Modulation of population density and size of silver nanoparticles embedded in bacterial cellulose via ammonia exposure: visual detection of volatile compounds in a piece of plasmonic nanopaper.

The localized surface plasmon resonance exhibited by noble metal nanoparticles can be sensitively tuned by varying their size and interparticle distances. We report that corrosive vapour (ammonia) exposure dramatically reduces the population density of silver nanoparticles (AgNPs) embedded within bacterial cellulose, leading to a larger distance between the remaining nanoparticles and a decrease in the UV-Vis absorbance associated with the AgNP plasmonic properties. We also found that the size distribution of AgNPs embedded in bacterial cellulose undergoes a reduction in the presence of volatile compounds released during food spoilage, modulating the studied nanoplasmonic properties. In fact, such a plasmonic nanopaper exhibits a change in colour from amber to light amber upon the explored corrosive vapour exposure and from amber to a grey or taupe colour upon fish or meat spoilage exposure. These phenomena are proposed as a simple visual detection of volatile compounds in a flexible, transparent, permeable and stable single-use nanoplasmonic membrane, which opens the way to innovative approaches and capabilities in gas sensing and smart packaging.

A DNA Aptasensor for Electrochemical Detection of Vascular Endothelial Growth Factor.

Electrochemical aptasensors can detect different cancer biomarkers to provide point-of-care diagnosis that is low cost, rapid, specific and sensitive. In this work, we described the development of an electrochemical single-use aptasensor for detection and analysis of vascular endothelial growth factor (VEGF). Gold nanostructured graphite screen-printed electrodes were firstly modified with a mixed monolayer of a primary thiolated DNA aptamer and a spacer thiol, 6-mercapto-1-hexanol. VEGF protein was then incubated with the aptasensor. An enzyme-amplified detection scheme, based on the coupling of a streptavidin-alkaline phosphatase conjugate and secondary biotinylated aptamer was then applied. The enzyme catalyzed the hydrolysis of the electroinactive 1 -naphthyl-phosphate to 1 -naphthol; this product is electroactive and has been detected by means of differential pulse voltammetry (DPV). The aptasensor response was found to be linearly related to the target concentration between 0 and 250 nmol L(-1); the detection limit was 30 nmol L(-1). The performance of the immunoassay in terms of reproducibility and selectivity has been also studied.

Enzyme entrapment by ß-cyclodextrin electropolymerization onto a carbon nanotubes-modified screen-printed electrode.

A novel enzyme entrapment approach based on an electropolymerization process utilizing multi-walled carbon nanotubes (MWCNT), ß-cyclodextrin (ß-CD) and glucose oxidase (GOx) is shown. Dopamine (DA) quantification is presented using a screen-printed electrode modified by electropolymerization of cyclodextrin with glucose oxidase, SPE/MWCNT/ß-CD-GOx. In order to show the relevance of the enzyme entrapment strategy controlled by electropolymerization to develop a specific and efficient biosensor, the various parts composing the electrode: SPE, SPE/ß-CD, SPE/GOx, SPE/ß-CD/GOx, SPE/MWCNT/ß-CD, SPE/MWCNT/GOx and SPE/MWCNT/ß-CD/GOx were tested separately. It was shown that although DA determination can be achieved with all of them, the electrodes modified with MWCNT presented better analytical features that those built without MWCNT, the best being the one including all components. This biosensor displayed good reproducibility, repeatability, and prolonged life-time under cold storage conditions. Its DA limit of detection (LOD) was 0.48±0.02 µA in a linear range of 10-50 µM with a sensitivity of 0.0302±0.0003 µA µM(-1) that makes it comparable or even better than many other electrodes reported in the literature. Moreover, it was also shown that using this electrode, DA quantification can be done in the presence of interfering agents such as ascorbic and uric acid. These findings demonstrate that the approach employed is feasible for enzyme entrapment and may find applications in other biosensing systems, where better sensitivity, stability and fast response are required.

Stable and sensitive flow-through monitoring of phenol using a carbon nanotube based screen printed biosensor.

A stable and sensitive biosensor for phenol detection based on a screen printed electrode modified with tyrosinase, multiwall carbon nanotubes and glutaraldehyde is designed and applied in a flow injection analytical system. The proposed carbon nanotube matrix is easy to prepare and ensures a very good entrapment environment for the enzyme, being simpler and cheaper than other reported strategies. In addition, the proposed matrix allows for a very fast operation of the enzyme, that leads to a response time of 15 s. Several parameters such as the working potential, pH of the measuring solution, biosensor response time, detection limit, linear range of response and sensitivity are studied. The obtained detection limit for phenol was 0.14 x 10(-6) M. The biosensor keeps its activity during continuous FIA measurements at room temperature, showing a stable response (RSD 5%) within a two week working period at room temperature. The developed biosensor is being applied for phenol detection in seawater samples and seems to be a promising alternative for automatic control of seawater contamination. The developed detection system can be extended to other enzyme biosensors with interest for several other applications.

Improvement of the electrochemical detection of catechol by the use of a carbon nanotube based biosensor.

Tyrosinase (Tyr) has been used frequently for the detection of phenolic compounds. The development of a biosensor based on this enzyme-integrated carbon nanotube (CNT) epoxy composite electrode (CNTECE) is described in order to perform measurements of catechol. The enzyme is immobilized into a matrix prepared by dispersion of multi-wall CNT (MWCNT) inside the epoxy resin forming a CNT epoxy-biocomposite (CNTEC-Tyr). The use of CNT improves the electronic transference between the enzyme and the electrode surface. The modified electrode was characterized electrochemically by amperometric and voltammetric techniques. An applied potential of -200 mV vs. Ag/AgC1 applied to the biocomposite based electrode was found to be optimal for electrochemical reduction of the enzymatic reaction products (quinones). The biosensor modified with MWCNT is also compared with a tyrosinase biosensor based on a graphite epoxy-composite (GECE-Tyr) showing a sensitivity of 294 microA/mM cm(2), a detection limit of 0.01 mM for a signal-to-noise ratio of 3 in a concentration range of 0.0-0.15 mM catechol with a response time of 20 s and an RSD of 8% (n = 3). The electrodes were stable for more than 24 h. A 90% increase of the signal indicated that the response is better with the biocomposite based on carbon nanotubes rather than with the graphite.

Electrochemical genosensors for biomedical applications based on gold nanoparticles.

Two gold nanoparticles-based genomagnetic sensors designs for detection of DNA hybridization are described. Both assays are based on a magnetically induced direct electrochemical detection of gold tags on magnetic graphite-epoxy composite electrodes. The first design is a two strands assay format that consists of the hybridization between a capture DNA strand which is linked with paramagnetic beads and another DNA strand related to BRCA1 breast cancer gene used as a target which is coupled with streptavidin-gold nanoparticles. The second genomagnetic sensor design is a sandwich assay format with more application possibilities. A cystic fibrosis related DNA strand is used as a target and sandwiched between two complementary DNA probes: the first one linked with paramagnetic beads and a second one modified with gold nanoparticles via biotin-streptavidin complexation reactions. The electrochemical detection of gold nanoparticles by differential pulse voltammetry was performed in both cases. The developed genomagnetic sensors provide a reliable discrimination against noncomplementary DNA as well against one and three-base mismatches. Optimization parameters affecting the hybridization and analytical performance of the developed genosensors are shown for genomagnetic assays of DNA sequences related with the breast cancer and cystic fibrosis genes.

Sensitive stripping voltammetry of heavy metals by using a composite sensor based on a built-in bismuth precursor.

A new graphite-epoxy composite electrode (GECE) containing Bi(NO(3))(3) as a built-in bismuth precursor for simultaneous and individual anodic stripping analysis of heavy trace metals like lead and cadmium is reported. The developed Bi(NO(3))(3)-GECE is compatible with bismuth film electrodes reported previously including the composite electrodes (Bi-GECE) recently reported by our group. Bi(NO(3))(3)-GECE displays the ability for the detection of both individual and simultaneous determination of heavy trace metals and exhibits well defined, reproducible and sharp stripping signals. The sensitive response is combined with the minimal toxicity of Bi(NO(3))(3). This novel sensor would be an appropriate alternative tool to sensors using bismuth in solution during their utilization in environmental quality monitoring as well as other applications.

Rapid electrochemical genosensor assay using a streptavidin carbon-polymer biocomposite electrode.

A sensor capable of detecting a specific DNA sequence was designed by bulk modification of a graphite epoxy composite electrode with streptavidin (2% w/w). Streptavidin is used to immobilise a biotinylated capture DNA probe to the surface of the electrode. Simultaneous hybridisation occurs between the biotin DNA capture probe and the target-DNA and between the target-DNA and a digoxigenin modified probe. The rapid binding kinetic of streptavidin-biotin allows a one step immobilisation/hybridisation procedure. Secondly, enzyme labelling of the DNA duplex occurs via an antigen-antibody reaction between the Dig-dsDNA and an anti-Dig-HRP. Finally, electrochemical detection is achieved through a suitable substrate (H2O2) for the enzyme-labelled duplex. Optimisation of the sensor design, the modifier content and the immobilisation and hybridisation times was attained using a simple nucleotide sequence. Regeneration of the surface is achieved with a simple polishing procedure that shows good reproducibility. The generic use of a modified streptavidin carbon-polymer biocomposite electrode capable of surface regeneration and a one step hybridisation/immobilisation procedure are the main advantages of this approach. In DNA analysis, this procedure, if combined with the polymerase chain reaction, would represent certain advantages with respect to classical techniques, which prove to be time consuming in situations where a simple and rapid detection is required. This innovative developed material may be used for the detection of any analyte that can be coupled to the biotin-streptavidin reaction, as is the case of immunoassays.

Dot-blot amperometric genosensor for detecting a novel determinant of beta-lactamase resistance in Staphylococcus aureus.

A new electrochemical hybridisation genosensor for the detection of resistant bacteria has been developed. This device relies on the immobilisation of a 50-mer oligonucleotide target, unique to a novel determinant of beta-lactamase resistance in Staphylococcus aureus, onto an electrochemical transducer. This genosensor is based on a concept adapted from classical dot-blot DNA analysis, but implemented in an electrochemical biosensor configuration. Amperometric transduction and an enzyme label method, that increases the genosensor sensitivity, are the main features of this new approach. In addition to the adapted dot-blot format, a double hybridisation assay, in which two different labelled probes were used, is reported. This procedure, if combined with polymerase chain reaction (PCR), allows determination of the genotype of an antibiotic-resistant organism in a shorter time than that required to perform traditional phenotypic susceptibility testing. Its characteristics are ideal for implementation in a kit form.

Classical dot-blot format implemented as an amperometric hybridisation genosensor.

A new electrochemical hybridisation genosensor has been designed. This genosensor is based on a concept adapted from classical dot-blot DNA analysis, but implemented in an electrochemical biosensor configuration. The use of amperometric transduction and the enzyme label method--that increases the genosensor sensitivity--are the main features of this new approach. The analytical procedure consists of five steps: DNA target immobilisation by adsorption onto a nylon membrane, hybridisation between DNA target and biotin-DNA probe, complexation reaction between biotin-DNA probe and an enzyme (horseradish peroxidase) streptavidin conjugate; integration of the modified membrane onto an electrochemical transducer; and finally, amperometric detection using a suitable substrate for the enzyme labelled duplex. Besides the adapted dot-blot format, a competitive assay in which the target is in solution is reported as well. This procedure, based on amperometric transduction, represents certain advantages with respect to dot-blot analysis: labelled hybrid detection is far simpler, quicker and requires more ordinary or simple reactives; the response obtained is a direct analytical signal via low-cost instrumentation, a nonisotopic labelling is used, and the membranes can be reused. These characteristics are ideal in implementing the procedure developed in kit form.

Comparison of chromium speciation by CZE and ion exchange followed by AAS.

The hydrogen chromate anion (HCrO4-), which is the predominant species in acidic solutions and solutions with low chromium concentration, was determined by capillary zone electrophoresis (CZE) using UV detection on-column at 200 nm. A fused-silica capillary (55 cm x 50 microm i.d.) was employed with a high negative voltage of 20 kV. Total chromium was determined after reduction by H2O2 and its complexation by EDTA. The use of H2O2 as reducing agent is advantageous, as it does not increase the conductivity of the solution. Detection limits achieved (for 200 s injection time) were 30 and 8 microg/L for Cr(VI) and Cr(III), respectively. The CZE results obtained for Cr(III) and Cr(VI) were compared with those obtained by ion exchange with subsequent AAS.

Electrochemical genosensor design: immobilisation of oligonucleotides onto transducer surfaces and detection methods.

The present report reviews immobilisation techniques of purified oligonucleotides on electrochemical transducers and their corresponding detection techniques. Most of the literature reviewed was published in the 1990s. The immobilisation techniques of a DNA probe to the surface of an electrochemical transducer made from carbon, gold, platinum or polypyrrole, ranged from simple adsorption to covalent bonding. Recent efforts to couple the recognition layer containing the immobilised nucleic acid recognition layer with the electrochemical signal transducer are discussed. Special attention is given to hybridisation biosensing based on electroactive indicators.

Potentiometric characterization of weak acids by multiple sample addition II. The effect of chemical interferences and the practical performance of linearization methods.

Potentiometric multiple addition of a sample containing a pure weak acid to a solution of supporting electrolyte has been previously shown [C. Maccà and A. Merkoci, Talanta, 41 (1994) 2033] to be formally suitable for the determination of the dissociation constant and concentration of the sample acid. Linear equations have been developed for the treatment of experimental data to yield, simultaneously or separately, the chemical parameters of the acid solution. These equations are now tested on real samples together with analogous equations for titrations, and the results are compared with those obtained with rigorous statistical methods. For the determination of the acidity constant with samples of known concentration, multiple samples addition is comparable with titration. When the sample concentration is unknown and must be determined simultaneously to the acidity constant, the results obtained by linearized multiple sample addition can be seriously affected by impurities present, even at low level, either in the sample or in the supporting solution. Linear equations accounting for the effects of basic or acidic impurities in the sample or in the supporting solution are developed. Sample addition is confirmed to be a useful complement to pH-metric titration for the determination of acidity constants of moderately weak acids by non-linear regression; linearization of data is a convenient technique for screening purposes and a powerful means of detecting and correcting some common pitfalls, interferences and contaminations, whose effects are enhanced in linearized sample addition.

Potentiometric characterization of weak acids by multiple sample addition-I. Linear equations and intrinsic performance of the method.

The suitability of linearized multiple sample-addition for the potentiometric characterization of monoprotic weak acids is examined. Linear equations for the separate or simultaneous determination of the acidity constant and of the sample concentration by treatment of experimental data are introduced. The intrinsic performance of the method and the application range of the different equations are discussed with reference to the theoretical effect of measurement errors on the values of the quantities to be determined.