A novel dinuclear iridium(III) complex as a G-quadruplex-selective probe for the luminescent switch-on detection of transcription factor HIF-1α

Crosstalk between G-quadruplex and ROS

The excessive production of reactive oxygen species (ROS) can lead to single nucleic acid base damage, DNA strand breakage, inter- and intra-strand cross-linking of nucleic acids, and protein-DNA cross-linking involved in the pathogenesis of cancer, neurodegenerative diseases, and aging. G-quadruplex (G4) is a stacked nucleic acid structure that is ubiquitous across regulatory regions of multiple genes. Abnormal formation and destruction of G4s due to multiple factors, including cations, helicases, transcription factors (TFs), G4-binding proteins, and epigenetic modifications, affect gene replication, transcription, translation, and epigenetic regulation. Due to the lower redox potential of G-rich sequences and unique structural characteristics, G4s are highly susceptible to oxidative damage. Additionally, the formation, stability, and biological regulatory role of G4s are affected by ROS. G4s are involved in regulating gene transcription, translation, and telomere length maintenance, and are therefore key players in age-related degeneration. Furthermore, G4s also mediate the antioxidant process by forming stress granules and activating Nrf2, which is suggestive of their involvement in developing ROS-related diseases. In this review, we have summarized the crosstalk between ROS and G4s, and the possible regulatory mechanisms through which G4s play roles in aging and age-related diseases.

Interference Reduction Biosensing Strategy for Highly Sensitive microRNA Detection

MicroRNAs are potential biomarkers for human cancers and other diseases due to their roles as post-transcriptional regulators for gene expression. However, the detection of miRNAs by conventional methods such as RT-qPCR, in situ hybridization, northern blot-based platforms, and next-generation sequencing is complicated by short length, low abundance, high sequence homology, and susceptibility to degradation of miRNAs. In this study, we developed a nicking endonuclease-mediated interference reduction rolling circle amplification (NEM-IR-RCA) strategy for the ultrasensitive and highly specific detection of miRNA-21. This method exploits the advantages of the optical properties of long-lived iridium(III) probes, in conjunction with time-resolved emission spectroscopy (TRES) and exponential rolling circle amplification (E-RCA). Under the NEM-IR-RCA-based signal enhancement processes, the limit of detection of miRNA-21 was down to 0.0095 fM with a linear range from 0.05 to 100 fM, which is comparable with the conventional RT-qPCR. Unlike RT-qPCR, the strategy was performed at a lower and constant temperature without heating/cooling cycles and reverse transcription. The strategy could clearly discriminate between matched and mismatched targets, demonstrating high specificity. Moreover, the potential application of this method was demonstrated in cancer cells and mouse serum samples, showing good agreement with RT-qPCR results. Apart from miRNA-21 detection, this platform could be also adapted for detecting other miRNAs, such as let-7a and miRNA-22, indicating its excellent potential for biomedical research and clinical diagnostics.

Ruthenium(II) Polypyridyl Complexes and Their Use as Probes and Photoreactive Agents for G-quadruplexes Labelling

Due to their optical and electrochemical properties, ruthenium(II) polypyridyl complexes have been used in a wide array of applications. Since the discovery of the light-switch ON effect of [Ru(bpy)2dppz]2+ when interacting with DNA, the design of new Ru(II) complexes as light-up probes for specific regions of DNA has been intensively explored. Amongst them, G-quadruplexes (G4s) are of particular interest. These structures formed by guanine-rich parts of DNA and RNA may be associated with a wide range of biological events. However, locating them and understanding their implications in biological pathways has proven challenging. Elegant approaches to tackle this challenge relies on the use of photoprobes capable of marking, reversibly or irreversibly, these G4s. Indeed, Ru(II) complexes containing ancillary π-deficient TAP ligands can create a covalently linked adduct with G4s after a photoinduced electron transfer from a guanine residue to the excited complex. Through careful design of the ligands, high selectivity of interaction with G4 structures can be achieved. This allows the creation of specific Ru(II) light-up probes and photoreactive agents for G4 labelling, which is at the core of this review composed of an introduction dedicated to a brief description of G-quadruplex structures and two main sections. The first one will provide a general picture of ligands and metal complexes interacting with G4s. The second one will focus on an exhaustive and comprehensive overview of the interactions and (photo)reactions of Ru(II) complexes with G4s.

Metal complexes as optical probes for DNA sensing and imaging

Transition and lanthanide metal complexes have rich photophysical properties that can be used for cellular imaging, biosensing and phototherapy. One of the applications of such luminescent compounds is the detection and visualisation of nucleic acids. In this brief review, we survey the recent literature on the use of luminescent metal complexes (including Re^I, Ru^II, Os^II, Ir^III, Pt^II, Eu^III and Tb^III) as DNA optical probes, including examples of compounds that bind selectively to non-duplex DNA topologies such as quadruplex, i-motif and DNA mismatches. We discuss the applications of metal-based luminescent complexes in cellular imaging, including time-resolved microscopy and super-resolution techniques. Their applications in biosensing and phototherapy are briefly mentioned in the relevant sections.

Recent Progress and Development of G-Quadruplex-Based Luminescent Assays for Ochratoxin A Detection

Ochratoxin A (OTA) is a mycotoxin that is widespread throughout the world. It contaminates foods such as vegetables, fruits, and rice. It harms human health and has potential carcinogenic effects. The G-quadruplex (G4) is a tetraplexed DNA structure generated from guanine-rich DNA that has found emerging use in aptamer-based sensing systems. This review outlines the status of OTA contamination and conventional detection methods for OTA. Various G4-based methods to detect OTA developed in recent years are summarized along with their advantages and disadvantages compared to existing approaches.

Novel cyclometalated iridium(iii) phosphine-imine (P^N) complexes: highly efficient anticancer and anti-lung metastasis agents in vivo

Iridium(iii)-based complexes with phosphine-imine (P^N) ligands are synthesized and authenticated. The combined treatment with Ir(iii) and BIX01294 potently inhibited tumour growth and lung metastasis in vitro and in vivo.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Targeting G-Rich DNA Structures with Photoreactive Bis-Cyclometallated Iridium(III) Complexes

The synthesis and characterisation of three novel iridium(III) bis-cyclometallated complexes is reported. Their photophysics have been fully characterised by classical methods and revealed charge-transfer (CT) and ligand-centred (LC) transitions. Their ability to selectively interact with G-quadruplex telomeric DNA over duplex DNA has been studied by circular dichroism (CD), bio-layer interferometry (BLI) and surface plasmon resonance (SPR) analyses. Interestingly, one of the complexes was able to promote photoinduced electron transfer (PET) with the guanine DNA base, which in turn led to oxidative damage (such as the formation of 8-oxoguanine) to the telomeric sequence. To the best of our knowledge, this is the first study of highly photo-oxidising bis-cyclometallated iridium(III) complexes with G-quadruplex telomeric DNA.

A ratiometric fluorescent assay for the detection and bioimaging of alkaline phosphatase based on near infrared Ag2S quantum dots and calcein

Herein, a ratiometric fluorescent method was developed for alkaline phosphatase (ALP) detection based on near-infrared (NIR) Ag₂S quantum dots (QDs) and calcein through the competitive approach. The system based on Ag₂S QDs and calcein shows green (maximum emission at 512 nm from calcein) and near infrared (NIR) fluorescence (maximum 798 nm from Ag₂S QDs) under the same excitation wavelength (468 nm). In the presence of Ce³⁺, the fluorescence intensity of calcein is decreased due to static quenching, while the fluorescence intensity of Ag₂S QDs is enhanced through aggregation induced emission (AIE). The p-nitrophenyl phosphate is hydrolyzed by ALP, and the yield phosphate ions bind with Ce³⁺ with higher affinity than these of Ag₂S QDs and calcein. Therefore, the green fluorescence from calcein is recovered while NIR fluorescence from Ag₂S QDs is decreased. On the basis of these findings, a ratiometric fluorescence assay was developed for the measurement of ALP activity. The ratio of fluorescence intensity at 512 and 798 nm (F₅₁₂/F₇₉₈) was well associated with the ALP concentration ranging from 2 to 100 mU/mL with the detection limit of 1.28 mU/mL. The method was successfully applied for detecting ALP in human serum with an acceptable recovery and bioimaging intracellular ALP with good performance. In addition, the approach was also employed for the screening ALP inhibitor.

Ultrasensitive detection of the androgen receptor through the recognition of an androgen receptor response element and hybridization chain amplification

An ultrasensitive electrochemical detection of the androgen receptor (AR) was developed based on the protection of a DNA duplex by the AR from restriction endonuclease-mediated digestion and a subsequent hybridization chain reaction (HCR). Two partially complementary DNA probes P1 and P2 were designed to form an androgen receptor binding probe (ARBP) through hybridization. The ARBP contains a duplex at one end and two single-stranded tails at the other end. The duplex part containing the recognition sites of the AR and NspI restriction endonuclease was immobilized on an Au electrode, whereas the single-stranded parts served as capture probes to activate the HCR. In the absence of the AR, NspI can cleave the duplex and release the capture probes, and thus, no HCR occurs. However, the AR can bind to the ARBP and protect the duplex from cleavage; therefore, the capture probes can trigger the HCR between four carefully designed G-quadruplex forming hairpin probes and the capture probes, resulting in the formation of numerous G-quadruplexes. Finally, differential pulse voltammetry (DPV) was carried out to quantify the AR. The assay revealed a detection limit of 7.64 fM. The verification of its high specificity and practicability in serum samples indicated its potential applications in the fields of clinical examination and disease diagnosis.

Development of a Luminescent Dinuclear Ir(III) Complex for Ultrasensitive Determination of Pesticides

To improve the G-quadruplex specificity of Ir(III) complexes, a novel dinuclear Ir(III) complex (Din Ir(III)-1) was designed and synthesized through connecting two mononuclear Ir(III) complexes via a diphenyl bridge. Din Ir(III)-1 presents 3.4-4.1-fold enhancements for G-quadruplex relative to ssDNA and 4.3-5.3-fold enhancements relative to dsDNA in luminescence intensity, respectively, demonstrating an excellent G-quadruplex selectivity. Ascribed to its superior specificity to G-quadruplex, Din Ir(III)-1 was employed to construct a highly sensitive luminescent pesticides' detection platform. The detection is based on acetylcholinesterase (AChE)-catalyzed hydrolysis product-induced DNA conformational transformation and subsequent terminal deoxynucleotidyl transferase (TdT) directed G-quadruplex formation. The assay exhibited a linear response between the emission intensity of Din Ir(III)-1 and the pesticide concentration in the range of 0.5-25 μg/L ( R² = 0.994), and the limit of detection for the pesticide was as low as 0.37 μg/L when using aldicarb as the model pesticide. Moreover, this strategy demonstrates good applicability for the pesticide detection in real samples. It is also versatile for the detection of other organophosphate or carbamate pesticides, which have the inhibition ability toward AChE. Therefore, the proposed approach is scalable for practical application in food safety and environmental monitoring fields and will provide promising solutions for the assay of pesticide residues.

Determination of hypoxia-inducible factor-1 by using a ratiometric colorimetric test based on click-mediated growth of gold nanoparticles

The authors describe a significantly improved colorimetric nanoprobe for the determination of transcription factors (TFs). It is making use of click-mediated growth of gold nanoparticles (AuNPs) to amplify the signal-to-noise ratio. Hypoxia-inducible factor-1 (HIF-1) is an important TF that acts as a mediator of cell response to hypoxia. So, the detection of HIF-1 was chosen as the model analyte. Specifically, target HIF-1 is designed to bind to the hypoxia response element within DNA duplex. The click chemistry between the DNA duplex and alkynyl-functionalized AuNPs (AF-AuNPs) is then inhibited because of significant steric hindrance. As a result, the AF-AuNPs grow into larger-sized highly-aggregated irregular nanostructures, which in turn enable colorimetric determination. The ratio of absorbances at 620 and 560 nm increases in the 0.5 to 10 nM HIF-1 concentration range, and the detection limit is 0.27 nM. This is better by a factor of 100 than that of aggregation-based colorimetric assays. The nanoprobe is selective and can be used in complex samples. Conceivably, it may also be extended to the determination of other TFs by simply changing the used DNA duplex. Graphical abstract Schematic of a nanoprobe for detecting hypoxia-inducible factor-1 (HIF-1). Three concepts are involved: the binding of HIF-1 and hypoxia response element, the Cu⁺-catalyzed click chemistry between P1/P2 duplex and alkynyl-functionalized AuNPs (AF-AuNPs), and the AuNPs growth with hydroxylamine and HAuCl₄.

Nitrogen-doped Carbon Dots Mediated Fluorescent on-off Assay for Rapid and Highly Sensitive Pyrophosphate and Alkaline Phosphatase Detection

In this report, a novel fluorescent sensing platform using nitrogen-doped carbon dots (N-CDs) as probes for fluorescence signal transmission has been designed for the detection of significant biomolecules pyrophosphate (PPi) and alkaline phosphatase (ALP). The high fluorescent N-CDs could be selectively quenched by Cu2+, and recovered by the addition of PPi because PPi preferentially binds to Cu2+. Once ALP was introduced into the system, ALP can specifically hydrolyze PPi into Pi, the intense fluorescence of N-CDs could be quenched again due to the recombination of the as-released Cu2+ with N-CDs. So, fluorescence of N-CDs is regulated by an ALP-triggered reaction. Based on this strategy, we demonstrated that N-CDs could serve as a very effective fluorescent sensing platform for label-free, sensitive and selective detection of PPi and ALP with low detection limit of 0.16 μM and 0.4 U/L for PPi and ALP, respectively. Moreover, the assay time is just around 0.5 min for PPi and 30 min for ALP. This developed strategy shows remarkable advantages including sensitive, rapid, simple, convenient, and low-cost and so forth. Furthermore, this method was also successfully applied to monitor ALP in human serum, which indicates its great potential for practical applications in biological and clinical diagnosis.