Advances in element speciation analysis of biomedical samples using synchrotron-based techniques

Highlights

• Principle of direct element speciation with synchrotron X-ray absorption spectroscopy (XAS).

• Experimental modalities for bulk- and micro-XAS speciation and their limitations.

• Review (2012–2017) of XAS in pharmacology, metals and nanoparticles toxicology, physiopathology.

• Future directions and developments of XAS speciation for biomedical research.

Abstract

Synchrotron-radiation X-ray absorption spectroscopy (XAS) is a direct method for speciation analysis with atomic resolution, providing information about the local chemical environment of the probed element. This article gives an overview of the basic principles of XAS and its application to element speciation in biomedical research. The basic principle and experimental modalities of XAS are introduced, followed by a discussion of both its limitations, such as beam damage or detection limits, and practical advices to improve experiments. An updated review of biomedical studies involving XAS published over the last 5 years is then provided, paying special attention to metal-based drug biotransformation, metal and nanoparticle toxicology, and element speciation in cancer, neurological, and general pathophysiology. Finally, trends and future developments such as hyphenated methods, in situ correlative imaging and speciation, in vivo X-ray Absorption Near Edge Spectroscopy (XANES), full-field XANES, and X-ray Free Electron Laser (XFEL) XAS are presented.

Introduction

The IUPAC (International Union of Pure and Applied Chemistry) definition of speciation in chemistry is; ‘The distribution of an element amongst defined chemical species in a system’, and the definition for speciation analysis is; ‘Analytical activities of identifying and/or measuring the quantities of one or more individual chemical species in a sample’ [1]. In the field of speciation analysis, X-ray absorption spectroscopy (XAS) is a singular and powerful method with an increasing number of applications in biomedical research. The uniqueness of XAS compared with other analytical methods for speciation derives from its atomic resolution [2], [3], [4]. XAS is based on the measurement of the modulation of X-ray absorption by an atom around the core-level binding energies. XAS probes the core level electrons in an element specific manner, it is sensitive to the oxidation state, coordination chemistry, and the distances, coordination number and species of the atoms immediately surrounding the selected element. XAS is therefore a powerful tool for matter investigation in different scientific fields, from catalysis and geology to bioinorganic chemistry. The high sensitivity of XAS, with limits of detection in the tens of μg.g-¹ range or mM range, makes it applicable to trace element speciation in biology, where it can be performed on solids or liquids without the need for complex sample preparation. Accordingly, this review will focus only on XAS element speciation of biomedical samples.

To begin, the physical principle of XAS, the distinction between XANES (X-ray Absorption Near Edge Spectroscopy) and EXAFS (Extended X-ray Absorption Fine Structure), the various modalities of operation from bulk XAS to spatially resolved micro-XAS, and the strengths and limitations of the techniques with a special attention to biological samples analysis will be presented. Then XAS applications in various fields of biomedical research will be reviewed, over the last five years, to give an overview of the state of the art and the trends in this domain. For works published in previous periods the reader may refer to several valuable review articles [5], [6], [7], [8], [9]. This review will be limited to the application of XAS on biomedical samples, but interesting examples of XAS element speciation of bio-environmental samples such as plants or micro-organisms can be found in other recent articles [10], [11], [12], [13]. Due to the simultaneous measurement of many important parameters, XAS is among the best techniques for determining the “structure-to-function” relationship in metalloprotein studies. XAS structural studies of metal-binding sites from purified metalloproteins, which are often carried out jointly with X-ray crystallography, will not be considered in this review, information about this procedure can be found in other articles [14], [15], [16]. However, the emerging development of XAS in combination with chromatographic methods to characterize proteins from complex samples will be discussed.

This review will cover XAS element speciation in the fields of metal-based pharmacological compounds, metal and nanoparticle (NP) toxicology, trace element physiology, and trace element dyshomeostasis in the etiology of cancer, neurological disorders and other diseases. The aim of this article is to highlight which biomedical questions have been recently addressed by using XAS, and how this has been achieved.