This invention relates to an apparatus for conducting immunoassay test. The apparatus includes a groove unit having a groove along a vertical direction configured to hold a rod-shaped portion of a probe along the vertical direction, and a push pin configured to move along a horizontal direction, the push pin being capable of residing at a first position and a second position. A tip of the push pin is capable of pressing the rod-shaped portion of the probe against the groove when the push pin resides at the first position. The distance between the tip of the push pin and the groove is larger than a diameter of the rod-shaped portion of the probe when the push pin resides at the second position.