Contact Pin-Printing onto Porous Silicon for Creating Microarrays with High Chemical Diversity

We explore the size and spatial microheterogeneity of contact pin-printed spots formed on porous silicon (pSi). Glycerol was contact printed at room temperature onto as-prepared, hydrogen-passivated pSi (ap-pSi) using 50 or 200 µm diameter solid pins. The pSi was then subjected to a strong oxidizing environment (gaseous O₃) and washed to remove the glycerol masks. The glycerol-free regions were converted to oxidized pSi (ox-pSi); the glycerol-coated regions were protected from O₃, but not entirely. The final array is described as circularly shaped "ap-pSi" regions on a field of ox-pSi. When comparing the areas outside and inside the glycerol-masked pSi spots, one finds dramatic differences in the Si-O-Si, SiH_x (x = 1-3) and O_ySiH_x (y, x = 1-3) levels with a spatially dependent continuum of compositions across the spot diameter. Experimental conditions could be adjusted to tune the final ap-pSi spot diameter and edge widths from 90 µm to 520 µm and 20 µm to 130 µm, respectively. The resulting ap-pSi spot diameter is explained by using molecular kinetic theory and time-dependent glycerol imbibement into the pSi within a one-dimensional Darcy's law model.