The characteristic dual emission spectra of UCLNPs have been exploited to implement a ratiometric encoding strategy for multiplexed analyte detection, which is essential for such diverse applications as genotyping, screening combinatorial libraries, medical diagnostics or environmental monitoring. The luminescence intensity of one emission band is tuned in ten increments by surface-modification with selective filter dyes, while the second unfiltered emission band serves as a constant reference signal. The ratio of both emission bands defines the code. Combining several types of UCLNPs - each defining ten distinct ratiometric codes - increases the coding capacity exponentially. Ratiometric optical measurements are inherently resistant to fluctuations in the absolute luminescence intensity that may result from changes in the concentration, the intensity of the light source or the photodetector sensitivity. In combination with the background-free detection mode under near-infrared excitation, ratiometric encoding with UCLNPs thus enables the highly robust and reproducible detection of multiple analytes.