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[EN] Multicore optical fibers are of great interest in the optical sensing field. Their core diversity and spatial distribution enable the development of sensing mechanisms that are not possible in single-core fibers. In this paper, we study the use of the inter-core crosstalk phenomena for the implementation of a surrounding refractive index (SRI) sensor. The selective inscription of a tilted fiber Bragg grating (TFBG) intentionally increases the inter-core crosstalk between the inscribed cores and makes it sensitive to the SRI. With this technique we simplify the measurement of the SRI and improve the identification and tracking of the excited cladding modes, as compared with the analysis of the transmission spectrum of a TFBG in single-core fibers. The proposed device is also sensitive to temperature. Temperature is obtained from the crosstalk wavelength shift with a measured sensitivity of 9.75 pm/degrees C. The SRI is obtained from the measurement of the crosstalk optical power. For increasing SRIs the cladding modes gradually fade, reducing the crosstalk optical power. We observed that the higher the tilt, the higher the sensor sensitivity. For a 7 degrees TFBG the SRI sensitivity obtained is -74.2 dB/RIU from 1.31 to 1.39 and -250.8 dB/RIU from 1.39 to 1.44., This work was supported in part by the Spanish Ministry of Economy and Competitiveness under the DIMENSION TEC2017 88029-R Project and in part by the Generalitat Valenciana by PROMETEO 2017/103 research excellency award and IDI/FEDER/2018 GVA Infraestructura. The work of J. Madrigal was supported by Universitat Politecnica de Valencia scholarship PAID-01-18. The work of D. Barrera was supported by Spanish MICINN fellowship IJCI-2017-32476.