In response to the challenges of instrumental determinations carried out in the lab setting, attention has been increasingly focused on field-deployable electrochemical sensors which can be used on-site rather than in a centralized laboratory. The aim of this research is to investigate specific point-of-use electrochemical sensors based on interdigitated electrode arrays (IDEA) after modifying the electrode surface for on-site measurements of illicit chemicals and biomarkers of disease. Two device geometries and applications are described in this thesis: The first device realizes portable sensing of illicit compounds using closed bipolar electrochemistry (CBE) that marries amplified electrical signals from IDEAs and converts it to more easily understandable optical signals. The second device explores continuous portable sensing of biomarkers in human fluid matrices, such as sweat and saliva, that can be tuned to allow selective and specific detection at concentration ranges that are relevant to human physiology.