The original QTL technology was invented at Los Alamos National Laboratory. A broad technology and intellectual property platform QTL Lightspeed®  has since been developed at QTLBiosystems. The QTL Lightspeed® assays provide a simple fluorescence superquenching-based platform for selective identification of biological molecules.

 
Fluorescence Superquenching
Superquenching is accomplished by QTL’s conjugated polyelectrolytes. These are charged (anionic or cationic) polymers consisting of repeat units of chromophores that have a delocalized system of "pi" electrons that shuttle throughout or part of the polymer chain. Such conjugated polyelectrolytes are typically fluorescent, with significant quantum yields for florescence. These polyelectrolytes have been shown to produce thousand-fold or higher amplification of fluorescence quench response in the presence of electron and energy transfer quenchers. The amplification is the result of exciton migration along the backbone of the excited polyelectrolyte.

When a small molecule chromophore is quenched by a quencher (A), the corresponding signal change is 1:1. i.e. Fret Assays In contrast to “A”, hundreds to thousands of chromophores, when conjugated to form a polymer, can be quenched by only one quencher (B). i.e. Superquenching Assays

A fluorescent polyelectrolyte-based superquenching assay has been shown to offer several advantages over conventional small molecule based fluorescence assays. For example, conjugated polyelectrolytes, dye pendant polyelectrolytes, etc. can "harvest" light effectively both by absorption and by superquenching. The enhanced absorbing power of the polymers is indicated by the observation that even sub nanomolar solutions of some of these materials are visibly colored. The fluorescence of these polymers can be detected at even lower concentrations.

The result is that sensitivities of detection can be achieved that are hundreds to thousand times higher than those achieved with conventional fluorescent assays.

QTL Lightspeed Sensor
The polymer QTL approach provides a system for effective sensing of biological agents by observing fluorescence changes. The key scientific basis is the amplification of quenching of fluorescence that can be obtained with certain charged conjugated polymers and small molecule quenchers.

The key aspect to QTL technology is the QTL Sensor, which consists of a receptor molecule and the polymer deposited onto beads. Detection of a biological agent or activity is accomplished by addition of a Quencher-Tether-Ligand (QTL) conjugate. The ligand portion of the QTL is a specific molecule that binds the receptor on the Sensor. The ligand is separated from a quencher by the tether. When the QTL binds the Sensor, the quencher is brought into close proximity with the fluorescent polymer and fluorescence is quenched. With the use of different receptor molecules coupled to beads, assays have been developed for a variety of biological targets and many more are currently under development.

Below is a schematic that depicts a platform in which the receptor is a metal ion that can associate with a phosphoryl group on a peptide target. The phosphorylated product binds to the QTL Sensor resulting in superquenching of polymer fluorescence. Upon quenching, energy is transferred from the polymer to the dye, which allows ratiometric analysis of the two emission wavelengths.