Label-free DNAzyme assays for dually amplified and one-pot detection of lead pollution

DNAzyme-activated CRISPR/Cas assay for sensitive and one-pot detection of lead contamination

Hazardous lead ions (Pb2+) even at a minute level can pose side effects on human health, highlighting the need for tools for trace Pb2+ detection. Herein, we present a DNAzyme-activated CRISPR assay (termed DzCas12T) for sensitive and one-pot detection of lead contamination. Using an extension-bridged strategy eliminates the need for separation to couple the DNAzyme recognition and CRISPR reporting processes. The tandem design endowed the DzCas12T assay with high specificity and sensitivity down to the pM-level. This assay has been used to detect lead contamination in food and water samples, indicating the potential for monitoring lead-associated environmental and food safety.

Ultrasensitive label-free electrochemical aptasensor for Pb2+ detection exploiting Exo III amplification and AgPt/GO nanocomposite-enhanced transduction

Lead ion pollution has become a serious public health concern worldwide. Therefore, sensitive detection of Pb2+ is critical to control lead pollution, assess risks, and safeguard the health of vulnerable populations. This study reports a highly sensitive labelling-free electrochemical aptasensor for Pb2+ detection. The aptasensor employs silver-platinum nanoparticles/graphene oxide (AgPt/GO) and Exonuclease III (Exo III) for signal amplification. GO provides high surface area and conductivity for immobilizing AgPt NPs, facilitating the immobilization of aptamer (Apt) probes on the electrode surface. Exo III enzymatically cleaves DNA strands on the electrode surface, releasing DNA segments to amplify the signal further. The synergistic amplification by AgPt/GO and ExoIII enables an extremely wide linear detection range of 0.05 pM-5 nM for Pb2+, with a low detection limit of 0.019 pM. Additionally, the G-quadruplex structure ensures excellent selectivity for Pb2+ detection, resulting in high reproducibility and stability of the aptasensor. The aptasensor was successfully applied to detect spiked Pb2+ in tap water samples, achieving recovery rates ranging from 96 to 108.4 %. By integrating nanomaterials, aptamers and enzymatic amplification, the aptasensor facilitates highly sensitive and selective detection of Pb2+, demonstrating potential for practical applications in environmental monitoring.

Nanotechnology-based analytical techniques for the detection of contaminants in aquatic products

Food safety of aquatic products has attracted considerable attention worldwide. Although a series of conventional bioassays and instrumental methods have been developed for the detection of pathogenic bacteria, heavy metal residues, marine toxins, and biogenic amines during the production and storage of fish, shrimp, crabs et al., the nanotechnology-based analyses still have their advantages and are promising since they are cost-efficient, highly sensitive and selective, easy to conduct, facial design, often require no sophisticated instruments but with excellent detection performance. This review aims to summarize the advances of various biosensing strategies for bacteria, metal ions, and small molecule contaminants in aquatic products during the last five years, The review highlights the development in nanotechnologies applied for biorecognition process, signal transduction and amplification methods in each novel approach, the nuclease-mediated DNA amplification, nanomaterials (noble metal nanoparticle, metal-organic frameworks, carbon dots), lateral flow-based biosensor, surface-enhanced Raman scattering, microfluidic chip, and molecular imprinting technologies were especially emphasized. Moreover, this study provides a view of current accomplishments, challenges, and future development directions of nanotechnology in aquatic product safety evaluation.

A portable multi-channel fluorescent paper-based microfluidic chip based on smartphone imaging for simultaneous detection of four heavy metals

Due to the excessive contamination of heavy metals pollution, it is very urgent and necessary to develop a real-time detection method for the heavy metals in food. As a target sensing device, a paper-based microfluidic device (μPAD) has the advantages of simplicity, low-cost, and portability. In this study, a self-driven microfluidic paper-based chip was first developed for the simultaneous detection of four targets. The channels on the microfluidic chip were prepared by using wax printing and automatic screen printing on the filter paper, where liquid flowed by capillary force without pump assistance. Based on the specific binding ability of aptamers to heavy metals, a "turn-on" fluorescence aptasensor for the simultaneous detection of four heavy metal ions was developed on the proposed multi-channel device via smartphone imaging. The obtained fluorescence images were digitized into RGB color values by Image J software, and an M-mode was established to realize the quantitative detection of heavy metal ions. Under optimal conditions, the limits of detection for lead(II), mercury(II), cadmium(II), and arsenic(III) were 4.20 nM, 1.70 nM, 2.04 nM, and 1.65 nM, respectively. Furthermore, the aptasensor was successfully applied to the quantitative detection of four heavy metal ions in apple and lettuce samples with recovery rates of 84.0%-104.1%.

Electric field-enabled aptasensing interfacial engineering to simultaneously enhance specific preconcentration and electrochemical detection of mercury and lead ions

Aptamers with strong affinity to heavy metal ions (HMIs) allow fabrication of electrochemical sensors with high selectivity and sensitivity, while controllable regulation of aptamer-HMI recognition at the sensing interface, which is vital for better analytical performance, remains challenging. Here, an electric field-based strategy for engineering an aptasensing interface was proposed to realize the specific preconcentration and accurate detection of mercury (Hg2+) and lead (Pb2+) ions with a ratiometric electrochemical sensor. The working principle is to apply an electric field to drive HMIs to approach the aptamer and retain the orientation of the DNA structure. Anthraquinone-2-carboxylic acid (AQ)-labeled complementary DNA was designed to simultaneously bind a ferrocene (Fc)-labeled aptamer for Hg2+ and a methylene blue (MB)-labeled aptamer for Pb2+, and the sensing interface was fabricated with this presynthesized DNA structure. For preconcentration, an electric field of 3.0 V pushed HMIs to approach the aptamer and retained the orientation of DNA to favor the following recognition; for detection, the oriented DNA in 2.5 V electric field offered a stable current of AQ as a reference. In this way, currents of AQ, Fc and MB were used to produce ratiometric signals of IAQ/IFc and IAQ/IMB for Hg2+ and Pb2+, respectively. Such a strategy allowed the simultaneous detection of Hg2+ and Pb2+ within 30 min with detection limits of 0.69 pM and 0.093 pM, respectively. The aptasensor was applied for soil, water, and crayfish analysis in paddy fields. The electric field-enabled strategy offers a new way to fabricate high-performance electrochemical aptasensor for HMIs detection.

Statistical Analysis and Health Risk Assessment: Vegetables Irrigated with Wastewater in Kirri Shamozai, Pakistan

One of the primary environmental routes through which humans are exposed to metals and may be exposed to health risks is the food chain's contamination with heavy metals. The study observed the risks posed by contaminants in vegetables produced in soil that received wastewater irrigation, as well as their origins and the human health impacts. Eight harmful metals (Cu, Fe, Zn, Mn, Pb, Cd, Ni, and Cr) were tested for concentration levels in water, soil, and vegetable samples using analytical techniques and an atomic absorption spectrophotometer. The present study investigated the potential health implications associated with the consumption of vegetables irrigated using wastewater containing heavy metals. The results indicated a notable accumulation of heavy metals in plant and soil samples obtained from Kirri Shamozai, Pakistan. In comparison to vegetables cultivated in soil irrigated with fresh water, the concentration levels of heavy metals in vegetables grown on soil irrigated with untreated wastewater were considerably higher at (P ≤ 0.001) and above the World Health Organization (WHO) recommended limits. The results showed that heavy metals had significantly accumulated in the soil and had permeated into the crops. Heavy metal concentrations in vegetables cultivated on land irrigated with wastewater were more significant than those grown on land irrigated with freshwater. They exceeded US EPA and World Health Organization (WHO) limits. PCA results for Pb, Cu, and Cr are the main issues impacting water quality and health hazards. The PCA results show that the soil has an extensive loading of heavy metals Cd, Ni, and Mn.

Engineered DNA molecular machine for ultrasensitive detection of environmental lead pollution

Dynamic monitoring of environmental Pb2+ is of utmost importance for food safety and personal well-being. Herein, we report a novel, rapid, and practical fluorescence detection platform for Pb2+. The platform comprises two essential components: an engineered DNAzyme probe (EDP) and a responsive functionalized probe (RFP). The EDP demonstrates specific recognition of Pb2+ and the subsequent release of free DNA fragments. The released DNA fragments are then captured using the RFP to form DNA complexes, which undergo multiple cascade amplification reactions involving polymerases and nickases, resulting in the generation of a large number of fluorescence signals. These signals can detect Pb2+ at concentrations as low as 0.114 nmol/L, with a dynamic range spanning from 0.1 nmol/L to 50 nmol/L. Moreover, the platform exhibits excellent sensitivity and selectivity for Pb2+ detection. To further validate its effectiveness, we successfully quantitatively detected lead contamination in water from Chaohu Lake, and the results aligned closely with those obtained using inductively coupled plasma-mass spectrometry (ICP-MS). Moreover, this platform is suitable for detecting Pb2+ in seawater, soil, and fish samples. These findings confirm the suitability of the current detection platform for the dynamic assessment of Pb contamination in ecological environments, thereby contributing to environmental and food safety.

Ultralow background one-pot detection of Lead(II) using a non-enzymatic double-cycle system mediated by a hairpin-involved DNAzyme

A double-cycle system has been developed for specifically detecting trace amounts of Pb2+ by significantly decreasing the background signal. The detection involves two types of RNA cleavage reactions: one using a Pb2+-specific GR5 DNAzyme (PbDz) and the other utilizing a newly constructed 10-23 DNAzyme with two hairpins embedded in its catalytic center (hpDz). The ring-structured hpDz (c-hpDz) exhibits significantly lower activity compared to the circular 10-23 DNAzyme without hairpin structures, which plays a crucial role in reducing the background signal. When Pb2+ is present, PbDz cleaves c-hpDz to its active form, which then disconnects the molecular beacon to emit the fluorescent signal. The method allows for rapid and sensitive Pb2+ detection within 40 min for 10 fM of Pb2+ and even as short as 10 min for 100 nM of Pb2+. Additionally, visual detection is possible through the non-crosslinking assembly of Au nanoparticles. The entire process can be performed in one pot and even one step, making it highly versatile and suitable for a wide range of applications, including food safety testing and environmental monitoring.

Combination of RCA and DNAzyme for Dual-Signal Isothermal Amplification of Exosome RNA

The RNA contained in exosomes plays a crucial role in information transfer between cells in various life activities. The accurate detection of low-abundance exosome RNA (exRNA) is of great significance for cell function studies and the early diagnosis of diseases. However, their intrinsic properties, such as their short length and high sequence homology, represent great challenges for exRNA detection. In this paper, we developed a dual-signal isothermal amplification method based on rolling circle amplification (RCA) coupled with DNAzyme (RCA-DNAzyme). The sensitive detection of low-abundance exRNA, the specific recognition of their targets and the amplification of the detection signal were studied and explored. By designing padlock probes to specifically bind to the target exRNA, while relying on the ligation reaction to enhance recognition, the precise targeting of exosome RNA was realized. The combination of RCA and DNAzyme could achieve a twice-as-large isothermal amplification of the signal compared to RCA alone. This RCA-DNAzyme assay could sensitively detect a target exRNA at a concentration as low as 527 fM and could effectively distinguish the target from other miRNA sequences. In addition, this technology was successfully proven to be effective for the quantitative detection of miR-21 by spike recovery, providing a new research approach for the accurate detection of low-abundance exRNA and the exploration of unknown exRNA functions.

DNAzyme-templated exponential isothermal amplification for sensitive detection of lead pollution and high-throughput screening of microbial biosorbents

Very low concentrations of lead (Pb²⁺) pollution can have far-reaching adverse impacts on human health, due to the cumulative toxicity of Pb²⁺. Herein, we report a DNAzyme-templated exponential isothermal amplification strategy (termed DNAzymee) for the ultrasensitive detection of Pb²⁺ pollution and the high-throughput screening of microbial biosorbents to remove Pb²⁺ pollution. DNAzyme can specifically recognize Pb²⁺, and this recognition event can be amplified by the subsequent exponential isothermal amplification reaction (EXPAR) and monitored by a G-quadruplex specific dye. The proposed design showed a low limit of detection (95 pM) and could identify Pb²⁺ pollution in different real samples with high precision. In particular, the proposed assay was used to screen Pb²⁺ biosorbents, and the results showed that Leuconostoc mesenteroides is a promising microbial biosorbent for removing Pb²⁺ pollution. Thus, the DNAzymee assay can serve as a platform to monitor lead pollution in the environment and screen efficient biosorbents for the control of lead pollution.

Wastewater-Irrigated Vegetables Are a Significant Source of Heavy Metal Contaminants: Toxicity and Health Risks

Water contaminated with heavy metals constitutes an important threat. This threat is a real problem with a negative impact in some developing countries where untreated industrial effluents are used for irrigation. The present study examines heavy metals in wastewater-irrigated vegetables (apple gourd, spinach, cauliflower, sponge gourd, and coriander) water, and soil from Chenab Nagar, Chiniot, Pakistan. In particular, the metals quantified were cadmium (Cd), chromium (Cr), cobalt (Co), nickel (Ni), lead (Pb), and manganese (Mn). Among them, Cr and Co in crops irrigated -wastewater exceeded the levels recommended by the World Health Organization (WHO). In contrast, Ni, Cu, Pb, and Mn concentrations were in line with WHO standards. Compared with the limits established by the Food and Agriculture Organization of the United Nations (FAO), all the study vegetables presented higher (thus unsafe) concentrations of Cd (0.38 to 1.205 mg/Kg). There were also unsafe concentrations of Cr in coriander, sponge gourd, and cauliflower. Pb was found at an unsafe concentration (0.59 mg/Kg) in cauliflower. Conversely, Ni and Mn concentrations were below the maximum permissible limits by WHO, and FAO in all of the analyzed samples. The contamination load index (CLI) in soil, bioconcentration factor (BCF) in plants, daily intake of metals (DIM), and health risk index (HRI) have also been evaluated to estimate the potential risk to human health in that area. We have found an important risk of transitions of Pb, Cd, Cr, and Co from water/soil to the edible part of the plant. The highest HRI value associated with Cd (6.10-13.85) followed by Cr (1.25-7.67) for all vegetable samples presented them as high health risk metal contaminants. If the issue is not addressed, consumption of wastewater-irrigated vegetables will continue posing a health risk.

DNAzyme-Based Microscale Thermophoresis Sensor

Microscale thermophoresis (MST) technology has emerged as a powerful growing method in a molecular interaction study by measuring fluorescence responses of molecules inside a capillary to infrared (IR) laser heating with the benefits of rapid ratiometric measurement, separation-free, no immobilization, and low sample consumption. Combining the advantages of RNA-cleaving DNAzymes in target recognition and enzymatic catalysis and the strength of MST technology for fluorescence signaling, here, we reported a DNAzyme-based MST method for sensitive target detection. We introduced a fluorescein terminal label at the RNA-cleaving DNAzyme, and the substrate was linked to DNAzyme together with a poly-T sequence in a unimolecular design or not conjugated with DNAzyme in a bimolecular design. The presence of the cofactor activated DNAzyme to catalytically cleave the substrate, causing molecular structure alteration and significant changes in MST signals. This DNAzyme MST sensor enabled sensitively detecting activator targets Pb²⁺ and l-histidine, with a detection limit of 49 pM Pb²⁺ and 3.9 μM l-histidine. This biosensing strategy is universal and promising for wide applications.

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

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

A microplasma optical emission spectrometry pen for point-of-care diagnosis of child blood lead

Blood lead levels (BLL) of children have attracted considerable attention due to their putative impact on intelligence decline. However, most methods used for the determination of blood lead typically require expensive, bulky, high power and gas consuming instrumentation, limiting their application for a point-of-care diagnosis. Herein we report the development and testing of a portable ballpoint discharge microplasma optical emission spectrometer (BD-OES pen) device having the potential to fill this needed measurement capability. The BD-OES pen utilizes a compact ballpoint-pen format integrating point-discharge microplasma, which permits the determination of child BLL requiring no more than 100 μL blood while providing high specificity, sensitivity and satisfactory limit of detection (0.73 μg L^-1). The handheld BD-OES pen is successfully used to diagnose BLL of 16 asymptomatic children on-site, two of whom had excessive the normal BLL. The pen may aid the on-site and rapid diagnosis of childhood BLL, particularly in low-income areas.

Ferrocene-Doped Polystyrene Nanoenzyme and DNAzyme Cocatalytic SERS Quantitative Assay of Ultratrace Pb2+

A new, stable and high-catalytic activity ferrocene-doped polystyrene nanosphere (PN_Fer) sol was prepared by the hydrogel procedure and characterized by electron microscopy and molecular spectroscopy. Results show that the nanosol exhibits excellent catalysis of the new indicator nanoreaction between AgNO₃ and sodium formate to generate nanosilver with strong surface-enhanced Raman scattering (SERS), resonance Rayleigh scattering (RRS) and surface plasmon resonance absorption (Abs) trimode molecular spectral signals. This new nanocatalytic amplification trimode indicator reaction was coupled with the G-quadruplex DNAzyme catalytic amplification of Pb²⁺ aptamer to fabricate a new SERS quantitative/RRS/Abs assay platform for the determination of ultratrace amounts of Pb²⁺. The Pb²⁺ content in water samples was analyzed with satisfactory results.

Heavy Metal Ions Removal from Aqueous Solutions by Treated Ajwa Date Pits: Kinetic, Isotherm, and Thermodynamic Approach

In the current study we prepared cost-effective adsorbents based on ajwa date pits to remove Cu(II) ions from aqueous medium. Adsorbents were studied using scanning electron microscopy (SEM), FT-IR, and Brunauer-Emmett-Teller (BET) methods to characterize the surface functionalities, morphology, pore size, and particle size. The concentration of Cu(II) ions in the studied samples was determined by atomic adsorption spectrometry technique (AAS). Adsorption method was performed sequentially in a batch system followed by optimization by studying the numerous conditions, for instance the initial amounts of Cu(II) ions, dosages of the adsorbent, contact time, and pH of the solution. The ideal pH observed for maximum adsorption capacity was ~6.5. Langmuir and Freundlich isotherm models correctly predicted the investigation results, with the maximum monolayer adsorption capacities for Cu(II) ions at 328 K being 1428.57 mg/g (treated ajwa date pits, TADP) and 1111.1 mg/g for as produced ajwa date pits (ADP). It was revealed that TADP possess greater adsorption capability than ADP. Recovery investigations revealed that the saturated adsorbents eluted the maximum metal with 0.1 M HCl. Cu(II) ions adsorption was observed to be reduced by 80–89% after the second regeneration cycle. For the raw and chemically processed ajwa date pits adsorbent, the Langmuir model performed significantly better than the Freundlich model. The results demonstrated that the adsorbent made from ajwa date pits could be an economical and environmentally friendly alternative for removing Cu(II) ion pollutant from aqueous media.

An ultra-high-sensitivity electrochemiluminescence aptasensor for Pb2+ detection based on the synergistic signal-amplification strategy of quencher abscission and G-quadruplex generation

Signal amplification provides an effective way to improve detection performance. Herein, an ultrasensitive electrochemiluminescence (ECL) aptasensor for Pb²⁺ detection was developed based on a dual signal-amplification strategy of the abscission of a quencher and the generation of a G-quadruplex by one-step and simultaneous way. Nitrogen-doped carbon quantum dots linked with complementary DNA (cDNA-NCQDs) at the sensing interface was applied as the quencher of a tris(4,4'-dicarboxylic acid-2,2'-bipyridyl)ruthenium(II) (Ru(dcbpy)₃²⁺)/tripropylamine system to minimize the ECL signal due to the intermolecular hydrogen bond-induced energy-transfer process. Upon the addition of Pb²⁺, its specific binding with the aptamer triggered the abscission of cDNA-NCQDs, accompanied by the formation of G-quadruplex on the surface of the electrode, both of which amplified the intensity of the light emission. The ECL amplification efficiency induced by the above two mechanisms (78.6%) was valuably greater than that of their sum value (69.3%). This synergistic effect resulted in high detection sensitivity of the ECL aptasensor, which allowed to thereby obtain Pb²⁺ measurements in the range of 1 fM - 10 nM with an ultra-low detection limit of 0.19 fM. The Pb²⁺-mediated synergistic signal-amplification ECL strategy can provide a new approach for integrating various amplification strategies.

Inhibition of CRISPR-Cas12a trans-cleavage by lead (II)-induced G-quadruplex and its analytical application

In this work, the inhibition of clustered regularly interspaced short palindromic repeats (CRISPR) - CRISPR associated protein (Cas) trans-cleavage by Pb²⁺-induced G-quadruplex has been firstly explored to detect Pb²⁺ in tea beverage and milk. In absence of Pb²⁺, the Na⁺-induced G-quadruplex can be cleaved by CRISPR-Cas12a. In contrast, Pb²⁺ can competitively combine with G-quadruplex, resulting in its conformational changes and resistance to trans-cleavage of CRISPR-Cas12a. Therefore, the fluorescence resonance energy transfer can happen. Pb²⁺ can be detected in a linear range from 100 nM to 5 µM with a lowest detection limit of 2.6 nM and a relative standard deviation of 4.32%. In summary, this work not only provides a new method for Pb²⁺ detection based on its induced G-quadruplex inhibition on CRISPR-Cas12a trans-cleavage, but also broadens the application of CRISPR-Cas system for heavy metal analysis in the field of food safety.

Ultrasensitive detection of trace Hg2+ by SERS aptasensor based on dual recycling amplification in water environment

Due to the nonbiodegradability and accumulation of mercury ion, even in extremely small amount, it will cause varying degrees of harm to environment and human health. Although researchers have developed many strategies to detect and monitor trace Hg²⁺, only a few provide sensitivities of less than 1.0 pM. Surface Enhanced Raman Spectroscopy (SERS) is a common method to detect mercury ion due to its high sensitivity, rapid detection and easy operation. In this work, we report a new SERS aptasensor based on dual recycling amplification for the detection of trace mercury ion, which combines SERS with nucleic acid signal amplification through functional aptamer and elaborately designed hairpin DNA. Under the optimal experimental conditions, this SERS aptasensor exhibits excellent selectivity and high sensitivity. A linear range (0.2-125 fM) and a low detection limit (0.11 fM) are obtained. By using specific aptamers, the strategy will provide a new idea for the trace detection of toxic contaminants in water environment.