Combined TPEF and SHG Imaging for the Microstructural Characterization of Different Wood Species Used in Artworks

Alice Dal Fovo,Monica Anichini,Sara Mattana,Marco Marchetti,Enrico Baria,Alessio Giovannelli,Raffaella Fontana,Riccardo Cicchi

doi:10.3390/photonics9030170

Abstract

The morphological and chemical conformation of wood microstructures is characteristic of individual species and strongly influences the macromechanical properties of the material, as well as its sensitivity to deterioration factors. Noninvasive techniques enabling the visualization of wood microstructures, while simultaneously providing compositional information, can significantly facilitate the analysis of wooden artworks for conservation purposes. In this paper, we present the application of combined two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) imaging as a versatile diagnostic tool for the microcharacterization of three hardwood species never analyzed by this method. Multimodal mapping of the molecular constituents based on the detected nonlinear signals provides useful information for studying the biological and biochemical deterioration of wood, opening a new field of application for a well-established and widely used imaging technology.

Highlights

Given the importance of cell walls in the growth and morphogenesis of the microstructures, it is of great interest to investigate how molecular components are organized in the three dimensions [3]
The combined use of different nonlinear optical (NLO) microscopy modalities, such as two- and three-photon excited fluorescence (TPEF [10] and 3PF [11,12]) and second- and third-harmonic generation (SHG [13] and THG [14]), provides complementary information based on the detection of fluorophores, crystalline or highly organized structures without inversion symmetry, and local differences in refractive index and dispersion, i.e., interfaces [15]
Laminar crystallites of cellulose are organized in microfibrils, which are embedded in a complex matrix of variously amorphous polysaccharides, cross-linked with lignin, a condensed polymer of coniferyl, sinapyl, or p-coumaryl alcohol (Figure 1) [31,33]

Summary

Introduction

Wood has been used extensively since ancient times with both a structural and decorative function in a variety of art objects, ranging from sculptures to paintings and musical instruments. Given the importance of cell walls in the growth and morphogenesis of the microstructures, it is of great interest to investigate how molecular components are organized in the three dimensions [3]. The main chemical components of wood cells, namely cellulose, hemicellulose, and lignin, are characterized by nonlinear optical (NLO) properties, which can be exploited to obtain morphological, structural, and compositional information on the microstructures [6–8]. The combined use of different NLO microscopy modalities, such as two- and three-photon excited fluorescence (TPEF [10] and 3PF [11,12]) and second- and third-harmonic generation (SHG [13] and THG [14]), provides complementary information based on the detection of fluorophores (by TPEF and 3PF), crystalline or highly organized structures without inversion symmetry (by SHG), and local differences in refractive index and dispersion, i.e., interfaces (by THG) [15]. Nonlinear processes are generated through the interaction of atoms and/or molecules with two or more photons within the same quantum event

Methods

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Discussion

Conclusion