Polyaniline nanofibers response time

Figure 4.21 Normalized resistance of individual electrospun HCSA-doped polyaniline nanofibers to various alcohols (a) methanol (Ct), (b) ethanol (O) snd (c) 2-propanol (A). The response of a sensor made from several nanofibers to methanol (m) is also indicated in (a) and shows lower magnitude changes when compared to the response from individual nanofibers. The inset to (a) shows the normalized resistance of a cast film of the same polymer with and without the addition of PEO. The films have a slower response time, but similar overall behavior compared to that of the nanofiber indicating that PEO has no effect on the response to methanol. (Reprinted with permission from Sensors and Actuators B., Electric response of isolated electrospun polyaniline nanofibers to vapors of aliphatic alcohols by N. J. Pinto, i. Ramos, R. Rojas eta ., 129, 621-627. Copyright (2008) Elsevier Ltd)...

Response time is a central issue to sensing applications. Data shown in Figure 7.25 were taken at relatively low flow rates in a large volume cell hence the fastest data shown are flow rate limited. Data are plotted over many orders of magnitude on a log scale that makes the time responses appear slow even though they are not. Figure 7.26 shows the change in resistance of both the nanofiber and the polyaniline films plotted on a linear scale. The response time (90%) is measured to be 2 s for the nanofiber film and 30 s for the polyaniline film. 

The response time is another important parameter of a sensor. The response time (T90) is defined as the time it takes to reach 90% of the full value. Figure 4 shows the response time of polyaniline upon exposure to acid. The graph shows that a nanofiber film responds much faster than a conventional film, even though the nanofiber film is twice as thick as the conventional film. This is also related to the porosity of the film and small diameter of the nanofibers. 

FIGURE 7.26 Response of 0.3 p.m nanofiber (solid line) and conventional polyaniline (dotted line) thin films to 100 ppm HCl. The data are plotted on a linear scale instead of a log scale. The time response for the nanofiber data is 2 s but is instrument limited. (Reproduced from Virji, S., Huang, J.X., Kaner, R.B., and Weiller, B.H., Nano. Lett., 4, 491 96, 2004. With permission.)... 

Conventional polyaniline has been previously used as a chemical sensor but has been limited in its sensitivity and time response. Previous work on enhancing the detection capabilities of polyaniline has included the use of thinner films (6). The disadvantages of this method are the loss of robustness of the film and the difficulty in making such thin films with good control. Another way to increase sensitivity to gases is to change the morphology of the polyaniline. In particular, nanostructured forms of polyaniline, such as nanowires, nanofibers, or nanorods, have recently received much attention because their small diameters are expected to allow for fast diffusion of gas molecules into the structures (7). 

We have shown previously that the response of the nanofiber film is thickness independent, whereas, the conventional film is thickness dependent (2). That is, no matter how thick the nanofiber film is, it responds similarly to the analyte gas. However, for conventional polyaniline, thicker films give a slower and smaller response. This is an important property of the nanofiber sensors because we do not need to control the ftiickness of the film. A likely explanation of this property is that the nanofiber film is very porous while the conventional film is a dense film. The gas can, therefore, penetrate more easily through the nanofiber film than the conventional film. Also, the gas needs only diffuse into the 50 nm fiber as opposed to penetrating a micron thick film that will take much more time. 

---