The Bionanoprobe: Synchrotron-Based Hard X-ray Fluorescence Microscopy for 2D/3D Trace Element Mapping
Si Chen , 1 * Tatjana Paunesku , 2 Ye Yuan , 2 Qiaoling Jin , 3 Lydia Finney ,1 Benjamin Hornberger , 4 Claus Flachenecker , 4 Barry Lai , 1 Keith Brister , 5 Chris Jacobsen , 1 ,3 ,4 , 6 Gayle Woloschak , 2 and
Stefan Vogt 1 , 2 1 Advanced Photon Source , Argonne National Laboratory , Argonne , IL 60439 2 Department of Radiation Oncology , Northwestern University , Chicago , IL 60611 3 Department of Physics and Astronomy , Northwestern University , Evanston , IL 60208 4 Carl Zeiss X-ray Microscopy , Inc. , Pleasanton , CA 94588 5 Synchrotron Research Center , Northwestern University , Argonne , IL 60439 6 Chemistry of Life Processes Institute , Northwestern University , Evanston , IL 60208
*
sichen@aps.anl.gov Introduction
Trace elements, particularly metals, play an important role in a large variety of cellular processes in a biological system. In the context of biological organisms and tissues, the term trace element means that over the entire organism an element is present at only trace levels, 100 ppm or lower. Trace element distribution and content can be analyzed using several techniques, for example, visible light optical fl uorescence imaging, energy-dispersive X-ray spectroscopy on an electron microscope, synchrotron-based X-ray fl uorescence (XRF)
imaging, secondary-ion mass spectrometry, and laser ablation inductively coupled with mass spectrometry. Comprehensive reviews on these techniques are given by Lobinski et al. [ 1 ] and McRae et al. [ 2 ]. Among these techniques, synchrotron- based XRF microscopy, particularly using third-generation X-ray sources and advanced X-ray focusing optics, off ers the most suitable capabilities to perform trace element studies of biological samples: the penetrating power and non-destructive nature of X rays allows one to image many-micron-thick biological samples such as whole cells in a way that visible light or electron microscopes cannot; the sensitivity of X-ray-induced XRF is down to parts per million, several orders of magnitude better than standard electron-based techniques due to the absence of bremsstrahlung background in X-ray-induced X-ray emission. T e capability of imaging frozen samples in both 2D and 3D with sub-50 nm resolution in various X-ray modes has greatly advanced a broad range of scientifi c studies. T is article describes how this technique can be used to track the incorpo- ration of nanocomposites into cancer cells.
Figure 1 : Schematic of a synchrotron-based X-ray fl uorescence micro/ nanoprobe. A monochromatized X-ray beam is focused onto the sample using a zone plate. While the sample is raster scanned, X-ray fl uorescence spectra are recorded, forming 2D elemental maps and 3D reconstructions in tomography mode. Differential phase contrast images are produced from transmission signals recorded using a quadrant photodiode.
Materials and Methods
Figure 2 : Potassium (K) X-ray fl uorescence image (left side) and a differential phase contrast image (right side) of a mouse fi broblast cell. The count level for K signal (minimum to maximum range) is 0–267 counts/s. The images were acquired with the Bionanoprobe using a 50 nm step size.
26 doi: 10.1017/S1551929515000401
XRF analysis and elemental mapping . A typical setup of a synchrotron-based XRF micro/nanoprobe is shown in Figure 1 . A monochromatized X-ray beam is focused using an X-ray objective lens (a Fresnel zone plate in this case) onto a sample. While the sample is raster-scanned, a full X-ray fl uorescence spectrum is recorded for each pixel using an energy-dispersive detector (Vortex-ME4, Hitachi High- Technologies Science America, USA) located at 90 ° with respect to the incident beam. T is arrangement pro- duces 2D elemental maps of the specimen. Simultaneously, a transmis- sion signal is recorded using a quad- rant photodiode for diff erential phase contrast imaging. Figure 2 shows both the potassium X-ray fl uores- cence map and a diff erential phase contrast image of a mouse fi broblast
www.microscopy-today.com • 2015 May
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