An inclusion complex between water-soluble p-sulfocalix[n]arene (C-n, n = 4, 6, 8) and the chromophore trans4-[4-(dimethylamino)styryl]-1-methylpyridinium-p-toluenesulfonate (D) formed the basis for a highly sensitive sensor for the selective detection of neurotransmitter acetylcholine (ACh). Formation of the [C-n center dot D] complex (K-a = similar to 10(5) M-1) was accompanied by a drastic increase (up to 20-60-fold) in the chromophore relative quantum yield and by a large hypsochromic shift of the emission band maximum. The observed optical effects are fully reversible: ACh displaces the chromophore molecules from the calixarene cavity as shown by the reappearance of the free chromophore emission band. Formation and dissociation of the complex were studied by fluorescence, H-1 NMR, and UV-vis absorption spectroscopies. The [C-n center dot D] complex is capable of sensing ACh selectively in solution at sub-micromolar concentrations. Immobilization of monocarboxyl p-sulfocalix[4]arene (C-4m) on an oxide-containing silicon surface is in keeping with its properties, such as chromophore binding and the ability of the immobilized inclusion complex to detect ACh. The unique [C-m center dot D] complex optical, switching paves the way for application in ACh imaging and optoelectronic sensing.