NANO LETTERS
Spatial Compartmentalization of Nanoparticles into Strands of a Self-Assembled Organogel
2002 Vol. 2, No. 10 1037-1042
Blake Simmons,† Sichu Li,† Vijay T. John,*,† Gary L. McPherson,*,‡ Chad Taylor,‡ Daniel K. Schwartz,‡,| and Karol Maskos§ Department of Chemical Engineering and Department of Chemistry, and Coordinated Instrumentation Facility, Tulane UniVersity, New Orleans, Louisiana 70118 Received December 5, 2001; Revised Manuscript Received August 7, 2002
ABSTRACT The incorporation of nanoparticles into a novel organogel occurs spontaneously when nanoparticle-containing AOT reverse micelles come into contact with a solution of p-chlorophenol in isooctane. The organogel consists of strands of stacked phenols that self-assemble to fibers and further to fiber bundles. Atomic force microscopy clearly reveals the fiber bundles and the presence of nanoparticles incorporated into these structures. The gels of organic solvents obtained by linking small molecules through noncovalent interactions constitute a class of materials where the transition from gel to low viscosity liquids is sharp. The immobilization of superparamagnetic ferrites or semiconductor quantum dots of CdS into these gels confers magnetic and/or luminescent properties to the gels.
The organization of nanoparticles on surfaces and in threedimensional geometries remains a fascinating challenge in exploiting the unique properties of these materials. There are many novel techniques for depositing ordered arrays of nanoparticles onto surfaces through size selective precipitation techniques1,2 to attach them to synthetic or biopolymers to stabilize them in solution,3 or to incorporate them into solid structures of polymers4 or ceramics.5,6 In this paper, we describe a method to incorporate nanoparticles into the strands of a novel organogel material. In recent years there has been increasing interest in the design and development of gels that have the potential to respond to external stimuli. Such gels typically consist of polymeric networks in a solvent, with the viscosity or gel volume being responsive to external stimuli such as temperature, pH, electromagnetic fields, light, or mechanical force.7 Such sensitive responses to external stimuli present potential novel applications in sensors, controlled delivery systems, actuators, separation systems, and artificial muscles.8,9 While hydrogels have been the subject of extensive study, primarily due to their relevance in drug delivery, there is also significant interest in the discovery of species that can gelate organic liquids to organogels. A comprehensive review by Terech and Weiss10 lists several low molecular weight species * Corresponding authors: E-mail:
[email protected]. Ph: 504865-5883. Fax: 504-865-6744. † Department of Chemical Engineering. ‡ Department of Chemistry. § Coordinated Instrumentation Facility. | Currently at the University of Colorado, Department of Chemical Engineering. 10.1021/nl015691r CCC: $22.00 Published on Web 09/11/2002
© 2002 American Chemical Society
capable of gelling organic solvents and describes the underlying principles and potential applications of these materials. In the gelation of nonpolar organic solvents with small molecules, electrostatic interactions do not provide the driving force for network formation, as in the case of many hydrogels. On the other hand, networks in organogels may be derived from hydrogen bonding, organometallic coordination bonding, electron transfer, etc. between the gelating species.10 We have reported a class of self-assembling organogels formed by the addition of suitable phenols to a solution containing reverse micelles of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in isooctane.11-13 The driving force behind the formation of these gels appears to be hydrogen bonding between the phenolic species and the surfactant, which leads to a spontaneous phase transition from the liquid reverse micellar state to the organogel state. The organogel may be induced to form at low concentrations of the gel forming species, typically less than 2 wt %. For example, organogels with a 1:1 molar ratio of p-nitrophenol to AOT can form at concentrations as low as 3 mM of AOT and p-nitrophenol each. These gels are sensitive to temperature and exhibit very sharp phase transitions from a viscoelastic gel (with viscosities up to 105 Pa‚s at shear rates of 10-2 s-1) to the low-viscosity liquid phase upon heating. Our results also indicate that optimal gel formation occurs at a 1:1 molar ratio of the two gel forming species, with significant deviations from this ratio (3:1 or
