Sensors and Actuators B 45 (1997) 87–92 Polyelectrolyte and molecular host ion self-assembly to multilayer thin films: An approach to thin film chemical sensors X. Yang, S. Johnson, J. Shi, T. Holesinger, B. Swanson * Chemical Science and Technology, Los Alamos National Laboratory, CST-1, MS J565, Los Alamos, NM 87545, USA Received 31 January 1997; received in revised form 12 August 1997; accepted 18 August 1997 Abstract Multilayer molecular films of polyelectrolyte/calixarene and polyelectrolyte/cyclodextrin hosts were fabricated by alternating adsorption of charged species in aqueous solutions onto a substrate (quartz or silicon wafer). This layer-by-layer molecular deposition approach has been successfully used to integrate molecular recognition reagents into polymer films as chemically selective layers for surface acoustic wave (SAW) chemical sensing applications. The resulting sensors have high sensitivity and selectivity to the organic vapors studied. The films were characterized with SEM, infrared and UV-vis spectroscopy, and monitored by SAW devices. These measurements revealed that the deposition process is highly reproducible and the resulting films are uniform and stable. © 1997 Elsevier Science S.A. Keywords: Polyelectrolyte; Calixarene; Self-assembly; Thin-film; Sensor 1. Introduction The use of polymer-coated surface acoustic wave (SAW) chemical sensors for directly detecting organic vapors has attracted increasing attention [1 – 5]. Poly- mer coatings not only increase the sensitivity but also the selectivity of SAW sensors towards organic vapors at very low concentrations. However, the current tech- niques (dip-coating, spray, solution casting and brush- ing) of fabricating polymer thin films have minimum control of film properties such as film thickness, unifor- mity and stability. Moreover, there are problems associ- ated with the method currently used to control the thickness of the polymer by observing a total frequency shift, since the frequency shifts observed are not only the function of mass loading but also that of polymer elasticity and temperature. Polymers with various func- tional groups such as CF 3 , CN, pyridyl, OH, and perfluoro units have been used to promote selective vapor absorption for SAW chemical sensors [4]. How- ever, selectivity towards structurally similar hydrocar- bons is still low. In general, the polymers used are not commercially available and the polar groups in- corporated result in polymers with high glass transition points and low permeability [4]. Problems associated with film dewetting (island formation) and poor film adhesion were also reported. An alternative approach in developing selective sensor coatings relies on the use of molecular recognition reagents such as cyclodextrins [6] calixarenes [7–9], cavitands [10], metallopor- phyrins[11]. Although it has been demonstrated that selectivity of the sensing layer is improved, it is difficult to fabricate ordered multilayer molecular films incorpo- rating these organic host species. Solution casting of molecular species results in films with low coverage [10], poor uniformity [11], low stability and low reproduci- bility. Decher et al. recently developed a technique for polymer thin film growth using a polyelectrolyte self-as- sembly approach [12 – 14]. Alternating adsorption of anionic and cationic electrolytes on a charged substrate by sequential dipping of the substrate into aqueous polyelectrolyte solutions yields a uniform thin film that is both durable and reproducible in terms of film thick- ness. The driving force for film formation is multi-point electrostatic interactions between polyions. The result- ing thin film has a well organized bilayer structure, high thermal stability and high uniformity. Multilayer thin films with precisely controlled thickness and molecular * Corresponding author. Tel.: +1 505 6674686; fax: +1 505 6654631. 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)00 2 7 4- 8