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Abstract We develop for the first time a new electrochemical biosensor for signal-on detection of T4 polynucleotide kinase (PNK) based on the enhanced quasi-reversible redox signal of prussian blue generated by self-sacrificial label of iron metal-organic framework (FeMOF). The DNA hairpin probe modified with FeMOF@AuNPs at the 3'-thiol end acts as the substrate of PNK. The presence of PNK enables the 5'-phosphorylation of hairpin probe, which may subsequently function as the substrate of lambda exonuclease. Lambda exonuclease removes the 5' mononucleotides from the stem, unfolding the hairpin structure and releasing the single-stranded DNA (ssDNA). The resultant FeMOF@AuNP-modified ssDNA may specifically hybridize with the capture probe to form the double-strand DNA (dsDNA) duplex, enabling the immobilization of FeMOF on the electrode surface. The reaction of Fe3+ in the MOF with K 4 Fe(CN) 6 leads to the formation of prussian blue on the electrode surface and consequently the generation of a high electrochemical signal. This assay is very simple without the involvement of the pre-synthesized prussian blue which is unstable in neutral buffer, and the self-sacrificial label of FeMOF can achieve great signal amplification without the involvement of any extra functional materials. Due to the introduction of self-sacrificial label of FeMOF and the formation of prussian blue nanolayers with good quasi-reversible cyclic voltammetric electrochemical activity, this biosensor exhibits high sensitivity and a large dynamic range. Moreover, this biosensor can be used to screen the PNK inhibitors, holding great potential in disease diagnosis and drug discovery. Highlights • We develop a new electrochemical biosensor for signal-on detection of T4 polynucleotide kinase (PNK). • This electrochemical biosensor is based on the enhanced quasi-reversible redox signal of prussian blue generated by self-sacrificial label of iron metal-organic framework (FeMOF). • This assay is very simple without the involvement of the pre-synthesized prussian blue which is unstable in neutral buffer. • The self-sacrificial label of FeMOF can achieve great signal amplification without the involvement of any extra functional materials. [ABSTRACT FROM AUTHOR]

We develop a reusable ratiometric electrochemical biosensor on the basis of the binding of methylene blue (MB) to DNA with alternating AT base sequence for sensitive detection of adenosine. We design a strand 1 with MB-modified thymine (T) base in the proximal 3′ termini as the capture probe for its immobilization on the gold electrode and a 3′ termini ferrocene (Fc)-modified aptamer for the recognition of adenosine. The hybridization of strand 1 with the aptamer leads to the formation of a double-stranded DNA (dsDNA) and consequently the away of MB from the electrode surface and the close of Fc to the electrode surface, generating a small value of I MB /I Fc ( I MB and I Fc are the peak currents of MB and Fc, respectively). In the presence of adenosine, its binding with the aptamer induces the release of Fc from the electrode surface and the close of MB to the electrode surface, generating a large value of I MB /I Fc . As a result, adenosine may be accurately quantified by the measurement of ratiometric signal ( I MB / I Fc ). This ratiometric electrochemical biosensor can be simply fabricated and exhibits high sensitivity with a limit of detection of as low as 90.8 pM and a large dynamic range from 0.1 nM to 100 μM. Moreover, this biosensor demonstrates good performance with excellent selectivity, regeneration capability, high reliability and good reproducibility, and may become a universal platform for the detection of various biomolecules which can be recognized by aptamers, holding great potential for further applications in biomedical research and clinical diagnosis. [ABSTRACT FROM AUTHOR]

Polynucleotide kinase (PNK) plays an essential role in cellular nucleic acid metabolism and the cellular response to DNA damage. However, conventional methods for PNK assay suffer from low sensitivity and involve multiple steps. Herein, we develop a simply electrochemical method for sensitive detection of PNK activity on the basis of Au nanoparticle (AuNP)-mediated lambda exonuclease cleavage-induced signal amplification. We use [Ru(NH 3 ) 6 ] 3+ as the electrochemically active indicator and design two DNA strands (i.e., strand 1 and strand 2) to sense PNK. The assembly of strand 2 on the AuNP surface leads to the formation of AuNP-strand 2 conjugates which can be subsequently immobilized on the gold electrode through the hybridization of strand 1 with strand 2 for the generation of a high electrochemical signal. The presence of PNK induces the phosphorylation of the strand 2-strand 1 hybrid and the subsequent cleavage of double-stranded DNA (dsDNA) by lambda exonuclease, resulting in the release of AuNP-strand 2 conjugates and [Ru(NH 3 ) 6 ] 3+ from the gold electrode surface and consequently the decrease of electrochemical signal. The PNK activity can be simply monitored by the measurement of [Ru(NH 3 ) 6 ] 3+ peak current signal. This assay is very sensitive with a detection limit of as low as 7.762 × 10 −4 U mL −1 and exhibits a large dynamic range from 0.001 to 10 U mL −1 . Moreover, this method can be used to screen the PNK inhibitors, and it shows excellent performance in real sample analysis, thus holding great potential for further applications in biological researches and clinic diagnosis. [ABSTRACT FROM AUTHOR]

A signal-on aptasensor was fabricated for highly sensitive and selective electrochemical detection of arsenite with a label-free Ars-3 aptamer self-assembled on a screen-printed carbon electrode (SPCE) via Au–S bond. The Ars-3 aptamer could adsorb cationic polydiallyldimethylammonium (PDDA) via electrostatic interaction to repel other cationic species. In the presence of arsenite, the change of Ars-3 conformation due to the formation of Ars-3/arsenite complex led to less adsorption of PDDA, and the complex could adsorb more positively charged [Ru(NH 3 ) 6 ] 3+ as an electrochemically active indicator on the aptasensor surface, which produced a sensitive “turn-on” response. The target-induced structure switching could be used for sensitive detection of arsenite with a linear range from 0.2 nM to 100 nM and a detection limit down to 0.15 nM. Benefiting from Ars-3 aptamer, the proposed system exhibited excellent specificity against other heavy metal ions. The SPCE-based aptasensor exhibited the advantages of low cost and simple fabrication, providing potential application of arsenite detection in environment. [ABSTRACT FROM AUTHOR]

As heavy metal ions severely harm human health, it is important to develop simple, sensitive and accurate methods for their detection in environment and food. Electrochemical detection featured with short analytical time, low power cost, high sensitivity and easy adaptability for in-situ measurement is one of the most developed methods. This review introduces briefly the recent achievements in electrochemical sensing of heavy metal ions with inorganic, organic and bio-materials modified electrodes. In particular, the unique properties of inorganic nanomaterials, organic small molecules or their polymers, enzymes and nucleic acids for detection of heavy metal ions are highlighted. By employing some representative examples, the design and sensing mechanisms of these electrodes are discussed. [ABSTRACT FROM AUTHOR]

Abstract: A highly sensitive and selective electrochemical sensor was developed for the detection of Cu2+ by the assembly of Ag nanoparticles (AgNPs) at dithiobis[succinimidylpropionate] encapsulated Au nanoparticles (DSP-AuNPs), which was regulated by copper-catalyzed oxidation of cysteamine (Cys). The electrochemical sensor was constructed by layer-by-layer modification of glassy carbon electrode with carbon nanotubes, poly(amidoamine) dendrimers and DSP-AuNPs. In the absence of Cu2+, Cys could bind to the surface of citrate-stabilized AgNPs via Ag–S bond, thus AgNPs could be assembled on the sensor surface through the reaction between DSP and Cys. In contrast, the copper-catalyzed oxidation of Cys by dissolved oxygen in the presence of Cu2+ inhibited the Cys-induced aggregation of AgNPs, leading to the decrease of the electrochemical stripping signal of AgNPs. Under the optimized conditions, this method could detect Cu2+ in the range of 1.0–1000nM with a detection limit of 0.48nM. The proposed Cu2+ sensor showed good reproducibility, stability and selectivity. It has been satisfactorily applied to determine Cu2+ in water samples. [Copyright &y& Elsevier]

Abstract: An enzymeless biosensor was explored from Cu–Mg–Al calcined layered double hydroxide (CLDH) modified electrode in this study. The Cu–Mg–Al CLDH greatly promotes the electron transfer between H2O2 and GCE, and it is exemplified toward the non-enzymatic sensing of H2O2. The results indicate that the Cu–Mg–Al CLDH exhibits excellent electrocatalytic property, high sensitivity, good reproducibility, long-term stability, and fast amperometric response toward reduction of H2O2, thus is promising for the future development of man-made mimics of enzyme in H2O2 sensors. This work opens a way to utilize simply Cu–Mg–Al CLDH as an electron mediator to fabricate an efficient H2O2 biosensor, which exhibits great potential applications in varieties of simple, robust, and easy-to-make analytical approaches in the future. [Copyright &y& Elsevier]

Sensitive detection of cancer cells is essential to early clinic diagnosis, and the photoelectrochemical (PEC) sensors with high sensitivity and good selectivity may provide new approaches for cytosensing. Herein, we demonstrate the development of a new cathodic PEC cytosensor based on the integration of covalent organic polymer (COP) with palladium nanoparticles (PdNPs). The COP films are in-situ grown at room temperature on the transparent indium tin oxide-coated glass substrates, and they subsequently assemble with PdNPs to immobilize aptamers via palladium-sulfur chemistry. PdNPs can catalyze the oxidation of dopamine to produce aminochrome and its derivative, which may function as the electron acceptors of COP for the generation of an enhanced photocurrent. In the absence of cancer cells, the electrons on the conduction band of COP on the electrode transfer to the aminochrome and O 2 , while the electrons on the electrode transfer to the hole of valence band, resulting in a high cathodic photocurrent. In the presence of cancer cells, the trapped cancer cells efficiently cover the electrode to reduce the surface of COP/PdNPs, resulting in the decrease of catalytic precipitation on the electrode and consequently the generation of a low PEC signal. This PEC cytosensor exhibits high sensitivity with a detection limit of 8 cells mL−1 and a large dynamic range from 10 to 106 cells mL−1. Moreover, this PEC cytosensor has distinct advantages of high selectivity, good reproducibility and excellent stability, and it can be extended to directly detect various cancer cells through the integration with corresponding specific aptamers. • We develop a new cathodic photoelectrochemical (PEC) cytosensor to sensitively detect cancer cells. • This cytosensor is based on the integration of covalent organic polymer (COP) with palladium nanoparticles (PdNPs). • This cytosensor exhibits high sensitivity, high selectivity, good reproducibility and excellent stability. • This cytosensor can be extended to directly detect various cancer cells with the integration of corresponding aptamers. • This cytosensor does not involve any extraction of nucleic acids and proteins. [ABSTRACT FROM AUTHOR]

We developed a novel electrochemical biosensor for uracil-DNA glycosylase (UDG) detection based on enzyme-free and substrate-free electrocatalytic signal amplification by porphyrin-based covalent-linked nanomaterial (OAPS-Por). This OAPS-Por could not only absorb much Thionine (Thi), but also possess obvious electrocatalytic activity toward the reduction of Thi without involvement of H 2 O 2. Sequentially, the functionalized OAPS-Por with Thi, Au nanoparticles and single-stranded DNA (OAPS-Por/Thi@AuNPs-ssDNA) was ingeniously designed as the signal probe. Meantime, the hairpin DNA (hDNA) with four uracil bases was immobilized on AuNPs/GCE via an Au–S bond. When UDG was present, the uracil in hDNA was removed and hairpin structure was unfolded. Next, the signal probes binded with the unfolded hDNA by DNA hybridization. The Thi in signal probes could generated an original electrochemical signal, which could be further amplified and output due to the robust electrocatalytic activities of OAPS-Por toward Thi. As a result, the as-constructed electrochemical biosensor had a broad linear range from 0.005 to 1 U mL-1. It also exhibited a low detection limit of 6.97 × 10-4 U mL-1. Moreover, this biosensor could be used to assay the inhibition of UDG (UGI) and the UDG activity in real samples (HeLa cell lysates and human blood serums), and demonstrated great prospect in clinical diagnostics and biomedical research. • A novel electrochemical biosensor is developed for sensitive detection of uracil-DNA glycosylase (UDG). • This biosensor is based on nonenzymatic nanomaterial (OAPS-Por) signal amplification. • OAPS-Por exhibited intrinsic electrocatalytic activity toward the reduction of absorbed Thi without involvement of H 2 O 2. • This biosensor shows low limit of detection and wide linear range of UDG activity. [ABSTRACT FROM AUTHOR]

We develop a low-background electrochemical biosensor for one-step detection of uracil DNA glycosylase (UDG) based on the host-guest interaction and iron-embedded nitrogen-rich carbon nanotube (Fe–N–C) that mimics enzyme-mediated electrocatalysis to achieve signal amplification. In this work, Fe–N–C is initially immobilized on a glassy carbon electrode, followed by the immobilization of β –cyclodextrin (β –CD). We construct the signal probes by assembling the methylene blue (MB)-labeled hairpin DNAs onto the surface of Au nanoparticles (AuNPs) to form the MB-hairpin/AuNP probes. Due to the steric effect of AuNPs and the stem–loop structure of hairpin DNA, MB is prevented from entering the cavity of β -CD on the electrode. In contrast, UDG enables the removal of uracil from the U•A pairs in the stem of hairpin DNA probe to generate apurinic/apyrimidinic (AP) sites, leading to the assembly of MB-hairpin/AuNP probes on the electrode based on host–guest reaction between β -CD and MB. Meanwhile, L-cysteine (RSH) is oxidized by O 2 to disulfide L-cystine (RSSR) and H 2 O 2. In the presence of H 2 O 2 , Fe–N–C catalyzes the oxidation of MB to generate an amplified electrochemical signal. Notably, the Fe–N–C-catalyzed oxidation of MB is mediated by the oxidation of RSH by O 2 instead of external H 2 O 2 , greatly simplifying the experimental procedures and improving the electrochemical signal. Due to the introduction of host–guest recognition, this electrochemical biosensor displays a low-background signal and high signal-to-noise ratio, enabling the one-step sensitive measurement of UDG with a detection limit of 7.4 × 10−5 U mL−1. Moreover, this biosensor can measure UDG in crude cell extracts and screen the inhibitors, providing a new platform for biomedical research. • We develop a low-background electrochemical biosensor for sensitive detection of uracil DNA glycosylase. • This biosensorinvolves the host–guest interaction and the FeCN mimic enzyme-based electrocatalysis signal amplification. • The FeCN-catalyzed oxidation of methylene blue is mediated by the oxidation of L-cysteine by O 2 instead of external H 2 O 2. • This biosensor exhibits high sensitivity and good selectivity. [ABSTRACT FROM AUTHOR]

Abstract A sensitive and specific photoelectrochemical (PEC) immunosensor was fabricated for subgroup J avian leukosis viruses (ALV-J) analysis based on a dual signal-on strategy. Gold nanoparticles (AuNPs) decorated graphitic carbon nitride (AuNPs/g-C 3 N 4) as photoelectrochemical species and primary antibody (Ab 1) against ALV-J were immobilized onto ITO electrode in turn. An ALP-CdTe-Ab 2 bio-conjugant was fabricated by assembling second antibody (Ab 2) and alkaline phosphatase (ALP) to CdTe quantum dots (QDs) surface. The PEC immunosensor was fabricated by successively anchoring the target ALV-J and ALP-CdTe-Ab 2 bio-conjugants onto electrode surface via the immune recognition. By virtue of the matched energy levels between CdTe QDs and AuNPs/g-C 3 N 4 , ALP-CdTe-Ab 2 bio-conjugants could serve as the PEC active probes for photocurrent enhancement. Moreover, the photocurrent response could be further enhanced attributed to the ALP catalytic chemistry to in situ produce ascorbic acid for electron donating, achieving an effective dual signal-on mode for PEC assay. On the basis of the ALV-J titers-dependent photocurrent increment, the fabricated PEC immunosensor showed high sensitivity, specificity and stability for ALV-J assay in a wide linear range with a low detection limit of 85 TCID 50 /mL. This PEC immunosensor with the dual signal-on strategy may open up a promising platform for more target analytes in novel immune analysis and clinical diagnostics. Highlights • A dual signal-on strategy was proposed for PEC immunosensing target avian virus. • AuNPs/g-C 3 N 4 hybrids coupling with CdTe QDs as PEC probes revealed improved photocurrent. • ALP could catalyze AAP to in situ generate electron donor for further photocurrent enhancement. • The PEC immunosensor displayed high specificity and sensitivity for ALV-J detection. [ABSTRACT FROM AUTHOR]

A novel pH-responsive sandwich-type electrochemiluminescence immunosensor was successfully developed for the detection of avian leukosis virus subgroup J (ALV-J). The immunosensor consisted of graphene oxide functionalized with tannic acid (rGO-TA) and primary antibodies (Ab 1 ) as the main ALV-J sensing platform. For signal amplification, pH-responsive hollow MnO 2 (hMnO 2 ) nanospheres encapsulating Ru(bpy) 3 Cl 2 and carrying secondary antibodies (Ab 2 ) were employed. Under optimal conditions, the immunosensor exhibited a wide ALV-J detection range of 10 1.80 to 10 4.30 TCID 50 /mL (TCID 50 : 50% tissue culture infective dose) and a low detection limit of 10 1.71 TCID 50 /mL (S/N = 3). Importantly, the immunosensor showed high sensitivity, good reproducibility and excellent operational stability. Finally, the immunosensor was evaluated using a real avian serum sample containing ALV-J, with excellent results being achieved. [ABSTRACT FROM AUTHOR]

Nanozymes have been widely used in biosensors instead of natural enzymes because of low cost, high stability and ease of storage. However, few works use oxidase-like nanozymes to fabricate electrochemical biosensors. Herein, we proposed a sensitive electrochemical biosensor to detect uracil-DNA glycosylase (UDG) based on the hollow Mn/Ni layered doubled hydroxides (h-Mn/Ni LDHs) as oxidase-like nanozyme. Briefly, the h-Mn/Ni LDHs, which was prepared by a facile hydrothermal method, exhibited excellent oxidase-like activity because the hollow structure provided rich active sites and high specific surface area. Then, the signal probes were constructed by assembling the hairpin DNA (hDNA), single DNA1 and DNA2 on the h-Mn/Ni LDHs, respectively. In the presence of UDG, the uracil bases in the stem of hDNA were specifically excised, generating apyrimidinic (AP) sites and inducing the unwinding of the hDNA. Afterwards, the h-Mn/Ni LDHs@Au-hDNA/DNA1 was connected on the electrode via hybridization between unwinded hDNA and capture DNA (cDNA). Subsequently, the self-linking process allowed the retention of numerous h-Mn/Ni LDHs through simple DNA hybridization to amplify the signal of o -phenylenediamine (o -PD). Unlike many peroxidase-like nanozyme-based electrochemical biosensors, there is no need to add H 2 O 2 during the experimental process, which effectively reduced the background signal as well as improved the stability of the biosensor. As expected, the biosensor exhibited excellent performance with a wide linear range and a low detection limit. This work highlights an appealing opportunity to develop a no H 2 O 2 platform based on h-Mn/Ni LDHs for future application in biological analysis and clinical diagnosis. • An electrochemical biosensor was developed for UDG detection with high sensitivity. • The h-Mn/Ni LDHs shows excellent oxidase-like activity due to the hollow structure. • The background signal effectively reduced by the absence of unstable H 2 O 2. • Self-linking process enhanced the signal, and improved the sensitivity. [ABSTRACT FROM AUTHOR]