Images, chemically-selective

Figure Bl.14.8. Time course study of the arrival and accumulation of labelled sucrose in the stem of a castor bean seedling. The labelled tracer was chemically, selectively edited using CYCLCROP (cyclic cross polarization). The first image in the upper left comer was taken before the incubation of the seedlmg with enriched hexoses. The time given in each image represents the time elapsed between tire start of the incubation and the acquisition. The spectmm in the lower right comer of each image shows the total intensity...

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

Evans, C. L., Xu, X., Kesari, S., et al. 2007. Chemically-selective imaging of brain structmes with CARS microscopy. Opt Express 15 12076. 

Beyond imaging, the combination of CRS microscopy with spectroscopic techniques has been used to obtain the full wealth of the chemical and the physical structure information of submicron-sized samples. In the frequency domain, multiplex CRS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CRS microscopy allows the recording of the localized Raman free induction decay occurring on the femtosecond and picosecond time scales. CRS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. 

The chemical specificity of CARS microscopy is readily combined with other nonlinear optical image contrast mechanisms, such as two-photon fluorescence (TPF), SHG, and THG, resulting in a multimodal CARS microscopy [88, 118, 117, 43]. In multimodal nonlinear optical imaging, TPF, SHG, and THG signals all benefit from the use of femtosecond laser pulses of high peak intensities, whereas the contrast and chemical selectivity of CARS benefits from the use of picosecond (narrow-bandwidth) pulses (see discussion in Sect. 6.2.3). As demonstrated by Pegoraro et al. [43], this apparent... 

Some of the most dramatic STM images have been recorded for the Si(l 11) 7X7 reconstruction, as depicted in Figure 3.14.138 These images have been recorded at several different biases (see Spectroscopy and Chemical Selectivity, below) and provide one of the best examples of how STM can be used to better understand the chemistry of such surfaces ( specifically, the electrophilicity and nucleophilicity of the individual surface atoms). Clearly depicted in these results are the orbitals associated with surface atoms, rest atoms, and backbonds. Such studies have continued and been greatly extended into the exploration of a variety of chemical reactions that occur on silicon surfaces. These studies are described in detail in a recent review.135... 

To date, a number of chemically selective near-field imaging methods have been demonstrated. Near-field contrast mechanisms that rely on electronic spectroscopy (UV-visible absorption and fluorescence),204 vibrational spectroscopy (IR absorption and Raman spectroscopies), dielectric spectroscopy (microwave dispersion), and nonlinear spectroscopy (second harmonic generation) have all been demonstrated at length scales well below the diffraction limit of light. 

Fourier-Transform Infrared (FTIR) spectroscopy as well as Raman spectroscopy are well established as methods for structural analysis of compounds in solution or when adsorbed to surfaces or in any other state. Analysis of the spectra provides information of qualitative as well as of quantitative nature. Very recent developments, FTIR imaging spectroscopy as well as Raman mapping spectroscopy, provide important information leading to the development of novel materials. If applied under optical near-field conditions, these new technologies combine lateral resolution down to the size of nanoparticles with the high chemical selectivity of a FTIR or Raman spectrum. These techniques now help us obtain information on molecular order and molecular orientation and conformation [1],... 

Fig. 25. General structure of gelating terpyridine-based Pt(II) gelators described by Yam et al. TEM (left) and SEM (right) images of selected xerogels prepared from a DMSO gel (top), and a benzene gel (bottom). Reproduced with the permission of the American Chemical Society (220).

Ramanathan and Ackerman (1999) have shown that solid-state 31P NMR imaging can be used to measure quantitatively the mass of hydroxyapatite in the presence of bone hence to follow non-invasively the resorption and remodeling of calcium phosphate implants in vivo. A three-dimensional projection reconstruction technique has been used to record NMR images in the presence of a fixed amplitude field gradient, the direction of which was varied uniformly over the unit sphere. Chemical selection was achieved using differences in T1 relaxation time of neighbouring protons as the synthetic hydroxyapatite has a shorter T1 (1.8 s at 4.7 T) compared to bone (approximately 15 s at 4.7 T in vivo, 42 s ex vivo). The experimental results demonstrated that a linear relationship exists between image intensity and HAp density. 

In many instances, it is important that some form of chemical selectivity be applied in magnetic resonance imaging so as to distinguish nuclei in one or more specific molecular environment(s). There are many ways of doing this and we discuss here just three. The first option is to ensure that one of the excitation RF pulses is a narrow bandwidth, frequency selective pulse applied in the absence of any gradient [22]. Such a pulse can be made specific to one particular value of the chemical shift and thereby affects only nuclei with that chemical shift. In practice this can be a reasonable method for the specific selection of fat or oil or water in a mixed hydocarbon/water system. 

---