pubs.acs.org/Langmuir © 2009 American Chemical Society
Determination of the Surface Coverage of Adsorbed Dextran and β-Cyclodextrin Derivatives on Gold by Surface Titration )
Stephanie Hornig,†,‡ Tim Liebert,† Alan R. Esker,‡ Sarah L. Stoll,§ Julie Mertzman,§ Wolfgang G. Glasser, and Thomas Heinze*,† †
)
Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Jena 07743, Germany, ‡ Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, § Department of Chemistry, Georgetown University, Washington, D.C. 20057, and Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received February 5, 2009. Revised Manuscript Received March 28, 2009 The self-assembly of thiophene-containing dextran and cyclodextrin derivatives on gold surfaces was investigated. Morphological studies (AFM) and the elemental characterization (XPS) of the surfaces show that the carbohydrate derivatives form either aggregates or uniform films depending on the structure and the solvent used. The real coverage of the surface, and hence the amount of unmodified free gold, was examined by a “titration” of the surface with a carboxyl-terminated SAM (11-mercaptoundecanoic acid, MUA) and with Mn-12, a manganese oxocluster. Each carboxyl group reacts with one acetate ligand of the manganese cluster, with each Mn-12 cluster capable of binding multiple MUAs, leading to defined manganese-functionalized surfaces. The weight percentage of manganese and consequently the coverage area of the carboxyl-terminated SAM is examined by XPS.
The functionalization of surfaces is of considerable interest in applications such as sensors and biocompatible surfaces. In particular, poly- and oligosaccharides such as dextran and cyclodextrin are promising coating materials in the biomedical field. Dextran is a polyglucan composed of R-(1f6)-linked D-glucose units and may be conveniently used as a coating to prevent nonspecific protein adsorption.1 β-Cyclodextrin is a cyclic oligosaccharide of seven R-(1f4)-linked D-glucose units that are able to form inclusion complexes with various compounds. The immobilization of these molecules on a surface is feasible with high selectivity.2-5 Derivatization of the carbohydrates with sulfur-containing functional groups facilitates self-assembly onto gold surfaces. Several spectroscopic and microscopic techniques give information about the composition and morphology of such modified surfaces. The main objective of this letter is to demonstrate a simple strategy to determine the surface coverage of adsorbed substances at gold surfaces and hence the residual amount of unmodified free gold. Common gold surfaces are typically polycrystalline, making AFM imaging unreliable for probing the uniformity of the adsorbed films. Issues of adsorbed contaminants and X-ray beam damage make it difficult to obtain uniformity from X-ray photoelectron spectroscopy (XPS) with carbohydrate self-assembled monolayers (SAMs) that are only subtly different in terms of chemical composition. The current approach circumvents the complications of the characterization of functionalized gold *Corresponding author. E-mail:
[email protected]. (1) Lemarchand, C.; Gref, R.; Couvreur, P. Eur. J. Pharm. Biopharm. 2004, 58, 327–341. :: (2) Beulen, M. W. J.; Bugler, J.; deJong, M. R.; Lammerink, B.; Huskens, J.; :: :: Schonherr, H.; Vancso, G. J.; Boukamp, B. A.; Wieder, H.; Offenhauser, A.; Knoll, W.; vanVeggel, F. C. J. M.; Reinhoudt, D. N. Chem.-Eur. J. 2000, 6 1176–1183. (3) Samitsu, S.; Shimomura, T.; Ito, K.; Hara, H. Appl. Phys. Lett. 2004, 85, 3875–3877. (4) Rojas, M. T.; Koniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336–343. (5) Maeda, Y.; Fukuda, T.; Yamamoto, H.; Kitano, H. Langmuir 1997, 13, 4187–4189.
Langmuir 2009, 25(9), 4845–4847
surfaces with respect to the covered area. The uniformity of the surface and therefore the amount of unmodified free gold will be examined by a “titration” of the surface with a carboxylterminated SAM (11-mercaptoundecanoic acid, MUA) and with Mn-12, a manganese oxocluster (Scheme 1). The selective chemisorption of the Mn-12 cluster is based on a ligand exchange with carboxyl-terminated SAMs.6,7 Manganese can be used as a distinctive element in the XPS spectrum. The weight percentage of manganese correlates quantitatively with the number of carboxyl groups at the surface. Thus, the coverage area of carboxyl-terminated SAMs can be examined by XPS. The application of this procedure will be tested with weakly adsorbed thiophene-containing carbohydrates. The manganese oxocluster, [Mn12O12(O2CMe)16(H2O)4] 3 2(HO2CMe) 3 4(H2O) (Mn-12), forms monolayer films on potentially any surface covered with carboxyl groups. A well-ordered carboxyl-terminated SAM can be achieved by the adsorption of 11-mercaptoundecanoic acid (MUA).8 According to control experiments, there is no specific adsorption of Mn-12 onto alkyl-terminated SAMs. Using binary mixtures of ethanolic MUA and 1-dodecanethiol (MUCH) solutions, the adsorption behavior of Mn-12 from 10 μM ethanolic solutions on partially carboxyl-covered surfaces is investigated. The elemental composition of the functionalized gold surfaces is examined by XPS. The weight percentage on manganese at the modified surface shows a linear dependence on the molar ratio of MUA/MUCH (Figure 1). The linear increase in the manganese value with increasing MUA leads to the assumption that each carboxyl group reacts with one acetate ligand of the manganese cluster. This assumption is supported by the fact that the O/Mn atomic ratio is 4 within (6) Steckel, J. S.; Persky, N. S.; Martinez, C. R.; Barnes, C. L.; Fry, E. A.; Kulkarni, J.; Burgess, J. D.; Pacheco, R. B.; Stoll, S. L. Nano Lett. 2004, 4 399–402. (7) Clement-Leon, M.; Coronado, E.; Soriano-Portillo, A.; Mingotaud, C.; Dominguez-Vera, J. M. Adv. Colloid Interface Sci. 2005, 116, 1–3. (8) Ito, E.; Konno, K.; Noh, J.; Kanai, K.; Ouchi, Y.; Seki, K.; Hara, M. Appl. Surf. Sci. 2005, 244, 284–287.
Published on Web 4/6/2009
DOI: 10.1021/la900454g
4845
Letter Scheme 1. Adsorption of 11-Mercaptoundecanoic Acid (MUA) and the Manganese Oxocluster [Mn12O12(O2CMe)16(H2O)4] 3 2(HO2CMe) 3 4 (H2O), (Mn-12), onto Carbohydrate-Functionalized Gold Surfacesa
a
One Mn-12 is assumed to bind two to four MUAs.6
Figure 1. Weight percentage of manganese depending on the molar ratio of adsorbed 11-mercaptoundecanoic acid (MUA) to MUCH as determined by XPS.
experimental error for all compositions. If the Mn-12 cluster were adsorbing onto the MUA surface without ligand exchange, then a higher O/Mn ratio would be observed. This conclusion does not mean that the binding is 1:1 but rather that one Mn-12 cluster is capable of binding multiple MUA molecules. In fact, a previous quartz crystal microbalance study6 has shown that the likely binding ratio is ∼4 MUA per Mn-12 cluster. Furthermore, the previous study used ellipsometry to show that the MUA plus the adsorbed Mn-12 cluster was only ∼2.8 nm thick. Hence, the XPS X-ray beam can see the entire film and part of the underlying gold layer. For the 100% MUA film, it is possible to recalculate the weight percentage of Mn without the gold (∼20%). For the case where four MUAs exchange with acetate ligands on the Mn-12 cluster, the theoretical value would be (∼28 wt % Mn). In contrast to prior studies showing a preferred adsorption of MUA from solution,9 the linear function demonstrates the uniform distribution of the methyl- and carboxyl-terminated SAMs after adsorption at the surface. Moreover, the gold and oxygen weight percentages follow a linear decrease and increase, respectively, with increasing molar fraction of MUA. Similar results were obtained elsewhere showing a linear dependence of the water contact angles on the surface mole fraction of MUA.10 The calibration function is used for the determination of the surface coverage of dextran and β-cyclodextrin-functionalized gold surfaces. The formation of uniform monolayers of thiolated dextrans and cyclodextrins is well investigated and used in the field of surface functionalization (e.g., for molecular recognition and surface plasmon resonance).11,12 In our (9) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6561–6562. (10) Mendez, S.; Ista, L. K.; Lopez, G. P. Langmuir 2003, 19, 8115–8116. (11) Banerjee, I. A.; Yu, L.; Matsui, H. J. Am. Chem. Soc. 2003, 125, 9542–9543. (12) Nelles, G.; Weisser, M.; Back, R.; Wohlfart, P.; Wenz, G.; Mittler-Neher, S. J. Am. Chem. Soc. 1996, 118, 5039–5046.
4846
DOI: 10.1021/la900454g
present work, we want to examine the adsorption behavior of thiophene-functionalized dextran and cyclodextrin derivatives because thiophene is also known to form well-ordered monolayers.13 One effective synthesis strategy for the defined introduction of various substituents onto anhydroglucose units (AGU) is the esterification of carboxylic acids via in situ activation with N,N 0 -carbonyldiimidazole.14,15 In this way, derivatives containing thiophene groups with different spacer lengths attached to the AGU can be obtained.16 The degree of substitution (DS) of the samples examined in the present study ranges from 0.87 to 1.66 thiophene groups per AGU. Previously cleaned gold slides were stored for 2 days in dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF) solutions of the thiophene derivatives. The coverage of the surface with the derivatives is examined by AFM and XPS measurements after washing procedures. The reduction of the Au amount by about 20 wt%, the increase in the O value, and the occurrence of the S 2p peak at 162 eV provide evidence of the modification of the surface. AFM studies show an immobilization of the derivatives as aggregates or smooth films (Supporting Information). We observed that the tendency to form aggregates on the surface is higher in DMSO solutions, which might be due to the coadsorption of DMSO itself.17 The carbohydrate-functionalized gold surfaces are treated with 1 mM ethanolic MUA solution for 1 day for the binding of carboxyl moieties on free gold spaces. Whereas the carbohydrate derivatives are bound to the gold surface through multiple weak interactions and ethanol can be regarded as a nonsolvent, it is not possible to completely exclude partial ligand exchange between the polymer and the MUA. If partial ligand exchange is occurring, it would cause the percent coverage values for the thiophene derivatives to be smaller than their true values. After subsequent treatment of the surface with a 10 μM ethanolic Mn-12 solution for 2 h, the manganese content can be determined by XPS (Table 1). The selective chemisorption of the manganese cluster was proven by the treatment of the following substrates with Mn-12: 11-mercapto-1-undecanol, regenerated cellulose, and carbohydrate samples 1-3 before MUA treatment. Only manganese values of up to 1.3 wt % could be observed for any of these surfaces, which is comparable to the intercept in Figure 1. The intercept in Figure 1 is consistent with the noise in the XPS measurements for manganese where the background noise is 1.6 wt %. (13) Sako, E. O.; Kondoh, H.; Nakai, I.; Nambu, A.; Nakamura, T.; Ohta, T. Chem. Phys. Lett. 2005, 413, 267–271. (14) Heinze, T.; Liebert, T.; Heublein, B.; Hornig, S. Adv. Polym. Sci. 2006, 205, 199–291. (15) Liebert, T.; Heinze, T. Biomacromolecules 2005, 6, 333–340. (16) Hornig, S.; Liebert, T.; Heinze, T. Polym. Bull. 2007, 59, 65–71. :: (17) Schroter, C.; Roelfs, B.; Solomun, T. Surf. Sci. 1997, 380, L441–L445.
Langmuir 2009, 25(9), 4845–4847
Letter Table 1. Degree of Substitution (DS) of the Adsorbed Oligo- and Polysaccharides, Weight Percentage of Manganese after MUA/ Mn-12 Adsorption, and Calculated Surface Coverage (X) of the Carbohydrates no.
compound
DS wt % Mnc %X
6.5 35.3 1 thiophene-2-carboxylic acid cyclodextrin ester 0.87a 7.4 23.4 2 thiophene-2-acetic acid dextran ester 1.14b 7.7 19.5 3 thiophene-2-butyric acid cyclodextrin ester 1.66a a Determined by elemental analysis. b Determined by means of 1H NMR after perpropionylation. c Determined by XPS.
By using this selective patterning of carboxyl SAMs with the Mn-12 cluster, the calculation of the molar ratio of MUA and consequently the determination of the amount of preliminarily adsorbed material are realizable. Table 1 shows the results of the carbohydrate-functionalized gold surfaces in DMF solutions after Mn-12 titration. The coverage ratios vary from 19.5 to 35.3%. Although the microscopic and spectroscopic experiments lead to the conclusion that the surface is covered with a uniform layer, the dextran and cyclodextrin derivatives are loosely packed and cover only about one-third to one-fifth of the gold surface. A comparison of the carbohydrates leads to the assumption that an increase in the DS does not consequently yield a higher degree of coverage. Furthermore, the covered area is influenced by the chain length between AGU and thiophene. The degree of coverage is lower with an alkyl spacer consisting of three methylene groups (sample 3) than with the directly bound thiophene (sample 1). The low coverage of the gold surfaces with the carbohydrate derivatives is advantageous for multiple functionalization (e.g., with single-molecule magnets, which are an interesting approach to the development of nanoscale data storage).
Langmuir 2009, 25(9), 4845–4847
In summary, the adsorption behavior of thiophene-containing dextran and cyclodextrin derivatives was investigated. A method for the determination of the surface coverage of the functionalized gold surfaces was developed. The procedure is based on the adsorption of MUA and 1-dodecanethiol to form uniform SAMs with compositions that match that of the bulk solution. The quantitative titration of these model surfaces provides a calibration curve for titrating other surfaces. The gold surfaces functionalized with carbohydrate derivatives were treated with MUA and Mn-12. Subsequent XPS measurements reveal a carbohydrate surface coverage of 20-35%. Moreover, this strategy might be a general tool for the characterization of functionalized gold surfaces. In addition, the multifunctionalized surfaces present an interesting combination of single-molecule magnets with biopolymers and complexing compounds (e.g., for magnetic resonance imaging contrast agents).18 Acknowledgment. We thank Mr. Steven McCartney for his training and assistance with AFM. This research was supported by National Research Initiative Competitive Grant 2005-35504-16088 from the USDA Cooperative State Research, Education, and Extension Service. Supporting Information Available: Detailed synthesis procedure and product characterization of the dextran and cyclodextrin derivatives, experimental procedure for the functionalization of gold surfaces, XPS results, and AFM studies. This material is available free of charge via the Internet at http://pubs.acs.org. (18) Mertzman, J. E.; Kar, S.; Lofland, S.; Fleming, T.; Van Keuren, E.; Tonga, Y. Y.; Stoll, S. L. Chem. Commun. 2009, 788–790.
DOI: 10.1021/la900454g
4847
