Cation recognition by self-assembled monolayers of oriented helical peptides having a crown ether unit

General Signal Amplification Strategy for Nonfaradic Impedimetric Sensing: Trastuzumab Detection Employing a Peptide Immunosensor

A label-free and reagent-free peptide mimotope capacitive biosensor has been developed for cancer drug (trastuzumab) quantification based on nonfaradic readout. The low sensitivity issue of capacitive biosensors was overcome with two innovations: peptide mimotope mixed self-assembled monolayer (SAM) biointerface and dilution of the analysis buffer. Signal amplification was achieved through dilution of phosphate-buffered saline (PBS) to tune C_dl to dominate the overall capacitance change upon target binding, which contribution is often negligible without dilution. After 1000× dilution, the limit of detection was lowered 500-fold (0.22 μg/mL) and the sensitivity was increased 20-fold [0.04192 (μg/mL)^-1] in comparison with undiluted PBS. The proposed signal amplification strategy is more straightforward and practical compared to biorecognition element engineering and other strategies. The proposed method was further applied to planar electrodes for optimizing sensing response time to less than 1 min.

A generic expression system to produce proteins that co-assemble with alkane thiol SAM

Surface biology aims to observe and control biological processes by combining bio-, surface, and physical chemistry. Self-assembled monolayers (SAM) on gold surfaces have provided excellent methods for nanoscale surface preparation for such studies. However, extension of this work requires the specific immobilization of whole protein domains and the direct incorporation of recombinant proteins into SAM is still problematic. In this study a short random coil peptide has been designed to insert into thioalkane layers by formation of a hydrophobic helix. Surface plasmon resonance (SPR) studies show that specific immobilization via the internal cysteine is achieved. Addition of the peptide sequence to the terminus of a protein at the genetic level enables the production of a range of recombinant fusion-proteins with good yield. SPR shows that the proteins display the same gold-binding behavior as the peptide. It is shown that cell growth control can be achieved by printing the proteins using soft lithography with subsequent infilling with thio-alkanes The expression plasmid is constructed so that any stable protein domain can be easily cloned, expressed, purified and immobilized.

Molecular rectification of a helical peptide with a redox group in the metal-molecule-metal junction

A helical hexadecapeptide immobilized on gold via a thiophenyl group at the N-terminal was analyzed by scanning tunneling microscopy under ultrahigh vacuum to obtain the I-V response at a molecular level. The attenuation factor of the electron transfer through the hexadecapeptide was determined by applying the Simons model to the I-V response to show better molecular conductance of the hexadecapeptide than dodecanethiol. Chemical modification at the C-terminal of the hexadecapeptide with a ferrocene unit, on the other hand, brought about significant changes in the I-V response, where the helical peptide became more conductive at the negative bias voltage. The molecular rectification behavior is due to the ferrocene unit regulating the direction of the electron transfer at the metal-molecule junction.

Label-Free Impedance Biosensors: Opportunities and Challenges

Impedance biosensors are a class of electrical biosensors that show promise for point-of-care and other applications due to low cost, ease of miniaturization, and label-free operation. Unlabeled DNA and protein targets can be detected by monitoring changes in surface impedance when a target molecule binds to an immobilized probe. The affinity capture step leads to challenges shared by all label-free affinity biosensors; these challenges are discussed along with others unique to impedance readout. Various possible mechanisms for impedance change upon target binding are discussed. We critically summarize accomplishments of past label-free impedance biosensors and identify areas for future research.

Effects of Dipole Moment, Linkers, and Chromophores at Side Chains on Long-Range Electron Transfer through Helical Peptides

Octadecapeptides carrying a ferrocene moiety at the molecular terminal were self-assembled on gold, and long-range electron transfer from the ferrocene moiety to gold was investigated by electrochemical methods. Effects on electron transfer of dipole moment of helical peptides, linkers connecting the peptide to gold, and chromophores introduced into the side chains were discussed. Cyclic voltammetry of the monolayers in an aqueous solution revealed that long-range electron transfer over 40 A occurred along the peptide molecule. Chronoamperometry showed that the long-range electron transfer should be ascribed to a hopping mechanism with use of amide groups as hopping sites. Electron transfer through the long peptide was not significantly accelerated by the dipole moment. However, the linker remarkably affected electron transfer depending on whether it was a methylene chain or a phenylene group, suggesting that local electron transfer between gold and the peptides should be the slowest step to determine the overall rate. Pyrenyl groups introduced into the side chains in the middle of the peptide molecule did not noticeably change electron transfer, probably because pyrenyl groups were too distant to allow direct electron transfer between them. Electrostatic potential profiles across the peptide monolayers were also calculated to explain reasonably the several interesting features in the present peptide systems.