Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Systematic Investigation of the Wavelength-Dependent Upconversion Enhancement Induced by Single Plasmonic Nanoparticles Gao Yi,†,⊥ Byeong-Seok Moon,‡,§,⊥ Xu Wen,† Young-Jin Kim,*,† and Dong-Hwan Kim*,§ †
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 63979, Singapore Department of Materials Science and Engineering, Seoul National University, 08826 Seoul, Korea § School of Chemical Engineering, Sungkyunkwan University, 16419 Suwon, Korea ‡
ABSTRACT: A convenient method was developed to systematically study the wavelength-dependent upconversion enhancement induced by single plasmonic nanoparticles to overcome unavoidable variations of nanoparticle geometry in practical synthesis techniques. Gold nanospheres and gold nanotriangles coupled to an upconversion nanoparticle monolayer were selected to compare emission and excitation resonance couplings, respectively. The emission intensity of a upconversion nanoparticle monolayer coupled with gold nanospheres (i.e., emission coupling) was found to be slightly lower than that of a reference upconversion nanoparticle monolayer, independent of pumping power. In turn, the emission intensity of a upconversion nanoparticle monolayer coupled with a gold nanotriangle (i.e., excitation coupling) showed an enhancement factor of the emission intensity of about 3.26 at low pumping power, which decreased to 0.48 at high pumping power because of a local heating effect. Our method could provide a convenient strategy for massive data collection of coupled upconversion nanoparticles and plasmonic nanoparticles in a single-particle level and a guideline for systematic investigations involving plasmonic nanoparticles.
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emission light,16 as well as the increase of the emission rate17 in the upconversion process.18 To push the limits of understanding, pioneer studies were conducted in pairing systems of UCNPs and plasmonic nanoparticles in a single-nanoparticle level, which is the ideal experimental setup to access its full parameterization.18 Despite remarkable achievements utilizing a marked substrate,14 electron lithography-based templates,8 and atomic force microscope,19 there has been a lack of concern about the intrinsic variations of plasmonic nanoparticles such as their size and shape, which ultimately induce variations of localized surface plasmon resonance (LSPR) peak wavelengths.20 This could be a potential problem for practical nanoparticle applications because the plasmonic electric field varies with morphological changes of plasmonic nanoparticles.18,21 Therefore, it is important to investigate the dependence of upconversion enhancement on variations of the LSPR peak in a systematic manner.22
INTRODUCTION Upconversion refers to a phenomenon in which two or more photons in the near-infrared (NIR) regime are sequentially absorbed and converted into light with shorter wavelengths including visible and ultraviolet (UV).1 Light with NIR frequencies is an excellent energy source for luminescent materials because of its deep tissue-penetration depth, low phototoxicity, and culmination in solar photon flux.2 While upconversion nanoparticles (UCNPs) have attracted significant interest in different areas including bioimaging,3,4 controllable drug delivery,5,6 and solar cells,7 the luminescence of UCNPs has been significantly limited by the upconversion energy efficiency.8 To overcome this issue, great effort has been devoted to selecting novel host materials,9 optimizing the crystal field,10 and utilizing band gap resonance of photonic crystals.11,12 However, the energy conversion efficiency still remains low.1 One of the representative methods to enhance the upconversion emission consists of taking advantage of plasmonic nanoparticles in the vicinity of UCNPs,13 that is, using the so-called plasmonic coupling effect. The plasmon coupling has been proven to enhance the upconversion luminescence by one or two orders of magnitudes8,14,15 by the amplification of the excitation source,8 the resonance of the © XXXX American Chemical Society
Received: March 12, 2018 Revised: May 30, 2018
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DOI: 10.1021/acs.jpcc.8b02437 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Experimental setup and sample characterization. (a,b) Experimental setup (a) and investigation scheme (b) of the UCNP monolayer coupled with a gold plasmonic nanoparticle. (c,d) SEM images of the UCNP monolayer coupled with AuNTs (c) and AuNSs (d). (Inset: SEM images of synthesized AuNTs and AuNSs.) (e,f) UV−vis/NIR spectra of AuNTs matched with the UCNP excitation wavelength (e) and AuNSs matched with the UCNP emission wavelength (f).
hydrogen peroxide) for 12 h. The slides became hydrophilic with OH group functionalization after treatment. The OHfunctionalized slides were then thoroughly cleaned with DI water and dried under N2. After that, the OH-functionalized slides were modified with an NH2 group by immersion in a 10% 3-aminopropyltriethoxysilane (APTES) ethanol solution for 30 min. The slides were then sonicated in ethanol for 5 min to remove the unreacted APTES. Finally, the slides were baked at 120 °C for 3 h and stored in ethanol for future usage. Structure Fabrication. The synthesized AuNPs were diluted 10 times in DI water. The OH- and NH2-functionalized coverslips were immersed in the diluted AuNT and AuNS solution, respectively, for 3 h, and then rinsed with DI water and dried with N2. The AuNPs were then immobilized on the coverslips. UCNP monolayers were formed using an interfacial assembly method.17 A 2.5 × 2.5 × 1 cm Teflon well was half-filled with diethylene glycol, and 36 μL of a 15 mg/mL UCNP solution was cast onto the surface of the solution. Then, the Teflon well was quickly covered by a coverslip to allow for slow evaporation of hexane. After 1 h, AuNP-immobilized coverslips were inserted slightly beneath the diethylene glycol interface and then lifted up. A large area of the UCNP monolayer film was then transferred to the AuNP-immobilized coverslips. The transferred UCNP monolayer was dried at 150 °C to remove the excess diethylene glycol. Structure and Optical Measurement. Transmission electron microscopy images were taken using a JEOL 2010 UHR microscope operating at 200 kV. Field-emission scanning electron microscopy images were obtained on a JEOL JSM6710F microscope operating at 5 kV. The UV−vis/NIR absorption spectra were collected from 400 to 1400 nm on a spectrophotometer (Varian Cary 5000). Dark-Field Image and Spectrum Collection for UCNPs and AuNPs. The dark-field images for AuNPs and UCNPs were taken by an optical dark-field microscope system (Olympus IX71) coupled to a motorized stage (Ludl Flat-top inverted stage 96S106-N3-LE2, BioVision Technologies). A line-imaging spectrometer (Acton Research SpectraPro2150i) was used to collect spectra information for AuNP scattering and UCNP luminescence. The spectral data of the 20 μm × 20 μm area was scanned by the motorized stage and reconstructed into
Here, we study the dependence of upconversion enhancement on the LSPR peak variation in a single-particle level by massive data collection which has not been attainable with the conventional particle-to-particle coupling system. A monodispersed UCNP was uniformly self-assembled as a monolayer on top of single plasmonic nanoparticles to provide a homogeneous coupling system. The LSPR peak wavelength of individual plasmonic nanoparticles and the enhanced upconversion luminescence were investigated simultaneously using a dark field optical microscope. From as many as 50 coupling sites, we found that the upconversion enhancement is highly sensitive to the LSPR peak variation when coupling took place through excitation resonance rather than emission resonance. Also, we confirmed that the LSPR peak variation had a significant effect not only on the upconversion enhancement process but also on the quenching process. In this study, the most effective coupling sites were carefully chosen for further investigation to eliminate undesirable variation effects. Our results could provide a guideline for single-particle level investigations involving plasmonic nanoparticles to overcome unavoidable nanoparticle variations in practical synthesis techniques.
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EXPERIMENTAL SECTION
Nanoparticle Synthesis. UCNPs were synthesized according to the previous work.23 The synthesized colloidal particles were centrifuged at 8000 rpm for 10 min, and then the sediment was redispersed in a hexane and ethanol 1:1 mixture and centrifuged again at 8000 rpm for another 10 min. The sediment was then redispersed in hexane. Gold nanospheres (AuNSs) were synthesized based on a method previously reported.24 The synthesized nanospheres were centrifuged at 6000 rpm for 10 min and then redispersed in water. Gold nanotriangles (AuNTs) were synthesized following a previously published work.25 The AuNTs were then centrifuged at 3000 rpm and redispersed in deionized (DI) water before use. Gold Nanoparticle (AuNP) Immobilization. AuNSs and AuNTs were immobilized on a coverslip modified with NH2 and OH groups, respectively. Glass coverslips were functionalized with NH2 and OH groups using a similar published procedure.26 The slides were cleaned thoroughly with a piranha solution (a mixture of 3:1 concentrated sulfuric acid and B
DOI: 10.1021/acs.jpcc.8b02437 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Dark-field image of AuNT (a) and AuNS (b) coupled with the UCNP monolayer. (c,d) Scattering spectra of selected AuNTs (c) and AuNSs (d) identified by white circles in (a,b), respectively. (e,f) Luminescence spectra of a UCNP monolayer coupled with AuNTs (e) and AuNSs (f). The green curves show the reference spectra and the inset in (f) shows a magnified view of spectrum at 656 nm.
weak shoulder peak of AuNT at around 650 nm is neglected despite a spectral overlap with 4F9/2 → 4I15/2 upconversion (UC) emission peak. Therefore, AuNSs were used for emission coupling and AuNTs were used for excitation coupling, respectively. The emission at 656 nm of UCNPs appears to be stronger than that in the green region at around 550 nm because of the energy transfer among Yb3+ and energy back transfer from Er3+ to Yb3+.28−30 Figure 2a,b shows dark-field scattering images of plasmonic nanoparticles coupled with the UCNP monolayer. Even though the peak positions of ensemble LSPR, that is, the averaged LSPR peak of many plasmonic nanoparticles in a solution, are matched well with the excitation and the emission wavelengths of UCNPs (Figure 1e,f), the individual plasmonic nanoparticles have an apparent variation in the LSPR peak positions because of their geometric inhomogeneity (Figure 2c,d), shown as the various colored spots in dark-field images under white light illumination (Figure 2a,b). The scattering spectra of individual plasmonic nanoparticles and the luminescence spectrum of the UCNP monolayer were collected from 50 coupling spots. To investigate how the LSPR peak variation of plasmonic nanoparticles affects the enhancement of the luminescence spectrum of UCNPs, the collected data points were divided into 5 groups based on the degree of mismatch between the LSPR peak and the luminescence spectrum of UCNPs for both excitation and emission cases. Group 1 contains the plasmonic nanoparticles exhibiting LSPR peaks that deviated by 10 nm or less from the peak for the ensemble as-synthesized solution (their scattering spectra are shown in Figure 2c,d for excitation and emission coupling, respectively). For AuNTs, because of the limit of quantum efficiency of the CCD camera beyond 900 nm (
