A method for removing self-assembled monolayers on gold

Self-assembled monolayers (SAMs) have been widely used in studying interfacial phenomena, biological processes, electrochemistry, photoelectrochemistry, photoactivity and molecular interaction. Much research has been carried out in fabricating and removing SAMs on different substrates. In this work, we report for the first time, to our knowledge, that SAMs of thiolates on gold can be removed by immersing SAMs in 0.5 M NaBH 4 solution for 10 min. The procedure of removing thiolates was very convenient. Cyclic voltammetry, surface-enhanced Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize this process. The results indicated that the SAMs of thiolates on gold can be removed efficiently by NaBH 4.     

Adsorption of alkylthiol-capped gold nanoparticles onto alkylthiol self-assembled monolayers: an SPR study

The kinetics of alkylthiol-capped gold nanoparticle (RS/Au-NP) adsorption to alkylthiol/Au self-assembled monolayers (RS/Au-SAMs) has been studied using SPR (surface plasmon resonance) spectroscopy. Variation of the alkylthiol chain terminus (CH3, COOH) and solvent (H2O, hexane) provides insight into the relative importance of solvation energies (in the context of surface energies) and RS/Au-NP structure on adsorption kinetics. The kinetics, fitted to the Langmuir kinetic model, yield adsorption and desorption rate constants. DeltaG(ads) derived from kinetic data are compared to calculated values of work of adhesion (W(adh)), derived from the corresponding surface energies. The absence of a deltaG(ads) - W(adh) correlation arises because the measured kinetics are not reporting the approach to equilibrium and/or because there are factors (i.e., local chain packing defects, surface hydration differences, or particle-particle interactions) not accounted for in calculated W(adh) values.     

Electrochemical biosensor based on self-assembled monolayers modified with gold nanoparticles for detection of HER-3

We have developed a new immunological biosensor for ultrasensitive quantification of human epidermal growth factor receptor-3(HER-3). In order to construct the biosensor, the gold electrode surface was layered with, hexanedithiol, gold nanoparticles, and cysteamine, respectively. Anti-HER-3 antibody was covalently attached to cysteamine by glutaraldehyde and used as a bioreceptor in a biosensor system for the first time by this study. Surface characterization was obtained by means of electrochemical impedance spectroscopy and voltammetry. The proposed biosensor showed a good analytical performance for the detection of HER-3 ranging from 0.2 to 1.4 pg mL⁻¹. Kramers–Kronig transform was performed on the experimental impedance data. Moreover, in an immunosensor system, the single frequency impedance technique was firstly used for characterization of interaction between HER-3 and anti-HER-3. Finally the presented biosensor was applied to artificial serum samples spiked with HER-3.     

Dipodal ferrocene-based adsorbate molecules for self-assembled monolayers on gold

1,1'-Difunctionalised ferrocene derivatives have been studied, which contain groups suitable for chemisorption on gold substrates, namely -NC, -PR(2) as well as a range of sulfur-containing units like -NCS, -SR, and thienyl. Thin films on gold have been fabricated from solution with most of these adsorbate species. Film thickness, composition and structure were investigated primarily by X-ray photoelectron and near-edge X-ray absorption fine-structure spectroscopy. The quality of self-assembled monolayers fabricated from 1,1'-diisocyanoferrocene (1) and 1,1'-diisothiocyanatoferrocene (2) turned out to be superior to that of films based on the other adsorbate species investigated. In addition to the surface coordination behaviour of 1 towards gold substrates, relevant aspects of the molecular coordination chemistry of 1 have also been addressed, including the synthesis and characterisation of [(mu-1){Cr(CO)(5)}(2)], [Ag(2)(mu-1)(2)](NO(3))(2) x H(2)O and [(mu-1)(AuCl)(2)]. The crystal structure of the gold complex is governed by aurophilic interactions and can be taken as a model for the arrangement of 1 in self-assembled monolayers on gold.     

Mixed charged zwitterionic self-assembled monolayers as a facile way to stabilize large gold nanoparticles

Here we report a facile way of stabilizing large gold nanoparticles (AuNPs) by mixed charged zwitterionic self-assembled monolayers (SAMs). The citrate-capped AuNPs with diameters ranging from 16 nm to even ～100 nm are well stabilized via a simple place exchange reaction with a 1:1 molar ratio mixture of negatively charged sodium 10-mercaptodecanesulfonic acid (HS-C10-S) and positively charged (10-mercaptodecyl)-trimethyl-ammonium bromide (HS-C10-N4). The 16 nm AuNPs protected by mixed charged zwitterionic SAMs not only show much better stability than the single negatively or positively charged AuNPs, but also exhibit exciting stability as well as those modified by monohydroxy (1-mercaptoundec-11-yl) tetraethylene glycol (HS-C11-EG4). Importantly, 16 nm AuNPs protected by mixed SAMs exhibit good stability in cell culture medium with 10% FBS and strong protein resistance, especially with excellent resistance against plasma adsorption. Moreover, the mixed charged zwitterionic SAMs are also able to well-stabilize larger AuNPs with a diameter of 50 nm, and to help remarkably improve their stability in saline solution compared with HS-C11-EG4 protected ones. When it comes to AuNPs with a diameter of 100 nm, the mixed charged zwitterionic SAM protected nanoparticles retain a smaller hydrodynamic diameter and even better long-term stability than those modified by mercaptopolyethylene glycol (M(w) = 2000, HS-PEG2000). The above results demonstrated that the mixed charged zwitterionic SAMs are able to have a similar effect on stabilizing the large gold nanoparticles just like the single-component zwitterionic SAMs. Concerning its ease of preparation, versatility, and excellent properties, the strategy based on the mixed charged zwitterionic SAM protection might provide a promising method to surface tailoring of nanoparticles for biomedical application.     

Electroformation of peptide self-assembled monolayers on gold

The application of a potential to deposit a monolayer of 3-mercaptopropionic acid-histidinyl-histidinyl-histidinyl-aspartyl-aspartyl (3-MPA-HHHDD-OH) controls the density and molecular structure of the peptide monolayer, which results in different wettabilities of the surface, surface density, orientation of the molecule (extended or bent), and nonspecific adsorption of serum proteins. 3-MPA-HHHDD-OH must be deposited at 200 mV to maintain an extended configuration, which promoted low biofouling properties.     

Microelectrodes with gold nanoparticles and self-assembled monolayers for in vivo recording of striatal dopamine

Electrochemical determination of in vivo dopamine (DA) using implantable microelectrodes is essential for monitoring the DA depletion of an animal model of Parkinson's disease (PD), but faces substantial interference from ascorbic acid (AA) in the brain area due to similar electroactive characteristics. This study utilizes gold nanoparticles (Au-NPs) and self-assembled monolayers (SAMs) to modify platinum microelectrodes for improving sensitivity and specificity to DA and alleviating AA interference. With appropriate choice of ω-mercaptoalkane carboxylic acid chain length, our results show that a platinum microelectrode coated with Au-NPs and 3-mercaptopropionic acid (MPA) has approximately an 881-fold specificity to AA. During amperometric measurements, Au-NP/MPA reveals that the responsive current is linearly dependent on DA over the range of 0.01-5 μM with a correlation coefficient of 0.99 and the sensitivity is 2.7-fold that of a conventional Nafion-coated electrode. Other important features observed include fast response time (below 2 s), resistance to albumin adhesion and low detection limit (7 nM) at a signal to noise ratio of 3. Feasibility of in vivo DA recording with the modified microelectrodes is verified by real-time monitoring of electrically stimulated DA release in the striatum of anesthetized rats with various stimulation parameters and administration of a DA uptake inhibitor. The developed microelectrodes present an attractive alternative to the traditional options for continuous electrochemical in vivo DA monitoring.     

Alkanephosphonates on hafnium-modified gold: a new class of self-assembled organic monolayers

A new method for assembling organic monolayers on gold is reported that employs hafnium ions as linkers between a phosphonate headgroup and the gold surface. Monolayers of octadecylphosphonic acid (ODPA) formed on gold substrates that had been pretreated with hafnium oxychloride are representative of this new class of organic thin films. The monolayers are dense enough to completely block assembly of alkanethiols and resist displacement by alkanethiols. The composition and structure of the monolayers were investigated by contact angle goniometry, XPS, PM-IRRAS, and TOF-SIMS. From these studies, it was determined that this assembly strategy leads to the formation of ODPA monolayers similar in quality to those typically formed on metal oxide substrates. The assembly method allows for the ready generation of patterned surfaces that can be easily prepared by first patterning hafnium on the gold surface followed by alkanephosphonate assembly. Using the bifunctional (thiol-phosphonate) 2-mercaptoethylphosphonic acid (2-MEPA), we show that this new assembly chemistry is compatible with gold-thiol chemistry and use TOF-SIMS to show that the molecule attaches through the phosphonate functionality in the patterned region and through the thiol in the bare gold regions. These results demonstrate the possibility of functionalizing metal substrates with monolayers typically formed on metal oxide surfaces and show that hafnium-gold chemistry is complementary and orthogonal to well-established gold-thiol assembly strategies.     

Adsorption kinetics of p-nitrobenzenethiol self-assembled monolayers on a gold surface

The adsorption kinetics of octadecanethiol (ODT) and p-nitrobenzenethiol (NBT) from ethanol solutions has been studied by means of contact angle, optical ellipsometry, angle-resolved X-ray photoelectron spectroscopy (ARXPS), and grazing angle attenuated total reflection Fourier transform infrared (FTIR) measurements. ODT data were used as a reference for the kinetics studies of film growth. The growth of self-assembled monolayers from dilute solutions follows Langmuir isotherm adsorption kinetics. A saturated film is formed within 5 h after immersion in solutions of concentrations ranging from 0.0005 to 0.01 mM. The density of the monolayer depends on the concentration of the solution.     

Electron transfer of quinone self-assembled monolayers on a gold electrode

Dialkyl disulfide-linked naphthoquinone, (NQ-C_n-S)₂, and anthraquinone, (AQ-C_n-S)₂, derivatives with different spacer alkyl chains (C_n: n = 2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH₂) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were −0.48 and −0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E_1/2 (mV) = E′ − 59pH for 2H⁺/2e⁻ process in the pH range 3–11. In the range higher than pH 11, the value was estimated with E_1/2 (mV) = E′ − 30pH , which may correspond to H⁺/2e⁻ process. The tunneling barrier coefficients (β) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH₂), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.     

Adsorption of self-assembled monolayers of mercaptan on gold

XPS and AES are suitable techniques for studying organic monolayers on metals if radiation doses are kept low. The adsorption of self-assembled (SA) mercaptan monolayers on gold is a process in two stages. The adsorption to near completeness is very rapid. However, the process of orientation of the carbon chains, which is responsible for the blocking of electrochemical reactions takes much longer, as could be shown by ARXPS (angle resolved X-ray photo electron spectroscopy). Adsorption under potential control allows electrochemical experiments during the adsorption process as e.g. the measurement of the capacity of the electric double layer. Furthermore the control of the potential guarantees sure that the metal/liquid interface is well defined during the adsorption process.     

FTIR-ATR study of self-assembled thiol monolayers on gold

Self-assembled monolayers of 1-teradecanethiol on gold were characterized by means of FTIR-ATR measurements, XPS and contact angle measurements. Linear dichroism measurements using FTIR-ATR are used to estimate the orientation of the alkyl chains. An equation for calculating the orientation angles of the alkyls chains was deduced.     

Blocking behavior of self-assembled monolayers on gold electrodes

Self-assembled monolayers (SAMs) with metal electrodes, especially thiols on gold, are the subject of this investigation because of the unique properties of SAM-modified surfaces. Normal alkanethiols are used to modify the surface of a conventional gold electrode to block certain ions such as Pb(II) and Cu(II) from the surface of the electrode. Normal alkanethiols are also used to study the SAM-gold interfacial adsorption-desorption behavior of the self-assembled monolayer. The effects of varying chain length of SAMs, varying concentration of the alkanethiol solutions, immersion time of the pure gold electrode in the SAM solution, and the stability of a SAM-modified gold electrode in fresh chloroform are investigated using the oxidation-reduction peaks of gold. Conditions that optimize the surface coverage and the uniformity of the SAMs have been determined. Normal alkanethiols proved to be a good insulator on the electrode surface. Received: 16 January 1997 / Accepted: 4 March 1997     

Reactivity of acetylenyl-terminated self-assembled monolayers on gold: triazole formation

We report the reactivity of acetylenyl-terminated self-assembled monolayers (SAMs) on gold toward "click" chemistry, Huisgen 1,3-dipolar addition, leading to the formation of triazoles. After the formation of acetylenyl-terminated SAMs, the triazole formation was performed on the SAMs and the reaction was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and contact angle goniometry. "Click" chemistry has offered a versatile strategy for the functionalization in solution chemistry with mild reaction conditions and a high compatibility in functional groups, and our result shows that the reaction could be applied to acetylenyl-terminated SAMs for the introduction of useful functional groups to the surfaces.     

Adsorption of self-assembled monolayers of mercaptan on gold

XPS and AES are suitable techniques for studying organic monolayers on metals if radiation doses are kept low. The adsorption of self-assembled (SA) mercaptan monolayers on gold is a process in two stages. The adsorption to near completeness is very rapid. However, the process of orientation of the carbon chains, which is responsible for the blocking of electrochemical reactions takes much longer, as could be shown by ARXPS (angle resolved X-ray photo electron spectroscopy). Adsorption under potential control allows electrochemical experiments during the adsorption process as e.g. the measurement of the capacity of the electric double layer. Furthermore the control of the potential guarantees sure that the metal/liquid interface is well defined during the adsorption process.     

Stability of self-assembled monolayers on titanium and gold

Methyl- and hydroxyl-terminated phosphonic acid self-assembled monolayers (SAMs) were coated on Ti from aqueous solution. Dodecyl phosphate and dodecyltrichlorosilane SAMs were also coated on Ti using solution-phase deposition. The stability of SAMs on Ti was investigated in Tris-buffered saline (TBS) at 37 degrees C using X-ray photoelectron spectroscopy, contact angle goniometry, and atomic force microscopy. For comparison purposes, a hydroxyl-terminated thiol SAM was coated on Au, and its stability was also investigated under similar conditions. In TBS, a significant proportion of phosphonic acid or phosphate molecules were desorbed from the Ti surface within 1 day, while the trichlorosilane SAM on Ti or thiol SAM on Au was stable for up to 7 days under similar conditions. The stability of hydroxyl-terminated phosphonic acid SAM coated Ti and thiol SAM coated Au was investigated in ambient air and ultraviolet (UV) light. In ambient air, the phosphonic acid SAM on Ti was stable for up to 14 days, while the thiol SAM on Au was not stable for 1 day. Under UV-radiation exposure, the alkyl chains of the phosphonic acid SAM were decomposed, leaving only the phosphonate groups on the Ti surface after 12 h. Under similar conditions, decomposition of alkyl chains of the thiol SAM was observed on the Au surface accompanied by oxidation of thiolates.     

Reactivity of acid fluoride-terminated self-assembled monolayers on gold

This report describes the reactivity of acid fluoride (AF)-terminated self-assembled monolayers (SAMs) on gold toward amine and alcohol compounds and the potentiality of AF as a reactive intermediate for surface functionalizations. The AF group was generated in situ on a gold surface by reacting the terminal carboxylic acid group in the SAM of 16-mercaptohexadecanoic acid with cyanuric fluoride and pyridine under the optimized conditions. AF was found to be highly reactive toward various amine groups, such as primary and secondary amines, but it did not react effectively with alcohol. In addition, the amide coupling reaction by microcontact printing (microCP) was compared with the solution-based reaction: when amine-derivatized ferrocene compound was used for 1-min microCP on the AF-activated surface, the surface coverage of the reaction product was about 83% of 3.45 x 1014 cm-2, the coverage obtained in the solution-based reaction. On the basis of the high reaction efficiency of microCP, the AF-activated surface was also used as a platform for patterning a biological ligand, biotin.     

Direct patterning of self-assembled monolayers on gold using a laser beam

The development of a methodology to manipulate surface properties of a self-assembled monolayer (SAM) of alkanethiol on a gold film using direct laser patterning is the objective of this paper. The present study demonstrates proof of the concept for the feasibility of laser patterning monolayers and outlines theoretical modeling of the process to predict the resulting feature size. This approach is unique in that it eliminates the need for photolithography, is noncontact, and can be extended to other systems such as SAMs on silicon wafers or potentially polymeric substrates. A homogeneous SAM made of 1-hexadecanethiol is formed on a 300-A sputtered film of gold (supported by a soda lime glass substrate). Localized regions are then desorbed by scanning the focal spot of a 488-nm continuous-wave argon ion laser beam under a nitrogen atmosphere. The desorption occurs as a result of a high substrate temperature produced by the moving laser beam with a Gaussian spatial profile at a constant speed of 200 microm/s. After completing the scans, the sample is dipped into a dilute solution of 16-mercaptohexadecanoic acid and a hydrophilic monolayer self-assembles along the previously irradiated regions. The resultant lines are viewed, and line widths are measured using both wetting with tridecane under a light microscope and scanning electron microscopy. Using the direct laser patterning method, we have produced straight line patterns with widths of 28-170 microm. A thermal model was constructed to predict the line width of the desorbed monolayer. The effect of the laser power, beam waist, and temperature dependence of the substrate conductivity on the theoretical predictions is considered. It is shown that the theoretical predictions are in good agreement with the experimental results, and, thus, the model can effectively be used to predict experimental results.     

Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.     

Protein-resistant self-assembled monolayers on gold with latent aldehyde functions

In the present study, oligo(ethylene glycol) (OEG)-linked alkanethiols were synthesized which carry a vicinal diol on one end of the OEG chain. After self-assembled monolayer (SAM) formation on gold, the vicinal diols were converted into aldehyde functions by exposure to aqueous NaIO4, as previously used for SAMs with OEG chains buried in the center of the SAM [Jang et al. Nano Lett. 2003, 3, 691-694]. Mixed SAMs with latent aldehydes on 5% of the OEG termini showed high protein resistance, which greatly slowed the kinetics of protein coupling on the time scale of minutes. Small bioligands (such as biocytin hydrazide) or small heterobifunctional crosslinkers (maleimidopropionyl hydrazide, pyridyldithiopropionyl hydrazide) with hydrazide functions were efficiently bound to the aldehyde functions on the SAM, providing for specific capture of streptavidin or for fast covalent binding of proteins with free thiols or maleimide functions, respectively. In conclusion, OEG-terminated SAMs with latent aldehydes serve as protein-resistant sensor surfaces which are easily functionalized with small ligands or with heterobifunctional crosslinkers to which the bait molecule is attached in a subsequent step.