Sage Journals: Discover world-class research

Abstract

Upconversion nanoparticles (UCNs) find application in nanomedicine as biolabels to identify cancer cells. In this work, Y₂O₃: Er³⁺/Yb³⁺ and Gd₂O₃: Er³⁺/Yb³⁺ systems were fabricated by combustion synthesis (CS) and sol-gel (SG) methods to obtain nanoparticles with different shapes and sizes after post-annealing treatments. The crystallinity and morphology were analysed by x-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively, and the luminescent properties by spectrofluorescence measurements. The luminescence varied depending on the doping level. In this study, for the host Y₂O₃, with Er³⁺/Yb³⁺ (1%/1% mol), the green emission was more strongly attributed to the ²H _11/2→⁴I _15/2, ⁴S_3/2→⁴I _15/2 transitions of Er³⁺ ions. Red emission was present in Yb³⁺ (5%/10% mol) due to the ⁴F_9/2→⁴I_15/2 transition for both hosts. For the host Gd₂O₃, green emission was present in Er³⁺ (2% mol) and Yb³⁺ (3% mol). The surface of some UCNs was coated with a thin silica layer and analysed in order to compare the luminescence with the uncoated UCNs. The purpose of the coating was to prepare the samples for further functionalization for biolabel use. Results show that the emission intensity of all samples strongly depends on the synthesis method. SG was more reliable than CS synthesis in producing UCNs with better morphology and luminescent properties.

Keywords

Upconversion Luminescence Nanoparticles Sol-gel Combustion Synthesis

1. Introduction

Luminescent materials have been studied in the last decade due to their multiple applications in biomedicine [1, 2]; for example, as biolabels to identify cancer cells [3]. In the last five years, attention has been paid to upconversion luminescence because it can be applied to a variety of devices, such as lasers, displays and fluorescence imaging [4]. Y₂O₃ and Gd₂O₃ UCNs co-doped with Er³⁺/Yb³⁺ possess the ability to absorb near-infrared (NIR) radiation and upconvert it to emit within the visible range. Yb³⁺ acts as the sensitizer that transfers the NIR energy to the activator Er³⁺, with the advantage that the emission wavelength is shorter than the NIR excitation.

Several authors have prepared different UCNs with the hosts Y₂O₃ and Gd₂O₃. Li [5] prepared Gd₂O₃ co-doped with Er³⁺/Yb³⁺ (3%, 20% mol) by the precipitation and combustion method, while Hemmer [1] used the same host with the doping Er³⁺ 1% mol and Yb³⁺ 1% mol. Amongst other authors, Chen [6] co-doped the host Y₂O₃ with Er³⁺/Yb³⁺ (1%, 10% mol) and Kong [7] compared the hosts Y₂O₃, La₂O₃ and Gd₂O₃ co-doped with several concentrations of Er³⁺/Yb³⁺, whereas Kumar [17] studied the thermoluminescence effects of Gd₂O₃ co-doped with Er³⁺/Yb³⁺. In this context, UCNs of Y₂O₃ and Gd₂O₃ co-doped with Er³⁺/Yb³⁺ have been synthesized. The focus of this work, then, is that the annealing temperatures and the co-dopings were varied in order to obtain the better UCNs to be used as biolabels to detect cancer cells after proper functionalization. The upconversion process happens when Yb³⁺ absorbs NIR radiation (980 nm) and it transfers a first photon to Er³⁺, so that its electron in the ⁴I_15/2 level is promoted to ⁴I_11/2 level. The process continues when Yb³⁺ transfers a second photon to Er³⁺, which is the same electron that is raised to a higher level ⁴S_3/2. From here it rapidly decays to its base state, emitting green light. Depending on the percentage of the doping, the electron can also be promoted to level ⁴F_9/2 and then decays, emitting red light [8]. The electrons of Er³⁺ have a metastable energy level with longer excited states; meanwhile, another advantage of the doping is that the electrons of Yb³⁺ have a cross-section with large absorption in the NIR spectra and, in turn, can transfer the excitation energy to Er³⁺ [9].

The luminescent properties of diverse UCNs were studied in order to determine their efficacy as cellular biolabels. NIR-absorbing UCNs are adequate fluorescence labels because NIR light shows reduced phototoxicity compared with UV light, while its penetration into biological tissues can be deeper [10]. In this work, the nanoparticles were synthesized and characterized in order to compare their properties. In addition, the silica-coated UCNs that were obtained had strong luminescence and size, such that they can be used as biolabels with the appropriate functionalization

2. Materials and Methods

The UCNs of Y₂O₃: Er³⁺/Yb³⁺ and Gd₂O₃: Er³⁺/Yb³⁺ were prepared by two methods: combustion synthesis (CS) and sol-gel (SG), with several concentrations of Yb³⁺ and Er³⁺ and annealing temperatures. The UCNs were also ultrasonicated in order to obtain less agglomeration. Afterwards, the nanoparticles were characterized with different techniques. The UCN crystallographic structure was analysed by XRD and the morphology by TEM. The luminescence properties were analysed with a spectrophotometer (Hitachi 4500). The UCNs coated with silica were also analysed with the Energy-dispersive X-ray spectroscopy (EDS) in order to study the morphology and uniformity of the silica coating. We found that the synthesis method is relevant to the luminescence intensity and also the annealing temperatures.

2.1 Combustion synthesis

The solution for CS of Gd₂O₃: Er³⁺/Yb³⁺ were prepared with stoichiometric amounts of the precursors dissolved in 20 ml of de-ionized water. The precursors were as follows: Gd(NO₃)₃ (Alfa Aesar 99.9965%), Yb(NO₃)₃ (Alfa Aesar 99.9%) and Er(NO₃)₃ (Alfa Aesar 99.9%). The mixture was stirred in a magnetic stirrer to obtain a homogeneous product with no visible residue. The solution changed into a transparent gel, while carbohydrazide (CH₆N₄O Alfa Aesar 97%) was added to the solution as fuel. The stoichiometry is given below:

\begin{array}{l} 2 (1 - x - y) Gd {(N O_{3})}_{3} + 2 xYb {(N O_{3})}_{3} + 2 yEr {(N O_{3})}_{3} + C H_{6} N_{4} O \to \\ {({Gd}_{1 - x - y} Y b_{x} E r_{y})}_{2} O_{3} + 3 H_{2} O + 5 N_{2} + C O_{2} + \frac{11}{2} O_{2} \end{array}

(1)

where x varies between 0, 0.02 and 0.10, which are the percentages of Ytterbium, while y varies between 0.01 and 0.03 and represents the percentage of Erbium. This mixture was then transferred into a muffle furnace and maintained at a fixed temperature (500°C). The mixture of nitrates (oxidizers) and the fuel (carbohydrazide) underwent spontaneous combustion under heating, while the chemical energy from the exothermic reaction heated the precursor mixture to a high temperature. Consequently, large amounts of gases were produced in the reaction together with the flame due to gas-phase reaction of the combustion gases [11]. The solution underwent an annealing treatment consisting of exposure to a high temperature for variable periods. The reaction mixture was removed from the furnace and annealed at a temperature of 700 to 900°C for Gd₂O₃:Er³⁺/Yb³⁺, and 900 to 1200°C for Y₂O₃:Er³⁺/Yb³⁺ for two hours. The same process was repeated to prepare Y₂O₃:Er³⁺/Yb³⁺, where the precursor changed for Y(NO₃)₃ (Alfa Aesar 99.9%) instead of Gd(NO₃)₃. The stoichiometry was:

\begin{array}{l} 2 (1 - x - y) Y {(N O_{3})}_{3} + 2 xYb {(N O_{3})}_{3} + 2 yEr {(N O_{3})}_{3} + C H_{6} N_{4} O \to \\ {(Y_{1 - x - y} Y b_{x} E r_{y})}_{2} O_{3} + 3 H_{2} O + 5 N_{2} + C O_{2} + \frac{11}{2} O_{2} \end{array}

(2)

where x varies between 0.01, 0.05 and 0.10, which are the percentages of Ytterbium, while y is 0.01 and represents the percentage of Erbium. See Table 1 for doping and annealing temperatures.

Table 1.

Doping and annealing temperatures for the UCNs of Gd₂O₃ and Y₂O₃

Number	Host	% Er y	% Yb X	Temperatures for annealing (°C)	Time of annealing (hr.)
1	Y₂O₃	1	1	900, 1000, 1100, 1200	2

2	Y₂O₃	1	5	900, 1000, 1100, 1200	2

3	Y₂O₃	1	10	900, 1000, 1100, 1200	2

4	Gd₂O₃	1	0	800,900	2

5	Gd₂O₃	0	2	900	2

6	Gd₂O₃	1	10	700,800,900	2

7	Gd₂O₃	3	2	800,900	2

2.2 Sol-gel method

The precursors for this method were the same nitrates as used for the CS. The precursors were diluted with HNO₃ (14% mol). Tartaric acid (C₄H₆O₆ Aldrich, USA) was dissolved in de-ionized water and the individual solutions were mixed under constant stirring. The molar concentration of tartaric acid was 1:2 for the metal ions present in the Gd₂O₃ or Y₂O₃. Both solutions were mixed and stirred for 24 hours at room temperature. Thereafter, the pH of 5.0 was verified and the mixture was heated under constant stirring at 80°C for two hours. This yielded a denser sol. Subsequently, the sol was heated at 120°C until a gel was produced and dried to form the xerogel [12]. The xerogel was collected and annealed (Table 1). The stoichiometries for Y₂O₃ and Gd₂O₃ respectively, were:

\begin{array}{l} 2 (1 - x - y) Y {(N O_{3})}_{3} + 2 xYb {(N O_{3})}_{3} + 2 yEr {(N O_{3})}_{3} + C_{4} H_{6} O_{6} \to \\ {(Y_{1 - x - y} Y b_{x} E r_{y})}_{2} O_{3} + 6 N O_{2} + \frac{3}{2} O_{2} \end{array}

(3)

\begin{array}{l} 2 (1 - x - y) Gd {(N O_{3})}_{3} + 2 xYb {(N O_{3})}_{3} + 2 yEr {(N O_{3})}_{3} + C_{4} H_{6} O_{6} \to \\ {(G d_{1 - x - y} Y b_{x} E r_{y})}_{2} O_{3} + 6 N O_{2} + \frac{3}{2} O_{2} \end{array}

(4)

The variations of x and y and the annealing temperatures are shown in Table 1.

2.3 Ultrasonication

All the UCNs used were ultrasonicated with a high-intensity ultrasonic processor (Sonics & Materials, Inc.) for about 30 minutes with 20 ml of isopropanol/ethanol before the analysis for TEM or silica-coating in order to avoid agglomeration of the nanoparticles.

2.4 Silica-coating

When producing UCNs for use as biolabels in order to identify cancer cells, a fine silica coating was made to prepare the samples for further functionalization. Silica-coating of the particles was done by Stöber synthesis [13]. Distilled water (80 ml) was mixed with 0.21 gr of UCNs (see Table 2) under constant magnetic stirring for 20 minutes. Also 1 ml of Ludoxam (Aldrich, USA) was mixed with 9 ml of distilled water. The second mixture is added slowly to the first one. The final solution was stirred for between 20 and 24 hours. The silica-coated UCNs were precipitated with acetone and centrifuged three times. Finally, the UCNs were filtered and then annealed at 900°C for two hours.

Table 2.

Silica-coated UCNs

UCN	Synthesis method
Y₂O₃:Er³⁺, Yb³⁺ (1%, 1% mol)	SG

Y₂O₃:Er³⁺, Yb³⁺ (1%, 5% mol)	CS

Y₂O₃:Er³⁺, Yb³⁺ (1%, 10% mol)	CS

Gd₂O₃:Er³⁺, Yb³⁺ (1%, 10% mol)	SG

2.5 Characterization

The crystallinity and morphology of the UCNs were analysed by XRD with the Phillips X′Pert-MPD, equipped with Cu Kα radiation (λ= 0.15406 nm). Measurements in a 2θ = 10–80 degree range were taken with a step-size of 0.1° and a one-second dwell per point. The results obtained were compared with the database PCPDFWIN. The TEM used was JEOL JEM-2100-F in order to study the morphology and the nanoparticle size. The photoluminescence of the UCNs was analysed with a fluorescence spectrophotometer (PL, Hitachi® FL-4500) with a wavelength of 980 nm for excitation. The silica-coated UCNs were analysed with the EDS in order to study the morphology and uniformity of the silica coating.

3. Results

This section shows the XRD results as well as the morphology analysis and photoluminescence displayed by all UCNs synthesized in this work.

3.1 XRD and morphology of Y₂O₃:Er³⁺/Yb³⁺ and Gd₂O₃:Er³⁺/Yb³⁺ UCN

The composition obtained for the UCNs synthesized by CS and SG methods was (Gd_1-x-yYb_xEr_y)₂O₃ and (Y_1-x-yYb_xEr_y)₂O₃ (Table 1). The XRD patterns of the UCN are in Figure 1. All the diffraction peaks are consistent with JCPDS No. 89-5592 (Y₂O₃) and JCPDS No. 88-2165 for the Gd₂O₃ database and demonstrate that the samples have a cubic structure. The highest annealing temperatures, 800–900°C for Gd₂O₃ and 1100–1200°C Y₂O₃, were chosen in order to transform the Gd₂O₃:Er³⁺/Yb³⁺ and Y₂O₃:Er³⁺/Yb³⁺ nanoparticles into a crystalline structure. Consistent with Singh [20], the nanoparticles of Gd₂O₃:Er³⁺/Yb³⁺ annealed at 600°C were transformed into a multiphase form, with a monoclinic and cubic phase, while at 900°C, the phase was only cubic, as confirmed herein. The 900°C temperature was, therefore, selected in order to continue.

Figure 1.

XRD patterns of Y₂O₃: Er³⁺/Yb³⁺ and Gd₂O₃: Er³⁺/Yb³⁺ compared with the standard data JCPDS no. 89-5592 [Y₂O₃] and JCPDS no. 88-2165 [Gd₂O₃]: a) JCPDS no. 89-5592 [Y₂O₃]; cubic structure: b) Y₂O₃:Er, Yb (1%, 1%)1100°C (CS); c) Y₂O₃:Er, Yb(1%,5%) 1100°C (CS); d) Y₂O₃: Er, Yb (1%,10%)1200°C (SG); e) JCPDS 88–2165:Gd₂O₃ cubic structure; f) G₂O₃: Er, Yb (1%,10%) 900°C (CS); g) G₂O₃: Er, Yb (2%,3%) 800°C (CS); and h) G₂O₃: Er 1% 900°C (SG).

Figure 2 shows the TEM images of bare Gd₂O₃:Er³⁺/Yb³⁺ and Y₂O₃:Er³⁺/Yb³⁺ UCNs prepared by SG, while Figure 3 shows those prepared by CS. UCNs obtained by both synthesis methods were mostly of a spherical shape. The average size of the Gd₂O₃:Er³⁺/Yb³⁺ nanoparticles were 50 nm (+/-10 nm), while for the UCN Y₂O₃:Er³⁺/Yb³⁺ it was 70 nm (+/-10 nm). Agglomerates were observed mostly on the UCNs prepared by CS. UCNs tend to agglomerate; indeed, some authors have presented agglomerated UCNs [17–18]. The morphology did not change due of the different doping concentrations.

Figure 2.

TEM images of bare Y₂O₃: Er³⁺/Yb³⁺ and Gd₂O₃: Er³⁺/Yb³⁺ synthetized by SG: a) Y₂O₃: Er³⁺, Yb³⁺ (1%, 1%) 1200°C SG; b) Er 1%, Yb 10% 1200°C SG; c) Er 1%, Yb 5% 1200°C SG; d) Gd₂O₃: Er³⁺, Yb³⁺ (1%, 10%) 900°C SG; e) Yb 2% 900°C SG; f) Er 2%, Yb 3% 900°C SG; and g) Er 1% 900°C SG.

Figure 3.

TEM images of bare Y₂O₃: Er³⁺/Yb³⁺ and Gd₂O₃: Er³⁺/Yb³⁺ synthetized by CS: a) Y₂O₃: Er³⁺, Yb³⁺ (1%, 1%) 1200°C CS;b) Er 1%, Yb 10% 1200°C CS; c) Er 1%, Yb 5% 1200°C CS; d) Gd₂O₃: Er³⁺, Yb³⁺ (1%, 10%) 900°C CS; e) Yb 2% 900°C CS; f) Er 2%, Yb 3% 900°C CS; and g) Er 1% 900°C CS.

Figure 4 shows the TEM images of the silica-coated UCN. The size increased only by about 1–3 nm due to the core-shell formed of silica. Some of the UCNs presented the spherical shape. In Figures 4 b) and c), the crystallographic planes are clearly shown and the silica coating can be seen as amorphous. The UCNs tend to agglomerate after and under coating. The coated UCNs were also analysed with the EDS technique to confirm the presence of silica (Figure 5).

Figure 4.

TEM images of UCN with silica coating: a) Y₂O₃: Er³⁺, Yb³⁺ (1%, 10%); b) Y₂O₃: Er³⁺, Yb³⁺ (1%, 5%); c) Y₂O₃: Er³⁺, Yb³⁺ (1%, 1%); and (d) Gd2O3: Er³⁺, Yb³⁺ (1%, 10%).

Figure 5.

EDS technique to confirm the presence of silica on UCNs Y₂O₃: Er³⁺, Yb³⁺ (1%, 1%) (1200°C annealing)

3.2 Upconversion luminescence process

In Figure 6 is shown a classic diagram of the upconversion process [8,14]. On receiving the excitation energy of 980 nm, the Yb³⁺ ion transfers energy via two photons to the Er³⁺ ion, which is excited by the first photon from the ground state ⁴I_15/2 to the state ⁴I_11/2. With the second photon, the Er³⁺ ion is excited by the ⁴F_7/2 state. With no further radiation, it may decay to the ²H_11/2 and ⁴S_3/2 levels. The green emission is caused by the decay from ⁴S_3/2→⁴I_15/2 and the red emission by the ⁴F_9/2→⁴I_15/2 transition.

Figure 6.

Energy-level diagram for Yb³⁺ and Er³⁺ upconversion emission (980 nm excitation)

3.2.1 Upconversion luminescence properties of Gd₂O₃:Er³⁺, Yb³⁺

Inorganic host materials Gd₂O₃ and Y₂O₃ doped with Er³⁺ and Yb³⁺ ions are upconverting phosphors that absorb light in the near-infrared range (980 nm) and emit in the visible spectra. Upconversion luminescence spectra of samples under 980 nm laser excitation are shown in Figures 7, 8 and 9 for all doping concentrations and annealing temperatures (Table 1) for Gd₂O₃:Er³⁺/Yb³⁺, Gd₂O₃:Er³⁺ and Gd₂O₃:Yb³⁺. The spectra show two groups of upconversion emission peaks at 561–563 nm and 661–663 nm. These peaks are attributed to ⁴S_3/2→⁴I_15/2 transition and ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ions, respectively. For doping with Yb³⁺ (2%), the red emission is clearly much stronger than the green emission. For doping with Er³⁺ (1%), the green emission (661 nm) is stronger. The emission intensity for samples synthesized by the SG method is greater than the CS method.

Figure 7.

The upconversion emission spectra for Gd₂O₃:Er³⁺/Yb³⁺ (1%, 10% mol) under 980 nm excitation at three annealing temperatures: 700, 800 and 900°C. a) CS and b) SG method. Red emission (660–662 nm) obtained with both synthesis methods and low or no emission at 700°C annealing.

Figure 8.

The upconversion emission spectra for: a) Gd₂O₃:Er³⁺ (1% mol) at 800 and 900°C annealing temperatures, and b) Gd₂O₃:Yb³⁺ (2% mol) at 900°C annealing. Both under 980 nm excitation. In (a) green emission (563 nm) is stronger than red emission (661 nm). In (b) red (661 nm) and green emission (560 nm) are present but the red emission is the strongest.

Figure 9.

The upconversion emission spectra for Gd₂O₃:Er³⁺/Yb³⁺ (2%, 3% mol) under 980 nm excitation at three annealing temperatures: 800 and 900°C. a) CS and b) SG method. Red emission present (661 nm) with both methods, but green emission is also present with lower intensity. The best luminescence was with the 900°C annealing on all the samples.

3.2.2 Upconversion luminescence properties of Y₂O₃:Er³⁺, Yb³

Upconversion luminescence spectra for Y₂O₃:Er³⁺/Yb³⁺ UCN samples are shown in Figures 10, 11 and 12 for the different doping percentages and annealing temperatures (Table 1). Under laser excitation of 980 nm, transitions ²H_11/2→ ⁴I _15/2 (550 nm), ⁴S_3/2→⁴I _15/2 (564 nm) and ⁴F_9/2→⁴I_15/2 (660 nm) are marked in Figure 9. The highest intensity for the Er³⁺/Yb³⁺ (1%, 1% mol) doping is in the green emission (564 nm). For the doping Er³/Yb³⁺ (1%, 5% mol), the same transitions are present, but the highest is the red emission of the ⁴F_9/2→⁴I_15/2 (660 nm) transition (Figure 11). All samples doped (1%, 5% mol) emit orange due to the combination of emissions of several colours. For the Er³⁺/Yb³⁺ (1%, 10% mol) doping (Figure 12), the highest emission is in red (663 nm) and corresponds to the transition ⁴F_9/2→⁴I_15/2; the green transitions are present but with lower emission.

Figure 10.

The upconversion emission spectra for UCNs under excitation with 980 nm of Y₂O₃:Er³⁺/Yb³⁺ (1%, 1% mol) a) by SG and b) CS. Green emission (564 nm) with both synthesis methods is present

Figure 11.

Shows the upconversion emission spectra for UCN under excitation with 980 nm wavelength of Y₂O₃:Er³⁺/Yb³⁺ (1%, 5% mol) a) by SG and b) CS

Figure 12.

The upconversion emission spectra for UCNs under excitation with 980 nm of Y₂O₃:Er³⁺/Yb³⁺ (1%, 10% mol) a) by SG and b) CS. Red emission (661 nm) present with both methods

The spectra results of all the UCNs under laser excitation (980nm) are shown on Table 3. The doping Y₂O₃:Er³⁺/Yb³⁺ (1%, 1% mol) presents green emission, the same as Gd₂O₃:Er³⁺ (1% mol). The transitions of ²H _11/2→ ⁴I _15/2 (550 nm), ⁴S _3/2→4I _15/2 (564 nm) and ⁴F_9/2→⁴I_15/2 (660 nm) are present in all the UCNs of Y₂O₃:Er³⁺/Yb³⁺ with different intensities and in the Gd₂O₃:Er³⁺/Yb³⁺ samples.

Table 3.

Spectra results for all the UCNs

Number	Host	% Er y	% Yb X	Emission wavelength (nm)	Color of emission
1	Y₂O₃	1	1	564	Green

2	Y₂O₃	1	5	661	Orange

3	Y₂O₃	1	10	663	Red

4	Gd₂O₃	1	0	563	Green

5	Gd₂O₃	0	2	661	Orange

6	Gd₂O₃	1	10	661	Red

7	Gd₂O₃	3	2	661	Red

3.2.3 Upconversion luminescence properties of the silica-coated UCNs

Upconversion luminescence spectra for silica-coated Y₂O₃:Er³⁺,Yb³⁺ and Gd₂O₃:Er³⁺,Yb³⁺ samples are shown in Figure 13. Under laser excitation of 980 nm, transitions of ²H _11/2→⁴I _15/2 (550 nm), ⁴S _3/2→⁴I _15/2 (564 nm) and ⁴F_9/2→⁴I_15/2 (660 nm) are indicated. The intensity of some samples is lower than that of the non-coated samples, but still the luminescence is good. The same transitions are present.

Figure 13.

Silica-coated UCNs. The same transitions are present as much as the non-coated UCNs, with great intensity: a) Y₂O₃:Er³⁺/Yb³⁺ (1%, 10% mol) CS 1200°C; b) Y₂O₃:Er³⁺/Yb³⁺ (1%, 5%) CS 900°C, in which the transition ²H _11/2→ ⁴I _15/2 is not present, but the ⁴F_9/2→⁴I_15/2 is with lower intensity; c) Y₂O₃:Er³⁺/Yb³⁺ (1%, 1% mol) SG 1100°C, in which all transitions are present and almost with the same intensity as the original UCN; and d) Gd₂O₃:Er³⁺/Yb³⁺ (1%, 10%) SG 900°C.

4. Discussion

UCNs prepared by CS and SG methods were (Gd_1-x-yYb_xEr_y)₂O₃ and (Y _1-x-yYb_xEr_y)₂O₃ (Table 1). The XRD analysis showed that the crystal structure of UCNs is a cubic lattice in agreement with JCPDS nos. 89–5592 (Y₂O₃) and 88–2165 (Gd₂O₃) database. According to the morphological analysis, the average size of the Gd₂O₃:Er³⁺/Yb³⁺ UCN was about 50 nm (+/-10 nm deviation) and the UCNs Y₂O₃:Er³⁺/Yb³⁺ were about 70 nm (+/- 10 nm deviation). The SG method offered better morphology results than the CS method. UCNs calcined with the highest annealing temperatures, which in this work were 900°C for gadolinium oxide and 1100–1200°C for yttrium oxide, show more intense luminescence due to the better formation of the crystalline structure and a more efficient insertion of Er³⁺ and Yb³⁺ into the lattice. Yttrium oxide is more stable at higher temperatures than gadolinium oxide; this is why higher temperatures were used for annealing yttrium oxide. For the UCNs with the host Gd₂O₃, the annealing temperature of 900°C produced more intense UCNs luminescence due to the complete formation of the crystalline structure. At 700°C, the crystalline structure is in the process of forming [15] and the luminescence intensity of some UCNs is low or none: for example, the Gd₂O₃:Er³⁺/Yb³⁺ (2%,3%) (Figure 9) and Gd₂O₃:Yb³⁺ (2%) (Figure 8 b).

According to Li [5], the Yb³⁺ ions have a much larger absorption cross-section relative to that of Er³⁺ ions around 980 nm, while the Er³⁺ ion can be excited from the ground-state ⁴I_15/2 to the excited state ⁴I_11/2 by energy transfer from the excited Yb³⁺ ions. In this study, for the host Y₂O₃ with Er³⁺ (1%) and Yb³⁺ (1%) concentrations, the green emission was stronger due to the electron population in level ⁴S_3/2→⁴I_15/2 At higher concentrations of Yb³⁺ (5% and 10%), the ⁴F_9/2→⁴I_15/2 level was more electron-populated. With both synthesis methods, the red emission is much stronger than the green emission. This suggests that the ⁴F_9/2 level is more electron-populated than the ⁴S_3/2 level. However, present results agree with Li [5], in that the emission intensity strongly depends on the synthesis method. In the present study, the SG method produced UCNs with higher luminescence intensity than CS. We conclude that this is because the SG method produces a more homogeneous and purer solid outcome than the CS method, which generates a solid with more defects and impurities. For example, no green emission was found in the Gd₂O₃:Er³⁺/Yb³⁺ (2%, 3%) sample synthesized by CS; however, with the SG, the sample did emit in green. This may be because the CSUCNs have centres of non-radiative relaxation that releases a phonon instead of a photon [8]. That is why we propose that, for UCNs, the CS method is less reliable than SG. In this field, other authors, such as Li Yanhong [5], have compared synthesis with two methods, CS and precipitation, of only one host, Gd₂O₃:Er³⁺/Yb³⁺ (3%, 20%), with only one annealing temperature, 950°C; Singh [15] analysed only Gd₂O₃:Er³⁺/Yb³⁺, but varied the molar concentration of Yb₂O₃ from 0% to 0.6% and of Er₂O₃ from 0% to 3.5%. Meanwhile, Vetrone [16] prepared Y₂O₃: Er³⁺/Yb³⁺ by CS and hydrolysis methods. In addition, Kong [7] compared three different hosts – Y₂O₃, La₂O₃, and Gd₂O₃ – by flame synthesis co-doped with Er³⁺ and Yb³⁺. The effects on Gd₂O₃:Er³⁺/Yb³⁺ with several doping concentrations of Er³⁺ (0.5% to 2%) and Yb³⁺ (0% to 15%) were also reported by Kumar [17–18]. To the best of our knowledge, the present study is the first to compare two hosts with several doping concentrations of Er³⁺ and Yb³⁺ and various annealing temperatures for UCNs synthesized with the methods of CS and SG.

Hemmer [1] studied the application of UPNs (Gd₂O₃:Er³⁺, Yb³⁺) in biomedicine, such as in in vivo fluorescence bioimaging. Improving the UCNs synthesis method and luminescence properties can lead to the production of nanoparticles, which may be applied as biolabels for cancer cell detection. Further studies to determine cytotoxicity of silica-coated nanoparticles and functionalization of UCNs are needed.

5. Conclusions

The emission intensity depends of the synthesis method: the SG method produces more intense samples than CS. We conclude that this is because the SG method produces a more homogeneous and purer solid than the CS method, which generates a solid with more defects and impurities. The average size of the Gd₂O₃:Er³⁺/Yb³⁺ UCNs were about 50 nm (±10 nm deviation) and the UCNs Y₂O₃:Er³⁺/Yb³⁺ were about 70 nm (±10 nm deviation). The Er³⁺ and Yb³⁺ are on small percentages and did not affect the structure of the hosts. The red emission with high intensity was present in the UCNs Gd₂O₃:Er³⁺/Yb³⁺ (1%, 10% mol) (900°C annealing) and also in the UCNs Y₂O₃:Er³⁺/Yb³⁺ (1%, 10% mol) (1200°C annealing). The green emission with high intensity was present on the UCNs Y₂O₃:Er³⁺/Yb³⁺ (1%, 1% mol) (1200°C annealing). Silica-coated UCNs show good emission spectra. The UCNs synthesized in this work may be potentially used as biolabels to detect cancer cells given that, with the silica coating, the UCNs had a core/shell that can be functionalized with various ligands, which can be used on biological systems for imaging applications. Further experiments are in progress, such as cytotoxicity evaluations and functionalization studies, before these UCNs can be used in vitro.

6. Conflicts of interest

The authors declare no conflicts of interest.

Footnotes

7. Acknowledgements

The authors wish to acknowledge the support of the Dirección General Asuntos del Personal Académico UNAM DGAPA under grant no. 109913, and the Consejo Nacional de Ciencia y Tecnología CONACyT under grant no. 232608. The technical support provided by Eloisa Aparicio, Francisco Ruiz and Marcelino Ponce is also gratefully acknowledged.

References

Hemmer

Yamano

Kishimoto

Venkatachalam

Hyodo

Soga

(2013) Cytotoxic aspects of gadolinium oxide nanostructures for up-conversion and NIR bioimaging. Acta Biomaterialia. 9:4734–4743.

Lin

Zhao

Wang

Liu

Duan

Chen

(2011) Recent advances in and surface modification of lanthanide-doped upconversion nanoparticles for biomedical applications. Biotechnol. Adv. 30:1551–1561.

Nie

Xing

Kim

Simons

(2007) Nanotechnology Applications in Cancer. Ann. Rev. Med. 9:257–288.

Xie

Duan

Zhang

(1998) Research on Quenching Concentration of Nanocrystalline Y₂SiO₅: Eu. J. of Luminiscence. 19:19–23.

Hong

Zhang

Dai

(2008) Preparation and Upconversion Luminescence of Nanocrystalline Gd₂O₃: Er³⁺, Yb³⁺. Journal of Wuhan University of Technology-Mater. Sci. 23:4.

Chen

Liu

Zhang

Aghahadi

Somesfalean

Sun

Wang

(2007) Four-photon upconversion induced by infrared diode laser excitation in rare-earth-ion-doped Y₂O₃ nanocrystals. Chem. Phys. Lett. 448:127–131.

Kong

Shan

(2010) Flame synthesis and effects of host materials on Yb³⁺/Er ³⁺ co-doped upconversion nanophosphors. Materials Letters. 64:668–691.

Blasse

Grabmaier

(1994) Luminescent Materials. 1st Ed. Telos, Berlin: Springer-Verlag; 218.

Matsuura

(2010) Green and Blue Upconversion Luminescence of Trivalent-rare-earth Ion-doped Y₂O₃ Nanocrystals. Appl. Phys. Lett. 81:4526–4528.

10.

Anderson

Parrish

(1981) The optics of human-skin. J. Invest. Dermatol. 77:13–9.

11.

Hirata

G A

Bosze

McKittrick

(2008) Development of Nanostructured EuAl₂O₄ Phosphors with Strong Long-UV Excitation. J. Nanosci. Nanotechnol. 8:1–5.

12.

Taxak

Khatkar

Han

Kumar

(2009) Tartaric acid assisted sol gel synthesis of Y₂O₃:Eu³⁺ nanoparticles. J. Alloys Compds. 469:224.

13.

Stober

Fink

(1968) Colloid Interface. Science. 26:62.

14.

Yang

Dai

Jiang

(2003) Upconversion Emission of Rare Earth Ions and its Recent Developments. Progress in Phys. 23:284–298.

15.

Singh

Kumar

Rai

(2009) Multifunctional Er³⁺/Yb³⁺ codoped Gd₂O₃ nanocrystalline phosphor synthesized through optimized combustion route. Appl. Phys. B. 94:165–173.

16.

Vetrone

Boyer

Capobianco

(2003) Effect of Yb³⁺ Codoping on the Upconversion Emission in Nanocrystalline Y₂O₃: Er³⁺. J. Phys. Chem. 107:1107–1112.

17.

Kumar

R T

Bisen

D P

(2015) Thermoluminescence studies of ultraviolet and gamma Irradiated erbium(III)-and ytterbium(III)-doped gadolinium oxide phosphors. Materials Science in Semiconductor Processing. 33:169–188.

18.

Kumar

R T

Bisen

D P

Upadhyay

Prasad

(2015) Comparative Study and Role of Er³⁺ and Yb³⁺ Concentrations on Upconversion Process of Gd₂O₃:Er³⁺ Yb³⁺ Phosphors Prepared By Solid-State Reaction and Combustion Method. J. Phys. Chem. C. 119:21072–21086.

Synthesis and Upconversion Luminescence of Nanoparticles Y 2 O 3 and Gd 2 O 3 Co-doped with Yb 3+ and Er 3+

Abstract

Keywords

1. Introduction

2. Materials and Methods

2.1 Combustion synthesis

2.2 Sol-gel method

2.3 Ultrasonication

2.4 Silica-coating

2.5 Characterization

3. Results

3.1 XRD and morphology of Y2O3:Er3+/Yb3+ and Gd2O3:Er3+/Yb3+ UCN

3.2 Upconversion luminescence process

3.2.1 Upconversion luminescence properties of Gd2O3:Er3+, Yb3+

3.2.2 Upconversion luminescence properties of Y2O3:Er3+, Yb3

3.2.3 Upconversion luminescence properties of the silica-coated UCNs

4. Discussion

5. Conclusions

6. Conflicts of interest

Footnotes

7. Acknowledgements

References

3.1 XRD and morphology of Y₂O₃:Er³⁺/Yb³⁺ and Gd₂O₃:Er³⁺/Yb³⁺ UCN

3.2.1 Upconversion luminescence properties of Gd₂O₃:Er³⁺, Yb³⁺

3.2.2 Upconversion luminescence properties of Y₂O₃:Er³⁺, Yb³