Immunosensors show great potential for the direct detection of biological molecules. The sensitivity of these affinity-based biosensors is dictated by the amount of receptor molecules immobilized on the sensor surface. An enlargement of the sensor area would allow for an increase of the binding capacity, hence a larger amount of immobilized receptor molecules. To this end, we use electrochemically deposited "gold black" as a porous sensor surface for the immobilization of proteins. In this paper, we have analyzed the different parameters that define the electrochemical growth of porous gold, starting from flat gold surfaces, using different characterization techniques. Applied potentials of -0.5 V versus a reference electrode were found to constitute the most adequate conditions to grow porous gold surfaces. Using cyclic voltammetry, a 16 times increase of the surface area was observed under these electrochemical deposition conditions. In addition, we have assessed the immobilization degree of alkanethiols and of proteins on these different porous surfaces. The optimized deposition conditions for realizing porous gold substrates lead to a 11.4-fold increase of thiol adsorption and a 3.3-fold increase of protein adsorption, using the quartz crystal microbalance (QCM-D) as a biological transducer system. Hence, it follows that the high specific area of the porous gold can amplify the final sensitivity of the original flat surface device.
The theory of the potentiostatic transient for 3D nucleation with diffusion-controlled growth is discussed. It is shown that the theoretical model of Mirkin and Nilov [J. Electroanal. Chem., 283 (1990) 35] and Heerman and Tarallo [J. Electroanal. Chem., 470 (1999) 70] predicts too high values of the current, which becomes very apparent for high values of the site density and low values of the nucleation rate constant (progressive nucleation). For example, the model then predicts that the current in the limit of long times will be higher than the Cottrell limit by a factor of 4/3 which is physically unacceptable. Therefore, a modification to this model is proposed which is based on a careful analysis of the Kolmogorov-Avrami theorem. The ''extended area'' in the KolmogorovAvrami theorem includes contributions from ''phantom nuclei'' that are born inside already existing zones but do not exist physically. This is necessary to preserve the randomness of the system and allows the correct calculation of the appearance rate of the nuclei and the nucleus saturation density. The ''extended current'', defined in analogy with the ''extended area'', then also attributes current to the phantom nuclei. It follows that the ratio j ex ðtÞ=h ex ðtÞ which appears in the model of Mirkin and Nilov and Heerman and Tarallo does not correspond to the actual number of nuclei formed on the electrode. Therefore, the ''extended quantities'' in this ratio must be replaced with quantities that relate directly to the real number of clusters (this implies what is fairly obvious, that the appearance rate of the clusters must be calculated first). This makes it is possible to derive an equation that predicts correctly the current in the limits of both short and long times which is directly linked to the N a ðtÞ vs. time relation (where N a ðtÞ is the actual number of nuclei on the electrode). Experiments for the nucleation of silver on glassy carbon electrodes, with the simultaneous recording of both jðtÞ vs. time and N a ðtÞ vs. time relations, are described. The experimental results obtained from the transients and the direct visual counting of nuclei are compared with the theoretical predictions.
The electrodeposition of rhodium on different polycrystalline gold substrates from Na 3 RhCl 6 •12H 2 O ϩ NaCl solutions was investigated by electrochemical quartz crystal microbalance and voltammetric techniques. A study of the electrodeposition of rhodium from the concentrated chloride solutions used in this work show several features that are associated with potentiostatic transients with growth of the clusters controlled by mixed kinetics, charge transfer and diffusion. The results in this paper offer a clear warning against the blind interpretation of potentiostatic transients with models based on simple diffusion controlled growth. At low overpotentials the electrodeposition of rhodium is characterized by very slow charge transfer kinetics and starts with the formation of a submonolayer. Even at more negative potentials current transients and massograms recorded at constant potential exhibit an apparent induction time, indicating that growth initially is controlled by mixed kinetics, charge transfer and diffusion. Bulk deposition of rhodium is shifted to more negative potentials compared with other solutions, e.g., H 2 SO 4 -based electrolytes, but the exact influence of rhodium speciation in the plating solutions remains unknown.
This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.
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