Surface‐Functionalized Nanoparticles with Liquid‐Like Behavior

Abstract

Surface engineering of nanoparticles has fueled the evolution of nanoscience and nanotechnology through the design of new functional materials with novel electronic, magnetic, optical, and biological properties. Depending on the surface modifier, which can vary from a simple molecule to a complex biomolecule, nanoparticles with a range of functional properties and potential applications can be produced.[1±6] In all these instances, in the absence of any solvent the functionalized nanoparticles appear and behave in a solid-like manner.In the present communication, we describe a novel family of functionalized nanoparticles that exhibit liquid-like behavior in the absence of any solvent. Surface modification of silica or maghemite (c-Fe 2 O 3 ) nanoparticles with a charged organosilane moiety renders the resulting hairy nanoparticles cationic. A counterion is typically present to balance the charge on the nanoparticles (henceforth described as nanosalts), as shown in Figure 1. Depending on the nature of the counterion, it is possible to isolate the nanosalts in liquid form, even at room temperature. These hybrid nanoparticles represent a unique class of solvent-free colloids that are distinguished from conventional colloidal suspensions in a solvent. Their fluidity in the absence of any solvent and zero vapor pressure offers significant new scientific and technological opportunities. For instance, such systems can address some environmental concerns associated with solid nanoparticles. They can also provide a better means to process nanoparticles into films or other functional forms. In addition, they can offer new, solvent-free conducting, magnetic, or electrorheological fluids, among others. Lastly, space restrictions imposed by the dimensions of the nanoparticles, in conjunction with their intrinsic physicochemical properties, make this class of materials particularly attractive as new reaction media.Silica nanoparticles (diameter 7 nm) were modified by condensation of (CH 3 (R = alkyl chain) to obtain the corresponding nanosalts. When chloride is the counter anion, the nanosalt is isolated in a powder form. No melting is observed even after heating to 150 C, above the surface decomposition temperature of the organic-surface modifier. In contrast, replacement of the chloride by R(OCH 2 CH 2 ) 7 O(CH 2 ) 3 SO 3 ± ions yields a clear liquid at room temperature (Fig. 2a). The liquid-like nature of the nanosalt is demonstrated by its ability to dissolve the polar dye methylene blue as a common solvent would. Non-polar dyes (e.g., coumarin derivatives) can also be dissolved in the nanosalt, affording transparent, colored liquids. Furthermore, we were able to dissolve and polymerize pyrrole to form polypyrrole in the nanosalt medium (Fig. 2b). The high organic content of the material (vide infra), nanometer size, and density of the silica all play a key role in producing the liquid nanosalt. The differential scanning calorimetry (DSC) profile of the sulfonate nanosalt shows a reversible first-order endothermic phase ...

Diffusivities, viscosities, and conductivities of solvent-free ionically grafted nanoparticles

“…1,17,18,25,26 Among these prior studies, ref. 10 and 27 are the most comprehensive, covering local to global dynamics of NIMs through analyses of dielectric spectroscopy, shear moduli, and viscosities.…”

“…[1][2][3][4] Experiments and modeling suggest that their self-suspending, solvent-free character is a key determining factor for the structure and dynamics of these systems. 3,[5][6][7][8][9] While progress has been made in theoretical understanding of the simpler versions of these systems, in which chains are covalently graed on the particles giving rise to "Nanoscale Organic Hybrid Materials," there have been relatively few modeling attempts of their ionically graed counterparts -oen referred to as "Nanoparticle Ionic Materials" (NIMs).…”

Magnetic resonance in nanoparticles: between ferro- and paramagnetism

“…To check whether the narrow component was related to the organic groups on the nanoparticle surface, we ran the ESR spectra of the systems containing similar surface modified silica nanoparticles, fabricated by the same method [20]. No signal has been observed.…”

Dynamics of solvent-free grafted nanoparticles

“…Experimental realization of such solvent-free composite systems has been recently demonstrated in the form of "nanoparticle organic hybrid materials" (NOHMs). [10][11][12][13][14][15] These are core-shell systems consisting of a hard (inorganic) core and a soft (organic) oligomer corona. By varying the molecular weight and grafting density of the arms (corona), and the core particle size, they can display properties that span from glasses, stiff waxes, and gels (at high volume fractions), to simple liquids comprised of molecular building blocks (at a) Author to whom correspondence should be addressed.…”