Photothermal Efficiencies of Nanoshells and Nanorods for Clinical Therapeutic Applications

Joseph R. Cole,†,| Nikolay A. Mirin,‡,| Mark W. Knight,†,| Glenn P. Goodrich,⊥ andNaomi J. Halas*,†,‡,§,|

Department of Electrical and Computer Engineering MS-366, Department of Chemistry MS-60, Department ofBioengineering MS-142, and Laboratory for Nanophotonics, Rice UniVersity, 6100 Main Street,Houston, Texas 77005, and Nanospectra Biosciences, Inc., 8285 El Rio Street, Suite 150,Houston, Texas 77054

ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: May 7, 2009

With clinical trials for photothermal tumor ablation using laser-excited tunable plasmonic nanoparticles alreadyunderway, increasing understanding of the efficacy of plasmonic nanoparticle-based photothermal heatingtakes on increased urgency. Here we report a comparative study of the photothermal transduction efficiencyof SiO2/Au nanoshells, Au2S/Au nanoshells, and Au nanorods, directly relevant to applications that rely onthe photothermal response of plasmonic nanoparticles. We compare the experimental photothermal transductionefficiencies with the theoretical absorption efficiencies for each nanoparticle type. Our analysis assumes adistribution of randomly oriented nanorods, as would occur naturally in the tumor vasculature. In our study,photothermal transduction efficiencies differed by a factor of 3 or less between the different types of nanoparticlestudied. Both experiment and theory show that particle size plays a dominant role in determining transductionefficiency, with larger particles more efficient for both absorption and scattering, enabling simultaneousphotothermal heating and bioimaging contrast enhancement.

I. Introduction

The technological impact of plasmonic nanoparticles in fieldssuch as medicine,1-3 sensing,2,3 and advanced functional materi-als and devices4-6 is imminent. Clinical trials are currentlyunderway to test tunable plasmonic particles such as nanoshellsand nanorods as photothermal cancer therapies.7-9 This type oftherapy exploits the fact that the nanoparticle resonance can betuned to the infrared “water window” where biological tissueis highly transparent. Nanoparticles are injected into thebloodstream and accumulate at tumor sites, where they heat theirlocal environment when irradiated with laser light whosewavelength coincides with the resonant wavelength of thenanoparticle. Adjacent healthy tissue without embedded nano-particles is unaffected by the laser light alone, but cancer cellsin the direct vicinity of the nanoparticles undergo hyperthermiaand death, resulting in drug-free tumor remission.7,10

As this effect transitions from the research laboratory toclinical applications, it is important to identify which nanopar-ticle sizes and shapes meet the requirements of this type ofapplication. A strong absorption cross section, leading toefficient photothermal heating in the direct vicinity of thenanoparticle in a physiologically compatible wavelength range,is critically important. Other nanoparticle properties are alsoimportant to consider, however. Light scattering, for example,which does not lead to local heating but does allow the observerto easily locate the presence of nanoparticles and hence is ahighly desirable property for providing contrast in bioimagingapplications, needs to be considered. Both absorption and

scattering cross sections depend on nanoparticle size for anygiven nanoparticle shape.11,12 In addition to optical properties,the size and shape of candidate nanoparticles for therapeuticapplications may determine their tumoral or intracellular uptake,as well as their post-treatment biodistribution. Thus a study andanalysis of the transfer of optical energy into heat by nanopar-ticles of potential use in clinical therapies is an important firststep in the ultimate selection of which nanoparticles may bestbe suited for the development of any specific therapeuticapplication.

Here we investigate how the physical properties of threedifferent types of plasmonic nanoparticlessAu/SiO2 nanoshells,Au/Au2S nanoshells, and Au nanorodsscould be advantageousin photothermal cancer therapy. By use of a controlled thermalenvironment, we measure the photothermal transduction ef-ficiency of these three types of nanoparticles and compare it totheir absorption efficiency as calculated by Mie theory fornanoshells and the finite element method (FEM) for nanorods.These measurements provide important quantitative metrics forunderstanding how photothermal heating efficiencies are con-trolled by nanoparticle size and shape.

II. Experimental Methods

Electromagnetic Simulations. To estimate the photothermaltransduction efficiency of our three nanoparticle species wecalculate their effective extinction, absorption, and scatteringcross sections. The ratio of the absorption to extinction crosssection is the fraction of luminous energy that the nanopar-ticles are expected to convert to thermal energy. We refer tothis ratio as the absorption efficiency, and it should be equalto the photothermal transduction efficiency in the absenceof any physical effects that may alter the electromagneticresponse of the nanoparticle, such as nanoparticle aggregation.

For spherically symmetric nanoparticles such as the Au/Au2S or Au/SiO2 nanoshells used in these experiments, the

* To whom correspondence should be addressed. E-mail: halas@rice.edu.Phone: 713-348-5746. Fax: 713-348-5686.

† Department of Electrical and Computer Engineering, Rice University.‡ Department of Chemistry, Rice University.§ Department of Bioengineering, Rice University.| Laboratory for Nanophotonics, Rice University.⊥ Nanospectra Biosciences, Inc.

J. Phys. Chem. C 2009, 113, 12090–1209412090

10.1021/jp9003592 CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/11/2009

cross sections are most straightforwardly calculated by usingMie theory. The effective cross sections of the two nanoshelltypes in an aqueous medium (n ) 1.33) are shown in partsa and b of Figure 1. The size of the two nanoshell specieswas estimated by adjusting the core and shell radii tominimize the difference between Mie calculations andspectrophotometric measurements. For Au/SiO2 nanoshells,we found [r1, r2] ) [62, 77] nm and for Au/Au2S shells[r1, r2] ) [21, 25] nm. These values are in agreement withparticle size statistics from scanning electron microscopy(SEM) and transmission electron microscopy (TEM) (Figure2). Unlike nanorods, the spherical symmetry of nanoshellsmakes their spectrum insensitive to the polarization ofincident light, so the plasmon mode will be excited equallyin all particles in solution. From Mie calculations, we findthat the Au/SiO2 nanoshells are predominantly scattering andthat the Au/Au2S nanoshells are highly absorbing. However,the large physical size of the Au/SiO2 nanoshells makes theactual absorption cross section an order of magnitude higherthan that of the other two particle species considered in thisstudy.

The finite element method was used to calculate theabsorption and scattering cross sections of Au nanorods(Figure 1c), again in aqueous medium (n ) 1.33). The rodswere modeled as cylinders with hemispherical end caps, asis consistent with other reported studies,13 using averagedimensions ([l, d] ) [44, 13] nm) based on statistics fromTEM measurements. The absorption cross section is calcu-lated by integrating the resistive heating over the volume ofthe particle and dividing by the incident power density, asin eq 1

Here, Uav is the power absorbed by the particle in the formof ohmic losses, n is the refractive index of the embeddingmedium, ε0 and µ0 are the permittivity and permeability offree space, and |EFinc| is the amplitude of the incident electricfield. Similarly, the scattering cross section is the integral ofthe far field component of the outgoing electromagneticenergy flux over an arbitrary boundary surrounding theparticle. The result is normalized by the power density ofthe incident field

In eq 2, EFfar is the far field component of the outgoing wave,calculated by using the Stratton-Chu formula. Details of theparameters used for our FEM model are similar to those usedby Knight et al.14

For nanorods, the optical response of the nanoparticlesdepends on their orientation relative to the polarization ofincident light. The optical response of a nanorod suspension inan aqueous medium is essentially an ensemble average overthe random orientations of the nanorods. Fortunately, it ispossible to model any particle orientation knowing the spectralresponse of only two key orientations: that for light polarizedalong the longitudinal axis of the nanorod and that for the

Figure 1. Calculated ensemble average extinction (blue line), absorp-tion (red line), and scattering (black line) spectra: (a) Au/SiO2 nanoshells([r1, r2] ) [62, 77] nm) and (b) Au/Au2S nanoshells ([r1, r2] )[21, 25] nm) in water, calculated by using Mie theory. Since theseparticles are spherically symmetric, the polarization of incoming lighthas no effect on the spectra. (c) Ensemble of randomly orientednanorods ([l, d] ) [44, 13] nm) in aqueous solution calculated by usingFEM. These can be obtained by simulating a particle oriented so thatthe incoming electric field projects equally onto all three axes (inset).

Figure 2. Images of particles used for photothermal transductionefficiency measurements. Variations in size and shape can mediateextremely absorbing or scattering particles. (a) SEM image of Au/SiO2

nanoshells. (b) TEM image of Au/Au2S nanoshells. (c) TEM image ofnanorods.

Cabs )2

n√ε0/µ0| EFinc|2∫UavdV (1)

Cscat )∫ | EFfar|

2dΩ

| EFinc|2

(2)

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perpendicular polarization (see the Supporting Information).Since a nanoparticle oriented so that the electric field vector ofan incoming electromagnetic wave projected equally onto allthree axes (the [111] direction) is equivalent to many randomlyoriented particles (Figure SI1 in Supporting Information), wewere able to efficiently calculate the average spectrum forrandomly oriented nanorods in solution.

Our model predicts the Au nanorod plasmon resonancemaximum at a wavelength of 740 nm, slightly shorter than theresonance wavelength (∼780 nm) measured by UV/vis spec-troscopy of the nanorod solution from which size statistics werederived. The difference may arise from a slight size selectivitybias in the deposition of the nanorod population onto the TEMgrid15 or from variations in the population’s end-cap geometryresulting in significant deviations of the resonance from thehemispherical end-cap model.16,17 A representative TEM imageof the nanorods (Figure 2c) reveals that a significant fractionof the nanoparticles are slightly dog-bone shaped. Regardlessof the model’s prediction of resonance peak, it confirms thatextinction by nanorods is primarily due to absorption of thelight, as opposed to scattering. According to our calculations,the nanorods used in this study are expected to absorb 93% ofthe incident light.

The dielectric function of bulk gold reported by Johnson andChristy18 was used for Au/SiO2 nanoshells, in accordance withprevious single nanoparticle optical measurements and analy-sis.19 For the smaller Au/Au2S nanoshells, the finite mean freepath of electrons was taken into account in a modified dielectricfunction for Au.20 The calculated cross section of Au/Au2Snanoshells agrees well with the experimentally obtained spectrawhen this modification is included. Although surface scatteringalso affects the plasmon line width of nanorods,21 inhomoge-neous broadening of the line shape of nanorods in solution isfar greater than the anticipated effect of surface scattering onthe line shape, and this contribution to the line broadening wasneglected.

Particle Fabrication. Au/SiO2 nanoshells were fabricated aspreviously described.22 Briefly, 120-nm diameter silica nano-particles were obtained (Precision Colloids, Inc.) and suspendedin ethanol. The particle surface was then terminated with aminegroups by reaction with 3-aminopropyltriethoxysilane (Gelest).Very small gold colloid (1-3 nm diameter) was grown via themethod of Duff et al.23 This colloid was aged for 4-14 days at6-8 °C. The aminated silica particles were then added to thegold colloid suspension. Gold colloid adsorbs to the aminegroups on the silica surface resulting in a silica nanoparti-cle covered with islands of gold colloid. Au/SiO2 nanoshellswere then grown by reacting HAuCl4 (Sigma-Aldrich) with thesilica-colloid particles in the presence of formaldehyde.Nanoshell surfaces were coated with PEG by adding 1 mMPEG-SH (Lysan Technologies, LLC) to the nanoshell solutionovernight followed by diafiltration with a solution of trehalose(10% w/v) to remove residual PEG-SH from the nanoshellformulation.

To make Au/Au2S nanoshells,24 a solution of Na2S2O3 (1.5mM) was added to 2 mM HAuCl4. The resulting mixture wasstirred for 30 min. The particles obtained were PEGylated bythe addition of PEG-SH with stirring overnight. Excess PEG-SH was removed via centrifugation and redispersion into asolution of trehalose (10% w/v). Thiolated PEG is a universallyused coating of gold nanoparticles for in vivo applications: whenabsent or for cases of partial PEG coverage, unwanted proteinaggregation may be induced by adsorbed and misfolded proteins

adhering to the available gold nanoparticle surface at physi-ological pH.25

Nanorods were synthesized by using the method developedby Jana et al.,26 with minor changes. In brief, gold seed particleswere prepared by adding 250 µL of 10 mM HAuCl4 ·3H2O to7.5 mL of 100 mM cetyltrimethylammonium bromide (CTAB)with brief, gentle mixing. A 600 µL sample of freshly prepared,ice-cold 10 mM NaBH4 solution was added, followed by mixingfor 2 min. The nanorod growth solution was prepared by adding40 mL of 100 mM CTAB, 1.7 mL of 10 mM HAuCl4 ·3H2O,and 250 µL of 10 mM AgNO3 followed by 270 µL of 100 mMascorbic acid. To initiate nanorod growth, 840 µL of the seedsolution was added to the growth solution, which was thenmixed gently and left still for 40 min. Excess reactants wereremoved by centrifugation and resuspension in deionized (DI)water. The nanorods were PEGylated by the addition of 1 mMthiol terminated methoxypoly(ethylene glycol) mPEG-SH, andthen the solution was left to stir overnight. The final PEGylatedrod solution was cleaned by diafiltration of the solution into DIH2O. After cleaning, the particles were transferred via diafil-tration into a 10% (w/v) solution of trehalose.

Photothermal Conversion Efficiency Measurements. Tofind the photothermal transduction efficiency of our three typesof nanoparticles, we measured the temperature of the nanopar-ticle solution as a function of time while it was heated withinfrared laser irradiation, reached equilibrium, and subsequentlycooled to ambient temperature. Following Roper et al.,27 thefollowing energy balance equation can be used to describe thisprocess

where m and Cp are the mass and heat capacity of eachcomponent of the sample cell, T is the sample cell temperature,QI is the energy input by nanoparticles, Q0 is the baseline energyinput by the sample cell, and Qext is the outgoing energy. Amodel that is linear with temperature for the outgoing thermalenergy is used, resulting in

when there is no laser source incident on the sample. Here Tamb

is the ambient temperature, A is the surface area of the samplecell, and h is the heat transfer coefficient. Following this model,the cooling portion of the thermal cycle has an exponential timedependence. Similarly, the heating portion of the cycle also hasan exponential time dependence because both QI and Q0 aretime independent. By finding the best exponential fit to thetemperature data, we determine the characteristic thermal timeconstant for the system. The heat transfer coefficient of thesample cell is inversely proportional to the time constant

and this value should be the same regardless of whether it iscalculated by using the heating or the cooling data. Once theheat transfer coefficient is known, it can be used to calculate

∑i

miCp,idTdt

) QI + Q0 + Qext (3)

∑i

miCp,idTdt

) hA(T - Tamb) (4)

h )∑

i

miCp,i

τA(5)

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the amount of heat energy accumulated in or lost from thesample cell, given the current temperature inside.

When the sample cell reaches an equilibrium temperature,the power flowing into the sample cell is equivalent to the poweroutflow

where Tmax is the equilibrium temperature. Therefore, themaximum temperature achieved by the cell is directly relatedto the power absorbed by the particles inside. The photothermaltransduction efficiency is defined as the ratio of photothermalpower transduced by the nanoparticles to the incident opticalpower

where I is the power of the incident laser radiation afterattenuation by any elements in the optical path and Aλ is theoptical density of the sample solution at the laser wavelength.

A schematic of the apparatus used to measure the photother-mal transduction efficiency is depicted in Figure 3. Thenanoparticle solution is contained in a sealed cuvette with amagnetic stir bar and a T-type thermocouple probe inside. Thesample is irradiated with an 815 nm diode-laser (OptopowerCorp., Model No. OPE-BO15-815-FCPS) through a 32 mm2

aperture. The entire cuvette is contained in a vacuum chamberheld below 1 Torr to minimize sample cooling pathways throughthe surrounding atmosphere. The cuvette is sealed by a layerof paraffin wax to prevent nanoparticle aggregation followedby a layer of epoxy. Further details of sample construction areincluded in the Supporting Information. Only two coolingpathways remain: conduction through the thermocouple andblackbody radiation. Even though net energy loss throughblackbody radiation has a quartic temperature dependence, theerror resulting from our linear model for outgoing energy isnegligible for the small temperature changes in our system.

III. Results and Discussion

Representative curves showing the photothermal temperaturechange of the sample cell and its contents for our threenanoparticle types shown are in Figure 4. Detailed experimentalvalues associated with the data in Figure 4 can be found in theSupporting Information. The optical density of the samples wasadjusted to be equivalent at the laser wavelength of 815 nm.Maintaining a constant optical density required widely varyingnanoparticle concentrations because of the dramatic differencein the effective cross sections of the nanoparticles. We used an

approximately 3 pM solution of Au/SiO2 nanoshells, relativeto approximately 100 and 300 pM for nanorods and Au/Au2Snanoshells, respectively. The observed temperature changefollows a trend in qualitative agreement with predictions basedon electromagnetic calculations alone, with Au/SiO2 nanoshellsshowing the smallest increase and Au/Au2S shells and nanorodsshowing somewhat larger temperature changes. The photother-mal transduction efficiencies, calculated with eq 7 and averagedover several runs, show that nanorods have the highest ef-ficiency, but the advantage over Au/Au2S nanoshells is notstatistically significant (Figure 5a). On the other hand, whencomparing strictly on the basis of how much light energy asingle particle is expected to convert to heat, the photothermaltransduction cross section of Au/SiO2 nanoshells is overwhelm-ingly higher than either of the other particle types (Figure 5b).We calculate the photothermal transduction cross section bymultiplying the empirical efficiency of the particle by thetheoretical extinction cross section.

Despite expectations based on the calculated absorptionefficiencies, the nanorod photothermal transduction efficiencyis only about twice as large as the efficiency obtained for Au/SiO2 nanoshells. The difference between the absorption andphotothermal transduction efficiencies may seem surprising atfirst glance. Low intensity white light experiments usingphotoacoustic techniques28 or an integrating sphere29 to isolateabsorption from extinction show good agreement with Mietheory predictions. However, closer analysis of their experi-mental technique reveals that the current measurement is notprobing the same regime as those studies for two reasons. First,the particles used in this experiment are different. We are usingAu nanoshells and Au nanorods instead of Ag colloid. Withregard to nanoshells in particular, it is known that an asymmetric

Figure 3. Apparatus for measuring photothermal transduction ef-ficiency: (a) 815-nm diode laser; (b) vacuum chamber held below 1Torr; (c) aperture; (d) sample cell sealed with paraffin wax and epoxy;(e) T-type thermocouple; (f) Teflon-coated magnetic stir bar to eliminatetemperature gradients in the cell.

QI + Q0 ) hA(Tmax - Tamb) (6)

η )hA(Tmax - Tamb) - Q0

I(1 - 10-Aλ)(7)

Figure 4. Experimental data used to calculate photothermal transduc-tion efficiency of nanorods and nanoshells in aqueous solution. (Left)Temperature change of representative sample cells as a function of timewith exponential fits of heating (red lines) and cooling (blue lines).The time constant of the exponential fit allows calculation of the heattransfer coefficient of the sample cell and the photothermal transductionefficiency of the particles within. (Right) UV/vis absorbance spectraof solutions used, including (a) Au/SiO2 nanoshells, (b) Au/Au2Snanoshells, and (c) nanorods, shown with the laser line used to heatthe particles (815 nm, red line).

Figure 5. Comparison of heating by nanoparticles. (a) Photothermaltransduction efficiency of nanoshells and nanorods, calculated by eq 7and averaged over several runs. (b) Photothermal transduction crosssection (black bar) and absorption cross section (red bar) of nanoshellsand nanorods.

Nanoparticle Photothermal Efficiency J. Phys. Chem. C, Vol. 113, No. 28, 2009 12093

distribution of metal around the core, such as a dielectric coreoffset (nanoeggs30), can cause the particle to morph frompredominantly scattering to predominantly absorbing.14 Othergeometric imperfections, such as surface roughness, may alsocontribute to a particle’s tendency to absorb more than predictedby Mie theory.12,31 Second, in contrast to white light sources,laser illumination can deliver significantly more power to theparticles. At the extreme, pump/probe experiments can deliverenough power to induce phase changes and particle reshaping.32-34

The intermediate power levels used in this experimentpossibly caused local temperature increases sufficient to alterthe optical environment of the nanoparticles.35 The temperaturedependence of the solution’s refractive index could be respon-sible for a resonance shift by the particles, particularly if thelocal temperature were high enough to permit boiling. Water’srefractive index decreases gradually before it discontinuouslyjumps to n ) 1 at the boiling point.36,37 Nanorods in particularwould be sensitive to this effect because of their narrow linewidth. Their plasmon peak need not shift very far before it nolonger overlaps the laser line at all. Furthermore, nanoscalepockets of steam surrounding the nanoparticles38 may act asRayleigh scattering centers not accounted for by the presenttheory. This hypothesis is consistent with the observation byRoper et al. that mechanically chopping the laser beam resultsin higher photothermal transduction efficiencies.27 The reducedduty cycle allows adequate time for any hot spots around theparticles to dissipate. Unfortunately, it is extremely difficult topinpoint the surface temperature of the particles because itdepends on the thermal conductance of the particle/solventinterface. Reported values range over 2 orders of magnitudedepending on particle surface chemistry, thermal properties ofthe embedding medium, and the rate at which energy is deliveredto the nanoparticles.39-41 Further study is indicated in order toconfirm these effects and find their relative magnitudes.

IV. Conclusions

The optimal nanoparticle species to use for heating applica-tions ultimately depends on the specifics of the application inmind. While small nanoparticles such as nanorods and Au/Au2Snanoshells provide the highest transduction efficiencies, largerparticles such as Au/SiO2 shells can satisfy given heatingrequirements with fewer particles. The substantial scatteringcross section of Au/SiO2 nanoshells also makes them ideal forapplications that benefit from simultaneous image enhancement.These measurements probe a regime of photothermal heatingwhere electromagnetic calculations alone may not fully captureall the physics necessary to understand the system. Severalhypotheses are put forward to explain the discrepancy betweenthe electromagnetic calculations and the measured photothermalheating. Understanding the benefits or drawbacks of the largeand increasing plurality of nanoparticles available for photo-thermal applications adds versatility to our ability to utilize theseunique nano- and mesostructures well in other future applications.

Acknowledgment. The authors thank NASA’s GraduateStudent Researchers Program (Grant No. NNX07AO45H) forfunding this work. We also appreciate the efforts of RiziaBardhan and Felicia Tam, who obtained some of the TEMimages used in this work.

Supporting Information Available: Tabulated values ofexperimental parameters associated with the data shown inFigure 4; derivation of absorption and scattering spectra forensemble of randomly oriented nanorods; a figure demonstrating

the combination of basis spectra to find the spectrum forrandomly oriented nanorod; detailed description of samplepreparation. This material is available free of charge via theInternet at http://pubs.acs.org.

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