Temperature Response of Rhodamine B-Doped Latex Particles. From

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Temperature response of rhodamine B-doped latex particles. From solution to single particles Antonin Soleilhac, Marion Girod, Philippe Dugourd, Beatrice Burdin, Julien Parvole, PierreYves dugas, François Bayard, Emmanuel Lacôte, Elodie Bourgeat-Lami, and Rodolphe Antoine Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00647 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016

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Temperature response of rhodamine B-doped latex particles. From solution to single particles Antonin Soleilhac,a Marion Girod,b Philippe Dugourd,a Béatrice Burdin,c Julien Parvole,d Pierre-Yves Dugas, d François Bayard,d Emmanuel Lacôte,e Elodie Bourgeat-Lamid* and Rodolphe Antoinea*

a

Institut lumière matière, UMR5306 Université Claude Bernard Lyon1-CNRS, Université de Lyon 69622 Villeurbanne cedex, France. [email protected] b

Institut des Sciences Analytiques, UMR 5280/CNRS, ENS Lyon, UCB Lyon 1, Université de Lyon, Villeurbanne, France. c

Centre Technologique des Microstructures, Université Claude Bernard Lyon1, Bâtiment Darwin B, 5 rue Raphaël Dubois, 69622 Villeurbanne Cedex, France d

Université de Lyon, Univ. Lyon 1, CPE Lyon, CNRS, UMR 5265, Laboratoire de Chimie, Catalyse, Polymères et Procédés (C2P2), 43, Bd. du 11 Novembre 1918, F-69616 Villeurbanne, France. [email protected] e

Hydrazines, et Composés Energétiques Polyazotés (LHCEP), UMR 5278, CNRS, UCBL, CNES, HERAKLES-SAFRAN, Bâtiment Berthollet, 22 Avenue Gaston Berger, 69622 Villeurbanne Cedex, France

ACS Paragon Plus Environment

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ABSTRACT. Nanoparticle-based temperature imaging is an emerging field of advanced applications. Herein, the sensitivity of the fluorescence of rhodamine B-doped latex nanoparticles towards temperature is described. Sub-micrometer size latex particles were prepared by a surfactant-free emulsion polymerization method that allowed a simple and inexpensive way to incorporate rhodamine B into the nanoparticles. Also, rhodamine B-coated latex nanoparticles dispersed in water were prepared in order to address the effect of the dye location in the nanoparticles on their temperature dependence. A better linearity of the temperature dependence emission of the rhodamine B-embedded latex particles, as compared to that of free rhodamine B dyes or rhodamine B-coated latex particles, is observed. Temperaturedependent fluorescence measurements by fluorescent confocal microscopy on individual rhodamine B-embedded latex particles were found similar to those obtained for fluorescent latex nanoparticles in solution, indicating that these nanoparticles could be good candidates to probe thermal processes as nanothermometers.


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INTRODUCTION

Temperature is a fundamental property of matter and is probably the most fundamental parameter

in

science.

The

development

of

thermometry

on

a

sub-micron

scale

(nanothermometry) is tremendously stimulated by the current technological demands in areas such as microelectronics, microfluidics, and nanomedicine.1-2 Luminescence nanothermometry exploits the relationship between temperature and luminescence properties through variations of intensity, band-shape, spectral position, polarization, lifetime or bandwidth.3-4 Among the plethora of optical-based nanothermometry approaches, luminescence within nanoparticles (NPs) has attracted attention, with luminescent nanothermometers typically encompassing dyesensitized polymer dots,5-6 semiconducting quantum dots7-12 and lanthanide-doped NPs.13-17 The incorporation of organic components into polymer nanoparticles (PNPs) has attracted interest for the creation of functional materials with various applications. For example, polymer particles incorporating dye molecules (dyed polymer particles) are applicable to cell labeling,18 sensitive diagnostic reagents,19 multicolor optical coding for anti-counterfeiting20 and electronic inks.21 Dye molecules can be combined with PNPs using two strategies: embedding dye molecules during22 or after23-24 PNPs formation, or adsorbing dye molecules onto the surface of preformed PNPs.25-26 Encapsulation of hydrophobic molecules into hydrophobic particles, on both the nano- and microscale, has been applied in drug-delivery, imaging and diagnostic systems.27 Encapsulation isolates the dye from the disturbance of external environments, which may influence their spectral properties.28 The encapsulation of luminescent dyes is also a straightforward way to prepare temperature-sensitive nanomaterials. Rhodamine and its derivatives have long been known for the sensitivity of their fluorescence to temperature.29 The most common dye used for laser-induced fluorescence (LIF) temperature


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measurements, particularly in microfluidic systems,30-31 is rhodamine B (RhB).32-34 The LIF approach exploits the drop in RhB's emission quantum yield with increasing temperature, a consequence of the rotation of diethylamino groups on the xanthene ring.35-36 Three different strategies have been developed to resolve the temperature distribution in a solution containing RhB probes: the first utilizes the emission lifetime of the dye,37 the second uses a ratiometric evaluation of the fluorescence intensity,29 while the third relies on the change in the fluorescence bandwidth.38 The ratiometric approach generally involves the addition of a second fluorescent probe whose emission intensity does not vary with temperature.39 However, the two dyes must be chosen carefully such that their responses to factors other than temperature are closely aligned. Very recently, nanothermometers comprising two organic fluorophores encapsulated in a crosslinked polymethacrylate nanoshell were proposed.40 The role of the polymer shell around the fluorophores is to form a well-defined and stable microenvironment to prevent other factors besides temperature from affecting the fluorescence of the dyes. Hydrophobic rhodamine dyes can be directly entrapped within nanoparticles synthesized in situ by emulsion or miniemulsion polymerization of a hydrophobic vinylic monomer in the presence of the dye. To ensure that the dye is incorporated randomly and uniformly along the latex polymer backbone, rhodamine-labelled monomers are often used.41-43 Hydrophilic water-soluble dyes like RhB28, 44-45 or rhodamine 6G46-47 can also be directly incorporated into latex particles by emulsion polymerization. The process relies on in situ interaction of the dye molecules with the monomer and/or the growing polymer through hydrophobic and electrostatic interactions resulting in the dye being physically entrapped within the forming NPs. It is an operationally simple one-step straightforward procedure that does not require any post-synthetic treatment of the polymer nanoparticles nor the need for complex synthesis steps. As an alternative to in situ


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polymerization, hydrophobic and electrostatic interactions was also used to drive the adsorption of positively charged rhodamine dyes like rhodamine 6G or RhB onto preformed negatively charged latex beads using an ex situ approach.28, 48 In the present work, surfactant-free emulsion polymerization (SFEP) of styrene49 was used to produce isodisperse dye-doped particles using both in situ and ex situ approaches. SFEPs of hydrophobic monomers with ionic initiators are nice examples of self-regulating systems with respect to colloidal stability. During the course of the polymerization, the essential properties of the system (surface charge density, particle size, particle size distribution and morphology) develop in such a way that the free energy is minimized to ensure stability.50-51 Polystyrene (PS) latex particles with RhB located either in the core or at the particle surface were synthesized by SFEP. We then explored the temperature dependence and reversibility of the optical properties namely emission intensity, bandshift and bandwidth - of the free dye, the RhB-embedded latex particles as well as RhB-coated latex particles over the temperature range of 30 °C to 100 °C. Finally, temperature dependent-fluorescence measurements by fluorescent confocal microscopy on single RhB-embedded latex particles were reported in this work, indicating that these NPs could be good candidates to probe thermal processes as nanothermometers.

EXPERIMENTAL.

Materials. Styrene (99+%, Sigma) was used as received. Rhodamine B (RhB pure, Acros), sodium chloride (NaCl, Sigma) and the initiator: potassium persulfate (KPS, 98% from Acros, Fig. S1 in the Supporting Information), were used without further purification. Deionized water (PureLab Classic UV, Elga Lab Water) was used for latex syntheses and analyses.


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Fluorescent latex synthesis. RhB-embedded latex particles were produced by SFEP using KPS as initiator. Unless stated otherwise, emulsion polymerizations were carried out in a batch-wise process at 70°C under nitrogen atmosphere in a 300 mL glass reactor fitted with a condenser and a nitrogen gas inlet. Degassing of an aqueous solution of initiator was carried out for 30 minutes. At the same time, RhB was dissolved first in water and mixed with styrene, degassed and added in one portion to the aqueous phase with vigorous stirring before increasing the temperature to start the polymerization (sample 1 in Table 1). A similar procedure was used for synthesis of pure PS latex (sample 2 in Table 1) except that no dye was present and that the reaction was performed in the presence of NaCl to tune the particle size. The latter was dissolved in the aqueous solution at the same time as KPS (sample 2: [NaCl] = 0.04 mol L-1). The PS latexes were dialyzed against deionized water using a 23 mm-diameter cellulosic dialysis membrane (Spectra/Por1, 6-8k MWCO) before use. RhB-coated latex particles were obtained by adding 0.06 g of a 50 g L-1 aqueous solution of RhB to 5 g of the dialyzed latex suspensions (final RhB concentration = 0.6 g L-1, 1.25 mM), and kept under agitation. The RhB-coated latexes for fluorescence measurements were washed by successive centrifugation/redispersion cycles in deionized water. The experimental conditions for the polymerizations performed in this study are displayed in Table 1.


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Table 1. Experimental conditions and characterizations of all SFEPs performed in this study. The temperature was fixed at 70 °C. Reaction time was 7h for all experiments.

Entry

Zav. (DLS) Styrene KPS RhB (wt%/water) (wt%/styrene) (mol%/styr.) (µm)

Dn (TEM) Poly (DLS) (µm)

Dw/Dn (TEM)

1

11.1

5

0.1

0.89

0.09

0.93

1.2

2a

10.5

2

/

1.05

0.07

0.90

1.01

a

[NaCl] = 0.04 mol L-1

Size characterizations. Hydrodynamic particle diameters (Zav, nm) were measured by Dynamic Light Scattering (DLS) using a Malvern Zetasizer Nano ZS. A 633 nm wavelength laser beam was sent to an infinitely diluted sample and the scattered signal intensity analyzed at a 173° angle, at 25 °C. Note that the excitation wavelength is out of the absorption band of RhB doped latex particles. DLS allowed access to the particle sizes and the broadness of the size distribution (indicated by the poly value - the higher this value, the broader the size distribution) by computation using the cumulant analysis method. Transmission Electron Microscopy (TEM) was carried out at the Centre Technologique des Microstructures (CTµ, Claude Bernard University, Villeurbanne, France). For TEM measurements, a drop of the diluted latex suspension was deposited on a formvar-coated copper grid, and allowed to evaporate before observation with a Philips CM120 microscope operating at 80 kV. The number-average diameter (Dn = ΣniDi/Σni) was determined by counting more than 500 particles and the polydispersity index was calculated according to: PDI = Dw/Dn, with Dw = ΣniDi4/ΣniDi3, the mass-average diameter. From TEM data, particle numbers were calculated using equation (1).


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−1 ሻ

ܰ‫ ݌‬ሺ‫ܮ‬

6 × ߬ × 103 = ߨ × ‫ × ݎ݁݉ݕ݈݋݌݀ × ݊ܦ‬10−21

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ሺ1ሻ

With Np, the number of particles (L-1), τ the polymer concentration determined by gravimetry (g L-1), Dn (nm) the number average particle diameter determined by TEM and dpolymer the polymer density (g cm-3). Here, dpolymer was set to 1.05 g cm-3, the density of polystyrene.

Temperature-dependent fluorescence experiments

Temperature-controlled cell. The cell is an aluminum temperature-controlled tank and comprises three quartz windows sealed by O-rings (see Fig. S2 in the Supporting Information). The two in axis windows allow the injection and ejection of the laser. The collection of fluorescence can be performed either in an epi-fluorescence configuration or by using a third window at 90° and coupled to an optical fibre via a SMA connector. A heating finger (Thermocoax, Redring) and a PT100 thermocouple were inserted into the thermostat. This system allows accurate temperature control (from room temperature to ~150 °C) in the sample after temperature equilibration. The cell volume is about 3 mL.

Static emission spectra. The excitation laser was a 473 nm continuous wavelength laser (cw) (ACAL BFI, Evry, France) with output power ~500 mW and beam diameter 1.5 mm (divergence 1 mrad). Briefly, the laser beam was focused into the custom-build thermostated aluminum cell and the fluorescence collected by an objective used in an epi-fluorescence configuration. A fluorescence cube was placed into the objective, which along with filters, allowed collection of the emitted fluorescence in the same axis without background noise by separating the excitation and emission light components. Fluorescence spectra of fluorescent dyes were recorded at room


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temperature, using an ultra-compact Econic spectrophotometer (B&WTek Inc., Newark, DE, USA)52 and are presented in Fig. S3 in SI.

Laser scanning confocal microscopy (LSCM).

Micrography. Laser-scanning confocal microscopy analyses were performed at the Centre Technologique des Microstructures (CTµ, Claude Bernard University, Villeurbanne, France), using a confocal laser-scanning microscope (Zeiss LSM510META) with a 543 nm excitation wavelength. The emission signal was collected between 558 and 708 nm, and micrographs were collected on slices of 0.8 µm with an apochromatic ×63/1.4NA objective lens. As a reference, a PS latex was also analyzed and no fluorescence signal was observed. The settings of the emission source transmission power as well as the pinhole aperture, gain, and offset of the photomultiplier were accurately adjusted on the fluorescent latex and maintained for the characterization of the reference sample.

Temperature-dependent LSCM. The microscope is equipped with a thermo-regulated chamber that permits to change temperature from 25 °C to 50 °C. Analyses were performed with a 543 nm excitation wavelength. For spectral acquisitions, the emission signal was collected in the spectral configuration from 558 to 708 nm (with a wavelength step of 10 nm), with the same objective lens (Pinhole aperture = 2.5 Airy Unit). Spectra have been obtained by integrating the signal over the whole Point Spread Function (PSF) and summing the signal along the z axis (every 2 µm, over 10 µm) with an “open” pinhole. For single NP measurements, we took a confocal microscopy image (62.5µmx62.5µm) containing hundreds of NPs. Fluorescence spectra from 6 single rhodamine B latex nanoparticles were extracted and measured as a function of the temperature.


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Fluorescence spectrum extracted from a single Rhodamine B Latex Nanoparticles (812 nm) at T = 25 °C and its respective confocal microscopy image

RESULTS AND DISCUSSION

Synthesis and characterization of fluorescent RhB latex particles.

RhB-embedded latex particles were produced by SFEP in the presence of an anionic initiator (KPS) as described in the experimental section. In solution, RhB exists in a number of ionic and neutral forms depending on pH.53 In acidic conditions (due to the persulfate decomposition, known to result in a significant pH drop during polymerization), RhB is positively charged and prone to interact with KPS or with the negatively charged primary particles formed in the early stages of the polymerization, allowing their efficient incorporation into the final latex particles by means of electrostatic interactions. Such electrostatic interaction did not however affect the colloidal stability of the latex particles, which was ensured by the negative charges of the initiator fragments, which are in excess with regards to the amount of dye molecules introduced. The particle diameter can thus be easily adjusted from several hundreds of nanometers to few microns by simple change of the initiator concentration, the ionic strength (through salt addition) and/or the initial monomer composition and/or concentration (data not shown). The diameters of the latex particles used in this study are summarized in Table 1, and are around 900 - 930 nm as determined by TEM. The fluorescent properties of the particles were investigated using confocal microscopy. The confocal microscopy image of 930 nm diameter RhB latex particles synthesized via SFEP using KPS as initiator (sample 1 in Table 1) is shown in Fig. 1a. Uniform RhB latex particles displaying high fluorescent signal with red color are observed. Since the image represents a slice through the center of the beads, it is clear that RhB is homogeneously


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distributed throughout the polymeric particles and not merely adsorbed on their surface. Furthermore, analysis of the supernatant solution after latex centrifugation showed quantitative incorporation of RhB in the latex particles. Fluorescence spectroscopy performed on the supernatant solution shows an extremely weak signal. In other words, no free dye molecules could be detected in solution, indicating 100% RhB encapsulation efficiency. Calculations based on the feed ratios and the particle size thus gave an average number of dye molecules per particle of ~2x103.

a)

b)

Figure 1. Confocal microscopy images of : a) RhB-embedded latex particles (sample 1, Dn. = 930 nm) and b) RhBcoated on the PS surface, appearing as rings around the beads (sample 2, Dn = 900 nm, with adsorbed RhB).

For further validation, particles with only a surface coating of RhB were produced by adsorbing the cationic dye on the control particles (sample 2 in Table 1), and imaged in an identical fashion. They exhibited a distinctly different fluorescence, which was confined to the particles peripheries (Fig. 1b). This analysis confirmed the suitability of our approach to produce uniform


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fluorescent PNPs with controlled dye localization (either inside or at the particles surface) in a very simple manner.

Temperature response of RhB-embedded latex particles.

The luminescence of molecules is temperature-dependent due to the Boltzmann distribution of electrons within the excited state level.4 The dependence on T is a function of the specific vibronic band structure of the material. Thermal energy will excite the electrons within the excited state to the different vibrational states which are overlapping at different energy levels, and thus nonradiative transitions become possible at higher temperatures.4

Figure 2a shows the fluorescence spectra of the RhB-embedded latex particles (sample 1 in Table 1) in water as a function of temperature when excited at 473 nm. The dye-embedded particles undergo a significant change in luminescence intensity, peak position and bandwidth when heated. The right arrow shows the evolution of the maximum of fluorescence emission wavelength, which is temperature dependent. This temperature dependence might be due to the rotation of the diethylamino groups, which enables the formation of an alternative charge-shift quenching channel and a drop in the quantum yield.54 The intensity of the peak has a linear response to temperature changes over the range of 30-100 °C with high response sensitivity (– 0.93% per °C) (Fig. 2b).


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a)

b) 60

T = 30°C

1.0 Normalized Intensity

Intensity (a.u.)

50 40 30 20 10

T = 130°C

0 500

550

600

650

700

0.8 0.6 0.4

750

30

40

Wavelength (nm)

c)

50

60

70

80

90 100

Temperature (°C)

d) 54 595.0

53 FWHM (nm)

Maximum Wavelength (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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594.5

594.0

52 51 50 49 48

593.5

47 30

40

50

60

70

80

90 100

30

40

Temperature (°C)

50

60

70

80

90 100

Temperature (°C)

Figure 2. a) Fluorescence spectra of the temperature-sensitive RhB-embedded latex particles dispersed in water in the temperature range 30-130 °C. b) Intensity as a function of the temperature. c) Maximum peak position as a function of the temperature, or respectively, d) FWHM bandwidth as a function of the temperature. (red lines are linear fits of data sets).

Concomitantly, the maximum peak position exhibits a blue shift with a temperature sensitivity of –0.019 nm per °C, and a linear response between 30 °C and 100 °C, as shown in Fig. 2c. The decrease in fluorescence intensity is not uniform across the fluorescence band, with the fluorescence to the red side of the peak maximum showing a marginally smaller decrease. This change in shape can be used to determine the effect of temperature on the fluorescence


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bandwidth.38 The change in the width of the fluorescence profile is typically most prominent at a half of the maximum intensity (FWHM), and this is the relative intensity ratio at which we measured the bandwidths. The FWHM has a linear response to temperature changes over the range of 30-70 °C with a moderate response sensitivity (0.067 nm per °C in the linear part), as displayed in Fig. 2d. These changes both in shape and in maximum peak position represent a good alternative to the ratiometric approach, which as mentioned previously, consists in comparing intensities at two wavelengths, using two different dyes.

Effect of the dye location in the nanoparticles on the temperature response and reversibility.

To evaluate the effect of the dye location in the polymer particle, we compared the latex with the embedded dye to the RhB-coated particles. For that purpose, the cationic RhB was adsorbed on the anionic surface of the PS particles (sample 2, see Table 1). The RhB-coated particles display a similar fluorescence intensity dependence with the temperature (in the temperature range 3050 °C) than the free RhB dye (Fig. 3). However, at higher temperature (>50°C), the fluorescence intensity dependence of RhB-coated particles displays a lower slope. For the dye-embedded latex particles, the dependence is different and presents a better linear response over the range of 30100 °C as displayed in Figs. 3 and 2b. This means that the latex environment seems to have an effect on the energy gap between the lowest levels of the excited state. In other words, the rotation of diethylamino groups on the xanthene ring appears affected by the latex matrix. On the other hand, the dyes on the surface of particles are less affected by the polymer matrix and have a similar dependence with the temperature as the free dyes.


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An important issue is the reversibility of the temperature dependence emission of such fluorescent latex NPs. NPs show an irreversible alteration of their fluorescence after one heating/cooling cycle (when heated up to 100 °C), as shown in Fig. S4 in the Supporting Information. We performed DLS measurements of the RhB-embedded sample before and after heating (from 30°C to 100°C). The resulting size distribution of NPs after heating evidences signs of important agglomeration of the NPs (increases in size and polydispersity). However, in the 30-50 °C range, RhB-embedded particles presents a good reversibility both after one heating/cooling cycle (between 30 and 50 °C) or after 4 heating/cooling cycles (see in Fig. S5 in the Supporting Information). Additionally, we did fluorescence spectra on supernatants of RhB NPs solutions before and after heating (data not shown). As mentioned above, in the case of the RhB-embedded nanoparticles, there was no dye in solution indicating 100% encapsulation efficiency. Besides, the fluorescence intensity of the supernatant was unchanged after heating indicating the absence of leakage, while for the supernatant of RhB coated NPs, an increase of ~3.5 in the fluorescence intensity was observed showing that leaching of the dyes occured when the dyes was interacting electrostatically with the NP's surface.


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Normalized Intensity (a.u.)

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1.0 0.8 0.6 0.4 RhB-embedded NPs RhB-coated NPs RhB in solution

0.2 30

40

50

60

70

80

90 100

Temperature (°C)

Figure 3. Normalized fluorescence intensity as a function of the temperature in the temperature range 30-100 °C of RhB in water solution compared to the temperature response of RhB-embedded and RhB-coated latex suspensions. The dyed latex particles are dispersed in water and were diluted to give a final RhB aqueous solution concentration of 1 µM. Inset: Confocal microscopy images of RhB-embedded particles (Dn. = 930 nm) and RhB-coated particles (Dn = 900 nm).

Fluorescence response of temperature-sensitive RhB-embedded latex particles at the single particle level.

Thanks to spectral acquisition mode by fluorescence microscopy (as described in the experimental section), the temperature dependence of the fluorescence response of individual nanosensors can be determined for different temperatures (see Fig. S6 in the Supporting Information for further details). Fig. 4 shows that the averaged intensity of fluorescence, over the 6 individual NPs decreases when the temperature of the thermo-regulated chamber of our confocal microscope increases. When the fluorescence intensity is normalized, the response of the individual nanosensors is not statistically different from the fluorescence response of the


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dispersed nanosensors in solution. Similar to fluorescence measurements of NPs in solution (see Fig. S5 in SI), individual RhB-embedded particles presents a good reversibility after one heating/cooling cycle (between 30 and 50 °C), see Fig. S7 in SI. These RhB-embedded latex particles are thus good candidates to probe temperature changes between 30°C and 50°C with a spatial resolution in the micron range. These submicron particles could therefore find potential applications in the measurement of temperature in biological systems. Thermal imaging in cells is an emerging and exciting field of research.55 The thermoresponsive luminescent nanoparticles described in this work, in combination with gold nanoparticles, for example, would enable simultaneous targeting of cancer cells and optical imaging of temperature. Similarly, intracellular heat mapping was accomplished via the use of a polymeric nanogels.56 This intracellular heat mapping with sub-micron resolution would permit to show non-uniformity of cellular T distribution under heating of gold nanoparticles excited with laser light and so acted as localized heat sources.


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Normalized intensity (a. u.)

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1.1 Average over 6 single RhB-embedded NPs RhB-embedded NPs dispersed in Water

1.0 0.9 0.8 30

35 40 45 Temperature (°C)

50

Figure 4: Averaged intensity of fluorescence, over 6 individual NPs (see Fig. S6a in Supporting Information), as a function of the temperature compared to the temperature response of the same sample dispersed in water.

CONCLUSIONS

We have introduced a simple and versatile route to produce highly fluorescent RhB-doped latex particles. Temperature dependence of their steady-state fluorescence can be exploited to develop thermo-sensing devices at the micrometric scale. RhB-embedded NPs revealed a nearly linear and reversible response over the temperature (between 30°C and 50°C). The RhB-embedded latex nanosensors, were dispersed and dried on the surface of a conventional slide holder of a fluorescence confocal microscope. Temperature dependent changes in the fluorescence response of the individual nanosensors were evaluated and were found similar to those obtained for the RhB-embedded NPs in solution. Through modification of the synthesis protocol (by simple


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change of the initiator concentration, the ionic strength and/or the initial monomer concentration) and the introduction of super-resolution fluorescence microscopy,57 temperature mapping could be achieved using sub-100 nm temperature-sensitive nanosensors. Temperature-sensitive RhBembedded latex particles are simple and inexpensive to manufacture and could find potential application in the measurement of temperature in biological, micro-fluidic and micro-electrical systems.

ACKNOWLEDGMENTS

The authors are grateful for the financial support of Université de Lyon through the Program "Investissements d'Avenir" (ANR-1 1-IDEX-0007). We thank Vincent Joly and Xavier Dagany for the development of data acquisition for fluorescence, and Laure Cohendet and Mathilde Ghafar (MSc student) for their contribution to this work. Jacques Maurelli and Christian Clavier are acknowledged for their invaluable technical assistance.

Supporting Information. Chemical structure of potassium persulfate. Design and photo of the temperature-controlled cell. Fluorescence emission spectra of free RhB dye, and RhB-doped NPs in water. Reversibility of the temperature dependence emission of fluorescent latex NPs. Details of fluorescence response of individual nanosensors for different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.


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Temperature Response of Rhodamine B-Doped Latex Particles. From

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