Something to Bragg about Sanford A. Asher and his co-workers at the University of Pittsburgh have developed a Bragg-diffracting material that can be switched on and off in a matter of nanoseconds. The material is a polymerized crystalline colloidal array (PCCA) consisting of dyed, highly charged poly(lH, lH-heptafluorobutyl methacrylate) particles. The array does not diffract light of low incident intensity but irradiating the srrsy with 3. tiiffhintensity pump beam within the absorption band of the dye molecules causes the particles to rapidly heat up and their refractive index to decrease meeting the conditions for Braee diffraction of a probe beam A 1.6-cm circular PCCA disk was used to diffract the 594-nm beam from a dye laser. A 532-nm beam from a YAG laser was used to pump the dye laser and to switch on the Bragg diffraction of the PCCA by heating the dye molecules. The ratio of the diffracted beam intensities, R„n (with the pump beam on)/R„ff (with the pump beam off), was measured and used to characterize the device. For an array with a refractive index lower than that of the hydrogel
Understanding metal nanoparticles Metal and semiconductor nanoparticles have garnered much attention because of the hope that they will be suitable for optoelectronic nanodevices and chemical sensors. One class of these nanoparticles is alkanethiolate monolayerprotected metal clusters (MPCs). Gold MPCs are particularly stable and can be prepared in sizes ranging from 1.1 to 5 nm. These MPCs have been shown to exhibit an electrochemical "ensemble Coulomb staircase". Royce W. Murray, Robert L. Whetten, and co-workers at the University of North Carolina at Chapel Hill and the Georgia Institute of Technology studied this Coulomb staircase phenomenon by varying the size of gold MPCs in solution from 8 to 38 kDa. They found that larger MPCs displayed a double-layer capacitive charging, whereas the smaller MPCs demonstrated molecular redoxlike behavior. For larger gold cores the peak spacing in normalized differential pulse vol-
matrix, the diffraction efficiency increased as the incident pump energy increased. When the refractive index of the array was higher than that of the medium, the opposite trend was observed. Calculations indicate that for this type of colloidal particle, the maximum diffraction efficiency should occur for a particle volume fraction of 40.9%.
(A) Optical extinction spectrum of 138 nm diameter dyed
Up to a threshold poly(1H, 1H-heptafluorobutyl methacrylate) PCCA in water. pump beam intensity Peak 1 is from the absorption of the dye. Peak 2 is the PCCA (which depends on Bragg diffraction band. (B) Extinction of dyed PCCA as a things such as the beam function of medium refractive index. size and the particle absorption cross section), the PCCA response is reproducible with no Theoretical calculations indicate that a evidence of degradation. Above the threshperfect PCCA lattice should have a diffracold value, the PCCA is irreversibly damaged tion efficiency of 97%. However, the present and the diffraction efficiency decreases. The device switches only 2% of the probe beam. authors attribute this to damage to particles The researchers attribute this to disorder in themselves because damage is also obthe PCCA lattice, which they believe can be served at high incident intensities in a liquid eliminated with better PCCA films. (J. Am. dispersion of the particles Chem. Soc. 1998,120,6525-30)
tammograms (DPVs) corresponded to 3 or 4 uF/cm 2 , which is similar to the capacitance of macroscopic monolayercoated gold surfaces and of polydisperse MPC solutions. In contrast, the DPVs of smaller MPCs (8 and 14 kDa) had much larger central spacings and resembled the electrochemical charging of an electroactive molecule. The trends with decreasing core size are enhanced strength of optical transitions in the near-IR the appearance of bandlike spectral features, and an increasing gap energy. Double potential step chronocoulometry showed that the DPV peaks corresponded to single-electron transfers The authors suggest that because the staircase behavior is closely related to the MPC core electronic energy structure, it may help in the understanding of other nanoscale properties. They also suspect that, despite the differences in fundamental properties of metal-like and molecule-like charging behaviors, the electrochemical, thermodynamic, and kinetic properties of the MPCs will fit within a common formal representation. (Science 1998 280,2098-101)
Finding viral mutants Identifying mutant viruses requires sequencing the organism's genome to determine the mutation. That process becomes much easier in well-characterized organisms if there is some indication prior to sequencing of where the mutation exists. Roger N. Beachy, Gary Siuzdak, and colleagues at the Scripps Research Institute and Purdue University identified mutant viruses using mass mapping of structural viral proteins to narrow the region of the genome to be searched. Their studies compared wild-type with mutant forms of tobacco mosaic virus (TMV) and human rhinovirus 14 (HRV14). The procedure put the virus first through a tryptic digest followed by MALDI time-offlight MS. Close comparisons of mass spectra of the digests of wild-type and mutant viruses reveal small changes that are characteristic of the mutation. For example, HRV14 and mutant HRV14-Cysll99Tyr had most ion signals in common with the exception of a peak at 4700.5 Da, which was shifted by 83 Da in the mutant; this is possible only with a Cys->Tyr amino acid change. The mutation was linked to the VP1 protein a
