Construction of single fluorophore ratiometric pH sensors using dual-emission Mn(2+)-doped quantum dots.

We present a novel ratiometric pH sensor design using water-soluble, dual-emission, Mn(2+)-doped quantum dots (Qdots) decorated with D-penicillamine (DPA-MnQdots). In contrast to more commonly used ratiometric pH sensors that rely on the coupling of two fluorophores, our design uses only a single emitter, which simplifies ratiometric sensing and broadens the applications of the sensor. Our single-emitter DPA-MnQdots exhibit two emission bands, at 510nm (green) and 610nm (red), which are, respectively, attributable to exciton recombination and emission of the Mn(2+) dopants. The emission intensity ratio (I510/I610) of the DPA-MnQdots depends linearly on surrounding pH values within physiological conditions (from pH 4.5 to 8.5). Moreover, the biocompatible DPA-MnQdots were used for long-term monitoring of local pH values in HeLa cells.

Local pH tracking in living cells.

Continuous and simultaneous 3D single-particle movement and local pH detection in HeLa cells were demonstrated for the first time by combining fluorescent mesoporous silica nanoparticles (FMSNs) and a single-particle tracking (SPT) technique with a precision of ∼10 nm. FMSNs, synthesized by the co-condensation of both pH-sensitive and reference dyes with a silica/surfactant source, allow long-term reliable ratiometric pH measurements with a precision better than 0.3 pH unit because of their excellent brightness and stability. pH variation in the surrounding area of FMSNs during endocytosis was monitored in real-time. Acidification and low mobility of FMSNs were observed at the early endocytic stage, whereas basification and high mobility of FMSNs were observed at the late stage. Our results indicate that it is possible to monitor local pH changes in the environments surrounding nanoparticles during the cellular uptake process of FMSNs, which provides much needed information for designing an efficient drug delivery nanosystem.

Discovery of protein- and DNA-imperceptible nanoparticle hard coating using gel-based reaction tuning.

The seemingly inevitable protein corona appears to be an insurmountable obstacle to wider application of functional nanomaterials in biotechnology. The accumulation of serum proteins can block targeting functionalities and alter the in vivo fate of these nanomaterials. Here we demonstrate a method to generate non-stick, robustly passivated functional nanoparticles (NPs) using a tailored silica coating. We apply agarose gel electrophoresis to sensitively evaluate protein binding to NPs with different surface chemistry. Using gel banding and retardation as a read-out for protein adsorption, we optimize the surface chemistry to yield a mixed charge surface which displays remarkable binding resistance to a wide range of serum proteins and nucleic acids. The hard silica shell also protects the functional NP core in harsh environments (down to pH 1) while still showing the ability to be targeted for cellular uptake with little or no non-specific binding.

Mechanodelivery of nanoparticles to the cytoplasm of living cells.

Nanotechnology has opened up the opportunity to probe, sense, and manipulate the chemical environment of biological systems with an unprecedented level of spatiotemporal control. A major obstacle to the full realization of these novel technologies is the lack of a general, robust, and simple method for the delivery of arbitrary nanostructures to the cytoplasm of intact live cells. Here, we identify a new delivery modality, based on mechanical disruption of the plasma membrane, which efficiently mediates the delivery of nanoparticles to the cytoplasm of mammalian cells. We use two distinct execution modes, two adherent cell lines, and three sizes of semiconducting nanocrystals, or quantum dots, to demonstrate its applicability and effectiveness. As the underlying mechanism is purely physical, we anticipate that such "mechanodelivery" can be generalized to other modes of execution as well as to the cytoplasmic introduction of a structurally diverse array of functional nanomaterials.

Effect of surfactant and solvent on spin-lattice relaxation dynamics of magnetic nanocrystals.

The effect of varying the surfactant and solvent medium on the dynamics of spin-lattice relaxation in photoexcited Fe3O4 nanocrystals has been investigated by measuring the time-dependent magnetization employing pump-probe transient Faraday rotation technique. The variation of the surfactants having surface-binding functional groups modified not only the static magnetization but also the dynamics of the recovery of the magnetization occurring via spin-lattice relaxation in the photoexcited Fe3O4 nanocrystals. The variation of the polarity and size of the solvent molecules can also influence the spin-lattice relaxation dynamics. However, the effect is limited to the nanocrystals having sufficiently permeable surfactant layer, where the small solvent molecules (e.g., water) can access the surface and dynamically modify the ligand field on the surface.

An accessible approach to preparing water-soluble Mn2+-doped (CdSSe)ZnS (core)shell nanocrystals for ratiometric temperature sensing.

A new synthetic scheme allowing structural modifications to temperature-sensitive and water-soluble D-penicillamine-passivated Mn(2+)-doped (CdSSe)ZnS (core)shell nanocrystals (MnQDs) was reported using air-stable chemicals. The temperature-dependent optical properties of the nanocrystals were tuned by changing their structure and composition--the ZnS shell thickness and the Mn(2+)-dopant concentration. Thick ZnS shells significantly reduce the interference of nonradiative transitions on ratiometric emission intensities. High-dopant concentration affords consistent temperature sensitivity. In addition to the new base structure for quantum dot ratiometric temperature sensing via flexible, glovebox-free routes, the results also underscore the generalizability of the emission intensity ratio scheme for temperature sensing, originally proposed for rare-earth-doped materials.

Time-resolved study of surface spin effect on spin-lattice relaxation in Fe3O4 nanocrystals.

The role of surface spins in the relaxation of magnetization in optically excited Fe(3)O(4) nanocrystals has been investigated via time-resolved Faraday rotation measurements. The time scale of the magnetization recovery following the optically induced demagnetization increased from 250 to 350 ps as the size of the nanocrystals increased from 5 to 15 nm. From analysis of the data with a simple model relating the spin-lattice relaxation rate to the average spin-orbit coupling strength of the interior and surface spins, we estimated the relative efficiency of surface in spin-lattice relaxation with respect to the interior spins. Our analysis indicates that, in Fe(3)O(4) nanocrystals passivated with oleic acid, the surface is 3 times more efficient in spin-lattice relaxation than the interior.

Effects of ion solvation and volume change of reaction on the equilibrium and morphology in cation-exchange reaction of nanocrystals.

The effects of cation solvation and the volume change (Delta V) of reaction on the equilibrium and the morphology change in the cation-exchange reactions of metal chalcogenide nanocrystals, CdE --> M(x)E(y) (E = S, Se, Te; M = Pd, Pt), were investigated. Since the solvation of cations is an important controllable factor determining the free energy of the reaction, the effect of varying cation solvation conditions on the equilibrium of the reaction was examined. A two-phase solvent environment, where the cations involved in the exchange reaction were preferentially solvated in different phases by using selective cation complexing molecules, was particularly efficient in increasing the thermodynamic driving force. The effect of Delta V of reaction on the morphology of the product nanocrystals was also investigated. Depending on the stress developed in the lattice during the reaction, product nanocrystals underwent varying degrees of morphological changes such as void formation and fragmentation in addition to the preservation of the original morphology of the reactant nanocrystals. The knowledge of the effect of ion solvation and Delta V of reaction on the equilibrium and product morphology provides a new strategy and useful guides to the application of cation-exchange reactions for the synthesis of a broader range of inorganic nanocrystals.

Size-dependent ultrafast magnetization dynamics in iron oxide (Fe3O4) nanocrystals.

Optically induced ultrafast demagnetization and its recovery in superparamagnetic colloidal iron oxide (Fe3O4) nanocrystals have been investigated via time-resolved Faraday rotation measurements. Optical excitation with near-infrared laser pulse resulted in ultrafast demagnetization in approximately 100 fs via the destruction of ferrimagnetic ordering. The degree of demagnetization increased with the excitation density, and the complete demagnetization reached at approximately 10% excitation density. The magnetization recovered on two time scales, several picoseconds and hundreds of picoseconds, which can be associated with the initial reestablishment of the ferrimagnetic ordering and the electronic relaxation back to the ground state, respectively. The amplitude of the slower recovery component increased with the size of the nanocrystals, suggesting the size-dependent ferrimagnetic ordering throughout the volume of the nanocrystal.

Ultrafast energy transfer and strong dynamic non-condon effect on ligand field transitions by coherent phonon in gamma-Fe2O3 nanocrystals.

Relaxation dynamics of an optically excited ligand field state and strong modulation of oscillator strengths of ligand field transitions by coherent acoustic phonon in gamma-Fe(2)O(3) nanocrystals were investigated through transient absorption measurements. A near-infrared pump beam prepared the lowest excited ligand field state of Fe(3+) ions preferentially on the tetrahedral coordination site. A time-delayed visible probe beam monitored the dynamics of various ligand field transitions and modification of their oscillator strengths by a coherent lattice motion. Transient absorption data exhibited dynamic features of a few distinct time scales, 100 fs, 1 ps, and 17-100 ps, as well as intense oscillatory features resulting from a coherent acoustic phonon. The initial decay of the induced absorption in 100 fs has been attributed to the exchange interaction-mediated energy transfer from the tetrahedral to octahedral Fe(3+) sites. The dynamics of slower time scales were assigned to the vibrational and electronic relaxations. Excitation of the ligand field state created a coherent acoustic phonon resulting in unusually intense modulation of the transient absorption signal despite its predominantly local nature and relatively small vibronic coupling. Excitation of each Fe(3+) ion in the nanocrystal was estimated to modulate up to 60% of its contribution to the total absorption intensity of the nanocrystal. The intense modulation of the absorption has been attributed to the strongly modulated oscillator strength of the ligand field transitions rather than oscillating Frank-Condon overlap. Dynamic modification of the metal-ligand orbital overlap and exchange interaction between the neighboring metal ions are the main factors responsible for the modulation of the oscillator strength.

In situ generation of the silica shell layer--key factor to the simple high yield synthesis of silver nanowires.

a-Silica encapsulated silver nanowires (diameter of 25 +/- 5 nm, average length of 10 mum) have been synthesized by reacting (Me3Si)4Si with AgNO3 in nearly quantitative yield. Formation of the a-silica shell layer (1-3 nm) in situ appears to be one of the most important factors in this simple process.