Technical Note pubs.acs.org/ac
Identifying Pigment Mixtures in Art Using SERS: A Treatment Flowchart Approach Joo Yeon Roh,† Mary K. Matecki,† Shelley A. Svoboda,‡ and Kristin L. Wustholz*,† †
Department of Chemistry, The College of William and Mary, Williamsburg, Virginia 23187, United States Department of Conservation, The Colonial Williamsburg Foundation, Williamsburg, Virginia 23187, United States
‡
S Supporting Information *
ABSTRACT: A novel treatment flowchart approach for surface-enhanced Raman scattering (SERS) is used to identify both blue and yellow organic pigments in a single microscopic sample from a series of reference oil paints as well as an actual 18th century oil painting. In particular, several treatment strategies using acids and solvents are integrated into a specific flowchart designed to enable the minimally invasive identification of unknown blue (i.e., indigo, Prussian blue) and yellow organic (i.e., Reseda lake, Stil de Grain, gamboge) pigments in one sample. We demonstrate the first successful identification of a yellow lake pigment in a historic painting using SERS as well as the utility of our treatment flowchart approach for identifying pigments of varying resonance conditions, surface affinities, and treatment requirements in a single microscopic sample from a historic oil painting.
A
in historic oil paintings represents one of the most difficult challenges in conservation. Oil paint is a complex mixture of organic and inorganic colorants, binders, varnishes, and glues that are all present within an exceedingly limited sample size (i.e., approximately ng scale). The natural aging, degradation, and restoration processes further complicate attempts at pigment identification. Although previous SERS studies have demonstrated the unambiguous identification of individual organic pigments in minute samples from historic oil paintings,4,6,7,10,12−14,17,19,22,27−29 existing SERS methodologies do not provide for the identification of pigment mixtures in oil paint. Because paint is an amalgamation of organic and inorganic pigments of various hues, chemical properties, and photosensitivities, the development of SERS into a reliable, broad-spectrum method for art conservation requires the ability to detect pigment mixtures. For example, artists and suppliers frequently mixed blue pigments (e.g., indigo, Prussian blue) with yellow organic pigments (e.g., Reseda lake from weld, Stil de Grain lake from Buckthorn berries, and gamboge resin) to obtain green hues in oil paintings.30,31 Because of the fugitive nature of the yellow organic pigments in these green pigment mixtures, the identification of both blue and yellow colorants in a single minute sample is especially important and challenging. Colorant detection using SERS is dependent on the properties of the enhancing substrate, resonance Raman effects (i.e., pigment hue), and the affinity of the pigment for the metal
n ideal analytical method for identifying pigments in historic oil paintings should be exceedingly sensitive, selective, minimally invasive, and applicable to a wide range of color bodies. However, analytical techniques such as UV/vis, fluorimetry, high-performance liquid chromatography (HPLC), reflectance spectroscopy, and Raman spectroscopy have significant disadvantages for the study of pigments in art that include a large sample requirement (i.e., ∼1 mg for HPLC), poor sensitivity, poor selectivity, and the inability to definitively identify many natural organic colorants. For example, although Raman spectroscopy measures the unique vibrational fingerprint of analytes,1,2 Raman cross sections are modest, and strong molecular fluorescence from natural organic dyes and pigments often precludes the measurement of Raman scattering.3 To take advantage of the selectivity of Raman while simultaneously circumventing the problems of molecular fluorescence and weak scattering signals, recent studies have focused on surface-enhanced Raman scattering (SERS) methods to detect colorants in art.3−8 SERS is especially effective for the ultrasensitive detection of organic chromophores because the noble metal substrate both enhances the Raman signal while quenching competing molecular fluorescence.9 Indeed, SERS has proven to be a reliable and effective method for the identification of red anthraquinones (e.g., carmine lake),5,10−14 yellow flavonoids, curcuminoids, alkaloids, anthraquinones (e.g., Reseda lake,8,15−17 berberine18), and blue indigoids (e.g., indigo,19,20 Tyrian purple21) in minute samples from cultural heritage objects.6,8,21−26 Although SERS studies of art objects have demonstrated high sensitivity and selectivity, the identification of organic pigments © 2016 American Chemical Society
Received: January 5, 2016 Accepted: January 22, 2016 Published: January 22, 2016 2028
DOI: 10.1021/acs.analchem.6b00044 Anal. Chem. 2016, 88, 2028−2032
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substrate.3,9,32−34 Because SERS requires close contact between the analyte and the enhancing substrate, sample treatment strategies are often required to solubilize pigments and facilitate adsorption of chromophores to the metal surface. For example, improvements in SERS sensitivity are observed when red lake pigments are pretreated with HF to hydrolyze the water-soluble dye from the insoluble lake pigment.12,28,35 High-quality SERS spectra of indigo and Prussian blue are elucidated from paintings following sample pretreatment with H2SO4.19 In this approach, sulfuric acid acts to solubilize Prussian blue as well as perform an in situ conversion of insoluble indigo to soluble indigo carmine. More recently, we established a HCl/MeOH treatment method to identify several yellow organic dyestuffs and pigments (i.e., turmeric, old fustic, Buckthorn berries, Reseda lake, Stil de Grain) in paint using SERS.16 However, this treatment procedure is ineffective for gamboge, a well-known and frequently used yellow resinous pigment.36 Moreover, when samples containing yellow lake pigments are treated with sulfuric acid, the colorant is irreversibly degraded and cannot be identified using SERS. These observations suggest that different classes of pigments require different pretreatment strategies. Thus, the successful identification of colorants with various optical and chemical properties in a single microscopic paint sample is quite challenging. Consequently, SERS has identified more than one pigment in a single sample only under special circumstances: (1) when the mixture is comprised of red pigments with similar surface affinity and solubility (e.g., alizarin, purpurin, and carminic acid,4,10,12 Indian purple and vermillion27), (2) when a single treatment protocol works for two pigments of similar hue (i.e., Prussian blue and indigo),19 or (3) in textiles, where relatively large samples can be obtained.8,20 In the ideal scenario, SERS will identify multiple pigments in a single microscopic paint sample rather than merely the predominant signal of one component due to resonance, solubility, or surface-affinity effects. To identify pigment mixtures in real, complex oil paint samples, we focus on integrating multiple treatment strategies to develop a novel flowchart approach for SERS. In particular, motivated by the problem of fading in paints containing yellow organic pigments,30 we investigate a series of green reference paints comprised of optical mixtures of blue (i.e., indigo, Prussian blue) and yellow organic pigments (i.e., Reseda lake, Stil de Grain lake, and gamboge) using SERS. Both novel and previously reported16,19 treatment strategies are integrated into a reliable flowchart protocol for identifying both blue and yellow organic pigments in a single minute paint sample using SERS. In addition to a series of reference paints, we analyze a sample from the Portrait of Elizabeth Burwell Nelson (Mrs. William Nelson) by Robert Feke, a painting from the Colonial Williamsburg Foundation collection that is by the earliest native-born American artist of European descent. The SERS treatment flowchart approach provides a uniquely sensitive and specific methodology for conservation science that is able to identify pigment mixtures in a single microscopic sample from reference paints as well as actual art. Ultimately, this work demonstrates the first SERS-based detection of a yellow lake pigment in a historic oil painting as well as the utility of our treatment flowchart approach for identifying pigments of varying resonance conditions, surface affinities, and treatment requirements in a single microscopic art sample.
Technical Note
EXPERIMENTAL SECTION
Materials and Art Objects. Yellow organic pigments, natural indigo, Prussian blue, and cold-pressed linseed oil were used as received from Kremer Pigments (New York, NY). Acids and reagent-grade solvents were obtained from Fisher Scientific. Green reference paints comprised of various yellow and blue pigments were prepared in linseed oil on a glass plate with a glass muller and painted onto glass microscope slides (Fisher). Microscopic samples were obtained from reference and historic paints using surgical blades (Feather Safety Razor Company, #15). Sample sizes were measured using a color camera (Edmund Optics, EO-0413C) and were on the order of ∼10 and ∼100 μm for art and reference samples, respectively. Samples from the rose stem in Portrait of Elizabeth Burwell Nelson (Mrs. William Nelson) (by Robert Feke, oil on canvas, probably 1749−1751, 49-5/8 × 39-5/8″; CWF 1986-246) were investigated. Nanoparticle Synthesis and Sample Treatment. Glassware was cleaned with aqua regia and thoroughly rinsed with deionized water (Fisher EasyPure, 18.2 MΩ cm). Silver nitrate (Alfa Aesar, 99.9999%) and sodium citrate (Fisher Scientific) were used to synthesize citrate-reduced silver colloids.37 The colloids were centrifuged (Eppendorf, MiniSpin, 1 mL aliquots with ∼0.98 mL of supernanant removed) for two cycles at a relative centrifugal force of ∼12000g at 15 min per cycle to concentrate the colloids and remove excess citrate. Sample treatment of paints containing insoluble organic pigments is required before proceeding with SERS measurements. Solvent extraction of gamboge-containing samples is accomplished using acetonitrile (ACN) as follows.38 A microscopic paint sample containing gamboge was treated with 4 μL of a 1:3 solution of H2O:ACN and mixed using a micropipetter. Next, 1.0 μL aliquots of the resulting solution were spotted onto a clean glass coverslip (Fisher), and 0.5 μL of centrifuged silver colloids were immediately deposited atop the extracted solutions. Microscopic paint samples containing yellow lake pigments (i.e., Stil de Grain, Reseda lake) were treated with 5 μL of 1:10 HCl:MeOH for 6 min in a covered dish. Next, 0.2 μL aliquots of the resulting solution were spotted onto a clean glass coverslip and allowed to dry for 24 h. To the dried spots were applied 0.75 μL of centrifuged silver colloids. Following the identification of yellow pigments in microscopic samples of green paint using SERS, the remaining (undissolved) paint sample was treated with concd H2SO4 to solubilize the blue pigment, consistent with our previously reported procedure.19 Instrumentation. SERS measurements were performed on an inverted microscope (Nikon, TiU) as described elsewhere.13,16,19,39 Briefly, excitation at 632.8 nm from a HeNe laser (Research Electro-Optics, LHRP-1701) was filtered (Semrock, LL01-633-25) and focused to the sample using a 20× objective (Nikon CFI, N.A. = 0.5). Scattering from the sample was filtered (Semrock, LP02-633RS-25) and focused to the entrance slit of the spectrograph (Princeton Instruments, SP2356, 600 g mm−1 grating blazed at 500 nm). The observed Raman frequencies were calibrated using a cyclohexane standard. Typical excitation powers (Pexc) of 10−30 μW and 30 μW to 2 mW were used for SERS and normal Raman measurements, respectively. Acquisition times were varied from 30−120 s for SERS measurements to maximize signal while avoiding damage of the photosensitive yellow pigments. 2029
DOI: 10.1021/acs.analchem.6b00044 Anal. Chem. 2016, 88, 2028−2032
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RESULTS AND DISCUSSION Previous SERS studies of yellow lake pigments and indigo have demonstrated the need for sample pretreatment with various solvents and acids to obtain high-quality, reproducible SERS spectra of these insoluble colorants.16,19 However, the direct application of these methods to green paint samples containing optical mixtures of both blue and yellow pigments can reveal the identity of one pigment but not both. For example, H2SO4 treatment of indigo-containing green paint reveals the SERS signal of indigo but not the yellow organic component (Supporting Information). Thus, to identify both blue and yellow organic pigments, we developed a sample treatment flowchart that integrates both novel and previously reported16,19 procedures to provide stepwise instructions for the unambiguous identification of both pigments in a single microscopic paint sample. Figure 1 shows the experimental
series of green reference paints comprised of optical mixtures of blue and yellow organic pigments bound in linseed oil. Figure 2 presents the SERS spectra of green paints containing indigo with Stil de Grain, Reseda lake, or gamboge
Figure 2. SERS spectra of reference green paints containing indigo with (a) Reseda lake obtained after treatment with HCl/MeOH (i.e., Step 2A). Labeled peaks are consistent with Reseda lake;16 (b) Stil de Grain obtained after Step 2A with peaks consistent with Stil de Grain;16 (c) gamboge obtained after treatment with ACN/H2O (i.e., Step 2B), exhibiting peaks consistent with gamboge.1,2,15 (d) Corresponding normal Raman spectrum of green paint containing indigo and gamboge with modest peaks at 1628 and 1591 cm−1 indicating the presence of gamboge.1,2 (e) Representative SERS spectrum of a green paint containing indigo obtained following H2SO4 treatment (i.e., Step 3). Labeled peaks are consistent with converted indigo carmine.19 (f) SERS spectrum of blank silver colloids. Discriminant peaks for the pigments are labeled, and peaks due to citrate are indicated with asterisks.
Figure 1. Sample treatment flowchart for SERS studies of the blue and yellow organic pigments that comprise a single microscopic green paint sample.
flowchart for SERS studies of green paints containing blue pigments (i.e., indigo, Prussian blue) and yellow organic pigments (i.e., Reseda lake, Stil de Grain, gamboge). In the first step, a normal Raman measurement is performed, which serves two purposes: (1) the potential identification of an inorganic blue pigment (e.g., Prussian blue) and (2) to determine which treatment approach (i.e., Step 2A or Step 2B) to pursue for identification of the yellow organic component. That is, if broad molecular fluorescence is observed during the initial normal Raman experiment, users will proceed to Step 2A (i.e., treatment with HCl/MeOH to identify the yellow lake pigments). Otherwise, modest Raman scattering from gamboge at ∼1633 and ∼1592 cm−1 is observed,1,2 and users will proceed to Step 2B (i.e., treatment with ACN/H2O to extract gamboge). Following sample treatment in Step 2A or 2B and subsequent identification of the yellow organic pigment using SERS, users proceed to Step 3 (i.e., treatment with H2SO4 to identify indigo) or stop if indigo is not indicated. Both normal Raman and visible reflectance spectroscopy40,41 can indicate the presence of indigo in a noninvasive manner. However, the presence of the yellow organic pigments in these mixtures prevents the unambiguous identification of indigo and/or the corresponding yellow component using either approach (Supporting Information). To highlight the advantages of the SERS treatment flowchart approach, we first investigated a
obtained using the treatment flowchart approach. For paint samples containing Reseda lake and Stil de Grain, attempts at normal Raman measurements during Step 1 reveal broad molecular fluorescence. Therefore, according to the treatment flowchart (Figure 1), Step 2A is performed on these samples. Panels a and b in Figure 2 show the resulting SERS spectra of green paint containing indigo/Reseda lake and indigo/Stil de Grain, respectively, obtained following sample treatment with HCl/MeOH (i.e., Step 2A). Discriminant SERS peaks for Reseda lake are observed at 1582, 1504, 1299, 1129, 738, 695, 604, 532, and 420 cm−1.16 The reference green paint containing Stil de Grain exhibits discriminant SERS bands at 1594, 1520, 1349, 1210, 1101, 678, 590, 479, 460, and 411 cm−1 consistent with Stil de Grain pigment and paint.16 For paint samples containing gamboge, attempts at normal Raman measurements during Step 1 reveal modest Raman scattering at 1628 and 1591 cm−1, indicating the presence of gamboge. Therefore, the reference green paint containing gamboge is treated with ACN/H2O (i.e., Step 2B) to extract the principal chromophore 2030
DOI: 10.1021/acs.analchem.6b00044 Anal. Chem. 2016, 88, 2028−2032
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Analytical Chemistry
Figure 3. (a) Portrait of Elizabeth Burwell Nelson (Mrs. William Nelson) by Robert Feke, probably 1748−1750, CWF 1986-246. (b) Photomicrograph of the rose stem held by the sitter that now appears blue, indicating the presence of a yellow organic pigment that has undergone fading. Paper label marks approximate sample location. (c) Corresponding SERS spectrum of the sample obtained after treatment with HCl/MeOH (i.e., Step 2A) is presented with the SERS spectra of (d) green paint containing Reseda lake obtained after Step 2A and (e) blank silver colloids. Labeled peaks are discriminant bands for Reseda lake (RL) and Prussian blue (PB). Asterisks denote bands due to adsorbed citrate.
pigment that has faded with time and exposure to light. Indeed, microscopic examination of an associated cross-section sample revealed the presence of blue and, to a lesser extent, yellow colorants. To identify these components, we examined a microscopic sample from the stem using the SERS treatment flowchart approach. During Step 1, the sample exhibited normal Raman peaks at 2097, 505, and 456 cm−1, consistent with previous studies of Prussian blue.1,2,19 In yellow regions of the art sample visualized under the Raman microscope, molecular fluorescence is observed with no evidence for normal Raman scattering from gamboge. Therefore, consistent with the treatment flowchart, the sample was treated with HCl/ MeOH (i.e., Step 2A). Figure 3c presents the resulting SERS spectrum of the art sample following Step 2A, which exhibits bands at 1585, 1494, 1296, 1118, 735, 607, and 429 cm−1, consistent with the reference spectrum of Reseda lake (Figure 3d) and previous SERS investigations.16 The spectrum in Figure 3c also reveals peaks due to Prussian blue. To our knowledge, this data represents the first identification of a yellow lake pigment in a historic painting and the first SERS identification of multiple pigments with different hue, chemical, and electronic properties in a single microscopic art sample. Although the presence of indigo was not indicated (i.e., the Raman spectrum of Prussian blue is evident throughout the experiment), we tested the sample using Step 3 for completeness and found no evidence of indigo (Supporting Information). The treatment flowchart approach for SERS is a powerful tool for the study of pigment mixtures in reference paints as well as actual art. Using this method, an exceptionally small sample is adequate to provide definitive identification of both blue and yellow organic pigments. These results highlight the need for further studies of mixed pigment samples to generate a comprehensive SERS flowchart for various dyestuffs and pigments that will address numerous questions in conservation. Ultimately, this work demonstrates the utility of our treatment flowchart approach for identifying pigments of varying resonance conditions, surface affinities, and treatment requirements in a single microscopic art sample using SERS.
(i.e., gambogic acid). Panels c and d in Figures 2 present the resulting SERS spectrum and the normal Raman spectrum of green paint containing indigo/gamboge, respectively. The data demonstrate that significant signal enhancement is observed for gamboge upon sample treatment in Step 2B (i.e., from approximately 30 to 5000 counts mW−1 s−1 for the ∼1590 cm−1 band of gamboge). Moreover, additional discriminant bands for gamboge at 1290, 1211, 756, 578, and 538 cm−1 are observed following sample treatment, consistent with previous normal Raman1,2 and SERS studies.15 The SERS spectrum of gamboge presented in Figure 2c is in excellent agreement with previous Raman studies1,2 with peaks at approximately 1410, 1365, 1300, and 923 cm−1 attributed to adsorbed citrate from the SERS substrate (i.e., citrate-reduced silver colloids).10,14,16,19,27,42,43 After identifying the yellow organic pigments in samples from the green reference paints, Step 3 (i.e., treatment with H2SO4 to convert insoluble indigo to soluble indigo carmine) is performed. Figure 2e presents a representative SERS spectrum of a green paint sample after Steps 2 and 3 have been performed. Major SERS peaks for converted indigo carmine at 1695, 1632, 1584, 1450, 1360, 1316, 1248, 1147, 863, 726, 632, 603, 574, and 555 cm−1 are observed, consistent with previous calculations44 and SERS measurements.44−46 In all cases, the experimental treatment flowchart (Figure 1) provides for the unambiguous identification of both indigo and Reseda lake, Stil de Grain, or gamboge in a single microscopic paint sample. Corresponding SERS studies of reference green paints comprised of Prussian blue and yellow organic pigments are presented in the Supporting Information. Briefly, high-quality SERS spectra of Stil de Grain, Reseda lake, and gamboge in these mixed paints are readily obtained using the treatment flowchart methodology. The Portrait of Elizabeth Burwell Nelson (Mrs. William Nelson) (Figure 3a) by Robert Feke is an oil painting by the earliest native-born American artist of European descent. Figure 3b shows a photomicrograph of the sitter’s hand holding a rose stem, which now appears blue. The blue hue of the rose stem suggests the presence of a photosensitive yellow organic 2031
DOI: 10.1021/acs.analchem.6b00044 Anal. Chem. 2016, 88, 2028−2032
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(21) Bruni, S.; Guglielmi, V.; Pozzi, F. J. Raman Spectrosc. 2010, 41, 175−180. (22) Brosseau, C. L.; Rayner, K. S.; Casadio, F.; Grzywacz, C. M.; Duyne, R. P. Anal. Chem. 2009, 81, 7443−7447. (23) Harroun, S. G.; Bergman, J.; Jablonski, E.; Brosseau, C. L. Analyst 2011, 136, 3453−3460. (24) Jurasekova, Z.; del Puerto, E.; Bruno, G.; Garcia-Ramos, J. V.; Sanchez-Cortes, S.; Domingo, C. J. Raman Spectrosc. 2010, 41, 1165− 1171. (25) Leona, M.; Lombardi, J. R. J. Raman Spectrosc. 2007, 38, 853− 858. (26) Ajiki, H.; Pozzi, F.; Huang, L. L.; Massa, L.; Leona, M.; Lombardi, J. R. J. Raman Spectrosc. 2012, 43, 520−525. (27) Brosseau, C. L.; Casadio, F.; Van Duyne, R. P. J. Raman Spectrosc. 2011, 42, 1305−1310. (28) Pozzi, F.; Lombardi, J. R.; Bruni, S.; Leona, M. Anal. Chem. 2012, 84, 3751−3757. (29) Canamares, M. V.; Leona, M.; Bouchard, M.; Grzywacz, C. M.; Wouters, J.; Trentelman, K. J. Raman Spectrosc. 2010, 41, 391−397. (30) Harley, R. D. Artists’ Pigments c. 1600−1835, 2nd ed.; Archetype Publications: London, 1982. (31) Berrie, B. In Artists’ Pigments: A Handbook of Their History and Use; Fitzhugh, E., Ed.; Oxford University Press: National Gallery of Art, 1997. (32) Dieringer, J. A.; Wustholz, K. L.; Masiello, D. J.; Camden, J. P.; Kleinman, S. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2009, 131, 849−854. (33) Sisco, P. N.; Murphy, C. J. J. Phys. Chem. A 2009, 113, 3973− 3978. (34) Pierre, M. C. S.; Mackie, P. M.; Roca, M.; Haes, A. J. J. Phys. Chem. C 2011, 115, 18511−18517. (35) Teslova, T.; Corredor, C.; Livingstone, R.; Spataru, T.; Birke, R. L.; Lombardi, J. R.; Canamares, M. V.; Leona, M. J. Raman Spectrosc. 2007, 38, 802−818. (36) Eastaugh, N.; Walsh, V.; Chaplin, T.; Siddall, R. Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments; Butterworth-Heinemann: Oxford, 2008. (37) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391−3395. (38) Hou, W.; Chen, B.; Xiao, W. Zhongguo Zhong Yao Za Zhi 2011, 36, 775−779. (39) Mayhew, H. E.; Frano, K. A.; Svoboda, S. A.; Wustholz, K. L. J. Chem. Educ. 2015, 92, 148−152. (40) Miliani, C.; Romani, A.; Favaro, G. Spectrochim. Acta, Part A 1998, 54, 581−588. (41) Leona, M.; Winter, J. Stud. Conserv. 2001, 46, 153−162. (42) Canamares, M. V.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Appl. Spectrosc. 2006, 60, 1386−1391. (43) Sanchez-Cortes, S.; Garcia-Ramos, J. V. J. Raman Spectrosc. 1998, 29, 365−371. (44) Peica, N.; Kiefer, W. J. Raman Spectrosc. 2008, 39, 47−60. (45) Shadi, I. T.; Chowdhry, B. Z.; Snowden, M. J.; Withnall, R. Spectrochim. Acta, Part A 2003, 59, 2201−2206. (46) Shadi, I. T.; Chowdhry, B. Z.; Snowden, M. J.; Withnall, R. Chem. Commun. 2004, 1436−1437.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00044. Normal Raman measurements at three excitation wavelengths, reflectance studies of green and blue paints, and SERS measurements of pigment mixtures containing Prussian blue (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Fax: (757) 221-2715. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge support of this work by the Jeffress Memorial Trust (J-1027), The Eppley Foundation for Research, and the Charles Center at William and Mary for summer funding to M.K.M. through a 1943 Fiftieth Reunion Fund. Painting images are courtesy of the Colonial Williamsburg Foundation. The authors thank John A. Kean for assistance with Raman measurements.
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