We report the first self-referenced sensor based on the extraordinary optical transmission (EOT) of metal-nanoslits array in the near-infra-red (NIR) telecommunication window of the electromagnetic spectrum. The nanoslits array shows two enhanced transmission peaks, out of which one shows a red shift with an increase in the refractive index of the analyte medium, while the other remains fixed. We demonstrate the detection of small amounts of water in ethanol using the nanoslits array chip. The present study might be useful in developing ultra-small biosensor chips integrated to optical fibers for online monitoring and remote sensing applications.