2225 r 0002-78631941r5t6-2225$04.50/0 @
Copyright
Reprinted from the Journal of the American Chemical Society, 1994, 116. O 1994 by the American Chemical Society'and reprinted by' permission of the copyright
Wetting and Protein Adsorption of Self-Assmebled Monolayers of Alkanethiolates Supported on TransparentFilms of Goldr
Sr\l\4sof IIStCtl,tlilCll-r
PaulA. DiMilla,2JohnP. Folkers,HansA. Biebuyck, GabrielP. L6pez,aand RalphH?irter,3 GeorgeM. Whitesides' Departmentof Chemisty, Haruard Uniuersity 02I 38 Cambridge,Massachusetts Receiued September7, i,993 monolayers(SAMs) of We have formed self-assembled on5-nm-andl0-nm-thickfilmsof goldsupported alkanethiolates films on titanium-primedglass. Theseelectrically-conductive transmitvisiblelight,ssupportSAMsthathavewettingproperties superiorto thoseof SAMs supportedon opaque200-nm-thick andhave filmscommonlyusedin thisareaof surfacechemistr),6 thanthoseof thickerfilms. We present surfaces thataresmoother the combinationof flexibility in two examplesdemonstrating controlof surfacechemistryofferedby SAMs of alkanethiols providedby thesethin films: meaand opticaltransparency of RNaseA [EC 3.L27.51labeled surements of the adsorption (EITC) onmethyl-andhexaethylene witheosin-5-isothiocyanate glycol-terminated and spectroscopy, SAMs usingtransmission of thedifferentialattachmentandgrowthof living observations cellson patternedSAMs of hydroxyl-and MG63 osteosarcoma using conventionalphasealkanethiolates methyl-terminated contrastmicroscopy. filmsof goldon titaniumTo prepare5-nm-and 10-nm-thick primedglass,we sequentially depositedI nm of titaniumat 0.I nm/s and goldat 0.1 nm/s; thick filmsof gold(200nm) were deposited at 0.5 nm/s on glassprimedwith 5 nm of titaniumat nm of titanium of proteinadsorption,0.5 Forstudies 0.2nm/s.7'E on eachsideof glasssubstrates. and5 nm of goldweredeposited The continuityof thesefilms of gold wasverifiedusingX-ray (XPS).eTheyalsowereelectrically photoelectron spectroscopy (7.1 + 0.3) x lO{ O-cm(5with the resistivities conductive, Au), and nm-thickAu), (4.8+ 0.4) x l0-6O-cm(10-nm-thick (3.7+ 1.5)x l0{O-cm (200-nm-thickAu).r0 SAMswereformed of co-functionfrom I mM ethanolicsolutions by chemisorption alizedalkanethiolsfor 2 h at room temperature. of SAMs on thin films of gold increased The transparency gold(Figurel). Filmsbecame more withdecreasing thicknessof (200-nm-thick of golddecreased blueandgray asthe thickness goldhad the samecolorasbulk gold). Formationof SAMs on or color.rr Transthesefilms did not affecttheir transparency parencycould be increasedusing a (3-mercaptopropyl)tripromoter layerratherthantitaniumasanadhesion methoxysiloxy goldandglass,but evaporation of titaniumwasa more between ( I ) Partial supportwas providedby the Office of Naval Researchand the AdvancedResearchProjectsAgency. (2) NIH postdoctoralfellow, 1992-1993. (3) SwissNational ScienceFoundationfellow, 1992-1993. (4) Ford Foundationpostdoctoralfellow, 1992-1993. (5) For a reviewof transparentmetallic films, see: Vossen,J. L. In Physics of Thin Films, Vol 9',Haas,G., Francombe,M. H., Hoffman, R. W., Eds.; AcademicPress:New York, 1977;pp I -71. For otherexamplesof transparent gold,see: Johnson,P. B.; Christy,R. W. Phys. Reu.B 1972,6.4370_4379. J. S. E. M.; Granqvist,C. G. ,I. Smith, G. B.; Niklasson,G. A.; Svensson, A p p l . P h y s . 1 9 8 6 ,J 9 , 5 7 1 - 5 8 1 . (6) For reviewssee: Whitesides,G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembl/;AcademicPress,Inc.: San Diego,CA, l99l. (7) It is also possibleto preparediscontinuoustransparentmetallic films (e.9.,Porter,J. D.;Heller,A.;Aspnes,D.E. Nature by photoelectrodeposition 19t5, 3/J, 664466). (8) Glassslides(l-in. X 3-in., 1.2pg thick, VWR Scientific,catalogno. 48300-036)were cleanedwith 70/30 v/v concentratedHzSOr/30VoHzOz. (9) The intensitiesof the Au(4f), Ti(2p), and Si(2p) peaksby XPS were moreconsistentwith the intensitiespredictedby a model in which continuous films of gold werepresenton titanium than with a modelin which gold formed islandson titanium.l2
owner.
2225
SAlvlso1' llS{CH21'OH
r 5 nnr
Tltickners ofAu
Iunrrl
2fX)nm
I
tI I
F
IL
Figure l. Photographof 100-pL dropsof water on methyl- and films SAMs on 5-nm-,l0-nm-,and 200-nm-thick hydroxyl-terminated glassslides. on titanium-primed of gold supported reliableand convenienttechniquethan silanization.l2Thin gold films (5 and 10 nm) depositedin the absenceof either adhesion promoter were significantly grayer and lessconductivethan gold supported on titanium-primed glass and did not support wellorderedSAMs. The advancingcontactanglesof water and hexadecane(HD) for SAMs of undecanethioland ll-(hexaethylene glycol)undecane-1-thiol were relatively independentof the thicknessof the supportinggold (Table I ) . r3 Hysteresis,however,wassmaller tbr the SAMs on 5-nm- and lO-nm-thick gold than for these SAMs on 200-nm thick gold becausethe recedingcontactangles for these liquids increasedwith decreasingthicknessof gold.tc The thicknessof the gold on hysteresisalso correlateswith the morphology of the surface: atomic force microscopy (AFM) demonstratedthat the mean absoluteroughnessof films of gold gold to 0.2 nm for 10decreasedfrom 1.2 nm for 1OO-nm-thick nm-thick gold and 0.09 nm for 5-nm-thick gold.t5 We measured the adsorption of RNase A on methyl- and hexaethyleneglycol-terminatedSAMs by observingthe change in optical absorbanceof SAMs after incubationwith I mg/ml solutionsof protein covalentlylabeledwith EITC (Figure 2).tf'rt The visible spectra for both types of SAMs before and after exposure to unlabeled RNase A were comparable, but the spectrum for EITC-RNase A on a methyl-terminated SAM showeda distinct peak at 532 nm.r8'reThis peak corresponded of EITC-RNase A;20we estimatethat a closeto 0.35 LLglcm2 packed monolayer of RNase A would contain 0.214.37 pgl cm2.2t In contrast, no peak was observedfor a hexaethylene glycol-terminated SAM exposedto EITC-RNase A; the demonstrationthat theseSAMs resistprotein adsorptionagreeswith previousresults.l6.l8 ( l0) Resistivities weredeterminedby measuringthevoltagedrop at constant applied current using a four-point probe with osmium tips (tip spacingof 0.0625in., tip radiusof 0.005in. Alessi,Irvine,CA) and correctingfor the finite sizeof the film. The resistivityof bulk gold is 2.4 x lW O-cm(Kittel, C. Introduction to Solid-State Physics;John Wiley and Sons: New York, 1976). ( I I ) Allara, D. L.; Hebard,A. F.; Padden,F. J.; Nuzzo, R. G.; Falcone, D. R. "/. Vac. Sci. Technol.A 19t3. 1.376-382. ( 1 2 ) D i M i l l a , P . A . ; F o l k e r s J, . P . ; B i e b u y c k H , . A . ; W h i t e s i d e sG, M . , unpublisheddata. (13) Bain, C. D.;Troughton,E. B.; Tao, Y.-T.; Evall, J.; Whitesides,G. M. J. Am. Chem.Soc. 1989,I I 1,321-335. Troughton,E. B.; Bain, C. D.; Whitesides, G. M.;Nuzzo, R. G.; Allara, D. L.; Porter,M.D. Langmlir 1988, 4,365-385, (14) We define hysteresisin the contact angle as the differencebetween the cosinesof the recedingand advancingcontact angles. (15) Values for mean absoluteroughnesswere measuredusing a TMXcarbontip 2000 AFM (Topometrix,Sunnyvale,CA) with a electrodeposited and silicon cantilever. AFM rather than STM was used becausethe total conductanceof our films was limited by the total amount of metal present.12 The roughnessof bare glassand 5 nm of gold on glassprimed with I nm of titanium was indistinguishable;we exp€ctthat 20O-nm-thickgold is at least gold. as rough as 1O0-nm-thick
@1994AmericanChemicalSociety 0002-78631941r5t6-2225$04.50/0
2226 J. Am. Chem.Soc.,Vol. 116,No.5, 1994
Communications to the Editor
Table l. Wetting Propertiesof Water and Hexadecane(HD) on SAMs Prepared by Adsorption of Methyl- and Hexaethylene Glycol-Terminated Alkanethiols on Thin Films of Gold Supported on Titanium-Primed Glass contact angles(advancing,0., receding,dr, in deg)oand hysteresis (-A cos d = cos d, - cos d.) in wetting of H2O and HD on SAMs
HS(CH2)roCH3 HzO
HS(CH2)rr(OCH2CHz)oOH HD
HzO
thicknessof gold(nm)
0 ^ ,0 ,
-A cos d
0^,0,
-A cos d
5 l0 200
tt2, t04 I 1 09 ,8 l 13,98
0 . 13 3 0.203 0.252
44,39 44,39 4 5 ,3 0
0.058 0.058 0 . 15 9
a^,0, 36,3l 1 A
1 1
HD6 -A cos d
0.048 0.062 0.141
0u,0,
Reprinted from the Journal of the American Chemical Society, 1994, 116. O 1994 by the American Chemical Society'and reprinted by' permission of the copyright
Wetting and Protein Adsorption of Self-Assmebled Monolayers of Alkanethiolates Supported on TransparentFilms of Goldr
Sr\l\4sof IIStCtl,tlilCll-r
PaulA. DiMilla,2JohnP. Folkers,HansA. Biebuyck, GabrielP. L6pez,aand RalphH?irter,3 GeorgeM. Whitesides' Departmentof Chemisty, Haruard Uniuersity 02I 38 Cambridge,Massachusetts Receiued September7, i,993 monolayers(SAMs) of We have formed self-assembled on5-nm-andl0-nm-thickfilmsof goldsupported alkanethiolates films on titanium-primedglass. Theseelectrically-conductive transmitvisiblelight,ssupportSAMsthathavewettingproperties superiorto thoseof SAMs supportedon opaque200-nm-thick andhave filmscommonlyusedin thisareaof surfacechemistr),6 thanthoseof thickerfilms. We present surfaces thataresmoother the combinationof flexibility in two examplesdemonstrating controlof surfacechemistryofferedby SAMs of alkanethiols providedby thesethin films: meaand opticaltransparency of RNaseA [EC 3.L27.51labeled surements of the adsorption (EITC) onmethyl-andhexaethylene witheosin-5-isothiocyanate glycol-terminated and spectroscopy, SAMs usingtransmission of thedifferentialattachmentandgrowthof living observations cellson patternedSAMs of hydroxyl-and MG63 osteosarcoma using conventionalphasealkanethiolates methyl-terminated contrastmicroscopy. filmsof goldon titaniumTo prepare5-nm-and 10-nm-thick primedglass,we sequentially depositedI nm of titaniumat 0.I nm/s and goldat 0.1 nm/s; thick filmsof gold(200nm) were deposited at 0.5 nm/s on glassprimedwith 5 nm of titaniumat nm of titanium of proteinadsorption,0.5 Forstudies 0.2nm/s.7'E on eachsideof glasssubstrates. and5 nm of goldweredeposited The continuityof thesefilms of gold wasverifiedusingX-ray (XPS).eTheyalsowereelectrically photoelectron spectroscopy (7.1 + 0.3) x lO{ O-cm(5with the resistivities conductive, Au), and nm-thickAu), (4.8+ 0.4) x l0-6O-cm(10-nm-thick (3.7+ 1.5)x l0{O-cm (200-nm-thickAu).r0 SAMswereformed of co-functionfrom I mM ethanolicsolutions by chemisorption alizedalkanethiolsfor 2 h at room temperature. of SAMs on thin films of gold increased The transparency gold(Figurel). Filmsbecame more withdecreasing thicknessof (200-nm-thick of golddecreased blueandgray asthe thickness goldhad the samecolorasbulk gold). Formationof SAMs on or color.rr Transthesefilms did not affecttheir transparency parencycould be increasedusing a (3-mercaptopropyl)tripromoter layerratherthantitaniumasanadhesion methoxysiloxy goldandglass,but evaporation of titaniumwasa more between ( I ) Partial supportwas providedby the Office of Naval Researchand the AdvancedResearchProjectsAgency. (2) NIH postdoctoralfellow, 1992-1993. (3) SwissNational ScienceFoundationfellow, 1992-1993. (4) Ford Foundationpostdoctoralfellow, 1992-1993. (5) For a reviewof transparentmetallic films, see: Vossen,J. L. In Physics of Thin Films, Vol 9',Haas,G., Francombe,M. H., Hoffman, R. W., Eds.; AcademicPress:New York, 1977;pp I -71. For otherexamplesof transparent gold,see: Johnson,P. B.; Christy,R. W. Phys. Reu.B 1972,6.4370_4379. J. S. E. M.; Granqvist,C. G. ,I. Smith, G. B.; Niklasson,G. A.; Svensson, A p p l . P h y s . 1 9 8 6 ,J 9 , 5 7 1 - 5 8 1 . (6) For reviewssee: Whitesides,G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembl/;AcademicPress,Inc.: San Diego,CA, l99l. (7) It is also possibleto preparediscontinuoustransparentmetallic films (e.9.,Porter,J. D.;Heller,A.;Aspnes,D.E. Nature by photoelectrodeposition 19t5, 3/J, 664466). (8) Glassslides(l-in. X 3-in., 1.2pg thick, VWR Scientific,catalogno. 48300-036)were cleanedwith 70/30 v/v concentratedHzSOr/30VoHzOz. (9) The intensitiesof the Au(4f), Ti(2p), and Si(2p) peaksby XPS were moreconsistentwith the intensitiespredictedby a model in which continuous films of gold werepresenton titanium than with a modelin which gold formed islandson titanium.l2
owner.
2225
SAlvlso1' llS{CH21'OH
r 5 nnr
Tltickners ofAu
Iunrrl
2fX)nm
I
tI I
F
IL
Figure l. Photographof 100-pL dropsof water on methyl- and films SAMs on 5-nm-,l0-nm-,and 200-nm-thick hydroxyl-terminated glassslides. on titanium-primed of gold supported reliableand convenienttechniquethan silanization.l2Thin gold films (5 and 10 nm) depositedin the absenceof either adhesion promoter were significantly grayer and lessconductivethan gold supported on titanium-primed glass and did not support wellorderedSAMs. The advancingcontactanglesof water and hexadecane(HD) for SAMs of undecanethioland ll-(hexaethylene glycol)undecane-1-thiol were relatively independentof the thicknessof the supportinggold (Table I ) . r3 Hysteresis,however,wassmaller tbr the SAMs on 5-nm- and lO-nm-thick gold than for these SAMs on 200-nm thick gold becausethe recedingcontactangles for these liquids increasedwith decreasingthicknessof gold.tc The thicknessof the gold on hysteresisalso correlateswith the morphology of the surface: atomic force microscopy (AFM) demonstratedthat the mean absoluteroughnessof films of gold gold to 0.2 nm for 10decreasedfrom 1.2 nm for 1OO-nm-thick nm-thick gold and 0.09 nm for 5-nm-thick gold.t5 We measured the adsorption of RNase A on methyl- and hexaethyleneglycol-terminatedSAMs by observingthe change in optical absorbanceof SAMs after incubationwith I mg/ml solutionsof protein covalentlylabeledwith EITC (Figure 2).tf'rt The visible spectra for both types of SAMs before and after exposure to unlabeled RNase A were comparable, but the spectrum for EITC-RNase A on a methyl-terminated SAM showeda distinct peak at 532 nm.r8'reThis peak corresponded of EITC-RNase A;20we estimatethat a closeto 0.35 LLglcm2 packed monolayer of RNase A would contain 0.214.37 pgl cm2.2t In contrast, no peak was observedfor a hexaethylene glycol-terminated SAM exposedto EITC-RNase A; the demonstrationthat theseSAMs resistprotein adsorptionagreeswith previousresults.l6.l8 ( l0) Resistivities weredeterminedby measuringthevoltagedrop at constant applied current using a four-point probe with osmium tips (tip spacingof 0.0625in., tip radiusof 0.005in. Alessi,Irvine,CA) and correctingfor the finite sizeof the film. The resistivityof bulk gold is 2.4 x lW O-cm(Kittel, C. Introduction to Solid-State Physics;John Wiley and Sons: New York, 1976). ( I I ) Allara, D. L.; Hebard,A. F.; Padden,F. J.; Nuzzo, R. G.; Falcone, D. R. "/. Vac. Sci. Technol.A 19t3. 1.376-382. ( 1 2 ) D i M i l l a , P . A . ; F o l k e r s J, . P . ; B i e b u y c k H , . A . ; W h i t e s i d e sG, M . , unpublisheddata. (13) Bain, C. D.;Troughton,E. B.; Tao, Y.-T.; Evall, J.; Whitesides,G. M. J. Am. Chem.Soc. 1989,I I 1,321-335. Troughton,E. B.; Bain, C. D.; Whitesides, G. M.;Nuzzo, R. G.; Allara, D. L.; Porter,M.D. Langmlir 1988, 4,365-385, (14) We define hysteresisin the contact angle as the differencebetween the cosinesof the recedingand advancingcontact angles. (15) Values for mean absoluteroughnesswere measuredusing a TMXcarbontip 2000 AFM (Topometrix,Sunnyvale,CA) with a electrodeposited and silicon cantilever. AFM rather than STM was used becausethe total conductanceof our films was limited by the total amount of metal present.12 The roughnessof bare glassand 5 nm of gold on glassprimed with I nm of titanium was indistinguishable;we exp€ctthat 20O-nm-thickgold is at least gold. as rough as 1O0-nm-thick
@1994AmericanChemicalSociety 0002-78631941r5t6-2225$04.50/0
2226 J. Am. Chem.Soc.,Vol. 116,No.5, 1994
Communications to the Editor
Table l. Wetting Propertiesof Water and Hexadecane(HD) on SAMs Prepared by Adsorption of Methyl- and Hexaethylene Glycol-Terminated Alkanethiols on Thin Films of Gold Supported on Titanium-Primed Glass contact angles(advancing,0., receding,dr, in deg)oand hysteresis (-A cos d = cos d, - cos d.) in wetting of H2O and HD on SAMs
HS(CH2)roCH3 HzO
HS(CH2)rr(OCH2CHz)oOH HD
HzO
thicknessof gold(nm)
0 ^ ,0 ,
-A cos d
0^,0,
-A cos d
5 l0 200
tt2, t04 I 1 09 ,8 l 13,98
0 . 13 3 0.203 0.252
44,39 44,39 4 5 ,3 0
0.058 0.058 0 . 15 9
a^,0, 36,3l 1 A
1 1
HD6 -A cos d
0.048 0.062 0.141
0u,0,
