Establishing the physical and rheological properties, such as density and viscosity, of biological fluids, cells and tissues remains a central challenge in biology due to the liquid operating environment. In this work, we introduce reflector-free acoustic resonators that generate highly confined shear-horizontal surface modes through phononic crystal engineering. Starting from a 90° X-axis rotated ST-cut quartz piezoelectric substrate, which intrinsically does not support the electromechanical excitation of surface acoustic wave, we demonstrate that surface phononic crystal patterning enables conversion of a pseudo-surface mode into a pure surface acoustic mode with non-zero electromechanical coupling. We exploit this engineered mode conversion to achieve strong shear-horizontal surface confinement without the need of reflectors. The devices were fabricated and experimentally evaluated for in-liquid sensing. We demonstrate that the phononic surface resonance exhibits in-liquid quality factors from 170 to 285, the highest reported for in-liquid test environment, with a mass sensitivity up to 187 Hz.cm²/ng. The resonators were characterized using different liquid solutions and mixtures with varying concentrations, demonstrating their capability to discriminate between distinct liquids and to detect variations in physical and rheological properties. These results establish the proposed acoustic resonators as a sensitive platform for probing physical and rheological properties in liquid environments, which could potentially be of particular relevance for biological and biomedical applications.