Monitoring the Seed-Mediated Growth of Gold Nanoparticles Usingin Situ Second Harmonic Generation and Extinction SpectroscopyRami A. Khoury, Jeewan C. Ranasinghe, Asela S. Dikkumbura, Prakash Hamal, Raju R. Kumal,Tony E. Karam, Holden T. Smith, and Louis H. Haber*

Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States

*S Supporting Information

ABSTRACT: In situ second harmonic generation (SHG) coupled with extinctionspectroscopy is used for real-time monitoring of seed-mediated growth dynamicsof colloidal citrate-stabilized gold nanoparticles in water. The time-dependent insitu SHG results capture an early stage of the growth process where a largeenhancement in the SHG signal is observed, which is attributed to the formationof plasmonic hot spots from a rough and uneven nanoparticle surface. Thetemporal peak in the SHG signal is followed by a decay that is fit to an exponentialfunction to characterize the size-dependent nanoparticle growth lifetime, whichvaries from 0.45 to 1.7 min for final nanoparticle sizes of 66 and 94 nm,respectively. This early growth stage also corresponds to a broadening of the plasmon spectra, as monitored using time-dependent in situ extinction spectroscopy. Over the course of the seed-mediated growth reaction, the nanoparticle becomesmore thermodynamically stable through surface reconstruction resulting in a smoother, more uniform surface, corresponding tolower, stable SHG signals and narrower plasmon spectra. With real-time monitoring of nanoparticle formation, in situ SHGspectroscopy combined with in situ extinction spectroscopy provides an important insight for controlling nanoparticle synthesisand surface morphology for potential nanoscale engineering of different colloidal nanomaterials.

■ INTRODUCTION

Colloidal gold nanoparticles have received considerableattention due to their potential applications in molecularsensing, photovoltaics, catalysis, imaging, nanomedicine, andphotothermal therapy.1−14 These applications rely heavily onthe localized surface plasmon resonance (LSPR) fromcoherent oscillations of free electrons that lead to opticalfield enhancements at the nanoparticle surface.15 The opticalproperties of gold nanoparticles can be tuned depending onthe desired application by changing the nanoparticle size,shape, and surrounding medium.16,17 Varying the nanoparticlesize causes only minor changes in the extinction spectra forgold nanospheres of diameters ranging from approximately 12to 200 nm.13 However, varying the gold nanosphere shape byelongating it into a nanorod18 or by creating a bumpy “urchin-like” surface19 leads to pronounced changes in the associatedoptical properties. Therefore, developing synthetic protocolsfor greater control and tunability over the associated surfacechemistry is critical for developing gold nanoparticle-basedapplications.13,16

Citrate-stabilized gold nanoparticles prepared by thereduction of a gold chloride salt (HAuCl4) with sodiumcitrate (Na3C6H5O7) in water remains one of the mostcommon synthetic methods for producing spherical goldnanoparticles.20 By varying reaction conditions, such as thecitrate to gold salt ratio and the aqueous pH, it is possible toobtain gold nanoparticles ranging from 5 to 150 nm indiameter.21 However, the gold nanoparticle sample polydisper-sity is often very large with nonuniform surfaces when using

this method to prepare nanoparticles larger than 30 nm indiameter.22,23 To gain better control over the nanoparticle sizeand shape distributions, an additional seed-mediated growthstep can be used where additional gold salt is selectivelyreduced on the surface of the colloidal citrate-stabilized goldseeds using hydroquinone. This seed-mediated nanoparticlegrowth procedure results in spherical gold nanoparticles withsizes ranging from 50 to 200 nm while minimizing unwantedsecondary nucleation for improved monodispersity anduniform surface morphologies.24

Characterization techniques such as transmission electronmicroscopy (TEM), scanning electron microscopy (SEM), andatomic force microscopy (AFM) are excellent for studyingnanoparticle size and shape distributions ex situ and postsyn-thesis. However, monitoring nanoparticle surface formation insitu can provide important additional insight into the growthdynamics for better control over surface and optical properties.The nucleation and growth of gold nanoparticles by citratereduction in water have been studied using in situ extinctionspectroscopy22 as well as X-ray absorption near-edge spectros-copy (XANES) and small-angle X-ray scattering (SAXS),25

where the results indicate that the gold nanoparticle formationoccurs by a four-step mechanism. Previous work has also usedin situ extinction spectroscopy combined with in situ SAXS tomonitor the growth of gold nanoparticles synthesized in

Received: July 25, 2018Revised: September 28, 2018Published: October 1, 2018

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2018, 122, 24400−24406

© 2018 American Chemical Society 24400 DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

Dow

nloa

ded

via

NA

TL

CH

UN

G C

HE

NG

UN

IV o

n D

ecem

ber

3, 2

018

at 1

6:07

:45

(UT

C).

Se

e ht

tps:

//pub

s.ac

s.or

g/sh

arin

ggui

delin

es f

or o

ptio

ns o

n ho

w to

legi

timat

ely

shar

e pu

blis

hed

artic

les.

toluene with borohydride and surfactants.26 Additionally, goldnanoshells grown on silica microparticles have been studiedusing in situ nonlinear second-harmonic scattering (SHS)where the large SHS signal during the growth process isattributed to electric-field enhancements in gaps of incompleteor uneven nanoshells.27 Here, large nonlinear optical fieldenhancements are dependent on the metal nanoparticle shapeas well as the surface roughness. For example, fundamentalstudies have investigated the SHG electric field enhancementsfrom rough metal surfaces and sharp metal tips, bothexperimentally and theoretically, where the localized plasmonresonance contributes to the electrostatic “lightning-rodeffect”.28,29 Another nonlinear spectroscopic technique usingin situ four-wave mixing has been used to investigate growthlifetimes of gold nanoparticles in water using borohydride andcitrate reduction.30 Obtaining real-time information aboutnanoparticle growth in situ poses a challenge in that thesynthesis must be scaled to a suitable volume for sufficientoptical detection which typically utilizes a specializedexperimental setup.Second-harmonic generation (SHG) is a noninvasive

nonlinear spectroscopic technique in which two incidentphotons of frequency ω coherently add to produce a thirdphoton of frequency 2ω.31−35 This process is forbidden in bulkmedia with a center of inversion symmetry. However, SHG isallowed at interfaces, making SHG a powerful tool for studyingcolloidal nanoparticle surfaces. Previous work has used SHG toinvestigate TiO2 microparticles,36 liposomes,37,38 and metalnanoparticles made of gold, silver, and gold−silver al-loys.3,4,39−45 Additionally, in situ SHS or SHG coupled within situ extinction spectroscopy27 provides important informa-tion that can be used to characterize the nanoparticle growthdynamics for potential advances in colloidal nanoparticleengineering.In this article, we demonstrate the versatility of in situ SHG

and extinction spectroscopy by monitoring the seed-mediatedgrowth process of gold nanoparticles in water under varyingsynthesis conditions. Changes in the SHG signal as a functionof the reaction time are fit to exponential functions to obtainthe associated growth lifetimes under different initial gold seedconcentrations. The final nanoparticle sizes are determinedusing extinction spectroscopy and TEM measurements. Themeasured growth lifetimes depend on the final nanoparticlediameter where larger nanoparticles have corresponding longer

growth lifetimes. By comparing the in situ SHG results with insitu extinction spectroscopy, an intermediate growth stageattributed to a rough, nonuniform nanoparticle surface isobserved which is characterized by inhomogeneous spectralbroadening followed by a narrowing and blue-shifting of theplasmon spectrum. This correlates with the in situ SHG resultswhere a size-dependent maximum in the SHG signal ismeasured at early times and is attributed to a rough, bumpysurface during the initial nanoparticle growth process.Combining the in situ SHG and extinction spectroscopyresults with the ex situ TEM and Mie theory analysis providesimportant information about the size-dependent reactiondynamics and the associated mechanism, which cannot beobtained from ex situ measurements alone. The versatility of insitu SHG measurements coupled with in situ extinctionspectroscopy in combination with conventional ex situtechniques enables a wide range of investigations for studyingnanoparticle growth and functionalization in real time fordeveloping new nanomaterials and associated applications.

■ EXPERIMENTAL SECTIONGold Nanoparticle Synthesis. The colloidal gold nano-

particle samples are prepared by a seeded-growth techniqueusing hydroquinone (HQ) as the reducing agent.23,24 The goldseeds are prepared by adding 900 μL of 34 mM sodium citrate(Na3C6H5O7) to 30 mL of 290 μM gold tetrachloride(HAuCl4) in water under boiling and vigorous stirringconditions. Five gold nanoparticle samples of different sizesare synthesized by adding 25, 30, 35, 40, and 50 μL of theprepared gold seeds to 25 μL of 29 mM HAuCl4 which are alldiluted to a final volume of 2.5 mL with ultrapure water. Theaddition of the reducing agents using 25 μL of 0.03 M HQ and5 μL of 34 mM sodium citrate initiates the growth process. Foreach sample, the final solution is left to stir at roomtemperature for 60 min.

In Situ Second Harmonic Generation and ExtinctionSpectroscopy. The SHG setup uses an ultrafast oscillatorlaser, an optical setup, and a high-sensitivity spectroscopydetector, modified from our previous work.3,4 A titanium:sap-phire oscillator laser produces 75 fs pulses centered at 800 nmwith a repetition rate of 80 MHz and an average power of 2.7W. The laser beam is attenuated to 600 mW and is focusedinto a 1 cm quartz cuvette containing the gold seeds, HAuCl4,HQ, and sodium citrate in the various gold nanoparticle

Figure 1. Representative TEM images of gold nanoparticles with average sizes of (a) 92.2 ± 4.0, (b) 86.1 ± 4.3, (c) 72.6 ± 5.3, (d) 71.3 ± 4.8, and(e) 65.6 ± 5.0 nm prepared using 25, 30, 35, 40, and 50 μL of precursor gold seeds, respectively.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24401

reaction conditions. An optical filter is placed in front of thecuvette to remove any residual SHG prior to the sample.Another filter is placed after the sample to remove thefundamental light while transmitting the SHG, which iscollected in the forward direction and refocused into a high-sensitivity charge-coupled device (CCD) detector coupled to amonochromator-spectrograph. This optical setup allows for thedetection of the SHG signal as a function of time to capturenanoparticle reaction dynamics. At the same time, in situextinction spectroscopy is measured using a low-intensitybroadband tungsten filament lamp which is collimated using apair of lenses and focused orthogonal to the SHG beam andinto the quartz cuvette with the resulting spectra detectedusing a fiber-optic spectrometer detector. For each trial, theprecursor gold seeds and HAuCl4 are added in ultrapure waterfor a baseline SHG measurement. At time zero (t = 0), HQand sodium citrate are added simultaneously to initiate thenanoparticle growth process. For each sample, 5 in situ SHGspectra and background are taken followed by 10 in situextinction spectra at 0.5 and 1 s acquisition times, respectively,in repeating iterations to track the nanoparticle growthdynamics. A schematic diagram of the in situ SHG andextinction spectroscopy setup is shown in Figure S7 of theSupporting Information.

■ RESULTS AND DISCUSSIONAfter the nanoparticle synthesis is completed, TEM images areacquired surveying ∼200 nanoparticles for each nanoparticlesample. The nanoparticle sizes are measured and fit using alog-normal distribution to obtain the average nanoparticlediameter, as shown in the Supporting Information, for eachsample. Figure 1 shows TEM images that are representative ofeach gold nanoparticle sample with sizes of 92.2 ± 4.0, 86.1 ±4.3, 72.6 ± 5.3, 71.3 ± 4.8, and 65.6 ± 5.0 nm synthesizedfrom 25, 30, 35, 40, and 50 μL of precursor gold seeds,respectively. The average nanoparticle size is also confirmed byfitting each final extinction spectrum using Mie theory withcorresponding sizes of 94, 89, 76, 72, and 66 nm, respectively,which are within the standard deviations of the sizes measuredfrom TEM. Additional TEM images from the other nano-particle samples are shown in the Supporting Information.The in situ extinction spectra from gold nanoparticle growth

provide important information about the seed-mediatednanoparticle reaction. Representative extinction spectra atvarious times during the reaction for the 25, 35, and 50 μLprecursor gold seed samples are shown in Figure 2. Prior to thestart of the nanoparticle growth reaction, a baseline extinctionspectrum is taken showing the absorption and scattering profileof the 15 nm gold precursor seeds along with the added goldchloride solution. Immediately after the addition of thereducing agents, a broad plasmon peak centered near 600nm is observed, which is attributed to a polydispersedistribution and an uneven, rough nanoparticle surface.19,22

As the nanoparticle growth reaction proceeds, this broadplasmon peak narrows to a wavelength centered near 575 nm,indicating a more uniform surface and reduction inpolydispersity. During the final stage of the reaction theplasmon peak continues to narrow and blue-shifts as thenanoparticle surface reaches its final, smooth morphology witha final spectrum that is very stable over time. A similar trend inthe in situ extinction spectra is observed for the remainingnanoparticle samples, as shown in the Supporting Information.The final extinction spectrum using 25 μL of precursor gold

seeds is shown in Figure 3 with the corresponding Mie theoryfit which is in agreement to the sizes measured from TEM.

Previous studies have investigated the formation of an “urchin-like” or “blackberry-like” intermediate stage in gold nano-particles, which is characterized by a red-shifted plasmonicspectrum.19,46 This spectrum blue-shifts as the rough nano-particle surface becomes more smooth during the nanoparticlegrowth reaction,19 which is consistent with our in situextinction spectroscopy and SHG results, as discussed inmore detail below.The in situ SHG results provide additional and comple-

mentary surface-sensitive information about the seed-mediatednanoparticle growth process. The temporal evolution of theSHG electric field for each nanoparticle sample is shown inFigure 4. Before the growth process is initiated at time zero, abaseline SHG intensity is measured for all samples, whichcontains contributions of SHG from the precursor seeds41 aswell as hyper-Rayleigh scattering47 from the aqueous solvent.

Figure 2. In situ extinction spectra of gold nanoparticles synthesizedwith (a) 25, (b) 35, and (c) 50 μL of precursor gold seeds at differenttimes during the reactions.

Figure 3. Final extinction spectrum (red line) of gold nanoparticlesprepared using 25 μL of precursor gold seeds compared tocorresponding fit (black line) from Mie theory.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24402

After adding the reducing agents HQ and sodium citrate, asharp increase in the SHG electric field is observed followed bya size-dependent exponential decay for all samples. To separatethe time-dependent linear extinction response from thenonlinear response of the nanoparticles, a correction to thein situ SHG signal is applied, where ISHG,corr = ISHG exp(ε800 +1/2ε400), and where ISHG, ε800, and ε400 are the measured SHGintensity, extinction at 800 nm, and extinction at 400 nm,respectively.27 The SHG electric field is taken as the squareroot of the integrated SHG signal, with =E ISHG SHG,corr

where ESHG allows for precise fitting for the exponential profile.Representative in situ SHG spectra of the 25 μL precursor goldsample at selected reaction times are shown in Figure 5.

Additional in situ SHG spectra from the other samples aredisplayed in the Supporting Information. The in situ SHGspectra are fit with a Gaussian function to obtain a centralwavelength of 400 nm and a full width at half-maximum of 4.8nm, which are constant values throughout the growth processfor each nanoparticle sample.The SHG electric-field time profile allows for the

determination of the seed-mediated nanoparticle growthlifetime for the different prepared samples. For each nano-particle sample, a maximum in the SHG electric field isobserved in the initial stage of growth, shortly after time zero.The peak SHG near time zero shows approximately an order ofmagnitude increase in SHG intensity, which is attributed to theinitial stage in the reaction that is characterized byinhomogeneous growth of the nanoparticles with roughsurfaces giving rise to plasmonic hot spots.19,27−29,46 As thereaction proceeds, the sharp SHG feature exponentially decayswhere the characteristic growth lifetime is observed to belonger for larger nanoparticles. This indicates that the secondstage in the reaction is described by the nanoparticle surfacebecoming more smooth and uniform over time, leading to adecrease in plasmonic hot spots and a lower SHG intensity.Finally, the nanoparticles reach their final morphologycharacterized by a relatively smooth surface where the reactionis complete and the SHG signal remains constant over time.The size-dependent growth dynamics of the gold nano-

particles are analyzed in more detail to obtain thecorresponding growth lifetimes. The SHG electric field timetrace for each nanoparticle sample is fit using an exponentialfunction given by = + τ−E t y A( ) e t

SHG 0/ , where t is the

reaction time after the addition of the reducing agents, τ is thegrowth lifetime, A is the amplitude, and y0 is the offset. The fitsfor each nanoparticle sample are shown in Figure 4, and thebest fit parameters are tabulated in the SupportingInformation. The resulting growth lifetimes are plotted as afunction of the final nanoparticle diameter (Figure 6a). Thesegrowth lifetimes decrease as the final nanoparticle diameterdecreases with values of 1.67 ± 0.03, 1.57 ± 0.03, 1.05 ± 0.08,0.95 ± 0.03, and 0.45 ± 0.04 min for the 94, 90, 75, 72, and 66nm nanoparticles, respectively. The general trend in theselifetimes is in agreement with previous studies observing thatthe growth rate is proportional to the amount of precursorseeds.48 Because the SHG signal decreases as a function oftime for each of the seed-mediated nanoparticle growth

Figure 4. SHG electric field (red squares) as a function of reactiontime of gold nanoparticles using (a) 25, (b) 30, (c) 35, (d) 40, and(e) 50 μL of precursor gold seeds compared to the fits (black lines).

Figure 5. SHG spectra for gold nanoparticles prepared using 25 μL ofprecursor gold seeds at different times during the reaction.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24403

reactions, the dominant contribution of the change in SHGsignal is attributed to the surface morphology becoming moresmooth over time, corresponding to a decrease in plasmonichot spots.The in situ SHG time profiles and corresponding extinction

spectra of the seed-mediated gold nanoparticle growthdynamics are consistent with a two-step process. The firststep is characterized by a rapid growth resulting in an uneven,bumpy surface and a high surface concentration of plasmonichot spots. The second step is characterized by the nanoparticlesurface becoming more smooth over time resulting inincreased monodispersity, narrowing plasmonic spectra, andlower, more stable SHG signals as the nanoparticle surfacereaches thermodynamic equilibrium under a characteristic size-dependent exponential growth lifetime. The peak SHG electricfield, shown in Figure 6b, is observed to increase as the finalnanoparticle diameter increases. The addition of sodium citratereduces AuIII to AuI followed by the selective reduction by HQof AuI adsorbed on the seed surface.19,24,49 Lower seedconcentrations here correspond to higher concentrations ofAu0 adsorbed to each nanoparticle surface to produce largergold nanoparticles. A higher Au0 to seed ratio has beenobserved to promote the formation of urchin-like goldnanoparticles with high surface energy facets.19 These highersurface energy facets produce plasmonic hot spots correspond-ing to the large SHG signals observed shortly after time zero.These uneven surface energy facets that form under highconcentrations of Au0 in the presence of HQ become lessbumpy in morphology over time,19 reaching a thermodynamicequilibrium corresponding to a relatively smooth gold

nanoparticle surface according to a size-dependent exponentialtime profile. Additional future studies, including different insitu scattering measurements combined with computationalsimulations, can be done to further characterize the nano-particle size, shape, and surface morphology distributionsduring the nanoparticle growth reaction. Using the baselineSHG for each nanoparticle sample after the reaction iscomplete, the final contribution to the SHG electric field pernanoparticle is also determined, as plotted in Figure 6c. Theseresults demonstrate that the larger gold nanoparticles have ahigher SHG signal, in agreement with previous theoreticalwork on the enhancement of the second-harmonic field fromdifferent spherical nanoparticles.50,51 Overall, the in situ SHGand extinction spectroscopy measurements provide crucialinsight for monitoring the colloidal seed-mediated goldnanoparticle growth reaction under varying synthesis con-ditions.

■ CONCLUSIONIn summary, we have demonstrated the versatility of in situSHG and extinction spectroscopy in the investigation of size-dependent seed-mediated gold nanoparticle growth dynamicsin water. The in situ SHG and extinction spectroscopy resultsare consistent with a two-step growth process. During the firststep of the nanoparticle growth reaction, rough and unevensurfaces are formed rapidly giving rise to plasmonic hot spotsthat dramatically enhance the SHG electric field and havecorresponding broad, red-shifted plasmonic spectra. In thesecond step, the nanoparticle surface becomes smoother,reaching a thermodynamic equilibrium over a correspondingsize-dependent exponential growth lifetime that results in afinal nanoparticle sample with a lower, more stable SHG signaland corresponding narrower, blue-shifted plasmonic spectrathat agrees with Mie theory. The seed-mediated nanoparticlegrowth lifetimes measured using in situ SHG are faster forsmaller final gold nanoparticle sizes, varying from 0.45 to 1.7min for final nanoparticle sizes of 66 and 94 nm, respectively.The combination of in situ SHG and extinction spectroscopyprovides complementary information that can be used tomonitor colloidal nanoparticle growth dynamics for improvingnanomaterial synthesis and characterization.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b07176.

TEM images of the gold nanoparticles with theircorresponding size distributions, additional Mie theoryfitting, and discussion on nanoparticle concentrations;additional in situ SHG and in situ extinction spectra aswell as the tabulated fitting values for the growthlifetimes (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail lhaber@lsu.edu (L.H.H.).

ORCIDTony E. Karam: 0000-0002-1244-5141Holden T. Smith: 0000-0001-5487-8431Louis H. Haber: 0000-0001-7706-7789

Figure 6. (a) Analysis of growth lifetime, τ, as a function of finalnanoparticle diameter. (b) Peak SHG electric field as a function offinal nanoparticle diameter. (c) Final SHG electric field pernanoparticle.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24404

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank Louisiana State University and theConsortium for Innovation and Manufacturing in Materials(CIMM) under Award #OIA-1541079 for generous financialsupport for this work. The authors also thank the LSU MaterialCharacterization Center (LSUMCC) and Ying Xiao forassistance with transmission electron microscopy.

■ REFERENCES(1) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Surface plasmonresonance scattering and absorption of anti-EGFR antibodyconjugated gold nanoparticles in cancer diagnostics: applications inoral cancer. Nano Lett. 2005, 5, 829−834.(2) Haes, A. J.; van Duyne, R. P. A nanoscale optical biosensor:Sensitivity and selectivity of an approach based on the localizedsurface plasmon resonance spectroscopy of triangular silver nano-particles. J. Am. Chem. Soc. 2002, 124, 10596−10604.(3) Karam, T. E.; Haber, L. H. Molecular adsorption and resonancecoupling at the colloidal gold nanoparticle interface. J. Phys. Chem. C2014, 118, 642−649.(4) Kumal, R. R.; Karam, T. E.; Haber, L. H. Determination of thesurface charge density of colloidal gold nanoparticles using secondharmonic generation. J. Phys. Chem. C 2015, 119, 16200−16207.(5) Kamat, P. V. Meeting the clean energy demand: Nanostructurearchitectures for solar energy conversion. J. Phys. Chem. C 2007, 111,2834−2860.(6) Kholmicheva, N.; Moroz, P.; Rijal, U.; Bastola, E.; Uprety, P.;Liyanage, G.; Razgoniaev, A.; Ostrowski, A. D.; Zamkov, M.Plasmonic nanocrystal solar cells utilizing strongly confined radiation.ACS Nano 2014, 8, 12549−12559.(7) Narayanan, R.; El-Sayed, M. A. Catalysis with transition metalnanoparticles in colloidal solution: nanoparticle shape dependenceand stability. J. Phys. Chem. B 2005, 109, 12663−12676.(8) Kamat, P. V. Photophysical, photochemical and photocatalyticaspects of metal nanoparticles. J. Phys. Chem. B 2002, 106, 7729−7744.(9) Sperling, R. A.; Gil, P. R.; Zhang, F.; Zanella, M.; Parak, W. J.Biological applications of gold nanoparticles. Chem. Soc. Rev. 2008, 37,1896−1908.(10) Kumal, R. R.; Landry, C. R.; Abu-Laban, M.; Hayes, D. J.;Haber, L. H. Monitoring the photocleaving dynamics of colloidalmicrorna-functionalized gold nanoparticles using second harmonicgeneration. Langmuir 2015, 31, 9983−9990.(11) Prigodich, A. E.; Lee, O.-S.; Daniel, W. L.; Seferos, D. S.;Schatz, G. C.; Mirkin, C. A. Tailoring DNA structure to increasetarget hybridization kinetics on surfaces. J. Am. Chem. Soc. 2010, 132,10638−10641.(12) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancercell imaging and photothermal therapy in the near-infrared region byusing gold nanorods. J. Am. Chem. Soc. 2006, 128, 2115−2120.(13) Link, S.; El-Sayed, M. A. Shape and size dependence ofradiative, non-radiative and photothermal properties of gold nano-crystals. Int. Rev. Phys. Chem. 2000, 19, 409−453.(14) Karam, T. E.; Smith, H. T.; Haber, L. H. Enhancedphotothermal effects and excited-state dynamics of plasmonic size-controlled gold-silver-gold core-shell-shell nanoparticles. J. Phys.Chem. C 2015, 119, 18573−18580.(15) Chandra, M.; Dowgiallo, A.-M.; Knappenberger, K. L., Jr.Controlled plasmon resonance properties of hollow gold nanosphereaggregates. J. Am. Chem. Soc. 2010, 132, 15782−15789.(16) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly,supramolecular chemistry, quantum-size-related properties, andapplications toward biology, catalysis, and nanotechnology. Chem.Rev. 2004, 104, 293−346.

(17) Hu, M.; Novo, C.; Funston, A.; Wang, H.; Staleva, H.; Zou, S.;Mulvaney, P.; Xia, Y.; Hartland, G. V. Dark-field microscopy studiesof single metal nanoparticles: Understanding the factors that influencethe linewidth of the localized surface plasmon resonance. J. Mater.Chem. 2008, 18, 1949−1960.(18) Nikoobakht, B.; El-Sayed, M. A. Preparation and growthmechanism of gold nanorods (NRs) using seed-mediated growthmethod. Chem. Mater. 2003, 15, 1957−1962.(19) Li, J.; Wu, J.; Zhang, X.; Liu, Y.; Zhou, D.; Sun, H.; Zhang, H.;Yang, B. Controllable synthesis of stable urchin-like gold nano-particles using hydroquinone to tune the reactivity of gold chloride. J.Phys. Chem. C 2011, 115, 3630−3637.(20) Frens, G. Controlled nucleation for the regulation of theparticle size in monodisperse gold suspensions. Nature, Phys. Sci.1973, 241, 20−22.(21) Turkevich, J.; Stevenson, P. C.; Hillier, J. Nucleation andgrowth process in the synthesis of colloidal gold. Discuss. Faraday Soc.1951, 11, 55−75.(22) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.;Plech, A. Turkevich method for gold nanoparticle synthesis revisited.J. Phys. Chem. B 2006, 110, 15700−15707.(23) Brown, K. R.; Walter, D. G.; Natan, M. J. Seeding of colloidalau nanoparticle solutions. 2. Improved control of particle size andshape. Chem. Mater. 2000, 12, 306−313.(24) Perrault, S. D.; Chan, W. C. Synthesis and surface modificationof highly monodispersed, spherical gold nanoparticles of 50−200 nm.J. Am. Chem. Soc. 2009, 131, 17042−17043.(25) Polte, J.; Ahner, T. T.; Delissen, F.; Sokolov, S.; Emmerling, F.;Thunemann, F.; Kraehnert, R. Mechanism of gold nanoparticleformation in the classical citrate synthesis method derived fromcoupled in situ XANES and SAXS evaluation. J. Am. Chem. Soc. 2010,132, 1296−1301.(26) Abecassis, B.; Testard, F.; Spalla, O.; Barboux, P. Probing insitu the nucleation and growth of gold nanoparticles by small-angle x-ray scattering. Nano Lett. 2007, 7, 1723−1727.(27) Sauerbeck, C.; Haderlein, M.; Schurer, B.; Braunschweig, B.;Peukert, W.; Klupp Taylor, R. N. Shedding light on the growth ofgold nanoshells. ACS Nano 2014, 8, 3088−3096.(28) Boyd, G. T.; Rasing, T.; Leite, J. R. R.; Shen, Y. R. Local-fieldenhancement on rough surfaces of metals, semimetals, and semi-conductors with the use of optical second-harmonic generation. Phys.Rev. B: Condens. Matter Mater. Phys. 1984, 30, 519−526.(29) Bouhelier, A.; Beversluis, M.; Hartschuh, A.; Novotny, L. Near-field second-harmonic generation induced by local field enhancement.Phys. Rev. Lett. 2003, 90, 013903.(30) Wu, J.; Xiang, D.; Gordon, R. Monitoring gold nanoparticlegrowth in situ via the acoustic vibrations probed by four-wave mixing.Anal. Chem. 2017, 89, 2196−2200.(31) Eisenthal, K. Second harmonic spectroscopy of aqueous nano-and microparticle interfaces. Chem. Rev. 2006, 106, 1462−1477.(32) Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. New method fordetermination of surface potential of microscopic particles by secondharmonic generation. J. Phys. Chem. B 1998, 102, 6331−6336.(33) Jen, S.-H.; Gonella, G.; Dai, H.-L. The effect of particle size insecond harmonic generation from the surface of spherical colloidalparticles. I: Experimental observations. J. Phys. Chem. A 2009, 113,4758−4762.(34) Shen, Y. R. Surface properties probed by second-harmonic andsum-frequency generation. Nature 1989, 337, 519.(35) Hayes, P. L.; Malin, J. N.; Jordan, D. S.; Geiger, F. M. Getcharged up: Nonlinear optical voltammetry for quantifying thethermodynamics and electrostatics of metal cations at aqueous/oxideinterfaces. Chem. Phys. Lett. 2010, 499, 183−192.(36) Liu, Y.; Dadap, J.; Zimdars, D.; Eisenthal, K. B. Study ofinterfacial charge-transfer complex on TiO2 particles in aqueoussuspension by second-harmonic generation. J. Phys. Chem. B 1999,103, 2480−2486.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24405

(37) Liu, J.; Subir, M.; Nguyen, K.; Eisenthal, K. B. Second harmonicstudies of ions crossing liposome membranes in real time. J. Phys.Chem. B 2008, 112, 15263−15266.(38) Kumal, R. R.; Nguyenhuu, H.; Winter, J. E.; McCarley, R. L.;Haber, L. H. Impacts of salt, buffer, and lipid nature on molecularadsorption and transport in liposomes as observed by secondharmonic generation. J. Phys. Chem. C 2017, 121, 15851−15860.(39) Vance, F. W.; Lemon, B. J.; Hupp, J. T. Enormous hyperRayleigh scattering from nanocrystalline gold particle suspensions. J.Phys. Chem. B 1998, 102, 10091−10093.(40) Gan, W.; Gonella, G.; Zhang, M.; Dai, H.-L. Reactions andadsorption at the surface of silver nanoparticles probed by secondharmonic generation. J. Chem. Phys. 2011, 134, 041104.(41) Haber, L. H.; Kwok, S. J. J.; Semeraro, M.; Eisenthal, K. B.Probing the colloidal gold nanoparticle/aqueous interface with secondharmonic generation. Chem. Phys. Lett. 2011, 507, 11−14.(42) Hao, E. C.; Schatz, G. C.; Johnson, R. C.; Hupp, J. T. HyperRayleigh scattering from silver nanoparticles. J. Chem. Phys. 2002, 117,5963−5966.(43) Russier-Antoine, I.; Benichou, E.; Bachelier, G.; Jonin, C.;Brevet, P. F. Multipolar contributions of the second harmonicgeneration from silver and gold nanoparticles. J. Phys. Chem. C 2007,111, 9044−9048.(44) Jin, R.; Jureller, J.; Kim, H.; Scherer, N. Correlating secondharmonic optical responses of single ag nanoparticles withmorphology. J. Am. Chem. Soc. 2005, 127, 12482−12483.(45) Bachelier, G.; Butet, J.; Russier-Antoine, I.; Jonin, C.; Benichou,E.; Brevet, P.-F. Origin of optical second-harmonic generation inspherical gold nanoparticles: Local surface and nonlocal bulkcontributions. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82,235403.(46) Ziegler, C.; Eychmuller, A. Seeded growth synthesis of uniformgold nanoparticles with diameters of 15−300 nm. J. Phys. Chem. C2011, 115, 4502−4506.(47) Roke, S.; Gonella, G. Nonlinear light scattering andspectroscopy of particles and droplets in liquids. Annu. Rev. Phys.Chem. 2012, 63, 353−78.(48) Jana, N. R.; Gearheart, L.; Murphy, C. J. Evidence for seed-mediated nucleation in the chemical reduction of gold salts to goldnanoparticles. Chem. Mater. 2001, 13, 2313−2322.(49) Kumar, S.; Gandhi, K. S.; Kumar, R. Modeling of formation ofgold nanoparticles by citrate method. Ind. Eng. Chem. Res. 2007, 46,3128−3136.(50) Dadap, J. I.; Shan, J.; Eisenthal, K. B.; Heinz, T. F. Second-harmonic Rayleigh scattering from a sphere of centrosymmetricmaterial. Phys. Rev. Lett. 1999, 83, 4045−4048.(51) de Beer, A. G. F.; Roke, S. Nonlinear Mie theory for second-harmonic and sum-frequency scattering. Phys. Rev. B: Condens. MatterMater. Phys. 2009, 79, 155420.

The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b07176J. Phys. Chem. C 2018, 122, 24400−24406

24406