The Effects of Psi Morphology on the Detection Efficiency of the Au-Nps Hot Spot SERS Sensors

This work is related to the investigation of the effects of porous silicon (PSi) morphologies on the performance of plasmonic gold nanoparticles (Au-NPs) hot spot SERS sensors for the detection of amoxicillin molecules. Two Si wafers with different resistivity values of 10 and 100 Ω.cm were used to synthesize a PSi layer of poresand mud-like structures, respectively, by pulsed photo chemical etching process. The hot spot SERS sensors were synthesized by incorporating the Au-NPs within the PSi morphologies of poresand mudlike structures which are characterized by high density of nucleation sites. Plasmonic AuNPs with different sizes and hot spot regions were incorporated into the porous structures by the ion reduction process. It was recognized that the PSi morphologies have a considerable influence on the fabrication process of the detection sensors and, consequently, on the detection process. The plasmonic Au-NPs hot spot SERS sensor based on the PSi morphology of the mud-like structures shows higher sensitivity than that of pores-like structures, even at very low concentrations of amoxicillin solutions.


1-Introduction
The surface-enhanced Raman scattering (SERS) process was used to enhance the scattering effect and overcome the low sensitivity of the common Raman method. This process significant to distinguish the chemical and biochemical segments, by acting as a powerful process for the detection of various analytes [1,2]. Recently, SERS has been also efficiently used in evaluating food safety and quality, including fast detection of different types of antibiotics [3][4][5][6][7]. SERS is an enhancement of the Raman signal activity to several orders of magnitude. This enhancement is mainly due to the interaction between light and metallic nanoparticles, which is linked to the effects of Plasmon nanoparticles and hot spot regions [8]. Typically, in SERS sensors, the plasmonics nanoparticles such as gold (Au) or silver (Ag) were employed to enhance Raman signal intensity and lower the detection limits of the fabricated sensors [9][10][11].
The SERS sensor mainly consists of plasmonics nanoparticles incorporated on a based substrate. One of the most popular and significant substrates for developing this type of chemical sensors is the PSi [12,13]. The structural properties (surface morphology, silicon nanocrystallites, and the amount of nucleation sites) are the very specific parameters for the formation of the plasmonic nanoparticles for the sensor. The PSi morphology is the most particular formation factor for controlling the size of the plasmonic nanoparticle and the density of the hot spot regions, since the density of the dangling bonds (metallic nucleation sites) varies with the type of the morphologies [14,15]. The pores-like and mudlike structures of PSi have different topographical patterns and vary with the formation conditions of the PSi layer [16,17]. In this study, efficient SERS plasmonic Au-NPs hot spot sensors were fabricated and the effects of Psi morphologies on the performance of these sensors were studied at very low concentrations of amoxicillin molecules.

2-Experimental part
The formation of the PSi substrate was carried out through the etching process of n-type Si in a cell made of teflon at room temperature. The etching solution consists of ethanol (C 2 H 5 OH) and hydrofluoric acid (HF) in a ratio of 2 to 1. The PSi layer of pores-and mud-like structures were synthesized via the pulsed photochemical etching process by using two different silicon wafer resistivity values of 10 and 100 Ω.cm, respectively. For the n-type doped Si, light must be provided since holes are wasted during the etching process. The holes play a significant role in Si etching. The reaction of dissolution creates defects on the surface of the silicon wafer surface, pores will be formed, and their walls will be eroded until they are removed from the holes. Laser pulse duty cycle, 30% of laser wavelength of 405 nm, and laser intensity of 100mW/cm 2 were employed in the etching process at a fixed etching time of 40 min. The schematic representation of the pulsed photochemical etching process is illustrated in Figure-1. Plasmonic Au-NPs with different sizes and hot spot regions were incorporated into the porous structures by ion reduction (immersion) process. This process was accomplished at room temperature with a fixed concentration of HAuCl 4 of about 5x10 -3 M and an immersion time of 100 sec. The characterization of the based PSi substrate and SERS plasmonic Au-NPs hot spot sensors was carried out by measuring the FE-SEM images, photoluminescence's spectrum PL, and SERS spectra. The FE-SEM was investigated via a MIRA3 TESCAN fieldemission scanning electron microscope, the PL spectrum was scanned by PL (Cary Eclipse FL 0912M014), and Raman measurements were examined using RENISHAW Raman scope 2000 device.

3-Results and discussions 3-1 Morphological properties of the based PSi layer
Morphological aspects (pore shape, size, and nature) of the PSi layer were studied according to the values of the silicon wafer resistivity. Two specific types of morphologies of ultra-fine pores -like and mud-like structures were achieved for two different values of Si wafer resistivity (10 and 100 Ω.cm, respectively), as shown in Figure-  For silicon wafer with resistivity of 100 Ω.cm, Figure-2a shows that the surface of bulk Si was reconstructed into micro island regions named mud-like structures. This type of PSi morphology is characterized by high density of Si hydrogenated bonds (Si-H x ) with x=2, 3. [19,20]. This structure is related to the doping of the Si wafer. As the doping decreases with increasing the wafer resistivity, the etching process was carried out strongly on the surface of silicon in x and y directions rather than deeply across the depth of wafer (in Z-direction). For Si wafer of resistivity of 10 Ω.cm, the surface of the Si was converted to pores -like structures (Figure-2b). One can observe that the surface morphology includes the existence of very tiny pores (pores -like structures). Among these pores, there are silicon nano column regions with different sizes. The formation of this type of morphology is due to the increase in the etching rate in the z-direction deeply into the silicon wafer. As the doping increases with the decrease of the wafer resistivity, the availability of silicon atoms in the laser irradiated area was dropped and, therefore, the etching rate was maintained in the lower silicon layer.

3-2 Optical properties of the based PSi layer
The optical properties of the based PSi layer were examined by analyzing the photoluminescence spectra. The Si nano crystallite size (L) and the energy gap of the PSi layer (E gPSi ) are related with the bulk Si energy gap (1.12 eV) by the following equation [18]: (1) The E gPSi is realized directly from the peak of the PL emission corresponding to the maximum wavelength (λmax) and, thus, E gPSi , (eV) = hc/λ max . Figure-3 shows the PL spectra of both types of PSi morphologies (mud-and pores-like structures). From this figure, it is observed that the PL peaks are located at wavelengths of 638 and 680 nm, corresponding to the emission spectrum of PSi. The intensity and the full width at half maximum (FWHM) of the PL spectrum vary with the types of porous morphology. The PL spectra of the mud-like structure sample have lower FWHM compared with that of the PL spectra of the pores-like structure sample, while the PL peak intensity of the mudlike structure is higher than that of the pores-like structure sample. This broadening of the FWHM of PL spectra is increased with the dispersion of Si nano crystallites within the porous matrix. The PL peak intensity is increased with increasing the density of silicon nano crystallites. The values of L, λ max , and EgPSi for mud-and pores-like structures are tabulated in Table-1. This variation in the obtained results from PL spectra is related intensively with the morphology of the based PSi layer. The formed mud-like PSi structure is a specific form of PSi morphology that includes high density of silicon nano crystallites (high density of gold nucleation sites) [18]. The high density of nucleation sites indicates that the silicon nanocrystallites are very small in size but their density is high. The energy gap is increased by decreasing the size of silicon nanocrystallites.

3-3 Characterization of plasmonic Au-NPs hot spot SERS sensors
The characterization of plasmonic Au-NPs/PSi hot spot SERS sensors is very important to recognize the performance of these sensors. The performance of theses sensors was extensively studied through the analysis of the morphological and structural properties as well as Raman spectroscopy of the sensors at the fixed amoxicillin concentration of 10 -3 M. The SERS sensors were dipped into the prepared solution of amoxicillin for ten hours in dark at room temperature.

3-4 Morphological aspects of plasmonic Au-NPs hot spot SERS sensors
Plasmonic Au-NPs hot spot layer was examined as an efficient SERS sensor for the detection of amoxicillin based morphological aspects of both types of PSi (pores-like and mud-like structures). The influence of the size of the Au-NPs and the density of the hot spot regions on the performance of the SERS sensor was investigated. The sensor behavior varies with the structures of the base porous layer. Figure-4 shows the SEM of plasmonic Au-NPs/PSi hot spot SERS sensors based on the poreslike structures. The morphological aspect of the deposited Au-NPs over the Si of pores-like structures shows that the form of the nanoparticles appears as spherical, semi-isolated, nanoparticles with different sizes over the nano columns layer. These formed nanoparticles complete only a single layer over the pore structures which subsequently form a homogeneous growth, where the layer contains low density of Si-H x bond. For the pores-like structures, the dimension sizes of plasmonic Au-NPs and the density of hot spot regions among the nanoparticles fluctuated with the morphological aspects. Figure-(5 a, b) illustrates the size of plasmonic Au-NPs and the hot spot regions among the Au-NPs, respectively. The formation and density of the hot spot regions depends on the PSi morphology, since the structure has high nucleation site and high density of the hot spot regions. The histogram of plasmonic Au-NP size ranged from 10 to 70 nm, whereas the peak of the size was about 20 nm with a percentage of about 35%. The histogram of the hot spot gaps (vacancies) among the plasmonic Au-NPs ranged from 46 to 58 nm with the highest peak being located at 58 nm with a percentage of about 23%.  The size histogram of plasmonic Au-NPs and the density of the hot spot gaps among the nanoparticles deposited above the mud layer are illustrated in Figure-7 a, b, respectively. The size of the Au-NPs ranged from 20 to 120 nm, whereas the highest proportion of these NPS (39%) was of the size of 10 nm. Also, the size of the hot spot gaps (vacancies) among the Au-NPs ranged from 36 to 45 nm, while the highest peak was located at 45 nm with a percentage of about 23%. These sizes of the formed hot spot gaps for mud-like structures were lower than those of the pores-like structures. The density of the synthesized nano-gaps (Raman active sites) for the porous layer of mud-like structures, which was originated by 30% laser duty cycles, was higher than that of the others types of porous layers. Thus, we expect that the resulted SERS spectrum is increased with the percentage of this active site.

3-5 Detection of the performance of plasmonic AuNPs hot spot SERS sensors based on the bare PSi
The sensing performance of the bare pores-like and mud-like structures was firstly inspected at the higher concentration of amoxicillin of about 10 -1 M in order to recognize the effects of PSi morphologies on the detection process of amoxicillin solution. Figure-8(a, b) illustrates the measured Raman Spectra of bare PSi at a fixed concentration of amoxicillin of about 10 -1 M for both types of morphologies. The measured Raman spectra of mud-like structures for the amoxicillin solution were higher than those of pores-like structures. The analysis of Raman spectrum of amoxicillin showed the presence of main peak intensity of about 1592 cm -1 and 968cm -1 . These characteristic peaks can be used for a qualitative analysis of amoxicillin due to the positive correlation with its concentration. In addition to these characteristics, there were other Raman peaks of wave numbers of 850 and 1420 cm -1 . These measured peaks are in excellent agreement with those obtained by Ji et al. [21]. The intensity of the main peak is very small and, thus, the incorporation of bare PSi with Au-NPs was employed to develop SERS enhancement ability.

Mahmood and Alwan
Iraqi Journal of Science, 2021, Vol. 62, No. 1, pp: 157-166 861 The sensing performance of the plasmonic Au-NPs hot spot SERS sensors was inspected at a lower concentration of about 10 -3 M of amoxicillin for depositing Au-NPs on both PSi morphologies (pores-like and mud-like structure), as shown in Figure-9(a,b). From this figure, we observe that the increase of Raman intensity of the main peak of hot spot SERS sensor based on the pores-like structure was much lower than that of the hot spot SERS sensor based on the mud-like structure. This is strongly related with the density and the size of the formed hot

Mahmood and Alwan
Iraqi Journal of Science, 2021, Vol. 62, No. 1, pp: 157-166 865 spot regions. Based on the analysis of the obtained micro images of the scanning electron microscopy, the hot spot gaps had a higher population at higher gap sizes. This would inhibit or reduce the coupling efficiency between the local antenna and the target molecules inside the hot spot regions. The decrease of this efficiency would lead to decrease the obtained Raman signal due to the decrease in the absorbed photon by the target molecules. The density and size of the hot spot gaps among the plasmonic Au-NPs play a considerable role in the performance of the SERS sensors [22,23]. Assume that two nanoparticles of size (D) are separated by gap (hot spot gap) of size (d). If these nanoparticles are presented in an uniform electrostatic field (E 0 ) of Raman signal , the contingent local electric field (E L ) will vary with the nanoparticle size and the hot spot gap size, as given by the following equation [23] .
(( ) ) ( ) When a molecule of antibiotics is located in these hot spot gaps, an improvement in the intensity of the Raman signal will be attained by Surface Plasmonics (SP). This dependence of the performance of SERS sensor on the hot spot gaps is in excellent agreement with the results observed by Wali et al. [22]. In order for the comparison to be complete, it is better to calculate the enhancement factor (EF) using the following equation [22]: ( ) ( ) ⁄ ( ) where I SERS is the SERS intensity of plasmonic hot spot SERS sensors and I R is the Raman intensity of the bare PSi (mud-like s and pores-like structures) at the amoxicillin concentrations of C SERS and C R , respectively. The EF values of hot spot SERS sensor based on both types of PSi morphologies were about 250 and 175 for 10 -3 M of amoxicillin at the main peak of 1592 cm -1 , respectively. This confirms that the plasmonic Au-NPs hot spot SERS based on the mud-like structures has higher sensitivity than that based on the pores-like structures.

Conclusions
Efficient, low cost, and easy to fabricate plasmonic Au-NPs hot spot SERS sensors were developed based on both PSi morphologies (pores-like and mud-like structures) for the detection of amoxicillin. The best detection conditions which involve higher sensitivity and excellent linearity were achieved for the sensor based on PSi morphology with mud-like structures.