  Liquid spectroscopy in the mid-infrared spectral range is a very powerful,
yet premature technique for selective and sensitive molecule detection. Due to
the lack of suitable concepts and materials for versatile miniaturized sensors,
it is often still limited to bulky systems and offline analytics. Mid-infrared
plasmonics is a promising field of current research for such compact and
surface-sensitive structures, enabling new pathways for much-needed photonic
integrated sensors. In this work, we focus on extending the concept of
Ge/Au-based mid-infrared plasmonic waveguides to enable broadband liquid
detection. Through the implementation of high-quality dielectric passivation
layers deposited by atomic layer deposition (ALD), we cover the weak and
water-soluble Ge native oxide. We show that approximately 10 nm of e.g. Al2O3
or ZrO2 can already protect the plasmonic waveguides for up to 90 min of direct
water exposure. This unlocks integrated sensing schemes for broadband molecule
detection based on mid-infrared plasmonics. In a proof-of-concept experiment,
we further demonstrate that the ZrO2 coated waveguides can be activated by
surface functionalization, allowing the selective measurement of diethyl ether
at a wavelength of 9.38 {\mu}m.
