Antonio Riul

Exploiting Distinct Molecular Architectures of Ultrathin Films Made with Iron Phthalocyanine for Sensing

The possibility of generating distinct film properties from the same material is crucial for a number of applications, which can only be achieved by controlling the molecular architecture. In this paper we demonstrate as a proof-of-principle that ultrathin films produced from iron phthalocyanine (FePc) may be used to detect trace amounts of copper ions in water, where advantage was taken of the cross sensitivity of the sensing units that displayed distinct electrical properties. The ultrathin films were fabricated with three methods, namely physical vapor deposition (PVD), Langmuir-Blodgett (LB), and electrostatic layer-by-layer (LbL) techniques, where for the latter tetrasulfonated phthalocyanine was used (FeTsPc). PVD and LB films were more homogeneous than the LbL films at both microscopic and nanoscopic scales, according to results from micro-Raman spectroscopy and atomic force microscopy (AFM), respectively. From FTIR spectroscopy data, these more homogeneous films were found to have FePc molecules oriented preferentially, tilted in relation to the substrate surface, while FeTsPc molecules were isotropically distributed in the LbL films. Impedance spectroscopy measurements with films adsorbed onto interdigitated gold electrodes indicated that the electrical response depends on the type of film-forming method and varies with incorporation of copper ions in aqueous solutions. Using principal component analysis (PCA), we were able to exploit the cross sensitivity of the sensing units and detect copper ions (Cu(2+)) down to 0.2 mg/L, not only in ultrapure water but also in distilled and tap water. This level of sensitivity is sufficient for quality control of water for human consumption, with a fast, low-cost method.

Coupling Surface-Enhanced Resonance Raman Scattering and Electronic Tongue as Characterization Tools to Investigate Biological Membrane Mimetic Systems

The surface-enhanced Raman scattering (SERS) effect and sensor and biosensor analyses are widely applied to investigate drug-biomolecule interactions or to detect trace amount of analytes. In this work, surface-enhanced resonance Raman scattering (SERRS) and an electronic tongue system using impedance spectroscopy were brought together, combining sensitivity and structural level information. Taking advantage of the use of layer-by-layer (LbL) films of phospholipids as biological membrane mimetic systems, cardiolipin (CLP) and dipalmitoyl phosphatidyl glycerol (DPPG) were applied as transducers onto Pt interdigitated electrodes forming an array of sensing units. This e-tongue system was able to detect the phenothiazine methylene blue (MB) below nanomolar concentrations. SERRS was applied to investigate the MB molecular arrangement (monomers or aggregates) when in contact with the phospholipids at trace levels of concentration. The key point was the adsorption of Ag nanoparticles (AgNPs) within the phospholipid LbL films. This approach did not compromise the e-tongue performance and allowed the recording of in situ SERRS spectra for the LbL films after immersion into MB aqueous solutions. The detection of MB through SERRS gave similar results to those reported in the literature but now with an unprecedented sensitivity.

Strategies to Optimize Biosensors Based on Impedance Spectroscopy to Detect Phytic Acid Using Layer-by-Layer Films

Impedance spectroscopy has been proven a powerful tool for reaching high sensitivity in sensor arrays made with nanostructured films in the so-called electronic tongue systems, whose distinguishing ability may be enhanced with sensing units capable of molecular recognition. In this study we show that for optimized sensors and biosensors the dielectric relaxation processes involved in impedance measurements should also be considered, in addition to an adequate choice of sensing materials. We used sensing units made from layer-by-layer (LbL) films with alternating layers of the polyeletrolytes, poly(allylamine) hydrochloride (PAH) and poly(vinyl sulfonate) (PVS), or LbL films of PAH alternated with layers of the enzyme phytase, all adsorbed on gold interdigitate electrodes. Surprisingly, the detection of phytic acid was as effective in the PVS/PAH sensing system as with the PAH/phytase system, in spite of the specific interactions of the latter. This was attributed to the dependence of the relaxation processes on nonspecific interactions such as electrostatic cross-linking and possibly on the distinct film architecture as the phytase layers were found to grow as columns on the LbL film, in contrast to the molecularly thin PAH/PVS films. Using projection techniques, we were able to detect phytic acid at the micromolar level with either of the sensing units in a data analysis procedure that allows for further optimization.

Toward the Optimization of an e-Tongue System Using Information Visualization: A Case Study with Perylene Tetracarboxylic Derivative Films in the Sensing Units

The wide variety of molecular architectures used in sensors and biosensors and the large amount of data generated with some principles of detection have motivated the use of computational methods, such as information visualization techniques, not only to handle the data but also to optimize sensing performance. In this study, we combine projection techniques with micro-Raman scattering and atomic force microscopy (AFM) to address critical issues related to practical applications of electronic tongues (e-tongues) based on impedance spectroscopy. Experimentally, we used sensing units made with thin films of a perylene derivative (AzoPTCD acronym), coating Pt interdigitated electrodes, to detect CuCl(2) (Cu(2+)), methylene blue (MB), and saccharose in aqueous solutions, which were selected due to their distinct molecular sizes and ionic character in solution. The AzoPTCD films were deposited from monolayers to 120 nm via Langmuir-Blodgett (LB) and physical vapor deposition (PVD) techniques. Because the main aspects investigated were how the interdigitated electrodes are coated by thin films (architecture on e-tongue) and the film thickness, we decided to employ the same material for all sensing units. The capacitance data were projected into a 2D plot using the force scheme method, from which we could infer that at low analyte concentrations the electrical response of the units was determined by the film thickness. Concentrations at 10 μM or higher could be distinguished with thinner films--tens of nanometers at most--which could withstand the impedance measurements, and without causing significant changes in the Raman signal for the AzoPTCD film-forming molecules. The sensitivity to the analytes appears to be related to adsorption on the film surface, as inferred from Raman spectroscopy data using MB as analyte and from the multidimensional projections. The analysis of the results presented may serve as a new route to select materials and molecular architectures for novel sensors and biosensors, in addition to suggesting ways to unravel the mechanisms behind the high sensitivity obtained in various sensors.

Layer-by-layer assembly of functionalized reduced graphene oxide for direct electrochemistry and glucose detection

We report an electrochemical glucose biosensor made with layer-by-layer (LbL) films of functionalized reduced graphene oxide (rGO) and glucose oxidase (GOx). The LbL assembly using positively and negatively charged rGO multilayers represents a simple approach to develop enzymatic biosensors. The electron transport properties of graphene were combined with the specificity provided by the enzyme. rGO was obtained and functionalized using chemical methods, being positively charged with poly(diallyldimethylammonium chloride) to form GPDDA, and negatively charged with poly(styrene sulfonate) to form GPSS. Stable aqueous dispersions of GPDDA and GPSS are easily obtained, enabling the growth of LbL films on various solid supports. The use of graphene in the immobilization of GOx promoted Direct Electron Transfer, which was evaluated by Cyclic Voltammetry. Amperometric measurements indicated a detection limit of 13.4μmol·L(-1) and sensitivity of 2.47μA·cm(-2)·mmol(-1)·L for glucose with the (GPDDA/GPSS)1/(GPDDA/GOx)2 architecture, whose thickness was 19.80±0.28nm, as determined by Surface Plasmon Resonance (SPR). The sensor may be useful for clinical analysis since glucose could be detected even in the presence of typical interfering agents and in real samples of a lactose-free milk and an electrolyte solution to prevent dehydration.

On the distinct molecular architectures of dipping- and spray-LbL films containing lipid vesicles

The introduction of spraying procedures to fabricate layer-by-layer (LbL) films has brought new possibilities for the control of molecular architectures and for making the LbL technique compliant with industrial processes. In this study we show that significantly distinct architectures are produced for dipping and spray-LbL films of the same components, which included DODAB/DPPG vesicles. The films differed notably in their thickness and stratified nature. The electrical response of the two types of films to aqueous solutions containing erythrosin was also different. With multidimensional projections we showed that the impedance for the DODAB/DPPG spray-LbL film is more sensitive to changes in concentration, being therefore more promising as sensing units. Furthermore, with surface-enhanced Raman scattering (SERS) we could ascribe the high sensitivity of the LbL films to adsorption of erythrosin.

Information Visualization and Feature Selection Methods Applied to Detect Gliadin in Gluten-Containing Foodstuff with a Microfluidic Electronic Tongue

The fast growth of celiac disease diagnosis has sparked the production of gluten-free food and the search for reliable methods to detect gluten in foodstuff. In this paper, we report on a microfluidic electronic tongue (e-tongue) capable of detecting trace amounts of gliadin, a protein of gluten, down to 0.005 mg kg-1 in ethanol solutions, and distinguishing between gluten-free and gluten-containing foodstuff. In some cases, it is even possible to determine whether gluten-free foodstuff has been contaminated with gliadin. That was made possible with an e-tongue comprising four sensing units, three of which made of layer-by-layer (LbL) films of semiconducting polymers deposited onto gold interdigitated electrodes placed inside microchannels. Impedance spectroscopy was employed as the principle of detection, and the electrical capacitance data collected with the e-tongue were treated with information visualization techniques with feature selection for optimizing performance. The sensing units are disposable to avoid cross-contamination as gliadin adsorbs irreversibly onto the LbL films according to polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) analysis. Small amounts of material are required to produce the nanostructured films, however, and the e-tongue methodology is promising for low-cost, reliable detection of gliadin and other gluten constituents in foodstuff.

A study on Langmuir monolayers of methacrylate homo- and copolymers derivatized with disperse red dyes

We report on the organization of different disperse red dye derivatized methacrylate homo- and copolymers as Langmuir monolayers at the air-water interface. The monolayers were investigated using surface pressure and surface potential isotherms. Methacrylic homopolymers containing disperse red-1 (HPDR1) and disperse red-13 (HPDR13) and methacrylic copolymers with different mole percentages of the dye and hydroxyethyl spacer groups were synthesized and characterized. A comparison of the monolayer characteristics of HPDR1 and HPDR13 revealed the influence of the chlorine substitution in the aromatic ring of the dye. Studies with copolymers indicated a clear transition in the monolayer behavior with the change of mole percentage of the dye incorporated in the polymer. While copolymers with low dye content (up to about 5 mole%) presented an expanded monolayer, the copolymers containing higher dye content (12% and above) formed a condensed monolayer similar to that of the homopolymer. These results indicated the critical role of the dye component in the polymer chain in forming the monolayer at the air-water interface.

Ultra-Thin Films of Reduced Graphene Oxide (RGO) Nanoplatelets Functionalized with Different Organic Materials

This work aims the functionalization of reduced graphene oxide nanoplatelets with chitosan (G-chitosan) and also with poly(styrenesulfonic acid) (GPSS), thus forming stable, dispersed aqueous solut ...

Experimental and computational investigation of reduced graphene oxide nanoplatelets stabilized in poly(styrene sulfonate) sodium salt

The production of large-area interfaces and the use of scalable methods to build up designed nanostructures generating advanced functional properties are of high interest for many materials science applications. Nevertheless, large-area coverage remains a major problem even for pristine graphene, and here we present a hybrid, composite graphene-like material soluble in water that can be exploited in many areas such as energy storage, electrodes fabrication, selective membranes and biosensing. Graphene oxide (GO) was produced by the traditional Hummers’ method being further reduced in the presence of poly(styrene sulfonate) sodium salt (PSS), thus creating stable reduced graphene oxide (rGO) nanoplatelets wrapped by PSS (GPSS). Molecular dynamics simulations were carried out to further clarify the interactions between PSS molecules and rGO nanoplatelets, with calculations supported by Fourier transform infrared spectroscopy analysis. The intermolecular forces between rGO nanoplatelets and PSS lead to the formation of a hybrid material (GPSS) stabilized by van der Waals forces, allowing the fabrication of high-quality layer-by-layer (LbL) films with poly(allylamine hydrochloride) (PAH). Raman and electrical characterizations corroborated the successful modifications in the electronic structures from GO to GPSS after the chemical treatment, resulting in (PAH/GPSS) LbL films four orders of magnitude more conductive than (PAH/GO).