Interfacial adsorption and the assembly of gold nanoparticle embedded peptoid nanosheets
Christine Kang, Ellen Robertson, Menglu Qian, Ronald N. Zuckermann.
Oregon State University, Molecular Foundry at Lawrence Berkeley National Laboratory.
Introduction: Bio-inspired polymers have attained considerable interest as they can mimic the structural characteristics of natural compounds like proteins. Peptoids, or N-substituted glycines, are especially exciting due to their ability to collapse into bilayer “nanosheets” at interfaces.1 Surface functionalized nanosheets also have molecular recognition capabilities,2 so peptoid nanosheets are being developed as plasmonic nanosensors by embedding gold nanoparticles (AuNPs) inside the sheets. To assemble these materials, AuNPs will adsorb to the self-assembled peptoid monolayer. Then, the interface will be laterally compressed, allowing the peptoids to reach an irreversible collapse point, and the monolayer will fold into sheets. Based on preliminary findings,3 we hypothesize that AuNP monolayer formation is dependent on the concentration of gold used and can be related to decreases in interfacial tension.
Materials and Methods: Peptoids were synthesized using a solid-phase submonomer method4 and suspended in an aqueous solution. Dodecanethiolated AuNPs were purchased and suspended in hexane. The hexane phase was layered on top of the aqueous phase, and lateral compression was applied to collapse the peptoid monolayer at the interface into nanosheets. SEM was used to verify the presence and relative amount of AuNPs inside the nanosheets and pendant drop tensiometry was used to monitor adsorption of peptoid and AuNPs to the interface.
Results and Discussion: Our data shows that high AuNP concentrations result in lower steady state interfacial tension values. Since we know that interfacial self-assembly of a peptoid monolayer decreases the interfacial free energy by decreasing unfavorable interactions between the hexane and aqueous phases, these surface tension decreases can be correlated to increases in AuNP concentration. Figure 1 shows that the interfacial tension reaches a lower value when AuNPs adsorb to the hexane-peptoid interface (c) compared to either when only AuNPs (b) or peptoids (a) were allowed to adsorb. This indicates that cooperative interactions are occurring between the peptoid monolayer and AuNPs. Furthermore, we believe that increased concentrations of AuNPs will further decrease the interfacial tension between the two phases by adsorbing onto the peptoid monolayer, which will allow for more complete AuNP packing inside the nanosheets after compression.
Figure 1. Interfacial tension measurements over time at the hexane-aqueous interface with a) only adsorbed peptoid, b) only adsorbed AuNP, and c) both AuNP and peptoid adsorbed. The right-hand portion of the figure also shows a sample SEM image of the corner of a nanosheet with embedded AuNPs.
Conclusions: Surface tension is a powerful tool for investigating the interfacial assembly of nanostructures via adsorption kinetics, and can shed new light on optimizing the conditions for the assembly of 2D nanomaterials. Future studies will involve investigating broader concentrations of AuNPs to better understand the how they interact with the peptoid monolayer and using adsorption rates to determine the AuNP concentration at which we are able to achieve the most ordered and closest packed monolayer.
Acknowledgements: This research was supported in part by the HS-STEM Summer Internship Program sponsored by the U.S. Department of Homeland Security (DHS) Science & Technology Directorate Office of University Programs. This program is administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the U.S. Department of Energy (DOE) and DHS. ORISE is managed by Oak Ridge Associated Universities under DOE contract number DE-AC05-06OR23100.
(1) Robertson, E.J.; et al., Proc. Natl. Acad. Sci. U.S.A., 111, 13284–13289 (2014).
(2) Olivier, G.K.; et al., ACS Nano, 7, 9276-9386, (2013).
(3) Qian, M. UCB Honors Thesis. 1-43 (2013).
(4) Zuckermann, R.N. Pept. Sci., 96, 545-555 (2011).
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