Gold Nanowires Based on Photonic Crystal Fiber by Laser Ablation in Liquid to Improve Colon Biosensor

In this work, gold nanoparticles (NPs) have been synthesized using second harmonic generation ND-YAG laser ablation in ethanol employing 532 nm and 1.064 nm wavelengths. Field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and UV–Vis absorption were employed to examine the structural, chemical, and optical properties of Au NPs. XRD results showed that all synthesized Au nanoparticles are crystalline in nature. The optical band gap upon ablation using the higher wavelength (1064 nm) was about 4.02 eV. The value of the optical band gap increases to reach a value of 4.22 eV at the shorter wavelength (532 nm). The FESEM results reveal the formation of smaller nanorod size at second harmonic generation (SHG) wavelength was found to be about 30 nm at wavelength 532 nm and 44 nm at 1064 nm. After that, finite element analysis is used to simulate the photonic crystal fiber (PCF) as biosensors, depending on the surface plasmon resonance (SPR) phenomenon using the COMSOL multiphasic program. The hollow core photonic crystal fiber (HC-PCF) HC-800 was thus overfilled with water. The confinement loss (CL) of the fundamental mode for the proposed PCF with and without the gold nanoparticles was calculated; we observe without gold NPs the CL at the water and colon tissue about 1.08E + 03 and 5.46E + 02 dB/m, while at use the deposited PCF with gold NPs, the results ensured the decrease more than in the confinement loss at the water and colon tissue about 1.01E + 03 and 3.16E + 02 dB/m with a clear shift toward higher wavelengths, also when calculating the sensitivity, to increase HC-PCF biosensor performance by using wavelength interrogation and amplitude measurements; it found the bio-liquid (colon tissue) has the best electric field using the gold nanowire (NW) layer in the biosensor equal to 68.8 V/m by comparison when without using the gold NW layer which leads to high sensitivity.


Introduction
Throughout the past few decades and due to their excellent sensing properties and convenient operations, surface plasmon resonance (SPR)-based sensors were the research trending topics [1][2][3].Many applications could be obtained using this device such as glucose monitoring sensors, bioimaging, medical diagnostics, solution concentration measurement, food testing, and antibody-antigen investigation [4][5][6][7].Due to their design flexibility, improved containment, evanescent field manipulation, compact size, and controllable birefringence characteristics, photonic crystal fibers (PCFs) are highly explored for SPR sensing [8][9][10][11].In addition, sensitivity and measurement range are both important criteria for the SPR that can be increased by intended to select appropriate plasmatic material.Because of its chemical stability and very large wavelength shift, gold is the most attractive material for SPR phenomena [12][13][14][15].As the most conductive, plasmatic metal, silver is also used [16][17][18][19].Despite the above, it has been found that there are a lot of unusual characteristics, for example, chemical instability and oxidation susceptibility [20][21][22][23].While AuNP has many of the properties, nanometal colloids make them especially suitable for biological applications; namely, they are highly versatile in scale, shape, and surface chemistry; they are stable in a wide variety of environments; they are inert and non-toxic; and they have optical-electronic controllable properties [24][25][26][27].In Au nanowire (NW) synthesis, the control of size, shape, and surface functionality are very important issues.With the Au NW synthesis, good control was achieved by the chemical reduction of the gold ions in the solution [28][29][30][31].Au NW creation in this strategy results from removing a strong target put into a fluid medium.Changing the laser parameters and the idea of the comprehensive fluid medium enables NP size, focus, chemical composition, and utilitarian properties to be controlled; for this reason, numerous types of lasers have been proposed including nano-, pico-, and femtosecond lasers at different wavelengths from infrared (1064 nm) to ultraviolet (248 nm) [32][33][34][35].This paper explores laser P ablation of prepared gold nanoparticles (Au NWs) in ethanol at wavelengths 532 and 1064 nm [36,37] and the formation of a simple plasmonic biosensor consisting of photonic crystal fiber HC-800.Both the metal layer and sensing medium are placed on the front surface of the photonic crystal fiber [38][39][40].As a plasmonic material, gold was selected because of its stability in the aqueous solution since it shows a large sensor performance and resonance peak [41][42][43].These fibers have microchannels filled with fluid samples with a hollow core and cladding.HC-PCF can be filled in two ways and used as a sensor.
Selective filling of the fiber core with a sample while sealing off the cladding holes is the first method [44][45][46].The main drawback of this method is that the fiber-filling process is cumbersome and limits the choice of the sample to prevent highly multi-mode behavior [47][48][49][50].The other technique is the non-selective one, i.e., the core and the cladding holes are filled with the sample [51][52][53][54].In this case, the light remains guided by the effect of the band gap, and the wavelength band of the fiber transmission shifts depending on the sample's refractive index [55][56][57][58].The finite element method (FEM) using COMSOL Multiphysics is also used in the simulation.To prevent reflections from the outer borders and replace an infinite structure with a finite domain, a perfectly matched layer (PML) boundary is used.In this work, we used a non-selective method for hollow core photonic crystal fiber (HC-800 PCF) as a biosensor by studying the effect of light (laser beam) on an HC-PCF filling with gold nanowire to enhance the sensitivity.Mathematical analysis based on COMSOL Multiphysics was adopted to investigate the confinement loss and amplitude sensitivity and also presented the calculated amplitude sensitivity for different cases with and without deposited the gold NWs for the HC-PCF sensor.

Experimental Setup
High purity (99.999) gold plate of 2 cm diameter immersed in 5-ml ethanol was ablated using fundamental and Q-Switched Nd-YAG laser wavelength via 12-cm lens.The energy of 2 J/pulse and 500 pulses was used for gold nanoparticle preparation as shown in Fig. 1.Several seconds of ablation lead to a purple coloration of the solution indicating the formation of gold NWs.The dropcasting technique was used for the deposition of gold NPs on porous silicon.
The prepared Au NWs were characterized using an X-ray diffractometer system of X'Pert pro-MRD PW3040 type.An ultraviolet-visible photometer was used to investigate the optical absorption and transmission.Nanostructure morphology was measured using the FESEM.
The photonic crystal fiber (PCF) modification process includes an etching process using HF (30%) from Sigma Aldrich for 15 s.The obtained PCF sample was immersed in the prepared colloidal of gold nanowires.The deposition process was done with the help of a 405-nm laser for 4-h time duration.This step was taken during the immersing of the PCF-HC-800 in gold NWs to form a coated region on the etched part.The process has been presented in Fig. 1.The colon tissue has been immobilized on gold nanowire after dipping in colon tissue colloidal.

Results and Discussion
Figure 2a-c shows the XRD of the obtained gold nanostructure using the PLA technique at 1064 and 532 nm. Figure 2a shows the result of the gold target; one peak could be recognized that appeared at 2 = 38.1 corresponding to the (111) diffraction plain related to gold material.
Figure 2b shows the x-ray diffraction results of the prepared nanostructure using 1064-nm laser wavelength; three particular peaks could be shown at 2θ = 34.05,38.1, and 44.22 degree which are related to the (110), (111), and (200) diffraction plains, respectively.
Figure 2c presents the XRD at 532-nm laser wavelength; three particular peaks were shows up at 2θ = 34.05,38.1, and 44.22.Every one of the three peaks corresponded to the standard Bragg reflections (110), (111), and (200) of the Fcc lattice.The peak of 34.05 alludes to the SiO2 which shows up because of silicon collaboration with ethanol.From the obtained results, the enhancement in crystallization using a second harmonic generation laser at 532 nm is clear where the higher intensity was obtained as shown in Fig. 2c.
Transmission electron microscopy (TEM) was applied to investigate the morphology results of the ablated gold nanowires at two different wavelengths.The lowmagnification images of TEM of the gold nanowires showed the morphology of the nanowires with varying diameters but not exceeding 40 nm and lengths varying, sometimes reaching 3 µm in some wires, as shown in Fig. 3.The apparent difference in the two wavelengths used in the ablation is that more homogeneous and more diffuse gold nanowires have been reformulated and prepared with an average length of 3 μm and a diameter not exceeding 50 nm when ablation using the smaller wavelength (532 nm) and as described in part (b).From Fig. 3, this is done as a result of doubling the laser energy from bisecting the wavelength as a result of using the second harmonic generation.
Figure 4 illustrates the SPR transmission and the absorption spectrum of the gold NWs as a wavelength function for 1064 and 532 nm wavelengths.From this figure, it can be observed that the SPR band shifts to longer wavelengths as the Au NWs size increases.From Fig. 3a, the gold NWs at 1064 nm have a larger size than the 532 nm laser beam.As a result, the highest transmission peak has been achieved by 1064; the transmission peaks reached 1064 and 532 nm around 1064 nm (81.5) of the transmission.And the effective transmission of approximately 469 nm and the transmission of approximately 532 nm (72.78) and the effective transmission of approximately 455 nm indicate that if we lower the laser wavelengths to ablate the gold nanoparticles, we obtain a blue shift, and this means that the grain size will be lower.
From Fig. 5, the highest absorption peak resulted in the effective absorption of approximately 0.34 by the applied 1064-nm laser beam, while the absorption peak was obtained at 530 nm, and the effective absorption was approximately 0.4 by using 532-nm laser.SPR absorption of gold NPs depends on the size and shape of gold NPs where the larger the absorption volume, in order to give maximum absorption from 500 nm to the near-infrared spectrum area, the surface plasmon resonance can be easily tuned.For example, spherical colloidal gold cytodiagnostics in the area between 450 and 500 nm have the maximum absorbance as shown above, but irregular particles such as gold nanorods and urchin form gold nanoparticles (also known as gold nano-stars) have the maximum absorbance in the spectra's near-infrared area.This is similar to the results recorded by Nguyen Ngoc Long et al. [59,60].
The change in the red shift of the band gap with the presence of the gold NWs layer is related to the resonance effect resulting from the nanoparticle presence; this layer induced an immobilization process with biological tissue.The sensing activity is related to the difference in refractive index in the fiber structure after adding the gold layer according to Snell's law.And we also note the effect of the surface plasmon resonance phenomenon on the fiber after adding the gold layer, as it led to a reduction of confinement losses, and this leads to an increase in the efficiency of the sensor.
Figure 6 shows the reflectance of gold nanoparticles in the wavelength 532, 1064 nm at room temperature.The optical reflectance (R %) of gold nanoparticles was measured.In general, reflection is the variation in electromagnetic radiation direction at the interface between two mediums such that the electromagnetic radiation returns back to the same medium.Figure 5 shows the reflectance of gold nanoparticles in the wavelength 532, 1064 nm at room temperature.We notice the reflection of short wavelengths more than long wavelengths, so the highest reflectance peak has been achieved by 532 nm (62.24), and at 1064 nm, the reflectance peak has been achieved by 17.32.
Figure 7 shows the change in the index of refractive with the wavelength.The reduction in the refraction index value is clear as the wavelength increase using the two wavelengths at λ = 532 nm and λ = 1064 nm.A relative reduction in the value of the refractive index could be seen using the standard wavelength compared with the second harmonic generation of 532 nm, which may be related to the frequency doubling.
Figure 8 shows the variation of absorption coefficient with wavelength.The highest absorption coefficient peak resulted in the effective absorption of approximately 790,117.6 by the applied 1064-nm laser beam, while the absorption coefficient peak was obtained at 530 nm, and the Figure 9 presents the variance of (αhv) 2 versus as a function of the photon energy (hv) for the gold nanowires ablated in ethanol for 1064-nm and 532-nm lasers.The optical band gap result proved that the transmission type is direct, and the optical band gap value can be determined by extrapolating the linear portion of the curve in the figure to hv = 0 points.From here, we found that the value of the optical band gap upon ablation using the higher wavelength (1064 nm) is about (4.02 eV).The value of the optical band gap increases to reach a value of 4.22 eV at the shorter wavelength (532 nm).This value is better in the case of transmission at the shorter wavelength (532 nm) than that which was ablated at the higher laser wavelength (1064 nm).
Figure 10 explains the photoluminescence (PL) spectra of gold NPs.The PL emissions peak appears at 532 nm with an energy band gap of around (2.48 eV) and at 1064 nm the energy band gap around (2.41 eV).Due to the decreasing grain size and rising energy gap of the prepared material, it is determined that there were blue shifts in the PL spectra.The prepared gold NP morphological characteristics at a wavelength of 532 nm were better than those obtained at 1064 nm by wavelength.It is attributed to the high strength of short wavelengths [61].
Figure 11 presents the topographic structures (AFM) of the gold nanorods that ablated at different wavelengths and deposited using the drop-casting method on the quartz substrates.At the lower wavelength (532 nm), small size of particles and like a needle or rod were found.The values of the roughness (mean = 17.69 nm) and also the structures are more distributed and smooth than those nanostructures ablated using a high wavelength of pulsed laser (1064 nm) which was the roughness (mean = 6.93 nm), as a result of the effect of doubling the laser energy due to the use of the second harmonic generation (532 nm) in the ablation of Fig. 5 UV-Vis absorption spectrum of the gold NPs embedded in ethanol for 1064-and 532-nm lasers Fig. 6 The reflectance of the gold NPs embedded in ethanol for 1064-and 532-nm lasers gold atoms and its interaction more with the gold surface than with its interaction with the wavelength of the original laser (1064 nm); these values are highly consistent with the results of XRD and TEM that were previously presented and discussed above.All measured surface constants are tabulated in Table 1.
Figure 12 presents the images of the FESEM of ablated gold NWs at different wavelengths, where the presented nanostructures that ablated at the lower wavelength (532 nm) clearly show that a smooth surface and more distributed, and the average diameter of the deposited nanostructure is ranged from 30 to 40 nm.The images also clearly showed that the nanostructure of the deposited gold as a result of its ablation at the higher wavelength (1064 nm) was well distributed and somewhat smoothed but with less quality than what was obtained when using the simultaneous harmonic generation of the laser, and this result is consistent with what was previously discussed in the structural, optical, and the morphological results.The EDX results also confirmed the emergence of gold that was deposited after the ablation process in addition to the emergence of silicon and oxygen as a combination of silicon oxide (as a used substrate).

Sensor Design and Theoretical Basis
The two-dimension (2D) cross-sectional view of the photonic crystal fiber HC-800 as a biosensor is depicted in Fig. 13.The photonic crystal fiber (HC-800) has the following physical properties: a core diameter of 7.5 ± 1 μm, a cladding diameter of 130 ± 5 μm, a region diameter of 45 ± 5 μm, a pitch of 2.3 ± 0.1 μm, and an air-filling fraction of > 90% in the hollow region [62][63][64].A thin gold layer is selected as the plasmonic material having a diameter g = 32.55 nm, which is placed at the outer part of the PCF by the used etching process, the PCF HC-800.Therefore, the surface plasmon propagates through the metal dielectric surface effectively.In this structure, the fused silica is selected as the background material, and the material dispersion is obtained from Equation Sell Meier [65][66][67].
From the imaginary part of the effective mode index, one can calculate the confinement loss (CL) based on Eq. (1) [68,69]: In(10) = 8.686 × k 0• I m n eff Fig. 7 The refractive index of the gold NPs embedded in ethanol for 1064-and 532-nm lasers Fig. 8 The absorption coefficient of the gold NPs embedded in ethanol for 1064-and 532-nm lasers where λ is the operating wavelength measured in μm which is proportional to the Im (neff) (imaginary part of effective refractive index).Figure 14 presents the confinement loss of the fundamental mode for the proposed HC-800 PCF infiltrated with water and colon tissues, before adding a ring or layer of a gold nanoparticle to the proposed HC-800 PCF.In Fig. 14, the band gap noted that it is shifting to blue shift confinement and not only shifts, but also decreases to lower wavelengths, but also decreases confinement loss values as the refractive index increases in air holes.
In Fig. 15, the proposed sensor deposited with gold NWs prepared the confinement loss which decreases further, and we notice the band gap heads toward the longer wavelengths (redshift).By changing the cladding refractive index, eventually, we compared the value of the confinement loss for different liquids (water and colon tissue), between Fig. 11 and Fig. 15, so the confinement loss in Fig. 9 The energy gap of the gold NWs ablated in ethanol at 1064-and 532-nm lasers Fig. 10 The photoluminescence (PL) results the gold Nws colloidal prepared using 1064and 532-nm laser wavelengths Fig. 11 The FESEM images of the gold Nws prepared using a 532-nm and b 1064-nm laser wavelengths Fig. 14 is about 1.08E + 03 and 5.46E + 02 dB/m, and as seen in Fig. 15 when using the deposited PCF with gold NPs, the confinement loss is less than about 1.01E + 03 and 3.16E + 02 dB/m.
When comparing various characterizations of the results of the fundamental mode for the different cases with the deposited gold NWs and without deposited gold NWs, it finds the losses decreased when adding the gold layer.
There are also other calculations to increase HC-PCF biosensor performance, including sensitivity.We evaluated the proposed fiber's sensitivity using wavelength interrogation and amplitude measurements.The following formula can calculate wavelength sensitivity in wavelength interrogation [70], such as amplitude sensitivity, resolution, and wavelength sensitivity.So the amplitude sensitivity can be computed by the following equation [66,71]:  where Δ peak represents the difference between wavelength peak shifts, and n a is the difference between two (RI), where a change in the analyte RI is 1.3290-1.3351and then the calculated wavelength sensitivities are 18,032 nm/RI for colon tissue.And the following equation provides the proposed sensor's resolution [74,75]: where Δn a represented the difference between the two refrac- tive indices, Δ min = 0.1 [66,76].The standard assuming a value for the minimum spectral resolution, Δ peak , is the Figure 16 shows the proposed sensor's amplitude sensitivity for the same analyte RI values.At 0.98 μm, we discovered a maximum amplitude sensitivity of approximately 137.67 RIU-1 for an analyte refractive index (RI) of 1.3351.In addition to the comparison between other properties to increase the performance of the biosensor, the calculations of sensitivity for the sensor noted the amplitude sensitivity for the same analyte RI values.At 0.98 μm, we discovered a maximum amplitude sensitivity of approximately 137.67 RIU-1 for an analyte refractive index (RI) of 1.3351, as shown in Fig. 16.
We compare the results of the fundamental mode for the different cases with the deposited gold NWs and without deposited gold NWs.We find the colon tissue has the best electric field when using the gold layer on the sensor than the sensor without the gold layer as presented in Figs. 17  and 18.
Figures 17 and 18 explain the fundamental mode of liquids compared to their electric field.We found the bio-liquid (colon tissue) has the best electric field using the gold NW layer in the biosensor equal to 68.8 V/m by comparison when without using the gold NW layer which leads to high sensitivity.
There are many studies that depend on surface plasmonic phenomena.So when we make a comparison between this study and other studies, we note the results depend on the materials and the structure of the design for each study.When we compare between most recent works, we note this work has the best wavelength sensitivity equal to 13,725.49(nm/ RIU) as shown in Table 2 [77][78][79][80][81].

Conclusions
A high-quality gold nanowire could be obtained successfully by employing the highest intensity of shorter wavelengths at the 532-nm laser wavelength.Which reflects better properties than that achieved by the 1064nm laser?The theoretical investigation refers that the confinement loss decreased when the refractive index of liquids increased, where the value of confinement loss was decreased when using water and colon tissue.When using the proposed sensor deposited with gold NPs, the confinement loss decreased further than before depositing the gold NWs on the proposed sensor.Additionally, it was discovered that the bio-liquid (colon tissue) has the best electric field using the gold NW layer in the biosensor, equal to 68.8 V/m by comparison when not using gold NW layer, which leads to high sensitivity when calculating the sensitivity to increase HC-PCF biosensor performance by using wavelength interrogation and amplitude measurements.

Fig. 1
Fig. 1 Experimental set-up by laser ablation in solution for the gold Nws colloid preparation

Fig. 3
Fig. 3 TEM of the gold NWs embedded in ethanol for 1064and 532-nm lasers

Fig. 12 Fig. 13 Fig. 14
Fig.12 The FESEM images of the gold NPs embedded in ethanol for a 532-and b 1064-nm lasers

( 4 )Fig. 15 Fig. 16
Fig. 15 Confinement loss of the fundamental mode for different refractive indices infiltrated HC-800 after being deposited with gold NPs

Fig. 17
Fig. 17Fundamental mode for different cases of HC-PCF biosensor before adding the gold NWs layer on a sensor Fig. 17Fundamental mode for different cases of HC-PCF biosensor before adding the gold NWs layer on a sensor

Fig. 18
Fig. 18 Fundamental mode for different cases of HC-PCF biosensor after adding the gold NW layer

Table 1
Root mean square values, average roughness, and the average diameter of the gold nanostructure ablated at different wavelengths

Table 2
49velength sensitivity equal to 13,725.49