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Virtual Journal for Biomedical Optics

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  • Editor: Gregory W. Faris
  • Vol. 5, Iss. 10 — Jul. 19, 2010
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Average enhancement factor of molecules-doped coreshell (Ag@SiO2) on fluorescence

Jiunn-Woei Liaw, Chuan-Li Liu, Wei-Min Tu, Chieh-Sheng Sun, and Mao-Kuen Kuo  »View Author Affiliations


Optics Express, Vol. 18, Issue 12, pp. 12788-12797 (2010)
http://dx.doi.org/10.1364/OE.18.012788


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Abstract

Average enhancement factor (AEF) of a coreshell (Ag@SiO2) on the fluorescence of molecules doped within the silica shell is proposed and studied to estimate the overall performance of a large number of coreshells. Using Mie theory and dyadic Green’s functions, the enhancement factor (EF) of a coreshell is first calculated for any arbitrarily oriented and located electric dipole embedded in the shell. AEF is then obtained by averaging the individual EF over all possible orientations and positions of the electric dipoles. AEF of a FITC-doped coreshell (radius of Ag core: 25 nm, thickness of shell: 15 nm) irradiated by a laser of 488 nm for FITC’s emission at 518 nm is 2.406. It is much smaller than the maximum EF (30.114) of a coreshell containing a single molecule with a radial orientation at its optimal position. For Alexa 430-doped coreshell excited at 428 nm, AEF is 12.34 at the emission of 538 nm.

© 2010 OSA

1. Introduction

Recently, numerous metallic nanostructures have been developed for the applications of surface-enhanced fluorescence (SEF) [1

1. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]

8

8. F. Tam, G. P. Goodrich, B. R. Johnson, and N. J. Halas, “Plasmonic enhancement of molecular fluorescence,” Nano Lett. 7(2), 496–501 (2007). [CrossRef] [PubMed]

] (or called metal-enhanced fluorescence [9

9. J. R. Lakowicz, “Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission,” Anal. Biochem. 337(2), 171–194 (2005). [CrossRef] [PubMed]

]) and surface-enhanced Raman spectroscopy (SERS) [10

10. W. Wang, Z. Li, B. Gu, Z. Zhang, and H. Xu, “Ag@SiO2 core-shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering,” ACS Nano 3(11), 3493–3496 (2009). [CrossRef] [PubMed]

,11

11. C. Fernandez-Lopez, C. Mateo-Mateo, R. A. Alvarez-Puebla, J. Perez-Juste, I. Pastoriza-Santos, and L. M. Liz-Marzan, “Highly controlled silica coating of PEG-capped metal nanoparticles and preparation of SERS-encoded particles,” Langmuir 25(24), 13894–13899 (2009). [CrossRef] [PubMed]

] by utilizing the surface plasmon resonance (SPR) effect of metals. Several studies showed that the enhancement factor (EF) of a metallic nanoparticle (MNP) on the fluorescence of a single molecule depends on the orientation and the location of the molecule relative to the MNP, the polarization of the incident light [12

12. T. Härtling, P. Reichenbach, and L. M. Eng, “Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle,” Opt. Express 15(20), 12806–12817 (2007). [CrossRef] [PubMed]

], and the distance from the molecule to the MNP [13

13. M. H. Chowdhury, S. K. Gray, J. Pond, C. D. Geddes, K. Aslan, and J. R. Lakowicz, “Computational study of fluorescence scattering by silver nanoparticles,” J. Opt. Soc. Am. B 24(9), 2259–2267 (2007). [CrossRef] [PubMed]

,14

14. G. Colas des Francs, A. Bouhelier, E. Finot, J. C. Weeber, A. Dereux, C. Girard, and E. Dujardin, “Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes,” Opt. Express 16(22), 17654–17666 (2008). [CrossRef] [PubMed]

]. Moreover, there are several other factors affecting EF, e.g. the size of the MNP [15

15. O. Stranik, R. Nooney, C. McDonagh, and B. D. MacCraith, “Optimization of nanoparticle size for plasmonic enhancement of fluorescence,” Plasmonics 2(1), 15–22 (2007). [CrossRef]

] and the overlap of the absorption and the emission spectra of the molecule with the SPR band of the metal [3

3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

] etc. On the other hand, there are a few experimental researches focused on the overall performance (enhancement or quenching) of a large number of MNPs on a large amount of molecules [3

3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

,6

6. D. Cheng and Q.-H. Xu, “Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles,” Chem. Commun. (Camb.) 2007(3), 248–250 (2007). [CrossRef]

], rather than a single MNP on a single molecule. In order to confine the molecules in the proximity of a MNP with a preciously controlled location, coreshell (Au@SiO2 and Ag@SiO2) structures [3

3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

6

6. D. Cheng and Q.-H. Xu, “Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles,” Chem. Commun. (Camb.) 2007(3), 248–250 (2007). [CrossRef]

] were then developed, where metal cores were coated by molecules-doped silica shells. Sometimes, a silica spacer, in between the molecules-doped shell and the metal core, is used to separate the molecules and the metal in order to avoid the quenching effect, which is due to a too short distance resulting in a strong energy transfer.

In order to identify the EF of a large number of molecules-doped coreshells, we propose an idea of average enhancement factor (AEF) by theoretically considering the effects of all possible orientations and locations of these molecules in the shell. The theoretical work is based on the analytical solutions of Mie theory of a plane wave [10

10. W. Wang, Z. Li, B. Gu, Z. Zhang, and H. Xu, “Ag@SiO2 core-shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering,” ACS Nano 3(11), 3493–3496 (2009). [CrossRef] [PubMed]

] interacting with a spherical coreshell, as well as dyadic Green’s functions [16

16. C.-T. Tai, Dyadic Green's Functions in Electromagnetic Theory (IEEE, 1971).

,17

17. X.-W. Chen, W. C.-H. Choy, S. He, and P. C. Chui, “Highly efficient fluorescence of a fluorescing nanoparticle with a silver shell,” Opt. Express 15(11), 7083–7094 (2007). [CrossRef] [PubMed]

] of an electric dipole (embedded in the shell) interacting with the coreshell. Using these solutions, the individual EF of a coreshell with any arbitrarily oriented and located molecule is calculated. The AEF is then obtained by averaging the individual EF over all possible orientations and positions of the molecules.

2. Theory

Figure 1
Fig. 1 Configuration of a coreshell (Ag@SiO2) containing a molecule (with an arbitrarily oriented dipole moment) embedded in the silica shell, irradiated by a x-polarized plane wave. The radii of the Ag core and the coreshell, are denoted bya2 and a1, respectively. There are four typical embedded positions (A, B, C and D, they are on x, −z, y and z axes, respectively) of a single molecule in the middle of the silica shell.
shows the configuration of a molecule (an electric dipole) embedded in the silica shell of a coreshell irradiated by an incident polarized EM wave. The radii of the Ag core and the coreshell are denoted bya2anda1, respectively, and the thickness of the silica shell is ts=a1a2. Without loss of generality, the incident wave is assumed to be x-polarized and propagate in the z-direction. The origin of the coordinate system locates at the center of the coreshell. The multi-scatterings of light among multi-coreshells are neglected for a dilute colloid. In addition, the interaction of multi-molecular fluorescence is also neglected.

2.1 Excitation rate

When an incident plane wave illuminates a spherical coreshell, a strong local electric field is induced in the proximity. Since the molecules are doped within the silica shell, we use the Mie theory [10

10. W. Wang, Z. Li, B. Gu, Z. Zhang, and H. Xu, “Ag@SiO2 core-shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering,” ACS Nano 3(11), 3493–3496 (2009). [CrossRef] [PubMed]

,17

17. X.-W. Chen, W. C.-H. Choy, S. He, and P. C. Chui, “Highly efficient fluorescence of a fluorescing nanoparticle with a silver shell,” Opt. Express 15(11), 7083–7094 (2007). [CrossRef] [PubMed]

] of a multi-layered sphere to calculate the electric field in the shell. The intensified field can raise the probability for exciting molecules in the near field. The excitation rate for a molecule at a specific excitation wavelengthλexis defined as|E(xd;λex)ep|2/|Ei|2, where xd and ep are the position vector of the molecule and the unit vector of the molecular dipole moment, respectively, and the denominator is the intensity of the incident wave.

2.2 Apparent quantum yield

Once the molecule is excited, it behaves as an oscillating electric dipole for the following emissions. Since the dipole is in the proximity of a MNP, its radiative and nonradiative decay rates will be affected dramatically by the SPR of the MNP. The dipole moment vector ep can always be decomposed into its radial (normal) and tangential components. In order to systematically analyze the field induced by an arbitrarily oriented dipole interacting with a coreshell, two sets of dyadic Green’s functions are analytically derived by using the method of Ref [16

16. C.-T. Tai, Dyadic Green's Functions in Electromagnetic Theory (IEEE, 1971).

]; they are for a unit radial dipole and a unit tangential dipole, respectively, located within the shell. The EM fields induced by these two types of dipoles are in series forms of spherical wave functions at an emission wavelengthλem. The total EM fields generated by an arbitrarily oriented dipole in the presence of the coreshell are then the linear combinations of the EM fields of these two sets of analytical dyadic Green’s functions. With these EM fields at hand, the radiative decay rate Pr and the nonradiative decay rate Pnr of the dipole in the presence of the coreshell can be computed by evaluating the corresponding surface integrals of the EM field [18

18. J.-W. Liaw, J. H. Chen, C. S. Chen, and M.-K. Kuo, “Purcell effect of nanoshell dimer on single molecule’s fluorescence,” Opt. Express 17(16), 13532–13540 (2009). [CrossRef] [PubMed]

,19

19. J.-W. Liaw, J.-H. Chen, and C.-S. Chen, “Enhancement or quenching effect of metallic nanodimer on spontaneous emission,” J. Quant. Spectrosc. Radiat. Transf. 111(3), 454–465 (2010). [CrossRef]

]. Furthermore, the apparent quantum yield η, which is a function of xd, ep, and the emission wavelengthλem, is defined as η(ep,xd;λem)=Pr/(Pr+Pnr).

2.3 Enhancement factor

To assess the overall effect of a coreshell on the molecular fluorescence, enhancement factor (EF), α, is defined as the product of the excitation rate and the apparent quantum yield [1

1. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]

,14

14. G. Colas des Francs, A. Bouhelier, E. Finot, J. C. Weeber, A. Dereux, C. Girard, and E. Dujardin, “Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes,” Opt. Express 16(22), 17654–17666 (2008). [CrossRef] [PubMed]

]
α(ep,xd;λex,λem)=|E(xd;λex)ep|2|Ei|2η(epxd;λem)
(1)
The mean enhancement factor (MEF), αM, for a molecule locating at xd is defined as the average of EFs over all possible orientations ep
αM(xd;λec,λem)=14π04πα(ep,xd;λex,λem)dΩ
(2)
where Ω’ is the solid angle of ep. We can also use Eq. (2) to estimate the MEF of the surrounding medium on the fluorescence of a single free molecule stimulated by a polarized light in the absence of the coreshell. Since the apparent quantum yield is unit one, for the case without coreshell, its MEF is then 1/3.

The effective enhancement factor (EEF), αE, is defined as the ratio of the MEF of a coreshell in the surrounding medium to the MEF of the medium without the coreshell, in order to take further into account the relative effect of a surrounding medium with and without a coreshell on a single molecular fluorescence. Therefore the EEF of a coreshell on the fluorescence at position xd excited by a polarized light of wavelength λex and emitting a fluorescence of wavelengthλem, is αE(xd;λex,λem)=3αM(xd;λex,λem).

If the molecules are uniformly distributed on the surface of a spherical layer with a distance d to the spherical core, the average enhancement factor (AEF), α^A, of a coreshell containing an infinitely thin molecules-layer in the shell with a constant distance d away from the metal core is defined as
α^A(a2+d;λex,λem)=14π04παE(a2+d,   Ω;λex,λem)dΩ
(3)
where Ω is the solid angle of the position vector of the molecule, xd, with the distance r=a2+dto the center of the coreshell. Furthermore, if these molecules are uniformly distributed over the whole shell, the AEF, αA, of the coreshell with a molecules-doped layer of a finite thickness is further defined as
αA(λex,λem)=4πV0ria1α^A(r  ;λex,λem)r2dr
(4)
In Eq. (4), the integral is integrated from the inner radius of the molecules-doped layerri=a2+d0 to the outer radius of the layera1, and V0=4π(a13ri3)/3. Here d0 is the thickness of the inner spacer to keep the molecules away from the metal core. If there is no spacer, thend0=0, and ri=a2. It is worthwhile to mention that, for certain isotropic emitters (e.g. quantum dots [20

20. Y. Jin and X. Gao, “Plasmonic fluorescent quantum dots,” Nat. Nanotechnol. 4(9), 571–576 (2009). [CrossRef] [PubMed]

]; QDs), there is no orientation dependence for the excitation rate. Hence, Eqs. (1) and (2) need to be modified for this type of isotropic emitters, and the AEF can still be applied to evaluate the overall performance of QDs-doped coreshell structure.

3. Numerical results and discussion

Several studies have shown that the EF of Ag@SiO2 on a single molecule depends on several conditions [3

3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

]. On one hand, it is sensitive not only to the molecular location with respect to the coreshell and the incident direction of the illuminating light, but also to the dipole’s orientation relative to the polarization of the illuminating light. On the other hand, the EF of Ag@SiO2 is also dependent on the excitation and the emission spectra of the fluorescent molecule with respect to the SPR spectrum of Ag NP. In order to illustrate the frequency-dependence of AEF of Ag@SiO2 on the molecular fluorescence, we will discuss cases with and without Stokes shift, separately. In the following calculation, the coreshell is assumed to be submerged in a surrounding medium (water), and the frequency-dependent permittivity of silver of Ref [21

21. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

]. is adopted.

3.1 Non-Stokes shift

In this section, we assume there is no Stokes shift between the excitation and the emission spectra of molecules; i.e. λem=λex. Consider a typical coreshell (a2=25  nm,   ts=15  nm) containing a single molecule embedded in the middle of the silica shell. Four typical molecule positions (A, B, C and D) are considered, which locate on x, −z, y and z-axes, respectively, but have the same distance (d = 7.5 nm) to the surface of the Ag core, as shown in Fig. 1. When the coreshell is irradiated by a x-polarized light of wavelength λex=420  nm propagating in the z-direction, the 3D profile of the EF for an electric dipole versus dipole orientations at the position A is depicted in a spherical plot as shown in Fig. 2(a)
Fig. 2 (a) Spherical plot of the 3D profile of EF versus dipole orientations at λex=λem=420    nm for a coreshell (a2=25  nm,   ts=15  nm) containing a molecule located at point A, where EEF is 55.052. (b) x-z plane, (c) y-z plane, and (d) x-y plane cross sections of (a).
, where the EEF, αE(xd;λex,λem), is 55. The x-z, y-z, and x-y plane cross sections of this profile are shown in Figs. 2(b), 2(c), and 2(d), respectively. In this dumbbell-shape profile, the maximum EF of the position A occurs when the dipole orients itself along the x-axis, parallel to the polarization of the incident light.

To illustrate that the EEF is sensitive to the relative position, let us consider molecules located at a fix distance (d = 7.5 nm) to the surface of the Ag core. The 3D profile of the EEF versus the angles of molecular positions at λex=λem=420    nmis depicted in a spherical plot as shown in Fig. 3(a)
Fig. 3 (a) Spherical plot of the 3D profile of EEF versus molecule positions for a coreshell (a2=25  nm,   ts=15  nm) atλex=λem=420    nm, where molecules locate at the middle layer of the shell (d = 7.5 nm). AEF is 23.485. (b) x-z plane, (c) y-z plane, and (d) x-y plane cross sections of (a). EEF = 56.55 (A′), 55.052 (A), 8.819(B), 7.512(C), 6.281(D). In the 3D profile, the maximum EEF occurs at the point A′, and the minimum EEF at the point D.
. The x-z, y-z, and x-y plane cross sections of this profile are shown in Figs. 3(b), 3(c), and 3(d), respectively. The x-y plane cross section is the distribution of the EEF along the equator of the coreshell. Both positions A and C are then on the equator. As shown in Fig. 3(d), however, the EEF (7.512) of the position C is much less than EEF (55.052) of the position A. This is due to the fact that the incident wave is x-polarized, so the SPR oscillation makes the electric field at the position A much stronger than that at the position C. Figures 3(a) and 3(c) indicate also that the minimum EEF (6.281) occurs at the position D. Notice that the maximum EEF (56.55) occurs at the position A′ making an angle 10° from the position A, instead of right at the position A, as shown in Fig. 3(b). This finding is in agreement with Ref [12

12. T. Härtling, P. Reichenbach, and L. M. Eng, “Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle,” Opt. Express 15(20), 12806–12817 (2007). [CrossRef] [PubMed]

]. According to Eq. (3), the AEF of Fig. 3(a) is 23.485 for d = 7.5 nm at λex=420  nm.

Figure 4
Fig. 4 AEF versus distances d between the molecules-layer and the Ag core of a coreshell (a2=25  nm,   ts=15  nm) atλex=λem=420    nm. The maximum AEF (34.37) occurs at d = 4 nm. The AEF of the entire shell is 18.78.
shows the AEF, α^A, versus d which is the distance between the molecules-doped layer and the Ag core of a coreshell (a2=25  nm,   ts=15  nm) at λex=λem=420    nm. The AEF of the entire shell is 18.78, according to Eq. (4), for the case ofd0=0. Figure 4 illustrates that the maximum AEF (34.37) occurs at d = 4 nm; while when d < 1 nm, the AEF is less than 1. It suggests that a quenching phenomenon [22

22. E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar, J. Feldmann, S. A. Levi, F. C. J. M. van Veggel, D. N. Reinhoudt, M. Möller, and D. I. Gittins, “Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects,” Phys. Rev. Lett. 89(20), 203002 (2002). [CrossRef] [PubMed]

] will be caused for those molecules in this region. It also indicates that, for a fixed layer thickness, using a proper spacer [3

3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

,6

6. D. Cheng and Q.-H. Xu, “Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles,” Chem. Commun. (Camb.) 2007(3), 248–250 (2007). [CrossRef]

] in between the molecule-doped silica layer and the Ag core (say d0=1  nm) will help to raise the AEF of Ag@SiO2.

For simplicity, in the following discussion we will focus only on cases without spacer, i.e d0=0. Figure 5(a)
Fig. 5 (a) AEF versus wavelengths for a coreshell of a2=25  nmwith different shell thicknessests. (b) AEF versus wavelengths for a coreshell of ts=15  nmwith different core radiia2.
shows the AEF, αA, versus wavelengths for a coreshell of a2=25  nm with different shell thicknesses ts calculated by using Eq. (4). The results indicate that the optimal thickness of the silica shell is about 10 nm to have maximum AEF, which agrees with the results of Ref [4

4. K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Fluorescent core-shell Ag@SiO2 nanocomposites for metal-enhanced fluorescence and single nanoparticle sensing platforms,” J. Am. Chem. Soc. 129(6), 1524–1525 (2007). [CrossRef] [PubMed]

,5

5. K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Metal enhanced fluorescence solution-based sensing platform 2: fluorescent core-shell Ag@SiO2 nanoballs,” J. Fluoresc. 17(2), 127–131 (2007). [CrossRef] [PubMed]

]. The size effect of the Ag core on the AEF versus wavelengths is shown in Fig. 5(b), for a coreshell of ts=15  nmwith different core radiia2. Figure 5(b) indicates the smaller the Ag core, the narrower the spectrum of AEF is. Moreover, there is an optimala2, about 20 nm, for obtaining a narrowband (400-430 nm) AEF with a maximum peak at 420 nm. On the other hand, if the radius of the Ag core is larger than 40 nm, the AEF spectrum of coreshell is partitioned into two different bands: a narrow shorter-wavelength band (shorter than the SPR band of an Ag NP) and a broad longer-wavelength band. However, for a larger Ag@SiO2, the values of AEF in both bands become much smaller, as shown in Fig. 5(b). Therefore, from the aspect of AEF, the wavelength-selective property of a molecules-doped Ag@SiO2 highly depends on the size of the Ag core.

3.2 Stokes shift

Since every specific type of molecule has its own unique excitation and emission spectra, the Stokes-shift effect of these two spectra on the AEF needs to be further identified. For simplicity, again, we focus only on cases without spacer, i.e. d0=0, ri=a2. Using Eq. (4), the AEF versus the emission wavelengths λem for a coreshell ofa2=25  nmand ts=15  nm irradiated by several different excitation-wavelength lasers (λex = 405, 428, 458, 488, 561, and 633 nm) are shown in Fig. 6
Fig. 6 AEF versus emission wavelengthsλemfor a molecules-doped Ag@SiO2 of a2=25  nm,   ts=15  nmexcited by different lasers (λex = 405, 428, 458, 488, 561, and 633 nm).
. It is easily to see that, the AEF of λex=428  nm is much higher than the others. This is due to the fact that λex = 428 nm is near the peak of SPR (420 nm) of the Ag core. Therefore, Ag@SiO2 has a high selectivity for enhancing molecular fluorescence. For example, for the case of Alexa 430 dye-doped Ag@SiO2, the AEF is 12.34 atλex=428  nm, λem=538  nm, whereas for the case of CYe-doped one, the AEF is 5.046 atλex=405nm, λem=550nm. Notice that the AEF is much lower than the maximum EF of a coreshell containing a single molecule with a radial oriented dipole moment at its optimal position; e.g. for FITC-doped one, the AEF is only 2.406 atλex=488  nm,   λem=518  nm, but the maximum EF is 30.114 at the position A′.

To verify our model, the experimental data of Ag-Au@SiO2 [15

15. O. Stranik, R. Nooney, C. McDonagh, and B. D. MacCraith, “Optimization of nanoparticle size for plasmonic enhancement of fluorescence,” Plasmonics 2(1), 15–22 (2007). [CrossRef]

] on the fluorescence of a specific molecule, Ru(dpp)32+, is adopted for comparison. In Ref [15

15. O. Stranik, R. Nooney, C. McDonagh, and B. D. MacCraith, “Optimization of nanoparticle size for plasmonic enhancement of fluorescence,” Plasmonics 2(1), 15–22 (2007). [CrossRef]

], Ag-Au (4:1) alloy was synthesized as the core, and then coated with a 5 nm thick of silica layer, where a monolayer of molecule was attached on the outer surface of silica. Since Ru(dpp)32+ has a large Stokes shift with an excitation peak at 450 nm and an emission peak at 620 nm, the self-quenching between nearby molecules can be neglected. Because the frequency-dependent permittivity of the Ag-Au alloy is not known, the permittivity of Ag [21

21. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

] is adopted here for calculation. Figure 7
Fig. 7 AEF of Ag@SiO2 of a 5 nm silica layer with molecules attached on the outer surface excited by a laser ofλex = 450 nm for emission of λem = 620 nm, compared with the experimental data of Ref [15].
shows the theoretical AEF, α^A, of d = 5 nm by Eq. (3), compared with the experimental data for different a2. The theoretical AEF is not only in the same order of magnitude but also roughly in good accordance with the experimental data. The error bars of the experiment come from the distribution of the size of Ag core, which the discrepancy between the theoretical and experimental data may be attributed to.

4. Conclusion

For a molecule-doped coreshell (Ag@SiO2), EF on molecular fluorescence strongly depends on the molecular orientation and location. AEF has been proposed by considering all possible orientations and locations of molecules. We have demonstrated that AEF is more reasonable and useful than EF to evaluate the performance of a large number of coreshells on molecular fluorescence irradiated by a polarized or unpolarized light. In fact, AEF is much smaller than the maximum EF of a coreshell for a single molecule with a radial orientation at an optimal position. For Ag@SiO2, AEF is narrowband, if a230  nm. This is to say Ag@SiO2 is a molecule-selective fluorescence enhancer; if the excitation and emission spectra of the molecule overlap the SPR band of the Ag core (≈420 nm), the maximum AEF is obtained for an optimal dimension of a2=20 - 25 nm, ts=10 - 15 nm. For the other molecules with an excitation spectrum longer than that from the SPR band of the Ag NP, a larger Ag core (e.g. a260  nm) can benefit the fluorescence by providing a broadband AEF to cover this excitation spectrum. In this paper, we have not considered the improvement of the intrinsic quantum yield of molecule by the reduction of fluorescence lifetime, caused by SPR. However, for a real measurement of AEF of coreshell, this factor could be also significant, thus a further study is needed. Moreover, AEF will also be useful for the study of the surface-enhanced fluorescence resonance energy transfer of two nearby molecules (donor and acceptor) [23

23. H. Y. Xie, H. Y. Chung, P. T. Leung, and D. P. Tsai, “Plasmonic enhancement of Förster energy transfer between two molecules in the vicinity of a metallic nanoparticle: Nonlocal optical effects,” Phys. Rev. B 80(15), 155448 (2009). [CrossRef]

,24

24. M. Lessard-Viger, M. Rioux, L. Rainville, and D. Boudreau, “FRET enhancement in multilayer core-shell nanoparticles,” Nano Lett. 9(8), 3066–3071 (2009). [CrossRef] [PubMed]

] by using coreshells. In addition, the coherent light of a spaser-based nanolaser has been demonstrated recently by using molecules-doped Au@SiO2 structure [25

25. M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, “Demonstration of a spaser-based nanolaser,” Nature 460(7259), 1110–1112 (2009). [CrossRef] [PubMed]

], where our model of AEF could also be applied.

Acknowledgment

The research was supported by NSC, Taiwan, R.O.C. (NSC 98-2221-E-002-002, NSC 96-2221-E-182 021, NSC 97-2221-E-182-012-MY2).

References and links

1.

S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]

2.

P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). [CrossRef] [PubMed]

3.

O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]

4.

K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Fluorescent core-shell Ag@SiO2 nanocomposites for metal-enhanced fluorescence and single nanoparticle sensing platforms,” J. Am. Chem. Soc. 129(6), 1524–1525 (2007). [CrossRef] [PubMed]

5.

K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Metal enhanced fluorescence solution-based sensing platform 2: fluorescent core-shell Ag@SiO2 nanoballs,” J. Fluoresc. 17(2), 127–131 (2007). [CrossRef] [PubMed]

6.

D. Cheng and Q.-H. Xu, “Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles,” Chem. Commun. (Camb.) 2007(3), 248–250 (2007). [CrossRef]

7.

R. Bardhan, N. K. Grady, J. R. Cole, A. Joshi, and N. J. Halas, “Fluorescence enhancement by Au nanostructures: nanoshells and nanorods,” ACS Nano 3(3), 744–752 (2009). [CrossRef] [PubMed]

8.

F. Tam, G. P. Goodrich, B. R. Johnson, and N. J. Halas, “Plasmonic enhancement of molecular fluorescence,” Nano Lett. 7(2), 496–501 (2007). [CrossRef] [PubMed]

9.

J. R. Lakowicz, “Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission,” Anal. Biochem. 337(2), 171–194 (2005). [CrossRef] [PubMed]

10.

W. Wang, Z. Li, B. Gu, Z. Zhang, and H. Xu, “Ag@SiO2 core-shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering,” ACS Nano 3(11), 3493–3496 (2009). [CrossRef] [PubMed]

11.

C. Fernandez-Lopez, C. Mateo-Mateo, R. A. Alvarez-Puebla, J. Perez-Juste, I. Pastoriza-Santos, and L. M. Liz-Marzan, “Highly controlled silica coating of PEG-capped metal nanoparticles and preparation of SERS-encoded particles,” Langmuir 25(24), 13894–13899 (2009). [CrossRef] [PubMed]

12.

T. Härtling, P. Reichenbach, and L. M. Eng, “Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle,” Opt. Express 15(20), 12806–12817 (2007). [CrossRef] [PubMed]

13.

M. H. Chowdhury, S. K. Gray, J. Pond, C. D. Geddes, K. Aslan, and J. R. Lakowicz, “Computational study of fluorescence scattering by silver nanoparticles,” J. Opt. Soc. Am. B 24(9), 2259–2267 (2007). [CrossRef] [PubMed]

14.

G. Colas des Francs, A. Bouhelier, E. Finot, J. C. Weeber, A. Dereux, C. Girard, and E. Dujardin, “Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes,” Opt. Express 16(22), 17654–17666 (2008). [CrossRef] [PubMed]

15.

O. Stranik, R. Nooney, C. McDonagh, and B. D. MacCraith, “Optimization of nanoparticle size for plasmonic enhancement of fluorescence,” Plasmonics 2(1), 15–22 (2007). [CrossRef]

16.

C.-T. Tai, Dyadic Green's Functions in Electromagnetic Theory (IEEE, 1971).

17.

X.-W. Chen, W. C.-H. Choy, S. He, and P. C. Chui, “Highly efficient fluorescence of a fluorescing nanoparticle with a silver shell,” Opt. Express 15(11), 7083–7094 (2007). [CrossRef] [PubMed]

18.

J.-W. Liaw, J. H. Chen, C. S. Chen, and M.-K. Kuo, “Purcell effect of nanoshell dimer on single molecule’s fluorescence,” Opt. Express 17(16), 13532–13540 (2009). [CrossRef] [PubMed]

19.

J.-W. Liaw, J.-H. Chen, and C.-S. Chen, “Enhancement or quenching effect of metallic nanodimer on spontaneous emission,” J. Quant. Spectrosc. Radiat. Transf. 111(3), 454–465 (2010). [CrossRef]

20.

Y. Jin and X. Gao, “Plasmonic fluorescent quantum dots,” Nat. Nanotechnol. 4(9), 571–576 (2009). [CrossRef] [PubMed]

21.

P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

22.

E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar, J. Feldmann, S. A. Levi, F. C. J. M. van Veggel, D. N. Reinhoudt, M. Möller, and D. I. Gittins, “Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects,” Phys. Rev. Lett. 89(20), 203002 (2002). [CrossRef] [PubMed]

23.

H. Y. Xie, H. Y. Chung, P. T. Leung, and D. P. Tsai, “Plasmonic enhancement of Förster energy transfer between two molecules in the vicinity of a metallic nanoparticle: Nonlocal optical effects,” Phys. Rev. B 80(15), 155448 (2009). [CrossRef]

24.

M. Lessard-Viger, M. Rioux, L. Rainville, and D. Boudreau, “FRET enhancement in multilayer core-shell nanoparticles,” Nano Lett. 9(8), 3066–3071 (2009). [CrossRef] [PubMed]

25.

M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, “Demonstration of a spaser-based nanolaser,” Nature 460(7259), 1110–1112 (2009). [CrossRef] [PubMed]

OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(260.2160) Physical optics : Energy transfer
(260.2510) Physical optics : Fluorescence
(260.3910) Physical optics : Metal optics

ToC Category:
Optics at Surfaces

History
Original Manuscript: April 16, 2010
Revised Manuscript: May 20, 2010
Manuscript Accepted: May 21, 2010
Published: May 28, 2010

Virtual Issues
Vol. 5, Iss. 10 Virtual Journal for Biomedical Optics

Citation
Jiunn-Woei Liaw, Chuan-Li Liu, Wei-Min Tu, Chieh-Sheng Sun, and Mao-Kuen Kuo, "Average enhancement factor of molecules-doped coreshell (Ag@SiO2) on fluorescence," Opt. Express 18, 12788-12797 (2010)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-18-12-12788


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References

  1. S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna,” Phys. Rev. Lett. 97(1), 017402 (2006). [CrossRef] [PubMed]
  2. P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenching of single-molecule fluorescence,” Phys. Rev. Lett. 96(11), 113002 (2006). [CrossRef] [PubMed]
  3. O. G. Tovmachenko, C. Graf, D. J. van den Heuvel, A. van Blaaderen, and H. C. Gerritsen, “Fluorescence enhancement by metal-core/silica-shell nanoparticles,” Adv. Mater. 18(1), 91–95 (2006). [CrossRef]
  4. K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Fluorescent core-shell Ag@SiO2 nanocomposites for metal-enhanced fluorescence and single nanoparticle sensing platforms,” J. Am. Chem. Soc. 129(6), 1524–1525 (2007). [CrossRef] [PubMed]
  5. K. Aslan, M. Wu, J. R. Lakowicz, and C. D. Geddes, “Metal enhanced fluorescence solution-based sensing platform 2: fluorescent core-shell Ag@SiO2 nanoballs,” J. Fluoresc. 17(2), 127–131 (2007). [CrossRef] [PubMed]
  6. D. Cheng and Q.-H. Xu, “Separation distance dependent fluorescence enhancement of fluorescein isothiocyanate by silver nanoparticles,” Chem. Commun. (Camb.) 2007(3), 248–250 (2007). [CrossRef]
  7. R. Bardhan, N. K. Grady, J. R. Cole, A. Joshi, and N. J. Halas, “Fluorescence enhancement by Au nanostructures: nanoshells and nanorods,” ACS Nano 3(3), 744–752 (2009). [CrossRef] [PubMed]
  8. F. Tam, G. P. Goodrich, B. R. Johnson, and N. J. Halas, “Plasmonic enhancement of molecular fluorescence,” Nano Lett. 7(2), 496–501 (2007). [CrossRef] [PubMed]
  9. J. R. Lakowicz, “Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission,” Anal. Biochem. 337(2), 171–194 (2005). [CrossRef] [PubMed]
  10. W. Wang, Z. Li, B. Gu, Z. Zhang, and H. Xu, “Ag@SiO2 core-shell nanoparticles for probing spatial distribution of electromagnetic field enhancement via surface-enhanced Raman scattering,” ACS Nano 3(11), 3493–3496 (2009). [CrossRef] [PubMed]
  11. C. Fernandez-Lopez, C. Mateo-Mateo, R. A. Alvarez-Puebla, J. Perez-Juste, I. Pastoriza-Santos, and L. M. Liz-Marzan, “Highly controlled silica coating of PEG-capped metal nanoparticles and preparation of SERS-encoded particles,” Langmuir 25(24), 13894–13899 (2009). [CrossRef] [PubMed]
  12. T. Härtling, P. Reichenbach, and L. M. Eng, “Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle,” Opt. Express 15(20), 12806–12817 (2007). [CrossRef] [PubMed]
  13. M. H. Chowdhury, S. K. Gray, J. Pond, C. D. Geddes, K. Aslan, and J. R. Lakowicz, “Computational study of fluorescence scattering by silver nanoparticles,” J. Opt. Soc. Am. B 24(9), 2259–2267 (2007). [CrossRef] [PubMed]
  14. G. Colas des Francs, A. Bouhelier, E. Finot, J. C. Weeber, A. Dereux, C. Girard, and E. Dujardin, “Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes,” Opt. Express 16(22), 17654–17666 (2008). [CrossRef] [PubMed]
  15. O. Stranik, R. Nooney, C. McDonagh, and B. D. MacCraith, “Optimization of nanoparticle size for plasmonic enhancement of fluorescence,” Plasmonics 2(1), 15–22 (2007). [CrossRef]
  16. C.-T. Tai, Dyadic Green's Functions in Electromagnetic Theory (IEEE, 1971).
  17. X.-W. Chen, W. C.-H. Choy, S. He, and P. C. Chui, “Highly efficient fluorescence of a fluorescing nanoparticle with a silver shell,” Opt. Express 15(11), 7083–7094 (2007). [CrossRef] [PubMed]
  18. J.-W. Liaw, J. H. Chen, C. S. Chen, and M.-K. Kuo, “Purcell effect of nanoshell dimer on single molecule’s fluorescence,” Opt. Express 17(16), 13532–13540 (2009). [CrossRef] [PubMed]
  19. J.-W. Liaw, J.-H. Chen, and C.-S. Chen, “Enhancement or quenching effect of metallic nanodimer on spontaneous emission,” J. Quant. Spectrosc. Radiat. Transf. 111(3), 454–465 (2010). [CrossRef]
  20. Y. Jin and X. Gao, “Plasmonic fluorescent quantum dots,” Nat. Nanotechnol. 4(9), 571–576 (2009). [CrossRef] [PubMed]
  21. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]
  22. E. Dulkeith, A. C. Morteani, T. Niedereichholz, T. A. Klar, J. Feldmann, S. A. Levi, F. C. J. M. van Veggel, D. N. Reinhoudt, M. Möller, and D. I. Gittins, “Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects,” Phys. Rev. Lett. 89(20), 203002 (2002). [CrossRef] [PubMed]
  23. H. Y. Xie, H. Y. Chung, P. T. Leung, and D. P. Tsai, “Plasmonic enhancement of Förster energy transfer between two molecules in the vicinity of a metallic nanoparticle: Nonlocal optical effects,” Phys. Rev. B 80(15), 155448 (2009). [CrossRef]
  24. M. Lessard-Viger, M. Rioux, L. Rainville, and D. Boudreau, “FRET enhancement in multilayer core-shell nanoparticles,” Nano Lett. 9(8), 3066–3071 (2009). [CrossRef] [PubMed]
  25. M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, and U. Wiesner, “Demonstration of a spaser-based nanolaser,” Nature 460(7259), 1110–1112 (2009). [CrossRef] [PubMed]

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