Sc, Zr, Hf, and Mn Metal Nanoparticles: Reactive Starting Materials for Synthesis Near Room Temperature.

Zerovalent scandium, zirconium, hafnium, and manganese nanoparticles are prepared by reduction of ScCl3, ZrCl4, HfCl4, and MnCl2 with lithium or sodium naphthalenide in a one-pot, liquid-phase synthesis. Small-sized monocrystalline nanoparticles are obtained with diameters of 2.4 ± 0.2 nm (Sc), 4.0 ± 0.9 nm (Zr), 8.0 ± 3.9 nm (Hf) and 2.4 ± 0.3 nm (Mn). Thereof, Zr(0) and Hf(0) nanoparticles with such size are shown for the first time. To probe the reactivity and reactions of the as-prepared Sc(0), Zr(0), Hf(0), and Mn(0) nanoparticles, they are exemplarily reacted in the liquid phase (e.g., THF, toluene, ionic liquids) with different sterically demanding, monodentate to multidentate ligands, mainly comprising O-H and N-H acidic alcohols and amines. These include isopropanol (HOiPr), 1,1'-bi-2-naphthol (H2binol), N,N'-bis(salicylidene)ethylenediamine (H2salen), 2-mercaptopyridine (2-Hmpy), 2,6-diisopropylaniline (H2dipa), carbazole (Hcz), triphenylphosphane (PPh3), N,N,N',N'-tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bipy), N,N'-diphenylformamidine (Hdpfa), N,N'-(2,6-diisopropylphenyl)-2,4-pentanediimine ((dipp)2nacnacH), 2,2'-dipydridylamine (Hdpa), and 2,6-bis(2-benzimidazolyl)pyridine (H2bbp). As a result, 22 new compounds are obtained, which frequently exhibit a metal center coordinated only by the sterically demanding ligand. Options and restrictions for the liquid-phase syntheses of novel coordination compounds using the oxidation of base-metal nanoparticles near room temperature are evaluated.

Np(V) Retention at the Illite du Puy Surface.

In this study, Np(V) retention on Illite du Puy (IdP) was investigated since it is essential for understanding the migration behavior of Np in argillaceous environments. The presence of structural Fe(III) and Fe(II) in IdP was confirmed by Fe K-edge X-ray absorption near-edge structure (XANES) and 57Fe Mössbauer spectroscopy. In batch sorption experiments, a higher Np sorption affinity to IdP was found than to Wyoming smectite or iron-free synthetic montmorillonite. An increase of the relative Np(IV) ratio sorbed onto IdP with decreasing pH was observed by solvent extraction (up to (24 ± 2)% at pH 5, c0(Np) = 10-6 mol/L). Furthermore, up to (33 ± 5)% Np(IV) could be detected in IdP diffusion samples at pH 5. Respective Np M5-edge high-energy resolution (HR-) XANES spectra suggested the presence of Np(IV/V) mixtures and weakened axial bond covalency of the NpO2+ species sorbed onto IdP. Np L3-edge extended X-ray absorption fine structure (EXAFS) analysis showed that significant fractions of Np were coordinated to Fe─O entities at pH 9. This highlights the potential role of Fe(II/III) clay edge sites as a strong Np(V) surface complex partner and points to the partial reduction of sorbed Np(V) to Np(IV) via structural Fe(II).

Spatiotemporal Mineral Phase Evolution and Arsenic Retention in Microfluidic Models of Zerovalent Iron-Based Water Treatment.

Arsenic (As) is a toxic element, and elevated levels of geogenic As in drinking water pose a threat to the health of several hundred million people worldwide. In this study, we used microfluidics in combination with optical microscopy and X-ray spectroscopy to investigate zerovalent iron (ZVI) corrosion, secondary iron (Fe) phase formation, and As retention processes at the pore scale in ZVI-based water treatment filters. Two 250 µm thick microchannels filled with single ZVI and quartz grain layers were operated intermittently (12 h flow/12 h no-flow) with synthetic groundwater (pH 7.5; 570 µg/L As(III)) over 13 and 49 days. Initially, lepidocrocite (Lp) and carbonate green rust (GRC) were the dominant secondary Fe-phases and underwent cyclic transformation. During no-flow, lepidocrocite partially transformed into GRC and small fractions of magnetite, kinetically limited by Fe(II) diffusion or by decreasing corrosion rates. When flow resumed, GRC rapidly and nearly completely transformed back into lepidocrocite. Longer filter operation combined with a prolonged no-flow period accelerated magnetite formation. Phosphate adsorption onto Fe-phases allowed for downstream calcium carbonate precipitation and, consequently, accelerated anoxic ZVI corrosion. Arsenic was retained on Fe-coated quartz grains and in zones of cyclic Lp-GRC transformation. Our results suggest that intermittent filter operation leads to denser secondary Fe-solids and thereby ensures prolonged filter performance.

Towards Heteroleptic Dicoordinate CuII Complexes.

In this work we detail our efforts to systematically generate stable dicoordinate CuII complexes. Initial experiments via metathesis reactions of a bulky potassium carbazolide (RK) with copper(II) salts indeed yielded a stable product, RCuOTf (1). However, subsequent attempts to grasp systematic synthetic access to complexes of the type RCuX (X=monoanionic ligand) proved difficult as many of the complexes rapidly decomposed in solution. By using triflate-related ligands such as ethyl sulfate and bistriflimide, the additional dicoordinate copper complexes RCuOSO3 Et (2), [RCu(THF)][Cu(NTf2 )2 ] (3) and RCuNTf2 (4) could be isolated. Spectroscopic indications corroborate more CuI than CuII character in all RCuX derivatives.

Lattice Dynamics and Structural Phase Transitions in Eu2O3.

Using the density functional theory, we study the structural and lattice dynamical properties of europium sesquioxide (Eu2O3) in the cubic, trigonal, and monoclinic phases. The obtained lattice parameters and energies of the Raman modes show a good agreement with the available experimental data. The Eu-partial phonon density of states calculated for the cubic structure is compared with the nuclear inelastic scattering data obtained from a 20 nm thick Eu2O3 film deposited on a YSZ substrate. A small shift of the experimental spectrum to higher energies results from a compressive strain induced by the substrate. On the basis of lattice and phonon properties, we analyze the mechanisms of structural transitions between different phases of Eu2O3.

Transformation of Hexagonal Birnessite upon Reaction with Thallium(I): Effects of Birnessite Crystallinity, pH, and Thallium Concentration.

We examined the uptake of Tl(I) by two hexagonal birnessites and related phase transformations in laboratory experiments over 12 sequential additions of 0.01 M Tl(I)/Mn at pH 4.0, 6.0, and 8.0. The Tl-reacted Mn oxides were characterized for their structure, Tl binding, and morphology using X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies, and transmission electron microscopy. Very limited Tl oxidation was observed in contrast to previous works, where equal Tl(I)/Mn was added in a single step. Instead, both birnessites transformed into a 2 × 2 tunneled phase with dehydrated Tl(I) in its tunnels at pH 4, but only partially at pH 6, and at pH 8.0 they remained layered. The first four to nine sequential Tl(I)/Mn additions resulted in lower residual dissolved Tl+ concentrations than when the same amounts of Tl(I)/Mn were added in single steps. This study thus shows that the repeated reaction of hexagonal birnessites with smaller Tl(I)/Mn at ambient temperature triggers a complete phase conversion with Tl(I) as the sole reacting cation. The novel pathway found may be more relevant for contaminated environments and may help explain the formation of minerals like thalliomelane [Tl+(Mn7.54+Cu0.52+)O16]; it also points to the possibility that other reducing species trigger similar Mn oxide transformation reactions.

Effects of metal cation substitution on hexavalent chromium reduction by green rust.

Chromium contamination is a serious environmental issue in areas affected by leather tanning and metal plating, and green rust sulfate has been tested extensively as a potential material for in situ chemical reduction of hexavalent chromium in groundwater. Reported products and mechanisms for the reaction have varied, most likely because of green rust's layered structure, as reduction at outer and interlayer surfaces might produce different reaction products with variable stabilities. Based on studies of Cr(III) oxidation by biogenic Mn (IV) oxides, Cr mobility in oxic soils is controlled by the solubility of the Cr(III)-bearing phase. Therefore, careful engineering of green rust properties, i.e., crystal/particle size, morphology, structure, and electron availability, is essential for its optimization as a remediation reagent. In the present study, pure green rust sulfate and green rust sulfate with Al, Mg and Zn substitutions were synthesized and reacted with identical chromate (CrO42-) solutions. The reaction products were characterized by X-ray diffraction, pair distribution function analysis, X-ray absorption spectroscopy and transmission electron microscopy and treated with synthetic δ-MnO2 to assess how easily Cr(III) in the products could be oxidized. It was found that Mg substitution had the most beneficial effect on Cr lability in the product. Less than 2.5% of the Cr(III) present in the reacted Mg-GR was reoxidized by δ-MnO2 within 14 days, and the particle structure and Cr speciation observed during X-ray scattering and absorption analyses of this product suggested that Cr(VI) was reduced in its interlayer. Reduction in the interlayer lead to the linkage of newly-formed Cr(III) to hydroxyl groups in the adjacent octahedral layers, which resulted in increased structural coherency between these layers, distinctive rim domains, sequestration of Cr(III) in insoluble Fe oxide bonding environments resistant to reoxidation and partial transformation to Cr(III)-substituted feroxyhyte. Based on the results of this study of hexavalent chromium reduction by green rust sulfate and other studies, further improvements can also be made to this remediation technique by reacting chromate with a large excess of green rust sulfate, which provides excess Fe(II) that can catalyze transformation to more crystalline iron oxides, and synthesis of the reactant under alkaline conditions, which has been shown to favor chromium reduction in the interlayer of Fe(II)-bearing phyllosilicates.

Synthesis and Structural Characterization of a Pure ZnAl4(OH)12(SO4)·2.6H2O Layered Double Hydroxide.

The phase purity of a series of ZnAl4(OH)12SO4· nH2O layered double hydroxides (ZnAl4-LDH) obtained from a reaction of bayerite (Al(OH)3) with an excess of zinc(II) sulfate under hydrothermal conditions was investigated as a function of the reaction temperature, the duration of the hydrothermal treatment, and the zinc(II) concentration. The product quality, i.e., crystalline impurities, Al impurities, and bulk Zn:Al ratio, were assessed by powder X-ray diffraction (PXRD), 27Al MAS NMR, and elemental analysis. Structural characterization of a stoichiometric ZnAl4-LDH (120 °C, 9 days, and 2.8 M Zn(II)) showed a well-defined structure of the metal ion layer as evidenced by a single, well-defined Zn environment: i.e., no Zn substitution on the Al sites according to Zn k-edge EXAFS and PXRD. Furthermore, nearly all of the 12 different 1H atoms in the -OH groups and 4 27Al resonances could be assigned using 1H,27Al NMR correlation experiments recorded with ultrafast MAS. The interlayer water content is variable on the basis of thermogravimetric analysis and changes in the 1H MAS NMR spectra with temperature. A composition of ZnAl4(OH)12(SO4)·2.6H2O was obtained from a combination of these techniques and confirmed that ZnAl4-LDH is isostructural with the mineral nickelalumite (NiAl4(OH)12SO4·3H2O).

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles.

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M4 high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V), and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L3 XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to nonstoichiometric UO2+x nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M4 HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L3 XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).

Towards Printed Organic Light-Emitting Devices: A Solution-Stable, Highly Soluble CuI -NHetPHOS.

The development of iridium-free, yet efficient emitters with thermally activated delayed fluorescence (TADF) was an important step towards mass production of organic light-emitting diodes (OLEDs). Progress is currently impeded by the low solubility and low chemical stability of the materials. Herein, we present a CuI -based TADF emitter that is sufficiently chemically stable under ambient conditions and can be processed by printing techniques. The solubility is drastically enhanced (to 100 g L-1 ) in relevant printing solvents. The integrity of the complex is preserved in solution, as was demonstrated by X-ray absorption spectroscopy and other techniques. In addition, it was found that the optoelectronic properties are not affected even when partly processing under ambient conditions. As a highlight, we present a TADF-based OLED device that reached an efficiency of 11±2 % external quantum efficiency (EQE).

Electron transfer budgets and kinetics of abiotic oxidation and incorporation of aqueous sulfide by dissolved organic matter.

The reactivity of natural dissolved organic matter toward sulfide and has not been well studied with regard to electron transfer, product formation, and kinetics. We thus investigated the abiotic transformation of sulfide upon reaction with reduced and nonreduced Sigma-Aldrich humic acid (HA), at pH 6 under anoxic conditions. Sulfide reacted with nonreduced HA at conditional rate constants of 0.227-0.325 h(-1). The main transformation products were elemental S (S0) and thiosulfate (S2O3(2-)), yielding electron accepting capacities of 2.82-1.75 µmol e- (mg C)(-1). Native iron contents in the HA could account for only 6-9% of this electron transfer. About 22-37% of S reacted with the HA to form organic S (Sorg). Formation of Sorg was observed and no inorganic transformation products occurred for reduced HA. X-ray absorption near edge structure spectroscopy supported Sorg to be mainly zerovalent, such as thiols, organic di- and polysulfides, or heterocycles. In conclusion, our results demonstrate that HA can abiotically reoxidize sulfide in anoxic environments at rates competitive to sulfide oxidation by molecular oxygen or iron oxides.

Thallium speciation and extractability in a thallium- and arsenic-rich soil developed from mineralized carbonate rock.

We investigated the speciation and extractability of Tl in soil developed from mineralized carbonate rock. Total Tl concentrations in topsoil (0-20 cm) of 100-1000 mg/kg are observed in the most affected area, subsoil concentrations of up to 6000 mg/kg Tl in soil horizons containing weathered ore fragments. Using synchrotron-based microfocused X-ray fluorescence spectrometry (µ-XRF) and X-ray absorption spectroscopy (µ-XAS) at the Tl L3-edge, partly Tl(I)-substituted jarosite and avicennite (Tl2O3) were identified as Tl-bearing secondary minerals formed by the weathering of a Tl-As-Fe-sulfide mineralization hosted in the carbonate rock from which the soil developed. Further evidence was found for the sequestration of Tl(III) into Mn-oxides and the uptake of Tl(I) by illite. Quantification of the fractions of Tl(III), Tl(I)-jarosite and Tl(I)-illite in bulk samples based on XAS indicated that Tl(I) uptake by illite was the dominant retention mechanism in topsoil materials. Oxidative Tl(III)uptake into Mn-oxides was less relevant, probably because the Tl loadings of the soil exceeded the capacity of this uptake mechanism. The concentrations of Tl in 10 mM CaCl2-extracts increased with increasing soil Tl contents and decreasing soil pH, but did not exhibit drastic variations as a function of Tl speciation. With respect to Tl in contaminated soils, this study provides first direct spectroscopic evidence for Tl(I) uptake by illite and indicates the need for further studies on the sorption of Tl to clay minerals and Mn-oxides and its impact on Tl solubility in soils.

Physico-chemical properties of the new generation IV iron preparations ferumoxytol, iron isomaltoside 1000 and ferric carboxymaltose.

The advantage of the new generation IV iron preparations ferric carboxymaltose (FCM), ferumoxytol (FMX), and iron isomaltoside 1000 (IIM) is that they can be administered in relatively high doses in a short period of time. We investigated the physico-chemical properties of these preparations and compared them with those of the older preparations iron sucrose (IS), sodium ferric gluconate (SFG), and low molecular weight iron dextran (LMWID). Mössbauer spectroscopy, X-ray diffraction, and Fe K-edge X-ray absorption near edge structure spectroscopy indicated akaganeite structures (ß-FeOOH) for the cores of FCM, IIM and IS, and a maghemite (γ-Fe2O3) structure for that of FMX. Nuclear magnetic resonance studies confirmed the structure of the carbohydrate of FMX as a reduced, carboxymethylated, low molecular weight dextran, and that of IIM as a reduced Dextran 1000. Polarography yielded significantly different fingerprints of the investigated compounds. Reductive degradation kinetics of FMX was faster than that of FCM and IIM, which is in contrast to the high stability of FMX towards acid degradation. The labile iron content, i.e. the amount of iron that is only weakly bound in the polynuclear iron core, was assessed by a qualitative test that confirmed decreasing labile iron contents in the order SFG ≈ IS > LMWID ≥ FMX ≈ IIM ≈ FCM. The presented data are a step forward in the characterization of these non-biological complex drugs, which is a prerequisite to understand their cellular uptake mechanisms and the relationship between the structure and physiological safety as well as efficacy of these complexes.

Labile or stable: can homoleptic and heteroleptic PyrPHOS-copper complexes be processed from solution?

Luminescent Cu(I) complexes are interesting candidates as dopants in organic light-emitting diodes (OLEDs). However, open questions remain regarding the stability of such complexes in solution and therefore their suitability for solution processing. Since the emission behavior of Cu(I) emitters often drastically differs between bulk and thin film samples, it cannot be excluded that changes such as partial decomposition or formation of alternative emitting compounds upon processing are responsible. In this study, we present three particularly interesting candidates of the recently established copper-halide-(diphenylphosphino)pyridine derivatives (PyrPHOS) family that do not show such changes. We compare single crystals, amorphous bulk samples, and neat thin films in order to verify whether the material remains stable upon processing. Solid-state nuclear magnetic resonance (MAS (31)P NMR) was used to investigate the electronic environment of the phosphorus atoms, and X-ray absorption spectroscopy at the Cu K edge provides insight into the local electronic and geometrical environment of the copper(I) metal centers of the samples. Our results suggest that--unlike other copper(I) complexes--the copper-halide-PyrPHOS clusters are significantly more stable upon processing and retain their initial structure upon quick precipitation as well as thin film processing.

Structural incorporation of As5+ into hematite.

Hematite (α-Fe2O3) is one of the most common iron oxides and a sink for the toxic metalloid arsenic. Arsenic can be immobilized by adsorption to the hematite surface; however, the incorporation of As in hematite was never seriously considered. In our study we present evidence that, besides adsorption, the incorporation of As into the hematite crystals can be of great relevance for As immobilization. With the coupling of nanoresolution techniques and X-ray absorption spectroscopy the presence of As (up to 1.9 wt %) within the hematite crystals could be demonstrated. The incorporated As(5+) displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe4As2O11), a triclinic iron arsenate with structural relations to hematite, can epitaxially intergrow along the (210) plane with the (0001) plane of hematite. This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As(5+) by iron oxides.

Characteristic of phosphorus rich compounds in the incinerated sewage sludge ashes: a case for sustainable waste management.

Growing concern over mineral resources supply forces us to search for alternative sources of Phosphorus. The possibility to recover phosphorus from incinerated sewage sludge ashes appears to be an important aspect in anthropogenic phosphorus cycle and sustainable economy. To make phosphorus recovery efficient it is important to learn the chemical and mineral composition of ash and phosphorus speciation. The phosphorus content in the ash was over 7%, what corresponds to medium rich phosphorus ores. The main phosphorus rich mineral phases were phosphate minerals. The most widespread was tri-calcium phosphate Whitlockite with various Fe, Mg and Ca proportions. In minority Fe-PO4 and Mg-PO4 were detected. Whitlockite commonly overgrown with hematite, influences negatively mineral solubility and thus recovery potential and indicates low bioavailability of phosphorus. Considerable amount of phosphorus was found in the low crystalline matrix where phosphorus content was around 10 wt% however low crystallinity and dispersed phosphorus also does not strengthen the potential to recover this element.

Evaluation of thallium isotopic fractionation during the metallurgical processing of sulfides: An update.

In this study, we report combined Tl isotopic and Tl mineralogical and speciation data from a set of Tl-rich sulfide concentrates and technological wastes from hydrometallurgical Zn extraction. We also present the first evaluation of Tl isotopic ratios over a cycle of sulfide processing, from the ore flotation to pyro- and hydrometallurgical stages. The results demonstrate that the prevailing Tl form in all samples is Tl(I), without any preferential incorporation into sulfides or Tl-containing secondary phases, indicating an absence of Tl redox reactions. Although the Tl concentrations varied significantly in the studied samples (~9-280 mg/kg), the overall Tl isotopic variability was small, in the range of -3.1 to -4.4 ± 0.7 (2σ) ε205Tl units. By combining present ε205Tl results with the trends first found for a local roasting plant, it is possible to infer minimum Tl isotopic effects throughout the studied industrial process. As a result, the use of Tl isotopic ratios as a source proxy may be complicated or even impossible in areas with naturally high/extreme Tl background contents. On the other hand, areas with two or more isotopically contrasting Tl sources allow for relatively easy tracing, i.e., in compartments which do not suffer from post-depositional isotopic redistributions.

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring.

A high-repetition-rate pump-probe experiment is presented, based on the asynchronous sampling approach. The low-α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time-domain sampling of the coherent THz emission as well as for hard X-ray pump-probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.

Phonon confinement and interface lattice dynamics of ultrathin high-k rare earth sesquioxide films: the case of Eu2O3 on YSZ(001).

The spatial confinement of atoms at surfaces and interfaces significantly alters the lattice dynamics of thin films, heterostructures and multilayers. Ultrathin films with high dielectric constants (high-k) are of paramount interest for applications as gate layers in current and future integrated circuits. Here we report a lattice dynamics study of high-k Eu2O3 films with thicknesses of 21.3, 2.2, 1.3, and 0.8 nm deposited on YSZ(001). The Eu-partial phonon density of states (PDOS), obtained from nuclear inelastic scattering, exhibits broadening of the phonon peaks accompanied by up to a four-fold enhancement of the number of low-energy states compared to the ab initio calculated PDOS of a perfect Eu2O3 crystal. Our analysis demonstrates that while the former effect reflects the reduced phonon lifetimes observed in thin films due to scattering from lattice defects, the latter phenomenon arises from an ultrathin EuO layer formed between the thin Eu2O3 film and the YSZ(001) substrate. Thus, our work uncovers another potential source of vibrational anomalies in thin films and multilayers, which has to be cautiously considered.

Photoluminescence of Fully Inorganic Colloidal Gold Nanocluster and Their Manipulation Using Surface Charge Effects.

Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal-organic ligand-capped counterparts. As their synthesis by chemical methods is challenging, details about their optical properties remain widely unknown. In this work, laser fragmentation in liquids is performed to produce fully inorganic and size-controlled colloidal gold NCs with monomodal particle size distributions and an fcc-like structure. Results reveal that these NCs exhibit highly pronounced photoluminescence with quantum yields of 2%. The emission behavior of small (2-2.5 nm) and ultrasmall (<1 nm) NCs is significantly different and dominated by either core- or surface-based emission states. It is further verified that emission intensities are a function of the surface charge density, which is easily controllable by the pH of the surrounding medium. This experimentally observed correlation between surface charge and photoluminescence emission intensity is confirmed by density functional theoretical simulations, demonstrating that fully inorganic NCs provide an appropriate material to bridge the gap between experimental and computational studies of NCs. The presented study deepens the understanding of electronic structures in fully inorganic colloidal gold NCs and how to systematically tune their optical properties via surface charge density and particle size.