We investigated the immobilization of the estrogen receptor hER(alpha) on silanized SiO2 surfaces for biosensor applications. The conjugation of the estrogen receptor hER(alpha) to the quantum dot dye QD655 was achieved. In order to obtain an optimal immobilization of the estrogen receptor hER(alpha) on the functionalized SiO2 surface, the bioconjugate hER(alpha)-QD655 (Rcpt-qd655) solution was prepared with a higher molar ratio of 10-15 between the QDs and the receptors. A blue laser with an excitation wavelength of 405 nm was used for photoluminescence spectroscopy (PL) investigations to monitor the bioconjugate Rcpt-qd655 immobilization on the silanized SiO2 surfaces with three different functional groups, namely NH2, -COO-, and -SH. Several wash processes were applied to remove the excess receptors from the surface after the immobilization. A Fourier transform infrared spectroscopy (FTIR) was used to control the biofilm background after each wash of the receptor-coated surface which allows the optimization of the immobilization protocol. In order to test its stability a quartz crystal microbalance (QCM) was employed and the receptor density was calculated. Finally the optimal biolayer (silane film + hER(alpha) receptor) was tested for measurements of 17 beta-estradiol (E2) with a concentration of 1 mu M in waterish solution. The measurement concept outlined in [L Rebohle et al.. Vacuum 83 (2009) 24-28] was applied. The whole system was investigated by PL, which exhibits two color signals, namely from the receptor and the detected E2 molecules. (C) 2010 Elsevier Inc. All rights reserved.