Gold Nanoparticles as Selective and Concentrating Probes for Samples in MALDI MS Analysis Chien-Hsun Teng, Kun-Chan Ho, Ya-Shiuan Lin, and Yu-Chie Chen* Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan MALDI mass spectrometry is used widely in various fields because it has the characteristics of speed, ease of use, high sensitivity, and wide detectable mass range, but suppression effects between analyte molecules and inter- ference from the sample matrix frequently arise during MALDI analysis. The suppression effects can be avoided if target species are isolated from complicated matrix solutions in advance. Herein, we proposed a novel method for achieving such a goal. We describe a strategy that uses gold nanoparticles to capture charged species from a sample solution. Generally, ionic agents, such as anionic or cationic stabilizers, encapsulate gold nanoparticles to prevent their aggregation in solution. These charged stabilizers at the surface of the gold particles are capable of attracting oppositely charged species from a sample solution through electrostatic interactions. We have em- ployed this concept to develop nanoparticle-based probes that selectively trap and concentrate target species in sample solutions. Additionally, to readily isolate them from solution after attracting their target species, we used gold nanoparticles that are adhered to the surface of magnetic particles through S-Au bonding. A magnet can then be employed to isolate the Au@magnetic particles from the solution. The species trapped by the isolated particles were then characterized by MALDI MS after a simple washing. We demonstrate that Au@magnetic par- ticles having negatively charged surfaces are suitable probes for selectively trapping positively charged proteins from aqueous solutions. In addition, we have employed Au@magnetic particle-based probes successfully to con- centrate low amounts of peptide residues from the tryptic digest products of cytochrome c (10 -7 M). Matrix-assisted laser desorption/ ionization (MALDI) mass spectrometry has become a very powerful tool for biochemical analysis 1,2 in the years since its initial development. 3 Mixtures can be analyzed simultaneously using the MALDI MS technique without the requirement of any tedious pretreatment. Suppression effects between analyte molecules and matrix interference of samples may arise, however, and limit the power of this analytical tool. Several clever strategies have been proposed to improve the results of analysis. 4-18 On-probe cleanup methods have been used frequently to remove unwanted salts or surfactants during MALDI MS analyses. Xu et al. provided a detailed review of the develop- ment of nonspecific, on-probe cleanup methods for MALDI MS samples. 16 Hydrophobic polymers such as polyethylene (PE), 17 polypropylene, 14 polyurethane, 10 paraffin, 9 and Teflon 18 have been used for effectively desalting sample substrates. For example, Blackledge and Alexander employed a PE-modified MALDI probe as a sample support and obtained a mass spectrum of bovine serum albumin, from a sample that contained 0.73% SDS, after vortexing the sample-coated probe in 50%aqueous methanol for 30 s. 17 Most of the studies to date have focused on methods for on-probe treatment. 5-18 An alternative method that avoids sup- pression effects in MALDI MS analysis is to isolate trace amounts of target species from complicated samples prior to their analysis. Gold nanoparticles are used widely in various fields because of their extremely small size, visible colors, and ease of chemical modification. 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