Density of Grafted Chains in Thioglycerol-Capped CdS Quantum Dots Determines Their Interaction with Aluminum(III) in Water

Abstract

We aimed to quantify the interaction of water-soluble-functionalized CdS quantum dots (QDs) with metal cations from their composition and physical properties. From the diameter of thioglycerol-capped nanoparticles (TG-CdS QDs) measured by electronic microscopy ( D = 12.3 ± 0.3 nm), we calculated the molecular mass of the individual particle MA = (3 ± 0.5) × 10 g·mol and its molar absorption coefficient ε = 21 × 10 M·cm. We built a three-dimensional model of the TG-CdS QDs in agreement with the structural data, which allowed us to quantify the number of thioglycerol grafted chains to ∼2000 per QD. This value fully matches the saturation binding curve of Al cations interacting with TG-CdS QDs. The reaction occurred with a slow association rate ( k = 2.1 × 10 M·s), as expected for heavy QDs. The photophysical properties of the functionalized QDs were studied using an absolute QD concentration of 7 nM, which allowed us to investigate the interaction with 14 metallic cations in water. The fluorescence intensity of TG-CdS QDs could be quenched only in the presence of Al ions in the range 0.2-10 μM but not with other cations and was not observed with other kinds of grafting chains.

Raman Tweezers Microspectroscopy of Functionalized 4.2 nm Diameter CdSe Nanocrystals in Water Reveals Changed Ligand Vibrational Modes by a Metal Cation

“…We successfully recorded the Raman spectra of ligands grafted to very small colloidal QDs at a low concentration in water thanks to the effect of optical trapping. We first recorded (Figure ) the Raman spectrum of TG grafted to the surface of two kinds of semiconductor nanoparticles, TG–CdS and TG–CdSe, which have diameters of 12.3 and 2.5 nm, respectively. , We cannot measure the Raman spectrum of QDs in water without optical tweezers (e.g., using a 10× long-focal length objective). In “classical” Raman, the constraint for a high concentration of QD colloidal suspension results in a largely dominating Rayleigh scattering which precludes reliable measurement of the informative Raman signal on top of water background.…”

“…The synthesis was adapted from previous protocols. , Aqueous solution of cadmium acetate dehydrate (1.54 g) was first mixed with l -Cys (1.53 g) as the stabilizer in 100 mL of deionized water in a beaker under stirring. The pH of the resulting solution was adjusted to 11.2 by using aliquots of 1 M NaOH.…”

“…We calculated the molecular mass of individual QDs from their average diameter measured by high resolution transmission electron microscopy, , assuming a spherical shape. From the volume of one crystal unit (hexagonal for CdS and cubic for CdSe), we estimated the number of atoms in one QD (e.g., 668 each of Cd and Se for a 4.2 nm size QD).…”

“…These quantum dots (QDs) were widely studied for numerous applications, especially as probes for imaging and for the detection of diverse analytes in solution − rendered possible by means of their fluorescence properties, which are modulated through molecular interactions. The development of these QD-based optical sensors in solution takes advantage of the grafting of organic ligands at the surface of the semiconductor nanoparticles which allowed to solubilize the QDs and made them susceptible to molecular and ionic interactions with analytes. , Since the first reported study of CdS QD fluorescence quenching by metal ions, semiconductor QDs of various materials (CdS, CdSe, CdTe, ZnTe), whose surface was functionalized with different grafted organic ligands, were reported to detect metal cations in water. − The binding of metallic cations induced the fluorescence quenching of QDs but very few studies were aimed to rationalize the quenching mechanism.…”

Mechanistic Insights into Selective Sensing of Pb2+ in Water by Photoluminescent CdS Quantum Dots

“…The present contribution attempts to elucidate the mechanism of PL quenching selectively by Pb 2+ . Although prior reports have shown that the nature of the QD core, surface states, and capping ligands influence the observed response, − there are considerable lacunas in exact quantification. This issue has been addressed by a systematic analysis of observed PL and transient absorption (TA) spectral signatures.…”