Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 218

to the surface [ 90 , 101 , 102 ] and are loaded onto the fiber in a simple dipping and
evaporation technique. A typical fiber bundle consists of 5,000-50,000 individual
fibers which are read out with an imaging system based on microscope optics and a
CCD camera. The system has separated color channels to identify each bead and
detect the reporter fluorophores or indicator dyes. For example, in a highly efficient
DNA assay [ 103 ] the individual beads are decoded by sequential hybridization of
fluorescent complementary oligonucleotide sequences. However, reversible bind-
ing or sensing reaction is mandatory for this decoding method.
Randomly-ordered bead arrays can be divided into analyte-specific and cross-
reactive sensor arrays. In the first, each element responds with high selectivity using
specific receptors. In cross-reactive sensor arrays, the analytical information is
gained by the pattern of the different sensing elements responding with a broad
specificity. The scheme is based on the principles of the mammalian olfactory
system. Analyte specific random bead arrays are applied according to the classic
lock-and-key principles for nucleic acid detection and protein detection [ 85 ].
Systems for genotyping, sequencing and gene expression are commercially avail-
able from Illumina ( http://www.illumina.com ) and BeadArray Solutions ( http://
www.immucor.com ). Fiber optic bead arrays for ions based on the extraction into
the lipophilic phase by ion carriers were reported by Wygladacz et al. [ 104 , 105 ].
Cross-reactive sensing arrays were developed to detect odors and vapors in an
artificial nose manner. Solvatochromic dyes such as Nile Red are adsorbed on the
surface or embedded into various polymeric or porous silica beads. The beads
respond to analyte vapor by a change in fluorescence maxima or/and intensity
due to changes of polarity inside the bead. A portable instrument and preliminary
field test for the detection of petroleum products was recently described [ 106 ].
5.5 Magnetic Dye-Doped Polymeric Beads
Dye-doped polymeric beads with magnetic properties (either para- or ferromag-
netic) became increasingly popular in the past decades. Compared to conventional
micro- and nanobeads, these particles can be magnetically guided to a place of
interest by an external magnetic field, or even rotated by an alternating field. On one
hand, the magnetic separation from the medium can increase the signal strength,
reduce the required amount of fluorescent dyes, speed up separation in sedimenta-
tion arrays, or localize an effect of the particle to a specific point of a biological
material (e.g., targeted drug delivery). On the other hand, the modulated rotation of
such particles can greatly extend the signal-to-noise ratio.
The magnetic beads usually consist of inorganic or metallic nanoparticles which
are either homogeneously or inhomogeneously (e.g., core-shell) distributed in a
dye-doped polymeric bead. Since the dye properties (e.g., luminescence brightness,
solubility, etc.) are affected by the presence of magnetic additive the particle
synthesis becomes more challenging and its optimization time consuming. There-
fore, it is often the best to start with commercially available systems. Bangs