TECHNICAL NOTE Compatibility of quantum dots with immunobuffers, and its effect on signal/background of quantum dot-based immunoassay Xiaoshan Zhu & Dayue Duan & Steen Madsen & Nelson G. Publicover Received: 28 September 2009 / Revised: 26 October 2009 / Accepted: 3 November 2009 / Published online: 28 November 2009 # Springer-Verlag 2009 Abstract In this work, the compatibility of quantum dots (QDs) with immunobuffers was studied by investigating the fluorescence stability of QDs in immunobuffers (in this research immunobuffers were defined as buffers for immunoaffinity binding or separation). Experimentally, the fluorescence signals of QDs with different surface chemis- tries (amine-terminated, streptavidin-coated, or antibody- conjugated) in commonly used immunobuffers were monitored versus time. The effect of some buffer compo- sition on the compatibility of QDs with these buffers was also explored. Based on experimental data, the QD com- patibility with these buffers is summarized, and it is found that a trace amount of bovine serum albumin added to most of these buffers helps QDs to achieve compatibility with them. Moreover, with QD as fluorescence label and C- reactive protein as a model analyte, a magnetic bead-based assay was performed using compatible and incompatible QD–immunobuffer systems. It is shown that compatible QD–immunobuffer systems can be used to achieve a higher assay signal/background ratio. Keywords Nanoparticles/nanotechnology . Fluorescence/luminescence . Immunoassays/ELISA Introduction Compared with conventional organic fluorophores, quan- tum dots (QDs) as inorganic fluorophores have brighter fluorescence, narrower symmetric emission bands, larger Stokes shift, and greater stability against photobleaching. They have been widely used in imaging and biosensing in replacement of organic fluorophores [1–8]. For these applications, QDs are usually biofunctionalized in two steps—first QDs are made water-dispersible by ligand exchange with the hydrophobic ligands present from QD synthesis, then water-dispersible QDs are bound to bio- molecules by covalent cross-linking, biotin–avidin interac- tion, physical absorption, or other approaches [9–21]. The spectral position or shape of the absorption and emission of biofunctionalized QDs are barely different from those before biofunctionlization. However, the biofunctionalized QDs are not as compatible as organic fluorophores with many buffers. This means QDs in buffers usually suffer from fluorescence instability. The detailed reasons for the incompatibility or the fluorescence instability are not clear. A general thought is that buffer chemical compositions can make QD surface charge or chemistry more complex and induce QD colloidal instability. As a result, QDs aggregate and the total effective surface of QDs for photon excitation and emission is reduced, causing loss of the QD fluores- cence signal [3, 21–23]. It has been reported that a salt X. Zhu (*) : N. G. Publicover Department of Electrical and Biomedical Engineering, University of Nevada Reno, 1604 N Virginia ST, Reno, NV 89557-0260, USA e-mail: xzhu@unr.edu D. Duan Department of Pharmacology, School of Medicine, University of Nevada, 1664 N. Virginia Street, Reno, NV 89557, USA S. Madsen Department of Health Physics and Diagnostic Sciences, University of Nevada Las Vegas, 4505 S Maryland Pkwy, Las Vegas, NV 89154, USA Anal Bioanal Chem (2010) 396:1345–1353 DOI 10.1007/s00216-009-3291-x