From GISR 2012: present status and future perspectives of Raman researches in Italy

Authors


Overview of the conference

On June 2012, Bologna hosted the II Italian Conference of the National Group of Raman Spectroscopy and Non-Linear Effects (GISR). The conference was held in the excellent location of the Congress Centre of the National Research Council (CNR) in Bologna and was chaired by M. G. Giorgini, University of Bologna and A. Torreggiani, ISOF (CNR). In spite of the dramatic earthquake occurred in the vicinity of Bologna some days before the conference, a substantial group of researchers coming from different regions of Italy and some from abroad gathered in this site to discuss and share their scientific experiences in the field of Raman spectroscopy and nonlinear optical effects. Three plenary talks were given, each at the opening of each day, followed by oral (27) and poster (38) contributions discussed in two dedicated sessions. A committee awarded a prize to four of them, selected from young researchers. Some of the most significant works are collected in this issue of the Journal of Raman Spectroscopy.

Historical note

Two historical events intimately link Bologna and Italy with the birth and the development of Raman spectroscopy. The Raman effect was studied in Bologna since its discovery (1928), as witnessed by some papers on this subject written by G. B. Bonino (University of Bologna) that appeared in Nature (1930). Even more, Franco Rasetti (the physicist of the Fermi's group in Rome) started measurements of Raman spectra of gaseous molecules immediately after the announcement of the discovery of the effect. Interestingly, thanks to the observation of the Raman spectrum of 14N2, performed in 1928, an incontrovertible experimental evidence was gained, supporting the existence, in the atomic nuclei, of the neutron, the particle that would have been discovered some years later (1932) and on which our today's view of atomic nucleus is founded.

Contributions to the conference

Very interesting topics on innovative experimental methodologies were presented in the plenary lectures. Bruno Pettinger (Fritz-Haber Institute, Berlin, Germany) in the conference Recent progress in Tip-Enhanced Raman Spectroscopy enlightened the advantages offered by the use of an optical near-field technique, the Tip-Enhanced Raman Spectroscopy, TERS, in the enhancement of the sensitivity of Raman spectroscopy and of its spatial resolution, primary conditions for the detection of species adsorbed on solid surfaces at a nanometric scale and, thanks to recent progress, for the single molecule sensitivity.

Paolo Foggi (LENS, Florence, Italy) in the conference Non-linear vibrational spectroscopy: expanding infrared capabilities emphasized the peculiarities of two-dimensional infrared (2D-IR) spectroscopy and the merits of this technique over the conventional one by showing how the spread of the vibrational spectrum over two frequency axes allows to disentangle the dynamical and structural information, generally buried in the inhomogeneously broadened bands, from analysis of the diagonal peaks, and to unveil the coupling between vibrations encoded in the cross-peaks.

Guglielmo Lanzani (IIT, Milan, Italy), with the conference entitled Organic Photonics in life sciences and technology, attracted the attendees attention on the many facets for exploiting the photophysical properties of conjugated polymers in the field of opto-electronics such as photovoltaic cells and in the field of life enhancing technology such as opto-neuronal interfaces, a central issue in the field of neurosciences, where ambitious projects are close to ripen and, among them, that for the artificial retina prosthesis.

Solid state and materials science

The advent of sophisticated spectrometers with micrometric lateral resolution and high signal sensitivity has increased the range of applications and highlighted this nondestructive technique in order to obtain information on solid state phases and phase transitions, functional groups and nanoscopic features in materials with reduced dimensionalities. This can be carried out even if the sample under study is embedded in a working device.

During the meeting, many presentations were centered on a large number of ‘nanocarbons’ such as graphenes and carbon nanotubes. As an example, the peculiar properties of graphene films prepared by the chemical vapor deposition of hydrocarbons on oriented Pt(111) surfaces have been reported, evaluating the amount of compressive stress induced in the graphene lattice during the growth.[1] Other contributions have regarded the production of graphenes by laser ablation and the interaction of defect free single graphene layers with proteins and water molecules.

Specific studies have also involved polyyne chains. One of the works has been centered on Density Functional Theory calculations in symmetrically and unsymmetrically end-capped polyynes.[2] In this frame, we also mention some studies on the nonlinear optical properties of polyynes and polycumulenes through z-scan techniques.

Concerning the characterization of polymorphism in organic molecular crystals, some works have been presented to clarify the connection between molecular structure, crystal packing and lattice phonon dynamics. The correlation among this aspect, charge transport and optical properties, is of primary importance for all those molecules to be applied in micro- and opto-electronics. This is the case of 1,1,4,4-tetraphenyl-butadiene, a well-known blue-emitting molecule, which retains its luminescent properties in the crystalline state. The crystal has been studied with a refined methodology based on low wavenumber Raman spectroscopy coupled to lattice dynamics.[3] Some of the contributions at the GISR meeting have highlighted the relevance of Raman spectroscopy in many industrially scalable processes as in the case of the physico-chemical properties of semiconductor micro-structures and micro-devices. In particular, the determination of the crystallinity degree in thin films and the local temperature in devices under operational conditions can be obtained with a microscopic resolution with modern spectrometers.

Some researchers have studied the distribution profile of Al implanted in crystalline germanium by exploiting the optical penetration depths of different excitation wavelengths. By merely using laser lines of different wavelengths, it is then possible to probe the vibrational dynamics of implanted samples at different depths beneath the sample surface, without bevel of the sample surface.[4]

Finally, advances in solid state spectroscopy have been also presented for the characterization of zirconim dodecarborate thin films, stibnite, superconducting borates and orthophosphate (LuPO4) single crystals. In this last case, 11 of the 12 independent components of the polarizability tensor, expected on the basis of the group theory for LuPO4, were selected in turn and assigned in symmetry.[5]

Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) was one of the words most frequently resounded within the GISR conference for the open debate on the physical origins of the Raman signal enhancement as well as for its use in chemical analysis of traces and sensor technology. Some contributions have discussed the challenging issue of finding out methods for improving specific nanoparticle aggregation and the role of their shape and dimensions in the signal enhancement. As reported in a contribution to the conference on the SERS investigation of 4-anisole, a typical push–pull molecule with an electron-drawing and electron-donor group, the surface effect may appear even more complicated. When illuminated by the laser green line and in the presence of chloride ion, in fact, the surface promotes the photoreduction of the nitro-group to azoderivative as proved by the appearance of –N = N– stretching frequency in the surface enhanced Raman spectrum.[6] The peculiarity of SERS as a chemical analytical methodology has motivated its application to the detection and characterization of molecules present at bioactive concentrations such as drugs. The discovery of sport doping drugs in urine is a topical analytical subject nowadays. A SERS contribution to this conference was specifically dedicated to probenecid (PB), a drug reducing renal excretion of doping drugs, hence masking the athlete effective condition in antidoping tests. As suggested, for analytical purposes, several parameters have to be under control in SERS experiments: the metal used (PB presents affinity only to Ag surfaces), the preparation method of the nanoparticles, the pH and the drug concentration.[7]

Biological molecules

Also, the applications of Raman spectroscopy in the biological field were well represented during the GISR conference. As already commented, the field of chemical sensors applied to biological molecules has greatly exploited the surface enhancement of their, usually low, Raman signals. This application has been extensively described in a contribution [8] aimed at the detection of benzophenone and 4-methylbenzophenone, present in a food complex matrix, such as that of breakfast cereals, as a consequence of their package to food migration. It shows, specifically, that silver nanoparticles, functionalized with lucigenin as molecular assembler, may assure quantitative analytes detection at concentrations down to 70 μM and generally suggests that for the realization of chemical sensors with these performances, expertises in basic chemical procedures are, additionally, required.

To remain in the field of Raman characterization of biological molecules occurring in foods, we mention a contribution centered on the interactions between gliadins, the major proteins of the wheat flour and possible strong food allergens, and some anthocyanins. By Raman monitoring the β-turns content of gliadins, a structural element related to their gluten allergenicity, these investigations have revealed which of the studied anthocianins are able to significantly reduce this phenomenon. Very interestingly, these results can indicate to biotechnologists the directions along which to modify gliadins in order to reduce the allergen immunoreactivity they provoke.[9]

Among the contributions based on micro-Raman spectroscopy addressed to biomedical issues, it is in particular worth mentioning one [10] which, being addressed to the identification of crocidolite and erionite in tissue sections, deserves attention for its high social impact. It is shown, in fact, that this technique can reliably identify the presence of a definite type of mineral fiber directly on histopathological routine tissue specimens and, hence, that interests may be manifested for its potential application in the determination of fiber clearance or retention in populations environmentally exposed to mineral fibers/dusts.

Characterization of biomaterials is another field of application of Raman spectroscopy. We mention in particular that one addressed to enlighten the structural modifications induced on eight oligopeptides, representing basic components of biomimetic coatings, by oxidative free radical stress. Because the latter simulates a common physiological condition occurring under inflammatory response, the results achieved in these investigations may offer precious indications in the field of tissue engineering.[11] Specifically, this study has indicated that only half of the eight considered sequences are certainly adequate for their use as biomimetic coatings.

Cultural heritage

Nowadays, this subject cannot be missing in the agenda of any Raman conference and specifically of an Italian conference, for the huge amount of ancient monuments, paintings, statues and illuminated manuscripts present in our country. Several contributions have stressed the appeal of Raman Spectroscopy as a nondestructive analytical technique and the advancements produced by the availability of bench top micro-Raman instrumentations in addition to flexible portable apparatuses for in situ investigations. Some participants have reported a combined resonance Raman and UV–vis fluorescence study on two fragments derived from ancient objects for the identification of orcein, ‘the purple of the poor’, diagnosed by their Raman spectra reproducing the spectral pattern observed in a photochemicallly aged orcein, substantially given by a mixture of compounds with a common phenoxazone structure.[12]

Complex liquids and soft matter

In the last two decades, a new kind of materials, such as polymers, colloids, surfactants, membranes, biomaterials and their composites, known as soft matter, has attracted the interests of the scientific and industrial communities. A couple of contributions report Raman studies regarding different types of cyclodextrins nanosponges (CDNS) obtained by cross-linking cyclodextrins (CD, a class of polymers with a structure appropriate to bioactive molecules transport) through different polydentate ligands [ethylenediaminetetraacetate (EDTA), pyromellitic dianhydride (PMA)] containing the carbonyl group. One [13] is aimed at the structural and dynamical characterization of the type of CDNS cross-linked through EDTA by studying the intra-molecular C = O stretching vibration and the Boson peak. The other,[14] regarding the type of CDNS cross-linked through PMA, reports on the vibrational dynamics of water molecules into the swollen CDNS by analyzing the water vibrational bands through Fourier transform infrared-attenuated total reflectance and Raman spectra. Studies of molecular dynamics in complex liquids such as organic electrolytic and proteins solutions have been as well reported and discussed in this conference.

2D-IR spectroscopy

Some contributions have highlighted the merits of this technique for its efficiency to monitor the dynamics and the energy transfer in condensed systems and to provide new information on the system local's environment and of the homogeneous bandwidth. In the contribution,[15] the 2D-IR results obtained for dipeptide glycine-alanine-methylamide are reported, which reveal structural solvent effects on the C = O…C = O vibrational couplings and the different local dynamics around the two amide I groups.

Acknowledgements

As a final note and on behalf of the organizing and scientific boards, I wish to thank all the conference attendees for their interest and stimulating contribution to the success of the meeting and the authors who made possible the realization of the present issue. A special thank goes to Prof. Laurence Nafie for his kind help and wise advice in guiding us through this work and to Paul Trevorrow (Executive Journals Editor, Wiley) for his friendly support.

I am indebted to all the reviewers for their prompt and devoted professional evaluations that are important to maintain the standard of the issue.

The organizers are also grateful to many instrument producers (Renishaw, Horiba, LOT Oriel, WITec, Thermo Fischer, Laserpoint, Coherent, Madatec, Nordtest) for their financial support and for the technical reports during the meeting. To Prof. Giuseppe Compagnini (President of GISR) and Dr. Armida Torreggiani (Vice-chair of GISR2012) my warm appreciation for their contribution to the organization of GISR2012 Conference.

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