Imparting Stability to Chiral Helical Gold Nanoparticle Superstructures.

Realizing the promise of chiral inorganic nanomaterials hinges on improving their structural stability under various chemical and environmental conditions. Here, we examine the stability of 1-D gold nanoparticle (Au NP) single helices prepared using the amphiphilic peptide conjugate Cx-(PEPAuM-ox)2 (PEPAuM-ox = AYSSGAPPMoxPPF; x = 16-22). We present a general template-independent strategy of tuning helix stability that relies on controlling the dimensions of constituent NPs. As NP dimensions increase, Au NP single helices become both more thermally stable and more stable in the presence of chemical denaturants and protein digestion agents (e.g., urea and proteinase K, respectively). We use this strategy for imparting helix stability to create colloidal suspensions of thermally robust Au NP single helices which maintain their plasmonic chiroptical activity up to ∼80 °C.

Resonance Raman intensity analysis of photoactive metal-organic frameworks.

Metal-organic frameworks (MOFs) are promising candidate materials for photo-driven processes. Their crystalline and tunable structure makes them well-suited for placing photoactive molecules at controlled distances and orientations that support processes such as light harvesting and photocatalysis. In order to optimize their performance, it is important to understand how these molecules evolve shortly after photoexcitation. Here, we use resonance Raman intensity analysis (RRIA) to quantify the excited state nuclear distortions of four modified UiO-68 MOFs. We find that stretching vibrations localized on the central ring within the terphenyl linker are most distorted upon interaction with light. We use a combined computational and experimental approach to create a picture of the early excited state structure of the MOFs upon photoactivation. Overall, we show that RRIA is an effective method to probe the excited state structure of photoactive MOFs and can guide the synthesis and optimization of photoactive designs.

Reversible solvent interactions with UiO-67 metal-organic frameworks.

The utility of UiO-67 Metal-Organic Frameworks (MOFs) for practical applications requires a comprehensive understanding of intermolecular host-guest MOF-analyte interactions. To investigate intermolecular interactions between UiO-67 MOFs and complex molecules, it is useful to evaluate the interactions with simple polar and non-polar analytes. This problem is approached by investigating the interactions of polar (acetone and isopropanol) and non-polar (n-heptane) molecules with functionalized UiO-67 MOFs via temperature programmed desorption mass spectrometry and temperature programmed Fourier transform infrared spectroscopy. We find that isopropanol, acetone, and n-heptane bind reversibly and non-destructively to UiO-67 MOFs, where MOF and analyte functionality influence relative binding strengths (n-heptane ≈ isopropanol > acetone). During heating, all three analytes diffuse into the internal pore environment and directly interact with the µ3-OH groups located within the tetrahedral pores, evidenced by the IR response of ν(µ3-OH). We observe nonlinear changes in the infrared cross sections of the ν(CH) modes of acetone, isopropanol, and n-heptane following diffusion into UiO-67. Similarly, acetone's ν(C=O) infrared cross section increases dramatically when diffused into UiO-67. Ultimately, this in situ investigation provides insights into how individual molecular functional groups interact with UiO MOFs and enables a foundation where MOF interactions with complex molecular systems can be evaluated.

Single Amino Acid Modifications for Controlling the Helicity of Peptide-Based Chiral Gold Nanoparticle Superstructures.

Assembling nanoparticles (NPs) into well-defined superstructures can lead to emergent collective properties that depend on their 3-D structural arrangement. Peptide conjugate molecules designed to both bind to NP surfaces and direct NP assembly have proven useful for constructing NP superstructures, and atomic- and molecular-level alterations to these conjugates have been shown to manifest in observable changes to nanoscale structure and properties. The divalent peptide conjugate, C16-(PEPAu)2 (PEPAu = AYSSGAPPMPPF), directs the formation of one-dimensional helical Au NP superstructures. This study examines how variation of the ninth amino acid residue (M), which is known to be a key Au anchoring residue, affects the structure of the helical assemblies. A series of conjugates of differential Au binding affinities based on variation of the ninth residue were designed, and Replica Exchange with Solute Tempering (REST) Molecular Dynamics simulations of the peptides on an Au(111) surface were performed to determine the approximate surface contact and to assign a binding score for each new peptide. A helical structure transition from double helices to single helices is observed as the peptide binding affinity to the Au(111) surface decreases. Accompanying this distinct structural transition is the emergence of a plasmonic chiroptical signal. REST-MD simulations were also used to predict new peptide conjugate molecules that would preferentially direct the formation of single-helical AuNP superstructures. Significantly, these findings demonstrate how small modifications to peptide precursors can be leveraged to precisely direct inorganic NP structure and assembly at the nano- and microscale, further expanding and enriching the peptide-based molecular toolkit for controlling NP superstructure assembly and properties.

Crystallographic Mapping and Tuning of Water Adsorption in Metal-Organic Frameworks Featuring Distinct Open Metal Sites.

Crucial steps toward designing water sorption materials and fine-tuning their properties for specific applications include precise identification of adsorption sites and establishment of rigorous molecular-level insight into the water adsorption process. We report stepwise crystallographic mapping and density functional theory computations of adsorbed water molecules in ALP-MOF-1, a metal-organic framework decorated with distinct open metal sites and carbonyl functional groups that serve as water anchoring sites for seeding the nucleation of a complex water network. Identification of an unusual water adsorption step in ALP-MOF-1 motivated the tuning of metal ion composition to adjust water uptake. These studies provide direct evidence that the identity of the open metal sites in MOFs can dramatically affect water adsorption behavior between 0 and ∼20% RH and that multiple proximal water anchoring sites along the MOF skeleton facilitate water uptake which could be potentially useful for applications requiring rapid and energetically facile water sorption.

Leveraging Peptide Sequence Modification to Promote Assembly of Chiral Helical Gold Nanoparticle Superstructures.

Peptide conjugate molecules comprising a gold-binding peptide (e.g., AYSSGAPPMPPF) attached to an aliphatic tail have proven to be powerful agents for directing the synthesis and assembly of gold nanoparticle superstructures, in particular chiral helices having interesting plasmonic chiroptical properties. The composition and structure of these molecular agents can be tailored to carefully tune the structure and properties of gold nanoparticle single and double helices. To date, modifications to the ß-sheet region (AYSSGA) of the peptide sequence have not been exploited to control the metrics and assembly of such superstructures. We report here that systematic peptide sequence variation in a series of gold-binding peptide conjugate molecules can be leveraged not only to affect the assembly of peptide conjugates but also to control the synthesis, assembly, and optical properties of gold nanoparticle superstructures. Depending upon the hydrophobicity of a single-amino acid variant, the conjugates yield either dispersed gold nanoparticles or helical superstructures. These results provide evidence that subtle changes to peptide sequence, via single-amino acid variation in the ß-sheet region, can be leveraged to program structural control in chiral gold nanoparticle superstructures.

Size Discrimination of Carbohydrates via Conductive Carbon Nanotube@Metal Organic Framework Composites.

Traditional chemical sensing methodologies have typically relied on the specific chemistry of the analyte for detection. Modifications to the local environment surrounding the sensor represent an alternative pathway to impart selective differentiation. Here, we present the hybridization of a 2-D metal organic framework (Cu3(HHTP)2) with single-walled carbon nanotubes (SWCNTs) as a methodology for size discrimination of carbohydrates. Synthesis and the resulting conductive performance are modulated by both mass loading of SWCNTs and their relative oxidation. Liquid gated field-effect transistor (FET) devices demonstrate improved on/off characteristics and differentiation of carbohydrates based on molecular size. Glucose molecule detection is limited to the single micromolar concentration range. Molecular Dynamics (MD) calculations on model systems revealed decreases in ion diffusivity in the presence of different sugars as well as packing differences based on the size of a given carbohydrate molecule. The proposed sensing mechanism is a reduction in gate capacitance initiated by the filling of the pores with carbohydrate molecules. Restricting diffusion around a sensor in combination with FET measurements represents a new type of sensing mechanism for chemically similar analytes.

Breast Cancer Targeting of a Drug Delivery System through Postsynthetic Modification of Curcumin@N3-bio-MOF-100 via Click Chemistry.

Metal-organic frameworks (MOFs) offer many opportunities for applications across biology and medicine. Their wide range of chemical composition makes toxicologically acceptable formulation possible, and their high level of functionality enables possible applications as delivery systems for therapeutics agents. Surface modifications have been used in drug delivery systems to minimize their interaction with the bulk, improving their specificity as targeted carriers. Herein, we discuss a strategy to achieve a tumor-targeting drug-loaded MOF using "click" chemistry to anchor functional folic acid (FA) molecules on the surface of N3-bio-MOF-100. Using curcumin (CCM) as an anticancer drug, we observed drug loading encapsulation efficiencies (DLEs) of 24.02 and 25.64% after soaking N3-bio-MOF-100 in CCM solutions for 1 day and 3 days, respectively. The success of postsynthetic modification of FA was confirmed by 1H NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR), and liquid chromatography-mass spectrometry (LC-MS). The stimuli-responsive drug release studies demonstrated an increase of CCM released under acidic microenvironments. Moreover, the cell viability assay was performed on the 4T1 (breast cancer) cell line in the presence of CCM@N3-bio-MOF-100 and CCM@N3-bio-MOF-100/FA carriers to confirm its biological compatibility. In addition, a cellular uptake study was conducted to evaluate the targeting of tumor cells.

Ship-in-a-Bottle Preparation of Long Wavelength Molecular Antennae in Lanthanide Metal-Organic Frameworks for Biological Imaging.

While metal-organic frameworks (MOFs) have been identified as promising materials for sensitizing near-infrared emitting lanthanide ions (Ln3+) for biological imaging, long-wavelength excitation of such materials requires large, highly delocalized organic linkers or guest-chromophores. Incorporation of such species generally coincides with fewer Ln3+ emitters per unit volume. Herein, the excitation bands of ytterbium-based MOFs are extended to 800 nm via the postsynthetic coupling of acetylene units to form a high density of conjugated π-systems throughout MOF pores. The resulting long wavelength excitation/absorption bands are a synergistic property of the composite material as they are not observed in the individual organic components after disassociation of the MOFs, thus circumventing the need for large organic chromophores. We demonstrate that the long wavelength excitation and emission properties of these modified MOFs are maintained in the biological conditions of cell culture (aqueous environment, salts, heating), pointing toward their promising use for biological imaging applications.

Luminescence "Turn-On" Detection of Gossypol Using Ln3+-Based Metal-Organic Frameworks and Ln3+ Salts.

Gossypol (Gsp), a natural toxin concentrated in cottonseeds, poses great risks to the safe consumption of cottonseed products, which are used extensively throughout the food industry. In this work, we report the first luminescence "turn-on" sensors for Gsp using near-infrared emitting lanthanide (Ln3+) materials, including Ln3+ MOFs and Ln3+ salts. We first demonstrate that the Yb3+ photoluminescence of a Yb3+ MOF, Yb-NH2-TPDC, can be employed to selectively detect Gsp with a limit of detection of 25 µg/mL via a "turn-on" response from a completely nonemissive state in the absence of Gsp. The recyclability and stability of Yb-NH2-TPDC in the presence of Gsp was demonstrated by fluorescence spectroscopy and PXRD analysis, respectively. A variety of background substances present in practical samples that would require Gsp sensing, such as refined cottonseed oil, palmitic acid, linoleic acid, and α-tocopherol, did not interfere with the Yb3+ photoluminescence signal. We further identified that the "turn-on" of Yb-NH2-TPDC photoluminescence was due to the "antenna effect" of Gsp, as evidenced by spectroscopic studies and supported by computational analysis. This is the first report that Gsp can effectively sensitize Yb3+ photoluminescence. Leveraging this sensing mechanism, we demonstrate facile, highly sensitive, fast-response detection of Gsp using YbCl3·6H2O and NdCl3·6H2O solutions. Overall, we show for the first time that Ln3+-based materials are promising luminescent sensors for Gsp detection. We envision that the reported sensing approach will be applicable to the detection of a wide variety of aromatic molecules using Ln3+ compounds including MOFs, complexes, and salts.

Atom-by-Atom Evolution of the Same Ligand-Protected Au21, Au22, Au22Cd1, and Au24 Nanocluster Series.

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au22(SAdm)16 and Au22Cd1(SAdm)16 (SAdm = adamantanethiolate) which link up with Au21(SAdm)15 and Au24(SAdm)16 NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au22(SAdm)16 has an Au3(SAdm)4 surface motif which is longer than the Au2(SAdm)3 on Au21(SAdm)15, whereas Au22Cd1(SAdm)16 lacks one staple Au atom compared to Au24(SAdm)16 and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au22 with a 10-atom bioctahedral kernel to Au22Cd1 with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties.

Tuning the Structure and Chiroptical Properties of Gold Nanoparticle Single Helices via Peptide Sequence Variation.

Just as peptide function is determined by the position, sequence, and overall arrangement of constituent amino acids, the optical properties of nanoparticle (NP) assemblies are influenced by the size, dimensions, and arrangement of constituent NPs. In this work, we demonstrate that peptide sequence can be programmed to direct the structure and chiroptical activity of chiral helical gold NP (AuNP)superstructures, a growing class of chiral nanomaterials with potential in sensing, detection, and optics-based applications. Gold-binding peptide conjugate families, C18-(PEPAuM,x)2 and C18-(PEPAuM-ox,x)2, that differ in the position (x = 7, 9, and 11) of methionine (M)/methionine sulfoxide (M-ox) within the peptide sequences (PEPAu = AYSSGAPPMPPF/PEPAuM-ox = AYSSGAPPMoxPPF) are employed to control the aspect ratio and size of AuNPs within helical NP assemblies. Computational modeling reveals that the amino acid variations have a profound effect on peptide-AuNP interactions that ultimately lead to control over NP size. C18-(PEPAuM,x)2 (x = 7, 9, and 11) yield irregular double-helical superstructures comprising spherical AuNPs, while C18-(PEPAuM-ox,x)2 (x = 9, 11) yield single-helical assemblies comprising oblong or rod-shaped AuNPs. Further, component AuNPs are larger when M/M-ox is placed at x = 11, while smaller component AuNPs are observed when M/M-ox is placed at x = 7. Changes in nanoscale structures manifest themselves in observable differences in chiroptical signal intensity. Ultimately, we achieve dramatic variance in the structure and properties of chiral AuNP superstructures via simple molecular-level tuning of peptide primary sequence.

Multivariate Stratified Metal-Organic Frameworks: Diversification Using Domain Building Blocks.

We introduce the concept of domain building blocks (DBBs) as an effective approach to increasing the diversity and complexity of metal-organic frameworks (MOFs). DBBs are defined as distinct structural or compositional regions within a MOF material. Using the DBB approach, we illustrate how an immense number of multivariate MOF materials can be prepared from a small collection of molecular building blocks comprising the distinct domains. The multivariate nature of the MOFs is determined by the sequence of DBBs within the MOF. We then apply this approach to the construction of a rich library of UiO-67 stratified MOF (sMOF) particles consisting of multiple concentric DBBs. We discuss and highlight the negative consequences of linker exchange reactions on the compositional integrity of DBBs in the UiO-67 sMOFs and propose and demonstrate mitigation strategies. We also demonstrate that individual strata can be specifically postsynthetically addressed and manipulated. Finally, we demonstrate the versatility of these synthetic strategies through the preparation of sMOF-nanoparticle composite materials.

Designing Open Metal Sites in Metal-Organic Frameworks for Paraffin/Olefin Separations.

Incorporating open metal sites (OMS) into metal-organic frameworks allows design of well-defined binding sites for selective molecular adsorption, which has a profound impact on catalysis and separations. We demonstrate that Cu(I) sites incorporated into MFU-4l preferentially adsorb olefins over paraffins. Density functional theory (DFT) calculations show that the OMS are independent, with no dependence of binding energy on olefin loading up to one olefin per Cu(I). Experimentally, increasing Cu(I) loading increased olefin uptake without affecting the binding energy, as predicted by DFT and confirmed by temperature-programmed desorption. The potential of this material for olefin/paraffin separation under ambient conditions was investigated by gas adsorption and column breakthrough experiments for an equimolar ratio of olefin/paraffin. High-grade propylene and ethylene (>99.999%) can be generated using temperature-concentration swing recycling from a Cu(I)-MFU-4l packed column with no measurable paraffin breakthrough.

Au130-x Agx Nanoclusters with Non-Metallicity: A Drum of Silver-Rich Sites Enclosed in a Marks-Decahedral Cage of Gold-Rich Sites.

The synthesis and structure of atomically precise Au130-x Agx (average x=98) alloy nanoclusters protected by 55 ligands of 4-tert-butylbenzenethiolate are reported. This large alloy structure has a decahedral M54 (M=Au/Ag) core. The Au atoms are localized in the truncated Marks decahedron. In the core, a drum of Ag-rich sites is found, which is enclosed by a Marks decahedral cage of Au-rich sites. The surface is exclusively Ag-SR; X-ray absorption fine structure analysis supports the absence of Au-S bonds. The optical absorption spectrum shows a strong peak at 523 nm, seemingly a plasmon peak, but fs spectroscopic analysis indicates its non-plasmon nature. The non-metallicity of the Au130-x Agx nanocluster has set up a benchmark to study the transition to metallic state in the size evolution of bimetallic nanoclusters. The localized Au/Ag binary architecture in such a large alloy nanocluster provides atomic-level insights into the Au-Ag bonds in bimetallic nanoclusters.

A Correlated Series of Au/Ag Nanoclusters Revealing the Evolutionary Patterns of Asymmetric Ag Doping.

Doping of metal nanoclusters is an effective strategy for tailoring their functionalities for specific applications. To gain fundamental insight into the doping mechanism, it is of critical importance to have access to a series of correlated bimetal nanoclusters with different doping levels and further reveal the successive transformations. Herein, we report asymmetric doping of Ag into an Au21 nanocluster to form a series of new Au/Ag bimetal nanoclusters and the effects of doping on the evolution of size, structure, and properties based upon X-ray crystallography and optical spectroscopy analyses. The asymmetric doping discovered in the series reveals two important rules. First, the heteroatom doping-induced kernel transformation mechanism is revealed, explaining the successive conversions from Au21(S-Adm)15 with an incomplete cuboctahedral kernel to Au20Ag1(S-Adm)15 with a complete cuboctahedral Au12Ag1 kernel and then to Au19Ag4(S-Adm)15 with an icosahedral Au10Ag3 kernel. The electron density accumulated on the central Au atom(s) is rationalized to force an expansion of radial metal-metal bond angles, which triggers the cuboctahedral-to-icosahedral kernel conversion. This mechanism is generalized by elucidating several other cases. Second, through comparison of a series of seven nanoclusters (all protected by adamantanethiolate), we find that the unit cell symmetry of their crystals is correlated with the symmetry of the cluster's kernel. Specifically, we observe a sequential change from triclinic to monoclinic to trigonal unit cell in the series with increasing kernel symmetry. The kernel structure-dependent optical properties are also discussed.

Total Structure Determination of Au16(S-Adm)12 and Cd1Au14(S tBu)12 and Implications for the Structure of Au15(SR)13.

Ultrasmall nanoclusters (e.g., Au15(SR)13) are crucial in not only real applications such as bioapplication but also in understanding the structure transition from gold complexes to gold nanoclusters. However, the determination of these transition-sized gold nanoclusters has long been a major challenge. In this work, two new nanoclusters in the transition regime, including the thus far smallest thiolated alloy nanocluster Cd1Au14(S tBu)12 and the homogold nanocluster Au16(S-Adm)12, are obtained and their atomic structures are fully determined by single crystal X-ray diffraction. Moreover, based on the structures of Cd1Au14(SR)12 and Au16(SR)12, we perform DFT calculations to predict the structure of the "transformation" nanocluster, Au15 (Au15(SR)12- and Au15(SR)13). Overall, this work bridges the gaps between gold complexes and nanoclusters.

Programmable Topology in New Families of Heterobimetallic Metal-Organic Frameworks.

Using diverse building blocks, such as different heterometallic clusters, in metal-organic framework (MOF) syntheses greatly increases MOF complexity and leads to emergent synergistic properties. However, applying reticular chemistry to syntheses involving more than two molecular building blocks is challenging and there is limited progress in this area. We are therefore motivated to develop a strategy for achieving systematic and differential control over the coordination of multiple metals in MOFs. Herein, we report the design and synthesis of a diverse series of heterobimetallic MOFs with different metal ions and clusters severally distributed throughout two or three inorganic secondary building units (SBUs). By taking advantage of the bifunctional isonicotinate linker and its derivatives, which can coordinatively distinguish between early and late transition metals, we control the assembly and topology of up to three different inorganic SBUs in one-pot solvothermal reactions. Specifically, M6(µ3-O) n(µ3-OH)8- n(CO2)12 (M = Zr4+, Hf4+, Dy3+) SBUs are formed along with metal-pyridyl complexes. By controlling the geometry of the metal-pyridyl complexes, we direct the overall topology to produce eight new MOFs with fcu, ftw, and previously unreported trinodal pfm crystallographic nets.

Systematic Adjustment of Pitch and Particle Dimensions within a Family of Chiral Plasmonic Gold Nanoparticle Single Helices.

Systematically controlling the assembly architecture within a class of chiral nanoparticle superstructures is important for fine-tuning their chiroptical properties. Here, we report a family of chiral gold nanoparticle single helices, varying in helical pitch and nanoparticle dimensions, that is assembled using a series of peptide conjugate molecules Cx-(PEPAuM-ox)2 (PEPAuM-ox = AYSSGAPPMoxPPF; x = 16-22). We demonstrate that the aliphatic tail length (i) can be used as a handle to systematically tune the helical pitch from 80 to 130 nm; and (ii) influences the size, shape, and aspect ratio of the component nanoparticles. Certain members of this family of materials exhibit intense plasmonic chiroptical activity. These studies highlight the multiple levels of structural control that can be achieved within a class of chiral nanoparticle superstructures via careful design and selection of peptide conjugate precursor.

Tailoring the Structure of 58-Electron Gold Nanoclusters: Au103S2(S-Nap)41 and Its Implications.

We report the synthesis and crystal structure determination of a gold nanocluster with 103 gold atoms protected by 2 sulfidos and 41 thiolates (i.e., 2-naphthalenethiolates, S-Nap), denoted as Au103S2(S-Nap)41. The crystallographic analysis reveals that the thiolate ligands on the nanocluster form local tetramers by intracluster interactions of C-H···π and π···π stacking. The herringbone pattern formation via intercluster interactions is also observed, which leads to a linearly connected zigzag pattern in the single crystal. The kernel of the nanocluster is a Marks decahedron of Au79, which is the same as the kernel of the previously reported Au102(pMBA)44 (pMBA = -SPh-p-COOH); this is a surprise given the much bulkier naphthalene-based ligand than pMBA, indicating the robustness of the decahedral structure as well as the 58-electron configuration. Despite the same kernel, the surface structure of Au103 is quite different from that of Au102, indicating the major role of ligands in constructing the surface structure. Other implications from Au103 and Au102 include (i) both nanoclusters show similar HOMO-LUMO gap energy (i.e., Eg ≈ 0.45 eV), indicating the kernel is decisive for Eg while the surface is less critical; and (ii) significant differences are observed in the excited-state lifetimes by transient absorption spectroscopy analysis, revealing the kernel-to-surface relaxation pathway of electron dynamics. Overall, this work demonstrates the ligand-effected modification of the gold-thiolate interface independent of the kernel structure, which in turn allows one to map out the respective roles of kernel and surface in determining the electronic and optical properties of the 58e nanoclusters.