Letters

Single-particle studies of the plasmonic fluorescence in gold nanocubes

[+] Author Affiliations
Jane Huang, Pyng Yu, Chi-Tsu Yuan, Hsien-Chen Ko, Jau Tang

Academia Sinica, Research Center for Applied Sciences, 128 Academia Road, Section 2, Taipei, 115 Taiwan

Tao-Shih Hsieh

Academia Sinica, Institute of Cellular and Organismic Biology, 128 Academia Road, Section 2, Taipei, 115 Taiwan

J. Nanophoton. 6(1), 069502 (Jun 20, 2012). doi:10.1117/1.JNP.6.069502
History: Received October 8, 2011; Revised April 23, 2012; Accepted April 27, 2012
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Abstract.  Fluorescence of gold nanocubes is investigated down to the single-particle level using time-correlated single photon (SP) counting techniques with a combined confocal and atomic force microscopy system. In comparison with most fluorophores, gold nanocubes exhibit nonblinking and nonbleaching behavior at ambient environment with much faster fluorescence decay. Also, the photon statistics of single gold nanocubes follows a Poissonian distribution, unlike the SP emission characteristics of single semiconducting quantum dots. Therefore, from the single-particle perspective, the fluorescence of gold nanocubes is attributed to the radiative emission from excited localized surface plasmons.

Figures in this Article

Fluorophores, as biological labels, have always been a popular field of study.13 A suitable fluorescence label should have high quantum yield (QY), low cytotoxicity, and ability to attach to a specific location on the target molecule.4 Organic fluorescence molecules5 and colloidal semiconductor quantum dots (QDs) have attracted much attention in biological spectroscopy area due to its high QY. However, high toxicity, fast photobleaching, and complex blinking behavior have significantly limited their practical performance.68 Fluorescence from metallic bulk materials has been observed for more than 40 years; however, the low QY (1010) has degraded its potentials as fluorescence labels.9 A variety of metallic nanostructures can be fabricated on demand from the advanced development in chemical synthesis methods.1013 Metallic nanoparticles possess both optical scattering and fluorescence properties; in addition, they exhibit fast photon-heat energy conversion efficiency.14,15 Two major mechanisms have been proposed for fluorescence properties of metallic nanoparticles, plasmon enhanced interband recombination,16,17 and surface plasmon emission.18,19 Interband recombination is related to recombination of d-holes and sp-band electrons releasing photon radiatively. The lifetime of a few femtoseconds was reported for metallic nanoparticles for both mechanisms.20,21 High fluorescence QYs have been observed for small metallic nanoparticles (10100nm).16,22 The photoluminescence (PL) QY of gold nanorod is about 104, which is attributed to the strong surface plasmon resonance (SPR) enhancing the interband recombination.22 Recently, Wu and co-workers demonstrated that the PL QY of gold nanocubes reaches 4×102, about 200 times higher than that of nanorods. This large enhancement can be explained through the lightning rod effect and the spectra overlapping of SPR and PL band.23

Most of the previous reports have mainly focused on two-photon (TP) induced fluorescence from gold nanoparticles by infrared femtosecond (fs) laser. However, such nonlinear optical processes are more complex, and the study would require more expensive equipments. By contrast, single-photon (SP) induced fluorescence from gold nanostructures is easier to perform. In addition, the SP emission has no limitation on the power density, while TP emission has the high threshold and such high power would readily melt the nanoparticles. Furthermore, the QY of gold nanocubes with SP excitation is higher than TP excitation.23 In this paper, we study the fluorescent behavior and photon statistics of gold nanocubes on the single-particle level using time correlated SP counting (TCSPC) techniques.

Gold nanocubes were synthesized using the method reported by Sau and Murphy.24 For single gold nanocube experiments, the coverslip was thoroughly cleaned using Prinha solution and deionized water. Au nanocubes were diluted using acetone and dispersed onto coverslip for single nanocube measurement. Combined atomic force microscope (AFM) and confocal microscope system was used. Morphological measurements were carried out using an Asylum Research MFP-3D BIO AFM. The image was taken using Olympus AC 240TS cantiliver (resonance frequency 70 kHz, spring constant 2N/m). For confocal measurements, a pulsed diode laser at 467nm was used for excitation, which was focused to a diffraction limited spot through oil-immersion objective (Olympus, N.A.=1.4). The same objective was used to collect the fluorescence and guided through a confocal pinhole to eliminate the out-of-focus light. Then, the fluorescence was splited by a beam splitter cube into two beams and detected by a pair of SP counting module (PicoHarp 300, PicoQuant) to perform second-order correlation function.

An Olympus IX-71 inverted optical microscope with oil immersion dark-field condenser (Olympus U-DCW) and 50× objective was used for dark-field microscopy. The illumination light comes from the output of a 100 W halogen lamp. The Rayleigh scattering from a single nanoparticle was collected by the objective and directed to the entrance slit of an image spectrograph (Acton Research MicroSpec 2150i) equipped with an electron multiplying charge coupled device (EMCCD) camera thermoelectrically (TE)-cooled (Acton Princeton Instruments ProEm: 512BK). To choose the target particle, the scattering light was first imaged on the charge-coupled device (CCD) with a mirror, and then it is replaced by a grating (600grooves/mm, blaze wavelength 500 nm) to disperse the collected light. Generally, the acquisition time for the spectra was around 3 and 5 s.

Transmission electron microscope (JEOL, JEM-1200EX II) operating at 80 kV was used to take images of the synthesized nanoparticles. Ultraviolet (UV)-visible-infrared (IR) absorption spectra of gold nanocube solutions were measured by a spectrophotometer (JASCO V-670) with a light path of 10 mm.

The transmission electron microscope (TEM) image and size distribution of gold nanocubes are shown in Fig. 1(a). The edge length of gold nanocubes was determined to be 41.4±2.2nm after averaging over 100 nanocubes as presented in Fig. 1(b). A combined confocal microscope and AFM system was used to measure fluorescence properties while monitoring the morphology of the sample. Figure 1(c) shows a fluorescence imaging under excitation of 467nm pulsed laser. Each bright spot represents fluorescence emission from a single Au nanocube. Figure 1(d) is the AFM image obtained simultaneously with the fluorescence image to assure the fluorescence properties measured was from single gold nanocube. The edge length acquired from AFM is consistent with the result from TEM. Since Fig. 1(c) and 1(d) is closely matched, it allows us to validate our claim that the nonblinking behavior of nanocube is contributed by single gold nanocube instead of aggregation of many nanoparticles. Once a specific single nanocube was found, the fluorescence properties are probed through confocal microscopy and scattering spectrum based on dark-field microscopy.

Graphic Jump LocationF1 :

(a) TEM image of gold nanocubes. (b) The edge length distribution of gold nanocubes. (c) Confocal fluorescence image (15×15μm2) of gold nanocubes. (d) AFM image of gold nanocubes.

As shown in Fig. 2(a), the extinction of ensemble Au nanocubes (black) exhibits both interband absorption (400nm) and plasmon absorption (540nm). The fluorescence (cyan) and scattering (magenta) spectra of single Au nanocube are also presented in Fig. 2(a). Both the peaks of extinction and fluorescence spectrum are at 540 nm. Dulkeith et al.19 observed similar behavior indicating plasmonic nature of the PL. Also, as reported by Bouhelier et al.,18 the overlapping of the peaks for fluorescence and scattering indicates a strong relationship between PL and surface plasmon. In this study, fluorescence and scattering spectrum closely coincide with slight red-shift in scattering spectrum. The peak position of extinction and scattering spectrum are 540 and 560 nm, respectively. The slight red-shift of scattering spectrum could be explained by the change in the surrounding environment. According to Hu et al.,25 when in the water environment, large red-shift in localized surface plasmon resonance (LSPR) is observed; and it would return to its original position when the sample is dried.

Graphic Jump LocationF2 :

(a) Extinction, fluorescence, and scattering spectrum for single gold nanocube. (b) Typical fluorescence intensity time trajectory for single gold nanocubes on glass with 10 ms bin time. (c) Second-order autocorrelation function of single gold nanocubes showing absence of anti-bunching.

Over 50 Au nanocubes were observed and a typical fluorescence intensity time trajectory for a single Au nanocube is shown in Fig. 2(b). Unlike most fluorophores, Au nanocubes exhibit continuous nonblinking behavior. Many mechanisms were suggested for the blinking behavior of various nanomaterials and fluorescent molecules. For example, blinking of QDs is mainly caused by Auger recombination. And in the case of some single molecules, the main reason of blinking involves transitions between singlet and triplet states. Despite of the distinct mechanism, all blinking occurs when light emitters are at excited state and interacting with other electronic states or extra charge carriers.26,27 Since Au nanocubes exhibit nonblinking behavior, the fluorescence observed here is not associated with the radiative emission from electronic state transition. Furthermore, single nanocube fluorescence is very stable at ambient environments and no photoinduced bleaching was observed. With the advantages of nonblinking and nonbleaching, Au nanocubes demonstrate a greater potential as biological label.

To further explore Au nanocube emission, fluorescence decay profile was recorded by using time-correlated SP counting techniques. However, fluorescence lifetime is comparable or faster than our instrument response function of 300ps. The short fluorescence lifetime is considered reasonable since the lifetime of gold nanoparticle was determined to be 50fs.21 Such fluorescence lifetime is much shorter than tiny Au nanoclusters.28 Furthermore, in order to directly understand the photon statistics of Au emissions, we also performed photon correlation measurements based on Hanbury Brown Twiss experimental setup. A typical second-order autocorrelation function was shown in Fig. 2(c). The photon statistics followed Poissionian statistics instead of photon anti-bunching signature. The Poissionian statistics indicate such emission is classical and continuous emission opposing to discrete SP emission.

Based on the experimental observations on single Au nanocube, the fluorescence should be attributed to the radiative emission of the excitation of LSPR. The characteristics of LSPR emission are strongly related to the light source. For example, if a SP source was used to generate surface plasmon, the emission would behave like quantum light. Also, the photon anti-bunching signature would be preserved when such surface plasmons were converted into photons.29 On the contrary, in this study, a coherent laser was used to excite surface plasmons, the photon beam emitted would preserve the original plasmonic properties, including sub-nanosecond fluorescence lifetime, continuous emission without blinking and bleaching, and a Poissonian photon statistics. All the LSPR characteristics were observed in this study.

Emission of Au nanocubes possesses advantages of both plasmon, nonblinking and nonbleaching, and fluorescence, large Stoke shift. In this case, the plasmonic emission has fluorescence characteristics, which is excited at short-wavelength laser and detected at longer wavelength. Previously proposed mechanism for ensemble Au nanospheres can be used to explain the experimental observation here. When short-wavelength laser illuminated a single Au nanocube, d-band electron is promoted to sp conduction band. Then, the electron was efficiently relaxed to Fermi-levels via electron-electron or electron-photon scattering. In this case, the remaining energy is transferred to excitation of LSPR. Upon LSPR excitation, the energy damping could occur either through radiative or nonradiative processes. Radiative damping would occur when oscillated plasmon was converted into photons. Therefore, such emission behaved like the original pumping sources but with large Stoke shift.

High-yield florescence from metallic nanostructures is a fascinating property. Fluorescence from gold nanocubes was investigated down to single-particle level. The fluorescence exhibits nonblinking and nonbleaching behavior at ambient environment. In addition, extinction and single Au nanocube fluorescence spectra were measured and the peak positions of the spectra overlap. At the same time, scattering and fluorescence spectra of single Au nanocube also coincide with slightly red-shift in the scattering peak position. Moreover, the photon statistics follow a Poissonian distribution, which is very different from the SP emission characteristics of single QDs, which often exhibit anti-bunching behavior. From single-particle perspective, we suggest the fluorescence is attributed to plasmon emission.

We acknowledge Instrumental Center of National Taiwan University for TEM. J. Tang would like to thanks the support of the Academia Sinica and National Science Council of Taiwan under the program No. 99-2113-M-001-023-MY3.

This paper is a result of effort from many people. Jane Huang is responsible for synthesis and confocal measurements. Dr. Pyng Yu is responsible for the synthesis of the sample. Dr. Chi-Tsu Yuan has helped out a lot on the confocal measurements. Dr. Hsien-Chen Ko helped out with the AFM measurements, and Dr. Jau Tang and Dr. Tao-Shih Hsieh have supervised this project.

Pantazis  P. et al., “Second harmonic generating (SHG) nanoprobes for in vivo imaging,” Proc. Natl. Acad. Sci.. 107, (33 ), 14535 –14540 (2010), CrossRef. 0369-3236 
Tong  L. et al., “Bright three-photon luminescence from gold/silver alloyed nanostructures for bioimaging with negligible photothermal toxicity,” Angew. Chem.. 122, (20 ), 3563 –3566 (2010), CrossRef. 0044-8249 
Jun  Y. W. et al., “Continuous imaging of plasmon rulers in live cells reveals early-stage caspase-3 activation at the single-molecule level,” Proc. Natl. Acad. Sci.. 106, (42 ), 17735 –17740 (2009), CrossRef. 0369-3236 
Giljohann  D. A. et al., “Gold nanoparticles for biology and medicine,” Angew. Chem.. 49, (19 ), 3280 –3294 (2010), CrossRef. 0044-8249 
Waggoner  A., “Covalent labeling of proteins and nucleic acids with fluorophores,” Methods Enzymol.. 246, , 362 –373 (1995), CrossRef. 0076-6879 
Bruchez  M.  Jr. et al., “Semiconductor nanocrystals as fluorescent biological labels,” Science. 281, (5385 ), 2013 –2016 (1998), CrossRef. 0036-8075 
Dahan  M. et al., “Diffusion dynamics of glycine receptors revealed by single-quantum dot tracking,” Science. 302, (5644 ), 442 –445 (2003), CrossRef. 0036-8075 
Cichos  F., von Borczyskowski  C., Orrit  M., “Power-law intermittency of single emitters,” Curr. Opin. Colloid Interf. Sci.. 12, (6 ), 272 –284 (2007), CrossRef. 1359-0294 
Mooradian  A., “Photoluminescence of metals,” Phys. Rev. Lett.. 22, (5 ), 185 –187 (1969), CrossRef. 0031-9007 
Alkilany  A. M., Murphy  C. J., “Gold nanoparticles with a polymerizable surfactant bilayer: synthesis, polymerization, and stability evaluation,” Langmuir. 25, (24 ), 13874 –13879 (2009), CrossRef. 0743-7463 
Metraux  G. S., Mirkin  C. A., “Rapid thermal synthesis of silver nanoprisms with chemically tailorable thickness,” Adv. Mater.. 17, (4 ), 412 –415 (2005), 10.1002/adma.200401086. 0935-9648 
Nikoobakht  B., El-Sayed  M. A., “Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method,” Chem. Mater.. 15, (10 ), 1957 –1962 (2003), CrossRef. 0897-4756 
Skrabalak  S. E. et al., “Gold nanocages: synthesis, properties, and applications,” Acc. Chem. Res.. 41, (12 ), 1587 –1595 (2008), CrossRef. 0001-4842 
Hartland  G. V., “Coherent excitation of vibrational modes in metallic nanoparticles,” Ann. Rev. Phys. Chem.. 57, , 403 –430 (2006), CrossRef. 0066-426X 
Tai  P. T. et al., “Selective acoustic phonon mode excitation of multi-mode silver nanoprisms,” Chem. Phys. Lett.. 496, (4–6 ), 326 –329 (2010), CrossRef. 0009-2614 
Mohamed  M. B. et al., “The ‘Lightning’ gold nanorods: fluorescence enhancement of over a million compared to the gold metal,” Chem. Phys. Lett.. 317, (6 ), 517 –523 (2000), CrossRef. 0009-2614 
Wang  H. et al., “In vitro and in vivo two-photon luminescence imaging of single gold nanorods,” Proc. Natl. Acad. Sci.. 102, (44 ), 15752 –15756 (2005), CrossRef. 0369-3236 
Bouhelier  A. et al., “Surface plasmon characteristics of tunable photoluminescence in single gold nanorods,” Phys. Rev. Lett.. 95, (26 ), 267405  (2005), CrossRef. 0031-9007 
Dulkeith  E. et al., “Plasmon emission in photoexcited gold nanoparticles,” Phys. Rev. B. 70, (20 ), 205424  (2004), CrossRef. 1098-0121 
Sönnichsen  C. et al., “Drastic reduction of plasmon damping in gold nanorods,” Phys. Rev. Lett.. 88, (7 ), 077402-1-4  (2002), CrossRef. 0031-9007 
Varnavski  O. P. et al., “Relative enhancement of ultrafast emission in gold nanorods,” J. Phys. Chem. B. 107, (14 ), 3101 –3104 (2003), CrossRef. 1520-6106 
Eustis  S., El-Sayed  M. A., “Aspect ratio dependence of the enhanced fluorescence intensity of gold nanorods: experimental and simulation study,” J. Phys. Chem.. 109, (34 ), 16350 –16356 (2005), CrossRef. 0022-3654 
Wu  X. et al., “High-photoluminescence-yield gold nanocubes: for cell imaging and photothermal therapy,” ACS Nano. 4, (1 ), 113 –120 (2010), CrossRef. 1936-0851 
Sau  T. K., Murphy  C. J., “Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution,” J. Am. Chem. Soc.. 126, (28 ), 8648 –8649 (2004), CrossRef. 0002-7863 
Hu  M. et al., “Dark-field microscopy studies of single metal nanoparticles: understanding the factors that influence the linewidth of the localized surface plasmon resonance,” J. Mater. Chem.. 18, , 1949 –1960 (2008), CrossRef. 0959-9428 
Tang  J., Marcus  R. A., “Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles,” Phys. Rev. Lett.. 95, (10 ), 107401  (2005), CrossRef. 0031-9007 
Tang  J., Marcus  R. A., “Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots,” J. Chem. Phys.. 123, (5 ), 054704  (2005), CrossRef. 0021-9606 
Zheng  J., Zhang  C., Dickson  R. M., “Highly fluorescent, water-soluable, size-tunable gold quantum dots,” Phys. Rev. Lett.. 93, (7 ), 077402  (2004), CrossRef. 0031-9007 
Akimov  A. V. et al., “Generation of single optical plasmons in metallic nanowires coupled to quantum dots,” Nature. 450, , 402 –406 (2007), CrossRef. 0028-0836 
© 2012 Society of Photo-Optical Instrumentation Engineers

Citation

Jane Huang ; Pyng Yu ; Chi-Tsu Yuan ; Hsien-Chen Ko ; Jau Tang, et al.
"Single-particle studies of the plasmonic fluorescence in gold nanocubes", J. Nanophoton. 6(1), 069502 (Jun 20, 2012). ; http://dx.doi.org/10.1117/1.JNP.6.069502


Figures

Graphic Jump LocationF2 :

(a) Extinction, fluorescence, and scattering spectrum for single gold nanocube. (b) Typical fluorescence intensity time trajectory for single gold nanocubes on glass with 10 ms bin time. (c) Second-order autocorrelation function of single gold nanocubes showing absence of anti-bunching.

Graphic Jump LocationF1 :

(a) TEM image of gold nanocubes. (b) The edge length distribution of gold nanocubes. (c) Confocal fluorescence image (15×15μm2) of gold nanocubes. (d) AFM image of gold nanocubes.

Tables

References

Pantazis  P. et al., “Second harmonic generating (SHG) nanoprobes for in vivo imaging,” Proc. Natl. Acad. Sci.. 107, (33 ), 14535 –14540 (2010), CrossRef. 0369-3236 
Tong  L. et al., “Bright three-photon luminescence from gold/silver alloyed nanostructures for bioimaging with negligible photothermal toxicity,” Angew. Chem.. 122, (20 ), 3563 –3566 (2010), CrossRef. 0044-8249 
Jun  Y. W. et al., “Continuous imaging of plasmon rulers in live cells reveals early-stage caspase-3 activation at the single-molecule level,” Proc. Natl. Acad. Sci.. 106, (42 ), 17735 –17740 (2009), CrossRef. 0369-3236 
Giljohann  D. A. et al., “Gold nanoparticles for biology and medicine,” Angew. Chem.. 49, (19 ), 3280 –3294 (2010), CrossRef. 0044-8249 
Waggoner  A., “Covalent labeling of proteins and nucleic acids with fluorophores,” Methods Enzymol.. 246, , 362 –373 (1995), CrossRef. 0076-6879 
Bruchez  M.  Jr. et al., “Semiconductor nanocrystals as fluorescent biological labels,” Science. 281, (5385 ), 2013 –2016 (1998), CrossRef. 0036-8075 
Dahan  M. et al., “Diffusion dynamics of glycine receptors revealed by single-quantum dot tracking,” Science. 302, (5644 ), 442 –445 (2003), CrossRef. 0036-8075 
Cichos  F., von Borczyskowski  C., Orrit  M., “Power-law intermittency of single emitters,” Curr. Opin. Colloid Interf. Sci.. 12, (6 ), 272 –284 (2007), CrossRef. 1359-0294 
Mooradian  A., “Photoluminescence of metals,” Phys. Rev. Lett.. 22, (5 ), 185 –187 (1969), CrossRef. 0031-9007 
Alkilany  A. M., Murphy  C. J., “Gold nanoparticles with a polymerizable surfactant bilayer: synthesis, polymerization, and stability evaluation,” Langmuir. 25, (24 ), 13874 –13879 (2009), CrossRef. 0743-7463 
Metraux  G. S., Mirkin  C. A., “Rapid thermal synthesis of silver nanoprisms with chemically tailorable thickness,” Adv. Mater.. 17, (4 ), 412 –415 (2005), 10.1002/adma.200401086. 0935-9648 
Nikoobakht  B., El-Sayed  M. A., “Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method,” Chem. Mater.. 15, (10 ), 1957 –1962 (2003), CrossRef. 0897-4756 
Skrabalak  S. E. et al., “Gold nanocages: synthesis, properties, and applications,” Acc. Chem. Res.. 41, (12 ), 1587 –1595 (2008), CrossRef. 0001-4842 
Hartland  G. V., “Coherent excitation of vibrational modes in metallic nanoparticles,” Ann. Rev. Phys. Chem.. 57, , 403 –430 (2006), CrossRef. 0066-426X 
Tai  P. T. et al., “Selective acoustic phonon mode excitation of multi-mode silver nanoprisms,” Chem. Phys. Lett.. 496, (4–6 ), 326 –329 (2010), CrossRef. 0009-2614 
Mohamed  M. B. et al., “The ‘Lightning’ gold nanorods: fluorescence enhancement of over a million compared to the gold metal,” Chem. Phys. Lett.. 317, (6 ), 517 –523 (2000), CrossRef. 0009-2614 
Wang  H. et al., “In vitro and in vivo two-photon luminescence imaging of single gold nanorods,” Proc. Natl. Acad. Sci.. 102, (44 ), 15752 –15756 (2005), CrossRef. 0369-3236 
Bouhelier  A. et al., “Surface plasmon characteristics of tunable photoluminescence in single gold nanorods,” Phys. Rev. Lett.. 95, (26 ), 267405  (2005), CrossRef. 0031-9007 
Dulkeith  E. et al., “Plasmon emission in photoexcited gold nanoparticles,” Phys. Rev. B. 70, (20 ), 205424  (2004), CrossRef. 1098-0121 
Sönnichsen  C. et al., “Drastic reduction of plasmon damping in gold nanorods,” Phys. Rev. Lett.. 88, (7 ), 077402-1-4  (2002), CrossRef. 0031-9007 
Varnavski  O. P. et al., “Relative enhancement of ultrafast emission in gold nanorods,” J. Phys. Chem. B. 107, (14 ), 3101 –3104 (2003), CrossRef. 1520-6106 
Eustis  S., El-Sayed  M. A., “Aspect ratio dependence of the enhanced fluorescence intensity of gold nanorods: experimental and simulation study,” J. Phys. Chem.. 109, (34 ), 16350 –16356 (2005), CrossRef. 0022-3654 
Wu  X. et al., “High-photoluminescence-yield gold nanocubes: for cell imaging and photothermal therapy,” ACS Nano. 4, (1 ), 113 –120 (2010), CrossRef. 1936-0851 
Sau  T. K., Murphy  C. J., “Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution,” J. Am. Chem. Soc.. 126, (28 ), 8648 –8649 (2004), CrossRef. 0002-7863 
Hu  M. et al., “Dark-field microscopy studies of single metal nanoparticles: understanding the factors that influence the linewidth of the localized surface plasmon resonance,” J. Mater. Chem.. 18, , 1949 –1960 (2008), CrossRef. 0959-9428 
Tang  J., Marcus  R. A., “Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles,” Phys. Rev. Lett.. 95, (10 ), 107401  (2005), CrossRef. 0031-9007 
Tang  J., Marcus  R. A., “Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots,” J. Chem. Phys.. 123, (5 ), 054704  (2005), CrossRef. 0021-9606 
Zheng  J., Zhang  C., Dickson  R. M., “Highly fluorescent, water-soluable, size-tunable gold quantum dots,” Phys. Rev. Lett.. 93, (7 ), 077402  (2004), CrossRef. 0031-9007 
Akimov  A. V. et al., “Generation of single optical plasmons in metallic nanowires coupled to quantum dots,” Nature. 450, , 402 –406 (2007), CrossRef. 0028-0836 

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