José Manuel Costa

Inorganic mass spectrometry as a tool for characterisation at the nanoscale

AbstractInorganic mass spectrometry techniques may offer great potential for the characterisation at the nanoscale, because they provide unique elemental information of great value for a better understanding of processes occurring at nanometre-length dimensions. Two main groups of techniques are reviewed: those allowing direct solid analysis with spatial resolution capabilities, i.e. lateral (imaging) and/or in-depth profile, and those for the analysis of liquids containing colloids. In this context, the present capabilities of widespread elemental mass spectrometry techniques such as laser ablation coupled with inductively coupled plasma mass spectrometry (ICP-MS), glow discharge mass spectrometry and secondary ion/neutral mass spectrometry are described and compared through selected examples from various scientific fields. On the other hand, approaches for the characterisation (i.e. size, composition, presence of impurities, etc.) of colloidal solutions containing nanoparticles by the well-established ICP-MS technique are described. In this latter case, the capabilities derived from the on-line coupling of separation techniques such as field-flow fractionation and liquid chromatography with ICP-MS are also assessed. Finally, appealing trends using ICP-MS for bioassays with biomolecules labelled with nanoparticles are delineated. FigureInorganic mass spectrometry: an emerging tool for nanotechnology

Characterization of a simple glow discharge coupled to a time of flight mass spectrometer for in-depth profile analysis

The capabilities of a simple glow discharge (GD) ion source coupled to an on-axis time-of-flight (TOF) mass spectrometer (MS) have been investigated in terms of several parameters of interest for in-depth profile quantification, including crater shapes, sample sputtering rates, sensitivity and mass resolving power. The GD ion source used here is a simple prototype constructed in our laboratory and designed to be easy and quickly exchanged with the inductively coupled plasma (ICP) source of a commercial ICP-(TOF)MS. Experiments showed that good crater shapes, with walls “perpendicular to” and base “parallel to” the samples surface can be obtained for samples with different matrices, provided that a “sampler cone” in the interface between the GD chamber and the TOF is used; otherwise, an inhomogeneous sputtering in the sample is observed, ruining the crater shapes. The GD-(TOF)MS system developed offers a mass resolving power (full width at half maximum) of 2800 for 207Pb in the digital detection mode and 1700 in the analog mode. Furthermore, promising results have been obtained for possible use of polyatomic ions (formed between the argon plasma gas and a major element in the sample) as a quantitative indicator of the corresponding element. This approach could be very useful as an alternative strategy for the simultaneous determination of major, minor and trace elements, without saturation of the detector, by GD-(TOF)MS.

Flow-through optosensing of 1-naphthaleneacetic acid in water and apples by heavy atom induced-room temperature phosphorescence measurements

A sensitive and selective phosphorimetric method for the determination of 1-naphthaleneacetic acid (1-NAA) based on a flow-injection system connected to a flow cell packed with a solid support and placed in the sample compartment of a conventional luminescence spectrometer is described. A non-ionic solid polymeric resin Amberlite XAD-7 is used for the packing. After injection of the sample, 1-NAA is on-line retained in the packed resin and measurements of the heavy atom induced (HAI)-room temperature phosphorescence (RTP) emission (lambda(ex)/lambda(em)=292/490nm) from this native luminescent compound are taken. The optimum experimental conditions were investigated by injecting 2ml samples of an aqueous solution of 1-NAA in the flow system. A concentration 0.15moll(-1) of thallium(I) ions, as heavy atom, both in the samples and the carrier flow, was finally selected. Also, a concentration of 6mmoll(-1) of sulphite was optimal for ensuring the necessary deoxygenation of the system at the selected flow rate of 0.8mlmin(-1). After measurement, the solid support was efficiently regenerated by injecting 1ml of a mixture water:acetone in a ratio 1:1 (v/v) into the flow. The detection limit (3sigma criterion) was 1.2ngml(-1) of 1-NAA. The repeatability (R.S.D.) for five replicates of a sample containing 50ngml(-1) of analyte turned out to be +/-3% and the calibration graphs proved to be linear up to 500ngml(-1) of 1-NAA (maximum concentration assayed). The effect of potential interferences from other organic species which can be also used as plant growth regulators, as well as from various inorganic cations and anions, has been investigated as well. The method was successfully applied to the determination of low levels of this plant growth regulator in natural waters (river and fountain waters) and apples.

A Ratiometric Method for Oxygen Measurement Using a Luminescent Sensor

A great variety of methods for oxygen sensing using luminescent sensors have been proposed in recent years based on intensity or in lifetime quenching. Like lifetime measurements, ratiometric techniques are insensitive to the variations of the excitation light, optical path and photo-bleaching. In this work, we present a ratiometric method based on the phosphorescence-fluorescence spectral overlap emission of a phosphorescent chemical sensor. This dual emission makes ratiometric measurements possible without need of adding a reference luminophore. The ratio is calculated by measuring the phase shift between the excitation and the emission signal at two different frequencies. Theoretical aspects of the proposed methodology and the design and construction of a fiber-optical measuring system are discussed. Finally, the performance of the proposed measurement method has been assessed using the metal chelate Al-Ferron immobilized in an inorganic sol-gel support (an oxygen indicator which displays a strong fluorescence emission overlapping significantly with the measured phosphorescence emission)

Measurement of Polycyclic Aromatic Hydrocarbons by using Molecularly Imprinted Polymers

Polycyclic Aromatic Hydrocarbons (PAHs) comprise a large group of organic compounds whose molecular structure contain two or more fused aromatic rings; they are usually formed during the incomplete combustion of organic substances. Because of their low water-solubility and hydrophobic nature, PAHs tend to be associated with material in suspended and bed sediments. Due to its nature, polycyclic aromatic hydrocarbons are a ubiquitous pollutant and food can be contaminated with PAHs present in air, soil and water. Benzofajpyrene (BaP) is one of the most carcinogenic Polycyclic Aromatic Hydrocarbons and it is usually an indicator of the presence of other PAHs. On the other hand, molecularly imprinted polymers (MIPs) based sensors are a new way to measure PAHs. These polymers are made using the molecule to be measured measure as a template that is removed at the final stage of the preparation of the sensor. The resulting cavities have the shape of the molecule to be detected and thus, the selectivity of these sensors is high. In this paper, a MIP sensor is presented and evaluated by measuring BaP in water by using a simple prototype. The system is based on measuring the phosphorescence intensity of the light emitted by the BaP when excited with UV light.

In-line measurement of dissolved acetone using a nanoestructured optical sensor

In this paper we present a method for measuring dissolved acetone by employing a new sensor based on luminescent nanocrystals and a ratiometric measurement. The developed sensor shows an excitation peak at 280 nm and a long lived emission at 590 nm by adding Mn in the synthesis process. The sensor was immobilized in a low luminescent sol-gel matrix. In presence of acetone, the intensity of the long lived emission decreases; however, its decay time is always the same. A measurement method based on employing the luminescence of the solid support as reference is presented. Experimental results have been obtained using a deep UV-LED as the excitation light source and a lab prototype for measuring intensities and phase shifts using GPIB instruments.

Characterization of photoluminescence activation of semiconductor nanoparticles for optical sensors

Application of nanomaterials to the design of optical sensors has generated great interest in sensing applications because of their unique optical properties in comparison with traditional organic dyes. Nevertheless, not many commercial sensor instruments based on semiconductor nanoparticles are yet available. One reason is the instability of the intensity of the luminescence emission observed in some of these kinds of sensors, particularly with nanoparticles of CdSe. There are several reasons to these variations. One of them is the initial activation of the nanoparticles. In order to improve the stability of these sensors, a study about the photo-activation of two types of semiconductor nanoparticles, core-shell and only core, is presented in this manuscript.