Investigation of fine atmospheric particle surfaces and lung lining fluid interactions using XPS
Introduction
It has generally been established that exposure to urban airborne particulate matter (PM) with an aerodynamic diameter less than 10 μm (PM10), and especially less than 2.5 μm (PM2.5), adversely affects human health with increased respiratory and cardiovascular diseases and observed mortality in exposed populations [1], [2], [3]. However, it is still not clearly understood which particles are responsible for these adverse effects or the mechanisms involved. This is due to the complexity of urban PM which can originate from a multitude of sources and may also undergo physical and chemical transformations in the atmosphere. Low levels of exposure to urban PM can result in the onset of adverse health effects which are not observed for much higher levels of exposure in industrial workplaces [4].
The toxicity of PM2.5 is hypothesised to result from particle surface area, particle number, surface chemistry, oxidative stress and interstitialisation of particles [5]. The efficiency of removal of ultra-fine particles (<0.1 μm) deposited in the lung is decreased due to the large number of particles present in this fraction. Macrophages are cells invoked as an immune response to remove particles from the alveolar surface and there is evidence of damage to such macrophages [6]. The biological interaction and fate of inhaled particles will be in part dependent on the surface chemistry. There is evidence that trace metals such as Al and Fe may be important in the biological interactions with particles [7]. Particles may also interact with the epithelium and cause increased epithelial permeability [8]. Carbon for example is taken up by lung epithelial cells in vitro, causing lesions closely resembling those present in tumour cells [9]. Johnson et al. [10] showed that PTFE particles were taken up by pulmonary epithelial cells inducing a significant immune response, probably via catalysis of free radical generation and causing oxidative stress leading to lung injury.
Most of the literature describing fine particle characterisation has employed bulk analytical techniques [11]. However, atmospheric chemical transformations occur at the particle surface and it is this surface that directly interacts with biological fluids after inhalation or ingestion. Therefore, the surface composition of particles is important in the formulation of hypotheses for the initiation of respiratory and cardiovascular diseases, as well as in determining PM sources. X-ray photoelectron spectroscopy (XPS) has been successfully used in determining the surface composition of urban airborne particulates in a number of studies [12], [13], [14], [15] and its suitability for analysis of a wide range of environmental samples, including atmospheric particles has been discussed [16]. XPS has previously been used to characterise airborne particles from London environments [17] and localised variations in chemistry across particle surfaces has been observed using imaging XPS [18]. This paper describes the use of XPS to characterise the particle surface chemistry of PM collected from outdoor urban, indoor cigarette smoke and clean air sites. The effect of the variation in surface chemistry of particles from different sources was then examined in terms of their interaction with lung lining fluid. This study highlights the applicability of surface sensitive techniques such as XPS for probing the processes occurring at particulate surfaces in biological environments.
Section snippets
Experimental
Non-fibrous membrane filters are used to collect PM to ensure that a layer of particles are deposited on the filter surface, which can then be analysed. Since the main constituent of atmospheric PM is carbon [19], it is important to ensure that XPS peaks arising from the collection substrate are well separated from those arising from particulate species in the C 1s region. Common substrates such as polycarbonate membrane filters cannot therefore be used. Metallic samplers have been used for
Results and discussion
XPS survey spectra showed that carbon dominates the particulate surface and C 1s spectra for the untreated blank, clean air, urban air and cigarette smoke samples are shown in Fig. 1. Spectra were referenced to the binding energy of the F 1s peak as this was inherent in the PTFE substrate and was shown to be unaffected by the type of particles collected; normalised F 1s peaks were seen to have similar peak widths for all samples (not shown). The C 1s signal from the particles (284.3–289.0 eV)
Conclusions
PTFE was shown to be an excellent substrate for particulate collection and subsequent XPS analysis with control samples showing very low contamination levels with photoelectron peaks from PTFE occurring at different binding energies to those of the particles. No adsorbate species could be identified after blank immersion in saline and only low levels of hydrocarbon and oxygen were observed after blank immersion in BALF. Particle loads were only significant for urban air and smoke samples.
The
Acknowledgements
The authors would like to thank David Williams for use of the XPS facility at UCL for this work, DoH for finding this project and Tim Carney, Thermo VG Scientific for useful discussions throughout this work.
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