Buoyancy-Driven Micro/-Nanomotors: From Fundamentals to Applications.

The progression of self-powered micro/-nanomotors (MNMs) has rapidly evolved over the past few decades, showing applications in various fields such as nanotechnology, biomedical engineering, microfluidics, environmental science, and energy harvesting. Miniaturized MNMs transduce chemical/biochemical energies into mechanical motion for navigating through complex fluidic environments with directional control via external forces fields such as magnetic, photonic, and electric stimuli. Among various propulsion mechanisms, buoyancy-driven MNMs have received noteworthy recognition due to their simplicity, efficiency, and versatility. Buoyancy force-driven motors harness the principles of density variation-mediated force to overcome fluidic resistance to navigate through complex environments. Restricting the propulsion in one direction helps to control directional movement, making it more efficient in isotropic solutions. The changes in pH, ionic strength, chemical concentration, solute gradients, or the presence of specific molecules can influence the motion of buoyancy-driven MNMs as evidenced by earlier reports. This review aims to provide a fundamental and detailed analysis of the current state-of-the-art in buoyancy-driven MNMs, aiming to inspire further research and innovation in this promising field.

A Ratiometric and Visual Sensing of Phosphate by White Light Emitting Quantum Dot Complex.

Ratiometric and visual sensing of phosphate by using a white light emitting quantum dot complex (WLE QDC) is reported herein. The WLE QDC comprised of Mn2+-doped ZnS quantum dot (with λem = 585 nm) and surface zinc quinolate (ZnQS2) complex (with λem= 480 nm). The limit of detection was estimated to be of 5.9 nM in the linear range of 16.6-82.6 nM. This was accomplished by monitoring the variations in the photoluminescence color, intensity ratio (I480/I585), chromaticity and hue of the WLE QDC in the presence of phosphate. The high selectivity and sensitivity of WLE QDC toward phosphate was observed. The chemical interaction of ZnQS2 (present in WLE QDC) with phosphate might have led to the observed specificity in photoluminescence changes. The presented WLE QDC was successfully employed for the quantification of phosphate in samples prepared using environmental water and commercial fertilizer.

Physical insights into the facilitation of an unprecedented complexation reaction on the surface of a doped quantum dot leading to white light generation.

Herein, we report a complexation reaction between Zn2+ ions present on the surface of an orange-red-emitting environmentally sustainable Mn2+-doped ZnS QD and a non-emitting copper quinolate (CuQ2) complex, which leads to the formation of a greenish blue-emitting surface zinc quinolate (ZnQ2) complex. The synchronous contribution of the surface ZnQ2 complex and Mn2+-doped ZnS QD is directed towards the generation of photostable bright white light (at λex - 355 nm) with chromaticity coordinates of (0.34, 0.42), color rendering index (CRI) of 71 and color-correlated temperature (CCT) of 5046 K. The ZnQ2 complexed Mn2+-doped ZnS QD is herein called as quantum dot complex (QDC). The excitation- and time-dependent tunability in emission, chromaticity, CRI and CCT of QDC revealed their futuristic applications in light-emitting devices with an anticipated color output. The current work also shows the catalytic behavior of Mn2+-doped ZnS QDs towards facilitating the formation of surface ZnQ2 from CuQ2, which is not feasible with regard to the reactivity of CuQ2 under normal conditions according to the Irving-William series. The rate of the reaction was observed to be first order with respect to CuQ2 at 20 °C, and the complexation constant for the formation of ZnQ2 was estimated to be 8.3 × 105 M-1. This is important for understanding the surface chemistry of metal chalcogenide QDs towards complexation reactions.

Chemical Reactions Involving the Surface of Metal Chalcogenide Quantum Dots.

This invited feature article focuses on the chemical reactions involving the surface ions of colloidal quantum dots (Qdots). Emphasis is placed on ion-exchange, redox, and complexation reactions. The pursuit of reactions involving primarily the cations on the surface results in changes in the optical properties of the Qdots and also may confer new properties owing to the newly formed surface species. For example, the cation-exchange reaction, leading to systematic removal of the cations present on the as-synthesized Qdots, enhances the photoluminescence quantum yield. On the other hand, redox reactions, involving the dopant cations in the Qdots, could not only modulate the photoluminescence quantum yield but also give rise to new emission not present in the as-synthesized Qdots. Importantly, the cations present on the surface could be made to react with external organic ligands to form inorganic complexes, thus providing a new species defined as the quantum dot complex (QDC). In the QDC, the properties of Qdots and the inorganic complex are not only present but also enhanced. Furthermore, by varying reaction conditions such as the concentrations of the species and using a mixture of ligands, the properties could be further tuned and multifunctionalization of the Qdot could be achieved. Thus, chemical, magnetic, and optical properties could be simultaneously conferred on the same Qdot. This has helped in externally controlled bioimaging, white light generation involving individual quantum dots, and highly sensitive molecular sensing. Understanding the species (i.e., the newly formed inorganic complex) on the surface of the Qdot and its chemical reactivity provide unique options for futuristic technological applications involving a combination of an inorganic complex and a Qdot.

The nature of binding of quinolate complex on the surface of ZnS quantum dots.

We report that the Z-type binding rather than X-type binding was favored when 8-hydroxyquinoline (HQ) reacted with presynthesized ZnS quantum dots (Qdots) to form surface zinc quinolinate complexes having a preferred stoichiometry of 1 : 2 (surface Zn2+ : HQ). Importantly, the higher solubility in polar solvents and high desorption coefficient (following Langmuir binding isotherm) of HQ-treated ZnS Qdot in DMSO solvent compared with those in methanol clearly indicated the favorable Z-type binding of HQ and thus the formation of surface octahedral ZnQ2 complex. Furthermore, the characteristics peaks in the 1H-nuclear magnetic resonance (NMR) spectrum of the desorbed species and the ligand density calculation of the surface complex (formed due to the reaction between HQ and ZnS Qdot) supported the octahedral ZnQ2 complex formation. Interestingly, the presence of dangling sulphide and the loss of planarity of ZnQ2 complex on the surface of ZnS Qdots (in turn gaining structural rigidity) may be the reasons for the Z-type binding of HQ. The specific binding might be the reason for superior optical properties and thermal stability of the surface ZnQ2 complex compared to the free ZnQ2 complex as such. The results can be considered important towards understanding the coordination chemistry of inorganic complex on the surface of Qdots and thus for their application potential.

A White Light-Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile.

The interaction of the neurotransmitter dopamine is reported with a single particle white light-emitting (WLE) quantum dot complex (QDC). The QDC is composed of yellow emitting ZnO quantum dots (Qdots) and blue emitting Zn(MSA)2 complex (MSA = N-methylsalicylaldimine) synthesized on their surfaces. Sensing is achieved by the combined changes in the visual luminescence color from white to blue, chromaticity color coordinates from (0.31, 0.33) to (0.24, 0.23) and the ratio of the exponents (αon /αoff ) of on/off probability distribution (from 0.24 to 3.21) in the blinking statistics of WLE QDC. The selectivity of dopamine toward ZnO Qdots, present in WLE QDC, helps detect ≈13 dopamine molecules per Qdot. Additionally, the WLE QDC exhibits high sensitivity, with a limit of detection of 3.3 × 10-9 m (in the linear range of 1-100 × 10-9 m) and high selectivity in presence of interfering biological species. Moreover, the single particle on-off bilking statistics based detection strategy may provide an innovative way for ultrasensitive detection of analytes.

Surface Complexed ZnO Quantum Dot for White Light Emission with Controllable Chromaticity and Color Temperature.

We report the formation of blue emitting Zn(MSA)2 complex on the surface of a yellow emitting ZnO quantum dot (Qdot)-out of a complexation reaction between N-methylsalicylaldimine (MSA) and ZnO Qdot. This led to formation of a highly luminescent, photostable, single-component nanocomposite that emits bright natural white light, with (i) chromaticities of (0.31, 0.38) and (0.31, 0.36), (ii) color rendering indices (CRI) of 74 and 82, and (iii) correlated color temperatures (CCT) of 6505 and 6517 K in their solution and solid phases, respectively. Importantly, the control over the chromaticity and CCT-depending upon the degree of complexation-makes the reported nanocomposite a potential new advanced material in fabricating cost-effective single-component white light emitting devices (WLED) of choice and design in the near future.

Gold Nanocluster Embedded Albumin Nanoparticles for Two-Photon Imaging of Cancer Cells Accompanying Drug Delivery.

Gold nanoclusters in albumin nanoparticles (nanovehicles) are used for single-photon and two-photon imaging of cancer cells following the delivery of doxorubicin through the nanovehicle. NIR excitation and emission wavelengths in the biological window (650-900 nm) make the nanovehicle an ideal potential platform for imaging guided drug delivery.

Double channel emission from a redox active single component quantum dot complex.

Herein we report the generation and control of double channel emission from a single component system following a facile complexation reaction between a Mn(2+) doped ZnS colloidal quantum dot (Qdot) and an organic ligand (8-hydroxy quinoline; HQ). The double channel emission of the complexed quantum dot-called the quantum dot complex (QDC)-originates from two independent pathways: one from the complex (ZnQ2) formed on the surface of the Qdot and the other from the dopant Mn(2+) ions of the Qdot. Importantly, reaction of ZnQ2·2H2O with the Qdot resulted in the same QDC formation. The emission at 500 nm with an excitation maximum at 364 nm is assigned to the surface complex involving ZnQ2 and a dangling sulfide bond. On the other hand, the emission at 588 nm-with an excitation maximum at 330 nm-which is redox tunable, is ascribed to Mn(2+) dopant. The ZnQ2 complex while present in QDC has superior thermal stability in comparison to the bare complex. Interestingly, while the emission of Mn(2+) was quenched by an electron quencher (benzoquinone), that due to the surface complex remained unaffected. Further, excitation wavelength dependent tunability in chromaticity color coordinates makes the QDC a potential candidate for fabricating a light emitting device of desired color output.

Surface complexation reaction for phase transfer of hydrophobic quantum dot from nonpolar to polar medium.

Chemical reaction between oleate-capped Zn(x)Cd(1-x)S quantum dots (Qdots) and 8-hydroxyquinoline (HQ) led to formation of a surface complex, which was accompanied by transfer of hydrophobic Qdots from nonpolar (hexane) to polar (water) medium with high efficiency. The stability of the complex on the surface was achieved via involvement of dangling sulfide bonds. Moreover, the transferred hydrophilic Qdots--herein called as quantum dot complex (QDC)--exhibited new and superior optical properties in comparison to bare inorganic complexes with retention of the dimension and core structure of the Qdots. Finally, the new and superior optical properties of water-soluble QDC make them potentially useful for biological--in addition to light emitting device (LED)--applications.

Surfactant-Assisted Förster Resonance Energy Transfer from a Quantum Dot Complex for Highly Stable White Light Emission.

Herein we report the fabrication of a surfactant modified quantum dot complex (S-QDC, having λem = 485 nm) nanocomposite (composed of cetyltrimethylammonium bromide surfactants and a zinc-quinolate complex attached ZnS quantum dot), the donor capability of S-QDC in Förster resonance energy transfer (FRET) with an acceptor organic molecule (λem = 573 nm), and finally their utilization in the FRET-based white light emission having features near to mid-day sunlight. The Förster distance, energy transfer efficiency, donor-acceptor distance, number of binding sites, and binding constant are evaluated to be 3.48 nm, 85.74%, 2.58 nm, 0.94, and 1.87 × 104 M-1, respectively, for the current electrostatically driven FRET pair. The solid polymer coated FRET pair composite emits white light having chromaticity color coordinates of (0.33, 0.33) and correlated color temperature of 5350 K and also shows long-term atmospheric white luminescence stability up to 30 days, photostability, and thermal stability with preservation of their pristine morphology.

Nanometal surface energy transfer (NSET) from biologically active heterocyclic ligands to silver nanoparticles induces enhanced antimicrobial activity against gram-positive bacteria.

Herein we report the formation of a nanometal surface energy transfer (NSET) pair between a donor biologically active heterocyclic luminescent ligand such as 3-(1,3-Dioxoisoindolin-2-yl)-N, N-dimethylpropan-1-ammonium perchlorate (S4PNL; λem-408 nm) and an acceptor silver nanoparticle (Ag NP; λabs-406 nm). When the S4PNL ligand interacts with Ag NPs, the quenching in their luminescence intensity at 408 nm is noticed, with a Stern-Volmer constant of 0.8 × 104 M-1. The present donor-acceptor pair displays a binding constant of 2.8 × 104 M-1 and binding sites of 1.12. The current work shows the energy transfer from a molecular dipole (S4PNL) to a nanometal surface (Ag NP) and thus follows the nanometal surface energy transfer (NSET) ruler with an energy transfer efficiency of 80.0%, 50% energy transfer efficiency distance (d0) of 4.9 nm, donor-acceptor distance of 3.4 nm. The alteration in the zeta potential value of S4PNL upon interaction with AgNP clearly demonstrates the strong electrostatic interaction between donor and acceptor. Importantly, the current NSET pair shows enhanced antimicrobial activity against gram-positive bacteria such as Bacillus cereus (B. cereus) in comparison to their parent components i.e. S4PNL ligand and Ag NP. The NSET pair shows maximum inhibition against B. cereus (9202.21 ± 463.26 CFU/ml.) at 10% while minimum inhibition is observed at 0.01% of it (39,887.19 ± 242.67 CFU/ml.).

Chemical coupling of halide perovskite nanocrystals with a metal quinolate complex for white light generation.

Herein we report the construction of a white light emitting (WLE) nanocomposite by chemically coupling halide perovskite nanocrystals (HPNCs; e.g., orange-emitting Mn2+-doped CsPbCl3) with a metal quinolate complex (e.g., a cyan-emitting calcium quinolate (CaQ2) complex) while keeping their distinct features. The surface chloride of HPNCs coupled with the Ca-metal center of the CaQ2 complex without altering the morphology, size, and dopant oxidation state of the HPNCs and provided additional environmental stability of the WLE nanocomposite. The photostable solid WLE nanocomposite displays chromaticity of (0.33, 0.32), color rendering index (CRI) of 80, correlated color temperature (CCT) of 5483 K, and quantum yield of 54.1%. This clearly indicates their bright WLE nature with properties close to those of bright midday sunlight. The current work will bring new surface chemistry between HPNCs and inorganic complexes and new paradigm toward advanced light emitting applications.

Halide-Driven Halogen-Hydrogen Bonding versus Chelation in Perovskite Nanocrystals: A Concept of Charge Transfer Bridging.

The choice of surface functionalized ligands to encapsulate semiconductor nanocrystals (NCs) is important for tailoring their optoelectronic properties. We use a small bidentate 8-hydroxyquinoline (HQ) molecule to surface functionalize CsPbX3 perovskite NCs (X = Cl, Br, I), along with traditional long-chain monodentate ligands. Our experimental results using optical and ultrafast spectroscopy depict a halogen-hydrogen bonding formation in the HQ functionalized CsPbCl3 and CsPbBr3 NCs, which act as a charge transfer (CT) bridging for the interfacial hole transfer from the NCs to the HQ molecule as fast as 540 fs. In contrast, weak chelation is observed for HQ-coupled CsPbI3 NCs without an active CT process. We explain two distinct surface coupling mechanisms via the polarizability of halides and larger PbI64- octahedral cage size. Control of two contrasting halide-dependent surface coupling phenomena of a small molecule that further regulate the CT process may have significant implications in their development in optoelectronics.

Surfactant-mediated enhanced FRET from a quantum-dot complex for ratiometric sensing of food colorants.

Herein we report that a surfactant modified quantum dot-complex (S-QDC; with λem-515 nm) nanocomposite, as a donor fluorophore, exhibits enhanced Förster resonance energy transfer (FRET) efficiency to an acceptor organic dye (λem-576 nm) in comparison to only the QDC. The proposed S-QDC (consisting of a ZnS quantum dot, zinc quinolate inorganic complex and cetyltrimethylammonium bromide (CTAB) surfactant) provides the unique and selective ratiometric visual detection of organic dyes present as food colorants in commercial chili powder, tomato ketchup and mixed fruit jam. Notably, the S-QDC shows a limit of detection (LOD) as low as 2.2 nM in the linear range of 0.17-4.89 µM for food colorants. Furthermore, the present work will bring new possibilities to unravelling the chemistry among surfactants, inorganic complexes and quantum dots to make newer optical materials with futuristic scope of utilization ranging from optical sensors to light emitting devices.

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 Mn(2+)-doped ZnS Qdots, when treated with different amounts of CB, resulted in two kinds of changes in the emission due to Mn(2+) 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 Mn(2+) 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 Mn(2+) (and also Zn(2+)) ions present on the surfaces of Qdots in the form of clusters as well as individual ions.

Hybrid Lead Bromide Perovskite Single Crystals Coupled with a Zinc(II) Complex for White Light Emission.

Herein we report the fabrication of green emitting hybrid lead bromide perovskite single crystals (HLBPSCs), their anion exchange mediated tunable yellow luminescence and thereby their coupling ability with blue emitting inorganic complex leading to generation of a photostable white light emission, with properties close to bright day sunlight. The partial anion exchange reaction to green emitting HLBPSCs led to formation of yellow emitting anion exchanged HLBPSCs─which are termed as AE-HLBPSCs herein. Then, AE-HLBPSCs were chemically combined with blue emitting Zn-aspirin complex to produce white light with a photoluminescence quantum yield (PLQY) of 47.7%. The solid form of the white light emitting (WLE) composite (followed by coating with poly methyl methacrylate─PMMA) showed color coordinates of (0.34, 0.33), color rendering index of 76 and correlated color temperature of 5282 K. Furthermore, the PMMA coated inorganic complex coupled AE-HLBPSCs showed the preservation of their WLE nature and luminescence stability in their solid form.

The quantum dot-FRET-based detection of vitamin B12 at a picomolar level.

Herein we report the picomolar level detection of vitamin B12 (VB12) using orange-red emitting ligand-free Mn2+-doped ZnS quantum dots (QDs; λ em = 587 nm) in an aqueous dispersion. Sensing was achieved following the quenching of the luminescence of the Mn2+-doped ZnS QDs with an increasing concentration of VB12. The Stern-Volmer constant was determined to be 5.2 × 1010 M-1. Importantly, the Mn2+-doped ZnS QDs exhibited high sensitivity towards VB12, with a limit of detection as low as 1.15 ± 0.06 pM (in the linear range of 4.9-29.4 pM) and high selectivity in the presence of interfering amino acids, metal ions, and proteins. Notably, a Förster resonance energy transfer (FRET) mechanism was primarily proposed for the observed quenching of luminescence of Mn2+-doped ZnS QDs upon the addition of VB12. The Förster distance (R o) and energy transfer efficiency (E) were calculated to be 2.33 nm and 79.3%, respectively. Moreover, the presented QD-FRET-based detection may bring about new avenues for future biosensing applications.

Luminescence Enhancement based Sensing of L-Cysteine by Doped Quantum Dots.

The interaction of a presynthesized orange emitting Mn2+ -doped ZnS quantum dots (QDs) with L-Cysteine (L-Cys) led to enhance emission intensity (at 596 nm) and quantum yield (QY). Importantly, the Mn2+ -doped ZnS QDs exhibited high sensitivity towards L-Cys, with a limit of detection of 0.4±0.02 µM (in the linear range of 3.3-13.3 µM) and high selectivity in presence of interfering amino acids and metal ions. The association constant of L-Cys was determined to be 0.36×105  M-1 . The amplified passivation of the surface of Mn2+ -doped ZnS QDs following the incorporation and binding of L-Cys is accounted for the enhancement in their luminescence features. Moreover, the luminescence enhancement-based detection will bring newer dimension towards sensing application.

A two-target responsive reversible ratiometric pH nanoprobe: a white light emitting quantum dot complex.

Herein we report the use of a white light emitting quantum dot complex (comprising an orange emitting Mn2+-doped ZnS quantum dot and greenish-blue emitting zinc-quinolate complex) as a two-target responsive ratiometric reversible pH nanosensor in the physiological range of 6.5-10.3, following changes in their luminescence intensity ratio, color and chromaticity.