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

Abstract

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 Mn 2+ -doped ZnS QDs excited by two-photon absorption (2PA) in near-infrared window II (NIR-II: 1000−1350 nm). Multiphoton-absorptioninduced 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 Mn 2+ emission at 586 nm on account of the 4 T 1 − 6 A 1 transition is attributed to the transitions from the valence subband of ZnS QDs (or ground states of Mn 2+ ions) to the excited states of Mn 2+ 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), Mn 2+ -doped ZnS QDs appear to be a promising candidate for deep tissue imaging applications.

High potential of Mn-doped ZnS nanoparticles with different dopant concentrations as novel MRI contrast agents: synthesis and in vitro relaxivity studies

“…Research on the potential of manganese zinc sulphide as a fluorescent bio-label has been extensive (Baruah et al 2012;Subha et al 2013;Zhang et al 2011), but the efficiency of Mn:ZnS as an MRI contrast enhancement is less well studied. Furthermore, the majority of Mn-doped QDs have a coreshell structure and Mn atoms are doped on either the core or the shell of QDs.…”

Strong two-photon absorption of Mn-doped CsPbCl3 perovskite nanocrystals

“…16 Doping may also change multiphoton absorption (MPA) properties of NCs by forming defect levels in the bandgap. 17 Hence, the large MPA of Mn 2þ -doped semiconductor NCs is appropriate for highresolution cellular imaging and in vivo tumor-targeting imaging. 16 However, the role of Mn 2þ defect levels in modifying the MPA properties of perovskite NCs has not been investigated.…”

Origin of Luminescence‐Based Detection of Metal Ions by Mn–Doped ZnS Quantum Dots

“…The QDs have an absorption edge at ∼ 320 nm due to the host ZnS. Upon excitation at 320 nm or lower wavelength, an intense orange colored emission was observed with peak maximum at 585 nm, which is the characteristic of Mn dopant emission . Upon excitation, the electron is promoted to the conduction band of ZnS, and then transferred to 4 T 1 state of Mn 2+ , followed by the relaxation of electron through the well‐known d‐d transition of Mn 2+ ( 4 T 1 → 6 A 1 ) generating the orange color emission .…”