Dynamically Tunable Protein Microlenses

Abstract

Proteins have been utilized in numerous photonic and optoelectronic devices, for example, in optical computation, organic light emitting diodes (OLEDs), waveguides, biomicro/nanolasers, organic field effect transistors (OFETs), and memory devices, because their unique optical, mechanical, electrical, and chemical properties are easily tailored to each application. The performance of as-prepared proteinbased photonic devices has been demonstrated to exceed that of devices that are made with currently available organic materials. 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). 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. Recently, pioneering work from Shear and co-workers demonstrated that protein hydrogel-based microstructures fabricated by FsLDWexhibit 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. 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 mgmL 1 in aqueous solution) and a photosensitizer (methylene blue, MB, 0.6 mgmL ) 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 (O2). 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, proteins with photooxidizable groups, such as Tyr, Trp, His, Met, and Cys, can absorb infrared or UV light to form reactive or ionized species that are capable of cross-linking with other oxidizable moieties. This mechanism appears to play a role in the formation of some types of cataracts and in the aging of skin. BSA and other proteins with oxidizable side chains “inherit” this photo-cross-linking ability, and can thus be used for multiphoton fabrication (Figure 1). Proof-of-concept protein microlenses were fabricated by using a FsLDW system of our own construction (Figure 1). The system was composed of a femtosecond titanium/ sapphire laser (Spectra Physics 3960-X1BB), a piezo stage with a precision of 1 nm (Physik Instrumente P-622.ZCD), and a set of two galvano mirrors. The 3D shapes of the microstructures were designed by using 3Ds Max and then the designs were converted into computer processing programs. Prior to the photo-cross-linking of the proteins at the focal spot, the beam from the femtosecond laser (80 MHz repetition rate, 120 fs pulse width, 780 nm central wavelength) was tightly focused by a high-numerical-aperture (NA= 1.35) oilimmersion objective lens (60 ). The horizontal and vertical scanning movements of the focused laser spot were achieved simultaneously by the two-galvano-mirror set and the piezo stage. After cross-linking, the sample was rinsed in water several times to remove unreacted proteins. Then the asformed protein microstructures were left on the chip. Surface topography (shape and roughness) plays an important role in the optical properties of protein microoptics. However, the surface roughness of BSA microstruc[*] Y. L. Sun, Prof. Dr. W. F. Dong, R. Z. Yang, X. Meng, L. Zhang, Prof. Dr. Q. D. Chen, Prof. Dr. H. B. Sun State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University, 2699 Qianjin Street Changchun 130012 (China) E-mail: dongwf@jlu.edu.cn hbsun@jlu.edu.cn