Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

Summary

Here, we present a protocol to obtain luminescent hyperspectral imaging data and to analyze optical anisotropy features of lanthanide-based single crystals using a Hyperspectral Imaging System.

Abstract

In this work, we describe a protocol for a novel application of hyperspectral imaging (HSI) in the analysis of luminescent lanthanide (Ln³⁺)-based molecular single crystals. As representative example, we chose a single crystal of the heterodinuclear Ln-based complex [TbEu(bpm)(tfaa)₆] (bpm=2,2’-bipyrimidine, tfaa^– =1,1,1-trifluoroacetylacetonate) exhibiting bright visible emission under UV excitation. HSI is an emerging technique that combines 2-dimensional spatial imaging of a luminescent structure with spectral information from each pixel of the obtained image. Specifically, HSI on single crystals of the [Tb-Eu] complex provided local spectral information unveiling variation of the luminescence intensity at different points along the studied crystals. These changes were attributed to the optical anisotropy present in the crystal, which results from the different molecular packing of Ln³⁺ ions in each one of the directions of the crystal structure. The HSI herein described is an example of the suitability of such technique for spectro-spatial investigations of molecular materials. Yet, importantly, this protocol can be easily extended for other types of luminescent materials (such as micron-sized molecular crystals, inorganic microparticles, nanoparticles in biological tissues, or labelled cells, among others), opening many possibilities for deeper investigation of structure-property relationships. Ultimately, such investigations will provide knowledge to be leveraged into the engineering of advanced materials for a wide range of applications from bioimaging to technological applications, such as waveguides or optoelectronic devices.

Introduction

Hyperspectral Imaging (HSI) is a technique that generates a spatial map where each x-y coordinate contains a spectral information that could be based on any kind of spectroscopy, namely photoluminescence, absorption and scattering spectroscopies^1,2,3. As a result, a 3-dimensional set of data (also called “hyperspectral cube”) is obtained, where the x-y coordinates are the spatial axes and the z coordinate is the spectral information from the analyzed sample. Therefore, the hyperspectral cube contains both spatial and spectral information....

Protocol

CAUTION: It is recommended to use safety goggles specific for the excitation wavelength being used at all times when operating the imager.

1. Configuration of the hyperspectral microscope

NOTE: An overview of the hyperspectral imaging system is given in Figure 2a, with the main components of the imager being described. The imaging system can be used for the detection of the visible or the near-infrared (NIR) emission from a sample. Depending on which detection is desired (visible or NIR), the light goes through two different light paths (Figure 2e). A comb....

Results

To illustrate the configuration of the hyperspectral microscope for the data acquisition on a Ln-based, molecular single crystal (i.e., [TbEu(bpm)(tfaa)₆], Figure 1a), Figure 2 shows an overview of the system as well as the right placement of the optical cubes in the setup. Figure 3 shows a screen shot of the PHySpec software containing the menus used during the HSI acquisition. Figure 4 and .......

Discussion

The hyperspectral imaging protocol here described provides a straightforward approach that allows to obtain spectroscopic information at precise locations of the sample. Using the described setup, the spatial resolution (x and y mapping) can reach down to 0.5 µm while the spectral resolution can be of 0.2 nm for the mapping at the visible range and 0.6 nm for the NIR range.

In order to conduct hyperspectral mapping on a single crystal, sample preparation follows an easy .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Microscope glass slides	FisherBrand	12-550-15	Glass slides used for sample preparation
Visible and Near Infrared Hyperspectral Confocal Imager	PhotonETC		Microscope used for the analysis, builted according to the user needs, therefore it is no catalog number