The key challenge for fabricating electrogenerated chemiluminescence (ECL) biosensors still lies in the improvement of the sensing performance of ECL luminophores. Graphite-like carbon nitride (g-C3N4) nanosheets were prepared and the cathodic ECL performance was investigated. The ECL emission of g-C3N4 nanosheets was greatly enhanced and higher sensing sensitivity towards copper ions was achieved by a "space effect", the incorporation of g-C3N4 nanosheets into a top-down prepared nanoporous matrix. Label-free ECL immunosensors were fabricated based on this simple sensing platform. The unique signal amplification strategy and simple fabrication procedure guarantee the sensor stability and production reproducibility. Nanoporous matrix supported ECL probes have potential as a versatile sensing strategy for the fabrication of ECL biosensors.