Application of L-Aspartic Acid-Capped ZnS:Mn Colloidal Nanocrystals as a Photosensor for the Detection of Copper (II) Ions in Aqueous Solution

Water-dispersible ZnS:Mn nanocrystals (NCs) were synthesized by capping the surface with polar L-aspartic acid (Asp) molecules. The obtained ZnS:Mn-Asp NC product was optically and physically characterized using the corresponding spectroscopic methods. The ultra violet-visible (UV-VIS) absorption spectrum and photoluminescence (PL) emission spectrum of the NCs showed broad peaks at 320 and 590 nm, respectively. The average particle size measured from the obtained high resolution-transmission electron microscopy (HR-TEM) image was 5.25 nm, which was also in accordance with the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charge and degree of aggregation of the ZnS:Mn-Asp NCs were determined by electrophoretic and hydrodynamic light scattering methods, respectively. These results indicated the formation of agglomerates in water with an average size of 19.8 nm, and a negative surface charge (−4.58 mV) in water at ambient temperature. The negatively-charged NCs were applied as a photosensor for the detection of specific cations in aqueous solution. Accordingly, the ZnS:Mn-Asp NCs showed an exclusive luminescence quenching upon addition of copper (II) cations. The kinetic mechanism study on the luminescence quenching of the NCs by the addition of the Cu²⁺ ions proposed an energy transfer through the ionic binding between the two oppositely-charged ZnS:Mn-Asp NCs and Cu²⁺ ions.

Surface Properties and Photocatalytic Activities of the Colloidal ZnS:Mn Nanocrystals Prepared at Various pH Conditions

Water-dispersible ZnS:Mn nanocrystals (NC) were synthesized by capping the surface with mercaptoacetic acid (MAA) molecules at three different pH conditions. The obtained ZnS:Mn-MAA NC products were physically and optically characterized by corresponding spectroscopic methods. The UV-Visible absorption spectra and PL emission spectra showed broad peaks at 310 and 590 nm, respectively. The average particle sizes measured from the HR-TEM images were 5 nm, which were also supported by the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charges and the degrees of aggregation of the ZnS:Mn-MAA NCs were determined by electrophoretic and hydrodynamic light scattering methods, indicating formation of agglomerates in water with various sizes (50–440 nm) and different surface charge values accordingly the preparation conditions of the NCs (−7.59 to −24.98 mV). Finally, the relative photocatalytic activities of the ZnS:Mn-MAA NCs were evaluated by measuring the degradation rate of methylene blue (MB) molecule in a pseudo first-order reaction condition under the UV-visible light irradiation. As a result, the ZnS:Mn-MAA NC prepared at the pH 7 showed the best photo-degradation efficiency of the MB molecule with the first-order rate constant (k_obs) of 2.0 × 10⁻³·min⁻¹.

Cytotoxicity and cellular uptake of ZnS:Mn nanocrystals biofunctionalized with chitosan and aminoacids

Highly luminescent, manganese doped, zinc sulphide (ZnS:Mn) nanocrystals biofunctionalized with chitosan and various aminoacids such as L-citrulline, L-lysine, L-arginine, L-serine, L-histidine and glycine were synthesized by chemical capping co-precipitation method at room temperature, which is a simple and cost effective technique. The synthesized nanocrystals were structurally characterized by TEM, XRD, EDXS and FT-IR spectroscopy techniques. They possess high colloidal stability with strong orange red photoluminescence emission at 598 nm. The intensity of orange red emission has been observed to be maximum in L-citrulline capped ZnS:Mn nanocrystals in which the emission at 420 nm is effectively quenched by surface passivation due to capping. Taking into consideration the prospects of these highly luminescent, bio-compatible ZnS:Mn nanocrystals in bio-imaging applications, cytotoxicity studies were conducted to identify the capping combination which would accomplish minimum toxic effects. ZnS:Mn nanocrystals biofunctionalized with chitosan, L-citrulline, glycine, L-artginine, L-serine and L-histidine showed least toxicity up to 10 nM concentrations in mouse fibroblast L929 cells, which further confirms their cytocompatibility. Also the ZnS:Mn nanocrystals biofunctionalized with l-arginine showed maximum uptake in in vitro studies carried out in human embryonic kidney cells, HEK-293T, which shows the significant role of this particular amino acid in fetoplacental nutrition. The present study highlights the suitability of aminoacid conjugated ZnS:Mn nanocrystals, as promising candidates for biomedical applications.

Highly luminescent and biocompatible, L-citrulline-capped ZnS:Mn nanocrystals for rapid screening of metal accumulating Lysinibacillus fusiformis bacteria

Biocompatible and highly luminescent manganese doped zinc sulfide (ZnS:Mn) nanocrystals of average particle size 10 nm have been synthesized by capping with a novel amino acid ligand, L-citrulline. Though there are many reports on the bioimaging applications of nanostructured semiconductors, the present study focused on the detection of a special type of metal accumulating bacteria, Lysinibacillus fusiformis. This bacterium has significant applications in the disposal of metal components from industrial effluents. In this context, the detection of this bacterium is quite important and the present work demonstrates a novel technique for this bacterial detection. The synthesized nanocrystals were attached to Lysinibacillus fusiformis and characteristics of the bioconjugated system were studied. The blue shift observed in the ultraviolet-visible absorption and photoluminescence spectra of the bioconjugated system, confirms conjugation of the Lysinibacillus fusiformis with L-citrulline-capped ZnS:Mn. When the bioconjugated system (capped ZnS:Mn + bacteria) was observed using a fluorescent microscope under excitation wavelengths 365.4 nm (ultraviolet), 435.8 nm (blue) and 546.1 nm (green), fluorescence emissions were obtained in yellow, green and red regions respectively. The study of relative growth of Lysinibacillus fusiformis in the presence of L-citrulline-capped ZnS:Mn proves biocompatible property of these nanocrystals and their tunable color properties under different excitation wavelengths make them ideal for biolabeling applications.

Attachment of CdSe nanoparticles to TiO2 via aqueous linker-assisted assembly: influence of molecular linkers on electronic properties and interfacial electron transfer

CdSe nanoparticles (NPs) capped with cysteinate (Cys), 3-mercaptopropionate (MP), and mercaptosuccinate (MS) were adsorbed to TiO(2) from basic aqueous dispersions. Native capping groups served as molecular linkers to TiO(2). Thus, the materials-assembly chemistry was simplified and made more reproducible and environmentally benign. The electronic properties of CdSe and the electron-transfer reactivity at CdSe-linker-TiO(2) interfaces varied with the structure and functionality of the capping groups. Cys-capped CdSe NPs exhibited a narrow and intense first excitonic absorption band centered at 422 nm, suggesting that they were magic-sized nanocrystals (MSCs) with diameters less than 2 nm. MP- and MS-capped CdSe NPs had broader and lower-energy absorption bands, which are typical of regular quantum dots. Photocurrent action spectra of nanocrystalline TiO(2) films functionalized with Cys-CdSe, MP-CdSe, and MS-CdSe overlaid closely with absorption spectra, indicating that excitation of CdSe gave rise to the injection of electrons into TiO(2). Under white-light illumination, the global energy-conversion efficiency for Cys-capped CdSe ((0.45 ± 0.11)%) was 1.2-to-6-fold greater than for MP- and MS-capped CdSe. Similarly, the absorbed photon-to-current efficiency was 1.3-to-3.3-fold greater. These differences arose from linker-dependent variations of electron-injection and charge-recombination reactivity. Transient absorption measurements indicated that electron injection from Cys-capped CdSe was more efficient than from MS-capped CdSe. In addition, charge recombination at CdSe-MS-TiO(2) interfaces was complete within hundreds of nanoseconds, whereas the charge-separated-state lifetime at CdSe-Cys-TiO(2) interfaces was on the order of several microseconds. Thus, Cys-capped CdSe MSCs are readily attached to TiO(2) and exhibit unusual electronic properties and desirable electron-transfer reactivity.

Size-selective growth and stabilization of small CdSe nanoparticles in aqueous solution

Using cysteine and its derivatives as capping molecules, we investigated the influence of the physical structure and chemical nature of capping molecules on the selective growth and stabilization of small CdSe nanoparticles (NPs) in aqueous solution at room temperature. Our investigations revealed specific roles for each functional group of cysteine, and we could correlate this structure and nature of the capping molecules with the size, size restriction, size distribution, and stability of the NPs. For selective growth and stabilization of the NPs in aqueous solution, their capping molecules should have at least one functional group with strong nucleophilicity as well as another free, charged functional group. Capping molecules acting as a monodentate ligand were more effective than those acting as a bidentate ligand for restricting the NPs to a smaller size, whereas the former was less effective than the latter for getting a narrower NP size distribution. Capping molecules with relatively bulky spatial geometry near the ligand-NP interface resulted in the formation of NPs with poor short- and long-term stabilities, whereas those having relatively compact spatial geometry near the interface led to NPs with at least moderate short-term stability. We saw that capping molecules having relatively compact outermost spatial geometry led to NPs with excellent long-term stability, whereas those having relatively bulky outermost spatial geometry produced NPs with at most only moderate long-term stability. Our results clearly showed general trends for the possibility of selective growth of stable semiconductor NPs with particular sizes in aqueous solution.