Quantification of quantum dot concentration using inductively coupled plasma-mass spectrometry (ICP-MS)

As quantum dot (QD) bioconjugates are increasingly being used for biomedical in vitro and in vivo studies, validated methods for the quantitative determination of QD concentration are of considerable potential value. In this work, we have assessed inductively coupled plasma mass spectrometry (ICP-MS) as a method for the quantitative detection of QDs and QD bioconjugates. We have established a linear relationship between the concentration of unconjugated QD and the mass of cadmium, selenium and zinc detected by ICP-MS. Furthermore, ICP-MS was used to quantitatively estimate the unknown concentration of a QD-antibody bioconjugate. Quantitative measurement of QD bioconjugate concentration was also attempted by optical methods, including fluorescence and absorbance, and compared to ICP-MS. Consistent with previous literature, the fluorescence of the nanoparticle construct was reduced after functionalization with a biomolecule (biotin or streptavidin). Optical absorbance of the QD is unaffected by chemical modifications in this study and is a reliable method to determine the concentration. Optical absorption in this application achieves nanomolar concentrations but is not suitable for most biomedical studies that require a nanoparticle detection limit in the sub-nanomolar region. Unlike optical absorbance and fluorescence, ICP-MS can reliably detect the concentration of QD bioconjugates in the nanomolar range, making ICP-MS a quantitative, sensitive method for QD concentration measurements even after surface conjugation and consequent changes in fluorescence characteristics.