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1、Synthesis of Core-Shell Surface-Enhanced Raman Tags for BioimagingXiangjiang Liu, Maria Knauer, Natalia P. Ivleva, Reinhard Niessner, and Christoph Haisch*Chair for Analytical Chemistry, Technische Universita t Mu nchen, Marchioninistrasse 17, D 81377 Munich, GermanyWe present a rapid and straightfo
2、rward procedure for the synthesis of core-shell surface-enhanced Raman scatter- ing (SERS) tags. SERS tags are combinations of Raman active dyes with metallic nanoparticles inducing theenhancement. Because of their significantly higher stabil- ity to environmental conditions, the majority of SERS ta
3、gs are designed in core-shell geometry. The main disad- vantage of this approach up to now was the time-consum- ing (normally days) and complex preparation of these labeled core-shell particles. The presented approach can be performed within hours on a daily base. The main characteristics and the st
4、ability of the fabricated SERS tags in extreme pH values, high ion strength, or in organic solvent is demonstrated in this communication. Further- more, the SERS tags are functionalized with anti-Salmo- nellaantibody as a model to present a potential applica- tion of the core-shell particles in bioi
5、maging.Surface-enhanced Raman scattering (SERS) holds vast potentialas a highly sensitive and selective tool for the identification of biological or chemical analytes. The fabrication of the SERS substrates or SERS tags is generally considered one of the most critical aspects of a SERS experiment. A
6、s one of the two pillars of a SERS experiment, the SERS tag is a labeled nanoparticle, usually consisting of gold or silver. Combined with resonant Raman reporter molecules, SERS tags can provide 106-1014-fold en- hancement of the Raman scattering intensity, which can evenbe sufficient for single-mo
7、lecule detection.1-3Compared to other tags, such as plasmon-resonant particles (PRPs), quantum dots(QDs), and fluorescent dyes, a unique advantage of SERS tags isthat they are highly specific. Hence, a wide range of different SERS tags can be used simultaneously in single detection systems. Most SER
8、S tags developed recently are based on two principal approaches. One is the direct attachment of Raman reporter molecules and recognition elements on silver or gold nanopar-ticles. The first works were presented by Cao et al.4The SERS tags they prepared enabled multiplex DNA and RNA detection. One l
9、imitation of this approach is the instability of the SERS tags caused by the direct exposure of tag surface to the environment.The second, improved approach is to cover the surface of nanoparticles with a layer of silica or polymers, in which the Raman reporter molecules are embedded (so-called core
10、-shellSERS tags). The first attempt was made by Doering et al.5and Mulvaney et al.6The silica shells protect the SERS tags from aggregation and facilitate functionalization. Even some more complex multilayer core-shell SERS tags have been developed recently through a layer-by-layer deposition proced
11、ure.7-9Gener- ally, this procedure can improve the stability and other optical properties of SERS tags. However, this improvement comes at the cost of preparation time: obviously, the more complex the structure is, the longer preparation time it needs (usually days or more), the more chemicals it co
12、nsumes, and the harder the process is to handle.6Especially cumbersome, a vitrophilic (“affine to silica”) pretreatment needs to be performed usually on silveror gold nanoparticles to make their surfaces affine to silica. Merely this process can take days; so, a more straightforward and simpler synt
13、hesis method is highly desired. Here, we describe a fast, daily use synthesis method of core-shell SERS tags. The whole process takes less than 2 h, and there is no need for vitrophilic pretreatment. Both Ag and Au nanoparticles can be applied to fabricate the tags. Moreover, the choice of Raman rep
14、orter molecules is broad. Organic dyes with an isothiocyanate (sNdCdS) group or with multiple sulfur atoms can be employed, since they bind strongly on the core particles with silica encapsulation. The core particles can be directly coated by a sol-gel procedure after the absorption of the reporter
15、molecules. Although the initial purpose of this coating method was to cover Au nanoparticles with conformal silica shells,10we found it effective also in coating Ag nanoparticles. The coating method involves two steps: the generation of silica sols with base-catalyzed hydrolysis of tetraethyl orthos
16、ilicate (TEOS), followed by nucleation and condensation of these sols onto the surface of tagged core particles. The stability of the fabricated SERS tags under extreme pH values, high ion strength, or in organic solvent was investigated.* To whom correspondence should be addressed. Christoph Haisch: phone, +49 89 2180 78242; fax, +49 89 2180 99 78242; e-mail, Christoph.Haisch ch.tum.de. (1) Nie, S. M.; Emory, S. R. Science 1997, 275, 11021106. (2) Kneipp, K.; Kneipp, I. I.; Dasari, R. R.; F
