7138 DOI: 10.1021/la9042099 Langmuir 2010, 26(10), 7138–7147 Published on Web 12/15/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society pH-Controlled Assembly and Properties of LbL Membranes from Branched Conjugated Poly(alkoxythiophene sulfonate) and Various Polycations Veronika Kozlovskaya, † Eugenia Kharlampieva, † Keith Jones, † Zhiqun Lin, ‡ and Vladimir V. Tsukruk* ,† † School of Materials Science and Engineering and School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 and ‡ School of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011 Received November 5, 2009. Revised Manuscript Received November 16, 2009 We report on multilayer layer-by-layer (LbL) films of the conjugated polymer sodium poly[2-(3-thienyl)ethyloxy-4- butylsulfonate] (PTH) assembled with polycations: poly(diallyldimethylammonium chloride) (PDDA), 20% quaternized poly(N-ethyl-4-vinylpyridinium bromide) (Q20), poly(ethylene imine) (PEI), and poly(allylamine hydrochloride) (PAH). These films were prepared through spin-assisted LbL assembly under various pH conditions. We demonstrated a crucial role of the deposition pH in formation of PTH/polycation films and showed that decrease in the deposition pH from 7.5 to 2.5 limits the PTH multilayer formation to Q20/PTH and PDDA/PTH films due to reduced charge density in the poly(thiophene) chains. We show that optical and surface properties of the resulting PTH/polycation films can be tuned by varying a polycation component and/or by varying the deposition pH. The fluorescence properties of the Q20/PTH, PEI/ PTH, and PDDA/PTH films are pH-dependent, and the films exhibit the drastic changes in photoluminescent intensity when transferred into solutions with different pH values, which may find useful in optical sensing applications. Introduction Chemical sensing with optical readout is considered to be promising for facile colorimetric-based detection scheme. 1-3 Ultrathin films with responsive optical properties have been demonstrated to be of a potential use for this field due to multifunctional physicochemical properties, tunability of the composition, and triggered response to external stimuli. 4-8 It is worth noting that for practical applications it is important to be able to fabricate films which can be later transferred to various microfabricated substrates of actual devices. 9,10 This ability is important as such free-standing films possess a faster response and permeability due to their short diffusion-limited path for incoming analytes as well as versatility in integration with various microfabricated substrates. Conjugated polymers comprise a class of functional materials with unique conducting and optical properties such as easily quenchable photoluminescence which can be explored for bio- and chemo-sensing applications. 11-13 Controlling optical proper- ties has been shown possible through changes of the geometric conformation of the conjugated polymers via electrostatically combining the conjugated polyelectrolyte with a charged surfac- tant, thereby resulting in enhanced fluorescence of such com- plexes. 14 The presence of charged compounds has been demonstrated to affect fluorescence of the conjugated polyelec- trolytes in solutions. 15 However, loss of fluorescence or significant changes in optical spectra has been observed when some con- jugated polymers were transferred from solutions to solid sub- strates due to crystallization, aggregation, or interaction with solid substrates. 16,17 Solubility of poly(thiophene)s in water is considered an im- portant property for future applications of these materials, and water-soluble poly(thiophene)s have recently received much at- tention due to their potential applications in bio- and chemo- sensing. 18-22 Introducing various modified side chains to the conjugated backbones of the poly(thiophenes) can result in new materials with useful functionalities. 23 For example, a carboxy- functionalized poly(thiophene) has been utilized as a sensor for *To whom correspondence should be addressed. 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