Notes

Strategy for Site-Selective Patterning ofOrganic Monolayers via Borohydride-Directed

Reduction: Surface-Enhanced RamanSpectroscopic Feasibility Study

Kwan Kim,* Hyun Sik Kim, and Seung Joon Lee

Laboratory of Intelligent Interfaces, School of Chemistry andMolecular Engineering and Center for Molecular Catalysis,

Seoul National University, Seoul 151-742, Korea

Received June 30, 2003. In Final Form: October 9, 2003

IntroductionMicrofabrication is essential for much of modern science

and technology.1 Two-dimensional nano- to mesoscalestructures can be used in sensitive optoelectronic devices,sensors, and devices that mimic biological functions.Patterning of the self-assembled monolayers (SAMs) oforganic molecules is an excellent strategy for preparingsuch templates possessing variable surface chemicalproperties.2-5 Patterning or modification of organic mono-layers can be accomplished either by utilizing conventionalphotolithography or by destructive atomic beam andproximal-probe lithographical processes.6-9 In recentyears, a number of nondestructive processes have alsobeen reported. For instance, chemical reactions can beinduced for the terminal groups of organic SAMs by acatalytically active platinum- or palladium-coated atomicforce microscope (AFM) tip,10 and selective borohydridereduction can be performed using AFM tips functionalizedwith sodium triacetoxyborohydride.11 On the other hand,lines with a width of only a few nanometers can be drawnwith a conducting AFM tip by inducing spatially defined

electro-oxidation.12 However, all of these currently avail-able methods have their own weaknesses. The destructiveatomic beam and proximal-probe lithographic processeshave to be conducted in an expensive ultrahigh vacuumsystem. AFM-tip-based lithography is time-consuming,and it has to be performed on SAMs on flat substrates. Inthis regard, it is highly desirable to develop simple butcost-effective chemical lithographic processes that can beapplied even to SAMs on uneven solid substrates underambient conditions. As one simple example, amine-group-terminated binary monolayers can be prepared on silverby inducing a surface-induced photoreaction for a SAM of4-nitrobenzenethiol on Ag.13

In our earlier surface-enhanced Raman scattering(SERS) study using silver electrodes,14 the characteristicpeaks of aromatic and aliphatic sulfides were clearlyidentified at -0.2 V or higher versus SCE (saturatedcalomel electrode); all the potentials quoted herein areversus SCE. Those peaks lose intensity progressively,however, as the electrode potential is lowered. Forinstance, the characteristic peaks of benzyl phenyl sulfide(BPS) and dibenzyl sulfide (DBS) disappear completelyat -0.6 V while those of dimethyl sulfide (DMS) disappearat -0.7 V. Instead, the resulting spectra are virtually thesame as those for benzenethiolate (BT), benzyl mercaptide,or methyl mercaptide. These electrochemical reactionsare irreversible in the potential range between -1.0 and0.2 V, and the observed potential dependence of the SERSspectral pattern can be correlated with the cyclic volta-mmetry (CV) behavior.

Another remarkable observation in our earlier SERSstudies was that the addition of BH4

- ions to aqueoussilver sol could lower the surface potential, facilitatingelectrochemical reduction of organic sulfides to thiolates.15

Such an electrochemical reduction may be applied to thepatterning of organic SAMs. To demonstrate its feasibility,we show herein that the SAMs of BPS on gold can beconverted to those of benzenethiolate by coming in contactwith BH4

- in ambient conditions. We also show the readyconversion of nitro groups to amine groups in the nitro-substituted aromatic monolayers, for example, the reduc-tion of 4-nitrobenzenethiol (4-NBT) to 4-aminobenzene-thiol (4-ABT). The experimental scheme is presented inFigure 1, in which borohydride can be transferred to thedesired area either using an ink-jet printer, a dip-pen, anappropriate stamp, or a microdispenser. Once a minuteamount of borohydride is transferred, the reductionreaction can be induced simply by adjusting the humidityof the area. Such a reaction is simple but effective andbenign, and also possible under ambient conditions. Thepresent strategy can thus be adopted as an environmen-tally friendly means of fabricating functional patternedmonolayers. It can also be applied to a variety of substratesincluding nonflat and nonmetallic surfaces. This is surelythe advantage of the present method over the existinglithographic methods.

* To whom correspondence should be addressed. Tel: +82-2-8806651. Fax: +82-2-8743704. E-mail: kwankim@plaza.snu.ac.kr.

(1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550.(2) Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Langmuir

1996, 12, 5509.(3) Maoz, R.; Yam, R.; Berkovic, G.; Sagiv, J. In Organic Thin Films

and Surfaces: Directions for the Nineties; Ulman, A., Ed.; Thin Films,Vol. 20; Academic Press: San Diego, CA, 1995; p 41. Duschl, C.; Liley,M.; Corradin, G.; Vogel, H. Biophys. J. 1994, 67, 1229.

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(5) Rieke, P.; Tarasevich, B. J.; Wood, L. L.; Engelhard, M. H.; Baer,D. R.; Fryxell, G. E.; John, C. M.; Laken, D. A.; Jaehnig, M. C. Langmuir1994, 10, 619.

(6) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829.Behm, J. M.; Lykke, K. R.; Pellin, M. J.; Hemminger, J. C. Langmuir1996, 12, 2121. Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem.,Int. Ed. 1997, 36, 1080.

(7) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara,D. L. Appl. Phys. Lett. 1996, 68, 1504. Delamarche, E.; Schmid, H.;Michel, B.; Biebuyck, H. Adv. Mater. 1997, 9, 741. Gupta, V. K.; Abbott,N. L. Science 1997, 276, 1533.

(8) Thywissen, J. H.; Johnson, K. S.; Younkin, R.; Dekker, N. H.;Berggren, K. K.; Chu, A. P.; Prentiss, M.; Lee, S. A. J. Vac. Sci. Technol.,B 1997, 15, 2093.

(9) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457.(10) Muller, W. T.; Klein, D. L.; Lee, T.; Klarke, J.; McEuen, P. L.;

Schultz, P. G. Science 1995, 268, 272. Blackledge, C.; Engebretson, D.A.; McDonald, J. D. Langmuir 2000, 16, 8317.

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(12) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55.(13) Han, S. W.; Lee, I.; Kim, K. Langmuir 2002, 18, 182.(14) Yim, Y. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 2552.(15) Lee, S. B.; Kim, K.; Kim, M. S. J. Phys. Chem. 1992, 96, 9940.

10985Langmuir 2003, 19, 10985-10988

10.1021/la0351663 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 11/20/2003

Experimental SectionAll the chemicals otherwise specified were reagent grade, and

triply distilled water of resistivity greater than 18.0 MΩ cm wasused in making aqueous solutions. A SERS-active Au substrate(0.05 mm thick foil) was prepared via oxidation-reduction cycles(ORCs) in 0.1 M KCl by sweeping consecutively at 1 V/s between-0.8 and +1.0 V. The self-assembly of BPS or 4-NBT onto Auwas conducted in 1 mM ethanolic solution overnight. To preparethe benzenethiol SAMs on Au, the Au substrate was soaked in1 mM ethanolic solution for 3 h. The SERS enhancement factorfor our Au substrate was estimated to be ∼4.8 × 104 from the1574 cm-1 peak of BPS, ∼7.2 × 104 from the 1574 cm-1 peak ofBT, ∼5.0 × 104 from the 1576 cm-1 peak of 4-ABT, and ∼3.1 ×104 from the 1572 cm-1 peak of 4-NBT; these values werereproducibly obtained in six different measurements.

SERS spectra were obtained using a Renishaw Raman System2000 spectrometer equipped with a holographic notch filter andan integral microscope (Olympus BH2-UMA). The 632.8 nmradiation from a 17 mW air-cooled He/Ne laser (Spectra Physicsmodel 127) was used as the excitation source. Raman scatteringwas detected with 180° geometry using a Peltier-cooled (-70 °C)CCD camera (400 × 600 pixels). The holographic grating (1800grooves/mm) and the slit permitted a spectral resolution of 1cm-1. The Raman band of a silicon wafer at 520 cm-1 was usedto calibrate the spectrometer, and the accuracy of the spectralmeasurement was estimated to be better than 1 cm-1. The typicallaser power at the sampling position was 0.2 mW with an averagespot size of 1 µm in diameter. The integration time was 30 s. TheRaman spectrometer was interfaced with an IBM-compatiblePC, and the spectral data were analyzed using Renishaw WiREsoftware, version 1.2, based on the GRAMS/32C suite program(Galactic Industries).

CV measurements were conducted in a 1 mM solution of BPSor 4-NBT in 0.1 M Na2SO4 in a 1:1 volume mixture of H2O andCH3OH on a CH Instruments model 600 electrochemical system.The working electrode was a Au foil (Aldrich, 0.05 mm thick).The reference electrode was a Ag/AgCl electrode, and a spiralplatinum wire was used as the counter electrode. Freshlyprepared sodium borohydride solution was stored under an Aratmosphere before use.

A poly(dimethylsiloxane) (PDMS) stamp with stripe patterns(width of 2 µm and interdistance of 5 µm) was fabricated followinga published procedure.1 An aliquot of 1 M methanolic NaBH4solution was spin-coated onto the PDMS stamp at 3000 rpm for30 s. The coated borohydride was transferred to the 4-NBT SAMsby microcontact printing for 10 s. The modification reaction withstearic acid (Aldrich, STA) was performed by overnight incubationof the patterned substrate in 100 mM STA solution in N,N-dimethylformamide (DMF) containing 100 mM 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) as a coupling agent.16

An atomically flat Au substrate was prepared by vacuumevaporation of Ti and Au onto a mica substrate. Atomic force andlateral force microscope (AFM/LFM) images were acquired usinga Digital Instruments model Nanoscope IIIa scanning probemicroscope; more specifically, they were measured using aV-shaped 200 µm long Si3N4 cantilever with a nominal springconstant of 0.12 N/m (Nanoprobe, Digital Instruments) at a scan

rate of 2-5 Hz for the AFM and 1 Hz for the LFM. The typicalroot-mean-square (rms) roughness (standard deviation of theheight values determined using a software provided by DigitalInstruments) of the evaporated Au film on mica was 2-3 nm inthe sampling area of 30 µm × 30 µm; in the 100 nm × 100 nmsized terraces, it was <1 nm.

Results and DiscussionAccording to CV measurements using a Au electrode

(see Figure 2), the reduction of BPS to BT commences at-0.2 V and continues up to -0.8 V. Once reduced, noelectrochemical reaction takes place in the potential rangebetween at least -0.9 and 0.1 V. On the other hand, thereduction of 4-NBT to 4-ABT begins at -0.3 V andcompletes at -0.7 V. The oxidation of 4-ABT does nottake place, however, below 0.1 V. If the electrode potentialis lowered to -1.2 V, BT and 4-ABT desorb from the Auelectrode. Separately, we measured the electrochemicalpotential of a gold wire electrode as a function of time inthe ambient conditions after being dipped in variouslydiluted aqueous NaBH4 solutions. Immediately afterdipping in 1, 10, 100, 300, and 500 mM aqueous NaBH4solutions, the potential of the Au electrode was measuredto be -0.69, -0.92, -1.06, -1.15, and -1.23 V, respectively(data not shown). These initial potential values aremaintained for at least approximately 60 s. Recalling thecyclic voltammograms of BPS and 4-NBT, a drop of 200mM NaBH4 will then reduce BPS immediately to BT and4-NBT to 4-ABT. This effect can be confirmed from theSERS measurements.

Figure 3A(a) shows the SERS spectrum of 4-NBT onAu, and its spectral assignment is summarized in Table1. All the peaks in Figure 3A(a) can be attributed to

(16) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392. Checkik,V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161.

Figure 1. A diagram showing the strategy for inducingchemical reduction of BPS or 4-NBT on Au by virtue ofborohydride ions spread thereon.

Figure 2. Cyclic voltammograms of BPS and 4-NBT. The solidand dashed lines show the first and second cycles, respectively.

Figure 3. (A) SERS spectra of 4-NBT on Au (a) before and (b)after coming in contact with a 200 mM NaBH4 droplet (1 µL).(c) SERS spectrum of 4-ABT on Au taken for reference. (B)SERS spectra of BPS on Au (a) before and (b) after coming incontact with a 200 mM NaBH4 droplet (1 µL). (c) SERS spectrumof BT on Au taken for reference.

10986 Langmuir, Vol. 19, No. 26, 2003 Notes

4-nitrobenzenethiolate. The complete absence of the S-Hstretching peak in the SERS spectrum, observable at 2548cm-1 for pure 4-NBT, indicates that 4-NBT is adsorbed onAu as thiolate after the S-H bond cleavage. The prominentSERS peak at 1346 cm-1 can be assigned to the symmetricstretching vibration of the nitro group (νs(NO2)). Figure3A(b) shows the SERS spectrum of 4-NBT on Au obtainedafter coming in contact with a 1 µL droplet of 200 mMNaBH4. The SERS feature in Figure 3A(b) is substantiallydifferent from that in Figure 3A(a). The νs(NO2) peak wasno longer identifiable, and several new peaks appeared,for instance, at 1436, 1392, 1192, and 1143 cm-1. Thisindicates that chemical reaction has taken place for 4-NBTby borohydride. Although the exact nature of the speciesresponsible for Figure 3A(b) is a matter of conjecture, theSERS spectral feature is surprisingly coincident with thatof 4-ABT on Au, shown in Figure 3A(c). This suggeststhat 4-NBT has been reduced on Au to 4-ABT by aborohydride droplet. Although other molecules such asnitroso-, hydroxylamine-, and azo-compounds would alsobe produced by reduction of aromatic nitro molecules,17

no peak due to any such compound was identified at allin our SERS spectra. The peak positions in curves b andc of Figure 3A are also collectively listed in Table 1 alongwith the appropriate vibrational assignments.

Using SERS, the borohydride-directed chemical reduc-tion can also be confirmed to occur for BPS on Au. Figure3B(a) shows the SERS spectrum of BPS on Au, and all thepeaks therein can be attributed to either the benzylthioor phenylthio moieties of BPS, indicating that the speciesresponsible for the SERS spectrum is BPS; the peakpositions and their spectral assignments are summarizedin Table 1. However, the SERS spectrum of BPS takenafter treatment with a droplet of NaBH4 is the same asthat of pure BT; see curves b and c of Figure 3B as wellas Table 1. This clearly indicates that BPS is reduced onAu to BT by a borohydride droplet. These observationssuggest that 2-D patterning can be readily accomplished

simply by dispensing droplets of borohydride ions ontothe selected regions of monolayers; droplets may be spreadonto monolayers using ink-jet, microdispenser, PDMSstamp, or dip-pen methodologies, and after reaction, anyunwanted products can be removed by brief washing withalcohol or other organic solvent.

When borohydride is delivered onto SAMs by a micro-contact printing (µCP) method, borohydride remains onthe SAMs as fine particles. In dry conditions, no chemicalreaction is induced by those particles but the surfacereduction can be initiated simply by raising the humidityof the area near the SAMs. As an example, we show hereinthe fabrication of 2-D monolayers from the SAMs of 4-NBTon Au. Curves a and b of Figure 4A present SERS spectraobtained for 4-NBT SAMs on Au taken before and afterexposure to a wet atmosphere after µCP of 1 M alcoholicNaBH4 onto the SAMs. Spectral variation is indeednoticeable in wet conditions, while no apparent change isidentified in dry conditions. The SERS spectral featuresin Figure 4A(b) are therefore the same as those of 4-ABT,implying that 4-NBT is reduced to 4-ABT by the actionof borohydride. The nitro-to-amine conversion can also beconfirmed from the amide coupling reaction of the pat-terned amine group with STA in DMF. In fact, the 4-NBTSAMs on Au did not react with STA, but the borohydride-treated 4-NBT SAMs readily reacted with STA. This maybe clearly demonstrated by LFM measurements since theH2O contact angle of the CH3-terminated SAMs (>110°)is far greater than that of the NO2-terminated SAMs(∼60°).20 The surface friction experienced by a LFM tipwill be dependent on the surface free energy and thus thecomposition of the surface functional groups.21 In this light,we show in Figure 4B the LFM image of a 4-NBT/Au/micafilmthathasbeensubjectedbeforehandtodry-borohydridepatterning (with a PDMS stamp) followed by couplingreaction with STA. Dark regions with a width of ∼2 µm

(17) Tomilov, A. T.; Mairanovskii, S. G.; Foishin, M. Y.; Smirronov,V. A. Electrochemistry of Organic Compounds; Halstead: New York,1972.

(18) Skadtchenko, B. O.; Aroca, R. Spectrochim. Acta, Part A 2001,57, 1009. Futamata, M. J. Phys. Chem. 1995, 99, 11901. Osawa, M.;Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702.

(19) Joo, S. W.; Han, S. W.; Kim, K. Appl. Spectrosc. 2000, 54, 378.(20) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G.

Y. Langmuir 2001, 17, 95.(21) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G.

M. Langmuir 1995, 11, 827.

Table 1. Vibrational Assignments for the SERS Peaks inFigure 3a

4-NBT/AuFigure 3A(a)

4-NBT/Aub

Figure 3A(b)4-ABT/Au

Figure 3A(c) assignmentc

1572 1576 1576 8b1437 1437 19b1392 1392 3

1345 νsNO2

1142 1142 9b1110 9b1082 1075 1075 7a

BPS/AuFigure 3B(a)

BPS/Aub

Figure 3B(b)BT/Au

Figure 3B(c) assignmentd,e

1601 8a (B)1574 1574 1574 8a (P)1236 CH2 twisting1201 13 (B)1077 1076 1076 1 (P)1023 1023 1023 18a (B,P)1001 1001 1001 12 (B,P)803 CH2 rocking (B)659 CS stretching (B)416 416 416 7a (P)

a Units in cm-1. b After coming in contact with a 200 mM NaBH4droplet (1 µL). c Assigned based on refs 13 and 18. d Abbreviations:

B ) benzyl moiety; P ) phenyl moiety. e Assigned based on refs5, 14, 15, and 19.

Figure 4. (A) SERS spectra of 4-NBT SAMs taken before andafter exposure to a wet atmosphere (water-vapor treatment)after µCP of 1 M alcoholic NaBH4 onto the SAMs. (B) LFMimage (30 µm × 30 µm) of 4-NBT SAMs on vacuum-evaporatedAutakenafterdry-borohydridepatterning (withaPDMSstamp)followed by coupling reaction with STA.

Notes Langmuir, Vol. 19, No. 26, 2003 10987

are clearly identified in the LFM image. The dark stripesmust be associated with the low-friction regions termi-nated with -CH3 groups of STA, while the brighterinteriors reflect the -NO2-terminated regions. However,their boundaries are not sharply defined. This may beunderstood by presuming that the sharpness of boundariesis determined by the extent and/or the fineness ofborohydride particles remaining on the patterned areas.

For the present method to be used in the developmentof biosensors, the efficiency of the nitro-to-amine groupconversion has to be evaluated. It is difficult, however, toassess quantitatively the number of amine groups pro-duced. Regarding this matter, we are currently attemptingto quantify the surface density of amine groups throughthe fluorescence-based titration method using, for in-stance, 9-anthraldehyde. Nonetheless, we presume at themoment that a near-complete nitro-to-amine conversionis possible by a borohydride droplet. To support thepresumption, we have conducted a control experiment inwhich 1 µL of 200 mM NaBH4 was spread over the 4-NBTSAMs on Au to form a ∼2 mm diameter droplet and thenSERS spectra were measured at ∼80 slightly differentpositions of the droplet; the Raman excitation beam sizewas ∼1 µm. (When an alcoholic borohydride droplet (1 M)was spread on the 4-NBT SAMs on flat Au and then keptin a wet atmosphere to induce the nitro-to-amine conver-sion, we obtained the subsequent SERS spectra by virtueof the electromagnetic enhancement mechanism mediatedvia Ag nanoparticles;22 laser-ablated Ag nanoparticles (1µL) were physically overlaid onto the reacted area of the4-NBT SAMs to obtain SERS spectra at ∼80 slightlydifferent positions.) In fact, all the SERS features observedthereby were exactly the same as those in curves b andc of Figure 3A, implying the near-complete nitro-to-amineconversion by borohydride. Hence, the number of amine

groups that can be produced by the present method issupposed to be sufficient to interact further with biologicalmolecules for biosensor applications, albeit that the exactconversion efficiency is surely dependent on the extentand/or the fineness of borohydride particles remaining onthe patterned areas.

In summary, we have demonstrated that site-selectivereduction can be readily induced to occur for organic SAMsby injecting or spreading borohydride droplets or particlesonto the desired regions of SAMs. Since the reaction iseffective and benign, a strategy making use of such areducing agent should be adopted as an environmentallyfriendly means of fabricating functional patterned mono-layers. As another advantage, one can recall that normalsolid substrates such as gold and silicon are not affectedby borohydride, and the flatness of substrates is not arequirement for the occurrence of a surface reaction. Otherreducing or oxidizing agents can also be used at the sametime. The finest resolution that can be achieved for 2-Dpatterns via ink-jet printing is now of the order of ∼50µm, while nanoscaled patterns are possible via a dip-penmethodology. Nonetheless, a rapid improvement of drop-on-demand devices in recent years may allow the readyfabrication of nanoscaled patterns using redox chemicalssuch as borohydride.

Acknowledgment. This work was supported in partby the Ministry of Health & Welfare (Korea Health 21R&D Project, 01-PJ11-PG9-01NT00-0023), the Ministryof Science and Technology (Nano Project, M10213240001-02B1524-00210) of the Republic of Korea, and the KoreaScience and Engineering Foundation (KOSEF, 1999-2-121-001-5). K.K. is also supported by KOSEF throughthe Center for Molecular Catalysis at Seoul NationalUniversity.

LA0351663(22) Kim, K.; Lee, I.; Lee, S. J. Chem. Phys. Lett. 2003, 377, 201.

10988 Langmuir, Vol. 19, No. 26, 2003 Notes