We present a new scheme for sensing biomolecules by combining an enzyme hydrogel with a photonic crystal hydrogel layer that responds to ionic strength and pH changes. We demonstrate this unique combination by successfully detecting acetylcholine (ACh) and acetylcholinesterase (AChE) inhibitors. Specifically, the sandwich assembly is composed of layers of photonic crystals and a polyacrylamide hydrogel functionalized with AChE. The photonic crystal film has a red color and turns dark purple within 2-6 minutes of the enzymatic reaction upon analyte addition. This 3D photonic crystal sensor responds to acetylcholine in the 1 nM to 10 mu M concentration range (which includes the relevant range of ACh concentrations in human body fluids). Michaelis-Menten kinetics of the enzyme were determined which correlated well with literature data demonstrating the uninhibited reactivity of the immobilized enzyme. Furthermore, the presence of the acetylcholinesterase inhibitor neostigmine at concentrations as low as 1 fM was demonstrated, which is even below the necessary detection limit for clinical diagnostics. We suggest that this novel concept will find its application in clinical diagnostics, for pesticide and nerve agent detection.