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Disintegration of Hexagonal NaYF:Yb ,Er Upconverting Nanoparticles in Aqueous Media: the Role of Fluoride in Solubility Equilibrium Satu Lahtinen, Annika Lyytikäinen, Henna Päkkilä, Emmy Hömppi, Niina Perälä, Mika Lastusaari, and Tero Soukka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09301 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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The Journal of Physical Chemistry

Disintegration of Hexagonal NaYF4:Yb3+,Er3+ Upconverting Nanoparticles in Aqueous Media: the Role of Fluoride in Solubility Equilibrium Satu Lahtinen† Annika Lyytikäinen,† Henna Päkkilä, † Emmy Hömppi, † Niina Perälä, † Mika Lastusaari,‡,§ and Tero Soukka*,† †

University of Turku, Department of Biotechnology, Tykistökatu 6A, 50520 Turku, Finland ‡

University of Turku, Department of Chemistry, Laboratory of Materials Chemistry and Chemical Analysis, Vatselankatu 2, 20014 Turun yliopiston, Turku, Finland §

Turku Centre for Materials and Surfaces (Matsurf),Turku, Finland

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ACS Paragon Plus Environment


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ABSTRACT The disintegration of hexagonal NaYF4:Yb3+,Er3+ upconverting nanoparticles (UCNP) was studied by incubating various nanoparticle concentrations in aqueous suspensions over time while monitoring the upconversion emission intensity and measuring the dissolved particle-constituting ion concentrations. The results revealed that the ions dissolved into water resulting apparently in anisotropic structural disintegration of the UCNPs as observed with transmission electron microscopy. The UCNP disintegration caused partial loss of active ions Yb3+ and Er3+ from the host matrix and therefore decrease in the upconversion luminescence intensity. The decrease, however, was strongly dependent on the UCNP concentration and dramatic drop in the intensity was observed especially at diluted nanoparticle suspensions, where the nanoparticles disintegrated almost completely until the solubility equilibrium was achieved. At the concentrated suspensions the equilibrium was achieved already with minimal disintegration and the change in the luminescence intensity was negligible. Further, due to the high impact of fluoride ions on the solubility equilibrium the disintegration of the UCNPs could be prevented by adding fluoride to the suspension. The reported disintegration of NaYF4:Yb3+,Er3+ nanoparticles in diluted aqueous suspensions should be taken into consideration when the UCNPs are used at low concentrations in analytical applications and in guiding the design of improved shell-stabilized UCNPs.

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1. INTRODUCTION Upconverting nanoparticles (UCNPs) have attracted wide interest as reporters in bioanalytical applications due to their unique optical properties. The UCNPs have the capability to convert nearinfrared excitation to emission at shorter wavelength upon sequential multiphoton absorption which enables autofluorescence free measurements. Initially, advantages of the photon upconverting reporters were demonstrated with sub micrometer sized particles, which were considered to be unaffected by the environment.1, 2 However, many applications would benefit from small and uniform nanosized particles due to their high colloidal stability, better cell permeability and reduced sterical issues. Therefore, after the development of direct synthesis of nanosized upconverting particles3-5 they have been widely used in bioanalytical applications.6, 7 Nevertheless, the upconversion luminescence of the nanosized UCNP is significantly lower than with microsized particle, since the brightness of an individual particle is dependent on the amount of active ions participating in the upconverting process. Further, the UCNPs have high surface-tovolume ratio, which increases the luminescence quenching due to the surface defects.8-11 In addition to the size of the UCNP, the composition and structure of the crystalline host lattice affect the upconversion emission efficiency. Hexagonal phase NaYF4 host matrix doped with Yb3+ as a sensitizer and Er3+ (green and red emission) or Tm3+ (blue and near infrared emission) as an activator ion is presently considered to be the most efficient upconverting crystalline material.12-14 Fluoride ion containing host matrices are preferred due to their low phonon energies.15 Na+ and Y3+ are commonly used as host matrix cations because their ionic radii are close to the ionic radii of lanthanides and therefore prevent crystal defects and lattice stress.3, 16-19 The crystal structure

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has a major effect on the upconversion efficiency and upconversion emission intensity is higher in hexagonal than in the cubic NaYF4 particles. To achieve control over the morphology and size, the UCNPs are generally synthesized in organic solvents and therefore they must be rendered water dispersible to be utilized in bioanalytical applications. Different protocols for transferring the UCNPs into water suspensions have been demonstrated in a recent review.20 However, in water based suspensions upconversion emission intensity is highly quenched by the OH-vibrations of water.11, 21 Other problems have also emerged in aqueous UCNP suspensions. Although UCNPs composed of fluoride materials have been formerly considered to be chemically stable, low degree of particle-constituting ion dissolution from cubic-phase nanoparticles have been observed in water.22-24 The hexagonal phase NaYF4 nanocrystals have been considered to be thermodynamically more stable than the cubicphased crystals, but still fluoride ion dissolution along with the other constituents of hexagonal NaYF4:Yb3+,Tm3+ particles has been observed in water.24 The dissolution of the nanoparticleconstituting ions can be expected to significantly affect the structural integrity and upconversion luminescence of the UCNPs, and have a significant impact on the use of UCNPs in bioanalytical applications. However, the effect of ion dissolution on the nanoparticle luminescence and integrity at various UCNP concentrations has not been studied before. Therefore, our aim was to study the dissolution of the ionic constituents from the highly luminescent hexagonal NaYF4:Yb3+,Er3+ nanoparticles in aqueous suspensions and the impact of their dissolution on the structural integrity and the upconversion luminescence. In addition, we studied the effect of ion dissolution on the upconversion luminescence intensity at different UCNP concentrations. The UCNPs were coated with polyacrylic acid (PAA), thin silica shell or thick silica shell to render them colloidally stable and to study if the coating has an effect on the disintegration. The surface functionalization is also 4


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necessary for biomolecule conjugation. Furthermore, the dissolution mechanism of the constituting ions was studied and an effective prevention of the NaYF4:Yb3+,Er3+ nanoparticle disintegration was demonstrated by addition of fluoride ions to the suspension. 2. EXPERIMENTAL 2.1 Upconverting nanoparticles. The UCNPs composed of NaYF4:Yb3+,Er+3 (XYb = 0.17, XEr = 0.03) were synthesized by thermal decomposition in organic oils25 and coated with PAA (Mw 2000)26, thin silica shell with thickness lower than two nanometers27 or thick silica shell of around 5 nm28. In case of silica coating (N-(3-trimethoxysilyl)-propyl)ethylene diamine was added to the coating reaction to introduce primary amino groups, which were subsequently converted to carboxylic acid groups.29 After coating the UCNPs with PAA or silica shell the nanoparticles were suspended in 5 mM Tris-HCl, pH 8, and stored at 3 rpm slow rotation at room temperature. The coatings of the UCNPs are described in detail in the supporting information. 2.2 Characterization of ion dissolution and disintegration of the nanoparticles. To confirm that the fluoride ion was dissolved from the UNCPs over time, its concentration was measured from water suspensions of 5, 15 and 50 µg/ml of PAA coated UCNPs. The dissolved fluoride ion concentration was determined with perfectIONTM combination fluoride electrode (Mettler Toledo, Switzerland) after incubating the PAA coated UCNPs in water at 3 rpm rotation for 0, 24 or 96 h. Before the measurements TISAB III concentrate (Mettler Toledo) was used to adjust the ionic strength of the suspensions measured according to manufacturer’s instructions. The dissolution of other nanoparticle constituents Na+, Y3+, Yb3+ and Er3+ from the UCNPs were studied with an inductively coupled plasma mass spectrometer (ICP-MS, Perkin Elmer Elan 6100 DRC Plus, MDS, SCiex Concord, ON, Canada). Before the analysis, a suspension of 5 µg/ml of PAA coated UCNPs in water with or without 1 mM KF was incubated for 24 h in slow shaking. 5



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The possible remnants of the nanoparticles were removed by centrifugation (20 000 g, 20 min) and the supernatant was taken for the analysis. The elemental composition was also determined from as-prepared UCNPs for comparison. Before ICP-MS analysis the nanoparticles were first degraded in a microwave digestion unit with a mixture of acids30 and then diluted with deionized water. The ICP-MS instrument was calibrated with multi-standard IMS-1 and IMS-2 solutions from Ultra Scientific (RI, USA). Transmission electron microscopy (TEM) images were taken in order to visually characterize the disintegration of UCNPs. For the TEM images 10 µg/ml dilution of PAA coated UCNPs were made to water or 1 mM KF in water and incubated in slow shaking for 0 or 24 h. The samples were concentrated with Nanosep 30K Omega Centrifugal devices (PALL Life sciences, NY, USA). The samples were added to the Centrifugal device and centrifuged 7000 g until all the solvent had passed through the filter. The UCNPs were washed twice by adding 500 µl of water and centrifuging as before after which the nanoparticles were removed from the filter by adding 60 µl of water and sonicating for 20 cycles of 0.5 s with 100 % amplitude with Vial Tweeter ultrasonicator (Hielscher Ultrasonics GmbH, Germany). The concentrated dispersions were pipetted onto silicon monoxide copper grids (Ted Pella Inc., CA, USA) and the images were taken with JEM-1400 Plus TEM with 80 kV electron beam (JEOL, Massachusetts, USA). 2.3 Time-course measurements of upconversion emission intensity. The effect of ion dissolution from the UCNPs on the upconversion emission intensity was studied by measuring the time course of emission intensity after diluting the UCNPs at different concentrations in water. UCNP dilutions were also made to KF solutions in order to demonstrate that the addition of fluoride ion to the solution prevents the emission intensity decrease. All emission intensity studies were performed similarly. The solutions with or without KF in a volume of 97 µl was added to 6


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clear Greiner polypropylene microtiter plate (Sigma-Aldrich, MO, USA) in four replicates after which 3 µl of UCNP dilution was added to the wells and then incubated in slow shaking. The emission intensity of the UCNPs was monitored over time between 0 and 24 h with modified Plate Chameleon fluorometer equipped with a 980 nm laser30 (Hidex Oy, Finland). For the emission intensity measurement a 2 s read out was used and the green emission of the UCNPs was selected by using a 535/40 nm band-pass filter (Chroma Technology Corp., Vermont, USA). The emission was damped tenfold with an OD1 neutral optical density filter. The adequate KF concentration to prevent the disintegration of UCNPs was studied by using KF concentrations from 0 to 100 mM. The upconversion emission intensity of UCNPs was also measured in the presence of KCl and NaF with concentrations of 0−100 mM to demonstrate that the fluoride ion of the KF prevents the emission intensity decrease, not potassium. To prevent the UCNP disintegration the effect of other particle matrix constituting ions was tested by adding NaCl or YCl3 to UCNP suspensions to a final concentration of 1 mM. In addition, the effect of different buffer ions on the dissolution was studied with 50 mM Tris-HCl pH 7.7, 50 mM borate pH 8.0, and 50 mM phosphate pH 7.7, with or without 1 mM KF and compared to the emission intensity in water with or without 1 mM KF. For all these tests PAA coated UCNPs in a final concentration of 5 µg/ml were used. The UCNP concentration dependency of the nanoparticle disintegration was studied in UCNP concentration range from 0.5 µg/ml to 200 µg/ml. PAA, thin and thick silica shell coated UCNPs were used in order to study the effect of surface coating. The UCNP dilutions were made to water or to 1 mM KF in water. The UCNP concentration dependency test was also performed with PAA coated UCNPs in heavy water with or without 1 mM KF to ensure that the decrease in the emission intensity was not caused by water based quenching. 7



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2.4 Upconversion emission lifetime measurements. The disintegration of the UCNPs was also studied by measuring the upconversion luminescence lifetimes over time. The PAA coated UCNPs were diluted to a final concentrations of 10 µg/ml. The luminescence lifetimes were measured with a lifetime measurement mode of a modified Plate Chameleon fluorometer after 0, 4 and 24 h incubation in slow shaking. The lifetimes were measured by exposing the samples repeatedly to 2-ms wide pulsed excitation at 980 nm and measuring the decays with a 535/40 nm band-pass filter. The data was analyzed with Origin 8 (OriginLab, Northampton, MA) using a second order exponential decay fitting. 3. RESULTS AND DISCUSSION The as-prepared NaYF4:Yb3+,Er3+ UCNPs used in this study were 25−31 nm in dimensions according to the TEM images (Figure S1 in the ESI) and their pure hexagonal crystal structure was confirmed with XRD analysis (Figure S2 in the ESI). The elemental composition of the nanoparticles after the synthesis was confirmed with ICP-MS. The mole ratios of Na+, Y3+, Yb3+ and Er3+ were 100, 82, 15 and 2.9 mol %, respectively, thus corresponding well with ratios present in the synthesis. 3.1 Ion dissolution from the upconverting nanoparticles. The fluoride ion dissolution from the nanoparticles was studied by measuring the fluoride ion concentration from aqueous suspensions of PAA coated UCNPs with the fluoride ion electrode. The dissolved fluoride ion concentration at different time points and in different UCNP concentrations is presented in Table 1. At the time point zero, the dissolved fluoride ion concentration was measured immediately after the UCNP dilution to water. The dissolved fluoride ion concentration increased significantly until 24 h of incubation in water and reached 0.08 mM concentration in the two highest UCNP concentrations tested. With the lowest UCNP concentration the dissolved fluoride concentration 8


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was slightly lower. Shorter incubation times were not studied and therefore the dissolution is likely to be more rapid as indicated by the time course upconversion luminescence measurements. After 24 h there was no further increase in the fluoride ion dissolution from the UCNPs and the fluoride concentration in water remained in a constant level when measured after 96 h incubation. More importantly, the maximum dissolved fluoride ion concentration achieved after 24 h incubation in water was independent of the UCNP concentration, although different UCNP concentrations contain naturally different total amounts of fluoride. This implicates that solubility equilibrium and saturated solution of particle-constituting ions is achieved in the solvent after reaching a certain concentration. The observation that fluoride ion dissolves from the UNCPs is in accordance with a previous reports, where less than 5 mol % dissolution was demonstrated from fluoride ion containing cubic and hexagonal phased nanoparticles after three days of incubation in water.23, 24 According to our study, however, the dissolved mole fractions of fluoride are much higher even for hexagonal-phased nanoparticles in neutral water suspensions especially at the lowest UCNP concentration tested, which is due to the lower UCNP concentrations used in our study.

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Table 1. Dissolved fluoride ion concentrations and upconversion luminescence at different time points in aqueous suspensions with different UCNP concentrations UCNP concentration (µg/ml) 5

15

50

0h

0.004

0.008

0.015

24 h

0.07

0.08

0.08

96 h

0.07

0.08

0.08

66.4

24.2

7.3

0h

100

100

100

24 h

2.9

29

64

96 h

1.2

28

50

[Fluoride] (mM)

Dissolved F- at 96 h (mol %) Intensity / Intensity at 0 h (%)

Furthermore, the effect of ion dissolution to the upconversion luminescence was not studied in the previous reports. We measured the upconversion emission intensity from the same UCNP suspensions used for the fluoride ion concentration measurement (Table 1). The upconversion emission intensity decreased in conjunction with fluoride ion increase indicating that the Yb3+ and Er3+ ions, which participate in the upconversion process, actually dissolve along with the fluoride ion. The dissolution of particle-constituting ions and the resulting emission intensity decrease highly indicated partial or complete disintegration of the UCNP structural integrity. The upconversion luminescence decrease was high when compared to the dissolved mole fractions of fluoride especially at the lowest UCNP concentration. The disintegration of the nanoparticles affects the surface-to-volume ratio increasing the luminescence quenching due to surface defects. 10


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Nevertheless, since the fluoride ion was dissolved from the UCNPs until the equilibrium was achieved in water, the dissolution might be prevented by adding fluoride ions into the UCNP suspensions. Therefore, the solubility equilibrium could be achieved in the NaYF4:Yb3+,Er3+ suspension without nanoparticle disintegration. The loss of active ions Yb3+ and Er3+ was confirmed with ICP-MS analysis. The amounts of Yb3+ and Er3+ ions as well as Na+ and Y3+ were determined from water solutions, where PAA coated UCNPs were incubated for 24 h. In addition, the hypothesis that the addition of fluoride ion into the UCNP suspensions prevents the dissolution of the nanoparticle ions was tested by measuring the Na+, Y3+, Yb3+ and Er3+ from a sample which was incubated in 1 mM KF solution for 24 h. According to the ICP-MS analysis, the dissolved total molar amount of Y3+, Yb3+ and Er3+ after 24 h incubation was 20 times smaller in the dilution containing 1 mM KF in water compared to the dilution incubated in water only (Table 2). Therefore, the addition of KF to the UCNP suspension prevented effectively the dissolution of the Yb3+ and Er3+ ions. Small amounts of the ions detected in the KF control reactions may originate as an impurity from the synthesis or from the UCNP stock solution, where some dissolution of the ions has already occurred before addition of KF. Na+ ion was also detected in both solvents but the amount was very high due to NaOH used to adjust pH of the UCNP stock buffer (data not shown). Therefore, no conclusions could be made from Na+ concentrations. The mole ratios of dissolved ions differentiated in water suspension when compared to the mole ratios of the as-prepared nanoparticles. The mole ratio of Y3+ was higher and the mole ratio of Yb3+ was lower. This might be due to uneven distribution of ions in the nanoparticle or alternatively the Y3+ ion might be more prone to dissolution than other ions. According to the previous report by Dong et. al. the ions inside NaGdF4:Y3+,Tb3+ nanoparticles were nonstatistically distributed.31 The Gd3+ ions were more concentrated towards 11



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the center and Y3+ towards the surface of the nanoparticles. If this is the case also in the nanoparticles used in our study, the Y3+ ions concentrated on the surface of the nanoparticles are dissolved excessively compared to other ions.

Table 2. The total molar amount of rare-earth ions and percentages of dissolved Y3+, Yb3+ and Er3+ individual ions after 24 h incubation in water or in 1 mM KF in water determined with ICPMS analysis. H2O

1 mM KF in H2O

Total Y3+, Yb3+, Er3+ (x10-6 M)

4.18

0.196

Y3+ (mol %)

90.1

80.4

Yb3+ (mol %)

7.74

15.8

Er3+ (mol %)

2.19

3.80

The dissolution of fluoride ion and other particle-constituting ions can be described with equilibrium equation 1 where R3+ denotes for rare-earth ions Y3+, Yb3+ and Er3+, assuming that the ions would dissolve stoichiometrically. H2O

NaRF4 (s) ↔ Na+ (aq) + R3+(aq) + 4F- (aq)

(Eq. 1)

Based on the equation 1 the equilibrium constant i.e. solubility product Ksp can be described for NaRF4 nanoparticles (Equation 2.) Ksp = [Na+][R3+][F-]4

(Eq. 2)

If we further exclude the possible complexation of the dissolved ions a crude estimate of the solubility product can be calculated from the equation 2 based on the fluoride ion equilibrium concentration of 0.08 mM measured with the electrode. The solubility product of the NaRF4 12


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nanoparticles was calculated to be 1.6x10-26. The solubility data available in the literature for ternary fluorides is very limited24 and therefore more studies are required. According to the equation 2 fluoride ion has the highest impact on the solubility product and therefore the fluoride ion has a major role on the disintegration of the nanoparticles. Nevertheless, it is important to acknowledge that the presence and ionic strength of other ions or molecules in the solution can influence the solubility equilibrium by binding or forming complexes with the particle-constituting ions. The particle disintegration, however, is most likely dependent on the particle diameter as indicated by the theoretical studies on nanoparticle synthesis.32,33 According to these studies the growth of the nanoparticles in the synthesis is dependent on the radius of the particles. Smaller particles dissolve on the expense of larger particles. The smaller particles are energetically less stable than larger particles due to their larger surface-to-volume ratios and therefore large surface energies. Thus, the dissolution is decreased when particle size increases. The disintegration kinetics are also slower with large particles because of their low surface-to-volume ratio. Therefore, the results presented here directly concern only the particle sizes closely related to the size utilized in this study. Nanosized particles in this size range or even smaller are nevertheless preferred in bioanalytical applications due their high colloidal stability and reduced sterical issues. 3.2 Visual detection of structural integrity with TEM. The effect of particle-constituting ion dissolution to the structural integrity of the PAA coated UCNPs was visually studied with TEM. The disintegration of the UCNPs could be clearly observed in TEM images after 24 h incubation in water (Figure 1). According to TEM images the corrosion was anisotropic and detectable only on two sides of the nanoparticles. The nanoparticles seem to have corroded most from the center of the crystal lattice even creating distinctive holes in the middle. According to our hypothesis the 13



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dissolution proceeds from the (001) crystal faces of the hexagonal lattice, while the (100) and (010) faces remain intact. Such face-dependent differences in dissolution are rather common for crystalline drugs. For example, in aspirin the (100) face dissolves five times as fast as the (001) one.34 There seem to be no clear crystallographic difference between the (001) and (100) faces, however, that could explain the differences in surface energy and the resulting differences in observed dissolution rate. Further, PAA ligand on the UCNP surface might induce the anisotropic disintegration. According to the studies reported on anisotropic synthesis of UCNPs the oleic acid preferentially adsorbs on the (010) crystal plane and when the amount of oleic acid exceeds a critical value the (010) plane is stabilized resulting in anisotropic morphology.35 The PAA ligand on the UCNP surface might also stabilize the (010) crystal plane and thus, the dissolution of ions from the (001) plane is favored. The observation needs to be yet confirmed with high resolution TEM study. In the zero time point, the TEM-sample was prepared immediately after UCNP dilutions and therefore, disintegration of UCNPs was not observed in the images. The images of UCNPs incubated in 1 mM KF in water for 24 h were equivalent in their appearance compared to the zero time point confirming visually the earlier conclusion that the addition of fluoride ion prevents the UCNP disintegration.

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Figure 1. TEM-images from NaYF4:Yb3+,Er3+ nanoparticles incubated at 10 µg/ml concentration for 0 hours (left) or 24 hours (middle) in water only and for 24 h in 1 mM KF in water (right). Five and a half time magnification of one particle is presented at the inset. 3.3 Time course of upconversion emission and the effect of added KF on the disintegration of the nanoparticles. The adequate concentration of fluoride ions to prevent the disintegration of UCNPs was studied by monitoring the upconversion emission intensity of 5 µg/ml suspension of PAA coated UCNPs in the presence of different KF concentrations. The prevention of the disintegration was strongly dependent on the concentration of the added KF in the suspension (Figure 2). The upconversion luminescence of the PAA coated UCNPs in aqueous suspension without KF decreased almost 90 % during 24 h incubation. The lowest KF concentration that prevented the disintegration of UCNPs was 0.1 mM, which was close to the dissolved fluoride ion concentration equilibrium determined with the fluoride ion electrode. However, even with high KF concentrations there was a slight decrease of emission intensity up to 20 % during 24 h incubation. This is most likely a combined effect of both precipitation and surface adsorption of UCNPs as well as potential dissociation of nanoparticle aggregates upon dilution and subsequent incubation. For subsequent studies, KF concentration of 1 mM was chosen to ensure sufficient concentration of fluoride ions in the solution.

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Figure 2. Time course of upconversion luminescence of polyacrylic acid coated NaYF4:Yb3+, Er3+ nanoparticles at 5 µg/ml concentration in water at KF concentrations of 0 mM (black), 0.032 mM (red), 0.1 mM (blue), 0.32 mM (magenta), 1 mM (olive), 3.2 mM (orange), 10 mM (green), 32 mM (violet) and 100 mM (pink). KCl and NaF were also added to the UCNP suspension to demonstrate that fluoride in the KF solution was solely responsible for preventing disintegration of the UCNPs and that the potassium did not have an effect (Figure S3 in ESI). The upconversion emission intensity decrease was independent on the KCl concentration while the results with NaF were similar to the results obtained with KF and therefore, it was concluded that fluoride is the preventing ion. In addition, the effect of sodium and yttrium, which are the other constituents of the host matrix, were studied. The emission intensities of UCNP suspensions in 1 mM NaCl and YCl3 were comparable to the emission intensities in water suspensions (Figure S4 in ESI). According to the solubility product the Na+ and Y3+ ions should have at least some effect on the prevention of the disintegration but 16


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most likely at the added 1 mM concentration the effect is so small that it was not detected with the upconversion intensity measurement. Due to the solubility product, where the fluoride ion is in the power of four, significantly higher concentrations of Na+ and Y3+ ions should be added to the UCNP suspensions in order to achieve the same effect as with 1 mM KF. Hence, from all the NaYF4: Yb3+, Er3+ nanoparticle constituents only the fluoride ion and its equilibrium had major effect on the dissolution and prevention of the UCNP disintegration in water. In the previous dissolution studies concentrated UCNP suspension of 1 mg/ml was tested and therefore only small molar ratios of dissolved particle-constituting ions were observed in water.23, 24

According to our studies fluoride and other nanoparticle-constituting ions dissolve until the

solubility equilibrium is achieved in water. Thus, the extent of the nanoparticle disintegration is dependent on the UCNP concentration, which was demonstrated by measuring upconversion emission of different UCNP concentrations in water with or without 1 mM KF as a function of time. PAA, thin silica shell and thick silica shell coated UCNPs were utilized in the study because coating renders the UCNPs water dispersible, keeps the nanoparticle colloidal in aqueous media and surface functionalization is further needed for bioconjugation in order to use the nanoparticles in applications. Also, differently coated UCNPs were tested in order to study whether the coating has a preventing effect on the dissolution by protecting the particle from water. However, the surface ligand might have a negative effect on the nanoparticle stability especially at high concentrations as demonstrated by Cross et. al with 5%Eu:LaF3 nanoparticle in the presence of dipicolinate ligand.36 The time course of upconversion emission using different UCNP concentrations is illustrated in Figure 3. The zero time point denotes the time, when the UCNP dilution was made. The decrease of the upconversion emission as a function of time was indeed dependent on the UCNP concentration in water suspensions. The lower the UNCP concentration 17



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was the more and the faster the emission intensity was decreased during incubation. However, in high UCNP concentrations down to 50 µg/ml the emission intensity decrease was negligible. With high UCNP concentrations fewer ions are dissolved per nanoparticle before the ion equilibrium in the solvent is reached. With low UCNP concentration the nanoparticles have to be disintegrated almost completely before the equilibrium is reached resulting in dramatic loss in the upconversion emission.

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Figure 3. Time course of upconversion luminescence of a) polyacrylic acid, b) thin silica shell and c) thick silica shell coated NaYF4:Yb3+,Er3+ nanoparticle in water at 0 mM (left) and 1 mM KF (right). The nanoparticle concentrations were 0.5 µg/ml (black), 1 µg/ml (red), 5 µg/ml (blue), 10 µg/ml (magenta), 50 µg/ml (green), 100 µg/ml (dark blue) and 200 µg/ml (orange). The emission intensity decrease was observed with all the differently coated nanoparticles but the decrease was slower with UCNPs coated with thick silica shell. It has been also demonstrated before in a water based luminescence quenching study that the silica shell does not fully protect the UCNPs from water.21 Nevertheless, the coatings most likely have some stabilizing effect on the UCNPs when compared to particles completely without coating. The stabilizing effect of oleic acid during synthesis of NaYF4 nanoparticles has been reported before.35,37 The oleic acid favors the bonding with Y3+ ion and therefore reduces the energy barrier for phase transformation from cubic to hexagonal phase.37 PAA coating might also favor the binding with Y3+ resulting in the stabilization of the UCNPs and therefore, inhibition of ion dissolution. However, because the PAA probably stabilize only the (010) crystal plane as discussed previously in the section 3.2, the other crystal planes are prone to dissolution of ions. The thick silica shell protected the UCNPs more effectively than the thin coatings but did not completely prevent the dissolution effect. Silica shells are amorphous and therefore the thick silica shell may still not completely block the solvent molecules from accessing the nanoparticle surface. Further, the uniformity of the shell thickness may vary and thus part of the surface can be more vulnerable to the solvent. At 1 mM KF in water, the upconversion emission did not decrease significantly with any UCNP concentration or differently coated nanoparticles. The highest emission intensity decrease observed at 1 mM KF in water after 24 h incubation was 20 %. 20


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Previously, it has been demonstrated that upconversion emission is highly quenched in water.11, 21

We measured the upconversion emission of PAA coated UCNPs in heavy water or in 1 mM KF

in heavy water as a function of time in order to demonstrate that the observed emission intensity decrease in water over time is not due to interaction with water molecules and nonradiative relaxations by OH-vibrations. Intensity decrease with different UCNP concentrations in heavy water was found to be almost equal to water (Figure S5 in ESI). Furthermore, the addition of KF in to heavy water prevented the emission intensity decrease. Thus, the intensity decrease over time in diluted UCNP concentrations in aqueous suspensions can be associated with nanoparticle disintegration and is not due to water based luminescence quenching. In addition to UCNP concentration and fluoride ion, the buffer ion used in the application has an effect on the upconversion emission intensity decrease of PAA coated UCNPs during incubation (Figure 4). Intensity decrease was faster over time in phosphate buffer than in water and the upconversion emission intensity was significantly decreased even in the presence of added KF. The observation of increased ion dissolution in phosphate buffer is concordant with the previous report.23 In borate buffer, on the other hand, the emission intensity was stable and the intensity was equivalent with and without KF. Emission intensity in Tris buffer suspension was similar than in water suspension. The significant luminescence decrease in phosphate buffer even in the presence of KF might be explained by complexation of the phosphate to the rare-earths and exchange of the PAA on the UCNP surface to phosphate. PAA is assumed to be coordinated to several rare earth ions located on the UCNP surface weakly protecting the particle from disintegration and thus the loss of coordinated PAA in a buffer with high phosphate concentration could partially explain the negative effect observed. The positive effect of borate buffer to prevent the disintegration is interesting and the mechanism needs more research. 21



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Figure 4. Time course of upconversion luminescence of polyacrylic acid coated NaYF4:Yb3+,Er3+ nanoparticles at 5 µg/ml concentration suspended in water (black), Tris-HCl buffer (red), phosphate buffer (blue) and borate buffer (green) without KF (left) and with 1 mM KF (right). 3.4 Time course of upconversion luminescence lifetime. The effect of UCNP disintegration on upconversion luminescence decay was measured from PAA coated UCNPs suspended in water with or without KF after 0, 4 and 24 h incubation (Figure 5 and Table 3). The emissions were collected at 535 nm for 8 ms after 2-ms wide pulsed excitation at 980 nm. All the collected data was fitted with second order exponential decay function. The fitting was started 50 µs after the excitation was turned off in order to exclude the delayed energy transfer from Yb3+ to Er3+ ions. The upconversion luminescence lifetimes decreased over time in water suspensions and a significant decrease was observed especially in the longer decay component, which is suspected to originate from the core of the particles. This indicates that due to the disintegration of the UCNPs the core of the nanoparticles is also exposed to the environment, which results in quenching of the 22


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core in addition to the surface. In KF suspensions significant decrease in lifetimes were not detected and the lifetimes in KF suspensions were equivalent to the zero time point in water suspension. Thus, in KF suspensions the cores of the nanoparticles are not exposed to the environment, therefore confirming that the UCNPs are not disintegrated in the presence of added KF.

Figure 5. Upconversion luminescence lifetimes of polyacrylic acid coated NaYF4:Yb3+,Er3+ nanoparticles at 10 µg/ml concentration measured at time points 0 (black), 4 hours (red) and 24 hours (blue) and suspended in water (left) or in 1 mM KF in water (right).

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Table 3. The upconversion luminescence lifetimes of polyacrylic acid coated NaYF4:Yb3+,Er3+ nanoparticles measured at different time points in water or in 1 mM KF in water. H2O

1 mM KF in H2O

τ1 (µs)

67.3 ± 0.2

68.3 ± 0.2

τ2(µs)

664 ± 4

675 ± 5

τ1(µs)

62.6 ± 0.3

68.5 ± 0.3

τ2(µs)

584 ± 5

660 ± 6

τ1(µs)

61.9 ± 0.3

69.1 ± 0.3

τ2(µs)

559 ± 6

689 ± 6

0h

4h

24 h

4. CONCLUSIONS The results represented in this study strongly indicate that the disintegration of upconverting nanoparticles in aqueous suspensions is due to dissolution of particle-constituting ions. The ions dissolved from the NaYF4:Yb3+,Er3+ nanoparticles until a solubility equilibrium was achieved. Thus, the disintegration of the nanoparticles is concentration dependent and at concentrated UCNP suspensions only minor disintegration per nanoparticle is needed before the ion equilibrium is reached. However, at diluted UCNP suspensions most of the particles must be almost completely disintegrated before the equilibrium is reached. The disintegration of the nanoparticles resulted in the loss of active Yb3+ and Er3+ ions, which caused the decrease in the upconversion luminescence because the ions participating in the upconversion process are no longer in close distance to one another. According to the solubility product the fluoride ion has a major role in the nanoparticle 24


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disintegration. This conclusion is also supported by the observation that the disintegration was prevented by adding fluoride ions to the aqueous suspensions thus forcing the equilibrium to the solvent without the UCNP disintegration. Furthermore, disintegration of the nanoparticle surface causes more surface defects, which also exposes the particle cores to enhanced luminescence quenching mechanism such as OH-vibrations of water. The results of this study will have an impact on research of bioanalytical applications of UCNPs, where diluted nanoparticles concentrations are utilized and in guiding the design of improved shell-stabilized UCNPs. Especially during long incubation times majority of the emission intensity might be lost in aqueous environment. Due to the UCNP concentration dependency, UCNP storage at high concentration in aqueous solutions should not cause a major concern although longer than 96 h time periods were not included in this study. Nevertheless, the buffer used for the storage should be carefully considered since phosphate buffer accelerated the disintegration. Addition of KF to the buffer or water prevented the decrease in the upconversion luminescence, but the applicability of KF in bioanalytical assays requires additional studies. Although the addition of KF to the UCNP suspensions had beneficial effect on the emission intensities, KF certainly cannot be used in imaging of live cells because fluoride can induce cell death.38 In addition to fluoride and borate, other way of protecting the nanoparticles from the dissolution could be implementation of a protective shell on the UCNPs. Previously, it has been suggested that UCNPs could be protected from water based quenching with a NaYF4 shell.39, 40 However, our results raise a concern that the NaYF4 shell may disintegrate in aqueous media due to dissolution of the shell-constituting ions. According to the results, thick silica shell slowed down the disintegration of the nanoparticles and therefore, possibly even thicker silica or other polymer shell with complete coverage might prevent the UCNPs completely from disintegration. 25



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ASSOCIATED CONTENT Supporting Information. Transmission electron microscopy image and X-ray powder diffraction pattern of as-prepared UCNP, Upconversion luminescence in the presence of KCl, NaF, NaCl and YCl3, upconversion luminescence in heavy water at different particle concentrations. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by Tekes the Finnish Funding Agency for Innovation and Doctoral Program of Molecular Life Sciences. The authors wish to thank Johan Bobacka and Paul Ek from Åbo Akademi for the ICP-MS analysis and Jaana Rosenberg from University of Turku for providing the nanoparticles. REFERENCES 1.

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Disintegration of Hexagonal NaYF - ACS Publications

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