Cerium(iii)-directed assembly of glutathione-capped gold nanoclusters for sensing and imaging of alkaline phosphatase-mediated hydrolysis of adenosine triphosphate

Peptide-Directed Synthesis of Aggregation-Induced Emission Enhancement-Active Gold Nanoclusters for Single- and Two-Photon Imaging of Lysosome and Expressed αvβ3 Integrin Receptors

This study explores the synthesis and characterization of aggregation-induced emission enhancement (AIEE)-active gold nanoclusters (AuNCs), focusing on their near-infrared luminescence properties and potential applications in biological imaging. These AIEE-active AuNCs were synthesized via the NaBH4-mediated reduction of HAuCl4 in the presence of peptides. We systematically investigated the influence of the peptide sequence on the optical features of the AuNCs, highlighting the role of glutamic acid in enhancing their quantum yield (QY). Among the synthesized peptide-stabilized AuNCs, EECEE-stabilized AuNCs exhibited the maximum QY and a pronounced AIEE effect at pH 5.0, making them suitable for the luminescence imaging of intracellular lysosomes. The AIEE characteristic of the EECEE-stabilized AuNCs was demonstrated through examinations using transmission electron microscopy, dynamic light scattering, zeta potential analysis, and single-particle imaging. The formation of the EECEE-stabilized AuNCs was confirmed by size-exclusion chromatography and mass spectrometry. Spectroscopic and electrochemical examinations uncover the formation process of EECEE-stabilized AuNCs, comprising EECEE-mediated reduction, NaBH4-induced nucleation, complex aggregation, and subsequent cluster growth. Furthermore, we demonstrated the utility of these AuNCs as luminescent probes for intracellular lysosomal imaging, leveraging their pH-responsive AIEE behavior. Additionally, cyclic arginylglycylaspartic acid (RGD)-modified AIEE dots, derived from cyclic RGD-linked peptide-induced aggregation of EECEE-stabilized AuNCs, were developed for single- and two-photon luminescence imaging of αvβ3 integrin receptor-positive cancer cells.

Loading of AuNCs with AIE effect onto cerium-based MOFs to boost fluorescence for sensitive detection of Hg2. Talanta 2024;273:125843. [PMID: 38492285 DOI: 10.1016/j.talanta.2024.125843] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Ligand-protected gold nanoclusters (AuNCs) have become promising nanomaterials in fluorescence (FL) methods for mercury ions (Hg2+) monitoring, but low FL efficiency hinders their widespread application. Herein, AuNCs/cerium-based metal-organic frameworks (AuNCs/Ce-MOFs) were prepared by loading 6-aza-2-thiothymine-protected AuNCs (ATT-AuNCs) with aggregation-induced emission (AIE) effect on the surface of Ce-MOFs by electrostatic attraction. This strategy improved the FL intensity of AuNCs through two aspects: (i) the AIE effect of ATT-AuNCs and (ii) the confinement effect of Ce-MOFs, which improved the restriction of intramolecular motion (RIM) of ATT-AuNCs. In addition, Ce-MOFs could adsorb and aggregate Hg2+ during detection, which might increase the local concentration. Therefore, based on the high FL signal of AuNCs/Ce-MOFs and enriched Hg2+, sensitive detection of Hg2+ could be achieved. More importantly, the strong specific recognition between AuNCs and Hg2+ could guarantee selectivity. The developed FL sensor exhibited superior detection performances with a wide linear range of 0.2-500 ng mL-1 and a low detection limit of 0.067 ng mL-1. Furthermore, the FL sensor used for sensitive and selective detection of Hg2+ in real samples, and the results agreed well with the standard method. In summary, this work proposed an effective and generalized strategy for improving the FL efficiency of AuNCs, which would greatly facilitate their application in pollutant monitoring.

Strategic framework for harnessing luminescent metal nanocluster assemblies in biosensing applications

The distinctive physicochemical attributes of ultra-small metal nanoclusters (MNCs) resembling those of molecules make them versatile constituents for self-assembled frameworks. This critical review scrutinizes the influence of assembly on the photoluminescence (PL) properties of MNCs and investigates their utility in biosensing applications. The investigation is initiated with an assessment of the shift from individual MNCs to assemblies and its repercussions on PL efficacy. Subsequently, two distinct biosensing modalities are explored: assembly-driven detection mechanisms and detection predicated on structural modifications in assembled MNCs. Through meticulous examination, we underscore the potential of self-assembly methodologies in tailoring the PL behavior of MNCs for the detection of diverse biological analytes and diseases.

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

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

Metal Ion-Regulated Fluorescent Sensor Array Based on Gold Nanoclusters for Physiological Phosphate Sensing

The detection of physiological phosphates (PPs) is of great importance due to their essential roles in numerous biological processes, but the efficient detection of different PPs simultaneously remains challenging. In this work, we propose a fluorescence sensor array for detecting PPs based on metal-ion-regulated gold nanoclusters (AuNCs) via an indicator-displacement assay. Zn2+ and Eu3+ are selected to assemble with two different AuNCs, resulting in quenching or enhancing their fluorescence. Based on the competitive interaction of metal ions with AuNCs and PPs, the fluorescence of AuNCs will be recovered owing to the disassembly of AuNC-metal ion ensembles. Depending on different PPs' distinct fluorescence responses, a four-channel sensor array was established. The array not only exhibits good discrimination capability for eight kinds of PPs (i.e., ATP, ADP, AMP, GTP, CTP, UTP, PPi, and Pi) via linear discriminant analysis but also enables quantitative detection of single phosphate (e.g., ATP) in the presence of interfering PPs mixtures. Moreover, potential application of the present sensor array for the discrimination of different PPs in real samples (e.g., cell lysates and serum) was successfully demonstrated with a good performance. This work illustrates the great potential of a metal ion-regulated sensor array as a new and efficient sensing platform for differential sensing of phosphates as well as other disease-related biomolecules.

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Fluorometric and colorimetric dual-mode sensing of α-glucosidase based on aggregation-induced emission enhancement of AuNCs

Herein, a fluorometric and colorimetric dual-mode assay platform used for α-glucosidase (α-Glu) activity sensing based on aggregation-induced emission enhancement (AIEE) of AuNCs was developed for the first time. The quantum yield (QY) and fluorescence lifetime of AuNCs were successfully ameliorated by Ce3+-triggered AIEE (Ce@AuNCs). Subsequently, on the basis of the inner filter effect (IFE) and dynamic quenching effect (DQE) between 2,6-dichlorophenolindophenol (DCIP) and Ce@AuNCs as well as the reduction of DCIP by ascorbic acid (AA) generated from α-Glu-catalyzed hydrolysis of L-ascorbic acid-2-O-α-D-glucopyranosyl (AA2G), the marriage of fluorometric and colorimetric modes applied for α-Glu activity monitoring was achieved. Besides, the feasibility of this dual-mode sensing system was confirmed by the assays versus potential interfering substances and in real samples. In particular, this system was further applied to evaluate natural α-Glu inhibitors (AGIs) including luteolin, apigenin, and hesperidin. Overall, the multi-mode optical sensor newly designed here has the potential for the accurate discovery of natural anti-diabetes drugs and the therapy of diabetes.

Prolonged near-infrared fluorescence imaging of microRNAs and proteases in vivo by aggregation-enhanced emission from DNA-AuNC nanomachines

Developing a comprehensive strategy for imaging various biomarkers (i.e., microRNAs and proteases) in vivo is an exceptionally formidable task. Herein, we have designed a deoxyribonucleic acid-gold nanocluster (DNA-AuNC) nanomachine for detecting tumor-related TK1 mRNA and cathepsin B in living cells and in vivo. The DNA-AuNC nanomachine is constructed using AuNCs and DNA modules that incorporate a three component DNA hybrid (TD) and a single-stranded fuel DNA (FD). Upon being internalized into tumor cells, the TK1 mRNA initiates the DNA-AuNC nanomachine through DNA strand displacement cascades, leading to the amplified self-assembly and the aggregation-enhanced emission of AuNCs for in situ imaging. Furthermore, with the aid of a protease nanomediator consisting of a mediator DNA/peptide complex and AuNCs (DpAuNCs), the DNA-AuNC nanomachine can be triggered by the protease-activated disassembly of the DNA/peptide complex on the nanomediator, resulting in the aggregation of AuNCs for in vivo protease amplified detection. It is worth noting that our study demonstrates the impressive tumor permeability and accumulation capabilities of the DNA-AuNC nanomachines via in situ amplified self-assembly, thereby facilitating prolonged imaging of TK1 mRNA and cathepsin B both in vitro and in vivo. This strategy presents a versatile and biomarker-specific paradigm for disease diagnosis.

Adjustable luminescence copper nanoclusters nanoswitch based on competitive coordination of samarium ions for cascade detection of adenosine triphosphate and acid phosphatase activity

Benefit from the strong coordination property, lanthanide metal ions have been used as competitive reagents to modulate the fluorescence changes of the system. However, lanthanide metal ions as inducers for aggregation-induced emission enhancement in nanosystems is rare. Herein, we report a "turn on-off-on" fluorescent switch for cascade detection of acid phosphatase (ACP) and adenosine triphosphate (ATP) based on the competitive coordination of samarium ions (Sm3+). Novel copper nanoclusters (CuNCs) with long wavelength emission (614 nm) stabilized by glutathione (GSH) and glycylglycine (Gly-Gly) have been confirmed to have AIE property. With the continuous aggregation of GSH/Gly-Gly CuNCs under the induction of Sm3+, the fluorescence of the system increased to achieve the "turn-on" process. The coordinated behaviour between Sm3+ and GSH/Gly-Gly CuNCs is discussed. Due to the strong metal coordination ability of ATP, the Sm3+ coordinated with the GSH/Gly-Gly CuNCs is competed out, resulting in the fluorescence "turn-off" process of the system. As the substrate of enzymatic hydrolysis of ACP, with the continuous hydrolysis of ATP by ACP, Sm3+ coordinates with GSH/Gly-Gly CuNCs again, which leads to the AIE effect and realize the fluorescence "turn-on" process of the system. This strategy results in ATP linear range of 0.508 ~ 120.0 μM with a detection limit of 0.508 μM (S/N = 3) and ACP linear range of 0.011 ~ 30.0 U·L-1 with a detection limit of 0.011 U·L-1 (S/N = 3). Application to biologic samples was successful.

Umbelliferone and Its Synthetic Derivatives as Suitable Molecules for the Development of Agents with Biological Activities: A Review of Their Pharmacological and Therapeutic Potential

Umbelliferone (UMB), known as 7-hydroxycoumarin, hydrangine, or skimmetine, is a naturally occurring coumarin in the plant kingdom, mainly from the Umbelliferae family that possesses a wide variety of pharmacological properties. In addition, the use of nanoparticles containing umbelliferone may improve anti-inflammatory or anticancer therapy. Also, its derivatives are endowed with great potential for therapeutic applications due to their broad spectrum of biological activities such as anti-inflammatory, antioxidant, neuroprotective, antipsychotic, antiepileptic, antidiabetic, antimicrobial, antiviral, and antiproliferative effects. Moreover, 7-hydroxycoumarin ligands have been implemented to develop 7-hydroxycoumarin-based metal complexes with improved pharmacological activity. Besides therapeutic applications, umbelliferone analogues have been designed as fluorescent probes for the detection of biologically important species, such as enzymes, lysosomes, and endosomes, or for monitoring cell processes and protein functions as well various diseases caused by an excess of hydrogen peroxide. Furthermore, 7-hydroxy-based chemosensors may serve as a highly selective tool for Al3+ and Hg2+ detection in biological systems. This review is devoted to a summary of the research on umbelliferone and its synthetic derivatives in terms of biological and pharmaceutical properties, especially those reported in the literature during the period of 2017-2023. Future potential applications of umbelliferone and its synthetic derivatives are presented.

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.

Calixarene-based supramolecular assembly with fluorescent gold-nanoclusters for highly selective determination of perfluorooctane sulfonic acid

The present study developed an efficient fluorescent approach, based on a supramolecular assembly between gold nanoclusters and calix[4]arene derivatives (C4A-Ds), to detect sever pollutant of perfluorooctane sulfonic acid (PFOS). For that, a series of C4A-Ds with different chain lengths and positive charges at the wider rim were designed and synthesized. Cytidine-5' phosphate protected gold nanoclusters (AuNCs@CMP) were then assembled with calix[4]arene (LC4AP) to form AuNCs/LC4AP assembly, leading to 8-fold luminescence enhancement via the AIEE effect. However, further binding with PFOS reconstituted the as-formed assembly hrough a competitive effect, generating a fluorescence quenching. Particularly, the linear fluorescence response of AuNCs/LC4AP to PFOS realized a highly sensitive determination of the pollutant PFOS in a wide range (2.0-100 μM). In addition, the developed method successfully detected PFOS in pool water near a fire drill field, being good enough for the practical PFOS determination. The calixarene mediated method, based on the fluorescence "on-off" strategy of metal nanoclusters, is sensitive, rapid-responsive, economical, particularly, suitable for the PFOS determination in practice. It takes full advantage of the molecular recognition and self-assembly of artificial macrocyclic host molecules as a promising strategy for the PFOS determination, and will be highlight to develop new detection methods for PFOS and other poisonous compounds in environments.

Construction of Metal Organic Framework-Derived Fe-N-C Oxidase Nanozyme for Rapid and Sensitive Detection of Alkaline Phosphatase

Alkaline phosphatase (ALP) is a phosphomonoester hydrolase and serves as a biomarker in various diseases. However, current detection methods for ALP rely on bulky instruments, extended time, and complex operations, which are particularly challenging in resource-limited regions. Herein, we synthesized a MOF-derived Fe-N-C nanozyme to create biosensors for the coulometric and visual detection of ALP. Specifically, we found the Fe-N-C nanozyme can efficiently oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue-colored tetramethyl benzidine (TMBox) without the need for H2O2. To construct the biosensor, we incorporated the ALP enzymatic catalytic reaction to inhibit the oxidation of TMB by Fe-N-C oxidase nanozyme. This biosensor showed rapid and highly sensitive detection of ALP in both buffer and clinical samples. The limit of detection (LOD) of our approach could be achieved at 3.38 U L-1, and the linear range was from 5 to 60 U L-1. Moreover, we also developed a visual detection for ALP by using a smartphone-based assay and facilitated practical and accessible point-and-care testing (POCT) in resource-limited areas. The visual detection method also achieved a similar LOD of 2.12 U L-1 and a linear range of 5-60 U L-1. Our approach presents potential applications for other biomarker detections by using ALP-based ELISA methods.

Gold nanocluster-based fluorescent sensors for in vitro and in vivo ratiometric imaging of biomolecules

Gold nanoclusters (AuNCs) are promising nanomaterials for ratiometric fluorescent probes due to their tunable fluorescence wavelengths dependent on size and structure, as well as their biocompatibility and resistance to photobleaching. By incorporating an additional fluorescence spectral peak, dual-emission AuNC-based fluorescent probes have been developed to enhance the signal output reproducibility. These probes can be fabricated by integrating various luminescent nanomaterials with AuNCs. This review focuses on the preparation methods and applications of ratiometric fluorescent probes derived from AuNCs and other fluorescent nanomaterials or fluorescent dyes for both in vitro and in vivo bioimaging of target analytes. Additionally, the review delves into the sensing mechanisms of AuNC-based ratiometric probes, their synthetic strategies, and the challenges encountered when using AuNCs for ratiometric bioimaging. Moreover, we explore the application of protein-stabilized AuNCs and thiolate-capped AuNC-based ratiometric fluorescent probes for biosensing and bioimaging. Two primary methods for assembling AuNCs and fluorophores into ratiometric fluorescent probes are discussed: triggered assembly and self-assembly. Finally, we address the challenges and issues associated with ratiometric bioimaging using AuNCs and propose future directions for further advancing AuNCs as ratiometric imaging agents.

Ratiometric fluorescent detection of total phosphates in frozen shrimp samples using catalytic active Zr(IV) modified gold nanoclusters

Phosphate salts are important food additives in a variety of foods. In this study, the Zr(IV) modified gold nanoclusters (Au NCs) were prepared for ratiometric fluorescent sensing of phosphate additives in seafood samples. Compared with bare Au NCs, the synthesized Zr(IV)/Au NCs showed stronger orange fluorescence at 610 nm. On the other hand, the Zr(IV)/Au NCs retained the phosphatase-like activity of Zr(IV) ions and could catalyze the hydrolysis of fluorescent substrate 4-methylumbelliferyl phosphate to produce blue emission at 450 nm. The addition of phosphate salts could effectively inhibit the catalytic activity of Zr(IV)/Au NCs, resulting the fluorescence decrease at 450 nm. However, the fluorescence at 610 nm almost unchanged upon the addition of phosphates. Based on this finding, the ratiometric detection of phosphates using the fluorescence intensity ratio (I₄₅₀/I₆₁₀) was demonstrated. The method has been further applied for sensing total phosphates in frozen shrimp samples with satisfactory results.

Facile preparation of bimetallic Au-Cu nanoclusters as fluorescent nanoprobes for sensitive detection of Cr3+ and S2O82- ions

Metallic nanoclusters (NCs) have attracted special attention from researchers due to their interesting optical properties. In this experiment, we proposed a facile one-step method for the synthesis of bimetallic gold-copper nanoclusters (AuCuNCs). The prepared AuCuNCs were characterized by fluorescence spectroscopy (FL), UV-vis absorption spectrum, transmission electron microscopy (TEM), etc. The emission peak of the prepared AuCuNCs was located at 455 nm and showed blue luminescence under the excitation of 365 nm UV light. Furthermore, after the addition of Cr³⁺ and S₂O₈^2- ions, the FL emission intensity of AuCuNCs was significantly reduced at 455 nm and there was a color change of diminished blue luminescence under UV lamp. The AuCuNCs exhibited excellent linearity and sensitivity for the detection of Cr³⁺ and S₂O₈^2- ions. The limits of detection (LOD) for the Cr³⁺ and S₂O₈^2- ions were calculated to be 1.5 and 0.037 μM, respectively. Finally, the recoveries of Cr³⁺ and S₂O₈^2- ions in Runxi Lake and tap water were measured by standard addition recovery test and were 96.66 ∼ 116.29 %, 95.75 ∼ 119.4 %.

Fluorescence turn-on detection of human serum albumin based on the assembly of gold nanoclusters and bromocresol green

As the most abundant protein in plasma, human serum albumin plays a vital role in physiological processes, such as maintaining blood osmotic pressure and carrying small-molecule ligands. Since the content of albumin in the human serum can reflect the status of liver and renal function, albumin quantitation is significant in clinical diagnosis. In this work, fluorescence turn-on detection of human serum albumin (HSA) had been performed based on the assembly of gold nanoclusters and bromocresol green. Gold nanoclusters (AuNCs) capped by reduced glutathione (GSH) were assembled with bromocresol green (BCG), and the assembly was used as a fluorescent probe for HSA. After BCG assembling, the fluorescence of gold nanoclusters was nearly quenched. In acidic solution, HSA can selectively bind to BCG on the assembly and recover the fluorescence of the solution. Based on this turn-on fluorescence, ratiometric HSA quantification was realized. Under optimal conditions, HSA detection by the probe possessed a good linear relationship in the range of 0.40-22.50 mg·mL^-1, and the detection limit was 0.27 ± 0.04 mg·mL^-1 (3σ, n = 3). Common coexisting components in serum and blood proteins did not interfere with the detection of HSA. This method has the advantages of easy manipulation and high sensitivity, and the fluorescent response is insensitive to reaction time.

A ratiometric fluorescent sensor for the detection of phosphate

Over the past few years, ratiometric fluorescent nanoprobes have garnered substantial interest because of their self-calibration characteristics. This research developed a ratiometric fluorescent sensor to detect phosphate. Through encapsulating luminescent materials, gold nanoclusters (AuNCs) and carbon dots (CDs) into a zeolitic imidazolate framework-8 (ZIF-8), the fluorescence signal of AuNCs was enhanced, while that of CDs was suppressed. After phosphate was added, it could decompose ZIF-8, and AuNCs and CDs were released, which weakened the fluorescence signal of the AuNCs while restoring that of the CDs. Thereby, this makes CDs/AuNCs@ZIF-8 a potential fluorescent sensor for phosphate determination. The ratiometric sensor had facile synthesis, good selectivity, and a low detection limit. Therefore, this sensor was an effective tool for the detection of phosphate.

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.

An Oligopeptide-Protected Ultrasmall Gold Nanocluster with Peroxidase-Mimicking and Cellular-Imaging Capacities

Recent decades have witnessed the rapid progress of nanozymes and their high promising applications in catalysis and bioclinics. However, the comprehensive synthetic procedures and harsh synthetic conditions represent significant challenges for nanozymes. In this study, monodisperse, ultrasmall gold clusters with peroxidase-like activity were prepared via a simple and robust one-pot method. The reaction of clusters with H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB) followed the Michaelis-Menton kinetics. In addition, in vitro experiments showed that the prepared clusters had good biocompatibility and cell imaging ability, indicating their future potential as multi-functional materials.

Ce3+ and Fe2+ co-enhanced ratiometric fluorescence probe utilizing copper nanoclusters and coumarin for sensitive assay of hydrogen peroxide and glucose

A novel ratiometric fluorescent probe was constructed for sensitive assay of hydrogen peroxide (H₂O₂) and glucose, which utilized the synergistically enhanced effects of Ce³⁺ and Fe²⁺ on copper nanoclusters (CuNCs) and coumarin. In the CuNCs-Ce³⁺/Fe²⁺-coumarin system, Ce³⁺ triggered the aggregation-induced emission phenomenon of CuNCs, and Fe²⁺ catalyzed the Fenton reaction to efficiently yield hydroxyl radical (•OH). In the presence of H₂O₂, the 625-nm red fluorescence of CuNCs was sharply quenched owing to the oxidation of CuNCs to Cu(II) by •OH, but the 460-nm blue fluorescence of 7-hydroxycoumarin from the oxidation of coumarin by •OH dramatically increased. Based on the reversible changes in two fluorescence signals, a satisfactorily ratiometric probe was constructed for H₂O₂ assay with a detection limit (LOD) of 0.6 μM accompanied by a visual color variation from red to blue. For glucose assay, this ratiometric probe gave a linear range of 3.2-160 μM and LOD of 0.96 μM owing to the oxidization of glucose to yield H₂O₂ in the presence of glucose oxidase and O₂. Overall, the newly developed ratiometric probe shows a great prospect in real applications for visual assay of H₂O₂ and glucose by our naked eyes.

Silver ions involved fluorescence "on-off" responses of gold nanoclusters system for determination of carbendazim residues in fruit samples

Herein, a fluorescence "on-off" system was developed for monitoring carbendazim (CBZ) residues in fruit samples, based on glutathione-gold nanoclusters (GSH-Au NCs) and silver ions (Ag⁺). First, the fluorescence intensity of GSH-Au NCs was greatly enhanced (turn on) with aggregation-induced emission enhancement (AIEE) effect in the presence of Ag⁺, then fluorescence quenching occurred (turn off) with adding CBZ by the chelation between CBZ and Ag⁺. The quenching degree was well linearly dependent on CBZ concentration covering from 0.5 to 20 μM. Moreover, the GSH-Au NCs-Ag⁺ system exhibited superior selectivity towards CBZ and was sensitive for the determination of CBZ in apple and orange juices with a low detection limit of 0.12 μM. The recoveries of CBZ spiked in fruit samples ranged from 81.0 % to 111.4% with the relative standard deviations less than 6.6%, demonstrating its great potential for monitoring CBZ residues in fruit samples.

Metal-Organic Enzyme Nanogels as Nanointegrated Self-Reporting Chemobiosensors

A fluorometric glucose biosensor based on fine-tuned chemoenzymatic nanohybrids is herein proposed. The successful integration of an engineered glucose oxidase enzyme and an optically responsive polymeric nanogel in a single entity has led to the fabrication of a highly efficient glucose chemobiosensor. The optical responsiveness has been achieved by the loading of preactivated polymeric hydrogel with fluorescent lanthanide, i.e., cerium (III), cations. A comprehensive investigation of the responsiveness of the biomaterial revealed the interplay between the oxidation state of the cerium lanthanide and the fluorescence emission of the polymer. Finally, a full structural, chemical, and biochemical characterization of the reported system supports the chemobiosensors as robust, specific, and sensitive materials that could be utilized to faithfully quantify the amount of glucose in tear fluids.

Catechin-inspired gold nanocluster nanoprobe for selective and ratiometric dopamine detection via forming azamonardine

The sensitive and selective perception of dopamine (DA, a typical neurotransmitter) is important to evaluate the biological environment. In this study, a catechin-functionalized gold nanocluster (C-Au NC) nanoprobe has been explored for the ratiometric DA sensing. The detection mechanism is based on the formation of azamonardine via selective DA-catechin chemical reaction and subsequent enhanced fluorescence emission. Using Au NC emission as the internal reference, ratiometric fluorescence variation is realized, which allows sensitive DA analysis with a limit of detection of 1.0 nM (S/N = 3) and linear response concentration range from 0 to 500 nM. The characteristic chemical reaction between catechin and DA affords favorable selectivity over other amino acids, metal ions and small molecules. In addition, the practical application of the proposed nanoprobe is validated by the accurate detection of DA content in urea and cell lysate samples.

A novel ratiometric nanoprobe based on copper nanoclusters and graphitic carbon nitride nanosheets using Ce(III) as crosslinking agent and aggregation-induced effect initiator for sensitive detection of hydrogen peroxide and glucose

Herein, glutathione-capped copper nanoclusters (CuNCs) and graphitic carbon nitride nanosheets (g-C₃N₄ NSs) were synthesized by a facile one-pot chemical reduction and directly thermal pyrolysis following ultrasonic exfoliation approaches, respectively. The introduction of Ce(III) (Ce³⁺) played dual functions in constructing a fluorescence-enhanced ratiometric nanoprobe (g-C₃N₄ NSs-Ce³⁺-CuNCs), i.e., triggering aggregation-induced emission of CuNCs and conjugating g-C₃N₄ NSs with CuNCs by virtue of electrostatic and coordination interactions. The as-fabricated nanohybrid displayed 460 and 625 nm dual-emitting peaks, attributing to the emission of g-C₃N₄ NSs and CuNCs, respectively. Upon addition of H₂O₂, the 625 nm emission was dramatically quenched, whereas the 460 nm emission remained nearly unchanged, thereby causing obvious color changes from purple to blue under a 365-nm UV lamp. A ratiometric fluorescent assay, based on g-C₃N₄ NSs-Ce³⁺-CuNCs, was devised for sensitive and visual detection of H₂O₂, which spanned the linear range of 2-100 μM with a detection limit of 0.6 μM. In the presence of glucose oxidase, the ratiometric nanoprobe could be simultaneously employed to detect glucose across the linear range of 1.6-320 μM with a detection limit of 0.48 μM. In milk and human serum samples, the fortified recoveries for H₂O₂ and glucose by the nanoprobe were in the range of 95.5-103.6% with RSDs <3.8%. The real detection levels for glucose are consistent with those by a standard glucometer. As such, the ratiometric nanoprobe offers a promising methodology for several practical applications, such as point-of-care diagnosis and workplace health evaluations.

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.

Phosphate-triggered ratiometric fluoroimmunoassay based on nanobody-alkaline phosphatase fusion for sensitive detection of 1-naphthol for the exposure assessment of pesticide carbaryl

The excessive use of carbaryl has resulted in the risk of its exposure. In this study, we isolated six nanobodies (Nbs) from a camelid phage display library against the biomarker of carbaryl, 1-naphthol (1-NAP). Owing to its characteristics of easy genetic modifications, we produced a nanobody-alkaline phosphatase (Nb-CC4-ALP) fusion protein with good stability. A dual-emission system based ratiometric fluoroimmunoassay (RFIA) for quick and highly sensitive determination of 1-NAP was developed. Silicon nanoparticles (SiNPs) was used as an internal reference and for aggregation-induced emission enhancement (AIEE) of gold nanoclusters (AuNCs), while AuNCs could be quenched by MnO₂ via oxidation. In the presence of ALP, ascorbic acid phosphate (AAP) can be transformed into ascorbic acid (AA), the later can etch MnO₂ to recover the fluorescence of the AuNCs. Based on optimal conditions, the proposed assay showed 220-fold sensitivity improvement in comparison with conventional monoclonal antibody-based ELISA. The recovery test of urine samples and the validation by standard HPLC-FLD demonstrated the proposed assay was an ideal tool for screening 1-NAP and provided technical support for the monitoring of carbaryl exposure.

A novel dual-emission fluorescent probe for ratiometric and visual detection of Cu2+ ions and Ag+ ions

In this work, the biomolecule glutathione was used to prepare cyan fluorescent carbon dots (GSH@CDs) by a hydrothermal method. The GSH@CDs were adopted as the scaffolds to synthesize fluorescent gold nanoclusters (GSH@CDs-Au NCs) with two independent emission peaks at 430 nm and 700 nm. A fluorescent method for the Cu²⁺ and Ag⁺ ion assay was established based on the fluorescence quenching or enhancement at 700 nm of GSH@CDs-Au NCs. The fluorescent test strips were successfully prepared for visual detection of Cu²⁺ ions and Ag⁺ ions based on GSH@CDs-Au NCs. In addition, GSH@CDs-Au NCs were found to possess well peroxidase-like activity.

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.

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.

Ratiometric fluorescent sensor based on MoS2 QDs and AuNCs for determination and bioimaging of alkaline phosphatase

Herein, a ratiometric fluorescent sensor based on MoS₂ quantum dots (QDs) and glutathione-capped gold nanoclusters (AuNCs) was developed for determination and imaging of alkaline phosphatase (ALP). The sensor was developed by covalently linking QDs with AuNCs to form stable MoS₂@AuNCs nanohybrids that exhibited the blue fluorescence of MoS₂ QDs and the red fluorescence of AuNCs. In the presence of Ce³⁺, the fluorescence intensity of AuNCs was increased due to the aggregation-induced emission enhancement (AIEE), while that of MoS₂ QDs remained unchanged, thus could be used as a reference signal. After adenosine 5'-monophosphate (AMP) and ALP were introduced into the system, AMP was hydrolyzed to adenosine and phosphate ions (PO₄^3-). Owing to higher affinity between Ce³⁺ and PO₄^3-, the AIEE effect was inhibited, in turn resulting in the decrease of AuNCs fluorescence. The developed ratiometric fluorescent sensor had a linear response to ALP concentration ranging from 0.5 to 50 U L^-1 with a detection limit (LOD) of 0.08 U L^-1. Moreover, the sensor had low cytotoxicity and was successfully employed in lysosome localization and bioimaging of intracellular ALP in living cells.

Metal-Based Aggregation-Induced Emission Theranostic Systems

Efficient theranostic systems can realize better outcomes in disease treatment because of precise diagnosis and the concomitant effective therapy. Aggregation-induced emission luminogens (AIEgens) are a unique type of organic emitters with intriguing photophysical properties in the aggregate state. Among the AIEgens studied for biomedical applications, so far, metal-based AIE systems have shown great potential in theranostics due to the enhanced multimodal bioimaging ability and therapeutic effect. This research field has been growing rapidly, and many rationally designed systems with promising activities to cancer and other diseases have been reported recently. In this review, we summarized the recent progress of metal-based AIE materials in bioimaging and biological theranostics, and deciphered the pertinent design strategies. We hope that this review can offer new insights into the development of this growing field.

Driving Forces and Routes for Aggregation-Induced Emission-Based Highly Luminescent Metal Nanocluster Assembly

The development of ultrasmall, luminescent metal nanoclusters (MNCs) with aggregation-induced emission (AIE) characteristics is a relatively new research area that has gained significant attention in various multidisciplinary applications such as optoelectronics, sensing, imaging, and therapy. The numerous scientific breakthroughs in the AIE field provide many tools that, if incorporated into MNCs design strategies, could help realize various new and exciting MNC-based avenues that maximize the utilization of the AIE phenomenon. Indeed, leveraging the aggregation strategies from the AIE community with the judicious use of various covalent and noncovalent interactions has been demonstrated to be effective for constructing several MNC-based hybrid assemblies with enhanced AIE characteristics. In this Perspective, we summarize the key driving forces and routes of MNC assembly together with their impact on deciphering the working mechanism behind the AIE process. These strategies can inspire the design of highly luminescent MNC-based hierarchical functional materials across multiple length scales.

Glutathione Disulfide as a Reducing, Capping, and Mass-Separating Agent for the Synthesis and Enrichment of Gold Nanoclusters

Water-soluble nanoclusters, which are facilely enrichable without changes in the original properties, are highly demanded in many disciplines. In this contribution, a new class of gold nanoclusters (AuNCs) was synthesized using glutathione disulfide (GSSG) as a reducing and capping agent under intermittent heating mode. The as-prepared GSSG–AuNCs had a higher quantum yield (4.1%) compared to the conventional glutathione-protected AuNCs (1.8%). Moreover, by simply introducing the GSSG–AuNC solution to acetonitrile at a volume ratio of 1:7, a new bottom phase was formed, in which GSSG–AuNCs could be 400-fold enriched without changes in properties, with a percentage recovery higher than 99%. The enrichment approach did not need additional instruments and was potentially suitable for large-scale enrichment of nanoclusters. Further, density functional theory calculations indicated that the hydrogen bonding between GSSG and acetonitrile plays a key role for the bottom phase formation. Our work suggests that the highly emissive GSSG–AuNCs possess great potential not only in fluorescent measurements but also in other scenarios in which high-concentration AuNCs may be needed, such as catalysis, drug delivery, and electronic and optical industries.

Fluorescence sensing of heparin and heparin-like glycosaminoglycans by stabilizing intramolecular charge transfer state of dansyl acid-labeled AG73 peptides with glutathione-capped gold nanoclusters

Sensors that can specifically and accurately detect glycosaminoglycans are rare. Here, a dual-mode platform for fluorescence intensity and lifetime sensing of plasma heparin and fluorescence imaging of heparan sulfate proteoglycan-expressed cancer cells was developed by stabilizing the intramolecular charge transfer (ICT) state of dansyl acid-labeling AG73 (DA-AG73) peptide with glutathione-capped gold nanoclusters (GSH-AuNCs). DA-AG73 peptides, including an electron-donor dimethylamino group and an electron-withdrawing sulfonamide moiety in the labeled DA molecules, emitted weak fluorescence due to the formation of the twisted ICT excited state. The complexation of heparin with DA-AG73 peptides followed by interacting with the GSH-AuNCs could restrict the rotation of the dimethylamino groups of the labeled DA molecules, triggering the transition from their twisted ICT state to ICT excited state. As a result, the fluorescence intensity and lifetime of the labeled DA molecules in DA-AG73 peptides were gradually enhanced with increasing the heparin concentration. The proposed platform provided excellent selectivity toward heparin and heparan sulfate and exhibited two linear calibration curves for quantifying 20-800 nM and 20-1000 nM heparin in the fluorescence intensity and lifetime modes, respectively. The proposed platform was practically applied for the fluorescence intensity and lifetime determination of plasma heparin and for the selective imaging of heparan sulfate proteoglycan-expressed cells.

Nucleic acid-driven aggregation-induced emission of Au nanoclusters for visualizing telomerase activity in living cells and in vivo

Visual monitoring of telomerase activity in living cancer cells and in vivo is essential for clinical diagnosis and treatment. However, most detection methods were performed in vitro due to the difficulty of probes entering cells and the interferences from complex biological environments. Herein, we developed a novel probe based on Au nanoclusters (AuNCs) with a nucleic acid-driven aggregation-induced emission (AIE) property for the first time. The probe was applied for detection of telomerase with high sensitivity. Importantly, the probe could achieve telomerase imaging in living cells and in solid tumor tissue in vivo. The study provided a specific connection fashion of metal nanoclusters for AIE generation. It holds great potential for the development of AIE-active metal nanoclusters as a diagnostic tool for disease detection in vitro as well as in vivo.

Determination of dopamine based on its enhancement of gold-silver nanocluster fluorescence

Dopamine (DA) is one of the most important neurotransmitters in human bodies and its sensitive detection remains a challenge. Herein, protein stabilized gold-silver nanoclusters (Au-AgNCs) were synthesized at first. It was found that the introduction of dopamine lead to a significant enhancement of the fluorescence from the nanoclusters, together with a red-shift of the peak. Through related spectroscopic and electrochemical studies, the fluorescence enhancement was attributed to the reduction of the nanoclusters by dopamine. This enhancement was then adopted for quantitative measurements, and linear responses toward dopamine in the ranges 0.01-1.7 μM and 1.7-10 μM were constructed. A limit of detection was obtained at 6.9 nM. The present study provided a facile and efficient method for the determination of dopamine, and the method was successfully applied for related measurements in serum samples.

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.

Fluorescent and visual assay of H2O2 and glucose based on a highly sensitive copper nanoclusters-Ce(III) fluoroprobe

Herein, we synthesized and characterized glutathione-capped copper nanoclusters (CuNCs) using a convenient one-pot chemical reduction approach based on glutathione as capping and reducing agents. The Ce(III) induced aggregation-induced emission of CuNCs to form a CuNCs-Ce³⁺ fluoroprobe due to electrostatic and coordination interactions between Ce³⁺ and CuNCs. In contrast to CuNCs, the fluorescent intensities (FLs) of CuNCs-Ce³⁺ were enhanced by ~ 40-fold concomitant with 20-nm blue-shift of the maximum emission, and a 3.45-fold lengthening of the average fluorescent lifetime. The FLs of CuNCs-Ce³⁺ were selectively quenched at 650 nm by hydrogen peroxide (H₂O₂) via the redox reaction. Based on this phenomenon, the sensitive assay of H₂O₂ was realized, and the linear range spanned over the range of 14-140 μM. Notably, the visualization of the fluorescence quenched effect of H₂O₂ could be easily attained. Additionally, glucose could be specifically oxidized by glucose oxidase to produce H₂O₂, and thus the detection of glucose was achieved according to changes in the concentrations of H₂O₂. Under optimized conditions, the fluorescent assay of glucose based on the CuNCs-Ce³⁺ system offered the linear range of 8-48 μM with detection limit of 2.4 μM. Meanwhile, high selectivity of the as-constructed fluorescent assay allows the sensitive detection of H₂O₂ and glucose in real-world care products and human serum samples, showing a great application potential in their conventional monitoring.

Design of ratiometric monoaromatic fluorescence probe via modulating intramolecular hydrogen bonding: A case study of alkaline phosphatase sensing

Monoaromatic molecules are a category of molecules containing a single aromatic ring which generally emit light in the ultraviolet (UV) region. Despite their facile preparation, the UV emission greatly limits their application as organic probes. In this study, we developed a general method to red shift the emission of monoaromatic molecules. Significant fluorescence red-shift (∼100 nm per intramolecular hydrogen bonding) can be achieved by introducing intramolecular hydrogen bonding units to benzene, a typical monoaromatic molecule. Upon increasing the number of hydrogen bonding units on the benzene ring, UV, blue, and green emissions are screened, which are switchable by simply breaking/restoration the intramolecular hydrogen bonding. As a demonstration, with the breaking of one intramolecular H-bonding, the green emission (λ_em^max = 533 nm) of 2,5-dihydroxyterephthalic acid (DHTA) changed to cyan (λ_em^max = 463 nm) upon the formation of its phosphorylated form (denoted as PDHTA), which, in the presence of alkaline phosphatase (ALP), hydrolyzed and recovered the green emission. By taking advantage of the switchable emission colors, ratiometric in vitro and endogenous ALP sensing was achieved. This general approach offers a great promise to develop organic probes with tunable emissions for fluorescence analysis and imaging by different intramolecular hydrogen bonding.

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.