Polyurethane films modified with polyaniline-zinc oxide nanocomposites for biofouling mitigation
Graphical abstract
Introduction
In the aquatic environment, submerged surfaces are significantly affected by the growth of micro/macro organisms via biofouling process. The ensuing biocorrosion is responsible for annual economic damages to the tune of ∼500 billion USD [1]. Biofouling adversely affects the performance as well as the efficiency of marine equipment or vessels by lowering its speed, increasing the consumption of fuel, enhancing metallic corrosion, emission of greenhouse gases and unexpected catastrophe related to structural failures [2], [3], [4], [5]. Apart from these issues, biofouling has detrimental impacts on water treatment plants, power generation stations, and also in aquaculture. In addition, biofouled surfaces can serve as reservoirs for invasive species in marine ecosystems. This problem tends to be persistent as biofouling-associated organisms often evade predation and resist conventional control agents.
The process of biofouling involves three major steps [6]. The first step is the formation of a conditioning film on surfaces by organic moieties via van der Waals interactions. This process is relatively rapid and occurs as soon as the surface is in contact with water. The second step involves the formation of biofilms by the colonization of microorganisms. This in turn, provides a source of nutrients and triggers the adhesion of other unicellular eukaryotes. Later, in the third step, macro-fouling occurs when multicellular microorganisms such as molluscs or barnacles adhere on the surface. The process of biofouling can be retarded by employing commercial protective coatings that suppress one or more of the above steps.
Earlier, the commercially available protective coatings were based on toxic biocidal pigments such as tributyltin (TBT). It has harmful impacts on marine life and therefore its use in the paint industry has been banned [7]. Later, these toxic compounds were replaced by copper-based pigments within different polymer matrices. High leaching rates (due to gradual erosion of the matrix), and long persistence of the toxic metal-based pigments in water has led to adverse ecological effects and hence paused their prolonged use [8], [9]. The next generation of coatings involved the use of ablative material as well as modified silicones such as poly dimethyl siloxanes (PDMS). Low compatibility of the constituents of these coatings and poor mechanical stability of silicone based coatings hinder their utility to a great extent [10], [11].
During the process of biofilm formation, features of the attaching surface such as hydrophilicity/hydrophobicity, roughness, energy, and morphology influence cell attachment. Studies on engineered topographies to prevent biofilm formation have also been initiated [12]. Specifically, zwitterionic polymer brushes are being employed to prevent bacterial adhesion and biofilm formation effectively [13], [14]. For commercial applications, the ease of synthesis and improved properties are the two important factors which need to be focused on. Literature survey reveals that most of the present methodologies involve tedious chemical treatments, multifarious steps, complex techniques, expensive materials, and additionally cause environmental hazards. Therefore, there is an urgent need for simple antifouling technologies based on environmentally-benign compounds that ensure protection of marine systems.
The usefulness of nano semiconductor metal/metal oxides is inevitable in this field due to their unique physico-chemical properties such as excellent photon absorption capacity, efficient transport of charge carriers and antibacterial activities [15], [16], [17]. However, such materials alone could bring about disruption of non-targeted species in marine systems owing to its cytotoxicity [18]. Short term action is also one of the major drawbacks. Therefore, the reinforcement of nanotechnology and polymer science is found to be an effective way out.
In the present work, a novel, and simple strategy, based on embedding nanocomposites of polyaniline-ZnO in commercially available polyurethane (PU) has been investigated. Owing to inherent conductivity of PANI and photocatalytic effect of ZnO, their nanocomposites were observed to be effective in decreasing adhesion of marine bacteria [15], [19]. Ou et al. have extensively investigated biodegradable PU coatings modified with graphene, fluorinated diols and PDMS for superior antibacterial properties [20], [21], [22]. Herein, thermoplastic PU has been chosen on the basis of its excellent hydrolytic stability as well as chemical and microbial resistance that it offers. The biocompatibility and mechanical properties of PANI are anticipated to improve by incorporating small amount of nano ZnO. Additionally, a naturally abundant chelating acid, namely, phytic acid has been chosen as a dopant for PANI. Phytic acid is well-known as an anti-nutrient since it decreases the bioavailability of iron, zinc, and copper due to its exceptional chelating effects [23]. Hence, this property has been employed to achieve excellent binding between the filler and the PU matrix, thereby minimizing the leaching rate of ZnO from polymer nanocomposites as well as enhancing the environmental stability for long term applications. The polyaniline-ZnO nanocomposites particles embedded in PU matrices were evaluated for their antibiofilm and antifouling properties by using representative Gram negative (Vibrio harveyi) and Gram positive (Bacillus licheniformis) bacteria, which are hitherto unattempted. This preliminary lab-scale investigation is necessary prior to their incorporation in marine based paints. Herein, the primary focus was specifically to develop hydrophilic films which can offer passive technology emphasizing on prevention of surface bio-adhesion rather than destroying the natural flora and fauna of marine eco system.
Section snippets
Materials
Zinc acetate, potassium hydroxide, ethanol, ammonium persulfate, aniline, tetrahydrofuran were of AR grade and purchased from SD Fine chemicals. Aniline was distilled twice under vacuum prior to use, whereas, all other chemicals were used as obtained. Phytic acid (50%) was purchased from Sigma Aldrich and thermoplastic polyurethane (Elastollan 1185 A) was from BASF. V. harveyi (MTCC 7771) and B. licheniformis D1, (an epibiotic marine bacterium isolated from the surface of green mussel and
Infrared spectroscopy
FTIR spectra were recorded for ZnO, PANI and PANI-ZnO nanocomposites (Fig. 1a). Characteristic bands of wurtzite (ZnO) in the range between 400 and 600 cm−1 are observed. They are more prominent in the pristine sample (Z) as compared to the nanocomposites with polyaniline (PNZ1 and PNZ2). In case of sample Z, a band observed at 474 cm−1 is attributed to Zn-O stretching mode, while the band at 553 cm−1 correlates to oxygen deficiency in ZnO [26]. Additionally, two bands observed between 893 and
Conclusions
In the present study, polyurethane based films were modified with a novel phytic acid doped PANI-ZnO nanocomposites by an effective time-saving process. The nanocomposites particles were homogenously dispersed within the matrix. It is observed that the films modified with these fillers show good mechanical strength with moderate elasticity and Young’s modulus as compared to pristine PU. Further, the films exhibited good antifouling properties against marine bacteria V. harveyi and B.
Notes
The authors declare no conflict of interest.
Acknowledgements
Authors thank the Department of Chemistry and Physics, Savitribai Phule Pune University, Pune, India for the characterization facilities and financial support by UGC-UPE-Phase-II, Govt. of India for this work. The central surface analytical facility of IIT, Bombay is acknowledged for XPS analysis. Authors thank Dr. Sreekumar Kurungot and Dr. C.V. Avadhani (NCL, Pune) for their timely help during this work. Special thanks to Ms. Daraksha M. Ansari, Mr. Adam Fergusan, Anurag Kanse and Suraj
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