Upconversion-based nanosystems for fluorescence
sensing of pH and H
2
O
2
†
Chunning Sun *and Michael Gradzielski *
Hydrogen peroxide (H
2
O
2
), a key reactive oxygen species, plays an important role in living organisms,
industrial and environmental fields. Here, a non-contact upconversion nanosystem based on the
excitation energy attenuation (EEA) effect and a conventional upconversion nanosystem based on the
joint effect of EEA and fluorescence resonance energy transfer (FRET) are designed for the fluorescence
sensing of H
2
O
2
. We show that the upconversion luminescence (UCL) is quenched by MoO
3x
nanosheets (NSs) in both systems due to the strong absorbance of MoO
3x
NSs in the visible and near-
infrared regions. The recovery in UCL emissions upon addition of H
2
O
2
enables quantitative monitoring
of H
2
O
2
. Benefiting from the non-contact method, hydrophobic OA-NaYF
4
:Yb,Er can be used as the
luminophore directly and ultrahigh quenching efficiency (99.8%) is obtained. Moreover, the non-contact
method exhibits high sensitivity toward H
2
O
2
with a detection limit of 0.63 mM, which is lower than that
determined by simple spectrophotometry (0.75 mM) and conventional upconversion-based
nanocomposites (9.61 mM). As an added benefit, the same strategy can be applied to the sensing of pH,
showing a broad pH-responsive property over a range of 2.6 to 8.2. The successful preparation of
different upconversion-based nanosystems for H
2
O
2
sensing using the same material as the quencher
provides a new design strategy for fluorescence sensing of other analytes.
Introduction
Hydrogen peroxide (H
2
O
2
), an important bioactive molecule in
living systems, plays an essential role in the physiological
process including signal transduction, cell proliferation,
differentiation, and maintenance.
1,2
Abnormal production or
accumulation of H
2
O
2
will lead to severe damage to DNA and
proteins, causing a series of serious diseases,
3–7
such as dia-
betes, Alzheimer's and Parkinson's disease, cardiovascular
disorders, and even cancer. Additionally, H
2
O
2
is widely used as
a bleaching agent and sterilant in industrial and environmental
elds,
8,9
such as food processing, drinking water treatment,
packaging, and organic pollutant degradation. However, expo-
sure to high concentrations of H
2
O
2
is a great threat to organ-
isms.
10,11
Therefore, quantitative detection of H
2
O
2
is of great
importance for monitoring its potential risk.
Optical methods via uorescence changes have attracted
considerable attention, as the uorometric approach is a non-
destructive method that can be simply and rapidly performed
with high sensitivity and selectivity.
12
In contrast to conven-
tional uorescence probes (such as organic dyes, carbon
nanomaterials, and semiconductor quantum dots), upconver-
sion nanoparticles featuring large anti-Stokes shis, excellent
chemical- and photo-stability, sharp multicolor emissions, and
low toxicity have been regarded as a promising class of
luminophores.
13
Up to now, a variety of functional materials including
organic dyes,
14–16
noble metals,
17–19
quantum dots,
20–22
carbon
nanomaterials,
23–25
and two-dimensional materials
26–28
has been
employed to couple with upconversion nanoparticles to
construct uorescence probes, realizing quantitative detection
of inorganic ions,
29–31
pH,
32–34
small molecules,
35–38
and nucleic
acids.
39–42
Most of the upconversion-based probes rely on the
uorescence resonance energy transfer (FRET) process, in
which a very short distance between the upconversion nano-
particles and absorbers is required. Moreover, in order to obtain
high-sensitivity detection, high-quality upconversion nano-
particles with strong emission and high upconversion efficiency
are employed, which are commonly prepared by applying oleic
acid (OA) as the ligand. The oleate-capped upconversion
nanoparticles are hydrophobic and prone to disperse in
nonpolar solvents, whereas hydrophilic upconversion nano-
particles are required for typical sensing applications of
Stranski-Laboratorium f¨
ur Physikalische und Theoretische Chemie, Institut f¨
ur
Chemie, Technische Universit¨
at Berlin, Strasse des 17. Juni 124, 10623 Berlin,
Germany. E-mail: chunning.sun@campus.tu-berlin.de; michael.gradzielski@
tu-berlin.de
†Electronic supplementary information (ESI) available: XRD pattern of
OA-UCNPs, TEM images of ligand-free and PEI-UCNPs, FT-IR spectra of OA-,
ligand-free, and PEI-UCNPs, zeta potential of bare UCNPs, PEI-UCNPs, and
MoO
3x
NSs, absorbance of MoO
3x
NSs solution at 980 nm with different pH
and addition of different H
2
O
2
concentrations at pH 4.5, UCL spectra of
PEI-UCNPs in the absence and presence of MoO
3x
NSs solution in the
non-contact mode, and UCL spectra of PEI-UCNPs under excitation at 980 nm
with different power. See DOI: 10.1039/d0na01045f
Cite this: Nanoscale Adv.,2021,3,2538
Received 15th December 2020
Accepted 18th March 2021
DOI: 10.1039/d0na01045f
rsc.li/nanoscale-advances
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interest. Therefore, the hydrophobic-to-hydrophilic transition
of upconversion nanoparticles is essential.
43
Herein, we propose different upconversion nanosystems for
H
2
O
2
sensing using MoO
3x
nanosheets (NSs) as the energy
acceptor based on either the excitation energy attenuation (EEA)
effect or the joint effect of the EEA and FRET, owing to the
strong absorbance of MoO
3x
NSs in both visible and near-
infrared (NIR) regions. By coupling of MoO
3x
NSs solution
and oleate-capped NaYF
4
:Yb,Er upconversion nanoparticles
(abbreviated as OA-UCNPs) solution, a EEA-based upconversion
nanosystem for sensing of H
2
O
2
in the non-contact mode is
designed, where MoO
3x
NSs act as the energy acceptor of the
incident light for the activation of UCNPs. Additionally, this
system can be used for pH sensing as well. Beneting from the
non-contact method, hydrophobic OA-UCNPs can be used
directly for the sensing and ultrahigh quenching efficiency
(99.8%) can be reached. Meanwhile, by the integration of
hydrophilic UCNPs and MoO
3x
NSs, we are able to prepare
conventional upconversion-based nanocomposites for H
2
O
2
sensing via the joint effect of the EEA and FRET, where MoO
3x
NSs act as the energy acceptor of not only the 980 nm exciting
light for UCNPs but also uorescence emissions of UCNPs. To
the best of our knowledge, this is the rst upconversion-based
nanoprobe for the sensing of one analyte by two different
systems while using the same material as an energy acceptor.
Experimental section
Materials
Yttrium(III) acetate tetrahydrate (99.9%), ytterbium(III) acetate
hydrate (99.9%), erbium(III) acetate hydrate (99.9%), MoO
3
(99.95%) were purchased from Alfa Aesar, 1-octadecene (ODE,
90%), oleic acid (OA, 90%), sodium hydroxide (NaOH, $98%),
ammonium uoride (NH
4
F, $98%), methanol (99.8%), cyclo-
hexane (99.5%), ethanol ($99.8%), formic acid ($98%), poly-
ethylenimine (PEI, branched, M
w
25 000) were obtained from
Sigma-Aldrich. Milli-Q water (18.2 MUcm at 25 C) was used in
all experiments.
Characterization
Fourier transform infrared (FT-IR) spectra were recorded in
transmission mode on a Thermo Scientic Nicolet iS5 FT-IR
spectrometer with the KBr method. X-ray photoelectron spec-
troscopy (XPS) was measured with a Thermo Fisher Scientic
ESCALAB 250Xi instrument. Transmission electron microscopy
(TEM) and energy-dispersive X-ray spectroscopy (EDS) were
performed on the FEI Tecnai G2 20 S-TWIN with a LaB
6
cathode
operated at 200 kV. UV-vis absorption spectra were acquired on
a CARY 50 spectrophotometer. Powder X-ray diffraction (XRD)
measurements were performed on a Philips X'Pert MPD Pro X-
ray diffractometer at a scanning rate of 4min
1
in the 2qrange
from 10to 80(Cu Karadiation, l¼0.15406 nm). z-Potential
measurements were carried out on an Anton Paar Litesizer™
500 instrument. Upconversion luminescence (UCL) emission
spectra were obtained on a ber-coupled spectrometer (Ocean
HDX, Ocean Optics) with an external 980 nm continuous-wave
(CW) laser (0–5 W, Roithner Lasertechnik GmbH) at room
temperature (RT). Quartz cuvettes (0.7 mL, 10 mm 2 mm light
path) were used for UV-vis absorption and UCL measurements.
Synthesis of MoO
3x
NSs
MoO
3x
NSs were prepared according to the previous publica-
tion with minor modications.
44,45
In a typical process, 1.5 g
bulk MoO
3
powder was ground with 0.3 mL of acetonitrile for
30 min and then added to a water/ethanol solution (25 mL, v/v ¼
1/1). The dispersion was then probe-sonicated for 2 h at 100 W
(Branson Digital Sonier W-250D) at a 5 s ON and 2 s OFF pulse.
To avoid overheating of the solvent, the beaker lled with MoO
3
dispersion was immersed in an ice bath during sonication. The
light blue supernatant containing a high concentration of MoO
3
NSs (denoted as S-MoO
3
NSs) was collected via centrifugation at
7000gfor 30 min. For the preparation of MoO
3x
NSs, the
supernatant dispersion was lled into a quartz glass vial and
irradiated with a UV lamp (254 & 365 nm, 15 W) for 5 h, dark
blue MoO
3x
NSs solution was nally obtained, and the MoO
3x
NSs solution was then diluted to 2 mg mL
1
by water and
ethanol (v/v ¼1/1) solution, and stored at 4 C for further use.
Synthesis of OA-UCNPs
As previously reported, the synthesis of oleate-capped NaYF
4
:
20 mol% Yb, 2 mol% Er was carried out by employing OA as
ligand via a high-temperature coprecipitation method.
46
Briey,
in a 100 mL round ask, 3.12 mL of Y(CH
3
COO)
3
(0.2 M), 0.8 mL
of Yb(CH
3
COO)
3
(0.2 M) and 0.8 of mL Er(CH
3
COO)
3
(0.02 M)
were mixed with 6 mL of OA and 14 mL of ODE at RT. The
mixture solution was rst heated to 110 C for 30 min to evap-
orate the water and then heated to 160 C for 40 min to form
lanthanide-oleate complexes, followed by cooling down to
50 C. A methanolic solution (10 mL) containing 3.2 mmol of
NH
4
F and 2.0 mmol of NaOH was slowly added and then stirred
at 50 C for 30 min. Aer evaporating the methanol, the solution
was heated to 310 C at a rate of 10 C min
1
and maintained for
30 min under nitrogen atmosphere. Aer cooling down to RT,
OA-UCNPs were precipitated out with the addition of excess
ethanol, collected aer washing three times with the ethanol,
and nally dissolved in cyclohexane for further use.
Preparation of ligand-free UCNPs
Ligand-free UCNPs were prepared using our previously reported
method.
47
5 mmol of formic acid was directly added to 2 mL of
cyclohexane solution containing 20 mg of OA-UCNPs, ligand-
free UCNPs were precipitated out aer shaking for 10 s at
3000 rpm on a vortex mixer. Bare UCNPs were obtained aer
centrifugation and washing once with ethanol and three times
with water and nally dissolved in water.
Synthesis of UCNPs/MoO
3x
nanocomposites
To synthesize UCNPs/MoO
3x
nanocomposites, PEI-capped
UCNPs (abbreviated as PEI-UCNPs) was rst prepared. Typi-
cally, 4 mL ligand-free UCNPs solution (5 mg mL
1
) were added
to a vial containing 4 mL PEI solution (10 mg mL
1
), followed by
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overnight stirring. PEI-UCNPs were collected aer centrifuga-
tion at 16 000gfor 30 min and washing three times with water,
and nally dispersed in water with a concentration of 1 mg
mL
1
. UCNPs/MoO
3x
nanocomposites were prepared by mix-
ing 0.5 mL PEI-UCNPs solution with an appropriate amount of
MoO
3x
NSs solution, the mixture was rst shaken for 3 min
(3000 rpm) on a vortex mixer and then ultrasonicated for 5 min.
UCNPs/MoO
3x
nanocomposites were then collected by centri-
fugation at 7000gfor 30 min, washed three times with water,
and redispersed in water.
Non-contact uorescence sensing of pH
To detect pH in the non-contact mode, OA-UCNPs dispersed in
cyclohexane with a concentration of 1 mg mL
1
were sealed in
a quartz cuvette, the cuvette was then aligned with the other
cuvette containing 1 mg mL
1
MoO
3x
NSs solution with
different pH. The pH was adjusted by either 50 mM NaOH or
50 mM HCl ethanol/H
2
O (v/v ¼1/1) solution. The cuvette con-
taining MoO
3x
NSs was put in front of the other one containing
OA-UCNPs solution, and the UCL spectra were collected under
the excitation of a 4 W 980 nm CW laser.
Non-contact uorescence sensing of H
2
O
2
The non-contact sensing procedure for the H
2
O
2
was similar to
that of the non-contact pH sensing, except that MoO
3x
NSs
were dissolved in acetate buffer (50 mM, pH 4.5, ethanol/H
2
O, v/
v¼1/1) with different concentrations of H
2
O
2
.
Fluorescence sensing of H
2
O
2
by UCNPs/MoO
3x
nanoassemblies
To detect H
2
O
2
, 0.5 mg mL
1
of UCNPs/MoO
3x
aqueous solu-
tion (0.35 mg mL
1
MoO
3x
NSs) and different concentrations
of H
2
O
2
(0.4 mL) were added to 0.1 mL acetate buffer (50 mM,
pH 4.5, DMF/H
2
O, v/v ¼1/1), The mixture was then incubated at
RT for 2 h, and the UCL spectra were measured under the
excitation of a 4 W 980 nm CW laser.
Results and discussion
Design principle of upconversion-based nanosystems for
H
2
O
2
and pH
The design strategy of UCNPs/MoO
3x
nanocomposites for
uorescence sensing of H
2
O
2
is based on the modulation of
MoO
3x
NSs-induced reduction in UCL emissions by H
2
O
2
through the joint effect of EEA and FRET. In contrast, the pH
and H
2
O
2
dual-responsive upconversion-based nanosystem is
realized by the direct adjustment of the excitation energy for
UCNPs in the non-contact mode (Fig. 1a).
Without modications, UCNPs give rise to green and red
luminescence emissions under 980 nm excitation. Aer the
reduction of MoO
3
by UV light, the oxygen-decient MoO
3x
NSs exhibit strong absorption in both visible and NIR regions,
overlapping well with the UCL emissions of UCNPs and the
excitation wavelength for UCNPs of 980 nm (Fig. 1b). Owing to
the strong NIR absorption of MoO
3x
NSs attached on UCNPs,
the EEA will rst take place in the UCNPs/MoO
3x
system when
activated by the 980 nm light, resulting in a lowered intensity of
excitation light arriving at the UCNPs, thus weakening the
resulting luminescence emissions. Moreover, the efficient FRET
process occurs through the spectral overlap between the
absorption of MoO
3x
NSs and the UCL of UCNPs in the visible
region, leading to a further decrease in the intensity of lumi-
nescence emissions. Thus, the quenching in UCL of UCNPs is
efficiently achieved by the joint effect of the EEA and FRET.
However, upon the addition of H
2
O
2
, the oxygen-decient
MoO
3x
NSs can be oxidized back to MoO
3
(denoted as H-
MoO
3
), leading to the decrease of absorption in the visible and
NIR regions (Fig. 1b), resulting in the recovery of UCL emissions
via the reduction in EEA and FRET. Additionally, XPS was per-
formed to evaluate the valence state of Mo in these nanosheets.
As shown in Fig. 1c, the doublet peaks (235.9 eV and 232.8 eV) in
the pristine MoO
3
sample are assigned to the binding energies
of the 3d
3/2
and 3d
5/2
orbital electrons of Mo
6+
.Aer treatment
by tip-sonication, two new peaks at lower binding energies
(234.7 eV and 231.6 eV) appear in the obtained S-MoO
3
NSs,
which can be assigned to the Mo
5+
oxidation state, and the
integral area ratio of Mo
5+
/Mo
6+
is calculated to be 17.1% from
the XPS spectrum. This phenomenon indicates that the MoO
3
is
slightly reduced during the exfoliation process, showing weak
absorption ability of S-MoO
3
NSs in visible and NIR regions
(Fig. 1b). Furthermore, the peak area ratio of Mo
5+
/Mo
6+
increases to 47.9%, suggesting that oxygen-decient MoO
3x
NSs are formed, where one-third of the Mo
6+
is reduced upon
UV irritation. However, the peaks at lower binding energies
disappear aer the addition of H
2
O
2
, conrming that MoO
3x
NSs have been oxidized. Thus, H
2
O
2
-involved oxidation of
Fig. 1 (a) Schematic illustration of the design principle of upconver-
sion-based nanosystems for the sensing of pH and H
2
O
2
. (b) UCL
spectrum of OA-UCNPs under 980 nm excitation and UV-vis spectra
of S-MoO
3
, MoO
3x
, and H-MoO
3
NSs with concentration of 1 mg
mL
1
. (c) The Mo 3d XPS spectra of (i) pristine MoO
3
, (ii) S-MoO
3
NSs,
(iii) MoO
3x
NSs, and (iv) H-MoO
3
NSs.
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MoO
3x
enables the ability of UCNPs/MoO
3x
nanoprobes for
H
2
O
2
sensing with high sensitivity. Additionally, the adjustment
of pH or addition of H
2
O
2
in the acidic environment will lead to
the variation of MoO
3x
NSs in NIR absorption, and thus uo-
rescence sensing of pH and H
2
O
2
can be achieved through the
direct modulation of MoO
3x
absorption-induced EEA in the
non-contact mode.
Characterization of UCNPs, MoO
3x
NSs, and UCNPs/MoO
3x
nanocomposites
Hydrophobic OA-UCNPs are synthesized by employing OA as
the ligand via the high-temperature coprecipitation method.
46
OA-UCNPs present uniform hexagonal shape with a mean
diameter of about 28 nm, which is revealed by the TEM
measurement (Fig. 2a). The XRD pattern of the obtained OA-
UCNPs with well-dened diffraction peaks agrees well with
the standard data of hexagonal-phase NaYF
4
(JCPDS no. 28-
1192), demonstrating their high crystallinity (Fig. S1†). Ligand-
free UCNPs are prepared by direct addition of formic acid to the
cyclohexane solution containing OA-UCNPs through the vor-
texing method and sequential modication with PEI to obtain
PEI-UCNPs.
47
TEM images demonstrate unchanged morphology
and size aer ligand removal and polymer functionalization
(Fig. S2†). The transition of OA-UCNPs to ligand-free UNCPs
and further to PEI-UCNPs are conrmed by FT-IR. As shown in
Fig. S3,†the transmission bands at 2926 and 2852 cm
1
can be
assigned to asymmetric and symmetric methylene (–CH
2
–)
stretching, and those at 1561 and 1460 cm
1
can be attributed
to the vibrations of the carboxylate groups, indicating the
presence of oleate ligand on the surface of OA-UCNPs. However,
the disappearance of these characteristic peaks conrms the
removal of surface ligand aer treatment by formic acid. When
further modied by PEI, new peaks appear at 3396 cm
1
(N–H
stretching), 2930 and 2854 cm
1
(asymmetric and symmetric
–CH
2
stretching), and 1545 cm
1
(N–H bending). Accordingly,
the FT-IR results verify the success in ligand removal of OA-
UCNPs and further attachment of PEI on bare UCNPs. Aer
ligand exfoliation and polymer modication, ligand-free UCNPs
and PEI-UCNPs are easily dispersed in water, and the z-poten-
tials are measured to be +35.7 mV and +32.8 mV, respectively
(Fig. S4†), indicating the formation of stable colloidal solutions.
To prepare UCNPs/MoO
3x
nanoassemblies, MoO
3
NSs are
rstly prepared by tip sonication of bulk MoO
3
, and oxygen-
decient MoO
3x
NSs are easily obtained by UV irritation.
45
As
shown in Fig. 2b, the nanostructure of the MoO
3x
sample is
comprised of NSs with lateral diameters in the range of 20–
300 nm. UCNPs/MoO
3x
nanoassemblies are then constructed
by assembling the positive charged PEI-UCNPs and negatively
charged MoO
3x
NSs (Fig. S4†)via electrostatic interactions, as
characterized by TEM (Fig. 2c). Furthermore, the EDS spectrum
of UCNPs/MoO
3x
nanocomposites implies the presence of Na,
F, Y, Yb, Er, Mo, and O. These results prove the successful
assembling of UCNPs and MoO
3x
NSs (Fig. 2d).
Next, the optical properties of UCNPs and MoO
3x
NSs are
investigated. OA-UCNPs generate green (524 and 543 nm) and
red (658 nm) luminescence emissions originating from the
2
H
11/2
/
4
I
15/2
,
4
S
3/2
/
4
I
15/2
, and
4
F
9/2
/
4
I
15/2
transitions of
Er
3+
ions when activated by a 980 nm CW laser. The UV-vis
spectroscopy of MoO
3
NSs shows only slight absorption in
visible and NIR regions. In contrast, MoO
3x
NSs strongly
absorb in both visible and NIR regions, ascribed to the
enhancement of the free electron concentration and the
increased oxygen vacancies in the MoO
3x
NSs aer exposure to
UV light. The absorption of MoO
3x
NSs overlaps well with not
only UCL emissions of UCNPs but also the excitation wave-
length for UCNPs, namely 980 nm. Additionally, the absorption
in the visible and NIR regions disappears aer the addition of
H
2
O
2
, as shown in Fig. 1b. The loss in the absorption intensity is
due to the oxidative effect of H
2
O
2
in the acidic medium, lling
up the oxygen vacancies of MoO
3x
NSs.
48
Non-contact uorescence sensing of pH
The optical properties of MoO
3x
NSs solutions (1 mg mL
1
)at
different pH are rst investigated by UV-vis spectroscopy. As
represented in Fig. 3a, the absorption intensity in the visible
and NIR regions becomes weakened with increasing pH, and
the maximum of the absorption peak gradually redshis from
744 to 866 nm. However, no absorption peak is found in the
visible and NIR region above pH 7. Moreover, the absorption at
980 nm shows the same trend as well (Fig. S5a†). This
phenomenon arises from the reduction of Mo in the reduced
state (returning to the Mo
VI
state) by the addition of OH
to the
MoO
3x
NSs solution, leading to the reduction of free carrier
concentration, and thus reducing the absorption in visible and
NIR regions.
48,49
Next, the luminescence properties are investigated by
placing MoO
3x
NSs solutions (1 mg mL
1
) with different pH in
front of the OA-UCNPs solution (1 mg mL
1
) and illuminate it
then with the light of 980 nm wavelength at RT, where the
Fig. 2 TEM images of (a) OA-UCNPs, (b) MoO
3x
NSs and (c) UCNPs/
MoO
3x
nanocomposites. (d) EDS spectrum of UCNPs/MoO
3x
nanocomposites.
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980 nm light rst passes through the MoO
3x
NSs solution and
then reaches OA-UCNPs (Fig. 1a). The luminescence intensity
rises generally with increasing pH and remains constant above
pH 8.2, as is presented in Fig. 3b. The luminescence intensity at
658 nm grows slowly when pH < 4.4, then increases remarkably
in the range of 5.0 to 8.2, and the UCL shows no signicant
change aerward. However, the UCL intensity at 658 nm shows
a nonlinear relationship with the pH, which is different from
typical upconversion sensors based on the FRET process.
32–34
Notably, we nd that the logarithm of luminescence intensity at
658 nm exhibits three-separate linear regions with pH, and the
linear correlation coefficient of each calibration curve is calcu-
lated to be 0.992 (pH 2.6–4.4), 0.988 (pH 5–6), and 0.998 (6.3–
8.2), respectively (Fig. 3c). Thus, this upconversion-based sensor
shows broad pH responsiveness in the range of 2.6 to 8.2. To
investigate the reversibility of this pH sensor, the pH value of
MoO
3x
NSs was adjusted from 8.2 to 2.6 and back to 8.2 by
NaOH and HCl solutions for 5 cycles. As shown in Fig. S6,†the
uorescence intensity shows good reversibility of the two-way
switching processes aer the second cycle of pH adjustment.
A slight increase in the uorescence intensity at pH 2.6 was
noticed aer the rst pH adjustment from 8.2, which may result
from a lower reduction degree of Mo(VI) in the acidic environ-
ment than under exposure to the UV light.
Non-contact uorescence sensing of H
2
O
2
The sensing ability of the upconversion-based nanosystem for
H
2
O
2
in the non-contact mode is evaluated by the UV-vis
absorption and UCL spectroscopy. As can be seen in the
absorption spectrum (Fig. 4a), the MoO
3x
NSs solution shows
a broad absorption in both visible and NIR regions, and the
overall absorption intensity of MoO
3x
NSs solution decreases
with the increasing amount of H
2
O
2
, and absorbance is barely
observed aer the addition of 0.8 mM H
2
O
2
. Notably, the
maximum absorbance of MoO
3x
NSs at 722 nm decreases
substantially when a low amount of H
2
O
2
is added (<0.3 mM).
Then the absorption intensity reduces gradually and no further
variation in absorption is found aer the addition of 0.8 mM
H
2
O
2
, indicating the completion in the conversion of MoO
3x
to
MoO
3
. The change in absorption intensity at 722 nm (denoted
as (A
0
A)/A
0
, where A
0
and Arefer to the MoO
3x
NSs solution
in the absence and presence of H
2
O
2
, respectively) shows
a linear relationship with the H
2
O
2
concentration in two-
separated regions (Fig. 4b). The linear correlation coefficients
of these two calibration curves are larger than 0.99, and the
limit of detection (LOD) is calculated to be 0.75 mM.
Fig. 3 (a) UV-vis absorption spectra of MoO
3x
NSs solution (1 mg mL
1
)atdifferent pH values. (b) UCL spectra of OA-UCNPs in the presence of
MoO
3x
NSs solutions with different pH in the non-contact mode under 4 W 980 nm excitation. (c) Relationship between the logarithm of
luminescence intensity of OA-UCNPs at 658 nm and pH. Error bars represent the standard deviations of three independent measurements.
Fig. 4 (a) UV-vis spectra of MoO
3x
NSs (1 mg mL
1
) upon addition of
different H
2
O
2
concentrations. (b) Relationship between the change in
absorbance of MoO
3x
NSs at 722 nm and H
2
O
2
concentration. (c)
UCL spectra of OA-UCNPs (1 mg mL
1
) in the presence of MoO
3x
NSs
solutions containing different H
2
O
2
concentrations at pH 4.5 under
4 W 980 nm excitation. (d) Relationship between the logarithm of
luminescence intensity of OA-UCNPs at 658 nm and the H
2
O
2
concentration. (e) UCL spectra of OA-UCNPs in the presence of
MoO
3x
NSs solutions containing 3 mM various interfering species at
pH 4.5 under 4 W 980 nm excitation. Inset: UCL spectrum of OA-
UCNPs in the presence of MoO
3x
NSs solutions containing 0.6 mM
H
2
O
2
at pH 4.5. (f) Changes in the logarithm of luminescence intensity
of OA-UCNPs at 658 nm upon addition of 0.6 mM H
2
O
2
and 3 mM
other interfering species to MoO
3x
NSs solution at pH 4.5. Green bars
represent changes in the logarithm of luminescence intensity at
658 nm upon addition of various species in MoO
3x
NSs solution, red
bars represent the subsequent addition of 0.6 mM H
2
O
2
to the above
MoO
3x
NSs solution. Error bars represent the standard deviations of
three independent measurements.
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The luminescence properties are then studied using similar
procedures as the above-mentioned pH sensing, except that
MoO
3x
solutions (1 mg mL
1
in acetate buffer, pH 4.5) with
different added H
2
O
2
concentrations are placed in front of the
OA-UCNPs solution. The quenching efficiency (denoted as (F
0
F)/F
0
, where Fand F
0
represent the luminescence intensity at
a specic wavelength in the presence and absence of MoO
3x
NSs, respectively) at 658 nm reaches 99.8% when 1 mg mL
1
MoO
3x
NSs solution is aligned in front of 1 mg mL
1
OA-
UCNPs solution. When H
2
O
2
is added in the range from 0 to
0.8 mM, the absorption intensity of MoO
3x
NSs solution at
980 nm shows a continuous decrease (Fig. S5b†). As a result, the
UCL intensity of OA-UCNPs experiences a gradual uptrend in
both red and green regions upon 980 nm excitation with the
increasing addition of H
2
O
2
(Fig. 4c). This can be ascribed to the
oxidation of MoO
3x
to MoO
3
by H
2
O
2
, leading to the reduction
in excitation energy depletion by MoO
3x
NSs at 980 nm, and
resulting in more excitation energy reached by OA-UCNPs.
Similarly, like the above-discussed pH sensing in non-contact
mode, the uorescent intensity exhibits a nonlinear relation-
ship with the H
2
O
2
concentration as well. In addition, the
logarithm of luminescence intensity at 658 nm is linearly
correlated with the H
2
O
2
concentration in the range of 0–200
mM(R
12
¼0.993) and 250–500 mM(R
22
¼0.997), respectively
(Fig. 4d). According to the 3srule, the detection of H
2
O
2
can be
down to 0.63 mM, providing a lower detection limit than those
reported by other upconversion-based nanoprobes (Table 1).
To further estimate the selectivity for H
2
O
2
in the non-
contact mode, the uorescence responses of the nanosystem
toward various interfering species including cations, anions,
and amino acids were investigated. As shown in Fig. 4e, only the
addition of H
2
O
2
results in the recovery of the UCL emission,
whereas no obvious change in luminescence intensity is
observed aer the addition of large excesses of the other
interfering species, such as Na
+
,K
+
,Ca
2+
,Mg
2+
,Zn
2+
,F
,Cl
,
CO
32
,NO
3
,SO
42
, cysteine (Cys), glutamine (Gln), glycine
(Gly), leucine (Leu), proline (Pro), serine (Ser), threonine (Thr),
and valine (Val). Furthermore, competition experiments exhibit
the recovery in UCL intensities at 658 nm, performed by adding
H
2
O
2
to MoO
3x
NSs solutions containing other interfering
species (Fig. 4f). The results indicate that the sensing of H
2
O
2
is
barely affected by these coexistent species. Therefore, this
system can serve as an upconversion uorescence nanoprobe
for H
2
O
2
with high selectivity in the non-contact mode.
Application in real sample analysis
For a practical application of the non-contact upconversion-
based sensor, we studied the detection of H
2
O
2
residue in
contact lens solution, as H
2
O
2
is usually applied in the contact
lens disinfection processes and is harmful to human eyes. The
results are summarized in Table 2. The recoveries of H
2
O
2
in
contact lens solutions range from 96.56% to 102.04% and the
relative standard deviation (RSD, n¼3) values are lower than
4.45%, suggesting the efficient practical applicability of the
proposed sensor.
Conventional uorescence sensing of H
2
O
2
by UCNPs/MoO
3x
nanocomposites
To quantitatively analyze the quenching ability of MoO
3x
NSs
on PEI-UCNPs, a series of MoO
3x
NSs modied PEI-UCNPs
nanocomposites (the concentration of PEI-UCNPs is xed at
0.5 mg mL
1
) is prepared by changing the MoO
3x
NSs content
(from 0 to 0.4 mg mL
1
). The overlap integral (J(l)) between the
normalized emission spectrum of the donor (UCNPs) and the
absorption spectrum of the acceptor (MoO
3x
NSs) is dened by
the equation as follows:
JðlÞ¼ðN
0
FDðlÞ3AðlÞl4dl
Table 1 Comparison of various upconversion-based nanoprobes for H
2
O
2
sensing
Sensors Mechanisms LOD (mM) Ref.
Benzopyrylium–coumarin-functionalized
UCNPs
FRET 4.37 14
DNA-Ag/UCNPs nanocomposites FRET 1.08 17
MnO
2
-nanosheets-modied UCNPs FRET 0.9 26
Squaric acid-Fe(III) & UCNPs Inner lter effect 2.3 50
UCNPs-PDA nanosystem FRET 0.75 51
UCNPs & MoO
3x
(non-contact mode) EEA 0.63 This work
UCNPs/MoO
3x
nanoassemblies EEA & FRET 9.61 This work
Table 2 Detection of H
2
O
2
in contact lens solution
a
Contact lens solution Detected (mM) Added (mM) Found (mM) Recovery (%) RSD (%)
1 ND 50 48.64 97.28 1.47
2 ND 100 102.04 102.04 4.45
3 ND 200 193.11 96.56 3.32
a
ND ¼no detection.
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where lis the wavelength in nm, F
D
is the 980 nm laser-
activated UCL spectrum of PEI-UCNPs normalized to an area
of 1, 3
A
is the extinction coefficient spectrum of MoO
3x
NSs in
units of M
1
cm
1
. The J(l) value for the donor acceptor pair is
calculated to be 2.79 10
13
M
1
cm
1
nm
4
. The effect of
different MoO
3x
NSs loading on PEI-UCNPs is evaluated by
UCL spectra. As shown in Fig. 5a, the red emission intensity of
UCNPs/MoO
3x
nanoassemblies experiences a signicant
decrease with the increasing addition of MoO
3x
NSs. Addi-
tionally, the green emission intensity of upconversion-based
nanoassemblies with different loading of MoO
3x
NSs shows
a similar tendency, but with a slower downward trend. As shown
in Fig. 5b, the quenching efficiency at 658 nm enhances rapidly
with increasing addition of MoO
3x
NSs, and shows no obvious
changes aer the addition of 0.35 mg mL
1
MoO
3x
NSs solu-
tion. Compared with unmodied PEI-UCNPs, the highest uo-
rescence quenching efficiency at 658 nm reaches 95.6% with the
addition of 0.35 mg mL
1
MoO
3x
NSs. Moreover, the
quenching efficiency at 543 nm shows the same trend, but with
a maximum value of 87.3%. Correspondingly, UCNPs/MoO
3x
nanocomposites with the addition of 0.35 mg mL
1
MoO
3x
NSs are selected for the subsequent sensing experiments.
To elucidate the effect of EEA-induced reduction in the UCL
of UCNPs by MoO
3x
NSs, a non-contact mode is designed. The
MoO
3x
NSs solution (0.35 mg mL
1
) is sealed in the quartz
cuvette, aligning in front of another quartz cuvette containing
0.5 mg mL
1
PEI-UCNPs solution, and the UCL spectra are
depicted in Fig. S7.†Upon the excitation by a 980 nm CW laser,
the incident light rst passes through the MoO
3x
NSs solution,
and the energy-reduced light then reaches the UCNPs, resulting
in the loss of intensity in UCL emissions. Ideally, the UCL
intensity at 658 nm reduces by 72.3% compared with the control
experiment.
Notably, the EEA effect will affect the intensity in all emis-
sions, and the red-to-green emission ratio (R/G, where the red
emission is integrated from 600–700 nm and the green emis-
sion is integrated from 500–600 nm) keeps its stability, which is
conrmed by the activation of PEI-UCNPs (0.5 mg mL
1
) with
different power of the 980 nm laser. As presented in Fig. S8,†the
red and green emission intensities increase with increasing
laser power, and the R/G remains stable. However, a gradual
decrease in the R/G values is observed for the UCNPs/MoO
3x
nanocomposites with the increasing loading content of MoO
3x
NSs (inset of Fig. 5a). This phenomenon can be attributed to the
FRET-induced uorescence quenching by MoO
3x
NSs, where
the quenching ability by MoO
3x
NSs in red emission is more
pronounced than in the green region. As discussed above, the
uorescence quenching of UCNPs by MoO
3x
NSs is achieved by
the joint effect of EEA and FRET, owing to the strong absor-
bance ability of MoO
3x
NSs in both visible and NIR regions.
The sensing performance of UCNPs/MoO
3x
nano-
assemblies toward H
2
O
2
is investigated by UCL emission spec-
troscopy. As shown in Fig. 5c, the UCL emission intensity in red
and green regions increases with the increasing addition of
H
2
O
2
solution. As discussed above, the addition of H
2
O
2
leads
to the oxidation of MoO
3x
, resulting in the reduction in the
absorption in both visible and NIR regions, and thus inhibiting
the EEA effect at 980 nm and FRET process from the UCL of
UCNPs to absorption of MoO
3x
in the visible region, corre-
sponding to the enhancement of UCL emission intensity.
However, the uorescence intensity shows no obvious changes
if more than 3.0 mM H
2
O
2
are added. The uorescence intensity
at 658 nm exhibits a linear correlation to the H
2
O
2
concentra-
tion in the range of 0–0.8 mM (R
12
¼0.990) and 1.0–2.5 mM (R
22
¼0.996), respectively (Fig. 5d). The detection limit of H
2
O
2
is
calculated to be 9.61 mM according to the 3srule. Intriguingly,
the addition of a low concentration of H
2
O
2
only leads to slight
UCL recovery, while signicant UCL recovery takes place with
the addition of a large amount of H
2
O
2
, showing an opposite
trend when compared to the sensing of H
2
O
2
in the non-contact
mode (Fig. 4d). Moreover, much more H
2
O
2
is required for the
recovery of UCL in the conventional UCNPs/MoO
3x
system.
This phenomenon may be attributed to the structure of the
stacked MoO
3x
NSs on UCNPs, slowing H
2
O
2
to ll up the
oxygen vacancies in MoO
3x
NSs, and thus more H
2
O
2
is
consumed for complete oxidation of MoO
3x
.
Conclusions
In summary, we have designed two different methods (i.e.,
a non-contact method and a conventional method) for upcon-
version uorescence sensing of H
2
O
2
. The non-contact method
relies on the MoO
3x
NSs absorption-induced EEA effect and
operates by placing the MoO
3x
NSs solution in front of UCNPs
solution, whereas the conventional upconversion-based
Fig. 5 (a) UCL spectra of 0.5 mg mL
1
UCNPs upon the addition of
different contents of MoO
3x
NSs at pH 4.5 under 4 W 980 nm exci-
tation. Inset: R/G values of UCNPs/MoO
3x
nanocomposites with
different MoO
3x
NSs contents. The black line in the inset serves as
a guide to the eye. (b) Fluorescence quenching efficiency of UCNPs/
MoO
3x
nanocomposites at 658 nm upon the addition of different
MoO
3x
NSs concentrations at pH 4.5. The black line serves as a guide
to the eye (c) UCL spectra of 0.5 mg mL
1
UCNPs/MoO
3x
nano-
composites (0.35 mg mL
1
MoO
3x
NSs) upon the addition of different
H
2
O
2
concentrations at pH 4.5 under 4 W 980 nm excitation. (d)
Relationship between the fluorescence intensity of UCNPs/MoO
3x
nanocomposites at 658 nm and the H
2
O
2
concentration. Error bars
represent the standard deviations of three independent
measurements.
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uorescence nanoprobe, based on the joint effect of EEA and
FRET, was constructed by the integration of UCNPs and MoO
3x
NSs via electrostatic interactions. An advantage of the non-
contact method is that the valuable sensor particles do not
become consumed or contaminated during the measurement
and can be reused for a long time. The MoO
3x
NSs act as the
quencher in both nanosystems, owing to the strong absorptive
capacity of MoO
3x
in both visible and NIR regions. However,
the addition of H
2
O
2
leads to the oxidation of MoO
3x
, resulting
in the recovery of UCL emissions, and thus enabling the
quantitative detection of H
2
O
2
by both methods. Beneting
from the non-contact method, hydrophobic OA-UCNPs can be
applied as the luminophore directly and ultrahigh uorescence
quenching (99.8%) is obtained. Moreover, the non-contact
method exhibits high sensitivity toward H
2
O
2
down to 0.63
mM, which is lower than that determined by the spectropho-
tometry of MoO
3x
(0.75 mM) and conventional UCNPs/MoO
3x
nanocomposites (9.61 mM). Additionally, pH sensing can be
achieved by employing the non-contact mode as well, which has
shown a broad pH-responsive range from 2.6 to 8.2. We believe
that these results could provide new insights into the design of
upconversion-based nanosystems for uorescence sensing of
other analytes.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
C. Sun acknowledges the nancial support from the China
Scholarship Council (CSC, No. 201404910463) and TU Berlin.
We are grateful to Ina Speckmann for the XRD measurement,
S¨
oren Selve and Jan Ron Justin Simke for the TEM measure-
ments, and Jin Yang for the XPS measurement.
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2546 |Nanoscale Adv.,2021,3, 2538–2546 © 2021 The Author(s). Published by the Royal Society of Chemistry
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