Towards the design of self-sorting nanomaterials through kinetically directed chemoselective control over interfacial surface chemistry

A gold nanoparticle platform is described in which post-synthesis surface modifications can be conducted using kinetically-tunable strain-promoted cycloaddition chemistry, which is dependent on the electronic properties of the complementary dipolar species. This permits chemoselective reactivity with one reactive dipole over another less reactive dipole, providing exciting opportinities for kinetically-directed self-sorting strategies.

Strained alkyne polymers capable of SPAAC via ring-opening metathesis polymerization

We present a strategy that combines the attractive traits of chain-growth polymerization and strain-promoted azide–alkyne cycloaddition chemistry for the production of functional polymers.

Anhydride Post-Synthetic Modification in a Hierarchical Metal-Organic Framework

Metal-organic frameworks (MOFs) are important porous materials. Post-synthetic modification (PSM) of MOFs via the pendant groups or secondary functional groups of organic linkers has been widely used to introduce new or enhance existing properties of MOFs for various practical applications. In this work, we have constructed, for the first time, a novel platform for PSM of MOFs by introducing an anhydride functional group into a hierarchically porous MOF (MIL-121) as an effective anchor. We have demonstrated that the combination of the high reactivity of anhydride and hierarchical porosity makes this protocol particularly novel and important, as it led to excellent opportunities of incorporating not only a wide variety of organic molecules with different sizes and chemical nature but also the noble metal complexes in MOFs. Specifically, we show that the anhydride group decorated in the MOF exhibits a high reactivity toward covalently binding 10 different guest molecules including alcohols, amines, thiols, and noble metal (Pt(II)/Pt(IV)) complexes, whereas the hierarchical pores created in the MOF allow the incorporation of guest species varying in size from methanol to larger molecules such as polyaromatic amines. This novel approach provides the community with a new avenue to prepare MOF-based materials for targeted applications. To illustrate this point, we furnish an example of using this new platform to prepare a Pt-based electrocatalyst which shows excellent catalytic activity toward the oxygen reduction reaction (ORR), a pivotal half-reaction in hydrogen-oxygen fuel cells and other energy storage and conversion devices.

Investigation of Au SAMs Photoclick Derivatization by PM-IRRAS

In this work, we present a clean one-step process for modifying headgroups of self-assembled monolayers (SAMs) on gold using photo-enabled click chemistry. A thiolated, cyclopropenone-caged strained alkyne precursor was first functionalized onto a flat gold substrate through self-assembly. Exposure of the cyclopropenone SAM to UVA light initiated the efficient photochemical decarbonylation of the cyclopropenone moiety, revealing the strained alkyne capable of undergoing the interfacial strain-promoted alkyne-azide cycloaddition (SPAAC). Irradiated SAMs were derivatized with a series of model azides with varied hydrophobicity to demonstrate the generality of this chemical system for the modification and fine-tuning of the surface chemistry on gold substrates. SAMs were characterized at each step with polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to confirm the successful functionalization and reactivity. Furthermore, to showcase the compatibility of this approach with biochemical applications, cyclopropenone SAMs were irradiated and modified with azide-bearing cell adhesion peptides to promote human fibroblast cell adhesion, and then imaged by live-cell fluorescence microscopy. Thus, the "photoclick" methodology reported here represents an improved, versatile, catalyst-free protocol that allows for a high degree of control over the modification of material surfaces, with applicability in materials science as well as biochemistry.

Nitrone-Modified Gold Nanoparticles: Synthesis, Characterization, and Their Potential as 18F-Labeled Positron Emission Tomography Probes via I-SPANC

A novel bioorthogonal gold nanoparticle (AuNP) template displaying interfacial nitrone functional groups for bioorthogonal interfacial strain-promoted alkyne-nitrone cycloaddition reactions has been synthesized. These nitrone-AuNPs were characterized in detail using 1H nuclear magnetic resonance spectroscopy, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy, and a nanoparticle raw formula was calculated. The ability to control the conjugation of molecules of interest at the molecular level onto the nitrone-AuNP template allowed us to create a novel methodology for the synthesis of AuNP-based radiolabeled probes.

Highly Electron-Deficient Pyridinium-Nitrones for Rapid and Tunable Inverse-Electron-Demand Strain-Promoted Alkyne-Nitrone Cycloaddition

Highly accelerated inverse-electron-demand strain-promoted alkyne-nitrone cycloaddition (IED SPANC) between a stable cyclooctyne (bicyclo[6.1.0]nonyne (BCN)) and nitrones delocalized into a C_α-pyridinium functionality is reported, with the most electron-deficient "pyridinium-nitrone" displaying among the most rapid cycloadditions to BCN that is currently reported. Density functional theory (DFT) and X-ray crystallography are explored to rationalize the effects of N- and C_α-substituent modifications at the nitrone on IED SPANC reaction kinetics and the overall rapid reactivity of pyridinium-delocalized nitrones.

Loading across the Periodic Table: Introducing 14 Different Metal Ions To Enhance Metal-Organic Framework Performance

Loading metal guests within metal-organic frameworks (MOFs) via secondary functional groups is a promising route for introducing or enhancing MOF performance in various applications. In this work, 14 metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Co²⁺, Mn²⁺, Ag⁺, Cd²⁺, La³⁺, In³⁺, and Pb²⁺) have been successfully introduced within the MIL-121 MOF using a cost-efficient route involving free carboxylic groups on the linker. The local and long-range structure of the metal-loaded MOFs is characterized using multinuclear solid-state NMR and X-ray diffraction methods. Li/Mg/Ca-loaded MIL-121 and Ag nanoparticle-loaded MIL-121 exhibit enhanced H₂ and CO₂ adsorption; Ag nanoparticle-loaded MIL-121 also demonstrates remarkable catalytic activity in the reduction of 4-nitrophenol.

Fluorogenic Gold Nanoparticle (AuNP) Substrate: A Model for the Controlled Release of Molecules from AuNP Nanocarriers via Interfacial Staudinger-Bertozzi Ligation

The ability to regulate small-molecule release from metallic nanoparticle substrates offers unprecedented opportunities for nanocarrier-based imaging, sensing, and drug-delivery applications. Herein we report a novel and highly specific release methodology off gold nanoparticle (AuNP) surfaces based on the bioorthogonal Staudinger-Bertozzi ligation. A thiol ligand bearing the molecular cargo, a Rhodamine B dye derivative, was synthesized and used to modify small water-soluble 5 nm AuNPs. Upon incorporation into the AuNP monolayer, we observed efficient quenching of the dye emission, resulting in a very low level of fluorescence emission that provided the baseline from which cargo release was monitored. We examined the ability of these AuNPs to react with azide molecules via Staudinger-Bertozzi ligation on the nanoparticle surface by monitoring the fluorescence emission after the introduction of an organic azide. We observed an immediate increase in emission intensity upon azide addition, which corresponded to the release of the dye into the bulk solution. The ³¹P NMR spectrum of the AuNP product also agrees with the formation of the ligation product. Thus this system represents a novel and highly specific release methodology off AuNP surfaces that can have potential applications in drug delivery, sensing, and materials science.

Small gold nanoparticles for interfacial Staudinger-Bertozzi ligation

Small gold nanoparticles (AuNPs) that possess interfacial methyl-2-(diphenylphosphino)benzoate moieties have been successfully synthesized (Staudinger-AuNPs) and characterized by multi-nuclear MR spectroscopy, transmission electron microscopy (TEM), UV-Vis spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy (XPS). In particular, XPS was remarkably sensitive for characterization of the novel nanomaterial, and in furnishing proof of its interfacial reactivity. These Staudinger-AuNPs were found to be stable to the oxidation of the phosphine center. The reaction with benzyl azide in a Staudinger-Bertozzi ligation, as a model system, was investigated using (31)P NMR spectroscopy. This demonstrated that the interfacial reaction was clean and quantitative. To showcase the potential utility of these Staudinger-AuNPs in bioorganic chemistry, a AuNP bioconjugate was prepared by reacting the Staudinger-AuNPs with a novel azide-labeled CRGDK peptide. The CRGDK peptide could be covalently attached to the AuNP efficiently, chemoselectively, and with a high loading.

A nanoaggregate-on-mirror platform for molecular and biomolecular detection by surface-enhanced Raman spectroscopy

A nanoaggregate-on-mirror (NAOM) structure has been developed for molecular and biomolecular detection using surface-enhanced Raman spectroscopy (SERS). The smooth surface of the gold mirror allows for simple and homogeneous functionalization, while the introduction of the nanoaggregates enhances the Raman signal of the molecule(s) in the vicinity of the aggregate-mirror junction. This is evidenced by functionalizing the gold mirror with 4-nitrothiophenol, and the further addition of gold nanoaggregates promotes local SERS activity only in the areas with the nanoaggregates. The application of the NAOM platform for biomolecular detection is highlighted using glucose and H2O2 as molecules of interest. In both cases, the gold mirror is functionalized with 4-mercaptophenylboronic acid (4-MPBA). Upon exposure to glucose, the boronic acid moiety of 4-MPBA forms a cyclic boronate ester. Once the nanoaggregates are added to the surface, detection of glucose is possible without the use of an enzyme. This method of indirect detection provides a limit of detection of 0.05 mM, along with a linear range of detection from 0.1 to 15 mM for glucose, encompassing the physiological range of blood glucose concentration. The detection of H2O2 is achieved with optical inspection and SERS. The H2O2 interferes with the coating of the gold mirror, enabling qualitative detection by visual inspection. Simultaneously, the H2O2 reacts with the boronic acid to form a phenol, a change that is detected by SERS.

Gold nanosponges (AuNS): a versatile nanostructure for surface-enhanced Raman spectroscopic detection of small molecules and biomolecules

Prepared by simple pour and mix chemistry, gold nanosponges (AuNS) are versatile structures for surface-enhanced Raman spectroscopy (SERS).

High-resolution Raman imaging of bundles of single-walled carbon nanotubes by tip-enhanced Raman spectroscopy

Bundles of single-walled carbon nanotubes (SWCNTs) prepared by plasma torch method and further purified, are deposited over a glass coverslip to estimate the spatial resolution of tip-enhanced Raman spectroscopy measurements. For this purpose, near-field Raman maps and spectra of isolated bundles of carbon nanotubes are collected using optimized experimental conditions such as a tightly focused beam using a 1.4 numerical aperture oil immersion microscope objective and a gold coated atomic force microscope probe illuminated by a radially polarized 632.8 nm wavelength to selectively excite the localized surface plasmon confined at the extremity of the tip. The near-field nature of the collected Raman signals is evaluated through measuring the decay of the Raman signal with respect to the tip-sample separation.

Expanding the scope of strained-alkyne chemistry: a protection–deprotection strategy via the formation of a dicobalt–hexacarbonyl complex

A protection–deprotection strategy for strained alkynes is reported. A strained alkyne can be protected with dicobalt–octacarbonyl and we demonstrate for the first time that the a strained alkyne can be re-formed and isolated under mild conditions for further bioorthogonal reactivity.

Synthesis of small water-soluble diazirine-functionalized gold nanoparticles and their photochemical modification

Dual water and organic solvent soluble 3-aryl-3-(trifluormethyl) diazirine-functionalized gold nanoparticles (AuNPs) were prepared through a place exchange reaction from triethylene glycol monomethyl ether (EG3-Me) capped AuNPs. These nanoparticles were fully characterized using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). TGA along with 1H NMR data allowed the determination of 15% incorporation of diazirine (Diaz) ligands onto mixed monolayer AuNPs, while TEM images showed an average diameter of 2.3 ± 0.5 nm. This information led to the estimated molecular formula of Au400 (S-EG4-Diaz)40 (S-EG3-Me)230 for these AuNPs. It is noteworthy that high-resolution XPS was a powerful tool for quantitative analysis. Irradiation of the diazirine capped AuNPs resulted in nitrogen extrusion and the formation of a highly reactive carbene with evidence of a portion of the reaction proceeding via the diazo intermediate and thus requiring a second photon for activation. The carbene species generated was utilized to tether the attached AuNPs via insertion into C=C or O–H functionality inherent on various substrates. Here, we demonstrated that photolysis of the diazirine modified AuNPs in the presence of a variety of model carbene scavengers led to clean and efficient insertion products while maintaining their solubility in polar solvents.

Thermodynamic and kinetic origins of Au25(0) nanocluster electrochemiluminescence

Au clusters with protecting organothiolate ligands and core diameters less than 2 nm are molecule-like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au25(0) (Au25[(SCH2CH2Ph)18], SCH2CH2Ph = 2-phenylethanethiol) allowed us to construct a Latimer-type diagram for the first time, which revealed a rich photoelectrochemistry of the cluster and the unique relationship to its various oxidation states and corresponding excited states. The occurrence of cluster electrochemiluminescence (ECL) was examined in the presence of tri-n-propylamine (TPrA) as a co-reactant and was discovered to be in the near-infrared (NIR) region with peak wavelengths of 860, 865, and 960 nm, emitted by Au25(+*), Au25(0*), and Au25(-*), respectively. The light emissions, with an efficiency up to 103% relative to that of the efficient Ru(bpy)3(2+)/TPrA system, depended on the kinetics of the reactions between the electrogenerated TPrA radical and Au25(z) (z = 2+, 1+, 1-, and 2-) in the vicinity of the electrode or the bulk Au25(0). These thermodynamic and kinetic origins were further explored by means of spooling ECL and photoluminescence spectroscopy during a sweep of the potential or at a constant potential applied to the working electrode. NIR-ECL emissions of the cluster can be tuned in wavelength and intensity by adjusting the applied potential and TPrA concentration based on the above discoveries.

Tessellated gold nanostructures from Au144(SCH2CH2Ph)60 molecular precursors and their use in organic solar cell enhancement

We report for the first time the fabrication of nanocomposite hole-blocking layers consisting of poly-3,4-ethylene-dioxythiophene:poly-styrene-sulfonate (PEDOT:PSS) thin films incorporating networks of gold nanoparticles assembled from Au144(SCH2CH2Ph)60, a molecular gold precursor. These thin films can be prepared reproducibly on indium tin oxide by spinning on it Au144(SCH2CH2Ph)60 solutions in chlorobenzene, annealing the resulting thin film at 400 °C, and subsequently spinning PEDOT:PSS on top. The use of our nanocomposite hole-blocking layers for enhancing the photoconversion efficiency of bulk heterojunction organic solar cells is demonstrated. By varying the concentration of Au144(SCH2CH2Ph)60 in the starting solution and the annealing time, different gold nanostructures were obtained ranging from individual gold nanoparticles (AuNPs) to tessellated networks of gold nanostructures (Tess-AuNPs). Improvement in organic solar cell efficiencies up to 10% relative to a reference cell is demonstrated with Tess-AuNPs embedded in PEDOT:PSS.

New polydentate trimethylsilyl chalcogenide reagents for the assembly of polyferrocenyl architectures

A series of polychalcogenotrimethylsilane complexes Ar(CH2ESiMe3)n, (Ar = aryl; E = S, Se; n = 2, 3, and 4) can be prepared from the corresponding polyorganobromide and M[ESiMe3] (M = Na, Li). These represent the first examples of the incorporation of such a large number of reactive -ESiMe3 moieties onto an organic molecular framework. They are shown to be convenient reagents for the preparation of the polyferrocenylseleno- and thioesters from ferrocenoyl chloride. The synthesis, structures, and spectroscopic properties of the new silyl chalcogen complexes 1,4-(Me3SiECH2)2(C6Me4) (E = S, 1; E = Se, 2), 1,3,5-(Me3SiECH2)3(C6Me3) (E = S, 3; E = Se, 4) and 1,2,4,5-(Me3SiECH2)4(C6H2) (E = S, 5; E = Se, 6) and the polyferrocenyl chalcogenoesters [1,4-{FcC(O)ECH2}2(C6Me4)] (E = S, 7; E = Se, 8), [1,3,5-{FcC(O)ECH2}3(C6Me3)] (E = S, 9; E = Se, 10) and [1,2,4,5-{FcC(O)ECH2}4(C6H2)] (E = S, 11 illustrated; E = Se, 12) are reported. The new polysilylated reagents and polyferrocenyl chalcogenoesters have been characterized by multinuclear NMR spectroscopy ((1)H, (13)C, (77)Se), electrospray ionization mass spectrometry and, for complexes 1, 2, 3, 4, 7, 8, and 11, single-crystal X-ray diffraction. The cyclic voltammograms of complexes 7-11 are presented.

Facile nucleation of gold nanoparticles on graphene-based thin films from Au₁₄₄ molecular precursors

We demonstrate a facile and cost effective method to obtain gold nanoparticles on graphene by dispersing Au₁₄₄ molecular nanoclusters by spin coating them in thin layers on graphene-based films and subsequent annealing in a controlled atmosphere. The graphene-based thin films used for these experiments are prepared by solvent-assisted exfoliation of graphite in water in the presence of ribonucleic acid as a surfactant and by subsequent vacuum filtration of the resulting graphene-containing suspensions. Not only is this method easily reproducible, but it leads to gold nanoparticles that are not dependent in size on the number of graphene layers beneath them. This is a distinct advantage over other methods. Plasmonic effects have been detected in our gold nanoparticle-decorated graphene layers, indicating that these thin films may be useful in applications such as plasmonic solar cells and optical memory devices.

NIR electrochemiluminescence from Au25− nanoclusters facilitated by highly oxidizing and reducing co-reactant radicals

By means of spooling spectroscopy, we demonstrate our discovery on near-infrared electrochemiluminescence of Au₂₅⁻ nanoclusters at 950 and 900 nm, which can be attributed to the Au₂₅⁻*, Au₂₅⁰* and Au₂₅⁺* excited species being controllable chemically and electrochemically.

Versatile strained alkyne modified water-soluble AuNPs for interfacial strain promoted azide–alkyne cycloaddition (I-SPAAC)

Versatile water-soluble AuNPs that incorporate an interfacial strained alkyne were synthesized and their reactivity towards the I-SPAAC reaction was demonstrated by using azide-decorated polymersomes as bioorthogonal reaction partners.

Near-infrared electrochemiluminescence from Au25(SC2H4Ph)18+ clusters co-reacted with tri-n-propylamine

The wavelength and intensity of electrochemiluminescence in the Au₂₅⁺ clusters/tri-n-propylamine (TPrA) co-reactant system can be tuned by varying the TPrA concentration and applied potential.

Anion-exchange reactions on a robust phosphonium photopolymer for the controlled deposition of ionic gold nanoclusters

UV curing (photopolymerization) is ubiquitous in many facets of industry ranging from the application of paints, pigments, and barrier coatings all the way to fiber optic cable production. To date no reports have focused on polymerizable phosphonium salts under UV irradiation, and despite this dearth of examples, they potentially offer numerous substantial advantages to traditional UV formulation components. We have generated a highly novel coating based on UV-curable trialkylacryloylphosphonium salts that allow for the fast (seconds) and straightforward preparation of ion-exchange surfaces amenable to a roll-to-roll process. We have quantified the surface charges and exploited their accessibility by employing these surfaces in an anion exchange experiment by which [Au25L18](-) (L = SCH2CH2Ph) nanocrystals can be assembled into the solid state. This unprecedented application of such surfaces offers a paradigm shift in the emerging chemistry of Au25 research where the nanocrystals remain single and intact and where the integrity of the cluster and its solution photophysical properties are resilient in the solid state. The specific loading of [Au25L18](-) on the substrates has been determined and the completely reversible loading and unloading of intact nanocrystals to and from the surface has been established. In the solid state, the assembly has an incredible mechanical resiliency, where the surface remains undamaged even when subjected to repeated Scotch tests.

Photolysis and thermolysis of pyridyl carbonyl azide monolayers on single-crystal platinum

The photochemical and thermal reactivity of a number of acyl azide-substituted pyridine compounds, namely nicotinyl azide, isonicotinyl azide, picolinyl azide and dinicotinyl azide with investigated as saturated monolayers on a single-crystal Pt(111) surface in an ultrahigh vacuum chamber. Multilayers of the substrates exhibited a maximum rate of desorption at 270 K, above which, stable saturated monolayers formed as characterized by reflection-absorption infrared spectroscopy by observation of C=O and N3 bands at 1700 cm(-1), and 2100 and 1300 cm(-1) respectively. The monolayers were stable up to 400 K. Photolysis of the monolayer (or heating above 400 K) results in the formation of the respective isocyanate intermediate after loss of nitrogen as evidenced by the appearance of a new infrared band at 2260 cm(-1) with concomitant loss of the azide bands. The resulting isocyanate saturated monolayer is stable in absence of nucleophiles, but can be quenched with appropriate nucleophiles.

Interrogating near-infrared electrogenerated chemiluminescence of Au25(SC2H4Ph)18(+) clusters

The electrochemistry, near-infrared photoluminescence (NIR-PL) spectroscopy, and electrogenerated chemiluminescence (ECL) of Au(25)(SC(2)H(4)Ph)(18)(+)C(6)F(5)CO(2)(-) (Au(25)(+)) clusters were investigated. For the first time, NIR-ECL emission was observed in both annihilation and coreactant paths. Our newly developed spooling spectroscopy was employed during the ECL evolution and devolution processes along with explicit NIR-PL spectroscopy to elucidate light generation mechanisms. It was discovered that the electronic relaxation of the Au(25)(-) excited state to the ground state plays a key role in giving off ECL at 893 nm, while intermediate, strong, and weak NIR-PL emissions at 719/820, 857, and 1080 nm can be attributed to the excited states higher than the HOMO-LUMO gap, across the HOMO-LUMO gap, and of semi-rings, respectively.

Improved methodology for the preparation of water-soluble maleimide-functionalized small gold nanoparticles

Improved methodology to prepare maleimide-functionalized, water-soluble, small (<3 nm) gold nanoparticles using a retro-Diels-Alder strategy that we developed for similar organic-soluble AuNP's is described. Importantly, our results suggest that a recent paper by Zhu, Waengler, Lennox, and Schirrmacher describing a similar strategy gave results inconsistent with the formation of the titled maleimide-modified AuNP (Zhu, J.; Waengler, C.; Lennox, R. B.; Schirrmacher, R. Langmuir2012, 28, 5508) as the major product, but consistent with the major product being an adduct derived from the hydrolysis of maleimide formed under the conditions used for the required deprotection of the maleimide. Our methodology provides an efficient and accessible route to pure maleimide-modified small AuNP's that circumvents the formation of the hydrolysis product. The maleimide-modified small AuNP's are versatile because they are soluble in water and in a wide range of organic solvents and their reactivity can now be properly exploited as a reactive moiety in Michael addition for bioconjugation studies in aqueous solution.