Modified Gold Surface Research Articles

One-step formation of bifunctionnal aryl/alkyl grafted films on conducting surfaces by the reduction of diazonium salts in the presence of alkyl iodides.

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction with surfaces takes place at low driving force (∼-0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN2(+)), at a potential where the latter is reduced. By comparison to the direct grafting of ICH2CH2C6F13, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl-alkyl groups (Ar = 3-CH3-C6H4, 4-NO2-C6H4, and 4-Br-C6H4, R = C6H13 or (CH2)2-C6F13). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.

A sensitive molecularly imprinted polymer based quartz crystal microbalance nanosensor for selective determination of lovastatin in red yeast rice

Lovastatin (LOV) is a statin, used to lower cholesterol which has been found as a hypolipidemic agent in commercial red yeast rice. In present study, a sensitive molecular imprinted quartz crystal microbalance (QCM) sensor was prepared by fabricating a self-assembling monolayer formation of allylmercaptane on QCM chip surface for selective determination of lovastatin (LOV) in red yeast rice. To prepare molecular imprinted quartz crystal microbalance (QCM) nanosensor, LOV imprinted poly(2-hydroxyethyl methacrylate–methacryloylamidoaspartic acid) [p(HEMA–MAAsp)] nanofilm was attached on the modified gold surface of QCM chip. The non-modified and improved surfaces were characterized by using contact angle, atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy. The imprinted QCM sensor was validated according to the ICH guideline (International Conference on Harmonisation). The linearity range was obtained as 0.10–1.25nM. The detection limit of the prepared material was calculated as 0.030nM. The developed QCM nanosensor was successfully used to examine red yeast rice. Furthermore, the stability and repeatability of the prepared QCM nanosensor were studied. The spectacular long-term stability and repeatability of the prepared LOV-imprinted QCM nanosensor make them intriguing for use in QCM sensors.

Detection of 17β-estradiol in water samples by a novel double-layer molecularly imprinted film-based biosensor

This study reports a novel double-layer molecularly imprinted film (MIF)-based biosensor for rapid, sensitive and highly selective detection of small molecule 17β-estradiol (E2) that is frequently detected in environmental water samples. In this system, the modification of gold surface of SPR chip was performed by 1-dodecanethiol. Then double-layer MIF was generated on the 1-dodecanethiol modified gold surface. The non-modified and imprinted surfaces were characterized by atomic force microscopy (AFM), scanning electron microscope (SEM) and contact angle measurements. Analysis of surface plasmon resonance (SPR) spectroscopy showed that the imprinted sensing film displayed good selectivity for E2 compared to other analog molecules and NIF. A good linear relationship was obtained between the SPR angle and E2 concentrations over a range of 2.50×10−13–2.50×10−9mol/L (R2=0.993) with the lowest measurable concentration of 2.50×10−13mol/L. The sensor can be regenerated with the mixture of acetic acid and PBS buffer (v/v=1:9) as a desorption agent over tens of times without significant deterioration of the sensor performance. Potential interference of real environmental sample matrix was assessed by spiked samples in several waste seawater effluents. This portable sensor system can be successfully applied for on-site real-time inexpensive and easy-to-use monitoring of E2 or other small molecule pollutants in environmental samples such as effluents or water bodies.

Study of sulfur adlayers on Au(1 1 1) from basic hydrolysis of piperazine bis(dithiocarbamate) sodium salt

Sulfur adlayers on Au(111) were obtained after the interaction of a gold substrate with an alkaline solution of piperazine bis(dithiocarbamate) sodium salt. Characterization of the sulfur modified gold surface was performed by means of X-Ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations. XPS signals indicated the presence of S–Au bonds, monomeric and polymeric sulfur, and absence of nitrogen and sodium. Images from STM showed the formation of quasi-rectangular octomers in coexistence with another phase. A DFT model using the arrangement of sulfur dimers on the Au(111) surface effectively reproduced the experimental STM images.

A novel double-layer molecularly imprinted polymer film based surface plasmon resonance for determination of testosterone in aqueous media

This work aimed to prepare a novel double-layer structure molecularly imprinted polymer film (MIF) on the surface plasmon resonance (SPR) sensor chips for detection of testosterone in aqueous media. The film was synthesized by in-situ UV photo polymerization. Firstly, the modification of gold surface of SPR chip was performed by 1-dodecanethiol. Then double-layer MIF was generated on the 1-dodecanethiol modified gold surface. The non-modified and imprinted surfaces were characterized by atomic force microscopy (AFM), fourier transform infrared (FTIR) spectroscopy and contact angle measurements. Analysis of SPR spectroscopy showed that the imprinted sensing film displayed good selectivity for testosterone compared to other analogues and the non-imprinted polymer film (NIF). Within the concentrations range of 1×10−12–1×10−8mol/L, the coupling angle changes of SPR were linear with the negative logarithm of testosterone concentrations (R2=0.993). Based on a signal/noise ratio of three, the detection limit was estimated to be 10−12mol/L. Finally, the developed MIF was successfully applied to the seawater detection of testosterone. The results in the experiments suggested that a combination of SPR sensing with MIF was a promising alternative method for detection of testosterone in aqueous media.

Site-specific immobilization of protein layers on gold surfaces via orthogonal sortases

We report a site-specific, sortase-mediated ligation to immobilize proteins layer-by-layer on a gold surface. Recombinant fluorescent proteins with a Sortase A recognition tag at the C-terminus were immobilized on peptide-modified gold surfaces. We used two sortases with different substrate specificities (Streptococcus pyogenes Sortase A and Staphylococcus aureus Sortase A) to immobilize layers of GFP and mCherry site-specifically on the gold surface. Surfaces were characterized using fluorescence and atomic force microscopy after immobilizing each layer of protein. Fluorescent micrographs showed that both protein immobilization on the modified gold surface and protein oligomerization are sortase-dependent. AFM images showed that either homogenous protein monolayers or layers of protein oligomers can be generated using appropriately tagged substrate proteins. Using Sortase A variants with orthogonal peptide substrate specificities, site-specific immobilization of appropriately tagged GFP onto a layer of immobilized mCherry was achieved without disruption of the underlying protein layer.

Targeted images of KB cells using folate-conjugated gold nanoparticles.

Mercaptosuccinic acid-coated gold (GM) nanoparticles were prepared and characterized by transmission electron microscopy and dynamic light scattering. Folic acid (F) was then conjugated to the GM to preferentially target oral squamous cancer (KB) cells with folate receptors expressed on their membranes and facilitate the transit of the nanoparticles across the cell membrane. Finally, a fluorescence dye (Atto) was conjugated to the nanoparticles to visualize their internalization into KB cells. After culture of the cells in a medium containing GM and folate-conjugated GM (GF), the interaction of surface-modified gold nanoparticles with KB cells was studied.

Global transcriptomic analysis of model human cell lines exposed to surface-modified gold nanoparticles: the effect of surface chemistry.

Gold nanoparticles (Au NPs) are attractive for biomedical applications not only for their remarkable physical properties, but also for the ease of which their surface chemistry can be manipulated. Many applications involve functionalization of the Au NP surface in order to improve biocompatibility, attach targeting ligands or carry drugs. However, changes in cells exposed to Au NPs of different surface chemistries have been observed, and little is known about how Au NPs and their surface coatings may impact cellular gene expression. The gene expression of two model human cell lines, human dermal fibroblasts (HDF) and prostate cancer cells (PC3) was interrogated by microarray analysis of over 14,000 human genes. The cell lines were exposed to four differently functionalized Au NPs: citrate, poly(allylamine hydrochloride) (PAH), and lipid coatings combined with alkanethiols or PAH. Gene functional annotation categories and weighted gene correlation network analysis were used in order to connect gene expression changes to common cellular functions and to elucidate expression patterns between Au NP samples. Coated Au NPs affect genes implicated in proliferation, angiogenesis, and metabolism in HDF cells, and inflammation, angiogenesis, proliferation apoptosis regulation, survival and invasion in PC3 cells. Subtle changes in surface chemistry, such as the initial net charge, lability of the ligand, and underlying layers greatly influence the degree of expression change and the type of cellular pathway affected.

Low Biotinyl Glycogen: A Model for Single-Molecule Force Analysis of Branched Biological Macromolecules

Characterization of polysaccharides in their native-like forms is of great importance for their subsequent application in the pharmaceuticals and biomedicine fields. Commercial glycogen molecules immobilized on a modified gold surface were characterized using a combination of Single Molecule Force and InfraRed spectroscopies. For this, low biotinylated glycogen was synthetized and subsequently immobilized on a Bovine Serum Albumin/Streptavidin-coated gold modified surface. The presence of glycogen was ascertained by vibrational spectra and confirmed based on the specific interaction of concanavalin A functionalized AFM tips with -glucose residues. Theoretical values of the particle size of this glycogen fitted well with the structural observations. Conformational properties were deduced from the force curves using the FJC model. The studied glycogen showed a compact slightly ramified structure with molecular elongations up to 1000 nm. The investigation of other conformational properties showed similarities with bacterial glycogen, however some differences that are possibly related to the different biosynthesis pathways were observed.

Real-Time Impedimetric Detection of DNA Amplification By PCR Using Electrochemical Impedance Spectroscopy

DNA sensors are innovative diagnostic tools that have gained broad acceptance in recent years. Different procedures have been used to develop these sensors. One of the most frequently used method is the self-assemble monolayer (SAMs) that provides advantages in terms of simplicity, efficiency and cost of electrode design. Gold surface modification with SAMs change the electrochemical capacitive behavior of their surface which can be measured by Electrochemical Impedance Spectroscopy (EIS). Tethering of ssDNA via SAMs techniques onto metallic Au surface provide a biosensing probe which is sensitive to changes on the surface, and these changes on the ssDNA conformation correlate to changes in the double layer capacitance (Cdl). Using this as the basis, a novel microchip with an interdigitated array electrodes was done and used as a real-time electrochemical biosensor for the detection of DNA amplification directly on its surface. The microchip was designed using a silicon/SiO2 substrate and gold thin film patterned by photolithography. The microchip is connected by a USB type A connector. In this device, the polymerase chain reaction (PCR) takes place on the surface interface, and the primer ssDNA tethered to the surface is elongated. Every modification, variation or alteration of the DNA on the surface of the electrodes result in changes in the Cdl and measured by EIS having a non-faradaic and label-free detection. The recognized sensitivity of electrochemical platforms and the well-known specificity of exclusive primers provide portability, reliability, rapidness, and accuracy in a robust biosensor microchip. Moreover, by avoiding the use of optical labels we are able to measure complex or unpurified biofluid samples without the need of extensive purification. All these features will have a tremendous impact on the biomedical science filling the absence of methods that can detect DNA in real-time and in a direct readout of crude biological fluids.

Investigation of the Antifouling Properties of Phenyl Phosphorylcholine‐Based Modified Gold Surfaces

AbstractLow impedance, antifouling coatings on gold electrodes based on three new zwitterionic phenyl phosphorylcholine (PPC)‐based layers namely 1) reductively adsorbed PPC diazonium salt, 2) dithiocarbamate PPC SAM and 3) lipoamide PPC SAM (PPC coupled to α‐lipoic acid) were evaluated. The layers were assessed for their ability to limit nonspecific adsorption of proteins to electrode surface with some significant differences observed compared with previously studied PPC diazonium salts reductively adsorbed on glassy carbon. Fluorescence microscopy and electrochemical impedance spectroscopy results suggest that protein adsorption is sensitive to the difference in the structure of the PPC molecules and the charge neutrality of the layers. The lipoamide PPC SAM was shown to be the most effective at resisting nonspecific protein adsorption and this layer was as effective as the ‘gold standard’ of oligo(ethylene oxide) SAMs on gold and PPC diazonium salts reductively adsorbed on glassy carbon.

Functional Interfaces for Biomimetic Energy Harvesting: CNT-DNA Matrix for Enzyme Assembly

The development of 3D structures exploring the properties of nano-materials and biological molecules allows us to mimic natural metabolic processes such as Krebs or Calvin cycle by creating “artificial” metabolic pathways called metabolons. The utilization of metabolons in biomimetic energy-harvesting systems leads to increased fuel oxidation and as a result increased Columbic efficiency. The specific order and minimized distance between the enzymes from those metabolons are the main reasons for subsequent and more effective fuel consumption.In this study a complex carbon nanotubes (CNTs) – DNA matrix for specific enzyme immobilization was created. It composed of “brush” type aligned CNTs covalently attached on electrochemically modified gold surface. This forest like CNTs structure was used as a substrate for the development of a DNA scaffold. This DNA scaffold had a designed sequence that allows strong attachment of DNA to the CNTs surface by wrapping the nanotubes through pi-pi stacking and at the same time has a unique nine base pair motif that was further hybridized. The hybridized part of the DNA was used to specifically attach zinc finger (Znf268) that was genetically fused to small laccase (SLAC).Zinc fingers are small protein structural motifs that can recognize dsDNA and specifically bind to it. We used this property to control the SLAC immobilization by fusing the Znf268 to the enzyme and attaching it to the CNTs-DNA scaffold (Fig. 1). Due to the specific interactions of zinc fingers with DNA we will be able to immobilize enzymes in particular order and distance creating metabolons.Figure 2 shows Atomic Force Microscopy (AFM) images of the gold surface at the different stages of its modification to the creation of the CNTs forest. The modification procedures were also confirmed with x-ray photoelectron spectroscopy (XPS).To prove the concept we used SLAC, which is an enzyme from the family of well-known multi-copper oxidases that are able to reduce oxygen and form water [1]. The enzyme was immobilized by the described approach and characterized by XPS (Fig. 3). The presence of copper in the XPS spectrum unambiguously proofs the success of the immobilization procedure. The developed herein approach for specific enzyme immobilization can be applied for all types of enzymes for the development of bio-cathodes as well as bio-anodes in the design of energy-harvesting systems.The developed herein approach for specific enzyme immobilization can be applied for all types of enzymes for the development of bio-cathodes as well as bio-anodes in the design of energy-harvesting systems.[1] Szilvay, G., S. Brocato, D. Ivnitski, C. Li, P. De La Iglesia, C. Lau, E. Chi, M. Werner-Washburne, S. Banta P. Atanassov Chem. Commun., 2011, 47, 7464–7466.

Determination of amikacin in human plasma by molecular imprinted SPR nanosensor

The molecular imprinting technique depends on the molecular recognition. It is polymerization method occuring around the target molecule. Thus this technique creates specific cavities in the cross-linked polymeric matrices. Amikacin (AMIK) is an important aminoglycoside (AG) antibiotic used to treat different types of bacterial infections. In this study, we prepared surface plasmon resonance (SPR) sensor for the determination of AMIK in human plasma. After the modification of gold surface of SPR chip was performed by allyl mercaptane, AMIK-imprinted poly(2-hydroxyethyl methacrylate-methacryloylamidoaspartic acid) [p(HEMA–MAAsp)] film was generated on the allyl mercaptane modified gold surface. The non-modified and imprinted surfaces were characterized by Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM) and contact angle measurements. The developed method was validated according to the ICH guideline. The linearity range and the detection limit were obtained as 0.01–0.15μg/mL and 0.0025μg/mL (4.3×10−9mol/L), respectively. The developed AMIK-imprinted nanosensor was successfully applied to the human plasma samples for the determination of AMIK.

Protein Corona Fingerprinting Predicts the Cellular Interaction of Gold and Silver Nanoparticles

Using quantitative models to predict the biological interactions of nanoparticles will accelerate the translation of nanotechnology. Here, we characterized the serum protein corona 'fingerprint' formed around a library of 105 surface-modified gold nanoparticles. Applying a bioinformatics-inspired approach, we developed a multivariate model that uses the protein corona fingerprint to predict cell association 50% more accurately than a model that uses parameters describing nanoparticle size, aggregation state, and surface charge. Our model implicates a set of hyaluronan-binding proteins as mediators of nanoparticle-cell interactions. This study establishes a framework for developing a comprehensive database of protein corona fingerprints and biological responses for multiple nanoparticle types. Such a database can be used to develop quantitative relationships that predict the biological responses to nanoparticles and will aid in uncovering the fundamental mechanisms of nano-bio interactions.

Label-free electrochemical monitoring of protein addressing through electroactivated “click” chemistry on gold electrodes

In this work, using electrochemical impedance spectroscopy (EIS), we have, for the first time, label-free monitored protein immobilization on a gold surface through a strategy of electroaddressing, compatible with the production of microarrays for multi-detection. This functionalization is achieved via the alkyne/azide cycloaddition, better known as the “click” reaction. The electroaddressing was applied to a polythiol hexynyl derivative previously grafted onto the gold surface. This compound consists of two dithiol phosphate groups and a hexynyl function and was synthesized through a supported synthesis approach, from a dithiol reagent, phosphoramidite (DTPA), and a hexynyl phosphoramidite. Next, an azide-PEG3-biotin derivative was grafted onto the modified gold surface by electro-chronocoulometry. The “click” reaction was controlled by electrochemical impedance spectroscopy, showing the change in impedance only when the electroaddressing was performed at −300mV. No effect on the EIS signal was observed when a positive potential was applied, confirming the specificity of the electroactivation. Biotin-modified electrodes were used to fix streptavidin and the immobilization was monitored using EIS. Fluorescent streptavidin-functionalized silica nanoparticles were also specifically grafted onto the biotinylated gold surface in order to confirm the “click” reaction using fluorescence microscopy. The obtained streptavidin platform was used to detect the surface coverage by biotinylated human serum albumin (HSA). The lowest detectable concentration is 10pg/mL, and surface saturation is obtained with concentrations higher than 100ng/mL.

Optical and Electrochemical Combination Sensor with Poly‐Aniline Film Modified Gold Surface and Its Application for Dissolved Oxygen Detection

AbstractPoly‐aniline (PAn) film can be oxidized by contacting oxygen in an electron‐transfer reaction and its redox state, for instance the open circuit potential and the dielectric constant, is quantitatively related to oxygen concentration. This feature may contribute to an improved application of dissolved oxygen (DO) detection with the combined optical and electrochemical sensor. In this paper, PAn is used as a sensing surface to detect DO by the combination sensor composed of electrochemistry and total internal reflection imaging ellipsometry (EC‐TIRIE). Results demonstrate that both optical and electrical signals show a logarithmic correlation with DO concentration (0–20 ppm). Compared with the results obtained only with the gold surface, both the optical and electrical signals of the EC‐TIRIE sensor for DO detection are amplified with the PAn modified gold surface.

In situ Raman spectroscopic studies on potential-induced structural changes in polyaniline thin films synthesized via surface-initiated electropolymerization on covalently modified gold surface

The work describes the in situ Raman spectroelectrochemical studies on polyaniline (PANi) thin films covalently bound to the modified gold surface. The PANi thin films were synthesized via surface-initiated electrochemical polymerization on gold substrates initially modified by the formation of self-assembled monolayer of 4-aminothiophenol, or comparatively by the electrografting of 4-nitrobenzenediazonium salt followed by the electrochemical reduction of aromatic nitro-groups to form aniline-containing monolayer. Due to the defined surface-chain bonding and the ordered polymer structure, the obtained thin PANi films exhibit a narrow peak separation of the polymer redox transitions and negatively shifted potential of its full oxidation, as revealed by the PANi Raman spectra recorded at controlled potentials and cyclic voltammetry measurements.

Molecular imprinted nanosensor based on surface plasmon resonance: Application to the sensitive determination of amoxicillin

In this report, we developed surface plasmon resonance (SPR) sensor for the sensitive determination of amoxicillin (AMOX) in chicken egg and human plasma. Firstly, the modification of gold surface of SPR chip was performed by allyl mercaptane. Then, AMOX-imprinted poly(2-hydroxyethyl methacrylate–methacryloylamidoglutamic acid) [p(HEMAGA)] nanofilm was generated on the allyl mercaptane modified gold surface. The unmodified and imprinted surfaces were characterized by fourier transform infrared (FTIR) spectroscopy, ellipsometry and contact angle measurements. The developed method was validated according to the ICH guideline. The linearity range and the detection limit were obtained as 0.1–2.0ng/mL and 0.022ng/mL, respectively. The developed imprinted nanosensor was applied to the chicken egg and human plasma samples for the determination of AMOX. In addition, isotherm models were applied to data to explain adsorption process.

Self-Assembly of Proteins into Three-Dimensional Structures Using Bio-Conjugation

ABSTRACTSelf–assembly of molecular building blocks provides an interesting route to produce well-defined chemical structures. Tailoring the functionalities on the building blocks and controlling the time of self-assembly could control the properties as well as the structure of the resultant patterns. Spontaneous self-assembly of biomolecules can generate bio-interfaces for myriad of potential applications. Here we report self-assembled patterning of human serum albumin (HSA) protein in to ring structures on a polyethylene glycol (PEG) modified gold surface. The structure of the self-assembled protein molecules and kinetics of structure formation entirely revolved around controlling the nucleation of the base layer. The formation of different sizes of ring patterns is attributed to growth conditions of the PEG islands for bio-conjugation. These assemblies might be beneficial in forming structurally ordered architectures of active proteins such as HSA or other globular proteins.

A novel molecular imprinted nanosensor based quartz crystal microbalance for determination of kaempferol

The molecular imprinting technique relies on the molecular recognition. It is a kind of polymerization which is formed around the target molecule. Hence this technique forms specific cavities in the cross-linked polymeric matrices. In this report, we developed quartz crystal microbalance (QCM) nanosensor for the real-time detection of kaempferol (KAE). Firstly, the modification of gold surface of QCM chip was carried out by self-assembling monolayer formation of allyl mercaptane to introduce polymerizable double bonds on the chip surface. Then, KAE imprinted poly(2-hydroxyethyl methacrylate–methacryloylamidoaspartic acid) [p(HEMA–MAAsp)] film was generated on the gold surface. The non-modified and KAE-imprinted p(HEMA–MAAsp) surfaces were characterized by using atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and ellipsometry. The linearity range and the detection limit were obtained as 2.0×10−10 to 1.5×10−9M and 6.0×10−11M, respectively. The developed method was applied to real samples such as orange and apple juices for the determination of KAE in the presence of quercetin (QR), myricetin (MYR) and apigenin (API). In addition, isotherm models were applied to data to explain adsorption process.