Fabrication of Superhydrophilic TEOS-Lactic acid Composite Films and Investigation of Biofouling Behaviour

Abstract

Keywords: Sol-gel; Composite; Microalgae; Bioadhesion; Lactic acid; Eco friendly; Sol-gel; Composite; Microalgae; Lactic acid

1. Introduction

Stepping up day by day after the industrial revolution worldwide, industrialization has caused ecological problems such as environmental pollution, climate change, and various diseases. This situation has led scientists to

research how to produce sustainable, eco-friendly, and economical materials. Usage of microalgae to bring down resource consumption has been one of the most common green engineering trends of recent times. Microalgae are unicellular photosynthetic eukaryotic creatures, and there are approximately 40000 defined species worldwide. They

*Corresponding author. *E-mail address: ucengiz@comu.edu.tr Published by Yıldız Technical University Press, İstanbul, Türkiye This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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are lived in oceans, lakes, surface and ground water. Diatom, phytoplankton, blue-green algae, brown and green flagella, and dinoflagellate are some of the species known as microalgae [1]. According to some investigations, they have been found that microalgae are unicellular; however, some species are multicellular. For example, Heimann and Huerlimann [2] reported that microalgae are multicellular because they possess one plastid and chlorophyll a and b, mucilage sheath. Furthermore, their high nutritional value and organic components are used as feed for aquarium fish, poultry, cats, and dogs, as a natural dye in buildings, biocatalyst in genetic engineering applications [3]. In addition, they are utilized as the raw material of the bioplastic to decrease the petroleum-based plastics' adverse environmental impact [4]. Adhesion consists of three stages: primary adhesion, irreversible adhesion, and biofilm density. Depending upon the negative or positive surface charges of all microorganisms are affected by the adhesion strength between the cell and surface via physicochemical interactions [5]. Microalgae harvesting occurs in closed and open systems, flat panels, stirred tanks, hybrid type, helical type, airlift, bubble column photobioreactors (PBRs), and raceway ponds [6]. Some optimum conditions are required for microalgae harvesting to take place. These can be listed as the light, CO2, and PBR designs. As a result of limited scattering of light, biofilm layer and microalgae adhesion inside the PBR [7]. Besides little light, cell-surface interaction, microalgae surface free energy, and hydrophilicity or hydrophobicity of microalgae caused adhesion on any surface [8, 9]. Microalgae adhesion formation is decreased by altering the thin films' wettability properties [10–12]. Superhydrophilic surfaces were defined as water contact angles lower than 10o and showed antifogging, self-cleaning, and antireflective properties [12, 13]. However, microalgae behavior on superhydrophilic surfaces is also exciting [14]. Koschitzli et al. [14] reported that the superhydrophilic surface successfully results against the microalgae adhesion, especially the pure culture of microalgae harvesting, due to the absence of the silt. In this study, the TEOS-LA super hydrophilic composite thin films were fabricated containing different LA content having high light transmittance. The super hydrophilic TEOS-LA composite surfaces were also tested in pure microalgae cultures of Chlorella sorokiniana (C. sorokiniana), Nannochloropsis sp., and Chlorella vulgaris (C. vulgaris). Increasing the composite surface's LA content decreased the water contact angle from 79 to 8o. In addition, the surface hardness values of the composite surface changed from 9H hardness to 6B softness with increasing the LA content. The microalgae adhesion test indicated that the biofilm formation is decreased on superhydrophilic TEOS-LA surfaces.

3. Results And Discussions

The light transmittance of the composite surfaces is given in Figure 2. This figure indicated the optical transmission of the TEOS-LA composite films with a bare glass slide. All composite films have exhibited higher optical transmittance, almost as bare glass.

Figure 2. Optical transmission of the TEOS-LA composite films.

Figure 3. Changed with WCA and SFE values depending on the LA content of the composite films. The contact angle of the water, MeI2, α-BN and F result on the composite surface, and the SFE of the composite films are given in Table 2. This table indicated no linear connection with the rising LA content of the composite films. The lactic acid molecular structure has two –OH groups, so the LA surface tends to be hydrophilic. However, the number of the – OH group decreased with the sol-gel reaction's progression,

J Sustain Const Mater Technol, Vol. 7, Issue. 4, pp. 316–321, December 2022

Figure 4. The biofouling performance of TLA18.7 interaction with microalgae species. The left side is coated, and the right side is uncoated on images a) Nannochloropsis sp. b) C. vulgaris, and c) C. sorokiniana, respectively. especially the LA's lower content. Because the sol-gel reaction between TEOS and LA continues with the attack silanol structure of TEOS on the –OH group of LA. While the water con-

tact angle value is high at low LA content, the water contact angle values decrease at high LA content due to the free –OH group, and superhydrophilic surfaces were obtained in Table 2.

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The variation of WCA depending on LA content decreased sharply up to 10% (wt) LA content. However, a constant WCA plateau was obtained despite increasing the LA content from 10 to 25% (wt) as given in Figure 3. In addition, total SFE values (γTot) of the composite films were calculated by the Van-Oss Good method using the test liquids tabulated in Table 2. Usually, the variation of SFE values changes opposite the contact angle values of the test liquids. An increasing contact angle results in a decreasing SFE value, as given in Figure 3. The mechanical stability of the composite film was tested by scratching [13], and the hardness test results are shown in Table 3. The findings showed that while the hardest coating is T0.0 having a 9H value, the softest coating is TLA23.5 having 6B due to the rising organic part of the composite surface depending on LA content. An increasing organic amount of the composite layer resulted in a decrease in the hardness [18]. Miller et al. [18] reported that higher PLA content results in softer films. The TLA18.7 surface was selected to investigate microalgae adhesion due to the lowest contact angle values. The test was carried on for 14 days in 3 types of microalgae. The microalgae concentration on day 14 is measured as 3.2 g/L for C. vulgaris, 2.8 g/L for C. sorokiniana, and

4. Conclusions

The dip coating method successfully fabricated superhydrophilic TEOS-LA composite films in this study. The sol-gel reaction between TEOS and LA was prepared at constant TEOS content with increasing LA. The superhydrophilic composite films having 8o of the contact angle were obtained without nanoparticle adding. This point of view is important in terms of environmental sustainability for developing biocompatible materials with reduced chemical content. The Van-Oss Good method calculated the SFE values of the composite films. The SFE results increased from 28.5 to 54.5 by increasing the LA content due to the rising OH unit on the composite films. However, a decrease in the WCA and an increase in the SFE did not have a linear relationship to LA content. The hardness test indicated that an increase in the organic part of the composite films resulted in a decreasing hardness. As a result of the bioadhesion test showed that the superhydrophilic TLA18.7 thin film reduced the biofouling formation in the microalgae.

Ethics

There are no ethical issues with the publication of this manuscript.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Financial Disclosure

This research was financially supported by Çanakkale Onsekiz Mart University The Scientific Research Coordination Unit, Project number: FYL-2020-3256.

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