Md. Aminur Rahman

Electrochemical Sensors Based on Carbon Nanotubes

This review focuses on recent contributions in the development of the electrochemical sensors based on carbon nanotubes (CNTs). CNTs have unique mechanical and electronic properties, combined with chemical stability, and behave electrically as a metal or semiconductor, depending on their structure. For sensing applications, CNTs have many advantages such as small size with larger surface area, excellent electron transfer promoting ability when used as electrodes modifier in electrochemical reactions, and easy protein immobilization with retention of its activity for potential biosensors. CNTs play an important role in the performance of electrochemical biosensors, immunosensors, and DNA biosensors. Various methods have been developed for the design of sensors using CNTs in recent years. Herein we summarize the applications of CNTs in the construction of electrochemical sensors and biosensors along with other nanomaterials and conducting polymers.

Amplified electrochemical detection of a cancer biomarker by enhanced precipitation using horseradish peroxidase attached on carbon nanotubes.

An electrochemical nanoimmunosensor based on multiwall carbon nanotubes (MWCNTs)/gold nanoparticles (AuNPs) was developed for the amplified detection of prostate specific antigen (PSA). The amplified detection was achieved by the enhanced precipitation of 4-chloro-1-naphthol (CN) using a higher number of horseradish peroxidase (HRP) molecules attached on MWCNTs. The PSA nanoimmunosensor was fabricated by immobilizing a monoclonal anti-PSA antibody (anti-PSA) on the AuNP-attached thiolated MWCNT on a gold electrode. The sensor surface was characterized using scanning electron microscope, transmission electron microscope, quartz crystal microbalance, and electrochemical techniques. Cyclic and square wave voltammetric techniques were used to monitor the enhanced precipitation of CN that accumulated on the electrode surface and subsequent decrement in the electrode surface area by monitoring the reduction process of the Fe(CN)(6)(3-)/Fe(CN)(6)(4-) redox couple. Under the optimized experimental condition, the linear range and the detection limit of PSA immunosensor were determined to be 1.0 pg/mL to 10.0 ng/mL and 0.40 ± 0.03 pg/mL, respectively. The validity of the proposed method was compared with an enzyme-linked immunosorbent assay method in various PSA spiked human serum samples.

Gold Nanoparticles Doped Conducting Polymer Nanorod Electrodes: Ferrocene Catalyzed Aptamer-Based Thrombin Immunosensor

Au nanoparticles-doped conducting polymer nanorods electrodes (AuNPs/CPNEs) were prepared by coating Au nanorods (AuNRs) with a conducting polymer layer. The AuNRs were prepared through an electroless deposition method using the polycarbonate membrane (pore diameter, 50 nm, pore density, 6 x 10(8) pores/cm(2)) as a template. The AuNPs/CPNEs combining catalytic activity of ferrocene to ascorbic acid were used for the fabrication of an ultrasensitive aptamer sensor for thrombin detection. The AuNPs/3D-CPNEs were characterized employing cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Sandwiched immunoassay for alpha-human thrombin with NH(2)-functionalized-thrombin binding aptamer (Apt) immobilized on AuNPs/3D-CPNEs was studied through the electrocatalytic oxidation of ascorbic acid by the ferrocene moiety that was bound with an antithrombin antibody and attached with the Apt/3D-CPNEs probe through target binding. Various experimental parameters affecting thrombin detection were optimized, and the performance of the thrombin aptamer sensor was examined. The Apt/AuNPs/3D-CPNEs based thrombin sensor exhibited a wide dynamic range of 5-2000 ng L(-1) and a low detection limit of 5 ng L(-1) (0.14 pM). The selectivity and the stability of the proposed thrombin aptamer sensor were excellent, and it was tested in a real human serum sample for the detection of spiked concentrations of thrombin.

Increased Electrocatalyzed Performance through Dendrimer-Encapsulated Gold Nanoparticles and Carbon Nanotube-Assisted Multiple Bienzymatic Labels: Highly Sensitive Electrochemical Immunosensor for Protein Detection

A highly sensitive electrochemical carcinoembryonic antigen (CEA) immunosensor was fabricated by covalently immobilizing a monoclonal CEA antibody (anti-CEA, Ab(1)) and a mediator (thionine, Th) on a gold nanoparticle (AuNP)-encapsulated dendrimer (Den/AuNP). Multiwalled carbon nanotube (MWCNT)-supported secondary antibody (Ab(2))-conjugated multiple bienzymes, glucose oxidase (GOx), and horseradish peroxidase (HRP) (Ab(2)/MWCNT/GOx/HRP) were used as electrochemical labels. The highly sensitive detection was achieved by the increased HRP-electrocatalyzed reduction of hydrogen peroxide, which was locally generated by the enzyme GOx. The immunosensor surface was characterized using electrochemical impedance spectroscopy, atomic force microscopy, and quartz crystal microbalance techniques. The Den/AuNP and Ab(2)/MWCNT/GOx/HRP bioconjugates were characterized using high-resolution transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Cyclic voltammetry and square wave voltammetry techniques were used to monitor the increased electrocatalyzed reduction of hydrogen peroxide by HRP. The linear dynamic range and the detection limit were determined to be 10.0 pg/mL to 50.0 ng/mL and 4.4 ± 0.1 pg/mL, respectively. The validity of the immunosensor response was tested in various CEA-spiked human serum samples, and the results were compared to those of an enzyme-linked immunosorbent assay method.

Direct Electrochemistry of Laccase Immobilized on Au Nanoparticles Encapsulated-Dendrimer Bonded Conducting Polymer: Application for a Catechin Sensor

The direct electrochemistry of laccase was promoted by Au nanoparticle (AuNP)-encapsulated dendrimers (Den), which was applied for the detection of catechin. To increase the electrical properties, AuNPs were captured in the interiors of the dendrimer (Den-AuNPs) as opposed to attachment at the periphery of dendrimer. To prepare Den-AuNPs, the Au(III) ions were first coordinated in the interior of dendrimer with nitrogen ligands and then reduced to form AuNPs. The size of AuNPs encapsulated within the interior of the dendrimer was determined to be 1.7 +/- 0.4 nm. AuNPs-encapsulated dendrimers were then used to covalently immobilize laccase (PDATT/ Den(AuNPs)/laccase) through the formation of amide bonds between carboxylic acid groups of the dendrimer and the amine groups of laccase. Each layer of the PDATT/Den(AuNPs)/laccase probe was characterized using CV, EIS, QCM, XPS, SEM, and TEM. The PDATT/Den(AuNPs)/laccase probe displayed a well-defined direct electron-transfer (DET) process of laccase. The quasi-reversible redox peak of the Cu redox center of the laccase molecule was observed at -0.03/+0.13 V vs Ag/AgCl, and the electron-transfer rate constant was determined to be 1.28 s (-1). A catechin biosensor based on the electrocatalytic process by direct electrochemistry of laccase was developed. The linear range and the detection limit in the catechin analysis were determined to be 0.1-10 and 0.05 +/- 0.003 microM, respectively. Interference effects from various phenolic and polyphenolic compounds were also studied, and the general applicability of the biosensor was evaluated by selective analysis of real samples of catechin.

A cytochrome c modified-conducting polymer microelectrode for monitoring in vivo changes in nitric oxide.

A nitric oxide (NO) microbiosensor based on cytochrome c (cyt c), a heme protein, immobilized onto a functionalized-conducting polymer (poly-TTCA) layer has been fabricated for the in vivo measurement of NO release stimulated by an abuse drug cocaine. Based on the direct electron transfer of cyt c, determination of NO with the cyt c-bonded poly-TTCA electrode was studied using cyclic voltammetry and chronoamperometry. Interferences for the sensory of NO by foreign species such as oxygen and hydrogen peroxide were minimized by covering a Nafion film on the modified electrode surface. Cyclic voltammograms taken using the cyt c/poly-TTCA electrode with NO solutions show a reduction peak at -0.7 V. The calibration plot showed the hydrodynamic range of 2.4-55.0 microM. The detection limit was determined to be 13+/-3 nM based on S/N=3. The microbiosensor was applied into the rat brain to test fluctuation of NO evoked by the abuse drug cocaine. The concentrations of NO levels by acute and repeated injections of cocaine were determined to be 1.13+/-0.03 and 2.13+/-0.05 microM, respectively, showing high sensitivity of the microbiosensor in monitoring NO concentrations in the in vivo intact brain.

Water sensor for a nonaqueous solvent with poly(1,5-diaminonapthalene) nanofibers.

A water sensor for a nonaqueous solvent was fabricated using poly(1,5-diaminonapthalene (DAN) nanofibers, which were prepared through a catalytic chemical polymerization of the DAN monomer using Fe(III) salt as the catalyst. Poly(1,5-DAN) nanofibers were characterized by atomic force microscope (AFM), transmission electron microscope (TEM), scanning electron microscope (SEM), and UV-vis spectroscopy. The electrochemical properties of poly(1,5-DAN) nanofibers were investigated using cyclic voltammetry (CV). The electrochemical activity of poly(1,5-DAN) nanofibers was utilized for water sensing. The fabrication of water sensor was followed by placing one drop (about 2 microL) of 0.01% poly(1,5-DAN) nanofibers solution in the gap between two split gold electrodes (PBSA) and completely dried. The response of the water sensor in an acetonitrile solution was evaluated under optimized conditions. The linear dynamic range was from 0.05 to 20%, and the detection limit was determined to be 0.01%. The response of this sensor was shown to be comparable to that obtained with the Karl Fischer titration method.

Stability and Sensitivity Enhanced Electrochemical In Vivo Superoxide Microbiosensor Based on Covalently Co-immobilized Lipid and Cytochrome c

Enhanced stability and sensitivity of a superoxide anion radical (O(2)(•-)) microbiosensor were achieved through the sequential immobilization of lipid and cytochrome c (Cyt c) covalently bonded onto a conducting polymer layer that showed a clear quasi-reversible direct electron transfer (DET) process. The formal potential and the apparent standard rate constant were determined to be -0.24 V and 0.62 ± 0.05 s(-1), respectively. The detection of O(2)(•-) was attained through the catalytic activity of the haem group of Cyt c stabilized by coimmobilized lipid molecules (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-n-dodecanylamine (DGPD)). The linear dynamic range and the detection limit of the O(2)(•-) analysis were determined to be 0.2-6.0 nM and 30.0 ± 0.9 pM, respectively. The in vivo microbiosensor implanted into rat brain successfully determined the extracellular level of O(2)(•-) produced by acute and repeated injections of cocaine. The present O(2)(•-) microbiosensor could be an effective tool for monitoring the change in extracellular O(2)(•-) levels in response to stimulant drug exposure.

Repeated cocaine administration increases N-methyl-D-aspartate NR1 subunit, extracellular signal-regulated kinase and cyclic AMP response element-binding protein phosphorylation and glutamate release in the rat dorsal striatum

This study was conducted to determine the phosphorylation state of N-methyl-d-aspartate (NMDA) NR1 subunit on serine residues 896 (Ser896) and 897 (Ser897), the extracellular signal-regulated kinase 1/2 (ERK1/2), and the cyclic AMP response element-binding protein (CREB) after repeated exposure to cocaine (20 mg/kg, once daily for 9 days) in the dorsal striatum of rats. The real-time changes of glutamate concentration evoked by repeated cocaine injections were examined using a glutamate biosensor in order to evaluate the correlation between glutamate concentration and the change in these phosphoproteins. The results of this study showed that the immunoreactivity of phosphorylated (p)NMDA NR1 subunit at Ser896 and Ser897 as well as pERK1/2, but not pCREB, in the dorsal striatum was increased at 30 min and then returned to basal levels 4 h after repeated cocaine injections. Similarly, glutamate responses evoked by repeated cocaine injections were also increased 30 min after repeated cocaine injections for 3 days and were prolonged by the 9th day of treatment. However, the glutamate responses were not detected at 4 h after repeated cocaine injections for 5 days. In addition, the elevated immunoreactivity of the phosphoproteins 2 h after repeated cocaine injections was attenuated by the blockade of dopamine D1 receptors and NMDA receptors with the SCH23390 or MK801 antagonists, respectively. These findings suggest that glutamate release and dopamine D1 and NMDA receptor stimulation after repeated exposure to cocaine are associated with NMDA NR1 subunit, ERK1/2 and CREB phosphorylation in the dorsal striatum.