Editorial pubs.acs.org/cm
Best Practices for the Reporting of Colloidal Inorganic Nanomaterials
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analyzing only the crystalline portion of any sample and, thus, cannot be used as the sole method to determine composition. 2. Determination of Nanoparticle Size Distribution. Transmission or scanning electron micrographs with histograms (in which at least 300 particles are measured per sample) and, if possible, a solution technique to characterize size distributions (dynamic light scattering, DLS, or a correlation technique to measure diffusion coefficients which then produce an average size) are most useful. Authors cannot claim that their material has a specific size, or dispersity, without such good quality, statistically validated data. A standard deviation with respect to size below 20% would be considered acceptable for most syntheses, but nanoparticles cannot be termed “monodisperse” if their size distribution is not within ±5% of the mean size.24 The inclusion of a histogram for size analysis is important for reasons beyond determination of size. It is also possible that a sample of nanoparticles has a bimodal distribution of particle size, and in that case, reporting of only average size would be misleading. Histograms showing measured size distributions should be provided for readers to see for themselves the nature of the sample. For very complex samples (e.g., a core−corona assembly of one type of nanoparticle surrounding another type of nanoparticle), a representative image is required as a figure, but then further data discussed in the text ought to quantify the extent of assembly and its scalability (e.g., “40% dimers, 20% trimers, 10% oligomers, remainder apparently monomers, made at the 10-mg scale”). A histogram analysis would be very useful in quantifying and sharing this data with the reader. 3. Shape Distribution. Nanorods, nanowires, octapods, cubes, stars, to name a fewthese are all interesting shapes of inorganic nanomaterials that can provide increased functionality for optical, electronic, biological, and catalytic applications, for instance. Therefore, it is imperative that the readers of such papers see evidence for both the purity and the yield of such shapes. It must be clear in the paper how “yield” is defined: Are 90% of the objects in the electron microscope the interesting shape (“shape yield”), or did 90% of the initial reagents result exclusively in these materials with the interesting shape (“overall or absolute yield”)? Shape yield and overall yield are frequently two different numbers and should be reported in the main text, with additional details, such as the raw data and calculations, in the Supporting Information. 4. Surface Characterization. Many methods of colloidal synthesis use surfactants, capping agents, and other stabilizers, and even the solvent can play the role of the stabilizing agent. Therefore, papers that report that there are “no” capping agents when performing solution-based syntheses and imply that there is “nothing” on the surface are irritating and incorrect. Efforts should be made to provide a molecular-level picture of what adsorbates (ions, solvent, surfactants, etc.) are most likely to be adsorbed to nanoparticles in solution, considering that there may be a combination of surface-bound species. Again, XPS,
etallic nanoparticles have been used by humanity for centuries, with the classic examples from antiquity represented by the Roman Lycergus Cup,1 medieval stained glass windows,2 and Faraday’s work on colloidal gold in the 1870s.3 In the twentieth century, colloidal metal nanoparticles were then heavily used in the area of catalysis, long before the term “nanoparticle” was coined, an application that remains an extremely active area of research.4−8 In the last three decades, since the first description by Brus and co-workers at Bell Laboratories,9,10 semiconductor quantum dots have been the focus of a great deal of interest for optical and imaging applications11,12 and now for energy capture in photovoltaics, among others.13 With respect to the synthesis of colloidal inorganic nanoparticles, the first well-defined and now iconic preparations of silica particles by Stöber14 and gold nanoparticles by Turkevich15 paved the way for new routes that led to the development of a myriad of colloidal semiconductor quantum dots16 and size- and shape-controlled metallic nanoparticles.17,18 Gold nanoparticles are now being used for a variety of applications, including medical therapies,19 control of light (plasmonics),20 sensing (SERS),21 and many others. At present, thousands of papers are published every year that report new, expanded, or improved colloidal syntheses of increasing complex metallic, semiconducting, and oxide nanoparticles, with control over size, shape, composition, and surface chemistry. With increasing concerns regarding the reproducibility of new syntheses,22 reviewers and readers of papers that describe the synthesis of colloidal nanoparticles now have rising expectations of what characterization methods need to be used when new or modified materials are reported in the literature.23 While the number of possible combinations of materials prevents us from capturing every possible scenario, we do believe that the following represent the minima that should be included; additional spectroscopic data or experimental/ procedural information will almost certainly be required by different fields. There will be cases when a specific technique cannot be performed due to limited material, high reactivity, or other reasons, and in these situations authors should be clear as to why such data cannot be provided and what the resulting uncertainties are (with regards to, for instance, composition, size, etc.).
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BASIC REQUIREMENTS FOR REPORTING A NEW OR MODIFIED COLLOIDAL NANOMATERIAL 1. Thorough Compositional Analysis. Ensure that you determine what your material is made of. For example, gold nanoparticles on a titania surface should show clear energy dispersive X-ray analysis (EDX/EDAX/EDS) and/or X-ray photoelectron spectroscopy (XPS) evidence that what appear to be dark spots on a lighter background in a specific electron microscopy experiment really are Au and Ti. Bulk X-ray diffraction (XRD) data may help to substantiate claims regarding composition, but XRD is, of course, limited to © 2015 American Chemical Society
Published: July 28, 2015 4911
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(2) Brown, T. E.; LeMay, E. H.; Bursten, B. E.; Woodward, P.; Murphy, C. J. Chemistry: The Central Science, 12th ed.; Pearson College Div.: 2011; p 499. (3) Faraday, M. Experimental Relations of Gold (and Other Metals) to Light. Philos. Trans. R. Soc. 1857, 147, 145 DOI: 10.1098/ rstl.1857.0011. (4) Schauermann, S.; Nilius, N.; Shaikhutdinov, S.; Freund, H.-J. Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights. Acc. Chem. Res. 2013, 46, 1673−1681. (5) Li, G.; Jin, R. Atomically Precise Gold Nanoclusters as New Model Catalysts. Chem. Res. 2013, 46, 1749−1758. (6) Zhang, H.; Jin, M.; Lim, B.; Xia, Y. Shape-Controlled Synthesis of Pd Nanocrystals and Their Catalytic Applications. Acc. Chem. Res. 2013, 46, 1783−1794. (7) Mahmoud, M. A.; Narayanan, R.; El-Sayed, M. Enhancing Colloidal Metallic Nanocatalysis: Sharp Edges and Corners for Solid Nanoparticles and Cage Effect for Hollow Ones. Acc. Chem. Res. 2013, 46, 1795−1805. (8) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181− 190. (9) Rossetti, R.; Nakahara, S.; Brus, L. E. Quantum Size Effects in the Redox Potentials, Resonance Raman Spectra, and Electronic Spectra of CdS Crystallites in Aqueous Solutions. J. Chem. Phys. 1983, 79, 1086− 1088. (10) Rossetti, R.; Brus, L. Electron-Hole Recombination Emission as a Probe of Surface Chemistry in Aqueous CdS Colloids. J. Phys. Chem. 1982, 86, 4470. (11) Ma, G. Background-Free In vivo Time Domain Optical Molecular Imaging Using Colloidal Quantum Dots. ACS Appl. Mater. Interfaces 2013, 5, 2835−2844. (12) Bischof, T. S.; Correa, R. E.; Rosenberg, D.; Dauler, E. A.; Bawendi, M. G. Measurement of Emission Lifetime Dynamics and Biexciton Emission Quantum Yield of Individual InAs Colloidal Nanocrystals. Nano Lett. 2014, 14, 6787−6791. (13) Beard, M. C.; Luther, J. M.; Semonin, O. E.; Nozik, A. J. Third generation photovoltaics based on multiple exciton generation in quantum confined semiconductors. Acc. Chem. Res. 2013, 46, 1252− 1260. (14) Stö ber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (15) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55−75. (16) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; et al. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012−1057. (17) Lohse, S. E.; Murphy, C. J. The Quest for Shape Control: A History of Gold Nanorod Synthesis. Chem. Mater. 2013, 25, 1250− 1261. (18) Lohse, S. E.; Murphy, C. J. Applications of Colloidal Inorganic Nanomaterials: From Medicine to Energy. J. Am. Chem. Soc. 2012, 134, 15607−15620. (19) Murphy, C. J.; Gole, A. M.; Stone, J. W.; Sisco, P. N.; Alkilany, A. M.; Goldsmith, E. C.; Baxter, S. C. Gold Nanoparticles in Biology: Beyond Toxicity to Cellular Imaging. Acc. Chem. Res. 2008, 41, 1721− 1730. (20) Odom, T. W.; Schatz, G. C. Introduction to Plasmonics. Chem. Rev. 2011, 111, 3667−3668. (21) Casadio, F.; Leona, M.; Lombardi, J. R.; Van Duyne, R. Identification of Organic Colorants in Fibers, Paints, and Glazes by Surface Enhanced Raman Spectroscopy. Acc. Chem. Res. 2010, 43, 782−791. (22) Korgel, B. A.; Buriak, J. M. Chem. Mater. The Experimental Section: The Key to Longevity of Your Research. Chem. Mater. 2014, 26, 1765−1766.
EDX/DAX/EDS, vibrational spectroscopy (such as FTIR and Raman), and other techniques to characterize surface composition are extremely helpful. Zeta potential analysis to determine effective surface charge in polar solvents is qualitatively useful, but tightly bound counterions may be present and therefore lead to spurious results as to the “true” nature of the nanoparticle. To quantify ligands bound to a nanoparticle surface, however, XPS suffers from curvature effects that affect relative peak intensities,23 although there are now protocols for correcting this.25 In case of experimental difficulties in this regard, it may be possible to use “total − free = bound” data to quantify (i) the initial ligand concentration and (ii) the remaining ligand concentration by UV−visible spectroscopy, gas chromatography−mass spectrometry (GC/ MS), or similar, to arrive at an estimation of the quantity of bound ligand/nanoparticle. Fluorescent ligands may have altered quantum yields or lifetimes when bound on a surface, and therefore, simple measurements of relative quantum yields on nanoparticles compared to that of the free ligands may be misleading. 5. Nanoparticle Concentration. Separation of unreacted reagents from a solution or suspension of prepared nanoparticles can be challenging and is best done by some combination of centrifugation, dialysis, or tangential-flow filtration, depending on the system. Digestion of the inorganic components of the purified nanoparticles and analysis of inorganic content by ICP-OES or ICP-MS can then lead to absolute concentrations that be converted into extinction coefficients or other more ready measures of nanoparticle concentration in solution. The more monodisperse a nanoparticle sample is, the more accurate such measurements will be. Concentration units vary in the literature; it is best to use molarity or number of nanoparticles per volume, as opposed to mg/mL, ppb, or similar measures to make comparisons to molecular data. 6. Dopant Concentration. Early papers on doped quantum dots frequently assumed that the quantity of dopants that went into the synthesis pot was equal to the concentration of dopants that ended up in the nanoparticle products. This assumption must be experimentally verified, either by calibrated optical methods or by digestive ICP-OES or ICP-MS, depending on the system. To reiterate, these six items cannot fully encapsulate every possible scenario of nanomaterial that will be prepared in the future. By requesting that authors address these issues, however, the community will benefit from the reporting of more reliable and reproducible syntheses, which is critical for the long-term health of materials science and nanoscience.
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Catherine J. Murphy, Deputy Editor, Journal of Physical Chemistry C Jillian M. Buriak, Editor-in-Chief, Chemistry of Materials
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) http://www.britishmuseum.org/explore/highlights/highlight_ objects/pe_mla/t/the_lycurgus_cup.aspx. 4912
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(23) Baer, D. R.; Gaspar, D. J.; Nachimuthu, P.; Techane, S. D.; Castner, D. G. Application of Surface Chemical Analysis Tools for Characterization of Nanoparticles. Anal. Bioanal. Chem. 2010, 396, 983−1002. (24) Baalousha, M.; Lead, J. R. Nanoparticle Dispersity in Toxicology. Nat. Nanotechnol. 2013, 8, 308−309. (25) Torelli, M. D.; Putans, R. A.; Tan, Y.; Lohse, S. E.; Murphy, C. J.; Hamers, R. J. Quantitative Determination of Ligand Densities on Nanomaterials by X-Ray Photoelectron Spectroscopy. ACS Appl. Mater. Interfaces 2015, 7, 1720−1725.
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DOI: 10.1021/acs.chemmater.5b02323 Chem. Mater. 2015, 27, 4911−4913
