Desorption/ionization on silicon mass spectrometry (DIOS-MS) is a matrix-free technique that allows for the direct desorption/ionization of low-molecular-weight compounds with little or no fragmentation of analytes. This technique has a relatively high tolerance for contaminants commonly found in biological samples. Desorption/ionization on porous silicon (PSi) or silicon nanowires (SiNWs) mass spectrometry is a technique related to matrix-free laser desorption/ionization. In DIOS, PSi and SiNWs are used as sample support and substrate for the generation of intact gas-phase ions of small molecules upon irradiation with a pulse laser using standard MALDI (matrix-assisted laser desorption/ionization) instrumentation.
In this paper, we report the first use of “click chemistry” to couple functional alkynes to azide-terminated porous silicon and silicon nanowires and the subsequent analysis using DIOS-MS. Porous silicon (PSi)¹ is prepared by electrochemical anodization of crystalline silicon in HF-based solution while silicon nanowires (SiNWs) are obtained by chemical etching² of crystalline silicon or using chemical vapor deposition (CVD) techniques.^3,4
The Cu(I)-catalyzed 1,3-diploar cycloaddition of azides and alkynes, the premier example of “click chemistry” is an efficient route to make a covalent connection among diverse molecules and materials. Azide-terminated monolayers were prepared in a two step procedure starting from oxidized PSi and SiNWs surfaces, involving surface amination followed by subsequent coupling of the terminal NH₂ groups with 4-azidobenzoic acid. Amine termination was obtained by the reaction of the oxide with 3-aminopropyltrimethoxysilane at room temperature. The resulting NH₂-terminated surfaces were reacted with 4-azidobenzoic acid in the presence of N,N-dicyclohexylcarbodiimide to yield azide-modified substrates. The “clicking” between the azide groups and functional terminal alkynes in the presence of Cu(I) catalyst yields 1,2,3,-triazoles via the Huisgen 1,3-dipolar cycloaddition reaction. Terminal acetylene bearing different moieties such as ferrocene and sugar were successfully coupled to the PSi and SiNWs surfaces in high yield. The sequential reactions on the surface leading to alkyne immobilization were monitored using different surface analysis techniques, electrochemical measurements and DIOS-MS.

References

1. R. Boukherroub, S. Morin, D. D. M. Wayner, F. Bensebaa, G. I. Sproule, J.-M. Baribeau, and D. J. Lockwood, Chem. Mater. 13 (2001) 2002-2011.

2. K.Peng, Y.Wu, H.Fang, X.Zhong, Y.Xu, J.Zhu, Ang. Chem. Int. Ed. 44 (2005) 2737

3. B. Salhi, B. Grandidier and R. Boukherroub, J. Electroceram.16 (2006) 15-21.

4. N. Verplanck, E. Galopin, J.-C. Camart, V. Thomy, Y. Coffinier and R. Boukherroub Nano Lett. 7 (2007) 813-817.

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