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ANALYTICAL CURRENTS
SLEDs in a fluorimeter
Nanotubes as SPM probe tips
Superluminescent light-emitting diodes In slightly more than a decade, fullerenes (SLEDs) can provide power in the milliwatt have moved from "buckyballs" to nanorange and a spectral bandwidth of more wires and nanotubes. Some of these unique than 100 nm, making them more powerful molecules could provide well-defined tips and more tunable than conventional lightfor scanning probe microscopy (SPM). emitting diodes but still relatively inexpen- J. A. Harrison and co-workers at the U.S. sive. SLEDs were originally available for Naval Academy, IUPUI University, and the the red and IR regions of the spectrum, but University of Oxford (U.K.) use molecular SLEDs for the blue region have recently dynamics simulations of interactions bebeen developed. Chieu D. Iran and cotween nanotubes and a hydrogen-termiworkers at Marquette University combined nated diamond surface to explore the use SLEDs with acoustooptic tunable filters of capped single-walled nanotubes as SPM (AOTFs) to develop a compact and inextips. pensive fluorimeter that they use to analyze In the simulation, a [10,10] nanotube chlorophylls ci and b was capped with a C5v hemifullerene dome with a single pentagon centered on a fiveThe fluorimeter was constructed with fold symmetry axis at the vertex. The nanotwo SLEDs that were used in a countertube, which contained 850 carbon atoms, propagation configuration to further inwas approximately 53 A from base to tip. crease the excitation intensity. Fluorescence emitted from the sample was foThe simulated force trace shows distinct cused onto a noncollinear Te0 2 AOTF, regions in which the force increases linearly which was placed between two cross-axis as the tube is pushed toward the surface. polarizers to block the undiffracted light. However, these regions are separated by The chlorophyll solutions were prepared sharp discontinuities, which mark distinct changes in the tube's conformation. Three of with surfactants to prevent the chlorophyll the conformational changes occur as the molecules from aggregating. The two chlorophyll species could be determined simulta- tube is brought toward the diamond surface, neously, but because the shape and intensity and the other three occur during retraction. of the fluorescence vary with changes in the During thefirstchange, the 20 atoms at the end of the cap "pop" approximately 1.0 A into relative chlorophyll concentrations, partialthe tube. By the time of the cap inversion, least-squares analysis was required. The . the tube has been compressed 2.5 A A secresearchers could obtain detection limits of ond inversion occurs at a compression of 3.5 2.30 x 10~9 and 1.10 x 10"9 M ffo rhlorophylls a and b, respectively. .Appl. Spectrosc.A, but additional stresses beyond that are 1997,51,1603-06)
released by "finning", in which opposing walls of the nanotube collapse inward to form an elliptical cross section. The conformational changes are completely reversible and demonstrate that the nanotubes can recover when pushed onto a hard substrate, such as diamond, with a force as great as 43 nN. The information provided by such simulations should help in the quest to use carbon nanotubes as SPM tips by providing the physical parameters to make nanotubes useful probes of surface structure. (J. Phys. Chem. B 1997, 101, 9682-85)
A snapshot of the molecular dynamics simulation of the interaction of a capped [10,10] nanotube with a hydrogen-terminated diamond (111) surface.
tubular structures or ion channels, which become embedded in the gold monolayer. The orientation of the nanotube assemDynamic sensors, which depend on adsorp- blies was found to depend on the sample tion or diffusional proceeses, are capable oo preparation and the nature of the organodetecting analytes in rapid real time. M. Reza sulfur compounds assembled on the gold surface. Two SAMs containing dodecyl Ghadiri and Kianoush Motesharei of The thiols and thioethers were compared to Scripps Research Institute have designed a determine which was more effective in new diffusion-limited size-selective ion senforming channeled monolayers. In addisor based on a self-assembled monolayer (SAM) of organosulfur on gold. A cycllc pep- tion, two procedures were followed for incorporating the peptides into the monotide wiih allernattng D-leucine aan d-tryptoo layer systems. In the stepwise method, orphan amino acid residues and a flat ringganosulfur monolayers are formed onto shaped conformation self-assembles into
Self-assembled peptide nanotubes
Schematic of the SLEDs-AOTF fluorimeter. (Adapted with permission. Copyright 1997 Society for Applied Spectroscopy.)
Analytical Chemistry News & Features, February 1, 1998 87 A
