A miniaturized and fully integrated probe for analytical assays based on electrochemiluminescence (ECL) is proposed. It combines both, the electrode transducer and the photodetector in a single 5x6 mm2 silicon chip. The device accommodates two identical cells, each consisting of a gold interdigitated microelectrode array (IDA) resting on top of a pn photodiode, and of which one is used as an active cell and the other as a dummy. Light- intensity measurements are taken in a differential mode to increase the overall system performances. The device was characterized using ruthenium (II) tris(2,2'-bipyridyl) and tripropylamine as model system. The influence of both, the pH and the electrode to electrode distance on the response was investigated. The highest signal intensifies were detected in a phosphate buffer solution of pH 7.5 and an IDA gap of 0.8 μm. Excellent correlation was achieved between the peak light intensity and the concentration of the ruthenium complex over a 0.5-50 μM concentration range.