Templating using molecular gradients

The production of structures comprising organized arrays of nanoparticles embedded in material matrices represents one of the most important challenges facing today’s materials scientists and engineers. The macroscopic properties of the nanoparticle-based composites will reflect both the physical characteristics which are specific to nanoobjects (e.g., the ability to form ferromagnetic monodomains, optical microcavities or to generate third order harmonic optical waves) and those which are characteristic of large mesh periodic structure of the nanoparticles in the matrix (e.g., a possible coherent response to electromagnetic radiation). From a technological point of view, the unusual optical, electric, magnetic, adsorption, and other important materials characteristics of nanoparticle/matrix hybrids can be utilized in a large variety of applications, including high-density information storage media, magnetic fluids, medical diagnostics, molecular semiconductors, selective membranes and catalysts, etc.

We envision that molecular and macromolecular gradients can be used as templates for assembling nanosized objects. The gradient arrangement not only provides a combinatorial” approach for exploring the phase behavior of such systems, but perhaps more importantly, can lead to unprecedented technological applications that fully utilize the dual character of the gradients (discrete on nano/micro scales and continuous on the meso scale). Examples of such applications may include sensors, molecular electronic devices, protein separators, etc.

The Genzer group has recently shown that molecular gradients can be used as templates for assembling nanosized objects. They initiated a project aiming at the formation of gradient assemblies of gold nanoparticles on solid substrates. Bhat and coworkers utilized the vapor deposition method and formed gradients of aminopropyl triethoxy silane (APTES) on silica-covered substrates. Such gradient nanotemplate-containing substrates were immersed into an aqueous solution (pH~6.5) of gold nanoparticles (~17 nm diameter); the gold sol was prepared by citrate reduction of HAuCl4.

illustration of density of nanoparticles on the substrate

Negatively charged citrate molecules on the surfaces of the gold nanoparticles were electrostatically attracted to the APTES surfaces consisting of NH3+. Experiments using atomic force microscopy (AFM) revealed that the number density of nanoparticles on the substrate varied continuously as a function of the position on the substrate (see figure below). Studies using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy confirmed that the nanoparticle number density gradient was closely correlated with the concentration gradient of –NH2 groups anchored to the substrate. Bhat et al. demonstrated that the number density of nanoparticles within the gradient and the length of the gradient can be tuned by controlling the vapor diffusion of organosilane molecules.

top-image: afm images from the nanoparticle samples; bottom-image: concentration of gold nanoparticles

(top) AFM images from the nanoparticle gradient samples recorded at various distances along the substrate. (bottom) Concentration of gold nanoparticles deposited from colloidal gold solution (pH=6.5) onto a molecular gradient formed from APTES evaporated for 3 and 5 minutes. The solid line shows the concentration of the -NH2 groups in the sample measured by NEXAFS. The data points represent an average from 3 transverse scans along the gradient taken at the center of the sample (y=0 mm), y=-3 mm, and y=+3 mm. The area around the PEY NEXAFS line denotes the measurement uncertainty (based on 7 line scans along the gradient taken between – 3 mm and +3 mm from the center of the sample).

The potential of the vapor diffusion technique in forming planar molecular gradients can be utilized to prepare other interesting architectures of nanoparticles. For example, as can be seen from the figure below, we can construct “double gradient” architecture of nanoparticles by diffusing silane molecules simultaneously from both ends of the substrate, thus producing a “valley in nanoparticle concentration”. This architecture also shows an excellent agreement between the particle gradient and the underlying silane gradient.

Particle number density (left) and NEXAFS PEY (right) profile for a “double gradient” prepared by evaporating APTES/PO mixture for 3 min from both ends of the substrate followed by immersion in colloid gold solution for 24 hrs. The data points represent an average from 3 transverse scans along the gradient taken at the center of the sample (y=0 mm), y=-3 mm, and y=+3 mm. The area around the PEY NEXAFS line denotes the measurement uncertainty (based on 7 line scans along the gradient taken between – 3 mm and +3 mm from the center of the sample).

image of Particle number density (left) and NEXAFS PEY (right) profile
The silane diffusion technique of preparing molecular gradients allows one to have some control over the characteristics of the gradient. Effect of one such controlling factor viz. the time of diffusion of silane molecules in vapor phase, was already discussed. Some other parameters, which may be varied to tune the concentration gradient of APTES on the substrate and hence the gold particle gradient, include: i) composition of the silane/PO mixture, ii) temperature of the silane diffusion source, iii) temperature of the substrate, iv) concentration of the hydroxyl groups on the substrate, and v) humidity. In addition to these factors, we expect pH of the gold colloid solution and the time of immersion of the substrate in the colloid solution to have a substantial effect on the gradient characteristics. Work is in progress to carefully study the effect of these parameters.