Fabrication Of Metal Microstructures By Direct Laser Writing Based On Two-Photon Absorption

Electrophysiological signals at the microscale have attracted significant attention with the research and application of bioelectrical phenomena.To realize the electrical interaction with tissues and even single cells,conductive microstructures with editability and feature size close to the biological target are needed.The existing preparation methods have many disadvantages.For example,conductive-materials-doped structures built by extrusion 3D printing have low resolution and complexity.Photolithography needs the assistance of a mask,and the process is cumbersome and inflexible.In this paper,we proposed and demonstrated the fabrication of metal microstructures with high editability and resolution with direct laser writing（DLW）based on two-photon absorption（2PA）.Two DLW recipes were developed,i.e.,the ink containing metal salt and laser dye and the ink containing metal salt and ionic liquid.In addition,the conductivity and surface-enhanced Raman scattering（SERS）performance of the microstructures were tested,which illustrated the possibility of our method in preparing microstructures with high resolution,complexity and conductivity.Here are the works in detail:（1）The ink containing Ag NO₃（silver salt）and Coumarin 440（laser dye）was developed.The writing parameters,including laser wavelength,the concentration of laser dye,and the composition of the solvent,were studied.It was found that the 2PA of laser dye contributed to reducing silver ions.Besides,the influence of exposure power on the linewidth of the metal structure was studied to obtain the optimal DLW power and scanning speed.However,it was found that the writing process was heavily dependent on thermal reduction.（2）To overcome the dependence of thermal reduction during DLW,the ink containing Ag BF₄（silver salt）and 1-butyl-3-methylimidazolium tetrafluoroborate（[Bmim][BF₄],ionic liquid）was developed.Suitable metal salt and ionic liquid were selected through the solubility screening and spectrum analysis.The optimal parameters were determined by changing laser power and scanning speed in the DLW.The obtained microstructures showed better performance than the recipe containing silver salt and laser dye.In addition,the reaction mechanism was studied,which could be extended to the ink containing gold salt for generating gold microstructure.（3）The conductivity and SERS performance of the fabricated metal microstructure were characterized.The chemical modification of interdigital electrodes was applied to support the conductivity measuring of DLW microstructures.Then,the destructive effect induced by applying voltage or current to the metal microstructure was found.With the SERS probe molecule of Nile blue,the SERS signal was checked from the metal microstructure.It was found that signal intensity from the silver microstructure is better than that of the gold.