The NaYF4:Yb,Er,Tm/NaYF4 core/shell nanosized phosphorswere coated with anti-HER2 DARPin DARPin 9_29 and used for targeted delivery toa HER2-positive cancer cell culture and for the imaging of tumor xenografts inanimals for at least 24 h. Here, ERBB2 is linked to cancer.