Investigation of silver nanoparticles embedded in extracted gelatins from camel, bovine, and fish bones for possible use in radiation dosimetry

The silver nanopowder with a purity of 99.99% and average size of 30–50 nm was purchased from US Research Nanomaterials, Inc., USA. Distilled water was used for the preparation of a silver colloidal solution-based gel dosimeter. Other chemicals used in processing the gel, such as hydrochloric acid and ammonium sulfate, were of analytical grade without further purification.

To ensure the highest level of reproducibility in the preparation of the Gel/AgNPs, the samples were created on a benchtop in a fume hood with normal atmospheric conditions. Aqueous gelatin (30% by weight) silver colloidal solutions at low concentrations (1.25, 2.5, and 5 mM) were prepared in distilled water at room temperature. Freestanding films of gelatin entrained with silver nanoparticles (Gel/AgNPs) were synthesized as radiation dosimeters. In our case, thin films for dosimetry applications do not refer to the types of films commonly used in dosimetry, such as radiographic films also called X-ray film [30], which require chemical development, and radiochromic films, which are mainly used for 2D dose verification. After self-development, the latter is used to test and characterize X-ray devices such as CT scanners and radiotherapy linear accelerators (LINACs). Instead, our dose gelatin films are suitable as a human tissue equivalent for radiotherapy diagnosis and treatment. In this direction, other emerging polymeric materials may find application as ionization dose sensors. These include polar organic polymers such as polyvinyl alcohol (PVA) that contain salts of rare earth elements like gadolinium chloride (GdCl₃). When such composites are exposed to radiation doses, their optical properties change [31,32]. PVA doped with 1 wt% Ni, NiO, and Fe₂O₃ nanoparticles was prepared and irradiated for radiation protection. PVA samples containing 1% Ni showed the highest mass attenuation coefficient values [33]. Cu/Zn reinforced polymer composites that had been exposed to alpha, proton, neutron, and gamma radiation and are more suitable for neutron shielding [34]. Gamma-ray attenuation parameters have also been studied in matrix composites such as high-density polyethylene doped with bismuth oxide nanopowders and polymer composites with BaTiO₃ and CaWO₄ [35,36]. A polymer fiber Bragg grating (PFBG) was irradiated with fast neutrons at different doses from 24 to 720 Gy. Transmission and reflection studies have shown that PFBGs with high Bragg wavelengths are good candidates for use in dosimetry systems [37].

A 450 W U.S. solid ultrasonic homogenizer (Cleveland, USA) was used to dissolve and disperse the AgNPs in gelatin. The power of the ultrasonic homogenizer was kept between 30% and 100% of the maximum power of 450 Watts for up to 10 minutes of mixing time. These mixtures were then pipetted into a 30-mm glass dish and allowed to form transparent gels at room temperature where the slippery surface of the glass makes it easy to peel off the gel mixture. The thickness of the free-standing film was controlled by the amount of mixture poured into the glass dish as shown in Figure 3. The black round part in the center was used to lift the Gel/AgNPs films after they had solidified. Only a 1-mm thickness was required for this study of free-standing membranes. It is very difficult to control the thickness of raw gelatin in a research laboratory setup. In most published works, silver nanoparticles are introduced into a gelatin solution, which is poured into a vial of a certain thickness. The rheological properties of gelatin films are strongly influenced by and depend on the viscoelasticity, viscosity, and processing temperature of the film. The well-known process of producing gelatin in sheet or leaf form requires expensive equipment and is very time-consuming. This method of obtaining a gelatin film, especially a gelatin sheet, from powdered gelatin includes the following steps: plasticizing the powdered gelatin to which water is added by applying a shearing force at elevated pressure and elevated temperature; the softened fabric is extruded as a film through a slit die, the film is drawn from the slit, stretched, and the stretched film is dried. In this case, a large amount of air is drawn into the solution, so after the gelatin is completely dissolved, the air must be evacuated in a degassing apparatus to obtain a final product free of air bubbles.

Fig. 3.

The morphology and crystallinity of the nanocomposites were investigated by using Philips XL 30 ESEM scanning electron microscope on an Au substrate, optical absorption, and Fourier-transform infrared spectroscopy (FTIR). The dose enhancement was assessed by using X-ray irradiations having beam energies below and above the silver K-edge, which is about 25.5 keV. The term “K-edge” describes the sharp increase in X-ray photon photoelectric absorption that is seen at an energy level just above the binding energy of the atom’s k-shell electrons. The beam was configured by setting the X-ray apparatus (Leybold®554-800) at 15, 25.5, and 35 kV potential and a beam current of 1 mA. An X-ray detector is used to detect the number of electrons in the air and after passing through Gel/AgNPs samples with a 2 × 2 cm² field size and a collimator to sample distance of 10 cm. The characterization of the free-standing film dosimeters was also carried out at room temperature by means of UV-visible absorption spectroscopy using Jenway 6800 Double-Beam (2-nm slit width) and ATR-FTIR (4000–225 cm^–1) spectrometers. The spectrometers were calibrated daily using the machines’ “AutoCalibrate”? air calibration.

Figure 4 shows a typical SEM micrograph of fish gelatin loaded with 1.25 mM silver nanoparticles. The highly conductive AgNPs are highly reflective under electron bombardment. They are not highly dispersive under ultrasonic mixing, and they tend to show some degree of agglomeration and aggregation as indicated by the arrows, leading to the formation of submicron-sized silver particle clusters ranging from 150 to 250 nm, which negatively impacts the final product. The viscosity of the gel solution during processing affects the degree of agglomeration of infused nanoparticles. Therefore, decreasing the binder “gelatin” viscosity will certainly lower the formation of agglomerates. Agglomerates can be disrupted by sonication techniques to a certain degree, but aggregates cannot [38]. The addition of a surfactant, biological in particular, in this case may be needed to prevent both agglomeration and aggregation. It is stated in the sample preparation section above that once the gelatin has completely dissolved, a significant amount of air is drawn into the solution. The air bubbles have an obvious impact on mixing, and it is believed to give rise to agglomeration to some extent. We believe that if mixing is performed under vacuum for a reduction of air pockets, highly dispersed nanoparticles may be achieved.

Fig. 4.

Figure 5 shows the FTIR spectrum of a typically extracted fish gelatin in the form of a free-standing film. The obtained FTIR spectrum reveals that the peaks of gelatin transmittance are consistent with those reported in the literature [39–45].

Fig. 5.

FTIR measurements were performed to identify the possible biomolecules and bonds responsible for the structural and functional stability of fish bone-derived gelatin. Proteins are made up of amino acids linked by amide bonds. Peptide and protein repeat units give rise to nine characteristic infrared (IR) absorption bands, namely amides A, B, and I-VII [46]. Amide bands represent different vibrational modes of the peptide bond. The absorption band of gelatin in the infrared spectrum is in the region of the amide band; amide-I represents the C=O stretching/hydrogen bond coupling with COO, amide-II represents the bending vibration of the N-H group and the stretching vibration of the C-N group, and amide-III represents the coupling. The plane vibrations of the C-N and N-H groups of amides are coupled [47]. The peaks of the gelatin at 3305 cm^–1 attributed to the presence of hydrogen bond water and amide-A, 1632 cm^–1 peaks correspond to the occurrence of amide-I, a peak at 1542 cm^–1 is indicating amide-II, a band at 1241 cm^–1 indicates the amide-III, and peak ranges from 1445 cm^–1 to 1335 cm^–1 were attributed to the symmetric and asymmetric bending vibrations of the methyl group. The FTIR spectra of extracted gelatin from camel and bovine bones, which are not presented, show very similar typical transmittance peaks that correspond to the respective nine characteristic IR absorption bands.

Figure 6 shows the optical absorption spectrum of a 1-mm thick free-standing film made of AgNP-entrained fish gelatin. As a result of surface plasmon resonance, silver nanoparticles are known to exhibit maximum optical absorption in the 350 nm–450 nm range depending on the size and shape of the nanoparticle [48]. In our case, the peaks are centered in the 395 nm–400 nm range, where pure gelatin exhibits relatively low absorption. As the amount of silver nanoparticles added to gelatin increases, however, the optical absorption of the mixture increases. The use of AgNPs embedded in gelatin caused the enhancement of X-ray radiation absorption and the highest percentage of linearity for the dosimeter is found to be 90% in the optical range of 385 nm to 425 nm. The camel and bovine gelatin samples tested showed qualitatively similar behavior, but slightly different amounts of optical absorption were recorded. In order to monitor the radiation dose in the synthesized nanocomposites, silver colloidal concentration is considered a good strategy in light of the works referred above [31–36].

Fig. 6.

Figure 7 shows the X-ray absorption of freestanding films made of AgNPs entrained camel (a), fish (b), and bovine (c) gelatin. The thickness of the films was 1 mm each and the potential was set to 15, 25.5, and 35 kV potential with a beam current of 1 mA. When exposed to 25.5 kV, silver nanoparticles exhibit their highest level of absorbance, indicating that the photon’s energy is just slightly higher than the silver nanoparticles’ K-edge binding energy. Additionally, there is a direct correlation between the absorption and the amount of gelatin and silver nanoparticles. Tested samples showed qualitatively similar behavior, but different amounts of X-ray absorption were recorded. Bovine gelatin/AgNPs demonstrated the highest absorption, followed by camel and fish gelatin.

Fig. 7.

The attenuation coefficient μ which describes how easily the X-ray beam can penetrate a volume of the gelatin/AgNPs. It measures the total loss of the beam intensity, including scattering. It is estimated by using the Beer-Lambert law: I=Ioe−μx and μ=ln(IoI)x$$I = {I_o}{e^{ - \mu x}}\quad {\rm{\;and\;}}\quad \mu = {{\ln \left( {{{{I_o}} \over I}} \right)} \over x}$$

Where I_o is the incident X-ray beam intensity, I is the intensity of the collected passing beam through the gelatin/AgNPs sample, and x is the thickness of the sample.

Figure 8 shows the variation of μ with respect to the beam’s energy. As expected, μ decreases as the energy of the X-ray source increases. The highest value of μ is found to be the one corresponding to the highest amount of infused silver nanoparticles in gelatin.

Fig. 8.

The absorbance of X-rays at three different discrete energies is shown as a continuous curve showing a maximum at about 25 keV, but the absorption coefficient shows a continuous decrease with beam energy. The absorption coefficient is defined as the rate at which the absorption is taking place, this entity measures how quickly the X-ray beam would lose dose due to absorption alone while the attenuation coefficient μ describes how easily the X-ray beam can penetrate a volume of the gelatin/AgNPs. This corresponds to the rate at which the beam attenuates, or loses intensity, at depth. Because it measures the total loss of the beam intensity, including scattering, however, it can serve as a qualitative or a quantitative measurement entity depending on how strong or weak scattering tissue is. In short, this is because attenuation is a continuous energy loss, while absorption is a sudden energy loss when the X-ray beam encounters an absorbing object. The attenuation coefficient varies with the type of material and the energy of the radiation. In general, for electromagnetic radiation, the higher the energy of the incident photons and the less dense the material, the lower the corresponding attenuation coefficient [49]. The increase in loading silver nanoparticles in gelatin may also have caused the density of the nanocomposite to increase, leading to relatively higher values of the attenuation coefficient.

This preliminary finding shows that the freestanding films prepared using gelatin/AgNPs can be recommended as reliable materials for the fabrication of radiation dosimetry devices for low-energy external radiotherapy sources. The challenge in preparing high-sensitive, reliable 3D or 2D dosimeters is related mainly to sensitivity, precision, reproducibility, accuracy, resolution, and time for measurement, independent of the radiation quality of the final dosimeter. The three main steps of gel dosimetry are the following: (1) the radiation-sensitive gel solution is made and placed into a container, phantoms, and associated vials for calibration, (2) irradiation and polymerization of the gel solution, and (3) scanning of the polymerized gel by an appropriate imaging technique and subsequent analysis of acquired images. The second step—gel dosimetry—will be to expose the free-standing Gel/AgNPs composites to X-ray radiation. No solution and no polymerization are needed. However, aiming at step 3 necessitates exposing the free-standing Gel/AgNP films to high doses of x-ray radiation which can be controlled by the current beam value and exposure duration while setting the beam energies below and above the silver K-edge. Using an X-ray generator with a high beam current of 10 mA which is ten times the beam setting that is currently in use will certainly result in a high dose. This anticipated work will lead to more details and interpretation of the results with regard to the potential use of the intended dosimeter. To that end, a procedure on how gelatin can be used, and the values of radiation read with precision and accuracy for the materials under study will be reported.