Monitoring the Ligand-Nanopartcle Interaction for the Development of SERS Tag Materials Prepared by: George Franklin McKinney Jr. Faculty Advisors: Dr. Stanley May REU Site Director, Department of Chemistry Dr. Chaoyang Jiang Faculty Advisor, Department of Chemistry Mrs. Xianghua (Lily) Meng Graduate Student Mentor, Department of Chemistry Program Information: National Science Foundation Grant NSF EEC-1263343 Research Experience for Undergraduates Summer 2014 University of South Dakota 414 E Clark Street Vermillion, SD 57069 Page 1
Table of Contents Table of Contents... 2 Table of Figures... 3 Abstract... 4 Introduction... 5 Broader Impact... 6 Procedure... 6 Synthesis Silver nanoparticles... 6 Concentrations of 4-MBA... 8 Combining Silver Nanoparticles with 4-MBA... 9 UV-Vis... 9 Surface-Enhance Raman Scattering... 10 Results... 11 UV-vis... 12 SERS... 18 Discussion... 20 Conclusion... 22 Summary... 22 Future work... 22 References... 24 Acknowledgments... 25 Page 2
Table of Figures Figure 1: Colloid solution of 45 nm silver nanoparticles created using the seed meditated leemeisel method.... 8 Figure 2: 1 ml silver nanoparticles mixed with 0.5 ml 4-MBA concentrations... 9 Figure 3: Spectrometer used for UV-vis... 10 Figure 4: Raman Spectrometer used to measure SERS... 10 Figure 5: UV-vis Spectra of the silver nanoparticles... 12 Figure 6: UV-vis Spectra of different concentrations of 4-MBA with silver nanoparticles after 2 minutes of the reaction.... 13 Figure 7: Zoom in, Background corrected UV-vis Spectra... 14 Figure 8: Absorbance of 266 & 300 nm with various 4-MBA concentrations... 15 Figure 9: Changes in the absorbance of the 266 nm peak during the reaction with various concentrations.... 16 Figure 10: Changes in the slope (266 nm peak) during the reaction... 17 Figure 11: SERS comparison of Ag NPs with different 4-MBA concentrations.... 18 Figure 12: Intensity of different concentrations of 4-MBA... 19 Page 3
Abstract This research provides a fundamental understanding of surface interaction of silver nanoparticles, which are important for the development of surface-enhanced Raman scattering (SERS) active materials. The effects of ligands on silver nanoparticles were studied via UVvisible spectroscopy. Interaction between 4-methoxybenzoic acid (4-MBA) and silver nanoparticles results in several interesting optical behaviors. With the addition of the 4-MBA ligands, a shift at the silver nanoparticles major peak (440 nm) was observed. Furthermore, a new absorption peak is observed as its intensity increases with the increase in concentration of 4- MBA. We will continue to study their SERS behavior and explore their possible application as tag materials. Page 4
Introduction Nanoparticles (NPs) of different sizes and shapes absorb light at different wavelengths and display different Plasmon resonances. Silver nanoparticles (AgNPs) have unique optical, electrical, and thermal properties which is why they are incorporated into products ranging from photovoltaics to biological and chemical sensors. 4-mercaptobenzoic acid is a white crystalline solid which is insoluble in water, highly soluble in alcohols, and soluble in ether and ethyl acetate. During this project, we chose a different concentration (mostly under 1mM) of 4-MBA with Ethanol to be used in the AgNPs solution. In this study, surface-enhanced Raman spectroscopy (SERS) and UV-Vis will be used to collect the data. When light hits an atom, the photons scatter away in both an elastic and inelastic scattering. The unchanged photons are known as Rayleigh scattering and the changed photons are known as Raman scattering. For this project, we will be concentrating on the Raman scattering. Raman scattering or the Raman Effect is the inelastic scattering of a photon. It was discovered by C. V. Raman, who won the 1930 Noble Prize in Physics for the discovery. Surfaceenhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) are surface-sensitive techniques that enhance Raman scattering. Scattering is enhanced by extra molecules being absorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silica nanotubes. The enhancement factor can be as much as 10 10 to 10 11. This study will provide a fundamental understanding on silver nanoparticles with the use of Surface-Enhanced Raman spectroscopy (SERS) and UV-Vis. Page 5
Broader Impact Every year, counterfeit goods account for about 7-8% of the world s trade. This is according to FBI, Interpol, World Customs Organization, and International Chamber of Commerce. This results in a $512 billion loss in global sales every year. United States companies lose between $200 billion - $250 billion every year. In addition, 2.5 million jobs have been lost as a result of fake products. The ICC predicts that by the year 2015 the value of counterfeit goods globally will exceed 1.7 trillion, which is 2% of the world s total current output. Additionally, there have also been a number injuries and deaths due to counterfeit materials, such as fake pharmaceutical medicines. At a glance, this summer s research does not seem as if it will have an impact on protecting people and businesses from harm caused by counterfeit goods. However, our research is just the beginning and could lead to new, more effective types of security printing and anti-counterfeiting technologies. Procedure Synthesis silver nanoparticles Before being able to measure the silver nanoparticles and 4-MBA interaction, the silver nanoparticles need to be synthesized. The silver nanoparticles were synthesized using the Seed Meditated Lee-Meisel Method. This method is a step seed growth where you start by creating the starter seeds. Then, with the use of the starter seeds, the next size seeds can be synthesized (for this project, it was 28.5nm seeds). This can continue until the final size nanoparticle is created. Page 6
In a round-bottom flask, 20 ml of 1% (w/v) citrate solution and a 75 ml of water were added and then mixed at 70 C for 15 minutes. After 15 minutes, 1.7 ml of 1% (w/v) AgNO3 solution was introduced to the mixture. This was followed by the addition of 2 ml of 0.1% (w/v) NaBH solution. The solution was stirred and kept at 70 C for one hour. After one hour, the solution was cooled to room temperature. Next, cold to room temperature water was added to bring the volume of the solution to 100 ml. The resulting AgNPs were used as starter seeds. To reach the next size of silver nanoparticles (28.5nm), the starter seeds will be used. In another 250 ml round-bottom flask, 2 ml of 1% citrate solution was mixed with 75 ml of water. Then, this was brought to a boil for 15 minutes. Next, 10.0 ml of starter seed solution was added while stirring. Then, the addition of 1.7 ml of 1% AgNO3 solution was placed in the solution. Stirring continued for one hour while keeping reflux. 2 ml of 1% citrate solution and 1.7 ml of 1% AgNO3 solution was added to the solution. Reflux continued for another hour, and then the process was repeated with the addition of citrate solution and AgNO3 solution. Afterwards, the reaction solution was cooled to room temperature and water was added to bring the volume to 100 ml. To reach the next size of silver nanoparticles (45nm), the starter seeds used will be the 28.5nm silver nanoparticles. A 2 ml of 1% citrate solution was mixed with 80 ml of water and brought to 80 C for 15 minutes. Next, 10.0 ml of the seed solution was added while stirring, followed by the addition of 1.7 ml of 1% AgNO3 solution. The solution was kept at 80 C while stirring for two hours. Then, the solution was cooled to room temperature and water was added to bring the volume to 100 ml. Page 7
Figure 1: Colloid solution of 45 nm silver nanoparticles created using the seed meditated leemeisel method. Concentrations of 4-MBA For this project, different concentrations of 4-MBA were used to interact with the silver nanoparticles. The 4-MBA begins as a white crystal, which is then mixed with ethanol to make it into a solution. The 4-MBA was made into a 20 mm solution in a 20 ml vial. To begin, we add (64.19g) to 20ml of ethanol. From there, different concentrations of 4-MBA could be made with different amounts of the 20mM concentration and ethanol. Some concentrations that were made for this project are 0.1 mm, 0.5 mm, 1 mm, and 5mM. All of those concentrations, in addition to a few others, were used for this project. To figure out the amount of 20 mm concentration needed for each of the other concentrations, MiVi = MfVf was used. For example: 20 mm * 5 ml = 5 mm *20 ml), or to further explain, 5 ml of 20 mm with 15ml of ethanol would make 20 ml of 5 mm 4-MBA. Page 8
Combining silver nanoparticles with 4-MBA In order to start the reaction process, the silver nanoparticles and 4-MBA need to be combined. To do this, a 20 ml vial and 1 ml pipette are needed. Using the 1 ml pipette, 1 ml of 45 nm silver nanoparticles is added to the 20 ml vial. Then, using the 1000 ml pipette with a different tip, 0.500 ml of a 4-MBA concentration was added to the 1 ml of 45nm silver nanoparticles inside the 20 ml vial. This begins the reaction between the 4-MBA and silver nanoparticles. Figure 2: 1 ml silver nanoparticles mixed with 0.5 ml 4-MBA concentrations UV-Vis A spectrometer was used in the lab to study the UV-vis spectra of the interactions. 0.150 ml of the mixture of silver nanoparticles and 4-MBA was place in a 4 window cuvette with 3 ml of water. Throughout the reaction, we scanned each solution of silver nanoparticles and 4-MBA. (An example of this will be seen in the results section.) After gathering all of the data from the reaction with the spectrometer, the data was analysis on origin. Page 9
Figure 3: Spectrometer used for UV-vis Surface-Enhance Raman Scattering A Raman spectrometer was used in the lab to study the SERS from the interactions. 0.150 ml of the mixture of silver nanoparticles and 4-MBA was placed in a 4 window cuvette with 1.5 ml of water. A scan was completed on each solution of silver nanoparticles and 4-MBA throughout the reaction. (An example of this will be seen in the results section.) After gathering all of the data from the reaction with the Raman spectrometer, the data was analysis on origin. Figure 4: Raman Spectrometer used to measure SERS Page 10
Results As stated above, this section will show the results from the UV-vis and the SERS which have been analyzed on origin. The parameters that were involved in this reaction were the concentration of the 4-MBA and the reaction time. The images below show the absorbance from the spectrometer for the silver nanoparticles and different solutions of silver nanoparticles and 4- MBA. The Raman spectrometer was used to measure the intensity of the silver nanoparticles 4- MBA solutions. During this project, 200 nm to 800 nm wavelengths were used for the UV-vis spectrum. There were three main peaks that were looked at on the UV-vis spectrum. The peaks used were the 266 nm peak, the 300 nm peak, and the 416 nm peak. The 266 nm peak is believed to be the 4-MBA peak. It is believed that the 300 nm peak (more visible on the lower concentrations of 4- MBA) affects the interaction between the silver nanoparticles and 4-MBA. The 416nm peak is the peak for the silver nanoparticles. The SERS were measured from 1220 cm -1 to 1920 cm -1. The peak that was looked at during this project was at 1584 cm -1. This peak is believed to have been the 4-MBA peak. To find the real intensity of this peak, the intensity of the peak had to be subtracted from the base intensity since it was higher than zero. Page 11
UV-vis 0.6 0.5 George's AgNPs 11-18-13 09-08-13 0.4 Abs (a. u.) 0.3 0.2 0.1 300 450 600 750 Wavelength (nm) Figure 5: UV-vis Spectra of the silver nanoparticles The plot of UV-vis spectra for silver nanoparticles shows that all the peaks are around 416 nm. This spectrum was used to prove that the silver nanoparticles produced in the lab are, on average, 45 nm. This technique was used because the lab does not have another way to measure the silver nanoparticles. Silver nanoparticles from the earlier dates were made by another lab member. Those silver nanoparticles were measured using a TEM to find the average size. Therefore, by comparing the absorbance of the different silver nanoparticles, it is assumed that the silver nanoparticles are the same sizes. Page 12
2.5 Abs (a. u.) 2.0 1.5 1.0 0.5 0.0 300 450 600 750 Wavelength (nm) 0.0 mm 0.1 mm 0.2 mm 0.25 mm 0.3 mm 0.4 mm 0.5 mm 0.6 mm 0.7 mm 0.8 mm 1 mm 2 mm 3 mm 3.5 mm 4 mm 5 mm 6 mm 7 mm 8 mm 10mM Figure 6: UV-vis Spectra of different concentrations of 4-MBA with silver nanoparticles after 2 minutes of the reaction. This spectrum shows the affect that 4-MBA has on the silver nanoparticles at different concentrations. As seen, the 266 nm peak increases with the increase in 4-MBA while the 416 nm peak decreases with the increase of 4-MBA. The 300 nm doesn t change much after the 1 mm concentration of 4-MBA. Page 13
Abs (a. u.) 2.0 1.5 1.0 0.5 266 300 0.0mM-0.0mM 0.1mM-0.0mM 0.2mM-0.0mM 0.25mM-0.0mM 0.3mM-0.0mM 0.4mM-0.0mM 0.5mM-0.0mM 0.6mM-0.0mM 0.7mM-0.0mM 0.8mM-0.0mM 1mM-0.0mM 2mM-0.0mM 3mM-0.0mM 3.5mM-0.0mM 4mM-0.0mM 5mM-0.0mM 6mM-0.0mM 7mM-0.0mM 8mM-0.0mM 10mM-0.0mM 0.0 240 270 300 330 Wavelength (nm) Figure 7: Zoom in; Background corrected UV-vis Spectra The main concerns during this project were the 266 nm and the 300 nm peaks. To evaluate these peaks, the background from unwanted materials had to be removed. Taking 0.0 mm concentration and subtracting it from the rest of the concentration resulted in a background subtracted spectrum. The spectrum was zoomed in to get a better look at the peaks. At that point, each of the peak absorbencies were measured and mapped out to see clearly what occurred. Page 14
2.0 First peak (266 nm) Second peak (300 nm) 1.5 Abs (a. u.) 1.0 0.5 0.0 0 2 4 6 8 10 Concentration (mm) Figure 8: Absorbance of 266 & 300 nm with various 4-MBA concentrations Data gathered from the UV-vis spectrum shows the 266 nm peak increases linearly with increments of 4-MBA in the solution. Additionally, the data shows that the 300 nm peak increases non-linearly. This information was the basis for running tests with a spectrometer to look at the UV-vis spectrum of the reaction. Page 15
0.4 0.3 Abs (a. u.) 0.2 0.1 0.0 0.0 0.4 0.8 1.2 1.6 2.0 Concentration (mm) 1min 30min 60min 90min 120min Linear Fit of Sheet1 Abs Linear Fit of Sheet1 D Linear Fit of Sheet1 F Linear Fit of Sheet1 H Linear Fit of Sheet1 J Figure 9: Changes in the absorbance of the 266 nm peak during the reaction with various concentrations. As time was increased, the absorbance of the 266 nm peak changed. There were 5 different times (1 min, 30 min, 60 min, 90 min, and 120 min) used for this trial. The higher concentration changed more dramatically than any of the lower concentrations. The slope between the concentrations should still be linear even with the change in time. However, the slope shouldn t be changing. Page 16
0.195 Model Equation Reduced Chi-Sqr ExpDec1 y = A1*exp(-x/t1) + y0 9.31358E-7 Slope (Abs) 0.180 0.165 Adj. R-Square 0.99684 Value Standard Error Slope y0 0.12343 0.00708 Slope A1 0.06495 0.00672 Slope t1 107.41553 19.52908 Slope k 0.00931 0.00169 Slope tau 74.45477 13.53653 0.150 Slopes ExpDec1 Fit of Sheet1 Slope 0 30 60 90 120 Time (mins) Figure 10: Changes in the slope (266 nm peak) during the reaction After noticing the slope change, the value of each slope for the 5 times was analyzed. It was found that the slope was decreasing at a non-linear rate. It looks as if it were decreasing exponentially. It is unclear as to why the slope was decreasing, but more tests should verify what is happening. Page 17
SERS Intensity (a. u.) 1.0 mm 0.9 mm 0.8 mm 0.7 mm 0.6 mm 0.5 mm 0.3 mm 1300 1400 1500 1600 1700 1800 1900 Raman Shift (cm -1 ) Figure 11: SERS comparison of Ag NPs with different 4-MBA concentrations. The intensity of the SERS at 1580 cm-1 is elevated with increment of 4-MBA concentration. This shows us that the intensity of the SERS increases with increasing amounts of 4-MBA concentration. Page 18
Normalization to laser Intensity (a. u.) 300 250 200 150 100 50 10 mins 0 0.0 0.2 0.4 0.6 0.8 1.0 Concentration (mm) 06-24 07-07 07-08 Figure 12: Intensity of different concentrations of 4-MBA The change in concentration should result in a linear increase. However, Figure 12 shows the intensities are not changing linearly. We repeated this data on three different days. Additionally, each had a reaction time of only ten minutes. Page 19
Discussion The concentration of 4-MBA had many effects on the silver nanoparticles and resulted in several interesting optical behaviors. Currently, the 266 nm absorbance peak will increase by increasing the concentration of 4-MBA. However, it will also decrease throughout the reaction. In Figure 6, it is seen that the concentrations of 4-MBA affects different parts of the spectrum. Figure 7 shows the background subtracted peak absorbance of the silver nanoparticle with different concentrations of 4-MBA at the 266 nm and 300 nm peaks. For the 266nm peak, it increases linearly with the increments of 4-MBA concentration. The 300 nm peak begins by increasing, but then will almost level off at the higher peaks. The 416 nm peak decreases with the increase of 4-MBA. This is due to the silver nanoparticles aggregating together. Also, in Figure 2 it can be seen that the 2 mm has changed colors. As the reaction time changed, it was noticed that the absorbance at the 266 nm peak was also changing. Figure 9 displays that, as the reaction time continues, each of the silver nanoparticles 4-MBA solutions are changing. However, the 2 mm experiences the most dramatic change. Even though the higher mm is having a greater change than the lower concentration, the slope of their peaks is still linear. In Figure 10, it shows that the slope of the 266 nm peak changes with the reaction time. However, it is decreasing at a non-linear rate. Although it remains unclear why this is happening, there are a few possible causes, many of which cannot be tested during this time. A few of the possibilities are dimmers, oxidation, and possible deterioration of the material. Figure 11 shows the intensities of the SERS from the silver nanoparticle 4-MBA solutions. It shows that, with the increase of the 4-MBA concentration, the intensity will also Page 20
increase. To analyze the data, the intensity peaks need to be subtracted from the base intensity. Next, the intensities needed to be normalized. In order to normalize the intensity, the real intensity would be divided by the laser intensity, and then multiplied by 1000 so the results are easier to read. These results are seen in Figure 12. Page 21
Conclusion Summary The concentration of 4-MBA had a variety of effects on the Ag NPs and resulted in several interesting optical behaviors. Currently, it is difficult to say which of the concentrations of 4-MBA will be used later on. The solution of the silver nanoparticles and 4-MBA need to be looked at differently to be able to understand what is truly happening. As the reaction time progresses, the 266 nm peak slope (abs. vs. conc.) decreases non-linearly. The SERS intensities of 4-MBA adsorbed to Ag NPs show a non-linear relationship with the increase of 4-MBA concentration. Future work To gain a better understanding of the relationship between the 266 nm peak and the reaction time, more tests will be completed with the spectrometer to see what is happening during this reaction. It will be necessary to use higher concentrations of 4-MBA and longer reaction times for testing. Also, further investigation on the SERS spectra with different concentrations of 4-MBA will be done in order to see the change in the intensities. From this point, the rest of the Raman spectrum will need to be used to test all of the other peaks found. At these different peaks, the concentration of 4-MBA will be changed and the reaction time will be monitored. After looking at all the different concentrations of 4-MBA in the solution, a particular concentration will be chosen then held constant. With a constant concentration of 4-MBA, more characterizations can be completed, but with different concentrations of silver nanoparticles. All Page 22
of the peaks in the UV-vis spectrum will be observed for changes. The entire SERS spectrum will have to be re-observed. Page 23
References Oldenburg, S. J. (n.d.). Silver nanoparticles: Properties and Applications.. Retrieved June 6, 2014, from http://www.sigmaaldrich.com/materials-science/nanomaterials/silvernanoparticles.html Rabin, O. Dispersion in the SERS Enhancement with Silver Nanocube Dimers. ACS Nano, 5763-5772. Jiang, C. Formation and Optical Properties of Compression-Induced Nanoscale Buckles on Silver Nanowires. ACS Nano, 1795-1802. Hernandez-Rivera, S. Surface Enhanced Raman Scattering (SERS) Studies of Gold and Silver nanoparticles Prepared by Laser Ablation. Nanomaterials, 158-172. Gu, N. Quasi-spherical silver nanoparticles: Aqueous synthesis and size control by the seedmediated Lee Meisel method. Journal of Colloid and Interface Science, 263-268. Kitaev, V. Synthesis of Silver Nanoprisms with Variable Size and Investigation of Their Optical Properties: A First-Year Undergraduate Experiment Exploring Plasmonic Nanoparticles. Journal of Chemical Education, 1098-1101. Hargreaves, Steve. "Counterfeit goods market is booming and becoming more dangerous." CNNMoney. Cable News Network, 27 Sept. 2012. Web. 3 July 2014. <http://money.cnn.com/2012/09/27/news/economy/counterfeit-goods/>. Ling, X. Y. Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications. Nanoscale, 282. "How serious a problem is counterfeiting and piracy?." STOPfakes.gov. N.p., n.d. Web. 2 July 2014. <http://www.stopfakes.gov/learn-about-ip/ip/how-serious-problem-counterfeiting-andpiracy>. Smekal, Adolf. "Zur Quantentheorie der Dispersion." Die Naturwissenschaften: 873-875. Print. Page 24
Acknowledgments The author would like to thank the following for their help over the summer: a big thanks to Dr. Grant Crawford, Dr. Stanley May, and other SPACT Faculty. Another thank you goes to the CY group members for their help with the project. A special thank you goes to Dr. Jiang for including the author in this project and assisting every step of the way. The author would also like to thank the Chemistry department at the University of South Dakota. Finally, thanks to Dr. Boysen for his guidance this summer and the wonderful help that he provided. The project was funded by the National Science Foundation grant #EEC-1263343. Page 25