Akhtar Hayat

Disposable Screen Printed Electrochemical Sensors: Tools for Environmental Monitoring

Screen printing technology is a widely used technique for the fabrication of electrochemical sensors. This methodology is likely to underpin the progressive drive towards miniaturized, sensitive and portable devices, and has already established its route from “lab-to-market” for a plethora of sensors. The application of these sensors for analysis of environmental samples has been the major focus of research in this field. As a consequence, this work will focus on recent important advances in the design and fabrication of disposable screen printed sensors for the electrochemical detection of environmental contaminants. Special emphasis is given on sensor fabrication methodology, operating details and performance characteristics for environmental applications.

Aptamers: A Promising Tool for Ochratoxin A Detection in Food Analysis

The contamination of food and feed by mycotoxins has become an increasingly serious problem. Mycotoxins represent a major risk to human and animal health, as well as economics. Herein, we focus on Ochratoxin A (OTA), which is one of the most common mycotoxins contaminating feed and foodstuffs. OTA is a secondary metabolite produced by various Aspergillus and Penicillium strains. Upon ingestion, OTA has a number of acute and chronic toxic effects. It is nephrotoxic, teratogenic, immunosuppressive, and carcinogenic (group 2B). As a consequence, some regulatory limits have been introduced on the levels of OTA in several commodities. The toxic nature of OTA demands highly sensitive and selective monitoring techniques to protect human and animal health. As alternative to traditional analytical techniques, biochemical methods for OTA analysis have attained great interest in the last few decades. They are mainly based on the integration of antibodies or aptamers as biorecognition elements in sensing platforms. However, aptamers have gained more attention in affinity-based assays because of their high affinity, specificity, stability, and their easy chemical synthesis. In this brief review, we present an overview of aptamer-based assays and their applications in OTA purification and detection, appeared in the literature in the last five years.

Enzyme-linked immunosensor based on super paramagnetic nanobeads for easy and rapid detection of okadaic acid.

Okadaic acid (OA), a lipophilic phycotoxin highly toxic to humans is produced by toxigenic dinoflagellates. The need to develop high performing methods for OA analysis able to improve the traditional ones is evident. In this work, competitive indirect enzyme-linked electrochemical immunosensor based on super paramagnetic nanobeads has been developed for the detection of OA. Streptavidin-coated magnetic beads were used as support to immobilize the biotinylated OA. Preliminary, colorimetric tests were performed in order to optimize different experimental parameters. Electrochemical detection was carried out by differential pulse voltammetry (DPV). The limit of detection (LOD) (0.38 μg L(-1)), the mid point value (IC(50)) (3.15 μg L(-1)) and the time needed (60 min) for analysis of a real sample validated the developed electrochemical immunosensor as a promising tool for routine use. The matrix effect and the recovery rate were also assessed, showing an excellent percentage of recovery.

Aptamer based electrochemical sensors for emerging environmental pollutants

Environmental contaminants monitoring is one of the key issues in understanding and managing hazards to human health and ecosystems. In this context, aptamer based electrochemical sensors have achieved intense significance because of their capability to resolve a potentially large number of problems and challenges in environmental contamination. An aptasensor is a compact analytical device incorporating an aptamer (oligonulceotide) as the sensing element either integrated within or intimately associated with a physiochemical transducer surface. Nucleic acid is well known for the function of carrying and passing genetic information, however, it has found a key role in analytical monitoring during recent years. Aptamer based sensors represent a novelty in environmental analytical science and there are great expectations for their promising performance as alternative to conventional analytical tools. This review paper focuses on the recent advances in the development of aptamer based electrochemical sensors for environmental applications with special emphasis on emerging pollutants.

Highly sensitive ochratoxin A impedimetric aptasensor based on the immobilization of azido-aptamer onto electrografted binary film via click chemistry.

The aptamer immobilization onto organized mixed layers of diazonium salts via click chemistry was explored. The immobilized aptamer was employed in the fabrication of a highly sensitive and reusable electrochemical impedimetric aptasensor for the detection of ochratoxin A (OTA). The screen-printed carbon electrodes (SPCEs) were first modified by electrografting of a protected 4-((trimethylsilyl)ethynyl) benzene (TMSi-Eth-Ar) layer followed by a second one of p-nitrobenzene (p-NO(2)-Ar) by means of electrochemical reduction of their corresponding diazonium salts, (TMSi-Eth-Ar-N(2)(+)) and (p-NO(2)-ArN(2)(+)). After deprotection, a layer with active ethynyl groups was obtained. In the presence of copper (I) catalyst, the ethynyl groups reacted efficiently with aptamer bearing an azide function, thus forming a covalent 1,2,3-triazole linkage. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in the presence of ferri/ferrocyanide redox probe [Fe(CN)(6)](4-/3-) were used to characterize each step in the aptasensor development. The increase in electron-transfer resistance (R(et)) values due to the specific aptamer-OTA interaction was proportional to the concentration of OTA in a range between 1.25 ng/L and 500 ng/L, with a detection limit of 0.25 ng/L.

Sensitive quantitation of Ochratoxin A in cocoa beans using differential pulse voltammetry based aptasensor

In this work, we propose for the first time a sensitive Ochratoxin A (OTA) detection in cocoa beans using competitive aptasensor by differential pulse voltammetry (DPV). In the proposed method, biotin labeled and free OTA competed to bind with immobilized aptamer onto the surface of a screen printed carbon electrode (SPCE), and percentage binding was calculated. The detection was performed after adding avidin-ALP to perform avidin-biotin reaction; the signal was generated through a suitable substrate 1-naphthyl phosphate (1-NP), for alkaline phosphatase (ALP). The cocoa samples were extracted and purified using molecular imprinted polymer (MIP) columns specifically designed for OTA. The developed aptasensor showed a good linearity in the range 0.15-5 ng/mL with the limit of detection (LOD) 0.07 ng/mL and 3.7% relative standard deviation (RSD). The aptasensor displayed good recovery values in the range 82.1-85% with 3.87% RSD, thus, demonstrated the efficiency of proposed aptasensor for such matrices.

Aptasensor and genosensor methods for detection of microbes in real world samples.

The increasing concerns about food and environmental safety have prompted the desire to develop rapid, specific, robust and highly sensitive methods for the detection of microorganisms to ensure public health. Although traditional microbiological methods are available, they are labor intensive, unsuitable for on-site and high throughput analysis, and need well-trained personnel. To circumvent these drawbacks, many efforts have been devoted towards the development of biosensors, using nucleic acid as bio-recognition element. In this review, we will focus on recent significant advances made in two types of DNA-based biosensors, namely genosensors, and aptasensors. In genosensor approach, DNA or RNA target is detected through the hybridization reaction between DNA or RNA and ssDNA sensing element, while in aptasensor method, DNA or RNA aptamer, capable of binding to a target molecule with high affinity and specificity, plays the role of receptor. The goal of this article is to review the innovative methods that have been emerged in genosensor and aptasensor during recent years. Particular attention is given to recent advances and trends in selection of biorecognition element, DNA immobilization strategies and sensing formats.

Current Trends in Nanomaterial-Based Amperometric Biosensors

The last decade has witnessed an intensive research effort in the field of electrochemical sensors, with a particular focus on the design of amperometric biosensors for diverse analytical applications. In this context, nanomaterial integration in the construction of amperometric biosensors may constitute one of the most exciting approaches. The attractive properties of nanomaterials have paved the way for the design of a wide variety of biosensors based on various electrochemical detection methods to enhance the analytical characteristics. However, most of these nanostructured materials are not explored in the design of amperometric biosensors. This review aims to provide insight into the diverse properties of nanomaterials that can be possibly explored in the construction of amperometric biosensors.

Design of PEG-aptamer two piece macromolecules as convenient and integrated sensing platform: Application to the label free detection of small size molecules

A novel strategy for the fabrication of electrochemical label free aptasensor for small size molecules is purposed, and the strategy has been demonstrated by the development of an aptasensor for ochratoxin (A). A long spacer chain of polyethylene glycol (PEG) was immobilized on screen printed carbon electrode (SPCE) via electrochemical oxidation of its terminal amino-group. The amino-aptamer was covalently linked to carboxy end of immobilized PEG to form two piece macromolecules. The designed immobilized macromolecules resulted in the formation of long tunnels on SPCE surface, while aptamer acted as gate of the tunnels. The aptamer gates were closed due to change in conformation of aptamer upon target analyte binding, decreasing the electrochemical signal. The decrease in electrochemical signal was used for the detection of target molecule.

Nanoceria Particles As Catalytic Amplifiers for Alkaline Phosphatase Assays

We propose a novel system to enhance detection sensitivity of alkaline phosphatase (ALP) in electrochemical assays by using nanoceria particles as redox active catalytic amplifiers of ALP signals. The catalytic activity of nanoceria particles attributed to their dual oxidation state Ce(4+)/Ce(3+) and high oxygen mobility enabled oxidation of the products of the ALP-catalyzed reaction. A suite of spectroscopic and electrochemical methods, including UV-vis spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), and cyclic voltammetry (CV) were used to characterize the interaction of nanoceria with the ALP-generated products. Spectrometric experiments demonstrate change in the oxidation state of nanoceria upon exposure to the hydrolytic products of ALP. Three enzymatically generated products of commonly used ALP substrates were detected at a screen printing electrode surface in the presence of nanoceria. Electrochemical experiments demonstrate signal amplification of the ALP activity assay by nanoceria for all three products, demonstrating remarkable sensitivity of this assay. The assay was optimized with respect to pH and buffer composition. Analytical characterization of the nanoceria-based ALP activity assay was established using a 1-naphthyl phosphate substrate. The proposed strategy can find widespread applications in sensing schemes involving ALP.