Elsevier

Polymer Testing

Volume 86, June 2020, 106481
Polymer Testing

Material Properties
Chitosan aerogel containing silver nanoparticles: From metal-chitosan powder to porous material

https://doi.org/10.1016/j.polymertesting.2020.106481Get rights and content

Highlights

  • Chitosan is modified using silver organosols produced via metal vapor synthesis.

  • Silver nanoparticles are spherical and have dominant size fraction below 4 nm.

  • Modified silver-containing chitosan is used to form silver-chitosan aerogel.

  • New states of nitrogen in the aerogel indicate silver-chitosan chemical interaction.

  • Specific surface area of the formed aerogels is up to 100 m2/g.

Abstract

The work presents a synthetic approach that combines methods of metal vapor synthesis (MVS), gelation and supercritical drying in order to obtain chitosan aerogels containing silver nanoparticles. On the first stage, two types of silver organosols were prepared via the eco-sustainable MVS method. Then the prepared silver organosols were used to modify chitosan powders for producing metal-chitosan powder composites. Gelation of the powder composites was performed in oxalic acid at elevated temperatures. Supercritical drying of the gels was implemented in order to preserve the formed porous structures. Thus, the chitosan powders modified with MVS-produced silver nanoparticles were used to prepare metal-chitosan aerogels. Characterization of the structure and the morphology of both powder and aerogel silver-chitosan composites was conducted by means of low temperature nitrogen adsorption, X-ray photoelectron spectroscopy, X-ray powder diffraction, small-angle X-ray scattering, SEM and TEM. Changes in the structure and morphology of silver nanoparticles between powder and aerogel composites were analyzed.

Introduction

Chitosan is a deacetylated derivative of a polysaccharide chitin which is commonly found in the shells of insects and crustaceans, as well as cell walls of some fungi, and is known as the second most abundant biopolymer in nature after cellulose [1]. Its abundance, biocompatibility, biodegradability and antimicrobial activity make chitosan a perspective material for the use in biomedical applications [2,3]. Chitosan is known to form both chemical and physical gels that can be further modified via several drying techniques to yield chitosan aerogels [4,5]. Combination of bioactive properties of chitosan and unique physical properties of aerogels, i.e. their high porosity and low density, can be utilized in thermal insulation applications [6,7], CO2 capture [8], oil-water separation process [9], as well as in drug-delivery [10].

A significant difference between chemical and physical gels is that different types of inter-molecular interactions are involved in their formation. Interactions in physical gels mainly occur due to the formation of hydrogen or ionic bonds and hydrophobic associations. Thus, the bonds in physical gels are reversible. For the chemical gels, on the other side, bonds are covalent in nature and cross-linking is difficult to break. As a rule, special cross-linking agents, such as aldehydes are used to obtain chemical gels [11,12]. For example, the synthesis of transparent nanofibrous chemically cross-linking aerogels from chitosan using aldehydes with various chain length, including glutaric aldehyde, was suggested [5,13,14]. Possible applications for chemically cross-linked chitosan aerogels are mainly thermal insulation and adsorbents [4,6]. Their use in biomedical applications can be problematic due to possible cytotoxic effects caused by residual cross-linking chemicals and by-products in the end materials.

For the formation of physical gels of chitosan, the dependence of chitosan solubility on the pH can be exploited. It is reported that if chitosan is dissolved in acetic acid and then coagulated in an alkaline bath with subsequent supercritical drying, chitosan aerogels with specific surface area up to 175 m2/g are formed [15]. Another study describes chitosan dissolution in acetic acid with subsequent cooling of the solution to −20 °C and solvent exchange to acetone. The resulted chitosan aerogel scaffolds are suggested for the use in tissue engineering [16].

Another way to form a physical chitosan gel is by increasing the ionic strength of the solution. For example, if chitosan is dissolved in a multivalent acid at a certain pH, ionic strength and at elevated temperature and then the solution is cooled down, it is possible to obtain a physical chitosan hydrogel [17,18]. Oxalic acid, a cheap and widely used reagent in the food and biomedical fields, can act as such a multivalent acid [19]. The described ionotropic cross-linking was implemented to produce strong porous chitosan membranes with high water uptake, electrolyte membranes [20] and hydrogels [21].

For some medical-related applications it is considered beneficial to introduce silver nanoparticles into chitosan-based materials [[22], [23], [24], [25]], including chitosan gels [26]. Indeed, metallic silver has intrinsic antimicrobial activity [27], which in combination with chitosan properties can lead to a synergetic effect [28,29]. The most common way to produce silver nanoparticles is by chemical reduction of silver nitrate with special additives such as sodium borohydride, sodium citrate or sodium ascorbate [30]. When using this method to introduce silver nanoparticles specifically into biopolymer aerogels, the silver nitrate was reported to be added either prior to gelation [31] or after the gel structure was formed [32].

The described conventional chemical reduction method may be associated with some difficulties, mainly regarding the level of control of metal reduction and the necessity to deal with by-products and residues of the precursors. Yet, both complete metal conversion and absence of any biohazardous residues are crucial for biomedical applications. An alternative approach to metal nanoparticles synthesis is metal vapor synthesis (MVS) which can provide metal nanoparticles via metal evaporation and co-condensation with a chosen solvent on a cooled reactor wall [33]. We have successfully demonstrated that the resulted metal-containing organosols can be used to modify biopolymer powders, such as microcrystalline cellulose [34], chitosan [35], porous collagen-chitosan scaffold [36], and bacterial cellulose film [37]. Some important features of MVS, e.g. the absence of any by-products, zero valences of the forming metal nanoparticles and the ability to use medical-grade solvents with no solvent contamination, make the proposed approach promising for the preparation of materials for biomedical applications [38,39].

In the present research, we expand the possible use of MVS in introducing metal nanoparticles to biopolymer composites. More specifically, the method is used to obtain metal-containing chitosan powder, which is then used for gelation. The work is aimed at the study of formation of aerogels from chitosan powders containing MVS-produced silver nanoparticles. For this purpose, the morphology, phase and surface composition of chitosan-based materials are studied.

Section snippets

Materials and reagents

Chitosan (Chit) with viscosity of 1% solution in 1% acetic acid 200 сP and degree of acetylation less than 25% was supplied by Wirud, Germany. Acetone and isopropyl alcohol with special purity grade of 99.5%; Ag foils (99.99%) cut on small pieces; oxalic acid dihydrate of analytical grade were used in this work. Before all preparation procedure chitosan powder was degassed for 12 h under vacuum of 1 Pa at 40 °C.

Preparation of Ag/Chit composites

Formation of the metal-chitosan powder composite occurred through two main steps.

Preparation procedure and elemental analysis

Metal-containing aerogels were prepared according to the general synthetic route schematically described in Fig. 1. At first, silver-containing organosols were prepared by means of MVS. Either isopropanol or acetone was used as a solvent. The prepared organosols were then used to modify chitosan powder (Chit) by introducing silver nanoparticles. Two types of metal-containing chitosan powders were obtained – using isopropanol-based organosol and acetone-based organosol (AgIP/Chit and AgAC/Chit,

Conclusions

Chitosan-based silver-containing aerogels were successfully synthesized in the present work using a multi-step process that included MVS, chitosan powder modification, ionic gelation and supercritical drying. X-ray powder diffraction detected the presence of metallic silver in both modified chitosan powders and aerogels. X-ray fluorescence analysis data show that silver nanoparticles are not leached as a result of the transition from powders to porous aerogel materials. Microscopy analysis

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources

This work was supported by Russian Foundation for Basic Research grant № 18-33-01094.

CRediT authorship contribution statement

Margarita S. Rubina: Conceptualization, Investigation, Writing - original draft, Writing - review & editing. Igor V. Elmanovich: Investigation, Writing - original draft, Writing - review & editing. Alexandra V. Shulenina: Investigation. Georgy S. Peters: Investigation. Roman D. Svetogorov: Investigation. Alexander A. Egorov: Investigation. Alexander V. Naumkin: Investigation, Writing - review & editing. Alexander Yu Vasil'kov: Conceptualization, Investigation, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by Russian Foundation for Basic Research grant № 18-33-01094. This work was performed with the financial support from Ministry of Science and Higher Education of the Russian Federation. X-ray fluorescence analysis was performed with the financial support from Ministry of Science and Higher Education of the Russian Federation using equipment of Center for molecule composition studies of INEOS RAS. The authors acknowledge support from M.V. Lomonosov Moscow State University

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