Synchrotron-based techniques for studying the environmental health effects of heavy metals: Current status and future perspectives

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Highlights

  • Synchrotron techniques for environmental health effects of heavy metals are summarized.

  • The quantification, speciation and structure characterization by synchrotron techniques are presented.

  • Challenges and future aspects are discussed.

Abstract

This review summarizes the state-of-the-art synchrotron-based techniques for studying the environmental health effects of heavy metals exposure. Synchrotron radiation based X-ray fluorescence (SRXRF) is widely applied in quantification of metals in different biological and environmental samples. X-ray absorption spectrometry (XAS) is used for speciation of heavy metals. With high energy resolution fluorescence detected (HERFD) XAS, it is possible to study heavy metals in biological samples at realistic concentrations. The focused synchrotron-based X-ray is applied to image metals down to nm resolution at 2- or 3- dimension (2D or 3D). The combination of XAS with SRXRF can realize 2D or 3D spatial speciation, along with other techniques like scanning transmission X-ray microscopy (STXM) and full field XAS. The structure of metal-binding biomolecules can be characterized by Protein X-ray Diffraction (PX) and/or XAS, together with neutron scattering. The future aspects of multimode detection, new imaging methods, fast detector technologies and big data strategies in synchrotron-based techniques were also discussed.

Introduction

Environmental health science is a transdisciplinary field, which tackles public health threats from environmental exposures and estimate the magnitude of health risks related to environmental contaminants by investigating the mechanisms of biological response, and use this information to develop solutions to mitigate environmental problems [1]. Heavy metals are one of the many environmental exposure factors, which are metallic elements that have a density over 4.0 or 5.0 g/cm3. Metalloids, such as arsenic, can also induce toxicity at low level of exposure, which are regarded as heavy metals, too [2,3]. Although heavy metals are naturally occurring, most heavy metals come from anthropogenic activities like mining, smelting, industrial production and domestic and agricultural application [4].

Many heavy metals play vital roles in the chemistry of life. For example, copper serves as a cofactor in many redox enzymes like Cytochrome C oxidase, which influence respiratory electron transport chain of mitochondria [5]. On the other hand, many heavy metals including those essential metals are toxic to living organisms at high concentrations. For example, lead, arsenic, manganese and methylmercury (MeHg) cause injury to the nervous system in humans [6]. The toxicity of heavy metals to organisms and human is highly linked to their concentration, speciation and distribution. Therefore, the study on the absorption, distribution, metabolism and excretion of heavy metals in biological systems will help us to understand the mechanisms underlying their environmental health effects.

Different approaches have been applied to study the concentration, speciation and distribution of heavy metals [[7], [8], [9], [10]]. Among all the techniques, synchrotron based techniques are versatile and sometimes indispensable to study the environmental health effects of heavy metals [11,12]. Synchrotron radiation (SR) is an advanced light source providing from infrared up to hard X-rays with high brilliance (many orders of magnitude more than conventional light sources) and pulsed light emission (pulse durations at or below one nanosecond). Synchrotron based techniques are developed through the interactions of synchrotron radiation with matters, i.e. absorption and scattering. Fourier transformed infrared spectroscopy (FTIR), X-ray absorption spectrometry (XAS), and X-ray computed tomography (X-CT), etc. are based on the absorption of synchrotron radiation by matters. X-ray diffraction (XRD), protein X-ray crystal diffraction (PX), and small angle X-ray scattering (SAXS) etc are based on the scattering of the synchrotron radiation. Besides, the detection of the emission of secondary particles like X-ray fluorescence spectrometry (XRF) and X-ray photoelectron spectroscopy (XPS), are also developed using synchrotron radiation [12,13].

The light produced by SR facility is highly polarized, tunable, and collimated, which make SR-based techniques outstanding tools in multidisciplinary research areas including physics, chemistry, life science, materials science, environmental science and environmental health science [[14], [15], [16], [17]]. With the rapid growth in the number of SR facilities around the world, synchrotron-based techniques are more and more widely applied. To the best of our knowledge, there is no review focusing on the application of synchrotron based techniques for environmental health effects of heavy metals although there are references that summarized synchrotron based techniques like X-ray fluorescence imaging in biological systems [18]. This review summarizes the recent advances of synchrotron-based techniques in quantification, speciation, mapping, and spatial speciation of heavy metals and structure characterization of metal-binding biomolecules in environmental health study. Future aspects on the application of synchrotron based techniques to study the environmental health effects of heavy metals will also be discussed.

Section snippets

Multielemental quantification of heavy metals

Knowing the concentrations of heavy metals in biological systems is the first step to understand the environmental health effects of them. The quantification of heavy metals can be achieved through different element-specific techniques like atomic fluorescence spectrometry (AFS), atomic absorption spectrometry (AAS), etc, which are generally suitable for the determination of one or several specific metals in one run. Inductively coupled plasma optical emission spectrometry (ICP-OES) and

Direct speciation of heavy metals

The environmental health effects of heavy metals are not only dependent on the concentration, but also to their chemical forms or species. For example, mercuric selenide (HgSe) has a relatively low toxicity and accumulates as an apparently non-toxic detoxification product in marine animals while dialkylmercury derivatives are extremely toxic, and methylmercury cysteinate was much less toxic than methylmercury chloride in a zebra fish larvae model system [33].

For the speciation analysis, samples

Spatial distribution of heavy metals

Besides quantification and speciation of heavy metals in samples, it is also important to know their distribution in the body, which can give information on their trafficking and deposition. Labeling methods like radioactive or stable isotopic tracers or artificial dyes can only give information on the trafficking and distribution of specific metals introduced into the samples [49,50].

The distribution of heavy metals with no introduction of isotopic tracers or artificial dyes can be achieved

Spatial speciation of heavy metals

The spatial speciation analysis gives the information on the distribution of different chemical species of heavy metals in the sample.

In the soft X-ray region, scanning transmission X-ray microscopy (STXM) is used in an approach called XANES “stack” imaging, as a stack of repeated 2D scans is acquired. Changing the incident photon energy and taking images with other photon energies gives an image sequence that includes both chemical information and topographical information. Through the third

Structure characterization of metal-binding biomolecules

Protein X-ray crystallography (PX) is one of the most powerful tools for the determination of macromolecular 3D structure at a resolution as low as 0.1 nm of metal-binding biomolecules [102]. Crystal structures characterization using synchrotron radiation provide high-resolution snapshots of biomolecules. The higher brightness, flux, and availability of advanced detectors for synchrotron radiation have led to better quality data and higher-resolution crystal structures. A notable example is the

Conclusion and future perspectives

In this review, the state-of-the-art synchrotron-based techniques in quantification, speciation, mapping, and spatial speciation of heavy metals and structure characterization of metal-binding biomolecules in biological samples are reviewed (Fig. 3). SRXRF is a non-destructive, multielemental analytical technique, which has been used in quantification of metals in different biological samples. XAS has been used for speciation of heavy metals while HERFD XAS, can be used to study heavy metals in

Acknowledgements

We thank Professor Herman Autrup for his critical reading and constructive comments. We thank the financial support from National Natural Science Foundation of China (11975247; 11475196; 41877405), Ministry of Science and Technology of China (2016YFA0201600), and Guizhou Department of Science and Technology (No. QKH-2016-2804). We gratefully acknowledge the staff from Beijing Synchrotron Radiation Facility, Shanghai Synchrotron Radiation Facility and National Synchrotron Radiation Laboratory

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