In the CheckNano project, we have developed two alternatives and scalable approaches to engineer multi-scale particle arrays, based on capillary force assisted nanoparticle assembly (CAPA) and rapid template-assisted trapping of silver nanospheres with topographically patterned polydimethylsiloxane moulds (PDMS). The latter is a modified version of the template-assisted self-assembly (TASA) approach, where the overall time of the process was reduced to less than 5 min (rTASA) – Fig. 1. Instead of trapping single particles, which would result in a relatively low signal to noise ratio, we are trapping the particles into close-packed arrangements. Close-packed particle poses rich plasmonic resonances, which enable tailoring the optical response, on both the nano- and the macroscale.
In both assembly methods, topographically structures PDMS moulds of nanosized holes are utilized to direct the assembly of monodisperse nanoparticles into structured cluster arrays. While the CAPA ensures the highest precision in particle positioning, leading to full controlled tailoring of the optical properties (compare also here), the rTASA brings speed and user-friendliness, eliminating steps requiring precise meniscus control.
Figure 1 Rapid Template assisted particle assembly (rTASA)
For demonstration, a mixture of 50 µL of concentrated Ag (5 mg/mL, 50 nm diameter) nanoparticle diluted with 200 µL DI water and 200 µL ethanol was deposited on a two-dimensional square lattice of PDMS. Finally, the optical transmission data of both CAPA and rTASA colloidal patterning were explored in the VIS-NIR spectral range. Depending on the array pitch distance and number of particles per trap, the average transmission drop is between 20 – 80%, which makes the detection possible using the most simple spectroscopic solutions.