Towards a FRET-based immunosensor for continuous carbohydrate monitoring

In this report we have evaluated the potential of using fluorescence/Förster resonance energy transfer (FRET) in a competitive immunosensor for continuous monitoring of the carbohydrate hapten maltose. The cyanine dyes Cy5 and Cy5.5 were used as a donor–acceptor pair by conjugation to maltose-labeled bovine serum albumin (BSA) and the monoclonal antibody IgG 39.5, giving Cy5–BSA–maltotriitol (3.1/1/18) and Cy5.5–mAb39.5 (2.2/1), respectively. This antibody with weak affinity towards maltose showed full reversibility to both the free maltose and the maltose-labeled conjugate. It allowed us to measure continuously the maltose content by monitoring the FRET signal change over time due to displacement of Cy5–BSA–maltotriitol from Cy5.5–mAb39.5 inside a semipermeable capsule. A near 22% total increase was seen in the fluorescence intensity ratio I670/I700 in the presence of maltose, with a calculated EC50 = 1.87 ± 0.13 mM (R2 = 0.9984) from the sigmoidal dose–response curve at 25 °C. Specificity of the immunosensor was shown with the structural analog to maltose, cellobiose, and it generated no detectable response. A minor drift in the sensor baseline was seen with 0.4% per 24 h, which was in the same magnitude as the signal-to-noise ratio, during the 4 weeks of measurements. The immunosensor was applied to crude samples of oat drinks for direct quantification of the maltose content. Overall, this work demonstrates the potential to use an immunosensor based on weakly binding antibodies and FRET technology for remote and non-invasive carbohydrate monitoring.