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Identification by Raman microprobe (LRM) of phenyl groups in thin surface coatings
Applied Surface North-Holland
Science 6S/h6
(1993) 362-36s
Identification by Raman in thin surface coatings
applied surface science
microprobe
(LRM) of phenyl groups
C.A. Davis, Zhao Rupeng Faculty
ofScience and Technology, Gtiffith Uttiewity,
J.A.A. Crossley,
Received
29 June
P.R. Graves
1992; accepted
Nathan,
and S. Myhra
for publication
12 August
Qld 41 I I, Australia
’
1992
Thin biological/organic surface coatings have been investigated by LRM. Phenyl groups in monolayer coatings are detectahlc hy either surface enhanced standard Raman techniques, or with the aid of high quantum efficiency parallel detectors. without surface enhancement. The spectra suggest that the phenyl groups are oriented parallel to the substrate.
1. Introduction The properties of heterostructures involving the deposition of organic and biologically active materials on inorganic substrates are becoming increasingly relevant for a number of technological applications. For instance, the attachment of organic molecules to solid substrates have applications for liquid chromatography [l] and adhesion [2]. Likewise, interaction between biological molecules and inorganic surfaces are germane to biofouling in biotechnology [3] and to the enhancement of compatibility between tissue and implants in medical technology. Silane compounds are of particular interest for the former group of applications, as well as being used in microelectronics, while amino acids, being the fundamental building blocks of biological molecules, are most relevant for the latter. We have chosen dimethoxydiphenylsilane, (MeO),Si(Ph&. and L-phenylalanine, PhCH,(CH(NH ,))COOH,
’ Permanent address: Faculty of Science and Technology. Griffith University, Nathan, Qld 411 I, Australia. 0169.4332/93/$Oh.O0
0 1993 - Elsevier
Science
Publishers
as representative examples of the respective generic groups of compounds. In many instances the organic/ biological overlayer may be only a few, or even one, monolayer thick. As a result only a small subset of the full inventory of techniques for materials charactcrisation will provide useful information about the interface and the overlayer. This subset is limited to those techniques which have either exceedingly good dispersive discrimination (such as NMR in the chemical shift mode) or a shallow depth-ofinteraction-volume (such as many of the surface sensitive techniques, e.g. XPS or AES). Laser Raman spectroscopy has good dispersive discrimination, typically 1 part in 103 cm ‘. and gcnerally provides positive spectral “finger-prints”. Also, it is non-destructive, capable of in situ investigations in most environments, and is relatively convenient. However, it has tended to bc regarded as a bulk technique. In some casts it is a true surface technique, when surface cnhancement (SERS) can be exploited (e.g. ref. [4]). Moreover, recent developments in photon detection technology has opened up avenues for more general exploitation of Raman spectroscopy as 21
B.V. All rights reserved
CA. Davis et al. / Identification
surface science tool. The state-of-the-art is now at the level of detectability of 10h macromolecules within the interaction volume, in favourable cases. The aims of the present study were two-fold. Firstly, to establish some of the general characteristics of thin deposited organic/biological overlayers on inorganic substrates; and secondly, to demonstrate the merits of modern Raman spectroscopy for the study of such systems.
2. Experimental
Table 1 Summary groups
1030 1024
1001
1027
01
si IPh
0’
‘Ph
Si
OH
Si
OH
FOH
+
I Fig. 1. Schematic
representation
1035
The amino acid was obtained from Sigma Chemicals. The crystalline specimen was analysed in its as-received form. Spectra from species in solution were obtained by dissolving the amino acid in O.lM K,SO,, with adjustment of pH by addition of either KOH or H,SO,. The SERS spectra were obtained in situ from adsorbed species on an Ag electrode substrate. The application of SERS to adsorption of macromolecules on metallic substrates has been discussed by Nabiev et al. [6]. Accordingly, in the present experiment the electrode surface was subjected to cyclic oxidation/reduction in 1M KCI in an electrochemical cell. The amino acid coating was subsequently deposited under various conditions of electrode potential. Further details of those procedures and of the experimental arrangements are published elsewhere [3]. The Raman microprobe/ spectrometer was configured in the i( backscatter mode, as described in the Porto notation. The primary excitation source was an Ar+ laser (Spectra Physics Model 2025), which was filtered with a premonochromator in order to remove unwanted plasma lines. The line at 514.5 nm was used for all measurements. The beam was focussed to a spot of 2 pm diameter by a metallurgical micro-
Si
OH
of phenyl
999 993 1005
Si
-
modes
(MeO),Si(Ph), Ph on surface L-phenylalanine Ph on surface
OH
(MeO)LSi(Ph)p in solution
vibrational
1’18. Anti-symmetric mode (cm-‘)
OH +
Si
for the main
VI? Symmetric mode (cm-‘)
The dimethoxydiphenylsilane compound (> 97%) was obtained commercially from Huls America Inc. (Cat. No. D6010). The substrate for this compound was silica gel with 5-25 pm particle size. The large BET surface area (500 m2/g) was indicative of open porosity. The procedure for deposition and production of a thin continuous surface layer has been described generically elsewhere [5]. Essentially it amounted to suspension of the specimen grains in a solution of reactant with high purity toluene as a dilutant under inert nitrogen gas and subjection of the exposed surfaces to a reflux reaction at 80°C for periods of up to 6 h. The reaction is depicted schematically in fig. 1. The as-received neat compound was analysed by LRM by focusing the laser beam into a drop of liquid. The coated gel particles were distributed on a microscope slide. Single particles tended to produce weak and noisy spectra, while signals from a “thicker” specimen volume (several particles being traversed) resulted in more definitive data. It was estimated that adequate signal-to-noise ratios within practical data acquisition times required a traversal of approximately 5 grains (or the equivalent of 20-50 layers of surface coating, allowing for porosity).
Si
results
Compound
details
Si
363
by LRM of phenyl groups in thin surface coat@
of the surface
silylation
reactions.
2MeOH in solution
scope fitted with a X40 0.65 NA objective lens. The incident power of the fully focussed beam was restricted to less than 3 mW (other surface measurements have shown that the phenyl groups are relatively weakly bound to the surface). Dispersion of the backscattered light was effected with a grating spectrometer (Spex Model Triplemate). Parallel detection and counting was done in the case of the silane coating with a high quantum efficiency two-dimensional charge-coupled liquid nitrogen cooled array (420 x 600 pixels), the data acquisition was under computer control with proprietary software (ATI CCD Imaging System, Wright Instruments Ltd.). The SERS studies of the amino acid were carried out with an earlier version of the Raman instrument. The significant difference was that this version used an intensified 1024 element photodiode array detector (0-SMA, Spectroscopy Instruments GmbH). The diode array has less quantum efficicncy and a smaller active arca in comparison with the charge-coupled array.
3. Results and discussion The spectra for the free silane molecule in solution and for the silane compound attached to silica gel are shown in fig. 2. Likewise, the results of conventional LRM analysis of the as-received crystalline L-phenylalanine and of SERS analysis of the adsorbed molecule are shown in fig. 3. The spectrum shown was obtained for deposition under conditions near the point of zero charge for the silver substrate ( - 0.67 V) and at an electrode potential of 0.8 V versus Ag/AgCl. Surface enhanced Raman has been used as a probe for a study by Gao and Weaver [7] of the adsorption of aromatic ring molecules on Au electrodes. They identified a number of useful characteristics of such processes; many of these are relevant to the present study. (i) The most distinctive characteristics of the Raman modes for both “free” and adsorbed monosubstituted (i.e., only one group attached to the aromatic ring) ring molecules tend to be the ring modes at = 1000 (u,~) and 1025 cm- ’ (v,~,~) using the so-called Wilson number notation. Mul-
Fig. 2. Spectra of (MeO),Si(Ph),. Neat solution (upper) and wrfacc coating (lower). The broad futures at = 45tl and SOtI cm ’ arc consistent with excitation in the amorphous hilic;t gel whstrate.
tiply substituted aromatic molecules tend to have the corresponding frequencies shifted down. Likewise, Varsanyi et al. [X] have identified the vibrational modes of the Ph,Si molecule and assigned the two features at - 670 and = 620 cm ’ to uhc,and v,,,,, respectively. These arc the most likely analogues of the modes found at 660 and 612 cm-’ for the neat solution of the diphenylsilane. Furthermore. it is plausible to ascribe the peak at I I10 cm ’ to the U, mode (cf. 1107 cm-’ for Ph,Si). Finally, the least intcnsc
C.A. Daris et al. / Identification
by LRM
features at = 1160 and = 1180 cm- ’ are likely to be the u,+, and vqb modes. (ii) Downward shifts following adsorption of the two major ring modes are generally observed and are taken to indicate that the rings take on orientations parallel to that of the substrate. For instance, Gao and Weaver [7] have investigated a number of monosubstituted benzene compounds and found that Av,, = (v,,(free) - v,,(ads)) ranges from 0 (for C,H,CN and C,H,NO,) to 11-12 cm-’ (for C,H5CH, and C,H,C(CH,),). The proposition is that there should be a transfer of charge density from the substrate to the rr* antibonding orbitals, if there is parallel orientation of the aromatic rings. The diffuse bonding of the ring to the substrate should in turn damp the ring modes and result in the downward shift. We found in the present experiments that there were shifts of = 5 cm-’ for both compounds; these are in the middle of the range, and suggest that the C,H, groups are indeed adsorbed parallel to the substrate, as well as being weakly bound.
of phenyl groups in thin surface coatirlgs
365
SERS conditions (i.e., any substrate will be sufficient, in principle). (4) Measurable shifts in the most prominent ring modes between the free molecules and the adsorbed species can be analysed for the purpose of determining the orientations and interactions of the rings with the substrate. The characteristics above suggest that Raman spectroscopy is a more convenient and definitive technique for the analysis of interface phenomena of many organic and biological overlayers than other surface techniques (e.g. XPS, AES and SIMS).
Acknowledgements This research was funded in part by the Harwell Corporate Research Programme and by the Australian Research Council. One of the authors (S.M.) wishes to acknowledge support and hospitality extended during a period of attachment. Another author (Z.R.) was supported by an Australian Overseas Student Fellowship.
4. Conclusions (1) The spectra of Raman active aromatic ring compounds have distinctive features (the vr2 and vlHa modes, in particular). These can be exploited for purposes of rapid and positive molecular identification for a wide range of organic and molecular compounds. (2) When SERS conditions apply, surface coatings of aromatic ring molecules down to monolayer thicknesses can be monitored by standard parallel detection methods. (3) With high quantum efficiency parallel detectors, coatings down to near-monolayer thicknesses are detectable without the imposition of
References [l] J.J. Kirkland, J.L. Glajch and R.D. Farlee. Anal. Chem. 61 (lY89) 2. [2] T.L. Weeding. W.S. Veeman, L.W. Jenneskens, H. Angad Gaur, H.E.C.Schuurs and W.G.B. Huysmans. Macromolecules 22 (1989) 706. [3] A. Crossley and P.R. Graves, Biofouling (1992), in press. [S] V. Dudler, L.F. Lindoy, D. Sallin and C.W. Schalaepfer, Aust. J. Chem. 40 (1987) 1557. [6] I.R. Nabiev, V.A. Savchenko and E.S. Efremov, J. Raman Spectrosc. 14 (1983) 375. [7] P. Gao and M.J. Weaver, J. Phys. Chem. X9 (1985) 5040. [X] G. Varsanyi, B. Zelei, S. Dobos and M. Gal, Spectrochim. Acta 40 A (1984) 52’).
Science 6S/h6
(1993) 362-36s
Identification by Raman in thin surface coatings
applied surface science
microprobe
(LRM) of phenyl groups
C.A. Davis, Zhao Rupeng Faculty
ofScience and Technology, Gtiffith Uttiewity,
J.A.A. Crossley,
Received
29 June
P.R. Graves
1992; accepted
Nathan,
and S. Myhra
for publication
12 August
Qld 41 I I, Australia
’
1992
Thin biological/organic surface coatings have been investigated by LRM. Phenyl groups in monolayer coatings are detectahlc hy either surface enhanced standard Raman techniques, or with the aid of high quantum efficiency parallel detectors. without surface enhancement. The spectra suggest that the phenyl groups are oriented parallel to the substrate.
1. Introduction The properties of heterostructures involving the deposition of organic and biologically active materials on inorganic substrates are becoming increasingly relevant for a number of technological applications. For instance, the attachment of organic molecules to solid substrates have applications for liquid chromatography [l] and adhesion [2]. Likewise, interaction between biological molecules and inorganic surfaces are germane to biofouling in biotechnology [3] and to the enhancement of compatibility between tissue and implants in medical technology. Silane compounds are of particular interest for the former group of applications, as well as being used in microelectronics, while amino acids, being the fundamental building blocks of biological molecules, are most relevant for the latter. We have chosen dimethoxydiphenylsilane, (MeO),Si(Ph&. and L-phenylalanine, PhCH,(CH(NH ,))COOH,
’ Permanent address: Faculty of Science and Technology. Griffith University, Nathan, Qld 411 I, Australia. 0169.4332/93/$Oh.O0
0 1993 - Elsevier
Science
Publishers
as representative examples of the respective generic groups of compounds. In many instances the organic/ biological overlayer may be only a few, or even one, monolayer thick. As a result only a small subset of the full inventory of techniques for materials charactcrisation will provide useful information about the interface and the overlayer. This subset is limited to those techniques which have either exceedingly good dispersive discrimination (such as NMR in the chemical shift mode) or a shallow depth-ofinteraction-volume (such as many of the surface sensitive techniques, e.g. XPS or AES). Laser Raman spectroscopy has good dispersive discrimination, typically 1 part in 103 cm ‘. and gcnerally provides positive spectral “finger-prints”. Also, it is non-destructive, capable of in situ investigations in most environments, and is relatively convenient. However, it has tended to bc regarded as a bulk technique. In some casts it is a true surface technique, when surface cnhancement (SERS) can be exploited (e.g. ref. [4]). Moreover, recent developments in photon detection technology has opened up avenues for more general exploitation of Raman spectroscopy as 21
B.V. All rights reserved
CA. Davis et al. / Identification
surface science tool. The state-of-the-art is now at the level of detectability of 10h macromolecules within the interaction volume, in favourable cases. The aims of the present study were two-fold. Firstly, to establish some of the general characteristics of thin deposited organic/biological overlayers on inorganic substrates; and secondly, to demonstrate the merits of modern Raman spectroscopy for the study of such systems.
2. Experimental
Table 1 Summary groups
1030 1024
1001
1027
01
si IPh
0’
‘Ph
Si
OH
Si
OH
FOH
+
I Fig. 1. Schematic
representation
1035
The amino acid was obtained from Sigma Chemicals. The crystalline specimen was analysed in its as-received form. Spectra from species in solution were obtained by dissolving the amino acid in O.lM K,SO,, with adjustment of pH by addition of either KOH or H,SO,. The SERS spectra were obtained in situ from adsorbed species on an Ag electrode substrate. The application of SERS to adsorption of macromolecules on metallic substrates has been discussed by Nabiev et al. [6]. Accordingly, in the present experiment the electrode surface was subjected to cyclic oxidation/reduction in 1M KCI in an electrochemical cell. The amino acid coating was subsequently deposited under various conditions of electrode potential. Further details of those procedures and of the experimental arrangements are published elsewhere [3]. The Raman microprobe/ spectrometer was configured in the i( backscatter mode, as described in the Porto notation. The primary excitation source was an Ar+ laser (Spectra Physics Model 2025), which was filtered with a premonochromator in order to remove unwanted plasma lines. The line at 514.5 nm was used for all measurements. The beam was focussed to a spot of 2 pm diameter by a metallurgical micro-
Si
OH
of phenyl
999 993 1005
Si
-
modes
(MeO),Si(Ph), Ph on surface L-phenylalanine Ph on surface
OH
(MeO)LSi(Ph)p in solution
vibrational
1’18. Anti-symmetric mode (cm-‘)
OH +
Si
for the main
VI? Symmetric mode (cm-‘)
The dimethoxydiphenylsilane compound (> 97%) was obtained commercially from Huls America Inc. (Cat. No. D6010). The substrate for this compound was silica gel with 5-25 pm particle size. The large BET surface area (500 m2/g) was indicative of open porosity. The procedure for deposition and production of a thin continuous surface layer has been described generically elsewhere [5]. Essentially it amounted to suspension of the specimen grains in a solution of reactant with high purity toluene as a dilutant under inert nitrogen gas and subjection of the exposed surfaces to a reflux reaction at 80°C for periods of up to 6 h. The reaction is depicted schematically in fig. 1. The as-received neat compound was analysed by LRM by focusing the laser beam into a drop of liquid. The coated gel particles were distributed on a microscope slide. Single particles tended to produce weak and noisy spectra, while signals from a “thicker” specimen volume (several particles being traversed) resulted in more definitive data. It was estimated that adequate signal-to-noise ratios within practical data acquisition times required a traversal of approximately 5 grains (or the equivalent of 20-50 layers of surface coating, allowing for porosity).
Si
results
Compound
details
Si
363
by LRM of phenyl groups in thin surface coat@
of the surface
silylation
reactions.
2MeOH in solution
scope fitted with a X40 0.65 NA objective lens. The incident power of the fully focussed beam was restricted to less than 3 mW (other surface measurements have shown that the phenyl groups are relatively weakly bound to the surface). Dispersion of the backscattered light was effected with a grating spectrometer (Spex Model Triplemate). Parallel detection and counting was done in the case of the silane coating with a high quantum efficiency two-dimensional charge-coupled liquid nitrogen cooled array (420 x 600 pixels), the data acquisition was under computer control with proprietary software (ATI CCD Imaging System, Wright Instruments Ltd.). The SERS studies of the amino acid were carried out with an earlier version of the Raman instrument. The significant difference was that this version used an intensified 1024 element photodiode array detector (0-SMA, Spectroscopy Instruments GmbH). The diode array has less quantum efficicncy and a smaller active arca in comparison with the charge-coupled array.
3. Results and discussion The spectra for the free silane molecule in solution and for the silane compound attached to silica gel are shown in fig. 2. Likewise, the results of conventional LRM analysis of the as-received crystalline L-phenylalanine and of SERS analysis of the adsorbed molecule are shown in fig. 3. The spectrum shown was obtained for deposition under conditions near the point of zero charge for the silver substrate ( - 0.67 V) and at an electrode potential of 0.8 V versus Ag/AgCl. Surface enhanced Raman has been used as a probe for a study by Gao and Weaver [7] of the adsorption of aromatic ring molecules on Au electrodes. They identified a number of useful characteristics of such processes; many of these are relevant to the present study. (i) The most distinctive characteristics of the Raman modes for both “free” and adsorbed monosubstituted (i.e., only one group attached to the aromatic ring) ring molecules tend to be the ring modes at = 1000 (u,~) and 1025 cm- ’ (v,~,~) using the so-called Wilson number notation. Mul-
Fig. 2. Spectra of (MeO),Si(Ph),. Neat solution (upper) and wrfacc coating (lower). The broad futures at = 45tl and SOtI cm ’ arc consistent with excitation in the amorphous hilic;t gel whstrate.
tiply substituted aromatic molecules tend to have the corresponding frequencies shifted down. Likewise, Varsanyi et al. [X] have identified the vibrational modes of the Ph,Si molecule and assigned the two features at - 670 and = 620 cm ’ to uhc,and v,,,,, respectively. These arc the most likely analogues of the modes found at 660 and 612 cm-’ for the neat solution of the diphenylsilane. Furthermore. it is plausible to ascribe the peak at I I10 cm ’ to the U, mode (cf. 1107 cm-’ for Ph,Si). Finally, the least intcnsc
C.A. Daris et al. / Identification
by LRM
features at = 1160 and = 1180 cm- ’ are likely to be the u,+, and vqb modes. (ii) Downward shifts following adsorption of the two major ring modes are generally observed and are taken to indicate that the rings take on orientations parallel to that of the substrate. For instance, Gao and Weaver [7] have investigated a number of monosubstituted benzene compounds and found that Av,, = (v,,(free) - v,,(ads)) ranges from 0 (for C,H,CN and C,H,NO,) to 11-12 cm-’ (for C,H5CH, and C,H,C(CH,),). The proposition is that there should be a transfer of charge density from the substrate to the rr* antibonding orbitals, if there is parallel orientation of the aromatic rings. The diffuse bonding of the ring to the substrate should in turn damp the ring modes and result in the downward shift. We found in the present experiments that there were shifts of = 5 cm-’ for both compounds; these are in the middle of the range, and suggest that the C,H, groups are indeed adsorbed parallel to the substrate, as well as being weakly bound.
of phenyl groups in thin surface coatirlgs
365
SERS conditions (i.e., any substrate will be sufficient, in principle). (4) Measurable shifts in the most prominent ring modes between the free molecules and the adsorbed species can be analysed for the purpose of determining the orientations and interactions of the rings with the substrate. The characteristics above suggest that Raman spectroscopy is a more convenient and definitive technique for the analysis of interface phenomena of many organic and biological overlayers than other surface techniques (e.g. XPS, AES and SIMS).
Acknowledgements This research was funded in part by the Harwell Corporate Research Programme and by the Australian Research Council. One of the authors (S.M.) wishes to acknowledge support and hospitality extended during a period of attachment. Another author (Z.R.) was supported by an Australian Overseas Student Fellowship.
4. Conclusions (1) The spectra of Raman active aromatic ring compounds have distinctive features (the vr2 and vlHa modes, in particular). These can be exploited for purposes of rapid and positive molecular identification for a wide range of organic and molecular compounds. (2) When SERS conditions apply, surface coatings of aromatic ring molecules down to monolayer thicknesses can be monitored by standard parallel detection methods. (3) With high quantum efficiency parallel detectors, coatings down to near-monolayer thicknesses are detectable without the imposition of
References [l] J.J. Kirkland, J.L. Glajch and R.D. Farlee. Anal. Chem. 61 (lY89) 2. [2] T.L. Weeding. W.S. Veeman, L.W. Jenneskens, H. Angad Gaur, H.E.C.Schuurs and W.G.B. Huysmans. Macromolecules 22 (1989) 706. [3] A. Crossley and P.R. Graves, Biofouling (1992), in press. [S] V. Dudler, L.F. Lindoy, D. Sallin and C.W. Schalaepfer, Aust. J. Chem. 40 (1987) 1557. [6] I.R. Nabiev, V.A. Savchenko and E.S. Efremov, J. Raman Spectrosc. 14 (1983) 375. [7] P. Gao and M.J. Weaver, J. Phys. Chem. X9 (1985) 5040. [X] G. Varsanyi, B. Zelei, S. Dobos and M. Gal, Spectrochim. Acta 40 A (1984) 52’).