Evaluation of Luminescence Properties of Single Hydrophilic

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C: Physical Processes in Nanomaterials and Nanostructures

Evaluation of Luminescence Properties of Single Hydrophilic Upconversion Nanoparticles by Optical Trapping Ya-Feng Kang, Bei Zheng, Chong-Yang Song, Chengyu Li, Zhi-Liang Chen, Qiongshui Wu, Yi Yang, Dai-Wen Pang, and Hong-Wu Tang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00430 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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The Journal of Physical Chemistry

Evaluation of Luminescence Properties of Single Hydrophilic Upconversion Nanoparticles by Optical Trapping

Ya-Feng Kang[a], Bei Zheng[a], Chong-Yang Song[a], Cheng-Yu Li[b], Zhi-Liang Chen[a], Qiong-Shui Wu[b], Yi Yang[c], Dai-Wen Pang[a], and Hong-Wu Tang*[a]

[a] Ya-Feng Kang, Bei Zheng, Chong-Yang Song, Cheng-Yu Li, Zhi-Liang Chen, Dai-Wen Pang, and Hong-Wu Tang Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China E-mail: [email protected] [b] Qiong-Shui Wu Electronic information school, Wuhan University, Wuhan 430072, China [c] Yi Yang Key Laboratory of Artificial Micro- and Nano- Structures of Ministry of Education, School of Physics & Technology, Wuhan University, Wuhan 430072, China.

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ABSTRACT By using multifunctional optical tweezers (OT) equipped with a 980 nm continuous-wave single-mode diode laser and multiple detectors, we are able to trap single upconversion nanoparticles stably as well as synchronously analyze their luminescence properties. Successive trapping of individual octylamine-modified poly(acrylic acid) encapsulated UCNPs (OPA-UCNPs) is proved by real-time monitoring of forward scattering (FSC), luminescence intensity and spectra, and the results verify that these nanoparticles possess excellent colloidal stability and uniform luminescence properties. Besides, ligand/solventdependent surface quenching effect of single UCNPs is investigated, and the results show that

OPA-UCNPs by hydrophobic encapsulation

strategy

possess outstanding

luminescence properties and the property of OPA to reduce the quenching effect of ligands and water molecules are well identified. Upconversion luminescence decay lifetime also explains the mechanism that the presence of OPA molecules reduces surface quenching effect, thus OPA-UCNPs exhibit longer luminescence decay lifetime. Therefore, we not only provide a new method to evaluate the luminescence properties of single nanoparticles by using multifunctional OT but also prove that encapsulating hydrophobic UCNPs with amphiphilic molecules is an alternative strategy to prepare monodisperse hydrophilic UCNPs while significantly maintaining their luminescence properties.

Introduction 2


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Lanthanide (Ln3+)-doped upconversion nanoparticles that convert low-energy near-infrared (NIR) irradiation into high energy visible emission are gaining increasing interest and it has been used as luminescence probes in a number of bioassays1-4 and bioimaging fields.5-10 Based on sensitizer ions with relatively large absorption cross-sections, it can sequentially transfer energy to activator ions generate visible luminescence by using a 980 nm CW laser excitation. Nowadays, the most widely adopted synthesis strategies of hexagonal NaYF4 UCNPs are based on thermal decomposition method to fine control the growth of the nanocrystals. Notably, recent synthetic approaches have studied various aspects to enhance upconversion luminescence, for instance, fine-tuning the size,11-14 composition,1517

phase of nanoparticles,18 which are key components for synthesizing highly bright

UCNPs. However, the hydrophobic nanoparticles lack water-soluble ligand to bioanalytical applications remains a formidable challenge, therefore, surface modification is essential for biological applications.10, 19, 20 A large number of methods for surface modification of hydrophobic nanoparticles are reported,21-25 with the same feature that the modification ligands must be rich in carboxyl or amino groups in order to possess high colloidal stability as well as the possibility of subsequent bioconjugation. In particular, the solvent-dependent surface quenching effect, luminescence intensities of nanoparticles decline sharply when they are transferred to water by surface modification,26,27 is proposed and correlated to low upconversion efficiency due to the dopant ions will exposure to different chemical environments 28,29. Some of the previous reports have suggested that the core-shell structure,30-32 size of nanoparticles,11,33 dopant concentrations,16,34 surface molecule26-28 are the primary 3



influence factors of surface quenching effect that deteriorate the upconversion luminescence efficiency. Most of the approaches to judge the quenching efficiency of UCNPs luminescence are still limited to the use of a fluorescence spectrometer obtaining ensemble-averaged measurements relying on either same concentration of UCNPs or stable colloidal suspensions. Generally, the concentration of nanoparticles applied in bioimaging or single-molecule studies is low, so the luminescence property of the single nanoparticle is extremely important. Unfortunately, so far, the measurement of the luminescence intensity and spectrum generated by a single UCNP suspended in solution has been a challenging task.16,34 To overcome this drawback, it is crucial to conclusively establish a new method to investigate the quenching effect of ligands and solvent molecules on luminescence intensity of single UCNPs. Optical tweezers (OT), which provides tiny force to trap micro/nanoscale objects by using a tightly focused Guassian laser beam, has attracted great interests and validated to be a new technique for single particles research since Arthur Ashkin, the father of optical tweezers, applied OT for accelerating and trapping micron-sized particles.35 Recently, several studies have been committed to trap and manipulate individual nanoparticles as small as 10 nm, such as quantum dots,36 metallic nanoparticles37,38 and UCNPs,39,40 which have overcome the light diffraction limit. In particular, Study on the combination of UCNPs and OT has become one of the up-to-date active research area. On one hand, the various kinds of OT were used to detect biomarker and bacteria in complex biological samples.41,42 On the other hand, the properties of the different shaped UCNPs in the optical trap were also investigated, Such as, electrostatic effects,43 polarized luminescence.44 These studies 4


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have shown potential applications of OT in intracellular microrheolog,45,46 thermal scanning47 and detection platform construction.48 Although OT has been applied in trapping and manipulation in various fields, the photophysical and photochemistry performance of the trapped nanoparticles are of great interests. By changing of the surrounding chemical environment and using various sensors to explore more properties of individual nanoparticles has become an essential approach for deeper understanding of single nanoparticles. In this study, the concept of a multifunctional OT to accurately evaluate the luminescence properties and surface quenching effect of single UCNPs is proposed, by using a 980 nm near-infrared laser acting as the trapping beam and simultaneously exciting visible upconversion luminescence of the trapped UCNPs. Single or multiple OPA-UCNPs are successively loaded into the optical trap with real-time monitoring of FSC signals, luminescence intensities and luminescence spectra of the nanoparticles. Importantly, the luminescence intensities of three types of ligands modified OA-UCNPs dispersed in H2O and D2O are quantitatively measured based on single-particle trapping, to further investigate the surface quenching mechanism of the ligands and solvent molecules, and give a direct evidence of a surface quenching effect on ligand/solvent-dependent luminescence of single UCNPs and the encapsulated monolayer of OPA will reduce the surface quenching.

RESULTS AND DISCUSSION Characterization of hydrophobic UCNPs. The β-phase NaYF4 as host material doped 5



with 20% Yb3+ sensitizer and 2% Er3+ emitter, which is regarded as an optimized composition for high efficient upconversion luminescence, is employed as the model particle in this study and synthesized based on the thermal decomposition method15. Transmission electron microscopy (TEM) image (Figure 1B) of the as-synthesized oleic acid coated UCNPs (OA-UCNPs) shows that the UCNPs are uniform hexagonal plate in shape with an average diameter of approximately 370 nm. The upconversion luminescence spectrum of OA-UCNPs dispersed in cyclohexane exhibits peaks at 525, 547 and 660 nm (Figure 1C). The inserted photograph of Figure 1C shows the transparent non-colored OA-UCNPs solution exhibits bright green luminescence under the 980 nm laser excitation. In addition, the powder X-ray diffraction (XRD) pattern of OA-UCNPs is analyzed by positions and intensities of the peaks, which match well with that of β-phase UCNPs (Figure 1D). Therefore, the high-quality oleic acid coated NaYF4: Yb/Er (20/2 mol%) is synthesized and used for further modifications.

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Figure 1. (A) Schematic illustration of the home-built multifunctional optical tweezers setup equipped with a 980 nm continuous-wave single-mode diode laser, QPD, EMCCD, fiber optical spectrometer and digital camera. (B) TEM image of the hydrophobic UCNPs. (C) Upconversion luminescence spectra and corresponding digital images of the hydrophobic UCNPs emitting visible luminescence excited by a 980 nm laser. Scale bar: 2 μm. (D) Powder XRD patterns of the hydrophobic UCNPs.

Two Strategies for OA-UCNPs Surface modification. According to hydrophobic encapsulation strategy, amphiphilic polymer OPA intercalates spontaneously to the surface ligands of hydrophobic, and for ligand-exchange strategy, hydrophobic molecules are removed to form ligand-free UNCPs (LF-UCNPs) followed by the introduction of hydrophilic ligands to the surface of UCNPs (Figure S1). The modification of different ligands on the surface of UCNPs is verified by Fourier transform infrared (FTIR) spectroscopy (Figure S2)

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and zeta potential (Figure 2D). The FTIR spectroscopy, in contrast to that of the OA-UCNPs absorption band of 1559 cm-1 and 1458 cm-1, show clear absorption bands of OPA at 1732 cm-1 and 1643 cm-1, which can be attributed to the stretching vibrations of C=O in amide and carboxyl, the peak of 1631 cm-1 and 1560 cm-1 can be attributed to asymmetric stretching vibrations of COO- and C=O of PAA coated UCNPs, respectively. The absorption bands of 1574 cm-1 and 1471 cm-1 indicate that the modification of amino groups on the surface of UCNPs. The measured zeta potentials show that PAA-UCNPs and OPA-UCNPs possess negative charges (-23 mV and -22 mV respectively), because their surfaces are rich in water-soluble carboxyl groups. In contrast，the PEI-UCNPs exhibit positive charges due to its surface amino groups. Therefore, all types of surface-modified UCNPs possess the features of stable optically transparent colloids, good water solubility, stable luminescence as well as uniform shape and size distributions (Figure 2A-2C). Upon continuous excitation at 980 nm, UCNPs modified with these ligands in water exhibit predominantly green emissions without any changes in spectra (Figure S3).

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Figure 2. (A-C) TEM image of PAA-UCNPs, PEI-UCNPs and OPA-UCNPs. The inserted photographs show the transparent solutions emit visible luminescence while excited with a 980 nm laser. Scale bar: 1 μm. (B) Zeta potentials of LF-UCNP, PAA-UCNP, PEI-UCNP, OPA-UCNP. Error bar in each sample indicates the mean value of three measurements.

Single-nanoparticles optical trapping experiments. Although there have been some work presenting the optical trapping of UCNPs with diameter ~26 nm, the particle size was hardly controlled. As the result of size difference and irregular movement of the trapped particle, luminescence intensity of single particle cannot be quantified and compared with each other.39 In this work, in order to precisely quantify the luminescence intensities from the trapped particles, they should be firmly bound in the optical trap. Sub-micron OPAUCNPs are good for stable trapping and therefore are employed as model to evaluate the feasibility of single-particle trapping and successive multiple-particle trapping. Figure 1A illustrates the construction of the set-up which is based on the platform of an inverted microscope. A single regulated 980 nm Gaussian laser beam is expanded to match the back aperture of a 100ⅹmicroscope objective with numerical aperture 1.30. The 980 nm laser beam is used to trap single UCNPs of interest by tightly focusing the laser beam with 9



the objective while upcoversion luminescence of these particles are excited with the same laser. Under the performance of optical force, which is used to overcome Brownian motion, the suspension UCNPs can be always successively trapped at the laser focus to realize a fairly stable excitation process. The trapping force exerted on trapped nanoparticle is defined as F=-kx, where k is the trap stiffness characterizing the optical trap and x is the distance deviation from the center of equilibrium position. Position signal is recorded by a data acquisition card and analyzing results give a positional power spectrum (Figure S4), which is well fitted by Lorentzian function. The corner frequency are fcx=73 Hz and fcy=131 Hz respectively, to this end, kx=11.0710-12 N/μm, ky=19.8610-12 N/μm. In addition, Figure 4A shows that sequential multiple individual UCNPs incorporation into optical trap with time elapses, as can be observed, the FSC signal detected by the quadrant photodiode (QPD) gradually increases with similar intensity jumps, providing the evidence of trapping single OPA-UCNPs with uniform size and shape. To evaluate the photostability of single UCNPs under conditions for single-particle experiment, the luminescence of a single UCNP is monitored while it is optically trapped for 600 seconds (Figure 3A), and the data show no significant emission attenuation, suggesting that the satisfactory anti-photobleaching and exceptional photostability behavior differ from other fluorophores. Therefore, the luminescence intensity of single UCNP is highly stable and the UCNPs are reliable for long-term fluorescence imaging and single-particle tracking in complex biological systems.

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Figure 3. (A) Record luminescence intensity from trapping single OPA-UCNPs every 5 s for 600 s. Stepwise increase of (B) QPD signals, (C) luminescence intensities and (D) spectra of successively trapped single OPA-UCNPs recorded by QPD, EMCCD and fiber optic spectrometer, respectively. The insets in (C) are the luminescence images of the sequential loading of different numbers of trapped UCNPs into the trapping center.

In order to examine the uniform luminescence intensities of individual OPA-UCNPs in colloidal suspension, successive trapping of individual OPA-UCNPs is conducted. As shown in Figure 3C, at the beginning of the trapping experiment, no luminescence signal are detected, denoting no OPA-UCNPs enter the trapping center. As the trapping time increases, significant stepwise signal increase is observed, revealing sequential loading of particles into the trap within different time intervals. The number of particles in the trap change from single to multiple due to Brownian motion of nanoparticles is also recorded by camera as well (movie S1). Clearly, the increasing luminescence intensity from the 11



observed trapping center is ascribed to the stepwise increase from zero to five in terms of the number of OPA-UCNPs entering the trap while they are excited by the same laser. Therefore, each similar increment of luminescence intensity recorded by EMCCD implies that these UCNPs exhibit uniform luminescence intensities. In addition, another notable tendency of stepwise signal increase, which is in agreement with EMCCD response, is observed in the luminescence spectra of the trapped particles. As shown in Figure 3D, upconversion luminescence spectra are acquired by a fiber-optic spectrometer during optical trapping of these UCNPs. The four emission spectra with increasing intensities show that the peak positions do not change although more and more UCNPs enter the trap, and the luminescence intensities at 553 nm and 653 nm are linearly depend on the number of the trapped particles (Figure S5). Moreover, by carefully analyzing the emission spectra from 640 nm to 680 nm, we find that the spectral shape changes noticeably: two main emission peaks centered at 652-658 nm, and the ratio between peaks of 652 nm and 658 nm change from 0.949 to 0.970. Such difference can be found in some recent work,40,49 which have shown that the observed spectral changes are caused by the interparticle energy transfer from UCNPs collision in optical trap. Taken together, the relationship between the number of UCNPs in the optical trap and emission spectra are in good agreement with that between particle number and FSC or luminescence intensities. Such features of monodispersibility attributed to surface modification of the uniform-sized UCNPs with OPA in aqueous solution ensure that the single particles can be successively trapped easily and stably by controlling the laser power density in the trapping center.

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Figure 4. The comparison of the luminescence intensity of three types of ligands (PAA, PEI and OPA) modified UCNPs dispersed in H2O and D2O. For each type of UCNPs 50 particles are counted.

Evaluation of luminescence properties of single hydrophilic upconversion nanoparticles. As is known, the luminescence intensity of UCNPs, to some extent, is actually decreased by modification with certain ligands and surrounding environment. Therefore, it is of great significance to find out the surface quenching effect on ligands/solvent-dependent of the luminescence properties of the UCNPs. Upon above experiment, single UCNPs modified with three types of ligands dispersed in H2O and D2O are trapped and their luminescence intensities are detected by the EMCCD. As compared in Figure 4, the intensities are quite distinct from each other. When compare different types of UCNPs in the same solvent, we found that single OPA-UCNPs exhibit the most intense emission when they are dispersed in H2O. Furthermore, the solvent is also a vital factor that affects the luminescence of the UCNPs. Water molecules are known as a surface oscillator that causes significant effect on surface quenching. In this experiment, D 2O is 13



also used as the solvent for the comparison with H2O. The luminescence intensities of each type of UCNPs in the two solvents show that the intensities of all ligands modified UCNPs in H2O exhibit marked decrease compared with the same particles in D2O, and OPA-UCNPs in D2O are the substantially brightest among all types of UNCPs. We conclude that there is certain amount of surface -OH groups on the surface, which would strongly quench the excited states of Er3+ ions by multiphonon relaxation, thus leading to a great influence on the upconversion processes. However, this quench effect is obviously minimized in D2O solution. Therefore, the above result demonstrates that the inconsistent luminescence intensity of UCNPs is resulted from the quenching affect caused by different ligands and solvent molecules. Since OPA-UCNPs emit brighter luminescence than PAAUCNPs and PEI-UCNPs, we conclude that the difference between three types of ligands is attributed to carboxyl and amino groups, that is, the strongest luminescence of OPAUCNPs is ascribed to oleic acid separating surface carboxyl groups from lanthanide ions, and the PAA and PEI molecules inevitably influence the surface dopant ions and quench the luminescence intensities of UCNPs.

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Figure 5. Upconversion luminescence decay curves of Er3+ emission centered at 539 nm upon different types UCNPs dispersed in D2O and H2O.

Table 1. The upconversion luminescence decay lifetime of different types of ligands modified UCNPs dispersed in D2O and H2O. PAA-UCNPs

PEI-UCNPs

OPA-UCNPs

H2O

303.5 ± 0.7 s

305.5 ± 0.5 s

328.2 ± 0.7 s

D2O

339.5 ± 0.8 s

336.1 ± 0.8 s

352.1 ± 1.2 s

To validate our hypothesis about quenching effect and gain further insight into the upconversion luminescence mechanisms through ligands and solvent molecules. Upconversion luminescence decay lifetime is measured, which is considered to be explicit and convincing evidence among radiative relaxation, multiphonon relaxation and energy transfer processes of the excited energy levels of lanthanide ions. Upconversion luminescence decay lifetimes of three kinds of UCNPs dispread in the solvents H 2O and 15



D2O are measured at 539 nm, respectively (Figure 5 and Table 1). As shown in Table 1, the lifetimes of all the UCNPs in D2O are longer than those in H2O. Importantly, the lifetime of the OPA-UCNPs is the longest among the three types of ligands modified UCNPs in D2O. As the energy state diagram shown in Figure 5A, the upconversion luminescence spectra appear green emission due to 4H11/2, 4S3/2 → 4I15/2 transitions of Er3+ and red emission from the 4F9/2→4I15/2 transition. The energy gap between 2H11/2/4S3/2 and 4F9/2 is nearly 3200 cm-1, coincidently, the stretching vibrational modes of surface hydroxyl (∼ 3530–3700 cm–1). Thus, the vibration relaxation can be an efficient pathway to remove the excitation of the surface dopant ions. The dispersion of UCNPs in H2O or modification with carboxyl groups on their surfaces, will increase the probability of multiphonon relaxation between 2H11/2/4S3/2 and 4F9/2. Whereas in D2O solvent, the OD stretching vibrations is 2600 cm-1, the multiphonon relaxation processes are obviously less than that in H2O, Furthermore, the increase of D+ concentration results in an increase of the upconversion luminescence. This observation is consistent with the reported work.26 Therefore, the above results and mechanism demonstrate that lanthanide ions can be quenched by the ligands on the surface and solvent molecules, especially hydroxyl groups. For OPA-UCNPs, hydrophobic OA layer is an effective protection for dopant ions, and it increases the distance between surface ions and the surrounding chemical environment, leading to a low possibility of energy relaxation from 2H11/2/4S3/2 to 4F9/2. Thereby, we provide clear evidence to support that the ligands and solvent molecules will lead to surface quenching so that the interaction between OA and OPA molecules shortens the surface quenching effect. Taken together, both the luminescence intensity and lifetime of OPA-UCNPs are observed to be 16


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substantially more resistant to surface quenching effect by the modification ligands and the solvent molecules, subsequently retaining outstanding luminescence properties. CONCLUSIONS In conclusion, we present a new approach to evaluate luminescence properties and surface quenching effect of monodisperse hydrophilic UCNPs by means of single-particle trapping using multifunctional OT. Single and multiple OPA-UCNPs entering the trapping center successively are detected by monitoring FSC, luminescence intensity and spectra, respectively, to determine the number of particles in the trap. The results show that OPA encapsulated UCNPs is efficiently trapped by multifunctional OT and reveal that individual OPA-UCNPs possess outstanding luminescence properties compared with PAA-UCNPs and PEI-UCNPs. Furthermore, OPA-UCNPs dispread in the solvents of D2O exhibiting longer upconversion luminescence decay lifetime explains the mechanism of ligands and solvent molecules related quenching effects on luminescence intensity. Additionally, the home-built multifunctional OT provides a facile platform for determining a new insight for the evaluation of high-quality hydrophilic UCNPs and is anticipated to open up a wide range of applications in nanotechnology.

METHODS Synthesis of hydrophobic UCNPs. The high-quality oleic acid capped NaYF4: Yb3+ 20%, Er3+ 2% nanoparticle were synthesized via the thermal decomposition under standard argon protection conditions.15 Yb(CF3COO)3 and Er(CF3COO)3 were prepared according to a previously reported method At first, 6.37 mmol CF3COONa, 2.60 mmol Y(CF3COO)3, 17



0.68 mmol Yb(CF3COO)3 and 0.068 mmol Er(CF3COO)3 were dissolved with 15 mL of ODE and 15 mL of oleic acid. The mixture was added into a 100 mL three-necked flask and then heated at 120 °C for 45 min under vacuum to form a primrose yellow solution. Subsequently, the temperature of the reaction was heated to 340 °C rapidly under Ar protection and maintained 33 min for nanocrystal grown with vigorous magnetic stirring. After cooling to room temperature, the nanoparticles were isolated by adding ethanol and centrifugation and purified by washing three times with hexane/ethanol (v/v = 1:4). The asprepared hydrophobic UCNPs were dispersed in chloroform for further modification.

Synthesis of hydrophilic UCNPs modified with PAA, PEI and OPA. The as-prepared OA-UCNPs (100 mg) re-dispersed in 10 mL ethanol by centrifugation and then the reaction was performed sonication for 1 h to remove oleate ligands while adjusting the pH to 4 by adding hydrochloric acid.25 After the reaction, the ligand-free UCNPs were collected by centrifugation and washed three times. The new ligands PAA and PEI were dissolved in deionized water while maintaining the pH at weak basic, then, the ligand-free nanoparticles were added into the above solution dropwise under vigorous stirring. After reaction for 1 h, the hydrophilic UCNPs were collected by centrifugation, washed three times with deionized water, and redispersed in deionized water. Hydrophobic UCNPs and a certain amount of OPA were dissolved in chloroform. Hydrophilic UCNPs were obtained by evaporation chloroform, purified from excess polymer by centrifugation and then dissolved in pH 10 borate saline buffer.

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Schematic illustration of the home-built OT setup and trapping experiment. In order to investigate the monodispersion nature and luminescence performance of the UCNPs encapsulated with different ligands, a home-built multifunctional OT set-up is used for optically trapping single UCNPs. Luminescence intensities of the trapped particles are collected by the same objective and detected by an Electron Multiplying CCD (EMCCD), and the luminescence spectra of the particles by a fiber-optic spectrometer (QE6500, Ocean Optics). FSC signal is collect by a 10ⅹ objective and focused on a QPD (PDQ80A, Thorlabs)through a dichroic mirror. The movies are recorded by a digital camera (Canon EOS 600D). The 980nm laser is focused by a high numerical objective to from optical trap with diameter and power intensity estimated to be approximately 0.9 μm and 7.8ⅹ106 W/cm2, respectively. The concentration of OPA-UCNPs in the colloidal suspension is diluted to ensure the accuracy of the single-particle trapping experiment.

Supporting Information Materials, instrumentations, modification strategy (Figure S1), FTIR spectra (Figure S2), upconversion luminescence spectra (Figure S3), power spectra (Figure S4), linear fitting curves between luminescence intensities at peaks 553 nm and 653 nm (Figure S5), and the number of particles in the trap change from single to multiple is recorded by camera (Movie S1)

ACKNOWLEDGEMENTS 19



This work was supported by the National Natural Science Foundation of China (81572086, 81772256 and 21827808).

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Evaluation of Luminescence Properties of Single Hydrophilic

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