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ANALYTICAL CURRENTS
Go for the gold During surface-enhanced Raman spectroscopy (SERS), the Raman signals of molecules attached to metal surfaces are increased manyfold. Multiple factors, including electromagnetic and chemical effects, contribute to the overall enhancement. In the past, total enhancement factors on the order of 1014 have been observed in SERS experiments with near-IR excitation on colloidal silver particles. Now, Katrin Kneipp and co-workers at the Massachusetts Institute of Technology and Technical University Berlin (Germany) have found similar enhancement factors using colloidal gold as a SERS-active substrate. Solutions containing gold as single colloidal particles about 60 nm in diameter were used as the SERS-active substrate. The addition of NaCl induced aggregation of the gold into colloidal clusters. Crystal violet served as the target molecule. On the basis of its absorption spectrum, molecular resonance should contribute to the overall enhancement at 514-nm excitation but not at 830 nm.
Enhancement factors were calculated in two ways—by comparing the SERS spectrum to the 1030-cnr1 methanol normal Raman band and by a method based on
Electron micrographs of typical colloidal gold particles in colloidal solution, including (a) isolated particles, (b) aggregates after addition of NaCl, and (c) and (d) typical colloidal gold clusters exhibiting extremely strong SERS enhancement. (Adapted with permission. Copyright 1998 Society for Applied Spectroscopy.)
Subattomole detection of catecholamines To investigate communication between neurons, researchers are trying to determine what is inside individual secretory vesicles—the "packages" that carry neurotransmitters. But detecting neurotransmitters is difficult because a vesicle may contain only 103—106 molecules. For neurotransmitters such as serotonin, the attomole detection limit has been approached using CE and UVexcited fluorescence detection, but the technique is several orders of magnitude less sensitive for measuring catecholamines. Now, Jason B. Shear and Michael L. Gostkowski at the University of TexasAustin have achieved attomole detection for two catecholamines using CE fractionation and two-photon excitation (2PE) from a titanium:sapphire laser instead. The mass detection limits of
