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Porous silicon (PSi) thin films on silicon substrates have been extensively investigated in the context of biosensing applications, particularly for achieving label-free optical detection of a wide range of analytes. However, mass transport challenges have made it difficult for these biosensors to achieve rapid response times and low detection limits. In this work, we introduce an approach for improving the efficiency of molecule transport in PSi by using open-ended PSi membranes atop paper substrates in a flow-through sensor scheme. The paper substrate provides structural support as well as an efficient means of draining solutions from the PSi membrane without the use of an external pump and microfluidic channels. Distinct changes in the reflectance properties of the PSi membrane are measured when molecules are captured in the membrane. A concentration dependent response of the sensor for protein detection is demonstrated. Factors influencing the interaction time of molecules in the PSi membrane and the drying time of the membrane, which directly affect the detection sensitivity and overall testing time, are discussed. The demonstrated performance of the PSi-on-paper sensor establishes the feasibility of a platform for low-cost rapid diagnostic tests with a highly sensitive, quantitative readout.