Surface Enhanced Raman Scattering (SERS) of Gold and Silver Nanoparticles 15 Surface Enhanced Raman Scattering (SERS) of Gold and Silver Nanoparticles Name: Course: Tutor: University: City/ State: Date: Introduction Nanotechnology has become a very important segment of science and research. Today, it is applied in medicine, industrial productions and in catalytic processes. Different properties of nanomaterials including their shapes, sizes and chemical properties are very crucial for future applications. Gold and silver nanoparticles are metallic and there are different ways through which they can be synthesized. The methods of synthesis can either be chemical or physical. The most commonly used chemical methods include the use of metallic salts, microemulsion process, thermal decompositions, and alcohol reduction procedures among others. On the other hand, physical methods of synthesis include plasma, microwave irradiation, sono chemical reductions, laser ablation among others.
In the experiments conducted, residuals and counter ions were non-existent on the reducing agents applied on the surface of the nanoparticles. Colloidal suspensions of the nanoparticles were placed on the gold slides in order to test them for explosives using the SERS. It is important to note that the use of spectroscopic techniques in the augmentation of nanostructured metals has become very popular in the last couple of years. This can be attributed to the ability of the method to enable Raman scattering of light, which is generally inelastic for certain molecules in nanostructures that are either roughened or discontinuous. The increase the Raman signal can be explained as being triggered by electromagnetic interaction model (EM) or resulting from charge transfer which is a chemical process.
In this research, various measurements were conducted for Surface Enhanced Raman Spectroscopy (SERS) with silver and gold nanoparticles colloids at different concentrations of the colloids of. During the synthesis, the optical properties and surface plasmon resonance (SPR) of both silver and gold were determined. It was observed that different concentrations of the colloidal particles had different sizes and shapes which ultimately affected the sensitivity of the SER process. Rhodamine-6G was used as a probe for evaluating the activities of silver and gold substrates. The experimental procedure and the results obtained are discussed in the sections that follow.
Experimental This section outlines the procedures followed in the synthesis, characterization and measurement of SERS of Ag and Au nanoparticles. a. Synthesis of Gold nanospheres Using the Turkevich-Frens procedure, synthesis was done on gold nanoparticles to achieve a size of 26 ± 5 nm. Thereafter, 10mg of HAuCl4 were measured and put in a 20 ml of deionised water until it dissolved. The solution was then boiled under a flame.
1% sodium citrate solution was then prepared and 20 mg of it added to deionised water.
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1% sodium citrate solution was then prepared and 20 mg of it added to deionised water. The solution was then added to HAuCl4 solution. The mixture was boiled for approximately an hour while stirring to ensure that the particles were evenly dispersed. The resulting solution had a deep red colour. It was observed that solution had a maximum absorption peak of 520 nm which was similar to that of AuNPs.
b. Synthesis of Silver nanospheres Using the Turkevich-Frens procedure, silver nanoparticles were produced. To start with, the PH of the aqueous solution was adjusted to 7.7 using a 0.1 M tri-sodium citrate solution of sodium hydroxide. The resulting solution was then heated until it reached boiling point.
Thereafter, 1 mL of 0.1 M aqueous silver nitrate was added to the boiled solution.
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Thereafter, 1 mL of 0.1 M aqueous silver nitrate was added to the boiled solution. The mixture was stirred for a period of 5 minutes and HNO3 was added to increase the pH to 6. This was done to control the reaction and slow down the rate of reaction, manipulate the size of the particles and achieve better distribution of particles.
The reaction was completed within 30 minutes. The resulting particles had a size of 58 nm and a standard deviation of 14 nm.
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The reaction was completed within 30 minutes. The resulting particles had a size of 58 nm and a standard deviation of 14 nm. The resulting maximum absorption peak was 460nm which corresponded to AgNPs.
c. SERS substrates preparation In the preparation of the SERS substrate, both silver and gold nanoparticles were used. First, about 100µL of the analyte were mixed with 50µL of the colloid. The mixture was then cast drop by drop in the microscope slides. In preparation for the Raman measurement, the mixture was exposed to air and left to dry. Thereafter, the dried mixture was aggregated into different clusters and aggregates. The process helped in the creation of spots where the molecules of the analyte could be adsorbed into the nanoparticles. This resulted in high scattering of the incident radiation. To enhance the process, a modified electric field of the nanoparticles ensured that there was increased interaction with the incident radiation.
The figure below illustrates the process used to prepare the SERS substrates.
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The figure below illustrates the process used to prepare the SERS substrates.
Figure 1: The process used to prepare the SERS substrates d.
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Figure 1: The process used to prepare the SERS substrates d. Characterisation of the nanoparticles The process of characterizing the gold and silver nanoparticles was conducted using several instruments. First, to conduct UV-Vis spectroscopy, the Lambda 25 UV-Vis spectrophotometer was used for the nanoparticles contained in the solution. Recordings were made for the 300 -800nm region for 50µL of the colloidal solution in 4mL DI water. Also recorded was the maximum absorption of (lmax) of silver and gold colloids which were about the average size of the particles. It was also determined that the Au colloids had a maximum absorption band of about 520nm.
This corresponded to the SPR of the gold nanoparticles which had a full width at half maximum (FWHM) of 50nm.
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This corresponded to the SPR of the gold nanoparticles which had a full width at half maximum (FWHM) of 50nm. Such recordings were indicators of a low dispersion of the particle sizes in the solution. Additionally, the absorption of gold nanoparticles at longer wavelengths was non-existent.
This shows that the colloids did not have any large particles.
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This shows that the colloids did not have any large particles.
For the silver colloids, the same procedure was followed and a maximum absorption band at a λmaxof 460nm was measured. The colloid had a FWHM of 124 nm. This was an indication that there were large particles in the solution. This can possibly be attributed to the aggregation that occurred in the silver nanoparticles.
e. SERS measurements of explosives by silver and gold nanoparticles The SERS spectra were recorded for the explosives using both the Au and Ag colloidal solutions as the substrate materials. An excitation wavelength of 633nm (He-Ne) from the source of the laser was applied with an exposure time of 10 seconds. Each measurement was given a time accumulation of 1. Prior to recording of the Raman and SERS spectra, recordings were done for the Raman spectra of the 50µL of the gold colloidal solution on the microscope slide. It was then air dried for 10 different spots for the Raman spectra. The goal of these measurements was to determine the effect of impurities and aggregation on the Raman signals. The
Results The results obtained show the characterisation of SERS activities of the Ag and Au nanoparticles, Trinitrotoluene (TNT), Cyclotrimethylene trinitramine (RDX) and Pentaerythritol tetranitrate (PETN). a. Characterisation of SERS activity of Ag and Au nanoparticles
The SERS activities of each of the substrates were characterised using unique Raman active modes. Rhodamine-6G has a unique Raman active mode and it was therefore used for the characterization of the sensitivity and activity of the SERS. Solutions of Rhodamine-6G were prepared at different concentrations including 10-5, 10-7 and 10-9M. They were then mixed with colloidal solutions of Au and Ag mixed with ethanol. The resulting mixture was in the ratio of 2:1 for the Rh-6G: colloidal solution. This was used to develop final concentrations of the analyte of 10-5, 10-7 and 10-9M. For Rh-6G, the solvent used was ethanol while for Au and Ag nanoparticles DI water was used and then re-dispersed in water. Thereafter, the resulting mixture was drop cast on glass slides and then air dried for SERS measurements. The results showed a high specificity and sensitivity for Rh-6G on Au nanoparticles. The intensity of the signal decrease in a linear manner as the levels of concentration increases. The figure below illustrates the Raman spectrum for Rh-6G at concentrations of 10-5, 10-7 and 10-9M.
Figure 2: The Raman spectrum for Rh-6G at concentrations of 10-5, 10-7 and 10-9M.
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Figure 2: The Raman spectrum for Rh-6G at concentrations of 10-5, 10-7 and 10-9M. The SERS measurements for Rh-6G indicated a very strong level of enhancement where the peaks had similar signatures in all the samples. This can be attributed to the fact that Rh-6G tends to hand a large Raman cross-section which resulted in massive scattering of the Au nanoparticles.
A procedure similar to that used in the characterization of the SERS activity of the Au nanoparticles was in Ag nanoparticles. The figure below shows the results obtained at different concentrations of 10-5, 10-7 and 10-9M of RH-6G of Ag nanoparticles.
Figure 3: The results obtained at different concentrations of 10-5, 10-7 and 10-9M of RH-6G of Ag nanoparticles. It was observed that the SERS spectrum of Rh-6G on Ag nanoparticles was strongly enhanced. This was so especially for sample solution with a 10-5 M concentration. The Ag nanoparticles also had a high sensitivity and had all the characteristic peaks of Rh-6G strongly enhanced including the nanomolar concentrations. The intensity of the signal decreased as the intensity of the solution decreased. A similar observation was made for the Au nanoparticles. The concentration and the intensity of the Ag nanoparticles had a linear relationship which can be attributed to the decreased number of number of adsorbed molecules in the Ag nanoparticles. A reduction in the concentration levels of the Ag nanoparticles results in the reduction of the number of molecules being adsorbed into the substrate. This eventually affects the levels of enhancement of the Ag nanoparticles.
b. Measurement of the SERS of Trinitrotoluene (TNT) SERS measurements were conducted for Trinitrotoluene (TNT) solutions at different concentrations including 10-5to 10-9M. The figure below illustrates the Raman spectrum of the different TNT samples with molar concentrations of 10-5to 10-9M of Ag nanoparticles substrates.
Figure 4: Raman spectrum of the different TNT samples with molar concentrations of 10-5to 10-9M of Ag nanoparticles substrates. From the Raman spectrum above, it is indeed clear that 10-5M of TNT had the highest enhancement. Strong enhancements were also evident on the weak modes, a phenomenon which was as a result of the out of plane bending of the C-H. Peaks with lower intensities were also observed at 791 and 822 cm-1on the SERS spectrum. The lower intensity can be attributed to their orientation towards the adsorbed molecules on the Au nanoparticles. This was the point at which the spectrum was recorded. At a concentration of 10-7M of TNT, the SERS spectrum showed a number of enhanced signals where strong Raman active modes in the spectrum were. For the 10-9M, the SERS measurements showed small enhancements on the peaks which included 823 cm-1, 1360 cm-1 and 1615 cm-1. This occurrence was mainly as a result of symmetric and asymmetric stretching of nitrogen dioxide. 10-9M solution showed the lowest concentration and showed enhanced Raman signals. TNT SERS measurements were mainly dependent on factors such as the orientation of the analyte and the nanoparticles. Molecule orientation contributes towards the enhancement of the molecules.
Regarding the Ag nanoparticles, the same procedure was conducted in making the measurements. At a concentration of 10-5M of TNT, a strong enhancement was observed. However, at 10-9M, no enhancement was observed. Several other peaks were observed whereby some matched the active Raman modes while others corresponded to the inactive modes. At 10-7M, the signal intensity was much lower than that of the 10-5M sample which was mainly as a result of the adsorbed molecules in the Ag nanoparticles. The higher enhancement observed in the Au substrates compared to the Ag nanoparticles was mainly as a result of the red-shift of the SPR of the nanoparticles after they had been mixed with the analyte.
The figure below shows the Raman and SERS spectra for TNT substrates at different concentration of 10-5, 10-7 and 10-9M for Ag nanoparticles.
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The figure below shows the Raman and SERS spectra for TNT substrates at different concentration of 10-5, 10-7 and 10-9M for Ag nanoparticles.
Figure 5: Raman and SERS spectra for TNT substrates at different concentration of 10-5, 10-7 and 10-9M for Ag nanoparticles.
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Figure 5: Raman and SERS spectra for TNT substrates at different concentration of 10-5, 10-7 and 10-9M for Ag nanoparticles. c. Measurement of Cyclotrimethylene trinitramine (RDX) Cyclotrimethylene trinitramine (RDX) is used in military operations and in terror attacks. In this measurement, SERS spectra were collected from different samples of the Ag and Au nanoparticles substrates. The results showed that at a 10-7M, the sensitivity levels were reasonable but below this level, the substrates struggled. The same procedure followed in the previous measurements was applied and readings were taken at a wavelength of 633 nm and an exposure time of 10 seconds. The concentrations of the RDX samples were at 10-5M, 10-7 M and 10-9 M. The figure below shows the recorded spectra of the RDX samples on Au nanoparticles and the resulting Raman spectrum of RDX.
Figure 6: Recorded spectra of the RDX samples on Au nanoparticles and the resulting Raman spectrum of RDX. The 10-5M sample showed a good enhancement of the signature peaks in its SERS spectra of the RDX. This was an indication that the sample had good intensities. The most intense peak was observed at approximately 882cm-1 and can be attributed to the asymmetric stretch breathing mode of the RDX ring which was strongly enhanced. There was only a small shift at 877cm-1 with another strong peak being recorded at 1215 cm-1. However, it was not as enhanced as the previous one. Other enhanced peaks were recorded at 1316cm-1(CH2 wagging) and 1364cm-1 (n(NO2)). Low intensity peaks for the SERS spectrum for the 10-7M of RDX showed strong enhancements for peaks at 880, 1315, 1364 and 1648 cm-1. There were no peaks observed for 10-9M except for a very weak one at 1610 cm-1. Good enhancements were observed at 10-5M and 10-7M but no enhancement was observed at 10-9M. Reproducibility which is an important factor was achieved by collecting the SERS spectra at different spots. The figure below illustrates the Raman spectrum for RDX and the SERS spectra for 10-5, 10-7, and 10-9M of RDX in Ag nanoparticles substrate.
Figure 7: Raman spectrum for RDX and the SERS spectra for 10-5, 10-7, and 10-9M of RDX in Ag nanoparticles substrate. d. Measurement of the SERS of Pentaerythritol tetranitrate (PETN) Pentaerythritol tetranitrate (PETN) is also an explosive nanoparticle material. In this measurement, three different solutions with concentrations 10-5, 10-7 and 10-9 M were prepared with Ag and Au colloids. They were then washed and re-dispersed with ethanol. A reasonable amount of enhancement was observed for the 10-5M PETN sample. At 10-7M, the sample showed limited sensitivity and had three different characteristic peaks with low intensity. The 10-9 M concentrated sample had a poor sensitivity for the SERS spectrum. Only one small peak was recorded at 871 cm-1. The figure below shows the Raman spectra for concentrations 10-5, 10-7 and 10-9 M of the PETN sample.
Figure 8: Raman spectra for concentrations 10-5, 10-7 and 10-9 M of the PETN sample. From the 10-5 M of PETN, the SERS spectrum showed specify and reasonableness. It enhanced most of the characteristics of the Raman active modes of PETN. Small shifts were observed at 871 cm-1 and 1290 cm-1, a phenomenon that results from C-C-C deformation and CH2 Wagging vibrational modes. Also observed was enhancement of four signals at 588, 622, 870 and 1290 cm-1 for the SERS spectrum of 10-7 M of PETN on AuNPs. They all had strong active Raman modes, except for the peak observed at 588 cm-1.
There were only two peaks with high frequencies and their Raman active modes matched at 1650 cm-1, a value which corresponded to the asymmetric stretching of NO2.
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There were only two peaks with high frequencies and their Raman active modes matched at 1650 cm-1, a value which corresponded to the asymmetric stretching of NO2. The figure below illustrates the SERS spectra of PETN recorded using Ag colloidal solutions at concentrations of 10-5 M and 10-7 M and 10-9 M.
Figure 9: SERS spectra of PETN recorded using Ag colloidal solutions at concentrations of 10-5 M and 10-7 M and 10-9 M. Discussion There are a number of factors that affect the sensitivity and reproducibility of SERS. The most important one discussed in this research are the properties of the nanoparticles. The shape and size of the nanoparticles as distributed in the substrate affect the sensitivity of the SERS. In the tests conducted, the nanoparticles of Ag and Au were randomly distributed and later deposited on the glass slides for air drying and SERS measurements. As a result, the detection sensitivity of the samples was reduced with the limit being 10-7M for the Rh-6G samples. Since Rh-6G samples have large molecules, they resulted in a high scattering with a lower cross section area of the molecules.
The sensitivity of the SERS could be improved through close packing of the nanoparticles. An example of this can be seen on the SERS spectra for 10-5M of RDX on the Ag nanoparticles which showed this characteristic. The enhancement of the nanoparticles is mainly affected by electromagnetic enhancement mechanism (EM) and chemical enhancement (CT). The two processes involve the generation and transfer of energy of the nanoparticles across the analyte. For lower concentrations, the enhancement factors were recorded to be very high. It is a common phenomenon to have EFs that are higher with low concentrated solutions. This can be attributed to the nature of the EF formula. The recorded values for the concentrations show varying values of the EF across different concentrations.
Conclusion The objectives of this research were attained. Measurements for various variables were taken and analysed accordingly. It is important to point out that among the colloids measured; only the 10-7M solutions were found to be good candidates for the explosives. Reasonable enhancements were observed high-to-medium concentration. For solutions less than 10-7 M, it was difficult to detect the specificity and the sensitivity. In RDX, it was show that the reproducibility is highly dependent on the molecules and the concentrations. The 10-5M sample showed good reproducibility. The sensitivity of the SERS can be improved by co

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