Langmuir 1993,9, 141-149
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Infrared Study of Thiol Monolayer Assemblies on Gold Preparation, Characterization, and Functionalization of Mixed Monolayers L. Bertilsson and B. Liedberg' Laboratory of Applied Physics, Linkaping University, S-58183 Linkaping, Sweden Received June 1,1992. In Final Form:September 4,1992
We report on the preparation and characterization of self-assembling monolayers (SAM's) on polycrystalline old surfaces. The aimof thework is to developmodel surfacesfor applicationsin biomolecule interactionstufIies. Our attention is primariiy focused on mixed monolayersof alkanethiolswith different chainlengthsand tailgroup. Mixed monolayera of 16-thiohexadecanol(HS-(CH2)leOH)and n-alkanethiola (H&$CH2)n 2920 cm-l).' Our infrared data (20)(a) Snyder, R.G.;Straw, H.L.J . Phys. Chem. 1$82,86,61455160. (b)Snyder, R.0.;Maroncelli, M.; Straw, H. L.;Hallmark, V. M. J . Phys. Chem. 1986,90,6623-6630.
of HS-(CH2),,-CH3monolayers,Figurelb-e, confirmtheae results and show the expected increase of the v,(CH2) frequency with decreasing chain length. Furthermore, a significant increase in the u,(ip) absorption at 2966 cm-1 is seen for even methylene chain lengths, Figure 1. This intensificationsuggeststhat the transition dipole moment of the v,(ip) mode, which is known to be aligned perpendicular to the C-CH3 bond? is oriented more parallel to the surface normal for even thanfor odd CH2 chains. Such an orientation gives rise to a stronger coupling with the incoming EM wave and thereby to a stronger mode intensity in the reflection-absorption (R-A)spectrum.21 The chain orientation for pure HS-(CH&&H monolayers has a b been determined and we find a chain tilt, a, of about 22O from the surface normal, and a chain rotation, 8, of about 50° for the C-C-C plane, Figure 2. These angles are in fair agreement with a = 28O and B = 50° obtained by Nuzzo and co-workers.3 Thus,the resulta from the present study of single component monolayers confinn the well-establiahed picture of a self-organizing layer with the alkyl chains in the all trans (zigzag) conformation,forming quasi crystal structures with high packing density. The effect on the alkyl chain absorption spectrum upon molecular mixing and further functionalization of the tail group (OH) will be discussed later in this paper. Mixed Monolayers. The question of whether mixed thiols in solution adsorb a~ mixed monolayers on gold has been addressed in severalpapers. Bain et al.*l0 concluded from XPS and wettability experiments of coadsorbed OH and CH3 terminated thiols and disulfides, with a chain length of 10-11 methylene groups, that none of these mixtures formed single-component domains or islands of any significant size. Figure 3 showa the R-Aspectra of a pure HS-( CH2)1gOHmonolayerand monolayeraprepared from different HS-(CH2)leOHHS-(CH2),-CHsmixtures. In the spectrum of the pure HS-(CH2)l&H monolayer, Figure 3a, a broad absorption band appears in the 3100-3500 cm-l region. This band together with a broad feature in the 750-500 cm-l region (not shownhere) are characteristic for intermolecularly hydrogen bonded alcohols,suggesting that lateral hydragen bonding occum between neighboringOH groupson the outermoetsurface. The 3100-3600 cm-l band is rtill present, but with reduced (21) (a) Greenler, R.G. J. Chem. Phys. 1066,44,91&996. (b)Fnncir, S.A.; Ellieon, A. H.J. Opt. SOC.Am. 1960,49, 131-138.
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formation of single component domains or islands, we conclude that true molecular mixing occurs for the HS(CH2)16-OH:HS-(CH2),-CH3 monolayers investigated here. We recently became aware of a paper by Laibinis et al.22 who studied the coadsorptionof n-alkanethiolswithdiffent chain lengths on gold. They found that the portion of the monolayer close to the head group (Au-S) consisted of a highly ordered alkyl chain with a structure virtually indistinguishablefrom that of a single-componentmonolayer, i.e., a fully entended all trans structure. The outermostportion of the monolayer displayed,on the other hand, alkyl chains with an increasing amount of gauche conformers. At very high surface concentrations of the longer alkanethiol, the amount of gauche conformers was found to decrease,a behavior which was taken as evidence for the formation of small (microscopic)clusters. Folkers et al.23 have recently investigated the coadsorption of alkanethiolswith dissimilar chain lengths and tailgroups. They also found evidence for phase separation into microscopicclusters under certain experimentalconditions and proposed that the driving force for aggregation was due to interactions between adjacent methylene chains of the same length. The reason for not obtaining any clear evidence for cluster formation in the present work is most 1 likely due to the small difference in chain length of the 360032002800 1400 1200 loo0 Wavenumber (cm-1) constituents used as compared to those in refs 22 and 23, making aggregation into clusters less favorable. Most Figure 3. R-A spectra showing mixed monolayers of OH- and importantly, however, none of the above investigaCHpterminatedthioln adsorbed on gold. In the preparation the following solution mixtures were used (a) pure HS-(CHz)lr tors.+10922t23suggest phase separation into large single O H (b) 1:1 mixture of HS-(CH&e-OH and HS-(CH~)II-CH~; component domains or microscopic islands. (c) 1:l mixture of HS-(CHz)rOH and HS-(CHz)ls-CHs; (d) The R-A spectra of the mixed monolayers of thiolswith 1:lO mixture of HS-(CHdls-OH and HS-(CHz)ls-CHs. the same chain length, Figure 3c,d, exhibit no signs of intensity, in the spectrum of the 1:l mixture of the HSstructural disturbances of the alkyl chains. The v,(CHd (CH~)~~OH:HS-(CH~)I~-CH~ monolayer, Figure 3b. Acfrequency occurs at 2918 cm-l, which is identical to that cordingly, HS-(CH~)I~OH:HS-(CHZ),-CH~ mixtures of observed for the pure HS-(CH2)lgOH monolayer,Figure different chains lengths, where n < 16, appear to allow for 3a. A shortening of the chain length of the HS-(CHz),lateral hydrogen bonding as long as the intermolecular CH3 thiol to n = 11shifts the va(CH2)frequency toward distance between neighboring OH species is short enough, slightly higher frequencies -2920 cm-l, Figure 3b. Ala phenomenonwhich most likely reflects the mobile (liquid though, this shift is consistent with a transition toward a like) character of the outermost methylene bonds in the less ordered monolayer structure, it is still lese than that HS-(CH&g-OH molecule. The situation is dzferent for obtained for the pure HS-(CH~)I~-CHI monolayer (2922 the 1:l mixture of HS-(CH~)I~OH:HS-(CH~)I~CH~, cm-l), Figure le. Recent electrochemical measurements Figure 3c, where the 3100-3500 cm-l band no longer is (1:l)monolayer,ls of HS-(CH~)I~-OH:HS-(CH~)~~-CH~ present. The only band present in the high frequency Figure 3b, furthermore reveal that it has essentially the range is avery weak feature near 3660 cm-l which we assign same blocking characteristics as the pure HS-(CH2)1,3to a free (non-hydrogen-bonded)OH stretching vibration. OH monolayer,whereas the HS-(CH2)11-CH3 monolayer, Thus, the lateral hydrogen bonding pattern appears to be Figure le, only exhibits about 2 4 % of the blocking completely disrupted already at a 1:l mixture of OH and efficiency obtained for the pure HS-(CH2)lg-OH monoCHa terminated thiols of equal chain length. However, layer. We concludetherefore that the coadsorptionprocess there is still an open questionof whether the disappearance in itself will not necessarily introduce any significant of the 3100-3500 cm-l band is due to a more or less perfect amount of defecta or disordered domains within the screening of the OH terminals caused by true molecular monolayer. This statement is consistent with the obsermixing, or to a significant dilution of the HS-(CH2)leOH vations by Chidsey et al.243who studied the electromolecules on the surface. Bain et al.10 studied mixed chemical activity of ferrocene-terminated alkanethiols in monolayersof shorter OH and CH3 terminated thiols and mixed n-alkanethiol monolayers on Au(ll1) crystal faces. disulfidesand proposed that the adsorptionof CH3species The ferrocene/methyl monolayers were found to be very was strongly favored. Intensity data from the low frestable and able to withstand oxidation/reduction cycles quency range in Figure 3 gives, however, a clear answer without influencingthe underlayingalkyl chain structure. to that question. By using the integrated area of the C-O In fact, the mixed monlayers were inferred to have “the stretching peak near 1060cm-’ as a measure of the surface same overlayer structure as the uneubstituted alkanethiol concentration of HS-(CHdl&H thiols within the monomonolayer on Au(ll1)”. layer, we estimate that about 50% of the original C-0 stretching intensity (see Figure 3a) still is present in the (22) Laibinie, P. E.; Nuzzo, R. C.; Whiteeidea, C. M. J. Phys. Chem. R-A spectrum of the mixed monolayer, Figure 3c. Thus, 1992,96,6097-6106. a 1:l mixture in solution seems to give a molecular (23) Folkere, J. P.; Laibinie, P. E.; Whiteaiden, G. M. Longmuir 1992, 8, 1330-1341. composition close to 1:l on the gold surface. From the (24) Chideey, C. E.D.Science 1991,251,819-922. above intensity consideration and the fact that the lack (26) Chidsey, C.E.D.;Berbzzi,C.R.;Putvinrki,T.M.;Muh, A.M. of tail to tail group interaction is hard to combine with the J . Am. Chem. SOC.1990,112,4301-4306. 10.5
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Mixed Alkanethiol Monolayer Assemblies on Gold I1
3100
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lo00
Figure 4. R-A spectra of a pure HS-(CH&&H surface after reaction with trifluoroacetic anhydride, TFAA. The R-A spectrum of an unreacted HS-(CH*)le-OH surface is shown in the C-O stretching region for comparison.
In coadsorption experimenta when adsorption from mixed solutions with low HS-(CH2)lgOH concentration is made, the C-O signature at 1060 cm-l becomes very weak (cf. Figure 3d). In order to be able to study the monolayers prepared under such conditione, CF3 groups were introduced on the surface of the mixed monolayers by reacting the OH groups with trifluoroacetic anhydride (TFAA), eq 1. This reaction gives rise to a strongly
k
O-CO-CF, + CF3-COOH
(1)
absorbing trifluoromethyl ester group which has several characteristic C-F stretching bands in the 1100-1300cm-l region. Figure 4 shows the R-A spectrum of a pure HS(CH2)lgOH monolayer after reaction with TFAA, according to eq 1. The strongest bands occur at 1260,1197, and 1158cm-l and are assigned to the u(FCF2) (a'), u(CF3) (a'), and u(CF2) (a") stretching modes,%respectively. The trifluoromethyl ester group also has a strong and asymmetrically shaped c-0 stretching vibration peaking at 1807 cm-l. This mode is shifted upward -60 cm-l as compared to a normal methyl ester group, making it very useful for identification purposes. A very important observation in Figure 4 is that the frequenciesand relative intensities of the u,,(CH3 stretching vibrations at 2919 and 2851 cm-l, respectively, are almost identical to those obtained for the pure HS-(CH2)lgOH monolayer, Figure 1, suggesting that the alkyl chain conformation and orientation remain unaffected by the TFAA reaction. Another important observation is that no traces of unreacted OH groups can be seen in Figure 4. By comparing the peak height of the 1060 cm-l band in the R-A spectrum obtained after the TFAA reaction (set equal to the noise near 1060cm-l since no such band can be seen in the R-A spectrum) with the peak height obtained in the R-A spectrum of the pure HS-(CH2)1gOH monolayer, inserted in Figure 4,we estimate the yield to be about 80-9095 for the TFAA reaction, eq 1. It is also clear from Figure 4 that the TFAA reaction enhances the C-O signatureby approximately a factor of 10,which in practice (26) Redington, R. L. Spectrochim. Acta 1976,31A, 1690-1705.
lsoo
1600 1400 1200 loo0 Wavenumber (cm.1) Figure 5. R-A spectra of different x:y mixtures of HS-(CHz)lg
OHHS-(CH2)l&H3 monolayers after reaction with "FAA: (a) 1:0 (1.0);(b)9 1 (0.9);(c) 1:1(0.6); (d) 1:s (0.17);(e) 1:20 (0.048). The values given within parenthesesare the correspondingmole cf. Figure 8. The spectrum of amodel fractions in solution xloloH; molecule trifluoroethylacetate (TFEA) diluted in n-hexaneand measured in transmission is shown in part f.
allows us to detect one OH function surrounded by approximately 60-100 CHs groups in a mixed monolayer. Figures 6 and 6 show R-A spectra of mixed monolayers after the TFAA reaction. The chemical composition of the monolayer and the mobile character of the tail group are both expected to influence the interaction with surroundingmedia, in this case TFAA. We have therefore chosen to study two differnt mixtures, one of equal chain Figure 6, and lengths HS-(CH~)~~OHHS-(CH~)I~-CH~, one with unequal chain lengths HS-(CH2)lgOHHS(CH2)13-CHs,Figure 6, where the OH group is more freely stickingout from the surface. Also shown in Figure 6 and 6 is the absorbance spectrum of a model molecule trifluoromethylethylacetate (TFEA) diluted in n-hexane. This solvent is used to simulate the hydrocarbon environment in mixed monolayers with low HS-(CH&&H content. Some general observations can be made from the R-A spectra in Figures 5 and 6. First, the spectral signature in the C-F stretchingregion, 1100-1300cm-l, is essentiallythe same if the two mixtures are prepared at high mole fractions xIoloH= ( ~ o H / ( ~ H + ~ C H & ) ~ Icf. ; the mixtures in Figures 6a,b and 6a,b. A commontrend for both mixturesis also that the vibrational frequencies for the C-F modes gradually decrease, about 10-16 cm-l, toward the values obtained for TFEA in n-hexane, Figures 6f and 6f, upon decreasing a l o H . This result is not unexpected since n-hexane is used to model the hydrocarbon environmentfor the trifluoromethylester group in mixed monolayers at low mole fractions. More pronounced differences can be observed in the relative intensities between the two mixtures at low mole fractions. The relative intensities for the C-F modes in the R-A spectra of the HS-(CH2)lgOHHS-(CH2)lgCHs mixture, Figure 6, remain almost unaffectedupon decreasing x,oloH
146 Langmuir, Vol. 9, No. 1, 1993
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Figure 6. R-A spectra of different x y mixtures of HS-(CH2)16OH:HS-(CH2)134!H3 monolayers after reaction with TFAA: (a) 1:0 (1.0); (b)9:1(0.9);(c) 1:l (0.5);(d) 1:5 (0.17); (e) 1:20 (0.048). See legend of Figure 5 for further details.
and agree very well with those obtained for TFEA. This observation suggests not only that TFEA in n-hexane is an excellent model molecule but also that the introduced trifluoromethyl ester group exhibits an orientation and conformation which appears to be almost independent of the composition of the monolayer. However, the relative intensities of the C-F modes are not independent of xsolOH for the HS-(CH~)~&H:HS-(CH~)~S-CH~ monolayer, Figure 5. The C-F stretchingpeak at 1197cm-l disappears almost completely at low xsoloH,indicating the formation of an ordered structure different from that obtained for the H S - ( C H ~ ) I ~ ~ H : H ~ - ( C H ~monolayer. ) ~ ~ - C H ~This result is not unexpected since the trifluoromethyl ester group occupies a site with a more rigid environment of CH3 groups when the alkyl chains have the same length. The outermost methylene bonds in such a monolayer are also expected to be less flexible, and space-filling models clearly show that the rotational freedom around the outermost methylene bonds is reduced because of steric hindrance, Figure 7. Second, the carbonyl stretching vibration of the trifluoromethylester group behaves essentially in the same way for the two monolayers, Figures5 and 6. Its frequency is slightly higher than that expected at high xsoloH(e15 cm-l) but approaches the value obtained for TFEA with decreasing xsoloH.Furthermore, the intensity of the C 4 mode in the R-A spectra is considerably reduced as compared to the C-F modes. We assign this change in relative intensity to an orientation effect, where the transition dipole moment of the C=O mode, M c ~ is, aligned nearly parallel to the metal surface; cf. Figure 7. The only main difference between the two mixtures is that the C = O mode seems to disappear faster with decreasing xsoloHin the R-A spectra of the HS-(CH&OH:HS-(CH2)1&H3 monolayer, Figure 5. Another interesting observation is the change in line shape of the C=O stretching vibration with xsoloH.The appearance of
Figure Space filling models showing a top view of a IS(CH2)?6- H monolayer after reaction the TFAA and with (top) and wthout (bottom) an unreacted OH center.
a low frequency shoulder on the sharp C=O band at high xsoloHis characteristic for hydrogen bonded carbonyls. Even though there is a small number of unreacted OH groups (yield80-90% ) present on the surface at high xmloH, spacefillingmodels, Figure 7, indicatethat it is not unlikely that some OH groups remain unreacted because of steric effects and thus can act as hydrogen bond donators for the carbonyl oxygen in the trifluoromethyl ester group. On the other hand, we have not been able to mimic this type of hydrogen bonding by studying different mixtures of TFEA in methanol, suggesting perhaps that there is another type of tail group interaction that is responsible for this carbonyl shoulder, as well as for the upward shift of the C=O and C-F modes at high xsoloH. Although, we have not been able to give a conclusive interpretation of the changes in tail group interaction with composition of the monolayers, Figures 5 and 6, they seem to follow the behavior of the OH groups in the unreacted monolayers very nicely (cf. Figure 3). Both the line shape of the C==O mode and the relative intensities and frequencies of the CF3 modes show a significant change for monolayers prepared in the 9:l to 1:l regime, Figure 5b,c, which shouldbe directlycompared with the hydrogenbond dissociation observed in the same regime, Figure 3a,c. The monolayers prepared with unequal chain lengths, Figure 6, show a similar trend in the 1:l to 1:5 regime, but the transition is not as sharp as in Figure 5. Again, we attribute this behavior to a more flexible alkyl chain,which allowsthe tailgroupsto interact over distances greater than the interchain spacing between neighboring molecules on the gold surface; cf. Figure 3b. Thus, the chemical picture emerging from this tagging experiment
Mixed Alkonethiol Monolayer Assemblies on Gold
Langmuir, Vol. 9, No. 1,1993 147
ester groups and how these relatively large tailgroups will influence the TFAA reaction sterically,Figure 7. The size of the trifluoromethyl ester group as compared to the hydroxylgroup is most likely responsiblefor the saturation effect observed in the high mole fraction regime, %loH 8: 0.8, Figure 8, where both mixtures behave analogously 0.8 and exhibitthe same x l d Hvalues asfor a pure OH surface. The x s d O H values for the two mixtures deviate, however, more stronglyin the low mole fraction regime (0.1 < x.,J'H < 0.7), where steric hindrance from neighboring trifluoromethyl ester groups is assumed to be of less importance for a complete functionalization. The x I d H values for an identical tagging experiment on a HS-(CH2)l&H: HS-(CH2)1&Hs m~nolayerl~ are also plotted (X) in the low mole fraction regime x ~ O HI0.5 for comparison,Figure 8. This monolayer shows, as expected, nearly the same xsoloH-dependenceas the HS-(CH~)I~OH:HS-(CH~)~~CH3 monolayer. T h e higher &doH values obtained for 0.0 and HS-(CH2)1r the HS-(CH2)1gOH:HS-(CH2)lgCHs 0.0 0.2 0.4 0.6 0.8 1.0 OH:HS-(CH2)1&H3 monolayers are again assigned the liquid-like character of the outermost methylene groups; Figure 8. Diagram showing the mole fraction of OH groups in i.e., a flexible alkoxide group available for attack on the mixed monolayers as a function of mole fraction in solution: ( 0 ) carbonyl carbon in the TFAA molecule (eq 1)is from a H S - ( C H ~ ) ~ ~ ~ H : H S - ( C H ~( 0 ) I) ~HS-(CH&-OH:HS~H~; mechanistic point of view certainly more advantageous (CH2)ldHs; (X) HS-(CHn)1rOHHS-(CHzhdH3 taken from than and alkoxide group embedded in a hydrophobic ref 19. The mole fraction of OH species in the monolayer is determined as x.UlrOH = tJ~1OP'"logRo(R)oH/cH~~ v / ~ ~ ~ ~ 1 3 0 0 (pocket l o g of CH3 groups. In summary so far the tagging experiment using TFAA R ~ R ) o&I. H The error bars refer to "mum dewatlone. Note that the spread in the x.doHvaluea is much larger for the mixture supporta our previous propoealthat true molecular mixing of equal chain length. ( 0 )X P S data for the HS-(CHz)lgOH occuz[~for the combination of HS-(CH2>m4HWCH2),,HS-(CH&-CHs monolayers based on the integrated XPS CH3monolayersinvestigatedin this study. Quantitatively intensity for the O(1s) p d q see text for further details. the infrared method used for compositional characterization appears to overestimate x,&H as compared to the isconsistentwith our previoueproposalthat true molecular mixing occurs for the combinationsof tailgroups and chain XPS method."J A smaller tail group than the trifluoromethyl ester group, but with similar oscillator strength lengths used here. would perhaps resolve this problem. However, the real Third,the strong C-F stretching signature in the 11001300cm-l region also has been used to obtain a correlation advantage with infrared method over XPS is that we can between x,lOH and xaurpOH. xau,tOH is obtained by inteperform compositionalstudies at low mole fractions with a resonable accuracy. Mixed monolayer with grating the peak area of the C-F stretching vibrations 0.02, i.e., 1OH surrounded by 60 CH3 groups can easily (1100-1300 cm-l) and normalizing it to the average area obtained from four TFAA reacted HS-(CH2)lgOH surbe studied with our infrared apparatus. In fact, we faces. Thereeultsfrom HS-(CH~)I~OH:HS-(CH~)IE,-CH~ estimate the detection limit to be approximately 1OH/ 100 CH3. and HS-(CH2)lgOH:HS-(CH2)13~H3 monolayers are plotted in Figure 8. Most of the experimental points in Functionalization of SAMs. We have also inveetiFigure 8 lie close or just above the line of unity slope, a gated the possibilities of using HS-(CH&gOH.HSbehavior which appears to favor adsorption of hydroxyl (CH2)13-CH3 monolayers as a base for further functionalization. The main purpose is to develop new and wellterminated thiols. This observation does not agree with characterizedsurfacesfor biomolecule interaction studiee. previous XPS studies of thiols and sulfides by Bain et al.,lo who reported a strong preference for adsorption of The reactivity and accessibility of OH groups have been methyl-terminated disulfides and thiols, especially at low studied by reacting them with chlorosulfonic acid ClSOr OH in an attempt to form sulfate groups on the surface, x,,,lOH. We also have u88d XPS,in the same way as Bain et al,,'O to determine the composition of the monolayers, and Figure 9 shows the results after such a reaction for three monolayers, with different composition. Characand the results from such a determination for the HSteristic vibrations for the sulfate group are seen in the (CH~)I~OH:HS-(CH~)~~-CH~ monolayers are plotted in 12W1220 cm-1 region and at 1069 cm-l, which we assign Figure 8 for comparison. T h e mole fraction at the surface has been determined by ratioing the O(1s) intensity for to asymmetric and symmetric 03S=ostretching vibrations, respectively.n The peaks near 1006 and 816 cm-' the mixed monolayer with that obtained for the pure HS(CH2)lgOH monolayer. The measured intensity (inteare assignedto asymmetxicand symmetric C-0-sstrectchgrated) of the O(1s) line was normaliied to the Au 4f(7/2) ing vibrations. Furthermore,the R-A spectra of the sulfate surfaces,Figure 9, showno shift of the v,(CH2) and &H2) lineintensity at 83.8 eV for each sample (mixture)in order to account for variations in the alignment of the sample. stretching modes (not shown here) as compared to the The XPS data points lie all close to line of unity slope spectra of untreated surfaces, Figure 1, indicating that rev& that the infrared method slightly overestimates the alkyl chains remain conformationally intact during -OH. It should, however, be kept in mind that the the reaction. For the 1:2 and 1:6 mixtures, Figure 9b,c, infrared data presented in Figure 8 are based on a tagging the spectral signatures are essentially the same as for the experiment which not necessarily gives the true mole pure monolayer, and again the CH2 stretching region fraction of OH species on the surface, but rather the suggests that the alkyl chains are conformationallyintact. fraction of OH species accessible for the TFAA reaction. Thus, the data undoubtedly reveal that sulfate groups Thus, the data in Figure 8 will ale0 contain information (27) Cabmi, F.;Cmu, B.;Perlin, A. 8. Carbohydr.Rsr. 1978,63,1-11. about the size and shape of the introduced trifluoromethyl
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Figure 9. R-A spectra of OH containing monolayers after reaction with chlorosulfonic acid (a) a pure HS-(CH2)l&H monolayer; monolayere prepared from (b) a 1:2 and (c) a 1:5 mixture of H S - ( C H ~ ) ~ ~ H H S - ( C H Z ) ~ ~ ~ H ~ .
m
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Figure 10. R-A spectra of OH-containing monolayers after reaction with phosphorus oxychloride: (a) a pure HS-(CHdlg OH monolayer;monolayers prepared from (b)a 1:2 and (c) a 1:5 mixture of H S - ( C H ~ ) ~ ~ ~ H H S - ( C H Z ) ~ ~ - C H ~ .
tabulated the frequency limita for the P = O group in 900 can be introduced on the surface in a simple and organophosphoruscompoundsand proposed the following reproducible way without influencing the alkyl chain frequency limita for the P 4 vibration in compounds conformation. with the subgroups (1-1111, where R, R’, and R” are The same type of HS-(CH~)I~OH.HS-(CH~)I~-CH~ monolayers were also immersed into a solution of phosI I1 111 phorus oxychloride, Cl-, in THF. The idea was to introduce-CH+POC12 groups and then hydrolyzethem into -CH+POsH2 groups. However, the reaction schemefor the C l s p s o moleculeis more complicatedthan for chloroeulfonicacid because it has three chlorineswhich i 2 5 a i 2 cm-‘ ~ 1258-1266 m-’ 1280-1299 m-’ allcen be replaced by surfacehydroxyls. Thus, aquestion of prime interest is whether the Cl3P-O molecule reacts aliphatic groups. The presence of the P - C l peak at 590 with one surface hydroxyl to form - C H 2 4 P 4 ( O H h cm-l in Figure 10a immediately excludes structure In. species or if it forms bridges like WH2-032 P 4 ( O H ) Moreover,the exact frequencyof the P-0 mode strongly or (-CH+)@=O between neighboring hydroxyl terfavors structure 11, indicating that a bridging reaction minals. Such bridges will influence not only the chemical occurs with neighboring surface hydroxyla on a pure HSidentity of the functionalizedsurface but also the mobility (CH2)ldH monolayer. of the alkylchains. The likelihood for bridging is of course Figure 10b shows the R-A spectrum of the sample in dependent on the distribution of OH functions on the Figure 10a after it has been immersed in an ultraaonic surface,or more correctlythe distance between OH groups bath of pure water for 10 min. The most pronounced on the surface. changes in the R-Aspectrum, Figure lob, are the disapFigure 1Oa shows the R-A spectrum of a pure HSpearance of the P-Cl stretching mode and the lowering (CH2)rOH monolayer after treatment in C 1 H in of the P 4 stretching frequency to 1269 cm-l. A weak THF, but before hydrolysis. This spectrum is characand very broad peak occurs also in the 2200-2600 cm-’ terized by strong peaks at 1284,1072,and 590 cm-l. The region (not shown here) after hydrolysis. This peak is peak at 690 cm-l is assigned to the P - C l stretching assigned to the OH stretching mode of a P - O H groups, vibration.* The assignment of the peak at 1072 cm-l is, revealing that >-(OH) is formed on the surface. The due to ita position and shape, more uncertain. However, presence of such groups explains also the behavior of the the P - 0 - C group is expected to adsorb strongly in this P=O mode, which is known to move downward in region. Other groups like P - O H are also expected to frequencyin a hydrogen bond donating environment.The absorb in that region, but they occur generally at slightly mixed monolayersexhibit also a completely different, and lower frequencies. The peak at 1284cm-1 is undoubtedly continuously changing, spectral signature in the 1100assigned to the phosphoryl (P-0) group. Thomas et al.29 (28) Corbridge, D. E. C. J. Appl. Chem. 1966,6, 456-466.
(29) Thomaa, L. C.; Chittenden, R. A. Spectrochim. Acta 1964,20, 467-487.
Mixed Alkanethiol Monolayer Assemblies on Gold 950 cm-l region as compared to the pure HS-(CH&gOH monolayer. The originally very strong P-0-C mode at -1070 cm-l is drastically reduced in intensity for the 1:2 monolayer,Figure lOc, and appears only as a weak shoulder on a new, and differently shaped, peak centered at 1066 cm-l. This new peak disappears upon further dilution, and the only remaining peake in the R-Aspectrum of the 1:s mixture, Figure lOd, are the P - 0 - C mode at 1078 cm-l and anew peak near 990 cm-l. An additional lowering of the OH content to 1:lO gives a less intense but otherwise identical spectrum as the one in Figure 1Od. Although, we cannot explain all spectral features, especially not in the 1100-950cm-l region, some conclusions can be drawn from Figure 10. Most importantly, the ability to form strong hydrogen bonds ( P e O . - H 4 ) appears to be more pronounced in the mixed monolayers, as revealed by the downward shift of the P=O mode from 1269cm-l for the pure HS-(CHz)lgOH monolayer to 1236 cm-l for the 1:2 and 1:sHS-(CH~)I~OH:HS-(CH~)~~-CH~ mixtures. The only reasonable interpretation to this behavior is that the distance between neighboring surface hydroxyls becomes too large for briding (11)and that most C l 8 4 molecules react with only one OH group to form -CH&P=O(OH)2 species on the surface. One extra hydrogen bond donating group per introduced tail group will certainly increase the probability for strong (internal) hydrogen bond formation, and thereby result in a downward shift of the p-0 mode. The CH2 stretching region is also unchanged, indicating that the alkyl chains remain conformationally intact after the C l a p 4 reaction. Biologically important ligands like sulfates and phosphates can easily be introduced on HS-(CHz)lg-OH and HS-(CH~)I~OH:HS-(CH~)~~-CH~ monolayers. The sulfate reaction is straightforward and the product formed appears to be independent of the composition of the monolayer. The situation is more complex for the phosphate reaction since the C18=0 molecule can react and form bridges with more than one surface hydroxyl. Thus, the final product on the surface is strongly dependent on the composition of the monolayer, and a considerable amount of research effort must, therefore, be focused on the preparation and characterization of such monolayers before we can employ them in our biologically oriented work.
Langmuir, Vol. 9, No. 1, 1999 149
Conclusions Self-assembledmonolayersof pure and binary mixtures of methyl- and hydroxyl-terminated alkanethiols with different chain lengths are prepared on gold and analyzed with infrared reflection-absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). Our infrared data reveal that the lateral hydrogen bonding between neighboring hydroxyls in a mixed monolayer is very sensitive to variations in the alkyl chain lengths of its constituents and to the molecular composition. The complete lack of lateral hydrogen bonding in the HS(CH3le~H:HS-(CH2)1S~H3 monolayer, already at a monolayer compositionclose to 1:1,is consistent with true molecular mixing. Thus, the present infrared work gives stating that strong support to previous propOeals~10*22~~ Single-component domains or macroscopic ia2cmnds do not form”. Important information about the distribution of accessible OH groups as a function of composition and chain length of the methyl-terminatedthiol is also obtained by reacting them with trifluoro acetic anhydride. This taggingexperimentenhancesthe OH signatureby an order of magnitude, which allows us to extend the range of detection from 1 OH/5-10 CH3 groups to 1 OH/WlOO CH3 groups in a mixed monolayer. It provides also information about the steric requirements for the reaction. The combination of methyl- and hydroxyl-terminated thiols gives us also the opportunity to introduce specific functionalities on the monolayer surface. We have, for example, illustrated the versatility and simplicityof using SAM’s as the basis for more advanced bio-related modifications involving covalent attachment of sulfate and phosphate groups.
Acknowledgment. This work was supported by the Swedish Board for Technical Development (STU) and the Engineering Research Council of STU (STUF). We thank Pharmacia Bioeensor AB for the generous gift of the 16-thiohexadecanol molecules. We also express our sincere thanks to Dr. Stefan L6fAs for critical reading of the manuscript, Dr. K. Uvdal for the X P S measurements, and Mr. Magnus Lestelius for technical assistance during preparation of the sulfate and phosphate surfam.
