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Analytical performance (selectivity, sensitivity, limit of detection) of a sensor is ultimately connected to the interfacial architecture of the sensing surface that needs to be optimized. Self‐assembly of a mixed monolayer of thiolated probes and diluent onto gold surfaces used in the design of biosensing platforms is nowadays commonly performed by a two‐step immobilization procedure. In this work, the merits of a one‐step coadsorption procedure are emphasized, and it is shown that the ionic strength is a very efficient tool to fine control the surface concentration of the immobilized probe. Probe densities were obtained by alternating current voltammetry or fluorescence measurements depending on the type of labeling of the probes. With unlabeled DNA sequences, chronocoulometric measurements were performed in the presence of dissolved hexaammineruthenium(III) cations. The maximum probe density achievable at high ionic strength by the one‐step coadsorption procedure is limited by steric constraints and depends on the structure of the sequence. Various thiolated DNA sequences (single‐stranded DNA, double‐stranded DNA, linear, hairpin, quadruplex) coadsorbed with a common diluent, 4‐mercaptobutan‐1‐ol, were used to discuss the structural factors affecting the maximum surface densities obtained for the different types of probe. Application of the one‐step coadsorption procedure to the optimization of the analytical performance of a protein aptasensor is illustrated by building a sensor for the detection of thrombin on the basis of the thrombin binding aptamer sequence.