Nd/Yb/Ho-doped hexagonal NaYF4 nanocrystals were synthesized and excited at 808 nm to avoid an overheating effect under 980 nm laser excitation. A core/double-shell nanocomposite (NaYF4:Nd/Yb/Ho@SiO2@Ag nanoparticles) was then developed to enhance upconversion luminescence intensity. The fabricated system exhibited a metal-induced effect and was used to determine the influence of SiO2 layer thickness on upconverting optical properties. SiO2 layer with different thicknesses were used to adjust the distance between NaYF4 nanocrystals and the outer layer of Ag nanocrystals. The largest enhancement of upconversion luminescence occurred when 10 nm-thick SiO2 shell was used, with enhancement factors of 15 and 7.5 for green and red emissions, respectively. Results indicated that the degree of luminescence enhancement was correlated with the competition of the three interactions occurred between fluorophores and metal NPs. After coating with Ag nanoparticles, some modifications were found on the upconversion processes of NaYF4 core nanoparticles and the underlying mechanisms were discussed respectively. The controllable upconversion luminescence renders metal-upconverted nanocomposites as suitable for bioassay, bio-imaging, and energy conversion applications, which require high sensitivity and low background.
© 2016 Optical Society of America
Rare earth-doped upconversion (UC) emission nanoparticles (NPs) have been widely studied and developed because of their considerable potential in biological sensing [1–6], biological imaging [7–10], drug delivery [11,12], photodynamic therapy , and photoactivation . Compared with other fluorescent materials, such as organic dyes and quantum dots, upconversion nanoparticles (UCNPs) display anti-Stokes emission; this property allows minimization of autofluorescence and photodamage and enables high penetration depth in biospecies when excited with a near-infrared (NIR) light source [15–17]. In particular, lanthanide-doped fluorides are suitable materials for biological applications because of their excellent physicochemical stability and high UC efficiency . β-NaYF4 is used as an optimal matrix because of its low phonon energy and high chemical stability . These UCNPs, which consist of sensitizer and activator ions, have been the focus of various studies. In most cases, UCNPs are simultaneously doped with Yb3+ and Ln3+ (Ln = Er3+, Tm3+, Ho3+) to provide visible emissions under 980 nm laser excitation.
Despite these advances, several challenges must be addressed to ensure widespread biological applications of UCNPs. One notable question is that the physically unaltered excitation band of Yb3+ ions centered at 980 nm (the peak absorption of Yb3+ ions) overlaps the absorption band of water molecules in biological samples, as shown in Fig. 1; as such, the 980 nm excitation source will lead to overheating, thereby inducing significant cell death and tissue damage [20,21].
Numerous studies were performed to determine the appropriate excitation wavelength within the “biological window”. As shown in Fig. 1, the absorption intensity of water at 808 nm is about 18 timers lower than that at 980 nm. Among rare earth ions, Nd3+ features a sharp absorption band centered around 800 nm. Yb3+ ions, which act as a bridge not a sensitizer ions, can transfer energy from the Nd3+ ions to the activators. Previous studies used core/shell structures to separate Nd3+ and activator ions and therefore avoid quenching effects . However, the interface between the layers may decrease the luminescence intensity. In the present study, we subjected NaYF4 to triple doping, which will not lead to adverse cross relaxation because of low concentrations of Nd3+ and activator ions. To confirm our assumption, we performed a set of comparative experiments and presented the results in Fig. 2. The luminescence intensity of NaYF4:Nd/Yb/Ho co-doped with Yb3+ ions is much higher than that of NaYF4:Nd/Ho. This finding indicates that Yb3+ ions can act as the bridging center to induce energy transfer from Nd3+ ions to Ho3+ ions under 808 nm excitation.
In addition to the overheating effect, another important challenge is that UCNPs suffer from low emission efficiency because of surface quenching. Extensive efforts have been made to increase the excitation and emission efficiencies of UCNPs [23–28]. One way is to introduce metal NPs, such as Au and Ag. Nevertheless, previous studies many focused on NPs deposited on the substrates , or embedded in glasses , which rarely occur in biological systems. Therefore, an efficient approach must be developed to synthesize materials with optimum emission enhancement for biological applications.
This paper is the first to report the controlled synthesis and the UC luminescence property of NaYF4:Nd/Yb/Ho@SiO2@Ag core/double-shell nanocomposites under 808 nm laser excitation. Yb3+ ions act as a bridge to transfer energy from Nd3+ to Ho3+. The SiO2 shell prevents the phase change of NaYF4 and yields high UC efficiency by reducing the surface defects of NPs. SiO2 layer with different thicknesses are also used to adjust the distance between the core nanocrystals and the outer layer of Ag NPs to obtain the optimal UC emission. The two-photon process is the main UC mechanism for NaYF4:Nd/Yb/Ho NPs. However, the three-photon process has been included in our core/double-shell nanocomposites due to the presence of Ag NPs. This novel core/double-shell design combines the plasma resonance effect of precious metals and the excellent UC luminescence property of rare earth ions. What’s more, the prepared particles are hydrophilic and possess good stability in water. Therefore, the proposed design cannot only achieve high UC efficiency, but also satisfies the material requirements for biological applications.
Synthesis of β-NaYF4:Nd/Yb/Ho nanocrystals. All chemicals were of analytical grade and used without further purification. β-NaYF4:Nd/Yb/Ho (5%/20%/2%) NPs with a partical size of 26.5 nm were synthesized using a previously reported method . Typically, 0.73 mmol of YCl3, 0.05 mmol of NdCl3, 0.20 mmol of YbCl3, and 0.02 mmol of HoCl3 were dissolved into 2 mL of DI water in a flask and then vigorously stirred to form a clear solution. The solution was added with 6 mL of oleic acid and 15 mL of 1-octadecene, heated to 160°C for 30 min to form a homogeneous solution, and then cooled to room temperature. Briefly, 10 mL of a methanol solution containing NaOH (2.5 mmol) and NH4F (4 mmol) was slowly added into the flask and stirred for 30 minutes. After methanol was evaporated, the solution was heated to 300 °C under argon atmosphere for 1 h and then cooled to room temperature. Nanocrystals were precipitated and washed with ethanol solution for several times. The as-prepared nanocrystals could be easily redispersed in various nonpolar organic solvents such as cyclohexane, toluene, and chloroform.
Synthesis of β-NaYF4:Nd/Yb/Ho@SiO2 nanoparticles. Briefly, 0.2 mL of CO-520, 6 mL of cyclohexane, and 4 mL of 0.01 M NaYF4:Nd/Yb/Ho nanoparticle solution in cyclohexane were mixed and stirred for 10 minutes. Subsequently, 0.8 mL of CO-520 and 0.07 mL of 25 wt% ammonia were added to the container. After sonicating for 20 min, TEOS was slowly added into the microemulsion under constant stirring for 2 days. The obtained NaYF4:Nd/Yb/Ho@SiO2 NPs were washed with ethanol/water (1:1, v/v) and re-dispersed in 6 mL of water. The NPs were added with 0.01, 0.02, 0.03, and 0.06 mL of TEOS to prepare 3, 5, 10, and 20 nm silica-coated NaYF4 NPs, respectively.
Synthesis of β-NaYF4:Nd/Yb/Ho@SiO2@Ag nanocomposites. Briefly, 1.0 mL of silica-coated NaYF4:Nd/Yb/Ho NPs were added into 4.5 mL of 1.5% (w/w) 3-mercaptopropyl-triethoxysilane (MPS) in ethanol and stirred for 20 h at room temperature. The modified UCNPs were centrifuged and washed with ethanol twice. The NPs were then redispersed in 3.0 mL of 1.0 mM AgNO3 (ethylene glycol) solution, added with 60 μL of 100 mM ethanolamine (ethylene glycol) solution, and then stirred for 1 h. The NPs were centrifuged, washed with ethanol/water (1:1, v/v) several times, and dispersed in water.
Characterization of nanoparticles. The crystalline structures of the as-prepared samples were investigated by XRD analysis using a Rigaku D/max 2200 diffractometer with Ni-filter Cu Kα radiation (λ = 1.542Å) operated at 36 kV and 30 mA. Absorption spectra were obtained using a UV-visible spectrophotometer in the 600-1060 nm regions. Photoluminescence emission spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer. Luminescent decay curves were detected by Compact Fluorescence Lifetime Spectrometer C11367 under 808nm excitation. TEM images and EDS images were obtained using a JEOL 2100 transmission electron microscope operating at an acceleration voltage of 200 kV.
3. Results and discussions
Structures and morphologies of As-prepared samples. Fig. 3 shows the synthetic procedures of NaYF4:Nd/Yb/Ho@SiO2@Ag core/double-shell nanocomposites. The as-synthesized NaYF4:Nd/Yb/Ho NPs and NaYF4:Nd/Yb/Ho@SiO2 NPs were investigated by X-ray powder diffraction (XRD) studies (Fig. 4). The XRD patterns of NPs exhibit peak positions and intensities that can be indexed to hexagonal NaYF4 crystals (JCPDS file number: 16-0334). The XRD pattern of the core/shell sample consists of a broad band centered at a low-angle region, which is due to the diffraction among the irregular molecule layers of amorphous SiO2. The average crystallite size of the core nanocrystals was calculated according to the Scherrer equation (Eq. (1)):
The representative transmission electron microscopy (TEM) images at different magnifications reveal the uniform spherical shape of the core nanocrystals (Figs. 5(a)-5(c)). The size histograms corresponding to low-resolution TEM images are given in Figs. 5(d). The mean particle size d is about 26.5 nm with a standard deviation of σ/d = 4%; this finding confirms the uniform size and shape of the particles. Moreover, the high-resolution TEM (HRTEM) image of an individual nanocrystal (Figs. 5(c)) exhibits lattice fringes of the (100) planes with a d-spacing of 0.51 nm. NaYF4 NPs were then coated with a silica shell of controllable thickness through reverse microemulsion, which provided uniform silica coating around the NaYF4:Nd/Yb/Ho NPs (Figs. 6(a)-6(f)). The HRTEM image further indicates the amorphous structure of the SiO2 shell, which can be attributed to the absence of lattice fringes above the shell. After silica coating, the core NPs become hydrophilic and exhibit good chemical and photochemical stabilities, thereby allowing the growth of silver NPs on the outer shell in the aqueous phase. This core/double-shell design can be used to easily adjust the distance between NaYF4 NPs and Ag NPs by controlling the thickness of the SiO2 layer to achieve the maximum luminescence enhancement. After adding different amounts of TEOS, SiO2 coatings with external thicknesses of 20 (Figs. 6(a)), 10 (Figs. 6(c)), and 5 nm (Figs. 6(e)) were successfully synthesized. The use of thicker shells results in better particle dispersion. The fabricated NPs also maintain good uniformity and monodispersion property.
After silica coating, the core/shell NPs were modified with thiol groups by using MPS to introduce Ag NPs into the nanocomposites. With the coordination between Ag and –SH, the thiol groups allow NaYF4:Nd/Yb/Ho@SiO2 NPs to effectively capture Ag nanocrystals. The prepared Ag nanocrystals possess negative surface charges, whereas the modified NaYF4@SiO2 NPs are positively charged; as such, a strong electrostatic attraction occurs between these particles. The TEM images of NaYF4@SiO2@Ag nanostructures at different magnifications are shown in Figs. 7(a) and 7(b). Most Ag nanocrystals are attached to the surface of NaYF4@SiO2 NPs, and no free Ag nanocrystals exist. The HRTEM image of Ag NPs (Figs. 7(c)) shows the lattice fringes with a lattice spacing of about 0.202 nm, which is consistent with the (200) plane of Ag NPs. The UV-vis absorption spectrum of NaYF4@SiO2@Ag NPs (Figs. 7(d)) displays a peak at about 415 nm, which correspond to the surface plasmon resonance (SPR) band of Ag NPs. The plasmonic peak also shows considerable spectral overlap with the luminescence peaks of UCNPs, indicating that a large resonant enhancement of UC emission is possible. The EDS results (Fig. 8) confirm the presence of Ag, Si, O, F, Y, Na and lanthanides (Nd, Yb, Ho) and correspond to those in the core/double-shell sample.
UC luminescence properties. The UC photoluminescence spectra (Fig. 9) were determined using an 808 nm CW diode laser as the excitation source. The pure NaYF4 nanocrystals display bright green UC emission centered at 540 nm, which is due to the transition from 5S2/5F4 to 5I8 energy levels, as well as weak red UC emission centered at 646 nm, which correspond to the transition from 5F5 to 5I8. After the NaYF4 nanocrystals were coated with SiO2 layer of different thicknesses, the luminescence intensity decreases to some extent. As we know, the SiO2 shell can make the nanoparticles surface passivation. However, when the shell reaches a certain thickness, the amorphous SiO2 will not only absorb the excited light, but also the upconversion emission. So the emission intensity decreases. Insets (a) and (b) show the UC luminescence from the cyclohexane solution containing pure UCNPs and UCNPs@SiO2(10 nm), respectively, under 808 nm laser excitation. Similar results were observed under illumination from a 980 nm continuous wave (Fig. 10). Moreover, Figs. 11(a)-11(c) show that coating of silver NPs does not change the peak position and the spectra shape of UC luminescence. Interestingly, the emission intensity is strongly enhanced by the presence of Ag NPs. The extent of modulation depends on the distance between emitters and Ag NPs. The largest enhancement of UC luminescence occurred when 10 nm-thick SiO2 shell was used, with enhancement factors of 15 and 7.5 for green and red emissions, respectively, compared with those from the core/shell sample without Ag nanostructures. Meanwhile, insets (a) and (b) in Figs. 11(a) display the samples before and after coating with Ag NPs, respectively. Low enhancement factors are obtained in the UC luminescence for separation distances smaller or larger than 10 nm (Figs. 11(b) and 11(c)). These results are consistent with those detected under 980 nm excitation, as shown in Figs. 12. The UC emission spectra of UCNPs, UCNPs@SiO2(10 nm) and UCNPs@SiO2(10 nm)@Ag are displayed in Figs. 13(a). These findings, as well as the digital luminescent photos shown in Figs. 13(b), confirm that the proposed core/double-shell design exhibits increased UC emission intensity. The nanoscale fluorophore-metal interactions give rise to the process called metal-enhanced fluorescence, which is caused by the interaction between excited fluorophores and SPR in metals. What’s more, this effect is particularly strong in metal nanostructures. This phenomenon produces positive effects, such as increased quantum yield of fluorophores, decreased lifetimes, increased photostability, and remarkable potential for improved energy transfer.
In the present case, three interactions occur between fluorophores and metal NPs. First, enhanced radiative decay rate increases the quantum yield and therefore increases the UC emission efficiency. Second, an improved light harvesting by sensitizer ions can be expected, thereby leading to enhanced upconversion luminescence. Third, the electron transfer from the fluorophores to Ag NPs introduces an additional non-radiative deactivation channel, which decreases the UC emission intensity. Therefore, the degree of luminescence enhancement is correlated with the competition of the above three kinds of effects. The radiative decay rate increases because of the coupling of plasmon resonance of Ag NPs and emission of UCNPs. While the enhancement of the absorption of the sensitizer is the result of the effective coupling between the electric field of the plasmon with the transition dipole of the UCNP. Both of the effects can increase emission intensity. Moreover, the non-radiative decay rate increases because of electron transfer from UCNPs to Ag NPs, which reduces the UC efficiency. For samples with a very thin SiO2 layer, the non-radiative decay caused by electron transfer plays a dominant role and leads to less luminescence enhancement or even quenching. The silica coating also ensures the nanoscale separation between NPs and metal nanostructures, which is required to prevent metal quenching. All the three effects decelerate with increasing separation distance, and the electron transfer exhibits a higher decrease rate. The enhanced radiative decay rate and enhanced excitation efficiency prevail, thereby enhancing the emission efficiency. At very large separation distances, the radiative decay rate and excitation efficiency are not affected and the electron transfer does not occur. Under this condition, luminescence efficiency is also not affected by the presence of metal NPs. Consequently, with increasing separation distance, the overall effect of luminescence intensity initially increases, reaches the optimum enhancement, and then decreases. We also found that the luminescence enhancement of the three samples for the green band is higher compared with the red band because Ag plasmon exhibits a higher overlap.
Figure 14 shows the luminescence decay rates for NaYF4@SiO2(10 nm) NPs and NaYF4@SiO2(10 nm)@Ag NPs. The green emission in Ag-uncoated sample demonstrates longer lifetime (22.5 μs) than that in the coated sample (16.8 μs). As we know, the coupling of Ag NPs and UCNPs can increase the total radiative decay rate, thereby leading to significant effects, such as an increase in quantum yield and decrease in lifetime, which is consistent with that obtained from the UC luminescence lifetime measurements. The lifetime of Ag-coated NPs is attributed to the combined plasmonic and surface recombination effects. Meanwhile, the decreased decay times suggests that UC enhancement is indeed a metal-induced effect and is due to the electromagnetic coupling between metal NPs and UCNPs. The optimal distance for radiation enhancement arises from the balance between quenching and metal-induced enhancement. However, the intensity gain is much stronger compared to luminescence lifetime decrease. This result also proves that the existence of the second way to improve the emission intensity, which has be mentioned above.
Mechanisms for UC emissions. The dependence of UC luminescence intensity on the 808 nm excitation power of UCNPs was investigated to identify the relative mechanisms of UC (Figs. 15). Generally, the UC luminescence intensity IUCL increases in accordance with the nth power of the pumping power P, that is (Eq. (2)),Figs. 15(c)), which are higher than the n values for the NaYF4@SiO2 samples. The same behavior was observed under 980 nm excitation (Figs. 16). These results indicate that the emissions are two- and three-photon processes in UCNPs before and after Ag NPs coating, respectively. Therefore, the mechanism of UC radiation is affected by introducing Ag NPs.
Basing on the analysis of the UC emission spectra, measured decay times, and dependence of luminescence intensity on excitation power, we propose the mechanisms of the Nd/Yb/Ho system under 808 nm excitation (Figs. 17). As shown in Figs. 17(a), the UC excitation mechanism without Ag NPs can be interpreted as follows. Nd3+ ions are excited by 808 nm radiation, which corresponds to the Nd3+: 4I9/2→4F5/2 transitions. The excited ions at the 4F5/2 level are then rapidly relaxed to the lower level (4F3/2) by multi-phonon relaxation. Energy is efficiently transferred (ET) form the Nd3+ ions to the Yb3+ ions, 2F7/2 (Yb3+) + 4F3/2 (Nd3+) → 2F5/2 (Yb3+) + 4I9/2 (Nd3+). For the UC emissions of Ho3+, the excitation power dependence of green and red luminescent intensities exhibits a square relationship; hence, the emissions of the excited 5S2, 5F4 and 5F5 levels of Ho3+ are achieved through absorption of the two photons. The Ho3+ ion at the ground state can be excited to 5S2 and 5F4 through ET from the Yb3+ ion: 2F5/2 (Yb3+) + 5I8 (Ho3+) → 2F7/2 (Yb3+) + 5I6 (Ho3+) and 2F5/2 (Yb3+) + 5I6 (Ho3+) → 2F7/2 (Yb3+) + 5F4/5S2(Ho3+), thereby producing a green emission. In addition, a red emission (645 nm) can be acquired through the transition of Ho3+ ions from the 5F5 level to the ground state. After coating with Ag NPs, the UC mechanism varies to some extent (Figs. 17(b)). Once the distance between the emitters and the Ag NPs is extremely close, the precious metallic nanostructure displays a plasmon resonance arising from the collective oscillation of migrated electrons on its surface. The electromagnetic field around the emitters is significantly changed through coupling with the metal plasmon resonance, causing a change of the excitation properties and impacting on a multiphoton absorption involved in the UC process. Therefore, compared with the samples without Ag nanostructures, the intensity of UC luminescence is enhanced . Three successive energy transfers from the excited Yb3+ ions to the Ho3+ ions bring the latter directly to the 5G6 state. Several Ho3+ ions in the 5G6 state relax to the lower 5F4/5S2 and 5F5 states through multi-phonon relaxation; then, the electrons in the 5F4/5S2, and 5F5 states jump to the ground state 5I8 and yield green (540 nm) and red (645 nm) emissions.
The controlled synthesis and enhanced UC luminescence of NaYF4:Nd/Yb/Ho@SiO2@Ag core/double-shell nanocomposites under 808 nm laser excitation were successfully achieved for the first time. The silica coating facilitated the nanoscale separation between NPs and metal nanostructures, which is required to prevent metal quenching. The effect of separation distance on UC luminescence enhancement was also investigated and ascribed to the competition of the three interactions occurred between fluorophores and metal NPs. The optimum UC luminescence enhancement was observed when the thickness of SiO2 layer was 10 nm, with enhancement factors of 15 and 7.5 for green and red emissions, respectively. Furthermore, we demonstrated that Ag nanostructures reduced luminescence lifetime, thereby confirming the metal-induced effect on UC luminescence enhancement. The coating of Ag NPs led to the appearance of three-photon population process and the energy transfer mechanisms responsible for UC luminescence were also discussed. This study proposes that core/double-shell UCNPs are a novel tool for various biological applications.
This work was supported by the National Natural Science Foundation of China (Nos. 51272097, 61265004 and 61307111), and the Nature and Science Fund from Yunnan Province Ministry of Education (No. KKJA201432042).
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