Applying laser induced breakdown spectroscopy (LIBS) and elemental imaging on marine shells for archaeological and environmental research

Using LIBS for the analysis of archaeological and geological marine mollusc shells is a growing research area that relies on customised instrumentation and specific workflows that can accommodate the variety and precision of the required sampling parameters. However, the increased efficiency offered by LIBS, which enables the study of a larger quantity of shell samples for temperature variation, ecological parameters, and human consumption practices, outweighs the initial efforts required to develop customised instrumentation and workflows. In this work, we present detailed specifications and parameters for the development of a LIBS system capable of generating Mg/Ca images on marine shells that directly correlate with seasonal sea temperatures. Our main objective was to develop specifications that enable easy adaptation of LIBS systems to existing laboratories for studying hard-tissue samples. These specifications were used to develop a customised micro-LIBS system and apply it to a real-world example of an archaeological study to better understand its efficiency on the marine mollusc shells and demonstrate its potential for broader applications in interdisciplinary research. In total 101 shell specimens have been analysed within a time frame of approximately 71 h of machine time, producing 234 images (100 µm resolution: 100 images, 30 µm resolution: 134 images). SEM analysis of the irradiated sections of the shell revealed a primary ablated area of 10-15 µm in diameter, while a secondary affected area of the shell's crystal fabric extended to 30-50 µm after repeated shots. Overall, this new customised system reliably and efficiently analysed marine mollusc specimens without major destructive effects, enabling additional analyses for other proxies to be carried out. This study highlights the potential of the LIBS method for interdisciplinary research, encompassing applications in paleoclimatology, marine ecology, and archaeology.

Low Energy Pulsed Laser Excitation in UV Enhances the Gas Sensing Capacity of Photoluminescent ZnO Nanohybrids

Nanohybrids, composed of luminescent zinc oxide (ZnO) nanoparticles dispersed in an inert polydimethylsiloxane (PDMS) matrix, exhibit an excellent ability to follow changes in the type and composition of their surrounding atmosphere. These changes are found to affect the UV photoluminescence (PL) emission of the ZnO-PDMS hybrids measured at room temperature. The influence of irradiation parameters, such as excitation intensity and wavelength, on the response of the ZnO-PDMS sensor against ethanol and oxygen, have been systematically investigated in a comparative study performed employing pulsed excitation at 248 and 355 nm. This study represents the first demonstration that the sensing performance of the PL-based ZnO sensors can be optimized by tuning the excitation parameters and it particularly illustrates that maintaining a low pump energy density is crucial for enhancing the sensitivity of the sensor achieving response values approaching 100%.

Extensive elemental mapping unlocks Mg/Ca ratios as climate proxy in seasonal records of Mediterranean limpets

Elemental analysis of biogeochemical archives is an established technique used to study climate in a range of applications, including ocean circulation, glacial/interglacial climates, and anthropogenic climate change. Data from mollusc archives are especially important because of their global abundance and sub-annual resolution. Despite this potential, they are underrepresented among palaeoclimate studies, due to enigmatic physiological influences skewing the elemental record. Understanding the patterns behind these influences will improve data interpretation and lead to the development of new climate proxies. Here, we show for the first time that extensive spatial mapping of multiple mollusc specimens using Laser Induced Breakdown Spectroscopy (LIBS) across a wider region can resolve enigmatic patterns within the elemental record caused by physiological influences. 2D elemental (Mg/Ca) maps of whole limpet shells (Patella caerulea) from across the Mediterranean revealed patterns of variability within individual mollusc records as well as within isochronous parts of specimens. By registering and quantifying these patterns, we established previously uninterpretable correlations with temperature (R² > 0.8, p < 0.01). This outcome redefines the possibilities of accessing sub-annual climate proxies and presents the means to assess annual temperature ranges using oxygen isotope analysis requiring only 2 samples per shell.

Egg yolk identification and aging in mixed paint binding media by NMR spectroscopy

NMR spectroscopy is a powerful analytical tool for the identification and quantitative analysis of organic materials in a cultural heritage context. In this report, we present an analytical NMR protocol for the identification and semiquantification of egg yolk binders and mixed binding media that also contain a drying oil, namely linseed oil. The samples studied have been artificially and/or naturally aged in order to simulate the composition of organic materials in paintings. Analysis of the 1D and 2D NMR spectra showed that egg yolk can be identified even in binding media of considerable age via signals originating from cholesterol and/or cholesterol oxidation products present in the aged binding medium. Based on cholesterol-related and other lipid signals in the NMR spectra of egg yolk binders, a molecular marker (R/F) that suggests the presence of egg yolk in paint binders is proposed. Via this marker, the presence of egg yolk in the organic material obtained from an early 18th century Greek icon is confirmed, and this is further verified by 2D NMR spectroscopy. It is demonstrated that NMR molecular markers developed to estimate the hydrolysis/oxidation state of oil paintings are also suitable for the analysis of egg yolk and mixed medium (egg yolk-linseed oil) binders, indicating the generality of the NMR methodological approach in the analysis of organic materials in a cultural heritage context.

Depth-resolved multilayer pigment identification in paintings: combined use of laser-induced breakdown spectroscopy (LIBS) and optical coherence tomography (OCT)

A detailed feasibility study on the combined use of laser-induced breakdown spectroscopy with optical coherence tomography (LIBS/OCT), aiming at a realistic depth-resolved elemental analysis of multilayer stratigraphies in paintings, is presented. Merging a high spectral resolution LIBS system with a high spatial resolution spectral OCT instrument significantly enhances the quality and accuracy of stratigraphic analysis. First, OCT mapping is employed prior to LIBS analysis in order to assist the selection of specific areas of interest on the painting surface to be examined in detail. Then, intertwined with LIBS, the OCT instrument is used as a precise profilometer for the online determination of the depth of the ablation crater formed by individual laser pulses during LIBS depth-profile analysis. This approach is novel and enables (i) the precise in-depth scaling of elemental concentration profiles, and (ii) the recognition of layer boundaries by estimating the corresponding differences in material ablation rate. Additionally, the latter is supported, within the transparency of the object, by analysis of the OCT cross-sectional views. The potential of this method is illustrated by presenting results on the detailed analysis of the structure of an historic painting on canvas performed to aid planned restoration of the artwork.

An ethanol vapor detection probe based on a ZnO nanorod coated optical fiber long period grating

A new ethanol vapor detection probe based on an optical fiber long period grating overlaid with a zinc oxide (ZnO) nanorods layer is presented. The ZnO nanorod layer was developed onto the cladding of the fiber using aqueous chemical growth, seeded by a thin layer of metallic Zn. The growth of the ZnO nanorods overlayer onto the long period grating cladding is monitored in real time for investigating its effect on the spectral properties of the device and its subsequent role in the sensing mechanism. Results are presented, on the correlation between the growth time of the ZnO layer and the ethanol vapor detection performance. Reversible spectral changes of the notch extinction ratio of more than 4 dB were recorded for ~50 Torr of ethanol vapor concentration. In addition, photoluminescence emission studies of the ZnO overlayer performed simultaneously with the optical fiber spectral measurements, revealed significant ethanol induced changes in the intensity of the bandgap peak.

Random Lasers Based on Organic-Inorganic Hybrids

Laser action is demonstrated in organic/inorganic nanocomposites consisting of pristine and surface-modified ZnO nanoparticles dispersed in an optically inert polymer matrix. The semiconductor particles feature simultaneously gain behavior and strong scattering ability, whereas the polymer matrix provides better processability and mechanical robustness. When excited by a laser pulse of appropriately short duration (pico- to nanoseconds), the composites give off fluorescence emission, which is strongly amplified as a result of light trapping due to multiple scattering. This laser action is manifested as a dramatic increase in the emitted light intensity accompanied by a significant spectral and temporal narrowing above a threshold value of the pumping laser intensity. Applications may include brighter flat-panel displays, inexpensive lasers for communications, military countermeasures, and lasers for search and rescue operations.

An optimization of parameters for application of a laser-induced breakdown spectroscopy microprobe for the analysis of works of art

Laser-induced breakdown spectroscopy (LIBS) provides many advantages for analysis of works of art. Both qualitative and semi-quantitative information about the elemental composition of an object can be rapidly obtained using LIBS. The time of response is on the order of a few seconds and no sample preparation is required. The possibility of performing analysis in museums and archeological sites makes LIBS particularly important; the combination of laser ablation and analysis as encountered in LIBS provides means to effectively carry out depth profiling of samples. By combining the use of a microscope, the dimension of the spot of the laser used to carry out analysis can be reduced and the spatial resolution highly improved; in this way, a very small area of the sample (on the order of 10 miccrom) can be analyzed. The aim of this work is to investigate the best working conditions in order to obtain the least amount of material removal during analysis, and, at the same time, the best quality in the spectral response for rapid and reliable identification of the elemental composition of an object. First, investigations were undertaken using metal alloys; second, the optimized LIBS conditions were used for analysis of models of painting layers prepared in the laboratory. Finally, a painting cross-section and a 19th century painted icon were analyzed using the micro-LIBS setup.

Longitudinal coherence of organic-based microcavity lasers

We report on the measurement of the longitudinal coherence of organic microcavity lasers based on a conjugated polymer. By using a modified Michelson interferometer configuration enabling single-shot measurements of the coherence length, the transition from spontaneous emission to lasing is investigated. The measured coherence length grows upon increasing the pumping fluence, saturating around 45 microm above threshold. At large fluences, possible thermal and photo-oxidation processes occurring in the gain medium limit the further increase of the coherence length. Our results are important for understanding lasing emission in organic microcavities and optimizing the device design and performances.

Analysis of protein-based media commonly found in paintings using synchronous fluorescence spectroscopy combined with multivariate statistical analysis

The spectrofluorimetric analysis of protein-based binding media, which are commonly found as painting materials, is based on the detection of emissions from amino acids, as well as fluorescent degradation products that develop with aging. Laser-induced fluorescence spectroscopy, fluorescence excitation emission spectroscopy, and time-resolved fluorescence spectroscopy have all been employed in efforts to discriminate between commonly found proteinaceous binding media, including egg white, egg yolk, milk, and casein, as well as collagen-based glues from rabbit skin, ox bone, parchment, and fish. However, synchronous fluorescence spectroscopy (SFS), a rapid means of recording fluorescence properties of samples, has not been reported for the differentiation between binding media. This work focuses on the analysis of a large set of naturally aged films of different protein-based binding media using SFS with a range of different offsets between excitation and emission monochromators between 30-60 nm. An interpretation of synchronous fluorescence spectra of binding media is presented and is followed by an assessment and classification of a database of recorded spectra using multivariate analysis. Importantly, following SFS analysis of films of binding media, principal component analysis is used to differentiate among all the proteinaceous media considered on the basis of clustering of data. This application is thus a novel and nondestructive means for differentiation between protein-based binding media.

Visual perception of colourful petals reminds us of classical fragments

Colour has attracted the interest and attention of many of the most gifted intellects of all time. Ideas of early thinkers were not -and could not have been- grasped on a scientific level without knowledge of a kind that lay far in the future. One character that is being considered is the colourful surfaces of living tissues, which could hardly have been visualized without a corresponding reference to the microscale parallel. Millions of years before man made manipulated synthetic structures, biological systems were using nanoscale architecture to produce striking optical effects. Here we show the microsculpture of the adaxial surface of flower petals from the asphodel, the Stork's-bill and the common poppy by using optical, scanning electron and atomic force microscopy. Microsculpture has been studied in leaves and pollen grains of higher plants. To the best of our knowledge imaging and nanoscale morphometry of petals has not been reported hitherto. Our findings on flower petals' microsculpture may be linked with aspects on colour revealed from ancient literature.

Raman Spectra of Proteinaceous Materials Used in Paintings: A Multivariate Analytical Approach for Classification and Identification

This work presents Raman spectra obtained from thin films of protein materials which are commonly used as binding media in painted works of art. Spectra were recorded over the spectral range of 3250-250 cm(-1), using an excitation wavelength of 785 nm, and several bands have been identified in the fingerprint region that correspond to the various proteins examined. Differences in the C-H vibrations located between 3200 and 2700 cm(-1) can be accounted for with reference to the amino acid composition of the protein-based binding media as well as the presence of fatty acid esters, in the case of egg yolk. In addition, the discrimination of different proteins on the basis of variations in spectra between 3200 and 2700 cm(-1) can be achieved following multivariate analysis of a large data set of spectra, providing a novel and nondestructive alternative based on Raman spectroscopy to other methods commonly used for the analysis of proteins.

Time-resolved fluorescence spectroscopy and imaging of proteinaceous binders used in paintings

The differentiation of proteins commonly found as binding media in paintings is presented based on spectrally resolved and time-resolved laser-induced fluorescence (LIF) and total emission spectroscopy. Proteins from eggs and animal glue were analysed with pulsed laser excitation at 248 nm (KrF excimer) and 355 nm (third harmonic of Nd:YAG) for spectrally resolved measurements, and at 337 nm (N2) and 405 nm (N2 pumped dye laser) for spectrally resolved lifetime measurements and fluorescence lifetime imaging (FLIM). Total emission spectra of binding media are used for the interpretation of LIF spectra. Time-resolved techniques become decisive with excitation at longer wavelengths as fluorescence lifetime permits the discrimination amongst binding media, despite minimal spectral differences; spectrally resolved measurements of fluorescence lifetime have maximum differences between the binding media examined using excitation at 337 nm, with maximum observed fluorescence at 410 nm. FLIM, which measures the average lifetime of the emissions detected, can also differentiate between media, is non-invasive and is potentially advantageous for the analysis of paintings.

Laser-induced breakdown spectroscopy (LIBS) in archaeological science--applications and prospects

Laser-induced breakdown spectroscopy (LIBS) has emerged in the past ten years as a promising technique for analysis and characterization of the composition of a broad variety of objects of cultural heritage including painted artworks, icons, polychromes, pottery, sculpture, and metal, glass, and stone artifacts. This article describes in brief the basic principles and technological aspects of LIBS, and reviews several test cases that demonstrate the applicability and prospects of LIBS in the field of archaeological science.

Analysis of protein-based binding media found in paintings using laser induced fluorescence spectroscopy

Laser induced fluorescence (LIF) spectroscopy of intrinsic fluorophores from organic media found in paintings (casein, animal glue and egg proteins) provides novel non-invasive means of characterisation of general classes of media on the basis of fluorescence emission arising from the presence of certain amino acids and their degradation byproducts. Proteins from traditionally employed binding media include collagen, casein, albumin and other egg proteins, of animal sources (skins, milk and egg respectively). Wavelength dependence of the spectra is presented for analyses of thin films of protein-based binding media.

Ultraviolet laser filaments for remote laser-induced breakdown spectroscopy (LIBS) analysis: applications in cultural heritage monitoring

We report experiments with subpicosecond UV laser filaments for the remote analysis of samples related to objects of cultural heritage. The classic laser-induced breakdown spectroscopy (LIBS) technique finds new avenues through femtosecond filamentation, and dynamic remote LIBS becomes possible. Advantages such as self-regulated laser intensity deposition on the target suggest that there is significant potential for using UV femtosecond filaments for the remote analysis of sculpture and large monuments.

Study of Aging in Oil Paintings by 1D and 2D NMR Spectroscopy

Nuclear magnetic resonance spectroscopy is proposed as an efficient analytical tool in the study of painted artworks. The binding medium from two original oil paintings, dated from the early 20th and the late 17th century, was studied via high-resolution 1D and 2D NMR, establishing the advanced state of hydrolysis and oxidation of the oil paint. Studies of the solvent-extractable component from model samples of various drying oils, raw oil paints, and aged oil paints allowed the definition of several markers based on the integral ratios of various chemical species present in the 1H and 13C NMR spectra. These markers are sensitive to hydrolytic and oxidative processes that reflect the extent of aging in oil paintings. The rapidity, simplicity, and nondestructive nature of the proposed analytical NMR methodology represents a great advantage, since the usually minute sample quantities available from original artwork can be subsequently analyzed further by other analytical techniques, if necessary.

Investigation of the laserlike behavior of polymeric scattering gain media under subpicosecond laser excitation

The narrowing effects of scatterers on the lifetime and the spectral width of the laser-induced fluorescence of organic dyes hosted in poly(methyl methacrylate) polymer sheets were studied. The excitation source was a distributed-feedback dye laser emitting 0.5-ps pulses at 496 nm. Spectral and temporal features were recorded simultaneously on a spectrograph-streak-camera detection system. The results were then compared with those obtained from dye solutions in methanol that were recorded in previous studies. The effects of the different host environments on the fluorescence characteristics of the dye were thus investigated. These effects are currently studied when the dye is inserted into human tissue in an attempt to boost tumor detection and photodynamic-therapy efficiency. Some initial results are presented.

Corneal Hydration Monitored by Laser-induced Breakdown Spectroscopy

BACKGROUND

Corneal hydration is an important factor in laser corneal ablation. In photorefractive keratectomy (PRK), corneal ablation rate and final ablation surface quality are strongly dependent on corneal hydration. We used a spectroscopic technique for monitoring corneal hydration during PRK.

METHODS

Hydroxyethlymethacrylate (HEMA) was employed for corneal hydration modeling. Hydrated HEMA samples were irradiated with a pulsed Nd:YAG laser (1064 nm, 10 mJ/pulse, pulse duration 15 nsec). Successive emission spectra corresponding to different degrees of hydration were recorded on a gated optical multichannel analyzer. The weight of the sample and hence its water content was monitored during the entire procedure with a sensitive balance. One rabbit and one human cornea were used to demonstrate the spectral analogy between the model and corneal tissue.

RESULTS

The most noticeable dependence on water content of the substrate was that of atomic emission lines of Ca at 393 nm and 396 nm.

CONCLUSION

Plasma emission spectra exhibited significant dependence on sample hydration. This dependence can be used for estimation of water content of irradiated model material and real cornea.

Fluorescence quenching in a strongly helical peptide series: the role of noncovalent pathways in modulating electronic interactions

The very strong helical propensity of peptides rich in alpha-aminoisobutyric acid (Aib) has enabled the design of a set of helices containing as guest amino acids one fluorescent chromophore, beta-(1'-naphthyl)-L-alanine, and one heavy atom perturber, p-bromo-L-phenylalanine. The fluorescence of the chromophoric residue was monitored in this set to explore heavy atom induced enhanced intersystem crossing as a potentially useful tool for exploring remote electronic interactions in biomolecules. The peptides in this set were sequence isomers of each other and were designed such that the chromophore and the perturber were separated by two, one, or zero Aib residues. The respective distances between the aromatic side chains are then modulated by the twist of the helix. All peptides showed steady-state fluorescence quenching, and on the basis of further time-resolved triplet-triplet absorption experiments, two mechanisms for the heavy atom induced fluorescence quenching were established: (i) a weak and nominally spin-forbidden singlet-triplet energy transfer and (ii) the remote heavy atom effect (RHAE) on the intersystem crossing within the fluorophore. Both the rate of singlet-triplet energy-transfer and the RHAE are at their maxima in the peptide with the largest sequence separation but the smallest direct distance between the chromophore and the perturber. Thus neither quenching mechanism is controlled by the length of the intervening covalent pathway. Subtle factors arising from the structure of the intervening peptide backbone apparently contribute to the RHAE for the peptides with shorter sequence separation. Because the sensitivity to the remote heavy atom is a measure of electronic delocalization, this result may have significance for the understanding of the role of helices in biological electron-transfer interactions.