Aptamer-Mediated Energy Acceptor Sensitization Strategy Driving Reverse Dual-Signal Change for Constructing High-Contrast Upconversion Nanoprobes

Upconversion nanoprobes show great potential in sensing applications due to low background interference and near-infrared excitation. Most upconversion nanoprobes are constructed based on the principle of luminescence resonance energy transfer, with only a single sensing signal. However, due to the structural characteristics of upconversion nanoparticles (UCNPs), the LRET efficiency is limited, which severely reduces the signal contrast and becomes a critical bottleneck for improving the sensitivity. This study proposes an aptamer-mediated energy acceptor sensitization strategy to realize the reverse dual-signal change for the construction of a highly sensitive upconversion nanoprobe with high signal contrast. Specifically, the probe is fabricated by conjugating a Cy3 dye-labeled aptamer to the surface of UCNPs. As a proof of concept, the nanoprobe is further applied for the detection of Pb²⁺, whose trace concentration in soil or agricultural products would cause irreversible harm in the human body. In the presence of Pb²⁺, the aptamer undergoes a structural transition into a G-quadruplex, which shortens the distance between Cy3 and UCNPs and activates the LRET process from UCNPs to the Cy3 dye. As a result, the green upconversion luminescence at 540 nm is quenched while Cy3 fluorescence at 565 nm is sensitized, enabling reverse sensing signal changes. Using the <i>I</i>₅₄₀/<i>I</i>₅₆₅ ratio as the detection signal, the nanoprobe achieves a detection limit as low as 51 pM, exhibiting high selectivity and anti-interference capability and demonstrating excellent performance in complex samples. This work proposes a versatile strategy to overcome the signal contrast limitations of conventional upconversion nanoprobes.