Gangli Wang

Mixed dithiolate durene-DT and monothiolate phenylethanethiolate protected Au130 nanoparticles with discrete core and core-ligand energy states.

A new type of gold nanoparticle with interesting energetics has been created by employing a mixture of dithiol durene (Durene-DT) and monothiol phenylethanethiol (PhC2S) in the synthesis. The average composition of these mixed thiolate clusters is characterized to be Au(130)(Durene-DT)(29)(PhC2S)(22). Continuous quantized core charging behaviors were observed at lower potentials in voltammetric measurements, while ligand reaction and core-ligand interactions were observed at higher potentials. The absorbance spectrum displays discrete absorption bands at ca. 355, 490, 584, and 718 nm. The electrochemical and absorbance features are correlated through the determined energy states and charging energy. Broad near-IR luminescence was observed, associated with significant relaxation of excitation energy. Such interesting optical and electrochemical properties are attributed to the nanoparticle core size, ligand composition, and core-ligand charge delocalization determined by the dithiolate molecular structure.

Surface Charge Density Determination of Single Conical Nanopores Based on Normalized Ion Current Rectification

Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the nanodevices. In this article, an interesting trend is observed in steady-state current-potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate-solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by solving Poisson and Nernst-Planck equations in the simulation of the experimental results. The flux distribution inside the nanopore and the SCD of individual nanopores are reported. The respective diffusion and migration translocations are found to vary at different positions inside the nanopore. This knowledge is believed to be important for resistive pulse sensing applications because the detection signal is determined by the perturbation of the ion current by the analytes.

Structure of the thiolated Au130 cluster.

The structure of the recently discovered Au130-thiolate and -dithiolate clusters is explored in a combined experiment-theory approach. Rapid electron diffraction in scanning/transmission electron microscopy (STEM) enables atomic-resolution imaging of the gold core and the comparison with density functional theory (DFT)-optimized realistic structure models. The results are consistent with a 105-atom truncated-decahedral core protected by 25 short staple motifs, incorporating disulfide bridges linking the dithiolate ligands. The optimized structure also accounts, via time-dependent DFT (TD-DFT) simulation, for the distinctive optical absorption spectrum, and rationalizes the special stability underlying the selective formation of the Au130 cluster in high yield. The structure is distinct from, yet shares some features with, each of the known Au102 and Au144/Au146 systems.

Transmembrane Potential across Single Conical Nanopores and Resulting Memristive and Memcapacitive Ion Transport

Memristive and memcapacitive behaviors are observed from ion transport through single conical nanopores in SiO(2) substrate. In i-V measurements, current is found to depend on not just the applied bias potential but also previous conditions in the transport-limiting region inside the nanopore (history-dependent, or memory effect). At different scan rates, a constant cross-point potential separates normal and negative hysteresis loops at low and high conductivity states, respectively. Memory effects are attributed to the finite mobility of ions as they redistribute within the negatively charged nanopore under applied potentials. A quantative correlation between the cross-point potential and electrolyte concentration is established.

Fluorescence Intensity and Lifetime Cell Imaging with Luminescent Gold Nanoclusters

In this article, luminescent properties of gold nanoclusters (AuNCs) were studied at the single nanoparticle level and also used as novel imaging agents in cell media. Two types of water-soluble AuNCs which were stabilized with a monolayer composed of either mercaptosuccinic acid (MSA) or tiopronin thiolate ligands were synthesized by a chemical reduction reaction. These AuNCs were determined to have an average core diameter of less than 2 nm. On a time-resolved confocal microscope, the emission signals from the single AuNCs were distinctly recordable. The quantum yields of these AuNCs were measured to be ca. 5%. The lifetime of these AuNCs is also much longer than the lifetime of cellular autofluorescence in lifetime cell imaging as well as the lifetime of organic dye Alexa Fluor 488. After being derivatized with polyethylene glycol (PEG) moieties, the AuNCs were uploaded efficiently in the HeLa cells. Fluorescence intensity and lifetime cell images were recorded on the time-resolved confocal microscope in which the emission from the AuNCs was readily differentiated from the cellular autofluorescence background because of their relatively stronger emission intensities and longer lifetimes. These loaded nanoclusters in the cells were observed to widely distribute throughout the cells and especially densely loaded near the cell nucleuses. The AuNCs in the cells were also tested to have a better photostability relative to the organic fluorophores under the same conditions. We thus conclude that the AuNCs have a great potential as novel nanoparticle imaging agents, especially as lifetime imaging agents, in fluorescence imaging applications. We also prospect much broader applications of these AuNCs after further improvements of their luminescence quantum yields.

Quantification of Steady-State Ion Transport through Single Conical Nanopores and a Nonuniform Distribution of Surface Charges

Electrostatic interactions of mobile charges in solution with the fixed surface charges are known to strongly affect stochastic sensing and electrochemical energy conversion processes at nanodevices or devices with nanostructured interfaces. The key parameter to describe this interaction, surface charge density (SCD), is not directly accessible at nanometer scale and often extrapolated from ensemble values. In this report, the steady-state current-voltage (i-V) curves measured using single conical glass nanopores in different electrolyte solutions are fitted by solving Poisson and Nernst-Planck equations through finite element approach. Both high and low conductivity state currents of the rectified i-V curve are quantitatively fitted in simulation at less than 5% error. The overestimation of low conductivity state current using existing models is overcome by the introduction of an exponential SCD distribution inside the conical nanopore. A maximum SCD value at the pore orifice is determined from the fitting of the high conductivity state current, while the distribution length of the exponential SCD gradient is determined by fitting the low conductivity state current. Quantitative fitting of the rectified i-V responses and the efficacy of the proposed model are further validated by the comparison of electrolytes with different types of cations (K(+) and Li(+)). The gradient distribution of surface charges is proposed to be dependent on the local electric field distribution inside the conical nanopore.

Noninvasive Surface Coverage Determination of Chemically Modified Conical Nanopores that Rectify Ion Transport

Surface modification will change the surface charge density (SCD) at the signal-limiting region of nanochannel devices. By fitting the measured i-V curves in simulation via solving the Poisson and Nernst-Planck equations, the SCD and therefore the surface coverage can be noninvasively quantified. Amine terminated organosilanes are employed to chemically modify single conical nanopores. Determined by the protonation-deprotonation of the functional groups, the density and polarity of surface charges are adjusted by solution pH. The rectified current at high conductivity states is found to be proportional to the SCD near the nanopore orifice. This correlation allows the noninvasive determination of SCD and surface coverage of individual conical nanopores.

Transitions in Discrete Absorption Bands of Au130 Clusters upon Stepwise Charging by Spectroelectrochemistry

Rich and tunable physicochemical properties make noble metal clusters promising candidates as novel nanomolecules for a variety of applications. Spectroelectrochemistry analysis is employed to resolve previously inaccessible electronic transitions in Au130 clusters stabilized by a monolayer of di- and monothiolate ligands. Well-defined quantized double-layer charging of the Au core and oxidizable ligands make this Au130 nanocluster unique among others and enable selective electrolysis to different core and ligand charge states. Subsequent analysis of the corresponding absorption changes reveals that different absorption bands originate from different electronic transitions involving both metal core energy states and ligand molecular orbitals. Besides the four discrete absorption bands in the steady-state UV-visible-near-IR absorption spectrum, additional transitions otherwise not detectable are resolved upon selective addition/removal of electrons at cores and ligand energy states, respectively, upon electrolysis. An energy diagram is proposed that successfully explains the major features observed in electrochemistry and absorption spectroscopy. Those assignments are believed applicable and effective to explain similar transitions observed in some other Au thiolate clusters.

Physical Origin of Dynamic Ion Transport Features through Single Conical Nanopores at Different Bias Frequencies

Ionic transport through nanometer scale channels or interfaces is the physical origin of the detection signals in stochastic single molecular sensing, DNA sequencing and nano-structured electrodes. Dynamic transport regulated by systematically varying the bias frequency has not been explored. In this report, ion transport through single conical nanopore platforms is studied by applying an alternating electrical field at a wide range of scan rates. Rich frequency-dependent features of the measured transport current are discovered. The complete transition of characteristic transport features from low to high scan rates or bias frequencies is demonstrated experimentally. Key dynamic features include: multiple hysteresis loops separated by one or two non-zero cross points in the I–V curves, shifts in cross point potentials at different scan rates, and growth and diminishment in the hysteresis loops with normal and negative phase shifts. Combined theoretical and experimental studies reveal different processes contributing to and dominating the total current responses on different time scales. The respective contributions of each type of transport process to the overall measured current signals are quantitatively separated based on the fundamental insights gained. Although these exciting experimental observations are successfully simulated using an optimized model by solving PNP equations, the experimental studies at this nanoscale dimension suggest substantial deviations from a continuum regime. Inputs from molecular theory are needed to further validate the proposed physical mechanism.

Correlation of Ion Transport Hysteresis with the Nanogeometry and Surface Factors in Single Conical Nanopores

Better understanding in the dynamics of ion transport through nanopores or nanochannels is important for sensing, nucleic acid sequencing and energy technology. In this paper, the intriguing nonzero cross point, resolved from the pinched hysteresis current-potential (i-V) curves in conical nanopore electrokinetic measurements, is quantitatively correlated to the surface and geometric properties by simulation studies. The analytical descriptions of the conductance and potential at the cross point are developed: the cross-point conductance includes both the surface and volumetric conductance; the cross-point potential represent the overall/averaged surface potential difference across the nanopore. The impacts by individual parameter such as pore radius, half cone angle, and surface charges are systematically studied in the simulation that would be convoluted and challenging in experiments. The elucidated correlation is supported by and offer predictive guidance for experimental studies. The results also offer more quantitative and systematic insights in the physical origins of the concentration polarization dynamics in addition to ionic current rectification inside conical nanopores and other asymmetric nanostructures. Overall, the cross point serves as a simple yet informative analytical parameter to analyze the electrokinetic transport through broadly defined nanopore-type devices.