Fabrication of Surface Enhanced Raman Spectroscopy based sensor using Ag NPs on copper tape flexible substrate

In this work, the authors have investigated the properties of thermally evaporated silver nanoarrays on copper tape (Ag/Cu) as flexible and reusable sensors for organic contaminants. Uniformly deposited silver nanoparticles (Ag NPs) with well-controlled sizes have formed by thermal evaporation. The deposition time is varied to obtain different morphology of the Ag nanoparticles. The Surface-enhanced Raman spectroscopy (SERS) efficiency has been optimized using Rhodamine 6G (R6G) as a test molecule. Morphological studies have also been carried out using scanning electron and atomic force microscopes. The Ag NPs on the copper tape-based sensor show a substantial enhancement in the order of 8.3x107 in the SERS signal for a deposition time of 10 s. Furthermore, we observe that as-prepared Ag/Cu tape can detect micro-molar alizarine red S (ARS), which exhibited excellent reproducibility as well. Hence, the flexible copper substrate proves to be a promising material for detecting environmental hazards at a meager cost.


Introduction
Surface-enhanced Raman spectroscopy (SERS) is an essential tool for identifying molecules in different areas like food quality assessment, environmental monitoring, pharmaceutical research, and clinical diagnosis [1][2][3][4].In SERS, the intensity of scattered Raman signals can be enhanced when analyte molecules are deposited on metal nanoparticle surfaces.Local surface Plasmon Resonance (LSPR) of metal nanostructures is responsible for electromagnetic enhancement (EM), whereas the charge transfer between the metal nanoparticles to analyte molecules leads to chemical enhancement (CM) [5][6][7].In the case of EM, the intensification in the Raman signal is observed by 10 6 to 10 8 folds of its magnitude when the analyte has adsorbed on SERS active 'hotspot'.Also, the SERS effect can be greatly affected by interparticle distance, shape, and size [8].Therefore, it has been considered that correlating the LSPR of the SERS substrate with the energy of incident radiation is highly crucial.Many research strategies have been employed for developing highly efficient and reproducible substrates, whereas ultra-low-level detection limits are reported for many molecules [9].It has already been reported the detection of para amino benzoic acid (PABA) using silver nanoparticles (Ag NPs) deposited on a silicon substrate [10].
Different organic pollutants have been used in industries, usually discharged into the environment, which can cause serious water contamination issues [15].Since many organic dyes are carcinogenic, it causes severe health issues to human and living organisms in the water [16][17][18].Alizarin Red S (ARS) is a water-soluble organic dye used in fabric industries, and it is thermally and optically stable [19,20].A complex metal form for ARS is also used as mordant cotton, wool, and silk dyes.In addition, ARS has been employed for various laboratory uses such as pH indicators, spot test reagents, etc.Also, it has a very high capacity to bind to metal ions [21].ARS has been recognized as the most stable dye that can enter humans, which can lead to cancer and severe allergic reactions [22].Hence, decomposing this dye is very important before it gets discharged to nature from any industry.Traditional water treatments like adsorption and desalination have many limitations in degrading ARS due to their high stability.
In this study, we are reporting the detection of hydroxy anthraquinone, also known as alizarin red S (ARS, 1,2-dihydroxy-3-sulfonate-9,10-anthraquinone), over silver nanoarrays deposited on a copper flexible substrate.Highly flexible copper tape decorated with silver nanoparticles was fabricated by a thermal evaporation process, and the efficacy of ARS detection was investigated.

Experimental
Silver nanoparticles were deposited on a copper flexible substrate (Ag/Cu) by a thermal evaporation method.A current of 250 A was applied for 5, 10, 15, 20, 25, and 30 seconds to a high-purity silver target bought from a local jewellery shop.The instrument used for this experiment was from Denton benchtop thermal evaporator, and a tungsten boat was used to hold the silver target.A vacuum of 2×10 - 5 torr was created inside the chamber, and the substrates were allowed to rotate at a speed of 15 RPM.
The morphological studies were carried out on the prepared samples using Thermo scientific Apreo 2C field emission scanning electron microscope (FESEM) and Nanosurf Flex axiom 3000 atomic force microscope (AFM).The presence of silver deposition was confirmed using the Oxford Instruments UltimMax 100 energy dispersive spectrometer (EDS) detector attached to the FESEM.The surfaceenhanced Raman spectroscopy studies were carried out using a Renishaw Invia Raman microscope with an objective of 50x.An excitation wavelength of 514 nm was used with a power of 100 µwatt for an integration time of 10 seconds.

Results and Discussion
FESEM micrographs show a uniform deposition of spherical silver nanoparticles on the surfaces of flexible copper tape (Figure 1a) and the corresponding EDS spectra, which confirms the presence of the silver (Figure 1b). Figure 1c represents the morphology obtained from the AFM image.Both the FESEM and AFM show a uniform distribution of the silver nanoparticles with uniform sizes.The deposition process was done by applying a very high current for a short duration.Hence the particles were deposited instead of continuous film which is proved from the SEM image.The SERS activities of fabricated Ag/Cu tape sensors have been analysed using both R6G and ARS (Figure 2a).One can see the selectivity of SERS analysis as both dyes show distinct fingerprint bands [24,25].The main Raman modes such as C-C ring bending (612 cm -1 ), C-H bending (773, 1128, and 1187 cm -1 ), C-C stretching (1373 cm -1 ), C-N stretching (1449 cm -1 ), C-C stretching (1508, 1572 and 1649 cm -1 ) are observed for R6G.In the case of ARS, the characteristics of vibrational modes such as SO3 stretching (935 and 1063 cm -1 ), 1248 (C=O stretching), C-C stretching (1318 cm -1 ), C-C stretching (1318 cm -1) and C-OH bending (1420 cm -1 ), C=O stretching (1635 cm -1 ) are observed.
The SERS activities of Ag nanoarrays deposited on copper are investigated using R6G.It has been found that the intensity of the Raman signal is significantly affected by the deposition time.In the thermal evaporator process, the time has been varied to obtain and tune Ag nanoparticles' morphological characteristics.The Ag-NPs sizes are estimated and plotted against SERS intensity, as shown in Figure 2b.It is observed that the highest SERS intensity is obtained for the evaporation duration of 10 seconds.The superior SERS enhancement can correlate to the confined nanoparticle morphology, which can provide better LSPR effects [26].In comparison with copper tape, we have evaluated Ag NPs on glass substrates as well which we have already demonstrated on our earlier work [10].
The enhancement factor (EF) of Ag/Cu can be calculated from the concentrations of Raman and the SERS spectra of the R6G by taking the counts for the peak position at 612 cm -1 .This is estimated about 8.3x10 7 using the equation [27] The optimized Ag/Cu tape sensor has been evaluated for reproducibility of Raman signals by performing Raman 2D mapping.From the 2D map, we have selected 15 random spectra of ARS and plotted them, as shown in Figure 3a.Moreover, the intensities at 1250 cm -1 are plotted, and the relative standard deviation (RSD) is estimated (Figure 3b).It has been observed that an RSD value of 11% is obtained for the as-fabricated sensor.Thus, Ag/Cu tape demonstrates excellent spot-to-spot reproducibility for being used as chemo-sensors for environmental applications.In the present work, we have attempted to investigate the feasibility of SERS sensing of stable dye like ARS using Ag/Cu tape as a flexible and cost-effective sensor (Figure 4a).The electromagnetic mechanism can mainly account for the SERS enhancement observed for Ag/Cu tape sensor.In addition, the charge transfer mechanism can also occur between silver nanoparticles and ARS, as represented in Figure 4b.Upon laser exposure, the free electrons present in silver can be excited to the fermi level (Ef), while the electrons present in the HOMO of ARS can also be excited to LUMO.SPR electrons of silver can be further transferred to LUMO of ARS, resulting in a charge transfer mechanism.EM and CM resulted in superior SERS activities of Ag/Cu tape flexible substrates, which can be employed to efficiently detect organic pollutants.

Conclusions
Ag nanoparticles/Cu tape has been successfully fabricated by thermal evaporation.The highest SERS activity is observed for 10 s deposition time.SERS sensing of micromolar solutions of R6G and ARS are demonstrated.The mechanism of sensing has been illustrated in detail.Reproducibility analysis has shown that as-fabricated sensor has high repeatability for being used as excellent sensors for environmental applications.

Figure 1 :
Figure 1: (a) FESEM image of Ag NPs on copper substrate, (b)corresponding EDS spectra that show the presence of silver and copper, and (c) represents the AFM image of the sensor.

Figure 2 :
Figure 2: (a) SERS spectra depicting vibrational peaks of Rhodamine 6G (R6G) (in red) and ARS (in black), (b) variation of SERS intensity of R6G with respect to the thickness of Ag NPs, (c) Raman spectra of ARS on a pure copper substrate (in Black) and ARS on Ag NPs (in Red) [23].

Figure 4 :
Figure 4: (a) Schematic representation of Ag/Cu tape flexible sensor for ARS sensing (b) charge transfer mechanism of ARS sensing.