There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

This critical review highlights supermolecular building blocks (SBBs) in the context of their impact upon the design, synthesis, and structure of metal–organic materials (MOMs). MOMs, also known as coordination polymers, hybrid inorganic–organic materials, and metal–organic frameworks, represent an emerging class of materials that have attracted the imagination of solid-state chemists because MOMs combine unprecedented levels of porosity with a range of other functional properties that occur through the metal moiety and/or the organic ligand. First generation MOMs exploited the geometry of metal ions or secondary building units (SBUs), small metal clusters that mimic polygons, for the generation of MOMs. In this critical review we examine the recent (<5 years) adoption of much larger scale metal–organic polyhedra (MOPs) as SBBs for the construction of MOMs by highlighting how the large size and high symmetry of such SBBs can afford improved control over the topology of the resulting MOM and a new level of scale to the resulting framework (204 references).

Design and synthesis of metal-organic frameworks using metal-organic polyhedra as supermolecular building blocks.

Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.

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Crystallization by particle attachment in synthetic, biogenic, and geologic environments

Biogenic calcium carbonate forms the inorganic component of seashells, otoliths, and many marine skeletons, and its formation is directed by an ordered template of macromolecules. Classical nucleation theory considers crystal formation to occur from a critical nucleus formed by the assembly of ions from solution. Using cryotransmission electron microscopy, we found that template-directed calcium carbonate formation starts with the formation of prenucleation clusters. Their aggregation leads to the nucleation of amorphous nanoparticles in solution. These nanoparticles assemble at the template and, after reaching a critical size, develop dynamic crystalline domains, one of which is selectively stabilized by the template. Our findings have implications for template-directed mineral formation in biological as well as in synthetic systems.

The initial stages of template-controlled CaCO3 formation revealed by Cryo-TEM

Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC⇒anhydrous ACC ∼ biogenic anhydrous ACC⇒vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.

https://www.pnas.org/content/pnas/107/38/16438.full.pdf

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy.

Nanotubular materials have unique water transport and storage properties that have the potential to advance separations, catalysis, drug delivery, and environmental remediation technologies. The development of novel hybrid materials, such as metal–organic nanotubes (MONs), is of particular interest, as these materials are amenable to structural engineering strategies and may exhibit tunable properties based upon the presence of inorganic components. A novel metal–organic nanotube, (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON) (ida = iminodiacetate), that demonstrates the possibilities of these types of hybrid compounds has been synthesized via a supramolecular approach. Single-crystal X-ray diffraction of the compound revealed stacked macrocyclic arrays that contain highly ordered water molecules with structural similarities to the “ice channels” observed in single-walled carbon nanotubes. Nanoconfinement of the water molecules may be the cause of the unusual exchange properties observed for UMON, includin...

Development of Metal–Organic Nanotubes Exhibiting Low-Temperature, Reversible Exchange of Confined “Ice Channels”

Carbon-based fullerenes have received considerable attention since their discovery and are currently being produced on an industrial scale. Fullerenes are cage structures composed of 12 pentagons and several hexagons. Many fullerene topologies are possible, but selection criteria favor a small subset. C60 is the classic and most stable fullerene. [1] No smaller fullerene can be built without its topology containing adjacent pentagons, which destabilize the structure through increased curvature. For fullerenes with less than 60 C atoms, it is thought that those with the fewest adjacent pentagons will be the most stable because they minimize strain. 5] Fullerenes with less than 60 C atoms exist, for example C50, which can be stabilized as C50Cl10 by addition of Cl ligands. This structure does follow the minimal pentagon adjacency rule, but theoretical calculations suggest that pure C50 will not because sphericity is substantially increased when the number of adjacent pentagons is increased to six, as compared to five in the case of C50Cl10. [7] There is also considerable interest in inorganic fullerenelike materials; particular emphasis is on compounds such as MoS2 that tend to form onion-skin-like structures with useful materials properties. Molybdenum–oxygen heteropolyhedra have previously been found to form clusters with fullerene topologies, termed keplerates. Our earlier discovery of spherical clusters of uranyl peroxide polyhedra containing 24, 28, or 32 hexagonal bipyramids suggests that large clusters of metal–oxygen isopolyhedra with fullerene topologies may be synthesized. Of the three uranyl peroxide clusters described earlier, the 28-metal-ion cluster U28 has a fullerene topology with 12 pentagons and 4 hexagons, whereas the other two also contain squares in their topologies. We propose that assembly of metal–oxygen isopolyhedra into nanoscale fullerene topologies may be facilitated by judicious selection of structural building units. The rules that govern the formation of metal–oxygen cages are of interest, as such materials could have a variety of applications, including catalysis and synthesis of advanced materials. Assembly of metal–oxygen isopolyhedra into conventional fullerene topologies can only occur if at least the following conditions are met: 1) Each polyhedron must link to exactly three other polyhedra, and the most stable structures will occur when the connections between the metal–oxygen polyhedra are by the sharing of polyhedral edges. 2) The polyhedra must be geometrically compatible with forming topological pentagons and hexagons. 3) The three linkages emanating from any given polyhedron should be approximately coplanar to facilitate the cage geometry. 4) Linkages between polyhedra should be consistent with the bond-valence requirements of the shared polyhedral elements within the cage.

Metal–Oxygen Isopolyhedra Assembled into Fullerene Topologies

The radius of curvature of gold (Au) nanostar tips but not the overall particle dimensions can be used for understanding the large and quantitative surface-enhanced Raman scattering (SERS) signal of the uranyl (UO2)2+ moiety. The engineered roughness of the Au nanostar architecture and the distance between the gold surface and uranyl cations are promoted using carboxylic acid terminated alkanethiols containing 2, 5, and 10 methylene groups. By systematically varying the self-assembled monolayer (SAM) thickness with these molecules, the localized surface plasmon resonance (LSPR) spectral properties are used to quantify the SAM layer thickness and to promote uranyl coordination to the Au nanostars in neutral aqueous solutions. Successful uranyl detection is demonstrated for all three functionalized Au nanostar samples as indicated by enhanced signals and red-shifts in the symmetric U(VI)–O stretch. Quantitative uranyl detection is achieved by evaluating the integrated area of these bands in the uranyl fingerprint window. By varying the concentration of uranyl, similar free energies of adsorption are observed for the three carboxylic acid terminated functionalized Au nanostar samples indicating similar coordination to uranyl, but the SERS signals scale inversely with the alkanethiol layer thickness. This distance dependence follows previously established models assuming that roughness features associated with the radius of curvature of the tips are considered. These results indicate that SERS signals using functionalized Au nanostar substrates can provide quantitative detection of small molecules and that the tip architecture plays an important role in understanding the resulting SERS intensities.

Tori Z. Forbes

Papers

Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy.

Development of Metal–Organic Nanotubes Exhibiting Low-Temperature, Reversible Exchange of Confined “Ice Channels”

Metal–Oxygen Isopolyhedra Assembled into Fullerene Topologies

SERS detection of uranyl using functionalized gold nanostars promoted by nanoparticle shape and size.