the activation of the HCR process. In the HCR, a pair of hairpin structured DNA probes (H1 and H2) with sticky ends self-assembles to form a long DNA polymer. Subsequently, the output and amplification of the SERS signal were performed by conjugating 4-ATP modified Au@Ag NPs with the obtained DNA polymer via a specific Ag-S bond, and further collected through a self-administered polydimethylsiloxane (PDMS) cone-shaped support array. The precise quantification of S. aureus was performed in the concentration range of 28 to 2.8 × 10⁶ cfu/mL, achieving a detection limit of 0.25 cfu/mL. This strategy was further applied to S. aureus detection in spiked milk samples with good recoveries (91-102%) and the relative standard deviation (4.35-8.41%). The sensing platform also showed satisfactory validation results (p > 0.05) using the traditional plate counting method. The proposed HCR-assisted SERS probe can be extended to other foodborne pathogenic bacteria types via engineering appropriate Apt and DNA initiators, thus, inspiring widespread applications in food safety and biomedical research.

Achieving Ultrasensitive Point-of-Care Assay for Mercury Ions with a Triple-Mode Strategy Based on the Mercury-Triggered Dual-Enzyme Mimetic Activities of Au/WO3 Hierarchical Hollow Nanoflowers

The exploration of new strategies for portable detection of mercury ions with high sensitivity and selectivity is of great value for biochemical and environmental analyses. Herein, a straightforward, convenient, label-free, and portable sensing platform based on a Au nanoparticle (NP)-decorated WO₃ hollow nanoflower was constructed for the sensitive and selective detection of Hg(II) with a pressure, temperature, and colorimetric triple-signal readout. The resulting Au/WO₃ hollow nanoflowers (Au/WO₃ HNFs) could efficaciously impede the aggregation of Au NPs, thus significantly improving their catalytic activity and stability. The sensing mechanism of this new strategy using pressure as a signal readout was based on the mercury-triggered catalase mimetic activity of Au/WO₃ HNFs. In the presence of the model analyte Hg(II), H₂O₂ in the detection system was decomposed to O₂ fleetly, resulting in a detectable pressure signal. Accordingly, the quantification of Hg(II) was facilely realized based on the pressure changes, and the detection limit could reach as low as 0.224 nM. In addition, colorimetric and photothermal detection of Hg(II) using the Au/WO₃ HNFs based on their mercury-stimulated peroxidase mimetic activity was also investigated, and the detection limits were calculated to be 78 nM and 0.22 μM for colorimetric and photothermal methods, respectively. Hence, this nanosensor can even achieve multimode determination of Hg(II) with the concept of point-of-care testing (POCT). Furthermore, the proposed multimode sensing platform also displayed satisfactory sensing performance for the Hg(II) assay in actual water samples. This promising strategy may provide novel insights on the fabrication of a multimode POCT platform for sensitive, selective, and accurate detection of heavy metal ions.

Ultrasensitive electrochemical sensor for mercury ion detection based on molybdenum selenide and Au nanoparticles via thymine-Hg2+-thymine coordination

An ultrasensitive and specific-selection electrochemical sensor was constructed for Hg²⁺ detection based on Au nanoparticles and molybdenum selenide (Au NPs@MoSe₂) as well as the thymine-Hg²⁺-thymine (T-Hg²⁺-T) coordination. Herein, Au NPs@MoSe₂ not only could improve the sensitivity due to the large surface area and good electrical conductivity but also offered more sites to immobilize thiol-labeled T-rich hairpin DNA probes (P-1), which has a specific recognition for Hg²⁺ and methylene blue-labeled T-rich DNA probes (MB-P). When Hg²⁺ and MB-P exist, P-1 and MB-P can form a stable T-Hg²⁺-T complex. Then, methylene blue can be close to the electrode and detectable via differential pulse voltammetry (DPV). Benefiting from the specific recognition of T-Hg²⁺-T and the merits of Au NPs and MoSe₂, the fabricated biosensor presented an ultrasensitive and highly selective performance. The DPV responses had a positive linear relationship with Hg²⁺ concentrations over ten orders of magnitude from 1.0 × 10^-16 to 1.0 × 10^-7 mol L^-1. The detection limit was down to 1.1 × 10^-17 mol L^-1. Moreover, the developed sensor exhibited a promising application for trace Hg²⁺determination in water samples.

Signal enhancing strategies in aptasensors for the detection of small molecular contaminants by nanomaterials and nucleic acid amplification

Small molecular contaminants (such as mycotoxins, antibiotics, pesticide residues, etc.) in food and environment have given rise to many biological and ecological toxicities, which has attracted worldwide attention in recent years. Meanwhile, due to the advantages of aptamers such as high specificity and stability, easy synthesis and modification, as well as low cost and immunogenicity, various aptasensors for the detection of small molecular contaminants have been flourishing. An aptasensor as a whole is composed of an aptamer-based target recognizer and a signal transducer, which are fields of concentrated research. In the practical detection applications, in order to achieve the quantitative detection of small molecular contaminants at low abundance in real samples, a large number of signal enhancing strategies have been utilized in the development of aptasensors. Recent years is a vintage period for efficient signal enhancing strategies of aptasensors by the aid of nanomaterials and nucleic acid amplification that are applied in the elements for target recognition and signal conversion. Therefore, this paper meticulously reviews the signal enhancing strategies based on nanomaterials (including the (quasi-)zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanomaterials) and nucleic acid amplification (including enzyme-assisted nucleic acid amplification and enzyme-free nucleic acid amplification). Furthermore, the challenges and future trends of the abovementioned signal enhancing strategies for application are also discussed in order to inspire the practitioners in the research and development of aptasensors for small molecular contaminants.

Nanomaterials for electrochemical detection of pollutants in water: A review

The survival of living beings, including humanity, depends on a continuous supply of clean water. However, due to the development of industry, agriculture, and population growth, an increasing number of wastewaters is discarded, and the negative effects of such actions are clear. The first step in solving this situation is the collection and monitoring of pollutants in water bodies to subsequently facilitate their treatment. Nonetheless, traditional sensing techniques are typically laboratory-based, leading to potential diminishment in analysis quality. In this paper, the most recent developments in micro- and nano-electrochemical devices for pollutant detection in wastewater are reviewed. The devices reviewed are based on a variety of electrodes and the sensing of three different categories of pollutants: nutrients and phenolic compounds, heavy metals, and organic matter. From these electrodes, Cu, Co, and Bi showed promise as versatile materials to detect a grand variety of contaminants. Also, the most commonly used material is glassy carbon, present in the detection of all reviewed analytes.

Bio-inspired Track-Etched Polymeric Nanochannels: Steady-State Biosensors for Detection of Analytes

Bio-inspired polymeric nanochannel (also referred as nanopore)-based biosensors have attracted considerable attention on account of their controllable channel size and shape, multi-functional surface chemistry, unique ionic transport properties, and good robustness for applications. There are already very informative reviews on the latest developments in solid-state artificial nanochannel-based biosensors, however, which concentrated on the resistive-pulse sensing-based sensors for practical applications. The steady-state sensing-based nanochannel biosensors, in principle, have significant advantages over their counterparts in term of high sensitivity, fast response, target analytes with no size limit, and extensive suitable range. Furthermore, among the diverse materials, nanochannels based on polymeric materials perform outstandingly, due to flexible fabrication and wide application. This compressive Review summarizes the recent advances in bio-inspired polymeric nanochannels as sensing platforms for detection of important analytes in living organisms, to meet the high demand for high-performance biosensors for analysis of target analytes, and the potential for development of smart sensing devices. In the future, research efforts can be focused on transport mechanisms in the field of steady-state or resistive-pulse nanochannel-based sensors and on developing precisely size-controlled, robust, miniature and reusable, multi-functional, and high-throughput biosensors for practical applications. Future efforts should aim at a deeper understanding of the principles at the molecular level and incorporating these diverse pore architectures into homogeneous and defect-free multi-channel membrane systems. With the rapid advancement of nanoscience and biotechnology, we believe that many more achievements in nanochannel-based biosensors could be achieved in the near future, serving people in a better way.

Functionalized Fe3O4 Nanoparticles as Glassy Carbon Electrode Modifiers for Heavy Metal Ions Detection-A Mini Review

Over the past few decades, nanoparticles of iron oxide Fe3O4 (magnetite) gained significant attention in both basic studies and many practical applications. Their unique properties such as superparamagnetism, low toxicity, synthesis simplicity, high surface area to volume ratio, simple separation methodology by an external magnetic field, and renewability are the reasons for their successful utilisation in environmental remediation, biomedical, and agricultural applications. Moreover, the magnetite surface modification enables the successful binding of various analytes. In this work, we discuss the usage of core-shell nanoparticles and nanocomposites based on Fe3O4 for the modification of the GC electrode surface. Furthermore, this review focuses on the heavy metal ions electrochemical detection using Fe3O4-based nanoparticles-modified electrodes. Moreover, the most frequently used electrochemical methods, such as differential pulse anodic stripping voltammetry and measurement conditions, including deposition potential, deposition time, and electrolyte selection, are discussed.