Electroanalysis and biosensors

Innovative polymer engineering for the investigation of electrochemical properties and biosensing ability

Subtle engineering for the generation of a biosensor from a conjugated polymer with the inclusion of fluorine-substituted benzothiadiazole and indole moieties is reported. The engineering includes the electrochemical copolymerization of the indole-6-carboxylic acid (M1) and 5-fluoro-4,7-bis(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (M2) on the indium tin oxide and graphite electrode surfaces for the investigation of both their electrochemical properties and biosensing abilities with their copolymer counterparts. The intermediates and final conjugated polymers, Poly(M1) [P-In6C], Poly(M2) [P-FBTz], and copoly(M1 and M2) [P-In6CFBTz], were entirely characterized by 1H NMR, 13C NMR, CV, UV-Vis-NIR spectrophotometry, and SEM techniques. HOMO energy levels of electrochemically obtained polymers were calculated from the oxidation onsets in anodic scans as -4.78 eV, -5.23 eV, and -4.89 eV, and optical bandgap (Egop) values were calculated from the onset of the lowest-energy π-π* transitions as 2.26 eV, 1.43 eV, and 1.59 eV for P-In6C, P-FBTz, and P-In6CFBTz, respectively. By incorporation of fluorine-substituted benzothiadiazole (M2) into the polymer backbone by electrochemical copolymerization, the poor electrochemical properties of P-In6C were remarkably improved. The polymer P-In6CFBTz demonstrated striking electrochemical properties such as a lower optical band gap, red-shifted absorption, multielectrochromic behavior, a lower switching time, and higher optical contrast. Overall, the newly developed copolymer, which combined the features of each monomer, showed superior electrochemical properties and was tested as a glucose-sensing framework, offering a low detection limit (0.011 mM) and a wide linear range (0.05-0.75 mM) with high sensitivity (44.056 μA mM-1 cm-2).

Aptamers used for biosensors and targeted therapy

Aptamers are single-stranded nucleic acid sequences that can bind to target molecules with high selectivity and affinity. Most aptamers are screened in vitro by a combinatorial biology technique called systematic evolution of ligands by exponential enrichment (SELEX). Since aptamers were discovered in the 1990s, they have attracted considerable attention and have been widely used in many fields owing to their unique advantages. In this review, we present an overview of the advancements made in aptamers used for biosensors and targeted therapy. For the former, we will discuss multiple aptamer-based biosensors with different principles detected by various signaling methods. For the latter, we will focus on aptamer-based targeted therapy using aptamers as both biotechnological tools for targeted drug delivery and as targeted therapeutic agents. Finally, challenges and new perspectives associated with these two regions were further discussed. We hope that this review will help researchers interested in aptamer-related biosensing and targeted therapy research.

Detection and Identification of Hematologic Malignancies and Solid Tumors by an Electrochemical Technique

Purpose

Develop and evaluate an electrochemical method to identify healthy individuals, malignant hematopathic patients and solid tumor patients by detecting the leukocytes in whole-blood.

Methods

A total of 114 individual blood samples obtained from our affiliated hospital in China (June 2015- August 2015) were divided into three groups: healthy individuals (n = 35), hematologic malignancies (n = 41) and solid tumors (n = 38). An electrochemical workstation system was used to measure differential pulse voltammetry due to the different electrochemical behaviors of leukocytes in blood samples. Then, one-way analysis of variance (ANOVA) was applied to analyze the scanning curves and to compare the peak potential and peak current.

Results

The scanning curve demonstrated the specific electrochemical behaviors of the blank potassium ferricyanide solution and that mixed with blood samples in different groups. Significant differences in mean peak potentials of mixture and shifts (ΔEp (mV)) were observed of the three groups (P< = 0.001). 106.00±9.00 and 3.14±7.48 for Group healthy individuals, 120.90±11.18 and 18.10±8.81 for Group hematologic malignancies, 136.84±11.53 and 32.89±10.50 for Group solid tumors, respectively. In contrast, there were no significant differences in the peak currents and shifts.

Conclusions

The newly developed method to apply the electrochemical workstation system to identify hematologic malignancies and solid tumors with good sensitivity and specificity might be effective, suggesting a potential utility in clinical application.

Design of immunoprobes for electrochemical multiplexed tumor marker detection

Many approaches have been developed for simultaneous detection of multiple tumor markers. Among these approaches, the electrochemical immunoassay has the advantage of high sensitivity and specificity and could be easily expanded into multiplex detection platform. For the simultaneous multianalyte electrochemical immunosensor, performance is closely related with the characteristics of the immunoprobes and substrate. In order to construct a multilabeled immunoprobe platform, the most important issue is how to discriminate each signal for each analyte from the multiple antigen-antibody reactions. Currently, enzyme-based, noble metal nanomaterials, carbonmaterials and polymer-based nanomaterial immunoprobes have been used for dual- or three-analyte detections. However, there are still some challenges in developing sensitive method to detect three or more tumor markers owing to the lack of redox-active species that can produce three or more distinctive peaks. Additionally, for the immunosensing substrate, good conductivity, high specific surface area and good biocompatibility are further necessities.

Functionalized solid electrodes for electrochemical biosensing of purine nucleobases and their analogues: a review

Interest in electrochemical analysis of purine nucleobases and few other important purine derivatives has been growing rapidly. Over the period of the past decade, the design of electrochemical biosensors has been focused on achieving high sensitivity and efficiency. The range of existing electrochemical methods with carbon electrode displays the highest rate in the development of biosensors. Moreover, modification of electrode surfaces based on nanomaterials is frequently used due to their extraordinary conductivity and surface to volume ratio. Different strategies for modifying electrode surfaces facilitate electron transport between the electrode surface and biomolecules, including DNA, oligonucleotides and their components. This review aims to summarize recent developments in the electrochemical analysis of purine derivatives, as well as discuss different applications.

Methylene blue covalently attached to single stranded DNA as electroactive label for potential bioassays

Methylene blue is an electroactive molecule that has been employed for the detection of the DNA hybridization event in electrochemical sensors. However, its use as a covalent label is very scarce and in most of the cases, non-covalent interactions (hydrophobic, electrostatic) are employed. Although it has advantages as simplicity and fewer number of procedure steps, the covalent attachment is less exploited in the development of these sensors. In this article, the electrochemical behavior of methylene blue attached to different DNA-strands is studied. Several lengths (15- and 30-mer) and different degree of DNA modification (MB-DNA, MB-DNA-MB and MB-DNA-SH) have been studied. The highest signals were obtained for longer strands with two MB molecules. In all the cases the signal is enhanced by CNT-nanostructuration of the electrode. Adsorption on these modified screen-printed electrodes allowed the amplification by employing an accumulation time. In this way, a sensitivity of -0.2864 μA μM-1 and a limit of detection of 800 nM for a 120 s accumulation time were obtained.

Chitosan encapsulated quantum dots platform for leukemia detection

We report results of the studies relating to electrophoretic deposition of nanostructured composite of chitosan (CS)-cadmium-telluride quantum dots (CdTe-QDs) onto indium-tin-oxide coated glass substrate. The high resolution transmission electron microscopic studies of the nanocomposite reveal molecular level coating of the CdTe-QDs with CS molecules in the colloidal dispersion medium. This novel composite platform has been explored to fabricate an electrochemical DNA biosensor for detection of chronic myelogenous leukemia (CML) by immobilizing amine terminated oligonucleotide probe sequence containing 22 base pairs, identified from BCR-ABL fusion gene. The results of differential pulse voltammetry reveal that this nucleic acid sensor can detect as low as 2.56 pM concentration of complementary target DNA with a response time of 60s. Further, the response characteristics show that this fabricated bioelectrode has a shelf life of about 6 weeks and can be used for about 5-6 times. The results of experiments conducted using clinical patient samples reveal that this sensor can be used to distinguish CML positive and the negative control samples.

An amperometric glucose biosensor based on titania sol-gel/Prussian Blue composite film

An improved amperometric glucose biosensor was constructed by immobilizing glucose oxidase (GOD) in a titania sol-gel film, which was prepared by a vapor deposition method, on a Prussian Blue (PB)-modified electrode. The method combined the merits of immobilizing biomolecules in the titania sol-gel film by vapor deposition method and the synergic catalysis effects of PB and GOD molecules. Results showed that the fabricated titania sol-gel/PB membrane possessed high surface area, good mechanical stability, and good hydrophilicity, which provided a biocompatible microenvironment for maintaining the bioactivity of the immobilized enzyme and prevented the enzyme from leaking out of the film. Therefore, the present biosensor exhibited fast response time (10 s), high sensitivity (12.74 muA cm(-2) mM(-1)), long-term operational stability, good suppression of interference, and a wide linear range from 0.02 to 15 mM with a low detection limit of 5 muM for the detection of glucose. In addition, this simple and controllable method could fabricate biosensors in batches with a very small amount of enzyme.

Synthesis and Characterization of High Integrity Solid-Contact Polymeric Ion Sensors

High integrity solid-contact (SC) polymeric ion sensors have been produced by using spin casting and electropolymerization techniques in the preparation of the SC employing the conductive polymer, poly(3-octylthiophene) [POT]. The physical and chemical integrity of the POT SCs have been evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). Furthermore, the electrochemical stability of SC polymeric ion sensors has been investigated using electrochemical impedance spectroscopy (EIS). The results of this study demonstrate that electropolymerization and spin casting methods also comprising annealing of the synthesized SC film are capable of producing SCs that are relatively free of imperfections such as pores and pinholes. This leads to electrochemically stable and robust polymeric ion sensors where the SC/sensor interface is resistant to the formation of a detrimental water layer that normally gives rise to spurious ion fluxes and a degradation in the sensitivity and selectivity of the SC polymeric ion sensor.

Structure and activity assay of nanozymes prepared by the coimmobilization of practically useful enzymes and hydrophilic block copolymers on gold nanoparticles

Enzyme/polymer/gold nanoparticle hybrids, called "nanozymes", were prepared and structurally analyzed by dynamic light scattering (DLS), ultraviolet-visible spectroscopy, and zeta-potential and transmission electron microscopy (TEM) measurements, which showed that the nanozyme particles were mainly composed of a single gold nanoparticle, on whose surface the enzyme and polymer were coimmobilized. This kind of structure resulted in the high dispersion stability of the nanozyme under various conditions, accompanied by improved thermal stability of the enzyme.

Electrochemical Interrogation of Interactions between Surface-Confined DNA and Methylene Blue

In this work, we reported a systematic investigation on the interactions between methylene blue (MB) and surface-confined DNA by using electrochemical methods. We demonstrated that the redox potential of MB and binding and dissociation kinetics of MB to DNA differed significantly for single-stranded DNA (ss-DNA) and double-stranded DNA (ds-DNA) immobilized on gold electrodes. This was possibly due to the different binding mechanism between MB and ss- or ds-DNA. This work might provide useful information for developing MB-based sequence-specific electrochemical DNA sensors.

Label-free electrochemical detection of DNA using ferrocene-containing cationic polythiophene and PNA probes on nanogold modified electrodes

A label-free electrochemical method for the detection of DNA-PNA hybridization using a water-soluble, ferrocene-functionalized polythiophene transducer and single-stranded PNA probes on the nanogold modified electrode is investigated. Nanogold modified electrodes can largely increase the immobilization amount of ss-PNA capture probe and lead to an increase of the electrical signal. The ferrocene-containing cationic polythiophene do not interact electrostatically with the PNA probes due to the absence of the anionic phosphate groups on the PNA probes. But after DNA-PNA hybridization, cationic polythiophene is adsorbed on the DNA backbone, giving a clear hybridization detection signal in differential pulse voltammetry (DPV). Very good discrimination against non-complementary DNA and four-base mismatch DNA is observed. These studies show that the proposed method can provide an alternative for expanding the range of detection methods available for DNA hybridization.

Successively amplified electrochemical immunoassay based on biocatalytic deposition of silver nanoparticles and silver enhancement

A successively signal-amplified electrochemical immunoassay has been reported on the basis of the biocatalytic deposition of silver nanoparticles with their subsequent enlargement by nanoparticle-promoted catalytic precipitation of silver from the silver-enhancer solution. The immunoassay was carried out based on a heterogeneous sandwich procedure using polystyrene microwells to immobilize antibody. After all the processes comprising the formation of immunocomplex, biocatalytic deposition of silver nanoparticles and following silver enhancement were completed, the silver on polystyrene microwells was dissolved and quantified by anodic stripping voltammetry (ASV). The effect of relevant experimental conditions, including the concentration of ascorbic acid 2-phosphate (AA-p) substrate and Ag(I) ions, the biocatalytic deposition time, and of crucial importance, the silver enhancement time, were investigated and optimized. The anodic stripping peak current was proportional to the concentration of human IgG in a dynamic range of 0.1-10 ng ml(-1) with a detection limit of 0.03 ng ml(-1). Scanning electron microscope (SEM) was applied to characterize the silver nanoparticles before and after silver enhancement on the surface of polystyrene microplates. By coupling the highly catalytic effect of enzyme and nanoparticles to successively amplify the analytical signal, the sensitivity of immunoassay was enhanced so dramatically that this approach would be a promising strategy to achieve a lower detection limit for bioassays.

Nucleic acid biosensor for detection of hepatitis B virus using 2,9-dimethyl-1,10-phenanthroline copper complex as electrochemical indicator

In this study, an electrochemical DNA biosensor was developed based on the recognition of target DNA by hybridization detection. The study was carried out using glassy carbon electrode (GCE) modified with lable-free 21-mer single-stranded oligonucleotides related to hepatitis B virus sequence via covalent immobilization and [Cu(dmp)(H(2)O)Cl(2)] (dmp=2,9-dimethyl-1,10-phenanthroline) as an electrochemical indicator, whose sizes are comparable to those of the small groove of native double-duplex DNA. The method, which is simple and low cost, allows the accumulation of copper complex within the DNA layer. Electochemical detection was performed by cyclic voltammetry and differential pulse voltammetry over the potential range where the [Cu(dmp)(H(2)O)Cl(2)] was active. Numerous factors affecting the probe immobilization, target hybridization, and indicator binding reactions were optimized to maximize the sensitivity and speed the assay time. With this approach, a sequence of the hepatitis B virus could be quantified over the ranges from 8.82 x 10(-8) to 8.82 x 10(-7) M with a linear correlation of r=0.9937 and a detection limit of 7.0 x 10(-8) M. The [Cu(dmp)(H(2)O)Cl(2)] signal observed from probe sequence before and after hybridization with four bases mismatch containing sequence is lower than that observed after hybridization with complementary sequence.

Hairpin DNA probe based electrochemical biosensor using methylene blue as hybridization indicator

In this paper, a label-free, rapid and simple method was proposed to study the hybridization specificity of hairpin DNA probe using methylene blue (MB) as a hybridization indicator. Thiolated hairpin DNA probe was immobilized on the gold electrode by self-assembly. The voltammetric signals of MB were investigated at these modified electrodes by means of cyclic voltammetry (CV) detection. Single-base mutation oligonucleotide and random oligonucleotide can be easily discriminated from complementary target DNA. The effect of mismatch position in target DNA was investigated. Experimental results showed that mutation in the center of target DNA had greatest effect on the hybridization with hairpin DNA probe. The relationship between electrochemical responses and DNA target concentration was also studied. The reduction current of MB intercalation decreased with increasing the concentration of target DNA. Taken together, these experiments demonstrate that the hybridization indicator MB provides great promise for rapid and specific measurement of target DNA.

The effect of the layer structure on the activity of immobilized enzymes in ultrathin films

The molecular engineering capability of the layer-by-layer (LbL) method for fabricating thin films has been exploited in order to immobilize glucose oxidase (GOD) in films with alternating layers of chitosan. Chitosan was proven a good scaffolding material, as GOD molecules preserved their catalytic activity towards glucose oxidation. Using electrochemical measurements, we showed that chitosan/GOD LbL films can be used to detect glucose with a limit of detection of 0.2 mmol l-1 and an activity of 40.5 microA mmol-1 L microg-1, which is highly sensitive when compared to other sensors in previous reports in the literature. The highest sensitivity of the LbL film was achieved when only the top layer contained GOD, thus indicating that GOD in inner layers did not contribute to glucose oxidation, probably because it hampers analyte diffusion and electron transport through the deposited layers. This may be explained by the dense packing of GOD molecules in the LbL films with chitosan, as inferred from estimates of the amount of GOD adsorbed per layer using a quartz crystal microbalance.

A novel electrochemical detection method for aptamer biosensors

A beacon aptamer-based biosensor for the detection of thrombin was developed using electrochemical transduction method. Gold surface was modified with a beacon aptamer covalently linked at 5'-terminus with a linker containing a primary aliphatic amine. Methylene blue (MB) was intercalated into the beacon sequence, and used as an electrochemical marker. When the beacon aptamer immobilized on gold surface encounters thrombin, the hairpin forming beacon aptamer is conformationally changed to release the intercalated MB, resulting a decrease in electrical current intensity in voltamogram. The peak signal of the MB is clearly decreased by the binding of thrombin onto the beacon aptamer. The linear range of the signal was observed between 0 and 50.8 nM of thrombin with 0.999 correlation factor. This method was able to linearly and selectively detect thrombin with a detection limit of 11 nM.

Development of liposomal immunosensor for the measurement of insulin with femtomole detection

The monitoring of insulin is of great relevance for the management of diabetes, the detection of pancreatic islet-cell malfunction, the definition of hypoglycemia, and the diagnosis of insulinoma. A liposomal immunosensing system for the determination of insulin was developed in this study. The insulin sensor was constructed by the immobilization of anti-insulin antibodies on the inner wall of the microcapillary immunoseparator. Liposomes tagged with anti-insulin and encapsulating a fluorescent dye were used as the detectable label. In the presence of insulin, sandwich immunocomplexes were formed between the immobilized antibodies in the column, the sample of insulin, and the antibody-tagged sulforhodamine B-dye-loaded liposomes. Signals generated by lysing the bound liposomes with 30 mM n-octyl-beta-D-glucopyranoside were measured by a fluorescence detector. The detected signal was directly proportional to the amount of insulin in the test sample. The liposomal immunosensing system successfully detected as low as 136 attomole. MeOH (30%) was used for the regeneration of antibody-binding sites in the microcapillary after each measurement, which allowed the immunoseparator to be used for at least 70 repeated assays. The antibody activity in this proposed microcapillary immunoseparator could be well maintained for at least 1 week. The calibration curve for insulin in Tris-buffered saline had a linear dynamic range of 10 pM-10 nM, and the total assay time was less than 30 min. The coefficient of variation for triplicate measurements was <5.00%, which indicated that well-reproducible results can be obtained by this newly developed method.

Reagentless amperometric immunosensor for human chorionic gonadotrophin based on direct electrochemistry of horseradish peroxidase

A novel amperometric immunosensor for determination of human serum chorionic gonadotrophin (HCG) was constructed by immobilization of HCG with titania sol-gel on a glassy carbon electrode and the direct electrochemistry of horseradish peroxidase (HRP) labeled to HCG antibody (HRP-anti-HCG). The morphologies of the HCG membrane were characterized to be chemically clean, porous and homogeneous. HRP-anti-HCG was functionally conjugated with the immobilized HCG after incubation in phosphate buffer (PBS) containing HRP-anti-HCG. A direct electron transfer of HRP with a rate constant of 1.35+/-0.40 s(-1) was observed at the HRP-anti-HCG-HCG/titania sol-gel membrane modified electrode in 0.1 M PBS pH 7.0. With a competitive mechanism the differential pulse voltammetric peak current of the immobilized HRP decreased linearly with an increasing HCG concentration from 2.5 to 12.5 mIU/ml in the incubation solution. The HCG immunosensor showed a detection limit of 1.4 mIU/ml, a good accuracy and acceptable precision and reproducibility with an intra-assay CV of 4.7% at 5.0 mIU/ml and an inter-assay precision of 8.1% obtained at 10 mIU/ml. The biosensor displayed a good stability in a storage period of 30 days.

Electrochemical DNA sensing using osmium complexes as hybridization indicators

A surface-based method for the study of the interactions of DNA with redox-active osmium complexes is described. The study was carried out using gold electrodes modified with DNA by adsorption and [Os(bpy)3]3+/2+ (bpy=2,2'-bipyridyl) or [Os(phen)3]3+/2+ (phen=1,10-phenantroline) as electrochemical indicators. The method, which is simple and reagent saving, allows the accumulation of osmium complexes on the DNA layer. The amount of osmium complex bound by the layer of double-stranded (dsDNA) or single-stranded DNA (ssDNA) adsorbed at gold electrodes was estimated from the cyclic voltammetric (CV) peak charge of osmium complex reduction. The dissociation constants (K) for the oxidized and reduced forms of a bound species are also estimated. [Os(phen)3]3+/2+ was applied to a probe for electrochemical DNA sensing. A thiol-linked single-stranded DNA probe was immobilized through the S-Au bonding to 70 pmol/cm2 on a gold electrode. Following hybridization with the complementary DNA, the osmium complex was electrochemically accumulated on the double-stranded DNA layer and the differential pulse voltammogram for this electrode gave an electrochemical signal due to the redox reaction of [Os(phen)3]3+/2+ that was bound to the double-stranded DNA on the electrode.

Novel potentiometric immunosensor for determination of diphtheria antigen based on compound nanoparticles and bilayer two-dimensional sol?gel as matrices

A novel method for fabrication of a diphtheria potentiometric immunosensor has been developed by means of self-assembling compound nanoparticles to a thiol-containing sol-gel network. A cleaned gold electrode was first immersed in a 3-mercaptopropyltrimethoxysilane (MPS) sol-gel solution to assemble a silica sol-gel monolayer. The silane entities were then polymerized into a two-dimensional sol-gel network (2D network) by dipping into aqueous NaOH. The second silane layer was formed by re-immersion in the MPS sol-gel solution overnight. The compound nanoparticles (nanocompounds) containing gold nanoparticles and silver nanoparticles were then chemisorbed on to the thiol groups of the second silane layer. Finally, diphtheria antibody (anti-Diph) was adsorbed on to the surface of the compound nanoparticles. The modified process was characterized by cyclic voltammetry (CV). Detection is based on the change in the potentiometric response before and after the antigen-antibody reaction. A direct potentiometric response to diphtheria antigen (Diph) was obtained from the immobilized diphtheria antibody. The potentiometric response of the resulting immunosensor was rapid and the linear range was from 22 to 800 ng mL-1 with the linear regression equation DeltaE=-79.5+69.4 log [Diph] and a detection limit of 3.7 ng mL-1 (at 3delta). Up to 19 successive assay cycles with retention of sensitivity were achieved for probes regenerated with 0.2 mol L-1 glycine-hydrochloric acid (Gly-HCl) buffer solution. Moreover, analytical results from several serum samples obtained using the developed technique were in satisfactory agreement with those given by the ELISA method, implying a promising alternative approach for detecting diphtheria antigen in clinical diagnosis.