Elsevier

Carbohydrate Polymers

Volume 115, 22 January 2015, Pages 238-245
Carbohydrate Polymers

Green chitosan–carbon dots nanocomposite hydrogel film with superior properties

https://doi.org/10.1016/j.carbpol.2014.08.021Get rights and content

Highlights

  • Synthesis of carbon dots (CD) from tea by a greener approach.

  • Preparation of stable, soft but tough chitosan–carbon dots nanocomposite hydrogel films.

  • Carbon dots improve the strength, thermostability and wettability of chitosan hydrogel films.

Abstract

In this work we report novel chitosan–carbon dots nanocomposite hydrogel films. A new green source “tea” was used as precursor for carbon dots (CDs). The electrostatic interaction of positive charge on chitosan and negative charge on CDs prepared from tea was used for the successful preparation of a stable and robust chitosan–carbon dots nanocomposite hydrogel film. The hydrogel films were characterized by UV–visible spectroscopy, X-ray diffraction (XRD), Fourier transformed infra-red spectroscopy (FTIR), scanning electron microscope (SEM), fluorescent microscopy, thermogravimetric analysis (TGA) and contact angle analysis. It was observed that chitosan–carbon dots hydrogel films are soft but tough with superior UV–visible blocking, swelling, thermal and mechanical properties in comparison to chitosan hydrogel film. Moreover chitosan–carbon dots films are more water repellent (hydrophobic) as indicated by their high contact angle values. Thus, fabrication of such green soft but tough biocompatible chitosan–carbon dots nanocomposite hydrogel films offers tremendous bio-medical and industrial applications.

Introduction

Chitosan, the cationic (1-4)-2-amino-2-deoxy-<beta>-D-glucan partly acetylated to the typical extent close to 0.25, is industrially produced from marine chitin (Muzzarelli, 2010, Muzzarelli, 2012, Muzzarelli et al., 2012). It has attracted attention owing to its superior features such as biocompatibility, atoxicity and biodegradability (Bottegoni et al., 2014, Busilacchi et al., 2013, Muzzarelli, 2009). Solubilisation of chitosan occurs in acidic aqueous medium by protonation of the single bondNH2 functional group present on the C-2 position of the D-glucosamine repeating unit. This makes chitosan the only pseudo-natural polymer which can be applied as solution, film, fibers as well as in hydrogel form (Rinaudo, 2006, Illum, 1998). Chitosan hydrogel, the most commonly used form of chitosan can be prepared by crosslinking with glycerol in an acidic medium (Chowdhury, Gogoi, & Majumdar, 2012). Hydrogel is a three dimensional network structure of polymer which can entrap water without losing their network structure (Kiuchi, Kai, & Inoue, 2008). Kiuchi et al. reported preparation of chitosan hydrogel film cross-linked with DiepoxyPoly(ethylene glycol) with improved mechanical and swelling behavior compared to the chitosan hydrogel film cross-linked with poly(ethylene glycol) (Kiuchi et al., 2008). Khan et al. demonstrated the fabrication of improved nanocrystalline cellulose reinforced chitosan based nanocomposite films (Khan et al., 2012). So, this polymer has been used in past tuning its properties via fabrication of nanocomposites.

There are reports of using different nanomaterials like gold, silver, carbon nanotube etc. for the preparation of chitosan hydrogel nanocomposite. Recently carbon quantum dots also known as carbon dots (CDs) having sizes below 10 nm have emerged as the most alternative fluorescent nanoprobe (Qian et al., 2014) due to their easy availability, cost-effectiveness, thermal stability and relatively non-cytotoxic nature (De, Voit & Karak, 2013). Many new carbon sources for preparation of carbon dots are continuously reported since its first report by Sun et al. (2006). Sahu et al. showed carbon dots prepared from hydrothermal treatment of orange juice (Sahu, Bhhera, Maiti & Mahapatra, 2012). Zhang et al. demonstrated the preparation of fluorescent CDs directly via simple hydrothermal method using bovine serum albumin (BSA) as a carbon source in the presence of surface passivating agent (Zhang, Hao, Zhang, Zhang & Tang, 2012). Recently Hsu, Shih, Lee & Chang, (2012) showed a simple method for the preparation of carbon dots from coffee grounds. Other methods of preparing of carbon dots from egg yolk or egg white (Wang, Wang & Chen, 2012) and ascorbic acid (Jia, Li, & Wang, 2012) has also been reported recently. We have also demonstrated a simple and convenient strategy to prepare fluorescent carbon dots from chitosan gel and also various methods developed over the years for the preparation of carbon dots finds mention in our earlier paper (Chowdhury et al., 2012).

In this report we present a new green source “tea” as a suitable precursor for synthesis of fluorescent carbon dots. These tea carbon dots are successfully utilized in preparing a chitosan–tea carbon dots nanocomposite hydrogel film with improved physico-mechanical properties compared to that of chitosan hydrogel film cross-linked only with glycerol. Tea is one of the major agro-commercial products of India. There are three main kinds of tea produced in India: Assam tea from north-eastern part of India (state of Assam), Darjeeling tea from the foothills of the Himalayas and Nilgiri tea from southern India. In this work we have successfully prepared carbon dots from Assam tea. As per our knowledge, there is only one report of preparation of chitosan nanocomposite using carbon dots. Huang et al. reported preparation of CDs nanocomposite film for dopamine biosensing (Huang et al., 2013). Here we have focused on the preparation of a green soft but tough biocompatible hydrogel film from chitosan and tea carbon dots and its varied applicability due to possession of many important properties like UV–visible blocking, excessive swelling, thermal stability and mechanical strength.

Section snippets

Materials

Chitosan (M.W  193400, degree of deacetylation = 77.7%) obtained from Sigma Aldrich, Germany was vacuum dried before every use. Glycerol (density = 1.255 at 20 °C), glacial acetic acid and sodium hydroxide (pellets) obtained from Merck, India were used without any further purification. CTC (Crush, tear, curl) tea was purchased from Jalan Golaghat Tea Co. (P) Ltd., Assam, India.

Preparation of chitosan hydrogel film

Chitosan hydrogel was prepared by following a simple protocol. A solution of glycerol and 0.1 M acetic acid solution was

Results and discussions

Chitosan and chitosan–carbon dots nanocomposite hydrogel films were successfully prepared by film casting method as described in the experimental section. We have used commercially available “Assam tea” as carbon source for the first time for preparing fluorescent carbon dots in two dispersion medium, viz., ethanol and 0.1 M acetic acid. Tea which is abundant in eastern part of India was used as a green and cost-effective material in preparing carbon dots.

Fig. 1A shows the schematic

Conclusion

In conclusion, we have reported herein fabrication of a soft but tough chitosan–carbon dots nanocomposite hydrogel film possessing interesting properties. The carbon dots used in making the nanocomposite was prepared from a new green source “tea”. The basic criteria for the preparation of such hydrogel films were the successful electrostatic interaction between positive charge on chitosan and negative charge on carbon dots. These electrostatic interactions helped the chitosan–carbon dots films

Acknowledgment

The authors would like to thank Council of Scientific and Industrial Research (CSIR), New Delhi for the project No. 01(2488)/11/EMR-II), All India Council For Technical Education, New Delhi for the project 8023/RID/RPS-3/(NER)2011-12, Science and Engineering Research Board (SERB), New Delhi for project grant SR/S1/PC/0058/2010 and SB/S1/PC-69/2012. Authors would like to acknowledge help from Dr. Tridib Sarma for SEM images and Dr. Nandana Bhardwaj for fluorescent microscopy. AK thanks IASST for

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