Energy Transfer from a Cationic Conjugated Polyelectrolyte to a DNA Photonic Wire: Toward Label-Free, Sequence-Specific DNA Sensing

We demonstrate a label-free, sequence specific DNA sensor based on fluorescence resonant energy transfer (FRET) occurring between a cationic conjugated polyelectrolyte and a small intercalating dye, malachite green chloride. The sensor combines (1) conjugated polymer chain conformation changes induced by the binding with DNA, with the conjugated polymer wrapping/twisting around the DNA helical duplex and experiencing a 3-fold increase in its photoluminescence quantum yield and (2) FRET from the conjugated polymer to the intercalated DNA. Owing to its small size, the dye intercalates at maximal, one-to-one dye-to-base pair load, making the intercalated DNA a molecular photonic wire with dyes excitonically coupled and chiroptically active. Any sequence mismatch between probe and target DNA degrades the intercalated DNA photonic wire by decreasing its brightness, excitonic coupling, and chiroptical properties, and this provides a signal transduction method for the DNA sensor. Coupling of intercalated DNA with the conjugated polymer via FRET provides target signal amplification and increased sensitivity toward sequence mismatch, with the FRET efficiency decreasing with added DNA sequence mismatch

Shell Thickness Dependent Photoinduced Hole Transfer in Hybrid Conjugated Polymer/Quantum Dot Nanocomposites: From Ensemble to Single Hybrid Level

Photoinduced hole transfer is investigated in inorganic/organic hybrid nanocomposites of colloidal CdSe/ZnS quantum dots and a cationic conjugated polymer, poly(9,9′-bis(6-<i>N</i>,<i>N</i>,<i>N</i>-trimethylammoniumhexyl)fluorene-alt-phenylene, in solution and in solid thin film, and down to the single hybrid level and is assessed to be a dynamic quenching process. We demonstrate control of hole transfer rate in these quantum dot/conjugated polymer hybrids by using a series of core/shell quantum dots with varying shell thickness, for which a clear exponential dependency of the hole transfer rate <i>vs</i> shell thickness is observed, for both solution and thin-film situations. Furthermore, we observe an increase of hole-transfer rate from solution to film and correlate this with changes in quantum dot/polymer interfacial morphology affecting the hole transfer rate, namely, the donor–acceptor distance. Single particle spectroscopy experiments reveal fluctuating dynamics of hole transfer at the single conjugated polymer/quantum dot interface and an increased heterogeneity in the hole-transfer rate with the increase of the quantum dot’s shell thickness. Although hole transfer quenches the photoluminescence intensity of quantum dots, it causes little or no effect on their blinking behavior over the time scales probed here

Light-Harvesting Nanoparticle Core–Shell Clusters with Controllable Optical Output

We used DNA self-assembly methods to fabricate a series of core–shell gold nanoparticle–DNA–colloidal quantum dot (AuNP–DNA–Qdot) nanoclusters with satellite-like architecture to modulate optical (photoluminescence) response. By varying the intercomponent distance through the DNA linker length designs, we demonstrate precise tuning of the plasmon–exciton interaction and the optical behavior of the nanoclusters from regimes characterized by photoluminescence quenching to photoluminescence enhancement. The combination of detailed X-ray scattering probing with photoluminescence intensity and lifetime studies revealed the relation between the cluster structure and its optical output. Compared to conventional light-harvesting systems like conjugated polymers and multichromophoric dendrimers, the proposed nanoclusters bring enhanced flexibility in controlling the optical behavior toward a desired application, and they can be regarded as controllable optical switches <i>via</i> the optically pumped color

Evolution of Excited-State Dynamics in Periodic Au₂₈, Au₃₆, Au₄₄, and Au₅₂ Nanoclusters

Understanding the correlation between the atomic structure and optical properties of gold nanoclusters is essential for exploration of their functionalities and applications involving light harvesting and electron transfer. We report the femto-nanosecond excited state dynamics of a periodic series of face-centered cubic (FCC) gold nanoclusters (including Au₂₈, Au₃₆, Au₄₄, and Au₅₂), which exhibit a set of unique features compared with other similar sized clusters. Molecular-like ultrafast S_n → S₁ internal conversions (i.e., radiationless electronic transitions) are observed in the relaxation dynamics of FCC periodic series. Excited-state dynamics with near-HOMO–LUMO gap excitation lacks ultrafast decay component, and only the structural relaxation dominates in the dynamical process, which proves the absence of core–shell relaxation. Interestingly, both the relaxation of the hot carriers and the band-edge carrier recombination become slower as the size increases. The evolution in excited-state properties of this FCC series offers new insight into the structure-dependent properties of metal nanoclusters, which will benefit their optical energy harvesting and photocatalytic applications

Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide

The combination of zero-dimensional (0D) colloidal CdSe/ZnS quantum dots with tin disulfide (SnS₂), a two-dimensional (2D)-layered metal dichalcogenide, results in 0D–2D hybrids with enhanced light absorption properties. These 0D–2D hybrids, when exposed to light, exhibit intrahybrid nonradiative energy transfer from photoexcited CdSe/ZnS quantum dots to SnS₂. Using single nanocrystal spectroscopy, we find that the rate for energy transfer in 0D–2D hybrids increases with added number of SnS₂ layers, a positive manifestation toward the potential functionality of such 2D-based hybrids in applications such as photovoltaics and photon sensing

Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution

The ability of cesium lead halide (CsPbX₃; X = Cl^–, Br^–, I^–) perovskite nanoparticles (P-NPs) to participate in halide exchange reactions, to catalyze Finkelstein organohalide substitution reactions, and to colorimetrically monitor chemical reactions and detect anions in real time is described. With the use of tetraoctylammonium halide salts as a starting point, halide exchange with the P-NPs was performed to calibrate reactivity, stability, and extent of ion exchange. The exchange of CsPbI₃ with Cl^– or Br^– causes a significant blue-shift in absorption and photoluminescence, whereas reacting I^– with CsPbBr₃ causes a red-shift of similar magnitudes. With the high local halide concentrations and the facile nature of halide exchange in mind, we then explored the ability of P-NPs to catalyze organohalide exchange in Finkelstein like reactions. Results indicate that the P-NPs serve as excellent halide reservoirs for substitution of organohalides in nonpolar media, leading to not only different organohalide products, but also a complementary color change over the course of the reaction, which can be used to monitor kinetics in a precise manner. The merits of using P-NP as spectrochemical probes for real time assaying is then expanded to other anions which can react with, or result in unique, classes of perovskites

Tin DisulfideAn Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS₂), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS₂ in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with <i>ab initio</i> calculations and photoluminescence spectroscopy show that SnS₂ is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS₂ supported by SiO₂/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on–off current ratios >10⁶, as well as carrier mobilities up to 230 cm²/(V s), minimal hysteresis, and near-ideal subthreshold swing for devices screened by a high-<i>k</i> (deionized water) top gate. SnS₂ transistors are efficient photodetectors but, similar to other metal dichalcogenides, show a relatively slow response to pulsed irradiation, likely due to adsorbate-induced long-lived extrinsic trap states

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties