**2. Technology of applying the detector layer PANI and (RR)—P3HT and sensing mechanism**

The bilayer structure of Nafion-Polyanliline was examined by the acousto-electric method (**Figure 2a**). An electrometer Keithley was used in the study due to the high resistance of the structure. The main purpose of the Nafion application was the possibility of controlled protonation of the second layer based on polyaniline to increase its electrical conductivity. Nafion (thickness of approx. 300 nm) and polyaniline (for the thickness of the layers: 100, and 180 nm) were used as sensing structures [2]. A thin layer of Nafion was deposited on the surface of the LiNbO3 waveguide by the spin-coating method at speed of 7500 rpm. Thin layers of Nafion were annealed at 40°C for 2 hours and then for 15 minutes at 120°C to harden them. Polyaniline (PANI) layers of thicknesses of 100 nm and 180 nm were made in a vacuum evaporation process. The whole process was carried out in the residua atmosphere of argon (Ar), in connection with the oxidation of the polymer during the time of its application, which affected the sensing properties of the layers. To achieve the desired quality of the atmosphere in the vacuum chamber, the set-up was rinsed three times with argon. Then, in order to evaporate the water vapor trapped in the PANI, the structure was annealed at 200°C for 10 minutes. The polyaniline application process was made at a temperature of 350–400°C. To obtain the desired thickness, the sublimation process lasted about 45 minutes.

The detector layer (**Figure 3**) of the (RR)—P3HT type polymer with a thickness of 350 nm, in the empty space of one of the delay lines of the quartz module with SAW was created. The thickness using the atomic force microscope (AFM) profile analysis was estimated. A field fragment polymer plate to use a suitably designed mask was exposed. Through the open window spraying method (nozzle thickness—0.4 mm) a pre-prepared polymer solution (RR)—P3HT was applied. Compressed synthetic air at a pressure of 1 atm. Was used. Solution by dissolving about 1 mg (RR)—P3HT in 1 ml chloroform was prepared. The distance of the nozzle from the substrate during the process was: about 40 mm, settling time was about 3 s (see **Figure 4**) [1, 8].

*Numerical Analysis of the Steady State in SAW Sensor Structures with Selected Polymers… DOI: http://dx.doi.org/10.5772/intechopen.109367*

#### **Figure 3.**

*Layer topography/morphology (RR) P3HT quartz substrate—Fragment of polymer layer (RR)-P3HT on quartz crystal from SAW—View under magnification (a), AFM images of the P3HT-RR sensing layer measured on a laminate: The polymer surface—resistive detectors research (b) AFM images of the P3HT-RR sensing layer on metallization: The polymer surface—Resistive detectors research (c) [1].*

#### **Figure 4.**

*View of a fragment of the applied polymer layer (RR)-P3HT on the quartz crystal of the AFP module. View of the boundary of the formation of a porous layer from the crystal edge visible from above 100x magnification [1].*

The sensing mechanism in PANI or PPY (Polipyrrole) and P3HT detectors layers is very interesting. The interaction of CO molecules with PANI is primarily of electrical nature. CO is an oxidizing gas—CO molecules attach electrons from the PANI structure. Polyaniline is a p-type semiconductor. The binding of PANI electrons to CO molecules results in an increase in the electrical resistance of the structure (the difference between the concentration of holes and electrons increases). This is manifested by a decrease in the frequency of the measuring resonator and, as a result, a decrease in Δf relative to the situation without CO particles in the atmosphere. In the measurements, the sensing structure was tested for the presence of very low concentrations of CO (5, 10, 15, and 20 ppm) in the synthetic air.

The P3HT contains delocalized π bonds, which permit the easy flow of electrons within the delocalized π system. The relative response of P3HT to analyte species depends on their Lewis acidity or basicity: stronger acids or bases have a larger effect on polymer resistance, with acids decreasing resistance and bases increasing resistance. For DMMP, there is a phosphorous-oxygen double bond. The electronegativity of the oxygen atom is stronger than that of the phosphorus atom, resulting in the increase of the electron density of the oxygen, so DMMP shows alkalescence. The p-type semiconducting behavior of P3HT promotes holes in the valence band of P3HT which play a key role in sensing properties. The DMMP is a strong electron donor which depletes holes from the valence band of P3HT, resulting in an increase of resistance after being adsorbed on the P3HT surface. The number of charge carriers in the P3HT film is increased by the light excitation, resulting in enhanced sensing properties, like sensitivity, the limit of detection, and response time. In addition, for the highly developed surface of the sensing layer deposited on the porous substrate, we obtain more active adsorption sites, and the scheme of the sensing mechanism described above is presented in [15].

### **3. Experimental investigations**

In the research, a measuring stand with resonators on acoustic surface waves SAW with a positive feedback loop was used in historical measurements (**Figure 2a**). The system consists of two identical delay lines—DLs (or resonators). One of the delay lines (or resonator) is isolated from the influence of the external atmosphere. The second line (resonator) is exposed to an external gas environment. Changes in the chemical composition of the atmosphere change the resonance frequency of the active line (resonator). At the output of the set-up (DLs or resonators), their high-frequency signals are electronically mixed. The frequency of the delay line without the structure of the detectors was 43.60 MHz, while the sensing structure was lower from several dozen to even one hundred kHz (as a result of its mass loading by the sensing structure). The normal mode frequency configuration (NMFC) occurs when the measuring frequency (f) is lower than the reference ones (f0). In the investigations as mentioned above, the bilayer sensing structures of Nafion+Polyaniline were examined by means of the acoustoelectric method in the NMFC case and the difference frequency Δf is determined as f–f0 (see **Figures 5** and **6**).

**Figure 5.** *Response (Δf) of the bilayer detector structure PANI (100 nm) + Nafion, to CO gas (5, 10, 15 ppm) in air, T = 34°C.*

*Numerical Analysis of the Steady State in SAW Sensor Structures with Selected Polymers… DOI: http://dx.doi.org/10.5772/intechopen.109367*

#### **Figure 6.**

*Response (Δf) of the bilayer detector structure PANI (180 nm) + Nafion, and CO gas (5, 10, 15, 20 ppm),T = 42° C (a), response (Δf) of the detector layer PANI (180 nm) + Nafion, CO gas (5, 10, 15, 20 ppm),T = 35°C (b) [2].*

The results of the studies prepared for the numerical acoustoelectric analysis (NNA) studies for the PANI layer below were shown.

The collective responses (Δfmax-Δf)/Δfmax) SAW detector on CO gas at thicknesses 100 nm and 180 nm (PANI), for the concentration from 5 to 15 ppm in synthetic air at a temperature of 34° C (100 nm) and 35° C (180 nm) in **Figure 7** were showed (**Figure 8**) [2].

The work presents comparative measurements for a polymer layer based on Polypyrrole. The results of the experiment below were shown (**Figure 9**).

Comparison of the detection properties of Polyaniline + Nafion layers (film thickness of 100 nm and 180 nm) and Polipirol layer (thickness 80 nm) of the concentration of 25 ppm, 37.5 ppm, 50 ppm in **Figure 10** were shown. It is clear that the Polypyrrole layers will be suitable for the measurement of higher concentrations.

The main aim of researching selected detector structures with a surface acoustic wave was an empirical verification of the response with numerical analysis. We must emphasize that performing empirical research was possible within limits because of

#### **Figure 7.**

*Relative changes of the response of a detector normed to maximum differential frequency for each layer PANI +Nafion: 100, 180 nm, CO gas (5, 10, 15 ppm), in synthetic air—measuring system Figure 2a.*

#### **Figure 8.**

*Relative changes of the response of a detector normed to maximum differential frequency for PANI+Nafion 180 nm, CO gas (5, 10, 15, 20 ppm), at temperature: 35°C and 42°C—measuring system Figure 2a.*

*Numerical Analysis of the Steady State in SAW Sensor Structures with Selected Polymers… DOI: http://dx.doi.org/10.5772/intechopen.109367*

#### **Figure 9.**

*Acoustical response (Δf) and resistance response (R) of the detector layer PPy (Polypyrrole 80 nm), CO gas (25, 37.5, 50 ppm),T = 36°C—measuring system Figure 2a.*

#### **Figure 10.**

*A comparison of the detection properties of PANI + NAFION with empirical results of Polypyrrole layer (80 nm) at temperature = 35°C—measuring system Figure 2a.*

the wide range of work and the complex technological processes connected with the practical feasibility of the detector structure [1, 16].

A practical system for testing the acoustic-electrical properties of polymer layers based on (Regio-Regular)-P3HT in **Figure 11** was shown.

Empirical results for illuminating LEDs (200 mA) with different wavelengthsof the layer (RR)-P3HT are shown in **Figure 12**. The research system from **Figure 2b** was used. The measuring system uses diodes—1 W SMD 350 mA ProLight Opto.

#### **Figure 11.**

*The practical implementation of the invention of the patent no. PL 230526 B1—A measuring chamber for testing using SAW detectors—measuring system Figure 2b.*

Results of measurements DMMP in interaction with polymer layer RR-P3HT illuminated with a diode light of 200 mA in the form of a histography are shown in **Figure 13**.

In order to optimize numerically and compare the experiment with the theory, the results of the experiment depending on the concentration were normalized as follows in **Figure 14**.

Three measurement series were made. The diodes were driven with 100, 200, 300 mA current. The layer was additionally activated by a small incandescent lamp with white light (with a maximum of about 750 nm) and different illuminance (see **Figure 13**). Exposure time was about 10 min for each 100, 200, and 300 mA current.

#### **Figure 12.**

*Experiment—Detector layer (RR)-P3HT, thickness 500 nm, gas DMMP (1.5; 2; 3 ppm, illumination by diode 200 mA (selected wavelengths) relative change of velocity vs. time (concentration)—measuring system Figure 2b.*

#### **Figure 13.**

*Experiment—Layer (RR)-P3HT, thickness 500 nm, gas DMMP (1.5; 2; 3 ppm, illumination by diode 200 mA (selected wavelengths)—Histography—relative change of velocity vs. time (concentration)—measuring system Figure 2b.*

#### **Figure 14.**

*Experiment—Layer (RR)-P3HT, thickness 500 nm, gas DMMP (1.5; 2; 3 ppm, illumination by diode 200 mA (yellow light) normalized results relative change of velocity vs. time (concentration)—measuring system Figure 2b.*

*Numerical Analysis of the Steady State in SAW Sensor Structures with Selected Polymers… DOI: http://dx.doi.org/10.5772/intechopen.109367*

In order to optimize numerically and compare the experiment with the theory, the results of the experiment were normalized as follows in **Figure 14**—depending on the concentration. Results with a DMMP concentration of 2 ppm in the air were shown.
