State-of-the-Art of (Bio)Chemical Sensor Developments in Analytical Spanish Groups

(Bio)chemical sensors are one of the most exciting fields in analytical chemistry today. The development of these analytical devices simplifies and miniaturizes the whole analytical process. Although the initial expectation of the massive incorporation of sensors in routine analytical work has been truncated to some extent, in many other cases analytical methods based on sensor technology have solved important analytical problems. Many research groups are working in this field world-wide, reporting interesting results so far. Modestly, Spanish researchers have contributed to these recent developments. In this review, we summarize the more representative achievements carried out for these groups. They cover a wide variety of sensors, including optical, electrochemical, piezoelectric or electro-mechanical devices, used for laboratory or field analyses. The capabilities to be used in different applied areas are also critically discussed

Carbon Paste Macrocycle Doped Composite Electrodes for the Selective Electrochemical Detection of Dopamine

The neurotransmitter dopamine (DA) has shown to play a very important role in the functioning of the central nervous system. Thus, the determination of DA is of great importance in the fields of neurochemistry and biomedical chemistry. In this thesis, a number of carbon paste electrodes modified with macrocycles for the electrochemical detection of DA is reported. The different macrocycles employed were based on cyclodextrin derivatives and consist of sulfated β-CD (S-β-CD), carboxymethyl β-CD (CM-β-CD), Ferrocene complex β-CD (Fc-β-CD), Heptakis 6-deoxy-6-(1-(4,5-dicarboxyl)-1,2,3-triazolyl)-β-CD (CD6.6) and Heptakis (6-(4-hydroxymethyl-1H-[1, 2, 3] triazol-1-yl)-6-deoxy)-β-cyclodextrin (CD6.7). The fabricated electrodes were characterized by using surface techniques and electrochemical methods such as energy dispersive X-ray (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (ESI) and cyclic voltammetry (CV). The detection of DA at all modified electrodes (except for CD6.7) resulted in an enhancement of the oxidation signal response over that of the bare electrode. High performance of the electrochemical detection of DA was obtained at S-β-CD modified CPE such as a wide concentration range (from 5 × 10-7 M to 5 × 10-4 M) and low detection limit (1.33 × 10-7 M). It was shown that the sensitivity of the developed sensor towards the detection of DA depends on the amount of S-β-CD incorporated within the paste. The optimum sensor architecture was made by impregnating 0.545 g S-β-CD in a carbon paste containing 0.71 g graphite and 200 μL silicone oil. In addition, graphite was replaced by graphene and the electrochemical behaviour of DA at the S-β-CD modified graphene electrode was investigated by DPV. The results showed that the S-β-CD modified graphene electrode exhibited excellent electrochemical oxidation of DA in phosphate buffer solution (pH 6.8) compared to the bare graphite paste electrode. CD6.6 and CD6.7 were synthesised in order to exhibit further increase in the signal responses of DA. The formation of inclusion complexes of CD6.6 and CD6.7 with DA was studied by 1H-NMR spectroscopy. A 2:1 and 1:1 stoichiometry was obtained in the case of CD6.6:DA and CD6.7:DA, respectively. The bare GPE was modified with CD6.6 and the electrode was miniaturised from macro to micro-size level. This configuration also was shown to improve the detection of DA (3 × 10-7 M). This thesis also aimed to utilise these modified electrodes to enhance the selectivity for DA over two interferents, ascorbic acid (AA) and serotonin (5-HT). Improvements in the selectivity of DA were obtained at the S-β-CD modified GPE as AA was excluded at the electrode surface. Moreover, the results showed that DA, AA and 5-HT in coexisting solutions can be simultaneously oxidised at significantly different potentials in the presence of S-β-CD modified CPE. The voltammetric response of the three compounds could be completely separated at the modified electrode using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques at optimal conditions. DPV provided larger peak potential separations and higher response sensitivities of DA, AA, and 5-HT compared to CV

Polymer based electrospun nanofibers as diagnostic probes for the detection of toxic metal ions in water

The thesis presents the development of polymer based electrospun nanofibers as diagnostic probes for the selective detection of toxic metal ions in water. Through modification of the chemical characteristics of nanofibers by pre- and post-electrospinning treatments, three different diagnostic probes were successfully developed. These were the fluorescent pyridylazo-2-naphthol-poly(acrylic acid) nanofiber probe, the colorimetric probe based on glutathione-stabilized silver/copper alloy nanoparticles and the colorimetric probe based on 2-(2’-Pyridyl)-imidazole functionalized nanofibers. The probes were characterized by Fourier transform infrared spectroscopy (FTIR), Energy dispersive x-ray spectroscopy (EDX), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The fluorescent nanofiber probe was developed towards the determination of Ni²⁺. Covalently functionalized pyridylazo-2-naphthol-poly(acrylic acid) polymeric nanofibers were employed. The solid state Ni²⁺ probe exhibited a good correlation between the fluorescence intensity and nickel concentration up to 1.0 mg/mL based on the Stern-Volmer mechanism. The detection limit of the nanofiber probe was found to be 0.07 ng/mL. The versatility of the fluorescent probe was demonstrated by affording a simple, rapid and selective detection of Ni²⁺ in the presence of other competing metal ions by direct analysis without employing any sample handling steps. For the second part of the study, a simple strategy based on the in-situ synthesis of the glutathione stabilized silver/copper alloy nanoparticles (Ag/Cu alloy NPs) in nylon 6 provided a fast procedure for fabricating a colorimetric probe for the detection of Ni²⁺ in water samples. The electrospun nanofiber composites responded to Ni²⁺ ions but did not suffer any interference from the other metal ions. The effect of Ni²⁺ concentration on the nanocomposite fibers was considered and the “eye-ball” limit of detection was found to be 5.8 μg/mL. Lastly, the third probe was developed by covalently linking an imidazole derivative; 2-(2′-Pyridyl)-imidazole (PIMH) to Poly(vinylbenzyl chloride) (PVBC) and nylon 6 nanofibers by post-electrospinning treatments using a wet chemical method and graft copolymerization technique, respectively. The post-electrospinning modifications of the nanofibers were achieved without altering their fibrous morphology. The color change to red-orange in the presence of Fe²⁺ for both the grafted nylon 6 (white) and the chemically modified PVBC (yellow) nanofibers was instantaneous. The developed diagnostic probes exhibited the desired selectivity towards the targeted metal ions

Cyclodextrins as Supramolecular Recognition Systems: Applications in the Fabrication of Electrochemical Sensors

Supramolecular chemistry, although focused mainly on noncovalent intermolecular and intramolecular interactions, which are considerably weaker than covalent interactions, can be employed to fabricate sensors with a remarkable affinity for a target analyte. In this review the development of cyclodextrin-based electrochemical sensors is described and discussed. Following a short introduction to the general properties of cyclodextrins and their ability to form inclusion complexes, the cyclodextrin-based sensors are introduced. This includes the combination of cyclodextrins with reduced graphene oxide, carbon nanotubes, conducting polymers, enzymes and aptamers, and electropolymerized cyclodextrin films. The applications of these materials as chiral recognition agents and biosensors and in the electrochemical detection of environmental contaminants, biomolecules and amino acids, drugs and flavonoids are reviewed and compared. Based on the papers reviewed, it is clear that cyclodextrins are promising molecular recognition agents in the creation of electrochemical sensors, chiral sensors, and biosensors. Moreover, they have been combined with a host of materials to enhance the detection of the target analytes. Nevertheless, challenges remain, including the development of more robust methods for the integration of cyclodextrins into the sensing unit

Diffusion of tin from TEC-8 conductive glass into mesoporous titanium dioxide in dye sensitized solar cells

The photoanode of a dye sensitized solar cell is typically a mesoporous titanium dioxide thin film adhered to a conductive glass plate. In the case of TEC-8 glass, an approximately 500 nm film of tin oxide provides the conductivity of this substrate. During the calcining step of photoanode fabrication, tin diffuses into the titanium dioxide layer. Scanning Electron Microscopy and Electron Dispersion Microscopy are used to analyze quantitatively the diffusion of tin through the photoanode. At temperatures (400 to 600 °C) and times (30 to 90 min) typically employed in the calcinations of titanium dioxide layers for dye sensitized solar cells, tin is observed to diffuse through several micrometers of the photoanode. The transport of tin is reasonably described using Fick\u27s Law of Diffusion through a semi-infinite medium with a fixed tin concentration at the interface. Numerical modeling allows for extraction of mass transport parameters that will be important in assessing the degree to which tin diffusion influences the performance of dye sensitized solar cells

Development of glucose biosensors using nanostructured composites

Developing a biosensor capable of measuring glucose and other whole blood analytes for monitoring diabetes has been a major challenge for over four decades. In this thesis, an attempt has been made to develop three different novel metallophthalocyanine-based (MPcs) biosensors for this purpose. Three novel enzymatic biosensors have been fabricated to monitor glucose concentration and other whole blood analytes in vitro. The first fabricated biosensor is based on conducting multifunctional hydrogel (PAA-rGO/VS-PANI/LuPc2/GOx-MFH) utilising reduced graphene oxide (rGO) and lutetium phthalocyanine (LuPc2) dissolved in chloroform to produce three-dimensional (3D) matrix as platform to enhance the sensing performance. Utilising the aqueous properties of a novel water-soluble iron phthalocyanine (FePc) derivative on the other hand, added more simplicity for the fabrication process of another biosensor (PAA-CP/GPL-FePc/GOx-CH) for enzymatic detection of glucose. Graphene nanoplatelets (GPL) have been attached non-covalently to the FePc resulting in a conducting hydrogel-based platform (CH). A third novel bioprobe (SiO2(LuPc2)-PANI(PVIA)/GOx-CNB) based on silica nanoparticles (SiO2) grafted polyaniline (PANI) has formed a conducting nanobeads (CNB)-based biosensor. The latter is employed as an enzymatic biosensor platform for the detection of glucose with enhanced sensitivity. Full characterisation has been carried out for all the raw studied materials as well as prepared biosensing platforms. UV-Visible and FT-IR spectroscopies as well as SEM, TEM, XRD, and EDX have been employed in order to help gaining full understanding of the nature and properties of the studied materials. The electrochemical properties of the newly developed biosensing platforms have been fully studied using common analytical methods such as cyclic voltammetry (CV), amperometry, and electrochemical impedance spectroscopy (EIS). The freeze dry system alongside the Brunauer-Emmett-Teller (BET) method were employed to characterise the surface area of the produced platforms. The sensitivity of MFH biosensor studied in the range 2-12 mM of glucose is found to fall in the region of 15.31 μA mM−1 cm−2 with low detection limit of 25 μM. The biosensor based on CH platform exhibited a broad linear behaviour when glucose in the range 1-20 mM is studied, with high sensitivity of 18.11 μA mM−1 cm−2 and low detection limit of 1.1503 ng/mL. The conducting nanobeads-based biosensor (CNB) has led to a further improvement the sensitivity of glucose detection (38.53 μA mM−1 cm−2) with wide linear range of 1-16 mM and detection limit of 0.1 mM. All three biosensing platforms have exhibited excellent selectivity when examined against whole blood components

Ionophoric and aptameric recognition-modulated electroactive polyaniline films for the determination of tetrodotoxin

Philosophiae Doctor - PhDTetrodotoxin (TTX) is a nonpeptidic neurotoxin with a high rate of food poisoning mortality (60%) that has been associated with the consumption of diets from puffer fish and mud snails harbouring TTX-producing bacteria. As this neurotoxin has no known antidote and could not be mitigated by cooking, the only way for safety appears to be the detection of TTX-contaminated fishes at the points of harvest and control. The overall aim of this study was to develop amperometric and impedimetric sensors for TTX based on ionophores and aptamer immobilised on the modified conducting electroactive polyaniline (PANI)/electrode. The undoped polyaniline and poly(4-styrenesulfonic acid) (PSSA) doped electroactive polyanilines were prepared in perchloric acid/acetonitrile and phosphoric acid respectively by electrochemical oxidative polymerisation. Two types of electropolymerisation were applied to prepare the neutral and p-doped PANI−PSSA films composites. The dynamic electroinactivity of TTX was studied which revealed that TTX is not electrochemically active on bare Au, GC, Pt, PG, Ni, Ti and BDD (Boron dopeddiamond) electrodes in acetate buffer pH 4.8. Using ion transfer voltammetry and UV-Vis analysis, the complexation of TTX with two neutral ionophores (sodium ionophore X (NaX) and dibenzo-18-crown6 (B18C6)) was investigated. The cyclic voltammograms (CVs) recorded from ion transfer voltammetry presented no redox peak and no increasing/decreasing current was observed which indicates that no TTX ions transfer from the liquid to the organic phase. In addition, the absorption spectra of the mixture of TTX/NaX and TTX/B18C6 presented the same absorption bands recorded for NaX and B18C6 respectively. Three absorptions bands at 250.4, 278.3, and 370.6 nm for NaX and two at 222.03 and 274.10 nm for B18C6 were observed before and after mixing TTX with NaX and TTX with B18C6 separately. No chemical reaction occurred between the TTX and both ionophores, therefore, sodium ionophore X and dibenzo-18-crown-6 did not form a complex with TTX. Thus, TTX ion sensor cannot be developed based on these two neutral compounds. The electrodynamics of the PANI and PANI−PSSA films electropolymerised on the bare precious metal electrodes were also investigated through various electrochemical techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies in sodium phosphate (SPB) and acetate (OAc) buffer revealed that both neutral and p-doped films synthesized were thin (thickness L < 5 nm in acetate buffer and L < 10 nm in sodium phosphate buffer) film polymers

Advanced Electrochemical and Opto-Electrochemical Biosensors for Quantitative Analysis of Disease Markers and Viruses

The recent global events of the SARS-CoV-2 pandemic in 2020 have alerted the world to the urgent need to develop fast, sensitive, simple, and inexpensive analytical tools that are capable of carrying out a large number of quantitative analyses, not only in centralized laboratories and core facilities but also on site and for point-of-care applications. In particular, in the case of immunological tests, the required sensitivity and specificity is often lacking when carrying out large-scale screening using decentralized methods, while a centralized laboratory with qualified personnel is required for providing quantitative and reliable responses. The advantages typical of electrochemical and optical biosensors (low cost and easy transduction) can nowadays be complemented in terms of improved sensitivity by combining electrochemistry (EC) with optical techniques such as electrochemiluminescence (ECL), EC/surface-enhanced Raman spectroscopy (SERS), and EC/surface plasmon resonance (SPR). This Special Issue addresses existing knowledge gaps and aids in exploring new approaches, solutions, and applications for opto-electrochemical biosensors in the quantitative detection of disease markers, such as cancer biomarkers proteins and allergens, and pathogenic agents such as viruses. Included are seven peer-reviewed papers that cover a range of subjects and applications related to the strategies developed for early diagnosis