Conversion of Pollen Particles into Three-Dimensional Ceramic Replicas Tailored for Multimodal Adhesion W. Brandon Goodwin, †,∥ Ismael J. Gomez, ‡,∥ Yunnan Fang, † J. Carson Meredith,* ,‡ and Kenneth H. Sandhage* ,†,§ † School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States * S Supporting Information ABSTRACT: We report the first syntheses of three-dimen- sional (3D) nanocrystalline all-oxide replicas of pollen microparticles tailored for multimodal (bioenabled and synthetic) adhesion via use of a scalable, highly conformal surface sol−gel (SSG) coating process. High-fidelity replication allowed the pollen-shaped oxide microparticles to be utilized for adhesion via tailorable short-range (∼10 nm) van der Waals (VDW) attraction, with the magnitude of such VDW-based adhesion influenced by the nanoscale topography of surface features retained by the replicas. Conversion of the pollen into ferrimagnetic (Fe 3 O 4 ) microparticle replicas allowed the use of magnetic attraction at short and long ranges (up to ∼1 mm). By selecting pollen particles with particular surface features and by SSG-enabled conversion of such pollen into 3D nanocrystalline replicas composed of an appropriate type and amount of magnetic oxide, adhesive microparticles with tunable short- and long-range attractive forces can be generated. KEYWORDS: surface sol−gel coating, pollen, three-dimensional replicas, magnetic oxides, van der Waals, adhesion ■ INTRODUCTION Adhesion by or on microparticles plays a critical role in a wide range of developing and mature technologies, including drug delivery, catalysis, water/chemical purification, sensing, anti- fouling coatings and membranes, semiconductor device processing, composite processing, paints, printing, and xerography. 1 Although predominant models and mechanistic experimental studies for understanding adhesion have been based on smooth spherical particles, 2 microparticles with rough surfaces and nonspherical shapes are desired for a number of such technologies. However, the scalable fabrication of microparticles with well-controlled surface asperities in a variety of three-dimensional (3D) morphologies and with tailorable chemistries to allow for tunable adhesion remains a difficult synthetic challenge. A rich sustainable source of 3D microparticles, with complex morphologies affecting dispersion and adhesion in nature, is pollen. Pollen particles come in a wide variety of 3D shapes and surface topographies 3 and are produced in large and increasing quantities worldwide by plants. 4 Atomic force microscopy (AFM)-based adhesion measurements have recently 5 shown that the van der Waals (VDW) attraction of pollen particles to various inorganic and organic surfaces scales directly with the contact radii of asperities on the pollen surface; that is, the selection of pollen particles with particular surface structural features may be used to affect such VDW-based adhesion. The purpose of the present article is to show, for the first time, how such pollen particles may be converted into 3D ceramic replicas endowed with tunable multimodal adhesion. In this demon- stration, native pollen particles have been converted, via the use of a highly conformal surface sol−gel (SSG)-coating process, 6 into 3D replicas composed of ferromagnetic hematite (α- Fe 2 O 3 ) or ferrimagnetic magnetite (Fe 3 O 4 ). The nanoscale surface topography and the magnetic oxide content of such high-fidelity replicas can provide for multimodal attraction to surfaces via both short-range VDW and short-to-long-range magnetic forces. Although other authors have used coating or infiltration methods to chemically modify/transform pollen 7 and other biological microparticles 8 for desired (bio)chemical, optical, electrical, structural, or fluidynamic properties, the conversion of sustainable biogenic particles (like pollen) into all-inorganic 3D replicas for the purpose of achieving tunable multimodal adhesion has not been reported. ■ EXPERIMENTAL SECTION Pollen Preparation. The conversion of sunflower (Helianthus annuus) pollen (Greer Laboratories, Lenoir, NC) into iron oxide replicas has been examined in this work. The pollen grains were first Received: July 5, 2013 Revised: October 20, 2013 Published: October 21, 2013 Article pubs.acs.org/cm © 2013 American Chemical Society 4529 dx.doi.org/10.1021/cm402226w | Chem. Mater. 2013, 25, 4529−4536