Journal of Colloid and Interface Science 283 (2005) 153–159 www.elsevier.com/locate/jcis A polyaniline-containing filter paper that acts as a sensor, acid, base, and endpoint indicator and also filters acids and bases Dipak Dutta, Tridib Kumar Sarma, Devasish Chowdhury, Arun Chattopadhyay ∗ Indian Institute of Technology Guwahati, Guwahati 781 039, India Received 28 February 2004; accepted 9 August 2004 Available online 15 January 2005 Abstract In this paper we report a new idea for synthesizing polyaniline in an ordinary filter paper. The synthesis was carried out by a process in which aqueous acidic aniline solution and the oxidizing agent H 2 O 2 was added to the paper drop by drop and in sequence. Uniform polymerization could be obtained with the addition of reagents in either sequence. The polymer formation led to a green coloration of the paper. Formation of the emeraldine salt of polyaniline was confirmed by UV–vis and FTIR spectroscopy. Scanning electron microscopic measurements were made for surface characterization of the polymer formed in the paper. The same paper was used as a sensor for ammonia in vapor and in solution, for acid and base as well as endpoint indication, and also to filter acids and bases. We found that, using the polymer-containing paper, ammonia concentrations in a solution as low as 14 ppm could be measured.  2004 Elsevier Inc. All rights reserved. Keywords: Polymerization; Filter paper; Conducting polyaniline; Sensor; Spectroscopic methods; Materials 1. Introduction Polyaniline (PANI), a preeminent electrically conduct- ing polymer with tunable electrical conductivity, has been the subject of intense study due to its application poten- tial in diverse fields such as microelectronics [1–5], displays [6,7], electrodes (for example, [8–10]), sensors and actuators [11–14], membranes for gas separations [15–17], solar cells [18,19], fuel cells [20,21], and electromagnetic interference (EMI) shielding [22,23]. Also, PANI undergoes a nonredox reversible doping/dedoping process [24] based on simple acid–base chemistry, making possible control over proper- ties such as optical activity, electrical conductivity, and sen- sor activity, thus making it unique in the class of conjugated polymers. As a natural consequence of its demand, a large number of synthetic protocols have been developed that incorporate application-specific functionalization of PANI backbone [25–29], enhancement of biocompatibility of the * Corresponding author. Fax: +91-361-2690762. E-mail address: arun@iitg.ernet.in (A. Chattopadhyay). polymer using enzymes as catalysts for synthesis [30,31], and synthesis of PANI nanofibers, nanotubes [32–34], metal nanoparticle–PANI composite [35], and carbon-nanotube- doped PANI [36]. In addition, an inclusion complex of PANI in cyclodextrin has been made to form “insulated molecular wire” [37]. Prominent applications of PANI as a sensor are its use as an ammonia vapor sensor [38–40], glucose biosen- sor [41,42], and ascorbic acid sensor [43]. The prevalent way of constructing a sensor involves fabricating a thin film of PANI on a substrate. The usual ways of making thin films are by spin coating, spraying, fiber spinning, electrospinning, electrochemical polymerization on the surface, dip coating, and the Langmuir–Blodgett technique. Further, PANI thin films and microtubules have been grown in porous mem- branes for application as sensors [11,44–46]. In addition, PANI has been synthesized electrochemically inside ordered macroporous carbon [47]. However, it would be even more advantageous if these thin films not only were to act as sen- sors but also were able to filter acids and bases and other species based on the acid–base properties of the polymer and perform chiral separation based on the chiral properties of a 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.051