Aptamer-based and sensitive label-free colorimetric sensing of phenylalanine via cascaded signal amplifications

The concentration variation of phenylalanine (Phe), an essential amino acid in humans, can cause metabolism disorders and even mental disability. Sensitive and convenient monitoring of Phe is therefore important for disease diagnosis. We describe here the establishment of a new aptamer-based, sensitive and label-free colorimetric Phe detection strategy by integrating catalytic hairpin assembly (CHA) and Mg²⁺-dependent DNAzyme amplification cascades. The target Phe coordinates with pentamethylcyclopentadienyl rhodium(III) chloride dimer [(Cp*RhCl₂)₂] to form a complex that has a high affinity to the corresponding aptamer sequence. Upon its binding to the aptamers in DNA duplex probes, ssDNA strands are released to trigger subsequent CHA reactions for the formation of many DNAzymes, which cleave the substrate signal probes to liberate lots of CHA initiation strands and free G-quadruplexes to realize the cascaded amplifications. Hemin further associates with the many G-quadruplexes to yield hemin/G-quadruplex mimicking peroxidases, which catalyze solution of substrate to exhibit highly enhanced UV-vis adsorption for detecting Phe at 0.19 μM level. At the meantime, the monitoring of Phe in diluted serums with high selectivity has also been demonstrated by the developed method, indicating its potential for simple diagnosis of Phe-related diseases.