Yichi_Wang

Life scientists often desire to display the signal from two different molecular probes as a single colour image, so as to convey information about the probes’ relative concentrations as well as their spatial corelationship. Traditionally, such colour images are created through a merge display, where each greyscale signal is assigned to different channels of an RGB colour image. However, human perception of colour and greyscale intensity is not equivalent. Thus, a merged image display conveys to the typical viewer only a subset of the absolute and relative intensity information present in and between two greyscale images. The Commission Internationale de l'Eclairage L*a*b* colour space (CIELAB) has been designed to specify colours according to the perceptually defined quantities of hue (perceived colour) and luminosity (perceived brightness). Here, we use the CIELAB colour space to encode two dimensions of information about two greyscale images within these two perceptual dimensions of a single colour image. We term our method a Perceptually Uniform Projection display and show using biological image examples how these displays convey more information about two greyscale signals than comparable RGB colour space-based techniques.

It is often useful to display the fluorescent signal from two different molecular probes as a single colour image so that information about the probes’ relative concentrations as well as their spatial corelationship can be visualized. A common way to accomplish this effect is to assign each greyscale signal to a different channel of an RGB colour image. However, this method fails to convey much of the information about the two greyscale images, because humans’ perception of colour and monochrome values is not equivalent. We have used the Commission Internationale de l'Eclairage L*a*b* colour space (CIELAB) to display two greyscale images as a single colour image. Our method encodes two dimensions information about the greyscale images as two, perceptually independent and uniform quantities – hue and luminosity – in the colour image. Our technique, which we termed a perceptually uniform projection display, conveys more information about two greyscale signals than comparable RGB colour space-based techniques. We apply this method to display several types of information commonly encountered in cell biology images.

Noble metal nanocrystals that are free of capping ligands promise significantly enhanced activities in surface-enhanced Raman scattering (SERS) and catalytic applications. Conventional physical and chemical processes to remove the capping ligands are usually too harsh to retain the morphology and surface structure of the noble metal nanocrystals. In this work, a mild, effective, and robust strategy is presented to remove the capping ligands from the surface of noble metal nanocrystals. Polyvinylpyrrolidone and oleylamine, which are generally known to adsorb strongly on the metal surface, have been successfully removed from the Au and Pt nanocrystals, respectively, by a convenient ligand-exchange process with diethylamine. The resulting surface-clean Au nanospheres and nanoflowers show significantly enhanced activity in SERS with great potential in single-particle SERS applications. The surface-clean Pt nanocrystals show highly improved electrocatalytic activity, compared with those cleaned by conventional methods. It is believed that this novel ligand-exchange strategy opens up new opportunities in eliminating the effect of the capping ligands for optimal activities of colloidal noble metal nanocrystals in a variety of applications.

A delicate ligand-exchange strategy has been developed to remove capping ligands (polyvinylpyrrolidone and oleylamine for example) effectively from colloidal noble metal nanocrystals. It leads to clean metal surface and thus its high accessibility by analytes or reactants, which enables significantly enhanced activity of the metal nanocrystals in surface-enhanced Raman scattering and catalytic applications.

Electron tomography in transmission electron microscopy provides valuable three-dimensional structural, morphological and chemical information of condensed matter at nanoscale. Current image acquisitions require at least tens of minutes, which prohibits the analysis of nano-objects evolving rapidly such as under dynamic environmental conditions. Reducing the acquisition duration to tens of seconds or less permits to follow in 3D the same object during its evolution under varying temperatures and pressures. We report Operando Electron nanotomography using image series acquired in less than 230 seconds instead of typically 15 min in the best cases so far. The in situ calcination of silica zeolites encaging silver nanoparticles, a catalytic nanosystem of potential interest for, e.g., nuclear waste treatments or selective heterogeneous catalysis, was successfully studied. Kinetic environmental Operando 3D electron microscopy becomes possible, as well as real time observation of beam sensitive samples (polymers, biological objects) without prior preparation, which reduces their contrast and reactivity.

The development of nanotechnologies leads to a constant need of better characterisation of shapes and morphologies of nano-objects, nanomaterials, nanoparticles at the appropriate scale that is a submicrometre or nanometre level. These sizes impose transmission electron microscopy technique, an unsurpassed technique to study condensed matter at such a spatial resolution. Because these systems are aimed to be used in ‘real life’, it is further of the greatest importance to perform this analysis in their natural environment, or as close as possible to the conditions under which they are intended to be used. As a typically relevant example here, catalytic systems are developed for making chemical reactions possible, or improving their efficiency, by really manipulating gas molecules, most generally at high temperatures and of course a given gas pressure. Ideally, these investigations should be conducted in 3D: flawed interpretation or measurements are current in 2D images where a tridmensional object is simply projected in a viewing direction, which does not give access to the position of elementary features along, precisely, this viewing direction (this may be up to the point where the projection evokes a totally different object from the real one and this has been used for entertainment for many years – see the nice example of hand shadows by Henry Bursill in the 19th century www.gutenberg.org/files/12962/12962-h/12962-h.htm). 3D analyses can be performed in different ways, a current approach with microscopy techniques is the so-called ‘tilted tomography’, where 2D projections of the object are recorded under several orientations over a large angular amplitude, which allows the volume to be numerically rebuilt with the help of dedicated reconstruction softwares.

Summarising this introduction leads to the goal of the present work: can we perform in situ 3D analyses of nanocatalysts under environmental conditions (temperature and gas pressure) in the transmission electron microscope (TEM)? A positive demonstration will be made using a specific nanocomposite catalyst, which will be literally burnt under oxygen inside an Environmental TEM. The ambitious goal of following in situ the morphological evolution of a nanometric object during an ongoing chemical reaction (or any other dynamic process) raises the question of the speed of image acquisition (during the sample rotation required in ‘tilted tomography’). The duration of the whole acquisition must obviously be faster that the morphological changes of the sample in order to allow correct 3D reconstructions, and this is also what we focus on in the present contribution.