Desorption Of Monolayers Research Articles

A Minimally Arbitrary Numerical Method for the Separation of Faradaic and Background Currents of Interest

Various self-limiting adsorptive processes occur that modify the electrode―electrolyte interface in a well-defined way. A few of these processes include reductive desorption of self-assembled monolayers, underpotential deposition of metals, and oxidation of adsorbed CO. The characterization of these systems requires determining the adsorbate coverage. The most common electrochemical technique for this is to perform voltammetry on the adlayer, which is integrated to obtain the charge. However, to obtain accurate coverages, it is necessary to subtract background currents due to unrelated processes. The development and application of a minimally arbitrary numerical background subtraction scheme is the subject of this article.

Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(1 1 1) electrode was investigated. The monolayer desorption in 0.1 M KClO 4 is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa −0.65 V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a wider potential range. This suggests that electrochemically-formed monolayers obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30 μF cm −2 after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1 M NaOH).

Toposelective Electrochemical Desorption of Thiol SAMs from Neighboring Polycrystalline Gold Surfaces

We describe a method for the selective desorption of thiol self-assembled monolayers from gold surfaces having micrometer-scale separations on a substrate. In an electrolyte solution, the electrical resistance between the adjacent areas can be much lower than the resistance between a surface and the counter electrode. Also, both reductive and oxidative thiol desorption may occur. Therefore, the potentials of the surfaces must be independently controlled with a multichannel potentiostat and operating windows for a given thiol/electrolyte system must be established. In this study operating windows were established for 1-dodecanethiol-based SAMs in phosphate buffer, phosphate-buffered saline, and sodium hydroxide solution, and selective SAM removal was successfully performed in a four-electrode configuration.

Synthesis and Stability of Monolayer-Protected Au38 Clusters

A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust-Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and scanning electrochemical microscopy to unequivocally demonstrate that Au38 can be reversibly oxidized to charge states z = +1 or +2; however, reduction to z = -1 leads to desorption of the protecting thiolate monolayer. Although this reductive desorption of thiol from the cluster surface is superficially analogous to electrochemical desorption of planar self-assembled monolayers (SAMs) from macroscopic electrodes, the molecular details of the process are likely to be complicated based on the current view that the thiolate monolayer in clusters is in fact composed of polymeric Au-S complexes.

In-situ fluorescence spectroscopy of self-assembled monolayers of HS-(CH 2) n-fluorescein and HS-(CH 2) 6-poly(dT) 18-fluorescein at gold electrodes under cyclic voltammetric conditions

We present the dynamics of electrochemical desorption of self-assembled monolayers of HS-(CH 2) n -fluorescein ( n = 3 and 12; C 3FAM and C 12FAM) and HS-(CH 2) 6-poly(dT) 18-fluorescein (ssDNAFAM) as monitored in-situ by fluorescence spectroscopy. Surface coverage and fluorescence intensity were also correlated, as sense during electrochemical desorption of the SAM. Changes in potential scan rate did not impact the fluorescence emission. Co-adsorption of mercaptohexanol (MCH) could be detected electrochemically and by in-situ fluorescence spectroscopy. Based on these results, energy transfer from an excited dye to the metal can be used to study interfacial dynamics. The transition of a single stranded DNA self-assembled monolayer from a condensed to an elongated state was monitored by in-situ fluorescence. Its electrochemical desorption from the electrode surface was measured as the electrochemical potential was applied above −1.0 V vs. Ag/AgCl. The DNA transition from condensed to elongated state can be exploited to study DNA hybridization process and other possible recognition events at the interface.

Infrared Reflection Absorption Spectroscopy Study of CO Adsorption and Reaction on Oxidized Pd(100)

The adsorption, desorption, and reaction of CO on the (√5×√5)R27° surface oxide on Pd(100) grown by two methods was investigated with infrared reflection absorption spectroscopy. CO multilayer desorbs at ∼40 K, whereas monolayer desorption is complete at ∼210 K. The surface oxide formed at 575 K with a 600 Langmuir (L) exposure of O2 exhibits a stronger interaction with CO compared with a surface oxide formed at 575 K with a 4500 L O2 exposure, presumably due to defects in the former. The surface oxide formed with a lower exposure of O2 exhibits enhanced reactivity with CO at 400 K. Below 60 K, O2 blocks CO adsorption on the monolayer oxide.

Counterion binding induces attractive interactions between negatively-charged self-assembled monolayer of 3-mercaptopropionic acid on Au(111) in reductive desorption

The electrochemical reductive desorption of the self-assembled monolayers of 3-mercaptopropionic acid in an aqueous alkaline solution gives a sharp peak with the full width at half maximum of about 20 mV irrespective of the type of cations in a linear scan voltammogram. This suggests that a strong attractive interaction exists between negatively charged carboxylate groups in the self-assembled monolayer surface due to the counterion binding, which not only simply stabilizes the adsorbed carboxylates but also makes the interaction between the adsorbed thiolates even attractive possibly by forming a two-dimensional ionic crystal. The effect of tetraalkylammonium ions on the shape of the voltammograms was also examined.

Comportamento higroscópico do resíduo seco de camarão-rosa

Estudou-se o comportamento higroscópico do resíduo seco de camarão-rosa (Peneaus subtilis), o qual foi caracterizado como sendo rico em proteínas totais (42,59%) e resíduo mineral fixo (22,01%). Foram construídas isotermas de adsorção e dessorção de umidade a 10, 25, e 40 °C. Determinou-se o valor da monocamada e o calor de dessorção, e testou-se a aplicabilidade de treze modelos matemáticos na predição dos dados de sorção. O produto apresentou isotermas do Tipo II. Os dados de adsorção mostraram que para que o produto apresente estabilidade microbiológica (a w < 0,6), o resíduo seco necessitará estar com umidade inferior a 13,0 g H2O.100 g -1 b.s. na adsorção e 14,0 g H2O.100 g-1 b.s. na dessorção. Os valores da monocamada, para a dessorção, indicam que o resíduo não deve ser seco a níveis de umidade inferiores a 7,29 g H2O.100 g -1 b.s., para evitar gasto desnecessário de energia. Os calores de dessorção evidenciaram a não necessidade de grandes quantidades de energia para secar o resíduo até níveis de umidade que o tornem microbiologicamente estável (a w< 0,6). De acordo com os ajustes os modelos de Oswin e Halsey (bi-paramétricos) e GAB, BET modificada, Hailwood-Horrobin, Anderson, Anderson-Hall e Gascoyne-Pethig (tri-paramétricos), podem ser utilizados na predição das isotermas de sorção do resíduo.

1-Phenyl-1-propyne on Cu(111): TOFMS TPD, XPS, UPS, and 2PPE Studies

The bonding properties of 1-phenyl-1-propyne (PP, C6H5CCCH3) on Cu(111) at 100 K have been studied using temperature-programmed desorption (TPD), and X-ray, ultraviolet, and two-photon photoemission spectroscopies (XPS, UPS, and 2PPE). In TPD, there is no evidence for dissociation. Multilayer desorption occurs at 187 K, and monolayer desorption occurs at 320 (83.5 kJ/mol) and 390 K (102.4 kJ/mol), with the latter dominating. Based on the calibrated C(1s) XPS, the saturation monolayer coverage is one PP per four surface Cu atoms. The broad and asymmetric C(1s) intensity profile of the monolayer can be resolved into three symmetric components, with peaks at 283.6, 284.5, and 285.2 eV and intensities of 2:6:1, respectively. These are attributed, respectively, to acetylenic carbons bound to Cu, phenyl, and methyl carbons. The monolayer valence band ultraviolet photoemission spectrum profile contains four resonances attributable to PP perturbed by interactions with the Cu(111) substrate. With the exception of the highest occupied molecular orbital (HOMO) that is shifted by 0.4 eV, these are uniformly shifted by 1 eV further from the Fermi level for the multilayer. Calculated electron density plots of the occupied orbitals coupled with UPS profiles suggest a spectator role for the phenyl group and bonding to Cu via the acetylenic carbons. The adsorption of 1.0 monolayer (ML) of PP on Cu(111) lowers the work function by 0.85 eV. Using 2PPE, two unoccupied orbitals were identified at 1.0 (U1*-LUMO) and 0.6 eV (U2*-image state) below the vacuum level. A chemisorption model consistent with these spectroscopic results and the major chemisorption peak in TPD involve di-sigma-bonding of the acetylenic carbons to a pair of second-nearest neighbor surface Cu atoms (cross-bridge).

Hydrodebromination of Bromobenzene over Pt(111)

The surface chemistry of benzene and bromobenzene over Pt(111) has been studied by temperature-programmed XPS/MS and NEXAFS. Time-resolved XPS shows that benzene adopts a single chemically distinguishable environment during low-temperature adsorption within the monolayer, with a saturation coverage at θC6H6 = 0.2 ML. Around 20% of a benzene monolayer desorbs molecularly, while the remainder dehydrogenates to surface carbon. Bromobenzene likewise adsorbs molecularly at 90 K, giving rise to two C 1s environments at 284.4 and 285.3 eV corresponding to the C−H and C−Br functions, respectively. The saturation C6H5Br monolayer coverage is 0.11 ML. NEXAFS reveals that bromobenzene adopts a tilted geometry, with the ring plane at 60 ± 5° to the surface. Bromobenzene multilayers desorb at ∼180 K, with higher temperatures promoting competitive molecular desorption versus C−Br scission within the monolayer. Approximately 30% of a saturated bromobenzene monolayer either desorbs reversibly or as reactively formed hydr...

Laser-Induced Chemistry and Desorption of Organic Monolayers

Laser-induced desorption of organic monolayers (LIDOM) is performed to analyze the chemical nature, thermal stability, and fragmentation pattern of organic silicon terminations based on calibration of surface heating introduced with a XeCl laser (308 nm). Results are presented for trimethylsilyl- (TMS) and 1-norbornen-2-yl(ethyl)dimethylsilyl- (NDMS) terminated surfaces which were prepared by silanization with suitable chloro compounds. Depending on laser fluence, either essentially intact end groups are emitted from the surface or specific fragmentation patterns are generated preferentially at higher energy densities. Clean retro Diels−Alder cleavage (rDA) was initiated in a norbonen compound at low fluences (110−200 mJ/cm2), providing reactive surface moieties such as double bonds that can be used as a starting point for controlled secondary surface reactions. At fluences above 200 mJ/cm2 fragmentation of the NDMS group takes place.

Enhanced thermal stability and structural ordering in short chain n-alkanethiol monolayers on gold probed by vibrational spectroscopy and EQCM

Monolayers of alkanethiols with varied chain lengths, CH3(CH2)nSH where n=3, 5, and 7, on gold substrates have been prepared by adsorption from (1) neat thiol, (2) millimolar thiol solution in alcohol (conventional method), and (3) potential-controlled adsorption. Reflection absorption infrared spectroscopy (RAIRS) and electrochemical quartz crystal microbalance (EQCM) have been used to characterize the integrity of the monolayers. Methylene and methyl stretching modes along with C-S stretching modes have been used as benchmarks to follow the order-disorder transitions in the monolayer structure, in the temperature range from 25 to 175 degrees C. Monolayers adsorbed from neat thiol show superior quality in terms of stability and structural arrangement. Short chain thiols with n=3, 5, and 7 do show substantial stability. The possibility of multilayer formation is ruled out by EQCM studies comparing the frequency and mass change associated with the monolayer desorption. Self-assembled monolayers (SAMs) adsorbed under potential control behave very similarly to the monolayers adsorbed from neat thiol as far as stability and structural orientation are concerned, irrespective of the chain length. The adsorption from neat thiol gets rid of the solvent-induced defects and arrests the propagation of defects under temperature constraints.

Adsorption and Desorption of Stearic Acid Self-Assembled Monolayers on Aluminum Oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.

A Method for Patterning Multiple Types of Cells by Using Electrochemical Desorption of Self‐Assembled Monolayers within Microfluidic Channels

Fenced in: Selective electrochemical desorption of self-assembled monolayers of HS(CH2)11(OCH2CH2)6OH was carried out on parts of a gold substrate restricted to microfluidic channels. This chemical transformation activates parts of the surface for adhesion of multiple types of cells with well-controlled geometry.

Electrochemical Release of Immobilized IgG Protein

AbstractIn this paper, we present the electrochemically programmed release of immobilized IgG protein molecules that have been attached to gold coated surfaces via a thiol-gold linkage. Fluorescence microscopy has been used to image the release of fluorescently tagged IgGs in phosphate buffered saline. In this technique, the reductive desorption of self-assembled monolayers is employed for the release of proteins, which are immobilized on the surface either by non-covalent or covalent interactions. The voltage applied for the release of proteins is in a range of -1.5V to -60V.

Phase transition behavior of two-component viologen adsorption layers on a HOPG electrode surface

Phase transition behavior of two-component viologen adsorption layers at a HOPG electrode was described using the results of voltammetric measurements. In the coexistence of heptyl viologen (HV) and its bis-carboxylated derivative in the solution phase, a well-mixed condensed monolayer of the radical cations was formed at any molar fraction. In sharp contrast, the binary system of HV and butyl viologen (BV) exhibited phase separation in the molar fraction range where BV is saturated in the predominantly formed condensed phase of HV. It was, however, found that this separation, being opposed to the prediction based on the adsorption free energy, occurs only when the time period enough for full condensation of HV is not given. The significant features of phase transition of two-component viologen adsorption layers on a HOPG electrode surface were highlighted in comparison with the formation and reductive desorption of the self-assembled monolayers of alkanethiols.

Electrochemical Desorption of n-Alkylthiol SAMs on Polycrystalline Gold: Studies Using A Ferrocenylalkylthiol Probe

The reductive voltammetric desorption of n-alkylthiol self-assembled monolayers (SAMs) was studied using ferrocenyldodecanethiol (FcC12SH) as a probe. A short (10 s) incubation in a 2 mM FcC12SH solution labels the pre-existing defect sites present in a tetradecanethiol (C14S-Au) SAM. Additional defects in a C14S-Au SAM are then created by voltammetric cycling to reductive potentials. The effects of changing the desorption potential and the hold time at the desorption potential were investigated by monitoring the change in the two characteristic FcC12S-Au peaks. These two peaks are associated with FcC12S-Au filling individual single site defects (peak I at 260 mV) and pinhole defects (peak II at 380 mV). The reductive desorption potential applied to a binary (FcC12S-/C14S-Au) SAM results in a partial desorption and a potential "remixing" of the phase-separated state.

Dynamics of NF 3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF 3 on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF 3 adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF 3 is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F + central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF 3 (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F +, N +, NF +, NF 2 + and NF 3 + ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the NF 2 + and NF 3 + ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF 3 molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF 3 crystal. In this process, some NF 3 molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.

Electrically-Assisted Formation and Desorption of Dodecyl Phosphate Self-Assembled Monolayers on Indium Tin Oxide Surfaces

Molecularly homogenous and hydrophobic surfaces play an important role in a number of technological applications such as tribology and protein/cellular adhesion. Self-assembled monolayers of dodecyl phosphates (DDPO4) have previously been demonstrated to spontaneously form on titanium oxide, producing surfaces with excellent hydrophobic properties. In this paper, we report on the adsorption of DDPO4 onto a transparent electronic material, indium tin oxide (ITO), under both open circuit and applied potential conditions. We have used two complementary surface characterization techniques: variable angle scanning ellipsometry and contact angle measurements to investigate the adsorption of DDPO4 on an ITO surface. Under open circuit condition, both methods consistently confirmed the formation of DDPO4 monolayers on the ITO surface. The presence of an electrical field increased the amount of DDPO4 adsorbed on the ITO surface. An applied anodic electrical stimulus of 1800 mV resulted in an exponential loss of the monolayer as confirmed by X-ray photoelectron spectroscopy and electrochemical optical waveguide lightmode spectroscopy (EC- OWLS). This electrically stimulated selective adsorption/desorption of DDPO4 opens up new ways to tailor the physico-chemical properties of surfaces.

Electrochemically Programmed Release of Biomolecules and Nanoparticles

The controlled release of molecules or nanoparticle conjugates is an important tool for a wide range of applications in science and engineering. Here we demonstrate electrochemically programmed release of biomolecules and nanoparticles immobilized on patterned gold electrodes using the thiol-gold linkage. This technique exploits the reductive desorption of self-assembled monolayers and allows both spatially controlled release and regeneration of small molecules (e.g., drugs), biopolymers (e.g., peptides, proteins, DNA), protein assemblies (e.g., viruses), and nanoparticles (e.g., particle-DNA conjugates). Fluorescence microscopy is used to image the release of avidin and nanoparticles in phosphate-buffered saline and to determine the kinetics of desorption. We also demonstrate that the electrodes can be regenerated using the same conjugation scheme.