Fluorescent Gold Nanocluster-Based Sensor Array for Nitrophenol Isomer Discrimination via an Integration of Host-Guest Interaction and Inner Filter Effect

Surface-Engineered Nanomaterials for Optical Array Based Sensing

Array based sensing governed by optical methods provides fast and economic way for detection of wide variety of analytes where the ideality of detection processes depends on the sensor element's versatile mode of interaction with multiple analytes in an unbiased manner. This can be achieved by either the receptor unit having multiple recognition moiety, or their surface property should possess tuning ability upon fabrication called surface engineering. Nanomaterials have a high surface to volume ratio, making them viable candidates for molecule recognition through surface adsorption phenomena, which makes it ideal to meet the above requirements. Most crucially, by engineering a nanomaterial's surface, one may produce cross-reactive responses for a variety of analytes while focusing solely on a single nanomaterial. Depending on the nature of receptor elements, in the last decade the array-based sensing has been considering as multimodal detection platform which operates through various pathway including single channel, multichannel, binding and indicator displacement assay, sequential ON-OFF sensing, enzyme amplified and nanozyme based sensing etc. In this review we will deliver the working principle for Array-based sensing by using various nanomaterials like nanoparticles, nanosheets, nanodots and self-assembled nanomaterials and their surface functionality for suitable molecular recognition.

A Multifunctional CaII-EuIII Heterometallic Organic Framework with Sensing and Selective Adsorption in Water

With increasing global industrialization, it is urgent and challenging to develop multifunctional species for detection and adsorption in the environment. For this purpose, a novel anionic heterometallic organic framework, [(CH3)2NH2][CaEu(CAM)2(H2O)2]·4H2O·4DMF (CaEuCAM), is hydrothermally synthesized based on chelidamic acid (H3CAM). Single crystal analysis shows that CaEuCAM features two different oxygen-rich channels along the c-axis in which one CAM3- bridges two sextuple-coordinated Ca2+ and two octuple-coordinated Eu3+ with a μ4-η1: η1: η1: η1: η1: η1 new chelating and bridging mode. The characteristic bright red emission and superior hydrostability of CaEuCAM under harsh acidic and basic conditions benefit it by acting as a highly sensitive sensor for Fe3+ and 3-nitrophenol (3-NP) with extremely low LODs through remarkable quenching. The combination of experiments and theoretical calculations for sensing mechanisms shows that the competitive absorption and interaction are responsible for Fe3+-induced selective emission quenching, while that for 3-NP is the result of the synergism of host-guest chemistry and the inner filter effect. Meanwhile, the assimilation of negative charge plus channels renders CaEuCAM a highly selective adsorbent for methylene blue (MB) due to a synergy of electrostatic affinity, ion-dipole interaction, and size matching. Of note is the reusability of CaEuCAM toward Fe3+/3-NP sensing and MB adsorption besides its fast response. These findings could be very useful in guiding the development of multifunctional Ln-MOFs for sensing and adsorption applications in water media.

Recent Advances in Gold Nanocluster-Based Biosensing and Therapy: A Review

Gold nanoclusters (Au NCs) with bright emission and unique chemical reactivity characters have been widely applied for optical sensing and imaging. With a combination of surface modifications, effective therapeutic treatments of tumors are realized. In this review, we summarize the recently adopted biosensing and therapy events based on Au NCs. Homogeneous and fluorometric biosensing systems toward various targets, including ions, small molecules, reactive oxygen species, biomacromolecules, cancer cells, and bacteria, in vitro and in vivo, are presented by turn-off, turn-on, and ratiometric tactics. The therapy applications are concluded in three aspects: photodynamic therapy, photothermal therapy, and as a drug carrier. The basic mechanisms and performances of these systems are introduced. Finally, this review highlights the challenges and future trend of Au NC-based biosensing and therapy systems.

Highly selective detection of 2,4-dinitrophenol by fluorescent NH2-MIL-125(Ti) via dual-parameter sensing technology

The organic contaminant 2,4-dinitrophenol (2,4-DNP) is widely prevalent and poses significant risks to human health. Although numerous in-depth studies having been reported on the highly sensitive detection of 2,4-DNP, there are still challenges to its selective detection. Here, the fluorescence intensity ratio (I0/I) and emission peak shift (Δλ) were utilized for selective detection of 2,4-DNP by NH2-MIL-125(Ti). Notably, the emission peak of the NH2-MIL-125(Ti) suspension exhibited a remarkable red shift in the presence of 2,4-DNP (Δλ = 26 nm), accompanied by the blue shift or weak red shift of analogs, which provided a solid basis for selective detection of 2,4-DNP. Meanwhile, the I0/I ratio of the NH2-MIL-125(Ti) suspension exhibited a robust linear correlation with 2,4-DNP at the low concentration range (0-70 μM). The interaction of the analyte with NH2-MIL-125(Ti) was revealed to involve intermolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) through XPS, FTIR, and UV-vis absorption spectroscopy. Additionally, we achieved the detection of 2,4-DNP using a smartphone by recognizing both the blue (B) values and the luminance (L) values. The obtained results demonstrated that the NH2-MIL-125(Ti) probe based on dual-parameter sensing technology exhibited excellent potential for selectively detecting 2,4-DNP in water environments, thereby offering significant prospects for its application in water quality assessment.

Antimicrobial Agent Functional Gold Nanocluster-Mediated Multichannel Sensor Array for Bacteria Sensing

Urinary tract infections (UTIs) have greatly affected human health in recent years. Accurate and rapid diagnosis of UTIs can enable a more effective treatment. Herein, we developed a multichannel sensor array for efficient identification of bacteria based on three antimicrobial agents (vancomycin, lysozyme, and bacitracin) functional gold nanoclusters (AuNCs). In this sensor, the fluorescence intensity of the three AuNCs was quenched to varying degrees by the bacterial species, providing a unique fingerprint for different bacteria. With this sensing platform, seven pathogenic bacteria, different concentrations of the same bacteria, and even bacterial mixtures were successfully differentiated. Furthermore, UTIs can be accurately identified with our sensors in ∼30 min with 100% classification accuracy. The proposed sensing systems offer a rapid, high-throughput, and reliable sensing platform for the diagnosis of UTIs.

A single recognition unit-based virtual sensor Array: Applying 3D fluorescence spectroscopy to inner filter effect-based sensing

A convenient, fast, low-cost detection and discrimination method is demanded for environmental monitoring but still it remains more technological challenges. Herein, we demonstrate that the inner filter effect (IFE), in combination with three-dimensional fluorescence spectroscopy, can offer a virtual sensor array (VSA) as apropersolution. And with the aid of pattern recognition techniques, it is feasible to recognize compounds with structural similarities economically and effectively. In this study, with the help of visual clustering plots of principal component analysis (PCA), a prediction model based on hierarchical strategy was made using support vector machine (SVM) method for the qualitative profiling of aromatic pollutants. The VSA was constructed by a single metal-organic framework (MOF) recognition unit (MOF-74 (Zn)) with the excitation wavelength as external regulatory factors. Pattern characteristics of four aromatics with very similar structures (phenylamine, chlorobenzene, nitrobenzene, and phenol), both single analyte and binary mixtures, were acquired. The primary constituents of multi-dimensional spectral signals were subsequently extracted and fed into a vector machine to construct a prediction model through 10-fold cross-validation optimization, resulting in a classification accuracy of 100% for single analytes and 96% for mixtures. Quantitative research has shown that, except for chlorobenzene, all three other analytes can be predicted in concentration within an acceptable error range, and the mixture can be predicted proportionally. Moreover, the VSA can be used to distinguish these pollutants in tap and river water also. We propose for the first time a new tack for the construction of VSA in a general manner, namely using three-dimensional full range fluorescence scanning for IFE based sensing to get multiple times of information resulting from different weak interaction between analyte and sensor for decision-making.

Mechanistic elucidation of the catalytic activity of silver nanoclusters: exploring the predominant role of electrostatic surface

The core and the ligand shell of metal nanoclusters (MNCs) have an influential role in modulating their spectroscopic signatures and catalytic properties. The aspect of electrostatic interactions to regulate the catalytic properties of MNCs has not been comprehensively addressed to date. Our present work conclusively delineates the role of the metal core and the electrostatic surface of MNCs involved in the reduction of nitroarenes. A facile surface modification of mercaptosuccinic acid (MSA)-templated AgNCs has been selectively achieved through Mg2+ ions (Mg-AgNCs). Microscopic studies suggest that the size of Mg-AgNCs is ∼3.3 nm, which is considerably higher than that of MSA-templated AgNCs (∼1.75 nm), confirming the formation of a nano-assembled structure. Our spectroscopic and microscopic experiments revealed that the negatively charged AgNCs efficiently catalyze the reduction of 4-nitrophenol (4-NP) with a rate constant of 0.23 ± 0.01 min-1. However, upon surface modification, the catalytic efficiency almost doubles due to the formation of Mg-AgNCs. Catalysis through AgNCs and Mg-AgNCs collectively portrays the role of the core and electrostatic surfaces. Furthermore, the role of electrostatic interaction has been substantiated by varying the ionic strength of the medium, as well as employing different molecular systems. A quantitative assessment of the Debye screening length asserts the correlation between the ionic strength of the medium and the role of electrostatic interactions involved herein. This highly enhanced catalytic aspect has been utilized for the real sample analysis, wherein AgNCs unexpectedly outperform Mg-AgNCs. This approach of real sample analysis also emanates the role of electrostatics involved. This comprehensive investigation represents the influential role of the core and ligand shell of MNCs as well as the role of electrostatics on its catalytic activities, which is relevant for the rational design of highly efficient catalysts.

A white-light-emitting sensor array based on cucurbit[8]uril for quantitative detection of multicomponent nitroaniline isomers

A white-light-emitting supramolecular complex through supramolecular interactions has been assembled; the white luminescent supramolecular complex exhibits two emission spectra. Based on this, a dual-channel white-light array sensor was constructed. The results show that it can quickly identify and detect nitroaniline isomer pollutants (p-nitroaniline, m-nitroaniline, o-nitroaniline). When these three nitroaniline isomers were added to the supramolecular white-light array sensor, the fluorescence intensity of the white-light complex decreased to varying degrees. Linear discriminant analysis (LDA) showed that the supramolecular white-light array sensor could recognize and distinguish three nitroaniline isomers and could classify mixtures containing different concentrations. Factor 1 of the array had a good linear relationship with the concentration of pollutants, and the detection limit (LOD) was as low as 0.7 μM. The method has good reproducibility and stability. In addition, it can also qualitatively detect the nitroaniline isomers in river water and contaminated rice seedling extract. It provides an ideal platform for constructing multiresponse sensors.

Rapid detection of carbamate nerve agent analogues using dually functionalized gold nanoclusters

Carbamate nerve agents (CMNAs) are a type of lethal cholinesterase inhibitor with one or more quaternary amine centres and aromatic rings. CMNAs have been recently added to the Annex on Chemicals of the Chemical Weapons Convention (CWC) and Schedules of Controlled Chemicals of China. In this study, a rapid, sensitive and selective method was developed for the fluorescence detection of ambenonium chloride (AC) through host-guest and electrostatic dual interactions between AC and cyclodextrin/11-mercaptoundecanoic acid (CD/MUA) dually functionalized gold nanoclusters (AuNCs). Through this method, AC was detected with a limit of detection of 10.0 ng/mL. Method evaluation showed high selectivity towards AC over other related compounds. The practical applicability was verified, as satisfactory recoveries were obtained for AC spiked in river water and urine, as well as Proficiency Test samples from Organisation for the Prohibition of Chemical Weapons (OPCW). In addition, a fluorescence sensing array comprising four AuNCs was designed to distinguish six carbamates and structurally similar compounds. This method provides a potential approach for the rapid, sensitive and selective recognition and detection of CMNAs.

Facile synthesis of yellow-green fluorescent silicon nanoparticles and their application in detection of nitrophenol isomers

A clear formation mechanism is essential for the controllable synthesis of nanomaterials with different optical properties, which is also one of the challenges facing the preparation of fluorescent silicon nanomaterials. In this work, a one-step room temperature synthesis method was established to prepare yellow-green fluorescent silicon nanoparticles (SiNPs). The obtained SiNPs exhibited excellent pH stability, salt tolerance, anti-photobleaching ability and biocompatibility. Based on X-ray photoelectron spectroscopy, transmission electron microscopy, ultra high performance liquid chromatography tandem mass spectrometry and other characterization data, the formation mechanism of the SiNPs was proposed, which provided a theoretical basis and important reference for the controllable preparation of SiNPs and other fluorescent nanomaterials. In addition, the obtained SiNPs illustrated excellent sensitivity for nitrophenol isomers, the linear range of o-nitrophenol, m-nitrophenol, p-nitrophenol was 0.05-600 μM, 20-600 μM and 0.01-600 μM under the λ_ex and λ_em were set as 440 nm and 549 nm, and related limit detection was 16.7 nM, 6.7 μM and 3.3 nM, respectively. The developed SiNP-based sensor achieved satisfactory recoveries in detecting nitrophenol isomers in a river water sample, showing great promise in practical applications.

Selective Supramolecular Recognition of Nitroaromatics by a Fluorescent Metal-Organic Cage Based on a Pyridine-Decorated Dibenzodiaza-Crown Macrocyclic Co(II) Complex

Two isomorphous fluorescent (FL) lantern-shaped metal-organic cages 1 and 2 were prepared by coordination-directed self-assembly of Co(II) centers with a new aza-crown macrocyclic ligand bearing pyridine pendant arms (L_py). The cage structures were determined using single-crystal X-ray diffraction analysis, thermogravimetric, elemental microanalysis, FT-IR spectroscopy, and powder X-ray diffraction. The crystal structures of 1 and 2 show that anions (Cl^- in 1 and Br^- in 2) are encapsulated within the cage cavity. 1 and 2 bear two coordinated water molecules that are directed inside the cages, surrounded by the eight pyridine rings at the "bottom" and the "roof" of the cage. These hydrogen bond donors, π systems, and the cationic nature of the cages enable 1 and 2 to encapsulate the anions. FL experiments revealed that 1 could detect nitroaromatic compounds by exhibiting selective and sensitive fluorescence quenching toward p-nitroaniline (PNA), recommending a limit of detection of 4.24 ppm. Moreover, the addition of 50 μL of PNA and o-nitrophenol to the ethanolic suspension of 1 led to a significant large FL red shift, namely, 87 and 24 nm, respectively, which were significantly higher than the corresponding values observed in the presence of other nitroaromatic compounds. The titration of the ethanolic suspension of 1, with various concentrations of PNA (>12 μM) demonstrated a concentration-dependent emission red shift. Hence, the efficient FL quenching of 1 was capable of distinguishing the dinitrobenzene isomers. Meanwhile, the observed red shift (10 nm) and quenching of this emission band under the influence of a trace amount of o- and p-nitrophenol isomers also showed that 1 could discriminate between o- and p-nitrophenol. Replacement of the chlorido with a bromido ligand in 1 generated cage 2 which was a more electron-donating cage than 1. The FL experiments showed that 2 was partially more sensitive and less selective toward NACs than 1.

Recent Progress in Sensor Arrays: From Construction Principles of Sensing Elements to Applications

The traditional sensors are designed based on the "lock-and-key" strategy with high selectivity and specificity for detecting specific analytes, which however are not suitable for detecting multiple analytes simultaneously. With the help of pattern recognition technologies, the sensor arrays excel in distinguishing subtle changes caused by multitarget analytes with similar structures in a complex system. To construct a sensor array, the multiple sensing elements are undoubtedly indispensable units that will selectively interact with targets to generate the unique "fingerprints" based on the distinct responses, enabling the identification among various analytes through pattern recognition methods. This comprehensive review mainly focuses on the construction strategies and principles of sensing elements, as well as the applications of sensor array for identification and detection of target analytes in a wide range of fields. Furthermore, the present challenges and further perspectives of sensor arrays are discussed in detail.

Multi-excitation wavelength of gold nanocluster-based fluorescence sensor array for sulfonamides discrimination

Sulfonamides (SAs) are widely used in many fields because of their advantages, including low price, wide antibacterial spectrum, and high stability. However, their accumulation in the human body leads to a variety of serious diseases. Therefore, it is necessary to design a convenient, effective, and sensitive method to detect SAs. Moreover, the fluorescence excitation spectrum has rich information characteristics, especially for the interaction between fluorophore and quencher via various mechanisms. However, the excitation wavelength-guided sensor array construction does not draw proper attention. To address these issues, we used BSA-AuNCs as a single probe to construct a sensor array for the detection of five SAs. The selected SAs showed different quenching effects on the fluorescence intensities of BSA-AuNCs. The changes in the fluorescence intensity at different excitation wavelengths (λ = 230, 250, and 280 nm) have been applied to construct our sensor array and address the distinguishability between the selected SAs. With helping of pattern recognition methods, five different SAs have been identified at three different concentrations. Additionally, qualitative analysis at different moral ratios and quantitative analysis at nanogram concentrations have been considered. Moreover, the proposed sensor array was successfully used to distinguish between different SAs in commercial milk with an accuracy of 100 %. This study provides a simple and powerful approach to SAs detection. Also, it shows a broad application prospect in the field of food and drug monitoring.

A mesoporous silica-based probe with a molecularly imprinted polymer recognition and Mn:ZnS QDs@rhodamine B ratiometric fluorescence sensing strategy for the analysis of 4-nitrophenol

In this study, a mesoporous silica fluorescence probe co-doped with manganese-doped zinc sulfide quantum dots (Mn:ZnS QDs) and rhodamine B (RB) and coated with molecularly imprinted polymer (MIP) has been prepared by sol-gel methods. The morphology and structure were characterized in detail by transmission electron microscopy (TEM), Fourier transform-infrared absorption spectroscopy (FT-IR) and ultraviolet-visible absorption spectroscopy (UV-vis). The probe exhibited two characteristic emission peaks at 411 nm and 582 nm, and the synchronous ratiometric fluorescence responses F₄₁₁/F₅₈₂ to different concentrations of 4-nitrophenol (4-NP) showed a good linear correlation in the range of 0.01-10 μmol L^-1 besides achieving the sensitive detection of 4-NP with a detection limit as low as 3.0 nmol L^-1 (3σ). The probe possesses the advantages of selectivity toward the target molecular structure, self-stability in the detection time domain and anti-interference ability, exhibiting excellent potential for application in 4-NP detection in different water environments.

Visualization of Polymer–Surfactant Interaction by Dual-Emissive Gold Nanocluster Labeling

Polymer-surfactant interaction decides the performance of corresponding complexes, making its rapid and intuitionistic visualization important for enhancing the performance of products and/or processing in related fields. In this study, the fluorescence visualization of the interaction between cationic hyperbranched polyethyleneimine and anionic sodium dodecyl sulfonate surfactant was realized by dual-emissive gold nanocluster labeling. The sensing mechanism was due to the interaction-induced polymer conformation change, which regulated the molecular structure and subsequent photoradiation process of the gold nanoclusters. All three inflection points of the interactions between the polymers and the surfactants were obtained by the change in fluorescence emission ratio of the designed dual-emissive gold nanoclusters. Moreover, these inflection points are verified by the hydrodynamic diameter and zeta potential measurements.

Identification of milk quality and adulteration by surface-enhanced infrared absorption spectroscopy coupled to artificial neural networks using citrate-capped silver nanoislands

Milk is one of the most important multicomponent superfoods owing to its rich macronutrient composition. It requires quality control at all the production stages from the farm to the finished products. A localized surface plasmon resonance optical sensor based on a citrate-capped silver nanoparticle (Cit-AgNP)–coated glass substrate was developed. The fabrication of such sensors involved a single-step synthesis of Cit-AgNPs followed by surface modification of glass slides to be coated with the nanoparticles. The scanning electron microscope micrographs demonstrated that the nanoparticles formed monolayer islands on glass slides. The developed surface-enhanced infrared absorption spectroscopy (SEIRA) sensor was coupled to artificial neural networking (ANN) for the qualitative differentiation between cow, camel, goat, buffalo, and infants’ formula powdered milk types. Moreover, it can be used for the quantitative determination of the main milk components such as fat, casein, urea, and lactose in each milk type. The qualitative results showed that the obtained FTIR spectra of cow and buffalo milk have high similarity, whereas camel milk resembled infant formula powdered milk. The most difference in FTIR characteristics was evidenced in the case of goat milk. The developed sensor adds several advantages over the traditional techniques of milk analysis using MilkoScan™ such as less generated waste, elimination of pre-treatment steps, minimal sample volume, low operation time, and on-site analysis.

Array-based chemical warfare agent discrimination via organophosphorus-H2O2 reaction-regulated chemiluminescence

It has been a challenge to achieve rapid, simple, and effective discrimination of organophosphorus nerve agents (typical chemical warfare agents) due to the similar chemical properties of the targets such as sarin, soman, cyclosarin and VX. In this study, we propose a chemiluminescence sensor array that can effectively discriminate organophosphorus nerve agents by organophosphorus-H₂O₂ reaction, which produces peroxyphosphonate intermediate and regulates the chemiluminescence intensity. A simple chemiluminescence sensor array based on different chemiluminescence characteristics of the four organophosphorus nerve agents in the luminol–H₂O₂ system and layered double hydroxide–luminol–H₂O₂ system has been constructed. Four agents can be well distinguished at a concentration of 1.0 mg L⁻¹ when linear discriminant analyses and hierarchical cluster analyses are smartly combined. The high accuracy (100%) evaluation of 20 blind samples demonstrates the practicability of this proposed chemiluminescence sensor array.

A chemical warfare agent sensor array based on organophosphorus-H₂O₂ reaction-regulated chemiluminescence is proposed.

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 nanozyme-based colorimetric sensor array as electronic tongue for thiols discrimination and disease identification

Thiol analysis is of vital significance due to the essential roles in disease diagnosis, while the highly similar structures of thiols are a major challenge in practical determination. Herein, a nanozyme-based colorimetric sensor array has been proposed as electronic tongue for excellent discrimination and sensitive quantitation of thiols. The sensing units are fabricated by integrating the terephthalic acid modified graphene quantum dots (TPA@GQDs) with three transition metal ions (Fe²⁺, Cu²⁺ and Zn²⁺) via coordination, respectively, which not only provide sufficient substrate binding sites but also form the metal ion-regulated catalytic active centers. In this way, disparate promotion degrees on the peroxidase-like catalytic activity have been achieved in different metal ion-TPA@GQD ensembles. Based on the strong binding affinity between metal ions and thiols, the catalytic active centers are removed from TPA@GQDs, which inhibits the catalytic activity of sensing unit to diverse degrees. Accordingly, using 3, 3', 5, 5'-tetramethylbenzidine (TMB) as chromogenic substrate in the presence of hydrogen peroxide (H₂O₂), each sensing unit can generate differential colorimetric signals (fingerprints) for six thiol analytes, which can be accurately discriminated through linear discriminant analysis (LDA) with a detection limit of 50 nM. In addition, the discrimination of the same thiol with different concentrations and thiol mixtures have also been achieved. Furthermore, inspired by the distinct levels of thiols in practical samples, the proposed sensor array enables the identification of thiol-associated diseases by means of machine learning algorithm, which makes a positive contribution to medical diagnosis.

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

Surfactant stabilized gold nanomaterials for environmental sensing applications - A review

Surfactant stabilized Gold (Au) nanomaterials (NMs) have been documented extensively in recent years for numerous sensing applications in the academic literature. Despite the crucial role these surfactants play in the sensing applications, the comprehensive reviews that highlights the fundamentals associated with these assemblies and impact of these surfactants on the properties and sensing mechanisms are still quite scare. This review is an attempt in organizing the vast literature associated with this domain by providing critical insights into the fundamentals, preparation methodologies and sensing mechanisms of these surfactant stabilized Au NMs. For the simplification, the surfactants are divided into the typical and advanced surfactants and the Au NMs are classified into Au nanoparticles (NPs) and Au nanoclusters (NCs) depending upon the complexity in structure and size of the NMs respectively. The preparative methodologies are also elaborated for enhancing the understanding of the readers regarding such assemblies. The case studies regarding surfactant stabilized Au NMs were further divided into colorimetric sensors, surface plasmonic resonance (SPR) based sensors, luminescence-based sensors, and electrochemical/electrical sensors depending upon the property utilized by the sensor for the sensing of an analyte. Future perspectives are also discussed in detail for the researchers looking for further progress in that particular research domain.

Copper ions assisted fluorescent detection of some dithiocarbamates based on nickel nanocluster with aggregation-induced emission enhancement behavior

Pesticide residue contamination has become an urgent issue since it threatens both the natural environment and public health. In this study, a fluorescent method for detecting dithiocarbamate (DTC) compounds was constructed based on novel nickel nanoclusters (Ni NCs) and copper ions (Cu²⁺). The water-soluble fluorescent Ni NCs were synthesized for the first time through a one-pot method using glutathione as stabilizer and ascorbic acid as reducing agent. The as-prepared Ni NCs exhibited a maximum fluorescence emission at 445 nm when excited by 380 nm. And they displayed aggregation-induced emission enhancement when ethylene glycol was introduced into the nanocluster aqueous solution. Based on the Ni NCs, a label-free fluorescence quenching sensor was established for sensitive and selective detection of DTC compounds with the assistance of Cu²⁺. The complex formed by DTC and Cu²⁺ led to fluorescence quenching of Ni NCs through inner filter effect. The sensing method was successfully applied to two typical DTC compounds, thiram and disulfiram, with good linearity over a wide linear range and a low detection limit. Moreover, the proposed approach was capable of thiram detection in real samples, which confirms the potential of this sensing method as a platform for DTC compound detection.

Surface-Engineered Gold Nanoclusters for Stimulated Emission Depletion and Correlated Light and Electron Microscopy Imaging

Stimulated emission depletion (STED) nanoscopy is an emerging super-resolution imaging platform for the study of the cellular structure. Developing suitable fluorescent probes of small size, good photostability, and easy functionalization is still in demand. Herein, we introduce a new type of surface-engineered gold nanoclusters (Au NCs) that are ultrasmall (1.7 nm) and ultrabright (QY = 60%) for STED bioimaging. A rigid shell formed by l-arginine (l-Arg) and 6-aza-2-thiothymine (ATT) on the Au NC surface enables not only its strong fluorescence in aqueous solution but also its easy chemical modification for specific biomolecule labeling. Au NCs show remarkable performance as STED nanoprobes, including high depletion efficiency, good photobleaching resistance, and low saturation intensity. Super-resolution imaging has been achieved with these Au NCs, and targeted nanoscopic imaging of cellular tubulin has been demonstrated. Moreover, the circular structure of lysosomes in live cells has been revealed. As a Au NC is also an ideal probe for electron microscopy, dual imaging of Aβ₄₂ aggregates with the single labeling probe of Au NCs has been realized in correlative light and electron microscopy (CLEM). This work reports, for the first time, the application of Au NCs as a novel probe in STED and CLEM imaging. With their excellent properties, Au NCs show promising potential for nanoscale bioimaging.

Novel dual-emissive fluorescent immunoassay for synchronous monitoring of okadaic acid and saxitoxin in shellfish

In this work, we present a novel dual-emissive fluoroimmunoassay for synchronous monitoring of okadaic acid (OA) and saxitoxin (STX) using multicolor fluorescent labels composed of sulfur, phosphorous co-doped graphene quantum dots (S, P-GQDs), and ovalbumin (OVA)-coated gold nanoparticles (OVA-AuNPs). The novel OVA-AuNPs were prepared by the reduction of chloroauric acid under alkaline conditions using OVA as a reducing agent. Both S, P-GQDs and OVA-AuNPs exhibit bright fluorescence, more importantly, a large emission wavelength difference (Δλ = 156 nm) under an excitation of 400 nm and relatively independent fluorescence behavior, which are essential to realizing the dual-signal marks in a directly mixing system. Using a competitive fluorescence-linked immunosorbent assay (cFLISA) format, the dual-emissive cFLISA was successfully utilized to measure OA and STX contents in Alectryonella plicatula (commonly named as fingerprint oyster) and the detection results were in good agreement with the commercial enzyme-linked immunosorbent assay (ELISA) kits.

Ultrafast Ratiometric Detection of Aflatoxin B1 Based on Fluorescent β-CD@Cu Nanoparticles and Pt2+ Ions

Rapid detection of aflatoxin B1 (AFB1) is a very important task in food safety monitoring. However, it is still challenging to achieve highly sensitive detection without antibody or aptamer biomolecules. In this work, a rapid detection of aflatoxin B1 was achieved using a ratiometric fluorescence probe without antibody or aptamer for the first time. In the ratiometric fluorescence system, the fluorescence emission of AFB1 at 433 nm was significantly enhanced due to the β-cyclodextrin-AFB1 host-guest interaction and the complexation of AFB1 and Pt²⁺. Meanwhile, the inclusion of aflatoxin B1 also quenched the fluorescence emission of β-CD@Cu nanoparticles (NPs) at 650 nm based on inner filter effect mechanism. On the basis of the above effects, the ratiometric detection of aflatoxin B1 was achieved in the range of 0.03-10 ng/mL with a low detection limit of 0.012 ng/mL (3σ/s). In addition, the β-CD@Cu NPs based nanoprobe could achieve stable response within 1 min to AFB1. The above ratiometric detection also demonstrated excellent application potential in the rapid on-site detection of AFB1 in food due to the advantages of convenience, rapidness, and high accuracy.

A poly(arylene ethynylene)-based microfluidic fluorescence sensor array for discrimination of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) were discriminated using a microfluidic paper-based sensor array device.

Ultrasensitive Electrochemical Impedance Chiral Discrimination and Sensing of Tryptophan Isomers Based on Core-Shell-Structured Au-Ag Nanoparticles

Au-Ag nanoparticles (Au-Ag NPs) with a core-shell structure are prepared and used for ultrasensitive electrochemical impedance (EI) discrimination of the isomers of tryptophan (Trp). As revealed by circular dichroism, rotary polarization caused by the Au-Ag NPs is consistent with D-Trp but opposite to L-Trp, and thus, the Au-Ag NPs can selectively combine with D-Trp through preferential interactions. Compared with Au-Ag NPs, the composites of D-Trp and Au-Ag NPs (Au-Ag NPs/D-Trp) display significantly increased charge transfer resistance (R_ct); differently, the R_ct of Au-Ag NPs/L-Trp remains almost unchanged because the Au-Ag NPs exhibit poor affinity toward L-Trp. Therefore, ultrasensitive EI enantiodiscrimination of the isomers of Trp is realized even at an extremely low concentration of the Trp isomers (0.1 nM). In addition, it is successfully applied in the ultrasensitive determination of D-Trp at a low concentration level (0.1 nM∼10 μM).

Fluorometric discrimination of tyrosine isomers based on the inner filter effect of chiral Au nanoparticles on MoS2 quantum dots

A fluorescent chiral sensor is proposed based on the inner filter effect (IFE) of chiral Au nanoparticles (AuNPs) on MoS2 quantum dots (MoS2 QDs), which can be used for the discrimination of the isomers of tyrosine (Tyr). l-Tyrosine (l-Tyr) can induce obvious agglomeration of the chiral AuNPs, leading to an attenuated IFE of the chiral AuNPs and greatly restored fluorescence of the MoS2 QDs, and thus the enantioselective recognition of the Tyr isomers can be achieved. Also, l-Tyr but not d-Tyr induced agglomeration of the chiral AuNPs is confirmed by the larger association constant between l-Tyr and the chiral sensor.

A Fluorescent Detection for Paraquat Based on β-CDs-Enhanced Fluorescent Gold Nanoclusters

In this report, a fluorescent sensing method for paraquat based on gold nanoclusters (AuNCs) is proposed. It was found that paraquat could quench both glutathione-capped AuNCs (GSH-AuNCs) and β-cyclodextrin-modified GSH-AuNCs (GSH/β-CDs-AuNCs). The modification of β-CDs on the surface of GSH-AuNCs obviously enhanced the fluorescence intensity of GSH-AuNCs and improved the sensitivity of paraquat sensing more than 4-fold. This sensibilization was ascribed to the obvious fluorescence intensity enhancement of GSH-AuNCs by β-CDs and the “host–guest” interaction between paraquat and β-CDs. The fluorescence quenching was mainly due to the photoinduced energy transfer (PET) between GSH/β-CDs-AuNCs and paraquat. With the optimized β-CDs modification of the GSH-AuNC surfaces and under buffer conditions, the fluorescent detection for paraquat demonstrated a linear response in the range of 5.0–350 ng/mL with a detection limit of 1.2 ng/mL. The fluorescent method also showed high selectivity toward common pesticides. The interference from metal ions could be easily masked by ethylene diamine tetraacetic acid (EDTA). This method was applied to the measurement of paraquat-spiked water samples and good recoveries (93.6–103.8%) were obtained. The above results indicate that host molecule modification of fluorescent metal NC surfaces has high potential in the development of robust fluorescent sensors.

Diagnosis of cancer at early stages based on the multiplex detection of tumor markers using metal nanoclusters

Traditional cancer diagnosis strategies are not considered by most people until the last resort, which delays many cancer treatments leading to advanced stages. Tumor marker sensors show great potential for detecting cancer because of its cost-effective and harmless checking procedures. Normally, one tumor marker is detected each time by using one type of sensor, but the accuracy to declare cancer is not always satisfied. Metal nanoclusters are ultra-small nanomaterials with low toxicity, distinct optical properties, catalytic activities, and cost-effective performance. Some metal nanoclusters have been designed to detect more than one tumor marker in a single step. The consideration of combined parameters using such facile sensing strategies has the potential to simplify the test procedure, and increase the diagnostic accuracy of early cancer. Therefore, various sensing strategies for the multiplex detection of tumor markers using metal nanoclusters are summarized.

Fluorescent and colorimetric determination of glutathione based on the inner filter effect between silica nanoparticle-gold nanocluster nanocomposites and oxidized 3,3',5,5'-tetramethylbenzidine

Determination of glutathione (GSH) is closely related to the clinical diagnosis of many diseases. Thus, a fluorescent and colorimetric dual-readout strategy for the sensitive determination of glutathione was proposed. The mesoporous silica nanoparticle-gold nanocluster (MSN-AuNC) nanocomposites with significantly enhanced emission and effectively improved photostability characteristics were used as fluorescent probes. Based on the inner filter effect (IFE), the fluorescence of MSN-AuNCs at 570 nm can be effectively quenched by oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) with absorption in the wavelength ranges of 330-470 nm and 500-750 nm. However, the addition of GSH could cause the reduction of blue oxTMB to colorless TMB, resulting in the inhibition of IFE and the recovery of the fluorescence of MSN-AuNCs. Therefore, using oxTMB as both quencher and color indicator, a dual-readout oxTMB/MSN-AuNC sensing system for the sensitive determination of GSH was constructed. As signal amplification is caused by the fluorescence enhancement of MSN-AuNCs, the detection limits as low as 0.12 μM and 0.34 μM can be obtained for fluorescent and colorimetric assay, respectively. This method may not only offer a new idea for the sensitive and effective determination of GSH, but also broaden the applications of AuNCs in fluorescent and colorimetric dual-readout bioanalysis.

A colorimetric aptasensor for the simple and rapid detection of human papillomavirus type 16 L1 proteins

In this study, a novel colorimetric aptasensor was developed for the rapid detection and visual screening of HPV16 L1 proteins using gold nanoparticles (AuNPs) and an RNA aptamer against HPV16 L1 protein (APTHPV16 L1). The AuNP-APTHPV16 L1 conjugates could be aggregated by the addition of a salt in the presence of HPV16 L1 proteins at the ppb level. At the same time, the surface plasma resonance absorption peaks of AuNPs shifted to a short wavelength, and an observable change in color from red to blue occurred. The relative absorbance (Ablank - Asample/Ablank) at 520 nm exhibited a stable response to HPV16 L1 proteins over a concentration range from 9.6 to 201.6 ng mL-1. The visual detection limit of HPV16 L1 proteins was found to be 9.6 ng mL-1. Finally, the proposed colorimetric aptasensor was successfully applied for the rapid and effective detection of HPV16 L1 proteins in clinical samples and vaccine samples. The validity and reliability of the proposed colorimetric aptasensor were verified by the enzyme-linked immunosorbent assay method. The proposed colorimetric aptasensor provided a promising indicator for screening and quantitative detection of HPV16 L1 proteins in clinical samples.

Machine Learning-Assisted Array-Based Detection of Proteins in Serum Using Functionalized MoS2 Nanosheets and Green Fluorescent Protein Conjugates

Abnormal concentrations of a specific protein or the presence of some biomarker proteins may indicate life-threatening diseases. Pattern-based detection of specific analytes using affinity-regulated receptors is one of the potential alternatives to specific antigen-antibody-based detection. In this report, we have schemed a sensor array by using various functionalized two-dimensional (2D)-MoS2 nanosheets and green fluorescent protein (GFP) as the receptor and the signal transducer, respectively. Two-dimensional MoS2 has been used as a promising candidate for recognition of the bioanalytes because of its high surface-to-volume ratio compared to those of other nanomaterials. Easy surface tunability of this material provides additional advantages to analyze the target of interest. The optimized 2D-MoS2-GFP conjugates are able to discriminate 15 different proteins at 50 nM concentration with a detection limit of 1 nM. Moreover, proteins in the binary mixture and in the presence of serum were discriminated successfully. Ten different proteins in serum media at relevant concentrations were classified successfully with 100% jackknifed classification accuracy, which proves the potentiality of the above system. We have also implemented and discussed the implication of using different machine learning models on the pattern recognition problem associated with array-based sensing.

Single gold nanocluster probe-based fluorescent sensor array for heavy metal ion discrimination

There is a continuing high demand to design effective sensors for the determination of heavy metal ions (HMIs) since they are hazardous to both human health and the environment. In this study, we reported a facile fluorescent sensor array for rapid discrimination of HMIs based on a single gold nanocluster (AuNC) probe. This AuNC probe was prepared by using 2-mercapto-1-methylimidazole (MMI) as a ligand and polyvinypyrrolidone (PVP) as a dispersing agent. The fluorescence emission of PVP/MMI-AuNC was observed to be closely related to the pH value of the aqueous solution, which displays yellow (λ_max = 512 nm) and red (λ_max = 700 nm) fluorescence at pH 12.0 and 6.0, respectively. Further experiments indicated that different HMIs can produce differential effects on the photoluminescence of PVP/MMI-AuNC and thus generate distinct fluorescent responses at 512 and 700 nm. On the basis of this phenomenon, a fluorescent sensor array based on the PVP/MMI-AuNC was then built by simply changing pH value in the sensor element. A total of seven HMIs had their unique response patterns and were successfully distinguished by hierarchical cluster analysis and linear discriminant analysis both in buffer solution and spiked water samples, achieving 100% identification accuracy. This study provides a simple and powerful fingerprinting sensing platform for multiple HMIs, showing broad application prospects in the field of environmental monitoring.

Colorimetric recognition of aromatic amino acid enantiomers by gluconic acid-capped gold nanoparticles

In this work, we prepared gold nanoparticles (AuNPs) by employing gluconic acid (GlcA) as reducing-cum-stabilizing agent. The proposed GlcA-AuNPs successfully worked as a colorimetric sensor for visual chiral recognition of aromatic amino acid enantiomers, namely tyrosine (D/L-Tyr), phenylalanine (D/L-Phe), and tryptophan (D/L-Trp). After adding L-types to GlcA-AuNPs solution, the color of the mixture changed from red to purple (or gray), while no obvious color change occurred on the addition of D-types. The effect can be detected by naked eyes. The particles have been characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, zeta potential, the dynamic light scattering analysis as well as UV-Vis spectroscopy. This assay can be used to determine the enantiomeric excess of L-Trp in the range from 0 to + 100%. The method has advantages in simplicity, sensitivity, fast response, and low cost.

Recent advances of plasmonic nanoparticle-based optical analysis in homogeneous solution and at the single-nanoparticle level

Plasmonic nanoparticles with special localized surface plasmon resonance (LSPR) characters have been widely applied for optical sensing of various targets. With the combination of single nanoparticle imaging techniques, dynamic information of reactions and biological processes is obtained, facilitating the deep understanding of their principle and design of outstanding nanomaterials. In this review, we summarize the recently adopted optical analysis of diverse analytes based on plasmonic nanoparticles both in homogeneous solution and at the single-nanoparticle level. A brief introduction of LSPR is first discussed. Colorimetric and fluorimetric homogeneous detection examples by using different sensing mechanisms and strategies are provided. Single plasmonic nanoparticle-based analysis is concluded in two aspects: visualization of chemical reactions and understanding of biological processes. The basic sensing mechanisms and performances of these systems are introduced. Finally, this review highlights the challenges and future trend of plasmonic nanoparticle-based optical analysis systems.

Ratiometric and sensitive cyanide sensing using dual-emissive gold nanoclusters

The detection of cyanide anion (CN^-), a highly toxic pollutant, has attracted growing attention in the past years. In this work, a nanosensor composed of hyperbranched polyethyleneimine (hPEI)-assisted dual-emissive gold nanoclusters (DE-Au NCs) is proposed for ratiometric detection of CN^- based on surface valence state-driving etch. The ratiometric color change of fluorescence is based on a fact that the red-emissive Au NCs with a high content of surface Au(I) can be easily etched by CN^-, while the blue-emissive Au NCs with nearly neutral character can resist CN^-. Because of the specific gold-CN^- chemistry and electrostatic attraction between the positively charged hPEI protecting layer and the negatively charged CN^-, the DE-Au NC-based nanosensor provides high selectivity toward CN^- over other anions with a limit of detection of 10 nM. Practical application of the proposed DE-Au NC nanosensor is verified by satisfying recoveries of CN^- determination in river water and urine samples. Graphical abstract.

A label-free multifunctional nanosensor based on N-doped carbon nanodots for vitamin B12 and Co2+ detection, and bioimaging in living cells and zebrafish

Multifunctional N-doped carbon nanodots (N-CNDs) with a fluorescence (FL) quantum yield (QY) of 13.6% have been synthesized via a facile one-step hydrothermal process using Artemisia annua and 1,2-ethylenediamine as precursors. As-prepared N-CNDs showed excellent FL properties and were developed as a multifunctional sensing platform for vitamin B₁₂ (VB₁₂) and Co²⁺ determination, and bioimaging in living cells and zebrafish. The FL of N-CNDs is quenched efficiently in the presence of VB₁₂ on the basis of the inner filter effect (IFE) or Co²⁺ by static quenching, respectively. EDTA as a masking agent enables Co²⁺ to be effectively eliminated and N-CNDs were used to selectively detect VB₁₂ in the presence of both VB₁₂ and Co²⁺. The present FL nanosensor can detect VB₁₂ and Co²⁺ in the linear ranges of 0.5-35 μM and 2.5-25 μM with the corresponding detection limits of 47.4 nM and 230.5 nM, respectively. The study proved that the determination of Co²⁺ was based on the static quenching to form a complex between the amino group of N-CNDs and Co²⁺. Inspired by these outstanding properties, practical applications of this nanosensor for the detection of VB₁₂ in actual samples (human serum, egg yolk, VB₁₂ tablets and VB₁₂ injection) and Co²⁺ in water samples were further verified with satisfactory results. The as-constructed N-CNDs have negligible toxicity and good biocompatibility, which facilitates utilization of N-CNDs in bioimaging of A549 cells and zebrafish, and sensing VB₁₂ in living cells.