Activity-enhanced DNAzyme for design of label-free copper(II) biosensor

Metal ion-driven, DNA-cleaving DNAzymes are characterised by high selectivity and specificity. However, their use for metal ion sensing remains largely unexplored due to long reaction times and poor reaction yields relative to RNA-cleaving DNAzymes and other sensing strategies. Herein we present a study demonstrating a significant rate enhancement of a copper-selective DNA cleaving DNAzyme by both polydopamine (PDA) and gold (Au) nanoparticles (NPs). PDA NPs enhance the reaction through the production of hydrogen peroxide, while for AuNPs the enhancement is aided by the presence of citrate surface moeities, both of which drive the oxidative cleavage of the substrate. A 50-fold enhancement for PDA NPs makes the combination of PDA and DNAzyme suitable for a practical application as a sensitive biosensor for Cu(II) ions. Using DNAzyme deposition onto a gold electrode followed by Polydopamine Assisted DNA Immobilisation (PADI), we achieve a cost-effective, label-free and fast (within 15 min) electrochemical biosensor with a limit of detection of 180 nmol (11 ppm), thus opening a route for the rational design of a new generation of hybrid DNAzyme-based biosensors.

Metabolic engineering of the violacein biosynthetic pathway toward a low-cost, minimal-equipment lead biosensor

Metabolic engineered bacteria have been successfully employed to produce various natural colorants, which are expected to be used as the visually recognizable signals to develop mini-equipment biological devices for monitoring toxic heavy metals. The violacein biosynthetic pathway has been reconstructed in Escherichia coli (E. coli). Here the successful production of four violacein derivatives was achieved by integrating metabolic engineering and synthetic biology. Lead binding to the metalloregulator enables whole-cell colorimetric biosensors capable of assessing bioavailable lead. Deoxyviolacein-derived signal showed the most satisfied biosensing properties among prodeoxyviolacein (green), proviolacein (blue), deoxyviolacein (purple), and violacein (navy). The limit of detection (LOD) of pigment-based biosensors was 2.93 nM Pb(II), which is lower than that of graphite furnace atomic absorption spectrometry. Importantly, a good linear dose-response model in a wide dose range (2.93-6000 nM) was obtained in a non-cytotoxic deoxyviolacein-based biosensor, which was significantly better than cytotoxic violacein-based biosensor (2.93-750 nM). Among ten metal ions, only Cd(II) and Hg(II) exerted a slight influence on the response of the deoxyviolacein-based biosensor toward Pb(II). The deoxyviolacein-based biosensor was validated in detecting bioaccessible Pb(II) in environmental samples. Factors such as low cost and minimal-equipment requirement make this biosensor a suitable biological device for monitoring toxic lead in the environment.

A critical review on microbe-electrode interactions towards heavy metal ion detection using microbial fuel cell technology

Implicit interaction of electroactive microbes with solid electrodes is an interesting phenomenon in nature, which supported development of bioelectrochemical systems (BESs), especially the microbial fuel cell (MFCs) for valorization of low-value waste streams into bioelectricity. Intriguingly, the metabolism of interacted microbes with electrode is affected by the microenvironment at electrodes, which influences the current response. For instance, when heavy metal ions (HMIs) are imposed in the medium, the current production decreases due to their intrinsic toxic effect. This event provides an immense opportunity to utilize MFC as a sensor to selectively detect HMIs in the environment, which has been explored vastly in recent decade. In this review, we have concisely discussed the microbial interaction with electrodes and mechanism of detection of HMIs using an MFC. Recent advancement in sensing elements and their application is elaborated with a future perspective section for follow-up research and development in this field.

A novel self-cleaning electrochemical biosensor integrating copper porphyrin-derived metal-organic framework nanofilms, G-quadruplex, and DNA nanomotors for achieving cyclic detection of lead ions

A self-cleaning electrochemical biosensor based on two-dimensional Cu-porphyrin (Cu-TCPP) metal-organic framework nanofilms, novel super G-quadruplex (G4), and DNA nanomotors was developed for the cyclic detection of Pb²⁺ ions. The Cu-TCPP framework with inherent peroxidase activity can create an ultra-thin nanofilm that functioned as a carrier to support the metastable G4 comprising four individual DNA strands. The introduction of Pb²⁺ and the intercalation of hemin can help it to form stable G4-hemin DNAzymes, which exhibits strong catalytic H₂O₂ reduction activity, and its number will be directly related to the amount of the introduced Pb²⁺. Moreover, a DNA nanomotor system is introduced to achieve cyclic detection, and the addition of the fuel DNA strands enables G4 to perform a "complete-dissociation-complete" process for achieving self-cleaning of the electrode interface and the cycle detection of Pb²⁺. The synergistic effects of Cu-TCPP and G4-hemin DNAzymes, which exhibits efficient and catalytic H₂O₂ reduction, enhance the performance of the electrochemical sensing system. The linear range of this sensor to Pb²⁺ is 5 nM-5 μM, and the detection limit is 1.7 nM. Compared with the best system in reported studies, its linear range is five times wider and its detection limit is lower than the previously lowest one. Taking advantage of the Pb²⁺ stabilized G4, the proposed sensor can selectively detect Pb²⁺ in the presence of other metal ions. The results presented herein comprise a valuable reference for constructing DNA nanoelectronic devices and establish sensitive and cyclic detection of the target and preparing of self-cleaning electrode interfaces.

Magnetically induced self-assembly DNAzyme electrochemical biosensor based on gold-modifiedα-Fe2O3/Fe3O4heterogeneous nanoparticles for sensitive detection of Ni2

A magnetically induced self-assembly DNAzyme electrochemical biosensor based on gold-modifiedα-Fe₂O₃/Fe₃O₄heterogeneous nanoparticles was successfully fabricated to detect Nickel(II) (Ni²⁺). The phase composition and magnetic properties ofα-Fe₂O₃/Fe₃O₄heterogeneous nanoparticles controllably prepared by the citric acid (CA) sol-gel method were investigated in detail. Theα-Fe₂O₃/Fe₃O₄heterogeneous nanoparticles were modified by using trisodium citrate as reducing agent, and the magnetically induced self-assemblyα-Fe₂O₃/Fe₃O₄-Au nanocomposites were obtained. The designed Ni²⁺-dependent DNAzyme consisted of the catalytic chain modified with the thiol group (S1-SH) and the substrate chain modified with methylene blue (S2-MB). The MGCE/α-Fe₂O₃/Fe₃O₄-Au/S1/BSA/S2 electrochemical sensing platform was constructed and differential pulse voltammetry was applied for electrochemical detection. Under the optimum experimental parameters, the detection range of the biosensor was 100 pM-10μM (R² = 0.9978) with the limit of detection of 55 pM. The biosensor had high selectivity, acceptable stability, and reproducibility (RSD = 4.03%).

Biosensing with DNAzymes

This article provides a comprehensive review of biosensing with DNAzymes, providing an overview of different sensing applications while highlighting major progress and seminal contributions to the field of portable biosensor devices and point-of-care diagnostics. Specifically, the field of functional nucleic acids is introduced, with a specific focus on DNAzymes. The incorporation of DNAzymes into bioassays is then described, followed by a detailed overview of recent advances in the development of in vivo sensing platforms and portable sensors incorporating DNAzymes for molecular recognition. Finally, a critical perspective on the field, and a summary of where DNAzyme-based devices may make the biggest impact are provided.

Bio- and Biomimetic Receptors for Electrochemical Sensing of Heavy Metal Ions

Heavy metals ions (HMI), if not properly handled, used and disposed, are a hazard for the ecosystem and pose serious risks for human health. They are counted among the most common environmental pollutants, mainly originating from anthropogenic sources, such as agricultural, industrial and/or domestic effluents, atmospheric emissions, etc. To face this issue, it is necessary not only to determine the origin, distribution and the concentration of HMI but also to rapidly (possibly in real-time) monitor their concentration levels in situ. Therefore, portable, low-cost and high performing analytical tools are urgently needed. Even though in the last decades many analytical tools and methodologies have been designed to this aim, there are still several open challenges. Compared with the traditional analytical techniques, such as atomic absorption/emission spectroscopy, inductively coupled plasma mass spectrometry and/or high-performance liquid chromatography coupled with electrochemical or UV-VIS detectors, bio- and biomimetic electrochemical sensors provide high sensitivity, selectivity and rapid responses within portable and user-friendly devices. In this review, the advances in HMI sensing in the last five years (2016-2020) are addressed. Key examples of bio and biomimetic electrochemical, impedimetric and electrochemiluminescence-based sensors for Hg2+, Cu2+, Pb2+, Cd2+, Cr6+, Zn2+ and Tl+ are described and discussed.

A label-free lead(II) ion sensor based on surface plasmon resonance and DNAzyme-gold nanoparticle conjugates

Detection of lead(II) (Pb²⁺) ions in water is important for the protection of human health and environment. The growing demand for onsite detection still faces challenges for sensitive and easy-to-use methods. In this work, a novel surface plasmon resonance (SPR) biosensor based on GR-5 DNAzyme and gold nanoparticles (AuNPs) was developed. Thiolated DNAzyme was immobilized on the gold surface of the sensor chip followed by anchoring the substrate-functionalized AuNPs through the DNAzyme-substrate hybridization. The coupling between the localized surface plasmon (LSP) of AuNPs and the surface plasmon polaritons (SPP) on the gold sensor surface was used to improve the sensitivity. The substrate cleavage in the presence of Pb²⁺ ions was catalyzed by DNAzyme, leading to the removal of AuNPs and the diminished LSP-SPP coupling. The optimal detection limit was 80 pM for the sensor fabricated with 1 μM DNAzyme, corresponding to two or three orders of magnitude lower than the toxicity levels of Pb²⁺ in drinking water defined by WHO and USEPA. By tuning the surface coverage of DNAzyme, the sensitivity and dynamic range could be controlled. This sensor also featured high selectivity to Pb²⁺ ions and simple detection procedure. Successful detection of Pb²⁺ ions in groundwater indicates that this method has the prospect in the onsite detection of Pb²⁺ ions in water. Given the variety of AuNPs and metal-specific DNAzymes, this detection strategy would lead to the development of more sensitive and versatile heavy metal sensors. Graphical abstract.

Influence of Saline Buffers over the Stability of High-Annealed Gold Nanoparticles Formed on Coverslips for Biological and Chemosensing Applications

Herein, coverslips were used as solid supports for the synthesis of gold nanoparticles (AuNPs) in three steps: (i) detergent cleaning, (ii) evaporation of 4 nm gold film and (iii) exposure at high annealing temperature (550 °C) for 3 h. Such active gold nanostructured supports were investigated for their stability performances in aqueous saline buffers for new assessments of chemical sensing. Two model buffers, namely saline-sodium phosphate-EDTA buffer (SSPE) and phosphate buffer saline (PBS), that are often used in the construction of (bio)sensors, are selected for the optical and microscopic investigations of their influence over the stability of annealed AuNPs on coverslips when using a dropping procedure under dry and wet media working conditions. A study over five weeks monitoring the evolution of the localized surface plasmon resonance (LSPR) chemosensing of 1,2-bis-(4-pyridyl)-ethene (BPE) is discussed. It is concluded that the optimal sensing configuration is based on annealed AuNPs exposed to saline buffers under wet media conditions (overnight at 4 °C) and functionalized with BPE concentrations (10^-3-10^-11 M) with the highest LSPR spectra after two weeks.

Ultrasensitive and Portable Assay for Lead(II) Ions by Electronic Balance as a Readout

Lead ions (Pb²⁺) cause harm to human health. Therefore, the development of fast, effective, and convenient sensors for Pb²⁺ monitoring has received great attention. In this study, a portable method has been proposed for Pb²⁺ detection using normal electronic balance as a readout. Magnetic bead-catalytic strand is hybridized with platinum nanoparticles (Pt NPs) functioned substrate strand (Pt-Sub) to form double-stranded DNA first. In the presence of Pb²⁺, the DNAzyme is activated and cleaved at the ribo-adenosine site of the substrate strand and hence causes Pt NPs to be released into the supernatant, which can be easily separated from the Pt-Sub by a magnet. The separated Pt NPs can effectively catalyze the decomposition of H₂O₂ to produce O₂. In a sealed bottle, the pressure inside the bottle is increased by the generation of oxygen so that the water is discharged from the drainage device, and the weight of the water can be easily and precisely measured by a normal electronic balance. The weighting water has a linear relationship with the concentration of Pb²⁺ in the range of 2.5-100 nM and the detection limit of 0.83 nM (S/N = 3). The proposed method has been applied to detect Pb²⁺ in water with satisfactory results. Because the electronic balance is one of the most commonly used analytical tools for the laboratory, it is very practical and convenient without the need for expensive instruments and complicated data processing.

Superwettable Microwell Arrays Constructed by Photocatalysis of Silver-Doped-ZnO Nanorods for Ultrasensitive and High-Throughput Electroanalysis of Glutathione in Hela Cells

Superwettable microwell arrays were constructed for the first time by the synergic photocatalysis of silver-doped-ZnO (Ag-ZnO) nanorods patterned on indium tin oxide (ITO) electrodes for electroanalysis of glutathione (GSH) in hela cells through the signal output of AgCl electrochemistry. The newly prepared Ag-ZnO nanorods with high photocatalysis were dispersed into an octadecyltrichlorosilane (OTS) matrix to be deposited onto ITO substrates, yielding superhydrophobic Ag-ZnO-OTS coatings. Superhydrophilic microwells were further created by the Ag-ZnO photocatalysis under UV irradiation to produce Ag-ZnO microwell arrays featuring the superwettability profile. The resulting Ag-ZnO microwell-modified ITO electrodes were employed further for electroanalysis of GSH through solid-state AgCl electrochemistry, in which the specific Ag-GSH interactions would trigger a rational decrease in the sharp AgCl peak currents at the potential approaching zero. Moreover, benefitting from the superwettability feature, the microwells on the ITO electrodes could facilitate the condensing enrichment of GSH analytes from the sample droplets, achieving improved analysis sensitivity. The as-developed electroanalysis strategy was subsequently demonstrated for the detection of GSH in hela cell supernatant with levels down to about 27.30 pM. Additionally, this synergic photocatalysis-based preparation route can be tailored for the large-scale fabrication of various array platforms with the superwettability feature for high-throughput and sensitive biological analysis.