nanoparticles: Optical properties

wari Sivaraj a,, Rajendran Venckatesh biences, Kllege, Ud

Cupric oxide (CuO) nanoparticles are of signicant technologicalinterest and have attractedmore attention due to its unique proper-ties. This transition metal oxide with a narrow band gap (Eg 1.2 eV)forms the basis of several high temperature superconductors [14],gas sensors [57], giant magneto resistance materials [8,9],solar energy transformation and preparation of organicinorganic

microbial, anti-fouling, anti-biotic and anti-fungal agent whenincorporated in coatings, plastics and textiles [11]. Copper and cop-per-based compounds, due to their potent biocidal properties [12],are now routinely used in pesticidal formulations [13] and severalhealth related areas applications are being explored and/or imple-mented. Therefore, on the basis of the fundamental and practicalimportance of CuOnanomaterials, well-denedCuOnanostructureswith various morphologies have been fabricated. There are severalroutes through which CuO nanoparticles can be formed, like Sono-chemical [14], microwave irradiations [15], alkoxide basedroute[16], solgel [17] technique, one step solid-state reaction

Corresponding author. Tel./fax: +91 4222611146.E-mail addresses: rajeshwarishivaraj@gmail.com (R. Sivaraj), rvenckat@gmail.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 11401144

Contents lists available at

Spectrochimica Acta PS

w.com (R. Venckatesh).Keywords:Copper oxide nanoparticlesBiosynthesisSpectroscopySurface plasmon resonanceElectron microscopy

TEM techniques. The particles are crystalline in nature and average sizes were between 15 and 30 nm.The morphology of the nanoparticles can be controlled by tuning the amount of Aloe vera extract. Thisnew eco-friendly approach of synthesis is a novel, cheap, and convenient technique suitable for largescale commercial production and health related applications of CuO nanoparticles.

2012 Elsevier B.V. All rights reserved.

Introduction nanostructure composites [10]. Applications include as an anti-synthesized." The particle sizes are controlled by

varying the Aloe leaf brothconcentrations.

" Biologically synthesizednanoparticles are ecofriendly andhave wide applications in biomedicaland cosmetic industries.

a r t i c l e i n f o

Article history:Received 26 March 2012Received in revised form 23 July 2012Accepted 26 July 2012Available online 8 August 20121386-1425/$ - see front matter 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.07.096a b s t r a c t

In this paper, we report on the synthesis of nanostructured copper oxide particles by both chemical andbiological method. A facile and efcient synthesis of copper oxide nanoparticles was carried out withcontrolled surface properties via green chemistry approach. The CuO nanoparticles synthesized aremonodisperse and versatile and were characterized with the help of UVVis, PL, FT-IR, XRD, SEM, andstable CuO nanoparticles areSangeetha Gunalan a, RajeshaDepartment of Biotechnology, School of Life ScbDepartment of Chemistry, Government Arts Co

h i g h l i g h t s

" This paper reports on the synthesisof monodisperse CuO nanoparticlesby both chemical and biologicalmethods.

" Crystalline, versatile and highlyarpagam University, Eachanari Post, Coimbatore 641 021, Tamilnadu, Indiaumalpet 642 126, Tamilnadu, India

g r a p h i c a l a b s t r a c tAloe barbadensis Miller mediated green synthesis of mono-disperse copper oxideBiomolecularjournal homepage: wwll rights reserved.SciVerse ScienceDirect

art A: Molecular andpectroscopy

elsevier .com/locate /saa

precipitation-pyrolysis [20], thermal decomposition of precursor

ical processes has led to a desire to develop an eco-friendly approach

lar afor the synthesis of nanoparticles. The use of environmentally be-nign materials like plant leaf extract [22], bacteria [23], fungi [24]and enzymes [25] for the synthesis of silver nanoparticles offersnumerous benets of eco-friendliness and compatibility for phar-maceutical and other biomedical applications as they do not usetoxic chemicals for the synthesis protocol. Although biosynthesisof gold nanoparticles by plants such as alfalfa [26,27], Cinnamomumcamphora [28], neem [29], Emblica ofcinalis [30], lemon grass [31],and tamarind [32] have been reported, the potential of plants asbiological materials for the synthesis of nanoparticles is yet to befully explored. However, studies on the synthesis of CuO nanoparti-cles by biological method are sparse. The present investigationdescribes for the rst time, synthesis, characterization and opticalproperties of CuO nanoparticle prepared by both chemical and bio-logical (green) techniques using Aloe vera (Aloe barbadensis Miller).Herewe report a simple route for the successful synthesis of crystal-line nanoparticles of CuO by Aloe vera.

Aloe vera (Aloe barbadensis Miller) is a perennial succulentbelonging to the Liliaceae family, and it is a cactus-like plant thatgrows in hot, dry climates [33]. For many years, Aloe vera has beenreported to possess anti-inammatory, UV protective, antibacte-rial, and wound healing properties [3437]. Aloe vera has beenused for medicinal purposes in several cultures for millennia andit contains 75 potentially active constituents: vitamins, enzymes,minerals, sugars, salicyclic acids, lignins, saponins, amino acids[3840] and has been used for medicinal purposes in several cul-tures for millennia. Recently, the extract of Aloe vera plant has beensuccessfully used to synthesize single crystalline triangular goldnanoparticles and spherical silver nanoparticles in high yield bythe reaction of aqueous metal source ions with the extract of theAloe vera plant [41]. In order to nd the efciency of Aloe vera insynthesis of nanoparticles, leaf extracts with varying concentrationhave been used for the study. The effect of leaf broth concentrationon synthesis rate and particle size of the copper oxide nanoparti-cles has also been investigated.

Experiment

Materials

All the chemical reagents used in this experiment were ofanalytical grade. The Aloe vera leaves were collected from in andaround Coimbatore, Tamilnadu, India. Thoroughly washed leaves(350 g) were ground and boiled with 700 ml of de-ionized waterfor 10 min in a medium ame. The resulting product was lteredand stored in refrigerator for further experiments.

Synthesis of copper oxide nanoparticles[21] orby combinationof electrodepositionandself-catalyticmech-anism etc. Chemical synthesis methods lead to presence of sometoxic chemical absorbed on the surface thatmay have adverse effectin the medical application. Synthesis of high-quality nanomaterialswith respect to chemical purity, phase selectivity, crystallinity, andhomogeneity in particle size with controlled state of agglomerationin a cost-effective procedure is still a challenge tomaterial chemists.Moreover chemical synthesis methods lead to presence of sometoxic chemical absorbed on the surface thatmay have adverse effectin the medical application.

Increasing awareness towards green chemistry and other biolog-method at room temperature [18], electrochemical methods [19],

S. Gunalan et al. / Spectrochimica Acta Part A: MolecuWe used two different methods for synthesizing nanocrystal-line CuO nanoparticle of varying particle sizes [42]. In synthesis Iand stored in air tight container for further analysis.

Physical techniques

The optical properties of synthesized CuO nanoparticles werecharacterized by UVVis spectrophotometer (UV-2450, Shimadzu)and Photoluminescence (PL) spectra by Perkin-Elmer LS-55 B. Fou-rier Transform Infrared (FTIR) spectrumwas recorded by Perkin-El-mer 1725X in the range of 4000400 cm1 and X-ray diffraction(XRD) analysis by diffractometer (SEIFERT PTS 3003). The shapeand grain size were characterized by using scanning electronmicroscopy (SEM, Hitachi S-4700 with EDS analyzer NORAN) andtransmission electron microscopy (TEM, JEOL, JEM 3010). Allexperiments were done in triplicates to check for accuracy andthe data were analyzed using Origin Pro 7.5 SRO software (Orgin-Lab Corporation, USA).

Results and discussion

The study reports the formation of nanoparticles when exposedto Aloe extract by monitoring changes in color from green to gray-ish black. Black precipitate formed was the end product in synthe-sis I whereas in synthesis II, grayish black precipitate appeared. Thecolor changes arise because of the excitation of surface plasmonvibrations (SPR) with the metal nanoparticles [43].

UVVis and PL studies

Fig. 1 shows UV-spectra of CuO particles synthesized from withand without ALE at room temperature. It is observed that the CuOSPR bands are centered between 265 and 285 nm. It is generally rec-ognized that UVVis spectra could be used to examine the size andshape controlled nanoparticles in aqueous suspension [44]. Thepeaks (Fig. 1bd) show blue shift for lower concentrations of ALEand broadened red shift above 25% ALE. The observed red shift im-plies that the particle size increases with increased ALE concentra-tions. On the other hand, sample without ALE (Fig. 1a) shows a redshift of theUVVisible absorptionwhich canbe attributed to the lar-ger particle size. The optical absorption spectra of noblemetal nano-(chemical method), analytical grade CuSO4 was dissolved in dis-tilled water under constant stirring. While at room temperature,aqueous solution of sodium hydroxide was added dropwise. Ini-tially a blue precipitate of Cu(OH)2 was produced, which thenchanged to black CuO with continual addition of NaOH solution.After stirring for 23 h, the black CuO precipitate formed wasallowed to settle for overnight and the supernatant liquid was dis-carded. The black precipitate formed was washed thoroughly forthree times with double distilled water to remove all the ionsand then centrifuged at 2500 rpm for 10 min. The obtained precip-itate was dried in a hot air oven at 100 C for 5 h and then grindedfor use.

In synthesis II (biological method), a typical procedure was em-ployed, where Aloe leaf extracts were diluted to different concen-trations (50%, 25%, 10%) with distilled water and the volume wasmade up to 250 ml. Later, analytical grade copper sulfate was dis-solved in the Aloe extract solution under constant stirring usingmagnetic stirrer. After complete dissolution of the mixture, thesolution was kept under vigorous stirring at 130 C for 7 h, allowedto cool at room temperature and the supernatant was discarded.The black solid product obtained was centrifuged twice at3500 rpm for 20 min after thorough washing and dried at 120 Cfor 6 h. The resulting dried precursor was crushed into powder

nd Biomolecular Spectroscopy 97 (2012) 11401144 1141particles are known to exhibit unique optical properties due to theproperty of Surface Plasmon Resonance (SPR), which shift to longerwavelengths with increasing particle size. The position and shape of

lar aFig. 1. UVVis spectra of CuO nanoparticle with (a) 0% ALE, (b) 10% ALE, (c) 25% ALEand (d) 50% ALE.1142 S. Gunalan et al. / Spectrochimica Acta Part A: Molecuplasmon absorption of silver nanoclusters are strongly dependenton the particle size, dielectric constant of the medium and surfaceadsorbed species. The number of SPR peaks increases as the symme-try of the nanoparticle decreases [4547].

Fig. 2 corresponds to PL spectra of CuO particles obtained fromsynthesis I to II. All the samples with different concentrations ofALE show similar excitation and emission spectra but emissionintensities gradually decreases and may be likely due to concentra-tion quenching. Two sharp emission bands appear at about 275 and670 nm are generally assigned as a near-band-edge (NBE) emissionband and another broad deep-level band extending from 600 to610 nm. CuO nanoparticles synthesized from without ALE (Fig. 2a)shows the lowest emission while ALE (Fig. 2bd) shows decreasingintensity with increasing concentration ranging from 10% to 50%.The deep level green emission band at around 600 nm is assignedto the singly ionized oxygen vacancies [48]. The emission shiftsfrom green to near yellow at around 670 nm due to interstitial inCuO and increase in particle sizes. The visible luminescence origi-nates from the radioactive recombination of a photo-generated holewith an electron occupying the oxygen vacancy [49].

FTIR and XRD studies

FTIR analysis of nanoparticles synthesized by chemical and bio-logical method (Fig. 3ad) revealed strong band at 1110 cm1;

Fig. 2. PL spectrum of CuO nanoparticle with (a) 0% ALE, (b) 10% ALE, (c) 25% ALEand (d) 50% ALE.nd Biomolecular Spectroscopy 97 (2012) 11401144however, peaks at 610, 499 and 415 relate to MO bond vibrationalfrequencies that support the presence of monoclinic phases [50].The bands at 3405, 1538 and 944 cm1 show the presence of OH, C@O and CC stretching of alkanes [28,51]. Similar spectraare obtained for the nanomaterials produced via synthesis II(Fig. 3bd). The bands observed at 3495, 3154 and 2910 cm1 havebeen assigned to stretching vibrations of the primary amines, alco-hols and alkanes. The peaks around 1640 cm1 are due to the amideI bonds of proteins/enzymes [52]. The intense bands observed at710, 1053, 1125 and 1399 cm1 have been assigned to alcoholsand phenolic groups, CN stretching vibrations of aliphatic and aro-matic amines [28], respectively. The overall observation proves the

Fig. 3. FTIR spectra of CuO nanoparticle with (a) 0% ALE, (b) 10%ALE, (c) 25% ALEand (d) 50% ALE.

Fig. 4. XRD patterns of CuO nanoparticle with (a) 0% ALE, (b) 10% ALE, (c) 25% ALEand (d) 50% ALE.

Fig. 5. SEM images of CuO nanoparticles with (a) 0% ALE, (b) 10% ALE, (c) 25% ALE and (d) 50% ALE.

Fig. 6. TEM images of CuO nanoparticles with (a) 0% ALE, (b) 10% ALE, (c) 25% ALE and (d) 50% ALE.

S. Gunalan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 11401144 1143

lar aexistence of some phenolic compounds, terpenoids or proteins thatare bound to the surface of CuO nanoparticles that remaineddespite repeated washing. The stability of CuO nanoparticle maybe due to the free amino and carboxylic groups that have interactedwith the copper surface. Moreover the proteins present in the med-ium prevent agglomeration and aids in the stabilization by forminga coat, covering the metal nanoparticles.

XRD analysis showed intense peaks corresponding to (110),(111), (200), (202), (020), (202), (113), (311), (220) and (400)Braggs reection based on the crystallinity of CuO nanoparticles(Fig. 4). All diffraction peaks can be indexed as the typical mono-clinic structure and no extra diffraction peaks of other phases areobserved. Similar crystalloid structure and XRD pattern has beenreported for the silver nanoparticles formed by the reduction ofAg+ ions by the Gliricidia leaf extract [28]. Preferred orientationalong the (200) plane was observed for all Aloe extract samplesand orientation was slightly shifted to the left when compared toCuO prepared without Aloe extract (Fig. 4). As the concentrationincreased, the typical tendency of increasing intensity of sampleswas determined using the Scherrers equation by determining thewidth of the (111) Bragg reection [53,54]. The particle size ofthe products was around 20 nm for copper oxide nanoparticle.

SEM and TEM studies

The SEM and TEM micrographs in Figs. 5 and 6(ad) clearlyshow well dispersed, versatile and spherical shape distribution ofCuO nanoparticles prepared with or without Aloe extract with par-ticle sizes ranging from 15 to 30 nm. Addition of Aloe vera extractto the copper sulfate did not change nanoparticles shape, but itcaused an increase in nanoparticle size especially for higher con-centration (50%) and with limited aggregation for 0% ALE. Thenanoparticles appear to have assembled into very open, quasi-lin-ear superstructures rather than a dense closely packed assembly[52]. The TEM images at higher resolution also reveal that nanopar-ticles are not in physical contact but are separated by uniforminterparticle distance. The images clearly show the presence of sec-ondary material capping with a thickness of 5 nm (Fig. 6c and d),this capping may be assigned to bioorganic compounds present inthe leaf broth [29]. However, it is not yet clear which protein orcompound is responsible for bioreduction of copper.

Conclusion

To the best of our knowledge, this biosynthetic route has notbeen extended to the preparation of oxidematerials. This is becauseplants serve as incredibly rich sources of naturally synthesizedchemical compounds that are environmentally acceptable, inex-pensive, readily available and renewable source of materials. Thisrapid biological synthesis of copper oxide nanoparticles using Aloevera leaf broth provides a simple, ecofriendly and efcient routefor synthesis of nanoparticle with tunable optical properties direc-ted by particle size. Maximum nanoparticles show particle size of20 nm with distinct cap, which may be due to the avonoids, pro-teins and other functional groups present in the leaf broth of Aloevera and are likely to be responsible for the formation of copperoxide nanoparticles. The explored biosynthetic route has potentapplications in biomedical and biotechnological applications withseveral advantages such as cost effectiveness and pharmaceuticalapplications as well as for large scale commercial productions.

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Aloe barbadensis Miller mediated green synthesis of mono-disperse copper oxide nanoparticles: Optical propertiesIntroductionExperimentMaterialsSynthesis of copper oxide nanoparticlesPhysical techniques

Results and discussionUVVis and PL studiesFTIR and XRD studiesSEM and TEM studies

ConclusionReferences