Bioconjugated Gold Nanodots and Nanoparticles for Protein Assays Based on Photoluminescence Quenching Chih-Ching Huang, Cheng-Kang Chiang, Zong-Hong Lin, Kun-Hong Lee, and Huan-Tsung Chang* Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan This study describes the first instance of the use of two differently sized Au nanoparticles (Au NPs), acting sepa- rately as donor and acceptor, in homogeneous photolu- minescence quenching assays developed for the analysis of proteins. Introduction of a breast cancer marker protein, platelet-derived growth factor AA (PDGF AA), to a solution of 11-mercaptoundecanoic acid-protected, 2.0- nm photoluminescent Au nanodots (L AuND ) led to the preparation of PDGF AA-L AuND as the donor. Thiol- derivative PDGF binding aptamers (Apt) and 13-nm spherical Au NPs were used to synthesize the Apt-Q AuNP acceptor. The photoluminescence of PDGF AA-L AuND at 520 nm decreased when photoluminescence quenching occurred between Apt-Q AuNP and PDGF AA-L AuND . We used the PDGF AA-L AuND /Apt-Q AuNP -based molecular light switching system to analyze PDGFs and PDGF r-receptor in separate homogeneous solutions. In the presence of PDGFs, the interaction between Apt-Q AuNP and PDGF AA-L AuND decreased as a result of competitive reactions between the PDGFs and Apt-Q AuNP . Similarly, the interaction between Apt-Q AuNP and PDGF AA-L AuND reduced as a result of competitive reactions between PDGF r-receptor and PDGF AA-L AuND . The limits of detection (LODs) for PDGF AA and PDGF r-receptor were 80 pM and 0.25 nM, respectively, resulting from a low background photoluminescence signal. When using the Apt-Q AuNP as selectors for (a) the enrichment of PDGF AA and (b) the removal of matrixes possessing intense background fluorescence from cell media and urine samples, the LOD for PDGF AA decreased to 10 pM. Unlike quantum dots, the L AuND provide the advantages of biocompatibility, ease of bioconjugation, and minimal toxicity. The application of photoluminescence quenching is a powerful technique for protein analysis. 1 Intermolecular and intramolecular photoluminescence quenching between the donor and acceptor through specific binding of fluorophore-labeled probes toward target analytes is sensitive and of interest. 2 One interesting example is the use of fluorophore-labeled aptamers for the analysis of proteins such as platelet-derived growth factor (PDGF), 2g,h which was associated with a variety of disorders, including atherosclerosis; fibroproliferative diseases of lungs, kidneys, and joints; and neoplasia. 3 The PDGF also found is an important protein for cell transformation and tumor growth and progression. 4 Aptamers are advantageous over antibodies because they can be engineered, are more easily labeled with signal moieties such as fluorophores and enzymes, are more stable, and are of a relatively lower cost. 5-7 When using fluorophore-labeled aptamers, tedious * To whom correspondence should be addressed. 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