Photo-Deformable Micro-Shells of Nanometer Thick

Azobenzene is a chemical structure that directly transmits energy of ultraviolet (UV) light, through an isomerization between the trans and cis states of a double bond between two nitrogen atoms, to a mechanical effect that reduces its molecular length from 9.0 Å to 5.5 Å [1]. This phenomenon has been applied in the fabrication of photo-deformable polymers, where monomers containing azobenzene were polymerized to form gel-like materials [1, 2, 3]. The mechanism of photo-isomerization could be very promising for the field concerning nano/micro-electro-mechanical-system (MEMS/NEMS) [4]. Layer-by-layer (LBL) method [5, 6] is a method to form thinfilms of nanometer-scale thickness through a series of adsorptions of poly-anions and poly-cations onto a charged template. 3-dimensional (3D) hollow shells can be formed by casting the LBL film onto small particles and subsequently etching away the particles. [7–11]. This report is about a combination between the photoisomerization phenomenon and LBL method. Silica micro-spheres (6.84 μm in diameter) were used as templates. In a series of LBL procedures, poly- (1- 4- 3 carboxy- 4- hydroxyphenyl azobenzene sulfonaide-1, 2-ethanediyl, sodium salt) (PAZO) and poly-allylamine hydrochloride (PAH, a poly-cation) solutions were alternatively deposited onto the surfaces of the micro-spheres. Details of the procedures followed reference [9]. After 20 PAZO/PAH double layers were formed, we etched away the SiO₂ cores by using 6:1 buffered hydrofluoride acid (BHF) and successfully formed 3-dimemsional (3D) hollow shells (Figure 1). We used a 355 nm Nd:YAG pulse-laser to irradiate the micro-shells. The microshells contracted anisotropically under the laser irradiation, and the shape of microshells changed from spherical to ellipsoidal. The long axes of the contracted microshell are parallel to the direction of the polarization of the UV laser (Figure 2). We believe that this anisotropic deformation is generated from a polarization dependence of the photo-isomerization rate of the azobenzene groups in the microshells. The deformed micro-shells were tested by irradiation of a 532nm pulse laser. The contraction, however, is not recoverable even through cis-trans isomerization was shown generated by visible-light irradiation [3]. To solve this problem, further study is needed to investigate the details of the micro-shells’ deformation. Nevertheless, we think it is quite possible that the irreversibility was resulted from the migrations and re-adhesions between the polyelectrolyte molecules during the UV irradiation. We believed that this kind of technique is very promising for the development micro and nanomachines (MEMS/NEMS). For instance, the large volumic shrinkage of the micro-shells could be applied to the field concerning nano-robots, artificial muscles, or drug delivery systems.