Abstract

The wet chemical synthesis of nanostructures has many crucial advantages over high-temperature methods, including simplicity, low-cost, and deposition on almost arbitrary substrates. Nevertheless, the density-controlled solution growth of nanowires still remains a challenge, especially at the low densities (e.g. 1 to 10 nanowires/100 μm2) required, as an example, for intracellular analyses. Here, we demonstrate the solution-growth of ZnO nanowires using a thin chromium film as a nucleation inhibitor and Au size-selected nanoclusters (SSNCs) as catalytic particles for which the density and, in contrast with previous reports, size can be accurately controlled. Our results also provide evidence that the enhanced ZnO hetero-nucleation is dominated by Au SSNCs catalysis rather than by layer adaptation. The proposed approach only uses low temperatures (≤70 °C) and is therefore suitable for any substrate, including printed circuit boards (PCBs) and the plastic substrates which are routinely used for cell cultures. As a proof-of-concept we report the density-controlled synthesis of ZnO nanowires on flexible PCBs, thus opening the way to assembling compact intracellular-analysis systems, including nanowires, electronics, and microfluidics, on a single substrate.

Highlights

  • Boards (PCB)[6], a compact lab-on-printed circuit boards (PCBs) for intra-cellular analysis, comprising electronics, microfluidics and nanowires, can in principle be fabricated on a single PCB; though nanowires on conventional PCBs have already been reported[7,8,9] a critical task for fabricating such a system would be the density-controlled solution-growth of nanowires at the low density levels required for intracellular analyses

  • An accurate control of the density of ZnO nanowires has been achieved using Au-PMMA nanoparticles as catalysts[17], resulting in nanowires densities ranging from about 25 nanowires/100 μ m2 to about 1500 nanowires/100 μ m2; the density could be further reduced by lowering the concentration of Au-PMMA nanoparticles, allowing to achieve the low densities required for intracellular studies[2]

  • Gold nanoclusters have been applied to the wet-chemistry synthesis of ZnO nanostructures; for instance, 6–8 nm sized gold nanoclusters were inglobated into ZnO pyramidal structures with 15 nm to 25 nm side-edge length, with the gold nanoparticle seed located centrally at the basal surface[16]

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Summary

Introduction

Boards (PCB)[6], a compact lab-on-PCB for intra-cellular analysis, comprising electronics, microfluidics and nanowires, can in principle be fabricated on a single PCB; though nanowires on conventional PCBs have already been reported[7,8,9] a critical task for fabricating such a system would be the density-controlled solution-growth of nanowires at the low density levels required for intracellular analyses. Since, for a given number of nanoparticles, smaller particles resulted in higher densities of ZnO nanowires, we conclude that, first, the enhanced ZnO hetero-nucleation is dominated by catalysis rather than by layer-adaptation and, second, that the catalytic properties of the nanoparticles are stronger when shrinking the particle size, as expected Among other factors, such as SSNC aggregation or atomic structure variations, a general argument to explain the observed trend would be that the surface to volume ratio increases with downscaling, so the smaller the nanocluster, the larger the fraction of superficial atoms[25] and, the greater the expected catalytic activity[26]. Though our method is general, as a proof-of-concept we have used conventional flexible PCB-substrates which would, be the ultimate low-cost solution for assembling into a single substrate a complete intracellular analysis system comprising nanowires, electronics, and microfluidics

Objectives
Methods
Results

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