Monolayer Of 11-mercaptoundecanoic Acid Research Articles

Surface Plasmon Resonance Immunosensor for Direct Detection of Antibodies against SARS-CoV-2 Nucleocapsid Protein.

The strong immunogenicity of the SARS-CoV-2 nucleocapsid protein is widely recognized, and the detection of specific antibodies is critical for COVID-19 diagnostics in patients. This research proposed direct, label-free, and sensitive detection of antibodies against the SARS-CoV-2 nucleocapsid protein (anti-SCoV2-rN). Recombinant SARS-CoV-2 nucleocapsid protein (SCoV2-rN) was immobilized by carbodiimide chemistry on an SPR sensor chip coated with a self-assembled monolayer of 11-mercaptoundecanoic acid. When immobilized under optimal conditions, a SCoV2-rN surface mass concentration of 3.61 ± 0.52 ng/mm2 was achieved, maximizing the effectiveness of the immunosensor for the anti-SCoV2-rN determination. The calculated KD value of 6.49 × 10-8 ± 5.3 × 10-9 M confirmed the good affinity of the used monoclonal anti-SCoV2-rN antibodies. The linear range of the developed immunosensor was from 0.5 to 50 nM of anti-SCoV2-rN, where the limit of detection and the limit of quantification values were 0.057 and 0.19 nM, respectively. The immunosensor exhibited good reproducibility and specificity. In addition, the developed immunosensor is suitable for multiple anti-SCoV2-rN antibody detections.

Multiphysics Modeling of Electrochemical Impedance Spectroscopy Responses of SAM-Modified Screen-Printed Electrodes

In this work, we present a multiphysics modeling approach capable of simulating electrochemical impedance spectroscopy (EIS) responses of screen-printed electrodes (SPEs) modified with self-assembled monolayers of 11-Mercaptoundecanoic acid (MUA). Commercially available gold SPEs are electrochemically characterized through experimental cyclic voltammetry and EIS measurements with 10 mM [Fe(CN)6]3−/4− redox couple in phosphate buffered saline before and after the surface immobilization of MUA at different concentrations. We design the multiphysics model through COMSOL Multiphysics® based on the 3D geometry of the devices under test. The model includes four different physics considering the metal/solution interface electrochemical phenomena, the ion and electron potentials and currents, and the measurement set-up. The model is calibrated through a set of experimental measurements, allowing the tuning of the parameters used by the model. We use the calibrated model to simulate the EIS response of MUA-modified SPEs, comparing the results with experimental data. The simulations fit the experimental curves well, following the variation of MUA concentration on the surface from 1 µM to 100 µM. The EIS parameters, retrieved through a CPE-modified Randles’ circuit, confirm the consistency with the experimental data. Notably, the simulated surface coverage estimates and the variation of charge transfer resistance due to MUA-immobilization are well matched with their experimental counterparts, reporting only a 2% difference and being consistent with the experimental electrochemical behavior of the SPEs.

Experimental Evaluation of Quantum Dots and Antibodies Conjugation by Surface Plasmon Resonance Spectroscopy.

The application of antibody-functionalized quantum dots (QDs) in different areas has been widely described in the literature. However, a standard routine method for obtaining information on the conjugation efficiency of QDs with antibodies in terms of the interaction of the functionalized QDs with a specific antigen is still lacking. Herein, surface plasmon resonance (SPR) spectroscopy is proposed for this purpose. Gold-coated SPR sensor disks were modified with a self-assembled monolayer of 11-mercaptoundecanoic acid, and carbodiimide cross-linker chemistry was used to covalently immobilize the CD44 biomarker on the premodified surface (Au/CD44). Meanwhile, QDs functionalized with amine-derivatized polyethylene glycol (PEG) (QDs-NH2) were chosen for conjugation with antibodies because of their low non-specific adsorption on the Au/CD44 surface. Prior to conjugation, the surface binding capacity (Bmax) and equilibrium dissociation constant (KD) of the specific antibodies against CD44 (anti-CD44) were found to be 263.32 ± 2.44 m° and 1.00 × 10−7 ± 2.29 × 10−9 M, respectively. QDs-NH2 and anti-CD44 were conjugated at their initial molar ratios of 1:3, 1:5, 1:10 and 1:12. SPR measurements showed that the conjugates (QDs-anti-CD44) prepared using 1:10 and 1:12 molar ratios interacted comparably with immobilized CD44 biomarkers. The equilibrium angles in the case of 10- and 12-fold concentrations of anti-CD44 were calculated to be 60.43 ± 4.51 and 61.36 ± 4.40 m°, respectively. This could be explained by the QDs-NH2 and anti-CD44 having a similar surface loading (about four molecules per QDs-NH2) and similar hydrodynamic diameters, which were 46.63 ± 3.86 and 42.42 ± 0.80 nm for the 1:10 and 1:12 ratios, respectively. An initial QDs-NH2: anti-CD44 molar ratio of 1:10 was chosen as being optimal. SPR spectroscopy proved to be the right choice for QDs-anti-CD44 conjugation optimization, and can be used for the evaluation of conjugation efficiency for other nanostructures with various bio-recognition molecules.

Rapid sensing of Tilletia indica – Teliospore in wheat extract by a piezoelectric label free immunosensor

‘Tilletia indica’, a fungal pathogen causes Karnal bunt disease in wheat. It has been renowned as a quarantine pest in more than 50 countries, therefore, urged a threat to wheat in the international market. To date, conventional methods employed to detect the disease involve the tentative identification of spores (teliospores) based on morphology. For effective and specific disease control, it is essential to get the specific protein of the analyte (teliospore) to target. In present study, a label-free immunosensor has been developed to detect Karnal bunt disease. A specifically synthesized anti-teliosporic monoclonal antibody (mAb) was immobilized on a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA) to detect teliospore. All modified electrodes were morphologically characterized by scanning electron microscopy (SEM), atomic force microscopy(AFM), Fourier transform infra-red spectroscopy (FT-IR) techniques and analytically characterized by quartz crystal microbalance (QCM) and cyclic voltammetry (CV). The linearity range was 19 pg mL−1–10 ng mL−1, while the detection limit (LOD) was 4.4 pg mL−1 and 12.5 pg mL−1, respectively. The stability, reproducibility, and repeatability of the immunoelectrode was examined by CV, and found stable upto 18 days with negligible variation. The binding affinity (association constant (Ka)) of the developed immunoelectrode was 1.9 × 10−2 ng mL−1. The real sample has been tested in spiked wheat samples and found about 95–103 % recovery with 2.8–4.4 % relative error.

Surface Plasmon Resonance Immunosensor with Antibody-Functionalized Magnetoplasmonic Nanoparticles for Ultrasensitive Quantification of the CD5 Biomarker

A surface plasmon resonance (SPR) immunosensor signal amplification strategy based on antibody-functionalized gold-coated magnetic nanoparticles (mAuNPs) was developed for ultrasensitive and quantitative detection of the CD5 biomarker using an indirect sandwich immunoassay format. The gold surface of the SPR sensor disk and mAuNPs was modified with a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA), and the coupling method using N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide was used to immobilize capture antibodies against human CD5 (anti-CD52A) and detection antibodies against human CD5 (anti-CD52B), respectively. The mAuNPs and anti-CD52B conjugates (mAuNPs–anti-CD52B) were separated by an external magnetic field and used to amplify the SPR signal after the formation of the anti-CD52A/CD5 immune complex on the SPR sensor disk. Compared to the direct CD5 detection with a limit of detection (LOD) of 1.04 nM and a limit of quantification (LOQ) of 3.47 nM, the proposed sandwich immunoassay utilizing mAuNPs–anti-CD52B significantly improved the LOD up to 8.31 fM and the LOQ up to 27.70 fM. In addition, it showed satisfactory performance in human blood serum (recovery of 1.04 pM CD5 was 109.62%). These results suggest that the proposed signal amplification strategy has superior properties and offers the potential to significantly increase the sensitivity of the analysis.

Surface functionalization strategy to enhance the antibacterial effect of nisin Z peptide

One of the main challenges when building antibacterial surfaces with antimicrobial peptides (AMPs) is to preserve their antimicrobial activity after stable immobilization of the peptides. Among all parameters, order/conformation of self-assembled monolayers, used as spacer, is one the most important. Herein we report the covalent immobilization of the nisin Z peptide on a gold surface functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) alone or mixed with 6-mercaptohexanol, used as a spacer. The MUA acid is activated by treatment with carbodiimide/N-hydroxysuccinimidine and then reacts with nisin Z to form amide bonds via the N terminal part of the peptide. We have characterized each step of the surface modification using X-ray photoelectron spectroscopy, FTIR-ATR spectroscopy and contact angle measurements. The combined results show the success of each functionalization step. Additionally, SFG brings information on the orientation and conformational ordering of the self-assembled monolayers. Indeed, a better order of MUA25 layers compared to MUA was observed due to the spacing of carboxylic acid groups. The antibacterial activity of the immobilized AMPs against Staphylococcus aureus is evaluated using confocal microscopy and bacterial counting: it increases with a better order of the SAMs rather than a greater peptide concentration. This study provides fundamental insights on how to engineer AMPs and substrate to produce efficient biocidal surfaces.

An impedimetric immunosensor for the selective detection of CD34+ T-cells in human serum

Chimeric antigen receptor T (CAR-T) cell therapy has led to a paradigm change in cancer treatment. To allow widespread use of this therapy, it is important to reduce the costs associated with cells manufacturing whilst ensuring a high-quality production. Online biosensors can help monitor in real time the effectiveness of engineering CAR-T cells, with the possibility to be integrated on the workflow without the need to extract samples for off line analysis that require specialised personnel and expensive analytical equipment. In this context, we present an innovative label-free impedimetric immunosensor for the detection of CD34+ T-cells in human serum. The sensor is based on screen-printed gold electrodes functionalised with anti-CD34 antibody, via a mixed self-assembled monolayer of 11-mercaptoundecanoic acid and 6-mercapto-1-hexanol, which recognizes the CD34 antigen expressed onto the T-cells surfaces. The immunosensor exhibits a high selectivity and a wide working range in human serum, both in Faradaic and non-Faradaic detection conditions, being respectively of 230 – 1.4 × 103 cell mL−1 and 50 – 1 × 105 cell mL−1. This result, along with performance validation both in batch and flow-through operations, demonstrates the applicability of the developed immunosensor for clinical use.

Quantitative Assessment of Periodontal Bacteria Using a Cell-Based Immunoassay with Functionalized Quartz Crystal Microbalance

Periodontal disease is an inflammatory disorder that is triggered by bacterial plaque and causes the destruction of the tooth-supporting tissues leading to tooth loss. Several bacteria species, including Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans, are considered to be associated with severe periodontal conditions. In this study, we demonstrated a quartz crystal microbalance (QCM) immunoassay for quantitative assessment of the periodontal bacteria, A. actinomycetemcomitans. An immunosensor was constructed using a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA) on the gold surface of a QCM chip. The 11-MUA layer was evaluated using a cyclic voltammetry technique to determine its mass and packing density. Next, a monoclonal antibody was covalently linked to 11-MUA using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide to act as the biorecognition element. The specificity of the monoclonal antibody was confirmed by an enzyme-linked immunosorbent assay. A calibration curve, for the relationship between the frequency shifts and number of bacteria, was used to calculate the number of A. actinomycetemcomitans bacteria in a test sample. Based on a regression equation, the lower detection limit was 800 cells, with a dynamic range up to 2.32 × 106 cells. Thus, the QCM biosensor in this study provides a sensitive and label-free method for quantitative analysis of periodontal bacteria. The method can be used in various biosensing assays for practical application and routine detection of periodontitis pathogens.

Gold nanoparticles decorated reduced graphene oxide nanolabel for voltammetric immunosensing.

This study describes the development and testing of a simple and novel enzyme-free nanolabel for the detection and signal amplification in a sandwich immunoassay. Gold nanoparticles decorated reduced graphene oxide (rGOAu) was used as the nanolabel for the quantitative detection of human immunoglobulin G (HIgG). The rGOAu nanolabel was synthesised by one pot chemical reduction of graphene oxide and chloroauric acid using sodium borohydride. The pseudo-peroxidase behaviour of rGOAu makes the nanolabel unique from other existing labels. The immunosensing platform was fabricated using self-assembled monolayers of 11-mercaptoundecanoic acid (11-MUDA) on a gold disc electrode. The covalent immobilisation of antibody was achieved through the bonding of the carboxyl group of 11-MUDA and the amino group of the antibody using chemical linkers [1-ethyl-3-(3-dimethylaminopropyl)carbodiimide] and N-hydroxysuccinimide. The fabricated immunosensor exhibited a linear range that included HIgG concentrations of 62.5-500 ng ml-1. The sensor was also used for the testing of HIgG in the blood sample.

Label-Free Detection of Chikungunya Non-Structural Protein 3 Using Electrochemical Impedance Spectroscopy

Chikungunya is a mosquito-borne infectious disease, which causes acute fever and joint pain, and can trigger long term arthritis. In the present study, we demonstrate a label-free detection of chikungunya virus non-structural protein 3 (CHIKV-nsP3), which is responsible for alphavirus replication, using electrochemical impedance spectroscopy (EIS). This is hitherto, the first work addressing the detection of CHIKV-nsP3 using biosensors. The immunosensors were developed by modifying a gold surface using self-assembled monolayer of 11-mercaptoundecanoic acid and 6-mercaptohexanol. Anti-CHIKV-nsP3 antibodies were attached to the sensor surface via cabodiimide coupling reaction. The impedance spectra were fit to an equivalent circuit, and the percentage change in charge transfer resistance was correlated to the antigen concentration, and a linear working range from 25 ng/ml to 1 μg/ml of CHIKV-nsP3 was obtained. The limit of detection was 8 ng/ml and the limit of quantification was 17 ng/ml. The biosensor was also found to be highly specific to CHIKV-nsP3. The sensor was further tested for matrix effect from human serum, and appreciable recoveries ranging from 97% to 106% was observed. The results illustrate the potential of EIS based immunosensors with CHIKV-nsP3 as a suitable biomarker for direct and sensitive detection of chikungunya.

An Optically-Transparent Aptamer-Based Detection System for Colon Cancer Applications Using Gold Nanoparticles Electrodeposited on Indium Tin Oxide.

In this paper, a label-free aptamer based detection system (apta-DS) was investigated for detecting colon cancer cells. For this purpose, we employed an aptamer specific to colon cancer cells like HCT116 expressing carcinoembryonic antigen (CEA) on their surfaces. Capture aptamers were covalently immobilized on the surface of gold nanoparticles (GNPs) through self-assembly monolayer of 11-mercaptoundecanoic acid (11-MUA) activated with EDC (1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide)/N-hydroxysuccinimide (NHS). The cyclic voltammetry (CV) and chronopotentiometry (CP) methods were used for electrodeposition of GNPs on the surface of indium tin oxide (ITO). In this work, the CV method was also used to demonstrate the conjugation of GNPs and aptamers and identify the cancer cell capturing events. Additionally, Field Emission Scanning Electron Microscopy (FE-SEM) confirmed the deposition of GNPs on ITO and the immobilization of aptamer on the apta-DS. The electrodeposited GNPs played the role of nanoprobes for cancer cell targeting without losing the optical transparency of the ITO substrate. A conventional optical microscope also verified the detection of captured cancer cells. Based on this study’s results relying on electrochemical and optical microscopic methods, the proposed apta-DS is reliable and high sensitive with a LOD equal to 6 cell/mL for colon cancer detection.

Covalent immobilization of molecularly imprinted polymer nanoparticles on a gold surface using carbodiimide coupling for chemical sensing

One challenging task in building (bio)chemical sensors is the efficient and stable immobilization of receptor on a suitable transducer. Herein, we report a method for covalent immobilization of molecularly imprinted core–shell nanoparticles for construction of robust chemical sensors. The imprinted nanoparticles with a core–shell structure have selective molecular binding sites in the core and multiple amino groups in the shell. The model Au transducer surface is first functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid. The 11-mercaptoundecanoic acid is activated by treatment with carbodiimide/N-hydroxysuccinimide and then reacted with the core–shell nanoparticles to form amide bonds. We have characterized the process by studying the treated surfaces after each preparation step using atomic force microscopy, scanning electron microscopy, fluorescence microscopy, contact angle measurements and X-ray photoelectron spectroscopy. The microscopy results show the successful immobilization of the imprinted nanoparticles on the surface. The photoelectron spectroscopy results further confirm the success of each functionalization step. Further, the amino groups on the MIP surface were activated by electrostatically adsorbing negatively charged Au colloids. The functionalized surface was shown to be active for surface enhanced Raman scattering detection of propranolol. The particle immobilization and surface enhanced Raman scattering approach described here has a general applicability for constructing chemical sensors in different formats.

Photopatterning of stable, low-density, self-assembled monolayers on gold.

Photoinitiated thiol-yne chemistry is utilized as a click reaction for grafting of acid-terminated alkynes to thiol-terminated monolayers on a gold substrate to create stable, low-density monolayers. The resulting monolayers are compared with a well-packed 11-mercaptoundecanoic acid monolayer and the analogous low-density monolayers prepared through a solution phase synthetic approach. The overall structuring of the monolayer prepared by solid-phase grafting is characterized by contact angle goniometry and Fourier transform infrared spectroscopy. The results show that the product monolayer has an intermediate surface energy and a more disordered chemical structuring compared to a traditional well-packed self-assembled monolayer, showing a low-packing density of the chains at the monolayer surface. The monolayer's structure and electrochemical stability were studied by reductive desorption of the thiolates. The prepared low-density monolayers have a higher electrochemical stability than traditional well-packed monolayers, which results from the crystalline structure at the gold interface. This technique allows for simple, fast preparation of low-density monolayers of higher stability than well-packed monolayers. The use of a photomask to restrict light access to the substrate yielded these low-density monolayers in patterned regions defined by light exposure. This general thiol-yne approach is adaptable to a variety of analogous low-density monolayers with diverse chemical functionalities.

A regenerating ultrasensitive electrochemical impedance immunosensor for the detection of adenovirus

We report on the development of a regenerable sensitive immunosensor based on electrochemical impedance spectroscopy for the detection of type 5 adenovirus. The multi-layered immunosensor fabrication involved successive modification steps on gold electrodes: (i) modification with self-assembled layer of 1,6-hexanedithiol to which gold nanoparticles were attached via the distal thiol groups, (ii) formation of self-assembled monolayer of 11-mercaptoundecanoic acid onto the gold nanoparticles, (iii) covalent immobilization of monoclonal anti-adenovirus 5 antibody, with EDC/NHS coupling reaction on the nanoparticles, completing the immunosensor. The immunosensor displayed a very good detection limit of 30 virus particles/ml and a wide linear dynamic range of 105. An electrochemical reductive desorption technique was employed to completely desorb the components of the immunosensor surface, then re-assemble the sensing layer and reuse the sensor. On a single electrode, the multi-layered immunosensor could be assembled and disassembled at least 30 times with 87% of the original signal intact. The changes of electrode behavior after each assembly and desorption processes were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques.

A Sensitive capacitive immunosensor for direct detection of human heart fatty acid-binding protein (h-FABP)

The fabrication of a capacitive interdigitated immunosensor (CID) based on a mixed self-assembled monolayer (mSAM) film for the direct detection of heart fatty-acid binding protein (h-FABP) without any labeling is described. The capacitance changes of mSAMs vs. homogenous ordered self-assembled monolayers (hSAMs) on gold work electrodes/covalently bonded antibodies/buffered medium are utilized for monitoring the specific antibody–antigen interaction. Capacitance measurements in the absence and presence of Faradaic currents were performed. The electrochemical properties of mixed monolayers were compared with those of a pure monolayer of 11-mercaptoundecanoic acid (MUA) self-assembled on gold surfaces. Taking into account the stability of the studied monolayers during the electrochemical experiments with the Faradaic process, the best SAM functionalization method was used for developing a sensitive capacitive immunosensor with a non-Faradaic process for direct immune detection of human h-FABP. Under the optimized conditions, the proposed mixed self-assembled monolayer (mSAM1) on gold electrode exhibited good insulating properties such as a capacitive behavior when detecting h-FABP from human serum in the range of 98pgml−1–100ngml−1, with a detection limit of 0.836ngml−1 comparative with a homogenous self-assembled monolayer (hSAM).

An amperometric immunosensor for diagnosis of celiac disease based on covalent immobilization of open conformation tissue transglutaminase for determination of anti-tTG antibodies in human serum

A new amperometric immunosensor based on the covalent immobilization of tissue transglutaminase enzyme in its open conformation (open-tTG) was developed and optimized for determination of anti-tissue transglutaminase antibodies (anti-tTG) in human serum. Experimental design allowed us to find the optimal conditions for quantification of both IgA and IgG isotypes of anti-tTG in order to assess suitability of the device for diagnostic purposes.The glassy carbon electrodic substrate was electrochemically functionalized with gold nanoparticles and subsequently derivatized with a self-assembled monolayer of 11-mercaptoundecanoic acid for the covalent anchoring of the enzyme. This step was performed under carefully controlled conditions in order to keep the open conformation of the tTG.The immunosensor showed good analytical performance with limit of detection levels (1.7AUmL−1 for IgA and 2.7AUmL−1 for IgG) below the diagnostic threshold value (3.0AUmL−1) and inter-sensor reproducibility giving RSD lower than 10%. The developed sensor was validated in serum samples from pediatric patients for clinical applications, using two ELISA kits specific for the determination of anti-tTG IgA and IgG antibodies as reference methods; good recovery rates ranging from 74% to 117% were calculated.

Potentiostatic infrared titration of 11-mercaptoundecanoic acid monolayers

In situ IR difference spectra of 11-mercaptoundecanoic acid monolayers deposited on gold have been recorded as a function of both solution pH⁎ and substrate's potential. IR spectra recorded under voltammetric conditions indicate that the potential-induced ionization of the monolayer is a slow process, involving a simultaneous rearrangement of the hydrocarbon chains, and that a significant population of carboxylic groups are not dissociated at positive potentials in a pH⁎9 solution. Steep potentiostatic IR titration curves, consistent with a monolayer pKa=4.3, are obtained at negative potentials. As the potential is made more positive, titration curves become lower, broader and are shifted towards more basic pHs. Rather than acting directly on the electrostatic energy of the carboxylate groups, the electrode potential seems to control the ionization of the monolayer by attracting (or repelling) cations from the electrolyte and by reorienting the thiol head groups inside and outside the low permittivity layer of methylene chains.

Recycling gold nanohole arrays

The authors report the impact of common cleaning methods on the stability of gold nanohole arrays used as extraordinary optical transmission surface plasmon resonance sensors. Their optical sensitivity, physical structure, and surface contamination levels were measured before and after multiple cycles of monolayer adsorption and removal with various wet chemicals (sulfochromic acid, piranha, or ammonium hydroxide: hydrogen peroxide) and dry oxygen plasma etchants. While these oxidative chemical and plasma etches remove organic monolayers and other contaminants, the oxidation and associated heating also damages the gold nanostructures to varying degrees. The authors observed decreases in the arrays' optical sensitivities via changes in the shapes and positions of their surface plasmon resonance peaks. The optimum recycling process was a room temperature, aqueous ammonium hydroxide: hydrogen peroxide treatment (15 min) commonly referred to as Radio Corporation of America Clean 1, followed by immersion in dilute nitric acid (0.1M, 30 min). This method was effective at removing an alkanethiol self-assembled monolayer of 11-mercaptoundecanoic acid; after six recycles, no loss in optical sensitivity was detected with minimal changes in the gold film thickness (−10%), hole area (−10%), and hole circularity (+6%).

Spatial Organization of Hydroxyapatite Nanorods on a Substrate via a Biomimetic Approach

The growth of uniform single-crystal HAp (hydroxyapatite) nanorods on specific sites on a substrate is achieved. HAp nanorods were synthesized under hydrothermal conditions in a solution containing calcium and phosphate ions, urea as a source of base, and with gelatin as a structure directing agent. The nanorods are hundreds of micrometers in length and 50–200 nm in width. Without gelatin, platelike HAp crystals were formed. To grow these uniform HAp nanorods on specific sites on a substrate, the surface of a gold-coated glass slide was coated by a self-assembled monolayer of 11-mercaptoundecanoic acid by stamping with a patterned PDMS (polydimethylsiloxane) stamp. The HAp nanorods were grown exclusively in the carboxylate functionalized regions. Selective area electron diffraction analyses show that the single-crystal HAp nanorods formed on the substrate grow along the crystallographic c-axis. In comparison to the nonpatterned sample, an increase of the X-ray diffraction intensity ratio of (00l)/(h00) peaks in the patterned sample indicates that nanorods are preferentially oriented with their c-axes perpendicular to the substrate, which was confirmed by SEM cross-sectional images. In this strategy, both the morphology and the deposition sites of HAp nanorods are controlled.

Analytical Expressions for Proton Transfer Voltammetry: Analogy to Surface Redox Voltammetry with Frumkin Interactions

Theory for interfacial proton transfer voltammetry of a molecular film containing any acid/base loading has been developed under equilibrium conditions. Diagnostic criteria to disentangle the interplay between diffuse layer and ionization effects are outlined. Easy-to-use analytical expressions for the voltammetric features are derived for the particular case of an invariant diffuse layer effect, which turn out to be entirely analogous to those for a surface redox conversion with Frumkin interactions. It is demonstrated that, regardless of the electrolyte concentration, significant ionization of the external acid groups located nearby the diffuse layer is sufficient for the fulfillment of this relevant particular case. A strategy is outlined to determine the amount, the intrinsic pKa, and the burial depth of the voltammetrically active groups from the surface concentration dependence of the main voltammetric features. Self-assembled monolayers of 11-mercaptoundecanoic acid deposited on Au(111), containing higher amounts of buried carboxylic groups than previously reported, have been studied to assess more critically the influence of electrostatic effects on the ionization process. Preliminary evidence suggests that the protonation/deprotonation voltammetric wave involves physisorbed rather than chemisorbed thiol molecules. Application of the present theoretical approach to this system reveals that the voltammetrically active carboxylic groups are located close to the electrode surface and become more acidic upon increasing their surface concentration.