Ratiometric fluorescent probe based on gold nanoclusters and alizarin red-boronic acid for monitoring glucose in brain microdialysate

In-situ quantification of lipids in live cells through imaging approaches

Lipids are important molecules that are widely distributed within the cell, and they play a crucial role in several biological processes such as cell membrane formation, signaling, cell motility and division. Monitoring the spatiotemporal dynamics of cellular lipids in real-time and quantifying their concentrations in situ is crucial since the local concentration of lipids initiates various signaling pathways that regulate cellular processes. In this review, we first introduced the historical background of lipid quantification methods. We then delve into the current state of the art of in situ lipid quantification, including the establishment and utility of fluorescence imaging techniques based on sensors of lipid-binding domains labeled with organic dyes or fluorescent proteins, and Raman and magnetic resonance imaging (MRI) techniques that do not require lipid labeling. Next, we highlighted the biological applications of live-cell lipid quantification techniques in the study of in situ lipid distribution, lipid transformation, and lipid-mediated signaling pathways. Finally, we discussed the technical challenges and prospects for the development of lipid quantification in live cells, with the aim of promoting the development of in situ lipid quantification in live cells, which may have a profound impact on the biological and medical fields.

Molecularly Imprinted Nanomedicine for Anti-angiogenic Cancer Therapy via Blocking Vascular Endothelial Growth Factor Signaling

The VEGF-VEGFR2 (VEGF = vascular endothelial growth factor) signaling has been a promising target in cancer therapy. However, because conventional anti-angiogenic therapeutics suffer from drawbacks, particularly severe side effects, novel anti-angiogenic strategies are much needed. Herein, we report the rational engineering of VEGF-targeted molecularly imprinted polymer nanoparticles (nanoMIP) for anti-angiogenic cancer therapy. The anti-VEGF nanomedicine was prepared via a state-of-the-art molecular imprinting approach using the N-terminal epitope of VEGF as the template. The nanoMIP could target the two major pro-angiogenic isoforms (VEGF165 and VEGF121) with high affinity and thereby effectively block the VEGF-VEGFR2 signaling, yielding a potent anti-angiogenic effect of "killing two birds with one stone". In vivo experiments demonstrated that the anti-VEGF nanoMIP effectively suppressed tumor growth via anti-angiogenesis in a xenograft model of human colon carcinoma without apparent side effects. Thus, this study not only proposes an unprecedented anti-angiogenic strategy for cancer therapy but also provides a new paradigm for the rational development of MIPs-based "drug-free" nanomedicines.

Biosensors with Boronic Acid-Based Materials as the Recognition Elements and Signal Labels

It is of great importance to have sensitive and accurate detection of cis-diol-containing biologically related substances because of their important functions in the research fields of metabolomics, glycomics, and proteomics. Boronic acids can specifically and reversibly interact with 1,2- or 1,3-diols to form five or six cyclic esters. Based on this unique property, boronic acid-based materials have been used as synthetic receptors for the specific recognition and detection of cis-diol-containing species. This review critically summarizes the recent advances with boronic acid-based materials as recognition elements and signal labels for the detection of cis-diol-containing biological species, including ribonucleic acids, glycans, glycoproteins, bacteria, exosomes, and tumor cells. We also address the challenges and future perspectives for developing versatile boronic acid-based materials with various promising applications.

A turn-off fluorimetric -aptasensor for early detection of apoptosis inside the cells

To detect cytochrome c (Cyt c) as an important biomarker of apoptosis inside the cells, a simple, label-free, fluorometric detection method has been presented. For this purpose, an aptamer/gold nanocluster probe (Aptamer@AuNCs) was produced which could specifically bind to Cyt c leading to fluorescence quenching of AuNCs. The developed aptasensor showed two linear ranges of 1-80 μM and 100-1000 μM and a detection limit of 0.77 μM and 297.5 μM, respectively. This platform was successfully used to assay Cyt c release inside the apoptotic cells and their cell lysate. Aptamer@AuNC due to its enzyme-like properties could replace antibodies in Cyt c detection by conventional blotting techniques.

Near Infrared-II Photothermal and Colorimetric Synergistic Sensing for Intelligent Onsite Dietary Myrosinase Profiling

Myrosinase (Myr) is a type of critical β-thioglucosidase enzyme activator essential for sustaining many functional foods to perform their health-promoting functions. An accurate and reliable Myr test is meaningful for food quality and dietary nutrition assessments, whereas the currently reported methods do not guarantee specificity and have high reliance on instrumentation, which are not suitable for rapid and onsite Myr screening especially in complex systems from various sources. Herein, we present a unique NIR-II absorption-based photothermal-responsive colorimetric biosensor for anti-interference onsite Myr determination and realization of rapid visualized outputs with the aid of smartphone calculation. Typically, assisted by glucose oxidase (GOx), Myr specifically converts the sinigrin substrate into hydrogen peroxide (H₂O₂) that can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) catalyzed by AuNPs to form a charge transfer complex (CTC) with NIR-II absorption and photothermal characters. Delightfully, such a proposed method is able to determine Myr within a wide range of 0 to 172.5 mU/mL with a detection limit down to 2.96 mU/mL. Moreover, simple, rapid, and real-time visual Myr identification in actual food-sourced samples could also be readily achieved by smartphone readout processing, with the promising advantages of anti-interference, high accuracy, and low cost as well as labor-saving and intelligence engagement, thus providing great feasibility for precise measurement in complex and dynamic dietary sample analysis. Overall, our proposed method presents a novel technology for onsite dietary Myr enzyme profiling, which is promising to be applied in the food industry for nutritional composition profiles, freshness evaluation, and quality assessment.

Water-Soluble Photoluminescent Adenosine-Functionalized Gold Nanoclusters as Highly Sensitive and Selective Receptors for Riboflavin Detection in Rat Brain

Simple, selective, and sensitive detection of cerebral riboflavin is of great significance due to the vital roles of riboflavin in physiological and pathological processes. In the work, water-soluble photoluminescent adenosine-functionalized gold nanoclusters (Ade-AuNCs) are exploited as highly sensitive and selective receptors for cerebral riboflavin detection. The Ade-AuNCs are prepared under aqueous conditions by the one-step "synthesis-functionalization integration" strategy, using chloroauric acid as gold precursors and adenosine as outer-shell ligands. During the Ade-AuNCs synthesis process, adenosine and ascorbic acid are demonstrated to respectively serve as a stabilizer and a reductant, and citrate buffer plays multiple roles including a pH regulator, reductant, and complexing agent. The added riboflavin causes photoluminescence quenching of Ade-AuNCs, and the quenching photoluminescence is applied for well quantifying riboflavin in the range of 0.005-0.1 nM with a detection limit of 0.002 nM. The detailed analytical characterizations reveal that the photoluminescence quenching results from the static photoinduced electron transfer process from the surface functional Ade-AuNCs to riboflavin and the strong affinity between Ade-AuNCs and riboflavin. Moreover, the Ade-AuNC-based sensor exhibits a high selectivity for riboflavin over metal ions, anions, amino acids, and biological substances that possibly exist in the rat brain. Finally, by coupling the microdialysis technique, the proposed sensor is successfully applied to detect riboflavin in living rat brain microdialysates with a basal value of 13.1 ± 2.5 nM (n = 3), and the results are comparable well with those from a reference high-performance liquid chromatography method.

Glucose determination in human serum by applying inner filter effect quenching mechanism of upconversion nanoparticles

Accurate blood glucose determination is essential to the clinical diagnosis and management of diabetes. This work establishes an inner filter effect (IFE) strategy between upconversion nanoparticles (UCNPs) and quinone-imine complex for glucose monitoring in human serum simply and efficiently. In this system, the enzyme glucose oxidase (GOx) catalyzes the reaction of glucose into hydrogen peroxide (H₂O₂) and gluconic acid when compulsion by oxygen. In the presence of horseradish peroxidase (HRP), the produced H₂O₂ can catalytically oxidize phenol and 4-amino antipyrine (4-AAP) to generate quinone-imine products. The purple-colored quinone-imine complex effectively absorbed the fluorescence of NaYF₄:Yb³⁺, Er³⁺ UCNPs, leading to the strong fluorescence quenching of UCNPs through IFE. Thus, a new approach was established for glucose monitoring by determining the fluorescence intensity. Under the optimal condition, this approach shows better linearity to glucose from 2-240 μmol/L with a low detection limit at 1.0 μmol/L. Owing to the excellent fluorescence property and background-free interference of the UCNPs, the biosensor was applied for glucose measurements in human serum and got a satisfactory result. Furthermore, this sensitive and selective biosensor revealed great potential for the quantitative analysis of blood glucose or different kinds of H₂O₂-involved biomolecules for the application of clinical diagnosis.

Water-Dispersible Gold Nanoclusters: Synthesis Strategies, Optical Properties, and Biological Applications

Atomically precise gold nanoclusters (AuNCs) are an emerging class of quantum-sized nanomaterials. Intrinsic discrete electronic energy levels have endowed them with fascinating electronic and optical properties. They have been widely applied in the fields of optoelectronics, photovoltaics, catalysis, biochemical sensing, bio-imaging, and therapeutics. Nevertheless, most AuNCs are synthesized in organic solvents and do not disperse in aqueous solutions; this restricts their biological applications. In this review, we focus on the recent progress in the preparation of water-dispersible AuNCs and their biological applications. We first review different methods of synthesis, including direct synthesis from hydrophilic templates and indirect phase transfer of hydrophobic AuNCs. We then discuss their photophysical properties, such as emission enhancement and fluorescence lifetimes. Next, we summarize their latest applications in the fields of biosensing, biolabeling, and bioimaging. Finally, we outline the challenges and potential for the future development of these AuNCs.

Charge Neutralization Strategy to Construct Salt-Tolerant and Cell-Permeable Nanoprobes: Application in Ratiometric Sensing and Imaging of Intracellular pH

Intracellular pH homeostasis is essential for the survival and function of biological cells. Negatively charged molecular probes, such as pyranine (HPTS), tend to exhibit poor salt tolerance and unsatisfactory cell permeability, limiting their widespread use in intracellular assays. Herein, we explored a charge neutralization strategy using multicharged cationic nanocarriers for an efficient and stable assembly with the pH-sensitive HPTS. Through immobilization and neutralization with poly(allylamine hydrochloride)-stabilized red-emitting gold nanoclusters (PAH-AuNCs), the resulting nanoprobes (HPTS-PAH-AuNCs) offered improved salt tolerance, satisfactory cell permeability, and dual-emission properties. The fluorescence ratio exhibited a linear response over the pH range of 3.0-9.0. Moreover, the proposed HPTS-PAH-AuNCs were successfully applied to determine and visualize lysosomal pH variations in living cells, which indicated great potential for biosensing and bioimaging applications in living systems. Benefiting from the charge neutralization strategy, various types of probes can be expected to achieve broader analytical applications.

Design of a dual-signal sensing platform for d-penicillamine based on UiO-66-NH2 MOFs and APBA@Alizarin Red

Herein, we designed a diversified sensing platform for d-penicillamine based on amino-functionalized Zr-based metal-organic frameworks (UiO-66-NH₂ MOFs) and 3-aminophenylboronic acid (APBA)@Alizarin Red (ARS). The boronic acid group of 3-aminophenylboronic acid could react with Alizarin Red to form an APBA@ARS complex with a yellow fluorescence emission at 580 nm and ultraviolet absorption at 435 nm. APBA@ARS can greatly quench the fluorescence of UiO-66-NH₂ MOFs at 450 nm via fluorescence resonance energy transfer (FRET). When copper ions were present in the reaction system of APBA and Alizarin Red, the copper ions could complex with Alizarin Red to prevent the generation of APBA@ARS, and the absorption of Cu@ARS at 530 nm occurred. Thus, the absorbance of APBA@ARS at 435 nm declined, restoring the fluorescence of UiO-66-NH₂ MOFs. Nevertheless, when d-penicillamine and copper ions coexist in the APBA and Alizarin Red reaction system, the copper ions would complex with the sulfhydryl group of d-penicillamine and no longer hinder the generation of APBA@ARS, and the fluorescence of UiO-66-NH₂ MOFs is quenched again. Meanwhile, the absorbance of APBA@ARS at 435 nm enhanced and the absorbance at 530 nm decreased. Thus, a fluorescence and colorimetric dual-signal sensing platform was constructed for d-penicillamine detection, which could detect d-penicillamine in the 1-20 μM and 2-50 μM ranges with the limit of detection (LOD) values of 0.46 μM and 1.38 μM, respectively. Furthermore, this sensing platform could also realize the intelligent RGB detection via mobile phones due to the obvious color change of the reaction system.

Recent advances in templated synthesis of metal nanoclusters and their applications in biosensing, bioimaging and theranostics

As a kind of promising nanomaterials, metal nanoclusters (MNCs) generally composed of several to hundreds of metal atoms have received increasing interest owing to their unique properties, such as ultrasmall size (<2 nm), fascinating physical and chemical properties, and so on. Recently, template-assisted synthesis of MNCs (e.g., Au, Ag, Cu, Pt and Cd) has attracted extensive attention in biological fields. Up to now, various templates (e.g., dendrimers, polymers, DNAs, proteins and peptides) with different configurations and spaces have been applied to prepare MNCs with the advantages of facile preparation, controllable size, good water-solubility and biocompatibility. Herein, we focus on the recent advances in the template-assisted synthesis of MNCs, including the templates used to synthesize MNCs, and their applications in biosensing, bioimaging, and disease theranostics. Finally, the challenges and future perspectives of template-assisted synthesized MNCs are highlighted. We believe that this review could not only arouse more interest in MNCs but also promote their further development and applications by presenting the recent advances in this area to researchers from various fields, such as chemistry, material science, physiology, biomedicine, and so on.

The chain-like Au/carbon dots nanocomposites with peroxidase-like activity and their application for glucose detection

The nanocomposites with highly synergistic effect show important potential application as nanozymes. Herein, the chain-like Au/carbon dots (CDs) (GCDs) nanocomposites were prepared by self-assembly method. The negatively charged Au nanoparticles (NPs) and positively charged CDs were connected by the electrostatic interaction. Then, the electron transfer between Au and CDs induces the strong catalytic effect of GCDs nanocomposites. The cross-linking reaction occurred between amino groups and carboxyl groups on the surface of CDs, which led to form the chain-like Au aggregation surrounded by carbon shells. By FDTD simulation, the aggregation of Au NPs may enhance the electromagnetic field so that the surface-enhanced Raman scattering (SERS) signal can be increased based on GCDs nanocomposites as substrate. Otherwise, the GCDs nanocomposites can also be used to catalyze the oxidation of colourless3,3^',5,5'-tetramethylbenzidine (TMB) to blue oxTMB in the presence of H₂O₂, which displays the enhanced peroxidase-like activity compared with alone Au NPs or CDs. The obvious oxidation process of TMB molecules may be monitored by the change of SERS signal during the catalytic reaction. On this basis, GCDs nanocomposites can be further used for detection of glucose. The detection level of glucose is obtained as low as 5 × 10^-7 M. Therefore, this provides a method to detect the glucose based on GCDs nanocomposites as an enzyme mimic.

Layer-by-Layer Assembly of Electroactive Dye/LDHs Nanoplatelet Matrix Film for Advanced Dual Electro-optical Sensing Applications

It proved that the most destructive effects of the toxic Al3+ ion on the human nervous system and disease that are involved with this system, such as Alzheimer's. The development of solid-state electrodes is still in its infancy during the sensor-based detection methods for Al3+. Hence, in this study, a novel flexible ITO/PET-based electrochemical solid-state sensor was designed and constructed. Modification of the surface of electrode bedding was done by layer-by-layer (LbL) assembly of Mg-Al LDH. nanoplatelets along with alizarin red S (ARS) in an interconnected matrix film. In the molecular design of sensing base of the electrode, the electroactive organic units (ARS molecules) present in the ITO/PET-layered (ARS/LDHs)n matrix are involved in electrochemical reactions when exposed to the target molecule (Al3+ ion), so the electrochemical changes of the new formed Al-chelated system are detectable. This type of sensor is used for sensitive and selective detection of Al3+. The minimum sheet resistance, morphology and high electrocatalytic activity of the modified matrix film are obtained in the fifth cycle of LbL assembly technique. In this electrochemical sensor, both electrochemical and optical methods were detected with high sensitivity and selectivity of Al3+, so that in a cyclic voltammetry electrochemical method, the lower detection limit of 10.1 nM with a linear range of [0.2-120 μM] was obtained compared to the fluorescence-based optical method.

β-Lactoglobulin amyloid fibril-templated gold nanoclusters for cellular multicolor fluorescence imaging and colorimetric blood glucose assay

β-Lactoglobulin amyloid fibril (BLGF)-capped gold nanoclusters (Au NCs) with red, green and blue emissions were fabricated via pH-dependent reduction strategy. The BLGF-Au NCs exhibited 3.2 times enhancement of fluorescence (λex = 500 nm, λem = 684 nm), a significant 42 nm red shift, a 11.57% quantum yield and a 1.4 μs decay time compared with native β-lactoglobulin (BLG)-stabilized Au NCs. Meanwhile, the multicolor Au NCs were employed for cell imaging via incubation with A549 cells for 14 h. According to the Michaelis-Menten equation, the kinetic parameters of the BLGF-Au NCs showed a lower Km value (66 μmol L-1) for 3,3,5,5-tetramethylbenzidine (TMB) and a higher vmax (3.74 × 10-8 M s-1) for H2O2, which are comparable with other artificial nanoenzymes and natural peroxidases. Based on the highly intrinsic peroxidase-like activity of the BLGF-Au NCs, a colorimetric method was developed for glucose determination with a detection limit of 1.5 μmol L-1 by determining the variation of the absorption at 652 nm, ranging from 5 to 100 μmol L-1. In addition, the glucose assay method also revealed a 101.02 to 104.16% recovery in a real human serum sample.

Carbon dots based ratiometric fluorescent sensing platform for food safety

Food safety has become a major global concern and the rapid detection of food nutritional ingredients and contaminants has aroused much more attention. Nanomaterials-based fluorescent sensing holds great potential in designing highly sensitive and selective detection strategies for food safety analysis. Carbon dots (CDs) possess tremendous prospects in fluorescent sensing food ingredients and contaminants due to their superior properties of chemical and photostability, highly fluorescence with tunability, and no/low-toxicity. Numerous endeavors are demanded to contribute to overcoming the challenge of lower sensitivity and selectivity of the sensors interfered by various components in intricate food matrices to ensure food safety and human health. Nanohybrid CDs based ratiometric fluorescent sensing with self-calibration is regarded as an efficient strategy for the CDs based sensors for the specific recognition of target analyte in the food matrices. This work is devoted to reviewing the development of nanohybrid CDs based ratiometric fluorescent sensing platform and the perspectives of the platform for food safety. The applications of nanohybrid CDs in sensing are summarized and the sensing mechanisms are briefly discussed.

Detection of Glucose in Human Serum Based on Silicon Dot Probe

Background:

Diabetes Mellitus (DM) is a major public metabolic disease that influences 366 million people in the world in 2011, and this number is predicted to rise to 552 million in 2030. DM is clinically diagnosed by a fasting blood glucose that is equal or greater than 7 mM. Therefore, the development of effective glucose biosensor has attracted extensive attention worldwide. Fluorescence- based strategies have sparked tremendous interest due to their rapid response, facile operation, and excellent sensitivity. Many fluorescent compounds have been employed for precise analysis of glucose, including quantum dots, noble metal nanoclusters, up-converting nanoparticles, organic dyes, and composite fluorescent microspheres. Silicon dot as promising quantum dots materials have received extensive attention, owing to their distinct advantages such as biocompatibility, low toxicity and high photostability.

Methods:

MnO2 nanosheets on the Si nanoparticles (NPs) surface serve as a quencher. Si NPs fluorescence can make a recovery by the addition of H2O2, which can reduce MnO2 to Mn2+, and the glucose can thus be monitored based on the enzymatic conversion of glucose by glucose oxidase to generate H2O2. Therefore, the glucose concentration can be derived by recording the fluorescence recovery spectra of the Si NPs.

Results:

This probe enabled selective detection of glucose with a linear range of 1-100 μg/mL and a limit of detection of 0.98 μg/mL. Compared with the commercial glucometer, this method showed favorable results and convincing reliability.

Conclusion:

We have developed a novel method based on MnO2 -nanosheet-modified Si NPs for rapid monitoring of blood glucose levels. By combining the highly sensitive H2O2/MnO2 reaction with the excellent photostability of Si NPs, a highly sensitive, selective, and cost-efficient sensing approach for glucose detection has been designed and applied to monitor glucose levels in human serum with satisfactory results.

Fluorescent Gold Nanoclusters for Biosensor and Bioimaging Application

With the rapid development of materials technology, fluorescent gold nanoclusters (AuNCs) are emerging as novel functional materials for diagnostic applications including the detection of biomarkers and bioimaging due to the advantages of their ultra-small size, tunable emissions, size-dependent fluorescence and excellent biocompatibility. In this review, we introduced the synthetic methods, and physical and chemical properties of AuNCs. Subsequently, we described the AuNCs-based design strategies for the detection of biomarkers including small molecules, DNA and proteins. The applications of AuNCs for tumor imaging in vitro and in vivo were also presented. Finally, we discussed the challenges and potential solutions of AuNCs-based nanosensors.

A strategy for visual optical determination of glucose based on a smartphone device using fluorescent boron-doped carbon nanoparticles as a light-up probe

Boronic acid-doped carbon nanoparticles were prepared and are shown to undergo aggregation induced emission (AIE). The nanoparticle composite is a viable fluorescent probe for glucose determination by using the RGB technique and a smartphone. The structure and the chemical composition of the doped carbon nanoparticles were confirmed by SEM, TEM, FTIR and UV-vis spectroscopy. The combination of 4-carboxyphenylboronic acid with o-phenylenediamine and rhodamine B endowed the hybrid with high fluorescence intensity (quantum yield 46%). Compared with conventional two-step preparation of boronic acid-based fluorescent probes for glucose, the present one step synthesis strategy is simpler and more effective. The addition of glucose causes the formation of covalent bonds between the cis-diols group of glucose molecules and boronic acid moiety. Fluorescent intensity can be quantified using dual wavelengths simultaneously, where both increases, as the target analytes bind to the bronic acid. These variations was monitored by the smartphone camera, and the green channel intensities of the colored images were processed by using the RGB option of a smartphone. The assay works in the 32 μM to 2 mM glucose concentration range and has an 8 μM detection limit. The method was successfully used for the assay of glucose in diluted human serum. Graphical abstractThe fluorometric method was developed for determination of glucose using boron doped carbon nanoparticles (BCNBs). The BCNPs aggregate after covalent binding between the cis-diols of glucose and boronic acid. The green channel of the images is recorded by a smartphone camera.

The synthesis of metal nanoclusters and their applications in bio-sensing and imaging

Noble metal nanomaterials have been studied by many researchers for their ultra-small size, excellent photophysical properties and good biocompatibility. Metal nanoclusters are a kind of nanoscale ultrafine particle, which have completely different properties from macroscopic metals. In the visible region, they do not usually show the characteristic surface plasmon resonance absorption but instead show fluorescence in the visible to near infrared region. In particular, the noble metallic (Au, Ag, Cu, etc) nanoclusters (NMNCs) have broad application prospects in the field of biomedicine as probes for fluorescence sensing. Their strong photoluminescence, living cell compatibility, and easy availability make up for the shortcomings of traditional fluorescent probes such as organic fluorescent dyes, fluorescent proteins, and fluorescent quantum dots. In this review, we summarize the synthetic method and application of metal nanoclusters as fluorescent probes in bio-sensing and imaging, especially in the early diagnosis of cancer cells.

Ratiometric fluorescent nanoprobes for visual detection: Design principles and recent advances - A review

Signal generation techniques for visual detection of analytes have received a great deal of attention in various sensing fields. These approaches are considered to be advantageous when instrumentation cannot be employed, such as for on-site assays, point-of-care tests, and he althcare diagnostics in resource-constrained areas. Amongst various visual detection approaches explored for non-invasive quantitative measurements, ratiometric fluorescence sensing has received particular attention as a potential method to overcome the limitations of intensity-based probes. This technique relies on changes in the intensity of two or more emission bands (induced by an analyte), resulting in an effective internal referencing which improves the sensitivity of the detection. The self-calibration, together with the unique optophysical properties of nanoparticles (NPs) have made the ratiometric fluorescent nanoprobes more sensitive and reliable, which in turn, can result in more precise visual detection of the analytes. Over the past few years, a vast number of ratiometric sensing probes using nanostructured fluorophores have been designed and reported for a wide variety of sensing, imaging, and biomedical applications. In this work, a review on the NP-based ratiometric fluorescent sensors has been presented to meticulously elucidate their development, advances and challenges. With a special emphasis on visual detection, the most important steps in the design of fluorescent ratiometric nanoprobes have been given and based on different classes of analytes, recent applications of fluorescent ratiometric nanoprobes have been summarized. The challenges for the future use of the technique investigated in this review have been also discussed.

Rapid synthesis of Au/Ag bimetallic nanoclusters with highly biochemical stability and its applications for temperature and ratiometric pH sensing

Herein, we developed a simple and rapid strategy to synthesize gold/silver bimetallic nanoclusters (Au/Ag NCs) with highly biochemical stability by a one-pot route. The Au/Ag NCs were obtained via a chemical reduction procedure in alkaline aqueous solution at 75 °C within only 20 min by employing bovine serum albumin (BSA) as both ligand and reductant. The as-obtained Au/Ag NCs displayed bright orange fluorescence with an emission peak located at 570 nm and temperature-dependent fluorescence property, which were utilized as fluorescent thermometer directly. More intriguingly, the Au/Ag NCs were very stable against various pH values, ions, biothiols, H₂O₂, fetal bovine serum (FBS), RPMI 1640 medium and amino acids. Taking advantage of the excellent biochemical stability, a ratiometric fluorescence biosensor, fluorescein-5-isothiocyanate (FITC)-Au/Ag NCs, was constructed for pH sensing based on the incorporation of FITC into the Au/Ag NCs. Furthermore, the ratiometric pH sensor was also successfully applied on the model of HeLa cells.

Development of ratiometric sensing and white light-harvesting materials based on all-copper nanoclusters

Herein, we developed a special strategy for the fast sensitization of red emitting copper nanoclusters with the assistance of green emitting copper nanoclusters. Compared to most previous methods based on AIE, which do not maintain the water solubility or tiny size of nanoclusters, the charming features of the copper nanoclusters were retained after the fabrication. Furthermore, the product was employed for the detection of sulphide, which revealed its ratiometric sensing ability in water since the ratio of the intensity change for green and red emission was related to the sulphide concentration. In addition, after the addition of Zn²⁺, the green and red emission was either enhanced or quenched via the corresponding mechanism. This enables the facile fabrication of promising white light-harvesting materials since the species of the emitting color can be simply tuned.

"ON-OFF-ON" fluorescent probes based on nitrogen-doped carbon dots for hypochlorite and bisulfite detection in living cells

Although hypochlorite (ClO-) and bisulfite (HSO3-) play important roles in the biological immune system and the aging process of living organisms, they are also classified as a third type of carcinogens. Hence, a convenient and efficient method to monitor ClO- and HSO3- in ecological environments is highly desired. In this article, an "ON-OFF-ON" fluorescent probe based on nitrogen-doped carbon dots (N-CDs) for the detection of ClO- and HSO3- has been demonstrated successfully. Because of the destruction of the surface passivation layer, the fluorescence of the N-CDs was quenched by ClO-. Furthermore, the quenched fluorescence of the N-CDs was restored efficiently through the increase in conjugation from the attached sulfo groups, indicating the feasibility of ClO- and HSO3- detection. This fluorescent probe exhibited excellent sensitivity and selectivity to ClO- and HSO3- detection with the limits of detection (LODs) of 3.4 μM and 0.27 μM in aqueous solution, respectively. In addition, the as-prepared N-CDs were successfully applied to detect both ClO- and HSO3- in living cells due to their low toxicity and fast response speed.

From inorganic precipitation to organic aggregation: solubility product constant-mediated specific metal-ion lighting-up using a triazolium iodide organic fluorescence tag

Innovation in sensing strategies is a continual goal pursued by analytical chemists.

Fluorescent Neoglycoprotein Gold Nanoclusters: Synthesis and Applications in Plant Lectin Sensing and Cell Imaging

Carbohydrate-protein interactions mediate fundamental biological processes, such as fertilization, cell signaling, or host-pathogen communication. However, because of the enormous complexity of glycan recognition events, new tools enabling their analysis or applications emerge in recent years. Here, we describe the first preparation of neoglycoprotein functionalized fluorescent gold nanoclusters, containing a biantennary N-glycan G0 as targeting molecule, ovalbumin as carrier/model antigen, and a fluorescent gold core as imaging probe (G0-OVA-AuNCs). Subsequently, we demonstrate the utility of generated G0-OVA-AuNCs for specific sensing of plant lectins and in vitro imaging of dendritic cells.

Gold-carbon dots for the intracellular imaging of cancer-derived exosomes

As a novel fluorescent nanomaterial, gold-carbon quantum dots (GCDs) possess high biocompatibility and can be easily synthesized by a microwave-assisted method. Owing to their small sizes and unique optical properties, GCDs can be applied to imaging of biological targets, such as cells, exosomes and other organelles. In this study, GCDs were used for fluorescence imaging of exosomes. Tumor-specific antibodies are attached to the GCDs, forming exosome specific nanoprobes. The nanoprobes can label exosomes via immuno-reactions and thus facilitate fluorescent imaging of exosomes. When incubated with live cells, exosomes labeled with the nanoprobes can be taken up by the cells. The intracellular experiments confirmed that the majority of exosomes were endocytosed by cells and transported to lysosomes. The manner by which exosomes were taken up and the intracellular distribution of exosomes are unaffected by the GCDs. The experimental results successfully demonstrated that the presented nanoprobe can be used to study the intrinsic intracellular behavior of tumor derived exosomes. We believe that the GCDs based nanoprobe holds a great promise in the study of exosome related cellular events, such as cancer metastasis.

Mesoporous Iron Oxide Synthesized Using Poly(styrene-b-acrylic acid-b-ethylene glycol) Block Copolymer Micelles as Templates for Colorimetric and Electrochemical Detection of Glucose

Herein, we report the soft-templated preparation of mesoporous iron oxide using an asymmetric poly(styrene-b-acrylic acid-b-ethylene glycol) (PS-b-PAA-b-PEG) triblock copolymer. This polymer forms a micelle consisting of a PS core, a PAA shell, and a PEG corona in aqueous solutions, which can serve as a soft template. The mesoporous iron oxide obtained at an optimized calcination temperature of 400 °C exhibited an average pore diameter of 39 nm, with large specific surface area and pore volume of 86.9 m² g^-1 and 0.218 cm³ g^-1, respectively. The as-prepared mesoporous iron oxide materials showed intrinsic peroxidase-like activities toward the catalytic oxidation of 3,3',5,5'-tertamethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂). This mimetic feature was further exploited to develop a simple colorimetric (naked-eye) and electrochemical assay for the detection of glucose. Both our colorimetric (naked-eye and UV-vis) and electrochemical assays estimated the glucose concentration to be in the linear range from 1.0 μM to 100 μM with a detection limit of 1.0 μM. We envisage that our integrated detection platform for H₂O₂ and glucose will find a wide range of applications in developing various biosensors in the field of personalized medicine, food-safety detection, environmental-pollution control, and agro-biotechnology.

Fluorescence Enhancement of Terminal Amine Assembled on Gold Nanoclusters and Its Application to Ratiometric Lysine Detection

Ratiometric fluorescent sensors have emerged as an attractive tool for analytical sensing and optical imaging due to their providing a built-in self-calibration for environmental effects. However, cumbersome processes of nanoparticles modified with fluorophores for constructing traditional ratiometric sensors limit their further application. Herein, we report a facile and label-free strategy for constructing a ratiometric sensor based on an aggregation-induced-emission (AIE)-active amine-terminated small molecule on the surface of gold nanoclusters (AuNCs). Intrinsic fluorescence of the terminal primary amine of the small molecule lysine resulting from AIE was first observed in the presence of glutathione-stabilized gold nanoclusters (GSH-AuNCs). Using lysine as both the fluorophore and the analyte, the synthesized GSH-AuNCs showed a good lysine-responsive ratiometric property. The AIE-active dual-emitting fluorescence property of the GSH-AuNCs/lysine complex made it feasible to achieve ratiometrically detection of the analyte without conjugated fluorogen. This AIE-active GSH-AuNC-based biosensor possesses high selectivity, rapid response, and excellent photostability. Moreover, the strategy opens a new pathway for the construction of a label-free ratiometric fluorescent sensor with various applications.

Turn-on fluorescent sensor for the detection of glucose using manganese dioxide-phenol formaldehyde resin nanocomposite

Monitoring blood glucose has attracted considerable attention because diabetes mellitus is a global public health problem. Herein, we reported a turn-on fluorescence detection strategy based on manganese dioxide (MnO₂)-phenol formaldehyde resin (PFR) nanocomposite for rapid, sensitive, and selective detection of glucose levels in human blood. In this biosensing system, MnO₂ nanoshell on the PFR nanoparticle surfaces serve as a quencher. PFR fluorescence can make a recovery in the presence of H₂O₂, reducing MnO₂ to Mn²⁺. The sensor shows a linear range from 50nM to 90μM with a low detection limit of 20nM for H₂O₂ detection. Thus, the glucose can be detected on the basis of the enzymatic conversion of glucose by glucose oxidase to produce H₂O₂. This method exhibits a wide linear range from 5μM to 1mM with a low detection limit of 1.5μM. Because of the excellent photostability offered by PFR, the developed strategy has been successfully applied for the diagnosis of diabetes mellitus in human blood samples. Compared with commercial glucometer, our method showed satisfactory results, indicating the significant reliability. The developed turn-on fluorescent sensor might hold great promise in nanomedicine and bioanalysis.

A Self-Assembled Ratiometric Polymeric Nanoprobe for Highly Selective Fluorescence Detection of Hydrogen Peroxide

In this study, a dual-emission fluorescence resonance energy transfer (FRET) polymeric nanoprobe by single-wavelength excitation was developed for sensitive and selective hydrogen peroxide (H₂O₂) detection. Polymeric nanoprobe was prepared by simple self-assembly of functional lipopolymers, which were 4-carboxy-3-fluorophenylboronic acid (FPBA)-modified DSPE-PEG (DSPE-PEG-FPBA) and 7-hydroxycoumarin (HC)-conjugated DSPE-PEG (DSPE-PEG-HC). Subsequent binding of alizarin red S (ARS) to FPBA endowed the nanoprobe with a new fluorescence emission peak at around 600 nm. Because of the perfect match of the fluorescence emission spectra of HC with the absorbance spectra of ARS-FPBA, FRET was achieved between them. The sensing strategy for H₂O₂ was based on H₂O₂-induced deboronation reaction and boronic acid-mediated ARS fluorescence. Interaction between phenylboronic acid and ARS was revisited herein and it was found that electron-donating or -withdrawing group on phenylboronic acid (PBA) has significant influence on the fluorescence property of ARS, which enabled sensitive and selective H₂O₂ sensing. The nanoprobe displayed two well-separated emission bands (150 nm), providing high specificity and sensitivity for ratiometric detection of H₂O₂. Further application was exploited for the determination of glucose and the results demonstrated that the proposed strategy showed ratiometric response capability for glucose detection. The current method does not involve complicated organic synthesis and opens a new avenue for the construction of multifunctional polymeric fluorescent nanoprobe.

One-step synthesis of nitrogen, boron co-doped fluorescent carbon nanoparticles for glucose detection

Heteroatom-doped carbon nanoparticles (CNPs) have attracted considerable attention due to an effective improvement in their intrinsic properties. Here, a facile and simple synthesis of nitrogen, boron co-doped carbon nanoparticles (NB-CNPs) from a sole precursor, 3-aminophenylboronic acid, was performed via a one-step solid-phase approach. Because of the presence of boronic acid, NB-CNPs can be used directly as a fluorescent probe for glucose. Based on a boronic acid-triggered specific reaction, we developed a simple NB-CNP probe without surface modification for the detection of glucose. When glucose was introduced, the fluorescence of NB-CNPs was suppressed through a surface-quenching states mechanism. Obvious fluorescence quenching allowed the highly sensitive determination of glucose with a limit of detection of 1.8 μM. Moreover, the proposed method has been successfully used to detect glucose in urine from people with diabetes, suggesting potential application in sensing glucose.

Rational Design of a Stimuli-Responsive Polymer Electrode Interface Coupled with in Vivo Microdialysis for Measurement of Sialic Acid in Live Mouse Brain in Alzheimer's Disease

Sensitive and selective monitoring of sialic acid (SA) in cerebral nervous system is of great importance for studying the role that SA plays in the pathological process of Alzheimer's disease (AD). In this work, we first reported an electrochemical biosensor based on a novel stimuli-responsive copolymer for selective and sensitive detection of SA in mouse brain. Notably, through synergetic hydrogen-bonding interactions, the copolymer could translate the recognition of SA into their conformational transition and wettability switch, which facilitated the access and enrichment of redox labels and targets to the electrode surface, thus significantly improving the detection sensitivity with the detection limit down to 0.4 pM. Besides amplified sensing signals, the proposed method exhibited good selectivity toward SA in comparison to potential interference molecules coexisting in the complex brain system due to the combination of high affinity between phenylboronic acid (PBA) and SA and the directional hydrogen-bonding interactions in the copolymer. The electrochemical biosensor with remarkable analytical performance was successfully applied to evaluate the dynamic change of SA level in live mouse brain with AD combined with in vivo midrodialysis. The accurate concentration of SA in different brain regions of live mouse with AD has been reported for the first time, which is beneficial for progressing our understanding of the role that SA plays in physiological and pathological events in the brain.