Coating of silver nanoparticles (AgNPs) on glass fibers by a chemical method as plasmonic surface-enhanced Raman spectroscopy (SERS) sensors to detect molecular vibrations of Doxorubicin (DOX) drug in blood plasma

2.1 Fabrication and detection methods

2.2 FDTD simulation

Finite-difference time-domain (FDTD) simulation was conducted using FDTD solutions (version 8.19) software provided by Lumerical Solutions, Inc (Kim et al., 2013). For performing this simulation, three spherical AgNPs of diameters 80 nm with a distance of 60 nm from together, deposited on the glass substrate, were modeled, and then the DOX analyte was added to the substrate coated with AgNPs. In addition, a gap between the AgNPs and the substrate was established and fixed to be 1 nm for modeling the “hotspot” phenomenon.

3.1 UV–Vis absorption spectrum and XRD spectrum of the colloidal solution of AgNPs

The Tollens' method and sucrose, as a reductant, were used to chemically fabricate the AgNPs; their absorption spectra are shown in Fig. 1-(A). The formation of AgNPs is confirmed by the emergence of a plasmonic resonance peak at 418 nm (Ngumbi et al., 2018); spherical or quasi-spherical morphologies of silver and gold nanoparticles are shown by the appearance of a peak in the absorption spectra of nanoparticles (Baytekin et al., 2015). The XRD pattern of AgNPs is shown in Fig. 1-(B). It is noteworthy that this pattern indicates the FCC structure. At 2θ, the significant and main peaks are equal to 38.22, 44.38, 64.58, and 77.49°and are respectively related to the formed crystal plates 111, 200, 220, and (3 1 1). Using the Debye Scherrer equation defined as Eq. (1), the average grain size was roughly equal to 28 ± 1 nm.

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Fig. 1

(A)- Absorption spectrum of AgNPs with 0.05 M AgNO₃ at 23 ± 1 °C (fabrication temperature) and the sucrose as a reductant with a maximum absorption (wavelength = 418 nm) at 2 h; (B)- XRD pattern from AgNPs fabricated with 0.05 M AgNO₃ at 23 ± 1 °C (fabrication temperature) prepared by the sucrose as reductant and the Tollens' method.

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3.2 UV–Vis extinction spectrum, XRD spectrum, FE-SEM images, and energy-dispersive X-ray (EDX) of colloidal solution of AgNPs deposited on a glass fiber substrate

Fig. 2-(A) and Fig. 2-(B) indicates the FE-SEM images of glass fiber substrates in the scalebars of 50 μm and 10 μm, respectively. In Fig. 2-(C), Fig. 2-(D), Fig. 2-(E), and Fig. 2-(F), FE-SEM images are relevant to the AgNPs-coated glass fiber substrate as a SERS plasmonic sensor at scalebars of 50 μm, 10 μm, 2 μm, and 1 μm, respectively. Moreover, Fig. 2-(G) demonstrates the EDX spectrum of AgNPs-coated glass fiber substrate as a SERS plasmonic sensor that indicates the elements and their mass and atomic percentages in the sample. In Fig. 2-(F), through magnifying on a scalebar of 1 μm, it is confirmed that AgNPs are present on the glass fiber substrate. In Fig. 2-(F), it is clear that the concentration of AgNO₃ and the amount of AgNPs are key parameters in the formation of AgNPs on glass fiber substrates since the full coating of AgNPs on the glass fiber substrate is not performed in the case of low concentrations of AgNO₃. The selective growth of spherical AgNPs on the glass fiber substrate has been caused by the concentration of AgNPs and AgNO₃. As shown in Fig. 2-(G), through analyzing the EDX spectrum, it is confirmed that AgNPs are present on the glass fiber substrate. The larger particles appear whiter and brighter, and significant electric near-fields are created by smaller AgNPs around themselves, which are the results of the resonance of silver surface plasmons. Various chemical variants of DOX in and out of blood plasma will experience electric near-fields if they are placed at these positions. The light emitted to larger particles with insignificant electric near-fields strengthens the electric far-field or scatters from the surface of these particles (Ding et al., 2017). Fig. 2-(H) indicates the size distribution of deposited AgNPs on a glass fiber substrate as SERS biosensors for Fig. 2-(E) that were measured by Digimizer software (version 4.2). As shown in this figure, the AgNPs have sizes between 100 nm and 1700 nm, and the number of nanoparticles with sizes in the range of 1400–1500 nm is more than other nanoparticles. Fig. 2-(I) shows the extinction spectrum of AgNPs coated on SAS. As indicated in Fig. 1-(C), through depositing AgNPs on the glass fiber substrate and the appearance of a plasmon resonance peak at 430 nm, it is confirmed that AgNPs are formed on the glass fiber substrate. By changing the media containing these particles, which change from water to glass fiber and air, the displacement occurs at the wavelength of the plasmonic peak, its height decreases, and the width of the peaks increases since the position of the plasmonic peak depends on the refractive index of the media (Bohren et al., 1983). According to Fig. 1-(C), in contrast to a stable colloidal solution, in which AgNPs are dispersed in aqueous solution at certain distances from each other, by depositing AgNPs on glass fibers, the particles are placed next to each other during the drying process, and clumps of nanoparticles are formed on the substrates so that these clumps can be considered to be larger particles leading an increase in the width of the spectrum (Bohren et al., 1983). Moreover, the decrease in peak intensity is also due to the scattering of light from the clumps (Bohren et al., 1983). Due to the reflection and scattering of light from the surface of the glass fiber, the background of the extinction spectrum (absorption spectrum + scattering spectrum) of SAS has occurred at higher values compared to the absorption spectrum. Fig. 2-(J) shows the XRD pattern of AgNPs deposited on glass fibers as SAS (Olusanya et al., 2017). As shown in Fig. 2-(J), all AgNPs have similar structures and morphologies. The high crystallinity of the substrate is also shown in Fig. 2-(J). It is noteworthy that the Ag deposited on the glass fiber substrate oxidizes after some time in the laboratory and the peak appearing at 2θ (equal to 24.11°) is related to Ag₂O.

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Fig. 2

(A)- FE-SEM images of glass fiber substrates at a scalebar of 50 μm; (B)- FE-SEM images of glass fiber substrates at a scalebar of 10 μm; (C)- FE-SEM images of the AgNPs-coated glass fiber substrate (as a SERS plasmonic sensor) at a scalebar of 50 μm; (D)- FE-SEM images of the AgNPs-coated glass fiber substrate at a scalebar of 10 μm; (E)- FE-SEM images of the AgNPs-coated glass fiber substrate at a scalebar of 2 μm; (F)- FE-SEM images of the AgNPs-coated glass fiber substrate at a scalebar of 1 μm; (G)- EDX spectrum of the AgNPs-coated glass fiber substrate; (H)- Size distribution of deposited AgNPs on a glass fiber substrate; (I)- Extinction spectrum of AgNPs deposited on glass fibers; (J)- XRD spectrum of glass fibers and AgNPs deposited on glass fibers.

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The formed AgNPs were found to have FCC structures based on the number of plates. Using the Debye Scherrer formula, the average grain size on the plate 111 was roughly equal to 129 ± 1 nm (1). The angle 2θ of the plate 111 is 38.12°, and the full width at high maximum (FWHM) is equal to 0.579. The significant peaks are at 38.12, 44.35, 64.56, and 77.49°, and respectively correspond to the crystal plates 111, 200, 220, and (3 1 1). The presence of AgNPs is confirmed by the crystal plates that appeared on the glass fibers (Olusanya et al., 2017; Nangmenyi et al., 2009).

3.3 Calibration curve for intensity variations of Raman spectrum for detecting DOX using AgNPs coated on glass fiber substrates as plasmonic sensors

The Raman spectrum of glass fiber substrates, the Raman spectrum of 10⁻¹ M DOX deposited on glass fiber substrates, and the SERS spectrum of 10⁻¹ M DOX deposited on SAS are presented in Fig. 3-A. In the Raman spectrum of the DOX deposited on glass fiber substrates (blue curve), there are no molecular vibrations related to DOX. Thus, in practice, the detection of these chemicals, even in the concentration of 10⁻¹ M and using Raman spectroscopy, is significantly difficult and somehow impossible. Molecular vibrations of DOX appear by depositing 10⁻¹ M DOX on the SAS. Molecular vibrations of DOX are indicated as dashed lines (Gautier et al., 2013) on the spectra of the DOX in Fig. 3-A. In the case of DOX deposited on the SAS, Ring stretch CO, Ring stretch C = O, Skeletal deformation, Skeletal deformation, Skeletal deformation, Skeletal deformation, Ring stretch HO, Skeletal deformation, Ring stretch symmetry A1 + Ring stretch OH, Ring stretch symmetry B1 + Ring stretch OH, Ring stretch CO, Ring stretch CC-O, Ring stretch C = O, Ring stretch C = O appear at Raman signals of 439 cm⁻¹, 507 cm⁻¹, 608 cm⁻¹, 770 cm⁻¹, 878 cm⁻¹, 916 cm⁻¹, 1007 cm⁻¹, 1081 cm⁻¹, 1208 cm⁻¹, 1243 cm⁻¹, 1297 cm⁻¹, 1440 cm⁻¹, 1573 cm⁻¹, and 1640 cm⁻¹, respectively. Raman signal is enhanced because of employing SAS due to light scattering from roughness spots. The Raman signals of the larger silver particles observed in Fig. 2-C are enhanced by scattering the incident laser light and reaching the scattered light to the DOX. The surface plasmon resonance of smaller silver particles, or similarly, the strong electric fields around these nanoparticles, is another cause for the enhancement of the Raman signal, in addition to the scattering of light from larger silver particles. Fig. 2-(C) illustrates that the AgNPs behave as optical lenses, concentrating incident laser light around themselves. As a result, the electric field's intensity rises near the nanoparticles. The nanoparticles are exposed to a higher electric field intensity and become more polarized when the DOX is placed around them; therefore, their molecular vibrations are enhanced, and stronger signals are generated (Chen et al., 2015; Granger et al., 2016).

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Fig. 3

(A)- Raman spectrum of glass fiber substrates (blue curve), the Raman spectrum of 10⁻¹ M DOX deposited on glass fiber substrates (red curve), and the SERS spectrum of 10⁻¹ M DOX deposited on SAS (pink curve); (B)- SERS spectra of DOX detected by SAS at different concentrations; (C)- Calibration curve of intensity variations (I) of the SERS signal related to Ring stretch C-O at the wavenumber of 439 cm⁻¹ in logarithmic changes of DOX concentration (C); (D)- SERS spectra of 10 distinct spots of 10 μL of the 10⁻¹⁰ M DOX deposited on the SAS and the variations diagram for the ten continuous experiments for 10⁻¹⁰ M DOX drug; (E)- Bar charts of the variations in the calculation of RSD for the peaks at 439 cm⁻¹ (red diagram), 1208 cm⁻¹ (green diagram), and 1440 cm⁻¹ (blue diagram).

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The SERS spectra of DOX detected by SAS at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M are indicated in Fig. 3-(B). The intensity of the DOX drug's peak molecular vibrations decreases as its concentration decreases, which is attributed to the reduction in the number of its molecules, and therefore the number of its molecular vibrations so that the DOX drug's vibrations are not easily visible at concentrations lower than 10⁻¹⁰ M. Hence, the SAS is sensitive enough to detect DOX at concentrations as low as 10⁻¹⁰ M. The calibration curve of intensity variations (I) of the SERS signal related to Ring stretch C-O at the wavenumber of 439 cm⁻¹ in logarithmic changes of DOX concentration (C) is indicated in Fig. 3-(C) and follows the fitting by Eq. (2).

Ten continuous experiments were conducted on the 10⁻¹⁰ M DOX over one day to achieve the repeatability of the fabrication of SAS. The SERS spectra of 10 distinct spots of 10 μL of the 10⁻¹⁰ M DOX deposited on the SAS are shown in Fig. 3-(D). At a concentration of 10⁻¹⁰ M, all SERS spectra of the DOX drug are well-matched, and the position and characteristics of the spectra do not change. Eq. (3) was used to calculate the relative standard deviation (RSD) to assess the repeatability of SAS fabrication at a concentration of 10⁻¹⁰ M of DOX drug and for the peaks at 439 cm⁻¹, 1208 cm⁻¹, and 1440 cm⁻¹ (Granger et al., 2016).

3.4 Calibration curve of area under the curve of intensity variations of Raman spectrum for detecting DOX using AgNPs coated on glass fiber substrates as plasmonic sensors

The SERS spectra of DOX at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M detected by SAS plasmonic sponsors are shown in Fig. 4-(A). Moreover, the calibration curve of intensity variations (I), including the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX (C) on SAS, which is extracted from Fig. 4-(A), is shown in Fig. 4-(B). The SERS spectra demonstrated in Fig. 4-(A) for DOX at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M deposited on AgNPs coated on glass fiber substrates as plasmonic sensors are presented with the areas under the curve calculated by Arean software for Fig. 4-(C),4-(D), 4-(E), 4-(F), 4-(G), 4-(H), 4-(I), 4-(J), 4-(K), and 4-(L) to be equal to 1024482.77 cm², 682988.55 cm², 455325.67 cm², 303550.45 cm², 202366.96 cm², 134911.31 cm², 89940.87 cm², 59960.58 cm², 39973.72 cm², 266491.14 cm², respectively, which are represented only for the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹. By employing this diagram, it is possible to identify the concentrations of unknown amounts of DOX by observing the intensity of the peak molecular vibrations that only fall within the range of 400–478 cm⁻¹. The intensity of the DOX drug's peak molecular vibrations decreases as its concentration decreases, which is attributed to the decrease in the number of its molecules, and therefore the number of its molecular vibrations so that the DOX drug's vibrations are not easily visible at concentrations lower than 10⁻¹⁰ M. Hence, the SAS is sensitive enough to detect DOX at concentrations as low as 10⁻¹⁰ M. The calibration curve for intensity variations (I), which includes the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX (C) on SAS, which is extracted from Fig. 4-(A) and follows the curve fitting of Eq. (4).

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Fig. 4

(A)- SERS spectra of DOX at different concentrations detected by SAS plasmonic sponsors; (B)- Calibration curve of intensity variations (I), including the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX (C) on SAS; (C)-(I)- Diagrams used for calculation of areas under the curve for presenting the SERS spectra of DOX at different concentrations deposited on AgNPs coated on glass fiber substrates.

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Since the objective of fabricating SAS plasmonic sensors is the detection of significantly low concentrations of DOX, the calibration curve diagram, which is presented in the current research, is extremely important. This diagram can also be employed to measure DOX concentrations by observing the intensity of peak molecular vibration in the Raman spectrum.

3.5 Calibration curve of intensity variations of Raman spectrum for detecting DOX in blood plasma using AgNPs coated on glass fiber substrates as plasmonic sensors

The SERS spectrum of blood plasma deposited on SAS, the SERS spectrum of 10⁻¹ M DOX dissolved in DI water and deposited on SAS, and the SERS spectrum of 10⁻¹ M DOX in blood plasma deposited on SAS are presented in Fig. 5-A. In the Raman spectrum of the DOX deposited on glass fiber substrates (blue curve), there are no molecular vibrations related to DOX. Thus, in practice, the detection of these chemicals, even in the concentration of 10⁻¹ M and using Raman spectroscopy, is significantly difficult and somehow impossible. Molecular vibrations of DOX appear by depositing 10⁻¹ M DOX on the SAS. Raman signal is enhanced because of employing SAS due to light scattering from roughness spots. The Raman signals of the larger silver particles observed in Fig. 2-C are enhanced by scattering the incident laser light and reaching the scattered light to the DOX, the blood plasma, and the DOX in blood plasma. The surface plasmon resonance of smaller silver particles, or similarly, the strong electric fields around these nanoparticles, is another cause for the enhancement of the Raman signal, in addition to the scattering of light from larger silver particles. Fig. 2-(C) illustrates the AgNPs behave as optical lenses, concentrating incident las er light around themselves. As a result, the electric field's intensity rises near the nanoparticles. The nanoparticles are exposed to a higher electric field intensity and become more polarized when the DOX in blood plasma is placed around them; therefore, their molecular vibrations are enhanced, and stronger signals are generated (Chen et al., 2015; Granger et al., 2016).

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Fig. 5

(A)- SERS spectrum of blood plasma deposited on SAS (blue curve), the SERS spectrum of 10⁻¹ M DOX dissolved in DI water and deposited on SAS (red curve), and the SERS spectrum of 10⁻¹ M DOX in blood plasma deposited on SAS (pink curve); (B)- SERS spectrum of DOX in blood plasma detected by SAS at different concentrations; (C)- Calibration curve of intensity variations (I) of the SERS signal related to Ring stretch C-O at the wavenumber of 439 cm⁻¹ in logarithmic changes of DOX concentration (C); (D)- SERS spectra of 10 distinct spots of 10 μL of the 10⁻¹⁰ M DOX in blood plasma deposited on the SAS and the variations diagram for the 10 continuous experiments for 10⁻¹⁰ M DOX drug in blood plasma; (E)- Bar charts of the variations in the calculation of RSD for the peaks at 439 cm⁻¹ (red diagram), 879 cm⁻¹ (green diagram), and 1453 cm⁻¹ (blue diagram).

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The SERS spectra of DOX in blood plasma detected by SAS at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M is indicated in Fig. 5-(B). The intensity of peak molecular vibrations of DOX drug in blood plasma decreases as its concentration decreases, which is attributed to the decrease in the number of its molecules, and therefore the number of its molecular vibrations so that the vibrations DOX drug in blood plasma are not easily visible at concentrations lower than 10⁻¹⁰ M. Hence, the SAS is sensitive enough to detect DOX in blood plasma at concentrations as low as 10⁻¹⁰ M. The calibration curve of intensity variations (I) of the SERS signal related to Ring stretch C-O at the wavenumber of 439 cm⁻¹ in logarithmic changes of DOX concentration (C) is indicated in Fig. 5-(C) and follows the fitting by Eq. (5).

Ten continuous experiments were conducted on the 10⁻¹⁰ M DOX in blood plasma over one day to achieve the repeatability of the fabrication of SAS. The SERS spectra of 10 distinct spots of 10 μL of the 10⁻¹⁰ M DOX in blood plasma deposited on the SAS are shown in Fig. 5-(D). At a concentration of 10⁻¹⁰ M, all SERS spectra of the DOX drug in blood plasma are well-matched, and the position and characteristics of the spectra do not change. Eq. (3) was used to calculate the RSD to assess the repeatability of SAS fabrication at a concentration of 10⁻¹⁰ M of DOX drug in blood plasma and for the peaks at 439 cm⁻¹, 897 cm⁻¹, and 1453 cm⁻¹.

In this case and regarding Eq. (3), n = 10 implies the number of tested Raman spectra, I_i denotes the Raman signal intensity at each specified peak, and I implies the average Raman signal intensity of the specified peaks. The variations diagram for the ten continuous experiments for 10⁻¹⁰ M DOX drug in blood plasma for the peaks at 439 cm⁻¹, 879 cm⁻¹, and 1453 cm⁻¹ with the RSD values ​​of 3.39%, 3.49%, and 3.97%, respectively, is indicated in Fig. 5-(D). Moreover, Fig. 5-(E) demonstrates the bar charts of the variations in the calculation of RSD to be 3.39% for the peak at 439 cm⁻¹, 3.49% for the peak at 879 cm⁻¹, and 3.97% for the peak at 1453 cm⁻¹. The RSD obtained in this experiment was 3.61% on average, which shows that the procedure used to determine concentrations of DOX in blood plasma performs well. Fig. 5-(F) demonstrates the diagram of SERS spectrum intensity of the DOX in blood plasma adsorbed on SAS relative to the changes in the concentrations 10⁻¹, 10⁻², 10⁻³, 10⁻⁴, 10⁻⁵, 10⁻⁶, 10⁻⁷, 10⁻⁸, 10⁻⁹, and 10⁻¹⁰ M of DOX drug in blood plasma for the peaks at the wavenumbers of 439 cm⁻¹, 879 cm⁻¹, and 1453 cm⁻¹. Several other methods such as electrochemical sensors (Soleymani et al., 2017), the spectrophotometric method (Eman and El-Yazbi, 2016), mass spectrometry/liquid chromatography mass spectrometry (MS/LC-MS) (Ibsen et al., 2013), high-performance liquid chromatography (HPLC) (Daeihamed et al., 2015), etc. are employed for determining the amount of DOX in blood plasma. However, they are not cost-effective, sometimes provide low limits of detection, and require complicated sample preparation procedures and a significant amount of time to decompose the samples.

3.6 Calibration curve of area under the curve of intensity variations of Raman spectrum for detecting DOX in blood plasma using AgNPs coated on glass fiber substrates as plasmonic sensors

The SERS spectra of DOX drug in blood plasma at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M detected by SAS plasmonic sponsors are shown in Fig. 6-(A). Moreover, the calibration curve of intensity variations (I), including the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX in blood plasma (C) on SAS, which is extracted from Fig. 6-(A), is shown in Fig. 6-(B). The SERS spectra demonstrated in Fig. 6-(A) for DOX in blood plasma at concentrations of 10⁻¹ M, 10⁻² M, 10⁻³ M, 10⁻⁴ M, 10⁻⁵ M, 10⁻⁶ M, 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, and 10⁻¹⁰ M deposited on AgNPs coated on glass fiber substrates as plasmonic sensors are presented with the areas under the curve calculated by Arean software for Fig. 6-(C),6-(D), 6-(E), 6-(F), 6-(G), 6-(H), 6-(I), 6-(J), 6-(K), and 4-(L) to be equal to 560703.81 cm², 448563.05 cm², 386692.28 cm², 287540.41 cm², 243784.26 cm², 215655.31 cm², 193346.14 cm², 108832.26 cm², 72414.28 cm², 26699.26 cm², respectively, which are represented only in the range of 400–478 cm⁻¹. By using this diagram, it is possible to identify the concentrations of unknown amounts of DOX in blood plasma by observing the intensity of the peak molecular vibrations that only fall within the range of 400–478 cm⁻¹. The intensity of peak molecular vibrations of DOX in blood plasma decreases as its concentration decreases, which is attributed to the decrease in the number of its molecules and therefore the number of its molecular vibrations so that the vibrations of DOX drug in blood plasma are not easily visible at concentrations lower than 10⁻¹⁰ M. Hence, the SAS is sensitive enough to detect DOX drug in blood plasma at concentrations as low as 10⁻¹⁰ M. The calibration curve for intensity variations (I), which includes the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX in blood plasma (C) on SAS, which is extracted from Fig. 6-(A) and follows the curve fitting of Eq. (6).

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Fig. 6

(A)- SERS spectra of DOX in blood plasma at concentrations detected by SAS plasmonic sponsors; (B)- Calibration curve of intensity variations (I), including the area under the curve of the SERS signal related to molecular vibrations at the wavenumbers ranging from 400 cm⁻¹ to 478 cm⁻¹ in logarithmic changes of the concentration of DOX in blood plasma (C) on SAS; (C)-(I)- Diagrams used for calculation of areas under the curve for presenting the SERS spectra of DOX in blood plasma at different concentrations deposited on AgNPs coated on glass fiber substrates.

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Since the objective of fabricating SAS plasmonic sensors is the detection of significantly low concentrations of DOX in blood plasma, the calibration curve diagram, which is presented in the current research, is extremely important. This diagram can also be employed to measure the concentrations of DOX in blood plasma by observing the intensity of peak molecular vibration in the Raman spectrum.

3.7 Calculation of enhancement factor

3.7.2

3.7.2 FDTD simulation results

As described in Section 2.2, three spherical AgNPs of diameters 80 nm with a distance of 60 nm from together, deposited on the glass substrate, were modeled, and then the DOX analyte was added to the substrate coated with AgNPs. Afterward, an optical source or laser was applied to the modeled substrate to simulate the SERS effect and investigate the electric field distribution as well as EF. The schematic of this model is illustrated in Fig. 7-(A). The EF was calculated using a standard equation as |E_loc/E₀|⁴, where E_loc is the local maximum electric field and E₀ is the amplitude of the input source electric field (here, E₀ is equal to 1) (Li et al., 2011). The value of the calculated EF was equal to 24.95 × 10³ which was in line with the value calculated by the experimental method. The electric field distribution, intensity in the XZ plane, and E⁴ in the XZ plane are presented in Fig. 7-(B), Fig. 7-(C), and Fig. 7-(D), respectively. Moreover, a simulated video of electric field distribution around the modeled substrate is presented as Video S1 in the supplementary information.

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Fig. 7

(A)- Schematic of the modeled SERS-active substrate coated with AgNPs; (B)- Electric field distribution; (C)- Intensity in the XZ plane; (D)- Enhancement factor calculated as E⁴ in the XZ plane.

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