S. Sofia M. Rodrigues

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%.

Enhancing reactive species generation upon photo-activation of CdTe quantum dots for the chemiluminometric determination of unreacted reagent in UV/S2O82− drug degradation process

A new chemiluminescence (CL) flow method for persulfate determination was developed based on luminol oxidation by in-line generated radicals. Reactive oxygen species (ROS) generated by CdTe quantum dots (QDs) under a low energetic radiation (visible light emitted by LEDs) promoted the decomposition of persulfate ion (S2O8(2-)) into sulfate radical (SO4(∙-)), leading to subsequent radical chain reactions that yield the emission of light. Due to the inherent radical short lifetimes and the transient behavior of CL phenomena an automated multi-pumping flow system (MPFS) was proposed to improve sample manipulation and reaction zone implementation ensuring reproducible analysis time and high sampling rate. The developed approach allowed up to 60 determinations per hour and determine S2O8(2-) concentrations between 0.1 and 1 mmol with good linearity (R=0.9999). The method has shown good repeatability with relative standard deviations below 2.5% (n=3) for different persulfate concentrations (0.1 and 0.625 mmol L(-1)). Limits of detection (3σ) and quantification (10σ) were 2.7 and 9.1 µmol L(-1), respectively. The MPFS system was applied to persulfate determination in bench scale UV/S2O8(2-) drug degradation processes of model samples showing good versatility and providing real time information on the persulfate consumption in photo-chemical degradation methodologies.

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.

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.

Study of the quenching effect of quinolones over CdTe-quantum dots using sequential injection analysis and multicommutation.

The field of light-emitting nanoparticles has experienced an enormous development over the past two decades. The fluorescence of these nanometer-size crystalline particles, called quantum dots (QDs), can be both quenched and enhanced by different compounds. Since a high percentage of articles related to QDs are focused on theoretical studies, the development of analytical methods with real applications is an important step in order to progressively demonstrate the versatility of these particles. Moreover, taking into account that most of the QDs-based analytical methods are non-automated, the development of automated flow methodologies is still a field that presents an important analytical potential. With this purpose, two automatic methodologies, multicommutated flow injection analysis and sequential injection analysis, have been here applied to the analysis of quinolones in pharmaceutical formulations, making use of the quenching effect caused by the analytes over mercaptopropionic acid-capped CdTe QDs fluorescence. Both methodologies were compared in terms of versatility, sample throughput, sensitivity, etc., and applied to the determination of five quinolones in pharmaceutical preparations available in the Spanish Pharmacopoeia. The detection limits ranged between 26 and 50μmolL(-1), and Relative Standard Deviations lower than 3% were observed in all cases.

A novel multi-commutated method for the determination of hydroxytyrosol in enriched foods using mercaptopropionic acid-capped CdTe quantum dots

Hydroxytyrosol (HXT) has been reported to have beneficial effects for human health, such as antioxidant and antimicrobial properties and an important contribution to the prevention of cardiovascular disease. Hence, exhaustive research is currently being performed to prepare functional foods, such as tomato juice or milk, with HXT. This paper presents a multi-commutated flow method based on the quenching effect that HXT has on the fluorescence of water-soluble mercaptopropionic acid-capped CdTe quantum dots. Under optimal conditions a linear working range was obtained for concentrations between 10 and 250 ng µl−1. In order to demonstrate the suitability of the proposed method for the determination of HXT, HXT-enriched samples were prepared. Using a QuEChERS (quick, easy, cheap, effective, rugged and safe) procedure for extraction, HXT was determined in the prepared functional foods (milk, infant formula, tomato juice and tomato soup). Recoveries of 100% ± 8%, relative standard deviations (RSDs) lower than 5% and high sample throughput of 70 samples per h show the potential of the system for the analysis of HXT in food samples.

Fluorescence probe for mercury(II) based on the aqueous synthesis of CdTe quantum dots stabilized with 2-mercaptoethanesulfonate

Manipulation of the QD surface by changing morphology and surface capping ligands, as well as adjusting nanocrystal size, plays a crucial role on the selectivity and sensitivity exhibited by QDs towards a given target analyte. In this study, a novel aqueous synthesis of CdTe QDs capped with a thiol compound containing a sulfonate (SO3−) terminal group, viz., 2-mercaptoethanesulfonate (MES), was thoroughly investigated with the aim of obtaining a fluorescent probe for chemical analysis. The results obtained with the prepared CdTe-MES QDs in the determination of various metal ions demonstrated the high efficiency of these nanomaterials for determining Hg(II) levels. Upon optimization, linear working calibration curves for Hg(II) concentrations of up to 0.5 μmol L−1 were obtained, with a determination coefficient of 0.9984. The detection limit was 0.0095 μmol L−1 and the quantification limit was 0.0324 μmol L−1. When applied to the determination of Hg(II) in tap water samples, the developed method provided analytical results similar to those obtained with a reference method. The accuracy and precision of both methods were comparatively evaluated using the Students t-test and the Fischer test, and the tabulated values showed a good agreement at a 95% confidence level.