Visualizing Chemistry The Progress and Promise of Advanced Chemical Imaging (2006) / Chapter Skim
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2 Utilizing Chemical Imaging to Address Scientific and Technical Challenges: Case Studies
2 Utilizing Chemical Imaging to Address Scientific and Technical Challenges: Case Studies
Pages 21-58
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More technical information about specific imaging techniques is provided in greater detail in Chapter 3. A GRAND CHALLENGE FOR CHEMICAL IMAGING Chemical imaging helps us to answer difficult questions, especially when these questions occur in complex chemical environments.
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, as well as capture images at appropriate time dimensions to acquire necessary information. An overarching objective for future breakthroughs using chemical imaging techniques is to gain a fundamental understanding and control of these complex chemical structures and processes.
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From low-energy radio waves that tickle the states of nuclei, to infrared light that captures the nature and energies of chemical bonds, to visible light that probes electronic structure, to high-energy X-rays and electrons that report on electron density, spectroscopy provides detailed information and generally does so in a spatially and temporally patterned way. Scanning probe microscopy, while still largely an in vitro approach, adds an additional dimension in which mechanical and electrical probes can be applied directly.
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A more sophisticated approach that has recently become possible is to specifically turn genes on or off with light, giving complete control of gene expression within a population of cells as a function of both space and time. In general, the concept of refitting our molecular imaging probes to become "full-duplex" molecules, functioning both to report on the environment that surrounds them and to manipulate that environment in an externally controlled way, is an idea that is just taking form and provides new vistas both for fundamental research in biology and for environmental, medical, and synthetic applications.
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IMAGING TECHNIQUES The development of multiple imaging techniques provides researchers with powerful tools to probe multiple aspects of chemical problems. A more detailed discussion of these techniques is provided in Chapter 3; however, the techniques are introduced briefly here.
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Electron Microscopy, X-rays, Ions, and Neutrons With wavelengths that are about 1,000 times smaller than that of visible light, electrons provide a high-resolution probe of chemical and structural information below surfaces of materials. Images of atomic arrangements over a large range of length scales can be obtained using electron microscopy (EM)
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In addition, computational methods, particularly when applied to computer modeling and simulation, extend imaging capabilities to address problems that have not or cannot be addressed using standing imaging techniques. CASE STUDIES A series of real-life "case studies" is presented to illustrate the importance of the technical issues that have been introduced in this chapter and show how chemical imaging can contribute to understanding them.
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28 VISUALIZING CHEMISTRY FIGURE 2.1 MCM-41 units are formed from self-assembly to create honeycomb structures that can be functionalized on the inside (light blue) to create confined catalytic sites.
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Opportunities for Imaging Development Opportunities to develop imaging techniques for this application would include the following: · Optical imaging at microsecond to nanosecond time scales per consecutive image · One instrument for imaging the entire length scale from nanometers to millimeters · Single-molecule imaging without fluorescence labeling
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domains, electronic coupling between individual molecules in the aggregate, and the mechanisms of electrical charge generation, injection, transport, and recombination. Microscopic methods will continue to provide vital information on molecular- to micrometer-length scales for both existing and emerging materials.
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2001. Imaging molecular and nanoscale order in conjugated polymer thin films with near-field scanning optical microscopy.
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Electron microscopy provides valuable information on nanometer and larger structures patterned within their films. Force microscopy and STM provide valuable data on organization and electronic structure on angstrom-to nanometer-scale distances, while nearfield optics provides high resolution spectroscopic data with sub 50-nm spatial resolution and subnanosecond time resolution.
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An important challenge here involves the development of methods that can image beneath the electrodes between which the materials are sandwiched, with depth discrimination capabilities. A further challenge involves implementation of techniques that provide clear chemical information (i.e., Raman, IR, or other vibrational imaging techniques)
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Optical imaging is important to study nondestructively the development of the disease and changes in cellular structure that occur in animal models. The full range of in vitro chemical imaging techniques (e.g., electron microscopy, mass spectrometry imaging)
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(b) Atomic representation of the structure with colors representing side chain type (green, hydrophobic; magenta, polar; red, negatively charged; blue positively charged)
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Insights Obtained Using Chemical Imaging A large amount is now known about the location of plaque formation in the brain, the pathophysiology of Alzheimer's disease, the molecular and cellular basis of the disease, and the genetic basis of the disease. All of these developments have relied on the use of chemical imaging techniques.
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, binds specifically to amyloid plaque and can be used to image Alzheimer's disease using PET. The image on the left is the brain of a normal person, and the blue colors indicate little accumulation of PIB.
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Along with the development of basic imaging technologies, it is critical to develop new imaging agents that will give greater chemical, molecular, and cellular specificity to imaging techniques. In particular, the development of optical imaging probes for detailed studies of animal models and new MRI and PET agents for human studies will greatly expand the capabilities of chemical imaging for the human brain.
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FIGURE 2.10 A general approach for site-specific incorporation of unnatural amino acids into proteins in vivo. NOTE: AMP = adenosine 5 monophosphates; ATP = adenosine 5-triphosphate; PPi = pyrophosphate.
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However, broadening the applicability of these significant improvements calls for focused research efforts to reach broad applicability. Opportunities for Imaging Development High-resolution chemical imaging methods utilizing these diverse fluorescent markers would strongly enhance capability in analyzing molecular patterns, mobility, and interactions important in biological and materials science research.
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in a chip. SOURCE: Courtesy of the Cancer Research Microscopy Facility, University of New Mexico Hospital; the W.M.
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Because details of the flow profiles in individual nanochannels are below the resolution limit of optical microscopy, only the average velocities of dye fronts can be monitored. Significant improvements in the lateral resolution of analytical imaging methods are required to study the transport of molecules in an individual channel.
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Case Study 6: Biological Imaging Involving Multiple Length Scales To observe the chemistry of the human body down to the molecular details of individual cells (Figure 2.13) , imaging instruments with significantly better resolving power are needed.
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Chemical Imaging Technique Involved Light microscopy (confocal, multiphoton) , X-ray microscopy, and electron microscopy are the mainstays of imaging instrumentation for biological imaging.
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, such as the phase-field model of Goldenfeld in which the dynamics are carried out at mesoscopic length scales.21,22,23 Figure 2.15 illustrates this in the case of a two-dimensional crystal. A key advantage of such a representation is that its dynamical structures can be computed so quickly that there is no need to store them; rather, one need only store the results by way of the parameterizations of the model and its parameterization for a particular material.
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at the atomic length scale to important components at mesoscopic or macroscopic length scales · Multiscaling and first-principles computational approaches to predict the structure and dynamics of materials given their atomistic or molecular composition
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Insights Obtained Using Chemical Imaging Using theoretical and computational techniques, one can identify the mesoscopic structures leading to a requisite function. Once identified, these structural motifs can be used to guide experimental chemical imaging probes.
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It is important to study how individual molecules work together, ultimately in living cells. Opportunities for Imaging Development Single-molecule imaging techniques with improved temporal and spatial resolution have to be developed.
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This movement has been observed using total internal reflection fluorescence (TIRF) spectroscopy of individual myosin molecules.
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Case Study 9: Reverse Imaging In addition to using imaging as a technique to obtain spatially and temporally patterned chemical data from a sample, one can also pattern chemical reactions in space and time using similar methods. Figure 2.18 demonstrates an example in which multiphoton scanning with ultrafast laser pulses was used to polymerize a photoresist resin with about 120 nm spatial resolution in three dimensions.
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Involved Multiphoton microscopy has been used to initiate photopolymerization of a photoresist material in three dimensions with resolution in the hundred-nanometer range. Insights Obtained Using Chemical Imaging It is clearly possible to use chemical imaging not only to observe the structure and dynamics of chemical systems, but also to manipulate them at high resolution.
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to about 300 cm1 (10 THz) .25 The radiation source may be generated from either continuous wave or short-pulsed lasers; the latter source of radiation allows TRTS studies to take place with subpicosecond temporal resolution.
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Figure 2.20 shows the general scheme of dye sensitization of TiO2. The photon energy of sunlight is not strong enough to excite an electron from the TiO2 valence band to the conduction band in bulk; as a result, the surface of the TiO2 film on a photovoltaic device is coated with a monolayer of a charge-transfer dye in order to photoexcite dye molecules that then inject
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B 106:7146-7159. electrons into the TiO2 semiconductor.28 TRTS can be used to dynamically measure the mobilized electrons on a picosecond time scale within the conduction band without being affected by the dye molecules.29 In studies of TiO2 conduction, TRTS has several advantages over fluorescence and other optical methods of spectroscopy.
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Pulsed laser excitation using pulse widths in the range of 10-100 femtoseconds has enabled the use of time-resolved terahertz spectroscopy, which is capable of capturing dynamic information at subpicosecond time scales. Insights Obtained Using Chemical Imaging Time-resolved terahertz imaging is capable of providing information about the dynamics of chemical reactions in materials science, chemistry, and biology.
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2000. Molecular organization of bis-urea substituted thiophene derivatives at the liquid/solid interface studied by scanning tunneling microscopy.
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2001. Imaging molecular and nanoscale order in conjugated polymer thin films with near-field scanning optical microscopy.
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1998. Direct observation of ultrafast electron injection from coumarin 343 in TiO2 nanoparticles by femtosecond infrared spectroscopy.
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