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We calculate the dust emission expected at 3.43 and 3.53 microns ifmeteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of thefar-UV break observed in quasars.  We integrate the UV flux that hydrogenated nanodiamonds must absorb toreproduce the far-UV break. Based on laboratory spectra of H-terminated diamondsurfaces, we analyse the radiative energy budget and derive theoretically theIR emission profiles expected for possible C-H surface stretch modes of thediamonds.  Using as test case a spectrum of 3C298 provided by the Spitzer Observatory,we do not find evidence of these emission bands.  While diamonds without surface adsorbates remain a viable candidate forexplaining the far-UV break observed in quasars, hydrogenated nanodiamondsappear to be ruled out, as they would giverise to IR emission bands, which havenot been observed so far.Comment: To appear in A&A Letters, 4 pages, 2 figure

Quasar 3C 298: a test-case for meteoritic nanodiamond 3.5$\mu{\rm m}$ emission