Single-sided inkjet functionalization of silicon photonic microcantilevers

We investigate inkjet deposition of protein-containing fluid to individually functionalize top surfaces of silicon photonic microcantilevers (PMCLs) in a microcantilever array. An array is first prepared with vapor-deposited epoxysilane (3-glycidoxypropyl) trimethoxysilane (GOPS), followed by inkjet deposition of functionalization fluid containing a model receptor molecule, biotinylated bovine serum albumin (bBSA). We find that a 61% (w/w) solution of glycerol achieves sufficient viscosity for reliable jetting, along with either Tween 20 or Triton X-100 as surfactant to prevent pooling of liquid on inkjet nozzles. We demonstrate that increasing relative humidity during incubation can be used to achieve uniform coverage of each PMCL surface with functionalization fluid through droplet swelling. Such swelling can also be designed to create a region off the base of each PMCL at which dissolved salts aggregate during droplet drying, thereby avoiding interference with or fouling of receptor molecule sites on PMCLs for significant improvement in active site density and uniformity. After functionalization, incubation, and droplet drying, a silicon PMCL die is integrated with polydimethylsiloxane (PDMS) microfluidics such that a PMCL array forms the bottom surface of a flow channel. A solution containing Alexa Fluor 514 labeled streptavidin is introduced into the flow channel to bind to bBSA on PMCLs. Fluorescence imaging is used to comparatively evaluate the uniformity and density of active receptor molecule attachment to the PMCLs for functionalization fluids with different pH and buffer species. In each case uniformity is enhanced by formation of a bulb at the microcantilever base during droplet swelling.