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Cyclodextrin-Modified Gold Nanospheres Jian Liu,† Winston Ong,† Esteban Roma´n,† Matthew J. Lynn,‡ and Angel E. Kaifer*,† Center for Supramolecular Science and Department of Chemistry, and Center for Advanced Microscopy, University of Miami, Coral Gables, Florida 33124-0431 Received November 22, 1999. In Final Form: January 18, 2000 A new procedure for the modification of gold nanospheres (2-7 nm diameter) with thiolated cyclodextrin receptors is described. The resulting monolayer-protected nanoparticles behave as multisite hosts in aqueous media, engaging in host-guest interactions with guest molecules in the solution.
Metal colloid chemistry has experienced a renaissance in the last few years driven by substantial improvements in the methods used to modify the surface of colloidal particles. It is now possible to modify colloidal particles with monolayers of organic molecules that can be used to affect or even control their properties.1 Recently, several groups have started research work on metal colloidal particles derivatized with molecules that can engage in host-guest interactions with solution partners.2 Here we describe the preparation and characterization of gold nanoparticles modified with surface-attached cyclodextrin (CD) receptors. The resulting CD-modified gold nanospheres can be used as multisite hosts for binding of guests in the solution. In this communication, we also report electrochemical evidence for the binding of water-soluble ferrocene derivatives to the CD-modified gold nanospheres.
Figure 1. 1H NMR (400 MHz, D2O) spectra of (A) β-CD and (B) β-SH-CD immobilized on gold nanoparticles (average diameter 2.7 nm).
The CD-modified gold nanospheres were prepared by reduction of AuCl4- with NaBH4 in DMSO solution also containing one of the perthiolated CDs (R-, β-, or γ-).3 The reaction mixture becomes deep brown immediately after the addition of the reducing agent. This single-phase procedure is similar to that reported by Brust and coworkers for the preparation of gold clusters protected with p-mercaptophenol monolayers.4 The CD-modified gold particles can be isolated by addition of CH3CN5 and characterized by 1H NMR and UV-vis spectroscopies as * To whom correspondence should be addressed. Tel: (305) 2843468. Fax: (305) 444-1777. E-mail:
[email protected]. † Center for Supramolecular Science and Department of Chemistry. ‡ Center for Advanced Microscopy. (1) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid & Interface Sci. 1997, 2, 42-50. (2) (a) Liu, J.; Xu, R.; Kaifer, A. E. Langmuir 1998, 14, 7337-7339. (b) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304-4305. (c) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 1147-1150. (d) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914-4915. (3) The perthiolated CDs were prepared as described previously: Rojas, M. T.; Ko¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336-343. (4) Brust, M. F.; Bethell, J.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656.
well as transmission electron microscopy (TEM). The 1H NMR spectrum (D2O, 400 MHz) of the CD-modified gold particles shows sharp intense peaks corresponding to DMSO and D2O molecules plus broad peaks that can be assigned to immobilized thiolated CDs (Figure 1). The UV-vis spectrum of these particles dissolved in aqueous solution shows relatively weak plasmon resonance absorption maxima in the range 507-520 nm (see Figure 2). The excellent solubility of these modified colloids in aqueous media is an important result by itself as relatively few water-soluble, monolayer-coated gold colloids (or clusters) have been reported.6 Size measurements were performed using TEM,7 and the results are summarized in Table 1 (For sample TEM (5) To prepare the gold modified gold nanospheres, a 50 mg sample of HAuCl4 was dissolved in 20 mL DMSO. This solution was quickly mixed with another 20 mL of DMSO containing 75.5 mg NaBH4 and a variable weight of the perthiolated CD receptor. The reaction mixture turned deep brown immediately but the reaction was allowed to continue for 24 h. At this point, 40 mL CH3CN was added to precipitate the colloid, which was collected by centrifugation, washed with 60 mL of CH3CN: DMSO (1: 1 v/v) and 60 mL of ethanol, isolated by centrifugation, and dried under vacuum (60 oC) for 24 h. (6) See, for instance: (a) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (b) Chen; S.; Kimura, K. Langmuir 1999, 15, 1075-1082. (7) TEM measurements were performed with a Philips EM 300 microscope.
10.1021/la991519f CCC: $19.00 © 2000 American Chemical Society Published on Web 03/08/2000
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Figure 3. SWV response on a glassy carbon electrode (0.018 cm2) of a 0.2 mM FcOH solution in 0.1 M NaCl also containing the following concentrations of gold particles (2.7 nm diameter) modified with β-SH-CD: (A) 0 mg/mL, (B) 1.0 mg/mL, (C) 2.1 mg/mL, and (D) 10.4 mg/mL. The dotted line corresponds to the last set of conditions after addition of 1.0 mM adamantanol. Scheme 1. Competitive Binding of Ferrocenemethanol and 1-adamantanol to the CD Hosts Immobilized on Gold Nanospheres
Figure 2. Electronic absorption spectra of aqueous solutions of gold nanoparticles modified with β-SH-CD. (A) Average diameter ) 2.7 nm, λmax ) 507 nm. (B) Average diameter ) 3.6 nm, λmax ) 516 nm. (C) Average diameter ) 4.9 nm, λmax ) 518 nm. (D) Average diameter ) 6.4 nm, λmax ) 520 nm. Table 1. TEM Diameters of Gold Particles Prepared by Reduction of AuCl4- in the Presence of Thiolated Cyclodextrins cyclodextrin
[CD]/ [AuCl4-]
diametera (nm)
std. deviationa (nm)
β-SH-CD β-SH-CD β-SH-CD β-SH-CD R-SH-CD γ-SH-CD
0.20/1 0.11/1 0.077/1 0.059/1 0.077/1 0.077/1
2.7 3.6 4.9 6.4 5.5 3.9
(0.5 (0.5 (0.6 (0.8 (0.7 (0.6
a
The reported mean diameters and std. deviations were determined by measuring a minimum of 100 randomly selected particles for each preparation. Sample TEM images and particle size distributions are shown in the supporting information.
images, see Supporting Information). Several key findings must be pointed out. First, the diameters of the gold particles produced in all cases range from 2 to 7 nm, a size range that is widely considered the frontier between clusters and colloids. We performed a series of experiments designed to assess the effect of the concentration of β-SHCD in the reaction mixture. From the results of these experiments (performed at constant [AuCl4-]), we conclude that the average size of the gold particles depends on the concentration of the host. Similar results have been obtained with simple alkanethiols.8 Finally, the nature of the perthiolated CD (R-, β-, or γ-) also affects the average particle size of the final modified gold colloid (Table 1). We have previously reported a completely different procedure for the preparation of CD-modified gold colloids, which relies on the preparation of 13-nm citrate-capped gold nanospheres followed by exposure to dissolved β-SHCD.2b The procedure described here is fundamentally different because both Au3+fAu0 reduction and attach(8) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 4, 17-30.
ment of the perthiolated CDs to the surface of the developing Au particles take place in the same step. The interplay between the kinetics of the two processes is probably the key factor that determines the average diameter of the resulting gold nanospheres. Perhaps the most important property of CDs is their ability to form inclusion complexes with hydrophobic guests.9 Do CD receptors immobilized on the surface of gold nanospheres maintain the host properties that they exhibit in homogeneous aqueous solution? To answer this question, we selected ferrocenemethanol (FcOH) as the guest of choice, owing to its aqueous solubility, electrochemical properties, and the fact that the host-guest pair composed by β-CD and ferrocene has been extensively investigated by our group3,10 and others.11 As anticipated, the square wave voltammogram12 (SWV) of FcOH (0.2 mM in 0.1 M NaCl) shows a single peak having a halfwave potential (E1/2) of 0.264 V vs Ag/AgCl (see Figure 3). Addition of β-CD-modified gold nanospheres (average diameter: 2.7 nm) has two pronounced effects: (i) it decreases the current of the voltammetric wave, and (ii) (9) For recent reviews, see: (a) Connors, K. A. Chem. Rev. 1997, 97, 1325-1357. (b) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 18751917. (10) (a) Isnin, R.; Salam, C.; Kaifer, A. E. J. Org. Chem. 1991, 56, 35-41. (b) Godı´nez, L. A.; Patel, S.; Criss, C. M.; Kaifer, A. E. J. Phys. Chem. 1995, 99, 17449-17455. (c) Wang, Y.; Kaifer, A. E. J. Phys. Chem. B 1998, 102, 9922-9927. (11) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc. 1985, 107, 3411-3417.
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Figure 4. Comparison of the changes induced by free and gold nanosphere immobilized β-CD receptors on the (A) half-wave potential and (B) anodic peak current measured in SWV with 0.2 mM FcOH solutions in 0.1 M NaCl. Open squares or circles represent data points obtained upon addition of free β-CD, while filled squares or circles represent data points obtained upon addition of CD-modified gold nanospheres.
it shifts the apparent half-wave potential to more positive values. Both effects clearly indicate that FcOH is forming inclusion complexes with the CD receptors immobilized on the gold nanospheres.10,11 This is further verified by addition of 1-adamantanol to the same solution. Adamantanol, being also an excellent guest for inclusion complexation by β-CD,13 competes effectively with ferrocenemethanol for the available CD binding sites on the surface of the gold particles (see Scheme 1). As a result, (12) The voltammetric measurements were recorded with a BAS 100 B/W electrochemical analyzer using a single compartment cell equipped with a glassy carbon working electrode (0.018 cm2), a Pt flag auxiliary electrode and a Ag/AgCl reference electrode. The parameters used to generate the excitation function for square wave voltammetric experiments were as follows: step size, 4 mV.; pulse amplitude, 25 mV.; frequency, 15 Hz. (13) Godı´nez, L. A.; Schwartz, L.; Criss, C. M.; Kaifer, A. E. J. Phys. Chem. B 1997, 101, 3376-3380.
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most of the ferrocenemethanol molecules are displaced from their binding sites and released back to the bulk solution, giving rise to a voltammetric response very similar to that recorded in the absence of the CD-modified gold nanospheres (Figure 3). Figure 4 provides a quantitative comparison of the effects on the voltammetric response of FcOH induced by free β-CD and β-SH-CD attached to the gold particles. Note that while the observed shift in the half-wave potential values follows a similar pattern as free or particle-immobilized receptors are added to the solution, the CD-induced current decrease is substantially more pronounced in the case of the nanosphere-immobilized CDs. Binding of ferrocenemethanol to a CD receptor attached to a gold particle decreases its effective diffusion coefficient much more than its binding to a free CD receptor, which affords a reasonable explanation for the experimental observations. In conclusion, we have shown that the reduction of AuCl4- in the presence of perthiolated CD receptors yields gold nanospheres (with diameters in the range 2-7 nm) modified with covalently attached CDs. These modified gold particles are soluble in water and behave as hosts toward conventional CD guests, as evidenced by our voltammetric data. This work suggests that CD-modified gold nanospheres may be useful as multisite hosts in aqueous media. Their potential analytical applications are currently being explored in our laboratory. Acknowledgment. This work was supported by the National Science Foundation (to A.E.K., CHE-9633434 and CHE-9982014). E.R. gratefully acknowledges a Maytag graduate fellowship from the University of Miami. Supporting Information Available: Six pages containing TEM images and size distribution histograms for gold nanoparticles modified with thiolated R-, β-, and γ-CD (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA991519F
