3.1 UV-Visible Results
The absorption spectra of Zn(II)Pc chloroform solution using different concentration and Zn(II)Pc LB thin film multilayers prepared onto quartz glass substrate was obtained by UV-vis spectrometer and results represented in Fig. S3 and Fig. 3, respectively. Two absorption bands attributed to π–π* transition (the Soret (B) and the Q-band) of the Zn(II)Pc molecules were observed at 269 nm and 684 nm both Zn(II)Pc solution and LB thin film multilayers. The inset in Fig. 3 shows the correlation between number of layers and the change of absorption values. This linear proportionality between them has supported that deposition takes place during coating for each layer and coating is very sustainable for each bilayer.
3.2 QCM results
In this work, the Y-type Zn(II)Pc LB films were fabricated in a symmetrical mode onto quartz crystal substrate (Fig. 4). The inset in Fig. 4 presents linearity between the frequency change and Zn(II)Pc LB layers with a linear regression of 0.9923. This linearity demonstrated that each Zn(II)Pc bilayer could be successfully deposited and almost equal mass was coated onto the substrate. The frequency change for each layer (27.17 Hz/layer) and the mass coated onto the substrate for each layer (435.124 ng/layer) could be determined from the data of the inset in Fig. 4 and by utilizing Sauerbrey equation [13].
3.3 SPR results
SPR system was utilized to evidence the deposition of Zn(II)Pc LB monolayer onto gold-coated substrate. The SPR curves of Zn(II)Pc LB thin films are represented in Fig. S4 and these curves (from 2 layers to 10 layers) shift to from the left to the right. The inset in Fig. S4 presents the linear relationship between the shifts in the angle of incidence and Zn(II)Pc LB layers. A linear regression of 0.9884 indicates that Zn(II)Pc LB films were fabricated onto the substrate successfully and homogenously. The experimentally measured SPR curves of Zn(II)Pc LB films were fitted via Winspall software to determine the values of refractive index and thickness for Zn(II)Pc-coated thin films. The fitted SPR curves for Zn(II)Pc-coated four LB film layers and bare gold were given in Fig. 5. Similar fitting process was fixed for other Zn(II)Pc LB layers. The inset in Fig. 5 provided similar results compared with UV-Vis and QCM results by obtaining of the linear relationship. As seen in Table 1 and the inset of Fig. 5, the values of Zn(II)Pc LB film thickness or refractive index demonstrate a rise depending on the Zn(II)Pc LB layers.
Table 1 The thickness and refractive index of Zn(II)Pc thin films.
|
Number of LB layers
|
Thickness (nm)
|
Refractive index
|
Zn(II)Pc
LB thin film
|
2 layer
|
3.2
|
1.41
|
4 layer
|
5.6
|
1.53
|
6 layer
|
7.4
|
1.61
|
8 layer
|
9.5
|
1.66
|
10 layer
|
10.9
|
1.72
|
3.4 AFM and SEM results
AFM measurements were taken to analyze the surface morphology of Zn(II)Pc LB thin film with 2D and 3D AFM images in the surface area of 10x10 μm2 (Fig. 6). The RMS roughness, mean roughness and maximum height values of the surface were observed as 3.8, 2.85 and 14.96 nm for the image recorded with the dimensions 10 μm x 10 μm, respectively. The Zn(II)Pc LB film surfaces, containing some roughness structure, provide VOCs molecules to penetrate into deeper layer in vapor sensing application. SEM images of non-coated bare glass and Zn(II)Pc LB film coated glass were obtained for the supporting of the LB films fabricated. While Fig. S5a displays the SEM image for bare glass (non-coated), the SEM image given in Fig. S5b proves the matrix of Zn(II)Pc can be formed in the thin film.
3.5 Kinetic Measurements of the Optical/Mass Chemical Sensor
SPR technique is used to monitor a host-guest interaction between the Zn(II)Pc-based chemical sensor and VOCs by recording the photodetector responses (Fig. 7). Some harmful VOCs, namely, toluene, m-xylene, carbon tetrachloride and dichloromethane were released into the space of gas cell for 120 seconds, in order of air-vapor-air-vapor-…-air, periodically. The responses of LB film to all vapors suddenly increased by several seconds (adsorption process) and then an exponential decrease was observed due to the diffusion process. This rapid change can be resulted from two important reasons. The first, the shift of background refractive index, which depends on the concentration of vapor, may be asserted. The surface effect between Zn(II)Pc LB film surface and vapor molecules can be stated as the second reason. Similar kinetic study was carried out using QCM technique for supporting SPR kinetic results. From the Fig. 7 and Fig. S6, the comparatively large response values obtained from SPR and QCM measurements for dichloromethane are noteworthy among used vapors at saturated concentration. The SPR/QCM kinetic results of the Zn(II)Pc-based chemical sensor to dichloromethane vapor was presented in Fig. S7 and Fig. S8 to observe the repeatability and renewability properties of Zn(II)Pc LB film chemical sensor by running three times of this process. These kinetic results show that Zn(II)Pc chemical sensor are reproducible for dichloromethane vapor. Similar the host-guest interaction was recorded at different concentration of dichloromethane represented in Fig. S9.
The results of all kinetic measurements can be expressed through the vapor pressure and the molar volume of VOCs given in Table 2, and dichloromethane vapor has the biggest vapor pressure (at 20 ℃) and molar volume among used in this work. Since dichloromethane molecules have the highest vapor pressure and the lowest molar volume among VOCs used in this work, their diffusions into the Zn(II)Pc LB film is the easiest. The values of the kinetic responses for the other vapors (carbon tetrachloride, toluene and m-xylene) are lower than dichloromethane vapor due to the effect of these physical properties. Therefore, the other vapors with the highest molar volume cannot easily penetrate into the Zn(II)Pc LB thin film sensor material when compared with the diffusion of dichloromethane vapor into the same thin film.
Table 2 Physical properties of organic vapors and kinetic responses.
Organic vapors
|
Molar volume
(cm3 mol−1)
|
Vapor pressure (kPa, 20 ℃)
|
Photodetector
response change, (∆I)
|
Frequency shift,
(∆f)
|
Dichloromethane
|
64.10
|
46.50
|
0.148
|
78
|
Carbon tetrachloride
|
97.10
|
12.00
|
0.044
|
29
|
Toluene
|
107.10
|
2.91
|
0.037
|
27
|
m-xylene
|
122.00
|
0.80
|
0.016
|
24
|
|
|
|
|
|
|