Representative LDI mass spectra of AuNPs in mouse spleens. a, AuNP 1 (molecular ion m/z 436). b, AuNP 2 (molecular ion m/z 492). c, AuNP 3 (molecular ion m/z 548). Other ions in the spectra correspond to common fragments of the phosphatidylcholine headgroup (e.g. m/z 184), one of which is indicated with an *, and Au+ (m/z 197), indicated by a +.