shows the wavelength dependent radiated power enhancement for a dipole located s
= 5 nm away from single aluminum nanoparticle with diameters d
= 20, 40 and 80 nm respectively. In these calculations, the dipole was oriented perpendicular to the tangential aluminum surface (along the x
-axis). We see that the enhancement in the radiated power peaks at approximately λ = 155 nm for the d
= 20 nm particle size [8
]. For the d
= 40 nm particle size, the radiated power enhancement shows a dipolar peak at λ = 180 nm and we also see a quadrupolar enhancement peak at λ = 137 nm [8
]. This indicates that higher order radiation modes become relevant when the radiating dipole and the metal surface are in very close proximity. Finally, for the d
= 80 nm particle size the radiated power enhancement peaks at λ = 310 nm - a 130 nm red shift from the corresponding peak of the d
= 40 nm particle. Additionally we also see sharp higher order mode peaks at λ = 172 nm and λ = 141 nm [8
]. This result further corroborates the fact that higher order radiation modes arise when the radiating dipole and the metal surface are in very close proximity. The enhancements observed in can be expected because when the dipole is oriented perpendicular to the metal surface, the fluorophore’s dipole induces a dipole in the aluminum nanoparticle in a configuration that allows the dipoles to align along the x
-axis head to tail, leading to a much larger effective radiating dipole than in the case of an isolated fluorophore – and hence the enhancements. We have deliberately included FDTD results with dipoles oriented perpendicular to the metal surface to present the best case scenario. Previous studies done from laboratory have demonstrated that dipoles oriented parallel to the metal surface induce significant quenching of the emission and hence we did not choose the parallel orientation case for this study [8
]. The radiation enhancement of a spherical nanoparticle/dipole system can also be analytically solved and we have validated our FDTD results by comparison with those solutions [8
]. However the dimer systems to be discussed later do not admit such analytical solutions.
Wavelength dependent radiated power enhancement for dipoles spaced s = 5 from d = 20, 40 and 80 nm aluminum nanoparticles calculated using FDTD. The dipoles are oriented perpendicular to the metal surface (oriented along the x-axis).
It is important to investigate the effect of the aluminum nanoparticle size on the enhancement of the radiated power of fluorophores in the biologically relevant UV range. This was done by integrating the area under the radiated power enhancement spectra for the fluorophrore near the d
= 20 nm, 40 nm, 80 nm, 100 nm and 140 nm aluminum nanoparticles between the wavelength range of 300–420 nm and dividing it by the corresponding area under the curve for the isolated dipole [8
]. This wavelength region was chosen because it is the typical region for the intrinsic fluorescence emission of biological molecules. For these calculations, the dipole-aluminum distance was kept constant at s
= 5 nm. shows these results which indicate that there is a clear dependence of the radiation enhancement with particle size, with the 80 nm aluminum nanoparticle giving the maximum enhancement of about 12-fold [8
Total radiated power enhancement in the λ = 300–420 nm range for dipoles at a distance of s = 5 nm from the surface of aluminum nanoparticles of varying diameters (d = 20, 40, 80, 100 and 140 nm).
We also studied the effect of fluorophore-aluminum distance on the extent of enhancement observed for the case of a fluorophore next to a single aluminum as well as in between two aluminum particles in a dimer system. shows a visual comparison of the enhancement in the radiated power for all the d
= 80 nm aluminum particle monomer and dimer systems studied with the perpendicular dipole orientation. The calculations were performed in a manner similar to i.e. in the wavelength range of 300–420 nm. We investigated fluorophore-metal distances of s
= 1 nm, 2nm, 5 nm, 10 nm and 20 nm for the single particle case and dimer spacing of 2s
= 2nm, 4nm, 10 nm, 20 nm and 40 nm respectively. For the dimer case, the fluorophore is located exactly the midpoint of the dimer axis. i.e. equidistant from each aluminum nanoparticle. The first five values in the x
= 1, 2, 5, 10 and 20 nm) of denotes the single nanoparticle system or monomer, whereas the last five values (2s
= 2, 4, 10, 20 and 40 nm) represents the dimer case. The y
-axis of is plotted in a semi-logarithmic scale (base 10 along the y-axis) for convenience of observation. We see that extent of enhancement decreases with increasing fluorophore-metal distance – an observation that is expected as the increasing distance will reduce the strength of the induced dipole in the aluminum nanoparticles. The major conclusion that can be drawn from is that the presence of the dimer creates a significant increase in the enhancement of the radiated power when compared to an isolated dipole, up to a maximum of over 3,500-fold for the dimers with a surface-surface spacing of 2s
= 2 nm [8
]. Additionally it is also seen that the degree of enhancement observed from the dimer system decreases with increasing dimer spacing. This huge enhancement for the dimer case occurs because in the perpendicular orientation the fluorophore’s dipole induces two dipoles, one in each of the nanoparticles in the dimer system: in this configuration allows all the three dipoles to align along the x-axis head-to-tail, leading to a much larger effective radiating dipole than in case of the isolated dipole and the single nanoparticle system and hence we observe the higher enhancements. The enhancement values for all the monomer and dimer cases plotted in are tabulated in .
Figure 4 Computed total radiated power enhancement of emission (integrated around a closed surface containing the system) for different d = 80 nm monomer and dimer aluminum nanoparticles with the perpendicular fluorophore orientation. For the dimer systems, the (more ...)
Table 1 Computed enhancement/quenching of the total power radiated in the 300–420 nm range (integrated around a closed surface containing the Al-dipole system) by the various d = 80 nm aluminum nanoparticle systems studied with the dipoles oriented perpendicular (more ...)
The results discussed thus far have been instructive in providing us with an insight on the possible use of aluminum nanoparticles as MEF substrates for the label free detection of biological molecules. But this effort cannot be complete without an attempt to examine of the electromagnetic near-field distributions around the aluminum nanoparticles that are created by both excitation light as well as excited-state fluorophores. shows the enhancement in the near-fields (E2 = Ex2 + Ey2 + Ez2) around a d = 60 nm dimer spaced 2s = 10 nm apart that was created by the interaction with a plane wave of wavelength 280 nm. All the near-field calculations shown are performed along a single plane, that is, the x-y plane running through the center of the fluorophore-aluminum nanoparticles. A wavelength of 280 nm was chosen because it is typical for excitation of protein fluorescence. shows there is intense near-field enhancements in the region in between the particles. In this case the plane wave was propagating along the z-axis and polarized along the x-axis. The areas of high near-field enhancements in between the particles means that a protein or any other biomolecule once localized between the dimers will experience a much higher excitation field than if it were isolated and directly excited only by the incident light. This will lead to higher excitation rates of the fluorophore, which leads to greater excitation-emission cycles in a given time period. In we present the near-field enhancements around a d = 80 nm aluminum dimer system spaced 2s = 10 nm apart that are induced by a fluorophore radiating at 350 nm. We chose a fluorophore radiating at 350 nm because it is the typical emission region for proteins and nucleotides. In , only a small area between the two aluminum nanoparticles shows quenching. Interestingly this is the area where the excited-state fluorophore is physically located. The overwhelmingly large portion of the image depicts significant near-field enhancements in the region immediately surrounding the dimer. It is important to note that the near-fields calculated in do not necessarily represent propagating radiation. They could either be propagating fields or localized evanescent fields that are non-propagating. However, we believe these near-field enhancements are somehow related to the enhancements in the far-field propagating emission (radiation) as shown in –. Such spatial variations in the near-field enhancements are not easily inferred from calculations involving changes in the total radiated power. They provide a unique 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.
Figure 5 (a) Near-field image of the field enhancement around a d = 60 nm aluminum nanoparticle dimer spaced 2s = 10 nm apart by its interaction with the plane wave with wavelength 280 nm propagating along the z-axis and polarized along the x-axis; (b) Near-field (more ...)
We also performed experiments to corroborate the theoretical predictions presented above concerning the efficiency of aluminum nanostructures to detect unlabeled biomolecules such as amino acids and/or nucleotides using their intrinsic emission. We spin-coated a 10 nm layer of PVA containing dissolved NATA and GMP separately on a 10 nm thick aluminum film (sitting on top of a quartz substrate) and compared its fluorescence emission intensity an identical sample on a quartz substrate [8
]. shows that for a 10 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). Similarly shows that for a 10 nm thick PVA film containing GMP, the 10 nm thick aluminum substrate gives an emission intensity enhancement of approximately 11-fold when compared to the quartz (control). We have also reported the emission enhancement from a number of amino acids and DNA bases in addition to NATA and GMP [8
]. The experimental results of corroborate the validity of our theoretical predictions that aluminum nanoparticles can be used to efficiently enhance the intrinsic emission of various biomolecules in the UV region, and so suggest the possibility of designing bioassays based on intrinsic biomolecule fluorescence, thus potentially avoiding the pitfalls involved in additional external labeling steps that are currently limit biological experiments.
Experimental fluorescence spectra of a 10 nm thick PVA film on top of a 10 nm thick aluminum film, and on a quartz control that contain: (a) NATA; (b) GMP.