The band edge photoluminescence (PL) of bulk single-crystal n-CdSe is perturbed by adsorption of tri-n-octylphosphine oxide (TOPO) from toluene solution onto the crystal's 0001, 0001̄, and 112̄0 faces. These are three of the crystal faces observed in studies of CdSe nanocrystals, which are commonly capped with TOPO surfactant molecules to control their properties. At low concentration, where monomeric TOPO dominates the toluene solution composition, reversible PL enhancements are observed, indicating that the adsorbate is acting as a labile Lewis base toward the surface. However, above ∼10 mM concentration, there is an abrupt reversal in the PL signature, such that net quenching of PL is observed relative to the PL intensity in the toluene reference ambient. The PL changes at concentrations above 10 mM are not reversible with toluene rinsing, and are associated with a strongly bound species of Lewis acidic character that requires exposure to a strong base such as pyridine for desorption. XPS data are consistent with PL measurements in identifying experimental conditions associated with strong and weak binding of TOPO to CdSe. The PL changes can be fit to a dead-layer model in both the low and high concentration regimes, permitting an estimate for TOPO-induced contractions and expansions of the dead-layer thickness of about 100−300 Å, for the 0001 face, which generally yielded the largest PL changes. Equilibrium binding constants were estimated from the Langmuir adsorption isotherm model as being ∼104 M-1 and at least 102 M-1 in the low and high concentration regimes, respectively. The concentration at which the PL signature reversal occurs corresponds to incipient aggregate formation in solution based on both 31P NMR and IR spectral changes. MacroModel calculations indicate that TOPO dimer formation is energetically favorable and that a surface adduct formed from the dimer could be stabilized by multiple surface interactions.

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When initially asked to prepare this Viewpoints piece on materials science, we selected the computer for the story line because of the breadth of materials it encompasses and its profound effect upon our lives. In no way, however, were we prepared for just how rapidly the materials and technologies associated with this remarkable tool are changing. During the four months over which this article was written-from our initial outline in August 1997 until the time it was completed in December 1997-it seemed that each time we developed a draft, a new technology was announced: IBM, for example, trumpeted a shift from aluminum to copper conductors in integrated circuits and introduced new magnetic read heads based on the giant magnetoresistance phenomenon during this period. Both of these advances will lead to faster, more powerful computers. Thus, writing this article has given us a distinctly unsettling but nonetheless exhilarating sense of how rapidly materials science is moving. We will not be surprised if, by the time you read this article, some of the materials and processes described will already have been outperformed and displaced, as suggested by the logarithmic plots that dot the landscape of this article. But what we hope will be conveyed is the breakneck pace at which this field is moving, and the vast potential and excitement it holds for all of us.

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For veterinary medications administered per os, animal health companies strive to develop highly palatable dosage forms that are voluntarily accepted by animals to improve compliance and convenience. Achieving high palatability is often complex and difficult even without the presence of an active ingredient. This work compared acceptance and preference studies, as standardized methods are not established for informing formulation development or for more routine testing. Formulation development was followed by an acceptance study completed with laboratory Beagle dogs. One acceptance study and one preference study were completed in mixed breed dogs, also laboratory-housed, to gain wider representation of dog breed and age. Through these studies, we have evaluated both formulation parameters and palatability study conduct. In general, more complex palatants that have appealing taste, smell, and mouth feel enhance voluntary uptake. However, dosage forms that are too chewy may not be freely consumed even with complex palatants. The addition of aroma can entice dogs to prehend the tablet, as observed in one preference study. Preference studies in the veterinary pharmaceutical field identify the preferred first choice or first prehend, but not which product will be routinely voluntarily and fully consumed. Acceptance studies with cross-over treatment groups are used to quantify the full consumption of a dosage form when a dog is not given two choices at once. Since all dogs in acceptance studies are offered all treatment groups throughout the study, a comparison between degrees of consumption could suggest that one formulation might be preferred over another.