Polymeric Ligand Research Articles

Unraveling chemical processes during nanoparticle synthesis with liquid phase electron microscopy and correlative techniques.

Liquid phase transmission electron microscopy (LPTEM) has enabled unprecedented direct real time imaging of physicochemical processes during solution phase synthesis of metallic nanoparticles. LPTEM primarily provides images of nanometer scale, and sometimes atomic scale, metal nanoparticle crystallization processes, but provides little chemical information about organic surface ligands, metal-ligand complexes and reaction intermediates, and redox reactions. Likewise, complex electron beam-solvent interactions during LPTEM make it challenging to pinpoint the chemical processes, some involving exotic highly reactive radicals, impacting nanoparticle formation. Pairing LPTEM with correlative solution synthesis, ex situ chemical analysis, and theoretical modeling represents a powerful approach to gain a holistic understanding of the chemical processes involved in nanoparticle synthesis. In this feature article, we review recent work by our lab and others that has focused on elucidating chemical processes during nanoparticle synthesis using LPTEM and correlative chemical characterization and modeling, including mass and optical spectrometry, fluorescence microscopy, solution chemistry, and reaction kinetic modeling. In particular, we show how these approaches enable investigating redox chemistry during LPTEM, polymeric and organic capping ligands, metal deposition mechanisms on plasmonic nanoparticles, metal clusters and complexes, and multimetallic nanoparticle formation. Future avenues of research are discussed, including moving beyond electron beam induced nanoparticle formation by using light and thermal stimuli during LPTEM. We discuss prospects for real time LPTEM imaging and online chemical analysis of reaction intermediates using microfluidic flow reactors.

A phosphorus-based olefin linked fully conjugated polymeric ligand for palladium-catalyzed trans-selective dicarbofunctionalization of internal alkynes

Phosphorus is introduced in sp2 carbon (CC) linked fully conjugated porous organic polymers (POPs) and has been exploited in synthetically challenging and selectively competitive multicomponent dicarbofunctionalization reactions.

Visualizing formation of high entropy alloy nanoparticles with liquid phase transmission electron microscopy.

High entropy alloy (HEA) nanoparticles hold promise as active and durable (electro)catalysts. Understanding their formation mechanism will enable rational control over composition and atomic arrangement of multimetallic catalytic surface sites to maximize their activity. While prior reports have attributed HEA nanoparticle formation to nucleation and growth, there is a dearth of detailed mechanistic investigations. Here we utilize liquid phase transmission electron microscopy (LPTEM), systematic synthesis, and mass spectrometry (MS) to demonstrate that HEA nanoparticles form by aggregation of metal cluster intermediates. AuAgCuPtPd HEA nanoparticles are synthesized by aqueous co-reduction of metal salts with sodium borohydride in the presence of thiolated polymer ligands. Varying the metal : ligand ratio during synthesis showed that alloyed HEA nanoparticles formed only above a threshold ligand concentration. Interestingly, stable single metal atoms and sub-nanometer clusters are observed by TEM and MS in the final HEA nanoparticle solution, suggesting nucleation and growth is not the dominant mechanism. Increasing supersaturation ratio increased particle size, which together with observations of stable single metal atoms and clusters, supported an aggregative growth mechanism. Direct real-time observation with LPTEM imaging showed aggregation of HEA nanoparticles during synthesis. Quantitative analyses of the nanoparticle growth kinetics and particle size distribution from LPTEM movies were consistent with a theoretical model for aggregative growth. Taken together, these results are consistent with a reaction mechanism involving rapid reduction of metal ions into sub-nanometer clusters followed by cluster aggregation driven by borohydride ion induced thiol ligand desorption. This work demonstrates the importance of cluster species as potential synthetic handles for rational control over HEA nanoparticle atomic structure.

Non-conjugated polymer ligand: stimulating charge transfer towards photocatalytic selective organic transformation

Non-conjugated polymer ligand and metal NCs as simultaneous electron transfer mediators are synergistically harnessed to stimulate photocatalytic selective organic transformation.

Amine-assisted catechol-based nanocoating on ultrasmall iron oxide nanoparticles for high-resolution T1 angiography.

Surface engineered iron oxide nanoparticles (IONPs) with catecholic ligands have been investigated as alternative T1 contrast agents. However, complex oxidative chemistry of catechol during IONP ligand exchange causes surface etching, heterogeneous hydrodynamic size distribution, and low colloidal stability because of Fe3+ mediated ligand oxidation. Herein, we report highly stable and compact (∼10 nm) Fe3+ rich ultrasmall IONPs functionalized with a multidentate catechol-based polyethylene glycol polymer ligand through amine-assisted catecholic nanocoating. The IONPs exhibit excellent stability over a broad range of pHs and low nonspecific binding in vitro. We also demonstrate that the resultant NPs have a long circulation time (∼80 min), enabling high resolution T1 magnetic resonance angiography in vivo. These results suggest that the amine assisted catechol-based nanocoating opens a new potential of metal oxide NPs to take a step forward in exquisite bio-application fields.

Polymeric ligands comprising sulfur-containing amino acids for targeting tumor-associated amino acid transporters

Various cancer cells overexpress L-type amino acid transporter 1 (LAT1) to take up a large number of neutral amino acids such as phenylalanine and methionine, and LAT1 transporter should be a promising target for cancer diagnosis and therapy. However, only a few studies reported drug delivery systems targeting LAT1 probably due to limited knowledge about the interaction between LAT1 and its substrate. Here, we developed polymers having methionine (Met)- or cysteine (Cys)-like structures on their side chains to examine their affinity with LAT1. While both the Met- and Cys-modified polymers exhibited efficient cellular uptake selectively in cancer cells, the Met-modified polymers exhibited higher cellular uptake efficiency in an LAT1-selective manner than the Cys-modified polymers. In the in vivo study, the intraperitoneally injected Met-modified polymers showed appreciable tumor-selective accumulation in the peritoneal dissemination model, and importantly, Met-modified polymers conjugated with photosensitizers exhibited significant therapeutic effects upon photoirradiation with reduced photochemical damage to normal organs. Our results may provide important knowledge about the polymer-LAT1 interaction, and the Met-modified polymers should offer a new concept for designing LAT1-targeting drug delivery systems.

Synthesis and Characterization of New Polymeric-Schiff Bases and Their Complexes

Complexations of three new polymeric Schiff base ligands to transition metals (Ni and Zn) were synthesized via the preparation of a Schiff base ligand on Styrene-allyl alcohol (SAA) and two types of polyvinyl alcohol (PVA). These polymers were a supporting agent for preparing Schiff base ligand on it, followed by complexation with transition metals. Modified SAA and PVA polymers with Ni (II) and Zn (II) have been synthesized in order to investigate some transition metal complexes roles in these polymer modifications. The prepared polymeric complexes were confirmed and characterized by FTIR spectroscopy and TGA instrument respectively. The transition metals (Ni and Zn) content was estimated using SEM-EDX analysis

Amorphous Porous Polyurethanes as Macromolecular Ligands for Palladium Catalysts

Porous organic polymers (POPs) having urethane bonds were prepared through polyaddition of 4,4′-bis(hydroxylmethyl)-2,2′-bipyridyl with 1,3,5-triisocyanato-benzene and 1,3,5-tris(4-isocyanatophenyl)benzene, resulting in POP1 and POP2, respectively. The resulting POPs were complexed with Pd(OAc)2 to form POP-bound pre-catalysts, i.e., Pd(OAc)2@POP1 and Pd(OAc)2@POP2. Pd(II)@POP1 showed higher catalytic activities in Suzuki-Miyaura cross-coupling reactions compared with 2,2′-bipyridyl-Pd(OAc)2, as the reference “monomeric” catalyst. The higher catalytic activities of Pd(OAc)2@POP1 were interpreted in terms of the rigid framework of POP1. Within the POP1 framework, Pd may be more efficiently protected from agglomeration, which eventually leads to inactive palladium black formation. Unlike covalent organic frameworks (COFs) with crystalline structure, the present POPs are amorphous but capable, as polymeric ligands, of leading to active catalytic species.

Clickable Polymer Ligand-Functionalized Iron Oxide Nanocubes: A Promising Nanoplatform for 'Local Hot Spots' Magnetically Triggered Drug Release.

Exploiting the local heat on the surface of magnetic nanoparticles (MNPs) upon exposure to an alternating magnetic field (AMF) to cleave thermal labile bonds represents an interesting approach in the context of remotely triggered drug delivery. Here, taking advantages of a simple and scalable two-step ligand exchange reaction, we have prepared iron oxide nanocubes (IONCs) functionalized with a novel multifunctional polymer ligand having multiple catechol moieties, furfuryl pendants, and polyethylene glycol (PEG) side chains. Catechol groups ensure a strong binding of the polymer ligands to the IONCs surface, while the PEG chains provide good colloidal stability to the polymer-coated IONCs. More importantly, furfuryl pendants on the polymer enable to click the molecules of interest (either maleimide-fluorescein or maleimide-doxorubicin) via a thermal labile Diels-Alder adduct. The resulting IONCs functionalized with a fluorescein/doxorubicin-conjugated polymer ligand exhibit good colloidal stability in buffer saline and serum solution along with outstanding heating performance in aqueous solution or even in viscous media (81% glycerol/water) when exposed to the AMF of clinical use. The release of conjugated bioactive molecules such as fluorescein and doxorubicin could be boosted by applying AMF conditions of clinical use (16 kAm-1 and 110 kHz). It is remarkable that the magnetic hyperthermia-mediated release of the dye/drug falls in the concentration range 1.0-5.0 μM at an IONCs dose as low as 0.5 gFe/L and at no macroscopical temperature change. This local release effect makes this magnetic nanoplatform a potential tool for drug delivery with remote magnetic hyperthermia actuation and with a dose-independent action of MNPs.

(Invited) Ligand Effect in Electroreduction of CO2: N-Heterocyclic Carbene-Capped Polymers to Promote Catalytic Efficiency

N-heterocyclic carbenes (NHCs) are excellent ligands that bind to a variety of transition metals with extremely high binding strength via σ donation. In my talk, I will highlight the use of two polymeric N-heretocyclic carbenes (NHC) (polydentate and monodentate) to stabilize metal nanocatalysts (Au and Pd) for efficient CO2 electroreduction in aqueous solution. Compared with other conventional ligands including thiols and amines, metal-NHC bonds are stable under reductive potentials prevent the nanoclustering of nanoparticles, which is of key importance for any practical application. In particular, Au nanocatalysts modified by polymeric NHC ligands show an activity retention of 86% after CO2 reduction at -0.9 V for 11 h, while it is less than 10% for unmodified Au. We demonstrate that the hydrophobicity of polymer ligands and the enriched surface electron density of metal NPs through σ-donation of NHCs substantially improve the selectivity for CO2 reduction over proton reduction. As a passive coating on the surface of metal nanocatalysts, NHC-terminated polymers can control the accessibility of substrates, i.e., CO2 vs. H2O, as a means to improve the Faradic efficiency toward CO2 reduction. Our results provide guideline to improve catalytic efficiency and stability of metal nanocatalysts for CO2 electroreduction and, more generally, as cathodic materials for other electroreduction reactions.

(Keynote) N-Heterocyclic Carbene-coated Gold Nanoparticles and Luminescent Quantum Dots

N-heterocyclic carbenes (NHCs) have generated much interest for use as versatile metal-coordinating groups, since they were first synthesized by Arduengo and coworkers in 1991. NHC molecules can be considered as L-type ligands, because they share their non-bonding electron pairs with the σ-accepting orbital of transition metals, and this endows them with strong coordination interactions. NHC-appended molecules have recently been actively exploited as potentially effective ligands for the surface passivation of various colloidal nanomaterials.We investigate the coordination interactions between a few representative colloidal nanocrystals, including gold nanoparticles (AuNPs) and luminescent quantum dots (QDs), and a NHC-based polymer ligand. The latter presents multiple NHC groups and several short poly (ethylene glycol) (PEG) chains as solubilizing blocks. We find that our NHC-decorated ligands rapidly coordinate onto both sets of nanocrystals, which we attribute to their soft Lewis base nature. These ideally match the soft Lewis acid character of transition metal colloid surfaces, promoting strong coordination bonding through soft‐soft interaction. We combine NMR spectroscopy, fluorescence spectroscopy, high-resolution transmission electron microscopy supplemented with dynamic light scattering to characterize the nature of the binding interactions. Furthermore, the long-term stability of the NHC-stabilized nanocolloids have been tested after phase transfer to water, a highly challenging chemical venue for such groups, due to the moisture sensitive nature of NHC molecules. Data show that our NHC-polymer-stabilized AuNPs and QDs exhibit long-term colloidal stability in buffer media while preserving their optical and fluorescing properties, with no sign of degradation or aggregation build up for at least one year of storage. We will discuss the ligand design and synthesis, characterization of the polymer-stabilized nanocrystals under various conditions, with a particular focus on the beneficial effects of ligand multi-coordination interactions onto the nanocolloid surfaces.

Preparation and Performance of PU/Cpsf Blend Ultrafiltration Membranes for Removal of Heavy Metal Ion Rejection Studies

The performance of membranes for a specific application can be determined with the help of structural properties such as molecular weight cut-off (MWCO), morphology, and pore statistics. Heavy metal ions from aqueous streams can be separated with the help of ultrafiltration membranes. In the presence and absence of the various components of the additive poly (ethylene glycol) 600, MWCOs and pore statistics of polyurethane (PU) and carboxylated polysulfone (CPSf) blend ultrafiltration (Total Polymer Concentration = 17.5 wt %) were studied with the help of dextran of different molecular weights ranging from 19 kDa to 150 kDa. The derived pore size, porosity, and the number of pores have a remarkable relationship with the MWCO, morphology, and the flux performance of the membranes. The blend membranes rejected certain toxic divalent heavy metal ions such as copper, cadmium, nickel, and zinc by complexing them into a polymeric ligand, poly(ethyleneimine) (PEI). The effect of polymer blend compositions and additive concentrations on metal ions' rejection and permeate flux are discussed

RAFT Hydroxylated Polymers as Templates and Ligands for the Synthesis of Fluorescent ZnO Quantum Dots.

The remarkable photoluminescent properties, biocompatibility, biodegradability, and antibacterial properties of zinc oxide quantum dots (ZnO QDs) coupled with their low cost and nanoscale size guarantee bio-related and technological applications. However, the effect of the polymeric ligand during synthesis has hardly been investigated compared to other less environmentally friendly QDs. Thus, the objective of this work was to focus on the synthesis of fluorescent hybrid ZnO QDs by the sol-gel method using different polymers with hydroxyl groups as templates and ligands to obtain stable particles in different media. For this purpose, well-defined hydroxylated statistical polymers and block copolymers were synthesized using reversible-addition fragmentation chain transfer (RAFT) polymerization to establish the influence of molecular weight, hydrophobic/hydrophilic balance, and polymer architecture on the colloidal and photophysical properties of the synthesized hybrid ZnO QDs. Dynamic light scattering (DLS), TEM, and X-ray diffraction measurements indicated the formation of stable nanoparticles of a few nanometers. A remarkable enhancement in terms of fluorescence was observed when ZnO QDs were synthesized in the presence of the hydroxylated homopolymers and even more so with block copolymers architecture. Organosilanes combined with the hydroxylated polymers were used to improve the colloidal stability of ZnO QDs in aqueous media. These samples exhibited uniform and stable enhanced photoluminescence for nearly five months of being investigated. Among other applications, the hybrid ZnO QDs synthesized in this work exhibit high selectivity to detect Cr6+, Fe2+, or Cu2+ in water.

Polymer ligands of taurine – New class of high selective sorbents for exctraction of silver from multicomponent solutions

The systematic study on the properties of the taurine polymer ligand has made it possible to determine the influence of the structure of the sorbents on their selective properties towards silver (I). It has been shown that an increase in the degree of modification of the sulfoethyl groups resulted in an increase in the selectivity of silver(I) sorption from multicomponent solutions onto the materials based on polyallylamine (the values of the KAg/Cu selectivity coefficients for the sorbent with the maximum degree of modification exceeded 103). Sorbents based on polyethyleneimine, unlike other sulfoethylated aminopolymers, are appropriate materials for the group extraction of a number of transition metal ions. The pKa values of amino groups in the composition of sorbents based on poly(N-2-sulfoethyl)ethylenimine and poly(N-2-sulfoethyl)allylamine were determined, and a correlation between the acid-base and selective properties of sorbents was established. A significant effect of the nature of the aminopolymer matrix on the selectivity of sorption was revealed, and recommendations were formulated for the use of N-derivative of taurine polymer ligand in separation and preconcentration processes.

Straightforward modulation of graphene nanosheets with magnetic nanoclusters for ultrathin microwave absorption materials

Facile regulation of magnetic–dielectric synergy is of great importance for the fabrication of microwave absorption (MA) materials. Herein, we propose a polymeric ligand–mediated interface self–assembly strategy to design a series of MA composites using magnetic nanoclusters and graphene nanosheets as building blocks. The employed aromatic polymeric ligand not only allows the assembly of individual nanocrystals into magnetic nanoclusters but also enables their attachment to graphene substrate, resulting in rapid integration of magnetic and dielectric modules at ambient temperature. The straightforward self–assembly enables consecutive MA performances across a wide range of minimum reflection loss that can be greatly increased by 4.3 times. Moreover, the self–assembled nanocomposites exhibit ultrathin MA characteristics with strong reflection loss at an ultrathin thickness of only 1.5 mm and effective frequency bandwidth at an ultrathin thickness of only 1.4 mm. This work provides insights into simple modulation of structure and properties toward ultrathin MA materials.

Heteroleptic Coordination Polymer Electrolytes Initiated by Lewis-Acidic Eutectics for Solid Zinc–Metal Batteries

Solid polymer electrolytes (SPEs) offer a viable path for overcoming the interfacial problems caused by side reactions and irregular deposition in rechargeable Zn (zinc)–metal batteries. However, this potential has been hampered by limited Zn2+ mobility in polymers; a central conundrum remains on how to solvate Zn2+ strongly enough to free it from anionic traps but weakly enough to minimize its migration barriers. Inspired by biologically dynamic Zn functions, we report a general strategy for constructing highly Zn2+-conductive SPEs by the engineering of heteroleptic coordination. Leveraging polymerization catalyzed by Lewis-acidic Zn2+ predissociated eutectics, we stoichiometrically integrate polymeric ligands (polyacrylamide) with kindred small-molecule co-ligands (acetamide) for Zn2+ centers. This heteroleptic configuration allows for the formation of entropy-increased ion channels with both labile Zn2+–polymer bonding and accelerated polymer mobility, warranting conductivity gains of 2 orders of magnitude and doubling the Zn2+-transference number to 0.44, compared with traditional SPEs. The applicability of the proposed heteroleptic coordination design is also demonstrated by the improved reversibility of Zn plating/stripping process (1200 h) and prolonged cycle life of solid Zn–metal batteries (350 cycles with Mo6S8 cathodes) with improved Coulombic efficiency (∼99%). This study underscores the importance of tailoring the coordination environment in improving cationic mobility in polymers.

Supramolecular Interfaces and Reconfigurable Liquids Derived from Cucurbit[7]uril Surfactants.

Nanoparticle surfactants (NPSs) offer a powerful means to stabilize the oil-water interface and construct all-liquid devices with advanced functions. However, as the nanoparticle size decreases to molecular-scale, the binding energy of the NPS to the interface reduces significantly, leading to a dynamic adsorption of NPS and "liquid-like" state of the interfacial assemblies. Here, by using the host-guest recognition between a water-soluble small molecule, cucurbit[7]uril (CB[7]) and an oil-soluble polymer ligand, methyl viologen-terminated polystyrene, a supramolecular NPS model, termed CB[7] surfactant, is described. CB[7] surfactants form and assemble rapidly at the oil-water interface, generating an elastic film with excellent mechanical properties. The binding energy of CB[7] surfactant to the interface is sufficiently high to hold it in a jammed state, transforming the interfacial assemblies from a "liquid-like" to "solid-like" state, enabling the structuring of liquids. With CB[7] surfactants as the emulsifier, O/W, W/O and O/W/O emulsions can be prepared in one step. Owing to the guest-competitive responsiveness of CB[7] surfactants, the assembly/disassembly and jamming/unjamming of CB[7] surfactants can be well controlled, leading to the reconfiguration of all-liquid constructs.

Mannosylated Polymeric Ligands for Targeted Delivery of Antibacterials and Their Adjuvants to Macrophages for the Enhancement of the Drug Efficiency.

Bacterial infections and especially resistant strains of pathogens localized in macrophages and granulomas are intractable diseases that pose a threat to millions of people. In this paper, the theoretical and experimental foundations for solving this problem are proposed due to two key aspects. The first is the use of a three-component polymer system for delivering fluoroquinolones to macrophages due to high-affinity interaction with mannose receptors (CD206). Cytometry assay determined that 95.5% macrophage-like cells were FITC-positive after adding high-affine to CD206 trimannoside conjugate HPCD-PEI1.8-triMan, and 61.7% were FITC-positive after adding medium-affine ligand with linear mannose label HPCD-PEI1.8-Man. The second aspect is the use of adjuvants, which are synergists for antibiotics. Using FTIR and NMR spectroscopy, it was shown that molecular containers, namely mannosylated polyethyleneimines (PEIs) and cyclodextrins (CDs), load moxifloxacin (MF) with dissociation constants of the order of 10−4–10−6 M; moreover, due to prolonged release and adsorption on the cell membrane, they enhance the effect of MF. Using CLSM, it was shown that eugenol (EG) increases the penetration of doxorubicin (Dox) into cells by an order of magnitude due to the creation of defects in the bacterial wall and the inhibition of efflux proteins. Fluorescence spectroscopy showed that 0.5% EG penetrates into bacteria and inhibits efflux proteins, which makes it possible to increase the maximum concentration of the antibiotic by 60% and maintain it for several hours until the pathogens are completely neutralized. Regulation of efflux is a possible way to overcome multiple drug resistance of both pathogens and cancer cells.

Magnetic Resonance Neuroimaging Contrast Agents of Nanomaterials.

Since the early 1980s when MRI imaging technology was put into clinical use, the number of MRI clinical tests has steadily increased by more than 10% every year. At the same time, exogenous MRI contrast agents have also been developed with the development of MRI technology. However, there are still challenges in the preparation of contrast agents for magnetic resonance imaging, such as how to prepare high-efficiency contrast agents with high stability and low biological toxicity. In order to study the contrast agent with simple preparation method, low cost, and good imaging effect, a magnetic resonance contrast agent was prepared by magnetic nanoparticles. By acting on magnetic resonance imaging detection method, and using polymer ligands to synthesize magnetic nanoparticles, experiments and tests of P(MA-alt-VAc) polymer ligand-modified magnetic nanoparticles were carried out. The experimental results showed that when nanoparticles containing different iron ion concentrations were incubated with DC 2.4 normal cells for 48 hours, the cell viability was still higher than 80% at concentrations up to 200 μm. It shows that the nanoparticle has high cell activity and good biological adaptability. The transverse relaxation (r2) value of the nanoparticles in aqueous solution at 37°C and 1.5 T magnetic field is 231.1 m−1 s−1, which is much higher than that of PTMP-PMAA (r2 = 35.1 mM−1 s−1), which is also more than five times the relaxation of SHU-555C (r2 = 44 mM−1 s−1). It shows that the nanoparticles prepared in this paper have good effect and can be used as a contrast agent in human brain for magnetic resonance imaging.

Incorporating Comonomers into Polymeric Phosphate Ligands Can Tune the Affinity and Capacity for Rare Earth Element, La

Incorporating Comonomers into Polymeric Phosphate Ligands Can Tune the Affinity and Capacity for Rare Earth Element, La