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Non-Radiation Cellular Thermometry based on Interfacial Thermally Induced Phase Transformation in Polymer Coating of Optical Microfiber

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A non-radiation based approach based on thermoresponsive polyer coated silica microfibers has been developed. A highly thermoresponsive and bio-compatible pNIPAM was surface functionlized to conjugate to the tapered silica microfiber with waist diameter of 7.5 μm. The interfacial phase transtition of coating triggered by the LCST cause a drastic molecular morphological change in body temperature range of 35-42 oC. This surface morphological change strongly modulates optical path difference between the high order and fundamental mode propagating in the microfiber due to the evanescent-field interaction, and therefore shifts the intermodal interference fringe. Owing to the non-radiation based nature, the thermoresponsive polymer coated microfiber enables an improved thermal sensitivity of 18.7 nm/oC and hence a high temperature resolution of milli-degree. Furthermore, the micrometer sized footprint enables its easy implantation in human organs for cellular thermometry and the potential of in-vivo applications.
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Nonradiation Cellular Thermometry Based on Interfacial Thermally
Induced Phase Transformation in Polymer Coating of Optical
Microber
Yunyun Huang, Tuan Guo, Zhuang Tian, Bo Yu, Mingfei Ding, Xiangping Li, and Bai-Ou Guan*
Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics Technology, Jinan
University, Guangzhou 510632, China
*
SSupporting Information
ABSTRACT: A nonradiation approach based on thermoresponsive polymer coated silica
microbers has been developed. A highly thermoresponsive and biocompatible poly(N-
isopropylacrylamide) (pNIPAM) was surface functionlized to conjugate to the tapered
silica microber with waist diameter of 7.5 μm. The interfacial phase transtition of coating
triggered by the lower critical solution temperature (LCST) causes a drastic molecular
morphological change in the body temperature range of 3542 °C. This surface
morphological change strongly modulates optical path dierence between the higher order
and the fundamental mode propagating in the microber because of the evanescent-eld
interaction and, therefore, shifts the intermodal interference fringe. Owing to the
nonradiation-based nature, the thermoresponsive polymer coated microber enables an
improved thermal sensitivity of 18.74 nm/°C and, hence, a high-temperature resolution of
millidegree. Furthermore, the micrometer-sized footprint enables its easy implantation in
human organs for cellular thermometry and for the potential of in vivo applications.
KEYWORDS: cellular thermometry, interface, thermoresponsive polymer, coating, silica microber, sensor
INTRODUCTION
Sensitive thermometry at the micro- and nanoscales represents
an outstanding challenge in various principles of modern
science and technology.
13
In particular, the development of
ultrasensitive thermometers capable of millidegree temperature
resolution at cellular levels within a living system ushers in an
emerging research eld, which could underpin many signicant
biological and life science research including gene expression,
4,5
tumor metabolism,
6
and pathogenesis of disease.
7
The
considerable demand has evoked onrushing research on
single-cell nanothermal probes such as organic dyes,
8
uorescent polymers,
9,10
green uorescent proteins,
11
and
nanoparticles,
1214
which exhibit strong temperature-depend-
ent uorescent radiations in quantum yield, peak position, and
lifetime. Although promising, the nature of temperature-
dependent features in the uorescent radiation associated
with the electronphonon interaction on excited-state
relaxation sets up a fundamental physical limit on the thermal
sensitivity to 0.1 nm/°C,
12,13
temperature resolution of 0.5 °C within in the temperature
change range of human organs of 3542 °C.
10,15
In real-world applications, it is essential to develop
nonradiation thermometries, which are biocompatible and
which have miniaturization and ultrasensitivity. Here, we
developed a nonradiation-based approach by thermoresponsive
polymer coated abruptly tapered silica microber which serves
as interferometer and which produces interferometric fringe in
the transmission spectrum. A highly thermoresponsive and
biocompatible poly(N-isopropylacrylamide) (pNIPAM) was
rst aminated and then surface functionalized to conjugate to
the silica microber (Figure 1a). The phase transition of
pNIPAM triggered by the lower critical solution temperature
(LCST) causes a drastic molecular morphological change in the
interface of silica ber and detected environment. Because of
the restriction of the optical microber surface, the phase
transformation of polymer coating occurs in the body
temperature range of 3542 °C. The interfacial morphological
change was captured by the tapered silica microber because of
the strong evanescent-eld interaction and was translated into a
signicant wavelength shift in the interferometric fringe (Figure
1b). Owing to the nonradiation-based nature, the thermores-
ponsive polymer coated microber enables an improved
thermal sensitivity of 18.74 nm/°C which is 2 orders higher
than typical radiation-based thermometry and, hence, a high-
temperature resolution of millidegree. Furthermore, the
micrometer-sized footprint enables its easy implantation in
human organs for cellular thermometry and the potential of in
vivo applications (Figure 1c).
RESULTS AND DISCUSSION
As a highly thermoresponsive smart polymer with biocompat-
ibility and nontoxity, the pNIPAM has been widely used as a
Accepted: February 22, 2017
Published: February 22, 2017
Research Article
www.acsami.org
ACS Appl. Mater. Interfaces XXXX, XXX, XXXXXX
promising material for temperature-modulating controlled
release,
1618
which is employed here as the coating material.
The abruptly tapered silica microber with waist diameter of
7.5 μm(Figure S1) was fabricated by ame-brushing technique.
To conjugate to the surface of the silica microber, electrostatic
attraction was employed in the surface functionalization as
follows: the end of the pNIPAM chain was modied by amino
(Figure S2), while the surface of the silica microber was
functionalized by hydroxyl group, and then the aminated
pNIPAM was attached on the hydroxylated surface of the
microber by electrostatic attraction, as shown in Figure 1a.
The key to the ultrahigh thermal sensitivity of the
thermoresponsive polymer coated microber thermometer is
the phase transition of pNIPAM and the consequent
morphological change when the temperature increases above
the LCST. The LCST of our synthesized pNIPAM solution
without conjugating to the silica microber is 31 °C, which was
obtained from the temperature dependence of elastic light
scattering intensity as shown in Figure S3. It was previously
demonstrated that the LCST could be raised when one end of
the pNIPAM chain was restricted.
19,20
Here, after being
conjugated to the surface of the silica microber, the LCST
of interfacial pNIPAM was raised to 35 °C, which is close to
the physical temperature. As shown in Figure 2a, at temperature
below 35 °C, the pNIPAM chains on the microber are
extended, becoming well-solvated random coils which exhibit
little change with increasing temperature. This was further
corroborated by the atomic force microscope (AFM) images in
Figure 2d and g where the morphology of pNIPAM chains
presented smooth skinning on the microber surface. However,
when temperature slightly increased to above 35 °C, the
pNIPAM chains began to dehydrate (Figure 2b). Their
conformation underwent a coilglobuletransition and started
to aggregate.
19
The swollen chains were dehydrated, shrunk
gradually, and gave rise to collapsed microgels, wrapping on the
microber surface. Such a dramatic phase behavior change is
evidenced in Figure 2e and h. After continuous heating to 42
°C, pNIPAM chains shrunk and were further dehydrated
(Figure 2c), becoming poorly solvated random coils,
21
and
formed a lumpy skin on the ber surface as shown in Figure 2f
and i. During this process, water was squeezed out from the
pNIPAM skin of the microber surface. The thickness of the
pNIPAM skin on the microber was gradually reduced
14
until
the nal collapsed one with less than 50% of that of the swollen
microgels.
20
Consequently, this dramatic interfacial morphology change
was captured by the tapered silica microber because of the
evanescent-eld interaction. The abruptly tapered silica micro-
ber excites the higher order mode (HE12) which interferes
with the fundamental mode (HE11)andwhichcreates
interferometric fringe in the ber transmission spectrum. As
the HE12 mode spreads into the pNIPAM coating, it feels the
change of the pNIPAM morphology, and translates it into a
wavelength shift in the interference fringe. Figure 3d shows the
transmission spectra of the pNIPAM coated tapered microber
at dierent temperatures in the range of 3042 °C in water.
Figure 3e shows the transmission dip wavelength as a function
of temperature. When the temperature was below 35 °C, the
dip wavelength was insensitive to temperature. The eld
emission scanning electron microscope (SEM) image (Figure
3a) reveals a smooth and even pNIPAM coating on the
microber (the pNIPAM chains are extended, becoming well-
solvated random coils and forming a smooth surface on the
microber) at temperature below 35 °C. However, when the
temperature was increased to above 35 °C, the temperature
sensitivity was signicantly improved to 18.74 nm/°C in the
temperature range of 3542 °C, which is 2 orders of magnitude
higher than that of the typical uorescent radiation based
methods such as quantum dots of 0.1 nm/°C.
13
The SEM
images illustrate an uneven surface at 35 °C(Figure 3b) and a
lumpy surface on microber at 42 °C(Figure 3c). They prove
that the pNIPAM chains undergo a drastic phase change in a
physiological temperature range from 35 to 42 °C.
The thermal sensitivity of the naked microber without the
pNIPAM skin is only 0.17 nm/°C within the temperature range
of 3042 °C(Figure S4). In essence, it is the surface refractive
index (sRI) change induced by the interfacial pNIPAM phase
Figure 1. (a) The scheme of surface polymer functionalized microber
(inset: scanning electron microscope (SEM) image of it), (b) the
morphological change induced immense shifts in the interferometric
fringe of the transmission spectrum, and (c) photo of the microber
biosensor needle (inset: SEM image of it).
Figure 2. (ac) The schematic of the temperature-induced
morphological change of the pNIPAM skin in water. (df) The
height images and (gi) phase images of AFM of pNIPAM chain
coating on the microber surface; a, d, g, below 35 °C; b, e, h, above
35 °C; and c, f, i, above 42 °C (after drying).
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.7b00049
ACS Appl. Mater. Interfaces XXXX, XXX, XXXXXX
B
transformation that enables the high thermal sensitivity of the
polymer coated tapered microber. At temperature below the
LCST, as there was a large amount of water between pNIPAM
chains, it can be deduced that the sRI is close to the solution
refractive index at 35 °C. With the increase of temperature, the
pNIPAM chains were gradually dehydrated and nally became
a dried skin tightly coated onto the microber surface at 42 °C.
This indicates that sRI of the microber coated with pNIPAM
undergoes a dramatic change as temperature increases from 35
to 42 °C. The tapered microber induces interference between
the HE12 and the HE11 mode. The HE12 mode is highly
sensitive to the sRI because its mode energy spreads into the
pNIPAM coating. As a result, the dramatic sRI change induced
by the pNIPAM morphology transformation modulates the
phase dierence between the HE12 and HE11 mode and,
therefore, the interference pattern. For optical wavelength
encoded sensors, the resolution is determined by both the
response sensitivity of the sensor element and the wavelength
resolution of the readout unit. Typically, the commercial optical
spectrum analyzers could achive a wavelength resolution of 0.02
nm. This indicates that the pNIPAM coated microber
thermometer could enable a temperature resolution up to 1
×103°C (temperature resolution = R/S;Ris the spectrum
resolution of the optical spectrum analyzer (OSA); Sis the
temperature sensitivity). This provides the potential for
biological research and life science applications requiring ne
monitoring of cellular temperature.
Figure 4 presents the reversibility of temperature response in
coolingheating cycles. The heating and cooling were realized
by a water bath. The sensor was employed under a simple
condition (nonbiological condition) just as that in Figure 3e.
The pNIPAM coating exhibits excellent reversibility between
the swollen and dehydrated states without any fatigue. The
reversible collapse and swelling of the pNIPAM coating realize
the reversibility of the sensors temperature detecting in the
range from 35 to 42 °C.
The micrometer-sized footprint of coated microber enables
the easy implantation for ultrasensitive cellular temperature
sensing. Figure 5 demonstrates the capability of cellular
temperature sensing by the pNIPAM coated microber. The
coated microber was implanted into a cluster of rat breast
carcinoma cells, and the cellular temperature information can
be captured by the interfacial polymer phase transformation
and can be translated to the optical signal. In the temperature
range from 35 to 42 °C, the temperature sensitivity of the
sensor was 14.32 nm/°C. It was slightly lower than that in the
simple distilled water condition because there might be some
loss caused by cells and the culture uid. Nevertheless, this real-
world application sensitivity is still 2 orders of magnitude higher
than that of typical uorescent radiation based methods.
13
The
temperature resolution reached 1 ×103°C, which seems high
enough for further applications in temperature uctuation at
cellular levels within a living system.
CONCLUSIONS
A nonradiation-based approach by thermoresponsive polymer
coated silica microbers tapered to micrometer has been
developed. A highly thermoresponsive and biocompatible
pNIPAM was conjugated to the tapered microber surface.
Interfacial phase transition of pNIPAM triggered by the LCST
causes obvious sRI change in the temperature range of 3542
°C. As a consequence, the intermodal coupling between the
fundamental and the higher-order modes at tapered regions can
produce a large phase abruptly, allowing immense shifts in the
interferometric fringe of the transmission spectrum. Owing to
the nonradiation-based nature, the thermoresponsive polymer
coated microber enables a 2-order magnitude improved
sensitivity of 18.74 nm/°C and, hence, a high-temperature
resolution of millidegrees. Moreover, the micrometer-sized
footprint enables its easy implantation in human organs for
Figure 3. SEM images of microber with pNIPAM-coating at (a) 30, (b) 35, and (c) 42 °C; (d) transmission spectra of the pNIPAM-coating silica
microber interferometer at 30, 35, and 42 °C; (e) measured wavelength shift of the pNIPAM-coating silica microber interferometer as a function
of temperature (3042 °C, in simple condition) (dots: measured results; curve: linear tting).
Figure 4. Reversibility of temperature response over several cooling
heating (3542 °C) cycles.
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.7b00049
ACS Appl. Mater. Interfaces XXXX, XXX, XXXXXX
C
cellular thermometry and for the potential of in vivo
applications.
EXPERIMENTAL SECTION
Materials and Characterization. The materials and
characterization is provided in the Supporting Information.
Briey, aminated pNIPAM (NH2-pNIPAM) was prepared
following our previous work
19
(as shown in Figure S2) and was
dissolved in diluted water to form a solution.
Nonadiabatic Tapered Microber Sensor. The fabrica-
tion of nonadiabatic tapered microber was as previously
described by our group.
22,23
mode ber was abruptly tapered down to micrometer scale in
diameter by using the ame-brushing method. The ame with a
width of 5 mm scanned across the ber once while slowly
stretching the ber with two linear stages. The geometrical
parameters including the diameter of the ber and the length of
the transition regions were mainly determined by the moving
speeds of the ame and the stages.
Conjugating of pNIPAM onto Microber Surface. The
conjugating method is provided in the Supporting Information.
Experimental Setup and Optical Conguration. The
experimental setup was congured to allow it to operate in the
transmission mode. During the experiments, the sensor was
xed in a polydimethylsiloxane (PDMS)-based microuidic
channel designed specically for the temperature-sensing tests.
Water was injected into the microuidic chip to form a
temperature probe. Heatings and coolings by water bath
realized the temperature changing from 26 to 50 °C and from
50 to 26 °C. The temperature interval was 1 °C.
The sensing taper was excited by a broad-band source (BBS)
with the beam ranging from 1250 to 1650 nm. The
transmission spectra were monitored by an OSA with minimum
wavelength resolution of 0.02 nm. The measurements were
recorded continuously at a rate of 1 spectrum every 30 s.
Temperature Sensing of Rat Breast Carcinoma. The
probe was placed in the culture uid of the rat breast carcinoma
cells and was surrounded by cells. A water bath was employed
to realize the temperature from 26 to 50 °C.
ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.7b00049.
Expermential details about materials and reagents,
characterization, conjugating of pNIPAM onto micro-
ber surface, schematic geometry and transmission
spectrum of silica microber, schematic representation
of the pNIPAM-NH2fabrication process, thermal
property of pure pNIPAM, and thermal response of
naked silica microber
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: tguanbo@jnu.edu.cn. Phone: +86-20-8522206.
ORCID
Yunyun Huang: 0000-0001-7528-1001
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the nal version of
the manuscript.
Notes
The authors declare no competing nancial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (No. 51403077), the National Science
Fund for Distinguished Young Scholars of China (No.
61225023), the Guangdong Natural Science Foundation
(Nos. S2013030013302, 2014A030313387), and the Youth
Science and Technology Innovation Talents of Guangdong
(No. 2014TQ01X539).We acknowledge Otsuka Electronics
Co., Ltd for the measurement of the RI of pure pNIPAM, and
we thank Prof. D. Ma in Jinan University for oering the cells.
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