Using aryldiazonium salts that are air-stable and easily synthesized, we describe here a one-step, room-temperature route to direct covalent bonds between π-conjugated organic molecules on three ...material surfaces: Si, GaAs, and Pd. The Si can be in the form of single crystal Si including heavily doped p-type Si, intrinsic Si, heavily doped n-type Si, on Si(111) and Si(100), and on n-type polycrystalline Si. The formation of the aryl−metal or aryl−semiconductor bond attachments was confirmed by corroborating evidence from ellipsometry, reflectance FTIR, XPS, cyclic voltammetry, and AFM analyses of the surface-grafted monolayers. A data-encompassing explanation for the mechanism suggests a diazonium activation by reduction at the open circuit potential, with aryl radical secondary products bonding to the surface. The synthetic details are included for preparing the surface-grafted monolayers and the precursor diazonium salts. This spontaneous diazonium activation reaction offers an attractive route to highly passivating, robust monolayers and multilayers on many surfaces that allow for strong bonds between carbon and surface atoms with molecular species that are near perpendicular to the surface.
The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its ...structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current−voltage (i − V) and current−distance (i − d) measurements. The i − V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i − V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i − V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V TH, for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor β of hexadecanethiol was high, ranging from 1.3 to 1.4 Å-1, and was nearly independent of the tip bias. The β-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 Å-1 for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 Å-1 for dinitro oligo(phenylene) thiol, VI, at a −3.0 V tip bias. Both the V TH and β values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, ∼25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.
A proof of concept procedure for the electroaddressable covalent immobilization of DNA and protein on arrayed electrodes along with simultaneous detection of multiple bioagents in the same sample ...solution is described. Carboxyphenyldiazonium was selectively deposited onto five of nine individually addressable electrodes in an array via bias assisted assembly. Amine functionalized DNA probes were covalently coupled to the carboxyl surface via carbodiimide chemistry. This was followed by the covalent immobilization of diazonium-antibody conjugates into the remaining four electrodes via cyclic voltammetry. Simultaneous electrochemical detection of a DNA sequence related to the breast cancer BRCA1 gene and the human cytokine protein interleukin-12, which is a substantial component in the immune system response and attack of tumor cells, is reported. These results demonstrate the possibility of selective patterning of diverse biomolecules on a single device and may have significant implications for future development of microarrays and biosensors.
A simple one-step procedure is introduced for the preparation of diazonium−enzyme adducts. The direct electrically addressable deposition of diazonium-modified enzymes is examined for electrochemical ...sensor applications. The deposition of diazonium−horseradish peroxidase leads to the direct electron transfer between the enzyme and electrode exhibiting a heterogeneous rate constant, k s, of 10.3 ± 0.7 s-1 and a ΔE p of 8 mV (v = 150 mV/s). The large k s and low ΔE p are attributed to the intimate contact between enzyme and electrode attached by one to three phenyl molecules. Such an electrode shows high nonmediated catalytic activity toward H2O2 reduction. Future generations of arrayed electrochemical sensors and studies of direct electron transfer of enzymes can benefit from protein electrodes prepared by this method.
The electronic properties of alkanethiolate CH3(CH2) n S−, n = 9 and 11 and alkaneselenolate CH3(CH2) n Se−, n = 9 and 11 self-assembled monolayers on Au{111} have been quantitatively compared. ...Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.
Presented here are several convergent synthetic routes to conjugated oligo(phenylene ethynylene)s. Some of these oligomers are free of functional groups, while others possess donor groups, acceptor ...groups, porphyrin interiors, and other heterocyclic interiors for various potential transmission and digital device applications. The syntheses of oligo(phenylene ethynylene)s with a variety of end groups for attachment to numerous metal probes and surfaces are presented. Some of the functionalized molecular systems showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects. Finally, the syntheses of functionalized oligomers are described that can form self‐assembled monolayers on metallic electrodes that reduce the Schottky barriers. Information from the Schottky barrier studies can provide useful insight into molecular alligator clip optimizations for molecular electronics.
Functionalized conjugated oligo(phenylene ethylene)s with donor groups, acceptor groups, porphyrin interiors, and heterocycle interiors (see scheme) have been synthesized by several convergent synthetic routes. These compounds have potential for wire and digital device applications: some showed linear, wirelike, current versus voltage (I(V)) responses, while others exhibited nonlinear I(V) curves for negative differential resistance (NDR) and molecular random access memory effects.
The direct electrically addressable deposition of diazonium-modified antibodies is examined for electrochemical immunosensing applications. The immobilized antibodies can be detected by the use of ...electroactive enzyme tags and nanoparticle-gold labeling. Control over antibody functionalization density and minimal spontaneous grafting of diazonium–antibody adducts is shown. The utility of the technique for a sandwich immunoassay as well as the ability to individually and selectively address closely spaced microelectrodes for multi-target protein detection in an array format is demonstrated.
We report the application of 4‐nitrophenyl diazonium modified electrodes towards the electrochemical detection of NADH. Selective activation of individual electrodes on a 5 element array by ...electro‐addressable conversion of nitro groups to amines and subsequent EDC/NHS crosslinking to the NADH oxidant, pyrroloquinoline quinone (PQQ), is demonstrated. Inactivated electrodes retained nitro functionality and were protected against non‐specific adsorption and mild chemical reactions. Electrodeposition conditions were used to control nitrophenyl film thickness and showed that while increased film thickness leads to greater functionalization density of PQQ, it also results in decreased electron transfer kinetics. The electrodeposition protocol can therefore serve as a method to control electrode functionalization density and film electron transfer kinetics. We believe this simple technique for selective electrode functionalization may facilitate the development of next generation multianalyte electrochemical sensors.
Many publications have examined the biodegradable polymer poly(propylene fumate) (PPF) for use in tissue engineering applications. We have examined a similar crosslinkable polymer system, ...poly(propylene fumerate)-co-(propylene maleate) (PPFcPM), derived from maleic anhydride (MA) and 1,2-propylene diol (PD). This copolymer system uses a less expensive monomer as well as leads to varied ratios of fumerate to maleate groups, allowing tuning of the crosslinked polymer properties such as degradation rate. Two different reaction conditions were used to synthesize the copolymer from MA and PD. In the first case (Method A), toluene was used as a solvent to azeotropically (85°C) remove water to drive the acid catalyzed esterification reaction. In the second case (Method B), the initial ring opening reaction was conducted, followed by addition of catalyst and removal of water to produce polymer of higher molecular weight. Both polymer systems had glass transition temperatures (Tg) below room temperature. The low Tg PPFcPM was dissolved in chloroform along with the photoinitiator phenylbis(2,4,6-trimethylbenzoyl)-phosphine oxide (BAPO) and electrospun. The polymer fibers were crosslinked soon after they formed to produce noncalendaring 3D porous scaffolds. Control experiments without the BAPO photoinitiator did not produce fiber mats.
Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate CH3(CH2) n Se, n = 9 and 11 self-assembled monolayers on Au{111} have been characterized by scanning tunneling ...microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moiré pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 Å, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the (√3 × 3√3)R30° unit cell differentiated by whether the molecules bind at 2- or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one Au atom lower than the rest. The structural conclusions are supported by excellent agreement of experimental lattice parameters and those derived from molecular packing models. Comparison of the alkaneselenolate data with analogous structural phases reported for alkanethiolate monolayers on Au{111} shows that differences between the two systems can be understood on the basis that self-assembly is guided both by headgroup−headgroup as well as headgroup−substrate interactions.
