Superparamagnetic Nanoparticles with Efficient Near-Infrared Photothermal Effect at the Second Biological Window

Abstract

Superparamagnetic nanoparticles (iron oxide nanoparticles—IONs) are suitable for hyperthermia after irradiating with radiofrequency radiation. Concerning the suitability for laser ablation, IONs present a low molar absorption coefficient in the near-infrared region close to 800 nm. For this reason, they are combined with other photothermal agents into a hybrid composite. Here, we show that IONs absorb and convert into heat the infrared radiation characteristic of the so-called second-biological window (1000–1350 nm) and, in consequence, they can be used for thermal ablation in such wavelengths. To the known excellent water solubility, colloidal stability and biocompatibility exhibited by IONs, an outstanding photothermal performance must be added. For instance, a temperature increase of 36 °C was obtained after irradiating at 8.7 W cm−2 for 10 min a suspension of IONs at iron concentration of 255 mg L−1. The photothermal conversion efficiency was ~72%. Furthermore, IONs showed high thermogenic stability during the whole process of heating/cooling. To sum up, while the use of IONs in the first bio-window (700–950 nm) presents some concerns, they appear to be good photothermal agents in the second biological window.

Towards the Standardization of Photothermal Measurements of Iron Oxide Nanoparticles in Two Biological Windows

“…Once the limit volume for consistent measurements has been determined, the laser intensity is another important factor to be discussed. The laser intensity ( I in W/cm 2 ) is calculated as the power deposited on an area, and this is commonly used as a reference for the measurement conditions [ 8 , 13 , 14 ] or as an extrinsic parameter for the evaluation of the SAR [ 10 , 20 , 23 , 34 ]. In order to determine whether I can be considered as an extrinsic parameter to compare different photothermia experiments, the laser power and the spot area were varied to obtain the same I .…”

“…A smaller area of incidence leads to a decrease in the power so as to maintain a constant intensity, and vice versa. The following Table 2 shows the powers and intensities used, which are within the range of values frequently used in the literature [ 10 , 13 , 18 , 22 , 23 ].…”

“…As noted previously, most of the photothermia experiments of iron oxide nanoparticles are performed either in Eppendorf tubes [ 11 , 13 ] or quartz cuvettes [ 6 , 23 ]. In this work, we compared the SARs for the colloids in Eppendorf tubes with those for the containers designed for this work.…”

“…Several studies have been performed in different laboratories with different particle sizes [ 12 , 14 , 19 , 20 ], shapes [ 12 , 14 , 20 , 21 ], clustering [ 12 , 13 , 14 , 20 , 22 ], coatings [ 6 , 8 ], iron oxidation states [ 13 , 20 ], irradiation intensities [ 11 , 18 , 20 ], and wavelengths [ 12 , 13 , 14 , 23 ], with the aim of determining the intrinsic and extrinsic parameters that maximize the heating efficiency under the infrared irradiation of the magnetic nanoparticles.…”

“…However, we are still missing a fundamental key that is required to understand the role played by the extrinsic and intrinsic physical parameters: the determination of a protocol that allows for the standardization of the measurements. For example, the measurements reported in the literature have been conducted using different colloidal containers (mainly Eppendorf tubes but also quartz cuvettes) [ 6 , 23 ], sample volumes (from 0.05 mL to 3.0 mL), and irradiation intensities (from 0.3 W/cm 2 to 8.7 W/cm 2 ) [ 6 , 13 , 14 , 18 , 23 ], without knowing the effects that these parameters have on the heating curves beyond the physical properties of the nanoparticles. It is worth noting that standardization protocols are a requirement for developing new techniques, especially those related to biomedical applications [ 24 , 25 ].…”