We discuss the properties and potential of slotted photonic crystals devices as small optical, label-free biosensors. This
approach combines slot waveguides, which guide light in a narrow air slot, with photonic crystals in which cavities and
slow light behaviour can be engineered. We use cavities based upon the heterostructure approach, demonstrating
experimental quality factors of up to 50,000 in air and 4,000 in water. As the peak of the cavity mode interacts with the
contents of the slot, small changes in refractive index can be inferred from the cavity resonant wavelength with high
sensitivity (~500 nm/RIU). We also integrate microfluidic channels, which when combined with the small footprint of
each sensor, allows potential for dense multiplexing with only micro-litres of analyte. As the dispersive properties of the
fundamental mode of a standard and slotted photonic crystal differ greatly, a suitable interface for coupling into the
device must be found. We here utilise a resonant defect approach, which preferentially couples into the slot mode.
Functionalising the surface of the device with antibodies allows us to detect specific binding of a target protein on the
sensor surface. As a proof of principle demonstration we show detection of dissolved avidin concentrations as low as 15
nM using biotin functionalised devices.