Direct laser writing of photoresponsive colloids for microscale patterning of 3D porous structures

Several routes have recently been introduced for microscale patterning of materials in three dimensions, including multilayer photolithography, nanotransfer printing, LiGA, a german acronym for Lithographie–Galvanoformung–Abformung (lithography-electroplating-molding), microstereolithography, and multiphoton polymerization. Each of these routes typically yields a solid structure, yet novel porous architectures such as those assembled from colloidal building blocks, are required for applications ranging from microfluidic filters and mixing elements to catalyst supports. Direct-write assembly of colloidal inks offers a pathway for creating the desired porous structures. However, their minimum dimensions must exceed 100mm to maintain continuous ink flow during deposition. To overcome this limitation, we harness the power of multiphoton direct laser writing to locally define the interactions between photoswitchable colloidal microspheres suspended in an organic solvent. Through this novel approach, we create porous-walled 3D structures includingmicroscale rectangular cavities that exhibit size-selective permeability. Direct laser writing of photoresponsive colloids consists of three basic steps (see Fig. 1a–c). First, we produce a dense colloidal suspension via sedimentation of a dilute solution of photoresponsive microspheres (Fig. 1a). Next, we locally induce colloidal gelation by photoswitching these microspheres from a repulsive to an attractive state (Fig. 1b). We achieve this transformation by rastering a high-intensity near-IR pulsed laser that alters the polymer brush chemistry, and hence colloidal stability, via a two-photon absorption process. Finally, we remove the unexposed microspheres through a simple rinsing step, leaving behind the desired 3D structure (Fig. 1c). The photoresponsive microspheres are formed by grafting a copolymer brush 55 nm thick (dry thickness) onto silica colloids with 927 nm diameter (Fig. 1d). The brush layer is grown using surface-initiated atom-transfer radical polymerization (ATRP), and is composed of methyl methacrylate (MMA) containing 20% spirobenzopyran (SP) pendant groups poly(SP-co-MMA), as shown in Figure 1e. In the SP-form, the microspheres are sterically stabilized when suspended in a nonpolar solvent such as toluene. However, upon irradiation with UV or high-intensity near-IR pulsed radiation (utilizing two-photon absorption), the SP side groups photoisomerize into the polar, zwitterionicmerocyanine (MC) form (see Fig. 1f). In toluene, the exposed colloids, now coated with the MC-rich form of the copolymer, undergo rapid flocculation when they come into contact via Brownian motion. It has been postulated that MC–MMA, MC–SP, and H-stacked MC–MC aggregates are all present, and contribute to the strong intermolecular and interparticle bonding in this system. While the reverse reaction back to the sterically stabilized SP-form can be induced upon exposure to visible wavelengths (lmax1⁄4 585 nm), the colloidal microspheres remain flocculated, and significant mechanical agitation is required to break up the particle network. Figure 2 depicts two simple structures fabricated using direct laser writing that illustrate the power of this approach. The ‘‘MRL’’ structure is composed of high-aspect-ratio walls (Fig. 2a–c), and the ‘‘mushrooms’’ are examples of complex 3D self-supporting features (Fig. 2d–i). Spatially defined C O M M U N IC A IO N