SnS₄^4– Metal Chalcogenide Ligand, S^2– Metal Free Ligand, and Organic Surface Ligand Toward Efficient CdSe Quantum Dot- Sensitized Solar Cells

Inorganic surface ligands such as metal chalcogenides ligand SnS₄^4– and metal free ligand S^2– were introduced for CdSe quantum dot sensitized solar cell (QDSSC) applications. The SnS₄^4– ligand QDs were successfully deposited onto TiO₂ photoanode through metal ion coordination. In solution, metal–ammonia complexes of Zn²⁺, Cd²⁺, and Cu²⁺ can be coordinated by the SnS₄^4– ligands and reverse the zeta potential. Similarly, the metal ions can be sandwiched by SnS₄^4– ligands and the photoanode. Using the metal ion bridged SnS₄^4– ligand QDs as the sensitizer, photovoltaic properties of the QDSSCs have been studied. Cd²⁺ mediated deposition case showed better photovoltaic performance than the cases of Zn²⁺ or Cu²⁺. To further investigate the surface ligand effect on QDSSC, organic/inorganic mixed surface CdSe QDs were introduced using partial ligand exchange after the deposition onto the TiO₂ photoanode. The postdeposition surface ligand exchange with inorganic ligands such as SnS₄^4– and S^2– is thought to retain the initial organic ligands between QDs and TiO₂ photoanode and selectively replace the QD ligands that would contact the electrolytes. Metal free S^2– ligand QDSSC showed the best photovoltaic performance recording 1.4 times enhanced photocurrent and 1.5 times enhanced photoconversion efficiency when compared with the initial organic surface ligand QDSSC. Comparison studies on the photovoltaic properties of QDSSCs with different surfaces suggest that (i) the Cd–SnS₄^4– complex and SnS₄^4– surface ligand act as efficient electron traps hurdling the photovoltaic performance severely, (ii) S^2– surface ligand works as an efficient hole trap only at the interface between the QD and TiO₂, and (iii) S^2– surface ligand blocks back electron transfers better than the initial organic ligand

Signal Amplification <i>via</i> Biological Self-Assembly of Surface-Engineered Quantum Dots for Multiplexed Subattomolar Immunoassays and Apoptosis Imaging

The parallel and highly sensitive detection of biomolecules is of paramount importance to understand biological functions at the single cell level and for various medical diagnoses. Surface-engineered semiconductor quantum dots (QDs) have been demonstrated to act as a signal amplifiable reporter in immunoassays. This takes advantage of the QDs’ robustness against self-quenching in proximity and the tunability of their surface properties. A streptavidin (SA) and biotin QD conjugate pair containing a zwitterionic surface modification was designed for QD self-assembly with minimal nonspecific adsorption. Typical sandwich-type immunoassay procedures were adopted, and the targeted protein binding events were effectively transduced and amplified by the fluorescence of the SA–biotin QD conjugates. The detection limit of myoglobin in 100% serum was determined to be at the subattomolar (tens of copies per milliliter) level, which was achieved by using 100 cycles of the layer-by-layer QD assembly. Adsorption kinetics studies and Monte Carlo simulations revealed that this highly sensitive signal amplification was accomplished by the zwitterionic surface, which gave equilibrium constants 5 orders of magnitude larger for specific binding than for nonspecific binding. The QD conjugates showed an effective multivalency of two, which resulted in a broad linear dynamic range spanning 9 orders of magnitude of target protein concentrations. The assay can be highly miniaturized and multiplexed, and as a proof-of-concept, parallel and rapid detection of four different cancer markers has been successfully demonstrated. To demonstrate that this QD signal amplification can be a universal platform, sensitive imaging and early detection of apoptotic cells were also showcased

Layer-by-Layer Quantum Dot Assemblies for the Enhanced Energy Transfers and Their Applications toward Efficient Solar Cells

Two different quantum dots (QDs) with an identical optical band gap were prepared: one without the inorganic shell and short surface ligands (BQD) and the other with thick inorganic shells and long surface ligands (OQD). They were surface-derivatized to be positively or negatively charged and were used for layer-by-layer assemblies on TiO₂. By sandwiching BQD between OQD and TiO₂, OQD photoluminescence showed seven times faster decay, which is attributed to the combined effect of the efficient energy transfer from OQD to BQD with the FRET efficiency of 86% and fast electron transfer from BQD to TiO₂ with the rate of 1.2 × 10⁹ s^–1. The QD bilayer configuration was further applied to solar cells, and showed 3.6 times larger photocurrent and 3.8 times larger photoconversion efficiency than those of the device with the OQD being sandwiched by BQD and TiO₂. This showcases the importance of sophisticated control of QD layer assembly for the design of efficient QD solar cells

Layer-by-Layer Assemblies of Semiconductor Quantum Dots for Nanostructured Photovoltaic Devices

A multilayer of quantum dots (QDs) is preferred for QD-sensitized solar cells over a monolayer counterpart to fully utilize the sunlight incident into a relatively thin-film-based photovoltaic device. A controlled assembly of QD multilayers such as layer-by-layer (LbL) assemblies can provide a model system to study the interactions between the QD layers and can offer an optimal device configuration for efficient solar power conversion. Recently, we have proposed a LbL QD assembly using electrostatic interactions of the surface charges and have successfully prepared a controlled multilayer of QD on the surface of mesoporous metal oxide films. The as-prepared tailor-made QD multilayers not only guaranteed the sufficient absorption of incident solar light but also provided a toolbox for the study and optimization of electron/energy transfers between QD layers

i‑Motif-Driven Au Nanomachines in Programmed siRNA Delivery for Gene-Silencing and Photothermal Ablation

The present work illustrates unique design, construction and operation of an i-motif-based DNA nanomachine templated on gold nanoparticles (AuNPs), which utilizes pH-responsive dynamic motion of i-motif DNA strands and aggregational behavior of AuNPs to elicit programmed delivery of therapeutic siRNA. The pH-sensitive nucleic acids immobilized on the AuNPs consisted of three functional segments, <i>i.e.</i>, an i-motif DNA, an overhanging linker DNA and a therapeutic siRNA. At neutral pH, the i-motif DNA is hybridized with the overhanging linker DNA segment of the therapeutic siRNA. However, in endosomal acidic pH, the i-motif DNA forms interstrand tetraplex, which could induce cluster formation of AuNPs resulting in endosomal escape of AuNP clusters, and produce a high gene silencing efficiency by releasing siRNA in the cytosol. Furthermore, the cluster formation of AuNPs accelerated photothermal ablation of cells when irradiated with laser. Precise and synchronized biomechanical motion in subcellular microenvironment is realized through judicious integration of pH-responsive behavior of the i-motif DNA and AuNPs, and meticulous designing of DNA

Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double Hydroxide Composites

We report a designed strategy for a synthesis of highly luminescent and photostable composites by incorporating quantum dots (QDs) into layered double hydroxide (LDH) matrices without deterioration of a photoluminescence (PL) efficiency of the fluorophores during the entire processes of composite formations. The QDs synthesized in an organic solvent are encapsulated by polymers, poly­(maleic acid-alt-octadecene) to transfer them into water without altering the initial surface ligands. The polymer-encapsulated QDs with negative zeta potentials (−29.5 ± 2.2 mV) were electrostatically assembled with positively charged (24.9 ± 0.6 mV) LDH nanosheets to form QD-polymer-LDH composites (PL quantum yield: 74.1%). QD-polymer-LDH composite films are fabricated by a drop-casting of the solution on substrates. The PL properties of the films preserve those of the organic QD solutions. We also demonstrate that the formation of the QD-polymer-LDH composites affords enhanced photostabilities through multiple protections of QD surface by polymers and LDH nanosheets from the environment

Self-Assembled Gold Nanoparticle–Mixed Metal Oxide Nanocomposites for Self-Sensitized Dye Degradation under Visible Light Irradiation

Gold nanoparticle (Au NP)–mixed metal oxide (MMO) nanocomposite photocatalysts for efficient self-sensitized dye degradations under visible light were prepared by an electrostatically driven self-assembly. Dihydrolipoic acid (DHLA)-capped Au NPs (building block I) were synthesized through a room temperature reaction. Their hydrodynamic size was determined as being around 4.9 nm by dynamic light scattering measurements. MMO nanoplates with lateral dimensions of 100–250 nm (building block II) were prepared by a calcination of zinc aluminum layered double hydroxides at 750 °C for 2 h in air. In a pH 7.0 aqueous solution, the DHLA-capped Au NPs had a negative zeta potential (−22 ± 3 mV); on the other hand, the MMO nanoplates had a positive zeta potential (15 ± 2 mV). Electrostatic self-assembly was achieved by stirring an aqueous solution (pH 7.0) containing DHLA-capped Au NPs and MMO nanoplates at room temperature for 1 h. The self-assembled and sequentially calcined nanocomposites exhibited the superior self-sensitized dye degradation efficiency under visible light to that of ZnO, TiO₂ (P25), or pure MMO nanoplates. The enhanced degradation efficiency could be attributed to strong coupling interactions of ZnO and ZnAl₂O₄ phases of the MMO and the role of Au as an electron sink and mediator for formations of reactive oxidation species and as a light concentrator

Strategy for Synthesizing Quantum Dot-Layered Double Hydroxide Nanocomposites and Their Enhanced Photoluminescence and Photostability

Layered double hydroxide-quantum dot (LDH-QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields and thermal- and photostabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites

Theragnostic pH-Sensitive Gold Nanoparticles for the Selective Surface Enhanced Raman Scattering and Photothermal Cancer Therapy

We report a nanoparticle-based probe that can be used for a “turn-on” theragnostic agent for simultaneous Raman imaging/diagnosis and photothermal therapy. The agent consists of a 10 nm spherical gold nanoparticle (NP) with pH-responsive ligands and Raman probes on the surface. They are engineered to exhibit the surface with both positive and negative charges upon mildly acidic conditions, which subsequently results in rapid aggregations of the gold NPs. This aggregation simultaneously provides hot spots for the SERS probe with the enhancement factor reaching 1.3 × 10⁴ and shifts the absorption to far-red and near-infrared (which is optimal for deep tissue penetration) by the coupled plasmon resonances; this shift was successfully exploited for low-threshold photothermal therapy. The theragnostic gold NPs are cancer-specific because they aggregate rapidly and accumulate selectively in cancerous cells. As the result, both Raman imaging and photothermal efficacy were turned on under a cancerous local environment. In addition, the relatively small hydrodynamic size can have the potential for better access to targeted delivery in vivo and facilitated excretion after therapy

Mesenchymal Stem Cells Aggregate and Deliver Gold Nanoparticles to Tumors for Photothermal Therapy

Gold nanoparticles (AuNPs) have been extensively studied for photothermal cancer therapy because AuNPs can generate heat upon near-infrared irradiation. However, improving their tumor-targeting efficiency and optimizing the nanoparticle size for maximizing the photothermal effect remain challenging. We demonstrate that mesenchymal stem cells (MSCs) can aggregate pH-sensitive gold nanoparticles (PSAuNPs) in mildly acidic endosomes, target tumors, and be used for photothermal therapy. These aggregated structures had a higher cellular retention in comparison to pH-insensitive, control AuNPs (cAuNPs), which is important for the cell-based delivery process. PSAuNP-laden MSCs (MSC-PSAuNPs) injected intravenously to tumor-bearing mice show a 37-fold higher tumor-targeting efficiency (5.6% of the injected dose) and 8.3 °C higher heat generation compared to injections of cAuNPs after irradiation, which results in a significantly enhanced anticancer effect