Polymer nanowires/nanofibers have a high surface to volume ratio so they seem interesting candidates for preparing sensors with a high sensitivity and a fast response [10]. All of our polymers used in this research (SPAN-Na, PEI) are difficult to process with standard lithography to pattern nanowires because they are well soluble in many solvents. Moreover, ultraviolet (UV) light exposure, electron beam or laser ablation cannot be applied because these processes can modify chemical structures leading to changing properties of the polymers.
A common solution is synthesis of polymer nanowires/nanofibers then deposition of these nanowires onto prefabricated interdigitated electrodes in thin film/sheet form [25, 38, 43-45].
PANI nanofibers with diameters of 30-50 nm and lengths from 500 nm up to several micrometers have been made, but tended to agglomerate into interconnected nanofiber networks, rather than bundles [38, 43, 44]. The PANI nanofiber thin film responded as
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anticipated much faster than the conventional film to both acid and base, even when the total nanofiber film was more than twice as thick. The small diameter of the nanofibers and increased high surface area within the film enabled a fast access by the gas vapors. It was also found that the nanofiber films showed essentially no thickness (0.2-2.5 m) dependence in their performance [38, 43]. However, the contact resistance between nanofibers and electrodes should be considered because nanofiber films exhibited lower sensitivity than conventional thin films due to imperfect contact between nanofibers and electrodes [46].
Electro or chemical polymerization inside an anodic aluminum oxide template has been used widely to make conductive polymer nanofibers such as PANI [47] and polypyrrole (PPy) nanowires [48]. PPy nanowire arrays with high density and small diameter (~50 nm) [49]
showed a relatively high response (10%) towards 1.5 ppm NH3 and a comparatively short response and recovery time.
Patterning by direct writing via dip-pen nanolithography can also be used to make polymer nanowires [50]. Poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires with a diameter of 300 nm across a 55 m gap between a pair of electrodes were fabricated by using dip-pen nanolithography [51]. The responses to nitric oxide (NO) were highly linear and reproducible with a minimal concentration of NO of 10 ppm.
Electrospinning is another method which has been used to make isolated and relatively long polymer nanofibers. This approach has also been used in fabricating conductive polymer nanowire such as sulfuric acid-doped PANI fibers [52] and isolated camphor sulfonic acid- doped PANI nanofibers [53]. Oriented PANI nanowires with diameters of 100 nm were deposited on gold electrodes by using a scanned-tip electrospinning [54]. The devices showed a rapid and reversible resistance change upon exposure to NH3 gas at low concentration of 0.5 ppm. The response times of nanowire sensors with various diameters corresponded to radius- dependent differences in the diffusion time of ammonia gas into the wires. Electrospinning has also been extensively used to fabricate composite nanowires of PANI and poly(methyl methacrylate) (PMMA) [55]. The composite nanowires showed a linear, reversible and reproducible response towards triethylamine vapors ranging from 20 ppm to 500 ppm. The PANI doped with toluene sulfonic acid exhibited the highest sensing magnitude. A gas sensor based on polypyrrole (PPy):PMMA composite fibers [56] exhibited greatly improved performances towards NH3 comparing with those of the device based on a PPy flat film.
133 The polymers used in this research including SPAN-Na, PEI were synthesized from available commercial polymers and then were post-modified as presented in the previous chapters.
Therefore, the synthesis of nanowires by aqueous/organic interfacial synthesis [38, 43, 44] or UV synthesis [45] or polymerization incorporated with anodic aluminum oxide template has not been applied. Hence, electrospinning is probably the most suitable way to make conductive polymer nanowires in our case.
Electrospinning method was tried in our research to make SPAN-Na nanowires deposited on micro comb-shaped Pt electrodes. The setup for electrospinning is illustrated in Figure 3.
SPAN-Na solution was filled in a 10 l glass syringe bearing a metal capillary with internal diameter of 130 m. This needle tip acted as an electrospinning source. The chip with interdigitated Pt electrodes was mounted as a fiber collector and the electrode contact pad was connected to the grounded counter electrode. The voltage of 5 kV was supplied from a high voltage power supply Teltron, England. The distance between the capillary tip and the chip was 500 m - 3 mm. To obtain nanofibers, the electrical field strength should be maintained at 107 V.m-1 [57]. So in this process utilizing 5 kV, the distance was set 0.5 mm. However, with short distance of 0.5-2 mm, there was arc between needle and the chip (connected to the ground electrode), resulting in burnt polymer dots on the chip. The arc disappeared when the distance increased to 3 mm. The flow rate of SPAN-Na solution (0.2 mL/hour) was controlled by a syringe pump (NE-1000X2, USA). The polymer jet was electrostatically extracted from the tip and dried on the way to the substrate with comb-shaped electrodes.
Figure 3. Setup for electrospinning process.
chip syringe
Power supply Syringe pump
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However, the electrospinning process did not yield adequate sensor material in our case. A spinning jet could be obtained but no suitable nanofibers were obtained. Co-spinning of SPAN-Na in DMSO with polyethylene oxide (PEO) to facilitate the formation of nanofibers [58, 59] was also employed, however not successful. Further research is needed to fabricate SPAN-Na, PEI and their blend nanofibers, probably a higher voltage of 10-15 kV should be used [52]. On the other hand the response time of our thin film sensor in the order of minutes seems sufficient for application in a greenhouse environment.