The authors would like to acknowledge the State of Washington for supporting this research through the University of Washington Clean Energy Institute Exploratory Fellowship Program, and National Science Foundation (NSF) Sustainable Energy Pathway (SEP) Award CHE-1230615.

Caution: Cadmium precursors are highly toxic and must be handled with great care. Wear proper protective equipment, use appropriate engineering controls and consult relevant materials safety data sheets (MSDS). In addition, the formation of nanoparticles may present additional hazards. The reactions described herein are conducted with a standard vacuum gas manifold, in order to conduct the experiments within an inert atmosphere. All chemicals were purchased commercially and used as received. This protocol is based upon a previously developed synthetic method, which we recently described elsewhere.5
1. Sulfur Copolymer Synthesis
<ol>
	<li>Preparation of Molten Elemental Sulfur
	<ol>
		<li>Place elemental sulfur (4 g, 124.8 mmol, S8, 99.5%) in a 50 ml three neck flask with an attached condenser and temperature probe. Perform pump and purge cycles with vacuum and nitrogen several times.</li>
		<li>Heat under nitrogen to 150 &#176;C with stirring, which will cause the sulfur to become a yellow colored liquid.</li>
	</ol>
	</li>
	<li>Preparation of Sulfur Copolymer
	<ol>
		<li>Once all of the sulfur has dissolved into the liquid, immediately inject &#945;-methylstyrene (330 &#181;l, 2.5 mmol, 99%) into the solution.</li>
		<li>Heat solution to 185 &#176;C with stirring at 500 rpm for 10 min. As the copolymer forms, the solution will change color from yellow to orange, finally producing a deep red color.</li>
		<li>Remove the solution from heat and cool to room temperature. As it cools, the copolymer will slowly crystallize to form an orange rubbery solid. At this stage, the copolymer can be stored at room temperature for a subsequent synthesis or it can be used immediately.</li>
	</ol>
	</li>
</ol>
2. CdS Nanoparticle Synthesis
<ol>
	<li>Add cadmium acetylacetonate (Cd(acac), 900 mg, 2.9 mmol 99.9%) to the three neck flask from the previous step, so that the powder is placed evenly on top of the solid sulfur copolymer (4.0 g, 116 mmol).</li>
	<li>Perform pump and purge cycles on the flask with nitrogen and vacuum several times.</li>
	<li>Heat the solution to 200 &#176;C under nitrogen with stirring. The sulfur copolymer will melt and mix with the cadmium precursor, and the nanoparticle nucleation and growth processes will begin.</li>
	<li>Allow the nanoparticles to grow for 30 min. 
	Note: Varying the reaction time will influence the growth of the nanoparticles, so it is possible to tune the final size of the nanoparticles.5 A 30 min reaction time will with a size range of 7-10 nm.5</li>
	<li>Remove the solution from heat and allow to cool to room temperature.</li>
	<li>Once cooled, remove the solid nanocomposite from the flask and store at room temperature.</li>
</ol>
3. Remove the Sulfur Copolymer and Isolate the Nanoparticles&#160;
<ol>
	<li>Removal of the Sulfur Copolymer
	<ol>
		<li>Place the nanocomposite (200 mg) in a 20 ml glass vial and add chloroform (20 ml).</li>
		<li>Place the vial in an ultrasonicator and sonicate for 1 hr, to break up the nanocomposite and suspend the nanoparticles within the solution.</li>
		<li>Separate the solution into two 30 ml centrifuge tubes and add another 20 ml of chloroform to each.</li>
		<li>Centrifuge the solution at 8,736 x g (relative centrifugal force) for 15 min.</li>
		<li>Decant the sulfur copolymer from the centrifuge tubes, making sure to not disturb the settled nanoparticles.</li>
	</ol>
	</li>
	<li>Isolation of the Nanoparticles
	<ol>
		<li>Re-disperse the settled nanoparticles by adding chloroform to each centrifuge tube (30 ml) and sonicate for 15 min.</li>
		<li>Repeat the steps described in sections 3.1.4, 3.1.5 and 3.2.1 three more times to ensure that all of the sulfur copolymer has been removed. Once all of the sulfur copolymer is removed, the decanted solution will no longer have an orange color.</li>
		<li>Collect the final nanoparticles by adding chloroform (2 ml) to each centrifuge tube.</li>
		<li>Combine the collected nanoparticles in one 20 ml glass vial (4 ml solution total) and place the glass vial under vacuum to remove all of the chloroform and to dry the nanoparticles. At this stage, the mass of the resulting nanoparticles can be determined and compared to the starting mass of the precursors in order to determine the yield of the reaction using molar ratios of the starting material and product.</li>
	</ol>
	</li>
</ol>
4. Characterize the CdS Nanoparticles
<ol>
	<li>Transmission Electron Microscopy
	<ol>
		<li>Dilute the isolated nanoparticles (20 mg) in chloroform (20 ml) and ultrasonicate for 1 hr.</li>
		<li>Dilute this solution in chloroform (5 drops/5 ml) and sonicate for 15 min.</li>
		<li>Drop the final solution onto an ultrathin carbon film substrate with holey carbon support films on a 400 mesh copper Transmission Electron Microscopy (TEM) grid.</li>
		<li>Place the TEM grid in a glass vial and hold under vacuum overnight, to remove any residual solvent from the sample.</li>
		<li>Once the drying is completed, acquire TEM images using a 200 kV accelerating voltage, a spot size of 3 and an attached Energy Dispersive X-ray Spectroscopy (EDS) detector.</li>
	</ol>
	</li>
	<li>X-ray Diffraction&#160;
	<ol>
		<li>Dilute the isolated nanoparticles in chloroform (10 mg/ml).</li>
		<li>Clean molybdenum coated soda lime glass substrates (1 cm2) by sonicating in detergent, deionized water, acetone and isopropyl alcohol, each for 10 min. Finally, clean the substrates in an air plasma cleaner for 10 min prior to drop casting.</li>
		<li>Drop cast the solution from 4.2.1 onto the substrates from 4.2.2 in 7 &#181;l increments.</li>
		<li>Once the films have dried, acquire X-ray Diffraction (XRD) data. Collect data using 7,000 data points at a scan rate of 1 data point per second with a Cu-K&#945; X-ray source and an incident wavelength of 1.54059 &#197;.</li>
	</ol>
	</li>
	<li>Solution Spectroscopy
	<ol>
		<li>Disperse the isolated nanoparticles (0.1 mg/ml) in chloroform and sonicate for 30 min and place samples in a sealed quartz cuvette.</li>
		<li>Disperse the nanocomposite from section 2.6 and the sulfur copolymer from section 1.2.3 in formamide (1 mg/ml), stir at 700 rpm, and heat to 70 &#176;C to facilitate suspension of the material.</li>
		<li>Acquire photoluminescence (PL) and absorbance spectra for all three samples. Conduct optical absorbance measurements using a spectrometer with a triple detector that extends across the ultraviolet, visible and near infrared ranges (UV-Vis-NIR). Conduct PL measurements using a fluorescence spectrophotometer with an excitation wavelength of 330 nm. 
		NOTE: The specific protocol for characterizing nanoparticles using the techniques discussed in sections 4.1.5, 4.2.4, and 4.3.2 varies widely depending on the nature of the specific equipment used, so we present only general characterization parameters here. The interested reader is directed to several review papers for more information regarding using these analysis techniques for CdS nanoparticles.17-19</li>
	</ol>
	</li>
</ol>

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P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Semiconductor+Clusters,+Nanocrystals,+and+Quantum+Dots.">Semiconductor Clusters, Nanocrystals, and Quantum Dots.</a> Science. 271 (5251), 933-937 (1996).</li><li>Alivisatos, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perspectives+on+the+Physical+Chemistry+of+Semiconductor+Nanocrystals.">Perspectives on the Physical Chemistry of Semiconductor Nanocrystals.</a> J. Phys. 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The authors disclose no competing financial interests.

We have developed a method to synthesize CdS nanoparticles within a sulfur copolymer matrix. This sulfur copolymer is composed of elemental sulfur and methylstyrene.5 An important feature of this method is that the copolymer can be used as both a high-temperature solvent and a sulfur source that reacts with a cadmium precursor to produce CdS nanoparticles in solution.5 The critical step in the procedure is the synthesis of the sulfur copolymer with a suitable ratio of methylstyrene and sulfur. The use of methylstyrene and in the appropriate concentration is important to form the copolymer that can be stored as a powder at room temperature, and then be heated to produce a liquid to allow the nanoparticle synthesis to occur. During the synthesis, only some of the sulfur reacts with the cadmium precursor, while the remainder stays within the sulfur copolymer.5 Once the reaction solution is cooled and solidified, this procedure produces CdS nanoparticles suspended within a sulfur copolymer matrix.5 The copolymer can subsequently be removed, producing nanoparticles that do not have conventional organic ligands.5 A limitation of our method however, is that once the copolymer is removed, the nanoparticles are prone to aggregation, which can be prevented when conventional organic ligands are present. This makes it difficult to control the self-assembly of our nanoparticles during film formation. The nanoparticle size can be tuned by varying the reaction time, although another limitation is that this method produces nanoparticles with a higher level of polydispersity in size than conventional techniques,5,18 which broadens the absorption and emission spectra of our nanoparticles.
This nascent synthesis method can be useful in a range of fields where the presence of conventional aliphatic ligands diminishes the charge transfer or surface reaction properties of the nanoparticles. Furthermore, this synthesis method is relatively simple in comparison to other methods that require multistep ligand exchange or ligand stripping procedures to remove conventional aliphatic ligands.1-4 For example, one set of nanoparticles could be functionalized with a range of ligands to produce nanoparticles with a spectrum of different properties.9 In addition, this method could be used to produce nanoparticle based devices where the presence of organic ligands would otherwise introduce unwanted carbon impurities.10,12

Although proven useful for synthesis, conventional aliphatic ligands present a number of challenges for the implementation of nanoparticles in photonic and electrochemical devices. Aliphatic ligands are highly insulating, hydrophobic, and constitute a significant barrier to electrochemical surface reactions.1 Accordingly, several studies have developed ligand exchange and ligand stripping protocols that replace these aliphatic ligands with functional moieties or that strip away the ligands to reveal a bare nanoparticle surface.1-3 These reactions, however, pose several intrinsic problems. They significantly add to the complexity of the synthetic process, do not always go to completion, and can deteriorate the surface of the nanoparticles, which can in turn impose significant problems during device fabrication when using these techniques.4
We have developed a sulfur copolymer that can be used as both a high temperature solvent and sulfur source during the synthesis of CdS nanoparticles.5 This copolymer is based on a network copolymer developed by Chung et al. that uses elemental sulfur and 1,3-diisopropenylbenzene (DIB).6 In our case, a methylstyrene monomer is implemented instead of DIB. The methylstyrene monomer limits cross-linking reactions, which would otherwise produce a high molecular weight network copolymer.5,6 The presence of only one vinylic functional group on the methylstyrene monomer promotes the formation of oligomeric radicals once heated, which allows the sulfur copolymer to operate as a liquid solvent and sulfur source in parallel during the nanoparticle synthesis.5 Specifically, the sulfur polymer is produced by heating elemental sulfur to 150 &#176;C, which causes the S8 rings to transition into a linearly structured liquid sulfur diradical form. Next, methylstyrene is injected into the liquid sulfur in a 1:50 molar ratio of methylstyrene molecules to sulfur atoms.5 The methylstyrene double bond reacts with the sulfur chains to produce the copolymer, as presented in Figure 1.5 The sulfur copolymer is then cooled and the cadmium precursor is added. This mixture is then reheated to 200 &#176;C, during which, the sulfur copolymer melts and the nanoparticle nucleation and growth processes are initiated within the solution.5 A 20:1 molar ratio of sulfur to cadmium precursor is used, so that only some of the sulfur is consumed during the reaction.5 This copolymer stabilizes the nanoparticles by suspending them within a solid polymer matrix once the reaction has been terminated.5 The copolymer can be removed after the synthesis, resulting in the production of CdS nanoparticles that do not have organic coordinating ligands, as depicted in Figure 2.5
The synthetic method presented in this work is relatively simple in comparison with other methods presented in the literature.1-3,7 It is applicable for a diverse range of applications where traditional ligated nanoparticles have proven problematic or undesirable. This technique can open doors to higher throughput testing, where one batch of nanoparticles can be used to examine a complete spectrum of subsequent functionalizations without the need for complex and time consuming ligand stripping or exchange procedures.2,4,8,9 These unligated nanoparticles also offer opportunities to reduce the number of carbon defects commonly observed in printed nanoparticle devices, by eliminating the carbon source.10-16 This detailed protocol is intended to help others implement this new method and to help spur its active use in a variety of fields that will find it of particular significance.

<table><tbody><tr><td>Sulfur (S&lt;sub&gt;8&lt;/sub&gt;), 99.5%</td><td>Sigma Aldrich</td><td>84683</td><td></td></tr><tr><td>&amp;alpha;-methylstyrene, 99%</td><td>Sigma Aldrich</td><td>M80903</td><td></td></tr><tr><td>Cadmium acetylacetonate (Cd(acac)), 99.9%</td><td>Sigma Aldrich</td><td>517585</td><td>Highly Toxic</td></tr><tr><td>Chloroform (CHCl&lt;sub&gt;3&lt;/sub&gt;), 99.5%</td><td>Sigma Aldrich</td><td>C2432</td><td></td></tr><tr><td>Hotplate / magnetic stirrer</td><td>IKA RCT&amp;nbsp;</td><td>3810001</td><td></td></tr><tr><td>Temperature controller with probe and heating mantle</td><td>Oakton Temp 9000</td><td>WD-89800</td><td></td></tr><tr><td>Centrifuge</td><td>Beckman Coulter Allegra X-22</td><td>392186</td><td></td></tr><tr><td>Centrifuge Tubes</td><td>Thermo Scientific</td><td>3114</td><td>Teflon for resistance to chlorinated solvents</td></tr><tr><td>TEM with attached EDS detector</td><td>FEI Tecnai G2 F-20 with EDAX detector</td><td></td><td></td></tr><tr><td>TEM Sample Grid</td><td>Ted Pella</td><td>1824</td><td>Ultrathin carbon film substrate with holey carbon support films on a 400 mesh copper grid</td></tr><tr><td>XRD</td><td>Bruker F-8 Focus Diffractometer</td><td></td><td></td></tr><tr><td>Molybdenum coated soda lime glass substrates</td><td></td><td></td><td>750 nm thick sputtered molybdenum layer</td></tr><tr><td>Quartz Fluorescence Cuvettes</td><td>Sigma Aldrich</td><td>Z803073</td><td>10 mm by 10 mm, 4 polished sides with screw top</td></tr><tr><td>UV-Vis-NIR</td><td>Perkin Elmer Lambda 1050 Spectrometer</td><td></td><td>With 3D WB Detector Module</td></tr><tr><td>PL</td><td>Horiba FL3-21tau Fluorescence Spectrophotometer</td><td></td><td></td></tr></tbody></table>

synthesis of ligand-free cds nanoparticles within a sulfur copolymer matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

The TEM image in Figure 3a shows small CdS nanoparticles (3-4 nm) that have nucleated within the sulfur copolymer before the sulfur copolymer has been completely removed. The image in Figure 3a was acquired by taking an aliquot of the nanoparticle solution immediately after the solution reached 200 &#176;C. Figure 3b shows larger nanoparticles (7-10 nm) that have grown in solution for 30 min before the sulfur copolymer has been completely removed. Figure 3c shows a higher magnification image of the highlighted area in Figure 3b. One nanoparticle in Figure 3c has a particularly clear atomic plane spacing that was measured to be 3.3 &#197;. The 3.3 &#197; atomic plane spacing is consistent with the (111) plane spacing of zinc-blende CdS or the (002) plane spacing of wurtzite CdS. Figure 3d shows that once the sulfur copolymer is completely removed and dissolved in chloroform, the nanoparticles will aggregate together. The EDS data presented in Figure 3d confirms the stoichiometry of CdS (nearly 1:1) and confirms that the sulfur copolymer has been effectively removed.
In a previous publication, we used Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy (1H NMR) to show that once the sulfur copolymer is completely removed, these nanoparticles do not have conventional organic ligands.5 One study by Nag et. al. showed that metal-sulfide nanoparticles can be effectively ligated with anionic sulfur species.7 We posit that the nanoparticles made using this method have similarly structured sulfur species on the nanoparticle surface. The EDS data presented in Figure 3d shows a slight stoichiometric excess of sulfur, which is in accordance with the presence of sulfur species on the nanoparticle surface.
The XRD pattern for the drop cast nanoparticle thin films is presented in Figure 4 and is consistent with the formation of wurtzite and possibly also zinc blende structured CdS. The peak located at 26.6 degrees corresponds to an atomic plane spacing of 3.3 &#197;, which is consistent with the (111) plane spacing of zinc-blende CdS or the (002) plane spacing of wurtzite CdS and corroborates the planar spacing seen in the TEM image of Figure 3c.
The sulfur copolymer and the nanocomposite were analyzed using UV-Vis spectroscopy as shown in Figure 5. Since the concentrations of each dispersion are equal, the data show that the nanocomposite exhibits a significantly enhanced absorbance in comparison to only the sulfur copolymer. The photoluminescence data presented in Figure 6 show that the nanocomposite exhibits a peak that is blue-shifted from the bulk bandgap of CdS (510 nm, 2.4 eV), while the sulfur copolymer peak is comparatively small.
The isolated nanoparticles were also examined using UV-Vis-NIR and PL spectroscopy once the sulfur copolymer was removed. Figure 7 shows both the UV-Vis-NIR and PL data. In Figure 6, the nanoparticles have a broad PL peak that is centered at the bulk bandgap of CdS and a correspondingly broad absorption spectrum with a small absorption edge that is in the range of 450-550 nm. Once the sulfur polymer is removed, the nanoparticle surface is no longer passivated and contains surface defects due to the presence of dangling bonds. This leads to the presence of new surface-mediated energy states that subsequently broaden and red-shift the PL and absorbance spectra in comparison to the spectra presented in Figures 5 and 6 for the nanocomposite material.18,20-22 In addition, the polydispersity of the nanoparticles results in a population of nanoparticles that exhibit quantum confinement effects, which also broadens the spectra.23 Therefore, in conjunction with our previous work, this data supports the claim that these nanoparticles do not have conventional defect passivating organic ligands once the sulfur copolymer is removed.5 Furthermore, this data shows that the sulfur copolymer passivates the surface defects of the CdS nanoparticles prior to its removal.
<img alt="Figure 1" src="/files/ftp_upload/54047/54047fig1.jpg" /> 
Figure 1. Synthesis and structure of sulfur copolymer. Elemental sulfur is heated to produce a linearly structured liquid sulfur diradical, which reacts with methylstyrene to produce the sulfur copolymer. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54047/54047fig2.jpg" /> 
Figure 2. Nanoparticle growth and Isolation. The CdS nanoparticles nucleate and grow within the sulfur copolymer. Once the reaction is complete, the sulfur copolymer is removed to produce ligand-free CdS nanoparticles. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54047/54047fig3.jpg" /> 
Figure 3. TEM images of isolated CdS nanoparticles. a) 3-4 nm CdS nanoparticles beginning to grow within the copolymer. b) 7-10 nm nanoparticles once the reaction is terminated. S copolymer is still present at this stage. c) Magnified image of highlighted region in b. d) TEM image after removal of S copolymer. Inset shows EDS data. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54047/54047fig4.jpg" /> 
Figure 4. XRD pattern for CdS nanoparticle films. XRD pattern is consistent with the formation of wurtzite or zinc-blende CdS. The molybdenum peak is due to the substrate. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54047/54047fig5.jpg" /> 
Figure 5. Absorbance spectroscopy for sulfur copolymer and nanocomposite. The nanocomposite exhibits a significantly enhanced absorption peak in comparison to the sulfur copolymer alone. Both materials have equal concentrations while in dispersion. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54047/54047fig6.jpg" /> 
Figure 6. Photoluminescence spectroscopy for sulfur copolymer and nanocomposite. The nanocomposite exhibits a peak that is blue-shifted from the bulk bandgap of CdS (510 nm, 2.4 eV), while the sulfur copolymer exhibits comparatively little photoluminescence. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/54047/54047fig7.jpg" /> 
Figure 7. Absorbance and Photoluminescence spectroscopy of ligand-free CdS nanoparticles. The PL data show a broad peak centered at 510 nm. The UV-Vis-NIR data show a broad absorption curve with a weak absorption shoulder in the range of 450-550 nm. <a href="https://www.jove.com/files/ftp_upload/54047/54047fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix at JoVE.com

Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature ...

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Nanyang Technological University

Research

JoVE Journal

Chemistry

9.4K Views.  University of Washington. Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Video: Synthesis of Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Engineering

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large ...

Synthesis of Cd-free InP/ZnS Quantum Dots Suitable for Biomedical Applications

Fluorescent nanocrystals, specifically quantum dots, have been a useful tool for many biomedical applications. For successful use in biological systems, quantum dots should be highly fluorescent and small/monodisperse in size. While commonly used cadmium-based quantum dots possess these qualities, they are potentially toxic due to the possible release of Cd2+ ions through nanoparticle degradation. Indium-based quantum dots, specifically InP/ZnS, have recently been explored as a viable alternative to cadmium-based quantum dots due to their relatively similar fluorescence characteristics and size. The synthesis presented here uses standard hot-injection techniques for effective nanoparticle growth; however, nanoparticle properties such as size, emission wavelength, and emission intensity can drastically change due to small changes in the reaction conditions. Therefore, reaction conditions such temperature, reaction duration, and precursor concentration should be maintained precisely to yield reproducible products. Because quantum dots are not inherently soluble in aqueous solutions, they must also undergo surface modification to impart solubility in water. In this protocol, an amphiphilic polymer is used to interact with both hydrophobic ligands on the quantum dot surface and bulk solvent water molecules. Here, a detailed protocol is provided for the synthesis of highly fluorescent InP/ZnS quantum dots that are suitable for use in biomedical applications.

In this protocol, the synthesis of Cd-free InP/ZnS quantum dots (QDs) is detailed. InP-based QDs are gaining popularity due to the toxicity of Cd2+ ions that may be released through nanoparticle degradation. After synthesis, QDs are solubilized in water using an amphiphilic polymer for use in biomedical applications.

Fluorescent nanocrystals, specifically quantum dots, have been a useful tool for many biomedical applications. For successful use in biological systems, ...

Advanced Compositional Analysis of Nanoparticle-polymer Composites Using Direct Fluorescence Imaging

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise composition of these materials is essential, especially in the design of surface catalysts, where the surface concentration of the active component determines the activity of the material. Antimicrobial materials which utilize nanoparticles are a particular focus of this technology. Recently swell encapsulation has emerged as a technique for inserting antimicrobial nanoparticles into a host polymer matrix. Swell encapsulation provides the advantage of localizing the incorporation to the external surfaces of materials, which act as the active sites of these materials. However, quantification of this nanoparticle uptake is challenging. Previous studies explore the link between antimicrobial activity and surface concentration of the active component, but this is not directly visualized. Here we show a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of CdSe/ZnS nanoparticles can be accurately visualized through cross-sectional fluorescence imaging. Using this method, we can quantify the uptake of nanoparticles via swell encapsulation and measure the surface concentration of encapsulated particles, which is key in optimizing the activity of functional materials.

Here we present a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of cadmium selenide quantum dots can be accurately visualized through cross-sectional fluorescence imaging.

The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise ...

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

Synthesis of a Thiol Building Block for the Crystallization of a Semiconducting Gyroidal Metal-sulfur Framework

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and porous metal-dithiolene network in the highly ordered single crystalline state. Unlike the highly reactive free-standing thiols, which tend to decompose and complicate the crystallization of metal-thiolate open frameworks, the thioester reacts in situ to provide the thiol species, serving to mitigate the reaction between the mercaptan units and the metal centers, and to improve crystallization consequently. Specifically, the thioester was synthesized in a one-pot procedure: an aromatic bromide (hexabromotriphenylene) reacted with excess sodium thiomethoxide under vigorous conditions to first form the thioether intermediate product. The thioether was then demethylated by the excess thiomethoxide to provide the thiolate anion that was acylated to form the thioester product. The thioester was conveniently purified by standard column chromatography, and then used directly in the framework synthesis, wherein NaOH and ethylenediamine serve to revert in situ the thioester to the thiol linker for assembling the single-crystalline Pb(II)-dithiolene network. Compared with other methods for thiol synthesis (e.g., by cleaving alkyl thioether using sodium metal and liquid ammonia), the thioester synthesis here uses simple conditions and economical reagents. Moreover, the thioester product is stable and can be conveniently handled and stored. More importantly, in contrast to the generic difficulty in accessing crystalline metal-thiolate open frameworks, we demonstrate that using the thioester for in situ formation of the thiol linker greatly improves the crystallinity of the solid-state product. We intend to encourage broader research efforts on the technologically important metal-sulfur frameworks by disclosing the synthetic protocol for the thioester as well as the crystalline framework solid.

Here, we present a one-pot, transition-metal-free synthesis of thiols and thioesters from aromatic halides and sodium thiomethoxide, followed by the preparation of single crystals of a metal-dithiolene network using thiol species generated in situ from the more stable and tractable thioester.

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and ...

Ligand-Mediated Nucleation and Growth of Palladium Metal Nanoparticles

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution of capping ligands during the synthesis reaction, their role in regulating the nucleation and growth rates of colloidal nanoparticles is not well understood. In this work, we demonstrate a mechanistic investigation of the role of trioctylphosphine (TOP) in Pd nanoparticles in different solvents (toluene and pyridine) using in situ SAXS and ligand-based kinetic modeling. Our results under different synthetic conditions reveal the overlap of nucleation and growth of Pd nanoparticles during the reaction, which contradicts the LaMer-type nucleation and growth model. The model accounts for the kinetics of Pd-TOP binding for both, the precursor and the particle surface, which is essential to capture the size evolution as well as the concentration of particles in situ. In addition, we illustrate the predictive power of our ligand-based model through designing the synthetic conditions to obtain nanoparticles with desired sizes. The proposed methodology can be applied to other synthesis systems and therefore serves as an effective strategy for predictive synthesis of colloidal nanoparticles.

The main goal of this work is to elucidate the role of capping agents in regulating the size of palladium nanoparticles by combining in situ small angle x-ray scattering (SAXS) and ligand-based kinetic modeling.

The size, size distribution and stability of colloidal nanoparticles are greatly affected by the presence of capping ligands. Despite the key contribution ...

The Effect of Ultraviolet Radiation on the Chemical Bath Deposition of Bis(thiourea) Cadmium Chloride Crystals and the Subsequent CdS Obtention

In this work, the effects on the preparation of bis(thiourea) cadmium chloride crystals when illuminated with ultraviolet (UV) light at a wavelength of 367 nm using the chemical bath deposition technique are studied comparatively. Two experiments are performed to make a comparison: one without UV light and the other with the aid of UV light. Both experiments are performed under equal conditions, at a temperature of 343 K and with a pH of 3.2. The precursors used are cadmium chloride (CdCl2) and thiourea [CS(NH2)2], which are dissolved in 50 mL of deionized water with an acidic pH. In this experiment, the interaction of electromagnetic radiation is sought at the moment the chemical reaction is carried out. The results demonstrate the existence of an interaction between the crystals and the UV light; the UV light assistance causes crystal growths in an acicular shape. Also, the final product obtained is cadmium sulfide and shows no evident difference when synthesized with or without the use of UV light.

The Effect of Ultraviolet Radiation on the Chemical Bath Deposition of Bisthiourea Cadmium Chloride Crystals and the Subsequent CdS Obtention

This article presents a protocol for the synthesis of bis(thiourea) cadmium chloride crystals by chemical bath deposition. Two experiments are described: one aided by ultraviolet light compared to one without ultraviolet light.

In this work, the effects on the preparation of bis(thiourea) cadmium chloride crystals when illuminated with ultraviolet (UV) light at a wavelength of ...