Due to their size and tailorable physicochemical properties, nanomaterials are an emerging class of structures utilized in biomedical applications. There are now many prominent examples of nanomaterials being used to improve human health, in areas ranging from imaging and diagnostics to therapeutics and regenerative medicine. An overview of these examples reveals several common areas of synergy and future challenges. This Nano Focus discusses the current status and future potential of promising nanomaterials and their translation from the laboratory to the clinic, by highlighting a handful of successful examples.

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Dip-pen nanolithography (DPN) is a nanofabrication technique that can be used to directly write molecular patterns on substrates with high resolution and registration. Over the past two decades, DPN has evolved in its ability to transport molecular and material "inks" (e.g., alkanethiols, biological molecules like DNA, viruses, and proteins, polymers, and nanoparticles) to many surfaces in a high-throughput fashion, enabling the synthesis and study of complex chemical and biological structures. In addition, DPN has laid the foundation for a series of related scanning probe methodologies, for example, polymer pen lithography (PPL), scanning probe block copolymer lithography (SPBCL), and beampen lithography (BPL), which do not rely on cantilever tips. Structures prepared with these methodologies have been used to understand the consequences of miniaturization and open a door to new capabilities in catalysis, optics, biomedicine, and chemical synthesis, where, in sum, a process originally intended to compete with tools used by the semiconductor industry for rapid prototyping has transcended that application to advanced materials discovery. This review outlines the major DPN advances, the subsequent methods based on the technique, and the opportunities for future fundamental and technological exploration. Most importantly, it commemorates the 20th anniversary of the discovery of DPN.

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Plasmon-induced interfacial charge separation (PICS) is one of the key processes responsible for the improved conversion efficiencies of energy-harvesting devices that incorporate metal nanostructures. In this Letter, we reveal a mechanism of PICS by visualizing (with nanometer-scale resolution) and characterizing plasmon-exciton coupling between p-type poly(pyrrole) (PPy) nanowires (NWs) and Ag nanoparticles (NPs) using light-irradiated Kelvin probe force microscopy (KPFM). Under blue-light irradiation, the Ag NPs are expected to donate electrons to the PPy NWs via a hot electron injection process. However, in this Letter, we observe that under blue-light irradiation the plasmonically and excitonically excited electrons in the semiconductor back-transfer to the metal. The PICS in this system can be explained by comparing it with a similar one where Au NPs are attached to n-type ZnO NWs; we observed a net electron transfer from the Au NPs to the ZnO NWs (an upward band bending is formed at the interface of the two materials, presumably obstructing electron back-transfer). Indeed, energy band matching between the metal and the semiconductor components of hybrid nanostructures influences PICS pathways. These experimental findings and our proposed mechanism consistently explain the PICS occurring in the PPy NW-Ag NP system with important implications on explaining their cooperative optoelectronic activities.

are guided to assemble into desired architectures over a large scale. [2] Over the past three decades, we and others have repurposed DNA for the programmable assembly of synthetic materials. [3][4][5][6] These efforts have led to designer architectures made entirely out of DNA (i.e., structural DNA nanotechnology) [7][8][9][10] as well as structures where DNA is used as a bonding element to position functional building blocks in one, two, and three dimensions with sub-nanometric precision. [3,11,12] In particular, the concept of programmable atom equivalents, or PAEs, has paved the way for the field of colloidal crystal engineering with DNA. [13] In this field, DNA is used to chemically program the assembly of colloidal particles into precise, and in many cases, crystalline architectures, where various aspects of the resulting structures (e.g., crystallographic symmetry, lattice parameters, and crystal sizes/habits) can be systematically controlled. [13] PAEs have led to key fundamental chemical insights and form the basis for a whole new field of chemistry based on nanoparticles as "atoms" and DNA as programmable "bonds"; many analogies can be made between traditional chemical bonding and this new form of bonding based on DNA. PAEs also are particularly useful in biomedicine as diagnostic probes and therapeutic modalities, [14][15][16][17][18] and colloidal crystals engineered with DNA have been used as catalysts, actuators, and optical/plasmonic devices. [19][20][21][22] In the following sections, the characteristics of PAEs, the physical and chemical parameters that control the growth of PAE superstructures into complex assemblies, the types and properties of colloidal crystals that have been realized from PAEs (note that crystals with over 50 different symmetries have been prepared, some of which do not exist in nature), and the applications enabled by PAEs and PAE assemblies are discussed. The similarities and key differences between atoms and nanoparticle "atoms" and electron-based bonds and DNA "bonds" are also discussed. An outlook on future directions in the field is provided at the end. Programmable Atom EquivalentsA PAE is comprised of a nanoparticle core that is densely functionalized with a radially oriented DNA shell (Figure 1). Typically, the DNA shell consists of "anchor" strands that are attached to the nanoparticle surface and complementary "linker" strands that terminate in short single-stranded regions Colloidal crystal engineering with DNA has led to significant advances in bottom-up materials synthesis and a new way of thinking about fundamental concepts in chemistry. Here, programmable atom equivalents (PAEs), comprised of nanoparticles (the "atoms") functionalized with DNA (the "bonding elements"), are assembled through DNA hybridization into crystalline lattices. Unlike atomic systems, the "atom" (e.g., the nanoparticle shape, size, and composition) and the "bond" (e.g., the DNA length and sequence) can be tuned independently, yielding designer materials with unique catalytic, optical, an...

Spherical nucleic acids (SNAs) represent an emerging class of nanoparticle-based therapeutics. SNAs consist of densely functionalized and highly oriented oligonucleotides on the surface of a nanoparticle which can either be inorganic (such as gold or platinum) or hollow (such as liposomal or silica-based). The spherical architecture of the oligonucleotide shell confers unique advantages over traditional nucleic acid delivery methods, including entry into nearly all cells independent of transfection agents and resistance to nuclease degradation. Furthermore, SNAs can penetrate biological barriers, including the blood-brain and blood-tumor barriers as well as the epidermis, and have demonstrated efficacy in several murine disease models in the absence of significant adverse side effects. In this chapter, we will focus on the applications of SNAs in cancer therapy as well as discuss multimodal SNAs for drug delivery and imaging.