This paper aims to study the green synthesis Ag-NPs using seaweed K

GREEN SONOCHEMICAL SYNTHESIS OF SILVER NANOPARTICLES USING MARINE SEAWEED AS BIOPOLYMER MEDIA

M. FARIED, K. SHAMELI*, M. MIYAKE, H. HARA, N. B. A. KHAIRUDIN Department of Environment and Green Technology, Malaysia-Japan

International Institute of Technology, Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra, 54100 Kuala Lumpur, Malaysia

Silver nanoparticles (Ag-NPs) were successfully synthesis from AgNO₃ through a green method under sonochemical irradiation by using seaweed Kappaphycus alvarezii (K.

alvarezii) as biopolymer media. Many methods for synthesis Ag-NPs reported such as chemical and physical techniques however many of them have environmental risk. This paper aims to study the green synthesis Ag-NPs using seaweed K. alvarezii due to an eco- friendly stabilizer and sonochemical irradiation method. The formations of Ag-NPs/K.

alvarezii were determined by the surface plasmon resonance (SPR) under UV-visible spectroscopy that was observed at 377-387 nm. Zeta potential results indicated that these nanoparticles have good stability after 720 min irradiation. SEM-EDX analysis confirmed the element of Ag and spherical shape of the Ag-NPs. The FT-IR spectrum described the presence of K. alvarezii and Ag-NPs.

(Received September 3, 2015; Accepted December 17, 2015)

Keywords: Silver nanoparticles, AgNO₃, Green synthesis, Kappaphycus alvarezii, Sonochemical irradiation.

1. Introduction

Numerous researchers and engineers in this period have extraordinary consideration regarding the field of nanotechnology. Nanotechnology has given many aspects for human life such as in the electronics, materials, computing, manufacturing, energy, catalysis, medicine, and transportation [1-2]. Nanoparticles are characterized as solid particles with size range of 1-100 nm.

They have unique chemical and physical properties because of their large proportion of high- energy surface atoms. Accordingly, the size of these particles has frequently properties which are not quite same like bulk samples in the same materials [3].

The Ag-NPs have been received enormous attention in various areas due to their applications for instance as antibacterial agents [4], catalysts [5], and surface enhance Raman spectroscopy (SERS) [6]. Base on their novel biological properties of Ag-NPs [7], these materials have been significantly different from macroscopic metal phases. Today, Ag-NPs can be synthesized through many technics such as photochemical method [8], chemical reduction [9], microwave irradiation [10], and sonochemical irradiation [11]. The greater part of synthesized Ag- NPs is using toxic chemical reducing agents which have potential for ecological risks.

Sonochemical method can be an alternative for the producing Ag-NPs because of this capability as the reducing agent. Since found of this method, it has been studied for yielding many kinds of nanomaterials particularly noble metal nanoparticles for example gold [12], silver [13], and platinum [14]. This method can rupture chemical bond under 20 kHz radiation that giving mechanism for the creation, growth, and collapse of the bubbles in the solution. Moreover, the bubbles grow in the solution and raise the local temperature to 5000 K and the pressure to a few hundred atmospheres. These extreme conditions of sonochemical cause the rupture of chemical

*Corresponding author: [email protected]

NPs/K. alvarezii have been optimized and characterized to study the effect of the sonochemical irradiation time on the optical properties, structures, stability, and morphology of Ag-NPs.

2. Experimental section

2.1 Materials

For this work, all reagents were used as received without any purification. The seaweed K.

alvarezii is a kind of red seaweeds that was obtained from Sabah, Malaysia. AgNO3 was obtained from Bendosen 99.89 % (C0721-2284551). The aqueous solution was prepared with deionized water from ELGA Lab-Water purification system, UK.

2.2. Synthesis of silver nanoparticles

The Ag-NPs were synthesized by reducing AgNO3 under sonochemical irradiation in the presence of K. alvarezii. A total of 100 ml AgNO3 (0.01 M) solution was added to 400 ml of K.

alvarezii (0.3 wt %). The K. alvarezii crashed to be finely using a grinder until uniform suspension was obtained. AgNO3/K. alvarezii solutions were divided into 12 samples. The samples were exposed to high-intensity sonochemical irradiation for 30, 60, 120, 180, 240, 360, 480, 540, 600, 660, and 720 min; at amplitude of 70% and 0.5 of cycle include of seaweed. Sonochemical irradiation was carried out with ultrasonic liquid processor (Hielscer Ultrasound Technology UP- 200S-RN, Germany, 50/60 Hz) which has probe that immersed directly into the reaction solution.

2.3. Characterization methods and instruments

The Ag-NPs/K. alvarezii were characterized using ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), zeta potential analyzer, energy dispersive X-ray (EDX), and scanning electron microscopy (SEM). The UV-Vis spectra were recorded over the range of 300-800 nm with UV-Vis spectrophotometer (UV-1800 SHIMADZU).

FT-IR spectra were recorded over the range of 200-4000 cm^-1 with a series Aligent Technologies Cary 660 Series FT-IR Spectrometer. The stability materials were recorded with Particulate Systems Nano-Plus Zeta/Nano Particle Analyzer, Japan. EDX was performed with the XL 30 Philips instrument to study the element of nanoparticles. SEM was carried out on a JEOL-Jsm- 7600F to study the morphology.

3. Results and discussion

The reactivity of Ag-NPs/K. alvarezii under different sonochemical irradiation times is shown in Figure 1. The colorless suspension of Ag-NPs/K. alvarezii under various sonochemical irradiation times was remarkable which changed the color from clear-color to brown and lastly to the dark brown. This changing color can be indicated the formation of Ag-NPs in the K. alvarezii suspension.

Fig. 1. Photograph of K. alvarezii and Ag-NPs/K. alvarezii suspension at different sonochemical irradiation times.

When the suspension exposed to the sonochemical irradiation, bubbles grew in this solution. These bubbles reached in maximum value and collapsed to the solution. This strong collapse raised the temperature and pressure in the solution which caused the rupture of chemical bonds and the formation of free radicals [19]. The mechanism of colloidal silver formation could be suggested as shown in Equations (1-7). After the AgNO3/K. alvarezii suspensions exposed by sonochemical wave, they produced radicals H• and OH• as described in Eq. (1).

nH2O Sonication H• + OH• (1)

AgNO3 separated to Ag⁺ and NO3ˉ ions in the aqueous solution as shown in Eq. (2). Then it reacted with the free radicals from sonochemical irradiation.

AgNO3 Hydrolysis

Ag⁺ + NO3ˉ (2)

The indirect effect of OH• radicals yielded free radicals inside the biopolymer groups of K. alvarezii in Eq. (3), this free radical reduced Ag⁺ to form Ag° and new group (R′) Eq. (4).

OH• + RH R• + H2O (3)

R• + Ag⁺ Ag° + R′ + H⁺ (4)

Additionally this H• radicals were as a strong reducing agent due to sonochemical irradiation. Hence, it could be reduced silver ions to the zero-valent state of the silver Eq. (5).

H• + Ag+ Reduction

Ag° + H⁺ (5)

Equation (6) referred to the direct reaction of Ag⁺ with water in the interfacial region between the cavitation bubbles and the liquid.

Ag⁺ + H2O Ag° + OH• + H⁺ (6)

In Eq. (7), silver atoms formed by the sonochemical irradiation and it made relatively stabilization of Ag clusters.

nAg° (Ag°)n (7)

(Ag°)n were the silver nanoclusters containing n silver atoms. After sonochemical irradiation of AgNO3/K. alvarezii aqueous suspension, many aqueous electrons (eaq‾) were produced and the Ag⁺ ions reduced into Ag-NPs.

Fig. 2. Time evolution of the UV-Vis absorption spectra from K. alvarezii until 720 min sonochemical irradiation.

UV-Vis spectra of the different time irradiation using biopolymer K. alvarezii is shown in Figure 2. This figure shows the characteristic of surface plasmon resonance (SPR) absorption bands at around 377 until 387 nm. It can be observed that the Ag-NPs have spherical shape [21], a finding of the shape would be confirmed by SEM images.

Time variation studies of green synthesis Ag-NPs using K. alvarezii was carried out with irradiation time of 30, 60, 120, 180, 240, 360, 480, 540, 600, 660, and 720 minutes. The spectra shows that apart from increasing time has effect to the increasing of SPR bands. However, after 480 min irradiation, the SPR peaks shift to the decreasing of the absorbance. This phenomenon is related with the increased size and agglomeration of Ag-NPs [22].

In addition, the UV-Vis spectra shows that the increasing time irradiation caused the increased SPR absorbance. Until 480 min irradiation, the SPR shifted to the high wavelength to 387 nm, a red-shift theory. This theory can be indicated an increase of the Ag-NPs size [23]. On the contrary, when irradiation time increased to 720 min, the SPR bands resulted absorbance at 384 nm, a blue-shift theory. This particularly condition is caused by the decreased particle size of the nanoparticles [24]. Therefore, the SPR band intensity increased with the increasing the time irradiation which was small together with the rising of time irradiation [25].

3.2. FTIR analysis

The biomolecule responsibility of the reduction Ag⁺ ion was identified with FT-IR spectra.

The solid residue was scraped and mixed with KBr powder, grounded, and pressed into a clear disc which was tested directly to the equipment. Figure 3 represents the FT-IR spectrum of Ag- NPs/K. alvarezii after sonochemical irradiation.

The peak in Fig 3(a) is represented of K. alvarezii which has region of 3366 cm^-1 and 2919 cm^-1 that can be described to stretching vibration of –OH and C–H group, respectively [26]. The absorption band at 1638 cm^-1 can be corresponded to polymer-bound water. The peak at 1410 cm^-1 can be attributed to the sulfate stretching due to sulfate atom in the chemical structure of K.

alvarezii. The band at 1146 cm^-1 can be assigned to SO3 stretching. The peak at 837 cm^-1 may be due to C–O–S stretching in B-D-galactose of this biopolymer.

Fig. 3. FTIR spectra for K. alvarezii (a), 480 min irradiation time (b), and 720 min irradiation time (c), respectively.

Meanwhile, after sonochemical irradiation for 480 in Fig. 3(b) and 720 min in Fig. 3(c), the FT-IR spectra does not show any obvious different with the biopolymer K. alvarezii. However, free carbonyl groups in the K. alvarezii macromolecules are formed by oxidation of carbohydrate radicals that is delivered in the biopolymer by the indirect effect of OH radicals [27]. The –OH stretching from K. alvarezii polysaccharides have been contributed in nanoparticle binding due to the negative charge for the silver. In addition, the new peak representing of Ag-NPs were evident in the region that less than 500 cm^-1 refers to the bonding of Ag-NPs with the oxygen from hydroxyl groups of K. alvarezii chains [28].

3.3 Zeta potential analysis

Zeta potential is an important parameter for understanding of nanoparticle stability. The good stability of nanomaterials can be indicated for durable properties in the long time. In Fig. 4, the Ag-NPs/K. alvarezii got a negative zeta potential value for 720 min irradiation. This aqueous suspension has value of -35.86 + 2.80 mV. The nano-suspension can be indicated stable when the zeta potential value is more than + 30 mV [29]. Therefore, this result can be clearly understood that the Ag-NPs/K. alvarezii are fairly stable with the increasing irradiation time until 720 minutes.

Fig. 4. Zeta potential result for Ag-NPs/K. alvarezii after 720 min sonochemical irradiation.

Fig. 5. EDX spectra of Ag-NPs/K. alvarezii after 720 min irradiation time.

The scanning electron microscopy (SEM) image study the morphology of the Ag-NPs. On Fig. 6 shows the Ag-NPs/K. alvarezii after 720 min under sonochemical irradiation. The SEM images display the changing on the surface of Ag-NPs/K. alvarezii when the irradiation time increased. Moreover, the shape of these NPs can be analized that the spherical shape of these Ag- NPs/K. alvarezii biopolymer media has been obtained [31].

Fig. 6. SEM image of Ag-NPs/K. alvarezii after 720 min sonochemical irradiation time.

4. Conclusions

In this study, Ag-NPs have been synthesized by green method using seaweed K. alvarezii biopolymer under different sonochemical irradiation times. The UV-Vis spectra shows the surface plasmon resonance (SPR) at 377-387 nm which indicates the creating of Ag-NPs. FT-IR shows the

interactions exist between molecules of K. alvarezii as the biopolymer media with surface charges of Ag-NPs. The stability of Ag-NPs after 720 min irradiation can be shown by zeta potential value which indicates good stability of these nanoparticles. The SEM image present that after long time sonochemical irradiation (720 min) the size and shape of nanoparticle changed to the small and spherical form. The EDX spectrum demonstrate that the Ag-NPs formation from the Ag ions after 720 min of irradiation time.

Acknowledgment

The authors would like to thank the Ministry of Education Malaysia for funding this research project through a Research University Grant of Universiti Teknologi Malaysia (UTM), with project title "Green sonochemical synthesis of silver nanoparticles using natural polymer media" and reference number PY/2015/04574 under PAS grant. Also, thanks to the Research Management Center (RMC) of UTM for providing an excellent research environment in which to complete this work.

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