| Matrix-assisted laser desorption/ionization mass spectrometry(MALDI-MS)is a soft ionization technology,with the significant advantages of low sample consumption,simple operation,high throughput,high salt tolerance and in situ analysis.The matrixes are usually organic small molecules,which can absorb laser energy and promote the desorption and ionization of analyte molecules.MALDI-MS is more suitable for the detection of large molecules,because there are serious interferences in the low m/z range caused by the self-desorption of matrixes.In addition,the detection repeatability also needs to be improved.With the rapid development of nanotechnology,surface-assisted laser desorption/ionization mass spectrometry(SALDI-MS)was proposed,which takes inorganic nanomaterials or nanostructured surfaces instead of organic matrixes to assist laser desorption/ionization(LDI)of analytes.This method not only inherits the above advantages of MALDI-MS,but also features low background interference and good detection reproducibility,which has been widely used in small molecule analysis.A variety of nanomaterials and nanostructured surfaces have been exploited for SALDI-MS.Though great progress has been made,the following issues are still needed to be investigated:(1)revealing the fundamental mechanisms of LDI in SALDI-MS;(2)developing new substrates with simple preparation and high detection sensitivity towards large-scale application.In this thesis,the following investigations were carried out based on the fabrication of micro/nano structures:Firstly,a superhydrophobic polymethyl methacrylate(PMMA)microspheres MALDI-MS substrate was prepared by self-assembly method to improve the repeatability and sensitivity for detecting peptide.The superhydrophobic surface with low-friction prevents the contact line of droplet from pinning onto the substrate,and the PMMA can capture peptides through branches chains to weaken their outward transport,thus eliminating the"coffee ring"effect;the uniformly distributed molecules provide homogeneous nucleation sites for the co-crystallization with matrixes,resulting in repeatable signals.The relative standard deviations of the three tested peptides were all less than 10%and their linear correlation coefficients between the concentrations and signal intensities were all higher than 0.99,which prove that the substrate is capable of rapid quantification without internal standard.In addition,the limit of detection was lowered by three orders of magnitude on the substrate compared to conventional steel targets thanks to the superhydrophobic enrichment effect.The applicability of this substrate in real samples was proved by quantitative analysis of bacitracin A in milk extract and bradykinin 1-7 in serum without internal standard.This substrate exhibits significant advantages in analysis of peptides and has great potential in proteomics research.Secondly,two typical Si nanostructures,porous Si(PSi)and thorny Si(TSi),were prepared by reactive ion etching and metal catalytic etching,respectively.The correlation of the LDI performance and the morphologies of Si nanostructures was investigated by keeping two kinds of substrates with the similar absorption at 355 nm.The results show that LDI efficiency varies with the structure morphology.The nanoporous structure of PSi can promote the ion desorption because of its strong thermal effect;the nanocone structure of TSi facilitate molecular ionization by promoted electron transfer.This hypothesis was further confirmed by detecting fourteen of standard samples and complex sample on PSi and TSi substrates.This work provides a theoretical basis for improving LDI performance by regulating the Si nanostructure,which is helpful to guide the design of Si substrates with high LDI performance.Thirdly,high-density Si nanotips SALDI-MS substrate was prepared via a simple template-free reactive ion etching.The dense tips act as“antennas”,not only limit energy in a small region,but also promote electron transfer.In addition,the stability of Si ensures low background interference.Therefore,the substrate exhibits high detection sensitivity,which can be used to detect various small molecules,such as fatty acids,amino acids,environmental pollutants and drugs.The substrate can also be used to detect trace small molecules in serum,lake water and fish skin.Moreover,it is compatible with MS imaging,enabling the visual analysis of endogenous and exogenous substances in fingerprints.The results show that the high-density Si tips,which can be prepared in large area,have great practical application value with the advantages of high detection sensitivity and high stability.Fourthly,a composite SALDI-MS substrate formed by high-density Si nanotips and Ag nanoislands was prepared by combining reactive ion etching and vacuum thermal evaporation.In which,the high-density Si nanotips can effectively absorb laser energy,and promote electron transfer;Ag nanoislands can enhance the thermal effect;the formation of Schottky barrier and conductive nanolayer between semiconductor and metal prolong electron lifetime and promote electron tunneling effect.Therefore,this substrate provides a high detection sensitivity(the limit of detection for sulfamethazine is as low as 10-12 M).In addition,the substrate can sensitively identify and rapidly quantify sulfa antibiotics in complex samples,indicating its great potential applications in food and environmental monitoring. |
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