Electroless deposition of noble metals on silicon has applications in a wide range of fields including electronic circuitry, metal plating industry, lithography, and other fabrication techniques. In addition, studies using self-assembled monolayers (SAMs) as resists for electroless deposition for controlled deposition have significant potential for aiding advancement in the fields of nanoelectronics, sensing applications, and fundamental studies. Herein, we discuss the development of appropriate plating solutions for controlled deposition of metallic gold and silver on Si(111) surfaces in the presence of an organic silane monolayer acting as a resist film for directed metal deposition to produce metal-monolayer hybrid surfaces while investigating microscopic plating trends. For this, plating solutions were optimized to deposit metal on bare silicon surfaces while avoiding deposition on the SAM protected areas. Trends in the electroless deposition of gold and silver on a Si(111) surface as a function of concentration of metal ions, NH4F, citric acid, sodium citrate, polyvinylpyrrolidone (PVP), and deposition time have been monitored under ambient conditions. The resulting surfaces were characterized using atomic force microscopy (AFM), and the stability of plating solutions was investigated by UV–vis spectroscopy. For both gold and silver, we observed an increase in metal deposition when the concentration of NH4F, citric acid, and deposition time increased. The addition of PVP and the pH of the solution were also shown to have a significant effect on the metal deposition. The octadecyltrichlorosilane (OTS) SAM films act as effective nanoscale resists when the NH4F concentration is reduced from typical plating conditions. In particular, NH4F concentrations from 0.02 to 0.50 M and metal ions concentrations from 0.001 to 0.020 M were found to allow deposition of metal nanostructures on a bare Si surface while preserving OTS protected areas.

Precision-controlled fabrication of metallic nanostructures is of great interest in applications such as sensing, optoelectronics, and high-capacity storage devices. However, the expense and throughput of the current methods limit the applicability of metal nanodot arrays for many of these applications. This issue is addressed by a method for generating periodic silver (Ag) and gold (Au) nanodot arrays in a straightforward, inexpensive, tunable way. Specifically, regularly placed hexagonal arrays of Ag and Au nanodots were fabricated on Si(111) surfaces via a nanosphere lithography-based approach followed by electroless deposition (ELD). Silicon surfaces with hexagonally packed nanospheres were reacted with octadecyltrichlorosilane (OTS) to form a self-assembled monolayer resist over the substrate, which leads to a hexagonal array of nanopores upon removal of the spheres. Different electroless plating solutions for Ag and Au were introduced onto the nanopore surfaces to selectively deposit metal in the nanopores, resulting in metal nanodots grown only in the nanopores, where the nanospheres were originally in contact with the substrate. Ag and Au nanodot heights can be effectively tuned from 20 to 100 nm by varying the plating time and the composition of the plating solution. Atomic force microscopy (AFM) was used to characterize the height and diameter of the nanopore and nanodot arrays along with energydispersive X-ray spectroscopy (EDS) to characterize the elemental composition distribution on the surface. This method provides control over the distance between nanodots and their size at the nanoscale with high reproducibility.

Applications such as biosensing, plasmonics, and nanoelectronics require nanoscale metal structures with controlled dimensions and placement. However, significant challenges remain in the fabrication of metal nanostructures of controlled size, shape, and placement on a solid support. Among these challenges are precise positional control at the nanoscale, flexibility and tunability in shape, and the cost and complexity of methods. This work presents the development and exploration of methods for the fabrication of copper, silver, and gold (Cu, Ag, and Au) nanostructures directly on silicon (Si) substrates through the use of atomic force microscopy (AFM)-based nanofabrication using a self-assembled monolayer (SAM) resist followed by metal deposition in the structure using electroless deposition (ELD). The importance of the role of the SAM resist layer is highlighted, as it is critical to prevent metal deposition on the areas of the substrate outside the desired pattern. We have found that octadecyltrichlorosilane (OTS) monolayers are much more robust and resistant films for the ELD process than either octadecyl SAMs, formed from alkenes on hydrogen-terminated Si, or octadecyldimethylchlorosilane (ODMS) SAM films. In addition, the patterning parameters used for the AFM-based fabrication, the ELD solution parameters, and the role of doping of Si have been explored and together the results suggest that with proper tuning of the ELD solution concentrations and the use of a robust SAM resist film, such as OTS, tunable metal nanostructures are achievable. This is demonstrated here for Cu, Ag, and Au, but the process should be adaptable to a variety of metals, as long as the redox potentials are compatible with the oxidation of Si. Importantly, this method exploits the exquisite tunability of AFM-based lithography to provide precise control over the size, shape, and position of the metal nanostructure. This provides significant advantages for prototyping of new structures, as well as fundamental investigations of the properties of such nanostructures.

Resin-based composite materials have been widely used in restorative dental materials due to their aesthetic, mechanical, and physical properties. However, they still encounter clinical shortcomings mainly due to recurrent decay that develops at the composite-tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal this interface, but the adhesive seal is inherently defective and readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite-tooth interface and bacterial by-products demineralize the tooth and erode the adhesive. These activities lead to wider and deeper gaps that provide an ideal environment for bacteria to proliferate. This complex degradation process mediated by several biological and environmental factors damages the tooth, destroys the adhesive seal, and ultimately, leads to failure of the composite restoration. This paper describes a co-tethered dual peptide-polymer system to address composite-tooth interface vulnerability. The adhesive system incorporates an antimicrobial peptide to inhibit bacterial attack and a hydroxyapatite-binding peptide to promote remineralization of damaged tooth structure. A designer spacer sequence was incorporated into each peptide sequence to not only provide a conjugation site for methacrylate (MA) monomer but also to retain active peptide conformations and enhance the display of the peptides in the material. The resulting MA-antimicrobial peptides and MA-remineralization peptides were copolymerized into dental adhesives formulations. The results on the adhesive system composed of co-tethered peptides demonstrated both strong metabolic inhibition of S. mutans and localized calcium phosphate remineralization. Overall, the result offers a reconfigurable and tunable peptide-polymer hybrid system as next-generation adhesives to address composite-tooth interface vulnerability.

Flavin oxidases are valuable biocatalysts for the oxidative synthesis of a wide range of compounds, while at the same time reduce oxygen to hydrogen peroxide. Compared to other redox enzymes, their ability to use molecular oxygen as an electron acceptor offers a relatively simple system that does not require a dissociable coenzyme. As such, they are attractive targets for adaptation as cost-effective biosensor elements. Their functional immobilization on surfaces offers unique opportunities to expand their utilization for a wide range of applications. Genetically engineered peptides have been demonstrated as enablers of the functional assembly of biomolecules at solid material interfaces. Once identified as having a high affinity for the material of interest, these peptides can provide a single step bioassembly process with orientation control, a critical parameter for functional immobilization of the enzymes. In this study, for the first time, we explored the bioassembly of a putrescine oxidase enzyme using a gold binding peptide tag. The enzyme was genetically engineered to incorporate a gold binding peptide with an expectation of an effective display of the peptide tag to interact with the gold surface. In this work, the functional activity and expression were investigated, along with the selectivity of the binding of the peptide-tagged enzyme. The fusion enzyme was characterized using multiple techniques, including protein electrophoresis, enzyme activity, and microscopy and spectroscopic methods, to verify the functional expression of the tagged protein with near-native activity. Binding studies using quartz crystal microbalance (QCM), nanoparticle binding studies, and atomic force microscopy studies were used to address the selectivity of the binding through the peptide tag. Surface binding AFM studies show that the binding was selective for gold. Quartz crystal microbalance studies show a strong increase in the affinity of the peptide-tagged protein over the native enzyme, while activity assays of protein bound to nanoparticles provide evidence that the enzyme retained catalytic activity when immobilized. In addition to showing selectivity, AFM images show significant differences in the height of the molecules when immobilized through the peptide tag compared to immobilization of the native enzyme, indicating differences in orientation of the bound enzyme when attached via the affinity tag. Controlling the orientation of surface-immobilized enzymes would further improve their enzymatic activity and impact diverse applications, including oxidative biocatalysis, biosensors, biochips, and biofuel production.