and give results for single aluminum nanoparticles with diameters d
= 20 and 80 nm respectively. Figures S-2 and S-3 (Supporting Information)
give results for diameters d
= 40 and 100 nm, respectively. In these figures, the upper panel has extinction, scattering, and absorption efficiencies [optical cross sections normalized by π
], and the lower panels contain radiated power enhancements when a fluorophore (radiating dipole) is placed near the nanoparticle. Two different fluorophore-metal surface distances, s
= 5 and 10 nm are considered.
Figure 2 Results for a d = 80 nm aluminum nanoparticle: (a) Extinction, scattering, and absorption efficiencies; (b) radiated power enhancement for dipoles spaced s = 5 and 10 nm from the nanoparticle. The dipole is oriented perpendicular to the metal surface; (more ...)
For a d
= 20 nm aluminum nanoparticle, the extinction peaks are at ≈150 nm (). In contrast, similar sized silver nanoparticles have plasmon resonances typically in the 350–375 nm region. The differing plasmon resonances of aluminum and silver are consistent with the small particle surface plasmon resonance condition41
) = −2, where ε
is the metallic dielectric constant. For aluminum we find λSP
≈ 150 nm, and for silver λSP
≈ 354 nm. The extinction for this particle size is dominated by absorption. shows the radiated power enhancement for a fluorophore placed at s
= 5 and 10 nm from the nanoparticle. The fluorophores are oriented perpendicular to the aluminum surface. (We define fluorophore orientation to be the oscillation direction of the dipole.) The enhancement in the radiated power peaks at ≈155 nm and parallels the form of the optical cross sections () indicating the role of surface plasmons. This enhancement is expected because in this orientation the fluorophore’s dipole induces a dipole in the aluminum nanoparticle such that the dipoles align head-to-tail, leading to a larger effective dipole than that for an isolated fluorophore. Note also that the degree of enhancement depends on the fluorophore-metal distance with s
= 5 nm showing significantly more enhancement than s
= 10 nm.
The problem of a radiating point dipole and a metal sphere can also be solved analytically. includes (dashed curves) our implementation of the exact radiated power enhancement calculated using eq 27 of ref 42
. The agreement with the FDTD results is very good, giving us confidence in our FDTD results for systems without exact analytical solutions such as the dimer system. We also considered simpler, more approximate analytical forms for the radiated power enhancement.43–45
A quasistatic (d λ
expression, eq 6 of ref 43
, is qualitatively correct but leads to peak positions that are blue-shifted by ≈10 nm relative to the exact positions and peak heights that are 1.5–2 times larger than the exact ones. An improved version45
of a quasistatic limit model due to Gersten and Nitzan44
is much better but still underestimates by 15–20% the peak enhancement and all enhancements on its blue side.
The radiated power enhancements in show long-wavelength limits of ≈1.6 for the s = 10 nm case and ≈2.6 for the s = 5 nm, that is, the enhancement factor does not approach unity as might be naively expected. In this long-wavelength limit, the metal behaves as a perfect electrical conductor (PEC). PECs do not to support surface plasmons. Nonetheless some enhancement via near-field interactions can still occur. Indeed, we carried out calculations assuming the metal is a PEC rather than aluminum and obtained enhancements for λ > 500 nm that are very close to the aluminum results of . However, the PEC enhancement factors rise only slightly with decreasing wavelength and reach values of just 2–3 in the region where the plasmon resonance dominates the aluminum results of .
= 40 nm case is discussed in the Supporting Information (Figure S-2)
. For the d
= 80 nm nanoparticles, the dipolar extinction peak is further red-shifted relative to the d
= 40 nm particle to ≈250 nm (). Higher order peaks at ≈170 and 140 nm are also observed. The extinction for this particle size is more dominated by the scattering, although there is a small absorptive component. shows the radiated power enhancement for fluorophores oriented perpendicular to the metal surface. The radiated power enhancements, like the previous cases, have features in common with the optical spectra, although with some red-shifting. The enhancement peaks at ≈310 nm, a 60 nm red shift from the corresponding extinction peak. As with the smaller nanoparticle cases, the s
= 5 nm fluorophore-surfce spacing shows significantly more enhancement than s
= 10 nm and can show sharper higher order mode features.
shows the radiated power enhancements for a dipole oriented parallel to the metal surface with spacings s = 5 and 10 nm from the d = 80 nm metal surface. The dashed line in is the boundary between fluorescence enhancement and quenching. Quenching clearly dominates aside from a small region between λ = 100–175 nm where there are modest enhancements. In the parallel orientation, the fluorophore’s dipole induces a dipole in the aluminum nanoparticle of the opposite polarity. This causes the dipoles to counteract each other, leading to a smaller effective radiating dipole than in case of the isolated dipole. These results also indicate that for most wavelengths the parallel dipole orientation is not conducive for fluorescence emission enhancements with aluminum nanoparticles.
See Figure S-3 (Supporting Information)
for a discussion of d
= 100 nm aluminum nanoparticles. In , , S-2 and S-3
, we observe a progressive red-shifting of the radiated power enhancement peak as the nanoparticles become larger. This correlates with the scattering cross sections of the nanoparticles which also show a distinct red-shifting with larger nanoparticles.
Figure S-4a–d in the Supporting Information
provides a discussion of the relationship between the wavelength of the extinction maximum peak and the wavelength of the radiated power peak for the various aluminum nanoparticle sizes studied. Our calculations also reveal that for emission wavelengths between 300–350 nm (the primary emission region for amino acids tryptophan and tyrosine, and all DNA bases), aluminum is a much more efficient MEF substrate than silver [Figure S-5a–d
and discussion in the Supporting Information
The wavelength region 300–420 nm is typical for intrinsic fluorescence in biomolecules. A unitless measure of enhancement in this region is the integral of the radiated power enhancement over the region and dividing by the integration range of 120 nm (this ratio is = 1 for the case of an isolated dipole, so any such ratio >1 for a dipole-aluminum nanoparticle system represents an enhancement and vice-versa). We use this enhancement measure to ascertain the effect of nanoparticle size, . For these calculations, the dipole-aluminum distance is kept constant at s = 5 nm. Three dipole orientations are represented: (i) perpendicular (P); (ii) parallel (L); (iii) orientation averaged = (P + 2L)/3. Two important trends are as follows: (a) There is a clear dependence of the enhancement with particle size, with d = 80 nm giving the maximum enhancement measure of ≈ 12.5; (b) The perpendicular orientation always leads to enhancement whereas the parallel orientation leads to quenching.
Figure 3 Enhancement measure for emission in 300–420 nm (see text) as a function of aluminum nanoparticle size (d = 20–140 nm). Three dipole orientations are represented: (a) Perpendicular (P); (b) Parallel (L); and Orientation Averaged = (P + (more ...)
shows the effect of the fluorophore-metal distance on enhancement for the d = 80 nm aluminum nanoparticle case. Like , we consider perpendicular, parallel, and orientationally averaged dipoles and use the same enhancement measure associated with the 300–420 emission region. The largest enhancement measure, ≈15, is obtained for the perpendicular orientation, at the shortest metal-fluorophore distance of 1 nm. This may be understood in terms of the head-to-tail dipole alignment argument concerning and the fact that as s increases the magnitude of the dipole induced in the metal decreases. However, for the parallel case, we see quenching of the radiated power for all metal-fluorophore separations, with the extent of quenching the greatest at s = 1 nm. The unfavorable dipole alignment argument concerning applies here, along with the fact that as s increases, the induced dipole in the metal is weaker and so cancels the fluorophore dipole a little less effectively.
Figure 4 (a) Enhancement measure for emission in 300–420 nm at various distances (s = 1–20 nm) from the surface of a d = 80 nm aluminum nanoparticle. Three dipole orientations are considered, as in the caption for . (b) Enhancement measure (more ...)
We consider the radiated power enhancement for an 80 nm aluminum nanoparticle dimer system in . The fluorophore is placed between the nanoparticles on the dimer axis. The surface-surface-spacing between particles (Figure S-1, Supporting Information
) are in the 2s
= 2–40 nm range. shows that for the perpendicular oriented fluorophore: (a) the dimer creates a significant increase in radiated power enhancement, up to ≈3500 with a surface-surface spacing of 2 nm; (b) the degree of enhancement for the dimer decreases with increasing dimer spacing; (c) each dimer system yields a greater enhancement than its corresponding monomer system. For example, dimer particles spaced by 2 nm give larger radiated power enhancements than a single nanoparticle spaced 1 nm from the dipole. This is because the fluorophore’s dipole now induces two dipoles, one in each of the metal nanoparticles. All three dipoles align head-to-tail, leading to a much larger effective radiating dipole. For the parallel oriented dipoles, shows the following: (a) the dimer creates significant quenching of radiated power compared to an isolated dipole, with up over 200-fold quenching for surface-surface spacing of 2 nm; (b) the degree of quenching in the dimers decreases with increasing dimer particle spacing; (c) each of the dimer shows significantly higher quenching of the radiated power than its corresponding monomer system. The large quenching in case of the dimers with parallel dipole orientation is explained by the fluorophore’s dipole inducing two dipoles of the opposite polarity, one in each of the aluminum nanoparticles. This causes the dipoles to strongly counteract each other, thus leading to a much smaller effective radiating dipole than in the single nanoparticle case. See also Table S-1 (Supporting Information)
. These results are similar to those for silver nanoparticles in the visible range.36
, inset, shows the wavelength-dependent radiation power enhancement of a d
= 80 nm aluminum nanoparticle dimer with particles spaced 4 nm apart. The fluorophore is oriented perpendicular and located in the middle of the dimer axis. The major enhancement peak is at ≈405 nm, with higher mode peaks at ≈210 and 170 nm. The enhancements in this case are larger than those in similar figures for single aluminum nanoparticles (, , S-2b, and S-3b
shows a comparison of the radiated power enhancement for all the 80 nm aluminum particle monomer and dimer systems studied with the perpendicular dipole orientation. It clearly reveals the higher enhancement factors that occur with the dimer systems when compared with single nanoparticles.
Figure 5 Radiated power enhancement of d = 80 nm aluminum nanoparticle systems with perpendicular fluorophore orientation. The horizontal axis denotes the system where “M” denotes an isolated monomer and “D” denotes a dimer. The (more ...)
It is interesting to examine the electromagnetic near-field distributions around the aluminum nanoparticles that are created by both excitation light as well as excited-state fluorophores. These fields provide insight into the nature of metal enhanced fluorescence that is interesting from the perspective of applications involving molecular spectroscopy and designing specific fluorophore-metal nanoparticle systems. A description of the effect of the 280 nm excitation plane wave on the near-fields around an 80 nm aluminum nanoparticle is in the Supporting Information (Figure S-6)
. A wavelength of 280 nm is typical for the excitation of protein fluorescence. All the near-field calculations shown are performed along a single plane, that is, the x-y
plane running through the center of the dipole and/or aluminum nanoparticles. Figure S-7
also presents the effect of an excited-state fluorophore emitting at 350 nm on the near-fields around an 80 nm aluminum nanoparticle. To obtain the wavelength-resolved result we keep the fluorophore or dipole oscillating at a fixed frequency corresponding to 350 nm throughout the entire simulation time, and construct a time average of the square of the electric field vector over the last period of evolution. show respectively the near-field intensity in the x-y
plane around an isolated fluorophore, the near-fields around a d
= 80 nm aluminum nanoparticle dimer system with a surface-to-surface distance of 4 nm and a perpendicularly oriented dipole located halfway between the particles, and the near-field enhancement and quenching image that was calculated in an identical manner to Figure S-7c
. From we see that the near-field is not enhanced in the gap between
the particles, but there are intense field enhancements around all other areas of the particles. The intense near-field enhancement also extends tens of nanometers from the edge of the particles into the free space as observed by the extent of the red areas in the image. The near-field enhancements of are much greater than those of the single aluminum nanoparticle (Figure S-7c
). The dark red regions of represent more than an order of magnitude greater enhancements than those of Figure S-7c
. It is important to note that the near-fields calculated in and S-7
do not necessarily represent propagating radiation. They could either be propagating fields or localized evanescent fields that are non-propagating.
Figure 6 Near-field intensities arising from a point dipole radiating at 350 nm: (a) Isolated dipole; (b) Dipole between two 80 nm diameter aluminum nanoparticles with surface-surface particle spacing 2s = 4 nm; (c) Near-field enhancement/quenching obtained by (more ...)
In , there is only an extremely small area between the two aluminum nanoparticles that shows quenching. The overwhelmingly large portion of the image is bright or dark red which depicts significant near-field enhancements in the region immediately surrounding the dimer which is induced by the excited fluorophore. These near-field enhancements can eventually lead to enhancements in the far-field propagating emission, for example, . Hence we see that nanoparticle systems displaying large enhancements in the radiated power also show very strong enhancements in the near-fields. Comparing the near-field distributions around an isolated fluorophore () and a fluorophore in between two aluminum particles () suggests a possible mechanism for MEF. Since the intensity of the excited-state fluorophore is actually decreased when it is in between the aluminum nanoparticles (), it suggests that the fluorophore alone is not the entity that is responsible for the enhanced emission. Rather it is the fluorophore coupled with the nearby aluminum nanoparticles, behaving as a single radiating entity, that is the source of the enhanced fluorescence signals. A similar effect is seen in the case of a flurophore in proximity to a single aluminum nanoparticle [Figure S-7c, Supporting Information
We also performed experiments to corroborate the theoretical predictions above concerning the efficiency of aluminum for MEF-based label-free biological detection. As detailed in the Supporting Information (Figure S-8)
, we spin-coated a 15 nm layer of polyvinyl alcohol (PVA) containing dissolved NATA and NATA-tyr separately on a thin film of aluminum on a quartz substrate and compared its fluorescence emission intensity, lifetime, and photostability with an identical sample on just the quartz substrate. The aluminum film is rough or nanostructured and so one might expect to see MEF similar to that predicted for the small nanoparticle limit here.
shows that for a 15 nm thick PVA film containing NATA, the 10 nm thick aluminum substrate gives an emission intensity enhancement of approximately 11-fold when compared to the quartz control. The lifetimes of 15 nm thick PVA film containing NATA on quartz and aluminum substrates are shown in the inset of . The solid lines are fits to the experimental decay curves. The intensity decay of NATA on the aluminum surface is faster than that on the quartz control. The intensity-decay of the NATA PVA film on quartz could be fit with a single exponential with a lifetime of 3.2 ns. NATA on the aluminum surface could only be fit with a double-exponential with lifetimes 3.3 ns (8%) and 1.4 ns (92%). The amplitude-weighted lifetime of NATA on aluminum was 1.6 ns. Hence, the intensity decays show that the lifetime was decreased ≈2-fold. In the case of NATA on aluminum, the multiexponential decay may reflect the breadth of the distribution of NATA molecules both in proximity to and distant from the aluminum. The shortening of lifetime on the aluminum nanostructured substrate supports the notion that the increase in observed fluorescence intensity is due to the radiation from the plasmon-fluorophore complex 21,46
that results when excited fluorophores interact with aluminum nanoparticles in the near-field. The near-field images of Figures S-7b,c (Supporting Information)
, and panels b and c of show it is difficult to differentiate what the origins of the enhanced fields around the aluminum nanoparticle are. They could be either from the fluorophore or from plasmons or both. This lends support to our radiating plasmon model where we believe the enhanced fluorescence emission observed in MEF experiments is due to radiation from the entire excited-state fluorophore-metal nanoparticle complex acting as a single radiating entity.21,46,47
The reduction of the lifetime of NATA on the aluminum surface together with the calculations showing aluminum nanoparticles causing an enhancement in the radiated power of a fluorophore in its proximity suggest an increase in the radiative decay rate of NATA emission due to the interaction of aluminum nanoparticles.46,47
Precise agreement between the increases in intensity and decreases in lifetime cannot be expected. This is because time-domain measurements often result in overweighting of the lifetime by the longer lifetime components in cases involving a heterogeneous decay, especially when the decay of the short components overlaps the instrument response function.
Figure 7 Fluorescence spectra of functionalized amino acids contained in a 15 nm thick PVA film on top of a 10 nm thick aluminum film and on a quartz control. Insets are the corresponding intensity decays: (a) Tryptophan (NATA); (b) Tyrosine (NATA-tyr). IRF is (more ...)
In fluorescence experiments, the photostability of the fluorophore is a factor that governs its detectability. We compare the photostability of NATA on aluminum films and on quartz in the Supporting Information (Figure S-9)
shows that for a 15-nm thick PVA film containing NATA-tyr, the 10 nm thick aluminum film gives emission intensity enhancement of ≈7 compared to the quartz control. The emission spectra of NATA-tyr was collected through a 300 nm long-pass filter to prevent the excitation beam from striking the detector. The inset in shows the intensity decays of 15 nm PVA films containing NATA-tyr on aluminum and quartz substrates. We observe a slightly faster decay for the NATA-tyr PVA film on aluminum when compared to quartz. The intensity-decay of the NATA-tyr PVA film on quartz could be fit with a sum of two exponentials with lifetimes 3.04 ns (32%) and 0.84 ns (68%). NATA-tyr on the aluminum surface also could only be fit with with two lifetimes of 0.62 ns (17%) and 0.93 ns (83%). The amplitude-weighted lifetimes of NATA-tyr on quartz and aluminum were 1.1 and 0.86 ns, respectively. Hence, the intensity decays in (inset) show that the NATA-tyr lifetime on aluminum substrates was decreased by only a modest factor of ≈1.3. The experimental results of and Figure S-9 (Supporting Information)
corroborate the validity of our theoretical predictions that aluminum nanoparticles can be used to efficiently enhance the emission of a fluorophore in the ultraviolet region and thus can potentially serve as a valuable tool in implementing a modality for the label free detection of biomolecules in a variety of sensing and imaging platforms.
The more significant theoretical enhancements, relative to the experimental ones discussed above, occur particularly for the dipoles oriented perpendicular to the metal. This is because in this orientation, the fluorophore’s dipole induces a dipole in the aluminum nanoparticle in a configuration that allows the dipoles to align head-to-tail, leading to a much larger effective radiating dipole than in the case of an isolated fluorophore. In the case of the experimental results, the signal observed is ensemble averaged over potentially millions of tryptophan or tyrosine molecules. Since they were spin coated in PVA over the thin aluminum film, their orientation and location could not be precisely controlled. It is expected that in such a system there is a broad distribution of the NATA or NATA-tyr molecules both in proximity to and distant from the aluminum. Additionally, it can be expected that many of these excited molecules are not oriented perpendicular to the metal surface, and might even be oriented parallel to the surface where we expect significant quenching to occur.