Silver-Titania Nanocomposites for Photothermal Applications

Abstract

Local temperature measurement is crucial for understanding nanoscale thermal transport and developing nanodevices for biomedical, photonic, and optoelectronic applications. The rise of photothermal therapy for cancer treatment has increased the demand for high-resolution nanothermometric techniques capable of non-contact intracellular temperature measurement and modification. Raman spectroscopy meets this need: the ratio of anti-Stokes to Stokes Raman intensities for a specific vibrational mode correlates with local temperature through the Boltzmann distribution. The present study proposes a novel photothermal therapy agent designed to advance the current state of the art while adhering to green chemistry principles, thereby favoring low-temperature synthesis involving limited energy consumption. A key challenge in this field is to achieve close contact between plasmonic nanosystems, which act as nanoheaters, and local temperature sensors. This is achieved by employing silver nanoparticles as a heat release agent, coated with anatase-phase titanium dioxide, as a local temperature sensor. The proposed synthesis, which combines refluxing and subcritical solvothermal treatments, enables direct anatase formation, despite its metastability under standard conditions, thus eliminating the need for a calcination step. Structural characterization through SAED-HRTEM and Raman spectroscopy confirms the successful crystallization of the desired phase. Moreover, the nanothermometry measurements conducted at various wavelengths ultimately demonstrate both the effectiveness of these nanomaterials as thermometric probes, with a relative sensitivity of about 0.24 K^-1%, and their capability as local heaters, with a release of a few tens of degrees. This work demonstrates a new synthetic strategy for these nanocomposites, which offers a promising pathway for the optimization of nanosystems in therapeutic applications.

Keywords: Raman thermometry; green chemistry; nanocomposites; photothermal therapy; sol-gel synthesis; solvothermal treatment.

Figure 1

(a) UV/Vis spectra recorded every 15 min by taking small aliquots from the OP3 one-pot synthesis of Ag@TiO₂ one-pot without dilution during its course. The spectrum of silver nanoparticles in DMF (solution 2) synthesized on the same day is shown for comparison (continuous light gray line). (b) UV/Vis spectra recorded every 20 min by taking small aliquots from the OP10 synthesis diluted.

Figure 2

Comparison of normalized Raman Stokes spectra of OP4 (red line), Ag (blue line), and TiO₂ (black line) nanoparticles between 107 and 1700 cm⁻¹, recorded with excitation at 514 nm. Dotted lines are only guide for eyes.

Figure 3

Raman spectra of OP4. (a) Stokes spectra of different positions of the sample (1–5), excited at 514 nm and normalized against the anatase signal at 143 cm⁻¹. (b) Anti-Stokes and Stokes Raman spectra of the different positions (A-I,T), excited at 800 nm. Raman spectra are recorded at input power of about 1.0 mW.

Figure 4

HRTEM dark field images of three different positions of OP4 sample.

Figure 5

EDS maps for the three regions explored by HRTEM reporting silver (green), oxygen (blue), and titania (red), respectively.

Figure 6

UV/Vis spectra of Ag seeds suspension at t = 0 (black line) and during the three weeks after the synthesis: t = 2 days (blue line), t = 3 days (red line), and t = 3 weeks (green line).

Figure 7

UV/Vis spectra of products at different steps of the synthesis: (a) silver seeds (black line) and Ag Nps (blue line) and (b) Ag@TiO₂ in ethanol, as synthetized (black line) and after 24 h (blue line).

Figure 8

Comparison between the XRD diffractograms, acquired in BB configuration, of Ag NPs (black) and Ag@TiO₂ in ethanol before (blue) and after (red) solvothermal treatment at 150 °C for 24 h, with the assignation of Ag-O (+), anatase (○), and silver (◇) diffraction peaks.

Figure 9

XRD diffractograms of Ag@TiO₂ after solvothermal treatment at 150 °C in ethanol for 24 h (black line) and 48 h (blue line), acquired in BB configuration. XRD diffractograms, acquired in GID configuration of Ag@TiO₂ after solvothermal treatment at 150 °C in ethanol for 48 h (red line) and in water for 24 h (green line). The reflection assignments are anatase (○), rutile (∗), Ag-O (+), cubic Ag (◇), and Na₂O (Δ).

Figure 10

TEM images of each reaction step product: (a) Ag seeds; (b) Ag NPs; (c) Ag@TiO₂ nanocomposite; (d) Ag@TiO₂ nanocomposite in ethanol treated at 150 °C for 24 h. It is helpful to point out that the scale bar of (a) and (b) is 50 nm, while that of (c) and (d) is 100 nm.

Figure 11

TEM images, acquired in diffraction mode, of each reaction step product: (a) Ag seeds, (b) Ag NPs, and (c) Ag@TiO₂ nanocomposite in ethanol in reciprocal space.

Figure 12

Ag@TiO₂ in ethanol for 24 h (a) TEM images; (b) Ag, (c) O, and (d) Ti elemental mapping.

Figure 13

Raman spectra of the reaction products: Ag NPs (black line) and Ag@TiO₂ after solvothermal treatment (blue line).

Figure 14

(a) Raman spectra at increasing powers of OP4 at 800 nm. (b) Thermometer indicator values versus power of the excitation laser. The line is a polynomial interpolation of data serving as an eye guide for the behavior.

Figure 15

(a) Raman spectra recorded at increasing temperatures. (b) Anti-Stokes/Stokes intensity ratio as a function of temperature. The data were collected in the temperature range from 20 to 50 °C, at 800 nm, with input power of 1.0 mW.

Figure 16

Raman spectra of two-steps Ag@TiO₂ sample in water, measured at (a) 530 nm and (c) 633 nm, and anatase NPs, measured at (b) 530 nm and (d) 633 nm, at different input powers.

Figure 16

Raman spectra of two-steps Ag@TiO₂ sample in water, measured at (a) 530 nm and (c) 633 nm, and anatase NPs, measured at (b) 530 nm and (d) 633 nm, at different input powers.

Figure 17

Reaction scheme for one-pot syntheses followed by solvothermal treatment.

Figure 18

General scheme of two-steps Ag@TiO₂ synthesis.

Figure 19

(a) Image and scheme of the micro-Raman of the second set-up (Triple Raman). (b) Image with laser spot incident on the sample Ag@TiO₂ in water. (c) Thermostat employed with magnification on the laser spot incident on the sample OP4 inside the thermostat.