Fluorescent metal nanoclusters: From synthesis to applications

Single-Ligand Modulated Size-Dependent Multi-Color Au/Os Nanoclusters for Multi-Analyte Detection

The development of nanoclusters (NCs) capable of multicolor emissions for simultaneous detection of multiple analytes has aroused tremendous interest. However, the current methods for synthesizing NCs with multicolor emissions mainly depend on a multiple ligand strategy, which not only compromises the stability of the NCs but also alters their physicochemical properties. Herein, we propose a novel strategy for designing single-ligand capped bimetallic Au/Os NCs with multicolor fluorescence by adjusting the size of the NCs. This size-controlled, single-ligand encapsulation strategy not only enhances their stability and compatibility but also ensures uniformity in their physicochemical properties, thereby overcoming limitations inherent in multiligand systems. By meticulously modulating the reaction parameters, we achieved precise tuning of the NCs size, resulting in the synthesis of multicolor fluorescent NCs displaying blue (465 nm), green (507 nm), and yellow (560 nm) emissions. These multicolor Au/Os NCs were then incorporated into an array system for the differentiation of tetracyclines (TCs) by virtue of their interaction with TCs through the inner filter effect (IFE). Finally, each TC elicited a unique fluorescent response, which was subsequently analyzed by principal component analysis. The sensor array has been successfully employed for detection of TCs in milk, urine, and water, demonstrating its practical application potential. The strategy developed in this work holds great promise for the development of multicolor NCs.

Intrinsically chiral thermoresponsive assemblies from achiral clusters: enhanced luminescence and optical activity through tailor-made chiral additives

Chiral metal clusters, due to their intriguing optical properties and unique resemblance in size to biomolecules, have attracted a lot of attention in recent times as potential candidates for application in bio-detection and therapy. While several strategies are reported for the synthesis of optically active clusters, a facile approach that enhances a multitude of properties has remained a challenge. Herein, we report a simple strategy wherein the use of a chiral cationic surfactant, during the synthesis of achiral clusters, leads to the fabrication of chiral assemblies possessing enhanced luminescence and optical activity. The structural resemblance of the capping ligand, mercaptoundecanoic acid (MUA) to the co-surfactant, (+/-)-N-dodecyl-N-methylephedrinium bromide (DMEB), assisted specific interactions that led to the formation of chiral assemblies exhibiting unique thermoresponsive behavior and tunable optical activity. The symmetry breaking in the aggregates driven by the electrostatic interaction between the positively charged chiral surfactant and negatively charged clusters led to enhanced luminescence through aggregation induced enhanced emission (AIEE). The work highlighting the generation and tuning of cluster chirality provides newer insights into the fundamental understanding of nanocluster optical activity thereby opening newer avenues for the development of materials for application in the field of chiral biosensing and enantioselective catalysis.

Sensing Platform Based on Gold Nanoclusters and Nanoporous Anodic Alumina for Preeclampsia Detection

Preeclampsia is a pregnancy-specific hypertensive syndrome recognized as the leading cause of maternal and fetal morbidity worldwide. Early diagnosis is crucial for mitigating its adverse effects, and recent investigations have identified endoglin as a potential biomarker for this purpose. Here, we present the development of a hybrid biosensor platform for the ultrasensitive detection of endoglin, aimed at enabling the early diagnosis of preeclampsia. This platform integrates the high surface area properties of nanoporous anodic alumina (NAA) with the unique optical characteristics of gold nanoclusters (AuNCs) to achieve enhanced detection capabilities. The NAA surface functionalized to promote attachment of AuNCs, which then was functionalized with specific antibodies to confer selectivity towards endoglin. Photoluminescence (PL) analysis of the biosensor demonstrated a linear detection range of 10-50 ng/mL, with a detection limit of 5.4 ng/mL and a sensitivity of 0.004 a.u./(ng/mL). This proof-of-concept study suggests that the NAA-AuNCs-based biosensing platform holds significant potential for the development of ultrasensitive, portable, and cost-effective diagnostic tools for preeclampsia, offering a promising avenue for advancing prenatal care.

Progress and challenges in bacterial infection theranostics based on functional metal nanoparticles

The rapid proliferation and infection of bacteria, especially multidrug-resistant bacteria, have become a great threat to global public health. Focusing on the emergence of "super drug-resistant bacteria" caused by the abuse of antibiotics and the insufficient and delayed early diagnosis of bacterial diseases, it is of great research significance to develop new technologies and methods for early targeted detection and treatment of bacterial infection. The exceptional effects of metal nanoparticles based on their unique physical and chemical properties make such systems ideal for the detection and treatment of bacterial infection both in vitro and in vivo. Metal nanoparticles also have admirable clinical application prospects due to their broad antibacterial spectrum, various antibacterial mechanisms and excellent biocompatibility. Herein, we summarized the research progress concerning the mechanism of metal nanoparticles in terms of antibacterial activity together with the detection of bacterial. Representative achievements are selected to illustrate the proof-of-concept in vitro and in vivo applications. Based on these observations, we also give a brief discussion on the current problems and perspective outlook of metal nanoparticles in the diagnosis and treatment of bacterial infection.

Trimetallic nanocomposites developed for efficient in vivo bimodal imaging via fluorescence and magnetic resonance

Despite several attempts, in vivo bimodal imaging still represents a challenge. Generally, it is accepted that dual-modality in imaging can improve sensitivity and spatial resolution, namely, when exploiting fluorescence (FI) and magnetic resonance imaging (MRI), respectively. Here, a newly developed combination of (i) protein-protected luminescent Au-Ag nanoclusters (LGSN) manifesting themselves by fluorescent emission at 705 nm and (ii) superparamagnetic iron oxide nanoparticles (SPION) embedded within the same protein and creating contrast in MR images, has been investigated in phantoms and applied for in vivo bimodal imaging of a mouse as a proof of principle. Unique LGSN-SPION nanocomposites were synthesized in a specific sequential one-pot green preparation procedure and characterized thoroughly using many physicochemical experimental techniques. The influence of LGSN-SPION samples on the viability of healthy cells (RPE-1) was tested using a calcein assay. Despite the presence of Ag (0.12 mg mL-1), high content of Au (above 0.75 mg mL-1), and moderate concentrations of Fe (0.24 mg mL-1), LGSN-SPION samples (containing approx. 15 mg mL-1 of albumin) were revealed as biocompatible (cell viability above 80%). Simultaneously, these concentration values of all components in the LGSN-SPION nanocomposite were used for achieving both MRI and fluorescence signals in phantoms as well as in a living mouse with sufficiently high resolution. Thus, the LGSN-SPION samples can serve as new efficient bimodal FI and MRI probes for in vivo imaging.

Silver Nanoclusters as Label Free Non-enzymatic Fast Glucose Assay with the Fluorescent Enhancement Signal

A simple and low-cost green preparation method was used for BSA capped silver nanoclusters (BSA-Ag NCs) as turn on fluorescent probe for glucose. Non-enzymatic fast glucose detection assay with a widest concentration range was proposed which requires neither nanoclusters (NCs) modification nor complicated enzyme immobilization. The DLS analysis, HRTEM patterns, fluorescence and UV-visible measurement well supported the synthesis product. The advantages of the fabricated glucose sensor based on fluorescence increasing of probe compared to other established optical techniques was inspected and summarized as well. The glucose sensor exhibited a high sensitivity, fast response time (in seconds), satisfactory selectivity, well stability (at least two months), low detection limit (31 µmol L- 1) and a wide concentration response (three orders of magnitudes) to glucose between 0.1 and 92 mmol L- 1 as calibration plot. A theoretical model of the sensing mechanism based on the binding interaction of glucose to BSA-Ag NCs is proposed and data fitting demonstrated a good agreement between the experimental and theoretically calculated fluorescence data. The facile preparation and excellent sensing performance of BSA-Ag NCs in the real samples (plasma and juice) make sure that synthesized probe material is a promising candidate for advanced enzyme-free glucose sensing approach.

Recent advances towards point-of-care devices for fungal detection: Emphasizing the role of plasmonic nanomaterials in current and future technologies

Fungal infections are a significant global health problem, particularly affecting individuals with weakened immune systems. Moreover, as uncontrolled antibiotic and immunosuppressant use increases continuously, fungal infections have seen a dramatic increase, with some strains developing antibiotic resistance. Traditional approaches to identifying fungal strains often rely on morphological characteristics, thus owning limitations, such as struggles in identifying several strains or distinguishing between fungal strains with similar morphologies. This review explores the multifaceted impact of fungi infections on individuals, healthcare providers, and society, highlighting the often-underestimated economic burden and healthcare implications of these infections. In light of the serious constraints of traditional fungal identification methods, this review discusses the potential of plasmonic nanoparticle-based biosensors for fungal infection identification. These biosensors can enable rapid and precise fungal pathogen detection by exploiting several readout approaches, including various spectroscopic techniques, colorimetric and electrochemical assays, as well as lateral-flow immunoassay methods. Moreover, we report the remarkable impact of plasmonic Lab on a Chip technology and microfluidic devices, as they recently emerged as a class of advanced biosensors. Finally, we provide an overview of smartphone-based Point-of-Care devices and the associated technologies developed for detecting and identifying fungal pathogens.

A Ratiometric Fluorescence Probe Based on Silver Nanoclusters and CdSe/ZnS Quantum dots for the Detection of Hydrogen Peroxide by Aggregation and Etching

In this study, a ratiometric fluorescence nanoprobe is developed for the analysis of hydrogen peroxide (H2O2). Silver nanoclusters (AgNCs) were synthesized by chemical reduction method using sodium borohydride (NaBH4) as reducing agent, and were coupled with CdSe/ZnS quantum dots (QDs) to form the ratiometric fluorescence nanoprobe silver nanoclusters-quantum dots (AgNCs-QDs). The effect of the volume ratio of CdSe/ZnS QDs to AgNCs on the fluorescence ratio of AgNCs-QDs was investigated. The fluorescence characterization results show that two emission peaks of AgNCs-QDs are located at 473 nm and 661 nm, respectively. Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) results show that H2O2 can cause the fluorescence probe to aggregate, while etching AgNCs to produce silver ions, which together cause the fluorescence of the QDs in the ratiometric fluorescent probe to be quenched. Based on this strategy, the fluorescence intensity ratio of the two emission peaks F473/F661 exhibits a strong linear correlation with the concentration of H2O2. The detection range is 3.32 µM ~ 2.65 mM with a detection limit of 3.32 µM. In addition, the ratiometric fluorescence probe can specifically recognize H2O2 and has excellent anti-interference performance and good fluorescence stability. Importantly, the probe was utilized for the detection of H2O2 in serum, showing the possibility of the probe in clinical detection applications.

Fluorescent Enzymatic Sensor Based Glucose Oxidase Modified Bovine Serum Albumin-Gold Nanoclusters for Detection of Glucose

An enzymatic fluorescent probe is developed for the selective detection of glucose. In this work, a Bovine Serum Albumin stabilized gold nanocluster (BSA-AuNCs) was synthesized by microwave assisted method, and it is modified with glucose oxidase, thereby a fluorescent enzymatic sensor (BSA-AuNCs@GoX) was designed for the sensitive detection of glucose with a limit of detection of 0.03 mM. The red fluorescence exhibited by the probe is quenched by the production of H2O2 on addition of glucose via. a static quenching mechanism from UV visible absorption and Fluorescence lifetime results. The developed probe exhibits good selectivity and sensitivity with other coexisting molecular species such as glycine, creatinine, methionine, histidine, uric acid, albumin, and ions such as sodium, potassium, calcium, magnesium, zinc etc. that appear in the body fluid. The practical applicability was studied in paper strip and extended its reproducibility in biological matrixes such as human serum and urine and found a good recovery percentage of 94-101 %. By this way, we have fabricated an effective fluorescent enzymatic "turn-off" sensing probe for the detection of glucose.

Fabrication of Au nanoclusters confined on hydroxy double salt-based intelligent biosensor for on-site monitoring of urease and its inhibitors

Sensitive and convenient sensing of urease and its inhibitors is exceptionally urgent in clinical diagnosis and new drug development. In this study, the gold nanoclusters (AuNCs) and hydroxyl double salt (HDS) were composited by a simple confinement effect to prepare highly fluorescent AuNCs@HDS composites to monitor urease and its drug inhibitors. HDS was used as a matrix to confine AuNCs (AuNCs@HDS), facilitating the emission intensity of AuNCs. However, acidic conditions (low pH) can disrupt the structure of HDS to break the confinement effect, and quench the fluorescence of AuNCs. Therefore, a sensing platform for pH-related enzyme urease detection was constructed based on the sensitive response of AuNCs@HDS to pH. This sensing platform had a linear response range of 0.5-22.5 U/L and a low limit of detection (LOD) of 0.19 U/L for urease. Moreover, this sensing platform was also applied to monitor urease inhibitors and urease in human saliva samples. Additionally, a portable hydrogel kit combined with a smartphone was developed for urease detection to achieve portable, low-cost, instrument-free, and on-site monitoring of urease.

Ultrasensitive Electrochemiluminescence Biosensor Based on Efficient Signal Amplification of Copper Nanoclusters Induced by CaMnO3 for CD44 Trace Detection

Metal nanoclusters (Me NCs) have become a research hotspot in the field of electrochemiluminescence (ECL) sensing analysis. This is primarily attributed to their excellent luminescent properties and biocompatibility along with their easy synthesis and labeling characteristics. At present, the application of Me NCs in ECL mainly focuses on precious metals, whose high cost, to some extent, limits their widespread application. In this work, Cu NCs with cathode ECL emissions in persulfate (S2O82-) were prepared as signal probes using glutathione as ligands, which exhibited stable luminescence signals and high ECL efficiency. At the same time, CaMnO3 was introduced as a co-reaction promoter to increase the ECL responses of Cu NCs, thereby further expanding their application potential in biochemical analysis. Specifically, the reversible conversion of Mn3+/Mn4+ greatly promoted the generation of sulfate radicals (SO4•-), providing a guarantee for improving the luminescence signals of Cu NCs. Furthermore, a short peptide (NARKFYKGC) was introduced to enable the fixation of antibodies to specific targets, preventing the occupancy of antigen-binding sites (Fab fragments). Therefore, the sensitivity of the biosensor could be significantly enhanced by releasing additional Fab fragments. Considering the approaches discussed above, the constructed biosensor could achieve sensitive detection of CD44 over a broad range (10 fg/mL-100 ng/mL), with an ultralow detection limit of 3.55 fg/mL (S/N = 3), which had valuable implications for the application of nonprecious Me NCs in biosensing analysis.

Selective fluorescence turn-on detection of combination cisplatin-etoposide chemotherapy based on N-CDs/GSH-CuNCs nanoprobe

Cisplatin (CIS) and etoposide (ETP) combination therapy is highly effective for treating various cancers. However, the potential for pharmacokinetic interactions between these drugs necessitates selective sensing methods to quantitate both CIS and ETP levels in patient's plasma. This work develops a dual fluorescence probe strategy using glutathione-capped copper nanoclusters (GSH-CuNCs) and nitrogen-doped carbon dots (N-CDs) for the simultaneous analysis of CIS and ETP. The fluorescence signal of GSH-CuNCs at 615 nm increased linearly with CIS concentration while the N-CD emission at 480 nm remained unaffected. Conversely, the N-CD fluorescence was selectively enhanced by ETP with no interference with the CuNC fluorescence. Extensive materials characterization including UV-vis, fluorescence spectroscopy, XRD, and TEM confirmed the synthesis of the nanoprobes. The sensor showed high sensitivity with limits of detection of 6.95 ng mL-1 for CIS and 7.63 ng mL-1 for ETP along with excellent selectivity against potential interferences in rabbit plasma. Method feasibility was demonstrated with application to real rabbit plasma samples. The method was further applied to estimate the pharmacokinetic parameters of CIS before and after ETP coadministration. The dual nanoprobe sensing strategy enables rapid and selective quantitation of CIS and ETP levels to facilitate therapeutic drug monitoring and optimization of combination chemotherapy regimens.

Accelerated size-focusing light activated synthesis of atomically precise fluorescent Au22(Lys-Cys-Lys)16 clusters

Atomically precise metal nanoclusters are promising candidates for various biomedical applications, including their use as photosensitizers in photodynamic therapy (PDT). However, typical synthetic routes of clusters often result in complex mixtures, where isolating and characterizing pure samples becomes challenging. In this work, a new Au22(Lys-Cys-Lys)16 cluster is synthesized using photochemistry, followed by a new type of light activated, accelerated size-focusing. Fluorescence excitation-emission matrix spectroscopy (EEM) and parallel factor (PARAFAC) analysis have been applied to track the formation of fluorescent species, and to assess optical purity of the final product. Furthermore, excited state reactivity of Au22(Lys-Cys-Lys)16 clusters is studied, and formation of type-I reactive oxygen species (ROS) from the excited state of the clusters is observed. The proposed size-focusing procedure in this work can be easily adapted to conventional cluster synthetic methods, such as borohydride reduction, to provide atomically precise clusters.

Towards Green Synthesis of Fluorescent Metal Nanoclusters

In the modern development of nanoscience and nanotechnology, metal nanoclusters have emerged as a foremost category of nanomaterials exhibiting remarkable biocompatibility and photo-stability having dramatically distinctive optical, electronic, and chemical properties. This review focuses on synthesizing fluorescent metal nanoclusters in a greener way to make them suitable for biological imaging and drug delivery application. The green methodology is the desired route for sustainable chemical production and should be utilized for any form of chemical synthesis including nanomaterials. It aims to eliminate harmful waste, uses non-toxic solvents, and employs energy-efficient processes for the synthesis. This article provides an overview of conventional synthesis methods, including stabilizing nanoclusters by small organic molecules in organic solvents. Then we focus on the improvement of properties, applications of green synthesized metal nanoclusters, challenges involved, and further advancement required in the direction of green synthesis of MNCs. There are plenty of problems for scientists to solve to make nanoclusters suitable for bio-applications, chemical sensing, and catalysis synthesized by green methods. Using bio-compatible and electron-rich ligands, understanding ligand-metal interfacial interactions, employing more energy-efficient processes, and utilizing bio-inspired templates for synthesis are some immediate problems worth solving in this field that requires continued efforts and interdisciplinary knowledge and collaboration.

Catalytic probes based on aggregation-induced emission-active Au nanoclusters for visualizing MicroRNA in living cells and in vivo

Highly sensitive monitoring of cancer-related miRNAs is of great significance for tumor diagnosis. Herein, catalytic probes based on DNA-functionalized Au nanoclusters (AuNCs) were prepared in this work. The aggregation-induced emission-active Au nanoclusters showed an interesting phenomenon of aggregation induced emission (AIE) affected by the aggregation state. Leveraging this property, the AIE-active AuNCs were used to develop catalytic turn-on probes for detecting in vivo cancer-related miRNA based on a hybridization chain reaction (HCR). The target miRNA triggered the HCR and induced aggregation of AIE-active AuNCs, leading to a highly luminescent signal. The catalytic approach demonstrated a remarkable selectivity and a low detection limit in comparison to noncatalytic sensing signals. In addition, the excellent delivery the ability of MnO₂ carrier made it possible to use the probes for intracellular imaging and in vivo imaging. Effective in situ visualization of miR-21 was achieved not only in living cells but also in tumors in living animals. This approach potentially offers a novel method for obtaining information for tumor diagnosis via highly sensitive cancer-related miRNA imaging in vivo.

A sensitive multicolor fluorescence sensing strategy for chlorotetracycline based on bovine serum albumin-stabilized copper nanocluster

Fluorescent probes with on-site visual detection function have received extensive attention in the detection of chlortetracycline (CTC), which was widely used in aquaculture and animal husbandry. Copper nanoclusters (Cu NCs) with excellent optical properties were prepared using bovine serum albumin (BSA) as a template, and a multicolor fluorescence strategy based on BSA-stabilized Cu NCs (BSA-Cu NCs) for detecting CTC was proposed. BSA-Cu NCs had a red emission at 640 nm. After the addition of CTC, the red emission of BSA-Cu NCs gradually decreased for internal filtering effect, while the green emission of CTC was significantly enhanced under the sensitization of BSA. This simple sensing process can be achieved in real time by directly mixing the target sample with BSA-Cu NCs, and the detection limit (LOD) of the system for CTC was 12.01 nM. Based on this sensing strategy, a fluorescence film sensing detection platform was constructed to achieve ultra-fast detection of CTC within 30 s. This work provided a fluorescent film sensor with the advantages of portability, ultra-fast and low cost, which provided a feasible alternative for on-site ultra-fast screening of CTC.

Delayed luminescence guided enhanced circularly polarized emission in atomically precise copper nanoclusters

Metal nanoclusters, owing to their intriguing optical properties, have captivated research interest over the years. Of special interest have been chiral nanoclusters that display optical activity in the visible region of the electromagnetic spectrum. While the ground state chiral properties of metal nanoclusters have been reasonably well studied, of late research focus has shifted attention to their excited state chiral investigations. Herein, we report the synthesis and chiral investigations of a pair of enantiomerically pure copper nanoclusters that exhibit intense optical activity, both in their ground and excited states. The synthesis of nanoclusters using l- and d-isomers of the chiral ligand led to the formation of metal clusters that displayed mirror image circular dichroism and circularly polarized luminescence signals. Structural validation using single crystal XRD, powder XRD and XPS in conjunction with chiroptical and computational analysis helped to develop a structure-property correlation that is unique to such clusters. Investigations on the mechanism of photoluminescence revealed that the system exhibits long excited state lifetimes. A combination of delayed luminescence and chirality resulted in circularly polarized delayed luminescence, a phenomenon that is rather uncommon to the field of metal clusters. The chiral emissive properties could be successfully demonstrated in free-standing polymeric films highlighting their potential for use in the field of data encryption, security tags and polarized light emitting devices. Moreover, the fundamental understanding of the mechanism of excited state chirality in copper clusters opens avenues for the exploration of similar effects in a variety of other clusters.

A dual-signal fluorometric and colorimetric sensing platform based on gold-platinum bimetallic nanoclusters for the determination of β-galactosidase activity

Herein, a novel dual-signal sensing system for the determination of β-galactosidase (β-Gal) activity was established, which was based on a dual-emission probe assembled from gold-platinum bimetallic nanoclusters (Au-Pt NCs) and rhodamine B. Under the catalysis of β-Gal, 4-nitrophenyl β-D-galactopyranoside (PNPG) was rapidly hydrolyzed to generate p-nitrophenol (PNP), which has an obvious UV absorption peak at 400 nm. The hydrolyzed product PNP can quench the fluorescence of Au-Pt NCs effectively by inner filter effect (IFE), and PNP had no impact on the fluorescence of rhodamine B, which will change the emission intensity ratio of Au-Pt NCs and rhodamine B. Therefore, the ratiometric fluorescent and colorimetric dual-signal sensor based on Au-Pt NCs and rhodamine B was successfully constructed for sensitive detection of β-Gal activity. The linear detection range for the ratiometric fluorescence and colorimetric methods were 2.5-25 U/L and 15-55 U/L with detection limits of 1.2 U/L and 5.2 U/L, respectively. The developed assay method has been used for quantitative detection of β-Gal in spiked serum samples and showed good performance. And the detection platform has high reliability and excellent selectivity, which opens a new avenue for the further application of Au-Pt NCs in chemical sensing and biological analysis.

Ligand-Mediated Control of the Surface Oxidation States of Copper Nanoparticles Produced by Laser Ablation

We report on studies that demonstrate how the chemical composition of the surface of copper nanoparticles (CuNPs) - in terms of percentage copper(I/II) oxides - can be varied by the presence of N-donor ligands during their formation via laser ablation. Changing the chemical composition thus allows systematic tuning of the surface plasmon resonance (SPR) transition. The trialed ligands include pyridines, tetrazoles, and alkylated tetrazoles. CuNPs formed in the presence of pyridines, and alkylated tetrazoles exhibit a SPR transition only slightly blue shifted with respect to CuNPs formed in the absence of any ligand. On the other hand, the presence of tetrazoles results in CuNPs characterized by a significant blue shift of the order of 50-70 nm. By comparing these data also with the SPR of CuNPs formed in the presence of carboxylic acids and hydrazine, this work demonstrates that the blue shift in the SPR is due to tetrazolate anions providing a reducing environment to the nascent CuNPs, thus preventing the formation of copper(II) oxides. This conclusion is further supported by the fact that both AFM and TEM data indicate only small variations in the size of the nanoparticles, which is not enough to justify a 50-70 nm blue-shift of the SPR transition. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies further confirm the absence of Cu(II)-containing CuNPs when prepared in the presence of tetrazolate anions.

Latest strategies for rapid and point of care detection of mycotoxins in food: A review

Mycotoxins contaminated in agricultural products are often highly carcinogenic and genotoxic to humans. With the streamlining of the food industry chain and the improvement of food safety requirements, the traditional laboratory testing mode is constantly challenged due to the expensive equipment, complex operation steps, and lag in testing results. Therefore, rapid detection methods are urgently needed in the food safety system. This review focuses on the latest strategies that can achieve rapid and on-site testing, with particular attention to the nanomaterials integrated biosensors. To provide researchers with the latest trends and inspiration in the field of rapid detection, we summarize several strategies suitable for point of care testing (POCT) of mycotoxins, including enzyme-linked immunoassay (ELISA), lateral flow assay (LFA), fluorescence, electrochemistry, and colorimetry assay. POCT-based strategies are all developing towards intelligence and portability, especially when combined with smartphones, making it easier to read signals for intuitive access and analysis of test data. Detection performance of the devices has also improved considerably with the integration of biosensors and nanomaterials.

Emerging ultrasmall luminescent nanoprobes for in vivo bioimaging

Photoluminescence (PL) imaging has become a fundamental tool in disease diagnosis, therapeutic evaluation, and surgical navigation applications. However, it remains a big challenge to engineer nanoprobes for high-efficiency in vivo imaging and clinical translation. Recent years have witnessed increasing research efforts devoted into engineering sub-10 nm ultrasmall nanoprobes for in vivo PL imaging, which offer the advantages of efficient body clearance, desired clinical translation potential, and high imaging signal-to-noise ratio. In this review, we present a comprehensive summary and contrastive discussion of emerging ultrasmall luminescent nanoprobes towards in vivo PL bioimaging of diseases. We first summarize size-dependent nano-bio interactions and imaging features, illustrating the unique attributes and advantages/disadvantages of ultrasmall nanoprobes differentiating them from molecular and large-sized probes. We also discuss general design methodologies and PL properties of emerging ultrasmall luminescent nanoprobes, which are established based on quantum dots, metal nanoclusters, lanthanide-doped nanoparticles, and silicon nanoparticles. Then, recent advances of ultrasmall luminescent nanoprobes are highlighted by surveying their latest in vivo PL imaging applications. Finally, we discuss existing challenges in this exciting field and propose some strategies to improve in vivo PL bioimaging and further propel their clinical applications.

Recyclable fluorescence sensing based on copper clusters for simultaneous determination of copper ions and ammonia

A one-step strategy for synthesizing fluorescent copper clusters stabilized by L-cysteine has been successfully established in aqueous solutions. The direct determination of copper ions was realized by the fluorescence enhancement phenomenon caused by the preparation and aggregation process. At the same time, ammonia treatment can lead to rapid fluorescence quenching, resulting from the influence on the aggregation behavior of Cu clusters, while the fluorescence can be recovered by the continuous addition of copper ions. Therefore, a recyclable fluorescence sensing system is constructed for the simultaneous determination of copper ions and ammonia. This method is simple, anti-interference and has been successfully applied to the determination of environmental samples.

Aggregation-Induced Emission-Active Nanostructures: Beyond Biomedical Applications

The discovery of aggregation-induced emission (AIE) phenomenon in 2001 has had a significant impact on materials development across different research disciplines. AIE-active materials have been widely exploited for various applications in optoelectronics, sensing, biomedical, and stimuli-responsive systems, etc. This is made possible by integrating AIE features with other fields of science and engineering, such as nanoscience and nanotechnology. AIE has been extensively employed, particularly for biomedical applications, such as biosensing, bioimaging, and theranostics. However, development of AIE-based nanotechnology for other applications is comparatively less, although there have been increasing research activities in recent years. Given the significance and potential of the marriage between AIE hallmark and nanotechnology in AIE-active materials development, this review article summarizes and showcases the latest research efforts in AIE-based nanomaterials, including nanomaterials synthesis and their nonbiomedical applications, such as sensing, optoelectronics, functional coatings, and stimuli-responsive systems. A perspective on the outlook of AIE-based nanostructured materials and relevant nanotechnology for nonbiomedical applications will be provided, giving an insight into how to design AIE-active nanostructures as well as their applications beyond the biomedical domain.

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

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

Recent advances in simultaneous detection strategies for multi-mycotoxins in foods

Mycotoxin contamination has become a challenge in the field of food safety testing, given the increasing emphasis on food safety in recent years. Mycotoxins are widely distributed, in heavily polluted areas. Food contamination with these toxins is difficult to prevent and control. Mycotoxins, as are small-molecule toxic metabolites produced by several species belonging to the genera Aspergillus, Fusarium, and Penicillium growing in food. They are considered teratogenic, carcinogenic, and mutagenic to humans and animals. Food systems are often simultaneously contaminated with multiple mycotoxins. Due to the additive or synergistic toxicological effects caused by the co-existence of multiple mycotoxins, their individual detection requires reliable, accurate, and high-throughput techniques. Currently available, methods for the detection of multiple mycotoxins are mainly based on chromatography, spectroscopy (colorimetry, fluorescence, and surface-enhanced Raman scattering), and electrochemistry. This review provides a comprehensive overview of advances in the multiple detection methods of mycotoxins during the recent 5 years. The principles and features of these techniques are described. The practical applications and challenges associated with assays for multiple detection methods of mycotoxins are summarized. The potential for future development and application is discussed in an effort, to provide standards of references for further research.

Label-Free and Bioluminescence-Based Nano-Biosensor for ATP Detection

A bioluminescence-based assay for ATP can measure cell viability. Higher ATP concentration indicates a higher number of living cells. Thus, it is necessary to design an ATP sensor that is low-cost and easy to use. Gold nanoparticles provide excellent biocompatibility for enzyme immobilization. We investigated the effect of luciferase proximity with citrate-coated gold, silver, and gold-silver core-shell nanoparticles, gold nanorods, and BSA-Au nanoclusters. The effect of metal nanoparticles on the activity of luciferases was recorded by the luminescence assay, which was 3-5 times higher than free enzyme. The results showed that the signal stability in presence of nanoparticles improved and was reliable up to 6 h for analytes measurements. It has been suggested that energy is mutually transferred from luciferase bioluminescence spectra to metal nanoparticle surface plasmons. In addition, we herein report the 27-base DNA aptamer for adenosine-5'-triphosphate (ATP) as a suitable probe for the ATP biosensor based on firefly luciferase activity and AuNPs. Due to ATP application in the firefly luciferase reaction, the increase in luciferase activity and improved detection limits may indicate more stability or accessibility of ATP in the presence of nanoparticles. The bioluminescence intensity increased with the ATP concentration up to 600 µM with a detection limit of 5 µM for ATP.

Engineering colloidally stable, highly fluorescent and nontoxic Cu nanoclusters via reaction parameter optimization

Metal nanoclusters (NCs) composed of the least number of atoms (a few to tens) have become very attractive for their emerging properties owing to their ultrasmall size. Preparing copper nanoclusters (Cu NCs) in an aqueous medium with high emission properties, strong colloidal stability, and low toxicity has been a long-standing challenge. Although Cu NCs are earth-abundant and inexpensive, they have been comparatively less explored due to their various limitations, such as ease of surface oxidation, poor colloidal stability, and high toxicity. To overcome these constraints, we established a facile synthetic route by optimizing the reaction parameters, especially altering the effective concentration of the reducing agent, to influence their optical characteristics. The improvement of the photoluminescence intensity and superior colloidal stability was modeled from a theoretical standpoint. Moreover, the as-synthesized Cu NCs showed a significant reduction of toxicity in both in vitro and in vivo models. The possibility of using such Cu NCs as a diagnostic probe toward C. elegans was explored. Also, the extension of our approach toward improving the photoluminescence intensity of the Cu NCs on other ligand systems was demonstrated.

Application of Nanomaterials for Coping with Mycotoxin Contamination in Food Safety: From Detection to Control

Mycotoxins, which are toxic secondary metabolites produced by fungi, are harmful to humans. Mycotoxin-induced contamination has drawn attention worldwide. Consequently, the development of reliable and sensitive detection methods and high-efficiency control strategies for mycotoxins is important to safeguard food industry safety and public health. With the rapid development of nanotechnology, many novel nanomaterials that provide tremendous opportunities for greatly improving the detection and control performance of mycotoxins because of their unique properties have emerged. This review comprehensively summarizes recent trends in the application of nanomaterials for detecting mycotoxins (fluorescence, colorimetric, surface-enhanced Raman scattering, electrochemical, and point-of-care testing) and controlling mycotoxins (inhibition of fungal growth, mycotoxin absorption, and degradation). These detection methods possess the advantages of high sensitivity and selectivity, operational simplicity, and rapidity. With research attention on the control of mycotoxins and the gradual excavation of the properties of nanomaterials, nanomaterials are also employed for the inhibition of fungal growth, mycotoxin absorption, and mycotoxin degradation, and impressive controlling effects are obtained. This review is expected to provide the readers insight into this state-of-the-art area and a reference to design nanomaterials-based schemes for the detection and control of mycotoxins.

DNA Templated Silver Nanoclusters for Bioanalytical Applications: A Review

Due to their unique programmability, biocompatibility, photostability and high fluorescent quantum yield, DNA templated silver nanoclusters (DNA Ag NCs) have attracted increasing attention for bioanalytical application. This review summarizes the recent developments in fluorescence properties of DNA templated Ag NCs, as well as their applications in bioanalysis. Finally, we herein discuss some current challenges in bioanalytical applications, to promote developments of DNA Ag NCs in biochemical analysis.

Supramolecular Self-Assembly of Atomically Precise Silver Nanoclusters with Chiral Peptide for Temperature Sensing and Detection of Arginine

Metal nanoclusters (NCs) as a new type of fluorescent material have attracted great interest due to their good biocompatibilities and outstanding optical properties. However, most of the studies on metal NCs focus on the synthesis, atomic or molecular assembly, whereas metal NCs ability to self-assemble to higher-level hierarchical nanomaterials through supramolecular interactions has rarely been reported. Herein, we investigate atomic precise silver NCs (Ag₉-NCs, [Ag₉(mba)₉], where H₂mba = 2-mercaptobenzoic acid) and peptide DD-5 were used to induce self-assembly, which can trigger an aggregation-induced luminescence (AIE) effect of Ag₉-NCs through non-covalent forces (H-bond, π–π stacking) and argentophilic interactions [Ag(I)–Ag(I)]. The large Stokes shift (~140 nm) and the microsecond fluorescence lifetime (6.1 μs) indicate that Ag₉-NCs/DD-5 hydrogel is phosphor. At the same time, the chirality of the peptide was successfully transferred to the achiral Ag₉-NCs because of the supramolecular self-assembly, and the Ag₉-NCs/DD-5 hydrogel also has good circularly polarized luminescence (CPL) properties. In addition, Ag₉-NCs/DD-5 luminescent hydrogel is selective and sensitive to the detection of small biological molecule arginine. This work shows that DD-5 successfully induces the self-assembly of Ag₉-NCs to obtain high luminescent gel, which maybe become a candidate material in the fields of sensors and biological sciences.

Near-Infrared II Gold Nanocluster Assemblies with Improved Luminescence and Biofate for In Vivo Ratiometric Imaging of H2S

Ultrasmall gold nanoclusters (AuNCs) are emerging as promising luminescent nanoprobes for bioimaging due to their fantastic photoluminescence (PL) and renal-clearable ability. However, it remains a great challenge to design them for in vivo sensitive molecular imaging in desired tissues. Herein, we have developed a strategy to tailor the PL and biofate of near-infrared II (NIR-II)-emitting AuNCs via ligand anchoring for improved bioimaging. By optimizing the ligand types in AuNCs and using Er³⁺-doped lanthanide (Ln) nanoparticles as models, core-satellite Ln@AuNCs assemblies were rationally constructed, which enabled 2.5-fold PL enhancement of AuNCs at 1100 nm and prolonged blood circulation compared to AuNCs. Significantly, Ln@AuNCs with dual intense NIR-II PL (from AuNCs and Er³⁺) can effectively accumulate in the liver for ratiometric NIR-II imaging of H₂S, facilitated by H₂S-mediated selective PL quenching of AuNCs. We have then demonstrated the real-time imaging evaluation of liver delivery efficacy and dynamics of two H₂S prodrugs. This shows a paradigm to visualize liver H₂S delivery and its prodrug screening in vivo. Note that Ln@AuNCs are body-clearable via the hepatobiliary excretion pathway, thus reducing potential long-term toxicity. Such findings may propel the engineering of AuNC nanoprobes for advancing in vivo bioimaging analysis.

The Multifarious Applications of Copper Nanoclusters in Biosensing and Bioimaging and Their Translational Role in Early Disease Detection

Nanoclusters possess an ultrasmall size, amongst other favorable attributes, such as a high fluorescence and long-term colloidal stability, and consequently, they carry several advantages when applied in biological systems for use in diagnosis and therapy. Particularly, the early diagnosis of diseases may be facilitated by the right combination of bioimaging modalities and suitable probes. Amongst several metallic nanoclusters, copper nanoclusters (Cu NCs) present advantages over gold or silver NCs, owing to their several advantages, such as high yield, raw abundance, low cost, and presence as an important trace element in biological systems. Additionally, their usage in diagnostics and therapeutic modalities is emerging. As a result, the fluorescent properties of Cu NCs are exploited for use in optical imaging technology, which is the most commonly used research tool in the field of biomedicine. Optical imaging technology presents a myriad of advantages over other bioimaging technologies, which are discussed in this review, and has a promising future, particularly in early cancer diagnosis and imaging-guided treatment. Furthermore, we have consolidated, to the best of our knowledge, the recent trends and applications of copper nanoclusters (Cu NCs), a class of metal nanoclusters that have been gaining much traction as ideal bioimaging probes, in this review. The potential modes in which the Cu NCs are used for bioimaging purposes (e.g., as a fluorescence, magnetic resonance imaging (MRI), two-photon imaging probe) are firstly delineated, followed by their applications as biosensors and bioimaging probes, with a focus on disease detection.

Ratiometric Detection of microRNA Using Hybridization Chain Reaction and Fluorogenic Silver Nanoclusters

miRNA (miR)-155 is a potential biomarker for breast cancers. We aimed at developing a nanosensor for miR-155 detection by integrating hybridization chain reaction (HCR) and silver nanoclusters (AgNCs). HCR serves as an enzyme-free and isothermal amplification method, whereas AgNCs provide a built-in fluorogenic detection probe that could simplify the downstream analysis. The two components were integrated by adding a nucleation sequence of AgNCs to the hairpin of HCR. The working principle was based on the influence of microenvironment towards the hosted AgNCs, whereby unfolding of hairpin upon HCR has manipulated the distance between the hosted AgNCs and cytosine-rich toehold region of hairpin. As such, the dominant emission of AgNCs changed from red to yellow in the absence and presence of miR-155, enabling a ratiometric measurement of miR with high sensitivity. The limit of detection (LOD) of our HCR-AgNCs nanosensor is 1.13 fM in buffered solution. We have also tested the assay in diluted serum samples, with comparable LOD of 1.58 fM obtained. This shows the great promise of our HCR-AgNCs nanosensor for clinical application.

Advances and Challenges of Fluorescent Nanomaterials for Synthesis and Biomedical Applications

With the rapid development of nanotechnology, new types of fluorescent nanomaterials (FNMs) have been springing up in the past two decades. The nanometer scale endows FNMs with unique optical properties which play a critical role in their applications in bioimaging and fluorescence-dependent detections. However, since low selectivity as well as low photoluminescence efficiency of fluorescent nanomaterials hinders their applications in imaging and detection to some extent, scientists are still in search of synthesizing new FNMs with better properties. In this review, a variety of fluorescent nanoparticles are summarized including semiconductor quantum dots, carbon dots, carbon nanoparticles, carbon nanotubes, graphene-based nanomaterials, noble metal nanoparticles, silica nanoparticles, phosphors and organic frameworks. We highlight the recent advances of the latest developments in the synthesis of FNMs and their applications in the biomedical field in recent years. Furthermore, the main theories, methods, and limitations of the synthesis and applications of FNMs have been reviewed and discussed. In addition, challenges in synthesis and biomedical applications are systematically summarized as well. The future directions and perspectives of FNMs in clinical applications are also presented.

AIE materials for lysosome imaging

The aggregation-induced emission (AIE) active bioprobes are known for their high photostability and extraordinary signal to noise ratio. In view of this, research efforts to synthesize new AIE bioimaging probes are at an incredible speed. In this chapter, we have summarized the various lysosome specific AIE active "turn-on" bioprobes having applications in lysosome imaging, monitoring of lysosome bioactivity and evaluation of their therapeutic effects. By discussing their design and operational mechanisms, we hope to provide more insight into designing new AIE bioprobes for specific sensing and imaging of lysosome having flexibility for broad range of biomedical applications.

Simple and Sensitive Multi-components Detection Using Synthetic Nitrogen-doped Carbon Dots Based on Soluble Starch

Although carbon dots (CDs) as fluorescent sensors have been widely exploited, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, two kinds of nitrogen-doped CDs (NCD-m and NCD-o) based on soluble starch (SS) as carbon source were prepared through one-pot hydrothermal process using m-phenylenediamine and o-phenylenediamine as nitrogenous dopant respectively. Through fluorescence "on-off" mechanism of CDs, NCD-m and NCD-o could be used as a fluorescence sensor for detection of Fe 3+ and Ag + with LOD of 0.25 and 0.51 μM, respectively. Additionally, NCD-m could be used for indirect detection of ascorbic acid (AA) with LOD of 5.02 μM. Moreover, fluorescence intensity of NCD-m also exhibited the sensitivity to pH change from 2 to 13. More importantly, Both NCD-m and NCD-o had potential application for analysis of complicated real samples such as tap water, Vitamin C tablets and orange juice. Ultimately, the small size of NCD-m could contribute to reinforcing intracellular endocytosis, which allowed them to be used for bacteria imaging. Obviously, these easily obtainable nitrogen-doped CDs were able to be used for multi-components detection. Strategy for synthesis of nitrogen-doped carbon dots (NCDs) and a schematic for fabrication of as-prepared NCDs for detection of Fe 3+, Ag + and ascorbic acid (AA).

Rapid field testing of mercury pollution by designed fluorescent biosensor and its cells-alginate hydrogel-based paper assay

With increasing industrial activities, mercury has been largely discharged into environment and caused serious environmental problems. The growing level of mercury pollution has become a huge threat to human health due to its significant biotoxicity. Therefore, the simple and fast means for on-site monitoring discharged mercury pollution are highly necessary to protect human beings from its pernicious effects in time. Herein, a "turn off" fluorescent biosensor (mCherry L199C) for sensing Hg²⁺ was successfully designed based on direct modification of the chromophore environment of fluorescent protein mCherry. For rapid screening and characterization, the designed variant of mCherry (mCherry L199C) was directly expressed on outer-membrane of  Escherichia coli cells by cell surface display technique. The fluorescent biosensor was characterized to have favorable response to Hg²⁺ at micromole level among other metal ions and over a broad pH range. Further, the cells of the fluorescent biosensor were encapsulated in alginate hydrogel to develop the cells-alginate hydrogel-based paper. The cells-alginate hydrogel-based paper could detect mercury pollution in 5 min with simple operation process and inexpensive equipment, and it could keep fluorescence and activity stable at 4 °C for 24 hr, which would be a high-throughput screening tool in preliminarily reporting the presence of mercury pollution in natural setting.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

A fluorescent PET probe based on polyethyleneimine-Ag nanoclusters as a reversible, stable and selective broad-range pH sensor

In this work, nanoclusters (NCs) of Cu and Ag capped with hyperbranched polyethyleneimine (PEI) were prepared using chemical reduction by a one-step hydrothermal method. The PEI coated-NCs were characterized by high-resolution transmission electron microscopy, ζ potential, thermogravimetric analysis, dynamic light scattering, Fourier-transform infrared, UV-visible, and fluorescence spectroscopy. The PEI-NCs exhibited strong absorption and fluorescence, high stability, and excellent water dispersibility. The resulting PEI-NCs showed a reversible and linear response of fluorescence intensity with pH over a wide range (3-11); however, PEI-AgNCs showed a better reversibility and sensitivity than PEI-CuNCs. Unlike several types of pH sensors based on modified NCs, which are based on a nanoparticle aggregation/disaggregation mechanism, the response of our sensor is based on a photoinduced electron transfer process, which gives it a high reversibility. This method was successfully applied in pH measurements in tap water and green tea samples, with excellent results, indicating its practical utility for these applications. A visual device was obtained by immobilizing PEI-AgNCs into agarose hydrogels at different pH values. The results show that the proposed sensor can be used as a pH visual detector. Besides, the light emission of the nanosensor was corroborated by fluorescence microscopy, confirming that the nanosensor based on PEI-AgNCs has great potential to be used in cellular imaging.