Functionalized self-assembled monolayers (SAMs) of alkanethiolates on gold were used to study citrated human plasma protein adsorption by ellipsometry and antibody techniques in vitro. The aspiration is to gain knowledge about how surface properties affect such events like coagulation and complement activation, which in turn influence cell activation at an implant site. Five functionalities, methyl (−CH3), trifluoromethyl ester (−O(CO)CF3), sulfate (−OSO3H), carboxyl (−COOH), and hydroxyl (−OH), were subject to characterization by ellipsometry, contact angle measurements, scanning force microscopy (SFM), and Fourier transform infrared reflection−absorption spectroscopy (IRAS). The low-energy surfaces, with methyl and trifluoromethyl ester terminations, showed affinity for fibrinogen. The trifluoromethyl ester also deposited lipoprotein (LP). The sulfate and the carboxyl surfaces deposited the coagulation proteins high molecular weight kininogen (HMWK), factor XII (F XII), and prekallikrein (PK), indicating contact activation of coagulation. Hydroxyl-functionalized SAMs showed low deposition of plasma proteins in general, although a low binding of antibodies against the contact activation proteins, complement factor 3c (C3c), and lipoproteins was observed. The morphology of the formed plasma protein layers was studied using scanning force microscopy. On the low-energy surfaces the proteins tended to cluster into large formations. In the case of the methylated surface the formations had the appearance of dendrite-like networks. On the high-energy surfaces the proteins retained a more uniform spreading in small rounded clusters.
Four different carbonyl-containing self-assembled monolayers (SAMs) of alkanethiolates on gold were studied to assess the impact of the functional group linked to the carbonyl upon its hydrogen bond accepting capability. These SAMs (HS(CH2)16O(CdO)-X, X ) CH3, CF3, or C6H5, and HS(CH2)15(CdO)OCH3) were thoroughly characterized with contact angle measurements, single wavelength ellipsometry, and infrared reflection-absorption spectroscopy (IRAS) prior to the studies of interaction with D2O. The first three monolayer compounds were introduced by reacting hydroxyl-terminated SAMs (HS(CH2)16OH) with either acetyl chloride, trifluoroacetic anhydride, or benzoyl chloride. The behavior of D2O ice on the SAMs was investigated at 100 K with IRAS and temperature programmed desorption (TPD). On all monolayers the D2O molecules were shown to interact with the carbonyl oxygen. The degree of interaction depended upon the termination of the thiol, where the size, structure, and electronegativity of the terminating groups in the molecules comprising the monolayer were found to be important factors. Indications of interaction with the C-O-C oxygen were seen for all compounds, as well as weak interaction between water molecules and the CF 3 group of one of the investigated SAMs. Common behavior for all four monolayers with an adsorbed D2O overlayer was a decrease in the number of hydrogen bonds to the substrate when the overlayer was annealed from amorphous ice at 100 K to polycrystalline-like ice at 140 K. The spectral changes accompanying the structural transition were consistent with a change from a mainly flat overlayer to condensed three-dimensional clusters. The bulk-to-surface molecular ratio of adsorbed ice clusters could be assessed by IRAS and correlated to macroscopic wetting properties. Our results infer that microscopic ice clusters on these SAMs qualitatively mimic the shape of macroscopic water drops on the same SAMs. Results of TPD measurements are also consistent with this view.
This paper explores the interfacial properties of one-dimensional molecular gradients of alkanethiols (HS -(CH 2 ) n -X) on gold. The kinetics and thermodynamics of monolayer formation are important issues for these types of mixed molecular assemblies. The influence of chain length difference on the contact angles with hexadecane (HD), q a and q r , and the hysteresis, has been studied by employing alkanethiols HS -(CH 2 ) n -CH 3 , with n=9, 11, 13, 15 and 17, in the preparation of the self-assembled monolayers (SAM) gradients. The contact angles with hexadecane, at the very extreme ends of the gradients, show characteristic values of a highly ordered CH 3 -like assembly: q a = 45 -50°. In the middle of the gradients q a drops noticeably and exhibits values representative for CH 2 -like polymethylenes, q a =20-30°, indicating a substantial disordering of the protruding chains of the longer component in the gradient assembly. As expected, the exposure of CH 2 -groups to the probing liquid increases with increasing differential chain length of the two n-alkanethiol used, in this case eight methylene units. However, the contact angles always display a non-zero value which means that even at a chain length difference of eight methylene units there is a substantial exposure of methyl (CH 3 ) groups to the probing liquid. With infrared reflection-absorption spectroscopy (IRAS) we have monitored the structural behavior of the polymethylene chains along the gradient. We find complementary evidence for disordered chains in the gradient region, and the IRAS results correlate well with the contact angle measurements.
Thiols with varying functionalities are well suited for immobilization onto gold. In the present study surfaces modified with L-cysteine (zwitterionic, neutral) and 3-mercaptoproprionic acid (MPA, negatively charged) were incubated in human plasma, and the antisera binding patterns of four proteins were determined by eUipsometry. Significant differences among the surfaces were observed. Plasma-treated L-cysteine surfaces bound low amounts of both anti-fibrinogen (a-FG) and anti high molecular weight kininogen (a-HMWK), while MPA surfaces bound increased amounts of a-HMWK but no a-FG. The results demonstrate that the surface biology in complex, but biologicaUy relevant, systems may be conveniently studied through a
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