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Dynamic Light Scattering and Optical Absorption in BiologicalNanofluids of Gold Nanoparticles in Poly(vinyl pyrrolidone) Molecules

A. Mishra, S. Ram, and G. GhoshJ. Phys. Chem. C, 2009, 113 (17), 6976-6982• DOI: 10.1021/jp8096742 • Publication Date (Web): 03 April 2009

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Dynamic Light Scattering and Optical Absorption in Biological Nanofluids of GoldNanoparticles in Poly(vinyl pyrrolidone) Molecules

A. Mishra,† S. Ram,*,† and G. Ghosh‡

Materials Science Centre, Indian Institute of Technology, Kharagpur-721 302, India, and UGC-DAEConsortium for Scientific Research, Trombay, Mumbai 400 085, India

ReceiVed: NoVember 2, 2008; ReVised Manuscript ReceiVed: February 16, 2009

Nanofluids of distinctive colors are obtained of gold nanoparticles (Au-NPs) embedded in biological moleculesof poly(vinyl pyrrolidone) (PVP) following in situ Au3+f Au reaction in 10 g/dL PVP in hot water. VaryingAu-NPs by 0.05, 0.1, 0.2, 0.5, or 1.0 wt % (as per the Au-PVP) tailors a pink, bluish-pink, reddish-blue, ora light-orange color of nanofluid. In dynamic light scattering, average hydrodynamic dimension (Dh) hasincreased from a few nanometers on early Au-NPs contents (e0.2 wt %) to as big as 180 nm, i.e., larger toaverage size Dp ≈ 100 nm for the transmission electron microscopic images of Au-NPs. Shear bands (servingas templates or beds to these Au-NPs) in counterpart PVP molecules, which are aligned in thin laminates(10-20 nm widths), can be seen in nanofluids on small Au-NPs (Dp ) 5-20 nm). Enhanced viscosity, asmuch as four times on the presence of such Au-NPs, relaxes slowly to the base value by increasing the shearrate (10 to 50 s-1), characterizing a non-Newtonian rheological fluid. Shear stress describes enhanced opticalabsorption, which red-shifts from 535 nm average value in Au-NPs of early sizes to as large value as 571 nmin bigger sizes. Possible applications include light-sensitive drug delivery and other biological devices.

1. Introduction

Nonequilibrium properties in nanofluids involve a rich varietyof behavior such as enhanced shear viscosity (η), light absorp-tion and emission, thermal conductivity (k) and diffusion rates,highly selective adsorption, or local structural changes. Under-standing of these properties is important both for fundamentaland applied research as almost any natural or industrial processinvolves transport phenomena in nanofluids. Examples includeadsorption, catalysts, lubricants, colored dyes, nanoink, livingcells metabolism, surface enhanced photonics, and many otherprocesses. Atomic and molecular clusters, i.e., nanofluids oninterfaces and quantum nanofluids in atomic traps are focalpoints in this discipline of materials for applications. Since theterm “nanofluid”, i.e., a fluid with dispersed nanoparticles (NPs),was coined by Choi1 in 1995, considerable theoretical, numer-ical, and experimental studies had been done on thermophysicalproperties in a variety of nanofluids.2-12 Only 1.3 × 10-4 %Au-NPs in water lead to promote the k value dramatically by20%.6 Stable size, shape, and distribution of NPs in a specificfluid with stable surface interfaces devise the performance.

Elaborate studies of nanofluids confined in oxides, carbonnanotubes (CNTs), or NPs of metals suspended in water,ethylene glycol (EG), or some organic liquids in terms ofthermal properties have been carried out in heat-transferfluids.1-4,6-12 A meshed heat pipe with Au-NPs in water dropsthermal resistance as much as 37%.7 Choi and co-workers3

reported a 40% increase in inherently poor k values in EG whendispersing 0.3 vol % Cu-NPs. In some nanofluids,10,13-15 shearviscosity increases more rapidly than predicted in the Einsteinmodel16 even at concentrations sufficiently small that the system

is truly dilute. The origin of the apparent failure of effectivemedium theories in describing both kinds of the properties isan open question.

In this article, we report on the rheological and opticalproperties of biological nanofluids Au-PVP consisting of 0-1wt % Au-NPs dispersed in polymer molecules of poly(vinylpyrrolidone) (PVP) in water. Dynamic light scattering (DLS)and microstructure are studied in elucidating the effects ofaverage hydrodynamic dimension (Dh) and geometrical size,shape, or dispersion of Au-NPs on these properties in thenanofluids. Aqueous PVP, a medicinal product, reacts with manydrugs in drug delivery17-19 or in a blood plasma substitute withdeintoxicating effects.17,20 It is used as a biocompatible binderin pharmaceutical drugs.21,22

2. Experimental Details

A simple in situ Au3+f Au reaction with PVP molecules inhot water yields Au-PVP nanofluids in a single process. In brief,a 100 mL precursor was prepared of 10 g/dL PVP in distilledwater by magnetic stirring at 60-70 °C in air. Pure PVP as99.99% (average molecular weight ∼40,000 and polymerizationnumber n ) 360) was used from Aldrich chemicals. To inducethe reaction, a 0.05 M aqueous solution in HAuCl4 ·3H2O(99.99% pure from Merck chemicals) was added drop by dropthrough a syringe with stirring the solution in the hot conditions.As we described earlier,23 hot PVP molecules act as both amodel reductant as well as a surface stabilizer in devising shape-controlled Au-NPs, which get dispersed instantaneously in themedium in a nanofluid. Some water if required is heated off toa stable sample, with ideal η values of 2-12 mPas and freefrom byproduct chlorides.

The optical absorptions of the Au-PVP nanofluids werestudied by a UV-visible spectrometer (Ocean Optics, modelSD2000). The shape and size of Au-NPs and those of polymerbeds were studied in these nanofluids using a high-resolution

* To whom correspondence should be addressed. E-mail: sram@matsc.iitkgp.ernet. Phone: (091) 3222 283980. Fax: (091) 3222-255303.

† Indian Institute of Technology.‡ UGC-DAE Consortium for Scientific Research.

J. Phys. Chem. C 2009, 113, 6976–69826976

10.1021/jp8096742 CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/03/2009

scanning electron microscope (SEM) (Oxford, model Leo1550)with an accelerated voltage of 1-20 kV as well as a JEOL JEM-2100 transmission electron microscope (TEM) operating at 200kV. TEM images were taken from a dilute sample of nanofluidon specific carbon coated copper grids. The η value and shearstress σ were measured at shear rates γ in the 10-50 s-1 rangeon a rotational rheometer (TA Instruments, model AR-1000)of parallel plate geometry, with a diameter of 40 mm of upperplate. This method provides estimating these values preciselyat given γ from a nanofluid in a few drops on the plate.

The DLS studies were carried out in a home-built setup usinga vertically polarized He-Ne laser beam of wavelength λ )632.8 nm (15-mW power) fixed at one arm of a goniometer. Ascattered beam sample is passed through a vertical polarizerand counted by a photomultiplier tube (PMT) at 90°, mountedon the other arm of the goniometer. A nanofluid in an opticalcell was placed inside a cuvette consisting of index matchingliquid (trans-decalene) and aligned with the axis of rotation ofthe goniometer. Scattered photocurrent from PMT was amplifiedsuitably and digitized before feeding to a 256 channel digitalcorrelator (Malvern, Model 4700 autosizer employing 7132digital correlator). Average decay rate Γ of the electric fieldautocorrelation function g1(τ) was estimated in the method ofcumulants.24

3. Results and Discussion

3.1. Optical Absorption in Au-NPs in Au-PVP Nanofluids.Figure 1 shows the surface plasmon resonance (SPR) absorptionspectra in Au-NPs in the Au-PVP nanofluids (of photographsgiven in the right corner) which comprise (a) 0.05, (b) 0.1, (c)0.2, and (d) 1.0 wt % Au-NPs. An exceptionally intense pinkcolor of nanofuid has formed on as early size of Au-NPs assample a. Bluish-pink, reddish-blue, and ultimately light-orangecolors turn up successively in increasing the Au-NPs contentin this series. To describe the visible colors, as marked overthe spectra in Figure 1, the maximum absorption wavelengthλmax follows a parabolic path AB from 571 nm in nanofluid dto as small value as 535 nm observed in nanofluid a. The λmax

values observed in individual samples are given in Table 1 alongwith other properties. A parabolic path (with a quadratic physicalvariable) infers that the surface area A and number density Fn

in Au-NPs (of a specific group of morphology) tailor the λmax

value. In a proposed empirical relation it can be expressed asfollows

where λ0max is a constant on a dilute sample of noninteracting

Au-NPs and Cg is a scaling parameter describing average contentof dispersed Au-NPs in the sample. Arbitrary values of n1, n2,and are selected with a correlation factor Φ = 100 (nm dL/g)n1 in such a way that eq 1 simulates the experimental points.As portrayed in Figure 2, the Fn value with λ0

max ) 528 nm,after extrapolating the λmax value against the Cg value (Au-NPcontent in fluid in g/dL) to the point C at which the absorbancevanishes, in eq 1 briefly describes the experimental curve AB,assuming n1 ) 0.5 and ) 106 (nm2 dL/g)n2. As given in Table2, the Fn values, which varied from 1.33 to 0.003 particles per108 nm3 in the various Au-PVP nanofluids, were determinedfrom average size and content of Au-NPs (near cuboids) as perthe TEM images. A value of n2 ) 1 is taken in cubical orspherical Au-NPs. Thin wires or platelets of effectively largerA values would share larger n2 values in preponderant effectsof the Au-NPs surfaces and surrounding.

In Au-PVP nanofluids, a deviation from a linear relation

arises above a critical Cg value 0.02 g/dL (Figure 2) in severalfactors, e.g., (i) a change in size and/or morphology in Au-NPs, (ii) a change in the medium structure of polymermolecules, (iii) the growth of a surface layer onto Au-NPsthrough the surface atoms, and (iv) the Au-NP-polymerinteraction. The first factor controls the Fn value and dynamicsof the optical species, i.e., SPR electrons oscillating with adistinct λ0

max value, in individual Au-NPs. A linear plot obtained(see the dashed line CBD in Figure 2) with λ0

max ) 528 nmprovides a slope ψ ) 1400 nm per unit Cg value over early Cg

values below 0.02 g/dL Au-NPs in primary effects of the SPR

Figure 1. Visible absorption spectra in Au-PVP nanofluids consistingof (a) 0.05, (b) 0.1, (c) 0.2, and (d) 1.0 w t% Au-NPs with photographsin the corner.

Figure 2. A variation in average position λmax in the SPR absorptionband (curve AB) as a function of the content of Au-NPs in Au-PVPnanofluids, showing a deviation from a linear plot CBD above 0.02g/dL Au-NPs. The model curve describes the nonlinearity in terms ofthe surface modifications and interactions in larger Au-NPs in eq 1 asdescribed in the text.

λmax ) λmax0 +ΦCg

n1 + (FnACg)n2 (1)

λmax ) λmax0 + ψCg (2)

Biological Nanofluids of Au-NPs in PVP Molecules J. Phys. Chem. C, Vol. 113, No. 17, 2009 6977

absorption in a dilute system of isolated Au-NPs of self-confinedsize. Qualitatively, in a simple form, the term ψ in eq 3 can becorrelated to that of Φ in eq 1 as ψ ) Φ(n1 + ν)- 1

. A derived υvalue of 0.14 ascribes to the nonlinear effects in the other termsin eq 1.

The Fn value more effectively controls the absorbance in theSPR bands in Au-NPs in such nanofluids. In Figure 3, anormalized value of the absorbance maximum Rmax per unit Cg

value in nanofluid of unit centimeter column (also called theextinction coefficient εmax) thus follows the same path as the Fn

value with Cg value in Au-PVP nanofluids. The initial εmax value,as large as 5.7 × 103 L/mol cm observed in the 535-nm bandon as early of Au-NPs content as 0.05 wt % is decreasingmonotonically with a decrease in the Fn value. As demonstratedin the dashed line in Figure 3, as Fn f 0 (or Cg f 0.1 g/dL),an equilibrium value ε0

max of 0.5 × 103 L/mol cm hasapproached. By use of observed Fn and Dh values in the modelrelationship

simulates (dashed line) the experimental curve in Figure 3,with fit parameters δ ) 2190 (g nm2/mol)m1, ) 10-3 (g nm2/mol)m2, m1 ) 0.8, and m2 ) 0.5. A physical basis of thisempirical relation is that the Fn and Dh values tailor the εmax

value in SPR absorption in Au-NPs dispersed via polymermolecules in a specific structure of a rheological nanofluid.

3.2. DLS in Au-PVP Nanofluids. The DLS is studied inorder to determine the Dh value of Au-NPs in the Au-PVPnanofluids. As described elsewhere,24 it is obtained from a fieldcorrelation function

where Γ ) q2D describes the particle relaxation with a delaytime τ and the diffusion coefficient D. The scattering wavevectorq ) 4πµ sin(θ/2)/λ is determined by the refractive index of themedium µ and the scattering angle θ. D ) D0 at infinite dilution

in the Stokes-Einstein relation, D0 ) kBT /(3πη0Dh), yields theDh value at temperature T after putting the solvent viscosity η0

) 1.002 mPa s and the Boltzmann constant kB ) 1.38 × 10-23

J K-1. Figure 4 portrays normalized g1(τ) values measured forAu-PVP nanofluids comprised of (a) 0, (b) 0.05, (c) 0.2, (d)0.5, and (e) 1.0 wt % Au-NPs. A linear Γ dependence had beenshown upon the q2 value in describing g1(τ) of a single(translational) diffusive relaxation mode. Progressively fasterrelaxation processes render the distribution intensities to decayrapidly over shorter time intervals until the Au-NP content doesnot exceed 0.5 wt %. A different process operates in sample eon the Fn value lowered down as much as 4 orders of magnitude.

As portrayed in Figure 5, the Au-PVP nanofluids, whichconsist of 0.05, 0.1, 0.2, 0.5, or 1.0 wt % Au-NPs, present 7.2,13.1, 29.2, 76, or 180 nm average Dh values. The distributionis rather broadened upon Au content above 0.5 wt % due tomodified microstructure. A nearly linear growth of the Dh value(Figure 6) is shown with the Au content, and those stand10-20% larger over the particle sizes (Dp) in the TEM images.The difference describes that the DLS signal includes localinteractions which modify the effective Dp value. Zande et al.25

reported a polymer layer of 10-15 nm thickness on Au nanorodsin aqueous PVP using DLS signal.

To estimate polydispersity of Au-NPs in the Au-PVP nanof-luids, the g1(τ) curves in Figure 4 are expressed on a lineartime scale with a single exponent through the cumulants24

with k1 ) τ-1 ) ⟨Dq2⟩ and k2 ) ⟨(D0q2 - ⟨Dq2⟩)2⟩ . Apolydispersity of k1/k2

2 ) 0.50 is estimated for the parentsolution of 10.0 g/dL PVP in water, indicating that PVP polymermolecules in such dilute solution are well dispersed apart. Sucha high value retains in Au-PVP nanofluids.

3.3. Rheology in Au-PVP Nanofluids. Figure 7 comparesan increase in η values in 10 g/dL PVP in water in the presenceof (a) 0, (b) 0.05, (c) 0.1, (d) 0.2, (e) 0.5, and (f) 1.0 wt %Au-NPs as a function of the γ value. In general, an Au-PVPnanofluid that has the maximum value for Fn as well as εmax

offers also a maximum η value, i.e., as much as four times thebase value. Over early γ values (below 10 s-1), although a

TABLE 1: SPR Absorption Bands with η and σ Values in Au-PVP Nanofluids

absorption γ ) 20 s-1 γ ) 30 s-1 γ ) 40 s-1

Au content (wt %) λmax (nm) Rmax (cm-1) η (mPa s) σ (Pa) η (mPa s) σ (Pa) η (mPa s) σ (Pa)

PVP 247 4.4 3.6 0.07 3.3 0.10 3.0 0.120.05 535 1.9 11.7 0.23 9.6 0.29 8.3 0.330.10 543 2.5 6.7 0.13 5.5 0.17 5.1 0.200.20 556 3.2 7.1 0.14 5.0 0.15 4.2 0.170.50 562 3.9 9.8 0.19 5.9 0.18 4.2 0.171.0 571 5.0 7.1 0.14 4.7 0.14 3.7 0.15

TABLE 2: Extinction Absorption Coefficient εmax, Number Density Gn, Hydrodynamic Dimension Dh, and Particle Size Dp inAu-PVP Nanofluids

concentration (g/dL)b

Au contenta (wt %) Cg Cp εmax (L/mol cm) Fn (10-8 nm-3) Dp (nm) A (102 nm2) Dh (nm)

0.05 0.005 10 5.7 × 103 1.33 6 2.2 7.20.10 0.010 10 2.7 × 103 1.05 8 3.8 13.10.20 0.020 10 1.5 × 103 0.21 17 17.3 29.20.50 0.050 10 1.0 × 103 0.05 36 77 761.0 0.100 10 0.7 × 103 0.003 128 983 180

a Au content in Au-PVP. b Au-NPs and PVP in nanofluid.

εmax ) εmax0 + δ(Fn

Cg)m1

+ ( ACgDh

)m2

(3)

g1(τ) ) ∫F(Γ)exp(-Γτ)dΓ (4)

ln|g1(q, t)| ) -k1t +1/2k2t

2 (5)

6978 J. Phys. Chem. C, Vol. 113, No. 17, 2009 Mishra et al.

similarly large η value exists even on a large Au-NP content of0.5 wt % in sample e, the final value decays readily against theγ value. In increasing γ values, the initial η value relaxes tothe base value exponentially in a rheological thinning ofnanofluid with forced hydrodynamics of Au-NPs. As given inthe inset in Figure 7, the σ-value is increasing nearly linearlywith the γ value, except in the sample e, which has adverselydecreasing value over γg15 s-1 in a rather viscous-flow.Evidently, small Au-NPs present in effectively large Fn-valuemediate intermolecular PVP interactions in the enhanced η andσ-values. In Figure 8, η-value plots a sharp peak with Au-contentas early as 0.05 wt %. A broad peak, which follows around 0.5wt % Au, collapses at as early γ value as 40 s-1. Possibly, Au-NPs coagulate by debonding from the polymeric chains andthus no longer induce so long shear waves. Molecularlydispersed PVP molecules in planar or spiral shapes26,27 via Au-NPs behave to be rigid enough to face such effects.

As portrayed in Figure 9, Au-NPs are highly efficient inenforcing hydrodynamics of Au-PVP nanofluids so that theaverage σ value decreases rapidly against the Au content. Theshear rate (applied to measure the data) controls the σ value as

per the reinforcement as can be seen with two distinct plotsobtained at (a) 40 and (b) 50 s-1 shear rates (Figure 9). The σvalue varies with Au-NPs as does the εmax value, implying thatit can be treated as a physical variable for tailoring εmax in suchnanofluids. As portrayed in the inset in Figure 9, a model relation

describes the experimental points, assuming arbitrary valuesκ ) 1010 (g/mol cm Pa)s1, ) 10-3 (g nm2/mol)s2, s1 ) 0.8,and s2 ) 0.5 along with the other parameters in Table 2. In adifference from eq 3, it describes the εmax value in SPR band inAu-NPs in terms of σ value in place of the Fn value. It accountsin the effect of rheology of the nanofluid on the εmax value.

In a Newtonian solvent (constant η value) as used here,Schmidt et al.15 observed as much as a 10% enhanced η valuein dilute suspensions of alumina (1 vol % of near cuboids ofDp ) 40 nm) in decane compared to continuum models of well-dispersed NPs. Dispersions of silica (3-12 vol %, Dp ) 8-25nm) in aqueous poly(ethylene oxide)28 have a Newtonian flownear zero shear rates. High shear rates invite a shear thickening

Figure 3. Variation of the εmax value in the absorption band againstthe Au-content in Au-PVP nanofluids: (a) experimental and (b) themodel using Fn value in eq 3 with Fn values plotted against Au-contentin the inset. The line AB describes the equilibrium ε0

max value.

Figure 4. Variations of normalized g1(τ) values against the time scaleof the measurement in Au-PVP nanofluids: (a) 0, (b) 0.05, (c) 0.2, (d)0.5, and (e) 1.0 wt % Au-NPs.

Figure 5. The size distributions of Au-NPs of model (near-spherical)shapes in Au-PVP nanofluids consisting of (a) 0.05, (b) 0.1, (c) 0.2,(d) 0.5, and (e) 1.0 wt % Au-NPs.

Figure 6. The variations of average (a) Dh and (b) Dp values of Au-NPs as a function of Au content in Au-PVP nanofluids.

εmax ) εmax0 + κ( σ

Cg)s1

+ ( ACgDh

)s2

(6)

Biological Nanofluids of Au-NPs in PVP Molecules J. Phys. Chem. C, Vol. 113, No. 17, 2009 6979

on bridging three-dimensional flock structures. In such nanof-luids, which are prepared by dispersing separately prepared NPsin a proper fluid, the η value increases with volume fractionand agglomeration of NPs.10,14,28,29 An in situ synthesis exploredhere displays a different rheological behavior of Au-PVPnanofluids from ex situ synthesized nanofluids. The fact thatthese Au-PVP nanofluids are extremely dilute and have noclustering or sedimentation suggests that aggregation is notresponsible for deviating the rheology from effective mediummodels and that models based on Brownian dynamics15,30,31 orother nanoscale phenomenon should be considered.

3.4. Microstructure in Au-PVP Nanofluids. The size,shape, and distribution of NPs and polymeric nature ofcounterpart liquid of a nanofluid determine its optical orrheological properties. This is studied with SEM and TEMimages from Au-PVP nanofluids on selective magnifications.Figure 10 presents marked variation in SEM images in polymerstructure. Small Au-NPs in early content such as 0.05 wt %(Figure 10a) cause cross-linking of elongated PVP blends (whichfavor an enhanced η value), i.e., 300-900 nm length (L) with50 nm average width (W). The structure, which changes oninteraction with the electron beam, can be imaged only at low

operating voltage 1-2 eV. Such long coils include roughly asmany monomers as N ) Ωn = πW2L/(LmWmδm) ≈ 1.8 × 108,assuming a monomer length Lm ≈ 0.5 nm, width Wm ≈ 0.2nm, and thickness δm ≈ 0.1 nm, i.e., the initial value ismultiplied by a factor Ω ≈ 105. Thin polymer blends are seenof poor contrasts on as much Au-NPs as 0.2 wt % (Figure 10b).On larger Au contents, polymer refines into small flocks, L )50-100 nm and W ) 20-40 nm, accounting for shear thinningin nanofluid as described above.

The polymer blends embed small Au-NPs, and those can beresolved on peeling off the polymer surface films. In Figure10b, such Au-NPs (mostly cuboids of 20 nm average size) couldbe imaged at rather high operating voltage 10 eV. The Au-NPsprecipitate readily from films on heating in the electron beamfor 90-120 s. More realistic view of bare Au-NPs can be studiedwith TEM images. Mostly cuboids of an average 6 nm size areshown (Figure 11a) in nanofluid which consists of 0.05 wt%Au-NPs. Complex shapes are grown in bigger Au-NPs. In Figure11b, selected area electron diffraction (SAED) has (111), (200),(220), (311), and (420) reflections of dhkl values (interplanarspacings) 0.2305, 0.2010, 0.1415, 0.1205, and 0.0895 nm, withlattice parameter a ) 0.4003 nm, i.e., 1.9% smaller than puregold of 0.4079 nm.32

The cuboids have grown in complex shapes on highercontents 0.5-1.0 wt % of Au-NPs in Au-PVP nanofluids. SAEDfrom the surfaces in Figure 11c describes an amorphous surfacestructure of Au-NPs with two halos q1 and q2 of 0.2235 nm(111) and 0.1385 nm (220) interatomic distances (carbonmodified gold). The core stands crystalline only. As shown inFigure 11d, well-resolved (111) diffraction arrays arise fromthe main parts of these Au-NPs, and this characterizes that theyare single crystallites. Average Dp values obtained from TEMimages of Au-NPs in the various sample are given in Table 2.

Phenomenologically, in in situ synthesis of Au-PVP nanofluidin this work, dispersed PVP molecules in water serve not onlyas a reductant or a stabilizer (for Au-NPs growing from theAu3+ f Au reaction) but also a polymer template. The initialshape and size of such templates monitor the Au3+ f Aureaction and growth of Au-NPs in support on the templates.PVP monomer consists of a backbone of hydrophobic polyvinyland a hydrophilic pyrrolidone group. The pyrrolidone groupsin a template act as head groups to adsorb the Au3+ species on

Figure 7. Variations of η-values in Au-PVP nanofluids consisting of(a) 0, (b) 0.05, (c) 0.1, (d) 0.2, (e) 0.5, and (f) 1.0 wt % Au-NPs as afunction of the shear rate. The inset displays the flow behaviors againstthe shear rate in respective samples.

Figure 8. Evolving η values in Au-PVP nanofluids as a function ofthe Au-NPs content. The data were measured at selective (a) 20, (b)30, and (c) 40 s-1 shear rates.

Figure 9. Variation of σ values in Au-PVP nanofluids as a functionof the Au-NP content. The data were measured at (a) 40 and (b) 50 s-1

shear rates. In the inset, the σ value (at shear rate of 40 s-1) modelsvariation of εmax value in the Au-PVP nanofluids.

6980 J. Phys. Chem. C, Vol. 113, No. 17, 2009 Mishra et al.

the surface and those ultimately grow as Au-NPs following thepolymer structure as per the experimental conditions. Thehydrophobic tail keeps a distance from such species so that theyhave preferential growth on early reaction stages. Au atoms from

the reaction cluster and grow primarily in [111] planes of theface-centered cubic Au lattice. In a counterpart effect, the Au-NPs induce cross-linking in PVP molecules in the templates,and those cultivate rather varied sizes and shapes of such Au-NPs. Diversity in Au-NPs results in the broad SPR absorption,especially on effectively large content of Au-NPs above 0.1 wt%. Controlled diversity of Au-NPs of cuboids renders intensecolor on the contents below this value.

4. Conclusions

Au-NPs embedded in polymer molecules of PVP in waterdevise optical nanofluids. On increasing size in Au-NPs over0.05-1.0 wt % content, the SPR absorption red-shifts sensitivelyover 535-571 nm in the quantum size effects. As largeabsorption extinction coefficient (εmax) as 5.7 × 103 L/mol cmhas been tailored with Au-NPs over early sizes (∼6 nm) and aslarge a content as 0.05 wt %. It is shown that the initial εmax

value decreases monotonically to a steady ε0max value 0.5 ×

103 L/mol cm if increasing Au content further. Number density(Fn) of Au-NPs and average shear stress in these nanofluidsdescribe briefly this nonlinear εmax behavior in model relations.Hydrodynamic dimension stands as larger as twice the averageparticle size in a non-Newtonian rheological fluid. In general,the sample of maximal Fn and εmax values also has a maximalshear viscosity, i.e., as much as four times the base value. Suchdilute nanofluids suggest that NPs mediate shear waves respon-sible for deviating rheology from effective medium model andthat models based on such shear-waves or other nanoscalephenomenon should be considered. Results may be useful indevising gold based biological nanofluids and applications.

Figure 10. SEM images from Au-PVP nanofluids: (a) 0.05, (b) 0.2, (c) 0.5, and (d) 1.0 wt % Au-NPs. Interconnecting polymer structure breaksdown on Au-NPs above 0.05 wt %.

Figure 11. (a) TEM images and (b) SAED pattern from Au-NPs ofnear cuboids from a 0.05 wt% Au-PVP nanofluid. Au-NPs are grownwith (c) an amorphous surface layer with two diffraction halos in 0.5wt % and (d) well-resolved arrays of SAED pattern in (d) 1.0 wt %contents.

Biological Nanofluids of Au-NPs in PVP Molecules J. Phys. Chem. C, Vol. 113, No. 17, 2009 6981

Acknowledgment. This work has been supported financiallyin part by the University Grant Commission, New Delhi, andthe Department of the Atomic Energy, Government of India.

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