A comparative study of capacitive immunosensors based on self-assembled monolayers formed from thiourea, thioctic acid, and 3-mercaptopropionic acid

A procedure was developed for the covalent coupling of anti-alpha-fetoprotein antibody (anti-AFP) to a gold surface modified with a self-assembled monolayer (SAM) of thiourea (TU). The performance of the SAM-antibody layer was compared to those of similar layers based on thioctic acid (TA) and 3-mercaptopropionic acid (MPA) by using flow injection capacitive immunosensor system. Covalent coupling of anti-AFP on self-assembled thiourea monolayer (SATUM) modified gold electrode can be used to detect alpha-fetoprotein with high efficiency, similar sensitivity, the same linear range (0.01-10 microgl(-1)) and detection limit (10 ngl(-1)) as those obtained from sensors based on self-assembled thioctic acid monolayer (SATAM) and self-assembled 3-mercaptopropionic acid monolayer (SAMPAM). The system is specific for alpha-fetoprotein and can be regenerated and reused up to 48 times. Therefore, self-assembled monolayer using thiourea which is cheaper than thioctic acid and 3-mercaptopropionic acid is a good alternative for biosensor applications when SAMs are used.     

Preparation and characterization of nitrilotriacetic-acid-terminated self-assembled monolayers on gold surfaces for matrix-assisted laser desorption ionization-time of flight-mass spectrometry analysis of proteins and peptides

On-target affinity capture, enrichment and purification of biomolecules improve detection of specific analytes from complex biological samples in matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) analysis. In this paper, we report a simple method for preparation of a self-assembled nitrilotriacetic acid (NTA) monolayer on gold surface which can be used as a MALDI-TOF-MS sample target specifically for recombinant oligohistidine-tagged proteins/peptides and phosphorylated peptides. The NTA functional groups are immobilized to the gold surface via the linkage of 1,8-octanedithiol which forms a self-assembled monolayer on gold. Characterization by X-ray photoelectron spectroscopy and MALDI analysis of the modified surface are described. The chemically modified surface shows strong affinity toward the analytes of interest, which allows effective removal of the common interferences, e.g. salts and detergents, and therefore leads to improved signal/noise ratio and detection limit. The use of the modified surface simplifies the sample preparation for MALDI analysis of these targeted analytes.     

Immobilization of rhodopsin on a self-assembled multilayer and its specific detection by electrochemical impedance spectroscopy

Rhodopsin, the G protein-coupled receptor (GPCR) which mediates the sense of vision, was prepared from calf eyes and used as receptor enriched membrane fraction. In this study it was immobilized onto gold electrode by two different techniques: Langmuir-Blodgett (LB) and a strategy based on a self-assembled multilayer. We demonstrated that Langmuir and LB films of rhodopsin are not stable. Thus, in this study a new protein multilayer was prepared on gold electrode by building up layer-by-layer a self-assembled multilayer. It is composed of a mixed self-assembled monolayer formed by MHDA and biotinyl-PE, followed by a biotin-avidin system which allows binding of biotinylated antibody specific to rhodopsin. The immobilization of rhodopsin in membrane fraction, by the specific antibody bound previously on self-assembled multilayer, was monitored with electrochemical impedance spectroscopy (EIS). In addition, the specificity and sensitivity of this self-assembled multilayer system to the presence of rhodopsin were investigated. No effect was observed when the system was in contact with olfactory receptor I7 in membrane fraction used for control measurements. All these results demonstrate that rhodopsin can be immobilized efficiently, specifically, quantitatively and stably on gold electrode through the self-assembled multilayer.     

Rapid and Sensitive Detection of Malachite Green and Melamine with Silver Film over Nanospheres by Surface-Enhanced Raman Scattering

Plasmonics - Silver film over nanospheres (AgFON) have been prepared by depositing Ag thin film onto the self-assembled monolayer arrays of polystyrene nanospheres with vacuum magnetron sputtering...     

Ultrasensitive electrical detection of protein using nanogap electrodes and nanoparticle-based DNA amplification

The present study describes an ultrasensitive protein biochip that employs nanogap electrodes and self-assembled nanoparticles to electrically detect protein. A bio-barcode DNA technique amplifies the concentration of target antigen at least 100-fold. This technique requires the establishment of conjugate magnetic nanoparticles (MNPs) and gold nanoparticles (AuNPs) through binding between monoclonal antibodies (2B2), the target antigen, and polyclonal antibodies (GP). Both GP and capture ssDNA (single-strand DNA) bonds to bio-barcode ssDNA are immobilized on the surface of AuNPs. A denature process releases the bio-barcode ssDNAs into the solution, and a hybridization process establishes multilayer AuNPs over the gap surface between electrodes. Electric current through double-layer self-assembled AuNPs is much greater than that through self-assembled monolayer AuNPs. This significant increase in electric current provides evidence that the solution contains the target antigen. Results show that the protein biochip attains a sensitivity of up to 1 pg/μL.     

Label-free capacitive immunosensor for microcystin-LR using self-assembled thiourea monolayer incorporated with Ag nanoparticles on gold electrode

A label-free immunosensor based on a modified gold electrode incorporated with silver (Ag) nanoparticles (NPs) to enhance the capacitive response to microcystin-LR (MCLR) has been developed. Anti-microcystin-LR (anti-MCLR) was immobilized on silver nanoparticles bound to a self-assembled thiourea monolayer. Interaction of anti-MCLR and MCLR were directly detected by capacitance measurement. Under optimum conditions, MCLR could be determined with a detection limit of 7.0pgl(-1) and linearity between 10pgl(-1) and 1mugl(-1). The immobilized anti-MCLR on self-assembled thiourea monolayer incorporated with silver nanoparticles was stable and good reproducibility of the signal could be obtained up to 43 times with an R.S.D. of 2.1%. Comparing to the modified electrode without silver nanoparticles it gave 1.7-fold higher sensitivity and lower limit of detection. The developed immunosensor was applied to analyze MCLR in water samples and the results were in good agreement with those obtained by high-performance liquid chromatography (HPLC) (P<0.05).     

Detecting penicillin G in milk with impedimetric label-free immunosensor

A label-free impedimetric flow injection immunosensor for the direct detection of penicillin G has been developed. Anti-penicillin G was immobilized on a gold working electrode modified with a self-assembled monolayer of thioctic acid. Real time monitoring of impedance was carried out at the optimum frequency of 160 Hz. Under optimum operating conditions the system provided a wide linear range between 1.0 x 10(-13) and 1.0 x 10(-8) M with a very low detection limit of 3.0 x 10(-15) M, much lower than the MRL of penicillin G in milk (1.2 x 10(-8) M). The immobilized anti-penicillin G on self-assembled thioctic acid monolayer gold electrode was very stable and provided good reproducible signal after regeneration up to 45 times with a relative standard deviation (R.S.D.) lower than 4%. Good recoveries and precisions were obtained when spiked raw milk samples were analyzed.     

Sum-frequency generation spectroscopy of DNA monolayers

The anchoring of thiolated single-stranded DNA (HS-ssDNA) monolayers onto platinum substrates was investigated by sum-frequency generation spectroscopy. Different buffer solutions were used for the preparation of the adlayers. Vibrational fingerprints in the 2700-3100 cm(-1) spectral range showed the intercalation of Tris/EDTA (TE) buffer molecules within the HS-ssDNA self-assembled monolayer. Buffer contribution to SFG can be quenched either by using SFG inactive molecules like KH(2)PO(4)/K(2)HPO(4)/NaCl (PBS) or by repeated rinsing of the DNA layer with pure water. Comparing the SFG spectra of HS-ssDNA and mercaptohexanol (MCH), which had been self-assembled onto the same substrate, enabled us to infer ordering of the anchor arms and strong disordering of the DNA strands of HS-ssDNA monolayers self-assembled on platinum.     

Surface immobilization of an organo-iridium complex through a carbon-metal bond

A new approach to the immobilization of organometallic complexes to monolayer surfaces is demonstrated using aldehyde-terminated self-assembled monolayers and a Cp^∗Ir complex. The iridium complex is anchored to the surface by direct attachment of the metal to the carbon chain of the monolayer film.     

Self-assembled monolayer coating of biological probes to avoid protein adhesion

Probes designed to locally illuminate structures within plant cells are described. The probes studied are etch-sharpened single mode optical fibers, coated with aluminum, similar to probes used for near-field scanning optical microscopy. We find that cellular material adheres to the probes that are not coated with a self-assembled monolayer octadecyltrichlorosilane. The hydrophobic monolayer coating enabled these probes to be inserted into and removed from plant cells with no protein adhesion to the probes. This allows probe reinsertion and it causes less damage to the target cell, greatly facilitating in vivo optical study of cells.     

Vectorially oriented membrane protein monolayers: profile structures via x-ray interferometry/holography.

X-ray interferometry/holography was applied to meridional x-ray diffraction data to determine uniquely the profile structures of a single monolayer of an integral membrane protein and a peripheral membrane protein, each tethered to the surface of a solid inorganic substrate. Bifunctional, organic self-assembled monolayers (SAMs) were utilized to tether the proteins to the surface of Ge/Si multilayer substrates, fabricated by molecular beam epitaxy, to facilitate the interferometric/holographic x-ray structure determination. The peripheral membrane protein yeast cytochrome c was covalently tethered to the surface of a sulfhydryl-terminated 11-siloxyundecanethiol SAM via a disulfide linkage with residue 102. The detergent-solubilized, photosynthetic reaction center integral membrane protein was electrostatically tethered to the surface of an analogous amine-terminated SAM. Optical absorption measurements performed on these two tethered protein monolayer systems were consistent with the x-ray diffraction results indicating the reversible formation of densely packed single monolayers of each fully functional membrane protein on the surface of the respective SAM. The importance of utilizing the organic self-assembled monolayers (as opposed to Langmuir-Blodgett) lies in their ability to tether specifically both soluble peripheral membrane proteins and detergent-solubilized integral membrane proteins. The vectorial orientations of the cytochrome c and the reaction center molecules were readily distinguishable in the profile structure of each monolayer at a spatial resolution of 7 A.     

Comparison of different protein immobilization methods on quartz crystal microbalance surface in flow injection immunoassay.

In this study, a quartz crystal microbalance (QCM) system operated repetitively in flow injection analysis (FIA) mode, is reported. Four immobilization approaches of seven different methods include: (i) physical adsorption; (ii) two thioamine thiolation methods, using cysteamine and cystamine for gold chemisorption and further coupling; (iii) two oxidized dextran spacer methods, coupling of cysteamine and cystamine thiolated QCM surface with periodate-oxidized dextran for further Schiff acid-base reaction; and (iv) two thiol-gold chemisorption-based self-assembled monolayer (SAM), applying short-chain, C(3), and long-chain, C(11), mercapto fatty acids to insolubilize human serum albumin (HSA) on QCM surface. Effects of these protein immobilization methods on FIA immunoassay of anti-HSA were compared. At the 0.01 mg/ml anti-HSA level, the lowest analyte concentration tested, the SAM using 11-mercaptoundecanoic acid as QCM surface activating agent generated a larger frequency shift than the other immobilization methods. This implied that the use of thiolated long-chain fatty acid constructed as self-assembled monolayer may thereby potentially be a useful protein immobilization method in QCM-FIA application.     

Comparison of a carboxylated terthiophene surface with carboxymethylated dextran layer for surface plasmon resonance detection of progesterone

Functionalization of a gold surface is usually accomplished by covalent binding via self-assembled monolayers (SAMs) on the gold surface, followed by attachment of flexible polymeric linker layers such as dextran hydrogels. However, these techniques require multiple steps and also have nonspecific interactions and steric problems. In this study, a self-assembled carboxylated terthiophene monolayer was formed onto a gold surface to create a sensitive and stable surface plasmon resonance (SPR) biosensing system. Compared with a commercial carboxymethyl dextran chip (CM5), the terthiophene SAM surface provided more than six times more antibody-binding signals and nearly three times the SPR assay sensitivity for progesterone (P₄).     

A DNA self-assembled monolayer for the specific attachment of unmodified double- or single-stranded DNA.

A novel method for DNA surface immobilization and a paradigm for the attachment of unmodified DNA of any length or sequence are described herein. The development of a DNA self-assembled monolayer (DNA-SAM) that incorporates a DNA-thiol into a monolayer of inert alkane thiolates is reported. This DNA-SAM specifically hybridized complementary oligonucleotides while resisting the nonspecific adsorption of noncomplementary DNA and irrelevant proteins. Duplex DNA, having a single-stranded "capture tail," specifically bound to the DNA-SAM if the sequence of the "tail" was complementary to DNA presented in the SAM. The sense strand of the hybridized duplex DNA could be covalently attached to the surface by an enzymatic ligation reaction (leaving the anti-sense strand dissociable). DNA-binding proteins specifically bound to these surfaces only if their cognate sites were present in the duplex DNA.     

A piezoelectric immunosensor using hybrid self-assembled monolayers for detection of Schistosoma japonicum

Background

The parasite Schistosoma japonicum causes schistosomiasis disease, which threatens human life and hampers economic and social development in some Asian countries. An important lesson learned from efforts to reduce the occurrence of schistosomiasis is that the diagnostic approach must be altered as further progress is made towards the control and ultimate elimination of the disease.

Methodology/Principal Findings

Using mixed self-assembled monolayer membrane (mixed SAM) technology, a mixture of mercaptopropionic acid (MPA) and mercaptoethanol (ME) was self-assembled on the surface of quartz crystals by gold-sulphur-bonds. Soluble egg antigens (SEA) of S. japonicum were then cross-linked to the quartz crystal using a special coupling agent. As compared with the traditional single self-assembled monolayer immobilization method, S. japonicum antigen (SjAg) immobilization using mixed self-assembled monolayers exhibits much greater immunoreactivity. Under optimal experimental conditions, the detection range is 1∶1500 to 1∶60 (infected rabbit serum dilution ratios). We measured several infected rabbit serum samples with varying S. japonicum antibody (SjAb) concentrations using both immunosensor and ELISA techniques and then produced a correlation analysis. The correlation coefficients reached 0.973.

Conclusions/Significance

We have developed a new, simple, sensitive, and reusable piezoelectric immunosensor that directly detects SjAb in the serum. This method may represent an alternative to the current diagnostic methods for S. japonicum infection in the clinical laboratory or for analysis outside the laboratory.     

Scanning Tunnelling Microscopy and Molecular Modelling of Xanthine Monolayers Self-assembled at the Solid-Liquid Interface: Relevance to the Origin of Life

The development of scanning tunnelling microscopy (STM) has allowed examination of inorganic crystalline surfaces and their interactions with organic adsorbates with unparalleled resolution. As a novel technique in origin of life studies, the application of STM is detailed with particular attention paid to the methods employed in the analysis of organic monolayer structures. STM imaging and molecular modelling of self-assembled monolayers of the purine base, xanthine, formed on the surfaces of graphite and molybdenum disulfide are presented as an example. The putative role of such structures in the origin of life is discussed.     

Patterning of photosensitive polyimide LB film and its application in the fabrication of biomolecular microphotodiode array

Ultra thin film of photosensitive polyimide having benzene and sulfonyloxyimide moieties in the main chain was prepared using a Langmuir-Blodgett (LB) technique, and then micro array pattern of the polyimide LB film on a gold substrate was obtained by deep UV lithographic technique. In order to array cytochrome c molecules along the micro-patterned gold substrate, the well-characterized monolayer of cytochrome c was immobilized with a mixed monolayer of 11-mercaptoundecanoic acid (11-MUDA) and decanethiol. The redox activity and electron transfer between cytochrome c molecular center and gold electrode interface for the self-assembled cytochrome c monolayer were investigated by cyclic voltammetry measurement. Biomolecular photodiode consisting of cytochrome c and green fluorescent protein (GFP) onto the patterned gold substrate was fabricated by self-assembly process. The integration and morphology of cytochrome c and GFP were studied from the measurements of atomic force microscopy (AFM) and fluorescence emission. Especially, current-voltage characteristics of the protein multilayers were investigated by scanning tunneling microscopy (STM) and its application in biomolecular photodiode was also examined.     

Covalent immobilization of native biomolecules onto Au(111) via N-hydroxysuccinimide ester functionalized self-assembled monolayers for scanning probe microscopy.

We have worked out a procedure for covalent binding of native biomacromolecules on flat gold surfaces for scanning probe microscopy in aqueous buffer solutions and for other nanotechnological applications, such as the direct measurement of interaction forces between immobilized macromolecules, of their elastomechanical properties, etc. It is based on the covalent immobilization of amino group-containing biomolecules (e.g., proteins, phospholipids) onto atomically flat gold surfaces via omega-functionalized self-assembled monolayers. We present the synthesis of the parent compound, dithio-bis(succinimidylundecanoate) (DSU), and a detailed study of the chemical and physical properties of the monolayer it forms spontaneously on Au(111). Scanning tunneling microscopy and atomic force microscopy (AFM) revealed a monolayer arrangement with the well-known depressions that are known to stem from an etch process during the self-assembly. The total density of the omega-N-hydroxysuccinimidyl groups on atomically flat gold was 585 pmol/cm(2), as determined by chemisorption of (14)C-labeled DSU. This corresponded to approximately 75% of the maximum density of the omega-unsubstituted alkanethiol. Measurements of the kinetics of monolayer formation showed a very fast initial phase, with total coverage within 30 S. A subsequent slower rearrangement of the chemisorbed molecules, as indicated by AFM, led to a decrease in the number of monolayer depressions in approximately 60 min. The rate of hydrolysis of the omega-N-hydroxysuccinimide groups at the monolayer/water interface was found to be very slow, even at moderately alkaline pH values. Furthermore, the binding of low-molecular-weight amines and of a model protein was investigated in detail.     

Development of surface plasmon resonance immunosensor through metal ion affinity and mixed self-assembled monolayer

An immunosensor based on surface plasmon resonance (SPR) with enhanced performance was developed through a mixed self-assembled monolayer. A mixture of 16- mercaptohexadecanic acid (16-MHA) and 1-undecanethiol with various molar ratios was self-assembled on gold (Au) surface and the carboxylic acid groups of 16-MHA were then coordinated to Zn ions by exposing the substrate to an ethanolic solution of Zn(NO(3))(2)d6H2O. The antibody was immobilized on the SPR surface by exposing the functionalized substrate to the desired solution of antibody in phosphatebuffered saline (PBS) molecules. The film formation in series was confirmed by SPR and atomic force microscopy (AFM). The functionalized surface was applied to develop an SPR immunosensor for detecting human serum albumin (HSA) and the estimated detection limit (DL) was 4.27 nM. The limit value concentration can be well measured between ill and healthy conditions.     

Fabrication and characteristics of MOSFET protein chip for detection of ribosomal protein

A metal oxide silicon field effect transistor (MOSFET) protein chip for the easy detection of protein was fabricated and its characteristics were investigated. Generally, the drain current of the MOSFET is varied by the gate potential. It is expected that the formation of an antibody-antigen complex on the gate of MOSFET would lead to a detectable change in the charge distribution and thus, directly modulate the drain current of MOSFET. As such, the drain current of the MOSFET protein chip can be varied by ribosomal proteins absorbed by the self-assembled monolayer (SAM) immobilized on the gate (Au) surface, as ribosomal protein has positive charge, and these current variations then used as the response of the protein chip. The gate of MOSFET protein chip is not directly biased by an external voltage source, so called open gate or floating gate MOSFET, but rather chemically modified by immobilized molecular receptors called self-assembled monolayer (SAM). In our experiments, the current variation in the proposed protein chip was about 8% with a protein concentration of 0.7 mM. As the protein concentration increased, the drain current also gradually increased. In addition, there were some drift of the drain current in the device. It is considered that these drift might be caused by the drift from the MOSFET itself or protein absorption procedures that are relied on the facile attachment of thiol (-S) ligands to the gate (Au) surface. We verified the formation of SAM on the gold surface and the absorption of protein through the surface plasmon resonance (SPR) measurement.