An imaging agent for detecting analytes in an environment includes functionalized nanodiamonds and functionalized magnetic particles that can selectively interact with an analyte. Each functionalized nanodiamond contains at least one color center configured emit light in response to illumination. At least one property of the light emitted by the color centers is related to the proximity of the functionalized magnetic particles to the color centers. This property can be detected to determine that the functionalized nanodiamonds are proximate to the functionalized magnetic particles, to determine that the functionalized nanodiamonds and the functionalized magnetic particles are interacting with the analyte, or other applications. Devices and methods for detecting properties of the analyte by interacting with the functionalized nanodiamonds and functionalized magnetic particles are also provided.