Chemistry Reference
In-Depth Information
The search for environmentally more acceptable and user friendly systems for field biosensing detection
has long been pursued. Various green approaches based on enzyme biosensors have been described. A recent
example is the development of a highly sensitive amperometric biosensor for organophosphate (OPs)
pesticides based on the immobilization of acetylcholinesterase (AChE) on multiwalled carbon nanotubes
(MWCNTs)-
-CD) composite-modified glassy carbon electrodes. The composite was
prepared using polymer wrapping strategy. The good dispersability and porous structure of MWCNTs-
β
-cyclodextrin (
β
-CD
composite provided a favourable microenvironment for maintaining AChE bioactivity for screening of OPs
exposure. MWCNTs promoted electron-transfer reactions at a lower potential and catalysed the electro-
oxidation of thiocholine, thus increasing detection sensitivity. Based on the inhibition of OPs on the AChE
activity, a detection limit of 2 nM (S/N
β
3) was achieved using dimethoate as a model pesticide [111].
The attractive green properties of microsystems have been also profited for the development of biosensing
methods. Nowadays, analytical microdevices have found application in the preparation of electrochemical
biosensors due to their versatility, low cost and minimum energy consumption as well as their inherent
characteristics of room saving and low volumes of sample, reagents and wastes [112]. In the particular case
of clinical applications, the use of protein chips is the best choice to analyse real samples with small volume.
Protein chip-based electrochemical sensors can be readily miniaturized and incorporated into microfluidic
components for the development of lab-on-a-chip devices. The smaller sample volumes and reduced reaction
distances in such devices would be expected to significantly increase the efficiency of immunological
reactions. In particular, immunosensors coupled with electrochemical detectors using microelectrodes arrays
have attracted extensive interest due to their powerful potential in simultaneously probing disease relevant
markers [113]. On-chip integration of immunological and enzymatic reactions, amperometric detection and
microchip operation allow immunoassays to perform more rapidly, easily and economically. An illustrative
example is a microchip-based electrochemical immunoassay prepared to determine toxicity due to pesticides
in a glass lab-on-a-chip based on the enzymatic inhibition of acetylcholinesterase immobilized on magnetic
beads. The reproducible insertion of a controlled amount of enzyme-coupled magnetic beads inside the chip
channel and their immobilization in a capture region with the aid of a magnetic field allowed the easy renewal
of the biosensing material after each determination in a highly reproducible manner. The experimental
conditions for the detection were optimized using thiocholine (TCh) as the target compound. Furthermore,
the method was applied to the determination of carbofuran down to the nanomolar level [114].
=
14.5.2
Direct electrochemical transfer of proteins
In recent years, studies on protein film electrochemistry have been widely reported. Direct electron transfer
of proteins with electrodes is a versatile and powerful strategy to construct third generation biosensors and
also to investigate the chemistry of redox proteins. However, achieving an efficient direct electron transfer is
not an easy task because the redox active prosthetic groups are usually buried in the protein molecule structure.
On one hand, adsorption of the protein on the electrode surface induces deactivation and denaturation.
Moreover, unfavourable orientations on the electrode surface difficult protein molecules to perform redox
reactions. In spite of these difficulties, various electrodes modified with diverse materials have demonstrated
to provide a suitable microenvironment to enhance direct electron transfer rate of proteins with electrodes.
Ionic liquids are such materials [75]. Furthermore, carbon nanotubes in combination or not with ILs have
been also used. For example, the direct electrochemistry of myoglobin (Mb) entrapped in a Nafion film on a
multiwalled carbon nanotubes (MWCNTs) modified carbon ionic liquid electrode (CILE) was investigated
using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF
4
) as the modifier. Spectroscopic results
revealed that the Mb molecule on the surface of MWCNTs/ CILE retained its native structure. Cyclic
voltammetry showed that a pair of well-defined quasi-reversible redox peaks appeared in the pH 7.0 phosphate
buffer solution (PBS), which was attributed to the direct electron transfer of Mb heme Fe(III)/Fe(II) redox
