ABSTRACT
Acknowledgments .........................................................................................................................246
References .....................................................................................................................................246
Upconverting phosphors are rare-earth-doped ceramic materials with the unique property of emit-
ting visible light upon excitation with near-infrared light. This process is called upconversion and
was first proposed in 1959 by Bloembergen [1] and experimentally demonstrated by Auzel in 1966
[2]. Following their discovery, these materials were studied for applications as light sources and as
detection materials for near-infrared diode laser light. The first use of downconverting phosphors as
labels in biological assays was in 1990 by Beverloo et al. [3] who first reported the advantages of
using this class of materials. The use of upconverting phosphors as labels in biological assays was
first considered by Zarling et al. in the early 1990s [4]. Since the mid-1990s, several groups have
extensively studied the use of upconverting phosphors in a variety of assay formats and detection
schemes. These include Tanke’s group at Leiden University as well as researchers at OraSure
Technologies (formerly STC) and SRI International. To date, upconverting phosphors have been
used primarily in lateral flow assays to detect protein [5], bacterial [6], and nucleic acid [7] targets.
However, they have also been used in microarray, microtiter plate, and bead-based assay formats.
