Gold nanoclusters based dual-emission hollow TiO2 microsphere for ratiometric optical thermometry

Article abstract of DOI:10.1039/c5ra13475g

In this work, we develop a novel dual-emitting hollow TiO₂ microsphere with high stability and attractive thermal sensitivity, which could work as nanosensor for accurately measuring temperature. This nanosensor is prepared through coating gold nanocluster (AuNCs) on the surface of carbon dots-doped hollow TiO₂ microsphere. As-prepared nanosensor shows characteristic dual-emitting property of carbon dots (CDs, blue fluorescence) and AuNCs (red fluorescence) under a single excitation wavelength. Moreover, upon increasing the temperature, the intensity of red emission from the AuNCs continuously quenched, whereas that of blue emission from the CDs remained nearly constant. The different response results in a continuous fluorescence color change from red to purple that can be clearly observed by the naked eyes. Thus, as-prepared dual-emitting hollow TiO₂ microsphere could be used as optical thermometry by taking the advantage of the temperature sensitivity of their fluorescence intensity ratio (red/blue) with high reliability and accuracy, which change considerably over the wide temperature range (20-80 °C) with small temperature resolution (～0.5 °C). Additionally, this nanosensor is successfully applied in 10 mM buffered saline (PBS) solution with physiological temperature ranging from 20 to 45 °C, which suggests as-prepared dual-emitting microspheres have promising applications in vivo temperature sensing.

Full text of DOI:10.1039/c5ra13475g

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PAPER
Gold nanoclusters based dual-emission hollow
TiO2 microsphere for ratiometric optical
Cite this: RSC Adv., 2015, 5, 61586
thermometry†
Chuanxi Wang, Yijun Huang,a Huihui Lin,a Zhenzhu Xu,a Jiapeng Wu,a
a
*
b
a
*
Mark G. Humphrey and Chi Zhang
In this work, we develop a novel dual-emitting hollow TiO2 microsphere with high stability and attractive
thermal sensitivity, which could work as nanosensor for accurately measuring temperature. This
nanosensor is prepared through coating gold nanocluster (AuNCs) on the surface of carbon dots-doped
hollow TiO2 microsphere. As-prepared nanosensor shows characteristic dual-emitting property of
carbon dots (CDs, blue fluorescence) and AuNCs (red fluorescence) under
a single excitation
wavelength. Moreover, upon increasing the temperature, the intensity of red emission from the AuNCs
continuously quenched, whereas that of blue emission from the CDs remained nearly constant. The
different response results in a continuous fluorescence color change from red to purple that can be
clearly observed by the naked eyes. Thus, as-prepared dual-emitting hollow TiO2 microsphere could be
used as optical thermometry by taking the advantage of the temperature sensitivity of their fluorescence
intensity ratio (red/blue) with high reliability and accuracy, which change considerably over the wide
temperature range (20–80 ꢀC) with small temperature resolution (ꢁ0.5 ꢀC). Additionally, this nanosensor
is successfully applied in 10 mM buffered saline (PBS) solution with physiological temperature ranging
from 20 to 45 ꢀC, which suggests as-prepared dual-emitting microspheres have promising applications
in vivo temperature sensing.
Received 9th July 2015
Accepted 13th July 2015
DOI: 10.1039/c5ra13475g
www.rsc.org/advances
biocompatibility and multifunctional surface chemistry.10,11
More importantly, AuNCs showed temperature sensitivity of
1. Introduction
their luminescence and could be used recently as versatile
nanothermometry devices in living cells.12,13 However, this kind
of intensity-based temperature sensors would be affected by
local environment including oxygen content, pH, concentration
and usually gave inaccurate temperature information.14 It is well
known that temperature is one of the most important param-
eters that governs biological reactions within a living cell.15,16
The accurate measurement of temperature and its gradient
inside a live cell can promote the advancements in cell biology
and biomedicine, for example, the cellular pathogenesis of
cancer is characterized by extraordinary heat production.17
Recently, dual-emitting temperature sensors have attracted
much attention since they operate as “non-contact” tools and
accurately measure temperature.18–20 With such a sensor, a
change in temperature leads to a measurable change in the
relative rather than absolute PL intensities of one or both the
states.21 This reduces the impact of extrinsic factors like uc-
tuations in excitation rate, detection efficiency, probe concen-
tration, optical occlusion, or other local inhomogeneity. To
date, several dual-emitting nanomaterials, such as CdSe/ZnS
nanocrystals, rare earth-doped nanoparticles and polymer
dots have already shown a great potential for nanothermometry
in biological systems, and especially live cells.22–25 For example,
Metal nanoclusters (NCs) with discrete energy levels possess
molecule-like electronic transitions within their conduction
bands, and may exhibit unique physical and chemical proper-
ties including strong luminescence, as well as have potential
practical applications in, for example, cell labelling, ion
sensing, and catalysis.1–4 Similar to quantum dots, these small
clusters exhibit tunable uorescence from visible to near-
infrared regions due to molecule-like electronic transitions
within the conduction bands or the charge-transfer transition
from the ligand to metal nanoparticles.5–7 In addition, these
clusters exhibit high photoluminescence (PL) quantum yield,
and could be handily synthesized.8,9 Among them, luminescent
gold NCs (AuNCs) are attractive for a wide variety of biomedical
applications, such as biosensing, in vitro and in vivo imaging,
and also in cancer therapy, owing to their low toxicity, good
aChina-Australia Joint Research Centre for Functional Molecular Materials, School of
Chemical & Material Engineering, Jiangnan University, Wuxi 214122, P. R. China.
E-mail: wangcx@jiangnan.edu.cn; chizhang@jiangnan.edu.cn
bResearch School of Chemistry, Australian National University, Canberra, ACT 0200,
Australia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra13475g
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CdSe/ZnS core–shell quantum dots have been demonstrated large particles. Resultant AuNCs were stored at 4 ꢀC for further
temperature-sensitive uorescence, which is used as thermal using.
nanoprobe for temperature sensing in liquids and HeLa cervical
The carbon dots (CDs) were prepared by a similar organic-
cancer cells.24 Unfortunately, the high toxicity or instability based surface-capping method.34 In typical process, the
limited their further application. Therefore, it is practical to mixture of CA (0.42 g, 2 mmol), OA (3 mL, 8.7 mmol), OLA
develop a novel dual-color uorescence sensor based on ratio- (0.3 mL, 0.62 mmol), and ODE (7 mL, 19.8 mmol) were degassed
metric signal output for rapid visual identication of tempera- at room temperature and heated up to 100 ꢀC under vacuum for
ture. On the other hand, carbon dots (CDs) and NCs, as two 1 h with stirring. The resultant homogeneous solution was
important class of PL nanomaterials, have been applied widely switched to a dry nitrogen atmosphere, heated to 250 ꢀC slowly
in the elds of bioimaging, printing inks, photocatalysis, and and maintained at this temperature for 1 h. The color of the
sensors.26–28 CDs or NCs based dual-emitting nanomaterials solution became transparent with raising temperature and
could work as nanosensors for Cu2+, Hg2+ and highly reactive nally turned to dark brown. The temperature of the solution
oxygen species,29–31 while no literature has reported CDs(NCs)- was dropped under 100 ꢀC. The resultant CDs were precipitated
based dual-emitting probes for ratiometric temperature by addition of methanol, rinsed with methanol, and then re-
detection.
Herein, the novel dual-emitting microspheres are designed
dispersed in 10 mL of chloroform for further using.
using the blue uorescent CDs and red uorescent AuNCs as 2.3 Preparation of dual-emitting microspheres
uorophores. Resultant microspheres showed dual-emitting
The blue PL CDs-doped TiO2 microspheres would be prepared
uorescence in blue and red region. The red uorescence
by sol–gel method.35 0.05 mL of TBOT was added to 10 mL
came from the AuNCs on the surface of microspheres which
ethylene glycol under stirring for 8 h at room temperature, then
showed temperature responsive optical properties; in contrary,
3 mL of the mixture of TBOT and ethylene glycol was added into
the blue uorescence of CDs in the core of microspheres would
the 40 mL acetone containing 0.2 mL Tween 20 and 2 mL CDs.
Aer agitation for 24 h, the resultant microspheres were
not change. Hence, such dual-emitting microspheres could be
used as nanosensors for accurate measurement of temperature.
collected by centrifugation and washed three times with
Moreover, owing to TiO2 layer as scaffolds, resultant dual-
ethanol, and then re-dispersed in 5 mL of ethanol for further
using.
To prepare the dual-emitting microspheres, we rstly
modied the TiO2 microspheres with APS as follows: the CDs-
emitting temperature sensors showed good biocompatibility
and high stability. These features suggested resultant dual-
emitting nanosensors could be promising nanothermometry
in living cells.
doped TiO2 microspheres (5 mL) were mixed with APS (10 mL),
and stirred for 12 h. The resultant products were centrifuged
and washed with ethanol and water several times, and then re-
dispersed in 5 mL water solution. The dual-emitting micro-
spheres were built through carbodiimide-activated coupling
reaction between GSH-AuNCs and amino-functionalized TiO2
2. Experimental
2.1 Materials
Reduced glutathione (GSH, molecular weight of 307 g molꢂ1),
1-octadecene (ODE, 90%), oleic acid (OA, 90%), oleyl amine
(OLA, 70%), (3-aminopropyl)trimethoxysilane (APS), 1-ethyl-3-
(3-dimethylaminopropyl)carboxylate (EDC) and N-hydroxy-
succinimide (NHS) were purchased from Aldrich. Tetra-
chloroauric(III) acid (HAuCl4), citric acid monohydrate (CA),
titanium butoxide (TBOT, 98%), ethylene glycol, acetone, Tween
20, chloroform, and methanol were purchased from Sinopharm
Chemical Reagent Co., Ltd (China). All reagents were used as
received without further purication. The solutions of 200 mM
NaCl, KNO3, Ca(NO3)2, MgSO4 were prepared, respectively. De-
ionized water was used in all experiments.
microspheres. Typically, 25 mg EDC and 15 mg of NHS were
added to 3 mL of PBS buffer (pH ¼ 6.86) containing 10 mg GSH-
AuNCs, and then mixed with 2 mL of aqueous solution con-
taining 20 mg of amino-functionalized TiO2 microspheres. This
solution was allowed to react under magnetic stirring for 24 h.
Finally, the resultant dual-emitting microspheres were collected
through centrifugation at 6000 rpm and washed three times
with water, and then re-dispersed in 5 mL aqueous solution for
further application.
2.4 Temperature-dependent uorescence of dual-emitting
microspheres
2.2 Preparation of uorescent carbon dots and gold
nanoclusters
To test the temperature dependence of uorescence, tempera-
ture was controlled in the range from 20 to 80 ꢀC and the
uorescence intensity of dual-emitting microspheres towards
According to the previously reported method, gold nanoclusters
(AuNCs) were synthesized by a green thermal reduction
approach with GSH as both reducing and stabilizing agents.32,33
Briey, 20 ꢃ 10ꢂ3 M HAuCl4 (0.5 mL) was added to deionized
water (15 mL) followed by the adding 5 mL GSH (0.0150 g)
different temperature values were monitored with
spectrouorimeter.
a
2.5 Calibration curve and temperature resolution of the
dual-emission uorescent nanosensor
ꢀ
solution. The mixture was stirred at 25 C for 5 min, and then
continuously stirred for 8 h at 100 ꢀC. The purication of The calibration curve for temperature with dual-emitting
resultant GSH-AuNCs was centrifuging at 12 000 rpm to remove microspheres was obtained by approximating the relationship
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between averaged ratio of uorescence intensity at 596 nm and
436 nm (n ¼ 3) and temperature to the third-order polynomial
2
ꢀ
ꢀ
(with OriginPro-2012, r ¼ 0.99, 20 C < T < 80 C)
R(T) ¼ 7.21699 ꢃ 10ꢂ8T3 + 7.06814 ꢃ 10ꢂ7T2
ꢂ 0.00896 ꢃ T + 1.15089
(1)
where T and R(T) represent temperature and ratio of uores-
ꢀ
cence intensity at 596 nm and 436 nm at T C, respectively.
The temperature resolution (dT) of dual-emitting micro-
spheres was evaluated using the following equation:
ꢀ
ꢁ
vT
vR
dT ¼
dR
(2)
where vT/vR and dR represent the inverse of the slope of the
uorescence ratio–temperature diagram and the SD of the
uorescence ratio, respectively. The SD value was obtained by
triplicate measurements of one sample at each temperature.
Fig. 1 Schematic illustration of the preparation of ratiometric dual-
emission fluorescent hollow microspheres.
2.6 Characterization methods
Photoluminescence (PL) experiments were performed with a
Cary Eclipse spectrouorimeter (Varian, America). X-ray
photoelectron spectroscopy (XPS) using Mg Ka excitation
(1253.6 eV) was collected in a VG ESCALAB MKII spectrometer.
Binding energy calibration was based on C 1s at 284.6 eV. The
morphology and mean diameter of the resultant samples were
characterized by scanning electron microscopy (SEM, Hitachi
S4800) and transmission electron microscopy (TEM, JEM-2100).
chloroform solution was slight yellow color under visible light
(Fig. S1a†), while it emitted blue uorescence under UV light
illumination (lex ¼ 365 nm) (inset of Fig. 1). Aer forming the
CDs/TiO2 hollow microspheres, the blue uorescence of CDs
was observed, thus as-prepared microspheres exhibited intense
blue emission under 365 nm UV light (inset of Fig. 1) and good
dispersion in aqueous solution (Fig. S1b†). The AuNCs were
prepared by a green thermal reduction approach without any
noxious reducing agent, such as NaBH4 or N2H4$2H2O.40,41 GSH,
a naturally occurring and readily available tripeptide, functions
as both reducing agent and stabilizing agent in the thermal
reduction process.32,33 As-prepared AuNCs showed good
dispersion in aqueous solution and red PL with an emission
peak at 596 nm (Fig. 2c), and a highly red uorescence can be
clearly observed under a UV lamp (lex ¼ 365 nm) (inset of Fig. 1).
3. Results and discussion
3.1 Synthesis and characterization of dual-emitting
microspheres
The fabrication process of the dual-emitting hollow micro-
spheres were shown in Fig. 1. In the rst step, the carbon dots
(CDs) were prepared by organic-based surface-capping
method.34 The citric acid (CA) worked as C source which
would carbonized under high temperature to form CDs and
gave off strongly blue photoluminescence (PL). Moreover, as-
prepared CDs exhibited good dispersion and high stability in
chloroform solution due to the OA and OLA as surface passiv-
ation. Using typical sol–gel method,35 the hollow CDs/TiO2
microspheres were prepared and as-prepared microspheres
displayed the blue PL of CDs. Moreover, the TiO2 surface can be
modied with suitable functional groups, such as thiol,
carboxyl, and amine groups.36–38 Herein, the surface of CDs/TiO2
hollow microspheres would be modied with amine groups
through conjugating with (3-aminopropyl)trimethoxysilane
(APS). Then the dual-emission uorescent hollow microspheres
could be formed through carbodiimide-activated coupling
reaction between glutathione stabilized gold nanoclusters
(GSH-AuNCs, red PL) and amino-functionalized CDs/TiO2
hollow microspheres (blue PL).39
The optical properties of resultant products were displayed
in Fig. 2. In chloroform solution, resultant CDs showed an
emission peak at 436 nm upon excitation at 365 nm (Fig. 2a).
Fig. 2 Fluorescence spectra of (a) CDs; (b) CDs/TiO2 hollow micro-
spheres; (c) AuNCs; (d) dual-emission fluorescent hollow
Moreover, the diluted solution of the as-prepared CDs in microspheres.
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Moreover, as a natural biomolecule, GSH owned several func- thicknesses of 66 nm. Seeing the SEM image (Fig. S3†), we
tional groups including carboxyl and amino groups, made NCs known that the surfaces of the resultant CDs/TiO2 microspheres
be benecial for their surface modication to achieve further were very smooth.
applications in wide range.11,42 Herein, dual-emission uores-
To build the dual-emitting ratiometric nanosensor, the GSH-
cent nanosensor can be formed by carbodiimide-activated AuNCs are conjugated with formed CDs/TiO2 hollow micro-
coupling of GSH-AuNCs (red PL) and amino-functionalized spheres. Their architecture of the ratiometric nanosensors are
CDs/TiO2 hollow microspheres (blue PL). Following excitation studied in detail. TEM image indicated that the as-prepared
at a single wavelength (365 nm), the hollow microspheres ratiometric nanosensors were well-dispersed with the average
exhibited two different luminescence peaks, one from the size of about 210 nm and the shell thicknesses were 58 nm
AuNCs (red PL) and the other from the CDs (blue PL), as shown (Fig. 3d). As can be seen, the surface of CDs/TiO2 hollow
in Fig. 2d.
microsphere was very smooth (Fig. 3b). Different from them, as-
Then the morphology and structure of resultant products prepared dual-emitting ratiometric nanosensor showed
a
were conrmed by transmission electron microscope (TEM) comparatively rough surface (Fig. 3d), suggesting the successful
and X-ray photoelectron spectroscopy (XPS). As shown in attachment of AuNCs on the surfaces of CDs/TiO2 hollow
Fig. 3a, as-prepared CDs were distributed in the range from 1.8 microspheres. Although the size of TiO2 microspheres was
to 3.6 nm with an average size of 2.5 nm. Moreover, there were larger than 200 nm, there was no PL signal in their solution with
no formation of larger size C nanoparticles or aggregation due various concentrations (Fig. S4a†). While the PL of dual-
to OA and OLA as surface passivation agent.34 And TEM image emission microspheres decreased with decreasing the concen-
indicated that the as-prepared AuNCs had a uniform dispersion tration, but the PL intensity ratio would not change with various
without apparent aggregation and particle diameters were 1.6– concentrations (Fig. S4b†), which conrmed the blue PL came
2.5 nm with the average size of 2.1 nm (Fig. 3c). The size from the doped CDs, rather than the scattering of TiO2 micro-
distribution of CDs and AuNCs were shown in Fig. S2.† It is well spheres. When the GSH-AuNCs were coated on the surface of
known that the organic-soluble CDs could be easily wrapped by the CDs/TiO2 hollow microspheres, a well-resolved dual emis-
the TiO2 shell to avoid the direct contact of CDs with compo- sion spectrum peaked at 436 nm and 596 nm was displayed at a
nents in the sample. In this procedure, CDs were extremely single excitation wavelength (365 nm) (Fig. 2d). The corre-
stable in chloroform solution as a nonpolar solvent. However, sponding uorescence color of the ratiometric nanosensor was
organic-soluble were unstable in acetone/chloroform mixture shown in Fig. 2d for a direct comparison with those of the blue
since acetone is a polar solvent and particles incline to self- emission CDs/TiO2 (Fig. 2b) and the red emission AuNCs
assemble into clusters in the mixture of solvents.43,44 This may (Fig. 2c). Clearly, a signicant change of uorescence color
cause the fact that emulsion drops would be formed by the under a UV (l ¼ 365 nm) lamp was observed (inset of Fig. 1).
acetone as the emulsier when chloroform-suspended nano- Moreover, the elemental composition of dual-emission uo-
particles were added to acetone.45 During the sol–gel process, rescent microspheres were conrmed by XPS. The binding
the hollow structure usually appeared.46,47 The morphology of energy of Au 4f located at 88.1 eV and 84.3 eV respectively
CDs/TiO2 microspheres was conrmed by TEM and SEM. As (Fig. S5a†) could explicitly demonstrate that the electronic
shown in Fig. 3b, the as-prepared CDs/TiO2 hollow micro- structure of AuNCs.48 The binding energy of C 1s was matched
spheres were fairly uniform in shape and size, and the average with the previously prepared CDs (Fig. S5b†).34,49 Thus, these
diameter of the microspheres was about 207 nm with shell results indicated that the red emission AuNCs were successfully
conjugated onto the surface of the blue CDs-embedded TiO2
hollow microspheres and resultant products showed dual-
emitting PL under a single excitation.
3.2 High stability of dual-emitting nanosensor
The stability of luminescent nanomaterials is an important
factor to assess their applications.11,50 Therefore, to demonstrate
effective detection performances of dual-emitting nanosensor,
the stability of the nanosensor was rstly investigated. As shown
in Fig. 4a, the photo-stability of the as-prepared ratiometric
nanosensor was systematically investigated by exposing them
under a 450 W xenon (Xe) lamp for various time spans. The
relative corresponding effect can be quantitatively evaluated by
analyzing the dependence of the PL intensity ratio (I596/I436).
Aer 12 consecutive illumination at 365 nm (10 min for each
time), the relative uorescence intensity ratio had no apparent
change, implying the photo-stability of the ratiometric uores-
cence nanosensor in aqueous solution (Fig. 4a). The uores-
cence spectra of the ratiometric uorescence nanosensor
Fig. 3 TEM image of (a) CDs; (b) CDs/TiO2 hollow microspheres; (c)
AuNCs; (d) dual-emission fluorescent microspheres.
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Fig. 4 The fluorescent stability of dual-emission nanosensor: (a)
photo-stability of as-prepared nanosensor radiated by a 450 W Xe light
at various time; (b) the pH stability of nanosensor in pH range from 4.9
to 8.3; (c) the metal stability of nanosensor in various metal ions with
the concentration of 200 mM.
Fig. 5 (a) PL spectra of dual-emission fluorescent nanosensor with
increasing temperature from 20 to 80 ꢀC (top to bottom); (b) the ratio
of the intensity at 596 nm and 436 nm (I596/I436) is plotted versus
temperature and good temperature resolution (blue line, right axis); (c)
fluorescence photographs of the ratiometric nanosensor are taken
under a UV lamp with an excitation wavelength at 365 nm.
irradiated by a Xe lamp at various time in detail were shown in
Fig. S6a.†
When the uorescence nanosensors are used for intracel-
lular nanothermometry, the materials must keep stable toward
intracellular environments. PL intensity ratio of the uores-
cence nanosensors toward physiological pH and physiological
ions in solution were measured. Fig. 4b presented the PL
intensities of uorescent nanosensor at different pH values. The
results revealed that the material displayed stable uorescence
intensity ratio with different pH values. We also investigated the
effect of ionic strength on the stability of uorescent nano-
sensor in the solution containing different ions. As shown in
Fig. 4c, even in the physiological ionic strength (200 mM of KCl
or NaCl), the PL intensity-ratio remained constant with different
ions. The uorescence spectra of the ratiometric uorescence
nanosensor toward physiological pH and physiological ions in
detail in Fig. S6b and c.† Compared to previously reported dual-
emitting nanosensors,18,25 as-prepared dual-emitting hollow Fig. 6 Temperature-induced switching of the emission intensities
ratio of the dual-emitting nanosensor (alternating between 20 ꢀC and
60 ꢀC).
microspheres indicated ultra-stable PL in solution environment
due to the TiO2 as a protective shell. Moreover, this novel dual-
emitting hollow microspheres could be easy purication.
Therefore, these advantages made them have particular
appropriate for practical applications.
intensity of red emission from the reactive GSH-AuNCs showed
continuous quenching, whereas that of blue emission from the
CDs still remained constant (Fig. 5a). As shown in Fig. 5b, there
3.3 Optical response of dual-emitting nanosensor toward
temperature
is a good linear relationship between temperature and uores-
cence intensity ratio. This linear relationship can be tted as a
function of R(T) ¼ 1.13286 ꢂ 8.3 ꢃ 10ꢂ3T with correlation
coefficient 0.998, where T and R(T) represent temperature and
ratio of uorescence intensity at 596 nm and 436 nm. Moreover,
the temperature resolution, evaluated as the product of the
inverse of the slope of intensity ratio versus temperature and the
standard deviation of the averaged uorescent intensity ratio,
was determined to be smaller than 0.5 ꢀC (Fig. 5b), which is
better than the resolution of previous uorescent thermome-
ters.24,25,51 Thus the uorescent nanosensor could be acted as an
To explore the potential applications of the dual-emitting
hollow microspheres as ratiometric optical thermometry, we
investigated their temperature-dependent response. The uo-
rescence responses toward temperature of them were measured
at the temperature ranged from 20 to 80 ꢀC (Fig. 5a). According
to previous research, GSH-AuNCs could work as versatile
nanothermometry devices by taking advantage of the tempera-
ture sensitivity of their uorescence lifetime and emission
intensity.13 Therefore, upon increasing the temperature, the
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ideal ratiometric temperature sensor with high reliability and emitting nanosensors could be promising in offering the dual
accuracy. Clearly, a distinguishable color change from original function of cellular imaging and temperature sensing at the
red to purple that can be clearly observed by the naked eyes molecular level.
(Fig. 5c). To further investigate the reproducibility of uorescent
nanosensor, the luminescence switching operations were
repeated for six consecutive cycles by multiple heating and
Acknowledgements
cooling cycles between 20 and 60 ꢀC. The results are presented This work was supported by the National Natural Science
in Fig. 6, and the corresponding spectra are shown in Fig. S7.† Foundation of China (No. 50925207 and No. 51432006), the
This fatigue-resistant performance suggests that the dual- Ministry of Science and Technology of China for the Interna-
emitting hollow microspheres may be applicable as PL ther- tional Science Linkages Program (No. 2011DFG52970), the
mometers and biosensors in cell monitoring.
Natural Science Foundation of Jiangsu Province, China (No.
This dual-emission uorescent temperature sensor could be BK20140157), the Changjiang Innovation Research Team
useful in the 10 mM phosphate buffered saline (PBS) solution (IRT14R23), the Programme of Introducing Talents of Disci-
(pH ¼ 7.4, Fig. S8†) and the ratios of uorescence intensities pline to Universities (111 Project B13025), and the Fundamental
showed a linear relationship with increasing temperature from Research Funds for the Central Universities (JUSRP11418).
20 to 45 ꢀC. Given this temperature range is larger than the M.G.H. thanks the Australian Research Council (ARC) for
physiological temperature, which suggests the as-prepared support.
uorescent nanosensor has promising applications in vivo
temperature sensing. Compared with traditional intensity-
based temperature sensors,52,53 this novel dual-emitting
Notes and references
sensors show resolvable luminescence emissions from two
different excited states. When temperature changed one or both
of these PL intensities, its effect is measurable from relative
instead of absolute PL intensities, reducing the impact of
extrinsic factors like uctuations in excitation rate, detection
efficiency, probe concentration, optical occlusion, or other local
inhomogeneities that alter absolute PL intensities.21 Although
the size of as-prepared dual-emitting microsphere is large,
similar microspheres have been applied in cellular
imaging.29,32,39,54 Moreover, this dual-emitting microspheres
show dual-emitting property under a single excitation wave-
length, small temperature resolution (0.5 ꢀC), high stability and
good dispersion in aqueous solution. Additionally, as-prepared
dual-emitting microspheres could be used as optical ther-
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ꢀ
mometry over the wide temperature range (20–80 C) aqueous
solution and in PBS solution with physiological temperature
ꢀ
ranging from 20 to 45 C, which suggests they have promising
applications in vivo temperature sensing.
10 H.-H. Wang, C.-A. J. Lin, C.-H. Lee, Y.-C. Lin, Y.-M. Tseng,
C.-L. Hsieh, C.-H. Chen, C.-H. Tsai, C.-T. Hsieh, J.-L. Shen,
W.-H. Chan, W. H. Chang and H.-I. Yeh, ACS Nano, 2011,
5, 4337–4344.
4. Conclusion
In summary, the novel dual-emitting microspheres were 11 C. X. Wang, Y. Wang, L. Xu, X. D. Shi, X. W. Li, X. Xu,
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AuNCs as uorophores. Resultant microspheres showed an 12 J. Bomm, C. Gunter and J. Stumpe, J. Phys. Chem. C, 2012,
ideal single-excitation, dual emission uorescence with peaks at
116, 81–85.
436 nm (blue PL) and 596 nm (red PL), respectively. The red 13 L. Shang, F. Stockmar, N. Azadfar and G. U. Nienhaus,
uorescence came from the AuNCs on the surface of micro- Angew. Chem., Int. Ed., 2013, 52, 11154–11157.
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microspheres would not change towards temperature. The 15 G. Kucsko, P. C. Maurer, N. Y. Yao, M. Kubo, H. J. Noh,
intensity ratio of the two emission wavelengths (I596/I436 P. K. Lo, H. Park and M. D. Lukin, Nature, 2013, 500, 54–59.
¨
)
showed a linear relationship with increasing temperature from 16 M. Mahmoudi, A. M. Abdelmonem, S. Behzadi,
ꢀ
20 to 80 C. Hence, such dual-emitting nanocomposites could
be used as nanosensors for accurate measurement of temper-
ature. Moreover, owing to TiO2 layer as scaffolds, resultant dual-
emitting temperature sensors showed good biocompatibility
and high stability. These features suggested resultant dual-
J. H. Clement, S. Dutz, M. R. Ejtehadi, R. Hartmann,
K. Kantner, U. Linne, P. Maffre, S. Metzler,
M. K. Moghadam, C. Pfeiffer, M. Rezaei, P. Ruiz-Lozano,
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