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RESEARCH

ARTIC

LE

Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 1141ndash1146 2011

A Fully Standardized Method of Synthesis ofGold Nanoparticles of Desired Dimension in

the Range 15 nmndash60 nm

Debanjana Ghosh Deboleena Sarkar Agnishwar Girigoswamilowast and Nitin Chattopadhyaylowast

Department of Chemistry Jadavpur University Kolkata 700032 India

The citrate reduction method of synthesis of gold nanoparticles (AuNPs) as introduced by Frenshas been standardized to enable one to prepare AuNPs of desired dimension by controlling thecomposition of the reactants The standardization has been made through characterization of thenanoparticles by UV-vis spectroscopy and from the transmission electron microscopic (TEM) mea-surements Linearity of the plot of the plasmon absorption maximum (max of the synthesizedAuNPs against their diameter as measured from TEM as well as the plot of max with the fractionalconcentration of citrate in the reaction mixture provides a convenient and easy route to dictatethe size of the synthesized AuNPs from a control on the composition of the reactants The stan-dardization reveals that a calculated composition of citrate (in terms of fractional concentration) inthe reaction mixture produces AuNPs of a desired dimension within the range of 15ndash60 nm Thediameter of the synthesized gold nanoparticles can be confirmed simply from the UV-vis spec-trophotometric technique This essentially makes the use of costly TEM unnecessary at least forthe primary purposes

Keywords Gold Nanoparticle Absorption Maximum Plasmon Band Citrate Reduction MethodTEM

1 INTRODUCTION

Metallic nanoparticles constitute a very important andfrequently used tool in nanoscience and nanotechnol-ogy chiefly due to their optical and radiative propertiesthe characteristic surface plasmon resonance (SPR) theireasy surface functionalization or bioconjugation and theirchemical stability and biocompatibility1ndash5 These nanopar-ticles have potential applications in analytical chemistryand have been used as probes in mass spectroscopy67 Ingeneral many of the physical and chemical properties ofan element are judged by the motions of the electrons8

In metals the electrons are highly delocalized over a largearea resulting in the low band gap between the valence andthe conduction bands But as the size of the metal particledecreases the band gap increases Eventually it becomescomparable to kT (k being the Boltzmann constant andT the absolute temperature) or even larger and the sys-tem behaves as a semiconductor9 Thus the nanoparticlesdisplay electronic band structures following the quantum-mechanical laws10 It is known that the interaction of a

lowastAuthors to whom correspondence should be addressed

noble metal nanoparticle with incident light of specificenergy induces intense localized fields at the surface ofthe particle This phenomenon occurs when the electronsof the conduction band of metal nanoparticles couple withthe electric field of the incident light at a resonant fre-quency This generates a plasmonic oscillation localizedon the surface of the nanoparticle known as the surfaceplasmon resonance (SPR)11 The enhanced surface plas-mon resonance of the nanometals at specific optical fre-quencies makes them excellent scatterers and absorbers ofvisible light1213 Because of their interesting optical andelectronic properties and consequent applications in pho-tonics and biomedicine the novel methods of synthesis ofmetal nanoparticles have found prominence in the contem-porary researchIntegration of nanotechnology with biology and

medicine is expected to produce major advances inmolecular diagnostics therapeutics molecular biologyand bioengineering In this regard gold surfaces andgold colloids are of great demand14 Gold nanoparticles(AuNPs) are known to be the most stable among themetal nanoparticles15 They present interesting behavioralcharacteristics as individual entities leading to size-related

J Nanosci Nanotechnol 2011 Vol 11 No 2 1533-48802011111141006 doi101166jnn20113090 1141

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IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

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A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

electronic magnetic and optical properties This recog-nizes AuNPs as the building blocks for nanoscale mate-rials and devices including conductive and optical hybridcomposites1617 and promotes their applications in diversefields818 The well-developed chemistry for linkage to thesurface the ease of preparation and the chemical stabil-ity of the surfaces intrigues the researchers to study theluminescence of fluorophores in the close vicinity of thegold nanoparticle surface This often results in a transferof energy from the excited fluorophore to the nanoparticlesurface leading to the quenching of the total fluorescenceThe phenomenon is coined as Nanoparticle Surface EnergyTransfer (NSET)In order to comply with the huge demand of gold

nanoparticles because of the explosive research in diversefields as mentioned above bulk quantities of AuNPs ofuniform sizes are necessary There are several methodsfor the synthesis of gold nanoparticles They include wetsynthetic methods like chemical reduction followed bycapping in homogeneous19ndash23 or microemulsion media24

biosynthesis using fungi25 use of block copolymers26

etc Recently in a different approach multilayered goldstructures have been designed through immobilization ofAuNPs27 However only few schemes produce particles ofuniform size192023ndash26 In the present work we deal withthe synthesis of AuNPs by the citrate reduction of HAuCl4introduced by Frens to produce gold nanoparticles within15 nmndash60 nm diameter2122

Transmission electron microscopy (TEM) is the mostreliable technique for obtaining pictorial and accurate dataabout the size of the gold nanoparticles However TEManalysis neither allows fast and real-time monitoring ofAuNP size nor does it provide information about AuNPconcentration28 Moreover sample preparation is nontriv-ial and can modify the size distribution and morphologyof the nanoparticles specially when they are included insolid matrices or reactive environments29ndash31 These diffi-culties can be avoided to a great extent if one takes thehelp of UV-vis spectrophotometer Since the surface plas-mon resonance results in an extinction spectrum whichdepends on the size shape and concentration of AuNPUV-vis spectroscopy is a useful technique to throw lighton the determination of the size of the gold nanoparticlesas well as its concentration83233 Compared to the exor-bitant costly TEM instruments UV-vis spectrometers arecheap and they are accessible in most of the laboratoriesThe spectrophotometric analysis does not alter the sampleand the registration of the spectrum is quite prompt Theexperimental results based on the measurement of the opti-cal absorbance of the nanoparticles can as well be inter-preted theoretically using the Mie theory for assigning theband to a specific size of AuNP34 Mie model is based onthe resolution of the Maxwell equations in spherical coor-dinates using the multipole expansion of the electric andmagnetic fields and accounting for the discontinuity of the

dielectric constant between the sphere and the surroundingmedium34

Characterization of the gold nanoparticles can also bemade with the dynamic light scattering (DLS) techniqueHowever TEM measurement is preferably accepted overthe DLS method since the latter presents the hydrodynamicsize of the nanoparticle and assumes the particle shapeto be spherical DLS is unable to discriminate betweenthe isolated and aggregated AuNPs35ndash37 Our objective ofthe present work is to standardize an accepted method ofsynthesis of AuNP with the help of TEM measurementsWe intend to find correlations between the fractional con-centration of the reactant in the reaction mixture and themaximum of the plasmon absorption (max as well as thediameter of the AuNPs produced so that the dimension ofthe synthesized gold nanoparticles can be dictated directlyfrom the composition of the reactants and can be con-firmed easily from UV-vis spectrophotometryHere we have synthesized the gold nanoparticles of

diameters in the range 15 nmndash60 nm by the citratereduction of HAuCl4 as introduced by Frens22 We haveexplored the variation of the absorption maximum of theAuNPs with their diameter and also with the fractionalconcentration (FC) of the citrate salt in the reaction mix-ture defined as FC = [Citrate]([Citrate]+[HAuCl4]) Thestudy standardizes the method of synthesis and yields cal-ibration curves that provide an easy route for the measure-ment of the particle diameter of the synthesized AuNPswith confidence

2 MATERIALS AND METHODS

21 Preparation of Gold Nanoparticles

Glassware was cleaned in aqua regia (nitric acidhydrochloric acid 13) rinsed with Milli-Q water andthen oven dried The reaction was performed in a three-necked 250 mL round-bottomed flask with the center neckattached to a reflux condenser The flask was placed ona hot plate fitted with a magnetic stirrer to provide asteady and continuous stirring Gold nanoparticles wereprepared by the citrate reduction of HAuCl4 as introducedby Frens223839 Different sizes of AuNPs were synthe-sized by controlling the fractional concentration of sodiumcitrate in the reaction mixture 05 mL of 1 stock solu-tion of hydrogen tetrachloroaurate (III) trihydrate (Sigma-Aldrich USA 999) was added to the flask containing50 mL of water The resulting solution was heated untilboiling and then the desired volume of 34 mM solution(stock) of trisodium citrate dihydrate (Merck India GR)was rapidly added to the flask (Scheme 1) The volume ofthe citrate solution (reductant) was varied from 025 mLto 20 mL to evaluate its effect on the size of the goldnanoparticles produced The color of the solutions changedrapidly from colorless to burgundy red or violet indicating

1142 J Nanosci Nanotechnol 11 1141ndash1146 2011

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

HAuCl4

REFLUX

Scheme 1 Schematic presentation of preparation of citrate stabilizedAuNP in water

the formation of the gold nanoparticles The colors devel-oped were characteristic to the size of the AuNPs Thesolutions were then allowed to reflux for further 15 minand slowly cooled to room temperature under stirring con-dition They were then centrifuged for 15 min washedtwice with water and stored at room temperature

22 Measurements and Characterization

Room temperature optical absorption spectra wereobtained with Shimadzu UV-2450 UV-vis spectrophotome-ter The TEM images were taken using a JEOL-TEM-2010 high resolution transmission electron microscope atan operating voltage of 200 kV

3 RESULTS AND DISCUSSION

31 Characterization of Gold Nanoparticles fromUV-Vis Spectroscopy

Figure 1 shows the absorption spectra of the synthesizedgold nanoparticles of four different dimensions reveal-ing the characteristic surface plasmon bands Dimensions

400 500 600 700

000

025

050

075

100

Nor

mal

ized

abs

orba

nce

Wavelength (nm)

AuNP λabs = 520 nmmax

maxAuNP λabs = 525 nm

AuNP λmax = 530 nm

AuNP λmax = 5345 nm

abs

abs

Fig 1 Absorption spectra of four of the synthesized spherical goldnanoparticles

of the AuNPs formed from the reduction of hydrogentetrachloroaurate (III) by sodium citrate of varying con-centrations were assessed with the help of the steady stateUV-visible spectra While varying the fractional concen-tration of citrate in the reaction mixture from 04 to 0728there was a hypsochromic shift in the absorption maxi-mum (max of the synthesized gold nanoparticles from536 nmndash520 nm The high extinction coefficients of theAuNPs were manifested in the intense absorption bands

32 Characterization of Gold Nanoparticles fromTEM Measurements

TEM micrographs of gold nanoparticles (Fig 2) revealhomogeneity of the samples as well as the expected spheri-cal shape of the small gold nanoparticles The diameters ofthe synthesized gold nanoparticles (four different images)were determined to be in the range 16 nm to 59 nmDiameters of different AuNPs as obtained from the TEMmeasurements are presented in Table I along with theirrespective absorption maxima and molar extinction coef-ficients ( the latter being taken from the literature39 Ithas already been reported by Frens although qualitativelythat an alteration of concentration ratio of sodium citrate totetrachloroaurate used for the reduction reaction changesthe particle size22 Our results corroborate it quantitatively(vide supra)AuNPs have surface plasmon resonance (SPR) band

maxima between 510 nmndash550 nm depending on the size ofthe nanoparticles40ndash42 The origin of the band is attributedto the collective oscillation of the free conduction electrons

Fig 2 TEM micrographs of gold nanoparticles of diameters 16 nm (a)30 nm (b) 46 nm (c) and 59 nm (d) synthesized from the citrate reductionof HAuCl4

J Nanosci Nanotechnol 11 1141ndash1146 2011 1143

Delivered by Ingenta toBhabha Atomic Research Centre

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RESEARCH

ARTIC

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A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

Table I Optical parameters and diameters of synthesized AuNPs

max (nm) (Mminus1 cmminus1)a Diameter (nm)

520 27 times 108 16525 7 times 108 30530 3 times 109 465345 15 times 1010 59

aFrom Ref [39]

induced by an interacting electromagnetic field Theseresonances are also denoted as surface plasmons Miewas the first to describe them quantitatively by solvingMaxwellrsquos equations with the appropriate boundary con-ditions for spherical particles34 The total extinction cross-section composed of absorption and scattering is givenas a summation over all electric and magnetic multipoleoscillations For nanoparticles which are small comparedto the wavelength of the exciting light ( 2r r beingthe radius of the nanoparticle and for gold 2r lt 25 nm)only the dipole absorption of the Mie equation contributesto the extinction cross-section of the nanoparticles Forlarger nanoparticles (2r gt 25 nm) the extinction cross-section is also dependent on higher order multipole modeswithin the full Mie equation and the extinction spectrum isthen dominated by quadrupole and octopole absorption aswell as scattering43 These higher oscillation modes explic-itly depend on the particle size The total plasmon bandabsorption is the superposition of all the contributing mul-tipole oscillations peaking at different energies The exci-tation of the higher order modes is explained in terms ofan inhomogeneous polarization of the nanoparticles by theelectromagnetic field as the particle size becomes compa-rable to the wavelength of the exciting radiation43

In order to find the diameter of the synthesized AuNPsdirectly from the composition of the reaction mixturethrough a reliable correlation we looked for a seriesof variations between different parameters related to theaspect First the variation of diameter of the AuNPs asdirectly measured from TEM was noticed as a function ofmax The variation of max was then studied as a functionof the fractional concentration of the reductant (citrate) inthe reaction mixture Finally the correlation for the vari-ation of particle diameter as a function of the fractionalconcentration of citrate was determined Interestingly thelast correlation was same as that calculated from the firsttwo correlations This agreement emphasizes the reliabil-ity of our strategy and enables one to assess the diameterof the synthesized AuNPs directly from the knowledge ofthe fractional concentration of the citrate in the reactionmixture while adopting Frens method of synthesisFigure 3 shows the plot of estimated diameter of the

AuNPs (from TEM) as a function of max This plotreveals a linear variation with a very good correlation fac-tor (R = 0999) and establishes that one can rely almostequally on either of the two factors This is consistent with

520 524 528 532 53610

20

30

40

50

60

Dia

met

er (

nm)

λmax (nm)

R = 0999

Y = 299 X ndash 1539

Fig 3 Plot of diameter of the gold nanoparticles measured from TEMagainst absorption maxima of AuNPs

the proposition of Mie theory34 This plot therefore pro-vides an easier and cheaper method to estimate the sizeof the nanoparticles from the knowledge of the max ofthe synthesized AuNPs TEM is a costly instrument andtherefore is not affordable or readily accessible to manyof the researchers Figure 3 suggests that dimensions ofthe AuNPs can also be known from spectrophotometricmeasurements once we have a calibration pattern on thevariation of diameter of AuNPs against the absorptionmaximum of the synthesized AuNPConsidering the fractional concentration of the reductant

as the controlling factor for the size of the synthesized goldnanoparticles and the proposition from Mie theory thatmax depends on the size of the nanoparticles we intendedto find a correlation between max and fractional concen-tration of the citrate in the reaction mixture Figure 4depicts a linear variation between the two factors for thesynthesized AuNPs giving again a very good correlation

040 048 056 064 072516

520

524

528

532

536

540

Fractional concentration of citrate

λ max

(nm

)

Y = ndash 498 X + 5559R = 0993

Fig 4 Plot of absorption maxima of the AuNPs with the fractionalconcentration of citrate in HAuCl4

1144 J Nanosci Nanotechnol 11 1141ndash1146 2011

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IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

(R= 0993) From this calibration plot one gets a directcorrelation between the maximum of the plasmon absorp-tion band and the fractional concentration of the reductantGaining confidence from the linear correlations (Figs 3

and 4) we intended to make a calibration plot directlybetween the diameter of the synthesized AuNPs and thefractional concentration of citrate in the reaction mix-ture Figure 5 depicts the interrelation between these twoparameters with a good correlation factor once again(R= 0997)This calibration plot allows one to synthesize gold

nanoparticles directly with a desired dimension within therange 15 nmndash60 nm from a control on the concentra-tion of citrate relative to HAuCl4 in the reaction mix-ture It is interesting to note that the product of theslopes of Figures 3 and 4 agrees well with the slope ofFigure 5 This corroborates the cross correlations betweenthe three parameters dealt with and encourages one toexploit the standardized method of synthesis of AuNPsto get the dimension of the nanoparticle formed directlyfrom the fractional concentration of the reductant FromFigure 5 we can directly propose the correlation betweenthe diameter of the synthesized AuNP and composition ofcitrate in the mixture as

Diameter of AuNP

=minus14706times(fractional concentration of citrate)+1223

This is a simple equation that provides the diameter ofAuNPs synthesized from the citrate reduction methoddirectly from the knowledge of the composition of thereaction mixture without going through any instrumen-tal technique The standardization of the citrate reductionmethod of synthesis of gold nanoparticles based exclu-sively on the instrumental support makes the method veryfriendly to use and provides added impact of the presentwork

040 048 056 064 072

10

20

30

40

50

60 Y = ndash 14706 X + 1223

Dia

met

er (

nm)

Fractional concentration of citrate

R = 0997

Fig 5 Plot of diameter of the AuNPs with the fractional concentrationof citrate in HAuCl4

4 CONCLUSION

Citrate reduction method of synthesis of gold nanoparti-cles (AuNPs) has been fully standardized to enable oneto prepare AuNPs of desired dimension (diameter within15 nmndash60 nm) simply by controlling the fractional concen-tration of the reductant in the reaction mixture Transmis-sion electron microscopy (TEM) and UV-vis spectroscopyhave been exploited for the standardization process Stan-dardization of the method projects that a calculated com-position of citrate in the reaction mixture produces AuNPsof a desired dimension which can be confirmed from thespectrophotometric technique

Acknowledgments Financial support from D B Tand C S I R Government of India is gratefullyacknowledged The authors thank I I T Kharagpur forproviding the instrumental facility of TEM DeboleenaSarkar and Agnishwar Girigoswami thank CSIR for theirresearch fellowship and associateship respectively

References and Notes

1 N R Jana L Gearheart and C J Murphy J Phys Chem B105 4065 (2001)

2 J Hu T W Odom and C M Lieber Acc Chem Res 32 435(1999)

3 S Link and M A El-Sayed J Phys Chem B 103 8410(1999)

4 W Cheng S Dong and E Wang Langmuir 19 9434 (2003)5 M Fukushima H Yanagi S Hayashi N Suganuma and

Y Taniguchi Thin Solid Films 438 39 (2003)6 K Shrivas and H-F Wu Anal Chem 80 2583 (2008)7 M M Kemp A Kumar S Mousa T-J Park P Ajayan

N Kubotera S A Mousa and R J Linhardt Biomacromolecules10 589 (2009)

8 M-C Daniel and D Astruc Chem Rev 104 293 (2004)9 M A El-Sayed Acc Chem Res 34 257 (2001)10 A P Alivisatos Science 271 933 (1996)11 C Tabor R Murali M Mahmoud and M A El-Sayed J Phys

Chem A 113 1946 (2009)12 W P McConnell J P Novak L C Brousseau III R R Fuierer

R C Tenent and D L Feldheim J Phys Chem B 104 8925(2000)

13 P K Jain K S Lee I H El-Sayed and M A El-Sayed J PhysChem B 110 7238 (2006)

14 P C Ray G K Darbha A Ray W Hardy and J Walker Nano-technology 18 375504 (2007)

15 C Tu G Li Y Shi X Yu Y Jiang Q Zhu J Liang Y GaoD Yan J Sun and X Zhu Chem Commun 3211 (2009)

16 A Borriello P Agoretti A Cassinese P DrsquoAngelo G T Mohanrajand L Sanguigno J Nanosci Nanotechnol 9 6307 (2009)

17 C-W Hu Y Huang and R C-C Tsiang J Nanosci Nanotechnol9 3084 (2009)

18 V G Praig H McIlwee C L Schauer R Boukherroub andS Szunerits J Nanosci Nanotechnol 9 350 (2009)

19 X-M Li M R de Jong K Inoue S Shinkai J Huskens and D NReinhoudt J Mater Chem 11 1919 (2001)

20 M M Maye S C Chun L Han D Rabinovich and C-J ZhongJ Am Chem Soc 124 4958 (2002)

21 N R Jana L Gearheart and C J Murphy Langmuir 17 6782(2001)

J Nanosci Nanotechnol 11 1141ndash1146 2011 1145

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

electronic magnetic and optical properties This recog-nizes AuNPs as the building blocks for nanoscale mate-rials and devices including conductive and optical hybridcomposites1617 and promotes their applications in diversefields818 The well-developed chemistry for linkage to thesurface the ease of preparation and the chemical stabil-ity of the surfaces intrigues the researchers to study theluminescence of fluorophores in the close vicinity of thegold nanoparticle surface This often results in a transferof energy from the excited fluorophore to the nanoparticlesurface leading to the quenching of the total fluorescenceThe phenomenon is coined as Nanoparticle Surface EnergyTransfer (NSET)In order to comply with the huge demand of gold

nanoparticles because of the explosive research in diversefields as mentioned above bulk quantities of AuNPs ofuniform sizes are necessary There are several methodsfor the synthesis of gold nanoparticles They include wetsynthetic methods like chemical reduction followed bycapping in homogeneous19ndash23 or microemulsion media24

biosynthesis using fungi25 use of block copolymers26

etc Recently in a different approach multilayered goldstructures have been designed through immobilization ofAuNPs27 However only few schemes produce particles ofuniform size192023ndash26 In the present work we deal withthe synthesis of AuNPs by the citrate reduction of HAuCl4introduced by Frens to produce gold nanoparticles within15 nmndash60 nm diameter2122

Transmission electron microscopy (TEM) is the mostreliable technique for obtaining pictorial and accurate dataabout the size of the gold nanoparticles However TEManalysis neither allows fast and real-time monitoring ofAuNP size nor does it provide information about AuNPconcentration28 Moreover sample preparation is nontriv-ial and can modify the size distribution and morphologyof the nanoparticles specially when they are included insolid matrices or reactive environments29ndash31 These diffi-culties can be avoided to a great extent if one takes thehelp of UV-vis spectrophotometer Since the surface plas-mon resonance results in an extinction spectrum whichdepends on the size shape and concentration of AuNPUV-vis spectroscopy is a useful technique to throw lighton the determination of the size of the gold nanoparticlesas well as its concentration83233 Compared to the exor-bitant costly TEM instruments UV-vis spectrometers arecheap and they are accessible in most of the laboratoriesThe spectrophotometric analysis does not alter the sampleand the registration of the spectrum is quite prompt Theexperimental results based on the measurement of the opti-cal absorbance of the nanoparticles can as well be inter-preted theoretically using the Mie theory for assigning theband to a specific size of AuNP34 Mie model is based onthe resolution of the Maxwell equations in spherical coor-dinates using the multipole expansion of the electric andmagnetic fields and accounting for the discontinuity of the

dielectric constant between the sphere and the surroundingmedium34

Characterization of the gold nanoparticles can also bemade with the dynamic light scattering (DLS) techniqueHowever TEM measurement is preferably accepted overthe DLS method since the latter presents the hydrodynamicsize of the nanoparticle and assumes the particle shapeto be spherical DLS is unable to discriminate betweenthe isolated and aggregated AuNPs35ndash37 Our objective ofthe present work is to standardize an accepted method ofsynthesis of AuNP with the help of TEM measurementsWe intend to find correlations between the fractional con-centration of the reactant in the reaction mixture and themaximum of the plasmon absorption (max as well as thediameter of the AuNPs produced so that the dimension ofthe synthesized gold nanoparticles can be dictated directlyfrom the composition of the reactants and can be con-firmed easily from UV-vis spectrophotometryHere we have synthesized the gold nanoparticles of

diameters in the range 15 nmndash60 nm by the citratereduction of HAuCl4 as introduced by Frens22 We haveexplored the variation of the absorption maximum of theAuNPs with their diameter and also with the fractionalconcentration (FC) of the citrate salt in the reaction mix-ture defined as FC = [Citrate]([Citrate]+[HAuCl4]) Thestudy standardizes the method of synthesis and yields cal-ibration curves that provide an easy route for the measure-ment of the particle diameter of the synthesized AuNPswith confidence

2 MATERIALS AND METHODS

21 Preparation of Gold Nanoparticles

Glassware was cleaned in aqua regia (nitric acidhydrochloric acid 13) rinsed with Milli-Q water andthen oven dried The reaction was performed in a three-necked 250 mL round-bottomed flask with the center neckattached to a reflux condenser The flask was placed ona hot plate fitted with a magnetic stirrer to provide asteady and continuous stirring Gold nanoparticles wereprepared by the citrate reduction of HAuCl4 as introducedby Frens223839 Different sizes of AuNPs were synthe-sized by controlling the fractional concentration of sodiumcitrate in the reaction mixture 05 mL of 1 stock solu-tion of hydrogen tetrachloroaurate (III) trihydrate (Sigma-Aldrich USA 999) was added to the flask containing50 mL of water The resulting solution was heated untilboiling and then the desired volume of 34 mM solution(stock) of trisodium citrate dihydrate (Merck India GR)was rapidly added to the flask (Scheme 1) The volume ofthe citrate solution (reductant) was varied from 025 mLto 20 mL to evaluate its effect on the size of the goldnanoparticles produced The color of the solutions changedrapidly from colorless to burgundy red or violet indicating

1142 J Nanosci Nanotechnol 11 1141ndash1146 2011

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

HAuCl4

REFLUX

Scheme 1 Schematic presentation of preparation of citrate stabilizedAuNP in water

the formation of the gold nanoparticles The colors devel-oped were characteristic to the size of the AuNPs Thesolutions were then allowed to reflux for further 15 minand slowly cooled to room temperature under stirring con-dition They were then centrifuged for 15 min washedtwice with water and stored at room temperature

22 Measurements and Characterization

Room temperature optical absorption spectra wereobtained with Shimadzu UV-2450 UV-vis spectrophotome-ter The TEM images were taken using a JEOL-TEM-2010 high resolution transmission electron microscope atan operating voltage of 200 kV

3 RESULTS AND DISCUSSION

31 Characterization of Gold Nanoparticles fromUV-Vis Spectroscopy

Figure 1 shows the absorption spectra of the synthesizedgold nanoparticles of four different dimensions reveal-ing the characteristic surface plasmon bands Dimensions

400 500 600 700

000

025

050

075

100

Nor

mal

ized

abs

orba

nce

Wavelength (nm)

AuNP λabs = 520 nmmax

maxAuNP λabs = 525 nm

AuNP λmax = 530 nm

AuNP λmax = 5345 nm

abs

abs

Fig 1 Absorption spectra of four of the synthesized spherical goldnanoparticles

of the AuNPs formed from the reduction of hydrogentetrachloroaurate (III) by sodium citrate of varying con-centrations were assessed with the help of the steady stateUV-visible spectra While varying the fractional concen-tration of citrate in the reaction mixture from 04 to 0728there was a hypsochromic shift in the absorption maxi-mum (max of the synthesized gold nanoparticles from536 nmndash520 nm The high extinction coefficients of theAuNPs were manifested in the intense absorption bands

32 Characterization of Gold Nanoparticles fromTEM Measurements

TEM micrographs of gold nanoparticles (Fig 2) revealhomogeneity of the samples as well as the expected spheri-cal shape of the small gold nanoparticles The diameters ofthe synthesized gold nanoparticles (four different images)were determined to be in the range 16 nm to 59 nmDiameters of different AuNPs as obtained from the TEMmeasurements are presented in Table I along with theirrespective absorption maxima and molar extinction coef-ficients ( the latter being taken from the literature39 Ithas already been reported by Frens although qualitativelythat an alteration of concentration ratio of sodium citrate totetrachloroaurate used for the reduction reaction changesthe particle size22 Our results corroborate it quantitatively(vide supra)AuNPs have surface plasmon resonance (SPR) band

maxima between 510 nmndash550 nm depending on the size ofthe nanoparticles40ndash42 The origin of the band is attributedto the collective oscillation of the free conduction electrons

Fig 2 TEM micrographs of gold nanoparticles of diameters 16 nm (a)30 nm (b) 46 nm (c) and 59 nm (d) synthesized from the citrate reductionof HAuCl4

J Nanosci Nanotechnol 11 1141ndash1146 2011 1143

Delivered by Ingenta toBhabha Atomic Research Centre

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RESEARCH

ARTIC

LE

A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

Table I Optical parameters and diameters of synthesized AuNPs

max (nm) (Mminus1 cmminus1)a Diameter (nm)

520 27 times 108 16525 7 times 108 30530 3 times 109 465345 15 times 1010 59

aFrom Ref [39]

induced by an interacting electromagnetic field Theseresonances are also denoted as surface plasmons Miewas the first to describe them quantitatively by solvingMaxwellrsquos equations with the appropriate boundary con-ditions for spherical particles34 The total extinction cross-section composed of absorption and scattering is givenas a summation over all electric and magnetic multipoleoscillations For nanoparticles which are small comparedto the wavelength of the exciting light ( 2r r beingthe radius of the nanoparticle and for gold 2r lt 25 nm)only the dipole absorption of the Mie equation contributesto the extinction cross-section of the nanoparticles Forlarger nanoparticles (2r gt 25 nm) the extinction cross-section is also dependent on higher order multipole modeswithin the full Mie equation and the extinction spectrum isthen dominated by quadrupole and octopole absorption aswell as scattering43 These higher oscillation modes explic-itly depend on the particle size The total plasmon bandabsorption is the superposition of all the contributing mul-tipole oscillations peaking at different energies The exci-tation of the higher order modes is explained in terms ofan inhomogeneous polarization of the nanoparticles by theelectromagnetic field as the particle size becomes compa-rable to the wavelength of the exciting radiation43

In order to find the diameter of the synthesized AuNPsdirectly from the composition of the reaction mixturethrough a reliable correlation we looked for a seriesof variations between different parameters related to theaspect First the variation of diameter of the AuNPs asdirectly measured from TEM was noticed as a function ofmax The variation of max was then studied as a functionof the fractional concentration of the reductant (citrate) inthe reaction mixture Finally the correlation for the vari-ation of particle diameter as a function of the fractionalconcentration of citrate was determined Interestingly thelast correlation was same as that calculated from the firsttwo correlations This agreement emphasizes the reliabil-ity of our strategy and enables one to assess the diameterof the synthesized AuNPs directly from the knowledge ofthe fractional concentration of the citrate in the reactionmixture while adopting Frens method of synthesisFigure 3 shows the plot of estimated diameter of the

AuNPs (from TEM) as a function of max This plotreveals a linear variation with a very good correlation fac-tor (R = 0999) and establishes that one can rely almostequally on either of the two factors This is consistent with

520 524 528 532 53610

20

30

40

50

60

Dia

met

er (

nm)

λmax (nm)

R = 0999

Y = 299 X ndash 1539

Fig 3 Plot of diameter of the gold nanoparticles measured from TEMagainst absorption maxima of AuNPs

the proposition of Mie theory34 This plot therefore pro-vides an easier and cheaper method to estimate the sizeof the nanoparticles from the knowledge of the max ofthe synthesized AuNPs TEM is a costly instrument andtherefore is not affordable or readily accessible to manyof the researchers Figure 3 suggests that dimensions ofthe AuNPs can also be known from spectrophotometricmeasurements once we have a calibration pattern on thevariation of diameter of AuNPs against the absorptionmaximum of the synthesized AuNPConsidering the fractional concentration of the reductant

as the controlling factor for the size of the synthesized goldnanoparticles and the proposition from Mie theory thatmax depends on the size of the nanoparticles we intendedto find a correlation between max and fractional concen-tration of the citrate in the reaction mixture Figure 4depicts a linear variation between the two factors for thesynthesized AuNPs giving again a very good correlation

040 048 056 064 072516

520

524

528

532

536

540

Fractional concentration of citrate

λ max

(nm

)

Y = ndash 498 X + 5559R = 0993

Fig 4 Plot of absorption maxima of the AuNPs with the fractionalconcentration of citrate in HAuCl4

1144 J Nanosci Nanotechnol 11 1141ndash1146 2011

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

(R= 0993) From this calibration plot one gets a directcorrelation between the maximum of the plasmon absorp-tion band and the fractional concentration of the reductantGaining confidence from the linear correlations (Figs 3

and 4) we intended to make a calibration plot directlybetween the diameter of the synthesized AuNPs and thefractional concentration of citrate in the reaction mix-ture Figure 5 depicts the interrelation between these twoparameters with a good correlation factor once again(R= 0997)This calibration plot allows one to synthesize gold

nanoparticles directly with a desired dimension within therange 15 nmndash60 nm from a control on the concentra-tion of citrate relative to HAuCl4 in the reaction mix-ture It is interesting to note that the product of theslopes of Figures 3 and 4 agrees well with the slope ofFigure 5 This corroborates the cross correlations betweenthe three parameters dealt with and encourages one toexploit the standardized method of synthesis of AuNPsto get the dimension of the nanoparticle formed directlyfrom the fractional concentration of the reductant FromFigure 5 we can directly propose the correlation betweenthe diameter of the synthesized AuNP and composition ofcitrate in the mixture as

Diameter of AuNP

=minus14706times(fractional concentration of citrate)+1223

This is a simple equation that provides the diameter ofAuNPs synthesized from the citrate reduction methoddirectly from the knowledge of the composition of thereaction mixture without going through any instrumen-tal technique The standardization of the citrate reductionmethod of synthesis of gold nanoparticles based exclu-sively on the instrumental support makes the method veryfriendly to use and provides added impact of the presentwork

040 048 056 064 072

10

20

30

40

50

60 Y = ndash 14706 X + 1223

Dia

met

er (

nm)

Fractional concentration of citrate

R = 0997

Fig 5 Plot of diameter of the AuNPs with the fractional concentrationof citrate in HAuCl4

4 CONCLUSION

Citrate reduction method of synthesis of gold nanoparti-cles (AuNPs) has been fully standardized to enable oneto prepare AuNPs of desired dimension (diameter within15 nmndash60 nm) simply by controlling the fractional concen-tration of the reductant in the reaction mixture Transmis-sion electron microscopy (TEM) and UV-vis spectroscopyhave been exploited for the standardization process Stan-dardization of the method projects that a calculated com-position of citrate in the reaction mixture produces AuNPsof a desired dimension which can be confirmed from thespectrophotometric technique

Acknowledgments Financial support from D B Tand C S I R Government of India is gratefullyacknowledged The authors thank I I T Kharagpur forproviding the instrumental facility of TEM DeboleenaSarkar and Agnishwar Girigoswami thank CSIR for theirresearch fellowship and associateship respectively

References and Notes

1 N R Jana L Gearheart and C J Murphy J Phys Chem B105 4065 (2001)

2 J Hu T W Odom and C M Lieber Acc Chem Res 32 435(1999)

3 S Link and M A El-Sayed J Phys Chem B 103 8410(1999)

4 W Cheng S Dong and E Wang Langmuir 19 9434 (2003)5 M Fukushima H Yanagi S Hayashi N Suganuma and

Y Taniguchi Thin Solid Films 438 39 (2003)6 K Shrivas and H-F Wu Anal Chem 80 2583 (2008)7 M M Kemp A Kumar S Mousa T-J Park P Ajayan

N Kubotera S A Mousa and R J Linhardt Biomacromolecules10 589 (2009)

8 M-C Daniel and D Astruc Chem Rev 104 293 (2004)9 M A El-Sayed Acc Chem Res 34 257 (2001)10 A P Alivisatos Science 271 933 (1996)11 C Tabor R Murali M Mahmoud and M A El-Sayed J Phys

Chem A 113 1946 (2009)12 W P McConnell J P Novak L C Brousseau III R R Fuierer

R C Tenent and D L Feldheim J Phys Chem B 104 8925(2000)

13 P K Jain K S Lee I H El-Sayed and M A El-Sayed J PhysChem B 110 7238 (2006)

14 P C Ray G K Darbha A Ray W Hardy and J Walker Nano-technology 18 375504 (2007)

15 C Tu G Li Y Shi X Yu Y Jiang Q Zhu J Liang Y GaoD Yan J Sun and X Zhu Chem Commun 3211 (2009)

16 A Borriello P Agoretti A Cassinese P DrsquoAngelo G T Mohanrajand L Sanguigno J Nanosci Nanotechnol 9 6307 (2009)

17 C-W Hu Y Huang and R C-C Tsiang J Nanosci Nanotechnol9 3084 (2009)

18 V G Praig H McIlwee C L Schauer R Boukherroub andS Szunerits J Nanosci Nanotechnol 9 350 (2009)

19 X-M Li M R de Jong K Inoue S Shinkai J Huskens and D NReinhoudt J Mater Chem 11 1919 (2001)

20 M M Maye S C Chun L Han D Rabinovich and C-J ZhongJ Am Chem Soc 124 4958 (2002)

21 N R Jana L Gearheart and C J Murphy Langmuir 17 6782(2001)

J Nanosci Nanotechnol 11 1141ndash1146 2011 1145

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

HAuCl4

REFLUX

Scheme 1 Schematic presentation of preparation of citrate stabilizedAuNP in water

the formation of the gold nanoparticles The colors devel-oped were characteristic to the size of the AuNPs Thesolutions were then allowed to reflux for further 15 minand slowly cooled to room temperature under stirring con-dition They were then centrifuged for 15 min washedtwice with water and stored at room temperature

22 Measurements and Characterization

Room temperature optical absorption spectra wereobtained with Shimadzu UV-2450 UV-vis spectrophotome-ter The TEM images were taken using a JEOL-TEM-2010 high resolution transmission electron microscope atan operating voltage of 200 kV

3 RESULTS AND DISCUSSION

31 Characterization of Gold Nanoparticles fromUV-Vis Spectroscopy

Figure 1 shows the absorption spectra of the synthesizedgold nanoparticles of four different dimensions reveal-ing the characteristic surface plasmon bands Dimensions

400 500 600 700

000

025

050

075

100

Nor

mal

ized

abs

orba

nce

Wavelength (nm)

AuNP λabs = 520 nmmax

maxAuNP λabs = 525 nm

AuNP λmax = 530 nm

AuNP λmax = 5345 nm

abs

abs

Fig 1 Absorption spectra of four of the synthesized spherical goldnanoparticles

of the AuNPs formed from the reduction of hydrogentetrachloroaurate (III) by sodium citrate of varying con-centrations were assessed with the help of the steady stateUV-visible spectra While varying the fractional concen-tration of citrate in the reaction mixture from 04 to 0728there was a hypsochromic shift in the absorption maxi-mum (max of the synthesized gold nanoparticles from536 nmndash520 nm The high extinction coefficients of theAuNPs were manifested in the intense absorption bands

32 Characterization of Gold Nanoparticles fromTEM Measurements

TEM micrographs of gold nanoparticles (Fig 2) revealhomogeneity of the samples as well as the expected spheri-cal shape of the small gold nanoparticles The diameters ofthe synthesized gold nanoparticles (four different images)were determined to be in the range 16 nm to 59 nmDiameters of different AuNPs as obtained from the TEMmeasurements are presented in Table I along with theirrespective absorption maxima and molar extinction coef-ficients ( the latter being taken from the literature39 Ithas already been reported by Frens although qualitativelythat an alteration of concentration ratio of sodium citrate totetrachloroaurate used for the reduction reaction changesthe particle size22 Our results corroborate it quantitatively(vide supra)AuNPs have surface plasmon resonance (SPR) band

maxima between 510 nmndash550 nm depending on the size ofthe nanoparticles40ndash42 The origin of the band is attributedto the collective oscillation of the free conduction electrons

Fig 2 TEM micrographs of gold nanoparticles of diameters 16 nm (a)30 nm (b) 46 nm (c) and 59 nm (d) synthesized from the citrate reductionof HAuCl4

J Nanosci Nanotechnol 11 1141ndash1146 2011 1143

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RESEARCH

ARTIC

LE

A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

Table I Optical parameters and diameters of synthesized AuNPs

max (nm) (Mminus1 cmminus1)a Diameter (nm)

520 27 times 108 16525 7 times 108 30530 3 times 109 465345 15 times 1010 59

aFrom Ref [39]

induced by an interacting electromagnetic field Theseresonances are also denoted as surface plasmons Miewas the first to describe them quantitatively by solvingMaxwellrsquos equations with the appropriate boundary con-ditions for spherical particles34 The total extinction cross-section composed of absorption and scattering is givenas a summation over all electric and magnetic multipoleoscillations For nanoparticles which are small comparedto the wavelength of the exciting light ( 2r r beingthe radius of the nanoparticle and for gold 2r lt 25 nm)only the dipole absorption of the Mie equation contributesto the extinction cross-section of the nanoparticles Forlarger nanoparticles (2r gt 25 nm) the extinction cross-section is also dependent on higher order multipole modeswithin the full Mie equation and the extinction spectrum isthen dominated by quadrupole and octopole absorption aswell as scattering43 These higher oscillation modes explic-itly depend on the particle size The total plasmon bandabsorption is the superposition of all the contributing mul-tipole oscillations peaking at different energies The exci-tation of the higher order modes is explained in terms ofan inhomogeneous polarization of the nanoparticles by theelectromagnetic field as the particle size becomes compa-rable to the wavelength of the exciting radiation43

In order to find the diameter of the synthesized AuNPsdirectly from the composition of the reaction mixturethrough a reliable correlation we looked for a seriesof variations between different parameters related to theaspect First the variation of diameter of the AuNPs asdirectly measured from TEM was noticed as a function ofmax The variation of max was then studied as a functionof the fractional concentration of the reductant (citrate) inthe reaction mixture Finally the correlation for the vari-ation of particle diameter as a function of the fractionalconcentration of citrate was determined Interestingly thelast correlation was same as that calculated from the firsttwo correlations This agreement emphasizes the reliabil-ity of our strategy and enables one to assess the diameterof the synthesized AuNPs directly from the knowledge ofthe fractional concentration of the citrate in the reactionmixture while adopting Frens method of synthesisFigure 3 shows the plot of estimated diameter of the

AuNPs (from TEM) as a function of max This plotreveals a linear variation with a very good correlation fac-tor (R = 0999) and establishes that one can rely almostequally on either of the two factors This is consistent with

520 524 528 532 53610

20

30

40

50

60

Dia

met

er (

nm)

λmax (nm)

R = 0999

Y = 299 X ndash 1539

Fig 3 Plot of diameter of the gold nanoparticles measured from TEMagainst absorption maxima of AuNPs

the proposition of Mie theory34 This plot therefore pro-vides an easier and cheaper method to estimate the sizeof the nanoparticles from the knowledge of the max ofthe synthesized AuNPs TEM is a costly instrument andtherefore is not affordable or readily accessible to manyof the researchers Figure 3 suggests that dimensions ofthe AuNPs can also be known from spectrophotometricmeasurements once we have a calibration pattern on thevariation of diameter of AuNPs against the absorptionmaximum of the synthesized AuNPConsidering the fractional concentration of the reductant

as the controlling factor for the size of the synthesized goldnanoparticles and the proposition from Mie theory thatmax depends on the size of the nanoparticles we intendedto find a correlation between max and fractional concen-tration of the citrate in the reaction mixture Figure 4depicts a linear variation between the two factors for thesynthesized AuNPs giving again a very good correlation

040 048 056 064 072516

520

524

528

532

536

540

Fractional concentration of citrate

λ max

(nm

)

Y = ndash 498 X + 5559R = 0993

Fig 4 Plot of absorption maxima of the AuNPs with the fractionalconcentration of citrate in HAuCl4

1144 J Nanosci Nanotechnol 11 1141ndash1146 2011

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

(R= 0993) From this calibration plot one gets a directcorrelation between the maximum of the plasmon absorp-tion band and the fractional concentration of the reductantGaining confidence from the linear correlations (Figs 3

and 4) we intended to make a calibration plot directlybetween the diameter of the synthesized AuNPs and thefractional concentration of citrate in the reaction mix-ture Figure 5 depicts the interrelation between these twoparameters with a good correlation factor once again(R= 0997)This calibration plot allows one to synthesize gold

nanoparticles directly with a desired dimension within therange 15 nmndash60 nm from a control on the concentra-tion of citrate relative to HAuCl4 in the reaction mix-ture It is interesting to note that the product of theslopes of Figures 3 and 4 agrees well with the slope ofFigure 5 This corroborates the cross correlations betweenthe three parameters dealt with and encourages one toexploit the standardized method of synthesis of AuNPsto get the dimension of the nanoparticle formed directlyfrom the fractional concentration of the reductant FromFigure 5 we can directly propose the correlation betweenthe diameter of the synthesized AuNP and composition ofcitrate in the mixture as

Diameter of AuNP

=minus14706times(fractional concentration of citrate)+1223

This is a simple equation that provides the diameter ofAuNPs synthesized from the citrate reduction methoddirectly from the knowledge of the composition of thereaction mixture without going through any instrumen-tal technique The standardization of the citrate reductionmethod of synthesis of gold nanoparticles based exclu-sively on the instrumental support makes the method veryfriendly to use and provides added impact of the presentwork

040 048 056 064 072

10

20

30

40

50

60 Y = ndash 14706 X + 1223

Dia

met

er (

nm)

Fractional concentration of citrate

R = 0997

Fig 5 Plot of diameter of the AuNPs with the fractional concentrationof citrate in HAuCl4

4 CONCLUSION

Citrate reduction method of synthesis of gold nanoparti-cles (AuNPs) has been fully standardized to enable oneto prepare AuNPs of desired dimension (diameter within15 nmndash60 nm) simply by controlling the fractional concen-tration of the reductant in the reaction mixture Transmis-sion electron microscopy (TEM) and UV-vis spectroscopyhave been exploited for the standardization process Stan-dardization of the method projects that a calculated com-position of citrate in the reaction mixture produces AuNPsof a desired dimension which can be confirmed from thespectrophotometric technique

Acknowledgments Financial support from D B Tand C S I R Government of India is gratefullyacknowledged The authors thank I I T Kharagpur forproviding the instrumental facility of TEM DeboleenaSarkar and Agnishwar Girigoswami thank CSIR for theirresearch fellowship and associateship respectively

References and Notes

1 N R Jana L Gearheart and C J Murphy J Phys Chem B105 4065 (2001)

2 J Hu T W Odom and C M Lieber Acc Chem Res 32 435(1999)

3 S Link and M A El-Sayed J Phys Chem B 103 8410(1999)

4 W Cheng S Dong and E Wang Langmuir 19 9434 (2003)5 M Fukushima H Yanagi S Hayashi N Suganuma and

Y Taniguchi Thin Solid Films 438 39 (2003)6 K Shrivas and H-F Wu Anal Chem 80 2583 (2008)7 M M Kemp A Kumar S Mousa T-J Park P Ajayan

N Kubotera S A Mousa and R J Linhardt Biomacromolecules10 589 (2009)

8 M-C Daniel and D Astruc Chem Rev 104 293 (2004)9 M A El-Sayed Acc Chem Res 34 257 (2001)10 A P Alivisatos Science 271 933 (1996)11 C Tabor R Murali M Mahmoud and M A El-Sayed J Phys

Chem A 113 1946 (2009)12 W P McConnell J P Novak L C Brousseau III R R Fuierer

R C Tenent and D L Feldheim J Phys Chem B 104 8925(2000)

13 P K Jain K S Lee I H El-Sayed and M A El-Sayed J PhysChem B 110 7238 (2006)

14 P C Ray G K Darbha A Ray W Hardy and J Walker Nano-technology 18 375504 (2007)

15 C Tu G Li Y Shi X Yu Y Jiang Q Zhu J Liang Y GaoD Yan J Sun and X Zhu Chem Commun 3211 (2009)

16 A Borriello P Agoretti A Cassinese P DrsquoAngelo G T Mohanrajand L Sanguigno J Nanosci Nanotechnol 9 6307 (2009)

17 C-W Hu Y Huang and R C-C Tsiang J Nanosci Nanotechnol9 3084 (2009)

18 V G Praig H McIlwee C L Schauer R Boukherroub andS Szunerits J Nanosci Nanotechnol 9 350 (2009)

19 X-M Li M R de Jong K Inoue S Shinkai J Huskens and D NReinhoudt J Mater Chem 11 1919 (2001)

20 M M Maye S C Chun L Han D Rabinovich and C-J ZhongJ Am Chem Soc 124 4958 (2002)

21 N R Jana L Gearheart and C J Murphy Langmuir 17 6782(2001)

J Nanosci Nanotechnol 11 1141ndash1146 2011 1145

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

Table I Optical parameters and diameters of synthesized AuNPs

max (nm) (Mminus1 cmminus1)a Diameter (nm)

520 27 times 108 16525 7 times 108 30530 3 times 109 465345 15 times 1010 59

aFrom Ref [39]

induced by an interacting electromagnetic field Theseresonances are also denoted as surface plasmons Miewas the first to describe them quantitatively by solvingMaxwellrsquos equations with the appropriate boundary con-ditions for spherical particles34 The total extinction cross-section composed of absorption and scattering is givenas a summation over all electric and magnetic multipoleoscillations For nanoparticles which are small comparedto the wavelength of the exciting light ( 2r r beingthe radius of the nanoparticle and for gold 2r lt 25 nm)only the dipole absorption of the Mie equation contributesto the extinction cross-section of the nanoparticles Forlarger nanoparticles (2r gt 25 nm) the extinction cross-section is also dependent on higher order multipole modeswithin the full Mie equation and the extinction spectrum isthen dominated by quadrupole and octopole absorption aswell as scattering43 These higher oscillation modes explic-itly depend on the particle size The total plasmon bandabsorption is the superposition of all the contributing mul-tipole oscillations peaking at different energies The exci-tation of the higher order modes is explained in terms ofan inhomogeneous polarization of the nanoparticles by theelectromagnetic field as the particle size becomes compa-rable to the wavelength of the exciting radiation43

In order to find the diameter of the synthesized AuNPsdirectly from the composition of the reaction mixturethrough a reliable correlation we looked for a seriesof variations between different parameters related to theaspect First the variation of diameter of the AuNPs asdirectly measured from TEM was noticed as a function ofmax The variation of max was then studied as a functionof the fractional concentration of the reductant (citrate) inthe reaction mixture Finally the correlation for the vari-ation of particle diameter as a function of the fractionalconcentration of citrate was determined Interestingly thelast correlation was same as that calculated from the firsttwo correlations This agreement emphasizes the reliabil-ity of our strategy and enables one to assess the diameterof the synthesized AuNPs directly from the knowledge ofthe fractional concentration of the citrate in the reactionmixture while adopting Frens method of synthesisFigure 3 shows the plot of estimated diameter of the

AuNPs (from TEM) as a function of max This plotreveals a linear variation with a very good correlation fac-tor (R = 0999) and establishes that one can rely almostequally on either of the two factors This is consistent with

520 524 528 532 53610

20

30

40

50

60

Dia

met

er (

nm)

λmax (nm)

R = 0999

Y = 299 X ndash 1539

Fig 3 Plot of diameter of the gold nanoparticles measured from TEMagainst absorption maxima of AuNPs

the proposition of Mie theory34 This plot therefore pro-vides an easier and cheaper method to estimate the sizeof the nanoparticles from the knowledge of the max ofthe synthesized AuNPs TEM is a costly instrument andtherefore is not affordable or readily accessible to manyof the researchers Figure 3 suggests that dimensions ofthe AuNPs can also be known from spectrophotometricmeasurements once we have a calibration pattern on thevariation of diameter of AuNPs against the absorptionmaximum of the synthesized AuNPConsidering the fractional concentration of the reductant

as the controlling factor for the size of the synthesized goldnanoparticles and the proposition from Mie theory thatmax depends on the size of the nanoparticles we intendedto find a correlation between max and fractional concen-tration of the citrate in the reaction mixture Figure 4depicts a linear variation between the two factors for thesynthesized AuNPs giving again a very good correlation

040 048 056 064 072516

520

524

528

532

536

540

Fractional concentration of citrate

λ max

(nm

)

Y = ndash 498 X + 5559R = 0993

Fig 4 Plot of absorption maxima of the AuNPs with the fractionalconcentration of citrate in HAuCl4

1144 J Nanosci Nanotechnol 11 1141ndash1146 2011

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

(R= 0993) From this calibration plot one gets a directcorrelation between the maximum of the plasmon absorp-tion band and the fractional concentration of the reductantGaining confidence from the linear correlations (Figs 3

and 4) we intended to make a calibration plot directlybetween the diameter of the synthesized AuNPs and thefractional concentration of citrate in the reaction mix-ture Figure 5 depicts the interrelation between these twoparameters with a good correlation factor once again(R= 0997)This calibration plot allows one to synthesize gold

nanoparticles directly with a desired dimension within therange 15 nmndash60 nm from a control on the concentra-tion of citrate relative to HAuCl4 in the reaction mix-ture It is interesting to note that the product of theslopes of Figures 3 and 4 agrees well with the slope ofFigure 5 This corroborates the cross correlations betweenthe three parameters dealt with and encourages one toexploit the standardized method of synthesis of AuNPsto get the dimension of the nanoparticle formed directlyfrom the fractional concentration of the reductant FromFigure 5 we can directly propose the correlation betweenthe diameter of the synthesized AuNP and composition ofcitrate in the mixture as

Diameter of AuNP

=minus14706times(fractional concentration of citrate)+1223

This is a simple equation that provides the diameter ofAuNPs synthesized from the citrate reduction methoddirectly from the knowledge of the composition of thereaction mixture without going through any instrumen-tal technique The standardization of the citrate reductionmethod of synthesis of gold nanoparticles based exclu-sively on the instrumental support makes the method veryfriendly to use and provides added impact of the presentwork

040 048 056 064 072

10

20

30

40

50

60 Y = ndash 14706 X + 1223

Dia

met

er (

nm)

Fractional concentration of citrate

R = 0997

Fig 5 Plot of diameter of the AuNPs with the fractional concentrationof citrate in HAuCl4

4 CONCLUSION

Citrate reduction method of synthesis of gold nanoparti-cles (AuNPs) has been fully standardized to enable oneto prepare AuNPs of desired dimension (diameter within15 nmndash60 nm) simply by controlling the fractional concen-tration of the reductant in the reaction mixture Transmis-sion electron microscopy (TEM) and UV-vis spectroscopyhave been exploited for the standardization process Stan-dardization of the method projects that a calculated com-position of citrate in the reaction mixture produces AuNPsof a desired dimension which can be confirmed from thespectrophotometric technique

Acknowledgments Financial support from D B Tand C S I R Government of India is gratefullyacknowledged The authors thank I I T Kharagpur forproviding the instrumental facility of TEM DeboleenaSarkar and Agnishwar Girigoswami thank CSIR for theirresearch fellowship and associateship respectively

References and Notes

1 N R Jana L Gearheart and C J Murphy J Phys Chem B105 4065 (2001)

2 J Hu T W Odom and C M Lieber Acc Chem Res 32 435(1999)

3 S Link and M A El-Sayed J Phys Chem B 103 8410(1999)

4 W Cheng S Dong and E Wang Langmuir 19 9434 (2003)5 M Fukushima H Yanagi S Hayashi N Suganuma and

Y Taniguchi Thin Solid Films 438 39 (2003)6 K Shrivas and H-F Wu Anal Chem 80 2583 (2008)7 M M Kemp A Kumar S Mousa T-J Park P Ajayan

N Kubotera S A Mousa and R J Linhardt Biomacromolecules10 589 (2009)

8 M-C Daniel and D Astruc Chem Rev 104 293 (2004)9 M A El-Sayed Acc Chem Res 34 257 (2001)10 A P Alivisatos Science 271 933 (1996)11 C Tabor R Murali M Mahmoud and M A El-Sayed J Phys

Chem A 113 1946 (2009)12 W P McConnell J P Novak L C Brousseau III R R Fuierer

R C Tenent and D L Feldheim J Phys Chem B 104 8925(2000)

13 P K Jain K S Lee I H El-Sayed and M A El-Sayed J PhysChem B 110 7238 (2006)

14 P C Ray G K Darbha A Ray W Hardy and J Walker Nano-technology 18 375504 (2007)

15 C Tu G Li Y Shi X Yu Y Jiang Q Zhu J Liang Y GaoD Yan J Sun and X Zhu Chem Commun 3211 (2009)

16 A Borriello P Agoretti A Cassinese P DrsquoAngelo G T Mohanrajand L Sanguigno J Nanosci Nanotechnol 9 6307 (2009)

17 C-W Hu Y Huang and R C-C Tsiang J Nanosci Nanotechnol9 3084 (2009)

18 V G Praig H McIlwee C L Schauer R Boukherroub andS Szunerits J Nanosci Nanotechnol 9 350 (2009)

19 X-M Li M R de Jong K Inoue S Shinkai J Huskens and D NReinhoudt J Mater Chem 11 1919 (2001)

20 M M Maye S C Chun L Han D Rabinovich and C-J ZhongJ Am Chem Soc 124 4958 (2002)

21 N R Jana L Gearheart and C J Murphy Langmuir 17 6782(2001)

J Nanosci Nanotechnol 11 1141ndash1146 2011 1145

Delivered by Ingenta toBhabha Atomic Research Centre

IP 59163306Mon 17 Jan 2011 051555

RESEARCH

ARTIC

LE

Ghosh et al A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm

(R= 0993) From this calibration plot one gets a directcorrelation between the maximum of the plasmon absorp-tion band and the fractional concentration of the reductantGaining confidence from the linear correlations (Figs 3

and 4) we intended to make a calibration plot directlybetween the diameter of the synthesized AuNPs and thefractional concentration of citrate in the reaction mix-ture Figure 5 depicts the interrelation between these twoparameters with a good correlation factor once again(R= 0997)This calibration plot allows one to synthesize gold

nanoparticles directly with a desired dimension within therange 15 nmndash60 nm from a control on the concentra-tion of citrate relative to HAuCl4 in the reaction mix-ture It is interesting to note that the product of theslopes of Figures 3 and 4 agrees well with the slope ofFigure 5 This corroborates the cross correlations betweenthe three parameters dealt with and encourages one toexploit the standardized method of synthesis of AuNPsto get the dimension of the nanoparticle formed directlyfrom the fractional concentration of the reductant FromFigure 5 we can directly propose the correlation betweenthe diameter of the synthesized AuNP and composition ofcitrate in the mixture as

Diameter of AuNP

=minus14706times(fractional concentration of citrate)+1223

This is a simple equation that provides the diameter ofAuNPs synthesized from the citrate reduction methoddirectly from the knowledge of the composition of thereaction mixture without going through any instrumen-tal technique The standardization of the citrate reductionmethod of synthesis of gold nanoparticles based exclu-sively on the instrumental support makes the method veryfriendly to use and provides added impact of the presentwork

040 048 056 064 072

10

20

30

40

50

60 Y = ndash 14706 X + 1223

Dia

met

er (

nm)

Fractional concentration of citrate

R = 0997

Fig 5 Plot of diameter of the AuNPs with the fractional concentrationof citrate in HAuCl4

4 CONCLUSION

Citrate reduction method of synthesis of gold nanoparti-cles (AuNPs) has been fully standardized to enable oneto prepare AuNPs of desired dimension (diameter within15 nmndash60 nm) simply by controlling the fractional concen-tration of the reductant in the reaction mixture Transmis-sion electron microscopy (TEM) and UV-vis spectroscopyhave been exploited for the standardization process Stan-dardization of the method projects that a calculated com-position of citrate in the reaction mixture produces AuNPsof a desired dimension which can be confirmed from thespectrophotometric technique

Acknowledgments Financial support from D B Tand C S I R Government of India is gratefullyacknowledged The authors thank I I T Kharagpur forproviding the instrumental facility of TEM DeboleenaSarkar and Agnishwar Girigoswami thank CSIR for theirresearch fellowship and associateship respectively

References and Notes

1 N R Jana L Gearheart and C J Murphy J Phys Chem B105 4065 (2001)

2 J Hu T W Odom and C M Lieber Acc Chem Res 32 435(1999)

3 S Link and M A El-Sayed J Phys Chem B 103 8410(1999)

4 W Cheng S Dong and E Wang Langmuir 19 9434 (2003)5 M Fukushima H Yanagi S Hayashi N Suganuma and

Y Taniguchi Thin Solid Films 438 39 (2003)6 K Shrivas and H-F Wu Anal Chem 80 2583 (2008)7 M M Kemp A Kumar S Mousa T-J Park P Ajayan

N Kubotera S A Mousa and R J Linhardt Biomacromolecules10 589 (2009)

8 M-C Daniel and D Astruc Chem Rev 104 293 (2004)9 M A El-Sayed Acc Chem Res 34 257 (2001)10 A P Alivisatos Science 271 933 (1996)11 C Tabor R Murali M Mahmoud and M A El-Sayed J Phys

Chem A 113 1946 (2009)12 W P McConnell J P Novak L C Brousseau III R R Fuierer

R C Tenent and D L Feldheim J Phys Chem B 104 8925(2000)

13 P K Jain K S Lee I H El-Sayed and M A El-Sayed J PhysChem B 110 7238 (2006)

14 P C Ray G K Darbha A Ray W Hardy and J Walker Nano-technology 18 375504 (2007)

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J Nanosci Nanotechnol 11 1141ndash1146 2011 1145

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RESEARCH

ARTIC

LE

A Fully Standardized Method of Synthesis of AuNPs of Desired Dimension in the Range 15 nmndash60 nm Ghosh et al

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Received 22 September 2009 Accepted 23 December 2009

1146 J Nanosci Nanotechnol 11 1141ndash1146 2011