Novel Synthesis of Thiolated Gold Nanoclusters Induced by Lanthanides for Ultrasensitive and Luminescent Detection of the Potential Anthrax Spores' Biomarker

A novel fluorescent sensing platform for glutathione based on Förster resonance energy transfer and aggregation-induced emission

Glutathione (GSH) plays essential roles in anti-oxidation and detoxification within the human body. An imbalance in its concentration can lead to serious health conditions. Therefore, accurate monitoring of GSH is critical for maintaining human health. In this study, we present a novel GSH detection method that enhances the fluorescence of α-lipoic acid-functionalized gold nanoclusters (LA@Au NCs) through aggregation induced by zinc and nitrogen co-doped carbon dots (Zn@N-CDs). Additionally, the fluorescence of Zn@N-CDs (donor) decreases upon adding LA@Au NCs (acceptor), indicating Förster resonance energy transfer (FRET) between them. In the presence of GSH, complexation with Zn2+ on the N-CD surface disrupts both the aggregation induced emission (AIE) and FRET mechanisms. This disruption leads to the restoration of N-CD fluorescence while simultaneously quenching the fluorescence of LA@Au NCs. Under optimized conditions, the fluorescence response ratio (F465/F670) is directly proportional to the concentration of GSH within a linear dynamic range of 0.1-90 µM, with a detection limit (S/N = 3) of 0.03 µM. This novel combination paves the way for the development of fluorescent probes for detecting various molecules and biomolecules.

Dual-Function Strategy for Enhanced Quercetin Detection Using Terbium(III) Ion-Bound Gold Nanoclusters

The engineering of metal nanoclusters (NCs) that exhibit bright emissions and high sensing performance under physiological conditions is still a formidable challenge. In this study, we report a novel design strategy for realizing excellent performance metal NC-based probes by leveraging both concerted proton-coupled electron transfer (PCET) and photoinduced electron transfer (PET) mechanisms, with terbium(III) (Tb3+) ions serving as a key modulator. Our findings indicate that the binding of Tb3+ ions to the 6-aza-2-thiothymidine (ATT) ligand effectively inhibits the proton-transfer step in the concerted PCET pathway of Au10(ATT)6 NCs, giving rise to over a 10-fold enhancement in fluorescence and a quantum yield of 7.2%. Moreover, the capped Tb3+ ions on the surface of Au10(ATT)6 NCs can act as a bridge to facilitate an efficient donor-linker-acceptor type PET reaction from quercetin (Que) to the excited Au10 core by specifically interacting with the bare 3-OH group. These advancements enable the Tb3+/Au10(ATT)6 NC-based probe to achieve a significantly lower limit of detection for Que, reduced by nearly 3 orders of magnitude to 2.6 nM, while also addressing the critical difficulty of selectively detecting Que in the presence of its glycosylated analogues. This work opens new opportunities for the precise control of photoluminescence in metal NC probes at the molecular level, potentially promoting the development of next-generation metal NC-based sensing technologies.

Developing a switch "OFF-ON" fluorescent probe for detection of melamine based on doubly-protected red emissive copper nanoclusters mediated by Hg2+ ions

Melamine, often used as an adulterant in infants' formula due to its high protein content, can be harmful when ingested in large amounts, leading to the formation of cyanurate-melamine co-crystals in infants and potentially causing kidney damage. In this study, we introduce a fluorescent method for the selective and reliable detection of melamine in milk and infants' formula. The fluorescent probe comprises copper nanoclusters (Cu NCs) functionalized with thiosalicylic acid (TSA) and polyvinylpyrrolidone (PVP) as double-protecting ligands. Upon the addition of Hg2+, the fluorescence emission of TSA-PVP@Cu NCs is diminished due to static quenching. Subsequently, the fluorescence emission of the TSA-PVP@Cu NCs + Hg2+ probe is restored upon the introduction of melamine, facilitated by the coordination interaction between melamine and Hg2+ and the formation of a stable chelate between them. Under optimized conditions, the fluorescence emission was recorded initially for the TSA-PVP@Cu NCs + Hg2+ probe (F°) and after melamine addition (F). The (F/F°) ratio increased with rising melamine concentrations within the range of 0.025-65 µM. The detection limit, calculated using a signal-to-noise ratio of 3, was determined to be 8.0 nM. The TSA-PVP@Cu NCs + Hg2+ probe was successfully employed to detect melamine in milk and infants' formula, yielding acceptable recovery percentages and relative standard deviations. These results underscore the reliability and efficacy of the proposed probe for the fluorometric detection of melamine in real-world samples.

Carbon dot-ZIF-67-infused tragacanth hydrogel films: leveraging tunable luminescence through displacement mechanisms for anthrax detection

A novel method for 2,6-Pyridine dicarboxylic acid (DPA) detection is introduced using red emissive carbon dots (r-CDs) derived from amaranth leaves and Eosin Yellow (EY), encapsulated within a Co-based MOF (ZIF-67). The approach utilizes ratiometric turn-on fluorescence and visual identification of DPA through a displacement assay, where the release of EY enhances its fluorescence intensity, while the fluorescence of r-CDs remains constant. The sensing platform demonstrates a wide detection range (10.0-280.0 μM) with limits of detection of 1.7 μM and 3.3 μM. The probe offers notable advantages, including simple preparation, high yield, stability, selectivity, and rapid detection. Additionally, a composite functional hydrogel, made from Tragacanth (TG), was developed for DPA detection in real samples (tap water and urine), showcasing excellent performance. Integration with smartphone-based on-site signal recording enables clear fluorescence visual changes, highlighting the potential for practical applications. This study presents an innovative r-CDs/EY-ZIF-67-based hydrogel film for fluorescence sensing of DPA, expanding the scope of high-performance fluorescent materials for anthrax monitoring and real-world applications.

Determination of Dipicolinic Acid through the Antenna Effect of Eu(III) Coordination Polymer

Bacillus anthracis is a Gram-positive bacterium that can cause acute infection and anthracnose, which is a serious concern for human health. Determining Bacillus anthracis through its spore biomarker dipicolinic acid (DPA) is crucial, and there is a strong need for a method that is rapid, sensitive, and selective. Here, we created Eu(III)-coordination polymers (Eu-CPs) with surfaces that have abundant carboxyl and hydroxyl groups. This was achieved by using citric acid and europium nitrate hexahydrate as precursors in a straightforward one-pot hydrothermal process. These Eu-CPs were then successfully utilized for highly sensitive DPA determination. The fluorescence (FL) emission of Eu-CPs, which is typically weak due to the coordination of Eu(III) with water molecules, was significantly enhanced in the presence of DPA. This enhancement is attributed to the competitive binding between DPA's carboxyl or hydroxyl groups and water molecules. As a result, the absorbed energy of DPA, when excited by 280 nm ultraviolet light, is transferred to Eu-CPs through an antenna effect. This leads to the emission of the characteristic red fluorescence of Eu3+ at 618 nm. A strong linear relationship was observed between the enhanced FL intensity and DPA concentration in the range of 0.5-80 μM. This relationship allowed for a limit of detection (LOD) of 15.23 nM. Furthermore, the Eu-CPs we constructed can effectively monitor the release of DPA from Bacillus subtilis spores, thereby further demonstrating the potential significance of this strategy in the monitoring and management of anthrax risk. This highlights the novelty of this approach in practical applications, provides a valuable determination technique for Bacillus anthracis, and offers insights into the development cycle of microorganisms.

Raspberry-like Nanoheterostructures Comprising Glutathione-Capped Gold Nanoclusters Grown on the Lanthanide Nanoparticle Surface

Bare lanthanide-doped nanoparticles (LnNPs), in particular, NaYF4:Yb3+,Tm3+ NPs (UCTm), have been seeded in situ with gold cations to be used in the subsequent growth of gold nanoclusters (AuNCs) in the presence of glutathione (GSH) to obtain a novel UCTm@AuNC nanoheterostructure (NHS) with a raspberry-like morphology. UCTm@AuNC displays unique optical properties (multiple absorption and emission wavelengths). Specifically, upon 350 nm excitation, it exhibits AuNC photoluminescence (PL) (500-1200 nm, λmax 650 nm) and Yb emission (λmax 980 nm); this is the first example of Yb sensitization in a UCTm@AuNC NHS. Moreover, under 980 nm excitation, it displays (i) upconverting PL of the UCTm (at the blue, red and NIR-I, ca. 800 nm, regions); (ii) two-photon PL of AuNC; and (iii) down-shifting PL of thulium (around 1470 nm). The occurrence of energy transfer from UCTm to AuNCs in the UCTm@AuNC NHS was evidenced by the drastic lengthening of the AuNC PL lifetime (τPL) (from few hundred nanoseconds to more than one hundred microseconds). Initial biological assessment of UCTm@AuNC NHSs in vitro revealed high biocompatibility and bioimaging capabilities upon near-infrared excitation.

Amorphous amEu-NH2BDC and amTb-NH2BDC as ratio fluorescence probes for smartphone-integrated naked eye detection of bacillus anthracis biomarker

The abnormal concentration of anthrax spore biomarker 2,6-pyridinedicarboxylic acid (2,6-DPA) will seriously affect public health. Therefore, a sensitive and rapid assay for 2,6-DPA monitoring is of vital importance. In this work, novel nano-sized amorphous Eu-NH2BDC (amEu-NH2BDC) and amorphous Tb-NH2BDC (amTb-NH2BDC) metal organic frameworks are prepared by adjusting the ratio of metal and ligand, respectively. Both of them exhibit highly sensitive and selective ratiometric fluorescence detection for 2,6-DPA with wider linear range and lower detection limit in aqueous solutions and human serum. Attributed to the coordination effect of 2,6-DPA in triggering the characteristic fluorescence emissions of Eu3+or Tb3+ by replacing coordinated solvent molecules, as evidenced by ultraviolet-visible spectroscopy, the fluorescence lifetimes analysis, thermal gravimetric analysis, Fourier-transform infrared spectroscopy, density functional theory (DFT) simulations and X-ray photoelectron spectroscopy. In addition, the amEu-NH2BDC or amTb-NH2BDC loaded paper-based microsensors are constructed for real-time and sensitive detection of 2,6-DPA and coupled with a smartphone-assisted visual portable device.

Development of a fluorescent sensor based on TPE-Fc and GSH-AuNCs for the detection of organophosphorus pesticide residues in vegetables

A novel dual-signal fluorescent sensor was developed for detecting organophosphorus pesticides (OPs). It relies on the catalytic activities of acetylcholinesterase (AChE) and choline oxidase (ChOx) to generate hydrogen peroxide (H2O2) through the conversion of acetylcholine (ACh) to choline·H2O2 then oxidizes ferrocene-modified tetraphenylethylene (TPE-Fc) to its oxidized state (TPE-Fc+), resulting in enhanced cyan fluorescence due to aggregation. Simultaneously, ferrocene oxidation generates hydroxyl radicals (•OH), causing a decrease in orange fluorescence of glutathione-synthesized gold nanoclusters (GSH-AuNCs). The presence of OPs restricts AChE activity, reducing H2O2 production. Increasing OPs concentration leads to decreased cyan fluorescence and increased orange fluorescence, enabling visual OPs detection. The sensor has a linear dynamic range of 10-2000 ng/mL with a detection limit of 2.05 ng/mL. Smartphone-based color identification and a WeChat mini program were utilized for rapid OPs analysis with successful outcomes.

Ce4+/Ce3+ as the switch of AIE-copper nanoclusters for highly selective detection of ascorbic acid in soft drinks

An ultrasimple "turn-on" sensor for indirectly detecting ascorbic acid (AA) was prepared using N-acetyl-L-cysteine stabilized copper nanoclusters (NAC-CuNCs) via the AIE (aggregation-induced emission) effect controlled by Ce4+/Ce3+ redox reaction. This sensor fully utilizes the different properties of Ce4+ and Ce3+. Non-emissive NAC-CuNCs were synthesized by a facile reduction method. NAC-CuNCs easily aggregate in the presence of Ce3+ due to AIE, resulting in fluorescence enhancement. However, this phenomenon cannot be observed in the presence of Ce4+. Ce4+ possesses strong oxidizing ability and produces Ce3+ by reacting with AA via a redox reaction, followed by switching on the luminescence of NAC-CuNCs. Moreover, the fluorescence intensity (FI) of NAC-CuNCs increases with the concentration of AA in the range of 4-60 µM, with the limit of detection (LOD) as low as 0.26 µM. This probe with excellent sensitivity and selectivity was successfully used in the determination of AA in soft drinks.

A highly stable chain-based EuIII metal-organic framework as a turn-on and blue-shift luminescent sensor for dipicolinic acid

The development of a rapid and selective method for the identification of dipicolinic acid (DPA), a specific biomarker in Bacillus anthracis spores, is of great importance for the avoidance of anthrax infection. Herein, a chain-based Eu^III metal-organic framework with the formula {[Eu₃(BTDB)₃(μ₃-OH)₃(H₂O)]·solvents}_n (JXUST-38, H₂BTDB = (benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid) was obtained using 2-fluorobenzoic acid as the pH regulator. JXUST-38 exhibits good chemical and thermal stability and can specifically recognize DPA in N,N-dimethylformamide solution through luminescence enhancement and blue-shift effects with a detection limit of 0.05 μM. Furthermore, the significant luminescence enhancement and blue shift under UV lamps are obviously observable by the naked eye. The luminescence sensing mechanism is attributed to absorbance-induced enhancement between JXUST-38 and DPA. Test paper and mixed-matrix membrane based on JXUST-38 are designed for DPA detection. In addition, the feasibility of using JXUST-38 in biosensing is discussed in detail.

Zirconium-Directed Supramolecular Self-Assembly of Coenzyme A@GNCs with Enhanced Phosphorescence for Developing Ultrasensitive Tracer Probe of Dipicolinic Acid, a Biomarker of Bacterial Spores

Luminescent gold nanoclusters (GNCs) are a class of attractive quantum-sized nanomaterials bridging the gap between organogold complexes and gold nanocrystals. They typically have a core-shell structure consisting of a Au(I)-organoligand shell-encapsulated few-atom Au(0) core. Their luminescent properties are greatly affected by their Au(I)-organoligand shell, which also supports the aggregation-induced emission (AIE) effect. However, so far, the luminescent Au nanoclusters encapsulated with the organoligands containing phosphoryl moiety have rarely been reported, not to mention their AIE. In this study, coenzyme A (CoA), an adenosine diphosphate (ADP) analogue that is composed of a bulky 5-phosphoribonucleotide adenosine moiety connected to a long branch of vitamin B5 (pantetheine) via a diphosphate ester linkage and ubiquitous in all living organisms, has been used to synthesize phosphorescent GNCs for the first time. Interestingly, the synthesized phosphorescent CoA@GNCs could be further induced to generate AIE via the PO32- and Zr4+ interactions, and the observed AIE was found to be highly specific to Zr4+ ions. In addition, the enhanced phosphorescent emission could be quickly turned down by dipicolinic acid (DPA), a universal and specific component and also a biomarker of bacterial spores. Therefore, a Zr4+-CoA@GNCs-based DPA biosensor for quick, facile, and highly sensitive detection of possible spore contamination has been developed, showing a linear concentration range from 0.5 to 20 μM with a limit of detection of 10 nM. This study has demonstrated a promising future for various organic molecules containing phosphoryl moiety for the preparation of AIE-active metal nanoclusters.

Light-driven uranyl-organic frameworks used as signal-enhanced photoelectrochemical sensors for monitoring anthrax

The semiconductor-like characteristics and light absorption ability of metal-organic frameworks (MOFs) make it have the potential for photoelectrochemical sensing. Compared with composite and modified materials, the specific recognition of harmful substances directly using MOFs with suitable structures can undoubtedly simplify the fabrication of sensors. Herein, two photosensitive uranyl-organic frameworks (UOFs) named HNU-70 and HNU-71 were synthesized and explored as the novel "turn-on" photoelectrochemical sensors, which can be directly applied to monitor the biomarker of anthrax (dipicolinic acid). Both sensors have good selectivity and stability towards dipicolinic acid with the low detection limits of 1.062 and 1.035 nM, respectively, which are far lower than the human infection concentration. Moreover, they exhibit good applicability in the real physiological environment of human serum, demonstrating a good application prospect. Spectroscopic and electrochemical studies show that the mechanism of photocurrent enhancement results from the interaction between dipicolinic acid and UOFs, which facilitates the photogenerated electron transport.

Determination of 2, 6-dipicolinic acid as an Anthrax biomarker based on the enhancement of copper nanocluster fluorescence by reversible aggregation-induced emission

The weak fluorescence efficiency of copper nanoclusters (Cu NCs) limits their wide applications in biosensing and bioimaging areas, while the aggregation-induced emission (AIE) effect is anticipated to increase their luminescence intensity. Herein, the weak red emission of Cu NCs is increased considerably by the addition of lanthanide Tb³⁺, ascribed to the AIE effect. Monitoring of spores contamination can be carried out by determining the level of 2, 6-dipicolinic acid (DPA), which is a marker of spores. Due to the stronger synergy between DPA and Tb³⁺ for its clamped configuration of adjacent pyridine nitrogen group with the carboxylic acid group, the addition of DPA leads Tb³⁺ to be taken away from Cu NCs through a stronger coordination effect, causing Cu NCs to return to the dispersed state and weakened fluorescence. Based on this, an "off-on-off" fluorescent probe for DPA sensing was built, in which Tb³⁺ was used as a bridge to achieve AIE enhanced fluorescence effect on Cu NCs as well as a specific recognizer of DPA. The detection range for DPA was 0.1-60 μM and the detection limit was 0.06 μM, which was much lower than the infectious dose of anthrax spores. Since DPA is a unique biomarker for bacterial spores, the method was applied to the detection of actual bacterial spores and satisfactory results were obtained with a detection limit of 4.9*10³ CFU mL^-1.

A self-designed device integrated with a Fermat spiral microfluidic chip for ratiometric and automated point-of-care testing of anthrax biomarker in real samples

A desirable lanthanide-based ratiometric fluorescent probe was designed and integrated into a self-designed Fermat spiral microfluidic chip (FS-MC) for the automated determination of a unique bacterial endospore biomarker, dipicolinic acid (DPA), with high selectivity and sensitivity. Here, a blue emission wavelength at 425 nm was generated in the Fermat spiral structure by mixing the europium (Eu³⁺) and luminol to form the Eu³⁺/Luminol sensing probe. DPA in the reservoir can be used to specifically bind to Eu³⁺ under the negative pressure and transfer energy from DPA to Eu³⁺ sequentially via an antenna effect, thus resulting in a significant increase in the red fluorescence emission peak at 615 nm. According to the fluorescence intensity ratio (F615/F425), a good linearity can be obtained with increasing the concentration of DPA from 0 to 200 μM with a limit of detection as low as 10.11 nM. Interestingly, the designed FS-MC can achieve rapid detection of DPA in only 1 min, reducing detection time and improving sensitivity. Furthermore, a self-designed device integrated with the FS-MC and a smartphone color picker APP was employed for the rapid automatic point-of-care testing (POCT) of DPA in the field, simplifying complex processes and reducing testing times, thus confirming the great promise of this ready-to-use measurement platform for in situ inspection.

Bifunctional ratiometric fluorescent probe for sensing anthrax spore biomarker and tetracycline at different excitation channels

Multifunctional fluorescent probes have received increasing attention for the sake of atom economy and high-density integration. Herein, CdTe quantum dots (QDs) modified with Eu³⁺ were synthesized as the bifunctional ratiometric fluorescent probe for sensing two hazardous substances tetracycline (TC) and anthrax spore biomarker 2,6-dipicolinic acid (DPA) at different excitation channels, based on the discrepant excitation wavelengths of Eu³⁺ and the fluorescence quenching of CdTe QDs after interaction with them. Both DPA and TC enhanced the red emission of Eu³⁺ via antenna effect but caused the green emission of CdTe QDs to quench. Interestingly, the excitation wavelengths of Eu³⁺ after coordinating with DPA and TC were 275 and 386 nm, respectively. On this basis, CdTe QDs-Eu³⁺ achieved the bifunctional ratiometric detection of DPA (λ_ex = 275 nm) and TC (λ_ex = 386 nm) with different excitation channels. Both DPA and TC were selectively detected by CdTe QDs-Eu³⁺ with rapid response (DPA-1 min, TC-1 min) and high sensitivity (DPA-LOD 0.3 μM, TC-LOD 2.2 nM). CdTe QDs-Eu³⁺ were applied to analyzing DPA and TC in food, biological and environmental samples. Satisfactory spiked recoveries (80.0-119.0 %) and relative standard deviations (0.5-8.4 %) were obtained for measuring DPA and TC in these samples.

Ce3+-induced Fluorescence Amplification of Copper Nanoclusters Based on Aggregation-induced Emission for Specific Sensing 2,6-pyridine Dicarboxylic Acid

A straightforward, cost-effective and biocompatible reduction approach was applied to fabricate soluble but non-luminous glutathione-stabilized copper nanocluster (GSH-CuNCs). Surprisingly, as high as 1 × 10³ times fluorescence enhancement was acquired when Ce³⁺ was injected at an extremely low concentration of only 18 µM. Ce³⁺ outperformed other rare-earth metal ions in terms of inducing fluorescence amplification of the non-luminous GSH-CuNCs. Furthermore, Ce³⁺ was employed as inducer for aggregation-induce emission (AIE) effect as well as reactant to coordinate with target of 2,6-pyridine dicarboxylic acid (DPA) due to the stronger coordination ability between Ce³⁺ and DPA than that of Ce³⁺ and GSH. As a result, the Ce³⁺/GSH-CuNCs ensemble was developed as a novel sensor to detect DPA in the "on-off" mode. When DPA was introduced into the sensor, Ce³⁺ failed to interact with GSH and detached from the surface of GSH-CuNCs, leading to fluorescence quenching. In addition, static quenching process and internal filtration effect (IFE) between Ce³⁺/GSH-CuNCs and DPA were also responsible for fluorescence quenching effect. A good linear relationship was obtained from 0.3 µM to 18 µM, with a limit of detection (LOD) of 0.19 µM. The as-proposed probe displayed high specificity to DPA and provided a simple, fast rapid and cheap method for construction this type of ensemble sensors to detect other targets.

Advances in Ultra-small Fluorescence Nanoprobes for Detection of Metal Ions, Drugs, Pesticides and Biomarkers

Identification of trace level chemical species (drugs, pesticides, metal ions and biomarkers) plays key role in environmental monitoring. Recently, fluorescence assay has shown significant advances in detecting of trace level drugs, pesticides, metal ions and biomarkers in real samples. Ultra-small nanostructure materials (metal nanoclusters (NCs), quantum dots (QDs) and carbon dots (CDs)) have been integrated with fluorescence spectrometer for sensitive and selective analysis of trace level target analytes in various samples including environmental and biological samples. This review summarizes the properties of metal NCs and ligand chemistry for the fabrication of metal NCs. We also briefly summarized the synthetic routes for the preparation of QDs and CDs. Advances of ultra-small fluorescent nanosensors (NCs, QDs and CDs) for sensing of metal ions, drugs, pesticides and biomarkers in various sample matrices are briefly discussed. Additionally, we discuss the recent challenges and future perspectives of ultra-small materials as fluorescent sensors for assaying of wide variety of target analytes in real samples.

One-step synthesized antimicrobial peptide-functionalized gold nanoclusters for selective imaging and killing of pathogenic bacteria

The development of multifunctional nanomaterials with bacterial imaging and killing activities is of great importance for the rapid diagnosis and timely treatment of bacterial infections. Herein, peptide-functionalized gold nanoclusters (CWR11-AuNCs) with high-intensity red fluorescence were successfully synthesized via a one-step method using CWR11 as a template and by optimizing the ratio of CWR11 to HAuCl4, reaction time, pH, and temperature. The CWR11-AuNCs bound to bacteria and exhibited selective fluorescence microscopy imaging properties, which is expected to provide a feasible method for locating and imaging bacteria in complex in vivo environments. In addition, CWR11-AuNCs not only retained the antibacterial and bactericidal activities of CWR11 but also exhibited certain inhibitory or killing effects on gram-negative and gram-positive bacteria and biofilms. The MICs of CWR11-AuNCs against Escherichia coli and Staphylococcus aureus were 178 and 89 μg/ml, respectively. Surprisingly, cell viability in the CWR11-AuNC-treated group was greater than that in the CWR11-treated group, and the low cytotoxicity exhibited by the CWR11-AuNCs make them more promising for clinical applications.

Aggregation-induced emission (AIE)-Based nanocomposites for intracellular biological process monitoring and photodynamic therapy

As a non-invasive visualization technique, photoluminescence imaging (PLI) has found its huge value in many biological applications associated with intracellular process monitoring and early and accurate diagnosis of diseases. PLI can also be combined with therapeutics to build imaging-guided theragnostic platforms for achieving early and precise treatment of diseases. Photodynamic therapy (PDT) as a quintessential phototheranostics technology has gained great benefits from the combination with PLI. Recently, aggregation-induced emission (AIE)-active materials have emerged as one of the most promising bioimaging and phototheranostic agents. Most of AIEgens, however, need to be chemically engineered to form versatile nanocomposites with improved their photophysical property, photochemical activity, biocompatibility, etc. In this review, we focus on three categories of AIE-active nanocomposites and highlight their application progresses in the intracellular biological process monitoring and PLI-guided PDT. We hope this review can guide further development of AIE-active nanocomposites and promote their practical applications for monitoring intracellular biological processes and imaging-guided PDT.

Solvo-thermal synthesis of a unique cluster-based nano-porous zinc(II) luminescent metal-organic framework for highly sensitive detection of anthrax biomarker and dichromate

In this work a flexible multi-dentate 4,4'-(1H-1,2,4-triazole-1-yl) methylene-bis(benzonic acid) (H₂L) ligand has been employed, a unique cluster-based nano-porous luminescent zinc(II) metal-organic framework {[Zn(μ₆-L)]·(DMAC)₂}_n (1) (DMAC = Dimethylacetamide) has been isolated under solvo-thermal conditions. The H₂L ligand adopts hexa-dentate coordination modes via one triazole nitrogen atom and four aromatic carboxylate oxygen atoms, which bridge the neighboring six-coordinated Zn^II centers, leading to a three-dimensional (3D) nano-porous metal organic framework. A PLATON program analysis suggests the total potential solvent area volume is 2028.9 Å³, which occupy 62.5% percent of the unit cell volume (3248.4 Å³). PXRD Patterns of the as-synthesized samples 1 have been determined confirming the purity of the bulky samples. Photo-luminescent properties indicate strong fluorescent emissions of 1 at the room temperature. Further photo-luminescent measurements show that 1 can exhibit highly sensitive real-time luminescence sensing of anthrax biomarker dipicolinic acid (DPA) with high quenching efficiency (K_SV = 1.48 × 10⁵ M^-1) and low detection limit (0.298 μM (S/N = 3)). Meanwhile 1 also exhibits highly selective and sensitive luminescence sensing for Cr₂O₇^2- ions in aqueous solutions with high quenching efficiency K_SV = 1.22 × 10⁴ L·mol^-1 and low detection limit (0.023 μM (S/N = 3)). Therefore 1 can be used a unique multi-functional 3D cluster-based metal organic material in sensitive detection and effective detection of environment pollutants and biomarker molecules.

Rapid and Reliable Excitation Wavelength-Dependent Detection of 2,6-Dipicolinic Acid Based on a Luminescent Cd(II)-Tb(III) Nanocluster

A Cd(II)-Tb(III) nanocluster {[Cd₁₀Tb₉L₈(OH)₁₆(OAc)₂₃(H₂O)₃][Cd₁₀Tb₉L₈(OH)₁₆(OAc)₂₃(H₂O)₄]}·3H₂O (1), which contains two crystallographically independent components, was constructed from a tridentate ligand (HL, 3-ethoxysalicylaldehyde). It exhibits rapid and reliable excitation wavelength-dependent luminescence response to 2,6-dipicolinic acid (DPA) [limit of detection = 0.23 nM], which is not influenced by aromatic carboxylates, amino acids, and ions. The test papers of 1 can be used to check DPA in solution. The equation I_Ex272nm/I_Ex329nm = 0.0109 × [DPA]2 + 0.106 × [DPA] + 2.39 of 1 for the luminescence response could be used to quantitatively measure the concentration of DPA in tap water. 1 displays rapid and stable luminescence response to DPA, with the sensing times shorter than 5 s and no changes for the lanthanide luminescence over 24 h.

Luminescent gold-peptide spheric aggregates: selective and effective cellular targeting

Although the restriction of intramolecular motion has been well recognized as the fundamental of aggregation induced emission enhancement (AIEE), the regulation mechanism of gold nanoclusters (AuNCs) based AIEE system are still unclear. In this paper, we have investigated the Zn²⁺-induced AIEE process of thiolate ligands (i.e., cysteine, glutathione and an 8-mer peptide) protected AuNCs, which shows a pH-dependent evolution from single AuNCs to spheric aggregates to irregular network. Using photoluminescent enhancement ratio as an index, the concept of "mid-pH" is proposed to indicate the optimal pH for the formation of spheric AuNCs aggregates. Importantly, the surface ligands allow the formation of spheric AuNCs aggregates at tunable mid-pH between 5.7 and 7.5. Owing to the appropriate size and surface peptide targetability, the spheric AuNCs aggregates can be successfully screened for targeted tumor cell uptake and imaging at physiological pH. The cell uptake mechanism study showed that AuNCs aggregates was specifically recognized by arginine-glycine-aspartic acid (RGD) sequence on the ligand and integrin αvβ3 on the cell surface, thus mainly through clathrin-mediated endocytosis. This work provides new sight to artificially regulate the construction of efficient cellular imaging probes.

I2/I--mediated fluorescence quenching of an Ag+-doped gold nanocluster-based immunoassay for sensitive detection of Escherichia coli O157:H7 in milk

Escherichia coli O157:H7 is a type of hazardous bacteria in the field of food safety. A sensitive and effective method is urgently needed to detect it, avoiding enormous harm for the human health. In this study, we synthesized stable Ag⁺-doped gold nanoclusters (Ag-AuNC) with a fluorescence intensity 4.8 times stronger than that of AuNC. It was further demonstrated that Ag⁰ existing in the AuNC core and a fraction of Ag⁺ anchored on the AuNC shell eliminated the surface defects and improved the luminescent properties of AuNC. A combination of I₂ and I^- was used to quench fluorescence-enhanced Ag-AuNC, which was first applied in ELISA for detecting E. coli O157:H7 to improve the sensitivity. In the presence of E. coli O157:H7, the biotinylated anti-E. coli O157:H7 mAb and streptavidin-alkaline phosphatase would be immobilized and catalyze l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate to produce ascorbic acid. After addition of KIO₃, I₂/I^- were generated. The I₂ could trigger oxidative etching of Ag-AuNC and I^- could combine with Ag⁺ to decrease the Ag⁺ concentration of Ag-AuNC, which resulted in fluorescence quenching of Ag-AuNC. Under optimal conditions, the linear range of I₂/I^--mediated fluorescence quenching of Ag-AuNC-based immunoassay for detecting E. coli O157:H7 was 3.3 × 10³ to 10⁶ cfu/mL, with a detection limit of 9.2 × 10² cfu/mL, 10.7-fold lower than that of the traditional ELISA. The proposed immunoassay exhibits excellent sensitivity, specificity, recovery, and accuracy, which is useful for quantitative detection of E. coli O157:H7 in food safety.

In situ synthesis of chiral AuNCs with aggregation-induced emission using glutathione and ceria precursor nanosheets for glutathione biosensing

Au(i)–SG/Ce(CO3)2 NS conjugated nanoprobe was developed for selective GSH detection. The redox reaction between GSH and the NS could release Ce3+ ions to initiate the intense AIE of Au(i)–SG oligomers.

Smart intercalation and coordination strategy to construct stable ratiometric fluorescence nanoprobes for the detection of anthrax biomarker

L@Mg-Al-Ln-LDHs (Ln = Tb, Eu) constructed by the intercalation coordination strategy exhibited a strong and stable fluorescence reference signal and achieved reliable ratiometric detection of DPA in complex environments and actual spores.

Three novel metal-organic frameworks with different coordination modes for trace detection of anthrax biomarkers

Dipicolinic acid (DPA) is an anthrax biomarker. Its serious consequences make its detection a great need. In this paper, three novel metal-organic frameworks (MOFs) with different coordination modes were synthesized by a simple solvothermal method, which can be used as highly efficient fluorescence sensors for the highly selective and sensitive trace detection of DPA. MOFs 1-3 showed rapid responses to DPA (<30 s), and the limits of detection (LODs) were calculated to be 1.01 × 10^-6 M^-1 (MOF 1), 1.17 × 10^-6 M^-1 (MOF 2) and 2.07 × 10^-6 M^-1 (MOF 3). DPA detection based on MOFs 1-3 in fetal bovine serum is highly reliable based on the high recovery rates (90% to 115%). Hence, the three MOF-based sensors can be used in the real-time detection of DPA.

Cation Crosslinking-Induced Stable Copper Nanoclusters Powder as Latent Fingerprints Marker

Luminescent copper nanoclusters (Cu NCs) have shown great potential in light-emitting devices (LEDs), chemical sensing, catalysis and biological fields. However, their practical use has been restricted by poor stability, and study on the stability of Cu NCs solid powder along with the mechanism is absent. In this study, stablized Cu NCs powder was first obtained by cation crosslinking method. Compared with the powder synthesized by solvent precipitation method, the stability of Cu NCs powder crosslinked by ionic inducer Ce3+ was enhanced around 100-fold. The storage time when the fluorescence intensity decreased to 85% (T85) was improved from 2 h to 216 h, which is the longest so far. The results of characterizations indicated that the aggregation structure was formed by the binding of Ce3+ with the capping ligands of Cu NCs, which helped in obtaining Ce-Cu NCs powder from aggregate precipitation in solution. Furthermore, this compact structure could avoid the destruction of ambient moisture resulting in long-lasting fluorescence and almost unchanged physical form. This demonstrated that phosphor, with excellent characteristics of unsophisticated synthesis, easy preservation and stable fluorescence, showed great potential in light sources, display technology and especially in latent fingerprints visualization on different substrates for forensic science.

Lanthanide upconversion and downshifting luminescence for biomolecules detection

Biomolecules play critical roles in biological activities and are closely related to various disease conditions. The reliable, selective and sensitive detection of biomolecules holds much promise for specific and rapid biosensing. In recent years, luminescent lanthanide probes have been widely used for monitoring the activity of biomolecules owing to their long luminescence lifetimes and line-like emission which allow time-resolved and ratiometric analyses. In this review article, we concentrate on recent advances in the detection of biomolecule activities based on lanthanide luminescent systems, including upconversion luminescent nanoparticles, lanthanide-metal organic frameworks, and lanthanide organic complexes. We also introduce the latest remarkable accomplishments of lanthanide probes in the design principles and sensing mechanisms, as well as the forthcoming challenges and perspectives for practical achievements.

Multifunctional nano-biosensor based on metal-organic framework for enhanced fluorescence imaging of intracellular miRNA-122 and synergistic chemo-photothermal therapy of tumor cells

A rationally designed multifunctional polydopamine (PDA)-coated metal-organic frameworks (MOFs) biosensors for detection of miRNA-122 with Zn²⁺-triggered aggregation-induced enhancement (AIE) and synergistic chem-photothermal therapy in vitro was developed for the first time. Further, it was successfully used for enhanced fluorescence imaging of miRNA-122 in living cells. The pH-responsive MOFs structure was decomposed under the influence of acidic environment, and a large amount of free Zn²⁺ was released as the trigger agent for AIE signal amplification, realizing the ultra-sensitive detection of miRNA-122 and the accurate discrimination of the cells with different expression levels of miRNA-122, with the detection limit as low as 12.5 pM. Meanwhile, ZIF-8 nanoparticles with high loading rate can effectively deliver therapeutic drugs to achieve responsive release. In addition, the modification of versatile PDA-coating provides the biosensor with a faster drug release capability and photothermal conversion performance, demonstrating its superior synergistic chem-photothermal therapy performance. It is expected to play an important role in the integration of cancer diagnosis and synergistic therapy.

Gold Nanomaterials as a Promising Integrated Tool for Diagnosis and Treatment of Pathogenic Infections-A Review

This review summarizes research on functionalized gold nanomaterials as pathogen detection sensors and pathogen elimination integrated tools. After presenting the challenge of current severe threat from pathogenic bacteria and the increasingly serious growth rate of drug resistance, the first section mainly introduces the conspectus of gold nanostructures from synthesis, characterization, physicochemical properties and applications of gold nanomaterials. The next section deals with gold nanomaterials-based pathogen detection sensors such as colorimetric sensors, fluorescence sensors and Surface-Enhanced Raman Scattering sensors. We then discuss strategies based on gold nanomaterials for eliminating pathogenic infections, such as the dual sterilization strategy for grafting gold nanomaterials with antibacterial substances, photothermal antibacterial and photodynamic antibacterial methods. The fourth part briefly introduces the comprehensive strategy for diagnosis and sterilization of pathogen infection based on gold nanomaterials, such as the diagnosis and treatment strategy for pathogen infection using Roman signals real-time monitoring and photothermal sterilization. A concluding section that summarizes the current status and challenges of the novel diagnosis and treatment integrated strategy for pathogenic infections, gives an outlook on potential future perspectives.

Amphiphilic Au Nanoclusters Modulated by Magnetic Gemini Surfactants as a Cysteine Chemosensor and an MRI Contrast Agent

Cationic magnetic Gemini surfactants (mag-G-surfs), [C₁₄H₂₉(CH₃)₂N(CH₂)₂N(CH₃)₂C₁₄H₂₉]²⁺·2[XCl₃Br]^- (14-2-14·2X, X = Ce, Gd, or Ho), efficiently induce the aggregation of glutathione-protected Au nanoclusters (NCs) (GSH-Au NCs). These magnetic luminescent aggregates not only possess aggregation-induced emission (AIE) behavior but also display aggregation-induced magnetic enhancement. In particular, 14-2-14·2Ce and 14-2-14·2Gd have a better effect on boosting the luminescence intensity, quantum yield (QY), and luminescence lifetime (τ). The luminescent aggregates of GSH-Au NCs triggered by 14-2-14·2Gd or 14-2-14·2Ho exhibit more favorable paramagnetic behavior. Other Au NCs containing a Au(I)-thiolate complex shell also exhibit the obvious AIE phenomenon after introducing 14-2-14·2Gd, demonstrating the luminescence enhancement effect of mag-G-surfs. The luminescent aggregate 14-2-14·2Ce@GSH-Au NCs can serve as a "light up" fluorometric probe to detect cysteine selectively with the detection limit (DL) of 36 μM, and the magnetic luminescent aggregate 14-2-14·2Gd@GSH-Au NCs has the potential to be a novel contrast agent in T₁-weighted magnetic resonance (MR) imaging due to its satisfactory contrasting ability.

Biomarker sensing platforms based on fluorescent metal nanoclusters

Metal nanoclusters (NCs) and their unique properties are increasing in importance and their applications are covering a wide range of areas. Their remarkable fluorescence properties and easy synthesis procedure and the possibility of functionalizing them for the detection of specific targets, such as biomarkers, make them a very interesting biosensing tool. Nowadays the detection of biomarkers related to different diseases is critical. In this context, NCs scaffolded within an appropriate molecule can be used to detect and quantify biomarkers through specific interactions and fluorescence properties of the NCs. These methods include analytical detection and biolocalization using imaging techniques. This review covers a selection of recent strategies to detect biomarkers related to diverse diseases (from infectious, inflammatory, or tumour origin) using fluorescent nanoclusters.

Rare-Earth Eu3+/Gold Nanocluster Ensemble-Based Fluorescent Photoinduced Electron Transfer Sensor for Biomarker Dipicolinic Acid Detection

The use of metal ions to bridge the fluorescent materials to target analytes has been demonstrated to be a promising way to sensor design. Herein, the effect of rare-earth ions on the fluorescence of l-methionine-stabilized gold nanoclusters (Met-AuNCs) was investigated. It was found that europium (Eu³⁺) can significantly suppress the emission of Met-AuNCs, while other rare-earth ions showed a negligible impact. The mechanism on the observed fluorescence quenching of Met-AuNCs triggered by Eu³⁺ was systematically explored, with results revealing the dominant role of photoinduced electron transfer (PET). Eu³⁺ can bind to the surface of Met-AuNCs by the coordination effect and accepts the electron from the excited Met-AuNCs, which results in Met-AuNC fluorescence suppression. After introducing dipicolinic acid (DPA), an excellent biomarker for spore-forming pathogens, Eu³⁺ was removed from the surface of Met-AuNCs owing to the higher binding affinity between Eu³⁺ and DPA. Consequently, an immediate fluorescence recovery occurred when DPA was present in the system. Based on the Met-AuNC/Eu³⁺ ensemble, we then established a simple and sensitive fluorescence strategy for turn-on determination of biomarker DPA, with a linear range of 0.2-4 μM and a low limit of detection of 110 nM. The feasibility of the proposed method was further validated by the quantitative detection of DPA in the soil samples. We believe that this study would significantly facilitate the construction of metal-ion-mediated PET sensors for the measurement of various interested analytes by applying fluorescent AuNCs as detection probes.

Recent Advances in Electrochemiluminescence and Chemiluminescence of Metal Nanoclusters

Metal nanoclusters (NCs), including Au, Ag, Cu, Pt, Ni and alloy NCs, have become more and more popular sensor probes with good solubility, biocompatibility, size-dependent luminescence and catalysis. The development of electrochemiluminescent (ECL) and chemiluminescent (CL) analytical methods based on various metal NCs have become research hotspots. To improve ECL and CL performances, many strategies are proposed, from metal core to ligand, from intermolecular electron transfer to intramolecular electron transfer. Combined with a variety of amplification technology, i.e., nanostructure-based enhancement and biological signal amplification, highly sensitive ECL and CL analytical methods are developed. We have summarized the research progresses since 2016. Also, we discuss the current challenges and perspectives on the development of this area.

Hemoglobin-Conjugated Gold Nanoclusters for Qualitative Analysis of Haptoglobin Phenotypes

Designing a facile and rapid detection method for haptoglobin (Hp) phenotypes in human blood plasma is urgently needed to meet clinic requirements in hemolysis theranostics. In this work, a novel approach to qualitatively analyze Hp phenotypes was developed using a fluorescent probe of gold nanoclusters (AuNCs). Hemoglobin-conjugated (Hb)-AuNCs were successfully synthesized with blue-green fluorescence and high biocompatibility via one-pot synthesis. The fluorescence of Hb-AuNCs comes from the ligand-metal charge transfer between surface ligands of Hb and the gold cores with high oxidation states. The biocompatibility assays including cell viability and fluorescence imaging, demonstrated high biocompatibility of Hb-AuNCs. For the qualitative analysis, three Hp phenotypes in plasma, Hp 1-1, Hp 2-1, and Hp 2-2, were successfully discriminated according to changes in the fluorescence intensity and peak position of the maximum intensity of Hb-AuNCs. Our work provides a practical method with facile and rapid properties for the qualitative analysis of three Hp phenotypes in human blood plasma.

Selective determination of Ag+ in the presence of Cd2+, Hg2+ and Cu2+ based on their different interactions with gold nanoclusters

In this work, a fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters (AuNCs). That is, bovine serum albumin (BSA) templated AuNCs with double emission peaks were synthesized using BSA as a protective agent. AuNCs with uniform distribution and average size between 2.0 and 2.2 nm were synthesized using a green and simple method, and showed bright orange-red fluorescence under ultraviolet light. AuNCs have two emission peaks at 450 nm and 630 nm with an excitation wavelength of 365 nm. Under alkaline conditions, Cd²⁺ can combine with the surface sulfhydryl groups of BSA–AuNCs to form Cd–S bonds, which cause AuNCs to aggregate, resulting in an increase in fluorescence intensity at 630 nm. Conversely, due to the d¹⁰–d¹⁰ metal affinity interaction, the addition of Hg²⁺ can reduce the fluorescence peak at 630 nm. Ag⁺ was reduced to Ag⁰ by gold nuclei in AuNCs, forming a stable hybrid Au@ AgNCs species with blue-shifted and enhanced fluorescence. Finally, the paramagnetic behavior of Cu²⁺ combined with BSA causes the excited electrons of the gold cluster to lose their energy via ISC, eventually leading to simultaneous quenching of the two emission peaks. The results show that the limit of detection (LOD) of Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.

A fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters. The limit of detection for Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.