Hydrogel Microdomain Encapsulation of Stable Functionalized Silver Nanoparticles for SERS pH and Urea Sensing.

Conceptual and commercial examples of implantable sensors have been limited to a relatively small number of target analytes, with a strong focus on glucose monitoring. Recently, surface-enhanced Raman spectroscopy (SERS) pH sensors were demonstrated to track acid-producing enzymatic reactions targeting specific analytes. We show here that SERS pH tracking in the basic regime is also possible, and can be used to monitor urea concentration. To accomplish this, we developed a hydrogel consisting of polyelectrolyte multilayer microcapsules containing a SERS-sensitive pH reporter (4-mercapopyridine capped silver nanoparticles modified with bovine serum albumin). This pH sensing material exhibited a sensitive Raman scattering response to a wide range of pH from 6.5-9.7. By incorporating urease into the hydrogel matrix, the new sensor was capable of distinguishing urea concentrations of 0, 0.1, 1, and 10 mM. We also found that bovine serum albumin (BSA) prevented severe aggregation of the nanoparticle-based pH sensor, which improved sensing range and sensitivity. Furthermore, BSA safeguarded the pH sensor during the encapsulation procedure. Together, the combination of materials represents a novel approach to enabling optical sensing of reactions that generate pH changes in the basic range.

Multidomain-Based Responsive Materials with Dual-Mode Optical Readouts.

Responsive materials designed to generate signals for both surface-enhanced Raman spectroscopy (SERS) and phosphorescence lifetime-"dual-mode"-measurements are described. To demonstrate this concept, we incorporated pH-sensitive and oxygen-sensitive microdomains into a single hydrogel that could be interrogated via SERS and phosphorescence lifetime, respectively. Microdomains consisted two populations of discrete microcapsules containing either (1) gold nanoparticles capped with pH-sensitive Raman molecules or (2) oxygen-sensitive benzoporphyrin phosphors. While the microdomain-embedded hydrogels presented an expected background luminescence, the pH-sensitive SERS signal was distinguishable for all tested conditions. Response characteristics of the dual sensor showed no significant difference when compared to standalone single-mode pH and oxygen sensors. In addition, the feasibility of redundant multimode sensing was proven by observing the reaction produced by glucose oxidase chemically cross-linked within the corresponding alginate matrix. Each optical mode showed a signal change proportional to glucose concentration with an opposite signal directionality. These results support the promise of micro-/nanocomposite materials to improve measurement accuracy using intrinsic multimode responses and built-in redundancy, concepts that have broad appeal in the chemical sensing and biosensing fields.

Composite Hydrogels Containing Bioactive Microreactors for Optical Enzymatic Lactate Sensing.

Continuously monitoring specific biomarkers offer a promising method to interrogate disease status and progression. In this work we have demonstrated a composite hydrogel-based sensing platform that may be used for optical detection of lactate. The sensor design consists of microsized enzymatic sensors that are embedded in an outer hydrogel matrix. In these engineered microdomains, encapsulated lactate oxidase serves as the bioactive component, phosphorescent metalloporphyrin acts as the optical transducer, and polyelectrolyte multilayers coated on the enzymatic microsensors control the permeation of lactate into the microsensors. The response of the composite hydrogel-based lactate sensors was characterized by subjecting the sensors to lactate concentration challenges at low physiological oxygen levels. The analytical range and the mean sensitivity were determined to be 9.2 ± 0.83 mg/dL and 11 ± 0.90% dL mg-1, respectively. Repeated cyclic exposure to high levels of lactate revealed that these sensors were extremely stable, with no significant loss in sensor response after 20 cycles. These preliminary results support the premise that these composite hydrogels are capable of continuous lactate tracking and have the potential for use as fully implantable optical lactate sensors.

Layer-by-layer modification of high surface curvature nanoparticles with weak polyelectrolytes using a multiphase solvent precipitation process.

The layer-by-layer modification of ≈5 nm mercaptocarboxylic acid stabilized gold nanoparticles was studied in an effort to illustrate effective means to overcome practical issues in handling and performing surface modification of such extremely small materials. To accomplish this, each layer deposition cycle was separated into a multi-step process wherein solution pH was controlled in two distinct phases of polyelectrolyte adsorption and centrifugation. Additionally, a solvent precipitation step was introduced to make processing more amenable by concentrating the sample and exchanging solution pH before ultracentrifugation. The pH-dependent assembly on gold nanoparticles was assessed after each layer deposition cycle by monitoring the plasmon peak absorbance location, surface charge, and the percentage of nanoparticles recovered. The selection of solution pH during the adsorption phase was found to be a critical parameter to enhance particle recovery and maximize surface charge when coating with weak polyelectrolytes. One bilayer was deposited with a high yield and the modified particles exhibited enhanced colloidal stability across a broad pH range and increased ionic strength. These findings support the adoption of this multi-step processing approach as an effective and generalizable approach to improve stability of high surface curvature particles.

Identification of multiple dityrosine bonds in materials composed of the Drosophila protein Ultrabithorax.

The recombinant protein Ultrabithorax (Ubx), a Drosophila melanogaster Hox transcription factor, self-assembles into biocompatible materials in vitro that are remarkably extensible and strong. Here, we demonstrate that the strength of Ubx materials is due to intermolecular dityrosine bonds. Ubx materials auto-fluoresce blue, a characteristic of dityrosine, and bind dityrosine-specific antibodies. Monitoring the fluorescence of reduced Ubx fibers upon oxygen exposure reveals biphasic bond formation kinetics. Two dityrosine bonds in Ubx were identified by site-directed mutagenesis followed by measurements of fiber fluorescent intensity. One bond is located between the N-terminus and the homeodomain (Y4/Y296 or Y12/Y293), and another bond is formed by Y167 and Y240. Fiber fluorescence closely correlates with fiber strength, demonstrating that these bonds are intermolecular. To our knowledge, this is the first identification of specific residues that participate in dityrosine bonds in protein-based materials. The percentage of Ubx molecules harboring both bonds can be decreased or increased by mutagenesis, providing an additional mechanism to control the mechanical properties of Ubx materials. Duplication of tyrosine-containing motifs in Ubx increases dityrosine content in Ubx fibers, suggesting these motifs could be inserted in other self-assembling proteins to strengthen the corresponding materials.

Extraction of quantitative parameters for describing the microstructure of solid oxide fuel cells.

Digital quantification of a two-dimensional structure was applied to a GDC(Gd2O3-doped CeO2)/LSM(La0.85Sr0.15MnO3) composite cathode employed for solid oxide fuel cells. With the aid of high-resolution imaging capability based on secondary and backscattered electron images, two-dimensional electron micrographs were converted to digital binary files using an image processing tool combined with the line intercept method. Statistical analysis combined with a metallurgical tool was employed to determine microstructural factors, i.e., volume fraction, size distribution, and interconnectivity. The current work reports the quantification of the two-dimensional structural images of GDC/LSM composites applicable to solid oxide fuel cells, with the aim of obtaining the volume fraction, size distribution, and interconnectivity as functions of composite composition. The volume fractions of the solid constituent phases exhibit compositional dependence in cathodes; however, LSM interconnectivity increases gradually as a function of LSM composition, whereas that of GDC decreases significantly at 50 wt% LSM.

Metal-induced crystallization of amorphous Si thin films assisted by atomic layer deposition of nickel oxide layers.

Atomic layer deposition (ALD) of nickel oxide was applied to the nickel-induced crystallization of amorphous Si thin films. The nickel-induced crystallization was monitored as a function of annealing temperature and time using Raman spectroscopy. Since Raman spectroscopy allows for the numerical quantification of structural components, the incubation time and the crystallization rates were estimated as functions of the annealing temperature. The spatial locations of a nickel-based species, probably NiSi2, were investigated using X-ray photoelectron spectrometry. The formed NiSi2 seeds appeared to accelerate the crystallization kinetics in amorphous Si thin films deposited onto glass substrates. The ramifications of the atomic layer deposition are discussed with regard to large-panel displays, with special emphasis on the sophisticated control of the catalytic elements, especially nickel.