Quantum Detectors

We introduce an image-contrast mechanism for scanning transmission electron microscopy (STEM) that derives from the local symmetry within the specimen. For a given position of the electron probe on the specimen, the image intensity is determined by the degree of similarity between the exit electron-intensity distribution and a chosen symmetry operation applied to that distribution. The contrast mechanism detects both light and heavy atomic columns and is robust with respect to specimen thickness, electron-probe energy, and defocus. Atomic columns appear as sharp peaks that can be significantly narrower than for STEM images using conventional disk and annular detectors. This fundamentally different contrast mechanism complements conventional imaging modes and can be acquired simultaneously with them, expanding the power of STEM for materials characterization.

A scanning precession electron diffraction system has been integrated with a direct electron detector to allow the collection of improved quality diffraction patterns. This has been used on a two-phase α–β titanium alloy (Timetal® 575) for phase and orientation mapping using an existing pattern-matching algorithm and has been compared to the commonly used detector system, which consisted of a high-speed video-camera imaging the small phosphor focusing screen. Noise is appreciably lower with the direct electron detector, and this is especially noticeable further from the diffraction pattern center where the real electron scattering is reduced and both diffraction spots and inelastic scattering between spots are weaker. The results for orientation mapping are a significant improvement in phase and orientation indexing reliability, especially of fine nanoscale laths of α-Ti, where the weak diffracted signal is rather lost in the noise for the optically coupled camera. This was done at a dose of ~19 e−/Å2, and there is clearly a prospect for reducing the current further while still producing indexable patterns. This opens the way for precession diffraction phase and orientation mapping of radiation-sensitive crystalline materials.

Microsecond (μs) time-resolved extended X-ray absorption fine structure spectroscopy (EXAFS) has been developed using an energy-dispersive EXAFS (EDE) setup equipped with a silicon Quantum Detector ULTRA. The feasibility was investigated with a prototypical thermally driven redox reaction, the thermal decomposition of (NH₄)₂[PtCl₆]. EXAFS data were collected with snapshots every 60 μs during the course of the thermolysis reaction, then averaged for 100 times along the reaction to get better signal to noise ratio which reduces the time resolution to 6 millisecond (ms). Our results provide direct structural evidence of cis-PtCl₂(NH₃)₂ as the intermediate, together with continuous electronic and geometric structure dynamics of the reactant, intermediate and final product during the course of the thermolysis of ((NH₄)₂[PtCl₆]. The thermal effect on EXAFS signals at high temperatures is considered in the data analysis, which is essential to follow the reaction process correctly. This method could also be applied to other reaction dynamics.

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The demand for broad-spectrum antibacterial agents continues with increasing rates of resistance of microbial pathogens to traditional antibiotics. Peptides and lipopeptides are gaining traction as promising novel, class-reference antibiotics for tackling difficult-to-treat infections caused by multi-drug resistant bacteria. To identify novel candidates and expand treatment options in clinical settings, we explored the in vitro antibacterial potential and mode of action of a short octapeptide combining a cationic block of four lysines and a highly hydrophobic segment of four phenylalanines (K4F4), and two K4F4-inspired lipopeptides (Palmitoyl-K4F4 and K4-NH-Palmitoyl). Preliminary AI-based screening had revealed the antimicrobial potential of the K4F4 peptide coupled with limited haemolytic activity. Broth dilution and haemolytic assays have confirmed these in silico predictions. Overall, our lipidated peptides were more active at lower MIC values compared to non-lipidated species, indicating the beneficial impact of tailing lipidation on design of peptide-based antimicrobials. An integrated view of the membrane-active mechanism of these novel therapeutic templates was obtained using a combination of flow cytometry, fluorescence microscopy and dye-based permeabilization assays. K4F4 and its lipidated derivatives act via a fast-disrupting mechanism without inducing bacterial resistance mechanisms in a long-term exposure assay. A K4F4-inspired lipopeptide together with its shorter version (K4-NH-Palmitoyl), were more stable in environments closer emulating physiological conditions, showing a higher antibacterial response in physiological salts and serum than their parent peptide. Our findings reveal the antibacterial and antibiofilm potential of a novel polylysine-polyphenyalanine peptide and highlight the significant contribution of lipidation and shortening as molecular engineering strategies to improve and guide the future design of next-generation membrane-targeting antibiotics.

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Journal Article First Results from a Novel CMOS Detector Optimised for 100keV CryoEM Get access Deividas Krukauskas, Deividas Krukauskas Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Campus, Didcot, UK Search for other works by this author on: Oxford Academic Google Scholar Tobias Starborg, Tobias Starborg Rosalind Franklin Institute, Harwell Campus, Didcot, UK Search for other works by this author on: Oxford Academic Google Scholar Roger Goldsbrough, Roger Goldsbrough Quantum Detectors Ltd, Harwell Campus, Didcot, UK Search for other works by this author on: Oxford Academic Google Scholar Liam O'Ryan, Liam O'Ryan Quantum Detectors Ltd, Harwell Campus, Didcot, UK Corresponding author: liam@quantumdetectors.com Search for other works by this author on: Oxford Academic Google Scholar Angus I Kirkland, Angus I Kirkland Rosalind Franklin Institute, Harwell Campus, Didcot, UK Search for other works by this author on: Oxford Academic Google Scholar Nicola Guerrini Nicola Guerrini Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Campus, Didcot, UK Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1174–1175, https://doi.org/10.1017/S1431927622004925 Published: 01 August 2022

Scanning/transmission electron microscopes (S/TEMs) are invaluable tools for the characterisation of nanosized matter, in no small part due their flexibility to be configured to perform a wide variety of experiments. Most well known are the conventional high-resolution imaging modes with a static parallel electron beam or a scanning converged electron probe. In recent years, hybrid pixel direct electron detectors (HPDEDs) have revolutionised modern electron microscopy (EM) applications, owing to their high detective quantum efficiencies (DQEs), readout speeds, dynamic ranges and radiation hardnesses. For example, Quantum Detectors’ MerlinEM was designed for 4D scanning transmission electron microscopy (4D STEM). 4D STEM is a data collection protocol in which the entire 2D diffraction pattern is collected at each probe position in a 2D scan [1]. Therefore, compared to a conventional 2D STEM image, the 4D STEM dataset is richer in crystallographic information and is ripe for processing and analysis by a host of algorithms and software. In fact, data analysis is only limited by the user’s imagination: the simplest data processing strategy uses virtual detectors and apertures to remove the constraints of their physical counterparts. Moreover, researchers have used 4D STEM to measure the electric and magnetic fields of materials [2], and to map the crystalline phase and orientations of polycrystalline domains [3], as well as the sample strain [4]. And most recently, these detectors have afforded data with sufficiently high quality to enable amplitude and phase image reconstruction from electron- beam sensitive weak phase materials using electron ptychography algorithms [5]. Indeed, the qualities of HPDEDs also make them highly suitable for micro electron diffraction (microED), which is an umbrella term for a set of protocols that involves the collection of three dimensional electron diffraction data collected from several crystallographic orientations for structural elucidation [6]. MicroED is complementary to X-ray and neutron diffraction methods, with its primary benefit being structural elucidation from nanosized volumes at accessible facilities. Certainly, the versatility of S/TEMs necessitates a versatile detector. MerlinEM is a universal detector, capable of both 4D STEM and microED data acquisition, empowering researchers to maximise the information they collect from their samples on a single S/TEM. Here, we demonstrate the MerlinEM technology, and describe how it can be incorporated into a typical microscope setup. Then, we highlight some recent work conducted by researchers across several labs, including both 4D STEM and microED.

The MerlinEM/Medipix3 is a versatile detector widely used for scanned electron diffraction, precession, dynamic transmission electron microscopy (TEM) and scanning transmission electron microscopy (4D STEM).It can be used to image direct probes (up to 300kV) while still retaining the ability to count single electrons for the weaker diffraction spots [1].It contains two 12-bit counters, enabling zero dead time acquisitions and noiseless readout.This makes MerlinEM an attractive solution for low-dose techniques in many areas of electron microscopy.We are particularly interested in the application of micro electron diffraction (3DED, microED) techniques for structure determination of small molecules of biological and pharmaceutical importance [2].3DED has been growing in popularity in recent years thanks to the development of more sensitive detectors and stable automated diffraction setups.Electron diffraction has the essential advantage over X-ray crystallography when crystal size is concerned.It produced multiple medium to high resolution structures from nano crystals and from artificially thinned (by focused ion beam milling) crystals [3,4].Furthermore, electrons have an advantage over X-rays as they interact orders of magnitude more strongly with samples.This allows hydrogen atoms to be located within the structure to gain insights into the chemical properties of small compounds.This is essential in chemical research, pharmaceutical drug discovery and other industries [5].Furthermore, performing electron diffraction tomography on multiple crystals in an automated manner enables efficient collection of large datasets.For this reason, we worked closely with the team at Quantum Detectors.A recently available plugin for MerlinEM to the academic SerialEM software environment [6] has enabled low-dose 3DED data collections on the MerlinEM Quad 4S installed on the FEI Tecnai F30 cryo electron microscope at the Rosalind Franklin institute.We present data obtained on small compounds of pharmaceutical interest by HeXI at Diamond Light Source using 3DED with MerlinEM.