To develop a zinc(II)-selective emission ratiometric probe suitable for biological applications, we explored the cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT) with a series of 2-(2'-benzenesulfonamidophenyl)benzimidazole derivatives. In the absence of Zn(II) at neutral pH, the fluorophores undergo ESIPT to yield a highly Stokes' shifted emission from the proton-transfer tautomer. Coordination of Zn(II) inhibits the ESIPT process and yields a significant hypsochromic shift of the fluorescence emission maximum. Whereas the paramagnetic metal cations Cu(II), Fe(II), Ni(II), Co(II), and Mn(II) result in fluorescence quenching, the emission response is not altered by millimolar concentrations of Ca(II) or Mg(II), rendering the sensors selective for Zn(II) among all biologically important metal cations. Due to the modular architecture of the fluorophore, the Zn(II) binding affinity can be readily tuned by implementing simple structural modifications. The synthesized probes are suitable to gauge free Zn(II) concentrations in the micromolar to picomolar range under physiological conditions.

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We report on the development of a digital-micromirror-device (DMD)-based multishot snapshot spectral imaging (DMD-SSI) system as an alternative to current piezostage-based multishot coded aperture snapshot spectral imager (CASSI) systems. In this system, a DMD is used to implement compressive sensing (CS) measurement patterns for reconstructing the spatial/spectral information of an imaging scene. Based on the CS measurement results, we demonstrated the concurrent reconstruction of 24 spectral images. The DMD-SSI system is versatile in nature as it can be used to implement independent CS measurement patterns in addition to spatially shifted patterns that piezostage-based systems can offer.

Two-photon excitation fluorescence microscopy (TPEM) has rapidly evolved into a widely used tool in biological and biomedical research. 1 Compared to traditional fluorescence microscopy, TPEM offers intrinsic 3D resolution combined with reduced phototoxicity, increased specimen penetration, and negligible background fluorescence. At present, most fluorophores used as labels or sensor platforms in TPEM have been adopted from linear microscopy and are not optimized for two-photon excitation. 2 Notably, the fluorescence brightness (ηδ), defined by the product of TPA cross section (δ) and emission quantum yield (η), is typically low due to a modest δ. 3 The development of new TPEM-optimized fluorophores is particularly vital in the context of biological metal-ion sensing since most of currently available ratiometric sensors, including the widely used dyes fura-2 and indo-1, 4 exhibit a low brightness that decreases even further upon cation binding. In addition, the majority of ratiometric metal-ion sensors offer only a large shift of the excitation peak but not emission energy. If only a single two-photon excitation source is on hand, such sensors are not suitable for dynamic ratiometric TPEM imaging with temporal resolution. In this communication, we address these problems with a molecular design approach that yields both an increase in δ and a shift of the peak emission energy upon metal-ion binding in a polar environment.Molecular design strategies of organic molecules with large δ are well-established. 5,6 In general, the magnitude of δ increases with an increasing degree of intramolecular charge transfer (ICT) upon excitation. For example, centrosymmetric fluorophore architectures consisting of a conjugated linear π-system with an acceptor moiety sandwiched between two electron donors (D-π-A-π-D) exhibit exceptionally large δ values. 6 However, in water, the highly polarized excited state gives rise to enhanced solvent-solute interactions, which in turn leads to a reduced δ, more efficient nonradiative deactivation, and thus to a drastically reduced brightness. 7 The centrosymmetric architecture poses additional challenges for the design of metal-ion sensors: (1) the interpretation of the sensor response is complicated due to the presence of two metal binding sites (Scheme 1a); (2) metal binding induces a reduction of ICT which in turn results in a smaller δ accompanied by a strongly blue-shifted peak excitation energy; and (3) partial decomplexation or ejection of the metal cation in the excited state is presumably responsible for typically small emission shifts that are not suitable for ratiometric sensing. 8 Here, we propose to overcome these problems by using a simplified D-A 9 motif where the metal-ion binds to the acceptor rather than donor site (Scheme 1b). Besides eliminating the second binding site, such an arrangement should yield an increase rather than decrease of ICT upon excitation. As a consequence, metal-ion binding is expected to result in an increased δ, enhanced fluorescence brightness, and a ...