We present the design, fabrication and optical investigation of photonic crystal (PhC) nanocavity drop filters for use as optical biosensors. The resonant cavity mode wavelength and Q-factor are studied as a function of the ambient refractive index and as a function of adsorbed proteins (bovine serum albumin) on the sensor surface. Experiments were performed by evanescent excitation of the cavity mode via a PhC waveguide. This in turn is coupled to a ridge waveguide that allows the introduction of a fluid flow cell on a chip. A response of partial derivative lambda/partial derivative c = (4.54 +/- 0.66) x 10(5) nm/M is measured leading to a measured detection limit as good as Delta m = 4.0 +/- 0.6 fg or Delta m/Delta A = (4.9 +/- 0.7) x 10(2) pg/mm(2) in the sensitive area. (C) 2009 Elsevier B.V. All rights reserved.