Colloidal Cd Research Articles

Tracing the vertical migration of exogenous cadmium in soil by seasonal freeze-thaw event using rare earth elements

Environmental behaviors of heavy metal in soil are strongly influenced by seasonal freeze-thaw events at the mid-high altitudes. However, the potential impact mechanisms of freeze-thaw cycles on the vertical migration of heavy metal are still poor understood. This study aimed to explore how exogenous cadmium (Cd) migrated and remained in soil during the in-situ seasonal freeze-thaw action using rare earth elements (REEs) as tracers. As a comparison, soil which was incubated in the controlled laboratory (25 °C) was employed. Although there was no statistically significant difference in the Cd levels of different soil depths under different treatments, the original aggregate sources of Cd in the 5–10 cm and 10–15 cm soil layers differed. From the distributions of REEs in soil profile, it can be known that Cd in the subsurface of field incubated soil was mainly from the breakdown of >0.50 mm aggregates, while it was mainly from the <0.106 mm aggregates for the laboratory incubated soil. Furthermore, the dissolved and colloidal Cd concentrations were 0.47 μg L−1 and 0.62 μg L−1 in the leachates from field incubated soil than those from control soil (0.21 μg L−1 and 0.43 μg L−1). Additionally, the colloid-associated Cd in the leachate under field condition was mainly from the breakdown of >0.25 mm aggregates and the direct migration of <0.106 mm aggregates, while it was the breakdown of >0.50 mm and the direct migration of <0.106 mm aggregates for the soil under laboratory condition. Our results for the first time provided insights into the fate of exogenous contaminants in seasonal frozen regions using the rare earth element tracing method.

The impact of heteroaggregation between nZVI and SNPs on the co-transport of Cd(II) in saturated sand columns

This study investigated the co-transport behaviors of nano zero-valent iron (nZVI) and Cd(II) in the presence of soil nanoparticles (SNPs) under various SNPs/nZVI mass ratios. It was illustrated that the mobility of colloidal Cd(II) was highly dependent on the nZVI-SNPs heteroaggregation behavior. In the case of 40 mg/L nZVI with SNPs/nZVI mass ratios > 1, the formation of stable SNPs-nZVI heteroaggregates with hydrodynamic diameters (Dh) < 500 nm facilitated the nZVI and colloidal Cd(II) transport at their effluent mass recoveries of 34.76–37.82 % and 9.81–17.17 %, respectively. However, in the case of 100 mg/L nZVI with SNPs/nZVI mass ratios of 0.4–2, the interception of nZVI-SNPs heteroaggregates with Dh > 1500 nm by quartz sands led to almost complete retention of nZVI and colloidal Cd(II) in the columns. Combined with analytical results of zeta potentials and XRD spectrum, it was revealed that the Cd(II) ions could accelerate nZVI corrosion. The positively charged Fe3O4 and γ-FeOOH on corroded nZVI surface could facilitate the heteroaggregation of nZVI-SNPs by the patch-charge attraction, which further reduced the environmental risk of colloidal Cd(II) transport. These findings revealed the important effects of heteroaggregation between nZVI and SNPs on the transport risk of Cd(II) in groundwater.

Colloids facilitate transport of cadmium and uranium in arable soils, which is undetected by suction cups in the field

AbstractColloids can facilitate the transport of trace elements in soil, but earlier studies fell short either because of sampling artefacts or because no contrasting soils were used; both factors hamper a better understanding of where this occurs. Here, leaching in undisturbed and unsaturated soil columns was observed to identify colloidal transport of cadmium (Cd) and uranium (U) in soils with contrasting properties. Crucially, it was examined how such column data relate to more traditional in situ data obtained with porous suction cups. Soils were collected from three agricultural fields in Switzerland (soil pH 5.1–7.3), leached during 36 days (0.6 pore volumes, ~1.6 mm day−1), and elements trapped in the PTFE suction plates were recovered and were ascribed to colloids. Of the total mobile mass (sum percolate and plate), 18%–53% (Cd) or 8–89% (U) was retained by plates, the range depending on the field where soil was sampled and that fraction depends on soil pH and for U also on dissolved inorganic carbon. The total mobile Cd or U mass divided by the percolated volume indicates the metal concentration in the average leachate and was logically highest for Cd in the most acidic soil and for U in the most alkaline soil. The concentrations of Cd and U in these column leachates were &gt;10‐fold higher than corresponding concentrations in the suction cups in the field for those soils and elements where colloids dominated leaching. In contrast, they were within a factor of two where colloids are &lt;20% of the mobile mass. Flow Field‐Flow Fractionation (FlFFF) analyses showed an association of colloidal U with oxyhydroxides, clays and organic matter and, for colloidal Cd, an association with organic matter. These data show that suction cups or filtered percolates can underestimate the leaching of Cd and U by up to an order of magnitude.

Mercapto–palygorskite decreases the Cd uptake of wheat by changing Fe and Mn fraction in Cd contaminated alkaline soil

Wheat is one of the major sources of dietary nutrition for human body, and excessive Cadmium (Cd) in wheat poses a serious risk to human health. Mercapto–palygorskite (MP) has been proven to efficiently reduce Cd stress in wheat, but the contribution of major soil oxides to the stability of soil Cd under MP treatment is unclear. Here, a wheat pot experiment was conducted to evaluate the effects of soil Fe and Mn oxides on the Cd fraction under MP treatment. The results showed that the application of MP decreased the diethylenetriaminepentaacetic acid (DTPA)–extractable Cd by 24.0–34.7 %, increased the Fe and Mn oxide-bound Cd (OX–Cd) by 5.0–12.0 % and significantly reduced the Cd content in wheat grains (17.9–69.5 %). It also decreased the soil’s free Fe and Mn oxides by 6.1–11.7 %and 6.9–12.4 %, respectively, and increased the amorphous Fe and Mn oxides by 19.3–32.5 % and 21.9–24.1 %, respectively. In addition, the application of MP increased the concentration of Fe and Cd in soil colloid by 19.3–33.3 % and 11.3–45.3 %, respectively. Correlation analysis indicated that the amorphous Fe, Mn oxides in soil had a significant positive correlation with the content of Fe and Mn bound Cd in soil (P < 0.01). Primary component analysis (PCA) analysis shows colloidal Fe and Mn on the soil available Cd had relatively high positively loadings. Meanwhile, soil colloidal Fe exhibited a significant positive correlation with colloidal Cd (P < 0.01) and amorphous Fe oxides (P < 0.01). These results indicated that MP remediated Cd-contaminated soil by promoting an increase in amorphous Fe, Mn oxides and their adsorption and precipitation of Cd on these oxides.

Colloid-facilitated mobilization of cadmium: Comparison of spring freeze-thaw event and autumn freeze-thaw event

Freeze-thaw action has the potential to facilitate the mobilization of colloid-associated contaminants in soil. However, the differences in colloid-associated contaminants following autumn freeze-thaw (AFT) events and spring freeze-thaw (SFT) events remain unclear. In this study, the potential influence mechanisms of AFT and SFT on both the generation and migration of colloids and colloid-associated cadmium (Cd) in soil were explored. Higher aggregate stabilities were found in soils after AFT compared with after SFT. After SFT, lower Cd concentrations were found in soil aggregates of 0.25–0.50 mm and <0.106 mm and higher concentrations were found in 0.106–0.25 mm aggregates. Moreover, SFT generated higher amounts of colloidal Cd than AFT, while AFT increased the total Cd concentration in leachates. Additionally, compared with SFT, AFT led to higher Cd concentrations in dissolved and colloid-associated forms in leachates. These findings demonstrate that higher amounts of colloid and fewer loadings of Cd in colloids in Cd contaminated soil can be found after SFT events. Thus, to better understand the environmental risk of contaminants in areas subject to seasonal freeze-thaw cycles, the differences between freeze-thaw processes in spring and autumn should be considered.

Cotransport of nano-hydroxyapatite and different Cd(II) forms influenced by fulvic acid and montmorillonite colloids

Soil colloids can affect the cotransport of nanoparticles and pollutants. In this study, the influencing mechanisms of organic fulvic acid (FA) and inorganic montmorillonite colloid (MONT) on the cotransport of nHAP and Cd(II) were investigated. Column experiments combined with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, attachment efficiency calculation and two-site kinetic retention model were applied to study the mechanisms. Results showed that the co-existence of FA or MONT made the transport of nHAP improved by 58–75% and 33–59%, respectively. Both of them could improve the stability of nHAP particles and enhance electrostatic repulsion between nHAP particles and sand. Retention of nHAP in the sand was mainly caused by secondary energy minimum and physical straining. The co-existence of FA or MONT changed the amount of adsorbed species of Cd(II) and decreased the retardation effect of nHAP on Cd(II) transport. With increasing FA concentration, soluble FA·Cd and suspended nHAP·FA·Cd complexes in the system increased. Transport of soluble Cd(II) and total Cd(II) were strengthened due to the concentration effect of FA and the improved stability of nHAP particles. With increasing MONT concentration, the amount of soluble Cd(II) decreased, but that of colloidal Cd(II) (nHAP·Cd and MONT·Cd) increased. Due to the stronger effect of colloidal Cd(II) change than that of the soluble Cd(II) change, the transport of total Cd(II) was improved by 34–57%. The findings of this study can help to understand the fate of nanoparticles and Cd(II) in natural water and soil.

Effects of Different Particle Sizes of Hydroxyapatite on the Distribution and Migration of Trace Elements (Copper and Cadmium) in a Smelter-Impacted Soil.

To study the remediation effect of hydroxyapatite with different particle sizes, a field in situ experiment was carried out by adding conventional hydroxyapatite (0.25 mm) and microhydroxyapatite (3 μm) and nanohydroxyapatite (40 nm) to the contaminated soil and planting Elsholtzia splendens. The distribution and migration of copper (Cu) and cadmium (Cd) in soil were investigated after 4 years. The results show that the application of three different particle sizes of hydroxyapatite significantly raise the soil pH, total phosphorus, and soil organic carbon. Moreover, the addition of hydroxyapatite can reduce the EXC fraction of Cu and Cd by 73.7%–80.1% and 20.8%–35.2%, respectively. In addition, the concentrations of Cu and Cd in >2 mm, 0.25–2 mm, 0.053–0.25 mm, and <0.053 mm aggregate are significantly increased. This improvement indicates that there are risks which may cause the increasing of total Cu and Cd in the soil where the pollution sources still exist. Furthermore, the content of soil colloid is significantly increased, and the colloidal Cu and Cd distribution percentage have been significantly increased by 49.9%–120% and 30.3%–181%. This result illustrates that the application of hydroxyapatite may greatly increase the possibility of colloid and dust migration of Cu and Cd.

Soil cadmium mobilisation by dissolved organic matter from soil amendments

Dissolved organic matter (DOM) release from Cd contaminated soils been linked to mobilisation of the metal as Cd-DOM complexes and this may be exacerbated by organic matter-rich soil amendments. The quantity and quality of the DOM can determine the proportion of dissolved Cd that partitions to mobile complexes and their stability and, thus, the potential for Cd transport from contaminated soils. The aim of this work was to examine differences in Cd mobilisation from soils to which different types of soil amendments/conditioners have been applied and the importance of DOM characteristics in determining the extent to which this can happen. Three soils were spiked with Cd to 2 mg kg−1, allowed to equilibrate and then treated with compost and peat. These soils and an untreated subsample of each soil were then adjusted to three different pHs: 5.6, 6.4 and 7.4, using lime. The amount of Cd mobilised from each soil was tested using a column leaching experiment. Ultrafiltration and speciation modelling were used to determine amounts of Cd as DOM-complexed, “truly” dissolved (<5 kDa) and colloidal species, while DOM quality was assessed using UV–Vis and fluorescence spectroscopy. Most colloidal Cd was mobilised from the compost treated soils (50%–60%), followed by the peat treated soils (20–44%). The relationships between colloidal Cd, DOC concentration and soil pH, together with the spectroscopic and modelling results showed that structural properties of DOM are an important factor in mobilising Cd from contaminated soils.

Liming effects on dissolved and colloid-associated transport of cadmium in soil under intermittent simulated rainfall

Liming has been regarded as an effective measure to reduce the bioavailability and mobility of cadmium (Cd) in soil. However, its effect on Cd transport in colloid-associated form remains unclear. In this study, relative importance of dissolved and colloid-associated transport of Cd was explored in columns packed with moist soil aggregates (diameter <2 mm) under intermittent simulated rainfall of distilled water or 5 mmol L−1 CaCl2 solution. The Cd2+/Ca2+ exchange selectivity coefficient determined in column experiments displayed gradual decreases with decreasing ionic strength. It is proposed that the exchange selectivity coefficient determined by repeated batch extraction can be used to predict Cd discharge in dissolved form in column effluent. Colloid-associated Cd was the main Cd form in the first flushing effluent sample upon resuming infiltration of distilled water. Otherwise, Cd was transported mainly in dissolved form, accounting for 81–93 % and 54–72 % of total Cd discharge for unlimed soils and limed soils, respectively. Liming remarkably reduced dissolved Cd concentration but only slightly enhanced colloidal Cd transport. Cd was enriched on colloids, and the enrichment factor was enhanced by liming. Colloidal Cd transport through preferential pathways (e.g., macropores, shrinkage cracks, tile drains) should be paid due attention.

Detection of Luminescence Centers in Colloidal Cd\(_{0.3}\)Zn\(_{0.7}\)S Nanocrystals by Synchronous Luminescence Spectroscopy

With the advantages of selectivity, spectral resolution and reduction of interference on account of light scattering, synchronous luminescence spectroscopy (SLS) is successfully applied to analyze complex mixtures with overlapped emission and/or excitation spectra. In fact, it is difficult to clearly distinguish the contributions of various luminescence centers to low-energy band of semiconductor nanocrystals (NCs). Herein, we report the application of SLS method to detect luminescence centers in colloidal Cd&lt;sub&gt;0.3&lt;/sub&gt;Zn&lt;sub&gt;0.7&lt;/sub&gt;S NCs. Their conventional luminescence and synchronous luminescence spectra were comparatively investigated. Differently from conventional luminescence spectrum, the emission peaks at 460 and 515 nm were found using SLS method. They are attributed to the emission transitions related to sulfur and zinc/cadmium vacancies. The obtained results are useful to clarify the nature of luminescence centers as well as relaxation mechanism in Cd&lt;sub&gt;x&lt;/sub&gt;Zn&lt;sub&gt;1-x&lt;/sub&gt;S NCs.

Colloidal toxic trace metals in urban riverine and estuarine waters of Yantai City, southern coast of North Yellow Sea

The environmental characteristics of colloidal toxic trace metals Cd, Cu and Pb in riverine and estuarine waters collected from two urban rivers of Yantai City in eastern China, the Guangdang and Xin'an Rivers, were investigated using a modified centrifugal ultrafiltration (CUF) method in conjunction with acid extraction and inductively coupled plasma mass spectrometry. The target metals in dissolved pool were divided into four CUF fractions, i.e. <1 kDa, 1–3 kDa, 3–10 kDa and 10 kDa–0.2 μm, and the results showed that colloidal Cd, Cu and Pb were dominated by 1–10 kDa (1–3 and 3–10 kDa), 1–3 kDa and 10 kDa–0.2 μm fractions, respectively. The coagulation/flocculation of low-molecular-weight (1–10 kDa) colloidal Cd and Cu in the estuaries was obvious and strong, while the enrichment of dissolved Pb in the 10 kDa–0.2 μm fraction may be mainly related to its biogeochemical interactions with Fe-oxides, which is easy to occur in macromolecular colloids. In addition, the actual molecular weight cutoffs (MWCOs) of the three used CUF units with nominal MWCOs of 1, 3 and 10 kDa were determined to be 4.9, 8.5 and 33.9 kDa, respectively, indicating that membrane calibration is essential for explaining the actual fraction of dissolved trace metals and verifying the integrity of ultrafiltration membrane. Overall, the results in this study provide a further understanding of the heterogeneity in biogeochemical features, migration and fate of toxic trace metals in aquatic ecosystems, especially that of the river-sea mixing zone.

Separation and determination of colloidal trace metals in seawater by cross-flow ultrafiltration, liquid-liquid extraction and ICP-MS

In this study, a method was developed for the desalination and pre-concentration of three colloidal trace metals in seawater, namely Cd, Cu and Pb, using diethylammonium diethyldithiocarbamate and ammonium 1-pyrrolidine dithiocarbamate as chelates. Seawater samples were first treated with cross-flow ultrafiltration (CFUF) for the separation of the colloidal matter of different molecular weights, i.e. 1–3 kDa, 3–10 kDa and 10 kDa-0.22 μm. Then, after acid digestion, the organic chelates and the extracting agent chloroform were added at pH = 3.8. Finally, the organic phase was back extracted, and the target metals in all samples were measured using inductively coupled plasma mass spectrometry (ICP-MS) with low detection limits in the range of 0.13–1.18 pmol L−1. The extraction efficiencies for the three metals were in the range of 92.2%–100.3% with an enrichment factor of 40, and their mass recoveries in the studied colloidal fractions were in the range of 83.1%–103.5%. The developed method has been successfully applied for the analysis of seawater samples collected from the sites off the south coast of North Yellow Sea in Shandong Peninsula, China. For the first time, the distribution of colloidal Cd, Cu and Pb in different molecular weights in this area is reported. The results obtained from real samples demonstrate that the developed method is simple, efficient and convenient for routine analysis with low detection limit.

Colloidal Cd x Zn1-x S nanocrystals as efficient photocatalysts for H2 production under visible-light irradiation.

CdxZn1−xS nanocrystals with sizes ranging from 3–11 nm were synthesized by a simple organic solution method. The nanocrystals possess a cubic zinc-blende structure and the bandgap blue-shifts from 2.1 eV to 3.4 eV by increasing the composition of Zn ions in the solid solutions. After a facile ligand exchange process, the photocatalytic activity for H2 production of the CdxZn1−xS nanocrystals was investigated under visible-light irradiation (λ ≥ 420 nm) with Na2SO3/Na2S as the electron donor. It was found that the Cd0.8Zn0.2S had the highest photoactivity with H2 evolution rate of 6.32 mmol g−1 h−1. By in situ adding Pt precursors into the reaction solution, inhomogenous Pt–CdxZn1−xS nanoheterostructures were formed, which accounted for a 30% enhancement for the H2 evolution rate comparing with that of pure Cd0.8Zn0.2S nanocrystals. This work highlights the use of facile organic synthesis in combination with suitable surface modification to enhance the activity of the photocatalysts.

Relationship between structural and optical properties of colloidal Cd x Zn 1 − x S quantum dots in gelatin

Aqueous synthesis of mixed cadmium and zinc sulfides colloidal quantum dots (QDs) has been successfully realized. Colloidal CdxZn1−xS QDs are formed in a cubic crystal lattice with particle size of ∼2 nm. The blueshift of optical absorption spectra from 420 to 295 nm and recombination photoluminescence from 646 to 483 nm with increasing zinc content in QDs was observed. Optimum photoluminescence intensity occurs for QDs with Cd0.3Zn0.7S composition. With increasing zinc content up to Cd0.3Zn0.7S, luminescence intensity increases and decreases when zinc content is larger than 0.7. The increase in photoluminescence intensity is explained by the increase in the number of point defects, such as complexes of interstitial metal atoms–metal vacancies [Mei−VMe]. Such complexes occur due to displacement of the metal atom at the center of the elementary tetrahedron due to substitution of one of the four sulfur atoms by an impurity atom, such as an oxygen atom.

Absorption and Magnetic Circular Dichroism Analyses of Giant Zeeman Splittings in Diffusion-Doped Colloidal Cd(1-x)Mn(x)Se Quantum Dots.

Impurity ions can transform the electronic, magnetic, or optical properties of colloidal quantum dots. Magnetic impurities introduce strong dopant-carrier exchange coupling that generates giant Zeeman splittings (ΔEZ) of excitonic excited states. To date, ΔEZ in colloidal doped quantum dots has primarily been quantified by analysis of magnetic circular dichroism (MCD) intensities and absorption line widths (σ). Here, we report ΔEZ values detected directly by absorption spectroscopy for the first time in such materials, using colloidal Cd(1-x)Mn(x)Se quantum dots prepared by diffusion doping. A convenient method for decomposing MCD and absorption data into circularly polarized absorption spectra is presented. These data confirm the widely applied MCD analysis in the low-field, high-temperature regime, but also reveal a breakdown at low temperatures and high fields when ΔEZ/σ approaches unity, a situation not previously encountered in doped quantum dots. This breakdown is apparent for the first time here because of the extraordinarily large ΔEZ and small σ achieved by nanocrystal diffusion doping.

One-pot synthesis of colloidal Cdx:CuInS2 quaternary quantum dots used as sensitizers in photovoltaic cells

Cdx:CuInS2 quaternary QDs were obtained in a one-pot reaction; these quaternary QDs exhibited enhanced photovoltaic performance compared to pristine CuInS2 ternary QDs.

Metals removal during estuarine mixing of Arvand River water with the Persian Gulf water

AbstractIn the present study, the removal of dissolved and colloidal Cd, Co, Cu, Ni and Zn in Arvand River water during estuarine mixing with the Persian Gulf water is investigated. The flocculation process was investigated for a series of mixtures with salinities ranging from 0.48 to 30.3^. The flocculation rates were indicative of the non-conservative behavior of studied metals during estuarine mixing. Rapid flocculation in the low salinity regimes was observed. The order of the final flocculation rate of metals in the river water was as follows: Co (91.2%)&gt; Cd (86.9%)&gt; Zn (83%)&gt; Cu (75.2%)&gt; Ni (74.3%). Salinity, pH, EC and dissolved oxygen do not govern the flocculation of metals during estuarine mixing. The results of the present investigation show that estuarine processes can be considered as an effective mechanism in self purification of colloidal metals that are anthropogenically introduced into the fresh water ecosystem.

A Solution Chemistry Approach for the Selective Formation of Ultralong Nanowire Bundles of Crystalline Cd(OH)2 on Substrates

Nanowires of different inorganic materials represent unique systems for exploring interesting nanoscale phenomena, and have consequently been synthesized with an eye towards applications in different fields. [1] Inorganic nanowires are also expected to play a critical role in future electronic and optoelectronic devices. [2] Since increasing emphasis has been placed recently on low cost, high throughput, high volume, and ease of production, various template-based syntheses have emerged as the method of choice for the synthesis of nanowires. [3] However, the main drawbacks associated with template-based methods arise from the production and removal of templates, and thus there has been a great deal of interest in developing template-less strategies. Furthermore, there are two prerequisites for the realization of nanodevices: the development of simple and economical methods for synthesizing nanomaterials in bulk quantities and the ability to control the dimensions of the nanostructures. [4] In recent years, nanostructures of metal hydroxides such as Ni(OH)2, Cu(OH)2, Mg(OH)2, and Cd(OH)2 have been synthesized as potential templates or precursors for the corresponding oxide materials. [5] Among the various nanostructured metal hydroxides, Cd(OH)2 is an important precursor for the eventual synthesis of functional materials such as CdS and CdSe by reaction with appropriate compounds. Previous reports of the synthesis of Cd(OH)2 nanostructures in the literature include the preparation of nanowires, [6] nanodisks, [7] and nanoflakes. [8] A colloidal Cd solution obtained by the addition of an alkali solution to a cadmium salt (pH<10) has been treated hydrothermally in an autoclave at elevated temperatures higher than 473 K. To synthesize Cd(OH)2 nanorings, the Cd(OH)2 precipitate obtained from the Cd salt has been irradiated using a highly intense ultrasonic horn. [9] Ichinose et al. [10] have reported the formation of Cd(OH)2 nanostrands in water by raising the pH of the Cd-salt solution to 9. However, not much effort has been focused on studying the directed assembly and selective production of these nanostructures on substrates. Here, we present a one-step, template-free, and seedless method for the selective growth of ultralong nanowire bundles of crystalline Cd(OH)2 on glass substrates at low temperatures through a simple method involving controlled chemical precipitation based on principles of ionic and solubility products. [11] Through controlled precipitation, as the ionic product of the supersaturated solution exceeds the solubility product, the selective growth of nanowire structures occurs on the substrate via heterogeneous nucleation. In contrast, homogeneous nucleation proceeds in the solution phase. The chemistry involved in the formation of Cd(OH)2 nanowires on substrate surfaces is discussed below. The morphology and size of the as-synthesized Cd(OH)2 nanostructures grown on glass substrates have been characterized by field-emission scanning electron microscopy (FES

New approach to synthesis of semimagnetic semiconductor nanoparticles

The technique of colloidal chemistry has been applied for synthesis of nonmagnetic CdS and Cd 1− x Mn x S semimagnetic semiconductor nanocrystals. The Cd 1− x Mn x S nanocrystals have been synthesized by means of substitution reaction. The performed TEM analysis suggests of spherical shape of nanoparticles for the colloidal Cd 1− x Mn x S nanoparticles. The optical properties of the obtained nanocomposite solution have been investigated by absorption and emission spectroscopy. Optical spectra demonstrate features which are associated with confinement effect and change of Mn content x in the Cd 1− x Mn x S nanocrystals. Compositional information have been obtained on the base of the model of weak confinement regime and estimation the energy gap for a given composition of the II 1− x Mn x VI alloys at room temperature.

Amphiphilic Cyclodextrin Carriers Embedding Porphyrins: Charge and Size Modulation of Colloidal Stability in Heterotopic Aggregates

The interaction between the anionic 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphyrin (TPPS) and cationic vesicles formed by heptakis(2-omega-amino-O-oligo(ethylene oxide)-6-hexylthio)-beta-cyclodextrin (SC6CDNH2) has been investigated in detail through a combination of elastic light scattering (ELS), quasi-elastic light scattering (QELS), zeta potential measurements, and time-resolved fluorescence anisotropy. ELS experiments provided the first structural characterization of these cationic vesicles both in the absence and in the presence of TPPS porphyrin, modeling the system as a spherical particle described by a single thin shell form factor. The structure of mixed hetero-aggregates is modulated by charge and size of the two components as function of different porphyrin/cyclodextrin (CD) molar ratios. At the limiting molar ratio studied, the absolute value of zeta potential (/zeta/ = 12.5 mV) seems to be a reference value for the formation of stable colloidal CD vesicular aggregates at thermodynamic equilibrium. New insights on the structure of these heterotopic colloids have been obtained by analysis of rotational correlation times at different molar ratios exploiting time-resolved fluorescence anisotropy experiments. At high porphyrin loads, the anisotropy decays behave as monoexponentials and the rotational correlation times (1-2 ns) together with the r(0) values close to zero suggest the presence of small amounts of TPPS embedded in a hydrophobic environment either in monomeric or in aggregated form. At the lower porphyrin/CD molar ratios, the anisotropy decays exhibit a double-exponential behavior showing a predominant component with a slow rotational correlation time (20-25 ns) and limiting anisotropy values of approximately 0.15. This component has been assigned to molecules that are more stabilized onto the CD vesicles, that is, porphyrins embedded into the oligo-ethylene "wall" of the CD vesicles. Scanning near-field optical microscopy of the samples evaporated on glass surfaces gave further insights on the morphology and optical properties of these systems, confirming the embedding of TPPS on the vesicles and evidencing the role of the solvent.