When Fluorescent Sensing Meets Electrochemical Amplifying: A Powerful Platform for Gene Detection with High Sensitivity and Specificity

Optical Bioassays Based on the Signal Amplification of Redox Cycling

Optical bioassays are challenged by the growing requirements of sensitivity and simplicity. Recent developments in the combination of redox cycling with different optical methods for signal amplification have proven to have tremendous potential for improving analytical performances. In this review, we summarized the advances in optical bioassays based on the signal amplification of redox cycling, including colorimetry, fluorescence, surface-enhanced Raman scattering, chemiluminescence, and electrochemiluminescence. Furthermore, this review highlighted the general principles to effectively couple redox cycling with optical bioassays, and particular attention was focused on current challenges and future opportunities.

Chemical-chemical redox cycling for improving the sensitivity of the fluorescent assay: A proof-of-concept towards DNA methylation detection

Ultrasensitive analytical methods are still urgent for the discovery of trace level biomarkers and the early clinical diagnosis of disease. In this work, an ultrasensitive universal sensing platform was constructed by integrating fluorescent assay with chemical-chemical redox cycling signal amplification strategy. Using Ru@SiO2 nanoparticles wrapped by MnO2 nanosheets (Ru@SiO2@MnO2) as fluorescent probe, the chemical-chemical redox cycling system was conducted upon ascorbic acid (AA) and tris(2-carboxyethyl)phosphine (TCEP) as reductants and MnO2 nanosheets as oxidant. The MnO2 nanosheets not only could quench the fluorescence of Ru@SiO2 nanoparticles to reduce the background, but also could serve as oxidants to react with AA, generating dehydroascorbic acid (DHA). The DHA was reduced by TCEP in turn to form AA that participated in the next cycling of chemical-chemical redox reaction. Thus, the constantly released AA from the chemical-chemical redox cycling system could massively etch MnO2 nanosheets on Ru@SiO2 surface, making the fluorescence of Ru@SiO2 nanoparticles greatly recovered. It was shown that the sensitivity of the fluorescent assay was improved almost 52 times by utilizing the chemical-chemical redox cycling signal amplification strategy. This strategy was further employed to detect DNA methylation with the aid of AA-encapsulated liposomes that were modified with 5 mC antibodies to bind with the methylated DNA captured in 96-well plate. A detection of limit down to 16.2 fM was achieved for the detection of methylated DNA. It's believed that the incorporation of chemical-chemical redox cycling signal amplification strategy into fluorescent sensing paves a new way for ultrasensitive detection of biomarkers.

NIR Plasmonic Nanozymes: Synergistic Enhancement Mechanism and Multi-Modal Anti-Infection Applications of MXene/MOFs

Nanozymes are considered as the promising antimicrobial agents due to the enzyme-like activity for chemo-dynamic therapy (CDT). However, it remains a challenge to develop novel nanozyme systems for achieving stimuli-responsive, and efficient nanozyme catalysis with multimodal synergistic enhancement. In this work, a near-infrared (NIR) plasmonic-enhanced nanozyme catalysis and photothermal performance for effective antimicrobial applications are proposed. A Ti3 C2 MXene/Fe-MOFs composite (MXM) with NIR plasmonic-enhanced CDT combined with photothermal properties is successfully developed by loading metal-organic framework (MOF) nanozymes onto Ti3 C2 MXene. The mechanism of NIR induced localized surface plasmon resonance (LSPR)-enhanced CDT and photothermal therapy (PTT) is well explained through activation energy (Ea ), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), fluorescence analysis experiments, and finite element simulation. It reveals that MXene nanosheets exhibit NIR plasmon exciters and generate hot electrons that can transfer to the surface of Fe-MOFs, promoting the Fenton reaction and enhances CDT. While the photothermal heating of MXene produced by LSPR can also boost the CDT of Fe-MOFs under NIR irradiation. Both in vitro and in vivo experimental results demonstrate that LSPR-induced MXM system has outstanding antimicrobial properties, can promote angiogenesis and collagen deposition, leading to the accelerated wound healing.

High index dielectric films on metals: An island of emission

Fluorescent emitters are quenched near the surfaces of metals via rapid energy transfer to the metal, via surface plasmons, waveguide modes, and absorption. Commonly, this quenching is reduced by introducing a polymeric or dielectric spacer but requires large distances, at least a fraction of the wavelength, between the metal and chromophore. Using the classical theory for a dipole above a metal/dielectric substrate, we investigate the fluorescent yield for emitters above a wide range of metals and spacers. For metals with low loss and low plasma frequencies, a high index spacer is shown to be advantageous for obtaining higher fluorescent yield in an "island of emission" at finely tuned spacer thickness just 20-30 nm from the metal surface. For such metal-dielectric combinations, fluorophores can be placed surprisingly close to the metal surface while remaining significantly emissive.

Chemical Cyclic Amplification: Hydroxylamine Boosts the Fenton Reaction for Versatile and Scalable Biosensing

Nucleic acid detection is undoubtedly one of the most important research fields to meet the medical needs of genetic disease diagnosis, cancer treatment, and infectious disease prevention. However, the practical detection methods based on biological amplification are complex and time-consuming and require highly trained operators. Herein, we report a simple, rapid, and sensitive method for the nucleic acid assay by fluorescence or naked eye using chemical cyclic amplification. The addition of hydroxylamine (HA) during the Fenton reaction can continuously generate hydroxyl radicals (^•OH) via Fe³⁺/Fe²⁺ cycle, termed as "hydroxylamine boosts the Fenton reaction (Fenton-HA system)". Meanwhile, the reducing substances, such as terephthalic acid or o-phenylenediamine, react with ^•OH to generate oxidized substances that can be recognized by the naked eye or detected by fluorescence so as to realize the detection of Fe³⁺. The concentration of Fe³⁺ has a good linear relationship with fluorescence intensity in the range of 0.1 to 100 nM, and the limit of detection is calculated to be 0.03 nM (S/N = 3). Subsequently, Fe was introduced into the nucleic acid hybridization system after the Fe source was transformed into Fe³⁺, and the nucleic acids were indirectly determined by this method. This Fenton-HA system was used for sensing HIV-DNA and miRNA-21 to verify the validity of this method in nucleic acid detection. The detection limits were as low as 2.5 pM for HIV-DNA and 3 pM for miRNA-21. We believe that our work has unlocked an efficient signal amplification strategy, which is expected to develop a new generation of highly sensitive chemical biosensors.

A novel electrochemical biosensor based on MIL-101-NH2 (Cr) combining target-responsive releasing and self-catalysis strategy for p53 detection

A novel electrochemical biosensor was constructed to detect p53 gene based on MIL-101-NH₂ (Cr) by combining target-responsive releasing and self-catalysis strategy. MIL-101-NH₂ (Cr) with suitable pore structure was used to encapsulate methylene blue (MB) as signal probe. The hairpin DNA (HP) containing rich-G sequences was used as gatekeeper to seal up the pores and avoid MB leakage through covalent immobilization. The p53 gene could hybridize with the loop portion of HP for the formation of dsDNA, which had the specific nicking site of the nicking endonuclease (Nt.BstNBI). Then Nt.BstNBI recognized the specific recognition site and cleaved HP to open the pore for releasing of MB. Meanwhile, the cleavage of HP released the target DNA to trigger the target recycling for signal amplification. More importantly, the plentiful rich-G sequences were exposed to form Hemin/G-quadruplex DNAzymes, which could unite MIL-101-NH₂ (Cr) to catalyze redox reaction of MB released by itself for signal amplification. The biosensor for p53 had wide linear range from 1 × 10^-14 to 1 × 10^-7 M and a low detection limit of 1.4 × 10^-15 M. The combination of target-responsive releasing and self-catalysis strategy provided a promising way for constructing ultrasensitive and simple biosensor.

Boron and Nitrogen-Codoped Carbon Dots as Highly Efficient Electrochemiluminescence Emitters for Ultrasensitive Detection of Hepatitis B Virus DNA

In this work, boron and nitrogen-codoped carbon dots (BN-CDs) as highly efficient electrochemiluminescence (ECL) emitters with advantages of low excitation potential and high ECL efficiency were prepared to establish a novel ternary ECL system for ultrasensitive detection of HBV-DNA. Especially, both platinum nanoflowers (Pt NFs) and boron radicals (B^•) from the BN-CDs could accelerate the reduction of coreactant S₂O₈^2- to abundant SO₄^•- simultaneously, making the BN-CDs have outstanding ECL performance. Impressively, the ECL efficiency of BN-CDs is much higher than that of nondoped CDs and single-doped CDs. In addition, by combining the novel ECL ternary system with the exonuclease III (Exo III)-induced target DNA amplification strategy, an ECL biosensor was constructed to realize the ultrasensitive detection of HBV-DNA from 100 aM to 1 nM, while the limit of detection was 18.08 aM. Therefore, a promising highly efficient ECL emitter was offered to develop a novel ECL detection method for clinical disease analysis.

Swelling of Serum-Stable DNA Nanoparticles upon Target-Induced Conformational Rearrangement of Sensing Probes for the Signal-On Detection of Cancer-Related Genes

Nuclease-resistant assay probes are of significant importance for biochemical analysis and disease diagnosis. In this contribution, a reconfigurable lipidic moiety-attached DNA nanoparticle (LDN) is constructed from a cholesterol-conjugated multifunctional hairpin-type DNA probe (Chol-DP) by hydrophobicity-mediated self-assembly. The LDN holds high serum stability and displays a low false-positive signal even in a complex biological milieu. The hydrophobic cholesterol moiety enables the hydrophobicity-mediated assembly, while hydrophilic DNA sequence serves as a recognition element and a polymerization template. The initiator-activated strand displacement amplification (SDA) reaction can convert the hairpin-shaped probe into rigid double-stranded DNA (dsDNA), causing the conformational rearrangement-based LDN swelling that can be used to reliably and fluorescently signal the cancer-related p53 gene. The size increase and structural reconfiguration are confirmed by dynamic light scattering (DLS) analysis and confocal microscopy imaging, respectively. Target p53 is specifically detected down to 10 pM. The whole assay process involved only several simple mixing steps. Recovery test and blind test further confirm the feasibility of the use of the LDN for the detection of target DNA in a complex biological milieu, indicating a promising nanotool for biomedical applications.