pubs.acs.org/Langmuir © 2010 American Chemical Society
Understanding the Mechanism of Amino Acid-Based Au Nanoparticle Chain Formation Manish Sethi and Marc R. Knecht* Department of Chemistry, University of Kentucky, 101 Chemistry-Physics Building, Lexington, Kentucky 40506-0055 Received January 15, 2010. Revised Manuscript Received March 17, 2010 Understanding the surface orientation and interactions between biomolecules and nanoparticles is important in order to determine their effects on the final structure and activity. At present, limited analytical techniques are available to probe these interactions, especially for materials dispersed in solution. We recently demonstrated that arginine, a simple amino acid, is able to bind to the surface of Au nanoparticles in a segregated pattern, which produces an electronic dipole across the structure. As a result, the formation of linear chains of Au nanoparticles occurred that was dependent upon of the concentration of arginine. Here, we present new information concerning the mechanism of assembly and demonstrate unique reaction conditions that can be used to directly control the assembly rate, and thus the size of the final superstructure that is produced. The assembly process was modulated by the arginine/Au nanoparticle ratio, the temperature of the system, the dielectric of the solvent, and the solution ionic strength, all of which can be used in combination to control the process. These effects were monitored using UV-vis spectroscopy, transmission electron microscopy, and dynamic light scattering. From these results, it is suggested that the second step of the assembly process, which is the formation of nanoparticle chains mediated by Brownian motion, controls the overall assembly rate and thus the size and orientation of the final superstructure produced. Furthermore, the reaction kinetics of the system have been studied from which rate constants and activity energies have been extracted for electrostaticbased nanoparticle assembly. This analysis indicates that the assembly/organization step is likely broken into two substeps with the formation of nanoparticle dimers occurring in solution first, followed by the oligomerization of the dimers to form the linear and branched chains. The dimerization step follows traditional second-order kinetics and is relatively fast, while the oligomerization process is quite complex and is anticipated to be slower than the dimerization step. These results are important, as they lay the basis for the subsequent use of this technique for the possible fabrication of electronic device components or as sensitive assays to probe the surface structure of nanomaterials.
Introduction The use of nanomaterials for commercial and industrial applications is becoming increasingly important. These applications rely upon the enhanced properties that are achieved by the quantum confinement effects that are observed at the nanoscale, which can dramatically alter the activity of the structure as compared to their bulk counterparts.1-5 Such effects are evident with Au nanoparticles that possess vibrant plasmon bands and surfaces that are easily functionalized with a variety of ligands ranging from oligonucleotides to hydrophobic chains.1 The surface functionalization is typically achieved using thiol-based chemistries. One highly employed technique uses the exchange of ligands onto the surface of citrate capped Au nanoparticles, which has been used to display proteins, oligonucleotides, and organometallic complexes, as well as many *To whom correspondence should be addressed. Phone: (859) 257-3789, email:
[email protected]. (1) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (2) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243– 18253. (3) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578–1586. (4) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Stone, J. W.; Sisco, P. N.; Alkilany, A.; Kinard, B. E.; Hankins, P. Chem. Commun. 2008, 544–557. (5) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (6) Laaksonen, P.; Kivioja, J.; Paananen, A.; Kainlauri, M.; Kontturi, K.; Ahopelto, J.; Linder, M. B. Langmuir 2009, 25, 5185–5192. (7) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547–1562. (8) Huo, F.; Lytton-Jean, A. K. R.; Mirkin, C. A. Adv. Mater. 2006, 18, 2304– 2306. (9) Lee, J.-S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093– 4096.
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other species.6-11 The surface display of these ligands is critical to their subsequent function, which can be especially true for different applications where the ligands possess a significant degree of control over the activity.12-19 For instance, for catalytically active nanomaterials, the ligands must present a sufficient fraction of the metallic surface to solution from which the reaction is processed while maintaining the particle stability.13,17-19 For a different capability, nanoparticle assembly, the ligands must be designed in such a way that controlled assembly of the component structures is achieved without the formation of bulk or uncontrolled aggregates.20-22 To address this issue, some groups have employed asymmetric surface functionalization techniques from which assembly can occur from (10) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462–463. (11) Balci, S.; Noda, K.; Bittner, A. M.; Kadri, A.; Wege, C.; Jeske, H.; Kern, K. Angew. Chem., Int. Ed. 2007, 46, 3149–3151. (12) Jakhmola, A.; Bhandari, R.; Pacardo, D. B.; Knecht, M. R. J. Mater. Chem. 2009 [Online early access]. (13) Pacardo, D. B.; Sethi, M.; Jones, S. E.; Naik, R. R.; Knecht, M. R. ACS Nano 2009, 3, 1288–1296. (14) Sethi, M.; Joung, G.; Knecht, M. R. Langmuir 2009, 25, 1572–1581. (15) Sethi, M.; Joung, G.; Knecht, M. R. Langmuir 2009, 25, 317–325. (16) Sethi, M.; Knecht, M. R. ACS Appl. Mater. Interfaces 2009, 1, 1270–1278. (17) Astruc, D. Inorg. Chem. 2007, 46, 1884–1894. (18) Diallo, A. K.; Ornelas, C.; Salmon, L.; Aranzaes, J. R.; Astruc, D. Angew. Chem., Int. Ed. 2007, 46, 8644–8648. (19) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692–704. (20) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914–13915. (21) Joseph, S. T. S.; Ipe, B. I.; Pramod, P.; Thomas, K. G. J. Phys. Chem. B 2006, 110, 150–157. (22) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066–13068.
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spatially registered ligands.8,23,24 While such techniques do produce a level of control for the organization process, the synthetic strategies used to achieve the required surface ligand display can be complicated and the assembly of the materials can be limited to a single site, which can minimize the complexity of the final superstructure. For many years, numerous synthetic strategies have been developed for the production of nanomaterials of various compositions using judiciously selected ligands.1,17,19,25 Unfortunately, the ligand set must be designed initially to achieve materials that are fully stable in solution. This, however, can minimize the subsequent activity of the structures by poisoning or disrupting the inorganic surface. As an alternative, biomimetic strategies have been developed that are modeled on biological routes toward inorganic materials.25-27 While the number of biologically observed inorganic minerals is limited, phage display techniques13,28-36 have been used to isolate peptides with the ability to produce nanomaterials of technologically interesting compositions such as BaTiO3,37 FePO4,29 and Pd.13 Here, mixing of the peptide with appropriate precursors can initiate and modulate the growth of their respected materials, which are usually controlled by binding of the peptide to the growing nanoparticle surface.38 For instance, using the Pd4 peptide, production of nearly monodisperse Pd nanoparticles is achieved, which can be used as highly reactive C-coupling catalysts.13 The activities of these bioinspired nanomaterials are likely controlled by the peptide surface, which dictates the nanoparticle interactions in solution. Recent studies suggest that the bioligands are able to form patterns on inorganic surfaces as a result of their complex binding motifs.16,39,40 Such patterns could be used to direct and control the activity of the resultant nanomaterials, but a more complete understanding of the ligand arrangement and binding events at the mechanistic level is required. Ideally, by knowing the binding strength and kinetics, control over the final (23) Xu, X.; Rosi, N. L.; Wang, Y.; Huo, F.; Mirkin, C. A. J. Am. Chem. Soc. 2006, 128, 9286–9287. (24) Sardar, R.; Heap, T.; Shumaker-Parry, J. S. J. Am. Chem. Soc. 2007, 129, 5356–5357. (25) Dickerson, M. B.; Sandhage, K. H.; Naik, R. R. Chem. Rev. 2008, 108, 4935–4978. (26) Mann, S. Biomineralization: Principals and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2002. (27) Uchida, M.; Klem, M. T.; Allen, M.; Suci, P.; Flenniken, M.; Gillitzer, E.; Varpness, Z.; Liepold, L. O.; Young, M.; Douglas, T. Adv. Mater. 2007, 19, 1025– 1042. (28) Reiss, B. D.; Mao, C.; Solis, D. J.; Ryan, K. S.; Thomson, T.; Belcher, A. M. Nano Lett. 2004, 4, 1127–1132. (29) Lee, Y. J.; Yi, H.; Kim, W.-J.; Kang, K.; Yun, D. S.; Strano, M. S.; Ceder, G.; Belcher, A. M. Science 2009, 324, 1051–1055. (30) Lee, S.-W.; Mao, C.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892– 895. (31) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665–668. (32) Mao, C.; Solis, D. J.; Reiss, B. D.; Kottmann, S. T.; Sweeney, R. Y.; Hayhurst, A.; Georgiou, G.; Iverson, B.; Belcher, A. M. Science 2004, 303, 213– 217. (33) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885–888. (34) Naik, R. R.; Stringer, S. J.; Agarwal, G.; Jones, S. E.; Stone, M. O. Nat. Mater. 2002, 1, 169–172. (35) Dickerson, M. B.; Jones, S. E.; Cai, Y.; Ahmad, G.; Naik, R. R.; Kr€oger, N.; Sandhage, K. H. Chem. Mater. 2008, 20, 1578–1584. (36) Dickerson, M. B.; Naik, R. R.; Stone, M. O.; Cai, Y.; Sandhage, K. H. Chem. Commun. 2004, 1776–1777. (37) Ahmad, G.; Dickerson, M. B.; Cai, Y.; Jones, S. E.; Ernst, E. M.; Vernon, J. P.; Haluska, M. S.; Fang, Y.; Wang, J.; Subramanyam, G.; Naik, R. R.; Sandhage, K. H. J. Am. Chem. Soc. 2008, 130, 4–5. (38) Diamanti, S.; Elsen, A.; Naik, R.; Vaia, R. J. Phys. Chem. C 2009, 113, 9993–9997. (39) So, C. R.; Kulp, J. L.; Oren, E. E.; Zareie, H.; Tamerler, C.; Evans, J. S.; Sarikaya, M. ACS Nano 2009, 3, 1525–1531. (40) So, C. R.; Tamerler, C.; Sarikaya, M. Angew. Chem., Int. Ed. 2009, 48, 5174–5177.
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Article Scheme 1. Proposed Mechanism for the Arginine-Based Assembly of Au Nanoparticlesa
a Step 1 includes the binding and self-segregation of arginine on the surface of the Au nanoparticles, while step 2 incorporates the assembly of the materials across the surface electronic dipole.
activity could be achieved. This is especially true in systems where the reaction conditions can be altered to modulate the final activity of the materials. Such control could be of great importance in the directed assembly of nanomaterials to achieve desirable complex superstructures. We have recently reported the formation of branched linear chains of Au nanoparticles in response to citrate surface replacement by the amino acid arginine.16 Upon an incomplete surface place exchange reaction, segregation of the arginine from the remaining citrate molecules on the surface occurs. This process is facilitated by the formation of a charged electrostatic network between the surface-bound amino acids based upon the interactions of the zwitterions of the arginine headgroups.14,16,41 As a result, an electronic dipole is imparted across the system surface from which assembly can occur with other nanoparticles in solution to form branched linear chains.16,42,43 Formation of this patchy charged layer is likely to be driven thermodynamically and has been observed previously using other alkylthiolates.42-46 In the present case, these results indicate that biomolecules may surface segregate, or bind in a specific fashion, due to molecular electronic effects or other biological forces. At present, it is somewhat unclear how this process occurs with the individual nanoparticles or what the controlling step is to achieve the linear structures. Such information is critical not only to the understanding of bioligand surface binding, but also for the control over the size, shape, and distribution of assembled superstructures in response to the ligand exchange. In the proposed mechanism of nanoparticle assembly based upon the arginine surface exchange reaction, multiple steps are envisioned to mediate superstructure formation.16 As shown in Scheme 1, this process could be controlled at two different (41) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. J. Phys. Chem. B 2004, 108, 4046–4052. (42) Bonell, F.; Sanchot, A.; Dujardin, E.; Pechou, R.; Girard, C.; Li, M.; Mann, S. J. Chem. Phys. 2009, 130, 034702. (43) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. Adv. Mater. 2005, 17, 2553–2559. (44) Carney, R. P.; DeVries, G. A.; Dubois, C.; Kim, H.; Kim, J. Y.; Singh, C.; Ghorai, P. K.; Tracy, J. B.; Stiles, R. L.; Murray, R. W.; Glotzer, S. C.; Stellacci, F. J. Am. Chem. Soc. 2008, 130, 798–799. (45) Jackson, A. M.; Myerson, J. W.; Stellacci, F. Nat. Mater. 2004, 3, 330–336. (46) Singh, C.; Ghorai, P. K.; Horsch, M. A.; Jackson, A. M.; Larson, R. G.; Stellacci, F.; Glotzer, S. C. Phys. Rev. Lett. 2007, 99, 226106.
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points: ligand exchange/surface adsorption of arginine, followed by nanoparticle chain formation in solution that is mediated by Brownian motion.16 Furthermore, similar assembly results have been observed by exchanging 2-mercaptoethanol onto Au nanoparticles,42,43 thus suggesting that partial ligand exchange processes may serve as a general mechanism for nanoparticle superstructure generation. The formation of such arrangements in a controlled manner is important for the further development of nanoscale components for electronic devices, medical diagnostics, and therapeutic applications. Unfortunately, a full understanding of the mechanistic aspects that lead to the assembled motifs remains unclear. By manipulating this process via judicious changes to the reaction conditions, it may be possible to specifically direct the spatial growth of the materials. This effect would likely be a function of the size, arrangement, binding strength, and surface concentration of the two ligands on the nanoparticle surface. Such control could also arise from the reaction conditions employed. For instance, by altering the temperature or solvent dielectric, changes in the reaction rate of assembly or nanoparticle arrangements could occur, thus allowing for increased control over the mechanism. Additionally, by controlling these solution conditions, valuable kinetic data such as reaction rate constants and activation energies concerning isotropic nanoparticle assembly could be determined to assist in controlling the superstructures that are generated.22 This kinetic information may also be directly related to the binding strength of the ligands in solution, thus possibly allowing for the development of sensitive analytical assays to characterize three-dimensional, in solution ligand binding events. Here, we present an in-depth analysis focused on the mechanism and kinetics of the assembly process mediated by the arginine surface exchange reaction with citrate capped Au nanoparticles. In the proposed mechanism of Scheme 1, the amino acid first binds to the Au nanoparticle surface in a segregated fashion to form a patchy charged network (step 1) from which the nanoparticle assembly process in solution can be achieved through electrostatic interactions (step 2). To fully examine this process, we have employed time- and temperature-resolved UV-vis spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS) as a function of four specific parameters: the concentration of arginine in solution, the temperature of the assembly process, the solvent dielectric, and the solvent ionic strength. By judicious selection of these conditions, the process can be directly modulated to control the rate of assembly from which kinetic rate constants and the activation energy of the formation of nanoparticle dimers can be extracted. Overall, the results indicate that the limiting step of the process is at the level of particle motion/orientation rather than the ligand exchange reaction. Furthermore, based upon the spectroscopic evidence, the assembly step appears to follow a process in which nanoparticle dimers form first, followed by the formation of longer chains by oligomerization of the dimers in solution. Such results are important for three key factors. First, we demonstrate a more complete understanding of the mechanism at play, which is envisioned for use with other biological molecules for the development of binding assays. Second, by using these methods, the kinetic rate of assembly can be readily controlled; therefore, it may be possible to direct the size and orientation of the final self-assembled structure in solution. Such a level of control may make it possible to design and grow specifically selected lengths, shapes, and orientations of nanoparticle superstructures. Third, the concepts learned from this study employing biomacromolecules may be applicable to other complex and/or nonbiological systems, which could be used 9862 DOI: 10.1021/la100216w
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as a basis to understand general trends in electrostatic materials assembly.
Experimental Section Chemicals. HAuCl4 3 3H2O, sodium citrate tribasic dihy-
drate, and L-arginine were purchased from Sigma-Aldrich (St. Louis, MO). Ethanol (95%, ACS grade) was purchased from Pharmco-AAPER. All chemicals were used as received. Milli-Q water (18 MΩ cm; Millipore, Bedford, MA) was used throughout. Preparation of Citrate-Capped Au Nanoparticles. Au nanoparticles were synthesized using the citrate reduction method.47 Prior to the reaction, all glassware was thoroughly washed using aqua regia (3:1 HCl/HNO3) and then fully rinsed with deionized water to remove any acidic species. For the reaction, a 50.0 mL aqueous solution of 1.00 mM HAuCl4 was refluxed while vigorously stirring. Once refluxing of the solution was achieved, 5.00 mL of an aqueous 38.8 mM sodium citrate solution was added in a single injection. Immediately, the solution changed from pale yellow to colorless. The reaction was allowed to continue to reflux for 15.0 min from which a final solution color of wine red was developed. After the reaction, the solution was allowed to cool to room temperature before use.
Arginine-Based Assembly of Citrate-Capped Au Nanoparticles. For this analysis, various volumes of a 400 μM aqueous arginine stock solution were added in a 1.00 cm path length quartz cuvette, to result in final amino acid concentrations of 0, 20.0, 40.0, 80.0, 120, 160, and 200 μM. These concentrations were selected as they represent a 0-, (1.00 104)-, (2.00 104)-, (4.00 104)-, (6.00 104)-, (8.00 104)-, and (1.00 105)-fold excesses of arginine as compared to the Au nanoparticles, respectively. These samples are designated as 0, 10K, 20K, 40K, 60K, 80K, and 100K throughout the text, where K = 1000. The reaction volume was then diluted to 2.40 mL for each sample before adding 600 μL of the prepared Au nanoparticle solution to each cuvette. As a result, the final volume of the reaction solution was 3.00 mL with a 2.00 nM concentration of Au nanoparticles.16 The reaction was allowed to proceed for 1.00 h while being monitored using UV-vis spectroscopy at various temperature and dielectric conditions as discussed below. Analysis of the Reaction Temperature. Each reaction condition described above was studied as a function of temperature at 10.0 °C, 20.0 °C, 30.0 °C, 40.0 °C, 50.0 °C, 60.0 °C, and 70.0 °C. For this analysis, the UV-vis cuvette holder containing eight wells was thermally controlled using an Isotemp 3016S recirculating chiller (Fisher Scientific). After addition of the solvent and arginine solutions to the cuvettes, the mixtures were allowed to equilibrate with a set temperature value for 15.0 min before the addition of Au nanoparticles. Immediately after addition of the nanoparticle solution, UV-vis spectra were obtained for 1.00 h at 30.0 s intervals. Identical procedures were also employed for DLS analysis. Analysis of the Solvent Dielectric. To ascertain the effects of the solution dielectric, identical reaction analyses were completed as described above; however, various volumes of the aqueous solvent were replaced with EtOH. Three separate analyses were completed where 0.50 mL, 1.00 mL, and 1.50 mL of the aqueous medium was replaced with EtOH at the appropriate temperature conditions. For each analysis, a control study was conducted simultaneously in neat water to ensure that the observed results were the effect of the lower dielectric solution based upon the added EtOH. Analysis of the Solution Ionic Strength. To determine the effect of solution ionic strength on the nanoparticle assembly process, reaction solutions containing 400 μM and 4.00 mM NaCl (47) Frens, G. Nat. Phys. Sci. 1973, 241, 20–22.
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were used. For each analysis, a positive control was conducted simultaneously at the appropriate NaCl reaction concentration to ensure that the observed results were the effect of arginine. Characterization. Time-resolved UV-vis spectra were obtained using an Agilent 8453 UV-vis spectrometer, employing 1.00 cm path length quartz cuvettes (Starna). Each cuvette was washed with aqua regia and rinsed with water prior to the analysis. All spectra were background subtracted against water, which is the main reaction solvent. Over a period of 1.00 h, spectra were collected at 30.0 s intervals. TEM images of the assembly process were obtained using a JEOL 2010F transmission electron microscope having a resolution of 0.19 nm and operating at 200 kV. A total volume of 5.00 μL of the reaction solution was pipetted onto the surface of a 400 mesh Cu grid coated in a thin layer of carbon (EM Sciences) and allowed to dry in a desiccator. DLS analyses were conducted on a Zetasizer Nano ZS System (Malvern Inc.) at 1.00 min intervals over a total time of 1.00 h.
Results and Discussion To understand the effects of amino acid surface binding, the assembly process was monitored over a variety of reaction conditions to probe the mechanism of superstructure formation and extract relevant kinetic information. The proposed mechanism of Scheme 1 invokes a first step of arginine/Au nanoparticle surface binding and segregation, followed by a second step of electrostatically mediated assembly controlled by the electronic dipole on nanoparticle surface. This second step is likely to be facilitated by Brownian motion and electrostatic effects of the individual particles in solution, which would control the orientation of the dipole on the nanoparticles in solution for subsequent interactions and assembly. By probing this synthetic process, it may also be possible to enhance the ability to control the final size, shape, and orientation of the Au nanoparticle superstructures to form desirable multidirectional materials. Such structures may prove to be significant in the formation of device components for optical and electronic applications. At present, the chemical information about these two events is both minimal and challenging to study due to available techniques, which further demonstrates the need to fundamentally understand the observed process. Initially, the arginine-based Au nanoparticle assembly process was monitored at different reaction temperatures between 10.0 and 70.0 °C using UV-vis spectroscopy. For all spectroscopic studies, triplicate analyses verified the reproducibility of the results. For this analysis, the system temperature was maintained employing a recirculating water bath, which minimized thermal fluctuations. As shown in Figure 1b, when the analysis was conducted at 30.0 °C, similar results were obtained as previously described.16 The UV-vis spectra of the reactions with different arginine concentrations after 1.0 h are presented on the left, while a plot of the absorbance intensity at 665 nm as a function of time is shown on the right. When no arginine is added to the system, a single plasmon band from the independent Au nanoparticles is observed at 520 nm, which is consistent with particles of approximately 15 nm in diameter.47 As the concentration of arginine in the reaction increases, the growth of a second peak at 665 nm is observed, which is associated with the formation of the assembled structures using arginine. 16 Additionally, the absorbance at 520 nm is maintained throughout the assembly process with only a decrease in the intensity over time; therefore, the samples that demonstrate a degree of assembly possess two plasmon bands. This effect is likely due to the formation of the one-dimensional (48) Sun, Z.; Ni, W.; Yang, Z.; Kou, X.; Li, L.; Wang, J. Small 2008, 4, 1287– 1292.
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Figure 1. UV-vis analysis of the temperature effects on the arginine concentration dependent assembly of Au nanoparticles studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. The plots on the left demonstrate the UV-vis spectra obtained after 1.00 h of reaction for each arginine/Au nanoparticle ratio studied, while the plots on the right present the absorbance intensity of the 665 nm peak as a function of time.
structures, where two absorbances would be anticipated due to the shape of the assembled motif producing both a transverse and longitudinal plasmon.2,48 Furthermore, the growth of the assembled structure is dependent upon the concentration of arginine such that the 100K sample demonstrated the generation of the strongest absorbance at 665 nm. A plot of the 665 nm absorbance intensity over time for each sample produced linear growth, which became saturated for the reactions at the highest arginine concentrations (80K and 100K). For instance, minimal to no growth was observed for the 0, 10K, and 20K samples over the 1.0 h time frame; however, for the 40K sample, a clear linear increase in the absorbance was observed. This increase over time was further noted for the 60K sample, which also followed a similar linear growth trend, but at a higher rate. This trend was continued for the 80K and 100K samples, where the 100K sample reached a maximum absorbance that saturated after 40.0 min. Figure 1a displays the same analysis of arginine effects on Au nanoparticle assembly; however, the reaction temperature was lowered to 10.0 °C. At this temperature, a noticeable shift in the assembly rate was observed for all of the arginine concentrations studied. For the 40K sample and those at lower arginine concentrations, no change was observed over the reaction time in the UV-vis spectra of the materials; no growth at 665 nm was demonstrated, which suggested that the materials remained unassembled at this temperature. It was not until the 60K sample that an observable shift in the optical properties of the Au nanoparticles was demonstrated. With this sample, a minimal increase in the absorbance at 665 nm was observed, with no clear peak formation. A nearly identical result was observed for the DOI: 10.1021/la100216w
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Figure 2. TEM analysis of the temperature effects on the assembly process employing arginine/Au nanoparticle ratios of 0, 20K, 40K, and 80K studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. TEM images were obtained after a reaction time of 1.00 h. The scale bars in the figures represent 50 nm, while the scale bars in the insets represent 5 nm. The graphs on the right demonstrate the percentage of nanoparticles that are independent, linearly assembled, or randomly oriented.
80K sample, as compared to the 60K sample; however, the absorbance intensity was greater over the specified time frame. The only sample at 10.0 °C that produced a semiresolved peak shoulder at 665 nm was the 100K sample, where the growth of the assembled band did not reach saturation over the time of the analysis. For the 100K sample, while peak growth was evident, it resulted in only a linear absorbance growth over time that reached a final intensity that was significantly reduced as compared to the same analysis conducted at 30.0 °C. UV-vis studies of the reaction at a system temperature of 60.0 °C are presented in Figure 1c. For these reactions, a dramatic increase in the assembly rate is apparent as compared to the studies at lower temperatures. No change is observed in the control study of Au nanoparticles in the absence of arginine, which indicates that the materials are stable at the elevated temperature. A noticeable increase in the absorbance was detected for both the 10K and 20K samples, above the background of the Au nanoparticles, at 665 nm, which suggests that some degree of assembly may be occurring for these materials at the higher temperature. As the concentration of arginine increased, a larger degree of assembly is demonstrated. For the 40K sample, the absorbance at 665 nm increases over time, to which the rate of the absorbance growth slows considerably after 25.0 min; however, an increase persists after this time point at a slower rate. For the 60K sample, saturation of the 665 nm peak intensity occurs at 15.0 min and the absorbance is maintained for the duration of the experiment. As the concentration of the amino acid is further increased in the 80K and 100K samples, the absorbance growth rate is initially rapid for the first 10.0 min; however, after this time point, the intensity begins to decrease. At 1.0 h, the 80K and 100K samples demonstrate broad absorbances that are red-shifted in proportion to the arginine concentration: the higher the arginine concentration, the further to the red the peak is shifted. This suggests that larger chains and/or bulk-like materials develop as the reaction progresses, which may result from the increased rate 9864 DOI: 10.1021/la100216w
of assembly. It is known that, at higher arginine concentrations, bulk materials are prepared due to the higher coverage of arginine on the nanoparticle surface, which minimizes the electrostatic stability of the materials in solution.16 Similar results are likely to occur at shorter time frames at lower arginine/Au nanoparticle ratios if the rate of assembly is increased at the higher temperatures. This would be observed with a decrease in the intensity of the 665 nm absorbance due to peak shifting and materials precipitation. Indeed, such results are observed as shown in Figure 1c. TEM analysis of the materials at the different arginine concentrations as a function of reaction temperature is presented in Figure 2. Specifically, Figure 2b displays the results at 30.0 °C for the 0, 20K, 40K, and 80K samples. These arginine/Au nanoparticle ratios were chosen to demonstrate the results over a range of conditions to fully observe the effects of both arginine concentration and temperature. In addition, the degree of assembly for each of the materials was extracted from the TEM images and is presented in the bar chart of the figure. For this statistical analysis, materials were considered linearly assembled when three Au nanoparticles were aligned with an interparticle spacing of e1.0 nm, while random orientations were populated in the other category. For the 0 control sample, mostly independent Au nanoparticles were observed on the TEM grid surface as anticipated. This conforms directly to the UV-vis results, which indicated that the nanoparticles remained independent as the 520 nm plasmon band remained unchanged. For the 20K sample, two different sets of materials were observed: individual as well as short linear chains of Au nanoparticles. As the arginine/Au nanoparticle ratio increased in the 40K sample, a preference for a larger set of linear chains was observed with a degree of branching over random orientations. Furthermore, as shown in the inset of the 40K sample, in the nanoparticle linear chained structures, while the materials are assembled in a controlled fashion, minimal to no necking is observed between the particles. Langmuir 2010, 26(12), 9860–9874
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For the 80K sample, larger linear structures are observed in addition to random structures. In these samples, a high degree of branching is noted, as well as a degree of nanoparticle aggregation, which is demonstrated in the inset. The necking between particles occurs when two species are assembled in solution and the degree of repulsion at the assembled region is significantly minimized to allow for mixing of the metallic components, as discussed below. This is likely a function of the surface electronic dipole as an effect of the arginine display. Over time, as this process ensues, bulk materials precipitation would be anticipated. Indeed, a dark black precipitate is observed after allowing the reaction to proceed overnight. Figure 2a presents the TEM analysis of the materials studied at 10.0 °C. Consistent with the UV-vis results, no assembly was observed in the 0 control sample; however, at higher arginine/Au nanoparticle ratios, linear assembly was observed. For instance, in the 20K sample, linear chains were noted on the TEM grid surface in addition to random structures. Furthermore, such effects were also observed for the 40K and 80K samples with higher degrees of linear assembly and branching. These assembly results are similar to those observed for the same samples at 30.0 °C; however, they are inconsistent with the UV-vis results, which indicated a lack of assembly at low ratios with minimal degrees of assembly at the highest ratios. The observed TEM effects are likely due to TEM sample preparation. For this, 5.00 μL of the reaction sample at 10.0 °C is pipetted onto the TEM grid surface and the solution is allowed to evaporate overnight in a desiccator in the refrigerator at 4.00 °C. While the temperature is maintained, due to the evaporation process, the particle concentration drastically increases over time, which causes the particles to come closer together. As a result, the particles could assemble during the evaporation process, thus resulting in the observed chain formation. Similar evaporation effects have previously been observed for Au nanorods, where the sample preparation process resulted in the formation of organized structures.49 Furthermore, since no assembly was observed in the arginine-free control, this suggests that the arginine is able to bind and segregate on the surface at the low temperature. While the TEM results are unanticipated, DLS studies, discussed below, confirm minimal to no assembly for the nanoparticles in solution at 10.0 °C, which is consistent with the UV-vis analysis. TEM examination of the assembly process at a temperature of 60.0 °C is displayed in Figure 2c. For these materials, a higher degree of bulk and random aggregation is observed, as compared to those samples studied at lower temperatures; however, a clear preference for linear assembly dominates the system. From this analysis, the control again demonstrated mostly independent Au nanoparticles in the absence of arginine. For the 20K sample, a large degree of assembly and some aggregation are observed to form Au agglomerates. The larger structures likely arise from the rapid assembly process, which could lead to the observed aggregation. For the 40K sample, linear branched Au nanoparticle chains are observed, with a significant degree of neck formation between the aligned particles. The HR-TEM inset demonstrates the formation of four interconnected particles that are linearly aligned with a significant degree of agglomerization between the species. The mixing of the metallic components of the particles is likely due to the surface electronic dipole and individual nanoparticle spacings that are minimized for neck formation at the elevated temperatures. Similar effects are observed with the 80K (49) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635–8640.
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Figure 3. UV-vis analysis of the effects of the amount of EtOH added to the reaction system for the assembly of Au nanoparticles using the (a) 0, (b) 20K, (c) 40K, and (d) 80K samples. The plots on the left demonstrate the UV-vis spectra obtained after 1.00 h of reaction for each volume of EtOH studied, while the plots on the right present the absorbance intensity of the 665 nm peak as a function of time.
sample where linear necked structures are observed with additional bulk-like aggregates. In addition to temperature effects, changes to the solvent dielectric are likely to alter the ability to assemble the Au nanoparticles in the presence of arginine. By lowering the dielectric of the solvent, the degree of charge shielding of the electronic dipole along the nanoparticle surface should be minimized, which would increase the rate of assembly; therefore, it is anticipated that the solvent dielectric and the rate of assembly should be inversely proportional. To study this factor, the Au nanoparticle surface exchange process was monitored where 0.00, 0.50, 1.00, and 1.50 mL of the aqueous solvent was replaced with EtOH at 20.0 °C. As such, the dielectric of the solvent system should decrease for those samples that possess a higher volume of EtOH. The effects of lowering the solvent dielectric on the assembly and optical properties of the materials for the 0, 20K, 40K, and 80K samples are shown in Figure 3, while the rest are presented in Supporting Information Figure S2. Specifically, the control analysis in the absence of arginine using the various volumes of EtOH is presented in Figure 3a. For these materials, DOI: 10.1021/la100216w
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no changes in the UV-vis spectra over 1.00 h are observed using the solvents of different dielectrics. This result is important, as it indicates that the materials are stable in solution and do not aggregate in response to EtOH; thus, any changes observed in the presence of EtOH can be directly attributed to the effects of the amino acid. Figure 3b displays the effects of decreasing the solvent dielectric for the 20K sample. For this analysis, as shown in the plot on the left, minimal to no changes in the UV-vis spectra of the 20K materials after 1.00 h in a solvent of pure water or with 0.50 mL of EtOH are observed; only a single plasmon band at 520 nm is detected, which suggests that no assembly occurs for these reactions. When 1.00 mL of water is replaced with EtOH in the reaction, formation of the 665 nm assembled peak is observed for the 20K sample within the reaction time frame. For this analysis, the absorbance increases linearly over 1.00 h and results in the generation of an absorbance shoulder at the higher wavelength when the reaction is complete. Furthermore, when the amount of EtOH used in the reaction is increased to 1.50 mL, the rate of assembly is even faster. Under these conditions, a clear and sharp plasmon band is observed at 665 nm after 1.00 h that rapidly grows during the analysis. The peak continues to grow for approximately 30.0 min, after which it begins to saturate, indicating that the assembly process is nearly complete. Further studies for the effect of solvent dielectric for the 40K and 80K samples demonstrated nearly identical results as compared to the 20K sample, only faster assembly rates of spectroscopic change were noted for the higher ratio analyses. As shown in Figure 3c, for the 40K studies, minimal assembly was observed from the water-only control sample; however, when 0.50 mL of EtOH was used in the reaction, a new peak shoulder was observed to grow over the time frame of the reaction. The shoulder intensity at 665 nm grew linearly throughout the reaction at a slower rate as compared to the reactions that possessed 1.00 and 1.50 mL of EtOH. For these reactions, the 665 nm absorbance grew rapidly in intensity and demonstrated distinct plasmon bands after 1.00 h (shown in the left panel of the figure). Again, the rate of assembly was directly related to the amount of EtOH in the solution; as the EtOH volume increased, the rate of assembly increased. For instance, with the reaction containing only 1.00 mL of EtOH, 665 nm peak growth occurred linearly for the first 30.0 min of the reaction, but trailed off at a slower rate after this time point; however, for the reaction possessing 1.50 mL of EtOH, rapid linear peak growth occurred for the first 15.0 min and then was saturated at longer time points. Nearly identical results are achieved for the 80K sample, Figure 3d, with various amounts of EtOH, but the main difference was a faster rate of assembly as compared to those samples with lower arginine/Au nanoparticle ratios. Additionally, minor assembly was observed for the 80K reaction in the water-only solution, consistent with the abovedescribed results, and a sharp 665 nm absorbance peak was observed for all reactions completed in the presence of EtOH. The rate of assembly was again inversely proportional to the solvent dielectric, which is consistent with the 20K and 40K studies. The combination of temperature and solvent dielectric effects for the assembly of Au nanoparticles for all of the arginine/Au nanoparticle ratios was further studied and is presented in Figure 4 for the results obtained using 1.00 mL of EtOH at 10.0 °C, 30.0 °C, and 60.0 °C, while the other temperatures are presented in Supporting Information Figure S4. The studies employing 0.50 and 1.50 mL of EtOH follow an identical trend with respect to temperature and are presented in Supporting Information Figures S3 and S5, respectively. For the studies at 10.0 °C, Figure 4a, 9866 DOI: 10.1021/la100216w
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Figure 4. UV-vis analysis of the temperature effects on the arginine concentration-dependent assembly of Au nanoparticles in the presence of 1.00 mL of EtOH studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. The plots on the left demonstrate the UV-vis spectra obtained after 1.00 h of reaction for each arginine/Au nanoparticle ratio studied, while the plots on the right present the absorbance intensity of the 665 nm peak as a function of time.
it is evident that the rate of arginine-mediated assembly in the presence of 1.00 mL of EtOH is increased as compared to the reactions studied using a water-only solvent (Figure 1a). With the addition of 1.00 mL of EtOH, all reactions at arginine/Au nanoparticle ratios of g10K demonstrate a degree of nanoparticle assembly at 10.0 °C. Even for the lowest ratio, 10K, an increase in the absorbance at 665 nm is observed that increases linearly during the reaction as compared to the 0 control. Again, the rate of absorbance growth follows a linear trend for the materials as a function of arginine concentration to prepare the assembled structures. Furthermore, at the 1.00 h time point, sharp plasmon bands are observed for the materials at ratios of g40K (left panel), which further indicate the rapid rate of assembly. Note that for the materials studied at the same temperature in the water-only solvent demonstrated minimal to no assembly and that even at the highest ratio, 100K, only a slight peak shoulder was observed. The rates of assembly are further increased for the reactions at higher temperatures employing the solvent that contains 1.00 mL of EtOH. For the reactions studied at 30.0 °C, Figure 4b, assembly is observed for all samples that possess arginine and the rate of assembly increases for reactions with higher amino acid concentrations. For instance, for the 20K sample, an absorbance shoulder develops during the 1.00 h reaction time frame from which the intensity increases linearly. For all of the other samples with ratios g40K, the rate of assembly is rapid, which, after a certain time period, saturates at roughly the same absorbance value. In fact, at the end of the 1.00 h reaction time, all samples Langmuir 2010, 26(12), 9860–9874
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Figure 5. TEM analysis of the temperature effects on the assembly process in the presence of 1.00 mL of EtOH with arginine/Au nanoparticle ratios of 0, 20K, 40K, and 80K studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. TEM images were obtained after a reaction time of 1.00 h. The scale bars in the figures represent 50 nm, while the scale bars in the insets represent 5 nm. The graphs on the right demonstrate the percentage of nanoparticles that are independent, linearly assembled, or randomly oriented.
with ratios g40K demonstrate very similar UV-vis spectra with nearly identical 665 nm absorbance peaks, which suggests that the assembly rate is significantly increased for these samples, with minimal formation of bulk materials on the time scale of the study. For the reactions processed at 60.0 °C, presented in Figure 4c, the reaction rate is extremely fast and generates UV-vis spectra for the samples at ratios g20K that demonstrate broad red-shifted spectra, indicative of large superstructures and/ or bulk material formation. In fact, the reaction rate is so fast that the materials in the 80K and 100K samples are completely aggregated before their first UV-vis spectrum can be obtained, which is evident by the high absorbance value at 665 nm at the initial time point. Together, the observed UV-vis results suggest that the solvent dielectric can significantly alter the rate of assembly such that, as the dielectric decreases, the rate of arginine-based Au nanoparticle assembly increases. TEM analysis of the 0, 20K, 40K, and 80K materials reacted for 1.00 h at 10.0 °C, 30.0 °C, and 60.0 °C in the presence of 1.00 mL of EtOH are displayed in Figure 5. For all of the materials, regardless of the reaction conditions, those systems processed in the absence of arginine were dominated by independent Au nanoparticles. Figure 5a specifically presents the TEM images of the materials studied at 10.0 °C. For the samples where arginine was present, branched linear chains are observed for all reactions with a minor degree of random assemblies. For the most part, nearly all of the nanoparticles tend to form linear chains regardless of the ratios; however, for the 20K sample, individual Au nanoparticles not aligned in chains are occasionally observed. For the materials studied at 30.0 °C, Figure 5b, linear assemblies were again preferentially observed; however, a slight increase in the degree of random orientations were noted. For these materials, while most of the nanoparticles in the assembled state remained unagglomerated, meaning lacking neck formation between multiple particles, a degree of larger bulk-like aggregates were observed for the 80K reaction. When the analysis was studied at a temperature of 60.0 °C, shown in Figure 5c, branched linear Langmuir 2010, 26(12), 9860–9874
chains were again detected, but a higher degree of nanoparticle necking was noted. For all samples, many of the nanoparticles demonstrated mixing of the metal atoms between particles to form the long linear structures, which were likely produced based upon the initial nanoparticle linear chain formation. This is again caused by the increased assembly rate and changes to the dipole shielding, which rapidly position the nanoparticles in sufficiently close contact to allow for direct agglomerization between the materials. While the UV-vis and TEM results suggest that the Au nanoparticles are assembling in response to the effects of the arginine addition, DLS studies were conducted to monitor the aggregation process in solution under the various reaction conditions. This is important to rule out the effects of solvent evaporation on the assembly process as observed by TEM imaging, which has been shown to affect the assembly state of nanomaterials.49 It should be noted that DLS analysis is dependent upon aggregate size, shape, dispersity, and hydrodynamic radius, as well as the index of refraction of the solution, all of which are quite complex and unable to be directly accounted for in the present system. As such, the results attained from this technique should be viewed as a qualitative analysis of the aggregation process and not as an accurate reflection of the individual assembly size in solution. Furthermore, the materials presented a high degree of polydispersity. For the presented data, the average aggregate size is presented; however, a significant number of smaller-sized materials associated with nanoparticle dimers are present within the system. The overall analysis is plotted in Figure 6 for the 0, 20K, 40K, and 80K samples at 10.0 °C, 30.0 °C, and 60.0 °C with 0.00 to 1.50 mL of EtOH. Individual plots of the DLS studies at all reaction conditions are additionally presented in the Supporting Information Figures S6-S8. Figure 6a specifically displays the results obtained at 10.0 °C. When considering the reactions processed using an aqueousonly solvent, the materials in the absence of arginine displayed a particle size of 17.5 nm, which is consistent with the 15.0 nm DOI: 10.1021/la100216w
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Figure 6. DLS-based determination of aggregate size in solution after 1.00 h. The three-dimensional graphs are plotted as functions of the arginine/Au nanoparticle ratio, volume of EtOH in the system, and size of the aggregates achieved at temperatures of (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C.
particle size observed via TEM. As the concentration of arginine increased, a small increase in the observed size occurred over the 1.00 h time period, with a maximum aggregate size of 31.5 nm for the 80K sample. This confirms that a small degree of assembly for the materials happens in an aqueous only solution at 10.0 °C, as is consistent with the UV-vis results. When the same reaction conditions were used with 0.50 mL of EtOH, an increase in the aggregate dimension was observed as compared to the results achieved for the water-only solvent. For instance, for the 20K, 40K, and 80K samples, aggregate sizes of 53.4 nm, 62.1 nm, and 103 nm, were noted at 1.00 h, respectively, as compared to the sizes of 22.5 nm, 26.1 nm, and 31.5 nm achieved in the water-based system. Both trends of increasing aggregate sizes for increasing arginine/Au nanoparticle ratios, as well as increasing sizes for solvents of lower dielectric constants (higher volumes of EtOH), were conserved for all samples across the analysis. As anticipated, based upon these trends, the 80K sample with 1.50 mL of EtOH produced the largest aggregate size of 2438 nm at 10.0 °C. DLS analysis of the arginine/Au nanoparticle system at a temperature of 30.0 °C is presented in Figure 6b, which displays similar sizing trends as compared to those observed at 10.0 °C. For materials studied under identical conditions of arginine/Au nanoparticle ratio and solvent composition, larger aggregate sizes were observed for the reactions processed at the higher temperature. For instance, for the 80K materials in an aqueous-only 9868 DOI: 10.1021/la100216w
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solvent, an aggregate size of 45.6 nm was detected, as compared to the value of 31.5 nm achieved at 10.0 °C. Furthermore, the trends previously determined for the effects of arginine concentration and solvent dielectric were maintained; however, the size of the aggregate after 1.00 h for the 80K sample using 1.50 mL of EtOH (2283 nm) does not fit this trend, as it is clearly smaller in size as compared to the 80K sample with 1.00 mL of EtOH (2348 nm) or the 40K sample with 1.50 mL of EtOH (2630 nm). This change from the anticipated trend is likely due to the formation of excessively large structures that precipitate from solution, thus leaving only smaller assembled materials dispersed in solution. Indeed, as shown in Supporting Information Figure S7d, the 80K sample in 1.50 mL EtOH shows an increasing aggregate size over time that maximizes to 3035 nm at 40.0 min, after which the size decreases to 2283 nm at 1.00 h. In addition, a black precipitate is also noted from this sample after 1.00 h. Further studies of the same set of materials at 60.0 °C (Figure 6c) demonstrates identical trends; however, a larger degree of materials assembly/aggregation followed by precipitation is observed. For this temperature, the reactions processed at 80K using 1.00 and 1.50 mL of EtOH and at 40K studied with 1.50 mL of EtOH showed precipitation due to the rapid assembly process, which resulted in the observed deviations from the expected sizes; the other samples that were examined demonstrated anticipated aggregate sizes as dictated by the hypothesized trends. From these results, a few key trends can be elucidated that could assist in understanding the Au nanoparticle assembly process occurring in solution as a result of amino acid surface exchange. First, the rate of assembly is dependent upon the arginine/Au nanoparticle ratio such that, as the concentration of the amino acid increases, a more rapid assembly process is observed. Second, the temperature of the reaction system affects the process in such a way that, as the temperature increases, the formation of linear structures occurs faster. Third, as the dielectric constant of the solvent employed in the reactions decreases, the rate of the assembly of the nanoparticles increases. Fourth, by combining the effects of temperature and solvent composition, the rates of assembly can be further tuned (i.e., by raising the temperature and lowering the solvent dielectric). Taken together, these results suggest that the second step of the amino acid-based assembly process presented in Scheme 1, which is facilitated by Brownian motion, controls the overall mechanism and may be able to be manipulated by the reaction conditions to dictate the rate of assembly and, subsequently, the aggregate size. The initial temperature-based studies change the assembly process specifically by increasing the Brownian motion of the nanoparticles dispersed in solution, which directly affects the kinetic assembly rate discussed below. To that end, as the temperature increases, the velocity and tumbling of the particles increases as well.50 As a result, the nanoparticles can more readily orient themselves, with respect to the electronic dipole of neighboring materials, and form the interactions responsible for nanoparticle alignment. In addition, by lowering the temperature, the Brownian motion of the materials should significantly decrease;50 thus, the materials assembly process should be minimized or prevented, especially when approaching the freezing point of the solvent system. The observed results correspond to this theory, thus suggesting that the reaction temperature is directly affecting the second step of the process. A second possible temperature effect could alter the first step of the process, which could change the ability of the amino acids on the surface to (50) Mazo, R. M. Brownian Motion: Flucuations, Dynamics, and Applications; Oxford University Press: New York, 2002; Vol. 112.
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segregate and form the charged network/electronic dipole required for assembly. In this event, at elevated temperatures, sufficient thermal energy would be available to overcome the thermodynamic stability of the electrostatic network between the amino acid residues, thus causing a scrambling of the surface patterns to form a mixed monolayer, which would disrupt the formation of the electronic dipole required for assembly. Previous results have demonstrated this effect for the electrostaticbased assembly and disassembly of nanomaterials.51 Under this hypothesis, the assembly process should decrease at higher temperatures; however, this is exactly the opposite of the observed trend, which suggests that the temperature effects do not alter the ability to form the surface segregated layer and electronic dipole. Furthermore, this also suggests that the electrostatic interactions of the amino acid residues to form this type of electronic network are relatively strong to maintain the assembled structures at higher temperatures. The effects of changing the solvent dielectric are also likely to change the ability of the nanoparticles to assemble at the second step by changing the shielding of the electronic dipole. By adding increasing volumes of EtOH, the dielectric constant of the solvent will decrease. As a result, the shielding of the charged patchy surface of the nanoparticles will decrease, which will enhance the electrostatic interactions between the individual particles in solution.52 By increasing these interactions, the species will orient more quickly in a fashion that minimizes electrostatic repulsions, which is anticipated to occur through the assembly of the materials across the electronic dipole. This event has been directly observed by the studies presented using EtOH, thus suggesting that the Brownian motion and electronic character of the surface of the materials dictate the assembly rate. Furthermore, the combination of both temperature variations and solvent composition could be used to control the assembly process without interfering with each other. This suggests that, by judiciously selecting the appropriate reaction conditions, the ability to fabricate nanoparticle chains of certain dimensions may be possible using this method. Since it appears that the assembly process is controlled mainly by electrostatic interactions, changes to the solution ionic strength should also directly affect the mechanism via charge screening. To study this effect, the assembly process was monitored in the presence of two different NaCl concentrations, 400 μM and 4.00 mM, as such levels do not adversely affect the stability of the initial citrate-capped Au nanoparticles.53 Figure 7 presents the UV-vis spectra after a 1.0 h reaction time for the 4.00 mM NaCl reaction on the left with a plot of the 665 nm absorbance increase on the right. As shown in Figure 7a, when the reaction was processed at 10.0 °C, formation of the somewhat broad 665 nm plasmon band was observed that was again dependent upon the arginine/Au nanoparticle ratio. The new absorbance was broad, compared to the previous studies; however, the rate of formation was faster than those samples processed in an aqueous-only environment. When the assembly results were studied at 30.0 °C, the newly formed plasmon band was noticeably broadened and red-shifted. This trend of broadening and shifting was further exacerbated in the reactions studied at 60.0 °C, consistent with a more rapid assembly process occurring in solution. This is again different from the above water and EtOH studies, especially at (51) Mandal, S.; Gole, A.; Lala, N.; Gonnade, R.; Ganvir, V.; Sastry, M. Langmuir 2001, 17, 6262–6268. (52) Somasundaran, P.; Markovic, B.; Krishnakumar, S.; Yu, X. In Handbook of Surface and Colloid Chemistry, Birdi, K. S., Ed.; CRC Press: New York, 1997. (53) Lim, I.-I. S.; Ip, W.; Crew, E.; Njoki, P. N.; Mott, D.; Zhong, C.-J.; Pan, Y.; Zhou, S. Langmuir 2007, 23, 826–833.
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Figure 7. UV-vis analysis of the temperature effects on the arginine concentration-dependent assembly of Au nanoparticles in the presence of 4.00 mM NaCl studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. The plots on the left demonstrate the UV-vis spectra obtained after 1.00 h of reaction for each arginine/Au nanoparticle ratio studied, while the plots on the right present the absorbance intensity of the 665 nm peak as a function of time.
30.0 °C. For these reactions, such a degree of shifting was not observed until the 60.0 °C sample; the 30.0 °C samples demonstrated sharp and well-resolved absorbances at 665 nm. Similar results were observed when the assembly was studied in 400 μM NaCl, as presented in Supporting Information Figure S9; however, the growth of the 665 nm peak was observed to be significantly slower as compared to the 4.00 mM study. The main difference between 400 μM and 4.00 mM studies was observed at 30.0 °C where the 400 μM sample demonstrated broadened peaks at 665 nm rather than the red-shifted and broadened bands at 4.00 mM NaCl. These results suggest that the concentration of NaCl does indeed affect the organization process such that the rates of assembly increase proportionally to the NaCl concentration. TEM and DLS characterization of the 4.00 mM NaCl reaction materials is presented in Figure 8. Specifically, Figure 8a presents the materials assembled at 10.0 °C. For the 20K sample, small linear chains were observed; however, many independent Au nanoparticles are also detected on the TEM grid surface. When the arginine/Au nanoparticle ratio was increased to 40K, the sample presented linear aggregates that demonstrated a small degree of branching. This trend was continued for the 80K sample where large branching assemblies of Au nanoparticles were observed. DLS analysis of the NaCl-based assembly process is shown in the plot to the right of the figure. From this study, it is evident that the formation of larger aggregates occurs at higher NaCl concentrations; however, the sizes of these aggregates are smaller than those that are formed in the presence of EtOH. For instance, for the 80K sample at 10.0 °C with 400 μM and 4.00 mM DOI: 10.1021/la100216w
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Figure 8. TEM analysis of the temperature effects on the assembly process in the presence of 4.00 mM NaCl with arginine:Au nanoparticle ratios of 20K, 40K, and 80K studied at (a) 10.0 °C, (b) 30.0 °C, and (c) 60.0 °C. TEM images were obtained after a reaction time of 1.00 h. The scale bars in the figures represent 50 nm. The plot on the right presents the DLS size-based analysis for reactions at that specific temperature using 400 μM or 4.00 mM NaCl in the reaction medium.
NaCl, an aggregate size of 49.4 and 82.8 nm, respectively, was determined. This is indeed larger than that of 31.5 nm for the water-only sample; however, it is smaller than the aggregate size of those reactions studied in the presence of EtOH. For these materials, a trend of increasing aggregate size with both increasing arginine/Au nanoparticle ratios and NaCl concentrations was maintained. For the materials studied at 30.0 °C, Figure 8b, larger aggregates were achieved; however, a significant change in the material’s morphology was identified. For the 20K sample, linear chains and independent Au nanoparticles were observed. When the 40K sample was studied, short and branched ribbonlike nanostructures were achieved. For many of these linear materials, wire-like structures were observed; however, for a significant fraction, bulges were noted, which may be remnants of the original Au nanoparticles. Such structures may be the result of an increased amount of necking between linearly assembled Au nanoparticles, which could result in wire-like materials under appropriate conditions. Similar structures were observed in the 80K sample; however, longer wire-like materials were noted for this sample in the presence of individual Au nanoparticles and bulging-based materials. Due to this significant change in morphology, determination of the ratio of nanoparticles assembled in chains, random orientations, and independent materials could not be generated. DLS analysis of these materials suggested that similar assembly size trends were observed as compared to the 10.0 °C sample. Again, as the concentration of both arginine and NaCl in the reaction increased, larger aggregates were generated, but these materials were again significantly smaller than those observed with the EtOH-based 9870 DOI: 10.1021/la100216w
studies. Similar results were also observed for the samples studied at 60.0 °C; however, a more drastic change in structure toward wire-like materials was observed (Figure 8c). For the 20K sample, independent Au nanoparticles were noted, but the materials were mostly oblong-shaped structures, rather than the spherical materials typically prepared. At higher ratios, 40K and 80K, only linear networked structures were observed. In these materials, while bulging points were noted, the materials were generally wire-like in shape, consistent with nanoparticle networks previously observed for Pd, Pt, and Au.54-56 These structures were highly integrated with a large degree of networking and branching. Additionally, these changes in morphology likely play a significant role in the observed alterations in the UV-vis spectra that were obtained, as the absorbances are based upon the shape, size, and orientation of the materials.57 DLS studies of these materials indicated the formation of larger aggregates, as compared to those prepared at 30.0 °C, that followed the anticipated trend of larger aggregates at higher NaCl and arginine concentrations; however, the 80K sample with 4.00 mM NaCl was smaller in size as compared to the sample processed in 400 μM NaCl. This is likely due to materials precipitation, which can result in the detection of smaller than expected aggregate size. (54) Song, Y.; Garcia, R. M.; Dorin, R. M.; Wang, H.; Qiu, Y.; Coker, E. N.; Steen, W. A.; Miller, J. E.; Shelnutt, J. A. Nano Lett. 2007, 7, 3650–3655. (55) Jakhmola, A.; Bhandari, R.; Pacardo, D. B.; Knecht, M. R. J. Mater. Chem. 2010, 20, 1522–1531. (56) Ramanath, G.; D’Arcy-Gall, J.; Maddanimath, T.; Ellis, A. V.; Ganesan, P. G.; Goswami, R.; Kumar, A.; Vijayamohanan, K. Langmuir 2004, 20. (57) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677.
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The above changes to both the assembly rate and structural changes are likely due to the effects of NaCl on the reaction. Colloidal Au nanoparticles are stabilized by electrostatic repulsion interactions that prevent bulk aggregation and subsequent precipitation.52,58 While lowering the solution dielectric with EtOH can maximize these effects, addition of NaCl can lead to partial neutralization of the nanoparticle surface, which shortens the Debye length and allows for the Au nanoparticles to come closer together in solution.15,52,58 At a critical NaCl concentration, the electrostatic repulsions will be minimized to a point to allow for bulk material aggregation; however, at lower concentrations, the materials can come more closely together, but remain at interparticle distances where they are stable in solution. Such events are observed in the present study at NaCl concentration of e4.00 mM. Upon arginine addition, the charged patchy ligand network is generated on the nanoparticle surface, again forming an electronic dipole from which linear assembly can occur. The rate of assembly is likely to be increased, as compared to the aqueous samples, due to the fact that the particles are closer together in solution. Upon the basis of this consideration, since the interparticle distance is decreased at higher salt concentrations, a more rapid assembly process should be observed at higher NaCl concentrations, which was observed and is consistent with previous studies.53 Furthermore, while the rate of chain formation is increased as compared to the aqueous samples, it is slower than those observed for the EtOH-based studies. This suggests that the observed changes in shielding due to the solvent dielectric play a more significant role in the assembly process as compared to the interparticle distance effects associated with the NaCl. An interesting effect of the NaCl is the dramatic change in morphology that was observed. For these materials, a more wirelike structure was achieved at higher temperatures and arginine/ Au nanoparticle ratios, as compared to the EtOH and waterbased studies. This is likely due to the necking process, which could result in the observed linear structures. In the presence of NaCl, the necking process could be significantly increased based upon partial charge neutralization effects. For these materials, it is likely that the formation of linear chains of Au nanoparticle is achieved, as was determined for the study at 10.0 °C. When this process rapidly occurs in solution, i.e., at higher temperatures and arginine/Au nanoparticle ratios, this directly positions the particles in the chains at shorter time periods. To remain as distinct Au nanospheres while assembled, the dipole on the structure must be constantly centered at the interface between the two particles, while the negative charges of the citrate species around this interface are likely to repel each other when placed in close proximity. As such, the dipolebased interactions must be stronger than the electrostatic repulsions to maintain assembly; however, the repulsion effects at the perimeter of the assembled region must be strong enough to prevent neck formation and eventual agglomeration. If these two forces are appropriately balanced, individual Au nanoparticles in chains are observed. When NaCl is added to the system, this could change significantly the electrostatic effects. In this system, chain formation does occur as a result of a decrease in interparticle distances; however, the electrostatic effects of the negative regions of the two Au nanoparticles in the chain also decrease. While chain formation occurs based upon the nanoparticle dipole, the repulsion effects of the negative regions on the perimeter of the assembly location decrease. Upon the basis of this decrease, the ability to form necks between the particles can occur. Over time,
this process is likely to continue, which could result in the observed ribbon-like structures. Kinetic Analysis of the Assembly Process. Upon the basis of the obtained experimental results, it is possible to model the assembly process using standard chemical kinetics.21,59 From this information, a deeper understanding about the mechanistic steps, assembly rates, and rate constants can be achieved, which could prove to be useful in the adaptation of this method to devicecomponent fabrication. To model the biomimetic nanoparticle assembly mechanism, parameters must be set. First, the rate of arginine binding to the surface is anticipated to be significantly faster than the individual nanoparticle assembly process. As discussed above, this is highly likely based upon the observed assembly rate changes by alterations to the reaction conditions. Furthermore, generation of assembled species occurs instantly, within the limits of resolution for UV-vis spectroscopy. Should the rate of arginine binding the surface be sufficiently slow, an induction period in which no changes to the UV-vis spectrum would be observed. Since the formation of an absorbance at 665 nm appears immediately for those species that demonstrate assembly, this suggests that the limiting step of the process lies during the formation of the linear aggregates, as shown in Scheme 1. Second, UV-vis can only monitor the assembly process and not the direct binding of arginine to the Au nanoparticles. At present, we are unaware of a technique capable of monitoring the initial ligand binding to the Au nanoparticle, especially on the time scale suggested by the present study. Third, the assembly process likely follows two individual steps. Immediately prior to formation of chains, the nanoparticles exist as independent species in solution. Once assembly begins, it is likely that the formation of nanoparticle dimers, or to a lesser extent trimers, occurs first, followed by the formation of the longer branching chains through the assembly of the dimers in solution. The formation of dimers is supported by the UV-vis results that present a distinct plasmon band at 665 nm that demonstrates a red shift at longer time periods when a faster assembly rate is observed. Upon the basis of known molar absorptivity values for one-dimensional Au nanorods, materials with an aspect ratio of two (i.e., dimers) would likely present a longitudinal plasmon band around this location in the UV-vis spectrum.60 Eventually, the oligomerization process would overtake dimer formation, to generate a second regime in the assembly process. Fourth, based upon the arginine/Au nanoparticle ratio, a higher percentage of surface coverage is anticipated at higher ratios. This is also likely an effect of the on-off equilibrium of ligand surface binding. To that end, each system should be modeled separately based upon this ratio, as the assembling materials are chemical and compositionally different immediately prior to chain formation. As such, the rates of assembly are likely to be a function of the surface character of the materials, which is related to the selected ratio. From these parameters, it is then possible to monitor the assembly process using standard second-order chemical kinetics to understand the initial dimerization process.21,59 To achieve this, we employed the model of Joseph et al., which was used to understand the formation of chains of Au nanorods using dithiols.21 For this, eq 1 can be used to determine the second-order rate constant, k, where a0 represents the initial concentration of Au nanoparticles in solution, x is the concentration of nanoparticles that were reacted at time t, and a0 - x represents the concentration of independent Au nanoparticles that remain
(58) Somasundaran, P.; Markovic, B.; Krishnakumar, S.; Yu, X. Handbook of Surface and Colloid Chemistry; CRC Press: New York, 1997.
(59) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Row: New York, 1987. (60) Orendorff, C. J.; Murphy, C. J. J. Phys. Chem. B 2006, 110, 3990–3994.
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DOI: 10.1021/la100216w
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at time t.59 x a0 ða0 - xÞ
kdim t ¼
ð1Þ
Furthermore, the initial and reaction concentration values of the individual Au nanoparticles in the reaction mixture at all time points can be calculated using Beer’s law since the molar absorptivity value ε is known for these species.61 Using this consideration, eq 1 can be rearranged into the following expression:21 kdim t ¼
εðA0 - At Þ A0 At
ð2Þ
Here, A0 and At represent the absorbance value of the plasmon band at 520 nm before the reaction and at time t, respectively. As a result, eq 2 can thus be used to ascertain the rate constant of the assembly process using arginine in solution based upon the 520 nm plasmon absorbance intensity. Kinetic analysis for the materials studied in water at 30.0 °C is presented in Figure 9. Figure 9a specifically shows the decrease in absorbance values for the reactions at 520 nm over the 1.0 h time frame. We specifically chose the 520 nm peak as the molar absorptivity of the individual nanoparticles is known (ε = 3.6 108 M-1 cm-1),61 which can be used to calculate the concentrations of free nanoparticles and dimers. As is evident, the decrease was most rapid for those samples with an arginine/Au nanoparticle ratio of g40K. Furthermore, these samples demonstrated a clear absorbance increase at 665 nm (Figure 1b), which suggests that dimerization was indeed occurring for these materials. While a minor decrease was observed for the 10K and 20K sample, no clear formation of a new band at 665 nm was evident. Under such conditions where assembly was not abundantly obvious, either from UV-vis or DLS analyses, no rate constants were determined for these samples. Figure 9b shows a plot of the data using eq 2. From this, the slope of the linear region of the plot represented the second-order rate constant, k, of the reaction, at the specific temperatures, which are listed in Table 1. For each reaction, the R2 value was >0.97 to ensure linearity. From this analysis, it was evident that the rate constant increased proportionally to the ratio of arginine/Au nanoparticles used in the system as expected. For the 40K sample where assembly was evident, a second-order k-value of 8.70 103 M-1 s-1 was determined, which increased to 65.36 103 M-1 s-1 for the 100K sample. Upon completion of the dimerization process, deviation from linearity was observed, which is likely due to the oligomerization process. Changes to the assembly beyond this point were unable to be modeled as the order of the reaction can rapidly change depending on the individual components that form the specific chains in solution. Further characterization of the reaction kinetics was employed to determine the energy of activation (Ea) for the formation of nanoparticle dimers using the biomimetic method. For each system at the specified reaction temperature between 10.0 and 70.0 °C, a rate constant was determined using the described method. These values were then used to produce an Arrhenius plot of the assembly process, which is presented in Figure 9c. From the slope of the regression line used to fit the data, the Ea value can be determined for every reaction system, which is also listed in Table 1. Interestingly, for each arginine/Au nanoparticle ratio studied, similar Ea values were calculated, which varied (61) Lee, J.-S.; Stoeva, S. I.; Mirkin, C. A. J. Am. Chem. Soc. 2006, 128, 8899– 8903.
9872 DOI: 10.1021/la100216w
Figure 9. Kinetic analysis of the biomimetic dimerization of Au nanoparticles in water at 30.0 °C. Part (a) presents the decrease in absorbance of the surface plasmon band at 520 nm for the independent Au nanoparticles, while part (b) presents the kinetic plot of the materials as determined using eq 2. Part (c) displays the Arrhenius plot of the materials, where the slope of the regression line is used to determine the Ea value for each system.
between 46.70 and 54.29 kJ/mol. The close proximity in the activation energy for each system likely lies in the fact that the values are related to the formation of dimers, which occurs in every system. The rate of formation can vary as a result of the degree of surface arginine coverage of the Au nanoparticle, which can alter the probability of dimer formation. For instance, at a higher fraction of arginine surface coverage, the likelihood of two Au nanoparticles colliding in appropriate orientations to generate a dimer is greater as compared to two particles with lower arginine surface coverage. Such chemical information is adjusted for by the pre-exponential factor for the system, which increases for the systems with higher arginine surface coverage (higher arginine/Au nanoparticle ratios). Furthermore, the kinetic analysis suggests that the formation of dimers can occur at or above a critical amino acid/Au nanoparticle ratio. At this value, formation of a sufficiently large patchy charged surface network occurs that allows dimer formation that can subsequently be followed by the oligomerization processes. This value is likely to be controlled by Langmuir 2010, 26(12), 9860–9874
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Table 1. Second-Order Rate Constants, k, and Activation Energies, Ea, for the Biomimetic Dimerization of Au Nanoparticles in the Presence of Argininea 103 k (M-1 s-1) Arg/Au NP Ratio
10 °C
20 °C
30 °C
60 °C
70 °C
kJ/mol
20K ------2.14 3.09 6.04 40K ----8.70 11.20 39.77 38.91 60K --7.86 11.78 28.05 97.76 106.54 80K 5.62 15.29 31.65 55.35 142.90 166.16 100K 10.52 31.38 65.36 120.94 199.69 271.60 a The k-values are listed for samples that demonstrated detectable dimerization at the specified temperature.
10.04 69.64 150.71 238.43 357.69
47.21 46.84 54.29 50.95 46.70
the binding strength of the ligand to the Au surface. Additionally, we attempted an identical kinetic analysis using the materials with various additions of EtOH and NaCl; however, nonlinear results were obtained under second- and lower-ordered kinetics. This is due to the rapid assembly process that quickly leads to transitions from dimerization to oligomerization at rates that are too fast to monitor. While we are unable to kinetically model the oligomerization process, a collective analysis of the experimental results suggests that it is slower than the dimerization process. The formation of the linear chains is envisioned to occur from oligomerization of the Au nanoparticle dimers in solution, which is anticipated to follow higher-ordered kinetics that would vary based upon the size and number of dimers and/or chains assembling to form the larger structure. Furthermore, no clear point is observed at which dimerization is complete from which oligomerization occurs. To that end, at longer time points, the formation of larger chains may occur in the presence of individual Au nanoparticles and dimers, thus further complicating the system. Indeed, such results are confirmed by the DLS studies, which showed smaller materials in the presence of larger aggregates. This likely causes the deviations from linearity at higher time points observed in Figure 9b. The rate of oligomer formation can be qualitatively addressed relative to the dimerization process from the UV-vis and DLS results in the time frame after which linear deviation occurs. As the number of longer nanoparticle chains increases, shifting of the plasmon band further toward the red is anticipated and is observed for samples at higher temperatures and EtOH concentrations. For many samples, however, only a single 665 nm peak is observed in the UV-vis spectrum, while the DLS results demonstrate assemblies larger than dimers. This is evident for the 80K sample at 30.0 °C with 1.0 mL of EtOH. The UV-vis spectrum of these materials after 1.0 h of assembly, as shown in Figure 4b, presents a relatively sharp 665 nm plasmon band, while the DLS results for this sample at this time point present an average aggregate size of 2348 nm. These two results appear contradictory as the UV-vis analysis suggests most of the materials are present as dimers, while the DLS results suggest formation of larger aggregates in solution. While the large nanoparticle chains are present, they exist in solution with a significant number of nanoparticle dimers, which the UV-vis detects. The larger aggregates may possess a plasmon band that is red-shifted beyond the detection limit of the instrument, or may be in such low concentrations, as compared to the dimers, that their absorbance is undetected. As a result, a sharp 665 nm band is possible in the presence of larger aggregates. In addition, while the plasmon band is relatively sharp, nanoparticle dimers and other short aggregates may possess similar peak positions, which would collectively add to form the 665 nm peak. As the reaction progresses to longer times, more assembly of the dimers would occur to produce a detectable shift in the absorbance, within the detection limits, which was indeed observed. Taken together, this suggests that the rate of formation of longer Langmuir 2010, 26(12), 9860–9874
40 °C
Ea 50 °C
chains is slower than dimerization and likely decreases over time, which is a direct effect of the concentration of the assembling species in solution. To that end, as linear chain formation continues, the concentration of the assembling components in solution decreases. As these components decrease, the likelihood of the materials becoming in close enough proximity and correctly arranged to lead toward assembly decreases, thus lowing the oligomerization rate as a function of time.
Summary and Conclusions In summary, the amino acid-based capping and assembly of Au nanoparticles has been extensively probed to further resolve the mechanism and assembly rates by which the formation of branched linear superstructures is achieved. Understanding this event is important, as determining how the individual surface and electronic structure of the materials affects their function is critical for the incorporation of such organization methods into in situ device fabrication or for use as sensitive assays for detection methods. At present, very little information is readily known about biomolecular interactions with dispersed, three-dimensional nanomaterials due to instrumental limitations; therefore, new methods must be developed to monitor this process. These methods, such as the present technique, must be extensively characterized and validated to fully understand the mechanism at play to ensure correct results. Furthermore, the assembly ability of the materials in a linear fashion is also attractive for use as approaches to produce controlled arrangements of nanomaterials. As such, determining the method of assembly, rates of the individual steps, and developing techniques to control the process could prove to be highly important. From the results of the present study, it is indicated that the first step of the process, which incorporates arginine surface binding and selfsegregation, is relatively rapid and not easily perturbed; however, the second step of assembly can be directly manipulated by the reaction conditions. Furthermore, this study further resolves the level of understanding of the organization step, which suggests that it can be broken into two substeps: formation of nanoparticle dimers followed by dimer oligomerization to form the longer chains. These results suggest that the dimer step can be relatively fast, on the order of minutes to hours, but that the oligomerization step is likely to be slower. Changing of the reaction temperature or electronic shielding capability of the solvent can directly alter the rate of assembly, from which the size and orientation of the final prepared structure may be able to be manipulated. These results were confirmed via the optical (UV-vis) and scattering (DLS) properties of the Au nanoparticles, which is an additional attractive component, as such characteristics can also be manipulated based upon the assembly of the materials. Further research to apply this process to other sets of materials is currently under study. Acknowledgment. Funding from the University of Kentucky is gratefully acknowledged. The authors also wish to acknowledge the UK Microscopy Center for assistance in obtaining the TEM DOI: 10.1021/la100216w
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images and Professors Younsoo Bae (College of Pharmacy) and Thomas Dziubla (Department of Chemical and Materials Engineering) for assistance with the DLS studies. Finally, we wish to thank Professors Yinan Wei and Yuguang Cai for helpful discussions.
9874 DOI: 10.1021/la100216w
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Supporting Information Available: Additional UV-vis and DLS analyses of the assembly process at various temperatures, EtOH, and NaCl conditions. This material is available free of charge via the Internet at http://pubs. acs.org.
Langmuir 2010, 26(12), 9860–9874
