A miniature fiber optic spectrometer enclosed within a semipermeable (dialysis) membrane is proposed for in vivo interstitial sensing applications. The semipermeable membrane acts as a molecular filter, allowing only small molecules to pass through to the sampling volume. This filtering, in principle, should enable continuous in vivo drug sensing, removing the necessity for complex microdialysis systems. We use a biological phantom to examine the reliable detection of a fluorescence signal from small dye molecules in the presence of large fluorophores and scatterers. We find that spectral artefacts arising from scatterers and large fluorophores are substantially suppressed, simplifying the spectral analysis. In addition, the measured sampling rate of 157 s is superior to existing in vivo tissue assaying techniques such as microdialysis, which can take tens of minutes.