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.     

Probing Adsorption Behaviors of BSA onto Chiral Surfaces of Nanoparticles

Chiral properties of nanoscale materials are of importance as they dominate interactions with proteins in physiological environments; however, they have rarely been investigated. In this study, a systematic investigation is conducted for the adsorption behaviors of bovine serum albumin (BSA) onto the chiral surfaces of gold nanoparticles (AuNPs), involving multiple techniques and molecular dynamic (MD) simulation. The adsorption of BSA onto both L‐ and D‐chiral surfaces of AuNPs shows discernible differences involving thermodynamics, adsorption orientation, exposed charges, and affinity. As a powerful supplement, MD simulation provides a molecular‐level understanding of protein adsorption onto nanochiral surfaces. Salt bridge interaction is proposed as a major driving force at protein–nanochiral interface interaction. The spatial distribution features of functional groups (? COO^?, ? NH₃⁺, and ? CH₃) of chiral molecules on the nanosurface play a key role in the formation and location of salt bridges, which determine the BSA adsorption orientation and binding strength to chiral surfaces. Sequentially, BSA corona coated on nanochiral surfaces affects their uptake by cells. The results enhance the understanding of protein corona, which are important for biological effects of nanochirality in living organisms.     

Nanostructured microcantilevers with functionalized cyclodextrin receptor phases: self-assembled monolayers and vapor-deposited films

It is shown that the performance of microcantilver-based chemical sensors in a liquid environment is affected by altering cantilever surface morphology and receptor phase type and thickness. Self-assembled monolayers of thiolated beta-cyclodextrin (HM-beta-CD) and thin films of vapor-deposited heptakis (2,3-O-diacetyl-6-O-tertbutyl-dimethylsilyl)-beta-cyclodextrin (HDATB-beta-CD) were studied on smooth and nanostructured (dealloyed) gold-coated microcantilever surfaces. The dealloyed surface contains nanometer-sized features that enhance the transduction of molecular recognition events into cantilever response, as well as increase film stability for thicker films. Improvements in the limits of detection of the compound 2,3-dihydroxynaphthalene as great as 2 orders of magnitude have been achieved by manipulating surface morphology and film thickness. The observed response factors for the analytes studied varied from 0.02-604 nm/ppm, as determined by cantilever deflection. In general, calibration plots for the analytes were linear up to several hundred nanometers in cantilever deflections.     

Fabrication of a surface plasmon resonance biosensor based on gold nanoparticles chemisorbed onto a 1,10-decanedithiol self-assembled monolayer

We have investigated the fabrication of surface plasmon resonance (SPR) biosensors using self-assembled monolayers (SAMs) and adsorbed gold nanoparticles. The SAM of 1,10-decanedithiol was first fabricated onto a gold substrate. Gold nanoparticles were then chemisorbed onto the SAM surface by bonding with the terminal thiol groups, forming a sensor that can be used to immobilize proteins. Bovine serum albumin (BSA) was used as a test protein in this study. Several spectroscopic and microscopic techniques were used to investigate both the SAM and the chemisorption of gold nanoparticles at the SAM surface. Our results confirm the covalent bonding of the gold nanoparticles onto the SAM. Surface plasmon resonance (SPR) was used to study both the adsorption of BSA to the SAM surface and to the gold nanoparticle-coated SAM. For SAM surfaces with adsorbed gold nanoparticles a larger SPR response to BSA than to the sensors with a bare SAM is observed.     

Graphene-nanoplatelet-based photomechanical actuators

This paper reports large light-induced reversible and elastic responses of graphene nanoplatelet (GNP) polymer composites. Homogeneous mixtures of GNP/polydimethylsiloxane (PDMS) composites (0.1-5 wt%) were prepared and their infrared (IR) mechanical responses studied with increasing pre-strains. Using IR illumination, a photomechanically induced change in stress of four orders of magnitude as compared to pristine PDMS polymer was measured. The actuation responses of the graphene polymer composites depended on the applied pre-strains. At low levels of pre-strain (3-9%) the actuators showed reversible expansion while at high levels (15-40%) the actuators exhibited reversible contraction. The GNP/PDMS composites exhibited higher actuation stresses compared to other forms of nanostructured carbon/PDMS composites, including carbon nanotubes (CNTs), for the same fabrication method. An extraordinary optical-to-mechanical energy conversion factor (η(M)) of 7-9 MPa W(-1) for GNP-based polymer composite actuators is reported.     

Nonexclusive Fluorescent Sensing for l/d Enantiomers Enabled by Dynamic Nanoparticle-Nanorod Assemblies

Fluorescence sensing of enantiomers is a much needed yet very challenging task due to nearly identical chemical and physical properties of the chiral isomers also known as chiral equivalence. In this study, we propose a novel strategy for fluorescence sensing of enantiomers using chiral nanoparticles and their ability to form dynamic assemblies. Fluorescence resonance energy transfer (FRET) in nanoscale assemblies consisting of either l-cysteine- or d-cysteine-modified quantum dots (QDs) and gold nanorods (GNRs) was found to be strongly dependent on traces of cysteine. This occurs due to high sensitivity of dynamic assemblies to the weak internanoparticle interactions that can exponentially increase energy transfer efficiencies from QDs to GNRs. Comprehensive analysis of the fluorescence responses in the two types of chiral nanoscale assemblies enables accurate determination of both concentration and enantiomeric composition of the analyte, i.e., cysteine. The described method can quantify the composition of a chiral sample, even the content of one enantiomer is as low as 10% in the mixture. Exceptional selectivity in respect to d/l-cysteine in comparison to analogous small molecules was observed. Versatility of nanoparticle-nanorod assemblies and tunability of intermolecular interactions in them open the road to adaptation of this sensing platform to other chiral analytes.     

Formation of chiral surface with enantiomeric tartaric acid on gemini-structured self-assembled monolayers

Chiral surfaces were prepared by L, D, and Meso-tartaric acids (TAs) adsorbed on gemini-structured self-assembled monolayers (SAMs) composed of ethylenebis [(12-mercaptododecyl) dimethyl ammonium bromide] (HS-gQA-SH). The formation and structure of the chiral surfaces were characterized by surface plasmon resonance spectroscopy (SPR) and Fourier transform infrared-reflection adsorption spectroscopy (FTIR-RAS). The thickness of enantiomeric TA layers on the HS-gQA-SH SAM was estimated to be c.a. 5-6 angstroms regardless of their chirality, in good agreement with the height of TA molecules anchoring on the surface with two COOH groups. All the TAs on the HS-gQA-SH SAM exhibit the same ionization state independent of their chirality in their vibration bands of carboxylic groups. We attempted a second-layer adsorption of the enantiomeric TAs on L-TA monomolecular layer (L-TA SAM) precomposed on the HS-gQA-SH. A strong affinity between first and second TA layers resulted in the film growth when their chirality is identical (i.e., L-TA on L-TA SAM). We found the structure of second L-TA layer was completely different from that of the first layer, where a crystalline-like L-TA phase was found as a predominant component. Our results imply a preferential crystalline growth of chiral molecules on the same chiral surface, which may lead to a work for optical resolution into two enantiomers at a solid-liquid interface.     

Quantification of proteins on gold nanoparticles by combining MALDI-TOF MS and proteolysis

Protein-coated nanoparticles have been used in many studies, including those related to drug delivery, disease diagnosis, therapeutics, and bioassays. The number and density of proteins on the particles' surface are important parameters that need to be calculable in most applications. While quantification methods for two-dimensional surface-bound proteins are commonly found, only a few methods for the quantification of proteins on three-dimensional surfaces such as nanoparticles have been reported. In this paper, we report on a new method of quantifying proteins on nanoparticles using matrix assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS). In this method, the nanoparticle-bound proteins are digested by trypsin and the resulting peptide fragments are analyzed by MALDI-TOF MS after the addition of an isotope-labeled internal standard (IS) which has the same sequence as a reference peptide of the surface-bound protein. Comparing the mass intensities between the reference peptide and the IS allows the absolute quantification of proteins on nanoparticles, because they have the same molecular milieu. As a model system, gold nanoparticles were examined using bovine serum albumin (BSA) as a coating protein. We believe that our strategy will be a useful tool that can provide researchers with quantitative information about the proteins on surfaces of three-dimensional materials.     

Enantiomeric interactions between nucleic acid bases and amino acids on solid surfaces

Molecular interaction between nucleic acid bases and amino acids is a fundamental process in biology. The adsorption of the molecules on surfaces provides the opportunity to study such interactions in great detail by exploiting the high-resolution imaging capabilities of scanning tunnelling microscopy (STM). The chemisorption of prochiral molecules, such as adenine, on a metal surface causes the adsorbed species to become chiral. Subsequent interactions with inherently chiral molecules may then lead to the formation of diastereoisomers, if the enantiomeric interaction process is sufficiently strong. In the case of adenine adsorption on Cu[110], the chiral adsorbates form homochiral chains. Here, we show that the adenine chain direction is fully correlated with the chirality, and that the alpha-amino acid, phenylglycine, shows a strong chiral preference in its interaction with these chains. STM images clearly demonstrate that S-phenylglycine (R-phenylglycine) binds only to chains rotated 19.5 degrees (anti-) clockwise from the [001] direction. Closer examination reveals that the enantiomeric interaction involves double rows of phenylglycine molecules and the adenine chains. This is the first observation at the molecular level of diastereoisomeric interaction, and demonstrates that STM is a powerful method for studying the details of these interactions.     

Analysis of rat plasma proteins desorbed from gold and methyl- and hydroxyl-terminated alkane thiols on gold surfaces

It is believed that adsorbed blood or plasma components, such as water, peptides, carbohydrates and proteins, determine key events in the concomitant inflammatory tissue response close to implants. The aim of the present study was to develop a procedure for the collection and analysis of minor amounts of proteins bound to solid metal implant surfaces. The combination of a sodium dodecyl sulfate washing method coupled with a polyacylamide gel electrophoretic protein separation technique (SDS–PAGE), Western blot and image analysis enabled the desorption, identification and semiquantification of specific proteins. The analyzed proteins were albumin, immunoglobulin G, fibrinogen and fibronectin. Concentration procedures of proteins were not required with this method despite the small area of the test surfaces. The plasma proteins were adsorbed to pure gold and hydroxylated and methylated gold surfaces, which elicit different tissue responses in vivo and plasma protein adsorption patterns in vitro. The image analysis revealed that the pure gold surfaces adsorbed the largest amount of total and specific proteins. This is in accordance with previous ellipsometry/antibody experiments in vitro. Further, the principles described for the protein analysis can be applied on implant surfaces ex vivo. ©©2000 Kluwer Academic Publishers     

Dynamic Adsorption of Albumin on Nanostructured TiO(2)Thin Films

Spectroscopic ellipsometry was used to characterize the optical properties of thin (<5 nm) films of nanostructured titanium dioxide (TiO(2)). These films were then used to investigate the dynamic adsorption of bovine serum albumin (BSA, a model protein), as a function of protein concentration, pH, and ionic strength. Experimental results were analyzed by an optical model and revealed that hydrophobic interactions were the main driving force behind the adsorption process, resulting in up to 3.5 mg/m(2) of albumin adsorbed to nanostructured TiO(2). The measured thickness of the adsorbed BSA layer (less than 4 nm) supports the possibility that spreading of the protein molecules on the material surface occurred. Conformational changes of adsorbed proteins are important because they may subsequently lead to either accessibility or inaccessibility of bioactive sites which are ligands for cell interaction and function relevant to physiology and pathology.     

Chiral ionic liquid that functions as both solvent and chiral selector for the determination of enantiomeric compositions of pharmaceutical products

We have successfully synthesized both enantiomers of a novel chiral ionic liquid, (R)- and (S)-[(3-chloro-2-hydroxypropyl) trimethylammonium][bis((trifluoromethyl)sulfonyl)amide] ((R)- and (S)-[CHTA]+[Tf2N]-) in optically pure form by a simple ion exchange reaction from corresponding chloride salts that are commercially available. In addition to the ease of preparation, this chiral IL has relatively high thermal stability (up to 300 degrees C), is liquid at room temperature (glass transition temperature of -58.4 degrees C), and exhibits strong enantiomeric recognition. The high solubility power and strong enantiomeric recognition ability make it possible to use this chiral IL to solubilize an analyte and to induce diastereomeric interactions for the determination of enantiomeric purity. In fact, we have successfully developed a novel method based on the near-infrared technique with this chiral IL serving both as solvent and as a chiral selector for the determination of enantiomeric purity. Enantiomeric compositions of a variety of pharmaceutical products and amino acids with different shape, size, and functional groups can be sensitively (milligram concentration) and accurately (enantiomeric excess as low as 0.6%) determined by use of this method.     

Chiral separation of leucovorin with bovine serum albumin using affinity capillary electrophoresis.

A method for the determination of the (6R)- and (6S)-stereoisomers of leucovorin using electrokinetic chromatography (EKC) in the affinity mode has been developed. Bovine serum albumin (BSA) is used as a run buffer additive to incorporate enantiomeric selectivity into the system. Protein-wall interactions are minimized by using a poly(ethylene glycol) (PEG) coated capillary. Chiral resolution is obtained in 12.5 min with efficiencies greater than 200,000 theoretical plates using BSA as an additive, while no resolution is obtained in the absence of BSA. A general equation is derived to calculate the free energy of interaction between the leucovorin isomers and the BSA molecule. This method represents a new means of obtaining thermodynamic data for substrate binding interactions and for the general study of drug cross-reactions and interactions of drugs with serum and other proteins.     

Tertiary amine appended derivatives of N-(3,5-dinitrobenzoyl)leucine as chiral selectors for enantiomer assays by electrospray ionization mass spectrometry

Derivatives of the chiral selector N-(3,5-dinitrobenzoyl)leucine were prepared and used as chiral selectors for enantiomer discrimination in single-stage electrospray ionization mass spectrometric experiments. The chiral selectors were designed to remove the ionization site from the sites required for effective chiral recognition. Addition of a chiral analyte to a solution of the two pseudoenantiomeric chiral selectors, which differ in absolute stereochemistry and the length of the tether connecting the tertiary amine site used for ionization via protonation and the rest of the chiral selector, affords selector-analyte complexes in the electrospray ionization mass spectrum where the ratio of these complexes is dependent on the enantiomeric composition of the analyte. The relationship between the ratio of the selector-analyte complexes in the electrospray ionization mass spectrum and the enantiomeric composition of the analyte can be used to relate the extent of enantioselectivity that is being observed and for quantitative enantiomeric composition determinations. Investigations into the scope and limitations of this method, plus a comparison to the enantioselectivities observed by chiral HPLC using a N-(3,5-dinitrobenzoyl)leucine-derived chiral stationary phase, is presented.     

Aluminum nanostructured films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region

Particulate aluminum films of varied thicknesses were deposited on quartz substrates by thermal evaporation. These nanostructured substrates were characterized by scanning electron microscopy (SEM). With the increase of aluminum thickness, the films progress from articulate toward smooth surfaces as observed by SEM images. To date, metal-enhanced fluorescence (MEF) has primarily been observed in the visible - NIR wavelength region using silver or gold island films or roughened surfaces. We now show that fluorescence could also be enhanced in the ultraviolet-blue region of the spectrum using nanostructured aluminum films. Two probes, one in the ultraviolet and another one in the blue spectral region, have been chosen for the present study. We observed increased emission, decrease in fluorescence lifetime, and increase in photostability of a DNA base analogue 2-aminopurine and a coumarin derivative (7-HC) in 10-nm spin-casted poly(vinyl alcohol) film on Al nanostructured surfaces. The fluorescence enhancement factor depends on the thickness of the Al films as the size of the nanostructures formed varies with Al thickness. Both probes showed increased photostability near aluminum nanostructured substrates, which is consistent with the shorter lifetime. Our preliminary studies indicate that Al nanostructured substrates can potentially find widespread use in MEF applications particularly in the UV-blue spectral regime. Furthermore, these Al nanostructured substrates are very stable in buffers that contain chloride salts compared to usual silver colloid-based substrates for MEF, thus furthering the usefulness of these Al-based substrates in many biological assays where high concentration of salts are required. Finite-Difference Time-Domain calculations were also employed to study the enhanced near-fields induced around aluminum nanoparticles by a radiating fluorophore, and the effect of such enhanced fields on the fluorescence enhancement observed was discussed.     

Simultaneous separation of coplanar and chiral polychlorinated biphenyls by off-line pyrenyl-silica liquid chromatography and gas chromatography. Enantiomeric ratios of chiral congeners

A method for the unambiguous determination of 41 key polychlorinated biphenyls (PCBs) (including coplanar and chiral congeners) and the enantiomeric ratio of chiral congeners is described. The method includes a fractionation step using a 2-(1-pyrenyl) ethyldimethylsilylated silica column for separating the polychlorinated biphenyls according to the number of chlorine atoms in the ortho positions. High-resolution gas chromatography with an electron capture detector and an achiral column was used to determine the PCB congener content in each fraction. The enantiomeric ratio of chiral congeners was calculated by high-resolution gas chromatography with a mass spectrometry detector using a chiral column. The method was found to be inexpensive, rapid, effective, and reliable under the operational conditions proposed. It eliminates the main coelution problems among the polychlorinated biphenyl congeners. It also makes it possible to determine the enantiomeric ratio of nine chiral congeners using monodimensional gas chromatography. The method was applied successfully to the analysis of the coplanar and atropisomeric polychlorinated biphenyl congeners in dolphin liver samples. The enantiomeric ratio of nine chiral congeners is also reported for the first time.     

A reversible gel for chiral separations

The use of a guanosine gel as a chiral selector in capillary electrophoresis is introduced. Guanosine gels are reversible organized media that are formed through the self-association of guanosine compounds. Their degree of organization and their physicochemical properties can be modulated through variations in guanosine monomer concentration, pH, temperature, and cation content. Baseline resolution of the d and l enantiomers of propranolol was achieved using a reversible biogel formed by 5'-guanosine monophosphate as the run buffer in capillary electrophoresis. Conditions were optimized to provide enantiomeric resolution of 2.1-2.3 in less than 5 min. The reversibility of the gel network offers potential advantages for chiral separations, including the possibility of using thermal or chemical dissociation of the gel network to remove the nucleoside monomers from the separated enantiomers, thereby eliminating the chiral selector as a source of physical contamination of the enantiomerically pure products and spectral background in UV absorbance detection.     

High-performance taste sensor made from Langmuir-Blodgett films of conducting polymers and a ruthenium complex

A sensor array made up of nanostructured Langmuir-Blodgett (LB) films is used as an electronic tongue capable of identifying sucrose, quinine, NaCl, and HCl at the parts-per-billion (ppb) level, being in some cases 3 orders of magnitude below the human threshold. The sensing units comprise LB films from conducting polymers and a ruthenium complex transferred onto gold interdigitated electrodes. Impedance spectroscopy is used as the principle of detection, and the importance of using nanostructured films is confirmed by comparing results from LB films with those obtained from cast films.     

Ultrasensitive detection of CrO4(2-) using a microcantilever sensor

Microcantilevers modified with a self-assembled monolayer respond sensitively to specific ion concentrations. Here, we report the detection of trace amounts of CrO4(2-) using microcantilevers modified with a self-assembled monolayer of triethyl-12-mercaptododecylammonium bromide. The self-assembled monolayer was prepared on a silicon microcantilever coated with a thin layer of gold on one side. The microcantilever undergoes bending due to sorption of CrO4(2-) ions on the monolayer-modified side. It was found that a concentration of 10(-9) M CrO4(2-) can be detected using this technology in a flow cell. Other anions, such as Cl-, Br-, CO3(2-) (or HCO3-), and SO4(2-), have minimal effect on the deflection of this cantilever. The mechanics of the bending and the chemistry of cantilever modification are discussed.     

Concurrent zero-dimensional and one-dimensional biomineralization of gold from a solution of Au3+ and bovine serum albumin

Abstract

A technique was developed for preparing a novel material that consists of gold nanoparticles trapped within a fiber of unfolded proteins. These fibers are made in an aqueous solution that contains HAuCl₄ and the protein, bovine serum albumin (BSA). By changing the ratio of gold to BSA in solution, two different types of outcomes are observed. At lower gold to BSA ratios (30–120), a purple solution results after heating the mixture at 80 °C for 4 h. At higher gold to BSA ratios (130–170), a clear solution containing purple fibers results after heating the mixture at 80 °C for 4 h. UV–Vis spectroscopy and light scattering techniques show growth in nanocolloid size as gold to BSA ratio rises above 100. Data indicate that, for the higher gold to BSA ratios, the gold is sequestered within the solid material. The material mass, visible by eye, appears to be an aggregation of smaller individual fibers. Scanning electron microscopy and transmission electron microscopy indicate that these fibers are primarily one-dimensional aggregates, which can display some branching, and can be as narrow as 400 nm in size. The likely mechanism for the synthesis of the novel material is discussed.