Characterization of single functionalized quantum dots using combined atomic force and confocal fluorescence microscopy

Fluorescent nanocrystals (quantum dots or QDs) have a number of unique properties that overcome the limitations of conventional organic dyes. However, the optical properties of QDs have been observed to be strongly dependent upon their local chemical environment including novel surface coatings, which have been developed to render QDs water soluble and conjugation ready leading to their use as fluorescent tags and optical sensors for a variety of biological and biomedical applications. The quantitative utility of QDs in complex biological systems requires that their optical properties be well understood and interrelated with their chemical functionalization on the surface and their interactions with surface-conjugated materials. In this report, quantitative measurement of adhesion forces between a hydrophilic or a hydrophobic AFM probe and an amine-functionalized single QD or a hydrophilic substrate were obtained to demonstrate the utility of the atomic force microscopy (AFM) as a tool to probe surface functionalities of single functionalized QDs. We also present procedures to combine AFM and confocal fluorescence microscopy in an effort to simultaneously probe optical characteristics and physical/chemical properties of single or clustered functionalized QDs at the nanoscale.