Catalytic Synthesis and Characterization of Biodegradable ...

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Polymerization and Applications of Biodegradable Polyesters
observable responses. A key advantage of CP-based sensors is the potential of the CP to
exhibit collective properties that are sensitive to very minor chemical or physical changes.
The most important aspect of biosensor is how to combine the electrical component (i.e., CP)
with the biological recognition components. An effective and widely used approach was to
immobilize bioactive molecules in or on CPs. 141-143
Physical adsorption is the simplest way to introduce bioactive molecules on CPs for
biosensors. For example, glucose oxidase was directly adsorbed onto PPy for a biosensor
which can detect glucose from concentrations of 2.5 to 30 mM using dimethylferrocene as an
electron transfer mediator. 144 However, this method is not reliable because the amount of the
absorbed compound is not repeatable and this immobilization is not stable. 145 An alternative
way is to entrap the biomolecule into the polymers substrate. For example, poly(3,
4-ethylenedioxythiophene) (PEDOT) exhibited a 5% shrinkage in thickness when it is rinsed
with ethanol, which can be utilized to incorporate horseradish peroxidase. 146 Besides enzymes,
this method has also been used for immobilization of antibodies and DNA. 147-149 Just like
the absorption method, this approach also has some limitations such as denature of the
proteins and the low accessibility of analytes to the sensing element. Moreover, entrapment
methods need a high concentration of the biomolecule (~0.2-3.5 mg/mL), which is not
suitable for industry application due to the higher cost.
Compared to entrapment and absorption, affinity binding seemed to be more stable
bonding techniques. The avidin-biotin complex was the most studied affinity binding pair due
to the extremely specific and high-affinity interactions between biotin and the glycoprotein
avidin (Ka = 1 × 10 15 mol -1 L). 150 This method requires the synthesis of biotinylated CPs
firstly. For example, the pyrrole monomers was copolymerized with biotinylated hydrophilic
pyrrole monomers with different PEG lengths as spacer arms to create the bonding site. 151
And the amount of anchored avidin can be tuned by the biotin density or the length of the
PEG spacer in the copolymer. There are also other affinity binding complexes that have been
studied for the immobilization of DNA onto CP surfaces. For example, a unique intercalator
(i.e., a molecule that inserts itself into double-stranded DNA)-based immobilization technique
has been developed. 152 Target DNA strands are detected when they form DNA duplexes with
labeled DNA probes and are subsequently immobilized by the affinity binding of the
intercalator, which is covalently bound to PPy. And more importantly, the detection limits can
lower to 1 pg/mL.