Therefore, charge distribution and charge density over Ag layer significantly differ from those of Ab layer, which results in local changes of the pH making immunoreaction possible. low pH sample matrix is usually briefly discussed. Keywords:hydrophobic surface modification, immunosensor, direct detection, electrochemical impedance, nano-porous aluminum, antibody immobilization, biological toxin, ricin, acidic foods == 1. Introduction == Detection of biological toxins is around the practical front line to keep public health and food safety from foodborne illness and Amorolfine HCl bioterrorism [1]. As the efficient protection is usually of concern to public health and food safety, the detection method has to be sensitive, rapid, and applicable to diverse sorts of diagnostic samples. However, it is the challenge to develop the immunosensor for the direct detection of biological toxins in the acidic diagnostic samples since acids interfere with immunoreaction [2]. Although the acidic environment is regarded as microbiologically safe, the acidic environment is the proper condition where some of biological toxins Amorolfine HCl strongly maintain their configurations and toxicities. Additionally there still exist the potentials of foodborne illness from acidic foods, hence Food and Drug Administration of United States (FDA) regulates acidic foods by classifying into low acid foods and acid foods [3,4]. Generally immunoreaction occurs around neutral pH, and the rate of immunoreaction is extremely low in acidic environment. Accordingly traditional immunosensors such as ELISA (enzyme-linked immunosorbent assay), RIA (radioimmunoassay), and electrophoretic immunoassay frequently include the step of pH adjustment prior to the analysis of target toxins in order to avoid the interference by acids on immunoreaction. In the immunosensing system, the detection of biological toxin is accomplished through the continuous sequence of physicochemical processes: (1) the diffusion of the biological toxin from the solution of a diagnostic sample to the solid-liquid interface between immunosensor and the diagnostic answer, (2) the immunoreaction between the toxin and the molecular recognition elements,i.e., antibody (Ab), antigen (Ag) ligand, andetc., and (3) the transduction of the immunoreaction to analytical signals in the immunosensing system. The Amorolfine HCl diffusion of biological toxin from hucep-6 the solution to a solid-liquid interface is usually mediated by solvent. Ions dissolved in the diagnostic sample are diffused to the solid-liquid interface along with biological toxin. Diffused ions including acids would affect the immunoreaction of the molecular recognition elements. The behavior of ions at the solid-liquid interface shows the different tendency opposed to the surface properties. Ions are assimilated on the surface that has high dielectric constant. They also would be repelled from the surface having low Amorolfine HCl dielectric constant, such as hydrophobic surface [5]. Hydroxide, however, interestingly tends to adsorb at the water-hydrophobic interface [57]. As a consequence of different actions of acids and hydroxides around the hydrophobic surface, we assumed that the effect of acids around the immunological actions of molecular recognition elements would be offset or reduced by the hydrophobic modification of immunosensors surface. In this study, ricin and anti-ricin were selected to demonstrate the Ag-Ab reaction as a model immunosensing system since ricin, a lethal biological toxin, which has been used as a biological warfare, maintains its stability and toxicity in acidic environment. Aluminum substrates were hydrophobically altered with 3-aminopropyltriethoxysilane (APTES), then the immunosensor for the detection of ricin was prepared by the covalent immobilization of anti-ricin on APTES treated aluminum substrate. Immunoreaction between Ab on immunosensor and Ag in acidic environment was monitored based on electrochemical impedance spectroscopy (EIS) since electrochemical impedimetric immunosensor (EII) has been shown an advantage to directly detect broad organic substances including DNA [8], proteins Amorolfine HCl [9], microbes [10], and biological toxin [11] without labeling. The accessibility of ions onto Ag immunosensor was analyzed.