Paul K. Eggers

Formation of Efficient Electron Transfer Pathways by Adsorbing Gold Nanoparticles to Self-Assembled Monolayer Modified Electrodes

The influence of the length of a self-assembled monolayer (SAM) linker on the electrochemical performance of electrode-linker-gold nanoparticle molecular constructs is investigated. Electrodes were first modified with amino-1-alkanethiols of four different lengths (C=2, 6, 8, and 11). The SAM showed progressively greater blocking ability to ruthenium hexamine as the length of the alkyl chain increased to the point where no significant Faradaic peak was observed for the amino-1-undecanethiol SAM. Upon the attachment of gold nanoparticles, distinct Faradaic electrochemistry of the ruthenium hexamine was observed for all four length SAMs with the electrochemistry being similar to that observed on a bare electrode. The charge transfer resistance to this Faradaic process was observed to be insensitive to the length of the intervening SAM, indicating it is electron transfer between the redox species and the nanoparticles, rather than tunneling across the SAM, which is the rate-limiting step. Some comments on the mechanism of charge transfer are provided. When forming multilayers of the linker-nanoparticle constructs, fabricated in a stepwise manner, whenever the distal species was the SAM the Faradaic process was blocked and whenever it was the nanoparticle a distinct Faradaic process was observed. With up to five layers of linker-nanoparticles, there was little increase in charge transfer resistance and again the charge transfer resistance was insensitive to the length of the linker.

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers.

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.

p-Phosphonic acid calix[8]arene assisted exfoliation and stabilization of 2D materials in water

Exfoliated 2D materials including graphene, BN, MoS(2) and WS(2) are accessible in water over a wide range of pH for a synergistic process involving sonication in the presence of p-phosphonic acid calix[8]arene.

Non-covalently modified graphene supported ultrafine nanoparticles of palladium for hydrogen gas sensing

A facile aqueous based method of decorating p-phosphonic acid calix[8]arene functionalized graphene with well-dispersed ultrafine palladium nanoparticles (∼2 nm) has been developed. The electrocatalytic Pd-NP–graphene nano-composite has been incorporated into a functional hydrogen sensing device using a simple drop casting technique on interdigitated electrodes.

Nitrate uptake by p-phosphonic acid calix[8]arene stabilized graphene

In situ sonic probe exfoliated graphene sheets in the presence of various concentrations of p-phosphonic acid calix[8]arene are effective in removing nitrate from aquatic effluents, with the efficiency increasing for higher ratios of calixarene to graphite. Mild sonication of the nitrate-adsorbed material releases some nitrate ions back to the effluent.

Shear flow assisted decoration of carbon nano-onions with platinum nanoparticles

Aqueous based controlled decoration of platinum nanoparticles on plasma treated carbon nano-onions (CNOs) occurs within the shear flow generated by a vortex fluidic device (VFD), using ascorbic acid as the reducing agent, with the electrocatalytic potential of the resulting Pt-NPs@CNOs nano-composites demonstrated.

Surface-bound molecular rulers for probing the electrical double layer

Herein, we report the first experimental investigation on the effect of varying the position of redox-active moieties, within the electrical double layer, on the apparent formal potential and on the electron transfer rate constant. This was achieved using a rigid class of molecules, norbornylogous bridges, to place redox species (ferrocene) at a fixed position above the surface of the electrode. Cyclic voltammetry and alternating current voltammetry were used to calculate the apparent formal potential and the electron transfer rate constant for the electron transfer between the ferrocene and the gold electrode. We use the effect of electric field on the apparent formal potential measurement of the surface-bound redox species to calculate the potential drop from the initiation of the electrical double layer to different distances above it. It was found that self-assembled monolayers formed from ω-hydroxyalkanethiol have a potential profile very similar to that described by classical theories for bare metal electrodes. A steep drop in potential in the Stern layer was observed followed by a smaller potential drop in the Gouy-Chapman layer. The electron transfer rate constant was found to decrease as the distance between the ferrocene moiety and the initiation of the double layer is increased. Thus, the electron transfer rate constant appears to be dependent on ion concentration.

Shear induced carboplatin binding within the cavity of a phospholipid mimic for increased anticancer efficacy

Vesicles 107 ± 19 nm in diameter, based on the self-assembly of tetra-para-phosphonomethyl calix[4]- arene bearing n-hexyl moieties attached to the phenolic oxygen centres, are effective in binding carboplatin within the cavity of the macrocycle under shear induced within a dynamic thin film in a continuous flow vortex fluidic device. Post shearing the vesicles maintain similar diameters and retain carboplatin within the cavity of the calixarene in a hierarchical structure, with their size and morphology investigated using DLS, TEM, SEM and AFM. Location of the carboplatin was confirmed using NMR, FTIR, ESI-MS and EFTEM, with molecular modelling favouring the polar groups of carboplatin hydrogen bonded to phosphonic acid moieties and the four member cyclobutane ring directed into the cavity of the calixarene. The loading efficiency and release profile of carboplatin was investigated using LC-TOF/MS, with the high loading of the drug achieved under shear and preferential released at pH 5.5, offering scope for anti-cancer drug delivery. The hierarchical structured vesicles increase the efficacy of carboplatin by 4.5 fold on ovarian cancer cells, lowered the IC50 concentration by 10 fold, and markedly increased the percent of cells in the S-phase (DNA replication) of the cell cycle.

Antioxidant phospholipid calix[4]arene mimics as micellular delivery systems

Amphiphilic calix[4]arenes were designed as phospholipid mimics by incorporating PO3H2 or NMe3(+) head groups. Using PC12 cells and three stressors (H2O2, menadione and glutamate), we established safe calix[4]arene levels that are able not only to deliver antioxidant payloads of curcumin, but intriguingly also have inherent antioxidant properties. The calix[4]arenes appear to be potent synthetic antioxidants that could be used as nano-carriers for drug delivery.

Self-assembled monolayers formed using zero net curvature norbornylogous bridges: the influence of potential on molecular orientation.

A new class of electroactive norbornylogous bridges, with no net curvature, that form self-assembled monolayers on gold electrodes were studied by electrochemistry and in situ infrared spectroscopy. The influence of the electrode potential on the structure and conformation of the self-assembled monolayers (SAMs) was investigated. This was performed using two different lengths of rigid norbornylogous bridges with terminal ferrocene moieties and ω-hydroxyalkanethiols. It was found that single component monolayers of the rigid norbornylogous bridges changed their tilt angle with their transition from the ferrocene to ferricinium. However, when the norbornylogous SAMs were diluted with ω-hydroxyalkanethiols the tilt angle remained unchanged upon oxidation of ferrocene to ferricinium. It was also observed that the tilt angle of the diluent, ω-hydroxyalkanethiols changed at potentials exceeding 500 mV.