Efficient Photoluminescence of Mn2+-Doped ZnS Quantum Dots Excited by Two-Photon Absorption in Near-Infrared Window II

Highly fluorescing biological labels with excitation in near-infrared window II have attracted the interest of scientific community as they are capable of increasing both penetration depth and imaging quality. However, studies on the utilization of quantum dots (QDs) in biological imaging appear to be rather limited to the near-infrared window I (NIR-I: 650-950 nm). We herein report on the observation of efficient photoluminescence (PL) in Mn2+-doped ZnS QDs excited by two-photon absorption (2PA) in near-infrared window II (NIR-II: 1000-1350 nm). Multiphoton-absorption-induced PL measurements indicate that these biocompatible QDs exhibit a two-photon action cross-section of 265 GM at 1180 nm, the highest value reported to date among conventional fluorescent probes on excitation in NIR-II. This value is 1-2 orders of magnitude higher than that for organic dye molecules excited by NIR-I photons and 3-4 times greater than that of fluorescent proteins excited in the NIR-II. The underlying NIR-II excitation mechanism for the Mn2+ emission at 586 nm on account of the T-4(1)-(6)A(1) transition is attributed to the transitions from the valence subband of ZnS QDs (or ground states of Mn2+ ions) to the excited states of Mn2+ ions by direct two-photon absorption. Transient PL measurements reveal single exponential decay with a PL lifetime of 0.35 +/- 0.03 ms irrespective of excitation wavelength, which are 4-5 orders longer than that of conventional fluorescent probes. With the excitation in NIR-II window and the unique combination of photophysical properties such as a greater two-photon action cross-section, a longer PL lifetime, and larger anti-Stokes shift (450 nm or more), Mn2+-doped ZnS QDs appear to be a promising candidate for deep tissue imaging applications.