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During the recent years fluorescence spectroscopy applications focused on using longer wavelength fluorophores and new classes of vehicles delivering fluorophores such as nanoparticles. Silica nanoparticles synthesis is relatively straight forward using any of the well established methods (Strober, Reverse Emulsion, etc.,). The basis of these methods is the use of TEOS to form the silica nanoparticle. In addition to TEOS a wide range of modified TEOS is available commercially or can be synthesized using published synthetic schemes. Many different moieties, molecules can be used as substitution in the TEOS molecules and subsequently using these substituted analogs during the polymerization process of the silica nanoparticle. Following this process virtually any new property can be introduced in the silica nanoparticle. One of these is fluorescence using a wide range of fluorophores including visible or near-infrared dyes due to the wide availability of modified TEOS reactive analogues and reactive dyes. Silica nanoparticles containing copolymerized molecules can be utilized as fluorescence sensors if the copolymerized dye has sensing moieties, such as –COOH for pH measurements. One drawback of the direct facile synthesis using one or more modified TEOS in the same reaction is that the whole silica nanoparticle will have the property as determined by the modified TEOS. For example in the case of dyes containing functional moieties that can complex metal ions dye molecules will be located on the surface and the inner structure as well. For many biomedical and bioanalytical applications it may be desired the silica nanoparticle to have different surface and inner core properties. This can be achieved by using surface re-growth technique. For example the surface of the silica nanoparticle can be made very hydrophobic or hydrophilic opposite of the inner core; two different dyes can be present in these regions or make the silica nanoparticle surface to contain optically active enantiomers. This presentation compares the use of longer wavelength fluorophores to shorter wavelength counterparts. Surface properties can be modified by utilizing molecules that are themselves fluorescent, such as modifying surface hydrophobicity using hydrophobic surface dyes or non-hydrophobic dyes in the core with adding surface hydrophobicity with non-fluorescent hydrophobic molecules. Several additional surface modification moieties will be discussed along with their applications.
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