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Materials Letters 217 (2018) 33–38 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue Green synthesized PLA/silver nanoparticle probe for sensing of hydrogen peroxide in biological samples Shebi Alipilakkotte, Lisa Sreejith ⇑ Soft Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut 673601, KL, India a r t i c l e i n f o Article history: Received 13 July 2017 Received in revised form 15 December 2017 Accepted 7 January 2018 Available online 10 January 2018 Keywords: Polylactic acids Nanoparticles Piper nigrum Biological reduction Sensors Hydrogen peroxide a b s t r a c t In this paper, the physio-chemical characteristics of green synthesised PLA/silver nanoparticles (PLAAgNPs) system was investigated for H2O2 sensing. The precursor, silver nitrate (AgNO3) was reduced to nanosilver (n-Ag) by Piper nigrum leaves extract in dual phase medium containing PLA (Capping agent). X-ray photo electron spectroscopy (XPS) analysis was carried out to evaluate the interaction of PLA with silver nanoparticles. The morphological analysis by Transmission electron microscopy (TEM) obtained images of spherical silver particles with average size of about 10–20 nm. The H2O2 sensing was confirmed from the colour fading of the solution and vanishing of the characteristic surface plasmon resonance (SPR) absorbance peak of PLA-AgNPs on adding H2O2. The proposed sensor can detect even very low concentrations (5 lM) of hydrogen peroxide and finds applicability in biological and environmental analysis. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen peroxide (H2O2) is known to be a strong oxidant, which has its applicability in many fields. An accurate and highly sensitive detection of hydrogen peroxide (H2O2) is of practical importance in various fields ranging from bioanalysis to environmental protection [1]. There are several reported methods such as spectroscopic, colorometric, luminescent and electrochemical for H2O2 sensing. The colorometric detection is user-friendly due to its simplicity, cost-effectiveness, fast response and high detection limit [2]. Metal nanoparticles are of considerable interest in various fields due to their smaller size, large surface area, interesting morphologies and thereby showing enhanced catalytic, optical, electrical and sensing properties. Among metal nanoparticles, silver is widely accepted due to its attractive stability [3] and facile synthesis. The silver nanoparticles (AgNPs) were found to exhibit good sensing property against H2O2 [4]. AgNPs synthetic route involves both chemical and biological methods. Chemical reduction method exploits reducing agents specifically sodium borohydride [5], hydrazine hydrate, tri sodium citrate etc, of which sodium borohydride is known to be a stronger one. The biological synthesis of ⇑ Corresponding author. E-mail address: shebi_p130041cy@nitc.ac.in (L. Sreejith). https://doi.org/10.1016/j.matlet.2018.01.034 0167-577X/Ó 2018 Elsevier B.V. All rights reserved. colloidal AgNPs using various plant extracts like M. charantia, Carica papaya, Aloe vera, Moringa oleifera, lemon, Eucommis ulmoides and Piper nigrum Lin (P. nigrum) [6–9] were reported. The biological synthesis is of keen interest, as it can synthesise particles in a non toxic and cost- effective manner. The growth and agglomeration of AgNPs can be controlled by using stabilizing agents. The increase in surface tension and collision of particles can be prevented by using suitable stabilizers such as PVA, PVP, PEG, PLA, gelatin, chitosan [10,11] etc. Among these, PLA can strongly control the size, shape and also prevents agglomeration of AgNPs by capping the silver through hydroxyl group in PLA. Among the biodegradable polymers, PLA has received much attention due to its impressive biocompatibility. We synthesised PLA/Ag nanocomposites by biological reduction with P. nigrum leaves extract followed by colorimetric detection of H2O2 using UV-Visible spectroscopy. 2. Materials and methods 2.1. Preparation of P. Nigrum leaf extract 50 g of P. nigrum leaves were immersed in 50 mL distilled water in a 250 mL RB flask and was heated at 90 °C for about 30 min. The mixture was then cooled to room temperature and was filtered to obtain a yellowish extract and was stored in refrigerator at 10 °C to avoid contamination. 34 S. Alipilakkotte, L. Sreejith / Materials Letters 217 (2018) 33–38 Table 1 The formulations of samples with sample code. Sample code Sample description PLA-05Ag PLA-10Ag PLA-15Ag 95% PLA + 5% Silver nitrate 90% PLA + 10% Silver nitrate 85% PLA + 15% Silver nitrate 2.2. Preparation of PLA capped AgNPs using P. Nigrum leaf extract 5, 10 and 15 wt% (with respect to PLA, Mw = 40,000) of silver nitrate were added to three different DCM solutions each containing 10 wt% of PLA and their formulations are shown in Table 1 (abbreviated to PLA-05Ag, PLA-10Ag and PLA-15Ag respectively). 10 mL of the above solutions were separately stirred in ice cold condition for about 48 hr. Then 2.5, 5 and 7.5 mL of the leaves extract were respectively added drop wise to the ice-cold solutions under vigorous stirring. The reaction mixture was then heated to 50 °C under pH 10 for 30 min. The alkaloids present in the P. nigrum leaves extract reduces the silver ions to nanosilver, which gets capped by hydroxyl group of PLA (Scheme 1). 2.3. Fabrication of PLA/Ag nanocomposites film The reaction mixtures after the above procedure were collected and the organic phase containing PLA capped AgNPs were separated from aqueous phase. After drying at room temperature, the PLA/Ag nanocomposites were redissolved in DCM and the samples were solution-casted after ultrasonication for 15 min. The UV-VIS spectra were recorded over a range of 300–700 nm with UV spectrophotometer (UV-2600 SHIMADZU). The morphological analyses were performed using field-emission scanning electron microscope (FE-SEM; Hitachi Su 66,000) at an accelerating voltage of 5 kV and the samples coated on copper grids were subjected to transmission electron microscopy (TEM, JEM-2010). XPS spectra were measured with Auger Electron Spectroscopy (AES) module PHI 5000 versa prob II spectrometer. 3. Results and discussion Formation of PLA capped AgNPs in solution stage was confirmed by UV-Visible spectroscopy. The absorbance peaks at the vicinity of 400 nm were the characteristic peak for AgNPs [12]. PLA/Ag nanoparticles, which was biologically synthesized using p. nigrum extract was found to exhibit a single sharp intense peak at 416, 419 and 453 nm for PLA-05Ag, PLA-10Ag and PLA-15Ag (Fig. 1a) respectively. The presence of single SPR peak indicated that the particles were spherically shaped, further confirmed by TEM analysis. The increase in particle size of nanosilver (agglomeration) caused a red shift for AgNPs in the absence of PLA [13]. The stability of PLA capped AgNPs was analysed by UV-Visible spectroscopy at different time intervals (0 h to 3 months). These results convey the fact that incorporation of PLA may enhance the stability of nanoparticle through van der waals force of attraction. It was found that the absorbance peak slightly shifts to longer wavelength as the storage time increases. Scheme 1. AgNPs stabilization by PLA matrix. S. Alipilakkotte, L. Sreejith / Materials Letters 217 (2018) 33–38 35 Fig. 1. (a) Absorption spectra of PLA-AgNPs (b) FTIR spectra (c) X-ray diffraction pattern of PLA and PLA-10Ag (d) XPS spectrum of PLA-10Ag. Fourier transform infrared spectroscopy (AT-IR) were analysed for studying the interaction of nanosilver with PLA and to identify the functional groups present in the P. nigrum extract involved in the reduction of AgNO3 (Fig. S1). The increase in broadness of OH peak may be due to the Van der waals interaction between partial positive charge on nanosilver and hydroxyl group in PLA [14]. The interaction of nanosilver with the carbonyl group in the PLA is also evident from the shift in the carbonyl peak from 1748 cm 1 in PLA to 1751 cm 1 in PLA-Ag system (Fig. 1b). The phase analysis of the PLA and PLA-10Ag system was carried out to confirm the presence of silver particles in the PLA matrix. Three crystalline peaks (Fig. 1c) were obtained for PLA-10Ag system at 44.1°, 64.3°, 77.3°, which corresponds to (2 0 0),(2 2 0),(3 1 1) crystallographic planes of face-centered cubic (fcc) phase of the silver and no trace of Ag2O or AgO was found [15]. The PLA-10Ag system was subjected to XPS analysis, and the peaks corresponding to the 3 d orbitals of silver particles were evaluated. As shown in the Fig. 1d, the doublet peak in which an intense peak at 367.91 eV and the other peak at 373.86 eV can be attributed to Ag 3d5/2 and 3d3/2 electronic states, respectively. The reported literatures suggest that the XPS for pure metallic silver was found to exhibit two peaks at 368.3 and 374.3 eV. A slight variation in the obtained peak might be due to the change in chemical environment around the silver particles in presence of PLA [16]. The surface morphology of bio-synthesized PLA-AgNPs films was shown in Fig. 2. The spherical AgNPs with uniform size appeared as discrete particles indicating the stabilizing effect of PLA matrix. In the case of PLA-5Ag system, nanoparticle concentration was less in comparison with the PLA-10Ag system and has less probability for agglomeration in both the systems. On increasing the silver content, i.e., in PLA-15Ag system, the agglomeration tendency was high even in the presence of the stabilizing agent, PLA. The SEM images of PLA-AgNPs (Fig. 2a, d and g) revealed that the silver particles were in the range of 10–15 nm, 20–30 nm and 300–600 nm for PLA-5Ag, PLA-10Ag and PLA-15Ag respectively. The TEM image (Fig. 2b and e) of both PLA-05Ag and PLA-10Ag system shows discrete spherical silver particles, which hasn’t 36 S. Alipilakkotte, L. Sreejith / Materials Letters 217 (2018) 33–38 Fig. 2. SEM and TEM images of PLA-05Ag (a & b–c), PLA-10Ag (d & e–f) and PLA-15Ag (g & h–i) respectively. agglomerated due to the influence of PLA. Interestingly, the particles were uniformly distributed throughout the PLA matrix for PLA-10Ag system. The average size of the silver particles were 10 ± 3.19, 23.44 ± 5.23, and 80 ± 17.79 nm for PLA-05Ag, PLA-10Ag and PLA-15Ag system respectively (Fig. S2). The PLA-15Ag contains agglomerated silver particles (as depicted in Fig. 2h) due to high concentration of silver loading. 3.1. Colorimetric detection of H2O2 Further investigation was carried out to check the sensitivity of the proposed sensor. The PLA capped AgNPs were studied for their H2O2 sensitivity, with different concentrations of H2O2. It was observed from the Fig. 3e, that with the increase in H2O2 concentration, the absorbance for the nanosilver decreases. A linear plot was obtained for SPR peak change (DA) against H2O2 concentration as shown in Fig. 3f (R2 = 0.9775). Even a smaller concentration of about 10 lM H2O2 was sensitively detected by the system (Table S1), which is evident from the UV-VIS spectrum [17]. The sensitivity of the silver nanoparticles as an H2O2 sensor depends on their morphology [18]. The obtained spherical nanoparticles has shown 8.57% decrease in the absorbance of the bands at 419 nm at a low concentration of H2O2 (10 lM), and about 98.53% decrease for high concentration of H2O2 (100 lM). The colorimetric sensing of H2O2 by the PLA-10Ag system was supported by the morphological change observed in the TEM image (Fig. 3a and b), which shows an observable decrease in the average size of nanoparticle from 23.44 ± 5.23 nm to 5.25 ± 1.42 nm on etching, revealing the effectiveness of H2O2 in etching of silver particles present in PLA-10Ag system. 4. Conclusion The developed system i.e., PLA capped spherical Ag-NPs were characterized and studied for their sensing against H2O2. An optimal temperature of 50 °C and a pH of 10 were set for obtaining the silver nanoparticles with narrow size distribution in PLA matrix. Thus a cost-effective and sensitive sensor for H2O2 was developed by green conditions without imposing any complicated technique or strong chemical reducing agents. S. Alipilakkotte, L. Sreejith / Materials Letters 217 (2018) 33–38 37 Fig. 3. (a) and (c) TEM image and size distribution histogram of PLA-10Ag before and (b) and (d) after etching reaction respectively (e) SPR absorption spectra of the spherical PLA-10Ag before and after reaction with H2O2 at various concentrations for 40 min (f) Plots of the SPR peak change of spherical PLA-10Ag vs. concentrations of H2O2. Acknowledgement Appendix A. Supplementary data This research was financially supported by Ministry of Human Resource Development (MHRD), India. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.matlet.2018.01.034. 38 S. Alipilakkotte, L. Sreejith / Materials Letters 217 (2018) 33–38 References [1] Y.A. Liu, X.B. Liao, Curr. Org. Chem. 17 (2013) 654–669. 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