Highly sensitive photoelectrochemical assay for DNA methyltransferase activity and inhibitor screening by exciton energy transfer coupled with enzyme cleavage biosensing strategy

Detection of rabies virus via exciton energy transfer between CdTe quantum dots and Au nanoparticles

Rabies is a fatal encephalitis caused by the rabies virus. The diagnosis of the disease depends in large part on the exposure history of the victim and clinical manifestations of the disease. Rapid rabies diagnosis is an important step in its prevention and control. Therefore, for accurate and timely diagnosis and prevention of rabies, we developed nanomaterials for a novel photoelectrochemical biosensing approach (PBA) for the rapid and reliable diagnosis of rabies virus. This approach uses high-efficiency exciton energy transfer between cadmium telluride quantum dots and Au nanoparticles and is low cost, and easy to miniaturize. By constructing PBA, rabies virus can be detected quickly and with a high degree of sensitivity and specificity; the minimum detection concentration limit for rabies virus is approximately 2.16 ffu/mL of rabies virus particles, or 2.53 × 10¹ fg/mL of rabies virus RNA. PBA could also detect rabies virus in the brain and lung tissue from rabid dogs and mice with better sensitivity than RT-PCR.

Ratiometric Electrochemical Biosensing of Methyltransferase Activity

In this work, a novel ratiometric electrochemical readout platform was proposed and developed for the fast and flexible analysis of M.SssI methyltransferase (MTase) activity. In this platform, two hairpin DNAs (H1 and H2) were designed. H1 contains the palindromic sequence of 5′-CCGG-3′ in its stem which could be methylated and hybridize with H2 labeled by methylene blue (MB) as one of the signal reporters on a gold electrode (GE) in the presence of M.SssI MTase. Additionally, a specific immunoreaction was introduced by conjugating an anti-5-methylcytosine antibody, a DNA CpG methylation recognition unit, with 1,3-ferrocenedicarboxylic acid (Fc) as the second signal reporter. The results showed that when the Fc tag approaches, the MB tag was far from the gold electrode surface, resulting in a decrease in the oxidation peak current of MB (IMB) and an increase in the oxidation peak current of Fc (IFc). The ratiometric electrochemical method above shows the linear range of detection was 0 U/mL 40 U/mL with a detection limit of 0.083 U/mL (the mean signal of blank measures þ3s).

Ultrasensitive Photoelectrochemical Biosensor for microRNA-155 Based on Energy Transfer between Au Nanocages and Red Emission Carbon Dot-Assembled Nanosheets Coupled with the Duplex-Specific Nuclease Enzyme-Assisted Target Recycling Strategy

Energy transfer (ET) is an effective tool to construct photoelectrochemical (PEC) biosensors for its high sensitivity. Since the materials to develop ET systems are limited, exploring new and universal ET systems is significant. Herein, new photoactive nanosheets (R-CDs NS) formed by self-assembling of red emission carbon dots (R-CDs) have been synthesized, which exhibit wide visible light absorption and stable photocurrent response and have an obvious sensitization effect for TiO₂. Gold nanocages (AuNCs), whose absorption overlap well with the R-CDs' emission, were synthesized and served as PEC quenchers for the photosensitized system that consists of TiO₂ and R-CDs. The ET between R-CDs and AuNCs can boost the recombination of photogenerated electron-hole pairs of R-CDs and results in a quenched photocurrent of this system. MicroRNA-155 was chosen as a model target. First, the nanocomposite containing R-CDs NS and AuNCs was prepared through DNA modification and hybridization. In the absence of the target, AuNCs and R-CDs were close enough for ET, with TiO₂-modified FTO serving as the working electrode, and a quenched photocurrent was detected. In the presence of the target, the disintegration of the nanocomposite was induced through target hybridization and DNA hydrolyzation, leading to the separation of AuNCs and R-CDs NS, and the ET disappeared and led to a high photocurrent. With duplex-specific nuclease enzyme-assisted target recycling, the high sensitivity enabled the sensor to monitor the target in cancer cells. The sensor has a low detection limit of 71 aM. The sensing platform has high sensitivity, good selectivity, and reproducibility.

Tailored Biosensors for Drug Screening, Efficacy Assessment, and Toxicity Evaluation

Biosensors have been flourishing in the field of drug discovery with pronounced developments in the past few years. They facilitate the screening and discovery of innovative drugs. However, there is still a lack of critical reviews that compare the merits and shortcomings of these biosensors from a pharmaceutical point of view. This contribution presents a critical and up-to-date overview on the recent progress of tailored biosensors, including surface plasmon resonance, fluorescent, photoelectrochemical, and electrochemical systems with emphasis on their mechanisms and applications in drug screening, efficacy assessment, and toxicity evaluation. Multiple functional nanomaterials have also been incorporated into the biosensors. Representative examples of each type of biosensors are discussed in terms of design strategy, response mechanism, and potential applications. In the end, we also compare the results and summarize the major insights gained from the works, demonstrating the challenges and prospects of biosensors-assisted drug discovery.

Cyclometalated Iridium(III) Complex-Sensitized NiO-Based-Cathodic Photoelectrochemical Platform for DNA Methyltransferase Assay

This work reports an efficient [(C6)₂Ir(dppz)]⁺PF₆^- (C6 = coumarin 6 and dppz = dipyridophenazine)-sensitized NiO photocathode and its application in photoelectrochemical (PEC) bioanalysis field for the first time. This dye-sensitized NiO photocathode was found to exhibit a markedly enhanced cathodic photocurrent. A sensitive cathodic PEC platform was proposed integrating the as-prepared photocathode with enzyme-free cascaded amplification strategies of the catalytic hairpin assembly (CHA) and the hybridization chain reaction (HCR) for DNA methyltransferase (MTase) assay. A hairpin DNA(H_Dam) with specific recognition site of Dam MTase in its stem was designed. The site of H_Dam was methylated in the presence of Dam MTase and then cut by endonuclease DpnI. The released loop fragment, as an initiator, triggered the CHA circuit and the follow-up HCR circuit, resulting in long dsDNA concatemers on the ITO electrode. Numerous [(C6)₂Ir(dppz)]⁺PF₆^- were intercalated into dsDNA, and highly efficient signal amplification was realized. Benefiting from the superior iridium(III) complex-sensitized NiO photocathode and effective amplification strategy, a detection limit of 0.0028 U/mL for the determination of Dam MTase was achieved. Moreover, this work further demonstrated that these proposed tactics could be applied to screen Dam MTase activity inhibitors.

A photoelectrochemical sensor based on a reliable basic photoactive matrix possessing good analytical performance for miRNA-21 detection

The basic photoactive matrixes on transparent electrodes are essential for the performance of photoelectrochemical (PEC) biosensors. Herein, we demonstrate an optimized fabrication strategy toward a reliable ITO/TiO2/AuNP photoanode by sequential deposition of TiO2/Au nanoparticles (Au NPs) on indium tin oxide (ITO) substrates. The identified fabrication conditions include spin-coating tetraisopropyl titanate on ITO slices followed by in situ electrodeposition of Au NPs and finally the thermal annealing treatment. By the conjugation of the thiolated hairpin NH2-DNA sequence and CdTe quantum dots (QDs) onto the thus-prepared photoanodes, a novel PEC sensor for the ultrasensitive detection of miRNA was constructed. The proposed PEC sensor offered advantages including simple structure, storage stability and excellent detection reproducibility as well as sensitivity and specificity toward miRNA-21. Finally, we found that this PEC displayed a broad detection linear range of 1.0 fM to 1.0 nM with a low detection limit of 0.37 fM. This PEC sensor can also excellently discriminate the mismatched miRNA. Moreover, the PEC sensor also showed a satisfactory result in normal human serum sample analysis. These findings emphasized the importance of basic photoactive matrixes for the fabrication of PEC sensors, providing solid fundamental insights for future application of metal oxide substrates for other PEC applications, especially PEC biosensors.

From bench to bedside: Biosensing strategies to evaluate endocrine disrupting compounds based on epigenetic events and their potential use in medicine

The relationship between endocrine system disorders and health risks due to chemical environmental compounds has become a growing concern in recent years. Involuntary exposure to endocrine disruptors (EDCs) is associated with the worldwide increase of diseases such as cancer, obesity, diabetes, and neurocortical disorders. EDCs are compounds that target the nuclear hormonereceptors (NHR) leading to epigenetic changes. Consequently, the use of biosensing strategies based on epigenetic events have a great potential to provide outstanding information about the exposition of EDCs and their evaluation in human health. This review addresses the novel trends in biosensing EDCs evaluation based on DNA methylation assays associated with different human diseases.

Photoelectrochemical aptasensor with low background noise

In photoelectrochemical (PEC) detection, enhancing the PEC signal and depressing the blank signal are conducive to improve the sensitivity. Because the carbon nanotube (CNT) effectively transfers photogenerated electrons from SnSe to the electrode, the composite nanomaterial CNTs/SnSe generates a strong PEC signal. Methionine (Met), AuNPs, and probe DNA are woven together forming a nanoprobe which is used as a quencher to quench the PEC signal of CNTs/SnSe. When the nanoprobe and CNTs/SnSe are modified onto the electrode, there is a low blank signal. In the presence of metastatic breast cancer cells, the cells interact with the aptamer of dsDNA; concomitantly, cDNA is released to trigger catalytic hairpin assembly (CHA). As a result, a new dsDNA which has an overhang is formed. The nanoprobe on the surface of the electrode hybridizes with the newly formed dsDNA. Subsequently, the nanoprobe is released from the surface of the electrode and the quenching effect between the nanoprobe and the CNTs/SnSe disappears. The PEC aptasensor is linear in the concentration range of 300-5,000 cells/mL, and the detection limit is 180 cells/mL under optimized conditions. The relative standard deviation (RSD) is 3.6% at 10,000 cells/mL. This work demonstrates a promising strategy using CNTs/SnSe as the photoactive material and Met-AuNPs as the quencher to establish a PEC aptasensor with a high PEC response and low blank signal. It can be used to detect bioactive substances at ultralow levels prospectively. Graphical abstract.

Nanomaterial-based biosensors for DNA methyltransferase assay

We review the recent advances in the development of nanomaterial-based biosensors for DNA methyltransferase assay.

A dumbbell probe-based dual signal amplification strategy for sensitive detection of multiple DNA methyltransferases

Integration of a dumbbell probe with dual signal amplification enables simultaneously sensitive detection of multiple DNA methyltransferases.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Near-Infrared-Responsive Photoelectrochemical Aptasensing Platform Based on Plasmonic Nanoparticle-Decorated Two-Dimensional Photonic Crystals

The photoelectrochemical (PEC) analysis is an emerging and fast developing biosensing technique. However, the in vivo PEC biosensing in deep tissue is seriously hampered because of the shallow penetration depth of ultraviolet and visible light. Expanding the optical absorption wavelength of photoelectrodes from the visible light region into the near-infrared (NIR) light region is highly desirable because of its deep tissue penetrability and minimal invasiveness for organisms, but the exploration of the facile strategy to implement efficient NIR absorption with good biocompatibility is still challenging. Herein, a NIR PEC aptasensor is proposed by coupling plasmonic nanoparticles (NPs) into periodic two-dimensional nanocavity (NC) photonic crystals as photoelectrodes, where the Au NPs are sputtered on a periodic two-dimensional TiO₂ NC photonic crystal substrate to significantly enhance the NIR PEC response and successfully achieve sensitive PEC detection of Hg²⁺ under irradiation of NIR light in blood. We believe that the proposed NIR-responsive Au/TiO₂ NC-based PEC aptasensor will open a new in vivo biosensing model for a series of important biomolecules and pave up an avenue for the practical applications of PEC biosensing in deep tissue or even in organs and brain of the living body.

Quenched sandwich-type photoelectrochemical aptasensor for protein detection based on exciton energy transfer

This work proposes a quenched photoelectrochemical sensing method for highly selective and sensitive detection of protein via Energy Transfer (ET) effect between the AuNPs and CdS:Mn quantum dots. This detection was performed on a sandwich-type aptamer sensing interface. Chitosan modified CdS:Mn/TiO₂/ITO electrode was used to immobilize capture DNA (S1) via -CONH- bond. In the presence of target protein, AuNPs labeled DNA (AuNPs-S2) was further bonded to the protein to fabricate sandwich sensing platform, which forced the AuNPs away from the electrode surface. In this state, the photocurrent was greatly depressed, mainly due to two factors: (a) the ET effect produced by interparticle distance between CdS:Mn and AuNPs; (b) the steric hindrance of AuNPs-S2 partly obstructs the diffusion of the electron donor. The photocurrent decreased with the increasing concentration of the target protein. Using thrombin as a target, this sensitized method showed a detectable range of 0.1 pM to 8 nM and a detection limit of 30 fM. It possessed high selectivity and good stability for detection of thrombin. This method is extremely flexible and can be extended to varieties of protein targets.

Antimony selenide/graphene oxide composite for sensitive photoelectrochemical detection of DNA methyltransferase activity

An Sb₂Se₃/graphene oxide composite was applied as both the photoelectrochemical probe and substrate for biomolecule conjugation for the construction of a “signal-off” sandwich-type biosensor for DNA methyltransferase activity detection.

One-Pot Synthesis of N-Graphene Quantum Dot-Functionalized I-BiOCl Z-Scheme Cathodic Materials for "Signal-Off" Photoelectrochemical Sensing of Chlorpyrifos

A Z-scheme I-BiOCl/N-GQD (i.e., nitrogen-doped graphene quantum dot) heterojunction was prepared by a one-pot precipitation method at room temperature. The doped iodine decreased the band gap of BiOCl, the introduced N-GQDs enhanced light harvesting and prolonged the photogenerated electron lifetime, and the resultant Z-scheme heterojunction promoted the spatial separation of interfacial charges. Thus, the composite showed high photoelectrochemical activity and a big cathodic photocurrent signal. On the basis of the coordination of chlorpyrifos with surface Bi(III) of the composite, a cathodic photoelectrochemical sensor was constructed for the selective detection of chlorpyrifos. In this case, chlorpyrifos decreased the lifetime of photogenerated electrons, so the photocurrent became small. Furthermore, the photocurrent changed and the logarithm of chlorpyrifos concentration presented a linear relationship. The linear range was 0.3-80 ng mL^-1, and the limit of detection was estimated to be 0.01 ng mL^-1 (defined as S/N = 3). The present strategy can also be used for the design and fabrication of other PEC sensors suitable for different analytes.

Oligonucleotide-modulated photocurrent enhancement of a tetracationic porphyrin for label-free homogeneous photoelectrochemical biosensing

This work reports the first demonstration of an oligonucleotide-modulated label-free homogeneous photoelectrochemical (PEC) biosensing platform based on the adsorption of tetracationic porphyrin (denoted as TMPyP here) onto 1-naphthalenesulfonate anion (NS^-)-grafted indium tin oxide electrode (denoted as TMPyP-NS^--ITO), which generates a stable and rapid photocurrent response. We found that when NS^--ITO electrode was subjected to single-stranded oligonucleotide (ssON) before TMPyP adsorption, a remarkable enhancement of photocurrent intensity was observed from the resulted TMPyP-ssON-NS^--ITO electrode with high specificity towards oligonucleotide. A series of investigations were carried out to understand the mechanism of this oligonucleotide-modulated photocurrent enhancement phenomenon. Moreover, the studies of this robust photocurrent enhancement mechanism was successfully extended to develop a signal-on homogeneous PEC biosensing platform for, as a proof-of-concept, label-free M.SssI methyltransferase activity analysis through a judiciously and compatibly engineered signal transduction strategy consisted of hairpin-shaped oligonucleotide probe, restriction endonuclease HpaII, and Exonuclease I. The rationally designed homogeneous PEC biosensor exhibit sensitive PEC response toward M.SssI methyltransferase with a low detection limit of 3.5 mU/mL and a wide linear range from 0.01 to 120 U/mL. Additionally, we show that our homogeneous PEC biosensing platform can be also utilized to screen methyltransferase inhibitors. Therefore, this work will provide a distinctive paradigm for versatile homogeneous PEC biosensing platform that can be used as potential powerful tool toward innovative label-free bioanalytical purposes.

An ultrasensitive "on-off-on" photoelectrochemical aptasensor based on signal amplification of a fullerene/CdTe quantum dots sensitized structure and efficient quenching by manganese porphyrin

In this work, an ultrasensitive "on-off-on" photoelectrochemical (PEC) aptasensor was proposed based on the signal amplification of a fullerene/CdTe quantum dot (nano-C60/CdTe QDs) sensitized structure and efficient signal quenching of nano-C60/CdTe QDs by a manganese porphyrin (MnPP).

Electrochemical signal-amplified detection of 5-methylcytosine and 5-hydroxymethylcytosine in DNA using glucose modification coupled with restriction endonucleases

A sensitive electrochemical assay based on glucose modification coupled with restriction endonucleases was fabricated for the detection of 5-methylcytosine and 5-hydroxymethylcytosine in DNA.

Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples

Photoelectrochemical (PEC) bioanalysis is a newly developed methodology that provides an exquisite route for innovative biomolecular detection. Quantum dots (QDs) are semiconductor nanocrystals with unique photophysical properties that have attracted tremendous attentions among the analytical community. QDs-based PEC bioanalysis comprises an important research hotspot in the field of PEC bioanalysis due to its combined advantages and potentials. Currently, it has ignited increasing interests as demonstrated by increased research papers. This review aims to cover the most recent advances in this field. With the discussion of recent examples of QDs-PEC bioanalysis from the literatures, special emphasis will be placed on work reporting on fundamental advances in the signaling strategies of QDs-based PEC bioanalysis from 2013 to now. Future prospects in this field are also discussed.

An electrochemiluminescence assay for sensitive detection of methyltransferase activity in different cancer cells by hybridization chain reaction coupled with a G-quadruplex/hemin DNAzyme biosensing strategy

A sensitive electrochemiluminescence assay coupled HCR with a G-quadruplex/hemin DNAzyme was fabricated for the detection of DNMT1 activity in cancer cells.

Recent advances in the use of quantum dots for photoelectrochemical bioanalysis

Photoelectrochemical (PEC) bioanalysis is a newly developed technique for innovative biomolecular detection. Quantum dots (QDs) with unique photophysical properties are key components in realization of various exquisite PEC bioanalyses. Particularly, significant progress has been made in the QD-based PEC bioanalysis. In this work, we briefly summarize the most recent and important developments in the use of traditional and newly emerging QDs for novel PEC bioanalytical applications. The future prospects in this dynamic field are also highlighted.

Microfluidic platforms for DNA methylation analysis

In the field of genetics, epigenetics is the study of changes in gene expression without any change in DNA sequences. Chemical base modification in DNA by DNA methyltransferase, and specifically methylation, has been well studied as the main mechanism of epigenetics. Therefore, the determination of DNA methylation of, for example, 5'-methylcytosine in the CpG sequence in mammals has attracted attention because it should prove valuable in a wide range of research fields including diagnosis, drug discovery, and therapy. Methylated DNA bases and DNA methyltransferase activity are analyzed using conventional methods; however, these methods are time-consuming and require complex multiple operations. Therefore, new methods and devices for DNA methylation analysis are now being actively developed. Furthermore, microfluidic technology has also been applied to DNA methylation analysis because the microfluidic platform offers the promising advantage of making it possible to perform thousands of DNA methylation reactions in small reaction volumes, resulting in a high-throughput analysis with high sensitivity. This review discusses epigenetics and the microfluidic platforms developed for DNA methylation analysis.