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RESEARCH FEATURES Super-resoluton imaging using MeV ions Imaging with energetc ions can reveal sub-cellular structure Introducton In 1873, German physicist Ernst ABBE frst described a fundamental limit to the resoluton of an optcal system which became more commonly known as the difracton limit. The difracton limit states that two objects closer than roughly half the illuminaton wavelength cannot be resolved. In practce, if visible light is used for illuminaton, the resoluton limit is approximately 250 nm. Techniques that are able to break this difracton limit are called super-resoluton techniques. At the Centre for Ion Beam Applicatons (CIBA), we have been developing the Figure 1: Schematc of the signals produced by a MeV ion beam interactng with a use of mega-electron volt (MeV) ion sample. The analytcal technique for each signal is denoted in red. The techniques beams for super-resoluton imaging. highlighted in yellow are those which have been developed for nano-imaging applicatons. Nuclear Microscopy resoluton. This is because MeV ions probe have a specifc absorpton lose energy as they penetrate a sample band for excitaton to occur. MeV At CIBA, we have been performing by interactng with the target electrons ions therefore excite all fuorescent imaging using microscopy techniques which are approximately 1,800 tmes probes, including any fuorescent that are based on energetc ions for lighter than the impinging proton or molecules that make up the biological several decades. These techniques rely alpha partcle beams. If the energy lost sample (endogenous fuorescence on the ability to focus MeV beams of by the ion as it transmits through the or autofuorescence). In our system, protons or alpha partcles down to spot sample is measured and mapped, an Proton (or alpha) Induced Fluorescence sizes of several 10’s of nanometers. image of the density of the sample can (PIF) imaging is performed by focusing When these beams are scanned be obtained. This technique is called the MeV ion beam down to about over a sample, the signals generated Scanning Transmission Ion Microscopy 20 nm and then scanning it over by the ion beam can be mapped to (STIM). The STIM technique can be the sample and measuring the light form an image. These signals include used to map the density of whole intensity that is emited as a functon electromagnetc radiaton in the form cells at high resoluton (about 20 nm) of positon. of ultra-violet, visible and infra-red because cells are thin enough for light, high energy radiaton such as MeV ions to pass through them, and At CIBA, we have developed a dedicated x-rays and gamma rays or primary every single ion and its energy can be beam-line for performing simultaneous ions that have been scatered by the detected. structural and fuorescence imaging sample. Figure 1 shows a schematc of whole cells at spatal resolutons of the processes that occur when an Optcal fuorescence imaging is by far well below the optcal difracton energetc ion interacts with a material. the most common imaging technique limit. Figure 2 shows a photograph The yellow highlights (see Figure 1) used in biology, and in partcular, cell of our cell imaging beam-line. This indicate the signals that are of most biology. MeV ions are also able to beam-line has a target chamber with interest for high resoluton imaging. excite light emission from commonly a microscope and various detectors for used fuorescent probes. The excitaton measuring transmited ions, photons Structural and Fluorescence Imaging process for MeV ions is diferent from and electrons. The microscope has the processes that occur for excitaton been modifed so that it can be used One of the advantages that MeV ion by light. Unlike optcal excitaton, in a vacuum system. The objectves are beams have over electron beams is excitaton by MeV ions is not selectve located in the vacuum and the flter their ability to penetrate deep into and does not require that a fuorescent sets, eye-pieces and camera ports are a sample while maintaining their ADVANCES IN SCIENCE | VOL. 22 | NUMBER 1 | JUNE 2017 6
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