Effect of silica coating on luminescence and temperature sensing properties of Nd3+ doped nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2017.11.048

Structural and luminescence properties of uncovered YVO₄:Nd³⁺ and core-shell YVO₄:Nd³⁺@SiO₂ nanoparticles synthesized via modified Pechini technique were studied. Effect of silica shell thickness on steady state and kinetics luminescence parameters was investigated. Synthesized nanoparticles were used as non-contact optical thermometers operating in the second biological window upon heating-free 808 nm light by monitoring fluorescence intensity ratio between Nd³⁺ emission bands as a temperature-dependent parameter. Benefits of silica coating on the temperature sensing properties were clearly demonstrated. The best thermal sensitivity was found to be 0.40% K^?1 at 299 K, the minimum temperature uncertainty was less than 0.5 K, which is enough for accurate temperature evaluation in the real systems.

Full text of DOI:10.1016/j.jallcom.2017.11.048

Accepted Manuscript
3+
Effect of silica coating on luminescence and temperature sensing properties of Nd
doped nanoparticles
I.E. Kolesnikov, M.A. Kurochkin, A.A. Kalinichev, D.V. Mamonova, E. Yu. Kolesnikov,
A.V. Kurochkin, E. Lähderanta, M.D. Mikhailov
PII:
S0925-8388(17)33794-5
10.1016/j.jallcom.2017.11.048
JALCOM 43741
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 August 2017
Revised Date: 2 November 2017
Accepted Date: 4 November 2017
Please cite this article as: I.E. Kolesnikov, M.A. Kurochkin, A.A. Kalinichev, D.V. Mamonova, E.Y.
Kolesnikov, A.V. Kurochkin, E. Lähderanta, M.D. Mikhailov, Effect of silica coating on luminescence and
3+
temperature sensing properties of Nd doped nanoparticles, Journal of Alloys and Compounds (2017),
doi: 10.1016/j.jallcom.2017.11.048.
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ACCEPTED MANUSCRIPT
3
+
Effect of silica coating on luminescence and temperature sensing properties of Nd doped
nanoparticles
*
,a,b
a
a
c
d
I.E. Kolesnikov , M.A. Kurochkin , A.A. Kalinichev , D.V. Mamonova , E.Yu. Kolesnikov ,
a
b
c
A.V. Kurochkin , E. Lähderanta , M.D. Mikhailov
a
St. Petersburg State University, 7/9 Universitetskaya nab., 199034, St. Petersburg, Russia
Lappeenranta University of Technology LUT, Skinnarilankatu 34, 53850, Lappeenranta,
b
Finland
c
Scientific and Technological Institute of Optical Material Science, VNTs S. I. Vavilov State
Optical Institute, Babushkina 36-1, 192171, St. Petersburg, Russia
d
Volga State University of Technology, Lenin sqr. 3, 424000, Yoshkar-Ola, Russia
Contact information
e-mail address: ie.kolesnikov@gmail.com (I. Kolesnikov)
Abstract
3
+
Structural and luminescence properties of uncovered YVO :Nd
and core-shell
4
3
+
YVO :Nd @SiO nanoparticles synthesized via modified Pechini technique were studied.
4
2
Effect of silica shell thickness on steady state and kinetics luminescence parameters was
investigated. Synthesized nanoparticles were used as non-contact optical thermometers operating
in the second biological window upon heating-free 808 nm light by monitoring fluorescence
3
+
intensity ratio between Nd emission bands as a temperature-dependent parameter. Benefits of
silica coating on the temperature sensing properties were clearly demonstrated. The best thermal
-1
sensitivity was found to be 0.40% K at 299 K, the minimum temperature uncertainty was less
than 0.5 K, which is enough for accurate temperature evaluation in the real systems.
Keywords: Lanthanides, Nanothermometry, Core-shell, Thermal sensitivity, Biological
windows
1
. Introduction
Nanostructured materials have become a hot topic in almost all branches of science and
technology [1–3]. It is well known that such materials often demonstrate significant changes in
their properties compared with bulk materials due to the surface effect, quantum confinement
effect or lattice contraction. The geometric structure, chemical bonds, ionization potential,
electronic properties, optical properties, mechanical strength, thermal properties, magnetic
properties etc. are affected by particle sizes in nanometer range, so all of them can be improved
when switching from bulk to nanomaterials [4].
3
+
Trivalent lanthanide (Ln ) doped nanoparticles (NPs), due to their unique electronic and optical
properties, can be used as high-performance luminescence devices, magnets, catalysts, medical
diagnostics, biological fluorescence labels, etc. [5–9]. Recently, luminescence thermometry
3
+
based on fluorescence intensity ratio (FIR) method using Ln doped NPs emerged as useful
noninvasive technique with unique advantages ranging from high spatial resolution and detection
sensitivity to rapid response, opening new possibilities for the comprehension of the rules
governing the processes in biological fluids and fast objects [10–16]. One of the most attractive
1

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3
+
3+
Ln ion for thermometry is neodymium since Nd ions efficiently emit light in the NIR region
and particularly in the first (I-BW, 650–950 nm) and in the second (II-BW, 1000–1350 nm)
biological windows [17,18]. These optical regions are specially interesting for biomedical
application as radiation absorption by biological tissues is very low, permitting a deeper
3
+
penetration of the exciting and emitted radiations [19]. Moreover, the Nd ion can be excited at
about 800 nm, in the I-BW where the absorption by biological tissues is approximately 25 times
3
+
lower than for excitation at 980 nm, typically used to excite the Yb ion in up-conversion
3
+
3+
3+
3+
system such as Yb –Er or Yb –Tm [20].
3
+
Most prominent luminescence bands of Nd doped NPs are attributed to the transitions from
4
4
4
F3 metastable level. Energy gap between F metastable level and nearest lower lying I
15/2
/2
3/2
-1
–
level is about 5600 cm , which corresponds to the 1-2 high-energy phonons of OH vibrations.
Therefore, one can conclude that hydroxyl groups are strong quenchers for Nd luminescence.
In order to minimize vibration-induced deactivation processes, the luminescence centers should
3
+
–
be separated from OH groups typically situating on the surface of NP. Application of the core-
3
+
3+
shell design of Nd doped NPs protects Nd ion from solvent interactions. Such shell also
contributes to reduce collision-induced deactivation in solvent.
In this paper, we studied morphology, structure and luminescence properties of uncovered
3
+
3+
YVO :Nd and core-shell YVO :Nd @SiO NPs. Effect of silica shell thickness on
4
4
2
luminescence intensity and lifetimes was investigated. The synthesized NPs were used as
ratiometric luminescence nanothermometers operating in the biological windows. It was found
that the silica shell have enhanced the thermometric performance.
2
. Experimental
3
+
YVO :Nd nanocrystalline powders were synthesized with modified Pechini method that allows
4
obtaining of low agglomerated nanoparticles [21–23]. Firstly, yttrium (Y O ) and neodymium
2
3
(
Nd O ) oxides were dissolved in concentrated nitric acid (HNO ), then heated to form
2
3
3
corresponding yttrium and neodymium nitrates solution. As a next step the saturated citric acid
solution was prepared and added to mentioned nitrates mixture (with volume ratio 1:1) according
to the following reaction:
Me(NO ) +3C H O =[Me(C H O ) ](NO )
3 3
3
3
6
8
7
6
8
7 3
Further the solution containing vanadium ions (VO(C H O ) ) was performed by dissolving
6
7
7 2
vanadium (V) oxide (V O ) in citric acid.
2
5
V O + 5C H O = 2VO(C H O ) +C H O + ½ CO + H O
2
5
6
8
7
6
7
7 2
6
8
7
2
2
After that the ethylene glycol was added; volume ratio of ethylene glycol and citric acid solution
was 1:4. Polymer citrate transparent gel was formed by the reaction of etherification between
ethylene glycol and metals citrate complex. The obtained polymer gel was calcined at 500 °C for
1
hour into furnace to burn off the organic components, resulting in the formation of powder,
which was grinded in mortar with adding the potassium chloride in weight ratio 1:1. Then the
resulting powder mixture was calcined for the second time at 950 °C for 1.5 hours. The
3
+
calcination temperatures, durations and Nd doping concentration were investigated and
optimized in our previous research works [24,25]. After the second calcination procedure the
powder was centrifuged and washed three times with distilled water to remove potassium
3
+
chloride and, finally, dried. Thus, YVO :Nd 3 at.% nanocrystalline powders were synthesized.
4
2

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Silicon oxide coating on the fluorescent particles was made using Stöber process [26].
Isopropanol was distilled in two stages. Most of the water was expelled out of alcohol by zeolite.
Then isopropanol was distilled and the remaining water is extracted with sodium metal.
3
+
YVO :Nd 3 at.% powder (0.3 g) was immersed in 200 ml of dehydrated alcohol. Powder
4
particles were dispersed in the solution by sonicator for 5 min. Coarse particle fraction was
sedimentated during 24 hours then residue was educed out of the system. The obtained colloidal
3
+
solution of YVO :Nd 3 at.% NPs was divided into four equal parts. To prepare core-shell
4
particles with various shell thickness, calculated amounts of tetraethyl orthosilicate (TEOS) (25,
4
0 and 60 ꢀl) were added to the three aforementioned colloidal solutions, then these solutions
were stirred with magnetic stirrer for 24 hours. When the particles are coated with ethyl silicate
molecules, calculated amount of deionized water was added. The hydrolysis was carried out in
3
+
the system until an amorphous silicon oxide is obtained. As a result, uncovered YVO :Nd
(
4
3
+
VNd) and YVO :Nd @SiO NPs with different silica shell thicknesses were synthesized
4 2
(VNd_S25, VNd_S40 and VNd_S60).
Electron micrograph images were obtained with Zeiss Libra 200FE transmission electron
microscope (TEM) equipped with energy-dispersive X-ray spectroscopy detector. FTIR spectra
were measured using multiple attenuated total reflection (mATR) accessory (ZnSe) at
IRPrestige-21. Steady state and kinetics luminescence parameters were studied with modular
fluorescence spectrometer Fluorolog-3, whereas 808 nm single mode laser Coherent MBR-110
and T64000 Raman Spectrometer were used for thermal sensing experiments. The laser beam
was focused into the sample by using a 4x long working distance microscope objective (NA 0.1).
The fluorescence was collected by using the same microscope objective and was spectrally
analyzed by single spectrometer and a liquid nitrogen cooled Synapse CCD detector. The
thermal experiments were carried out using heating stage controlled with ThorLabs TC200 with
a resolution of 0.1 °C.
3
. Results and discussion
3
+
Fig. 1 shows TEM images at low and high magnification of YVO :Nd @SiO NPs. As can be
4
2
3
+
seen, the average size of YVO :Nd core is about 100 nm. 10 nanoparticles were analyzed to
4
obtain the average particle size. Thickness of silica shell depends on TEOS amount added during
synthesis. SiO layer thickness was derived from 20 measurements of silica shell for 5 different
2
nanoparticles. It was found that average thickness of SiO layer was determined to be (12±1) nm,
2
(14±2) nm and (18±2) nm for VNd_S25, VNd_S40 and VNd_S60 NPs, respectively. As can be
seen, an increase of TEOS amount during synthesis led to the monotonic increase of silica shell
thickness.
3
+
XRD pattern of YVO :Nd NPs presented in Fig. 1e can be indexed as pure tetragonal phase
4
without any structural impurities. All diffraction peaks match well with the standard YVO4
sample peaks (space group I41/amd, JCPDS No. 17-0341). Crystal structure of core-shell
3
+
YVO :Nd @SiO NPs was confirmed with the selected area electron diffraction (SAED)
4
2
technique (Fig. 1f).
3

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3
+
Fig. 1. TEM images of YVO :Nd @SiO NPs with different SiO thickness: a), b) VNd_S25, c)
4
2
2
3
+
VNd_S40; d) VNd_S60. e) XRD patterns of YVO :Nd NPs and standard card of YVO4
JSPDS 17-0341). f) SAED pattern of YVO :Nd @SiO NPs.
4
3
+
(
4 2
To study elemental analysis of VNd and VNd_S40 NPs EDX technique was used (Fig. 2). The
signals from yttrium, vanadium and oxygen were observed in both samples, whereas the signal
from silicon was found only for core-shell NPs. Presence of carbon ions is due to the special
sample preparation for this analysis.
3
+
3+
Fig. 2. EDX analysis of a) uncovered YVO :Nd NPs and b) core-shell YVO :Nd @SiO NPs
4
4
2
(VNd_S40).
Fig. 3 shows the FTIR spectra for the uncovered VNd and core-shell VNd_S40 NPs measured
-1
using mATR system. Spectral signatures from the VNd NPs include a peak at 800 cm which is
assigned to the V-O bonds of the VO group. Core-shell VNd_S40 NPs also demonstrates V-O
4
-1
-1
vibration at 800 cm as well as strong band centered at 1100 cm with shoulders at 1160 and
4

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-1
1
205 cm which are assigned to the Si-O-Si vibrations: 4- and 6-fold SiO rings, linear chain of
4
Si-O-Si and 4-fold SiO rings, respectively [27]. FTIR results display strong evidence of
4
3
+
successful covering of YVO :Nd NPs with silica shell and no modifications of the
4
luminescence NPs themselves.
3
+
3+
Fig. 3. FTIR spectra of uncovered YVO :Nd and core-shell YVO :Nd @SiO NPs
4
4
2
(VNd_S40).
3
+
3+
Fig. 4a shows emission spectra of uncovered YVO :Nd and core-shell YVO :Nd @SiO NPs
4
4
2
colloidal solutions upon 315 nm excitation. As can be seen, observed spectra consist of
characteristic narrow lines attributed to the intra-configurational 4f–4f transitions in the
4
4
neodymium ions. The emission spectrum is dominated by line corresponded to the F – I
11/2
3
/2
transition with maximum at wavelength 1063 nm. Emission bands situated at 865–930 nm and
4
4
4
4
1
330–1400 nm are assigned to F3/2 – I and F – I
transitions, respectively. The most
9/2
3/2
13/2
prominent luminescence was demonstrated by VNd_S40 sample. Observed enhancement of
luminescence intensity with silica coating can be explained by more effective absorption of the
3
–
3+
excitation light and decrease of nonradiative decay rate during VO – Nd energy transfer due
4
–
to the reduction of OH groups quenching.
4
Excitation spectra of VNd and VNd_S40 NPs monitored at the most intensive transition F
I11/2 (λem = 1063 nm) are presented in Fig. 4b. These spectra consist of a broad band
–
/2
3
4
corresponding to the charge transfer (CT) from the oxygen ligands to the central vanadium atom
3
–
inside the VO4 ion (260–340 nm) and weaker peaks in the longer wavelength region
3+
corresponding to the transitions inside Nd ion. Excitation lines centered at 530, 591, 750, 807,
4
4
4
2
4
8
53 and 872 nm could be assigned to the following transitions I – G + G + K , I
–
9
/2
7/2
9/2
13/2
9/2
4
4
2
4
4
4
4
4
4
4
2
4
4
G + G + H , I – F + S , I – F , I – F + H and I – F [28–30]. It is
5/2
7/2
11/2
9/2
7/2
3/2
9/2
5/2
9/2
5/2
9/2
9/2
3/2
noteworthy that presence of silica shell resulted in slight shape change of charge transfer band.
5

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3
+
3+
Fig. 4. a) Emission spectra of YVO :Nd and YVO :Nd @SiO NPs (λ = 315 nm).
4
4
2
ex
3
+
3+
b) Excitation spectra of YVO :Nd and core-shell YVO :Nd @SiO NPs (λ = 1063 nm).
4
4
2
em
Excitation spectra are magnified by 500 times within spectral range of 380–950 nm. c) Energy
3
+
levels scheme of YVO :Nd NPs.
4
4
In order to study silica shell effect on kinetics parameters, lifetime of the F excited level at
3
/2
λem = 1063 nm was monitored. To obtain the best luminescence intensity we used excitation
through charge transfer (λ = 315 nm). The decay curves show a single exponential decay,
ex
which can be fitted by the equation:
t
−
τ f
I = I ⋅ e
(1)
0
4
where τ is lifetime of F level. As can be seen from inset of Fig. 5, presence and thickness of
f
3/2
4
silica shell does not affect observed lifetime. The average value of F lifetime was determined
to be 48 ꢀs. It should be noted that the observed lifetime of YVO :Nd 3 at.% NPs in colloidal
solution is similar to lifetime of Nd -doped YVO powders studied previously [25].
3
/2
3
+
4
3
+
4
6

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3
+
3+
Fig. 5. Decay curves of YVO :Nd and YVO :Nd @SiO NPs (λ = 315 nm, λ = 1063 nm)
4
4
2
ex
em
Nowadays, lanthanide doped nanoparticles attract much attention as thermal sensors [31–35].
Luminescence nanothermometry can be performed using various temperature-dependent
parameters such as emission intensity, spectral line position, lifetime, bandwidth, and
polarization [36]. Most frequently thermal sensing is based on comparing the relative intensity
between different spectral lines. This approach is called “ratiometric” and it does not require
additional reference measurements [37,38]. FIR prevents errors in measurements arising from
power fluctuations of the excitation source, variations on the concentration of luminescent
nanoparticles, and inhomogeneities of the material [39].
3
+
Recently, we reported that YVO :Nd NPs can be used as effective nanothermometers in both I-
4
3
+
BW and II-BW [40]. Effect of silica shell coating of YVO :Nd NPs for nanothermometry in II-
4
BW was studied and discussed below. Fig. 6 shows normalized emission spectra generated upon
8
08 nm excited VNd and VNd_S40 colloidal solutions as obtained at two different temperatures
o
(26 and 58 C). To avoid laser-induced heating of colloidal solutions, laser power was kept
4
4
4
minimal (10 mW). Usually, intensity ratio of F (1) – I (1) (1064.7 nm) and F (2) –
3
/2
11/2
3/2
4
I11/2(3) (1072 nm) transitions is used to determine local temperature. However, the new strategy
to enhance thermal sensitivity was recently suggested [41]. It was found that temperature
changes most noticeably affect relative intensity of “valley” centered at 1066.3 nm, and therefore
fluorescence intensity ratio between largest peak (1064.7 nm) and “valley” 1066.3 nm was used
for thermal calibration. It should be noted that FIR has been calculated using integral intensities
(spectral boundaries are shown in Fig. 6). The integration intervals have been chosen to
minimize the overlap between neighboring bands resulting from different transitions.
7

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Fig. 6. Normalized emission spectra of (a) VNd and (b) VNd_S40 colloidal solutions obtained at
different temperatures (λ =808 nm). The shadowed areas are used for the integral intensity ratio
ex
calculations.
3
+
3+
The variations of the FIR values of uncovered YVO :Nd and core-shell YVO :Nd @SiO NPs
4
4
2
colloidal solutions as a function of the temperature are shown in Fig. 7, together with the error
bars. It can be noted that FIR values decrease monotonically with increase of the temperature
demonstrating pseudo-linear trend in the bio-physical temperature range for all samples. The
dependence of the FIR on the temperature is based on the population redistribution of Stark
levels and is ruled by Boltzmann formula. Definition of local temperature from observed ratio
becomes easier, when linear function is used instead of exponential one.
8

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Fig. 7. Fluorescence intensity ratio of (a) VNd, (b) VNd_S25, (c) VNd_S40 and (d) VNd_S60
colloidal solution as a function of temperature. Red lines correspond to the best fitting.
The absolute thermal sensitivities (S), can be defined as [38,42]:
ꢂꢃ
ꢀ
= ꢁ ꢁ
(2)
ꢂꢄ
As FIR values demonstrate linear dependence on temperature in the studied range, the absolute
thermal sensitivity can be considered as constant that is equal to the slope of FIR vs T plot. The
obtained absolute sensitivity monotonically increased along with silica shell thickness from
–
1
–1
1
.66% K for VNd NPs to 2.14% K for VNd_S60 NPs.
To compare the performance of different thermometers independently of their nature, we
considered the figure of merit described by the relative sensitivity S , defined as [39]:
r
ꢆ ꢂꢃ
ꢀꢅ = ꢁ ꢁ
(3)
ꢃ ꢂꢄ
3
+
3+
The relative thermal sensitivities of uncovered YVO :Nd and core-shell YVO :Nd @SiO
2
4
4
NPs as a function of the temperature are presented in Fig. 8 with the relevant uncertainties. As
can be seen, S increases with the temperature for all samples: from a value of
r
−
1
−1
−1
0
0
.31% K @299 K to 0.35% K @331 K for VNd NPs and from 0.40% K @299 K to
.45% K @331 K for VNd_S60. Among Nd single doped nanothermometers operating in the
−
1
3+
I&II-BW upon I-BW excitation, the obtained S values are one of the highest reported in the
r
-1
3+
literature. For example, the thermal sensitivity was determined to be 0.15% K for Nd :YAG
NPs [43], 0.12% K for Nd :NaYF NPs [44], 0.26% K for Nd :LaF NPs [42], and 0.22% K
for LiNdP O NPs [45]. Some Nd -doped NPs excited with visible wavelength demonstrated
-1
3+
-1
3+
-
4
3+
3
1
4
12
9

ACCEPTED MANUSCRIPT
3
+
-1
3+
3+
-1
higher relative sensitivity (Gd O :Nd NPs – 1.75% K [18], SrF :Gd ,Nd NPs – 0.61% K
2
3
2
[
20]), however due to high extinction of excitation wavelength by biological tissue, possible
biomedical application of such nanothermometers is limited.
3
+
3+
Fig. 8. Relative thermal sensitivities for the YVO :Nd and YVO :Nd @SiO NPs colloidal
4
4
2
solutions as a function of the temperature.
A very relevant parameter which is commonly used to compare the thermometric performance is
the minimum temperature uncertainty (ꢁT). This parameter defines the accuracy of temperature
evaluation that can be achieved in real experimental conditions [46]:
ꢆ ꢊꢃ
∆
ꢇ = ꢈ
(4)
ꢃ
ꢉ
where δR/R is the relative uncertainty in the determination of the temperature. The temperature
uncertainty depends on the thermometer performance, quantified by S , as well as on the
r
experimental setup for the luminescence detection, since δR/R is directly linked to signal-to-
noise ratio [10]. ꢁT values were determined to be 0.44 K, 0.48 K, 0.30 K and 0.50 K for VNd,
VNd_S25, VNd_S40 and VNd_S60 NPs, respectively. It should be noted that all
nanothermometers can be used for thermal sensing with sub-degree resolution.
Repeatability is a key factor for the assessment of the precision of a thermometric system,
referring to the variation in repeating the measurement under identical conditions [10]. Fig. 9
presents a thermal cycling experiment, where the temperature was determined in consecutive
heating-cooling cycles. The thermal sensing was performed using two independent methods:
luminescence nanothermometry and thermocouple measurements. As can be seen, both methods
gave the same results taking into account thermal uncertainties.
1
0

ACCEPTED MANUSCRIPT
Fig. 9. Calculated and measured temperature in heating-cooling cycles for (a) VNd, (b)
VNd_S25, (c) VNd_S40 and (d) VNd_S60 colloidal solutions.
4
. Conclusion
3
+
In this work, YVO :Nd 3 at.% NPs have been prepared with modified Pechini method. Silicon
4
oxide coating of nanophosphors was carried out with Stöber process and proved using TEM,
3
+
3+
EDX and IR spectroscopy. YVO :Nd and YVO :Nd @SiO NPs showed typical narrow lines
4
4
2
attributed to the intra-configurational 4f–4f transitions in both emission and excitation spectra.
3
+
YVO :Nd @SiO (VNd_S40) NPs demonstrated almost 4-fold enhancement of luminescence
4
2
3
+
4
intensity comparing with uncovered YVO :Nd NPs, whereas F lifetime did not depend on
presence and thickness of silica shell. A ratiometric technique involving the Nd NIR emission
bands in the II-BW was used to provide thermal sensing in the 25–60 C temperature range. It
was found that covering of YVO :Nd NPs by silica shell noticeably increase the relative
thermal sensitivity from 0.31% K for VNd to 0.40% K for VNd_S60. All studied
nanothermometers demonstrated sub-degree resolution (ꢁT ≤ 0.5 K) which is low enough to
monitor the temperature variations during photothermal treatment. Thermal cycling experiments
showed good repeatability of both uncovered and core-shell nanothermometers.
4
3/2
3
+
o
3
+
4
−
1
−1
Acknowledgments
This research has been supported by the Russian Science Foundation (№ 17-72-10055).
Experimental investigations were carried out in “Center for Optical and Laser materials
research” and “Interdisciplinary Resource Center for Nanotechnology” (St. Petersburg State
University).
1
1

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Highlights
3
+
3+
•
•
•
•
•
YVO :Nd and YVO :Nd @SiO NPs was used as ratiometric optical thermal sensors.
4 4 2
SiO effect on steady-state and kinetics luminescence parameters was studied.
2
Thermal calibration was obtained for bio-physical temperature range.
The absolute and relative thermal sensitivities of nanothermometers were defined.
Sub-degree thermal resolution was achieved using all nanothermometers.