Here, we first find that Na+ can induce Pb2+-stabilized T30695 undergoing conformational transition from partly parallel to completely parallel, and further forming a dimeric G-quadruplex, which was characterized by circular dichroism (CD) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and native polyacrylamide gel electrophoresis (PAGE). Thermal denaturation experiments show that the transforming process is a thermodynamics-driven process. Furthermore, the presence of Na+ further improves the binding efficiency of Pb2+-stabilized T30695 with the fluorescent probe (such as ZnPPIX). Based on the fact, with a partially hybridized double-stranded DNA (ds-DNA) containing T30695 as a sensing probe and ZnPPIX as a fluorescence probe, the effect of Na+ on Pb2+ detection is subsequently investigated. The presence of Na+ (varied from 0.3 mM to 500 mM) simultaneously increases the read-out and background fluorescence, which results in a decreased signal-to-noise ratio and further leads to a decreased sensing performance (detection limits is increased to 120 nM). In order to avoid Na+ interference, a fully matched ds-DNA containing T30695 is utilized as a sensing probe to fix the background fluorescence, regardless of whether Na+ is present or not. Thus, a relatively lower detection limit (10 nM) in all Na+-containing real samples is achieved, respectively. Therefore, the paper provides a novel insight into the conformational changes in G-quadruplex and presents an efficient step to resolve the challenging problem about Pb2+ detection in Na+-containing real samples.

In recent years, graphene quantum dots (GQDs) have attracted tremendous attention for their potential for biological, optoelectronic and energy-related applications, owing to their excellent biocompatibility, low cytotoxicity, stable photoluminescence (PL) and resistance to photobleaching. Among these applications, GQD-based biosensors are rapidly developed due to their particular PL, electrochemiluminescence (ECL) and electrochemical properties, which are highly sensitive to minute perturbations. In this review, recent exciting progress in GQD-based biosensors, such as PL sensors, ECL sensors and electrochemical sensors is highlighted. First, the synthesis and the fundamental properties of GQDs are briefly introduced. Then, emerged applications of PL, ECL and electrochemical biosensors are mainly addressed. Finally, their future potential developments are discussed and speculated.

G-rich aptamers have been widely applied to develop various sensors for detecting proteins, small molecules, and cations, which is based on the target-induced conformational transfer from single strand to G-quadruplex. However, the transforming process is unclear. Here, with PW17 as an aptamer example, the forming process of G-quadruplex induced by K+ is investigated by circular dichroism spectroscopy, electrospray ionization mass spectroscopy, and native gel electrophoresis. The results demonstrate that PW17 undergoes a conformational transforming process from loose and unstable to compact and stable G-quadruplex, which is strictly K+ concentration-dependent. The process contains three stages: (1) K+ (<0.5 mM) could induce PW17 forming a loose and unstable G-quadruplex; (2) the compact and stable K+-stabilized G-quadruplex is almost formed when K+ is equal to or larger than 7 mM; and (3) when K+ ranges from 0.5 mM to 7 mM, the transformation of K+-stabilized PW17 from loose and unstable to compact and stable occurs. Interestingly, dimeric G-quadruplex through 5′-5′ stacking is involved in the forming process until completely formed at 40 mM K+. Moreover, the total process is thermodynamically controlled. With PW17 as a sensing probe and PPIX as a fluorescent probe for detection of K+, three linear fluorescent ranges are observed, which corresponds to the three forming stages of G-quadruplex. Clarifying the forming process provides a representative example to deeply understand and further design aptamer-based biosensers and logic devices.

Direct and rapid detection of 1-hydroxypyrene (1-OHP) is of great importance owing to its high carcinogenicity, teratogenicity and toxicity. In this study, a simple colorimetric assay for rapid determination of 1-OHP is reported, which is based on non-crosslinking aggregation of gold nanoparticles (Au NPs) induced by 1-OHP in the presence of formic acid (FA). Initially, Au NPs were synthesized with citrate as the capping agent and exhibited red color. Subsequently, the addition of FA did not cause aggregation of Au NPs, but a proton transfer process occurred from FA to carboxylic anions on the surface of Au NPs with a decreased zeta potential. The subsequent addition of 1-OHP resulted in a further decreased zeta potential and an intensely hydrophobic environment, which led to a strong and rapid non-crosslinking aggregation of Au NPs within 5 min with the color changing from red to violet blue. Based on this principle, sensitive and selective detection of 1-OHP was achieved. The detection limit was 3.3 nM. Finally, the colorimetric assay was successfully applied to detect 1-OHP in a urine sample. This strategy provides new insights into developing colorimetric methods for on-site and real-time detection of polycyclic aromatic hydrocarbons.

•Acetonitrile–water was selected as a medium to oxidize BaP;•The oxidation potential was negatively shifted 180 mV compared to that in acetonitrile;•The oxidation current was 22 times greater compared to that in acetonitrile;•Acetonitrile–water binary medium was applied for sensitive and selective detection of BaP.Electrochemical oxidation of adsorbed benzo(a)pyrene (BaP) on the glassy carbon electrode (GCE) was explored in acetonitrile–water. When the GCE was incubated in 100 nM BaP acetonitrile–water (Vwater:Vacetonitrile=1:1) for 10 min at open circuit, and then transferred into blank acetonitrile–water (Vwater:Vacetonitrile=1:1, pH= 0.70) for differential pulse voltammetry measurement, a distinct oxidation peak at 0.98 V (vs. Ag/AgCl) was observed. The peak potential was about 180 mV lower than that in acetonitrile. Importantly, the peak current was more than 22 times greater. The effects of water on BaP preconcentration on the electrode and electrochemical oxidation were revealed, respectively. Based on the results, an electrochemical assay for BaP detection was developed. The GCE was respectively incubated in acetonitrile–water (Vwater:Vacetonitrile=1:1)with BaP concentration ranged from 0 nM to 1000 nM, and then transferred into the corresponding blank acetonitrile–water (pH= 0.70) for DPV measurements. When the BaP concentration was increased, an increased oxidative current at 0.98 V (vs. Ag/AgCl) was observed, and a detection limit of 0.67 nM was achieved. Because all other priority polycyclic aromatic hydrocarbons could not be electrochemically oxidized at 0.98 V, the electrochemical assay showed very high selectivity to BaP. Finally, the developed electrochemical assay was successfully applied to determination of BaP in a series of real world samples, such as drinking water, tap water, lake water and river water.

Direct reduction of Pb2+ in self-assembled G-quadruplex on the gold electrode was first observed, which was applied in constructing a series of simple and reversible logic gates, such as one-input, two-input and three-input logic gates. Importantly, the largest scale of reversibility among two-input logic gates was achieved based on the reciprocal transformations of DNA.

Sulfur-doped graphene quantum dots (S-GQDs) with stable blue-green fluorescence were synthesized by one-step electrolysis of graphite in sodium p-toluenesulfonate aqueous solution. Compared with GQDs, the S-GQDs drastically improved the electronic properties and surface chemical reactivities, which exhibited a sensitive response to Fe3+. Therefore, the S-GQDs were used as an efficient fluorescent probe for highly selective detection of Fe3+. Upon increasing of Fe3+ concentration ranging from 0.01 to 0.70 μM, the fluorescence intensity of S-GQDs gradually decreased and reached a plateau at 0.90 μM. The difference in the fluorescence intensity of S-GQDs before and after adding Fe3+ was proportional to the concentration of Fe3+, and the calibration curve displayed linear regions over the range of 0–0.70 μM. The detection limit was 4.2 nM. Finally, this novel fluorescent probe was successfully applied to the direct analysis of Fe3+ in human serum, which presents potential applications in clinical diagnosis and may open a new way to the design of effective fluorescence probes for other biologically related targets.

The direct interaction of hairpin DNA with 9-hydroxyfluorene (9-OHFLU) through hydrogen bonds was investigated by electrochemical impedance spectroscopy (EIS), UV-Vis spectroscopy and 1H NMR spectra. Based on these results, an electrochemical hairpin DNA sensor was developed for the detection of 9-OHFLU by EIS. Upon 9-OHFLU interacting with hairpin DNA film on the gold electrodes, the charge transfer resistance (RCT) of the hairpin DNA film was significantly increased and remained constant after 30 min. Depending on the difference in charge transfer resistance (ΔRCT) before and after 9-OHFLU interaction with the hairpin DNA, 9-OHFLU could be detected with a concentration as low as 1 nM. However, only a much smaller ΔRCT appeared when eight selected hydroxyl polycyclic aromatic hydrocarbons (HO-PAHs) interacted with the hairpin DNA for 30 min, which demonstrated that 9-OHFLU could be discriminated from other HO-PAHs by EIS. The performance of the sensor in real lake water sample was further explored for the detection of 9-OHFLU with the detection limit of 4 nM.

A simple and sensitive chemiluminescence assay for iodide (I−) detection was reported, which was based on iodide extracting Hg2+ from DNA featuring a stem-loop structure containing T–Hg2+–T. Specifically, Hg2+ induced random coiled G-rich single-strand DNA to form a stem-loop structure containing T–Hg2+–T. Because the binding of Hg2+ and I− is much stronger than that of Hg2+ and thymine (T), I− could extract Hg2+ from the stem-loop structure, releasing the DNA, which then bound with K+ and transformed into a K+-stabilized G-quadruplex (with hemin as a cofactor), which catalyzed the H2O2-mediated oxidation of luminol. The produced chemiluminescence as a sensing signal was applied to sensitively and selectively detect iodide with a detection limit of 12 nM. This system exhibited the first DNAzyme-based iodide sensor. Finally, the sensor was successfully applied for iodide detection in real lake water samples.

A novel template-assisted deposition and etching strategy is proposed in this paper for preparing IrO2 nanotube arrays on ITO substrates, i.e., electrodepositing IrO2 nanoparticles onto ZnO nanorod surfaces to produce IrO2-coated core–shell nanorod arrays, and followed by wet chemical etching the ZnO nanorods away to generate IrO2 nanotube arrays. Well-aligned IrO2 nanotube arrays with high purity and uniform size are produced by using this synthetic strategy. Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) are employed to examine the morphology, fine structure and composition of the IrO2 nanotube arrays intensively. Furthermore, electrocatalytic water oxidization experiments are performed to assess the catalytic performance of the as-prepared IrO2 nanotube arrays toward oxygen evolution reaction (OER). The IrO2 nanotube arrays have been found to possess an excellent catalytic performance: high turnover frequency (3.3 s−1 for TOF at 1.2 V versus Ag/AgCl), low oxygen evolution overpotential (η = 0.15 V) and good catalytic stability (55% catalytic activity remaining after undergoing 400 times potential cycles).Graphical abstractWell-aligned IrO2 nanotube arrays are fabricated by using a template-assisted deposition and etching strategy, which demonstrates good catalytic performance toward oxygen evolution reaction (OER).Highlights► A simple an general strategy for fabricating nanotube arrays. ► Well-aligned IrO2 nanotube arrays on conductive substrates. ► Excellent catalytic performance toward oxygen evolution reaction (OER).

A highly sensitive, reusable G-rich DNA sensor was reported for the detection of α-naphthol by electrochemical impedance spectroscopy (EIS). Specifically, a single-stranded G-rich DNA was self-assembled on the electrode and transformed into K+-stabilized G-quadruplex, which could catalyze H2O2-mediated oxidation of α-naphthol (with hemin as a cofactor) to 1, 4-naphthoquinone precipitated on the DNA films. Due to the insolubility of 1, 4-naphthoquinone, the charge transfer resistance (RCT) was increased to maximum within 15 min. Depending on the difference in charge transfer resistance change (ΔRCT), the α-naphthol could be detected with the detection limit of 0.1 nM in Tris–ClO4 buffer solution (20 mM, pH=7.4). Moreover, the sensor demonstrated a high selectivity over other selected phenolic compounds. The performance of the sensor in the real lake water was also explored with the detection limit of 0.8 nM. Finally, the regeneration of the sensor was investigated, which allowed for reuse more than 4 cycles with a mean recovery of 94% of the original signal.Highlights► We provide a new strategy of G-DNA modified gold electrode to detect α-naphthol. ► UV–vis spectroscopy, fluorescence spectra, SEM and DPV confirm the mechanism. ► The sensor exhibits better selectivity, sensitivity and regenerability. ► The detection limit is 0.1 nM in buffer solution, or 0.8 nM in natural lake water.

•Hairpin DNA sensor was developed to study the amino-substituted naphthalene compounds.•Fluorescence spectra, Raman spectroscopy, DPV and EIS confirm the mechanism.•The ΔRCT is confirmed to be correlative to the binding constants (R2=0.99).•The sensor exhibits excellent selectivity and high sensitivity, nano-mole detection limit.•The sensor is successfully performed in natural water sample with a recovery of 96–102%.The amino-substituted naphthalene compounds, such as 1,8-diaminonaphthalene (1,8-DANAP), 2,3-diaminonaphthalene (2,3-DANAP), 1,5-diaminonaphthalene (1,5-DANAP), 1-naphthylamine (1-NAP) and 2-naphthylamine (2-NAP), were investigated by electrochemical impedance spectroscopy (EIS), which was based on the interaction with hairpin DNA immobilized on the gold electrodes. Upon hairpin DNA interacting with the target chemicals, the charge transfer resistance (RCT) of the hairpin DNA films was significantly decreased and the charge transfer resistance change (ΔRCT) decreased in a sequence of ΔRCT 1,8-DANAP>ΔRCT 2,3-DANAP>ΔRCT 1,5-DANAP>ΔRCT 1-NAP>ΔRCT 2-NAP. The ΔRCT changes were due to the difference in the binding constant (KSV) of the target chemicals to DNA. In addition, the interaction mechanism was further explored using 1,8-DANAP as a model analyte by fluorescence spectra, Raman spectroscopy, differential pulse voltammetry (DPV) and EIS, correspondingly. The results demonstrated that the amino-substituted naphthalene compounds intercalated into “stem” appearing in the hairpin DNA. Moreover, the hairpin DNA sensor exhibited high sensitivity to the amino-substituted naphthalene compounds with the detection limit of nano-mole, and maintained high selectivity over other selected environmental pollutants. Finally, the DNA sensor was challenged in natural water sample with a recovery of 96–102%, which offered a platform for prospective future development of a simple, rapid, sensitive and low-cost assay for the detection of target aromatic amine pollutants.

An electrochemical DNA sensor based on DNA conformational changes for simultaneous detection of Hg2+ and Pb2+ was reported. The sensor was consisted of a probe strand (DNA), a Pb2+-specific DNAzyme, and a substrate strand contains mercury-specific oligonucleotide (MSO). When Hg2+ and Pb2+ interacted with DNA, the induced conformational changes were tracked by electrochemical impedance spectroscopy (EIS), which led to a decreased RCT. The RCT difference (ΔRCT) was applied to selectively detect Hg2+ and Pb2+ with detection limit of 1 pM and 0.1 pM, respectively. Through using masking agents, such as cysteine (masking Hg2+) and G-DNA (CTG-GGA-GGG-AGG-GAG-GGA) (masking Pb2+), Hg2+ and Pb2+ were simultaneously detected in buffer solution, human serum and river water, respectively.

An unlabeled immobilized DNA-based sensor was reported for simultaneous detection of Pb2+, Ag+, and Hg2+ by electrochemical impedance spectroscopy (EIS) with [Fe(CN)6]4–/3– as redox probe, which consisted of three interaction sections: Pb2+ interaction with G-rich DNA strands to form G-quadruplex, Ag+ interaction with C–C mismatch to form C–Ag+–C complex, and Hg2+ interaction with T–T mismatch to form T–Hg2+–T complex. Circular dichroism (CD) and UV–vis spectra indicated that the interactions between DNA and Pb2+, Ag+, or Hg2+ occurred. Upon DNA interaction with Pb2+, Ag+, and Hg2+, respectively, a decreased charge transfer resistance (RCT) was obtained. Taking advantage of the RCT difference (ΔRCT), Pb2+, Ag+, and Hg2+ were selectively detected with the detection limit of 10 pM, 10 nM, and 0.1 nM, respectively. To simultaneously (or parallel) detect the three metal ions coexisting in a sample, EDTA was applied to mask Pb2+ and Hg2+ for detecting Ag+; cysteine was applied to mask Ag+ and Hg2+ for detecting Pb2+, and the mixture of G-rich and C-rich DNA strands were applied to mask Pb2+ and Ag+ for detecting Hg2+. Finally, the simple and cost-effective sensor could be successfully applied for simultaneously detecting Pb2+, Ag+, and Hg2+ in calf serum and lake water.

Conformational switch from hairpin DNA to G-quadruplex induced by Pb2+ is studied by electrochemical impedance spectroscopy (EIS) in the presence of [Fe(CN)6]3−/4− as the redox probe. In the presence of Pb2+, the G-rich hairpin DNA opens the stem-loop and forms G-quadruplex structure, which gives rise to a sharp increase in the charge-transfer resistance (RCT) of the film reflected by the EIS. This structural change is also confirmed by circular dichroism (CD) measurements and UV-Vis spectroscopic analysis and calculated by density functional theory (DFT). On the basis of this, we develop a label-free electrochemical DNA biosensor for Pb2+ detection. With increasing concentrations of Pb2+, the differences in the charge-transfer resistance RCT before and after the Pb2+ incubation is linearly dependent on the logarithm of Pb2+ concentration within a range from 50 μM to 0.5 nM. The biosensor also exhibits good selectivity for Pb2+ over other metal ions. This is a simple and label-free electrochemical method for Pb2+ detection making use of the G-quadruplex.

An electrochemical assay for the detection of silver ion was reported, which was based on the interaction of the Y-type, C-rich ds-DNA with Ag+. Upon addition of Ag+, Y-type, C-rich ds-DNA could form an intramolecular duplex, in which Ag+ can selectively bind to cytosine–cytosine (C–C) mismatches forming C–Ag+–C complex. The binding result was evaluated by electrochemical impedance spectroscopy (EIS) and analyzed with the help of Randles' equivalent circuits. The differences of charge transfer resistance, ΔRCT, after and before the addition of Ag+, allows the detection and quantitative analysis of Ag+ with a detection limit of 10 fM. Moreover, cysteine (Cys) was applied to remove Ag+ from the C–Ag+–C complex, which allowed the Ag+ sensor to be reproduced. In the same way, ΔRCT for the C–Ag+–C system in the absence and presence of Cys allows the detection of Cys at a concentration as low as 100 fM. Finally, the potential application of the Ag+ sensor was also explored, such as in lake and drinking water.

The presence of Ni2+ enables us to distinguish the presence of single-nucleotide mismatches in PNA (peptide nucleic acids)–DNA films on gold electrodes by electrochemical impedance spectroscopy (EIS). With the help of a modified Randles’ equivalent circuit, differences in the charge transfer resistance (ΔRCT) before and after the addition of Ni2+ are a diagnostic measure for the presence of single-nucleotide mismatch. The approach works under real-life conditions with concentrations of the DNA target strand down to 10 fM, and a PNA capture probe is used to genotype the single-nucleotide mismatch in apoE 4 related to Alzheimer's disease (AD).

The interactions of the metal ions Mg2+, Zn2+, Ni2+, and Co2+ with thin films of peptide nucleic acids (PNAs) were studied by electrochemical impedance spectroscopy (EIS), and the results show that Zn2+, Ni2+ and Co2+ interacted favorably with the PNA film involving the backbone and the nucleobases with the exception of Mg2+ for which the interaction with the backbone appears to be dominant.

The interaction of the metal ions Mg2+, Zn2+, Ni2+, and Co2+ with DNA−peptide nucleic acid (PNA) films on a gold surface is studied by electrochemical impedance spectroscopy in the presence of [Fe(CN)6]3−/4− as the redox probe. Impedance data were analyzed with the help of a modified Randles’ equivalent circuit. Changes in the charge-transfer resistance, RCT, decreases in the order of Ni2+ > Co2+ > Zn2+ > Mg2+. We interpret these results in terms of stronger interactions for Ni2+ with the DNA−PNA film compared to the other metal ions, potentially involving interactions with the nucleobases, presumably with the N7 of purines or the N3 of pyrimidines. On the basis of these observations, Ni2+ was chosen to probe the detection of a C-T mismatch in 15-mer PNA−DNA films. Using Ni2+, it is possible to detect a single C-T mismatch. The resulting ΔRCT is larger for the PNA−DNA hybrid compared to that for the identical 15-mer DNA−DNA hybrid.

The paper described a label-free assay for the detection of single-nucleotide mismatches in which an unlabeled hairpin DNA probe and a MutS protein conjugate (His6-MutS-linker peptide-streptavidin binding peptide (HMLS)) are exploited for the detection of mismatches by electrochemical impedance spectroscopy (EIS). We demonstrate this method for eight single-nucleotide mismatches. Upon hybridization of the target strand with the hairpin DNA probe, the stem-loop structure is opened forming a duplex DNA. In duplexes containing a single nucleotide mismatch, the mismatch is present at the solvent exposed side, enabling more effective HMLS recognition and binding. The binding event is evaluated by EIS and analyzed with the help of Randles’ equivalent circuits. The differences in the charge transfer resistance ΔRCT before and after protein binding to the duplex DNA allows the unequivocal detection of all eight single-nucleotide mismatches. ΔRCT allows the discrimination of a C-A mismatch with the concentration of the target strand as low as 100 pM.

An electrochemical DNA sensor based on DNA conformational changes for simultaneous detection of Hg2+ and Pb2+ was reported. The sensor was consisted of a probe strand (DNA), a Pb2+-specific DNAzyme, and a substrate strand contains mercury-specific oligonucleotide (MSO). When Hg2+ and Pb2+ interacted with DNA, the induced conformational changes were tracked by electrochemical impedance spectroscopy (EIS), which led to a decreased RCT. The RCT difference (ΔRCT) was applied to selectively detect Hg2+ and Pb2+ with detection limit of 1 pM and 0.1 pM, respectively. Through using masking agents, such as cysteine (masking Hg2+) and G-DNA (CTG-GGA-GGG-AGG-GAG-GGA) (masking Pb2+), Hg2+ and Pb2+ were simultaneously detected in buffer solution, human serum and river water, respectively.

Direct reduction of Pb2+ in self-assembled G-quadruplex on the gold electrode was first observed, which was applied in constructing a series of simple and reversible logic gates, such as one-input, two-input and three-input logic gates. Importantly, the largest scale of reversibility among two-input logic gates was achieved based on the reciprocal transformations of DNA.

The interactions of the metal ions Mg2+, Zn2+, Ni2+, and Co2+ with thin films of peptide nucleic acids (PNAs) were studied by electrochemical impedance spectroscopy (EIS), and the results show that Zn2+, Ni2+ and Co2+ interacted favorably with the PNA film involving the backbone and the nucleobases with the exception of Mg2+ for which the interaction with the backbone appears to be dominant.