Visualizing chromatographic separation of metal ions on a surface-enhanced Raman active medium

Phenylboronic Acid-Functionalized Ratiometric Surface-Enhanced Raman Scattering Nanoprobe for Selective Tracking of Hg2+ and CH3Hg+ in Aqueous Media and Living Cells

The development of appropriate molecular tools to monitor different mercury speciation, especially CH3Hg+, in living organisms is attractive because its persistent accumulation and toxicity are very harmful to human health. Herein, we develop a novel activity-based ratiometric SERS nanoprobe to selectively monitor Hg2+ and CH3Hg+ in aqueous media and in vivo. In this nanoprobe, a new bifunctional Raman probe bis-s-s'-[(s)-(4-(ethylcarbamoyl)phenyl)boronic acid] (b-(s)-EPBA) was synthesized and immobilized on the surface of gold nanoparticles via a Au-S bond, in which the phenylboronic acid group was employed as the recognition unit for Hg2+ and CH3Hg+ based on the Hg-promoted transmetalation reaction. In the presence of Hg2+ and CH3Hg+, a new surface-enhanced Raman scattering (SERS) peak aroused from of C-Hg appeared at 1080 cm-1, and the SERS intensity at 1002 cm-1 belonged to the B-O symmetric stretching decreased simultaneously. The quantitative tracking of Hg2+ and CH3Hg+ was realized based on the SERS intensity ratio (I1080/I1303) with rapid response (∼4 min) and high sensitivity, with detection limits of 10.05 and 25.13 nM, respectively. Moreover, the SERS sensor was used for the quantitative detection of Hg2+ and CH3Hg+ in four actual water samples with a high accuracy and excellent recovery. More importantly, cell imaging experiments showed that AuNPs@b-(s)-EPBA could quantitatively detect intracellular CH3Hg+ and had a good concentration dependence in ratiometric SERS imaging. Meanwhile, we demonstrated that AuNPs@b-(s)-EPBA could detect and image CH3Hg+ in zebrafish. We anticipate that AuNPs@b-(s)-EPBA could potentially be used to study the physiological functions related to CH3Hg+ in the future.

A Simple Live Cell Imaging "Turn-On" Fluorescence Probe for the Selective and Sensitive Detection of Aqueous Hg2+ Ions

A simple, efficient, and reversible fluorescent sensor probe, PBA (2,6-dimethyl pyrone barbituric acid conjugate), comprised of a pro-aromatic donor conjugated with a barbituric acid, was developed for the detection of highly toxic mercuric ions. The probe showed high selectivity and "Turn-On" fluorescence response towards Hg2+ among various metal cations such as Na+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ba2+, Hg2+, and Pb2+, in both homogeneous and microheterogeneous micelle medium sodium dodecyl sulphate (SDS). The binding stoichiometry, limit of detection (LOD), and binding constant for the PBA-Hg complex were determined. The mechanism of binding was ascertained using the N,N'-dimethylbarbituric acid conjugate of 2,6-dimethylpyran (PDMBA), where no binding interaction by deprotonation is possible. In the presence of cysteamine hydrochloride and trifluoroacetic acid (TFA), the complexation of Hg2+ with PBA was demonstrated to be reversible, indicating its potential for the development of reusable sensors. Moreover, the practical applicability of PBA in monitoring Hg2+ in living cells was also evaluated.

Raman and Surface-Enhanced Raman Scattering Detection in Flowing Solutions for Complex Mixture Analysis

Raman scattering provides a chemical-specific and label-free method for identifying and quantifying molecules in flowing solutions. This review provides a comprehensive examination of the application of Raman spectroscopy and surface-enhanced Raman scattering (SERS) to flowing liquid samples. We summarize developments in online and at-line detection using Raman and SERS analysis, including the design of microfluidic devices, the development of unique SERS substrates, novel sampling interfaces, and coupling these approaches to fluid-based chemical separations (e.g., chromatography and electrophoresis). The article highlights the challenges and limitations associated with these techniques and provides examples of their applications in a variety of fields, including chemistry, biology, and environmental science. Overall, this review demonstrates the utility of Raman and SERS for analysis of complex mixtures and highlights the potential for further development and optimization of these techniques.

A photoluminescent and electrochemiluminescent probe based on an iridium(III) complex with a boronic acid-functionalised ancillary ligand for the selective detection of mercury(II) ions

Exposure to mercury(II) ions (Hg2+) can cause various diseases such as Minamata disease, acrodynia, Alzheimer's disease, and Hunter-Russell syndrome, and even organ damage. Therefore, real-time and accurate monitoring of Hg2+ in environmental samples is crucial. In this study, we report a photoluminescent (PL) and electrochemiluminescent (ECL) probe based on a cyclometalated Ir(III) complex for the selective detection of Hg2+. The introduction of a reaction site, o-aminomethylphenylboronic acid, on the ancillary ligands allowed a prompt transmetalation reaction to take place between Hg2+ and boronic acid. This reaction resulted in significant decreases of the PL and ECL signals due to the photo-induced electron transfer from the Ir(III) complex to the Hg2+ ions. The probe was applied to the selective detection of Hg2+, and the signal changes revealed a linear correlation with Hg2+ concentrations in the range of 0-10 μM (LOD = 0.72 μM for PL, 8.03 nM for ECL). The designed probe allowed the successful quantification of Hg2+ in tap water samples, which proves its potential for the selective detection of Hg2+ in environmental samples.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Plasmonic 3-D wrinkled polymeric shrink film-based SERS substrates for pesticide detection on real-world surfaces

The continuous and excessive use of agrochemicals for crop improvement and protection has raised widespread concern, as they exert adverse effects on human health and the local environment. Surface Enhanced Raman Spectroscopy (SERS) provides a method for the quick identification and detection of such hazardous substances in a short amount of time due to its properties of being robust, accurate, sensitive and non-destructive. Despite the fact that several SERS substrates have been developed, the bulk of them are ineffective in terms of sample collection or providing reproducible results. In this study, we showed that a 3-D wrinkled polymeric heat-shrink film coated with Au bead@Ag nanorods (silver nanorods) serves as a potential SERS substrate for trace analysis. The surface of the heat-shrink film became wrinkled after heating, and this, along with the spatial arrangement of nanoparticles, significantly enhances the Raman signal of the analytes. The fabricated SERS substrate was able to sense two model analytes 1,4-benzenedithiol (BDT) and 2-naphthalenethiol (NT) up to 10^-13 M and 10^-11 M concentrations. The fabricated substrate was also effective in sensing thiram down to 10^-13 M concentration. Additionally, the SERS substrate was applied in a real-world setting for the detection of the pesticide thiram spiked onto apple skin surfaces. To collect the thiram residues, the substrate was simply swabbed across the surface of the apple. This allowed for the detection of thiram at concentrations as low as 10^-9 M (1 ppb). The fabricated SERS substrate can thus detect analytes in an efficient, sensitive, dependable and accurate manner, allowing for the sensing of trace analytes like pesticides in a real-world environment.

Organic Molecules Containing N, S and O Heteroatoms as Sensors for the Detection of Hg(II) Ion; Coordination and Efficiency toward Detection

Rapid detection of potentially toxic heavy metals like Hg(II) has attracted great attention in the last few decades due to the importance to maintain a safe and sustainable environment for human beings. Coordination chemistry and concepts therein, play an important role in the detection of Hg(II). Size, charge, and nature of the donor atom and the respective cation (metal ion), are crucial in selective interactions between the sensor and metal ions. The sensors designed for the purpose, coordinate to Hg(II) ion through various donor sites, coordination causes a change in the electron density in organic molecules and results in either visible color change or enhancing/quenching fluorescence intensity. Since Hg(II) is soft metal, with d10 electron system, so majority of the sensors have soft donor sites which prefer to coordinate with Hg(II). Oxygen is also present in some chelating ligands which is least preferred coordination site, due to its hard nature. There are several reports of replacing other ligating sites by sulfur for enhanced mercury sensing. In some cases, desulfurization is being detected as clear change in spectral behavior during the sensing process. Efforts are still in progress to design and introduce a sensor with utmost sensitivity and selectivity. In this review, we made an attempt to explain the coordination aspects of Hg(II) detectors, reasons for poor efficiency and possible suggestions to improve the selection criterion of various compounds. It will help researchers to know about important concepts in designing more sensitive and selective sensors for detection of Hg(II) in environmental and biological samples.

New Mechanism for Long Photo-Induced Enhanced Raman Spectroscopy in Au Nanoparticles Embedded in TiO2

The photo-induced enhanced Raman spectroscopy (PIERS) effect is a phenomenon taking place when plasmonic nanoparticles deposited on a semiconductor are illuminated by UV light prior to Raman measurement. Results from the literature show that the PIERS effect lasts for about an hour. The proposed mechanism for this effect is the creation of oxygen vacancies in the semiconductor that would create a path for charge transfer between the analyte and the nanoparticles. However, this hypothesis has never been confirmed experimentally. Furthermore, the tested structure of the PIERS substrate has always been composed of plasmonic nanoparticles deposited on top of the semiconductor. Here, gold nanoparticles co-deposited with porous TiO₂ are used as a PIERS substrate. The deposition process confers the nanoparticles a unique position half buried in the nanoporous semiconductor. The resulting PIERS intensity is among the highest measured until now but most importantly the duration of the effect is significantly longer (at least 8 days). Cathodoluminescence measurements on these samples show that two distinct mechanisms are at stake for co-deposited and drop-casted gold nanoparticles. The oxygen vacancies hypothesis tends to be confirmed for the latter, but the narrowing of the depletion zone explains the long PIERS effect.

Tetrahedral DNA-directed core-satellite assembly as SERS sensor for mercury ions at the single-particle level

To monitor the deteriorating mercury emissions, it is imperative to propose methods for detecting mercury ions (Hg²⁺) with sensitivity and selectivity. The SERS spectral-resolved single-particle detection approach can be carried out using dark-field optical microscopy (DFM) combined with Raman spectroscopy. Herein, we have designed a novel yet convenient single-particle detection assay for quantifying Hg²⁺ using DFM-correlated Raman spectroscopy. In the assay, a tetrahedral DNA-directed core-satellite nanostructure is used as the SERS probe. Especially, one edge of the tetrahedron is made up of a single-stranded DNA containing a Hg²⁺ aptamer, which reconfigures upon the specific recognition of Hg²⁺. As a result, the interparticle distance reduces from 4.5 to 1.2 nm, thus generating Raman signal enhancement. As a proof of concept, Hg²⁺ was detected in a linear range from 1 to 100 nM based on the variation in SERS intensity. Furthermore, the experimental results were supported by the finite difference time domain (FDTD) calculations. Owing to its high sensitivity and selectivity, this method was further employed to detect Hg²⁺ in practical tap water and lake water samples, revealing that the single-particle detection strategy holds great promise for Hg²⁺ analysis in real environment analysis.

Si@Ag@PEI substrate-based SERS sensor for rapid detection of illegally adulterated sulfur dioxide in traditional Chinese medicine

The illegal adulteration of sulfur dioxide in natural healthcare products may lead to serious health problems, which raise an urgent demand of straightforward approach for detecting sulfur dioxide. In this paper, surface-enhanced Raman scattering (SERS) sensor with sample preparation apparatus for headspace adsorption of SO₂ has been developed, which was successfully applied to detect illegal adulteration of sulfur dioxide in traditional Chinese medicine (TCM). Functional membrane substrate of Si@Ag@PEI composite was synthesized to enhance the adsorption and Raman signal of SO₂. A 3D-printed headspace extraction device was designed to adsorbed SO₂ by Si@Ag@PEI membrane after micro-extraction of TCM samples in 15 min. The content of sulfur dioxide was subsequently quantitatively measured by SERS sensor. The linear range of sensor is between 2.5 and 250 mg/kg with limit of detection of 0.25 mg/kg, which is lower than the strictest standard of Chinese Pharmacopoeia (10 mg/kg). The proposed approach was used to detect the SO₂ residue in TCMs including ginseng, Salvia miltiorrhiza, and bitter almonds. The fabricated sensor exhibited satisfied sensitivity and stability, which provide a simple approach for on-site detection of illegal adulteration of sulfur dioxide.

3D Printed Microfluidic Device for Magnetic Trapping and SERS Quantitative Evaluation of Environmental and Biomedical Analytes

Surface-enhanced Raman scattering (SERS) is an ideal technique for environmental and biomedical sensor devices due to not only the highly informative vibrational features but also to its ultrasensitive nature and possibilities toward quantitative assays. Moreover, in these areas, SERS is especially useful as water hinders most of the spectroscopic techniques such as those based on IR absorption. Despite its promising possibilities, most SERS substrates and technological frameworks for SERS detection are still restricted to research laboratories, mainly due to a lack of robust technologies and standardized protocols. We present herein the implementation of Janus magnetic/plasmonic Fe3O4/Au nanostars (JMNSs) as SERS colloidal substrates for the quantitative determination of several analytes. This multifunctional substrate enables the application of an external magnetic field for JMNSs retention at a specific position within a microfluidic channel, leading to additional amplification of the SERS signals. A microfluidic device was devised and 3D printed as a demonstration of cheap and fast production, with the potential for large-scale implementation. As low as 100 μL of sample was sufficient to obtain results in 30 min, and the chip could be reused for several cycles. To show the potential and versatility of the sensing system, JMNSs were exploited with the microfluidic device for the detection of several relevant analytes showing increasing analytical difficulty, including the comparative detection of p-mercaptobenzoic acid and crystal violet and the quantitative detection of the herbicide flumioxazin and the anticancer drug erlotinib in plasma, where calibration curves within diagnostic concentration intervals were obtained.

Charge Transfer on the Surface-Enhanced Raman Scattering of Ag/4-MBA/PEDOT:PSS System: Intermolecular Hydrogen Bonding

A sandwich-structured noble metal-probe molecule-organic semiconductor consisting of Ag nanoparticles (NPs), 4-mercaptobenzoic acid (4-MBA) and different concentrations of poly(styrenesulfonate:poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) was prepared by layer-by-layer assembly. Intermolecular hydrogen bonding was observed to have a significant effect on the surface-enhanced Raman scattering (SERS) of Ag/4-MBA/PEDOT:PSS. Upon increasing the PEDOT:PSS concentration, the characteristic Raman band intensity of 4-MBA was enhanced. In addition, the selected b2 vibration mode was significantly enhanced due to the influence of the charge transfer (CT) mechanism. The CT degree (ρCT) of the composite system was calculated before and after doping with PEDOT:PSS; when the concentration of PEDOT:PSS was 0.8%, the SERS intensity tended to be stable, and ρCT reached a maximum. Compared with that of the undoped PEDOT:PSS system, ρCT was significantly enhanced after doping, which can be explained by the CT effect induced by hydrogen bonds. These results indicate that hydrogen bonding transfers a charge from the Fermi energy level of Ag to the lowest unoccupied molecular orbital (LUMO) of 4-MBA, and due to the resulting potential difference, the charge will continue to transfer to the LUMO of PEDOT:PSS. Therefore, the introduction of organic semiconductors into the field of SERS not only expands the SERS substrate scope, but also provides a new idea for exploring the SERS mechanism. In addition, the introduction of hydrogen bonds has become an important guide for the study of CT and the structure of composite systems.

Surface-Enhanced Raman Scattering Optophysiology Nanofibers for the Detection of Heavy Metals in Single Breast Cancer Cells

Mercury(II) ions (Hg²⁺) and silver ions (Ag⁺) are two of the most hazardous pollutants causing serious damage to human health. Here, we constructed surface-enhanced Raman scattering (SERS)-active nanofibers covered with 4-mercaptopyridine (4-Mpy)-modified gold nanoparticles to detect Hg²⁺ and Ag⁺. Experimental evidence suggests that the observed spectral changes originate from the combined effect of (i) the coordination between the nitrogen on 4-Mpy and the metal ions and (ii) the 4-Mpy molecular orientation (from flatter to more perpendicular with respect to the metal surface). The relative intensity of a pair of characteristic Raman peaks (at ∼428 and ∼708 cm^-1) was used to quantify the metal ion concentration, greatly increasing the reproducibility of the measurement compared to signal-on or signal-off detection based on a single SERS peak. The detection limit of this method for Hg²⁺ is lower than that for the Ag⁺ (5 vs 100 nM), which can be explained by the stronger interaction energy between Hg²⁺ and N compared to Ag⁺ and N, as demonstrated by density functional theory calculations. The Hg²⁺ and Ag⁺ ions can be masked by adding ethylenediaminetetraacetate and Cl^-, respectively, to the Hg²⁺ and Ag⁺ samples. The good sensitivity, high reproducibility, and excellent selectivity of these nanosensors were also demonstrated. Furthermore, detection of Hg²⁺ in living breast cancer cells at the subcellular level is possible, thanks to the nanometric size of the herein described SERS nanosensors, allowing high spatial resolution and minimal cell damage.

A novel multidentate pyridyl ligand: A turn-on fluorescent chemosensor for Hg2+ and its potential application in real sample analysis

A novel pyridyl-based ligand with multiple binding sites was developed as potential turn on fluorescent probe for mercuric ion. In comparison with other transition metal ions, the ligand displayed a significant optical selectivity and sensitivity for Hg²⁺ in aqueous solution with a remarkable fluorescence enhancement. The obtained spectroscopic response was related to the inhibition of the photo-chemical mechanism known as photo-induced electron transfer (PET) in the ligand and CN isomerization by Hg²⁺ binding. A good linearity between fluorescence responses and Hg²⁺ concentration was obtained in the range 3.3 × 10^-9 M-1.6 × 10^-8 M and a nanomolar level limit of detection (LOD) (1.4 × 10^-9 M ~ 0.28 ppb) and limit of quantification (LOQ) (4.8 × 10^-9 M ~ 0.93 ppb) were obtained. Both LOD and LOQ values are very low compared to the reported permissible Hg²⁺ level in drinking water (2 ppb) by US Environmental Protection Agency (EPA). The possible binding mode between ligand and Hg²⁺ were determined using Job's plot analysis and density functional theory (DFT) calculations and a complex with 1:1 stoichiometric ratio was suggested. The response of the pyridyl ligand upon Hg²⁺ addition was noted to be fast without any time delay and reversible. The performance of the ligand at nanomolar level of Hg²⁺ and real sample application of the proposed method was investigated and satisfactory results were obtained.

Molecular hot spots in surface-enhanced Raman scattering

The chemical and electromagnetic (EM) enhancements both contribute to surface-enhanced Raman scattering (SERS). It is well-known that the EM enhancement is induced by the intense local field of surface plasmon resonance (SPR). This report shows that the polarizability of the molecules adsorbed on the metal surface can lead to another channel for the EM field enhancement. When aromatic molecules are covalently bonded to the Au surface, they strongly interact with the plasmon, leading to a modification of the absorption spectrum and a strong SERS signal. The effect is seen in both 3 nm-Au nanoparticles with a weak SPR and 15 nm-Au nanoparticles with a strong SPR, suggesting that the coupling is through both EM field and chemical means. Linear-chain molecules on the 3 nm-Au nanoparticles do not have a SERS signal. However, when the aromatic and linear molecules are co-adsorbed, the strong SPR/molecular polarizability interaction spatially extends the local EM field, leading to a strong SERS signal from the linear-chain molecules. The results show that aromatic molecules immobilized on Au can create "hot spots" just like plasmonic nanostructures.

Raman reporter-assisted Au nanorod arrays SERS nanoprobe for ultrasensitive detection of mercuric ion (Hg2+) with superior anti-interference performances

Ultra sensitive detection of mercuric ion (Hg²⁺) with superior anti-interference capability from natural water is of great significance for food safety, environmental protection, and human health. We herein develop Au ordered nanorod arrays (Au NRAs) as surface-enhanced Raman scattering (SERS) substrates to construct SERS-active and signal-reproducible sensing platforms modified with 4-mercaptophenylboronic acid (4-MBA) as multifunctional SERS reporters. The aqueous Hg²⁺ can be efficiently trapped by 4-MBA through electrophilic substitution reactions and precisely appraise its concentration based on the collective spectral changes of reporters including peak disappearance, emergence, and Raman shift. Based on this, the optical nanoprobe shows an ultrahigh detection sensitivity of 0.1 nM for Hg²⁺, which is two orders of magnitude lower than the U.S.A. environmental protection agency (EPA)-required maximum level of drinkable water. It also offers both an exceptional Hg²⁺ discrimination against other metal ions as well as organic ligands and perfect feasibilities of detecting solutions with ultra-wide pH ranges from 1.0-14.0 at varying temperatures. Moreover, the nanoprobe demonstrates an ability to identify different chemical forms of mercury and has a high repeatability, accuracy and reliability to meet the practical detection requirements in natural environments.

A New Highly Selective Colorimetric and Fluorometric Coumarin-based Chemosensor for Hg2

A novel colorimetric and fluorometric method based on coumarin as signalling unit was developed for Hg²⁺ recognition and quantification. Initially, the alkyne functionality was incorporated into a coumarin system and the resulting molecule showed higher specificity and sensitivity for Hg²⁺ over other cations in both absorption and emission sensing assays. The Hg²⁺ recognition was detected as visible colour change from colourless to yellow and as fluorescence quenching. The colour change was assigned to the increased intramolecular charge transfer (ICT) in the signalling unit upon Hg²⁺ binding whereas a decline in the fluorescence intensity was ascribed to the heavy atom effect from Hg²⁺. In order to generate a material with high sensing performance level, alkyne-functionalized molecule was hosted into a polymeric material. The resulting functionalized polymer showed higher sensitivity and selectivity for Hg²⁺ over its corresponding coumarin molecule. The investigation of the possible binding modes for Hg²⁺ suggested both alkyne and triazole functionalities as potential binding sites for Hg²⁺. The limit of detection (LOD) and limit of quantification (LOQ) of the proposed method were evaluated and values less than a recommended maximum level of Hg²⁺contaminant in drinking water (2.00 μg/L) were obtained (LOD = 0.44 μg/L and LOQ = 1.33μg/L). The real-life application of the method was investigated using natural water samples containing Hg²⁺ levels equivalent to the maximum tolerable concentration of Hg²⁺ in drinking water. The outcomes suggested that the method could be used in the sensing and determination of Hg²⁺ level of contaminant in the environment.

Manipulating chemistry through nanoparticle morphology

We demonstrate that the protonation chemistry of molecules adsorbed at nanometer distances from the surface of anisotropic gold nanoparticles can be manipulated through the effect of surface morphology on the local proton density of an organic coating. Direct evidence of this remarkable effect was obtained by monitoring surface-enhanced Raman scattering (SERS) from mercaptobenzoic acid and 4-aminobenzenethiol molecules adsorbed on gold nanostars. By smoothing the initially sharp nanostar tips through a mild thermal treatment, changes were induced on protonation of the molecules, which can be observed through changes in the measured SERS spectra. These results shed light on the local chemical environment near anisotropic colloidal nanoparticles and open an alternative avenue to actively control chemistry through surface morphology.

Multiplex SERS Chemosensing of Metal Ions via DNA-Mediated Recognition

The combination of molecular sensors and plasmonic materials is emerging as one of the most promising approaches for ultrasensitive SERS-based detection of metal ions in complex fluids. However, only a very small fraction of the large pool of potential chemosensors described in classical analytical chemistry has been successfully implemented into viable SERS platforms for metal ion determination. This is due to the molecular restrictions that require the chemosensor to adhere onto the plasmonic surface while retaining the capability to undergo large structural alterations upon metal ion binding. In this work, we demonstrate that the structural and functional plasticity of DNA for interacting with small aromatic molecules can be exploited to this end. DNA coating of silver nanoparticles modulates the interaction of the commercially available alizarin red S (ARS) chemosensor with the nanomaterial, translating its recognition capabilities from bulk solution onto the plasmonic surface, while simultaneously directing the particle assembling into highly efficient SERS clusters. The sensing approach was successfully applied to the multiplex, quantitative determination of Al(III) and Fe(III) in tap water in the subppb level.

One-step synthesis of boron-doped graphene quantum dots for fluorescent sensors and biosensor

Heteroatom doping can endow graphene quantum dots (GQDs) with various new or improved structural, optical and physicochemical properties. In contrast to the widely reported oxygen, nitrogen or sulfur doping in GQDs, simple and scalable synthesis of boron-doped GQDs (B-GQDs) with high yield and quantum yields remains challenge. In this work, B-GQDs are one-step synthesized and serve as the fluorescence probes for the fabrication of sensors towards Fe³⁺ ion or phosphate (Pi) as well as biosensor towards cytochrome C (Cyt C). The B-GQDs are facile synthesized using one-step bottom-up molecular fusion between 1,3,6-trinitropyrene and borax in sodium hydroxide under hydrothermal process. The synthesis can be performed using large volume autoclave (500 ml) with a high yield of 71%, indicating possibility for gram-scale production of B-GQDs. The as-prepared B-GQDs exhibit single or bilayer graphene structure, high crystallinity, uniform size, bright (absolute photoluminescence quantum yield of 16.8%) and excitation-independent green fluorescence (maximum excitation wavelength and emission wavelength of 480 nm and 520 nm, respectively). Successful doping of B atoms in the lattice of GQDs enables high selectivity towards Fe³⁺. Based on quenching of fluorescence of B-GQDs by Fe³⁺ (turn-off model), detection of Fe³⁺ (with limit of detection-LOD of 31.2 nM) and Fe³⁺-rich Cyt C (with LOD of 5.9 μg/ml) are demonstrated. As Pi can recover Fe³⁺-quenched fluorescence of B-GQDs (turn-off-on model), indirect fluorescent detection of Pi is also achieved with LOD of 340 nM. In addition, detection of Fe³⁺, Cyt C and Pi in real samples is achieved.

A highly selective and sensitive ESIPT-based coumarin–triazole polymer for the ratiometric detection of Hg2+

A reversible ESIPT based system for the detection of Hg²⁺ was developed. The system exhibited better properties compared to that of recently developed ratiometric fluorescent systems.

Modulating and probing the dynamic intermolecular interactions in plasmonic molecule-pair junctions

The intermolecular interactions, including hydrogen bonds, are electromechanically modulated and probed in metal–molecule pair–metal junctions.

Combination of liquid-based column separations with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy is a constantly developing analytical method providing not only high-sensitive quantitative but also qualitative information on an analyte. Thus, it is reasonable that it has been tested as a promising detection method in column separations. Although its implementation in analytical separations is not widespread, some surprising results, like enormous signal enhancement and demonstrations of single-molecule identifications, proved in only a few special examples, indicate the potential of the method. The high detection sensitivity and selectivity would be of paramount importance in trace analyses of biologically relevant molecules in complex matrices. However, the combination of surface-enhanced Raman spectroscopy with column separation methods brings two principal issues. Interactions of analytes with metal substrates can cause deteriorations of separations and the detection can be affected by background electrolytes or elution agents. Thus, in principle, this review is on the experimental and methodological solutions to these problems. First, theoretical and practical aspects of Raman scattering, and excitation of surface plasmon in colloid suspensions of nanoparticles and on planar nanostructured substrates are briefly explained. Advances in experimental arrangements of on-line and at-line couplings with column liquid phase separation methods, including microfluidic devices, are described together with chosen analytical applications.

Utilizing Molecular Hyperpolarizability for Trace Analysis: A Surface-Enhanced Hyper-Raman Scattering Study of Uranyl Ion

Surface-enhanced hyper-Raman scattering (SEHRS), the nonlinear analog of surface-enhanced Raman scattering (SERS), provides unique spectral signatures arising from the molecular hyperpolarizability. In this work, we explore the differences between SERS and SEHRS spectra obtained from surface-bound uranyl ion. Exploiting the distinctive SEHRS bands for trace detection of the uranyl ion, we obtain excellent sensitivity (limit of detection = 90 ppb) despite the extreme weakness of the hyper-Raman effect. We observe that binding the uranyl ion to the carboxylate group of 4-mercaptobenzoic acid (4-MBA) leads to significant changes in the SEHRS spectrum, whereas the surface-enhanced Raman scattering (SERS) spectrum of the same complex is little changed. The SERS and SEHRS spectra are also examined as a function of both substituent position, using 2-MBA, 3-MBA, and 4-MBA, and the carbon chain length, using 4-mercaptophenylacetic acid and 4-mercaptophenylpropionic acid. These results illustrate that the unique features of SEHRS can yield more information than SERS in certain cases and represent the first application of SEHRS for trace analysis of nonresonant molecules.

Electrospun nanofibers decorated with bio-sonochemically synthesized gold nanoparticles as an ultrasensitive probe in amalgam-based mercury (II) detection system

In this study, bio-ultrasound-assisted synthesized gold nanoparticles using Gracilaria canaliculata algae have been immobilized on a polymeric support and used as a glassy probe chemosensor for detection and rapid removal of Hg²⁺ ions. The function of the suggested chemosensor has been explained based on gold-amalgam formation and its catalytic role on the reaction of sodium borohydride and rhodamine B (RhB) with fluorescent and colorimetric sensing function. The catalyzed reduction of RhB by the gold amalgam led to a distinguished color change from red and yellow florescence to colorless by converting the amount of Hg²⁺ deposited on Au-NPs. The detection limit of the colorimetric and fluorescence assays for Hg²⁺ was 2.21 nM and 1.10 nM respectively. By exposing the mentioned colorless solution to air for at least 2 h, unexpectedly it was observed that the color and fluorescence of RhB were restored. Have the benefit of the above phenomenon a recyclable and portable glass-based sensor has been provided by immobilizing the Au-NPs and RB on the glass slide using electrospinning. Moreover, the introduced combinatorial membrane has facilitated the detection and removal of Hg²⁺ ions in various Hg (II)-contaminated real water samples with efficiency of up to 99%.

Gold nanoparticles and the corresponding filter membrane as chemosensors and adsorbents for dual signal amplification detection and fast removal of mercury(ii)

Nowadays, the development of a multifunction system for the simultaneous multiple signal amplification detection and fast removal of Hg²⁺ remains a major challenge. Herein, we for the first time used gold nanoparticles (Au NPs) and the corresponding filter membrane as chemosensors and adsorbents for dual signal amplification detection and fast removal of Hg²⁺. Such a system was based on the formation of gold amalgam and a gold amalgam-based reaction between rhodamine B (RhB) and NaBH₄ with fluorescence and colorimetric sensing functions. When the gold amalgam catalyzes the reduction of RhB, the red color and orange fluorescence of RhB gradually changed to colorless by switching the amount of Hg²⁺ deposited on 13 nm Au NPs. The detection limit of the fluorescence assay and colorimetric assay is 1.16 nM and 2.54 nM for Hg²⁺, respectively. Interestingly, the color and fluorescence of RhB could be recovered when the above colorless reaction solution was exposed to air for about 2 hours. Taking advantage of the above optical phenomenon, a recyclable paper-based sensor has been developed by immobilizing the Au NPs and RhB dye on filter paper and has been successfully used for detection of Hg²⁺ in real water samples. In addition, the filter membrane immobilized Au NPs could allow fast removal of mercury ions in Yellow river water and tap water with the removal efficiency close to 99%.

Facile synthesis of Au/Al2O3nanocomposites for improving the detection sensitivity of adenosine triphosphate

Alumina is widely recognized as chemically inert, and resistant to oxidation and high temperature.

Multi-functional, thiophenol-based surface chemistry for surface-enhanced Raman spectroscopy

This article highlights the recent advances of thiophenol-based surface chemistry for the applications in surface-enhanced Raman spectroscopy (SERS).

Highly Sensitive Fluorescence Detection of Hg2+ Based on a Water-soluble Conjugated Polymer with Carboxylate Groups

A label-free, highly selective, and highly sensitive fluorescent sensor to detect Hg²⁺ was developed using a water-soluble conjugated polymer with carboxylate groups (poly(2,5-bis(sodium 4-oxybutyrate)-1,4-phenylethynylene-alt-1,4-phenyleneethynylene, PPE-OBS) in this work. The fluorescence of PPE-OBS would be quenched because of the effect among the unique coordination-induced aggregation and electron transfers of PPE-OBS toward Hg²⁺. The linear relationship between the fluorescence intensity and concentration of Hg²⁺ was observed within the range from 6 × 10^-8 to 8 × 10^-5 mol L^-1 (R² = 0.9985), and the limit of detection was 2.10 × 10^-9 mol L^-1. The proposed method was applied to detect Hg²⁺ in environmental water samples, and satisfactory results were obtained.

Ce (III) - Porphyrin Sandwich Complex Ce2(TPP)3: A Rod-Like Nanoparticle as a Fluorescence Turn-Off Probe for Detection of Hg (II) and Cu (II)

In this study the researcher reports a novel, one step synthesized rod-like nanoparticles of cerium (III)-tetraphenylporphyrin sandwich complex as a spectrofluorometric sensor to measure trace amount of Hg (II) and Cu (II) metal ions. Moreover, the synthesized fluorescent probe was able to detect higher amounts (>10(-4) M) of Hg (II) in aqueous media by changing the color which can also be used as a selective mercury naked-eye sensor. The selectivity and sensitivity of the sensor based on its fluorescence quenching in the presence of Hg (II) and Cu (II) were studied according to the Stern-Volmer equation. The detection limit of the sensor was 16 nM for Hg (II) and about 2.34 μM for Cu (II) ions. Graphical Abstract Ce2(TPP)3 sandwich complex application as a fluorescent probe for measuring trace amounts of mercury and copper in real samples.

Highly uniform indicator-mediated SERS sensor platform for the detection of Zn2+

An indicator-mediated surface-enhanced Raman scattering (SERS) sensor platform with highly uniform SERS sensitivity was fabricated by modifying 4-mercaptopyridine (4-MPY) molecules as an indicator onto the surface of Ag nanoparticles that were anchored onto a silicon wafer.

Controlled Assembly of Gold Nanostructures on a Solid Substrate via Imidazole Directed Hydrogen Bonding for High Performance Surface Enhance Raman Scattering Sensing of Hypochlorous Acid

Here, we report an efficient and facile method for constructing plasmonic gold nanostructures with controlled morphology on a Si wafer and its use as a surface enhanced Raman scattering (SERS) reporting system for specific detection of HClO. To achieve this substrate, the core gold nanoparticles (AuNPs, ∼100 nm) with a monolayer of 4-mercaptoimidazole (MI) ligands were covalently linked to a thiol-derived Si wafer (MI-AuNPs@SH-Si). Taking advantage of the intermolecular NH···N hydrogen bond (HB) provided by the neighboring imidazole moiety, multiple satellite AuNPs (∼12 nm) decorated with both MI and a Raman reporter are assembled around the core MI-AuNPs at pH 5.0. The uniform morphology of the AuNP-based nanostructures on the Si wafer offer a high density of hot spots with good SERS performance for detecting HClO. The fast oxidation of the imidazole moieties by HClO causes HB destruction and therefore separation of the satellite AuNPs from the core AuNPs, which gives rise to SERS signal damping of the chip that is employed for HClO analysis. This simple and cost-effective method is highly selective for HClO over common interferences and several reactive oxygen/nitrogen species, and enabled rapid analysis at concentrations as low as 1.2 μmol L(-1). The present approach is applied to analyze water and human serum samples with satisfactory results.

Selective and Quantitative Detection of Trace Amounts of Mercury(II) Ion (Hg²⁺) and Copper(II) Ion (Cu²⁺) Using Surface-Enhanced Raman Scattering (SERS)

We report the development of a surface-enhanced Raman scattering (SERS)-based heavy metal ion sensor targeting the detection of mercury(II) ion (Hg(2+)) and copper(II) ion (Cu(2+)) with high sensitivity and selectivity. To achieve the detection of vibrational-spectroscopically silent heavy metal ions, the SERS substrate composed of gold nanorod (AuNR)-polycaprolactone (PCL) nanocomposite fibers was first functionalized using metal ion-binding ligands. Specifically, 2,5-dimercapto-1,3,4-thiadiazole dimer (di-DMT) and trimercaptotriazine (TMT) were attached to the SERS substrates serving as bridging molecules to capture Hg(2+) and Cu(2+), respectively, from solution. Upon heavy metal ion coordination, changes in the vibrational spectra of the bridging molecules, including variations in the peak-intensity ratios and peak shifts were observed and taken as indicators of the capture of the target ions. With rigorous spectral analysis, the coordination mechanism between the heavy metal ion and the corresponding bridging molecule was investigated. Mercury(II) ion primarily interacts with di-DMT through the cleavage of the disulfide bond, whereas Cu(2+) preferentially interacts with the heterocyclic N atoms in TMT. The specificity of the coordination chemistry provided both di-DMT and TMT with excellent selectivity for the detection of Hg(2+) and Cu(2+) in the presence of other interfering metal ion species. In addition, quantitative analysis of the concentration of the heavy metal ions was achieved through the construction of internal calibration curves using the peak-intensity ratios of 287/387 cm(-1) for Hg(2+) and 1234/973 cm(-1) for Cu(2+).

SERS Detection of Amyloid Oligomers on Metallorganic-Decorated Plasmonic Beads

Protein misfolded proteins are among the most toxic endogenous species of macromolecules. These chemical entities are responsible for neurodegenerative disorders such as Alzheimer's, Parkinson's, Creutzfeldt-Jakob's and different non-neurophatic amyloidosis. Notably, these oligomers show a combination of marked heterogeneity and low abundance in body fluids, which have prevented a reliable detection by immunological methods so far. Herein we exploit the selectivity of proteins to react with metallic ions and the sensitivity of surface-enhanced Raman spectroscopy (SERS) toward small electronic changes in coordination compounds to design and engineer a reliable optical sensor for protein misfolded oligomers. Our strategy relies on the functionalization of Au nanoparticle-decorated polystyrene beads with an effective metallorganic Raman chemoreceptor, composed by Al(3+) ions coordinated to 4-mercaptobenzoic acid (MBA) with high Raman cross-section, that selectively binds aberrant protein oligomers. The mechanical deformations of the MBA phenyl ring upon complexation with the oligomeric species are registered in its SERS spectrum and can be quantitatively correlated with the concentration of the target biomolecule. The SERS platform used here appears promising for future implementation of diagnostic tools of aberrant species associated with protein deposition diseases, including those with a strong social and economic impact, such as Alzheimer's and Parkinson's diseases.

Surface-enhanced Raman scattering of perchlorate on cationic-modified silver nanofilms - Effect of inorganic anions

Surface-enhanced Raman scattering (SERS) has emerged as one of the most sensitive spectroscopic analysis methods for the detection of environmental contaminants in water, including perchlorate (ClO4(-)). However, as with other commonly used analytical techniques, analysis of realistic environmental samples by SERS presents a challenge due to complex chemical components coexisting in the samples. In this work, we investigated the influence of inorganic anions (particularly oxyanions) on SERS spectra of ClO4(-) using a cationic thiol modified silver nanofilm substrate (Cys-Ag/rCu). The results show that the anions present in the samples did not shift the ClO4(-) characteristic band positions, but did decrease signal intensities due to their competitive binding with the -NH3(+) groups of cationic thiol molecules immobilized on the substrates. The pH changes caused by both the dissociation of H2PO4(-) and the hydrolysis of HCO3(-) may also play a non-negligible role. The selectivity of the Cys-Ag/rCu substrate towards these anions was determined to be in the following order: ClO4(-)>SO4(2-)>HCO3(-), NO3(-)>Cl(-)>H2PO4(-), indicating preferential adsorption of ClO4(-) ions. In the solutions with multiple anions present, the ClO4(-) SERS spectra were affected simultaneously by all the coexisting anions. Calibration curves with very good linear relationships were successfully obtained, demonstrating the great potential of quantitative detection of aqueous ClO4(-) in the matrix.

A rapid, sensitive and label-free sensor for Hg(ii) ion detection based on blocking of cysteine-quenching of fluorescent poly(thymine)-templated copper nanoparticles

A rapid fluorescence sensor was developed for Hg²⁺ detection based on blocking of cysteine-quenching of poly T templated Cu NPs.