Reversible luminescence switching of NaYF₄:Yb,Er nanoparticles with controlled assembly of gold nanoparticles

Abstract

Upon the tunable surface plasmon band of the pH-induced reversible assembly of gold nanoparticles mediated by cysteine, the manipulation of green and red upconversion emission, and the switching of the red emission of the NaYF₄:Yb,Er nanoparticles have been achieved in a facile and reproducible way.

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Design and realization of the optical manipulation for nano-scaled materials are of great challenge in nanoscience. An active and versatile method is especially required to achieve this goal for nanomaterials whose optical properties are difficult to be precisely control with preparation. NaYF₄:Yb,Er nanoparticles (NPs), a type of upconversion luminescent materials, can give out visible emission in the green and red region by absorbing two or three near-infrared (NIR) photons. Their upconversion luminescent behavior is desirable for the noninvasive detection with NIR light that falls within the “water window” of biological tissues and biolabels¹ which can improve signal-to-noise ratios.² However, the upconversion emission behaviour of NaYF₄:Yb,Er NPs is difficult to tune in an arbitrary and facile way.³ The surface plasmon band (SPB) of Au NPs together with their coupling from neighbouring Au NPs cover a spectral range from the visible to NIR with enhanced light absorption and scattering, which endows Au NPs with application in this broad range. As such they could provide a facile way to tune the optical properties of luminescent NPs through matching the SPB of Au NPs⁴ in the excitation or emission region without considering the complex factors in material preparation and modification. To manipulate the optical properties in an optional way, it is also expected that the SPB can be tuned repeatedly by reversible assembly and disassembly of Au NPs in a controllable way. Large shifts are especially desirable to match the requirements for different applications. However, few reports have realized SPB shifts larger than 100 nm. Furthermore, the reversibility in those studies is not satisfactory due to incomplete disassembly.⁵ Herein, we report a facile but efficient strategy to tune the upconversion emission of NaYF₄:Yb,Er NPs with the controllable shifting of the SPB of Au NPs by pH-controlled reversible assembly in the presence of cysteine.

Compared with relatively expensive polymers and biomolecules, the thiol-containing amino acidcysteine, which is abundant in nature, proved to be a good linker for Au NP assembly. It can be attached on the surface of Au NPs through its thiol group and therefore mediate the assembly through electrostatic interactions between the amino and carboxylic acid groups.⁶ Here we further develop the assembly into a reversible manner. The electrostatic interaction between the zwitterionic groups takes place reversibly by tuning the pH of the solution between acidic and basic. This strategy avoids the introduction of rigorous thermo-stimuli and organic solvents which might induce oxidation⁷ and aggregation of Au NPs. Fig. 1(a) shows the time-dependent extinction spectra (Hitachi U-3010 UV-Visible spectrophotometer) recorded during the assembly process by tuning the pH of the solution to 5 with HCl solution. The extinction spectrum of 9.4 nm Au NPs synthesized through a seed-mediated process⁸ exhibiting a SPB centered at 523 nm (red line in Fig. 1(a)) is presented for reference. By adding cysteine to a final concentration of 3.0 μM, the extinction at 523 nm is gradually decreased, and concomitantly a new peak appears at a longer wavelength. With increasing reaction time, the new peak becomes stronger and gradually red shifts to 619 nm (the black solid lines in Fig. 1(a)). The decreased extinction at 523 nm indicates the decreased number of isolated NPs. Meanwhile, a small red-shift which is caused by the aggregation of several NPs is observed (Fig. 1(a)). The appearance and gradual red-shift of the new extinction peak towards a longer wavelength indicate that the aggregates of Au NPs are developed in a large scale. Besides the extinction spectra, the clear color change of the colloidal solution under natural light is also recorded to identify the aggregation status of Au NPs. The colloidal solution is ruby red for isolated Au NPs, as shown in panel A of Fig. 1(b). Accompanied with the change of the extinction spectra, the color of the solution changes from ruby red to purple and becomes blue finally (panels B and C in Fig. 1(b)), which is well correlated to the aggregation status of Au NPs.⁹

Fig. 1 (a) Extinction spectral evolution of the assembly process (solid lines, pH = 5) and the disassembly behavior (dashed line, pH = 11) of 9.4 nm Au NPs. The extinction spectrum of the as-synthesized Au NPs is given as reference (red line). The extinction spectra were recorded every 3 min in the assembly process. (b) Digital picture of as-synthesized Au NPs (A), Au NPs assembled at pH = 5 for 45 and 60 min (B and C, respectively), and disassembled (at pH = 11) Au NPs (D). The concentration of cysteine is 3.0 μM.

As the pH of the solution is adjusted from 5 to 11 with the addition of NaOH solution, the electrostatic interaction between the zwitterionic groups is destroyed through the neutralization of the amino groups, resulting in the disassembly of the Au NPs. As a result, the extinction spectrum blue shifts, and the extinction peak stops at 530 nm rather than 523 nm (the dashed line in Fig. 1(a)). Moreover, compared with the as-synthesized Au NPs, disassembled Au NPs display a slightly broader and weaker extinction spectrum. Also a pink rather than a ruby red solution (panel D in Fig. 1(b)) is obtained in the end. These phenomena suggest that although a large number of Au NPs are separated into isolated particles, there still exist a small number of aggregates consisting of several NPs.

The above assembly and disassembly behavior of Au NPs was further confirmed with TEM observations (200CX microscope, JEOL, Japan). The as-synthesized Au NPs are randomly distributed as isolated particles (Fig. 2(a)). As the assembly develops with the addition of cysteine and acidity tuning, Au NPs first assemble into two-dimensional (2D) structures (Fig. 2(b)) and then three-dimensional (3D) architectures (Fig. 2(c)), causing the different extents of red-shifts in the extinction spectra. After the pH is adjusted to 11, the highly aggregated Au NPs separate into clusters of several NPs together with isolated ones (Fig. 2(d)). The incomplete disassembly of the Au NPs can be ascribed to the difficulty of OH⁻ to access into the interior of the densely packed cysteine and cetyltrimethylammonium bromide (CTAB) molecules around the Au NPs.

Fig. 2 TEM images of the as-synthesized (a), assembled (pH = 5, reaction time of 45 min (b) and 60 min (c), respectively), and disassembled (pH = 11, (d)) Au NPs. The concentration of cysteine is 3.0 μM.

To tune the luminescence of the NPs in a stable manner, the reversibility of the assembly and disassembly processes of Au NPs should be optimized. Elevated reaction temperature, cysteine concentration, and larger particle size endow Au NPs with higher assembly activity but lower disassembly activity (ESI† , Fig. S1−S3).

Upon the optimized reaction factors, the reversibility of the assembly and disassembly of Au NPs was studied (ESI† , Fig. S4). To consider the effectiveness of the tunable SPB on manipulating the upconversion emission of NaYF₄:Yb,Er NPs at 655 nm, the extinction intensity of Au NPs at this position through the evolution of the reversible assembly was investigated (Fig. 3). Along with the cycles of assembly and disassembly, the extinction of the assembly decreases gradually. This is supposed to be mainly induced by the dilution resulting from the alternate addition of NaOH and HCl solutions. In contrast, the extinction intensity of the disassembled Au NPs gradually increases due to the accumulation of Au NP aggregates from incomplete disassembly in each cycle.

Fig. 3 Extinction intensity at 655 nm during the reversible assembly and disassembly processes of 9.4 nm Au NPs.

Ethylenediamine tetraacetic acid (EDTA) stabilized NaYF₄:Yb,Er NPs were synthesized using hydrothermal reaction. A TEM image reveals that the average particle size is about 120 nm (Fig. 4(a)). To improve the hydrophilicity, a ∼20 nm thick silica shell is coated on the particles (ESI† , Fig. S5). Upon excitation with a 980 nm diode laser, the NPs show three upconversion emission bands at 525, 541 and 655 nm, corresponding to the recombination from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 to the ground state ⁴I_15/2, respectively (Fig. 4(b), Hitachi F-4500 fluorescence spectrophotometer).¹⁰ The emission intensities at 541 (green) and 655 nm (red) are almost equal. To manipulate the luminescence of NaYF₄:Yb,Er NPs by the SPB of Au NPs, 20 mL of 9.4 nm Au NPs was concentrated to 3 mL by centrifugation and then mixed with 10 mg of silica coated NaYF₄:Yb,Er NPs. The luminescence of the mixture before and after the addition of cysteine was investigated (Fig. 4(c)). After the NaYF₄:Yb,Er NPs were mixed with isolated Au NPs, the emission at 525 and 541 nm is greatly diminished, while the emission at 655 nm was still very strong as compared with that of Fig. 4(b). The dramatically weakened green emission is considered as the result of selective emission being masked by isolated Au NPs. After the addition of cysteine, the red emission at 655 nm is gradually reduced along with the assembly of Au NPs, while the green emission also remains depressed during this process. The red emission is recovered by untying the assembled Au NPs through adjusting the pH from 3 to 11 to blue shift the SPB away from the red light region. When the pH of the solution is tuned between 3 and 11, the reversible SPB leads to a reversible switching of the red emission (Fig. 4(d)). However, the emission intensity is not fully recovered because of the incomplete disassembly of Au NPs as discussed above (ESI† , Fig. S6). These results primarily demonstrate the ability of Au NPs in manipulating the luminescence of the NaYF₄:Yb,Er NPs upon external pH stimulation (Scheme 1).

Scheme 1 Schematic illustration of the pH-induced reversible assembly of Au NPs and its effect on the switching of the upconversion emission of NaYF₄:Yb,Er NPs.

Fig. 4 (a) TEM image of NaYF₄:Yb,Er NPs. (b) Upconversion emission spectrum of NaYF₄:Yb,Er NPs. (c) Emission spectra of NaYF₄:Yb,Er NPs obtained during the assembly and disassembly process of Au NPs. (d) Intensity of the emission at 655 nm measured during the reversible assembly and disassembly of Au NPs.

Since the Au NPs and NaYF₄:Yb,Er NPs were simply mixed in solution, the distance between them is not close enough to allow emission quenching from resonant energy transfer although despite spectral overlap.¹¹ As a reasonable explanation for the observed upconversion emission manipulation of NaYF₄:Yb,Er NPs with the assembly and disassembly of Au NPs, it is believed that the decrease of the upconversion emission is caused by the absorption and scattering of Au NPs in their isolated and assembly forms. To validate this explanation, control experiments are carried out by pouring NaYF₄:Yb,Er and Au NPs into two separate quartz cells. The cell containing Au NPs is located between that of NaYF₄:Yb,Er NPs and the spectral signal detector. As expected, the as-synthesized Au NPs act as a dense filter for green light, with absorption and scattering around 530 nm, and causes the reduction of green emission from the NaYF₄:Yb,Er NPs as observed in Fig. 4(c). Combined with the assembled and disassembled Au NPs, the red emission of NaYF₄:Yb,Er NPs could also be manipulated between the reduced and recovered status (ESI† , Fig. S7). In such a control experiment, luminescent quenching from Au NPs can be neglected, and the absorption and scattering of Au NPs with different aggregation states should be responsible for the emission manipulation. The protocol described in this work also can be extended into a universal idea to manipulate the luminescence of NPs or luminophores by controllably tuning the SPB of Au NPs to selectively filtrate the luminescence.

In summary, the upconversion emission of NaYF₄:Yb,Er NPs has been artificially controlled by the reversible shift of SPB achieved by the pH-dependent assembly and disassembly of Au NPs mediated by cysteine.

This work was supported by NSFC (20671005 and 20821091), NSFC-RGC (20731160001), MOST of China (2006CB601104).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The effects of the cysteine concentration, reaction temperature, and particle size on the assembly and disassembly processes. Extinction spectra of the reversible assembly and disassembly of 9.4 nm Au NPs at room temperature. TEM images of isolated, assembled and disassembled Au NPs mixed with NaYF₄:Yb,Er NPs. Upconversion emission of NaYF₄:Yb,Er NPs without and with isolated, assembled, and disassembled Au NPs, which were measured by pouring the two NPs into two separate quartz cells. See DOI: 10.1039/b823453a

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