Synthesis and characterization of novel poly(3-aminophenyl boronic acid-co-vinyl alcohol) nanocomposite polymer stabilized silver nanoparticles with antibacterial and antioxidant applications

Highlights

• Nanocomposite stabilized silver nanoparticles (PABA-PVA)AgNPs were synthesized.

• Poly(3-aminophenyl boronic acid) (PABA) binds to PVA through cis-diol interaction.

• AgNPs are anchored on the sheet-like surface of polymer (PABA-PVA).

• (PABA-PVA)AgNPs exhibited satisfactory antibacterial and antioxidant activities.

Abstract

In this work, the synthesis method and applications of nanocomposite polymer stabilized silver nanoparticles (AgNPs) are reported. 3-Aminophenyl boronic acid (3APBA) was used as a reductant of silver nitrate which acted as an oxidant for the polymerization of 3APBA through in situ chemical oxidative polymerization to poly(3-aminophenyl boronic acid) or PABA. The formation of PABA in the reaction mixture led to particle agglomeration owing to PABA poor solubility. However, in the presence of hydrophilic poly(vinyl alcohol) (PVA), PABA binds to the free hydroxyl group of PVA to form a composite polymer (PABA-PVA), which perfectly stabilized the formed AgNPs. Succinctly, PVA acted as a solubilizer and stabilizer for (PABA-PVA)AgNPs synthesis. Synthesis was optimized and sharp absorption peaks at 290 nm and 426 nm were observed, attributing to the π-π* transition of the benzenoid ring of PABA and the characteristic absorption spectrum of AgNPs, respectively. (PABA-PVA)AgNPs was characterized using UV–vis, TEM, FESEM, EDX, XRD, FTIR, TGA/DTG, DLS and zeta potential analysis. In addition, the antibacterial, antioxidant and metal chelating capacities of (PABA-PVA)AgNPs were evaluated. The (PABA-PVA)AgNPs exhibited significant antibacterial activity against Escherichia coli and Listeria monocytogenes, and good antioxidant and metal chelating properties of (PABA-PVA)AgNPs, thus validating its attractive biological applications.

Introduction

Metal nanoparticles stabilization in polymers has taken a central role in material sciences owing to the numerous applications such as sensor, conductive devices [1]. The preparation of polymer protected metal nanoparticles generally involve appropriate tuning of the reaction conditions of metal precursors and monomers or polymers, with a view to achieving control over growth, morphology and size distribution of the synthesized nanoparticles.

Poly(3-aminophenyl boronic acid) (PABA) is a conducting polymer with similar properties as polyaniline (PANI). Like PANI, PABA is often polymerized from its monomer, 3-aminophenyl boronic acid, while PANI is obtained from the polymerization of aniline [2,3]. On a comparative note, the presence of the boronic acid functionality on PABA imbues it with notable capacity for the electrochemical detection of cis-diol containing biomolecules such as glucose, fructose and dopamine [[3], [4], [5]], and also the boronic acid group itself is specific to sialic acid [6]. Moreover, PABA has been investigated to be more thermally stable than PANI and thus may find better applications in devices operating at extremely high temperatures [7]. Similar to PANI, PABA suffers from poor solubility in water. Preparation of PANI based nanoparticles in aqueous environment is mostly achieved with the aid of additional stabilizers or dopants. In this regard, different compounds was investigated such as dodecylbenzene sulfonic acid [2], mercaptocarboxylic acid [8], poly(vinyl alcohol) (PVA) [9], camphorsulfonic acid [10], poly(vinyl pyrrolidone) [11], where hydrogen bonding and electrostatic interaction between PANI and aforementioned materials furnished soluble nanoparticles with various applications. Thus, in view of the solubility challenge on PABA and PANI, appropriate stabilizer and solubilizer must be present in preparing highly stable metal nanoparticles. Interestingly, PVA seems to be good choice for this matter as it is popular water soluble synthetic polymer with extremely high flexibility and transparency. It has good compatibility with the human body and easy synthesis process, making it an excellent material for tissue engineering [12,13]. The presence of many hydroxyl groups in PVA permits effective binding between it and other numerous polymers, which can further guarantee fabrication of novel polymer blends.

Metal nanoparticles particularly of silver, gold, copper, iron have assumed a cornerstone position in nanotechnology as a result of their diverse applications. Importantly, they have well been investigated as an antimicrobial agent but silver nanoparticles (AgNPs) are the most effective in this regard. Amongst the different types of synthesis methods for AgNPs [[13], [14], [15], [16], [17], [18], [19]], the green synthesis method has been well accepted to be environmentally benign, which is often effected biomass and biocompatible polymers. The method is facile and cost effective, as readily available biomaterials are used and are easily scaled up. Polymer stabilized metal nanoparticles involving the use of biocompatible, low cost and hydrophilic polymers such as PVA fall under the green synthesis methodology. Excitingly, boronic acid containing polymers has not been found to be toxic to tissues in in vivo assays since the end product of boronic acid laden polymers is boric acid which is not particularly toxic to human [15,20].

Bacterial antibiotic resistance has been identified by the World Health Organization (WHO) as a serious threat to the health sector in view of the associated damages. It is the resistance of pathogens (virus, bacteria, fungi) to antimicrobial medicines that have been used and known to be effective for the treatment of infections. In the European Union, over 25,000 deaths are estimated to be caused annually by bacterial antibiotic resistance related cases [21]. AgNPs based nanocomposites are reported to be promising antimicrobial agent, depending on size, environmental conditions and capping agents [[13], [14], [15], [16], [17]]. PVA/silver hydrogel presented the greater antibacterial sensitivity of P. aeruginosa than that of S. aureus and E. coli [13]. The antibacterial activity of AgNPs-porphyrin hybrids against S. epidermidis and E. coli was enhanced by synergetic effect of AgNPs and porphyrin and light activation [16]. AgNPs embedded in poly(N-isopropylacrylamide) and poly(methyl methacrylate) blend was effective against S. aureus and P. aeruginosa [17]. Boronic acid impregnated with AgNPs was positive promise to the treatment of resistive bacteria owing to the synergistic effect, in view of the release of Ag⁺ on one hand and the boronic acid which is well suited for binding with the cis hydroxyl group of Gram-negative bacteria on the other hand [15]. Accordingly, the need to develop and explore new antimicrobial agent for the control of pathogens is highly welcome, especially combination with composite polymers of boronic acid, PVA and AgNPs.

In this contribution, polymer stabilized AgNPs synthesis is reported. 3-Aminophenyl boronic acid (3APBA) was used as the reductant for silver nitrate (AgNO₃), while PVA served dual roles as AgNPs stabilizer and solubilizer. 3APBA initially reduced silver salt and was concomitantly polymerized to its polymeric form as PABA through the in situ chemical oxidative polymerization method. In the absence of PVA, PABA aggregates the generated nanoparticles owing to its poor solubility. However, in the presence of PVA, PABA binds to PVA through cis-diol covalent bonding and forms a polymer chain of (PABA-PVA) around the AgNPs (called as (PABA-PVA)AgNPs). The as-synthesized nanocomposite was extensively characterized to obtain information on its physical, chemical and morphological properties. The biological applications of (PABA-PVA)AgNPs was demonstrated by investigating its antibacterial and antioxidant potentialities.