Experiments of in vivo HgSe nanoparticle formation demonstrate the potential of multi-element single particle icpTOF analysis for complex biological samples.
Olga Borovinskaya1, Zuzana Gajdosechova2, Eva Krupp2, Jörg Feldmann2
1. TOFWERK, Thun, Switzerland 2. University of Aberdeen, Scotland
Mechanism of in vivo HgSe Nanoparticle Formation
The mechanism of detoxifying MeHg, which is toxic to humans, is still not completely understood. Selenium is thought to play the part of defender in the detoxification scheme, transforming MeHg to species embodying lower toxicity, such as HgSe. Alternatively, HgSe nanoparticle formation is thought to exhaust the body’s Se pool, destabilizing the standard biological cycle and inhibiting Se-protein antioxidant activity.
The mechanism of in vivo HgSe nanoparticle formation was recently investigated in brain and liver specimens of stranded pilot whales, which can assimilate relatively high levels of Hg . HgSe clusters measuring up to 5 μm and embodying a Hg/Se molar ratio approaching 1 were located in adult animals with the use of synchrotron μ-XRF imaging with 800 nm lateral resolution.
Smaller aggregates surrounding large clusters represented an Hg/Se molar ratio of <1, suggesting the growth of HgSe nanoparticles on Se-rich core center structures. Single particle quadrupole inductively coupled plasma mass spectrometry (ICP-MS) analysis demonstrated multiplication of particle concentration and size as animal age increased.
Multi-Element Single Particle Analysis
To further understand the mechanism of HgSe nanoparticle formation, Hg/Se molar ratios were characterized in singular particles removed from whale liver with the use of multi-element single particle analysis, alongside the TOFWERK icpTOF. HgSe particles from the liver extract manifested sizes ranging from 40 to 100 nm and represented a mean Hg/Se molar ratio of 0.7.
A moderate growth of Hg/Se with enlargement in particle size was detected, which correlates with the findings of the synchrotron μ-XRF. As well as Hg and Se, the icpTOF also observed Cd and Fe at substantial levels, with molar ratios of Hg/Cd=1.1 and Hg/Fe=0.02.
Although it remains ambiguous whether the Cd and Fe examinations can shed additional light into HgSe crystal formation and growth, this discovery reveals the possibilities of multi-element single particle icpTOF analysis for complex biological samples.
Figure 1. Frequency distribution histograms of Hg/Se, Hg/Fe, and Hg/Cd molar ratios in individual particles extracted from a whale liver and a correlation plot of HgSe particle sizes vs Hg/Se molar ratios acquired with an icpTOF in multi-element single particle mode. The collision-reaction cell was pressurized with 3 ml/min hydrogen to suppress Ar2+ interference on 80Se+. For details on sample preparation and sample characterization refer to (1). An isotope-specific threshold was applied to discriminate nanoparticle signals from the ionic background. Element molar masses per particle and particle sizes were determined using Au 8013 NIST CRM nanoparticles, Au solutions, Hg, Se, Cd, Fe-containing calibration solutions and the method proposed by Pace et al. based on known size of 8013 Au particle standard (2). Sample specific size detection limits for HgSe particles were estimated to be 40 nm assuming the density of 13.5 g/cm3 that corresponds to 110 ag of Hg and 300 ag of Se. Fe is most likely bound to the particle in a form of metalloprotein. Co-accumulation of Cd with Hg has been already observed in the previous work (3).
 Z. Gajdosechova, M. M. Lawan, D. S. Urgast, A. Raab, K. G. Scheckel, E. Lombi, P. M. Kopittke, K. Loeschner, E. H. Larsen, G. Woods, A. Brownlow, F. L. Read, J. Feldmann, E. M. Krupp, Scientific Reports 2016, 6, 34361.
 H. E. Pace, N. J. Rogers, C. Jarolimek, V. A. Coleman, C. P. Higgins, J. F. Ranville, Anal. Chem. 2011, 83, 9361-9369.
 Z. Gajdosechova, A. Brownlow, N. T. Cottin, M. Fernandes, F. L. Read, D. S. Urgast, A. Raab, J. Feldmann, E. M. Krupp, Science of The Total Environment 2016, 545-546, 407-413.
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