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Adsorption Characteristics of Thionine on Gold Nanoparticles Yuanhua Ding,*,†,‡ Xumin Zhang,‡ Xiaoxia Liu,‡ and Rong Guo*,‡ School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China, and School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou 225002, P. R. China ReceiVed October 27, 2005. In Final Form: January 4, 2006 This paper was retracted on October 4, 2007 (Langmuir 2007, 23, 11342). Adsorption characteristics of thionine on gold nanoparticles have been studied by using UV-vis absorption spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), cyclic voltammetry and Fourier transform infrared spectroscopy. With the increasing concentration of gold nanoparticles, the absorption peak intensity of H-type dimers of thionine increases continuously, whereas that of monomers of thionine first increases and then decreases. The addition of gold nanoparticles makes the equilibrium between the monomer and H-type dimer forms of thionine move toward the dimer forms. Furthermore, the adsorption behavior of thionine on gold nanoparticles is also influenced by temperature. TEM images show that the addition of thionine results in an obvious aggregation, and further support the absorption spectral results. The fluorescence intensity of adsorbed thionine is quenched by gold nanoparticles due to the electronic interaction between thionine molecules and gold nanoparticles. Cyclic voltammetric and infrared spectroscopic studies show that the nitrogen atoms of both of the NH2 moieties of thionine strongly bind to the gold nanoparticle surfaces through the electrostatic interaction of thionine with gold nanoparticles. For 15-20 nm particles, the number of adsorbed thionine molecules per gold nanoparticle is about 7.66 × 104. Thionine molecules can not only bind to a particle to form a compact monolayer via both of the NH2 moieties, but they can also bind to two particles via their two NH2 moieties, respectively.
Introduction Significant efforts have been made in recent years to investigate the photophysical and photochemical behaviors of multicomponent nanostructured assemblies consisting of photoactive dyes, metals, and semiconductors.1-13 Such nanocomposite materials are especially useful for developing efficient light energy conversion systems, optoelectronic devices, and sensors.14-18 A burst of research activity in the field has been centered around the design of functionalized metal nanoparticles with optoelectronic properties.12 Tailoring the optoelectronic properties of metal nanoparticles by organizing organic molecules containing functional groups on metal nanoparticles can yield functionalized * To whom correspondence should be addressed.
[email protected] (Y.D.);
[email protected] (R.G.). † Nanjing University. ‡ Yangzhou University.
E-mail:
(1) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (2) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (3) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (4) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (5) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (6) Martino, A.; Yamanaka, S. A.; Kawola, J. S.; Loy, D. A. Chem. Mater. 1997, 9, 423. (7) Hornyak, G. L.; Patrissi, C. J.; Martin, C. R. J. Phys. Chem. B 1997, 101, 1548. (8) Henglein, A. Chem. Mater. 1998, 10, 444. (9) Xu, P.; Yanagi, H. Chem. Mater. 1999, 11, 2626. (10) Behar-Levy, H.; Avnir, D. Chem. Mater. 2002, 14, 1736. (11) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 7479. (12) George Thomas, K.; Kamat, P. V. Acc. Chem. Res. 2003, 36, 888. (13) Clapp, A. R.; Medintz, I. L.; Fisher, B. R.; Anderson, G. P.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 1242. (14) Chandrasekharan, N.; Kamat, P. V.; Hu, J.; Jones, G., J. Phys. Chem. B 2000, 104, 11103. (15) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (16) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (17) Ipe, B. I.; George Thomas, K.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 2002, 106, 18. (18) Sudeep, P. K.; Ipe, B. I.; George Thomas, K.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29.
organic-inorganic nanocomposite materials.12,19 However, all these properties and applications are related to the surface characteristics of metal nanoparticles.20 It is therefore important for the understanding of colloidal properties based on particle surface conditions to investigate the adsorption behaviors of functional molecules, especially photoactive molecules, on metal nanoparticles.20 Many recent investigations have focused on characterizing molecular adsorption for the understanding of the effect of adsorbate-surface interactions on colloidal properties as well as the nature of those interactions.20-23 There have been several literature reports on dye-metal nanoparticle interactions,12-14,24-27 but only limited information is available on the adsorption mechanism. Thionine is an important thiazine dye, and is usually chosen as the sensitizer since it yields both singlet and triplet excited states in detectable amounts.28 Recently, the dye thionine has been used for studies of the photosensitization of the large-band-gap semiconductor ZnO,28 and also for studies of electron transfer with DNA.29,30 However, little research has been reported about the adsorption characteristics of the dye on metal nanoparticles. On the other hand, gold nanoparticles have been drawing a great deal of attention because of their unique physicochemical propertiessespecially (19) George Thomas, K.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 3722. (20) Eckenrode, H. M.; Jen, S.-H.; Han, J.; Yeh, A.-G.; Dai, H.-L. J. Phys. Chem. B 2005, 109, 4646. (21) Aray, Y.; Marquez, M.; Rodrı´guez, J.; Coll, S.; Simo´n-Manso, Y.; Gonzalez, C.; Weitz, D. A. J. Phys. Chem. B 2003, 107, 8946. (22) Gonzalez-Garcia, C. M.; Gonzalez-Martin, M. L.; Gallardo-Moreno, A. M.; Gomez-Serrano, V.; Labajos-Broncano, L.; Bruque, J. M. J. Colloid Interface Sci. 2002, 248, 13. (23) Radovic, L. R. In Surfaces of Nanoparticles and Porous Materials; Schwarz, J. A., Contescu, C. I., Eds.; Marcel Dekker: New York, 1999; Vol. 78. (24) Kometuni, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578. (25) Franzen, S.; Folmer, J. C. W.; Glomm, W. R.; O’Neal, R. J. Phys. Chem. A 2002, 106, 6533. (26) Zhang, J.; Lakowicz, J. R. J. Phys. Chem. B 2005, 119, 8701. (27) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (28) Patrick, B.; Kamat, P. V. J. Phys. Chem. 1992, 96, 1423. (29) Reid, G. D.; Whittaker, D. J.; Day, M. A.; Turton, D. A.; Kayser, V.; Kelly, J. M.; Beddard, G. S. J. Am. Chem. Soc. 2002, 124, 5518. (30) Dohno, C.; Stemp, E. D. A.; Barton, J. K. J. Am. Chem. Soc. 2003, 125, 9586.
10.1021/la052897p CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
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Chart 1. Molecular Structure of Thionine
their special nonlinear optical properties and surface-enhanced Raman scattering effectsand potential applications in biology, catalysis, photocatalysis, and microelectronics.15,31-33 In view of these considerations, thionine and gold nanoparticles have been chosen to investigate the adsorption characteristics of dyes on metal nanoparticle surfaces. In the present work, the adsorption characteristics of thionine on gold nanoparticle surfaces were systematically investigated by means of UV-vis spectroscopy, fluorescence spectroscopy, transmission electron microscopy (TEM), cyclic voltammetry and Fourier transform infrared (FTIR) spectroscopy. The aggregation behavior of thionine on gold nanoparticle surfaces, the fluorescence quenching of adsorbed thionine by gold nanoparticles, the electrochemical properties of adsorbed thionine, and the surface coverage of gold nanoparticles were obtained. More importantly, information about the adsorption mechanism of thionine on gold nanoparticle surfaces was revealed by investigating the interaction sites. The above studies will contribute to understanding the surface binding properties of organic dyes on metal nanoparticle surfaces, and provide a theoretical basis for the design of functionalized organicinorganic nanocomposite materials. Experimental Section Materials. HAuCl4‚3H2O and trisodium citrate were purchased from Aldrich. 2-Propanol was from CDH chemicals (New Delhi, India). Thionine (Chart 1) was from Sigma and was used after purification with high-performance liquid chromatography (HPLC). Triply distilled water was used throughout the work. Synthesis of Gold Nanoparticles. Citrate-capped gold nanoparticles of 15-20 nm diameter were prepared by the reduction of HAuCl4 with trisodium citrate.34,35 An 18.5 mL volume of water and 0.5 mL of 1.0 × 10-2 mol/L trisodium citrate were mixed well, and 0.5 mL of chloroauric acid (1.0 × 10-3 mol/L) was then added to the system. The resulting mixture was stirred and then refluxed until the color changed to wine red. In this case, the trisodium citrate itself acted as the reducing agent. From the TEM measurements, the size of the prepared nanoparticles was determined to be mainly in the range of 15-20 nm. The concentration of the gold sol was 2.56 × 10-5 mol/L. Synthesis of Thionine-Coated Gold Nanoparticles. A 20 mL volume of citrate-capped gold nanoparticle solution was mixed with 5 mL of saturated thionine solution in 2-propanol and stirred effectively. Stirring was continued for 24 h with the wine red color fading gradually. The resulting solution was centrifuged at 12 000 rpm to obtain the precipitate of thionine-covered gold nanoparticles. The precipitate was washed several times with 2-propanol and cold water to remove any unadsorbed thionine and unreacted citrate. The washed precipitate can be used for infrared experiments, or be redispersed easily in organic solvents, such as dimethyl sulfoxide (DMSO) and 2-propanol, by sonication for electrochemical experiments. The dispersions were stable for several days. Absorption and Fluorescence Measurements. UV-vis absorption spectra were recorded with a Shimadzu UV-2550 spectropho(31) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (32) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549. (33) Barazzouk, S.; Kamat, P. V.; Hotchandani, S. J. Phys. Chem. B 2005, 109, 716. (34) Turkevitch, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (35) Tom, R. T.; Suryanarayanan, V.; Reddy, P. G.; Baskaran, S.; Pradeep, T. Langmuir 2004, 20, 1909.
Figure 1. UV-vis absorption spectra of thionine in aqueous solution containing different concentrations of gold nanoparticles at 25 °C. Thionine concentration: 1.25 × 10-5 mol/L. Gold nanoparticle concentration (mol/L): a ) 0; b ) 2.56 × 10-6; c ) 5.12 × 10-6; d ) 7.68 × 10-6; e ) 1.02 × 10-5; f ) 1.28 × 10-5; g ) 1.54 × 10-5; h ) 2.05 × 10-5. The dot curve is the absorption spectrum of the pure gold sol. tometer by varying the concentration of gold nanoparticles or the experimental temperature and keeping the thionine concentration constant in aqueous solution. The emission spectra of free thionine and the adsorbed species on gold nanoparticles were recorded with a Shimadzu 2501 spectrofluorimeter by keeping the thionine concentration constant in aqueous solution. TEM Measurement. The TEM images of gold nanoparticles before and after the addition of thionine were measured with a Philips Tecnai-12 transmission electron microscope. The TEM samples were prepared by dropping the gold sol onto a copper grid covered with Formvar film. The size distribution histograms of gold nanoparticles before and after the addition of thionine were obtained by averaging the sizes of 300 particles directly from the TEM images. Particle aggregates were removed manually prior to size measurements. The final thionine concentration in the gold sol was 1.25 × 10-5 mol/L. Cyclic Voltammetric Measurement. Cyclic voltammograms of pure thionine molecules and adsorbed species were obtained on an electrochemical analyzer (CH Instruments Model 600) in a standard three-electrode cell comprising a glass carbon electrode (GCE) as the working electrode, a platinum plate as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Voltammetry was performed at different sweep rates in 2-propanol containing 0.01 mol/L tetrabutylammonium hexafluophosphate (TBAPF6) as the supporting electrolyte. Infrared Spectral Measurement. The FT-IR spectra of free thionine and the adsorbed species on gold nanoparticle surfaces were recorded with a Bruker Tensor 27 FT-IR spectrometer. All the experiments were carried out at 25 ( 0.1 °C, unless otherwise mentioned.
Results and Discussion Absorption Characteristics. Figure 1 presents the UV-vis absorption spectra changes of thionine after the addition of different concentrations of gold nanoparticles at a given thionine
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concentration (1.25 × 10-5 mol/L). The dot curve in Figure 1 is the absorption spectrum of the prepared gold sol, in which the 520 nm absorption band is characteristic of the surface plasmon band of gold nanoparticles. As shown in Figure 1a, the spectrum of thionine in water exhibits two characteristic absorption bands at 598 and 565 nm. The 598 nm band is a characteristic absorption feature of monomeric form, and the 565 nm shoulder can be attributed to the H-type dimer aggregate.36 With the increasing concentration of gold nanoparticles (2.56 × 10-6 - 1.28 × 10-5 mol/L), as shown in Figure 1b-f, the intensities of the 598 and 565 nm bands increase gradually; the peak position of the 598 nm band is almost unchanged, whereas that of the 565 nm band is slightly blue-shifted. Moreover, the dimer peak becomes clear gradually, and the increase in the intensity of the dimer peak becomes larger than that of the monomer peak, which suggests that the equilibrium between the monomer and dimer forms of thionine moves gradually toward the dimer forms. When the concentration of gold nanoparticles is increased to 1.54 × 10-5 mol/L, as shown in Figure 1g, the intensity of the 565 nm band increases continuously, whereas that of the 598 nm band decreases slightly, which indicates that the equilibrium between the monomer and dimer forms of thionine moves significantly toward the dimer forms. With further addition of gold nanoparticles, the intensity of the 598 nm band decreases remarkably, whereas that of the 565 nm band increases continuously. Finally, the 565 nm band, gradually blue-shifted, merges with the surface plasmon band of the gold nanoparticles at 520 nm to form a broad absorption band (Figure 1h). The absorption spectra changes in Figure 1 show a strong interaction of the cationic dye with gold nanoparticles. It has been known that gold nanoparticles, prepared by the reduction of HAuCl4 with trisodium citrate, have a negative surface charge due to a weakly bound citrate coating.37 Thus, the cationic dye has opposite charges with respect to the surface charges of the gold particles. With the addition of gold nanoparticles, thionine molecules are adsorbed onto the gold nanoparticle surfaces due to the electrostatic attractive force between the cationic dye molecules and the particle surfaces. This often leads to close packing of dye molecules on a charged particle surface,12 and thus makes the local concentration of thionine on particle surfaces increase, and, correspondingly, its effective absorption crosssectional area increases. As a result, both the absorption intensities of the 598 and 565 nm bands increase with the addition of gold particles, which is similar to the sensitization of photoactive molecules in micelles.38,39 On the other hand, gold-dye assemblies coalesce to form larger clusters because of surface charge neutralization. This facilitates the face-to-face arrangement of two thionine molecules, resulting in the transition from the monomer to H-dimer forms of thionine. Strong electronic coupling between the molecules in the formed dye aggregates causes the blue shift of the 565 nm band. Because the dye aggregation can be seen at low dye concentrations (∼10-5 mol/L), it is evident that those dyes interact strongly with each other and exhibit spectral shifts arising from such interactions. Therefore, gold nanoparticles can be used to enhance the absorption sensitivity of thionine and modulate the arrangement of thionine molecules on the basis of the adsorption of thionine on gold nanoparticle surfaces. The influence of temperature on the adsorption behavior of thionine on gold nanoparticles can be revealed by the absorption (36) Das, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 209. (37) Lee, C.; Kim, S.; Yoon, C.; Gong, M.; Choi, B. K.; Kim, K.; Joo, S. W. J. Colloid Interface Sci. 2004, 271, 41. (38) Bunton, C. A.; Sepulveda, L. J. Phys. Chem. 1979, 83, 680. (39) Awan, M. A.; Shah, S. S. Colloids Surf., A 1997, 122, 97.
Ding et al.
Figure 2. UV-vis absorption spectra of (A) thionine in aqueous solution and (B) thionine in aqueous solution containing 2.56 × 10-6 mol/L gold nanoparticles at different temperatures. Thionine concentration: 1.25 × 10-5 mol/L. Temperature range: a ) 25 °C; b ) 30 °C; c ) 35 °C; d ) 40 °C; e ) 60 °C.
spectra in Figure 2. For comparison, the absorption spectra change of thionine in the aqueous solution with temperature is shown in Figure 2A, which indicates that the intensities of the monomer and dimer peaks increase with the increasing temperature. When the thionine aqueous solution contains a certain amount of gold nanoparticles, as can be observed from Figure 2B, the intensities of the monomer and dimer peaks decrease, and the H-dimer peak becomes unclear with the increasing temperature. The influence of temperature can be generalized as the following two factors. Upon raising the temperature of the gold-thionine suspension, (1) gold nanoparticles grow and melt with each other;14,40 and (2) the adsorbed thionine undergoes an intramolecular hydrophilic-hydrophobic transition due to the break of hydrogen bonds, resulting in an increase in the hydrophobic interaction of the colloidal golds. Both the factors cause the formation of compact gold clusters, and, simultaneously, the thionine molecules adsorbed on gold nanoparticle surfaces are immobilized inside the gold clusters. Since the compact gold clusters cannot transmit UV-vis light, no absorption occurs for the immobilized thionine molecules. TEM Images of Gold Sol. Figure 3 shows the TEM images and the corresponding size distribution histograms of gold nanoparticles before and after the addition of thionine. As can be seen in Figure 3A, the prepared gold nanoparticles are almost spherical shaped and separated from each other. The corresponding size distribution histogram shows that the size of most of the gold particles is 15-20 nm with an average diameter of 17.7 nm. After the addition of thionine, as shown in Figure 3B, the gold particles also exhibit similar particle diameter, but an obvious aggregation of gold nanoparticles occurs. From the corresponding size distribution histogram, it is found that the (40) Nath, N.; Chilkoti, A. J. Am. Chem. Soc. 2001, 123, 8197.
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Figure 3. TEM images and corresponding size distribution histograms of (A) the gold sol and (B) the gold sol with 1.25 × 10-5 mol/L thionine.
Figure 4. Emission spectra of (a) free thionine and (b) the adsorbed species on gold nanoparticles taken in aqueous solution. Thionine concentration in both the cases: 7.62 × 10-7 mol/L.
gold nanoparticles have an average diameter of 18.0 nm, indicating that the amount of large gold particles is increased slightly after the addition of thionine. Surface charge neutralization, which accompanies the adsorption of thionine on gold nanoparticles, induces such an aggregation. The formation of gold-dye assemblies and gold clusters further supports the explanations for the above absorption spectra changes. Fluorescence Quenching of Thionine by Gold Nanoparticles. Fluorescence is an excellent probe for revealing the electronic characteristics of nanomaterials. In Figure 4, the emission spectra of free thionine and the adsorbed species on gold nanoparticles taken in aqueous solution are displayed. Free thionine exhibits a maximum emission peak around 615 nm when excited at 550 nm (Figure 4a). The adsorbed species on nanogold surfaces also exhibits a maximum emission peak
around 615 nm at the same excitation wavelength, but the fluorescence intensity decreases greatly (Figure 4b). The concentration of thionine was the same in both cases, as determined by absorption spectroscopy. The decreased fluorescence indicates that a large fraction of excited thionine molecules are quenched by the gold nanocores, which can be attributed to the electronic interaction between thionine molecules and gold nanoparticles. The emission spectrum of Figure 4b represents the excited states that survive the deactivation by the metal surface.17 To check that the fluorescence intensity is only due to the adsorbed species, the solution used for the above measurement was centrifuged, and the thionine-adsorbed particles were removed. The fluorescence intensity of the centrifugate collected at different intervals of time was measured without any dilution, and it was found that there was no appreciable amount of thionine desorbed during the span of the measurement. The data (Figure 4) suggest that the bound thionine is fluorescent and the desorption of thionine from the nanoparticle surfaces is negligible on the experimental time scale. Since thionine is a sensitive photoactive dye, the thioninebound nanoparticles can be used as biomolecular labels, and also in the design of novel photo-based nanodevices. However, it is important to control the aggregate size of gold particles or to coat gold particles with thionine without inducing such an aggregation due to the fluorescence quenching of thionine by gold nanoparticles. This could be achieved by the careful control of synthetic conditions, such as the molar concentration ratio of particles to thionine, temperature, and reaction media, which is underway in our lab. Electrochemical Properties. The electrochemical properties of unadsorbed and adsorbed thionine molecules on gold
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Figure 6. Infrared spectra of (a) free thionine and (b) thionine adsorbed on gold nanoparticles.
The cyclic voltammograms of the adsorbed thionine on nanogold surfaces are shown in Figure 5B. By comparing Figure 5Ba with Figure 5Aa, it is found that, at the same sweep rate, the anodic peak potential is negatively shifted to -0.178 V,
whereas the cathodic peak potential is positively shifted to -0.277 V, and the ∆Ep value is decreased to 0.099 V. As the sweep rate is increased, the changing trends of the anodic and cathodic peaks are similar to that in Figure 5A (Figure 5Bb-e). But differently from Figure 5A, the increase in the peak currents becomes significant with the increasing sweep rate, and the ∆Ep value decreases at the same sweep rate. When the cathodic peak current is plotted against the square root of the sweep rate, a straight line is also obtained, but the slope value is increased to 1.01 (inset of Figure 5B), which also suggests that the electrontransfer process for the adsorbed thionine is diffusion controlled. The decrease in the ∆Ep value at the same sweep rate and the faster increase in the peak currents with the sweep rate indicate that the reversibility of the electrode reaction of the adsorbed thionine on nanogold surfaces is significantly improved, and the electron-transfer kinetics is rapidly enhanced.41 Meanwhile, they also indicate that gold nanoparticles can electrocatalyze the thionine adsorbed on their surfaces. However, the changing magnitude of the slope value of the i-V1/2 plot is relatively small when the inset of Figure 5B is compared with that of Figure 5A. This indicates that the oxidation-reduction properties of the nitrogen atom of the central heterocyclic ring of thionine is not fundamentally affected by the adsorption of thionine on gold nanoparticle surfaces, suggesting that the nitrogen atom of the heterocyclic ring does not directly bind to the nanogold surfaces. Infrared Spectral Features. The adsorption mechanism of thionine on gold nanoparticle surfaces was further characterized by FT-IR spectroscopy. The IR spectra of free thionine and adsorbed thionine are shown in Figure 6. The absorption bands at 1600 and 1500 cm-1 in Figure 6a are ascribed to the skeletal vibration of the phenyl ring of thionine.43 The above-mentioned bands are also shown by the adsorbed thionine (Figure 6b), confirming the presence of thionine on the nanoparticle surfaces. The absorption bands at 3310 and 3150 cm-1 are due to the N-H stretching vibration of the amino moieties,43 which are not observed in the IR spectrum of the adsorbed thionine (Figure 6b), indicating that the nitrogen atoms of both of the NH2 moieties of thionine bind strongly to gold nanoparticle surfaces. Furthermore, the absorption band at 850 cm-1 in Figure 6a, corresponding to the N-H bending vibration of the amino moieties of thionine, 43 is also not observed in the IR spectrum of the adsorbed thionine (Figure 6b), confirming the binding of the nitrogen atoms of both of the NH2 moieties on gold particle surfaces. In Figure 6b, the absorption band at 3427 cm-1 is due
(41) Raj, C. R.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2003, 543, 127. (42) Bauldreay, J. M.; Archer, M. D. Electrochim. Acta 1993, 38, 1619.
(43) Pretsch, E.; Bu¨hlmann, P.; Affolter, C. Structure Determination of Organic Compounds Table of Spectral Data, 3rd ed.; Springer-Verlag: Berlin, 2000.
Figure 5. Cyclic voltammograms of (A) thionine (1.25 × 10-5 mol/L) and (B) thionine adsorbed on gold nanoparticles taken on a GCE in 2-propanol/TBAPF6 at different sweep rates. Sweep rates (mV/s): a ) 20; b ) 40; c ) 60; d ) 80; e ) 100. The insets of panels A and B show the plots of cathodic peak current vs the square root of the sweep rate for (A) unadsorbed species and (B) adsorbed species.
nanoparticle surfaces were studied by cyclic voltammetry. Figure 5, panels A and B show the cyclic voltammograms of pure thionine and adsorbed thionine on gold nanoparticle surfaces in 2-propanol/ TBAPF6 (pH ) 6.5) at GCE, respectively. As can be seen in Figure 5Aa, when the sweep rate is 20 mV/s, thionine shows an oxidation peak at -0.020 V and a reduction peak at -0.397 V, the peak-to-peak separation (∆Ep) is found to be about 0.377 V, and the oxidation peak current is almost equal to the reduction peak current. As the sweep rate is increased, the anodic peak potential shifts to a more positive potential, whereas the cathodic peak potential shifts to a more negative potential (Figure 5Abe). When the cathodic peak current is plotted against the square root of the sweep rate, a straight line is obtained with a slope value of 0.57 (inset of Figure 5A), which suggests that the electrontransfer process at GCE is diffusion controlled.41 The above experimental phenomena indicate that the electrode reaction of thionine at GCE is a diffusion-controlled quasi-reversible electrochemical reaction at the neutral pH value, which can be ascribed to the oxidation-reduction reaction of the nitrogen atom of the central heterocyclic ring of thionine:42
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to traces of water in the nanoparticle sample, and the absorption bands at 2930 and 2856 cm-1 are due to traces of citrate impurity in the sample. The nitrogen and sulfur atoms of the central heterocyclic ring of thionine are less electron-rich due to delocalization of electrons to the electron-deficient phenyl ring. It has been reported that the electron deficiency of atoms make them unlikely to bind directly to the nanoparticle surfaces.35 The cyclic voltammetric experiments also suggest that the nitrogen atom of the central heterocyclic ring does not directly bind to the nanoparticle surfaces. Therefore, from the above data, it can be inferred that the nitrogen atoms of both of the NH2 moieties of thionine strongly bind to gold nanoparticle surfaces. The binding of the amino groups to the surfaces of gold nanoparticles has been reported in fabricating the nanogold self-assembly through an amine-terminated monolayer of cystamine,41 or in achieving the organization of pyrene chromophores around gold nanoparticles through the surface binding of 1-methylaminopyrene (Py-CH2NH2, molecular probes) to gold nanoparticles.44 Surface Coverage of Gold Nanoparticles. The surface coverage of gold nanoparticles (the number of adsorbed thionine molecules per gold nanoparticle) was determined by absorption spectroscopy. The intensity of the dominant peak at 598 nm of pure thionine in water was taken as the reference for measuring the molar concentration of adsorbed thionine. Assuming that Io is the initial intensity just after the mixing of thionine aqueous solution with the gold sol, and Ic is the intensity of the centrifugate after an equilibrium for 24 h, Ia is the intensity corresponding to the thionine molecules adsorbed on gold nanoparticle surfaces:
Io - Ic ) Ia
(2)
where the intensity Ia is proportional to the molar concentration of the adsorbed thionine (Ca). By measuring the intensity (Ip) of a known concentration of pure thionine (Cp), Ca can be calculated from eqs 3 and 4:
Ia ) KCa
(3)
Ip ) KCp
(4)
Assume that W is the weight of gold formed theoretically. From TEM measurements, the radius/volume of the nanoparticle can be calculated, and thus, the weight of each nanoparticle (Wnp) can be obtained. The number of nanoparticles is calculated from eq 5:
Np ) W/Wnp
(5)
From the number of adsorbed thionine molecules on gold nanoparticle surfaces (Nm; Nm ) Ca × 6.023 × 1023), we can obtain the number of the adsorbed thionine molecules per gold particle by Nm/Np. Herein, a standard thionine aqueous solution (Cp ) 1.25 × 10-5 mol/L) is taken for this calculation, which shows an absorption intensity of 0.0638 (Ip). The concentration of HAuCl4‚ 3H2O is 2.56 × 10-5 mol/L, so that, 1000 mL of the gold sol contains 5.07 × 10-3 g of Au. For 15-20 nm gold nanoparticles, after the addition of thionine, the average diameter of particles is known to be 18.0 nm from the previous discussion. The weight of each gold nanoparticle is 5.76 × 10-17 g, thus, the number of 18.0 nm nanoparticles (Np) present in 1000 mL of gold sol is 8.80 × 1013. For the present gold sol, Io ) 0.0713, Ic ) 0.0573, and hence, Nm ) 6.74 × 1018. From the above data, the number of the adsorbed thionine molecules per nanoparticle, Nm/Np ≈ (44) George Thomas, K.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655.
Figure 7. Schematic illustration of the surface binding of thionine to gold nanoparticles.
7.66 × 104. Assuming that the planar thionine molecules are adsorbed onto gold particle surfaces to form a monolayer with a perpendicular orientation occupying an area of 0.21 nm2, which was obtained from the dimension of the planar molecule (0.72 × 1.5 nm)45 and the radius of the nitrogen atom (0.07 nm)46 on the basis of the van der Waals radii, the maximum number of the adsorbed thionine molecules per nanoparticle is estimated to be about 4.84 × 103. The larger value of the surface coverage of gold nanoparticles compared to that in the case of a monolayer indicates strong binding between thionine and gold nanoparticles. As shown by the IR results, the disappearance of the N-H vibrations of amino moieties implies that both of the NH2 moieties of thionine almost completely bind to gold nanoparticle surfaces. On the other hand, the obtained adsorption amount of thionine on gold nanoparticles is significantly larger than that in the case of a monolayer. On these grounds, the plausible conclusion is that thionine molecules can not only bind to a particle to form a compact monolayer via both of the NH2 moieties, but can also bind to two particles via their two NH2 moieties, respectively, as schematically shown in Figure 7. It is noted that if thionine molecules are perpendicularly adsorbed on a particle surface, the nitrogen atoms of both of the NH2 moieties could not completely bind to the particle surface because of the planar molecular structure, and the similar bond distances of the C-N bonds at both the ends of the molecule and the C-C or C-S bonds of the central heterocyclic ring.47 Hence, it is possible that thionine molecules can be adsorbed on a particle surface at a certain angle of tilt against the particle surface, which makes the lone electron pairs of the nitrogen atoms of both of the NH2 moieties point to the particle surface. However, it is difficult to tell the difference between an H-dimer spanning two particles and a dimer formed on a particle due to their similar dimer structures, and further studies are needed. The unique surface binding mode of thionine on gold nanoparticles results in the formation of thionine-gold nanoassemblies and gold clusters, which agrees well with the absorption spectral results, and will find potential applications in developing functionalized metal nanoparticles.
Conclusions The adsorption characteristics of dye thionine on gold nanoparticle surfaces were studied by means of various analytical techniques. The nitrogen atoms of both of the NH2 moieties of thionine bind strongly to gold nanoparticle surfaces, as confirmed by cyclic voltammetric and infrared spectroscopic studies. The addition of gold nanoparticles into the thionine system facilitates the aggregation and arrangement of thionine molecules, and makes (45) Simoncic, P.; Armbruster, T.; Pattison, P. J. Phys. Chem. B 2004, 108, 17352. (46) Dean, J. A. Langes Handbook of Chemistry, 13th ed.; McGraw-Hill: New York, 1985. (47) Xu, W.; Aydin, M.; Zakia, S.; Akins, D. L. J. Phys. Chem. B 2004, 108, 5588.
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the equilibrium between the monomer and H-dimer forms of thionine move toward the dimer forms. Furthermore, the adsorption behavior of thionine on gold nanoparticles, revealed by absorption spectroscopy, is significantly influenced by temperature. TEM images support the explanations for the above absorption spectra changes. Based on the adsorption of thionine on gold nanoparticle surfaces, gold nanoparticles can be used to enhance the absorption sensitivity of thionine and modulate the molecular arrangement of thionine. The bound thionine is fluorescent, and this property suggests possible applications for the fabricating of biomolecular labels and the design of novel photo-based nanodevices for sensing, switching, and drug delivery. The number of adsorbed thionine molecules per gold
Ding et al.
nanoparticle is 7.66 × 104. The large value of surface coverage of gold nanoparticles indicates strong binding between thionine and gold nanoparticles. Thionine molecules can not only bind to a particle to form a compact monolayer via both of the NH2 moieties, but they can also bind to two particles via their two NH2 moieties, respectively. The above studies will contribute to understanding the interaction of organic dyes with metal nanoparticles, and provide a theoretical basis for the design of functionalized organic-inorganic nanocomposite materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20233010). LA052897P
