An electrochemical magneto biosensor for the rapid determination of biotin in 
food samples is reported. The affinity reaction was performed on 
streptavidin-modified magnetic microbeads as a solid support in a direct 
competitive format. The biotinylated horseradish peroxidase enzyme (biotin-HRP) 
competes with free biotin in the sample for the binding sites of streptavidin on 
the magnetic microbeads. The modified magnetic beads were then easily captured 
by a magneto graphite-epoxy composite electrode and the electrochemical signal 
was based on the enzymatic activity of the HRP enzyme under the addition of 
H(2)O(2) as the substrate and o-phenilendiamine as cosubstrate. The response was 
electrochemically detected by square wave voltammetry. The limit of detection 
was 8.4×10(-8) mol L(--1) of biotin (20 μg L(--1)) with a dynamic range from 
0.94 to 2.4×10(-7) mol L(--1). Biotin-fortified commercial dietary supplement 
and infant formula samples were evaluated obtaining good performances in the 
results. Total time of analysis was 40 min per 20 assays.