Zinc oxide–potassium ferricyanide composite thin film matrix for biosensing applications

Detection of Biological Molecules Using Nanopore Sensing Techniques

Modern biomedical sensing techniques have significantly increased in precision and accuracy due to new technologies that enable speed and that can be tailored to be highly specific for markers of a particular disease. Diagnosing early-stage conditions is paramount to treating serious diseases. Usually, in the early stages of the disease, the number of specific biomarkers is very low and sometimes difficult to detect using classical diagnostic methods. Among detection methods, biosensors are currently attracting significant interest in medicine, for advantages such as easy operation, speed, and portability, with additional benefits of low costs and repeated reliable results. Single-molecule sensors such as nanopores that can detect biomolecules at low concentrations have the potential to become clinically relevant. As such, several applications have been introduced in this field for the detection of blood markers, nucleic acids, or proteins. The use of nanopores has yet to reach maturity for standardization as diagnostic techniques, however, they promise enormous potential, as progress is made into stabilizing nanopore structures, enhancing chemistries, and improving data collection and bioinformatic analysis. This review offers a new perspective on current biomolecule sensing techniques, based on various types of nanopores, challenges, and approaches toward implementation in clinical settings.

Novel same-metal three electrode system for cyclic voltammetry studies

Conventional three-electrode systems used in electrochemical measurement demand time-consuming and maintenance intensive procedures to enable accurate and repeatable electrochemical measurements. Traditionally, different metal configurations are used to establish the electrochemical gradient required to acquire the redox activity, and vary between different electrochemical measurement platforms. However, in this work, we report using the same metal (gold) for the counter, working and reference electrodes fabricated on a miniaturized printed circuit board (PCB) for a much simpler design. Potassium ferricyanide, widely used as a redox probe for electrochemical characterization, was utilized to acquire cyclic voltametric profiles using both the printed circuit board-based gold-gold-gold three-electrode and conventional three-electrode systems (glassy carbon electrode or graphite foil as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode). The results show that both types of electrode systems generated similar cyclic voltammograms within the same potential window (-0.5 to +0.7 V). However, the novel PCB-based same-metal three-electrode electrochemical cell only required a few activation cycles and exhibited impressive cyclic voltametric repeatability with higher redox sensitivity and detection window, while using only trace amounts of solutions/analytes.

S-Layer Protein for Resistive Switching and Flexible Nonvolatile Memory Device

In this work, a flexible resistive switching memory device consisting of S-layer protein (Slp) is demonstrated for the first time. This novel device (Al/Slp/indium tin oxide/polyethylene terephthalte) based on a simple and easy fabrication method is capable of bistable switching to low resistive state (LRS) and high resistive state (HRS). This device exhibits bistable memory behavior with stability and a long retention time (>4 × 10³ s), being stable up to a 500 cycle endurance test and with significant HRS/LRS ratio. The device possesses consistent switching performance for more than 100 times bending, corresponding to desired applicability for biocompatible wearable electronics. The memory mechanism is attributed to a trapping/de-trapping process in S-layer protein. These promising results of the flexible memory device could find a way in the wearable storage applications like smart bands and sports equipments' sensors.

Sputter coated ZnO thin films on glass and polycarbonate: Evaluation of stability and interaction with Flavin adenine dinucleotide-dependent oxidases

Aqueous stability of sputter coated ZnO thin films were studied on two base materials, viz., polycarbonate (PC) and glass. The films showed higher stability on PC compared to glass, when exposed to aqueous buffered solution at pH-7.4, as studied by x-ray diffraction, surface reflectometry, and inductively coupled plasma-optical emission spectroscopy. Glucose oxidase (GOx) and cholesterol oxidase (Chl.Ox.) were used as model enzymes to study their electrochemical interaction with ZnO/PC. GOx showed a higher immobilization on ZnO/PC with an activity of 9.2 ± 1.7 mU cm^-2 compared to Chl.Ox. with an activity of 2.79 ± 0.5 mU cm^-2. This is attributed to the larger crystallite size and higher Zn per unit area on PC as compared to glass which enabled a higher activity of GOx on ZnO/PC compared to ZnO/glass. Immobilization was mainly dependent on the surface residue and the charge of the enzyme as indicated by zeta potential which showed -23 mV for GOx compared to -6 mV for Chl.Ox. under physiological conditions. Further under unstirred condition, the reaction was limited by diffusion of the substrate for the enzyme. Chl.Ox. showed a lower activity as compared to GOx on the surface due to low diffusional coefficient of the bulky cholesterol molecule as compared to glucose. It was confirmed by low charge transfer resistance in electrochemical impedance spectroscopy for GOx (1.51 ± 0.072 × 10⁵ Ω) as compared to Chl.Ox. (1.98 ± 0.09 × 10⁵ Ω). But under stirring condition, the diffusion limitation was overcome, and the sensitivity for Chl.Ox./ZnO was 11.2 μA cm^-2mM^-1 as compared to GOx/ZnO/PC with 3.5 μA cm^-2mM^-1. Thus, sputter coated ZnO thin films appeared to be good quality transducers for immobilization of oxidases with sensitivity dependent on the substrate diffusion and its potential application in biosensors.

Table top surface plasmon resonance measurement system for efficient urea biosensing using ZnO thin film matrix

The present report addresses the application of surface plasmon resonance (SPR) phenomenon for urea sensing. The former promises a high sensitivity, label-free detection, and real-time information by monitoring the refractive index change at the metal–dielectric interface. In the present report, a highly sensitive urea biosensor has been developed by integrating a ZnO thin film matrix with the SPR technique. Kretschmann configuration has been used to excite the surface plasmon (SP) modes at the ZnO–metal (gold) interface using an indigeneously developed table top SPR measurement setup. Urease (Urs), the urea-specific enzyme, has been immobilized on the surface of ZnO thin film by physical adsorption technique. The SPR reflectance curves were recorded for the prism/Au/ZnO/Urs system in angular interrogation mode with phosphate-buffered saline (PBS) solution as the liquid media. The SPR resonance angle is found to be shifted toward a lower angle from 49.1 to 42.0 deg with an increase in the urea concentration (0 to 300  mg/dl) in the PBS solution. The developed sensor (prism/Au/ZnO/Urs) is found to be highly sensitive [sensitivity=0.039  deg/(mg/dl) or 203  deg/RIU] with detection accuracy of 0.045(deg)(−1).

Influence of stress in ZnO thin films on its biosensing application

Highly c-axis oriented zinc oxide (ZnO) thin films deposited by radio frequency (RF) magnetron sputtering, under varying ambient atmosphere (oxygen and argon reactive gas mixture), were studied for biosensing application. The as-grown ZnO thin films were found to be under compressive stress. Glucose oxidase was chosen as model enzyme for studying biosensing response properties of the ZnO thin films. The present study reveals a good correlation between stress induced during thin film growth and its biosensing response characteristic. The bio-electrodes based on ZnO thin films which are under the influence of higher stress, show better sensitivity and higher enzyme loading along with a prolonged shelf life. The study highlights the importance of physical properties of thin film matrix on its biosensing application.

Reagentless uric acid biosensor based on Ni microdiscs-loaded NiO thin film matrix

The development of a noninvasive test for uric acid has been the holy grail of uric acid detection research over the last decade. In the present work, a novel matrix comprising of a NiO thin film (a biocompatible material) loaded with Ni microdiscs was prepared on an ITO-coated glass substrate (Ni/NiO/ITO) with the help of RF sputtering for the reagentless detection of uric acid. The bioelectrode was fabricated by immobilizing uricase using a physical adsorption technique on the surface of the Ni/NiO/ITO electrode. The prepared matrix was found to be efficient in sensing biological processes occurring on the surface of the bioelectrode (Ur/Ni/NiO/ITO) in the presence of the analyte (uric acid) to obtain an electronic output. The biosensor exhibits a high sensitivity (431.09 μA mM(-1)), low Km value (0.15 mM), high apparent enzyme activity (5.07 × 10(-2) units per cm(2)), high shelf life (20 weeks) and good selectivity for the detection of uric acid over a wide concentration range (0.05 mM to 1 mM) without any external mediator in the PBS buffer. The obtained results are encouraging for the realization of a reagentless uric acid biosensor with efficient sensing response characteristics.

Inducing electrocatalytic functionality in ZnO thin film by N doping to realize a third generation uric acid biosensor

A third generation uric acid biosensor has been developed by exploiting the electrocatalytic functionality of nitrogen (N) doped zinc oxide (ZnO:N) thin film matrix deposited using pulsed laser deposition technique. The electrochemistry of ZnO:N thin film based electrode is investigated by using electrochemical impedance spectroscopy and cyclic voltammetry. The obtained results demonstrate that nitrogen doping in ZnO matrix offers a striking electrocatalytic activity to the immobilized uricase towards the oxidation of analyte (uric acid) and promotes the direct transfer of electrons from active sites of enzyme onto the electrode without any mediator. In contrast to pure ZnO, ZnO:N (8% N) thin film based uric acid biosensor gives a high sensitivity of about 1.38 mA/mM in the absence of mediator. Moreover, ZnO:N derived bio-electrode exhibits excellent selectivity and outstanding analytical stability and reproducibility, which enables a reliable and sensitive determination of uric acid in the serum. The ZnO:N thin film based biosensor exhibits a linear sensing response in the range from 0 to 1.0mM of uric acid concentration and the apparent Michaelis-Menten kinetic parameter (Km) is estimated to be about 0.13 mM which indicates the high affinity of the prepared bio-electrode towards uric acid. The obtained results are encouraging and indicate that the ZnO:N thin film matrix offers a new and promising platform for the development of novel third generation biosensors without using any mediator.

Myelin basic protein immunosensor for multiple sclerosis detection based upon label-free electrochemical impedance spectroscopy

A novel highly sensitive impedimetric Myelin Basic Protein (MBP) immunosensor for the determination of a Multiple Sclerosis (MS) autoantibody, Anti-Myelin Basic Protein (Anti-MBP) was developed by immobilization of MBP on Gelatin and Gelatin-Titanium Dioxide (TiO₂) modified platinium electrode. Cyclic voltammetric (CV) and Electrochemical Impedance Spectroscopic (EIS) methods were employed in determination of the electrode responses and applicability. Gelatin-MBP and gelatin-TiO₂-MBP electrodes were prepared by chemical immobilization of the substrates onto the platinium electrodes. The formal potentials of MBP confined on gelatin-MBP and gelatin-TiO₂-MBP surfaces are estimated to be 195 and 205 mV, respectively. Thus, a little more reversible electron transfer reaction occurs on the gelatin-TiO₂-MBP immunosensor surface. The peak separations of MBP (150 mV and 110 mV s(-1) at 100 mV s(-1)) and the asymmetric anodic and cathodic peak currents indicate that the electron transfer between Anti-MBP and gelatin-MBP/gelatin-TiO₂-MBP immunosensor is quasireversible. Control samples containing a nonspecific human immunoglobulin G (hIgG) antibody were also studied, and calibration curves were obtained by subtraction of the responses for specific and nonspecific antibody-based sensors. Gelatin-MBP and gelatin-TiO₂-MBP immunosensors have detection limit of 0.1528 ng ml(-1) and 0.1495 ng ml(-1) respectively. This immunosensor exhibits high sensitivity and low response times (58 s for gelatin-MBP and 46 s for gelatin-TiO₂-MBP immunosensor). The developed label-free impedimetric immunosensors also provide a simple and sensitive detection method for the specific determination of Anti-MBP in human cerebrospinal fluid (CSF) and serum samples.

A novel amperometric biosensor based on ZnO nanoparticles-modified carbon paste electrode for determination of glucose in human serum

Zinc oxide nanoparticles-(ZnONPs)modified carbon paste enzyme electrodes (ZnONPsMCPE) were developed for determination of glucose. The determination of glucose was carried out by oxidation of H2O2 at +0.4 V. ZnONPsMCPE provided biocompatible microenvironment for GOx and necessary pathway of electron transfer between GOx and electrode. The response of GOx/ZnONPsMCPE was proportional to glucose concentration and detection limit was 9.1 × 10(-3) mM. Km and Imax, were calculated as 0.124 mM and 2.033 μA. The developed biosensor exhibits high analytical performance with wide linear range (9.1 × 10(-3)-14.5 mM), selectivity and reproducibility. Serum glucose results allow us to ascertain practical utility of GOx/ZnONPsMCPE biosensor.

Amperometric nitrate biosensor based on Carbon nanotube/Polypyrrole/Nitrate reductase biofilm electrode

This study describes the construction and characterization of an amperometric nitrate biosensor based on the Polypyrrole (PPy)/Carbon nanotubes (CNTs) film. Nitrate reductase (NR) was both entrapped into the growing PPy film and chemically immobilized via the carboxyl groups of CNTs to the CNT/PPy film electrode. The optimum amperometric response for nitrate was obtained in 0.1 M phosphate buffer solution (PBS), pH 7.5 including 0.1 M lithium chloride and 7 mM potassium ferricyanide with an applied potential of 0.13 V (vs. Ag/AgCl, 3 M NaCl). Sensitivity was found to be 300 nA/mM in a linear range of 0.44-1.45 mM with a regression coefficient of 0.97. The biosensor response showed a higher linear range in comparison to standard nitrate analysis methods which were tested in this study and NADH based nitrate biosensors. A minimum detectable concentration of 0.17 mM (S/N=3) with a relative standard deviation (RSD) of 5.4% (n=7) was obtained for the biosensor. Phenol and glucose inhibit the electrochemical reaction strictly at a concentration of 1 μg/L and 20 mg/L, respectively. The biosensor response retained 70% of its initial response over 10 day usage period when used everyday.

A Novel ZnO-Methylene Blue Nanocomposite Matrix for Biosensing Application

A novel hybrid matrix of zinc oxide-methylene blue (ZnO-MB) has been successfully developed for biosensing application. The introduction of methylene blue into the ZnO thin film leads to reduction in the charge transfer resistance and suggests an increase in the electron transfer capacity of the composite. Glucose oxidase (GOx) was chosen as the model enzyme and effectively immobilized on the surface of hybrid ZnO-MB nanocomposite matrix. Electrochemical measurements were employed to study biosensing response of the GOx/ZnO-MB/ITO bioelectrode as a function of glucose concentration. The low oxidation potential (−0.23 V) of the hybrid bioelectrode, in a mediatorless electrolyte, makes it resistant against interference from other bio-molecules. The low value of Michaelis-Menten constant (2.65 mM) indicates that immobilized GOx retains its enzymatic activity significantly on the surface of nanocomposite hybrid matrix that results in an enhanced affinity towards its substrate (glucose). The ZnO-MB nanocomposite hybrid matrix, exhibiting enhanced sensing response (0.2 μAmM−1cm−2) with long shelf-life (>10 weeks), has potential for the realization of an integrated biosensing device.