A tunable fluorescent probe for superoxide anion detection during inflammation caused by Treponema pallidum

Syphilis, caused by Treponema pallidum (T. pallidum), is associated with the oxidative stress due to its inflammation-like symptom, and detecting the reactive oxygen species (ROS) is crucial for monitoring the infectious process. Herein, we design and synthesize a perylene-based tunable fluorescent probe, PerqdOH, which can detect endogenous O₂˙^- during T. pallidum infection. The fluorescence peak shifted from 540 nm to 750 nm with increasing O₂˙^- levels. Besides, both decreased green fluorescence and enhanced red fluorescence could be observed simultaneously during the in vitro infection, providing the real-time monitoring of intracellular O₂˙^- caused by T. pallidum. Furthermore, the probe exhibited a remarkable signal in the treponemal lesions on the back of a rabbit model. Taken together, our synthesized PerqdOH holds great potential for application in clarifying the infectious process caused by T. pallidum in real time.

Near Infrared-Activatable Methylene Blue Polypeptide Codelivery of the NO Prodrug via π-π Stacking for Cascade Reactive Oxygen Species Amplification-Mediated Photodynamic Therapy

The application of photodynamic therapy (PDT) has attracted remarkable interest in cancer treatment because of the advantages of noninvasiveness and spatiotemporal selectivity. However, the PDT efficiency is considerably limited by photosensitizer (PS) quenching and severe hypoxia in solid tumors. Herein, a kind of near infrared (NIR)-activatable methylene blue (MB) peptide nanocarrier was developed for codelivery of nitric oxide (NO) prodrug JSK, expecting a cascade of reactive oxygen species (ROS) amplification-mediated antitumor PDT. In detail, MB was conjugated to water-soluble polyethylene glycol-polylysine (PEG-PLL) through NIR-photocleavable 10-N-carbamoyl bonds, and the subsequent amphiphilic conjugates (mPEG-PLL-MB) self-assembled into nanoparticles (NPs), which allowed JSK codelivery via π-π stacking interactions. MB in quenched state in mPEG-PLL-MB/JSK NPs could be photoactivated by NIR light locoregionally in a controlled manner due to the photocleavage of carbamoyl bonds. Apart from ROS production, assembly disturbance and even disintegration of mPEG-PLL-MB/JSK occurred along with MB activation that subsequently freed JSK, which was further triggered by intracellularly overexpressed glutathione (GSH) and glutathione S-transferase (GST) to sustain the release of NO. NO then served as a hypoxia relief agent through inhibition of cellular respiration to economize O₂, cooperating with MB activation and GSH depletion, which synergistically enabled a cascade of ROS amplification to augment PDT for mitochondrial apoptosis-mediated tumor inhibition in vitro and in vivo. Therefore, this pioneering strategy of cascade amplification of ROS addressed the key issues of PS inactivation, hypoxia resistance, and ROS neutralization in a three-pronged approach, which hold great promise in efficient antitumor PDT.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Target triggered ultrasensitive electrochemical polychlorinated biphenyl aptasensor based on DNA microcapsules and nonlinear hybridization chain reaction

In this work, we demonstrated an ultrasensitive detection platform for polychlorinated biphenyls (PCBs) based on DNA microcapsules and a nonlinear hybridization chain reaction (NHCR). In the process, first, electrochemical signal molecules (Methylene Blue, MB) were sealed in the prepared DNA microcapsules. In the presence of PCB-72, DNA microcapsules could be dissociated with the conjugation of the aptamer and target, and meanwhile, the released DNA strand could initiate the NHCR and trigger the chain branching growth of DNA dendrimers. Because the released MBs were intercalated into the DNA dendrimer, enhanced electrochemical responses could be detected. This method exhibited ultrahigh sensitivity to PCB-72 with a detection limit of 0.001 ng mL-1. Furthermore, the present aptasensor was also capable of discriminating different PCB congeners. Therefore, the devised label-free and enzyme-free amplification electrochemical aptasensor strategy has great potential for the detection of PCB-72 in real samples, and this strategy may also become an attractive alternative for sensitive and selective small molecule, protein, nucleic acid and nuclease activity detection.

A self-calibrating electrochemical aptasensing platform: Correcting external interference errors for the reliable and stable detection of avian influenza viruses

Conventional electrochemical biosensing systems rely on a single output signal, which limits their certain practical application, specifically from the viewpoint of external interference factors causing electrochemical signal errors. This study reports a self-calibrating dual-electrode based electrochemical aptasensor for the reliable and independent detection of avian influenza viruses (AIVs), which are the primary cause of highly contagious respiratory diseases, under external interference factors. Both electrodes were fabricated using tungsten rods surface-modified with a 3D nanostructured porous silica film (3DNRE). Subsequently, methylene blue (MB) was loaded as a redox-active material into the pores and capped with corresponding aptamer. One electrode was capped with an anti-AIV nucleoprotein (NP) aptamer (Apt_AIV-MB@3DNRE) allowing target-specific binding, resulting in changes in electrochemical signal upon diffusional release of the loaded redox molecules. The other electrode was capped with a control aptamer (Apt_con-MB@3DNRE), serving as a reference to correct false responses generated by nonspecific aptamer detachment and MB release under environmental changes in pH and ion strength and presence of nontarget molecules from cell lysis debris. In the dual-electrode platform, Apt_con-MB@3DNRE provides a corrected baseline for the fluctuating original output signals from Apt_AIV-MB@3DNRE. Consequently, this dual-electrode platform exhibits excellent output-signal stability (relative standard deviation, RSD: 5.86%) compared to a conventional single-electrode platform (RSD: 30.13%) at equivalent concentrations of AIV NP samples under different reaction buffer conditions. Moreover, no further purification and washing steps were required, indicating that the strategy may represent a universal and reliable platform for the electrochemical aptamer-based detection of various biomolecules.

Nanotechnology and nanomaterial-based no-wash electrochemical biosensors: from design to application

Nanotechnology and nanomaterial based electrochemical biosensors (ECBs) have achieved great development in many fields, such as clinical diagnosis, food analysis, and environmental monitoring. Nowadays, the single-handed pursuit of sensitivity and accuracy cannot meet the demands of detection in many in situ and point-of-care (POC) circumstances. More and more attention has been focused on simplifying the operation procedure and reducing detection time, and thus no-wash assay has become one of the most effective ways for the continuous development of ECBs. However, there are many challenges to realize no-wash detection in the real analysis, such as redox interferences, multiple impurities, non-conducting protein macromolecules, etc. Furthermore, the complex detection circumstance in different application fields makes the realization of no-wash ECBs more complicated and difficult. Thanks to the updated nanotechnology and nanomaterials, in-depth analysis of the obstacles in the detection process and various methods for fabricating no-wash ECBs, most issues have been largely resolved. In this review, we have systematically analyzed the nanomaterial based design strategy of the state-of-the-art no-wash ECBs in the past few years. Following that, we summarized the challenges in the detection process of no-wash ECBs and their applications in different fields. Finally, based on the summary and analysis in this review, we also evaluated and discussed future prospects from the design to the application of ECBs.

Highly ordered 3D electrochemical DNA biosensor based on dual orientation controlled rolling motor and graftable tetrahedron DNA

Herein, a robust and highly ordered three-dimensional electrochemical DNA (3D E-DNA) biosensor was proposed, and its orientation was controlled from top down by poly adenine oligonucleotides (polyA-ODNs)-mediated rolling motor (PRM) and graftable tetrahedron DNA (GTD). The GTD with a grafting domain was immobilized on the electrode surface to construct a well-organized sensing interface and controlled the orientation and distribution of the whole system at the "bottom" of this biosensor. The polyA-ODNs regulated the direction and density of the leg DNA attached on PRM at the "top" of the biosensor. The motion was achieved through the target induced cyclic cleaving, which triggered the motor rolling rather than walk. Impressively, the duplex strand DNA (dsDNA) formed after grafting, as a girder, provided a stable support to the soft long single strand (ssDNA), which facilitated the formation of the catalytic center, elevated the efficiency of the rolling cleavage. Under the optimal conditions, the designed biosensor exhibited a lower limit of 0.17 nM and wide linear range from 0.5 nM to 1.5 μM for adenosine rapid detection. Unique dual orientation regulated characteristics of the system increased the probability hybridization enormously and improved the motion efficiency significantly, which offered new avenue of DNA nanomachines development in biosensor platform.

Bioapplications of DNA nanotechnology at the solid-liquid interface

DNA nanotechnology engineered at the solid-liquid interface has advanced our fundamental understanding of DNA hybridization kinetics and facilitated the design of improved biosensing, bioimaging and therapeutic platforms. Three research branches of DNA nanotechnology exist: (i) structural DNA nanotechnology for the construction of various nanoscale patterns; (ii) dynamic DNA nanotechnology for the operation of nanodevices; and (iii) functional DNA nanotechnology for the exploration of new DNA functions. Although the initial stages of DNA nanotechnology research began in aqueous solution, current research efforts have shifted to solid-liquid interfaces. Based on shape and component features, these interfaces can be classified as flat interfaces, nanoparticle interfaces, and soft interfaces of DNA origami and cell membranes. This review briefly discusses the development of DNA nanotechnology. We then highlight the important roles of structural DNA nanotechnology in tailoring the properties of flat interfaces and modifications of nanoparticle interfaces, and extensively review their successful bioapplications. In addition, engineering advances in DNA nanodevices at interfaces for improved biosensing both in vitro and in vivo are presented. The use of DNA nanotechnology as a tool to engineer cell membranes to reveal protein levels and cell behavior is also discussed. Finally, we present challenges and an outlook for this emerging field.

A facile and label-free ratiometric optical sensor for selective detection of norepinephrine by combining second-order scattering and fluorescence signals

In this work, a facile and label-free ratiometric sensor is constructed for selective determination of norepinephrine (NE) by coupling second-order scattering (SOS) and fluorescence, two different and independent optical signals. Herein, polyethyleneimine (PEI) dilute solution medium shows an intensive SOS signal without any fluorescence response. Interestingly, NE can be selectively induced by PEI to emit bright fluorescence, and meanwhile causes an observable decrease in the SOS signal due to the interactions between NE and PEI. The simultaneous variation of the two independent signals can be used for ratiometric sensing of NE. Under the optimal conditions, the resultant ratiometric sensor displays high sensitivity and selectivity toward NE by simultaneously monitoring fluorescence and SOS signals with the same excitation wavelength. The proposed sensor exhibits a good linear relationship versus NE concentration in the range of 10.0 nM-45.0 μM with a detection limit of 2.0 nM (S/N = 3) and has been successfully applied to the determination of NE in real samples without the use of any extra reagent. The combination of fluorescence and SOS signals provides a new scheme for ratiometric sensor design, greatly simplifying experimental procedure and effectively enhancing detection accuracy. Moreover, the proposed analytical strategy further broadens the application of dilute solutions of polymers in research into optical sensor and green analytical chemistry. Graphical abstract.

Highly sensitive and selective "off-on" fluorescent sensing platform for ClO- in water based on silicon quantum dots coupled with nanosilver

We present a new "off-on" fluorescence probe for detecting hypochlorite (ClO^-) based on silicon quantum dots coupled with silver nanoparticles (SiQDs/AgNPs) as nanocomplexes. Via introducing N-[3-(trimethoxysilyl)propyl]ethylenediamine and catechol as initial reactants, silicon quantum dots (SiQDs) with excellent properties were synthesized through a simple hydrothermal method. Transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the morphology and structure of quantum dots. The fluorescence of SiQDs could be quenched by the silver nanoparticles (AgNPs) by surface plasmon-enhanced energy transfer (SPEET) from SiQDs (donor) to AgNPs (acceptor). The AgNPs could be etched by adding ClO^-, thus freeing the SiQDs from the AgNP surfaces and restoring the SiQDs' fluorescence. The sensing system exhibits many advantages, such as wide linear response range, high sensitivity, and excellent selectivity. Under optimized conditions, wide linear ranges (from 0.1 to 100.0 μM) and low detection limits (0.08 μM) were obtained for ClO^-. Graphical Abstract.

Diffusivity and intercalation of electroactive dyes-mediated truly ratiometric homogeneous electrochemical strategy for highly sensitive biosensing

A truly ratiometric homogeneous electrochemical biosensor was developed for miRNA detection based on the unique diffusion/intercalation properties of electroactive dyes.

Electron transfer characteristics of 2'-deoxy-2'-fluoro-arabinonucleic acid, a nucleic acid with enhanced chemical stability

The non-biological nucleic acid 2'-deoxy-2'-fluoro-arabinonucleic acid (2'F-ANA) may be of use because of its higher chemical stability than DNA in terms of resistance to hydrolysis and nuclease degradation. In order to investigate the charge transfer characteristics of 2'F-ANA, of relevance to applications in nucleic acid-based biosensors and chip technologies, we compare the electronic couplings for hole transfer between stacked nucleobase pairs in DNA and 2'F-ANA by carrying out density functional theory (DFT) calculations on geometries taken from molecular dynamics simulations. We find similar averages and distribution widths of the base-pair couplings in the two systems. On the basis of this result, 2'F-ANA is expected to have charge transfer properties similar to those of DNA, while offering the advantage of enhanced chemical stability. As such, 2'F-ANA may serve as a possible alternative to DNA for use in a broad range of nanobiotechnological applications. Furthermore, we show that the (experimentally observed) enhanced chemical stability resulting from the backbone modifications does not cause reduced fluctuations of the base-pair electronic couplings around the values found for "ideal" B-DNA (with standard step parameter values). Our study also supports the use of a DFT implementation, with the M11 functional, of the wave function overlap method to compute effective electronic couplings in nucleic acid systems.

Toehold integrated molecular beacon system for a versatile non-enzymatic application

A molecular beacon (MB) is an oligonucleotide hybridization probe with a hairpin-shaped structure that leads to specific and instantaneous nucleic acid hybridization, enabling a variety of applications. However, integration of additional module sequences interferes with the performance of MBs and increases the complexity of sequence design. Herein, we develop and characterize a toehold integrated molecular beacon (ToMB) strategy for nucleic acid hybridization, where the reaction rate can be flexibly regulated by a target-induced MB conformational switch. Using this basic mechanism, the ToMB is capable of identifying nucleic acids with high specificity and a wider linearity range compared with the conventional molecular beacon system. We further applied the ToMB to the construction of a hybridization chain reaction system and a basic OR logic gate VJHto explore its programmability and versatility. Our results strongly suggest that the novel ToMB can act as a powerful nano-module to construct universal and multifunctional biosensors or molecular computations. Graphical abstract Molecular beacon is employed as a flexible and switchable spacer to control the toehold-mediated strand displacement reaction.

DNA metallization: principles, methods, structures, and applications

This review summarizes the research activities on DNA metallization since the concept was first proposed in 1998, covering the principles, methods, structures, and applications.

Electro-Grafted Electrode with Graphene-Oxide-Like DNA Affinity for Ratiometric Homogeneous Electrochemical Biosensing of MicroRNA

This work demonstrated for the first time a simple and rapid approach to endow the electrode with the excellent discrimination ability over single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) through the robust electrochemical grafting of in situ generated 1-naphthalenesulfonate (NS^-) diazonium salt onto the surface of indium tin oxide (ITO) electrode. On the basis of understanding the influence of sequence and length on the binding affinity of ssDNA and dsDNA toward NS^- grafted ITO (NS^--ITO) electrode, these interesting findings were successfully employed to rationally develop a ratiometric homogeneous electrochemical biosensing platform for microRNA based on the affinity-mediated signal transduction. The achievement of ultrasensitive detection of microRNA lies in a compatibly designed T7 exonuclease-assisted isothermal amplification strategy, in which the presence of target microRNA initiated the continual and opposite affinity inversion of two rationally engineered electrochemical signal reporters, methylene blue (MB) labeled hairpin reporter and ferrocene (Fc) labeled dsDNA reporter, toward NS^--ITO electrode, thereby providing the ratiometric transduction and amplification of the homogeneous electrochemical output signal. By measuring the distinct variation in the peak current intensity ratios of Fc and MB tags, this ratiometric homogeneous electrochemical microRNA biosensing platform showed a detection limit of 25 aM, which is much lower than that of the reported homogeneous electrochemical biosensors. Therefore, we envision that the proposed approach will find useful applications in disease molecular diagnoses and biomedicine.