Paper-based electrodes modified with cobalt phthalocyanine colloid for the determination of hydrogen peroxide and glucose.

Cobalt(ii) phthalocyanine (CoPc) was suspended in aqueous medium and the colloidal system was used as catalyst for the electrochemical determination of hydrogen peroxide on paper-based electrodes modified with carbon nanomaterials. H2O2 was oxidised at 0.275 V vs. Ag pseudoreference electrode. This system was adapted to develop a glucose sensor with glucose oxidase immobilized on the cellulose electrode. CoPc suspended nanoparticles acted as nanoenzyme mimicking peroxidase activity and were combined with different carbon nanomaterials to form hybrids with optimised catalytic performance. GO-CoPc paper-based electrodes yielded the best results with a linear range of ∼12 µM to 49 mM for H2O2 and 0.1 mM to 1 mM for glucose. Glucose was determined in physiological serum and juice samples with recoveries of 93.3 and 94.2% respectively. CoPc could replace HRP for the catalytic sensing of H2O2, without the need to be dissolved. This material can be used in situ in a simple protocol with other nanomaterials for electrode modification. The sensor described has the advantage of easy preparation, using the catalyst in colloidal form, long term stability, and versatility to be adapted to other low cost and disposable enzymatic systems.

Implementation of an easy-to-follow standardized chart for free flaps clinical monitoring during the start of a head and neck microsurgery unit in a tertiary hospital.

Successful head and neck reconstructive microsurgery requires a multidisciplinary approach involving specialists such as maxillofacial surgeons, plastic surgeons, otolaryngologists, anaesthesiologists, and nurses. It requires strict perioperative management of the patient, with special attention paid to parameters such as blood pressure, ventilation, and bleeding, which can all impact flap perfusion. In high-volume centers, specially-trained microsurgery units are available for optimal control of flap monitoring and management of all aspects of patient care. However, during the beginning of a microsurgery unit at a hospital unfamiliar with the demands of this field, easy-to-follow, but effective, measures must be in place to ensure adoption and adherence to post-operative care principles that confer higher success rates in head and neck cancer patients undergoing free flap reconstruction. While frequent observations can be labor-intensive, clinical monitoring remains the most suitable method during this early period and requires appropriate experience to be effective. Our aim is to emphasize the importance of clinical monitoring during the beginnings of a microsurgery unit by introducing an easy-to-follow clinical monitoring chart that covers both intraluminal and extraluminal causes of free flap compromise.

Paper-based maskless enzymatic sensor for glucose determination combining ink and wire electrodes.

In this work we have developed an amperometric enzymatic biosensor in a paper-based platform with a mixed electrode configuration: carbon ink for the working electrode (WE) and metal wires (from a low-cost standard electronic connection) for reference (RE) and auxiliary electrodes (AE). A hydrophobic wax-defined paper area was impregnated with diluted carbon ink. Three gold-plated pins of the standard connection are employed, one for connecting the WE and the other two acting as RE and AE. The standard connection works as a clip in order to support the paper in between. As a proof-of-concept, glucose sensing was evaluated. The enzyme cocktail (glucose oxidase, horseradish peroxidase and potassium ferrocyanide as mediator of the electron transfer) was adsorbed on the surface. After drying, glucose solution was added to the paper, on the opposite side of the carbon ink. It wets RE and AE, and flows by capillarity through the paper contacting the carbon WE surface. The reduction current of ferricyanide, product of the enzymatic reaction, is measured chronoamperometrically and correlates to the concentration of glucose. Different parameters related to the bioassay were optimized, adhering the piece of paper onto a conventional screen-printed carbon electrode (SPCE). In this way, the RE and the AE of the commercial card were employed for optimizing the paper-WE. After evaluating the assay system in the hybrid paper-SPCE cell, the three-electrode system consisting of paper-WE, wire-RE and wire-AE, was employed for glucose determination, achieving a linear range between 0.3 and 15mM with good analytical features and being able of quantifying glucose in real food samples.

AC voltammetric carbon paste-based enzyme immunosensors.

Carbon paste electrodes, previously anodised in a basic media, are the basis for the development of a new voltammetric immunosensor device. Passive adsorption of the appropriate immunochemical reagent was performed onto the electrode surface. Alkaline Phosphatase labelled immunoglobulin was the tracer used in this work, 3-indoxyl phosphate being a very suitable enzymatic substrate for the electrochemical detection of the corresponding affinity reaction. The hydrolysis of this molecule generates indigo dimmer. This product was detected by alternating current voltammetry taking advantage of the adsorptive and inherent electrodic properties that it exhibits. The same electrochemical anodisation was used at the end of one assay to remove the entire protein layer attached to the carbon paste surface, allowing the formation of a new sensing phase and the use of the same support in several consecutive experiments. The methodology was applied to the design of two different immunoassays for the determination of human IgG. Good reproducibility of the electrodic signal and a limit of detection around 10(-10) M were achieved.

Silver electrodeposition catalyzed by colloidal gold on carbon paste electrode: application to biotin-streptavidin interaction monitoring.

A new electrochemical method to monitor biotin-streptavidin interaction on carbon paste electrode, based on silver electrodeposition catalyzed by colloidal gold, was investigated. Silver reduction potential changed when colloidal gold was attached to an electrode surface through the biotin-streptavidin interaction. Thus, the direct reduction of silver ions on the electrode surface could be avoided and therefore, they were only reduced to metallic silver on the colloidal gold particle surface, forming a shell around these particles. When an anodic scan was performed, this shell of silver was oxidized and an oxidation process at + 0.08 V was recorded in NH3 1.0 M. Biotinylated albumin was adsorbed on the pretreated electrode surface. This modified electrode was immersed in colloidal gold-streptavidin labeled solutions. The carbon paste electrode was then activated in adequate medium (NaOH 0.1 M and H2SO4 0.1 M) to remove proteins from the electrode surface while colloidal gold particles remained adsorbed on it. Then, a silver electrodeposition at -0.18 V for 2 min and anodic stripping voltammetry were carried out in NH3 1.0 M containing 2.0 x 10(-5) M of silver lactate. An electrode surface preparation was carried out to obtain a good reproducibility of the analytical signal (5.3%), using a new electrode for each experiment. In addition, a sequential competitive assay was carried out to determine streptavidin. A linear relationship between peak current and logarithm of streptavidin concentration from 2.25 x 10(-15) to 2.24 x 10(-12) M and a limit of detection of 2.0 x 10(15) M were obtained.

Colloidal gold as an electrochemical label of streptavidin-biotin interaction.

A new electrochemical method to monitor biotin-streptavidin interaction, based on the use of colloidal gold as an electrochemical label, is investigated. Biotinylated albumin is adsorbed on the pretreated surface of a carbon paste electrode (CPE). This modified electrode is immersed in colloidal gold-streptavidin labelled solutions. Adsorptive voltammetry is used to monitor colloidal gold bound to streptavidin, obtaining a good reproducibility of the analytical signal (R.S.D. = 3.3%). A linear relationship between peak current and streptavidin concentration from 2.5 x 10(-9) to 2.5 x 10(-5) M is obtained when a sequential competitive assay between streptavidin and colloidal gold-labelled streptavidin is carried out. On the other hand, the adsorption of streptavidin on the electrode surface was performed, followed by the reaction with biotinylated albumin labelled with colloidal gold. In this way, a linear relationship between peak current and colloidal gold labelled biotinylated albumin concentration is achieved with a limit of detection of 7.3 x 10(9) gold particles per ml (5.29 x 10(-9) M in biotin).

Comparative electrochemical behaviour of biotin hydrazide and photobiotin. Importance in the development of biosensors.

The cyclic voltammetric behaviour of biotin hydrazide and photobiotin on carbon paste electrodes has been studied. Biotin hydrazide presents an anodic and irreversible process, meanwhile photobiotin presents two, adsorptive in nature. This characteristic makes photobiotin desirable for following the interaction between biotin and streptavidin, being possible to detect a streptavidin concentration of 10(-12) M. The evidence of this reaction has been shown either directly in solution or on the electrode surface. Photobiotin as the molecule portable of analytical information and carbon paste as the solid support could be applied to the development of sensors based on the oxidation of this molecule.

Voltammetric study and determination of buprenorphine in pharmaceuticals.

The oxidation of buprenorphine on a carbon paste electrode has been studied using voltammetric techniques under both semi-infinite linear diffusion and hydrodynamic conditions. By applying a simple electrode pretreatment a good reproducibility of the current signal is obtained (R.S.D. = 0.85%, n = 6 for a 1.0 x 10(-5) M buprenorphine concentration). The limit of detection was found to be 2.0 x 10(-7) M. The voltammetric method developed for the determination of buprenorphine in pharmaceutical preparations was examined for its applicability to liquid and solid preparations.

Comparative voltammetric studies of immunoglobulins and DNP-labelled immunoglobulins.

A comparative electrochemical study of human immunoglobulins IgG(1) and IgG(3) carried out at a hanging mercury drop electrode shows that mechanisms other than the reduction of interchain disulphide linkages are responsible for the cathodic peaks observed for such proteins. Considering that the nature of the electrochemical process observed for immunoglobulins are poorly defined and not fully understood, a new approach to the electrochemical determination of such proteins, involving the use of 2,4-dinitrophenol (DNP) as a label, has been developed. Dynamic linear ranges of nearly two magnitudes and detection limits below 10(-10) M were achieved.

Adsorption of immunoglobulin G on carbon paste electrodes as a basis for the development of immunoelectrochemical devices.

The attachment of Immunoglobulin G (IgG) to a carbon paste electrode is investigated in this work. Studies of the immobilization of the immunoglobulin on this electrode were carried out. Alkaline phosphatase (AP) has been used as an enzyme label, linked to the antibody, for the determination of the adsorbed immunoglobulin. Various substrates for this enzyme have been tested and alpha-naphthyl phosphate was found to be the best because of the lower oxidation potential of the enzymatic product, alpha-naphthol, and greater sensitivity under the same experimental conditions. An electrodic pretreatment based on an anodic oxidation of the electrode surface in a phosphate media pH 9 was carried out prior to the adsorption step. This activation is also suitable for the removal of the IgG layer and the regeneration of the electrodic surface after each determination, with the intention of using the same electrode in the subsequent assays. A good reproducibility of the signal was achieved in this way (Relative Standard Deviation, R.S.D. = 3.7%). Using cyclic voltammetry, IgG labelled with AP has been quantified in a range from 2 x 10(-12) to 7 x 10(-11) M and a detection limit of 2.3 x 10(-12) M (signal-to-noise ratio = 3) was found. Finally, human IgG was quantified under non-optimized conditions on the electrodic surface through the reaction with AP labelled IgG using two different immunological designs.

Flow injection analysis with amperometric detection of naltrexone in pharmaceuticals.

Flow injection analysis (FIA) with amperometric detection using a carbon paste electrode is applied to the determination of naltrexone. The sample solution was injected into the carrier stream of 0.1 M perchloric acid, being determined by oxidation at +1.0 V vs. Ag/AgCl/sat. KCl using a flow rate of 4 ml min-1. A relative standard deviation of 1.5% was calculated for a concentration level of 10(-5) M (n = 17) without carrying out a carbon paste electrode pretreatment. Calibration curves were found to be linear between 2 x 10(-8) and 10(-5) M (almost three orders of magnitude) and the method has a detection limit of 2 x 10(-8) M. A simple and reproducible procedure is proposed for the determination of naltrexone in pharmaceuticals. The results compared favourably with those obtained by an HPLC-UV method.

Comparative voltammetric study of 2,4-dinitrophenol (DNP), albumin and DNP-albumin as an analytical approach to the use of DNP as a universal label in immunoelectrochemical assays.

A comparative electrochemical study of 2,4-dinitrophenol (DNP), albumin and DNP-albumin has been carried out at a hanging mercury drop electrode, in order to use DNP as a universal label in immunoelectrical assays. Several electrochemical techniques have been used. Differential pulse voltammetry has proved to be the most suitable. Wide dynamic linear ranges (more than three orders of magnitude for DNP-albumin) and low detection limits have been achieved (5 x 10(-10)M, 2 x 10(-10)M, 3 x 10(-12)M for DNP, albumin and DNP-albumin, respectively). Good reproducibility has been obtained in all cases (R.S.D. < 2.2%).

Phase-selective AC adsorptive stripping voltammetric assay for aminopterin and 10-Edam in human serum.

Aminopterin was studied as a model compound for its analogues which maintain the pteridine ring in their structure. Its adsorptive behaviour on mercury was studied and the DC adsorptive stripping and phase-selective AC adsorptive stripping conditions were optimized. 10-Edam, an aminopterin analogue, was studied and shown to behave similarly to aminopterin. Phase-selective AC voltammetry provided the best signal and gave a detection limit of 4 x 10(-12) M aminopterin in aqueous solution employing an accumulation time of 10 min. The optimized method was applied to the analysis of both aminopterin and 10-Edam respectively in human serum. After extraction with a C18 reversed-phase cartridge the detection limit of the method was 1 x 10(-8) M aminopterin and the overall assay percentage recovery was 73.5% (n = 5) at a concentration of 5 x 10(-7) M aminopterin in serum. The analysis of 10-Edam at the same concentration in serum yielded the higher percentage recovery of 94.46% (n = 5) following the same procedure.

Determination of mitoxantrone by flow injection analysis using an amperometric detector.

Mitoxantrone was determined by flow injection analysis using a flow cell modified in the laboratory and fitted with carbon paste as an amperometric detector. The sample solution (100 microliters, 5 x 10(-8)-1 x 10(-5) M) was injected into the carrier stream of 0.1 M perchloric acid (pH 1.12). Mitoxantrone was determined by oxidation at the carbon paste electrode (CPE) at +0.90 V. A 60-cm delay coil (0.5 mm i.d.) was incorporated just before the detector (a canal thin layer) and a flow rate of about 4 ml min-1 was used. The system was successfully applied to the determination of mitoxantrone in a pharmaceutical preparation; the method was fast and reproducible.

Adsorptive stripping voltammetric determination of pipemidic acid in human urine.

The electrochemical behaviour of pipemidic acid (8-ethyl-5,8-dihydro-5-oxo-2-(1-piperazinyl)-pyrido[2,3-d]pyrimidine-6- carboxylic acid), a well known antimicrobial agent used for urinary infections, was investigated by linear-sweep, differential-pulse and square-wave voltammetry at a hanging mercury drop electrode. Two reduction processes were observed in Britton-Robinson buffers at acid pH, whereas only one or two processes were observed in alkaline solutions, dependent on the pH of the buffer employed. Adsorptive effects were used to accumulate the drug on to the electrode. The adsorbed species were measured voltammetrically by using a cathodic process appearing at -0.76 V in 0.1 M HCIO4. Linear calibration graphs were obtained in the range 2.5 x 10(-9)-2.0 x 10(-7) M. A simple procedure of extraction was employed for the determination of the drug in urine samples.

Voltammetric assay of heroin in illicit dosage forms.

The oxidation of heroin on a carbon paste electrode has been studied by using voltammetric techniques under both semi-infinite linear diffusion and hydrodynamic conditions. By employing a simple and rapid in situ pre-treatment of the electrode, excellent reproducibility of the current signal was obtained. Subsequently, the current was measured and the concentration of total heroin present could be determined. The method compared favourably with instrumental methods that are more commonly used for the quantification of heroin in clinical laboratories such as chromatographic, spectrophotometric or radiometric techniques. Moreover, the proposed method showed good selectivity and was superior with respect to cost and time of analysis, permitting the determination of heroin in seized drug samples over a wide concentration range.

Voltammetric assay of methadone in urine.

A simple, rapid and sensitive voltammetric method for the determination of methadone in human urine is proposed, which permits the detection of concentrations of methadone as low as 0.3-0.4 microgram ml-1. The proposed method was compared with methods normally used in clinical trials, such as the enzyme multiplied immunoassay technique, and was found to be superior with regard to cost and to possess high sensitivity.

Adsorptive stripping voltammetric analysis of some pterines in human urine.

The combined content of biopterin and he separate folic acid content of human urine have been quantitated by phase-selective alternating-current stripping voltammetry at the static mercury drop electrode. The use of disposable C18 cartridge allowed the samples to be separated into two fractions, one containing biopterin and neopterin and the other folic acid. The average concentration of biopterin and neopterin together was 704 ng ml-1; precision, 6.6% (RSD, n=7); mean recovery, 96%. The average concentration of folic acid was 836 ng ml-1; precision, 8.2% (RSD, n=8); mean recovery, 91%.

Adsorptive stripping voltammetric assay of folic acid in human serum.

Folic acid was voltammetrically measured after preconcentration by adsorption at the static mercury drop electrode. Phase selective AC voltammetry provided the most sensitive stripping signal. After previous analyte extraction, using liquid-solid extraction with a C-18 reversed phase cartridge, the method was suitable for folic acid measurement in human serum. The detection limit was 5.9 x 10(-9) M with an overall precision of 9.9% (R.S.D.; n = 7) at the concentration level of 1.0 x 10(-7) M with a mean recovery of 57%.