Magnetic-Field-Induced Locomotion of Glass Fibers on Water Surfaces: Towards the Understanding of How Much Force One Magnetic Nanoparticle Can Deliver By Feng Shi, Shuhua Liu, Haitao Gao, Ning Ding, Lijie Dong, Wolfgang Tremel, and Wolfgang Knoll* Layer-by-layer (LbL) self-assembly is a powerful method to modify surfaces with predesigned compositions and tailored functional- ities. [1] Since the original report of Decher, [2] much progress has been made on more clear understanding of this technique. At the beginning, various water-soluble functional materials, such as colloid nanoparticles, [3] carbon nanotubes, [4] dendrimers, [5] enzymes, [6] polymer micelles, and others, [7] were used as building blocks in LbL fabrication, which can directly bring their functionalities to the multilayers. Besides electrostatic interactions, hydrogen bonging [8] and coordination bonding [9] have been developed to extend the concept of LbL from water to nonwater systems, which means that many water-insoluble molecules can be used in LbL fabrication. Moreover, many novel fabricating methods have been invented, such as the multiple-polyelectrolyte adsorp- tion/surface activation technique [10] and the electric-field-directed LbL assembly. [11] In order to develop a more efficient assembly technique, the spin self-assembly technique [12] and sprayed- polyelectrolyte multilayer [13] have been developed. Based on the above improvements, a lot of promising applications of polyelec- trolyte multilayers have been demonstrated in various research areas, such as chemical sensors/biosensors, [14] enzyme immobi- lization, [6] hollow capsules, [15] surface patterning, [16] separation membranes, [17] microporous films, [18] light-emitting diodes, [19] erasable films, [20] nanocomposite, [21] superhydrophobic coat- ings [22] biocompatible coatings, [23] and molecular imprinting multi- layers. [24] Recently, it has been exciting to see that the related research is not restricted to fundamental aspects, but is extended from laboratories to commercial production, for example, coating for contact lenses, antibacterial films, and conductive rubber. Moving an object on a water surface is very attractive for its potential applications, such as mimicking movement of insects, [25] exploring complexity of creatures, [26] and researching on fluidic drag. [27] The key point of the locomotion on water surfaces is how to harness the power generated on the object. Whitesides and coworkers employed the reaction between Pt and H 2 O 2 for the research on the complexity of creatures. [26] Moreover, Zhang’s group used a similar method for the research on fluidic drag. [27] Besides the chemical power, Hu et al. used an elastic thread, which ran the length of the object’s body and coupled to its driving legs through a pulley, to mimic the movement of water striders on water surfaces. [25] However, the above methods are limited to special systems or materials. Developing a general method to create power for driving an object on water surfaces is still a challenge. In this communication, we have developed a facile method to drive a glass fiber on a water surface through combining LbL self-assembly of Fe 3 O 4 magnetic nanoparticles (MNPs) induced by an external magnetic field. The magnetic-modification process of Fe 3 O 4 MNPs and polyelectrolytes is shown in Scheme 1d. First, the glass fiber was immersed in an aqueous solution of poly(diallyldimethy- lammonium chloride) (PDDA) for 30 min, to obtain a layer of positive charges. Second, the positively charged glass fiber was immersed in an aqueous solution of poly(acrylic acid) (PAA) for 30 min, to modify the surface with carboxyl groups. Third, the substrate modified with carboxyl groups was deposited in a dichloromethane solution of Fe 3 O 4 MNPs (0.2 mg mL 1 ) for 20 min, washed with dichloromethane, and dried with nitrogen. Finally, the substrate was immersed in an ethanol solution of PAA (1 mmol mL 1 ) for 20 min, washed with ethanol, and dried with nitrogen. The magnetic multilayer is obtained by repeating the last two steps to achieve desired bilayers. The stepwise assembly of the multilayer film was character- ized by UV-vis absorption spectroscopy. From Figure 1a, we can clearly see that a broad absorption peak spans the UV-vis wavelength range, centering on 320 nm. This broad absorption peak is primarily due to the mixture of the ligand field transition and charge-transfer transition of the Fe 3 O 4 MNPs. [28] The correlation between the absorbance intensities of Fe 3 O 4 MNPs at 320 nm and the number of bilayers shows a nice linear increase, implying an identical Fe 3 O 4 MNPs content upon build- up of the multilayer structure, as shown in Figure 1b. Moreover, after a 20-bilayer modification of PAA/Fe 3 O 4 , the resulting glass fiber is covered by a blend polyelectrolyte film with a light brown color, indicative of Fe 3 O 4 MNPs adsorption, as shown in the inset of Figure 1a. The as-prepared glass fiber was put on the water surface and we found that it could easily float. To understand why the glass fiber can float on water surface, we had fabricated 20 bilayers of PAA/ Fe 3 O 4 MNPs on a silicon wafer and measured its contact angle. We observed that the static contact angle is about 1008, which indicated the surface-wetting property of the glass fiber is hydrophobic. This is because the out-layer of the multilayer is COMMUNICATION www.advmat.de [*] Prof. W. Knoll, Prof. F. Shi, Dr. S. H. Liu, N. Ding, Dr. L. J. Dong Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) E-mail: knoll@mpip-mainz.mpg.de H. T. Gao, Prof. W. Tremel Institute of Inorganic and Analytical Chemistry Johannes Gutenberg University of Mainz Duesbergweg 10–14, 55099 Mainz (Germany) DOI: 10.1002/adma.200801346 Adv. Mater. 2009, 21, 1927–1930 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1927