Differentially ligand-functionalized microcantilever arrays for metal ion identification and sensing

A microcantilever array sensor with cantilevers differentially functionalized with self-assembled monolayers (SAMs) of thiolated ligands is prepared by simultaneous capillary coating. This array is described for the detection of metal ions including Li+, Cs+, Cu2+, Co2+, Fe3+, and Al3+. Binding of the charged metal cations to the surface of the microcantilever sensors produces surface stress that causes bending of the cantilevers that is detected as tip deflection using an array of vertical cavity surface emitting lasers and a position-sensitive detector. Optimization studies of the nanostructured dealloyed surface were performed for SAMs based on their response to Cu2+ cations. Sensor performance experiments demonstrate good sensitivity toward metal ions, with limits of detection as low as 10(-8) molar. A multiplex capillary coating method for cantilever array creation is demonstrated and validated based on surface-enhanced Raman spectra obtained from adjacent cantilevers that were functionalized with different thiolated SAMs. The cantilever array coated with a range of thiolated ligands was exposed to the group of metal ions. The response characteristics of each metal ion show substantial diversity, varying not only in response magnitude, but response kinetics. A pattern recognition algorithm based on a combination of independent component analysis and support vector machines was able to validate that the sensor array response profiles produced enough information content that metal ions could be reliably classified with probabilities as high as 89%.     

Enantioselective sensors based on antibody-mediated nanomechanics

The use of microfabricated cantilevers as bioaffinity sensors was investigated. Since many bioaffinity interactions involve proteins as receptors, we conducted studies of the magnitude, kinetics, and reversibility of surface stresses caused when common proteins interact with microcantilevers (MCs) with nanostructured (roughened) gold surfaces on one side. Exposure of nanostructured, unfunctionalized MCs to the proteins immunoglobulin G and bovine serum albumin (BSA) resulted in reversible large tensile stresses, whereas MCs with smooth gold surfaces on one side produced reversible responses that were considerably smaller and compressive. The response magnitude for nanostructured MCs exposed to BSA is shown to be concentration dependent, and linear calibration over the range of 1-200 mg/L is demonstrated. Stable, reusable protein bioaffinity phases based on unique enantioselective antibodies are created by covalently linking monoclonal antibodies to nanostructured MC surfaces. The direct (label-free) stereoselective detection of trace amounts of an important class of chiral analytes, the alpha-amino acids, was achieved based on immunomechanical responses involving nanoscale bending of the cantilever. The temporal response of the cantilever (delta deflection/delta time) is linearly proportional to the analyte concentration and allows the quantitative determination of enantiomeric purity up to an enantiomeric excess of 99.8%. To our knowledge, this is the first demonstration of chiral discrimination using highly scalable microelectromechanical systems.     

Nanostructured disposable impedimetric sensors as tools for specific biomolecular interactions: sensitive recognition of concanavalin A

The development of sensors to detect specific weak biological interactions is still today a challenging topic. Characteristics of carbohydrate-protein (lectin) interactions include high specificity and low affinity. This work describes the development of nanostructured impedimetric sensors for the detection of concanavalin A (Con A) binding to immobilized thiolated carbohydrate derivatives (D-mannose or D-glucose) onto screen-printed carbon electrodes (SPCEs) modified with gold nanoparticles. Thiolated D-galactose derivative was employed as negative control to evaluate the selectivity of the proposed methodology. After binding the thiolated carbohydrate to the nanostructured SPCEs, different functionalized thiols were employed to form mixed self-assembled monolayers (SAM). Electrochemical impedance spectroscopy (EIS) was employed as a technique to evaluate the binding of Con A to selected carbohydrates through the increase of electron transfer resistance of the ferri/ferrocyanide redox probe at the differently SAM modified electrodes. Different variables of the assay protocol were studied in order to optimize the sensor performance. Selective Con A determinations were only achieved by the formation of mixed SAMs with adequate functionalized thiols. Important differences were obtained depending on the chain lengths and functional groups of these thiols. For the 3-mercapto-1-propanesulfonate mixed SAMs, the electron transfer resistance varied linearly with the Con A concentration in the 2.2-40.0 μg mL(-1) range for D-mannose and D-glucose modified sensors. Low detection limits (0.099 and 0.078 pmol) and good reproducibility (6.9 and 6.1%, n=10) were obtained for the D-glucose and D-mannose modified sensors, respectively, without any amplification strategy.     

Cantilever-based sensor for the detection of different chromophore isomers

We report the use of microcantilevers (MCs) for the detection of three retinoid isomers: 9-cis-retinal, 13-cis-retinal and all-trans-retinal. Detection of synthetic and natural retinoids in topical cosmetic products is important, and their presence can be used to predict reactions with the skin surface. In this study the MC surfaces were functionalized in order to promote the formation of covalent bonds with the chromophores. The lowest mass shift we detected with the functionalized MCs was 1.2 ppt, which is in the range needed by the cosmetics industry. Our results indicate that properly designed and functionalized microcantilevers can be used to construct economical, fast, and sensitive sensors for quality control in cosmetics.     

Mixed alkylsilane functionalized surfaces for simultaneous wetting and homeotropic anchoring of liquid crystals

Surfaces functionalized with a self-assembled monolayer (SAM) formed from a mixture of two alkylsilanes with different chain lengths have been designed to simultaneously improve the liquid crystal (LC) wettability and promote homeotropic anchoring of the LC. Most chemically functionalized surfaces (e.g., long alkyl chain SAMs) that promote homeotropic alignment of LC possess low surface energy and result in poor LC wettability, inhibiting LC infiltration into microstructured surfaces and sometimes resulting in LC dewetting from the surface. However, a surface modified with a mixed SAM of octadecyltriethoxysilane (C18) and ethyltriethoxysilane (C2) exhibited very low LC contact angle while providing homeotropic anchoring. Ellipsometry was used to correlate the bulk concentration of C18 in the deposition solution to the surface coverage of C18 in the mixed monolayer; these bulk and surface concentrations were found to be equal within experimental uncertainty. The LC contact angle was found to depend nonmonotically with the surface coverage density, with a minimum (14.4 ± 0.1°) at a C18 surface coverage of 0.26 ± 0.08. Homeotropic LC anchoring was achieved at a C18 surface coverage of ≥0.11 ± 0.04, in the regime where a minimum in the LC contact angle was observed. The practical application of this approach to surface modification was demonstrated using a micropillar array sensor substrate. When the array was functionalized with a conventional C18 SAM, the LC did not infiltrate the array and exhibited a contact angle of 47.4 ± 0.5°. However, the LC material successfully infiltrated and wetted the same microstructured substrate when functionalized with a C18/C2 mixed SAM, while still exhibiting the desired homeotropic anchoring.     

Interaction control between endohedral metallofullerene and metal substrate by introducing alkanethiol self-assembled monolayer

The interaction control between endohedral metallofullerenes and a metal substrate has been demonstrated by introducing hexanethiol, octanethiol, and decanethiol self-assembled monolayers (SAMs) as the interlayer. We observe the electric properties of terbium endohedral metallofullerenes (Tb@C82) on alkanethiol SAMs with different chain lengths by scanning tunneling microscopy (STM) and spectroscopy (STS). Based on the comparison of the high-resolution STM images of a Tb@C82 molecule on hexanethiol and octanethiol SAMs, the interaction between Tb@C82 and a hexanethiol SAM is found to be larger than that between Tb@C82 and an octanethiol SAM; this is because at 68 K, the rotational states of Tb@C82 terminate only on the hexanethiol SAM. Furthermore, we find that the tunneling current-voltage characteristics of Tb@C82 on the hexanethiol SAM show the rectifying effects that are also caused by the molecular energy level shifts of Tb@C82 molecules due to the large interaction.     

Plasmonic-coupling-based sensing by the assembly and disassembly of dipycolylamine-tagged gold nanoparticles induced by complexing with cations and anions

A surface-plasmon-coupling-mediated sensor system is developed based on Au nanoparticles tagged with a coordinative dipycolylamine and lipoyl-anchored naphthalimide derivative (AuNP@DPA). The AuNPs with tailored ligands exhibit distinct sensing activity via sequential assembly into nanoparticle aggregates induced by metal ion complexing, and disassembly in the presence of pyrophosphate (PPi) anions, which is accompanied by a swift, reversible color change due to a surface plasmon resonance coupling effect. It is found that divalent metal ions are more effective than mono- or tri-valent ions in the aggregate formation process, Mn(2+)-induced aggregates are more sensitive to the capture of PPi anions than other AuNP aggregates, and the disassembly upon anion complexation exhibits a highly selective response. The AuNP@DPA-based molecular recognition system also demonstrates a viable performance for the detection of total selective metal ions present in different types of water analytes.     

Single DNA molecules as probes of chromatographic surfaces

YOYO-I-labeled lambda-DNA was employed as a nanoprobe for different functionalized surfaces to elucidate adsorption in chromatography. While the negatively charged backbone is not adsorbed, the 12-base unpaired ends of this DNA provide exposed purine and pyrimidine groups for adsorption. Self-assembled monolayers (SAMs) formed on gold substrate provide a wide range of choices of surface with well-defined and well-organized functional groups. Patterns of amino-terminated, carboxylic acid-terminated, and hydroxyl-terminated SAMs are generated by lithography. Patterns of metal oxides are generated spontaneously after deposition of metals. By recording the real-time dynamic motion of DNA molecules at the SAMs/aqueous interface, one can study the various parameters governing the retentivity of an analyte during chromatographic separation. Even subtle differences among adsorptive forces can be revealed.     

Variation of the surface charge of silica particles by functionalised self-assembled monolayers

In this study the synthesis of functionalized self-assembled monolayers (SAMs) with SH, SO₃H, NH₂ and CN functionality onto silica particles and their properties has been studied and characterized by infrared spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and measurement of the ζ potential of the resulting powder. The infrared and ToF-SIMS measurements of the samples clearly show the presence of CH₂ goups and the various functionalized head groups of the SAMs, indicating a successful deposition of the SAMs onto the silica particles. The measurements of the ζ potential of the samples show clear variations of their ζ potential depending on the type of SAM compared to the ζ potential of the pure silica powder used in this study. Even positive ζ potential could be measured for silica powder coated with NH₂ and CN SAM.     

A Thermodynamics Model for the Emergence of a Stripe‐like Binary SAM on a Nanoparticle Surface

It has been under debate if a self‐assembled monolayer (SAM) with two immiscible ligands of different chain lengths and/or bulkiness can form a stripe‐like pattern on a nanoparticle (NP) surface. The entropic gain upon such pattern formation due to difference in chain lengths and/or bulkiness has been proposed as the driving force in literature. Using atomistic discrete molecular dynamics simulations it is shown that stripe‐like pattern could indeed emerge, but only for a subset of binary SAM systems. In addition to entropic contributions, the formation of a striped pattern also strongly depends upon interligand interactions governed by the physicochemical properties of the ligand constituents. Due to the interplay between entropy and enthalpy, a binary SAM system can be categorized into three different types depending on whether and under what condition a striped pattern can emerge. The results help clarify the ongoing debate and our proposed principle can aid in the engineering of novel binary SAMs on a NP surface.     

Characterization of trypsin immobilized on the functionable alkylthiolate self-assembled monolayers: A preliminary application for trypsin digestion chip on protein identification using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Self-assembled monolayers (SAMs) on coinage metal provide versatile modeling systems for studies of interfacial electron transfer, biological interactions, molecular recognition and other interfacial phenomena. Recently the bonding of enzyme to SAMs of alkanethiols onto Au electrode surfaces was exploited to produce a bio-sensing system. In this work, the attachment of trypsin to a SAMs surface of 11-mercaptoundecanoic acid was achieved using water soluble N-ethyl-N -(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide as coupling agent. The thickness of SAMs was determined by optical ellipsometer; contact angles of the modified Au surfaces were measured in air using a goniometer. The Second Harmony Generation data displays the last few percents of the alkylthiol molecules adsorbed and produced the complete monolayer by inducing the transition from a high number of gauche defects to an all-trans conformation. Using X-ray Photoelectron Spectroscopy (XPS) and Fourier-Transformed Infrared Reflection-Absorption and Attenuated Total Reflection Spectroscopes (FTIR-RAS and ATR), we examined the chemical structures of samples with different treatments. By matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), we demonstrated the digestion of bovine serum albumin (BSA) on the trypsin-immobilized SAMs surface.Experimental results have revealed that the XPS C1s core levels at 286.3 and 286.5 eV (Amine bond), 288.1 eV (Amide bond) and 289.3 eV (Carboxylic acid) illustrate the immobilization of trypsin. These data were also in good agreement with FTIR-ATR spectra for the peaks valued at 1659.4 cm^{– 1} (Amide I) and 1546.6 cm^{– 1} (Amide II). Using MALDI-TOF MS observations, analytical results have demonstrated the BSA digestion of the immobilized trypsin on the functionalized SAMs surface. For such surfaces, BSA was digested on the trypsin-immobilized SAMs surface, which shows the enzyme digestion ability of the immobilized trypsin. The terminal groups of the SAMs structure can be further functionalized with biomolecules or antibodies to develop surface-base diagnostics, biosensors, or biomaterials.     

Molecular dynamics simulation of the enhancement of cobra cardiotoxin and E6 protein binding on mixed self-assembled monolayer molecules

Molecular dynamics simulations are performed on n-alkinethiol self-assembled monolayers (SAMs) and their mixture on a gold surface so that the orientations of the binding of cobra cardiotoxin and E6 protein molecules can be selected using the mixing ratio of CH3-terminated SAMs with different chain lengths. The simulations suggest that a SAM surface with different mixing ratios may provide a possible platform for aligning protein molecules with a desired orientation and for enhancing the binding energy of the protein on the designed surface.     

Nanostructured electrochemical sensors based on functionalized nanoporous silica for voltammetric analysis of lead, mercury, and copper

We have successfully developed electrochemical sensors based on functionalized nanostructured materials for voltammetric analysis of toxic metal ions. Glycinylurea self-assembled monolayers on mesoporous silica (Gly-UR SAMMS) were incorporated in carbon paste electrodes for the detection of toxic metal ions such as lead, copper, and mercury based on adsorptive stripping voltammetry (AdSV). The electrochemical sensor yields a linear response at a low ppb level of Pb2+ (i.e., 2.5-50 ppb) after a 2-min preconcentration period, with reproducible measurements (%RSD = 3.5, N = 6) and an excellent detection limit (1 ppb). By exploiting the interfacial functionality of Gly-UR SAMMS, the sensor is selective for the target species, does not require the use of a mercury film, and can be easily regenerated in dilute acid solution. The rigid, open, parallel pore structure, combined with suitable interfacial chemistry of SAMMS, also results in fast analysis times (2-3 min). The nanostructured SAMMS materials enable the development of miniature sensing devices that are compact and low cost, have low energy consumption, and are easily integrated into field-deployable units.     

Surface-based lithium ion sensor: An electrode derivatized with a self-assembled monolayer

Self-assembled monolayers (SAMs) of 21-(16-mercaptohexadecan-1-oyl)-4,7,13,16-tetraoxa-1,10,21-triazabicyclo[8.8.5]tricosane-19,23-dione were prepared on gold. Characterization of the SAMs was carried out by sessile drop contact angle, ellipsometry, grazing angle FT-IR spectroscopy, and electrochemical techniques. The cation recognition properties of the SAM were studied by cyclic voltammetry and impedance spectroscopy. The films show moderate selectivity for detection of Li+ ions in solution over K+ and Na+, with selectivity values calculated to be log K(Li+,Na+) approximately -1.30 and log K(Li+,K+) approximately -0.92. To the best of our knowledge, this is the first demonstration of a lithium sensor fabricated using self-assembled monolayer technology.     

A strategy for the generation of surfaces presenting ligands for studies of binding based on an active ester as a common reactive intermediate: a surface plasmon resonance study

This paper describes the immobilization of ten proteins and two low-molecular-weight ligands on mixed self-assembled monolayers (SAMs) of alkanethiolates on gold generated from the tri(ethylene glycol)-terminated thiol 1 (HS(CH2)11(OCH2CH2)3OH) (chi(1) = 1.0-0.0) and the longer, carboxylic acid-terminated thiol2(HS(CH2)11(OCH2-CH2)6OCH2CO2H) (chi(2) = 0.0-1.0). The immobilization was achieved by a two-step procedure: generation of reactive N-hydroxysuccinimidyl esters from the carboxylic acid groups of 2 in the SAM and coupling of these groups with amines on the protein or ligand. Because this method involves a common reactive intermediate that is easily prepared, it provides a convenient method for attaching ligands to SAMs for studies using surface plasmon resonance spectroscopy (and, in principle, other bioanalytical methods that use derivatized SAMs on gold, silver, and other surfaces). These SAMs were resistant to nonspecific adsorption of proteins having a wide range of molecular weights and isoelectric points. The pH of the coupling buffer, the concentration of protein, the ionic strength of the solution of protein, and the molecular weight of the protein all influenced the amount of the protein that was immobilized. For the proteins investigated in detail--carbonic anhydrase and lysozyme--the highest quantities of immobilized proteins were obtained when using a low ionic strength solution at a value of pH approximately one unit below the isoelectric point (pI) of the protein, at a concentration of approximately 0.5 mg mL-1. Comparisons of the kinetic and thermodynamic constants describing binding of carbonic anhydrase and vancomycin to immobilized benzenesulfonamide and N-alpha-Ac-Lys-D-Ala-D-Ala groups, respectively, on mixed SAMs (by methods described in this paper) and in the carboxymethyl dextran matrix of commercially available substrates yielded (for these systems) essentially indistinguishable values of Kd, koff, and kon.     

Immobilization of gold nanoparticles on solid supports utilizing DNA hybridization

Self-organization of colloidal metal nanoparticles into micro- and nanostructured assemblies is currently of tremendous interest promising to find new size- and structure-dependent physical properties. Owing to its unique recognition capabilities and physicochemical stability, DNA can be used as a molecular linker for gold nanoparticles and is a promising construction material for their precise spatial positioning. Due to the enormous specificity of nucleic acid hybridization, the site-specific immobilization of DNA-functionalized gold colloids (1–40 nm) to solid supports, previously functionalized with a complementary DNA array, allows the fabrication of novel nanostructured surface architectures. Scanning force microscopy (SFM), used to characterize the intermediate steps of the DNA-directed immobilization (DDI) on a gold substrate, provides initial insight into the specificity and efficiency of this technique.     

Electron permeable self-assembled monolayers of dithiolated aromatic scaffolds on gold for biosensor applications

Self-assembled monolayers (SAMs) of thiolated compounds are formed by the spontaneous chemisorption of thiolate groups on metal surfaces. In biosensors, they are most commonly used to covalently immobilize a biorecognition molecule onto the surface of the transducer, thus offering the possibility of controlling the orientation, distribution, and spacing of the sensing element while reducing nonspecific interactions. In this paper, self-assembled monolayers of dithiolated derivatives of 3,5-dihydroxybenzyl alcohol containing carboxyl and hydroxyl end groups have been prepared on gold surfaces and characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Impedance measurements revealed that SAM formation is essentially completed after 3-5 h of exposure by observing the successive blocking of the faradic response of ferricyanide anion due to the adsorption of the dithiol molecules. The surface coverage of these molecules, estimated by reductive desorption experiments, was in the range of (1.1-2.8) x 10-10 mol/cm2. To demonstrate the potential of the dithiol SAM, a model system for detection of a tumor marker, prostate-specific antigen (PSA), was developed. The carboxyl groups of the SAM were succinimide-activated, and an anti-PSA antibody was covalently immobilized via amide bonds. The modified SAM was used for the label-free detection of prostate-specific antigen using EIS with a detection limit of 9 ng/mL. The results described here demonstrate that this kind of dithiol-modified SAM can be used as supports in electrochemical biosensors and the results are explained in terms of the structural features of these dithiols.     

Noble metal nanoparticles deposited on self-assembled monolayers by pulsed laser deposition show coulomb blockade at room temperature

Nanometer-sized noble-metal clusters are fabricated on top of alkylthiolate self-assembled monolayers (SAMs) on annealed gold by pulsed laser deposition at elevated pressures. The size distribution of the clusters depends on the metal and on the pressure during the deposition. Scanning tunneling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM) showed that the metal clusters are insulated from the substrate on top of the SAM. Coulomb blockades could be measured at room temperature by STM for palladium clusters on decanethiol SAMs.     

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.     

硅基双层复合自组装分子膜结构特性及其润湿行为的分子动力学模拟

用分子动力学方法模拟了单晶硅（Si）表面N-3-（三甲氧基硅烷基）丙基乙二胺（DA）-月桂酰氯（LA）（DA-LA）双层复合自组装分子膜（SAMs）的结构特性,得到膜层中DA和LA分子的最佳覆盖率及分布情况。进一步讨论了水滴在DA-LA双层复合SAMs表面的润湿过程,通过接触角和径向分布函数等参量对其润湿行为进行了分析。研究表明：DA分子在Si上覆盖率为50%、LA分子在DA自组装单分子膜（DA SAM）上接枝率为100%时,分子膜呈有序排布,体系能量最低,从分子角度揭示了Si表面覆盖致密SAMs的形成机制。当取最佳覆盖率体系进行润湿机制模拟时,DA-LA双层复合SAMs表面水滴接触角与实验值相似,表现出良好的疏水性。而DA SAM表面由于DA分子短而稀疏,暴露出底层更亲水的羟基分子,从而导致所得接触角较实验偏小;经测量及计算得出,羟基化Si表面自由能最高,表现出较强的亲水性;DA表面次之;DA-LA表面自由能最低,表现出良好的疏水性。进一步分析发现：羟基化Si表面、DA SAM表面与水滴间存在氢键,加强了表面的亲水性,而DA-LA双层复合SAMs表面与水滴间只存在弱范德华力,有利于表面呈现疏水性。