Assays used in conjunction with a thermal contrast reader are disclosed. In the assay, the test strip includes materials that can develop a thermal response if a target analyte is present in a sample. Linear flow assays include nanoparticles with high affinity binding to the analyte. Binding of the nanoparticles with an analyte in the sample is detected using thermal contrast. Analytes over a broad range of concentrations are detected in the linear flow assays. Methods of detecting target analytes and kits comprising lateral flow assays and thermal contrast reader are also disclosed.