Correction: Electrochemical and X-ray structural evidence of multiple molybdenum precursor candidates from a reported non-aqueous electrodeposition of molybdenum disulfide.

[This corrects the article DOI: 10.1039/D3RA04605B.].

Electrochemical and X-ray structural evidence of multiple molybdenum precursor candidates from a reported non-aqueous electrodeposition of molybdenum disulfide.

A published report of electrodeposited molybdenum(iv) disulfide microflowers at 100 °C in the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethane)sulfonimide (PP13-TFSI) from 1,4-butanedithiol and the concentrated filtrate from a reaction mixture of molybdenum(vi) trioxide and ethylene glycol could not be reproduced reliably, affording numerous uniquely coloured reaction mixtures that precipitated a variety of crystalline molybdenum coordination complexes. Further attempts to use the same two of these filtrates to electrodeposit molybdenum(iv) disulfide from 0.1 M PP13-TFSI in tetrahydrofuran with 1,4-butanedithiol at room temperature were unsuccessful. Various crude reaction mixtures grew crystals of different identity from eight attempts to synthesize the reported molybdenum-precursor. Single crystal X-ray diffraction (SC-XRD) offered insight into a wide range of structural features from four candidate paramagnetic precursor compounds, including a novel organomolybdenum cluster. Electrochemical studies of the various molybdenum-precursor filtrates, ethylene glycol, and 1,4-butanedithiol were conducted in 0.1 M PP13-TFSI in tetrahydrofuran, offering insight into differences between preparations of the molybdenum-precursor and interference between ethylene glycol and 1,4-butanedithiol on platinum working electrodes. Molybdenum(iv) disulfide electrodeposition attempts included cyclic voltammetry and chronoamperometry on platinum and glassy carbon working electrodes, which led to either no deposited material, or molybdenum, carbon, oxygen, and sulfur containing amorphous and non-homogenous deposits, as indicated by SEM-EDS analysis.

Thiacloprid Detection by Silver Nanocubes-Based SERS Sensor.

The use of neonicotinoid insecticides leads to environmental problems such as accumulation and death of different insects and even bird species. In this work, we compared the SERS performance of Ag nanocubes- and nanospheres-based substrates for the analysis of thiacloprid, a neonicotinoid, reaching its detection at 1 mM using nanocubes as the active material.

Fabrication of high quality electrochemical SERS (EC-SERS) substrates using physical vapour deposition.

Screen-printed electrodes (SPEs) are an efficient and inexpensive substrate for electrochemical surface-enhanced Raman spectroscopy (EC-SERS) studies. Traditionally, the working electrode of the SPE is modified with either a colloidal paste of metal nanoparticles or an electrodeposited metallic film. These methods can be time-consuming and often produce non-uniform nanostructured films. Physical vapour deposition (PVD) is presented in this work as an efficient and effective alternative method for the production of SERS-active SPEs. SPEs coated with silver thin films via PVD show consistent and strong EC-SERS enhancement for the detection of two probe molecules, 4-aminothiophenol (p-ATP) and 5-(pyridine-4-yl)-1,3,4-oxadiazole-2-thiol (PYOT). The EC-SERS signal intensity for p-ATP and PYOT had coefficients of variation of 8.2% and 5.5%, respectively. More generally, these substrates show promise for reliable enhancement of molecules spanning diverse analytical applications.

Optimization of gold nanorod arrays for surface enhanced Raman spectroscopy (SERS) detection of atrazine.

Recently, there has been increasing concern over the widespread use of the herbicide atrazine which has been reported to have problematic side effects on local ecosystems. This has highlighted the need for rapid and accurate point-of-need assessment tools for analytical determination of herbicides in ground and surface waters. Surface enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique which has recently been employed for the analysis of a variety of analytes in water, ranging from pharmaceuticals to pesticides. In this work, SERS sensors constructed using gold nanorod (AuNR) arrays are optimized and then utilized for the rapid and sensitive detection of atrazine. In this study, the effect of relative humidity on the self-assembly of gold nanorods into arrays was explored, and the SERS performance was assessed using para-aminothiophenol as a SERS probe. Once the SERS performance of the substrates was deemed optimal, the detection of atrazine was highlighted. This work represents the first time that relative humidity has been explored as an optimization strategy for controlled alignment of gold nanorods for SERS analysis of atrazine.

Spectroelectrochemical and computational studies of tetrahydrocannabinol (THC) and carboxy-tetrahydrocannabinol (THC-COOH).

Rapid and accurate detection of tetrahydrocannabinol (THC) and its main secondary metabolite carboxy-tetrahydrocannabinol (THC-COOH) is important to ensure safe roadways and workplaces, particularly in regions of the world where cannabis use is legal. In this work, we seek to demonstrate the usefulness of electrochemical SERS (EC-SERS) for the rapid detection of both THC and THC-COOH, complemented by thorough ab initio calculations for both molecules. These results indicate that application of a voltage is essential for efficient SERS detection of cannabinoids at low concentrations in bodily fluids, allowing for the eventual development of sensitive and quantitative screening tools. To the best of our knowledge, this work represents the first EC-SERS study of both THC and THC-COOH.

Electrochemical Surface-Enhanced Raman Spectroscopy as a Platform for Bacterial Detection and Identification.

The field of bacterial screening is in need of a rapid, easy to use, sensitive, and selective platform for bacterial detection and identification. Current methods of bacterial identification lack time efficiency, resulting in problems for many sectors of society. Surface-enhanced Raman spectroscopy (SERS) has been investigated as a possible candidate for bacterial screening due to its demonstrated ability to detect biological molecules with a high degree of sensitivity. However, the field of bacterial screening using SERS is currently facing limitations such as signal irreproducibility, weak spectra, and difficulty differentiating between strains based on the SERS spectra of bacteria alone. The current study reports on the first ever use of electrochemical surface-enhanced Raman spectroscopy (EC-SERS) for bacterial screening. The results of this study demonstrate the ability of EC-SERS to greatly improve upon the SERS performance for the detection of Gram-positive and Gram-negative bacteria both in terms of improved peak intensities and spectral richness. EC-SERS shows great promise in its ability to advance SERS-based bacterial screening and could potentially be used for more efficient species discrimination at the point-of-need (PON).

Development of an electrochemical surface-enhanced Raman spectroscopy (EC-SERS) fabric-based plasmonic sensor for point-of-care diagnostics.

Early disease diagnosis is crucial for timely and effective healthcare monitoring and treatment. Demand for modern point-of-care (POC) technologies has increased during the past decade. Continuous monitoring of patient health status can be achieved through wearable sensors which can be incorporated into clothing and other wearables. While electronic textiles that monitor physical parameters (heart rate, blood pressure, etc.) are increasingly commonplace, smart textiles capable of monitoring chemical biomarkers are much less common. In this work, a conductive plasmonic electrochemical sensor was developed from a cotton blend fabric modified with silver nanoparticles and conductive inks. para-Aminothiophenol (pATP) was used as an initial probe molecule to evaluate the performance of the fabric-based electrode for electrochemical surface-enhanced Raman spectroscopic (EC-SERS) measurements. Further investigation was then carried out to detect levofloxacin, a commonly prescribed antibiotic, in both 0.1 M NaF and synthetic urine as supporting electrolyte. It was found that the fabric-based electrode provided excellent EC-SERS signals, comparable to commercial screen-printed electrodes, allowing for rapid detection of levofloxacin at clinically relevant concentrations. To the best of our knowledge, this is the first time a fabric-based electrode has been reported for EC-SERS investigations, highlighting a promising platform for wearable point-of-care sensors.

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) study of the interaction between protein aggregates and biomimetic membranes.

Human diseases characterized by the uncontrolled deposition of insoluble extracellular protein aggregates are collectively referred to as amyloidoses. Such diseases include Alzheimer's, Parkinson's, Huntington's, and prion disease. In Alzheimer's disease, it is believed that amyloid-ß proteins may be responsible for pore and defect formation within cellular membranes, leading to a breakdown of cellular homeostasis causing eventual neuronal death. This theory is referred to as the amyloid pore hypothesis of Alzheimer's disease. In this work, the interaction between a model amyloid-forming protein (insulin) and a biomimetic membrane was studied at the molecular level. Protein at different stages of aggregation was allowed to interact with a biomimetic membrane formed on a nanostructured substrate using Langmuir-Blodgett/Langmuir-Schaefer deposition. Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) was used to monitor the molecular level changes occurring as a result of this interaction. Based on the results it was observed that oligomers and protofibrils caused the most significant membrane deterioration whilst native protein appeared to play a protective role. To the best of our knowledge, this work represents the first EC-SERS investigation of protein aggregate-biomembrane interactions, and highlights the usefulness of this tool for studying complex biomolecular interactions.

The development of "fab-chips" as low-cost, sensitive surface-enhanced Raman spectroscopy (SERS) substrates for analytical applications.

The demand for methods and technologies capable of rapid, inexpensive and continuous monitoring of health status or exposure to environmental pollutants persists. In this work, the development of novel surface-enhanced Raman spectroscopy (SERS) substrates from metal-coated silk fabric, known as zari, presents the potential for SERS substrates to be incorporated into clothing and other textiles for the routine monitoring of important analytes, such as disease biomarkers or environmental pollutants. Characterization of the zari fabric was completed using scanning electron microscopy, energy dispersive X-ray analysis and Raman spectroscopy. Silver nanoparticles (AgNPs) were prepared, characterized by transmission electron microscopy and UV-vis spectroscopy, and used to treat fabric samples by incubation, drop-coating and in situ synthesis. The quality of the treated fabric was evaluated by collecting the SERS signal of 4,4'-bipyridine on these substrates. When AgNPs were drop-coated on the fabric, sensitive and reproducible substrates were obtained. Adenine was selected as a second probe molecule, because it dominates the SERS signal of DNA, which is an important class of disease biomarker, particularly for pathogens such as Plasmodium spp. and Mycobacterium tuberculosis. Excellent signal enhancement could be achieved on these affordable substrates, suggesting that the developed fabric chips have the potential for expanding the use of SERS as a diagnostic and environmental monitoring tool for application in wearable sensor technologies.

Quantitative detection of uric acid by electrochemical-surface enhanced Raman spectroscopy using a multilayered Au/Ag substrate.

Uric acid is a potential important biomarker in urine and serum samples for early diagnosis of preeclampsia, a life-threatening hypertensive disorder that occurs during pregnancy. Preeclampsia is a leading cause of maternal death, especially in developing nation settings. Quantitative detection of uric acid for rapid and routine diagnosis of early preeclampsia using electrochemical-surface enhanced Raman spectroscopy (EC-SERS) is presented herein. A uniform EC-SERS active Au/Ag substrate was developed by depositing nearly monodisperse gold and silver nanoparticles on the carbon working electrode surface of screen printed electrodes. The multilayered Au/Ag substrates were characterized by electron microscopy and used for quantitative detection of uric acid in 0.1 M NaF and synthetic urine at clinically relevant concentrations. These results showed a linear relationship between the EC-SERS signal intensity and the uric acid concentration. Relative errors calculated for selected concentrations were all within the Clinical Laboratory Improvement Amendments (CLIA) criterion for uric acid analysis (±17%). It is believed that routine and early diagnosis of disease could be possible through such quantitative detection of biomarkers in patient samples using this EC-SERS method.

A simple complex on the verge of breakdown: isolation of the elusive cyanoformate ion.

Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2](-), forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C-C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.

Electrochemical surface-enhanced Raman spectroscopy (E-SERS) of novel biodegradable ionic liquids.

Electrochemical SERS (E-SERS) was used for the first time to study the interfacial behavior of a class of pyridinium-based biodegradable ionic liquids at a silver nanoparticle (AgNP) electrode surface. An isomeric series of ionic liquids (IL) based on 3-butoxycarbonyl-1-methylpyridinium bis(trifluoromethanesulfonyl)imide were prepared, which have demonstrable biodegradability. It was found that all four of the isomeric ionic liquids studied exhibited excellent electrochemical stability as binary mixtures combined with methanol, with the absence of any specific redox processes occurring over nearly 3.0 V of applied potential. Normal Raman measurements of the neat isobutyl IL showed a signal rich in vibrational features, with strong contributions from both the anion and the bulky organic cation. E-SERS of the neat isobutyl IL was shown to exhibit excellent potential stability, with no potential-induced orientational change at the metal surface. When the ionic liquids were prepared as methanolic binary mixtures, dissociation of the IL ions was observed, and only the organic cation was shown to adsorb at the Ag/solution interface. The nature of the substituent on the ester group of the IL series was observed to have a significant effect on the orientation of the cation on the metal surface, based on the application of the metal surface selection rules combined with computational data. Notably, the isobutyl and sec-butyl isomers were observed to have an orientation wherein the pyridinium ring was oriented perpendicular to the surface, while the tert-butyl and n-butyl isomers were observed to have an orientation wherein the pyridinium ring was lying flat on the metal surface.

Electrochemical-surface enhanced Raman spectroscopy (E-SERS) of uric acid: a potential rapid diagnostic method for early preeclampsia detection.

An increased level of uric acid in urine and plasma is indicative of the development of preeclampsia, a hypertensive disorder that can occur during pregnancy. The preliminary steps towards developing a rapid tool for early diagnosis of preeclampsia using electrochemical SERS (E-SERS) for the detection of uric acid in urine are presented herein. Characterization of the uric acid species was completed using cyclic voltammetry, UV spectroscopy, Raman spectroscopy and electrochemical surface-enhanced Raman spectroscopy (E-SERS). E-SERS was capable of easily detecting uric acid directly at concentrations <1 mM in urine simulant, without the need for costly enzymes and bulky equipment, and thus demonstrates promise as a rapid point-of-care diagnostic tool for detection of early onset preeclampsia in developing nation settings.

Electrochemical and PM-IRRAS characterization of cholera toxin binding at a model biological membrane.

A mixed phospholipid-cholestrol bilayer, with cholera toxin B (CTB) units attached to the monosialotetrahexosylganglioside (GM1) binding sites in the distal leaflet, was deposited on a Au(111) electrode surface. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) measurements were used to characterize structural and orientational changes in this model biological membrane upon binding CTB and the application of the electrode potential. The data presented in this article show that binding cholera toxin to the membrane leads to an overall increase in the tilt angle of the fatty acid chains; however, the conformation of the bilayer remains relatively constant as indicated by the small decrease in the total number of gauche conformers of acyl tails. In addition, the bound toxin caused a significant decrease in the hydration of the ester group contained within the lipid bilayer. Furthermore, changes in the applied potential had a minimal effect on the overall structure of the membrane. In contrast, our results showed significant voltage-dependent changes in the average orientation of the protein α-helices that may correspond to the voltage-gated opening and closing of the central pore that resides within the B subunit of cholera toxin.

SERS of beta-thioglucose adsorbed on nanostructured silver electrodes.

Highly ordered microporous films of silver-containing regular arrays of spherical pores of differing diameters were prepared by electrochemical deposition into the interstitial spaces of a template formed by nanosphere lithography. These nanostructured electrodes in conjunction with a Raman microprobe spectrometer were used to obtain surface-enhanced Raman spectra (SERS) of beta-thioglucose (beta-TG) under potential control. The SERS results were compared with SERS of beta-TG on an electrochemically roughened silver electrode surface. The bands in the experimental spectra were assigned to particular vibrations with the help of ab initio predictions of the spectra. The results of this study show that beta-TG self-assembled at a silver electrode forms a hydrophilic film that may be used in biomimetic research to enhance interactions between the electrode and the hydrophilic portion of a model membrane, and to prevent interactions of proteins inserted into this membrane with the metal surface. However, in contact with the aqueous electrolyte an anomerization reaction takes place and the beta-TG film is a mixture of the alpha- and beta-anomers of thioglucose. A partial oxidation of the self-assembled molecules was also observed. In addition, the orientation of the adsorbed molecules changes as a function of the applied potential.

Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists' canvas.

This perspective presents recent surface-enhanced Raman spectroscopy (SERS) studies of dyes, with applications to the fields of single-molecule spectroscopy and art conservation. First we describe the development and outlook of single-molecule SERS (SMSERS). Rather than providing an exhaustive review of the literature, SMSERS experiments that we consider essential for its future development are emphasized. Shifting from single-molecule to ensemble-averaged experiments, we describe recent efforts toward SERS analysis of colorants in precious artworks. Our intention is to illustrate through these examples that the forward development of SERS is dependent upon both fundamental (e.g., SMSERS) and applied (e.g., on-the-specimen SERS of historical art objects) investigations and that the future of SERS is very bright indeed.

Surface-enhanced Raman spectroscopy: a direct method to identify colorants in various artist media.

Surface-enhanced Raman spectroscopy (SERS) has been developed as a direct, extractionless, nonhydrolysis tool to detect lake pigments and colorants of various classes used in a variety of artist materials. Presented first is the SERS analysis of the natural colorant turmeric (Curcuma longa L.), main component curcumin, as present in dry lake pigment grains, dyed textile yarns, and reference paint layers containing the lake pigment bound in animal glue painted on glass. This experiment demonstrated that it is possible to detect the chromophore in various matrixes of increasing complexity, allowing its unambiguous identification in a wide range of artists' materials, even at very low concentration and in the presence of binders such as glue. In addition, removal of the colorant from the complex with the inorganic substrate or mordanted yarn was not necessary for identification. This proof-of-concept study was then extended to include analysis of several pastel sticks from a historical pastel box and two samples from a pastel artwork, both attributed to American painter Mary Cassatt (1844-1926). This study represents the first extractionless, nonhydrolysis direct SERS study of multiple artist materials, including identification of natural and synthetic colorants and organic pigments contained in historic artists' pastels spanning a broad range of chemical classes: polyphenols, rhodamines, azo pigments, and anthraquinones. Successful identification is demonstrated on samples as small as a single grain of pigment.

Ad-hoc surface-enhanced Raman spectroscopy methodologies for the detection of artist dyestuffs: thin layer chromatography-surface enhanced Raman spectroscopy and in situ on the fiber analysis.

Tailored ad-hoc methods must be developed for successful identification of minute amounts of natural dyes on works of art using Surface-Enhanced Raman Spectroscopy (SERS). This article details two of these successful approaches using silver film over nanosphere (AgFON) substrates and silica gel coupled with citrate-reduced Ag colloids. The latter substrate functions as the test system for the coupling of thin-layer chromatography and SERS (TLC-SERS), which has been used in the current research to separate and characterize a mixture of several artists' dyes. The poor limit of detection of TLC is overcome by coupling with SERS, and dyes which co-elute to nearly the same spot can be distinguished from each other. In addition, in situ extractionless non-hydrolysis SERS was used to analyze dyed reference fibers, as well as historical textile fibers. Colorants such as alizarin, purpurin, carminic acid, lac dye, crocin, and Cape jasmine were thus successfully identified.

AFM studies of the effect of temperature and electric field on the structure of a DMPC-cholesterol bilayer supported on a Au(111) electrode surface.

Atomic force microscopy (AFM) was used to characterize a phospholipid bilayer composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, at a Au(111) electrode surface. Results indicate that addition of cholesterol relaxes membrane elastic stress, increases membrane thickness, and reduces defect density. The thickness and thermotropic properties of the mixed DMPC-cholesterol bilayer supported at the gold electrode surface are quite similar to the properties of the mixed membrane in unilamellar vesicles. The stability of the supported membrane at potentials negative to the potential of zero charge E(pzc) was investigated. This study demonstrates that the bilayer supported at the gold electrode surface is stable provided the applied potential (E - E(pzc)) is less than -0.3 V. At larger polarizations, swelling of the membrane is observed. Polarizations larger than -1 V cause electrodewetting of the bilayer from the gold surface. At these negative potentials, the bilayer remains in close proximity to the metal surface, separated from it by a approximately 2 nm thick layer of electrolyte.