Anal. Chem. 2008, 80, 7524–7533
Layer-by-Layer Self-Assembled Mutilayer Films of Gold Nanoparticles for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry Hideya Kawasaki,† Tsuyoshi Sugitani,† Takehiro Watanabe,† Tetsu Yonezawa,‡ Hiroshi Moriwaki,§ and Ryuichi Arakawa*,† Department of Applied Chemistry, Kansai University, Suita, Osaka 564-8680, Japan, Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Department of Applied Biology, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, Japan Layer-by-layer (LBL) self-assembled multilayer films of gold nanoparticles (AuNPs) on a silicon wafer were demonstrated to be promising substrates for surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) of peptides and environmental pollutants for the first time. LBL multilayer films, (AuNPs/PAHC)n, consisting of alternating layers of ammonium citrate capped AuNPs and poly(allylamine hydrochloride) (PAHC) were prepared on a silicon surface. Silicon plates with aggregated AuNPs were more suitable than those with dispersed AuNPs for the SALDIMS of peptides. The number of particle layers had a significant effect on the laser desorption/ionization of angiotensin I; the peak intensity of the peptide (molecular ion amount) increased with an increase in the number of layers of AuNPs. As a result, the (AuNPs/PAHC)5 multilayer films increased the sensitivity of the angiotensin I to subfemtomoles and raised the useful analyte mass range, thus making it possible to detect small proteins (a 12 kDa cytochrome c). The signal enhancement when using (AuNPs/ PAHC)5 may be due to (i) the high absorption of the UV laser light at 337 nm by the AuNP layers, (ii) the low thermal conductivity due to the AuNPs being covered with a thin monolayer of PAHC, and (iii) the increase in the surface roughness (∼100 nm) with the number of AuNP layers. Thus, laser-induced rapid high heating of AuNPs for effective desorption/ionization of peptides is possible. In addition, it was found that (AuNPs/PAHC)5 could be used to extract environmental pollutants (pyrene and dimethyldistearylammonium chloride) from very dilute aqueous solutions with concentrations less than 10-10 mg/mL, and the analytes trapped in the LBL film could be identified by introducing the film directly into the SALDI mass spectrometer without needing to elute the analytes out of the film. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has become a powerful tool for analyzing biomol* To whom correspondence should be addressed. E-mail:
[email protected] † Kansai University. ‡ The University of Tokyo. § Shinshu University.
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Analytical Chemistry, Vol. 80, No. 19, October 1, 2008
ecules and synthetic polymers.1-6 It is well-known that MALDIMS largely depends on the proper selection of matrixes, cationizing agents, and solvents. Organic acids, such as dihydroxybenzonic acid or R-cyano-4-hydroxycinnamic acid, are typical MALDI matrixes. Most organic matrices have a strong absorption coefficient at a wavelength of 337 nm. As energy mediators, MALDI matrixes effectively transfer photon energies absorbed from UV laser light (λ ) 337 nm) to the surrounding sample molecules. Although MALDI-MS has been successfully used to analyze large molecules, it has rarely been applied to low-molecular weight compounds (
