Janus nanoparticles (JNPs) have two faces with differing physical or chemical characteristics which endow them with unique properties. This anisotropy can lead to phenomena like self-assembly or interfacial activity, and JNPs have been under intense scientific scrutiny since the 1990s. Since then, powerful synthetic techniques have been developed which allow for JNPs with near limitless combinations of materials, properties, and morphologies. As such, JNPs have been featured in a number of applications such as drug delivery, catalysis, interfacial engineering, and optics. This perspective discusses the synthesis of these important anisotropic materials. We will focus exclusively on JNPs that involve polymers. In particular, we outline the various classes of JNP synthetic techniques including some historical context, describe recent developments in this area, and weigh the pros and cons of the different methods with an eye on future directions and applications. We hope this perspective will help acclimate those new to the field who want to make and/or use JNPs.

A novel, solution-based method is presented to prepare bifunctional gold nanorods (B-NRs), assemble B-NRs end-to-end in various solvents, and disperse linked B-NRs in a polymer matrix. The B-NRs have poly(ethylene glycol) grafted along its long axis and cysteine adsorbed to its ends. By controlling cysteine coverage, bifunctional ligands or polymer can be end-grafted to the AuNRs. Here, two dithiol ligands (C6DT and C9DT) are used to link the B-NRs in organic solvents. With increasing incubation time, the nanorod chain length increases linearly as the longitudinal surface plasmon resonance shifts toward lower adsorption wavelengths (i.e., red shift). Analogous to step-growth polymerization, the polydispersity in chain length also increases. Upon adding poly(ethylene glycol) or poly(methyl methacrylate) to chloroform solution with linked B-NR, the nanorod chains are shown to retain end-to-end linking upon spin-casting into PEO or PMMA films. Using quartz crystal microbalance with dissipation (QCM-D), the mechanism of nanorod linking is investigated on planar gold surfaces. At submonolayer coverage of cysteine, C6DT molecules can insert between cysteines and reach an areal density of 3.4 molecules per nm². To mimic the linking of Au NRs, this planar surface is exposed to cysteine-coated Au nanoparticles, which graft at 7 NPs per μm². This solution-based method to prepare, assemble, and disperse Au nanorods is applicable to other nanorod systems (e.g., CdSe) and presents a new strategy to assemble anisotropic particles in organic solvents and polymer coatings.