Article pubs.acs.org/ac
Sample Treatment Considerations in the Analysis of Organic Colorants by Surface-Enhanced Raman Scattering Federica Pozzi,†,‡ John R. Lombardi,§ Silvia Bruni,‡ and Marco Leona*,† †
Department of Scientific Research, The Metropolitan Museum of Art, New York, United States Dipartimento di Chimica Inorganica, Metallorganica e Analitica “Lamberto Malatesta”, Università degli Studi di Milano, Milano, Italy § Department of Chemistry, The City College of New York, CUNY, New York, United States ‡
S Supporting Information *
ABSTRACT: The introduction of surface-enhanced Raman spectroscopy (SERS) in the field of cultural heritage has significantly improved the analysis of the organic dyes and their complexes that have been used as textile dyes and pigments in paintings and other polychrome works of art since antiquity. Over the last five years, a number of different procedures have been developed by various research groups. In this Article, we evaluate the effect of pretreating samples by exposing them to hydrofluoric acid (HF) vapor prior to SERS analysis, a step designed to hydrolyze the dye−metal complexes and increase analyte adsorption on the nanosized metallic support, thus enhancing the SERS signal. Materials studied include pure colorants, commercial lake pigments, and fibers from dyed textiles, as well as actual aged samples, such as microscopic fragments of lakes on paper and ancient pigments and glazes from several works of art, covering a wide range of time, from the second century B.C. to the early 20th century. In each case, SERS spectra obtained with or without HF hydrolysis were critically evaluated. The pretreatment with HF vapor resulted in faster analysis and increased sensitivity in most cases, with the exception of dyed silk fibers, where silk protein hydrolyzates were found to interfere with SERS analysis. As a final point, a two-step procedure including SERS on untreated and treated samples is proposed as a standard approach: by analyzing a sample first without hydrolysis, and then, following removal of the colloid, upon HF treatment, the best and most reliable results for a great number of dyes and substrates are assured.
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supported glucose reduction of silver sulfate in the presence of sodium citrate as a capping agent.4 This colloid has a narrow particle size range and shows very high reproducibility and stability over the time. While SERS investigations can generally be carried out directly without any pretreatment steps on a range of materials,5,6 we will demonstrate that an increase in sensitivity can be obtained by acid treating the sample prior to analysis. Most natural dyes are in fact mordant dyes, fixed to the fabric by bridging metal atoms (mordants) bound to charged groups in the dye molecule and in the textile fiber. When used in painting, the dyes are similarly complexed to metal ions to form insoluble lake pigments. In high-performance liquid chromatography (HPLC) (the preferred, albeit sample intensive method for dye analysis), dye extraction is usually performed by treatment with hydrochloric acid and methanol.7 This method results in quantitative dye removal but causes extreme degradation of the substrate itself. As a milder alternative, a nonextractive gas−solid hydrolysis procedure performed by exposing the sample to hydrofluoric acid (HF) vapor in a
ne of the most challenging topics in the scientific analysis of cultural heritage materials is the detection of organic colorants, which are usually present in works of art in mixture with other substances and in very low concentrations. As dyes can be markers for the provenance and date of a work of art, analytical methods capable of reliably identifying dyes from microscopic samples are highly desirable. Surface-enhanced Raman spectroscopy (SERS) has found increasing application for this task,1 as the adsorption of organic molecules on nanosized metal substrates significantly enhances their Raman signals and quenches their fluorescence. Several SERS approaches combined with different sample treatment procedures have been developed so far for the identification of organic colorants in ancient textiles and works of art. Citrate-reduced silver colloids obtained according to the Lee-Meisel procedure2 have been the most popular substrate for SERS of cultural heritage materials, thanks to their ease of preparation and use. Silver nanoparticles produced by photoreduction of AgNO3 solutions using a laser/micro-Raman coupled system have been also tested as an alternative SERS substrate3 for the in situ detection of flavonoid dyes on hydrophilic supports. For most of the work recently carried out at the Metropolitan Museum of Art, we have used a monodisperse silver colloid synthesized by microwave© 2012 American Chemical Society
Received: February 8, 2012 Accepted: March 30, 2012 Published: March 30, 2012 3751
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concentrated Ag colloids without any pretreatment. Particular attention was dedicated to developing a safe and convenient HF hydrolysis procedure for sample pretreatment.8 Briefly, the sample, supported in a polyethylene holder, is exposed to HF vapor for 5 min in a closed microchamber. A drop of silver colloid is added to the sample following retrieval of the sample holder from the microchamber, and the analysis is carried out immediately as the sample holder is transferred to the Raman microscope. Further details about the HF treatment are given as Supporting Information (Figure S-1). Silver colloids synthesized by microwave reduction of Ag2SO4 in the presence of glucose and sodium citrate were prepared following a previously published recipe.4 These nanoparticles will be henceforth referred to as microwave colloid. A 5 times concentrated colloid, which will be referred to as 5× microwave colloid from now on, was also prepared for nonhydrolysis experiments by replacing 900 μL of the supernatant with 100 μL of ultrapure water. All the spectra were obtained by adding to the sample (or to the reference solution) in sequence 0.8 μL of Ag colloid and 0.1 μL of a 0.5 M KNO3 solution (to induce aggregation of the colloid). SERS spectra could be observed immediately after addition of the colloid and KNO3 but generally improved in quality as aggregation proceeded, before deteriorating when the liquid was fully evaporated. As pure dye references, 0.2 μL of 10−4 M solutions of alizarin, purpurin, carminic, and laccaic acids at pH = 2 were used. Spectra without hydrolysis and upon HF treatment were generally taken from different samples. However, when the available sample was not sufficient for two experiments, i.e., in the case of oil paintings, we followed a two-step procedure, in which the sample is first analyzed without hydrolysis and then, after removing the colloid, upon HF treatment. Colloid removal can be easily accomplished, as the 0.9 μL droplet (colloid + KNO3) naturally evaporates a few minutes after it has been deposited, and the residual nanoparticles can be then washed away upon deposition of a drop of water on top of the sample. Instrumentation. SERS spectra were obtained using a Bruker Senterra Raman instrument equipped with a chargecoupled device (CCD) detector and a 1800 rulings/mm holographic grating providing a resolution of 3−5 cm−1. The 488 nm radiation emitted by a Spectra Physics Cyan solid state laser was employed as the excitation source, with a power at the sample of about 0.5 mW. All the spectra were acquired with a single integration of 30 s, focusing just below the top surface of the drop with an Olympus 20× LMPlanFL long working distance microscope objective.
closed microchamber was developed specifically for SERS analysis at the Metropolitan Museum of Art.8 In the present work, we carry out a systematic evaluation of the HF treatment method for the ultrasensitive detection of red anthraquinone colorants by SERS. In this context, a number of works of art and ancient samples representative of several cultures and belonging to different historical periods, i.e., from the second century B.C. to the early 20th century, have been analyzed, including lakes from an original Winsor & Newton catalogue of watercolors on paper, colorants from dyed fabrics, and an ancient pink lake from Greece, as well as glazes from paintings and musical instruments. Several articles on SERS and Raman analysis of anthraquinone dyes are being published every year, both to elucidate their chemical properties9−12 and to demonstrate identification using newly developed procedures.13−16 The extensive and systematic study of the HF hydrolysis step presented in this Article provides evidence of the benefits and drawbacks that this procedure offers for the identification of anthraquinone red colorants from a wide variety of reference and art samples using SERS. The results presented here can be of great help to those who are interested in applying the SERS technique to the identification of an important class of dyes widely used in works of art.
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EXPERIMENTAL SECTION Chemicals, Reference Samples, and Art Objects. Silver nitrate, sodium citrate, sulfuric acid, glucose, and hydrofluoric acid were purchased from Fisher Scientific. Alizarin, purpurin, carminic acid, laccaic acid, and potassium nitrate were obtained from Sigma-Aldrich, while madder lake (lake of alizarin/ purpurin) and carmine naccarat (aluminum lake of carminic acid) were from Kremer Pigments. All the solutions were prepared using 18 MΩ ultrapure water (Millipore Simplicity 185 water purification system). Microscopic samples of roughly equivalent size were taken from several works of art and ancient objects and subsequently investigated. Samples studied include: fragments from an original Winsor & Newton handbook of watercolor pigments dating to the 20th century, containing a wide collection of swatches on drawing paper representing the firm’s colors; fibers from textiles dyed at the Metropolitan Museum of Art for reference purposes, i.e., wool dyed with Turkish madder and lac dye and silk dyed with cochineal and lac dye; particles from a pink pigment sample found in the excavation of a second century B.C. site in Corinth, Greece; paint samples from Cézanne’s The card players (oil on canvas, 1890−1892, 65.4 × 81.9 cm, The Metropolitan Museum of Art, accession number 61.101.1), Matisse’s The young sailor (oil on canvas, 1906, 101.6 × 83.2 cm, The Metropolitan Museum of Art, accession number 1999.363.41) and from a painted cloth depicting the celebration of the festival of cows (hide glue, India, late 18th to early 19th century, 248 × 262 cm, The Metropolitan Museum of Art, accession number 2003.177); glaze samples from a laboratory reproduction of a panel from the Nur al-Din room at the Metropolitan Museum of Art (linseed oil, Damascus, Syria, 1280−1924, accession number 1970.170) and from a mandolin made by Antonio Vinaccia (tempera, Naples, Italy, 1781, W. 19.1 × L. 58.4 cm, The Metropolitan Museum of Art, accession number 89.4.2140). SERS Methods: Ag Colloid Synthesis and Sample Preparation. In this study, the pretreatment of a sample with HF has been compared against SERS on regular and
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RESULTS AND DISCUSSION Unless otherwise specified, spectra acquired following HF hydrolysis and without any pretreatment are presented in this section without modifying their relative intensities, in order to allow an easy and effective comparison of the results obtained using these two methodologies. However, some spectra displayed very low relative enhancements and, as is, would not be legible. These spectra were thus multiplied by a certain factor to help in inspection, and such factor is reported in the corresponding figure panes for clarity. Five spectra were typically collected from each sample analyzed, to ensure that the results were reproducible in terms of band position. All the figures reported in the following display the most intense spectrum obtained from each set of 3752
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measurements. The wavenumbers observed for reference colorants are listed in the figure captions along with their relative intensities, as follows: vs = very strong; s = strong; m = medium; w = weak; vw = very weak; sh = shoulder. Reference spectra of commercial pure dyes and lake pigments, appearing as dashed lines in the graphs, are reported along with those collected with and without HF treatment from the art samples investigated for comparison purposes, i.e., to provide evidence of the colorants identified. Spectra with and without hydrolysis of madder and carmine lakes purchased from Kremer Pigments are shown and discussed in the Supporting Information (Figure S-2). Spectral differences including shifts in wavenumber and changes in relative intensities can be detected for HF-treated and untreated lakes. Such spectral differences have been previously observed both for alizarin/madder lake5,16,17 and for carminic acid/ carmine lake5,6 and can be mostly attributed to the presence of alizarin and carminic acid as free dyes following hydrolysis or complexed with the inorganic components of the lake when the sample had not been treated. Winsor & Newton Lakes. Five lakes on drawing paper, namely alizarin carmine, alizarin crimson, carmine, pink madder, and purple madder (alizarin), have been examined as representative samples from a historical Winsor & Newton catalogue of watercolor pigments belonging to the 20th century collection (Supporting Information, Figure S-3). For both alizarin carmine (Figure 1) and alizarin crimson (Figure 2), no
Figure 2. (a) SERS spectrum of reference alizarin at pH = 2 compared to those of alizarin crimson from the Winsor & Newton catalogue (b) on Ag microwave colloid upon HF treatment, (c) on 5× Ag microwave colloid without hydrolysis, and (d) on regular Ag microwave colloid without hydrolysis. Spectrum (d) was multiplied by a factor of 3. For signal wavenumbers with relative intensities of reference alizarin, refer to the peak list reported in Figure 1.
in the spectra of the pigments as such compared to those taken upon hydrolysis. The investigation of Winsor & Newton carmine (Figure 3) resulted in good SERS spectra on microwave colloids without hydrolysis. These spectra match untreated carmine naccarat purchased from Kremer very well. Using the same laser power at the sample, i.e., 0.5 mW, low quality results were achieved from the analysis of such lake upon HF treatment: indeed,
Figure 1. (a) SERS spectrum of reference alizarin at pH = 2 compared to those of alizarin carmine from the Winsor & Newton catalogue (b) on Ag microwave colloid upon HF treatment, (c) on regular Ag microwave colloid without hydrolysis, and (d) on 5× Ag microwave colloid without hydrolysis. Signals of reference alizarin were detected at 1624s, 1587s, 1558s, 1448vs, 1324vs, 1287s, 1185m, 1158 m, 1046w, 1009w, 895vw, 830vw, 681w, 658w, 629w, 580vw, 470w, 416vw, 391vw, and 341w cm−1. Figure 3. (a) SERS spectrum of reference carminic acid at pH = 2 compared to those of carmine from the Winsor & Newton catalogue (b) on Ag microwave colloid upon HF treatment, (c) on 5× Ag microwave colloid without hydrolysis, and (d) on regular Ag microwave colloid without hydrolysis. Spectra (c) and (d) were multiplied by a factor of 3. Signals of reference carminic acid were detected at 1635s, 1579s, 1448vs, 1324s, 1222s, 1069m, and 449m cm−1.
description is provided in the handbook concerning their chemical composition. Both lakes gave rise to excellent SERS spectra on microwave colloids upon HF treatment, showing a good correspondence with the spectral features of alizarin solution at pH = 2. As expected, a few spectral shifts and changes in terms of relative intensities of bands were observed 3753
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although some of the main spectral features of reference carminic acid are recognizable in the spectrum, a significant broadening of signals is encountered. It can be noticed that the spectral pattern observed, being at first glance dominated by two broad lines around 1580 and 1330 cm−1, shares distinctive features with that of amorphous carbon and, therefore, a degradation of the sample under the laser beam cannot be ruled out in the present case. This phenomenon could be possibly ascribed to a particular sensitivity of carminic acid to photodegradation depending on its chemical environment, as suggested in the literature.18,19 However, as will be further discussed in the following paragraphs, materials which are expected to have an overall composition comparable to that of Winsor & Newton carmine, e.g., carminic acid containing samples from a laboratory reproduction of a Nur al-Din room panel and from an Italian mandolin, gave rise to well resolved SERS spectra following HF hydrolysis, probably due to differences in terms of binding media and substrate materials used and, more in general, in terms of formulation. Spectra obtained for pink madder and purple madder (alizarin) are reported and discussed in the Supporting Information (Figure S-4). 2nd Century B.C. Pink Lake Pigment from Corinth, Greece. The investigation of a pink lake from Corinth, Greece, dating to the second century B.C. is an effective example of the issues that might be encountered when applying different analytical methodologies to the analysis of very ancient samples. Indeed, the unknown pigment could only be identified using the HF hydrolysis procedure, which allowed us to obtain from a microscopic lake particle of about 20 × 20 μm in size an excellent SERS spectrum perfectly matching the spectral features of madder lake subjected to the same treatment. On the contrary, very poor spectra were obtained from the sample as such, displaying a significantly lower intensity and strongly dominated by the bands due to citrate ions (Figure 4). Dyed Textiles. Satisfactory SERS spectra were taken from fragments of a wool fiber dyed with Turkish madder using both methodologies compared in the present study (Figure 5). Results achieved with and without HF treatment are indeed consistent with those obtained from reference madder lake upon hydrolysis and as such, respectively. It is worth mentioning that, among the materials here investigated, this is the only case where the use of a concentrated colloid for the analysis of a sample without any preliminary treatment gave rise to a spectrum displaying a slightly higher intensity in comparison to that obtained upon hydrolysis. As far as wool dyed with lac dye is concerned (Supporting Information, Figure S-5), the HF treatment allowed us to acquire a high quality SERS spectrum which turned out to be consistent with reference laccaic acid solution at pH = 2. Good spectra, even if of lower intensity, were obtained without hydrolysis as well especially on 5× microwave colloids. An interesting situation occurred for silk fabrics, the SERS spectra of which are characterized by a remarkable band broadening when applying the HF hydrolysis procedure. In particular, some of the main spectral features of carminic acid were identified in the spectrum taken from a cochineal-dyed silk fiber upon HF treatment, even though, as already observed for Winsor & Newton carmine, the resolution of the signals was rather low (Figure 6). On the other hand, the typical signals of laccaic acid were not even detected in the spectrum of lac dye on silk after hydrolysis, as only two broad bands located around 1351 and 1575 cm−1 appeared in the spectrum (Supporting
Figure 4. (a) SERS spectrum of reference madder lake upon HF treatment compared to those of a 2nd century B.C. pink pigment from Corinth, Greece, (b) on Ag microwave colloid upon HF treatment, (c) on 5× Ag microwave colloid without hydrolysis, and (d) on regular Ag microwave colloid without hydrolysis. Spectra (c) and (d) were multiplied by a factor of 3. Spurious bands due to citrate are marked with ∗. Signals of reference madder lake were detected at 1618m, 1575s, 1473sh, 1444vs, 1401s, 1326vs, 1272s, 1158m, 1063w, 1036w, 1015m, 961w, 822vw, 691vw, 648vw, 447w, and 422w cm−1.
Figure 5. (b) SERS spectrum of reference madder lake upon HF treatment compared to those of Turkish madder on wool (a) on 5× Ag microwave colloid without hydrolysis, (c) on Ag microwave colloid upon HF treatment, and (d) on regular Ag microwave colloid without hydrolysis. Spectrum (d) was multiplied by a factor of 3. For signal wavenumbers with relative intensities of reference madder lake, refer to the peak list reported in Figure 4.
Information, Figure S-6). A slightly more detailed spectral pattern was obtained for untreated cochineal-dyed samples both on 5× and regular microwave colloids, which does not correspond to carmine naccarat as such probably due to the different kind of substrate (Figure 6). Nonhydrolysis experiments were particularly successful in the case of silk dyed with lac dye, for which a well resolved SERS spectrum was obtained 3754
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Figure 6. (a) SERS spectrum of reference carminic acid at pH = 2 compared to those of cochineal on silk (b) on Ag microwave colloid upon HF treatment, (c) on 5× Ag microwave colloid without hydrolysis, and (d) on regular Ag microwave colloid without hydrolysis. For signal wavenumbers with relative intensities of reference carminic acid, refer to the peak list reported in Figure 3.
Figure 7. The young sailor by Henri Matisse, 1906. The Metropolitan Museum of Art, 101.6 × 83.2 cm, accession number 1999.363.41. Jacques and Natasha Gelman Collection, 1998.
with several sharp signals (Supporting Information, Figure S-6). This is consistent with the higher sensitivity to hydrolysis of silk fibers compared with wool. Indeed, silk proteins released into solution are adsorbed onto the silver nanoparticles interfering with dye adsorption.20 Oil Paintings. Glaze samples of less than 20 × 20 μm in size were taken from Cézanne’s The card players (Supporting Information, Figure S-7) and Matisse’s The young sailor (Figure 7). For each painting, the same sample was analyzed first without hydrolysis using 5× colloids and, following removal of the colloid and a rinse with a drop of water, upon HF treatment. In both cases, the hydrolysis procedure was the only approach that allowed us to identify the colorants. Indeed, an excellent SERS spectrum was acquired upon HF treatment from the Cézanne’s glaze, showing a remarkable correspondence with hydrolyzed madder lake purchased by Kremer. On the other hand, several spurious bands were noticed in the spectrum collected from the sample as such due to the interference of citrate ions, even if signals attributable to madder lake can be recognized (Supporting Information, Figure S-8). As far as Matisse’s The young sailor is concerned, carmine, or in general a carminic acid containing pigment, was detected as being responsible for the color of the glaze sample taken from the pink background of the painting. While no results could be obtained using the nonhydrolysis procedure, the HF approach allowed us to observe for the sample under investigation a well resolved spectrum of carmine lake (Figure 8). Although higher aggregation degrees are usually achieved when adding an electrolyte to the dye−colloid system, leading to improved signal enhancements, the addition of KNO3 resulted, in the present case, in a remarkable quenching of the SERS intensity. Therefore, for Matisse’s sample, a spectrum without KNO3 was also taken and this led one to observe a significantly higher enhancement. Moreover, it should be pointed out that the spectrum recorded upon HF treatment shows a greater similarity with the one acquired from reference
untreated carmine lake rather than to that of carminic acid. This could be ascribed to the fact that HF, which attacks the paint network surrounding the lake particles, exposing them and allowing adsorption on silver, may not be able, depending on
Figure 8. SERS spectra of (a) reference carminic acid at pH = 2 and (b) reference carmine lake without hydrolysis compared to those of a red glaze from Matisse’s The young sailor on Ag microwave colloid upon HF treatment (c) without and (d) with KNO3. Spectrum (d) was multiplied by a factor of 3. For signal wavenumbers with relative intensities of reference carminic acid, refer to the peak list reported in Figure 3. Bands of reference carmine lake without hydrolysis were observed at 1639s, 1463s, 1297vs, 1106vw, 1079vw, 1025vw, 955vw, 799vw, 656w, and 463w cm−1. 3755
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colorants from a number of ancient samples and works of art covering a period of time of about 2200 years. The experiments here conducted showed that spectra obtained without hydrolysis generally display lower intensity and worse signal-to-noise ratios in comparison to those taken upon HF treatment. Moreover, issues of reproducibility and variability of relative intensities of signals were sometimes encountered, even within the same measurement. Also, when dealing with very ancient samples, i.e., the second century B.C. pink pigment from Corinth, as well as in the case of Cézanne’s The card players, the nonhydrolysis methodology gave rise to spectra strongly affected by bands due to the colloid even when nanoparticles of regular concentration were used. On the other hand, this approach was found to be particularly suitable for dyed silk, for which very poor spectra were collected upon HF treatment probably due to the interference of proteins released into solution following hydrolysis. Also, the use of the HF procedure occasionally led to low quality results for carminic acid containing samples: in particular, for Winsor & Newton carmine, significant phenomena of band broadening were encountered, even if the laser power employed was rather low. Nevertheless, well resolved spectra could be obtained using the same methodology for other materials which were expected to have a similar composition to Winsor & Newton carmine, such as samples from a laboratory reproduction of a Nur al-Din room panel and from Vinaccia’s mandolin. The remarkable variability encountered in SERS results could be perhaps attributed to differences in terms of formulation and chemical composition of each lake, resulting in competitive adsorption on silver of certain constituents of the paint network surrounding the lake particles, which may cause significant interferences in the spectra. However, except for a few cases discussed above, notably silk textiles, the HF treatment has proven to be a very effective method, leading one to achieve a reliable fingerprint for the great majority of samples examined within very short times of analysis. The introduction of an additional step in the analytical procedure, i.e., the HF hydrolysis, does not result in slower analysis when compared to the nonhydrolysis approach: in fact, well resolved and reproducible spectra with high enhancements were obtained in less than 30 min from sampling in all cases, while several minutes were needed just for nanoparticle aggregation when the HF step was not used. In conclusion, because of the relative merits of both procedures and their mutual compatibility, it is recommended to adopt a two-step procedure for analysis of unknown samples. By analyzing a sample first without the hydrolysis step and then removing the colloid and exposing the same sample to HF, the best and most consistent results for a variety of dyes and substrates are assured.
the composition of each lake sample, to completely break the bond between the organic dye and its substrate in the complex. Nur al-Din Room Panel Reproduction, Mandolin, and Painted Cloth. Carminic acid was found to be responsible for the reddish color of samples taken from the laboratory reproduction of the Nur al-Din room panel (Supporting Information, Figure S-9) and the Italian mandolin (Supporting Information, Figure S-10): SERS spectra obtained upon HF treatment, better resolved and of higher quality with respect to those acquired from the glazes as such, showed in both cases a good correspondence with the reference spectrum of carminic acid solution at pH = 2. Spectra taken from both samples without hydrolysis match the spectral features of untreated carmine naccarat purchased from Kremer. Such spectra generally display lower enhancements in comparison to those obtained upon HF treatment, with the only exception of a sample taken from the Vinaccia mandolin, which gave rise, on 5× microwave colloids, to a SERS spectrum as intense as the one collected after hydrolysis (Supporting Information, Figures S-11 and S-12). An excellent SERS spectrum was obtained from a sample taken from the painted cloth depicting the celebration of the festival of cows (Supporting Information, Figure S-13) upon HF treatment, showing a remarkable similarity with that of laccaic acid solution at pH = 2; spectra of lower intensity and worse signal-to-noise ratio were collected without hydrolysis both on 5× and regular microwave colloids (Figure 9).
Figure 9. (a) SERS spectrum of reference laccaic acid at pH = 2 compared to those of a red glaze sample from the painted cloth (b) on Ag microwave colloid upon HF treatment, (c) on 5× Ag microwave colloid without hydrolysis and (d) on regular Ag microwave colloid without hydrolysis. Signals of reference laccaic acid were detected at 1633sh, 1577s, 1461vs, 1367m, 1324m, 1282m, 1226s, 1187sh, 1096m, 1050m, 1009w, 654vw, 453w, and 407w cm−1.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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CONCLUSIONS In the present study, two SERS procedures for the detection of organic dyes in cultural heritage investigations, i.e., SERS on Ag nanoparticles upon HF hydrolysis and without any preliminary treatment, were compared when applied to the identification of
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 3756
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ACKNOWLEDGMENTS This project was supported by NSF SCIART Award CHE1041832. We also acknowledge Award No. 2006-DN-BX-K034, National Institute of Justice, Office of Justice Programs, US Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect those of the Department of Justice.
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