David S.M. Ribeiro

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%.

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).

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 determination of ascorbic acid in pharmaceutical formulations exploiting photo-activation of GSH-capped CdTe quantum dots.

An automated multi-pumping flow system is proposed for the chemiluminometric determination of ascorbic acid in pharmaceutical formulations, relying on the ability of semiconductor nanocrystals to generate short-lived reactive species upon photo-irradiation. A photo-unit based on visible-light-emitting diodes is used to photo-excite cadmium telluride (CdTe) quantum dots capped with glutathione, leading to the generation of radicals that react with luminol under alkaline conditions, yielding the chemiluminescence. Ascorbic acid acts as a radical scavenger, preventing the oxidation of luminol, thus ensuring a concentration-dependent chemiluminescence quenching. After system optimization, a linear working range of 5.0 × 10(-7) to 5.0 × 10(-6) mol/L ascorbic acid (r = 0.9967, n = 5) was attained, with a detection limit of 3.05 × 10(-7) mol/L and a sampling rate of 200/h. The flow system was applied to the analysis of pharmaceutical formulations and the results were in good agreement with those obtained by the reference titrimetric procedure (RD < ± 4.3%, n = 7).

Antioxidant capacity automatic assay based on inline photogenerated radical species from l-glutathione-capped CdTe quantum dots

This work aimed at the development of a methodology implemented in an automatic flow system for determination of the antioxidant capacity in food samples, based on the luminol oxidation by inline photogenerated radical species from cadmium telluride nanoparticles capped with L-glutathione. Radical species were generated inline by a high-power visible light obtained by Light Emitting Diodes (LEDs) assembled in a multipumping flow system (MPFS). The use of visible light instead of UV radiation allowed the development of a new methodology for antioxidant capacity determination, more environment friendly and to circumvent the risk for UV photo-induced degradation of sample antioxidant compounds. Additionally, the formation of superoxide radical species was theoretically predicted considering the variation of the redox potential with the size of CdTe QDs and the values of redox potential of the oxidizing and oxidable species present in the irradiated medium. The obtained results of trolox equivalent antioxidant capacity (TEAC) from the analysis of commercial beverages were compared with the results of ABTS and DPPH batch assays through Spearmans-Rho correlation coefficients and no correlation was found (for ABTS: ρ=0.2, p<0.6 and for DPPH: ρ=0.5, p<0.1) since the mechanism of action of the proposed methodology was based on the scavenging capacity of ROS species rather than the reduction of a colored oxidant. An analytical linear response range between 0.0001 and 0.005mmol L(-1) of trolox and a limit of detection of 0.00005mmol L(-1) was found. The QDs based MPFS methodology allowed a determination rate of about 79h(-1), a total waste generation of 20.5mL h(-1) and the consumption of 0.100mg h(-1) of QDs and 2.1mg h(-1) of luminol.

pH-sensitive spectrophotometric control of nilutamide in an automatic micro-flow system

The drug nilutamide belongs to the pharmacological group of antiandrogens, clinically used to prevent tumor development observed in the first phase of the administration of analogues of gonadotrophin-releasing hormone in patients with prostate cancer. Due to the high toxicity of this drug, the chemical control in pharmaceutical formulations is extremely important. The aim of this work was to develop an automatic procedure, based on flow analysis, for the quantification of the drug nilutamide that could constitute a simpler alternative for routine quality control in industry that is normally performed by a chromatographic method. A new, very simple and high-throughput methodology was developed based on the spectrophotometric monitoring of the drug at the wavelength of 438 nm in a solution of sodium hydroxide, the only reagent used in this method. It was verified through spectrophotometric studies that by varying the pH conditions, electronic transitions from bonding orbitals π to anti-bonding orbitals π* occur in the molecular structure of nilutamide. In this work, the potentialities of the pulsed flow in the multipumping methodology were exploited taking into account the high viscosity of the sodium hydroxide solutions that impaired the flow of reagents and the development of reaction was expedited through a stopped-flow approach. The developed flow system allowed us to perform 24 determinations per hour, and the limit of detection was 2.26 mg L−1.