DNAzyme-based colorimetric sensing of lead (Pb(2+)) using unmodified gold nanoparticle probes

Metallopeptide nanoreservoirs for concurrent imaging and detoxification of lead (Pb) from human retinal pigment epithelial (hRPE1) cells

Inspired by natural metallopeptides, our work focuses on engineering self-assembling nanostructures of C2-symmetric metallopeptide conjugates (MPC) from a pyridine-bis-tripeptide bioprobe that uniquely detects lead (Pb2+) ions by emitting a fluorescence signal at 450 nm, which is further intensified in the presence of DAPI (λem = 458 nm), enhancing the bioimaging quality. This study enables precise lead quantification by modulating the ionic conformation and morphology. Experimental and theoretical insights elucidate the nanostructure formation mechanism, laying the groundwork for materials encapsulation and advancing lead detoxification. Our proof-of-principle experiment, demonstrating actin filament recovery in lead-treated cells, signifies therapeutic potential for intracellular lead aggregation and introduces novel avenues in biotechnological applications within biomaterials science.

Advances in colorimetric aptasensors for heavy metal ion detection utilizing nanomaterials: a comprehensive review

Heavy metal ion contamination poses significant environmental and health risks, necessitating rapid and efficient detection methods. In the last decade, colorimetric aptasensors have emerged as powerful tools for heavy metal ion detection, owing to their notable attributes such as high specificity, facile synthesis, adaptability to modifications, long-term stability, and heightened sensitivity. This comprehensive overview summarizes the key developments in this field over the past ten years. It discusses the principles, design strategies, and innovative techniques employed in colorimetric aptasensors using nanomaterials. Recent advancements in enhancing sensitivity, selectivity, and on-site applicability are highlighted. The review also presents application studies of successful heavy metal ion detection using colorimetric aptasensors, underlining their potential for environmental monitoring and health protection. Finally, future directions and challenges in the continued evolution of these aptasensors are outlined.

Picomolar Detection of Lead Ions (Pb2+) by Functionally Modified Fluorescent Carbon Quantum Dots from Watermelon Juice and Their Imaging in Cancer Cells

Water contamination due to the presence of lead is one of the leading causes of environmental and health hazards because of poor soil and groundwater waste management. Herein we report the synthesis of functionally modified luminescent carbon quantum dots (CQDs) obtained from watermelon juice as potential nanomaterials for the detection of toxic Pb²⁺ ions in polluted water and cancer cells. By introducing surface passivating ligands such as ethanolamine (EA) and ethylenediamine (ED) in watermelon juice, watermelon-ethanolamine (WMEA)-CQDs and watermelon-ethylenediamine (WMED)-CQDs exhibited a remarkable ~10-fold and ~6-fold increase in fluorescence intensity with respect to non-doped WM-CQDs. The relative fluorescence quantum yields of WMEA-CQDs and WMED-CQDs were found to be 8% and 7%, respectively, in an aqueous medium. Among various functionally-modified CQDs, only WMED-CQDs showed high selectivity towards Pb²⁺ ions with a remarkably good limit of detection (LoD) of 190 pM, which is less than that of the permissible limit (72 nM) in drinking water. The functionally altered WMED-CQDs detected Pb²⁺ metal ions in polluted water and in a human cervical cancer cell line (HeLa), thus advocating new vistas for eco-friendly nanomaterials for their use as diagnostic tools in the environment and biomedical research areas.

Advances in Colorimetric Assay Based on AuNPs Modified by Proteins and Nucleic Acid Aptamers

This review is focused on the biosensing assay based on AuNPs (AuNPs) modified by proteins, peptides and nucleic acid aptamers. The unique physical properties of AuNPs allow their modification by proteins, peptides or nucleic acid aptamers by chemisorption as well as other methods including physical adsorption and covalent immobilization using carbodiimide chemistry or based on strong binding of biotinylated receptors on neutravidin, streptavidin or avidin. The methods of AuNPs preparation, their chemical modification and application in several biosensing assays are presented with focus on application of nucleic acid aptamers for colorimetry assay for determination of antibiotics and bacteria in food samples.

A simple and cost-effective paper-based and colorimetric dual-mode detection of arsenic(iii) and lead(ii) based on glucose-functionalized gold nanoparticles

We report a simple and cost-effective paper-based and colorimetric dual-mode detection of As(iii) and Pb(ii) based on glucose-functionalized gold nanoparticles under optimized conditions. The paper-based detection of As(iii) and Pb(ii) is based on the change in the signal intensity of AuNPs/Glu fabricated on a paper substrate after the deposition of the analyte using a smartphone, followed by processing with the ImageJ software. The colorimetric method is based on the change in the color and the red shift of the localized surface plasmon resonance (LSPR) absorption band of AuNPs/Glu in the region of 200-800 nm. The red shift (Δλ) of the LSPR band observed was from 525 nm to 660 nm for As(iii) and from 525 nm to 670 nm for Pb(ii). The mechanism of dual-mode detection is due to the non-covalent interactions of As(iii) and Pb(ii) ions with glucose molecule present on the surface AuNPs, resulting in the aggregation of novel metal nanoparticles. The calibration curve gave a good linearity range of 20-500 μg L-1 and 20-1000 μg L-1 for the determination of As(iii) and Pb(ii) with the limit of detection of 5.6 μg L-1 and 7.7 μg L-1 for both metal ions, respectively. The possible effects of different metal ions and anions were also investigated but did not cause any significant interference. The employment of AuNPs/Glu is successfully demonstrated for the determination of As(iii) and Pb(ii) using paper-based and colorimetric sensors in environmental water samples.

Applications of liquid crystals in biosensing

Liquid crystals (LCs), as a promising branch of highly-sensitive, quick-response, and low-cost materials, are widely applied to the detection of weak external stimuli and have attracted significant attention. Over the past decade, many research groups have been devoted to developing LC-based biosensors due to their self-assembly potential and functional diversity. In this paper, recent investigations on the design and application of LC-based biosensors are reviewed, based on the phenomenon that the orientation of LCs can be directly influenced by the interactions between biomolecules and LC molecules. The sensing principle of LC-based biosensors, as well as their signal detection by probing interfacial interactions, is described to convert, amplify, and quantify the information from targets into optical and electrical parameters. Furthermore, commonly-used LC biosensing targets are introduced, including glucose, proteins, enzymes, nucleic acids, cells, microorganisms, ions, and other micromolecules that are critical to human health. Due to their self-assembly potential, chemical diversity, and high sensitivity, it has been reported that tunable stimuli-responsive LC biosensors show bright perspectives and high superiorities in biological applications. Finally, challenges and future prospects are discussed for the fabrication and application of LC biosensors to both enhance their performance and to realize their promise in the biosensing industry.

Limitations for colorimetric aggregation assay of metal ions and ways of their overcoming

The development of analytical methods for the determination of metal ions in water is one of the priority tasks for efficient environmental monitoring. The use of modified gold nanoparticles and the colorimetric detection of their aggregation initiated by ions binding with specific receptors on the nanoparticle surface has high potential for simple testing. However, the limits of this approach and the parameters determining the assay sensitivity are not clear, and the possibilities of different assay formats are estimated only empirically. We have proposed a mathematical description of the aggregation processes in the assay and have estimated the detection limits of an aptamer-based assay of Pb2+ ions theoretically and experimentally. In the studied assay, gold nanoparticles modified with G,T-enriched aptamer were used, and their aggregation caused by the interaction with Pb2+ ions was controlled via a color change. The experimentally determined limit of Pb2+ detection was 700 ppb, which was in good agreement with theoretical calculations. An examination of the model showed that the limiting parameter of the assay is the binding constant of the aptamer-Pb2+ ion interaction. To overcome this limitation without searching for alternate receptors, two methods have been proposed, namely additional aggregation-causing components or centrifugation. These approaches lowered the detection limit to 150 ppb and even to 0.4 ppb. The second value accords with regulatory demands for the permissible levels of water source contamination, and the corresponding approach has significant competitive potential due to its rapidity, simple implementation, and the visual assessment of the assay results.

Electrochemical device based on nonspecific DNAzyme for the high-accuracy determination of Ca2+ with Pb2+ interference

Calcium is one of the most abundant and indispensable elements in biology, as it is a vital component of nerves, bones, and muscles and maintains the excitability of normal neuromuscular muscles. However, it may be harmful to the human body and even damage the organs if the calcium content exceeds the standard value by several times. To evaluate the level of calcium ions (Ca²⁺), an electrochemical biosensor (FET/SWNTs/Cazyme) was developed using a nonspecific DNAzyme with high stability, which combined the unique advantage of field-effect transistors and single-walled carbon nanotubes, while being easy-to-use and having excellent sensitivity. The incubation time and voltage after optimization were 15 min and +0.02 V. The nonspecific DNAzyme-based biosensor was sensitive to Ca²⁺, but it was also interfered with by Pb²⁺, which affected the detection accuracy. To solve this shortcoming, an electrochemical device was proposed, in which FET/SWNTs/Cazyme combined with other specific biosensors for Pb²⁺, and then established some data processing models were established through support vector machine regression (SVMR) and artificial neural network fitting (ANNF). For the optimal SVMR, the electrochemical device can determine the Ca²⁺ concentration in the range of 7.5-1000 μM with a detection limit of 5.48 μM. Finally, the prepared electrochemical device was employed to detect the Ca²⁺ in different milk and water samples.

Interfacing DNA with Gold Nanoparticles for Heavy Metal Detection

The contamination of heavy metals (e.g., Hg, Pb, Cd and As) poses great risks to the environment and human health. Rapid and simple detection of heavy metals of considerable toxicity in low concentration levels is an important task in biological and environmental analysis. Among the many convenient detection methods for heavy metals, DNA-inspired gold nanoparticles (DNA-AuNPs) have become a well-established approach, in which assembly/disassembly of AuNPs is used for colorimetric signaling of the recognition event between DNA and target heavy metals at the AuNP interface. This review focuses on the recent efforts of employing DNA to manipulate the interfacial properties of AuNPs, as well as the major advances in the colorimetric detection of heavy metals. Beginning with the introduction of the fundamental aspects of DNA and AuNPs, three main strategies of constructing DNA-AuNPs with DNA binding-responsive interface are discussed, namely, crosslinking, electrostatic interaction and base pair stacking. Then, recent achievements in colorimetric biosensing of heavy metals based on manipulation of the interface of DNA-AuNPs are surveyed and compared. Finally, perspectives on challenges and opportunities for future research in this field are provided.

Dual Colorimetric Sensor for Hg2+/Pb2+ and an Efficient Catalyst Based on Silver Nanoparticles Mediating by the Root Extract of Bistorta amplexicaulis

Environmental pollution derivated from toxic metals and organic toxins is becoming a serious issue worldwide because of their harmful effects on the ecosystem and human health. Here we are reporting an extremely selective and cost-effective colorimetric sensor for simultaneous recognition of Hg2+ and Pb2+ by using green synthesized silver nanoparticles (AgNPs) mediated from the environmental friendly roots extract of Bistorta amplexicaulis. Biogenic synthesized AgNPs were well-characterized by various spectroscopic techniques e.g., UV-vis, FT-IR, XRD, AFM, and Zetasizer. The photophysical potential of synthesized AgNPs toward common metal cations was explored via absorption spectroscopy and colorimetric assay. The hypsochromic shift in the SPR band of AgNPs can easily be detected through naked eyes vision from dark brown to light yellow in the case of Hg2+. A substantial reduction in the absorbance of AgNPs was recorded upon mixing with Pb2+. AgNPs based colorimetric sensor is highly sensitive toward Hg2+ and Pb2+ with a limit of detection (LOD) of 8.0 × 10-7 M and 2.0 × 10-7 M for Hg2+ and Pb2+, respectively. Furthermore, AgNPs showed promising catalytic activity for the degradation of methyl orange dye. These results demonstrate that Bistorta amplexicaulis stabilized silver nanoparticles have potential applications as a colorimetric sensor and an effective catalyst for the degradation of methyl orange.

Dissecting the Effect of Salt for More Sensitive Label-Free Colorimetric Detection of DNA Using Gold Nanoparticles

Taking advantage of the protection effect of single-stranded DNA oligonucleotides, gold nanoparticles (AuNPs) remain dispersed and retain a red color with the addition of a low concentration of salt, while AuNPs would aggregate in the presence of double-stranded DNA. This difference has been used to design label-free colorimetric sensors for DNA detection. NaCl is the most commonly used salt to induce the aggregation of AuNPs. In this work, we aimed to test if other salts can provide even better sensor performance and to understand the effects of the cations and anions in salts. We first studied the effect of anions, including halides (NaF, NaCl, NaBr, and NaI), and other common salts (NaNO₃, NaClO₄, Na₂SO₄, Na₂S₂O₃, sodium phosphate, and sodium citrate). Among them, weakly adsorbing ones such as F^-, citrate, and phosphate appeared to yield better sensitivity than Cl^-. Anions can directly adsorb on the AuNPs and affect DNA adsorption. We then tested cations, and only group 1A metals (LiCl, NaCl, KCl, RbCl, and CsCl) can signal DNA adsorption, while divalent metals (MgCl₂, CaCl₂, MnCl_2, and NiCl₂) barely showed the effect of DNA. CsCl only works for strongly adsorbing DNA, such as A₁₅, but not weakly adsorbing T₁₅. Overall, NaF is a better salt than NaCl by having a 2.3-fold higher sensitivity, which was confirmed in a DNA sensing assay. This work has identified a better salt yielding higher sensitivity, and sensing work relying on the change of the aggregation state of AuNPs can benefit from this study.

Engineering Grey Nanosystem as Activatable Ratio-colorimetric Probe for Detection of Lead Ions in Preserved Egg

A grey system based on the principle of complementary colors was constructed as an activatable probe for the sensitive and specific ratio-colorimetric detection lead ions (Pb²⁺) in a complex food matrix. This grey system was prepared by mixing purple-red AuNP-capped glutathione (GSH) and green-blue sulfonated pigment green 7 (SPG7), to create the SPG7/AuNP probe. In the presence of Pb²⁺, the strong chelation of Pb²⁺ with GSH could trigger the aggregation of AuNPs, leading to the color activation of SPG7. Hence, the absorbance ratio A_523nm/A_628nm of AuNPs at ∼523 nm and SPG7 at ∼628 nm could be used for highly specific reporting of Pb²⁺ levels with a low detection limit of 0.33 μg/L. Moreover, this probe exhibited promising practical applications in real preserved egg samples with recoveries of 89.2 to 107.5% and relative standard deviations (RSD) in the range of 0.28 to 2.12%, revealing its great potential for harmful substance detection.

Dopamine and Melamine Binding to Gold Nanoparticles Dominates Their Aptamer-Based Label-Free Colorimetric Sensing

Target-directed aptamer adsorption by gold nanoparticles (AuNPs) has been widely used to develop label-free colorimetric biosensors. However, the potential interactions between target molecules and AuNPs have not been considered, which may lead to misinterpretation of analytical results. In this work, the detection of dopamine, melamine, and K⁺ was studied as model systems to address this problem. First, dopamine and two control molecules all induced the aggregation of citrate-capped AuNPs with apparent K_d's of 5.8 μM dopamine, 51.6 μM norepinephrine, and 142 μM tyramine. Isothermal titration calorimetry measured the aptamer K_d to be 1.9 μM dopamine and 16.8 μM norepinephrine, whereas tyramine cannot bind. Surface enhanced Raman spectroscopy confirmed direct adsorption of dopamine, and the adsorbed dopamine inhibited the adsorption of DNA. Using a typical salt-induced colorimetric detection protocol, a similar color response was observed regardless of the sequence of DNA, indicating the observed color change reflected the adsorption of dopamine by the AuNPs instead of the binding of dopamine by the aptamer. For this label-free sensor to work, the interaction between the target molecule and AuNPs should be very weak, while dopamine represents an example of strong interactions. For the other two systems, the melamine detection did not reflect aptamer binding either but the K⁺ detection did, suggesting melamine also strongly interacted with AuNPs, whereas K⁺ had very weak interactions with AuNPs. Since each target molecule is different, such target/AuNP interactions need to be studied case-by-case to ensure the sensing mechanism.

Sensitive colorimetric detection of Pb2+ by geometric field amplification and surface plasmon resonance visualization

Pb²⁺ is one of the major environmental pollutants, which can be visually detected by surface plasmon resonance of nanoparticles. Paper based analytical device, as a newly developed microfluidic detection platform, is featured in cost-effective and suitable for on-site analysis. In this paper, a sensitive and portable detection method for Pb²⁺ was proposed, in which Pb²⁺ was electrokinetically stacked on the paper fluidic channel by geometric field amplification effect and visualized online by glutathione-modified silver nanoparticles. Colorimetric quantification of the visualized stacking band was conducted by smart phone camera. To avoid unfavorable influence from pH change on the surface plasmon resonance visualization, field amplification effect was introduced by geometric design of the paper fluidic channel. The enriched Pb²⁺ was clearly visible on the paper substrate, and the stacking band intensity was about four orders of magnitude enhanced, comparing to the intensity without stacking. A linear response to Pb²⁺ was observed in the range of 0.3-7.0 μM (R² = 0.997) with a limit of detection of 86 nM and a limit of quantity of 0.28 μM. The established method was used in the detection of Pb²⁺ from river and lake water samples, and the results were confirmed by atomic absorption spectroscopy method.

Oxidase-Like Catalytic Performance of Nano-MnO2 and Its Potential Application for Metal Ions Detection in Water

Certain nano-scale metal oxides exhibiting the intrinsic enzyme-like reactivity had been used for environment monitoring. Herein, we evaluated the oxidase-mimicking activity of environmentally relevant nano-MnO₂ and its sensitivity to the presence of metal ions, and particularly, the use of MnO₂ nanozyme to potentially detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in water. The results indicated the oxidase-like activity of nano-MnO₂ at acidic pH-driven oxidation of 2,6-dimethoxyphenol (2,6-DMP) via a single-electron transfer process, leading to the formation of a yellow product. Notably, the presence of Cu²⁺ and Mn²⁺ heightened the oxidase-mimicking activity of nano-MnO₂ at 25°C and pH 3.8, showing that Cu²⁺ and Mn²⁺ could modify the reactive sites of nano-MnO₂ surface to ameliorate its catalytic activity, while the activity of MnO₂ nanozyme in systems with Zn²⁺ and Fe²⁺ was impeded probably because of the strong affinity of Zn²⁺ and Fe²⁺ toward nano-MnO₂ surface. Based on these effects, we designed a procedure to use MnO₂ nanozyme to, respectively, detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in the real water samples. MnO₂ nanozyme-based detecting systems achieved high accuracy (relative errors: 2.2-26.1%) and recovery (93.0-124.0%) for detection of the four metal ions, respectively. Such cost-effective detecting systems may provide a potential application for quantitative determination of metal ions in real water environmental samples.

Noble Metal Nanoparticles-Based Colorimetric Biosensor for Visual Quantification: A Mini Review

Nobel metal can be used to form a category of nanoparticles, termed noble metal nanoparticles (NMNPs), which are inert (resistant to oxidation/corrosion) and have unique physical and optical properties. NMNPs, particularly gold and silver nanoparticles (AuNPs and AgNPs), are highly accurate and sensitive visual biosensors for the analytical detection of a wide range of inorganic and organic compounds. The interaction between noble metal nanoparticles (NMNPs) and inorganic/organic molecules produces colorimetric shifts that enable the accurate and sensitive detection of toxins, heavy metal ions, nucleic acids, lipids, proteins, antibodies, and other molecules. Hydrogen bonding, electrostatic interactions, and steric effects of inorganic/organic molecules with NMNPs surface can react or displacing capping agents, inducing crosslinking and non-crosslinking, broadening, or shifting local surface plasmon resonance absorption. NMNPs-based biosensors have been widely applied to a series of simple, rapid, and low-cost diagnostic products using colorimetric readout or simple visual assessment. In this mini review, we introduce the concepts and properties of NMNPs with chemical reduction synthesis, tunable optical property, and surface modification technique that benefit the development of NMNPs-based colorimetric biosensors, especially for the visual quantification. The “aggregation strategy” based detection principle of NMNPs colorimetric biosensors with the mechanism of crosslinking and non-crosslinking have been discussed, particularly, the critical coagulation concentration-based salt titration methodology have been exhibited by derived equations to explain non-crosslinking strategy be applied to NMNPs based visual quantification. Among the broad categories of NMNPs based biosensor detection analyses, we typically focused on four types of molecules (melamine, single/double strand DNA, mercury ions, and proteins) with discussion from the standpoint of the interaction between NMNPs surface with molecules, and DNA engineered NMNPs-based biosensor applications. Taken together, NMNPs-based colorimetric biosensors have the potential to serve as a simple yet reliable technique to enable visual quantification.

Metal-Dependent DNAzymes for the Quantitative Detection of Metal Ions in Living Cells: Recent Progress, Current Challenges, and Latest Results on FRET Ratiometric Sensors

Many different metal ions are involved in various biological functions including metallomics and trafficking, and yet there are currently effective sensors for only a few metal ions, despite the first report of metal sensors for calcium more than 40 years ago. To expand upon the number of metal ions that can be probed in biological systems, we and other laboratories employ the in vitro selection method to obtain metal-specific DNAzymes with high specificity for a metal ion and then convert these DNAzymes into fluorescent sensors for these metal ions using a catalytic beacon approach. In this Forum Article, we summarize recent progress made in developing these DNAzyme sensors to probe metal ions in living cells and in vivo, including several challenges that we were able to overcome for this application, such as DNAzyme delivery, spatiotemporal control, and signal amplification. Furthermore, we have identified a key remaining challenge for the quantitative detection of metal ions in living cells and present a new design and the results of a Förster resonance energy transfer (FRET)-based DNAzyme sensor for the ratiometric quantification of Zn2+ in HeLa cells. By converting existing DNAzyme sensors into a ratiometric readout without compromising the fundamental catalytic function of the DNAzymes, this FRET-based ratiometric DNAzyme design can readily be applied to other DNAzyme sensors as a major advance in the field to develop much more quantitative metal-ion probes for biological systems.

Highly sensitive and selective optical sensor for lead ion detection based on liquid crystal decorated with DNAzyme

Lead ions (Pb²⁺) are one of the major environmental pollutants that are dangerous for human health, thus the detection methods of Pb²⁺ become very important as well. However, most reported techniques suffer from drawbacks such as long time, expensive equipment and complicated testing process, which prevent the use of real-time application. Herein, we demonstrate a novel liquid crystal optical sensor for detection of Pb²⁺ based on DNAzyme and its combined strand. The ordered and disordered configuration of liquid crystals, induced by complementary DNA strand and catalytically cleaved DNA in presence of lead ion separately, leads to dark and bright optical image under POM. The proposed naked-eye optical sensor possesses an extremely broad detection range of Pb²⁺ from 50 nM to 500 µM, with a low detection limit about 36.8 nM. The sensor also demonstrates high selectivity of Pb²⁺ from many other metal ions. The proposal LC sensor is highly sensitive and selective for Pb²⁺ detection, which provides a novel platform for other heavy metal, DNAs or antigen in biological and chemical fields by modifying sensing molecules.

A dual-model SERS and RRS analytical platform for Pb(II) based on Ag-doped carbon dot catalytic amplification and aptamer regulation

Several carbon dots doping with diferent elements (Ca, Ag, Au) were fabricated and their catalytic properties had been investigated in this paper. It was found that the Ag-doped carbon dots (CDAg) had played a role of mimic enzyme on the reaction of HAuCl4-H2O2 and generated nanogold particles with surface enhanced Raman scattering (SERS) and resonance Rayleigh scattering (RRS) effects. The aptamer (Apt) can be adsorbed on the CDAg surface and cause the catalysis weakening. When the target Pb(II) was added, it would combine with the Apt to produce firm complexes Pb-Apt and desorb CDAg, which caused its catalytic effect restore. The formed nanogold had a strong RRS peak (at 375 nm) and a high SERS peak (at 1615 cm-1) in the presence of molecular probe (Victoria blue B, VBB). The dual-model signals of SERS and RRS increased linearly with Pb(II) concentration increase within the scope of 0.006-0.46 μmol/L and 0.01-0.46 μmol/L. And their detection limits respectively were 0.0032 μmol/L and 0.0048 μmol/L Pb(II).

An investigation of solid-state nanopores on label-free metal-ion signalling via the transition of RNA-cleavage DNAzyme and the hybridization chain reaction

Recent advances have proven solid-state nanopores as a powerful analysis platform that enables label-free and separation-free single-molecule analysis. However, the relatively low resolution still limits its application because many chemicals or targets with small sizes could not be recognized in a label-free condition. In this paper, we provide a possible solution that uses solid-state nanopores for small species signaling via the transition of huge DNA assembly products. DNAzyme responding to metal ions and the hybridization chain reaction (HCR) generating nanopore-detectable dsDNA concatamers are used as the transition model set. By the two-step DNAzyme-HCR transition, Pb2+ that was too tiny to be sensed was successfully recognized by the nanopore. The whole process happened in a completely homogeneous solution without any chemical modification. During condition optimization, we also discussed one possible application challenge that may affect the HCR signal-background distinction. Solid-state nanopores provide a potential solution to this challenge due to its ability to profile product length or even 3D structure information.

Label-Free and Enzyme-Free Colorimetric Detection of Pb2+ Based on RNA Cleavage and Annealing-Accelerated Hybridization Chain Reaction

A label-free and enzyme-free colorimetric sensor for rapid detection of Pb²⁺ is reported, which is based on the strategy of DNAzyme-mediated RNA cleavage combined with an annealing-accelerated DNA hybridization chain reaction (HCR). As a trigger DNA, the substrate strand (S_TM) of DNAzyme can initiate HCR effectively. However, when it is cleaved by DNAzyme in the presence of Pb²⁺, the separation of DNA functional domains leads to a serious decrease in HCR efficiency. As a result, the difference in Pb²⁺ concentration converts into the difference of DNA assembly, which eventually leads to the color change of colloidal gold nanoparticles (AuNPs). In this work, a DNA strand (cGR5) completely complementary to the catalytic strand (GR5) of DNAzyme is used to improve the dissociation of S_TM to enhance the HCR efficiency. In addition, the simple operation of DNA annealing is first used to accelerate the HCR process, enabling the Pb²⁺ detection to be completed in about 30 min. As advantages of high sensitivity, good selectivity, strong anti-interference ability, and good practical performance are achieved, it is anticipated that the cheap and simple colorimetric sensor will be helpful for on-site detection of environmental and food samples.

Fluorescence Based Investigation of Temperature-Dependent Pb2+-Specific 8-17E DNAzyme Catalytic Sensor

The 8-17E DNAzyme is a temperature-dependent DNA metalloenzyme catalyzing RNA trans esterification in the presence of Pb²⁺ metal ions. Labeling the stems of the substrate and DNAzyme with the Cy3 and Cy5 respectively, the considered DNAzyme was studied by the fluorescence spectroscopy. The temperature-dependent variability of the Pb²⁺-specific 8-17E DNAzyme catalytic sensor was investigated trough a number of successive temperature fluctuations from 4 to 25 °C to obtain information. Investigating underlined biochemical system reveals that in this sensor, free single strands Enzyme (Cy5-E) and Substrate (Cy3-S) have higher fluorescence intensities than hybridized forms, suggesting that the fluorophores are in a contact quenched. Increasing the temperature has three effects: 1) Fluorescence intensities for the free fluorophores were reduced, 2) stability of the hybridized form was reduced and cleavage of substrate in presence of Pb²⁺was occurred, and 3) conformation of ES hybridized form was changed (before cleavage). As a result of conformation changes in ES, S was more affected than E in the ES. Pb²⁺ ion shows quenching effect on both fluorophores and in the absence of N₂(g) purge the effect was more considerable. A main goal that we had in mind was to find if significantly lower concentrations of Pb²⁺ and ES, compared to previous reports, can generate any observable cleavage in substrate. Analysis of the cleavage reaction for 50 nM ES indicates that S is cleaved at 25 °C in presence of N₂(g) and 0.5 μM Pb²⁺, while in same condition no apparent change occurs in the 4 or 10 °C. The rapid, sensitive and low cost strategy presented here can be applicable to study temperature-dependent behavior of other nucleic acid-based biosensors.

Solid-State Nanopore Single-Molecule Sensing of DNAzyme Cleavage Reaction Assisted with Nucleic Acid Nanostructure

The detection and investigation of biomolecules at a single-molecule level is important for improving diagnosis in biomedicine. Solid-state nanopores are a unique tool that have the potential to accomplish this task because they are label-free and require only low sample consumption. However, the event-readouts of current small polymer molecules are still limited because of its relatively large size and low signal-to-noise ratios. Here, we present a rapid sensing approach for the detection of GR-5 DNAzyme cleaving specific substrate reactions using relatively larger size silicon nitride nanopores by introducing a type of nucleic acid nanostructure (DNA tetrahedron) as a carrier. The proposed method is convenient and sensitive enough to detect the cleavage reactions by identifying translocation events before and after reactions with nanomolar concentrations of the target sample. Furthermore, this assay was also carried out by using larger size nanopores (60 nm diameter) to achieve the DNAzyme cleavage sensing with the same sample concentration. This approach can improve event detectability of other smaller molecules' translocation, which opens up a wide range of applications for analytes detection by incorporating solid-state nanopores. Nucleic acid nanostructure-assisted nanopore sensing can promote the development of single-molecule studies.

Multifunctional nanozymes: enzyme-like catalytic activity combined with magnetism and surface plasmon resonance

Over decades, as alternatives to natural enzymes, highly-stable and low-cost artificial enzymes have been widely explored for various applications. In the field of artificial enzymes, functional nanomaterials with enzyme-like characteristics, termed as nanozymes, are currently attracting immense attention. Significant progress has been made in nanozyme research due to the exquisite control and impressive development of nanomaterials. Since nanozymes are endowed with unique properties from nanomaterials, an interesting investigation is multifunctionality, which opens up new potential applications for biomedical sensing and sustainable chemistry due to the combination of two or more distinct functions of high-performance nanozymes. To highlight the progress, in this review, we discuss two representative types of multifunctional nanozymes, including iron oxide nanomaterials with magnetic properties and metal nanomaterials with surface plasmon resonance. The applications are also covered to show the great promise of such multifunctional nanozymes. Future challenges and prospects are discussed at the end of this review.

Chemical and Biological Sensing Using Hybridization Chain Reaction

Since the advent of its theoretical discovery more than 30 years ago, DNA nanotechnology has been used in a plethora of diverse applications in both the fundamental and applied sciences. The recent prominence of DNA-based technologies in the scientific community is largely due to the programmable features stored in its nucleobase composition and sequence, which allow it to assemble into highly advanced structures. DNA nanoassemblies are also highly controllable due to the precision of natural and artificial base-pairing, which can be manipulated by pH, temperature, metal ions, and solvent types. This programmability and molecular-level control have allowed scientists to create and utilize DNA nanostructures in one, two, and three dimensions (1D, 2D, and 3D). Initially, these 2D and 3D DNA lattices and shapes attracted a broad scientific audience because they are fundamentally captivating and structurally elegant; however, transforming these conceptual architectural blueprints into functional materials is essential for further advancements in the DNA nanotechnology field. Herein, the chemical and biological sensing applications of a 1D DNA self-assembly process known as hybridization chain reaction (HCR) are reviewed. HCR is a one-dimensional (1D) double stranded (ds) DNA assembly process initiated only in the presence of a specific short ssDNA (initiator) and two kinetically trapped DNA hairpin structures. HCR is considered an enzyme-free isothermal amplification process, which shows substantial promise and offers a wide range of applications for in situ chemical and biological sensing. Due to its modular nature, HCR can be programmed to activate only in the presence of highly specific biological and/or chemical stimuli. HCR can also be combined with different types of molecular reporters and detection approaches for various analytical readouts. While the long dsDNA HCR product may not be as structurally attractive as the 2D and 3D DNA networks, HCR is highly instrumental for applied biological, chemical, and environmental sciences, and has therefore been studied to foster a variety of objectives. In this review, we have focused on nucleic acid, protein, metabolite, and heavy metal ion detection using this 1D DNA nanotechnology via fluorescence, electrochemical, and nanoparticle-based methodologies.

Novel and label-free colorimetric detection of radon using AuNPs and lead(II)-induced GR5 DNAzyme-based amplification strategy

Radioactive radon decays into a stable daughter product, ²¹⁰Pb, which was used as the detection target to determine the radon radiation dose in a new technique. Pb²⁺ triggers DNAzyme to cleave a molecular beacon (MB), resulting in the stem-loop structure opening and forming two single DNA strands (ssDNA). The ssDNA binds to unmodified gold nanoparticles and effectively prevents their aggregation in a salt solution. The detached enzyme strands continue to complement the remaining MB to amplify the response signal. The method proposed in this study exhibited a good linear relationship for Pb²⁺ and radon concentrations in the range of 6.22 × 10²-1.02 × 10⁵ Bq h/m³ with a detection limit of 186.48 Bq h/m³ using an ultraviolet-visible spectrometer. In practical applications, this sensitive method can avoid radioactive damage in field testing, and the detection limit meets the national standard in China. Importantly, this simple, highly sensitive strategy uses simple equipment and has a strong anti-interference ability. Graphical abstract.

QCM-nanomagnetic beads biosensor for lead ion detection

A QCM biosensor combined with NMBs has been proposed for Pb²⁺detection with a lower detection limit of 0.3 pM.

Metal-induced aggregation of valine capped gold nanoparticles: An efficient and rapid approach for colorimetric detection of Pb2+ ions

In this study, we report a novel application of valine-capped gold nanoparticles for colorimetric and visual detection of lead ions. The -COO^- group of the hydrophobic valine molecules present efficient electrostatic repulsion resulting in generation of stable, well-dispersed and size-controlled GNPs. The GNPs were highly selective for Pb²⁺ ions and showed visible colour change in the assay mixture on addition of solution containing lead ions. Interestingly, a decrease in the intensity of original SPR peak at 530 nm was observed, with concomitant appearance of a new peak at longer wavelength due to agglomerated GNPs. The free -COO^- groups on GNP surface interacted with Pb²⁺ and ion-dependent chelation mechanism lead to cross-linking of particles and subsequent agglomeration. Binding of Pb²⁺ ions and valine-capped GNPs occur in a stochiometric ratio of 1:2. The GNPs displayed colorimetric sensing in the range of 0 to 100 ppm concentration with a very high selectivity towards lead even in the presence of other metal ions. The minimum detection limit (MDL) for Pb²⁺ was 30.5 µM. We anticipate that these valine-capped GNPs may be employed for lead detection in polluted water/wastewater through a cost-efficient, one-step assay protocol as it does not require additional functionalization with specific ligand molecules.

Colorimetric sensors for rapid detection of various analytes

Sensor technology for the rapid detection of the analytes with high sensitivity and selectivity has several challenges. Despite the challenges, colorimetric sensors have been widely accepted for its high sensitive and selective response towards various analytes. In this review, colorimetric sensors for the detection of biomolecules like protein, DNA, pathogen and chemical compounds like heavy metal ions, toxic gases and organic compounds have been elaborately discussed. The visible sensing mechanism based on Surface Plasmon Resonance (SPR) using metal nanoparticles like Au, Ag, thin film interference using SiO₂ and colorimetric array-based technique have been highlighted. The optical property of metal nanoparticles enables a visual color change during its interaction with the analytes owing to the dispersion and aggregation of nanoparticles. Recently, colorimetric changes using silica substrate for detection of protein and small molecules by thin film interference as a visible sensing mechanism has been developed without the usage of fluorescent or radioisotopes labels. Multilayer of biomaterials were used as a platform where reflection and interference of scattering light occur due to which color change happens leading to rapid sensing. Colorimetric array-based technique for the detection of organic compounds using chemoresponsive dyes has also been focused wherein the interaction of the analytes with the substrate coated with chemoresponsive dyes gives colorimetric change.

From boron clusters to gold clusters: new label-free colorimetric sensors

Based on boron clusters, AuNPs were successfully prepared and exhibited high performance in phase transfer and heavy-metal ion sensing.

Integrating Deoxyribozymes into Colorimetric Sensing Platforms

Biosensors are analytical devices that have found a variety of applications in medical diagnostics, food quality control, environmental monitoring and biodefense. In recent years, functional nucleic acids, such as aptamers and nucleic acid enzymes, have shown great potential in biosensor development due to their excellent ability in target recognition and catalysis. Deoxyribozymes (or DNAzymes) are single-stranded DNA molecules with catalytic activity and can be isolated to recognize a wide range of analytes through the process of in vitro selection. By using various signal transduction mechanisms, DNAzymes can be engineered into fluorescent, colorimetric, electrochemical and chemiluminescent biosensors. Among them, colorimetric sensors represent an attractive option as the signal can be easily detected by the naked eye. This reduces reliance on complex and expensive equipment. In this review, we will discuss the recent progress in the development of colorimetric biosensors that make use of DNAzymes and the prospect of employing these sensors in a range of chemical and biological applications.