Described herein are methods for separating one or more analytes present in a fluid sample. The methods involve passing the fluid through a filter having a microporous material, wherein the analytes are localized near the surface of the microporous material. Additional processing steps such as hybridization and amplification can be performed once the analyte is localized. In one method, once the analyte is localized, the analyte can be detected, counted, and correlated in order to determine the concentration of the analyte in the sample. Modified microporous materials and composite materials are also disclosed that can be used in any of the methods and articles described herein. The composite is composed of a microporous material and a pigment, wherein the pigment is incorporated in the microporous material. The pigments alter the optical properties of the microporous material, which enhances the detection of analyte once it is localized. In a further aspect, various kits and articles such as filtration devices containing any of the microporous materials described herein are provided.