Other Points and Future Directions

In terms of the study of protein at interfaces, the interaction between water molecules is one of the most important points. However, it is difficult to consider the influence of water accurately because water is a complex and poorly understood liquid. Souda (2004a,b,c), Kawanowa et al. (2004), and Gunster and Souda (2005) have reported the study of hydration of organic substances on the water-ice surface using temperature-programmed TOF-SIMS. This technique will contribute to aid protein study in future.

In order to analyze complicated biomaterials, for example living materi-alssuchasinternalorgansandtissuesaswellasmultifunctionalbiosensors, new TOF-SIMS techniques will be needed for the measurement of biopolymers. Among the innovations needed will be: (1) enhancement of the level of ionization of important secondary ions for the characterization and imaging of samples, and (2) production of intact or larger-fragment ions from biopolymers to allow the characterization and distinction of each protein and polymer. The enhancement of cationization by alkaline metals such as silver and gold (Bourdos et al. 2000; Wojciechowski et al. 2001) has been used for SIMS techniques and should also prove effective for protein measurement. Postionization by laser-secondary neutral mass spectrometry (SNMS; Dambach et al. 2004; Schnieders and Benninghoven 2000) and matrix-enhancement SIMS (Delcorte et al. 2004; Malyarenko et al. 2004; Wu and Odom 1996), based on matrix-assisted-laser-desorption/ionization-like sample preparation, are both expected to help provide more useful information.

Currently, cluster primary ions (Kollmer 2004; Thompson et al. 2004) such as C60+, Au3+ and Bi3+, which produce larger-fragment ions from proteins, are employed in the analysis of biomaterials (Xu et al. 2004). In particular, the C60+ ion source produces much larger fragment ions from macromolecules, such as proteins with measured intensities of up to 10,000 times greater than those obtained using a Ga+ ion source (Postawa et al. 2003). In terms of spatial resolution, however, the C60+ ion source does not have the submicron-scale but rather one of approximately several microns in size, and the Au+ ion source has the same scale as that of the Ga+ ion source (Kingshott et al. 2002; i. e., a submicron-scale spatial resolution), although its sensitivity for large fragment ions is not as high as the C60+ ion source.

Moreover, with integrating SIMS techniques including conventional techniques and novel ones, the protein depth profile, which provides information about the total chemical structure of a protein molecule immobilized on a device, will be developed in the near future

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