New Plastic Microparticles and Nanoparticles for Fluorescent Sensing and Encoding

We report on the progress that has been made in the area of luminescence sensing and encoding by makinguse of microparticles and nanoparticles prepared from plastic materials. These are quite different fromparticles built up from metal sulfides (such as the so-called quantum dots, “Q-dots”; see Michaletet al.,

Science

307:538, 2005), other semiconductor materials, metalnanoparticles (mainly gold) (see DL Feldheim, CA Foss (eds)

Metal Nanoparticles:Synthesis, Characterization, and Applications

, p 338, Marcel Dekker, 2002), or glass andits modifications including certain sol–gels. Plastic nanoparticles may contain magnetic beads inorder to facilitate separation from the sample solution. All the particles described here are doped withfluorescent dyes, which is in contrast to particles where the material itself displays intrinsic luminescence.Unlike the case of Q-dots, the color of plastic beads can be varied to a wide extent irrespective oftheir size, as can be the decay times and even anisotropy. This, in fact, is a most attractive featureof such beads and makes them superior in many cases despite the undisputed utility of other types of particlesin certain fields.

The area of beads was almost exclusively occupied until 10 years ago by polystyrene beads (alsoreferred to as latex beads) 0.1–5 μm in diameter. They are widely used in bioassays andflow cytometry because they can be manufactured with good reproducibility (usually by emulsion polymerization)and because they are rather inert. Other applications include agglutination tests, particle capture ELISAs(e.g., Abbott's IMx and AxSym), solid-phase assays (often used for pregnancy testing), scintillation proximityassays, luminescent oxygen channeling immunoassay (LOCI), and bead–FRET assays. Nanoparticles havebeen used for labeling purposes, particularly in the context of protein arrays and DNA arrays. In recentyears, beads have been fluorescently dyed for purposes of encoding, for example in combination with opticalfiber arrays and in microwells, and in methods for homogeneous multiplexed high-throughput screening. Theanalytical information may be the color of the fluorescence, its intensity (or—even better—theratio of two intensities), decay time, anisotropy, or combinations thereof.

In the first section we will describe dyes for doping plastic particles. The second section willreport on chemical sensing with addressable micro- and nanospheres, and the third on the use of dyed microparticlesin sensing pH values. We will also report on luminescence lifetime encoded microbeads as carriers for multiplexedbioassays (Sect. 4), the use of dyed polymer microparticles in simultaneous sensing of oxygen and temperature(Sect. 5), and on nanobead labels for homogeneous protein assays and protein arrays (Sect. 6).