Janus nanoparticles: reaction dynamics and NOESY characterization
- First Online:
- Cite this article as:
- Pradhan, S., Brown, L.E., Konopelski, J.P. et al. J Nanopart Res (2009) 11: 1895. doi:10.1007/s11051-008-9543-4
Janus nanoparticles were prepared by taking advantage of interfacial ligand exchange reactions of hydrophobic hexanethiolate-protected gold nanoparticles with hydrophilic 2-(2-mercaptoethoxy)ethanol (MEA). A monolayer of the particles was first formed at the air–water interface by the Langmuir technique and then deposited onto a substrate surface by the Langmuir–Blodgett method. The particle monolayer was then immersed into an aqueous solution of MEA for different periods of time. It was found that the exchange reactions occurred but were limited only to the top face of the nanoparticles and the reaction reached equilibrium in about 8 h. The resulting particles exhibited amphiphilic characters as confirmed by contact angle and UV–visible, FTIR and NMR spectroscopic measurements. Of these, the structural discrepancy between the Janus nanoparticles and bulk-exchanged particles was clearly manifested, in particular, by NOESY NMR measurements.
KeywordsJanus nanoparticles Langmuir–Blodgett Contact angle NOESY NMR Surface plasmon Exchange reactions Monolayer
Recently, nanoparticle materials with asymmetric chemical/physical properties or geometric structures have attracted much interest because of their unique performance that cannot be achieved by the homogeneous or symmetric counterparts. In particular, the formation of functional nanoarchitectures via controlled self assembly and the resulting wide range of possible applications in science and technology can be approached via asymmetric structures most efficiently (Feldheim et al. 1996; Storhoff et al. 1998; Ujihara et al. 2006; Whetten et al. 1996; Zhang and Cui 1998). Of these, spherical nanoparticles with two structurally and functionally asymmetric hemispheres, often referred to as Janus nanoparticles, are of significant interest as they are anticipated to behave as nanoscale analogs to conventional amphiphilic surfactant molecules (van Herrikhuyzen et al. 2008). In recent years, significant attention has been paid to Janus-type materials because of their potential applications in dual-functional devices (Perro et al. 2005), structural materials for organized assembly (Glotzer 2004; Nie et al. 2006), electronic paper and display applications (Nisisako et al. 2006), surfactants for emulsion systems (Binks et al. 2006), anisotropic imaging probes for both diagnostic and therapeutic purposes, bimetallic nanomotors (Wang et al. 2006b), nanoprobes (Takei and Shimizu 1997), and anisotropic plasmon materials (Wang et al. 2006a).
In our previous work (Pradhan et al. 2007; Xu et al. 2007), we developed an effective approach based on interfacial engineering for the preparation of Janus gold nanoparticles. First, a monolayer of hexanethiolate-protected gold nanoparticles was formed at the air–water interface by the Langmuir technique. The particle monolayer was then compressed to a selected surface pressure where ligand intercalation between adjacent particles occurred such that the interfacial mobility of the nanoparticles was impeded. A calculated amount of hydrophilic thiol derivatives (1,2-mercaptopropanediol, MPD) was then injected into the water subphase, where ligand place-exchange reactions were initiated and limited to the bottom face of the nanoparticle molecules that was in direct contact with water. Consequently, the resulting particles exhibit hydrophobic characters on one side and hydrophilic on the other. The amphiphilic nature of these particles was then confirmed by contact angle and other spectroscopic measurements, and the particles were found to form stable aggregate structures in an appropriate solvent medium.
In the present investigation, we describe a new procedure for the preparation of Janus nanoparticles again by taking advantage of the surface ligand exchange reactions of alkanethiolate-protected gold nanoparticles with another thiol derivative. In this approach, a monolayer of particles is first deposited onto a substrate surface by the Langmuir–Blodgett (LB) technique. The sample is then immersed into an aqueous solution of the hydrophilic ligands, where the ligand-exchange reactions are limited to the top face of the nanoparticles and hence the formation of Janus nanoparticles. With this experimental setup, the exchange dynamics can also be readily assessed by contact angle measurements, which has remained largely unexplored so far.
Additionally, by using a long hydrophilic ligand, 2-(2-mercaptoethoxy)ethanol (MEA) as compared to the short 1,2-mercaptopropandiol (MPD), used in earlier studies (Pradhan et al. 2007; Xu et al. 2007), the structural details of the Janus nanoparticles will be examined by nuclear Overhauser enhancement spectroscopy (NOESY), which we believe is the first of its kind in the investigation of Janus nanoparticles. This is a two-dimensional phase-sensitive NMR technique that detects the distance-dependent nuclear Overhauser effect between proton spins. Since NOESY signals depend inversely on the distance to the sixth power, only short-range dipolar couplings are resolved, and it is a very useful tool to examine the conformation of molecules. For instance, NOESY NMR spectroscopy has been used extensively in the field of proteomics to investigate micelle interactions (Bella et al. 1999; Emin et al. 2007). It has also been used to evaluate the statistical distribution of polymers as capping materials for semiconductor quantum dots (Guo and Moffitt 2007), and to study the amide proton exchange rates in metal nanoparticles that are capped with amidethiolates (Kohlmann et al. 2001). Thus, in this study, we would employ NOESY NMR spectroscopy to further verify the surface structural asymmetry of the Janus nanoparticles as compared to bulk-exchanged nanoparticles where the ligands are mixed rather homogeneously on the particles, by taking advantage of the different ligand distribution and the consequent ligand–ligand interactions.
Hydrogen tetrachloroauric acid (HAuCl4 · xH2O) was synthesized by dissolving ultra-high purity gold (99.999%, Johnson Matthey) in freshly prepared aqua regia followed by crystallization (Brauer 1963). Tetraoctylammonium bromide (Alfa Aesar, 98%), hexanethiol (C6SH, Acros, 96%), and sodium borohydride (NaBH4, Acros, 99%) were all used as received. 2-(2-Mercaptoethoxy)ethanol (MEA, HSCH2CH2OCH2CH2OH) was synthesized and characterized according to a literature procedure (Woehrle et al. 2004). Other solvents were purchased from typical commercial sources at their highest purity and used without further treatments. Water was supplied by a Barnstead Nanopure water system (18.3 MΩ cm).
Hexanethiolate-protected gold (AuC6) particles were synthesized and purified by using the Brust protocol (Brust et al. 1994). These particles then underwent fractionation by using a solvent–nonsolvent mixture of toluene and ethanol (Chen 2001a; Templeton et al. 2000), and the fraction with an average core diameter of 2.0 nm (corresponding approximately to a particle composition of Au314C691) (Hostetler et al. 1998) was used for the subsequent study. These particles were then subject to thermal annealing in toluene at 110 °C in an oil bath for 8 h to further reduce the dispersity of the core size and shape (Chen 2001b).
As a control measurement, exchange reactions of AuC6 nanoparticles with MEA were also carried out by mixing a calculated amount of AuC6 nanoparticles and MEA ligands in THF and stirred for 48 h. The solution was then dried under reduced pressure with a rotary evaporator and excessive ligands were removed by extensive rinsing with methanol. The resulting particles were denoted as bulk-exchange particles, and, similar to the Janus nanoparticles, soluble in THF and chloroform.
Contact angle measurements
Contact angles of nanoparticle thin films were evaluated with a Tantec CAM-PLUS Contact Angle Meter. For each sample, at least eight independent measurements were carried out for statistical analyses. Prior to deposition, a flat glass substrate was cleansed in aqua regia followed by extensive rinsing with water and ethanol, and blow-dried by ultrahigh-purity nitrogen. The substrate was then cleaned in a UV-ozone chamber (Jelight Model 42) for 15 min to remove any organic contaminants. For the Janus nanoparticles, the contact angle was monitored at different immersion time intervals of the AuC6 particle monolayer immersed in an MEA solution; whereas for the bulk-exchange particles, a monolayer was deposited onto a glass slide surface by the LB technique (the dipper speed was generally controlled at 1 mm/min) in both the upstroke and downstroke configurations, and their contact angles were then measured and compared.
FTIR measurements of the particles were performed with a Perkin–Elmer Precision Spectrum-1 FTIR Spectrometer. The particles were first dissolved in chloroform and a thick film was formed by dropcasting the solution onto a CsBr plate. The sample was then dried in a gentle stream of nitrogen. The UV–visible spectra were collected with a UNICAM ATI UV4 spectrometer at a particle concentration of 0.1 mg/mL in chloroform using a 1 cm quartz cuvette. 1H and NOESY NMR spectra were collected by using a VARIAN OXFORD 600 MHz spectrometer. The particles were dissolved in CDCl3 at a concentration of 1 mg/mL. Dynamic light scattering (DLS) measurements were carried out with a ProteinSolution Dynapro Temperature Controlled Microsampler. Typically an aliquot (12 µL) of the particle solutions at a concentration of 0.05 mg/mL in THF was introduced into a sample holder using a 20 µL micropipette. The results were reported in terms of %mass.
Results and discussion
In our previous studies (Pradhan et al. 2007; Xu et al. 2007), Janus nanoparticles were prepared by the exchange reactions of hydrophobic AuC6 nanoparticles with 1,2-propanediol (MPD) at the air–water interface. However, with MEA ligands, it is somewhat puzzling and surprising that this procedure was not very effective. Thus, a different synthetic scheme was designed as depicted in Scheme 1, where the Janus nanoparticles were produced by taking advantage of the interfacial exchange reactions of the nanoparticle LB monolayers with hydrophilic thiol derivatives.
Note that for bulk-exchange nanoparticles, the contact angle of the particle monolayer deposited by the LB technique is 55.8° ± 2.0°, which falls into the intermediate between that of the AuC6 nanoparticles and the Janus nanoparticles. Such a behavior was also observed earlier in the preparation of Janus nanoparticles based on MPD exchange (Pradhan et al. 2007; Xu et al. 2007). Comparison with two-dimensional self-assembled monolayers (2D SAMs) of varied thiol derivatives formed on flat gold film surfaces further confirms the asymmetric structure of the Janus nanoparticles. For instance, for 2D SAM of hexanethiols, the contact angle is 63.1° ± 3.4°, which is close to that of the LB monolayers of the original AuC6 nanoparticles (Fig. 1); for MEA monolayers, it is 47.8° ± 2.7°, very similar to that of the hydrophilic face of the Janus nanoparticles (e.g., Fig. 1); and for the mixed monolayer of hexanethiol and MEA, it becomes 53.2° ± 2.2°, which is consistent with that observed above with the bulk-exchange nanoparticles, suggesting a rather similar homogeneous mixing of the two mercapto-derivatives on the substrate surface.
The aggregation of nanoparticles in these solvent media was further verified by DLS measurements in THF. For the AuC6 nanoparticles, the average radius was 1.9 nm. Considering the nanoparticle core diameter of ca. 2 nm and the fully extended chain length of the hexanethiolate ligand of 0.77 nm, this is in good agreement with the particle physical diameter and hence indicative of isolated nanoparticles in the solution. For the bulk-exchanged nanoparticles, the DLC radius is 11 nm whereas for the Janus nanoparticles, 77 nm. Again, these results suggest different degrees of particle aggregation and are consistent with the UV–vis data shown above (Fig. 3).
Additionally, to further establish the Janus character of the MEA modified nanoparticles, nuclear Overhauser effect spectroscopy (NOESY) was employed to examine the spatial correlation between nuclear spins (Guo and Moffitt 2007; Kohlmann et al. 2001). NOESY is a two-dimensional NMR technique where cross peaks arise from dipole–dipole interactions (i.e., through-space coupling) between nuclear spins that are in close proximity (typically <0.4 nm), as the intensity of the cross peaks depends inversely on the sixth power of the distance between the protons (Morris et al. 2005). These unique features can thus be exploited for an estimation of the internuclear distance, and for monolayer-protected nanoparticles, a quantitative assessment of the packing (distribution) of ligands on particle surfaces. For instance, a recent study by the Murray and Johnson groups (Kohlmann et al. 2001) has demonstrated that NOESY could be used as a powerful tool in the examination of the packing of tiopronin ligands (CH3–CH(SH)–CO–NH–CH2–COOH) on gold nanoparticle surfaces by analyzing the cross peaks between the methyl and methylene protons. Since the intrachain distance between these protons exceeds the limit of 0.4 nm, the appearance of the cross peaks is most likely arising from the interchain contributions, from which a structural model for the packing of the tiopronin ligands can be built (Kohlmann et al. 2001).
Figure 4 depicts a typical NOESY spectrum for both the (a, bottom) Janus and (b, bottom) bulk-exchanged particles. From the diagonal profiles of both spectra, it can be seen that there are apparent polarization interactions between the methylene protons (highlighted in blue and orange circles of b’s and c’s), which most probably arises from intrachain interaction of the individual ligand chains as well as interchain interaction between adjacent ligands. Of these, the peak in the blue circle (b1 and b2) most likely reflects the contribution from the hexanethiolate ligands whereas that in the orange circle (c1 and c2), from the MEA ligands. In addition, the interactions between the methyl protons, and between methyl and methylene protons of the hexanethiolate ligands are clearly manifested in the spectra (green circles of a1 and a2).
In addition to these common features, remarkable discrepancy can also be observed. The most significant difference between these two spectra is the appearance of two cross peaks between the methyl/methylene protons of the hexanethiolate ligands and the methylene protons of the MEA ligands for the bulk-exchanged particles, which are highlighted in red circles (d2) in panel (b). Note that these cross peaks are totally absent in the Janus particles (panel (a)). Such an observation is consistent with the structural models for the surface distribution of the two types of ligands on the nanoparticle surface. In the Janus nanoparticles, the absence of these cross peaks clearly indicates that the hexanethiolate ligands are situated far away from the MEA ligands, because of the segregated distribution of these two ligands on two different hemispheres of the nanoparticle surface. In contrast, in the bulk-exchanged particles, the cross peaks were observed largely because of the homogeneous mixing of the two ligands and hence extensive interactions between the neighboring ligand protons (Jackson et al. 2004, 2006).
In summary, Janus nanoparticles were prepared which took advantage of the interfacial exchange reactions of alkanethiolate-protected gold nanoparticles with mercapto-alcohols. Experimentally, Langmuir–Blodgett monolayers of the hydrophobic nanoparticles were immersed into an aqueous solution of the hydrophilic thiols. The reaction dynamics was then examined by contact angle measurements. At equilibrium, contact angle and spectroscopic studies indicated that about 50% of the original hydrophobic ligands were replaced with the hydrophilic ones, and the resulting particles exhibited segregated distribution of the two kinds of ligands, as confirmed by NOESY NMR measurements, supporting the amphiphilic structural model of the Janus nanoparticles. Additionally, because of the anisotropic characteristics of the Janus nanoparticle surface, extensive aggregation was also observed in selected solvent media. These asymmetric structural units might be further exploited for the controlled assembly into organized functional nanoarchitectures.
This work was supported by a grant from the National Science Foundation (DMR-0804049) and the UC MEXUS-CONACYT program.