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6.5 Cross-section absorption of graphene oxide 149

using TAT protein. When the nanoparticles exposed to NIR laser irradiation, UCNPs
convert NIR light to UV. UV emission led to transfer azobenzene to trans configu-
ration had to do with releasing of encapsulated cargo [72]. In another study, Hu
et al. developed a novel UP-NIRSRS composed of NaYF4:Yb:Er coated with octa-
decyl-quaternized polyglutamic acid and polystyrene-block-poly(acrylic acid) which
further modified by zinc(II)phthalocyanine as a photosensitizer [73]. Similarly, in
another study, nanoimpellers composed of NaYF4:Yb and Er covered with meso-
porous silica were developed for chemo-PDT therapy. The cytotoxicity results dis-
played a synergistic effect of chemotherapy and PDT [74].

6.5 Cross-section absorption of graphene oxide
Besides the mathematical models described in Section 6.1, there are several experi-
ments to study optical properties of the materials. In 2018, Hashemi et al. obtained
the cross-section absorption of graphene oxide by calculating absorption coefficient
of the sample volume and normalized it to the graphene oxide concentration. When
laser emitted to the materials, the energy can be either reflect from the surface, dif-
fused via lateral parts of the sample holder, scatter or absorbed by the nanoparticles.
To calculate cross-section absorption, at first step heat loss trough lateral part of the
sample holder and scattering need to be minimized [75].

6.5.1 Minimization of lateral thermal diffusion and scattering

Lateral thermal diffusion was minimized by calculation of lateral thermal relaxation:
d 2
τ = (6.19)
16 x τ=d 16x
2
where d is the laser beam diameter (2 mm) and x is the thermal diffusivity of water
−1
2
(0.14 mm s ). Based on the equation above, the relaxation time was 1.78 s. So, by
choosing laser pulse with reputation rate of 2 Hz (50% duty cycle), the pulse duration
would be well below than lateral thermal relaxation time.
Scattering was minimized by comparing the temperature profile of various con-
−1
centrations of GO (0.25, 0.125, and 0.0625 mg mL ) with the temperature profile
of standard black absorber (Fig. 6.6). The concentration of 0.25 mg/mL which has a
similar temperature profile spread to the black absorber was chosen as a concentra-
tion with minimum scattering effect.

6.5.2 Obtaining cross-section absorption of GO
By reducing the effect of scattering and heat loss through the lateral part of sample
holder, the heat generated during pulsed laser irradiation is almost absorbed by the
nanoparticles:
dQ dT
dt = mC dt (6.20) dQdt=mCdTdt