Ultrasensitive nonenzymatic electrochemical glucose sensor based on gold nanoparticles and molecularly imprinted polymers.

A non-enzymatic electrochemical glucose sensor with high sensitivity and selectivity was developed using gold nanoparticles-decorated molecularly imprinted polymers (AuNP-MIPs). The AuNP-MIPs were synthesized on a gold surface by multistep amperometry using the optimized conditions and in-house synthesized gold nanoparticles in the presence of glucose as the template. The AuNP-MIPs were investigated by employing atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical techniques to confirm successful fabrication of the sensor. The electrochemical measurements for glucose binding on the AuNP-MIP sensor revealed a high affinity toward glucose with a dissociation constant (Kd) of 3 × 10-8 M whereas the MIPs without AuNPs could not detect even the highest concentration of the investigation range (1.25 nM-2.56 µM). The comparative rebinding studies with AuNP-MIP and non-imprinted polymer (AuNP-NIP) exhibited an excellent selectivity toward glucose. The specificity of AuNP-MIP sensor was further investigated by studying with interfering compounds (sucrose, dopamine, starch, and bovine serum albumin), resulting in negligible cross-reactivity except for sucrose. The behavior of imprinted polymers in fluid solvents was also investigated by employing the AFM for the first time. The sensor could detect glucose in human serum with a detection limit of 1.25 nM and preserved its stability up to around 95% during a storage time of 40 days. Hence, such a sensor demonstrates a promising future for the detection of clinically relevant small molecules with its facile, cheap, and highly sensitive nature.

Micropatterning of Au NPs on PEG Hydrogels Using Different Silanes To Control Cell Adhesion on the Nanocomposites.

Amino-silanization of silica-based substrates has proven to be effective in guiding the immobilization of citrate-stabilized Au NPs in a good, homogeneous fashion. This accomplishment has formed the basis of fabricating micropatterns of Au NPs on such substrates by patterning of oxidized silicon wafers with (3-aminopropyl)trimethoxysilane (amino-silane) using the microcontact printing (µCP) process. This micropattern of amino-silane is used to specifically adsorb Au NPs. To avoid unspecific adsorption to the nonsilanized areas on the silicon wafers, the nonstamped areas were backfilled with self-assembled monolayers of organosilanes, for example, with methyl- or perfluoro-end-groups. Finally, after having fabricated a micropattern of Au NPs on silicon wafers, the Au NP patterns were transferred onto poly(ethylene glycol) hydrogels by our newly developed procedures, and on these nanocomposite materials, controlled cell adhesion has been achieved. Furthermore, these materials are great candidates for plasmon-based biosensor applications and also for various medical applications, such as for drug delivery systems or photothermal therapies.

Micro-Patterning of PEG-Based Hydrogels With Gold Nanoparticles Using a Reactive Micro-Contact-Printing Approach.

In this work a novel, relatively simple, and fast method for patterning of gold nanoparticles (Au NPs) on poly(ethylene glycol) (PEG)-based hydrogels is presented. In the hereby exploited reactive micro-contact printing (reactive-µ-CP) process, the surface of a micro-relief patterned PDMS-stamp is first functionalized with an amino-silane self-assembled monolayer (SAM), which is then inked with Au NPs. The stamp is subsequently brought into conformal contact with thiol-functionalized PEG-based hydrogel films. Due to the strong gold-thiol interactions the Au NPs are adequately and easily transferred onto the surfaces of these soft, multifunctional PEG hydrogels. In this way, defined µ-patterns of Au NPs on PEG hydrogels are achieved. These Au NPs patterns allow specific biomolecular interactions on PEG surfaces, and cell adhesion has been studied. Cells were found to effectively adhere only on Au NPs micro-patterns and to avoid the anti-adhesive PEG background. Besides the cell adhesion studies, these Au NPs µ-patterns can be potentially applied as biosensors in plasmon-based spectroscopic devices or in medicine, e.g., for drug delivery systems or photothermal therapies.

Influence of Network Structure on the Crystallization Behavior in Chemically Crosslinked Hydrogels.

The network structure of hydrogels is a vital factor to determine their physical properties. Two network structures within hydrogels based on eight-arm star-shaped poly(ethylene glycol)(8PEG) have been obtained; the distinction between the two depends on the way in which the macromonomers were crosslinked: either by (i) commonly-used photo-initiated chain-growth polymerization (8PEG⁻UV), or (ii) Michael addition step-growth polymerization (8PEG⁻NH3). The crystallization of hydrogels is facilitated by a solvent drying process to obtain a thin hydrogel film. Polarized optical microscopy (POM) results reveal that, while in the 8PEG⁻UV hydrogels only nano-scaled crystallites are apparent, the 8PEG⁻NH3 hydrogels exhibit an assembly of giant crystalline domains with spherulite sizes ranging from 100 to 400 µm. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses further confirm these results. A model has been proposed to elucidate the correlations between the polymer network structures and the crystallization behavior of PEG-based hydrogels.

Surface Patterning of Gold Nanoparticles on PEG-Based Hydrogels to Control Cell Adhesion.

We report on a versatile and easy approach to micro-pattern gold nanoparticles (Au NPs) on 8-arm poly(ethylene glycol)-vinyl sulfone thiol (8PEG-VS-SH) hydrogels, and the application of these patterned Au NPs stripes in controlling cell adhesion. Firstly, the Au NPs were patterned on silicon wafers, and then they were transferred onto reactive, multifunctional 8PEG-VS-SH hydrogels. The patterned, micrometer-sized Au NPs stripes with variable spacings ranging from 20 µm to 50 µm were created by our recently developed micro-contact deprinting method. For this micro-contact deprinting approach, four different PEG-based stamp materials have been tested and it was found that the triblock copolymer PEG-PPG-PEG-(3BC) stamp established the best transfer efficiency and has been used in the ongoing work. After the successful creation of micro-patterns of Au NPs stripes on silicon, the patterns can be transferred conveniently and accurately to 8PEG-VS-SH hydrogel films. Subsequently these Au NPs patterns on 8PEG-VS-SH hydrogels have been investigated in cell culture with murine fibroblasts (L-929). The cells have been observed to adhere to and spread on those nano-patterned micro-lines in a remarkably selective and ordered manner.

Novel Wet Micro-Contact Deprinting Method for Patterning Gold Nanoparticles on PEG-Hydrogels and Thereby Controlling Cell Adhesion.

In the present work we introduce a novel method to create linear and rectangular micro-patterns of gold nanoparticles (Au NPs) on poly(ethylene glycol) (PEG) hydrogels. The strategy consists of removing Au NPs from defined regions of the silicon wafer by virtue of the swelling effect of the hydrogel. Using this method, which we denote as "Wet Micro-Contact Deprinting", well-defined micro-patterns of Au NPs on silicon can be created. This resulting pattern is then transferred from the hard substrate to the soft surface of PEG-hydrogels. These unique micro- and nano-patterned hydrogels were cultured with mouse fibroblasts L929 cells. The cells selectively adhered on the Au NPs coated area and avoided the pure PEG material. These patterned, nanocomposite biointerfaces are not only useful for biological and biomedical applications, such as tissue engineering and diagnostics, but also, for biosensor applications taking advantage of surface plasmon resonance (SPR) or surface enhanced Raman scattering (SERS) effects, due to the optical properties of the Au NPs.

Nano-Contact Transfer with Gold Nanoparticles on PEG Hydrogels and Using Wrinkled PDMS-Stamps.

In the present work, a soft lithographic process is used to create nanometer-sized line patterns of gold nanoparticles (Au NPs) on PEG-based hydrogels. Hereby nanometer-sized wrinkles on polydimethylsiloxane (PDMS) are first fabricated, then functionalized with amino-silane and subsequently coated with Au NPs. The Au NPs are electrostatically bound to the surface of the wrinkled PDMS. In the next step, these relatively loosely bound Au NPs are transferred to PEG based hydrogels by simple contacting, which we denote "nano-contact transfer". Nano-patterned Au NPs lines on PEG hydrogels are thus achieved, which are of interesting potential in nano-photonics, biosensor applications (using SERS) and to control nanoscopic cell adhesion events.