Simultaneous SERS detection of copper and cobalt at ultratrace levels

Extending the range of metal ions SERS detection using hybrid plasmonic/ZIF-8 particles

Nanosensors based on surface-enhanced Raman spectroscopy (SERS) have emerged as a class of promising optical tools for the ultrasensitive quantification of metal ions of environmental and biological interest. A central bottleneck in this field is the availability of suitable surface receptors able to convert the selective binding with these vibrationless analytes into measurable SERS signals. In this work, we tackle this issue by employing a hybrid substrate comprising a highly SERS-active plasmonic core and a ZIF-8 metal-organic framework (MOF) shell. The ZIF-8 shell firmly captures aromatic receptors close to the plasmonic structure regardless of their intrinsic affinity for the metallic surface and without altering their ability to coordinate metal ions. Furthermore, it imparts molecular sieving abilities enabling the direct use of the SERS sensing platform in complex media such as biological fluids. This was demonstrated by using different classes of chromogenic reagents (bathocuproine, a 2,6':2',2″-terpyridine derivative, and Arsenazo III) which were exploited for the SERS detection of both transition and alkaline earth metal ions (i.e., divalent copper, cobalt and calcium ions). Notably, we successfully applied this approach for the detection of Cu(II) in untreated urine samples for Wilson's disease diagnosis. Overall, we believe this class of multifunctional hybrid substrates will serve as a valuable material for expanding the applicability of SERS spectroscopy in real-life environmental and biomedical metal ions analysis.

High Selectivity and Sensitivity in Chemiresistive Sensing of Co(II) Ions with Liquid-Phase Exfoliated Functionalized MoS2 : A Supramolecular Approach

Chemical sensing of water contamination by heavy metal ions is key as it represents a most severe environmental problem. Liquid-phase exfoliated two-dimensional (2D) transition metal dichalcogenides (TMDs) are suitable candidates for chemical sensing thanks to their high surface-to-volume ratio, sensitivity, unique electrical characteristics, and scalability. However, TMDs lack selectivity due to nonspecific analyte-nanosheet interactions. To overcome this drawback, defect engineering enables controlled functionalization of 2D TMDs. Here, ultrasensitive and selective sensors of cobalt(II) ions via the covalent functionalization of defect-rich MoS2 flakes with a specific receptor, 2,2':6',2″-terpyridine-4'-thiol is developed. A continuous network is assembled by healing of MoS2 sulfur vacancies in a tailored microfluidic approach, enabling high control over the assembly of thin and large hybrid films. The Co2+ cations complexation represents a powerful gauge for low concentrations of cationic species which can be best monitored in a chemiresisitive ion sensor, featuring a 1 pm limit of detection, sensing in a broad concentration range (1 pm - 1 µm) and sensitivity as high as 0.308 ± 0.010 lg([Co2+ ])-1 combined with a high selectivity towards Co2+ over K+ , Ca2+ , Mn2+ , Cu2+ , Cr3+ , and Fe3+ cations. This supramolecular approach based on highly specific recognition can be adapted for sensing other analytes through specific ad-hoc receptors.

Synthesis of Temperature Sensing Nitrogen-Doped Carbon Dots and Their Application in Fluorescent Ink

With the discovery of research, many properties of carbon dots are getting better and better. People have taken advantage of this and utilized them interspersed in various fields. In the present study, water-soluble nitrogen-doped carbon dots (N-CDs) with excellent optical and fluorescence thermal properties were prepared by the hydrothermal method using 4-dimethylaminopyridine and N,N'-methylenebisacrylamide as precursors. Co2+ has a selective bursting effect on the fluorescence of N-CDs. The fluorescence of N-CDs is selectively burst by Co2+, and the high sensitivity is good in the range of 0-12 μM with a detection limit of 74 nM. In addition, the good temperature response (reversible and recoverable fluorescence in the temperature range of 20~90 °C) and excellent optical properties of the N-CDs also make them new potentials in the field of fluorescent inks and temperature sensing.

Optical Quantification of Metal Ions Using Plasmonic Nanostructured Microbeads Coated with Metal-Organic Frameworks and Ion-Selective Dyes

Herein, we designed and synthesized a hybrid material comprising polystyrene submicrobeads coated with silver nanospheres. This material provides a dense collection of electromagnetic hot spots upon illumination with visible light. The subsequent coating with a metal-framework and the adsorption of bathocuproine on it yield an optical sensor for SERS that can specifically detect Cu(II) in a variety of aqueous samples at the ultratrace level. Detection limits with this method are superior to those of induced coupled plasma or atomic absorption and comparable with those obtained with induced coupled plasma coupled with a mass detector.

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.

A novel and ultrasensitive fluorescent probe derived from labeled carbon dots for recognitions of copper ions and glyphosate

Labeling materials with special functional groups are very valuable for the creation of novel probes. Hence, a novel fluorescent probe was constructed by conjugating 4-butyl-3-thiosemicarbazide (BTSC) with carbon dots (CDs). The CDs labeled by BTSC (BTSC-CDs) displayed a strong capability for recognition of Cu²⁺ and Cu²⁺ could quench the emission of BTSC-CDs significantly. The fluorescence quenching was proved to be a static quenching which was resulted from the interaction between BTSC-CDs and Cu²⁺ to form a ground-state BTSC-CDs/Cu²⁺complex, and the fluorescence intensities showed a good linear correlation with Cu²⁺ concentrations in the range of 0.20-30 μM. What is more important, by adding glyphosate into the sensor system of BTSC-CDs/Cu²⁺ the fluorescence of the probe turned on again owing to the stronger chelating between glyphosate and Cu²⁺ than between BTSC-CDs and Cu²⁺. This could realize the specific detection of glyphosate and the limit of detection was low to 0.27 μM. Detecting glyphosate using the complex BTSC-CDs/Cu²⁺ system in actual samples with satisfactory outcomes indicated that a novel fluorescent probe for Cu²⁺ and subsequent glyphosate detections has been provided.

Microfluidic SERS devices: brightening the future of bioanalysis

A new avenue has opened up for applications of surface-enhanced Raman spectroscopy (SERS) in the biomedical field, mainly due to the striking advantages offered by SERS tags. SERS tags provide indirect identification of analytes with rich and highly specific spectral fingerprint information, high sensitivity, and outstanding multiplexing potential, making them very useful in in vitro and in vivo assays. The recent and innovative advances in nanomaterial science, novel Raman reporters, and emerging bioconjugation protocols have helped develop ultra-bright SERS tags as powerful tools for multiplex SERS-based detection and diagnosis applications. Nevertheless, to translate SERS platforms to real-world problems, some challenges, especially for clinical applications, must be addressed. This review presents the current understanding of the factors influencing the quality of SERS tags and the strategies commonly employed to improve not only spectral quality but the specificity and reproducibility of the interaction of the analyte with the target ligand. It further explores some of the most common approaches which have emerged for coupling SERS with microfluidic technologies, for biomedical applications. The importance of understanding microfluidic production and characterisation to yield excellent device quality while ensuring high throughput production are emphasised and explored, after which, the challenges and approaches developed to fulfil the potential that SERS-based microfluidics have to offer are described.

Robust, reliable and quantitative sensing of aqueous arsenic species by Surface-enhanced Raman Spectroscopy: The crucial role of surface silver ions for good analytical practice

Arsenic speciation analysis is important for pollution and health risk assessment. Surface-enhanced Raman Spectroscopy (SERS) is supposed to be a promising detection technology for arsenic species owing to the unique fingerprints. However, further application of SERS is hampered by its poor repeatability. Herein, the role of surface silver ions on colloidal Ag was revealed in SERS analysis of arsenic species. Arsenic species were adsorbed on Ag nanoparticles (Ag NPs) driven by surface silver ions and were simultaneously sensed by the SERS "hot spots" generated from the aggregation of Ag NPs. So, the inconsistent SERS activities of Ag NPs synthesized from different batches can be significantly improved by modifying external silver ions onto Ag NPs (AgNPs@Ag⁺), Specific binding affinity of surface silver ions to arsenic species generated higher sensitivity (detection limit, 4.0 × 10^-11 mol L^-1 for arsenite, 8.0 × 10^-11 mol L^-1 for arsenate), wider linear range, faster response, cleaner spectra background and better reproducibility. Batch-to-batch reproducibility was significantly improved with a variation below 3.1%. The method was also demonstrated with drinking and environmental water with adequate recovery and high interference resistance. Our findings displayed good analytical practice of the surface silver ions derived SERS method and its great potential in the rapid detection of hazardous materials.

Surface etching-dependent geometry tailoring and multi-spectral information of Au@AuAg yolk-shell nanostructure with asymmetrical pyramidal core: The application in Co2+ determination

In this paper, a novel Au@AuAg yolk-shell heterogeneous nanostructure is designed as plasmonic spectroscopic sensor based on surface etching for ultrasensitive detection of trace cobalt ions (Co²⁺). Due to the surface diffusion of gold atoms, the Ag at one end of the core gold nanobipyramids (Au NBPs) is retained, and Au@AuAg yolk-shell nanostructure with asymmetric core is prepared. The alloy shell is coupled to Au NBPs and the interface of asymmetric Ag respectively, the two local surface plasmon resonance bands will have obvious reverse changes depending on the surface morphology of the shell. By using this distinct plasmon response generated by Co²⁺ induced surface etching, which is driven by discrepancy of double-peaks, a sensing method has been established to realize multi-information spectral detection of Co²⁺. There is a good linear relationship between the intensity ratio and the Co²⁺ concentration in the range of 1-100 nM, in which the limit of detection is 0.2 nM. This method further improves the sensing capability by combining multiple pieces of strongly changing spectral information, and demonstrates great advantages and potential of Au@AuAg yolk-shell heterogeneous nanostructure as a multi-information plasmonic sensor based on etched shell surface for trace detection.

Harnessing selectivity in chemical sensing via supramolecular interactions: from functionalization of nanomaterials to device applications

Chemical sensing is a strategic field of science and technology ultimately aiming at improving the quality of our lives and the sustainability of our Planet. Sensors bear a direct societal impact on well-being, which includes the quality and composition of the air we breathe, the water we drink, and the food we eat. Pristine low-dimensional materials are widely exploited as highly sensitive elements in chemical sensors, although they suffer from lack of intrinsic selectivity towards specific analytes. Here, we showcase the most recent strategies on the use of (supra)molecular interactions to harness the selectivity of suitably functionalized 0D, 1D, and 2D low-dimensional materials for chemical sensing. We discuss how the design and selection of receptors via machine learning and artificial intelligence hold a disruptive potential in chemical sensing, where selectivity is achieved by the design and high-throughput screening of large libraries of molecules exhibiting a set of affinity parameters that dictates the analyte specificity. We also discuss the importance of achieving selectivity along with other relevant characteristics in chemical sensing, such as high sensitivity, response speed, and reversibility, as milestones for true practical applications. Finally, for each distinct class of low-dimensional material, we present the most suitable functionalization strategies for their incorporation into efficient transducers for chemical sensing.

Highly Selective and Sensitive Benzimidazole Based Bifunctional Sensor for Targeting Inedible Azo Dyes in Red Chilli, Red Food Color, Turmeric Powder, and Cu(Ii) in Coconut Water

In this study, a simple Benzimidazole based bifunctional chemosensor 4-(2-(3,4-dimethoxyphenyl)-1H-benzo[d]imidazol-6-yl) benzene-1,2-diamine, L was synthesized and characterized. The sensor proved to be selective and sensitive towards detecting banned azo dyes Sudan Dye I, II, and Metanil Yellow via fluorescence turn-off response. The proposed mechanism of fluorescence quenching was the inner filter effect. LODs for Sudan I, II, and Metanil Yellow were found to be 0.009 µM, 0.012 µM, and 0.0073 µM, respectively. The developed chemosensor also showed a colorimetric response towards Cu (II) ions via an apparent color change from yellow to pink. LOD for Cu (II) ions was found to be 1.2 µM. The synthesized benzimidazole based bifunctional chemosensor was adequately tested to determine Sudan I in Red chili powder and red Food color samples, Metanil yellow in turmeric powder, and Cu(II) packaged coconut water.

A nanosol SERS/RRS aptamer assay of trace cobalt(ii) by covalent organic framework BtPD-loaded nanogold catalytic amplification

The determination of heavy metal ions has always been a hot topic in the field of environmental analysis. In this study, a new covalent organic framework-loaded gold nanoparticle (AuCOF) nanocatalytic amplification signal strategy was developed to determine trace Co²⁺ in water. The COF of BtPD was synthesized from 1,3,5-benzene tricarboxaldehyde and p-phenylenediamine, and a new kind of AuBtPD nanosol was prepared by reduction of HAuCl₄ to AuNPs on the BtPD carrier. It has strong catalysis of the new indicator reaction of sodium formate reducing HAuCl₄ to AuNP sol with strong resonance Rayleigh scattering (RRS) at 370 nm and surface enhanced resonance Raman scattering (SERS) activity at 1614 cm^-1 in the presence of a Victoria blue 4R (VB4R) molecular probe. Combining the nanocatalytic reaction to amplify the dual-scattering signals and specific aptamer (Apt) of cobalt ions, a new, fast, stable, sensitive and specific dual mode method for detecting Co²⁺ was established; the RRS signal I _370nm and SERS signal I _1614cm^-1 show a linear relationship with the concentration of 0.033-1 nmol L^-1 Co²⁺ and with a limit of detection (LOD) of 0.02 nmol L^-1. The two methods have been applied to the determination of Co²⁺ in industrial wastewater, tap water and river water, and the results are satisfactory.

Surface-Enhanced Raman Scattering Sensing of Transition Metal Ions in Waters

In this mini-review, we provide a coherent discussion on the sensing schemes exploited in the surface-enhanced Raman scattering (SERS) analysis of transition metal ions in waters. A critical approach was used where illustrative examples are selected to discuss key drawbacks and challenges associated with various experimental configurations and the employed enhancing substrates.

Understanding active sites in molecular (photo)electrocatalysis through complementary vibrational spectroelectrochemistry

Highlighting vibrational spectroelectrochemistry for the investigation of synthetic molecular (photo) electrocatalysts for key energy conversion reactions.

Peroxymonosulfate decomposition by homogeneous and heterogeneous Co: Kinetics and application for the degradation of acetaminophen

Peroxymonosulfate (PMS) decomposition, hydroxyl radical (^•OH) generation, and acetaminophen (ACT) degradation by the Co/PMS system using homogeneous (dissolved cobalt) and heterogeneous (suspended Co₃O₄) cobalt were assessed. For the homogeneous process, >99% PMS decomposition was observed and 10 mmol/L of ^•OH generation was produced using 5 mmol/L of PMS and different dissolved cobalt concentrations after 30 min. A dissolved cobalt concentration of 0.2 mmol/L was used to achieve >99% ACT degradation using the homogeneous process. For the heterogeneous process, 60% PMS decomposition and negligible ^•OH generation were observed for 5 mmol/L of the initial PMS concentration using 0.1 and 0.2 g/L of Co₃O₄. Degradation of ACT greater than 80% was achieved for all experimental runs using 5 mmol/L of the initial PMS concentration independently of the initial Co₃O₄ load used. For the heterogeneous process, the best experimental conditions for ACT degradation were found to be 3 mmol/L of PMS and 0.2 g/L of Co₃O₄, for which >99% ACT degradation was achieved after 10 min. Because negligible ^•OH was produced by the Co₃O₄/PMS process, a second-order kinetic model was proposed for sulfur-based free radical production to allow fair comparison between homogeneous and heterogeneous processes. Using the kinetic data and the reaction by-products identified, a mechanistic pathway for ACT degradation is suggested.

A reactive probe for Co2+ ion detection based on a catalytic decomposition process and its fluorescence imaging in living cells

A novel reactive fluorescent probe for cobalt ions was prepared based on integration of thiourea functional groups, coumarin, and naphthalimide fluorophores. There was no fluorescence observed for the probe itself, however, in the presence of cobalt ions, catalytic decomposition occurred for the probe and coumarin molecular fragments were produced that emitted blue fluorescence. This enabled the probe to be used as a 'turn on' reagent for detection of cobalt ions. Under physiological pH conditions and in appropriate solvent systems, an obvious fluorescence enhancement for cobalt ions was observed in selective experiments. Competition experiments indicated that cobalt ions could still induce fluorescence enhancement in the presence of other metal ions. Sensitivity experiments showed that the detection limit for cobalt ions was 6.0 nM. Dynamics research demonstrated that the catalytic process was a pseudo-first-order reaction and the reaction constant (k_obs ) was calculated to be 1.49 × 10^-2 min^-1 . In addition, the mechanism of catalytic decomposition could be demonstrated using electrospray ionization mass spectrometry and thin layer chromatography experiments. Cell fluorescence imaging experiments demonstrated that the probe could be used to detect cobalt ions in living HeLa cells.

A frugal implementation of Surface Enhanced Raman Scattering for sensing Zn2+ in freshwaters - In depth investigation of the analytical performances

Surface Enhanced Raman Scattering (SERS) has been widely praised for its extreme sensitivity but has not so far been put to use in routine analytical applications, with the accessible scale of measurements a limiting factor. We report here on a frugal implementation of SERS dedicated to the quantitative detection of Zn²⁺ in water, Zn being an element that can serve as an indicator of contamination by heavy metals in aquatic bodies. The method consists in randomly aggregating simple silver colloids in the analyte solution in the presence of a complexometric indicator of Zn²⁺, recording the SERS spectrum with a portable Raman spectrometer and analysing the data using multivariate calibration models. The frugality of the sensing procedure enables us to acquire a dataset much larger than conventionally done in the field of SERS, which in turn allows for an in-depth statistical analysis of the analytical performances that matter to end-users. In pure water, the proposed sensor is sensitive and accurate in the 160-2230 nM range, with a trueness of 96% and a precision of 4%. Although its limit of detection is one order of magnitude higher than those of golden standard techniques for quantifying metals, its sensitivity range matches Zn levels that are relevant to the health of aquatic bodies. Moreover, its frugality positions it as an interesting alternative to monitor water quality. Critically, the combination of the simple procedure for sample preparation, abundant SERS material and affordable portable instrument paves the way for a realistic deployment to the water site, with each Zn reading three to five times cheaper than through conventional techniques. It could therefore complement current monitoring methods in a bid to solve the pressing needs for large scale water quality data.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

A New Fluorescent Chemosensor for Cobalt(II) Ions in Living Cells Based on 1,8-Naphthalimide

In this work, a highly selective fluorescent chemosensor N-(2-(2-butyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)hydrazine-1-carbonothioyl)benzamide (L) was prepared and characterized. An assay to detect the presence of cobalt(II) ions was developed by utilizing turn-on fluorescence enhancement with visual colorimetric response. Upon treatment with Co²⁺, a remarkable fluorescence enhancement located at 450 nm was visible to naked eyes accompanied with a distinct color change (from pink to colorless) in a CH₃CN/HEPES (4/1, v/v, pH = 7.4) solution due to the formation of a 1:1 complex at room temperature. In addition, the linear concentration range for Co²⁺ was 0–25 µM with the limit of detection down to 0.26 µM. Thus, a highly sensitive fluorescent method based on chelation-assisted fluorescence enhancement was developed for the trace-level detection of Co²⁺. The sensor was found to be highly selective toward Co²⁺ ions with a large number of coexisting ions. Furthermore, the L probe can serve as a fluorescent sensor for Co²⁺ detecting in biological environments, demonstrating its low toxic properties to organisms and good cell permeability in live cell imaging.

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.

Conformational sensitivity of surface selection rules for quantitative Raman identification of small molecules in biofluids

Biofluid analysis by surface-enhanced Raman scattering (SERS) is usually hindered by nonspecific interferences. It is challenging to drive targeted molecules towards sensitive areas with specific capture and quantitative recognition in complex biofluids. Herein, a highly specific and quantitative SERS analyzer for small molecule dopamine (DA) in serum is demonstrated on a portable Raman device by virtue of a transducer of mercaptophenylboronic acid (MPBA) and a site-directed decoration of plasmonic Ag dendrites on a superhydrophobic surface. Theoretical simulations of molecular vibrations and charge distributions demonstrate the predomination of Raman surface selection rules in molecular reorientation upon the binding of DA. This recognition event is translated into ratiometric changes in the spectral profile which evidences excellent capability on SERS quantitation. The rules can well distinguish DA from its common interferents including fructose, glucose, sucrose and ascorbic acid which all generate weak but completely opposite spectral changes. Moreover, benefitting from the wettability difference, the target DA in diluted serum can be specifically enriched on a transducer-capped Ag surface, and the adsorption of other interferences is resisted by superhydrophobic features. It paves a new way for labelling a single SERS tag to simultaneously realize the identification and quantification of small molecules in complex biological media.

SERS Sensors: Recent Developments and a Generalized Classification Scheme Based on the Signal Origin

Owing to its extreme sensitivity and easy execution, surface-enhanced Raman spectroscopy (SERS) now finds application for a wide variety of problems requiring sensitive and targeted analyte detection. This widespread application has prompted a proliferation of different SERS-based sensors, suggesting the need for a framework to classify existing methods and guide the development of new techniques. After a brief discussion of the general SERS modalities, we classify SERS-based sensors according the origin of the signal. Three major categories emerge from this analysis: surface-affinity strategy, SERS-tag strategy, and probe-mediated strategy. For each case, we describe the mechanism of action, give selected examples, and point out general misconceptions to aid the construction of new devices. We hope this review serves as a useful tutorial guide and helps readers to better classify and design practical and effective SERS-based sensors.

Silicon nanohybrid-based SERS chips armed with an internal standard for broad-range, sensitive and reproducible simultaneous quantification of lead(ii) and mercury(ii) in real systems

Lead ions (Pb²⁺) and mercury ions (Hg²⁺), the two commonly coexisting heavy metal ions, pose severe risks to environment and human health. To date, no surface-enhanced Raman scattering (SERS) sensor has been reported for the simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems. Herein, the first demonstration of SERS chips for simultaneous quantification of Pb²⁺ and Hg²⁺ in real systems is presented based on the combination of reproducible silicon nanohybrid substrates and a corrective internal standard (IS) sensing strategy. This chip was made of a silver nanoparticle-decorated silicon wafer via modification of the IS, i.e. 4-aminothiophenol, molecules. The as-prepared chip was further functionalized with Pb²⁺- and Hg²⁺- specific DNA strands capable of simultaneously detecting Pb²⁺ and Hg²⁺. Quantitatively, upon correction by the IS Raman signals, the broad dynamic ranges from 100 pM to 10 μM for Pb²⁺ and from 1 nM to 10 μM for Hg²⁺ were achieved, with the detection limit down to 19.8 ppt for Pb²⁺ and 168 ppt for Hg²⁺. For real applications, we further demonstrated that Pb²⁺ and Hg²⁺ spiked into industrial wastewater could be readily distinguished via the presented chip, and the relative standard deviation (RSD) value was less than ∼15%. More significantly, the resulting SERS chip can be well coupled with a hand-held Raman instrument and can then be used for the qualitative analysis of both Pb²⁺ and Hg²⁺ in real systems in a portable manner. Our results suggest that this high-quality SERS chip is a powerful tool for on-site detection of various heavy metal ions in real samples in the field of food safety and environment protection.

A new fluorescent–colorimetric chemosensor for cobalt(ii) ions based on bis-benzimidazolium salt with three anthraquinone groups

A novel colorimetric chemosensor was prepared, and its recognition performance for Co²⁺ was investigated by the fluorescence method and color change.

Multifunctional Biochar for Highly Efficient Capture, Identification, and Removal of Toxic Metals and Superbugs from Water Samples

According to the World Health Organization, more than two billion people in our world use drinking water sources which are not free from pathogens and heavy metal contamination. Unsafe drinking water is responsible for the death of several millions in the 21st century. To find facile and cost-effective routes for developing multifunctional materials, which has the capability to resolve many of the challenges associated with drinking water problem, here, we report the novel design of multifunctional fluorescence-magnetic biochar with the capability for highly efficient separation, identification, and removal of pathogenic superbugs and toxic metals from environmental water samples. Details of synthesis and characterization of multifunctional biochar that exhibits very good magnetic properties and emits bright blue light owing to the quantum confinement effect are reported. In our design, biochar, a carbon-rich low-cost byproduct of naturally abundant biomass, which exhibits heterogeneous surface chemistry and strong binding affinity via oxygen-containing group on the surface, has been used to capture pathogens and toxic metals. Biochar dots (BCDs) of an average of 4 nm size with very bright photoluminescence have been developed for the identification of pathogens and toxic metals. In the current design, magnetic nanoparticles have been incorporated with BCDs which allow pathogens and toxic metals to be completely removed from water after separation by an external magnetic field. Reported results show that owing to the formation of strong complex between multifunctional biochar and cobalt(II), multifunctional biochar can be used for the selective capture and removal of Co(II) from environmental samples. Experimental data demonstrate that multifunctional biochar can be used for the highly efficient removal of methicillin-resistant Staphylococcus aureus (MRSA) from environmental samples. Reported results also show that melittin, an antimicrobial peptide-attached multifunctional biochar, has the capability to completely disinfect MRSA superbugs after magnetic separation. A possible mechanism for the selective separation of Co(II), as well as separation and killing of MRSA, has been discussed.

A single colorimetric sensor for multiple targets: the sequential detection of Co2+and cyanide and the selective detection of Cu2+in aqueous solution

A colorimetric chemosensor was developed for simultaneous detection of Co²⁺and Cu²⁺and for sequential recognition of Co²⁺and CN⁻.

Detection of Copper(II) Ions Using Glycine on Hydrazine-Adsorbed Gold Nanoparticles via Raman Spectroscopy

A facile, selective, and sensitive detection method for the Cu²⁺ ions in environmental and biological solutions has been newly developed by observing the unique CN stretching peaks at ~2108 cm⁻¹ upon the dissociative adsorption of glycine (GLY) in hydrazine buffer on gold nanoparticles (AuNPs). The relative abundance of Cu species on AuNPs was identified from X-ray photoelectron spectroscopy analysis. UV-Vis spectra also indicated that the Au particles aggregated to result in the color change owing to the destabilization induced by the GLY-Cu²⁺ complex. The CN stretching band at ~2108 cm⁻¹ could be observed to indicate the formation of the CN species from GLY on the hydrazine-covered AuNP surfaces. The other ions of Fe³⁺, Fe²⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cr³⁺, Co²⁺, Cd²⁺, Pb²⁺, Ca²⁺, NH₄⁺, Na⁺, and K⁺ at high concentrations of 50 µM did not produce such spectral changes. The detection limit based on the CN band for the determination of the Cu²⁺ ion could be estimated to be as low as 500 nM in distilled water and 1 µM in river water, respectively. We attempted to apply our method to estimate intracellular ion detection in cancer cells for more practical purposes.

A Rapid In Situ Colorimetric Assay for Cobalt Detection by the Naked Eye

A simple, rapid, and convenient colorimetric chemosensor of a specific target toward the end user is still required for on-site detection and real-time monitoring applications. In this study, we developed a rapid in situ colorimetric assay for cobalt detection using the naked eye. Interestingly, a yellow to light orange visual color transition was observed within 3 s when a Chrysoidine G (CG) chemosensor was exposed to cobalt. Surprisingly, the CG chemosensor had great selectivity toward cobalt without any interference of other metal ions. Under optimized conditions, a lower detection limit of 0.1 ppm via a spectrophotometer and a visual detection limit of 2 ppm with a linear range from 0.4 to 1 ppm (R² = 0.97) were determined. Moreover, the CG chemosensor is reversible and maintains its functionality after treatment with chelating agents. In conclusion, we show the superior capabilities of the CG chemosensor, which has the potential to provide extremely facile handling, high sensitivity, and a fast response time for applications of on-site detection to real-time cobalt monitoring for the general public.

Detection of potentially toxic metals by SERS using salen complexes

Four metal ions have been clearly discriminated using SERS by coordinating each species to salen. The spectra acquired demonstrated characteristic differences, allowing identification of each metal ion.

Thiosemicarbazide chemical functionalized carbon dots as a fluorescent nanosensor for sensing Cu2+and intracellular imaging

Synthesis procedures for the CDs-based nanosensor and schematic diagram of Cu²⁺detection.

Regioselective Localization and Tracking of Biomolecules on Single Gold Nanoparticles

Selective localization of biomolecules at the hot spots of a plasmonic nanoparticle is an attractive strategy to exploit the light-matter interaction due to the high field concentration. Current approaches for hot spot targeting are time-consuming and involve prior knowledge of the hot spots. Multiphoton plasmonic lithography is employed to rapidly immobilize bovine serum albumin (BSA) hydrogel at the hot spot tips of a single gold nanotriangle (AuNT). Regioselectivity and quantity control by manipulating the polarization and intensity of the incident laser are also established. Single AuNTs are tracked using dark-field scattering spectroscopy and scanning electron microscopy to characterize the regioselective process. Fluorescence lifetime measurements further confirm BSA immobilization on the AuNTs. Here, the AuNT-BSA hydrogel complexes, in conjunction with single-particle optical monitoring, can act as a framework for understanding light-molecule interactions at the subnanoparticle level and has potential applications in biophotonics, nanomedicine, and life sciences.

Trace detection of tetrabromobisphenol A by SERS with DMAP-modified magnetic gold nanoclusters

Magnetic gold nanoclusters (MGNCs) functionalized with 4-dimethylaminopyridine (DMAP) enables the trace detection of tetrabromobisphenol A (TBBPA), an environmental pollutant, using surface-enhanced Raman scattering (SERS) spectroscopy. The synthesis, cleansing, and functionalization of MGNCs are conducted in aqueous solutions; SERS samples are prepared by magnetic precipitation in the presence of trace analyte. The limit of detection (LOD) for TBBPA is greatly increased by the use of DMAP as a reporter molecule: DMAP-modified MGNCs can detect TBBPA at 10 pM in water, whereas the LOD for TBBPA by unfunctionalized Au is 1 nM. The reproducibility of picomolar TBBPA detection with DMAP-modified MGNCs is confirmed by two-dimensional correlation analysis. The high SERS sensitivity for TBBPA can be attributed to its capacity to modulate the Raman spectrum of adsorbed DMAP. This indirect mode of detection can also be applied toward the detection of other hydrophobic analytes, each identifiable by its characteristic SERS identity.

Fenton-like reaction-mediated etching of gold nanorods for visual detection of Co(2+)

We have proposed a Fenton-like reaction-mediated etching of gold nanorods and applied it to the sensitive visual detection of Co(2+) ions. With the presence of bicarbonate (HCO3(-)) and hydrogen peroxide(H2O2), Co(2+) ions trigger a Fenton-like reaction, resulting in the generation of superoxide radical (O2(•-)). As a result, the gold nanorods are gradually etched by O2(•-) in the presence of SCN(-), accompanied by an obvious color change from green to red. The gold nanorods etching process preferentially occurs along the longitudinal direction, which is observed by transmission electron microscope. The etching mechanism is carefully proved by investigating the effects of different radical scavengers (e.g., dimethyl sulfoxide). The auto-oxidation of hydroxylamine assay further confirms the mechanism. Then, the main factors, including reactants concentrations, temperature, and incubation time, are specifically investigated. Under optimized conditions, we get an excellent sensing performance for Co(2+) with a lower detection limit of 1.0 nM via a spectrophotometer and a visual detection limit of 40 nM. In addition, this principle may provide a new concept of "intermediate-mediated etching of nanoparticles" for sensing.

Synthesis of photoluminescent carbon dots for the detection of cobalt ions

Photoluminescent carbon dots (C-dots) were prepared from l-cysteine through a simple hydrothermal process and used for selective detection of cobalt ions (Co²⁺), based on analyte induced aggregation and photoluminescence quenching of C-dots.

Determination of metal ion concentrations by SERS using 2,2′-bipyridyl complexes

SERS of bipyridyl complexes for the discrimination of six different metal ions.

Thiourea dioxide as a unique eco-friendly coreactant for luminol chemiluminescence in the sensitive detection of luminol, thiourea dioxide and cobalt ions

Thiourea dioxide, a unique reductant that can generate oxygen upon decomposition, has been investigated as an efficient, cost-effective, and stable coreactant.

Hydroxylamine-O-sulfonic acid as an efficient coreactant for luminol chemiluminescence for selective and sensitive detection

HOSA, a versatile synthetic reagent, has been explored as an effective chemiluminescence coreactant for the sensitive detection of luminol, HOSA, and Co²⁺, including selective Co²⁺ detection.

Precision synthesis: designing hot spots over hot spots via selective gold deposition on silver octahedra edges

A major challenge in plasmonic hot spot fabrication is to efficiently increase the hot spot volumes on single metal nanoparticles to generate stronger signals in plasmon-enhanced applications. Here, the synthesis of designer nanoparticles, where plasmonic-active Au nanodots are selectively deposited onto the edge/tip hot spot regions of Ag nanoparticles, is demonstrated using a two-step seed-mediated precision synthesis approach. Such a "hot spots over hot spots" strategy leads to an efficient enhancement of the plasmonic hot spot volumes on single Ag nanoparticles. Through cathodoluminescence hyperspectral imaging of these selective edge gold-deposited Ag octahedron (SEGSO), the increase in the areas and emission intensities of hot spots on Ag octahedra are directly visualized after Au deposition. Single-particle surface-enhanced Raman scattering (SERS) measurements demonstrate 10-fold and 3-fold larger SERS enhancement factors of the SEGSO as compared to pure Ag octahedra and non-selective gold-deposited Ag octahedra (NSEGSO), respectively. The experimental results corroborate well with theoretical simulations, where the local electromagnetic field enhancement of our SEGSO particles is 15-fold and 1.3-fold stronger than pure Ag octahedra and facet-deposited particles, respectively. The growth mechanisms of such designer nanoparticles are also discussed together with a demonstration of the versatility of this synthetic protocol.