- Email: [email protected]
gold
Chemical Physics Letters 371 (2003) 483–489 www.elsevier.com/locate/cplett
Supramolecular surface layer: coumarin/thiolated cyclodextrin/gold D. Velic
*,1,
G. K€ ohler
Institute of Theoretical Chemistry and Structural Biology, University of Vienna, Dr. Bohrgasse 7/b, Vienna 1030, Austria Received 30 August 2002; in final form 31 January 2003
Abstract A formation of supramolecular host–guest inclusion surface layer is reported. A nanostructure of this layer consists of an inclusion complex, where a guest molecule – coumarin-6 (3-(2-benzothiazolyl)-7-(diethylamino) coumarin, C20 H18 N2 O2 S) is placed into a ÔbucketÕ of a host molecule – thiolated cyclodextrin (6-monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S) which is attached on Au surface. The inclusion and the layer formation are determined by using time-resolved fluorescence anisotropy, second-harmonic generation (SHG), and fluorescence in combination with laserinduced thermal desorption from the surface. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Self-assembling phenomena have been attracting considerable attention in fields such molecular biochemistry or nanofabrication. Two specific cases of the phenomena were considered in this Letter, a host–guest inclusion complex [1–4] and a self-assembled monolayer [4–7]. The inclusion host was proposed to be cyclodextrin molecule. Cyclodextrins are cyclic oligosaccharides which can generally serve as a hydrophilic cover for the hydrophobic guest, and additionally provide mechanical, optical, and chemical protection. The
*
Corresponding author. Fax: +421-2-6542-3244. E-mail address: [email protected] (D. Velic). 1 Permanent address: Komensky University, Mlynska dolina CH-1, Bratislava 842 15, Slovakia.
inclusions of various molecules into cyclodextrins have been extensively demonstrated in liquid phase [1–4]. Our focus was on using the host–guest complex and on extending its utilization into a formation of the supramolecular self-assembled layer on a surface, similar to the self-assembled monolayer. The initial step in this direction was an attachment of the cyclodextrin host on the surface [4,8–12]. In this Letter, we used the thiolated bcyclodextrin (CD-SH), where OH group on the primary chain had been replaced with SH group. The structure of such a ÔbucketÕ is illustrated in Scheme 1. The CD-SH was then chemisorbed on the Au surface through the S–Au bond in the same way as alkanethiols in self-assembled monolayers on Au. However, due to the CD-SH ÔbucketÕ composition and geometry, the formation of only self-assembled monolayer-like film might be expected [4]. The inclusion of suitable guest
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00300-2
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D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489
Raman spectroscopy [9,11], plasmon surface polariton spectroscopy [10], cyclic voltammetry [8,10], contact-angle measurements [10], and atomic force microscopy [12]. The methods used in our experiment were a time-resolved fluorescence anisotropy [13], second-harmonic generation (SHG), and the fluorescence in combination with laser-induced thermal desorption from the surface. The time-resolved fluorescence anisotropy was suited for the purpose to conclusively observe the formation of the supramolecular inclusion structure with CD-SH. SHG signal is proportional to the nonlinear susceptibilities of surface atoms, therefore a modification of the surface results in a change of the SHG intensity, where a resonantly enhanced surface SHG was considered [14,15]. Due to a quenching effect of the Au surface [13], a direct fluorescence was not possible to use. Therefore we utilized a laser-induced thermal desorption from the surface, with following fluorescence, to our knowledge, a novel approach in a liquid phase, although this technique is well established in vacuum surface science.
2. Experimental
Scheme 1. The CD-SH molecule used in these experiments is 6monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S, consisting of seven a-1,4-glycosidically linked glucopyranose units with one SH group directly on the primary chain and forming a ÔbucketÕ structure.
molecules into the layer of CD-SH ÔbucketsÕ might then form a supramolecular surface layer, with a potential to tailor its functional response. As a model guest molecule we have chosen coumarin-6 (C6) due to its absorption and fluorescence in visible range for fluorescence detection, suitable resonant transition, and expected inclusion complexation with CD-SH. Only few experimental preparations have been so far attempted [8–12]. The layers were characterized by surface-enhanced
Gold film was prepared by using a thermal evaporation of gold on the mica (muscovite) substrate under vacuum of 10 7 Torr. The sample of Au surface was cooled under the vacuum for approximately 30 min and then placed in the quartz cell with a reagent solution. The CD-SH, 6-monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S, used in these experiments, was produced by Cyclolab. The C6, 3-(2-benzothiazolyl)-7-(diethylamino) coumarin, C20 H18 N2 O2 S, was produced by Radiant Dyes Chemie. Both used solvents, ethanol and water, had high-grade purity. All experiments were performed at room temperature. Femtosecond Ti–sapphire oscillator (Coherent, 100 fs, 76 MHz, 1.3 W, 860 nm) pumped with Argon ion laser was used to excite the system. For the SHG measurements, the sample surface and the laser beam were in 45° geometry. Two filters were employed in front and behind (with respect to laser beam direction) of the sample to filter the SHG response wavelength of 430 nm and the ex-
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citation wavelength of 860 nm, respectively. Noise of the SHG signal was proportional to square of the laser fluctuation of 3% which was approximately 9%. Note that no polarizers were used in SHG measurements and laser light was horizontally polarized (p-polarized). The SHG signal of 430 nm was collected through a monochromator with a photomultiplier tube connected to singlephoton counting setup. For the fluorescence measurements, the reagent solution was placed in the quartz cell and the geometry between the laser beam and the fluorescence collection direction was 90°. The excitation wavelength of 430 nm (vertically polarized) was produced by SHG in BBO crystal from the Ti–sapphire fundamental wavelength of 860 nm. The fluorescence signal of 505 nm was collected in the same way as SHG. The same setup, except the laser beam was focused with 50 mm lens, was used for laser-induced thermal desorption experiments from the surface.
3. Results and discussion Before a preparation of C6/CD-SH/Au layer can be considered, a formation of the supramolecular host–guest inclusion complex C6/CD-SH must be presented. The time-resolved fluorescence anisotropy was used to confirm, whether C6 forms the inclusion complex with CD-SH. The anisotropy was calculated, based on the G-factor, as a measure of the instrument sensitivity, and the measured fluorescence intensities in parallel and crossed polarizations [13]. Based on a rotational correlation time, the inclusion of the guest into the host was determined. Generally, the rotational correlation time of the guest molecule in pure solvent is expected to be shorter in comparison with the rotational correlation time of the guest molecule in the host–guest complex, due to interaction hindrance of the relaxation. The anisotropy dynamics of C6, measured in solution of ethanol (10 5 M), is shown in Fig. 1a. This anisotropy decay was fit by using a single exponential function resulting in a decay rate of 0:2 0:05 ns. The anisotropy dynamics of C6 and CD-SH mixture is shown in Fig. 1b. This mixture was prepared from aqueous CD-SH solution (10 4 M) by adding
Fig. 1. The anisotropy dynamics of C6 (a) and C6/CD-SH (b).
ethanol solution of C6 (10 4 M), so it was 5% of the whole mixture volume. The anisotropy decay was fit by using a single exponential function resulting in a decay rate of 1:4 0:1 ns. Note that the presence of ethanol molecule together with C6 molecule in the CD-SH ÔbucketÕ cannot be excluded. Additionally, the dynamics effects of clustering of C6 molecules in water are being currently investigated. Concluding, the slower anisotropy relaxation of the C6/CD-SH system in comparison with the C6/ethanol system suggests that the C6 molecule forms the supramolecular inclusion complex with the CD-SH molecule in aqueous solution. This result is also supported by the fluorescence of C6 where its maximum shifted 17 nm in wavelength, comparing C6 in water and in cyclodextrin solution which represent hydrophilic and hydrophobic environments, respectively [4].
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The fluorescence shift as a function of cyclodextrin concentration resulted in equilibrium shift from water into cyclodextrin [4]. The observations of the C6/CD-SH complex formation then propose that the formation of such a complex might be also feasible on the Au surface, resulting presumably in a C6/CD-SH/Au surface layer. The first step in a preparation of the C6/CDSH/Au layer was an adsorption of CD-SH ÔbucketsÕ on the Au surface. The detail adsorption kinetics were reported elsewhere [4]. The adsorption process was monitored by using SHG. SHG is the first-order nonlinear process where the frequency of incoming laser light is doubled, i.e., two photons of fundamental frequency are converted into one photon of doubled frequency. In the electric dipole approximation, symmetry requires that SHG can be generated only in noncentrosymmetric materials, what satisfies any surface [14,15]. The SHG response was first measured from the clean Au surface immersed in water and this SHG intensity served as a reference value of the SHG signal (100%). Then water was ejected from the cell and aqueous solution of CD-SH (10 3 M) was injected. After 20 min, the time sufficient to form a layer of CD-SH ÔbucketsÕ on Au surface [4], the solution was ejected and water was injected into the cell. The SHG intensity was 55 15% of the SHG intensity from the clean Au surface as shown in Fig. 2. The SHG from the surface depends on all three susceptibilities of the surface, the adsorbate–surface interaction, and the adsorbate [7]. The decrease in SHG intensity is due to the chemisorption of CD-SH on Au. Therefore, this decrease in SHG signal suggests that the susceptibility of Au–S interaction is smaller than the susceptibility of Au, while assuming no contribution from the rest of the CDSH molecule. Note that a role of the CD-SH surface structure is not clear with respect to how it contributes to a total surface SHG signal. The molecule of C6 was also chosen, because as a suitable adsorbate was proposed to resonantly enhance surface SHG [14]. The enhancement can be expected if the absorption transition of the adsorbate is in the resonance with second-harmonic frequency. The C6 has its first electronic absorption maximum at approximately 440 nm,
Fig. 2. Three stages of the C6/CD-SH/Au formation as functions of SHG intensity.
depending on solvent. Therefore, the resonance condition between this absorption and the SHG wavelength of 430 nm seemed to be satisfied, assuming no significant change upon the inclusion of C6 into CD-SH. The proposed inclusion of C6 into the CD-SH/Au layer was performed in two approaches. The first approach was based on adding the ethanol solution of C6 (10 4 M) into the cell with the CD-SH ÔbucketsÕ layer on Au surface placed in water, so it was 5% of the whole mixture volume. The inclusion process was allowed for 20 min and then this solution was ejected and after rinsing of the sample, water was injected into the cell. The second approach was based on injecting the ethanol solution of C6 (10 4 M) into the cell with the CD-SH ÔbucketsÕ layer on Au and on sequential replacing the ethanol solution of C6 with water in six steps, by ejecting 50% of volume of C6 solution and injecting 50% of water. Every mixture was homogenized for 3 min and at the end, the cell and the whole sample were carefully rinsed with water. The role of these ejection/injection steps was to shift the solubility gradient from ethanol into the CD-SH/Au layer through water. Note that C6 is well soluble in ethanol and almost insoluble in water, and forms the inclusion C6/CD-SH complex in solution, as was discussed in the previous paragraph. Both preparation approaches provided the same results which are summarized in Fig. 2. The SHG intensity from such a prepared C6/CD-SH/Au layer was 85 10% of the SHG intensity from the clean Au surface. This value represents 55% increase in
D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489
comparison with the response of CD-SH/Au layer, suggesting the enhancing role of C6. Such an increase of the enhancement factor of 1.5 seems to be very small in comparison with rhodamine films with the factor of 10 [14], in order to clearly describe our result based on the resonant enhanced surface SHG. However, the rhodamine films [14] and the C6/CD-SH/Au layer are very different. Rhodamine formed a homogeneous film of its own composition. In a case of C6/CD-SH/Au following considerations must be taken in account, three surface components, enhancement towards the Au film or the CD-SH layer itself, and distance of C6 from the relevant surface. Nevertheless, the crucial concern is about a presence of C6 on the CD-SH/ Au surface, what is discussed in the following paragraph. Concluding that a near resonance mechanism via C6 might be considered, but most probably a mechanism of increased surface susceptibility due to the adsorption of C6 is operative in this case. The change of SHG intensity, as shown in Fig. 2, upon exposure of CD-SH/Au with C6, clearly demonstrates that the C6 adsorbs on the CD-SH/ Au layer. An attempt, to confirm this observation by using C6 fluorescence signal from the C6/CDSH/Au layer, was negative. This result was in accordance with previous studies reporting on a quenching of fluorescence by Au atoms [13]. However, the presence of C6 in the CD-SH/Au layer was confirmed by an experiment described as a laser-induced thermal desorption from the surface and following fluorescence response in liquid phase. This experiment was designed for this purpose, where a focused laser light induced the desorption of C6 from the C6/CD-SH/Au layer into the aqueous solution in which the sample was placed. The laser photon energy was presumably transferred into heat and the C6, which is physisorbed in the CD-SH cavity, was then thermally desorbed from the C6/CD-SH/Au layer. The presence of C6 in water was then determined by fluorescence signal of 505 nm. The fluorescence intensity is shown in Fig. 3 as a function of a desorption time. Note that the intensity was very low, while the noise level was approximately 300 photon/s. Fig. 3 shows the time evolution from an induction period till the desorption–adsorption
487
Fig. 3. The laser-induced thermal desorption of C6 from C6/ CD-SH/Au and C6/Au systems. Fluorescence intensity of C6 as a function of desorption time.
equilibrium. Since our focus was only on a qualitative description of the process, a quantification, based on laser power, C6 concentration, and CDSH coverage, was not attempted. The C6 molecules presumably bind not only within the CD-SH on Au, as a case of specific binding, but also directly on the Au surface, as unspecific binding. Therefore, the desorption of C6 from the C6/Au film was also performed for a comparison and shown in Fig. 3. Note that the C6/Au film was prepared by C6 exposure of clean Au surface in the same approaches as the C6/CD-SH/Au layer was made, and was also intensively rinsed with water, as well as the same laser power was used for both desorptions. As shown in Fig. 3, the fluorescence intensity of C6 from C6/Au is almost constant and the equilibrium value is approximately 50% of the value from C6/CD-SH/Au. A similar result was observed by Weisser et al. [10] when the guest molecule was ferrocenecarboxylic acid. Comparing the shapes of the desorption curves in Fig. 3 suggests that the C6 is bond weaker on Au than on CD-SH/Au, based on the minimal induction period. Note that the used laser powers were assumed to be above both thresholds. Taking into an account the relative hydrophobicity of the Au surface in comparison with the CD-SH outer shell, the inclusion of C6 in the CD-SH/Au layer seems to only satisfy the stronger binding. Comparing the intensities of the desorption curves in Fig. 3 suggests that there are twice more C6 molecules in the CD-SH/Au layer than on the Au film. It is
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Scheme 2. The expected structure of supramolecular C6/CD-SH/Au layer. The CD-SH ÔbucketsÕ are attached on Au film and the C6 guests are either placed into the CD-SH hosts, or physisorbed on Au.
unlikely that the CD-SH/Au layer with the outer hydrophilic shell would accommodate more unspecific bond C6 than the clean Au. Therefore, there are approximately 50% of C6 molecules which are forced by the hydrophilicity of the outer CD-SH shell from possible hydrophobic sites on Au into the hydrophobic CD-SH ÔbucketsÕ on Au. Such a formation of the C6/CD-SH inclusion complex, as an indirect proof of the C6/CD-SH/ Au formation, was determined by using the timeresolved fluorescence anisotropy. Concluding that approximately 50% of C6 molecules bind inclusively into the CD-SH ÔbucketsÕ on Au and together form the supramolecular host–guest surface layer. The proposed structure of the C6/CD-SH/ Au layer is illustrated in Scheme 2.
4. Conclusion The supramolecular self-assembled inclusion complex, consisting of the C6 guest and the CDSH host on the Au surface was investigated. The rotational correlation times of C6 and C6/CD-SH systems were measured to be 0.2 and 1.4 ns, respectively. Comparing these anisotropy dynamics of C6 and C6/CD-SH suggests that the coumarin-6 forms the inclusion complex with the thiolated bcyclodextrin. SHG was used to monitor the formation of the C6/CD-SH/Au surface layer. The 55% enhancement of the surface SHG due to the C6 adsorption is a clear evidence for the C6
presence in the CD-SH/Au layer. To confirm the supramolecular inclusion on the surface, a novel experimental method was proposed, where C6 molecules were thermally laser-induced desorbed from the surface. This method of laser-induced thermal desorption and fluorescence detection compared the binding energies and surface populations of C6 on Au and on CD-SH/Au. The result suggests that at least 50% of C6 adsorbed molecules bind inclusively into the CD-SH ÔbucketsÕ on Au and together form the supramolecular C6/CDSH/Au surface layer. Acknowledgements DV gratefully acknowledges the financial support by FWF through Lise Meitner Fellowship (contract number M525-CHE) and by VEGA (contract number 1/7245/20). References [1] K.A. Connors, Chem. Rev. 97 (1997) 1325. [2] G. Grabner, K. Rechthaler, B. Mayer, G. Koehler, K. Rotkiewicz, J. Chem. Phys. A 104 (2000) 1365. [3] G. Li, L.B. McGown, Science 264 (1994) 249. [4] D. Velic, M. Knapp, G. Koehler, J. Mol. Struct. 598 (2001) 49. [5] A. Ulman, An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-assembly, Academic Press, Boston, 1991. [6] R.G. Nuzz, D.L. Allara, J. Am. Chem. Soc. 105 (1983) 4481.
D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489 [7] M. Buck, F. Eisert, J. Fischer, M. Grunze, F. Traeger, Appl. Phys. A 53 (1991) 551. [8] M.T. Rojas, R. Koeniger, J.F. Stoddart, A.E. Kaifer, J. Am. Chem. Soc. 117 (1995) 336. [9] Y. Maeda, H. Kitano, J. Phys. Chem. 99 (1995) 487. [10] M. Weisser, G. Nelles, G. Wenz, S. Mittler-Neher, Sensors and Actuators B 38-39 (1997) 58. [11] W. Hill, V. Fallourd, D. Klockow, J. Phys. Chem. B 103 (1999) 4707.
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[12] H. Schoenherr, M.J. Beulen, J. Buegler, J. Huskens, F.C.J.M. van Veggel, D.N. Reinhoudt, G.J. Vancso, J. Am. Chem. Soc. 122 (2000) 4963. [13] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 1999. [14] T.F. Heinz, C.K. Chen, D. Ricard, Y.R. Shen, Phys. Rev. Lett. 48 (1982) 478. [15] K.B. Eisenthal, Chem. Rev. 96 (1996) 1343.
Supramolecular surface layer: coumarin/thiolated cyclodextrin/gold D. Velic
*,1,
G. K€ ohler
Institute of Theoretical Chemistry and Structural Biology, University of Vienna, Dr. Bohrgasse 7/b, Vienna 1030, Austria Received 30 August 2002; in final form 31 January 2003
Abstract A formation of supramolecular host–guest inclusion surface layer is reported. A nanostructure of this layer consists of an inclusion complex, where a guest molecule – coumarin-6 (3-(2-benzothiazolyl)-7-(diethylamino) coumarin, C20 H18 N2 O2 S) is placed into a ÔbucketÕ of a host molecule – thiolated cyclodextrin (6-monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S) which is attached on Au surface. The inclusion and the layer formation are determined by using time-resolved fluorescence anisotropy, second-harmonic generation (SHG), and fluorescence in combination with laserinduced thermal desorption from the surface. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction Self-assembling phenomena have been attracting considerable attention in fields such molecular biochemistry or nanofabrication. Two specific cases of the phenomena were considered in this Letter, a host–guest inclusion complex [1–4] and a self-assembled monolayer [4–7]. The inclusion host was proposed to be cyclodextrin molecule. Cyclodextrins are cyclic oligosaccharides which can generally serve as a hydrophilic cover for the hydrophobic guest, and additionally provide mechanical, optical, and chemical protection. The
*
Corresponding author. Fax: +421-2-6542-3244. E-mail address: [email protected] (D. Velic). 1 Permanent address: Komensky University, Mlynska dolina CH-1, Bratislava 842 15, Slovakia.
inclusions of various molecules into cyclodextrins have been extensively demonstrated in liquid phase [1–4]. Our focus was on using the host–guest complex and on extending its utilization into a formation of the supramolecular self-assembled layer on a surface, similar to the self-assembled monolayer. The initial step in this direction was an attachment of the cyclodextrin host on the surface [4,8–12]. In this Letter, we used the thiolated bcyclodextrin (CD-SH), where OH group on the primary chain had been replaced with SH group. The structure of such a ÔbucketÕ is illustrated in Scheme 1. The CD-SH was then chemisorbed on the Au surface through the S–Au bond in the same way as alkanethiols in self-assembled monolayers on Au. However, due to the CD-SH ÔbucketÕ composition and geometry, the formation of only self-assembled monolayer-like film might be expected [4]. The inclusion of suitable guest
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00300-2
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Raman spectroscopy [9,11], plasmon surface polariton spectroscopy [10], cyclic voltammetry [8,10], contact-angle measurements [10], and atomic force microscopy [12]. The methods used in our experiment were a time-resolved fluorescence anisotropy [13], second-harmonic generation (SHG), and the fluorescence in combination with laser-induced thermal desorption from the surface. The time-resolved fluorescence anisotropy was suited for the purpose to conclusively observe the formation of the supramolecular inclusion structure with CD-SH. SHG signal is proportional to the nonlinear susceptibilities of surface atoms, therefore a modification of the surface results in a change of the SHG intensity, where a resonantly enhanced surface SHG was considered [14,15]. Due to a quenching effect of the Au surface [13], a direct fluorescence was not possible to use. Therefore we utilized a laser-induced thermal desorption from the surface, with following fluorescence, to our knowledge, a novel approach in a liquid phase, although this technique is well established in vacuum surface science.
2. Experimental
Scheme 1. The CD-SH molecule used in these experiments is 6monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S, consisting of seven a-1,4-glycosidically linked glucopyranose units with one SH group directly on the primary chain and forming a ÔbucketÕ structure.
molecules into the layer of CD-SH ÔbucketsÕ might then form a supramolecular surface layer, with a potential to tailor its functional response. As a model guest molecule we have chosen coumarin-6 (C6) due to its absorption and fluorescence in visible range for fluorescence detection, suitable resonant transition, and expected inclusion complexation with CD-SH. Only few experimental preparations have been so far attempted [8–12]. The layers were characterized by surface-enhanced
Gold film was prepared by using a thermal evaporation of gold on the mica (muscovite) substrate under vacuum of 10 7 Torr. The sample of Au surface was cooled under the vacuum for approximately 30 min and then placed in the quartz cell with a reagent solution. The CD-SH, 6-monodeoxy-6-monothio-b-cyclodextrin, C42 H70 O34 S, used in these experiments, was produced by Cyclolab. The C6, 3-(2-benzothiazolyl)-7-(diethylamino) coumarin, C20 H18 N2 O2 S, was produced by Radiant Dyes Chemie. Both used solvents, ethanol and water, had high-grade purity. All experiments were performed at room temperature. Femtosecond Ti–sapphire oscillator (Coherent, 100 fs, 76 MHz, 1.3 W, 860 nm) pumped with Argon ion laser was used to excite the system. For the SHG measurements, the sample surface and the laser beam were in 45° geometry. Two filters were employed in front and behind (with respect to laser beam direction) of the sample to filter the SHG response wavelength of 430 nm and the ex-
D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489
485
citation wavelength of 860 nm, respectively. Noise of the SHG signal was proportional to square of the laser fluctuation of 3% which was approximately 9%. Note that no polarizers were used in SHG measurements and laser light was horizontally polarized (p-polarized). The SHG signal of 430 nm was collected through a monochromator with a photomultiplier tube connected to singlephoton counting setup. For the fluorescence measurements, the reagent solution was placed in the quartz cell and the geometry between the laser beam and the fluorescence collection direction was 90°. The excitation wavelength of 430 nm (vertically polarized) was produced by SHG in BBO crystal from the Ti–sapphire fundamental wavelength of 860 nm. The fluorescence signal of 505 nm was collected in the same way as SHG. The same setup, except the laser beam was focused with 50 mm lens, was used for laser-induced thermal desorption experiments from the surface.
3. Results and discussion Before a preparation of C6/CD-SH/Au layer can be considered, a formation of the supramolecular host–guest inclusion complex C6/CD-SH must be presented. The time-resolved fluorescence anisotropy was used to confirm, whether C6 forms the inclusion complex with CD-SH. The anisotropy was calculated, based on the G-factor, as a measure of the instrument sensitivity, and the measured fluorescence intensities in parallel and crossed polarizations [13]. Based on a rotational correlation time, the inclusion of the guest into the host was determined. Generally, the rotational correlation time of the guest molecule in pure solvent is expected to be shorter in comparison with the rotational correlation time of the guest molecule in the host–guest complex, due to interaction hindrance of the relaxation. The anisotropy dynamics of C6, measured in solution of ethanol (10 5 M), is shown in Fig. 1a. This anisotropy decay was fit by using a single exponential function resulting in a decay rate of 0:2 0:05 ns. The anisotropy dynamics of C6 and CD-SH mixture is shown in Fig. 1b. This mixture was prepared from aqueous CD-SH solution (10 4 M) by adding
Fig. 1. The anisotropy dynamics of C6 (a) and C6/CD-SH (b).
ethanol solution of C6 (10 4 M), so it was 5% of the whole mixture volume. The anisotropy decay was fit by using a single exponential function resulting in a decay rate of 1:4 0:1 ns. Note that the presence of ethanol molecule together with C6 molecule in the CD-SH ÔbucketÕ cannot be excluded. Additionally, the dynamics effects of clustering of C6 molecules in water are being currently investigated. Concluding, the slower anisotropy relaxation of the C6/CD-SH system in comparison with the C6/ethanol system suggests that the C6 molecule forms the supramolecular inclusion complex with the CD-SH molecule in aqueous solution. This result is also supported by the fluorescence of C6 where its maximum shifted 17 nm in wavelength, comparing C6 in water and in cyclodextrin solution which represent hydrophilic and hydrophobic environments, respectively [4].
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The fluorescence shift as a function of cyclodextrin concentration resulted in equilibrium shift from water into cyclodextrin [4]. The observations of the C6/CD-SH complex formation then propose that the formation of such a complex might be also feasible on the Au surface, resulting presumably in a C6/CD-SH/Au surface layer. The first step in a preparation of the C6/CDSH/Au layer was an adsorption of CD-SH ÔbucketsÕ on the Au surface. The detail adsorption kinetics were reported elsewhere [4]. The adsorption process was monitored by using SHG. SHG is the first-order nonlinear process where the frequency of incoming laser light is doubled, i.e., two photons of fundamental frequency are converted into one photon of doubled frequency. In the electric dipole approximation, symmetry requires that SHG can be generated only in noncentrosymmetric materials, what satisfies any surface [14,15]. The SHG response was first measured from the clean Au surface immersed in water and this SHG intensity served as a reference value of the SHG signal (100%). Then water was ejected from the cell and aqueous solution of CD-SH (10 3 M) was injected. After 20 min, the time sufficient to form a layer of CD-SH ÔbucketsÕ on Au surface [4], the solution was ejected and water was injected into the cell. The SHG intensity was 55 15% of the SHG intensity from the clean Au surface as shown in Fig. 2. The SHG from the surface depends on all three susceptibilities of the surface, the adsorbate–surface interaction, and the adsorbate [7]. The decrease in SHG intensity is due to the chemisorption of CD-SH on Au. Therefore, this decrease in SHG signal suggests that the susceptibility of Au–S interaction is smaller than the susceptibility of Au, while assuming no contribution from the rest of the CDSH molecule. Note that a role of the CD-SH surface structure is not clear with respect to how it contributes to a total surface SHG signal. The molecule of C6 was also chosen, because as a suitable adsorbate was proposed to resonantly enhance surface SHG [14]. The enhancement can be expected if the absorption transition of the adsorbate is in the resonance with second-harmonic frequency. The C6 has its first electronic absorption maximum at approximately 440 nm,
Fig. 2. Three stages of the C6/CD-SH/Au formation as functions of SHG intensity.
depending on solvent. Therefore, the resonance condition between this absorption and the SHG wavelength of 430 nm seemed to be satisfied, assuming no significant change upon the inclusion of C6 into CD-SH. The proposed inclusion of C6 into the CD-SH/Au layer was performed in two approaches. The first approach was based on adding the ethanol solution of C6 (10 4 M) into the cell with the CD-SH ÔbucketsÕ layer on Au surface placed in water, so it was 5% of the whole mixture volume. The inclusion process was allowed for 20 min and then this solution was ejected and after rinsing of the sample, water was injected into the cell. The second approach was based on injecting the ethanol solution of C6 (10 4 M) into the cell with the CD-SH ÔbucketsÕ layer on Au and on sequential replacing the ethanol solution of C6 with water in six steps, by ejecting 50% of volume of C6 solution and injecting 50% of water. Every mixture was homogenized for 3 min and at the end, the cell and the whole sample were carefully rinsed with water. The role of these ejection/injection steps was to shift the solubility gradient from ethanol into the CD-SH/Au layer through water. Note that C6 is well soluble in ethanol and almost insoluble in water, and forms the inclusion C6/CD-SH complex in solution, as was discussed in the previous paragraph. Both preparation approaches provided the same results which are summarized in Fig. 2. The SHG intensity from such a prepared C6/CD-SH/Au layer was 85 10% of the SHG intensity from the clean Au surface. This value represents 55% increase in
D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489
comparison with the response of CD-SH/Au layer, suggesting the enhancing role of C6. Such an increase of the enhancement factor of 1.5 seems to be very small in comparison with rhodamine films with the factor of 10 [14], in order to clearly describe our result based on the resonant enhanced surface SHG. However, the rhodamine films [14] and the C6/CD-SH/Au layer are very different. Rhodamine formed a homogeneous film of its own composition. In a case of C6/CD-SH/Au following considerations must be taken in account, three surface components, enhancement towards the Au film or the CD-SH layer itself, and distance of C6 from the relevant surface. Nevertheless, the crucial concern is about a presence of C6 on the CD-SH/ Au surface, what is discussed in the following paragraph. Concluding that a near resonance mechanism via C6 might be considered, but most probably a mechanism of increased surface susceptibility due to the adsorption of C6 is operative in this case. The change of SHG intensity, as shown in Fig. 2, upon exposure of CD-SH/Au with C6, clearly demonstrates that the C6 adsorbs on the CD-SH/ Au layer. An attempt, to confirm this observation by using C6 fluorescence signal from the C6/CDSH/Au layer, was negative. This result was in accordance with previous studies reporting on a quenching of fluorescence by Au atoms [13]. However, the presence of C6 in the CD-SH/Au layer was confirmed by an experiment described as a laser-induced thermal desorption from the surface and following fluorescence response in liquid phase. This experiment was designed for this purpose, where a focused laser light induced the desorption of C6 from the C6/CD-SH/Au layer into the aqueous solution in which the sample was placed. The laser photon energy was presumably transferred into heat and the C6, which is physisorbed in the CD-SH cavity, was then thermally desorbed from the C6/CD-SH/Au layer. The presence of C6 in water was then determined by fluorescence signal of 505 nm. The fluorescence intensity is shown in Fig. 3 as a function of a desorption time. Note that the intensity was very low, while the noise level was approximately 300 photon/s. Fig. 3 shows the time evolution from an induction period till the desorption–adsorption
487
Fig. 3. The laser-induced thermal desorption of C6 from C6/ CD-SH/Au and C6/Au systems. Fluorescence intensity of C6 as a function of desorption time.
equilibrium. Since our focus was only on a qualitative description of the process, a quantification, based on laser power, C6 concentration, and CDSH coverage, was not attempted. The C6 molecules presumably bind not only within the CD-SH on Au, as a case of specific binding, but also directly on the Au surface, as unspecific binding. Therefore, the desorption of C6 from the C6/Au film was also performed for a comparison and shown in Fig. 3. Note that the C6/Au film was prepared by C6 exposure of clean Au surface in the same approaches as the C6/CD-SH/Au layer was made, and was also intensively rinsed with water, as well as the same laser power was used for both desorptions. As shown in Fig. 3, the fluorescence intensity of C6 from C6/Au is almost constant and the equilibrium value is approximately 50% of the value from C6/CD-SH/Au. A similar result was observed by Weisser et al. [10] when the guest molecule was ferrocenecarboxylic acid. Comparing the shapes of the desorption curves in Fig. 3 suggests that the C6 is bond weaker on Au than on CD-SH/Au, based on the minimal induction period. Note that the used laser powers were assumed to be above both thresholds. Taking into an account the relative hydrophobicity of the Au surface in comparison with the CD-SH outer shell, the inclusion of C6 in the CD-SH/Au layer seems to only satisfy the stronger binding. Comparing the intensities of the desorption curves in Fig. 3 suggests that there are twice more C6 molecules in the CD-SH/Au layer than on the Au film. It is
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Scheme 2. The expected structure of supramolecular C6/CD-SH/Au layer. The CD-SH ÔbucketsÕ are attached on Au film and the C6 guests are either placed into the CD-SH hosts, or physisorbed on Au.
unlikely that the CD-SH/Au layer with the outer hydrophilic shell would accommodate more unspecific bond C6 than the clean Au. Therefore, there are approximately 50% of C6 molecules which are forced by the hydrophilicity of the outer CD-SH shell from possible hydrophobic sites on Au into the hydrophobic CD-SH ÔbucketsÕ on Au. Such a formation of the C6/CD-SH inclusion complex, as an indirect proof of the C6/CD-SH/ Au formation, was determined by using the timeresolved fluorescence anisotropy. Concluding that approximately 50% of C6 molecules bind inclusively into the CD-SH ÔbucketsÕ on Au and together form the supramolecular host–guest surface layer. The proposed structure of the C6/CD-SH/ Au layer is illustrated in Scheme 2.
4. Conclusion The supramolecular self-assembled inclusion complex, consisting of the C6 guest and the CDSH host on the Au surface was investigated. The rotational correlation times of C6 and C6/CD-SH systems were measured to be 0.2 and 1.4 ns, respectively. Comparing these anisotropy dynamics of C6 and C6/CD-SH suggests that the coumarin-6 forms the inclusion complex with the thiolated bcyclodextrin. SHG was used to monitor the formation of the C6/CD-SH/Au surface layer. The 55% enhancement of the surface SHG due to the C6 adsorption is a clear evidence for the C6
presence in the CD-SH/Au layer. To confirm the supramolecular inclusion on the surface, a novel experimental method was proposed, where C6 molecules were thermally laser-induced desorbed from the surface. This method of laser-induced thermal desorption and fluorescence detection compared the binding energies and surface populations of C6 on Au and on CD-SH/Au. The result suggests that at least 50% of C6 adsorbed molecules bind inclusively into the CD-SH ÔbucketsÕ on Au and together form the supramolecular C6/CDSH/Au surface layer. Acknowledgements DV gratefully acknowledges the financial support by FWF through Lise Meitner Fellowship (contract number M525-CHE) and by VEGA (contract number 1/7245/20). References [1] K.A. Connors, Chem. Rev. 97 (1997) 1325. [2] G. Grabner, K. Rechthaler, B. Mayer, G. Koehler, K. Rotkiewicz, J. Chem. Phys. A 104 (2000) 1365. [3] G. Li, L.B. McGown, Science 264 (1994) 249. [4] D. Velic, M. Knapp, G. Koehler, J. Mol. Struct. 598 (2001) 49. [5] A. Ulman, An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-assembly, Academic Press, Boston, 1991. [6] R.G. Nuzz, D.L. Allara, J. Am. Chem. Soc. 105 (1983) 4481.
D. Velic, G. K€ ohler / Chemical Physics Letters 371 (2003) 483–489 [7] M. Buck, F. Eisert, J. Fischer, M. Grunze, F. Traeger, Appl. Phys. A 53 (1991) 551. [8] M.T. Rojas, R. Koeniger, J.F. Stoddart, A.E. Kaifer, J. Am. Chem. Soc. 117 (1995) 336. [9] Y. Maeda, H. Kitano, J. Phys. Chem. 99 (1995) 487. [10] M. Weisser, G. Nelles, G. Wenz, S. Mittler-Neher, Sensors and Actuators B 38-39 (1997) 58. [11] W. Hill, V. Fallourd, D. Klockow, J. Phys. Chem. B 103 (1999) 4707.
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[12] H. Schoenherr, M.J. Beulen, J. Buegler, J. Huskens, F.C.J.M. van Veggel, D.N. Reinhoudt, G.J. Vancso, J. Am. Chem. Soc. 122 (2000) 4963. [13] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 1999. [14] T.F. Heinz, C.K. Chen, D. Ricard, Y.R. Shen, Phys. Rev. Lett. 48 (1982) 478. [15] K.B. Eisenthal, Chem. Rev. 96 (1996) 1343.