Preparation of Chemically Modified Electrodes and Their Applications to Biochemical Sensors

In this dissertation, different chemical or biological molecules, including iodate, dopamine (DA), glycated hemoglobin (HbA1c), and nitrite were selected as the targets, and different materials were used to prepare the chemically modified electrodes (CMEs) for sensing them.
Iodate is often added in table salts to prevent goiter. A composite film composed of the conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) and the mediator, flavin adenine dinucleotide (FAD), was prepared for the first time for modifying the glassy carbon electrode (GCE). This modified electrode was designated as GCE/PEDOT-FAD. Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) analyses revealed that FAD was doped into the PEDOT film during the electrodepositon process. The optimal cycle number for preparing the modified electrode was determined to be 9. The amperometric detection of iodate was performed; the GCE/PEDOT-FAD showed a sensitivity of 0.78 μA μM-1 cm-2, a linear range of 4-140 μM, and a limit of detection (LOD) of 0.16 μM for iodate. Compared with the results in the literature obtained by using single FAD for sensing iodate, it can be said that the combination of PEDOT with FAD significantly improved the sensitivity and LOD. Eventually, the GCE/PEDOT-FAD was applied to detect iodate in a salt product.
DA is a vital neurotransmitter; its abnormal transmission has been associated with several neurological disorders such as Parkinson’s disease and Huntington’s chorea. Boron doped carbon nanotubes (BCNTs) were utilized for modifying the screen printed carbon electrode (SPCE). The BCNTs were synthesized by an atmospheric carbothermal reaction, in which ammonia (in argon atmosphere) was used as the etching gas to create defects in the multi-walled carbon nanotubes (MWCNT), and boron trioxide was used as the boron source. Each CNT sample was dispersed in 0.5 wt.% Nafion® solution. The relationship between the boron doped amount and the electrocatalytic activity of the BCNT was explored for the first time; it was found that the oxidation peak current of DA is the highest on the BCNT (B 2.1 at.%) modified SPCE. Rotating disk electrode (RDE) analysis revealed that doping of 2.1 at.% boron into the MWCNT upgrades the electroactive surface area (Ae) and the standard rate constant (k0) by ca. 13%, respectively. DA sensing on the BCNT (B 2.1 at.%) modified SPCE was conducted; higher sensitivity (35.65 μA cm-2 μM-1) and lower LOD (0.017 μM) were obtained by using the differential pulse voltammetry (DPV), with respect to those obtained by using CV. The interfering effects of ascorbic acid and uric acid on DA sensing were also studied.
HbA1c is an important index for assessing the long-term condition of diabetes monitoring. Screen printed gold electrode (SPGE) was chosen as the substrate, and Nafion® was dropped coated onto it as a selective material. Ferroceneboronic acid (FcBA) was utilized for recognizing HbA1c and providing the redox signal. Experimental results showed that the modification of Nafion® film effectively blocked the adsorption of hemoglobin (Hb) onto the SPGE; the main reason could be charge repulsion between Hb and Nafion®. The optimal layer of Nafion® film for modifying the SPGE was determined to be 3. Human whole blood was used for real sample test. The effect of HbA1c binding on the redox signal of FcBA was also investigated. It was verified that the binding with HbA1c is the main reason for causing the decrement of the redox signal. Furthermore, since the decrement of the reduction peak current is larger than that of the anodic peak current, it was deduced that HbA1c has stronger interaction with the oxidized FcBA (FcBA+).
Nitrite is a significant index for assessing the urinary tract infection (UTI). For the convenience of clinical use, we tried to fabricate an electrochemical sensor to detect nitrite in the undiluted human urine samples. The conducting polymer, poly(3,4-(2’,2’-diethylpropylene)dioxythiophene) (PProDOT-Et2), was utilized to modify the SPGE to enhance its sensor performance. Since nitrite does not exist in the urine from healthy persons, nitrite was spiked into the urine samples for detection. It was found that the lowest concentration of nitrite that can be detect in the human urine by using the CV method is 250 μM, which is higher than the value can be achieved by the test paper coloring method, 20 μM. Different materials and different electrochemical sensing methods will be used to improve the sensor performance.