Dye-Doped Polymeric Particles for Sensing and Imaging - Advanced Fluorescence Reporters in Chemistry and Biology - page 210

pH in microfluidic devices. Metabolic activity of Escherichia coli during cultiva-
tion was successfully monitored with help of the beads.
It was demonstrated that poly(styrene- block -vinylpyrrolidone) beads ( Ø 220 nm)
are suitable for preparation of pH nanosensors [ 12 ]. Various fluorescein derivatives
were embedded and did not leach out of the beads due to functionalization with
highly lipophilic octadecyl “anchor.” The p K a of the indicators inside the nanobeads
varied from 5.8 to 7.7 making them suitable for various biotechnological, biological
and marine applications. The beads based on a lipophilic 1-hydroxypyrene-3,6,
8-trisulfonate (p K a 6.9) were also manufactured.

5.1.3 Beads for Sensing and Imaging of Ionic Species

One can distinguish between the two main types of nanosensors for ionic species: the
beads relying on the indicators highly specific to a particular analyte and those
employing ionophores as recognition elements. The nanosensors ( Ø typically
20-200 nm) of the first type predominantly use cross-linked polyacrylamide as a
matrix where a fluorescent indicator and a reference dye are entrapped. For example,
Kopelman and co-workers reported nanosensors for Ca 2+ [ 35 ], Mg 2+ [ 40 ], Zn 2+ [ 41 ]
and Fe 3+ [ 42 ]. Evidently, the extent of leaching is determined by the lipophilicity of
the respective indicator and can vary from the very significant [ 35 , 40 ] to the virtually
negligible [ 41 ]. To prevent leaching, the reference dyes (such as Texas Red) are used
as macromolecular conjugates with dextrane [ 41 , 42 ]. Interestingly, such fluorescent
dyes as coumarin C343 (a laser dye) and Alexa Fluor 488 (commonly used as a label)
were found be highly selective toward Mg 2+ [ 40 ] and Fe 3+ [ 42 ], respectively.
The second type of the nanosensors relies on ionophores which are entrapped
into the polymeric bead along with a lipophilic pH-dependent chromionophore
(usually a phenoxazine derivative). These nanosensors are designed similarly to the
bulk optodes and ion-selective electrodes and usually possess very high selectivity.
Poly(decyl methacrylate) [ 43 , 44 ] and plasticized poly(vinyl chloride) [ 16 , 45 ] are
the polymers of choice for preparation of the nanobeads. For example, a nanosensor
reported by Kopelman and co-workers [ 43 ] was suitable for determination of
0.63 mM-0.63 M of K + although showed very poor storage stability due to the
leaching of the indicator. Chloride-selective neutral ionophores were used by
Brasuel et al. [ 44 ] and Ceresa et al. [ 45 ] to prepare the sensing beads operating in
similar dynamic ranges (0.4-190 mM and 0.3-300 mM, respectively) despite the
fact that different polymers and ionophores were employed.
Bychkova and Shvarev [ 16 ] recently prepared nanosensors (0.2-20 m) for

sodium, potassium and calcium using the precipitation method. Similarly to the
previous works, the plasticized poly(vinyl chloride) included a phenoxazine chro-
moionophore, a lipophilic ion exchanger and a cation-selective ionophore. The
dynamic range of the very selective sensors was 5

m

10 4 -0.5 M for sodium,

10 4 - 0.05 M for calcium. As was
demonstrated by Bakker and co-workers [ 45 ] a particle caster can be used can be
used for preparation of much larger beads ( Ø 11

10 5 -0.1 M for potassium and 2 1

m).

m