NANOPOLYMERS *
CHAPTER 6 CHAPTER 6
6.5 POLYMERIC NANOFIBERS AS SENSORS
6.5.4 TYPES OF SENSORS
There are many different types of sensors, measuring different things. This includes the following types of sensors:
● Piezoelectric—measuring mass
● Electrochemical—measuring electric conductivity and distribution (voltage)
● Optical—measuring the intensity of light or radiation
● Calorimetric—measuring the flux of heat into the sensor.
140 CHAPTER 6 NANOPOLYMERS
Polymeric nanosensors can be used in many applications associated with any of the above, which could include almost anything tangible. This can be applied to any industry that needs any type of sensory system.
The nanopolymer sensors have a wide variety of applications, which include measuring the:
● concentration of gaseous compounds in air: oxygen, carbon dioxide, ammonia, and volatile organic compounds, as well as others,
● humidity (water content) in air,
● concentrations of liquids in solution: mercury and various other compounds.
Not only can nanosensors be applied to a variety of processes, but they can replace older macromo- lecular sensors. This is because nanosensors analyze the concentration of various compounds on the molecular level. This means the nanosensor can measure concentrations on a very small scale, with a significantly higher sensitivity than macromolecular sensors. Nanosensors can accurately measure very small changes in a compound`s concentration.
Sensing makes use of an electric current to generate a signal. Therefore, to have a polymer conduct a signal, it must exhibit a certain amount of polar behavior. This is achieved by doping the polymer with conducting compounds or by using polymers that have moderately polar repeating chains. All of the conducting materials used in the following experiments exhibit some sort of molecular polarity.
A polymer comprised of poly(1,5-diaminonapthlene) (DAN) nanofibers was produced by Rahman et al. (2008) through a chemical catalyst polymerization, using Iron(III) salt as catalyst. Such nanofib- ers were used to sense water in a nonaqueous solution of acetonitrile.
The sensors were fabricated by recrystallizing the DAN nanofibers twice in an aqueous ethanol (50%v/v) solution under nitrogen and dried at room temperature under vacuum over P2O5 for 48 h.
Poly(1,5-DAN) nanofibers were fabricated using chemical catalytic polymerization. The fabrica- tion procedure had 2 g of DAN monomer being added to 100 mL methanol solution containing 3.4 g FeCl3. The mixture was stirred at 30°C for 24 h. The reaction was done in both oxygen and nitrogen atmospheres.
Various spectroscopic methods showed that the polymer was formed homogeneously. Fibers were found to be roughly 10–30 nm in diameter and 400 nm in length. Fig. 6.15 shows the experimental results obtained.
The graph on top (A) shows the variance of current with time, this is very important because having a constant current allows the sensors to receive a constant signal, so a proper display of the reading can be made. Graph (B) gives the current and applied voltage for various different acetonitrile solutions containing a water concentration of (starting from the bottom): (a) 1%, (b) 5%, (c) 10%, (d) 20%. The significance of graph (B) is it outlines the current expected for the applied voltage the circuit is operat- ing under. Finally, graph (C) gives the expected current generated at various temperatures, the percent- ages in graph (C) correlate to the water concentration in solution. Note that the highest concentration of water in solution gives the highest current, this is correlated to the fact water is a conductor, and having a greater presence of water in solution gives a greater amount of current during operation.
The concentration of FeCl3 and the solvent used was shown to have a significant effect on the reac- tion yield. Using methanol solvent also produced a very high reaction yield. The DAN sensor is very sensitive to water in a nonaqueous acrylonitrile solution. The detection limit of the sensor was found to be 0.01%.
141 6.5 POLYMERic NANOFiBERS AS SENSORS
The results were comparable to the Karl Fisher titration method. The Karl Fisher titration is prob- lematic because it is a time-consuming process, and it is difficult to determine the endpoint of the titration.
In a different study, Aussawasathien, Dong, and Dia investigated two different uses of polymeric nanofiber sensors: one is to sense the humidity, while the other is used to sense the presence of hydro- gen peroxide and glucose.
The nanofibers were prepared via electrospinning and had a diameter of between 400 and 1000 nm.
These sensors had significantly more sensitivity compared to their film-type counterparts. It was noticed that after use, the sensor measuring humidity had small morphological changes, while the sen- sor measuring hydrogen peroxide and glucose exhibited no changes.
They made a PEO/LiClO4 sensor that was used with a film-type sensor to measure humidity in a humidifier at 25°C at a range of 25–65% humidity. They also made a PA/PS/CSA sensor containing GOx that was used with a film-type sensor to measure H2O2 from the oxidation of glucose at hydrogen peroxide concentrations less than 25 mM.
FIGURE 6.15
Results of experimentation. (A) variance of current with time in acrylonitrile solution containing water at (a) 20%, (b) 10%, (c) 5%, (d) 1% concentrations. (B) current and applied voltage relationship.
(c) temperature dependence on current.
From Rahman, A., Won, M.-S., Kwon, N.-H., Yoon, J.-H., Park, D.-S., & Shim, Y.-B. (2008). Water sensor for a nonaqueous solvent with poly(1,5-diaminonapthalene) nanofibers. Department of Chemistry and Center for Innovative BioPhysio Sensor Technology.
142 CHAPTER 6 NANOPOLYMERS
The results of the experiment gave the following graphs: the first pertains to the PEO/LiClO4 sensor while the second pertains to the PA/PS/CSA sensor (Fig. 6.16).
Fig. 6.16 shows the change in resistivity with percentage humidity. The greater slope above shows the greater sensitivity of the nanopolymeric sensor compared to film sensors.
Fig. 6.17 shows the varying current with hydrogen peroxide concentration. It also shows that the nanopolymeric sensor is significantly more sensitive than the film sensor.
It was found that the polymeric nanosensor changes the current transmitted very quickly. This means that the nanosensor is more sensitive than the film sensor. A greater sensitivity is desired because it distinguishes the current conditions better than a less sensitive device. The sensitivity is improved by the contact surface area of the sensor.
Lee, Oh, Kang, and Kwak (2010) presented sensors that sense nitroaromatic compounds including:
2,4-dinitrotoluene and 2,6-dinitrotoluene. They also note that similar types of sensors may be applied to sense volatile organics. The polymers used were: poly[1-phenyl-2-(p-trimethylsilyl)phenylacetylene]
(PTMSPA) and poly[1-phenyl-2-p-(dimethyl octadecylsilylphenyl)acetylene] (PDMOSPA). Both of these polymers have high molecular weight and high polydispersity indices.
Polymer solutions were prepared by mixing benzene with one of the above mentioned polymers (the solution created is 3 mL) and put into a vail. The vail was deep-freezed in a deep freezer (−70°C) or with liquid nitrogen (−196°C). After freezing the sample was dried in a freeze dryer (−50°C at a pressure of 9 mmHg).
The photoluminescence spectra were analyzed on a spectrofluorometer. The fluorescence of PTMSPA was found by inserting the fibers into a vail containing the solid analytes (2,4-dinitrotolu- ene or 2,6-dinitrotoluene); this was at room temperature, with an excitation wavelength of 420 nm.
The structural coarsening was analyzed along two routes: first by freezing the solution and second FIGURE 6.16
Results for PEO/LiclO4 humidity sensor compared to the film sensor. (a) the results for the PEO/LiclO4 sensor and (b) the film sensor. Log “R” is the logarithm of resistivity and “% humidity” is the percentage humidity.
From Aussawasathien, D., Dong, J.-H., & Dia, L. (n.d.). Electrospun polymer nanofiber sensors. Department of Materials and Chemical Engineering and University of Dayton Research Institute.
FIGURE 6.17
Results for PA/PS/cSA hydrogen peroxide sensor. the vertical axis is milliamps/gram of PA/PS/cSA polymer and the horizontal axis is the concentration of H2O2 in mM.
From Aussawasathien, D., Dong, J.-H., & Dia, L. (n.d.). Electrospun polymer nanofiber sensors. Department of Materials and Chemical Engineering and University of Dayton Research Institute.
FIGURE 6.18
Picture of nanofibers at various mass fractions: (A) 0.1%, (B) 0.01%, (c) 0.001%, and (D) 0.0003% wt.
From Lee, W.-E., Oh, C.-J., Kang, I.-K., & Kwak, G. (2010). Diphenylacetylene polymer nanofiber mats fabricated by freeze drying:
preparation and application for explosive sensors. Macromolecular Journals.
144 CHAPTER 6 NANOPOLYMERS
by the sublimation of the frozen solution in a vacuum. The PTMSPA polymers were fabricated with four different mass fractions (in benzene): 0.1%, 0.01%, 0.001%, and 0.0003%. These different mass fractions had their properties analyzed after freeze drying in a deep freezer, and after they were frozen using liquid nitrogen.
PTMSPA and PDMOSPA polymer solutions were successfully prepared with a large fractional free volume. It was found that the morphology of the structure of the fibers are significantly dependent on the polymer solution, the composition of the polymer, and the freezing methodology (either using liquid nitrogen or a freeze drier). Fig. 6.18 shows the various thicknesses found when utilizing different mass fractions of polymer in solution.
These studies show the very interesting potential for nanofiber polymers to act as sensors. These sensors can accurately sense various compounds in the environment, such as: the concentration of water in nonaqueous liquids, the presence of nitroaromatic compounds in vapor, and the presence of hydrogen peroxide and glucose.