Hard X-ray fluorescence (XRF) microscopy offers unparalleled sensitivity for quantitative analysis of most of the trace elements in biological samples, such as Fe, Cu, and Zn. These trace elements play critical roles in many biological processes. With the advanced nano-focusing optics, nowadays hard X-rays can be focused down to 30 nm or below and can probe trace elements within subcellular compartments. However, XRF imaging does not usually reveal much information on ultrastructure, because the main constituents of biomaterials, i.e. H, C, N, and O, have low fluorescence yield and little absorption contrast at multi-keV X-ray energies. An alternative technique for imaging ultrastructure is ptychography. One can record far-field diffraction patterns from a coherently illuminated sample, and then reconstruct the complex transmission function of the sample. In theory the spatial resolution of ptychography can reach the wavelength limit. In this manuscript, we will describe the implementation of ptychography at the Bionanoprobe (a recently developed hard XRF nanoprobe at the Advanced Photon Source) and demonstrate simultaneous ptychographic and XRF imaging of frozen-hydrated biological whole cells. This method allows locating trace elements within the subcellular structures of biological samples with high spatial resolution. Additionally, both ptychographic and XRF imaging are compatible with tomographic approach for 3D visualization.
X-ray fluorescence offers unparalleled sensitivity for imaging the nanoscale distribution of trace elements in micrometer thick samples, while x-ray ptychography offers an approach to image weakly fluorescing lighter elements at a resolution beyond that of the x-ray lens used. These methods can be used in combination, and in continuous scan mode for rapid data acquisition when using multiple probe mode reconstruction methods. We discuss here the opportunities and limitations of making use of additional information provided by ptychography to improve x-ray fluorescence images in two ways: by using position-error-correction algorithms to correct for scan distortions in fluorescence scans, and by considering the signal-to-noise limits on previously-demonstrated ptychographic probe deconvolution methods. This highlights the advantages of using a combined approach.
Hard X-ray fluorescence microscopy is one of the most sensitive techniques to perform trace elemental analysis of
unsectioned biological samples, such as cells and tissues. As the spatial resolution increases beyond sub-micron
scale, conventional sample preparation method, which involves dehydration, may not be sufficient for preserving
subcellular structures in the context of radiation-induced artifacts. Imaging of frozen-hydrated samples under
cryogenic conditions is the only reliable way to fully preserve the three dimensional structures of the samples while
minimizing the loss of diffusible ions. To allow imaging under this hydrated “natural-state” condition, we have
developed the Bionanoprobe (BNP), a hard X-ray fluorescence nanoprobe with cryogenic capabilities, dedicated to
studying trace elements in frozen-hydrated biological systems. The BNP is installed at an undulator beamline at Life
Sciences Collaboration Access Team at the Advanced Photon Source. It provides a spatial resolution of 30 nm for
fluorescence imaging by using Fresnel zone plates as nanofocusing optics. Differential phase contrast imaging is
carried out in parallel to fluorescence imaging by using a quadrant photodiode mounted downstream of the sample.
By employing a liquid-nitrogen-cooled sample stage and cryo specimen transfer mechanism, the samples are well
maintained below 110 K during both transfer and X-ray imaging. The BNP is capable for automated tomographic
dataset collection, which enables visualization of internal structures and composition of samples in a nondestructive
manner. In this presentation, we will describe the instrument design principles, quantify instrument performance,
and report the early results that were obtained from frozen-hydrated whole cells.