The Impact of Ar or N2 Atmosphere on the Structure of Bi-Fe-Carbon Xerogel Based Composites as Electrode Material for Detection of Pb2+ and H2O2.

In this study, bismuth- and iron-embedded carbon xerogels (XG) were obtained using a modified resorcinol formaldehyde sol-gel synthesis method followed by additional enrichment with iron content. Pyrolysis treatment was performed at elevated temperatures under Ar or N2 atmosphere to obtain nanocomposites with different reduction yields (XGAr or XGN). The interest was focused on investigating the extent to which changes in the pyrolysis atmosphere of these nanocomposites impact the structure, morphology, and electrical properties of the material and consequently affect the electroanalytical performance. The structural and morphological particularities derived from X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) measurements revealed the formation of the nanocomposite phases, mostly metal/oxide components. The achieved performances for the two modified electrodes based on XG treated under Ar or N2 atmosphere clearly differ, as evidenced by the electroanalytical parameters determined from the detection of heavy metal cations (Pb2+) or the use of the square wave voltammetry (SWV) technique, biomarkers (H2O2), or amperometry. By correlating the differences obtained from electroanalytical measurements with those derived from morphological, structural, and surface data, a few utmost important aspects were identified. Pyrolysis under Ar atmosphere favors a significant increase in the α-Fe2O3 amount and H2O2 detection performance (sensitivity of 0.9 A/M and limit of detection of 0.17 µM) in comparison with pyrolysis under N2 (sensitivity of 0.5 A/M and limit of detection of 0.36 µM), while pyrolysis under N2 atmosphere leads to an increase in the metallic Bi amount and Pb2+ detection performance (sensitivity of 8.44 × 103 A/M and limit of detection of 33.05 pM) in comparison with pyrolysis under Ar (sensitivity of 6.47·103 A/M and limit of detection of 46.37 pM).

Ultrasensitive Electroanalytical Detection of Pb2+ and H2O2 Using Bi and Fe-Based Nanoparticles Embedded into Porous Carbon Xerogel-The Influence of Nanocomposite Pyrolysis Temperatures.

Multifunctional materials based on carbon xerogel (CX) with embedded bismuth (Bi) and iron (Fe) nanoparticles are tested for ultrasensitive amperometric detection of lead cation (Pb2+) and hydrogen peroxide (H2O2). The prepared CXBiFe-T nanocomposites were annealed at different pyrolysis temperatures (T, between 600 and 1050 °C) and characterized by X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption, dynamic light scattering (DLS), and electron microscopies (SEM/EDX and TEM). Electrochemical impedance spectroscopy (EIS) and square wave anodic stripping voltammetry (SWV) performed at glassy carbon (GC) electrodes modified with chitosan (Chi)-CXBiFe-T evidenced that GC/Chi-CXBiFe-1050 electrodes exhibit excellent analytical behavior for Pb2+ and H2O2 amperometric detection: high sensitivity for Pb2+ (9.2·105 µA/µM) and outstanding limits of detection (97 fM, signal-to-noise ratio 3) for Pb2+, and remarkable for H2O2 (2.51 µM). The notable improvements were found to be favored by the increase in pyrolysis temperature. Multi-scale parameters such as (i) graphitization, densification of carbon support, and oxide nanoparticle reduction and purification were considered key aspects in the correlation between material properties and electrochemical response, followed by other effects such as (ii) average nanoparticle and Voronoi domain dimensions and (iii) average CXBiFe-T aggregate dimension.

Anionic Polysaccharide Cryogels: Interaction and In Vitro Behavior of Alginate-Gum Arabic Composites.

In the present study, polysaccharide-based cryogels demonstrate their potential to mimic a synthetic extracellular matrix. Alginate-based cryogel composites with different gum arabic ratios were synthesized by an external ionic cross-linking protocol, and the interaction between the anionic polysaccharides was investigated. The structural features provided by FT-IR, Raman, and MAS NMR spectra analysis indicated that a chelation mechanism is the main process linking the two biopolymers. In addition, SEM investigations revealed a porous, interconnected, and well-defined structure suitable as a scaffold in tissue engineering. The in vitro tests confirmed the bioactive character of the cryogels through the development of the apatite layer on the surface of the samples after immersion in simulated body fluid, identifying the formation of a stable phase of calcium phosphate and a small amount of calcium oxalate. Cytotoxicity tests performed on fibroblast cells demonstrated the non-toxic effect of alginate-gum arabic cryogel composites. In addition, an increase in flexibility was noted for samples with a high gum arabic content, which determines an appropriate environment to promote tissue regeneration. The newly obtained biomaterials that exhibit all these properties can be successfully involved in the regeneration of soft tissues, wound management, or controlled drug release systems.

Insights into the Stability of Graphene Oxide Aqueous Dispersions.

Understanding graphene oxide's stability (or lack thereof) in liquid solvents is critical for fine-tuning the material's characteristics and its potential involvement in future applications. In this work, through the use of structural and surface investigations, the alteration of the structural and edge-surface properties of 2D graphene oxide nanosheets was monitored over a period of eight weeks by involving DLS, zeta potential, XRD, XPS, Raman and FT-IR spectroscopy techniques. The samples were synthesized as an aqueous suspension by an original modified Marcano-Tour method centred on the sono-chemical exfoliation of graphite. Based on the acquired experimental results and the available literature, a phenomenological explanation of the two underlying mechanisms responsible for the meta-stability of graphene oxide aqueous dispersions is proposed. It is based on the cleavage of the carbon bonds in the first 3-4 weeks, while the bonding of oxygen functional groups on the carbon lattice occurs, and the transformation of epoxide and hydroxyl groups into adsorbed water molecules in a process driven by the availability of hydrogen in graphene oxide nanosheets.

Thermal Evolution of C-Fe-Bi Nanocomposite System: From Nanoparticle Formation to Heterogeneous Graphitization Stage.

Carbon xerogel nanocomposites with integrated Bi and Fe particles (C­Bi­Fe) represent an interesting model of carbon nanostructures decorated with multifunctional nanoparticles (NPs) with applicability for electrochemical sensors and catalysts. The present study addresses the fundamental aspects of the catalyzed growth of nano-graphites in C­Bi­Fe systems, relevant in charge transport and thermo-chemical processes. The thermal evolution of a C­Bi­Fe xerogel is investigated using different pyrolysis treatments. At lower temperatures (~750°C), hybrid bismuth iron oxide (BFO) NPs are frequently observed, while graphitization manifests under more specific conditions such as higher temperatures (~1,050°C) and reduction yields. An in situ heating TEM experiment reveals graphitization activity between 800 and 900°C. NP motion is directly correlated with textural changes of the carbon support due to the catalyzed growth of graphitic nanoshells and nanofibers as confirmed by HR-TEM and electron tomography (ET) for the graphitized sample. An exponential growth model for the catalyst dynamics enables the approximation of activation energies as 0.68 and 0.29­0.34 eV during reduction and graphitization stages. The results suggest some similarities with the tip growth mechanism, while oxygen interference and the limited catalyst­feed gas interactions are considered as the main constraints to enhanced growth.

Preparation and Characterization of Carbon Xerogel Based Composites for Electrochemical Sensing and Photocatalytic Degradation.

In order to obtain a multifunctional nanocomposite material-for electrochemical sensors and photocatalytic applications, structures based on Bi, Fe and TiO2 were grown inside carbon xerogel supports (BiFeCX and BiFeCX-TiO2). First, a wet polymer containing Bi and Fe salts was obtained by following a modified resorcinol-formaldehyde based sol-gel route, followed by drying in ambient conditions, and pyrolysis under inert atmosphere. Then, through TiCl4 hydrolysis, TiO2 nanoparticles were deposited on the BiFeCX xerogel leading to BiFeCX-TiO2. The morphological and structural characterization of the investigated nanocomposites consisted in X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and N2 adsorption measurements, revealing porous carbon structures with embedded nanoparticles and the particularities driven by the pyrolysis and TiCl4 treatment. The new modified electrodes based on BiFeCX or BiFeCX-TiO2 nanocomposite materials, kept in a chitosan matrix (Chi) and deposited on a glassy carbon (GC) electrode surface (GC/Chi-BiFeCX or GC/Chi-BiFeCX-TiO2), were obtained and investigated for Pb(II) voltammetric detection and H2O2 amperometric detection. Moreover, the BiFeCX-TiO2 nanocomposite was tested for the photocatalytic degradation of methyl orange. The great potential of BiFeCX nanocomposite material for developing electrochemical sensors, or BiFeCX-TiO2 for sensors application and photocatalytic application was demonstrated.

Comparability of Raman Spectroscopic Configurations: A Large Scale Cross-Laboratory Study.

The variable configuration of Raman spectroscopic platforms is one of the major obstacles in establishing Raman spectroscopy as a valuable physicochemical method within real-world scenarios such as clinical diagnostics. For such real world applications like diagnostic classification, the models should ideally be usable to predict data from different setups. Whether it is done by training a rugged model with data from many setups or by a primary-replica strategy where models are developed on a 'primary' setup and the test data are generated on 'replicate' setups, this is only possible if the Raman spectra from different setups are consistent, reproducible, and comparable. However, Raman spectra can be highly sensitive to the measurement conditions, and they change from setup to setup even if the same samples are measured. Although increasingly recognized as an issue, the dependence of the Raman spectra on the instrumental configuration is far from being fully understood and great effort is needed to address the resulting spectral variations and to correct for them. To make the severity of the situation clear, we present a round robin experiment investigating the comparability of 35 Raman spectroscopic devices with different configurations in 15 institutes within seven European countries from the COST (European Cooperation in Science and Technology) action Raman4clinics. The experiment was developed in a fashion that allows various instrumental configurations ranging from highly confocal setups to fibre-optic based systems with different excitation wavelengths. We illustrate the spectral variations caused by the instrumental configurations from the perspectives of peak shifts, intensity variations, peak widths, and noise levels. We conclude this contribution with recommendations that may help to improve the inter-laboratory studies.

On the Stability of Glutaraldehyde in Biocide Compositions.

Glutaraldehyde (GA) is used as biocide in hospitals. Recent public investigations on the chemical composition of biocides used in Romania have in some cases found GA, as a key ingredient, to be apparently diluted. However, these data did not explicitly consider the complex chemical equilibria inherent to GA. An investigation of experimental and theoretical data is reported here, assessing the stability of GA solutions relevant for biocide compositions. GA solutions of various chemical composition and under varying circumstances were analyzed using spectroscopy (UV-VIS, Raman, NMR) coupled with density functional theory (DFT) calculations, as well as chemically, such as via the formation of imines in reaction/titration with glycine monitored at 270 nm; using LC-MS; or using SDS-PAGE analysis with GA as reagent in the polymerization of two test proteins- hemoglobin and myoglobin. The spectral properties of GA changed significantly over time, in a temperature-dependent manner; titration with glycine confirmed the spectral data. SDS-PAGE experiments demonstrated a non-linear and apparently unpredictable change in the reactivity of GA over time. The results may be relevant for the determination of GA concentration in various settings such as biocide analysis, hospital wastewaters, and others.

Optical Properties of Composites Based on Graphene Oxide and Polystyrene.

In this work, new optical properties of composites based on polystyrene (PS) microspheres and graphene oxide (GO) are reported. The radical polymerization of styrene in the presence of benzoyl peroxide, pentane and GO induces the appearance of new ester groups in the PS macromolecular chains remarked through an increase in the absorbance of the infrared (IR) band at 1743 cm-1. The decrease in the GO concentration in the PS/GO composites mass from 5 wt.% to 0.5 wt.% induces a diminution in the intensities of the D and G Raman bands of GO simultaneous with a down-shift of the D band from 1351 to 1322 cm-1. These variations correlated with the covalent functionalization of the GO layers with PS. For the first time, the photoluminescent (PL) properties of PS/GO composites are reported. The PS microspheres are characterized by a PL band at 397 nm. Through increasing the GO sheets' concentration in the PS/GO composite mass from 0.5 wt.% to 5 wt.%, a PS PL quenching process is reported. In addition, in the presence of ultraviolet A (UVA) light, a photo-degradation process of the PS/GO composite having the GO concentration equal to 5 wt.% is demonstrated by the PL studies.

Surface Enhanced Raman Spectroscopy for Quantitative Analysis: Results of a Large-Scale European Multi-Instrument Interlaboratory Study.

Surface-enhanced Raman scattering (SERS) is a powerful and sensitive technique for the detection of fingerprint signals of molecules and for the investigation of a series of surface chemical reactions. Many studies introduced quantitative applications of SERS in various fields, and several SERS methods have been implemented for each specific application, ranging in performance characteristics, analytes used, instruments, and analytical matrices. In general, very few methods have been validated according to international guidelines. As a consequence, the application of SERS in highly regulated environments is still considered risky, and the perception of a poorly reproducible and insufficiently robust analytical technique has persistently retarded its routine implementation. Collaborative trials are a type of interlaboratory study (ILS) frequently performed to ascertain the quality of a single analytical method. The idea of an ILS of quantification with SERS arose within the framework of Working Group 1 (WG1) of the EU COST Action BM1401 Raman4Clinics in an effort to overcome the problematic perception of quantitative SERS methods. Here, we report the first interlaboratory SERS study ever conducted, involving 15 laboratories and 44 researchers. In this study, we tried to define a methodology to assess the reproducibility and trueness of a quantitative SERS method and to compare different methods. In our opinion, this is a first important step toward a "standardization" process of SERS protocols, not proposed by a single laboratory but by a larger community.

Carbon Xerogel Nanostructures with Integrated Bi and Fe Components for Hydrogen Peroxide and Heavy Metal Detection.

Multifunctional Bi- and Fe-modified carbon xerogel composites (CXBiFe), with different Fe concentrations, were obtained by a resorcinol-formaldehyde sol-gel method, followed by drying in ambient conditions and pyrolysis treatment. The morphological and structural characterization performed by X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption/desorption porosimetry, scanning electron microscopy (SEM) and scanning/transmission electron microscopy (STEM) analyses, indicates the formation of carbon-based nanocomposites with integrated Bi and Fe oxide nanoparticles. At higher Fe concentrations, Bi-Fe-O interactions lead to the formation of hybrid nanostructures and off-stoichiometric Bi2Fe4O9 mullite-like structures together with an excess of iron oxide nanoparticles. To examine the effect of the Fe content on the electrochemical performance of the CXBiFe composites, the obtained powders were initially dispersed in a chitosan solution and applied on the surface of glassy carbon electrodes. Then, the multifunctional character of the CXBiFe systems is assessed by involving the obtained modified electrodes for the detection of different analytes, such as biomarkers (hydrogen peroxide) and heavy metal ions (i.e., Pb2+). The achieved results indicate a drop in the detection limit for H2O2 as Fe content increases. Even though the current results suggest that the surface modifications of the Bi phase with Fe and O impurities lower Pb2+ detection efficiencies, Pb2+ sensing well below the admitted concentrations for drinkable water is also noticed.

IR, Raman and SERS analysis of amikacin combined with DFT-based calculations.

Amikacin, a molecule formed by two glucosamine rings linked by α-linkages through a central deoxystreptamine, is an antibiotic often used in clinical treatments, with a special attention in the pediatric cases, due to the physiological activity of their renal system. In spite of its extensive use, no detailed information about the vibrational features of the molecule is available in the literature. Thus, in this study we performed a comprehensive vibrational investigation of amikacin from both an experimental and theoretical point of view. Raman and IR spectroscopy combined with DFT calculations conducted to a complete vibrational characterization of the molecule, with the assignment of the vibrational modes. Moreover, SERS spectrum was recorded and analyzed and provided information about the adsorption behavior of the amikacin on the silver nanoparticles surface.

The Comparison of the Photocatalytic Performance Shown by TiO2 and TiO2/WO3 Composites- A Parametric and Kinetic Study.

Kinetic and mechanistic related approaches for mostly titania were intensively studied in the literature. However, combined modelling and kinetic studies are few. Therefore, the present work focuses on modelling the dependence of the degradation kinetics of two model compounds (salicylic acid-SA and methyl orange-MO) on Evonik Aeroxide P25, hydrothermally prepared hierarchical TiO2 and P25/WO3 nanostars, obtained also by hydrothermal crystallization. The obtained individual semiconductors and the composites were characterized using XRD, DRS, SEM, while the photocatalytic degradation of the model pollutants were carried out varying the catalyst load, the initial pollutant concentration and incident light intensity. It was found that the degradation kinetics were independent from the hierarchical nature of the material, while significant dependencies of the degradation efficiency was found from the previously mentioned investigation parameters. All these parametric interdependences were successfully studied and a kinetic model was proposed for both bare TiO2 and TiO2/WO3 composite systems.

Metanephrine neuroendocrine tumor marker detection by SERS using Au nanoparticle/Au film sandwich architecture.

Neuroendocrine tumors, such as pheochromocytoma or paraganglioma, are dangerous tumors that constitute a potential threat for a large number of patients. Currently, the biochemical diagnosis of neuroendocrine tumors is based on measurement of the direct secretory products of the adrenomedullary-sympathetic system or of their metabolites, such as catecholamines or their metanephrine derivatives, from plasma or urine. The techniques used for analysis of plasma free metanephrines, i.e. high-performance liquid chromatography or high-performance liquid chromatography coupled with mass-spectrometry are technically-demanding and time consuming, which limit their availability. Here we demonstrate a simple, fast and low-cost method for detecting metanephrine by Surface Enhanced Raman Scattering (SERS). The protocol consists in using evaporation-induced self-assembly of gold (Au) nanoparticles incubated with the analyte, on planar gold films. The assembly process produces regions with a dense distribution of both inter-particle gaps and particle-film gaps. Finite-difference time-domain simulations confirm that both kinds of gaps are locations of enhanced electromagnetic fields resulting from inter-particle and particle-film plasmonic coupling, useful for SERS amplification. Metanephrine vibrational bands assignment was performed according to density functional theory calculations. Metanephrine metabolite was detected in liquid at concentration levels lower than previously reported for other similar metabolites. The obtained results demonstrate that the Au nanoparticle/Au film exhibits noticeable SERS amplification of the adsorbed metabolite and can be used in the design of efficient, stable SERS-active substrates for the detection and identification of specific tumor markers.

Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection.

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.

Multilayer structures of self-assembled gold nanoparticles as a unique SERS and SEIRA substrate.

Metallic multilayer structures consisting of self-assembled nanoparticles are prepared by successive immersion of a functionalized glass substrate in a gold colloidal suspension, and are characterized by UV/Vis spectroscopy and atomic force microscopy. The ability of the thin film to concomitantly enhance the Raman and IR signals of an adsorbed molecule is evaluated and a detailed analysis of the recorded spectra is provided, which emphasizes the capability of these complementary methods to provide information about the adsorbed species. This unique surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared absorption (SEIRA) substrate could be further used for applications in the elucidation of the structure of a great variety of adsorbed samples.