Improving the surface biocompatibility with the use of mixed zwitterionic self-assembled monolayers prepared by a proper solvent

In this study, the mixed self-assembled monolayers (SAMs) containing the mixture of long-chain alkanethiol, SH(CH(2))(11)NH(2) and SH(CH(2))(10)SO(3)H, was prepared as a model surface to examine the interaction between the biological environment and artificial surface. The 10% (v/v) NH(4)OH ethanolic solution and DMSO were chosen as the solvents for the preparation of these mixed SAMs and the "solvent effect" was discussed. X-ray photoelectron spectroscopy (XPS) has indicated that -SO(3)H/-NH(2) mixed SAMs formed from 10% (v/v) NH(4)OH ethanolic solution were surface "-SO(3)H poor", while a nearly equivalent amount of surface -SO(3)H functionality was presented on the mixed SAMs formed from DMSO. This has resulted from the different solvation capability between solvent molecules and the alkanethiol. Such solvent effects were also reflected in various surface properties such as surface wettability and surface zeta potential. The mixed SAMs formed from DMSO were more surface hydrophilic and less negatively surface charged than from 10% (v/v) NH(4)OH ethanolic solution. In addition, these mixed SAMs formed from DMSO exhibited the least amount of protein adsorbed as well as a better platelet compatibility than its counterpart from 10% (v/v) NH(4)OH ethanolic solution. These findings indicated that choosing a proper solvent for mixed zwitterionic SAM can greatly affect its surface properties and biocompatibility, such as to form a surface with near neutrality for reducing protein adsorption and subsequent platelet adhesion and activation.     

Surface characterization and platelet compatibility evaluation of the binary mixed self-assembled monolayers

This report describes a technique that used mixed self-assembled monolayer (SAM) as a model surface to evaluate the effect of steric hindrance on the SAM packing quality and its platelet compatibility. Two series of binary mixed SAMs were formed by mixing the bulky terminated alkanethiol (HS(CH2)10PO3H2) with a smaller terminated one (HS(CH2)9CH3 and HS(CH2)11OH) respectively. Surface characterization results showed the hydrophilicity on these two series of mixed SAMs changed with the solution mole fraction of PO3H2 terminated thiol, chi(PO3H2,soln), and reached to a nearly constant value as chi(PO3H2,soln) was 0.6 for PO3H2+CH3 SAM and 0.4 for PO3H2+OH SAM. This finding should be due to the gradual saturation of surface PO3H2 functionality on these mixed SAMs. The XPS analysis indicated the addition of the CH3 and OH terminated thiol could reduce the steric hindrance effect of PO3H2 functionality on monolayer formation and, henceforth, improve the SAM packing quality. In vitro platelet adhesion assay revealed the platelet compatibility on the PO3H2+OH SAMs was better than that on the PO3H2+CH3 and the pure PO3H2 ones. Moreover, the PO3H2+OH SAM with a low chi(PO3H2,soln) value exhibited the least platelet activating property of these two mixed SAM systems. These findings suggested that material's surface wettability and surface charge density should act collectively in affecting its platelet compatibility.     

Frictional dynamics of fluorine-terminated alkanethiol self-assembled monolayers

The frictional dynamics of fluorine-terminated alkanethiol (S(CH2)8CF3) self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. The simulations treat the interactions between two SAMs on flat surfaces. The structure and frictional behavior are investigated as a function of applied pressure (200 MPa to 1 GPa) for a shear velocity of 2 m/s and compared to methyl-terminated alkanethiol SAMs. The maximum adhesive pressure between the SAMs is 220 MPa for both end groups. In agreement with experiments on the molecular scale, the shear stress and the coefficient of friction for CF3-terminated alkanethiols are larger than for CH3-terminated alkanethiols. The main source for the difference is primarily the tighter packing of the fluorinated terminal group resulting in a higher degree of order. The molecular scale coefficient of friction is correlated with the degree of order among all the systems.     

Local packing environment strongly influences the frictional properties of mixed CH3- and CF3-terminated alkanethiol SAMs on Au(111)

Compositionally mixed, self-assembled monolayers (SAMs) derived from 16,16,16-trifluorohexadecanethiol and a normal alkanethiol, either hexadecanethiol or pentadecanethiol, were formed on Au(111) substrates. The relative composition of the films was determined using X-ray photoelectron spectroscopy and was found to approximately equal the equimolar composition of the isooctane solution from which they were formed. The frictional properties of the mixed films were measured on the nanometer scale using atomic force microscopy and were observed to decrease when the chain length of the CH(3)-terminated component was shortened by one methylene unit (i.e., when hexadecanethiol was replaced by pentadecanethiol). For comparison, the frictional properties of a mixed-chain-length CH(3)-terminated SAM derived from hexadecanethiol and pentadecanethiol in a 1:1 ratio was also examined. In contrast to the mixed CF(3)/CH(3) system, the latter mixed-chain-length system exhibited relatively higher friction when compared to single-component SAMs derived solely from either hexadecanethiol or pentadecanethiol. For both types of mixed films, the change in frictional properties that occurs as a result of modifying the position of neighboring terminal groups with respect to the surface plane is discussed in terms of the influence of local packing environments on interfacial energy dissipation (friction).     

Hydrophilicity of Polar and Apolar Domains of Amphiphiles

The hydrophilicity of polar and apolar domains of various amphiphiles was systematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilution. The homologues of protic alkyl H(CH(2))(n)-, perfluoroalkyl F(CF(2))(n)-, and alkylphenyl H(CH(2))(n)(C(6)H(5))- groups (n=0-10) were chosen to represent apolar hydrophobic domains. The polar hydrophilic domains tested were -SO(4)Na, -SO(3)Na, -COONH(4), -N(CH(3))(3)Br, N(C(m)H(2m+1))(4)Br (m=1-5), and -NH(CH(2))(n)SO(3) (n=3, 4) groups. Also tested were the tetraphenyl ionic compounds (C(6)H(5))(4)MX (M=B/X=Na, M=P/X=Cl, M=As/X=Cl) to study the effect of the ionic sign of the core atom across the tetraphenyl apolar shell, the polyethylene glycols H(OCH(2)CH(2))(m)OH (m=1-4) to study the role of apolar -CH(2)- units in the hydrophilic oxyethylene group, and the zwitterionic dimethylaminoalkylsulfonate (CH(3))(2)NH(CH(2))(n)SO(3) homologues to study the effect of intramolecular salt formation on the hydrophilicity of the zwitterion. The adiabatic compressibility of the solution was calculated from measurement of the sound velocity and density of solutions. The introduction of laboratory automation and the numerical control of the system improved the accuracies and efficiencies of the measurements a great deal. The range of the temperature scan was 0-40 degrees C with an effective accuracy of +/-0.001 degrees C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar substances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and analogues. The hydrophobic hydration of nonpolar substances was found to be very small at room temperature and was barely detected above 40 degrees C; however, it became large as the temperature was lowered and attained a maximum at 0 degrees C. The cationic charge of quaternary ammonium N(+)(C(n)H(2n+1))(4) was found to enhance the hydrophobic hydration of methylene groups located at a distance of 4 to 6 ? from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R-SO(4)(-), R-SO(3)(-), or R-COO(-) was found to decrease the hydrophobic hydration of -CH(2)- units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydrations. The hydrophilicity of the perfluoromethylene unit -CF(2)- was found to have a value comparable to that of the protic methylene unit -CH(2)-. The hydrophobic hydration seems to offer a good measure of the hydrophilicity of apolar substances; however, it does not necessarily represent the "hydrophobicity" of the apolar segment when the "surface activity" of the amphiphile is concerned. Copyright 2000 Academic Press.     

Experimental and theoretical studies of the effect of mass on the dynamics of gas/organic-surface energy transfer

The effect of mass on gas/organic-surface energy transfer is explored via investigation of the scattering dynamics of rare gases (Ne, Ar, and Kr) from regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) at 60 kJmol collision energy. Molecular-beam scattering experiments carried out in ultrahigh vacuum and molecular-dynamics simulations based on high-accuracy potentials are used to obtain the rare-gases' translational-energy distributions after collision with the SAMs. Simulations indicate that mass is the most important factor in determining the changes in the energy exchange dynamics for Ne, Ar, and Kr collisions on CH3- and CF3-terminated SAMs at 60 kJmol collision energy. Other factors, such as changes in the gas-surface potential and intrasurface interactions, play only a minor role in determining the differential dynamics behavior for the systems studied.     

Spontaneous Emulsification Produced by Chemical Reactions

Self-assembled monolayer (SAM) formation of alkanethiols with ionic, hydrophilic terminal functionalities onto various O(2) plasma/ethanol pretreated gold substrates was characterized to explore the effect of gold surface oxide on the SAM packing quality. Oxygen adsorption induced by the Au(2)O(3) surface residuals are observed on the plasma-oxidized and O(2) plasma/ethanol-rinsed pretreated Au surfaces while no obvious adsorbed oxygen is found on freshly coated and O(2) plasma/ethanol sonication pretreated Au substrates. A model for the formation of hydrophilic terminated SAMs, -OH, -COOH, and -PO(3)H(2) is proposed. According to this model, the ionic and/or other binding interactions between the surface Au(2)O(3) and the alkanethiol hydrophilic terminal end as well as the interactions between the terminal SAM functionalities could cause the packing disorder found on these three SAMs formed on Au substrates containing Au(2)O(3) surface species. Copyright 2001 Academic Press.     

Aggregation properties of supralong-chain surfactants with double or triple quaternary ammonium head groups

Double or triple quaternary ammonium head groups were designed to improve the solubility of supralong alkyl chain surfactants. In the surfactant head group, quaternary ammonium groups are connected by an ethylene spacer. Micellar shapes of divalent surfactants [C(n)H(2n)(+1)N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(3) 2Br(-): C(n)-2Am (n=18, 20, and 22)] and trivalent surfactants [C(n)H(2n)(+1)N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(2)-(CH(2))(2)-N(+)(CH(3))(3) 3Br(-): C(n)-3Am (n=18, 20, and 22)] were studied in aqueous solutions by means of dynamic light scattering (DLS) and transmission electron microscopy (TEM). Changes in the surfactant concentration have a small influence on the apparent hydrodynamic radii (r(h)) of the molecular aggregates in both surfactant series. Average values of r(h) of aggregates are 60-90 nm for C(n)-2Am (n=18, 20, and 22) and 2-40 nm for C(n)-3Am (n=18, 20, and 22). TEM micrographs showed that aggregates of C(n)-2Am (n=18, 20, and 22) typically formed rod-like micelles. In contrast, trivalent surfactants of C(n)-3Am (n=18, 20, and 22) formed spherical (C(18)-3Am) or ellipsoidal micelles (C(20)-3Am and C(22)-3Am). Moreover, the degree of micellar counterion binding for these surfactants was determined by using a bromide ion-selective electrode, which indicated relatively high values (0.8-0.9) for C(n)-2Am (n=18, 20, and 22) and more common values (0.5-0.8) for C(n)-3Am (n=18, 20, and 22). The size of the aggregates is closely related to the degree of counterion binding.     

Immunosensor interface based on physical and chemical immunoglobulin G adsorption onto mixed self-assembled monolayers

An immunosensor interface based on mixed hydrophobic self-assembled monolayers (SAMs) of methyl and carboxylic acid terminated thiols with covalently attached human Immunoglobulin G (hIgG), is investigated. The densely packed and organised SAMs were characterised by contact angle measurements and cyclic voltammetry. The effect of the non-ionic surfactant, Tween 20, in preventing nonspecific adsorption is addressed by ellipsometry during physical and covalent hIgG immobilization on pure and mixed SAMs, respectively. It is clearly demonstrated that nonspecific adsorption due to hydrophobic interactions of hIgG on methyl ended groups is totally inhibited, whereas electrostatic/hydrogen bonding interactions with the exposed carboxylic groups prevail in the presence of surfactant. Results of ellipsometry and Atomic Force Microscopy, reveal that the surface concentration of covalently immobilized hIgG is determined by the ratio of COOH / CH(3)-terminated thiols in SAM forming solution. Moreover, the ellipsometric data demonstrates that the ratio of bound anti-hIgG / hIgG depends on the density of hIgG on the surface and that the highest ratio is close to three. We also report the selectivity and high sensitivity achieved by chronoamperometry in the detection of adsorbed hIgG and the reaction with its antibody.     

Synthesis of a series of oligo(ethylene glycol)-terminated alkanethiol amides designed to address structure and stability of biosensing interfaces.

A strategy for the synthesis of a series of closely related oligo(ethylene glycol)-terminated alkanethiol amides (principally HS(CH(2))(m)CONH(CH(2)CH(2)O)(n)H; m = 2, 5, 11, 15, n = 1, 2, 4, 6, 8, 10, 12) and analogous esters has been developed. These compounds were made to study the structure and stability of self-assembled monolayers (SAMs) on gold in the prospect of designing new biosensing interfaces. For this purpose, monodisperse heterofunctional oligo(ethylene glycols) with up to 12 units were prepared. Selective monoacylation of the symmetrical tetra- and hexa(ethylene glycol) diols as their mesylates with the use of silver(I) oxide was performed. The synthetic approach was based on carbodiimide couplings of various oligo(ethylene glycol) derivatives to omega-(acetylthio) carboxylic acids via a terminal amino or hydroxyl function. SAM structures on gold were studied with respect to thickness, wettability (water contact angles approximately 30 degrees ), and conformation. A good fit was obtained for the relation between monolayer thickness (d) and the number of units in the oligo(ethylene glycol) chain (n): d = 2.8n + 21.8 (A). Interestingly, the corresponding infrared spectroscopy analysis showed a dramatic change in conformation of the oligomeric chains from all-trans (n = 4) to helical (n > or = 6) conformation. A crystalline helical structure was observed in the SAMs for n > 6.     

Mimicking protein-protein electron transfer: voltammetry of Pseudomonas aeruginosa azurin and the Thermus thermophilus Cu(A) domain at omega-derivatized self-assembled-monolayer gold electrodes

Well-defined voltammetric responses of redox proteins with acidic-to-neutral pI values have been obtained on pure alkanethiol as well as on mixed self-assembled-monolayer (SAM) omega-derivatized alkanethiol/gold bead electrodes. Both azurin (P. aeruginosa) (pI = 5.6) and subunit II (Cu(A) domain) of ba(3)-type cytochrome c oxidase (T. thermophilus) (pI = 6.0) exhibit optimal voltammetric responses on 1:1 mixtures of [H(3)C(CH(2))(n)()SH + HO(CH(2))(n)()SH] SAMs. The electron transfer (ET) rate vs distance behavior of azurin and Cu(A) is independent of the omega-derivatized alkanethiol SAM headgroups. Strikingly, only wild-type azurin and mutants containing Trp48 give voltammetric responses: based on modeling, we suggest that electronic coupling with the SAM headgroup (H(3)C- and/or HO-) occurs at the Asn47 side chain carbonyl oxygen and that an Asn47-Cys112 hydrogen bond promotes intramolecular ET to the copper. Inspection of models also indicates that the Cu(A) domain of ba(3)-type cytochrome c oxidase is coupled to the SAM headgroup (H(3)C- and/or HO-) near the main chain carbonyl oxygen of Cys153 and that Phe88 (analogous to Trp143 in subunit II of cytochrome c oxidase from R. sphaeroides) is not involved in the dominant tunneling pathway. Our work suggests that hydrogen bonds from hydroxyl or other proton-donor groups to carbonyl oxygens potentially can facilitate intermolecular ET between physiological redox partners.     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

Structure and unimolecular chemistry of protonated sulfur betaines, (CH3)2S(+)(CH2)(n)CO2H (n = 1 and 2)

The fixed charge zwitterionic sulfur betaines dimethylsulfonioacetate (DMSA) (CH(3))(2)S(+)CH(2)CO(2)(-) and dimethylsulfoniopropionate (DMSP) (CH(3))(2)S(+)(CH(2))(2)CO(2)(-) have been synthesized and the structures of their protonated salts (CH(3))(2)S(+)CH(2)CO(2)H···Cl(-) [DMSA.HCl] and (CH(3))(2)S(+)(CH(2))(2)CO(2)H···Pcr(-) [DMSP.HPcr] (where Pcr = picrate) have been characterized using X-ray crystallography. The unimolecular chemistry of the [M+H](+) of these betaines was studied using two techniques; collision-induced dissociation (CID) and electron-induced dissociation (EID) in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Results from the CID study show a richer series of fragmentation reactions for the shorter chain betaine and contrasting main fragmentation pathways. Thus while (CH(3))(2)S(+)(CH(2))(2)CO(2)H fragments via a neighbouring group reaction to generate (CH(3))(2)S(+)H and the neutral lactone as the most abundant fragmentation channel, (CH(3))(2)S(+)CH(2)CO(2)H fragments via a 1,2 elimination reaction to generate CH(3)S(+)=CH(2) as the most abundant fragment ion. To gain insights into these fragmentation reactions, DFT calculations were carried out at the B3LYP/6-311++G(2d,p) level of theory. For (CH(3))(2)S(+)CH(2)CO(2)H, the lowest energy pathway yields CH(3)S(+)=CH(2)via a six-membered transition state. The two fragment ions observed in CID of (CH(3))(2)S(+)(CH(2))(2)CO(2)H are shown to share the same transition state and ion-molecule complex forming either (CH(3))(2)S(+)H or (CH(2))(2)CO(2)H(+). Finally, EID shows a rich and relatively similar fragmentation channels for both protonated betaines, with radical cleavages being observed, including loss of ˙CH(3).     

In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition

Nano-structured CuS thin films were deposited on the functionalized -NH(2)-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the -NH(2)-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV-vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3Ω/□). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On -NH(2)-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On -OH- and -CH(3)-terminated surfaces, the dominant growth mechanism on the surface is cluster-by-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on -NH(2)-terminated SAMs regions of -NH(2)/-CH(3) patterned SAMs surface.     

The role of surface free energy on the formation of hybrid bilayer membranes

The interaction of small phospholipid vesicles with well-characterized surfaces has been studied to assess the effect of the surface free energy of the underlying monolayer on the formation of phospholipid/alkanethiol hybrid bilayer membranes (HBMs). The surface free energy was changed in a systematic manner using single-component alkanethiol monolayers and monolayers of binary mixtures of thiols. The binary surfaces were prepared on gold by self-assembly from binary solutions of the thiols HS-(CH(2))(n)()-X (n = 11, X = CH(3) or OH) in THF. Surface plasmon resonance (SPR), electrical capacitance, and atomic force microscopy (AFM) measurements were used to characterize the interaction of palmitoyl,oleoyl-phosphatidylcholine (POPC) vesicles with the surfaces. For all surfaces examined, it appears that the polar part of surface energy influences the nature of the POPC assembly that associates with the surface. Comparison of optical, capacitance, and AFM data suggests that vesicles can remain intact or partially intact even at surfaces with a contact angle with water of close to 100 degrees. In addition, comparison of the alkanethiols of different chain lengths and the fluorinated compound HS-(CH(2))(2)-(CF(2))(8)-CF(3) that characterize with a low value of the polar part of the surface energy suggests that the quality of the underlying monolayer in terms of number of defects has a significant influence on the packing density of the resulting HBM layer.     

Studies of mixed surfactant solutions of cationic dimeric (gemini) surfactant with nonionic surfactant C12E6 in aqueous medium

The interaction between the alkanediyl-alpha,omega-type cationic gemini surfactant, [(C(16)H(33)N(+)(CH(3))(2)(CH(2))(4)N(+)(CH(3))(2)C(16)H(33))2Br(-)], 16-4-16 and the conventional nonionic surfactant [CH(3)(CH(2))(10)CH(2)(OCH(2)CH(2))(6)OH], C(12)E(6) in aqueous medium has been investigated. The critical micelle concentrations of different mixtures have been measured by surface tension using a du Nouy tensiometer in aqueous solution at different temperatures (303, 308, and 313 K). Maximum surface excess (Gamma(max)) and minimum area per molecule (A(min)) were evaluated from a surface tension vs log(10)C (C is concentration) plot. The cmc value of the mixture was used to compute beta(m), the interaction parameter. The beta(sigma), the interaction parameter at the monolayer air-water interface, was also calculated. We observed synergism in 16-4-16/C(12)E(6) system at all concentration ratios. The micelle aggregation number (N(agg)) has been measured using a steady state fluorescence quenching method at a total surfactant concentration approximately 2 mM at 25 degrees C. The micropolarity and the binding constant (K(sv)) of mixed systems were determined from the ratio of intensity of peaks (I(1)/I(3)) of the pyrene fluorescence emission spectrum. The micellar interiors were found to be reasonably polar. We also found, using Maeda's concept, that the chain-chain interactions are very important in this system.     

KOH catalysed preparation of activated carbon aerogels for dye adsorption

Surfactants prevent the irreversible aggregation of partially refolded proteins, and they are also known to assist in protein refolding. A novel approach to protein refolding that utilizes a pair of low molecular weight folding assistants, a detergent and cyclodextrin, was proposed by Rozema and Gellman (D. Rozema, S.H. Gellman, J. Am. Chem. Soc. 117 (1995) 2373). We report the refolding of bovine serum albumin (BSA) assisted by these artificial chaperones, utilizing gemini surfactants for the first time. A combination of cationic gemini surfactants, bis(cetyldimethylammonium)pentane dibromide (C(16)H(33)(CH(3))(2)N(+)-(CH(2))(5)-N(+)(CH(3))(2)C(16)H(33)·2Br(-) designated as G5 and bis(cetyldimethylammonium)hexane dibromide (C(16)H(33)(CH(3))(2)N(+)-(CH(2))(6)-N(+)(CH(3))(2)C(16)H(33)·2Br(-) designated as G6 and cyclodextrins, was used to refold guanidinium chloride (GdCl) denatured BSA in the artificial chaperone assisted two step method. The single chain cationic surfactant cetyltrimethylammonium bromide (CTAB) was used for comparative studies. The studies were carried out in an aqueous medium at pH 7.0 using circular dichroism, dynamic light scattering and ANS binding studies. The denatured BSA was found to get refolded by very small concentrations of gemini surfactant at which the single chain counterpart was found to be ineffective. Different from the single chain surfactant, the gemini surfactants exhibit much stronger electrostatic and hydrophobic interactions with the protein and are thus effective at much lower concentrations. Based on the present study it is expected that gemini surfactants may prove useful in the protein refolding operations and may thus be effectively employed to circumvent the problem of misfolding and aggregation.     

Fabrication of biomolecular nanostructures by scanning near-field photolithography of oligo(ethylene glycol)-terminated self-assembled monolayers

The UV photo-oxidation of oligo(ethylene glycol) (OEG)-terminated self-assembled monolayers (SAMs) has been studied using static secondary ion mass spectrometry, X-ray photoelectron spectroscopy, contact angle measurement, and friction force microscopy. OEG-terminated SAMs are oxidized to yield sulfonates, but photodegradation of the OEG chain also occurs on a more rapid time scale, yielding degradation products that remain bound to the surface via gold-sulfur bonds. The oxidation of these degradation products is the rate-limiting step in the process. Photopatterning of OEG-terminated SAMs may be accomplished by using a mask and suitable light source or by using scanning near-field photolithography (SNP) in which the mask is replaced by a scanning near-field optical microscope coupled to a UV laser. Using SNP, it is possible to fabricate patterns in SAMs with a full width at half-maximum height (fwhm) as small as 9 nm, which is approximately 15 times smaller than the conventional diffraction limit. SNP-patterned OEG-terminated SAMs may be used to fabricate protein nanopatterns. By adsorbing carboxylic acid-terminated thiols into oxidized regions and converting these to active ester intermediates, it has been possible to fabricate lines of protein molecules with widths of only a few tens of nanometers.     

Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface

The properties of mixed monolayers composed of the cationic Gemini surfactant ([C(18)H(37)(CH(3))(2)N(+)(CH(2))(3)N(+)(CH(3))(2)C(18)H(37)],2Br(+), abbreviated as 18-3-18,2Br(-1)) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br(-1) and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br(-1) and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a composition X(SA)=0.67 and formed a "cationic-anionic surfactant," i.e., the carboxylate, with 18-3-18,2Br(-1) owing to the electrostatic interaction between the head groups.     

In-situ characterization of self-assembled monolayers of water-soluble oligo(ethylene oxide) compounds

In-situ spectroscopic ellipsometry (SE) was utilized to examine the formation of the self-assembled monolayers (SAMs) of the water-soluble oligo(ethylene oxide) [OEO] disulfide [S(CH(2)CH(2)O)(6)CH(3)](2) {[S(EO)(6)](2)} and two analogous thiols - HS(CH(2)CH(2)O)(6)CH(3) {(EO)(6)} and HS(CH(2))(3)O(CH(2)CH(2)O)(5)CH(3) {C(3)(EO)(5)} - on Au from aqueous solutions. Kinetic data for all compounds follow simple Langmuirian models with the disulfide reaching a self-limiting final state (d=1.2nm) more rapidly than the full coverage final states of the thiol analogs (d=2.0nm). The in-situ ellipsometric thicknesses of all compounds were found to be nearly identical to earlier ex-situ ellipsometric measurements suggesting similar surface coverages and structural models in air and under water. Exposure to bovine serum albumin (BSA) shows the self-limiting (d=1.2nm) [S(EO)(6)](2) SAMs to be the most highly protein resistant surfaces relative to bare Au and completely-formed SAMs of the two analogous thiols and octadecanethiol (ODT). When challenged with up to near physiological levels of BSA (2.5mg/mL), protein adsorption on the final state [S(EO)(6)](2) SAM was only 3% of that which adsorbed to the bare Au and ODT SAMs.