fluorescence colors of PNIPAM materials covalently labeled
with 3-hydroxyflavones change from a bluish to greenish
color, providing excellent sensitivity and reversibility.
Moreover, the sensing temperature range lying in the range
of 33 to 41 1C warrants applications in intracellular imaging.
The ratiometric nature of the nanothermometer may
successfully overcome certain difficulties encountered by
conventional fluorescent molecular thermometers, ultimately
providing a more robust signal. Further applications of this
fluorescent nanothermometer in cell imaging are underway
and will be reported in due course.
Fig. 4 (a) Single-run and (b) multiple-run reversibility experiments of
the fluorescence responses of Thermo-3HF to temperature variation.
All experimental conditions and data processing are the same as those
shown in Fig. 3.
Notes and references
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of Thermo-3HF, it swells well in the aqueous medium,
subsequently providing a highly polar and protic local
environment around 3HF and leading to an ESICT-dominant
blue fluorescence emission. Around the LCST, Thermo-3HF
undergoes a significant phase transition with an increase in
temperature, resulting in a more hydrophobic shrunken state,
as reflected in the intensity redistribution of two emission
bands. The ESICT-related blue fluorescence emission band
gradually decreases in intensity with an increasing temperature,
whereas the ESIPT-related green fluorescence emission band
gradually increases in intensity with an increasing temperature.
Eventually, Thermo-3HF exhibits an ESIPT-dominant green
fluorescence emission when above the LCST. The color
changes in the sensitive range are rather apparent rendering
the differentiation of the emission colors by visual detection
feasible. Additionally, the ratios of integrated intensities of the
ESIPT and ESICT bands can be envisioned as a sensitive
indicator of microenvironmental temperature.
The reversibility of Thermo-3HF was also examined. Fig. 4
shows the single-run and multiple-run reversibility experiments
of the fluorescence responses of Thermo-3HF to temperature
variation. The results indicate that no hysteresis exists during a
cycle of heating and cooling (Fig. 4a) and no declining signal
occurs during multiple-run tests, monitoring the ratios of
integrated fluorescence intensities at 25 and 50 1C (Fig. 4b).
The excellent reversibility can be attributed to a highly hydro-
philic, ionic surface of Thermo-3HF, which prevents individual
nanogels from incurring severe intermolecular aggregation
through electrostatic repulsion.2a High temperature resolution
as well as high availability are thus achieved.
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´
In summary, this work demonstrates for the first time the
feasibility of applying ratiometric fluorophores to stimuli-
responsive materials. With an increasing temperature, the
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c
996 Chem. Commun., 2011, 47, 994–996
This journal is The Royal Society of Chemistry 2011