ine monophosphate (AMP), n = 1, and adenosine triphosphate (ATP), n = 3) from aqueous solutions on the surface of multiwall CNTs. For adsorbate concen.
ISSN 1061933X, Colloid Journal, 2011, Vol. 73, No. 2, pp. 244–247. © Pleiades Publishing, Ltd., 2011. Original Russian Text © M.V. Manilo, T.A. Alekseeva, I.A. Ar’ev, N.I. Lebkova, 2011, published in Kolloidnyi Zhurnal, 2011, Vol. 73, No. 2, pp. 235–238.
Adenosine, Adenosine Monophosphate, and Adenosine Triphosphate Adsorption from Aqueous Solutions on the Surface of Multiwall Carbon Nanotubes M. V. Maniloa, T. A. Alekseevab, I. A. Ar’eva, and N. I. Lebovkaa a
Ovcharenko Institute of Biocolloid Chemistry, National Academy of Sciences of Ukraine, pr. Vernadskogo 42, Kiev, 03044 Ukraine b Kurdyumov Institute of Metal Physics, National Academy of Sciences of Ukraine, pr. Vernadskogo 36, Kiev, 03044 Ukraine Received April 2, 2010
Abstract—Adenosine, adenosine monophosphate, and adenosine triphosphate adsorption from aqueous solutions on the surface of carbon nanotubes is studied. Adsorption isotherms are plotted and adsorption free energies, as well as areas per molecules of the adsorbates, are calculated at their concentrations in solutions of 0–10–3 mol/dm3. It is shown that the monomolecular adsorption is characterized by rather loose packing of adsorbate molecules on the nanotube surface, with the packing density increasing in the presence and upon a rise in the concentration of phosphate groups in adsorbate molecules. In the range of the polymolecular adsorption, at high adsorbate concentrations in solutions (C > (5–6) × 10–4 mol/dm3), the adsorption decreases in a series adenosine > adenosine monophosphate > adenosine triphosphate, i.e., with an increase in the solubility of the examined compounds in water. DOI: 10.1134/S1061933X11010108
tration range C = 0–10–3 mol/dm3, adsorption iso therms were plotted and discussed in this work.
INTRODUCTION Carbon nanotubes (CNTs) possess a number of unique properties; they are successfully applied as fill ers for improving the mechanical, electric, thermo physical, and optical properties of composite materials [1]. At present, CNTs are intensively used in the devel opment of biosensor devices and materials designed for pharmaceutics and diagnostics [2–4]. The func tionalization of CNT surfaces with molecules playing important roles in biological processes, i.e., biologi cally important molecules (BIMs), including protein forming peptides, nucleic acids, etc., makes it possible to produce new systems capable of identifying biolog ical objects [5]. A CNT surface can serve as a platform for the targeted transport of different BIMs, including drugs (antibiotics or protein and carbohydrate substi tutes). The efficiency of this transport is governed by the unique adsorption ability of CNTs [7] and the spe cific interaction of BIMs with their surface [2]. CNTs are poorly soluble in aqueous systems; however, their content in water can be enhanced by adding solubiliz ing surfactants [6]. Phosphate groups contained in BIMs can strongly affect their solubilizing properties [8]. The goal of this work was to investigate the adsorp tion of a number of BIMs with different numbers of phosphate groups n (adenosine (AN), n = 0; adenos ine monophosphate (AMP), n = 1, and adenosine triphosphate (ATP), n = 3) from aqueous solutions on the surface of multiwall CNTs. For adsorbate concen
EXPERIMENTAL AN, AMP, and ATP produced by SigmaAldrich Co. (the United States) were applied in the experi ments. Table 1 lists the structural formulas, molecular masses M, solubilities in water g, and hydrophobicity indices logP for these compounds and the same parameters for adenine molecule, which constitutes the basic element in the structure of the molecules under investigation. The hydrophobicity of these com pounds is due to the presence of the adenine fragment, and it grows with a reduction in the number n of phos phate substituents. Multiwall CNTs (Spetsmash, Ukraine) had a spe cific surface area of 190 m2/g, a diameter of 10– 30 nm, a length of 1–10 µm, and a content of mineral impurities of less than 1% [9]. AN, AMP, and ATP were adsorbed from an aqueous physiological solution (infusion solution with NaCl concentration of 0.9 wt %, NovofarmBiosintez, Ukraine). Initial sam ples of adsorbate solutions were prepared by the gravi metric method as follows: initially, the most concen trated sample was prepared and all subsequent samples were obtained by its dilution. The adsorption was per formed in dark using 250ml flasks; the solutions were continuously stirred on a shaking table at 293 K. The BIM concentration was measured spectropho tometrically with a Specord UV Vis spectrophotome
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Table 1. Structural formulas, molecular masses M, solubility in water g (at 298 K), and hydrophobicity parameters logP calculated from the data on substance distribution between octane and water [11, 13] for adenine and the examined BIMs Substance
Molecular structure
M, g/mol
g, mg/ml
logP
135.1
1.03
–0.3
267.2
8.23
–1.6
347.2
high solubility
–3.1
507.2
≈1000 complete miscibility
–5.5
H2N N N
Adenine C5H5N5
N
N H
H2N N N AN C10H13N5O4
H O
O
N
N
OH OH
H2N N O H O P O O−
AMP C10H14N5O7P
N O
N
N
OH OH
H2N
ATP C10H16N5O13P3
−
O
O O O P O P O P O O− O− O−
N
N O
N
N
OH OH
ter (Germany). Spectra were registered in the wave number range ν = 28 × 103–40 × 103 cm–1 (wavelength λ = 250–357 nm). The measurements were carried out in semidemountable quartz cells; the optical path was fitted in a manner such that, at the absorption maximum (ν = 38.6 × 103 cm–1, which corresponds to λ = 260 nm), the optical density was in the range of 0.1–1. The temperature in the cells, T = 293 K, was stabilized with an accuracy of 0.1 K using a thermo stat. The initial region of a adsorption isotherms (in the range of the monolayer filling) was approximated by the Langmuir equation a/am = kC/(1 + kC), where am is the adsorption value that corresponds to the mono layer formation, k = exp(–G/RT) is the equilibrium constant, G is the adsorption free energy, R is the gas constant, and T is the absolute temperature. Note that, for AN, the initial region (before the monolayer is filled) of the a–C graphical dependence COLLOID JOURNAL
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represents a convex curve, and the data on the adsorp tion can be satisfactorily approximated by the Lang muir isotherm. However, for AMP and ATP, the pat tern of the a–C dependence in the initial region is dif ferent, which seems to reflect different mechanisms of the adsorption of AN molecules and molecules of AMP and ATP at submonolayer degrees of surface coverage. Therefore, equilibrium constant k was approximately estimated from the slope of the initial region of an sorption isotherm below C ≈ 2 × 10⎯4 mol/dm3, while the am value was assessed from the position of the horizontal region, which corresponds to the monolayer surface coverage. The surface area per a molecule was found from the relation s = A/amNA, where A is the specific surface area of nan otubes occupied by an adsorbate and NA is Avogadro’s number. For each substance, the free energy of adsorption in the initial regions of the isotherms was calculated for
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a/a∞
a × 104, mol/g 16
AN AMP ATP
1.0
AN AMP ATP
14 0.8
12 10
0.6
8 0.4
6 4
0.2
2 0 0
30
60
90
120 t, min
Fig. 1. Normalized adsorption values a/a∞ of AN, AMP, and ATP on CNT surface as functions of time t (a∞ is the adsorption value at t → ∞). All dependences are obtained at an initial solution concentration С of approximately 2.3 × 10–4 mol/dm3.
five points. The correlation coefficients turned out to be no less than 0.98. Reliably determined minimum concentrations were no higher than 2 × 10–5 g/dm3. RESULTS AND DISCUSSION Typical time dependences of the normalized values a/a∞ for AN, AMP, and ATP adsorption on CNTs are presented in Fig. 1. Here, a∞ corresponds to the adsorption value at t → ∞, and all dependences are obtained for initial solution concentrations C of approximately 2.3 × 10–4 mol/dm3. Similar a(t)/a∞ dependences were observed for other С values. As a rule, adsorption equilibrium was established within 1 h. Note that, during 2 h, the equilibrium of the reversible hydrolysis of AMP and ATP remained nonshifted, thus indicating a relative stability of the examined BIMs in aqueous solutions. Figure 2 illustrates the isotherms of AN, ATP, and AMP adsorption measured at room temperature. The Table 2. Surface areas s per a molecule and standard ad sorption free energies G for AN, AMP, and ATP molecules on CNT surface Adsor bate AN AMP ATP
s, nm2 1.80 ± 0.17 1.50 ± 0.12 1.71 ± 0.15
k × 105 4.13 ± 0.21 1.71 ± 0.18 1.65 ± 0.2
–G, kJ/mol 31.5 ± 0.1 29.4 ± 0.2 29.3 ± 0.1
0 0
2
4
6
8 10 12 с × 104, mol/dm3
Fig. 2. Isotherms of AN, AMP, and ATP adsorption from aqueous solutions on CNT surface.
initial regions of the isotherms correspond to the pro cess of the formation of adsorbate monolayers on the CNT surface. When equilibrium concentration C is increased to the value that corresponds to the com plete CNT surface coverage with the monolayer of a substance, a horizontal region is observed. As the adsorbate concentration in a solution is further increased, all three substances exhibit the behavior typical of polymolecular adsorption [10] (Fig. 2). In the case of AN, the onset of the monolayer for mation is observed at lower concentrations than for AMP and ATP. Polymolecular layers of AN, ATP and AMP are formed at С > 2.5 × 10–4, 3.5 × 10–4, and 4.5 × 10–4 mol/dm3. The longest horizontal region of the monomolecular adsorption is observed for AMP (n = 1), thus indicating the most complete coverage of the CNT surface with the adsorbate monolayer; for AN (n = 0) and ATP (n = 3) shorter regions are seen. Table 2 presents the calculated values of the surface areas per a molecule s and free energy G, which char acterize the monomolecular adsorption of the exam ined BIMs. After the CNT surface is completely covered with an adsorbate layer, the adsorption value drastically grows. A further increase in C causes the appearance of additional horizontal regions that seem to reflect the formation of adsorption bilayers, etc. (Fig. 2). At higher C values, a increases with C. The studied compounds are amphiphilic, and their hydrophobic–hydrophilic balance is governed by the size ratio between adenosine and phosphate frag ments. Hence, AN, AMP, and ATP are adsorbed on COLLOID JOURNAL
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the hydrophobic surface via the adenine fragments, which have a relatively high hydrophobicity parameter logP = –0.3 [11] (Table 1). The decrease in the adsorption free energy in the aforementioned series of the BIMs in the initial regions of the isotherms (Table 2), which is symbate to the increase in the sol ubility, confirms this conclusion. At high concentrations of adsorbate solutions (С > (5–6) × 10–4 mol/dm3), the adsorption diminished in a series AN > AMP > ATP, with the weakest a(C) depen dence being observed for ATP. This character of the polymolecular adsorption absolutely corresponds to the rise in the hydrophilicity and solubility of these compounds in the aforementioned series. Note that the obtained surface areas s per mole cules of AN and AMP exceed the theoretically esti mated value of the total area of the largest molecule (ATP), which is equal to nearly 1.0–1.2 nm2 [12]. Hence, the molecular packing of AN and AMP on the CNT surface in the case of the monomolecular cover age is rather loose. This fact seems to reflect the pecu liarities of the interaction between adenine fragments of these BIMs and the aromatic surface of CNTs. Taking into account that AN molecules have the smallest areas, it may also be concluded that that the packing of this adsorbate molecules on the CNT sur face is loosest. The analysis of the chemical shifts in the 1H NMR spectra of adenine and Dribose fragments [8] led us to conclude that hydrophobic and ππ interactions take place between adenine fragments and CNT surface. The denser packing of AMP and ATP molecules in the adsorption monolayers may be assumed to reflect a stronger lateral attraction between molecules of these adsorbates compared to AN mole cules due to the appearance and elongation of the phosphate fragment. The differences in the character of the polymolecular adsorption that are observed for AN, ATP, and AMP (Fig. 2) are likely to reflect the differences in the character of their molecular associ ation in aqueous solutions. CONCLUSIONS The experimentally observed positive adsorption of AN, ATP, and AMP from aqueous solutions on the surface of multiwall CNTs takes place because of the presence of adenine fragments in the studied com
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pounds. At the same time, the data obtained indicate that, in the case of monomolecular adsorption, the molecular packing is rather loose and the packing den sity is higher for compounds containing phosphate groups. The observed behavior of the adsorbates seems to reflect the intensification of lateral interactions in the adsorption layers. In the region of polymolecular adsorption at high adsorbate concentrations in solu tions (С > (5–6) × 10–4 mol/dm3), the adsorption diminishes in a series AN > AMP > ATP, i.e., with the enhancement of the hydrophilicity and the solubility of these compounds in water. REFERENCES 1. Ajayan, P.M., Chem. Rev., 1999, vol. 99, p. 1787. 2. Jagota, A., Diner, B.A., Boussaad, S., and Zheng, M., in NanoScience and Technology. Applied Physics of Car bon Nanotubes: Fundamentals of Theory, Optics and Transport Devices, Avouris, P., Bhushan, B., Klitzing, K., Sakaki, H., Wiesendanger, R., Rotkin, S.V., and Subra money, S., Eds., BerlinHeidelberg: Springer, 2005, p. 253. 3. Sinha, N. and Yeow, J.T.W, IEEE Trans. Nanobiosci., 2005, vol. 4, p. 180. 4. Pagona, G. and Tagmatarchis, N., Curr. Med. Chem., 2006, vol. 13, p. 1789. 5. Gao, H. and Kong, Y., Annu. Rev. Mater. Res., 2004, vol. 34, p. 123. 6. Lisunova, M.O., Lebovka, N.I., Melezhyk, O.V., and Boiko, Yu.P., J. Colloid Interface Sci., 2006, vol. 299, p. 740. 7. Eletskii, A.V., Usp. Fiz. Nauk, 2004, vol. 174, p. 1191. 8. Ikeda, A., Hamano, T., Hayashi, K., and Kikuchi, J., Org. Lett., 2006, vol. 8, p. 1153. 9. Melezhik, A.V., Sementsov, Yu.I., and Yanchenko, V.V., Zh. Prikl. Khim. (S.Peterburg), 2005, vol. 78, p. 938. 10. Koganovskii, A.M., Klimenko, N.A., Levchenko, T.N., and Roda, I.G., Adsorbtsiya organicheskikh veshchestv iz vody (Adsorption of Organic Substances from Water), Leningrad: Khimiya, 1990. 11. Mannhold, R., Poda, G.I., Ostermann, C., and Tetko, I.V., J. Pharm. Sci., 2009, vol. 98, p. 861. 12. Kashiwagi, E. and Rabinovitch, B., J. Phys. Chem., 1955, vol. 59, p. 498. 13. Myrdal, P., Ward, G.H., Dannenfelser, R.M., et al., Chemosphere, 1992, vol. 24, p. 1047.
