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Langmuir 2001, 17, 4836-4843
Stability and Exchange Studies of Alkanethiol Monolayers on Gold-Nanoparticle-Coated Silica Microspheres Michael S. Fleming and David R. Walt* The Max Tishler Laboratory for Organic Chemistry, Tufts University, Department of Chemistry, 62 Talbot Avenue, Medford, Massachusetts 02155 Received December 17, 1999. In Final Form: May 21, 2001 Self-assembled monolayers (SAMs) of carboxylate- and amine-terminated alkanethiols were formed on gold-nanoparticle-coated microspheres. The stability and place-exchange reactions of fluorescently labeled derivatives of these monolayers were studied as a function of time and storage conditions. Changes in the fluorescence intensity of the derivatized SAMs were monitored using fluorescence microscopy. Goldnanoparticle-coated microspheres were prepared by first derivatizing silica microspheres with a thiolcontaining silane followed by self-assembly of the gold nanoparticles onto the microsphere surfaces. Nanoparticle assembly was performed by mixing thiol-activated silica with a citrate-stabilized solution of gold nanoparticles in ultrapure water. The mean diameter of the gold particles was 14.5 ( 0.9 nm as determined by transmission electron microscopy (TEM). The mean diameter of the gold nanoparticles, after assembly onto the microsphere surfaces, was essentially unchanged (14.0 ( 2.8 nm). The relative surface coverage of the silica microspheres with gold was found to be dependent on the concentration of gold nanoparticles in solution and on the incubation time. Field-emission scanning electron microscopy (FE-SEM) was used to obtain high-resolution images of the microspheres before and after the gold coating procedure. As the surface coverage increased, the measured surface roughness of the silica microspheres also increased. Tapping mode atomic force microscopy (AFM) was used to measure the surface roughness of individual microspheres. The surface roughness of the microspheres and the chemical composition of the SAM were found to correlate with differences in SAM stability. SAMs on microspheres with relatively higher surface roughness were less stable and were more susceptible to place-exchange reactions. Carboxylate-terminated monolayers were found to be more stable than amine-terminated SAMs. A novel method for determining the relative amounts of exchange of fluorescently labeled alkanethiols between different monolayer-coated microspheres is reported. In this method, the transfer of fluorescently labeled alkanethiols composing SAMs on one set of gold-coated microspheres to nonlabeled SAMs on another set of gold-coated microspheres is monitored as a function of time using fluorescence microscopy. A collisiondependent mechanism was found to influence the rate and amount of exchange of alkanethiol between microspheres. The stability and exchange properties of SAMs on gold-nanoparticle-coated microspheres are presented in this paper.
Introduction Self-assembled monolayers (SAMs) provide a stable yet flexible method for immobilizing biological ligands on a surface. We were interested in determining whether goldnanoparticle-coated microspheres with an attached SAM could be used as a platform for biochemical assays. Traditionally, surface immobilization of biological ligands is performed using SAMs on a planar surface that has been coated with a noble metal (i.e., gold, silver, or platinum). When these surfaces are probed by an analytical method such as surface-enhanced Raman scattering (SERS) or surface plasmon resonance (SPR), ligandligand interactions may be evaluated without use of an external label (e.g., fluorescent dyes).1 Similarly, goldnanoparticle-coated microsphere substrates may be applicable to biological assay formats that rely on SERS or SPR for detecting ligand-ligand interactions. Self-assembled monolayers of alkanethiol derivatives on gold or other metal films have been well studied as substrates for biochemical analysis. For example, SAMs have been used as substrates for immobilizing ligands such as proteins2-8 (1) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. J. Am. Chem. Soc. 1999, 121, 9208-9214. (2) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem. 1999, 71, 777-790. (3) Jordan, C. E.; Corn, R. M. Anal. Chem. 1997, 69, 1449-1456. (4) Patel, N.; Davies, M. C.; Hartshorne, M.; Heaton, R. J.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 1997, 13, 6485-6490. (5) Frey, B. L.; Corn, R. M. Anal. Chem. 1996, 68, 3187-3193.
and antibodies.9-12 SAMs have also been used to immobilize various small molecule ligands for analyte detection.13-15 Stability and exchange studies of SAMs on gold-coated substrates have been performed previously.16-21 (6) Bladon, C. M.; Kindermann, J.; Sommerdijk, N.; Wright, J. D. J. Chem. Res., Synop. 1999, 1, 42-43. (7) Kroger, D.; Liley, M.; Schiweck, W.; Skerra, A.; Vogel, H. Biosens. Bioelectron. 1999, 14, 155-161. (8) Thomson, N. H.; Smith, B. L.; Almqvist, N.; Schmitt, L.; Kashlev, M.; Kool, E. T.; Hansma, P. K. Biophys. J. 1999, 76, 1024-1033. (9) Park, I. S.; Kim, N. Biosens. Bioelectron. 1998, 13, 1091-1097. (10) Feng, C. D.; Ming, Y. D.; Hesketh, P. J.; Gendel, S. M.; Stetter, J. R. Sens. Actuators, B 1996, 36, 431-434. (11) Lyon, L. A.; Musick, M. D.; Smith, P. C.; Reiss, B. D.; Pena, D. J.; Natan, M. J. Sens. Actuators, B 1999, 54, 118-124. (12) Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177-5183. (13) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413-1416. (14) Motesharei, K.; Myles, D. C. J. Am. Chem. Soc. 1998, 120, 73287336. (15) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055-2060. (16) Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639. (17) Schlenoff, J. B.; Li, M.; Hiep, L. J. Am. Chem. Soc. 1995, 117, 12528-12536. (18) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-1197. (19) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (20) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (21) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175.
10.1021/la9916575 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/13/2001
Stability and Exchange of Alkanethiol Monolayers
The majority of these studies, however, have been conducted on planar gold substrates. Many new applications of SAMs are being developed on spherical particles that either are composed of gold or are coated with gold.22-24 Recently, the mechanism of exchange reactions between monolayers formed around gold cluster molecules and solution alkanethiols has been examined.25,26 The stabilization of gold nanoparticles by the formation of alkanethiol monolayers has allowed the development of new sensor materials that rely on the variation in gold’s surface plasmon resonance as a function of interparticle distance. For example, Elghanian et al. and Storhoff et al. have developed a method for detecting oligonucleotides based on this principle.27,28 Recently, SAMs have been formed on gold deposited on silica gel using an electroless deposition process.29 It was demonstrated that SAMs formed in this way could be used as a support for biological ligands with application to various biochemical analyses.29 The substrates presented in this paper are similar to these materials except that the particles we used as templates for forming the gold surface are much smaller and have a smooth uniform surface as determined by tapping mode AFM. We have reported a multianalyte sensing approach in which a library of beads containing different sensing chemistries is used to create a sensor array.30 To examine the use of gold-nanosphere-coated microspheres as substrates for this sensing approach, we needed to study the stability and exchange properties of monolayers formed on the microspheres. When such microsphere substrates are used as multianalyte sensors, they must be synthesized well in advance of performing an experiment. Due to the exchange reactions that are known to occur between SAMs and the liquid phase,16,17,26 it was necessary to perform stability studies to verify that the integrity of individual sensors could be assured during the course of an analysis. We also wanted to determine if it was possible to observe the exchange of alkanethiols between SAMs on different microspheres. Exchange of alkanethiols between microspheres would compromise their use in multianalyte sensing. Prior to performing a multianalyte analysis, the different microspheres would be mixed and subsequently exposed to a sample solution. Quantification of analyte binding to each type of microsphere may then be accomplished by interrogating each microsphere sensor type. If exchange of ligand-derivatized alkanethiols were to occur at a relatively fast rate (i.e.,
