A novel reflection-based localized surface plasmon resonance (LSPR) fiber-optic probe has been developed to determine the refractive indices (RI) and chemical concentrations at high-pressure conditions. The key operating principle is the intensity measurement of the internal light reflection at a fixed wavelength from an optical fiber. The light attenuation caused by the absorption of self-assembled gold nanoparticles on the unclad portion of the optical fiber changes with different RI of the environment near the gold surface. The probe demonstrated a stable and repeatable response to within +/- 0.2% to detect 5 x 10(-4) M N i(2+) dissolved in pH 5.6 sodium acetate buffer solution for sequential operations of pressurization and depressurization in the range of 0.1-20.4 MPa at 308 K. A set of the normalized intensity change exhibited linear responses, indicating that the probe can distinguish the change of RI due to the variation in temperature or pressure under supercritical CO, (SCCO2) and SCCO2-ethanol mixture in the range of 10.2-20.4 MPa and 308-328 K. For chemical and biochemical sensing at different pressures, the probe has been shown to be capable of direct detection of the spectroscopically silent Ni2+, ion in the range of 0-9 x 10(-4) M and tabel-free detection of streptavidin and immunoassay of antibody at the picomolar level. At ambient conditions and 308 K, the sensitivity and the limit of detection (LOD) of the functionalized sensors are respectively, 35.9M(-1) and 1.72 x 10(-5) M for detecting Ni2+, 3.97 x 106 M-1 and 7.82 x 10(-10) M for detecting biotin, and 0.8 (ng/ml)(-1) and 0.1 ng/ml for detecting IgG. The developed LSPR sensor provides a basis as a detector for supercritical chromatography, although some efforts on the theory derivation for pressure effects are needed. (C) 2006 Elsevier B.V. All rights reserved.