Structural and surface studies of luminescent Ca/Eu phosphate nanomaterials: From the bulk to surface features.

Three luminescent Eu-containing phosphate materials (Ca-doped europium phosphate monohydrate, Eu-doped carbonated-apatite, and europium phosphate monohydrate) were prepared and analyzed on the level of bulk structure and surface properties and compared to the biomimetic non-luminescent counterpart hydroxyapatite. Europium-containing phosphate materials exhibited nanosized dimensions but different luminescence emissions and luminescence lifetimes depending on their crystalline structures (i.e., lanthanide phosphate or apatites) and chemical composition. The introduction of Eu in the crystal lattice leads to a notable decrease in the overall Lewis acidity of the surface cationic sites detected by CO probing. Further, the mixed Eu/Ca-containing materials surfaces were found to be very similar to the reference hydroxyapatite in terms of water adsorption energy, while the pure europium phosphate resulted to have the notably higher energy values of direct interaction of water molecules with the surface cations with no detected propagation of this effect towards water overlayers.

Polycyclic aromatic hydrocarbons in edible oils: An overview on sample preparation, determination strategies, and relative abundance of prevalent compounds.

Polycyclic aromatic hydrocarbons (PAHs) are food contaminants whose presence in foodstuffs is especially alarming due to their carcinogenic character. These substances are highly lipophilic and thus, unsafe levels of these compounds have been found in edible fats and oils. Efficient methodologies to determine such molecules in lipidic matrixes are therefore essential. In this review, a detailed description of the analytical methods for the determination of PAHs in vegetable oils from the last 15 years has been provided. Particular emphasis has been placed on innovative sample treatments, which facilitate and shorten the pretreatment of the oils. Finally, results from recent investigations have been reviewed and studied in depth, in order to elucidate which PAHs are most commonly found in vegetable oils.

Self-Referenced Multifrequency Phase-Resolved Luminescence Spectroscopy.

Phase-resolved luminescence chemical sensors provide the analyte determination based on the estimation of the luminescence lifetime. The lifetime is estimated from an analysis of the amplitudes and/or phases of the excitation and emission signals at one or several modulation frequencies. This requires recording both the excitation signal (used to modulate the light source) and the emission signal (obtained from an optical transducer illuminated by the luminescent sensing phase). The excitation signal is conventionally used as reference, in order to obtain the modulation factor (the ratio between the emission and the excitation amplitudes) and/or the phase shift (the difference between the emission and the excitation phases) at each modulation frequency, which are used to estimate the luminescence lifetime. In this manuscript, we propose a new method providing the luminescence lifetimes (based either on amplitudes or phases) using only the emission signal (i.e., omitting the excitation signal in the procedure). We demonstrate that the luminescence lifetime can be derived from the emission signal when it contains at least two harmonics, because in this case the amplitude and phase of one of the harmonics can be used as reference. We present the theoretical formulation as well as an example of application to an oxygen measuring system. The proposed self-referenced lifetime estimation provides two practical advantages for luminescence chemical sensors. On one hand, it simplifies the instrument architecture, since only one analog-to-digital converter (for the emission signal) is necessary. On the other hand, the self-referenced estimation of the lifetime improves the robustness against degradation of the sensing phase or variations in the optical coupling, which reduces the recalibration requirements when the lifetimes are based on amplitudes.

A Polynomial-Exponent Model for Calibrating the Frequency Response of Photoluminescence-Based Sensors.

In this work, we propose a new model describing the relationship between the analyte concentration and the instrument response in photoluminescence sensors excited with modulated light sources. The concentration is modeled as a polynomial function of the analytical signal corrected with an exponent, and therefore the model is referred to as a polynomial-exponent (PE) model. The proposed approach is motivated by the limitations of the classical models for describing the frequency response of the luminescence sensors excited with a modulated light source, and can be considered as an extension of the Stern-Volmer model. We compare the calibration provided by the proposed PE-model with that provided by the classical Stern-Volmer, Lehrer, and Demas models. Compared with the classical models, for a similar complexity (i.e., with the same number of parameters to be fitted), the PE-model improves the trade-off between the accuracy and the complexity. The utility of the proposed model is supported with experiments involving two oxygen-sensitive photoluminescence sensors in instruments based on sinusoidally modulated light sources, using four different analytical signals (phase-shift, amplitude, and the corresponding lifetimes estimated from them).

Eu-Doped Citrate-Coated Carbonated Apatite Luminescent Nanoprobes for Drug Delivery.

In the field of Nanomedicine, there is an increasing demand for new inorganic nanophosphors with low cytotoxicity and efficient loading-release ability of drugs for applications in bioimaging and drug delivery. This work assesses the potentiality of matured Eu-doped citrate-coated carbonated apatite nanoparticles to be used as theranostic platforms, for bioimaging, as luminescent nanoprobes, and for drug delivery applications, using Doxorubicin as a model drug. The drug adsorption isotherm fits the Langmuir-Freundlich (LF) model, showing that the Eu:cit-cAp nanoparticles can carry a maximum of 0.29 ± 0.02 mg Doxo mg Eu:cit-cAp-1 (Qmax). The affinity constant KFL for this binding is 44 ± 2 mL mg-1, and the cooperativity coefficient r is 6 ± 1. The nanoparticle suspensions presented charge reversion from negative to positive after loading with Doxo as revealed by the ζ-potential versus pH characterization. The release of drug from the loaded nanoparticles was found to be strongly pH-dependent, being around 5 wt % at physiological pH 7.4 and 20 wt % at pH 5, in experiments lasting 24 h. Luminescence spectroscopic measurements of Doxo-loaded nanoparticles revealed the increase of luminescence with a decrease in the amount of adsorbed Doxo, due to the so-called inner filter effect. The nanoparticles free of Doxo were cytocompatible when interacted with two human cell lines derived respectively from a gastric carcinoma (GTL-16), and a hepatocarcinoma (Huh7), while Doxo-loaded nanoparticles displayed significant toxicity in a dose-dependent relationship. Therefore, the new nanoassemblies might have a dual function, as nanoprobes in bioimaging by detecting the fate of the nanoparticles in biological environments, and for monitoring the delivery of the drug in such environments, by measuring the rise of the luminescence provided by the desorption of Doxo.

Development of a folic acid molecularly imprinted polymer and its evaluation as a sorbent for dispersive solid-phase extraction by liquid chromatography coupled to mass spectrometry.

This work shows the development of a molecularly imprinted polymer to determine folic acid (FA) in food extracts by using dispersive solid-phase extraction and liquid chromatography coupled to mass spectrometry (LC-MS). Herewith, combinations of monomers (methacrylic acid (MAA), 4-vinylpyridine (4VPy) and vinylbenzyl trimethylammonium chloride (VBTMAC)) and crosslinkers (ethylene glycol dimethacrylate (EGDMA) and divinyl benzene (DVB)) were tested in appropriate solvents. Isotherm tests revealed that the MIP with the highest affinity was obtained by combining VBTMAC and EGDMA. Having checked the appropriate template-monomer-crosslinker ratio, the FA MIP was analyzed for its kinetic and equilibrium binding properties, proving very high affinity (more than 2.5 mmol g-1) and MIP/NIP ratio (up to 37). The FA MIP was used to selectively isolate the compound of interest from lettuce and cookies matrices using a dispersive solid-phase extraction protocol (which exhibited appropriate recovery and repeatability, ≥79.50% and ≤13.41 (%RSD in terms of area values), respectively, as well as absence of matrix effect); the resulting extracts were analyzed by a rapid and reliable LC-MS method.

Evaluation of two sterically directed attachments of biomolecules on a coaxial nanofibre membrane to improve the development of optical biosensors.

In this study, we have optimised the sterically directed attachment of biomolecules on the surface of coaxial membranes prepared by co-electrospinning which have been proved to be a material with very high performance for the development of biosensors with optical oxygen transduction. Uricase has been used as model enzyme. Two sterically directed strategies: a) covalent attachment via maleimide, and b) affinity bonding via biotin-streptavidin interaction, have been tested in order to preserve the enzymatic activity of uricase and to improve the analytical figures of merits on the determination of uric acid. The best results were obtained with biotin-streptavidin affinity interaction and using a biotinylation reagent containing a polyethylene glycol chain. The developed biosensor showed high sensitivity towards uric acid with a detection limit of 0.5 µM, a quantification limit of 1.8 µM and linear range from 1.8 to 250 µM. The applicability of the membrane as biosensor with optical oxygen transduction was proved by determining uric acid in serum samples. The obtained results showed a good correlation (0.999) with those obtained by an external reference laboratory.

A multifunctional material based on co-electrospinning for developing biosensors with optical oxygen transduction.

A multifunctional material based on co-electrospinning has been developed as a basic material for the development of biosensors with optical oxygen transduction. It is based on coaxial nanofibres: inner fibres containing an oxygen sensitive dye and outer fibres containing aldehyde groups to allow the formation of Schiff bases with the amino groups of the enzyme. The resulting material preserves the oxygen sensing properties of the inner optical transducer as well as exhibits a high capacity for immobilizing molecules on its surface. Uricase has been selected as model enzyme and several parameters (temperature, pH, reaction time, buffer, and enzyme concentration) have been optimised to demonstrate the versatility of this novel multifunctional material in the development of biosensors with optical oxygen transduction for determining uric acid in serum samples. It suggests that the proposed multifunctional material can provide a promising multifunctional platform for biosensing applications.

High performance optical oxygen sensors based on iridium complexes exhibiting interchromophore energy shuttling.

A doubly pyrene-grafted bis-cyclometallated iridium complex with engineered electronically excited states demonstrates reversible electronic energy transfer between adjacent chromophores giving rise to extremely long-lived red luminescence in solution (τ = 480 µs). Time-resolved spectroscopic studies afforded determination of pertinent photophysical parameters including rates of energy transfer and energy distribution between constituent chromophores in the equilibrated excited molecule (ca. 98% on the organic chromophores). Incorporation into a nanostructured metal-oxide matrix (AP200/19) gave highly sensitive O2 sensing films, as the detection sensitivity was 200-300% higher than with the commonly used PtTFPP and approaches the sensitivity of the best O2-sensing dyes reported to date.

A novel optical biosensor for direct and selective determination of serotonin in serum by Solid Surface-Room Temperature Phosphorescence.

This paper describes a novel biosensor which combines the use of nanotechnology (non-woven nanofibre mat) with Solid Surface-Room Temperature Phosphorescence (SS-RTP) measurement for the determination of serotonin in human serum. The developed biosensor is simple and can be directly applied in serum; only requires a simple clean-up protocol. Therefore it is the first time that serotonin is analysed directly in serum with a non-enzymatic technique. This new approach is based on the covalent immobilization of serotonin directly from serum on a functional nanofibre material (Tiss®-Link) with a preactivated surface for direct covalent immobilization of primary and secondary amines, and the subsequent measurement of serotonin phosphorescent emission from the solid surface. The phosphorescent detection allows avoiding the interference from any fluorescence emission or scattering light from any molecule present in the serum sample which can be also immobilised on the nanofibre material. The determination of serotonin with this SS-RTP sensor overcomes some limitations, such as large interference from the matrix and high cost and complexity of many of the methods widely used for serotonin analysis. The potential applicability of the sensor in the clinical diagnosis was demonstrated by analysing serum samples from seven healthy volunteers. The method was validated with an external reference laboratory, obtaining a correlation coefficient of 0.997 which indicates excellent correlation between the two methods.

Novel optical sensing film based on a functional nonwoven nanofibre mat for an easy, fast and highly selective and sensitive detection of tryptamine in beer.

In this paper, the combination of Solid Surface-Room Temperature Phosphorescence (SS-RTP) and nanotechnology has led to a new approach in the detection of biogenic amines in complex matrices. This novel approach allows, for the first time, the direct determination of the concentration of tryptamine in beers. The novelty of the proposed optical sensor resides in its simplicity, rapidity, absence of complex chromatographic separation, sample clean-up, preconcentration, and derivatization protocols. Therefore, this novel methodology simplifies and reduces considerably the time and cost of the analysis, resolving the two major problems of the determination of tryptamine in beer up to now: low sensitivity and matrix effects. The proposed sensor is based on a novel white, uncharged, and non-luminescent functional nonwoven nanofibre mat (Tiss®-Link) formed by hydrophilic nanofibres of 300 nm of diameter functionalized with a high concentration of active vinyl groups (330 µmol g(-1)). It is used to carry out a kinetically controlled covalent immobilisation of tryptamine via Michael type-reaction. The transduction of the sensor is phosphorescence; the covalently immobilized tryptamine is quantified by SS-RTP, obtaining a detection limit of 6 ng mL(-1) with short response times (15 min). The applicability of the sensor was demonstrated by analysing tryptamine in 10 different varieties of beers, obtaining recovery percentages close to 100%.

Copper(I) complexes as alternatives to iridium(III) complexes for highly efficient oxygen sensing.

The complex [Cu(xantphos)(dmp)][PF6] (dmp = 2,9-dimethyl-1,10-phenanthroline) in a nanostructured metal oxyde matrix shows better sensitivity to oxygen (KSV = 9.74 ± 0.87 kPa(-1) between 0 and 1 kPa pO2 and 5.59 ± 0.15 kPa(-1) between 0 and 10 kPa pO2) than cyclometallated iridium complexes in the same conditions.

Improved multifrequency phase-modulation method that uses rectangular-wave signals to increase accuracy in luminescence spectroscopy.

We propose a novel multifrequency phase-modulation method for luminescence spectroscopy that uses a rectangular-wave modulated excitation source with a short duty cycle. It is used for obtaining more detailed information about the luminescence system: the information provided by different harmonics allows estimating a model for describing the global frequency response of the luminescent system for a wide range of analyte concentration and frequencies. Additionally, the proposed method improves the accuracy in determination of the analyte concentration. This improvement is based on a simple algorithm that combines multifrequency information provided by the different harmonics of the rectangular-wave signal, which can be easily implemented in existing photoluminescence instruments by replacing the excitation light source (short duty cycle rectangular signal instead of sinusoidal signal) and performing appropriate digital signal processing after the transducer (implemented in software). These claims have been demonstrated by using a well-known oxygen-sensing film coated at the end of an optical fiber [a Pt(II) porphyrin immobilized in polystyrene]. These experimental results show that use of the proposed multifrequency phase-modulation method (1) provides adequate modeling of the global response of the luminescent system (R(2) > 0.9996) and (2) decreases the root-mean-square error in analytical determination (from 0.1627 to 0.0128 kPa at 0.5 kPa O2 and from 0.9393 to 0.1532 kPa at 20 kPa O2) in comparison with a conventional phase-modulation method based on a sinusoidally modulated excitation source (under equal luminous power conditions).

Electrophoretic deposition as a new approach to produce optical sensing films adaptable to microdevices.

We report the fabrication of optical oxygen sensor films using electrophoretic deposition (EPD) of poly(styrene-co-maleic anhydride) nanoparticles containing the oxygen-sensitive dye platinum(ii) meso-tetra(pentafluorophenyl)porphine. Compared to other deposition methods, the EPD is simple and allows easy control over deposition, which is crucial for the implementation of optical sensing films in microdevices. By optimizing the synthesis of the functional nanoparticles, anodic EPD can be performed. The amount of deposited particles can be tuned by varying either the electrical potential or the deposition time. The sensing phases were characterized using a phase-modulation technique showing a Stern-Volmer constant (kSV1) between 45 and 52 bar(-1) for gas and of 20.72 bar(-1) in the aqueous phase without leaching of the particles from the surface. The small thickness of the layers lead to short response times (<0.4 s). This is the first time that polymeric optical sensing films have been obtained by EPD from dispersions of oxygen sensing nanoparticles.

New fluorescent pH sensors based on covalently linkable PET rhodamines.

A new class of rhodamines for the application as indicator dyes in fluorescent pH sensors is presented. Their pH-sensitivity derives from photoinduced electron transfer between non-protonated amino groups and the excited chromophore which results in effective fluorescence quenching at increasing pH. The new indicator class carries a pentafluorophenyl group at the 9-position of the xanthene core where other rhodamines bear 2-carboxyphenyl substituents instead. The pentafluorophenyl group is used for covalent coupling to sensor matrices by "click" reaction with mercapto groups. Photophysical properties are similar to "classical" rhodamines carrying 2'-carboxy groups. pH sensors have been prepared with two different matrix materials, silica gel and poly(2-hydroxyethylmethacrylate). Both sensors show high luminescence brightness (absolute fluorescence quantum yield Φ(F)≈0.6) and high pH-sensitivity at pH 5-7 which makes them suitable for monitoring biotechnological samples. To underline practical applicability, a dually lifetime referenced sensor containing Cr(III)-doped Al(2)O(3) as reference material is presented.

Synthesis of caffeic acid molecularly imprinted polymer microspheres and high-performance liquid chromatography evaluation of their sorption properties.

In the current work, a molecularly imprinted polymer (MIP) has been synthesised and used to enable the extraction of a naturally-occurring antioxidant from complex media. More specifically, we describe the first example of a caffeic acid (CA) MIP which has been synthesised in the form of well-defined polymer microspheres, and its use for the extraction of CA from fruit juice sample. The CA MIP was synthesised by precipitation polymerisation using 4-vinylpyridine as functional monomer, divinylbenzene-80 as crosslinker and acetonitrile:toluene (75/25, v/v) as porogen. The particle sizing and morphological characterisation of the polymers was carried out by means of scanning electron microscopy (narrow particle size distribution; ∼5 and 1.5 µm particle diameters for the MIP and NIP [non-imprinted polymer], respectively) and nitrogen sorption porosimetry (specific surface areas of 340 and 350 m(2)g(-1), and specific pore volumes of 0.17 and 0.19 cm(3)g(-1) for the MIP and NIP, respectively). The polymers were evaluated further by batch rebinding experiments, and from the derived isotherms their binding capacity and binding strength were determined (number of binding sites (N(K))=0.6 and 0.3 mmol g(-1) for the MIP and NIP, respectively, and apparent average adsorption constant (K(N))=10.0 and 1.6L mmol(-1) for the MIP and NIP, respectively). To evaluate the molecular recognition character of the MIP it was packed into a stainless steel column (50 mm × 4.6 mm i.d.) and evaluated as an HPLC-stationary phase. The mobile phase composition, flow rate, and the elution profile were then optimised in order to improve the peak shape without negatively affecting the imprinting factor (IF). Very interesting, promising properties were revealed. The imprinting factor (IF) under the optimised conditions was 11.9. Finally, when the imprinted LC column was used for the selective recognition of CA over eight related compounds, very good selectivity was obtained. This outcome enabled the direct extraction of CA in commercial apple juice samples with recoveries in excess of 81% and, rather significantly, without any need for a clean-up step prior to the extraction.

Synthesis of a novel polyurethane-based-magnetic imprinted polymer for the selective optical detection of 1-naphthylamine in drinking water.

The first polyurethane based magnetic-MIP for the selective detection of 1-naphthylamine (1-NA) in drinking water has been synthesised. The synthesis has been carried out in a two-step process: first,the incorporation of magnetite-coated-oleic acid nanoparticles (-Fe3O4-OA) into a lipophilic polymeric matrix (poly-MMA-co-EDMA) and second, the encapsulation of these magnetic seeds into the MIP structure by precipitation polymerisation. The mag-MIP was first RHTEM imaged showing a well-organised material with magnetite within the material and the imprinted polymer coating the magnetic core. Thereafter,it was evaluated by batch rebinding analysis and the derived Freundlich isotherm, calculating the number of binding sites (N(K(min)-K(max))=2.63 and 0.79 mmol g⁻¹, for mag-MIP and mag-NIP, respectively)and apparent average adsorption constant (K(K(min)-K(max))=3.31 and 3.06 mmol⁻¹, for mag-MIP and mag-NIP, respectively) showing a very effective imprinting process.We have also developed a magnetic optical sensor MIP by using an optical fiber coupled with a magnetic separator. An unexpected selectivity for 1-NA was revealed allowing the detection of this molecule in water, even in the presence of 4 structurally related compounds (2-naphthylamine, 1-naphthol, 2-naphthol and 1-naphthalenemethylamine), with a low limit of detection (LOD) = 18 ng mL⁻¹. Finally, we applied this new hybrid material to the analysis of 1-NA in tap and mineral waters, obtaining a 91.6%average recovery rate.

Oxygen sensing using microrobots.

We present a luminescence oxygen sensor incorporated in a wireless intraocular microrobot for minimally-invasive diagnosis. This microrobot can be accurately controlled in the intraocular cavity by applying magnetic fields. The microrobot consists of a magnetic body susceptible to magnetic fields and a sensor coating. This coating embodies Pt(II) octaethylporphine (PtOEP) dyes as the luminescence material and polystyrene as a supporting matrix, and it can be wirelessly excited and read out by optical means. The sensor works based on quenching of luminescence in the presence of oxygen. The excitation and emission spectrum, response time, and oxygen sensitivity of the sensor were characterized using a spectrometer. A custom device was designed and built to use this sensor for intraocular measurements with the microrobot. Due to the intrinsic nature of luminescence lifetimes, a frequency-domain lifetime measurement approach was employed. An alternative sensor implementation using poly(styrene-co-maleic anhydride) (PS-MA) and PtOEP was successfully demonstrated with nanospheres to increase sensor performance.

Octahedral iron(II) phthalocyanine complexes: multinuclear NMR and relevance as NO(2) chemical sensors.

The synthesis of new phthalocyanine iron(ii) (FePc) based coordination complexes , their structural characterization by multinuclear NMR ((1)H, (13)C, (15)N, (31)P, (57)Fe), and their use as improved sensitive and cheap optical NO(2) sensors is described. delta((15)N) and delta((57)Fe) values obtained via HMQC NMR methods show an interesting trend, the larger the chemical shift value the more the selectivity towards NO(2). Among all the sensing films prepared, the novel mixed ligand phosphite-amine [FePc(benzylamine)(P(OEt)(3)] () immobilized into AP200/19 showed the best sensitivity, reversibility (LOD and LOQ of 1.2 ppb and 4.0 ppb, respectively), and thermostability in the range of 4 to 25 degrees C.

A semi-empirical model to simplify the synthesis of homogeneous and transparent cross-linked polymers and their application in the preparation of optical sensing films.

We propose a simple, semi-empirical model based on Hansen's solubility parameters for simplifying the synthesis and the optimization of homogeneous and transparent cross-linked polymers in order to obtain optical sensing films. More than 740 experiments were undertaken to demonstrate the reliability of the model and several applications are proposed. We have demonstrated that our model can help in the synthesis and optimization (percentage of cross-linker, changes in hydrophilicity, selection of porogens, quantity of template etc.) of homogeneous and transparent MIPs and NIPs (molecularly imprinted polymers) with VOCs; after the synthesis of 440 polymers in the homogeneity zone only 4.32% of them (19 samples out of 440) were heterogeneous. We suggest a role for its use in the development of novel polymeric resins for detecting volatile organic compounds in water by measuring intrinsic fluorescence, in simplifying the synthesis of imprinted polymers and in decreasing the number of experiments required to optimize optical sensing membranes. In addition, it might also be used for synthesizing and optimizing MIPs with a non-volatile template.