Supra-Nanoparticle Assemblies of Quantum Dots for Extracellular Immunolabeling

With their exceptional fluorescence properties, colloidal semiconductor quantum dots (QDs) are powerful tools for bioanalysis and imaging. Examples of these properties include broad absorption spectra, narrow emission spectra, and both superior resistance to photobleaching and higher brightness than fluorescent dyes and proteins. Nevertheless, demanding applications, such as the detection of low-abundance biomarkers, single-particle tracking, and point-of-care diagnostics with low-cost portable devices, will benefit from even higher brightness. With their size-dependent optical properties, amplifying the brightness of QDs is not a simple matter of increasing the nanocrystal size. Instead, multiple copies of QDs must be assembled into a larger nanoparticle (NP) entity. In this chapter, we describe methods for the preparation and characterization of supra-nanoparticle assemblies of QDs (supra-QDs). The supra-QDs feature many individual QDs bound to a silica NP scaffold and are functionalized with dextran to improve colloidal stability. Both pure colors and composite colors of supra-QD are possible. We also describe two methods for the selective immunofluorescent labeling of an extracellular biomarker with supra-QDs: the use of tetrameric antibody complexes (TAC) and covalently linked supra-QD-antibody conjugates.