Layer-by-Layer Deposition of Chitosan/Polydiacetylene Vesicles for Convenient Preparation of Colorimetric Sensing Film A. Potisatityuenyong * , S. T. Dubas ** and M. Sukwattanasinitt * * Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand ** Metallurgy and Materials Science Research Institute (MMRI), Chulalongkorn University, Bangkok, 10330, Thailand ABSTRACT Photopolymerized vesicles of poly(10,12- pentacosadiynoic acid) (PPCDA) were successfully assembled into polyelectrolyte multilayer (PEM) thin films with polycationic chitosan by using a layer-by-layer deposition method. The linear relationship between the increase in absorbance at 639 nm and the number of deposited layers of chitosan and PPCDA vesicles was obtained. The stability of the vesicles was greatly enhanced when deposited into the PEM films. When assembled into PEM films, the desired colorimetric responses of the PPCDA vesicles to solvent (ethanol), pH and temperature were preserved with slight shifts in the color transition points. The shifts in transition points of the PPCDA vesicles assembled in PEM films from those of the vesicles dispersed in aqueous media is likely due to the highly electrostatic environment in the PEM films. Keywords: Polydiacetylene Vesicles, Polyelectrolyte multilayers, Sensor, Colorimetry 1. INTRODUCTION Polydiacetylenes (PDAs) [1] are an interesting class of conjugated polymers which exhibit dramatic color change upon exposure to various stimuli including light [2-4], heat [5-9], mechanical stress [10-12], solvent environment [13- 14] and binding of specific chemical/biological agents [15- 17], PDAs have thus been materials of choices for colorimetric sensing applications such as thermal [18-19], and biological sensors [20-21]. PDAs can be prepared from topochemical polymerization of monomeric diacetylenes in numerous forms: bulk solids [22-24], self-assembled films [25-26] and vesicles suspended in liquids [27-28]. Nanospherical vesicles or liposomes of PDAs homogeneously dispersed in aqueous media have been successfully used as colorimetric sensors [29-34]. The immobilization of uniformly distributed PDA vesicles into thin films should facilitate the storage stability and applicable expediency of the vesicles. Despite extensive research on PDA vesicles to be used as colorimetric sensors, reports on the fabrication of films containing PDA vesicles are scarce. Recently, monomeric diacetylene vesicles were covalently fixed onto a modified glass substrate followed by photopolymerization to form blue phase PDAs deposited as a monolayer film [35]. This approach is useful when monolayer deposition is desired but the fabrication of multilayer film with visible color can be quite difficult. Transfer of self-assembled PDA Langmuir-Blodgett films to flat substrates is somewhat simplet [36]. The Langmuir-Blodgett technique, relying mainly on the hydrophobic and hydrophilic interaction, is however not suitable for the assembly of PDA vesicles which have spherical hydrophilic surface. The polyelectrolyte multilayers (PEM) technique was developed as an alternative method for the preparation of multilayer thin films [37-43]. The PEM method has proven to be very simple and efficient to immobilize polyelectrolyte compounds onto a wide range of substrates. Early attempts to use the layer-by-layer technique to assemble diacetylene monomers or polydiacetylene chains with polyallylamine (PAH) resulted in thin films with a red color which excluded them from being used as sensor [44]. The PEM technique has recently been used successfully for deposition of unpolymerized vesicles on flat substrates that the spherical structure of vesicles being intact [45-47]. The aim of this paper is to report our development of PEM technique as a method for immobilization of PDA vesicles into thin films for colorimetric sensing. 2. EXPERIMENTAL 2.1 Materials The diacetylenic monomer, 10,12-Pentacosadiynoic acid (97%), was purchased from Fluka, USA. Poly(diallyldimethylammonium chloride) (PDADMAC) M w = 200,000-350,000 was purchased from Aldrich USA. Chitosan (M W ~8.0x10 5 , %DD ~84) was purchased from Seafresh Chitosan (Lab) Company, Thailand. Diethylether, analytical grade, was purchased from Lab-Scan, Thailand. De-ionized milliQ water with a resistance of 18.1 MΩ was used in all experiments. All solvents and chemicals were used as received without further purification, unless specified otherwise. 27 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006