The Optics and Lidar Laboratories
Group of Hans Hallen and Russell Philbrick, Physics Department, North Carolina State University

Up-Conversion Nano-Particles (UCNPs)

Introduction

Our work on UCNPs has been to use plasmonics to enhance the signal levels, as is described briefly on the nano-plasmonics page. Here, we give a few more details, and more information is in the paper [1]. There are two somewhat unexpected results here of interest besides the expected wavelength-dependent enhancements, plasmon tunability and predictability of the spectral enhancement. The first especially interesting result is how resonances of the emission wavelengths correspond to peaks in the emission, and we also have a nice visual on this. It constrasts with the correspondence of plasmon resonances to peaks in the excitation light. These plasmon resonances can be tuned by particle size and gold coating thickness, as we also show. The second especially interesting result is how proximity to the metal favors green emission over red emission of the UCNPs. We see this in the spectra overall, and in the modeling just for points close to the metal. The reason is that the metal increases the local photon density of states, so photon transition rates are sped up while phonon (non-radiative) rates remain fixed. A careful look at the possible transitions between levels in this material indicates that the bias towards photon transitions favors green emission.

Upconversion of infrared light to visible light has important implications for bioimaging. However, the small absorption cross-section of rare earth dopants has limited the efficiency of these anti-Stokes nanomaterials. We present enhanced excitation absorption and single particle fluorescent emission of sodium yttrium fluoride, NaYF4: Yb, Er based upconverting nanoparticles coated with a gold nanoshell through surface plasmon resonance. The single gold-shell coated nanoparticles show enhanced absorption in the near infrared, enhanced total emission intensity, and increased green relative to red emission. We also show differences in enhancement between single and aggregated gold shell nanoparticles. The surface plasmon resonance of the gold-shell coated nanoparticle is shown to be dependent on the shell thickness. In contrast to other reported results, our single particle experimental observations are corroborated by finite element calculations that show where the green/red emission enhancement occurs, and what portion of the enhancement is due to electromagnetic effects. We find that the excitation enhancement and green/red emission ratio enhancement occurs at the corners and edges of the doped emissive core.

Introduction

Contrast agents play an important role in the study of biological tissues and whole organisms, since they enable visualization of functional structures. Fluorescent contrast agents also enable specific targeting in therapeutic approaches. Developing optimized contrast agents is central to optimizing the performance of both imaging and therapy. The ideal contrast agent for optical microscopy combines high resolution, specific targeting of functional groups, 3D imaging (depth resolution), low toxicity, low bleaching (long observation time at high signal), and high signal to noise (low background). Traditional organic dyes and fluorescent proteins are excited in the ultraviolet (UV) or blue spectral region, and emit at a longer-wavelength that is Stokes-shifted. The use of the short-wavelength excitation leads to a short penetration depth of the excitation light and give rise to autofluorescence, photobleaching and photodamage to biological specimens. It is thus primarily suited to pathological and in-vitro imaging, but has limited applicability to live imaging. In contrast, near-infrared (NIR) multiphoton microscopy offers high penetration depth and high spatial and temporal resolution, with low autofluorescence backgrounds. However, conventional two-photon microscopy, using two-photon dyes, requires high excitation densities (106 – 109 W/cm2) attributed to the simultaneous absorption of two coherent NIR photons, and as a result, expensive pulsed lasers are required. Imaging using UCNPs combines conceptual strengths of both fluorescence microscopy and 2-photon imaging, while avoiding common drawbacks. In the co-doped rare earth ion upconverter system studied here, the Ytterbium and Erbium dopant couple, the upconversion occurs through an energy transfer upconversion (ETU) process, where the Yb3+ ion transfers its energy to the Er3+ ion. Hence, imaging using UCNPs exploit the low excitation intensity of fluorescence microscopy, such as that provided by a more affordable continuous wave (CW) diode laser, while retaining the benefits of the large penetration depth and background of two-photon fluorescence. These materials have been investigated for their bio-applications, as reporters for immunoassay, biolabels, bio-therapy and spectral conversion in solar cells, where they increase the absorption of sub-bandgap light. Despite using a real rather than virtual intermediate state, the brightness and upconversion efficiency of these nanoparticles is not comparable to that of semiconductor nanoparticles and dyes. The down-scaling of particle size that is needed to promote bio-distribution and the body clearance of such ultra-small nanoparticles also leads to a rapid loss of brightness. This has been attributed to the low absorption cross-section of the rare earth ion dopants. That is because transitions to the inner 4f-shell levels in rare earth ions are only very weakly allowed; hence their absorption coefficients are very small, limiting their maximum emission intensities. Although that shortcoming is partially compensated by its zero background fluorescence and its non-blinking and non-bleaching properties, we show that plasmonics can increase the absorption and emission efficiencies.

The Nanoparticles

The doped core and un-doped shell were grown in a 2-step process. For growth of the doped core, 2.1 mmol of sodium trifluoroacetate, 0.78 mmol of yttrium trifluoroacetate, 0.199 mmol of ytterbium trifluoroacetate, and 0.018 mmol of erbium trifluoroacetate were combined in 6.2 ml of oleic acid and 6.3 ml of octadecene. The mixture was heated at 120oC for 30 minutes, after which, the temperature was increased to 330oC for 55 minutes for core growth. After cooling, the doped nanocrystals were precipitated and washed 6 times with ethanol, and dried under filtered air flow.

The particles were surrounded by an undoped core to shield the emission from absorption by the metal coating. Absorption by the metal increases dramatically at distances less than a few nanometers, as the evanescent fields there have sufficient momentum to create electron-hole excitations in the metal, so all the emission energy is transferred to these excitations in the metal, and eventually absorbed by the metal. Thus, none scatters to reach the far-field and be detected. Particles and the coating are shown in Fig. 1.

(a)Fig bare UCNP (b)Fig Au seeds UCNP (c)Fig Au layer UCNP

Figure 1. (a) TEM image of the NaYF4: Yb3+, Er3+ center (335 nm)/undoped NaYF4 shell (60 nm)/silica–amine shell (10 nm), (b) view of the silica–amine shell with 3–5 nm diameter gold seeds attached on the silica–amine surface and (c), view of the gold seeds, (d) view of a gold shell (10–11 nm).

  • Excitation Enhancement

    The excitation is enhanced at a wavelength that is determined by a combination of the particle size and the metal thickness. For our particles, emission enhancement resonance conditions determined the particle size, and the excitation wavelength is set by the UCNP absorption, so we optimized the thickness of the gold layer to tune the plasmon to the excitation for this size particle. The result is sown in Fig. 2. The internal fields are shown for a symmetric section of the particles so that the origins of the resonance can be deduced. In practice, the gold film is not perfect as the calculations assume (it is granular), so we need to adjust the thickness based upon the volume fraction occupied by gold in an effective medium theory calculation.

    Fig UCNP gold thickness

    Figure 2. The scattered light signal is calculated as the gold thickness changes for a fixed particle size and incident wavelength.

  • The enhancement of the internal fields is largely near the edges, closer to the metal, and depends upon the orientation relative to the incident direction and light polarization. Since we study single particles that are bright, only the strongest case needs to be considered as those particles are 'self-selected.' Key examples are shown in Fig. 3.

    Fig excit UCNP

    Figure 3. Four different relative configurations between the incident light direction, excitation polarization and nanocrystal orientation. (a) The light is incident from above the plate with polarization apex–apex (the red ones). (b) The incident light is polarized vertically and enters towards the red apex. (c) The light incident from above is polarized face–face (perpendicular to the red faces). (d) The light incident towards the red face is polarized vertically.

  • Emission Enhancement

    When the emission wavelength 'fits' nicely into the particle in all three dimensions, then we call it a resonance and the light arranges itself to be a minimum at the metal. This reduces dissipation, since an electric field at a metal causes a current that will experience Ohmic dissipation. Note that the wavelength within the particle material should be used. An example of this is in Fig. 4(a), and the blue wavelength to a maximum in Fig. 4(c). Off resonance, the electric field is not well organized and higher fields will be present near the metal, resulting in increased Ohmic losses (dissipation), so less scattered light into the far-field. An example of this is in Fig. 4(b) in which the resonance in the vertical direction is quite good, but not in the plane of the hexagon. This red wavelength corresponds to a minimum in Fig. 4(c).

    (a)Fig emission resonance (b)Fig emission not resonant (c)Fig enh. calc.

    Figure 4. The magnitude of the electric field emitted by a dipole of unit strength near the apex of the doped core region is shown at the surface of the gold layer: (a) emission at 540 nm with resonance as indicated by the symmetric patterns on all surfaces, (b) emission at 660 nm not at resonance, as observed by the disorganized pattern on the top. Note the strong electric field levels at the front right of the top (and the color scale 7 times larger than in (a)). (c) The particle radiated power divided by the dipole source power calculated at several representative dipole positions within the doped UCNP volume and averaged. Two different polarizations were separately calculated. Note that there is always some Ohmic loss in the metal coating, so the numbers are all less than one.

  • Origins of Green/Red Enhancement Ratio

    Figure 5(a) shows the various optical and nonoptical (phonon) transitions in the UCNP system. On the right is the Yb absorber. The dashed down arrow is resonant with several transitions in the Er system, indicated by dashed up arrows that can take the energy from the Yb relaxation. The dotted down arrows are non-radiative and the colored down arrows are radiative decay. An inspection shows that a green emission can follow from two photon absorptions and one nonradiative decay, while a red photon can result from two photon absorptions and two nonradiative decays, via two different paths, one of which competes with green emission. Thus, if photon emission transitions are sped up, the green transition is favored over the nonradiative to red transition, and the green signal increases compared to red. This is expected near the metal but not far from it. The modeling shows this result in Fig. 5(b), where the emission points near the particle edge, closer to the metal, favor green emission while the points in the center of the particle, not strongly influenced by the metal, are essentially flat vs. wavelength. These plots also include the excitation gain (not in the last figure), so the factors are greater than one.

    (a)Fig transitions (b)Fig Enh. Positions

    Figure 5. (a) TEM image of the NaYF4: Yb3+, Er3+ center (335 nm)/undoped NaYF4 shell (60 nm)/silica–amine shell (10 nm), (b) Total averaged emission enhancement of a single gold- shell nanoparticle, from 500 nm to 700 nm for an optimal gold shell thickness of 10 nm is compared to the emission enhancements at several specific dipole source locations. Those near the edge (and metal) show a green/red enhancement difference.

  • Prediction of the Enhanced Spectrum

    Starting with the results of Fig. 5(b), the ratio of the emitted power to the total dipole power (measured on a surface within the cavity) of a single gold-shell nanoparticle calculated for each dipole position and wavelength, from 500 nm to 700 nm, and scaled by the excitation enhancement at that point. An optimal shell thickness of 10 nm is used. The overall average spectrum was obtained by a radius-weighted average over the 15 dipole locations and two orthogonal polarization directions within the doped nanocrystal core. The weighting accounts for the greater volume of nanocrystal at larger distances from the center. The calculations show an intensity enhancement for green 550 nm emission of ~2.6 times, which is comparable to the measured intensity enhancement of ~2.3 times. The discrepancy can be attributed to the assumption of a perfect continuous gold shell, the number of dipoles modeled, and the accuracy of the dielectric constants used. The green/red emission ratio is near 1 for an uncoated nanoparticle, whereas the measured ratio for our gold-shell coated nanoparticles is found to be ~1.9. The results in Fig. 6 indicate that finite element calculations can explain some of this increase. The emission spectrum was obtained by scaling the spectrum of an uncoated nanoparticle by the position-dependent, polarization and emission angle averaged, unit strength dipole spectrum, and by the position-dependent excitation power. The emission angles were limited to those captured by a collection lens similar to that used in the experiment, and an optimal nanoparticle orientation was used, since the brightest nanoparticles were chosen for experimental study. The experimental spectra for core nanoparticles with and without a gold shell are also plotted for comparison. These calculations assume constant transition probabilities in the Yb3+/Er3+ system, as a result of interposing a 60 nm thick undoped buffer layer between the doped upconverting center and the gold shell. Thus, the calculated enhancement must be due to electromagnetic effects. It must come from the emission process, since the excitation light, enhanced by plasmons, is the same for all emission. The electromagnetic effects, shown by the calculation, do a good job of explaining the enhancement of the green emission, suggesting that the undoped buffer layer is thick enough for this wavelength. However, the electromagnetic effects overestimate the enhancement of the red emission, suggesting that either the (ohmic) losses in the gold are higher than expected or that some modification of the rates in the Yb3+/Er3+ system reduced the red emission. We suspect that the edges of the doped core are close enough to the metal that the red (longer wavelength) optical processes are impacted (sped up) by the interaction. These Yb3+/Er3+ system changes would be more important in smaller particles and those without an undoped buffer layer.

    Fig calc spectraP

    Figure 6. Modeled emission spectrum of the gold-shell nanoparticles plotted against the experimental spectra for uncoated nanoparticles for comparison.

  • Conclusion

    In summary, we have synthesized upconverting gold-shell nanoparticles, demonstrated enhanced overall emission, and shown enhancement of the green compared to the red emission at the single particle level. Our absorption spectrum demonstrated the plasmon resonant coupling of the gold shell to the upconverting core. We also show that the electric field distribution and hence excitation enhancement is affected when the nanoparticles are aggregated. Our finite element calculations reinforce our experimental observations that the upconverting cores are indeed plasmonicly coupled to the gold nanostructures and experience electric field enhancement within the cores. We also show that the regions near the edge of the doped cores are responsible for most of the green/red emission ratio increase. Hence, the excitation enhancement is optimal with excitation normal to the hexagonal face, with polarization aligned diagonally to the hexagonal corners and also perpendicular to the edges.


  • [1] Ling Li, Kory Green, Hans Hallen and Shuang Fang Lim, "Enhancement of single particle rare earth doped NaYF4: Yb, Er emission with a gold shell," Nanotechnology 26, 2015 [online]. [local preprint]
  •  More info is in the papers.

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