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Concentrating nanoparticles Shuming Nie and Steven Emory of Indiana University recently discovered a new class of silver nanoparticles that improves the efficiencies of surface optical processes (such as surface-enhanced Raman spectroscopy, or SERS) by 14-15 orders of magnitude. Unfortunately, these optically active particles fall into a narrow size range of 90-100 nm and make up only one of every 100-1000 colloidal particles. A method is needed to enrich the population. Nie and Emory use size-selective fractionation with track-etched polycarbonate membranes to separate the heterogeneous colloid into different size fractions. The particles were divided into four fractions based roughly on size. Because the silver particles are irregularly shaped nanocrystals, the sizes are best described by the average value of the long and short axes. The short axial dimension determines whether a particle goes through a membrane. The authors occasionally observed rod-shaped particles as long as 500 nm in the
U V - a n d IR-MALDI go head to head Which laser wavelength is better for MALDI (matrix-assisted laser desorption/ionization) MS—UV or IR? Brian Chait and his co-workers at Rockefeller University don't actually answer that question, but they report the first headto-head comparison of the two techniques on the same sample surface. The Rockefeller group performed the MALDI experiments on a linear time-offlight laser desorption mass spectrometer that had been modified so that the samples could be irradiated by two alternative laser sources Cin this case, a frequencytripled Nd:YAG operating at 355 nm and an Nd:YAG-pumped optical parametric oscillator that was continuously tunable from 1.45 to 4.0 urn). It was possible to switch from one source to the other in
80-100 nm fraction, in which most of the SERS-active particles were concentrated. Even when the optically active particles were separated from other fractions, they repre-
sented only 10-15% of the particles, and they exhibited large variations in signal intensities. Transmission electron microscopy revealed that the optically active particles have a distinctly faceted shape, but it is unclear how the faceted shapes are related to surface-active sites. To make sure that the signals were from Raman scattering, they studied the wavelength-resolved spectra of rhoda mine 6G (R6G, a fluorescent dye) and 3-hydroxykynurenine (a nonfluorescent biomolecule). Although resonance enhancement contributed to the R6G spectrum at 514.5 nm, the large enhancement factors observed with near-IR excitation indicate that surface enhancement is the dominant effect. Other size-selective separation techniques, such as size-exclusion chromatography, could result in enrichment beyond the 15% demonstrated, but the authors doubt that those methods could achieve complete separation of the active colloids. They suggest that synthetic routes that produce more uniform colloidal particles must be developed. (J. Phys. Chem. B1998,102,493-97)
~ 1 min. The comparison between UVMALDI and IR-MALDI was facilitated by choosing matrices that produce highquality mass spectra of proteins in both the UV and the IR, such as 4-hydroxya-cyanocinnamic acid (4HCCA) and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapic acid). The mass spectra taken at 2.94 pun and 355 nm of equine cytochrome c in a 4HCCA matrix and of a mixture containing bovine ubiquitin and cytochrome c in a sinapic acid matrix appeared remarkably similar. On closer inspection, however, differences in the mass spectra were apparent. Die UVMALDI spectra of the protein mixture contain peaks corresponding to photoproducts. The lack of such photoproducts in the IRMALDI spectra is expected because the photon energy is too low. The peaks in the IR-MALDI mass spectra were broader than those in the UV-MALDI;
thus, a larger error in the molecular masses was calculated from the IRMALDI spectra. In addition, IR-MALDI provided intense spectra of higher mass proteins, such as bovine transferrin (~ 78 kDa) and human IgG (~ 150 kDa) that were also similar to the UV-MALDI spectra. IR-MALDI caused considerably less fragmentation of the larger proteins than did UV-MALDI. The authors speculate that the similarity between the UV- and IR-MALDI mass spectra means that the mechanisms of desorption and ionization may be the same, which would be more controversial for the ionization mechanism than for the desorption. These findings may mean that electronic excitation does not play a primary role in the UVMALDI ionization process, as previously was thought. (J. Am. Sue. Mass Sheetrom. 1998 9,1-7)
Enrichment of optically active nanoparticles by size-selective filtration. (A) Schematic of the filtration apparatus', (B) collection and resuspension of the fractionated colloidal particles.
Analytical Chemistry News & Features, March 1, 1998 171 A
