Nanofibers sensors

Composites of PANI-NFs, synthesized using a rapid mixing method, with amines have recently been presented as novel materials for phosgene detection [472]. Chemiresistor sensors with nanofibrous PANI films as a sensitive layer, prepared by chemical oxidative polymerization of aniline on Si substrates, which were surface-modified by amino-silane self-assembled monolayers, showed sensitivity to very low concentration (0.5 ppm) of ammonia gas [297]. Ultrafast sensor responses to ammonia gas of the dispersed PANI-CSA nanorods [303] and patterned PANI nanobowl monolayers containing Au nanoparticles [473] have recently been demonstrated. The gas response of the PANI-NTs to a series of chemical vapors such as ammonia, hydrazine, and triethylamine was studied [319,323]. The results indicated that the PANI-NTs show superior performance as chemical sensors. Electrospun isolated PANI-CSA nanofiber sensors of various aliphatic alcohol vapors have been proven to be comparable to or faster than those prepared from PANI-NF mats [474]. An electrochemical method for the detection of ultratrace amount of 2,4,6-trinitrotoluene with synthetic copolypeptide-doped PANI-NFs has recently been reported [475]. PANI-NFs, prepared through the in situ oxidative polymerization method, were used for the detection of aromatic organic compounds [476]. 

Nanofibers also used for sensors because their high specific area allows them to sorb and/or react rapidly with low levels of analytes in the air. It is reasonable therefore to expectbetter performance from nanofiber sensors. 

The sensing behaviors of PEDOT-PSS/PVP (Fig. 5.21) and PVP nanofibers on ethanol, methanol, THF, and acetone vapors are studied. The sensing was carried out for several q cles by repeated exposure of the nanofibers to saturated organic vapors and air alternately. Both PEDOT-PSS/PVP and PVP electrospun nanofiber sensors have exhibited good reversibility, reproducibility and response and recovery time. The response and recovery time of PEDOT-PSS/PVP nanofibers upon exposure to ethanol vapor are much faster than those of PVP nanofibers. 

Conducting Polymer-Vapor Interactions Enhanced Sensitivities of Polyaniline Nanofiber Sensors Thickness vs. Sensitivity Reversibility of Ammonia Vapor Sensors ... 

FIGURE 7.29 Response of polyaniline nanofiber sensors of different thicknesses upon exposure to 100 ppm of HCl vapor (left). The schematic diagram (right) illustrates that the vapor molecules (arrows) can interact rapidly with most of the polyaniline nanofibers (gray) since the films are porous. (Reproduced from Huang, J.X., Virji, S., Weiller, B.H., and Kaner, R.B., Chem. Eur. /., 10, 1314-1319, 2004. With permission.)... 

However, if the film is composed of long nanofibers (Figure 7.30b), is determined by the diameter of the nanofibers, rather than the film thickness. This may explain the thickness effects observed in the experiments. Since the surface area per unit mass of the nanofiber films is generally much greater than that of the conventional films (unless the thickness is reduced to less than the nanofiber diameter), a nanofiber sensor should have much better sensitivity than a conventional polyaniline sensor. 

FIGURE 7.31 (a) A nanofiber sensor responding to 378 ppm of NH3. (b) A first derivative plot of the sensor... 

The nanofiber sensors have a considerably lower detection limit than conventional polyaniline. An HQ doped nanofiber film would respond clearly and reversibly to 0.9 ppm of ammonia vapor... 

Similarly, Aussawasathien et al. [185] compared the performance of CSA-doped polyaniline-polystyrene electrospun nanofibers with a thin film of the same composition for the electrochemical detection of hydrogen peroxide. As expected, the thin-film sensor showed significantly weaker currents than those of the electrospun nanofibers, with both materials showing a linear response of the redox current as a function of hydrogen peroxide concentration. However, the electrospun nanofiber sensor showed a much higher sensitivity, as evidenced by the greater slope. It was also shown that the detection of... 

Figure 5. Humidity dependence of resistivity for PE0/LiCl04 sensors (a) the nanofiber sensor and (b) the film sensor (35).

Figure 7. (a) Cyclic voltammetric (CV) spectra for the GOX-immobilized HCSA-PANI/PS nanofiber sensors at different glucose concentrations, and (b) The current response of the GOX-immobilized HCSA-PANI/PS nanofiber andfilm sensors to various glucose concentrations. Note the current response has been scaled by the weight of the polymeric material deposited on the electrodes (35). 

Polyaniline nanofibers have recently received much attention as sensors because they have many advantages over conventional films. In particular, we have designed polyaniline nanofiber sensors for toxic gases and have shown that they give much faster and larger responses due to their small fiber diameters and hi surfrice areas (2, 5, 13). Subsequently, others have used arrays of oriented polyaniline nanofibers 14) as well as single nanowires (7) to detect gases. 

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. 

Ding B, Wang M, Yu J, Sun G (2009) Gas sensors based on electrospun nanofibers. Sensors 9 1609-1624 Dong B, Krutschke M, Zhang X, Chi LF, Fuchs H (2005a) Fabrication of polypyrrole wires between microelectrodes. Small 1 520-524... 

Fig. 11.3 (a) Gas sensing mechanisms of n-type emd p-type semiconductors and the different connecting configuration between long nanoflbers and nanopaiticles or short nanofibers, (b) Schematic model of the three steps for the sensitization mechanism of the as-prepared nanofiber sensor, (c) Response of Sn02 hollow nanofibers versus thin films serving as a reference (a Reprinted with permission from Yoon et al. [55]. 2012 Elsevier b Reprinted with permission from Zhang et al. [73]. 2010 Elsevier c Reprinted with permission from Cho et al. [74]. 2011 Elsevier)... 

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