Date Published: September 15, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Hyeon‐Ho Jeong, Mariana Alarcón‐Correa, Andrew G. Mark, Kwanghyo Son, Tung‐Chun Lee, Peer Fischer.
Nanoparticles composed of functional materials hold great promise for applications due to their unique electronic, optical, magnetic, and catalytic properties. However, a number of functional materials are not only difficult to fabricate at the nanoscale, but are also chemically unstable in solution. Hence, protecting nanoparticles from corrosion is a major challenge for those applications that require stability in aqueous solutions and biological fluids. Here, this study presents a generic scheme to grow hybrid 3D nanoparticles that are completely encapsulated by a nm thick protective shell. The method consists of vacuum‐based growth and protection, and combines oblique physical vapor deposition with atomic layer deposition. It provides wide flexibility in the shape and composition of the nanoparticles, and the environments against which particles are protected. The work demonstrates the approach with multifunctional nanoparticles possessing ferromagnetic, plasmonic, and chiral properties. The present scheme allows nanocolloids, which immediately corrode without protection, to remain functional, at least for a week, in acidic solutions.
Corrosion is a ubiquitous characteristic of metallic solids in which a base metal is converted into its ionic state or oxide form by electrochemical surface reactions.1 Nanoparticles are especially vulnerable because of their high surface‐to‐volume ratios.2 In many cases, since the reaction products are soluble, corrosion leads to complete destruction of the original nanostructure (see Figure1a). This means that in real world applications the dominant materials consideration has often been corrosion stability, and the functional property required by the application, e.g., optical response, mechanical strength, magnetic susceptibility—is secondary. With effective corrosion protection materials selection can be made on the basis of the functional property. This will permit better materials optimization and lead to nanoparticles that are less expensive or more effective at the targeted application.
Our method is based on a published wafer‐scale 3D nanofabrication scheme, which we call “nanoGLAD,”8 that combines block copolymer micelle nanolithography (BCML)9 with glancing angle deposition (GLAD).10” The method permits control over both the shape and material composition of 3D hybrid nanostructures that have been used as plasmonic nanoantennas for nanorheology11 and chiral sensing.12 However, many of the materials that can be grown with this technique are not chemically stable when exposed to air or water. This limits the scope of potential applications that require nanoparticles in a colloidal form.
In summary, we have shown that nanoparticles can be effectively protected against corrosion by combining a physical vapor shadow grown plug with an atomic layer deposited shell. The method permits the complete encapsulation of the nanoparticle and we have demonstrated stabilities lasting from a week to over a month without loss of magnetic or plasmonic functionality. The stability could be shown even when the nanoparticles are immersed in physiological buffers, acidic environments, and reactive solutions. We have shown protection of both nanorods and nanohelices of Co and Cu, and expect that the scheme can be extended to produce corrosion resistant nanoparticles with the full range of core shapes12, 32 and compositions[[qv: 8a,11,12]] that nanoGLAD14 is capable of. This opens up a number of potential applications for nanoparticles composed of materials that are, at present, considered too reactive or toxic for practical use. These include unstable magnetic and plasmonic materials as nanoparticles for sensing,[[qv: 19a,27a]] imaging,[[qv: 19b,c,33]] as catalysts,5, 34 and for biomedical applications.4, 11, 35
Block Copolymer Micelle Nanolithography: The array of Au nanoseeds was prepared using block‐copolymer micelle nanolithography as previously reported.9 Briefly, the block‐copolymer micelles were formed by self‐assembly and then spin‐coated onto a 2 in. Si wafer where the micelles form a quasihexagonally ordered monolayer (spacing ≈100 nm). Plasma treatment reduced the Au to form metallic nanoparticles with ≈10 nm in diameter. These acted as seeds for subsequent nanoGLAD growth.
The authors filed two patent applications related to the fabrication method.