Visualization of Diffusion within Nanoarrays

Abstract

The direct experimental characterization of diffusion processes at nanoscale remains a challenge that could help elucidate processes in biology, medicine and technology. In this report, two experimental approaches were employed to visualize ion diffusion profiles at the orifices of nanopores (radius (ra) of 86 ± 6 nm) in array format: (1) electrochemically assisted formation of silica deposits based on surfactant ion transfer across nanointerfaces between two immiscible electrolyte solutions (nanoITIES); (2) combined atomic force - scanning electrochemical microscopy (AFM-SECM) imaging of topography and redox species diffusion through the nanopores. The nature of the diffusion zones formed around the pores is directly related to the interpore distance within the array. Nanopore arrays with different ratios of pore center-to-center separation (rc) to pore radius (ra) were fabricated by focused ion beam (FIB) milling of silicon nitride (SiN) membranes, with 100 pores in a hexagonal arrangement. The ion diffusion profiles determined by the two visualization methods indicated the formation of overlapped or independent diffusion profiles at nanopore arrays with rc/ra ratios of 21 ± 2 and 91 ± 7, respectively. In particular, the silica deposition method resulted in formation of a single deposit encompassing the complete array with closer nanopore arrangement, whereas individual silica deposits were formed around each nanopore within the more widely spaced array. The methods reveal direct experimental evidence of diffusion zones at nanopore arrays and provide practical illustration that the pore-pore separation within such arrays has a significant impact on diffusional transport as the pore size is reduced to the nanoscale. These approaches to nanoscale diffusion zone visualization open up possibilities for better understanding of molecular transport processes within miniaturized systems.

Ion transfer at polarised liquid-liquid interfaces modified with adsorbed silica nanoparticles

“…The growing interest for the use of silica-based materials at the liquid-liquid interface is also due to their favourable adsorption kinetics and selectivity [28], which could be harnessed to the development of new sensing methods [29,30]. ITIES was modified by the electrogeneration of mesoporous silica deposits [31][32][33][34][35], which were the result of an electrochemically driven ion transfer followed by interfacial precipitation. Mesoporous silica was generated at the ITIES by the transfer of a cationic surfactant (e.g.…”

Decorating soft electrified interfaces: From molecular assemblies to nano-objects

“…Despite this improvement, the signal measured at such nanoarrays was only half of the value expected for such devices [186,187]. This discrepancy between experimental observations and theoretical calculations were due to the overlap of the diffusion zones for densely packed nano-interfaces (see Figure 9A), while widely spread interfaces showed independent diffusion profiles (see Figure 9B) [115]. By increasing the ratio pore-topore separation (S) / pore radius (r), the sensitivity increased and limits of detection improved until they reach a plateau for ratio values greater than 56 [188]; leading to the detection of 0.8 µM of propranolol using a moderately sensitive technique such as linear sweep voltammetry [189].…”

“…It was also found that silica deposits can be stabilized and reused once proper support was employed. Silicon wafers patterned with an array of micro or nano pores each supporting the ITIES were modified as shown on Figure 5B and Figure 6B,D respectively [113,115]. Interestingly, the shape of resulting silica deposits was found to be a print out of the diffusion zones of CTA + on the aqueous site of the liquidliquid interface.…”

Advances and Perspectives in Chemical Imaging in Cellular Environments Using Electrochemical Methods

“…SECM-AFM uses a probe that has topographical resolution based on the size and shape of the sharp AFM tip and electrochemical imaging resolution based on the geometry of the surrounding electrode (Figure 1f). This technique has been used to study ionic diffusion at nanopores, simultaneously with electrochemical currents associated with flux of species through the pores (SECM) and the pore topography/spacing (AFM) [41].…”