Nanostructured titanium nitride for highly sensitive localized surface plasmon resonance biosensing

Titanium nitride (TiN) as an alternative plasmonic ceramic material with superb properties including high hardness, outstanding corrosion resistance and excellent biocompatibility, has exhibited great potential for optical biochemical sensing applications. By sputtering about 35 nm–50 nm TiN on glass (f-TiN), the surface was found to provide sensing capability toward NaCl solution through the phenomenon of surface plasmon resonance. When the TiN film of about 27 nm–50 nm in thickness was sputtered onto a roughened glass surface (R–TiN), the sensing capability was improved. This was further improved when holes at nanoscale were created in the TiN film of about 19 nm–27 nm in thickness (NH–TiN). The roughened surface and nanohole patterns provided confinement of surface plasmons and significantly improved the sensitivity toward the local refractive index changes. In detail, the calculated refractive index resolution (RIR) of the optimal NH–TiN sensors for NaCl was found to be 9.5 × 10⁻⁸ refractive index unit (RIU), which had outperformed the f-TiN and R–TiN sensors. For biosensing, the optimized NH–TiN sensor was found to be capable to detect both small and large biomolecules, i.e. biotin (molecular weight of 244.3 g/mol) and human IgG (160,000 g/mol), in a label-free manner. Especially, the NH–TiN sensor significantly improved sensitivity in detecting small molecules due to the localized plasmonic confinement of electromagnetic field. Combining with the excellent mechanical and durability properties of TiN, the proposed NH–TiN can be a strong candidate for plasmonic biosensing applications.     

Response of ceramic microbial fuel cells to direct anodic airflow and novel hydrogel cathodes

The presence of air in the anode chamber of microbial fuel cells (MFCs) might be unavoidable in some applications. This study purposely exposed the anodic biofilm to air for sustained cycles using ceramic cylindrical MFCs. A method for improving oxygen uptake at the cathode by utilising hydrogel was also trialled. MFCs only dropped by 2 mV in response to the influx of air. At higher air-flow rates (up to 1.1 L/h) after 43–45 h, power did eventually decrease because chemical oxygen demand (COD) was being consumed (up to 96% reduction), but recovered immediately with fresh feedstock, highlighting no permanent damage to the biofilm. Two months after the application of hydrogel to the cathode chamber, MFC power increased 182%, due to better contact between cathode and ceramic surface. The results suggest a novel way of improving MFC performance using hydrogels, and demonstrates the robustness of the electro-active biofilm both during and following exposure to air.     

Impedance spectroscopy: Over 35 years of electrochemical sensor optimization

There is considerable interest in the development of electroanalytical sensors (i.e., potentiometric, amperometric, electrochemical biosensors) for the detection of a wide range of analytes. The success of many of these sensors is governed by the condition and stability of the membrane/electrode surface. In fact, the response mechanism is dictated primarily by the surface structure and a considerable amount of work has been undertaken to characterize the interfacial region. Consequently, electrochemical impedance spectroscopy (EIS) has played a pivotal role in the characterization of many types of sensors. EIS has been used to provide information on various fundamental processes (i.e., adsorption/film formation, rate of charge transfer, ion exchange, diffusion, etc.) that occur at the electrode–electrolyte interface. Understanding and manipulating these interfacial processes has assisted in the development of membranes/electrodes with new and improved response characteristics. This paper reviews some of the work that has been undertaken using EIS over the past 35 years. More importantly, it evaluates the power of EIS in characterizing a wide range of electrochemical sensor systems.     

Redox-active conducting polymers incorporating ferrocenes. Preparation, characterization and bio-sensing properties of ferrocenylpropyl and -butyl polypyrroles

The multi-step synthesis of the novel ferrocene-substituted pyrrole monomers, N-(3-ferrocenylpropyl)pyrrole (1), and 3-(4-ferrocenylbutyl)pyrrole (2), have been studied and optimized. A single crystal X-ray structure analysis has been performed on the synthetic intermediate 3-(4-ferrocenylbutyl)-N-(triisopropylsilyl)pyrrole. Monomers 1 and 2 can be electropolymerized to form the homopolymer, poly-2, and the copolymers, pyrrole-co-1 and pyrrole-co-2. The polymers have been characterized using cyclic voltammetry, UV-visible spectroscopy, scanning electron microscopy (SEM) and four-point probe conductivity measurements. The use of pyrrole-co-1 coatings for quantitative sensing and determination of the redox-active enzyme cytochrome C in solution has been demonstrated.     

Construction of an amperometric pyruvate oxidase enzyme electrode for determination of pyruvate and phosphate

In this study an amperometric biosensor based on pyruvate oxidase was developed for the determination of pyruvate and phosphate. For construction of the biosensor pyruvate oxidase was immobilized with gelatin and insolubilized in film by forming cross-linked bonds with glutaraldehyde. The film was fixed on a YSI type dissolved oxygen (DO) probe, covered with a teflon membrane which is high-sensitive for oxygen. The working principle of the biosensor depends on detection of consumed DO concentration related to pyruvate concentration which is used in enzymatic reaction catalyzed by pyruvate oxidase. The biosensor response shows a linearity with pyruvate concentration between 0.0025 and 0.05 μM and also response time of the biosensor is 3 min. In the optimization studies of the biosensor the most suitable enzyme activity was found as 2.5 U/cm² for pyruvate oxidase, and also phosphate buffer (pH 7.0; 50 mM) and 35 °C were established as providing the optimum working conditions. In the characterization studies of the biosensor some parameters such as reproducibility, substrate specificity, operational stability, determination of phosphate, and interference effects of some compounds on the pyruvate determination were investigated. Finally, the concentration of pyruvate was determined by using spectrophotometric method and the results obtained were compared to results obtained by the biosensor.     

On-chip glucose biosensor based on enzyme entrapment with pre-reaction to lower interference in a flow injection system

In this study, poly(3,4-ethylenedioxythiophene), PEDOT, was electropolymerized on a sensing chip to entrap glucose oxidase (GOD). The interdigitated array (IDA) electrode and microfluidic channel of the sensing chip was fabricated by photolithography. The IDA electrodes consist of two working electrodes in which one (WE1) was the enzyme-modified electrode and the other was a Pt (WE2) for eliminating noise effect. The microfluidic channel was formed on etched silicon by PDMS (poly(dimethylsiloxane)). In the flow injection analysis, a 0.7 V (vs. Ag/AgCl) was set on enzyme electrode to detect the catalytic product, H₂O₂, and the sensing signal was calibrated using the passed charge rather than the peak current. The linear relationship between the charges and the glucose concentrations, ranging from 1 to 10 mM, was obtained with a sensitivity of 157 μC cm⁻² mM⁻¹. Besides, the response time and the recovery time were about 15 and 35-75 s, respectively. In real human sample test, the error of single-potential and bi-potential were about 140% and 13%, respectively, comparing to the standard value, indicating that the WE2 can lower the interference effect in this system.     

Fabrication of novel chitosan nanofiber/gold nanoparticles composite towards improved performance for a cholesterol sensor

A novel amperometric cholesterol biosensor was fabricated by the immobilization of ChOx (cholesterol oxidase) onto the chitosan nanofibers/gold nanoparticles (designated as CSNFs/AuNPs) composite network (NW). The fabrication involves preparation of chitosan nanofibers (CSNFs) and subsequent electrochemical loading of gold nanoparticles (AuNPs). Field emission scanning electron microscopy (FE-SEM) was used to investigate the morphology of CSNFs (sizes in the range of ∼50-100 nm) and spherical AuNPs. Cyclic voltammetry, hydrodynamic voltammetry and amperometry were used to examine the performance of CSNF-AuNPs/ChOx biosensor. The CSNF-AuNPs/ChOx biosensor exhibited a wide linear response to cholesterol (concentration range of 1-45 μM), good sensitivity (1.02 μA/μM), low response time (∼5 s) and excellent long term stability. The combined existence of AuNPs within CSNFs NW provides the excellent performance of the biosensor towards the electrochemical detection of cholesterol.     

Rapid parallelized and quantitative analysis of five pathogenic bacteria by ITS hybridization using QCM biosensor

We developed a 2 × 5 model quartz crystal microbalance (QCM) DNA biosensor array for detection of five bacteria, which based on hybridization analysis of bacterial 16S-23S rDNA internal transcribed spacer (ITS) region. A pair of universal primers was designed for PCR amplification of the ITSs. The PCR products were analyzed by the biosensor. We used gold nanoparticles to amplify the frequency shift signals. Fifty clinical samples were detected by both the biosensor and conventional bacteria culture method. We found a linear quantitative relationship between frequency shift and logarithmic concentration of synthesized oligonucleotides or bacteria cells. The measurable concentration ranged from 10⁻¹² to 10⁻⁸ M for synthesized oligonucleotides and 1.5 × 10² to 1.5 × 10⁸ CFU/mL for bacteria. The 10⁻¹² M of synthesized oligonucleotides or 1.5 × 10² CFU/mL of Pseudomonas aeruginosa could be detected by the biosensor system. The detection could be completed within 5 h including the PCR amplification procedure. Compared with bacteria culture method, the detection sensitivity and specificity of the biosensor system were 94.12% and 90.91%, respectively. There was no significant difference between these two methods (P = 0.625 > 0.05). The biosensor system provides a rapid and sensitive method for parallelized and quantitative analysis of multiple pathogenic bacteria in clinical diagnosis.     

Amperometric detection of phenolic compounds with enzyme immobilized in mesoporous silica prepared by electrophoretic deposition

Laccase was immobilized in mesoporous silica powder with a 7.0 nm pore diameter (FSM7.0) coated onto a glassy carbon electrode using an electrophoretic deposition technique, and the electrode was then applied to the amperometric detection of catechol, which is a typical phenolic compound. The behavior of a biosensor attached to the electrode was examined in terms of pH, applied potential, sensitivity and operational range, selectivity, and storage stability. The sensor showed an optimum response at a pH of 5.0 and at an applied potential of −50 mV. The determination range and the response time for catechol were 2.0-100 μM and approximately 2 min, respectively. In addition, the sensor was quite stable and retained its initial response without notable change after being stored for over 50 days. This result suggests that our method is quite useful for the fabrication of a high-performance biosensor for practical use.     

Electrochemical DNA biosensor with nanometer scale using nano-patterning lithography machine

Major challenges in the field of electrochemical DNA hybridization biosensors are the immobilization of DNA and the detection of hybridization signals. The method of DNA immobilization using the nano-patterning machine and detection for DNA hybridization signals has been proposed. Here, two gold electrodes were deposited on SiO₂ layer and the gap between the electrodes was fabricated by electron beam lithography. 3-aminopropyltriethoxysilane (APTES) solution was selectively treated to immobilize the amino-modified oligonucleotides onto the SiO₂ layer between the electrodes. The recognition of DNA hybridization was accomplished by metallic aggregation of nano-particles. The results showed that DNA is immobilized with nanometer scales and the method for detecting hybridization signals is useful. The experimental results were verified by I-V curves. The conductance between two electrodes changed with the density of the Au-nanoparticles immobilized onto the oxide layer. These results can be applied to the DNA chip and the multi-functional sensors which will be researched in the further study.