Synthesis and electrografting of dendron anchored OEGylated surfaces and their protein adsorption resistance.

In this study, a series of electrochemically active oligo(ethylene glycol) (OEG) linear-dendrons have been synthesized and grafted onto electrode surfaces by cyclic voltammetry (CV) to improve protein resistance. Dendronized molecules with peripheral carbazole functionality and branching architecture enabled tethering of the poly(ethylene glycol) (PEG) or OEG group with a predictable number of electrochemical reactive groups affecting OEG distribution and orientation. It is possible that ample spacing between the OEG chains affects the intrinsic hydration of these layers and thus surface protein resistance. The films were characterized by CV, surface plasmon resonance (SPR), static contact angle measurements, and atomic force microscopy (AFM). This approach should enable improved nonbiofouling properties on biorelevant electrode surfaces (metal or metal oxides) by potentiostatic or potentiodynamic electrochemical methods, providing an alternative to the self-assembled monolayer (SAM) approach for anchoring PEG layers.

Morphological and chemical optimization of microcantilever surfaces for thyroid system biosensing and beyond.

The development of biosensors is vital in many areas of biotechnology and biomedical research. A prominent new class of label-free biosensors are those based on ligand-induced nanomechanical responses of microcantilevers (MCs). The interaction between biologically significant ligands with bioreceptors (e.g., antibodies or nuclear receptor proteins) immobilized on one side of the MC surface causes an apparent surface stress, resulting in static bending of the MC, which can be detected by an optical beam bending technique. The three key performance metrics of sensitivity, selectivity, and reversibility are foci of the work reported herein. The nature of the MC surface and the method by which the bioreceptor is immobilized influence these performance metrics and, hence, optimization studies involving these were conducted. In our work, the gold surface on one side of the MC is first activated via self-assembled monolayer formation with amino ethane thiol (AET) then reacted with glutaraldehyde (GA) as a crosslinker before finally functionalizing with the protein receptor. We report the effect of concentration, reaction time, and pH for these reagents on the magnitude of the nanomechanical responses using an anti-immunoglobulin G (anti-IgG) receptor: IgG ligand test system. By vapor depositing an alloy of silver and gold and then etching away the former, a nanostructured "dealloyed" MC surface is created that outperforms a smooth gold MC in terms of nanomechanical responses. Optimization of the dealloying parameters (thickness, metal ratio) is also reported herein using the aforementioned anti-IgG-IgG system. Maximum response was obtained with these conditions: 150 nm dealloyed surface, 1 mM aqueous solution of AET-incubation time 1h, 1% GA solution in 10mM pH 8 phosphate buffered saline (PBS)-incubation time 3h, and 0.5 mg mL(-1) of receptor protein solution in 10mM pH 7 PBS-incubation time 1h. Additionally, surprising results are reported when Protein A is immobilized first to properly orient the bioreceptor IgG molecules. We also report the application of optimum and non-optimum conditions to detect thyroid disrupting chemicals (TDCs) using MCs functionalized with the transport protein thyroxine-binding globulin. Selectivity patterns are reported for several TDCs and sensitive detection of thyroxin at sub-nM levels is demonstrated.

Development of a nanomechanical biosensor for analysis of endocrine disrupting chemicals.

A nanomechanical transducer is developed to detect and screen endocrine disrupting chemicals (EDCs) combining fluidic sample injection and delivery with bioreceptor protein functionalized microcantilevers (MCs). The adverse affects of EDCs on the endocrine system of humans, livestock, and wildlife provides strong motivation for advances in analytical detection and monitoring techniques. The combination of protein receptors, which include estrogen receptor alpha (ER-alpha) and estrogen receptor beta (ER-beta), as well as monoclonal antibodies (Ab), with MC systems employing modified nanostructured surfaces provides for excellent nanomechanical response sensitivity and the inherent selectivity of biospecific receptor-EDC interactions. The observed ranking of binding interaction of the tested EDCs with ER-beta is diethylstilbestrol (DES) > 17-beta-estradiol > 17-alpha-estradiol > 2-OH-estrone > bisphenol A > p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) with measurements exhibiting intra-day RSDs of about 3%. A comparison of responses of three EDCs, which include 17-beta-estradiol, 17-alpha-estradiol, and 2-OH-estrone, with ER-beta and ER-alpha illustrates which estrogen receptor subtype provides the greatest sensitivity. Antibodies specific to a particular EDC can also be used for analyte specific screening. Calibration plots for a MC functionalized with anti-17-beta-estradiol Ab show responses in the range of 1 x 10(-11) through 1 x 10(-7) M for 17-beta-estradiol with a linear portion extending over two orders of magnitude in concentration.

Independent component analysis of nanomechanical responses of cantilever arrays.

The ability to detect and identify chemical and biological elements in air or liquid environments is of far reaching importance. Performing this task using technology that minimally impacts the perceived environment is the ultimate goal. The development of functionalized cantilever arrays with nanomechanical sensing is an important step towards this goal. This report couples the feature extraction abilities of independent component analysis (ICA) and the classification techniques of neural networks to analyze the signals produced by microcantilever-array-based nanomechanical sensors. The unique capabilities of this analysis unleash the potential of this sensing technology to accurately identify chemical mixtures and concentrations. Furthermore, it is demonstrated that the knowledge of how the sensor array reacts to individual analytes in isolation is sufficient information to decode mixtures of analytes--a substantial benefit, significantly increasing the analytical utility of these sensing devices.

Facile hyphenation of gas chromatography and a microcantilever array sensor for enhanced selectivity.

The very simple coupling of a standard, packed-column gas chromatograph with a microcantilever array (MCA) is demonstrated for enhanced selectivity and potential analyte identification in the analysis of volatile organic compounds (VOCs). The cantilevers in MCAs are differentially coated on one side with responsive phases (RPs) and produce bending responses of the cantilevers due to analyte-induced surface stresses. Generally, individual components are difficult to elucidate when introduced to MCA systems as mixtures, although pattern recognition techniques are helpful in identifying single components, binary mixtures, or composite responses of distinct mixtures (e.g., fragrances). In the present work, simple test VOC mixtures composed of acetone, ethanol, and trichloroethylene (TCE) in pentane and methanol and acetonitrile in pentane are first separated using a standard gas chromatograph and then introduced into a MCA flow cell. Significant amounts of response diversity to the analytes in the mixtures are demonstrated across the RP-coated cantilevers of the array. Principal component analysis is used to demonstrate that only three components of a four-component VOC mixture could be identified without mixture separation. Calibration studies are performed, demonstrating a good linear response over 2 orders of magnitude for each component in the primary study mixture. Studies of operational parameters including column temperature, column flow rate, and array cell temperature are conducted. Reproducibility studies of VOC peak areas and peak heights are also carried out showing RSDs of less than 4 and 3%, respectively, for intra-assay studies. Of practical significance is the facile manner by which the hyphenation of a mature separation technique and the burgeoning sensing approach is accomplished, and the potential to use pattern recognition techniques with MCAs as a new type of detector for chromatography with analyte-identifying capabilities.

Characterization of ligand-functionalized microcantilevers for metal ion sensing.

A sensor for metal cations is demonstrated using single and binary mixtures of different thiolated ligands as self-assembled monolayers (SAMs) functionalized on silicon microcantilevers (MCs) with gold nanostructured surfaces. Binding of charged metal ions to the active surface of a cantilever induces an apparent surface stress, thereby causing static bending of the MC that is detected in this work by a beam-bending technique. A MC response mechanism based on changes in surface charge is discussed. The monodentated ligands arranged as SAMs on the MC surface are not expected to fully satisfy the coordination sphere of the detected metals. This leads to lower binding constants than would be expected for chelating ligands, but reversible responses. The modest binding constants are compensated in terms of the magnitudes of responses by the inherent higher sensitivity of the nanostructured approach as opposed to more traditional smooth surface MCs. Response characteristics are optimized in terms of SAM formation time, concentration of ligand solution, and pH of working buffer solution. Limits of detection for the tested mono-, di-, and trivalent metal ions are in low to submicromolar range. The results indicated that shapes and magnitudes of response profiles are characteristics of the metal ions and type of SAM. The response factors for a given SAM with the tested metal ions, or for a given metal with the tested SAMs, varied by roughly 1 order of magnitude. While the observed selectivity is not large, it is anticipated that sufficient ionic recognition contrast is available for selective metal ion identification when differentially functionalized arrays of MCs (different ligands on different cantilevers in the array) are used in conjunction with pattern recognition techniques.

Studies of the optical properties of metal-pliable polymer composite materials.

Polymer-nano-metallic-particle composites have demonstrated technological potential due to their unique optical and electrical properties. Herein, we report on composites prepared via physical vapor deposition of silver metal onto pliable poly(dimethylsiloxane) (PDMS) polymer. Rapid Ag diffusion and nano-metallic-particle formation in a phase-separated surface layer of the PDMS creates unique sub-surface-based composites whose properties vary based on rate of deposition and average Ag thickness. Additionally, nanometallic-particle spacing can be altered with fair reproducibility and reversibility by physically manipulating the Ag-PDMS composite. The optical properties of the materials are studied by visible wavelength optical extinction spectrometry and surface-enhanced Raman scattering (SERS), including studies performed during physical manipulation. Direct current (DC) conductivity measurements were made during Ag deposition to study percolation conditions for the materials. Depth-profiling was performed by X-ray photoelectron spectrometry. Sample Raman spectral data collected with the composite as a SERS substrate are included. A practical technological characteristic of these composite materials arises from their potential to be molded into functional devices.

Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles.

Octyl octanoate (O-OL) underwent hydrolysis in sodium octanoate (NaOA) reversed micelles in 85:15 = isooctane:octanol (OL) (v/v), containing w = [H2O]/[NaOA] = 40. The products of the hydrolysis, octanoic acid (OA) and octanol (OL), lead to the formation of additional (albeit smaller) reversed micelles; hence the process is considered to be self-reproducing. Self-reproduction was found to be catalyzed by lithium hydroxide, solubilized in the water pools, as well as by hydrogen sulfide, added to the solution of the reversed micelles. Addition of hydrogen sulfide to cadmium perchlorate containing self-reproducing reversed micelles resulted in the formation of cadmium sulfide (CdS) nanoparticles. Diameters of the CdS containing nanoparticles could be altered from 5.4 to 1.8 nm by changing the [Cd2+]/[H2S] ratios from 0.25 to 10. The CdS nanoparticles formed were capped by mercaptopropionic acid, isolated as solids, and could be repeatedly redispersed in water without changing their sizes. Additional CdS nanoparticles were generated in the supernatants removed from the precipitated capped CdS nanoparticles.