Development of a heterogeneous amperometric immunosensor for the determination of apolipoprotein E in serum

Metabolomic Alterations in the Tear Fluids of Patients With Superior Limbic Keratoconjunctivitis

Purpose

Superior limbic keratoconjunctivitis (SLK) is a bilateral, chronic inflammatory disease that recurs for up to several years; however, the fundamental processes involved in its pathogenic mechanisms remain unknown. We aimed to investigate the metabolomic alterations in the tear fluids of patients with superior limbic keratoconjunctivitis (SLK) compared with those of healthy volunteers (Ctrl group).

Methods

We performed a cross-sectional study involving 42 subjects. Tear fluid was taken from one eye of 24 SLK patients (40.13 ± 14.55 years, 83.33% female) and 18 healthy volunteers (Ctrl, 39.89 ± 9.2 years, 72.22% female) using Schirmer strips. After the liquid extraction of tear metabolites, samples were infused into the QE HFX Orbitrap mass spectrometer in both positive and negative ion mode. Metabolites were quantitatively analyzed and matched with entries in the HMDB database. Metabolic differences between the SLK group and the control group were identified based on multivariate statistical analysis. Open database sources, including SMPDB and MetaboAnalyst, were used to identify metabolic pathways.

Results

Among 179 metabolites retained for annotation, 133 metabolites were finally identified, among which 50 were found to be significantly changed in SLK patients. Of these 50 metabolites, 31 metabolites significantly increased and 19 metabolites decreased in SLK patients. The altered metabolites are mainly involved in α linolenic acid and linoleic acid metabolism, ketone body metabolism, butyrate metabolism, mitochondrial electron transport chain, carnitine synthesis, and so on. The most significantly changed pathway was linoleic acid metabolism. To explore the utility of tear biomarkers, a model combining 9 metabolites (phenol, ethyl glucuronide, eicosapentaenoic acid, 12-keto-leukotriene B4, linoleic acid, hypoxanthine, triethanolamine, 1-nitrohexane, and terephthalic acid) was selected as a candidate biomarker.

Conclusion

The results reveal that SLK has a specific metabolomic profile, of which some key elements can serve as potential biomarkers of SLK for diagnostic and prognostic purposes. The findings of this study are novel and provide a basis for further investigations of the mechanism of SLK.

Induced conformational change on ferrocenyl-terminated alkyls and their application as transducers for label-free immunosensing of Alzheimer's disease biomarker

ApoE Alzheimer's disease biomarker can be sensitively detected by a label-free platform using flexible ferrocene-terminated alkyl chains. The immunorecognition triggers conformational changes, which improve the rate constants of electron-transfer.

Over-the-Counter Biosensors: Past, Present, and Future

The demand for specific, low cost, rapid, sensitive and easy detection of biomolecules is huge. A well-known example is the glucose meters used by diabetics to monitor their blood glucose levels. Nowadays, a vast majority of the glucose meters are based on electrochemical biosensor technology. The inherent small size and simple construction of the electrochemical transducer and instrument are ideally suited for point-of-care biosensing. Besides glucose, a wide variety of electrochemical biosensors have been developed for the measurements of some other key metabolites, proteins, and nucleic acids. Nevertheless, unlike the glucose meters, limited success has been achieved for the commercialization of the protein and nucleic acid biosensors. In this review article, key technologies on the electrochemical detection of key metabolites, proteins, and DNAs are discussed in detail, with particular emphasis on those that are compatible to home-use setting. Moreover, emerging technologies of lab-on-a-chip microdevices and nanosensors (i.e., silicon and carbon nanotube field-effect sensors) offer opportunities for the construction of new generation biosensors with much better performances. Together with the continuous innovations in the basic components of biosensors (i.e., transducers, biorecognition molecules, immobilization and signal transduction schemes), consumers could soon buy different kinds of biosensing devices in the pharmacy stores.

Electrochemical biochips for protein analysis

Proteins bear important functions for most life processes. It is estimated that the human proteome comprises more than 250,000 proteins. Over the last years, highly sophisticated and powerful instruments have been developed that allow their detection and characterization with great precision and sensitivity. However, these instruments need well-equipped laboratories and a well-trained staff. For the determination of proteins in a hospital, in a doctor's office, or at home, low-budget protein analysis methods are needed that are easy to perform. In addition, for a proteomic approach, highly parallel measurements with small sample sizes are required. Biochips are considered as promising tools for such applications. The following chapter describes electrochemical biochips for protein analysis that use antibodies or aptamers as recognition elements.

Genosensor on gold films with enzymatic electrochemical detection of a SARS virus sequence

A hybridisation-based genosensor was designed on a 100 nm sputtered gold film. This material worked as an immobilisation and transduction surface. A 30-mer sequence that encodes a short lysine-rich region, unique to SARS (severe acute respiratory syndrome) virus, was chosen as target. A complementary strand (probe), labelled with a thiol group at the 3'-end, was immobilised on the film. After blocking the surface, hybridisation with the biotin-conjugated SARS strand (at the 3'-end) took place. Interaction with alkaline phosphatase-labelled streptavidin permits amplified indirect electrochemical detection. The analytical signal is constituted by an electrochemical process of indigo carmine, the soluble product of the enzymatic hydrolysis of 3-indoxyl phosphate. The use of a sensitive electrochemical technique such as square wave voltammetry allowed a detection limit of 6 pM to be obtained for this DNA sequence, lower than any other found in the bibliography. The parameters affecting the methodology were studied, with special attention being placed on selectivity. Specificity was clearly enhanced when interaction time and stringency (in the form of formamide percentage) were increased. With 1h of strand interaction and employing 50% of formamide in the hybridisation buffer, a 3-base mismatch strand was perfectly distinguished from the complementary.

Biosensors in clinical chemistry

Biosensors are analytical devices composed of a recognition element of biological origin and a physico-chemical transducer. The biological element is capable of sensing the presence, activity or concentration of a chemical analyte in solution. The sensing takes place either as a binding event or a biocatalytical event. These interactions produce a measurable change in a solution property, which the transducer converts into a quantifiable electrical signal. Present-day applications of biosensors to clinical chemistry are reviewed, including basic and applied research, commercial applications and fabrication techniques. Recognition elements include enzymes as biocatalytic recognition elements and immunoagents and DNA segments as affinity ligand recognition elements, coupled to electrochemical and optical modes of transduction. The future will include biosensors based on synthetic recognition elements to allow broad applicability to different classes of analytes and modes of transduction extending lower limits of sensitivity. Microfabrication will permit biosensors to be constructed as arrays and incorporated into lab-on-a-chip devices.

Immunosensors--principles and applications to clinical chemistry

INTRODUCTION

Immunosensors are affinity ligand-based biosensor solid-state devices in which the immunochemical reaction is coupled to a transducer. The fundamental basis of all immunosensors is the specificity of the molecular recognition of antigens by antibodies to form a stable complex. This is similar to the immunoassay methodology. Immunosensors can be categorized based on the detection principle applied. The main developments are electrochemical, optical, and microgravimetric immunosensors. In contrast to immunoassay, modern transducer technology enables the label-free detection and quantification of the immune complex.

METHODS

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors in these applications is most appropriate. Similarly, a series of clinical problems may be solved by continuous monitoring of certain analytes.

CONCLUSIONS

Clinical chemists should take advantage of immunosensors in clinical diagnostics. There are many recent developments in the immunosensor field which have potential impacts. The future role of this technique in intralaboratory, as well as bedside testing, will become even more important as the clinical laboratory is faced with increasing pressure to contain costs.

On applicability of laccase as label in the mediated and mediatorless electroimmunoassay: effect of distance on the direct electron transfer between laccase and electrode

Applicability of laccase as enzyme-label has been investigated. It was shown that the property of laccase to catalyze the oxygen electroreduction at an electrode allows to develop a mediatorless and pseudoreagentless electro-enzyme-immunoassay (EEIA). In this case the electrode acts as an electron-donor substrate. When the bioelectrocatalytic reaction takes place, some electric charge is collected on the electrode. A method of determination of the electrode charge as well as the concentration of oxidized form of the mediator at the electrode surface has been elaborated. For this aim a technique of the measurement of current-surge was employed. Human immunoglobulin G and insulin were taken as model in this investigation. A back titration schemes without any mediator and in the presence of o-carboxybenzoylferrocene as a mediator was applied. The antibody carbon-black and the antigen glassy-carbon electrodes were used. The limits of detection were found to be 0.3 and 1.6 nM, respectively. The advantage of the mediatorless assay is that the charge leakage is imperceptible by open circuit for a long time and the accumulation of the charge occurs linearly with time. The charge accumulation for a long time allows to diminish the limit of detection. However, there is a limitation of the method. The direct electron transfer slows down with increasing the distance between the enzyme molecule and the electrode surface. This effect reduces the sensitivity of the method. The decrease of the electron transfer rate with distance has been estimated. Monolayer of hemoglobin dividing the laccase molecule from the electrode surface decreases the rate by four times. The electron transfer rate for the antibody electrode with associated antigen-laccase conjugate is less than that for the analogous electrode, covered with monolayer of covalently attached laccase, by 210 times. The current-surge peak was expected to decrease with distance by an equation of the form I = I0 exp[-r/r0]. The parameter r0 is equal to 2.2 +/- 0.8 nm. The possibility of the sensitivity increase in the mediatorless mode by 'wiring' through the multilayer film of immunoproteins immobilized on the electrode is discussed.

Immunosensors: electrochemical sensing and other engineering approaches

This article overviews the engineering approaches and the recent trends in the development of alternative immunoassay systems. A brief description of the main principles and limitations of conventional immunoassay is given. Immunosensing approaches overcoming these limitations are discussed. Alternatives to traditional immunoassay systems are discussed in terms of the enhancement of immunointeraction processes and in terms of the various detection principles. Applications of flow-injection techniques to the development of immunosensing systems are presented. Immunosensors are categorized based on the detection principle employed, as immunoelectrodes (electrochemical immunosensors), piezoelectric immunosensors, or as sensors based on optical detection of the immunointeraction. The discussion focuses on electrochemical immunosensors. In conclusion, the engineering issues involved in immunosensor development are outlined and trends towards practical applications are discussed.

Amperometric immunosensors based on rigid conducting immunocomposites

Novel polishable immunosensors based on rigid biocomposite materials have been constructed. These biocomposites contain graphite powder, rabbit IgG, and methacrylate or epoxy resins. This material acts as a reservoir for the biological molecules and as a transducer at the same time. In order to study the potential analytical properties of this new type of material, a competitive binding assay was developed to determine the RIgG present in a sample with the aid of goat anti-rabbit IgG labeled with alkaline phosphatase. Using phenyl phosphate as a substrate, the phenol produced by the enzymatic reaction was amperometrically detected at 800 mV (vs Ag/AgC1). The surface of the immunosensor can be regenerated by simply polishing, obtaining fresh immunocomposite ready to be used in a new competitive assay.

Highly sensitive determination of hydrogen peroxide and peroxidase with tetrathiafulvalene-based electrodes and the application in immunosensing

Redox mediators enable an efficient electron transfer between redox enzymes and the electrochemical surface of amperometric sensors. A stable and highly sensitive signal was obtained using a tetrathiafulvalene (TTF)-modified graphite electrode. In the presence of horseradish peroxidase (HRP), hydrogen peroxide (H2O2) was monitored with a detection limit of 7 nM at a potential of +20 mV versus a saturated calomel electrode (SCE). With a constant concentration of H2O2, the detection limit for HRP was found to be 150 pM. The accuracy of consecutive measurement of HRP in flow-through systems was improved by short-time polarizing the TTF-modified graphite electrode at +100 mV versus Ag/AgCl/3 M KCl. Using the TTF-modified graphite electrode in an immuno-sandwich approach, rabbit-immunoglobulin G was monitored in the range 5-100 ng/ml.

On-line competitive immunoassay for insulin based on capillary electrophoresis with laser-induced fluorescence detection

An on-line competitive immunoassay for insulin has been developed and applied to monitoring insulin concentration in a flowing stream. In the assay, solutions of fluorescein-labeled insulin (FITC-insulin), monoclonal anti-insulin, and sample containing insulin are pumped into a cross where they begin to mix. The mixture flows through a fused silica reactor capillary to a flow-gated interface. During transfer to the interface, insulin and FITC-insulin compete to form a complex with the antibody. At the interface, a plug of the mixture is injected into a separation capillary, where the bound and free FITC-insulin are separated and detected by capillary electrophoresis with laser-induced fluorescence detection. The amount of bound FITC-insulin, amount of free FITC-insulin, or bound/free ratio can be used to quantify insulin concentration. Typical relative standard deviations of bound over free ratio are 5%. The detection limit of the immunoassay in the on-line mode is < 0.3 nM. Each separation requires as little as 3 s, and over 1600 consecutive assays can be acquired with no need to rinse the separation capillary. Thus, the system can be used to monitor insulin in a flowing stream for flow injection analysis or for sensor-like monitoring. Dilution and zone broadening during transfer of sample to the interface limit the response time of the on-line system to about 25 s. As a demonstration of the on-line immunoassay, the insulin content of single islets of Langerhans was determined by flow injection analysis.