Improving upconversion emission of NaYF4:Yb3+, Er3+ nanoparticles by coupling Au nanoparticles and photonic crystals: The detection enhancement of Rhodamine B

Article abstract of DOI:10.1016/j.jallcom.2019.02.331

The upconversion (UC) nanoparticles (NPs) based sensor exhibits the potential application for the detection of dyes, heavy metal ions and DNA et al. However, the application of upconversion NPs based sensor was limited due to its low UC luminescence intensity. In the present work, the SiO₂ opal photonic crystals were prepared by the self-assembly technology. After infiltration of SiO₂ opal by the transparent chloroauric acid solution and the following sintering at the high temperature, the Au NPs/opal hybrids were prepared. The Au NPs/opal hybrids result in the enhancement of excited electric field, which can be served as the template of UC emission enhancement of NaYF₄:Yb³⁺, Er³⁺ NPs exited at the 980 nm. The corresponding maximum UC emission enhancement was about 17-fold. The detection of Rhodamine B was realized by monitoring the changing of green UC intensity of NaYF₄:Yb³⁺, Er³⁺ NPs. In comparison with pure NaYF₄:Yb³⁺, Er³⁺ NPs, the sensor sensitivity of NaYF₄:Yb³⁺, Er³⁺ NPs deposited on the Au NPs/opal hybrid was increased by about the 35-factors, which exhibits a low detection limit of 164 μM and a high sensitivity of 1.15 μM^?1. The present work demonstrated that this solid sensor exhibits potential applications in heavy metal ions and DNA et al.

Full text of DOI:10.1016/j.jallcom.2019.02.331

Accepted Manuscript
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Improving upconversion emission of NaYF :Yb , Er nanoparticles by coupling Au
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nanoparticles and photonic crystals: The detection enhancement of Rhodamine B
Yingjin Ma, Jialun Zhu, Zhengwen Yang, Hailu Zhang, Jianbei Qiu, Zhiguo Song
PII:
S0925-8388(19)30803-5
DOI:
https://doi.org/10.1016/j.jallcom.2019.02.331
JALCOM 49763
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 4 December 2018
Revised Date: 18 January 2019
Accepted Date: 28 February 2019
Please cite this article as: Y. Ma, J. Zhu, Z. Yang, H. Zhang, J. Qiu, Z. Song, Improving upconversion
3+
3+
emission of NaYF :Yb , Er nanoparticles by coupling Au nanoparticles and photonic crystals:
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The detection enhancement of Rhodamine B, Journal of Alloys and Compounds (2019), doi: https://
doi.org/10.1016/j.jallcom.2019.02.331.
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Improving upconversion emission of NaYF4:Yb3+, Er3+ nanoparticles by coupling
Au nanoparticles and photonic crystals: the detection enhancement of
Rhodamine B
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Yingjin Ma1, Jialun Zhu2, Zhengwen Yang1*, Hailu Zhang1, Jianbei Qiu1, Zhiguo Song1
1College of Materials Science and Engineering, Kunming University of Science and Technology,
Kunming, 650093, China; 2Department of Nuclear Medicine, Tumor Hospital of Yunnan
Province, Kunming Medical University, Kunming, 650118, China
*Corresponding Email: yangzw@kmust.edu.cn; Jialun Zhu is the co-first author
Abstract: The upconversion (UC) nanoparticles (NPs) based sensor exhibits the
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potential application for the detection of dyes, heavy metal ions and DNA, et al.
However, the application of upconversion NPs based sensor was limited due its low
UC luminescence intensity. In the present work, the SiO2 opal photonic crystals were
prepared by the self-assembly technology. After infiltration of SiO2 opal by the
transparent chloroauric acid solution and the following sintering at the high
temperature, the Au NPs/opal hybrids were prepared. The Au NPs/opal hybrids result
in the enhancement of excited electric field, which can be served as the template of
UC emission enhancement of NaYF4:Yb3+, Er3+ NPs exited at the 980 nm. The
corresponding maximum UC emission enhancement was about 17-fold. The detection
of Rhodamine B was realized by monitoring the changing of green UC intensity of
NaYF4:Yb3+, Er3+ NPs. In comparison with pure NaYF4:Yb3+, Er3+ NPs, the sensor
sensitivity of NaYF4:Yb3+, Er3+ NPs deposited on the Au NPs/opal hybrid was
increased by about the 35-factors, which exhibits a low detection limit of 164 ꢀM and
a high sensitivity of 1.15 ꢀM-1. The present work demonstrated that this solid sensor
exhibits potential applications in heavy metal ions and DNA, et al.
Keyword: Upconversion luminescence; NaYF4:Yb3+, Er3+ nanoparticles; Rhodamine
B sensing; Au nanoparticles/opal hybrids

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1. Introduction
With the development of industries and economic, the organic rhodamine B dye
(RhB) was widely used in the food, printing, textile and water tracer fluorescent [1-4].
At present, lots of investigations demonstrated that the RhB has toxicity and
carcinogenicity to human health and can pollute the water source [5-7]. However, the
RhB is still applied in the textile and food, etc. because of its low cost and bright
coloration [8, 9]. Thus, the detection of RhB in various fields such as the food and
water is very important and necessary. Now, for the detection method of RhB, the
high-performance liquid chromatography, surface-enhanced resonance Raman
scattering, electrochemistry and ultraviolet-visible spectrophotometry method were
extensively used [10-12]. These detection approaches are complex and their
applications were limited.
Upconversion (UC) luminescent nanoparticles (NPs), which convert long
wavelength photons with low energy into a short wavelength photon with high energy,
have aroused much attention due to their wide applications in the various fields such
as the biological imaging, therapeutics and solar cells, etc. [13-16]. In recent years,
the UC NPs exhibits the potential application in the sensors for the detection of dyes,
heavy metal ions and DNA, et al [17-21]. However, the UC luminescent process
mainly arises from the 4f-f forbidden transition of rare earth (RE) ions, resulting in a
lower UC emission efficiency due to the small absorption and emission cross section.
For example, the NaYF4:Yb3+, Er3+ NPs are considered as one of the most efficient
UC materials in the previous investigations. However, its UC efficient is only up to

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0.3%, which limit their applications as the sensor. Thus, it is significant to improve
the UC luminescence of NaYF4:Yb3+, Er3+ NPs. To solve the problems, many methods
such as synthesis of core-shell structure, host selection and surface coating et al have
been developed to improve the UC emission of NaYF4:Yb3+, Er3+ NPs. Located
surface plasmons resonance (LSPR) of metal NPs such as Au and Ag has been widely
used to enhance UC luminescence of NaYF4:Yb3+, Er3+ NPs in previous investigations
[22-24]. For the UC luminescent enhancement based on the LSPR of metal NPs, it is
well-known that the 980 nm excitation light or UC luminescence wavelength is
needed to overlap with the LSPR of Au or Ag NPs to obtain the larger UC
luminescent enhancement of NaYF4:Yb3+, Er3+ NPs. However, in general, the LSPR
of Au or Ag NPs is in the visible light region. It is relatively difficulty for the
preparation of Au or Ag NPs with the LSPR of 980 nm. Therefore, a new approach
based on the coupling between photonic crystals and metal NPs with the visible LSPR
were developed in order to obtain larger UC luminescent enhancement [25, 26]. For
example, the inverse opal photonic crystal including Ag NPs with the visible LSPR of
Au or Ag NPs was applied to improve the UC luminescence of NaYF4:Yb3+, Er3+ NPs.
However, the NaYF4:Yb3+, Er3+ NPs can be entered into the macrospore in the inverse
opal structure, which cause the suppression the UC luminescent. Thus, the maximum
enhancement factor of UC luminescence was only 6.3 factor [27]. Additionally, there
have few investigations on the detection of Rhodamine B by the UC NPs/Au/PC
hybrids. In this work, the SiO2 opal photonic crystals (OPCs) were prepared by
self-assembly technology. After infiltration of OPCs by the transparent chloroauric

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acid solution and the following sintering at the high temperature, the Au NPs/OPCs
hybrids were obtained. The NaYF4:Yb3+, Er3+ NPs were deposited on the surface of
Au/OPCs hybrids. The influence of coupling metal NPs with photonic crystals on the
UC luminescence of NaYF4:Yb3+, Er3+ NPs has been investigated. The detection of
RhB was investigated by monitoring the changing of the green UC intensity of UC
NPs/Au/PC hybrids. The low detection limit and high detection sensitivity of Rh B
was obtained by the UC NPs/Au/OPCs hybrids.
2. Experimental
2.1 Synthesis of NaYF4:20%Yb3+, 2%Er3+ NPs.
High purity (99.99%) Yb2O3, Y2O3, Er2O3, and analytical reagent grade
1-octadecylene, NaOH, oleic acid, NH4F were used as raw materials to synthesize
NaYF4:Yb3+, Er3+ NPs. The hexagonal phase NaYF4:Yb3+, Er3+ NPs were prepared by
co-precipitation method according to the literatures [28]. Firstly, the RE2O3 (RE3+=
Y3+, Yb3+ and Er3+) were dissolved in the hot HCl solution to form corresponding
RECl3 compounds, and then the RECl3 compounds were dissolved in deionized water
to form the 0.2 mol/L RECl3 solution. The 1.56 mL YCl3, 0.4 mL YbCl3 and 0.04 mL
ErCl3 was added into the mixture of 12 mL 1-octadecylene and 8 mL oleic acid in a
three-necked flask. The mixture was heated to 150 oC for 40 min to remove water
with the argon gas under the stirring. After cooling to room temperature, the 2 mL
NaOH and 6.6 mL NH4F were added into the mixture, which was heated to 50 oC for
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30 min. Subsequently, the solution was heated to the 100 C for 10 min to remove
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methanol, then it was heated at the 300 C for 50 min under the argon shield. The

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resulting NaYF4:Yb3+, Er3+ NPs was obtained by centrifuging three times and
dispersing in cyclohexane.
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2.2 Fabrication of NaYF4:Yb3+, Er3+ NPs/Au/OPC hybrids.
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Monodisperse SiO2 microspheres with diameter of 250, 300 or 460 nm were
used to fabricate three-dimensional ordered OPCs on the quartz substrates through
vertical deposition method as the previous work [29]. The OPCs prepared by the 250,
300 or 460 nm SiO2 microspheres were denoted as the OPC-I, OPC-II and OPC-III,
respectively. The voids of OPC-I, OPC-II and OPC-III were infiltrated by using the
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0.05M HAuCl4 solution, respectively. After sintering at 450 C for 4h in air furnace,
the Au NPs and OPC-I~III composites were obtained, denoting as the Au/OPC-I,
Au/OPC-II and Au/OPC-III, respectively. For the comparison, the same HAuCl4
solution was deposited on the pure quartz substrate without the OPCs and was
sintered at the same temperature and time, which was denoted as the Au/Q. The
as-prepared NaYF4:Yb3+, Er3+ NPs were spin-coated on the pure quartz (denoted as
Q), Au/Q, Au/OPC-I~III substrates, and the NaYF4 NPs/Au/OPC-I~III hybrids were
obtained. Scheme 1 shows the preparation processes of NaYF4 NPs/Au/OPC-I~III
hybrids.
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Scheme 1 the preparation processes of NaYF4 NPs/Au/OPC hybrids
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The RhB was dissolved in the various volume ethanol to form 1.75*10-4,

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3.5*10-4, 5.25*10-4, 7*10-4 and 8.75*10-4 M solutions, respectively. Then, the 2 ꢀL
RhB solution with the different concentration was added into the NaYF4
NPs/Au/OPC-III hybrids and dried on the oven for the detection investigation of RhB.
2.3 Structure and optical characterization.
The crystal phase and morphology of NaYF4:Yb3+, Er3+ NPs were detected by an
X-ray diffraction diffractometer quipped with Cu Kα radiation and transmission
electron microscopy (TEM, JEOL 2100), respectively. The morphology of OPC-I~III,
Au/OPC-I~III and NaYF4 /Au/OPC-I~III hybrids were measured using a field
emission scanning electron microscopy (FESEM, QUANTA 650). To further
investigate the distribution of Au NPs on the OPC-I~III, the transmission electron
microscopy and energy dispersive X-ray analysis image of Au/OPC OPC-I~III
hybrids were obtained by transmission electron microscopy (TEM, JEOL 2100). The
absorption spectra were taken using a HITACHIU-4100 spectrophotometer. The UC
emission spectra of samples were characterized by a HITACHI F-7000
spectrophotometer with an excitation of 980 nm.
3. Results and discussions
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3.1 Morphology and absorption property of NaYF4 NPs/Au/OPC-I~III hybrids
Figure 1 (a) is the XRD pattern of NaYF4 NPs. It is clearly that all peaks are
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well-indexed to the standard PDF card (JCPDS No. 16-0334), indicating that the
prepared NaYF4 NPs are pure hexagonal phase. Figure 1 (b) shows the TEM image of
NaYF4 NPs, depicting the monodisperse and inerratic morphology of NaYF4 NPs
with average diameter of 25 nm. The high resolution TEM image of NaYF4 NPs
exhibits the clear lattice fringes as shown in Figure 1 (c). The space of two adjacent

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lattice fringes is 0.3 nm, which is consistent with the (110) plane of hexagonal NaYF4.
The selected area electron diffraction (SAED) pattern was observed in Figure 1 (d).
The diffraction rings are marked as (200), (111), (201), (210) and (002), which further
demonstrated that the NaYF4 NPs is the hexagonal phase.
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Figure 1 the XRD pattern (a), TEM image (b), high resolution TEM image (c) and selected area
electron diffraction pattern (d) of the prepared NaYF4: Yb3+, Er3+ NPs.
The SEM image of OPC-I~III were presented in the Figure 2 (a) and the Figure
S1~2 of supporting information, respectively. The silica microspheres form an ordered
face centered cubic (fcc) arrangement with the close packed 111 plane parallel to Q
substrate. After sintering of the silica OPCs with the infiltration of HAuCl4 solution,
the structure of Au/ OPC-I~III hybrids was characterized by the SEM, as shown in
Figure 2 (b) and Figure S1~2. The bright spots in the Fig. 2 (b) and Fig. S1~2 of
supporting information are the Au NPs with the irregular shape, and the homogeneous
distribution of Au NPs was observed in the positions of OPCs voids. The size

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distribution histogram of Au NPs was shown in the insert of Fig. 2 (b). It is clear that
the sizes of Au NPs were in the region of 10-100 nm and average size was about 53
nm. The TEM images and EDX pattern of Au/OPC-I~III hybrids were used to further
investigate the formation of Au/OPC-I~III hybrid structures and existence of Au NPs,
as shown in Fig. 2 (c) and Fig. S1~2. The silica microspheres surface attached lots of
small continuous bright spots (such as in the red circles) and some clear bigger bright
spots. The larger Au island NPs and a layer of Au continuous film were formed on the
surface of silica microspheres. These bright spots were verified as the Au element by
the EDX pattern (Fig. 2 (c)). The SEM micrograph of Au NPs on the Q substrate was
showed in the Figure S3 (a) of supporting information. It’s noted that the size of Au
NPs is in the region of 60-200 nm. Compared with the Au NPs on the OPC-I~III, the
size of Au NPs on the Q substrate is lager, which suggested that SiO2 microspheres of
OPCs limited the growth of Au NPs. Figure 2 (d)-(f) and Figure S1~2 exhibited the
SEM images of NaYF4: Yb3+, Er3+ NPs deposited on surface of OPC-I~III,
Au/OPC-I~III and Q substrate, respectively. It is obvious that the surface and voids of
OPC-I~III, Au/OPC- I~III and Q substrate were covered by a dense single layer of
NaYF4:Yb3+, Er3+ NPs.

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Figure 2 the SEM images of OPC-III (a) and Au/OPC-III hybrids (b); the insert in the (b) is the
size distribution histogram of Au NPs; the EDX pattern (c) of the Au/OPC-III hybrids; the insert in
the (c) is the TEM image of Au/OPC-III hybrids; the SEM images of NaYF4:Yb3+, Er3+ NPs
deposited on the OPC-III (d), Au/OPC-III substrate (e) and Q substrate (f).
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The absorption spectrum of Au NPs on the Q substrate is shown in the Figure S3
(b). It is clear that an obvious peak centered at 530 nm was exhibited in the absorption
curve, which arises from the LSPR of Au NPs. Figure 3 presented the absorption
spectra of OPC-I, OPC-II, OPC-III without and with Au NPs. Absorption curves of
OPC-I~III showed an obvious photonic band gaps located at 561, 672 and 996 nm,
respectively, which were overlapped with the green and red UC emission peaks of
Er3+ and excitation wavelength of 980 nm, respectively. It is noted that the silica
microsphere size would affect the position of photonic band gap of OPCs.
Theoretically, Bragg’s law: λ=1.633D(n2eff - sin2θ) could be used for calculating the
position of photonic band gaps. According to the equation, the photonic band gaps of
OPC-I~III are calculated as the 550, 659 and 1011 nm, respectively. Position of
photonic band gap shifts to the long wavelength with the increasing size of silica
microsphere. The calculation value of photonic band gap is consistent with the
measurement value. After sintering OPC-I~III samples with the infiltration of HAuCl4

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solutions, the photonic band gaps position of Au/OPC-I~III hybrids shifted slightly to
the shorter wavelength, and the photonic band gaps position of Au/OPC-I~III hybrids
were located at the 538, 650 and 970 nm, respectively. The blue shift of photonic band
gap may be due to the shrinking of microspheres and refractive index changing of
Au/OPC-I~III structure. As can be seen in the Figure 3, the absorption curve of
Au/OPC-II~III exhibited another peak centered at about 520 nm in contrast to the
OPC-II~III, arising from Au NPs surface plasmon absorption. For the position of
photonic band gap, OPC-I substrate is consistent with the LSPR of Au NPs, thus the
absorption curve of Au/OPC-I only showed an absorption peak at the 538 nm.
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Figure 3 Absorption spectra of OPC-I~III (a) and Au/OPC-I~III hybrids (b)
3.2 The UC emission enhancement of NaYF4:Yb3+, Er3+ NPs by the
Au/OPC-I~III hybrids
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The UC emission spectra of NaYF4: Yb3+, Er3+ NPs deposited on the various
templates were shown in Figure 4 excited at the 980 nm. All the UC
photoluminescence spectra exhibited the obvious green emission peaks at 525, 545
nm and a red emission peak at 660 nm of Er3+ ions, which arise from the electronic

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transition of H11/2→4I15/2, S3/2→4I15/2 and F9/2→4I15/2, respectively. There is an
exponential relationship between UC emission intensity (I) and pump power (P) in the
UC emission process of NaYF4:Yb3+, Er3+ NPs, which can be expressed as the I=Pn.
The n value in the equation is the number of absorbed infrared photons from ground
state to excited state required for an UC visible photon emission. Figure 5 is a log-log
spectrum of UC emission intensity as the function of the pump power on the Q, Au/Q,
OPC-III and Au/OPC-III substrates. The dependence of pump power and UC
luminescence of NaYF4:Yb3+, Er3+ NPs on the OPC-I, Au/OPC-I, OPC-II and
Au/OPC-II substrate are presented in Figure S4. It is clear from the Figure 5 and
Figure S4 that the n value of NaYF4:Yb3+, Er3+ on the four templates for 545 and 660
nm reveals that the two-photon UC emission processes were involved for the green
and red UC emissions. To better understand UC emission mechanisms of NaYF4:Yb3+,
Er3+ NPs under the excitation of 980 nm, the energy transfer between Yb3+ ions and
Er3+ ions was shown in Figure 6. Ground state Yb3+ ions absorb 980 nm excitation
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light, resulting in the transition from the F7/2 to excited F5/2 state. The absorption
cross section of Yb3+ ions at 980 nm excitation light are larger than Er3+ ions. The
Yb3+ ions served as sensitizer transfer their absorbed energy to adjacent Er3+ ions, and
the Er3+ ions are excited from I15/2 ground state to excited I11/2 state. Subsequently,
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the higher F7/2 excited state of Er3+ ions are populated due to the successive energy
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transfer from the Yb3+ ions to Er3+ ions in the excited 4I11/2 state. The 4F7/2 excited state
of Er3+ ions are non-radiatively relaxed to the 2H11/2 and 4S3/2 states. Excited Er3+ ions
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in the H11/2 and S3/2 state are jumped back to I11/2 state, leading to green UC

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emission at the 525 and 545 nm, respectively. As the electrons of Er3+ ions in the 4I11/2
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state undergo non-radiative relaxation to the lower I13/2 state, the I13/2 state of Er3+
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ions can be excited to the F9/2 state due to the energy transfer of Yb3+ ions.
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Additionally, the F9/2 state of Er3+ ions are populated by the non-radiative relaxation
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from higher the H11/2 and S3/2 states, The radiative transition from the 4F9/2 excited
state to 4I15/2 ground state of Er3+ ions result in the red UC emission located at the 660
nm.
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Figure 4 the UC emission spectra of NaYF4:Yb3+, Er3+ NPs deposited on the surface of Q, Au/Q,
OPC-I, Au/OPC-I (a), Q, Au/Q, OPC-II, Au/OPC-II (b) and Q, Au/Q, OPC-III, Au/OPC-III (c)
under the excitation of 980 nm.
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Figure 5 the dependence of pump power of 980 nm excitation light on the 545 and 660 nm UC
emission intensity of NaYF4:Yb3+, Er3+ NPs deposited on Q (a), Au/Q (b), OPC-III (c) and
Au/OPC-III (d) substrates.
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Figure 6 the UC emission mechanism of NaYF4:Yb3+, Er3+ NPs upon the 980 nm excitation.
The UC photoluminescence of NaYF4:Yb3+, Er3+ NPs on various substrate
presented the different emission intensity. The UC emission intensity on the various
substrates was enhanced in comparison with that of NaYF4:Yb3+, Er3+ NPs on the Q
substrate. The corresponding enhancement factor (EF) of UC emission has been
obtained, as shown in the Figure 7. The EF in Figure 7 (a) is the ratio of UC emission
intensity of NaYF4:Yb3+, Er3+ NPs on the Au/Q, OPC-I~III substrate to that of
NaYF4:Yb3+, Er3+ NPs on the Q substrate, respectively. Owing to the substrates coated
by the NaYF4:Yb3+, Er3+ NPs with the same amount. Thus the influence of thickness
and quantity of NaYF4:Yb3+, Er3+ NPs on the UC emission intensity could be avoided.
As seen in Figure 7, the EF of green and red UC emission on the OPC-I is 2.6 and 1.7,
respectively, and the corresponding EF values of red and green emission of
NaYF4:Yb3+, Er3+ NPs on the OPC-II substrate is 2.8 and 2, respectively. For the
OPC-III substrate, the EF values at green and red emission were about 4. It can be
seen that the UC emission of the NaYF4:Yb3+, Er3+ NPs was enhanced on the surface
of photonic crystal, attributing to the excitation enhancement caused by the silica
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microspheres induced the excitation light scattering. The electric field intensity of
OPC-I~III substrates along the polarization of X-axis and Y-axis was simulated with
the excitation light of 980 nm, showing the enhancement of electric field intensity, as
shown in Figure 8 and Figure S5. The EF values of green and red UC emissions of
NaYF4:Yb3+, Er3+ NPs on the surface of OPC-III are larger than these of NaYF4:Yb3+,
Er3+ NPs on the OPC-I~II. The photonic band gap of OPC-III is overlapped with the
980 nm excitation light, which can couple well with the excitation light of 980 nm.
Thus the larger enhancement of excitation field can obtained, resulting in the larger
UC emission enhancement on the OPC-III. Selective green UC enhancement of
NaYF4:Yb3+, Er3+ NPs on the OPC-I was presented in contrast to the red UC emission.
For the OPC-II substrate, the red UC emission enhancement of NaYF4:Yb3+, Er3+ NPs
is larger than its green UC emission. Photons cannot be propagated in the wavelength
range of photonic band gap. The selective enhancement of UC of NaYF4:Yb3+, Er3+
NPs on the OPC-I and OPC-II is due to the Bragg diffraction of photonic band gap
[24, 30].
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Figure 7 the UC emission enhancement factor of NaYF4:Yb3+, Er3+ NPs on the Au/Q, OPC-I,
OPC-II and OPC-III substrates (a) and Au/OPC-I, Au/OPC-II and Au/OPC-III substrates (b);
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The UC emission EF of 545 and 660 nm of NaYF4:Yb3+, Er3+ NPs on the Au/Q
substrate increased by 1.8 and 2.6 folds in contrast to the Q substrate, respectively. It’s
well-known that the LSPR of Au NPs can improve UC luminescence intensity due to
the emission rate increasing or excitation field enhancement [31, 32]. The
investigation of the enhancement mechanism of NaYF4 NPs on the Au/OPC hybrid
was carried out. The decay curves of NPs on the Q and Au/Q substrates were shown
in the Figure 9. The decay lifetimes of the UC emission of NaYF4 NPs on the Q and
Au/Q substrates exhibited no change, indicating decay rate increasing has no effect on
the UC emission of NaYF4 NPs on the Au/Q substrate. The electric field intensity and
distribution of the Au/Q substrate along the polarization of X-axis and Y-axis was
simulated by finite-difference time-domain (FDTD) software excited at 980 nm as
shown in Figure 8 (d) and Figure S5. The hot spots were formed around the Au NPs
regarding of the polarization direction. These results indicated that the enhancement
of UC emission on the Au/Q template could be from the electric field enhancement
around Au NPs. The relationship between the UC intensity (I) and the excitation field
intensity(E) can be expressed as the I∝En, where the n is the photons number. The n
value of red UC emission is larger that of green UC emission, as shown in Figure 5.
Thus the enhancement factor of red UC emission is larger than that of the green UC
emission.

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Figure 8 the simulated electric field of OPC-I (a), OPC-II (b), OPC-III substrate (c), Au/Q
substrate (d) and Au/OPC-III hybrids (e) along the polarization of X-axis under the excitation of
980 nm.
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The UC emission spectra of NaYF4 NPs/Au/OPC-I~III hybrids presented in the
Fig. 4 exhibited the further UC emission enhancement comparing with these of
NaYF4:Yb3+, Er3+ NPs on OPC-I~III or Au/Q substrates. The corresponding EF were
shown in the Figure 7 (b). The EF of green and red UC emissions on the surface of on
the Au/OPC-I hybrid is 11.7 and 11.4, respectively. On the Au/OPC-II, the EF of
green and red UC emissions is 9.5 and 16 folds, respectively. For Au/OPC-III hybrid,
the EF of green and red UC emission of NaYF4:Yb3+, Er3+ NPs is 12.11 and 16.8 folds,
respectively. It is clear that the UC emission intensity of NaYF4:Yb3+, Er3+ NPs on the
Au/OPC-I~III hybrid is larger than the total sum on the OPC and Au/Q substrate,
which indicated that the coupling of OPC and Au NPs improve greatly the UC
emission intensity in comparison with separate OPC or Au/Q substrate. As shown in

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Figure 9 (b), the decay lifetimes of the green and red UC emission of NaYF4 NPs on
the Au/OPC-I~III hybrids have no significant difference in contrast to the OPC and
Au/Q substrate, which indicated the enhanced UC emission on the Au/OPC-I~III
hybrids was not from the emission rate increasing. As presented in Figure 8 (e), for
Au/OPC hybrid structure, the simulated electric field distribution and intensity is
larger in compared with separated OPC or Au NPs substrates. Thus, the UC emission
enhancement of NaYF4:Yb3+, Er3+ NPs on the Au/OPC-I~III hybrids arise mainly
from the enhancement of excited electric field.
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Figure 9 Decay curves of 545 (a) and 660 nm (b) UC emission of NaYF4:Yb3+, Er3+ NPs on the Q,
Au/Q, OPC-III and Au/OPC-III substrates.
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3.3 Detection enhancement of RhB by the NaYF4/Au/OPC-III hybrid structure
The UC emission of NaYF4:Yb3+, Er3+ NPs on the prepared Au/OPC-I~III could
be improved, which can solve its low UC emission. The NaYF4/Au/OPC-III NPs can
served as a solid-state sensor to detect the RhB through the change of UC emission
intensity of NaYF4:Yb3+, Er3+ NPs. Figure 10 depicts the absorption spectra of the
RhB on the Q and NaYF4:Yb3+, Er3+/Au/OPC-III hybrid substrates, respectively. The
absorption peak at the 556 nm is from the absorption of the RhB, which is overlapped

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with the green UC emission peak of NaYF4:Yb3+, Er3+ NPs. Thus, the energy transfer
from the NaYF4:Yb3+, Er3+ NPs to the RhB may take place.
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Figure 10 the absorption spectra of RhB on the Q and NaYF4/Au/OPC-III hybrids
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The UC emission of NaYF4:Yb3+, Er3+ NPs on the Q and Au/OPC-III hybrid
substrates as the function of RhB concentration are shown in Figure 11 (a) and (b)
under the excitation of 980 nm, respectively. For the NaYF4:Yb3+, Er3+ NPs on the Q
and Au/OPC-III hybrid substrates, the decreasing of green UC emission and no
changing of red UC emission was observed with increasing the RhB concentration. It
is interesting that for the RhB on the NaYF4:Yb3+, Er3+/Au/OPC-III hybrids, the
emission peak located at the 581 nm form the RhB was observed. However, the 581
nm emission peak of the RhB was not observed obviously for the RhB on the
NaYF4:Yb3+,Er3+/Q substrate. The direct contact between the Au NPs and RhB can
influence the luminescence of RhB, which can result in the luminescence quenching
or enhancement. In present work, the luminescence enhancement of RhB was
observed. If the luminescence enhancement of RhB is from the influence of Au NPs,
the decreasing of green UC emission cannot observed with increasing the RhB

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concentration, which is inconsistent with the results. Therefore, the influence of the
direct contact between the Au NPs and RhB on the luminescence quenching or
enhancement of RhB can be removed. The green UC emission is overlapped with the
absorption of RhB, which demonstrated the energy transfer from the NaYF4:Yb3+,
Er3+ NPs to the RhB can occur. Therefore, the green UC emission intensity was
decreased because of the energy transfer from the NaYF4:Yb3+, Er3+ NPs to the RhB.
The UC emission enhancement of NaYF4:Yb3+, Er3+ NPs induced by the Au/OPC-III
hybrids enhance improve the energy transfer from NaYF4:Yb3+, Er3+ NPs to RhB.
Thus the 581 nm emission of the RhB was observed for the RhB on the NaYF4:Yb3+,
Er3+/Au/OPC-III hybrid in contrast to the NaYF4:Yb3+, Er3+/Q substrate.
The green UC emission intensity of the NaYF4:Yb3+, Er3+ NPs on the Q and
Au/OPC-III hybrid substrates are shown in Figure 11 (c) and (d) as the function of the
RhB concentration, respectively. For the NaYF4:Yb3+, Er3+ NPs on the Q substrate,
the linear relationship coefficient of R2 of the green UC emission intensity is 0.975
with the RhB concentration from 0 to 0.875 mM and the sensitivity is 0.033 ꢀM-1.
The linear relationship coefficient of higher R2 of the green UC emission intensity of
NaYF4:Yb3+, Er3+ NPs on the Au/OPC-III hybrids is 0.997 with a sensitivity of 1.15
ꢀM-1. The results suggested that the NaYF4 NPs with the UC emission as a solid-state
sensor can be used for detecting RhB. The UC emission enhancement of NPs caused
by Au/OPC-III hybrids improve the detection sensitivity of RhB. For the sensor, the
limit of detection (LOD) is one of the essential sensing parameters besides the
sensitivity [33, 34]. The LOD can be expressed by the 3s/m equation, where the s and

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m is the standard deviation and the slope of line plotted by the ratio of luminescence
intensity of acceptor (RhB) to that of donor(NaYF4:Yb3+,Er3+) [17, 35], respectively.
For the RhB on the NaYF4:Yb3+, Er3+/Q substrate, there was no the 581 nm emission
of the RhB. Therefore, the LOD could not be obtained. For the RhB on the
NaYF4:Yb3+, Er3+/Au/OPC-III hybrid, the 581 nm emission intensity of the RhB was
increased with the increasing of RhB concentration. The result means that the NaYF4
NPs on the Au/OPC hybrid is beneficial to detect the RhB. To obtain the LOD
parameter, the ratio of 581 nm emission intensity (I581) RhB to the 545 nm green UC
emission intensity (I545) is calculated as the function of the RhB concentration on the
Au/OPC-III hybrid as shown in Figure 11 (e). The I581/ I545 ratio was increased with
RhB concentration increasing from 0.175 to 0.875 mM and show the good linear
relationship. The LOD is calculated as the 164 ꢀM.

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Figure 11. the UC emission of NaYF4:Yb3+, Er3+ NPs deposited on the Q (a) and Au/OPC-III
substrates (b) as the function of RhB concentration; corresponding linear relationship of green UC
emission intensity with the increasing of RhB concentration on the Q (c) and Au/OPC-III substrate
(d); the calibration curve of I581/I545 versus RhB concentration on NaYF4/Au/OPC-III hybrid (e);
the stability curves of RhB/NaYF4/Au/OPC-III hybrid (f).
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For the RhB detection, it is necessary to investigate the photo-stability of the
sensor. Figure 11 (f) is the changing of green UC emission intensity of
RhB/NaYF4/Au/OPC-III as the function of the time under the continuous illumination

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of a 980 nm laser. The green emission intensity at 0 min is denoted as 1. It is clear that
the green UC emission intensity is not more than 1.05 under the continuous
illumination of a 980 nm laser for 1 h, exhibiting the unchanging. The result indicated
the NaYF4 NPs/Au/OPC-III hybrid is stable as a solid-state sensor of the RhB
detection, which may be used for the detecting RhB in water or food.
Conclusions
The hybrids constructed with the Au NPs and SiO2 opal photonic crystals were
prepared through a simple approach. NaYF4:Yb3+, Er3+ NPs were deposited on the
surface of Au/opal hybrids. The influence of coupling metal NPs with photonic
crystals on the UC luminescence of NaYF4:Yb3+, Er3+ NPs has been investigated. The
enhanced UC emission of NaYF4:Yb3+, Er3+ NPs excited at 980 nm can be observed
based on the coupling of Au NPs and photonic crystals, and the maximum UC
emission enhancement was about 17-fold. The UC luminescence enhancement on the
Au/OPC hybrid mainly arise from the enhancement of excited electric field. The
NaYF4 UC luminescence NPs can served as a solid-state sensor to detect the RhB
through the change of UC emission intensity of NaYF4:Yb3+, Er3+ NPs. Comparing
with the pure NaYF4:Yb3+, Er3+ NPs, the sensor of NaYF4:Yb3+, Er3+ NPs on the
Au/OPC hybrid was improved, existing a low detection limit of 164 ꢀM and a high
sensitivity of 1.15 ꢀM-1. For the NaYF4:Yb3+, Er3+ NPs deposited on the Au/opal
hybrids, the detection sensitivity of RhB was increased by about 35-factors.
Acknowledgments
This work was supported by the National Natural Science Foundation of China

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(51762029, 11674137), and the Applied Basic Research Key Program of Yunnan
Province (2018FA026).
References
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Highlights
1.The Au nanoparticles/opal photonic crystal hybrid was prepared by sintering
method.
2.Upconversion emission enhancement of NaYF4 nanoparticels has been largely
improved.
3.The detection of Rhodamine B by NaYF4:Yb3+, Er3+ nanoparticles was realized.
4.The sensor sensitivity of NaYF4 on the Au nanoparticles/opal hybrid was increased.