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Langmuir 1999, 15, 3390-3394
Self-Assembled Monolayers of Oligo(ethylene glycol)-Terminated and Amide Group Containing Alkanethiolates on Gold Ramu j nas Valiokas, Sofia Svedhem, Stefan C. T. Svensson, and Bo Liedberg* Graduate School Forum Scientum, Department of Physics and Measurement Technology, Linko¨ pings universitet, S-581 83 Linko¨ ping, Sweden Received November 24, 1998. In Final Form: March 15, 1999 Self-assembly of oligo(ethylene glycol)-terminated and amide group containing alkanethiols (HS(CH2)15CONH-EGn; n ) 1, 2, 4, 6) on gold are investigated by contact angle goniometry, ellipsometry, and infrared reflection-absorption spectroscopy. The compounds are shown to form highly ordered monolayers. The molecular conformation of the oligo(ethylene glycol) sublayer is found to depend on the oligomer chain length and intermolecular interactions within the layer, as evidenced by a sharp increase in the amount of helical conformers for n ) 6.
Introduction Self-assembled monolayers (SAMs) and LangmuirBlodgett (LB) films have been extensively used as a platform to modify the physicochemical properties of solid surfaces.1 The organosulfur SAMs, especially the alkanethiolates on gold, have attracted a continuously growing interest because of their promising structural properties and superior stability due to the strong pinning to gold via sulfur.2 Moreover, the potential use of these modifications as novel biomimetic interfaces has encouraged chemists to synthesize and assemble a broad range of bifunctional molecules bearing bioactive terminal groups.3 However, upon increase in the complexity of the terminal group, intermolecular contributions to the overall stability and structure of the SAMs are expected to become more and more important. Examination and characterization of these lateral interactions are therefore crucial for gaining a deeper insight in their structural and functional properties. Fragments (oligomers) of well-investigated polymers attached to SAMs can be considered as a class of sufficiently complex groups for such studies. Poly(ethylene glycol)4 (PEG) is an example of a polymer that has been extensively used to modify surfaces making them protein and cell repelling.5 Interactions between biomolecules and model surfaces consisting of oligomeric chains of ethylene glycols attached to alkanethiolate SAMs (so-called OEGSAMs) have also been studied.6-9 More recently, Harder et al.10 demonstrated the ability to select a specific * Corresponding author: tel, +46 13 281877; fax, +46 13 137568; e-mail,
[email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Go¨pel, W.; Heiduschka, P. Biosens. Bioelectron. 1995, 10, 853883. (4) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (5) Desai, N. P.; Hubbell, J. A. Biomaterials 1991, 12, 144-153 and references therein. (6) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (7) Mrksich, M.; Chen, C. S.; Xia, Y.; Dike, L. E.; Ingber, D. E.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 10775-10778. (8) Silin, V.; Weetall, H.; Vanderah, D. J. J. Colloid Interface Sci. 1997, 185, 94-103. (9) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 3464-3473. (10) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426-436.
molecular conformation of the oligomer portion in ether bonded OEG-SAM by a proper choice of the oligomer chain length and lattice parameters of the supporting substrate, and a new mechanism explaining the protein-repelling properties was proposed. Another interesting and potentially important area for the OEG-SAMs is to use them in the design of supported lipid membranes, where the OEG moieties serve as a spacer arm for tethering the lipid layer to the solid substrate.11-15 A highly relevant study on that topic was recently undertaken by Plant and co-workers16 who examined the assembly of an alkylated thiahexa(ethylene oxide) on gold. The above-mentioned bio-oriented applications of the OEG-SAMs are of interest for our group, and we have chosen this system to address the problem of the lateral interactions within the SAM. For this purpose, we have designed and synthesized a class of OEG-terminated alkanethiols, which differ from the previously reported compounds in terms of oligomer chain length and the presence of the amide group between the alkyl and OEG parts. This amide linkage creates a hydrogen bonding network within the SAM which is postulated to affect the conformation of the OEG portion as well as the overall packing of the alkyl chains. In this Letter we report on the preparation and characterization of four different OEG-SAMs. Experimental Section Compounds. A synthetic route to prepare new OEGterminated alkanethiols was developed (Scheme 1). Tetra- and hexa(ethylene glycol) were monomesylated and transferred to the corresponding monoazides. Reduction of the azide function to the amine was performed by catalytic hydrogenolysis with Pd/C in ethyl acetate. These amino-terminated ethylene glycols (11) Raguse, B.; BraachMaksvytis, V.; Cornell, B. A.; King, L. G.; Osman, P. D. J.; Pace, R. J.; Wieczorek, L. Langmuir 1998, 14, 648659. (12) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197-210. (13) Heyse, S.; Ernst, O. P.; Dienes, Z.; Hofmann, K. P.; Vogel, H. Biochemistry 1998, 37, 507-522. (14) Williams, L. M.; Evans, S. D.; Flynn, T. M.; Marsh, A.; Knowles, P. F.; Bushby, R. J.; Boden, N. Langmuir 1997, 13, 751-757. (15) Cheng, Y. L.; Boden, N.; Bushby, R. J.; Clarkson, S.; Evans, S. D.; Knowles, P. F.; Marsh, A.; Miles, R. E. Langmuir 1998, 14, 839844. (16) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916-6923.
10.1021/la981647o CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
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Langmuir, Vol. 15, No. 10, 1999 3391 Scheme 1a
Table 1. Ellipsometric Thickness d, Incremental Thickness d-dTOH, Advancing (θa) and Receding (θr) Contact Angles of Water of the OEG-Terminated Alkanethiolates on Gold EG1 EG2 EG4 EG6
a
Key: (i) MsCl, TEA, CH2Cl2. (ii) NaN3, EtOH. (iii) H2, Pd/ C, EtOAc. (iv) Ac2O, pyridine. (v) H2N-(CH2-CH2-O)n-H (n ) 1, 2, 4, 6), EDC/HOBt, DMF. (vi) NaOMe, MeOH, Dowex H+. and shorter purchased ones were coupled to thioacetylated 16mercaptohexadecanoic acid (1). Deprotection of the thioesters in methanolic sodium methoxide gave the final thiols (EGn, n ) 1, 2, 4, 6). A full description of the synthesis will be presented in a forthcoming paper. 16-Mercaptohexadecanol (TOH) used in comparative measurements was obtained as a generous gift from Dr. S. Lo¨fås, Biacore AB, Uppsala, Sweden. Sample Preparation. Standard, cleaned silicon (100) wafers were coated by a 25 Å titanium adhesion layer and, subsequently, by 2000 Å gold using electron beam evaporation (Balzers UMS 500 P system). The base pressure of at least 10-9 mbar and a pressure on the low 10-7 mbar scale were maintained during evaporation. The evaporation rate was 10 Å/s for gold. The gold surfaces were cleaned in a 5:1:1 mixture of MilliQ water, 25% hydrogen peroxide, and 30% ammonia for 5 min at 80 °C and then rinsed in MilliQ water. This treatment previously was shown to increase the size of the grains in the gold film.17,18 Ethanolic 1 mM solutions of the compounds were prepared and stored in plastic beakers at room conditions. The gold surfaces were incubated for 24 h, rinsed in ethanol, ultrasonicated in the ethanol for 5 min, then rinsed again. The samples were finally blown dry in nitrogen gas and immediately analyzed. Every experiment included ellipsometry, contact angle measurements, and infrared reflection-absorption spectroscopy (IRAS) measurements, enabling better evaluation of the quality of each batch of the samples. Separate samples were used for each of the three methods. Ellipsometry. Single-wavelength ellipsometry was performed using an automatic Rudolph Research AutoEL ellipsometer with He-Ne laser light source, λ ) 632.8 nm, at an angle of incidence of 70°. The freshly cleaned gold substrates were measured prior to their incubation, and the collected average values of the refractive index were later used in a model “ambient/organic film/gold”, assuming an isotropic, transparent organic layer19 with the refractive index of n ) 1.5.20-22 The film thickness was calculated as an average of measurements at three different spots on 7-14 samples for each compound. Contact Angle Goniometry. Contact angles were measured with Rame´-Hart NRL 100 goniometer, in air, i.e., without control on the humidity in the ambient, using fresh water from a MilliQ unit. Taking into account the high surface energy of the hydrophilic surfaces, only one measurement of the advancing and receding contact angle was done per sample. Infrared Spectroscopy. The reflection-absorption (RA) spectra were recorded on a Bruker IFS 66 system, equipped with a grazing angle (85°) infrared reflection accessory and a liquid nitrogen cooled MCT detector. The measurement chamber was continuously purged with nitrogen gas during the measurements. The acquisition time was around 10 min at 2 cm-1 resolution, and a three-term Blackmann-Harris apodization function was applied to the interferograms before Fourier transformation. A (17) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141-149. (18) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257-12267. (19) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (20) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (21) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (22) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90-94.
d (Å)a
d-dTOH (Å)
θa (deg)b
θr (deg)b
24.8 ( 0.8 28.4 ( 0.3 33.9 ( 0.5 38.9 ( 0.5
2.8 6.4 11.9 16.9
25 ( 2 27 ( 2 30 ( 1 28 ( 2
21 ( 2 25 ( 2 28 ( 2 25 ( 3
a Errors at 95% confidence level. b Errors for contact angles are given as maximum errors.
spectrum of a deuterated hexadecanethiol (HS(CD2)15CD3) SAM on a gold plate was recorded and used as a reference. The transmission infrared spectra of the compounds dispersed in KBr were recorded on a Bruker IFS 66v spectrometer at 5 mbar pressure. A DTGS detector was used, 500 interferograms were averaged at 2 cm-1 resolution.
Results and Discussion Ellipsometry and Contact Angle Measurements. The measurements of the ellipsometric thickness and contact angles are summarized in Table 1. The thicknesses of the SAMs correspond very well to the increase in the OEG chain length. Assuming a similar tilt angle of the alkyl chains to that in TOH, ∼30°,23 one can estimate the thickness of the EGn sublayers. TOH monolayers on gold are typically 22 Å thick. Let us also assume that this value is approximately valid for S(CH2)15CONH- part of the SAMs under investigation and that the tilt angle is roughly the same for all the compounds. Then the incremental thickness of the corresponding OEG sublayer can be calculated d-dTOH and is given in Table 1. The water contact angles for all four compounds were somewhat higher than that for the TOH SAM, which is an example of a hydrophilic, hydroxyl-modified surface (θa < 20 in our tests). This may be explained in terms of a somewhat different exposure of the EG hydroxyl groups. Although the contact angles exhibit only small oligomer chain-length-dependent variations, the highest advancing contact angle was obtained for EG4, which possibly indicates a slightly increased inhomogeneity of the outermost portion of this SAM. To summarize, the film thickness and contact angles indicate the presence of oriented films. However, the structural variations between the SAMs can be better understood by analyzing infrared spectra. Infrared Spectroscopy. The RA spectra of the OEGs are shown together with the spectrum of TOH SAM for comparison in Figures 1 and 3. The TOH compound has an identical alkyl chain of 15 methylenes and a terminal CH2-OH group. The nature of TOH spectral features is well-known;23 thus they represent a good reference for evaluation of the packing properties of the compounds under investigation. The RA spectra of the OEG-SAMs on gold show three characteristic spectral regions, having the origin in (1) the CH stretching modes of both alkyl and ether parts of the molecules, (2) the amide group, and (3) different inplane bending, wagging, twisting, rocking, as well as skeletal vibrations of the oligomer parts. In Table 2, the assignments are given for bands that display the largest sensitivity to conformational variations within the OEGs. The band assignments are taken from infrared studies of crystalline and molten PEG.4 Also, polarized infrared spectra of highly oriented crystalline films of PEG are used to evaluate orientation of the OEG groups.24 The (23) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
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Figure 1. CH-stretching region of EGn-terminated (n ) 1, 2, 4, 6) alkanethiolates and 16-mercaptohexadecanol (TOH) on Au. Figure 3. Fingerprint region of EGn-terminated (n ) 1, 2, 4, 6) alkanethiolates and 16-mercaptohexadecanol (TOH) on Au.
Figure 2. Comparison of amide I and II bands in KBr transmission spectra (dashed lines) and reflection absorption spectra of compounds EG6 and EG4.
weak negative peaks marked by “/” seen around 1100900 cm-1 in all spectra are due to the deuterated SAM on the reference plate, used as a protection against organic contamination from the air. The CH2 stretching region indicates a good crystalline structure of the alkyl chains in all OEG-SAMs (Figure 1). The position of the CH2 asymmetric stretch never exceeds 2918 cm-1 in all ∼30 measurements. The self-assembly of TOH obtained under identical conditions results also in well-packed monolayers, but larger variations are observed. The CH2 asymmetric stretch in a typical TOH spectrum shown here appears at 2919 cm-1, indicating a slightly increased amount of gauche defects along the alkyl chains, compared to the EGn SAMs. The CH2 symmetric stretch appears at 2851 cm-1 for EG6 and at 2850 cm-1 for the rest of the compounds. (24) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764-2776.
The 3000-2800 cm-1 region also contains methylene stretching modes typical for PEG. The broad shoulder around 2948 cm-1 is seen for all compounds and corresponds to the CH2 asymmetric stretch in PEG, while the bands of the CH2 symmetric stretch show changing features depending on the OEG chain lengths. EG6 has a strong unique peak at 2893 cm-1, and it corresponds to the parallel polarized mode at 2890 cm-1 in crystalline PEG. Further, a broad shoulder is seen at 2869 cm-1, which is present also in EG4. In EG2 and EG1 the oligomer bands are reduced, but they still contribute to the total intensity in the region and are mixed with a peak around 2879 cm-1. This latter peak is characteristic for TOH and is assigned to the methylene stretch of the outermost CH2 group of the alkyl chain, CH2-OH.23 Amides I and II. In previous infrared studies of alkanethiolate SAMs containing amide linkages the CdO and C-N bonds displayed a preferential alignment with respect to the surface normal.25-28 The amide I and II region of EG6 and EG4 SAMs on gold is compared to the pure compounds in KBr and shown in Figure 2. Both bands demonstrate very strong dichroism of the amide linkage. Only the C-N-H in-plane bend combined with C-N stretch (amide II, polarized parallel to the C-N axis) is active in the RA spectra, in contrast to the CdO stretch (amide I, polarized parallel to the CdO bond) which is normally seen in the transmission spectra at 1641 and 1633 cm-1 for EG6 and EG4, respectively. The absence of the amide I peak confirms an almost perfect alignment of the carbonyl bonds parallel to the surface in all four OEG compounds; see Figure 3 for EG2 and EG1. OEG Fingerprint Region. Assuming highly ordered alkyl chains and amide groups, one would also expect the OEG fingerprint bands, which display a more or less parallel (25) Tam-Chang, S. W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382. (26) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124-136. (27) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (28) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 46104617.
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Langmuir, Vol. 15, No. 10, 1999 3393
Table 2. IR Mode Assignments (cm-1) for OEG-Terminated Alkanethiolates on Gold and for Crystalline and Molten PEG4,24 PEG, crystallinea
2890 s 2865 s 1470 m/1463 m 1345 s 1244 m 1149 s 1119 s 1102 vs 963 s 947 m
PEG, moltena
2865 s 1249 m 1140 sh
mode assignmenta CH2 asym stretch, alkyl CH2 sym stretch, alkyl CH2 sym stretch CH2 sym stretch CH2 scissor CH2 wag CH2 twist C-O, C-C stretch
1107 s 945 m
CH2 rock, twist CH2 rock gauche, C-C stretch
polarizationb
| | ⊥/| | | ⊥ ⊥ | | ⊥
EG1
EG2
EG4
EG6
2918 2850
2918 2850
2918 2850
1466
∼2870 1466
2868 1465
2918 2851 2893 2869 1466 1349 1244/1252c ∼1146 1126 1114 964 944
1254c
1253c 1143
1253c 1146 ∼1108 ∼938
a
vs, very strong; s, strong; m, medium; w, weak; sh, shoulder; asym, asymmetric; sym, symmetric. b Transition dipole moment with respect to the helical axis in crystalline PEG. c May overlap with the amide III band.
alignment of the transition dipole moment with respect to the surface normal, to be dominating in the spectra,24 Figure 3. A broad peak around 1466 cm-1 in EG6 appears as a result of different CH2 scissoring modes from both the alkyl and OEG chains. The dominating contribution seems to be from the parallel mode seen in crystalline PEG at 1463 cm-1. With the OEG chain length reduced, the intensity of this mode drops, exposing another component of the peak around 1458 cm-1 for EG4 and EG2. Next, in the region of CH2 wagging modes, a sharp peak is seen at 1349 cm-1 for EG6. It is related to a characteristic peak in spectra of crystalline PEG, having origin in a wagging mode with a transition dipole moment oriented along helical axis of the polymer. This peak is clearly absent in the spectrum of EG4, as well as in the spectra of the shorter compounds. Several CH2 twisting modes are observed around 13001200 cm-1 in all compounds, possibly containing a contribution from the amide III (C-N stretch and N-H in-plane bend), which is expected to appear in the same frequency region.26 A different feature is seen again for EG6 at 1244 cm-1, while it is missing in spectra of the other OEGs. This is in a good agreement with the supposed crystalline helical structure of EG6.10 This band was shown to move toward higher frequencies for the molten PEG as a result of transition from a helical crystalline structure to an amorphous state. Thus, the peaks around 1252 cm-1 could indicate the presence of the amorphous phase in the OEGs. The suggested structural differences in relation to the chain length are most clearly seen for the skeletal modes of the OEGs. The EG6 spectrum shows a characteristic, very strong peak at 1114 cm-1 with a slightly weaker component at 1126 cm-1 and an additional shoulder appearing on the high-frequency side of it. The 1114 cm-1 peak can be identified as a parallel polarized C-O stretching mode, a very distinctive sharp feature in spectra of crystalline PEG. An essentially different shape of this band is seen for EG4 and EG2 indicating that a conformational transition occurs as a consequence of the reduced chain length of the oligomers. The strong resemblance between their RA spectra and the corresponding spectrum of the EG3 SAM on silver10,29 suggests that the EG4 and EG2 chains assemble predominantly in the all trans conformation. Furthermore, the EG4 SAM seems to consist of at least two phases, as evidenced by the appearance of a broad low-frequency shoulder on the main peak at 1146 cm-1. For EG1 the skeletal modes disappear, and the only
feature in this region may be assigned to a C-O stretching mode of the terminal COH group, seen also in TOH as a doublet at 1076/1061 cm-1. The weaker intensity of this peak in EG1 is probably due to different orientation of the C-O group. The sharp peak appearing at 964 cm-1 in EG6 is attributed to the CH2 rocking mode of helical crystalline chains. In the molten state, PEG is enriched with gauche conformers, resulting in another rocking mode, appearing as a weak and broad low frequency shoulder on the 964 cm-1 peak. It is present in both EG6 and EG4 at 944 and 938 cm-1, respectively. Finally, it is important to stress that we have not, under the present experimental conditions, been able to prepare EG4 and EG6 SAMs that can be represented by a single (pure) phase of helical, all trans, or amorphous OEG. Instead, the RA spectra typically show the presence of a dominating phase that coexists with one or two other less prominent phases. This observation is also consistent with conclusions drawn from studies on analogous systems.10
(29) Pertsin, A. J.; Grunze, M.; Garbuzova, I. A. J. Phys. Chem. 1998, 102, 4918-4926.
(30) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486-2487.
Conclusions We have reported on the self-assembly of four different oligo(ethylene glycol)-terminated and amide group containing alkanethiols on gold. The incremental film thicknesses (d-dTOH, Table 1) obtained from the ellipsometric measurements are in good agreement with the expected chain length differences between the EGn compounds. The ellipsometric thickness and contact angle measurements also correlate well with the excellent packing and ordering properties evaluated with infrared spectroscopy. The RA spectra of the EG6 SAM reveal the presence of an oriented and highly crystalline, helical OEG phase. This is consistent with recent findings for ether-bonded EG6-containing SAMs.10,16 Harder et al. stressed, however, in their study of ether-bonded EG6 SAM that the spectral features representing a high crystallinity (virtually identical with those shown in Figure 3 for EG6 at 964, 1114, 1244, and 1349 cm-1) were exceptional (6 out of ∼100).10 We always obtain spectra identical to the one in Figure 3, using similar experimental conditions. A possible explanation to the improved reproducibility for our EG6 SAM is that the introduced amide linkage contributes to the in-plane stabilization of the helical phase by providing a two-dimensional hydrogen bonding network.25-28,30 The longer alkyl chain may also in part explain the improved reproducibility. A different conformation of the OEG portion is observed in the RA spectra of the shorter compounds EG4 and EG2.
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A comparison with recent studies of EG3 SAM on silver10,29 suggests that the EG4 and EG2 chains adopt the all-trans conformation. The all-trans conformation of our EG4 SAM on gold was not expected as the helical phase was found to be dominating in ether-bonded EG3 SAMs on gold.10,29 The critical chain length for obtaining a helical conformation is apparently shifted from three ethylene glycol units for the ether-bonded EG3 SAMs10,29 to more than four units for the compounds investigated here. Thus, the present study demonstrates that specific intermolecular interactions, in this case lateral hydrogen bonds, can be used to control and manipulate the molecular conformation of the oligomer portion of the EGn SAMs. Our current research
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efforts are aiming at establishing a deeper understanding of the physical mechanisms driving the conformational transitions of the oligomer. A more detailed study on the role of lateral hydrogen bond for the molecular packing, conformation, and stability of OEG-terminated SAMs will be presented separately. Acknowledgment. The Graduate School Forum Scientum is founded by the Swedish Foundation for Strategic Research. This work was also supported by the Swedish Research Council for Engineering Sciences. LA981647O
