## 1 Introduction  
Surface plasmons (SPs) are collective oscillations of electrons at the boundaries between materials, which feature strongly confined and intensified fields well suitable for exploring fundamental electromagnetic phenomena [1; 2; 3; 4; 5] and for addressing challenges in optical sensing [6; 7; 8; 9; 10], and many other applications, such as photovoltaics and lasers [11; 12; 13]. Using either propagating or local surface plasmon resonances, which are sensitive to refractive index change of environmental medium, a variety of optical sensors have been demonstrated with sensitivity more than \(10^{3}\) nm/RIU in the structures made of metal films [14; 15], periodic gratings [16], and nanostructured metal surfaces [17; 18; 19; 20].  
Although SPs-based sensors thus far have been mostly developed in visible and near-infrared regions, the mid-infrared spectral band has recently attracted considerable interest due to two main reasons [21; 22; 23; 24]. One is the presence of the \"fingerprint region\", which refers to a spectral band having absorption lines of molecular vibration and rotation modes. The unique molecular absorptions can be used to access chemical bond information. The other one is related to the relatively longer wavelength of the mid-infrared. As resonant wavelength in a plasmonic structure is generally scaled with its feature size [25], by moving resonance to a longer wavelength in the mid-infrared, the sample feature size can be made larger, which reduces difficulties in fabrication. Using structures such as metallic nano-rods [26], nano-slits [27], split ring resonators [28], and patterned graphene [29; 30] in surface-enhanced infrared absorption spectroscopy (SEIRA), a number of molecular sensors have been reported in the mid-infrared, which revealed enhanced fingerprint absorptions of a wealth of molecules. Recently, [PERSON] et al. demonstrated asymmetric split H-shape nanoantennas, which showed not only amplified vibrational absorptions of C-H bonds but also a large spectral resonance shift with a high sensitivity value of 2335 nm/RIU [31]. These reported mid-infrared sensors all use local surface plasmon resonances (LSPRs), whose frequencies are generally required to match molecular resonances of target chemical bonds. This frequency matching condition imposes a strict requirement on control over sample feature size in nano-fabrication. One way without this resonance matching requirement is to detect refractive index change caused by molecules in a spectral band outside their fingerprint region, where a significantly wider wavelength range is accessible. This method is, thus, tolerant to frequency variations of plasmon resonance. In this paper, we demonstrate refractive index sensing of molecules outside their fingerprint region using both LSPRs and propagating surface plasmon resonances (PSPs) in a mid-infrared metagrating. Although the mid-infrared plasmon modes are generally considered less spatially-confined as compared to their counterparts in the visible and near-infrared regions, here they are shown to exhibit sensing ability with monolayer sensitivity. In response to a monolayer octadecanethiol (ODT) analyte, both LSPRs and PSPs exhibit clear redshift in wavelength. Our results offer a way of making mid-infrared sensors with reduced restriction on control over sample feature size in fabrication.