Regular Article
Development of fluorescent thermoresponsive nanoparticles for temperature monitoring on membrane surfaces

https://doi.org/10.1016/j.jcis.2016.09.059Get rights and content

Abstract

In this work, tris(phenantroline)ruthenium(II) chloride (Ru(phen)3) was immobilized in silica nanoparticles prepared according to the Stöber method. Efforts were devoted on the optimization of the nano-thermometer in terms of size, polydispersity, intensity of the emission and temperature sensitivity. In particular, the immobilization of the luminophore in an external thin shell made of silica grown in a second step on bare silica nanoparticles allowed producing fluorescent monodisperse silica nanoparticles (420 ± 20 nm). A systematic study was addressed to maximize the intensity of the emission of the fluorescent nanoparticles by adjusting the concentration of Ru(phen)32+ in the shell from 0.2 to 24 wt.%, whereas the thickness of the shell is affected by the amount of silica precursor employed.

The luminescent activity of the doped nanoparticles was found to be sensitive to the temperature. In fact, the intensity of the emission linearly decreased by increasing the temperature from 20 °C to 65 °C. The thermoresponsive nanoparticles were functionalized with long aliphatic chains in order to obtain hydrophobic nanoparticles. The developed nanoparticles were immobilized via dip-coating procedure on the surface of hydrophobic porous membranes, such as Polyvinylidene fluoride (PVDF) prepared via Non-Solvent Induced Phase Separation (NIPS), providing local information about the membrane surface temperature.

Introduction

Temperature is a key parameter affecting mostly all the chemical processes, included membrane processes. Nowadays, thermocouples are commonly used as accurate and inexpensive technologies with a rapid response time for temperature detection. However, the miniaturization of the sensors is the most critical issue of these technologies. In fact, monitoring the temperature with a high spatial resolution at micron and submicron scale is typically demanded in innovative research fields such as nanomedicine, biotechnology and microfluidics [1], [2], [3], [4], [5].

In the recent years, luminescent probes have been considered a powerful tool for the monitoring of the temperature at nanoscale, since the properties of their emissions (fluorescence or phosphorescence) are generally affected by the temperature [6], [7], [8]. Luminescent probes have been studied as a promising alternative since their immobilization in nanoparticles increases their optical properties (i.e. photostability, emission quantum yield, isolation from possible quenchers) [9], [10]. Luminescent probes silica-doped nanoparticles have been considered ideal for monitoring biochemical processes, since they are chemically inert, biocompatible, not subjected to microbial attack and with easy tailoring [11], [12]. Several luminescent probes such as rhodamine 6G, acridine orange, Ru(bpy)3 and Ru(phen)3 [13], [14], [15], [16], [17], were successfully immobilized on silica nanoparticles for the development of nano-thermometers. In fact, Ruthenium(II) polypyridyl complexes, in particular Ru(phen)3 and Ru(byp)3, are widely explored as luminescent probes for temperature detection because of the strong temperature dependence of their photochemical properties [18]. The thermal quenching of the emission is attributed to the thermal-driven non-radiative decay of the excited state, which leads to a decrease of the intensity of the emission and life time of the excited state, described by an Arrhenius-type model, approximated to a linear relationship at low temperature [19].

Temperature sensitive paints (TSP) based on the dispersion of emissive doped nanobeads or nanoparticles were successfully employed for two dimension temperature mapping in aerodynamics, providing information on aerodynamic performance and on heat transfer in structures [20], [21]. This technology could be of interest for the development of smart coatings for monitoring the temperature on membrane surfaces. Temperature plays a key role in membrane processes, in particular in those where the driving force is guaranteed by gradient of temperature across the membrane, such as direct contact membrane distillation. In this case, the heat transfer between the boundary layer of streams and membrane surface produces a temperature profile along the membrane surface (thermal polarization) which dramatically affects the global performance of the process [22], [23]. Efforts towards a better understanding of this phenomenon have been performed through the development of modelling studies. However, these studies have been limited by the lack of experimental data, since the non-invasive monitoring of temperature at the membrane surface with high enough spatial resolution conditions is a challenging task still not solved. Ali et al. studied the effect of various hydrodynamic and thermal conditions on heat and mass transport in direct contact membrane distillation using a cell containing different thermocouples [24]. Optical techniques present the advantage of a non-invasive monitoring and were employed for evaluation of temperature polarization in spacer-filled by means of Thermochromic Liquid Crystals [25].

The aim of this work is the development of a nano-thermometer for the measurement of the temperature on the surface of a membrane based on the attachment of fluorescent silica nanoparticles on its surface. Two different approaches will be evaluated for the immobilization of Ru(phen)32+ in silica nanoparticles: (i) immobilization of the probe in the silica matrix (Core NPs), (ii) immobilization of the probe in an external shell (Shell NPs) grown in a second step on bare silica nanoparticles (Bare NPs). Efforts have been devoted on finding a compromise between the photophysical properties and the nanoparticle morphology, in particular, size and polydispersity. Subsequently, Shell NPs will be functionalized to confer to the nanoparticles an hydrophobic behaviour and then immobilized on membrane surface via dip-coating procedure. The stability of the attached nanoparticles will be studied, as well as, the sensitivity to the temperature of the modified membrane.

In this work, silica nanoparticles doped with a luminescent molecule sensitive towards the temperature are proposed as an innovative technology for non–invasive, on-line, in-situ monitoring of the temperature in different processes. In particular, the membrane community will be benefit from this tool and will be able to validate theoretical models and answer important questions related to heat transfer and temperature polarization in membrane processes.

Section snippets

Materials

Tetraethoxysilane (TEOS), absolute ethanol (EtOH), ammonium hydroxide (ACS reagent 28%), Dodecyltriethoxysilane and the luminophore tris(phenantroline)ruthenium(II) chloride (Ru(phen)3) employed for the preparation of SiO2 NPs were purchased from Sigma Aldrich (Spain). Hydrophobic porous membranes made of poly (vinylidenefluoride) ((PVDF) Solef® 6012, Solvay Specialty Polymers, Italy) were prepared using the Non-Solvent Induced Phase separation (NIPS) technique. The solvent was N-

Optimization of fluorescent silica nanoparticles as nanothermomethers

Bare NPs were prepared using the well-known Stöber method based on the hydrolysis followed by the condensation of TEOS catalyzed by ammonia. According to SEM analysed micrographs (Fig. 2), monodispersed Bare NPs with an average diameter of 296 ± 19 nm were prepared, as confirmed by DLS measurements (275 ± 16 nm) reported in Table 2. Bare NPs showed a narrow size distribution with a polydispersity index of 0.05. On the other hand; Core NPs presented a wide size distribution showing a polydispersity

Conclusions

A nano-thermometer based on silica nanoparticles doped with Tris(1,10-phenanthroline)ruthenium(II) (Ru(phen)32+) was prepared. The immobilization of the luminophore in an external shell made of silica consented to prepare monodispersed nanoparticles. On the other hand the immobilization of the luminescent molecule in the core, made of silica, dramatically affect the polydispersity of the NPs. As expected, the thickness of the shell was affected by the concentration of the silica precursor,

Acknowledgements

Sergio Santoro would like to thank The Education, Audiovisual and Culture Executive Agency (EACEA) for the PhD grant under the Program “Erasmus Mundus Doctorate in Membrane Engineering” – EUDIME (http://www.eudime.unical.it). Artur Moro and Carla A.M. Portugal acknowledge the financial support of “Fundação para a Ciência e Tecnologia” (FCT-MCTES, Portugal) through the Post-Doc grants nr. SFRH/BPD/69210/2010 and SFRH/BPD/103619/2014. The People Program (CIG-Marie Curie Actions, REA grant

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