PLASMONIC MATERIALS OBTAINED IN NATURAL EXTRACT* In ...

PLASMONIC MATERIALS OBTAINED IN NATURAL EXTRACT* R.C. FIERASCU1,2, I. DUMITRIU1,2, R.-M. ION1,2 1

National Research & Development Institute for Chemistry and Petrochemistry – ICECHIM, Bucharest, Spl. Independentei, 202, sect. 6 E-mail: [email protected] 2 Valahia University, Targoviste, 2 Carol I Bd Received May 25, 2009

Gold nanoparticles as plasmonic materials, were prepared in this paper by using a simple, green, room-temperature, non-toxic route. The materials used are easy to obtain and the synthesis does not require any sophisticated equipment. The synthesized nanoparticles were characterized trough X-ray diffraction, UV-Vis spectroscopy and FT-IR. The influence of light on the synthesis is also discussed, as well as the molecules that are most probably involved in the synthesis. Key words: plasmons, gold nanoparticles, lemon grass extract.

1. INTRODUCTION

In recent years, plasmonics has emerged as a promising tool in the fields of analytical chemistry and biochemistry. In particular, surface plasmon resonance at the surfaces of gold nanostructures has led to the development of widespread interest in gold nanoparticles. A technological break-through in the fabrication of nanodimensional clusters and other metallic nanoparticles gave rise to a development of nano-technological and nanooptical branches such as nanoplasmonics, which is of great interest to physicists, chemists, material engineers, IT specialists, and biologists. The nanoplasmonics deals with conduction electron liquid oscillations in metallic nanostructures and nanoparticles, and an interaction of those oscillations with light (plasmon–polariton) [1, 2]. Such studies are also aimed at the oscillations of a crystal lattice (SiC, for example) in the nanoparticles, whose interaction with light has much in common with plasmon oscillations (phonon– polariton) [3]. The important feature of nanoplasmonic phenomena is the combination of a strong spatial localization and highfrequency (from ultraviolet to infrared) of electron oscillations. Strong localization leads to a giant enhancement of the local optical and electrical fields. These important features of plasmonic particles made it possible to discover quite a number of new effects. One of the most developed is the use of large local fields near plasmonic nanoparticles for *

Paper presented at the BRAMAT 2009, February 26–28, 2009, Braşov, Romania.

Rom. Journ. Phys., Vol. 55, Nos. 7–8, P. 758–763, Bucharest, 2010

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enhancement of the Raman scattering cross-section. Recent experiments have shown that such an increase may achieve 10–14 orders of magnitude of enhancement, which may help to resolve single molecules. The local enhancement of the fields can also be used to increase the fluorescence intensity and to determine the structure of a single DNA strand without using fluorescent labels. By using nanoparticles of more complex shapes one can provide enhancement of both the absorption and the emission of light by natural and artificial fluorophores. Besides, the plasmon nanoparticles are proposed to be used in nanolasers and to stimulate plasmonic oscillations in nanoparticles by means of the optical emission (SPASER). Beside these new applications of plasmonic nanoparticles, one can essentially increase the efficiency-cost ratio, for example, in solar batteries or light emitting diodes by using the achievements in nanoplasmonics. Finally, it is expected that nanoplasmonics will make it possible to create a new element base (genuine integrated optics and ultracompact optical components) for computers and data processing equipment by taking advantage of the small dimensions of metallic nanoparticles and fast speed of optical processes. An intricate spatial structure of the physical phenomena, which form the basis of nanoplasmonics, impedes the development of the latter. Very often, the numerical studies do not allow one to explain the physics of the observed phenomena, while the analytical studies are mostly devoted to the case of spherical and spheroidal nanoparticles, which are very far from the synthesized nanostructures from the viewpoint of geometry and physics [4]. There has been significant interest in gold nanoparticles over the past few decades, and particularly over the past several years, because of their unique shape- size-, and aggregation- (orientation-) dependent optical properties. These characteristics have been exploited for a variety of applications, including optical sensing, catalysis, and nanoscale electronics. Much of the focus on the spectroscopic applications of gold nanoparticles has been devoted to the intense plasmon bands appearing in the visible-to-near-IR region. A number of preparation procedures for gold nanospheres have been reported. Of these, the most familiar methods are via the chemical reduction of chloroauric acid with appropriate reducing agents such as citric acid and sodium borohydride in aqueous solutions [5]. Nanospheres from a few to several tenths of nanometers in diameter have been prepared [6]. Moreover, methods for the preparation of hydrophobic gold nanospheres have also been developed [7]. 2. EXPERIMENTAL

Materials used for the synthesis of gold nanoparticles are chloroauric acid (HAuCl4), and lemon grass (cymbopogon flexuosus). Both materials are commercially available. The chloroauric acid was purchased from Merck, while the lemon grass (used as spice all over the world) from specialized shop. The lemon

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grass extract prepared by mixing 100 g of thoroughly washed and finely cut cymbopogon flexuosus leaves, in a 500-mL Erlenmeyer flask, with 100 ml of distilled water and then boiling the mixture before finally decanting it. The extract was added to 3.5*10–4 M chloroauric acid aqueous solution, obtaining eight samples (the volumetric ratio mixture extract/chloroauric acid were: 4/100 referred to as sample (1), 6/100 – sample (2), 8/100 – sample (3), 10/100 – sample (4), 4/100 – sample (5), 6/100 – sample (6), 8/100 – sample (7), 10/100 – sample (8)) and one control sample (sample (9)), containing only chloroauric acid. Samples 1–4 and control sample 9 were exposed to natural light for 48 hours, while samples 5–8 were kept in the dark. UV–Vis spectra were recorded on a SPECORD M 400 Carl Zeiss Jena spectrophotometer. X-ray diffraction (XRD) measurements were carried out on films of the respective solutions drop-coated onto glass substrates on a Dron instrument with CuKα radiation. For Fourier transform infrared (FTIR) spectroscopy measurements, dry powders of the nanoparticles were obtained in the following manner. The Au nanoparticles synthesized after 24 h of reaction of the salt solutions with the lemon grass extract broth were centrifuged at 10,000 rpm for 15 min, following which the pellet was redispersed in sterile distilled water to get rid of any uncoordinated biological molecules. The process of centrifugation and redispersion in sterile distilled water was repeated three times to ensure better separation of free entities from the metal nanoparticles. The purified pellets were then dried and the powders (embedded in KBr) subjected to FTIR spectroscopy measurement. These measurements were carried out on a GX Perkin Elmer Fourier transform infrared spectrometer. 3. RESULTS AND DISCUSSIONS

Formation of the metal nanoparticles by reduction of the metal ions from the aqueous solution during the contact with cymbopogon flexuosus extract may be easily followed by UV–Vis spectroscopy, as well as from the apparition of a distinctive reddish colour. It is well known that gold nanoparticles exhibit a specific colour, in water, arising due to excitation of surface plasmon vibrations in the metal nanoparticles [7]. Figs. 1A and 1B are shown the picture of the solution containing the obtained nanoparticles and the UV–Vis spectra recorded for the aqueous chloroauric acid – cymbopogon flexuosus mixture for sample 3. It is observed that the band corresponding to the surface plasmon resonance occurred at 550 nm as shown in Fig. 1B. The best concentration for gold nanoparticles synthesis appeared to be in sample 3. As a proof of the influence of light on nanoparticles synthesis, in sample 7 (the same concentration as sample 3), no nanoparticles appeared. After the 48 hours, the samples 5–8 were exposed to natural light, and the formation of gold nanoparticles could be

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observed after 48 more hours in sample 7; these results are the prove that the natural light plays an essential role in the formation of gold nanoparticles.

A. Picture taken of the solution containing gold nanoparticles (sample 3)

B. UV-Vis spectra of the sample 3

Fig. 1 – The gold nanoparticles and their UV-VIS spectra.

The metal particles were observed to be stable in solution even 4 weeks after their synthesis. By stability, we mean that there was no observable variation in the optical properties of the nanoparticles solutions with time. Fig. 2 shows the XRD patterns obtained for gold nanoparticles synthesized using lemon grass. A number of Bragg reflections corresponding to the (111), (200), (220), (311), and (222) sets of lattice planes are observed. The XRD pattern thus clearly shows that the gold nanoparticles formed by the reduction of HAuCl4– by lemon grass leaves extract are crystalline in nature.

Fig. 2 – XRD pattern gold nanoparticles synthesized by treating lemon grass leaves extract with HAuCl4 aqueous solution.

FTIR measurements were carried out to identify the possible molecules responsible for capping and efficient stabilization of the metal nanoparticles synthesized by lemon grass leaves extract. The chloroauric acid solutions were

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centrifuged at 10,000 rpm for 15 min, after complete reduction of the ions and formation of gold nanoparticles. Figure 3 shows the FTIR spectra of the stabilized gold nanoparticles solution synthesized by using lemon grass leaves extract.

Fig. 3 – FTIR spectra of the obtained stabilized gold nanoparticles solution.

The stabilized gold nanoparticles solution shows peaks at 1725, 1615, 1401, 1228, 1140, and 1076 cm−1. There appear to be no peaks in the amide I and amide II regions characteristic of proteins/enzymes that have been found to be responsible for the reduction of metal ions when using microorganisms such as fungi for synthesis of metal nanoparticles [8]. The observed peaks are more characteristic of flavanones and terpenoids that are very abundant in lemon grass leaves extract [9]. The peaks observed for the stabilized gold nanoparticles solution at 1725 cm−1 (carbonyls of α, β unsaturated ketone and ester), 1615 cm−1 (C=C or aromatic groups), 1401 cm−1 (geminal methyls), and 1140 and 1076 cm−1 (ether linkages) confirm the presence of flavanones or terpenoids adsorbed on the surface of metal nanoparticles. Flavanones or terpenoids could be adsorbed on the surface of metal nanoparticles, possibly by interaction through carbonyl groups or π-electrons in the absence of other strong ligands in solution [10]. 4. CONCLUSION

In conclusion, we developed a simple, room-temperature, and efficient biological method for synthesis of gold nanoparticles using lemon grass (cymbopogon flexuosus) extract. It is probable that citral or citronellal, the main components of lemon grass, acted as molecules involved in the bioreduction and synthesis of gold nanoparticles. The synthesized nanoparticles could find applications in the field of nanomedicine. More quantitative experiments, as well as a more precise of the molecules involved in the synthesis of nanoparticles are presently undergoing.

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