- Using enhanced optical transmission (EOT) through periodic nanoslits in thin metal films we demonstrate sensing in water with highly enhanced sensitivity in the infrared range for the first time. This is explained as a result of the enhancement in the penetration depth in the analyte thus increasing the interaction volume which in turn increases the sensitivity and allows for sensing of large bio-entities. The penetration depth enhancement in the EOT case is found to be larger than the corresponding enhancement factor in the Kretschmann configuration.Research Highlights►The sensitivity of sensors based on enhanced transmission increases linearly with the wavelength. ►TThe penetration depth in the analyte gets enhanced significantly in the infrared in comparison to that in the SPR case in Kretschmann configuration. ►Detection of large bioentities using EOT sensors is possible when infrared wavelengths are used.