Renata Kelly Mendes

Characterization of self-assembled thiols monolayers on gold surface by electrochemical impedance spectroscopy

Thiols with different alkyl chain length and containing COOH terminal group were self-assembled on gold electrodes. The electron transfer of Fe(CN)63-/4- couple to the electrode was studied at different pH by means cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changes in solution pH resulted in the charge variation of the self-assembled monolayer (SAM) terminal group and, consequently, the electrostatic interaction of SAM with the electroactive species in the solution.

Aplicações de QCM, EIS e SPR na investigação de superfícies e interfaces para o desenvolvimento de (bio)sensores

The use of the quartz crystal microbalance process, electrochemical impedance spectroscopy and surface plasmon resonance for characterizing thin films and monitoring interfaces is presented. The theorical aspects of QCM, EIS and SPR are introduced and the main application areas are outlined. Future prospects of the combined applications of QCM, EIS and SPR methods in the studies of interfacial processes at surfaces are also discussed.

Surface plasmon resonance immunosensor for early diagnosis of Asian rust on soybean leaves.

Soybean rust (Asian rust) is a disease that occurs in soy cultures, negatively affecting pod formation and final grain weight and reducing value and product quality. Early identification of fungus in the plants prevents severe farming losses and spread to neighboring cultures. In this paper, a fast response sensor was developed based on surface plasmon resonance to detect Asian rust in soybean leaf extract at early stages of the disease. The antibody anti-Phakopsora pachyrhizi (pathogen) was covalently immobilized on a gold substrate via a self-assembled monolayer (SAM) of thiols using cysteamine-coupling chemistry. This immunosensor presented a linear response range for the antigen from 3.5 to 28.0 microg mL(-1) (r(2)=0.996). The effects of the antibody amount and the surface blocking to minimize non-specific adsorption on immunosensor response were evaluated. These studies provide new perspectives on using SPR technology for the development of a highly sensitive sensor for agricultural applications.

Development of graphite-polymer composites as electrode materials

Graphite powder was mixed to polyurethane, silicon rubber and Araldite® (epoxy) in order to prepare composite materials to be used in the preparation of electrodes. Results showed that voltammetric response could be obtained when at least 50% of graphite (w.w-1) is present in the material. SEM and thermogravimetry were also used in the characterization of the composites.

Development of a disposable amperometric biosensor for salicylate based on a plastic electrochemical microcell

The use of an amperometric biosensor for rapid salicylate determination in blood is described. Photolitography was used to make gold electrodes on a polyester film. The plastic microcell was characterized using cyclic voltammetry to demonstrate the electrochemical performance of the system. The biosensor was constructed by immobilizing salicylate hydroxylase onto the working electrode of the plastic electrochemical microcell. The optimized working conditions were 0.1 mol L(-1) phosphate buffer at pH 7.6 with 0.5 mmol L(-1) of NADH and 300 mV vs. Au as the applied potential. The resulting biosensor exhibited a high sensitivity (97.4 nA/mmol L(-1) salicylate) and an adequate linear response range (1.2x10(-4) to 1.0x10(-3)mol(-1)). The biosensor performance was verified by determining salicylate in spiked blood samples and the results were statistically equivalent to the values obtained from the standard Trinder spectrophotometric method, with a 95% confidence level. This study shows the potential development of a portable, inexpensive and disposable device for point-of-care monitoring.

Development of an electrochemical immunosensor for Phakopsora pachyrhizi detection in the early diagnosis of soybean rust

A ferrugem da soja e uma doenca que ocorre nas folhas da soja e e considerada muito agressiva, pois reduz a qualidade do grao final. A identificacao precoce do fungo na plantacao previne varias perdas para o agricultor bem como para as culturas vizinhas. Neste trabalho, um imunossensor foi desenvolvido sem a utilizacao de marcadores usando medidas de impedância para detectar a ferrugem asiatica nos extratos das folhas contaminadas nos estagios iniciais da doenca, antes do aparecimento dos sintomas. O anticorpo anti-micelio do fungo Phakopsora pachyrhizi (agente causador da doenca) foi imobilizado em substrato de ouro via formacao de monocamadas auto-organizadas de tiois e ligacoes covalentes com a biomolecula. O imunossensor apresentou um limite de deteccao de 385 ng mL-1. A otimizacao das condicoes experimentais e o bloqueio da superficie para minimizar ligacoes nao-especificas foram realizados. Este estudo proporcionou uma nova perspectiva da utilizacao do metodo para o diagnostico precoce da doenca.

Kinetic studies of HRP adsorption on ds-DNA immobilized on gold electrode surface by EIS and SPR

This paper describes the construction of a multicomponent film via layer-by-layer (LbL) method and the kinetic analysis of the interaction between horseradish peroxidase (HRP) enzyme on calf thymus double-stranded DNA layer on a modified gold surface. Surface plasmon resonance (SPR) and electrochemical impedance spectroscopy (EIS) are used to characterize the successful construction of the film on the gold surface. Surface plasmon resonance provided essential information for the study and characterization of protein and nucleic acid interaction and this method is label-free and monitors the interactions in real time. The kinetic studies determined by SPR of the horseradish peroxidase film formation on ds-DNA layer showed values of 24.7 L mol-1 s-1 and 1.2×10-3 s-1 for ka and kd, respectively. The Gibbs free energy obtained for the system was -23.1 kJ mol-1. The results obtained show that the interaction of the enzymes molecules on ds-DNA is kinetically and thermodynamically favourable and the interaction among the layers probably occurs mainly by attraction of opposite charges.

Nanomaterials for Advancing the Health Immunosensor

Nanotechnology has exerted a significant impact in the development of biosensors allowing more sensible analytical methods. In health applications, the main challenge of the immunoassay is to reach the suitable limit of detection, recognizing different analytes in complex samples like whole blood, serum, urine, and other biological fluids. Different nanomaterials, including metallic, silica and magnetic nanoparticles, quantum dots, carbon nanotubes, and graphene, have been applied, mainly to improve charge electron transfer, catalytic activity, amount of immobilized biomole‐ cules, low-background current, signal-to-noise ratio that consequently increase the sensitivity of immunosensors. Given the great impact of nanotechnology, this chapter intends to discuss new aspects of nanomaterials relating to immunosensor advance‐