Characterization of a Ruthenium(II) Complex in Singlet Oxygen-Mediated Photoelectrochemical Sensing.

A water-soluble ruthenium(II) complex (L), capable of producing singlet oxygen (1O2) when irradiated with visible light, was used to modify the surface of an indium-tin oxide (ITO) electrode decorated with a nanostructured layer of TiO2 (TiO2/ITO). Singlet oxygen triggers the appearance of a cathodic photocurrent when the electrode is illuminated and biased at a proper reduction potential value. The L/TiO2/ITO electrode was first characterized with cyclic voltammetry, impedance spectroscopy, NMR, and Raman spectroscopy. The rate constant of singlet oxygen production was evaluated by spectrophotometric measurements. Taking advantage of the oxidative process initiated by 1O2, the analysis of phenolic compounds was accomplished. Particularly, the 1O2-driven oxidation of hydroquinone (HQ) produced quinone moieties, which could be reduced back at the electrode surface, biased at -0.3 V vs Ag/AgCl. Such a light-actuated redox cycle produced a photocurrent dependent on the concentration of HQ in solution, exhibiting a limit of detection (LOD) of 0.3 µmol dm-3. The L/TiO2/ITO platform was also evaluated for the analysis of p-aminophenol, a commonly used reagent in affinity sensing based on alkaline phosphatase.

Optothermal properties of plasmonic inorganic nanoparticles for photoacoustic applications.

Plasmonic systems are becoming a favourable alternative to dye molecules in the generation of photoacoustic signals for spectroscopy and imaging. In particular, inorganic nanoparticles are appealing because of their versatility. In fact, as the shape, size and chemical composition of nanoparticles are directly correlated with their plasmonic properties, the excitation wavelength can be tuned to their plasmon resonance by adjusting such traits. This feature enables an extensive spectral range to be covered. In addition, surface chemical modifications can be performed to provide the nanoparticles with designed functionalities, e.g., selective affinity for specific macromolecules. The efficiency of the conversion of absorbed photon energy into heat, which is the physical basis of the photoacoustic signal, can be accurately determined by photoacoustic methods. This review contrasts studies that evaluate photoconversion in various kinds of nanomaterials by different methods, with the objective of facilitating the researchers' choice of suitable plasmonic nanoparticles for photoacoustic applications.

Adsorption Geometry of Alizarin on Silver Nanoparticles: A Computational and Spectroscopic Study.

The knowledge of the adsorption geometry of an analyte on a metal substrate employed in surface enhanced Raman scattering (SERS) spectroscopy is important information for the correct interpretation of experimental data. The adsorption geometry of alizarin on silver nanoparticles was studied through ab initio calculations in the framework of density functional theory (DFT) by modeling alizarin taking into account all the different charged species present in solution as a function of pH. The calculations allowed a faithful reproduction of the measured SERS spectra and to elucidate the adsorption geometry of this dye on the silver substrate.

Surface-Enhanced Raman Spectroscopy for Bisphenols Detection: Toward a Better Understanding of the Analyte-Nanosystem Interactions.

Silver nanoparticles functionalized with thiolated ß-cyclodextrin (CD-SH) were employed for the detection of bisphenols (BPs) A, B, and S by means of surface-enhanced Raman spectroscopy (SERS). The functionalization of Ag nanoparticles with CD-SH leads to an improvement of the sensitivity of the implemented SERS nanosensor. Using a multivariate analysis of the SERS data, the limit of detection of these compounds was estimated at about 10-7 M, in the range of the tens of ppb. Structural analysis of the CD-SH/BP complex was performed by density functional theory (DFT) calculations. Theoretical results allowed the assignment of key structural vibrational bands related to ring breathing motions and the inter-ring vibrations and pointed out an external interaction due to four hydrogen bonds between the hydroxyl groups of BP and CD located at the external top of the CD cone. DFT calculations allowed also checking the interaction energies of the different molecular species on the Ag surface and testing the effect of the presence of CD-SH on the BPs' affinity. These findings were in agreement with the experimental evidences that there is not an actual inclusion of BP inside the CD cavity. The SERS sensor and the analysis procedure of data based on partial least square regression proposed here were tested in a real sample consisting of the detection of BPs in milk extracts to validate the detection performance of the SERS sensor.

Combining cross-coupling reaction and Knoevenagel condensation in the synthesis of glyco-BODIPY probes for DC-SIGN super-resolution bioimaging.

Lectins are involved in a wide range of carbohydrate mediated recognition processes. Therefore, the availability of highly performant fluorescent tools tailored for lectin targeting and able to efficiently track events related to such key targets is in high demand. We report here on the synthesis of the glyco-BODIPYs 1 and 2, based on the efficient combination of a Heck-like cross coupling and a Knoevenagel condensation, which revealed efficient in addressing lectins. In particular, glyco-BODIPY 1 has two glycosidase stable C-mannose residues, which act as DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin) targeting modules. By using live-cell fluorescence microscopy, we proved that BODIPY-mannose 1 was efficiently taken up by immune cells expressing DC-SIGN receptors. Super-resolution stimulated emission depletion (STED) microscopy further revealed that the internalized 1 localized in membranes of endosomes, proving that 1 is a reliable tool also in STED applications. Of note, glyco-BODIPY 1 contains an aryl-azido group, which allows further functionalization of the glycoprobe with bioactive molecules, thus paving the way for the use of 1 for tracking lectin-mediated cell internalization in diverse biological settings.

Infinite supramolecular pseudo-polyrotaxane with poly[3]catenane axle: assembling nanosized rings from mono- and diatomic I- and I2 tectons.

Mono- and diatomic I- and I2 building blocks, despite their simplicity, can be used to generate complex hierarchical self-assembled architectures. Herein, the construction of a modular supramolecular poly[3]catenane and its conversion into the axle of an infinite supramolecular pseudo-polyrotaxane were achieved in a seamless process from the starting materials. The unique structural features, directionality, and iodine density of the obtained crystals demonstrate the benefits of a supramolecular design for polyiodide networks intended as solid-state conductors.

Highly Charged Ruthenium(II) Polypyridyl Complexes as Effective Photosensitizer in Photodynamic Therapy.

A comparative study between two novel, highly water soluble, ruthenium(II) polypyridyl complexes, [Ru(phen)2 L'] and [Ru(phen)2 Cu(II)L'] (L and L-CuII ), containing the polyaazamacrocyclic unit 4,4'-(2,5,8,11,14-pentaaza[15])-2,2'-bipyridilophane (L'), is herein reported. L and L-CuII interact with calf-thymus DNA and efficiently cleave DNA plasmid when light-activated. They also possess great penetration abilities and photo-induced biological activities, evaluated on an A375 human melanoma cell line, with L-CuII being the most effective. Our study highlights the key role of the Fenton active CuII center within the macrocycle framework, that would play a synergistic role with light activation in the formation of cytotoxic ROS species. Based on these results, an optimal design of RuII polypyridyl systems featuring specific CuII -chelating polyamine units could represent a suitable strategy for the development of novel and effective photosensitizers in photodynamic therapy.

Computational Investigation of the Selective Cleavage of Diastereotopic Cyclopropane Bonds in 5-Spirocyclopropane Isoxazolidines Rearrangement.

The complete path of the Brandi-Guarna rearrangement of 5-spirocyclopropane isoxazolidines has been investigated by means of density functional theory calculations to rationalize the competing formation of tetrahydropyridones and enaminones by the determination of the minimum energy reaction paths. Our calculations confirm that the rearrangement is triggered by the homolysis of the isoxazolidine N-O bond followed by cleavage of one of the two C-CH2 cyclopropane bonds as previously proposed by the Fabian group [ Eur. J. Org. Chem. 2001, 2001, 4223]. In addition, the results of this work suggest that in the presence of a stereogenic center at isoxazolidine C-4', the formation of a piperidinone or an enaminone as the final product depends on which of the two diastereotopic C-CH2 bonds of cyclopropane is cleaved in the second step of the process. The result can be of great interest for the understanding of other processes involving the opening of a cyclopropane ring.

A Photochromic Azobenzene Peptidomimetic of a ß-Turn Model Peptide Structure as a Conformational Switch.

The insertion of azobenzene moiety in complex molecular protein or peptide systems can lead to molecular switches to be used to determine kinetics of folding/unfolding properties of secondary structures, such as α-helix, ß-turn, or ß-hairpin. In fact, in azobenzene, absorption of light induces a reversible trans ↔ cis isomerization, which in turns generates a strain or a structure relaxation in the chain that causes peptide folding/unfolding. In particular azobenzene may permit reversible conformational control of hairpin formation. In the present work a synthetic photochromic azobenzene amino acid derivative was incorporated as a turn element to modify the synthetic peptide [Pro7,Asn8,Thr10]CSF114 previously designed to fold as a type I ß-turn structure in biomimetic HFA/water solution. In particular, the P-N-H fragment at positions 7-9, involved in a ß-hairpin, was replaced by an azobenzene amino acid derivative (synthesized ad hoc) to investigate if the electronic properties of the novel peptidomimetic analog could induce variations in the isomerization process. The absorption spectra of the azopeptidomimetic analog of the type I ß-turn structure and of the azobenzene amino acid as control were measured as a function of the irradiation time exciting into the respective first ππ* and nπ* transition bands. Isomerization of the azopeptidomimetic results strongly favored by exciting into the ππ* transition. Moreover, conformational changes induced by the cis↔ trans azopeptidomimetic switch were investigated by NMR in different solvents.

Surface Enhanced Raman Spectroscopy for In-Field Detection of Pesticides: A Test on Dimethoate Residues in Water and on Olive Leaves.

Dimethoate (DMT) is an organophosphate insecticide commonly used to protect fruit trees and in particular olive trees. Since it is highly water-soluble, its use on olive trees is considered quite safe, because it flows away in the residual water during the oil extraction process. However, its use is strictly regulated, specially on organic cultures. The organic production chain certification is not trivial, since DMT rapidly degrades to omethoate (OMT) and both disappear in about two months. Therefore, simple, sensitive, cost-effective and accurate methods for the determination of dimethoate, possibly suitable for in-field application, can be of great interest. In this work, a quick screening method, possibly useful for organic cultures certification will be presented. DMT and OMT in water and on olive leaves have been detected by surface enhanced Raman spectroscopy (SERS) using portable instrumentations. On leaves, the SERS signals were measured with a reasonably good S/N ratio, allowing us to detect DMT at a concentration up to two orders of magnitude lower than the one usually recommended for in-field treatments. Moreover, detailed information on the DMT distribution on the leaves has been obtained by Raman line- (or area-) scanning experiments.

A straightforward synthesis of phenyl boronic acid (PBA) containing BODIPY dyes: new functional and modular fluorescent tools for the tethering of the glycan domain of antibodies.

We report here on the efficient and straightforward synthesis of a series of modular and functional PBA-BODIPY dyes 1-4. They are an outstanding example of the efficient merge of the versatility of the 3,5-dichloro-BODIPY derivatives and the receptor-like ability of the PBA moiety. The potential bioanalytical applicability of these tools was assessed by measuring the binding to glycan chains of antibodies by a Quartz Crystal Microbalance (QCM).

SERS active Ag-SiO2 nanoparticles obtained by laser ablation of silver in colloidal silica.

Highly stable Ag-SiO2 nanoparticle composites were first obtained by laser ablation of a silver target in an aqueous colloidal dispersion of silica and examined by UV-vis absorption spectroscopy, transmission electron microscopy and Raman spectroscopy. The surface enhanced Raman scattering (SERS) activity of these nanocomposites was tested using 2,2'-bipyridine as a molecular reporter and excitation in the visible and near-IR spectral regions. The computational DFT approach provided evidence of ligand adsorption on positively charged adatoms of the silver nanostructured surface, in a very similar way to the metal/molecule interaction occurring in the corresponding Ag(I) coordination compound.

Correspondence between light-absorption spectrum and nonequilibrium work distribution as a mean to access free energy differences between electronic states.

The problem of recovering the free energy difference between two electronic states has been investigated by Frezzato [Chem. Phys. Lett. 533, 106 (2012)], exploring the equivalence between light-absorption spectra and work distribution, hence opening to the application of a spectroscopic version of the Jarzynski equality (JE) [Phys. Rev. Lett. 78, 2690 (1997)]. Here, assuming the validity of the time-dependent perturbation theory, we demonstrate that such equivalence does not lead to the known form of the JE. This is ascribed to the fact that light-absorption processes cannot be described as stochastic processes. To emphasize such an aspect, we devise a stochastic model for the UV-vis (ultraviolet and visible) absorption, suitable for determining the free energy difference between two generic quantum manifolds in a JE-like fashion. However, the model would require explicit knowledge of the transition dipole moments, which are in general not available. Nonetheless, we derive a spectroscopic version of the JE that allows us to recover the free energy difference between the ground and an excited electronic state when the latter state is the only one observed in the spectrum.

Silver nanowires as infrared-active materials for surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) is increasing in significance as a bioanalytical tool. Novel nanostructured metal substrates are required to improve performances and versatility of SERS spectroscopy. In particular, as biological tissues are relatively transparent in the infrared wavelength range, SERS-active materials suitable for infrared laser excitation are needed. Nanowires appear interesting in this respect as they show a very broad localized surface plasmon resonance band, ranging from near UV to near infrared wavelengths. The SERS activity of silver nanowires has been tested at three wavelengths and a fair enhancement at 1064 and 514 nm has been observed, whereas a very weak enhancement was present when exciting close to the nanowire extinction maximum. These experimentally measured optical properties have been contrasted with finite element method simulations. Furthermore, laser-induced optoacoustic spectroscopy measurements have shown that the extinction at 1064 nm is completely due to scattering. This result has an important implication that no heating occurs when silver nanowires are utilized as SERS-active substrates, thereby preventing possible thermal damage.

Photochemical Reactivity of 1,6-Methano[10]annulene.

1,6-Methano[10]annulene solutions in cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs, exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow one to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results in the critical species for methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6-methano[10]annulene.

Iodide and triiodide anion complexes involving anion-π interactions with a tetrazine-based receptor.

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = -11.6(4) kJ mol-1 for HL2+ + I- = (HL2)I and ΔG° = -13.4(2) kJ mol-1 for H2L22+ + I- = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3- complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I- and I3- with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion-π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3- and I- forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3- molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.

Bifunctional Fe3O4/Ag nanoparticles obtained by two-step laser ablation in pure water.

HYPOTHESIS: Bimetallic nanoparticles made of iron oxide and Ag could be fabricated by pulsed laser ablation of iron and silver targets in pure water by a two-step route. These nanoparticles could exhibit both magnetic and plasmonic properties. EXPERIMENTS: Bimetallic nanoparticles were fabricated by using a focused Nd:YAG nanosecond laser source emitting a 1064nm wavelength radiation and characterized with ζ-potential, Dynamic Light Scattering (DLS), UV-vis absorption, Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM), Energy Dispersive X-ray Spectrometry (EDX), and Selected Area Electron Diffraction (SAED). The magnetic character of the nanoparticles was ascertained by observing attraction by a magnet and complete removing from the water environment, while their SERS (surface-enhanced Raman scattering) response was checked by decorating them with 2,2'-bipyridine as molecular reporter and performing Raman tests with green (514.5nm) and far-red (785nm) excitation wavelengths. FINDINGS: The observed magnetic attraction was due to magnetite (Fe3O4), the only ferromagnetic iron oxide form evidenced by the characterization tests in the aqueous colloidal system, where silver nanoparticles were also embedded. UV-vis and SERS spectra confirmed the presence of nanostructured silver as plasmonic constituent of the fabricated metal nanoparticles.

Computing Free Energy Differences of Configurational Basins.

A simulation-based approach is proposed to estimate free energy differences between configurational states A and B, defined in terms of collective coordinates of the molecular system. The computational protocol is organized into three stages that can be carried on simultaneously. Two of them consist of independent simulations aimed at sampling, in turn, A and B states. In order to limit the evolution of the system around A and B, biased sampling simulations such as umbrella sampling can be employed. These simulations allow us to estimate local configuration integrals associated with A and B, which can be viewed as vibrational contributions to the free energy. Free energy evaluation is completed by the linking-path stage, in which the potential of mean force difference is estimated between two arbitrary points of the configurational surface, located the first around A and the second around B. The linking path in the space of the collective coordinates is arbitrary and can be computed with any method, starting from adaptive biasing potential/force approaches to nonequilibrium techniques. As an illustrative example, we present the calculation of free energy differences between conformational states of the alanine dipeptide in the space of backbone dihedral angles. The basic advantage of this method, that we term "path-linked domains" scheme, is to prevent accurate calculation of the whole free energy hypersurface in the space of the collective coordinates, thus limiting the statistical sampling to a minimum. Path-linked domains schemes can be applied to a variety of biochemical processes, such as protein-ligand complexation or folding-unfolding interconversion.

A fluorescent receptor for halide recognition: clues for the design of anion chemosensors.

The chemosensing properties of the polyaza-macrocycle 1-(6,7)-acridine-3,6,9,12-tetraaza-tridecaphane have been investigated by means of emission fluorescence spectroscopy, considering halide ions as substrates. As in the case of the free ligand, the fluorescence emission of the complexes is due to the acridinium species which are formed after photoinduced proton transfer reaction. The complexation constants have been obtained for the bi- and tri-protonated ligands in deoxygenated aqueous solutions. Two different emission behaviours have been observed varying the anion. Fluoride and chloride give rise to fluorescence enhancement whereas bromide and iodide strongly quench the emission. The macrocycle shows an unusual higher selectivity towards the chloride anion rather than fluoride. The fluorescence emission has been modelled considering a modified Stern-Volmer equation, taking into account the quenching effects of the largest anions, which can be considered negligible for fluoride and chloride anions. Ab initio calculations allow us to interpret the fluorescence emission of the complexes in terms of activation energy related to the proton transfer reaction responsible for the emission process.

Annealed importance sampling with constant cooling rate.

Annealed importance sampling is a simulation method devised by Neal [Stat. Comput. 11, 125 (2001)] to assign weights to configurations generated by simulated annealing trajectories. In particular, the equilibrium average of a generic physical quantity can be computed by a weighted average exploiting weights and estimates of this quantity associated to the final configurations of the annealed trajectories. Here, we review annealed importance sampling from the perspective of nonequilibrium path-ensemble averages [G. E. Crooks, Phys. Rev. E 61, 2361 (2000)]. The equivalence of Neal's and Crooks' treatments highlights the generality of the method, which goes beyond the mere thermal-based protocols. Furthermore, we show that a temperature schedule based on a constant cooling rate outperforms stepwise cooling schedules and that, for a given elapsed computer time, performances of annealed importance sampling are, in general, improved by increasing the number of intermediate temperatures.