A facile approach to patterning pollen microparticles for in situ imaging

Pollen is one of nature's most resilient materials and widely used as chemically stable environmental markers and in drug delivery. Recent findings show that a simple alkali treatment, similar to traditional soapmaking, can convert hard pollen grains into soft microgel particles, which exhibit stimuli-responsive changes in size and mechanical properties. Surface-based measurement approaches offer excellent potential to monitor these stimuli-responsive behaviors, however, such methods are encumbered by weakly adsorbing, free-flowing microparticles and a tendency for the microparticles to aggregate on substrates. Here, we demonstrate a facile approach to pattern soft pollen microparticles and track stimuli-responsive behaviors in situ based on a combination of chemical functionalization and scanning probe manipulation of individual microparticles. First, we prepared an amine-functionalized glass substrate that can covalently attach to carboxylic acid functional groups on the microparticle surface, tethering the particles stably. Second, we used a scanning probe cantilever to pick up and move individual microparticles to specific locations on the functionalized glass surface. Various patterns of well-separated, tethered microparticles were fabricated, including aligned arrays, hearts, and stars, and we could track the stimuli-responsive behavior of individual microparticles with in situ imaging. Taken together, this patterning concept can be broadly utilized to manipulate various classes of intelligent microparticles across fields such as diagnostics, biosensors, drug delivery, and chemo-mechanical actuators.