A Novel Approach for the Decoration of Parylene C Coatings Using Functional Silica-Triclosan Co-Polymer Nanoparticles Volume 3 - Issue 4

Rina B Binyamini¹, Edith Laux², Herbert Keppner² and Jean Paul Lellouche¹*

• ¹Department of Chemistry, Faculty of Exact Sciences, Institute of Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Israel
• ²Haute Ecole Arc Ingénierie (HES-SO), CH-2300 La Chaux-de-Fonds, Switzerland

Received: February 23, 2021;   Published: March 03, 2021

*Corresponding author: Jean-Paul Lellouche, Department of Chemistry, Faculty of Exact Sciences, Bar-Ilan University, Ramat Gan 5290002 Israel Institute of Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat Gan, 5290002, Israel

DOI: 10.32474/MAMS.2021.03.000173

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Abstract

Both pure silica (SiO₂) nanoparticles (SNPs) and functionalized hybrid triclosan (TCS)/silica nanocomposites (T-SNCs) were deposited onto nonfunctional Parylene C films using a novel, readily executed, one-step decoration method. Unlike previously known methods, this functionalization method of Parylene C films required neither any binding agent nor sophisticated equipment/devices. The SiO₂-based NPs anchored onto Parylene C substrates were formed via a common base-catalyzed hydrolytic sol-gel method. Regarding the mechanism, it has been assumed that the SiO₂ phase precursor (Si(OC₂H₅)₄), tetraethoxysilane (TEOS), was first adsorbed and 2D polymerized onto the parylene C film due to hydrophobic interactions that served as an anchor mechanism for further corresponding NPs growth [1] This assumption was investigated by comparing thermal behaviors (measured by differential scanning calorimetry, DSC) of parylene C coatings before and after the following specific surface treatment, i.e., first (i) first parylene C coating incubation with TEOS, followed by (ii) SNPs formation and growth from such a TEOS-modified coating surface. Following the same procedure, hybrid thiophene-containing H-SiO₂-TCS NPs were also successfully grown from the surface of a same TEOS-modified parylene C film and characterized using high resolution scanning electron microscopy (HR-SEM) and X-ray photoelectron spectroscopy (XPS). In order to obtain deeper insight into the overall functionalization process, the similar hybrid H-SiO₂-TCS NPs that formed in the bulk-contacting medium were also isolated and fully characterized for comparison needs. Resulting anti-bacterial biological experiments were also performed as well.

Keywords: Silica nanoparticles; hybrid silica/thiophene nanoparticles; functional parylene c coatings; hybrid silicaperylene c composites; antibacterial parylene c coatings

Introduction

The current deliverable issues with the subject of Parylene composite films modified with antibacterial coatings. In this study, we report on the design, synthesis, and characterization of covalently linked, triclosan silica-based nanocomposites (TCS-SNCs). In the context of increased bacterial resistance to common antibiotic treatments, nanoscale materials offer a unique opportunity to bring innovative and more effective solutions to bacterial disease control. Currently, several options have already been successfully explored using various types of organic/inorganic and composite nanomaterials. For example, such options include:

1. Silver (Ag) nanoparticles (NPs) [2].
2. Carbon-based nanomaterials like single- walled carbon nanotubes (SWCNTs),[3] C60 fullerenes, and graphene oxide;[4].
3. FDA-approved bioactive glasses (SiO₂–Na₂O–CaO–P₂O₅) of the 45S5 type, [2].
4. Metal oxide (TiO₂, MgO, ZnO) NPs, [2,4] magnesium fluoride (MgF₂) NPs, [5] and
5. Chemically modified (antibiotic-decorated) gold (Au) NPs [6,7].

Each of these optional nanoscale systems clearly possesses its own specific mechanism of action, cellular target(s), potential delivery capability, and overall therapeutic advantages and disadvantages. Triclosan (TCS) is a well-known commercial and Food and Drug Administration (FDA)-approved, synthetic, nonionic, broad-spectrum antimicrobial agent [6]. It mainly possesses antibacterial, but also some parallel antifungal and antiviral properties [7-9]. Numerous studies conducted on different bacteria strains showed that TCS acts on a defined bacterial target in the bacterial fatty acid biosynthetic pathway, the NADH-dependent enoyl- [acyl carrier protein] reductase (ENR) [10-15].

Loading triclosan into an organic or inorganic matrix to induce antibacterial properties has been extensively researched [16,17]. For this goal, different polymers and nanomaterials have been used, such as polystyrene, [18-20] TiO₂ particles to learn about the sustained release of antibacterial materials, [21-25] b-cyclodextrin bacteria-growth resistance [26,27] and loaded into poly(D,L-lactide-co- glycolide) (PLGA) [28-30], poly(D, L-lactide) (PLA) [31-35] and poly(vinyl alcohol) (PVAL) copolymers for periodontal disease, etc. [36,37] Triclosan-loaded NPs (TiO₂ nanocapsules) have also been prepared, but triclosan has never been covalently bound to an inorganic matrix. In this novel study, we developed a specific linker that will allow covalent binding between silica NPs and triclosan. The covalent bond between the linker and the biocide has been designed to be broken/hydrolyzed by enzymes, subsequently releasing the active triclosan, which will act upon its target inside the cell. Basically, the enzymes produced by the bacteria themselves will be responsible for the release of this antimicrobial agent. Such a covalent linkage ensures the above-mentioned properties and also prevents leaching, providing an improved mechanism for controlled release. In this context, parylene-type polymers are characterized by their high solvent resistance, low dielectric constant, good barrier properties, full biocompatibility, and ability to be readily deposited by chemical vapor deposition (CVD) via thermal cracking of (2,2)-paracyclophane monomers [38,39]. Furthermore, parylene C (poly(monochloro-para-xylylene)) has a high permeability resistance to common gases, i.e., H₂O, N₂, and O₂, while exhibiting a high elastic modulus [40]. These exceptional combined properties promote parylene C as an ideal coating polymer for microelectronic devices, medical instruments, implants, and numerous other applications [1, 41-46]. Herein, we report about the design and synthesis of parylene C composite films modified with antibacterial coatings, using the innovative linker triclosan-(3-(triethoxysilyl)propyl) carbamate (TTESPC), and the resulting nanosized, silica-based particles onto parylene films. Each particle contains the FDA-approved antibacterial agent triclosan, covalently linked within the matrix for its controlled slow release upon interaction. The particles were prepared according to a modified Stöber method, and the biocide-silanated linker was incorporated into the silica matrix during the particle-formation process.

Experimental Section

Triclosan-(3-(triethoxysilyl)propyl) carbamate (TSClinker) synthesis

Triclosan (5-chloro-2- (2,4-dichlorophenoxy) phenol) (1 g, 3.45 mmol, 1 eq.) and dry toluene (5.0 mL) were added to a three-necked round-bottom flask under an N2 atmosphere, to obtain a 0.7 M solution. 3-(Triethoxysilyl) propyl isocyanate (1.28 mL, 5.18 mmol, 1.5 eq.) and tetraoctyltin (3.02 ml, 5.18 mmol, 1.5 eq.) were added simultaneously to the reaction mixture, which was stirred at room temperature until no progress in the reaction could be observed by thin layer chromatography (TLC) (4:1 n-hexane: EtOAc) n-hexane: ethyl acetate (EtOAc)). Toluene was evaporated until off-white oil emerged. Upon crystallization overnight, white crystals were obtained. These were filtered with cold n-hexane to remove traces of the stannane complex and dried under vacuum to yield 63.5% (1.17 g) of a white crystalline powder. The TTESPC melting point is about 83-84˚C. (Scheme 1)

Parylene C films modified with Triclosan-silica nanocomposites (TCS-SNCs)

There are several ways to incorporate molecules into a solgel system [47-49]. The first is the physical incorporation of drug substances, drug loading for example, into sol-gel-derived silica materials. This method was first introduced in 1983 [50]. We designed and fabricated onto the parylene C surface novel hybridsilica nanoparticles containing the FDA-approved antimicrobial triclosan (Irgasan) covalently linked within the inorganic matrix for its controlled slow release upon interaction. The full characterization of the triclosan-silica nanocomposites (T-SNCs) onto the Parylene C film, triclosan-(3- (triethoxysilyl)propyl) carbamate (TCS linker), and their appropriate linkers is accomplished by thermogravimetric, microscopic, and spectroscopic techniques.

Parylene films modified with a different ratio of Triclosansilica nanocomposites (TCS-NCs): Parylene film (1.5×1.5cm), previously cleaned in an ultrasonic bath for 15 minutes with acetone, was placed in a reaction vessel containing 10 ml of ethanol. Then, 0.53 ml of ammonium hydroxide and 1.5 ml (6.72 mmol) of tetraethyl orthosilicate (TEOS) were added (Scheme 1). The reaction was mixed for 3 minutes and then different amounts of the TTESPC linker: 36 mg (1%), 108 mg (3%), and 180 mg (5%), which were previously dissolved in 2 mL ethanol, were added to the reaction vials. The reaction was performed at room temperature for 24 h with constant agitation using an orbital shaker. The resulting generated silica-modified film was washed with ethanol and then washed again in ethanol for 10 min using an ultrasonic bath (Elmasonic S 30 ultrasonic bath, 37 kHz at full power irradiation), three times, in order to remove physically adsorbed silica particles. The film was then air-dried. The decorated Parylene C was washed three times in an ultrasonic bath, each time for 10 minutes, using analytical grade EtOH. Thus, all the obtained composite washing solutions were UV tested (measurement scale: 200-800 nm, detection of conjugated triclosan chromophore/species) to detect the presence of triclosan-loaded silica nanocomposites. As clearly deduced from the (Figure 1) data, the second wash step already disclosed no UV-based evidence at all for the presence of any further free UV-absorbing triclosan molecules and/or triclosan silica nanocomposites. (Figure 2) shows micrographs of high-resolution scanning-electron microscopy (HRSEM) of the TCS-SNCs obtained in a typical experiment at room temperature with 2.5% (w/v) of TTESPC. One can appreciate from these micrographs the smooth, spherical morphology of the NPs. These nanocomposites were obtained with a narrow size distribution and an average diameter of 130 ± 30 nm (at dry measurements). DLS studies showed a hydrodynamic diameter of 164.3 nm (Figure 3), which is in a good accordance with the actual TEM size of similar dried particles, when considering the likely adsorption of water molecules onto the NC surface.

A comparison of the Parylene C film coverage by the triclosan-silica nanocomposites (T-SNC) using different TTESPC concentrations at room temperature revealed a proper coverage of the film by the nanocomposites. Nonetheless, the nanocomposites at the lower concentration (1%, Figure 2A) seem to be concentrated in clusters, hence there are bare areas of Parylene C film, while with other TTESPC concentrations (3% and 5%, Figures 1-B & 1-C respectively), the TCS- NC creates a complete coverage of the film. Triclosan-silica nanocomposites (TCS-NCs) were synthesized onto Parylene C film at room temperature. The content of the triclosan linker varied from 1%, 3%, and 5%. The diameter of the nanocomposites was measured using ImageJ software on HRSEM images. The size distribution of such nanocomposites assembled with a 1% triclosan linker was from 100 to 149nm, while when using a 3% triclosan linker, it yielded nanocomposites with the size range of 70 to 129 nm, while nanocomposites of the 5% one varied from 140 to 199nm.

3-Parylene films modified with different ratio of Triclosan-silica nanocomposites (TCS- NCs) at 60 ˚C: We were inspired to reduce the diameter and size distribution of the triclosan-silica NPs that were synthesized on the parylene film. Parylene film (10 µm thickness, third part of the microscope slide) previously cleaned in an ultrasonic bath for 15 minutes with acetone was placed in a reaction vessel containing 10 ml of ethanol. Then, 0.53 ml of ammonium hydroxide and 1.5 ml (6.72 mmol) of TEOS were added. The reaction was mixed for 3 minutes and then different amounts of TTESPC: 1%, 3%, and 5% (previously dissolved in ethanol), were added to the reaction vessel. The reaction was performed at 60˚C for 24h with constant agitation by an orbital shaker. The resulting silica modified film was washed with ethanol and then washed again in ethanol for 10 min using an ultrasonic bath (Elmasonic S 30 ultrasonic bath, 37 kHz at full power irradiation) three times, to remove physically adsorbed silica particles. The film was then air-dried. (Figures 4 & 5) An additional FTIR analysis (Figure 6) supports this data, since it discloses the similar chemical composition of the triclosan-silica nanocomposites synthesized onto Parylene C film (Figure 6) top). The IR spectrum of Parylene C film at 2830-3027 cm^-1 which correspond to C-H stretching bands are screened by the TCS-NCs, therefore almost invisible after the synthesis of the nanocomposites. The IR spectrum of the triclosan-silica nanocomposites (Figure 6 top) reveals peaks, which are characteristic of the SiO₂ phase (1110 cm^-1, 3000-3800 cm^-1, etc.) as well as a distinguished peak for C-Cl at 550 - 800 cm^-1 (see scheme 1). H-SiO₂-Th NPs produced in the bulk solution in presence of parylene film (Figure 7C) shows the same set of peaks as the spectrum of the H-SiO₂-Th NPs prepared in absence of parylene as opposed to the spectrum of SiO₂ NPs (Figure 7A), which exhibits only the peaks characteristic to SiO₂ NPs (1110 cm^-1, 3000-3800 cm^-1, etc.).

Nanoparticles from the bulk: During the synthesis of triclosansilica nanocomposites (TCS-NCs) onto Parylene C film (see section 3.2), triclosan-silica nanocomposites were being synthesized in the medium as well as onto the Parylene C film. The TCS-NCs were collected from the medium and characterized by varied methods such as: Zeta potential, elemental analysis, thermogravimetric analysis, Differential scanning calorimetry and High Resolution TEM (Figures 7 & 8). The FTIR image shows the Fourier transform IR spectra of triclosan linker-silica nanocomposites (TCS-SiO2/ NCs), which were isolated from the reaction solution. We can clearly identify characteristic peaks of the functional groups of the silica and the triclosan. The IR spectrum of the triclosan linker-silica nanocomposites (TCS-SiO2/NCs) shows a broad curve with a peak at 3390 cm−1, which corresponds to the carbamate NH and alkane CH2 asymmetric- stretching bands, a small peak at 1639 cm−1 which stands for the carbamate C=O stretching band, a broad curve with a peak at 1108 cm−1 (Si-O-C ether stretching bands, aromatic C=C stretching bands and phenolic symmetrical C-O stretching bands), a peak at 949 cm−1 (aromatic C-H stretching bands), and a peak at 789 cm−1 (C-Cl stretching bands) [51]. The elemental analysis summarized in (Figure 9), since these measurements relate to the Parylene C film, The T-SNC attached and not attached onto the film and to the triclosan linker itself. The Parylene C film consists of carbon rings,[1] so it is quite obvious that the highest elevation of the carbon element was found at the T-SNCs onto the Parylene C film samples and the lowest carbon content was found at the T-SNPs. This complete set of results clearly demonstrates the main advantages of this mild functionalization method for obtaining stable and efficiently grown grafted SiO2 NPs onto the parylene C substrate. This is a simple and mild one-step procedure that requires neither special equipment nor binding agent. It is entirely likely that such a method might be performed using various other kinds of silicabased precursors. These results clearly emphasize the advantages of this specific functionalization method, i.e., a simple one-step delivery of a stable and effective deposition/growth of bare silica NPs onto a parylene C substrate. Moreover, this wet chemistry procedure is operated without using any special equipment or binding chemical agents and might possibly be extended to other kinds of different silicate precursors as illustrated below.

Biological Results

The antimicrobial activity of NPs-coated Parylene C films was evaluated using both Escherichia coli ATCC 25922 and triclosanresistant strain RJH 108. Both bacteria were grown overnight in Nutrient Broth (NB, Sigma) media under shaking (250 rpm) at 37 ˚C. On the following day, the overnight cultures were each diluted in a fresh NB medium to obtain stock solutions with a working concentration of 105 colony-forming units (CFU) per ml. Each of the parylene-modified coatings (1 cm diameter) was exposed to 1 ml of either of the bacterial stock solutions in a 24- well plate (DE-GROOTH). The plates were then incubated at 37 °C for 24 h. On the following day, in order to determine the CFU parameter in each treatment, serial dilutions were carried out and the cells were spotted onto NB agar plates. The NB plates were incubated at 37 °C for 20 hours. The cell growth was monitored and determined by viable cell count. The antibacterial properties of triclosan-silica nanocomposites (TCS-NCs) coated surfaces were tested against E. coli, a common bacterial pathogen, which was either sensitive to triclosan or resistant to it. As shown in (Figure 10), TCS-NCscoated Parylene samples managed to kill all the tested bacteria within 2-4 hours as opposed to uncoated Parylene C coupons or to TCS-NCs coated Parylene samples that were incubated with triclosan-resistant E. coli (Figure 11), and as expected, did not kill the bacteria even within 24 hours.

Conclusion

Triclosan-silica nanocomposites (TCS-NCs) were deposited on the surface of Parylene C by a one-step simple effective decoration method. The size of the observed generated functional NPs and their distribution on the substrate are both affected by the solution temperature, the concentration of tetraethyl orthosilicate (TEOS) [1], and the triclosan linker part in the solution. The best deposition is obtained as respect to the elevation in the temperature (from room temperature to 60 ˚C) and at 3% amount level triclosan linker. The particle-size distribution study showed that, at 3% linker level and 60oC, the generated particles were in a 30-80 nm size range. The HRSEM images also show that when using these specific conditions for coating modification, the generated TCS-NCs are effectively and homogenously dispersed onto the surface of support Parylene C films. The IR analyses of both coated and uncoated Parylene C films confirm the presence of the triclosan species onto the hybrid-modified surface. These results highlight the potential use of Triclosan-silica nanocomposites-coated surfaces for various biomedical applications.

Acknowledgement

This research has been fully supported by the European Commission through the FP 7th collaborative RTD European Project PARYLENS (FP7-NMP-2009-SMALL-3 area, contract no 246362).

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