Performance of SiO2 - TiO2 Thin Films as Protective Layer to Chlorophyll in Medicinal Plants

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Performance of SiO2 - TiO2 Thin Films as Protective Layer to Chlorophyll in Medicinal Plants from UV Radiation: Influence of Dipping Cycles 1 M. Sankareswari1, B. Karunai Selvi1, K. Neyvasagam2 1 – V.V. Vanniaperumal College for Women, Virudhunagar, Tamilnadu, India 2 – PG and Research Department of Physics, The Madura College, Madurai, Tamilnadu, India DOI 10.2412/mmse.43.29.127 provided by Seo4U.link

Keywords: SiO2 - TiO2 thin films, XRD, UV radiation, chlorophyll, S.trilobatum, S.nigrum.

ABSTRACT. Titanium dioxide (TiO2) is a wide band gap semiconductor and efficient light harvester. SiO2 doped TiO2 (SiO2 - TiO2) thin films of different dipping cycles were prepared on glass substrate using sol gel method and annealed at 400°C for 3 hours. Thin films were characterized by various techniques such as X - ray diffraction (XRD), UV - visible spectroscopy and Scanning Electron Microscopy (SEM) with elemental analysis (EDAX). Ultraviolet rays constitute a very small fraction in solar spectrum but it influences much in all living organisms and their metabolisms. Plants use chlorophyll to trap light energy needed for photosynthesis. Increased exposure of UV light reduces the total chlorophyll in medicinal plants. TiO2 has strong Ultra Violet (UV) light absorbing capability because of its advantages like non – toxicity, chemical stability at high temperature and permanent stability under UV exposure. In the present study, the performance of SiO2 - TiO2 thin films as a protective layer to the chlorophyll contents present in the medicinal plants of Solanum trilobatum (Thuthuvaalai) and Solanum nigrum (Manathakkali) under UV radiation has been investigated. The results revealed that SiO2 - TiO2 thin films are good UV absorbers and chlorophyll content increases with the increase in number of dipping cycles.

Introduction. Transparent conducting oxide (TCO) materials are of great interest due to their distinctive physical, chemical, optical and opto electronic properties. Among the various TCO materials ZnO, CdO, SnO, SnO2 and TiO2 etc., TiO2 plays a promising role in several areas of research because of its high efficient photo catalytic activity, high refractive index, resistance to photo corrosion, chemical stability, low cost and non – toxicity [1]. Another importance of TiO2 is its implementation in self sterilizing surfaces and its usage in hospitals because of its reliable and stable characters under irradiation [2]. The phase structure and semiconducting properties of TiO2 thin films can be strongly modified by doping with impurities like Ag, Fe, Cu, SiO2, ZnO etc., [3]. SiO2 doped in TiO2 enlarges surface area and enhances the thermal stability and visible light photo activity of TiO2 [4]. Plants use chlorophyll to trap light energy needed for photosynthesis. Chlorophyll is more beneficial to human body in a numerous unique and distinct ways. It has anti-mutagenic and anticarcinogenic properties. A recommended intake of chlorophyll keeps the circulatory and digestive systems healthier [5]. Increased exposure of UV light reduces total chlorophyll in medicinal plants.

Medicinal plants like S.trilobatum and S.nigrum belongs to the family of Solanaceae that are well known for their medicinal properties across the world. S.trilobatum is an important plant in Siddha medicine which has anti-bacterial, anti-fungal and anti-tumor activities. It is a rejuvenator and has also been traditionally used to treat respiratory diseases.

S.nigrum is an important ingredient in traditional Indian medicine. Infusions are used in dysentery, stomach complaints and fever. The juice of this plant is used to treat on ulcers and other skin diseases.

Traditionally the plant has been used to treat tuberculosis. Since SiO2 - TiO2 thin films efficiently transform destructive UV light energy into heat, it can be used to protect chlorophyll content in S.trilobatum and S.nigrum medicinal plants from UV radiation. In our present work, the properties of 1

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

SiO2-TiO2 thin films prepared at various dipping cycles (4, 6 and 8) by sol-gel dip coating method have been studied elaborately. The effect of SiO2 - TiO2 thin films as a protective layer against UV radiation in medicinal plant is also reported.

Experimental. SiO2 - TiO2 thin films of various dipping cycles were prepared by using sol-gel dip coating technique. Titanium - tetra -iso -propoxide (TTIP) and Tetra- ethyl- ortho -silicate (TEOS) were used as starting materials. Ethanol was used as a solvent with acetic acid as a stabilizer. 4ml of TTIP was dissolved in 30 ml of ethanol. Then, 1 ml of acetic acid was added to stabilize the solution.

Finally, 1 mol % of TEOS was added to the solution and stirred for an hour. Obtained solution was deposited on microscopic glass slide by dip coating machine with a speed of 50 mm for 30 seconds.

The deposited film was pre annealed at 100° C for 10 min. This procedure was repeated to obtain films of 4, 6 and 8 dipping cycles. Finally, the obtained films were post annealed at 400°C for 3 hours. The annealed films were characterized by x-ray diffraction (XRD), UV – visible spectroscopy and Scanning Electron Microscopy (SEM) with elemental analysis (EDAX) studies. The thickness of the film was measured using stylus profilometer.

The chlorophyll content of S.trilobatum and S.nigrum leaves were determined under UV exposed condition. 1g of finely cut healthy fresh green leaves of S.trilobatum and S.nigrum were taken in 100 ml conical flasks. The control flasks containing S.trilobatum and S.nigrum leaves were completely unexposed to UV light i.e. they were maintained under room condition. Two flasks of S.trilobatum and S.nigrum were completely exposed to UV light and the other flasks were covered with thin films as prepared with 4, 6 and 8 dipping cycles. Then, they were exposed for 10 minutes with UV light of wavelength 260 nm at a distance of 40 cm. Chlorophyll was estimated using the method described by Arnon [6] on the UV exposed and unexposed leaves for both the plants.

1g of finely cut fresh leaves of S.trilobatum and S.nigrum was grounded to get a fine pulp with the addition of 20 ml of 80 % acetone using mortar and pestle. This solution was then centrifuged for 5 min at 5000 rpm. The supernatant was transferred to a volumetric flask. The residue was then grounded with 20 ml of 80 % acetone, and then centrifuged for 5 min at 5000 rpm. Finally, the supernatant was transferred to the same volumetric flask. This process was repeated for 4 times till the residues became almost colorless. The volume was made up to 100 ml with 80 % acetone. This procedure was repeated for all samples. The absorbance of the solution was observed at 663 and 645 nm by (systronics) UV spectrophotometer. 80% acetone is used as a blank for this experiment. The amount of chlorophyll which is present in the extract (i.e.mg of chlorophyll present per gram of tissue) was calculated using the following equations. mg chlorophyll a/g tissue = [12.7(A663) – 2.69(A645)] × V/(1000 × W) mg chlorophyll b/g tissue = [22.9(A645) – 4.68(A663)] × V/(1000 × W) mg total chlorophyll /g tissue = [20.2(A645) + 8.02(A663)] × V/(1000 × W) where A – is the absorbance at specific wavelength; V – is the final volume of chlorophyll extract in 80 % acetone which in this case is 100 ml; W – is the fresh weight of tissue extracted which is 1g.

Thus, V / (1000 × W) = 100 / (1000 × 1) = 0.1.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

The experiments were repeated thrice. The statistical software SPSS version 17.0 was used for analysis.

Results and Discussion. The XRD pattern of SiO2 - TiO2 thin films of different dipping cycles are shown in Fig.1.

Fig.1. XRD pattern of SiO2 - TiO2 thin films.

The dominant peak is observed at 25.25° for films of 4, 6 and 8 dip cycles. The films have tetragonal (101) crystal structure and anatase phase which are in agreement with standard JCPDs data (File No.89-4203).

The crystallite sizes of the films were determined using the well known Debye-Scherer formula

D 

k  Cos

(1)

The crystallite sizes are found to be 22.11 nm, 53.95 nm and 65.46 nm for different dipping cycles such as 4, 6 and 8 respectively which imply that the crystallinity of the films improves with increase in dipping cycles.

EDAX spectra and SEM analysis of 6 dipping cycles of SiO2 - TiO2 thin film annealed at 400° C is shown in Figure 2 (a) and (b). EDAX spectrum indicated the main peaks of Ti, Si and O elements.

The SEM micrograph of the SiO2 - TiO2 thin film indicated fractured structure. During drying and annealing process of the films, crack formation takes place as a result of contraction stress and different thermal coefficient of expansion of the over layer and substrate [7].

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

(a)

(b)

Fig. 2. (a) SEM and (b) EDAX spectra of SiO2 - TiO2 thin film of 6 dipping cycles.

Figure 3 shows the UV - visible transmittance spectra of SiO2 - TiO2 thin films of different dipping cycles. The transmittance spectra lie in the wavelength range 140 nm - 740 nm. Transmittance is mainly dependent on thickness and surface structure of the thin films [8]. The optical transmittance of the deposited film is low. It is observed that as the dipping cycle increases, the transmittance of the film decreases which is due to the increase in film thickness and the scattering effect originating from increased crystallite size [9]. The average transmittance values at 500 nm are 48 %, 35 % and 30 % for 4, 6 and 8 dipping cycles respectively.

Fig. 3. Transmittance spectra of SiO2 - TiO2 thin film.

In fresh S.trilobatum and S.nigrum leaves (unexposed to UV), total chlorophyll content is high whereas in the UV treated samples, the total chlorophyll content gradually increases as the dipping cycle increases due to increase in film thickness. In the UV completely exposed treatment the total chlorophyll content is minimum as shown in Fig. 4.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 4. Comparison of total chlorophyll (%) a) S.trilobatum, b) S.nigrum plants for different number of dipping cycles.

From Fig. 4, it can be observed that in S.nigrum, with thin film of 8 dipping cycles, the total chlorophyll content (99.3 ± 8.5 %) is equal to the control treatment (100 ± 8.6 %). Similarly, in S.trilobatum also with film of 8 dipping cycles, the chlorophyll content is maximum (72.6 ± 5.9 %).

Films of highest (8) dipping cycles absorb maximum UV light since increased thickness protects the leaves from UV damage leading to increase in chlorophyll content.

Summary. SiO2 - TiO2 thin films of various dipping cycles have been deposited on glass substrate using dip coating technique resulting in highly efficient UV absorbing film. The thin films exhibit anatase phase with tetragonal structure having a preferential orientation along (1 0 1) plane. SEM image reveals that the film has fractured structure. The presence of Si, Ti and O has been confirmed from EDAX spectra. XRD study reveals that crystallite size increases as the dipping cycle increases.

This study showed that SiO2 - TiO2 thin films are good absorber of UV light that protects total chlorophyll content in medicinal plants S.trilobatum and S.nigrum act as protective layer against UV radiation.

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