Upgrading Oryza sativa wastes into multifunctional antimicrobial and antibiofilm nominees; Ionic Metallo-Schiff base-supported cellulosic nanofibers

Highlights

• Rice straws were successfully recycled into cellulose nanofibers Schiff bases (CNFSBs)

• Practical synthetic routes of CNFSBs-Ag(I)/Zn(II) complexes were reported.

• New materials were fully characterized based on elemental, spectral, and microscopic analysis.

• Anti-Staphylococcal and anti-Escherichia activity of new CNF materials have been addressed.

• The anti-biofilm performance of the most potent antibacterial CNF materials was assessed.

Abstract

Biofilm formation acts as an important self-defense strategy used by bacteria to protect itself against external changes and antibiotics effects. Moreover, biofilms have serious environmental and health impacts. Potent and safe natural antimicrobials may offer an ideal solution to tackle this challenge. In this context, we have recycled rice wastes into amino-functionalized cellulose nanofibers (CNF-NH₂), ammonium-based cellulosic nanofibers Schiff bases (CNFSBs), and their corresponding Ag(I)/Zn(II) complexes. The antibacterial study of new cellulosic specimens revealed that the grafting of CNF-NH₂ with ammonium ionic liquids segments and metal ions has remarkably improved its antibacterial activity. CNFSB₁-Ag was the most potent antibiotic candidate against common medically-relevant strains, methicillin-resistant S. aureus (MRSA) and E. coli (MIC_{MRSA/E. coli} = 0.19/0.65 µg/mL). Moreover, it is more active than the clinical drug (vancomycin) (MIC_{MRSA/E. coli} = 1.02/0.85 µg/mL). Also, CNFSB₁-Ag was the most active in limiting of MRSA and E. coli biofilms formation with 97.9% and 81.0% biofilm reduction, respectively.

Introduction

Recently, cellulose-based materials were widely utilized in food packaging [1] and diverse biomedical fields such as drug vehicles, wound dressings, and sanitary products due to their unique properties including high abundant, renewability, biocompatibility, biodegradability, nontoxicity, and inexpensive [2], [3], [4]. However, because cellulose has not inherent biocidal activity [5], the cellulosic materials are antimicrobial inactive and thus easily attacked by microbes [6]. To overcome these shortcomings, chemical modification or grafting of cellulose surface with bioactive compounds to develop antibacterial cellulosic materials is urgently needed.

In this context, three approaches have been adopted to formulate antibacterial cellulose-based materials; (i) blending with natural antibiotics or proteins [5]; (ii) incorporating metal ions or metallic nanoparticles [7], [8]; (iii) chemical modification or grafting with antimicrobial segments such as Schiff bases [9], [10], [11]. Nevertheless, there are still some drawbacks with currently available approaches to fabricate antibacterial cellulose-based materials [5]. For instance, the complex extraction and production process along with high-cost for natural antimicrobials from plant sources. Conformational changes in proteins as a result of different interactions with cellulose network, resulting in significant loss of their antibacterial effects. Also, instability of metal nanoparticles and their higher propensity for agglomeration causing, not only loss of their antimicrobial affects but also toxicity for normal human cells.

Consequently, exploring of cellulose-based biomaterials with prolonged antibacterial efficacy, possess multiple antimicrobial moieties, is needed to overcome the aforementioned problems. Among certified antimicrobials, Schiff bases occupied an advanced position because of their facile synthesis, availability, structural and electronic features, and strong coordinative ability with transition metal ions. Besides interesting pharmacological sites found in their skeletons (such as azomethine group, H-bonding donors (HBD), H-bonding acceptors (HBA), … etc). These pharmacological groups can induce several antibacterial modes in the bacterial cell through interaction with cell-active centers, resulting in malfunctions in its normal biological processes [12], [13].

Noteworthy, for a long time, many silver(I) compounds have been used as antimicrobial agents [14]. For instance, Ag(I) sulfadiazine (SSD), is still widely used as an antiseptic to prevent bacterial infections during burns and skin wounds [15]. Not long ago, it was reported that the Ag(I) complexes of symmetrical Schiff bases (obtained by condensation ethylenediamine or 1,3-diaminopropane with p-anisaldehyde) are more effective than SSD against Mycobacterium tuberculosis [16].

Interestingly, coordination of Zn(II) ion with traditional antibiotics resulted in a significant enhancement in their antimicrobial performances. For example, Zn(II) ions were used to promote the antimicrobial action of tetracycline [17], macrolides [18], aminoglycosides [19], quinolones [20], and vancomycin [21].

The increasing interest in ammonium ionic liquids as pharmacological candidates [22], due to their ability to provoke different bioreactions within multiple biological targets; make them very attractive for many chemotherapeutic researchers in designing new bioactive materials.

All aforementioned outstanding facts along with our continuous program for exploring novel biopolymer-based pharmacological candidates [23], [24], [25] induced us to design an efficient and simple protocol for; (i) recycling rice straw wastes into cellulose nanofibers (CNFs), (ii) amino-functionalization of CNFs to yield amino cellulose nanofibers (CNF-NH₂), (iii) Schiff base condensation of CNF-NH₂ with ethoxysalicylaldehyde-functionalized ammonium ionic liquids (ESAILs) to fabricate poly(ethoxysalicyl ammonium) cellulose nanofibers Schiff bases (CNFSBs) which used as chelating ligands for Ag(I)/Zn(II) in preparation of their corresponding complexes. The aim of the fabrication of these materials is to target antibacterial and anti-biofilm applications. To the best of our knowledge, no work has been reported for recycling of this agricultural biomass (rice straw) into poly(ionic liquids) Schiff bases-functionalized cellulosic nanofibers.