Photonic crystal microcavity sensor for ultracompact monitoring of reaction kinetics and protein concentration

We report on time-resolved label-free monitoring of protein binding in a physiological buffer using a photonic crystal microcavity sensor of total area 50 μm² with an effective detection area of 0.272 μm². We use this ultracompact sensor to monitor the binding of anti-biotin to biotinylated-bovine serum albumin (b-BSA), and measure an affinity constant of 6.94 × 10⁷ M^-1. We show that this photonic crystal sensor can be used for anti-biotin detection at concentrations ranging from picomolar to micromolar. The lower limit of detection for anti-biotin is less than 20 pM, corresponding to less than 4.5 fg of bound material on the sensor surface and fewer than 80 molecules in the modal volume of the microcavity. The sensor also has the capability of measuring binding of small molecular species such as aromatic rings (98 Da). Furthermore, we show that the active surface of the sensor can be successfully regenerated and re-used in subsequent protein binding experiments. A comparison of experimental and theoretical data is given, and the current experimental limitations of the sensor with regard to noise are discussed.