Differentiating Plasmon-Enhanced Chemical Reactions on AgPd Hollow Nanoplates through Surface-Enhanced Raman Spectroscopy.

Plasmonic photocatalysis demonstrates great potential for efficiently harnessing light energy. However, the underlying mechanisms remain enigmatic due to the transient nature of the reaction processes. Typically, plasmonic photocatalysis relies on the excitation of surface plasmon resonance (SPR) in plasmonic materials, such as metal nanoparticles, leading to the generation of high-energy or "hot electrons", albeit accompanied by photothermal heating or Joule effect. The ability of hot electrons to participate in chemical reactions is one of the key mechanisms, underlying the enhanced photocatalytic activity observed in plasmonic photocatalysis. Interestingly, surface-enhanced Raman scattering (SERS) spectroscopy allows the analysis of chemical reactions driven by hot electrons, as both SERS and hot electrons stem from the decay of SPR and occur at the hot spots. Herein, we propose a highly efficient SERS substrate based on cellulose paper loaded with either Ag nanoplates (Ag NPs) or AgPd hollow nanoplates (AgPd HNPs) for the in situ monitoring of C-C homocoupling reactions. The data analysis allowed us to disentangle the impact of hot electrons and the Joule effect on plasmon-enhanced photocatalysis. Computational simulations revealed an increase in the rate of excitation of hot carriers from single/isolated AgPd HNPs to an in-plane with a vertical stacking assembly, suggesting its promise as a photocatalyst under broadband light. In addition, the results suggest that the incorporation of Pd into an alloy with plasmonic properties may enhance its catalytic performance under light irradiation due to the collection of plasmon-excitation-induced hot electrons. This work has demonstrated the performance-oriented synthesis of hybrid nanostructures, providing a unique route to uncover the mechanism of plasmon-enhanced photocatalysis.

Bacterial surface display of human lectins in Escherichia coli.

Lectin-glycan interactions sustain fundamental biological processes involved in development and disease. Owing to their unique sugar-binding properties, lectins have great potential in glycobiology and biomedicine. However, their relatively low affinities and broad specificities pose a significant challenge when used as analytical reagents. New approaches for expression and engineering of lectins are in demand to overcome current limitations. Herein, we report the application of bacterial display for the expression of human galectin-3 and mannose-binding lectin in Escherichia coli. The analysis of the cell surface expression and binding activity of the surface-displayed lectins, including point and deletion mutants, in combination with molecular dynamics simulation, demonstrate the robustness and suitability of this approach. Furthermore, the display of functional mannose-binding lectin in the bacterial surface proved the feasibility of this method for disulfide bond-containing lectins. This work establishes for the first time bacterial display as an efficient means for the expression and engineering of human lectins, thereby increasing the available toolbox for glycobiology research.

Histidine-Mediated Synthesis of Chiral Cobalt Oxide Nanoparticles for Enantiomeric Discrimination and Quantification.

Chiral transition metal oxide nanoparticles (CTMOs) are attracting a lot of attention due to their fascinating properties. Nevertheless, elucidating the chirality induction mechanism often remains a major challenge. Herein, the synthesis of chiral cobalt oxide nanoparticles mediated by histidine (Co3 O4 @L-His and Co3 O4 @D-His for nanoparticles synthesized in the presence of L- and D-histidine, respectively) is investigated. Interestingly, these CTMOs exhibit remarkable and tunable chiroptical properties. Their analysis by x-ray photoelectron, Fourier transform infrared, and ultraviolet-visible absorption spectroscopy indicates that the ratio of Co2+ /Co3+ and their interactions with the imidazole groups of histidine are behind their chiral properties. In addition, the use of chiral Co3 O4 nanoparticles for the development of sensitive, rapid, and enantioselective circular dichroism-based sensors is demonstrated, allowing direct molecular detection and discrimination between cysteine or penicillamine enantiomers. The circular dichroism response of the chiral Co3 O4 exhibits a limit of detection and discrimination of cysteine and penicillamine enantiomers as low as 10 µm. Theoretical calculations suggest that the ligand exchange and the coexistence of both species adsorbed on the oxide surface are responsible for the enantiomeric discrimination. This research will enrich the synthetic approaches to obtain CTMOs and enable the extension of the applications and the discovery of new chiroptical properties.

Common Reactivity and Properties of Heme Peroxidases: A DFT Study of Their Origin.

Electronic structure calculations using the density-functional theory (DFT) have been performed to analyse the effect of water molecules and protonation on the heme group of peroxidases in different redox (ferric, ferrous, compounds I and II) and spin states. Shared geometries, spectroscopic properties at the Soret region, and the thermodynamics of peroxidases are discussed. B3LYP and M06-2X density functionals with different basis sets were employed on a common molecular model of the active site (Fe-centred porphine and proximal imidazole). Computed Gibbs free energies indicate that the corresponding aquo complexes are not thermodynamically stable, supporting the five-coordinate Fe(III) centre in native ferric peroxidases, with a water molecule located at a non-bonding distance. Protonation of the ferryl oxygen of compound II is discussed in terms of thermodynamics, Fe-O bond distances, and redox properties. It is demonstrated that this protonation is necessary to account for the experimental data, and computed Gibbs free energies reveal pKa values of compound II about 8.5-9.0. Computation indicates that the general oxidative properties of peroxidase intermediates, as well as their reactivity towards water and protons and Soret bands, are mainly controlled by the iron porphyrin and its proximal histidine ligand.

2-Hydroxychalcone-ß-Cyclodextrin Conjugate with pH-Modulated Photoresponsive Binding Properties.

Stimuli-responsive supramolecular receptors are important building blocks for the construction of self-assembled functional materials. We report the design and synthesis of a pH- and light-responsive 2-hydroxychalcone-ß-cyclodextrin conjugate (1-Ct) and its characterization by spectroscopic and computational methods. 1-Ct follows the typical reaction network of trans-chalcone-flavylium photoswitches. Upon light irradiation, 1-Ct can be photochemically converted into the cis-chalcone/hemiketal forms (1-Cc/1-B) under neutral pH conditions or to the flavylium cation (1-AH+) at acidic pH values. This stimuli-responsive ß-cyclodextrin host, 1-Ct, was found to form stronger intramolecular self-inclusion complexes (Kintra = 14) than 1-AH+ (Kintra = 3) and weaker than 1-Cc/1-B (overall Kintra = 179), allowing control over their stability and binding properties by combinations of pH and light stimuli.

Experimental and Computational Studies on the Interaction of a Dansyl-Based Fluorescent Schiff Base Ligand with Cu2+ Ions and CuO NPs.

We studied the interaction of Cu2+ ions and CuO nanoparticles with the fluorescent Schiff base ligand H3L, which derives from the condensation of 4-formyl-3-hydroxybenzoic acid with N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide (DsA). A detailed assignment of the most significant bands of the electronic and infrared spectra of H3L and DsA was performed using DFT methods, based on both crystal structures. The affinity of H3L to react with Cu2+ ions in solution (KB = 9.01 103 L mol-1) is similar to that found for the Cu2+ ions present on the surface of CuO NPs (KB = 9.84 103 L mol-1). Fluorescence spectroscopic measurements suggest five binding sites for H3L on the surface of the CuO NPs used. The µ-XRF analysis indicates that a polycrystalline sample of CuO-H3L NPs contains 15:1 Cu:S molar ratio (CuO:H3L). ATR-FTIR spectroscopy, supported by DFT calculations, showed that the HL2- (as a phenolate and sulfonamide anion) is coordinated to superficial Cu2+ ions of the CuO NPs through their azomethine, sulphonamide, and phenolic groups. A solution of H3L (126 ppb) shows sensitive responses to CuO NPs, with a limit of detection (LOD) of 330 ppb. The working range for detection of CuO NPs with [H3L] = 126 ppb was 1.1-9.5 ppm. Common metal ions in water, such as Na+, K+, Mg2+, Ca2+, Fe3+, and Al3+ species, do not interfere significantly with the detection of CuO NPs.

Plasmonic MOF Thin Films with Raman Internal Standard for Fast and Ultrasensitive SERS Detection of Chemical Warfare Agents in Ambient Air.

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic technique for selective detection and quantification of molecules at extremely low concentrations. However, practical SERS applications for gaseous chemicals with small cross section is still in its early stages. We herein report a plasmonic-sorbent thin-film platform with integrated Raman internal standard with outstanding SERS sensing capabilities for chemical warfare agents (CWA) simulants. The thin film is constituted of close-packed core-shell Au@Ag nanorods individually encapsulated within a ZIF-8 framework (Au@Ag@ZIF-8). While the Au@Ag nanoparticles amplify the Raman signal of molecules located near their surface, the ZIF-8 framework plays a key role in the trapping of the dimethyl methylphosphonate (DMMP) or 2-chloroethyl ethyl sulfide (CEES) from the gas phase as well as Raman internal standard. The underlying adsorption mechanism of the molecules within the ZIF-8 framework as well as the interaction between DMMP and Ag surface are investigated by computational simulations. Outstanding SERS sensing capabilities of Au@Ag@ZIF-8 thin films, in terms of response time, quantification limit, reproducibility, and recyclability, are demonstrated for dimethyl methylphosphonate (DMMP) and 2-chloroethyl ethyl sulfide (CEES), selected as CWA simulants of sarin gas and mustard gas, respectively. A limit of detection (LOD) of 0.2 ppbV is reported for DMMP. Additionally, experiments performed with portable Raman equipment detect 2.5 ppmV for DMMP in ambient air and 76 ppbV for CEES in N2, with response times of 21 and 54 s, respectively. This proof of concept opens the door for handheld SERS-based gas sensing at ultralow concentrations in practical applications, such as homeland security, critical infrastructure protection, chemical process monitoring, or personalized medicine.

Highly porous palladium nanodendrites: wet-chemical synthesis, electron tomography and catalytic activity.

A simple procedure to obtain highly porous hydrophilic palladium nanodendrites in one-step is described. The synthetic strategy is based on the thermal reduction of a Pd precursor in the presence of a positively charged polyelectrolyte such as polyethylenimine (PEI). Advanced electron microscopy techniques combined with X-ray diffraction (XRD), thermogravimetry and BET analysis demonstrate the polycrystalline nature of the nanodendrites as well as their high porosity and active surface area, facilitating a better understanding of their unique morphology. Besides, catalytic studies performed using Raman scattering and UV-Vis spectroscopies revealed that the nanodendrites exhibit a superior performance as recyclable catalysts towards hydrogenation reaction compared to other noble metal nanoparticles.

Shape control in ZIF-8 nanocrystals and metal nanoparticles@ZIF-8 heterostructures.

Shape control in metal-organic frameworks still remains a challenge. We propose a strategy based on the capping agent modulator method to control the shape of ZIF-8 nanocrystals. This approach requires the use of a surfactant, cetyltrimethylammonium bromide (CTAB), and a second capping agent, tris(hydroxymethyl)aminomethane (TRIS), to obtain ZIF-8 nanocrystals with morphology control in aqueous media. Semiempirical computational simulations suggest that both shape-inducing agents adsorb onto different surface facets of ZIF-8, thereby slowing down their crystal growth rates. While CTAB molecules preferentially adsorb onto the {100} facets, leading to ZIF-8 particles with cubic morphology, TRIS preferentially stabilizes the {111} facets, inducing the formation of octahedral crystals. Interestingly, the presence of both capping agents leads to nanocrystals with irregular shapes and higher index facets, such as hexapods and burr puzzles. Additionally, the combination of ZIF-8 nanocrystals with other materials is expected to impart additional properties due to the hybrid nature of the resulting nanocomposites. In the present case, the presence of CTAB and TRIS molecules as capping agents facilitates the synthesis of metal nanoparticle@ZIF-8 nanocomposites, due to synergistic effects which could be of use in a number of applications such as catalysis, gas sensing and storage.

Imaging Bacterial Interspecies Chemical Interactions by Surface-Enhanced Raman Scattering.

Microbes produce bioactive chemical compounds to influence the physiology and growth of their neighbors, and our understanding of their biological activities may be enhanced by our ability to visualize such molecules in vivo. We demonstrate here the application of surface-enhanced Raman scattering spectroscopy for simultaneous detection of quorum-sensing-regulated pyocyanin and violacein, produced respectively by Pseudomonas aeruginosa and Chromobacterium violaceum bacterial colonies, grown as a coculture on agar-based plasmonic substrates. Our plasmonic approach allowed us to visualize the expression and spatial distribution of the microbial metabolites in the coculture taking place as a result of interspecies chemical interactions. By combining surface-enhanced Raman scattering spectroscopy with analysis of gene expression we provide insight into the chemical interplay occurring between the interacting bacterial species. This highly sensitive, cost-effective, and easy to implement approach allows spatiotemporal imaging of cellular metabolites in live microbial colonies grown on agar with no need for sample preparation, thereby providing a powerful tool for the analysis of microbial chemotypes.

Detection and imaging of quorum sensing in Pseudomonas aeruginosa biofilm communities by surface-enhanced resonance Raman scattering.

Most bacteria in nature exist as biofilms, which support intercellular signalling processes such as quorum sensing (QS), a cell-to-cell communication mechanism that allows bacteria to monitor and respond to cell density and changes in the environment. As QS and biofilms are involved in the ability of bacteria to cause disease, there is a need for the development of methods for the non-invasive analysis of QS in natural bacterial populations. Here, by using surface-enhanced resonance Raman scattering spectroscopy, we report rationally designed nanostructured plasmonic substrates for the in situ, label-free detection of a QS signalling metabolite in growing Pseudomonas aeruginosa biofilms and microcolonies. The in situ, non-invasive plasmonic imaging of QS in biofilms provides a powerful analytical approach for studying intercellular communication on the basis of secreted molecules as signals.

Encapsulation of Single Plasmonic Nanoparticles within ZIF-8 and SERS Analysis of the MOF Flexibility.

Hybrid nanostructures composed of metal nanoparticles and metal-organic frameworks (MOFs) have recently received increasing attention toward various applications due to the combination of optical and catalytic properties of nanometals with the large internal surface area, tunable crystal porosity and unique chemical properties of MOFs. Encapsulation of metal nanoparticles of well-defined shapes into porous MOFs in a core-shell type configuration can thus lead to enhanced stability and selectivity in applications such as sensing or catalysis. In this study, the encapsulation of single noble metal nanoparticles with arbitrary shapes within zeolitic imidazolate-based metal organic frameworks (ZIF-8) is demonstrated. The synthetic strategy is based on the enhanced interaction between ZIF-8 nanocrystals and metal nanoparticle surfaces covered by quaternary ammonium surfactants. High resolution electron microscopy and tomography confirm a complete core-shell morphology. Such a well-defined morphology allowed us to study the transport of guest molecules through the ZIF-8 porous shell by means of surface-enhanced Raman scattering by the metal cores. The results demonstrate that even molecules larger than the ZIF-8 aperture and pore size may be able to diffuse through the framework and reach the metal core.

A computational study of the interaction of graphene structures with biomolecular units.

Due to the great interest that biochemical sensors constructed from graphene nanostructures have raised recently, in this work we analyse in detail the electronic factors responsible for the large affinity of biomolecular units for graphene surfaces using ab initio quantum chemical tools based on density functional theory. Both finite and periodic graphene structures have been employed in our study. Whereas the former allows the analysis of the different energy components contributing to the interaction energy separately, the periodic structure provides a more realistic calculation of the total adsorption energy in an extended graphene surface and serves to validate the results obtained using the finite model. In addition, qualitative relations between interaction energy and electron polarization upon adsorption have been established using the finite model. In this work, we have analysed thermodynamically stable adsorption complexes formed by glycine, melamine, pyronin cation, porphine, tetrabenzoporphine and phthalocyanine with a 2D structure of ninety six carbons and periodic structures formed by cells of fifty and seventy two carbons. Differences in the electrostatic, Pauli repulsion, induction and dispersion energies among aromatic and non-aromatic molecules, charged and non-charged molecules and H-π and stacking interactions have been thoroughly analysed in this work.

Theoretical study of the adsorption of aromatic units on carbon allotropes including explicit (empirical) DFT dispersion corrections and implicitly dispersion-corrected functionals: the pyridine case.

The suitability of implicitly dispersion-corrected functionals, namely the M06-2X, for the determination of interaction energies and electron polarization densities in adsorption studies of aromatic molecules on carbon allotropes surfaces is analysed by comparing the results with those obtained using explicit dispersion through Grimme's empirical corrections. Several models of increasing size for the graphene sheet together with one-dimensional curved carbon structures, (5,5), (6,6) and (7,7) armchair single-walled nanotubes, and two-dimensional curved carbon structures, C60 fullerene, have been considered as substrates in this work, whereas pyridine has been chosen as an example for the adsorbed aromatic molecule. Comparison with recent experimental estimations of the adsorption energy and calculations using periodic boundary conditions on a supercell of 72 carbon atoms indicates that a finite model containing ninety six carbon atoms (C96) approaches quite well the adsorption on a graphene sheet. Analysis of the interaction energy components reveals that the M06-2X functional accounts for most of the dispersion energy implicitly, followed far by wB97X and B3LYP, whereas B97 and BLYP do not differ too much from HF. It has been found that M06-2X corrects only the energy component associated to dispersion and leaves the rest, electrostatic, Pauli and induction "unaltered" with respect to the other DFT functionals investigated. Moreover, only the M06-2X functional reflects the effect of dispersion on the electron polarization density, whereas for the remaining functionals the polarization density does not differ too much from the HF density. This makes the former functional more suitable a priori for the calculation of electron density related properties in these adsorption complexes.

Theoretical vibrational Raman and surface-enhanced Raman scattering spectra of water interacting with silver clusters.

In this study, we analyzed the Raman spectrum of a water molecule adsorbed on a cluster of 20 silver atoms, and the plasmonic electromagnetic effect of the silver surface was also considered to give a theoretical prediction of the surface-enhanced Raman scattering spectrum. The calculations were performed at the density functional theory (DFT) level by using both frozen and unfrozen silver clusters. Two different models were used to consider the plasmonic enhancement; one of them was a modified classical (dipole) model and the other was the coupled perturbed Hartree-Fock method with excitation frequencies obtained from time-dependent DFT calculations and with proper detuning of these frequencies. The importance of small geometrical distortions of the silver surface in the orientation of the adsorbed water was shown. Moreover, it was shown how the symmetry of the transition dipole moment and the symmetry of the vibrational modes influence the Raman intensities of the SERS spectrum.

Pillar[5]arene-mediated synthesis of gold nanoparticles: size control and sensing capabilities.

We present a simple procedure for the synthesis of quasi-spherical Au nanoparticles in a wide size range mediated by macrocyclic host molecules, ammonium pillar[5]arene (AP[5]A). The strategy is based on a seeded growth process in which the water-soluble pillar[5]arene undergoes complexation of the Au salt through the ammonium groups, thereby avoiding Au nucleation, while acting as a stabilizer. The presence of the pillar[5]arene onto the Au nanoparticle particle surface is demonstrated by surface-enhanced Raman scattering (SERS) spectroscopy, and the most probable conformation of the molecule when adsorbed on the Au nanoparticles surface is suggested on the basis of theoretical calculations. In addition, we analyze the host-guest interactions of the AP[5]A with 2-naphthoic acid (2NA) by using (1)H NMR spectroscopy and the results are compared with theoretical calculations. Finally, the promising synergetic effects of combining supramolecular chemistry and metal nanoparticles are demonstrated through SERS detection in water of 2NA and a polycyclic aromatic hydrocarbon, pyrene (PYR).

Electronegativity estimator built on QTAIM-based domains of the bond electron density.

The electron localization function, natural localized molecular orbitals, and the quantum theory of atoms in molecules have been used all together to analyze the bond electron density (BED) distribution of different hydrogen-containing compounds through the definition of atomic contributions to the bonding regions. A function, gAH , obtained from those contributions is analyzed along the second and third periods of the periodic table. It exhibits periodic trends typically assigned to the electronegativity (χ), and it is also sensitive to hybridization variations. This function also shows an interesting S shape with different χ-scales, Allred-Rochow's being the one exhibiting the best monotonical increase with regard to the BED taken by each atom of the bond. Therefore, we think this χ can be actually related to the BED distribution.

Structural characterization of anion-calcium-humate complexes in phosphate-based fertilizers.

Fertilizers based on phosphate-metal-humate complexes are a new family of compounds that represents a more sustainable and bioavailable phosphorus source. The characterization of this type of complex by using solid (31)P NMR in several fertilizers, based on single superphosphate (SSP) and triple superphosphate (TSP) matrices, yielded surprising and unexpected trends in the intensity and fine structure of the (31)P NMR peaks. Computational chemistry methods allowed the characterization of phosphate-calcium-humate complexes in both SSP and TSP matrices, but also predicted the formation of a stable sulfate-calcium-humate complex in the SSP fertilizers, which has not been described previously. The stability of this complex has been confirmed by using ultrafiltration techniques. Preference towards the humic substance for the sulfate-metal phase in SSP allowed the explanation of the opposing trends that were observed in the experimental (31)P NMR spectra of SSP and TSP samples. Additionally, computational chemistry has provided an assignment of the (31)P NMR signals to different phosphate ligands as well as valuable information about the relative strength of the phosphate-calcium interactions within the crystals.

Aromaticity in spin-polarized systems: can rings be simultaneously alpha aromatic and beta antiaromatic?

The partition of the multicenter electron delocalization indices and the nucleus independent chemical shift indices into alpha and beta contributions in open-shell systems has been performed. In general it is shown that a full understanding of aromaticity in these systems cannot be achieved by restricting the calculations to the global properties but by dissecting these properties into alpha and beta terms. The 4n+2- and 4n-aromaticity rules for singlet and triplet annulenes, respectively, reduce to a general aromaticity rule when the alpha and beta terms are studied separately. This new rule allows us to extend the concept of conflicting aromaticities to radical systems that are simultaneously alpha-aromatic and beta-antiaromatic or vice versa. The existence of such systems is demonstrated here by means of multicenter electron delocalization indices and nucleus independent chemical shifts. Finally, the global aromatic/antiaromatic character of these radical systems is estimated by means of aromatic stabilization energy, which is shown to be either slightly positive or slightly negative, thus reflecting the small aromatic/antiaromatic character of these radicals and reinforcing the conclusions obtained with aromaticity indices.

Theoretical study of the electronic structure of HXY/XYH radicals (X[Double Bond]C,Si;Y[Double Bond]O,S).

The electronic structures of the HXY/XYH compounds (X[Double Bond]C,Si;Y[Double Bond]O,S) on the (2)A(') electronic ground state were investigated by applying the natural bond orbital (NBO) method to the computed B3LYP6-311G(**) wave functions. Different localized structures are proposed for the HXY and XYH isomers and the central XY unit is described as intermediate between a double and a triple bond in HCO, HCS, HSiO, and HSiS, similar to a double bond in COH, CSH, and SiSH, and clearly a single bond in SiOH. Through the comparison between the NBO results for the diatomic and hydrogenated compounds, the energy preferences on each pair of isomers and the computed geometrical parameters are explained. According to the structures proposed, the HXY compounds are sigma radicals with the spin density distributed along the molecular framework, while the XYH compounds are pi radicals with most of the unpaired spin located on an almost pure p orbital of the X atom. Finally, the amounts of spin density on natural atomic orbitals provided by the NBO method are used to explain the computed values of the isotropic and anisotropic hyperfine coupling constants.