MARK SCHLOSSMAN

Water Interfaces Group – UIC Department of Physics

Nanoparticle Arrays

The development of functional materials as a result of the assembly of nanoparticles is the underlying motivation for much of the research on nanoparticles. Numerous self-assembly processes have been proposed during the past 10 years to design nanoparticle arrays, which are next generation materials with applications in the areas of plasmonics, electro-voltaic devices, catalysis, and biological sensing.

Voltage-tunable Nanoparticle Arrays at Interface

Nanoparticle arrays at interfaces are being explored by a number of scientific groups for novel optical, electrical, magnetic, and filtration properties, among others. In general, these arrays have a fixed spatial arrangement of nanoparticles, but arrays whose lattice spacing can be varied in situ can lead to novel applications, such as voltage tunable optics including mirrors and plasmonic filters, as well as tunable analyte sensors that rely on surface enhanced Raman scattering. More generally, tunable nanoparticle arrays self-assembled at liquid interfaces are applicable to the development of electro-variable optical devices and active elements that control the physical and chemical properties of liquid interfaces on the nanoscale

We have recently provided the first demonstration of a voltage-tunable interfacial array of nanoparticles [1]. Our experiments showed that nanoparticles form a nearly close-packed 2D array at the liquid-liquid interface between aqueous and organic electrolyte solutions, whose spacing can be tuned by varying the voltage. The tunability occurs because the voltage alters the interfacial distribution of supporting electrolytes, which mediate the interactions between nanoparticles (Figure 1d). MD simulations by our collaborators Petr Kral and Henry Chan reveal that sharing of condensed counterions contributes to the coupling between neighboring nanoparticles.

Nanoparticle 1
Nanoparticle 2

Figure 1. Grazing-incidence small angle X-ray scattering [1].

(a) Bragg rods determine the order within a 2-dimensional array of charged nanoparticles at the electrified water-DCE interface for a voltage \(\Delta \phi ^{w-o}\) across the interface of 30 mV, where \(Q_z\) is the wave vector transfer in the direction out of the plane of the interface and \(Q_{xy}\) is in the plane.

(b) Intensity integrated over \(Q_z\) from 0 Å-1 to
0.1 Å-1 plotted as function of \(Q_{xy}\) for different values of \(\Delta \phi ^{w-o}\). Three diffraction peaks are indicated.

(c) Change in the nearest-neighbor spacing \(a\) as a function of \(\Delta \phi ^{w-o}\) referenced to the spacing \(a_o\) measured at \(\Delta \phi ^{w-o}\) = 30 mV of the same sample (\(a_o\) = 67.2, 61.5 and 64.5 Å for samples 1, 2 and 3 respectively).
Inset: Cartoon of the 2D hexagonal close- packed structure of NPs with nearest-neighbor spacing \(a\) and lattice spacing \(d\).

(d) Varying the voltage alters the interfacial distribution of supporting electrolytes, which mediate the interactions between nanoparticles, thus tuning the array spacing.

Nanoparticle Transport from Aqueous to Organic Media

Multi-charged nanoparticles, including proteins, viruses, and synthetic constructs, interact strongly with counterions in electrolyte solutions. Electrostatic interactions produce spatial correlations between charged nanoparticles and counterions, which are responsible for the condensation of DNA and proteins, as well as for charge reversal in aqueous colloidal dispersions. Counterion condensation onto micrometer-sized colloidal particles in aqueous dispersions is enhanced in the presence of highly charged counterions, typically trivalent or greater, whose stronger electrostatic interactions lead to stronger particle-counterion correlations. Although ion condensation is not expected to occur with monovalent counterions in aqueous solutions, our recent experiments have demonstrated that condensation of monovalent counterions takes place on the organic side of an interface between aqueous and organic electrolyte solutions as a result of the stronger electrostatic interactions present in a low permittivity organic liquid [2].

Our experiments and computer simulations provide a new perspective that strong correlations with counterions can influence the interfacial localization of charged nanoparticles [1]. First, we demonstrated a method to measure the depth of nanoparticles at a liquid interface, a topic that has been discussed for a couple of decades. Then, we showed that ion condensation onto charged nanoparticles facilitates their transport from the aqueous-side of an interface between immiscible electrolyte solutions to the organic-side contiguous with the interface, thus overcoming the electrostatic barrier presented by the low permittivity organic material. This provides a mechanism to transport charged nanoparticles into organic phases with implications for the distribution of nanoparticles throughout the environment and within living organisms. Figure 2 presents a molecular dynamics simulation of this process [1]

Nanoparticle 4
Nanoparticle 5

Figure 2. Molecular dynamics simulations of a time sequence of snapshots taken from a 213 ns MD simulation show the submersion of a +100 charged nanoparticlefrom an aqueous (top) to an organic (bottom) electrolyte phase accompanied by the exchange of loosely bound Cl- ions (blue) in the aqueous phase for condensed organic organic TPFB+ ions (red) in the organic phase.

For lectures on Electrostatics at Soft Interfaces presented at the 7th Summer School on Complex Fluids and Soft Solids held at the University of Massachusetts at Amherst during June 2015, see here.

For an introduction to X-ray scattering techniques used for studying liquid surfaces and interfaces, see this link.

For a more detailed description of these techniques, see the book described here.

References

[1] Interfacial Localization and Voltage-Tunable Arrays of Charged Nanoparticles, M. K. Bera, H. Chan, D. F. Moyano, H. Yu, S. Tatur, D. Amoanu, W. Bu, V. M. Rotello, M. Meron, P. Král, B. Lin, and M. L. Schlossman, Nano Letters 14, 6816-6822 (2014).

[2] Tuning Ion Correlations at an Electrified Soft Interface, Nouamane Laanait, Miroslav Mihaylov, Binyang Hou, Hao Yu, Petr Vanysek, Mati Meron, Binhua Lin, Ilan Benjamin, and Mark L. Schlossman, Proceedings of the National Academy of Sciences (USA), 109, 20326-20331 (2012).