Characteristics and fabrication of oligophenyleneethynylene thiol self-assembled monolayers

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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 600–603

Characteristics and fabrication of oligophenyleneethynylene thiol self-assembled monolayers Se Young Oh a,b,∗ , Chan Moon Chung c , Dong Hwi Kim b , Seong Gu Lee b a b

Department of Chemical & Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Republic of Korea c Department of Chemistry, Yonsei University, Wonju-si, Gangwon-do 220-710, Republic of Korea Received 7 April 2007; accepted 30 April 2007 Available online 2 June 2007

Abstract Alkanethiols containing carboxylic acid in the tail group have been used as the receptor for the immobilization of biomolecules in biosensor and biomolecular device. However, the alkanethiol shows low electroactivity and sensitivity because of the flexibility of chain and poor electrical properties. In the present work, we have synthesized 4-(2-(4-(acetylthio)phenyl)ethynyl)benzoic acid (APBA) with conjugated structure, and then the self-assembled monolayer (SAM) was fabricated on a gold substrate. The electrical conductivity and electrochemical activity of the prepared SAMs were investigated from the measurements of scanning tunneling microscope (STM) and cyclicvoltammetry (CV). © 2007 Elsevier B.V. All rights reserved. Keywords: Conducting molecular wire; Self-assembled monolayer; Electrical conductivity; Electrochemical activity

1. Introduction Self-assembling is one of the convenient techniques for the formation of homogeneous organic monolayers through the interaction between the head group of molecule and substrate [1]. Recently, Self-assembled monolayers (SAMs) using conjugated aromatic thiol derivatives have attracted much attention because of their potential application as conducting molecular wire in molecular device [2,3]. The electrical conductivity of the molecular wire is the most fundamental and important property in the field of molecular electronics. Therefore, many interesting studies have been carried out in view of the characteristics of charge electron transfer. For example, Bumm et al. demonstrated that thiol-terminated conjugated oligomers inserted into n-alkanethiol SAMs on Au were probed by scanning tunneling microscopy (STM) to assess their electrical properties [4]. Kushmerick et al. measured the conductance of conjugated molecules in a SAM between two Au crossed molecular wires [5]. Frisbie and coworkers used conducting atomic force microscopy ∗ Corresponding author at: Department of Chemical & Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea. Tel.: +82 2 705 8681; fax: +82 2 714 3880. E-mail address: [email protected] (S.Y. Oh).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.125

(cAFM) to probe SAMs of alkanethiols on Au [6], alkane- or benzylthiols on Au [7], and alkane or oligophenylene thiols on Au [8]. In general, alkanethiols containing carboxylic acid in the tail group have been used in the fabrication of self-assembled biomolecular monolayers [9–11]. However, the electroactivity and sensitivity of biosensor or bioelectronic device using the alkanethiol were very low due to the flexible nature of carbon chains and poor electrical conductivity. On the other hand, the electrical conductivity of aromatic thiol is high due to the ␲-conjugated backbone system. Accordingly, in the previous work, we had synthesized aromatic thiol such as 4 -mercapto-biphenyl-4-carboxylic acid (MBCA) and then demonstrated that aromatic thiol shows good electrical and physical properties compared to the alkanethiols [12]. In the present work, we have synthesized oilgophenyleneethynylene thiol to improve the electrical conductivity due to the increase of coplanarity of ␲-conjugated structure. 4(2-(4-(Acetylthio)phenyl)ethynyl)benzoic acid (APBA) having conjugated structure was synthesized, and then the selfassembled monolayer (SAM) was fabricated on Au(1 1 1). The effect of conjugated structure of aromatic thiols on the electrical properties was investigated through the measurements of STM and CV.

S.Y. Oh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 600–603

2. Experiment 2.1. Chemical materials Acetylchloride, ethynyltrimethylsilane, triphenylphosphine, copper iodide, acetic acid, acetic anhydride, tetrabutylammonium fluoride (TBAF), 4-iodobenzoic acid, tert-butanol, triflouroaectic acid (TFA), 1-(3-(dimethylamino)propyl)-3ethylcarbodiimide hydrochloride (EDC), 11-mercaptoundecanoic acid (MUDA) and 3-mercaptopropanoic acid (MPA) were purchased from Aldrich Chemical Co. (USA). 4-Bromobenzenethiol, bis(triphenylphosphine)palladium dichloride, ditert-butyl dicarbonate (Boc2 O), dimethylaminopyridine (DMAP) were purchased from TCI Co. (Japan). Pyridine, triethylamine (TEA), tetrahydrofuran (THF), acetonitrile and ethanol were distilled from first grade solvents purchased on the market. Gold(1 1 1) substrate fabricated by sputering was purchased from INOSTEK Inc. (Korea). 2.2. Synthesis of 4-(2-(4-(acetylthio)phenyl)ethynyl)benzoic acid (APBA) Synthesis of 4-(2-(4-(acetylthio)phenyl)ethynyl)benzoic acid (APBA) was carried out following the synthetic routes as shown in Fig. 1. S-4-bromophenyl ethanethioate (1): 4-Bromobenzenethiol (7 g, 37 mmol) was dissolved in pyridine (120 mL, dried over NaOH pellets and degassed). Acetyl chloride (6 mL, 84 mmol) was syringed slowly with vigorous stirring under argon atmosphere. The mixture was stirred for an additional 1 h followed by the addition of H2 O and crushed ice. The precipitated yellow solid was obtained by suction–filtration, and then washed with H2 O. Column chromatography of the crude

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product on silicagel (CH2 Cl2 –hexane, 1:2) yielded compound 1 as a white solid (7.9 g, 92%). 1 H NMR (300 MHz, CDCl3 ); 2.43 (s, 3H); 7.26, 7.28 (d, 2H); 7.35, 7.55 (d, 2H). S-4-(2(trimethylsilyl)ethynyl)phenyl ethanethioate (2): Samples of 1 (7 g, 30 mmol), copper iodide (570 mg, 3 mmol), triphenylphosphine (790 mg, 3 mmol), bis(triphenylphosphine)palladium(II) chloride (2.1 g, 3 mmol) were dissolved in TEA (140 mL) under argon atmosphere, and ethynyltrimethylsilane (5 mL, 36 mmol) was added. The reaction was carried out at 70 ◦ C for 24 h. The mixture was filtered, and evaporated. The resulting dark brown solid was chromatographed (silicagel, EA–hexane, 1:15, then CH2 Cl2 –hexane, 1:5) to afford a slightly yellow solid at room temperature (4.2 g, 56%). 1 H NMR (300 MHz, CDCl3 ); 0.25 (s, 9H); 2.42 (s, 3H); 7.33, 7.35 (d, 2H); 7.47, 7.49 (d, 2H). S-4-ethynylphenyl ethanthioate (3): Acetic acid (0.5 mL) and acetic anhydride (0.5 mL) were added in a 70 mL THF solution of 2 (3.5 g, 14 mmol). The mixture was cooled to 0 ◦ C and a solution of TBAF (1 M, THF 70 mL) was added dropwise for 5 min. The reaction mixture was kept at 0 ◦ C for 1 h, and then poured on a silica pad, and eluted with CH2 Cl2 –hexane (1:2). The crude oil was chromatographed (silicagel, CH2 Cl2 –hexane 1:2) to afford yellowish oil (1.5 g, 60%). 1 H NMR (300 MHz, CDCl ); 2.42 (s, 3H); 3.15 (s, 1H); 7.36, 3 7.38 (d, 2H); 7.50, 7.53 (d, 2H). tert-Butyl 4-iodobenzoate (4): 4-Iodobenzoic acid (3 g, 12 mmol), DMAP (440 mg, 3.6 mmol) and di-tert-butyl dicarbonate (4.1 mL, 18 mmol) were dissolved in tert-butanol under argon atmosphere, and then reaction mixture was stirred for 48 h. The mixture was evaporated, and chromatographed (silicagel, CH2 Cl–hexane 1:2) to afford white oil (2.6 g, 71%). 1 H NMR (300 MHz, CDCl3 ); 1.57 (s, 9H); 7.66, 7.69 (d, 2H); 7.74, 7.77 (d, 2H). tert-Butyl 4-(2(4-(aectylthio)phenyl)ethynyl)benzoate (5): Compound 5 was

Fig. 1. Synthetic route of APBA compound.

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S.Y. Oh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 600–603

synthesized according to the synthetic method of compound 2. Compound 3 (1.2 g, 6.8 mmol), compound 4 (2.5 g, 8.2 mmol), copper iodide (130 mg, 0.68 mmol), triphenylphosphine (180 mg, 0.68 mmol), bis(triphenylphosphine)palladium(II) chloride (480 mg, 0.68 mmol), and TEA (50 mL) were used. The reaction residue was purified by silicagel flash chromatography by using CH2 Cl2 –hexane (1:3, then 1:1) to provide a white solid of 1.55 g (64%). 1 H NMR (300 MHz, CDCl3 ); 1.60 (s, 9H); 2.44 (s, 3H); 7.40, 7.43 (d, 2H); 7.55, 7.58 (d, 2H); 7.56, 7.59 (d, 2H); 7.96, 7.99 (d, 2H). 4-(2-(4-(Acetylthio)phenyl)ethynyl)benzoic acid (APBA): Suspension of compound 5 (1.5 g, 4.2 mmol) in TFA (17 mL), HO (4 mL) and acetonitrile (40 mL) was stirred vigorously at room temperature overnight under argon atmosphere. The reaction mixture was diluted with H2 O. The precipitated solid was washed with H2 O, and dried in vacuum to yield APBA compound (1.1 g, 87%) as a yellow solid. 1 H NMR (300 MHz, CDCl3 ); 2.45 (s, 3H); 7.41, 7.44 (d, 2H); 7.56, 7.59 (d, 2H); 7.61, 7.64 (d, 2H); 8.07, 8.10 (d, 2H). 2.3. Fabrication of SAMs and electrical measurements APBA (3 mg) was dissolved in a THF solution (10 mL) and 150 ␮L of concentrated H2 SO4 was added, and then the solution was incubated for 1 h in order to deprotect the thiol moiety. The cleaned gold substrate was immersed into the solution at room temperature for 24 h under nitrogen atmosphere in the dark. The substrate was rinsed thoroughly with THF, ethanol and H2 O, and finally blown dry with a nitrogen stream. Ferroceneamide was immobilized on the APBA SAM using EDC as a coupling agent. The electrochemical activity was investigated with a potentiostate measurement analyzer (IM6 system, Zahner Elektrik Co., Germany). The Ag/AgCl (3.0 M HCl) for reference electrode, a gold working electrode, a 1 cm2 Pt-gauze counter electrode were used, and scan rate was 0.2 V/s for the CV measurement. The CV measurement was carried out in potassium ferricyanide solution (0.01 M phosphate buffered saline, 0.138 M NaCl, 0.0027 M KCl and 0.1 mM K3 Fe(CN)6 in deionized water). I–V characteristics of APBA, MBCA, and MUDA SAMs were obtained by a STM (BT00642, Nano Surf AG, Switzerland) technique. The set point for Au tip approaching was 0.5 nA, and the scan range for conductivity measurement was −1 to 1 V. UV–vis spectra of APBA and MBCA compounds were obtained by using a UV–vis spectroscopy (Jasco V-570, Japan).

Fig. 2. XPS spectrum of APBA–SAM.

carbon, oxygen and sulfur atoms, are appeared in the spectrum of APBA SAM. It can be concluded that the successful preparation of APBA SAM was confirmed through the result of XPS. In the previous work, we already had reported that the electrical conductivity of aromatic thiol such as 4 -mercaptobiphenyl-4-carboxylic acid (MBCA) was higher than that of alkanethiol [12]. As shown in Fig. 3, the electrical conductivity of APBA SAM showed maximum value compared to the other SAMs. It should be noted that the electrical conductivity of APBA SAM as oilgophenyleneethynylene thiol was higher than that of MBCA as biphenyl thiol SAM because of the low energy gap due to the high coplanarity of ␲-conjugated backbone. The highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LOMO) gap of aromatic-based compound is a very important physical factor for charge transport through molecular backbone [13]. As shown in Fig. 4, HOMO–LUMO gaps of ␲-conjugated molecules were evaluated from the measurements of UV–vis spectroscopy. The lower HOMO–LUMO gap for APBA compound (3.5 eV), as compare to that of MBCA compound (3.9 eV), can explain high charge transport efficiency through the effective overlapping of ␲-orbital due to the coplanarity. Thus, it can be found that the coplanarity of ␲-conjugated

3. Results and discussion The synthesized APBA compound was identified through the measurements of elemental analysis, 1 H NMR and FT-IR. The result of elemental analysis was as follow; Calculated C: 68.90%; H: 4.08%; S: 10.82%; Found C: 68.93%; H: 4.06%; S: 10.86%. The FT-IR spectrum of APBA showed the absorption peaks corresponding to thioacetyl and carboxylic groups, e.g., O–H stretching at 2800–3100 cm−1 , C O stretching at 1700, 1680 cm−1 and C C stretching at 2210 cm−1 . Fig. 2 showed the X-ray photoelectron spectroscopy (XPS) spectrum of APBA SAM fabricated on a gold substrate. Peaks, binding energies of

Fig. 3. I–V characteristics of APBA, MBCA, MUDA and MPA monolayers.

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ductivity allowed high electrochemical activity compared to the other SAMs. The results of I-V and CV characteristics indicated that the electrical properties of aromatic thiol SAMs are significantly influenced on the difference in intramolecular charge transport. 4. Conclusion

Fig. 4. UV–vis spectra of APBA and MBCA compounds in THF solvent.

APBA compound as conducting receptor for the immobilization of biomolecules was synthesized, and then APBA SAM was fabricated using self-assembly technique. The prepared APBA SAM exhibited higher electrical conductivity and electrochemical activity than those of biphenyl thiol SAM due to the increase of coplanarity of ␲-conjugated backbone. Thus, it can be argued that the prepared APBA SAM showed the feasibility of application in promising bioreceptor as conducting molecular wire for biosensor or bioelectronic device. Acknowledgment This work was supported by Sogang University Special Research Grant in 2006. References

Fig. 5. Cyclicvoltammograms of bare gold and ferroceneamide immobilized onto the APBA, MBCA and MUDA monolayers.

backbone structure plays a role in the increase of electrical conductivity. In general, it was well known that the sensitivity of biosensor or biochip using ferrocene moiety immobilized onto the conventional SAM was significantly influenced on the intensity of ferrocene redox current. Fig. 5 showed the cyclicvoltammograms of ferroceneamide immobilized onto the each SAMs. The prepared APBA SAM having high electrical con-

[1] A. Ulman, Chem. Rev. 96 (1996) 1533. [2] F.-R.F. Fan, J. Yang, L. Cai, D.W. Price Jr., S.M. Dirk, D.V. Kosynkin, Y. Yao, A.M. Rawlett, J.M. Tour, A. Bard, J. Am. Chem. Soc. 124 (2002) 5550. [3] F.L. Carter, Molecular Electric Devices, Marcel Dekker, New York, 1982. [4] L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jone II, D.L. Allera, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705. [5] J.G. Kushmerick, J. Naciri, J.C. Yang, R. Shashidhar, NanoLett. 3 (2003) 897. [6] D.J. Wold, C.D. Frisbie, J. Am. Chem. Soc. 122 (2000) 2970. [7] D.J. Wold, C.D. Frisbie, J. Am. Chem. Soc. 123 (2001) 5549. [8] D.J. Wold, R. Haag, M.A. Rampi, C.D. Frisbie, J. Phys. Chem. B 106 (2002) 2813. [9] S.Y. Oh, H.S. Jie, H.S. Choi, J.W. Choi, Int. J. Nanosci. 1 (2002) 611. [10] S.Y. Oh, H.S. Choi, H.S. Jie, J.K. Park, Mater. Sci. Eng. C 24 (2004) 91. [11] S.Y. Lee, S.J. Lee, J. Jung, Ind. Eng. Chem. 9 (2003) 9. [12] S.Y. Oh, H.S. Choi, H.J. Kim, J.K. Park, Polymer (Korea) 29 (2005) 331. [13] A. Salmonon, D. Chaen, S. Lindsay, J. Tomfohr, W.B. Engelkes, A.D. Frisbie, Adv. Mater. 15 (2003) 1881.