Proteins in focus: Dynamically tunable protein microdevices were built up by a simple “top‐down”, maskless, femtosecond laser direct writing approach with bovine serum albumin. This technique was used to produce biocompatible microlenses that swell and shrink reversibly in response to changes in the pH of the surrounding solution. These responses to environmental stimuli can be used to focus the microlenses.
Proteins have been utilized in numerous photonic and optoelectronic devices, for example, in optical computation, [1] organic light emitting diodes (OLEDs), [2] waveguides, [3] biomicro/nanolasers, [3] organic field effect transistors (OFETs), [4] and memory devices, because their unique optical, mechanical, electrical, and chemical properties are easily tailored to each application. [5] The performance of as-prepared proteinbased photonic devices has been demonstrated to exceed that of devices that are made with currently available organic materials. [1][2][3][4][5][6][7][8][9][10][11][12] The underlying motivation for the use of proteins in microdevices is not only abundance, inexpensiveness, and biodegradability, but also biocompatibility and the capacity to tune their properties through appropriate external stimuli. These features are highly desirable for biologically inspired microdevices, for example delicate miniaturized lenses that are similar to the "camera-type" eyes of human beings, the compound eyes of insects, the photosensitive microlens arrays of brittlestars, or the infrared-sensitive microlens receptor arrays of the fire beetle (Melanophila acuminata). [13][14][15] Scientists have been highly motivated to fabricate these lens-like micro/nanostructures with the aim of producing small, multifunctional, artificial eyes by using dynamically adjustable and fully biocompatible proteins. However, the preparation of protein microlenses that have controlled geometry and precise positioning still poses a challenge.Herein, we report a promising approach for the production of biomimetic protein microlenses by facile and rapid maskless femtosecond laser direct writing (FsLDW). FsLDW is a well-known method for producing complicated 3D structures with nanometric resolution. [16] Recently, pioneering work from Shear and co-workers demonstrated that protein hydrogel-based microstructures fabricated by FsLDW exhibit a unique responsiveness to chemical signals. This responsiveness could result in rapid and reversible changes in the size and shape of the structures after stimulation by environmental triggers. [17] However, to our knowledge there are few reports on the development of practical and useful devices, such as a tunable microlens, that are made from this class of proteins.In this study, commercial bovine serum albumin (BSA, 300-500 mg mL À1 in aqueous solution) and a photosensitizer (methylene blue, MB, 0.6 mg mL À1 ) were used to fabricate micro/nanoarchitectures. The cross-linking reaction is initiated through the excitation of photosensitive molecules to their triplet states. The photoexcited molecules then react directly with oxidizable moieties (type I process) or transfer the energy to ground state molecular oxygen (type II process) to form a reactive oxygen species, such as singlet oxygen ( 1 O 2 ). In either case, excited-state intermediates catalyze the inter or intramolecular covalent cross-linking of oxidizable protein residues (see Scheme S1 in the Supporting Information). In other words, prote...
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