Ag/TiO₂ Nanocomposites for Nanothermometry in biological environment

Local temperature determination is essential to understand heat transport phenomena at the nanoscale and to design nanodevices for biomedical, photonic and optoelectronic applications [1]. In particular, with the emergence of photothermal therapy for the local treatment of cancerous tissues, the availability of nanothermometric techniques with high spatial resolution capable of remotely determining and modifying the intracellular temperature has become necessary. From this point of view, Raman spectroscopy is adequate for the purpose: the ratio between the intensity of the anti-Stokes and Stokes signals of a specific normal mode of vibration of an active Raman material, in fact, follows the Boltzmann distribution, linked to the temperature local. Titanium dioxide can be used as an optical material for temperature detection in biological samples, due to its high biocompatibility, already demonstrated in the literature [2], and to its strong Raman scattering signal. Moreover, the realisation of composite nanomaterials containing a metal and a Raman active material opens the way to their use in the field of photothermal therapy. The metal, silver nanoparticles, may act simultaneously as a nanoheater and as a plasmonic substrate, while anatase acts as a Raman nanothermometer [3].

In the present work, nanoparticles consisting of an Ag core, covered by a TiO₂ shell, Ag@TiO₂ core-shell, are suitably synthesised through a one-pot method. Silver nanoparticles synthesised in DMF are coated by controlled hydrolysis of titanium tetrabutoxide in the same reaction environment [4,5]. The synthesis led to nanocomposites where AgNPs are covered by a diffuse layer of anatase.

The nanocomposites are characterized by UV/Vis spectroscopy, Dynamic Light Scattering (DLS), X-Ray Diffraction (XRD), High Resolution Transmission Electronic Microscopy (HRTEM) and Raman spectroscopy. The samples obtained proved to be good Raman nanothermometers with a sensitivity superior to that of simple anatase nanoparticles. In particular, they showed maximum efficiency working at an excitation wavelength of 800 nm.

1. Bradac, C.; Lim, S.F.; Chang, H.-C.; Aharonovich I. Optical Nanoscale Thermometry: From Fundamental Mechanisms to Emerging Practical Applications. Optical Mater. 2020, 8, 200018.3. https://doi.org/10.1002/adom.202000183.
2. Yin, Z.F.; Wu, L.; Yang, H.G.; Su, Y.H. Yin, Z.F.; Wu, L.; Yang, H.G.; Su, Y.H. PCCP, 2013 , 15, 14, 4844. PCCP, 2013, 15, 14, 4844. https://doi.org/10.1039/c3cp43938k.
3. Zani, V.; Pedron, D.; Pilot, R.; Signorini, R. Contactless Temperature Sensing at the Microscale Based on Titanium Dioxide Raman Thermometry. Biosensors 2021, 11 (4). https://doi.org/10.3390/bios11040102.
4. Pastoriza-Santos, I.; Koktysh, D.; Mamedov, A.; Giersig, M.; Kotov N.; Liz-Marzán, L. One-Pot Synthesis of Ag@TiO2 Core-Shell Nanoparticles and Their Layer-by-Layer Assembly. Langmuir, 2000, 16, 2731-2735. https://doi.org/10.1021/la991212g.
5. Wang, P.; Wang, D.; Xie, T.; Li, H.; Yang M.; Wei, X. Preparation of monodisperse Ag/Anatase TiO2 core–shell nanoparticles. Materials Chemistry and Physics, 2008, 109, 108-183. https://doi.org/10.1016/j.matchemphys.2007.11.019