The Performance of Plasmonic Gold and Silver Nanoparticle-Based SERS Sensors

The influence of different types of plasmonic gold (Au-NPs) and silver (Ag-NPs) nanoparticles as well as aging on the performance of Surface-Enhanced Raman Scattering (SERS) sensors were studied. The average diameters of Au-NPs and AgNPs were about 23 nm and 15 nm, respectively, with a number of laser pulses of about 200. plasmonic nanoparticles were synthesized by laser ablation process in distilled water using a fixed energy laser fluence of about 14 J/cm 2 of Nd-YAG laser, with 1060 nm wavelength and 1 Hz pulse repetition rate. The SERS sensor was carried out by quick drop casting process of plasmonicplasmonic nanoparticles on glass substrates. The morphological aspects and the performance of SERS sensors were investigated by high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. All the results indicated the significant dependence of the performance of the sensor on the types of the plasmonic nanoparticles . The obtained Raman signal intensity of Ag-NPs was about 10 5 a.u. compared with 10 3 a.u. for Au-NPs. While, the stability of Au-NPs was much higher than that of Ag-NPs based on SERS sensors due to the normal oxidation process of Ag-NPs.


Introduction
The potential applications of nanostructures in chemical and biochemical sensors as well as industrial and nano photonics fields depend on the dimensions, forms, and physicochemical characteristics of the plasmonic nanomaterials [1][2][3][4][5]. A number of chemical and physical approaches are employed to produce these materials, such as ion reduction process, thermal dissociations of metal salts, electrochemical process, and pulsed laser ablation process [6,7]. The latter process is a green, fast, and simple physical technique for the production of colloidal suspensions of metallic plasmonic nanoparticles (gold and silver). Furthermore, the significant advantage of this technique over the other approaches is the formation of high surface-purity nanoparticles without counter ions or residuals of the reducing agents left over on the surfaces of the nanoparticles [8][9][10]. The surface enhanced Raman scattering (SERS) sensors were prepared via the ion reduction process of silver and gold ions by the dangling bonds of the porous silicon layer and, hence, the fabrication of porous silicon-based SERS sensors [11][12][13]. Colloidal suspensions of plasmonic nanoparticles were deposited on glass substrates immobilize them and to inspection those for possible request as an active substrate for sensing of Rohdamine 6G (R6G) via glass-based SERS sensors.
The consequences of SERS and hence the improvement of the Raman signal are mainly due to the inelastic scattering of specific target molecules in the presence of plasmonic metallic nanoparticles [13]. The main reason behind this improvement is the energy transfer process among the nanoparticles and the target molecules on the based substrate (porous silicon, rough surface and glass substrate). The rate of the transfer is increased with increasing the density of the plasmonic nanoparticles and hot spot regions [14][15][16][17][18][19].
The purpose of the present work is to investigate the influence of different types of plasmonic nanoparticles (Au-NPs and Ag-NPs) on the performance of as-prepared and air aged SERS sensors for detecting ultra-low concentrations of R6G molecules, which is a dye cytotoxic to Friend leukemia cells and doxorubicin-resistant variant cells.

Materials and Methods
Plasmonic nanoparticles (Au-NPs and Ag-NPs) were individually prepared via pulsed laser ablation technique for gold and silver plates (purity of 99.99%), located in a glass vessel filled with 5 mL distilled water (DW). Colloidal suspensions of Au-NPs and Ag-NPs were synthesized by laser ablation process using a fixed energy laser fluence of about 14 J/cm 2 of Nd-YAG laser with 1060 nm wavelength, 1 Hz pulse repetition rate, and 200 pulses, as shown in Figure-1. Nanoparticle layer of SERS was prepared by drop casting technique of suspensions of Au-NPs and Ag-NPs on a glass slide. A few droplets of an R6G solution of 10 -10 M concentration were dropped and left to be dried on the SERS substrate to study the performance of as-prepared and air aged SERS sensors after a period of 6 months. To characterize the morphological aspects and the performance of SERS sensors, high resolution transmission electron microscopy (HR-TEM, Philips CM30, USA), Raman microscope, and a UV-Vis beam spectrophotometer (Shemadze, Japan) were employed. Raman bands of R6G dye resulting from Au-NPs/glass and Ag-NPs /glass SERS sensors were recorded via dispersive Raman microscope (Senterra 2009, Bruker, Germany) through 750 nm excitation wavelength and 50 mW scanning power, as shown in Figure-2.

Results and discussion 3.1 Plasmonic features
The absorption spectra of the colloidal Au-NPs and Ag-NPs are illustrated in Figure-3. The resonance absorption peaks corresponding to the AuNPs and AgNPs were found to be at 525 nm and 425 nm, respectively. (1) where D p is the grain size of Ag-AuNPs and is the density of the bimetallic alloy of Ag-AuNPs (g/cm 3 ), which was calculated based on the density of gold and silver independently, as follows.
(2) where, a is wt. % Au, b is wt. % Ag, ρ Au is the density of gold, and ρ Ag is the density of silver. The values of S.S.A of Au-NPs and Ag-NPs were about 20.3 and 30.7 m 2 /gm, respectively. The achieved value of S.S.A of Ag-NPs of the as-prepared sample was higher than that of Au-NPs due to the fact that their size is smaller owing to their lower tendency of aggregation [20,21].

Figure 4-HR-TEM images of colloids (a) Ag-NPs and (b) Au-NPs.
Figure-5 illustrates the results of the energy-dispersive X-ray spectroscopy (EDS) analysis of asprepared Au-NPs and Ag-NPs (a and c) and those after 6 months of air ageing (b and d). These spectra showed definitively that the intensity of Au-NPs does not change with time, while it is clear that the peak for Ag-NPs was decreased with time due to the growth of the oxide layer and, hence, the presence of a new phase of silver nanoparticles in the form of Ag 2 O [22] specific peaks related to the presence of the gold and silver elements in addition to that of oxygen for the case of silver aged sample.
Figure5-EDS analysis of Au-NPs and Ag-NPs: (a and c) as-prepared, (b and d) after 6 months.

Alwan et al.
Iraqi Journal of Science, 2020, Vol. 61, No. 6, pp: 1320-1327 1231 3.2. Raman spectra of Au-NPs and Ag-NPs /glass SERS sensors Raman spectra measurements of Au-NPs and Ag-NPs/glass of SERS sensors, as depicted in Figure-6, show spectra consisting of five particular Raman peaks of high intestines which signify the R6G and located at 1650, 1574, 1517, 1364 and 1279 cm −1 . These particular peaks are intensely coupled to C-C and C-H bond stretching vibrations [22,23]. The activities of both as-prepared and air-aged Au-NPs and Ag-NPs/glass SERS sensors for the R6G target molecules, at a low concentration of about 10 −10 M, are shown in Figure-6. For Au-NPs /glass SERS sensors, the obtained Raman intensity of the main peaks at a wave number of about 1650 cm-1 did not change significantly with the aging period, with an intensity value of about I SERS =10 3 . While, for Ag-NPs /glass SERS sensors, the intensity of Raman peaks suffered a large degradation in intensity with the aging period by a factor of two order of magnitudes. This degradation in intensity is strongly related to the decrease in the density of hot spot regions with the aging time [11]. Also, the growth of the oxide layer leads to an increase in the size of the plasmonic nanoparticles and, hence, a decreased activity of the local antenna (plasmonic nanoparticles) [12]. The intensity of Raman signal resulting from as-prepared Au-NPs and Ag-NPs /glass SERS sensors was much higher than that of Au-NPs/glass SERS sensor. This performance is related to the exposure area (interaction volume), where the increasing of the S.S.A in the case of Ag-NPs will increase the probability of energy transfer among the plasmonic nanoparticles and the target molecules and, hence, increase the Raman signal [20,22] .

Conclusion
The drop casting process of plasmonic nanoparticles on glass substrates resulted in an inexpensive and easy to perform SERS sensor. The influences of different types of plasmonic nanoparticles (Au-NPs and Ag-NPs) as well as that of aging on the performance of SERS sensors were studied. We show that the Raman spectra of Au-NPs and Ag-NPs/glass sensors for R6G detection were efficiently improved. Furthermore, the specific surface area of Au-NPs and Ag-NPs as well their sizes and the aging process play significant roles on the performance of their glass SERS sensors. The SERS performance of the of Au-NPs and Ag-NPs/glass was assessed for a very low concentration, 10 −10 M, of R6G dye. The Raman signal was strongly enhanced by our SERS substrate. The performance of the