In Situ Synthesized Silver Nanoclusters for Tracking the Role of Telomerase Activity in the Differentiation of Mesenchymal Stem Cells to Neural Stem Cells

Fluorescent metal nanoclusters: From luminescence mechanism to applications in enzyme activity assays

Metal nanoclusters (MNCs) have outstanding fluorescence property and biocompatibility, which show widespread applications in biological analysis. Particularly, evaluation of enzyme activity with the fluorescent MNCs has been developed rapidly within the past several years. In this review, we first introduced the fluorescent mechanism of mono- and bi-metallic nanoclusters, respectively, whose interesting luminescence properties are mainly resulted from electron transfer between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Meanwhile, the charge migration within the structure occurs through ligand-metal charge transfer (LMCT) or ligand-metal-metal charge transfer (LMMCT). On such foundation, diverse enzyme activities were rigorously evaluated, including three transferases and nine hydrolases, in turn harvesting rapid research progresses within past 5 years. Finally, we summarized the design strategies for evaluating enzyme activity with the MNCs, presented the major issues and challenges remained in the relevant research, coupled by showing some improvement measures. This review will attract researchers dedicated to the studies of the MNCs and provide some constructive insights for their further applications in enzyme analysis.

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Cu2+-mediated turn-on fluorescence biosensor based on DNA-templated silver nanoclusters for label-free and sensitive detection of adenosine triphosphate

A Cu²⁺-mediated turn-on fluorescence biosensor based on the DNA-templated green-emitting silver nanoclusters (DNA@g-AgNCs) was developed for label-free and sensitive detection of adenosine 5'-triphosphate (ATP). Cu²⁺ was able to quench the bright green fluorescence of DNA@g-AgNCs because of the coordination and photoinduced electron transfer between DNA@g-AgNCs and Cu²⁺. Therefore, a unique and effective fluorescence biosensor can be constructed with the formation of DNA@g-AgNCs/Cu²⁺/ATP ternary-competition system. With the introduction of ATP, the DNA@g-AgNCs/Cu²⁺ fluorescence sensing system will be disrupted and the fluorescence of DNA@g-AgNCs was recovered due to higher affinity of ATP towards Cu²⁺. On the basis of this feature, the DNA@g-AgNCs/Cu²⁺ fluorescence sensing system demonstrated quantitative determination of ATP in the range 0.05 - 3 μM and a detection limit of 16 nM. Moreover, the fluorescence sensing system was successfully applied to the quantitative determination of ATP in human urine and serum samples with recoveries ranging from 98.6 to 106.5%, showing great promise to provide  a label-free, cost-efficient, and rapid platform for ATP-related clinical disease diagnosis.

Ultrasensitive fluorescent detection of telomerase activity based on tetrahedral DNA nanostructures as carriers for DNA-templated silver nanoclusters

Precise evaluation of telomerase activity is essential for the clinical diagnosis of early tumors. Herein, we have ingeniously designed a tetrahedral DNA nanostructure, with hairpin-shaped DNA probes rich in cytosine bases at four vertices for telomerase detection. The DNA-templated silver nanoclusters can be formed after the addition of Ag. Then the introduction of telomerase adds the single-strand TTAGGG extension, which can "turn on" the fluorescence of silver nanoclusters quickly by the proximity of the resulting guanine-rich sequences to silver nanoclusters and realize accurate detection of telomerase activity. In this study, integration of high stability tetrahedral DNA nanostructure and fluorescence signal amplification of four DNA-templated silver nanoclusters offers the advantage of high sensitivity, with a low detection limit of 1 cell. More than that, this method is low-cost, facile, and feasible for practical clinical applications.

In situ self-assembled Ag-Fe3O4 nanoclusters in exosomes for cancer diagnosis

Recently, exosomes have gained attention as an effective tool for early cancer detection. Almost all types of cells release exosomes, making them substantially important for disease diagnosis. In this study, we have utilized HepG2 cancer cells for the in situ biosynthesis of silver and iron oxide nanoclusters (NCs) from their respective salts (i.e., AgNO3 and FeCl2, respectively) in the presence of glutathione (GSH). The self-assembled biosynthesized silver and iron NCs were readily loaded on exosomes as payloads and secreted into the cell culture medium. The cargo loaded exosomes were then isolated and characterized by electron microscopy for nano-silver and iron oxide NC confirmation. Ag NCs have potential as a fluorescent probe and Fe3O4 NCs as a contrast agent for CT and MRI. Furthermore, these isolated exosomes from HepG2 cancer cells have a significant influence on cellular uptake and cell viability when exposed to both HepG2 and U87 cancer cells. These findings demonstrate that the biocompatible nature of these self-assembled NCs loaded on exosomes could be utilized to bioimage cancer in the initial stages through fluorescence imaging.

Functionalized nanoprobes for in situ detection of telomerase

Telomerase, a special ribonucleoprotein reverse transcriptase, can maintain the length and stability of telomeres and plays an important role in cell proliferation and differentiation. Due to the distinguishable expression level in normal cells and cancer cells, telomerase has become an important biomarker for cancer diagnosis and prognosis evaluation. Despite major breakthroughs in the field of telomerase detection, the extracts in the cell lysate are still the first choice as the analyte nevertheless, which will bring serious inaccuracies compared with the real intracellular activity. With the development of nanotechnology and nanomaterials, extraordinary progress has been made in telomerase detection by employing different versatile nanoprobes. In this review, we list the superiority of nanoprobes and systematically summarize the applications of nanoprobes in telomerase detection from the aspects of various nanomaterials and discuss the current challenges and potential trends in the future design of nanoprobes.

Biocompatible exosomes nanodrug cargo for cancer cell bioimaging and drug delivery

Therapy against cancer remains a daunting issue for human health, despite remarkable innovations in many areas of pathology. In situ biosynthesized nanoclusters bestow a novel remedy for carcinogenic cell imaging. Exosomes have received special attention as an efficient tool for the diagnosis of various diseases, including cancers. All types of cells (healthy or diseased) generate exosomes, making them significantly unique for relevant disease diagnosis and treatment. In this contribution, we exploit the possibility of utilizing the exosomes to facilitate chemotherapeutics, viz. the combination of doxorubicin (Dox) and biosynthesized silver nanoclusters in cancer cells. Our study showed a new facile way for bioimaging of cancer cells using biosynthesized silver-DNA nanoclusters, and thus further targeting cancer cells using the relevant cancer exosomes as drug delivery cargo. After isolating exosomes from neoplastic cells, i.e. HeLa, loaded with the drug, and treating other neoplastic cells with cargo-loaded isolated exosomes, we found that cargo-loaded isolated exosomes can readily enter into the targeted cancer cells and efficiently kill these neoplastic cells. This raises the possibility of acting as a novel facile modality for target cancer theranostics with high efficiency and biocompability.

Target-Driven Nanozyme Growth in TiO2 Nanochannels for Improving Selectivity in Electrochemical Biosensing

Nanozymes have been used in colorimetric and electrochemical sensing because of their low cost and high stability. However, the wide applications of nanozymes in sensing devices are largely limited due to their poor selectivity. In this study, unlike traditional methods using prepared nanozymes for target detection, we designed a target-driven nanozyme growth strategy in TiO₂ nanochannels to detect analytes. Using telomerase as an example, the established recognition event was used to expand the photocatalytic activity of TiO₂ to visible-light region, thus triggering Prussian blue nanoparticle (PBNP) growth in visible light. Benefiting from the peroxidase (POD)-like activity of PBNPs, the uncharged 3,5,3',5'-tetramethylbenzidine (TMB) is oxidized to positively charged oxTMB, which induces significant ionic transport changes in nanochannels, and thus in turn provides information about telomerase activity. Such a nanozyme-triggered sensing system exhibited excellent performance in telomerase detection in urine specimens from patients with bladder cancer. This innovative target-driven signal generation strategy might provide a new method for applying nanozymes in developing sensitive, rapid, and accurate biological sensing systems.

DNA-Templated Silver Nanocluster/Porphyrin/MnO2 Platform for Label-Free Intracellular Zn2+ Imaging and Fluorescence-/Magnetic Resonance Imaging-Guided Photodynamic Therapy

Developing a theranostic platform that integrates diagnosis and treatment in one single nanostructure is necessary for efficient tumor treatment. Here, we presented a novel theranostic nanoprobe for nonlabeled fluorescence imaging of Zn²⁺ and 635 nm red light-triggered photodynamic therapy (PDT) by a multifunctional DNA-templated silver nanocluster/porphyrin/MnO₂ nanoplatform. MnO₂ nanosheets adsorbed hairpin DNA-silver nanoclusters (AgNCs) and porphyrin (P) by facile physisorption, which accelerate the transfection of nanoprobes and P into tumor cells. After entering the cells, the biodegradation of MnO₂ nanosheets by glutathione and acidic hydrogen peroxide released AgNCs for label-free Zn²⁺ fluorescence imaging by the hairpin DNA-fueled dynamic self-assembly of three-way DNA junction architectures, and the released Mn²⁺ could act as an effective magnetic resonance imaging (MRI) contrast agent. In addition, MnO₂ was decomposed in the acidic H₂O₂-ample environment and produced O₂ to overbear hypoxia-related PDT resistance, highly efficient PDT was obtained by excess singlet oxygen (¹O₂) release of P-AgNCs-MnO₂ nanoprobes under light irradiation compared with free P. In vitro and in vivo studies confirmed that P-AgNCs-MnO₂ exhibited high fluorescence specificity, excellent PDT effect, and good biocompatibility and could be used as a contrast agent for MRI. This theranostic platform provided a new avenue for the fluorescence and MRI diagnosis of tumors and efficient tumor treatment.

Advances in optical assays for detecting telomerase activity

Telomerase uses its RNA as template and its protein unit as reverse transcriptase to synthesize TTAGGG repeats at the ends of the eukaryotic chromosome to maintain the lengths of telomeres. Telomerase activity up-regulates in about 85% of human tumors compared with somatic cells, which indicates that telomerase is a tumor biomarker. Reliable assay of telomerase activity is thus essential in diagnosis and management of malignant tumors. In this review, recent developed optical assays are summarized based on the readout signal, including chemiluminescence assay, colorimetric assay, and fluorescence assay.

Fluorescent Ag clusters conjugated with anterior gradient-2 antigen aptamer for specific detection of cancer cells

Anterior gradient protein 2 homolog (AGR) is a potential tumor biomarker and plays an important role in tissue development and regeneration. The intracellular detection of AGR is rarely reported. By conjugating the AGR aptamer with a cytosine base sequence as Ag cluster template, a highly fluorescent probe (MA@AgNCs) was synthesized for targeting intracellular AGR. The MA@AgNCs display a maximum fluorescence peak at 565 nm, and possess an excellent quantum yield (QY = 87.43%), small size, great biocompatibility, low toxicity, and good stability. Moreover, the as synthesized MA@AgNCs show high specificity on recognizing breast cancer cells.

Gold Nanobipyramids as Dual-Functional Substrates for in Situ "Turn On" Analyzing Intracellular Telomerase Activity Based on Target-Triggered Plasmon-Enhanced Fluorescence

Herein, we developed a novel plasmon-enhanced fluorescence (PEF)-based telomerase-responsive nanoprobe for in situ fluorescence "turn on" visualization of telomerase activity in live cells. The as-prepared nanoprobe was composed of a nicked molecular beacon (which contains Cy5.5-labeled hairpin-DNA sequences hybridized with telomerase primers)-functionalized gold nanobipyramids (Au NBPs). Au NBPs were selected as both fluorescence resonance energy-transfer and PEF dual-functional substrates, while DNA was selected to be the precise spacer to manage the interval between the Au NBPs and Cy5.5. On the basis of this target-triggered PEF probe, optimal fluorescence enhancement can be obtained with 49 DNA bases, which was higher than gold nanorods. The proposed method accomplishes sensitive telomerase activity detection down to 23 HeLa cells with a dynamic range of 40-1200 HeLa cells. On the basis of this, in situ fluorescence imaging of telomerase activity in live cells and real-time analysis of the variation in intracellular telomerase activity can be achieved. Moreover, cancer cells and normal cells can also be successfully discriminated even in their co-cultured mixtures, indicating promising potential in clinical diagnoses.