João A.V. Prior

Application of quantum dots as analytical tools in automated chemical analysis: A review

Colloidal semiconductor nanocrystals or quantum dots (QDs) are one of the most relevant developments in the fast-growing world of nanotechnology. Initially proposed as luminescent biological labels, they are finding new important fields of application in analytical chemistry, where their photoluminescent properties have been exploited in environmental monitoring, pharmaceutical and clinical analysis and food quality control. Despite the enormous variety of applications that have been developed, the automation of QDs-based analytical methodologies by resorting to automation tools such as continuous flow analysis and related techniques, which would allow to take advantage of particular features of the nanocrystals such as the versatile surface chemistry and ligand binding ability, the aptitude to generate reactive species, the possibility of encapsulation in different materials while retaining native luminescence providing the means for the implementation of renewable chemosensors or even the utilisation of more drastic and even stability impairing reaction conditions, is hitherto very limited. In this review, we provide insights into the analytical potential of quantum dots focusing on prospects of their utilisation in automated flow-based and flow-related approaches and the future outlook of QDs applications in chemical analysis.

Fluorescence enhancement of CdTe MPA-capped quantum dots by glutathione for hydrogen peroxide determination

The manipulation of the surface chemistry of semiconductor nanocrystals has been exploited to implement distinct sensing strategies in many analytical applications. In this work, reduced glutathione (GSH) was added at reaction time, as an electron-donor ligand, to markedly increase the quantum yield and the emission efficiency of MPA-capped CdTe quantum dots. The developed approach was employed in the implementation of an automated flow methodology for hydrogen peroxide determination, as this can oxidize GSH preventing its surface passivating effect and producing a manifest fluorescence quenching. After optimization, linear working calibration curve for hydrogen peroxide concentrations between 0.0025% and 0.040% were obtained (n=6), with a correlation coefficient of 0.9975. The detection limit was approximately 0.0012%. The developed approach was employed in the determination of H₂O₂ in contact lens preservation solutions and the obtained results complied with those furnished by the reference method, with relative deviations comprised between -1.18 and 4.81%.

Evidences of turbulent mixing in multi-pumping flow systems

Multi-pumping flow systems exploit pulsed flows delivered by solenoid pumps. Their improved performance rely on the enhanced radial mass transport inherent to the pulsed flow, which is a consequence of the establishment of vortices thus a tendency towards turbulent mixing. This paper presents several evidences of turbulent mixing in relation to pulsed flows, such as recorded peak shape, establishment of fluidized beds, exploitation of flow reversal, implementation of relatively slow chemical reactions and/or heating of the reaction medium. In addition, Reynolds number associated with the GO period of a pulsed flow is estimated and photographic images of dispersing samples flowing under laminar regime and pulsed flow conditions are presented.

Photoactivation by visible light of CdTe quantum dots for inline generation of reactive oxygen species in an automated multipumping flow system.

Quantum dots (QD) are semiconductor nanocrystals able to generate free radical species upon exposure to an electromagnetic radiation, usually in the ultraviolet wavelength range. In this work, CdTe QD were used as highly reactive oxygen species (ROS) generators for the control of pharmaceutical formulations containing epinephrine. The developed approach was based on the chemiluminometric monitoring of the quenching effect of epinephrine on the oxidation of luminol by the produced ROS. Due to the relatively low energy band-gap of this chalcogenide a high power visible light emitting diode (LED) lamp was used as photoirradiation element and assembled in a laboratory-made photocatalytic unit. Owing to the very short lifetime of ROS and to ensure both reproducible generation and time-controlled reaction implementation and development, all reactional processes were implemented inline by using an automated multipumping micro-flow system. A linear working range for epinephrine concentration of up to 2.28×10(-6) mol L(-1) (r=0.9953; n=5) was verified. The determination rate was about 79 determinations per hour and the detection limit was about 8.69×10(-8) mol L(-1). The results obtained in the analysis of epinephrine pharmaceutical formulations by using the proposed methodology were in good agreement with those furnished by the reference procedure, with relative deviations lower than 4.80%.

Automated determination of diazepam in spiked alcoholic beverages associated with drug-facilitated crimes

In this work, a multipumping flow system (MPFS) coupled to a photodegradation unit was developed, for the first time, for the determination of diazepam (a benzodiazepine) in spiked alcoholic beverages by fluorimetry. The main features of MPFS such as, high portability, versatility and straightforward automation and control combined with the efficiency and simplicity of photodegradation and the selectivity and sensitivity of fluorimetric detection makes the developed analytical methodology an attractive tool and a valuable contribution for the prevention of drug-facilitated crimes (DFC). Drug-facilitated crimes involve the unauthorized administration of strong central nervous system depressant drugs, which have the capability of preventing victims from resist to the action of the perpetrator or fighting off. Most often, the drugs identified as being used in DFC are surreptitiously placed in drinks served to potential victims in entertainment places, like night clubs. Five commercial alcoholic beverages (Eristoff, Smirnoff, Bacardi, Dry Gin and Brazilian Cachaça 51) spiked with diazepam were analyzed by the proposed methodology, and the results revealed good agreement with those obtained through a HPLC comparison procedure. Relative deviations comprised between -1.97 and 2.05% were achieved, and additionally, the application of a paired t-test, revealed the absence of any statistical difference for a confidence level of 95% (n=5). The detection limit was approximately 2.02 mg L(-1).

Exploiting the oxidative coupling reaction of MBTH for indapamide determination

The oxidative coupling reaction between aromatic amines and 3-methylbenzothiazolin-2-one hydrazone (MBTH) in the presence of cerium(IV) has been extensively used with quantitative analytical purposes. Nevertheless, a literature survey reveals that different wavelengths (visible range) can be used to monitor the reaction products when using MBTH and Ce(IV) as colour developing reagents. In the present work, the oxidative coupling reaction of indapamide (an oral antihypertensive diuretic drug) with MBTH in the presence of cerium(IV) was evaluated using distinct reaction approaches and was implemented in an automated multipumping flow system. The developed methodology was applied in the spectrophotometric control of the drug in pharmaceutical formulations. The optimization of the proposed multipumping flow system was performed by using an experimental design approach, namely by exploiting a Plackett-Burman factorial design and a central cubic faces design. This work revealed the formation of products with different reaction kinetics, chemical stabilities and absorbance spectra, depending on the sequence of addition of the reagents. By exploiting a specific sequence in the addition of reagents, the proposed automatic system allowed the rapid quantification of indapamide in pharmaceutical formulations, with a determination rate of about 25 h(-1), and a relative deviation under 2.1% when comparing with the reference procedure. Detection limit was approximately 1 mg L(-1).

Trimipramine determination in pharmaceutical preparations with an automated multicommutated reversed-flow system

In this work an automated multicommutated flow methodology was implemented for the spectrophotometric determination of trimipramine in pharmaceutical preparations by oxidation with ammonium monovanadate in acidic medium. The developed procedure exploits a new approach for sample/reagent intermixing by combining binary sampling with flow-reversal. Rather than inserting the sample as a single continuous volume the intercalation of multiple small sample and reagent aliquots, under a time-based control, created multiple reaction interfaces that promoted reaction zone homogenisation even in limited dispersion conditions. Additionally, the reaction interfaces were reversed, increasing mutual zone penetration, which contributed to a faster reaction development while assuring a low dispersion pattern. A linear range of determination was verified for trimipramine concentrations between 1.0 and 18.0 microg ml(-1) with a relative standard deviation (n=10) lower than 1.69% and a sample throughput of about 26 samples per hour. The results were in agreement with those obtained by the reference procedure with relative deviations lower then 2.37%.

Competitive metal–ligand binding between CdTe quantum dots and EDTA for free Ca2+ determination

In this work, a fluorometric approach for the selective determination of calcium by using CdTe nanocrystals as chemosensors, was developed. The quantum dots interacted not with the metal, but with a ligand that also bonded the metal. The fluorescence response was modulated by the extension of the competitive metal-ligand binding, and therefore the amount of free ligand. CdTe quantum dots (QDs) with different capping layers were evaluated, as the QDs surface chemistry and capping nature affected recognition, thus the magnitude of the ensuing fluorescence quenching. The developed procedure was automated by using a multipumping flow system. Upon optimization, thioglycolic acid (TGA) and EDTA were selected as capping and ligand, respectively, providing a linear working range for calcium concentrations between 0.80-3.20 mg L(-1), and a detection limit of 0.66 mg L(-1). A quenching mechanism relying on nanocrystal destabilization upon detachment of surface Cd by the ligand was proposed.

Selective determination of sulphide based on photoluminescence quenching of MPA-capped CdTe nanocrystals by exploiting a gas-diffusion multi-pumping flow method

In this study an automated flow-based methodology for the fluorometric determination of sulphide was reported. It relies on the utilization of CdTe nanocrystals as photoluminescent probes, which upon reaction with S2− are subject to a noteworthy concentration-related photoluminescence decrease. For lower S2− concentrations the photoluminescence quenching was based on dynamic processes while for higher concentrations the quenching mechanism was ascribed to the depassivation of the surface ligands, replaced by S2−, resulting in the aggregation of QDs. The developed approach was automated by resorting to a pulsed stream multi-pumping flow system guaranteeing a high versatility in terms of sample and reagent manipulation and reaction zone formation. The selectivity was ensured by means of the utilization of a gas-diffusion unit relying on a hydrophobic PTFE membrane that facilitated sulphide isolation from sample matrix interferences. Under optimal conditions, a good linear relationship between the photoluminescence quenching magnitude (ΔF) and the logarithmic of the S2− concentration within the range of 0.25–5.0 mmol L−1 were verified (R = 0.998, n = 5). The limit of detection (LOD) was found to be 0.19 mmol L−1. The sampling rate was of about 13 h−1.

Chemiluminometric evaluation of melatonin and selected melatonin precursors’ interaction with reactive oxygen and nitrogen species

Melatonin is a hormone, a derivative of tryptophan, that possesses a potent scavenging capacity for the most reactive and dangerous free radicals, being an important protection against oxidative stress. In this work, an automated flow-based procedure for assessment of melatonin, tryptophan, and 5-hydroxytryptophan scavenging capacity was developed. The presented methodology involved a multi-pumping flow system and exploited the ability of selected compounds to inhibit the chemiluminescence reaction of luminol with hydrogen peroxide, hydroxyl radical, and peroxynitrite anion. The system was based on the use of several solenoid actuated micro-pumps as the only active components of the flow manifold. This enabled the reproducible insertion and efficient mixing of very low volumes of sample and reagents as well as the transportation of the sample zone toward detection for monitoring the chemiluminometric response. Furthermore, the high versatility of the proposed multi-pumping flow system allowed the implementation of distinct reactions for the in-line generation of the different reactive species assayed without requiring physical reconfiguration. The results obtained demonstrated that 5-hydroxytryptophan is the most potent scavenger, followed by melatonin and tryptophan. The developed multi-pumping flow system exhibited good measurement precision (relative standard deviations typically <2%, n=10), low operational costs, and low reagent consumption.