Fabrication Of Si Micro/Nanostructures Based On Metal-Assisted Chemical Etching And Their Applications In SALDI MS

As the 2nd most abundant element on the earth,silicon?Si?has become one of the most important materials in modern semiconductor industry,due to its properties of low cost,abundance of storage,non-toxicity,advanced processing technology,and so on.Si micro/nanostructures have been widely applied in the fields of photovoltaic,photoelectrochemical catalysis,photochemical and biosensors,surface-assisted laser desorption/ionization mass spectrometry?SALDI MS?,etc.There are many methods for fabricating Si micro/nanostructures,such as“vapour-liquid-soild”growth,dry etching,wet etching,etc.Among these methods,metal-assisted chemical etching?MaCE?as a wet etching method has attracted more and more attentions due to its advantages of easy operation,low-cost,easy preparation of structures with high aspect ratio,etc.With the rapid development of modern science,SALDI MS becomes an important technology for trace analysis of small molecules.Many materials can be used as SALDI substrates,such as Si micro/nanostructures,carbon-based materials,metal and metal oxide nanoparticles,etc.Among these materials,Si micro/nanostructures play an important role in SALDI MS due to their strong absorption at UV region,high salt tolerance and good stability.Herein,we fabricated Si micro/nanostructures based on MaCE combined with colloidal lithography,thermal evaporation,photolithography,etc.We investigated the properties of the fabricated Si micro/nanostructures,and their applications in SALDI MS.This thesis includes the following four parts:Firstly,Si nanopillar arrays were fabricated by metal-assisted chemical etching?MaCE?combined with colloidal lithography.The Si nanopillars with different heights and diameters were obtained by controlling the durations of MaCE and that of the etching of colloids.The reflectivity of Si nanopillars decreased with the increasing of heights of Si nanopillars,and the reflectivity peaks blue shifted with the reducing nanopillar diameter.The results indicate that the antireflection performance can be tuned by adjusting the height and diameter of nanopillars.Secondly,we proposed a facile method to precisely regulate the tilt angle of nanocones with MaCE in one-step process,based on the systematical investigation of the formation mechanism of gradient.With the Au nanohole arrays as templates,the nanocone arrays with tilted angle ranging from 69.2°to 88.6°were fabricated by controlling the component of etchant.The tilt angle of nanocones was decreased with increasing the ratio of H₂O₂ and HF,or decreasing the concentrations of H₂O₂ and HF in a certain concentration range.The reflectivity of Si nanocones was decreased by reducing the tilt angle when the heights of Si nanocones are the same.Thirdly,we created hydrophilic spots on the hydrophobic nanocone arrays fabricated with the method described in third part.The designed substrates improved the reproducibility of SALDI MS by concentrating all the analyte molecules within a laser spot,which can eliminate the influence of coffee ring effect.The good reproducibilities were obtained by testing rhodamine 6G and malachite green.It also performs a good linear dependency?R²>0.98?in the log-log plot with the concentration in the range of 1 nM to 1?M,and the limit of detection for R6G is low to 1 fmol.However,it is a challenge to precisely find the sample spot without fluorescence due to the substrate without identifiable marks.A method applicable for any analytes still needs to be explored in future study.Finally,in order to overcome the limitation of the above method,we proposed a universal and efficient method to improve sensitivity and reproducibility of SALDI MS.The Si pillar with hydrophilic top,hydrophobic bottom and side wall was fabricated based on the combination of photolithography and MaCE.Then,silver nanoparticles were deposited on the top of Si pillar by electroless deposition,which was used as SALDI substrate.Analyte solution can be confined on the top of Si pillar due to its unique surface wettability.The size of the Si pillar is visible and smaller than the laser spot,which makes the sample spot easily be found and all the analytes under one laser shot.The sensitivity and reproducibility of SALDI TOF MS were improved.Four species of analytes,including tyrosine,R6G,bradykinin and PEG,were successfully detected with low concentrations using the created substrate.The good reproducibility and linearity of the substrate are further confirmed by detecting Ang III.The relative standard deviation of signal intensities is down to 5.39%,and good linearity?R²>0.98?is shown over the concentration range of 5 to 1000 fmol/?L.Therefore,localizing ananlyte spot on the visible Si pillar should be a promising method for high sensitive and reproducible detection in SALDI MS.