Natural Organic Matter and Disinfection By-Products - American

Chapter 24

Downloaded by UNIV OF ARIZONA on July 31, 2012 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0761.ch024

Environmental Applications of Novel, Highly Enhancing Substrates for Surface Enhanced Raman Scattering Lin H e , Shawn P. Mulvaney, Sarah K. St. Angelo, Bonnie E. Baker, and Michael J. Natan Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

Surface enhanced Raman scattering (SERS) has the potential to be an ultrasensitive technique for environmental analysis. Highly enhancing SERS substrates assembled by evaporation of a thin A g film over a Ag-coated, colloidal Au submonolayer have been developed. Optimal particle size and coverage was determined by combinatorial methods. Atomic force microscopy data and optical spectra have elucidated the island-like, discrete particle nature of the substrate surface. SERS data on 1,3,5-tri(pyridine)triazine and terbutryn illustrate possible applications.

Increased concern about the fate of pollutants in the biosphere mandates improvements in detection technology, both in ease of data acquisition and measurement sensitivity. Surface enhanced Raman scattering (SERS) is a technique for acquiring Raman spectra of analyte molecules in close proximity to roughened noble metal surfaces. Though largely untested, the notion of using Raman spectroscopy in environmental analysis is attractive for several reasons. First, Raman spectra comprise fingerprint-like collections of vibrational data that, like infrared spectra, are molecule-specific (/, 2). Second, H 0 is a poor Raman scatterer, simplifying measurements in "nature's solvent". Third, recent advances in instrumentation (3, 4) have made Raman spectroscopy both inexpensive and quite sensitive. Nevertheless, for detection of species present at the sub-microgram level, the additional signal available through SERS enhancement is required. SERS experiments typically involve adsorption of an analyte in aqueous solution onto a SERS-active substrate (followed by acquisition of the Raman spectra via laser excitation). There are a limited number of applications of SERS in analytical 2

Corresponding author. 366

© 2000 American Chemical Society

In Natural Organic Matter and Disinfection By-Products; Barrett, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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chemistry. This is due to the fact that, despite numerous approaches to substrate fabrication (5, 6), the requisite combination of reproducibility, ease of manufacture, low cost, simplicity of use, durability and enhancement factor have not yet been attained. Our research group has been focused on development of SERS as a technique for environmental analysis. Our efforts have been concentrated in two areas. The first is the integration of SERS detection into conventional capillary electrophoretic separations (7), with an eye toward analysis of disinfection byproducts. Results in this area will be described elsewhere (8). The second, which builds on our considerable experience with colloidal metal nanoparticles (9-11), is fabrication of substrates that meet the criteria delineated above. Aspects of our progress in this area are described herein; a more detailed study will be presented elsewhere (12).

Experimental Materials. Glass slides used were Fisher Premium Microscope Slides. 18.2 Μ Ω H 0 was distilled through a Barnstead Nanopure system. 3-Mercaptopropylmethyldimethoxysilane ( M P M D M S ) and 3-Mercaptotrimethoxysilane ( M P T M S ) were obtained from United Chemical. H A u C l ' 3 H 0 , A g N 0 , trisodium citrate dihydrate, NaOH, fnmy-l,2-bis(4-pyridy^ethylene (ΒΡΕ), and N,N-dimethyl-4-nitrosoaniline (p-NDMA) were obtained from Aldrich. ΒΡΕ was recrystallized in C H O H several times before use. A stock 10 m M ΒΡΕ solution was then prepared in a fresh H 0 : C H O H (9:1) solvent solution. Dilutions in H 0 were made as needed. 12-nm colloidal A u was prepared from H A u C l ' 3 H 0 reduced by citrate, as previously described. A l l references to "12-nm colloidal A u " particles in this text were nearly spherical, with a standard deviation less than 10%. Multiple preparations were used in this work. For each, nanoparticles were sized by transmission electron micrographs (>500 particles per image) using N I H Image software. L I A g solution purchased from Nanoprobes was used in the electroless plating process and used as directed. Concentrated HCI, H N 0 , H S 0 , and 30% H 0 were purchased from J. T. Baker Inc. Spectrophotometric grade C H O H was obtained from E M science. A g (99.99%) and A u (99.99%) used for thermal evaporation were obtained from Johnson-Matthey Corp. 2

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3-Laver Substrate Preparation. Glass slides were cut and cleaned as previously reported or through successively sonicating for 20 minutes in water, methanol, and acetone. Glass slides were functionalized in a 5-10% solution of M P M D M S in methanol for one hour. Copious rinsing with methanol and then water removed any unbound silane. Functionalized substrates were coated with a 12-nm colloidal A u solution for 90, 95, or 140 s, depending on colloid concentration and coverage desired. Substrates were then rinsed with water and partially dried under an A r stream. Further exposure to LI A g solution for 24 minutes produced a 2-layer SERSactive substrate. The first two layers of combinatorial substrates were assembled as previously reported. The third layer of non-continuous A g film was thermally evaporated on top of two-layer substrates. Optical spectra were taken after each step to monitor the surface quality.


368 Dislodgment of Particles from the Substrate. Dislodging the particles from the slides was accomplished through sonication for 60 min. either in H 0 or a NaOH solution (pH ca. 12). One slide was sonicated in a given solution at a given time. The concentration of the dislodged particles in solution was increased by sonication of three substrates in any given solution. Optical data was recorded for both the slide and solution after each step. The dislodged particle solutions were then centrifuged for 15 min. at 11.5 χ 10 rpm. 2

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Instrumentation. Film Evaporation: A g films were thermally evaporated in an Edwards Auto 306 evaporation system. Metal deposition occurred at a pressure of 2 χ 10* mbar and at various deposition rates, with constant sample rotation to ensure even evaporation. A l l substrates were used immediately after evaporation.


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Surface Enhanced Raman Scattering: SERS spectra were obtained on a Solution 633 micro-Raman spectrometer purchased from Detection Limit, Inc. Excitation was provided by a 30-mW, 632.8-nm HeNe laser with a 7 cm" bandpass. A l l spectra were taken with a 3-mm focal-length objective (~5-μιη diameter spot size). Spectra were collected by a C C D detector with TE-cooling system around -8.5 °C. The system was operated and monitored by Labview software on a Monorail laptop (Monorail Corp.). SERS response was optimized by manually adjusting the height of the probe that was attached to a translation stage. Spectral analysis was handled with either Grams 32 or Igor Pro software packages. 1

Atomic Force Microscopy: A l l A F M images were measured on a Digital Nanoscope Ilia instrument (Digital Instruments, C A ) in the tapping mode. Images were captured using Nanoscope Ilia version software. Samples coated with evaporated A g film were measured directly, while colloidal A u and 2-layer substrates were dried under an A r stream before measurement.

Results and Discussion

Substrate Morphology: Many materials can give rise to the enhancement effect in SERS experiments; however, the most often employed are roughened noble metal films of A g or A u (2). Scheme 1 is a cartoon depicting the assembly of a 3-layer, SERS-active substrate from A u and A g . First, monodisperse colloidal A u particles are immobilized on a silanized glass support.

Scheme 1. 3-Layer Substrate Fabrication.


369 Film integrity is monitored using UV-visible absorbance spectroscopy, with quality control dependent upon the shape and height of the A u plasmon band (//). Suitable substrates are then treated with a commercial A g plating solution, yielding a Ag-clad, colloidal A u microarray (2-layer substrate). A corresponding shift in the optical properties, namely the rise of a shoulder around 620 nm attributed to inter-particle coupling, is seen after the A g cladding is introduced. Finally, a discontinuous film of A g is evaporated over the 2-layer substrate giving rise to the final 3-layer geometry. Optical spectra for 3-layer substrates demonstrate a more pronounced shoulder due to increased particle coupling on the densely-covered surface. No special concerns beyond practicing basic laboratory safety need be considered when preparing these substrates.


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The general combinatorial approach to assembling 2-layer substrate was described in the previously published report (10). Briefly, this method comprises sequential, timecontrolled translation of substrates first into colloidal A u then a solution of A g . The translations produce gradients in particle coverage and particle size, respectively. Those nanostructures giving rise to highest SERS enhancement factors are elaborated via a positional map of SERS intensities for adsorbed analyte. Figure 1 (top) shows one such map for the 1198 cm* peak of ΒΡΕ. Note that the most active region of the surface is not that containing the highest density of large particles. A similar map can be generated after uniform gas-phase deposition of a A g film (Figure 1, bottom). It is interesting to note that evaporation of 20 nm of A g , previously determined to be optimal (12), changes the physical location of the most enhancing region of the surface, as well as increasing by at least a factor of two the overall SERS enhancement. Higher SERS intensity observed at the slightly smaller A u colloidal particle coverage can be explained as the result of the formation of small A g islands between the larger particles. +

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Figure 1. Combinatorial examination of 2-layer (top) and 3-layer (bottom) substrates with varying particle coverage and size. SERS intensity (cps) is plotted for the 1198 cm' peak of ΒΡΕ as a function of the sample position. Shaded boxes represent a 0.2 cm area. Acquisition parameters: 20 mW of 632.8 nm excitation, 2-s integration, 8 mm focal length, integration range - 1170 cm' to 1220 cm', reference range = 1120 cm' to 1170 cm' . 1

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The nanoscale surface morphology of a 3-layer substrate can be seen in atomic force micrographs illustrated in Figure 2. The 1 μιη χ 1 μηι image (left) shows the island-like nature

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Figure 2. Representative A FM images of 3-Layer substrates; (left) 1 μm χ / μτη image, (right) 3 μ/w x 3 μm image, of the metal film. Large surface features surrounded by small island-like features are clearly seen and assigned to Ag-elad, colloidal A u particles and inter-particle, evaporated A g islands, respectively. Regularity of surface structure can be seen in the 3 pm χ 3 pm image (right). This regularity helps explain the repeatability in SERS response observed in spectra taken at various places on the substrate surface. The surface architecture depicted in these representative micrographs has been confirmed with field emission scanning electron microscopy. The island-like surface features, which are responsible for the additional enhancement effect of 3layer substrates, appear to be discrete particles. A n effective method for tracking this characteristic is by monitoring the metal plasmon band with UV-visible spectroscopy. The upper panel of Figure 3 shows the U V visible spectra of 12-nm colloid (the first layer) in various states of relation to an M P T M S modified glass substrate. The trace a corresponds to the surface confined colloidal A u , while the other traces show colloidal A u dislodged from and remaining on the substrate Figure 3. (Upper) UV-visible spectra of 12-nm colloidal Au in solution and affixed to a MPTMS coated glass support both before and after J hr of sonication. (Lower) UVvisible spectra of Ag-clad, Au particles in solution and affixed to a MPTMS coated glass support both before and after Î hr of sonication.

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a) Au coated substrate b) Substrate after sonication (60 min, dHjO) c) Dislodged particles in solution

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a) Au-LI Ag coated substrate b) Substrate after sonication (60 min, dH o) c) Dislodged particles in solution a

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371 surface after 60 minutes of sonication. A significant percentage of the colloid has been dislodged from the substrate surface with sonication. The two post-sonication spectra demonstrate no shift in the plasmon band's X , indicating that sonication neither altered the particle size nor induced aggregation. The consistent position of the X also indicates that the particles are discrete and do not interact significantly on the glass surface nor when they are in solution. Finally, the sum of the absorbance maxima for the dislodged and the remaining surface confined particles approximately equals that of the original substrate, further implying little or no particle fragmentation or agglomeration during sonication. The lower panel of Figure 3 shows UV-visibie spectra that follows the dislodgment of Ag-clad, colloidal A u particles from a 2-layer substrate; the blue shifted plasmon band is attributed to the Ag-cladding, while the second band arises from inter-particle coupling as mentioned earlier. Again, sonication proves to be a viable means of dislodging these particles. However, the position of the plasmon band's À for both post-sonication spectra is blue-shifted with respect to the spectrum of the original substrate. This can be attributed to a lower degree of inter-particle interaction after the sonication treatment. Interestingly, this particular particle geometry would be difficult to recreate by solution synthesis; the portion of the A u particle affixed to the surface should not be coated by the A g plating solution. However, at this time we have no proof discrete A u and A g domains remain on the dislodged particles. In fact, the blue shift in the plasmon band could be attributed to the completion of the Ag-cladding. m a x

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Composition: The enhancement effect of substrates is not only morphology dependent, as demonstrated in Figures 2 and 3, but it depends critically on the material composition of the metal film. It is widely reported in the SERS literature that the coinage metals, Cu, A u , and A g have the highest enhancement factors, with the latter giving rise to the largest enhancement. This is demonstrated experimentally in Figure 4. SERS spectra are shown for 0.5 m M p - N D M A on a 3-layer substrate, a 2-layer substrate, and a 3-layer substrate where the evaporated A g layer is replaced with an evaporated A u layer of the same thickness. The data indicate that the two substrates with an outer Ag-cladding (either 2-layer or 3-layer) demonstrate a significant enhancement advantage over the A u clad substrate. This trend has been confirmed with several SERS active molecules. Figure 4. SERS spectra of 0.5 mM pNDMA on a Ag coated, 3-layer substrate, 2-layer substrate, and a Au coated, 3-layer substrate (12 nm colloidal Au, LI Ag layer, 20 nm evaporated Au). Acquisition parameters: 12 mW of 632.8 nm excitation, 1-s integration time, 3 mm focal length, and 20 μΕ samples.

Raman Shift (cm ) 1


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While a pure A g film might in theory lead to a more enhancing SERS substrate, it is not practical to construct 3-layer substrates without the colloidal A u monolayer. Monodisperse colloidal Au, readily synthesized with a standard deviation of less than 10%, provides a highly regular foundation for construction of 3-layer substrates. This regularity is not easily attained with colloidal A g because of the extreme difficulty of producing A g sols with a standard deviation less than 10%. A 3layer substrate with a polydispersed colloidal A g foundation may lead to some surface sites with a greater SERS enhancement than 3-layer substrates, but will simultaneously negate the ability to prepare reproducible substrates, as well as to compare data taken at different spots on the substrate. Thus, by taking advantage of the physical properties of colloidal A u and the enhancement properties of the A g cladding, 3-layer substrates exhibit great promise in SERS application.

Application: Measurements were conducted to evaluate the analytical merit of the newly developed SERS substrates using l,3,5-tri(pyridine)triazine, a common environmental waste compound (Figure 5). Only 5 seconds of integration time are needed to collect a well-resolved SERS spectrum of a 60 pmols deposited on a 3-layer substrate. While a peak at 1000 cm" was barely noticeable when the amount of deposited compound decreased to 60 fmols, a more discernible band can be observed by increasing the integration time. Analyte detection is further demonstrated with terbutryn, a commonly used pesticide. As can be seen in Figure 6, a well defined spectrum can be 1

400—'—8ϋϋ—'—rzbo—'—TBbn Raman Shift (cm") 1

Figure 5. SERS spectra of 1,3,5tri(pyridine) triazine on a 3-layer substrate. Acquisition parameters: 20 mW of632.8 nm excitation, 3 mm focal length, analyte concentration and integration time indicated on the graph.

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Figure 6. SERS spectrum of terbutryn on a 3-layer substrate. Acquisition parameters: 12 mW of 632.8 nm excitation, 30-s integration time, 3 mm focal length, and 20 μΐ sample.


373 gathered at a low excitation power with less than a minute of integration time when using 3-layer substrates. These two applications illustrate the promising future of 3layer substrates in ultrasensitive detection.


Conclusions

M.

These data have shown that 3-layer substrates comprise viable substrates for environmental SERS analyses. Evaporation of a 20-nm thick A g coating onto a combinatorially optimized two-layer architecture leads to increased SERS enhancement. The nanometer-scale morphology leading to increased enhancement was elaborated by atomic force microscopy. UV-vis spectroscopy was used to show how the particles comprising the discontinuous film could be dislodged from the substrate, leading to a loss in interparticle coupling. Finally, SERS spectra of representative analytes of environmental interest show that the requisite sensitivity can be obtained. Accordingly, our efforts in this area are continuing.

Acknowledgements N S F (CHE-9627338), N I H (DK48784-02), U S D A (96-35102-3840) and E P A (R825363-01-0) are gratefully appreciated for support.

References 1. 2.

Gerrard, D. L. Anal. Chem. 1994, 66, 547R-557R. Creighton, J. A. Surface Enhanced Raman Spectroscopy; Neagle, W. and Randell, D. R., Eds. The Royal Society of Chemistry: Cambridge, UK, 1990, pp 13-26. 3. Asher, S. Α.; Munro, C. H.; Chi, Z. Laser Focus World 1997, 33, 99-109. 4. Lyon, L. Α.; Keating, C. D.; Fox, A. P.; Baker, Β. E.; He, L.; Nicewarner, S. R.; Mulvaney, S. P.; Natan, M. J. Anal. Chem., 1998, 70, 341R-361R. 5. Maya, L.; Vallet, C. E.; Lee, Y. H. J. Vac. Sci. Technol, A 1997, 15, 238-242. 6. Hulteen, J. C.; Van Duyne, R. P. J. Vac. Sci. Technol., A 1995, 13, 1553-1558. 7. Engelhardt, H.; Beck, W.; Kohr, J.; Schmitt, T. Angew. Chem. Int. Ed. Engl. 1993, 32, 629-649. 8. He, L.; Natan, M. J. in preparation. 9. Freeman, R. G.; Grabar, K. G.; Allison, Κ. Α.; Bright, R. M.; Davis, J. Α.; Guthrie, A. P.; Hommer, M. B.; Jackson, Μ. Α.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1632. 10. Baker, Β. E.; Kline, N. J.; Treado, P. J.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 8721-8722. 11. Grabar, K. G.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S.-L.; Natan, J. Anal. Chem. 1997, 69, 471-477. 12. He, L.; Mulvaney, S. P.; Natan, M. J. in preparation.


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