Surface and Tip-Enhanced Raman Spectroscopy at the Plasmonic Hot Spot of a Coordination Complex-Conjugated Gold Nanoparticle Dimer

We report a synthetic method to generate surface enhanced Raman scattering (SERS) active gold nanodimer, by employing rationally designed co-ordination complex of europium (Eu) metal with 4-mercaptobenzoic acid (4-MBA) and heterocyclic phenanthroline ligands. The reaction between the Raman active 4-MBA ligand of the Eu-complex and citrate-stabilized Au nanoparticles (cit-Au NPs) led to selective dimerization in considerable yield and precise control over hot-spot engineering to maximize SERS. Additionally, scanning probe microscopy (SPM)-correlated tip-induced Raman enhancement using He–Ne laser focusing on the plasmonically coupled Au nanodimers helped obtain intense Raman signal at the spatial resolution of single dimer. The average SERS amplification was measured to be on the order of 10⁹, whereas that in the presence of tip-enhancement was found to be 10⁴. This extraordinary Raman activity of an inorganic complex-coupled gold nanodimer therefore augurs well for applying chemical synthetic strategies in designing plasmonically active assembled nanomaterials for applications in photovoltaics, photocatalysis, biosensing, and single molecule spectroscopy

Redox-Tuned Three-Color Emission in Double (Mn and Cu) Doped Zinc Sulfide Quantum Dots

The photoluminescence characteristics of colloidal Mn²⁺ and Cu²⁺ (double) doped zinc sulfide (ZnS) quantum dots (Qdots) could be drastically influenced by reactions with redox reagents. Importantly, experiments revealed Cu⁺ in ZnS nanocrystals rather than Cu²⁺, in conjunction with Mn²⁺, as the emitting dopant. Thus, as-synthesized aqueous Qdots emitted orange (with peaks at 460 and 592 nm) due to the host and Mn²⁺ dopant emissions. However, upon treatment with a reducing agent, the color changed to yellow with dual peaks positioned at 520 and 590 nm due to Cu⁺ and Mn²⁺ dopant emissions. The characteristics could be changed reversibly with appropriate redox reagents. Further, treatment with excess of an oxidizing agent led to blue emission with a single peak at 450 nm

Redox-Tuned Three-Color Emission in Double (Mn and Cu) Doped Zinc Sulfide Quantum Dots

The photoluminescence characteristics of colloidal Mn²⁺ and Cu²⁺ (double) doped zinc sulfide (ZnS) quantum dots (Qdots) could be drastically influenced by reactions with redox reagents. Importantly, experiments revealed Cu⁺ in ZnS nanocrystals rather than Cu²⁺, in conjunction with Mn²⁺, as the emitting dopant. Thus, as-synthesized aqueous Qdots emitted orange (with peaks at 460 and 592 nm) due to the host and Mn²⁺ dopant emissions. However, upon treatment with a reducing agent, the color changed to yellow with dual peaks positioned at 520 and 590 nm due to Cu⁺ and Mn²⁺ dopant emissions. The characteristics could be changed reversibly with appropriate redox reagents. Further, treatment with excess of an oxidizing agent led to blue emission with a single peak at 450 nm

Surface Ion Engineering of Mn²⁺-Doped ZnS Quantum Dots Using Ion-Exchange Resins

We report the engineering of surface ions present as defects in doped quantum dots (Qdots) following their synthesis. This was achieved by treating the Qdots with cation-exchange resin beads (CB). An aqueous dispersion of Mn²⁺-doped ZnS Qdots, when treated with different amounts of CB, resulted in two kinds of changes in the emission due to Mn²⁺ ions. First, the intensity increased in the presence of a smaller amount of CB, to the extent of a doubled quantum yield. With increased CB as well as incubation time, the emission intensity decreased systematically, accompanied by an increasing blue shift of the peak emission wavelength. Electron spin resonance results indicated the removal of clusters of Mn²⁺ present in the Qdots by the CB, which has been attributed to changes in the emission characteristics. Transmission electron microscopy studies revealed that for smaller amounts of CB there was no change in the particle size, whereas for greater amounts the particle size decreased. The results have been explained on the basis of the removal of Mn²⁺ (and also Zn²⁺) ions present on the surfaces of Qdots in the form of clusters as well as individual ions

Quantum Dot Surface Mediated Unprecedented Reaction of Zn²⁺ and Copper Quinolate Complex

We report the reaction between Zn²⁺ ions, being present on the surface of ZnS quantum dot (Qdot), and copper­(II) bis­(8-hydroxyquinoline) (copper quinolate; CuQ₂) complex, leading to the formation of luminescent ZnQ₂ on the surface. This is contrary to the reactivity of copper complex based on Irving–William series in the liquid medium and thus indicating catalytic role of the Qdot surface. The rate of the reaction was observed to be first order with respect to the concentrations of both the reactants, with activation energy measured to be 74.3 kJ mol^–1. Further, the reaction of the Zn²⁺ ions on the Qdot was observed to be the fastest with HQ, slower with MnQ₂, and the slowest with CuQ₂, all leading to the formation of ZnQ₂ on the surface of the Qdot. The enhanced stability of the product complex on the surface, in the presence of dangling sulfide ions, indicated a new type of chemical reaction deserving special attention

Probing Cancer Cells through Intracellular Aggregation-Induced Emission Kinetic Rate of Copper Nanoclusters

pH-responsive luminescent copper nanoclusters (Cu NCs) with aggregation-induced emission (AIE) characteristics have been synthesized. Upon internalization into living cells, the NCs displayed a cellular pH environment-dependent luminescence change with orange-red emission at pH_i 4.5, whereas bright green emission was observed over time at pH_i 7.4, through their AIE attributes. Furthermore, the intracellular AIE kinetics of the NC probe was measured in MCF-7 cells and compared to that of HEK-293 cells. Intriguingly, the intracellular rate constant value derived for AIE kinetics in MCF-7 cells was found to be 3-fold higher than that in HEK-293 cell lines, whereas the value was 2-fold higher than that observed in aqueous medium. This provided a new platform to study different cell lines based on intracellular AIE in living cells, with additional potential for future applications in cellular imaging, diagnostics, and disease detection

Complex Transfer Reaction from ZnO to ZnS Quantum Dots Driven by Surface Anions

This work demonstrates preferential transfer of 8-hydroxyquinoline-5-sulfonic acid (HQS) from surface of ZnO quantum dots to ZnS quantum dots in the form of Zn­(QS)₂ complex. This is an instance of inter quantum dot complex or Z-type ligand transfer that depends on solvent systemmore precisely, solubility of the migrating complex. The migration can be explained using Langmuir model by considering two parallel adsorption equilibria on ZnO and ZnS surfaces. We also propose that higher stability of the zinc quinolato complex on ZnS surface in comparison to that of ZnO is the driving force of the reaction

Conducting Carbon Dot–Polypyrrole Nanocomposite for Sensitive Detection of Picric acid

We report the conducting nature of carbon dots (Cdots) synthesized from citric acid and ethylene diamine. Chemically synthesized conducting nanocomposite consisting of Cdots and polypyrrole (PPy) is further reported, which showed higher electrical conductiviy in comparison to the components i.e., Cdots or PPy. The conductive film of the composite material was used for highly sensitive and selective detection of picric acid in water as well as in soil. To the best of our knowledge, this is the first report on the conductivity based sensing application of Cdot nanocomposite contrary to the traditional fluorescence based sensing approaches

Living Gut Bacteria Functionalized with Gold Nanoclusters and Drug for Facile Cancer Theranostics

Bacbots are potent self-propelling vehicles for targeted therapy that can be guided by chemical and biochemical stimuli of the host. In addition, they can be guided externally by the use of magnetic field or other physical forces. The challenge is to incorporate drugs and diagnostic tools in living bacteria with retention of theranostic activity until reaching the targets and easy clearance of the remainder following the treatment. We report that living Lactobacillus rhamnosus, when functionalized with photoluminescent Au nanoclusters and the anticancer drug methotrexate, was cytotoxic to monolayer and spheroids of cancer cells (HeLa and HT29) even at a low dose of bacteria used (107 cfu/mL). The observed cell death was nearly 90% in HeLa spheroids and 70% in HT29 spheroids. Further, functionalization of the bacterial surface with the nanoclusters helped incorporate the drug onto their cell surfaces. The drug and nanocluster-loaded bacteria annihilated the cells and the spheroids in a rather short time (6 h) that revealed the specificity and effectiveness of the bacbots. The bacbots exhibited synergistic toxicity on the cells as their effect was more than the drug and the bacteria individually. This higher toxicity could be associated with elevated levels of reactive oxygen species generated in the bacbot-treated cells. The multifunctional bacbots reported here provide an option for guided therapy with the natural variant of the human gut-friendly living bacteria without the need for attenuation or genetic modification

Living Gut Bacteria Functionalized with Gold Nanoclusters and Drug for Facile Cancer Theranostics

Bacbots are potent self-propelling vehicles for targeted therapy that can be guided by chemical and biochemical stimuli of the host. In addition, they can be guided externally by the use of magnetic field or other physical forces. The challenge is to incorporate drugs and diagnostic tools in living bacteria with retention of theranostic activity until reaching the targets and easy clearance of the remainder following the treatment. We report that living Lactobacillus rhamnosus, when functionalized with photoluminescent Au nanoclusters and the anticancer drug methotrexate, was cytotoxic to monolayer and spheroids of cancer cells (HeLa and HT29) even at a low dose of bacteria used (107 cfu/mL). The observed cell death was nearly 90% in HeLa spheroids and 70% in HT29 spheroids. Further, functionalization of the bacterial surface with the nanoclusters helped incorporate the drug onto their cell surfaces. The drug and nanocluster-loaded bacteria annihilated the cells and the spheroids in a rather short time (6 h) that revealed the specificity and effectiveness of the bacbots. The bacbots exhibited synergistic toxicity on the cells as their effect was more than the drug and the bacteria individually. This higher toxicity could be associated with elevated levels of reactive oxygen species generated in the bacbot-treated cells. The multifunctional bacbots reported here provide an option for guided therapy with the natural variant of the human gut-friendly living bacteria without the need for attenuation or genetic modification