Our paper presents a new approach to the facile synthesis of metallic nanostructures with hollow interiors. Typical examples include: nanoshells, nanoboxes, nanotubes, and nanocages. Due to their relatively thin walls and high surface areas, these nanostructures are superior to their solid counterparts in terms of their tuning range of photonic properties, mechanical strength, catalytic performance, and sensitivity to environmental changes. In the past, these structures could only be investigated theoretically. The availability of such nanostructures as monodispersed samples, and in copious quantities, has enabled many research groups to experimentally explore their new properties, as well as exploring a range of intriguing applications. For this reason, our paper has been highly cited in the past year.

Yes, our paper describes a conceptually new methodology for the large-scale synthesis of hollow nanostructures. Template-directed synthesis is probably the most widely used method for generating hollow nanostructures. In this approach, the template simply serves as scaffolds around which other materials are deposited. When the template is selectively removed, hollow nanostructures with voids complementary to the template will be obtained. The last step is usually an extremely slow process because constituents of the template have to diffuse through the coating layer. It is also nontrivial to remove the template without causing damage to the hollow nanostructures. We have solved these problems by employing templates that will be gradually consumed as the coatings are formed. We call this new approach "template-engaged synthesis" (see, Y. Sun, B. Mayers, Y. Xia, "Template-engaged replacement reaction: a one-step approach to the large-scale synthesis of metal nanostructures with hollow interiors," Nano Letters, 2: 481, 2002). The initial system we looked at involved the galvanic replacement reaction between silver nanospheres or nanocubes and an aqueous solution of chloroauric acid. In this paper published in Advanced Materials, we further extended this method to other types of silver nanostructures, as well as to other compounds such as platinum acetate and palladium nitrate. As a result, we could easily generate hollow nanostructures made of noble metals (e.g., Au, Pt, Pd, and their alloys) that are characterized by a variety of morphologies (e.g., triangular rings, prism-shaped boxes, cubic boxes, spherical or ellipsoidal shells, and long tubes). In principle, this method can be further extended to prepare hollow nanostructures from many other metals and other solid materials by choosing appropriate templates and reactions.

Since the publication of this paper in Advanced Materials, we have conducted a series of studies to achieve a better understanding and control of the templating process. It was found that the replacement reaction between silver nanostructures and chloroauric acid proceeds through at least two steps (Y. Sun and Y. Xia, "Alloying and dealloying processes involved in the preparation of metal nanoshells through a galvanic replacement reaction," Nano Letters 3: 1569, 2003; Sun and Y. Xia, "Mechanistic study on the replacement reaction between silver nanostructures and chloroauric acid in the aqueous medium," Journal of the American Chemical Society 126: 3892, 2004): a) formation of pinhole-free nanoboxes with homogeneous, uniform walls consisting of Au/Ag alloy through a combination of galvanic replacement reaction, alloying, and possibly Ostwald ripening; and b) development of pores in the shells (or cages) through dealloying, in which Ag atoms are selectively extracted from the alloy walls. As alloying and dealloying take place, the SPR peaks of resultant hollow and/or porous nanostructures can be continuously changed from blue (~425 nm) to near infrared (~1200 nm). Due to their strong scattering and absorption in the near infrared region (the transparent window for tissues), these porous, hollow nanostructures are potentially useful in optical coherence tomography (OCT) and in the photodynamic therapy of cancers. In another aspect, our recent work demonstrated that the replacement reaction could be combined with electroless plating of silver to fabricate hollow nanostructures with multifunctions and more complex configurations. Typical examples include multiple-walled nanotubes or nanoshells, and nanorattles with movable cores encapsulated in nanoshells. For those interested in these studies, please refer to the following publications: Y. Sun and Y. Xia, "Multiple-walled nanotubes made of metals," Advanced Materials 16: 264, 2004; Y. Sun, B. Wiley, Z.-Y. Li and Y. Xia, "Synthesis and optical properties of nanorattles and multiple-walled nanoshells/nanotubes made of metal alloys," Journal of the American Chemical Society 2004, 126, 9399-9406. It has also been demonstrated that silver nanowires coated with thin sheaths of palladium (through the galvanic replacement reaction between silver and palladium nitrate) could serve as excellent substrates for the reversible sorption/desorption of hydrogen at relatively low temperatures and pressures (Y. Sun, Z. Tao, J. Chen, T. Herricks and Y. Xia, "Ag nanowires coated with Ag/Pd alloy sheaths and their use as substrates for reversible adsorption and desorption of hydrogen," Journal of the American Chemical Society 126: 5940, 2004.

Our paper provides a simple and robust method for synthesizing metallic nanostructures with well-defined shapes and hollow interiors. It is also possible to control the chemical composition, crystallinity, and porosity associated with the walls of these hollow nanostructures. Combined together, this novel class of nanostructured materials presents some exciting opportunities for people from a variety of disciplines (e.g., chemistry, physics, photonics, materials science and engineering, mechanical engineering, biotechnology) to explore their peculiar properties and fascinating applications. Recent demonstrations indicate that these hollow nanostructures are superior to their solid counterparts in applications such as catalysis, energy conversion/storage, sensing, biomedical imaging, and photodynamic therapy.

Our group has been engaged in the synthesis of metallic nanostructures having well-controlled sizes, shapes, and properties since 2000. To this end, we have developed a wealth of chemical methods suitable for use with various solid materials. When we were asked by a professor in the Medical School if we could prepare metallic nanoparticles with large absorption coefficients in near-infrared region—a class of inorganic "pigments" that will find widespread use in optical diagnostics and photodynamic therapy of cancers—we began to search for new approaches to accomplish this goal. At that time, two demonstrations caught our attentions: a) it was shown by Professor Naomi Halas at Rice University that gold shells coated on dielectric spheres (e.g., silica colloids) exhibited surface plasmon resonance peaks tunable from visible to near infrared; b) it was illustrated in the freshman chemistry textbook (used for my teaching) that a zinc plate or iron nail could replace copper from a blue solution of copper sulfate to form a brown coating on the zinc plate or iron nail. These two phenomena triggered us to think about the possibility of generating gold nanoshells by reacting silver nanoparticles with chloroauric acid in an aqueous medium. The survey experiments performed by Dr. Yugang Sun, a postdoc in my group, proved the concept by showing that gold nanoshells could, indeed, be formed simply by refluxing an aqueous mixture of silver nanoparticles and chloroauric acid. More systematic studies confirm that this approach (i.e., template-engaged replacement reaction on the nanometer scale) represents a simple (one-step!) and versatile method for large-scale synthesis of hollow nanostructures of various metals with well-controlled parameters and properties.