ARTICLE pubs.acs.org/ac
Highly Sensitive Surface Enhanced Raman Scattering Substrates Based on Filter Paper Loaded with Plasmonic Nanostructures Chang H. Lee,† Mikella E. Hankus,‡ Limei Tian,† Paul M. Pellegrino,*,‡ and Srikanth Singamaneni*,† † ‡
Department of Mechanical Engineering and Materials Science, Washington University, St. Louis, Missouri 63130, United States U.S. Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, Maryland 20783-1197, United States
bS Supporting Information ABSTRACT: We report a novel surface enhanced Raman scattering (SERS) substrate platform based on a common filter paper adsorbed with plasmonic nanostructures that overcomes many of the challenges associated with existing SERS substrates. The paperbased design results in a substrate that combines all of the advantages of conventional rigid and planar SERS substrates in a dynamic flexible scaffolding format. In this paper, we discuss the fabrication, physical characterization, and SERS activity of our novel substrates using nonresonant analytes. The SERS substrate was found to be highly sensitive, robust, and amiable to several different environments and target analytes. It is also cost-efficient and demonstrates high sample collection efficiency and does not require complex fabrication methodologies. The paper substrate has high sensitivity (0.5nM trans-1,2-bis(4-pyridyl)ethene (BPE)) and excellent reproducibility (∼15% relative standard deviation (RSD)). The paper substrates demonstrated here establish a novel platform for integrating SERS with already existing analytical techniques such as chromatography and microfluidics, imparting chemical specificity to these techniques.
I
n recent years, the potential exposure of civilian and military populations to chemical, biological, and energetic hazards has become an increasingly likely threat. To efficiently and promptly assess potentially hazardous situations, the rapid and accurate onsite detection and identification of these threats is critical for first responders, military personnel, and environmentalists. A range of analytical techniques in combination with several types of sensor platforms is being used to assess chemical and biological hazards. Some of the more effective and common methodologies currently employed include various forms of bioassays and spectroscopic techniques. The effectiveness of these techniques is determined by considering analyte detection sensitivity, hazard specificity, sample preparation requirements (technician skill and time), robustness, and ease of use. Raman spectroscopy is one of the techniques well suited for the identification and characterization of unknown targets both hazardous and benign.1 16 It is particularly advantageous as it (i) does not suffer from interferences from water, (ii) requires little to no sample preparation, (iii) is robust and can be used in numerous environments, (iv) is relatively insensitive to the wavelength of excitation employed, and (v) produces a narrowband spectral signature unique to the molecular vibrations of the analyte. All of these advantages contribute to Raman spectroscopy’s capability to perform sample characterization, identification, and quantification. Despite such advantages, however, Raman spectroscopy has remained a marginalized technique for trace detection of hazardous materials in the field, mainly due to the extremely low scattering cross sections characteristic to many hazards. Surface enhanced Raman scattering (SERS) is a r 2011 American Chemical Society
technique that overcomes this shortcoming by greatly enhancing Raman scattering, which has been reported to detect a single molecule under ideal conditions.17 Compared to conventional Raman, the SERS enhancement has been reported to be as much as 14 orders of magnitude greater, although it is most commonly seen on the order of 6 to 8 orders of magnitude.18 20 The SERS phenomenon observed is mainly attributed to two main mechanisms (i) the electromagnetic fields generated at or near nanostructured surfaces and (ii) the physical or chemical adsorption of the analyte to a surface. With all the advantages offered by SERS, there is a research push to explore several different architectures that allow for the fabrication and use of a reproducible, uniformly enhancing SERS substrate.21 Colloids were some of the first types of commonly employed SERS substrates due primarily to ease of preparation and large SERS signal enhancement.22 29 However, the challenges with stability and reproducibility in potentially changing environmental conditions have given rise to more directed types of SERS architectures. Some of the newer techniques to address these challenges include film-overnanostructures (FONs), assembly, and lithographic techniques.30 38 In particular, filmovernanostructure (FON)-based SERS architecture is one of the common types of SERS substrates available.39 46 In summary, FON-based substrates are fabricated by depositing a layer of nanoparticles typically silica or polystyrene onto a rigid platform. Received: June 30, 2011 Accepted: October 21, 2011 Published: October 21, 2011 8953
dx.doi.org/10.1021/ac2016882 | Anal. Chem. 2011, 83, 8953–8958
Analytical Chemistry These nanoparticles can then be overcoated with the desired metal of choice (typically, silver or gold). The advantages of these types of SERS substrate architectures include increased stability as compared to colloids, ease of fabrication, low cost, rapid fabrication, some tailoring of substrate architecture (shape, size, and coinage metal) to match plasmon bands with irradiation source, and good detection of analytes of interest commonly seen with silver-coated substrates. The disadvantages of these types of SERS substrates include a rigid architecture resulting in low conformal contact with the sample and hence poor sample collection efficiency. Significant efforts have been made in the use of multilayer gold and silver coated biologically compatible substrates with high SERS signal enhancements.38,47 49 However, these substrates like most FONs still have the challenges of rigid scaffolding and reproducibility to overcome. Lithographically produced SERS substrates can be tailored by varying size, shape, and precise positioning of nanostructures on various support materials.12,35,50 These lithographically produced substrates are characterized as being highly reproducible and having repeatable arrayed nanostructure surfaces within a given area, allowing for increased quantifiable (and in some cases), highly sensitive SERS measurements. However, using this type of fabrication process is not cost efficient, requires a great deal of effort, time, and skill for fabrication, has a low sample collection efficiency, and does not necessarily result in a large uniform SERS active sensing area. Considering the many issues still associated with SERS substrates, there is a need for a sensitive, cost-efficient, reproducible, uniform, and flexible substrate. Such a substrate would have applications not only to first responders and military personal but also to several areas of medical, food analysis, and environmental research. We and others have introduced a new SERS substrate platform that overcomes many of the challenges associated with typical previous SERS substrates.51,52 Our novel SERS platform consists of plasmonic nanostructures adsorbed on flexible filter paper-based scaffolding. Our paper-based scaffolding results in a substrate that combines all of the advantages of a FON in a flexible scaffolding format. Our substrate is thus cost-efficient, highly sensitive, robust, and amiable to several different environments and target analytes. It also demonstrates high sample collection efficiency and does not require complex fabrication methodologies. Our paper substrate demonstrates high sensitivity (0.5 nM trans-1,2-bis(4-pyridyl)ethene (BPE)) and reproducibility (∼15% relative standard deviation (RSD)). In this paper, we discuss the fabrication and physical characterization of our SERS substrates and will briefly discuss our future efforts toward creating a highly selective, sensitive, and flexible SERS substrate for highly specific sensing.
’ RESULTS AND DISCUSSION The paper scaffolding used in this study was a common laboratory filter paper (Whatman #1). The choice of the paper substrate is based on the fact that the filter paper chosen is almost completely composed of α-cellulose (98%) and ensures minimal interference from other components (trace elements, coatings, etc.).53 Figure 1A shows the hierarchical fibrous morphology of the filter paper from SEM and AFM. The paper largely consisted of microscale (∼10 μm) cellulose fibrous strands interwoven together. Smaller microfibers (average diameter of ∼0.4 μm) made part of the large fibrous structure with nanofibers braided in between. The rms surface roughness of the paper was 72 nm
ARTICLE
Figure 1. SEM image of pristine paper showing the microfibers of the filter paper (inset shows the AFM image depicting the nanoscale fibers that are braided to form microfibers). (B) SEM image showing the uniform decoration of the paper surface with gold nanorods (inset: higher resolution image showing the gold nanorods decorated on the surface of paper).
over 5 5 μm2 area, indicating that the paper had a large surface area. Paper has gained much interest in recent years as a low-cost and ideal platform for building portable miniature diagnostic devices suitable for developing countries, resource-limited environments, and point-of-care treatment.54,55 Paper is a versatile and common material that finds many uses in consumer oriented products because its source (cellulose) is abundant in nature, renewable, inexpensively produced, and recycled.56 Paper is also biodegradable and biocompatible and has the ability to easily wick and absorb fluids.57 Recently, there have been extensive efforts to develop low-cost potable biomedical diagnostic devices by printing microfluidic patterns on paper, which create hydrophobic barriers that define channels and reaction zones to fabricate an analytical system.58 The gold nanorods were ∼60 nm long and ∼18 nm in diameter, making the aspect ratio approximately 3.3 (see Supporting Information, Figure S1). UV vis extinction spectra of the gold nanorod solution showed the two characteristic peaks at ∼515 and 740 nm, corresponding to the transverse and longitudinal plasmon resonances of gold nanorods, respectively (see Supporting Information, Figure S1).59 Extinction spectrum obtained from the filter paper adsorbed with gold nanorods showed the transverse and longitudinal plasmon blue-shifted compared to the gold nanorod solution. The observed blue shift can be attributed to the change in the dielectric ambient (from water to air/substrate) with an effective decrease in the refractive index. The blue shift of the longitudinal plasmon peak (∼45 nm) was larger than that of the transverse band (∼5 nm), due to the higher refractive index sensitivity of the longitudinal plasmon resonance compared to the transverse band.60 SEM images revealed uniform and dense adsorption of the gold nanorods on the surface without any signs of large scale aggregation (Figure 1B). Low magnification SEM image shows uniform speckled surface morphology of the paper, indicating highly uniform adsorption. Despite the inherent heterogeneity of the paper morphology (pores and fibers of different sizes), the adsorption of the nanorods was found to be highly uniform (Figure 1B). Higher magnification image shows the nanorods decorating the micro- and nanofibers of the paper surface (inset of Figure 1B). The number density of the gold nanorods on the filter paper was 98 ( 22/μm2, determined from a number of AFM images (not shown). Uniform and high density adsorption of CTAB (cetyl trimethyl ammonium bromide cationic surfactant) capped gold nanorods to polymer surfaces can be a significant challenge.61,62 However, we observed that once the gold nanorods 8954
dx.doi.org/10.1021/ac2016882 |Anal. Chem. 2011, 83, 8953–8958
Analytical Chemistry
Figure 2. (A) SERS spectra obtained from the paper substrate adsorbed with nanorods as they were exposed to different concentrations of BPE in ethanol. (B) Higher resolution spectra showing the detection limit down to 1 nM. (C) Plot showing the concentration vs intensity of the 1197 cm 1 Raman band showing the monotonic increase of intensity with concentration of the analyte.
were adsorbed on the filter paper, even vigorous rinsing with water or alcohol did not noticeably alter the gold nanorod density, suggesting the stability of the paper SERS substrate for deployment in liquid environments. Cellulose has a large number of hydroxyl groups, which are accessible for attaching positively charged species.63,64 The uniform, irreversible, and high density adsorption of the gold nanorods is possibly due to the electrostatic interaction between the positively charged nanorods and the filter paper. Autofluorescence can be a significant problem when paper as substrate is used in an optical analysis. Fluorescence images of filter paper revealed strong green (525 nm) fluorescence emission (see Supporting Information, Figure S2). On the other
ARTICLE
hand, in the case of filter paper adsorbed with gold nanorods, autofluorescence of the filter paper was significantly quenched due possibly to nonradiative energy transfer.65,66 Fluorescence spectra shows that the filter paper adsorbed with gold nanorods has virtually no fluorescence, and this feature is of a great advantage in the use of paper as a SERS substrate as the unwanted autofluorescence is self-subdued (see Supporting Information, Figure S2). Also for SERS using gold nanorods, a 785 nm wavelength laser is typically used as excitation source to place the longitudinal band of a gold nanorods between the wavelengths of the excitation source laser and vibrational band of interest to optimize SERS enhancement.35,43,67 This laser wavelength, which is far from the fluorescence excitation wavelength, also ensures weak autofluorescence. trans-1,2-Bis(4-pyridyl)ethene (BPE) was employed as a nonresonant analyte for investigating the efficiency of the paperbased SERS substrates. BPE is known to interact with gold through the pyridyl units. Figure 2A shows the SERS spectra for different concentrations of BPE in ethanol. The spectra clearly show the characteristic peaks of BPE at 1013 cm 1, 1197 cm 1, 1335 cm 1, 1605 cm 1, and 1636 cm 1. The Raman band at 1197 cm 1 was used for monitoring the trace detection ability of the paper substrates. The Raman band is clearly distinguishable (SNR > 5) down to 1 nM concentration as can be seen from the higher resolution and smoothed spectra (Figure 2B). The plot of the intensity of the 1197 cm 1 band vs concentration of BPE shows a monotonic increase of the intensity with concentration of BPE (Figure 2C). The SERS substrates exhibit excellent homogeneity with relative standard deviation of ∼15%, which is close to the values observed for commercially available microfabricated SERS substrates.68 This level of homogeneity is remarkable considering the simplicity of the fabrication approach and inherent heterogeneity of the paper substrates. In fact, the variation in the SERS signals was found to be largely due to the focal variations of the incident laser on the paper substrates as described below. One of the important considerations in the fabrication of paper-based SERS substrates is the effect of the exposure time of the paper substrates to the metal nanostructure solution. The extinction spectra of the paper substrates exposed to gold nanorod solution for different amounts of time show that the intensity of both transverse and longitudinal plasmon bands increase with the exposure time, indicating the progressive increase in the density of the nanorods adsorbed on the paper surface (see Supporting Information, Figure S3). It was also noted that the paper substrates progressively become darker over time with adsorption of nanorods. SERS intensity of the paper substrates exposed to AuNR solution for different amounts of time increased rapidly for the first 10 15 h, followed by a small increase for subsequent exposure (see Supporting Information, Figure S3). This trend closely agrees with the intensity variation of the extinction spectra obtained from the paper substrates exposed to AuNR solution for different durations. The number of nanorods adsorbed on the paper substrates rapidly increases for the first 10 15 h, followed by a saturation of the density of the adsorbed nanorods subsequently. In order to understand the source of spatial variation in the SERS intensity (heterogeneity) of the paper substrate, we performed confocal Raman mapping following the exposure to nonresonant analyte. 1,4-BDT employed for these studies is also widely employed as a model analyte for SERS, owing to its ability to readily adsorb on gold or silver particles and its distinct Raman fingerprint. The Raman spectrum of 1,4-BDT in neat solid state 8955
dx.doi.org/10.1021/ac2016882 |Anal. Chem. 2011, 83, 8953–8958
Analytical Chemistry
ARTICLE
Figure 3. Raman mapping of the paper substrate exposed to 1 mM 1,4-BDT (A) optical micrograph showing the region where the Raman mapping was performed. (B) Intensity map of 1058 cm 1 band within the region shown in (A). (C) Average Raman spectra obtained from regions indicated in (A). (D) Intensity variation along a single fiber shown by the dotted line in (A).
exhibits strong bands at 740, 1058, 1093, 1186, and 1573 cm 1. The optical micrograph of the region of the paper where the mapping was performed is characterized by porous and fibrous morphology (Figure 3A). Figure 3B shows the SERS intensity map of the 1058 cm 1 band. In general, the map shows a homogeneous SERS intensity with maximum difference between the brightest and darkest regions being close to 80%. There are several interesting features that can be noted from the Raman map. A close comparison between the optical image and the Raman map reveals that the variations in the Raman intensity are primarily related to the roughness of the paper substrates (Figure 3A,B). The brighter regions (indicated by square c) correspond to the areas where the incident laser was focused below the surface while the darker regions (indicated by square a) correspond to the areas where the laser was focused slightly above the surface. The regions which were in focus exhibited intermediate intensity between these two extreme cases. Figure 3C shows average spectra obtained from different regions (a, b, and c) marked in the optical image and the SERS map, quantitatively showing the variation in SERS intensity discussed above. Figure 3D shows the intensity of the 1058 cm 1 band along the length of a microfiber identified by the dotted line in the optical image. It can be seen that the variation in SERS intensity along the length of the fiber is within (15%. The variation even along the fiber is primarily due to the small variations in focus along the length of the microfiber, suggesting the high level uniformity of the adsorption of gold nanorods on paper surface. This observation clearly suggests that optimization
of the paper morphology (i.e., smoother surface) can significantly improve the homogeneity of the SERS intensity. Raman spectra obtained using larger laser spot would enable spatial averaging of the focal variations and result in enhanced homogeneity across the substrate. Highly efficient paper-based SERS substrates can be fabricated using other types of plasmonic nanostructures. As another example, we fabricated SERS substrate using gold bipyramids. Figure 4A shows the extinction spectrum of the gold bipyramids in solution and on paper substrates. It can be seen that the bipyramids exhibit transverse and longitudinal plasmons corresponding to the two possible ways in which electrons can be polarized within these nanostructures. The plasmon bands obtained from the paper substrates exhibit a blue shift compared to those observed in solution for similar reasons discussed in the case of AuNR. Inset at the bottom of Figure 4B shows the TEM image of the gold bipyramids with a waist diameter of ∼50 nm and length of ∼140 nm. SEM image of the paper substrates exposed to gold bipyramid solution shows highly uniform adsorption of gold bipyramids over large areas with no signs of aggregation or patchiness (Figure 4B). Inset at the top shows a higher magnification image of the bipyramids on paper. SERS spectra collected from the gold bipyramid adsorbed papers exposed to various concentrations of BPE show that BPE peaks can be resolved down to the concentration of 0.5 nM, which is two times smaller than that observed in the case of the nanorods adsorbed paper (Figure 4C,D). Higher SERS efficiency and lower detection limit of the gold bipyramid adsorbed paper is possibly due to 8956
dx.doi.org/10.1021/ac2016882 |Anal. Chem. 2011, 83, 8953–8958
Analytical Chemistry
ARTICLE
Figure 4. (A) Extinction spectrum of gold bipyramid solution. (B) SEM images of gold bipyramids adsorbed on paper substrate and TEM image of gold bipyramids. (C) SERS spectra obtained from the paper substrate adsorbed with gold bipyramids as they were exposed to different concentrations BPE in ethanol. (D) Higher resolution spectra showing the detection limit down to 0.5 nM.
the slightly better match of the longitudinal plasmon band of gold bipyramids with the source laser and the antenna effect (lightning rod effect) at the sharper tips of the bipyramids.
’ CONCLUSION We have demonstrated paper as a promising platform for the fabrication of a highly efficient SERS substrate for trace chemical detection. Our simple cost-effective approach of uniform adsorption of anisotropic plasmonic nanostructures onto paper matrix enabled a SERS enhancement factor of ∼5 106, excellent homogeneity, and better sample collection efficiency compared to conventional designs.51 The results presented here lay groundwork for a novel plasmonic platform in the form of paper (i.e., plasmonic paper), which offers numerous advantages for printable microfluidic SERS/LSPR-based chemical and biological sensors. The choice of paper substrates for the implementation of SERS is driven by numerous advantages such as (i) high specific surface area resulting in large dynamic range, (ii) excellent wicking properties for rapid uptake and transport of analytes to test domains, (iii) compatibility with conventional printing approaches, enabling low-cost multianalyte SERS substrates, (iv) significant reduction in cost, (v) smaller sample volume requirement, and (vi) easy disposability. The simple approach for the fabrication of highly efficient SERS substrate is expected to have numerous opportunities in integrating SERS with other (bio)analytical platforms such as
chromatography and microfluidics. The synergism of paper-based microfluidics and SERS-based detection is expected to be truly transformative by opening up novel avenues in multianalyte chemical and biological sensing. More importantly, in the light of the recent developments related to use of paper-based diagnostics for a resource-limited setting, plasmonic paper demonstrated here opens up a novel transduction platform for such biosensors. One of the important future challenges in the efficient application of SERS for trace detection is the functionalization of plasmonic nanostructures with selective species with specific affinity to the target analyte.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional informaion as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (P.M.P.); singamaneni@ wustl.edu (S.S.).
’ ACKNOWLEDGMENT The authors thank Prof. Younan Xia from Biomedical Engineering at Washington University for providing access to the 8957
dx.doi.org/10.1021/ac2016882 |Anal. Chem. 2011, 83, 8953–8958
Analytical Chemistry confocal Raman microscopy system and Dr. Ramesh Kattumenu at Washington University for technical help. The work was supported by Army Research Office (ARO) and Army Research Lab (ARL) under Contract No. W911NF-11-2-0091.
’ REFERENCES (1) Hankus, M. E; Stratis-Cullum, D. N.; Pellegrino, P. M. Proc. SPIE-Int. Soc. Opt. Eng. 2010, 7759, 77590G-1–77590G-13. (2) Holthoff, E. L.; Stratis-Cullum, D. N.; Hankus, M. E. Proc. SPIEInt. Soc. Opt. Eng. 2010, 7665, 76650W-1–76650W-9. (3) Hankus, M. E.; Cullum, B. M. Proc. SPIE-Int. Soc. Opt. Eng. 2006, 6380, 638004-1–638004-12. (4) Hankus, M. E.; Cullum, B. M. Proc. SPIE-Smart Biomedical and Physiological Sensor Technology V 2007, 6759, 675908-1–675908-10. (5) Hankus, M. E.; Gibson, G. J.; Cullum, B. M. Proc. SPIE-Int. Soc. Opt. Eng. 2005, 6007, 600704-1–600704-11. (6) Hankus, M. E.; Li, H.; Gibson, G. J.; Cullum, B. M. Anal. Chem. 2006, 78, 7535–7546. (7) Jian, S.; Hankus, M. E.; Cullum, B. M. Proc. SPIE-Int. Soc. Opt. Eng. 2009, 7313, 73130K-1–73130K-10. (8) Efrima, S.; Zeiri, L. J. Raman Spectrosc. 2009, 40, 277–288. (9) Guzelian, A. A.; Sylvia, J. M.; Janni, J. A.; Clauson, S. L.; Spencer, K. M. Proc. SPIE-Vib. Spectrosc.-Based Sens. Syst. 2002, 4577, 182–192. (10) Jarvis, R. M.; Goodacre, R. Chem. Soc. Rev. 2008, 37, 931–936. (11) Liu, Y. L.; Chen, Y. R.; Nou, X. W.; Kim, M. S.; Chao, K. L. Spectroscopy 2008, 23, 48–54. (12) Tripp, R. A.; Dluhy, R. A.; Zhao, Y. P. Nano Today 2008, 3, 31–37. (13) Guicheteau, J.; Argue, L.; Emge, D.; Hyre, A.; Jacobson, M.; Christesen, S. Appl. Spectrosc. 2008, 62, 267–272. (14) Driskell, J. D.; Shanmukh, S.; Liu, Y. J.; Hennigan, S.; Jones, L.; Zhao, Y. P.; Dluhy, R. A.; Krause, D. C.; Tripp, R. A. IEEE Sens. J. 2008, 8, 863–870. (15) Docherty, F. T.; Monaghan, P. B.; McHugh, C. J.; Graham, D.; Smith, W. E.; Cooper, J. M. IEEE Sens. J. 2005, 5, 632–640. (16) Holthoff, E. L.; Stratis-Cullum, D. N.; Hankus, M. E. Proc. of SPIE-Chem., Biol., Radiol., Nucl., Explos. (Cbrne) Sensing Xi 2010, 7665, 76650W-1–76650W-9. (17) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. Phys. 1999, 247, 155–162. (18) Kenipp, K.; Kneipp, H.; Bohr, H. G. Top. Appl. Phys. 2006, 103, 261–277. (19) Kneipp, J.; Kneipp, H.; Kneipp, K. Chem. Soc. Rev. 2008, 37, 1052–1060. (20) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102–1106. (21) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Small 2008, 4, 1576. (22) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536–1540. (23) Kneipp, K.; Haka, A. S.; Kneipp, H; Badizadegan, K; Yoshizawa, N.; Boone, C.; Shafer-Peltier, K. E.; Motz, J. T.; Dasari, R. R.; Feld, M. S. Appl. Spectrosc. 2002, 56, 150–154. (24) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (25) Eustis, S.; El-Sayed, M. A. Chem. Soc. Rev. 2006, 35, 209–217. (26) Kneipp, J; Kneipp, H.; McLaughlin, M.; Brown, D.; Kneipp, K. Nano Lett. 2006, 6, 2225–2231. (27) Freeman, R. G; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629–1632. (28) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735–743. (29) Jarvis, R. M.; Goodacre, R. Anal. Chem. 2004, 76, 40–47. (30) Doering, W. E.; Nie, S. M. J. Phys. Chem. B 2002, 106, 311–317. (31) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485–496. (32) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569–1574. (33) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267–297.
ARTICLE
(34) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 2002, 106, 9463–9483. (35) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426–7433. (36) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279–11285. (37) Qin, L. D.; Zou, S.; Xue, C.; Atkinson, A.; Schatz, G. C.; Mirkin, C. A. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 13300–13303. (38) Li, H. G.; Baum, C. E.; Sun, J.; Cullum, B. M. Proc. SPIE-Chem. Biol. Sensing VII 2006, 6218, 621804-1–621804-11. (39) Hulteen, J. C.; Young, M. A.; Van Duyne, R. P. Langmuir 2006, 22, 10354–10364. (40) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Annu. Rev. Anal. Chem. 2008, 1, 601–626. (41) Litorja, M.; Haynes, C. L.; Haes, A. J.; Jensen, T. R.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 6907–6915. (42) Felidj, N.; Lau Truong, S.; Aubard, J.; Levi, G.; Krenn, J. R.; Hohenau, A.; Leitner, A.; Aussenegg, F. R. J. Chem. Phys. 2004, 120, 7141–7146. (43) Haynes, C. L.; Yonzon, C. R.; Zhang, X. Y.; Van Duyne, R. P. J. Raman Spectrosc. 2005, 36, 471–484. (44) Lu, Y.; Liu, G. L.; Lee, L. P. Nano Lett. 2005, 5, 5–9. (45) Yonzon, C. R.; Stuart, D. A.; Zhang, X.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. Talanta 2005, 67, 438–448. (46) Braun, G.; Lee, S. J.; Dante, M.; Nguyen, T. Q.; Moskovits, M.; Reich, N. J. Am. Chem. Soc. 2007, 129, 6378–6379. (47) Klutse, C. K.; Li, H.; Cullum, B. M. Proc. SPIE-Smart Biomed. Physiol. Sens. Technol. VI 2009, 7313, 73130C-1–73130C-10. (48) Li, H.; Baum, C. E.; Cullum, B. M. Proc. SPIE-Smart Med. Biomed. Sens. Technol. IV 2006, 6380, 63800D-1–63800D-112. (49) Cullum, B. M.; Li, H.; Schiza, M. V.; Hankus, M. E. Nanobiotechnolgy 2007, 3, 1–11. (50) Malshe, A. P.; Virwani, K.; Rajurkar, K. P.; Deshpande, D. CIRP Ann.-Manuf. Technol. 2005, 54, 175–178. (51) Lee, C. H.; Tian, L.; Singamaneni, S. ACS Appl. Mater. Interfaces 2010, 2, 3429–3435. (52) Yu, W. W.; White, I. M. Anal. Chem. 2010, 82, 9626–9630. (53) http://www.whatman.com/CelluloseFilters.aspx. (54) Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Thomas, S. W., III; Sindi, H.; Whitesides, G. M. Anal. Chem. 2008, 80, 3699–3707. (55) Ellerbee, A. K.; Phillips, S. T.; Siegel, A. C.; Mirica, K. A.; Martinez, A. W.; Striehl, P.; Jain, N.; Prentiss, M.; Whitesides, G. M. Anal. Chem. 2009, 81, 8447–8452. (56) Samir, M. A. S. A.; Alloin, F.; Dufresne, A. Biomacromolecules 2005, 6, 612–626. (57) Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Whitesides, G. M. Anal. Chem. 2010, 82, 3–10. (58) Carrilho, E.; Martinez, A. W.; Whitesides, G. M. Anal. Chem. 2009, 81, 7091–7095. (59) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (60) Jain, P. K.; El-Sayed, M. A. Nano Lett. 2008, 8, 4347–4352. (61) Koo, H. Y.; Choi, W. S.; Kim, D.-Y. Small 2008, 4, 742–745. (62) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110–1121. (63) Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Angew. Chem., Int. Ed. 2007, 46, 1318–1320. (64) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Chem. Rev. 2010, 110, 3479–3500. (65) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; Levi, S. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; M€oller, M.; Gittins, D. I. Phys. Rev. Lett. 2002, 89, 203002-1–203002-4. (66) Singamaneni, S.; Jiang, C.; Merrick, E.; Kommireddy, D.; Tsukruk, V. V. J. Macromol. Sci., Part B: Phys. 2007, 46, 7–19. (67) Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17930–17935. (68) Hankus, M. E.; Stratis-Cullum, D. N.; Pellegrino, P. M. ARL-TR-4957 September 2009.
8958
dx.doi.org/10.1021/ac2016882 |Anal. Chem. 2011, 83, 8953–8958
