Rapid Size Determination of Quasispherical Gold Nanoparticles by Electrocatalysis Efficiency-Regulated Electrochemiluminescence.

The size of gold nanoparticles (AuNPs) largely decides their properties and applications, making the rapid screening of AuNP size important. Despite the fact that AuNP-amplified electrochemiluminescence (ECL) is widely used in various ECL sensing applications, the mechanism of ECL enhancement remains elusive, especially the quantitative relationship between the enhanced ECL intensity and the size of AuNPs. In this work, taking quasispherical and citrate-stabilized AuNPs as model nanoparticles, we have reported that the ECL intensity of the S2O82--O2 system enhanced significantly with the increasing AuNP size. AuNPs acted as bielectrocatalysts for reducing the S2O82- and O2. The further study of enhancement mechanism demonstrates that AuNPs with increasing size facilitate the electron transfer and promote the generation of radicals required for the ECL emission, which produces more emitters-singlet oxygen. Meanwhile, the high surface density of citrate on small AuNPs suppresses the ECL signal by forming an electrostatic barrier. On the basis of the above phenomena, an ECL-based rapid AuNP size screening approach has been established. The accuracy of this platform is verified by the consistent results in comparison to transmission electron microscopy (TEM) measurements. This work not only provides deep insight into the correlation between the AuNP size and the ECL enhancement but also contributes an alternative to the TEM technique for the rapid AuNP size screening. Additionally, this study also extends the exploration of ECL-based structure analysis techniques toward nanomaterials through clarifying the structure-electrocatalytic activity correlation.

Electron Transfer Efficiency-Regulated Electrochemiluminescence for Rapid Crystallinity Analysis in Layered Materials.

The electrochemiluminescence (ECL) signal is largely determined by the electron transfer efficiency. Therefore, in the nanomaterial-involved ECL system, the structure-related electron distribution could affect the electron transfer efficiency and further alter the ECL intensity. These features make the design of versatile ECL-based analytical techniques for probing the correlated structure possible. And it is generally accepted that the increased crystallinity of nanomaterials usually leads to a uniform electron distribution, which provides higher conductivity. Therefore, the crystallinity-improved conductivity could facilitate electron transfer, promote the electrochemical activity of support materials, and boost the efficiency of the ECL reaction. In this study, we have demonstrated that the ECL signal of the graphitic carbon nitride reporter was proportional to the crystallinity of layered double hydroxides (LDHs), which meets the supposition well. On the basis of this phenomenon, an ECL-based crystallinity analysis approach has been established using CdAl-LDHs as the model materials. The universality of this proposed technique was further validated by the rapid and accurate crystallinity determination of ZnAl-LDH samples with diverse crystallinities. This work not only contributes an alternative to the X-ray diffraction technique for the rapid screening of crystallinity in layered materials but also opens a new avenue for the design of ECL-based structure analysis techniques toward nanomaterials and even organic materials by involving electron transfer regulation correlation.

Probing the Alcoholysis Degree of Polyvinyl Alcohol by Synergistic Coordination-Regulated Fluorescence.

Polyvinyl alcohol (PVA) with abundant hydroxyl groups (-OH) has been widely used for membranes, hydrogels, and films, and its function is largely affected by the alcoholysis degree. Therefore, the development of rapid and accurate methods for alcoholysis degree determination in PVAs is important. In this contribution, we have proposed a novel fluorescence-based platform for probing the alcoholysis degree of PVA by using the (E)-N-(4-methoxyphenyl)-1-(quinolin-2-yl)methanimine (QPM)-Zn2+ complex as the reporter. The mechanism study disclosed that the strong coordination between -OH and Zn2+ induced the capture of the QPM-Zn2+ complex and promoted its subsequent immobilization into the noncrystalline area. The immobilization of the QPM-Zn2+ complex restricted its molecular rotation and reduced the nonirradiative transition, thus yielding bright emissions. In addition, the practical applications of this proposed method were further validated by the accurate alcoholysis degree determination of blind PVA samples with the confirmation of the National Standard protocol. It is expected that the developed fluorescence approach in this work might become an admissive strategy for screening the alcoholysis degree of PVA.

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.

Probing Coordination Number of Single-Atom Catalysts by d-Band Center-Regulated Luminescence.

It is essential to probe the coordination number (CN) because it is a crucial factor to ensure the catalytic capability of single-atom catalysts (SACs). Currently, synchrotron X-ray absorption spectroscopy (XAS) is widely used to measure the CN. However, the scarcity of synchrotron X-ray source and complicated data analysis restrict its wide applications in determining the CN of SACs. In this contribution, we have developed a d-band center-regulated acetone cataluminescence (CTL) probe for a rapid screening of the CN of Pt-SACs. It is disclosed that the CN-triggered CTL is attributed to the fact that the increased CN could induce the downward shift of d-band center position, which assists the acetone adsorption and promotes the subsequent catalytic reaction. In addition, the universality of the proposed acetone-CTL probe is verified by determining the CN of Fe-SACs. This work has opened a new avenue for exploring an alternative to synchrotron XAS for the determination of CN of SACs and even conventional metal catalysts through d-band center-regulated CTL.

3,5-Dihydroxybenzoic Acid-Based Selective Dopamine Detection via Subsititution-Enhanced Kinetics Differences.

The selective recognition of dopamine (DA) over other neurotransmitter analogues is difficult due to the similar molecular structure and chemical reactivity. In this study, substitution-regulated chemical reactivity of the sensing substrate is utilized to explore a novel DA detection probe with satisfying selectivity. As a case study, 3,5-dihydroxybenzoic acid (DHBA, carboxy-substituted resorcinol)-based probes have been explored for selective and ratiometric DA sensing. The carboxy substitution benefits the stabilization of the carbanion intermediate and the azamonardine product, which enhances the reaction kinetics and thermodynamics and subsequently facilitates selective DA recognition over other analogues and interferents. By exploring DHBA emission as the internal reference, ratiometric fluorescence variation is realized, which contributes to sensitive DA analysis. With the combination of logic gate and fluorometric analysis, DA detection in both low and high concentrations can be readily achieved. In addition, the DA analysis in biological samples and the enzymatic transformation of DA analogues in cerebrospinal fluid samples are achieved by the proposed DHBA probe.

Recent Advancements in Natural Plant Colorants Used for Hair Dye Applications: A Review.

There is an on-going demand in recent years for safer and "greener" hair coloring agents with the global consumer awareness of the adverse effects of synthetic hair dyes. The belief in sustainability and health benefits has focused the attention of the scientific community towards natural colorants that serve to replace their synthetic toxic counterparts. This review article encompasses the historical applications of a vast array of natural plant hair dyes and summarizes the possible coloration mechanisms (direct dyeing and mordant dyeing). Current information on phytochemicals (quinones, tannins, flavonoids, indigo, curcuminoids and carotenoids) used for hair dyeing are summarized, including their botanical sources, color chemistry and biological/toxicological activities. A particular focus is given on research into new natural hair dye sources along with eco-friendly, robust and cost-effective technologies for their processing and applications, such as the synthetic biology approach for colorant production, encapsulation techniques for stabilization and the development of inorganic nanocarriers. In addition, innovative in vitro approaches for the toxicological assessments of natural hair dye cosmetics are highlighted.

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.

11-Mercaptoundecanoic acid capped gold nanoclusters with unusual aggregation-enhanced emission for selective fluorometric hydrogen sulfide determination.

In present study, we discovered unusual solvent-mediated aggregation-enhanced emission (AEE) character of 11-mercaptoundecanoic acid capped gold nanoclusters (MUA-Au NCs). When aggregated in aqueous media, the MUA-Au NCs showed strong emission, which was weakened by adding ethanol. Interestingly, the suppressed emission was selectively enhanced in the presence of hydrogen sulfide (H2S) because H2S was absorbed onto Au NCs through the strong sulfur-gold bonding affinity. The hydrolyzed H2S, namely, HS-, made the Au NCs negatively charged, which aggregated again due to decreased solubility. The H2S-mediated fluorescence enhancement can be further amplified by introducing a hydrophilic thiolate (glutathione, GSH) onto the surface of Au NCs (GSH/MUA-Au NCs), which enabled sensitive determination of H2S. Under the optimized condition, a detection limit of 35 nM was achieved. The determination was not interfered by other anions such as F-, Cl-, Br-, I-, OAc-, N3-, NO3-, HCO3-, SCN-, SO32-, and SO42-. This excellent sensing performance allowed practical application of the GSH/MUA-Au NC-based sensing platform to accurate determination of H2S in human serum samples. Graphical abstractUnusual aggregation-enhanced emission character of 11-mercaptoundecanoic acid capped gold nanoclusters is discovered and has been applied for fluorometric hydrogen sulfide detection.

Propanol-Triggered Luminescence for Rapid Screening of Crystal Facets in Noble Metal.

The rapid and effective recognition techniques for crystal facets in noble metal nanocrystals (NCs) are highly desirable in evaluation of their catalytic performances. Herein, it was found that the variation trend of cataluminescence (CTL) intensity in the presence of propanol was proportional to the crystal facets index change of LDH-supported Pd@Au NCs. The mechanism investigation for the propanol-triggered CTL on the surface of layered double hydroxide (LDH)-supported Pd@Au NCs revealed that the abundant negative charge in the high-index facets of LDH-supported Pd@Au NCs facilitated electron transfer between NCs and volatile propanol, resulting in acceleration of the CTL reactions. Accordingly, the propanol-triggered CTL can be as an indicator for rapid screening of crystal facets in NCs. Its universality had further been verified by screening crystal facets in LDH-supported Pd NCs. The crystal facets detected by propanol CTL probe were well-matched with those obtained by conventional high-resolution transmission electron microscope. The proposed CTL strategy exhibits some advantages, such as convenient operation, rapid response, long-term stability, and low cost. And it is expected crystal facets of other kinds of NCs could be screened by replaceable CTL probe.

A novel homolateral and dicationic AIEgen for the sensitive detection of casein.

The exploitation of highly soluble and responsive AIEgens is essential for further expansion of their practical applications. In this study, dipropyltrimethylammonium bromide-substituted TPE (denoted as o-TPEDTA), a homolateral and dicationic AIEgen, was synthesized and applied for the turn-on detection of casein via hydrophobic interactions. The rapid and sensitive detection of casein was achieved using the designed o-TPEDTA probe with the limit of detection of 0.05 µg mL-1. The satisfying selectivity of over 1000-fold concentration of other probably existing chemicals, including amino acids, sugars and salts, was achieved due to the strong binding affinity between o-TPEDTA and casein. The evaluation of casein in milk powder samples with small relative standard deviations was realized using the o-TPEDTA probe. The accuracy of the o-TPEDTA probe-based detection method was validated by the consistency of the casein detection results with those obtained via a national standard casein evaluation approach.

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

The rapid discrimination of nitrophenol isomers has been a long-standing challenge because of the tiny structural differences among the isomers. In this study, a fluorescent sensor array based on three different-color emitting gold nanoclusters (Au NCs) that were functionalized with three different ligands and a cocapping ligand ß-cyclodextrin (ß-CD) has been constructed for the facile discrimination of three nitrophenol isomers via the linear discriminant analysis of isomer-induced fluorescence quenching patterns. The fluorescence quenching occurs in two steps: first, ß-CDs adsorb nitrophenol isomers onto the surface of Au NCs via a host-guest interaction; second, each nitrophenol isomer quenches the fluorescence of a specific type of Au NCs through diverse inner filter effect. The different binding affinities between ß-CD and each nitrophenol isomer, as well as the distinct quenching efficiencies of the isomers on the fluorescence of each Au NCs, enable an excellent discrimination of the three isomers at a concentration of 5 µM, when linear discriminant and hierarchical cluster analyses were smartly combined. In addition, even a mixture of two isomers could be distinguished with the proposed sensor array. The practicability of this developed sensor array is validated by a high accuracy (98.0%) examination of 51 unknown samples containing a single isomer or a mixture of two isomers.

Self-Assembled Chiral Gold Supramolecules with Efficient Laser Absorption for Enantiospecific Recognition of Carnitine.

Stereospecific recognition of chiral molecules is ubiquitous in chemical and biological systems, thus leading to strong demand for the development of enantiomeric drugs, enantioselective sensors, and asymmetric catalysts. In this study, we demonstrate the ratio of d-Cys and l-Cys playing an important role in determining the optical properties and the structures of self-assembled Cys-Au(I) supramolecules prepared through a simple reaction of tetrachloroaurate(III) with chiral cysteine (Cys). The irregularly shaped -[d-Cys-Au(I)] n- or - [l-Cys-Au(I)] n- supramolecules with a size larger than 500 nm possessing strong absorption in the near-UV region and chiroptical characteristics were only obtained from the reaction of Au(III) with d-Cys or l-Cys. On the other hand, spindle-shaped -[d/l-Cys-Au(I)] n- supramolecules were formed when using Au(III) with mixtures of d/l-Cys. Our results have suggested that Au(I)···Au(I) aurophilic interactions, and stacked hydrogen bonding and zwitterionic interactions between d/l-Cys ligands are important in determining their structures. The NaBH4-mediated reduction induces the formation of photoluminescent gold nanoclusters (Au NCs) embedded in the chiral -[d-Cys-Au(I)] n- or -[l-Cys-Au(I)] n- supramolecules with a quantum yield of ca. 10%. The as-formed Au NCs/-[d-Cys-Au(I)] n- and Au NCs/-[l-Cys-Au(I)] n- are an enantiospecific substrate that can trap l-carnitine and d-carnitine, respectively, and function as a nanomatrix for surface-assisted laser desorption/ionization mass spectrometry (LDI-MS). The high absorption efficiency of laser energy, analyte-binding capacity, and homogeneity of the Au NCs/-[Cys-Au(I)] n- allow for quantitation of enantiomeric carnitine down to the micromolar regime with high reproducibility. The superior efficiency of the Au NCs/-[d-Cys-Au(I)] n- substrate has been further validated by quantification of l-carnitine in dietary supplements with accuracy and precision. Our study has opened a new avenue for chiral quantitation of various analytes through LDI-MS using metal nanocomposites consisting of NCs and metal-ligand complexes.

Rapid Screening of Oxygen States in Carbon Quantum Dots by Chemiluminescence Probe.

The oxygen states (O-states) of carbon quantum dots (CDs) play an important role, with regard to their optical properties and analytical applications. However, the rapid screening of O-states in CDs is still a great challenge, because of the complicated surface composition. In this study, it is found that the chemiluminescence (CL) intensity of prepared CDs in the presence of peroxynitrite (ONOO-) is proportional to the content of C-O group-related O-states. The related mechanism discloses that the O-state-dependent CL is due to the fact that abundant C-O functional groups in CDs with high O-states could facilitate the electron transfer of the produced smaller energy gaps for strong CL emission. Hence, ONOO--induced CL can be utilized as a facile probe for the rapid screening of O-states in CDs with some advantages, such as rapid response, low cost, and easy operation. Its practicability is verified by detecting the CL of phosphorus-doped CDs with variable phosphorus-doping contents. The content of C-O group-related O-states in sulfur-/phosphorus-doped CDs measured by the proposed CL probe is consistent with X-ray photoelectron spectroscopy (XPS) characterization. This strategy can also be extended to distinguish O-states in different types of nanoparticles by tuning the CL probe molecules.

Recent advances in cataluminescence-based optical sensing systems.

Cataluminescence (CTL) is a specific chemiluminescence (CL) occurring on the surface of solid catalysts during heterogeneous catalytic oxidation reactions, and it is still a young technique compared to conventional CL. In recent years, CTL-based sensors have been widely used in the detection of various analytes. We have witnessed many developments in CTL-based sensing systems for the enhancement of sensitivity or selectivity. This review provides a thorough introduction to the history and development of CTL-based sensing systems. We have briefly discussed the working principles for CTL systems and related the influencing factors first. The development of CTL based on these factors and instruments has been subsequently introduced. Several excellent CTL-based sensing systems have been discussed to highlight their practicality for the analysis of complicated environmental samples. Discussions of the challenges and future trends of CTL-based sensing systems have been provided in the end of this review.

The effect of ligand-ligand interactions on the formation of photoluminescent gold nanoclusters embedded in Au(i)-thiolate supramolecules.

In this study, we prepared photoluminescent l-cysteine (Cys)-capped gold nanoclusters (Cys-Au NCs) via NaBH4-mediated reduction of aggregated coordination polymers (supramolecules) of -[Cys-Au(i)]n-. The -[Cys-Au(i)]n- supramolecules with interesting chiral properties were formed through simple reactions of chloroauric acid (HAuCl4) with Cys at certain pH values (pH 3-7). The -[Cys-Au(i)]n- polymers could self-assemble into -[Cys-Au(i)]n- supramolecules with irregular morphologies and diameters larger than 500 nm through stacked hydrogen bonding and zwitterionic interactions between Cys ligands and through Au(i)Au(i) aurophilic interactions in solutions with pH values ≤7. The photoluminescent Au NCs (quantum yield = 11.6%) dominated by a Au13 core were embedded in -[Cys-Au(i)]n- supramolecules after NaBH4-mediated reduction. The optical and structural properties of Cys-Au NCs/-[Cys-Au(i)]n- nanocomposites were investigated, revealing that the interaction between Cys ligands plays a critical role in the self-assembly of -[Cys-Au(i)]n- supramolecules and in the formation of photoluminescent Cys-Au NCs embedded in the supramolecules. To further demonstrate that the photoluminescence properties and structures of the nanocomposites are mediated by the intermolecular forces of thiol ligands, other thiol ligands (l-penicillamine, l-homocysteine, 3-mercaptopropionic acid, and l-glutathione) and a ligand-crosslinking agent [bis(sulfosuccinimidyl) suberate; BS3] were used. We concluded that the electrostatic interactions, hydrogen bonding and steric effects dominate the polymer self-assembly into thiol-ligand-Au(i) supramolecules and thus the formation of Au NCs. Our study provides insights into the bottom-up synthesis of photoluminescent Au NCs from thiol-ligand-Au(i) complexes, polymers, and supramolecules. The hybrid Au NCs/-[Cys-Au(i)]n- nanocomposites can potentially be employed as drug carriers and bioimaging agents.

Gold nanoparticles as sensitive optical probes.

Gold nanoparticles (Au NPs) have become one of the most popular materials for sensing of analytes of interest in the last decade, mainly because of their ease in preparation and conjugation, stability, biocompatibility, and size-dependent optical properties. We have witnessed many sensitive and selective Au NP based optical systems for the quantitation of metal ions, anions, proteins, and DNA, based on analyte induced changes in their absorption, fluorescence, and scattering. In this tutorial review, we briefly discuss wet chemical approaches for the preparation of Au NPs. Sensing mechanisms and strategies of Au NP based optical systems are provided to show basic concepts in designing sensitive and selective sensing systems. Strategies for signal amplification applied in Au NP based systems are emphasized for the analysis of trace amounts of analytes in real samples. Many excellent Au NP based optical sensing systems are discussed to highlight their practicality for the analysis of complicated biological and environmental samples. The tutorial review ends with the discussion of the challenges and future trends of Au NP based optical sensing systems.

Electrochemiluminescence detection of reduced and oxidized glutathione ratio by quantum dot-layered double hydroxide film.

The ratio of reduced and oxidized glutathione (GSH/GSSG ratio) is a greater first indication of disease risk than the total concentration of GSH. However, the interferences from thiolated biomolecules, especially cysteine (Cys), make the accurate detection of GSH/GSSG ratio a technical problem. In this work, we successfully used a mixture of quantum dots (QDs) and ZnAl-LDH nanosheets to fabricate a high electrochemiluminescence resonance energy transfer (ERET) efficiency sensor for GSH from the disturbances of amino acids, especially Cys and GSSG. The mechanisms of high ERET efficiency and selectivity were well investigated with spectroscopy analysis and theoretical calculation. The results showed that the interaction force between ZnAl-LDH nanosheets and molecules proved a long-range-ordered space and selective transmission for molecules. On the basis of these interesting phenomena, we successfully measured the GSH/GSSG ratio in whole blood and serum samples.

Layered-nanomaterial-amplified chemiluminescence systems and their analytical applications.

Layered nanomaterial has become a popular hierarchical material for amplifying chemiluminescence (CL) in recent years, mainly because of its ease of preparation and modification, large specific surface area, and high catalytic activity. In this review, we mainly discuss layered-nanomaterial-amplified CL systems based on graphene and its derivatives, layered double hydroxides, and clay. Detection mechanisms and strategies of layered-nanomaterial-amplified CL systems are provided to show the basic concepts for designing sensitive and selective sensing systems. Strategies for expanding the applications of layered-nanomaterial-amplified CL systems by combination with surfactants, quantum dots, organic dyes, and nanoparticles are introduced for the analysis of various analytes in real samples. The challenges and future trends of layered-nanomaterial-amplified CL systems are discussed at the end of the review. Graphical Abstract Schematic illustration of layered nanomaterial amplified chemiluminescence.