Determination of free tryptophan in serum with aptamer--comparison of two aptasensors.

Two aptasensors based on graphene oxide (GO) and molecular beacon were designed for the detection of L-tryptophan (L-Trp) using L-Trp aptamer (Trp3a-1). The fluorescein (FAM) labeled Trp3a-1 was absorbed by GO, which resulted in the fluorescence quenching, and exhibiting minimal background fluorescence. Upon the addition of L-Trp, Trp3a-1 was not absorbed quickly. This effect allows for a quantitative assay of L-Trp over the concentration range of 10-500 μM and with a detection limit of 6.84 μM. However, due to the unspecific adsorption of GO, the GO based aptasensor can't be applied in complex matrixes. In respect of molecular beacon based aptasensor, FRET between Trp3a-1 labeled with FAM and CS-Trp3a-1 labeled with BHQ-1(black hole quencher-1) which is partially complementary with the aptamer was used to detect L-Trp. L-Trp binding could induce the disassociation of CS-Trp3a-1, resulted in the enhancement of fluorescence in solution. With an excellent linear relationship in 10-500 μM and a detection limit of 6.97 μM in 25% serum, the aptasensor is expected to be improved for the detection of free L-Trp in other complex samples.