Abstract
Wood has been a crucial natural material for human civilization since prehistoric times. In archaeology, the examination of the wood microstructure is important for the study of architecture, musical instruments, sculptures, and so on. Scanning electron microscopy (SEM) examination is sometimes unsuitable for archaeological wood due to the limited amount of precious samples, which may be too small to be cut by microtomes and mounted on holders. Moreover, the conductive coating material cannot be uniformly deposited over uneven wood surfaces. To overcome these issues, a rapid and simple pretreatment method using room-temperature ionic liquids (RTIL) was proposed. Four common RTILs were evaluated for the pretreatment of wood chips for SEM examination. We found that water content, viscosity, density, and hydrophobicity of IL solutions were important factors affecting SEM image quality. A 7.5% solution of 1-butyl-1-methylpyrrolidium dicyanamide (BMP-DCA) in ethanol (v/v) was found to work very well. The IL pretreatment could be performed in a few minutes without special equipment. It is gentle enough to preserve delicate structures such as the torus/margo of pit membranes, even at elevated temperatures, without causing obvious damage or deformation. We successfully imaged hand-cut wood chips from 18th-century buildings, an 18th-century European violin, and a Chinese zither over 1000 years old. We therefore conclude that highly hydrophilic ionic liquids with low density and viscosity are suitable for use in SEM examinations of both modern and antique wood specimens.
Highlights
Scanning electron microscopy (SEM) is a vital technique for mapping out the morphologies and the structures of various materials
Four common ILs were tested as pretreatment reagents prior to the SEM examinations
The experimental results revealed that 7.5% butyl-1- methylpyrrolidium dicyanamide (BMP-DCA) solution was optimal, likely due to its lower viscosity, lower density and considerable hydrophilicity
Summary
Scanning electron microscopy (SEM) is a vital technique for mapping out the morphologies and the structures of various materials. SEM systems are usually operated within a high vacuum chamber, and the samples are typically coated with a conductive layer; the specimen must be nonvolatile and conductive Because of these restrictions, wet and liquid samples cannot be directly introduced into the chamber. RTILs have many unique properties compared to conventional solvents, in that they have good thermal stability, a high ionic conductivity, a wide electrochemical window and negligible vapor pressure These unique properties have enabled ILs to be applied to many areas of synthesis, separation, batteries, and electrodeposition[3,4,5,6,7]. Hyno et al reported the structure and size of human blood cells could be maintained without shrinkage when they are pretreated with a hydrophilic, choline-like IL17 These studies have shown that hydrophilic ILs can penetrate into a wet specimen and replace the water molecules therein. We hypothesize that further improvements in SEM imaging quality could be achieved if we systematically compare multiple ILs and explore associated parameters
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