Hybrid gel composites of melanin and tungsten oxide nanoparticles for enhanced electromagnetic radiation attenuation

2.1 Preparation of WO₃-NPs/PM composites

A solution of Pluronic P123–Melanin (PM) was prepared by dissolving Pluronic P123 in deionized water at concentrations ranging between 15% and 30% (w/v). Separately, a melanin solution was prepared by dissolving 1.0 g of melanin in 1.0 L of deionized water with continuous stirring to ensure complete dispersion. The melanin solution was subsequently added dropwise to the Pluronic P123 solution while maintaining continuous stirring to ensure uniform mixing. The temperature was incrementally raised to 35–40 °C during the addition process to ensure adequate homogenization of the components.

Commercial nanostructured tungsten oxide powder (Millipore-Sigma, St. Louis, MO, USA) was subsequently added to the prepared PM solution at varying weight ratios of 5%, 10%, 15%, and 20% relative to the total solution weight. The resulting mixtures were stirred continuously to ensure uniform dispersion of the tungsten oxide nanoparticles within the Pluronic P123–Melanin matrix. To prevent the agglomeration of nanoparticles, the mixture underwent sonication for 1 h. The composite solutions were labelled as WO₃-NPs/PM-x, with x being the weight fraction of nanostructured tungsten oxide in the PM solution. The synthesized WO₃-NPs/PM composites first displayed a viscous colloidal liquid form immediately after mixing. The samples were allowed to dry under ambient conditions for almost 10 days, resulting in a dense, gel-like consistency.

2.2 Gamma ray shielding measurement setup

The gamma shielding performance of the synthesized composites was evaluated using a NaI(Tl) scintillation detector (BICRON 2M2/2; crystal size: 2″ × 2″), coupled with a CAEN N6730B digitizer. The measurements employed point radioactive sources including Am-241 (178,222 Bq; 31 and 59.5 keV), Ba-133 (9,324 Bq; 81, 303, and 356 keV), Cs-137 (1.14 × 10⁵ Bq; 662 keV), and Co-60 (23,643 Bq; 1173 and 1332 keV). During the experiment, the shielding samples were placed 5 cm from the gamma source and positioned between the source and the detector, which was fixed at 15 cm from the source. Radiation counts were recorded over 30-min intervals, with each measurement repeated three times to ensure reproducibility. The mean values were calculated and used for subsequent analysis of linear attenuation coefficient (LAC), mass attenuation coefficient (MAC), half-value layer (HVL), and mean free path (MFP).

3.1 XRD patterns of the composites

The XRD patterns of the WO₃-NPs/PM composites (Fig. 1) exhibit prominent diffraction peaks at 2θ ≈ 23.1° (002), 23.6° (020), 24.3° (022), 33.6° (222), 49.8° (400), and 55.7° (420), which correspond to the crystalline planes of monoclinic WO₃, consistent with JCPDS card No. [83-0950]. These results confirm that the crystalline structure of WO₃ nanoparticles is retained after incorporation into the Pluronic-melanin matrix. No additional peaks were observed, indicating phase purity and the absence of crystalline by-products.

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Fig. 1.

XRD patterns of WO₃ nanoparticles (WO₃-NPs/M).

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Using the Debye-Scherrer equation, the average crystallite size of the WO₃ nanoparticles was calculated to be [169.84] nm, suggesting nanocrystalline dimensions favorable for electromagnetic interaction. No significant peak shift or broadening was detected, indicating minimal lattice strain from matrix interaction. The retention of crystallinity is crucial for effective radiation shielding, as it directly influences the electronic density and interaction probability with incoming photons.

3.2 Scanning electron microscopy

The SEM micrographs presented in Fig. 2 illustrate the surface morphology of WO₃-NPs/M composite at a magnification of 50,000x. The nanoparticles display primarily plate-like to rod-like nanostructures. The composite nanostructures exhibit lateral sizes generally between about 100 nm and 300 nm, with the presence of some smaller fragments noted, suggesting some degree of agglomeration or partial fragmentation. The observed plate thicknesses appear to be below 50 nm, indicating the formation of ultrathin nanostructures. This nanoscale morphology is expected to enhance surface area and interaction with the polymer-melanin matrix, potentially improving the composite’s radiation shielding performance.

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Fig. 2.

SEM image of tungsten trioxide (WO₃) nanoparticles dispersed in melanin WO₃-NPs/M. The image was captured at a magnification of 50,000 x with a scale bar of 100 nm.

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3.3 Elemental composition and surface analysis via EDX spectroscopy

Fig. 3 presents the SEM micrograph and the corresponding EDS analysis of the WO₃-NPs/PM-20 composite. The SEM image displays a textured, clustered surface morphology characterized by plate-like nanostructures, indicative of effective nanoparticle dispersion. The EDS spectrum confirms the presence of tungsten (W), oxygen (O), and carbon (C), which are consistent with the expected components of WO₃ nanoparticles and the Pluronic-melanin (PM) matrix. Quantitative analysis indicates tungsten constitutes 70.33 wt%, oxygen19.43 wt%, and carbon 10.25 wt%. The significant tungsten signal verifies the prevalent existence of WO₃ nanoparticles in the examined area, whereas the contributions from carbon and oxygen mainly stem from the polymer and melanin elements. The results confirm the effective integration and surface distribution of WO₃ within the composite.

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Fig. 3.

The SEM image and corresponding EDX spectrum of the WO₃-NPs/PM-20 composite, confirming the elemental composition and uniform dispersion of WO₃ nanoparticles within the pluronic-melanin matrix.

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3.4 UV-Vis absorption spectroscopy

Fig. 4 presents UV-Vis absorption spectra of WO₃-NPs composite solutions with varying tungsten oxide nanoparticle loadings: 5%, 10%, 15%, and 20% by weight. All samples exhibit a prominent absorption band in the UV region (approximately 190-200 nm) and additional absorption at 250-275 nm. As the WO₃ content increases, a slight enhancement in absorbance intensity is observed across the spectrum, particularly in the UV region.

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Fig. 4.

UV–Vis absorption spectra of WO₃-NPs with varying WO₃ nanoparticle concentrations.

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3.5 Photoluminescence spectroscopy analysis

Fig. 5 presents the photoluminescence (PL) spectra of WO₃-NPs composites, excited at 225 nm, with varying WO₃ nanoparticle loadings (5%, 10%, 15%, and 20% by weight). The spectra exhibit distinct emission peaks at approximately 410 nm. A clear concentration-dependent enhancement in PL intensity is observed, with the WO₃-NPs-20 sample showing the highest emission and WO₃-NPs/PM-5 the lowest. This trend confirms that the photoluminescence properties of the composites can be effectively tuned by varying the WO₃ nanoparticle content, potentially impacting their optoelectronic performance.

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Fig. 5.

PL spectra of WO₃ -NPs, excited at 225 nm, with varying WO₃ nanoparticle concentrations (5–20 wt%).

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3.6 FTIR analysis

Fig. 6 shows the Fourier transform infrared (FTIR) transmittance spectra of WO₃-NPs/PM composites with varying tungsten oxide nanoparticle loadings (5%, 10%, 15%, and 20% by weight). All spectra exhibit characteristic absorption bands corresponding to functional groups present in the Pluronic P123–melanin (PM) matrix. Prominent peaks are observed around 1122.8 cm⁻¹ (C–O–C stretching of Pluronic P123), 1725.1 cm⁻¹ (C=O stretching likely from the melanin or polymer matrix), 1872.8 cm⁻¹ (possible overtone or combination bands), and 3446.2 cm⁻¹ (O–H stretching of water or surface hydroxyl groups).

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Fig. 6.

FTIR transmittance spectra of WO₃-NPs/PM composites containing 5%, 10%, 15%, and 20% tungsten oxide nanoparticles by weight, illustrating the influence of increasing WO₃ content on vibrational band intensities within the polymer–melanin matrix.

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As the WO₃ nanoparticle content increases, a systematic decrease in transmittance intensity is observed across all bands, with the WO₃-NPs/PM-20 sample showing the most pronounced attenuation. This behavior suggests increased absorption and possible scattering effects due to the higher nanoparticle density, which may partially obscure the vibrational signals of the organic matrix. Importantly, the consistent positions of the characteristic peaks across all samples indicate that the incorporation of WO₃ does not significantly alter the chemical structure of the PM composite. Rather, the spectral changes are predominantly physical in nature, associated with increased optical density and potential particle–matrix interactions.

The FTIR spectra of the composites (WO₃-NPs/PM-10, WO₃-NPs/PM-10, WO₃-NPs/PM-15, and WO₃-NPs/PM-20) are consistent. Consequently, it was determined that the inclusion of the filler did not affect the functional group intensity of the polymer, as no chemical interaction was observed between the polymer and WO₃, indicating that only physical interaction took place.

3.7 Radiation attenuation performance

Fig. 7 presents the variation of the LAC (cm⁻¹) as a function of photon energy (keV) for tungsten trioxide nanoparticles (WO₃-NPs) dispersed in a polymer matrix at different weight fractions: 10%, 20%, 30%, and 40% (denoted as WO₃-NPs/PM-5, WO₃-NPs/PM-10, WO₃-NPs/P-15, and WO₃-NPs/P-20, respectively). The data show a significant decrease in the LAC as photon energy increases, particularly in the low-energy range (0–200 keV), where the photoelectric absorption is dominant. Higher WO₃ content corresponds to enhanced attenuation across all energy levels, with the WO₃-NPs/PM-20 composite demonstrating the highest LAC values. This trend highlights the role of WO₃ nanoparticles in improving photon interaction and shielding efficiency, especially at lower energies. At higher photon energies, the differences among the samples diminish, reflecting the reduced influence of material composition on photon attenuation in this range.

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Fig. 7.

LAC as a function of photon energy for WO₃-NPs/PM composites with varying tungsten oxide nanoparticle loadings (5%, 10%, 15%, and 20% by weight).

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Fig. 8 presents the MAC of WO₃-NPs/PM composites as a function of photon energy for polymer composites reinforced with different weight percentages of tungsten oxide nanoparticles (WO₃-NPs): 5, 10, 15, and 20 wt%. Across all samples, the MAC decreases with increasing photon energy, which is consistent with the general behavior of photon interaction with matter, wherein lower-energy photons are more readily absorbed. Attenuation is most effective in the low-energy region (below 200 keV), where the photoelectric absorption mechanism dominates. At higher energies, particularly beyond 500 keV, the attenuation performance significantly declines due to the reduced probability of photoelectric interactions and the increased dominance of Compton scattering. The data clearly show that composites with higher WO₃-NP content exhibit superior radiation shielding across the entire energy spectrum, with the WO₃_NPs/P-20 sample consistently achieving the highest MAC values. This improvement is attributed to the high atomic number and density of tungsten, which enhances photon attenuation through both photoelectric absorption and scattering processes (Sayyed et al., 2024; Hsieh and Taguchi, 2024; Almousa et al., 2024; Khattari, 2025). The results confirm that increasing the concentration of WO₃ nanoparticles substantially improves the shielding efficiency of the composites, particularly at lower photon energies, making them suitable candidates for radiation protection applications in medical, industrial, and electronic fields (Al-Ghamdi et al., 2022; Hannachi et al., 2023).

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Fig. 8.

MAC vs. photon energy for WO₃-NPs/PM composites with varying tungsten oxide nanoparticle concentrations (5%, 10%, 15%, and 20% by weight).

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Table 1 presents a comparison of the MAC values obtained from the fabricated composites with those of the conventional shield reported at 662 keV. We select 662 keV as a reference energy for comparison because it is linked to the distinct gamma emission from Cesium-137 (Cs-137), a commonly utilized and readily available radioactive isotope.

Fig. 9 illustrates the inverse relationship between the tungsten oxide (WO₃) content and the HVL in WO₃-NPs/PM composites as a function of photon energy. HVL, which represents the thickness of material required to reduce the intensity of incident radiation by 50%, is a critical parameter for assessing shielding effectiveness (Sayyed et al., 2022; Al-Hadeethi et al., 2020). HVL increases with increasing photon energy, indicating that higher-energy photons penetrate deeper into the material and require greater thickness for effective attenuation. The composites with higher WO₃ nanoparticle loading (WO₃-NPs/PM-20 and WO₃-NPs/PM-15) consistently demonstrate lower HVL values, reflecting superior radiation attenuation capability. This is attributed to the high atomic number and density of tungsten, which significantly enhances photon interaction probabilities, especially via photoelectric absorption and Compton scattering at lower and intermediate energies (Sartoretti et al., 2024). The separation between the HVL curves becomes more pronounced at photon energies below approximately 600 keV, where the influence of WO₃ loading is most significant. At higher photon energies (above ∼1000 keV), the HVL curves tend to converge slightly, suggesting that the relative difference in attenuation performance between samples diminishes, as high-energy photons primarily interact through pair production or are less effectively absorbed by all samples (Mohammed et al., 2024).

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Fig. 9.

HVL as a function of photon energy for WO₃-NPs/PM composites with varying concentrations.

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Fig. 10 illustrates the relationship between the mean free path (MFP) and photon energy for WO₃-NPs/PM composites containing different weight percentages of WO₃ nanoparticles (5%, 10%, 15%, and 20%). The mean free path (MFP) represents the average distance a photon travels within the material before interacting with it (via photoelectric absorption, Compton scattering, or pair production) (Basha et al., 2022). WO₃-NPs/PM-20 (highest concentration): Exhibits the shortest MFP values across the entire photon energy range, indicating the best photon attenuation efficiency. The high tungsten content significantly increases the probability of photon interactions due to tungsten’s high atomic number and density (Akhdar H., 2025). At low photon energies (0–200 keV), the differences in MFP between the samples are relatively small but still significant. At intermediate photon energies (200–600 keV), the separation between the MFP curves becomes more pronounced. As photon energy increases, Compton scattering dominates, and materials with higher tungsten content continue to offer improved attenuation. At high photon energies (above 600 keV), the MFP increases rapidly for all samples, but the composites with higher WO₃ loading consistently maintain lower MFP values, indicating superior shielding even at higher energies.

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Fig. 10.

Mean free path (MFP) as a function of photon energy for WO₃-NPs/PM composites with varying WO₃ nanoparticle loadings.

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