Preparation of Zinc(II) Phthalocyanine-Based LB Thin Film: Experimental Characterization, the Determination of Optical Properties and Harmful Organic Vapor Sensing Ability

Yaser Acikbas (  yaser.acikbas@usak.edu.tr ) University of Usak https://orcid.org/0000-0003-3416-1083 Matem Erdogan Balikesir University: Balikesir Universitesi Rifat Capan Balikesir University: Balikesir Universitesi Cansu Ozkaya Balikesir University: Balikesir Universitesi Yasemin Baygu Pamukkale Üniversitesi: Pamukkale Universitesi Nilgün Kabay Pamukkale Üniversitesi: Pamukkale Universitesi Yaşar Gök Usak University: Usak Universitesi


Introduction
Phthalocyanines have been greatly preferred in many technological and scienti c areas such as solar cells [1], LB lms [2] and chemical sensors [3]. Presence of planar delocalized large 18π electrons in the macrocycle and strong optical absorption in metallo phthalocyanines provide them a convenient sensing material for volatile organic compounds (VOCs) [4]. LB thin lm technique plays a critical role to fabricate a sensitive thin lm layer of phthalocyanine derivative for room temperature VOCs sensor applications due to its practical and controlled production [5]. Surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) methods are widely utilized as an optical sensitive and mass sensitive, respectively, for the VOCs detection using phthalocyanine LB lms [6,7]. Thanks to the high sensitivity and stability of the sensitive absorption layers, phthalocyanine-based thin lms have become a signi cant area of study in sensor applications [8].
In this paper, a new synthesized amphiphilic phthalocyanine (Zn(II)Pc) is selected to fabricate a sensitive and selective LB thin lm layer which is used for a detection of VOCs. Dichloromethane, carbon tetrachloride, toluene and m-xylene vapors are used for sensor applications. SPR and QCM results indicated that the selectivity of Zn(II)Pc LB layer to dichloromethane is approximately 3.39 times for SPR and 2.69 times for QCM results higher than other vapors. The results obtained from kinetic results stated that Zn(II)Pc-based mass or optical sensor yields a stable and reversible response to all vapors and a selective and sensitive response to dichloromethane vapor than others.

Experimental Details
Zn(II)Pc molecule (Fig. 1), which is a candidate for the use of a sensor active layer, was synthesized by the following literature [9]. All Zn(II)Pc thin lms were prepared with model 622 Nima-LB trough (area 1200 cm 2 ) onto three different substrates such as gold-coated glass, quartz glass and quartz crystal for SPR, UV-Vis, SEM, AFM and QCM measurements.
The properties of Langmuir for Zn(II)Pc molecules were examined using a NIMA 622 model LB trough. The surface pressure (π) measurements as a function of the area (A) covered by the Zn(II)Pc molecules dissolved in chloroform. This solution at a concentration of 1 mg mL -1 was slowly dripped into the trough, whose water surface was controlled for cleanliness (0 <π> 0.1), using microliter syringe (Hamilton). The LB trough (made of Te on materials) consists of Wilhelmy, which is used as a pressure sensor, and two barriers, one of which is movable. This barrier compression speed has been determined in the literature to be a suitable value to avoid instability effects [10][11][12]. After waiting approximately 15 minutes for the chloroform in solution to evaporate, the moving barrier was allowed to compress the monolayer at the air / water interface towards the center of the trough. Isotherms obtained as a result of compression of the Langmuir monolayer at the air / water interface are characterized by certain regions such as gas phase, liquid phase, solid phase and collapse. The process of all Zn(II)Pc LB layer deposition was xed by determining a convenient surface pressure value of 25 mN m -1 from isotherm graph (Fig.   S1).
The process of Zn(II)Pc LB layer deposition onto solid substrates was started after the determination of a suitable surface pressure. LB thin lm transfer curve is a critical parameter for monitoring the deposition process, and the rst four layer transfer graph of Zn(II)Pc monolayer within the scope of the paper is given in Fig. 2. The analysis of this process is carried out by calculating the transfer rate (TR) shown in A 1 is the decrease in the area covered by a monolayer on the water surface and A 2 is the area of the thin lm coated on the substrate. The TR value was calculated as ~0.93 for Zn(II)Pc LB thin lm. This value is proof that homogeneous LB thin lms have been successfully produced.
The chemical sensing abilities of Zn(II)Pc LB thin lm sensor were actualized via SPR and QCM technique. The detailed information about these techniques was reported in the part of Supplementary Data (Fig. S2).

UV-Visible Results
The absorption spectra of Zn(II)Pc chloroform solution using different concentration and Zn(II)Pc LB thin lm 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 lm 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.

QCM results
In this work, the Y-type Zn(II)Pc LB lms 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].

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 lms 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 lms were fabricated onto the substrate successfully and homogenously. The experimentally measured SPR curves of Zn(II)Pc LB lms were tted via Winspall software to determine the values of refractive index and thickness for Zn(II)Pc-coated thin lms. The tted SPR curves for Zn(II)Pc-coated four LB lm layers and bare gold were given in Fig. 5. Similar tting process was xed 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 lm thickness or refractive index demonstrate a rise depending on the Zn(II)Pc LB layers.

AFM and SEM results
AFM measurements were taken to analyze the surface morphology of Zn(II)Pc LB thin lm with 2D and 3D AFM images in the surface area of 10x10 μm 2 (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 lm 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 lm coated glass were obtained for the supporting of the LB lms 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 lm.

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, mxylene, 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 lm 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 rst, the shift of background refractive index, which depends on the concentration of vapor, may be asserted. The surface effect between Zn(II)Pc LB lm 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 lm 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 lm 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 lm sensor material when compared with the diffusion of dichloromethane vapor into the same thin lm.

Conclusions
The structural characterization and the preparation quality of Zn(II)Pc LB thin lm layers were examined by (i) the mass change onto quartz crystal (ii) the shift of the UV-Vis absorption spectra (iii) the change of resonance angle in the SPR curves, and (iv) SEM/AFM imaging techniques. The results obtained from all characterization techniques aforementioned in related parts prove that the fabrication of Zn(II)Pc-based thin lms onto the different substrates was successfully and uniformly achieved with linear regressions vary from 0.9884 to 0.9957. Some optical properties of Zn(II)Pc materials were also illuminated with this study. The sensing behaviors of Zn(II)Pc chemical sensor against to some VOCs were investigated by utilizing QCM and SPR techniques. These kinetic results present that Zn(II)Pc-coated chemical sensor exhibits higher sensitive and selective response to dichloromethane vapor than other vapors.