Dendritic Macromolecules at Interfaces

The structural state of dendritic macromolecules at air–water (Langmuir monolayers) and air–solid (adsorbed monolayers, self-assembled films, and cast films) interfaces is discussed. Examples of undistorted and compressed dendritic macromolecules within monolayers and multilayer films are presented. The high interaction strength between “sticky” surface groups of dendrimers and substrates is considered to be responsible for the formation of the compact monolayer structures and dendrimer compression. The interfacial behavior of dendrimers is compared to the surface states predicted by molecular dynamics simulations.     

Dendrimer-Mediated Adhesion between Vapor-Deposited Au and Glass or Si Wafers

Here, we report the use of amine-terminated poly(amidoamine) (PAMAM) dendrimers as adhesion promoters between vapor-deposited Au films and Si-based substrates. This method is relatively simple, requiring only substrate cleaning, dipping, and rinsing. Proof of concept is illustrated by coating glass slides and single-crystal Si wafers with monolayers of PAMAM dendrimers and then evaporating adherent, 150-nm-thick Au films atop the dendritic adhesion promoter. Scanning tunneling microscopy and cyclic voltammetry have been used to assess the surface roughness and electrochemical stability of the Au films. The effectiveness of the dendrimer adhesion layer is demonstrated using standard adhesive-tape peel tests.     

Investigation of the monolayer LB film properties of a silsesquioxane hybrid containing phenyl groups using SPM

In this study, we used a silsesquioxane hybrid material between phenyl group containing carbosilane dendrimer and siloxane polymer (G1-3Ph). As expected, G1-3Ph has a networked structure. Generally, it is difficult to generate uniform monolayers as common materials. However, if the materials have a networked structure, we can easily obtain uniform monolayers because of the molecular interactions. Moreover, this networked structure should have a cavity in which ions could be placed.We have fabricated Langmuir–Blodgett (LB) films of the G1-3Ph in a thin film state. We investigated the monolayer behavior using the π-A isotherm at the air–water interface. The π-A curves indicated that the surface pressure of this LB film from the liquid to the solid state falls between 5 and 8 mN/m. The monolayer was deposited onto highly oriented pyrolytic graphite (HOPG) substrates via Y-type deposition at a surface pressure of 5.5 mN/m. We also studied the morphological properties of the silsesquioxane hybrid LB films using SPM and determined the electrical properties of the LB films using STM. The I–V and the morphological characteristics of the sample were investigated at room temperature. The experimental results correlate with the calculated I–V curves when we use the parameters indicated in the caption. This study will enable us to apply a molecular electronic device to measure morphological and electrical characteristics on the nano scale.     

Interaction between calix[4]arene derivative bearing adenino units and complementary nucleosides at the air–water interface

The monolayers of the amphiphilic calix[4]arene derivatives at the lower rim with two adenino units on the surface of pure water, the aqueous subphases containing complementary nucleosides were studied by film balance measurement and relaxation experiments. LB films deposited from all subphases were investigated by UV spectra and FT-IR spectra. All the results indicate that the interaction between the adenino units in the headgroup of amphiphilic calix[4]arene derivatives and the complementary nucleosides in the subphase takes place through multiple complementary hydrogen bonding and the nucleosides in the subphases can be transferred to solid substrates along with their monolayers.     

Vapor growth of thin heteroepitaxial InP films on CdS

InP was heteroepitaxially deposited onto CdS single-crystal substrates by chemical vapor deposition using phosphine (PH₃), HCl and indium as reactants. Single-crystalline films were deposited at substrate temperatures as low as 450°C. Scanning Auger microscopy shows that the films are InP and that formation of solid solutions between the CdS and the InP is minimal. The as-grown films are n type with residual donor densities of 4 × 10¹⁶–4 × 10¹⁷ cm^-3.     

Gas adsorption gates based on ultrathin composite polymer films

High surface area alumina coatings were prepared on surface acoustic wave (SAW) mass balances. These coatings were fabricated by anodic etching of evaporated aluminum films. The coatings consisted of roughly collinear pores penetrating through the monolithic alumina film. The nanoporous (NP) coatings were characterized by scanning electron microscopy, and the pore number density and diameter were found to be (3.8 +/- 0.5) x 10(3) pores/microm2 and 6.8 +/- 4.8 nm, respectively. The mass of volatile organic compounds that adsorbed onto naked and chemically modified NP alumina coatings was measured using SAW mass balances and compared to the mass absorbed onto SAW devices having planar aluminum coatings. Thirty-four times more heptane adsorbed to the naked NP coating than to the planar coating. The mass loading response was also measured after modification with organic thin films (3-12 nm thick) that spanned the pores of the NP coating. These organic thin films were composed of sixth-generation, amine-terminated poly(amido amine) dendrimers and poly(maleic anhydride)-c-poly(methyl vinyl ether) (Gantrez). The key result of this study is that these organic thin films modulate adsorption of VOCs onto the pore walls of the NP alumina. Specifically, a single 3-nm-thick monolayer of the dendrimer reduces permeability of the VOCs by approximately 17%, whereas a 12-nm-thick G6-NH2/Gantrez composite reduces permeability by 100%. Thus, the polymer composite acts as a nonselective gate that controls access of VOCs to the underlying surface area of the pores.     

Structure and formation of self-assembled monolayers of helical poly (γ-benzyl l-glutamate) by surface plasmon resonance and infrared spectroscopy

The self-assembly of a rigid, rod-like, α-helical polypeptide was investigated on gold substrates. A disulfide labeled poly(γ-benzyl-l-glutamate) (PBLG) was prepared by a novel synthetic route wherein the polymerization of the N-carboxyanhydride was initiated with an amine terminated disulfide. Two different SSPBLG materials with molecular weight of 11 and 28 kDa were prepared and characterized using dielectric spectroscopy. The kinetics of self-assembly of the two polypeptides was studied using surface plasmon resonance (SPR) and the organization of the self-assembled monolayers (SAMs) was characterized by polarization-modulation infrared absorption spectroscopy (PM-IRRAS). SPR revealed that self-assembly of the high molecular weight polypeptides on gold substrates proceeds rapidly on a time-scale of minutes and is comparable to the time of assembly of simple n-alkanethiols. The increase in length of the polypeptide led to an increase in the dipole moment from 156 to 363 D. The enhanced electrostatic interactions in the SAMs were manifested as an increase in the average tilt of the helix axis from the normal to the surface and monolayers with smaller thickness.     

Amine-terminated TEG-derived PAMAM dendrimer as template for preparation of gold nanoparticles in water

Nano-sized monodisperse gold particles (AuNPs) have received significant attention in the past decade, due to their unique physical properties and good chemical stability, which can lead to a wide variety of potential applications. In this work, TEG-derived PAMAM dendrimers with amine-terminating groups were synthesized and characterized by ¹H NMR and FT-IR. These dendrimers were investigated as the templates for preparation of gold nanoparticles through the reduction of HAuCl₄ by NaBH₄ in water. Stable gold nanoparticles with diameters around 10 nm were obtained in the presence of G2.0 – G5.0 dendrimers and characterized by UV-Vis spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). The particle size of the produced AuNPs decreased with increasing dendrimer generations. A dendrimer of higher generation has a rigid structure with many end groups on the surface and may play a powerful role in the growth of the AuNPs, as well as having a solid stabilization effect on the AuNPs.     

Monolayer studies of sugar- and nucleoside-terminated dendrimers at the air–water interface

Sugar- and adenosine-terminated dendrimers, [1,2-o-Isopropylideneribosyl-(G₁-12acid), -(G₂-36acid)] and [Adenosyl-(G₁-12acid), -(G₂-36acid)], were synthesized using Newkome's dendrimer synthetic method. Langmuir and Langmuir–Blodgett (LB) monolayers of these dendrimers have been constructed and characterized at the air–water interface and on solid substrates by measuring surface pressure–molecular area (Π–A) isotherms, atomic force microscopy (AFM), ellipsometry and contact angle measurement. Π–A isotherms and AFM images showed that these dendrimers formed stable and homogeneous monolayers without aggregation on pure water surface. The first and second generation of sugar-terminated dendrimers show molecular areas of 647 and 1359 Ų, respectively. Ellipsometry measurement indicates that the thickness of both the first and the second generation of sugar-terminated dendrimers were about 10 Å. This reflects a flat orientation of both molecules at the air–water interface. On the other hand, the first generation of adenosine-terminated dendrimer shows an area of 105.6 Ų per molecule with a thickness of 16 Å, and for the second generation, the area was 738.4 Ų with a thickness of 27 Å. These results suggested that adenosine-terminated dendrimers maintain a spherical form at the air–water interface. It was found that small difference in the structure of thymine and uracil in the subphase critically affects the interaction of the molecules and conformation of the dendrimers at the interface.     

Stability, antimicrobial activity, and cytotoxicity of poly(amidoamine) dendrimers on titanium substrates

In this article, we present the first report on the antibacterial activity and cytotoxicity of poly(amidoamine) (PAMAM) dendrimers immobilized on three types of titanium-based substrates with and without calcium phosphate coating. We show that the amino-terminated PAMAM dendrimers modified with various percentages (0-60%) of poly(ethylene glycol) (PEG) strongly adsorbed on the titanium-based substrates. The resultant dendrimer films effectively inhibited the colonization of the Gram-negative bacteria Pseudomonas aeruginosa (strain PAO1) and, to a lesser extent, the Gram-positive bacteria Staphylococcus aureus (SA). The antibacterial activity of the films was maintained even after storage of the samples in PBS for up to 30 days. In addition, the dendrimer films had a low cytotoxicity to human bone mesenchymal stem cells (hMSCs) and did not alter the osteoblast gene expression promoted by the calcium phosphate coating.     

Vibrational circular dichroism in general anisotropic thin solid films: measurement and theoretical approach

In this study, the measurement of the true vibrational circular dichroism (VCD) spectrum is considered from an experimental and theoretical approach for any general anisotropic thin solid sample exhibiting linear as well as circular birefringence (LB, CB) and dichroism (LD, CD) properties. For this purpose, we have made use of a simple model alpha-helix polypeptide, namely, the poly(gamma-benzyl-L-glutamate) or PBLG, reference sample possessing a well-known VCD spectrum and giving rise to slightly oriented films by deposition onto a solid substrate. Also, we have used a different Fourier transform infrared modulation of polarization (PM-FTIR) optical setup with two-channel electronic processing in order to record the PM-VLD and PM-VCD spectra for various sample orientations in its film plane. All the corresponding general relations of the expected intensities in these experiments and the related properly designed calibration measurements were established using the Stokes-Mueller formalism; in addition, the residual birefringence of the optical setup and the transmittance anisotropy of the detector were estimated. From a comparative study of the results obtained in solution and in the solid state, we then propose a simple new experimental procedure to extract the true VCD spectrum of an oriented PBLG thin film: its consists of calculating the half-sum of two spectra recorded at theta and at theta +/- 90 degrees sample orientations. Moreover, the complete linear and circular birefringence and dichroism properties of the ordered PBLG thin film are estimated in the amide I and amide II vibrational regions. This allows us to establish for any sample orientation various theoretical simulations of the VCD spectra that agree nicely with the observed experimental results; this confirms that the measurement of LD and LB is in this case a prerequisite in simulating the true VCD spectrum of a partly oriented anisotropic sample. This validates our combined experimental and theoretical approach and opens the route to promising future vibrational CD studies on other macroscopic anisotropic thin film samples.     

Amine-terminated TEG-derived PAMAM dendrimer as template for preparation of gold nanoparticles in water

Nano-sized monodisperse gold particles (AuNPs) have received significant attention in the past decade, due to their unique physical properties and good chemical stability, which can lead to a wide variety of potential applications. In this work, TEG-derived PAMAM dendrimers with amine-terminating groups were synthesized and characterized by ¹H NMR and FT-IR. These dendrimers were investigated as the templates for preparation of gold nanoparticles through the reduction of HAuCl₄ by NaBH₄ in water. Stable gold nanoparticles with diameters around 10 nm were obtained in the presence of G2.0–G5.0 dendrimers and characterized by UV-Vis spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). The particle size of the produced AuNPs decreased with increasing dendrimer generations. A dendrimer of higher generation has a rigid structure with many end groups on the surface and may play a powerful role in the growth of the AuNPs, as well as having a solid stabilization effect on the AuNPs.     

Glucose biosensors based on dendrimer monolayers

The peculiarities of glucose biosensors based on different generation of dendrimers (G0, G1 and G4) have been studied by amperometry and QCM techniques. It is shown that stable glucose biosensor can be obtained with low generation of dendrimers. The sensor sensitivity, however considerable, increased with increasing number of generation of dendrimers. This can be due to the increased volume of the dendrimer interior as well as with increased number of binding sites for glucose oxidase (GOX). QCM experiments showed that immobilization of GOX resulted in formation of enzyme multilayers on a dendrimer surface. The enzyme turnover for this system (0.1-0.01 s(-1)) was lower then that for immobilization of GOX onto a supported lipid films by means of avidin-biotin technology (1.1 s(-1)). However, dendrimer based biosensors are more stable in comparison with sBLM based sensors and could be stored in a refrigerator in dry conditions over 15 days without substantial loss of sensitivity.     

Design and Application of Surface Modification at Molecular Scale

The structuring of surfaces on a nanoscale level-both chemically and topographically has become an increasingly relevant field of research in nanotechnology with widespread application potential in various fields of science ( e. g.surface engineering, electronics, biotechnology, optics). Two examples on surface modification at molecular scale with self-assembly monolayers are shown: ( 1 ) Chemically attaching ultra-thin polymer films through the self-assembly of silane fictionalized copolymer have been approved in this article. (2) The patterned films with microstructures on different substrates have been prepared through micro-contact printing technique and electro polymerization.     

Preparation and characterization of poly(amide-imide) multilayer films

Multilayer films of poly(amide-imide)s were prepared as follows: (1) monolayers of long alkyl amine salts of poly(amide-amic acid) were prepared at the air- water interface; (2) these monolayers were deposited on an appropriate plate by the Langmuir-Blodgett method; (3) the multilayers of poly(amide-amic acid) salts on the solid support were treated with acetic anhydride to afford poly(amide-imide) multilayer films. The monolayer thickness of the poly(amide-imide) multilayer films were 0.43–0.55 nm.     

Tin oxide nanocluster hydrogen and ammonia sensors

We have prepared sensitive hydrogen and ammonia sensors from thin films of tin nanoclusters with diameters between 3 and 10?nm. By baking the samples at 200?°C in ambient air the clusters were oxidized, resulting in very stable films of tin oxide clusters with similar diameters to the original Sn clusters. By monitoring the electrical resistance, it is shown that the cluster films are highly responsive to hydrogen and ammonia at relatively low temperatures, thereby making them attractive for commercial applications in which low power consumption is required. Doping of the films by depositing Pd on top of the clusters resulted in much improved sensor response and response times. It is shown that optimal sensor properties are achieved for very thin cluster films (a few monolayers of clusters).     

Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

Triple-Point Wetting of Molecular Hydrogen on Tailored Substrates

Triple-point wetting is a well-known phenomenon of many simple adsorbates on solid substrates. It implies that in the liquid phase above the triple-point temperature, T₃, complete wetting with the formation of arbitrarily thick films is observed, whereas below T₃ only a few monolayers of the solid phase are adsorbed at saturated vapour pressure. This effect is usually ascribed to substrate-induced strain in the solid film, which occurs due to lattice mismatch and/or the strong van der Waals pressure in the first monolayers. Molecular hydrogen is a suitable system to investigate this phenomenon, in particular by tailoring the adsorbate-substrate interaction by means of thin preplating layers of other adsorbates, and by introducing disorder into the system by using not only the pure systems H₂ and D₂, but also mixtures thereof. In particular the dependence of T₃ of the mixture on the mass ratio of its components is measured and deviations from the simple van der Waals law are discussed. The experiments show that triple-point wetting is a rather dominating effect, which in contrast to expectation persists even if the system parameters are widely varied, indicating that the present picture of this effect is incomplete.     

Method for quantification of a prostate cancer biomarker in urine without sample preparation

We describe a macrocantilever-based method for detecting a prostate cancer biomarker (alpha-methylacyl-CoA racemase; AMACR) directly in patient urine without a sample preparation step and without the use of labeled reagents. Clean catch voided urine specimens were prospectively collected from five confirmed prostate cancer patients 3 weeks postbiopsy. The presence of AMACR was measured in a blinded manner by exposing 3 mL of urine to the anti-AMACR-immobilized piezoelectric-excited millimeter-sized (PEMC) sensor. The resonance frequency of PEMC decreases as AMACR from sample binds to the antibody on the sensor. The resonance frequency changes for the five patients tested were 4,314 +/- 35 (n = 2), 269 +/- 17 (n = 2), 977 +/- 64 (n = 3), 600 +/- 31 (n = 2), and 801 +/- 81 (n = 2) Hz, respectively. Positive detection was observed within approximately 15 min. The responses to positive, negative, and buffer controls were -9 +/- 13, -34 +/- 18, and -6 +/- 18 Hz, respectively. Positive verification of AMACR attachment was confirmed by low-pH buffer release. The sensor response was quantitatively related to AMACR concentration in control urine, and the relationship was used in developing an in situ calibration method for quantifying AMACR in patient urine. Estimated concentrations of 42, 2, and 3 fg/mL AMACR were calculated for the three patients' urine, while absence of AMACR was confirmed in control urine (n = 13). Because of simplicity of measurement combined with high sensitivity and specificity, the method may be a useful adjunct in a point-of-care setting to identify men at increased risk for prostate cancer.     

Immobilization of dendrimers on Si-C linked carboxylic acid-terminated monolayers on silicon(111)

Poly(amidoamine) dendrimers were attached to activated undecanoic acid monolayers, covalently linked to smooth silicon surfaces via Si-C bonds. The resulting ultra-thin dendrimer films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray reflectometry (XR) and atomic force microscopy (AFM). XPS results suggested amide bond formation between the dendrimer and the surface carboxylic acid groups. XR yielded thicknesses of 10 Å for the alkyl region of the undecanoic acid monolayer and 12 Å for the dendrimer layer, considerably smaller than the diameter of these spherical macromolecules in solution. This was consistent with AFM images showing collapsed dendrimers on the surface. It was concluded that the deformation arose from a large number of amine groups on the surface of each dendrimer reacting efficiently with the activated surface, whereby the dendrimers can deform to fill voids while spreading over the activated surface to form a homogeneous macromolecular layer.