Janus Nanoparticles by Interfacial Engineering**

FULL PAPER DOI: 10.1002/adfm.200601034 Janus Nanoparticles by Interfacial Engineering** By Sulolit Pradhan, Li-Ping Xu, and Shaowei Chen* Nanosized Janus particles were prepared by ligand exchange reactions of a Langmuir monolayer of hydrophobic alkanethiolate-passivated gold nanoparticles at relatively high surface pressures with hydrophilic thiol derivatives injected into the water subphase. The ligand intercalation between adjacent particles led to impeded interfacial mobility of the particles. Consequently, ligand place-exchange reactions were limited only to the side of the particles facing the water phase, leading to the formation of amphiphilic nanoparticles which exhibited hydrophobic characters on one side and hydrophilic on the other, analogous to the dual-face Roman god, Janus. The unique amphiphilic characters of the Janus particles were confirmed by a variety of experimental measurements, including contact angle measurements, FTIR, UV-visible, and NMR spectroscopies. Interestingly, the Janus particles might be dispersed in water, forming micelle-like aggregates, as revealed in dynamic light scattering and AFM measurements. 1. Introduction sized Janus particles were prepared by biphasic electrified jet- ting of two polymer precursors.[7] In a different report,[8] Janus The intense research interest in nanoscience and nanotech- particles were prepared by sputtering gold onto the top face of nology is largely fueled by the unique properties of nanoscale a polymer bead array where further functionalization on the [11] materials that may deviate vastly from those of their constitu- gold surface might be achieved. Paunov and co-workers re- ent atoms and bulk forms.[1] In order to exploit these unprece- ported the fabrication of Janus particles by the replication of dented materials properties for the fabrication of next-genera- particle monolayers at liquid surfaces using a gel trapping tech- tion devices and circuitries, two key aspects that are intimately nique. It should be noted that in these earlier studies, the typi- related to each other have to be addressed: design and synthe- cal sizes range from a few hundred nanometers to a few mi- sis of nanoscale building blocks, and controlled assemblies of crometers; and reports on the synthesis of nanometer-sized [12] these structural units into functional architectures.[2] Thus, it is Janus particles are actually rather scarce. In fact, the major- of critical importance to look for additional nanomaterials de- ity of nanometer-sized Janus particles refer to bifunctional het- [13,14] sign parameters beyond size and shape. Among these, of par- erodimers consisting of two different particle cores. In ticular interest is the creation of amphiphilic nanoparticles these snowman or dumbbell-like nanostructures, the focus is which exhibit hydrophobic characters on one side and hydro- generally placed on the core materials rather than on the or- philic on the other, akin to the dual-face Roman god, Janus.[3,4] ganic capping shells. So the surface wettability of these hybrid These particles represent a unique nanoscale analog to the con- particles may actually be very similar to that of their respective ventional surfactant molecules and thus may be exploited in monomeric particles. the formation of functional superstructures by virtue of self-as- In the present investigation, we adopt the Langmuir method sembly.[5] to create nanosized Janus particles that exhibit hydrophobic Most previous studies of Janus particles are focused on poly- characters on one side and hydrophilic on the other, by taking [15] mer-based materials.[6–10] Several effective routes have been re- alkanethiolate-protected nanoparticles as the illustrating ex- ported towards the synthesis of these biphasic particles. For in- ample. These particles represent a unique class of nanomateri- stance, microfluidic flow systems have been used to prepare als. They consist of a nanosized metal core on which alkanethi- amphiphilic particles by the polymerization of the Janus drop- olates form a densely packed self-assembled monolayer by lets formed within the microfluidic channels.[9,10] Submicron- virtue of the strong affinity of the thiol group to transition metal surfaces. It has been demonstrated previously that ligand place-exchange reactions[16–20] (Scheme 1) can be employed to further functionalize the nanoparticle surface, which can then – be used as a point of departure for more complicated surface [*] Prof. S. Chen, S. Pradhan, Dr. L. Xu Department of Chemistry and Biochemistry, University of California functionalization, for instance, by surface coupling reactions. Santa Cruz, California 95064 (USA) However, typically, in these earlier studies,[16–20] the particles E-mail: [email protected] and the new ligands are mixed in the same phase. Thus, while [**] This work was supported in part by the NSF (CAREER Award the final composition (up to 100 % displacement) of the parti- CHE-0456130), the ACS-PRF (39729-AC5M) and UCSC. Supporting Information is available including a representative Langmuir iso- cle surface layers can be controlled by the initial concentra- therm and additional AFM images of Janus nanoparticle aggregates. tions of the particles and the new ligands, the incorporation of Adv. Funct. Mater. 2007, 17, 2385–2392 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2385 S. Chen et al./Janus Nanoparticles by Interfacial Engineering FULL PAPER Scheme 1. Schematic of nanoparticle surface place exchange reactions. the new ligands onto the particle surface is actually quite iso- tropic (it has been argued that the preferred sites are the core surface defects such as edges and vertexes which are distribut- ed rather symmetrically on the nanoparticle surface[16–20]), resulting in structurally symmetrical particles as the products. This is primarily because of the equal accessibility of the entire particle surface to the new ligands in solution. Therefore, in order to create nanosized Janus particles by ligand exchange reactions, the reactions must be confined to only one side of the particles. This can be achieved by dispers- ing the particles at an interface, e.g., at the air|water interface, where the particle interfacial mobility is impeded (e.g., by the Langmuir technique). Experimentally, as an illustrating exam- ple, the amphiphilicity was rendered to hexanethiolate-capped Scheme 2. Schematic of the preparation of Janus nanoparticles based on gold (AuC6) nanoparticles by subjecting them to place ex- the Langmuir technique. change reactions with hydrophilic mercaptopropanediol (MPD) at the air|water interface. First, the AuC6 particles were dispersed onto the water surface. Upon mechanical compres- sentative one was included in the Supporting Information).[21] sion, a compact monolayer was formed with ligand intercala- In a typical experiment, the interparticle edge-to-edge distance tion between adjacent particles. With the introduction of MPD was estimated by assuming a hexagonal close-packed structure ligands into the water subphase, exchange reactions occurred of the particle monolayer[21] and controlled at 1.22 nm which which were limited only to the side of the particles in direct was somewhat smaller than twice the hexanethiolate chain- contact with the water phase, resulting in the formation of Ja- length (0.77 nm each as estimated by Hyperchem®). Previous nus nanoparticles. A wide array of experimental characteriza- studies[21,22] have shown that by using particles of moderate dis- tions was then carried out to verify the amphiphilic structures. persity (<20 %) a compact and long-range ordered monolayer could be formed on the water surface, where the interparticle separation (and hence surface pressure) could be readily regu- 2. Results and Discussion lated by simple mechanical compression. It should be noted that the same surface pressure was main- In this study, Janus gold nanoparticles were prepared by ex- tained for a few hours (typically 8 h) after the injection of ploiting the Langmuir technique (Scheme 2). Specifically, at MPD molecules into the water subphase (corresponding to a fi- low surface pressures, the hydrophobic alkanethiolate-pro- nal concentration of 1 mM), where the exchange reactions were tected particles are dispersed on the water surface with signifi- initiated with the bottom half of the particles. The resulting cant interfacial mobility, corresponding to the two-dimensional particles exhibited hydrophobic characters on the top half be- gas state (top panel). In contrast, upon mechanical compres- cause of the original alkyl protecting ligands, whereas the sion, the nanoparticles will form a compact monolayer on the MPD hydroxyl functionalities rendered the bottom half of the water surface. Importantly, at high surface pressures, because particles hydrophilic, hence the formation of Janus particles of ligand intercalation between adjacent nanoparticles, the in- (Scheme 2). terfacial mobility of the particles is significantly impeded (mid- The amphiphilic characters of the Janus particle surface were dle panel). By injecting a new ligand into the water subphase, first examined by contact angle measurements of the particle ligand exchange reactions will occur at the particle surface that monolayers deposited onto a substrate surface. Here two inter- is in direct contact with the water phase, resulting in the forma- facial configurations were achieved by the Langmuir–Blodgett tion of Janus nanoparticles (bottom panel). (LB) method (Scheme 3). In the up-stroke

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