Nathaniel B. Zuckerman

Pyrene-Functionalized Ruthenium Nanoparticles as Effective Chemosensors for Nitroaromatic Derivatives

Pyrene-functionalized Ru nanoparticles were synthesized by olefin metathesis reactions of carbene-stabilized Ru nanoparticles with 1-vinylpyrene and 1-allylpyrene (the resulting particles were denoted as Ru=VPy and Ru=APy, respectively) and examined as sensitive chemosensors for the detection of nitroaromatic compounds, such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 1-chloro-nitrobenzene (CNB), and nitrobenzene (NB), by their effective quenching of the nanoparticle fluorescence. It was found that the detection sensitivity increased with increasing nitration of the molecules. Additionally, in comparison to monomeric pyrene derivatives, both Ru=VPy and Ru=APy nanoparticles exhibited markedly enhanced performance in the detection of nitroaromatic explosives, most probably as a result of the enhanced collision frequency between the fluorophores and the quencher molecules. Furthermore, Ru=VPy nanoparticles displayed much higher sensitivity (down to the nanomolar regime for TNT) than Ru=APy in the detection of these nitroaromatic explosives, which was ascribed to the extended intraparticle conjugation that provided efficient pathways for energy/electron transfer and consequently amplified the analyte binding events.

Alkyne-Stabilized Ruthenium Nanoparticles: Manipulation of Intraparticle Charge Delocalization by Nanoparticle Charge States†

Monolayer-protected transition metal nanoparticles are a unique family of functional nanomaterials in which the properties of the materials can be readily manipulated not only by the chemical nature of the metal cores and the organic protecting ligands, but also the metal–ligand interfacial bonding interactions. The latter is largely motivated by recent progress in nanoparticle passivation by metal–carbon covalent bonds, where intraparticle charge delocalization may occur as a result of the strong metal–carbon interfacial bonding interactions, in sharp contrast to nanoparticles that are functionalized by mercapto derivatives. For instance, when ferrocene moieties are bound onto a ruthenium nanoparticle surface by ruthenium–carbene p bonds, effective intervalence transfer occurs between the ferrocenyl metal centers at mixed valence, as manifested in electrochemical and near-infrared (NIR) spectroscopic measurements and density functional calculations. Furthermore, when fluorophores are attached onto the nanoparticle surface by the same conjugated linkage, novel emission characteristics emerge that are consistent with those of dimeric derivatives with a conjugated spacer. In a more recent study, effective intraparticle charge delocalization was also observed with ruthenium nanoparticles passivated by alkynyl fragments. This result was ascribed to the unique interfacial bonding interactions (Ru C ) formed by ruthenium and sphybridized carbon atoms of the ligands. In these studies, the nanoparticle metal cores serve as the conducting media to facilitate charge transfer between the functional moieties covalently bound onto the nanoparticle surface. Therefore it is anticipated that the extent of intraparticle conjugation may be readily controlled by the nanoparticle charge state, which is the primary motivation of the present study. Experimentally, by exploiting the molecular capacitor characters of Ru C nanoparticles, the charge states of the nanoparticles were varied by simple chemical reduction or oxidation. The impacts of the nanoparticle charge states on the particle optical and electronic properties were then carefully examined by FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence measurements, and compared to those of the as-prepared nanoparticles. The synthetic procedure for the preparation of ruthenium nanoparticles passivated by 1-octynyl fragments (Ru-OC) has been detailed previously. TEM measurements showed that the nanoparticles exhibited an average core diameter of (2.55 0.15) nm. The nanoparticle charge states were then varied by chemical redox reactions. Specifically, to render the nanoparticles negatively charged, in a typical reaction, 5 mg of Ru-OC nanoparticles were dissolved in dichloromethane (1 mL); a freshly prepared water solution of NaBH4 (1 mL, 5 mgmL ) was then added. The mixture was stirred for 30 min and then water was removed. The resulting nanoparticles exhibited negative net charges and were denoted as Ru-OCRed. Positively charged nanoparticles were prepared in a similar fashion by mixing the nanoparticle solution with an aqueous solution of saturated Ce(SO4)2 for 30 min. The resulting nanoparticles were denoted as Ru-OCOx. Transition metal nanoparticles passivated with a lowdielectric organic protecting layer have long been known to act as nanoscale molecular capacitors. In fact, based on a concentric structural model, the nanoparticle capacitance (CMPC) can be estimated by CMPC= 4pe0e(r+d) r d, where e0 is the vacuum permittivity, e is the effective dielectric constant of the organic protecting layer, r is the radius of the metal core, and d is the length of the organic protecting ligand. For the octyne-passivated ruthenium (Ru-OC) nanoparticles, r= 1.275 nm, d= 0.848 nm (estimated by Hyperchem), and e= 2.6. Thus, the nanoparticle capacitance can be estimated to be about 0.92 aF. To quantify the change of the nanoparticle charge state after reduction or oxidation, we measured the open circuit potentials of the nanoparticles electrochemically. It was found that the as-prepared Ru-OC nanoparticles exhibited an open circuit potential of + 0.140 V (versus Ag/AgCl). After reduction by NaBH4, it decreased to + 0.024 V, whereas after oxidation by ceric sulfate, it increased to + 0.250 V. This result indicated that the reduced nanoparticles (Ru-OCRed) exhibited an average charging of 0.67 electrons per nanoparticle, whereas the oxidized nanoparticles (Ru-OCOx) were formed by an average discharging of 0.63 electrons per nanoparticle. Interestingly, despite these subtle changes of nanoparticle charge states, rather drastic impacts were observed on the nanoparticle optoelectronic properties. Figure 1 depicts the FTIR spectra of the nanoparticles before and after reduction or oxidation. For the as-prepared Ru-OC nanoparticles, the C C stretching band appeared at 1965 cm 1 (inset). In comparison to octyne monomers, for which the C C stretch[*] X. W. Kang, N. B. Zuckerman, Prof. J. P. Konopelski, Prof. S. W. Chen Department of Chemistry and Biochemistry, University of California 1156 High Street, Santa Cruz, CA 95064 (USA) Fax: (+1)831-459-2935 E-mail: shaowei@ucsc.edu Homepage: http://chemistry.ucsc.edu/~ schen

Intraparticle charge delocalization of carbene-functionalized ruthenium nanoparticles manipulated by selective ion binding.

Olefin metathesis reactions of carbene-stabilized ruthenium nanoparticles were exploited for the incorporation of multiple functional moieties onto the nanoparticle surface. When the nanoparticles were cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-vinylpyrene, the resulting particles exhibited fluorescence characteristics that were consistent with dimeric pyrene with a conjugated chemical bridge, with three peaks observed in the emission spectra at 391, 410, and 485 nm. The behaviors were ascribed to intraparticle charge delocalization between the pyrene moieties afforded by the conjugated Ru═carbene interfacial linkages. Notably, upon the binding of metal ions in the crown ether cavity, the emission intensity of the nanoparticle fluorescence was found to diminish at 485 nm and concurrently increase at 391 and 410 nm rather markedly, with the most significant effects observed with K(+). This was accounted for by the selective binding of 18-crown-6 to potassium ions, where the positively charged ions led to the polarization of the nanoparticle core electrons that was facililated by the conjugated linkage to the metal surface and hence impeded intraparticle charge delocalization. Control experiments with a pyrene-crown ether conjugate (2) and with ruthenium nanoparticles cofunctionalized with 4-vinylbenzo-18-crown-6 and 1-allylpyrene suggested that the through-bond pathway played a predominant role in the manipulation of intraparticle electronic communication whereas the contributions from simple electrostatic interactions (i.e., through-space pathway) were minimal.

Manipulation of intraparticle charge delocalization by selective complexation of transition-metal ions with histidine moieties.

Ruthenium nanoparticles were cofunctionalized with pyrene and histidine moieties through Ru═carbene π bonds. The selective complexation of the histidine moiety with transition-metal ions led to a marked diminishment of the emission peak at 490 nm which arose from the nanoparticle-bridged pyrene moieties that behaved analogously to pyrene dimers with a conjugated spacer. This is accounted for by the polarization of the core electrons by the added positive charge that impacted the intraparticle charge delocalization between the particle-bound pyrene moieties. This electronic interaction was likely facilitated by the π interactions between the metal ions and the imidazole ring as well as by the conjugated molecular backbone that linked the imidazole ring to the nanoparticle cores. Within the present experimental context, of all the metal ions tested, the impacts were much more drastic with Pb(2+), Co(2+), and Hg(2+) than with Li(+), K(+), Rb(+), Mg(2+), Ca(2+), and Zn(2+) ions, with the most sensitive variation observed with Pb(2+). This is ascribed to the enhanced π interactions of the histidine moiety with the Pb(2+), Co(2+), and Hg(2+) ions because of their capability of donating d electrons, a behavior consistent with prior studies based on conventional histidine-metal ion complexes.

Structural Determination of NSC 670224, Synthesis of Analogues and Biological Evaluation

Follow my lead! NSC 670224, previously shown to be toxic to Saccharomyces cerevisiae at low micromolar concentrations, potentially acts via a mechanism of action related to that of tamoxifen (NSC 180973), breast cancer drug. The structure of NSC 670224, previously thought to be a 2,4-dichloro arene, was established as the 3,4-dichloro arene, and a focused library of analogues were synthesized and biologically evaluated.