Biofouling resistant polyethylene cage aquaculture nettings: A new approach using polyaniline and nano copper oxide

1 Introduction

Submerged aquaculture cage nets are highly susceptible to biofouling due to the nutrient rich environment around the cages which attract micro and macro foulers. Biofouling in aquaculture cage nets causes occlusion of mesh openings, thereby increasing weight and drag, deformation of cages due to the ensuing stress, reduction of volume, thereby decreased stocking density per area, anoxic condition due to disruption of dissolved oxygen flow, blocking of food waste diffusion, restriction of water exchange, increased hydrodynamic force, all of which adversely impacted fish health (Lai et al., 1993; Hodson and Burke, 1994; Braithwaite and McEvoy, 2004; Swift et al., 2006; Braithwaite et al., 2007; De Nys and Guenther, 2009; Lader et al., 2008; Fitridge et al., 2012). It has been reported that removal of fouling from a cage net costs 25% of the total project budget (Braithwaite et al., 2007). Cages are fabricated mainly with high density polyethylene (PE) netting whose non polar nature makes incorporation of antifouling biocides difficult. The surface of PE needs to be modified to develop strategies against fouling. The strategy employed in the present study was to synthesise a coating of polar or conducting molecule over PE and to use the surface to incorporate antifouling biocides. Polyaniline (PANI) is a well-known conducting polymer since it is easy to synthesise, cheaper, have excellent stability, ability to sense and good adhesion with organic films (Arenas et al., 2012; Ullah et al., 2013). Inherent conducting molecules like PANI and polypyrrole (PPY) are extensively employed as coating materials in metals to prevent corrosion in the aquatic environments (Biallozor and Kupniewska, 2005). Polyaniline shows moderate fouling resistance by virtue of hypochlorous acid generation over the surface (Chen et al., 2012). Functionalized PANI and PPY with hydrophobic thiols (Bergman and Hanks, 2000) and with dextran sulfate (Molino et al., 2013) showed reduced surface energy and protein cell binding respectively. Nano copper oxide-incorporated polyethylene glycol hydrogel over nylon netting materials exhibited excellent fouling resistance and it was due to the strong interaction of amide with hydrogel (Ashraf and Edwin, 2016). Bai et al. (2016) prepared PANI in situ over ZnO coated PET. Studies of different workers suggested that in situ polymerization of conducting materials over non conducting layers through chemical or photochemical means were more efficient (Strandwitz et al., 2010; Su and Wang, 2008; Li et al., 2005, 2012; Winther-Jensen et al., 2007). The novel approach employed in the present study is modification of the non-polar polyethylene surface to a polar surface which can adsorb nano biocides thereby rendering protection to protect the polyethylene aquaculture cage nets from biofouling. The present study aims at the synthesis of a stable conducting polymer coating of polyaniline over the non-conducting polyethylene which can be used as a platform for introducing antifouling biocides.

2 Materials and methods

High density polyethylene nettings (PE) of 2.5 cm mesh size and 0.05 cm diameter (1 × 3 ply) were purchased from Matsyafed, Cochin, India, a government owned net factory. Excelar grade aniline, ammonium persulphate and HCl were purchased from Merck India. Copper oxide nano particles with size 40–50 nm with purity >99%, was procured from Reinste Nano Ventures, India. Colored PE netting materials were preferred among aquaculture farmers but the color has no influence on the settlement of organisms over the nets (Braithwaite et al., 2007; Guenther et al., 2009). The present study was carried out by using blue colored polyethylene nettings.

Aniline was purified by passing through a freshly prepared charcoal column. Freshly purified aniline (0.3 M) in 0.1 M HCl was prepared, sonicated and the PE netting samples (size: 15 × 15 meshes) were immersed in the aniline solution overnight. On the following day, freshly prepared 0.3 M ammonium persulphate was added along the sides of the beaker of aniline – netting solution and the mixture was kept for 24 h in an undisturbed condition. Precipitates of polyaniline (PANI) were gradually formed over the netting material. The polyaniline coated nettings (PE-PANI) removed from the solution, washed with distilled water, dried in air and kept in a clean polyethylene bag for further studies. The polyaniline coated netting were immersed in a solution containing varying concentrations of aqueous nano copper oxide solution for 24 h, removed and dried in air. The concentration of nano copper oxide solution used were 0.01% (A2) and 0.02% (A3). Untreated (A0) and PE-PANI netting (A1) were also kept in parallel to compare the effect of treatments. Polyaniline-coated polyethylene (PE-PANI) was subjected to continuous stirring using magnetic stirrer for 10 days to test the coating stability and it was found to be intact after the experiment.

The surface characteristics of PE-PANI nettings and PE-PANI+nano copper oxide (PE-PANI+CuO) coated netting were analyzed using Atomic force microscope (AFM) and Scanning electron microscope (SEM). AFM topographic images of PE-nettings were recorded using Park XE 100 AFM in non-contact mode. The scanning probe was silicon with less than 10 nm. The AFM data was processed using the SEI software available with the instrument. The scanning electron micrographs of PE netting were recorded after sputtering with gold using JEOL Model JSM – 6390LV SEM. UV–VIS spectra of the materials were analyzed by immersing in water and scanned from 190 to 800 nm using Shimadzu 2450 double beam UV–Visible spectrophotometer. FTIR spectrographs were obtained by using Thermo Nicolet A10 FTIR. The images of the nettings were examined through Leica MZ16A stereo microscope at 25× magnification.

3 Results and discussion

The main challenge was to introduce a stable and stronger PANI coating over the PE. Direct application of PANI over PE showed poor adherence. The PANI synthesized in situ over PE netting showed thin coating with excellent adherance. The coating was intact and stable when subjected to continuous agitation. The PE-PANI and PE-PANI+CuO nettings showed blue green and dark blue–black colors respectively after treatment.

3.1

3.1 Characterization

3.1.1

3.1.1 Spectrophotometric evaluation

UV–Visible spectra of untreated polyethylene netting (A0), PE-PANI (A1) and PE–PANI+CuO (A3) are shown in Fig. 1. The A0 polyethylene netting material exhibited wider peaks at 555–656, 661–727, 365–420, 313–360 and 291–312 nm. The PE-PANI showed absorption bands at 662–742, 501–656, 413–460, 355–373 and 337–348 nm. The PE-PANI+CuO showed absorption at 450–740, 362–395 and 233–360 nm. PE absorption was expected near UV region and the additional absorption peaks were mainly due to the chromophore present in the PE. PANI coating over PE shifted the absorption to higher wavelength. In PE-PANI+CuO, the absorption peak was shifted further to red region and the peak at 413–460 nm of PE-PANI was absent, showing the adsorption of copper oxide in the matrix. The absorption at 337–348 nm in PANI was due to the π-π^∗ transition. The bands at 355–373 and 501–656 nm indicated the formation of polarons and bipolarons, respectively (Stejskal et al., 1993; Ghanbari and Babaei, 2016). The bipolaron absorption at 501–656 nm showed that PANI was in conductive state and emeraldine salt form. The chemical reaction responsible was due to the quinanoid ring of PANI (Saini and Basu, 2012) with polyethylene matrix. The absorption bands of CuO showed shrunken peaks compared to PE – PANI which highlights the fact that copper was adsorbed over the matrix and it was stable.

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Fig. 1

UV–Vis spectra of untreated polyethylene netting (A0), PANI coated polyethylene (A1) and PE-PANI+CuO (A3).

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3.2

3.2 Surface morphology

3.2.1

3.2.1 AFM

Atomic force micrographs of PE nettings and PE films (treated and untreated) are shown in Fig. 2. A uniform coat of PANI and nano copper oxide particles were held strongly over the surface. Due to the curved surface of PE nettings it was difficult to take the AFM micrographs and the features were not clearly represented. To get a precise scenario the experiment was repeated with polyethylene film and the same was shown in right side of Fig. 2. The results showed an almost flat image in the case of PE. The PE-PANI and PE-PANI+CuO showed well-structured and well differentiated projections with respect to the treatments. The AFM topography confirmed the adsorption of nano copper oxide over the PE-PANI matrix. The unvarying projections in the PE-PANI+CuO film highlighted the uniform distribution of nano copper oxide in the PE-PANI matrix. This was further evaluated by measuring the roughness of the matrices. Roughness Ra for PE, PE-PANI and PE-PANI+CuO films were 9.13, 2.65 and 4.64 nm respectively. Roughness Ra in PE-PANI film decreased significantly compared to untreated control. The PE-PANI+CuO matrix exhibited slightly higher Ra than that of PE-PANI film but less than PE film. This was due to the occurrence of nano copper oxide over polyaniline which resulted in higher number of projections in PE-PANI+CuO film compared to that of PE-PANI film. This showed copper oxide was attached over the quinanoid structure of the molecule (Jundale et al., 2013) and hence it manifested as projections which increased roughness. Copper oxide acts as a toxic hub, which prevents the approach and attachment of microorganism to the substrate.

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Fig. 2

Atomic force micrographs of PE untreated, PE-PANI and PE-PANI+CuO. The left shows the image taken on nettings and right the treatments were done on polyethylene film sheets.

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3.2.2

3.2.2 SEM

Scanning electron micrographs of PE nettings are shown in Fig. 3. A uniform layer of polyaniline and particles of nano copper respectively, were formed over the PE-PANI and PE-PANI+CuO. The rod like structures in the micrograph showed the formation of polyaniline rods over the matrix. The polyaniline formed over the PE matrix was a mixture of rods and spheres. The micrographs exhibited the presence of PANI and nano copper oxide over the PE and PE-PANI matrix respectively.

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Fig. 3

Scanning electron micrographs of PE untreated, PE-PANI and PE-PANI+CuO.

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3.3

3.3 FTIR evaluation

FTIR spectral bands of PE, PE-PANI and PE-PANI+CuO are shown in Fig. 4 and Table 1. The results showed that CH₂ wagging doublet bands of PE, 1373 and 1396 cm⁻¹, were shifted to 1362 and 1396 cm⁻¹ in PE-PANI+CuO. The characteristic quinanoid NH⁴⁺/NH⁺ band in pure polyaniline is 1027/1141 cm⁻¹. The quinanoid NH⁴⁺/NH⁺ bands in PE-PANI and PE-PANI+CuO nettings occur at 1047/1181 cm⁻¹ and 1070/1179 cm⁻¹ respectively. Similarly, benzenoid and quinoid ring bands, at 1500 and 1585 cm⁻¹, were shifted to 1508 and 1568 cm⁻¹ respectively. This shows the strong nature of the interaction or reaction between polyethylene and polyaniline. The most important changes in the FTIR spectra were (i) the characteristic PANI ⚌C—H in-plane vibration from 1141 to 1180 cm⁻¹ in the PE-PANI system, (ii) bending vibration of C—N from 1300 cm⁻¹ to 1312 cm⁻¹ in CuO coated system and (iii) the shift of 1585 cm⁻¹ corresponding to Quinanoid ring to 1568 cm⁻¹. The N—H group spectral absorptions were broadened in case of PE-PANI+CuO compared to PE-PANI. This shows the hydrogen bond formation of either O---H—N or N---H—N bonding in the matrix. The characteristic stretching vibration of CuO at 624 cm⁻¹ (Ghanbari and Babaei, 2016) was shifted to 579 cm⁻¹. This shows the presence of CuO in the matrix. FTIR evaluation showed a strong interaction between the PE and PE-PANI and PE-PANI and nano copper oxide.

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Fig. 4

FTIR spectra of PE, PE-PANI and PE-PANI+CuO.

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Table 1 FTIR characteristics of polyethylene, PE-PANI and PE-PANI+CuO.

Standard FTIR absorption peak (wavenumber cm−1)	FTIR absorption (wavenumber cm−1) showed in the present study			Description
	PE	PE-PANI	PE-PANI+CuO
Polyethylene (Gulmine et al., 2002)
731, 720	729	725	724	CH2 rocking deformation
1176	1180	1181	1179	CH2 wagging deformation
1306	1314	1314	1312	CH2 twisting
1351, 1366	1373, 1396	1373, 1396	1362/1396	CH2 wagging deformation
1377	1373	1373	1362	CH2 symmetric
1473, 1463	1471, 1488	1471, 1478	1471, 1456	CH2 bending
2851	2853	2853	2853	CH2 symmetric
2919	2924	2923	2921	CH2 assymmetric
Polyaniline (Ashraf et al., 2015)
695		725	724	C—C out of plane bending
829		830	831	Quinoid ring out of plane
1027		1047	1070	Quinoid ⚌NH4+
1141		1181	1179	Quinoid ⚌NH+
1300		1300	1312	υ (C—N) sec aromatic amine
1500		1508	1496	Benzenoid ring
1585		1568	1567	Quinoid ring
Copper
624			579	Stretching vibration of CuO (Ghanbari and Babaei, 2016)

3.4

3.4 Field evaluation

Antimicrobial and moderate antifouling property of nano copper oxide (Delgado et al., 2012) and poly aniline (Chen et al., 2012) respectively has been previously reported. Exposure of the treated cage nets in estuarine water expected to provide vital information on the efficiency of inhibition of foulers in the actual situation. The PE, PE-PANI and PE-PANI+CuO treated nettings were exposed to the estuarine waters of Cochin estuary for three months (12th December 2016 to 15th March 2017) (Fig. 5). The average hydrographic parameters and accumulation of fouling density at the test site has been described in detail elsewhere (Ashraf and Edwin, 2016). The untreated PE accumulated filamentous foulers and all other treatments showed only the light coating of polymer – calcareous like materials. The CuO treated nettings had significantly lower fouler accumulation compared to PE and PE-PANI. Samples retrieved after three months showed attachment of calcareous crystals and they were completely free from other foulers. The weight of foulers accumulated on 15 × 15 cm netting after three-month exposure to estuarine environment was 5.27, 4.07, 4.35 and 3.58 g for A0, A1, A2 and A3 respectively. The PE-PANI coated with 0.02% copper oxide showed excellent fouling resistance. This study showed that the PANI-coated PE acted as a platform to incorporate nano copper oxide and together these inhibited the accumulation of fouling organisms. Nano copper oxide treated in the matrix acted as the point source above the electron clouds of polyaniline, preventing initialization of biofilm formation and thereby fouling. Nano copper oxide interacted with the electron clouds of four nitrogen atoms of polyaniline. The surface of the netting had more active electron clouds of PANI and nano copper oxide that acted synergistically against the accumulation of microorganisms. Hypochlorous acid produced by PANI (Chen et al., 2012) over the surface and biocide activity of nano copper oxide synergistically acted against the attack of microbes, thereby rendering fouling resistance. The PANI+CuO coating over PE resulted in significant reduction in fouling density and the process could be adopted as for developing strategies to prevent fouling in submerged aquaculture cages.

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Fig. 5

Microscopic images (25×) of treated and untreated nettings after exposing in the estuary for 15, 60 and 90 days.

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Fig. 5

Microscopic images (25×) of treated and untreated nettings after exposing in the estuary for 15, 60 and 90 days.

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4 Conclusion

Aquaculture cage nets was susceptible to biofouling, which could be prevented by periodic cleaning and maintenance, a process that incurs huge expenditure. The study highlighted the in situ synthesis of PANI over PE nettings (PE-PANI) and application of nano copper oxide as a biocide to combat marine biofouling. The surface morphological studies showed the formation of uniform layer of PANI film over PE and the nano copper oxide. This was further confirmed with FTIR evaluation. The treated nettings exposed to the estuary for 90 days, showed no filamentous or other foulers compared to untreated PE nettings. The nano copper oxide present in the matrix acted as a point source above the electron clouds of polyaniline, preventing initialization of biofilm. The results highlighted the potential application of polyaniline to modify the non-polar surface of polyethylene to load active biocides to prevent fouling in cage aquaculture. The results need to be further evaluated by exposing cage nets for longer durations in the aquaculture fields of different regions.