Characterization of Pseudomonas aeruginosa Films on Different Inorganic Surfaces before and after UV Light Exposure.

The changes of the surface properties of Au, GaN, and SiO x after UV light irradiation were used to actively influence the process of formation of Pseudomonas aeruginosa films. The interfacial properties of the substrates were characterized by X-ray photoelectron spectroscopy and atomic force microscopy. The changes in the P. aeruginosa film properties were accessed by analyzing adhesion force maps and quantifying the intracellular Ca2+ concentration. The collected analysis indicates that the alteration of the inorganic materials' surface chemistry can lead to differences in biofilm formation and variable response from P. aeruginosa cells.

Persistent Photoconductivity, Nanoscale Topography, and Chemical Functionalization Can Collectively Influence the Behavior of PC12 Cells on Wide Bandgap Semiconductor Surfaces.

Wide bandgap semiconductors such as gallium nitride (GaN) exhibit persistent photoconductivity properties. The incorporation of this asset into the fabrication of a unique biointerface is presented. Templates with lithographically defined regions with controlled roughness are generated during the semiconductor growth process. Template surface functional groups are varied using a benchtop surface functionalization procedure. The conductivity of the template is altered by exposure to UV light and the behavior of PC12 cells is mapped under different substrate conductivity. The pattern size and roughness are combined with surface chemistry to change the adhesion of PC12 cells when the GaN is made more conductive after UV light exposure. Furthermore, during neurite outgrowth, surface chemistry and initial conductivity difference are used to facilitate the extension to smoother areas on the GaN surface. These results can be utilized for unique bioelectronics interfaces to probe and control cellular behavior.

Engineering the cell-semiconductor interface: a materials modification approach using II-VI and III-V semiconductor materials.

Developing functional biomedical devices based on semiconductor materials requires an understanding of interactions taking place at the material-biosystem interface. Cell behavior is dependent on the local physicochemical environment. While standard routes of material preparation involve chemical functionalization of the active surface, this review emphasizes both biocompatibility of unmodified surfaces as well as use of topographic features in manipulating cell-material interactions. Initially, the review discusses experiments involving unmodified II-VI and III-V semiconductors - a starting point for assessing cytotoxicity and biocompatibility - followed by specific surface modification, including the generation of submicron roughness or the potential effect of quantum dot structures. Finally, the discussion turns to more recent work in coupling topography and specific chemistry, enhancing the tunability of the cell-semiconductor interface. With this broadened materials approach, researchers' ability to tune the interactions between semiconductors and biological environments continues to improve, reaching new heights in device function.

Comparison of the Stability of Functionalized GaN and GaP.

Surface functionalization via 1 H,1 H,2 H,2H-perfluoro octanephosphonic acid was done in the presence of phosphoric acid to provide a simplified surface passivation technique for gallium nitride (GaN) and gallium phosphide (GaP). In an effort to identify the leading causes of surface instabilities, hydrogen peroxide was utilized as an additional chemical modification to cap unsatisfied bonds. The stability of the surfaces was studied in an aqueous environment and subsequently characterized. A physical characterization was carried out to evaluate the surface roughness and water hydrophobicity pre and post stability testing via atomic force microscopy and water goniometry. Surface-chemistry changes and solution leaching were quantified by X-ray photoelectron spectroscopy and inductively coupled plasma mass spectrometry. The results indicate a sensitivity to hydroxyl terminated species for both GaN and GaP under aqueous environments, as the increase of the degree of leaching was more significant for hydrogen peroxide treated samples. The results support the notion that hydroxyl species act as precursors to gallium oxide formation and lead to subsequent instability in aqueous solutions.

In situ chemical functionalization of gallium nitride with phosphonic acid derivatives during etching.

In situ functionalization of polar (c plane) and nonpolar (a plane) gallium nitride (GaN) was performed by adding (3-bromopropyl) phosphonic acid or propyl phosphonic acid to a phosphoric acid etch. The target was to modulate the emission properties and oxide formation of GaN, which was explored through surface characterization with atomic force microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL), inductively coupled plasma-mass spectrometry, and water contact angle. The use of (3-bromopropyl) phosphonic acid and propyl phosphonic acid in phosphoric acid demonstrated lower amounts of gallium oxide formation and greater hydrophobicity for both sample sets, while also improving PL emission of polar GaN samples. In addition to crystal orientation, growth-related factors such as defect density in bulk GaN versus thin GaN films residing on sapphire substrates were investigated as well as their responses to in situ functionalization. Thin nonpolar GaN layers were the most sensitive to etching treatments due in part to higher defect densities (stacking faults and threading dislocations), which accounts for large surface depressions. High-quality GaN (both free-standing bulk polar and bulk nonpolar) demonstrated increased sensitivity to oxide formation. Room-temperature PL stands out as an excellent technique to identify nonradiative recombination as observed in the spectra of heteroepitaxially grown GaN samples. The chemical methods applied to tune optical and physical properties of GaN provide a quantitative framework for future novel chemical and biochemical sensor development.

Surface characterization of gallium nitride modified with peptides before and after exposure to ionizing radiation in solution.

An aqueous surface modification of gallium nitride was employed to attach biomolecules to the surface. The modification was a simple two-step process using a single linker molecule and mild temperatures. The presence of the peptide on the surface was confirmed with X-ray photoelectron spectroscopy. Subsequently, the samples were placed in water baths and exposed to ionizing radiation to examine the effects of the radiation on the material in an environment similar to the body. Surface analysis confirmed degradation of the surface of GaN after radiation exposure in water; however, the peptide molecules successfully remained on the surface following exposure to ionizing radiation. We hypothesize that during radiation exposure of the samples, the radiolysis of water produces peroxide and other reactive species on the sample surface. Peroxide exposure promotes the formation of a more stable layer of gallium oxyhydroxide which passivates the surface better than other oxide species.

Wet-chemical passivation of InAs: toward surfaces with high stability and low toxicity.

In a variety of applications where the electronic and optical characteristics of traditional, siliconbased materials are inadequate, recently researchers have employed semiconductors made from combinations of group III and V elements such as InAs. InAs has a narrow band gap and very high electron mobility in the near-surface region, which makes it an attractive material for high performance transistors, optical applications, and chemical sensing. However, silicon-based materials remain the top semiconductors of choice for biological applications, in part because of their relatively low toxicity. In contrast to silicon, InAs forms an unstable oxide layer under ambient conditions, which can corrode over time and leach toxic indium and arsenic components. To make InAs more attractive for biological applications, researchers have investigated passivation, chemical and electronic stabilization, of the surface by adlayer adsorption. Because of the simplicity, low cost, and flexibility in the type of passivating molecule used, many researchers are currently exploring wet-chemical methods of passivation. This Account summarizes much of the recent work on the chemical passivation of InAs with a particular focus on the chemical stability of the surface and prevention of oxide regrowth. We review the various methods of surface preparation and discuss how crystal orientation affects the chemical properties of the surface. The correct etching of InAs is critical as researchers prepare the surface for subsequent adlayer adsorption. HCl etchants combined with a postetch annealing step allow the tuning of the chemical properties in the near-surface region to either arsenic- or indium-rich environments. Bromine etchants create indium-rich surfaces and do not require annealing after etching; however, bromine etchants are harsh and potentially destructive to the surface. The simultaneous use of NH(4)OH etchants with passivating molecules prevents contact with ambient air that can occur during sample transfer between solutions. The passivation of InAs is dominated by sulfur-based molecules, which form stable In-S bonds on the InAs surface. Both sulfides and alkanethiols form well-defined monolayers on InAs and are dominated by In-S interactions. Sulfur-passivated InAs surfaces prevent regrowth of the surface oxide layer and are more stable in air than unpassivated surfaces. Although functionalization of InAs with sulfur-based molecules effectively passivates the surface, future sensing applications may require the adsorption of functional biomolecules onto the InAs surface. Current research in this area focuses on the passivation abilities of biomolecules such as collagen binding peptides and amino acids. These biomolecules can physically adsorb onto InAs, and they demonstrate some passivation ability but not to the extent of sulfur-based molecules. Because these adsorbents do not form covalent bonds with the InAs surface, they do not effectively block oxide regrowth. A mixed adlayer containing a biomolecule and a thiol on the InAs surface provides one possible solution: these hybrid surfaces enhance passivation but also maintain the presence of a biomolecule on the surface. Such surface functionalization strategies on InAs could provide long-term stability and make these surfaces suitable for biological applications.

Aqueous stability of Ga- and N-polar gallium nitride.

The stability of III-nitride semiconductors in various solutions becomes important as researchers begin to integrate them into sensing platforms. This study quantitatively compares the stability of GaN surfaces with different polarities. This type of quantification is important because it represents the first step toward designing semiconductor material interfaces compatible with solution conditions. A stability study of Ga- and N-polar GaN was conducted by immersion of the surfaces in deionized H(2)O, pH 5, pH 9, and H(2)O(2) solutions for 7 days. Inductively coupled plasma mass spectrometry of the solutions was conducted to determine the amount of gallium leached from the surface. X-ray photoelectron spectroscopy and atomic force microscopy were used to compare the treated surfaces to untreated surfaces. The results show that both gallium nitride surface types exhibit the greatest stability in acidic and neutral solutions. Gallium polar surfaces were found to exhibit superior stability to nitrogen polar surfaces in the solutions studied. Our findings highlight the need for further research on surface passivation and functionalization techniques for polar III-nitride semiconductors.

Cell behavior on gallium nitride surfaces: peptide affinity attachment versus covalent functionalization.

Gallium nitride is a wide band gap semiconductor that demonstrates a unique set of optical and electrical properties as well as aqueous stability and biocompatibility. This combination of properties makes gallium nitride a strong candidate for use in chemical and biological applications such as sensors and neural interfaces. Molecular modification can be used to enhance the functionality and properties of the gallium nitride surface. Here, gallium nitride surfaces were functionalized with a PC12 cell adhesion promoting peptide using covalent and affinity driven attachment methods. The covalent scheme proceeded by Grignard reaction and olefin metathesis while the affinity driven scheme utilized the recognition peptide isolated through phage display. This study shows that the method of attaching the adhesion peptide influences PC12 cell adhesion and differentiation as measured by cell density and morphological analysis. Covalent attachment promoted monolayer and dispersed cell adhesion while affinity driven attachment promoted multilayer cell agglomeration. Higher cell density was observed on surfaces modified using the recognition peptide. The results suggest that the covalent and affinity driven attachment methods are both suitable for promoting PC12 cell adhesion to the gallium nitride surface, though each method may be preferentially suited for distinct applications.

Inorganic material coatings and their effect on cytotoxicity.

Inorganic materials have become an increasingly researched topic due to their applications in many areas especially health care. One major problem with them is the effect that their surface coatings have on cells. The same coatings that are meant to increase biocompatibility can actually invoke cytotoxicity. This tutorial review focuses on the various types of coatings and how their properties, such as electrostatic charge and hydrophobicity, affect the observed toxicity. The theorized mechanisms by which the coatings induce toxicity are also presented. Finally, the prospects for the future of this field are discussed.

Spore-terminated cantilevers for chemical patterning on complex architectures.

Atomic force microscope tips terminated with spore cells are used to directly pattern onto glass and tissue surfaces. The spore cells act as sponges and eliminate the need to use microfabricated ink reservoirs during lithography.

Molecular analysis of blood with micro-/nanoscale field-effect-transistor biosensors.

Rapid and accurate molecular blood analysis is essential for disease diagnosis and management. Field-effect transistor (FET) biosensors are a type of device that promise to advance blood point-of-care testing by offering desirable characteristics such as portability, high sensitivity, brief detection time, low manufacturing cost, multiplexing, and label-free detection. By controlling device parameters, desired FET biosensor performance is obtained. This review focuses on the effects of sensing environment, micro-/nanoscale device structure, operation mode, and surface functionalization on device performance and long-term stability.

Characterization of peptide adsorption on InAs using X-ray photoelectron spectroscopy.

The well-defined structure and high stability of peptides make them attractive biotemplates for low-temperature synthesis of semiconductor nanocrystals. Adsorbed peptide monolayers could also potentially passivate semiconductors by preventing regrowth of the oxide layer. In this work, the adsorption and passivation capabilities of different collagen-binding peptides on InAs surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). Before peptide functionalization, Br(2)- and HCl-based etches were used to remove the native oxide layer on the InAs surfaces. The presence of the N 1s peak for peptide-functionalized samples confirms the adsorption of peptides onto the etched InAs surfaces. Calculated coverages were similar for all peptide sequences and ranged from ∼20 to 40% of a monolayer using the deconvoluted C 1s spectra and from ∼2 to 5% for the N 1s spectra. The passivation ability of the peptides was analyzed by comparing the ratios of the oxide components to the nonoxide components in the XPS spectra. The thickness of the oxide layer was also approximated by accounting for the attenuation of the substrate photoelectrons through the oxide layer. We find that the oxide layer regrowth still occurs after peptide functionalization. However, the oxide layer thicknesses for peptide-functionalized samples do not reach as received levels, indicating that the peptides do have some passivation ability on InAs.

Detecting DNA methylation through changes in transverse proton relaxation.

We present a facile, simple method to detect DNA methylation by measuring the transverse proton relaxation behaviour. Positively charged nanoparticles are arranged along the negatively charged backbone of DNA strands through electrostatic interactions. The arrangement of NPs along DNA strands aids to amplify and compare the transverse proton relaxation signal for un-cut versus cut DNA strands cleaved by sequence specific restriction enzymes. Results from this study suggest that the presence of methylation on DNA can be detected using superparamagnetic NPs using NMR.

Tuning Microbial Activity via Programmatic Alteration of Cell/Substrate Interfaces.

A wide portfolio of advanced programmable materials and structures has been developed for biological applications in the last two decades. Particularly, due to their unique properties, semiconducting materials have been utilized in areas of biocomputing, implantable electronics, and healthcare. As a new concept of such programmable material design, biointerfaces based on inorganic semiconducting materials as substrates introduce unconventional paths for bioinformatics and biosensing. In particular, understanding how the properties of a substrate can alter microbial biofilm behavior enables researchers to better characterize and thus create programmable biointerfaces with necessary characteristics on demand. Herein, the current status of advanced microorganism-inorganic biointerfaces is summarized along with types of responses that can be observed in such hybrid systems. This work identifies promising inorganic material types along with target microorganisms that will be critical for future research on programmable biointerfacial structures.

Serial and parallel dip-pen nanolithography using a colloidal probe tip.

AFM tips terminated with PMMA colloids are used to pattern molecules in both serial and parallel modes by allowing the polymer on the tip to swell under different humidity conditions. This extension of the dip-pen nanolithography technique provides an easy methodology to place inks on different substrates without the need to perform specialized tip alignment.

Functional silica nanoparticle-mediated neuronal membrane sealing following traumatic spinal cord injury.

The mechanical damage to neurons and their processes induced by spinal cord injury (SCI) causes a progressive cascade of pathophysiological events beginning with the derangement of ionic equilibrium and collapse of membrane permeability. This leads to a cumulative deterioration of neurons, axons, and the tissue architecture of the cord. We have previously shown that the application of the hydrophilic polymer polyethylene glycol (PEG) following spinal cord or brain injury can rapidly restore membrane integrity, reduce oxidative stress, restore impaired axonal conductivity, and mediate functional recovery in rats, guinea pigs, and dogs. However there are limits to both the concentration and the molecular weight of the application that do not permit the broadest recovery across an injured animal population. In this study, PEG-decorated silica nanoparticles (PSiNPs) sealed cells, as shown by the significantly reduced leakage of lactate dehydrogenase from damaged cells compared with uncoated particles or PEG alone. Further in vivo tests showed that PSiNPs also significantly reduced the formation of reactive oxygen species and the process of lipid peroxidation of the membrane. Fabrication of PSiNPs containing embedded dyes also revealed targeting of the particles to damaged, but not undamaged, spinal cord tissues. In an in vivo crush/contusion model of guinea pig SCI, every animal but one injected with PSiNPs recovered conduction through the cord lesion, whereas none of the control animals did. These findings suggest that the use of multifunctional nanoparticles may offer a novel treatment approach for spinal cord injury, traumatic brain injury, and possibly neurodegenerative disorders.

Mechanism of proton relaxation for enzyme-manipulated, multicomponent gold-magnetic nanoparticle chains.

Longitudinal and transverse relaxation times of multicomponent nanoparticle (NP) chains are investigated for their potential use as multifunctional imaging agents in magnetic resonance imaging (MRI). Gold NPs (ca. 5 nm) are arranged linearly along double-stranded DNA, creating gold NP chains. After cutting gold NP chains with restriction enzymes (EcoRI or BamHI), multicomponent NP chains are formed through a ligation reaction with enzyme-cut, superparamagnetic NP chains. We evaluate the changes in relaxation times for different constructs of gold-iron oxide NP chains and gold-cobalt iron oxide NP chains using 300 MHz (1)H NMR. In addition, the mechanism of proton relaxation for multicomponent NP chains is examined. The results indicate that relaxation times are dependent on the one-dimensional structure and the amount of superparamagnetic NP chains present in the multicomponent constructs. Multicomponent NP chains arranged on double-stranded DNA provide a feasible method for fabrication of multifunctional imaging agents that improve relaxation times effectively for MRI applications.

Assessment of the passivation capabilities of two different covalent chemical modifications on GaP(100).

Gallium phosphide is a semiconductor material that can be used for the fabrication of optoelectronic devices. The report compares the ability of two similar organic molecules to form covalent bonds with the GaP(100) surface. Undecenoic acid (UDA) is a terminal alkene that can potentially form Ga-C bonds, and mercaptoundecanoic acid (MUA) is a thiol that can be used to generate Ga-S bonds. The chemical passivation capabilities of the functionalized surfaces exposed to different media were investigated by contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Toxicity levels, which are important for sensing applications, were evaluated by inductively coupled plasma mass spectrometry (ICP-MS) on the media in which surfaces were stored in order to identify any gallium leaching from the substrates. Both molecules formed fairly disordered monolayers demonstrated by comparable oxide thicknesses. The UDA molecules demonstrated better stability compared to MUA molecules based on contact angle measurements and tilt angle data extracted from XPS results. According to the XPS data, the UDA molecules formed a more dense adlayer compared to MUA molecules. With respect to toxicity, the UDA-functionalized GaP provided better passivation which was confirmed by less gallium leaching into water and saline solutions. Overall, the superior passivation provided by UDA demonstrates that alkene grafting has better potential for modifying GaP based devices such as implantable sensors.

Quantitative analysis of human internal limiting membrane extracted from patients with macular holes.

We report the study of the morphology, topography, and adhesion properties of internal limiting membrane (ILM) from patients with macular holes. The quantitative analysis of human ILM could provide essential information toward the improvement of existing surgical instruments for more efficient and safer surgical removal of ILM. Imaging in air revealed the presence of globular structures in most of the samples analyzed which were coupled with fibrillar structures in some of the samples. Modification of silicon nitride AFM tips with oppositely charged functional groups showed changes in adhesion force at the membrane-tip interface. Defining the surface characteristics of the human ILM is an initial step in the development of improved surgical tools that may allow nontraumatic stripping of ILM during surgery.