Graphene-Conjugated Upconversion Nanoparticles as Fluorescence

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Graphene-Conjugated Upconversion Nanoparticles as Fluorescence-Tuned Photothermal Nanoheaters for Desalination. Mukesh Kumar Thakur, Akash Gupta, Sandip Ghosh, and Surojit Chattopadhyay ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00186 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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ACS Applied Nano Materials

Graphene-Conjugated Upconversion Nanoparticles as Fluorescence-Tuned Photothermal Nanoheaters for Desalination

Mukesh Kumar Thakur, Akash Gupta, Sandip Ghosh, and Surojit Chattopadhyay * Institute of Biophotonics, National Yang Ming University, 155, Sec-2 Li Nong Street, Taipei 112, Taiwan.

*Corresponding author’s E-mail: [email protected]

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Abstract 980 nm multiphoton absorbing lanthanide doped up-conversion nanoparticles (UCNPs) are emerging as fluorophores, and photothermal agents, but limited by the low quantum yield of its visible fluorescence. In a way similar to metal enhanced fluorescence, we demonstrate that a monolayer of graphene could quench the 540 nm (green) fluorescence from the core-UCNPs by 3X, and a bi-layer graphene could enhance the green fluorescence from the Silica (SiO2) coated core-shell (cs) UCNPs by 30X. This graphene-aided fluorescence tuning in the engineered UCNPs can be translated to photothermal conversion using a 980 nm excitation. Infrared thermal imaging, and thermocouple measurements both indicate the local temperature to scale with fluorescence. From the dynamic response of local temperature, we could estimate the photothermal conversion efficiency of the core, and cs UCNP-graphene to be 65, and 46 %, respectively. The photothermally generated heat on these nanoheater surfaces can be used for desalination of salt water. We demonstrate > 96 % of salt recovery from saline water dispersed on the UCNP coated substrate under 980 nm irradiation. Around 9 mg/cm2 of water could be evaporated in 10 minutes from 1 cm2 area of a 1mL column of saturated saline water on the photothermally active UCNPgraphene material independent of the substrate (copper, or silicon) under 7.96 W/cm2 of 980 nm irradiation. Normal sunlight (~100 mW/cm2) could evaporate saturated saline water from the cs UCNP-graphene coated substrate in ~200 s, compared to ~600 s on the surface without the photothermal agent.

Keywords: Graphene, Upconversion nanoparticles, Fluorescence Quenching and Enhancement, Nanoheater, Desalination


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Introduction Energy is fundamental to sustainable development, and understandably bulk of the present day research is directed towards energy production, storage, and conversion.1 Beyond fossil fuels, the obvious source of energy is the sun, harvesting light from which can result in solar cells with efficiencies around 25%.2 Similarly, limited power may be generated following chemical routes, such as water-splitting, but challenged by low efficiency values.3 The resurgence of thermoelectrics also attracted lot of effort in a hope of meeting the challenges in the energy research.4 Here, heat is harnessed, and converted to electricity with materials requiring stringent, competing physical, and chemical properties. Such challenges would obviously result in poor efficiency values, or figure of merit.5 Interestingly, all the three examples presented above are energy conversion processes namely, photovoltaic, photoelectrochemical, and thermoelectric. Another conversion route is photothermal, which is commonly viewed as solar-thermal where solar energy is harvested for environmental purification, water distillation/desalination, catalytic production of fuels, and chemicals, water evaporation, and so on.5-6 Solar absorber materials include, plasmonic metals,7 narrow bandgap semiconductors,7 and carbon-based nanostructures6 with 60-81% photothermal conversion efficiencies with graphene foam8 topping the list. The reported graphene foam8 used sunlight, which is really helpful, for the desalination application. However, the infrared (IR) absorption, constituting ~40 % of the solar spectrum, is limited in the graphene foam. Hybrid materials have also been used. Plasmonic bi-metallic nanostructures, and semiconductor-metal nanoparticles populate this list.9 To increase optical absorption, novel structural design, such as plasmonic nanoparticles in one dimensional scattering media, has also been used.9 Optical absorption, and its conversion to light and heat is central to this work.



Multiphoton IR (980 nm) absorbing lanthanide-doped upconversion nanoparticles (UNCPs), emitting in the visible through an anti-Stokes process, has emerged as a key player in applications such as, solar cell,10 photodetector,11 photo thermal therapy,12 and others.13-17 Compared to other fluorescent nanoparticles, such as organic dyes, and quantum dots, UCNPs offer selective detectability, with low auto-fluorescence at the near IR (NIR) excitation. However, the major drawback of the UCNPs is their low quantum efficiency (4 for all the graphene used, but a maximum EF of 30x, for the 540 nm emission was observed when I2D/IG value is 1.36 indicating few layer or multilayer graphene (Figure 3d). Using graphene with I2D/IG ~1.05 resulted in a lowering of the fluorescence EF to ~4x (Figure 3 d). For subsequent fluorescence or photothermal measurements graphene with I2D/IG ~1.05 will be excluded. The result in Figure 3d may be attributed to graphene plasmons where the silica spacer on the UCNPs help to sustain the local electric field to support efficient radiative decay pathways, and prevent the charge transfer from the UCNP to the graphene.31,34 In fact, Rodrigo et al., have demonstrated that bilayer, and multilayer graphene express efficient plasmonic properties than single layer graphene.28 Localized plasmons in metallic nanostructures, with tunable optical 11 ACS Paragon Plus Environment


absorption bands, from visible to NIR (400 -1000 nm), have also demonstrated MEF in UCNPs.1922 The

EF (or QF) is dependent on many factors, such as the wavelength of emission, the material,

and morphology of the plasmonic nanostructures used, surface coverage of the fluorophores and so on, and are thus difficult to compare. We compared the EF of the green emission (540 nm) in core and cs UCNPs, among previously published works to this one being reported (ESI Table S1), and found that the 30X EF is among the highest reported so far using similar material, and enhancer design. In short, our observations suggest that single layer graphene is suitable for fluorescence quenching behavior, and bi-layer or multilayer graphene is better for fluorescence enhancement purposes. These graphene layers to maximize the QF, and EF shall be designated as optimized graphene from here on. The power dependent fluorescence study for the core, and cs UCNPs on different graphene coated substrates is presented in ESI Figure S3, and S4, respectively. From Figure S3, and S4 a double log plot of fluorescence intensity vs incident power density would yield the number of photons (n, from the slope) involved in the absorption/emission process for the core (ESI Figure S5 a-c), and cs UCNPs (ESI Figure S5 d-f). The data indicates a minimum, and maximum n value of 1.66 to 2.54, respectively, showing multiphoton absorption in all cases. The power dependence of the QF, and EF is shown in ESI Figure S6. Plasmonic fluorescence enhancement generally decreases the fluorescence lifetime. The lifetime data indicates a decreased lifetime when graphene enhances the fluorescence for the cs-UCNPs. An increase in lifetime is noted when graphene quenches the fluorescence in core-UCNPs (ESI Table S2, and references therein). The local temperature of the core, and cs UCNPs, with and without the optimized graphene, was measured as a function of incident laser power (Figure 4) using the NIR thermal camera (experimental setup is shown in ESI Figure S7). Figure 4 (a-d) shows the temperature rise in core 12 ACS Paragon Plus Environment

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UCNPs on SiO2/Si substrate, which is our (control), when irradiated with the 980 nm laser. The Tmax measured, at the laser incidence (encircled black), was found to be 54, 90, 125, and 145 °C, corresponding to increasing laser powers (Figure 4a-d). The observed photothermal conversion in UCNPs arises due to non-radiative transitions29 that have already been utilized for photothermal treatments in bio-applications.12,14 Graphene has also shown photothermal conversion in the NIR region.35 Figure 4e-h, shows the temperature of core-UCNPs on graphene (I2D/IG 1.72), irradiated similarly. Tmax of 48, 85, 118, and 139 °C was recorded.

Figure 4. Thermal images of (a-d) core-UCNPs on SiO2/Si, (e-h) core UCNPs/graphene, (i-l) cs UCNPs on SiO2/Si, and (m-p) cs UCNPs/graphene under increasing (3.98, 7.96, 11.94, and 15.58 W/cm2, left to right) powers of 980 nm laser exposure for 5 mins. The maximum temperature (Tmax) at the point of laser incidence is mentioned in each panel. Room temperature was 25 °C. For (e-h), graphene, that produced maximum fluorescence quenching, with I2D/IG~1.72, and for (m-p) graphene, that produced maximum fluorescence enhancement, with I2D/IG~1.36 was used.



This confirms that the local temperature of the core-UNCP/graphene layer is lower than that of the core-UNCPs (no graphene) under similar light exposure. As the fluorescence of the core UCNP is quenched in presence of graphene, a reduced photothermal effect resulted. The IR thermal camera image of the control SiO2/Si substrate without and with graphene (having I2D/IG ~ 1.36) is shown in (ESI Figure 8 a,b). Tmax was measured to be 46, 78, 108, and 119 °C on the cs UNCPs (Figure 4i-l) in absence of graphene. Upon the introduction of the optimized graphene (I2D/IG 1.36, that produced maximum fluorescence enhancement), elevated Tmax of 66, 121, 152, and 172 °C (Figure 4 m-p) was observed. In short, wherever the UCNP fluorescence was higher, the local temperature was higher be it because of higher excitation power,29-30 or the presence of fluorescence enhancing graphene.36-37 Hence, we may conclude that the photothermal heat generation is more where the fluorescence of the UCNPs is more. Figure 5 shows the dynamic temperature response of the UCNPs, on optimized graphene under different illumination powers, as measured by a thermocouple. In case of the core UCNPs on graphene (Figure. 5a), as the laser is turned ON the temperature quickly rose and attained a saturation. With the laser OFF, the temperature gradually dropped off to ambient values. The saturation temperature increased with laser power, and attains a maximum value of 233°C at 15.58 W/cm2. The temperature rise curve, when illumination is ON, is steeper or faster, than the temperature fall curve which is slower. For the cs UNCPs/graphene case, the temperature saturates at a higher limit of 243 °C at the same power of 15.58 W/cm2. The trend is in good agreement with the NIR thermal camera measurements (Figure. 4).


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Figure 5. Dynamic response of temperature in (a) core, and (b) cs UCNPs on graphene under 980 nm laser illumination. Irradiation ON, and OFF points are marked. Maximum rise of local temperature (ΔT) is also shown pictorially. For core-UCNPs case I2D/IG-1.72, and for cs UCNPs I2D/IG-1.36 was used. (c) Variation of photothermal conversion efficiency as a function of incident power density for core, and cs UCNPs on graphene. Data in (a), and (b) are used to estimate the photothermal conversion efficiency in (c). All data were obtained from thermocouple measurements.

The phothermal efficiency (η) of core, and cs UCNPs, both with graphene, was calculated using the cooling curves (ESI Figure S9) obtained from Figure 5a, b. η is found to be 65, and 46 % for the core, and cs UCNP (both with graphene), respectively. Similar values of η for UCNPs had been observed before.12,30,38-41 A longer duration of laser exposure has also been tried to demonstrate the thermal (7500 s) and photo stability (3600 s) of a random UCNP@SiO2/Graphene (ESI Figure S10). The rate of heating observed in this photothermal process was very high, and a temperature rise of >220 °C (assuming room temperature to be 23 °C) could be reached in less than 40 s, indicating a heating rate of 5.5 °C/s. Such rapid heating could be utilized in water evaporation6 leading to desalination. First, we demonstrate videographically (ESI Videos S1-S4) the rapid evaporation of water (2 µL) on core and cs UCNP-graphene surfaces under 980 nm illumination at 7.96 W/cm2. The experimental setup is shown in ESI Figure S11. It was found that complete evaporation of the 15 ACS Paragon Plus Environment


water droplet occurred within 30 s on the cs UCNP/graphene surface (ESI Video S4), and for all other surfaces the evaporation time to complete dryness was ~ 40 s.

Figure 6. Mass change of 1 mL saturated salt solution vs 980 nm laser irradiation time when the solution was placed on (a) copper (Cu) supported graphene (G), and UCNP bilayer, and (b) SiO2/Si supported graphene, and UCNP bilayer. Six different surfaces were used for each case. Plain substrate (Cu, or SiO2/Si), graphene on substrate, coreUCNP (C-UCNP) on plain, and graphene coated substrate, cs UCNP (CS-UCNP) on plain, and graphene coated substrate. A reference data of only saline water is also shown without any functional substrate. The arrow indicates the order in which the data appears in the graph from top to bottom. (c) Recovery (R %) of salt vs mass of the added salt (Ma) on CS-UCNP/graphene bilayer. The optical picture of a 2 µL saturated saline droplet (d) before, and (e) after complete photothermal recovery of the salt (encircled area), using 7.96 W/cm2 of 980 nm laser, on the core UCNPgraphene substrate. The optical picture of the saturated saline droplet (f) before, and (g) after complete photothermal recovery of the salt (encircled area), using 7.96 W/cm2 of 980 nm laser, on the cs UCNP- graphene substrate. The optical picture of a 10 µL saturated saline droplet (h) before, and (i) after complete photothermal recovery of the salt (~300 s), using ~100 mW/cm2 of sunlight, on the core- UCNP- graphene substrate. The optical picture of the saturated saline droplet (j) before, and (k) after complete photothermal recovery of the salt (200 s), using ~100 mW/cm2 of sunlight, on the cs UCNP- graphene substrate.


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Encouraged by the water evaporation experiment, we evaluated the desalination capability of the UCNP-graphene bilayer under 980 nm laser excitation using the setup shown in ESI, Figure S12. Figure 6a shows the mass change (per unit area of the surface) of 1 mL of saturated salt (5 g, in 10 mL) solution, as a function of time, when placed on the UCNP coated substrates, and irradiated with 980 nm laser. In the absence of the UCNP coated substrate, i.e. only in the presence of the 980 nm light, there was < 2.5 mg/cm2 decrease in the mass (Figure 6a). When a substrate, such as copper, was introduced, the decrease in the mass, i.e. evaporation of the saline water droplet, was more to ~17 mg/cm2 over a period of 10 minutes. The introduction of graphene on the copper resulted in an additional decrease in the mass to around 20 mg/cm2 over the same period of time. This may be due to the reflection of the 980 nm radiation through the water column resulting in enhanced optical absorption. The introduction of core, or cs UCNPs on the graphene coated surface resulted in a huge decrease of the mass of the saline water due to enhanced absorption of the 980 nm radiation, and its subsequent photothermal conversion to heat. It was observed that the cs UCNP/Graphene/Cu has the highest mass change at 26 mg/cm2 implying the highest evaporation rate. Compared to the bare copper substrate, an additional mass decrease of 9 mg/cm2 was observed for the cs UCNP/graphene coated substrate (Figure 6a). The same experiments were performed on a SiO2/silicon substrate that had absorption in the 980 nm range (Figure 6b) and results in 68, and 100 °C rise of local temperature without (ESI Figure S8a) and with graphene (ESI Figure S8b) cover under similar irradiation. The mass decrease of the saline water (on SiO2/silicon) under irradiation increased when compared to bare copper substrate. Here also, a similar trend of the change in the saline water mass was observed upon introduction of the UCNP, and graphene. Compared to the bare SiO2/silicon substrate, an additional mass decrease of 10 mg/cm2 was observed for the cs 17 ACS Paragon Plus Environment


UCNP/graphene coated substrate (Figure 6b). In both experiments, using copper (Figure 6a), or SiO2/silicon (Figure 6b), a substrate independent mass decrease of around 9-10 mg/cm2 was observed which can safely be attributed to the UCNP-graphene bilayer only. These results demonstrate the possibility of using cs UCNP/graphene bilayer for water evaporation, and desalination process. To quantify the desalination process, we have calculated the recovery (R %) (Equation 3, Experimental section) of salt, from its saturated aqueous solution, post photothermal evaporation. It was found that we could recover 96-99 % (Figure 6c) of the added salt (from its solution) post evaporation on the cs UCNP-graphene surface irradiated with 980 nm light at 7.96 W/cm2. The salt recovery process was videographed, and still shots taken from the video is presented in Figure 6d-g. Figure 6d, and Figure 6e shows the saturated saline droplet on the core-UCNP/graphene surface before, and after photothermal evaporation, respectively. Similar still shots were presented when the saline droplet was placed on the cs UCNP/graphene surface before (Figure 6f), and after (Figure 6g) photothermal evaporation. The videos of these experiments can be found in ESI (Videos S5 -S6). The desalination process using the graphene-UCNP surface with sunlight (~100 mW/cm2 power density) exposure have been videographed also (ESI Videos S7-S8). Still shots taken from the videos is presented in Figure 6h-k. Photographs show the saturated saline droplet (10 µL) on the core-UCNP/graphene (Figure 6h-i), and cs UCNP-graphene surface (Figure 6j-k), respectively, before, and after photothermal evaporation. In this case, as sunlight contains only ~40 % of IR component, the process takes around ~300 (on core-UCNP-graphene), and ~200 s (on cs UCNP-graphene) to complete, compared to ~30 s on the same substrate (on cs UCNPgraphene) when using 7.96 W/cm2 of 980 nm laser. In absence of the photothermal agents, this


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process will be substantially longer, around 600 s, on a bare SiO2/Si substrate. The evaporation rate, when using sunlight, may be accelerated by using higher concentration of the UCNPs. Experimental Section Materials Yttrium chloride (YCl3, 99.99 %), Ytterbium chloride (YbCl3, 99.99 %), Erbium chloride (ErCl3, 99.99 %), oleic acid (90 %), octadecene (90 %), sodium hydroxide (NaOH, 98.9 %), ammonium fluoride (NH4F, 98.9 %), methanol (99.9 %), tetraethylorthosilicate (TEOS) (98.99 %), polymethyl methacrylate (PMMA), 0.025 mm copper foil were purchased from Sigma- Aldrich (USA), and CO-520 (Igepal) (98.99 %), ammonia (NH3, 30 %), were purchased from Acros Organics (Taiwan). All the chemicals were used as-purchased without further purification or modification. Synthesis of NaYF4: Yb, Er nanoparticles NaYF4: Yb, Er nanoparticles were prepared via slightly modified thermal decomposition method according to a standard procedure.42-43 Typically, 0.8 mmol of YCl3, 0.18 mmol (18 %) of YbCl3, and 0.02 mmol (2 %) of ErCl3 were properly mixed with 12 mL of oleic acid, and 14 mL of octadecene. The solution was heated to 160 °C under nitrogen gas flow with 650 rpm stirring for 20 min to form a homogeneous solution, and then cooled down to 50 °C. 10 mL methanol solution of NaOH (2.5 mmol), and NH4F (4 mmol) were slowly added into the flask, and stirred for 30 min. The solution was then slowly heated to remove methanol, degassed at 100 °C for 12 min, followed by heating at 300 °C for 1 h under high purity nitrogen protection. After the solution was cooled naturally, the nanoparticles were precipitated from the solution with acetone, and washed with ethanol water (1:1) three times by sonication, and centrifugation (5000 rpm), and dispersed in cyclohexane.



Synthesis of silica coated NaYF4: Yb, Er (UCNP@SiO2) nanoparticles Tunable SiO2 shell outside the core UCNPs were synthesized by a reverse micro emulsion process.42 Typically, 0.5 mL of CO-520 (Igepal), 6 mL of cyclohexane, and 4 mL of 10 mM NaYF4: Yb, Er nanoparticles solution (dispersed in cyclohexane) were mixed, and stirred (500 rpm) for 15 min. Further, 81 µL of 30 wt. % NH3 was then added dropwise, and the container was sealed, and sonicated for 35 min until a transparent emulsion was formed. Finally, 10 µL of TEOS was then added into the solution. The solution was stirred for 48 hours at a speed of 600 rpm. We also varied the SiO2 shell thickness by changing the amount of TEOS. NaYF4: Yb, Er@SiO2 nanoparticles were precipitated by adding acetone, and cleaned with ethanol/ water (1:1) twice, and finally re-dispersed in 4 mL of water. The SiO2 shell thickness varied from 5-15 nm, however the 7 nm SiO2 shell on the UCNPs was used for this study. Synthesis of Graphene Graphene sheets were synthesized by standard thermal chemical vapor deposition (TCVD)44 technique using a single zone furnace. The 0.025 mm thick, high purity copper foils were cleaned in acetone for one minute, ethanol for one minute, and finally put into acetic acid for 10 minutes, and then dried using nitrogen jet.33 After cleaning, the foil was inserted into the middle of the furnace which would be heated up to 1000 ºC in H2 gas flow rate of 4 sccm. The foil was annealed for 30 min to remove residual oxide and prepare the surface for the growth, and subsequently CH4 gas was introduced as carbon precursor at flow rates within 15-45 sccm for 10 min, in presence of H2, to complete the growth.33,45 After growth (stoppage of CH4 flow) samples were cooled to room temperature in Ar, and H2 atmosphere. Transfer of Graphene on SiO2/Si


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In order to transfer the Graphene on SiO2/Si, polymethylmethacrylate (PMMA) was spin coated on graphene/copper. PMMA/graphene/Cu was kept on a hot-plate at 100 °C for 10 minutes for baking, and then immersed in aqueous solution of FeCl3 (0.5 M) to etch the copper. After etching process PMMA/graphene was transferred to DI-water using a glass slide to remove the FeCl3 solution. Finally, PMMA/graphene was transferred on SiO2/Si, and kept in acetone solution for the removal of PMMA from graphene. Details of this transfer process can be found elsewhere.46 Graphene/UCNP surface for fluorescence, and photothermal studies. Four sets of graphene samples, prepared with different CH4 flow rates, having different quality and layer numbers (determined by Raman spectroscopically estimated I2D/IG value discussed later) were prepared on Copper, and SiO2/Si. 10 µL of core, and cs UCNPs were dispersed on these surfaces and left to dry normally for 5h. Surfaces with core, and cs UCNPs, but without graphene, were used as control. All of these samples were used for the fluorescence and photothermal measurements under 980 nm excitation. Methods Raman spectroscopy measurements were done using Jobin Yvon HR 800 commercial Raman spectrometer with 532 nm excitation (Nd-Yag Laser, beam diameter - 4 μm, 10 mW) using 10 s exposure time. The intensity of the graphitic band (G), and second order of the defect band (2D) were used to calculate the I2D/IG, that can be used to estimate the layer numbers in the resultant CVD graphene. The Optical microscopy is done using Olympus (BX53) microscope to see the grains of graphene grown on copper. Transmission electron microscope (TEM) were carried out using a JEM-2000EX (JEOL, Japan) machine using an accelerating voltage of 100 KV, spot size 2. The high resolution TEM (HRTEM) images were recorded using JEM-1400 Plus, JEOL, Japan. TEM samples were prepared by transferring graphene on the 400 mesh Cu grid (Ted Pella Inc.,



USA) using the routine PMMA assisted method. 1 µL UCNPs from a solution in hexane (for coreUCNPs), or in water (for cs UCNPs) was drop coated on the Cu grid, and left to dry in a vacuum oven for 12 hours. UV-Vis absorption measurement was performed using UV-visible (vis)- near infrared (NIR) spectrophotometer (V-770, JASCO, Japan). Up conversion fluorescence emission spectra were measured using Fluorolog-3 (Jobin Yvon, USA) using 980 nm laser (SDL-5000 T, Shanghai China) with 60-degree angle of incidence, and power up to 3W as the excitation source. Photothermal imaging, efficiency, evaporation, desalination, and salt recovery NIR thermal camera (IRM-P384A, Ching Hsing Computer Tech, Taiwan) was used to measure, and image the local temperature distribution on the core, and cs UCNP dispersed surfaces coated with or without graphene. The experimental setup is shown in ESI Figure S7. A dual temperature sensor was used to monitor the surface temperature rise on the photothermal substrate (with the photothermal agent, under irradiation) with a thermocouple while monitoring the room temperature, simultaneously, with a platinum resistance thermometer as a function of time. The surface temperature monitoring thermocouple was embedded in the probe surface (1 cm2, SiO2/Si) coated with graphene, and dispersed with the UCNPs. The setup is shown in ESI Figure S11. The variation of temperature was monitored with the incident 980 nm light ON, and OFF under different power densities. All the experiments were performed under room temperature. The cooling curves of the samples after the incident radiation was turned OFF was used to estimate the photothermal conversion efficiency (η). For videographing photothermal evaporation experiments, optimized graphene coated SiO2/Si substrates were dispersed with core, and cs UCNPs, and a thermocouple was attached at the back of the substrate to measure the surface temperature. 2µL DI water droplet was placed on the surface, 22 ACS Paragon Plus Environment

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and irradiated with the 980 nm laser (7.96 W/cm2) normal to the surface. A common video camera (Canon 7D, with a 100 mm/f 2.8 macro lens) was placed in front to record the evaporation of the droplet (SI Video S1-S4) using the same setup as shown in Figure S11. In a similar way, we have placed 2µL of saturated saline water droplet on these surfaces and took photographs of the substrate before and after the evaporation of the droplet under laser irradiation (Figure 6 d-g, ESI Video S5, S6). Same visual experiments (ESI Video S7-8) were carried out with sunlight (~100 mW/cm2) on similarly prepared substrates where the volume of the saturated saline droplet was 10 µL (Figure 6h-k). Another desalination experiment was performed by measuring the mass loss of the saline water as done in a previous report.6 The experimental setup is shown in Figure S12, ESI. We used a cuvette (2.5 cm diameter) containing 1 mL of saturated salt (NaCl) solution (in de-ionized water), and kept it on an electronic weighing machine, and 980 nm laser (7.96 W/cm2) was irradiated normally on it. The mass loss of the water, due to evaporation, was measured as a function of time. Different substrates, such as bare SiO2/Si, graphene coated SiO2/Si, graphene coated SiO2/Si with core, and cs UCNPs, having the same area, was placed at the bottom of the cuvette, and the experiments repeated as before. As bare Si/SiO2 showed absorption of the incident radiation, we repeated the same experiments with the same graphene, core, and cs UCNPs on high purity copper substrates of the same size, to delineate the effect of the substrate, from the UCNP-graphene layer. For salt recovery experiments, 2-10 µL droplets of saturated saline water (5g salt in 10 mL DI water), was placed on the probe surface containing cs UCNP-Graphene-Si/SiO2 (known weight m1), and irradiated with 980 nm radiation at a power density of 7.96 W/cm2. After evaporation of the saline water droplet, dry salt was observed deposited on the surface whose total mass was



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measured (m2). The mass of the recovered salt is Mr = m2-m1. The recovery efficiency R (%) of the salt from the saline water is defined 47 as

(

𝑅(%) = 1 ―

(𝑀𝑎 ― 𝑀𝑟) 𝑀𝑎

)𝑥100

(3)

where Ma is the mass of the added salt in the saturated salt solution. If Mr= Ma, 100% recovery is attained. To observe the desalination process, videos of the photothermal evaporation of the 10 µL saturated saline droplet were taken on both core, and cs UCNP dispersed graphene/Si/SiO2 substrates. Optimized graphene was used for the core (I2D/IG ~ 1.72), and cs UCNPs (I2D/IG ~ 1.36). Still shots from the video before, and after the droplet evaporation was taken to show the recovered salt. Conclusions In summary, we demonstrate that fluorescence in lanthanide doped upconversion nanoparticles (UCNP, NaYF4: Yb, Er) can be quenched or enhanced by layer-controlled CVD graphene. Asgrown core-UCNPs have their 540 nm fluorescence quenched by 3 times when in contact with monolayer graphene. UCNPs with a 7 nm silica shell (UCNP@SiO2) have their green fluorescence enhanced by 30 times when in contact with bi-layer or few layer graphene. Thermal imaging data shows that the UCNP-graphene surface work as photothermal nano-heaters when irradiated with 980 nm light. The local temperature increased with increase in fluorescence intensity from the surface. The resultant temperatures on the UCNP-graphene surfaces could be ramped up as fast as 5 °C/s to reach as high as 243 °C under 7.96 W/cm2 of 980 nm irradiation. From the dynamic response of the local temperature variation of the irradiated UCNP-graphene, and UCNP@SiO2– graphene surfaces, we could estimate the photothermal conversion efficiency to be 65, and 46 %, respectively. This heat could be used to evaporate water aiding in desalination of saline water 24 ACS Paragon Plus Environment

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quickly (~30 s) using 7.96 W/cm2 of 980 nm laser, or relatively slowly (~200 s) using ~100 mW/cm2 of direct sunlight (having only 40 % of infrared light). A high mass loss of 60 mg/cm2 from 1 mL of saturated salt solution could be obtained on UCNP-graphene coated silicon surface irradiated with 7.96 W/cm2 of 980 nm light over a period of 10 minutes. More than 96 % of added salt, to an aqueous solution, could be recovered after photothermal evaporation of the water placed on the UCNP-graphene surface irradiated with 980 nm light. We hope that these UCNPs can be used in conjunction with graphene foam for high efficiency desalination application utilizing the infrared component of sunlight so that the IR laser may be redundant. Acknowledgements: This study was supported by the Ministry of Science and Technology, Taiwan under Grant No MOST 104-2112-M-010-002-MY3, and MOST 107-2112-M-010-003. MKT acknowledges Dr. Golam Haider, and Dr. Pardip Kumar Roy, National Taiwan University for helping in graphene transfer. Dr. K. H. Lin of Academia Sinica, Taiwan, is acknowledged for providing the data on the fluorescence lifetime. MKT acknowledges the support from National Yang Ming University Taiwan, for his PhD fellowship.



References: (1) Chattopadhyay, S.; Chen, L.-C.; Chen, K.-H. Energy production and conversion applications of one-dimensional semiconductor nanostructures. NPG Asia Materials 2011, 3 (8), 74. (2) Green, M. A. The path to 25% silicon solar cell efficiency: history of silicon cell evolution. Prog. Photovolt: Res. Appl. 2009, 17 (3), 183-189. (3) Gardecka, A. J.; Bishop, C.; Lee, D.; Corby, S.; Parkin, I. P.; Kafizas, A.; Krumdieck, S. High efficiency water splitting photoanodes composed of nano-structured anatase-rutile TiO2 heterojunctions by pulsed-pressure MOCVD. Applied Catalysis B: Environmental 2018, 224, 904911. (4) Zhang, X.; Zhao, L.-D. Thermoelectric materials: Energy conversion between heat and electricity. Journal of Materiomics 2015, 1 (2), 92-105. (5) Zhu, L.; Gao, M.; Peh, C. K. N.; Ho, G. W. Solar-driven photothermal nanostructured materials designs and prerequisites for evaporation and catalysis applications. Mater. Horiz. 2018, 5 (3), 323-343. (6) Liu, K.-K.; Jiang, Q.; Tadepalli, S.; Raliya, R.; Biswas, P.; Naik, R. R.; Singamaneni, S. Wood– graphene oxide composite for highly efficient solar steam generation and desalination. ACS Appl. Mater. Interfaces 2017, 9 (8), 7675-7681. (7) Wang, J.; Li, Y.; Deng, L.; Wei, N.; Weng, Y.; Dong, S.; Qi, D.; Qiu, J.; Chen, X.; Wu, T. High‐Performance Photothermal Conversion of Narrow‐Bandgap Ti2O3 Nanoparticles. Adv. Mater. 2017, 29 (3), 1603730. (8) Wang, G.; Fu, Y.; Guo, A.; Mei, T.; Wang, J.; Li, J.; Wang, X. Reduced graphene oxide– polyurethane nanocomposite foam as a reusable photoreceiver for efficient solar steam generation. Chem. Mater. 2017, 29 (13), 5629-5635.


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Jain, P. K.; Manthiram, K.; Engel, J. H.; White, S. L.; Faucheaux, J. A.; Alivisatos, A. P. Doped nanocrystals as plasmonic probes of redox chemistry. Angew.Chem. 2013, 125 (51), 13916-13920. (10) Yang, W.; Li, X.; Chi, D.; Zhang, H.; Liu, X. Lanthanide-doped upconversion materials: emerging applications for photovoltaics and photocatalysis. Nanotechnology 2014, 25 (48), 482001. (11) Zhou, N.; Xu, B.; Gan, L.; Zhang, J.; Han, J.; Zhai, T. Narrowband spectrally selective nearinfrared photodetector based on up-conversion nanoparticles used in a 2D hybrid device. Journal of Materials Chemistry C 2017, 5 (7), 1591-1595. (12) Gupta, A.; Lam, C. W.; Wu, C. T.; Yang, D. M.; Chattopadhyay, S. Photothermal Disintegration of 3T3 Derived Fat Droplets by Irradiated Silica Coated Upconversion Nanoparticles. Part. Part. Syst. Charact. 2018, 1800294. (13) Auzel, F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev. 2004, 104 (1), 139-174. (14) Chen, G.; Qiu, H.; Prasad, P. N.; Chen, X. Upconversion nanoparticles: design, nanochemistry, and applications in theranostics. Chem. Rev. 2014, 114 (10), 5161-5214. (15) Wei, R.; Xi, W.; Wang, H.; Liu, J.; Mayr, T.; Shi, L.; Sun, L. In situ crystal growth of gold nanocrystals on upconversion nanoparticles for synergistic chemo-photothermal therapy. Nanoscale 2017, 9 (35). (16) Wang, J.; Yao, C.; Shen, B.; Zhu, X.; Li, Y.; Shi, L.; Zhang, Y.; Liu, J.; Wang, Y.; Sun, L. J. T. Upconversion-Magnetic Carbon Sphere for Near Infrared Light-Triggered Bioimaging and Photothermal Therapy. Theranostics 2019, 9 (2), 608. (17) Sun, L.; Wei, R.; Feng, J.; Zhang, H. Tailored lanthanide-doped upconversion nanoparticles and their promising bioapplication prospects. Coordination Chemistry Reviews 2018, 364, 10-32.



(18) Wang, H.; Li, M.; Yin, Z.; Zhang, T.; Chen, X.; Zhou, D.; Zhu, J.; Xu, W.; Cui, H.; Song, H. Remarkable Enhancement of Upconversion Luminescence on Cap-Ag/PMMA Ordered Platform and Trademark Anticounterfeiting. ACS Appl. Mater. Interfaces 2017, 9 (42), 37128-37135. (19) Manurung, R. V.; Wu, C. T.; Roy, P. K.; Chattopadhyay, S. A plasmon-tuned ‘gold sandwich’for metal enhanced fluorescence in silica coated NaYF4: Yb, Er upconversion nanoparticles. RSC. Adv. 2016, 6 (90), 87088-87095. (20) Niu, W.; Chen, H.; Chen, R.; Huang, J.; Palaniappan, A.; Sun, H.; Liedberg, B. G.; Tok, A. Synergetically Enhanced Near‐Infrared Photoresponse of Reduced Graphene Oxide by Upconversion and Gold Plasmon. Small 2014, 10 (18), 3637-3643. (21) Lu, D.; Cho, S. K.; Ahn, S.; Brun, L.; Summers, C. J.; Park, W. Plasmon enhancement mechanism for the upconversion processes in NaYF4: Yb3+, Er3+ nanoparticles: Maxwell versus Förster. ACS Nano 2014, 8 (8), 7780-7792. (22) Yin, Z.; Zhou, D.; Xu, W.; Cui, S.; Chen, X.; Wang, H.; Xu, S.; Song, H. Plasmon-Enhanced upconversion luminescence on vertically aligned gold nanorod monolayer supercrystals. ACS Appl. Mater. Interfaces 2016, 8 (18), 11667-11674. (23) Kasry, A.; Ardakani, A. A.; Tulevski, G. S.; Menges, B.; Copel, M.; Vyklicky, L. Highly efficient fluorescence quenching with graphene. J. Phys. Chem. C. 2012, 116 (4), 2858-2862. (24) Wei, W.; He, T.; Teng, X.; Wu, S.; Ma, L.; Zhang, H.; Ma, J.; Yang, Y.; Chen, H. Nanocomposites of graphene oxide and upconversion rare‐earth nanocrystals with superior optical limiting performance. Small 2012, 8 (14), 2271-2276. (25) Chhabra, R.; Sharma, J.; Wang, H.; Zou, S.; Lin, S.; Yan, H.; Lindsay, S.; Liu, Y. Distancedependent interactions between gold nanoparticles and fluorescent molecules with DNA as tunable spacers. Nanotechnology 2009, 20 (48), 485201.


Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Lakowicz, J. R. Radiative decay engineering 3. Surface plasmon-coupled directional emission. Analytical Biochemistry 2004, 324 (2), 153-169. (27) Fei, Z.; Iwinski, E.; Ni, G.; Zhang, L.; Bao, W.; Rodin, A.; Lee, Y.; Wagner, M.; Liu, M.; Dai, S. J. Tunneling plasmonics in bilayer graphene. Nano Lett. 2015, 15 (8), 4973-4978. (28) Rodrigo, D.; Tittl, A.; Limaj, O.; De Abajo, F. J. G.; Pruneri, V.; Altug, H. Double-layer graphene for enhanced tunable infrared plasmonics. Light: Science & Applications 2017, 6 (6), 16277. (29) Zhang, W.; Li, J.; Lei, H.; Li, B. Plasmon-induced selective enhancement of green emission in lanthanide-doped nanoparticles. ACS Appl. Mater. Interfaces 2017, 9 (49), 42935-42942. (30) He, L.; Mao, C.; Cho, S.; Ma, K.; Xi, W.; Bowman, C. N.; Park, W.; Cha, J. N. Experimental and theoretical photoluminescence studies in nucleic acid assembled gold-upconverting nanoparticle clusters. Nanoscale 2015, 7 (41), 17254-17260. (31) Feng, A. L.; You, M. L.; Tian, L.; Singamaneni, S.; Liu, M.; Duan, Z.; Lu, T. J.; Xu, F.; Lin, M. Distance-dependent plasmon-enhanced fluorescence of upconversion nanoparticles using polyelectrolyte multilayers as tunable spacers. Sci. Rep. 2015, 5, 7779, DOI: 10.1038/srep07779. (32) Ma, T.; Ma, Y.; Liu, S.; Zhang, L.; Yang, T.; Yang, H.-R.; Lv, W.; Yu, Q.; Xu, W.; Zhao, Huang, W. Dye-conjugated upconversion nanoparticles for ratiometric imaging of intracellular pH values. J. Mater. Chem. C. 2015, 3 (26), 6616-6620. (33) Chattopadhyay, S.; Li, M.-S.; Roy, P. K.; Wu, C. T. Non-enzymatic glucose sensing by enhanced Raman spectroscopy on flexible ‘as-grown’CVD graphene. Analyst 2015, 140 (12), 3935-3941.



(34) Kwon, S. J.; Lee, G. Y.; Jung, K.; Jang, H. S.; Park, J. S.; Ju, H.; Han, I. K.; Ko, H. A Plasmonic Platform with Disordered Array of Metal Nanoparticles for Three‐Order Enhanced Upconversion Luminescence and Highly Sensitive Near‐Infrared Photodetector. Adv. Mater. 2016, 28 (36), 7809-7809. (35) Lin, J.; Huang, Y.; Huang, P. Graphene-Based Nanomaterials in Bioimaging. Biomedical Applications of Functionalized Nanomaterials; Elsevier: 2018; pp 247-287. (36) Kang, S.; Choi, H.; Lee, S. B.; Park, S. C.; Park, J. B.; Lee, S.; Kim, Y.; Hong, B. H. Efficient heat generation in large-area graphene films by electromagnetic wave absorption. 2D Mater. 2017, 4 (2), 025037. (37) Gan, X.; Zhao, C.; Wang, Y.; Mao, D.; Fang, L.; Han, L.; Zhao, J. Graphene-assisted all-fiber phase shifter and switching. Optica 2015, 2 (5), 468-471. (38) Savchuk, O. A.; Carvajal, J.; Massons, J.; Aguiló, M.; Díaz, F. Determination of photothermal conversion efficiency of graphene and graphene oxide through an integrating sphere method. Carbon 2016, 103, 134-141. (39) Li, Z.; Johnson, O.; Huang, J.; Feng, T.; Yang, C.; Liu, Z.; Chen, W. Enhancing the photothermal conversion efficiency of graphene oxide by doping with NaYF 4: Yb, Er upconverting luminescent nanocomposites. Materials Research Bulletin 2018, 106, 365-370. (40) Li, P.; Yan, Y.; Chen, B.; Zhang, P.; Wang, S.; Zhou, J.; Fan, H.; Wang, Y.; Huang, X. Lanthanide-doped upconversion nanoparticles complexed with nano-oxide graphene used for upconversion fluorescence imaging and photothermal therapy. Biomaterials Science 2018, 6 (4), 877-884.


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Zhu, X.; Feng, W.; Chang, J.; Tan, Y.-W.; Li, J.; Chen, M.; Sun, Y.; Li, F. Temperaturefeedback upconversion nanocomposite for accurate photothermal therapy at facile temperature. Nature Communications 2016, 7, 10437. (42) Yuan, P.; Lee, Y. H.; Gnanasammandhan, M. K.; Guan, Z.; Zhang, Y.; Xu, Q.-H. Plasmon enhanced upconversion luminescence of NaYF4: Yb, Er@ SiO2@ Ag core–shell nanocomposites for cell imaging. Nanoscale 2012, 4 (16), 5132-5137. (43) Li, Z.; Zhang, Y. An efficient and user-friendly method for the synthesis of hexagonal-phase NaYF4: Yb, Er/Tm nanocrystals with controllable shape and upconversion fluorescence. Nanotechnology 2008, 19 (34), 345606. (44) Obraztsov, A. N. Chemical vapour deposition: making graphene on a large scale. Nature Nanotech. 2009, 4 (4), 212. (45) Borah, M.; Singh, D. K.; Subhedar, K. M.; Dhakate, S. R. J. R. A. The role of substrate purity and its crystallographic orientation in the defect density of chemical vapor deposition grown monolayer graphene. RSC. Adv. 2015, 5 (85), 69110-69118. (46) Haider, G.; Roy, P.; Chiang, C. W.; Tan, W. C.; Liou, Y. R.; Chang, H. T.; Liang, C. T.; Shih, W. H.; Chen, Y. F. Electrical‐Polarization‐Induced Ultrahigh Responsivity Photodetectors Based on Graphene and Graphene Quantum Dots. Adv. Funct. Mater. 2016, 26 (4), 620-628. (47) Park, K.-N.; Hong, S. Y.; Lee, J. W.; Kang, K. C.; Lee, Y. C.; Ha, M.-G.; Lee, J. D. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Deselination 2011, 274 (1-3), 91-96.



Table of Contents (TOC) Graphene-Conjugated Upconversion Nanoparticles as Fluorescence-Tuned Photothermal Nanoheaters for Desalination

Mukesh Kumar Thakur, Akash Gupta, Sandip Ghosh, and Surojit Chattopadhyay * Institute of Biophotonics, National Yang Ming University, 155, Sec-2 Li Nong Street, Taipei 112, Taiwan. Keywords: Graphene, Upconversion nanoparticles, Fluorescence Quenching and Enhancement, Nanoheater, Desalination

Graphene was used to quench and enhance the green fluorescence from upconversion nanoparticles (UCNPs) under 980 nm excitation. The UCNPs produce heat by photothermal conversion which we show to scale with the fluorescence. This heat can be used for desalination of saturated salt solution by rapid (5 °C/s) evaporation of water.

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Graphene-Conjugated Upconversion Nanoparticles as Fluorescence

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