Adriano Santos

Comparing label free electrochemical impedimetric and capacitive biosensing architectures

The transducer faradaic signals of molecularly receptive interfaces associated with specific target binding can be sensitively monitored by electrochemical impedance and/or capacitance spectroscopies. A comparative evaluation of both impedimetric (associated with charge transfer resistance) and capacitive (associated with faradaic density of states) approaches was undertaken using C-reactive protein (CRP) antigen and antibody interaction as biomolecular binding model. Aiming at constructing redox free (impedimetric) and redox tethered receptive (capacitive) interfaces engineered by self-assembly monolayer, CRP sensitivity and limit of detections were comparatively assessed regarding biosensor capabilities. Binding affinity constant between CRP and anti-CRP interfacial receptor sites were additionally evaluated by the Langmuir adsorption model. Both the impedimetric and capacitive approaches reported similar values of experimental analytical parameters albeit the latter was found to have the advantage of requiring no solution redox reporter thus making it highly suitable for use in multiplexing affinity arrays.

Impedance-derived electrochemical capacitance spectroscopy for the evaluation of lectin-glycoprotein binding affinity

Characterization of lectin-carbohydrate binding using label-free methods such as impedance-derived electrochemical capacitance spectroscopy (ECS) is desirable to evaluate specific interactions, for example, ArtinM lectin and horseradish peroxidase (HRP) glycoprotein, used here as a model for protein-carbohydrate binding affinity. An electroactive molecular film comprising alkyl ferrocene as a redox probe and ArtinM as a carbohydrate receptive center to target HRP was successfully used to determine the binding affinity between ArtinM and HRP. The redox capacitance, a transducer signal associated with the alkyl ferrocene centers, was obtained by ECS and used in the Langmuir adsorption model to obtain the affinity constant (1.6±0.6)×10(8) L mol(-1). The results shown herein suggest the feasibility of ECS application for lectin glycoarray characterization.

Fundamentals and Applications of Impedimetric and Redox Capacitive Biosensors

Electroanalyses have brought a huge amount to our understanding of interfaces generally. When applied to surfaces which have been specifically engineered so as to recruit targets from an analytical solution, potent sensors can be derived. These may be based on a multitude of different analytical methods all typified by specific requirements and surface configurations. This short review examines the application of amperometric and impedimetric methods to the detection of biomarkers of clinical relevance. Basic principles are introduced with examples at both planar and “nanofunctionalised” interfaces comprising immobilized antibodies/antigens and oligonucleotide receptors. A particular focus is made of new developments in impedance and impedance derived capacitance spectroscopy.

Redox-tagged peptide for capacitive diagnostic assays.

Early detection assays play a key role in the successful treatment of most diseases. Redox capacitive biosensors were recently introduced as a potential electroanalytical assay platform for point-of-care applications but alternative surfaces (besides a mixed layer containing ferrocene and antibody receptive component) for recruiting important clinical biomarkers are still needed. Aiming to develop alternative receptive surfaces for this novel electrochemical biosensing platform, we synthesized a ferrocene redox-tagged peptide capable of self-assembly into metallic interfaces, a potentially useful biological surface functionalization for bedside diagnostic assays. As a proof of concept we used C-reactive protein (CRP), as a model biomarker, and compared the obtained results to those of previously reported capacitive assays. The redox-tagged peptide approach shows a limit of detection of 0.8 nmol L(-1) (same as 94 ng mL(-1)) and a linear range (R(2)∼98%) with the logarithm of the concentration of the analyte comprising 0.5-10.0 nmol L(-1), within a clinical relevant range for CRP.

Evaluating the Equilibrium Association Constant between ArtinM Lectin and Myeloid Leukemia Cells by Impedimetric and Piezoelectric Label Free Approaches.

Label-free methods for evaluating lectin–cell binding have been developed to determine the lectin–carbohydrate interactions in the context of cell-surface oligosaccharides. In the present study, mass loading and electrochemical transducer signals were compared to characterize the interaction between lectin and cellular membranes by measuring the equilibrium association constant, Ka, between ArtinM lectin and the carbohydrate sites of NB4 leukemia cells. By functionalizing sensor interfaces with ArtinM, it was possible to determine Ka over a range of leukemia cell concentrations to construct analytical curves from impedimetric and/or mass-associated frequency shifts with analytical signals following a Langmuir pattern. Using the Langmuir isotherm-binding model, the Ka obtained were (8.9 ± 1.0) × 10−5 mL/cell and (1.05 ± 0.09) × 10−6 mL/cell with the electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) methods, respectively. The observed differences were attributed to the intrinsic characteristic sensitivity of each method in following Langmuir isotherm premises.

Glycoprotein assay based on the optimized immittance signal of a redox tagged and lectin-based receptive interface.

Glycoproteins play important roles in biological systems such as in process related to cell binding, signaling and disease. Consequently, novel, potentially quantitative, and rapid electroanalytical approaches capable of detecting protein binding are welcome. Herein, we introduce a methodology that is both fast and sensitive, and capable of quantification of the binding affinity in glycoprotein-lectin molecular models. The proposed methodology is based on the electrochemical impedance spectroscopy technique focused on the immittance function approach, wherein a library of analytical parameters can be computed from the raw impedance data obtained, and automatically processed in a label-free, quantifiable and very sensitive assay platform. This approach also avoids redox probe pre-doping of the analytical sample. Avoiding redox pre-doping of the analytical sample is achievable designing an appropriate redox-tagging monolayer containing lectin interface (a carbohydrate binding protein, herein ArtinM) as the bio-receptor, endowing high sensitivity of electrochemical signal when specifically detecting glycoproteins of interest (presently horseradish peroxidase, HRP, a mannose glycoprotein) as the biochemical target for ArtinM. The electroanalytical curves demonstrated that the binding affinity constant could be evaluated as equivalent for all library (immittance function) parameters, allowing optimized single frequency (or a range of frequencies) assessment with high sensitivity. In other words, binding affinity constants between ArtinM and HRP for each of the parameters in the immittance function library at given optimized frequencies were similar, independently of the parameter. Thus, the feasibility of using this immittance function approach for electroanalytical glycoarrays by accessing bio-recognition processes on a rapid (optimized) single frequency and highly multiplexable platform was demonstrated.

The self‐assembly of redox active peptides: Synthesis and electrochemical capacitive behavior

The present work reports on the synthesis of a redox‐tagged peptide with self‐assembling capability aiming applications in electrochemically active capacitive surfaces (associated with the presence of the redox centers) generally useful in electroanalytical applications. Peptide containing ferrocene (fc) molecular (redox) group (Ac‐Cys‐Ile‐Ile‐Lys(fc)‐Ile‐Ile‐COOH) was thus synthesized by solid phase peptide synthesis (SPPS). To obtain the electrochemically active capacitive interface, the side chain of the cysteine was covalently bound to the gold electrode (sulfur group) and the side chain of Lys was used to attach the ferrocene in the peptide chain. After obtaining the purified redox‐tagged peptide, the self‐assembly and redox capability was characterized by cyclic voltammetry (CV) and electrochemical impedance‐based capacitance spectroscopy techniques. The obtained results confirmed that the redox‐tagged peptide was successfully attached by forming an electroactive self‐assembled monolayer onto gold electrode. The design of redox active self‐assembly ferrocene‐tagged peptide is predictably useful in the development of biosensor devices precisely to detect, in a label‐free platform, those biomarkers of clinical relevance.

Evidence for Conformational Mechanism on the Binding of TgMIC4 with β-Galactose-Containing Carbohydrate Ligand.

A deeper understanding of the role of sialic/desialylated groups during TgMIC4-glycoproteins interactions has importance to better clarify the odd process of host cell invasion by members of the apicomplexan phylum. Within this context, we evaluated the interaction established by recombinant TgMIC4 (the whole molecule) with sialylated (bovine fetuin) and desialylated (asialofetuin) glycoproteins by using functionalized quartz crystal microbalance with dissipation monitoring (QCM-D). A suitable receptive surface containing recombinant TgMIC4 for monitoring β-galactose-containing carbohydrate ligand (limit of quantification ∼ 40 μM) was designed and used as biomolecular recognition platform to study the binding and conformational mechanisms of TgMIC4 during the interaction with glycoprotein containing (fetuin), or not, terminal sialic group (asialofetuin). It was inferred that the binding/interaction monitoring depends on the presence/absence of sialic groups in target protein and is possible to be differentiated through a slower binding kinetic step using QCM-D approach (which we are inferring to be thus associated with β-galactose ligand). This slower binding/interaction step is likely supposed (from mechanical energetic analysis obtained in QCM-D measurements) to be involved with Toxoplasma gondii (the causative agent of toxoplasmosis) parasitic invasion accompanied by ligand (galactose) induced binding conformational change (i.e., cell internalization process can be additionally dependent on structural conformational changes, controlled by the absence of sialic groups and to the specific binding with galactose), in addition to TgMIC4-glycoprotein solely recognition binding process.

DETERMINAÇÃO DOS PARÂMETROS CINÉTICOS E TERMODINÂMICOS DA ADSORÇÃO DE L-CISTEÍNA EM OURO POR MEIO DA TÉCNICA DE MICROBALANÇA A CRISTAL DE QUARTZO

This article discusses the adsorption kinetics of a L-cysteine monolayer onto a gold surface by means of information obtained through the QCM technique. The results indicate that the adsorption process is rapid and follows the Langmuir isotherm, in which adsorption and desorption are considered. From these measurements the following parameter values were obtained: kd = (4.2 ± 0.4) x 10-3 s-1, ka = 75 ± 6 M-1 s-1, Keq=(1.8 ± 0.3) x 104 M-1 and ΔGads = - (5.8 ± 0.2) kcal mol-1.

A dual marker label free electrochemical assay for Flavivirus dengue diagnosis

Dengue is a RNA viral illness of the genus Flavivirus which can cause, depending on the pervasiveness of the infection, hemorrhagic dengue fever or dengue shock syndrome. Herein we present an electrochemical label free approach enabling the rapid sensitive quantification of NS1 and IgG (supporting an ability to distinguish primary and secondary infections). Using a bifunctional SAM containing PEG moieties and a tethered redox thiol, both markers are detectable across clinically relevant levels by label free impedance derived redox capacitance. A subsequent frequency specific immittance function approach enables assaying (within seconds) with no impairment of analytical quality (linearity, sensitivity and variance).