7436
Langmuir 2004, 20, 7436-7443
Preparation of Fluorescent Particles with Long Excitation and Emission Wavelengths Dispersible in Organic Solvents Hua Hu* and Ronald G. Larson Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received March 4, 2004. In Final Form: June 11, 2004 We introduce a fast and simple one-step method, a variation of the methods of Barrett and Campbell and Bartlett, to synthesize monodisperse fluorescent particles that can be dispersed in organic solvents and have long excitation (649 nm) and emission wavelengths (679 nm). A lipophilic fluorescent dye, 1,1′dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate, is directly incorporated into PMMA particles through dispersion polymerization. A poly(hydroxystearic acid) graft (poly) methyl methacrylate (MM) and methacrylic acid (MA) copolymer is used as a stabilizer to prevent the particles from aggregating and flocculating in the nonaqueous solvents. The fluorescent PMMA particles are very uniform in size, bleach at very low rate, and behave like hard spheres in their ordering on substrates. One important achievement in our synthesis protocol is that we are able to produce particles of a desired size by choosing the composition of the reactants according to a predetermined relationship between particle size and composition of reactants. In addition, the effects of fluorescent dye and polar solvent (ethanol) on the formation and size of particles are discussed.
1. Introduction The synthesis of nonfluorescent particles that are dispersible in organic media was well developed and documented as early as the 1970s.1 Later, Antl et al.2 modified the stabilizer and covalently attached it to the surfaces of colloidal particles. In recent years, with the rapid development of fluorescence microscopy and confocal microscopy, it has become increasingly important to synthesize fluorescent particles that are stable in organic media to measure microfluidic velocity fields as well as study dynamics and rheological behavior of colloids, gels, and nanomaterials in organic liquids. Dinsmore et al.3 swelled PMMA particles and absorbed into them a rhodamine fluorescent dye for use in a confocal microscopic study of colloidal dispersions. Although their method is very simple, after the particles are swelled to entrap the fluorescent rhodemine dye, the particles become softer and their behavior deviates slightly from that of hard spheres. Bosma et al.4 synthesized fluorescently dyed monomers, 4-methyulaminoethyl methacrylate-7-nitobenzo-2-oxa-1,3-diazol (NBD-MAEM) and RITC-aminostyrene (RAS) and copolymerized them with plain monomers to obtain fluorescent particles. These particles are very uniform in size and suitable for microscopy. Jardine and Bartlett5 adopted a similar approach, in which they first prepared a fluorescent dye-coupled monomer, 7-nitrobenzo-2-oxa-1,3-diazole-methyl methacrylate, and copolymerized it with the monomers methyl methacrylate and methacrylic acid to form fluorescent PMMA particles. Because this method involves many steps in preparing * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Barrett, K. E. J. Dispersion Polymerization in Organic Media; Wiley-Interscience: New York, 1975. (2) Antl, L.; Goodwin, J. W.; Hill, R. D.; Ottewill, R. H.; Owens, S. M.; Papworth, S.; Waters, J. A. Colloids Surf. 1986, 17, 67. (3) Dinsmore, A. D.; Weeks, E. R.; Prasad, V.; Levitt, A. C.; Weitz, D. A. Appl. Opt. 2001, 40, 4152. (4) Bosma, G.; Pathmamanoharan, C.; de Hoog, E. H.; Kedil, W. K.; van Blaaderen, A.; Lekkerkerker, H. N. W. J. Colloid Interface Sci. 2002, 245, 292. (5) Jardine, R. S.; Bartlett, P. Colloid Surf., A 2002, 211, 127-132.
the fluorescently stained monomers, the synthesis becomes complicated and the fluorescent dye bleaches rapidly and easily degrades due to the long synthesis time. Campbell and Bartlett6 therefore developed a “one-step” method, so called because once the stabilizer is synthesized, this and other ingredients, including the fluorescent dye, are all added into the reactor at the outset and converted into the final particles in a single reaction process. This method requires careful selection of a fluorescent dye that can be successfully entrapped in the monodiperse particles. The fluorescent particles obtained by this method have a low photobleaching rate and behave like hard spheres. Compared with the earlier methods, the method of Campbell and Bartlett is relatively simple and fast since it omits the synthesis of the fluorescent-dye-coupled monomers. Nevertheless, this method is not a true one-step method since it requires a locking stage to covalently attach stabilizers onto the particle surface. It takes at least 24 h to finish the locking stage. Long and complicated synthesis procedures may change the structure of fluorescent dyes and cause them to degrade and bleach rapidly. In addition, the above methods only produce fluorescent particles with rather short excitation and emission wavelengths (
