BaWO4:Ln Nanocrystals: Controllable Synthesis, Theoretical

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BaWO4:Ln3+ Nanocrystals: Controllable Synthesis, Theoretical Investigation on Substitution Site, and Bright Upconversion Luminescence as Sensor for Glucose Detection Di Wang, Kai Pan, Yang Qu, Guofeng Wang, Xiaofeng Yang, and Dingsheng Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00989 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Nano Materials

BaWO4:Ln3+ Nanocrystals: Controllable Synthesis, Theoretical Investigation on Substitution Site, and Bright Upconversion Luminescence as Sensor for Glucose Detection Di Wanga, Kai Pana, Yang Qua, Guofeng Wanga,*, Xiaofeng Yangb,*, Dingsheng Wangc

a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of

Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China E-mail: [email protected] b

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

E-mail: [email protected] c

Department of Chemistry, Tsinghua University, Beijing, 100084, China

ACS Paragon Plus Environment

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ABSTRACT: Simultaneous morphology and upconversion luminescence tuning of BaWO4 nanocrystals excited at 980 nm has been achieved by double doping of different Ln3+ successfully. Especially, the substitution sites of Ln3+ into BaWO4 have been investigated by the combination of experimental and DFT calculation for the first time. The Ba site is the most possible location for BaWO4:Yb3+, BaWO4:Er3+, BaWO4:Tm3+, and BaWO4:Ho3+. However, when co-doping Yb3+ with other Ln3+ species into the unit cell of BaWO4, the density functional theory calculations indicated that the Yb3+ is more possible to engage the W site of BaWO4, while the doped Ln3+ remains its location at the Ba site, with Yb3+ in the near W layer in c axis. In addition, a low-cost and sensitive BaWO4:Yb3+/Er3+@Au for glucose detection is designed through a luminescence resonance energy transfer process and characterized by fluorescence techniques. The limit of detection of BaWO4:Yb3+/Er3+@Au was 3.1 nM. BaWO4:Yb3+/Er3+@Au can be conveniently extended to detect many analytes involving H2O2 KEYWORDS:

BaWO4:Ln3+

nanocrystals,

DFT

calculations,

luminescence, glucose detection


upconversion

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INTRODUCTION High sensitivity detection of glucose is of great significance in many fields involving biology, chemistry, and clinical control.1-8 Over the years, many diverse methods for detecting glucose have been developed.7-9 However, most of the reported approaches have many disadvantages, such as sophisticated analytical instrumentation, and complicated operation. The luminescence from rare earth (RE) ions can offer the opportunity to reduce the detection limit and improve the sensitivity of glucose determination.10, 11 Recently, a great deal of effort has been made for controllable synthesis of lanthanide-doped nanocrystals owing to their potential applications.12-30 The luminescence of lanthanide-doped nanoparticles is divided into downconversion (DC) and upconversion (UC) processes. Especially, the UC nanocrystals are rapidly emerging as new bioprobes and conventional molecular probes due to their superior features.31-35 Up to now, many lanthanide-doped nanomaterials have been reported for diagnostic and analytical pathology.36-40 The host material is very important for obtaining UC emission with high luminescence efficiency. Among various materials, tungstates have been wide studied due to their excellent stability.41,42 Especially, tungstates have high quenching concentration of lanthanide ions. Therefore, further exploration of lanthanide ion doped tungstates with different shapes and sizes is still a significant research topic. BaWO4 is a promising host material for lanthanide ions due to its special chemical nature.43 The orthodox view was that the Ba site is the most possible



location for Ln3+ ions in BaWO4:Ln3+ because the radius of Ln3+ is smaller than that of Ba2+. However, the results of our studies showed that the Yb3+ is more possible to engage the W site of BaWO4 when co-doping Yb3+ with other Ln3+ species into the unit cell of BaWO4, and the doped Ln3+ remains its location at the Ba site, with Yb3+ in the near W layer in c axis. Therefore, the co-doping of Yb3+ and other Ln3+ species will leads to an unusual chemical environment of Yb3+, which would grant it great potentiality in new applications. In addition, simultaneous morphology and upconversion luminescence tuning of BaWO4 nanocrystals has been successfully achieved by doping with different Ln3+ ions. Water soluble BaWO4:Yb3+/Er3+@Au detection system for glucose (detection limit 3.1 nmol L−1) have been also developed, as shown in Scheme 1. Experimental part was shown in the Supporting Information.

Scheme 1. Design and principle for glucose detection.

RESULTS AND DISCUSSION Theoretical investigation on BaWO4 and BaWO4:Ln3+. It is well known that the BaWO4 with scheelite structure is centrosymmetric, which has an I41/a space group. The W and Ba atoms occupy S4 sites and the O atoms occupy C1 sites. Theoretically, the lattice parameters for the geometry optimized crystal structure of BaWO4 as a function of k-point sampling are tested first, with its results listed in Table S1 (Supporting Information). The unit cell of bulk BaWO4 was represented with


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its common crystal space group of I41/a, and the optimized cell dimensions are of a=b=5.71 Å and c=12.93 Å, respectively, which was consistent with the experimental measurements (Supporting Information, Figure 1 and Table S1).

Figure 1. The unit cell (a) and crystal structure (b) of bulk BaWO4 with different ionic sites.

With the optimized structure, the doping of an Yb3+ cation at either W or Ba site into the unit cell with an atomic loading of 25% was then determined by the combination of DFT calculation and experimental simulation, and the corresponding energies and optimized lattice parameters of BaWO4:Yb3+ are summarized in Table S2 and S3 (Supporting Information). In comparison, the Ba-site doping with a contraction at a, b and c directions (a=b=5.59 Å and c=12.52 Å), is much more consistent with the X-ray diffraction (XRD) experiment fitting (Table S4, Supporting Information), suggesting that the Ba site is the most possible location for Yb3+. Similarly, such favorite sites for Ln3+-substitution are also found over other BaWO4:Ln3+ composition including BaWO4:Er3+, BaWO4:Ho3+, and BaWO4:Tm3+. However, when co-doping Yb3+ with other Ln3+ species (Er3+, Ho3+, and Tm3+) into the unit cell of BaWO4, although there are multiple Ba-doping sites for both Yb3+ and other Ln3+ species, our DFT calculations indicated that Yb3+ and other Ln3+ species (Er3+, Ho3+, and Tm3+) prefer different doping sites. That is, as seen the total energies



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and optimized lattice parameters of BaWO4:Yb3+/Er3+, BaWO4:Yb3+/Ho3+, and BaWO4:Yb3+/Tm3+ in Table S5, S6, and S7 (Supporting Information), the Yb3+ is more possible to engage the W site of BaWO4, while the doped Ln3+ remains its location at the Ba site, with Yb3+ in the near W layer in c axis. Therefore, the co-doping of Yb3+ and other Ln3+ species will leads to an unusual chemical environment of Yb3+, which would grant it great potentiality in new applications. Crystal structures and morphologies of BaWO4:Ln3+ nanocrystals. The results of XRD patterns (Supporting Information, Figure S1) of the BaWO4:Yb3+/Er3+, BaWO4:Yb3+/Tm3+, and BaWO4:Yb3+/Ho3+ nanocrystals indicated that the positions of the peaks could be easily indexed as a pure tetragonal phase BaWO4 (JCPDS card No. 43-0646). Figure 2(a.b) shows the TEM and High-resolution transmission electron microscopy (HRTEM) of pure BaWO4 nanocrystals without doping. Obviously, BaWO4 display a wirelike shape with 100 nm in diameter. Typical HRTEM image in Figure 2(b) reveals an interplanar spacing of 0.289 nm, which corresponds

to

the

(107)

crystal

planes

of

pure

BaWO4.

However,

BaWO4:20%Yb3+/5%Er3+ and BaWO4:20%Yb3+/20%Er3+ nanocrystals display a sheetlike

shape,

as

shown

in

Figure

2(c,e).

The

HRTEM

image

of

BaWO4:20%Yb3+/5%Er3+ also reveals an interplanar spacing of 0.447 nm corresponding to the (501) plane, as shown in Figure 2(d,f). When the Er3+ concentration is 30%, the BaWO4:20%Yb3+/30%Er3+ nanocrystals tended to aggregate, as shown in Figure 2(g). Here, the Ln3+ concentration in BaWO4:Ln3+ is the mole ratio.


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The TEM images of BaWO4:20%Yb3+/Tm3+ with different Tm3+ concentration are shown in Figure 2(h-j). When the concentration was 20% or 40%, BaWO4:20%Yb3+/Tm3+ nanobelts was obtained. The Brunauer-emmett-teller (BET) surface areas increased with increasing Tm3+ concentrations (Figure S2, Supporting Information), further proving that BaWO4:Yb3+/Tm3+ nanocrystals shift to ribbon with increasing Tm3+ concentration. The HRTEM image of BaWO4:20%Yb3+/40%Tm3+ in Figure 2(k) shows an interplanar spacing of 0.461 nm corresponding to the (512) planes of BaWO4. Figure 2(l) shows the TEM image of BaWO4:20%Yb3+/30%Ho3+ nanocrystals.

Figure 2. TEM and HRTEM images of (a,b) BaWO4, (c,d) BaWO4:20%Yb3+/5%Er3+, (e,f) BaWO4:20%Yb3+/20%Er3+, (g) BaWO4:20%Yb3+/30%Er3+, (h) BaWO4:20%Yb3+/5%Tm3+, (i) BaWO4:20%Yb3+/20%Tm3+, (j,k) BaWO4:20%Yb3+/40%Tm3+, and (l) 3+ 3+ BaWO4:20%Yb /30%Ho .

Figure

S3

(Supporting

Information) shows

the

Raman spectrum

of

BaWO4:20%Yb3+/10%Er3+ nanocrystals. Four Raman bands centered at -354.1, 815.1, 853.4, and 947.5 cm−1 were observed, which were assigned to Ag+Bg, Eg, Bg, and Ag



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modes, respectively.44 Crystal structures and morphologies of BaWO4:Ln3+@Au.

Figure 3(a)

shows the TEM image of BaWO4:20%Yb3+/5%Er3+@Au core-shell composite nanocrystals with 10 mL Au nanoparticle (NPs) as raw materials, indicating the formation of core-shell composite nanocrystals. The HRTEM image of the BaWO4:Yb3+/Er3+@Au core-shell nanocrystals in Figure 3(b) shows two interplanar spacings of 0.314 and 0.203 nm corresponding to the (004) plane of BaWO4:Yb3+/Er3+ and (200) plane of Au, respectively. The corresponding TEM elemental mappings of BaWO4:Yb3+/Er3+@Au resulting from selected area show that the Ba, W, O, Yb, Er, and Au elements exist in the composited material, as shown in Figure 3(c).

Figure 3. (a,b) TEM and HRTEM images and (c) TEM mapping of BaWO4:Yb3+/Er3+@Au.

Figure 4(a) shows the XRD pattern of BaWO4:Yb3+/Er3+@Au core-shell nanocrystals. For comparison, the XRD pattern of BaWO4:Yb3+/Er3+ is also shown in


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Figure 4(a). The peaks of Au appear besides the peaks of BaWO4:Yb3+/Er3+, indicating the formation of BaWO4:Yb3+/Er3+@Au composite nanocrystals. To further prove the coexistence of BaWO4:Yb3+/Er3+ and Au in the composite nanocrystals, BaWO4:Yb3+/Er3+@Au composite were examined using energy dispersive X-ray (EDX) analysis under TEM (Figure 4), indicating that the elemental components are Ba, W, O, Er, Yb and Au.

Figure 4. (a) XRD patterns BaWO4:Yb3+/Er3+ and BaWO4:Yb3+/Er3+@Au core-shell nanocrystals. (b) EDX and (c) FT-IR analysis of BaWO4:Yb3+/Er3+@Au core-shell nanocrystals. (d) Absorption spectrum of Au nanoparticles and the UC luminescence spectrum of BaWO4:Yb3+/Er3+ under 980 nm excitation. Inset shows the luminescence photograph of BaWO4:Yb3+/Er3+ in cyclohexane excited at 980 nm.

Figure 4(c) shows Fourier transform infrared (FT-IR) spectrum at 400-4000 cm-1 of BaWO4:Yb3+/Er3+@Au. The absorption bands at ~3450 cm-1 are attributed to O-H stretching from the water and citrate adsorbed on the BaWO4:Yb3+/Er3+ surface, the bands at ~2927 and ~2850 cm-1 are attributed to the CH2 stretching vibration modes.



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The vibrations of carboxylate anion at ~1562 and ~1400 cm-1 indicated that the surfaces of BaWO4:Yb3+/Er3+@Au are capped by citrate.45 The W-O stretching bands were obtained, as shown in Figure 4(c). The results of the IR data well demonstrated that citrate adsorbed on the BaWO4:Yb3+/Er3+ surface. The prepared nanoparticles can be well dispersed in water because the citric acid molecule has a good hydrophilic group. As mentioned above, the Au nanoparticles exhibited a broad absorption band at ~520 nm, which overlaps with the UC luminescence of the BaWO4:Yb3+/Er3+ (Figure 4d), leading to quenching effect of the UC luminescence. The quenching effect of BaWO4:Yb3+/Er3+ induced by Au can be reversed by adding H2O2, which can result in the etching of Au NPs. Based on the above analysis, the design strategy for sensing of glucose are shown in Scheme 1.

Figure 5. XPS spectra of Ba, W, O, and Au in BaWO4:Yb3+/Er3+@Au composite nanocrystals. XPS (X-ray photoelectron spectroscopy) analysis provides an additional insight into

the

electronic

environment

and

chemical


composition

of

the

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BaWO4:Yb3+/Er3+@Au composite nanocrystals. Figure S4 (Supporting Information) presents the survey XPS spectrum of the BaWO4:Yb3+/Er3+@Au composite nanocrystals for the HAuCl4 aqueous solution of 10 ml, indicating that Ba, W, O, Yb, Er, and Au coexist in BaWO4:Yb3+/Er3+@Au. The high resolution XPS spectra in Figure 5 and Figure S5 (Supporting Information) demonstrate that the valence states of Ba, O, and Ln are +2, -2, and +3 respectively. Figure 5(a) shows the two strong peaks at 779.5 eV and 795.6 eV are assigned to the Ba 3d5/2 and Ba 3d3/2 binding energy, respectively. In the W 4f XPS spectrum, the four peaks at 34.5, 35.2, 36.7 and 37.6 eV were assigned to the 4f7/2 and 4f5/2 binding energy, indicated that W5+ and W6+ coexist on the surface of BaWO4:Yb3+/Er3+@Au. The peak at ~530.2 eV is from the lattice oxygen, and the peak at ~ 531.5 eV is from the adsorbed oxygen. The Au 4f XPS spectrum contains two peaks, which are attributed to 4f7/2 and 4f5/2. Similarly, the peak centered at 185.4 eV and 185.5 eV correspond to Yb3+ and Er3+, respectively. UC luminescence properties. Figure 6(a) shows the UC luminescence spectra of BaWO4:20%Yb3+/Er3+ nanocrystals under 980 nm excitation. The 4F9/2→4I15/2 (∼660 nm), 4S3/2→4I15/2 (∼553 nm), and 2H11/2 →4I15/2 (∼529 nm) transitions of Er3+ were observed. Obviously, the intensity of the UC luminescence changed with Er3+ concentration increasing. When the Er3+ concentration was 5%, the UC luminescence was the strongest. When the Er3+ concentration was 30%, the UC luminescence completely disappeared. The CIE (International Commission on Illumination) chromaticity coordinates were shown in Table S8 (Supporting Information). The CIE



coordinates were (0.306, 0.427), (0.287, 0.499), (0.315, 0.406), and (0.322, 0.358). Obviously, the CIE coordinates changed with Er3+ concentrations. Figure 6(b) shows the UC spectra of BaWO4:20%Yb3+/5%Er3+ under different excitation power density.

Figure 6. The UC luminescence spectra of (a,b) BaWO4:20%Yb3+/Er3+, (c,d) BaWO4:20%Yb3+/Tm3+, and (e,f) BaWO4:20%Yb3+/Ho3+ under 980 nm excitation. Inset shows the CIE coordinates of the BaWO4:Ln3+.

Figure 6(c) shows the UC spectra of BaWO4:20%Yb3+/Tm3+ with different Tm3+ concentrations under 980 nm excitation. The five emission lines could be assigned to the 1D2→3F4 (∼ 454 nm), 1G4→3H6 (∼ 475 nm), 1G4→3F4 (∼ 652 nm), 3F2,3→3H6


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(∼695 nm), and 3H4→3H6 (∼810 nm) transitions of Tm3+. Inset in Figure 6(c) shows the CIE coordinates of the UC luminescence: (0.243, 0.249), (0.289, 0.292), (0.326, 0.328), (0.333, 0.333), (0.333, 0.334), and (0.331, 0.332) for 1 mol%, 5 mol%, 10 mol%, 20 mol%, 30 mol%, and 40 mol% Tm3+, respectively. Figure S6 (Supporting Information) shows the UC emission spectra of BaWO4:Yb3+/Tm3+ in the range of 580-740 nm. The 3F2,3→3H6 transition was observed when the Tm3+ concentration was 10%. However, the 1G4→3F4 transition was observed then the Tm3+ concentration was 1% or 5%. Figure 6(d) shows the UC spectra of BaWO4:20%Yb3+/1%Tm3+ nanocrystals with different excitation power density. Figure 6(e) shows the UC spectra of BaWO4:20%Yb3+/Ho3+ with different Ho3+ concentrations. Two emission peaks around 547 and 660 nm were detected, which can be attributed to the 5S2/5F4→5I8 and 5F5→5I8 transitions of Ho3+. When the Ho3+ concentration was 5%, the the UC luminescence was the strongest. Inset shows the CIE coordinates: (0.361, 0.372), (0.364, 0.371), (0.349, 0.366), (0.338, 0.340), (0.338, 0.339), and (0.335, 0.339), for 1 mol%, 5 mol%, 10 mol%, 20 mol%, 30 mol%, and 40 mol% Ho3+, respectively. Figure 6(f) shows the UC luminescence spectra of BaWO4:20%Yb3+/5%Ho3+

nanocrystals.

The

emission

intensity

should

be

proportional to the power (n) of the infrared excitation power for an unsaturated UC process, and the power n is the number of infrared photons absorbed per visible photon emitted.41 The values of n for the green emissions were separately determined to be 1.08, 1.23, and 1.03 for 1 mol%, 5 mol%, and 10 mol% Er3+ (Figure 7(a,b)). The values of n for the red emission were separately determined to be 1.46, 1.79, and 1.29



for the 1 mol%, 5 mol%, and 10 mol% Er3+, as shown in Figure S7 (Supporting Information). When the Er3+ concentration is high enough, the cross-relaxation 4F7/2 + 4

I11/2 → 24F9/2 can occur, resulting in the enhanced red emission. Thus, the n value

changes with Er3+ concentration.

Figure 7. Plots (log-log) of emission intensity versus excitation power for (a,b) BaWO4:20%Yb3+/Er3+, (c,d) BaWO4:20%Yb3+/Tm3+, and (e,f) BaWO4:20%Yb3+/Ho3+ nanocrystals.

The values of n for the 1G4→3F4 transition were determined to be 2.49, 1.40, and 1.05 for the 1 mol%, 5 mol%, and 10 mol% Tm3+, respectively (Figure 7(c,d)). For


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the 3H4→3H6 transition, the n values were 2.39, 1.67, and 1.40 for the 1 mol%, 5 mol%, and 10 mol% Tm3+, respectively. Figure 7(e,f) shows the double logarithmic plots of the emission intensity as a function of excitation power for the BaWO4:20%Yb3+/Ho3+ nanocrystals with different Ho3+ concertrations. For the green mission, the n values of BaWO4:20%Yb3+/Ho3+ were separately determined to be 1.44, 1.42, 1.29 and 1.13 for the 1 mol%, 5 mol%, 10 mol% and 20 mol% Ho3+, as shown in Figure 7(e). For the red transitions, the n values of BaWO4:20%Yb3+/Ho3+ were separately determined to be 1.56, 1.62, 1.44, and 1.20 for the 1 mol%, 5 mol%, 10 mol%, and 20 mol% Ho3+, as shown in Figure 7(f). Detection of glucose based on BaWO4:Yb3+/Er3+@Au. The design strategy for glucose detection was shown in Scheme 1. First, the Au nanoparticles were formed and assembled on the BaWO4:Yb3+/Er3+ surface to form BaWO4:Yb3+/Er3+@Au. The fluorescence quenching occurred due to the energy transfer (ET) between BaWO4:Yb3+/Er3+ and Au. The glucose was oxidized by O2 to produce H2O2 when GOx was adding. Then Au was etched and transformed into Au+. And thus, the UC luminescence recovered. In addition, there is not leakage of metal ions during sensing testing. Figure 8(a) shows the UC luminescence spectra of BaWO4:Yb3+/Er3+@Au (powder) core-shell composite nanocrystals with different Au contents. The results indicated that the UC luminescence significant decreased with increasing Au contents, which can be attributed to the LRET from BaWO4:Yb3+/Er3+ (core) to Au (shell). The



influence of the Au contents on the quenching efficiency of BaWO4:Yb3+/Er3+@Au at 529 nm is shown in Figure S9 (Supporting Information). The quenching efficiency reached about 81.4% for the BaWO4:Yb3+/Er3+@Au prepared with 10 mL Au NPs as raw materials. To test the practicability of the obtained BaWO4:Yb3+/Er3+@Au nanosensor, the glucose detection was carried out in optimized conditions. Figure 8(b) shows the UC spectra of BaWO4:Yb3+/Er3+@Au in the presence of different glucose concentrations. The UC luminescence intensity was found to gradually increase with the glucose concentration increasing. And a linear response (R2=0.9717) was obtained over a wide glucose concentration range of 0-0.1336 µM. The linear regression equation is I=114.79C+9.1057, in which I is the emission intensity of the BaWO4:Yb3+/Er3+@Au at 529 nm and C is the concentration of glucose. It is noted that the assay is highly sensitive and 3.1 nM (Figure 8c) glucose could be easily detected. The results suggested that the BaWO4:Yb3+/Er3+@Au is appropriate for glucose detection in aqueous solution.


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Figure 8. (a) UC luminescence spectra of BaWO4:20%Yb3+/5%Er3+@Au (powder) with different Au contents. (b) The UC luminescence evolution of BaWO4:20%Yb3+/5%Er3+@Au with various glucose concentrations. (c) Plot of the UC luminescence intensity of 3+ 3+ 2 BaWO4:20%Yb /5%Er @Au at 529 nm versus the concentration of glucose (R =0.9717). (d) Selectivity of the proposed method for glucose detection. The concentrations are 0.5 mM for glucose, K+, Mg2+, Na+, SO42−, and NO3−.

The selectivity of the BaWO4:Yb3+/Er3+@Au nanosystem toward glucose over some possible interfering species was also tested, including K+, Mg2+, Na+, SO42−, and NO3−. The results indicated that all the species have little effect on the glucose detection, as shown in Figure 8(d). In addition, GOx cannot react with other sugars, such as maltose, tryptophan, and lactose, which have little effect on the detection of glucose. Of course, uric acid and sucrose have effect on the detection of glucose.

CONCLUSIONS In conclusion, BaWO4:Ln3+ nanocrystals with different morphologies have been synthesized. The effect of Ln3+ ions on the morphology of BaWO4 as well as the substitution of Ln3+ ions into BaWO4 has been investigated by the combination of density functional theory calculation and experimental simulation. It is imperative to point out that the Ba site is the most possible location for BaWO4 doped with single species of Ln3+ ions, such as BaWO4:Yb3+, BaWO4:Er3+, BaWO4:Ho3+, and



BaWO4:Tm3+. However, when co-doping Yb3+ with other Ln3+ species into the unit cell of BaWO4, the DFT calculations indicated that the Yb3+ is more possible to engage the W site of BaWO4, while the doped Ln3+ remains its location at the Ba site, with Yb3+ in the near W layer in c axis. Water soluble BaWO4:Yb3+/Er3+@Au with lower detection limit for glucose detection has been successfully developed, using BaWO4:Yb3+/Er3+ as the energy donors and Au as the quenchers through ET process. The limit of detections is found to be 3.1 nM within the linear range of glucose concentration from 0-0.1336 µM. More signifcantly, the BaWO4:Yb3+/Er3+@Au can be conveniently extended to detect many analytes involving H2O2. AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS No foundation was supported. REFERENCES (1) Wu, Y. Q.; Zeng, L. F.; Xiong, Y.; Leng, Y. K.; Wang, H.; Xiong, Y. H. Fluorescence ELISA Based on Glucose Oxidase-Mediated Fluorescence


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List of figure captions Figure 1. The unit cell (a) and crystal structure (b) of bulk BaWO4 with different ionic sites. Figure 2. TEM and HRTEM images of (a,b) BaWO4, (c,d) BaWO4:20%Yb3+/5%Er3+, (e,f) BaWO4:20%Yb3+/20%Er3+, (g) BaWO4:20%Yb3+/30%Er3+, (h) BaWO4:20%Yb3+/5%Tm3+, BaWO4:20%Yb3+/20%Tm3+,

(i)

(j,k)

BaWO4:20%Yb3+/40%Tm3+,

and

(l)

BaWO4:20%Yb3+/30%Ho3+. Figure 3. (a,b) TEM and HRTEM images and (c) TEM mapping of BaWO4:Yb3+/Er3+@Au. Figure 4. (a) XRD patterns BaWO4:Yb3+/Er3+ and BaWO4:Yb3+/Er3+@Au core-shell nanocrystals. (b) EDX and (c) FT-IR analysis of BaWO4:Yb3+/Er3+@Au core-shell nanocrystals. (d) Absorption spectrum of Au nanoparticles exhibiting significant spectral overlap with the emission spectrum of BaWO4:Yb3+/Er3+ under 980 nm excitation. Inset shows the luminescence photograph of BaWO4:Yb3+/Er3+ dissolved in cyclohexane excited at 980 nm. Figure 5. High resolution XPS spectra of Ba, W, O, and Au in BaWO4:Yb3+/Er3+@Au composite nanocrystals. Figure

6.

The

UC

luminescence

spectra

of

(a,b)

BaWO4:20%Yb3+/Er3+,

(c,d)

BaWO4:20%Yb3+/Tm3+, and (e,f) BaWO4:20%Yb3+/Ho3+ under 980 nm excitation. Inset: the CIE 1931 chromaticity coordinates of the BaWO4:Ln3+. Figure 7. Plots (log-log) of emission intensity versus excitation power for (a,b) BaWO4:20%Yb3+/Er3+, (c,d) BaWO4:20%Yb3+/Tm3+, and (e,f) BaWO4:20%Yb3+/Ho3+ nanocrystals. Figure 8. (a) UC luminescence spectra of BaWO4:20%Yb3+/5%Er3+@Au (powder) with different Au contents. (b) The UC luminescence evolution of BaWO4:20%Yb3+/5%Er3+@Au in the presence of various glucose concentrations. (c) Plot of the UC luminescence intensity of BaWO4:20%Yb3+/5%Er3+@Au at 529 nm versus the concentration of glucose (R2=0.9717). (d) Selectivity of the proposed method for glucose detection. The concentrations are 0.5 mM for glucose, K+, Mg2+, Na+, SO42−, and NO3−. F0 was the fluorescence intensity of BaWO4:Yb3+/Er3+@Au quenched by Au shells and F was the recovery fluorescence intensity in the presence of 0.334 µM glucose or other different substance.



Table of Contents (TOC)


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BaWO4:Ln Nanocrystals: Controllable Synthesis, Theoretical

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