Digestive Ripening Research Articles

Digestive ripening: a synthetic method par excellence for core–shell, alloy, and composite nanostructured materials

The solvated metal atom dispersion (SMAD) method has been used for the synthesis of colloids of metal nanoparticles. It is a top-down approach involving condensation of metal atoms in low temperature solvent matrices in a SMAD reactor maintained at 77 K. Warming of the matrix results in a slurry of metal atoms that interact with one another to form particles that grow in size. The organic solvent solvates the particles and acts as a weak capping agent to halt/slow down the growth process to a certain extent. This as-prepared colloid consists of metal nanoparticles that are quite polydisperse. In a process termed as digestive ripening, addition of a capping agent to the as-prepared colloid which is polydisperse renders it highly monodisperse either under ambient or thermal conditions. In this, as yet not well-understood process, smaller particles grow and the larger ones diminish in size until the system attains uniformity in size and a dynamic equilibrium is established. Using the SMAD method in combination with digestive ripening process, highly monodisperse metal, core–shell, alloy, and composite nanoparticles have been synthesized. This article is a review of our contributions together with some literature reports on this methodology to realize various nanostructured materials.

Self‐Assembled Ultra‐High Aspect Ratio Silver Nanochains

A simple chemical route to fabricating highly linear, well-aligned and unbranched one-dimensional arrays of silver nanoparticles with ultra-high aspect ratios is demonstrated. Starting with a polydisperse nanoparticle material, the application of ultrasound was found to promote digestive ripening and induce a thiolate-driven self-assembly process. The assembly method discovered has been extended to other systems, with initial findings showing the successful templating of gold nanoparticles into chains using Cu(I)-thiolates. This opens the possibility of a generic route which can be applied to many nanoparticle systems.

Gram‐Scale Synthesis of Multipod Pd Nanocrystals by a Simple Solid–Liquid Phase Reaction and Their Remarkable Electrocatalytic Properties

AbstractIn this paper, we describe how multipod Pd nanocrystals (NCs) have been synthesized on the gram scale by means of a simple solid–liquid phase reaction route (i.e., thermal reduction of solid Pd(CH3COO)2 in the liquid mixture of dodecylamine, oleic acid, and 1‐octadecene under a temperature‐programmed mode). The nanostructure evolves from the initially generated larger polyhedral NCs into smaller ones and then into the final multipods. The dodecylamine acts as both a mild reductant and a promoter, which affects the reduction rate and decreases the size of the initially formed polyhedral PdNCs. The morphology of the final NCs, such as tri‐ and tetrapods, may be determined by the number of growing points on each polyhedral NC. According to the temperature‐ and time‐dependent experiments, a multistep growth mechanism including digestive ripening, oriented attachment, and fusion process is proposed. This simple solid–liquid phase reaction route can be extended to prepare other multipod metal nanostructures. The multipod PdNCs are found to have a high electrochemically active surface area and possess excellent electrocatalytic performance toward the oxidation of formic acid. Relative to that of polyhedral Pd and commercial Pd black catalysts, the multipod PdNCs exhibit much higher catalytic activity and long‐term stability, which may make them a good candidate catalyst for direct formic acid fuel cells. This developed synthetic strategy together with the provided fundamental understanding of heterogeneous nucleation and growth has great potential for contriving a rational route to the preparation of advanced nanomaterials with specific morphology for catalytic and other functional applications.

Direct Synthesis of Aqueous Quantum Dots through 4,4′-Bipyridine-Based Twin Ligand Strategy

We report a new class of derivatized 4,4'-bipyridinium ligands for use in synthesizing highly fluorescent, extremely stable, water-soluble CdSe and CdTe quantum dots (QDs) for bioconjugation. We employed an evaporation-condensation technique, also known as solvated metal atom dispersion (SMAD), followed by a digestive ripening procedure. This method has been used to synthesize both metal nanoparticles and semiconductors in the gram scale with several stabilizing ligands in various solvents. The SMAD technique comprised evaporation condensation and stabilization of CdSe or CdTe in tetrahydrofuran. The as-prepared product was then digestively ripened in both water and dimethyl formamide, leading to narrowing of the particle size distributions. The ligands were synthesized by nucleophilic substitution (S(N)2) reactions using 4,4'-bipyridine as a nucleophile. Confocal microscopy images revealed the orange color of the nanocrystalline QDs with diameters of ~5 nm. The size has been confirmed by using transmission electron microscopy. As a part of our strategy, 85% of the 4,4'-bipyridinium salt was synthesized as propionic acid derivative and used to both stabilize the QDs in water and label basic amino acids and different biomarkers utilizing the carboxylic acid functional group. Fifteen percent of the 4,4'-bipyridinium salt was synthesized as N-propyl maleimide and used as a second ligand to label any protein containing the amino acid cysteine by means of a 1,4-Michael addition.

Transformation of Indium Nanoparticles to β-Indium Sulfide: Digestive Ripening and Visible Light-Induced Photocatalytic Properties

We report the transformation of polydispersed dodecanethiol stabilized indium nanoparticles, obtained from bulk indium shot by evaporation/condensation solvated metal atom dispersion (SMAD) technique, into highly monodispersed partially alkyl thiolate-capped β-indiumsulfide (In(2)S(3)) by a postpreparative digestive ripening in high boiling point t-butyltoluene (190 °C) solvent. Upon digestive ripening, the as-prepared polydispersed black indium nanoparticles showed a characteristic color transition from black to cream, pale yellow, yellow, and finally to brown, indicating the transformation of the indium metal nanoparticles into intermediates composed of indium thiolates, sulfides, and polysulfides, and finally into the product In(2)S(3) nanoparticles whose surfaces are partially capped with thiolates. The transformed product (In(2)S(3)) was characterized with UV-vis, XRD, EDX, SEM, XPS, and TEM. From XRD and TEM measurements, the average size of the transformed In(2)S(3) nanoparticles is 5 nm. The optical absorbance of the as-prepared sample showed absorption peaks around 538 and 613 nm; upon digestive ripening these two peaks disappeared and stabilized at 375 nm, providing evidence of strong quantum confinement of excitons. The visible light-induced photocatalytic activity test with the In(2)S(3) nanoparticles showed that 95% of Rhodamine B (RhB) dye degraded after 100 min of irradiation with visible light.

Effect of digestive ripening agent on nanoparticle size in the digestive ripening process

Digestive ripening is an efficient method that leads to monodispersed nanoparticles from polydispersed ones. A systematic investigation of digestive ripening with different agents viz. dodecanethiol and dodecylamine on different metallic systems is presented. It is shown that both the metallic system involved and the digestive ripening agent have influences on the final size distribution. Further, it is demonstrated that the interaction strength between the different metals and the digestive ripening agents based on Hard Soft Acid Base principles could be invoked to explain the variation in nanoparticle size.

Synthesis of CdSe/ZnS and CdTe/ZnS Quantum Dots: Refined Digestive Ripening

We report synthesis of CdSe and CdTe quantum dots (QDs) from the bulk CdSe and CdTe material by evaporation/co‐condensation using the solvated metal atom dispersion (SMAD) technique and refined digestive ripening. The outcomes of this new process are (1) the reduction of digestive ripening time by employing ligands (trioctylphosphine oxide (TOPO) and oleylamine (OA)) as capping agent as well as digestive ripening solvent, (2) ability to tune the photoluminescence (PL) from 410 nm to 670 nm, (3) demonstrate the ability of SMAD synthesis technique for other semiconductors (CdTe), (4) direct comparison of CdSe QDs growth with CdTe QDs growth based on digestive ripening times, and (5) enhanced PL quantum yield (QY) of CdSe QDs and CdTe QDs upon covering with a ZnS shell. Further, the merit of this synthesis is the use of bulk CdSe and CdTe as the starting materials, which avoids usage of toxic organometallic compounds, eliminates the hot injection procedure, and size selective precipitation processes. It also allows the possibility of scale up. These QDs were characterized by UV‐vis, photoluminescence (PL), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), X‐ray photoelectron spectroscopy (XPS), and powder XRD.

Colloidal calcium nanoparticles: digestive ripening in the presence of a capping agent and coalescence of particles under an electron beam

The nanochemistry of calcium remains unexplored, which is largely due to the inaccessibility of calcium nanoparticles in an easy to handle form by conventional methods of synthesis as well as its highly reactive and pyrophoric nature. The synthesis of colloidal Ca nanoparticles by the solvated metal atom dispersion (SMAD) method is described. The as-prepared Ca–THF nanoparticles, which are polydisperse, undergo digestive ripening in the presence of a capping agent, hexadecyl amine (HDA) to afford highly monodisperse colloids consisting of 2–3 nm sized Ca–HDA nanoparticles. These are quite stable towards precipitation for long periods of time, thereby providing access to the study of the nanochemistry of Ca. Particles synthesized in this manner were characterized by UV-visible spectroscopy, high resolution electron microscopy, and powder X-ray diffraction methods. Under an electron beam, two adjacent Ca nanoparticles undergo coalescence to form a larger particle.

Mechanistic Studies on Sintering of Silver Nanoparticles

Metal nanoparticles are known to form highly conductive films upon heating below 200 °C. We study the mechanism and morphological changes that occur as 3 nm, thiolate encapsulated, silver nanoparticles are annealed to form conductive films. We use X-ray diffraction (XRD), grazing incidence X-ray scattering (GIXS), and transmission electron microscopy (TEM) to monitor structural changes in the film. We show that the surfactant is present during the entire sintering process, that its presence greatly influences the grain size and crystallite orientation of the resulting film, and that the film becomes fully conductive in the presence of the surfactant. We show that particles that aggregate more rapidly form films that consist of smaller crystallites and are less textured. We further show that digestive ripening can lead to the degeneration of films back into particles, particularly when annealing in air versus an inert environment. Coalescence contributes to crystallite growth when particles are small but c...

Using Size-Exclusion Chromatography to Evaluate Changes in the Sizes of Au and Au/Pd Core/Shell Nanoparticles Under Thermal Treatment

In this study, we used size-exclusion chromatography (SEC) to evaluate the sizes of Au and Au/Pd core/shell nanoparticles (NPs) that had been subjected to thermal treatment, with the eluted NPs monitored through diode array detection (DAD) of the surface plasmon (SP) absorption of the NPs. In the absence of an adequate stabilizer, thermal treatment resulted in longer retention times for the Au NPs and shorter retention times for the Au/Pd core/shell NPs in the SEC chromatograms. Thus, thermal treatment influenced the sizes of these Au and Au/Pd core/shell NPs, through digestive ripening and Ostwald-type growth, respectively. In addition, the trends in the SP absorption phenomena of the NPs in the eluted samples, as measured using DAD, were consistent with the trends of their size variations, as measured from their elution profiles. In the presence of 3A-amino-3A-deoxy-(2AS,3AS)-β-cyclodextrin (H2N-β-CD) as a stabilizer, the retention times and SP absorptions of the eluted Au and Au/Pd NP samples remained constant. Thus, H2N-β-CD is a good stabilizer against size variation duration the thermal treatment of Au and Au/Pd core/shell NPs. A good correlation existed between the sizes obtained using SEC and those provided by transmission electron microscopy. Therefore, this SEC strategy is an effective means of further searching for suitable stabilizers for NPs, especially those exposed to harsh reaction conditions (e.g., in catalytic reactions).

Synthesis of Indium Nanoparticles: Digestive Ripening under Mild Conditions

Here we report the synthesis of monodispersed indium nanoparticles by evaporation/condensation of indium shot using the solvated metal atom dispersion (SMAD) technique, followed by digestive ripening in low boiling point (BP 38 °C) methylene chloride and in a high boiling point (BP 110 °C) toluene solvent. The as-prepared SMAD indium nanoparticles are polydispersed with particle size ranging from 25 to 50 nm, but upon digestive ripening (heating of colloidal material at the boiling point of solvent in presence of excess surface active ligands) in methylene chloride, a remarkable reduction of particle size was achieved. In higher boiling solvent (toluene), where the indium nanoparticles at reflux temperature are probably melted, it does not allow the best result, and less monodispersity is achieved. We employed different surface active ligands (amine, phosphine, and mixed ligands) to passivate these indium nanoparticles. The temporal evolution of the surface plasmon of indium nanoparticles was monitored by in situ UV-vis spectroscopy, and particles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The merits of this synthesis procedure are the use of bulk indium as starting material, tuning the particle size in low boiling point solvent, particle size adjustment with the choice of ligand, and a possible scale up.

Many manifestations of digestive ripening: monodispersity, superlattices and nanomachining

Digestive ripening has now been established as a very convenient route to obtain monodisperse nanoparticles from polydisperse ones by refluxing the latter in the presence of an excess ligand. Many ligands including long chain thiols, amines, or phosphines have been shown to be effective digestive ripening agents. It is hypothesized that the surface active groups of such digestive ripening agents bind and remove reactive surface atoms/clusters from big nanoparticles and redeposit them on smaller nanoparticles. In this way, large particles become smaller, while small particles become larger, and eventually, an equilibrium size is obtained that is specific to each of the digestive ripening agents used. Herein, the digestive ripening procedure is reviewed, discussed and its utility spanning the preparation of monodispersed metal nanoparticles, alloy nanoparticles, superlattice structures and the most interesting nano-machining (wherein the monodisperse particles can be reverted back to the polydisperse system) is demonstrated.

Synthesis of Au(Core)/Ag(Shell) Nanoparticles and their Conversion to AuAg Alloy Nanoparticles

Metal nanoparticles (NPs) are of great interest due to their special optical, [ 1–3 ] electronic, [ 4–8 ] and catalytic [ 9,10 ] properties. [ 11 ] Among metal NPs, Au NPs have been investigated most extensively because of their facile preparation, resistance to oxidation, and surface plasmon resonance (SPR) band that can absorb and scatter visible light. [ 3 ] Core/ shell and alloy bimetallic NPs are especially interesting because they provide opportunities to tune the NPs’ optical and catalytic properties [ 12–15 ] and are potentially useful as taggants for security applications. [ 2 ] The AuAg system is of particular interest because the SPR band is tunable between ∼ 520 nm for Au [ 11 ] and ∼ 410 nm for Ag. [ 16 ] Several syntheses for AuAg alloy, [ 15 , 17–37 ] Au(core)/Ag(shell), [ 22 , 25 , 31 , 33 , 35–45 ] and Ag(core)/Au(shell) NPs [ 28 , 31 , 33–35 , 42 , 46–48 ] have already been reported. Here, we report a facile, stoichiometrically controlled synthesis of Au(core)/Ag(shell) and AuAg alloy NPs through digestive ripening, [ 49–51 ] which is a potentially general method for synthesizing alloy NPs. [ 37 , 52,53 ] Au(core)/Ag(shell) NPs were synthesized and annealed to form AuAg alloy NPs, followed by elemental analysis and structural and optical characterization. Methods utilizing Au rather than Ag NPs as the seed particles are advantageous: obtaining monodisperse Ag NPs is signifi cantly more challenging [ 16 ] because it is harder to control the nucleation of Ag NPs and to avoid oxidation. Huang and co-workers recently demonstrated the conversion of Au(core)/Ag(shell) to AuAg alloy NPs as a part of a larger study showing the generality of digestive ripening for synthesizing alloy NPs, but very limited data without quantitative elemental analysis or optical characterization of the AuAg alloy NPs was provided. [ 37 ] In this study, we compare a twostep synthesis of Au(core)/Ag(shell) NPs and their conversion to AuAg alloy NPs through annealing with a one-step, direct conversion of Au NPs to AuAg alloy NPs. The products of

Bimetallic Ag/Au nanoparticles: A low temperature ripening strategy in aqueous solution

Bimetallic Ag/Au nanoparticles (NPs) with alloyed and core–shell structures were synthesized at a low temperature (75 °C) from the preformed gold and silver NPs in aqueous solution. The transmission electron microscope (TEM) mapping images, as well as the evolution of UV/vis spectroscopy, revealed that both the digestive ripening and Ostwald ripening were responsible for the formation of above structures during the heating process. Higher Ag ratios led to the growth of silver shell on the surface of Ag/Au alloy core, while lower Ag ratios tended to the formation of alloy particles. Interparticle diffusion, ligands and chemical potential were presumed to play important roles in the formation of alloyed and core–shell bimetallic Ag/Au NPs.

Synthesis and characterization of Pd(0), PdS, and Pd@PdO core–shell nanoparticles by solventless thermolysis of a Pd–thiolate cluster

Colloids of palladium nanoparticles have been prepared by the solvated metal atom dispersion (SMAD) method. The as-prepared Pd colloid consists of particles with an average diameter of 2.8±0.1 nm. Digestive ripening of the as-prepared Pd colloid, a process involving refluxing the as-prepared colloid at or near the boiling point of the solvent in the presence of a passivating agent, dodecanethiol resulted in a previously reported Pd–thiolate cluster, [Pd(SC 12H 25) 2] 6 but did not render the expected narrowing down of the particle size distribution. Solventless thermolysis of the Pd–thiolate complex resulted in various Pd systems such as Pd(0), PdS, and Pd@PdO core–shell nanoparticles thus demonstrating its versatility. These Pd nanostructures have been characterized using high-resolution electron microscopy and powder X-ray diffraction methods.

A facile “green” synthesis of ascorbic acid-capped ZnSe nanoparticles

A simple, green room temperature synthesis of ascorbic acid-capped ZnSe nanoparticles is hereby reported. By varying the pH of the solution, the temporal evolution of the optical properties and shape of the nanocrystals was investigated. The nanoparticles were characterized by UV–vis absorption and photoluminescence spectroscopy (PL), transmission electron microscope (TEM) and infra-red spectroscopy (IR). All the particles exhibited quantum confinement in their optical spectra. An atypical optical spectrum was observed at pH 11 after 5 h attributed to digestive ripening and shrinkage of ZnSe core. From the TEM image we inferred that the reaction is kinetically driven at pH 7 producing elongated particles as the reaction times increases, while spherical particles are produced at pH 4 and 11. The IR spectroscopy confirmed the capping of ascorbic acid and its deprotonation to give ascorbate ions.

Wet Chemical Synthesis of Monodisperse Colloidal Silver Nanocrystals Using Digestive Ripening

Silver metal nanocrystals with passivating ligand shells of dodecanethiol were synthesized by a one-phase wet chemical method. Postsynthesis thermochemical processing, known as digestive ripening, was used to focus the size distribution from a very broad distribution to 5.6 ± 0.4 nm. Chemical analysis of the final Ag nanocrystals showed a metallic core with a densely packed ligand shell. No oxidation was observed despite the entirely aerobic conditions. The size distribution was found to change most dramatically after the two steps proceeding digestive ripening, namely, ligand exchange and precipitation. Imaging also showed that the digestive ripening mechanism of Ag nanocrystals could be significantly different than that for Au, and involved a competitive silver thiolate side product. This synthesis was optimized to produce Ag nanocrystals with little to no silver thiolate materials.

The extended time evolution size decrease of gold nanoparticles formed by the Turkevich method

Gold nanoparticle (Au NP) solutions were synthesised by the Turkevich reduction method and stored in either the light or dark. All solutions were monitored daily using UV-visible absorption spectroscopy and displayed surface plasmon resonance (SPR), typical of Au nanoparticle colloids. An increase in SPR intensity, a narrowing of the SPR peak as well as a gradual shift towards lower wavenumbers over time indicated a decrease in average nanoparticle diameter and a more mono-dispersed particle size. After two weeks no further changes were observable by UV-visible absorption spectroscopy. A series of high resolution transmission electron micrographs (HRTEM) taken over the evolution period confirmed that the plasmon resonance shifts correlated to a decrease in nanoparticle size. A systematic size decrease in nanoparticle size was also observed for solutions even after centrifugation to remove the excess un-reacted citrate and auric acid. This indicated that the size evolution was independent of further excess reactant chemistries and charge stabilities. The gold nanoparticle evolution followed an inverse Ostwald type growth, by which the size of the NPs decreases, in effect a digestive ripening. The aging process provides a reliable route to fairly mono-dispersed gold nanoparticles of ca. 11.5–12.5 nm in size via the Turkevich method.

Combination of Digestive Ripening and Seeding Growth As a Generalized Route for Precisely Controlling Size of Monodispersed Noble Monometallic, Shell Thickness of Core−Shell and Composition of Alloy Nanoparticles

On the basis of combining the effect of digestive ripening and seeding growth, a generalized route for precisely controlling the size of noble monodisperse (MD) nanoparticles (NPs), the shell thickness of core−shell NPs, and composition of alloy NPs has been reported. First, small NPs are dispersed in high-boiling-point organic solvent as seeds; second, an alkyl amine-containing metal complex is used as the NP precursor; and finally, the metal atoms are reduced by the alkyl amine and grow on the surface of the seeds to form large NPs. The combining effect of seeding growth and digestive ripening render this route the ability to precisely control the size and maintain monodispersity. Through adjusting the reactive conditions, various MD noble NPs (Ag, Au, Pd), core−shell NPs (Ag@Pd, Pd@Au, Pd@Ag, Au@Pd, Au@Ag, Pt@Au, and Pt@Pd), and alloy nanoparticles(Ag/Au) can be prepared. The diameter, shell thickness, and composition of MD NPs can be precisely controlled by adjusting the ratio of the precursor/seed, o...

From Ultrafine Thiolate-Capped Copper Nanoclusters toward Copper Sulfide Nanodiscs: A Thermally Activated Evolution Route

This report shows that the size, shape, and composition of presynthesized copper nanoparticles can be nanoengineered through exploiting concurrent interparticle aggregative growth and interfacial carbon−sulfur cleavage in a thermally activated evolution route. This is demonstrated by thermally activated processing of ultrafine copper nanoclusters encapsulated with thiolate monolayer (Cun(SR)m) toward semiconducting copper sulfide (Cu2S) nanodiscs with controllable sizes and shapes. Under controlled temperatures (120−150 °C), the ultrafine Cun(SR)m nanoclusters, with a size of ∼0.5 nm evidenced by TEM, SAXS-WAXS, DCP-AES, and MALDI-TOF measurements, were shown to evolve into thiolate-capped Cu2S nanodiscs via thermally activated coalescence and copper-catalyzed interfacial C−S cleavage reactivities. The Cu2S nanodiscs, as confirmed by XPS and HRTEM analyses, exhibited controllable and monodispersed sizes depending on the thermal processing parameters, ranging from 5 to 35 nm in the disk dimension and 3−6 nm in the thickness dimension. These nanodiscs are stable and display remarkable 1D/2D ordering upon self-assembly. This process is not a simple digestive ripening of smaller particles because it involves an aggregative nucleation and growth process distinctively different from traditional ripening and a reactive carbon−sulfur bond cleavage controlled by the catalytic effect of copper under the specified temperatures. The coupling of the thermally activated coalescence and C−S bond cleavage to convert the ultrafine Cu nanoclusters toward the formation of Cu2S nanodiscs is highly effective for tuning nanoscale size, shape, and composition, and could find applications in nanoengineering a variety of semiconducting nanocrystals for applications in nanostructured electronic, sensing, and photochemical devices.