Fast, efficient and clean adsorption of bisphenol-A using renewable mesoporous silica nanoparticles from sugarcane waste ash.

Even with all the biological problems associated with bisphenol-A (BPA), this chemical is still being widely used, especially in thermal paper receipts. In this study, renewable mesoporous silica nanoparticles (MSN), obtained from sugarcane ash, functionalized with hexadecyltrimethylammonium (CTAB) were applied as an adsorbent in the removal of BPA from the aqueous solution. The versatility of this material and its BPA adsorption capacity were tested at different pH values, being practically constant at pH between 4 and 9, with a slight increase in pH 10 and a greater increase in pH 11. The removal time evaluation indicates a very fast adsorption process, removing almost 90% of BPA in the first 20 min of contact. The kinetic model indicates a monolayer formation of BPA molecules on the MSN-CTAB surface. The maximum adsorption capacity (Q max) was 155.78 mg g-1, one of the highest found in literature, and the highest for material from a renewable source.

Selecting the Mechanism of Surface-Enhanced Raman Scattering Effect using Shell Isolated Nanoparticles and an Oxo-Triruthenium Acetate Cluster Complex.

After more than 40 years, surface-enhanced Raman spectroscopy (SERS) stills attract much attention from chemists, not only because of the synthesis of plasmonic nanostructures but also due to the several simultaneous mechanisms which still remain unclear. One of the possibilities for a better understanding of the SERS mechanisms is the utilization of suitable inorganic complexes. The use of inorganic complexes makes it possible to observe the two main SERS mechanisms (electromagnetic and chemical) and to observe the intensification of Raman scattering due to the resonance Raman effect. In this publication, the observation of these mechanisms was possible utilizing an unpublished and very interesting complex with two oxo-triruthenium acetate clusters and an iron bis(terpyridine) in its structure (seven metals) and which interacted with bare gold nanoparticles and shell-isolated gold nanoparticles (SHIN), with a 1 nm silica shell. The utilization of SHIN allowed to quench the SERS chemical mechanism and led to a spectrum where iron-terpyridine peaks are absent and only the modes related to [Ru3O] center were observed (due to enhancement by resonance Raman, SERRS); it can be said that the the shell-isolated nanoparticles enhanced resonance Raman spectroscopy (SHINERRS) is observed. This approach led to a perfect selection of SERS mechanisms never seen before with any other molecule/complex. As can be seen in the UV-vis spectrum, this complex has a strong band around 700 nm, which suggests that silica shell enhances only surface-enhanced resonance Raman scattering, a long-distance phenomenon, different from chemical enhancement (a short-distance phenomenon). Additionally, along with the Raman spectroscopy results, cyclic voltammetry, UV-vis spectroelectrochemistry, resonance Raman (using 568 and 676 nm lasers), and density functional theory calculations of this new ruthenium cluster are presented.

Spectroscopic and electrophoresis study of substitution on the surface of gold nanoparticles by different mercaptoalkyl carboxylic acids and bioconjugation with bovine serum albumin.

To develop bioconjugated materials, it is necessary to understand how the various elements present in a conjugate interact with one another. To gain insights into nanoparticle-capping agent-protein interactions, gold nanoparticles (AuNPs) measuring 30 nm in diameter were coated with different molecules bearing a thiol group: 3-mercaptopropionic acid, 6-mercaptohexanoic acid, and 11-mercaptoundecanoic acid. The covalent conjugation of AuNPs to the protein bovine serum albumin (BSA) via a cross-linker reaction with N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide was systematically investigated under different reaction conditions with variation of the concentrations of the mercaptoalkyl carboxylic acid (MA) and BSA. All the products were analyzed by UV-vis spectroscopy, gel electrophoresis, and Raman spectroscopy in every modification step. From analysis of the UV-vis results, it is possible at low concentrations of MA to see strong coupling among AuNPs, observed when they are aggregated by KCl, which does not happen at higher concentration of MA, indicating an AuNP-to-MA ratio of 1:130,000 is best for bioconjugation purposes. Agarose gel electrophoresis, a classic technique for biomolecule characterization, indicated that BSA is capable of altering the mobility of AuNPs when it modifies completely the surface of AuNPs because of its high molecular mass (around 66 kDa). Principal component analysis of surface-enhanced Raman spectroscopy data was successfully used as a chemometric tool to assist the characterization of the nanoparticle modification with linker molecules in the absence and presence of different BSA concentrations, making it possible to clearly evaluate the gradual substitution/modification of AuNPs (1:13,000 < 1:65,000 < 1:130,000 AuNP-to-MA ratio) and the conjugation with BSA, which is homogenous at a concentration of 0.01 g L-1. Graphical abstract.

Highly Pure Silica Nanoparticles with High Adsorption Capacity Obtained from Sugarcane Waste Ash.

Silica nanoparticles (SiO2NPs) from renewable sources can be used in very different materials, such as paints, membranes for fuel cells, Li-ion batteries, adsorbents, catalysts, and so on. Brazil is the world's largest producer of sugarcane and generates huge amounts of sugarcane waste ash (SWA), which is a Si-rich source. This study investigates a method to produce highly pure SiO2NPs from SWA. The SiO2NPs were characterized by inductively coupled plasma optical emission spectroscopy, scanning and transmission electron microscopy (TEM), X-ray diffraction analyses, specific surface area and pore distribution, UV and Fourier transform infrared spectroscopy, and thermogravimetric analyses and applied as an adsorbent material in the removal of acid orange 8 (AO8) dye from aqueous solution. The SiO2 content was 88.68 and 99.08 wt % for SWA and SiO2NPs, respectively. TEM images of SWA and SiO2NPs exhibit drastic alterations of the material size ranging from several micrometers to less than 20 nm. The SiO2NPs showed a specific surface area of 131 m2 g-1 and adsorption capacity of around 230 mg g-1 for acid orange 8 dye. Furthermore, the recycling of the SiO2NPs adsorbent after AO8 adsorption was very satisfactory, with reuse for up to five cycles being possible. The results indicate that it was possible to obtain highly pure silica in a nanosize from the waste material and produce an adsorbent with high adsorption capacity and the possibility of reuse.

Localized surface plasmon resonance enhanced photocatalysis: an experimental and theoretical mechanistic investigation.

Titanium dioxide (TiO2) is an advantageous material in catalytic photodegradation due to its low cost, high stability, and considerably higher efficiency when compared to other semiconductors. However, the need for artificial radiation sources in the UV range is a limitation to its use in wastewater remediation. In this context, Localized Surface Plasmon Resonance (LSPR) has been shown to enhance the photoexcitation of charge carriers in the semiconductor. In the present work, the investigation of catalytic photodegradation of phenol solution under distinct excitation by UV-visible or just visible radiation, employing three TiO2 based plasmonic catalysts, was conducted. Spherical silver nanoparticles which present LSPR along the TiO2 bandgap energy and electrically insulated silver nanoparticles were employed. Gold nanoparticles, which present low energy LSPR, were also employed in order to compare the excitation efficiency. Discrete dipole approximation simulations were carried out in order to verify the electric field enhancement and penetration at the semiconductor surface of each plasmonic catalyst. The results presented here may help to shed some light with respect to the contribution of plasmonic photocatalysts and the charge transfer mechanism in catalysts containing plasmonic structures.

Elucidating Protein Involvement in the Stabilization of the Biogenic Silver Nanoparticles.

Silver nanoparticles (AgNPs) have been broadly used as antibacterial and antiviral agents. Further, interests for green AgNP synthesis have increased in recent years and several results for AgNP biological synthesis have been reported using bacteria, fungi and plant extracts. The understanding of the role and nature of fungal proteins, their interaction with AgNPs and the subsequent stabilization of nanosilver is yet to be deeply investigated. Therefore, in an attempt to better understand biogenic AgNP stabilization with the extracellular fungal proteins and to describe these supramolecular interactions between proteins and silver nanoparticles, AgNPs, produced extracellularly by Aspergillus tubingensis-isolated as an endophytic fungus from Rizophora mangle-were characterized in order to study their physical characteristics, identify the involved proteins, and shed light into the interactions among protein-NPs by several techniques. AgNPs of around 35 nm in diameter as measured by TEM and a positive zeta potential of +8.48 mV were obtained. These AgNPs exhibited a surface plasmon resonance (SPR) band at 440 nm, indicating the nanoparticles formation, and another band at 280 nm, attributed to the electronic excitations in tryptophan, tyrosine, and/or phenylalanine residues in fungal proteins. Fungal proteins were covalently bounded to the AgNPs, mainly through S-Ag bonds due to cysteine residues (HS-) and with few N-Ag bonds from H2N- groups, as verified by Raman spectroscopy. Observed supramolecular interactions also occur by electrostatic and other protein-protein interactions. Furthermore, proteins that remain free on AgNP surface may perform hydrogen bonds with other proteins or water increasing thus the capping layer around the AgNPs and consequently expanding the hydrodynamic diameter of the particles (~264 nm, measured by DLS). FTIR results enabled us to state that proteins adsorbed to the AgNPs did not suffer relevant secondary structure alteration upon their physical interaction with the AgNPs or when covalently bonded to them. Eight proteins in the AgNP dispersion were identified by mass spectrometry analyses. All these proteins are involved in metabolic pathways of the fungus and are important for carbon, phosphorous and nitrogen uptake, and for the fungal growth. Thereby, important proteins for fungi are also involved in the formation and stabilization of the biogenic AgNPs.

Cooperative hydrogen-bonding of the adenine-thymine pair as a strategy for lowering the limit of detection of thymine by surface-enhanced Raman spectroscopy.

The formation of cooperative hydrogen bonds between the thymine-adenine pair was used to indirectly determine thymine in aqueous solution by surface-enhanced Raman spectroscopy (SERS), therefore improving the limit of detection (LOD) values up to two orders of magnitude. The concentration of adenine was held constant and SERS spectra over gold nanoparticles were acquired through variable concentrations of thymine. The overall methodology followed a multivariate approach leading us to find the most suitable adenine concentrations to determine thymine and evidence the formation of new species whose response maintains a linear correlation with thymine nominal concentrations. Partial Least Squares (PLS) regression has been applied for modelling the data and close values of the root mean square error of cross-validation (RMSECV) and root mean square error of prediction (RMSEP) were obtained as indicators of model quality for modelling and prediction. The LOD for the thymine determination has been lowered from ∼20 to 0.278 mmol L(-1) with a mean prediction error of 3.3%. It was verified that the improvement in LOD is only possible if the base pair is formed prior to the addition of the plasmonic nanoparticles. Finally, the assessment of the effect of an interference species introducing uracil to the system showed that it was possible to isolate the analyte response from the overall signals.

Detection of Plasmon Coupling between Silver Nanowires Based on Hyperspectral Darkfield and SERS Imaging and Supported by DDA Theoretical Calculations.

We report the unprecedented observation of plasmon coupling between silver nanowires, showing how the surface-enhanced Raman scattering depends upon this interaction and how the spectrum can be shaped by the hot spot. Such observations were accomplished by Raman spectroscopy mapping of silver nanowires modified with rhodamine. The local spectra on the hot spots were measured by darkfield hyperspectral microscopy, a powerful but uncommonly used technique that is capable of determining the location, structure, and spectra of the hot spots. The result obtained by the simulation of two parallel nanowires based on the discrete dipole approximation (DDA) method was in excellent agreement with the results obtained experimentally.

Surface enhanced Raman spectroelectrochemistry of a µ-oxo triruthenium acetate cluster: an experimental and theoretical approach.

Surface enhanced Raman spectroelectrochemistry (SERS) spectroelectrochemistry provides a very sensitive technique to investigate the vibrational characteristics of coordination compounds and their particular behavior under the influence of plasmonic surfaces, concomitant with the exploitation of their redox properties and electronic spectra. The results, however, depend upon the mechanisms involved in the intensification of Raman spectra associated with the electromagnetic, resonance Raman and charge-transfer excitation at the Fermi levels. By probing the model complex [(Ru3O)(CH3COO)6(4,4'-bipy)3](n) (n = 1, 0, -1) adsorbed onto rough gold electrode surfaces, contrasting SERS profiles were obtained at several successive redox potentials and oxidation states, which enables a critical discussion on the role of the complex interaction with the gold surface, and the influence of the specific electronic bands in the triruthenium acetate cluster. Density functional theory (DFT) and time-dependent DFT calculations were carried out for the complex bound to an Au20 cluster to show the participation of active lowest unoccupied molecular orbital levels centered on the gold atoms. The corresponding charge-transfer band was predicted around 1200 nm, which supports a charge-transfer interpretation for the SERS response observed at λexc = 1064 nm. The selective enhancement of the vibrational modes was discussed based on the Raman theoretical calculations.

Catalytic oxidation of cinnamyl alcohol using Au-Ag nanotubes investigated by surface-enhanced Raman spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive detection of adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities on the same material. Here we report the application of well-defined Au-Ag nanotubes as both SERS substrates and catalysts for the oxidation of cinnamyl alcohol. The species adsorbed on the catalyst surfaces at different reaction times were analyzed by SERS. The bimetallic nanotubes prepared via a simple galvanic replacement reaction are highly active in the oxidation of cinnamyl alcohol, but do not avoid a radical-chain reaction and the cleavage of the carbon-carbon double bond. A comparison between changes in bulk composition and the nature of adsorbed species at the surface of the catalyst over time suggests that cinnamaldehyde is formed on the catalyst surface (metal-catalyzed oxidation) and benzaldehyde is probably formed in the bulk solution via a radical-chain pathway. In the presence of 2,6-di-tert-butyl-4-methylphenol, the radical-chain reaction is suppressed and the oxidation reaction produces cinnamaldehyde.

Cu nanoparticles enable plasmonic-improved silicon photovoltaic devices.

This work examines the effect of copper nanoparticles (Cu NPs) on the photocurrent efficiency of silicon photovoltaic (Si PV) devices. An optimized synthesis of stable Cu NPs is reported together with a procedure for their immobilization on the Si PV surface. A comprehensive analysis of the photocurrent and power dependence of the Cu NPs surface coverage and size is presented. A decrease in photoconversion was observed for wavelengths shorter than ~500 nm, due to the Cu interband absorption. In the low surface coverage limit, where the level of aggregation was found to be low, the surface plasmon resonance absorption dominates leading to a modest effect on the photocurrent response. As the number of aggregates increased with the surface coverage, the photocurrent efficiency also increased, and a maximum enhancement power conversion of 16% was found for a 54 ± 6 NPs per µm(2) PV cell. This enhancement was attributed to SPR light scattering and trapping into the Si PV device. Higher surface coverage yielded numerous aggregates which acted as a bulk coating and caused a decrease in both photocurrent and power measurements.

Surface-enhanced Raman spectroscopy studies of organophosphorous model molecules and pesticides.

This work reports the analytical application of surface-enhanced Raman spectroscopy (SERS) in the trace analysis of organophosphorous pesticides (trichlorfon and glyphosate) and model organophosphorous compounds (dimethyl methylphosphonate and o-ethyl methylphosphonothioate) bearing different functional groups. SERS measurements were carried out using Ag nanocubes with an edge square dimension of ca. 100 nm as substrates. Density functional theory (DFT) with the B3LYP functional was used for the optimization of ground state geometries and simulation of Raman spectra of the organophosphorous compounds and their silver complexes. Adsorption geometries and marker bands were identified for each of the investigated compound. Results indicate the usefulness of SERS methodology for the sensitive analyses of organophosphorous compounds through the use of vibrational spectroscopy.

High performance gold nanorods and silver nanocubes in surface-enhanced Raman spectroscopy of pesticides.

The behavior of Au nanorods and Ag nanocubes as analytical sensors was evaluated for three different classes of herbicides. The use of such anisotropic nanoparticles in surface-enhanced Raman scattering (SERS) experiments allows the one to obtain the spectrum of crystal violet dye in the single molecule regime, as well as the pesticides dichlorophenoxyacetic acid (2,4-D), trichlorfon and ametryn. Such metallic substrates show high SERS performance at low analyte concentrations making them adequate for use as analytical sensors. Density functional theory (DFT) calculations of the geometries and vibrational wavenumbers of the adsorbates in the presence of silver or gold atoms were used to elucidate the nature of adsorbate-nanostructure bonding in each case and support the enhancement patterns observed in each SERS spectrum.