Electrode Surface Charge Research Articles

Direct Operando Observation of Double Layer Charging and Early Solid Electrolyte Interphase Formation in Li-Ion Battery Electrolytes.

The solid electrolyte interphase (SEI) is the most critical yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF6 and organic carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, on-line electrochemical mass spectrometry, and electrochemical quartz crystal microbalance. The electric double layer is directly observed to charge as Li+ solvated by ethylene carbonate (EC) progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation, as evidenced by H2 evolution and electrode mass deposition. Electrolyte impurities, HF and H2O, are reduced early and contribute in a multistep (electro)chemical process to an inorganic SEI layer rich in LiF and Li2CO3. This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.

Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization.

The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

An electrochemical aptasensor for amyloid-β oligomer based on double-stranded DNA as "conductive spring".

In order to overcome the antibody-based sensor's shortcomings, an electrochemical aptamer (Apt)-based sensor was developedfor amyloid-β40 oligomer (Aβ40-O). The aptasensor was constructed by locating Apt and ferrocence (Fc) on streptavidin-modified gold (SA-gold) nanoparticles. The obtained AptFc@SA-gold nanoparticles were linked onto the Au electrode via the connection of double-stranded DNA (dsDNA) as a "conductive spring." The determination of Aβ40-O was performed with square-wave voltammetry (SWV). Upon bio-recognition between Apt and Aβ40-O, the conformation of Apt changed and the formed Apt/Aβ40-O complex separated from theSA-gold surface. As a result, the surface charge of SA-gold positively shifted, weakening the electrostatic attraction between the SA-gold and the positively charged Au electrode surface (at potential range of 0.1~0.5V, corresponding to the Fc redox transformation), and stretching the dsDNA chain. Based on the exponential decay of dsDNA's electron transfer efficiency on its chain stretching, the oxidation current density from Fc decreased and displayed linear correlation to the concentration of Aβ40-O. A wide linear range of 0.100nM to 1.00μM with a low detection limit of 93.0pM was obtained. The aptasensor displayed excellent selectivity toward Aβ40-O in contrast to other possible interfering analogs (Aβ40 monomer, Aβ42 monomer, and oligomer) at × 100 higher concentrations. The recoveries for Aβ40-O-spiked artificial cerebrospinal fluid and healthy human serum were 94.0~104% and 92.8~95.4%, respectively. The electrochemical aptasensor meets the demands of clinic determination of Aβ40-O, which is significant for the early diagnosis of AD. Graphical abstract Schematic representation of the electrochemical aptasensor for amyloid-β oligomer based on the surface charge change induced by target binding.

Label-free and immobilization-free electrochemiluminescent sensing platform for highly sensitive detection of As(III) by combining target-induced strand displacement amplification with polydopamine nanospheres

Strand displacement amplification (SDA) is a classic isothermal amplification technique, which usually requires one primer and one template. Here, we integrated the primer and the template to design a hairpin DNA. Using polydopamine nanospheres (PDANS) as a reversible and nonspecific inhibitor, an immobilization-free and label-free ECL sensor was fabricated. Without As(III), hairpin DNA is constrained by PDANS, and SDA process is inhibited. Ru(phen)32+, as ECL probe, can diffuse to the negatively charged ITO electrode surface to bring strong ECL response. While the presence of As(III) released the hairpin DNA, and triggered the SDA process with the help of polymerase and nicking endonuclease to generate lots of dsDNA, which interacted with Ru(phen)32+ to form the complex of dsDNA-Ru(phen)32+. The complex is difficult to approach the ITO electrode surface owing to the electrostatic repulsion, resulting in low ECL response. As a consequence, the detection limit of 1.2 × 10−3 ppb for As(III) can be achieved. Meanwhile, it is also applied to assay As(III) in flos sophorae. This strategy not only avoided the adjustment of primer-template ratio and improved amplification efficiency, but also reduced the treatment of the electrode and the immobilization of probe on the electrode. It is hopeful that this method was applied to assay food contaminants, environmental pollutions.

Real-time mass spectrometric characterization of the solid-electrolyte interphase of a lithium-ion battery.

The solid-electrolyte interphase (SEI) dictates the performance of most batteries, but the understanding of its chemistry and structure is limited by the lack of in situ experimental tools. In this work, we present a dynamic picture of the SEI formation in lithium-ion batteries using in operando liquid secondary ion mass spectrometry in combination with molecular dynamics simulations. We find that before any interphasial chemistry occurs (during the initial charging), an electric double layer forms at the electrode/electrolyte interface due to the self-assembly of solvent molecules. The formation of the double layer is directed by Li+ and the electrode surface potential. The structure of this double layer predicts the eventual interphasial chemistry; in particular, the negatively charged electrode surface repels salt anions from the inner layer and results in an inner SEI that is thin, dense and inorganic in nature. It is this dense layer that is responsible for conducting Li+ and insulating electrons, the main functions of the SEI. An electrolyte-permeable and organic-rich outer layer appears after the formation of the inner layer. In the presence of a highly concentrated, fluoride-rich electrolyte, the inner SEI layer has an elevated concentration of LiF due to the presence of anions in the double layer. These real-time nanoscale observations will be helpful in engineering better interphases for future batteries.

Electrocatalytic activity of Pd–Au nanoalloys during methanol oxidation reaction

Methanol fuel cells are very promising power source due to its high efficiency and low emissions of pollutants but their commercialization is hindered by development of the effective catalysts. Bimetallic nanostructured catalysts have been used to increase the effectiveness of methanol electrooxidation. Their high electrocatalytic activity can be accounted largely by the difference in electronegativity of two metals (e.g. Pd and Au), that resulting in gradual Auδ+→Auδ– transition with the increase in Pd content. Therefore, gold-enriched bimetallic Pd-Aunano were recommended as catalysts for oxidation processes since they are characterized by the presence of Auδ+ on their surface. Deposition of Pd, Au and Pd–Au nanoparticles (~50–350 nm) were carried out in dimethyl sulfoxide by pulsed mode of electrolysis directly on electrode surface. Cyclic voltammetry was the main method to study catalytic properties of the modified electrode in the anode oxidation process of methanol. It was found that oxidation rate on the electrode surface modified by bimetallic Pd–Au nanoparticles is ~1.5 times higher as compared to that in the case of electrodes modified by Pd or Au monometallic nanoparticles individually. In order to find highly active, selective, and stable catalysts for methanol electrocatalytic oxidation reaction additional studies are needed to understand the role of electrode surface charge and local OH− ions concentration from alkali solution.

Improving Electrochemical Stability and Low‐Temperature Performance with Water/Acetonitrile Hybrid Electrolytes

AbstractAlthough the “water‐in‐salt” electrolyte has significantly expanded the electrochemical stability window of aqueous electrolytes from 1.23 to 3 V, its inevitable hydrogen evolution under 1.9 V versus Li+/Li prevents the practical use of many energy‐dense anodes. Meanwhile, its liquidus temperature at 17 °C restricts its application below ambient temperatures. An advanced hybrid electrolyte is proposed in this work by introducing acetonitrile (AN) as co‐solvent, which minimizes the presence of interfacial water at the negatively charged electrode surface, and generates a thin and uniform interphase consisting of an organic outer layer based on nitrile (CN) and sulfamide (R‐S‐N‐S) species and an inner layer rich in LiF. Such an interphase significantly suppresses water reduction and expands the electrochemical stability window to an unprecedented width of 4.5 V. Thanks to the low freezing point (−48 °C) and low viscosity of AN, the hybrid electrolyte is highly conductive in a wide temperature range, and enables a LiMn2O4/Li4Ti5O12 full cell at both ambient and sub‐ambient temperatures with excellent cycling stability and rate capability. Meanwhile, such a hybrid electrolyte also inherits the nonflammable nature of aqueous electrolyte. The well‐balanced merits of the developed electrolyte make it suitable for high energy density aqueous batteries.

Electric-Field-Driven Molecular Recognition Reactions of Guanine with 1,2-Dipalmitoyl-sn-glycero-3-cytidine Monolayers Deposited on Gold Electrodes

Monolayers of 1,2-dipalmitoyl-sn-glycero-3-cytidine were incubated with guanine in a 0.1 M NaF electrolyte at the surface of a Langmuir trough and transferred to gold (111) electrodes using the Langmuir-Schaefer technique. Chronocoulometry and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate the influence of the static electric field on the orientation and conformation of the cytidine nucleolipid molecules on the metal surface in the presence of guanine and to monitor the molecular recognition of guanine with the cytosine moiety. When the monolayer is exposed to guanine solutions, the cytosine moiety binds to the guanine residue in either a Watson-Crick complex at positively charged electrode surfaces or a noncomplexed state at negative surface charges. The positive electrostatic field causes the cytosine moiety and the cytosine-guanine complex to adopt a nearly parallel orientation with respect to the plane of the monolayer with a measured tilt angle of ∼10°. The parallel orientation is stabilized by the interactions between the permanent dipole of the cytosine moiety or the Watson-Crick complex and the static electric field. At negative charge densities, the tilt of the cytosine moiety increases by ∼15-20°, destabilizing the complex. Our results demonstrate that the static electric field has an influence on the molecular recognition reactions between nucleoside base pairs at the metal-solution interface and can be controlled by altering the surface charge at the metal.

BSA‐Polysaccharide Interactions at Negatively Charged Electrode Surface. Effects of Current Density.

AbstractConstant current chronopotentiometric stripping (CPS) peak H due to catalytic hydrogen evolution reaction on Hg‐containing electrodes appeared useful in the analysis of protein complexes with single‐stranded and double‐stranded DNA as well as with peptides. In dependence on stripping current (Istr), structural transition of the protein alone or in complexes can be followed as a result of the protein exposure to electric field effects. For the first time we show here that the CPS analysis can be used for the study of the interaction of BSA with a polysaccharide namely sodium alginate (SA). BSA‐SA complex formation was accompanied by the shift of the structural transition of BSA to lower ‐Istr intensities. Another polysaccharide dextran did not alter Istr‐dependent structural transition of BSA. BSA‐SA complex can be disturbed by an electric field effect or high ionic strength confirming the electrostatic nature of BSA‐SA interaction.

Effect of Morphology of ϵ‐MnO2 on Hydrogen Peroxide Sensing

AbstractIn this work, solid spherical, solid cubic and hollow cubic MnO2 microparticles were successfully synthesized via a facile precursor conversion method. All the crystal structures were defined as rare ϵ‐phase. The ϵ‐MnO2 was modified onto the glassy carbon electrode (GCE) to fabricate sensor platform for hydrogen peroxide (H2O2) determination. The catalytic activity of the prepared materials was demonstrated to be in the order: hollow cubes > solid cubes > solid spheres ϵ‐MnO2. The higher electrochemical activity is related to the higher conductivity and more exposed highly active crystal planes instead of BET surface area, electrode active area and surface charge. Additionally, the cubic ϵ‐MnO2 possesses greater catalytic rate constant (K), which is approximately an order of magnitude higher than that of spherical morphology, due to the faster electron transfer rates caused by reduced contact resistance. The proposed sensor possesses the excellent sensitivity, selectivity and stability, and is successfully applied to determine H2O2 in real sample.

Wetting Behaviour of Gold Electrode and Molten Alkali Chlorides

Abstract The potential dependence of the contact angle between a gold electrode and lithium, sodium, potassium, rubidium, and caesium chloride melts was studied using the meniscus weight method to establish the patterns of wettability of solid surfaces by ionic melts when changing the composition of the salt phase and the jump of the electric potential. It is found that the forms of the contact angle versus the potential curve of Au change from a convex to a camel-like shape with two maxima upon replacing the lithium chloride with the caesium chloride melt. This phenomenon is explained by the assumption that the adsorption of the halide anions at the positively charged electrode surface has a chemical rather than electrostatic character. The adsorption process is accompanied by a charge transfer through the interface and the formation of covalent bonds between the adsorbent and adsorbate.

Off-center charge model revisited: Electrical double layer with multivalent cations.

The off-center charge model of ions is a relatively simple model for introducing asymmetry in Coulomb interaction while retaining the simplicity and convenience of the spherical hard core geometry. A Monte Carlo simulation analysis of the planar electric double layer formed by this ionic model for 1+:1- valence systems [S. Lamperski et al., Langmuir 33, 11554-11560 (2017)] is extended to include solutions of multivalent (2+, 3+) hard spherical cations and single valence (1-) hard spherical anions near a uniformly charged, planar electrode. The solvent is modelled as a uniform dielectric continuum with a dielectric constant equal to that of the pure solvent, viz., the primitive model. Results are reported for the ion density, the cation charge profile, and the electrostatic potential profile at 1 mol/dm3 salt concentration. Additionally, the double layer potential drop, that is, the electrode potential, and the integral and the differential capacitances are computed as functions of the electrode surface charge density. The latter two quantities show an expected asymmetry as long as the cation valence is not too great and the charge of the off-center ion cannot approach too close to the electrode surface. It is unusual that the integral and differential capacitances are negative for high valence cations and a negatively charged electrode when the off-center charge is large and can be very near the surface of the electrode. The corresponding electrode potential versus surface charge density curve becomes non-monotonic and shows a change of slope, and thus the resultant integral and differential capacitances can become negative. This nonphysical result is the result of an incipient singularity when a large positive charge is too near a negatively charged electrode. Overall, the off-center charge model suggests a useful recipe to model electrical asymmetry within the broader context of the primitive model provided that the off-center charge is not too near the surface of the electrode.

On the influence of physical parameters on the properties of the electric double layer modelled by soft potentials. A Monte Carlo study

Grand canonical Monte Carlo results are reported for an electric double layer formed by a planar electrode and spherical ions. The non-electrostatic interactions are the soft interactions approximated by the Lennard-Jones (LJ) potential. In particular, the ion-ion interactions are described by the 12-6 LJ potential, while the ion-electrode interactions are given by the 9-3 potential. The ion charge is located at the centre of a sphere. Ions are immersed in a continuous dielectric medium whose relative permittivity is equal to that of a solvent. The results of simulations for a 1:1 electrolyte and the same ion diameters show that the singlet distribution function of counter-ions has a single high maximum, while that of co-ions has two small maxima. The maxima of counter- and co-ions increase with increasing ion-ion or ion-electrode soft interactions, with decreasing temperature or electrolyte concentration. Analogous changes in the soft interactions, temperature and electrolyte concentration lead to formation at some distance from the negatively charged electrode surface of a negative minimum of the integrated charge, which indicates the overscreening or charge reversion effect. They also transform the shape of differential capacitance curve from that having a maximum (bell-like shape) at the point of zero charge to that showing a minimum surrounded by maxima (camel-like shape). Difference in ion charges or in ion diameters introduce asymmetry in the shape of differential capacitance curves. Increase in a counter-ion charge (the absolute value) or decrease in their diameters elevate the electric double layer capacitance.

Ion Sieving Effect of Pillared Graphene Materials in Electrochemical Capacitors

Electrical double-layer capacitors (EDLCs), commonly known as supercapacitors, store energy through reversible adsorption of electrolytic ions on the charged electrode surfaces.1 Extensive work on carbide-derived carbons (CDCs), ACs and templated carbons revealed that the sub-micro pores (< 1 nm) contribute significantly to the overall charge storage.2 , 3 Carbon materials with uni-modal pore size distribution curves (PSD) are essential for such a focused analysis.4 However, the difficulties in synthesizing such carbon materials and obtaining their PSDs reliably through gas sorption have hampered the progress.5 Meanwhile, research on graphene derivatives for charge storage in SCs has shown promising advances owing to good surface areas and electrical conductivities. However, much of the current research is devoted to improving the material porosity with a focus on PSDs, similar to the research on ACs.6 Surprisingly, little attention has been devoted towards exploring the possible confinement of the electrolytic ions into the unique layered structures of graphitic materials. An analysis of materials with varied inter-graphene sheets distance (d-spacing) in SCs would assess the possibility of such confinement. In this context, this study is aimed at understanding the consequences of interlayer spacing on electrochemical charge storage. A class of graphene-like materials with varying d-spacing have been synthesized using alkyl diamines cross-linking and are characterized for their application in SCs. Electrochemical analysis of the materials in SCs using electrolytes containing tetraalkylammonium cations of different sizes (alkyl: ethyl, propyl, butyl and hexyl) showed limitations of ion adsorption in different materials. The PSD curves of the materials show no presence of sub-micro pores (< 1 nm) and thus relate the observed ion limitation to d-spacing. Hence, this work demonstrates that the inter-layer galleries in graphene like materials indeed confine electrolyte ions based on the size constrictions. A correlation between the d-spacing and the electrolyte ion sizes is established and a condition for maximum capacitances versus d-spacing for a specific electrolyte is being investigated. [1] Simon, P.; Gogotsi, Y. Nat Mater 2008, 7 (11), 845–854. [2] Merlet, C.; Péan, C.; Rotenberg, B.; Madden, P. A.; Daffos, B.; Taberna, P.-L.; Simon, P.; Salanne, M. Nat. Commun. 2013, 4, 3701-3707. [3] Raymundo-Piñero, E.; Kierzek, K.; Machnikowski, J.; Béguin, F. Carbon 2006, 44 (12), 2498–2507. [4] Kondrat, S.; Pérez, C. R.; Presser, V.; Gogotsi, Y.; Kornyshev, A. A. Energy Environ. Sci. 2012, 5 (4), 6474–6479. [5] Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Adv. Mater. 2014, 26 (14), 2219–2251. [6] Li, C.; Shi, G. Adv. Mater. 2014, 26 (24), 3992–4012.

Homogeneous electrochemical aptasensor for mucin 1 detection based on exonuclease I-assisted target recycling amplification strategy

A sensitive and homogeneous electrochemical aptasensor was fabricated for the detection of mucin 1 (MUC1) by combining a well-designed DNA bulge-loop (L-DNA) structure with high-efficient exonuclease I (Exo I)-assisted target recycling amplification strategy. The L-DNA probe was constructed via the hybridization of the MUC1 aptamer and methylene blue (MB) labeled complementary DNA (cDNA) (cDNA-MB) and hence could not diffuse freely to the negatively charged ITO electrode surface due to the strong electrostatic repulsion, so small electrochemical signal was detected. The addition of MUC1 caused the dissociation of L-DNA structure due to the specificity between aptamer and MUC1. Then Exo I was implemented to digest the released cDNA-MB into mononucleotides and then produced short MB-labeled mononucleotides fragments (MB-MFs). As the MB-MFs contained few negative charges, it diffused easily to the negatively charged ITO electrode surface and resulted in the enhanced electrochemical signal. Meanwhile, the MUC1-aptamer complex was also specifically digested by Exo I, resulting in the liberation of MUC1 and hence realized the target recycling and then caused the amplification of the electrochemical signal. The enhanced electrochemical signal has a good linear relationship with logarithm of MUC1 concentration in the range of 1.0 pg mL−1–50 ng mL−1 with a limit of detection of 0.40 pg mL−1 (S/N = 3). Additionally, the fabricated aptasensor has been successfully applied to detect MUC1 in serum samples with satisfactory results and thereby it exhibits great potential in the practical application of clinical diagnosis.

Label-Free Detection of Salivary Pepsin Using Gold Nanoparticle/Polypyrrole Nanocoral Modified Screen-Printed Electrode.

Detection of salivary pepsin has been given attention as a new diagnostic tool for laryngopharyngeal reflux (LPR) disease, because saliva collection is non-invasive and relatively comfortable. In this study, we prepared polypyrrole nanocorals (PPNCs) on a screen-printed carbon electrode (SPCE) by a soft template synthesis method, using β-naphthalenesulfonic acid (NSA) (for short, PPNCs/SPCE). Gold nanoparticles (GNPs) were then decorated on PPNCs/SPCE by electrodeposition (for short, GNP/PPNCs/SPCE). To construct the immunosensor, pepsin antibody was immobilized on GNP/PPNCs/SPCE. Next, citric acid was applied to prevent non-specific binding and change the electrode surface charge before pepsin incubation. Electrochemical stepwise characterization was performed using cyclic voltammetry, and immunosensor response toward different pepsin concentrations was measured by differential pulsed voltammetry. As a result, our electrochemical immunosensor showed a sensitive detection performance toward pepsin with a linear range from 6.25 to 100 ng/mL and high specificity toward pepsin, as well as a low limit of detection of 2.2 ng/mL. Finally, we quantified the pepsin levels in saliva samples of LPR patients (n = 2), showing that the results were concordant with those of a conventional ELISA method. Therefore, we expect that this electrochemical immunosensor could be helpful for preliminarily diagnosing LPR through the detection of pepsin in saliva.

Modification of a Mercury Electrode with Different Thioalkanes: Structure‐Sensitive Bovine Serum Albumin Analysis

AbstractChronopotentiometric stripping (CPS) in combination with mercury electrodes is convenient for structure‐sensitive protein analysis. By using this method, we tested the discrimination between native and denatured bovine serum albumin (BSA) at various alkanethiol self‐assembled monolayers (SAMs). Our results showed that alkanethiol SAMs with different headgroups or different chain lengths had different influences on the efficiency of CPS in protecting BSA from the electric field‐induced denaturation. SAMs with longer alkanethiol chain lengths were more efficient in protecting BSA from denaturation at negatively charged electrode surface than those formed by short‐chain alkanethiols. Short alkanethiol SAMs could be prepared by co‐adsorption with the protein on the electrode surface, which was not possible with the long‐chain alkanethiols. CPS data were supported by impedance analysis. We believe that our work will be useful in the electrochemical sensing of nucleic acid‐protein and protein‐protein binding.

Mechanisms of Electrotunable Friction in Friction Force Microscopy Experiments with Ionic Liquids

Using molecular dynamics simulations and a coarse-grained model of ionic liquids (ILs), we study mechanisms of electrotunable friction measured in friction force microscopy experiments, where only one layer of IL is present between the tip and the electrode (substrate). We show that the variation of the friction force with the electrode surface charge density is determined by the regime of motion of the confined IL relative to the substrate and tip. The latter depends on the strengths of the ion–substrate and ion–tip interactions and on the commensurability between the characteristic ion dimensions and lattice spacings of the substrate and tip surfaces. Related with those factors, our simulations predict two strictly different scenarios for the variation of the friction force with the electrode surface charge. Revealing mechanisms of frictional energy dissipation in nanoscale IL films offers a way for controlling friction by tuning ion–substrate interactions and electrical polarization of sliding surfaces.

Homogeneous and label-free electrochemiluminescence aptasensor based on the difference of electrostatic interaction and exonuclease-assisted target recycling amplification

The difference of electrostatic interaction between free Ru(phen)32+ and Ru(phen)32+ embedded in double strand DNA (dsDNA) to the negatively charged indium tin oxide (ITO) electrode has been applied to develop a homogeneous and label-free electrochemiluminescence (ECL) aptasensor for the first time. Ochratoxin A (OTA) has been chosen as the model target. The OTA aptamer is first hybridized with its complementary single strand DNA (ssDNA) to form dsDNA and then interacted with Ru(phen)32+ via the grooves binding mode to form dsDNA-Ru(phen)32+ complex, which remains negatively charged feature as well as low diffusion capacity to the negatively charged ITO electrode surface owing to the electrostatic repulsion. Meanwhile, the intercalated Ru(phen)32+ in the grooves of dsDNA works as an ECL signal reporter instead of the labor-intensive labeling steps and can generate much more ECL signal than that from the labeling probe. In the presence of target, the aptamer prefers to form an aptamer-target complex in lieu of dsDNA, which induces the releasing of Ru(phen)32+ from the dsDNA-Ru(phen)32+ complex into the solution. With the assistance of RecJf exonuclease (a ssDNA specific exonuclease), the released ssDNA and the aptamer in the target-complex were digested into mononucleotides. In the meantime, the target can be also liberated from OTA-aptamer complex and induce target cycling and large amount of free Ru(phen)32+ present in the solution. Since Ru(phen)32+ contains positive charges, which can diffuses easily to the ITO electrode surface because of electrostatic attraction, causing an obviously enhanced ECL signal detected. Under the optimal conditions, the enhanced ECL of the system has a linear relationship with the OTA concentration in the range of 0.01–1.0 ng/mL with a detection limit of 2 pg/mL. This innovative system not only expands the immobilization-free sensors in the electrochemiluminescent fields, but also can be developed for the detection of different targets easily with the same strategy by changing the aptamer used.

A homogeneous electrochemical sensor for Hg2+ determination in environmental water based on the T-Hg2+-T structure and exonuclease III-assisted recycling amplification.

A simple, fast, sensitive, and homogeneous electrochemical sensor based on the T-Hg2+-T structure and exonuclease III-assisted recycling amplification has been constructed for mercury ion (Hg2+) detection. The cT and methylene blue-labeled DNA probes (MB-TDNA) were designed to contain poly T sequences, which were repulsed from the negatively charged indium tin oxide (ITO) electrode due to their abundant negative charges. Hg2+ could trigger the formation of double-stranded DNA (dsDNA) between two DNA probes owing to the stable T-Hg2+-T structure. Then, Exo III specifically recognizes the cleavage of the double-stranded structure to release a methylene blue-labeled mononucleotide fragment (MB-MF). Moreover, the release of the target Hg2+ induces new hybridization and produces a large number of MB-MFs; MB-MFs are not repulsed by the negatively charged ITO electrode surface, thus producing a significant current signal. Under optimal conditions, the differential pulse voltammetric (DPV) response had a linear relationship with the logarithm of Hg2+ concentration in the range of 1.0 nM-0.5 μM, and the proposed method displayed great applicability for detecting Hg2+ in tap-water samples.