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.

Gold Nanocluster Containing Polymeric Microcapsules for Intracellular Ratiometric Fluorescence Biosensing.

A new approach to sensing and imaging hydrogen peroxide (H2O2) was developed using microcapsule-based dual-emission ratiometric luminescent biosensors. Bovine serum albumin-capped gold nanoclusters (BSA-AuNCs) sensitive to H2O2 were coencapsulated with insensitive FluoSpheres (FSs) within polymeric capsules fabricated via the layer-by-layer method. Under single-wavelength excitation, the microcapsule-based biosensors exhibited emission bands at ∼516 and ∼682 nm resulting from the FSs and BSA-AuNCs, respectively. The polyelectrolyte multilayers lining the microcapsules were effective in protecting BSA-AuNCs from the degradation catalyzed by proteases (chymotrypsin, trypsin, papain, and proteinase K) and subsequent luminescent quenching, overcoming a key limitation of prior BSA-AuNC-based sensing systems. The luminescent response of the sensors was also found to be independent of local changes in pH (5-9). Quenching of the AuNCs in the presence of H2O2 enabled the spectroscopic quantification and imaging of changes in H2O2 concentration from 0 to 1 mM. The microcapsule sensors were easily phagocytized by murine macrophage cells (RAW 264.7), were effective as intracellular H2O2 imaging probes, and were successfully used to detect local release of H2O2 in response to an external chemical stimulus.

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.

Fabrication of nanocapsule carriers from multilayer-coated vaterite calcium carbonate nanoparticles.

Nanosized luminescent sensors were prepared as reagents for optical sensing and imaging of oxygen using ratiometric emission properties of a two-dye system. Polymeric capsules were fabricated utilizing poly(vinylsulfonic acid) (PVSA)-stabilized vaterite CaCO3 nanoparticles (CCNPs) as sacrificial templates. The buffer and polymeric surfactant requirements of the layer-by-layer (LbL) process were evaluated toward deposition of multilayer coatings and, ultimately, formation of hollow capsules using these interesting materials. CCNPs were found to be more stable in alkaline NaHCO3 buffer after repeated cycles of washing under sonication and resuspension. An intermediate PVSA concentration was required to maximize the loading of oxygen-sensitive porphyrin and oxygen-insensitive fluorescent nanoparticles in the CCNPs while maintaining minimal nanoparticle size. The CCNPs were then coated with polyelectrolyte multilayers and subsequent removal of the CaCO3 core yielded nanocapsules containing dye and fluorescent nanoparticles. The resulting nanocapsules with encapsulated luminophores functioned effectively as oxygen sensors with a quenching response of 89.28 ± 2.59%, and O2 (S = 1/2) = 20.91 µM of dissolved oxygen.

Poly (vinylsulfonic acid) assisted synthesis of aqueous solution stable vaterite calcium carbonate nanoparticles.

Calcium carbonate nanoparticles of the vaterite polymorph were synthesized by combining CaCl2 and Na2CO3 in the presence of poly (vinylsulfonic acid) (PVSA). By studying the important experimental parameters we found that controlling PVSA concentration, reaction temperature, and order of reagent addition the particle size, monodispersity, and surface charge can be controlled. By increasing PVSA concentration or by decreasing temperature CCNPs with an average size from ≈150 to 500 nm could be produced. We believe the incorporation of PVSA into the reaction plays a dual role to (1) slow down the nucleation rate by sequestering calcium and to (2) stabilize the resulting CCNPs as the vaterite polymorph, preventing surface calcification or aggregation into microparticles. The obtained vaterite nanoparticles were found to maintain their crystal structure and surface charge after storage in aqueous buffer for at least 5 months. The aqueous stable vaterite nanoparticles could be a useful platform for the encapsulation of a large variety of biomolecules for drug delivery or as a sacrificial template toward capsule formation for biosensor applications.

Processing and characterization of stable, pH-sensitive layer-by-layer modified colloidal quantum dots.

Quantum Dots (QDs) stabilized with dihydrolipoic acid (DHLA) were used as a template for layer-by-layer (LbL) modification to study the effect on the QD optical properties. We studied several different polyelectrolytes to determine that large quantities of monodisperse DHLA-QDs could only be obtained with the weak polyelectrolyte pair of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The key to this success was the development of a two-step method to split the LbL process into adsorption and centrifugation phases, which require different pH solutions for optimum success. Solution pH is highlighted as an important factor to achieve sufficient QD surface coverage and QD recovery during wash cycles. We optimized the process to scale up synthesis by introducing a solvent precipitation step before ultracentrifugation that, when coupled with the correct pH conditions, results in a mean QD recovery of 86-90% after three wash cycles. We found that adsorption of PAH had a negligible effect on the quantum yield and lifetime but an additional layer of PAA resulted in a substantial decrease in both quantum yield and lifetime that could not be recovered by the addition of more layers. The PAH coating provides a protective coating that extends DHLA-QDs stability, prevents photo-oxidation mediated aggregation, alleviates concerns over batch variability, and results in pH-dependent emission.