Distance Dependence of Gold-Enhanced Upconversion luminescence in Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ Nanoparticles

Abstract

We report a localized surface plasmon enhanced upconversion luminescence in Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺ nanoparticles when excited at 980 nm. By adjusting the silica spacer's thickness, a maximum 9.59-fold enhancement of the green emission was obtained. Effect of the spacer distance on the Au-Y₂O₃:Yb³⁺, Er³⁺ green upconversion mechanism was numerically simulated and experimentally demonstrated. In theory for radiative decay and excitation rates, they can be largely enhanced at the spacer thicknesses of less than 70 and 75 nm, respectively, and the quenching can be caused by the non-radiative energy transferring at the distance of less than 55 nm.

Introduction

Fluorescence imaging using nanoparticles could be a powerful tool for both biological studies and clinical medical applications due to its potential of convenience, high resolution and high sensitivity ¹. Conventional biolabels for imaging mainly include organic dyes and quantum dots (QD). Unfortunately, organic dyes are normally vulnerable to chemical and metabolic degradation, limiting the long-term cell tracking in experiments. On the other hand, possible toxicity of residual QDs has been a wide and persistent concern, even though much work has been underway to synthesize less harmful QDs or to enhance their biocompatibility ². Furthermore, excitation of these traditional biolabels usually requires the use of high energy photons such as UV radiation, which actually results in a series of drawbacks: (i) low signal-to-noise ratio due to the significant auto-fluorescence from biological samples under UV irradiation, (ii) low penetration depth due to the considerable absorption and scattering effects for short wavelength photons, and (iii) possible severe photo-damage on cells and even the cell death further caused by the long-term irradiation of such high energy photons ³. Therefore, it is desirable to obtain new fluorescent biolabels that can be excited by lower energy infrared (IR) light, which will be much safer to human body and can penetrate into tissues as far as several inches. Upconversion luminescent nanoparticles, which can convert long wavelength radiation into short wavelength fluorescence via a two- or multi-photon mechanism, are emerging as a new class of fluorescent biolabels ^4-7. Unfortunately, this upconversion process in general associates with low efficiency.

Past researches have been extensively focused on enhancing upconversion luminescence by using metallic nanoparticles or nanostructures ^8-12. In 2009, Yan et al obtained 2.3 and 3.7-fold enhancement for green and red emissions in NaYF₄:Yb³⁺, Er³⁺ nanocrystals by attaching them to Ag nanowires ¹³. In 2010, Benson et al. demonstrated a 3.8-fold upconversion enhancement by catching a single NaYF₄:Yb³⁺, Er³⁺ nanocrystal adjacent to an Au nanoparticle ¹⁴. Zhang et al. showed that the enhancement factor of approximately 2.6 can be achieved by attaching NaYF₄: Yb³⁺,Tm³⁺ hexaplate nanocrystals to gold nanoparticles ¹⁵, and a 4-fold upconversion enhancement was also found in Ag/Y₂O₃:Er³⁺ nanoparticles ¹⁶. Most impressively, a five-fold overall enhancement of upconversion emission was demonstrated in NaYF₄: Yb³⁺, Er³⁺ nanocrystals when coupled them with gold island films ¹⁷. However, all previously reported enhancement factors were less than 5-fold, and in many cases, quenching was unavoidable ^{18, 19}, mainly due to the following reasons: (1) frequency mismatching between the localized plasmon resonance (usually determined by the used metal, their shape, size, the dielectric environment, and the spacer distance) and the used emission/excitation light, (2) a competition of a few processes including a increase of the excitation rate by the local field enhancement (LFE) ²⁰, an enhancement of radiative decay rate by the surface plasmon-coupled emission (SPCE) and quenching that reduces the efficiency caused by the non-radiative energy transferring (NRET) from the upconversion material to the metal surfaces ^21,22, all of which will be greatly dependent on the spacing distance between the upconversion material and the metal ^23-26.

In this research, we have adopted a series of strategies with a goal to further enhance the upconversion luminescence: 1) instead of using Ag in many previous researches, we selected Au nanoparticle, considering a better overlapping between Au's intrinsic plasmon resonance and the visible emission; 2) the most widely used sensitizer Yb³⁺ was co-doped with the activator Er³⁺, first-timely in Y₂O₃ as shell to cap onto the SiO₂ spacer and the Au core in the Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺ (core/spacer/shell) nanostructure to be synthesized in this work. This was intended to increase the absorption cross section of Er³⁺ ions around 980nm; 3) the SiO₂ spacer thickness between Au and Y₂O₃:Yb³⁺, Er³⁺ was intentionally changed with a goal to optimize the core/spacer/shell structure, and also to compare with the previous results ¹⁶. In our results, a maximum 9.59-fold enhancement of green emission was obtained in ~30 nm Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺ nanoparticles with an optimized distance, and the distance's effect on the excitation rate, the radiative decay rate and the NRET were also simulated using a 3D-comsol in details, which would help us to further understand the enhancement process.

Experimental and characterizations

Synthesis of gold nanoparticles

Gold Nanoparticles were prepared using the method introduced by Frens ²⁷. A mean diameter of 30 nm Au nanoparticles were prepared by rapidly injecting a sodium citrate solution (1.3 mL, 38.8mM) into a boiling HAuCl₄ aqueous solution (100 mL, 0.3 mM) under vigorous stirring. After refluxing for 15 min., product was cooled to room temperature.

Synthesis of Au/SiO₂ nanoparticles

By a modified Stöber method ²⁸, the Au/SiO₂ nanoparticles were synthesized. The freshly prepared gold colloid solution (10 mL) was added to isopropanol (40 mL) in a 100 mL conical flask. Under vigorous stirring, ammonia (2 mL, 28~30%) and tetraethoxysilane (TEOS) were subsequently added until a homogeneous solution was formed. After stirring for 8h, the product was separated by centrifugation at 6000 rpm for 15 min. and washed with water until pH=7.

Synthesis of Au/SiO₂/Y₂O₃:20at%Yb³⁺,2at%Er³⁺ (20/2%) nanoparticles

The final product was synthesized via homogeneous precipitation and calcination ²⁹. The as-prepared 0.05g Au/SiO₂ nanoparticle was dissolved in 100 mL deionized water. After ultrasoniced for 30 min., 1.78 mL (0.1M) Y(NO₃)₃, 0.20 mL (0.1M) Yb(NO₃)₃ , 0.02 mL (0.1M) Er(NO₃)₃ solutions and 2.7g urea were subsequently added to the solution, then stirring 30 min. until uniform dispersion. The solution was heated to 80℃ for 5h with continuous stirring, then isolated by centrifugation and washed three times with water and ethanol. The final product was obtained after dried under vacuum at 80℃ for 12h, and calcined for 3h at 800℃. Figure 1 shows the detailed synthesis procedure of the Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ nanoparticles.

Figure 1.

Synthesis procedure of the Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺ nanoparticles.

Synthesis of Y₂O₃:Yb³⁺, Er³⁺ (20/2%) hollow nanoparticles

The Y₂O₃:Yb³⁺, Er³⁺ hollow nanoshells were synthesized by using NaOH solution (5M) to etch silica in the SiO₂/Y₂O₃: Yb³⁺, Er³⁺ nanoparticles.

Characterization of physical properties

The sizes and morphologies of all samples were observed by JEOL JEM-2011 transmission electron microscope (TEM) under a working voltage of 200 kV, as well as the EDX spectrum. The phase composition and crystallinity of the samples were characterized by XRD (Philips x'pert diffractometer employing Cu Kα radiation (λ=1.5405Å). Ultraviolet-visible-near infrared (UV-Visible-NIR) absorption spectra were recorded with a Shimadzu SolidSpec-3700.

The upconversion emission spectra were recorded by use of Zeiss CLSM 710 confocal microscopy with a 100x plan-Apochromat objective (1.4 oil) equipped with a 980nm laser. All Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ samples with different spacer thicknesses were prepared in the same concentration in ethanol solution. We then spun 20μL from the bulk solution on a cover glass substrate, in this effort we could suggest all the samples have the same concentration. The upconversion luminescence data of such nanoparticles was analyzed as follows: Firstly, we randomly choose a luminescent area of 84.85μm×84.85μm from the full image of total upconversion luminescence. Secondly, we calculated the emission intensity per μm² from the selected area. This process was repeated a few times on different selected area positions, and then we average the emission intensity with the repeated times. Finally, this averaged emission intensity per μm² was multiplied with the superficial area of a single core/spacer/shell nanoparticle, and then we obtained the approximate upconversion emission from a single nanoparticle. This entire averaging process is statistically meaningful, and is necessary in our work because the setup we used cannot distinguish a single nanoparticle.

Results and discussion

The nanoparticles were characterized by TEM, EDX, XRD and HRTEM, as shown in Figure 2. Figure 2(a) shows a typical TEM image of the samples in which well-dispersed Au nanoparticles with an average size of 30 nm can be clearly seen. After the surface modification with a ~40nm thick SiO₂ dielectric spacer, the Au/SiO₂ becomes more uniform than before and highly mono-dispersed (Figure 2(b)). Then, a layer of ~12nm Y₂O₃: Yb³⁺,Er³⁺was coated on the Au/SiO₂ surface (Figure 2(c)), which can be demonstrated by the energy-dispersive X-ray microanalysis (EDX, Figure 2(d)) and X-ray diffraction (XRD, Figure 2(e)) patterns. A HRTEM image of Y₂O₃:Yb³⁺,Er³⁺ layer indicates the adjacent lattice fringes distance of 0.306nm, which can be assigned to the {222} crystal plane of the Y₂O₃ bcc phase (the insert of Figure 2(c)). The XRD patterns show well-defined peaks, indicating the high crystallinity of the as-prepared nanoparticles. Except the hollow Y₂O₃:Yb³⁺, Er³⁺ (Figure 4(a)), all of the patterns show Au characteristic peaks with a fcc structure according to the PDF 04-0784 [Fm-3m (225)]. Because of the amorphous nature of the SiO₂ layer, there are no corresponding diffraction peaks. The Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ shows Y₂O₃ characteristic diffraction peaks indicating a cubic structure with a space group Ia-3(206) (PDF 01-0831), and the similar diffraction peaks also exist in the hollow Y₂O₃:Yb³⁺, Er³⁺ patterns. Besides, TEM images of all the ~30nm Au/SiO₂ and ~30nm Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ nanostructures with different silica thicknesses were shown in Figure 3, which indicated that the thickness of Y₂O₃ shells are about 12 nm in all the samples.

Figure 2.

TEM images of (a) ~30nm Au (b) ~30nm Au/~40nmSiO₂ (c) ~40nmAu/~40nmSiO₂/~10nmY₂O₃:Yb³⁺,Er³⁺ (the insert is HRTEM image of lattice fringes of the Y₂O₃:Yb³⁺, Er³⁺ layer) (d) EDX in a selected area of Au core and (e) XRD patterns of discussed samples (* represents Au diffraction peaks).

Figure 4.

(a) The upconversion spectra in Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺ nanoparticles with different SiO₂ thickness and the hollow Y₂O₃:Yb, Er (b) the plots of EF_green , EF_red and I_(G/R) verse silica thickness (the integral of green emission intensity from 500nm to 560nm, the integral of red emission intensity from 650nm to 750nm); (c) UV-VIS absorption spectra of all the discussed samples.

Figure 3.

TEM images of (a) ~30nmAu/~25nmSiO_2; (b) ~30nmAu/~25nmSiO₂/~12nmY₂O₃:Yb³⁺, Er³⁺; (c) ~30nmAu/~30nmSiO₂; (d) ~30nmAu/~30nmSiO₂/~12nmY₂O₃:Yb³⁺,Er³⁺; (e) ~30nmAu/~40nmSiO₂; (f) ~30nmAu/~40nmSiO₂/~12nmY₂O₃:Yb³⁺,Er³⁺; (g) ~30nmAu/~45nmSiO₂; and (h) ~30nmAu/~45nmSiO₂/~12nmY₂O₃:Yb³⁺,Er³⁺.

In order to investigate the distance-dependence of metal-enhanced upconversion luminescence in Au/SiO₂/Y₂O₃:Yb³⁺,Er³⁺nanoparticles with different SiO₂ thicknesses. The samples were measured by the Zeiss LCSM 710 confocal microscopy with a 100× plan-Apochromat objective (1.4oil) equipped with a 980nm laser. There are two main characteristic emission bands observed in Figure 4(a), corresponding to ²H_11/2/⁴S_3/2-⁴I_15/2 (green emission at ~524nm, ~549 nm) and ⁴F_9/2 -⁴I_15/2 (red emission at ~665 nm) radiative transitions, respectively. Here, we defined a parameter EF_green (enhancement factor for green emission) and EF_red (enhancement factor for red emission) by dividing integrated fluorescence intensities of Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ nanoparticles by those of hollow Y₂O₃:Yb³⁺, Er³⁺ nanoshells in the range of 500-560nm and 650-750 nm, respectively. In contrast to hollow Y₂O₃:Yb³⁺, Er³⁺nanoparticles, the green emission intensity of all core/spacer/shell nanoparticles is enhanced (EF_Green>1) (Figure 4(b)) and the corresponding red emissions of all samples are quenched (EF_red<1), which is attributed to the surface plasmon resonance frequency of Au nanoparticles having strong overlap with green emission band (Figure 4(c)), that is, an increase in emission rate by surface plasmon coupled emission (SPCE). SPCE is characterized by the increase of upconversion intensity accompanied by a decrease in the lifetime of upconversion nanoparticles located in the proximity of the metallic nanostructures. Therefore, decay curves of samples with and without Au core (Y₂O₃:Yb³⁺, Er³⁺ hollow shell) have been also carried out. As shown in Figure 5, Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ nanoparticles exhibit shorter lifetime compared with the corresponding Y₂O₃:Yb³⁺, Er³⁺ hollow shell, which supply direct proof for the conclusion that the enhancement originates from the increased emission rate. With the increase of SiO₂ thickness, EF_Green increased and reached the maximum 9.59 when SiO₂ thickness is ~40 nm, and the ratio of green to red emission I_(G/R) tracks well with the EF_Green across the range of SiO₂ spacers, demonstrating a preferential enhancement of the green emission band. As mentioned in the introduction, the metal-enhanced/quenching fluorescence resulted from a competition of processes including an increase of excitation rate by LFE ²⁰, an enhancement of radiative decay rate by surface plasmon-coupled emission (SPCE) and the quenching caused by NRET from upconversion materials to metal surfaces ^{21, 22}. Near the metal surface, the upconversion intensity fall rapidly as the NRET from the emitters to the metal occurred. With the increase of the distance, SPCE and LFE became dominant and the EF reached the maximum at about 40 nm. Because the enhancement of emission and excitation rate cause by SPCE and LFE is proportion to the square of localized electric field, and the electric field decreased with the distance increase, the upconversion luminescence became weaker beyond the 40 nm distance.

Figure 5.

Decay curves for (a) Y₂O₃:Yb³⁺,Er³⁺ hollow shells; (b)Au/~45nmSiO₂/Y₂O₃:Yb³⁺,Er³⁺; (c)Au/~40nmSiO₂/Y₂O₃:Yb³⁺,Er³⁺; (d)Au/~30nmSiO₂/Y₂O₃:Yb³⁺,Er³⁺ and (e)Au/~25nmSiO₂/Y₂O₃:Yb³⁺,Er³⁺ nanoparticles.

To understand deeply the distance-dependence of upconversion luminescence in Au/SiO₂/Y₂O₃:Yb³⁺, Er³⁺ nanoparticles, the electric field enhancement factor E′=|E_gold|/|E_o|, where E_gold and E_o were the electric field with and without gold, was calculated by 3D-comsol, as presented in Figure 6. We studied a very general case by 3D-comsol: the Au nanoparticles were imbedded in a homogeneous, lossless SiO₂ medium, supposing the Au nanosphere is small as compared to the light wavelength. In Figure 6, d is the distance to the surface of Au nanospheres. The wavelength-dependence of electric field is similar with the measured absorption spectra of Au nanoparticles (see Figure 4(c)), and the greatest electric field strength was found to 8.9 at 525 nm. With the increase of d, the electric field falls rapidly, and near to unity at d=90 nm. The detailed unit-less parameters: the electric field enhancement factor at 549 nm and 980 nm (E^′_549nm and E^′_980nm), and the overall enhancement factor of emission intensity at 549 nm, L_549nm, are listed in Table 1.

Figure 6.

Electric field enhancement (E) dependence as a function of the wavelength and distance to a ~30nm diameter Au nanoparticle surface.

Table 1.

The detailed list of the electric filed enhancement factor at 549 nm and 980 nm (E_549nm and E_980nm) and enhancement factor at 549 nm (L_549nm) for the Au/SiO₂/Y₂O₃:Yb,Er nanoparticle.

The enhancement factor in the excitation (L_ex) and emission (L_em) rates as a funtion of the distance between Au and Y₂O₃:Yb³⁺,Er³⁺ (d) could be expressed as ³⁰:

L_ex(d) = L_ex(d=0) exp(-d/R_ex) + 1 (1)

L_em(d) = L_em(d=0) exp(-d/R_em) + 1 (2)

R_em and R_ex is the characteristic distance over which L_em(d=0) and L_ex(d=0) decrease to 1/e exponentially, respectively.

The enhancement factor of excitation and emission rate by surface-plasmon-coupled emission is also proportional to the square of corresponding electric field enhancement ²⁶. Hereby, we analyzed our experimental upconversion intensity at 549 nm at various distance from the plasoninc metal-nanostructured surface. By fitting the experimental data with equation 1 and equation 2, the values of the free parameters L_em(d=0)=5.7, L_ex(d=0)=1.3, R_em=16.2 nm and R_ex=23.9 nm were obtained. Near the metal surface, the fluorescence intensity fall rapidly due to non-radiative energy transfer from the fluorescence materials to the metal surface. A complete, classical theory of NRET has been published by Campion et al.. When d≤λ, the expression of the unitless parameter quenching factor (Q) caused by NRET is as follows ³¹:

Q = ω_ET/ω_R = {ω_R×Ιm[(ε₂-ε₁)/(ε₂+ε₁)]×λ³} / [8π³×(ε₁)^3/2×4d³×ω_R] = {Ιm[(ε₂-ε₁)/(ε₂+ε₁)]×λ³} / [32π³×(ε₁)^3/2×d³] (3)

Where ω_ET is nonradiative rate, ω_R is the radiative rate constant, λ is the emission wavelength; ε₁(549nm)=1.46, is the dielectric constant of the spacer layer, and ε₂(549nm)=0.306+2.88i, is the complex dielectric constant of the metal ³².

The enhancement of excitation and radiative decay rate (L_ex, L_em) and the quenching factor by NRET (Q) are plotted as a function of Au-Y₂O₃ nanoparticle distance, as shown in Figure 7. It is apparent from distance-dependent plot that the radiative decay rate could be enhanced even at a distance of ~0 nm, and the enhanced excitation takes place below 75 nm. The quenching factor by NRET (Q) decreased rapidly as the distance increased, and near to L_emand L_ex at a distance of 55 nm.

Figure 7.

Changing of excitation rate L_ex(S^-1) and radiative decay rate L_em(S^-1), and the quenching factor by NRET (Q), as a function of the spacing distance from the metal surface.

Conclusions

In conclusion, plasmon-enhanced upconversion luminescence was observed in Au/SiO₂/Y₂O₃: Yb³⁺,Er³⁺ nanoparticles. With the increase of silica dielectric thickness, the green emission intensity increased and reached a maximum 9.59-fold enhancement when silica thickness is ~40 nm, which attributed to the competition of non-radiative energy transfer (NRET) from Y₂O₃:Yb³⁺, Er³⁺ to Au core, enhancement of radiative decay rate by surface plasmon-coupled emission (SPCE) and an increase of excitation rate by local field enhancement (LFE). The distance's effect on the enhancement factor of radiative decay and excitation rate and quenching factor by NRET was simulated. The radiative decay rate could be enhanced even at a distance of 70 nm, and the enhanced excitation rare takes place below 75 nm. The quenching factor by NRET (Q) decreased rapidly with the distance increase, and is near to L_em and L_exat a distance of 55 nm.