2228

Introduction

This paper describes studies of the preparation andcharacterization of self-assembled monolayers (SAMs)formed by the adsorption of an @-mercaptoalkylboronicacid, 1l-mercaptoundecanyl-l-boronic acid, 1, onto a goldsubstrate. This work is part of a program to study thephysicalorganic chemistry of organic interfaces.2 SAMsof alkanethiolates on gold are particularly useful sub-strates for studying organic surface chemistry becausethe high affinity of sulfur for gold3-6 allows the incorpora-tion of a wide range of tail groups, X, into precursoralkanethiols ofthe general formula HS(CH2)"X.7-15 SAMsof alkanethiolates on gold are useful model systems infundamental studies of electrochemistry,lG protein ad-

E Abstract published in Aduance ACS Abstracfs, May 15, 1994.(1) This research was supported in part by the Office of Naval

Research and the Defense Advanced Research Projects Agency.(2) For general reviews, see: Bain, C. D.;Whitesides, G.M.Angew.

Chem.,Int. Ed. Engl. L989,28, 506-512. Whitesides, G. M.; Laibinis,P. E. Langmuir L99O,6, 87-96.

(3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105,4481-4483.

(4) Porter, M.D.; Bright, T'. B.;Allara, D. L.; Chidsey, C. E. D. J.Arn.Chem. Soc. 1987, 109, 3559-3568.

(5) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides,G. M. J. Am. Chem. Soc. 1989. 111. 321-335.

(6) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc.1987. 109.733-740.

(7) Bain, C. D. ;Whitesides, G. M.J.Am.Chem. Soc. 1988, 110,6560-6561. Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989,111,7155-7164. Laibinis, P. E.;Whitesides, G. M. J. Am. Chem. Soc.1992, 114,1990-1995.

(8) Nuzzo, R. G.; Dubois, L. H.; Allara, D.L.,I. Am. Chem. Soc. 1990,112,558-569. Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am.Chem. Soc. 1990, 112,570-579. Chidsey, C. E. D.; l,oiacono, D. N.Langrnuir 1990, 6, 682-691.

(9) X: alkyl ether: Bain, C. D.; Whitesides, G. M. J. Am. Chem' Soc '1988, / t0, 5897-5898.

(10) X : carboxylic acid: Bain, C. D.; Whitesides, G. M. Langrnuir1989,5, 1370-1378.

(11) X : electroactive group: Chidsey, C. E. D.; Bettozzl C. R.;Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112,430L-4306. Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.;Wrighton, M. S. Science (Washingtan D.C.) lggl' 252,688-691.

copyright o 1ee4 by the A*""i"."%"fl;;li:"i ffJI"iSXTYJ*;13it;J0pu"-i",ion orthe copvright owner.

Self-Assembled Monolayers ContainingoMercaptoalkylboronic Acids Adsorbed onto Gold Form a

Highly Cross-Link€d, Thermally Stable Borate GlassSur{acer

Robert I. Carey, John P. Folkers, and George M. Whitesides*

Department of Ch.emistry, Haruard (Jniuersity, Cambridge, Massachusetts 02138

Receiued August 5, 1993. In Final Form: April 8, 1994@

This paper describes the preparation and properties of self-assembled monolayers (SAMs) obtained bythe ads'orptio.r of 11-mercaptoundecanyl-1-6oronic acid, HS(CHz)uB(OH)2, 1, onto the surface-of goidsubstrates. In solution in dry hydrocarbon solvents, 11-mercaptoundecanyl-1-boronic acid revelsibly for-rysa tri-ru-mercaptoalkylboroxine that adsorbs onto gold and folms a hydrophobic boroxine bilayer: thi-sbilayer is maiginally stablg in air; it can be completely hydrolyze!, to a monolayer in aqueous ethanol,yieliing a hydiophiiic, tz-A-thick SAM that preJents boronic acid grlups at its surface. Under water-laturat"ed.y.loo.tu.te, SAMs exposing the borbnic acid groups do not titrate appreciably over a pH rang_eof 0 to t t-ihat is, the contact angle ii independent of the pH of the contacting aqueous solution. At pq18 to 14, there is a sharp increas-e in the hydrophilicity ofthe SAM, indicating an-onset of ionization ofthe boronic acid groups. In dry hydrocarbon solvents or under vacuum, the boronic acid tail groups dehyllraterapidly and revJrsibiy and form a cross-linked borate glass at the surface. At 147 'C in hexadecane, SAMstravrni this cross-lini<ed borate glass as a surface are at least 5 times more stable to the;mal desorptionthan S'r\Ms of the structurally an-alogous 11-hydroxyundecane-1-thiol. The boronic acid/anhydride flouplof SAMs of 1 can be derivatized witl solutions of Certain cis-diols (especially pinanediol), catechols, andalkyltrichlorosilanes to yield hydrophobic structures conslltigg-of an inner monomer derived from 1 andan buter partial monolayer of the boronate ester or the alkylsiloxane.

sorption,l2 X-ray-induced damage to organic materials,l7wetting,z's'7-10'12'18 and corrosion'ie

Boronic acids are interesting groups to incorporate intothe surface of SAMs. In solution, alkylboronic acids reactwith simple alcohols, diols, and polyols and form covalentesters reversibly under mild and easily controllablereaction conditions.20-22 The ability of boronic acids toperform diol-specific molecular recognition in gels haslormed the basis of affrnity chromatography of carbohy-

Whitesides, G.

G. M. Science

0743-7463194124L0-2228$04.5010 O 1994 American Chemical Society

t993.114,

SAMs Containing HS( CH) I B( OH) z

drates in a variety ofboronate matrices.23-27 and boronateesters have played a major role in enantioselectivesynthesis.2s-33 In bulk solution in dry hydrocarbonsolvents, boronic acids dehydrate readily and form cyclictrimers (boroxines), eq 1. Dehydration of boronic acidtail groups constrained in position to the surface of a quasitwo-dimensional, ordered SAM would form a surfacecomprising cross-linked boronic anhydrides (a "borateglass").

Langmuir, Vol. 10, I{o. 7, 1994 2229

Scheme l. Synthesis of roMercaptoalkylboronicacids

a , b c , dBr-(CH2)e-CH=CH, --"-"+ HS-(CH2)e-CH=CHz + HS-(CH2)1r-B(OH)2

7 5 $ 8 0 t

o Thiolacetic acid and NaOMe in methanol, 2 h.b ExcessNaOMe in methanol, reflux, t h, N2." 2 equiv HBBr2.Me2S,CHzClz, Nz, 3 h.d Water, 10 min.

R = (CHr)r,-SH

Tl1Tz Boroxine<.414 =

Bit"y",

R = (CH2\ iS-

R\ / o \ /RB B

o ' " ' o

IB

dry hydrocarbon solventsor

vacuum( C H , ; t ' - 5 9

I^/\^I I

,a-ra",(\oA (cH2)r r -sH

H S - ( C H 2 ) 1 J . B ( O H ) 2

1

We have investigated the interfacial properties of SAMspresenting boronic acid groups at the surface ofan orderedalkanethiolate monolayer on gold. The first goal in thisstudy was to develop a convenient, gram-scale synthesisof the @-mercaptoalkylboronic acid, 1. The second goalwas to prepare and to characterize SAMs obtained from1 and to measure theirwettability, stability, and reactivity.We particularly wished to determine how hydrophilic asurface of boronic acid groups would be and at what pHthe boronic acid groups would begin to ionize. We alsowished to establish conditions under which the boronicacid tail groups would dehydrate and give a surface layerofcross-linked, boronic anhydride groups and to determineifthis cross-linked surface would be more stable to thermaldesorption than a structurally analogous SAM that wasnot cross-linked. The last goal was to determine ifboronicacid groups exposed on the surface of a SAM would reactwith cis-diols, catechols, simple alcohols, and alkyltrichlo-rosilanes in a manner similar to that of boronic acids inbulk solution.

Results and Discussion

Synthesis of HS(CHz)rrB(OH)2. We required a me-dium chain-length alkane substituted with a boronic acidat one end and a thiol at the other. The chemistrynecessary for introducing the thiol group has been wellstudied7 and many reagents for hydroboration are com-merically available.s4-40 The s5mthesis in Scheme 1 yields

(23) Poole, C. F.; Zlatkis, A. J. Chromatogr. 1980, 184,99-183.(24)Mazzeo, J. R.; Krull, L S. BioChromatography 1989, 4,I24-

130 .(25) Igloi, G. L.; Koessel,H. Methods EnzymoL 1987, 155,433-448.(26) Soundararajan, S.; Badawi, M.; Kohlrust, M.; Hageman, J. Anal.

Biochem. 1989, 178, 125-734.(27) Hageman, H. H.; Kehn, G.D. Methods in Molecular Biology, uol

11, Practical Protein Chromatography; Kenney, A., Fowell, S., Eds.;The Humana Press Inc.: Totowa, NJ, 1992; pp 45-7I.

(28) Matteson, D. S.; Jesthi, P. K.; Sadhu, K. M. Organometallics1984.3. 1284-1288.

(29) Matteson, D. S.; Ray, R.;Rocks, R. R.;Tsai, D. J. Organometallics1983 ,2 ,1536-1543 .

(30)Tsai, D. J. S.; Jesthi, P. K.; Matteson, D. S. Organometallics1983 .2 . 1543-1545 .

(31)Matteson, D. S.Acc, Chem. Ees. 1988.21.294-300.(32) Matteson, D. S. Pure Appl. Chem. 1991, 63, B3g-S44.(33) Matteson, D. S. Chem..Reu. 1989, 89, 1535-1551.(34)Brown, H. C.; Ravindran, N.; Kulkarni, S. U. J. Org. Chem.

1980. 45. 384-389.(35) Brown, H. C.; Bhat, N. G.; Somayagi, Y. Organometallics lg83.,

2 , 1 3 1 1 - 1 3 1 6 .(36) Racherla, U. S.; Khanna, V. V.;Brown, H. C. Tetrahedron Lett.

1992 .33 . 1037-1040 .(37) Brown, H. C.; Campbell, J. B. J. Org. Chem. L980, 45,g8g-ggb.(38)Lane, C. F.; Kabalka, G. W. Tetrahedron 1976,32. 981-990.(39) Brown, H. C. ; Gupta, S. K. J. Am. Chem. Soc. 1gZ1, gJ, 1816-

1 818 .(40) Brown, H. C.; Gupta, S. K. J. Am. Chem. Soc. lg7b, 97, S24g-

5255.

7/////tr-o.. -o.. -G. -O-r

n r B B B , n

Borate Glass

Figure l. Stylized illustrations ofmonolayer structures. (top)Proposed structure of the boroxine bilayer that is obtained bythe adsorption of2 from isooctane solution onto gold substrates.This bilayer is hydrolytically unstable, exhibits high hysteresisin its contact angles, and is prolably not well ordered. (middle)Proposed structure of the 17 A monolayer of the boronic acidformed by exposure of the boroxine bilayer or the monolayerof the borate glass to washes ryith alcohol and water. (bottom)Proposed structure of the 17-A borate glass surface formed bydehydration of the monolayer of the boronic acid undervacuumor in dry hydrocarbon solvents like hexadecane.

multigram quantities of c{r-mercaptoalkylboronic acidsfrom ar-bromoalkenes.

Preparation and Characterization of SAIVIs: Au/{[S(CHz)rr]e(BO)s], Au/S(CH2)uB(OH)2, and Au/tS-(CH2)rlBOl,. Figure 1 is a schematic illustration of themonolayer structures that are formed from 1. CompoundI exists in dry hydrocarbon solvents predominantly asthe boroxine 2. From solution in isooctane, the boroxine2 adsorbed onto gold substrates and formed a boroxinebilayer that was marginally stable to hydrolysis and thathad a thickness (as determined by both X-ray photoelec-tron spectroscopy (XPS) and ellipsometry) that varied withthe conditions used in its preparation. XPS analysisshowed a strong thiol signal, in addition to the thiolatesignal that is normally observed in SAMs. The surfaceof this boroxine bilayer was hydrophobic, and highhysteresis in contact angles measured w"ith drops ofbuffered water suggested that the bilayer was disorderedor reactive with water. Treatment ofthis boroxine bilayerwith ethanol and water yielded a hydrophilic monolayercomposed ofboronic acid tail groups thatwas consistently17 A thick as measured by ellipsometry. Dehydration ofthis monolayer under vacuum or by exposure to a dry

Ethanol I \ lsooctane

Hp I I HS{CH2h1-B(OH)2t l

a = B(oH),t l

| | = (cHr)11

I IS = s -

Boronic Acid

lsooctaneor

vacuumi I

Erhanol

I I H P

?v///////////////////////1////2

:Id" {ooo+

,*al. l

8{no a+I

6000 t

"t. 4ooo +T

,tot

or

2(X)00 r+t

lsooo+?ff

ct!1()ooo +ttt

sooo +t+

s L

l2ooo ft * T

8OO0 +

"t" eooo {*T

2000 +f

O I

2230 Langmuir, Vol. 10, No. 7, 1994

1000 800 600 tmo 2oo o

Carey et al.

We used the relative intensities ofthe two sets ofAu(4flpeaks to determine the difference in thickness betweenthe monolayer and the partial bilayer. The attenuationofthe signal was modeled as an exponential decay throughthe hydrocarbon overlayer using an attenuation length of40 A for the Au(4f) electrons.4r-43 For the layers whosedata are presented in Figure 2, the partial bilayer wasapproximately I A thicker than the monolayer. Thisdifference in thickness corresponded approximately to anadditional 50Vo of a monolayer.

For both of the layers, the binding energy of the B(1s)electrons was 192 €V, in good agreement with thepublished value forp-chlorobenzeneboronic acid of 191.beV.44 The intensity of the B(ls) peak was compared tothat of the corresponding O(1s) peak to obtain a ratio ofthe two elements in the samples. The oxygen signal couldbe attributed solelyto the boronic acid groups ofthe layers,because gold does not form a native oxide and becausespectra were taken on freshly prepared layers, thuslimiting potential oxidation of the sulfur atoms.as Theraw intensities for both samples were scaled by the Scofieldcross sections ofionization (0.486 for B(1s); 2.93 for O(1s))6and by the efliciency of the detector (or "6tendue"). Themagnitude of the latter correction is dependent on thekinetic energies of the electrons; we used the reciprocalof the kinetic energy as an approximation of this depen-dence.aT

In the case of the monolayer, we did not include theeffect of attenuation of the intensities since we assumedthat the boronic acid groups were at the monolayer-vacuum interface. Correcting the raw intensities for thecross sections ofionization and the 6tendue yielded a ratioof boron to oxygen of 1.1 in the monolayer. This resultsuggests that the surface of the monolayer is predomi-nantly composed of a cross-linked borate glass, ratherthan offree boronic acids groups, forwhich we would haveexpected a ratio ofboron to oxygen closer to 0.5.

To obtain a ratio of boron to oxygen for the partialbilayer, we needed to correct the data for attenuation ofthese signals through the 9 A of additional hydrocarbonin the partial bilayer, in addition to the cross sections andthe 6tendue. For this additional correction, we usedattenuation lengths of 37.5 and 30 A for the B(1s) andO(1s) electrons, respectively, which were estimated froman empirical equation that describes attenuation lengthsthrough hydrocarbon as a function of the kinetic enerryof the electrons.a2 Scaling the raw intensities by thesefactors yielded a boron to oxygen ratio of 1.1, suggestingformation of boroxine groups between the layers.

Figure 2 also summarizes XPS data for the S(2p) regionsfor both layers. The spectrum of the borate glass mono-layer exhibited the expected two peaks in a 2:1 ratio dueto spin-orbit coupling of the 2p electrons.a8 The binding

(41) Bain, C. D.; Whitesides, G. M. J. Phys. Chem.1989, 93, 1670-1673.

(42) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem.1991 ,95 , 7017-702r .

(43) Feldman,L.C.;Mayer, J. W.Fundamentals of Surfare andThinFiIm Analysis; Elsevier Science: New York, 1986; Chapter g.

(44) Hendrickson, D.N.;Hollander, J. M.;Jolly, W.L.Inorg. Chem.1970, 9, 612-615.

(45) Tarlov, M. J.; Newman, J. G. Langmuir 1992,8, 1398-1405.Huang, J. ; Hemmin ger, J. C. J . Arn. Chem. Soc. 1993, 1 1 5, 3342- 3343.

(46) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976,9,t24 - r37 .

(47) Seah, M. P. In Practbal Surface Analysis by Auger and X-rayPhotoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.;John Wileyand Sons: Chichester, 1983; Chapter 5. Using l/E may be a slightunderestimation of the 6tendue, but it does approximate the effect.Large exponents in the denominator would decrease the ratio of oxygento boron.

2q) 195 190 185Binding Enorgy In eV

173 168 163 158Bindlng Energy in eV

541 536 531 526Binding Energy in eV

ffi93 88 8il 78

Bindlng Energy in eV

Figure ?. XPS spectra for a monolayer and a partial bilayerderived from the adsorption of HS(CHz)rrB(OH)z onto gold. fhetop is a representative survey spectrum of the monolayer,insl-udilg labels tor the individual peaks. Note that the intensityofthe B(ls) peak at L92 eV is too low to be seen in a surveyspectrum. Below the survey spectrum are individual elementscans for boron, sulfur, gold, and oxygen. For the sulfurspectrum of the bilayer, two spectra are shown. Spectrum Ais the actual data. Spectrum B is a manipulation of the datain spectrum A to show the peaks due to thiol and disulfide. Thismanipulation was carried out by scaling the s^pectrum for themonolayer for attenuation through an extra 9 A of hydrocarbon(the thickness determined from the ratio of the Au(40 peaks)and subsequently subtracting these data from spectrum A.

hydrocarbon solvent left a boronic anhydride or "borareglass" monolayer; this SAM was 1Z A tni.t (by XpS) andhad an atomic ratio of oxygen to boron that was 0.9 asdetermined by XPS. The XPS spectra for four separatesamples of the monolayer had an average value of 1.0 *0.15 for the ratio of oxygen to boron. This value suggeststhat the species present on the surface under the ultiahighvacuum (UHV) conditions of XPS was indeed the ar*ry-dride and notthe boronic acid, forwhich one would expecta ratio of oxygen to boron of 2.0.

We note that all alkylboronic acids readilyinterconvertbetween the acid (in air) and the boroxine when exposedto heat or vacuum.20 SAMs of 1, which concentrate andpresent boronic acid tail groups at a monolayer surface,can be expected to behave in a similar manner, inter-converting between the anhydride form under vacuum_and the hydrated boronic acid form upon exposure tohumid air.

X-ray Photoelectron Spectroscopy CXPS). XpS wasuseful in the structural characterization of a boronicanhydride surface and a partial boroxine bilayer. XpSspectra were collected for boron (using the B(ls) peak),sulfur (s(2p) peaks), gold (Au(4f) peaks), and oxygenio(rr)peak). Spectra for a monolayer and a partial bilayer arepresented in Figure 2, along with a representative surveyspectrum of the monolayer.

I

SAMs Containing HS(CH) r B(OH)2

energy of the peak at lower enerry, the S(2pB l-2) peak,was 161.6 eV, the expected value for thiolates.s'6'15

For the biiayer, there are two sets of peaks in the S(2p)region, whose overlap gives rise to the appearance of atriplet. Superimposed on the two peaks for the thiolategroups are two peaks at higher binding energy. Peakfitting the spectrum with four peaks yielded a bindingenergy of 163.2 eV for the S(2p312) of the second set ofpeaks.ae The binding energJ of this peak corresponds tothiol and possibly disulfide groups in the second layer ofthe partial bilayer.6

The XPS data present a picture of the layers consistentwith the stylized illustrations drawn in Figure 1. For themonolayer, the sulfur atoms are adsorbed to the goldsurface as thiolates, and the boronic acid groups aredehydrated, forming a cross-linked borate-glass-like net-work. The partial bilayer has all of characteristics of themonolayer plus the presence of sulfur atoms not bound tothe gold surface. The partial second layer is stable in theultrahigh vacuum conditions of the XPS (10-e Torr)suggesting binding of the second layer to the monolayerpresumably through boroxine groups as observed forboronic acids in solution.

Interfacial Properties: Wetting. Figure 3 showsthat the fZ-A-thict- SAU bearing the boronic acid tailgroups had a hydrophilicity and hysteresis between theadvancing and receding contact angles that is similar tothe structurally analogous 11-hydroxyundecane-l-thiol(HUT). Contact angle titration under cyclooctanes0 sug-gested that the boronic acid groups of the SAM obtainedfrom 1 began to ionize at pH 12; this ionization wasreflected by a decrease in both advancing and recedingcontact angles. The SAM composed of HUT remainedun-ionized over the titration range of pH 0 to pH 14 (asreflected in the independence of both advancing andreceding contact angles to pH).

In air, the contact angles of buffered aqueous drops forthe SAM having terminal boronic acid groups wereconstant over a range of pH 2 to pH 10 (0uH'o : 16-24").At pH 11 or above, the SAM of the boronic acid was wetcompletely by buffered aqueous drops. The recedingcontact angles in air for the SAM of the boronic acid werezero at all values of pH.

These titration results in air and under cyclooctaneestablish that SAMs of alkylboronic acids do not begin toionize significantly until the aqueous drop is at least pH11. Thus, in analogy to our previous frndings for SAMsbearing carboxylic acid and phosphonic acid tail groups,lo'5ralkylboronic acids at the monolayer-water interfacerequire contact with stronger base for ionization thanalkaneboronic acids in bulk solution (ca. pK"t : 9.2 and

Langmuir, VoL 10, No. 7, 1994 223L

0.5

0.4

0.3

o.2acos0

-cos 0

0.1

0.0-0.0

-o.2

-0.4

-0.6

-0.8

-1 .0

. = 0" r = 0, AulS(cll2)irB(oHh

o = 0a o =0, aurs{cH2}rloH

(under cyclooctane)

^ a a a a t a t 'a u ^ ^ o ^ o ' o o o o o o

o " " - a

o

s e p E c g ! g o P l i o o : 30

(48) Briggs, D.; Rivi6re, J. C.InPractical Surface Analysis b1 Augerand X-ray-Fhotoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.;John Wiley and Sons: Chichester, 1983; Chapter 3. Althoughthe ratioof the peaks is expected to be 2:1 due to spin-orbit coupling, the slightapparent increase in the intensity of the S(2pvz) peak and the smallshoulder on the high energy side of these peaks is due predominantlyto sulfur-carbon bond cleavage during irradiation: the intensity of thehigher energ'y peaks increased with the amount of irradiation.

l+g) We frt the peaks by constraining the binding energy of S(2prn)peak for the thiolate to be 1.2 eV gxeater (or 162.8 eV) than the S(2prn)of the thiolate and also constraining its intensity to be half that of theS(2pgp) peak. For information on the spin-orbit coupling of the S(2p)peak, see: Salaneck, W.R.;Lipari, N.O.; Paston, A.;Zallen, R.; Liang,K. S. Phys. Reu. B 1975, 12,1493-1500.

(50) The contact angle titrations were performed under cyclooctane,Ca, in order to increase the contact angles and to reduce the rate ofcontamination of the surface ofboronic acids. Under air as the ambientmedium, a pure, clean surface of the boronic acid is wet (or very nearlywet) by water at all values of pH, but the high interfacial free energJcauses the surface composed of boronic acids to become contaminatedat a rate that is fast enough to interfere with the measurements. Cgis relatively nonvolatile and can be obtained in high purity at low cost.

(51) Lee, T. R.; Carey, R. I.;Whitesides, G. M. Unpublished results.

l l6

PrYFigure 3. (top) Hysteresis in the contact angle of bufferedaqueous solutions measured with drops under cyclooctane asa iunction of pH. Values of hysteresis cannot be determinedwhen the receding angle of water is zero t\^'hich, in practicalexperimental ter;s, iJ any value of 9,Hzo less than or equal to10;) and therefore there are no values of hysteresis for SAMsof L above pH : 11. (bottom) Advancing and receding contactangles ofbuffered aqueous solutjons measured urth drops-undercyc"looctane on monolayers of HS-(CH2)11-B(OHlz and HS-(eHz)rr-OH. The size of the symbols gives our best estimateof the error of the measurement.

pK^z:12.5 in water). The origin of the apparent shift inihe pK" to higher values has not been completely-estab-lished; the environment of the interface might influenceboth the properties of the functional groups and thepartitioni.tg olttydtoxide ion into the interfacial region.2'lo

Desorptlon of Au/tS(CH2)uBOl" in Contact withHexadecane. Manytechnological applications ofSAMs,particularly in microelectronic device fabncation, williequire greater thermal stability than that displayed bySAMs of simple alkanethiolates on gold. Our prior studiesof the thermal desorption of alkanethiolates from goldhave used methyl-terminated SAMs thathave low surfacefree energies;the SAMs of alkaneboronic acids have highsurface energies and contaminate more easily thanmethyl-terminated SlUVIs.2'10'51 Typically, thermal de-sorption is measured by exposing the SAM to a reservoirof hexadecane held at constant temperature and measur-ingthe change in ellipsometric constants ofthe SAM (after

removing and washing) as a function of time. Wedetermined the thermal stability of the SAIVIs in this studyinhexadecane at 147 "C: the choice oftemperature allowedus reproducibly to observe complete desorption of themonolayer in a short time.52

Figuie 4 is a plot of the rate of desorption vs time forSAMs of I and SAMs of HUT (used as a reference). HUT

(52) Although we observed thermal desorptior of the monolayers atg0 to 90 .c, quantitative analysis of the data was hinderqlby-atendencyofthe monol-ayers to contaminate as a function oftime. For this reason,we determined the thermal stability of the SAMs in this study at ahigher temperature, 147 oC.

l r l r l l0 2 4

l ' l ' l - T8 1 0 1 2 1 4

o

o a o o ' a a aa ^ . oO e - O O a

a O O O O O -o o - o

o Au/S(CHJfiB(OH)2

o Au/S(CH2)11OH

6 o 3 t . .

2232 Langmuir, Vol. 70, No. 7, 1994

Tlme (sec)

Figure 4. Thermal desorption of monolayers of thiols on goldin iontact with hexadecane at L47 'C: (O) 1l-mercaptoundec-anyl-1-boronic acid; (O) 1l-hydroxyundecane-1-thiol. In thefirit-order plot of the corrected thicknesses against time, ?r -

?- represents the difference in ellipsometric thickness betweentime 7 and long times. The inset shows a plot of ellipsometricthickness vs time.

and t have the same C11 alkyl chain length and similarsolubility in hexadecane. The primary difference in thechemical properties of the two SAMs is that the SAM ofthe boronic acid readily dehydrates to a cross-linked,borate glass surface that we expected to be more stabletoward thermal desorption than the similar, but non-cross-linked surface of a SAM of HUT. The data in Figure 4show that in hexadecane at 147 'C, a SAM of the boronicacid was approximately 5 times more stable to thermaldesorption than a SAM ofHUT. These data are consistentwith the hypothesis that cross-linking the tail groups inSAMs enhances their stability to thermal desorption, butit may also reflect differences in solubilities.

Reactions: Formation and Stability of BoronateEsters on SAMs. Alkylboronic acids in solution can formstrong covalent bonds reversibly under mild conditionswith a-hydroxyacids, catechols, y-hydroxyaldehydes, andcis-diols. In this section we describe the reactions of SAMsof 1 with pinanediol, catechol, simple alcohols and glycols,and alkyltrichlorosilanes (Table 1).

We compared the properties of SAMs prepared by theadsorption of 3 from solution in isooctane to SAMsprepared by the esterification in isooctane solution of apreexisting monolayer of I with pinanediol. We chosepinanediol as the diol constituent because the resultingboronate esters are very stable and easily manipulatedwithout complications due to hydrolytic instability.20'26'27 'ea

Compound 3 was prepared by refluxing equimolaramounts of 1 and (*)-pinanediol inheptane for 10 min (eq2). Compound 3 was stable to an aqueous extractiveworkup and to elution through a column of silica gel.

Carey et al,

Table 1. Wetting Properties and Thicknesses of SAIVIsDerived from I l-Mercaptoundecanylboronic Acido

monolayer system 9,Hro o,H"o thickness, A

Au/S(CHz)rrB(OH)zAu/t tS(CHz )r r le(BO)e)An/S(CHz)rrB(OH)z * ZtOClz *

C+HgB(OH)2"Au/S(CH2)rrB(OH)z * ZrOClz *

HzNCHzCHzPOsHz"Au/S(CH2)118(OH)z * CreHszSiClsdAu/S(CHz)rrB(OH)z *

CFs( CFz )s( CHz )zSiClsdAu/S(CHz)rrB(OH)z *

CFs(CFz)z(CHz)zOH/Au/S(CHz)uB(OH)z * catechofAu/S(CHz)118(OH)z * Pinanediol

n, , / - :Au/s(cHz)rr-B<: t'Yl'"' - -o"/-,v

20,0

^ 1 5 . 0

or 1 0 0oc

1 5 097 501 5 0

t02 65 37"110 74 25"

t 7b2gb17b

17b

F

15

3058

62 35 2V95 78 2lbe93 74 17bs

HS-(.H2)1r-B(oH)2 heptane'ref lux>

\-9tot- .r 3f7l\l/

SAMs formed from 3 and SAMs formed by reaction ofpinanediol with a preexisting SAM of 1 are both hydro-phobic, and each is stable toward water (pH : 7) for atIeast 24h. The SAM formed in the surface reaction wasactually slightly more hydrophobic and thicker than the

a Q^H2o and 9rH'o refer to advancing and receding contact anglesof water, respectively, using drops of water buffered at pH 7.b Thickness determined by XPS relative to a monolayer of undec-anethiol at 15 A. " These experiments attempted to build multilayersonto the surface of sAMs of I by the sequential deposition of zroclzand either an alkylboronic acid or an alkylphosphonic acid. (See

refs 52-54). By XPS, no zirconium was observed, and there wasno increase in the thickness of the films. d SAMS of l were treatedwith an alkyl- or perfluoroalkyltrichlorosilane in dry CHzClz or inthe vapor phase.-" Thickness determined by ellipsometry. f 11titboronate ester sAM and analogous sAMs derivatized with ethyleneglycol, octanol, and dodecanol were not stable to high vacuum'excessive washes with aqueous ethanol, or prolonged exposure toair. e The 21 A thick SAM of a pinanediol boronate ester wasprepared by the esterification of a preexistinq SAM o-f 1 withpinanediol in solution from isooctane. The 17 A thick SAM of apinanediol boronate ester was prepared by the adsorption of3 ontoi gold substrate. Both ofthese SAMs were stable to repeated washeswith water and to vacuum.

SAM obtained from 3 (see Table l-).53 The conclusion from

the experiments with pinanediol is that SAMs presenting

boronic acid tail groups react cleanly and rapidly (the

derivatization requires only 5 min in a 5 mM solution of

pinanediol in isooctane) if presented with the proper cis-

diol.In solution, boronate esters of catechols are much less

stable toward hydrolysis than esters of pinanediol.

Treatment ofa boronic acid-terminated SAM with catechol

from dilute solution in isooctane or from concentrated

solution in lHF yielded a derivatized monolayer surface

that was 3 A thicker than the underivatized boronic acid

SAM and that had an advancing contact angle with water

that varied from 62' to 65" depending on the sample.

Although several washes with ethanol and water did not

seem to change the wettability or thickness, a prolonged

exposure (overnight) to aqueous ethanol reversed the

esterification and yielded the surface exposing boronic

acid groups again. When an analogous esterification was

attempted with 3-fluorocatechol, very little change in

thickness or wettability was observed, and after one wash

with ethanol and water, no fluorine could be detected by

XPS. We conclude that SAMs of 1 form only moderately

stable boronate esters with catechols and that the greater

the electron drawing constituent on the catechol, the lower

the stability of the derivatized monolayer.sa

(53) The reason for the observed difference in thickness and wet-tability of the two SAMs is probably that the tail group of 3 is so bulky,that it prevents the formation of a complete monolayer. In_th_e_case ofthe SAM formed by the surface reaction, the underlying SAM of theboronic acid has already formed from I and is densely packed and highlyordered. Pinanediol can then react with these ordered boronic acids atthe surface, filling up all of the sites and making a more ordered, moredensely packed layei ofpinanediol boronate terminal groups than themonolayer formed from 3.

-1--/ ^X I

"l>t-(cH2)!l-sH

v, , ' "v (2 \

SAMs Containing HS(CH, r B( OH) z

Although multilayers can be formed from SAMs ofphosphonic acids by the sequential deposition of ZrOClzand o,ar-alkylphosphonic acids,ss-s? attempts to buildanalogous multilayer systems from ZrOClz and alkylbo-ronic acids were unsuccessful due to the incompleteformation or the hydrolytic instability of the zirconiumboronate compounds. XPS spectra contained no peaksfor zirconium and showed no increase in the thickness ofa SAM of 1 after it had been treated with ZrOClz andC4He-B(OH)z (see Table 1).

Formation of a Borosilicate Bilayer on SAIVIs:Reaction of Alkyltrichlorosilanes with Boronic AcidTail Groups. Partial bilayers attached by borosilicatebonds have been formed by treatment of SAMs of 1 witheither oftwo different alkyltricNorosilanes : C 16H3? - SiCl3and CFg(CF2)dCH2)z-SiCls. These reactions were carriedout both with the alkyltrichlorosilane in the vapor phaseand in solution in dry CHzClz. The advancing contactangles of water and hexadecane on these bilayers wereclose tothose oftypical long-chain alkyl and perfluorinatedalkyl-terminated SAMs (0^H"o: 115 and g,HD : 44). Theproperties ofthese surfaces also compare well with bilayersformed in a similar fashion from the reaction of alcohol-terminated SAMs with alkyltrichlorosilanes,ss althoughthe hysteresis in the contact angles of drops of bufferedwater is slightly higher for the borosilicate bilayer thanthat observed for analogously derived siloxane bilayer fromHUT.59

Conclusions

The ar-mercaptoalkylboronic acid, 1, adsorbs onto goldfrom isooctane solution and forms a boroxine bilayer thancan be hydrolytically cleaved by treatment with aqueousethanol and yield a 17 A thick monolayer bearing boronicacid tail groups. Under high vacuum or in dry organicsolvents, the boronic acid monolayer reversibly dehydratesand forms a cross-linked boronic anhydride or "borateglass" surface. This cross-linking leads to a 5-fold decreasein the rate of thermal desorption in hexadecane atl47 "Crelative to the rate of thermal desorption observed for astructurally analogous monolayer of hydroxyundecane-thiol. Boronic acid or boronic anhydride groups on thesurface of a SAM can be derivatized reversibly with cis-diols and formhydrophobic surfaces composed ofboronateesters;boronate ester groups formed from pinanediol are

(54) Attempts to derivatize SAMs of the boronic acid, 1, with simplealcohols or glycols such as ethylene glycol, octanol, or perfluorododecanolyielded derivatived monolayers that were at best transiently stable asjudged by XPS, ellipsometry, and contact angles. Afber treatment ofSAMs of l with simple alcohols and glycols in a dryhydrocarbon solvent,the contact angles with water and hexadecane increased and therewere small changes in the ellipsometric constants. In all such cases,however, the derivatized SAMs were highly unstabie in the presenceof moisture, and extensive washing with ethanol and water alwaysrestored the underivatized, surface ofboronic acids. The reasons forthis observation are that simple alcohols and glycols do not form boronateesters that are sufficiently stable to make a surface that is resistant tohydrolysis. Partial derivatization is typically not detected by XPSbecause if the alcohol is volatile, the small amount the alcohol adsorbedto the surface prior to an XPS experiment will be removed under thehigh vacuum conditions in the XPS chamber.

(55) Lee, H. ; Kepley, L. J. ;Hong, H.-G.;Mal louk, T.E.J.Am.Chem.Soc. 1988, 110,6L8-620. Lee, H.;Kepley, L. J.; Hong, H.-G.;Akhter,S.; Mallouk, T. E. J. Phys. Chem.1988,92,2597-260L.

(56) Putvinski, T. M.; Schill ing, M.L.; Katz,H. E.;Chidsey, C. E. D.;Mujsce, A. M.; Emerson, A. B. Langrnuir 199{J,6,1567-157t.

(57) Vermeulen, L.; Thompson, M. E. Nature 1992,358, 656-658.(58) Ulman, A.; fillman, N. Zongrnuir 1989, 5, l4L8-I420. (b)

Netzger, L.;Sagiv, J.J.Am.Chem. Soc. 1983, 105,674. (c) Pomeranz,M.; Segmuller, A.; Netzer, L.; Sagiv, J.Thin SolidFilms 1985,132,153.

(59) In our hands, treatment of monolayers of HLIT with octadecyl-trichlorosilane in dry CH2CI2 yielded bilayers that had hysteresis in thecontact angles with water consistently l}Volower than bilayers preparedin an analogous fashion from monolayers of I and octadecyltrichlo-rosilane.

Langmuir, Vol. 10, No. 7, 1994 2233

stable to hydrolysis. Derivatization of boronic acid orboronic anhydride monolayers with alkyltrichlorosilanesyields hydrophobic surfaces that are held together by aborosilicate network.

Experimental Section

Materials. Absolute ethanol (Quantum Chemical Corp.) wasdeoxygenated with Nz or Ar prior to use. Isooctane (Aldrich,99Vo) and hexadecane (Aldrich, 99Vo) were percolated twicethrough activated, neutral alumina (EM Science). Water wasdeionized and distilled in a glass and Teflon apparatus. Un-decanethiol (Pfalz & Bauer) was distilled prior to use. Un-decylenic bromide (Pfalz & Bauer) and dibromoborane dimethylsulfide complex (Aldrich) were purchased and used withoutfurther purifi cation. Hydroxyundecanethiol was available fromprevious studies.T

Preparation of Substrates. Gold substrates were preparedby electron-beam evaporation of -200 A of gold (MaterialsResearch Corp., Orangeburg, NY 99.999Vo) onto single crystalsilicon(100t test wafers (Silicon Sense: Nashua, NH; 100 mmdiameter, -500 rm thick) that had been precoated with 50-100A of titanium to improve adhesion. The substrates were storedin Fluoroware wafer holders until used in experiments, generallyas soon as possible afber evaporation.

Formation of Monolayers. Adsorptions were carried outin 10-mL glass weighing bottles that had been cleaned with"piranha solution" (7:3 concentrated HzSOq/3IVo HzOz) at 90 "Cfor t h, and rinsed first with distilled water and then with copiousamounts of deionized water. WARNING: "Piranha solution"should be handled with caution. It should not be allowed tocontact significant quantities of oxidizable organic materials. Insome circumstances (Most probably when mised with significantquantities of an oxidizable organic material), it has d,etonatedunexpectedly.6O The weighing bottles were stored in an oven atabove 100 'C until use, Adsorptions were carried out in 10 mLof deoxygenated solvent (ethanol or isooctane) at a total thiolconcentration of 1 mM.

Thermal Desorption Experiments. Desorptions werecarried out in an unstirred solution of hexadecane in a 25-mLweighing bottle that was partially immersed in an oil baththermostated at 153 + 1 "C. The hexadecane was percolatedtwice through a column of activated alumina and was purgedwith Nz immediately prior to use. The temperature of thehexadecane in the glass bottle was I47 + I "C. Slides (-1 cmx 3 cm) with preformed monolayers were immersed in 20 mL ofhot hexadecane, removed at the proper time intervals, rinsedwith heptane, and blown dry with a stream of nitrogen.Ellipsometry was used to follow the desorption ofthe monolayers;ellipsometric constants were measured on the complete mono-layer and then after each removal from solution. The thicknessof the adsorbed organic layer was calculated using the opticalconstants for clean gold. Corrected thicknesses (?, - ?-) werecalculated by subtracting from the ellipsometric thickness attime / the mean of several readings obtained at long times afterdesorption was essentially complete.

Instrumentation. Ellipsometric measurements were per-formed on a Rudolf Research Type 43E03-2008 ellipsometerequipped with a He-Ne laser (,1 : 6328 A) at an incident angleof 70'. Samples were rinsed and blown dry in a stream ofnitrogenprior to characterization. Values of thickness were calculatedusing a program written by Wasserman,6l following an algorithmby McCrackin and co-workers;62 in the calculation, we used arefractive index of 1.45 for the SAMs.63

Contact angles were measured on a Ram6-Hart Model 100goniometer at room temperature and ambient humidity for bothwater and hexadecane. Advancing and receding contact angleswere measured on both sides each of at least 3 drops of eachliquid per slide; data in the frgures represent the average ofthese

2234 Langmuir, Vol. 10, No. 7, 1994

measurements. A Micro-Electrapette syringe (Matrix Technolo-gres. Lowell, MA) was used for dispensinfremoving the liquidsonto/from the SAMs (ca. 1 ml/s). The method used for measuringthe advancing and receding angles has been described previ-ouslY.2'10

XPS was carried out using a SSX-100 spectrometer (SurfaceScience Instruments) using monochromatic Al Ka X-rays (photonenerry :1487 eV; the peaks are referenced to Au(4fid at 84.0eV). For quantitation of the individual elements in the SAMs,we used the 4f doublet for gold ("Au(40" at 84 and 87 eV forAu(4fzn') and Au(4fsrz), respectively), the ls peaks for carbon,oxygen, and boron at284,532, and 192 eV, respectively, and the2p peaks for sulfur between 161 and 164 eV. Data for gold,carbon, and sulfur were taken using a spot size of 600 4m anda pass energy of 50 eV. Data for oxygen and boron were takenusing a spot size of 1000 prm and a pass energ'y of 100 eV becauseof the low intensity ofthe boron peak; both regions were checkedat the lower pass enerry to show that there was only one peakin each region. T\vo scans were taken for gold, three scans forcarbon, eight scans for oxygen, thirty scans for boron, and frftyscans for sulfur. Each scan at the pass energy of 50 eV tookapproximately 1.5 min; each scan at the pass energJ of 100 eVtook approximately 2 min.

Peaks in the spectra were fitted using a 80Vo Gaussian/207oLorentzian peak shapes, using a Shirley background subtrac-tion.6a For each oxidation state present, the sulfur peaks werefi t using two peaks, corresponding to the S(2p3,2) and S(2p12)peaks due to spin-orbit coupling.a3'as The ratio of these peaksis 2:1, and the splitting between them is 1.2 eV.as In the fittingprogram, these values were used as constraints to obtain realisticfits to the data. For both the monolayer and the bilayer, two setsof peaks were used in the fit. In the former, the set of peaks athigher energT was shown to be due to X-ray-induced damagelTat the sulfur-carbon bond: the relative intensity of these peaksincreased with time of irradiation from 25Vo to 43Vo aft,er 4 h ofirradiation. For each sample, spectra in the S(2p) region weretaken first so as to minimize effects due to damage.

l0-Undecene-l-thiol. To a clear solution of NaOMe (465mg, 8.6 mmol) in degassed methanol (10 mL) was added thioaceticacid (0.70 mL, 1.15 equiv) and undecylenic bromide (2 g,8.6mmol). The reaction was refluxed under Nz for 3 h. AdditionalNaOMe (930 mg, 17.2 mmol) was added, and the reaction wasrefluxed for another t h. Most of the CH3OH was removed invacuo, and the reaction was quenched by addition ofdegassed,

(63) In the calculation ofellipsometric thickness, we assume an indexofrefraction of 1.45; the indices ofrefraction for various molecules similarto those used in this study vary from 1.42 to 1.46: undecane (liquid),1.42; undecanol (l iquid), 1.44; undecanethiol (l iquid), 1.46; octa-decanethiol (solid), 1.46; docosane (solid), 1.45. CRC Handbook ofChemistry and Physics, 65th ed.; Weast, R. C., Ed.; CRC Press, Inc.:Boca Raton, FL, 1984. This pqtential variation in the refractive indexaffects the thickness <*1.5 A, which is less than the variation wenormally observe for the SAMs of alkanethiolates on gold.

(64) Shir ley, D. A. Phys. Reu. B 1972,5,4709-4714.

Carey et al.

half-saturated ammonium chloride solution. The aqueous layerwas extracted with CHCIg (2x). The CHCI3 extracts werecombined, washed with HzO (2x), dried over MgSOa, andconcentrated in vacuo to yield a crude oil (1.5 g, 94Vo) that waspurified by flash chromatography using hexanes/ethyl acetate(95:5) as eluent to yield a clear colorless oil (1.2 g,75%o). lH NMR(400 MHz, CDCh) d 5.8 (m, 1 H), 4.95 (m, 2 H), 2.52 (q, J = 8Hz ,2H) ,2 .04 (q ,2 H ) , 1 .6 (m , 2H) , t . 2 -1 .4 ( - , 13 H ) . MS E I(pos): 185 (M+).

l-Mercapto-ll-undecanylboronic Acid. To a solution ofl0-undecene-1-thiol (1.1 g, 5.9 mmol) at 0 "C in freshly distilledCHzClz was added via syringe dibromoborane dimethyl sulfidecomplex (12.0 mL, 1.0 M in CHzClz, 12 mmol). The reaction wasallowed to warm to room temperature over 15 min and wasrefluxed for 3 h. Most of the CHzClz was removed in vacuo, andthe reaction was then transferred to a stirred mixture of water(10 mL) and ether (50 mL). The suspension was stirred for 10min and transferred to a separatory funnel. The aqueous layerwas extracted with ether (4 x ), and the ether layers were combinedand extracted with brine (2x), dried over MgSOa, arrd concen-trated in vacuo to yield a white solid (1.31 g, 96Vo) that waspurified by flash chromatography using hexanes/ethyl acetate(3:1) as eluent to yield a white sol id (1.1 g,80Vc). lH NMR (400MHz, CDCIg) 6 2 .48 (q , 8 Hz, 2 H) , 1 .55 (m, 8 Hz, 2H) , I .2-L4(m, 17 H), 0.82 ft , 8 Hz,2 H). FABMS (glycerol matr ix): 289f(NVglycerol ester)H*1, 576 f2(Wglycerol ester)H+]. FABMS(nitrobenzyl alcohol matrix): 350 l(ltl/mononitrobenzyl ester -

HzO )H*1, 503 [(tr&dinitrobenzyl ester)H+], 1006 t2(lwdinitroben-zyl ester)H'1.

(*)-Pinanediol-l-mercapto-11-undecanylboronate. Asolution of 1-mercapto-11-undecanylboronic acid (100 mg, 0.43mmol) and (*)-pinanediol (73 mg, 0.43 mmol) in heptane wasrefluxed for 10 min and then left at ambient temperature for 1h. The heptane was removed in vacuo, and the resulting oil waspassed through a column of silica gel using hexanes/EtOAc (97:3) as eluent. The appropriate fractions were collected to yielda clear oi l (130 mg,807c). TLC: fu: 0.8 (hexanes). lH NMR(300 MHz) 6 4.26-4.24 (1 H, dd, CHOB pinanyl); 2.54-2.47 (2H, q , CH2-Sl 2 .37-2.29 (1H, D, p inany l ) ; 2 .24-2.18 (1 H, m,p inany l t ; 2 .05-2.03 (1 h , t , p inany l ) ; 1 .92-1.88 (1 H, septet ,p inany l ) ; 1 .85 and 1.81 (1 H, m, p inany l ) ; I .64-1.52(2H,m,C l -Hz ) ; 1 .41 I . 39 (2 H , m , C , t -Hz ) ;1 .38 (3 H , s , CHe) ; 1 .33 (1H, t , SH) ; I .29 I H, s , CH3) ; 1 .25 (14 H, s) ; 0 .84 (B H, s , CHs) ;0.82-0.78 (2H,t, CH2-B). High resolut ion FABMS: calcd forCzrHssBOzS, 366.2764; found, 366.2760.

Acknowledgment. XPS spectra were obtained byusing instrumental facilities purchased under the DARPA/URI program and maintained by the Harvard UniversityResearch laboratory. NMR spectra were obtained in theBruker NMR Facility at Harvard. R.I.C. was an NIHPostdoctoral Fellow (GM-13678).