ABSTRACT
Electrochemistry offers great promise for such microsystems, with features that include remarkable sensitivity (approaching that of fluorescence), inherent miniaturization of both detector and control instrumentation, responses nondependent on the optical path length or sample turbidity, low cost, low power requirements, and high compatibility with advanced microfabrication and nanotechnologies. Also, electrochemical detection is compatible with both hydrodynamic and electrokinetic flows. Indeed, electrochemical detection has often been considered incompatible with electrokinetic flows because the combination of the high voltages applied in the electrophoretic separation and sensitive electrodes has been seen as a conflict. However, it has been found that, with appropriate designs of the detector cell, the separation voltage does not interfere with the electrochemical measurement. Electrochemical methods have therefore recently found wider
acceptation for conventional CE, and much of the know-how gained has been transferred to the microchip format [5-9]. The detector design should ensure well-defined mass transport, minimal band broadening, and electrical isolation (decoupling) from the high separation voltage (typically 1-5 kV). The latter is attributed to the fact that the current associated with the high separation voltage is usually several orders of magnitude larger than that measured at the electrochemical detector. High sensitivity, selectivity (via the applied potential and electrode material), simple handling, long-term stability, and rigidity are additional requirements. Taking into account the relative position between both working electrode separation channels [7], the configurations can be classified as end-channel, in-channel, and off-channel detection. In end-channel detection, the electrode is placed just outside the separation channel. For in-channel detection, the electrode is placed directly in the separation channel, and off-channel detection involves grounding the separation voltage before it reaches the detector, by means of a decoupler. The detector performance and the success of microfluidic system are strongly influenced by the material of the working electrode since it is placed where the electrochemical reaction of the analyte occurs. The selection of the working electrode depends primarily on the redox behavior of the target analytes and the background current over the applied potential region. Also, one important aspect is that selectivity in microfluidics can be obtained through a judicious choice of the working electrode material and the applied potential. Mainly, carbon, platinum, and gold have been used as electrode materials for microchips [9]. In these directions, solid successful achievements in the field of electrochemical microfluidics are expected. Indeed, the rapid progress in nanotechnology has opened a wide range of horizons for its applications in electrochemistry. Besides its high sensitivity and inherent miniaturization, an additional advantage of electrochemical detection is the opportunity to easily modify the electrode surface toward the use of novel nanomaterials. Without any question, carbon nanotubes (CNTs) are one of the most important nanomaterials in the analytical scene. Indeed, CNTs are a new group of nanomaterials with unique geometrical, mechanical, electronic, and chemical properties, which offer notably favorable features derived from the associated electron transfer enhancement and their strong sorption capacity [10, 11]. There are two main types
of CNTs characterized by high structural perfection-single-walled carbon nanotubes (SWCNTs), which consist of a single graphite sheet seamlessly wrapped into a cylindrical tube, and multiwalled carbon nanotubes (MWCNTs), which comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk. But the question is, Why use CNTs in microfluidics? Next, we are going to discuss the main analytical advantages expected and offered from the use of CNTs as electrochemical detectors in microfluidics [12-14]: (i) Lower detection potentials: The greater surface area of
CNT-based electrodes leads to lower current densities and therefore to lower “overpotentials” (see excellent literature [15-19]). The electrocatalytic effect of CNT materials might have a strong effect on the electrocatalysis of analytes and therefore on lowering the detection potentials and, in consequence, improving the overall selectivity of the analysis. (ii) Higher currents: The greater surface area of CNT detectors enable larger-scale redox conversion, increasing the analytical sensitivity. It is important to point out that higher sensitivity is not directly linked to lower detection limits, because in some cases the background noise levels increase with the same or greater magnitude than does the signal. In cases where the signal increases in greater magnitude than noise, the limit of detection is improved. (iii) Higher stability and resistance to passivation: It is originated from the greater surface area of the CNT-based detectors. This characteristic implies better reproducibility because the resulting signal is prone to fouling. (iv) In addition, one of the most unique perspectives derived from the use of CNTs as electrochemical detectors is the influence of them in separation performance. In wall-jet electrochemical detection (commonly used in end-channel configurations), nanomaterials can provide a higher heterogeneous electron transfer rate between the analyte and electrode surface, which results in sharper and less tailing peaks and consequently results in higher resolution power and higher peak capacity of the whole microchip electrophoresis-electrochemistry system [14].
