PT - JOURNAL ARTICLE AU - Lewis E. MacKenzie AU - Jack A. Goode AU - Alexandre Vakurov AU - Padmaja P. Nampi AU - Sikha Saha AU - Gin Jose AU - Paul A. Millner TI - The molecular weight of NaYF<sub>4</sub>:RE photonic up-conversion nanoparticles AID - 10.1101/114744 DP - 2017 Jan 01 TA - bioRxiv PG - 114744 4099 - http://biorxiv.org/content/early/2017/03/07/114744.short 4100 - http://biorxiv.org/content/early/2017/03/07/114744.full AB - Luminescence up-conversion nanoparticles (UCNPs) consisting of a NaYF4 crystal lattice doped with rare earth (RE) ions have found widespread application in bio-sensing, bio-imaging, and therapeutics; yet the molecular weight of UCNPs is not known. Lack of knowledge of molecular weight of UCNPs results in sub-optimal functionalisation and dosages of UCNPs. We present a simple method for calculating the molecular weight of NaYF4:RE UCNPs from arbitrary crystal lattice parameters and UCNP diameter measurements, and we apply this method to estimate the molecular weight of various NaYF4:RE UCNPs from the literature. UCNP molecular weight scales exponentially with UCNP volume (i.e. diameter cubed). UCNPs of 10 nm diameter are estimated to be a molecular weight of ~ 1 MDa, and 45 nm UCNPs are estimated to be ~100 MDa. Hexagonal lattice UCNPs were found to have a greater molecular weight than their cubic lattice UCNP counterparts. A Gaussian distribution of nanoparticle diameters was found to produce a lognormal distribution of nanoparticle molecular weights. We provide stand-alone graphic user interfaces to calculate UCNP:RE molecular weight. This approach can be generalised to estimate the molecular weight of crystalline nanoparticles of arbitrary size, geometry, and elemental composition where nanoparticle unit cell parameters are known.