Silver Nanocrescents with Infrared Plasmonic Properties As Tunable

Johnson , S. A., Pham , N. H., Novick , V. J. and Maroni , V. A. Appl. Spectrosc. 1997, 51, 1423– 1426. [Crossref], [CAS]. 16. Application of surfac...
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Silver Nanocrescents with Infrared Plasmonic Properties As Tunable Substrates for Surface Enhanced Infrared Absorption Spectroscopy Rostislav Bukasov and Jennifer S. Shumaker-Parry* Department of Chemistry, University of Utah, 315 S. 1400 E., RM 2020, Salt Lake City, Utah 84112 We exploit the unique infrared plasmonic properties of silver nanocrescents (AgNCs) in preparing tunable substrates for surface enhanced infrared absorption (SEIRA) spectroscopy. Fabrication provides good control over the crescents’ structural properties which enables tuning of the localized surface plasmon resonances (LSPRs) from the visible through the infrared (IR) regions of the spectrum. Using AgNCs as uniquely tunable IR LSPR substrates, we demonstrate the impact of spectral tuning on maximizing SEIRA signal enhancements measured for adsorbed alkylthiolates. The AgNCs demonstrate the largest reported area-normalized SEIRA signal enhancements which increase from 7,700 to 46,000 depending on the relative positions of the AgNC’s LSPR frequency and the molecular vibration frequency. The SEIRA enhancement increases and the absorption band line shape becomes more asymmetric as the AgNCs’ LSPR frequency overlaps more extensively with the frequency of the probed molecular vibration. The tunability of the LSPR properties will enable fundamental SEIRA studies and the development of optimized SEIRA substrates for detection and identification of molecular adsorbates. Significant research efforts have been devoted to the development of surface-enhanced Raman spectroscopy (SERS) since the phenomenon was first observed.1-3 The role of localized surface plasmon resonances (LSPRs) in noble metal nanostructures in producing enhanced responses from adsorbed molecules has led to both fundamental investigations and the development of nanoparticle-based SERS substrates with tunable LSPR properties. The observed enhancements in the measured Raman signals may be attributed to both electromagnetic and chemical mechanisms. While the role of chemical contributions is less understood, the general consensus is that electromagnetic contributions play a significant role. Tuning the LSPR wavelength and optimizing the locally amplified electromagnetic fields in nanoparticle-based substrates are both important in maximizing observed enhancements. For example, several SERS studies of various aspect ratio * To whom correspondence should be addressed. E-mail: [email protected]. (1) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1–20. (2) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215–5217. (3) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys.Lett. 1974, 26, 163–166. 10.1021/ac900477p CCC: $40.75  2009 American Chemical Society Published on Web 05/01/2009

nanoparticles such as nanoshells,4 nanoprisms,5,6 and nanorods7 showed that there is a substantial increase (e.g., 10-100 for gold and silver nanorods) in SERS enhancement upon optimizing the nanoparticle LSPR frequency relative to the frequencies of the excitation laser light and the inelastically scattered light in the measured SERS signal. Similar LSPR-related effects also have led to signal enhancements in surface enhanced infrared absorption (SEIRA) spectroscopy. However, because of the first order nature of SEIRA, the electromagnetic field enhancement should be less than what is observed for the second order SERS process. In addition, chemical enhancement can play a role in the generation of SEIRA signals.8-12 Enhancements in SEIRA spectroscopy appear if the probed vibration is along the dipole moment that is perpendicular to the substrate surface.13 Therefore SEIRA may provide not only chemical structure but also orientation information about the adsorbed molecules.14 Since the first vestiges of SEIRA were reported in 1980 by Harstein et al. with a substrate enhancement factor (EF) of 20,15 several groups have worked on developing the technique as a tool for chemical16-18 and biological sensing.19-22 For example, Plateck and co-workers reported specific SEIRA (4) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729–4735. (5) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426–7433. (6) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 102, 11279–11285. (7) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165–170. (8) Priebe, A.; Sinther, M.; Fahsold, G.; Pucci, A. J. Chem. Phys. 2003, 119, 4887–4890. (9) Heaps, D. A.; Griffiths, P. R. Vib. Spectrosc. 2006, 42, 45–50. (10) Krauth, O.; Fahsold, G.; Pucci, A. J. Chem. Phys. 1999, 110, 3113–3117. (11) Krauth, O.; Fahsold, G.; Magg, N.; Pucci, A. J. Chem. Phys. 2000, 113, 6330–6333. (12) Langreth, D. C. Phys. Rev. Lett. 1985, 54, 126–129. (13) Osawa, M.; Ataka, K.; Yushii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497–1502. (14) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992–6998. (15) Harstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. Rev. Lett. 1980, 45, 201–204. (16) Johnson, S. A.; Pham, N. H.; Novick, V. J.; Maroni, V. A. Appl. Spectrosc. 1997, 51, 1423–1426. (17) Aroca, R. F.; Bujalski, R. Vib. Spectrosc. 1999, 19, 11–21. (18) Leyton, P.; Domingo, C.; Sanchez-Cortes, S.; Campos-Vallette, M.; GarciaRamos, J. V. Langmuir 2005, 21, 11814–11820. (19) Seelenbinder, J. A.; Brown, C. W.; Pivarnik, P.; Rand, A. G. Anal. Chem. 1999, 71, 1963–1966. (20) Brown, C. W.; Li, Y.; Seelenbinder, J. A.; Pivarnik, P.; Rand, A. G.; Letcher, S. V.; Gregory, O. J.; Platek, M. J. Anal. Chem. 1998, 70, 2991–2996. (21) Ataka, K.; Heberle, J. Biopolymers 2006, 82, 415–419. (22) Enders, D.; Ruppa, S.; Ku ¨ llerc, A.; Pucci, A. Surf. Sci. Lett. 2006, 23, L305L308.

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fingerprints for Salmonella and glucose oxidase bound with their respective antibodies.20 SEIRA also has been used to investigate molecular adsorption on a range of surfaces23,24 and for in situ electrochemical studies.25-28 However, overall development of SEIRA spectroscopy has attracted considerably less attention than SERS, probably because of the lack of tunable substrates with infrared LSPR properties resulting in modest SEIRA signal enhancements.29 Most SEIRA studies have been based on vapor-deposited thin metal (Ag or Au) films typically 4-10 nm thick consisting of islands that provided SEIRA enhancements in the range of 10-100.30 SEIRA experiments conducted on somewhat larger wet chemically prepared Au island substrates achieved an area normalized SEIRA EF of approximately 3,600.31 Zhao and coworkers published one of the few studies based on an attempt to control the SEIRA substrate morphology. This group used an aligned Ag nanorod array with optimized p-polarization of the incident light. However, they achieved a moderate EF of 31 with an LSPR extinction maximum of the array at ∼900 nm which was very far from the nearest analyte vibration band of interest at 7407 nm (1350 cm-1).32 Van Duyne and co-workers investigated SEIRA EFs on substrates with some degree of plasmonic structure morphology control. The group prepared Ag particle periodic arrays on mica, Si, and Ge substrates, and found no significant increase in EF per unit of metal area in comparison to large Ag islands.33 Similar to other studies, the Ag particle array exhibited a relatively broad LSPR peak which was not tuned to any specific vibrational frequency of the adsorbates.33 More recently, Halas and co-workers reported SEIRA EFs of up to 60,000 and 38,000 on closed-packed nanoshell arrays29 and nanoshell aggregates,34 respectively. However these EF values were estimated by normalizing to small hot spot volumes rather than the total particle area. For the nanoshell arrays, the hot spots were estimated to be confined to 8-nm interparticle junctions between 344 nm diameter nanoshells from calculations of the near field behavior.29 Similar to other substrates, the LSPR spectrum for the nanoshell array is broad and does not exhibit a maximum close to the 822 cm-1 to 1590 cm-1 range of the probed vibrations. Here, we describe the LSPR properties of silver nanocrescents (AgNCs) and employ these structures as tunable substrates for (23) Nishikawa, Y.; Kunihiro, F.; Kenichi, A.; Osawa, M. Anal. Chem. 1993, 65, 556–562. (24) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967– 1974. (25) Ma, M.; Yan, Y.-G.; Huo, S.-J.; Xu, H.-X.; Cai, W.-B. J. Phys. Chem. B 2006, 110, 14911–14915. (26) Samjeske´, G.; Miki, A.; Osawa, M. J. Phys. Chem. B 2007, 111, 15074– 15083. (27) Jiang, X.; Ataka, K.; Heberle, J. J. Phys. Chem. B 2008, 112, 813–819. (28) Osawa, M. Electrocatalytic Reactions on Platinum Electrodes Studied by Dynamic Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS); Elsevier Science: London, England, 2007. (29) Wang, H.; Kundu, J.; Halas, N. J. Angew. Chem., Int. Ed. 2007, 46, 9040– 9044. (30) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58, 324A– 338A. (31) Enders, D.; Pucci, A. Appl. Phys. Lett. 2006, 88, 184104. (32) Leverette, C. L.; Jacobs, S. A.; Shanmukh, S.; Chaney, S. B.; Dluhy, R. A.; Zhao, Y. P. Appl. Spectrosc. 2006, 60, 906–913. (33) Jensen, T. R.; Van Duyne, R. P.; Johnson, S. A.; Maroni, V. A. Appl. Spectrosc. 2000, 54, 371–377. (34) Kundu, J.; Le, F.; Nordlander, P.; Halas, N. J. Chem. Phys. Lett. 2008, 452, 115–119.

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SEIRA spectroscopy. In comparison with other morphologies of tunable plasmonic nanoparticles, nanocrescents have unique LSPR properties consisting of narrow LSPR peaks with high extinction efficiency which are easily tuned in a broad range from the visible through the IR spectral regions.35 We use the IR plasmonic properties of AgNCs to investigate the effect of tuning the LSPR frequency with respect to the molecular vibration frequency probed in the SEIRA measurement. MATERIALS AND METHODS Materials. 1-Octadecanthiol (ODT, 98%) was purchased from Aldrich (St. Louise, MO). Ag pellets (99.99%) were purchased from K. J. Lesker (Philadelphia, PA). Polystyrene NIST Particle Size Standards (205.6 ± 2.6 nm, 410 ± 2.3 nm, 560 ± 4 nm, 659 ± 6.5 nm) were purchased from Polysciences, Inc. (Warrington, PA). Coverslips were purchased from Ted Pella (Redding, CA). All water was purified to 18 MΩ using a NANOpureDiamond system from Barnstead. Absolute ethanol (200 proof) was obtained from Aaper (Shelbyville, KY). Methods. AgNCs were fabricated on clean substrates using nanosphere template lithography (NTL) as described previously.35,36 Briefly, the polystyrene (PS) beads were diluted in ethanol and drop cast on clean glass coverslips to form a few percent of a monolayer surface coverage. Then those substrates coated with PS beads were mounted on homemade substrate holders which allow control of the deposition angle. The holders were installed in an electron-beam evaporator (Denton Vacuum U.S.A., Moorestown, NJ) chamber. A silver film was deposited and the metal film thickness was monitored with a quartz crystal microbalance (QCM). To remove the silver film that was not protected by the PS beads, we used an argon ion milling system (Plasmalab 80 Plus, Oxford Instruments, Witney, Oxfordshire, U.K.). Finally we lifted the PS sphere templates off the substrate by application of transparent tape (Magic Tape, 3M) leaving behind intact AgNCs on the glass substrate. The ODT SAMs on AgNCs were produced by overnight (10-14 h) exposure of the AgNC substrates to 1 mM ODT solution in absolute ethanol. Prior to optical and structural characterization, the substrates were individually rinsed with excess ethanol and dried under nitrogen flow. A field emission scanning electron microscopy (NanoNova SEM, FEI) was used for structural characterization of the AgNCs. The optical properties of the AgNCs were characterized by transmission VIS-NIR spectroscopy and transmission FTIR spectroscopy. Extinction spectra were measured over a wavelength range of 500-3200 nm with a Perkin-Elmer Lambda 9 UV/vis/ NIR spectrophotometer with light predominantly (∼80%) polarized in the horizontal plane. Extinction represents a measure of the loss of light because of scattering and absorbance and is defined as E )(1 -(I)/(I0)), where I0 is the intensity of the incident light and I is the measured light intensity after the beam passes through the sample. Extinction spectra also were measured over a wavelength range of 2500-4000 nm with a Perkin-Elmer Spectrum 100 FT-IR spectrometer with circularly polarized light. (35) Bukasov, R.; Shumaker-Parry, J. S. Nano Lett. 2007, 7, 1113–1118. (36) Shumaker-Parry, J. S.; Rochholz, H.; Kreiter, M. Adv. Mater. 2005, 17, 2131–2134.

Figure 1. SEM images of AgNCs fabricated on glass substrates. (A) AgNCs were prepared using 560-nm-diameter PS bead templates, a silver film thickness of 35 nm, and a deposition angle of approximately 30°. (B) SEM image of a single AgNC from the same substrate as shown in A.

An ODT SAM was prepared on a silver film to serve as a normalization reference for SEIRA analysis. A 40 nm thick silver film was deposited on a glass coverslip at the same deposition rate (0.5-0.6 Å/s) as was used for the fabrication of AgNCs. To form the SAM, the silver-coated substrate was immersed in a 1 mM ODT solution for 12 h. Before FTIR measurement, excess ODT was removed by copious rinsing with ethanol, and the substrate was dried in a stream of N2. For this purpose we employed a VeeMax2 variable angle specular reflectance angle accessory from Pike Technologies using two different angles of incidence: 45° and 60°. RESULTS AND DISCUSSION We fabricated AgNCs using a NTL process developed previously for preparing gold nanocrescents (AuNCs).35,36 SEM images of AgNCs are presented in Figure 1 and demonstrate the uniformity of the size, shape, and orientation of the structures prepared by NTL. The size of the crescents depends on the diameter of the polymer nanosphere used as a template in the fabrication process. Control of the angle of rotation between metal depositions may be used to control the size of the crescent opening, or the tip-to-tip distance, which can range from open (e.g., as shown in Figure 1) to nearly closed. For the studies presented here, a single silver deposition was done to prepare “open” crescent structures. The LSPR properties of the AgNCs were characterized using transmission spectroscopy. Absorbance was converted to extinction as described previously.35 As shown in Figure 2a, multiple LSPR extinction peaks extending from the visible into the mid-IR spectral regions are observed for AgNCs prepared from templates with diameters ranging from 206 to 659 nm. The uniform orientation of the AgNCs on the substrate (see Figure 1) and the polarization of the incident light in our spectrophotometer make it possible to assign the major peaks in the LSPR extinction spectra to stronger longitudinal and weaker transverse plasmon resonances that are excited in the AgNC structures. Additional quadrupole plasmon resonances also are observed in the near-IR and visible spectral regions for some structures. As shown in Figure 2b, the AgNCs’ longitudinal and transverse LSPR wavelengths are linearly proportional to the NC template diameter and therefore the size of the NC. In general, the tunability of the plasmon resonances in AgNCs is similar to that of AuNCs.35 The slopes of the transverse (1.7 nm λ/ nm diameter) and longitudinal (3.6 nm λ/ nm diameter) peak wavelength dependencies on the size of the AgNCs are approximately 5 and 14% less steep,

Figure 2. Characterization of AgNCs’ LSPR properties. (A) Representative LSPR extinction spectra of AgNCs prepared using PS templates with diameters indicated in the figure legend. (B) Sizedependent LSPR properties of AgNCs.

respectively, than the slopes for transverse (1.8 nm λ/ nm diameter) and longitudinal (4.2 nm λ/ nm diameter) resonances in AuNCs.35 We also characterized the AgNCs’ extinction efficiency (EE) which is an important parameter that measures the extent of interaction of incident electromagnetic radiation with the structures. The EE is the ratio of the optical cross section to the geometric cross section of the crescent structures and may be presented as the ratio of the measured extinction to the proportion of the probed area covered by the AgNCs. The former number is the ratio of the average substrate area per one nanostructure to the average area of a single nanostructure. The average area of a single AgNC was estimated from SEM images using the twodimensional NC cross section which excludes the area halfencircled by the NC. To calculate the average substrate area per NC we counted the number of particles using several SEM images (5000× or less magnification) and then divided the total area of the images in µm2 by the total number of NCs. For each sample, we counted between approximately 200 and 550 AgNCs. The extinction efficiency uncertainty is ∼15% due primarily to the variability in the number of AgNCs in the SEM images used in the calculation of the extinction efficiencies. In the case of AgNCs, the EE grows from 17 to 22 with increasing template diameter (e.g., from 206 nm to 659nm) and in-plane aspect ratio. Overall, for spectra obtained using predominantly favorable polarization (i.e., ∼80% of the incident light was polarized along the long axis of the particles), the EE of AgNCs is 10-25% Analytical Chemistry, Vol. 81, No. 11, June 1, 2009

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lower than that of similar diameter AuNCs (i.e., maximum EE of 30),35 but still larger than experimentally measured EEs reported for several other nanoparticle morphologies (e.g., 10 for rings,37 5 for shells,38 and 13 for ellipsoids39). The highly tunable LSPR properties and the high EEs make Au and AgNCs potentially useful substrates for a range of LSPRbased sensing and spectroscopy applications. In particular, the unique IR LSPR properties provide an opportunity to investigate the AgNCs on glass as a substrate that is tunable in the infrared (1705-3300 nm) for SEIRA measurements. We probed the SEIRA enhancements for octadecanethiol (ODT) adsorbed on AgNC substrates and the analysis of the symmetric and asymmetric methylene stretches in the SEIRA spectra which were in the vicinity of the LSPR peaks for some of the AgNCs. The FTIR spectra of AgNCs prepared using templates with diameters of 206, 410, 560, and 659 nm were collected using transmission mode before and after functionalization with ODT. Characteristic vibrations were observed for an asymmetric methylene stretch at ∼2915 cm-1 and a weaker symmetric methylene stretch at ∼2850 cm-1, in good agreement with spectral positions reported in literature for these vibrations.31 The region between the dashed lines in Figure 2A is expanded in Figure 3A to show the positions of the absorption peaks for the asymmetric and symmetric methlylene stretches relative to the AgNC LSPR peaks. Both sets of spectra were measured after AgNCs were functionalized with ODT. Interference fringes were removed over this narrow spectral region using a numerical process described by Neri et al.40 As shown in Figure 3A, we observed an increase in the asymmetry or the Fano-like nature of the absorption peaks as the extent of overlap between the LSPR frequency maxima and the vibrational band frequency increased. Also, as the LSPR peak shifts away from the frequency of the vibration (i.e., for smaller AgNCs), the absorption peaks become more symmetric. The observed asymmetry is likely due to the interference between the molecular dipole with the localized surface plasmon dipole when the LSPR frequency overlaps with the frequency of the molecular vibration.8,9 The interference that occurs leads to the spectral line shape changes and has been observed in other SEIRA studies.8-10 More systematic studies of how the line shape changes with different types of vibrations and for different LSPR modes should provide insight into the nature of any LSPR-molecule dipole interactions. The major parameter of all forms of surface enhanced spectroscopy is the enhancement factor (EF) which is the signal increase measured because of molecules near the surface of metal nanoparticles that are within the light induced localized electric field. The EF is typically normalized to the nanoparticle area or even to the nanoparticle “hot spot” area which is generally the region near the particle surface where the electric field may exceed the incident electric field by a factor >100 depending on the configuration (i.e., nanoparticle shape or multiparticle junction size). Clearly the EF normalized to the total nanoparticle area would appear to be much smaller than the EF normalized to the hot spot area for the same data because the area of hot spots is (37) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G. W.; de Abajo, F. J. G. Phys. Rev. Lett. 2003, 90, 057401. (38) Sun, Y.; Xia, Y. N. Anal. Chem. 2002, 74, 5297–5305. (39) Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 9846–9853. (40) Neri, F.; Saitta, G.; Chiofalo, S. J. Phys. E: Sci. Instrum. 1987, 20, 894–896.

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Figure 3. SEIRA spectroscopy using AgNCs as tunable substrates. (A) Extinction spectra of ODT-coated AgNCs showing the asymmetric and symmetric methylene stretch absorption peaks. The spectra are expanded from the infrared region bracketed by the dashed lines in Figure 2A. Each spectrum is for AgNCs made from PS beads with diameters of 206 (blue), 410 (red), 560 (cyan), and 659 (pink). (B) Dependence of SEIRA enhancement factors on the AgNC size which is related to the position of the longitudinal resonance peak as described by Figure 2B.

typically less than a few % of the total particle area. However, estimating an EF that is normalized to the total particle area has at least two practical advantages. First, typically analyte binding is not limited exclusively to nanoparticle hot spots, but rather the analyte may bind to any region of the nanoparticle which is usually uniformly functionalized. Second, it is easier to quantify the area of the nanoparticles compared to the area of the hot spots. Typically hot spots have rather subjective boundaries estimated from electromagnetic field simulations based on geometries that are similar but not necessarily identical to the actual structures used for experiments. To quantify the EF for the nanocrescent substrates, we first estimated the proportion of the substrate area covered with AgNCs from analysis of SEM images of each sample. This number is obtained as described above for the calculation of extinction efficiency. Next, we obtained the reference signal for EF calculations (e.g., 0.03% reflectivity at a 60° incident angle) from multiple variable angle reflectance measurements of an ODT SAM on a smooth 40 nm thick Ag film. We converted this signal to transmittance (e.g., 0.004% T) using the formula published by Enders and Pucci.31 We found the gross EF using the ratio of the IR transmission signal from the AgNC covered substrate to

the Ag film reference transmission signal. Finally, we divided the gross EFs by the proportion of area covered by NCs to obtain the nanoparticle area normalized SEIRA EFs. As shown in Figure 3B, the area-normalized EFs increased significantly (from 7,700 to 46,000) as the AgNC size became larger and the longitudinal dipole LSPR peak red-shifted. The relative uncertainty of our EF calculation is in the range of 10-20%. This error is dominated by the estimation of the AgNC coverage of the substrate surface area from analysis of multiple SEM images containing between 200 and 500 AgNCs for each sample. As the LSPR peak shifted, the extent of overlap of the LSPR peak with the absorption frequency of the dominant asymmetric methylene stretch vibration at 3430 nm (2915 cm-1) increased as shown in Figure 3A. The ability to tune the LSPR peak with respect to the molecular vibration peak provides an opportunity to investigate the nature of the SEIRA EFs in more detail, as well as to probe the origin of the peak asymmetry. As part of our ongoing studies, we will probe how far into the infrared the plasmon resonances extend for both Au and Ag NCs. Calculations are underway to better understand the nature of the plasmon resonances and to predict the near field properties of the structures, including the extent of localization as related to the NCs’ structural properties.

CONCLUSIONS In summary, AgNCs have LSPR properties that are tunable across a wide spectral region, from the visible to the IR. We applied the crescent-shaped structures as tunable substrates for SEIRA spectroscopy. As the longitudinal LSPR frequency was tuned to the frequency of a molecular vibration, a 6-fold increase in the area normalized SEIRA enhancement factor of up to 46,000 was observed, higher than any nanoparticle area normalized SEIRA EF reported in literature. The AgNCs’ broadly tunable LSPR properties make the structures excellent platforms for a range of spectroscopic sensing applications including SEIRA spectroscopy. The AgNCs will serve as tunable platforms for developing SEIRA spectroscopy for trace analysis of organic compounds for biological and chemical assays. Additional investigations are focused on mapping the near field enhancements for both Au and Ag NCs for potential applications in plasmonic waveguides and LSPR and surface enhanced spectroscopy applications.

Received for review March 4, 2009. Accepted April 12, 2009. AC900477P

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