Photogelling Colloidal Dispersions Based on Light-ActivatedAssembly of Nanoparticles
Kunshan Sun, Rakesh Kumar, Daniel E. Falvey, and Srinivasa R. Raghavan*,
Department of Chemical and Biomolecular Engineering and Department of Chemistry andBiochemistry, UniVersity of Maryland, College Park, Maryland 20742-2111
Received February 10, 2009; E-mail: firstname.lastname@example.org
Abstract: Photorheological (PR) fluids, i.e., fluids whose rheology can be tuned by light, have been arecent focus for our laboratory. We are interested in low-cost approaches to PR fluids using molecules ormaterials that are readily available. Toward this end, we report a new concept for such fluids based onlight-activated assembly of nanoparticles into a physical network (gel). Our system consists of disk-likenanoparticles of laponite along with a surfactant stabilizer (Pluronic F127) and the photoacid generator(PAG), diphenyliodonium-2-carboxylate monohydrate. Initially, the nanoparticles are sterically stabilizedby the surfactant, and the result is a stable, low-viscosity dispersion. Upon UV irradiation, the PAG getsphotolyzed, lowering the pH by 3 units. In turn, the stabilizing surfactant is displaced from the negativelycharged faces of the nanoparticle disks while the edges of the disks become positively charged. The particlesare thereby induced to assemble into a three-dimensional house-of-cards network that extends throughthe sample volume. The net result is a light-induced sol to gel transition, i.e., from a low, water-like viscosityto an infinite viscosity and yield stress. The yield stress of the photogel is sufficiently high to support theweight of small objects. The gel can be converted back to a sol by increasing either the pH or the surfactantcontent.
Numerous research groups1-13 have been interested increating fluids whose rheological properties, such as viscosity,can be tuned by irradiation with light (for reviews, see refs 2-5).Such photorheological (PR) fluids1,12 could be particularlyattractive for creating microvalves or flow sensors withinmicrofluidic systems. Indeed, compared to other external stimulisuch as heat or magnetic fields, light does offer distinctadvantages in microscale applications, notably, in its ability tobe directed at a precise spot with micron-level resolution. For
PR fluids to be used widely, researchers need to be able to createthem at will, preferably from inexpensive and commerciallyavailable components. However, most of the PR systemsdeveloped to date have been based on complex organicmolecules, such as photoresponsive surfactants,3,6,7 polymers,5,8,9
or gelators,4,10,11 most of which are not commercially availableand whose synthesis demands considerable skill in organicchemistry. There is a need for simpler approaches to makingPR fluids, and this has been the focus of our work.
Recently, we have demonstrated two PR fluids, a photo-thinning one12 and a photogelling one,13 both of which are basedon simple, widely available molecules. Both fluids employedthe photoisomerizable organic molecule, trans-ortho-methoxy-cinnamic acid (OMCA). The photothinning fluid combinedOMCA with a conventional cationic surfactant.12 In this case,UV irradiation of the fluid caused a substantial decrease inviscosity (factors of 1000 to 10 000). The photogelling fluidcombined OMCA with a zwitterionic surfactant, and in this case,the fluid viscosity increased dramatically upon UV irradiation.13
The mechanisms for the photorheological effects in both systemsinvolved changes in the length of cylindrical (wormlike)micelles. When the micelles were short, the viscosity was low;conversely, long micellar chains entangled with each other togive a high viscosity. The micelle length, in turn, was modulatedby the binding tendency of the OMCA isomer (trans vs cis). Inboth of the above systems, the light-induced viscosity changewas not reversible by irradiation with a different wavelengthof light, but it was reversible by changing the system composi-tion. Despite this limitation, our studies showed that significantlight-induced rheological changes are indeed possible in verysimple systems.
Department of Chemical and Biomolecular Engineering. Department of Chemistry and Biochemistry.
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Published on Web 04/30/2009
10.1021/ja9008584 CCC: $40.75 2009 American Chemical Society J. AM. CHEM. SOC. 2009, 131, 71357141 9 7135
In this paper, we report a new class of PR fluids based on anentirely different concept compared to the one discussed above.The key ingredient in our present fluids are nanoscale particlesof the synthetic clay, laponite (Figure 1a).14-17 These particlesare initially dispersed in water as individual, unconnected disks,thus forming a low-viscosity fluid. Upon irradiation with UVlight, the particles are induced to assemble into a three-dimensional network (called a house-of-cards structure in theliterature14-17). The resulting aqueous gel has an infiniteviscosity and a yield stress. In effect, our system shows a light-induced sol-gel transition, with the gel being held by physical(electrostatic) bonds. The gel can be converted back to a sol bychanging the pH, but not by irradiation with a differentwavelength of light.
How does this work? In addition to the laponite, the fluidshave two other components (Figure 1b, 1c): the nonionicsurfactant, Pluronic F127 (PF127), and the photoacid generator,diphenyliodonium-2-carboxylate monohydrate (abbreviated asPAG henceforth). Each of these components has a key roleto play. PF127 is known to be a stabilizing surfactant forlaponite: it adsorbs on the particle faces and provides stericstabilization.18 Photoacid generators (PAGs) are commerciallyavailable organic molecules that have a key property: whenirradiated with UV light, the molecules are photolyzed, withone of their photoproducts being an acidic moiety.19-22 As aresult, the solution pH drops by an amount proportional to the
PAG concentration (this can be as much as 3 pH units). Whilea variety of PAGs are available, the PAG chosen here has arelatively high solubility in water.21,23 Briefly, the mechanismbehind the photogelling is as follows: initially, in the sol state,the particles are covered by PF127. Upon UV irradiation, thepH drops and, in turn, the particle edges become positivelycharged.15,16 At the same time, the PF127 desorbs from theparticles and forms micelles in solution. The positively chargedparticle edges then bind with the negatively charged particlefaces and form a particulate network. Support for the abovemechanism is provided by a series of systematic experimentsalong with data from dynamic light scattering (DLS) and small-angle neutron scattering (SANS).
In closing this section, it is worth reiterating that the threecomponents of the present PR fluids (i.e., laponite, PAG, andPF127) are all commercially available and relatively inexpen-sive. Thus, our results can be replicated easily by otherresearchers who have an interest in PR fluids. Second, it is worthpointing out the analogy between our PR system with existingelectrorheological (ER) fluids24 and magnetorheological (MR)fluids.25 ER and MR fluids are those whose rheology can bemodulated by external electric or magnetic fields, respectively.Both types of fluids are dispersions of micron-sized particlesin a solvent; in the off state, the particles are unconnectedand the sample is of low viscosity, whereas when the field isswitched on, the particles get connected into a network and thesample develops a yield stress.24,25 The same mechanismunderlies the behavior of our PR system as well. To ourknowledge this is the first demonstration of a nanoparticle-basedphotorheological fluid.
2. Experimental Section
Materials. Laponite RD was obtained from Southern ClayProducts. The nanoparticles are disklike with a diameter of 25 nmand a thickness of 0.92 nm. Pluronic F127 (PF127) was reagentgrade and was purchased from Sigma Aldrich. The material is atriblock copolymer of the form PEO-PPO-PEO, where PEO refersto poly(ethylene oxide) and PPO to poly(propylene oxide). Thepercentages of PEO and PPO are 70% and 30%, respectively,and the overall molecular weight is 12 kDa. The PAG, diphenyl-iodonium-2-carboxylate monohydrate, was purchased from TCIAmerica (purity >98%). Ultrapure deionized water from a Milliporewater-purification system was used in preparing samples forrheological characterization, while D2O (99.95% deuterated, fromCambridge Isotopes) was used for the SANS studies.
Sample Preparation. Dispersions of laponite particles wereprepared by