Controlled synthesis and luminescent properties of monodispersed
PEI-modified YVO
4
:Bi
3+,Eu
3+nanocrystals by a facile hydrothermal
process†
Yi-Chin Chen, Yun-Chen Wu, De-Yin Wang and Teng-Ming Chen*
Received 8th February 2012, Accepted 22nd February 2012 DOI: 10.1039/c2jm30756a
A series of water-soluble YVO4:Bi3+,Eu3+nanocrystals, with surfaces functionalized by a branch
polyethylenimine (BPEI) polymer, have been synthesized via a one-pot hydrothermal method. It was found that the particle size and crystal morphology could be efficiently controlled by different reaction temperatures, pH values and molecular weights of the BPEI polymer. The surface modification of the nanocrystals was characterized using Fourier transform infrared spectroscopy (FT-IR). The highly crystalline YVO4:Bi3+,Eu3+nanoparticles, with an average diameter of 20 nm, can be dispersed in water
due to the presence of amino ligands. When conjugated with biomolecules, the YVO4:Bi3+,Eu3+
nanocrystals retain their strong red emission, peaking at 619 nm under near-ultraviolet (n-UV) excitation. The results indicate that YVO4:Bi3+,Eu3+nanocrystals can serve as a promising candidate
for biological imaging, and immunoassay applications.
1.
Introduction
Over the past few decades, increasing efforts have been made to explore the use of fluorescent materials for biological applica-tions.1–5Currently, the most common approach is the application
of fluorescent organic molecules and quantum dots (QDs). However, it is generally felt that organic dyes and QDs exhibit problematic weak photostability, a broad emission band, short luminescence lifetimes, photoblinking, and toxicity problems.6,7 These drawbacks limit fluorescent labeling in clinical diagnosis, DNA sequencing and immunoassays. Recently, several fluores-cence probes using lanthanide (Ln)-doped fluoride and oxide nanocrystals have been demonstrated to be effective for biolog-ical detection.8–11The fluoride nanocrystal has the advantages of a low phonon energy and a high up-conversion efficiency when pumped with infrared excitation.12,13 Nevertheless, it is also
characterized as having low chemical stability and low water dispersibility. Accordingly, it is important to start developing oxide nanocrystals for biological applications because of their chemical stability and nontoxic properties.
Yttrium orthovanadate doped with Ln3+ (YVO
4:Ln3+) is
a well-known, highly efficient optical material with a variety of emitting colors, which has been extensively used for lighting,
displays and laser technology.14–16Of the many Ln3+ions, the
Eu3+ion usually shows strong red line emissions due to the parity
forbidden characteristics of the 4f–5d transitions.17–19 The first scientific report on Eu3+-doped YVO
4as a new red phosphor for
cathode ray tubes (CRT) was in 1964.20 The combination of
a relatively red emission (615 nm) under UV excitation, good color purity and high luminescence efficiency (>70%) made this material ideally appropriate for fluorescent applications.21 Recently, a variety of synthesis methods of YVO4:Eu3+
crystalline nanoparticles have been reported, including co-pre-cipitation,15,22 microwave assistance, sol–gel thermolysis,23,24
micro-emulsion,25a hydrothermal/solvothermal process,26–29etc.
Modified syntheses via the hydrothermal/solvothermal method have also been developed in order to improve the size and dispersion of the YVO4nanocrystals.15,17,22–29However, it is still
a challenge to use the surface-functionalized and monodispersed nanocrystals for bio-imaging and drug delivery applications.
In this work, we present a facile one-step hydrothermal route for water-soluble and amino group functionalized YVO4:
Bi3+,Eu3+ nanocrystals, with controllable particle size and
crystal morphology. We also proposed a crystal growth mechanism related to the protonation behavior of the cationic surfactant branch-polyethylenimine (BPEI) polymer in different pH value environments. The relevant properties of YVO4:
Bi3+,Eu3+nanocrystals were investigated using X-ray diffraction
(XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FT-IR) and photoluminescence (PL) spectrometry.
Phosphors Research Laboratory, Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan. E-mail: [email protected]; Tel: +886-3-5731695 † Electronic supplementary information (ESI) available: EDX spectrum of YVO4:Bi3+,Eu3+nanocrystals synthesized with high molecular weight
BPEI polymer under different pH conditions. See DOI: 10.1039/c2jm30756a
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 7961
www.rsc.org/materials
PAPER
2.
Experimental
2.1 Materials and reagents
Y(NO3)3$6H2O (Aldrich), Eu(NO3)3$5H2O (Aldrich),
Bi(NO3)3$5H2O (Merck), NH4VO3 (Showa),
branched-poly-ethylenimine (BPEI) solution (Aldrich, Mw¼ 2000 and Mw ¼
20 000), bovine serum albumin (BSA) (Aldrich), 2-(N-morpho-lino)ethanesulfonic acid (MES) solution (Aldrich), sulfo-N-hydroxysuccinimide (Sulfo-NHS) (Thermo), and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (Thermo) were of analytical grade and were used as received without further purification.
2.2 Synthesis of YVO4:Bi3+,Eu3+nanocrystals
In a typical synthesis, 1 mmol Y(NO3)3$6H2O and the required
amount of Ln(NO3)3$5H2O (Ln¼ Bi and Eu) were dissolved in
10 ml deionized water (DIW) and stirred at room temperature to afford a transparent solution. In another vessel, 1 mmol NH4VO3was dissolved in 10 ml solution of sodium hydroxide at
pH¼ 12, and the aqueous solution was added dropwise to the Ln(NO3)3 solution and stirred at room temperature for 0.5 h
(Y : Bi : Eu : V ¼ 0.7 : 0.15 : 0.15 : 1). Next, 0.04 g ml1BPEI
solution was added to the mixed solution and 1 M NaOH aqueous solution was added dropwise to tune the final pH of the growth solution to 4, 7, and 11. After stirring for 1 h at 80C, the obtained mixture was transferred into a 100 ml Teflon-lined autoclave for a 24 h hydrothermal treatment at 180C and it was then cooled to room temperature naturally. The resulting prod-ucts were collected by centrifugation, washed with distilled water several times, and dried at 80C for 24 h in an oven.
2.3 Conjugation of YVO4:Bi3+,Eu3+nanocrystals with
biomolecules
BSA (10 mg) was dissolved in 100 mM MES buffer (pH¼ 5.9), and the amino group modified YVO4:Bi3+,Eu3+nanocrystals (0.1, 0.5,
1.0 mg) were resuspended in MES buffer. Sulfo-NHS (50 mM) and EDC (50 mM) solution were mixed with BSA solution. The mixture was incubated for 0.5 h at room temperature with vigorous shaking. After incubation, YVO4:Bi3+,Eu3+nanocrystals
were added to the mixture, and reacted for 24 h at 4C overnight. All impurities were removed by centrifugation and the products were washed with phosphate buffered saline (PBS). The samples were electrophoresed on a 10% sodium dodecyl sulfate poly-acrylamide gel electrophoresis (SDS-PAGE). The electrophoresis was performed with a 1X SDS-PAGE running buffer at 90 V for 30 min followed by 120 V for 1.5 h. The SDS-PAGE was stained with a stain buffer containing Coomassie Brilliant blue R-250 for 0.5 h and destained with destain buffer I (methanol–acetic acid– water¼ 4 : 1 : 5, v/v/v) for 20 min and followed by destain buffer II (methanol–acetic acid–water¼ 1.2 : 0.05 : 8.75) overnight. The protein concentrations of the samples were measured using a bicinchoninic acid (BCA) protein assay kit (Aldrich).
2.4 Characterization
The phase purity of the obtained products was analyzed by powder X-ray diffraction (XRD) using a Bruker AXS D8
advanced automatic diffractometer with Cu-Ka radiation (l ¼ 1.5405 A, 40 kV 20 mA). The surface components and composition of the samples were measured on a high resolution X-ray photoelectron spectrometer (PHI Quantera SXM; ULVAC-PHI). The particle sizes of the products were analyzed by a Brookhaven 90 plus dynamic light scattering (DLS) particle size analyzer equipped with a 35 mW He–Ne laser. All DLS measurements were carried out with a wavelength of 632.8 nm at 25 C with a 90 angle of detection. The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FE-SEM; JSM-7401F, JEOL) and a field emission gun transmission electron microscope (FEG-TEM; JEM-2100F, JEOL) with a Link ISIS 300 energy disper-sive X-ray analyzer (EDX). The selected area electron diffraction (SAED) patterns were obtained using FEG-TEM operating at 200 kV. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet Avatar 320 infrared spec-trometer using the KBr pellet technique. The photoluminescence (PL)/photoluminescence excitation (PLE) spectra and transient decays were recorded on a Spex FluoroLog-3 spectrofluorometer equipped with a 450 W Xe lamp and cutoff filters to avoid the second-order emissions of the source radiation.
3.
Results and discussion
3.1 Crystal structure and morphology
Fig. 1 shows the collected XRD data of the as-synthesized YVO4:Bi3+,Eu3+ nanocrystals with BPEI polymer of different
molecular weights, synthesized at pH¼ 4, 7 and 11, respectively. To ensure the phase purity, the diffraction peaks of the YVO4:
Bi3+,Eu3+nanocrystals are indexed on the basis of single crystal
YVO4data (ICSD No. 78074), with a tetragonal structure. The
results show a single phase with no unidentified diffraction peaks from impurities. In the crystal structure of YVO4, there is only
one crystallographically distinct site for the Y atoms. For the YVO4:Bi3+,Eu3+nanocrystals, as both the Eu3+and Y3+ions have
the same valence and atomic radius, there are no charge compensation issues for isovalent substitution. The calculated lattice parameters of a¼ b ¼ 7.15 A and c¼ 6.31 A are also consistent with the standard lattice contents a¼ b ¼ 7.12 A and c ¼ 6.29 A.30 As a whole, the sharp diffraction peaks of the
different nanocrystalline YVO4:Bi3+,Eu3+ samples suggest that
high crystallinity can be obtained using the BPEI polymer with different molecular weights at a relatively low reaction temper-ature (180C) during hydrothermal treatment.
XPS has been proven to be a successful technique for quali-tatively determining the surface components and composition of the sample. Fig. 2a shows the photoelectron survey spectrum of the YVO4:Bi3+,Eu3+ nanocrystals with the binding energy
ranging from 0 to 1100 eV. It can be seen that the YVO4:
Bi3+,Eu3+ nanocrystal contains Y, V and O elements, with no
other extant elements except for carbon. The carbon results from the hydrothermal process, in which some additives were employed. Fig. 2b and 2c show the respective element data for the XPS measurements. Two splitting lines: Y 3d5/2(1160.6
eV), and Y 3d3/2(162.6 eV), were observed in the binding energy
of the Y 3d states. Otherwise, the V 2p states were split into V 2p3/2(519.9 eV) and V 2p1/2(527.4 eV) with an energy splitting
of D ¼ 7.5 eV, due to the spin–orbit interactions. The doped elements: Bi 4f (166.8 eV) and Eu 4d (136.6 eV), were also detected in the YVO4:Bi3+,Eu3+nanocrystals.
Fig. 3 shows the SEM characterization of the particle size and morphology for the as-synthesized YVO4:Bi3+,Eu3+nanocrystals
using the BPEI polymer with a low molecular weight (Mw ¼
2000). To obtain insight into the influence of the polymer, we also investigated the particle using TEM. The sample (pH ¼ 4) synthesized with the low molecular weight polymer was composed of many irregularly shaped sheets. With the pH value increasing to 7, the sample starts to form regular granular-like particles with an average diameter of20 nm, and weak aggre-gation. When the pH value increases to 11, it could be seen from the SEM images that the obtained particles show an irregular rod-like shape. The TEM images show similar results compared to the SEM images; it is evident that most of the particles are affected by different pH conditions. This will be discussed later. As shown in Fig. 3b, 3d and 3f, the samples synthesized at pH ¼ 4 and 7 consist of single crystalline nanoparticles, whereas the sample synthesized at pH¼ 11 is made up of several gran-ular-like crystalline nanoparticles. The YVO4:Bi3+,Eu3+
nano-crystals using the BPEI polymer with a high molecular weight (Mw ¼ 20 000) at different pH values are also investigated in
Fig. 4. The as-prepared sample (pH¼ 4) is mainly composed of square nanoplates 50 nm thick. The SEM results for the
YVO4:Bi3+,Eu3+nanocrystal in acidic conditions show a typical
tetragonal-like morphology of the YVO4crystal, which can be
ascribed to the tetragonal I41/amd crystal structure. With
a further increase of the pH value to 7, the sample shows a granular-like morphology, which is similar to the sample using the low molecule weight BPEI polymer. The particle size esti-mated from the number average was also20 nm. When the pH value is increased to 11, the primary nanoparticles tend to agglomerate and form olive-like polycrystalline nanorods that are, on average,100 nm in length. In a sense, this suggests that the particle size and morphology of the samples (pH¼ 7 and 11) are not influenced by the molecular weight of the polymer;
Fig. 1 XRD patterns of YVO4:Bi3+,Eu3+ nanocrystals prepared by
a hydrothermal method using (a) low molecular weight BPEI polymer, (b) high molecular weight BPEI polymer at pH ¼ 4, 7, and 11. The standard data for tetragonal YVO4 (ICSD No. 78074) was used as
reference.
Fig. 2 XPS spectra of YVO4:Bi3+,Eu3+nanocrystal in the range of (a) 0–
1100 eV, (b) 150–170 eV, and (c) 515–535 eV.
however, the use of a high molecular weight polymer does lead to better uniformity and dispersion.
The TEM images also confirm the above results, as can be seen in Fig. 4b, 4d and 4f. According to the SEM and TEM results, the following discussion will focus on the samples synthesized using the BPEI polymer with a high molecular weight. The size and polydispersity of the as-prepared samples were characterized by DLS measurement. Fig. 5a demonstrates an illustration of the quantity of two dispersions for the tetragonal particle, i.e.,54.1 nm for height,338.2 nm for length and width, consistent with the observed size from the TEM image. Fig. 5b and 5c each show only one population: one has a peak diameter of20.3 nm, and the other has a peak diameter of98.2 nm. These results are also consistent with the observed sizes from the TEM images.
As shown in Fig. 6, the HRTEM images and the SAED patterns of the individual nanocrystal were used to study the crystallinity from its diffraction pattern. The HRTEM images show a perfect crystallite surface, in which the distances between adjacent lattice fringes are estimated for each sample. The calculated 0.35 and 0.46 nm lattice spacings, corresponding to the (200) and (101) interplanar distance of the samples synthe-sized at pH¼ 4 and 11, infer the preferential growth direction. Also, the sample synthesized at pH¼ 7 clearly demonstrates that the lattice fringes corresponding to the (220) and (200) planes were separated by 0.25 and 0.35 nm. The results can be indexed to diffractions from the (101), (200) and (220) planes of the tetragonal structure, in accordance with the XRD patterns. The sharp spots in the SAED patterns of the samples synthesized at
pH ¼ 4 and 7 exhibit highly ordered lattice ordering, which suggests the single crystalline nature of the YVO4:Bi3+,Eu3+
crystalline nanoparticles. Furthermore, the annular spots of the SAED patterns of the sample synthesized at pH ¼ 11 clearly verify the polycrystalline nature of the YVO4:Bi3+,Eu3+
crystal-line nanoparticles. The compositions of the obtained samples are confirmed by EDX analysis. Fig. S1† shows the EDX patterns of the YVO4:Bi3+, Eu3+ nanocrystals, which reveal the
homoge-neous distribution of Y/V/O/Bi/Eu in the solid. The details of the
Fig. 3 SEM and TEM images of YVO4:Bi3+,Eu3+nanocrystals
synthe-sized at 180C for 24 h and with pH values of (a,b) pH 4, (c,d) pH 7, and (e,f) pH 11. All samples were prepared with the low molecular weight BPEI polymer by hydrothermal treatment.
Fig. 4 SEM and TEM images of YVO4:Bi3+,Eu3+nanocrystals
synthe-sized at 180C for 24 h and with a pH value of (a,b) pH 4, (c,d) pH 7, and (e,f) pH 11. All samples were prepared with the high molecular weight BPEI polymer by hydrothermal treatment.
Fig. 5 DLS analysis of YVO4:Bi3+,Eu3+nanocrystals synthesized with
the high molecular weight BPEI polymer under different pH conditions. (a) pH 4, (b) pH 7, and (c) pH 11.
EDX spectra are summarized in Table S1.† It should be noted that the Bi atomic percentage of the sample (pH¼ 4) is much lower than those of the others.
To characterize the surface properties, the FT-IR absorption spectra of the YVO4:Bi3+,Eu3+nanocrystals were measured. The
bulk YVO4:Bi3+,Eu3+sample synthesized by solid-state reaction
was also presented as a comparison. As shown in Fig. 7, a strong absorption band at810 cm1and a weak band at451 cm1are
attributed to the V–O (from the VO43 group) and the Y–O
stretching vibrations of the host lattice, respectively. The broad band at3407 cm1corresponds to the O–H or N–H stretching vibrations, while the bands at 1633 and 1385 cm1are related to the bending vibrations of the N–H bond in the BPEI polymer. The weak bands at 2923 and 2849 cm1can be assigned to the
asymmetric and symmetric stretching vibrations of the –CH2in
the BPEI polymer. The details of the peak assignments for the samples prepared at pH¼ 4, 7 and 11 are presented in Table 1. These results indicate that the as-prepared YVO4:Bi3+,Eu3+
nanocrystals contain amino groups on the surfaces, which play an important role in conjugation with biomolecules for bio-applications.
3.2 Mechanism of crystal growth
In this work, the BPEI polymer acts as a chelating and protective agent that can further influence the size and morphology of the final products because the nitrogen atoms in the main and side chains of the polymer can serve as electron donors for chelating the metal ions. Therefore, due to the stronger chelating ability of the high molecular weight BPEI polymer, it is able to bind tightly to the surface of the nanoparticles, and hence control the
Fig. 6 HRTEM images and SAED patterns of YVO4:Bi3+,Eu3+
nano-crystals synthesized with high molecular weight BPEI under different conditions. (a,b) pH 4, (c,d) pH 7, and (e,f) pH 11.
Fig. 7 Comparison of the FT-IR spectra for (a) bulk YVO4:Bi3+,Eu3+prepared by the solid-state method; YVO4:Bi3+,Eu3+nanocrystals synthesized
with high molecular weight BPEI polymer under different pH conditions: (b) pH 4, (c) pH 7, and (d) pH 11.
growth of the YVO4:Bi3+,Eu3+ nanocrystals, and disperse the
nanoparticles.
In addition, the effects of the pH values on the size and morphology were investigated. First, it is noted that the protonation behaviors of the BPEI polymer at different pH values have been reported.31–33When the pH value decreases to a low range, the degree of protonation increases, and the polymer tends to form a loose structure with a rich positive charge. The deprotonated amines form a complex with the lanthanide metal ions, which are revealed on the crystal surface via coordination, and each protonated amino group is highly restricted for coor-dination to a metal ion because of strong neighboring electro-static repulsion. Therefore, the BPEI polymer acts as a capping agent to adsorb on the specific crystal facet, and restricts the crystal growth in order to form sheet- or square-like morphol-ogies, in this case. On the other hand, at a high pH range, the BPEI polymer tends to shrink from the reduced solvation power of the protonated polymer. Most of the deprotonated amines coordinate to metal ions, and the strong neighboring interactions of the polymer molecules cause the aggregation of crystal seeds, and form polycrystalline nanoparticles.
3.3 Spectroscopic characterization of the YVO4:Bi3+,Eu3+
nanocrystals
Fig. 8 shows the PLE/PL spectra of the YVO4:Bi3+,Eu3+
nano-crystals synthesized at pH¼ 4, 7 and 11. As can be seen from the PLE spectra (monitored at 619 nm), the absorption band peak-ing at285 nm for the YVO4:Bi3+,Eu3+nanocrystals synthesized
at pH¼ 4, 7 and 11 is attributed to the charge transfer from the oxygen ligands to the central vanadium atom within the VO43
group. The UV energy absorbed by the VO43groups is
subse-quently transferred to Eu3+by a nonradiative transfer process in
all of the samples. Noticeably, the band edges of the excitation band of the samples prepared at pH¼ 7 and 11 extend from 375 nm to 425 nm and the maximum of the excitation band shifts to 345 nm. These observations suggest that the extension of the excitation band originated from the metal–metal charge transfer transitions from the Bi3+to the V5+ion rather than the 6s2–6s6p
transition of Bi3+; then, the energy is transferred to the Eu3+ion.34
The sharp excitation band at 395 nm corresponds to the7F
0–5L6
transition within the 4f6configuration of the Eu3+ions.
The PL spectra of the YVO4:Bi3+,Eu3+ nanocrystals
synthe-sized at pH¼ 4, 7 and 11 show a typical linear feature for the Eu3+emission, peaking at 619 nm, and ranging from 580 nm to
730 nm, which can be ascribed to the radiative transitions from the 5D
0 to the 7FJ (J ¼ 1, 2, 3, 4) levels of the Eu3+ ion. By
comparing the maximum emission intensity at 619 nm, the obtained YVO4:Bi3+, Eu3+nanocrystals prepared at pH¼ 4 show
a much lower emission intensity than those prepared at pH¼ 7 and 11. In the present synthesis, the Bi(NO3)3reacted with water
and formed the intermediate product BiONO3, which further
reacted with VO3and generated Bi3+-doped YVO4precursors.
When the reaction is in acidic conditions, the formation of the intermediate product is regarded as restricted because of the inert
Table 1 The detailed FT-IR spectral peak assignments for (Y0.7Bi0.15Eu0.15)VO4NPs under different pH conditions
Assignment Band position/cm1 Annealed pH 4 pH 7 pH 11 N–H stretching vibration — 3407 3405 3407 CH2asymmetric stretching — 2923 2923 2923 CH2symmetric stretching — 2849 2849 2849 CH2deformation vibration — — — 1467 N–H bending vibration — 1633/1385 1633/1385 1633/1385 C–N stretching vibration — 1144 — —
V–O stretching vibration 815 800 813 809
Y–O stretching vibration 451 451 451 451
Fig. 8 (a) PLE spectra of YVO4:Bi3+,Eu3+nanocrystals. (b) PL spectra
of YVO4:Bi3+,Eu3+nanocrystals. The inset shows the relative integrated
PL intensity (lex¼ 345 nm) of the YVO4:Bi3+, Eu3+nanocrystals.
reactivity of the Bi3+ions. When the pH value rises to 11, the
polycrystalline nanoparticles show relatively weak emission in comparison to the YVO4:Bi3+,Eu3+nanocrystals synthesized at
pH ¼ 7, presumably because of the large surface area of the polycrystalline nanorods which introduces more defects into the YVO4:Bi3+,Eu3+nanocrystals. These defects provide
non-radia-tive recombination pathways as fluorescence quenching centers, and thus cause a decrease in the photoluminescence intensity. Fig. 9 shows the luminescence of the YVO4:Bi3+, Eu3+
nano-crystals under ambient light, 254 and 365 nm excitations. The sample prepared at pH¼ 7 exhibits a strong red emission under UV excitation.
The representative decay curves for the luminescence of the YVO4:Bi3+,Eu3+ nanocrystals are shown in Fig. 10. The decay
kinetics behavior depends on the number of different lumines-cent lumines-centers, defects, energy transfer and impurities in the host. In the case of the YVO4:Bi3+,Eu3+nanocrystals, the Eu3+ions
occupy the sites of the Y3+ions with D
2dsymmetry, implying
only one luminescence center. The room temperature decay dynamic monitors the5D
0–7F2 transitions at 619 nm for Eu3+
Fig. 9 The photos show the luminescence of different YVO4:Bi3+,Eu3+
nanocrystals under (a) daylight, (b) 254 nm, and (c) 365 nm excitations. Samples are prepared by hydrothermal methods at pH¼ 4 (sample 1), pH¼ 7 (sample 2), and pH ¼ 11 (sample 3), respectively. All samples are suspended in DIW.
Fig. 10 Photoluminescence decay curves for Eu3+emission (5D
0–7F2) under 345 nm and 395 nm excitations for YVO4:Bi3+,Eu3+nanocrystals. Samples
were prepared by the hydrothermal method at (a,b) pH¼ 4, (c,d) pH ¼ 7, (e,f) pH ¼ 11.
under 345 and 395 nm excitations. The excitation peaks at 345 and 395 nm correspond to the excitation band of Bi3+and the 7F
0–5L6 sharp line excitation of the Eu3+f–f transition,
respec-tively. The obtained decay curve can be well-fitted by the first-order exponential function, I ¼ Aexp(t/s), where I is the luminescence intensity at time t, A is a constant, and s is the decay lifetime for the exponential components. The measurement results of the luminescence lifetime of the as-synthesized samples at different pH values were reported with an accuracy (R2) of 95–
99%. For the 619 nm emission at a 345 nm excitation, the luminescence lifetimes are calculated to be 313.64, 1092.25 and 605.62ms, which correspond to the samples prepared at pH ¼ 4, 7 and 11, respectively. Moreover, the lifetime values are deter-mined to be 420.42, 751.59 and 470.46 ms for the samples prepared at pH¼ 4, 7 and 11, at a 395 nm excitation, respec-tively. The values are in accordance with those given in the literature for the bulk (525 ms)20and nanocrystalline (740ms)35 materials. It can be observed that the sample prepared at pH¼ 7 exhibits a longer decay time than the others. We suggest that the increase in the lifetime is due to the decrease in the surface effects, as well as an improvement of the crystallinity. The luminescence of Eu3+on the nanocrystallite surface sites is easily quenched by
combinations of solvent, or capping molecules. The poly-crystalline product prepared at pH ¼ 11 shows strong surface effects, leading to a decrease of the decay time value. The sample prepared at pH¼ 4 shows a much faster decay time of the Eu3+ 5D
0–7F2emission (619 nm) under the 345 nm excitation of Bi3+.
We assume that most of the Bi3+ions exhibit low reaction activities
to produce the YVO4:Bi3+,Eu3+nanocrystals at pH¼ 4, and the
concentration of Bi3+may involve the energy transfer rate from
Bi3+to the Eu3+ion, correlated to the decay lifetime value.
3.4 Biomolecule conjugation
The surface-modified nanophosphors synthesized at pH¼ 7 can be well dispersed in DIW, and they show strong fluorescence properties. The surface amino groups provided from BPEI are activated by EDC, together with Sulfo-NHS, and can then covalently conjugate to the carboxyl groups of biomolecules. In our report, the obtained YVO4:Bi3+,Eu3+ nanocrystals were
conjugated with BSA in order to evaluate the binding ability related to the conjugated biomolecules. The successful conjuga-tion of BSA with YVO4:Bi3+,Eu3+nanocrystals was proven by
SDS-PAGE and a BCA protein assay (Fig. 11). Lane 1 is pure BSA before conjugation, and the un-conjugated BSA (superna-tant) can be separated from the BSA-YVO4:Bi3+,Eu3+
nano-crystal conjugates (pellet) by centrifugation treatment. It can be seen that the concentration of supernatant BSA decreases, while the YVO4:Bi3+,Eu3+nanocrystal concentration increases from 0.1
mg ml1to 1.0 mg ml1. The quantity of conjugated BSA is determined by the difference in the UV absorbance values of the BCA solution before and after conjugation. This reveals that about 20 mg of BSA is associated per mg of YVO4:Bi3+,Eu3+
nanocrystals (Table 2).
4.
Conclusions
In this work, we have successfully synthesized YVO4:Bi3+,Eu3+
nanocrystals with surface functionalization via a one-step BPEI-modified hydrothermal method. The water-based system provides a simple, solvent-free and safe green chemical synthesis route for nanomaterial preparation. The crystal structures, morphological evolution, photoluminescence properties and surface character-ization were studied in detail. The crystal morphologies and particle sizes reveal diversities through a series of controlled experimental parameters, such as the reaction pH value and the concentration of capping agent. The obtained YVO4:Bi3+,Eu3+
YVO4:Bi3+,Eu3+ nanocrystals show a strong red luminescence
(619 nm) under near-ultraviolet (n-UV) excitation. The surface amino groups contributed from the capping agent BPEI not only improve the dispersibility and water/buffer stability of the YVO4:Bi3+,Eu3+nanocrystals, but can also specifically conjugate
with such biomolecules as folic acid, DNA or antibodies, for further applications in bioimaging, cell labeling and drug delivery. Moreover, the long luminescence lifetime and photoluminescence stability of the YVO4:Bi3+,Eu3+nanocrystals overcome the
limi-tations of organic dyes. In conclusion, the YVO4:Bi3+,Eu3+
nanocrystals with good water/buffer dispersibility, surface func-tionalization and long-lived red luminescence exhibit good potential for applications in bioimaging, DNA hybridization, fluorometric immunoassays and drug delivery.
Acknowledgements
This research was supported by the National Science Council of Taiwan (R.O.C.) under Contract no. NSC98-2113-M-009-005-MY3. We thank Dr Yu-Kuo Wang for assistance in the helpful suggestions.
Notes and references
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0 (lane 1), 0.1 (lane 2), 0.5 (lane 3), and 1.0 mg ml1(lane 4) of YVO
4:
Bi3+,Eu3+nanocrystals.
Table 2 The correlation between the YVO4:Bi3+,Eu3+NC concentration
and the conjugated BSA concentration
NCs/mg ml1 [BSA]/mg ml1
1 0 44.9
2 0.1 44.4
3 0.5 30.9
4 1.0 20.0
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