Publications by authors named "Jean M J Fréchet"

203 Publications

Reduction Triggered Polymerization in Living Mice.

J Am Chem Soc 2020 09 31;142(36):15575-15584. Epub 2020 Aug 31.

Molecular Imaging Program at Stanford, Bio-X Program, Department of Radiology, School of medicine, Stanford University, Stanford, California 94305, United States.

"Smart" biomaterials that are responsive to physiological or biochemical stimuli have found many biomedical applications for tissue engineering, therapeutics, and molecular imaging. In this work, we describe polymerization of activatable biorthogonal small molecules in response to a reducing environment change . We designed a carbohydrate linker- and cyanobenzothiazole-cysteine condensation reaction-based small molecule scaffold that can undergo rapid condensation reaction upon physiochemical changes (such as a reducing environment) to form polymers (pseudopolysaccharide). The fluorescent and photoacoustic properties of a fluorophore-tagged condensation scaffold before and after the transformation have been examined with a dual-modality optical imaging method. These results confirmed the polymerization of this probe after both local and systemic administration in living mice.
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http://dx.doi.org/10.1021/jacs.0c07594DOI Listing
September 2020

The effect of polymer backbone chemistry on the induction of the accelerated blood clearance in polymer modified liposomes.

J Control Release 2015 Sep 18;213:1-9. Epub 2015 Jun 18.

Department of Bioengineering, Therapeutic Sciences and Pharmaceutical Chemistry, University of California San Francisco, San Francisco, CA 94143, USA. Electronic address:

A variety of water-soluble polymers, when attached to a liposome, substantially increase liposome circulation half-life in animals. However, in certain conditions, liposomes modified with the most widely used polymer, polyethylene glycol (PEG), induce an IgM response resulting in an accelerated blood clearance (ABC) of the liposome upon the second injection. Modification of liposomes with other water-soluble polymers: HPMA (poly[N-(2-hydroxypropyl) methacrylamide]), PVP (poly(vinylpyrrolidone)), PMOX (poly(2-methyl-2-oxazoline)), PDMA (poly(N,N-dimethyl acrylamide)), and PAcM (poly(N-acryloyl morpholine)), increases circulation times of liposomes; but a precise comparison of their ability to promote long circulation or induce the ABC effect has not been reported. To obtain a more nuanced understanding of the role of polymer structure/MW to promote long circulation, we synthesized a library of polymer diacyl chain lipids with low polydispersity (1.04-1.09), similar polymer molecular weights (2.1-2.5kDa) and incorporated them into 100nm liposomes of a narrow polydispersity (0.25-1.3) composed of polymer-lipid/hydrogenated soy phosphatidylcholine/cholesterol/diD: 5.0/54.5/40/0.5. We confirm that HPMA, PVP, PMOX, PDMA and PAcM modified liposome have increased circulation times in rodents and that PVP, PDMA, and PAcM do not induce the ABC effect. We demonstrate for the first time, that HPMA does not cause an ABC effect whereas PMOX induces a pronounced ABC effect in rats. We find that a single dose of liposomes coated with PEG and PMOX generates an IgM response in rats towards the respective polymer. Finally, in this homologous polymer series, we observe a positive correlation (R=0.84 in rats, R=0.92 in mice) between the circulation time of polymer-modified liposomes and polymer viscosity; PEG and PMOX, the polymers that can initiate an ABC response were the two most viscous polymers. Our findings suggest that polymers that do not cause an ABC effect such as, HPMA or PVP, deserve further consideration as polymer coatings to improve the circulation of liposomes and other nanoparticles.
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http://dx.doi.org/10.1016/j.jconrel.2015.06.023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4684485PMC
September 2015

Solution-processed, molecular photovoltaics that exploit hole transfer from non-fullerene, n-type materials.

Adv Mater 2014 Jul 12;26(25):4313-9. Epub 2014 May 12.

Departments of Chemistry and Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA, 94720-1460, USA; Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA.

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http://dx.doi.org/10.1002/adma.201305444DOI Listing
July 2014

Changing attitudes in Saudi Arabia.

Nat Mater 2014 Apr;13(4):321-2

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http://dx.doi.org/10.1038/nmat3899DOI Listing
April 2014

On the efficiency of charge transfer state splitting in polymer:fullerene solar cells.

Adv Mater 2014 Apr 27;26(16):2533-9. Epub 2014 Feb 27.

Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany.

The field dependence and yield of free charge carrier generation in polymer:fullerene blends with varying energetic offsets is not affected when the excitation energy is varied from above band-gap to direct CT state excitation. Instead, the ability of the CT state to split is dictated by the energetic offset between the relaxed CT state and the charge separated (CS) state.
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http://dx.doi.org/10.1002/adma.201305283DOI Listing
April 2014

Enhanced solid-state order and field-effect hole mobility through control of nanoscale polymer aggregation.

J Am Chem Soc 2013 Dec 11;135(51):19229-36. Epub 2013 Dec 11.

Departments of Chemistry, ‡Chemical and Biomolecular Engineering, and #Physics, University of California , Berkeley, California 94720, United States.

Efficient charge carrier transport in organic field-effect transistors (OFETs) often requires thin films that display long-range order and close π-π packing that is oriented in-plane with the substrate. Although some polymers have achieved high field-effect mobility with such solid-state properties, there are currently few general strategies for controlling the orientation of π-stacking within polymer films. In order to probe structural effects on polymer-packing alignment, furan-containing diketopyrrolopyrrole (DPP) polymers with similar optoelectronic properties were synthesized with either linear hexadecyl or branched 2-butyloctyl side chains. Differences in polymer solubility were observed and attributed to variation in side-chain shape and polymer backbone curvature. Averaged field-effect hole mobilities of the polymers range from 0.19 to 1.82 cm(2)/V·s, where PDPP3F-C16 is the least soluble polymer and provides the highest maximum mobility of 2.25 cm(2)/V·s. Analysis of the films by AFM and GIXD reveal that less soluble polymers with linear side chains exhibit larger crystalline domains, pack considerably more closely, and align with a greater preference for in-plane π-π packing. Characterization of the polymer solutions prior to spin-coating shows a correlation between early onset nanoscale aggregation and the formation of films with highly oriented in-plane π-stacking. This effect is further observed when nonsolvent is added to PDPP3F-BO solutions to induce aggregation, which results in films with increased nanostructural order, in-plane π-π orientation, and field-effect hole mobilities. Since nearly all π-conjugated materials may be coaxed to aggregate, this strategy for enhancing solid-state properties and OFET performance has applicability to a wide variety of organic electronic materials.
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http://dx.doi.org/10.1021/ja4088665DOI Listing
December 2013

Efficient charge generation by relaxed charge-transfer states at organic interfaces.

Nat Mater 2014 Jan 17;13(1):63-8. Epub 2013 Nov 17.

Department of Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, California 94305, USA.

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.
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http://dx.doi.org/10.1038/nmat3807DOI Listing
January 2014

A mechanistic understanding of processing additive-induced efficiency enhancement in bulk heterojunction organic solar cells.

Adv Mater 2014 Jan 31;26(2):300-5. Epub 2013 Oct 31.

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA.

The addition of processing additives is a widely used approach to increase power conversion efficiencies for many organic solar cells. We present how additives change the polymer conformation in the casting solution leading to a more intermixed phase-segregated network structure of the active layer which in turn results in a 5-fold enhancement in efficiency.
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http://dx.doi.org/10.1002/adma.201303622DOI Listing
January 2014

Conjugation to Biocompatible Dendrimers Increases Lanthanide Relaxivity of Hydroxypyridinone (HOPO) Complexes for Magnetic Resonance Imaging (MRI).

Eur J Inorg Chem 2012 Apr;2012(12):2108-2114

Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 United States.

Magnetic resonance imaging (MRI) contrast agents represent a worldwide billion-dollar market annually. While relaxivity enhancement contrast agents receive greater attention and a significantly larger market share, the commercial potential for relaxivity enhancing contrast agents remains a viable diagnostic option due to their increased relaxivity at high field strengths. Improving the contrast and biocompatibility of MRI probes may enable new diagnostic prospects for MRI. Paramagnetic lanthanides have the potential to decrease and proton relaxation times, but are not commercially used in MRI diagnostics as agents. In this article, oxygen donor chelates (hydroxypyridinone, HOPO, and terephthalamide, TAM) of various lanthanides are demonstrated as biocompatible macromolecular dendrimer conjugates for the development of MRI probes. These conjugates have relaxivities up to 374 mms per dendrimer, high bioavailability, and low toxicity.
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http://dx.doi.org/10.1002/ejic.201101167DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3607537PMC
April 2012

Linear side chains in benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione polymers direct self-assembly and solar cell performance.

J Am Chem Soc 2013 Mar 19;135(12):4656-9. Epub 2013 Mar 19.

King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

While varying the size and branching of solubilizing side chains in π-conjugated polymers impacts their self-assembling properties in thin-film devices, these structural changes remain difficult to anticipate. This report emphasizes the determining role that linear side-chain substituents play in poly(benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers for bulk heterojunction (BHJ) solar cell applications. We show that replacing branched side chains by linear ones in the BDT motifs induces a critical change in polymer self-assembly and backbone orientation in thin films that correlates with a dramatic drop in solar cell efficiency. In contrast, we show that for polymers with branched alkyl-substituted BDT motifs, controlling the number of aliphatic carbons in the linear N-alkyl-substituted TPD motifs is a major contributor to improved material performance. With this approach, PBDTTPD polymers were found to reach power conversion efficiencies of 8.5% and open-circuit voltages of 0.97 V in BHJ devices with PC71BM, making PBDTTPD one of the best polymer donors for use in the high-band-gap cell of tandem solar cells.
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http://dx.doi.org/10.1021/ja400365bDOI Listing
March 2013

Clinical developments of chemotherapeutic nanomedicines: polymers and liposomes for delivery of camptothecins and platinum (II) drugs.

Wiley Interdiscip Rev Nanomed Nanobiotechnol 2013 Mar-Apr;5(2):130-8. Epub 2013 Jan 17.

College of Chemistry, University of California, Berkeley, CA, USA.

For the past 40 years, liposomal and polymeric delivery vehicles have been studied as systems capable of modulating the cytotoxicity of small molecule chemotherapeutics, increasing tumor bearing animal survival times, and improving drug targeting. Although a number of macromolecular-drug conjugates have progressed to clinical trials, tuning drug release to maintain efficacy in conjunction with controlling drug toxicity has prevented the clinical adoption of many vehicles. In this article, we review the motivations for and approaches to polymer and liposomal delivery with regard to camptothecin and cisplatin delivery.
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http://dx.doi.org/10.1002/wnan.1209DOI Listing
August 2013

Sensitivity to molecular order of the electrical conductivity in oligothiophene monolayer films.

Langmuir 2013 Jan 15;29(4):1206-10. Epub 2013 Jan 15.

Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA.

Using conducting probe atomic force microscopy (CAFM), we show that electrical conductivity in oligothiophene molecular films deposited on SiO(2)/Si wafers is extremely sensitive to degree of crystalline order in the film. By locally distorting the molecular order in the films through the controlled application of pressure with the AFM tip, the lateral charge transport was reduced by factors varying from 2 to 10, even when no changes in the height of the film could be observed.
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http://dx.doi.org/10.1021/la303609gDOI Listing
January 2013

Analysis of Lanthanide Complex Dendrimer Conjugates for Bimodal NIR and MRI Imaging.

Macromolecules 2012 Nov 12;45(22):8982-8990. Epub 2012 Nov 12.

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720.

Advances in clinical diagnostic instrumentation have enabled some imaging modalities to be run concurrently. For diagnostic purposes, multimodal imaging can allow for rapid location and accurate identification of a patient's illness. The paramagnetic and near Infra-red (NIR) properties of Dy(III) and Yb(III) are interesting candidates for the development of bimodal NIR and magnetic resonance imaging (MRI) contrast agents. To enhance their intrinsic bimodal properties, these lanthanides were chelated using the hexadentate-all-oxygen-donor-ligand TREN-bis-(1-Me)-3,2-HOPO-TAM-NX (NX, where X = 1, 2 or 3) and subsequently conjugated to the esteramide dendrimer (EA), to improve bioavailability, solubility, and relaxivity. Of these new complexes synthesized and evaluated, DyN1-EA had the largest ionic T(1) relaxivity, 7.60 mM(-1) s(-1), while YbN3-EA had the largest ionic T(2) relaxivity with a NIR quantum yield of 0.17 % when evaluated in mouse serum. This is the first Yb(III) bimodal NIR/T(2) MRI contrast agent of its kind evaluated.
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http://dx.doi.org/10.1021/ma302206gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3513780PMC
November 2012

Aerosolized antimicrobial agents based on degradable dextran nanoparticles loaded with silver carbene complexes.

Mol Pharm 2012 Nov 19;9(11):3012-22. Epub 2012 Oct 19.

College of Chemistry, University of California, Berkeley, California 94720-1460, United States.

Degradable acetalated dextran (Ac-DEX) nanoparticles were prepared and loaded with a hydrophobic silver carbene complex (SCC) by a single-emulsion process. The resulting particles were characterized for morphology and size distribution using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The average particle size and particle size distribution were found to be a function of the ratio of the organic phase to the surfactant containing aqueous phase with a 1:5 volume ratio of Ac-DEX CH(2)Cl(2) (organic):PBS (aqueous) being optimal for the formulation of nanoparticles with an average size of 100 ± 40 nm and a low polydispersity. The SCC loading was found to increase with an increase in the SCC quantity in the initial feed used during particle formulation up to 30% (w/w); however, the encapsulation efficiency was observed to be the best at a feed ratio of 20% (w/w). In vitro efficacy testing of the SCC loaded Ac-DEX nanoparticles demonstrated their activity against both Gram-negative and Gram-positive bacteria; the nanoparticles inhibited the growth of every bacterial species tested. As expected, a higher concentration of drug was required to inhibit bacterial growth when the drug was encapsulated within the nanoparticle formulations compared with the free drug illustrating the desired depot release. Compared with free drug, the Ac-DEX nanoparticles were much more readily suspended in an aqueous phase and subsequently aerosolized, thus providing an effective method of pulmonary drug delivery.
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http://dx.doi.org/10.1021/mp3004379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3579655PMC
November 2012

Conjugation chemistry through acetals toward a dextran-based delivery system for controlled release of siRNA.

J Am Chem Soc 2012 Sep 18;134(38):15840-8. Epub 2012 Sep 18.

College of Chemistry, University of California, Berkeley, California 94720-1460, United States.

New conjugation chemistry for polysaccharides, exemplified by dextran, was developed to enable the attachment of therapeutic or other functional moieties to the polysaccharide through cleavable acetal linkages. The acid-lability of the acetal groups allows the release of therapeutics under acidic conditions, such as that of the endocytic compartments of cells, regenerating the original free polysaccharide in the end. The physical and chemical behavior of these acetal groups can be adjusted by modifying their stereoelectronic and steric properties, thereby providing materials with tunable degradation and release rates. We have applied this conjugation chemistry in the development of water-soluble siRNA carriers, namely acetal-linked amino-dextrans, with various amine structures attached through either slow- or fast-degrading acetal linker. The carriers with the best combination of amine moieties and structural composition of acetals showed high in vitro transfection efficiency and low cytotoxicity in the delivery of siRNA.
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http://dx.doi.org/10.1021/ja305552uDOI Listing
September 2012

Branched polymeric media: perchlorate-selective resins from hyperbranched polyethyleneimine.

Environ Sci Technol 2012 Oct 20;46(19):10718-26. Epub 2012 Sep 20.

Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejon, Republic of Korea.

Perchlorate (ClO(4)(-)) is a persistent contaminant found in drinking groundwater sources in the United States. Ion exchange (IX) with selective and disposable resins based on cross-linked styrene divinylbenzene (STY-DVB) beads is currently the most commonly utilized process for removing low concentrations of ClO(4)(-) (10-100 ppb) from contaminated drinking water sources. However, due to the low exchange capacity of perchlorate-selective STY-DVB resins (∼0.5-0.8 eq/L), the overall cost becomes prohibitive when treating groundwater with higher concentration of ClO(4)(-) (e.g., 100-1000 ppb). In this article, we describe a new perchlorate-selective resin with high exchange capacity. This new resin was prepared by alkylation of branched polyethyleneimine (PEI) beads obtained from an inverse suspension polymerization process. Batch and column studies show that our new PEI resin with mixed hexyl/ethyl quaternary ammonium chloride exchange sites can selectively extract trace amounts of ClO(4)(-) from a makeup groundwater (to below detection limit) in the presence of competing ions. In addition, this resin has a strong-base exchange capacity of 1.4 eq/L, which is 1.75-2.33 times larger than those of commercial perchlorate-selective STY-DVB resins. The overall results of our studies suggest that branched PEI beads provide versatile and promising building blocks for the preparation of perchlorate-selective resins with high exchange capacity.
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http://dx.doi.org/10.1021/es301418jDOI Listing
October 2012

Preparation of porous polymer monoliths featuring enhanced surface coverage with gold nanoparticles.

J Chromatogr A 2012 Oct 11;1261:121-8. Epub 2012 Apr 11.

Department of Chemistry, University of California, Berkeley, CA 94720, USA.

A new approach to the preparation of porous polymer monoliths with enhanced coverage of pore surface with gold nanoparticles has been developed. First, a generic poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith was reacted with cystamine followed by the cleavage of its disulfide bonds with tris(2-carboxylethyl)phosphine, which liberated the desired thiol groups. Dispersions of gold nanoparticles with sizes varying from 5 to 40 nm were then pumped through the functionalized monoliths. The materials were then analyzed using both energy dispersive X-ray spectroscopy and thermogravimetric analysis. We found that the quantity of attached gold was dependent on the size of nanoparticles, with the maximum attachment of more than 60 wt% being achieved with 40 nm nanoparticles. Scanning electron micrographs of the cross sections of all the monoliths revealed the formation of a non-aggregated, homogenous monolayer of nanoparticles. The surface of the bound gold was functionalized with 1-octanethiol and 1-octadecanethiol, and these monolithic columns were used successfully for the separations of proteins in reversed phase mode. The best separations were obtained using monoliths modified with 15, 20, and 30 nm nanoparticles since these sizes produced the most dense coverage of pore surface with gold.
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http://dx.doi.org/10.1016/j.chroma.2012.04.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3424317PMC
October 2012

Improving T₁ and T₂ magnetic resonance imaging contrast agents through the conjugation of an esteramide dendrimer to high-water-coordination Gd(III) hydroxypyridinone complexes.

Contrast Media Mol Imaging 2012 Jan-Feb;7(1):95-9

Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA.

Commercial gadolinium magnetic resonance imaging (MRI) contrast agents are limited by low relaxivity (r₁) and coordination to only a single water molecule (q = 1). Consequently, gram quantities of these agents must be injected to obtain sufficient diagnostic contrast. In this study, MRI contrast agents for T(1) and T₂ relaxivity were synthesized using hydroxypyridinone and terephthalamide chelators with mesityl and 1,4,7-triazacyclononane capping moieties. When covalently conjugated to a highly biocompatible esteramide dendrimer, T₂ relaxation rates up to 52 mm(-1)  s(-1) and T₁ relaxation rates up to 31 mm(-1)  s(-1) per gadolinium were observed under clinically relevant conditions. These values are believed to be brought about by using a dendritic macromolecule to decrease the molecular tumbling time of the small molecule complexes. These agents also show high aqueous solubility and low toxicity in vitro. In this study we report six new compounds: three discrete complexes and three dendrimer conjugates.
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http://dx.doi.org/10.1002/cmmi.483DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3607543PMC
June 2012

Polyphosphonium polymers for siRNA delivery: an efficient and nontoxic alternative to polyammonium carriers.

J Am Chem Soc 2012 Feb 19;134(4):1902-5. Epub 2012 Jan 19.

College of Chemistry, University of California, Berkeley, California 94720-1460, USA.

A water-soluble polyphosphonium polymer was synthesized and directly compared with its ammonium analog in terms of siRNA delivery. The triethylphosphonium polymer shows transfection efficiency up to 65% with 100% cell viability, whereas the best result obtained for the ammonium analog reaches only 25% transfection with 85% cell viability. Moreover, the nature of the alkyl substituents on the phosphonium cations is shown to have an important influence on the transfection efficiency and toxicity of the polyplexes. The present results show that the use of positively charged phosphonium groups is a worthy choice to achieve a good balance between toxicity and transfection efficiency in gene delivery systems.
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http://dx.doi.org/10.1021/ja207366kDOI Listing
February 2012

Side-chain tunability of furan-containing low-band-gap polymers provides control of structural order in efficient solar cells.

J Am Chem Soc 2012 Feb 19;134(4):2180-5. Epub 2012 Jan 19.

Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley, California 94720, USA.

The solution-processability of conjugated polymers in organic solvents has classically been achieved by modulating the size and branching of alkyl substituents appended to the backbone. However, these substituents impact structural order and charge transport properties in thin-film devices. As a result, a trade-off must be found between material solubility and insulating alkyl content. It was recently shown that the substitution of furan for thiophene in the backbone of the polymer PDPP2FT significantly improves polymer solubility, allowing for the use of shorter branched side chains while maintaining high device efficiency. In this report, we use PDPP2FT to demonstrate that linear alkyl side chains can be used to promote thin-film nanostructural order. In particular, linear side chains are shown to shorten π-π stacking distances between backbones and increase the correlation lengths of both π-π stacking and lamellar spacing, leading to a substantial increase in the efficiency of bulk heterojunction solar cells.
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http://dx.doi.org/10.1021/ja2089662DOI Listing
February 2012

Porous polymer monoliths functionalized through copolymerization of a C60 fullerene-containing methacrylate monomer for highly efficient separations of small molecules.

Anal Chem 2011 Dec 21;83(24):9478-84. Epub 2011 Nov 21.

Department of Chemistry, University of California, Berkeley, California 94720, United States.

Monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) and poly(butyl methacrylate-co-ethylene dimethacrylate) capillary columns, which incorporate the new monomer [6,6]-phenyl-C(61)-butyric acid 2-hydroxyethyl methacrylate ester, have been prepared and their chromatographic performance have been tested for the separation of small molecules in the reversed phase. While addition of the C60-fullerene monomer to the glycidyl methacrylate-based monolith enhanced column efficiency 18-fold, to 85,000 plates/m at a linear velocity of 0.46 mm/s and a retention factor of 2.6, when compared to the parent monolith, the use of butyl methacrylate together with the carbon nanostructured monomer afforded monolithic columns with an efficiency for benzene exceeding 110,000 plates/m at a linear velocity of 0.32 mm/s and a retention factor of 4.2. This high efficiency is unprecedented for separations using porous polymer monoliths operating in an isocratic mode. Optimization of the chromatographic parameters affords near baseline separation of 6 alkylbenzenes in 3 min with an efficiency of 64,000 plates/m. The presence of 1 wt % or more of water in the polymerization mixture has a large effect on both the formation and reproducibility of the monoliths. Other factors such as nitrogen exposure, polymerization conditions, capillary filling method, and sonication parameters were all found to be important in producing highly efficient and reproducible monoliths.
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http://dx.doi.org/10.1021/ac202183gDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3418882PMC
December 2011

Efficient small molecule bulk heterojunction solar cells with high fill factors via pyrene-directed molecular self-assembly.

Adv Mater 2011 Dec 21;23(45):5359-63. Epub 2011 Oct 21.

Department of Chemistry, University of California Berkeley, 94720-1460, USA.

Efficient organic photovoltaic (OPV) materials are constructed by attaching completely planar, symmetric end-groups to donor-acceptor electroactive small molecules. Appending C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors. The use of end-groups to direct molecular self-assembly is an effective strategy for designing high-performance small molecule OPV devices.
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http://dx.doi.org/10.1002/adma.201103177DOI Listing
December 2011

Molecular design and ordering effects in π-functional materials for transistor and solar cell applications.

J Am Chem Soc 2011 Dec 22;133(50):20009-29. Epub 2011 Nov 22.

King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.

Organic electronics are broadly anticipated to impact the development of flexible thin-film device technologies. Among these, solution-processable π-conjugated polymers and small molecules are proving particularly promising in field-effect transistors and bulk heterojunction solar cells. This Perspective analyzes some of the most exciting strategies recently suggested in the design and structural organization of π-functional materials for transistor and solar cell applications. Emphasis is placed on the interplay between molecular structure, self-assembling properties, nanoscale and mesoscale ordering, and device efficiency parameters. A critical look at the various approaches used to optimize both materials and device performance is provided to assist in the identification of new directions and further advances.
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http://dx.doi.org/10.1021/ja2073643DOI Listing
December 2011

A monolithic lipase reactor for biodiesel production by transesterification of triacylglycerides into fatty acid methyl esters.

Biotechnol Bioeng 2012 Feb 26;109(2):371-80. Epub 2011 Sep 26.

Department of Chemistry, University of California, Berkeley, California 94720, USA.

An enzymatic reactor with lipase immobilized on a monolithic polymer support has been prepared and used to catalyze the transesterification of triacylglycerides into the fatty acid methyl esters commonly used for biodiesel. A design of experiments procedure was used to optimize the monolithic reactor with variables including control of the surface polarity of the monolith via variations in the length of the hydrocarbon chain in alkyl methacrylate monomer, time of grafting of 1-vinyl-4,4-dimethylazlactone used to activate the monolith, and time used for the immobilization of porcine lipase. Optimal conditions involved the use of a poly(stearyl methacrylate-co-ethylene dimethacrylate) monolith, grafted first with vinylazlactone, then treated with lipase for 2 h to carry out the immobilization of the enzyme. Best conditions for the transesterification of glyceryl tributyrate included a temperature of 37°C and a 10 min residence time of the substrate in the bioreactor. The reactor did not lose its activity even after pumping through it a solution of substrate equaling 1,000 reactor volumes. This enzymatic reactor was also used for the transesterification of triacylglycerides from soybean oil to fatty acid methyl esters thus demonstrating the ability of the reactor to produce biodiesel.
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http://dx.doi.org/10.1002/bit.23326DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3240714PMC
February 2012

Electrical transport properties of oligothiophene-based molecular films studied by current sensing atomic force microscopy.

Nano Lett 2011 Oct 13;11(10):4107-12. Epub 2011 Sep 13.

Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.
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http://dx.doi.org/10.1021/nl202720yDOI Listing
October 2011

Steric control of the donor/acceptor interface: implications in organic photovoltaic charge generation.

J Am Chem Soc 2011 Aug 14;133(31):12106-14. Epub 2011 Jul 14.

Department of Chemistry, University of California Berkeley, Berkeley, California 94720-1460, USA.

The performance of organic photovoltaic (OPV) devices is currently limited by modest short-circuit current densities. Approaches toward improving this output parameter may provide new avenues to advance OPV technologies and the basic science of charge transfer in organic semiconductors. This work highlights how steric control of the charge separation interface can be effectively tuned in OPV devices. By introducing an octylphenyl substituent onto the investigated polymer backbones, the thermally relaxed charge-transfer state, and potentially excited charge-transfer states, can be raised in energy. This decreases the barrier to charge separation and results in increased photocurrent generation. This finding is of particular significance for nonfullerene OPVs, which have many potential advantages such as tunable energy levels and spectral breadth, but are prone to poor exciton separation efficiencies. Computational, spectroscopic, and synthetic methods were combined to develop a structure-property relationship that correlates polymer substituents with charge-transfer state energies and, ultimately, device efficiencies.
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http://dx.doi.org/10.1021/ja203235zDOI Listing
August 2011

A facile approach to superhydrophilic-superhydrophobic patterns in porous polymer films.

Adv Mater 2011 Jul 20;23(27):3030-4. Epub 2011 May 20.

Institute of Toxicology and Genetics, Karlsruhe Institute of Technology, Germany.

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http://dx.doi.org/10.1002/adma.201101203DOI Listing
July 2011

Acid-degradable cationic dextran particles for the delivery of siRNA therapeutics.

Bioconjug Chem 2011 Jun 24;22(6):1056-65. Epub 2011 May 24.

College of Chemistry, University of California, Berkeley, California 94720-1460, United States.

We report a new acid-sensitive, biocompatible, and biodegradable microparticulate delivery system, spermine modified acetalated-dextran (Spermine-Ac-DEX), which can be used to efficiently encapsulate siRNA. These particles demonstrated efficient gene knockdown in HeLa-luc cells with minimal toxicity. This knockdown was comparable to that obtained using Lipofectamine, a commercially available transfection reagent generally limited to in vitro use due to its high toxicity.
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http://dx.doi.org/10.1021/bc100542rDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3152952PMC
June 2011

Mannosylated dextran nanoparticles: a pH-sensitive system engineered for immunomodulation through mannose targeting.

Bioconjug Chem 2011 May 25;22(5):949-57. Epub 2011 Apr 25.

College of Chemistry, University of California-Berkeley, CA 94720-1460, United States.

Biotherapeutic delivery is a rapidly growing field in need of new materials that are easy to modify, are biocompatible, and provide for triggered release of their encapsulated cargo. Herein, we report on a particulate system made of a polysaccharide-based pH-sensitive material that can be efficiently modified to display mannose-based ligands of cell-surface receptors. These ligands are beneficial for antigen delivery, as they enhance internalization and activation of APCs, and are thus capable of modulating immune responses. When compared to unmodified particles or particles modified with a nonspecific sugar residue used in the delivery of antigens to dendritic cells (DCs), the mannosylated particles exhibited enhanced antigen presentation in the context of major histocompatibility complex (MHC) class I molecules. This represents the first demonstration of a mannosylated particulate system that enables enhanced MHC I antigen presentation by DCs in vitro. Our readily functionalized pH-sensitive material may also open new avenues in the development of optimally modulated vaccine delivery systems.
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http://dx.doi.org/10.1021/bc100596wDOI Listing
May 2011

Long-term thermal stability of high-efficiency polymer solar cells based on photocrosslinkable donor-acceptor conjugated polymers.

Adv Mater 2011 Apr 22;23(14):1660-4. Epub 2011 Feb 22.

Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720-1460, USA.

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http://dx.doi.org/10.1002/adma.201004743DOI Listing
April 2011