Sensitive detection of NaYF4

Rare-earth doped upconversion nanocrystals are emerging as the next-generation luminescent biomaterials. Here we load NaYF₄: Yb/Er and NaYF₄: Yb/Tm upconversion nanocrystals into a soft-glass suspended-core optical fiber dip sensor, allowing sensitive measurements and power-dependent characterizations to be performed. This, in combination with negligible background autofluorescence from the glass fiber when using infrared excitation has provided a significant improvement in terms of sensitivity over what has previously been demonstrated using an optical fiber dip sensor. For detection we employ suspended-core optical fibers, which have found extensive use in sensing applications. These combine the high evanescent overlap comparable to that of a nanowire, with the robust handling characteristics and long interaction length of a conventional fiber. The fiber sensor platform allows measurements to be performed using minimal sample volumes (<20 nL) while still maintaining the sensitivity of the platform.