Effect of Temperature and Gold Nanoparticle Interaction on the

Jun 13, 2017 - In this paper, we measured the temperature dependence in the temporal response of the green emission for both the H band (2H11/2 → 4I...
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The Effect of Temperature and Gold Nanoparticle Interaction on the Lifetime and Luminescence of NaYF4:Yb3+: Er3+ Upconverting Nanoparticles Ali Rafiei Miandashti, Martin E. Kordesch, and Hugh H. Richardson ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00512 • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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The Effect of Temperature and Gold Nanoparticle Interaction on the Lifetime and Luminescence of NaYF4:Yb3+: Er3+ Upconverting Nanoparticles Ali Rafiei Miandashti,1 Martin E. Kordesch,2 and Hugh H. Richardson1* 1

Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701

2

Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701

Abstract In this paper, we measured the temperature dependence in the temporal response of the green emission for both the H band (2H11/2 → 4I15/2 transition) and the S band (4S3/2 → 4I15/2 transition) for β-NaYF4 nanocrystals and β-NaYF4 nanocrystals decorated with 10 nm gold nanoparticles and found that the emission is quenched with temperature. We measured the timeresolved green emission intensity for decorated upconverting nanoparticles and observed a biexponential decay that has a dominant decay time ~30 µs and a longer decay time ~300 µs that is similar to the single exponential decay observed for the undecorated UCNPs. The temperature dependence in the green emission for β-NaYF4 nanocrystals has a non-radiative quenching rate modeled with activation energy of 700 cm-1 assigned to multi-phonon relaxation to Er3+ lower energy levels. We measured the steady state emission from the H and S bands for temperatures between 300 K and 450 K and obtained a linear relationship between calculated and measured temperatures. Keywords Upconverting Nanoparticles, NaYF4: Yb3+: Er3+ Nanocrystals, Temperature dependent quenching, Luminescence thermometry, Nanoscale Thermometry

Lanthanide doped upconverting nanoparticles (UCNPs ) have been subject of extensive studies over the past decade due to their photo-physical properties.1–4 Nanocrystals doped with Yb3+ and Er3+ ions are one of the most widely used upconverting nanoparticles.3,5–12 NaYF4: Yb3+: Er3+ nanocrystals have green luminescence emission when irradiated with near-IR light. Photophysical properties of these nanoparticles make them unique candidates in life science,13–16 solar energy,17,18 and electronics.19,20 The green band emission of Er3+ doped nanocrystals are used for luminescence thermometry.7,21,22 However due to low quantum yield of these particles, their promising potential and applicability has been limited.23 There are different factors that can affect the natural luminescence emission of these UCNPs.5,6,10,23 One of the significant ways to influence the luminescence emission of lanthanide ions in crystalline structure is through functionalization with noble metals.24 Depending on the geometry and distance of noble metals, the luminescence emission undergoes enhancement or

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quenching. Great advances have been achieved by enhancement of luminescent emission through growing a shell around optically active upconverting nanocrystals,10,25,26 and decoration of optically active nanoparticles with gold nanoparticles.5,6,23 Encapsulation and decoration of UCNPs by an inert/active shell or plasmonic materials enhances the luminescence emission through isolation of surface defects or through increasing the absorption cross section of nanoparticles.1,3,10,21,25 The interaction of nanoparticles/shell with UCNPs leads to either increasing the number of energy transfers to excited states or reducing the number of rapid surface related non-radiative processes.25,27 Moreover, there has been some reports on the effect of temperature on emission lifetime of lanthanide ions.28 In one reports, Er3+ doped Zinc Tellurite glasses undergo an intrinsic quenching as a result of increased temperature. However, there is no report that investigates how the luminescence lifetime of Er3+ doped upconverting nanoparticles changes when interacting with a plasmonic nanocrystal at different temperatures. Since Er3+ ion luminescence thermometry is growing, there is a need to confirm if the temperature dependent quenching rate is different between the H (2H11/2 → 4I15/2 transition) and S (4S3/2 → 4I15/2 transition) bands. If the quenching rate is different, then the emission intensity no longer depends solely upon temperature but is a complicated function of temperature and how temperature changes the quenching rate for these two bands. Here, we investigate the effect of gold nanoparticles and temperature on the luminescence lifetime of NaYF4: Yb3+: Er3+ nanocrystals for their use in nanothermometry. We show that the excited states of Er3+ quenches at higher temperatures29 and explore the temperature dependence upon the quenching rate for the H and S bands to understand how quenching affects a temperature measurement using NaYF4: Yb3+: Er3+ nanocrystals. We synthesize large NaYF4: Yb3+: Er3+ nanoparticles (~ 300 nm diameter) and then attached 10 nm gold nanoparticles to them (UCNP/GNPs) through electrostatic interaction. We measured the time-dependent excitation and decay of luminescence emission from the H and S bands when excited at 980 nm for UCNPs and UCNP/GNPs to understand the effect of gold spheres on quenching rates and investigate the effect of temperature on the lifetime. Finally, we measured the temperature dependence in the steady-state emission of the H and S bands and compared the calculated temperature to the measured temperature to confirm that these UCNPs act as valid thermal sensors.

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Figure 1 a) TEM image of β-phase hexagonal NaYF4:Yb3+: Er3+ upconverting nanoparticle b) TEM image of hexagonal NaYF4:Yb3+: Er3+ upconverting nanoparticles decorated with gold nanoparticles, c and d) High-resolution TEM image of gold nanoparticle attached to NaYF4:Yb3+: Er3+ nanocrystal, f) EDS elemental mapping of Er3+ in a typical NaYF4:Yb3+: Er3+ nanocrystal

Figure 1a shows the TEM image of NaYF4:Yb3+: Er3+ nanocrystals synthesized through thermal decomposition method. NaYF4:Yb3+: Er3+ nanocrystals have average diameter of 300 nm and coated with hydrophobic layer of oleic acid as capping agent. In order to attach gold nanoparticles to the surface of NaYF4:Yb3+: Er3+ nanocrystals we removed the oleic acid from the surface via a protocol described in method section. NaYF4:Yb3+: Er3+ nanocrystals were attached to 10 nm gold nanoparticles through an electrostatic interaction (Figure 1b). The information regarding the enhancement and quenching of luminescence emission in response to the presence of gold is given in supporting information figure S2. Figure 1c and d show the high resolution TEM image of a gold nanoparticle attached onto a NaYF4:Yb3+: Er3+ nanocrystal with different magnifications. For a typical NaYF4:Yb3+: Er3+ nanocrystal the EDS elemental mapping and elemental spectrum of all the other elements are shown in S1. The luminescence intensity of NaYF4:Yb3+: Er3+ nanocrystal changes when decorated with gold nanoparticles (See supporting information S2). Figure 2a shows the response of NaYF4:Yb3+: Er3+ nanocrystals and NaYF4:Yb3+: Er3+ nanocrystals decorated with gold nanoparticle to the intensity of 980 nm laser. The temperature of the UCNPs is determined by measuring the relative peak areas of the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 bands. The temperature is calculated from this ratio using the Boltzmann equation,

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H −∆E = A exp( ) , where A is the pre-exponential factor and ∆E is an energy difference between S kT the H band (2H11/2 → 4I15/2 transition) and the S band (4S3/2 → 4I15/2 transition). The preexponential factor is determined through plotting the logarithm of the ratio of H to S band as a function of inverse temperature (T-1). More information regarding the calculation of temperature is given in supporting information (see S4). A slight increase in temperature with 980 nm light is observed for the UCNPs (~ 10 K) while a large temperature increase is observed for the UCNP/GNPs (~150 K). The linear increase in temperature for the UCNP/GNPs is due to close proximity of clusters of UCNP on the surface of cover slip. When a film of UCNP/GNPs are formed, the gold nanoparticles interact causing a change in the plasmonic bands of gold that shift toward 980 nm leading to an increase in absorption at 980 nm and subsequently an increase in temperature (Figure 3a). The Jablonski energy level diagram for Er+3 and Yb+3 ions are shown in Figure 2b. Upconversion occurs because the sensitizer (Yb3+) absorbs light at 980 nm and transfers electrons to Er3+ where a second photon is absorbed to promote the population of electron to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 levels.

Figure 2. a) The temperature response of UCNPs and UCNP/GNPs to the intensity of 980 nm laser. b) Jablonski energy diagram of UCNP/GNPs and the quenching mechanism due to the presence of gold nanoparticles. To understand the quenching process, we made time-resolved measurements of the green emission bands for UCNPs (Figure 1a) and UCNP/GNPs (Figure 1b). Figure 3a shows the timeresolved emission from the 2H11/2 → 4I15/2 band (~ 520 nm) for UCNPs and UCNP/GNPs with pulsed 980 nm laser excitation. The blue dotted line shows the pulse profile of the excitation laser. The solid red and black lines show the time-resolved emission for UCNPs and UCNP/GNPs in response to the laser pulse respectively. We will keep the color-coding that red data is for the UCNPs and black data is from UCNP/GNPs. Figure 3a shows that the time to reach steady-state is nearly the same for the UCNP/GNPs and UCNPs. The seed nanoparticles do

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not appear to alter the time needed to reach steady-state. However, we observe that the amount of emission is less for the UCNPs with gold nanoparticles compared to the UCNPs without gold suggesting that the gold nanoparticles open up a non-radiative channel that quenches the Er3+ excited state emission. Supporting this hypothesis we observe that after 980 nm light is turned off, the pathway for energy dissipation between the UCNP/GNPs and UCNPs is different. A faster decay is observed for the UCNP/GNPs than for the UCNPs. A plot of the natural log of the emission versus time is shown in Figure 3b for the UCNP/GNPs and UCNPs. The solid lines in the plot are fits to the data. The UCNPs have a single exponential fit while the UCNP/GNPs data is fitted with a double exponential. The lifetime for the UCNPs and UCNP/GNPs is compared in Figure 3c. The lifetime of the UCNPs is ~ 230 µs and the lifetimes for the UCNP/GNPs are 20 µs (weighting coefficient of ~ 0.75) and 180 µs (weighting coefficient of ~ 0.25). Table 1 gives a summary of the lifetimes and weighting coefficients for both the 2H11/2 → 4 I15/2 (~ 520 nm) and the 4S3/2 → 4I15/2 band (~ 540 nm).

Figure 3. a) The excitation and decay profile of luminescence emission of plain UCNPs and UCNP/GNPs under 980 nm laser. b) Single and double exponential decay of emission of bare and UCNP/GNPs under 980 nm laser, c) the lifetime emission for 520 nm band for plain UCNPs and UCNP/GNPs under 980 nm laser.

These NaYF4:Yb3+: Er3+ nanocrystals are used as temperature sensors by collecting the emission spectrum and evaluating the ratio of peak areas from the 2H11/2 → 4I15/2 and 4S3/2 → 4 I15/2 bands.21,29,31–33 We observe a change in the lifetime of the UCNPs when gold seed particles are attached to the UCNPs and there appears to be differences in the lifetime between 2H11/2 → 4 I15/2 and 4S3/2 → 4I15/2 bands when the UCNPs are decorated (see Table 1). This behavior led us to measure the temperature dependence in the lifetime for UCNP/GNPs and UCNPs to see if these UCNPs can be used as an effective temperature sensor. Figure 4a shows the temperature dependence in the lifetime for the H band (2H11/2 → 4I15/2 transition, black squares) and S band (4S3/2 → 4I15/2 transition, red circles) for UCNPs. The lifetime of these two bands decreases with temperature; suggesting that the emission from the H and S bands are quenched with temperature. This temperature dependence in the lifetime is also observed for UCNP/GNPs (see

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supporting information S8). This data shows that the lifetimes associated with the UCNP/GNPs, for both the H band and S band decrease with temperature. A decrease in the lifetime with temperature suggests that there is quenching of the Er3+ 2 H11/2 and 4S3/2 excited states due to energy transfer into a non-radiative energy level. We confirmed this hypothesis by measuring the temperature dependence in the steady-state emission from the H band for UCNPs shown in Figure 4b. The steady-state emission intensity is normalized to the area under the H band at 298 K. A nearly exponential drop in emission intensity is observed with temperature. We can model this behavior assuming two parallel pathways between a non-radiative and radiative path. The non-radiative path is given by a rate constant with a temperature dependence that can be expressed with an Arrhenius expression, −E knr = A exp( a ) . The steady-state emission is proportional to the quantum yield (Φ) given by kT o k φ = o r where kro is the intrinsic radiative rate constant (reciprocal of the intrinsic lifetime). kr + knr We use an initial guess of the room temperature quantum yield of 0.5% for our relatively large UCNPs to set the intrinsic lifetime at 17 ms. Our model fit is not sensitive to the initial guess in quantum yield as long as a value less than 5% is used. In this model, quenching of the excited state increases with temperature because the non-radiative rate constant increases with temperature. The solid red line in Figure 4b is the model fit to the temperature dependent drop in H band emission. The non-radiative rate constant can also be determined from the observed lifetime for the UCNPs. In the two-pathway model presented above, the observed rate constant is the sum of the intrinsic radiative rate constant and the non-radiative rate constant, i.e.,

kobs = kro + knr . The observed lifetime is the reciprocal of the observed rate constant given in Figure 4a. The non-radiative rate constant is then calculated from the observed and intrinsic rate constant The activation energy (Ea) for the model fit is 700 cm-1 with a pre-exponential factor (A) of 1.5 x 105. This low barrier value for the activation energy suggests that non-radiative relaxation is occurring by multi-phonon relaxation to lower Er3+ levels. We tested this hypothesis by measuring the visible spectrum of the green bands (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions) and red bands (2F5/2 → 4I15/2) as temperature is increased (see supporting information). A twodimensional correlation analysis is performed on the spectral variations and a negative correlation is observed between the green and red bands. A negative correlation reveals that as the green bands are decreasing with temperature, the red bands are increasing supporting the hypothesis that non-radiative relaxation is occurring by multi-phonon relaxation to the 2F5/2 state. The energy difference between the green bands and red bands is ~ 3500 cm-1. If a single phonon has the energy of ~ 700 cm-1, then it would take ~ 5 phonons to make the energy gap of 3500 cm1 .

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Figure 4. a) Lifetime drop for H and S bands as a function of temperature for UCNPs b) intensity drop of emission for H band as the temperature increases from 25 °C to 300 °C (black) and, c) non-radiative rate constant increase as a function of temperature for UCNPs under 980 nm excitation laser.

Table 1. lifetime and coefficient values for UCNP and UCNP/GNPs at room temperature

Because the emission bands used to calculate temperature are quenched when interacting with gold nanocrystals and also are quenched with temperature, it is possible that the quenching rate is different between the H and S bands. To probe this temperature effect, we measured the temperature dependence in the steady-state emission from the H and S bands for UCNPs and UCNP/GNPs. The sample temperature was recorded using a thermocouple and we compared the recorded temperature with the temperature calculated using the steady-state emission of the H and S bands using the identical set-up we measured the lifetime data. These results are shown in Figure 5 where the calculated temperature is plotted against the measured temperature. The calculated temperature for UCNPs and UCNP/GNPs is shown as red and black squares respectively. The uncertainty in the calculated temperature increases with temperature because

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the emission is quenched with temperature. The calculated temperature for the UCNP/GNPs has a temperature increase offset of ~60 K compared to the UCNPs because the UCNP/GNPs absorb more 980 nm light than the UCNPs. A linear relationship is observed, as expected, from this plot with unity slope and zero intercept. Higher temperature results were difficult to obtain because the signal is low and the uncertainty is large due to quenching of the emission. Figure 5 shows a good correlation between calculated and measured temperature, even for the UCNPs with gold nanocrystals, confirming that the H and S band temperature dependent quenching rates are very similar and that the UCNPs with and without gold nanocrystals can be used as temperature sensors even at a temperature as high as 450 K.

Figure 5. Calculated temperature as a function of measured temperature for UCNP/GNPs and UCNPs under 980 nm excitation laser irradiation

In this paper, we measured the temperature dependence in the temporal response of the green emission for both the H band (2H11/2 → 4I15/2 transition) and the S band (4S3/2 → 4I15/2 transition) for β- NaYF4:Yb3+: Er3+ nanocrystals and β- NaYF4:Yb3+: Er3+ nanocrystals decorated with 10 nm gold nanoparticles and found that the emission is quenched with temperature. Time-resolved measurements showed that the decay lifetime of UCNP/GNPs is biexponential with a dominant decay time an order of magnitude faster than the longer decay time around 300 µs. The UCNPs have a single exponential decay with a long decay time of ~ 175 µs. We further analyzed the temperature dependence in the green emission for UCNPs and found that a model using an Arrhenius expression for the non-radiative quenching rate gives an activation energy of 700 cm-1. This relatively low value for activation energy leads us to conclude that the dominant quenching mechanism is energy transfer by multi-phonon relaxation to Er3+ 2F5/2 state. We measured the steady-state emission from the H and S band for UCNPs and UCNP/GNPs for temperatures between 300 K and 450 K and obtain a linear relationship between the calculated and measured temperatures showing that quenching of the H and S bands does not affect the ability of UCNP/GNPs and UCNPs to be used as thermal sensors.

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Methods Synthesis of NaYF4:Yb3+: Er3+ nanocrystals All chemicals were purchased from Sigma-Aldrich and used without further purification. We synthesized NaYF4:18%Yb,2%Er nanoparticles via thermal decomposition method. In this protocol, we decomposed rare-earth/sodium trifluoroacetate oleic acid (OA) and octadecene (ODE) as reported in literature .34 In a typical protocol, we mixed 0.50 mmol of ytterbium(III) oxide (Yb2O3, 99.9%), 1.95 mmol of yttrium(III) oxide (Y2O3, 99.99%) and 0.050 mmol of erbium(III) oxide (Er2O3, 99.9%) in 10 ml trifluoroacetic acid (TFA, 99%) in a 200 ml threenecked flask. To remove water and excessive TFA, we heated the compounds to 80 ˚C with vigorous magnetic stirring under vacuum for 30 minutes. After drying the solvents, we added 5 mmol sodium trifluoroacetate (NaCOOCF3, 98%) to 15 mL of oleic acid (OA, 90%) and 15 ml of 1-octadecene (ODE, 90%) at 100 ˚C. Next, we increased the temperature to 330 ˚C with an increase rate of 30 ˚C per min and maintained at 330 ˚C for 60 minutes to complete the reaction. The nanoparticles were thoroughly washed and dispersed in toluene. Synthesis of Gold Nanoparticles In a typical procedure, we added 1 ml of aqueous solution of HAuCl4 trihydrate (0.01 M) and 1 ml of trisodium citrate (0.01 M) to 36 ml of deionized H2O and stirred for 1 minute. Next, we added 1 ml of ice-cold and freshly-prepared NaBH4 (0.1 M) to the solution and the color changed from colorless to orange. We continued the stirring stopped for 5 minutes and the solution was left undisturbed for 2 h.35 Synthesis of Gold decorated Upconverting Nanoparticles The attachment of gold nanoparticles (GNPs) and the growth of larger GNPs on the surface of UCNPs are carried out in aqueous solution. The attachment of gold nanoparticles (GNPs) and the growth of larger GNPs on the surface of UCNPs are carried out in aqueous solution. The upconverting nanoparticles are not colloidally stable in water because they are functionalized with oleic acid when they are prepared via our thermal decomposition method. Before decoration of upconverting nanoparticles with gold, oleic acid is removed from the surface. To make the nanoparticles water soluble, we performed two surface-modification steps. Firstly, we removed the hydrophobic oleic acid from the surface of UCNPs by reducing the pH of environment to 4 as reported in the literature.9 After removing the oleic acid from the surface, we used 10 ml of UCNP (1 wt %), 20 ml of Ethylenediamintetraacetic acid (EDTA) (1 wt %) as a multidentate ligand to displace the original hydrophobic oleic acid on the surface of UCNP. We stirred the solution for 6 hours before it was centrifuged at 1000 rpm for 10 min. We suspended the precipitate in water and stirred overnight with 10 ml of Poly(lysine) (1 wt %) to render a positive charge on the surface of UCNPs to positive. We centrifuged the particles for 10 min at 1000 rpm and suspended in deionized water. To 10 ml of UCNP (1 wt %), we added 2 ml of citrate stabilized GNPs and the solution was stirred for 2 hours. We separated the upconverting nanoparticles decorated with GNPs (UCNP/GNPs) from unattached GNPs through centrifuge.

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Due to the larger size of UCNP/GNPs, they are precipitated faster than GNPs and they are easily separable. Growth of Gold Nanoparticles on UCNPs To grow GNPs on the surface of UCNP, we added 1 ml of aqueous solution of HAuCl4.3H2O (0.01 M) and 0.5 ml of ascorbic acid (0.1M) as a mild reducing agent to the solution of gold decorated upconverting nanoparticles (UCNP/GNPs). The growth of GNPs on the surface of UCNPs was carried out in situ during the collection of emission spectrums. Associated Content Supporting Information: The supporting information is available free of charge on the ACS Publications website:. Additional details regarding procedures followed and equipment used, ancillary data, and expanded discussion of results (PDF) Author Information The Authors declare no competing financial interest Acknowledgement The authors acknowledge Ohio University Condensed Matter and Surface Science Program and Nanoscale Quantum phenomena Institute for financial support. We would also like to thank the Center for Electrochemical Engineering Research (CEER) at Ohio University, and the National Science Foundation through the Major Research Instrumentation Grant #CBET-1126350 for the Transmission Electron Microscopy images and measurements.

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