Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 215

particles [ 80 ]. Pt(II) and Pd(II) porphyrins used as oxygen indicators were embedded
in various polymeric beads (such as poly(styrene- co -acrylonitrile)) to optimize the
sensitivity. Luminescent microbeads also were used to design an optical sensor for
simultaneous measurements of pH and oxygen [ 7 , 81 ]. These materials relied on
oxygen-sensitive microbeads based on a Pt(II) porphyrin and pH-sensitive microbe-
ads with covalently immobilized 5(6)-carboxyfluorescein [ 7 ] or 8-hydroxypyrene-
1,3,6-trisulfonate [ 81 ]. The beads were dispersed in a layer of polyurethane hydrogel
D4 which is permeable to both oxygen and proton. Simultaneous sensing of three
analytes (pH, oxygen and temperature) with the help of indicator-doped beads was
also reported [ 82 ]. Due to the small size of the beads it was possible to manufacture
fiber-optic microsensors ( Ø 140 m) for simultaneous sensing of pH and oxygen or

oxygen and temperature [ 83 ]. Here, the tip of an optical fiber was covered with a
“cocktail” containing the bead probes.

m

5.4 Sensor Arrays Based on Dye-Doped Beads

Stained polymeric particles have become a useful tool in multiplexed analysis in
clinical diagnostics, drug discovery, gene-function analyzes, etc. In general, multi-
plexed analysis systems are divided into two classes: flat-surface and suspension
arrays [ 84 - 86 ] (Fig. 6 ). Spotted microarrays also called biochips are the major
representatives of the flat-surface type [ 87 , 88 ]. Suspension arrays or liquid arrays
consist of a multitude of particle carriers floating in solution. Each particle repre-
sents an array element that bears specific surface-bound receptor molecules. Sus-
pension arrays offer advantages in handling and higher flexibility in array creation.
Reaction rates of slide-bound assays are usually diffusion controlled, while bead-
based analyte-receptor reactions are limited by binding kinetics that are faster than
diffusion [ 89 ]. Additionally, bead arrays are intrinsically less prone to variations
resulting from poor spot reproducibility for two reasons. First, the same batch of
beads is split and used for calibration and sample analyzes. Thus, receptor immobi-
lization is similar for all readouts. Second, several dozens or hundreds of beads are
evaluated from one sample. Another type of array uses microspheres randomly
absorbed at the distal end of a bundle or fixed in cavities of silicon chips [ 90 ]. In
contrast to flat-surface arrays, where each type of assay is determined by its exact
position, each bead family - representing one type of assay - has to carry a
distinguishable code for identification. Various encoding methods including opti-
cal, chemical, graphical, electronic and physical are described in literature [ 85 , 86 ].

5.4.1 Suspension Arrays

Fluorescent encoded beads represent the largest group in multiplexed suspension
arrays (Fig. 6c ). Luminescent dyes are commonly incorporated by swelling. The