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Surface plasmon effects on two photonluminescence of gold nanorods

Da-Shin Wang,1,2

Fu-Yin Hsu,3

and Chii-Wann Lin,1,2,4

*

1Institute of Biomedical Engineering, National Taiwan University, No.1 Section 4,Roosevelt Road,Taipei,Taiwan 10617

2Center of Nano Science and Technology, National Taiwan University, No.1 Section 4,Roosevelt Road,Taipei,Taiwan 10617

3Institute of Bioscience and Biotechnology, National Taiwan Ocean University, 2 Pei-Ning Road,Keelung, Taiwan, 20224

4Department of Electrical Engineering, National Taiwan University, No.1 Section 4,Roosevelt Road,Taipei,Taiwan 10617*[email protected]

Abstract:Gold nanorods emit strong photoluminescence under two photonexcitation; the efficient two photon lumininescence (TPL) arises from thelocal field enhancement assisted by surface plasmons. The surface plasmoneffects on the TPL efficiency and spectrum are investigated by measuringthe TPL of gold nanorods with various aspect ratios. A large TPL efficiency

is found when incident light wavelength coincides with the longitudinalsurface plasmon mode of the gold nanorods. However, the emission spectraof nanorods with various aspect ratios look similar and exhibit modestsurface plasmon features, which implies a major non-radiative decay ofexcited surface plasmons.

2009 Optical Society of America

OCIS codes:(250.5230) Photoluminescence; (240.6680) Surface plasmons.

References and links

1. A. Mooradian, Photoluminescence of metals, Phys. Rev. Lett. 22(5), 185187 (1969).2. G. T. Boyd, Z. H. Yu, and Y. R. Shen, Photoinduced luminescence from the noble metals and its enhancement

on roughened surfaces, Phys. Rev. B 33(12), 79237936 (1986).3. G. T. Boyd, T. Rasing, J. R. R. Leite, and Y. R. Shen, Local-field enhancement on rough surfaces of metals,

semimetals, and semiconductors with the use of optical second-harmonic generation, Phys. Rev. B 30(2), 519526 (1984).

4. D. Yelin, D. Oron, S. Thiberge, E. Moses, and Y. Silberberg, Multiphoton plasmon-resonance microscopy,Opt. Express 11(12), 13851391 (2003).

5. H. Wang, T. B. Huff, D. A. Zweifel, W. He, P. S. Low, A. Wei, and J.-X. Cheng, In vitro and in vivo two-photon luminescence imaging of single gold nanorods, Proc. Natl. Acad. Sci. U.S.A. 102(44), 1575215756(2005).

6. R. A. Farrer, F. L. Butterfield, V. W. Chen, and J. T. Fourkas, Highly efficient multiphoton-absorption-inducedluminescence from gold nanoparticles, Nano Lett. 5(6), 11391142 (2005).

7. N. J. Durr, T. Larson, D. K. Smith, B. A. Korgel, K. Sokolov, and A. Ben-Yakar, Two-photon luminescenceimaging of cancer cells using molecularly targeted gold nanorods, Nano Lett. 7(4), 941945 (2007).

8. J. Park, A. Estrada, K. Sharp, K. Sang, J. A. Schwartz, D. K. Smith, C. Coleman, J. D. Payne, B. A. Korgel, A.K. Dunn, and J. W. Tunnell, Two-photon-induced photoluminescence imaging of tumors using near-infraredexcited gold nanoshells, Opt. Express 16(3), 15901599 (2008).

9. L. Bickford, J. Sun, K. Fu, N. Lewinski, V. Nammalvar, J. Chang, and R. Drezek, Enhanced multi-spectralimaging of live breast cancer cells using immunotargeted gold nanoshells and two-photon excitationmicroscopy, Nanotechnology 19(31), 315102 (2008).

10. M. B. Mohamed, V. Volkov, S. Link, and M. A. El-Sayed, The 'lightning' gold nanorods: fluorescenceenhancement of over a million compared to the gold metal, Chem. Phys. Lett. 317(6), 517523 (2000).

11. N. R. Jana, L. Gearheart, and C. J. Murphy, Wet chemical synthesisof high aspect ratio cylindrical goldnanorods, J. Phys. Chem. B 105(19), 40654067 (2001).

12. S. Link, and M. El-Sayed, Spectral Properties and Relaxation Dynamics of Surface Plasmon ElectronicOscillations in Gold and Silver Nanodots and Nanorods, J. Phys. Chem. B 103(40), 84108426 (1999).

13. S. Eustis, and M. El-Sayed, Aspect ratio dependence of the enhanced fluorescence intensity of gold nanorods:experimental and simulation study, J. Phys. Chem. B 109(34), 1635016356 (2005).

(C) 2009 OSA 6 July 2009 / Vol. 17, No. 14 / OPTICS EXPRESS 11350

#111292 - $15.00 USD Received 18 May 2009; revised 17 Jun 2009; accepted 18 Jun 2009; published 22 Jun 2009

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However, a great variation in the emission efficiency was observed among the nanorods withvarious aspect ratios. The variation in the TPL efficiency is also clearly seen when theambient refractive index is varied. This indicates that the surface plasmon plays an importantrole in TPL efficiency, but the emission spectrum is determined by the intrinsic electronicproperties of the rod particles.

2. Methods

2.1 Experiment

Gold nanorods were synthesized by using the seed-mediated methods as described elsewhere

[11]. In short, Au seeds were prepared by mixing solutions of 0.1M NaBH4 and 2.5 104M

HAuCl4, followed by vigorous stirring. For the rod formation, a sufficient amount ofsurfactant, cetyltrimetyl ammonium bromide (CTAB), was added into the solution. Byvarying the amount of surfactant, gold nanorods with different aspect ratios can be obtained.

The extinction spectra of gold nanorods with various aspect ratios were measured by aUV-Visible spectrophotometer (Cary 50Conc, Varian) and the longitudinal surface plasmon(LSP) peaks are found to be at 540, 590, 680, 740, 790, 820, and 930nm, which is equivalentto aspect ratios from 1.3 to 5.3 according to the linear dependence of LSP peaks on the aspectratios of gold nanorods [12] (see Fig. 1). The axial length of the nanorods is between 10 and15nm as indicated by the TEM (transmission electron microscopy) images. The concentrationof each nanorod solution was adjusted so the optical density (OD) of LSP band is about 1; the

nanorod solutions are then used for the TPL measurement.

Fig. 1. The extinction spectra for gold nanorods with various aspect ratios. The numbers at thetop and the right of each trace indicate the position of the longitudinal surface plasmon bandand the aspect ratio of that nanorod, respectively.

The setup for TPL measurements is schematically shown in Fig. 2. The light source was aTi:sapphire laser (Tsunami, Spectra Physics) at a 80 MHz repetition rate with a pulse width

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Horiba) connected to the backport; the TPL intensity at the selected wavelength (band-passfilter HQ660/50, Chroma Technology) was recorded by a highly sensitive and low-dark-countphotomultiplier (H7422 P, Hamamatsu) at the side port. The average incident power onsample was about 2 mW, corresponding to a peak intensity of about 1.25GW/m

2in the focus.

Fig. 2. Schematic diagram of the setup for the TPL measurement of gold nanorods.

2.2 Estimation of the local field factor

Mohamed et al. [10] measured the photoluminescence from gold nanorods (with aspect ratiosof 2.0-5.4) by single photon excitation at 480 nm. They observed a photoluminescenceenhanced by a factor of over a million compared to that of gold metal; this is due to theenhanced absorption facilitated by the transverse surface plasmon mode (~520 nm). Moreinterestingly, the efficiency increases and the emission wavelength peak red-shifts as thelength of the rods increases. To explain this phenomenon, they employed the local fieldcorrection idea used by Boyd et al. [2,3] to estimate the field enhancement on the rough metalsurface. The local field factor Lof a hemispheroid is the product of LLRand Lp, which are thelightning rod factor and the local plasmon factor, respectively. LLR is only related to thehemispheroid shape, given by the aspect ratio (a detailed formula for LLRand Lpcan be found

in the reference). The calculated local field factor L(), though successfully predicts certaintrends in the data, does not give a quantitatively accurate estimation. First, the theory predictsa fast increase in the emission strength as the rod length becomes very long, but in fact adecrease in emission is observed for very long nanorods. Second, it vastly overestimates theemission peak wavelength for nanorods with high aspect ratios [13]. Therefore, for aquantitative analysis in our study, the local field enhancement factor is evaluated with the helpof linear extinction spectroscopy, where the transmission minimum is associated with thelocalized surface plasmon [14]. The characterization technique by extinction spectroscopy,though lacking in spatial resolution, is considered quite reliable. It should be applicable to ourexperiment, as the extinction and TPL spectra were obtained on the same samples using far-field incident light, which completely smears the spatial information. The samples were goldnanorod aqueous solutions with rods of aspect ratios ranging from 1.3 to 5.3, and theconcentration was adjusted so that the OD of the LSP band was 1. For each gold nanorodsolution with OD~1, the exact particle concentration in each solution was slightly different;the nanorods with higher aspect ratios had slightly lower particle concentrations. In order tocompare the TPL efficiency on a single particle basis, we must at least know the relativeparticle concentration among the nanorod solutions. The relative concentrations of the goldnanorod solutions were calculated using the molar extinction coefficient of the LSP band [15].Since the reference only offers the molar extinction coefficient for nanorods with aspect ratios



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up to 4.5 (equivalent to LSP band at 850 nm), we only make comparisons in this range.Taking the particle concentration of the nanorod solution with LSP band at 820 nm as 1, weobtain a concentration ratio ranging from 3.1 to 0.7 for the nanorod solutions used in theexperiment.

Based on the same thinking as Boyd et al. [2,3], the local field factor L()is related to thelight extinction as follows:

22

0( ) ( ) ( ) ( ( ) ( )) ( ) ( ) ( ) ,ext abs sca inI I I N L E N L I = + + = (1)

whereext

I ,abs

I ,sca

I ,andin

I are the intensity of extinction, absorption, scattering and incident

light, respectively; ( ) and ( ) are the constants that include the intrinsic spectrum of

absorption and scattering; ( ) is the sum of ( ) and ( ) ; Nis the number of nanorods

in the light path; 0E is the incident electrical field. According to Eq. (1), the local field factor

for each sample can be deducted from the measured absorbance A

1/2 1/2 1/21 10( ) ( ) ( ) ( )A

ext in t

in in

I I IL

N I N I N

= = = , (2)

wheret

I is the transmission intensity. The L()for incident and outgoing light of each gold

nanorod sample can be computed with the measured absorbance.

2.3 Estimation of the TPL efficiency

Because the incident and the outgoing electrical fields are both enhanced by a local fieldfactor, the two-photon luminescence power from a volume of nanorod solution is given as[2,3]

4 4 2

2 2 0 1 2( ) ( ) ( ) ( )P N E L L = , (3)

where 2 and 1 are the luminescence and incident light frequencies, respectively, N is the

number of particles in the excitation volume, and is a factor related to the intrinsic

luminescence spectrum of the material. In the experiment measuring the TPL efficiency ofnanorods with different aspect ratios, the power and wavelength of the incident light was set

as constant (ext= 815 nm), and the emission intensity was measured at ~660 25 nm. In ourexperiment, the nanorod solution with LSPR at 820 nm was found to have the maximumemission. If we consider the TPL efficiency of a nanorod with LSPR at 790 nm and compareit to that of a nanorod with LSPR at 820 nm, we have the relative TPL efficiency Prfor thenanorod with LSPR at 790 nm

790 1

820 2

LSPR

r

LSPR

P NP

P N

=

=

= , (4)

Where PLSPR = 790 and PLSPR = 820 are the measured emission intensity for the gold nanorod

solutions (OD = 1) with LSP band at 790 and 820nm, respectively;1

N and2

N are the

numbers of gold nanorods in the excitation volume for solutions with LSP band at 820 nm and

790 nm, respectively. 2

1

N

N

is the concentration ratio mentioned in last section. Based on the

definition of Prin Eq. (4), we can integrate Eq. (3) into Eq. (4) and acquire a Prpredicted bythe local field model described above, wherePris



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4 2

1 2 790

4 2

1 2 820

( ) ( )

( ) ( )

LSPRr

LSPR

L LP

L L

=

=

= . (5)

According to Eq. (4), the Prcan also be experimentally obtained from the measured intensityand particle concentration ratio for each nanorod solution. The measured Pris compared to theone estimated by calculating the local field factor as described in Eq. (2) and Eq. (5).

3. Results and Discussion

3.1 The TPL efficiencies and spectra of gold nanorods with different aspect ratios

To verify the two photon origin of the observed luminescence, the TPL yield was measuredagainst the excitation power. A fit to the logarithm of the data reveals a slope of 2.05,indicative of a dominant two photon process. The relative TPL efficiency Prof the nanorodswith aspect ratios from 1.3 to 4.2 (equivalent to LSP modes from 540 to 820 nm) wasmeasured and is shown in Fig. 3. With the excitation wavelength set at 815 nm, it is notsurprising to see that the nanorod with LSP mode of 820 nm has the greatest TPL efficiency,due to its quadratic dependence on excitation power. For nonlinear processes such as TPL, theincoming field intensity plays a dominant role in PL efficiency; the largest enhancementhappens when the incoming light couples with the longitudinal surface plasmons of thenanorods and greatly increases the absorption cross section. Part of the excited surface

plasmons decay nonradiatively into electron-hole pairs via interband excitation [16]; thesubsequent relaxation and recombination of electron-hole pairs lead to the observed TPL. Theradiative recombination of electron-hole pairs may likewise excite surface plasmons andproduce a second enhancement, as the local plasmon effect would enhance both incoming andoutgoing electrical fields [2,3]. Equation (5) includes the enhancement of both the incidentand emission electric fields; the result is calculated and shown as the dotted line in Fig. 3. Theresult exhibits a larger TPL for the nanorods with LSPR at 680 and 740 nm, and the decreasein TPL is mild when the LSP mode varies from 790 to 680nm. However, we have consistentlymeasured a sharper TPL decrease in this range. In contrast, the calculation considering onlythe field enhancement of incident light gives a better estimation, as shown in Fig. 3. If asubstantial part of the emission is indeed enhanced by the support of surface plasmonexcitation, we would expect different spectrum shapes for nanorods with varied LSP modes.However, the observed spectra are quite similar in each case, with peaks at roughly the samepositions, as shown in Fig. 4. The observation suggests a likely weak coupling of the emission

to surface plasmons, or, if efficient coupling indeed occurs, the excited SPs may be subject tononradiative decay and thus are not detected as photons.



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Fig. 3. The measured and calculated TPL efficiency of gold nanorods with various aspect ratios(the equivalent LSP modes are at 540, 590, 680, 740, 790, and 820nm)

Fig. 4. The TPL spectra of gold nanorods with LSP modes at (i) 680nm, (ii) 740nm, (iii)790nm, (iv) 820nm, and (v) 930nm (Data is smoothed by averaging. Averaging interval: 3nm)

Although the gold nanorod spectrum in Fig. 4 is an ensemble average excited by far-fieldlight, it still bears much resemblance to the results obtained on a single nanorod. Imura et al.



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[17] have measured the spectrum of a single gold nanorod using near-field excitation, inwhich two peaks near 540 and 650 nm are found. The peak positions are close to the emissionpeaks of gold crystal, which is expected at 520 and 650 nm according to the calculated bandstructure. The emission peaks of gold crystal originate from the interband transitions near theX and L symmetry points of the first Brillouin zone due to the large density of states near thesymmetry points. The 650 and 540 nm peaks observed on a single nanorod are thus assignedto the electron-hole recombinations near the X and L symmetry points, respectively.

Similarly, we also observed peaks at 680 and 540 nm, which possibly represent the transitionat the X and L symmetry points. While the spectra in Fig. 4 look similar, the relativeintensities Ix/IL of these two spectral components vary for different nanorods. Imura et al.[17] reported a Ix/ILratio ranging from 0.5 to 2; they ascribed the variation of this ratio to thedifference in the plasmon modes excited. For nanorods with LSP modes at 680 to 930 nm, theIx/ILratio measured in our experiment is between 1 and 2, as listed in Table 1. The nanorodwith LSP mode at 680 nm has the highest Ix/IL ratio, 1.47, while the ratio for the nanorodwith LSP mode at 930 nm is the lowest. Apparently, the nanorods with LSP wavelengthsoverlapping the emission from the X region have higher Ix/IL ratios; this strongly suggeststhat the emission from X region is in resonance with the LSP of the nanorods.

Table 1. The Ix/ILratios for the TPL spectra of various gold nanorods

Gold nanorods with LSP mode at IX/ILRatio

680nm 1.47

740nm 1.29

790nm 1.27

820nm 1.21

930nm 1.12

Bouhelier et al. [18] have measured TPL spectra of relatively large gold nanorods; theTPL spectra they observed almost overlap with the scattering spectra, reflecting pronouncedsurface plasmon characteristics. Compared to their results, the TPL spectra observed in our

experiment do not exhibit prominent surface plasmon features. The discrepancy may arisefrom the different ways by which the excited surface plasmons (SPs) decay. The coupling ofTPL to SP can be large, but only part of the captured energy by SPs reradiates as photons,with part of the energy dissipated in the metal. The relative strength of these two processesdepends on the geometry of the nanoparticle, and the geometry is exactly the major differencebetween the nanorods used by Bouhelier et al. and those used by us. We used smallernanorods, with an axial length only 1/3 of that of their nanorods, and the nanorods producedby wet chemical methods may carry more shape irregularity compared to e-beam fabricatednanorods. As the dissipation dominates the plasmon decay for nanoparticles with small sizes,this is likely the reason why we do not see a clear surface plasmon feature in the emission.

3.2 The TPL efficiency of gold nanorods in medium with different refractive index

The sensitivity of TPL to the LSP mode of nanorods implies the possibility of sensing the

local refractive index change by TPL, as the ambient refractive index can modify the peakposition of the LSPR [12]. To verify this, 1 cc gold nanorod solutions were diluted with 1 ccDI water and 1 cc glycerol. The 50% glycerol solution had a refractive index of 1.4, whichcaused a red shift of about 20nm in the LSP wavelength of the gold nanorods, as shown inFig. 5(a). The TPL efficiency of the nanorods was measured at various wavelengths from 754nm to 860 nm. The TPL efficiency of the nanorods in the 50% glycerol solution is compared



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to that of the nanorods in 100% DI water, and the results are presented in Fig. 5(b). As theambient refractive changes from 1.33 to 1.40, depending on the choice of excitationwavelength, the TPL intensity can change up to 50%. If an event occurring near the nanorodinvolves drastic refractive index changes, then it can be detected simply by measuring theTPL intensity of the nanorod. According to Fig. 5(b), the excitation wavelength that producesan equal TPL efficiency is at ~815 nm. When excited at 815nm, the TPL spectra of goldnanorods in 100% DI water and 50% glycerol are nearly identical, as shown in Fig. 5(c). Note

that if we only consider the local field enhancement by surface plasmons, then the TPLefficiency should be roughly the same when the excitation wavelength is set at 800 nm.Apparently, the TPL intensity in 50% glycerol is slightly lower than expected. This can be dueto scattering losses of emitted light in a more dense medium like glycerol.

Fig. 5. (a)The absorption spectra for gold nanorods in water and 50% glycerol. (b) Thecomparison of TPL efficiency of gold nanorods in water and 50% glycerol when the excitationwavelength is varied. (c) The TPL spectra for gold nanorods in water and 50% glycerol almostoverlap each other when the excitation wavelength is set at 815nm (raw data is shown; no dataaverage is applied).

4. Conclusion and PerspectivesIn conclusion, we have measured the strong dependence of the TPL efficiency on the LSPmodes of nanorods. The enhanced TPL efficiency arises mostly from the enhanced absorptionby surface plasmons, which then increases the pumping rate of electron-hole pairs. Theplasmon-supported photoemission was observed, but the effects on the TPL efficiency werenot significant in our case. A possible explanation for the relatively weak plasmon-supported



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PL is the nonradiative dissipation of excited surface plasmons. We have also investigated thePL intensity change as the ambient refractive index changes from 1.33 to 1.40, and weobserved a large intensity change up to 50%, which is a quite an encouraging result. As somebiological events are generated by drastic ionic movement, which can induce a fluctuation inthe local refractive index, PL can be a promising tool to detect these events. An excellentexample for this type of applications is to monitor the fast membrane potential signals ofneurons; this requires a method to attach the nanoparticle directly to the cell membrane. Based

on our results, the biological use of gold nanorods is not limited to an imaging probe, but canbe simultaneously extended to monitor fast dynamic events, which are of great biologicalsignificance.

Acknowledgements

This work is financially supported by the Ministry of Education, Taiwan, R.O.C, under thegrant No.980061-01.



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