4.2.2. Photothermal Measurement in Water

The sample (Fe3O4 NPs in 0.5 mL of deionized water) contained in a 1.5 mL Eppendorf tube was irradiated by a NIR continuous laser at 810 nm (Photon Soft Tissue Diode Laser, Zolar Technology & MFG, Canada) with an external adjustable power, *P*<sup>0</sup> (0–3 W). The distance between the sample and the laser was 1–2 cm and the laser spot size was about 1 mm. The laser powers that were used was 0.5 and 1.0 W. Each sample was identically irradiated for 5 min. The resulting temperature rise was recorded by thermocouples (J-type, National Instrument, Austin, TX, USA) connected to a portable data acquisition system (NI USB-9222A, National Instruments, Austin, TX, USA) and recorded every 30 s with NI-DAQmx (National Instruments, Austin, TX, USA) and software (LabVIEW 8.6, National Instruments, Austin, TX, USA). All measurements were obtained in triplicate except stated otherwise.

The experimental photothermal conversion efficiency (*η*exp) of the NPs was calculated directly from steady-state temperature increase as follows:

$$\eta\_{\text{exp}} = \frac{Q\_{\text{exp}}}{P(1 - 10^{-A\_{\text{exp}}})} \tag{1}$$

where *Q*exp (W) was calculated with previously reported expression [47]: 16.855Δ*T* (mW), Δ*T* is the temperature change, *P* is the incident laser power and *A*exp is the absorbance of the Fe3O4 nanoparticles at 810 nm.
