Polyaniline Nanofiber Composites with Metal Salts: Chemical Sensors for Hydrogen Sulfide

Abstract

Polyaniline nanofibers show great promise as chemical sensors due to their high surface area and small diameters, which facilitate the diffusion of molecules and dopants into the nanofibers. [1, 2] Recently we reported a facile, practical, bulk synthetic method that yields pure, uniform polyaniline nanofibers with narrow size distributions that can be adjusted from 30 to 120 nm. [3] Other recent synthetic methods for nanostructured polyaniline include: induced self-assembly of nanostructures, [4] the use of nanostructured seeds, [5] electrochemical methods, [6] and electrospinning. [7] We have shown that polyaniline nanofibers respond much better than conventional films to a number of different gases including acids, bases, and reducing agents with large changes in conductivity-over eight orders of magnitude in some cases. [1, 2] However, polyaniline nanofibers do not respond well to certain classes of chemicals, such as hydrogen sulfide. In addition to providing a facile chemical route to polyaniline nanofibers with controlled distributions of diameters, our synthesis approach yields nanofibers that are dispersed in water. Water dispersions of nanofibers have several advantages for sensor applications including: 1) the simple formation of nanofiber films by drop casting, 2) the ability to modify the nanofibers by reaction with water-soluble inorganic or biological chemicals to create composite materials with different properties that can enable new applications, and 3) more environmentally benign processing than with organic solvents. Through the formation of new nanostructured conducting organic/inorganic hybrid materials, we demonstrate the ability to control and enhance the response of polyaniline nanofibers to molecules such as hydrogen sulfide. Specifically we show that nanofiber composites with transition-metal chlorides have a remarkable response to hydrogen sulfide that is four orders of magnitude greater than the corresponding unmodified nanofibers.Hydrogen sulfide is a weak acid that is important to detect because it is colorless, flammable, heavier than air, and dangerous at concentrations above 20 ppm. [8] There is also concern about its potential use in terrorist attacks. [9] Polyaniline gives a robust response to strong acids since they have the ability to fully dope polyaniline, which results in very large changes in conductivity. However, weak acids only partially dope the polymer and the response of polyaniline to hydrogen sulfide is minimal. [10] The approach we use here to enable the detection of hydrogen sulfide is by reaction with metal salts incorporated in a polyaniline nanofiber matrix. It is well known that hydrogen sulfide reacts with many metal salts in solution to form a metal sulfide precipitate and a strong acid as the by-product [11] as shown in Equation (1):where MCl 2 is a metal chloride salt and MS is a metal sulfide. The emeraldine base form of polyaniline can then detect the acid that is generated by becoming doped with a large resultant change in conductivity, as shown ...

Increased response/recovery lifetimes and reinforcement of polyaniline nanofiber films using carbon nanotubes

“…The surface to volume ratio of a material is an important property for active sensing materials as increasing this ratio can lead to dramatic enhancement of sensor sensitivity and response time [16][17][18][19][20]. Recently, interest has developed in the nanofiber form of polyaniline [21,22].…”

Covalent attachment of functional side-groups to polyaniline nanofibres

“…These one-dimensional objects combine the advantages of an organic conductor with those of a high surface area material, thus making them ideal for a diverse range of applications such as electronic devices, chemical sensors and actuators (Huang et al, 2003;Virji et al, 2004Virji et al, , 2005aVirji et al, , 2005bVirji et al, , 2006Small et al, 2007;Baker et al, 2008;Roh et al, 2002). Polyaniline (PAni) is an example of a conducting polymer, which can be switched between distinct states that exhibit dramatically different properties.…”

Synthesis and characterisation of controllably functionalised polyaniline nanofibres

“…More recently, interest has developed in the area of nanostructured polyaniline [15][16][17][18][19][20] . These one-dimensional objects 55 combine the advantages of an organic conductor and a high surface area material, thus making them suitable for a diverse range of applications such as chemical sensors, flash memory and electro-optic devices [21][22][23][24][25][26] . Derivatives of PAni have also been used to form 60 nanofibers, whereby monomers are first functionalised and then subsequently polymerised [27] .…”