Sondre Volden

Glucose sensors based on a responsive gel incorporated as a Fabry-Perot cavity on a fiber-optic readout platform

An optical sensor for detection of glucose is implemented by incorporating a carbohydrate sensitive hydrogel as a Fabry-Perot cavity at the end of optical fiber for high sensitivity readout of the gel length. The glucose sensing functionality was achieved by incorporating boronic acid moieties into an acrylamide-based hydrogel. The interaction between glucose and boronic acid changes the driving forces for gel swelling thus inducing a glucose sensitive hydrogel swelling. The effects on the carbohydrate swelling response, with respect to sensitivity and selectivity, by incorporation of a cationic monomer, dimethyl-aminopropyl acrylamide, into the boronic acid functionalized responsive gels were determined. The linear gel swelling response in aqueous solutions at aqueous 2.5mM carbohydrates were determined to -1760nm/mM for glucose whereas mannose, sucrose, fructose and galactose displayed a response of about 10% of the glucose response for the hydrogels containing 10mol% dimethylaminopropyl acrylamide. This gel composition with 10mol% dimethylaminopropyl acrylamide is the most promising for detection of glucose at physiological pH and ionic strength. A mechanism where carbohydrate specific stabilisation of the boronic acid group and possible carbohydrate mediated additional crosslinking of the elastically active polymer chains is suggested.

Temperature-dependent optical properties of gold nanoparticles coated with a charged diblock copolymer and an uncharged triblock copolymer.

We demonstrate that the optical properties of gold nanoparticles can be used to detect and follow stimuli-induced changes in adsorbed macromolecules. Specifically, we investigate thermal response of anionic diblock and uncharged triblock copolymers based on poly(N-isopropylacrylamide) (PNIPAAM) blocks adsorbed onto gold nanoparticles and planar gold surfaces in a temperature range between 25 and 60 degrees C. By employing a palette of analytical probes, including UV-visible spectroscopy, dynamic light scattering, fluorescence, and quartz crystal microbalance with dissipation monitoring, we establish that while the anionic copolymer forms monolayers at both low and high temperature, the neutral copolymer adsorbs as a monolayer at low temperatures and forms multilayers above the cloud point (T(C)). Raising the temperature above T(C) severely affects the optical properties of the gold particle/polymer composites, expelling associated water and altering the immediate surroundings of the gold nanoparticles. This effect, stronger for the uncharged polymer, is related to the amount of polymer adsorbed on the surface, where a denser shell influences the surface plasmon band to a greater degree. This is corroborated with light scattering experiments, which reveal that flocculation of the neutral polymer-coated particles occurs at high temperatures. The flocculation behavior of the neutral copolymer on planar gold surfaces results in multilayer formation. The observed effects are discussed within the framework of the Mie-Drude theory.

Same system-different results: the importance of protein-introduction protocols in Langmuir-monolayer studies of lipid-protein interactions.

For studies of protein-lipid interactions, thin films at the air-water surface are often employed as model systems for cell membranes. A convenient manner in which to study these interactions is the Langmuir technique, which allows for formation of monolayer phospholipid films together with a choice of where and how to introduce proteins, according to the desired response variable. Here, a distinction has been made between different interaction protocols and it is also commented upon to what extent introduction of protein to a solution prior to spreading of a lipid film affects the results. This paper describes commonly used methods when working with Langmuir monolayers as membrane mimics and compares the results of four different experimental protocols: formation of a lipid film on top of a protein-containing subphase, injection of protein under an existing, semicompressed phospholipid film (surface pressure 5 mN/m), and deposition of a protein solution on top of a lipid film contained at either surface pressure 0 mN/m or at surface pressure 5 mN/m. Results obtained from Langmuir isotherms and Brewster angle microscope clearly differentiate between these methods and give insight into under which conditions and at which interfaces the protein interactions are predominant (protein-air or protein-lipid).

Adsorption of cellulose derivatives on flat gold surfaces and on spherical gold particles

The adsorption of hydroxyethylcellulose (HEC), ethyl(hydroxyethyl)cellulose (EHEC), and their hydrophobically modified counterparts HM-HEC and HM-EHEC has been studied on planar gold and citrate-covered gold surfaces by means of quartz crystal microbalance with dissipation monitoring (QCM-D), and on citrate-covered gold particles with the aid of dynamic light scattering (DLS). The QCM-D results indicate that larger amounts of polymer are adsorbed from aqueous solutions of HM-HEC and HM-EHEC on both substrates than from solutions of their unmodified analogues. The adsorption affinity for all the polymers, except EHEC, is higher on the citrate-covered surfaces than on the bare gold substrate. This indicates that more adsorption sites are activated in the presence of the citrate layer. The experimental adsorption data for all the polymers can be described fairly well by the Langmuir adsorption isotherm. However, at very low polymer concentrations significant deviations from the model are observed. The value of the hydrodynamic thickness of the adsorbed polymer layer (delta h), determined from DLS, rises with increasing polymer concentration for all the cellulose derivatives; a Langmuir type of isotherm can be used to roughly describe the adsorption behavior. Because of good solvent conditions for HEC the chains extend far out in the bulk at higher concentrations and the value of delta h is much higher than that of HM-HEC. The adsorption of EHEC and HM-EHEC onto gold particles discloses that the values of delta h are considerably higher for the hydrophobically modified cellulose derivative, and this finding is compatible with the trend in layer thickness estimated from the QCM-D measurements.

Tunable photophysical properties, conformation and function of nanosized protein–gold constructs

Protein-stabilized gold nanoconstructs are widely studied due to their potential applications in biosensing, drug and gene delivery, and bioimaging. While a number of studies have focused on the novel properties of such materials emanating from the gold, there has been little focus on how the protein shell is affected by nanocluster formation with respect to conformation, stability and function. Herein, we show the synthesis of protein-stabilized gold nanoconstructs varying in size from small clusters (~8 Au atoms) dispersed within proteins to nanoparticles stabilized by multiple proteins by varying the concentration of gold precursor and reducing agent. Proteins used were bovine serum albumin (BSA), bovine α-lactalbumin (BLA) and lysozyme (LYZ). Photophysical properties of the gold nanostructures were monitored using UV-vis and fluorescence measurements, revealing that the gold constructs can be tuned from luminescent clusters to nanoparticles displaying localized surface plasmon resonance (LSPR). Conformational changes of the protein following conjugation to gold nanostructures were studied using steady-state and time-resolved Trp fluorescence measurements and circular dichroism. The degree of conformational perturbation varied greatly between the proteins used, with BLA being the most tunable in terms of gradual unfolding, whereas the conformational stability of LYZ was very sensitive to the reducing agent used. To assess the impact of the gold nanostructures as well as the reducing agent on protein function, the LYZ–gold nanoconstructs were subjected to an activity test by degradation of Micrococcus lysodeikticus cell walls, revealing that the activity of the LYZ–Au constructs was retained and tunable, albeit at attenuated levels.

Emergent membrane-affecting properties of BSA–gold nanoparticle constructs

By adsorbing bovine serum albumin (BSA) on gold nanoparticles (Aunps) with diameters 30 nm and 80 nm, different degrees of protein unfolding were obtained. Adsorption and adlayer conformation were characterized by UV-vis spectroscopy, ζ-potential measurements, steady-state and time-resolved fluorescence. The unfolding was also studied using 1-anilino-8-naphthalene sulfonate (ANS) as an extrinsic probe, showing that BSA unfolds more on 80 nm Aunp than on 30 nm Aunp. Langmuir monolayer studies using two distinct methods of introducing the BSA and BSA-Aunp constructs accompanied with Brewster Angle Microscopy (BAM) and Digital Video Microscope (DVM) imaging demonstrated that BSA-Aunp constructs induce film miscibility with L-α-phosphatidylethanolamine not seen for BSA or Aunp alone. The changes induced by partial unfolding clearly give better film-penetration ability, as well as disruption of liquid crystalline domains in the film, thereby inducing film miscibility. Gold or protein only does not possess the nanoscale film-affecting properties of the protein-gold constructs, and as such the surface-active and miscibility-affecting characteristics of the BSA-Aunp represent emergent qualities.

Use of cellulose derivatives on gold surfaces for reduced nonspecific adsorption of immunoglobulin G

This study addresses the design of protein-repellent gold surfaces using hydroxyethyl- and ethyl(hydroxyethyl) cellulose (HEC and EHEC) and hydrophobically modified analogues of these polymers (HM-HEC and HM-EHEC). Adsorption behavior of the protein immunoglobulin G (IgG) onto pure gold and gold surfaces coated with cellulose polymers was investigated and described by quartz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and contact angle measurements (CAM). Surfaces coated with the hydrophobically modified cellulose derivatives were found to significantly outperform a reference poly(ethylene glycol) (PEG) coating, which in turn prevented 90% of non-specific protein adsorption as compared to adsorption onto pure gold. HEC and EHEC prevented around 30% and 60% of the IgG adsorption observed on pure gold, while HM-HEC and HM-EHEC were both found to completely hinder biofouling when deposited on the gold substrates. Adsorption behavior of IgG has been discussed in terms of polymer surface coverage and roughness of the applied surfaces, together with hydrophobic interactions between protein and gold, and also polymer-protein interactions.

Interactions between bovine serum albumin and Langmuir films composed of charged and uncharged poly(N-isopropylacrylamide) block copolymers

The thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM) and NIPAAM block copolymer derivatives are attractive for drug delivery applications as they contract reversibly at lower critical solution temperatures (LCST) close to physiological conditions. In order to investigate biomaterial-protein compatibility, we have studied the interaction between PNIPAAM copolymer films spread at the air-water surface and bovine serum albumin (BSA) injected below the precompressed polymer films, using the Langmuir technique coupled with Brewster angle microscopy (BAM). A PNIPAAM homopolymer was applied together with a number of PNIPAAM-based di- and triblock copolymers, to assess effects of e.g., charge and hydrophobicity on protein-polymer interactions. The nature and strength of protein-polymer interaction was found to be tunable, ranging from complex formation (PNIPAAM homopolymer) to mixed monolayers and electrostatic cross-linking, according to the nature of the co-monomer. Results show that intercalation versus adsorption can be controlled through polymer composition.

Dendrimers and Hyperbranched Polyesters as Structure‐Directing Agents in the Formation of Nanoporous Silica

Anionic dendritic macromolecules (hyperbranched polyesters and polyamidoamine (PAMAM) dendrimers) and amine‐functionalized PAMAM dendrimers have been used as structure‐directing agents in the synthesis of nanoporous silica structures. When utilizing carboxylates as structure‐directing agents the incorporation of the negatively charged species into a SiO2 framework was achieved via a sol‐gel synthesis route using a quaternized aminosilane as co‐structure‐directing agent to yield silica materials of various morphologies and high surface areas. Amine dendrimers as porogens gave materials with high surface areas, revealing increasing pore sizes corresponding to the increasing size of the porogen. The resulting materials have been characterized by X‐ray diffraction (XRD), N2 adsorption/desorption (BET), scanning electron microscopy (SEM), infrared spectroscopy (IR), and thermogravimetric analysis (TGA).

Interactions of α-Lactalbumin and Cytochrome c with Langmuir Monolayers of Glycerophospholipids

Interaction of cytochrome c (Cyt c), α-lactalbumin III (α-La III) with Langmuir monolayers of pure and mixed glycerophospholipids was investigated using surface pressure-area (Π-A) isotherms. The general trend was that maximum interaction between protein and phospholipid is observed for mixed (1:1 molar ratio) phospholipid monolayers. Interaction between the protein and the charged phospholipid films was found to be independent of global protein charge. Our data indicate that the proteins interact with the phospholipid films by inserting themselves into the monolayer rather than anchoring to the phospholipid head groups.