Poly(N-isopropylacrylamide) (PNIPAM) microgel particles labeled with a fluorescent monomer 4-N-(2-acryloyloxyethyl)-N-dimethylaminosulfonyl-7-N,N-dimethylaminosulf onyl-2,1,3-benzoxadiazole (DBD-AE) were prepared by emulsion polymerization Under various crosslinker concentrations. The thermo-responsive behavior and the microenviroment of the microgel particles were studied in water by turbidimetric and fluorescence analyses. For file microgel particles prepared Under the crossfinker concentration of 1 mM, the turbidity began to increase at ca. 32.5 degrees C, but the relative fluorescence intensity dramaically increased and the wavelength.,th at the maximum fluorescence intensity (lambda(max)) was dramatically blue-shifted both at ca. 31.5 degrees C with increasina the temperature, Suggesting the hydrophobicity around the DBD-AE unit was dramatically increased and the Subsequent shrinking of the inicrogel particles occurred. As the crossfinker concentration increased from 0.5 to 20 mM, the transition ternperature determined by turbidimetric analysis was constant upto 2 mM, rose between 2 and 10 mM. leveled off above 10 mM, and was ca. 34 degrees C at 20 mM. The temperature-induced microenvironmental change inside the microgel particles was also reduced at high crosslinker concentractions i oils. The results obtained from the fluorescence of the DBD-AE unit and another fluorescent monomer unit 3-(2-propenyl)-9-(4-N,N-dimethylamino-phenyl)phenanthrene (VDP) suggested that the heterogeneity inside the rnicrogel particles prepared under the crosslinker concentration of 20 mM became high. (c) 2005 Elsevier Inc. All rights reserved.