Jeffrey S. Zabinski

Colorimetric Response of Peptide-Functionalized Gold Nanoparticles to Metal Ions†

The design of nanostructures with controlled surface chemistry for sensing, catalytic, and electronic applications is an important research challenge. Sensing platforms based on the optical properties of gold nanoparticles in combination with the molecular recognition of ligands, such as alkyl thiols, antibodies, nucleic acids, and proteins, are active areas of research. Detection of targets by functionalized gold particles has been performed by using surface-enhanced raman spectroscopy (SERS), quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR) spectroscopy, electrochemical and potentiometric detection, and colorimetric assays. The gold-nanaoparticle-based colorimetric sensors provide simplicity and excellent detection capability encompassing a variety of targets including metal ions, DNA, bacterial toxins, protein conformations, and enzyme activity. For example, Pb2þ was detected by a color change upon the dispersion of gold nanoparticles functionalized with DNAzyme. Recently, DNA-modified gold nanoparticles were used as a colorimetric sensor in the detection of Hg2þ.[14] The aggregation of the ligand-functionalized gold nanoparticles upon binding its target results in a colorimetric response caused by broadening and shifting of the plasmon resonance peak. This shift in the plasmon resonance frequency is employed in sensing strategies. To date, the majority of the colorimetric gold nanoparticle sensing strategies have used nucleic acids as the sensing element. Here, we demonstrate the potential of peptide-functionalized gold nanoparticles (PFNs) as a colorimetric sensor for metal ions. The PFNs were synthesized in a HEPES buffer using the Flg-A3 peptide (-Asp-Tyr-Lys-Asp-Asp-Asp-Asp-LysPro-Ala-Tyr-Ser-Ser-Gly-Pro-Ala-Pro-Pro-Met-Pro-Pro-Phe-). The synergistic contributions of both the Flg-A3 peptide and HEPES buffer result in the formation of peptidefunctionalized suspension of gold nanoparticles. The overall negative charge of the peptide (pI1⁄4 3.9) prevents aggregation of the particles by repulsive forces. The surface of the gold nanoparticles contains amino acid functional groups that can interact with metal ions. Charged, aromatic, and hydroxyl-

Nanocomposite tribological coatings with ''chameleon'' surface adaptation

Nanocomposite tribological coatings were designed to respond to changing environmental conditions by self-adjustment of their surface properties to maintain good tribological performance in any environment. These smart coatings have been dubbed “chameleon” because, analogous to a chameleon changing its skin color to avoid predators, the coating changes its “skin” chemistry and structure to avoid wear. The concept was originally developed using WC, diamondlike carbon, and WS2 material combination for adaptation to a humid/dry environment cycling. In order to address temperature variation, nanocomposite coatings made of yttria-stabilized zirconia (YSZ) in a gold matrix were developed with encapsulated nanosized reservoirs of MoS2 and diamondlike carbon (DLC). Coatings were produced using a combination of laser ablation and magnetron sputtering. They were characterized by x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, x-ray energy dispersive spectroscopy, and micro-Ram...

Recent advances in hard, tough, and low friction nanocomposite coatings

Abstract Nanocomposite coatings demonstrate improved friction and wear responses under severe sliding conditions in extreme environments. This paper provides a review how thin film multilayers and nanocomposites result in hard, tough, low-friction coatings. Approaches to couple multilayered and nanocomposite materials with other surface engineering strategies to achieve higher levels of performance in a variety of tribological applications are also discussed. Encapsulating lubricious phases in hard nanocomposite matrices is one approach that is discussed in detail. Results from state-of-the-art “chameleon” nanocomposites that exhibit reversible adaptability to ambient humidity or temperature are presented.

Effect of surface chemistry on the tribological performance of a MEMS electrostatic lateral output motor

The effect of surface chemistry on the tribological performance and reliability of a MEMS lateral output motor is reported. Relative humidity (RH) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) coatings were used to change surface chemistry. Electrical and tribological performance of uncoated and OTS-coated motors were found to be dependent on RH. For uncoated motors, excessive wear of sliding contacts and welding (permanent adhesion) of static contacts were observed at 0.1% RH. Degradation of electrostatic force and high static friction (stiction) forces limited dynamic performance and reliability and caused device sticking at and above 70% RH. Around 50% RH, uncoated motors exhibited negligible wear, low adhesion, and a wear life at least three orders of magnitude longer than in the dry environment (experiments were stopped without failure after about one billion cycles). Water vapor behaved as a gas phase replenishable lubricant by providing a protective adsorbed film. The OTS coating broadened the operating envelope to 30–50% RH and reduced stiction, which allowed better dynamic performance at high RH. The OTS coating improved durability at 0.1% RH, but it was still poor. At high RH, stiction problems reoccurred when the OTS coating was worn away. By controlling and balancing surface chemistry (adsorbed water and OTS), excellent performance, low friction and wear, and excellent durability were attained with the lateral output motor.

Lubrication of Microelectromechanical Systems (MEMS) Using Bound and Mobile Phases of Fomblin Zdol

A lubrication scheme for MEMS electrostatic lateral output motors based on a mixture of bound and mobile lubricant was studied. Lubrication by bound monolayer alone provided some increase in operational life, but after a short time, the film wore away and the device failed in the unlubricated mode. A mobile phase was used to provide lubricant replenishment. Tribological studies were conducted on Si(100) wafers, as well as on MEMS electrostatic lateral output motors, dip-coated with a mixture of bound and mobile phases of Fomblin Zdol. Accelerated screening tests on Si(100) wafers were undertaken using a pin on disk tribometer. However, the optimum balance of bound and mobile phases was determined by studies on the device itself. The fractional surface coverage of lubricant and the ratio of bound to mobile phase was varied through selection of reaction temperature and rinse chemistry. The mobile phase on model surfaces and devices acted as a source of lubricant replenishment, and together with the bound phase provided dramatic improvement in performance. The wide variation seen in the performance of individual devices suggests that dip coating does not provide a uniform coating on the contacting surfaces of these devices.

Mechanical and tribological properties of diamond-like carbon coatings prepared by pulsed laser deposition

Abstract The tribological properties of diamond-like carbon (DLC) coatings produced by pulsed laser deposition (PLD) are investigated. Films are grown onto steel substrates to 0.5 μm using a 248 nm laser to ablate graphite and polycarbonate targets in high vacuum. Chemical bonding is studied with Raman, XPS and EELS techniques; mechanical and tribological properties are evaluated using microindentation and ball-on-disk friction tests. Coatings grown from graphite targets are amorphous DLC (a-C), while those grown from polycarbonate targets are amorphous hydrogenated carbon (a-C:H). The hardness of the a-C coatings is 55–70 GPa and the hardness of the a-C:H coatings is 12–20 GPa depending on the substrate bias. Friction coefficients of the coatings against steel and sapphire balls are determined in several environments: in air as a function of relative humidity, in dry nitrogen, and in 10 Pa vacuum. For a-C coatings, the friction coefficients are typically below 0.1 and are as low as 0.03 in dry nitrogen. In wear tests, a critical contact pressure of 1.4 GPa led to catastrophic adhesive failure of a-C coatings, whereas failure of a-C:H coatings is by wear-through after 5 x 10 3 cycles. Extremely low wear rates of 10 −9 mm 3 N −1 m −1 are found for a-C coatings at the contact pressure of 0.8 GPa.

Structure and properties of diamondlike carbon films produced by pulsed laser deposition

Pulsed laser deposition was used to produce hydrogen‐free amorphous diamondlike carbon (a‐C) and hydrogenated amorphous diamondlike carbon (a‐C:H) from graphite and polycarbonate targets, respectively. Films were grown under identical conditions in high vacuum at low temperatures onto steel and Si substrates. The a‐C films were uniform, while a‐C:H films contained a great number of particles ejected from the target surface. The a‐C films have hydrogen contamination about 0.1 at.u2009%, while a‐C:H have about 25 at.u2009% H and 10 at.u2009% O. High percentages of sp3 bonding were found in both films. Film densities were estimated to be 3.0 gu2009cm−3 for a‐C films and 2.2 gu2009cm−3 for a‐C:H films. Chemical and structural characteristics of the films were correlated with their thermal stability and mechanical properties. Temperatures for starting graphitization were about 500u2009°C for a‐C and 350u2009°C for a‐C:H. The presence of hydrogen reduced film hardness from 60 GPa for a‐C films to 14 GPa for a‐C:H films. Hydrogen was also ...

Sliding behavior of multifunctional composite coatings based on diamond-like carbon

AbstractThe sliding behavior of several coatings based on non-hydrogenated diamond-like carbon (DLC) is described. Coatings were producedbyusingamagnetronsputter-assistedpulsedlaserdepositionprocessdevelopedattheAirForceResearchLaboratory.Resultsarecomparedfor two types of coatings: DLC with WC nanoparticles and DLC with both WC particles and WS 2 (“WCS”). Sliding tests were done in air,nitrogen and vacuum and for alternating periods in different environments, e.g., cycling between air and vacuum conditions. The frictionforce and the signal from an in situ Kelvin probe were monitored during sliding. Friction coefficients ranging from near 0.01 to 0.6 havebeen observed.The Kelvin probe detected transients lasting from ten minutes to more than one hour. Post-test characterization included SEM/EDS,Raman and TEM. The role of transfer and mixing is discussed.© 2003 Elsevier Science B.V. All rights reserved. Keywords: Composite coatings; DLC coatings; Sliding friction; WS 2 1. IntroductionTypical solid lubricant materials are relatively soft andtherefore not abrasion resistant. Hard coatings are often em-ployed to protect surfaces from abrasive and erosive wear.However, hard coatings may not have low friction and arefrequently brittle. There is a need to design hard coatingsthat also provide low friction, exhibit increased toughnessand remain compatible with their substrates. For aerospaceapplications, such coatings should also operate reliably overa wide range of loads, temperatures and environmental con-ditions and should survive cycling of these variables. Forexample, the coating might be required to operate well inhumid air at a launch site as well as for extended periodsin the vacuum of space. Coatings that maintain low frictionwould assure that energy use is minimized during extendedoperation or when resuming relative motion after periods ofrest during on–off cycles. They would also be expected toprovide increased lifetimes of tribological systems.To produce effective and reliable coatings that might sat-isfy the requirements outlined above, the multidisciplinarycoatings research group at AFRL has chosen a strategy thatincludes all of the following:

Tribology of pulsed laser deposited thin films of cesium oxythiomolybdate (Cs2MoOS3)

Abstract High temperature solid lubricants are required for use on silicon nitride bearings with application temperatures of 600 °C and beyond. In the present research, the tribological properties of cesium oxythiomolybdate (Cs 2 MoOS 3 ) thin films grown via pulsed laser deposition were studied as a function of temperature from 25 to 800 °C. When deposited on Si 3 N 4 , the coefficient of friction, μ, was 0.03 at 600 °C. When deposited on Al 2 O 3 , ZrO 2 and Inconel, μ≈0.15 at 600 °C. The friction coefficient generally increased with decreasing temperature. Wear rates were low on all substrates between 400 and 600 °C. Friction remained low up to 750 °C on Si 3 N 4 substrates, but failure occurred above 750 °C. The tribomechanisms are discussed.

CRYSTALLINE SIC THIN FILMS DEPOSITED AT ROOM TEMPERATURE USING PULSED LASER ABLATION OF GRAPHITE AND MAGNETRON SPUTTERING OF SILICON

Crystalline SiC films are typically deposited on substrates at elevated temperatures by sputtering, pulsed laser ablation, thermal chemical vapor deposition (CVD) and CVD plasma assisted. However, high temperature may adversely affect the substrate, especially when metal alloys are used. To maintain substrate properties (temper, dimensional tolerance, etc.), a low deposition temperature is required. In this work, silicon carbide is formed from simultaneous sputtering of silicon and laser ablation of graphite onto suitably biased substrates at room temperature. The advantage of this method lies in the independent selection of plasma characteristics of both magnetron sputtering and laser ablation to achieve the required stoichiometry and species energetics. Desirable film properties such as good adhesion and crystallinity normally requiring elevated substrate temperatures are obtained via the energetic bombardment of the growing film. In this study, films are grown on M50 steel substrates at biasing varied from 0 to −300 V permitting control over crystallinity, chemistry, and stoichiometry. X-ray photoelectron spectroscopy (XPS) analysis shows the existence of silicon carbide bonds and x-ray diffraction analysis demonstrated the growth of crystalline (prominently alpha polytypes (4H–SiC, 6H–SiC) films at room temperature. In addition, XPS is used to find percentage of SiC bonds in the films. The optimum bias −100 V was found to favor crystalline growth in these films. Special emphasis is given to control of film stoichiometry as it relates to mechanical properties.