Surface-Enhanced Raman Spectroscopy-and ... - ACS Publications

Chapter 10

Surface-Enhanced Raman Spectroscopy—

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0963.ch010

and Surface-Enhanced Infrared Absorption-Based Molecular Sensors Selective Detection of Polycyclic Aromatic Hydrocarbons Employing Different Silver Nanoparticle Functionalization Methods 1

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C. Domingo , L. Guerrini , P. Leyton , M. Campos-Vallette , J. V. Garcia-Ramos , and S. Sanchez-Cortes 1

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Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain Faculty of Sciences, University of Chile, P.O. Box 653, Santiago, Chile

Functionalization of metal nanoparticles usually employed for Surface Enhanced Raman and Infrared Spectroscopies, with adequate host molecules, has allowed the selective detection of some Polyaromatic Hydrocarbons (PAHs), considered as hazardous pollutants. The molecular sensing capabilities of these high sensitivity techniques, SERS and SEIRA, together with their complementarity, are here illustrated for the case of employing calix[4]arenes derivatives as host molecules in order to detect PAHs.

SERS (Surface Enhanced Raman Scattering) and, more recently, SEIRA (Surface Enhanced Infrared Absorption) spectroscopies have rapidly expanded their field of applications. These techniques are based on the use of noble metal nanoparticles (NPs) supporting Localized Surface Plasmons (LSP). They are extremely attractive due to unique high sensitivity and their potential for molecule-specific sensing technologies utilizing vibrational signatures (1,2).

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© 2007 American Chemical Society In New Approaches in Biomedical Spectroscopy; Kneipp, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


139 Significant effort, in many research laboratories, is directed towards improving the selectivity and reproducibility of the required nanostructured substrates, in order that these enhanced spectroscopies can be employed as true molecular chemosensing techniques. The main components involved in the vibrational signal enhancement in the presence of NPs are the metal nanostructure, the adsorbate and the interface between the metal surface and the surrounding medium. The nature of the interface determines the accessibility of the adsorbate to the surface as, firstly, it is necessary a diffusion of the molecule to the surface and, then, the interaction with the metal, directly or through ionic interactions when ions are present. The metal substrate is essential and the size and morphology of the metal nanostructures will determine the signal intensification. Correspondingly, a substantial part of the research work in our group is dedicated to fabrication and morphological characterization of nanostructured metal surfaces with controlled morphology and optical properties (3-5) using different methods like colloidal aggregation or photoreduction, and further characterization of their SERS and SEIRA properties. A crucial point in the application of these surface techniques is the nature of the adsorbate. Usually, the adsorption of the active molecules in SERS and SEIRA is driven by some affinity for the metal surface providing a close metalmolecule interaction and allowing for a significant vibrational enhancement. Many molecules are strongly adsorbed undergoing a chemical change at the surface, as we have seen for many polyphenols (6). In contrast, other molecules may not adsorbed at the metal surface due to their poor affinity to the metal. Among the latter are the Polycyclic Aromatic Hydrocarbons (PAHs), considered hazardous pollutants. We are specially interested in the detection of PAHs using enhanced vibrational techniques. The affinity of adsorbates toward the metal surface may be manipulated by introducing a metal interface. In the case of metal nanostructures the surface is negatively charged, the negative potential or zeta potential value depending on the method employed to prepare the nanostructures (5). This means that positively charged adsorbates could give the most enhanced vibrational spectra due to their high preference to adsorb onto the metal surface. The high polar nature of the interface makes difficult the adsorption of highly apolar molecules such as the PAHs mentioned above and many apolar pesticides, whose detection in trace quantitities is of tremendous interest. But the affinity of the adsorbate toward the metal can be increased by modifying the chemistry of the interface by a proper functionalization of the metal NPs. Lately, we have attempted to modify the surface of metal NPs by the adsorption of molecules displaying a double functionality, i.e. a high affinity to the metal and for the the target molecule (PAHs in our case). The molecules employed in this functionalization, i.e. the host molecules, can be classified into two main groups: natural hosts and specifically designed hosts.

In New Approaches in Biomedical Spectroscopy; Kneipp, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


140 Nucleic acids, proteins and humic acids are among the natural hosts we have employed in the surface functionalization of metal NPs. Since these molecules are the natural targets of many pollutants including PAHs (in fact their biological hazardous effect is due to their ability to interact with them) we have used them as host linkers in order to reach a twofold objective: the pollutant detection and the study of the interaction with these important natural targets (7). The most successful designed hosts we have found are the calixarenes that can be used in the molecular recognition of PAHs by SERS. Calixarenes are a class of synthetic cyclooligomers formed via a phenol-formaldehyde condensation. They exist in a "cup" like shape with a defined upper and lower rim and a central annulus (see Fig. 1). Calixarenes have interesting applications as host molecules as a result of their preformed cavities (8). By changing the chemical groups of the upper and/or lower rim it is possible to prepare different derivatives with differing selectivities for various guest ions and molecules. Adsorption and self-assembled monolayer formation of calixarenes is a prerequisite for the application of calixarenes in sensor devices (9-12). In previous works we have studied the adsorption and organization of calixarene molecules on metal surfaces, where charge-transfer phenomena can take place with potential for optical sensing. While the studies carried out by other authors on the adsorption and selfassembly of calixarenes on metal surfaces mainly dealt with thiol functionalized calixarenes (9, 75-75), we have proved that ester-fiinctionalized calixarene derivatives could also be effectively employed as hosts in the selective molecular recognition of PAHs using the SERS technique. Other kind of host molecules which has rendered good results in the detection pf PAHs are the quat molecules (16). These molecules include in their structure a positively charged quaternary nitrogen (quat) and an aromatic moiety. The quat moiety is tightly attached to the surface via an ionic pair formation with addition of chloride ions {17,18) and can interact with PAHs. As previously indicated, in our functionalization studies, we have centered our attention on the vibrational characterization and detection of PAHs. They constitute a family of different chemical compounds with a condensed multibenzene structure. PAHs are important environmental pollutants formed during the incomplete burning of coal, oil and gas, or other organic substances like tobacco or charbroiled meat (19). PAHs can be found as a mixture of different related molecular compounds in air, soil and water due to both natural processes and the human activity. Because many of them have been reported to be strong carcinogens (20-23) it is important to find an effective and selective method for their trace analysis. In this chapter, we report the application of several calix[4]arenes with different chemical functional groups in the lower rim, for the selective recognition of large PAHs molecules: anthracene (ANT), dibenzoanthracene


In New Approaches in Biomedical Spectroscopy; Kneipp, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 1

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Figure 1. Normalized areas (with respect to the 1405 cm' band ofPYR in the complex with DCEC) of the most intense SERS bands of PYR and BcP (10~ M) on the different assayed calixarenes


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(DBA), pyrene (PYR), benzo[c]phenantrene (BcP), triphenylene (TP), coronene (COR), chrysene (CHR) and rubicene (RUB). The detection is achieved applying both vibrational-enhanced and complementary techniques; S E R S and S E I R A t The complementary information provided by both techniques is very helpful in the study of complex systems such as the host/ligand presented in this work.


Experimental Details Colloidal silver nanoparticles were prepared by using hydroxylamine hydrochloride as reducing agent (24). These nanoparticles have the advantage, with respect to the citrate A g colloid, of a more uniform distribution of size and shape together with the absence of citrate excess and its oxidation products that could interfere in the SERS measurements (5). In addition, the surface properties of this colloid may be better suited for the detection of PAHs, since the total negative charge, mainly due to the residual chloride ions, should be much lower than in the case of citrate and borohydride colloids. In the latter cases, anions bearing more than one negative charge are adsorbed on the surface increasing the overall nanoparticle negative charge. On the other hand, these chloridecovered nanoparticles present improved adherence properties. This fact facilitates their immobilization on glass, giving rise to films provided with a uniform distribution of metal particles tightly linked to the glass by a simple deposition of an aliquot of the colloidal solution on a glass slide. The immobilized A g nanoparticles on substrates such as Ge or glass are systems which can be used as substrates for both SERS and SEIRA measurements. In some cases gold island films deposited on CaF substrates were also employed to obtain SERS and SEIRA spectra. They were prepared by metal evaporation as reported elsewhere (25). The calixarenes employed for this study were: 25,26,27,28-tetrahydroxy-p-/-butylcalix[4]arene (TOHC); 25,27-dicarboethoxy-26,28-dihydroxy-p-/-butylcalix[4]arene (DCEC); 25,26,27,28-tetracarboethoxy-p-i-butylcalix[4]arene (TCEC) 25,27-dicyano-26,28-dihydroxy-p-/-butylcalix[4]arene (DCNC) and 25,26,27,28-tetracyano-p-/-butylcalix[4]arene (TCNC). These calixarenes were synthesized following published procedures (26) and are functionalized with four /-butyl groups in the upper rim, while the two last calixarenes have a different functionalization in the lower rim with an alternate 1,3 substitution. SERS spectra at 785 nm were measured with a Renishaw Raman Microscope System RM2000 equipped with a diode laser, a Leica microscope, and an electrically refrigerated C C D camera. The laser power in the sample was 2


143 2.0 mW. FT-SERS spectra were obtained with a Bruker RFS 100/S instrument, using the 1064 nm line of a N d : Y A G laser and a Ge detector cooled by liquid nitrogen. The output laser power was 50 mW and a 180° geometry was employed. The spectral resolution was set to 4 cm" in both cases. SEIRA spectra were measured on a FTIR Bruker IFS 66 spectrometer provided with a D T G S detector. For the polarized reflection measurements, a variable angle reflectance accessory Specac SCI9650 with a KRS5 polarizer was employed. Spectra resolution was 8 cm" . 1

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Results and Discussion SERS of calix[4]arene/PAHs complexes: selectivity of the calix[4]arene molecule in function of the lower rim functionalization The SERS of PAHs cannot be obtained on A g colloids and in general on other metallic surfaces because these molecules do not manifest any affinity to be adsorbed on the metal surface. However, we succeeded in getting the SERS of calix[4]arene/PAHs complexes at different laser wavelengths excitation (27,28). Figure 1 shows the normalized areas, corresponding to the most intense SERS bands of P Y R and BcP (observed at 1405 and 1338 cm' , respectively) in the complexes with the different calixarene molecules with a different functionalization in the lower rim. P Y R and BcP have a different behaviour with respect to the calixarene selectivity. P Y R shows a much higher SERS intensity when it is interacting with D C E C compared with T O H C or D C N C . In contrast, BcP also gives rise to a high SERS intensity when interacting with D C E C (although lower relative intensity than that achieved for PYR), while it shows a significant SERS intensity when interacting with the other calixarenes. This result indicates the existence of a clear selectivity in the molecular recognition of PAHs by calixarenes thanks to the formation of a host/guest complex in which the chemical structure and the size of the PAHs play an important role (26). P Y R seems to interact with a high specificity with D C E C . Since the differences between the calixarenes are due to different chemical functionalization on the lower rim, through which the calixarene interacts with the surface, the specificity is attributed to a different adsorption affinity of the host/guest complex. In Figure 2 the normalized areas of the most intense SERS bands of the different PAHs studied here (the in-plane ring stretching band falling in the 1420-1320 cm" region) were quantified in their complexes with D C E C . As can be seen, all the 4-ringed PAHs assayed (PYR, TP and BcP) are selectively more enhanced. In particular, P Y R , followed by TP, are the PAHs molecules with the highest specificity to D C E C since they lead to the maximum SERS 1

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Figure 2. Normalized areas (with respect to the 1405 cm' band of PYR in the complex with DCEC) of the most intense SERS bands of the studied PAHs (ΙΟ M) on DCEC (10-4 M). All the intensities were registered by exciting at 1064 nm. 4

intensification. In contrast, the larger P A H s (COR, R U B and D B A ) exhibited the lowest SERS intensities. This size specificity is probably determined by the cavity of the calixarene and the /-butyl groups placed in the upper rim. It is noted that C H R , another phenantrene derivative, presented a much lower SERS intensity than BcP. This indicates that the binding properties of each P A H must change depending on their chemical structure, revealing the existence of a size and shape selectivity of D C E C to interact with P A H s molecules. Calixarenes bearing a dicarboethoxy ( - C H - C O O - C H - C H ) substituents in the lower rim are then good candidates in the design of optical sensors of PAHs based on enhanced vibrational spectroscopies. The presence of these groups in an alternate configuration in the lower rim together with the /-butyl groups in the upper rim ensures a strong interaction with both the ligand and the metal surface. 2

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145 Interestingly, the corresponding tetracarboethoxy calix[4]arene T C E C , which bears four carboethoxy groups in the lower rim, displays a much lower activity in the detection of PAHs.


Complementarity of SERS and SEIRA measurements Fig. 3 displays the SERS and SEIRA spectra of the D C E C / P Y R complex on the same Au/CaF2 substrate, compared to the Raman and infrared spectra of solid samples of P Y R and calixarene. The SERS spectrum shows intense P Y R bands at 1400 and 1235 cm-1 which correspond to the bands at 1407 and 1241 cm-1 in the solid, and which are assigned to in-plane ring stretching motions and in-plane 6(C-H) vibrations (29). The downwards shift of the latter bands is attributed to the interaction with calixarene (27). Moreover, a marked enhancement of the band at 590 cm-1 and an intensity decrease of that at 408 cm" are observed. In general, all the intensified bands correspond to symmetric ag modes (30,31) which are enhanced through a Frank-Condon resonance mechanism, most probably associated to a charge-transfer in the complexsurface system, as it also occurs for naphthalene interacting with TOHC (32). The complementary nature of the information provided by SEIRA and SERS is illustrated in the spectra of the D C E C / P Y R complex. SERS spectra are mainly dominated by P Y R bands. Thus, SERS can be used to follow the changes occurring in the PAHs as a consequence of the interaction with the calixarene host and for detection purposes. In contrast, SEIRA spectra display more intense bands corresponding to the more polar groups existing in the calixarene host molecule, while only a weak P Y R band is observed at 840 cm" . Thus, SEIRA seems to be more appropriate to follow structural changes occurring in the host due to the adsorption and the complexation with the analyte and to find a structure/selectivity relationship. Since SEIRA is a better technique to follow the structural changes undergone by the calixarene host molecule, in the following section we analyze the SEIRA of D C E C and T C E C in order to understand the reasons of the different activity of these molecules in the interaction with P Y R and detection of PAHs. (33). 1

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SEIRA of D C E C vs. T C E C : influence of lower rim functionalization on the adsorption and interaction with PAHs The IR spectrum of solid D C E C dispersed in K B r shows doublets at 1207/1188, 1125/1100 and 1060/1050 cm' (Fig. 4a). However, when this calixarene is adsorbed on A g (Fig. 4b) prominent SEIRA bands are seen at 1187, 1127 and 1056 cm" on A g and at 1188, 1127 and 1068 cm' on Au, indicating 1

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Figure 3. SERS and SEIRA spectra of the DCEC/PYR complex on Au/CaF2 substrate compared with normal Raman and infrared spectra of solid DCED and PYR. Illustration of the configuration adopted by the DCEC-PYR complex on Au.


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) SEIR of DCEC/PYR^ Λ ΛΑ

(d)SEIRofTCEC on Ag/Ge

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ο Ά < (e) TCEC in KBr

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F/gwre 4. Up: a) IR Absorption spectrum of DCEC in KBr; b) SEIRA of DCEC in Ag/Ge; c) SEIRA ofDCE/PYR complex in Ag/Ge; d) SEIRA of TCEC in Ag/Ge; and e) IR Absorption spectrum of TCEC in KBr.


148 that clear structural changes occur upon adsorption on the metallic surface. In addition, the upwards shift of the ester v(C-O) band from 1125 to 1127 cm" , and its intensity increase, is related to the downwards shift of the v(C=0) ester vibration at 1755 cm" . The shifts observed for the ester v(C-O) and v(C=0) motions indicate that both oxygen groups are interacting with the metal surface through a bidentate complex, as shown in Fig. 4, (bottom, left) due to the electric charge délocalisation occurring in the ester group. Moreover, this interaction induces a rearrangement of the lower molecular rim, responsible for the wavenumber shifts of the 1298, 1207 and 1060 cm' bands. The v(C-O) band at 1207 cm" seems to be very sensitive to calixarene structural changes involving the aromatic v(C-O) ether bond through which the carboethoxy group is linked to phenyl rings. This band appears at a lower wavenumber value in T C E C , in comparison to D C E C , due to a more open structure adopted by the four carboethoxy groups in the lower rim of the calixarene induced by the steric hindrance between them. Thus, the shift towards lower wavenumbers in D C E C when adsorbed on the metal can be also interpreted as due to an opening of the carboethoxy groups in the lower rim in order to facilitate the bidentate interaction of D C E C with the surface. 1

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SEIRA spectra of the D C E C / P Y R complex on A g displays a general frequency shifts with respect to the SEIRA of D C E C , mainly the involving carboethoxy group. The v(C-O) band at 1127 cm" shifts downwards to 1124 cm' both on A g and on Au. Moreover, the v(C-O) ether aromatic band at 1188 cm" is weakened, and that at 1298 cm" shifts downwards to 1271 cm" . These changes suggest a stronger interaction of carbonyl groups, and a weaker one of the C - 0 group in the presence of P Y R towards a rather monodentate interaction (Fig. 4, bottom, centre). This is attributed to a further opening of the carboethoxy groups in the lower calixarene rim induced by the interaction with P Y R . The induced structural change also influences the ether C - 0 bonds of the aromatic moiety, which could adopt a less perpendicular orientation, thus explaining the 1298 - » 1271 cm" shift and the intensity decrease of the 1188 cm' band. 1

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The structural changes undergone by D C E C upon adsorption and interaction with P Y R are also manifested in the 2800-3000 cm" region corresponding to CH stretchings (Fig. 4). A relative intensity decrease is seen for the band at 2958 cm' both on A g and Au. On the other hand, the interaction with P Y R induces an intensity increase of the band at 2917 cm" on A g and 2919 cm" on A u . The changes observed in this region are attributed to the reorientation of the molecule on the surface due to its complexation with P Y R . In fact, the most enhanced bands must correspond to C - H stretching vibrations with preferential perpendicular orientation with respect to the surface, according to the SEIRA selection rules (2). SEIRA spectrum of T C E C on Ag/Ge reveals slight changes in bands falling in the 1250-950 cm" region, indicating that also T C E C interacts with the metal 1

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149 surface through the ester group as in the case of D C E C . In particular the most intense v(C-O) band at 1191 cm" undergoes a significant intensity decrease and shift to 1184 cm" , while the band at 1128 cm' shift to 1124 cm" . However, the changes are not as strong as in the case of D C E C , thus indicating a different adsorption of T C E C with respect to D C E C . A detailed analysis of the C - H region and the 1250-950 cm' region indicates that T C E C is adsorbed through a monodentate interaction with the metal (Fig. 4, bottom, right), since a bidentate structure with both oxygen atoms interacting with the surface is not possible in T C E C due to the steric hindrance caused by the close carboethoxy groups. Interestingly, the spectral profile of the C - H stretching region in the SEIRA of T C E C is very similar to that found for the D C E C / P Y R complex, since also in D C E C a prominent band at 2919 cm" is seen in the complex with P Y R . Other similarities between the SEIRA spectra of T C E C and D C E C / P Y R complex are found in the lower wavenumber region, mainly corresponding to the position of bands at 1741, 1184 and 1124 cm" bands. The similarity found between SEIRA spectrum of T C E C and that of D C E C / P Y R complex could be the key to understand why T C E C is less active binding PAHs molecules, as demonstrated in previous experiments (27). In fact, both the steric hindrance of carboethoxy groups and the interaction with the metal may induce a shutdown of the calixarene upper rim, which may prevent the inclusion of P Y R . This structural change does not occur in D C E C , due to the lower steric effect of the two carboethoxy groups. Nevertheless, the interaction of D C E C with P Y R could lead to a shutdown of the upper rim and an opening of the calixarene lower rim similar to that occurring for non-complexed T C E C . 1

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Conclusions The functionalization of metal (silver and gold) nanoparticles with calix[4]arenes derivatives employed as host molecules allows the selective detection of some PAHs using SERS. The corresponding SEIRA spectra provide important information about the structural changes undergone successively by the calix[4]arenes, first when adsorbed onto metal nanoparticles and after when "lodging" the P A H molecule forming a complex. Analysis of SEIRA data allowed to find structure/selectivity relationships, complementing the information given by SERS. Both vibrational enhanced spectroscopies are promising molecular sensing techniques.

Acknowledgements Ministerio de Education y Ciencia of Spain (Project FIS2004-00108) and the Comunidad de Madrid (MICROSERES Program, S-0505/TIC-0191) are


150 gratefully acknowledged for financial support. Authors also acknowledge support received from the Convenio Conycit-CSIC (2003/2004 and 2006/2007).

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