Connecting the dots Michael A. Boles and Dmitri V. Talapin Science 344, 1340 (2014); DOI: 10.1126/science.1256197

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Connecting the dots Understanding nanocrystal surfaces helps to direct their assembly into novel material architectures By Michael A. Boles and Dmitri V. Talapin


anocrystals—nanometer-sized inorganic crystals containing hundreds to thousands of atoms—offer exciting opportunities in areas as varied as photonics, catalysis, biotechnology, and medicine. For example, nanocrystals enrich the display color palette in tablet computers and advanced TVs. Two reports in this issue, by Zherebetskyy et al. on page 1380 (1) and Boneschanscher et al. on page 1377 (2), investigate nanocrystal surfaces and show how these surfaces can be merged in a controllable fashion, allowing the assembly of novel material architectures. The ability to control matter at the nanometer scale was spurred by the discovery of advanced solution-based synthesis techniques in the 1990s (3). For example, lead sulfide (PbS) nanocrystals may be synthesized by reacting simple molecular species, such as lead acetate and hydrogen sulfide, in the presence of surfactant molecules such as oleic acid. The surfactant molecules bind to the nanocrystal surface, decorating the nanocrystals with a corona of organic chains (see the figure, panel A). It remains unclear, however, how the surfactant molecules bind to different nanocrystal facets. Zherebetskyy et al. use ab initio density functional theory (DFT) to study the facets of PbS nanocrystals, which have an inorganic core structure identical to that of common table salt. These nanocrystals, used as building blocks for solar cells, photodetectors, and transistors, mainly display two types of surfaces. The (001) surface has a checkerboard arrangement of alternating Pb2+ cations and S2⫺ anions, whereas the (111) surface presents a dense hexagonal arrangement of Pb2⫹ or S2⫺. Each nanocrystal has six (001) and eight (111) facets. How does oleic acid interact with these distinct surface patterns? And how does the nanocrystal preserve overall charge neutrality? Investigation of ligands on nanocrystal surfaces is conceptually related to the more established field of self-assembled monolayers on planar crystal surfaces (4). NanoDepartment of Chemistry and James Franck Institute, University of Chicago, Chicago, IL 60637, USA. E-mail: [email protected]


crystals add another element of complexity: They expose different types of crystallographic facets and have a high concentration of corner and edge sites. Zherebetskyy et al.’s calculations suggest that (001) surfaces are most stable when oleic acid binds as a neutral molecule. It is rather surprising that oleic acid does not lose a proton to form a covalent bond with lead ions, as it does in lead oleate and other salts. The results for the (111) surfaces are even more intriguing. Elemental analysis of PbS nanocrystals gives a cation-to-anion ratio of 1.2 to 1.4, with smaller particles having higher lead content (5); this suggests that all (111) surfaces are completely covered with Pb2⫹ (see the figure, panel A). However, oleic acid molecules are too bulky to bind all atoms on lead-terminated (111) surfaces. Zherebetskyy et al. propose a solution: Small OH⫺ anions sneak to the

nanocrystal surface and bind to sites not accessible to large surfactant molecules. The hydroxide anion, they suggest, is small enough to be interspersed in equal abundance with oleate (deprotonated oleic acid) ligands, providing charge compensation and eliminating broken bonds. Such small surface-bound anions tucked beneath large fatty acid ligands may be a general feature of most nanocrystal surfaces. Ligand molecules bound to the nanocrystal surface reduce surface energy. To fully eliminate energetically expensive surfaces, nanocrystals may align their atomic lattices and fuse into a single crystal through merging of identical facets. This “oriented attachment” (6) process allows nanocrystals to be used as building blocks to construct nanowires (7), crystalline rings (8), or twodimensional sheets (9). Boneschanscher et al. use this process to assemble lead selenide (PbSe) nanocrystals into a honeycomb structure similar to silicene (10), silicon’s graphene analog. By evaporating a solution of PbSe nanocrystals capped with oleic acid over a liquid ethylene glycol surface, the authors coaxed the particles to arrange themselves into an ordered superlattice. Upon gentle heating, the PbSe cores fused into a honeycomb architecture (see the figure, panel B). Electron




Pb Pb Pb Pb PbS(111) Strongly bound


PbS(001) Weakly bound


Ligand desorption from (001)

Oriented attachment

Directed assembly.(A) The theoretical and experimental investigation by Zherebetskyy et al. expands on the basic picture of ligand binding (center) by showing that lead sulfide nanocrystals coated with oleic acid have cationterminated (111) facets stabilized by strongly bound surface hydroxide groups and deprotonated oleate ligands (left). PbS (001) surfaces, on the other hand, are passivated by weakly bound oleic acid chains and may be left unprotected after ligand desorption (right). (B) Boneschanscher et al. show that selective removal of (001)-bound ligands and fusing of nanocrystal cores yields two-dimensional nanostructures through oriented attachment of nanocrystal facets. SCIENCE

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microscopy and modeling revealed oriented attachment through (001) facets. The honeycomb superlattices resemble atomic twodimensional crystals such as graphene or molybdenum disulfide and are remarkably robust: Lead ions may be exchanged for cadmium without disrupting the selenium anion lattice. How can such a complex structure form through attachment of nanocrystals that were originally separated by surface ligands? Given the similarity between PbSe and PbS, we can draw inspiration from the theoretical work of Zherebetskyy et al. For PbS nanocrystals, the binding energy of oleic acid to the (001) facet is too low to keep surfactant molecules in place for a long period of time. Binding of oleate/hydroxyl pairs to the (111) surface is much stronger. A simple calculation based on reported binding energies suggests that surfactants are less likely to desorb from a (111) surface than from a (001) surface by a factor of ~106. Furthermore, the ethylene glycol surface used by Boneschanscher et al. may not only act as the stage on which self-assembly takes place, but may also gently remove loosely bound (001) ligands from PbSe nanocrystals. In contrast, (111) surfaces remain protected by the oleate and hydroxyl ligands. Bare (001) facets become “sticky patches,” guiding the attachment of PbSe nanocrystals into buckled silicene-like two-dimensional sheets that may have exotic electronic properties (11). The work of Zherebetskyy et al. offers a new understanding of the nanocrystal surface and lays the foundation for targeted design of unusual nanocrystal architectures like that presented by Boneschanscher et al. Thoughtful use of surface ligands that bind strongly to some crystal facets and weakly to others may enable the construction of novel materials by merging crystalline particles through engineered surface patches (12). The first chapter of colloidal nanocrystal research was written by those who managed to tame the surface; the next challenge is to put it to work. ■



1. D. Zherebetskyy et al., Science 344, 1380 (2014). 2. M. P. Boneschanscher et al, Science 344, 1377 (2014). 3. C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc. 115, 8706 (1993). 4. J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides, Chem. Rev. 105, 1103 (2005). 5. I. Moreels et al., ACS Nano 3, 3023 (2009). 6. R. L. Penn, J. F. Banfield, Science 281, 969 (1998). 7. Z. Tang, N. A. Kotov, M. Giersig, Science 297, 237 (2002). 8. K. S. Cho, D. V. Talapin, W. Gaschler, C. B. Murray, J. Am. Chem. Soc. 127, 7140 (2005). 9. C. Schliehe et al., Science 329, 550 (2010). 10. P. Vogt et al., Phys. Rev. Lett. 108, 155501 (2012). 11. E. Kalesaki et al., Phys. Rev. X 4, 011010 (2014). 12. Z. Zhang, S. C. Glotzer, Nano Lett. 4, 1407 (2004). 10.1126/science.1256197


A cell death avenue evolved from a life-saving path Altruistic cell suicide model is challenged By Harm H. Kampinga


east metacaspases are the ancestral enzymes of caspases that execute cellular suicide (“programmed cell death”) in multicellular organisms. Studies on metacaspase 1 (Mca1) have suggested that single-cell eukaryotes can also commit programmed cell death (1, 2). However, on page 1389 of this issue,

Malmgren Hill et al. (3) show that Mca1 has positive rather than negative effects on the life span of the budding yeast Saccharomyces cerevisiae, especially when protein homeostasis is impaired. Mca1 helps to degrade misfolded proteins that accumulate during aging or that are generated by acute stress, and thereby ensures the continuous and healthy generation of daughter cells that are free of insoluble aggregates that otherwise would limit life span. Loss of Mca1 activity has been associated with a reduced appearance of programmed cell death markers (1, 4), implying that its overexpression should decrease the replicative life span of yeast (the number of daughter cells a mother cell can produce throughout its life). Cells lacking Mca1 have increased amounts of protein aggregates and oxidized proteins (4, 5). Malmgren Hill et al.


not only show that this is related to decreased survival, but also provide mechanistic insights into the mode of action of Mca1. Its pro-life action depends on the chaperone heat shock protein 104 (Hsp104), a protein that can disentangle protein aggregates and is crucial for the asymmetric segregation of protein aggregates in dividing cells. Mca1 deficiency does not affect life span of wildtype strains, but further decreases life span

in strains already compromised in protein quality control. In particular, replicative aging is accelerated in strains lacking the Hsp70 co-chaperone Ydj1. Mca1 does not improve protein folding but supports degradation of terminally misfolded proteins. Malmgren Hill et al. show that Mca1 requires proteasomes (protein structures that break down proteins) for all its effects. The study by Malmgren Hill et al. challenges the idea that caspases are activated as an altruistic suicide mechanism in single-cell eukaryotes as a means to provide nutrients for younger and fitter cells in the population (2). Rather, the data sugDepartment of Cell Biology, University Medical Center Groningen, University of Groningen, Groningen, Netherlands. E-mail: [email protected] 20 JUNE 2014 • VOL 344 ISSUE 6190

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