Surface Enhanced Raman Scattering of Electrospun Nanofibers Embedded with Silver Nanoparticles Prepared by: Albert Foster III Faculty Advisors: Dr. Chaoyang Jiang Department of Chemistry at USD Dr. Stanley May REU Site Director, Department of Chemistry at USD Dr. Alfred Boysen Department of Humanities at SDSMT Program Information: National Science Foundation Grant NSF #EEC-1263343 Research Experience for Undergraduates Summer 2013 University of South Dakota 414 E Clark St. Vermillion SD 57069 1
Table of Contents Abstract... 3 Introduction... 4 Broader Impact... 5 Procedure... 5 Materials... 5 Equipment... 6 Experimental... 8 Results... 10 PMMA Nanofibers... 10 Silver Nanoparticle Synthesis... 18 Electrospinning/Characterizing the Composite... 19 Raman Spectroscopy... 22 Discussion... 23 Conclusion... 24 Summary... 24 Future Work... 25 References... 26 Acknowledgments... 26 2
Abstract Surface enhanced Raman scattering (SERS) is a fairly recent discovery that has allowed for single molecule detection. With this extremely sensitive technique, it is possible to have unprecedented security in terms of anti-counterfeit technologies along with many other applications. The challenge is to create cheap, robust, and versatile substrates with SERS capabilities. Electrospinning provides a simple and quick method for obtaining submicron polymer fibers in large quantities. By combining the electrospun fibers with silver nanoparticles, we found that this technique is a practical method in developing SERS substrates. In this study, conditions for fabricating extremely thin (less than 500 nm) and continuous nanofibers with well dispersed silver nanoparticles were optimized. The Raman signals for the optimized composite nanofibers were measured to determine the amount of enhancement using 4-MBA (mercaptobenzoic acid) as a target molecule. The polymer studied was poly (methyl methacrylate) and the solvents were dimethylformamide, chloroform, and 1.1.2.2- tetrachloroethane. It was found that using chloroform and dimethylformamide as co-solvents yielded the optimum fibers. The silver nanoparticles did have a significant magnification effect on the Raman signal when embedded into the nanofibers. A rough calculation showed that the enhancement factor was approximately 10 8. 3
Introduction Surface enhanced Raman scattering (SERS) is an extremely sensitive detection technique which has resolution down to the single molecule. This method utilizes localized surface plasmon resonance (LSPR) to enhance the Raman single of a specific molecule. Silver nanoparticles have been found to produce very strong SERS if placed close together on a substrate. The problem is creating a method of producing these substrates cheap and quickly. Some current techniques require expensive masks and time intensive processing requirements [1]. Electrospinning, which is the method used in this paper, will allow for substrates that are tunable, quick to produce, and reproducible. Electrospinning is a technique in which very thin polymer fibers are drawn from a needle using a strong electric field. Silver nanoparticles can be dispensed in the polymer solution resulting in thin fibers that have the nanoparticles embedded inside. Since these fiber s diameters are incredibly small, they have a large surface area. This can result in a large number of the silver atoms being on the surface allowing them to interact with their environment. More silver atoms exposed on the surface could possibly result in increased SERS activity. In this study, silver nanoparticles are embedded into electrospun nanofibers to study the surface enhanced Raman scattering. This approach will provide a simple three-step process that has a substantial throughput. The three main steps of this process are silver nanoparticle synthesis, preparation of the polymer solution, and electrospinning the combination of the nanoparticles and the polymer solution. The goal is to electrospin a nanofiber that has uniform diameters and well-distributed silver nanoparticles. Also the fibers must be porous, continuous, and less than 500 nanometers in diameter. These criteria will hopefully yield adequate SERS enchantment. 4
Broader Impact Raman scattering is a detection technique that has been around for many years. Unfortunately the signal is extremely weak thus making this method ineffective. While the Raman signal was discovered by Sir Chandrasekhara Venkata Rāman in 1928 [2], it wasn t until forty years later that the effect of different surfaces on Raman scattering was studied. In 1973 Martin Fleischman found that by roughening the surface of silver in a certain way they observed SERS [3]. Just by altering the surface of metals such as silver or gold the Raman signal can be enhanced by up to 10 10 times [4]. Such a large enhancement factor allows for single molecule detection. This single molecule resolution is a very attractive aspect for anti-counterfeit technologies. For example, if a pharmaceutical is creating an expensive pill that they wish to make non-reproducible they could add a specific tagging molecule. The tagging molecule, which is unique to that specific type of pill, can then be detected with a SERS device. This would provide an additional layer of security that is extremely hard to counterfeit. Another application of these SERS devices would be airport security. It would be able to detect minute amounts of explosive or toxic chemicals which could save countless lives. Many more applications will be created as the technology continues to improve and new techniques are developed. Procedure Materials Poly (methyl methacrylate) (PMMA) with a molecular weight of 996,000 g/mol was used as the polymer throughout the entire study. This was chosen for its solubility in organic solvents and insolubility in water. The polymer was purchased from Sigma-Aldrich. The solvents used were dimethylformamide (DMF), chloroform (CHCl 3 ), and 1.1.2.2-tetrachloroethane (TCE), 5
which were all purchased from Sigma-Aldrich. The materials used for the nanoparticle synthesis were poly (vinyl pyrrolidone) (PVP) with a molecular weight of 1,300,000 g/mol, silver nitrate (AgNO 3 ), sodium chloride (NaCl), and sodium borohydride (NaBH 4 ). These were also purchased from Sigma-Aldrich, except for the PVP, which was bought from Alfa Aesar. The deionized water used in cleaning and the reactions was provided by Barnstead NANOpure Diamond. Equipment Electrospinning Apparatus The electrospinning apparatus that was used in this study consisted of two parts. The first was the syringe pump, which was purchased from SyringePump.com, and the second was a 40 kilovolt DC voltage source from Gamma High Voltage Research. Figure 1, on the next page, is an example of the apparatus used. Electrospinning utilizes a large electric field to attract a thin fiber from the end of the needle. This fiber is accelerated towards the ground and due to instabilities in the fibers; it is whipped back and forth creating an extremely thin fiber while evaporating any solvents that remain inside. This simple method is very effective at creating thin, reproducible fibers without heating up the polymer solution. Optical Microscope The optical microscope allows for characterization of the fibers and nanoparticles after they are created. The optical microscope used in this study was a Nikon Eclipse LV150, with a maximum magnification of 1000 times. This instrument allowed form good preliminary quality check because it was fast and required little to no sample preparation. However, limitations such as low magnification prevented advanced characterization to take place. 6
Figure 1: The above picture is an electrospinning apparatus consisting of a syringe (A), needle (B), voltage source (C), fiber (D), ground (E). (http://www.neotherix.com) Scanning Electron Microscope The scanning electron microscope (SEM) allowed for submicron resolution that wasn t available with the optical microscope. The SEM utilized accelerated electrons that interacted with the surface of the sample to precisely map the composition and topography. The SEM used in this study was a Hitachi VP-SEM S-3400N. Although the SEM has extremely high resolution, it also requires destructive sample preparation and is quite time intensive. 7
UV-Visible Spectrophotometer The UV-Vis provided this study with accurate measurements of the absorption of the nanoparticle solution. This allowed for determination of whether the particles were sythesized correctly. The tool used was a Cary 50 Bio UV-Visible Spectrophotometer. By matching the absorption spectra of the experimental nanoparticles to the theoretical spectra, it was possible to roughly determine the size and distribution of the silver nanoparticles. Raman Spectroscopy The Raman spectroscopy device from LabRAM ARAMIS utilizes a laser at specific wavelengths to interact with the surface of the sample. This laser excites molecules on the surface which vibrate in specific modes that correspond with its structure. The excitation emits a weak signal, or Raman signal, that is unique to each molecule. By using molecules that have a known Raman signal, it is possible to use this device as a detection tool. This piece of equipment can also be used to measure the amplitude of Raman signals of probe molecules on different substrates. If there is a substantial difference in amplitude then the substrate has Raman enhancement capabilities, which is the goal of this study. Experimental Polymer Solution First a specific weight percentage of PMMA was determined. Next solvents of varying type (chloroform, dimethylformamide, and 1.1.2.2-tetrachloroethane) and ratios were mixed together by stirring overnight to make a solution that was 10 grams. Next the solution were electrospun under different parameters such as voltage, pump rate, needle gauge, and distance 8
between needle and ground. The samples were characterized with an optical microscope and SEM to determine approximate diameter and continuity. Nanoparticle synthesis The first step in synthesizing silver nanoparticles was to create stock solutions of each of the components. First a 30 mm solution of silver nitrate (AgNO 3 ) in water was prepared. The next solution consisted of 30 mm of polyvinylpyrrolidone (PVP) and 17 mm of sodium chloride (NaCl) in water. The final solution was 15 mg of sodium borohydride (NaBH 4 ) in 10 ml of water. 3 ml of the PVP and NaCl solutions were added to 3 ml of the AgNO 3 solution. Then 1 ml of the NaBH 4 solution was added and stirred for 2 minutes. The mixture was then refluxed at 100 C for three hours. Then the nanoparticle solution was centrifuged three times at 10,000 rpm for 10 minutes and suspended in methanol [5]. The nanoparticles were then drop-casted for optical microscope characterization and the absorption spectra were measured with a UV-Vis Spectrophotometer. Silver Nanoparticle/PMMA Electrospinning After the optimization of the electrospun fibers and the synthesis of the silver nanoparticles, the next step is to incorporate the nanoparticles into the fibers. 1.5 ml of the silver nanoparticle solution was centrifuged at 10,000 rpm for 10 minutes and the solvent was removed. It was then replaced by chloroform and the nanoparticles were dispersed using sonication. 5% weight of PMMA was dissolved in the Ag NPs/CHCl 3 solution and co-solvent DMF at 8:2. This solution was stirred for 24 hours to ensure adequate nanoparticle dispersion and that all the PMMA was dissolved. Then the composite solution was loaded into a syringe and electrospun with a 21 gauge needle and a pump rate of.3 ml/hr. The voltage was 20 kv and 9
the distance between the needle and the ground was 13 cm. These were the optimum parameters for electrospinning. PMMA Nanofibers Results In order to prepare nanofibers that were both continuous and thin, a study of various solvents and parameters was conducted. The first task was to determine what solvents to use to dissolve PMMA. Chloroform and 1.1.2.2-tetrachloroethane were chosen because PMMA is soluble in both. DMF was chosen as a co-solvent because it is miscible with CHCl 3 and TCE along with being a slower evaporating solvent. Also the weight percent along with varied solvent ratios were studied (See Table 1). Table 1: The below table shows the different combinations of polymer solutions and their corresponding weight percentages and ratios. Trial # Polymer Polymer wt % Solvent 1 Solvent 2 Solvent Ratio (1:2) 1 PMMA 3% CHCl 3 DMF 6:4 2 PMMA 5% CHCl 3 DMF 8:2 3 PMMA 6% CHCl 3 DMF 8:2 4 PMMA 6% TCE DMF 9:1 5 PMMA 6% TCE DMF 8:2 6 PMMA 7% TCE DMF 9:1 7 PMMA 7% TCE DMF 8:2 8 PMMA 7% TCE DMF 7:3 9 PMMA 8% TCE DMF 9:1 10 PMMA 12% TCE DMF 8:2 11 PMMA 14% TCE DMF 8:2 Different parameters were tested to see which yielded the best fibers. The parameters were altered based on the observation of the polymer jet. For example if there were multiple jets 10
then the needle diameter was too large. The parameters which yielded the best fibers can be found in Table 2 below. The trail number corresponds to same trial number is Table 1. Table 2: The table below shows the parameters (pump rate, voltage, distance, and needle gauge) that yielded the best fibers for that sample Trial # Rate (ml/hr) Voltage (kv) Distance (cm) Needle Gauge 1 1 20 20 21 2 0.5 20 13 18 3 0.5 19 13 18 4 2 20 10 18 5 2 20 10 18 6 0.5 20 13 18 7 1 19 13 18 8 1 20 13 21 9 1 20 13 18 10 0.5 15 13 18 11 1 13 13 18 Even though the conditions in Table 2 created the best fibers, most of them still weren t optimum (< 500nm and no beading) for this study. Optical microscope images were taken of each trail sample. Figures 2 4 on the next couple pages show the images for trials 1 3, which had CHCl 3 as the primary solvent. Figures 5 12 are the optical microscope images for trials 4 11 and they all had TCE as the primary solvent. The observations of the optical microscope images for each trial conducted can be seen in Table 3 on the page 17. 11
Figure 2: The 100x Dark Field Optical Microscope image above is of Trial 1. Figure 3: The 100x Dark Field Optical Microscope image above is of Trial 2. 12
Figure 4: The 100x Dark Field Optical Microscope image above is of Trial 3. Figure 4: The 50x Dark Field Optical Microscope image above is of Trial 4. 13
Figure 6: The 50x Dark Field Optical Microscope image above is of Trial 5. Figure 7: The 100x Dark Field Optical Microscope image above is of Trial 6. 14
Figure 8: The 100x Dark Field Optical Microscope image above is of Trial 7. Figure 9: The 100x Dark Field Optical Microscope image above is of Trial 8. 15
Figure 10: The 100x Dark Field Optical Microscope image above is of Trial 9. Figure 11: The 100x Dark Field Optical Microscope image above is of Trial 10. 16
Figure 12: The 100x Dark Field Optical Microscope image above is of Trial 11. Table 3: The table below shows the observations corresponding to each of the trial numbers. Trial # Observations 1 Thin Diameter < 1µm 2 Thin Diameter < 1µm 3 Large Diameter 4 Beads, Uncontinuous Fibers 5 Beads, Large Diameter 6 Beads, Thin Diameter < 1µm 7 Minimal Beads, Thin Diameter 8 Beads, Thin Diameter 9 Minimal Beads, Holes, Thin 10 Minimal Beads, Large Diameter 11 Large Diameter 17
Silver Nanoparticle Synthesis The recipe of silver nanoparticles that was used was supposed to create nanospheres with a diameter of approximately 40nm [5]. The nanoparticles were characterized with UV-Vis spectroscopy and optical microscope. The absorption spectrum can be seen in Figure 13 below. Figure 13: The above figure shows the absorption spectrum of the Ag NPs in methanol. The nanoparticle solution was also diluted and drop-casted onto a glass slide. The glass slide was then dried for 24 hours then imaged using an optical microscope (See Figure 14). 18
Figure 14: The 100x Dark Field Optical Microscope image of Ag NPs. Electrospinning/Characterizing the Composite The next step was to electrospin the PMMA nanofibers with the Ag NPs dispersed inside. In each of Tables 1 3, trial 2 was highlighted in yellow. This indicated that the optimum fibers were produced using 5% PMMA in CHCl 3, which was then used as the basis for the nanofibers embedded with Ag nanoparticles. However, parameters had to be changed in order to electrospin acceptable fibers. Parameters like pump rate and distance were varied while the voltage was kept 19
at 20 kv and the needle gauge was 21. The optical microscope was then used to determine if beads were present. Table 4 shows the effects of the changing the parameters mentioned above. Table 4: The table below shows the effects of changing the pump rate along with the distance. Trial # Rate (ml/hr) Distance (cm) Beading 1 0.3 13 No 2 0.5 13 Yes 3 1 13 Yes 4 0.3 10 Yes 5 0.3 18 No Trial 1 yielded the best results because, while trial 5 did not have any beads, trial 5 fibers did not have consistent diameters (See Figure 15). However, trail 1 fibers did have a uniform diameter (See Figure 16). This sample was then sent the SEM for further characterization (See Figure 17). Figure 15: The 100x Dark Field Optical Microscope image above is of Trial 5 with 20px/µm. 20
Figure 16: The 100x Dark Field Optical Microscope image above is of Trial 1 with 20px/µm. Figure 17: The SEM image above is of Trial 1 nanofibers with measured diameters. 21
Raman Spectroscopy The Trial 1 sample from Table IV was used to test the Raman single along with trial 2 from Table 1. The probe molecule used was 4-mercaptobenzoic acid (4-MBA) in a concentration of 10-1 M for pure PMMA and 10-9 M for PMMA containing the silver nanoparticles. Figure 18 below shows the Raman spectra using the pure PMMA nanofibers along with the PMMA nanofibers with the Ag nanoparticles inside. A large amplitude change between each of the sample can be seen. Figure 18: The graph above shows the Raman spectra of the PMMA nanofibers with and without the embedded Ag nanoparticles. 22
Discussion The three main parts of this study consisted of optimizing the electrospun PMMA nanofibers, synthesizing silver nanoparticles, and electrospinning a composite fiber with SERS activity. By changing several of the parameters like solvent type, solvent ratio, voltage, pump rate, distance, and needle gauge it was possible to obtain nanofibers that were continuous and had a uniform diameter of less than 500 nanometers. As one can see in Figure 3, all of the conditions for the optimized fibers were met. An interesting point however, is that all of the nanofibers that had 1122-tetrachloroethane as the primary solvent result in fibers with beads. It wasn t until a large polymer weight percentage (approx. 14%) was spun that nanofibers without beads were created. This beading of TCE could be due to a number of factors such as low polymer conductivity, fast evaporation rate, or low surface tension. The 5 wt% PMMA with chloroform as the primary solvent and dimethylformamide as the co-solvent at a ratio of 8:2 yielded the necessary fibers. The silver nanoparticles were synthesized following a recipe that theoretically yielded ~40nm diameter nanospheres. By taking a UV-Vis absorption spectrum (See Figure 13) it was possible to compare the experimental data with the literature data. The absorption peak position and size obtained in this study were very similar to the data found in reference #5. Along with additional simulations using Matlab, it is possible to ascertain the approximate diameter of the silver nanoparticles. By indirect measurements, the nanoparticles were around 40 to 50 nm in diameter for this study. The next step of the process was optimizing the electrospun composite nanofibers. The parameters that yielded the best fibers can be seen for Trial 1 in Table 4. While Trial 5 also 23
yielded fibers without beads, it contained fibers that had diameters that were not uniform. Therefore Trial 5 was unsuccessful compared to Trial 1. The SEM image of the optimized composite nanofibers showed poor nanoparticle dispersion within the fiber (See Figure 17). Nanoparticle aggregations are the large white spots in the fibers. This is consistent with the optical microscope images where fibers with red spots, which are macroscopic aggregates, can be seen (See Figure 16). However, even though the silver nanoparticles were poorly dispersed there still is SERS activity. In Figure 18 the Raman spectra of 4-MBA on PMMA nanofibers can be seen in the red colored plot. The Raman spectra of 4-MBA on the composite (PMMA with Ag NPs) nanofibers can be seen in the blue colored plot. There is definitely a distinct enhancement of the Raman signal, which is most prevalent in the peaks associated with 4-MBA (located at around 1080, 1186, 1592 cm -3 ). A rough calculation showed that the enhancement factor was approximately 10 8. Conclusion Summary Optimum PMMA nanofibers with silver nanoparticles embedded inside were obtained. It was found that the solvent chloroform and DMF in an 8:2 ratio yielded fibers without any beads. 5% weight PMMA was also found to produce fibers with diameters less than 500nm. The silver nanoparticles inside the fibers were poorly distributed throughout and had visible aggregations. Even though there was poor nanoparticle uniformity a large enhancement factor was found. Preliminary calculations resulted in an enhancement factor of around 10 8, which is very large. Overall a successful SERS substrate was created. 24
Future Work There is plenty of work that will be done on this project in the future however. Since there wasn t any direct characterization of the silver nanoparticles, we will use a transmission electron microscope (TEM) along with an atomic force microscope (AFM) to look at them more carefully. This will also allow us to see the average diameter along with the shape of the nanoparticles. Another aspect of this study that was not completed due to time constraints was using different concentrations of silver nanoparticles in the nanofibers. By changing this concentration, it might be possible to get a larger SERS enhancement factor. This project was done for the Security Printing and Anti-Counterfeiting REU therefore anti-counterfeiting applications can also be research. By implementing the probe molecules directly into the composite polymer and silver nanoparticle solution then electrospinning the fiber, it might be possible to create a fiber with intrinsically enhanced known Raman signal. This fiber could then be electrospun onto sensitive documents or packaging which would allow for another level of authentication. This SERS substrate could also be used for direct detection, such as real-time minute detection of dangerous chemical or biological agents, which could be used in airport security. Much more work must be done on this preliminary study, however, considering the results obtained this has been a relatively successful study. 25
References [1] Jensen, T. R., Malinsky, M. D., Haynes, C. L., & Van Duyne, R. P. (2000). Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver. The Journal of Physical Chemistry B, 10549-10556. [2] Venkataraman, G. (1988). Journey into light: life and science of C.V. Raman. Bangalore, India: Indian Academy of Sciences in co-operation with Indian National Science Academy. [3] Fleischmann, M., Hendra, P., & McQuillan, A. (1974). Raman Spectra Of Pyridine Adsorbed At A Silver Electrode. Chemical Physics Letters, 26(2), 163-166. [4] Blackie, E. J., Ru, E. C., & Etchegoin, P. G. (2009). Single-Molecule Surface-Enhanced Raman Spectroscopy Of Nonresonant Molecules. Journal of the American Chemical Society, 131(40), 14466 14472. [5] Zhang, Y., Yang, P., & Zhang, L. (2012). Size- and shape-tunable silver nanoparticles created through facile aqueous synthesis. Journal of Nanoparticle Research, 15(1), 1-10. Retrieved July 19, 2013, from http://link.springer.com/content/pdf/10.1007/s11051-012- 1329-z.pdf Acknowledgments The funding for this research was provided by the National Science Foundation. Thanks to my advisors Dr. Chaoyang Jiang and REU site director Dr. Grant Crawford for both of their commitment and ideas, technical writing Professor Dr. Alfred Boysen for his guidance and wisdom, and the entire staff and faculty of University of South Dakota and everyone in the CY Research group. 26