Improve Detection Performance Of SALDI-MS By Micro-nano Composite Structure

Surface-assisted laser desorption/ionization mass spectrometry（SALDI-MS）is a soft ionization analytical technique,which is applicable for trace analysis of small molecules.To improve the performance of SALDI-MS detection of small molecules,a variety of substrates have been explored,including semiconductors,carbons,metals and composites.Among them,silicon-based micro/nano materials have unique properties of optics,electricity,as well as surface chemistry,which can be processed into various structures with controllable morphology,and the surface of silicon is easy to be functionalized.Carbon-based materials can absorb UV laser strongly by sp²hybridized carbon skeleton with delocalizedπ-electrons,and desorb/ionize the analyte through thermal energy and electron transfer.With the rapid development of society,qualitative and quantitative analysis of trace substances play an increasingly important role in various fields.Therefore,to enhance the desorption/ionization efficiency and improve its detection performance of SALDI-MS,the following research was carried out in this paper by combining various methods for micro/nano structures fabrication and surface functionalization:Firstly,for improving detection reproducibility and sensitivity of SALDI-MS,and achieving quantitative analysis and selective detection of small molecules,we prepared an ordered metal/semiconductor composite structure composed of Au nanoparticles（Au NPs）and silicon nanopillars（Si NPs）as SALDI-MS substrate based on self-assembly,metal-assisted chemical etching and electroless deposition techniques.This metal/semiconductor composite structure can facilitate the separation of electron-hole pair and interfacial charge transfer,thus improving charge utilization efficient.The substrate has the following advantages:（1）The hydrophobicity and order of Si NPs can provide excellent detection repeatability.When detecting malachite green（MG）at 10^-5mol/L,the relative standard deviation（RSD）are 1.96%for point-to-point and 2.40%for substrate-to-substrate.In the range of 10^-9～10^-5 mol/L,there is a good linear correlation between the concentration of MG and signal intensity in logarithmic coordinates（R²>0.99）.（2）Au NPs can capture specifically bio-thiol molecules through Au-S bond,thus the substrate can be used for selective detection.（3）The substrate is applicable for analyzing practical samples,such as bradykinin 1-7 and verapamil in human serum,methadone in human urine and MG in lake water.Secondly,to solve the problems of directly sample difficulty on irregular objects and background interference of SALDI-MS in small molecule detection,a flexible substrate composed of polyamide（PA6）and carbon soot nanoparticles（CS NPs）was prepared by electrospinning technique.The flexibility of nanofibers allows for sampling from irregular objects directly.CS NPs have excellent UV absorption,graphitization quality and high photothermal conversion efficiency,which is beneficial to heat transfer.CS NPs were uniformly encapsulated in PA6 nanofibers,and the interference of CS NPs on SALDI-MS detection was eliminated.The experimental results show that:（1）After hydrophobic treatment,the flexible substrate has high detection sensitivity,good reproducibility and no background peak interference in the low mass range.With this substrate,small molecules can be quantified successfully with the limit of detection as low as 10^-10 mol/L.（2）The detection results of methadone in human urine and MG in fish skin showed the promising applications of the substrate in analyzing real samples and directly sampling from irregular objects,which simplified sample pretreatment process.