Synthesis of Au Nanorod@ Amine-Modified Silica@ Rare-Earth

Au NR@A-silica@ LaF3: 20% Yb3+, 2% Er3+ (black line). ...... Boyer , J.-C.; Carling , C.-J.; Gates , B. D.; Branda , N. R. Two-Way Photoswitching Usin...
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Synthesis of Au Nanorod@Amine-Modified Silica@Rare-Earth Fluoride Nanodisk Core−Shell−Shell Heteronanostructures Chao Zhang and Jim Yang Lee* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore S Supporting Information *

ABSTRACT: We report here the synthesis of water-dispersible Au nanorod (NR)@ amine-modified silica@rare-earth fluoride (REF3) nanodisk (ND) heteronanostructures (HNs). The HN fabricated as such represents a rational approach to the concurrent improvements in luminescence, water dispersibility, and other application-specific properties (such as those useful for photothermal therapy). A Au NR is deployed as an internal plasmonic antenna to couple the emission of rare-earth (RE) ions to the surface plasmon resonance (SPR) of the Au NR for enhanced photoluminescence. An intervening amine-modified silica shell is used to attach the ultrathin REF3, to spatially separate them from the internal Au NR antenna, and to impart good water dispersibility all at the same time. Use of internal Au NR plasmonic antenna rather than the more common approach of external antenna may also be applied to design of other REF3 HNs to improve the application performance.

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plasmonic metal. Raman scattering, molecular fluorescence, fluorescence of quantum dots, and RE ions can all benefit from these effects.33,36,38,39 Thus, plasmonic metal NPs can be used as nanoantennas to transfer energy from the far field to around the metal for interaction with the emission of nearby emitters. Strategic placement of the plasmonic metal NP and the emitter is critical to achieving the desired outcome. Decreases in radiative rate and plasmonic field were observed in semiconductor−metal hybrids because metal NPs could also contribute to nonradiative pathways besides local field enhancement effects.40,41 Therefore, the HN structure has to be rationally designed to deliver the optimal optical response. RE emitters with a several nanometer-thick Au shell have been found to exhibit local field enhancements in some cases.29 The design, however, also introduces several limitations: First, an external Au nanoshell could hinder absorption and scattering of light by the internal RE ions and may even quench the RE luminescence when the Au shell is sufficiently thick.29 In addition, the likelihood of metal NP melting and coalescence under direct and prolonged light irradiation should be considered. Good water dispersibility is usually not achievable by the external Au nanoshell approach (because of the large size of the core UC NCs), which renders the HN unsuitable for biological applications. Therefore, new HNs should aim at optimizing optical performance as well as improving water dispersibility.

INTRODUCTION RE-doped luminescent materials have drawn significant research interest because of certain properties which are useful for LEDs, solar cells, biological labeling, and in vivo imaging.1−21 RE-based upconversion (UC) nanocrystals (NCs), in particular, can absorb near-infrared (NIR) photons and output visible light. In comparison with conventional downconversion-based fluorescence imaging agents, the UC NCs have less photodamage and autofluorescence, higher detection selectivity, and greater penetration depth. However, the quantum yield of RE-based NCs is not satisfactory and can be less than 1% in some cases. A higher excitation power density is often used to achieve acceptable brightness, but this brings about the undesirable side effect of overheating in the system. Such inadequacy can in principle be addressed by several approaches. The general method to increase quantum yield is to minimize nonradiative transitions and other energy losses (and hence stronger emission) through a prudent selection of host matrix materials, favorable dopant ratios, surface ligands, and core−shell constructions.22−27 Another strategy is to design HNs which can utilize external fields to enhance luminescence.28 The recent finding that RE’s fluorescence can be altered by the presence of a plasmonic metal is an encouraging development.28−33 The surface plasmon resonance of noble-metal nanoparticles (NPs) is now fairly well understood after two decades of extensive research.34−37 Localized surface plasmon resonance (LSPR) can significantly alter the electromagnetic field surrounding the metal NPs and couple it with the electromagnetic field of emitters which are in the vicinity of the © 2013 American Chemical Society

Received: March 30, 2013 Revised: June 26, 2013 Published: June 26, 2013 15253

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Figure 1. Schematic of Au NR@A-silica@REF3 NDs heteronanostructures.

was ultrasonicated until it was transparent. It was then transferred to a 15 mL Teflon-lined autoclave and heated at 140 °C for 20 h. The NCs prepared as such were recovered by centrifugation and washed thrice with ethanol and thrice with DI water. The final solid product was redispersed in DI water. Preparation of Au Seeds and Au NRs. Au seeds: 0.25 mL of 0.01 M HAuCl4 was added to 7.5 mL of 0.1 M CTAB and mixed by gentle inversion. The mixture turned bright brownyellow. A 0.6 mL amount of ice-cold 0.01 M NaBH4 was then added followed by rapid inversion for 2 min. The solution turned pale brown. The resulting mixture was aged in a water bath at 27 °C for at least 2 h before further use (to make the seeds more uniform and stable). Au NRs: 4.75 mL of 0.10 M CTAB, 0.200 mL of 0.01 M HAuCl4·H2O, and 30 μL of 0.01 M AgNO3 were introduced, one at a time and in that order, to a test tube, followed by gentle mixing by inversion. The solution at this stage appeared bright brown-yellow in color. A 32 μL amount of 0.10 M AA was then added to it. The solution became colorless after mixing with AA. Finally, 10 μL of the seed solution was added, and the reaction mixture was gently mixed for 10 s and left undisturbed for 12 h at 27 °C. Preparation of Silica-Coated Au NRs and NH 2 Modification. Silica coating: 5 mL of the Au NRs prepared above was centrifuged at 4000 rpm for 2 min and then at 12 000 rpm for 8 min. The supernatant was decanted. The precipitate was redispersed in 5 mL of DI water and centrifuged again (12 000 rpm for 8 min). The remaining precipitate was redispersed in 2.5 mL of DI water. Then 1 mL of the above Au NR solution was added to 5 mL of isopropanol under constant magnetic stirring at 100 rpm. A 0.15 mL amount of 25 wt % ammonium hydroxide and 10 μL of TEOS were then added to the solution. The mixture was stirred for 2 h and then transferred to the refrigerator where it was left undisturbed for 20 h. The resultant solution was light purple in color. The Au NR@silica formed was centrifuged at 4000 rpm for 10 min, washed by ethanol and DI water twice each, and then dispersed in absolute ethanol. NH2-modification: 3 μL of AMTPS was added into the ethanol solution containing the Au NR@silica, and then the solution was transferred to a vortex mixer for 20 h of continuous mixing. The solid phase was recovered by centrifugation, washed with ethanol and DI water, and then dispersed in 5 mL of DI Water. Deposition of REF3 NDs on Au@A-silica. In a typical experiment, 1 mL of an aqueous solution containing 2 mg of REF3 NDs was added to 5 mL of an aqueous solution of aminemodified Au NRs@silica. After intense ultrasonication for 20 min, the mixture was transferred to a vortex mixer and mixed

This study presents the synthesis of a new HN, where the plasmonic antenna is relegated to the interior of the HN. The antenna, a single Au NR, is embedded in an amine-modified silica (A-silica) core with a discontinuous shell of REF3 NDs (Figure 1). This HN with an internal plasmonic nanoantenna is distinctively different from HNs with an external Au shell. Localizing Au in the HN interior will not obstruct absorption of light by the REF3 NDs. The embedment of Au in silica also minimizes the likelihood of Au NR melting and coalescence under prolonged incident laser irradiation. Nonradiative relaxation of REF3 NDs that might be caused by the Au NR is abated by an intervening layer of dielectric (silica). Eradication of contact quenching also increases the possibility for creation of local hot spots. Last but not least, these HNs are chemically stable and surface modified for water dispersibility. Hence, they are suitable for bioimaging applications based on the up- or downconversion luminescence of the REF3 NDs.

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EXPERIMENTAL SECTION Materials. Lanthanum nitrate 6-hydrate (La(NO3)3·6H2O), cerium nitrate 6-hydrate (Ce(NO3)3·6H2O), terbium nitrate 6hydrate (Tb(NO3)3·6H2O), yttrium nitrate 6-hydrate (Y(NO3)3·6H2O), tetraethyl orthosilicate (TEOS), and isooctane were purchased from Alfa. Ytterbium nitrate 6-hydrate (Yb(NO 3 ) 3 ·6H 2 O) and erbium nitrate 6-hydrate (Er(NO3)3·6H2O) were purchased from Strem. Ammonium fluoride (NH4F), sodium bis(2-ethylhexyl)sulphosuccinate (NaAOT), cetyl-trimethylammonium bromide (CTAB), gold chloride tetrahydrate (HAuCl4·4H2O), and sodium borohydride (NaBH4) were purchased from Aldrich. 3-Aminopropyltrimethoxysilane (ATPMS) was purchased from Fluka. Silver nitrate (AgNO3), ascorbic acid (AA), and absolute alcohol (ethanol) were purchased from Merck. Deionized water (DI water) was used as the common solvent unless stated otherwise. Preparation of RE(AOT)3 Precursors and REF3 NCs. Precursors: In a typical synthesis, 1.05 mmol of RE(NO3)3·6H2O and 3 mmol of NaAOT were dissolved in 10 and 30 mL of DI water, respectively. The two solutions were mixed under constant stirring for 2 h. The solid product formed was isolated by centrifugation and washed at least twice with DI water. It was air dried in an oven at 70 °C for 10 h and then vacuum dried at 50 °C for 3 days. REF3 NDs: In a typical synthesis, 445 mg of NaAOT was dissolved in 8 mL of isooctane followed by addition of 1 mL each of 0.045 M Ce(AOT)3 and 0.005 M Tb(AOT)3. A 0.2 mL amount of 1 M NH4F was then introduced dropwise. The resultant mixture 15254

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CeF3 NDs were nearly structurally identical to hexagonal CeF3 (JCPDF 38-0452). Diffraction peaks could easily be indexed to the (002), (111), (112), (300), (113), (032), (221), (214), and (411) planes of CeF3. No other diffraction peaks or impurities were found. A slight shift to higher 2θ values was noted because of lattice contraction caused by lanthanide ion substitution. Diffraction peaks were relatively broad, thereby suggesting small crystallite size for the as-prepared NDs. The crystallite size according to the Scherrer equation is ∼13 nm. TEM examination (Figure 3) showed good uniformity and very high monodispersity of the Tb0.1Ce0.9F3 NDs. The NDs were about 15 nm in diameter and 5 nm in thickness and agree well with the XRD measurements. Clear lattice fringes indicative of high crystallinity were very visible in the TEM images of the NDs. The lattices fringes (∼0.36 nm, shown in Figure 3a) correspond well with the (0002) planes of Ce0.9Tb0.1F3. Figure 3d shows the TEM image of a single ND and its fast Fourier transform (FFT) patterns (inset). The FFT patterns and measurements of lattice fringes suggest that the NDs grew along their {0001} planes. The presence of selectively adsorbed AOT molecules on the ND {0001} planes was revealed by measuring the distance (around 1 nm) between two adjacent NDs. The distance matches well with the length of a AOT molecule (∼1.05 nm maximum). Hence, self-assembly of the NDs in the [0001] direction was promoted by the interaction between AOT molecules. For upconversion 20% Yb3+ and 2% Er3+ codoped LaF3 was synthesized (codopant levels were empirically optimized). Due to the size similarity between the dopant ions and the RE ions of the host and the crystal structure similarity between CeF3 and LaF3, the products here also featured a disk-like morphology. These NDs were morphologically similar to the Tb3+-doped CeF3 NDs (see the TEM image in Figure S1, Supporting Information). They were about 10−20 nm in size and very thin with a thickness of ∼5 nm. The XRD pattern of Yb 3+ - and Er 3+ -codoped LaF 3 (Figure S2, Supporting Information) also mirrors very well with that of hexagonal LaF3 (JCPDF 32-0483). Hence, AOT-assisted solvothermal synthesis is an effective method for fabricating high-quality ultrathin REF3 NDs. An efficient plasmonic antenna should have a large absorption cross section and a large local field enhancement effect to effectively collect incident electromagnetic radiation and transfer its energy to the local hot spots. Au NRs are particularly suitable for this purpose because their tunable plasmon resonance frequency, polarization sensitivity, and long dephasing time can lead to a strong and narrow spectral resonance.46 Fluorescence enhancement is known to depend strongly on the scattering quantum yield. The scattering efficiency of Au NRs is optimal at an aspect ratio of 3.4.47 A NR diameter in the region of 10−20 nm also provides the strongest near-field enhancement.48 Hence, Au NRs with diameters of 15−20 nm and an aspect ratio of 3−4 were synthesized in high yield by the seed-mediated growth method.49,50 Figure S3, Supporting Information, shows TEM images of the as-prepared Au NRs. These Au NRs gave rise to two SPR absorption bands in the visible to NIR region: transverse absorption around 530 nm, and longitudinal absorption at ∼800 nm (Figure 4). The Au NR plasmonic antenna was then connected to the luminescent NDs by an intervening silica shell. The silica shell was grown on each NR and modified to have NH2terminating surfaces to facilitate attachment of REF3 NDs in a subsequent step and increase water dispersibility of the HN.

for 24 h. During vortexing 1 mL of ethanol was added every 2 h for a total of 5 mL. The solid product was then recovered by centrifugation, washed by ethanol and DI water, and dispersed in 5 mL of DI water. Characterization. The morphology and size of the assynthesized nanoparticles were examined by a JEM-2100 transmission electron microscope at 200 kV. The structure and phase purity of the as-synthesized products were characterized by X-ray diffraction (XRD) on a Shimadu XRD-6000 diffractometer using Cu Kα radiation (λ = 1.5418 Å). UV−vis absorption spectra were recorded on a Shimadzu UV-2450 spectrometer. PL measurements were carried out on a Perkin-Elmer LS-55 equipped with a xenon lamp excitation source and a Hamamatsu (Japan) 928 PMT using 90° angle detection for solution samples. Fourier transform infrared (FTIR) spectra were collected at room temperature on a Bio-Rad FTS-3500 FT-IR spectrometer. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were carried out on an Agilent 7500 spectrometer. Upconversion luminescence was measured by a Hitachi F-500 fluorescence spectrophotometer equipped with a commercial CW IR laser (980 nm). Luminescence Measurements. Synthesized HNs were dispersed in 5 mL of DI water, 3.5 mL of which was used for DC and UC luminescence measurements. A 150 mW 980 nm laser was used for UC measurements.

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RESULTS AND DISCUSSION RE fluorides (REF3) are a better fluorescent host than RE oxides because their lower phonon energy can reduce the unintended quenching of the excited states of the doped RE ions.42,43 Tb3+ was selected as the dopant in this study for the preparation of downconversion CeF3 NDs. The dopant concentration was limited to 10% since concentration quenching was observed above this threshold. High-quality uniform ultrathin REF 3 NDs were first produced by an AOT-assisted solvothermal synthesis. AOT is a surfactant with two hydrophobic tails which can be used as microemulsions (micelles, reverse micelles, bicontinuous phases, etc.) or in solvothermal processes for synthesis of high-quality monodisperse NCs.44,45 The phase purity of the asprepared NDs was examined by XRD. The resulting XRD pattern (Figure 2) shows that the as-prepared 10% Tb3+-doped

Figure 2. XRD pattern of Ce0.9Tb0.1F3 NDs. 15255

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Figure 3. TEM images of Ce0.9Tb0.1F3 NDs (a) at low magnification. (b) Self-assembly of the NDs on the copper TEM grid. (c) HRTEM and FFT patterns (inset) of NDs standing on their lateral faces. (d) HRTEM and FFT patterns (inset) of an isolated ND standing on its (0001) face.

FTIR spectrum, indicating successful modification of the silica surface by amination. The monodispersity and ultrathin disk-like shape of REF3 NDs facilitated their attachment to the amine-modified Au NR@silica spheres to form the Au NR@A-silica@REF3 HN (Figure 5b−d). The AOT molecules on the NDs surface were removed during the patching process by adding ethanol to the mixture of ND and Au NR@A-silica solutions. With the gradual decrease in solution polarity, desorption of AOT capping molecules from the NDs surface occurred, allowing the NDs to be attached to the −NH2 groups on the Au NR@A-silica surface. Figure 5b show TEM images of the ND-patched Au NR@A-silica spheres. Figure 5c and 5d shows that attachment occurred on the {0001} facets of the NDs, where AOT was previously adsorbed. The patching process did not alter the morphology and crystallinity of the NDs in any way (Figure 5d), and no aggregation occurred either. Attachment of the NDs to the A-silica surface was strong enough to withstand 2 h of ultrasonication and centrifugation at 10 000 rpm. A thinner silica encapsulation would have been ideal since it could improve near-field enhancement and the electromagnetic coupling between the plasmonic NRs in neighboring HNs. Unfortunately, the resulting HNs were not robust to ultrasonication, which was applied during patching of REF3 NDs on the Au NR@A-silica spheres. For the HNs reported here, the distance between the central Au NR to the REF3 NDs varied from 30 to 90 nm, depending on the location of the central Au NR relative to the REF3 NDs. Since patching did not consume all of the amino and hydroxyl groups on the HN surface, the residual functional groups could contribute to the dispersibility of the HNs in water, which is definitely desirable for any biological application. The supernatant after patching and high-speed centrifugation was analyzed by ICP-MS. The RE ion concentrations in the supernatants of HNs with and without a Au antenna were all below 0.2 ppm. This is an indication of the stoichiometric conversion of RE ions in the preparation solution to REF3 NDs on the Au@A-silica spheres. Consequently, all HNs should

Figure 4. UV−vis spectra of Au NRs with (black) and without (red) silica coating.

The silica shell also served to increase the thermal stability of the plasmonic metal and prevent the metal from direct contact with the luminescent RE NDs (which causes quenching). The TEM image in Figure 5a shows a typical Au NR@silica core− shell nanostructure where each NR was completely encapsulated in a silica matrix to form an overall spherical particle about 150 −200 nm in size. The extinction spectra of Au NRs before and after silica encapsulation are shown in Figure 4. The red shifting of the longitudinal SPR mode of Au in Au NR@silica could be due to some weak coupling between the Au NRs and the different refractive index of the surrounding medium.51,52 The presence of surface −NH2, and −CH2 groups after amination of the silica shell was confirmed by FTIR (Figure S4, Supporting Information). Other than absorption due to Si−O− Si stretching (∼1103 cm−1), Si−O stretching (∼802 and ∼470 cm−1), Si−OH stretching (∼945 cm−1), −OH stretching (3000−3700 cm−1), N−H bending (1450−1550 cm−1), and −CH2 stretching (2830−2960 cm−1) were also detected in the 15256

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Figure 5. (a) TEM images of Au NR@silica. (b) TEM image of Au NR@[email protected] at low magnification. (c) Isolated Au NR@A-Silica@ Ce0.9Tb0.1F3 particle. (d) Magnified image of the patched NDs.

Figure 6. (a) Downconversion emission spectra of nanostructures: Au NR@A-silica@ Ce0.9Tb0.1F3 (black line) and [email protected] (red line). (b) Upconversion emission spectra of nanostructures. Au NR@A-silica@ LaF3: 20% Yb3+, 2% Er3+ (black line). A-silica@LaF3: 20% Yb3+, 2% Er3+ (red line).

contain the same concentration (number density) of REF3 NDs. Figure 6a shows the photoluminescence (PL) spectrum of the as-prepared Au NR@A-silica@CeF3 (10% Tb3+ (D1)) and A-silica@ CeF3 (10% Tb3+ (D2, the control, with TEM image in Figure S5, Supporting Information)) HNs. Their water dispersion emitted bright green emission when excited by 255 nm UV light. The four characteristic emission peaks of Tb3+ from 5D4−7F6, 5D4−7F5, 5D4−7F4, and 5D4−7F3 transitions are all present. Emission intensity was clearly higher in the presence of a plasmonic core. It could even be stronger if a thinner silica shell was used since electromagnetic field decays exponentially over distance. The intensity of the 5D4−7F5 transition for D1 was about 3.7 times that of D2. Likewise, the intensity of the 5D4−7F6 transition in D1 was 2.3 times that of D2 (more details in Table S1, Supporting Information). It is postulated that the enhancement was contributed mostly by the

Purcell effect resulting from the near-field enhancement of the embedded Au NR, although the NDs were not positioned for maximum field enhancement. The enhancement effect from the Au NR antenna was distance dependent and not the same for all REF3 NDs. NDs which were closer to the two ends of the NR would experience a stronger enhancement effect than the rest. There was good overlap between the plasmonic bands of the Au NR (530 and 800 nm) with the 5D4−7F6 and 5D4−7F5 transitions of Tb3+ (which peaked at 490 and 542 nm, respectively). The enhancement could be attributed to the effective coupling between the RE emission bands and the Au plasmonic bands.53 The greater overlap between the 5D4−7F5 transition and the plasmonic bands also resulted in greater intensity increase in this case. At the same time, some hot spots from the coupling of adjacent Au NRs in D1 could also be 15257

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The Journal of Physical Chemistry C formed, although the field enhancement effects would not be as strong as those of narrowly separated Au NPs.54 Upconversion luminescence was likewise enhanced by the HN. The as-prepared UC HNs emitted bright green emission when excited by a 980 nm laser. Figure 6b shows the upconversion emission spectra of Au NR@A-silica@ LaF3 (20% Yb3+, 2% Er3+ (U1)) and A-silica@LaF3(20% Yb3+, 2% Er3+ (U2, as control)). The two emission bands with maxima at 550 and 650 nm could be indexed to the2H11/2, 4S3/2−4I15/2, and 4F9/2−4I15/2 transitions of Er3+. The emission intensity of U1 in the 2H11/2, 4S3/2−4I15/2 transitions was 6.5 times of that of U2. As effective antennas, Au NRs with the optimized diameter and aspect ratio could offer the strongest near-field enhancement. The coupling of the emission bands (2H11/2, 4S3/2−4I15/2) to the Au SPR could increase the emission intensities. In addition, the pumped power density of the 980 nm excitation near the antennas could be increased by local field enhancement effects, leading to a greater number of excited Yb3+ ions.31 Consequently, more energy could be transferred to the Er3+ surrounding the Yb3+ ions. The resulting increase in the population of Er3+ ions in the excited states then contributed to emission enhancements. Enhancement effects could also be further improved by reducing the silica intervening layer thickness in the HN structure. Unfortunately, it was presently difficult to produce sufficiently robust HNs with a thinner silica shell. In comparison with RE NCs with an Au shell, these down/ upconversion HNs are more suited for bioimaging and biolabeling applications because of their good water dispersibility. One can also leverage on the SPR bands of Au NRs in the NIR region to use these HNs as photoacoustic nanoamplifiers55 and for photothermal therapy.

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CONCLUSION

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS

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REFERENCES

The authors gracefully acknowledge the assistance of Prof. Yong Zhang in measurement of upconversion luminescence.

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An interesting HN combining REF3 emitters and an embedded plasmonic antenna was used to provide enhanced luminescence and water dispersibility. By coupling the emission bands of RE ions to the SPR of Au NRs, luminescence intensity was increased relative to HNs without the plasmonic antennas. The HN uses an intervening dielectric (silica) to suppress nonradiative relaxations which are present if the Au NRs and the RE ions are in direct contact and promote generation of local hot spots for field enhancement effects. Besides, the good water dispersibility of these luminescent HNs with internal plasmonic antennas also improves their acceptance in biological applications as bioimaging markers and biolabels. In addition to improved luminescence, these HNs are also expected to be useful for photothermal therapy and photoaccoustic imaging. The principles presented here should be of interest to the design and preparation of other new HNs.

S Supporting Information *

TEM, XRD, and FTIR results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 15258

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dx.doi.org/10.1021/jp403147d | J. Phys. Chem. C 2013, 117, 15253−15259